Proceedings Volume 8600

Laser Resonators, Microresonators, and Beam Control XV

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Proceedings Volume 8600

Laser Resonators, Microresonators, and Beam Control XV

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Volume Details

Date Published: 29 March 2013
Contents: 17 Sessions, 47 Papers, 0 Presentations
Conference: SPIE LASE 2013
Volume Number: 8600

Table of Contents

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Table of Contents

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  • Front Matter: Volume 8600
  • CO2 Lasers and their Beams
  • Beam Quality Measurements and Improvement
  • Microresonators in Frequency Combs
  • Microresonators in Lasers, RF Photonics, THz, and Mid-IR I
  • Microresonators in Lasers, RF Photonics, THz, and Mid-IR II
  • Microresonators: Cavity QED, Optomechanics, and Frequency Conversion
  • Laser Beam Polarization and Laser Resonators
  • Laser Beam Formation
  • Beam Shaping and Phase Distortion I: Joint Session with Conferences 8600 and 8603
  • Microresonators: Novel Topologies, Materials, and Applications I
  • Microresonators: Novel Topologies, Materials, and Applications II
  • Microresonators in Photonic Lightwave Circuits
  • Microresonator Sensors I
  • Microresonator Sensors II
  • Microresonators: Cavity QED, Optomechanics, and Frequency Conversion II
  • Poster Session
Front Matter: Volume 8600
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Front Matter: Volume 8600
This PDF file contains the front matter associated with SPIE Proceedings Volume 8600, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and Conference Committee listing.
CO2 Lasers and their Beams
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Bistable behavior of a continuous optical discharge as a laser beam propagation effect
V. P. Zimakov, V. A. Kuznetsov, A. N. Shemyakin, et al.
Two stable configurations of a continuous optical discharge (COD) were observed in experiments with plasma sustained continuously in xenon at high pressure by radiation of a medium power CW ytterbium fiber laser. One is the plasmoid of relatively small length with one temperature maximum and laser beam absorption of 10-30%. The other one is the plasma formation stretched along the laser beam with two or three local temperature maxima. The laser beam absorption in the second plasma configuration is increased dramatically up to 70-80% due to increased plasma length. Both plasma shapes were obtained under close conditions, so that oscillations between the two states were possible and also have being observed. The effect was studied and explained on the base of simplified consideration of the laser beam propagation through lens-like plasma medium surrounded by refractive near-spherical bounds between cold and hot gas. Other experimental results on the sustaining conditions of COD and plasma properties are also presented.
Extremely high power CO2 laser beam correction
This paper presents the design of the closed loop adaptive system to measure and correct for the aberrations of CO2 laser radiation. We considered two wavefront sensors - one sensor is based on commercially available IR camera while the second one – on the so-called thing film sensors. Also we present the design of two bimorph deformable mirrors to be used under high power laser radiation. We discuss both positive and negative attributes of these devices and the possibility to use them in the real laser high-power systems.
Optimization of the intra-cavity optical flux in the unstable direction in RF excited annular CO2 laser in terms of power stability
Viktor Granson, Francisco J. Villarreal, Jochen Deile, et al.
Modern high power industrial CO2 lasers are the result of decades of technological advancements aimed to improve laser parameters such as gain and saturation intensity to obtain the best power extraction efficiency. In this paper a resonator optimization approach is presented that includes laser power stability as one of the criteria for selecting the best configuration. This approach is applied to a hybrid stable-unstable annular RF excited CO2 laser.
Beam Quality Measurements and Improvement
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Pulse compression and beam quality improvement of a single-frequency Nd:YAG MOPA system
Oliver Lux, Hristomir Stankov, Haro Fritsche, et al.
Stimulated Brillouin scattering phase conjugation (SBS-PC) in the fluorocarbon liquid FC-77 is investigated within the development of a single-frequency Nd:YAG master oscillator power amplifier system operating at 1064 nm wavelength. Focusing of the pulsed Nd:YAG laser radiation into a FC-77-filled SBS cell which acts as a phase conjugate mirror in a double-pass amplifier arrangement generates high peak power output radiation in the MW range. As a result of the SBS phase conjugation process, we observed an improvement of the spatial beam quality (M2 = 1.4) as well as pulse shortening to the sub-nanosecond regime (<800 ps). The latter is significantly governed by the pump energy and the focusing conditions.
