Acousto-optical imaging is an emerging biodiagnostic technique which provides an optical spectroscopic signature and a spatial localization of an optically absorbing target embedded in a strongly scattering medium. The transverse resolution of the technique is determined by the lateral extent of ultrasound beam focal zone while the axial resolution is obtained by using short ultrasound pulses. Although very promising for medical diagnostic, the practical application of this technique is presently limited by its poor sensitivity. Moreover, any method to enhance the signal-to-noise ratio must obviously satisfy the in vivo safety limits regarding the acceptable power level of both the ultrasonic pressure wave and the laser beam. In this paper, we propose to improve the sensitivity by using a pulsed single-frequency laser source to raise the optical peak power applied to the scattering medium and to collect more ultrasonically tagged photons. Such a laser source also allows illuminating the tissues mainly during the transit time of the ultrasonic wave to maintain the average optical power below the maximum permissible exposure. In our experiment, a single-frequency Nd:YAG laser emitting 500-μs pulses with a peak power superior to 100 W was used. Photons were tagged in few-cm thick optical phantoms with tone bursts generated by an ultrasonic transducer. Tagged photons were detected with a GaAs photorefractive interferometer characterized by a large optical etendue to process simultaneously a large number of speckle grains. When pumped by high intensity laser pulses, such an interferometer also provides the fast response time essential to obtain an apparatus insensitive to the speckle decorrelation due to mechanical vibrations or tissues movements. The use of a powerful long pulse laser appears promising to enhance the signal level in ultrasound modulated optical imaging. When combined with a photorefractive interferometer of large optical etendue, such a source could allow obtaining both the sensitivity and the fast response time necessary for biodiagnostic applications.

We present the first experimental study of self-forming synthetic lipids, trademarked as QuSomes^TM, using Raman spectroscopy in the spectral range of 500 to 3100 cm^-1. Raman spectra of these new artificial lipids composed of 1,2- dimyristoyl-rac-glycerol-3-dodecaethylene glycol (GDM-12) and 1,2-dioleoyl-rac-glycerol-3-dodecaethylene glycol (GDO-12) have been obtained in pure form and in aqueous suspensions with Phosphate Buffered Saline (PBS) by using an inverted confocal laser-tweezers-Raman-microscopy system. This spectrometer works with an 80 mW diode-pumped solid-state laser, operating at a wavelength of 785 nm in the TEM₀₀ mode. The laser is used both for optical trapping and Raman excitation. The two amphiphiles considered in this study, differ in their hydrophobic chain length and contain similar units of hydrophilic polyethylene glycol (PEG) head groups. Such synthetic PEG coated lipids exist in liquid form at room temperature and spontaneously form liposomes (nano type vesicles) upon hydration. In this work, we have focused on the band assignments for the spectra of single QuSomes^TM nano particles in pure form and in aqueous media acquired by means of Raman spectroscopy. In particular, we have found that the most prominent peaks in the studied spectral region are dominated by vibrational modes arising from C-C and C-H bonds. Furthermore, we have noticed that some of the distinct peaks observed below 1800 cm^-1 in pure sample are preserved in aqueous environment. These retained intense bands are located at 1449, 1128, 1079, and 1065 cm^-1. This effect might be due to the strong chain-chain interactions, because the chains have to orient themselves and become tightly packed in the vesicles wall rather than adopt random orientations in bulk. This technique has proven to be an excellent tool to establish the fingerprint region revealing the molecular structure and conformation of QuSomes^TM particles. The Raman spectroscopic data of these novel lipids and its vesicles formed in suspensions confirm high stability and are therefore considered as potential candidate for varieties of future applications including lipid based novel substances and drug delivery systems.

The study of phase transitions in lipids is important to understand various phenomena such as conformational order, trans-membrane diffusion, vesicle formation and fusion as well as drug-and protein-membrane interactions. Several techniques, including Raman spectroscopy, have previously been employed to investigate the phase behaviour of lipids. In this work, temperature-controlled Raman microscopy has been used to detect and analyze the phase transitions in two newly developed synthetic PEGylated lipids trademarked as QuSomes^TM and its nanovesicles in phosphate buffered saline (PBS) suspension. The amphiphiles considered in this study differ in their hydrophobic chain length and contain different units of polyethylene glycol (PEG) hydrophilic head groups. Raman spectra of these new artificial lipids and its nanovesicles have been recorded in the spectral range of 500-3100 cm^-1 by using a temperature-controlled sample holder attached to a Raman microscope. The gel to liquid-crystalline phase transitions of the sample lipids, composed of pure 1,2-dimyristoyl-rac-glycerol-3-dodecaethylene glycol (GDM-12) and 1,2-distearoyl-rac-glycerol-3-triicosaethylene glycol (GDS-23), have been detected by examining the changes in Raman spectra of the lipids caused by temperature variation. In the liquid phase both of the studied lipids spontaneously form liposomes (nanovesicles) upon hydration. In this study, we have demonstrated the efficacy of the temperature-controlled Raman microscope system to reveal the main phase transition temperature (T_m) profiles of our sample lipids and its nanovesicles in PBS suspension. The phase changes are detected by plotting peak intensity ratios in the C-H stretching region (~I₂₉₃₅/I₂₈₈₃) versus temperature. These ratios correlate with lateral or inter-chain interactions as well as intra-molecular interactions. In particular, we have found that phase transitions occur at a temperature of approximately 5.2°C and 21.2°C for pure GDM-12 and GDS-23, respectively. However, the phase transition temperature becomes significantly higher for lipid nanovesicles formed in aqueous suspensions. Such information about these PEG coated lipids might find applications in various studies including the development of lipid based novel substances and drug delivery systems.

Contact lenses manufacturers and ophthalmologists who perform laser surgery to correct visual problems depend on an accurate model of the anterior corneal surface. Several models have been suggested in the past, going from Gullstrand's initial idea to the aspheric profiles and to Bonnet's profile based on anatomical data. Clinical evidence shows, however, that the anterior corneal surface is characterized by a variable eccentricity, contrary to the hypothesis on which the current models are based. We present, in this work, a new model for the anterior corneal surface in terms of higher-order aspheric surfaces in which the eccentricity at a given point of the cornea is a continuous function of its distance to the optical axis. We also establish the conditions under which the different conic and Bonnet's profiles are recovered from our model. Finally, we present our preliminary results using this model.

We discuss the fabrication of optical phantoms that will serve as calibration and test standards for a diffuse optical tomographic system developed in our laboratory. For the fabrication of a phantom's matrix, two materials are compared: polyester and epoxy resins. We investigate the capacity to use either of these two resins in the fabrication of a phantom's matrix with different shapes and thicknesses. For the absorbing agent we use India ink and for diffusing agent we choose a fine titanium dioxide (TiO₂) powder. We test a collimated light measurements set-up to extract: the absorption coefficient μ_a of a purely absorbing medium, and the scattering coefficient μ_s of a purely diffusing medium. We also compare the way μ_a and μ_s add in a solution of absorbing and diffusing media.

We introduce an improved approach in the 3D localization of discrete fluorescent inclusions in a thick scattering medium. Previously our approach provided accurate localization of a single inclusion, showing the potential for direct time-of-flight fluorescence diffuse optical tomography. Here, we localize various combinations of multiple fluorescent inclusions. We resort to time-domain (TD) detection of emitted fluorescence pulses after short pulse laser excitation. Our approach relies on a signal processing technique, dubbed numerical constant fraction discrimination (NCFD), for extracting in a stable manner the arrival time of early photons emitted by one or many fluorescent inclusions from measured time-of-flight (TOF) distributions. Our experimental set-up allows multi-view tomographic optical TD measurements over 360 degrees without contact with the medium. It uses an ultra-short pulse laser and ultra-fast time-correlated single photon counting (TCSPC) detection. Fluorescence time point-spread functions (FTPSFs) are acquired all around the phantom after laser excitation. From measured FTPSFs, the arrival time of a fluorescent wavefront at a detector position is extracted with our NCFD technique. Indocyanine green (ICG; absorption peak = 780nm, emission peak = 830nm) is used for the inclusions. Various experiments were conducted with this set-up in a stepwise fashion. First, single inclusion experiments are presented to provide background information. Second, we present results using two inclusions in a plane. Then, we move on with two inclusions located in different planes. Finally, we show results with a plurality of inclusions (>2) distributed at arbitrary positions in the medium. Using an algorithm we have developed and tested on the acquired data, we successfully achieve to locate the inclusions. Here, results are obtained for discrete inclusions. In a close future, we expect to extend our method to continuous fluorescence distributions.