Wavefront measurement and data analysis of XUV HHG beam
P. Homer, B. Rus, J. Hrebicek, et al.
Extensive measurements of wavefront profile of the coherent XUV (eXtreme Ultra-Violet) HHG (High-order Harmonics Generation) beam at the wavelength of 30 nm have been performed. Unique results have been achieved using the PDI (Point Diffraction Interferometer) technique. The basic principle of the PDI is straightforward – ultrathin aluminium foil with a miniature pinhole – and it benefits from the self-referencing feature which is very important due to the measured wavelength. On the other hand, fabrication and experimental measurements are in general difficult in this spectral domain. In this paper we present basic principles, experimental setup, alignment techniques, obtained data and their analysis.
Microresonators in Frequency Combs
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Terabit/s data transmission using optical frequency combs
C. Koos, J. Leuthold, W. Freude, et al.
Terabit/s interconnects rely on advanced wavelength-division multiplexing (WDM) schemes. However, while efficient photonic-electronic interfaces can be efficiently realized on silicon-on-insulator chips, dense integration of WDM laser sources still represents a major challenge. Chip-scale frequency comb sources are an attractive alternative for providing optical carriers for WDM transmission. In this paper we give an overview on our recent work towards terabit/s data transmission using optical frequency combs. We demonstrate transmission of a 32.5 Tbit/s data stream using a modelocked solid-state laser as an optical source. Our current experiments aim at transmission schemes that exploit Kerr nonlinearities in high-Q microresonators for frequency comb generation.
On phase locking phenomena in Kerr combs
Aurélien Coillet, Irina Balakireva, Rémi Henriet, et al.
Based on a modal description of the evolution Kerr combs in a whispering-gallery mode resonator, we numerically investigate the phase bhavior of the different spectral lines of the spectrum. We show that a stable phase relation exists between adjacent modes in primary combs. This result is of great interest for metrological applications where one phase noise is an issue. For high input power however, chaotic signals are observed.
Microresonators in Lasers, RF Photonics, THz, and Mid-IR I
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A solid state ultraviolet lasers based on cerium-doped LiCaAlF6 crystal resonator
Nan Yu, Thanh Le, Steven J. Schowalter, et al.
We report the first demonstration of a UV laser using a high-Q whispering gallery mode (WGM) resonator of Ce3+: LiCaAlF6. We show that WGM resonators from LiCaAlF6 can achieve a Q of 2.6 x 107 at UV. We demonstrated a UV laser at 290 nm with a pulsed pump laser at 266 nm. The experiments showed the low pump threshold intensity of 7.5 x 109 W/m2 and slope efficiency of 25%. We have also observed lasing delay dynamics. These results are consistent with our modeling and theoretical estimates, and pave the way for a low threshold cw UV laser using WGM resonator cavity.
High gain selective amplification in whispering gallery mode resonators: analysis by cavity ring down method
P. Féron, A. Rasoloniaina, V. Huet, et al.
We study both theoretically and experimentally the dispersive properties of single whispering gallery mode resonators. We present a simple experimental protocol which allows us to obtain in detail its coupling regime and thus their dispersive properties. We demonstrate a compact optical amplifier with a gain up to 20dB in an Erbium doped fluoride microsphere of 135μm in diameter coupled via a tapered fiber. The model is also applied to analyze the dynamic behavior of the modal coupling between two degenerate resonances of the same cavity. In particular, this can be used to describe the coupling of counterpropagating whispering gallery modes (WGM) by Rayleigh scattering. The theory is successfully compared to experiments carried out in silica microspheres
Microresonators in Lasers, RF Photonics, THz, and Mid-IR II
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Chalcogenide glass planar photonics: from mid-IR sensing to 3-D flexible substrate integration
Chalcogenide glasses, namely the amorphous compounds containing sulfur, selenium, and/or tellurium, have emerged as a promising material candidate for integrated photonics given their wide infrared transparency window, low processing temperature, almost infinite capacity for composition alloying, as well as high linear and nonlinear indices. Here we present the fabrication and characterization of chalcogenide glass based photonic devices integrated on silicon as well as on flexible polymer substrates for mid-IR sensing, optical interconnect and nonlinear optics applications.
Microresonators: Cavity QED, Optomechanics, and Frequency Conversion
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Bose-Einstein condensation of photons in a microscopic optical resonator: towards photonic lattices and coupled cavities
Jan Klaers, Julian Schmitt, Tobias Damm, et al.