Fluorescence optical diffuse tomography (fDOT) is of much interest in molecular imaging to retrieve information from fluorescence signals emitted from specifically targeted bioprocesses deep within living tissues. An exciting application of fDOT is in the growing field of tissue engineering, where 3D non-invasive imaging techniques are required to ultimately grow 3D engineered tissues. Via appropriate labelling strategies and fluorescent probes, fDOT has the potential to monitor culture environment and cells viability non-destructively directly within the bioreactor environment where tissues are to be grown. Our ultimate objective is to image the formation of blood vessels in bioreactor conditions. Herein, we use a non-contact setup for small animal fDOT imaging designed for 3D light collection around the sample. We previously presented a time of flight approach using a numerical constant fraction discrimination technique to assign an early photons arrival time to every fluorescence time point-spread function collected around the sample. Towards bioreactor in-situ imaging, we have shown the capability of our approach to localize a fluorophore-filled 500 μm capillary immersed coaxially in a cylindrically shaped bioreactor phantom containing an absorbing/scattering medium representative of experiments on real tissue cultures. Here, we go one step further, and present results for the 3D localization of thinner indocyanine green labelled capillaries (250 μm and 360 μm inner diameter) immersed in the same phantom conditions and geometry but with different spatial configurations (10° and 30° capillary inclination).

Quadrature interferometry based on 3×3 fiber couplers could be used to double the effective imaging depth in Swept- Source Optical Coherence Tomography. This is due to its ability to suppress the complex conjugate artifact naturally. We present theoretical and experimental results for a 3×3 Mach-Zehnder interferometer using a new unbalanced differential optical detection method. The new interferometer provides simultaneous access to complementary phase components of the complex interferometric signal. No calculations by trigonometric relationships are needed. We demonstrate a complex conjugate artifact suppression of 27 dB obtained in a swept-source optical coherence tomography using our unbalanced differential detection. We show that our unbalanced differential detection has increased the signal to- noise ratio by at least 4 dB compared to the commonly used balanced detection technique. This is due to better utilization of optical power.

Quality and parameters of probing optical beams are extremely important in optical coherence tomography (OCT) systems both for image quality and light coupling efficiency considerations. For example, the shape, size, focal position, and focal range of such beams could have a great impact on the lateral resolution, penetration depth, and signal-to-noise ratio of the image. We present a design, construction and characterization of different variations of GRIN and ball fiber lenses, for ultra-small OCT probes. Those fiber probes are made of a single mode fiber and a GRIN or ball fiber lens with or without a fiber spacer between them. The probe diameters are smaller than 0.3 mm. We discuss design methods, fabrication techniques, and measured performance compared with modeling results. We demonstrate swept-source OCT images with different fiber probes.

High-resolution optical low-coherence reflectometry is applied to monitor biological samples. It has been found that the reflectivity of aged cow's milk is significantly lower than that of the fresh milk with a difference of 5.35dB. During the process of heating the fresh milk at a constant temperature of 80°C, the reflectivity of the milk gradually decreases with the increase of the heating duration. The technique is proved to be effective in monitoring the change in the refractive index of the sample.

Adaptive Optics (AO) technology has been used in confocal scanning laser ophthalmoscopes (CSLO) which are analogous to confocal scanning laser microscopes (CSLM) with advantages of real-time imaging, increased image contrast, a resistance to image degradation by scattered light, and improved optical sectioning. With AO, the instrumenteye system can have low enough aberrations for the optical quality to be limited primarily by diffraction. Diffraction-limited, high resolution imaging would be beneficial in the understanding and early detection of eye diseases such as diabetic retinopathy. However, to maintain diffraction-limited imaging, sufficient pixel sampling over the field of view is required, resulting in the need for increased data acquisition rates for larger fields. Imaging over smaller fields may be a disadvantage with clinical subjects because of fixation instability and the need to examine larger areas of the retina. Reduction in field size also reduces the amount of light sampled per pixel, increasing photon noise. For these reasons, we considered an instrument design with a larger field of view. When choosing scanners to be used in an AOCSLO, the ideal frame rate should be above the flicker fusion rate for the human observer and would also allow user control of targets projected onto the retina. In our AOCSLO design, we have studied the tradeoffs between field size, frame rate and factors affecting resolution. We will outline optical approaches to overcome some of these tradeoffs and still allow detection of the earliest changes in the fundus in diabetic retinopathy.

Nondiffracting or Bessel beams find applications in diverse fields like optical tweezers/spanners, microscopy, super-resolution and optical coherence tomography. An axicon, energywise, is the most efficient method for generating a diffraction-free beam. Yet one of the impediments to wide use of these optical elements is the cost related to the way they are manufactured. Recently we proposed a novel optical element - Fresnel axicon (Opt. Lett. 31, 1890, 2006) - which enables to overcome this problem while providing advantages such as compactness and very low bulk absorption. Prototypes of the Fresnel axicon were manufactured. In the present work we report the first experimental results characterising the optical properties of this novel device, which bridges the gap between diffractive axicons and their refractive counterparts.

Optical coherence tomography (OCT) is a non-invasive imaging technique invented in 1991 and allowing the observation of biological tissues with millimeter depth of penetration and a few micrometer resolution. In the standard time-domain OCT setup (TD-OCT), a broadband light source is used with a Michelson interferometer where one of the mirrors is replaced by the sample (which is mechanically moved transversally during data acquisition) while the other is axially vibrating. By analyzing the temporal signal at the exit of the interferometer, a high resolution tomographic cut of the sample can be obtained. A number of new OCT setups have been proposed since 1991 in order to improve the data acquisition speed. In particular, Fourier-domain OCT (FD-OCT) has allowed in vivo observation of samples by eliminating the necessity of the axial motion of the reference mirror in the setup. We propose in this paper new OCT setups having the same potential without requiring numerical treatment of the signal (as it is the case in FD-OCT). Because those setups are such that the axial information of the sample becomes linearly distributed at different points of space in an interference pattern, we call them spatial-domain OCT setups (SD-OCT). SD-OCT setups use a tilted mirror in a Michelson interferometer to produce an interference pattern which is imaged on a CCD detector. The pattern contains all the information on the sample and is obtained without mechanical motion or numerical treatment of the recorded signal. In order to validate the proposed scheme, prototypes of the setups have been made in the laboratories of COPL at Laval University; biological samples such as onion peels and phloem of trees have been tested in order to produce their tomographic images. Comparisons of some of our results with those from a commercial setup with the same samples had notably confirmed the capacity of ours prototypes to effectively image biological samples.

Numerous types of bacteria swim in their environment by rotating long helical filaments. At the base of each filament is a tiny rotary motor called the bacterial flagellar motor. A lot is already known about the structure, assembly and function of this splendid molecular machine of nanoscopic dimensions. Nevertheless many fundamental questions remain open and the study of the flagellar motor is a very exciting area of current research. We are developing an in vitro assay to enable studies of the bacterial flagellar motor in precisely controlled conditions and to gain direct access to the inner components of the motor. We partly squeeze a filamentous E. coli bacterium inside a micropipette, leaving a working flagellar motor outside. We then punch a hole through the cell wall at the end of the bacterium located inside the micropipette using a brief train of ultrashort (~60 fs) laser pulses. This enables us to control the rotation of the motor with an external voltage (for at least 15 minutes). In parallel, new methods to monitor the speed of rotation of the motor in the low load (high speed) regime are being developed using various nanoparticules.

Most of the time, arterial stenoses caused by atherosclerosis, hardening of the artery walls, or buildup of fatty deposits prevent the blood from flowing normally. Blood flow characteristics in the vicinity of a stenosis are therefore very important since the restriction may accelerate fatty deposits and thus quickly clog the artery. Doppler Optical Coherence Tomography (DOCT) is a biomedical technique that allows simultaneous structural imaging and flow monitoring inside biological tissues and materials with spatial resolution at least one order of magnitude better than ultrasound. This study deals with the application of a Near Infrared DOCT system for imaging and monitoring of liquid flow inside a stenosis phantom inserted in a glass tube. For the measurement of the Doppler frequency, we use a numerical method based on the detection of the zero-crossing points of the OCT signal.

The fabrication and experimental testing of polydimethylsiloxane (PDMS) optofluidic biochips with integrated solidcore polyepoxyacrylate (PEA). optical waveguides is described. The biochips were replicated from silicon masters made by 50-μm-deep reactive ion etching. Each biochip contained three 70-μm-wide solid-core waveguides focused on the middle section of a 50-μm-wide, Y-shaped, microchannel. Electroosmotic flow with a voltage range of 50 to 300 volts was used to drive fluorescent beads in the microfluidic channel. Using two excitation lasers at 532nm and 635nm with distinct modulation frequencies, two species of microparticles with different fluorophores were identified by capturing the fluorescence in a photomultiplier tube and analyzing the signal with a windowed Fourier transform analysis technique.

We present two optical systems that can transform a Gaussian beam into the lowest-order transverse magnetic laser beam (the so-called TM₀₁ laser beam). When a TM₀₁ laser beam is focused by an objective with a high numerical aperture (NA > 0.9), theory predicts the appearance of a bright central feature at focus; the width of this feature is smaller than that of the focal spot obtained with a Gaussian beam [1,2]. Our goal is to improve the lateral resolution of two-photon microscopy when a TM₀₁ laser beam is used.

In this paper, we present an approach toward the creation of coronary artery phantoms for optical coherence tomography (OCT). By mixing alumina powder in a matrix of transparent silicone, it is expected that the amplitude of the OCT signal and the soft tissue elasticity can be reproduced. The fabrication process to produce such multiple layer phantoms is presented along with optical and characterization experiments.

The aim of this paper is to improve the functionality and efficiency of a microfluidic device by optically simulating all of the components of the device and then identifying and optimizing the areas of the device where the optics and the microfluidic components overlap. Design of the device will incorporate current advanced methods, such as a state-of-the-art one-shot manufacturing processand noise reduction techniques utilizing 8 parallel waveguides. A suitable material to build this device is the polymer SU-8, which has a refractive index of 1.56 while borofloat glass, refractive index of 1.47, is used as a substrate, and an optical adhesive (Norland Optical Adhesive 74, index of 1.52) is used to seal the device (and serve as the waveguide cladding) by filling the gap in between waveguides reducing scattering losses and confining higher modes. As such, simulations take into account all these parameters. Design of a photomask will take into account three main sources of loss due to the integration of optics and microfluidics: wall thickness, channel thickness, and input angle. Simulations have yielded behaviours and values for these parameters. Wall thickness should be limited to 200um thick which will yield a -5.54dB attenuation (- 2.77dB at the particle) on the input (due to high angular propagation of higher order modes). Input angle of the waveguides (which is crucial to the elimination of signal noise on the output) has been found to reduce output signal noise to -9.60dB at an input angle of 74 degrees to the channel.

Laser beam shaping is an important subject in industrial and medical applications of lasers since different applications may require different laser intensity distributions. Recently we demonstrated successfully an all-fiber laser beam shaping device that could transform a Gaussian shaped laser beam into a uniform or ring-shaped beam in 1.5 μm wavelength region. In this paper we present the work of the beam shaping in 1.0 μm wavelength region to make it compatible to Yb-doped high power fiber laser. The device uses a long-period grating to couple a portion of core-mode, LP₀₁ into a low-order m^th-radial cladding mode LP_0m. Interference of the two modes could reduce field at the centre and enhance the field in the first or second ring of the cladding mode to transform the Gaussian-shaped laser beam to an intensity uniform beam. The design parameters that affect the beam shaping will be discussed and the results of the interference from two cladding modes, LP₀₃ and LP₀₄ will be presented.

The emergence of new fibers families induces considerable requirements in terms of characterization and metrology such as group delay, chromatic dispersion, birefringence, bending losses, etc. Unlike classical characterization techniques such as the well-known phase shift method, optical low-coherence reflectometry (OLCR) technique requires only short fiber samples (i.e. <50cm). Characterization results concerning different types of specialty fibers including erbium-doped fibers, few-modes fibers, photonic crystal fibers will be presented. Unique dispersive properties of higher-order mode fibers offer novel solutions for dispersion compensation and nonlinear effects management. OLCR can allow each LP mode characterization without the requirement of mode converters. A new method, called "Time-wavelength mapping," based on the process of the OLCR interferogram is demonstrated as a versatile method to determine chromatic dispersion of each guided LP mode whatever their group index. Different characterization results concerning photonic crystal fibers with guiding based on to the conventional total internal reflection principle - high index guiding - or photonic bandgap effect - low index guiding - will be presented. Finally, we show that the versatility and deep physical insight of OLCR technique can play a key role in the study of photonic crystal waveguides in terms of structural disorder, losses, group delay in highly dispersive region and emphasizes the unique role of this technique in the understanding of their properties.

We experimentally demonstrate the operation of a laser based on self-phase modulation followed by offset spectral filtering. This laser has three operation modes: a continuous-wave mode, a self-pulsating mode where the laser self ignites and produces pulses, and a pulse-buffering mode where no new pulse is formed from spontaneous emission noise but only pulses already propagating or pulses injected in the laser cavity can be sustained. In the self-pulsating and pulse-buffering modes, the laser is multi-wavelength and continuously tunable over the entire gain band of the amplifiers. The output pulse width is quasi transform-limited with respect to the spectral-width of the filters used in the cavity. Overall, this device provides a simple alternative to pulsed laser source and also represents a promising approach for signal buffering.

The increasing demand for broadband mobile communications has generated interest in exploring new frequency bands and modifying network structures. In such systems, photonic technologies can bring both cost reduction as well as an increase in performance, mainly due to the low-loss properties of optical fibers. An optical source capable of producing tunable, high-quality microwave/mm-wave signals would be of great interest not only in such communications systems, but in fiber sensors and numerous other applications as well. One potentially cost-effective method to fabricate such a system is via optical heterodyning. In this approach, the difficulties in generating a high-quality signal are two-fold. The first issue is in maintaining a specific frequency difference (i.e. microwave signal) between the lasers for an extended period of time. The second is in narrowing the inherent linewidth of the laser from the MHz values typically produced by conventional semiconductor lasers, down to values practical for a communications system. Both of the above requirements are facilitated by the newly developed doped-fiber, external cavity laser (DFECL), which offers relatively stable single-longitudinal-mode operation in addition to narrow linewidth operation. This paper will demonstrate frequency locking of a DFECL using a delay-line discriminator. The RF linewidth, initially 10-15MHz, is reduced to levels conducive to optical PLL locking. Optical power levels are approximately -3 dBm and unamplified microwave power output levels are typically -35 dBm, depending on photodetector responsivity. Carrier-to-noise ratios are generally 40-45 dB. The physical mechanisms underlying the observed laser dynamics are discussed, including laser-to-fiber alignment and thermal fluctuations.

We present a fiber long period grating based sensor for biological applications. Inscribed by using point-by-point method, the LPG has a resonant wavelength close to the emission wavelength of biological objects of interest. The part of the light, emitted from sensing area, is collected by both fiber core and cladding, while the majority of light is coupled into the cladding. Back coupling of light of certain wavelength can selectively be achieved from fiber's cladding to core thanks to the LPG. Such selective back coupling allows the increase of efficiency of remote detection of biological objects of interest, using excitation source and detector on the same side of the fiber.

We present a novel microstructured optical fiber (MOF) structure which consists of a 7-missing-holes core surrounded by a cladding formed by an Archimedean-like lattice of air holes, as opposed to the standard MOF cladding structure consisting of a triangular lattice. We demonstrate that this new cladding geometry can be achieved through the standard stack-and-draw MOF fabrication method, the main difference being the fact that the circular capillaries commonly used in conventional MOF are replaced by hexagonal canes comprising 7 air holes disposed in a centred hexagon (forming the so-called A7 unit cells). The Archimedean-like lattice MOF (ALMOF) is particularly interesting by the fact that it features a dodecagonal core, which is a shape much closer to a perfect circle than the hexagonally-shaped core characteristic of the conventional 7-missing-holes triangular-lattice MOF. As a result, we show that the improved core circularity of the ALMOF design leads to a significantly more circular fundamental mode profile, which can prove to be quite interesting for specific applications where the circularity of the mode profile is critical, such as precision laser micromachining for example.

The authors report a medium-power widely tunable single wavelength fiber laser. The laser wavelength was tuned from 1530 nm to 1575 nm. The gain medium consisted of a double-clad erbium-ytterbium codoped fiber. An un-pumped elliptical core erbium-doped fiber was used as a saturable absorber inside the cavity to reduce the laser linewidth and mode hopping. The output power of the laser at each lasing wavelength was more than 100 mW. The linewidth of the laser was 8 MHz, measured using a scanning Fabry-Perot spectrum analyzer of resolution 6.5 MHz. The laser was stable at room temperature with an intensity fluctuation of less than 0.2 dB.

A mechanism of Kerr-lens mode locking in fiber laser is proposed and analyzed. It could be realized in active photonic-crystal fibers with finite number of the air holes hexagonally arranged around the core with refractive index larger than refractive index of the glass matrix and relies at decrease of the mode confinement loss for higher intensity and mode transformation from leaky to guided when refractive index in the core increases owing to Kerr self-focusing.

The interaction of a fiber Bragg grating point like soliton with local defects has been thoroughly analyzed. The stability of trapped solitons has also been discussed. The main finding reveals the reversal of the sign of interaction from attraction to repulsion. It is also observed that the attraction depends on the accuracy of the numerical simulations.

Two dimensional multistage AOCA system based on generalized equations has been analyzed. Numerical results for Bragg angle as a function of acoustic frequency, deflection angles for different wavelengths, normalized power and diffraction efficiency with and without phase shift between consecutive arrays having different channel spacing based on generalized equations have been obtained.

We present results on a high-power, high-energy cladding-pumped fibre Raman laser. We first discuss fibre requirements and give a design rule. Then, experimentally, a pulsed cladding-pumped Raman fibre laser is demonstrated with up to 210 μJ high-brightness pulses. The fibre delivers pulses at the Stokes wavelength of 1112 nm, of 500 ns duration and 420 W peak power, with M² = 1.8. The linewidth is 6.5 nm without any wavelength selection. Our result is the highest reported energy and peak power from any fibre Raman source and illustrates the power scalability capability of such devices.

We report on a numerical investigation of the effect of self-steepening on the dynamics of a passively modelocked fiber laser containing a long period fiber grating. The numerical model is based on the normalized complex Ginzburg-Landau equation and the nonlinear coupled mode equations of the grating. The nonlinear dynamics of the laser are observed through plotting the pulse energy against the linearly increasing gain so obtaining bifurcation diagrams. The inclusion of self-steepening was found to result in a temporal walk-off with no significant pulse width or energy alternations, while exhibiting different regions of period doubling bifurcation.