Bose-Einstein condensation has in the last two decades been observed in cold atomic gases and in solid-state physics quasiparticles, exciton-polaritons and magnons, respectively. The perhaps most widely known example of a bosonic gas, photons in blackbody radiation, however exhibits no Bose-Einstein condensation, because the particle number is not conserved and at low temperatures the photons disappear in the system’s walls instead of massively occupying the cavity ground mode. This is not the case in a small optical cavity, with a low-frequency cutoff imprinting a spectrum of photon energies restricted to values well above the thermal energy. The here reported experiments are based on a microscopic optical cavity filled with dye solution at room temperature. Recent experiments of our group observing Bose-Einstein condensation of photons in such a setup are described. Moreover, we discuss some possible applications of photon condensates to realize quantum manybody states in periodic photonic lattices and photonic Josephson devices.
Bright squeezed light via second harmonic generation in a whispering-gallery mode resonator
Matt T. Simons, Irina Novikova
We investigate the conditions necessary for bright squeezed light generation through second harmonic generation inside a crystalline whispering-gallery mode resonator. We show that the variance of a coherent mode can be reduced by a factor of 9 due to low loss through a nonlinear medium. This results in a one-step process that can generate effcient bright squeeze light at a desired wavelength.
RF-induced change of optical refractive index in strontium barium niobate
It is well known that light is able to modify properties of solid state media. Photorefractivity is one of the brightest demonstrations of such an ability. The phenomenon is related to the change of refractive index resulting from light-mediated redistribution of charges within the material.1 This redistribution is particularly pronounced in a selected class of optical materials. The magnitude of photorefractivity depends on the energy of photons that induce the charge redistribution, and thus is generally not observed with infrared light. In this work we demonstrate experimentally that not only light, but also low power radio-frequency (RF) electromagnetic radiation results in a significant modification of the refractive index of strontium barium niobate (SBN), one of the widely used photorefractive material. To our knowledge, the observed effect cannot be explained using existing theories of photorefractivity in bulk material. We expect that the effect originates from the influence of the boundary of the material on the space charge distribution as well as RF field induced pyroelectric effect; however a more detailed study is required to completely unveil the origin of the phenomenon.
High-Q BBO whispering gallery mode resonators
Guoping Lin, Josef U. Fürst, Dmitry V. Strekalov, et al.
We report an investigation on optical whispering gallery mode (WGM) resonators made from non z-cut beta barium borate (BBO) crystals. We first fabricated high quality (Q) factor WGM resonators made of an angle-cut BBO crystal. Q factors of 1×108 level have been demonstrated at various wavelengths including UV. They led to new upper bounds for the absorption coefficients of BBO at 1560 nm, 980 nm and 370 nm. We observed only one set of ordinarily polarized WGMs with polarization rotating along the resonator circumference. We also fabricated xy-cut BBO WGM resonators, in which the optic axis is parallel to the resonator plane. In that case, two WGM families with different polarization exist, one with constant the other with oscillatory phase velocity. This enables a novel way of broadband phase matching in WGM resonators with cyclic gain. We experimentally demonstrated efficient second harmonic generation (SHG) to a wide harmonic wavelength range from 780 nm at near infrared to 317 nm in UV. It is also the first reported direct UV SHG in a high-Q WGM resonator. This work lays a foundation for further investigations of WGM properties of non-z cut birefringent resonators and their applications in nonlinear optics.
Laser Beam Polarization and Laser Resonators
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Compact laser beam analyzer with polarization independent optics and wide dynamic range of neutral density adjustment
Michael Scaggs, Gil Haas
An all passive optical design laser beam waist analyzer has been developed which can analyze in real time a focused beam waist independent upon its polarization state and facilitates a wide dynamic range of neutral density adjustment for optimizing the intensity on the sensor in an extremely compact size. The technique is applicable from the UV to the far infrared and tests in the visible and the far infrared are presented.
At the core of the design is a short Fabry-Perot resonator which produces spatial time slices of a focused laser beam and post the resonator is a pair of wire grid polarizers. The Fabry-Perot resonator optics provides a ~ 4.0 optical density for the incoming laser beam. As the intensity of the light following the Fabry-Perot resonator is sufficiently low, a very efficient and compact arrangement of a pair of wire gird polarizers are introduced to provide a wider dynamic range of focus intensity at the sensor plane without the need to add additional neutral density filters. This simple, but unique combination of optics, makes for a very compact and efficient means to evaluate focused laser beams from the ultra violet to the far infrared.
Recent advances in coupled laser cavity design
External cavity coherent beam combining represents a path forward to higher fiber laser radiance, with several groups demonstrating scalable approaches. In this paper, we review recent advances in coupled laser cavity design. In particular, we compare various designs and describe the pros and cons of each with regard to sensitivity to path length errors. Experimental measurements using a specially designed dual-core fiber demonstrate the modal loss from a superposition architecture. A second area of investigation is concerned with Q-switch suppression in coupled laser cavities. The increased cavity loss that accompanies path length errors in the laser arms can suppress lasing, causing an energy build-up in the laser inversion. When the path length errors are removed and the cavity resumes its low loss state, the stored energy can be released in a manner analogous to Q-switching, creating a giant laser pulse. Since the peak power of this pulse can be many orders of magnitude larger than the cw power, the high instantaneous intensity can cause irreparable damage to optical components. We investigate passive systems that are designed to suppress this unwanted Q-switching by allowing alternative lasing paths to clamp the gain.