Master oscillator power amplifier (MOPA) is becoming the obvious choice in order to develop some high power single frequency laser sources. Its simplicity, reliability, robustness have already allowed the demonstration of some tremendous increase of the output power. In this paper we will report our latest results in the development of high power single frequency, single mode and single polarization MOPA systems. We were able to obtain an output power as higher as 500 W with still keeping the narrow linewidth proprieties of the source.

A novel theoretical scheme for high-power Er³⁺ doped fiber amplifier assisted with a long period grating (LPG) is presented. This device consists of an Er³⁺ doped cladding pumped with a high power laser at a wavelength 1480 nm and un-doped core. The LPG imprinted into the fiber core at the beginning of the Er³⁺ doped region transfers a weak signal entering the amplifier in the core-mode of a single mode fiber into a cladding mode, dramatically increasing the effective mode-area of the signal and the threshold powers for unwanted nonlinear effects such as stimulated Raman and Brillouin scattering. The output fiber with the large core provides a high quality output beam.

The use of materials with a high thermo-optic coefficient would lead to significant improvements in energy consumption and thermal management in optical switching and sensing devices. Most liquids rank among materials having the highest thermo-optic coefficients, along with polymers and silicon. We have developed technology to directly incorporate liquids in integrated silica-on-silicon photonic device structures. Using this technology, we demonstrate experimentally integrated Mach-Zehnder interferometers (MZIs) comprising a liquid-core waveguide in one of the interferometer arms. Because of the large differential between the thermo-optic coefficients of silica and the liquid medium, the output of this device can be modulated through the thermal control of the device chip. A high contrast ratio (more than 20 dB) in the interferometer output modulation is obtained, demonstrating that the optical loss is well balanced between the two interferometer paths. The temperature variation required to fully cycle the output state is less than 0.5 degrees Celsius. Designs for low power thermo-optic switches based on these MZI structures with integrated heating electrodes are presented. The inclusion of a second liquid-core waveguide in the "passive" interferometer arm can enable athermal and polarization insensitive devices.

We present the cost impact of establishing new links in mesh optical networks. The analysis is based on an analytical study for quickly calculate the transmission and bandwidth management systems costs. Results show that each network has a mean nodal degree that minimizes the total cost.

In this work, we report continuous laser emission at 1064 nm from channel waveguides fabricated by carbon implantation on a Nd:YVO₄ crystal. A quasi single-mode intensity profile was observed, which was caused by defects in the cross-sectional geometry of the waveguides. An analysis of the main laser emission parameters is presented using different output couplers in the laser cavity and the waveguide losses were calculated from these parameters. The laser output shows high stability operating at room temperature, confirming the excellent optical properties of the yttrium vanadate host.

We present a waveguide with in-core Bragg grating and electro-optic liquid crystal cladding. The electric field induced reorientation of liquid crystal allows the reflection of guided light of desired wavelengths and polarizations at desired spatial positions.

This paper investigates the use of plasmonic optical waveguides in superconductive traveling-wave photodetectors (STWPDs) as a promising technique to efficiently couple the input optical field into the superconducting detecting structures. Field analysis is employed to study the propagation of the light through the integrated device and the coupling of optical power from the plasmonic waveguide to the superconducting film as a function of physical dimensions of the guide. A sample plasmonic waveguide, based on the LaAlO₃-YBCO-Au multilayer, will be discussed in detail and important design rules are addressed.

The Four Wave-Mixing (FWM) effect in a bulk Semiconductor Optical Amplifier (SOA) is widely used in all-optical communication networks for many applications such as wavelength conversion and fast optical switching. To best achieve such optical functions and bring them into practical stage, we present in this paper some performance criteria dealing with FWM phenomenon in both peaked and broad bandwidth gain SOA. The SOA with a large gain profile has given better performance by increasing the conversion efficiency and signal to background Ratio.

Photovoltaic solar cells are a route towards local, environmentally benign, sustainable and affordable energy solutions. Antireflection coatings are necessary to input a high percentage of available light for photovoltaic conversion, and therefore have been widely exploited for silicon solar cells. Multi-junction III-V semiconductor solar cells have achieved the highest efficiencies of any photovoltaic technology, yielding up to 40% in the laboratory and 37% in commercial devices under varying levels of concentrated light. These devices benefit from a wide absorption spectrum (300- 1800 nm), but this also introduces significant challenges for antireflection coating design. Each sub-cell junction is electrically connected in series, limiting the overall device photocurrent by the lowest current-producing junction. Therefore, antireflection coating optimization must maximize the current from the limiting sub-cells at the expense of the others. Solar concentration, necessary for economical terrestrial deployment of multi-junction solar cells, introduces an angular-dependent irradiance spectrum. Antireflection coatings are optimized for both direct normal incidence in air and angular incidence in an Opel Mk-I concentrator, resulting in as little as 1-2% loss in photocurrent as compared to an ideal zero-reflectance solar cell, showing a similar performance to antireflection coatings on silicon solar cells. A transparent conductive oxide layer has also been considered to replace the metallic-grid front electrode and for inclusion as part of a multi-layer antireflection coating. Optimization of the solar cell, antireflection coating, and concentrator system should be considered simultaneously to enable overall optimal device performance.

We report on the progress of our real-time entanglement based free-space quantum key distribution (QKD) system which uses polarization entangled photon pairs sent over a variety of free-space optical telescope links to distribute the key. An experiment with one photon from each pair sent over a 1,325 m long free-space link and the other photon detected locally next to the source is described. The system performs the complete QKD protocol including all error correction and privacy amplification algorithms. Over the course of 6.5 hours of communication at night an average raw key rate of 1,398 bits/s with an average quantum bit error rate (QBER) of 4.58% was observed producing an average final key rate of 244 bits/s. We also performed a Bell inequality experiment over two free-space links of 435 m and 1,325 m respectively, producing a total separation of 1,575 m between the two detectors in the experiment. During the Bell inequality experiment we observed an average Bell parameter of 2.51 ± 0.11.

The reliability and the expected lifetime of optical fibers used in telecommunication technologies are closely related to the chemical environment action on the silica network. To ensure the long-term mechanical strength of the optical fibers, a polymer coating was applied onto the fiber surface during fiber fabrication. This external coating is vital to ensure a long fiber lifetime. Its protective action includes several functions, such as to protect glass fiber from any external damage, to limit chemical attack, in particular that of water, and finally to ensure fatigue protection and bending insensitivity. Since the mechanical strength of the fiber is controlled by its surface characteristics, we propose a new method for increasing fiber strength. Submitted to a mechanical stretching, fibers were plunged into hot water and aged for several days. Then, the fibers were removed from the water and various weights were suspended on the fiber ends. Just before the fiber rupture, the fibers were unloaded and subjected to dynamic tensile tests at different velocities. Result analysis proved that the aging in hot water increased the fiber strength. The Weibull's diagram study shows a bimodal dispersion of defects on the fiber surface and the important role of polymer coating.

Thermal poling is an efficient way to induce optical second-order nonlinearity in different types of glasses, which typically have macroscopic inversion symmetry. In this paper we present a study on the current dynamics during thermal poling of glasses and relate these results to the formation dynamics of the depletion region, which is closely linked to the induced optical second order nonlinearity. Based on a simple theoretical viewpoint, supported by experimental results, we propose that thermal poling of glasses, and space-charge formation in dielectrics in general, can be viewed as an ionic RC circuit. This, to some extent modified view on thermal poling in glasses, opens up new opportunities to study and control the depletion layer dynamics subsequently leading to better control of the thermal poling induced optical second order nonlinearity in glasses.

Bulk-heterojunction photovoltaic devices consisting of poly (3-hexylthiophene) (P3HT) as a donor and [6,6]-phenyl- C61-butyric acid methyl ester (PCBM) as an acceptor are investigated in this paper. To achieve an efficient photo-induced charge transfer, the following aspects have been studied: (1) Selection of suitable solvent to obtain good morphology of the films and optimal absorption spectrum; (2) Determination of the donor/acceptor composition ratio that yields good film interface and high photon absorption; (3) Thermal annealing process to enhance the photon absorption, improve the short circuit current and the filling factor, and therefore the efficiency of the devices.

Interest in porous silicon (PS) has increased dramatically over the past two decades due to aspects such as photoluminescence due to quantum confinement, large surface area, and micro/nanoscale architecture. In this work, <111> p-type silicon wafers have been electrochemically etched with ethanolic solutions of hydrofluoric acid. Discrete surface domains showing luminescence were observed. The domains were typically many tens of micrometers in size and had a height of about 6-8 μm. Interestingly, central round wells of 10-30 μm diameter were observed to form within domains. Investigation of luminescence in depth profile of the wells was done using confocal fluorescence microscopy, and the results indicated that the domains were fully porous and luminescent throughout the entire depth. Spectrally, the peak fluorescence emission was in the range of 550-750 nm and the spectra had an average FWHM equal to about 150 nm. Covalent attachment of organic monolayers to the porous silicon surfaces was done to try and passivate against oxidation, and also to explore the possibilities of bioconjugation and tuning of the photoluminescence wavelength. A reaction of hydrogen terminated silicon with ω-undecylenyl alcohol was done using irradiation by a UV source, and successful derivatization was confirmed with IR spectroscopy. Bulk electrochemical etching of silicon provided a method to generate distributed luminescent structures suitable for compartmentalization of reactions within wells of micrometer dimensions without the use of spatially resolved fabrication methodologies such as epitaxial deposition, lithography, or ion beam technologies.

Finite-sized photonic crystal slabs have an effective refractive index that differs from that calculated for infinite structures. We investigate the imaging characteristics of such slabs with nonideal effective refractive index n_eff, namely |n_eff|<1 and |n_eff|>1 at the 2nd band of hexagonal lattice, and observe an approximately linear object-image relation and saturated imaging. We focus on the slab aperture effect on imaging and analyze the mechanisms of image formation. For the finite-sized photonic crystal slab with fixed object distance, due to beam leakage, there is a limiting aperture size beyond which the image resolution and image distance remain almost unchanged. A larger aperture does not necessarily lead to higher image resolution. In order to be focused by the slab with negative refraction, the physical source or the object must contain sufficient transversal wavevectors or higher transversal spatial frequencies. The analyses are for the cases where propagating waves dominate imaging formation.

We discuss a novel technique for accurately estimating the sensitivities of any desired response based on the finite difference Beam Propagation Method (BPM). Our technique utilizes the central adjoint variable method (CAVM) for estimating the response sensitivities. Using only one simulation of the photonic structure, the response and its sensitivities with respect to all the design parameters are obtained regardless of their number. This approach features accuracy comparable to that of the central finite difference approximation applied at the response level. The effectiveness of our approach is illustrated by using different response functions and different structures. Our approach utilizes virtual perturbations of the system matrices. Central difference scheme is utilized to calculate the sensitivity of these matrices with respect to the designable parameters. This sensitivity is then utilized to efficiently estimate the sensitivity of the objective function. This technique has been applied first to the scalar 2D BPM. It is also extended to calculate the sensitivities of 3D structures using full vectorial BPM. The proposed approach achieves a significant time saving in calculating the response and its sensitivities. The accuracy of our approach is verified through comparison with the expensive and accurate central finite difference applied directly at the response level. We also utilized the calculated sensitivities in gradient-based optimization algorithms to maximize the power coupling in 3D optical fiber coupler.

A new architecture of multiple private networks independent of optical line terminal (OLT) over Ethernet passive optical network (EPON) using ring topology is proposed. This architecture integrates the multiple private networks (PNs) with downstream/upstream EPON. Self determining private communications between optical network units (ONUs) are established using code-drop units. The use of optical code division multiple access (OCDMA) technique results in secure and reconfigurable PNs in the ring. Each ONU is assigned an appropriate codeword for private network communication and also the standard equipments for the up/downstream standard EPON communication. As an example, by using quadratic congruent (QC) codes with (p=5) leads to four optical private networks in the ring. To demonstrate the integration feasibility of multiple 622 Mb/s PNs over 1.25 Gb/s EPON using QC code-drop units, we analyze the network architecture by evaluating power budget, network dimensioning and BER performances.

In this paper we introduce a novel algorithm for the beamforming of a single channel retro-directive phased array system. Retro-directive array antennas are the best candidate for two-way communication, however there must be a large spectral difference between the send and receive carriers to reduce the interference and increase the isolation. When the send and receiver RF frequencies are far apart, e.g. 10 GHz, optical beamforming provides the unique solution to beamforming problem. Moreover single channel array antennas are hardware efficient and cost-effective. The core of the proposed structure is a cascaded ring resonator structure with two parallel waveguides, which can perform the roles of both a tunable optical delay line and a directional coupler. Hence, there is no need to use an optical circulator to separate the reception path from the transmission path. The received RF signal from the antenna is modulated with an optical carrier, ?C, and enters the delay line. The beamforming algorithm calculates the carrier wavelength and the coupling factors between the adjacent rings to maximize the received power from the desired source. Thermo-optics (TO) phase shifters are used to adjust the coupling factors. The algorithm calculates the optimum coupling factors based on the instantaneous feedback from the receiver array, hence it is robust and can compensate for the environmental changes or even the relative motion of the source and antenna platform. The delayed signal that leaves the delay line is demodulated by a photodiode (PD). The RF signal is amplified, filtered and delivered to the base-band receiver for decoding. A sample of the demodulated RF signal is used as the input to the beamforming algorithm to calculate the received power and signal to noise ratio. Based on the time-reversal property of the retro-directive arrays, the same amount of delay is required for the transmitter antenna, so the coupling factors do not need to change.

In opto-electronics and photonics, the severe disadvantage of an indirect band gap has limited the application of elemental silicon. Amongst a number of diverse approaches to engineering efficient light emission in silicon nanostructures, one system that has received considerable attention has been Si/SiO₂ quantum wells. Engineering such structures has not been easy, because to observe the desired quantum confinement effects, the quantum well thickness has to be less than 5 nm. Nevertheless, such nanometer thick structures have now been produced by a variety of techniques. The SiO₂ layers are amorphous, but the silicon layers can range from amorphous through nanocrystalline to single-crystal form. The fundamental band gap of the quantum wells has been measured primarily by optical techniques and strong confinement effects have been observed. A number of theories based primarily on ab initio approaches have been developed to explain these results with varying degrees of success. In this review, a detailed comparison is made between theoretical and experimental determinations of the band gap in Si/SiO₂ quantum wells.

Scattering resonances of metal nano-strip resonators are described as a consequence of formation of standing waves due to counter-propagating short-range (and slow) surface plasmon polaritons and gap plasmon polaritons, which are electromagnetic waves bound to and propagating along a nanometer-thin metal film, and a nanometer-sized gap between metal surfaces, respectively. Scattering spectra and resonant fields are presented for single-metal-nano-strip resonators and gap plasmon resonators with two closely spaced metal nano-strips. It is shown how strip resonators can be designed for any resonance wavelength in the range from 600nm to 1600nm.

Mode-hop-free operation of an extended-cavity diode laser is achieved with an original technique. A varied line-space diffraction grating is used as an external coupler. Continuous tuning is obtained by a simple linear translation of the grating. As of now, the measured continuous tuning range is as wide as 14 nm near 1568 nm with a relatively constant output power.

The focusing properties of orthogonal optical systems that include a varied line-space grating with curved lines can be analyzed efficiently with the ray matrices presented in this paper. These matrices are obtained by comparing the true optical path length truncated to the second order and the eikonal function (the phase of the kernel appearing in the Fresnel-Kirchhoff diffraction integral) expressed in terms of ABCD-matrix elements.

This paper studies the diffraction in the near-field of a monochromatic Gaussian beam by a sequence of parallel coaxial circular apertures. A method to design diffraction-based and wavelength-specific focusing systems through a sequence of circular apertures, using confocal Fresnel ellipsoids, is detailed. The results obtained with this research show that the design method introduced here is valid and can be used to focus electromagnetic beams without traditional refractive lenses.

The performance of three bidirectional modal methods the "classical" bidirectional eigenmode expansion propagation method, the aperiodic rigorous coupled wave analysis (known also as the Fourier modal method), and the mode expansion method based on harmonic expansion are mutually compared using modeling tasks that include eigenmode calculation of a relatively high-contrast planar waveguide, spectral transmittance of a one-dimensional "photonic crystal" filter in a photonic wire, spectral transmittance of a surface plasmon based optical sensor, and a reflectance from a double-groove structure in a high-contrast waveguide. All methods exhibit generally comparable performance, as follows from good mutual agreement of the results and generally comparable computational time. Although all methods use perfectly matched layers as absorbing boundary conditions, their implementation in the aperiodic rigorous coupled wave analysis exhibits significantly stronger attenuation than that used in the other two methods. Thus, significant improvement of the latter methods seems possible.

We review the theory and design of a novel type of stationary spectrometer in planar optical waveguides. These spatial heterodyne spectrometers are based on arrayed Mach-Zehnder interferometers and offer high resolution and increased optical throughput (etendue). A stationary Fourier technique is employed to reconstruct the input spectrum from the Mach-Zehnder outputs. Calibration mitigates waveguide fabrication errors and can be readily implemented in the spectra retrieval algorithm. Sensitivity to errors calibration measurements is numerically simulated.

In this paper, we interest on the synthesis of spectral phase OCDMA encoders/decoders based on step chirped Fiber Bragg Gratings to enhance their spectral and temporal efficiency. In this process, we use a real-coded Genetic Algorithm as a synthesis technique. The encoders/decoders are formed by step chirped fiber Bragg gratings with π phase shifts giving rise to bipolar code signatures. In fact, the device structure results from the concatenation of fiber Bragg gratings with different nominal periods and π phase shifts inserted between the gratings to implement the code behavior. Ecole Polytechnique de Montreal

In this paper, we compare the performances of a coherent versus incoherent Direct Sequence Optical Code Division Multiple Access (DS-OCDMA) system. Superstructured Fiber Bragg Grating (S-FBG) encoders/decoders are used to implement unipolar codes such as Prime Sequence (PS) and Extended Quadratic Codes (EQC) codes. We implement the Importance Sampling (IS) technique, which is a variant of the well-known Monte-Carlo (MC) method, to evaluate the Bit Error Rate (BER) performances of the system. Our simulation results depict that coherent system outperforms the incoherent one. The last system can be used but a BER floor is demonstrated due to the beat noise of the incoherent source. We show that increasing bit rate leads to a deterioration of the BER behavior and requiring an increase of the optical bandwidth of the signal.