Laser Beam Formation
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Unraveling light with digital holograms
A. Forbes, C. Schulze, S. Ngcobo, et al.
Modal decomposition of optical fields as a concept has been in existence for many decades, yet despite its clear applications to laser beam analysis it has nevertheless remained a seldom used tool. With the commercialization of liquid crystal devices, digital holography as an enabling tool has become accessible to all, and with it modal decomposition has come of age. Here we outline the basic principles of modal decomposition of laser beams with digital holograms, and review recent results on the modal decomposition of arbitrary optical fields. We show how to use the information to infer the intensity, phase, wavefront, Poynting vector and orbital angular momentum density of the light. In particular, we show how to achieve optimal modal decomposition even in the absence of key information about the field, such as its scale and wavefront. We demonstrate the techniques on optical fields from fibers, diode-pumped solidstate lasers, and structured light by laser beam shaping.
Modeling of semiconductor saturable absorber mirrors using dynamic mode analysis
Semiconductor saturable absorber mirrors (SESAMs) are used to produce passively Q-switched ultrashort pulsed lasers. Numerical modeling of physical effects of SESAM is required to effectively design this type of lasers. For this purpose, simulations are performed to study the dynamic behavior of Gauss modes, gain of modes and saturation of the saturable absorber mirror. The laser beam quality has to be good enough in order to avoid chaotic laser behavior. We extended our dynamic mode analysis (DMA) algorithm to calculate laser beam quality. This simulation technique is based on rate equations for a set of Gauss modes and population inversions. Gain of each mode can be calculated separately by solving the corresponding set of rate equations. We have assumed that the reflectivity of the mirror is spatially invariant in the SESAMs model. An additional rate equation is required to include the saturation of SESAM. This equation considers parameters such as modulation depth, saturation fluence and relaxation time. Simulation results show that our model can predict pulse energy and non-chaotic behavior of the laser.
Large deformable mirrors for beam control of high brightness lasers
Nicolas Lefaudeux, Xavier Levecq, Lionel Escolano, et al.
Adaptive Optics is now a standard feature to control the laser beam quality of the high power lasers facilities. The development of the next generation of high power and high brightness laser facilities comes along with the increase of the energy of the laser pulses. In these lasers, the size of the optical elements used at the end of the chain must be increased in order to withstand the higher energy of the laser pulses. Laser adaptive optics systems are based on the use of deformable mirrors and are usually located at the end of the laser chain. Therefore, along with the other optics, the size of the deformable mirror must be increased in order to withstand the energy of the laser. Mechanical deformable mirror technology is compatible with all the standard high power dielectric coatings and is easily scalable. Large mechanical deformable mirrors able to withstand high pulse energies can be manufactured without technological obstacle. We present characterization and beam shaping results obtained with two large mechanical deformable mirrors. One mirror has a 180mm circular clear aperture. The other is an elliptical deformable mirror with 270 x 190mm clear aperture and is used as a fold mirror at 45° incidence. These large deformable mirrors can withstand pulse energies around 10 kilojoules for chirped pulses. They are compatible with the needs of beam shaping and beam control of the next generation of high power and high brightness laser facilities.
Free space propagation without free space
Christian Schulze, Daniel Flamm, Michael Duparré, et al.
We present a novel, fast and easy measurement technique to measure the beam propagation factor M2 of laser beams using a spatial light modulator (SLM). Two different measurement procedures are outlined, that are both based on digitally simulating the free space propagation of the beam. Hence, the traditional scan in propagation direction can be avoided and no moving components are required. In the first approach the SLM is employed as variable focus lens, yielding differently focused beams in a fixed plane, in which the beam diameter is determined with a static CCD camera. Using standard Gaussian optics the measured data can be fitted with the theoretical curve yielding the M2 value as a fit parameter. The second approach is based on the principles of the angular spectrum method, after which the propagated (near) field is obtained after multiplication with a transfer function of free space in the far field and back transformation to the near field. In the experiment the angular spectrum method is easily implemented by transforming the field under test with a simple lens onto the SLM, displaying the mentioned transfer function on the SLM, and transforming back to a fixed plane, in which the changing beam diameter is recorded with a CCD camera. To prove both techniques, reference beams of Laguerre-Gaussian type with known M2 value were generated and the beam propagation ratio measured. The comparison with the theoretical predictions reveals excellent agreement and emphasizes the fidelity of the M2 measurement.