We review the applications of cladding stress induced birefringence for controlling the polarization dependent properties in SOI waveguide components. In particular, we discuss the phase index and group index birefringence and their dispersion in this high index contrast waveguide platform, and the influence of waveguide cross-section geometry and cladding stress on these properties. Design of broadband polarization independent ring resonators using stress engineering is presented.

To avoid the commonly required regrowth steps in conventional distributed feedback laser fabrication, laterally-coupled distributed feedback (LC-DFB) lasers lithographically pattern the grating out of the ridge waveguide. Using higher order gratings increases the lithographic tolerances, resulting in lasers that are more amenable to mass-manufacturing techniques, such as stepper lithography. We have extended the modified coupled-mode theory to a two-dimensional cross-section, and thereby identified grating geometries that are both fabrication-tolerant and provide high performance.

In this work the application of Fourier Decomposition Methods to analyze photonic passive devices is reviewed. The work will specifically study the limitations of these methods to deal with discontinuous fields and recent advances of these techniques which make them possible to deal with high contrast challenging structures, such as SoI photonic wires, with excellent results.

Factors that contribute to the temperature dependence of a resonant frequency in a low-expansion optical cavity are discussed, including deformation at the cavity ends due to different coefficients of thermal expansion (CTE) of the spacer, optically-contacted mirror substrate and coating. A model of the temperature dependence is presented that incorporates finite-element-analysis of the cavity ends. A measurement of frequency versus temperature of a cavity mode is used along with the model to deduce a spacer's CTE versus temperature profile. The measured profile correlates very well with a separate experiment utilizing a temporary surface-mounted Fabry-Perot cavity fabricated on the outside of the spacer with hydroxy-catalysis bonding.

Several studies have shown that the incremental introduction of disorder in photonic crystals results in the high frequency band gaps closing followed by the lower frequency band gaps. The level and type of disorder required to pinch off the lower band gap depends on the photonic crystal's initial dielectric layout. Our research has shown that a rotationally symmetric 12-fold quasi-crystal structure can be reached by introducing a relatively low level of dielectric disorder to the hexagonal array. A morphing algorithm has been developed that permits the transformation of the hexagonal rod array photonic crystal into a 12-fold quasi-crystal. The intermediate dielectric profiles generated are used to examine the evolution of the band gap and central defect states during the transformation. The resulting FDTD simulations display evidence that the underlying structure of the 12-fold quasi-crystal may be closely related to the hexagonal array.

A method is proposed to minimize the impact of spectral aberrations in a monochromator based on a rectilinear translation of a plane chirped grating. The chirped grating, that has a spatially variable groove spacing, is used to diffract and to spectrally focus the radiation. The expression of the width of the instrument line shape due to aberrations have been developed in order to obtain the optimal rectilinear trajectory required to operate the monochromator without significant spectral aberrations. Experimental measurements of the emission spectrum of a five-wavelength Helium-Neon laser are presented, as well as the sensitivity of the monochromator performance to different geometrical parameters.

The photonic crystal fibers (PCF) are considered as the future information support for the telecommunication system. In this paper, a multicriteria method is used for the design of the PCFs with the user-defined optical proprieties. This method combines the deductive and the inductive learning and it is introduced for the first time in the field of optical fibers. These simulation tools will be optimized for PCF structures in order to optimize the parameters necessary for the improvement of the communication system performances. The multicriteria decision analysis makes it possible to evaluate the optical proprieties of PCFs by determining the effects of attenuation and distortion caused by Physics Phenomena. This decision is done by the means of a relational model preferably. As a result, this method avoids the recourse to distances and makes it possible to use quantitative and/or qualitative criteria. Moreover, it defeat some difficulties encountered when data are expressed in different units. These advantages allow the new multicriteria classification method to be employed easily to the diagnosis and to the design of photonic-crystals fibers.

We use the method of moments to calculate the propagation of an arbitrarily shaped pulse in a nonlinear dispersive fiber. By assuming that the pulse is linearly chirped, we are able to determine analytically the evolution of the second order moments (representing the duration, bandwidth and chirp of the pulse) along propagation regardless of the initial pulse shape. The evolution of the moments is given by an implicit equation and several invariants. These invariants allow an easy estimation of the different pulse parameters. The linear chirp approximation implies that the arbitrary pulse shape remains invariant along propagation but allows to calculate the propagation in both dispersion regimes from the same solution. The solution show an oscillatory behavior in the anomalous dispersion regime and a monotonic behavior in the normal dispersion regime. In both regimes the calculations are compared to numerical split-step simulations and are shown to agree for propagation over many dispersion and nonlinear lengths. While this method describes well the evolution of the pulse duration, bandwidth and chirp, we need to proceed differently to find the evolution of the pulse shape. From these propagation equations for the moments, we derive an approximate implicit solution describing the propagation of a Gaussian pulse in the normal dispersion regime. This approximate solution describes the pulse shaping toward a parabola that the pulse undergoes along propagation. A good agreement is found between the pulse obtained from numerically solving the implicit equation and the split-step propagation of the same pulse. Numerically solving the implicit analytical function describing the pulse is much faster than using purely numerical simulations, which becomes time consuming for highly chirped pulses with large bandwidths over long propagation distances. These and other results suggest that pulse shaping along propagation is only adequately modeled by implicit functions.

We report on our recent progress in polarization control and polarization compensator designs in SOI-based planar reflective gratings for a range of silicon core thicknesses of 0.1 μm to 10 μm. The dispersion property of the silicon slab, without a compensator region, was found to limit the applicability of SOI gratings for achieving the polarizationinsensitive performance of wavelength division multiplexing systems based on planar gratings. We have found that in coarse wavelength division multiplexing systems, the birefringence of the uncompensated slab becomes impractical at core thicknesses below 1.7 μm. Our findings clearly show that shallow etched polarization compensators can effectively eliminate polarization dependence only in thick-core gratings and only in applications requiring free spectral ranges (FSRs) of no more than 80 nm. In silicon cores with thicknesses of less than 1.0 μm, the significantly different value of linear dispersion strength for the two polarization states make traditional compensator designs ineffective, since only the central wavelength can be fully compensated. We used our findings to construct a procedure for building compensators with a flat polarization response over wide FSRs (>80 nm). The results of our study were applied to the design of a polarization compensator in an 18-channel multiplexer for use in coarse wavelength division multiplexing. Our simulation results show that a careful selection of the silicon core thicknesses in the slab and compensator regions is essential for achieving low-cross talk and low insertion loss devices. The application of thin core planar silicon gratings to building silicon interconnects is discussed.

Dynamic response of lightweight structures is affected by the mass of a transducer. The additional mass of the transducer is defined as a mass loading for the system, which is comparable to the weight of the structures, such as composites and thin cantilever beam/bar. In this paper, we measure the bending and torsion mode frequencies with and without mass loading to the lightweight structures by using lightweight optical fiber sensor (OFS) and conventional accelerometer. The frequency difference is verified by the finite element method using ANSYS software. The OFS, benefited to its lightweight, has shown the advantage of measuring vibration frequency accurately without affecting the dynamic response of the structure.

Integrated optofluidic devices have many potential applications for on-chip analysis and sensing. The fabrication of single-mode liquid-core waveguides in an integrated format allows the implementation of robust and very sensitive interferometers that combine long optical paths (on the cm scale) with small volumes (less than a nanoliter). We have demonstrated the monolithic integration of microchannels and liquid-core waveguides with planar silica lightwave circuits, which allows a number of refractometer devices to be implemented. Of these, we demonstrate experimentally a monolithic Mach-Zehnder interferometer (MZI) comprising a 20 mm-long liquid-core waveguide. The liquid-core waveguide is quasi single mode at 1550 nm when filled with a liquid of nominal index of ~1.47 (such as toluene or an index matching fluid). In these conditions, the output of the MZI is a pure cosine function, as a function of a linear progression of the refractive index of the liquid medium. Furthermore, the high contrast ratio experimentally observed in the output function allows a precise monitoring of refractive index changes by tracking the position of the transmission minimum in the spectral domain. Refractive index variations can be measured to a precision on the order of 4x10^-6. The large differential in thermo-optic coefficients between liquid media and silica allows the structure to function as a temperature sensor with a precision on the order of 10^-2 degrees Celsius. The measurement of the spectral fringe spacing of the interferometer response allows absolute-value measurements of temperature and refractive index.

There is an increasing demand for the efficient collection of fluorescent light emitted from a very thin layer of polymer material such as the specifically synthesized polymer material used in TNT explosives detection. However, the enormous transparency of this thin film poses a severe challenge for any light detection system based on a traditional approach. For a simple two-fiber architecture with one fiber for excitation light delivery and another for emission collection, we report that by launching the excitation light to one corner edge of a planar glass substrate covered with a thin layer of polymer film, we are able to simultaneously dramatically enhance the collectable fluorescent signal level and reduce the level of stray excitation light. The proposed sensing architecture opens up an efficient way of light coupling and collecting for fluorescent-related chemical and biological sample assay.

We investigate the characteristics of 40-GHz all-optical clock recovery based on a distributed Bragg reflector (DBR) self-pulsating laser. With the injection of a low timing jitter clock signal, the timing jitter characteristics of the DBR self-pulsating laser are investigated using both time domain and frequency domain methods. The results reveal that the cause of the timing jitter in the recovered clock signal depends on the injected clock signal power. The system performance of the clock recovery is investigated by the injection of a 40 Gb/s return-to-zero on-off key (RZ-OOK) signal with a 2³¹ - 1 pseudo random bit sequence (PRBS) pattern.

In this work, a ridge waveguide Bragg grating pressure or touch sensor is proposed. The sensor consists of an open top ridge waveguide Bragg grating with a pressure sensing film: polydimethylsiloxane (PDMS) on the waveguide surface. Under pressure, the guided mode of the waveguide accesses the film by coupling of the evanescent field. A large shift of the Bragg wavelength occurs when the effective index of the waveguide is changed by stress-indued variations in the film refractive index that are caused by increases in pressure. By monitoring the shifts of Bragg wavelengths in TE and TM modes, respectively, the external pressure change and the internal strain change in the film are measured. The sensitivities of the sensor with different waveguide structures are investigated.

We investigated the use of a pulsed, distributed feedback (DFB) quantum cascade (QC) laser centered at 970 cm^-1 in combination with cavity ring-down spectroscopy (CRDS) and cavity enhanced spectroscopic (CES) techniques for the detection of ethylene. In these techniques, the laser is coupled to a high-finesse cavity formed by high reflectivity mirrors. In the CRDS application, the laser frequency was tuned at a rate of ~0.071 cm^-1/K by changing the heat sink temperature in the range between -20 and 50°C. For off-axis CES, the laser was excited with short current pulses (5-10 ns), and the pulse amplitude was modulated with an external current ramp which gave a frequency scan of ~0.3 cm^-1. We utilized a demodulation approach followed by numerical filtering to improve the signal-to-noise ratio. Basic instrument performance and optimizations of the experimental parameters for sensitivity improvement are discussed. We demonstrated a detection limit of ~130 ppb with CRDS and ~15 ppb with off-axis CES for ethylene.

A new compact standard single mode fiber Michelson interferometer deflection sensor was proposed, tested and simulated. The new interferometer consists of a symmetrical abrupt 3 dB taper region with a 40 μm waist diameter, a 700 μm length and a 500nm thick gold layer coating. Compared with similar interferometric devices based on long period gratings that need microfabrication technology and photosensitive fibers, the proposed sensor uses a much simplified fabrication process and normal single mode fiber, and has a linear response of 1.1nm/mm.

Silica based fibre-optic refractive index sensors are gaining acceptance over conventional refractometers and finding applications in chemical/biological sensing due to many of their desirable properties. Here we present an optical fibre-based refractive index sensor that uses the power transmission through etched D-shaped fibres. The sensor's operating point and resolution can be tailored for a specific application by selecting the correct combination of the operating wavelength and the cladding thickness of the etched fibre. The sensor's power transmission depends on the surrounding refractive index in which the sensor head is immersed. The sensor presented has a maximum resolution on the order of 10^-6 in its "high resolution" region and on the order of 10^-4 in its "low resolution" region. The refractive index at which the maximum resolution occurs in the high resolution region can be shifted by ~0.012 and by ~0.027 in the low resolution region. To date, such high resolutions have been reported over narrow ranges and/or for fibre Bragg grating based sensors, which require optical spectral analysis which typically, is costly.

We have developed optical fibers with a thin approximately 40 to 60 nm thick Lithium Niobate Layer at the glass core and cladding boundary. These Lithium Niobate Cylinder Fibers (LNCF) are now in the process of being commercialized as strain gauges for bridges, tunnels, pipe lines and aircraft components. An application as a sonar sensor is also being investigated. LNCF strain sensors use light amplitude detection rather than phase detection. Amplitude detection is easier to implement and is substantially less costly. LNCF's when used as light amplitude sensing sonar detectors are over 1000 times more sensitive than standard Single Mode Fibers. LNCFs use so called "Non Propagating" light modes rather than the conventional propagating modes.

A novel theoretical scheme is presented for a surface plasmon-polariton (SPP) planar refractive index sensor based on one of the simplest integrated optical devices available, the Mach-Zehnder interferometer (MZI), to monitor relative phase variations in waveguides. An SPP is excited with the Bragg grating imprinted into core and buffer layers of one of the arms of the MZI. The main principle of operation of this device is based on the large phase change of the waveguide mode transmitted through the grating during the SPP excitation caused by the change in the refractive index of the sensed layer in contact with the SPP supporting metal layer.

Laser-Induced Breakdown Spectroscopy (LIBS) technique combined with Laser-Induced Fluorescence (LIF) is known to be a high sensitivity and high selectivity analytical technique. Although sub-ppm limits of detection (LoD) have already been demonstrated, there is still a constant and urgent need to reach lower LoDs. Here, we report results obtained for the detection of lead trace in brass samples. The plasma was produced by a Q-switched Nd:YAG laser at 1064 nm and then re-excited by a nanosecond optical parametric oscillator (OPO) laser tuned at 283.31 nm. Emission from Pb atoms was then observed at 405.78 nm. The experiments were performed in air at atmospheric pressure. We found out that the optimal conditions were obtained for an ablation fluence of 2-3 J/cm² and inter-pulse delay of 8-10 μs. Also, excitation energy of about 200 μJ was required to maximize the Pb(I) 405.78 nm emission. Using the LIBS-LIFS technique, the LoD was estimated to be about 180 ppb over 100 laser shots, which corresponds to an improvement of about two orders of magnitude with that obtained using conventional LIBS.

Photodynamic Therapy (PDT) is a minimally invasive treatment that uses a photosensitive drug into convert triplet state oxygen (³O₂) to singlet oxygen (¹O₂) to destroy malignant tissue. A fiber-optic system based on frequency domain detection of phosphorescence quenching by ³O₂ is described which optically measures the distribution of ³O₂ in the treatment volume during PDT to permit adjustments of treatment parameters to improve outcome. A specially designed fiber optic probe containing phosphorescent sensors embedded along its length permit spatially resolved measurements. Each sensor is composed of a phosphorescent metalloporphyrin compound that emits a characteristic spectrum. Four candidate sensors with high absorption at the excitation wavelength of 405nm and emission in the 650nm to 700nm region are considered. The dependence of phosphorescence lifetime (τ) on ³O₂ concentration is described by the linearized Stern-Volmer relationship as being inversely proportional. Determination of τ, and hence ³O₂ concentration, is accomplished in the frequency domain by means of phase-modulation detection of the phosphorescence signal due to an amplitude modulated excitation. The τ's of each sensor are recovered by performing global non-linear least squares fit on the measured phase and modulation index over a range of frequencies and wavelengths. With the τ of each sensor known, the oxygen concentration at each sensor's location can be determined with the Stern-Volmer relationship.

A new fiber-optic sensor system consisting of a fiber Bragg grating (FBG) cantilever as a transducer is proposed and demonstrated to realize simultaneous measurement of magnitude and direction of deflection. For the FBG mounted on a stainless steel cantilever, a change in the bending deflection gives rise to a monotonous shift in the Bragg resonance wavelength of the grating while the deflection direction results in either a red- or blue-shift in the Bragg wavelength due to a stretched or shrunk state of the grating. As an application of this deflection sensor, the water flow rate was measured, which showed good agreement with the theoretical analysis.

Long-period grating assisted Michelson interferometer has been proved to be a perspective device for chemical and biochemical sensing based on refractive index measurement. However, the phase difference between the fiber core propagation and the fiber cladding propagation in the in-fiber Michelson interferometer is correlated to the common path length. As a result, the fringe spacing in the reflection spectrum drops with the interferometer cavity length, leading to reduced fringe wave length shift in refractometry measurement. In this paper, we proposed a long-period grating assisted Michelson interferometer with the core mode and the cladding mode propagation separated. By eliminating the residue core mode after the long period grating coupling, an interferometer arm consists of cladding mode propagation is realized. With the cavity length of the other arm controllable, the fringe spacing in the reflection spectrum of the Michelson interferometer can be precisely controlled. The proposed Michelson interferometer allows use of long cavity length of the cladding mode arm to accumulate phase change and large fringe spacing for wavelength interrogation in refractometry sensing.

Bio-security for health monitoring and diagnosis are the needs of the hour, for rapid detection of biological and chemical species. This calls for a necessity to develop a cost effective miniaturized and portable biosensor device for in-situ biomedical applications and Point-of-Care Testing (POCT). While portability of the biosensor is required for in-situ medical detections, miniaturization is essential for handling smaller sample volumes and high throughput. Thus, the above mentioned concerns cannot be addressed unless a fully integrated biosensor system is developed. In this work, an integrated opto microfluidic based Lab-on-a-chip device is proposed for carrying out fluorescence based biodetection. The input and output fibers were integrated with the microfluidic channel so as to make a robust setup. Fluorescence detection was carried out using Alexafluor 647 tagged antibody particles and the output was measured with a Spectrometer-on-Chip, integrated with the device. The experimental results prove that the proposed device is highly suitable for Lab-on-a-Chip applications.

A chemical sensor system consisting of a coated long period grating, which was spliced into a fiber loop cavity, has been prepared and characterized. Designer coatings based on polydimethylsiloxane and nanostructured organically modified silica (ORMOSIL) materials were prepared to provide enhanced sensitivity for a variety of key environmental pollutants. Upon microextraction of the analyte into the polymer matrix, an increase in the refractive index of the coating resulted in a change of the attenuation spectrum of the long period grating. The grating was interrogated using ring-down detection as a means to amplify the optical loss and to gain stability against misalignment and laser power fluctuations. Chemical differentiation of cyclohexane and xylenes was achieved and a detection limit of 300 ppm of xylenes vapour in air was readily realized for PDMS coatings. Ormosil-type coatings were capable of detecting lead cations at concentrations below 1 ppm in water.

Design, fabrication and characterization of a 16x1 programmable microshutter array are described. Each shutter controls the light transmitted through a microslit defined on the transparent substrate supporting the array. Two approaches were considered for the shutter array implementation: sweeping blades and zipping actuators. Simulation results and fabrication constraints led to the selection of the zipping actuators. The device was fabricated using a surface micromachining process. Each microshutter is basically an electrostatic zipping actuator having a curved shape induced by a stress gradient throughout the actuator thickness. When a sufficient voltage is applied between the microshutter and an actuation electrode surrounding the microslit area, the generated electrostatic force pulls the actuator down to the substrate which closes the microslit. Opening the slit relies on the restoring force due to the actuator deformation. Microshutter arrays were fabricated successfully. High light transmission through the slit area is obtained with the actuator in the open position and excellent light blocking is observed when the shutter is closed. Static and dynamic responses of the device were determined. A pull-in voltage of about 110 V closes the microslit and the response times to close and open the microslit are about 2 and 7 ms, respectively.

Optical Fiber Bragg Gratings (FBG) sensors have seen significant development in recent years. Such sensor technology developed initially for the civil infrastructure is currently attracting the aerospace industry due to the potential versatility of this technology and its measurement capability. The structural health monitoring and the diagnostics and prognostics health management communities are excited about such development and ready to embrace such capability. Sensors reliability and accuracy, however, continue to be two parameters critical to the eventual implementation of the technology in high value targets. Such parameters can be improved by different manufacturing techniques as well as optimum grating's coating selection. This paper presents an evaluation of the mechanical behavior of the FBG strain sensors. A simulated analysis, using finite element modeling, revealed the impact of coating material selection, coating thickness selection, and bonding effect on the strain transfer loss. Results illustrate that metallic fiber coatings are more suitable for improved strain transfer than their polymeric counterparts and acrylic coatings are least effective with adhesive layer as small as possible.

We observe the electric fields caused by charge distributions during femtosecond laser ablation from a silicon (100) surface. Femtosecond electron pulses passing near the ablation site serve as a probe of the electric field generated by the emitted charges and countercharges on the sample surface. The density map of the electron pulse downstream from the sample contains information about the charge distributions. We invert this information by fitting the beam maps using a simple charge distribution model. Under the present excitation conditions (390 nm, 150 fs, 5.6 J/cm²), we observe the emission of 5.3×10¹¹ electrons/cm² within 3 ps of the excitation pulse, leading to self-acceleration of the emitted electrons to 2% of the speed of light. Preliminary experiments on a metal sample display even faster dynamics.

We report on the development of a widely tunable high power optical parametric oscillator (OPO) based on a diode-pumped femtosecond Yb:KGW laser. The laser operates at a wavelength of 1030 nm and can produce up to 3.7 W of average power in 300 fs pulses at 61 MHz repetition rate. Owing to its high peak power (>200 kW) this laser is well suited for the development of the OPO in the near-infrared spectral region. Design considerations of the OPO are also discussed and include the choice of the nonlinear material, its dispersive characteristics, wavelength tuning methods and operating wavelength ranges, phase-matching properties for accurate crystal design as well as OPO pumping configuration.

Expressions for the fields of transverse-magnetic laser beams in free space that are rigorous solutions to Maxwell's equations are given in a closed form. The electric and the magnetic fields are both expressed in terms of nonparaxial elegant Laguerre-Gaussian beams that are exact solutions of the Helmholtz equation. These solutions involve spherical Bessel functions and associated Legendre functions. Radially polarized beams of arbitrary order are considered and the lowest-order radially polarized beam (TM₀₁ beam) is investigated in detail.

Optical waveguides have been inscribed in fused silica by focusing femtosecond laser pulses with an axicon. The axicon is a conical lens that allows obtaining an optical beam with a transverse intensity profile that follows a zero-order Bessel function. This profile is invariant along a certain distance (>1 cm). The advantage of using axicon is that the beam is focused along a narrow focal line of a few micron width. Therefore the inscription of waveguides can be done without moving the glass sample. The waveguides so fabricated exhibit low losses and no detectable birefringence due their excellent circular symmetry. By translating the glass sample during the inscription process, we have induced a refractive index change along a thin plane in order to fabricate planar waveguides.

We recently proposed a new class of two-dimensional (in x and t) theoretically invariant optical wave packets characterized by a spatiotemporal Bessel function (STB beams). In these particular field distributions, a balance between the diffraction and the dispersion of pulsed beams takes place. The solution describing the propagation of an STB beam is independent of the propagation distance, but the infinite dimensions of this beam make it impossible to generate. The dimensions of this beam can be limited by the use of a Gaussian envelope, resulting in a spatiotemporal Bessel-Gauss beam (STBG beam), that we have generated experimentally. We present the experimental setup used to generate STBG beams and experimental beam profiles which are in good agreement with those of theoretical STBG beams.

In this work we explore the use of intracavity spectral filtering to enhance the performance of ytterbium-doped mode-locked fiber lasers. This technique has been experimentally demonstrated with a single-mode ytterbium-doped fiber laser operated with a ring cavity in the stretched-pulse regime. We characterized experimentally the effect of the long-wavelength filtering on the noise of a stretched-pulse cavity with a slightly negative global dispersion.

In many countries around the world, ultra-intense laser facilities are being built. These state-of-the-art lasers are intended for innovative medical and technological applications, as well as for basic experiments at the frontiers of fundamental science. Laser particle acceleration is a promising new endeavor. Recently developed schemes using radially polarized beams could help in reaching unprecedentedly short electron pulse durations, well in the attosecond range and potentially in the subattosecond range.

We present new results on supercontinuum generation obtained with a high-power Er-doped femtosecond fiber laser. Our results cover many different types of optical fibers: silica, dispersion-shifted fibers, doped fibers, etc. We have obtained supercontinua covering a wide spectrum from the visible to the mid-infrared ( >2μm ). We also identified third harmonic generation phenomena and we present experimental results that may exhibit the signature of two-photon absorption in an Yb-doped fiber.

Vectorial laser beams propagating beyond the paraxial limit exhibit an intensity profile at focus that depends upon their field structure and the width of their plane wave spectrum. Under tight focussing conditions, the longitudinal component of the lowest order transverse magnetic laser beam has a field amplitude that becomes comparable to that of the transverse components of the beam; the global intensity profile is then narrower than that produced by a Gaussian beam, thus enabling hyperresolution. With a general polarization eigenmode approach for all propagating directions in anisotropic media, we can show that privileged propagating directions exist, allowing preservation of the transverse magnetic polarization state despite birefringence. Using wave functions satisfying the non-paraxial wave equation, we can also find exact expressions for the field components. During propagation of tightly focussed beams along those privileged directions inside an appropriate anisotropic nonlinear crystal, the longitudinal electric field component may then be used to take advantage of nonlinear tensor terms otherwise ineffective with a paraxial beam. In this work, spectral conversion rate and power conversion efficiency of second-harmonic generation are characterized as a function of effective and undepleted nonlinear pumping in the case of propagation along the anisotropic axis of an uniaxial nonlinear crystal. Even if the phase matching condition is not fully satisfied for propagation along this privileged direction, we show to which extent the nonlinear properties are preserved for a restricted interaction volume.

Fibre content of solid wood plays an important role in the wood products industry in terms of value. Additionally, fibre structure in composite wood products such as Oriented Strand Board (OSB) and paper products plays an important role in terms of strength properties. The effect of moisture content on wood properties is important in the manufacturing process and final product performance, and therefore its effect on the birefringence is of considerable interest. Since solid wood exhibits strong birefringence at terahertz frequencies, there may be potential applications of terahertz spectroscopy to fibre content and structure sensing. There are two potential sources for this strong birefringence: (i) form birefringence resulting from the porous structure of solid wood and (ii) intrinsic birefringence resulting from the dielectric properties of the material itself. In this report, the variability of birefringence within and between species, the dependence of the birefringence on moisture content and the relative contributions from form and intrinsic birefringence are examined. In order to clarify the role of these contributions to the measured birefringence, polarized terahertz reflection spectroscopy is examined and compared to the results obtained in a transmission geometry. Comparison of the birefringence measured in transmission and reflection geometries suggests that form birefringence may dominate.