Selective excitation of higher-order modes in diode-pumped solid-state laser resonators
In this paper we experimentally demonstrate the intra-cavity generation of selected higher-order Laguerre-Gaussian modes using a simple absorbing ring. First, we show selection of modes of variable radial order, from zero to five, with zero azimuthal order. Second, we select super-positions of azimuthal modes of zero radial order but high azimuthal index, up to eleven. In all cases we demonstrate high mode purity and a gain volume proportional to the order of the mode. Our results suggest a possible route to high-brightness diode-pumped solid-state laser sources.
Beam Shaping and Phase Distortion I: Joint Session with Conferences 8600 and 8603
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Brilliant green laser lines for surface processing
Mikhail Ivanenko, Wyacheslav Grimm, Lisa Kleinschmidt, et al.
Ultra-narrow line-shaped laser focuses are required for different material surface applications. We review the development of line-shaping optics for green DPSS lasers and report exemplary on several systems providing different line geometries and using different types of the lasers. These systems cover the line length range from 19 to 215 mm. One of the reported systems provides ultra-homogeny line-focus of 7.5 μm width and 215 mm length. It uses two rod Nd:YAG DPSS lasers and LIMO micro-optical anisotropic beam transformation technique to reach such a tight focusing and long depth of the focus. Contrarily another reported 200-mm green line is designed for bundling of eight Yb:YAG disc laser beams in a 100-μm wide line. The anisotropic beam transformation is not necessary for the shaping of this relatively broad line.
Refractive beam shapers for material processing with high power single mode and multimode lasers
Alexander Laskin, Vadim Laskin
The high power multimode fiber-coupled laser sources, like solid state lasers or laser diodes as well as single mode and multimode fiber lasers, are now widely used in various industrial laser material processing technologies like metal or plastics welding, cladding, hardening, brazing, annealing. Performance of these technologies can be essentially improved by varying the irradiance profile of a laser beam with using beam shaping optics, for example, the field mapping refractive beam shapers like piShaper. Operational principle of these devices presumes transformation of laser beam irradiance distribution from Gaussian to flattop, super-Gauss, or inverse-Gauss profile with high flatness of output wave front, conserving of beam consistency, providing collimated output beam of low divergence, high transmittance, extended depth of field. Important feature of piShaper is in capability to operate with TEM00 and multimode lasers, the beam shapers can be implemented not only as telescopic optics but also as collimating systems, which can be connected directly to fiber-coupled lasers or fiber lasers, thus combining functions of beam collimation and irradiance transformation. This paper will describe some features of beam shaping of high-power laser sources, including multimode fiber coupled lasers, and ways of adaptation of beam shaping optical systems design to meet requirements of modern laser technologies. Examples of real implementations will be presented as well.
New monolithic Gaussian-to-Tophat converter with integrated Fourier function and Gaussian-to-Tophat beam splitter
Aliaksei Krasnaberski, Lisa Kleinschmidt, Udo Fornahl, et al.
Optical Gaussian-to-tophat converters (g2T) that convert a Gaussian intensity distribution into a tophat profile find growing applications in different laser processing technologies. Usually, such refractive or diffractive g2T converters comprise of two or more optical components. For example, one aspherical component to form a tophat angular distribution followed by a Fourier lens that transforms it into the desired tophat intensity distribution in the focal plane. Here we report an optical design, which combines both optical functions in a single monolithic component. The component is designed and manufactured by LIMO as a free-form profile, providing the square tophat of 100-μm width at the distance of 125 mm. Compared to the traditional g2T-converters it is much more compact, easy to adjust, and less sensitive to alignment errors. In many industrial applications, not a single but multiple tophat foci are desirable for a fast parallel processing. For such applications we have developed a Gaussian-to-Tophat beam splitter. The beam splitting is done by a refractive-diffractive high-order grating with a smooth continuous pitch profile. Thanks to the smooth profile, such a Gaussian-to- Tophat beam splitter demonstrates very high efficiency of above 95% and high homogeneity between the diffraction orders.
Top hat beam shaping for a high power laser diode bar with a wavelength of 780 – 1100 nm
H. Moser, D. Kura, H. Forrer, et al.
Broad area high power laser diodes emit in fast axis and slow axis direction very different beam profiles. In fast axis direction a nearly diffraction limited Gaussian beam, but in the slow axis direction a multi-mode beam profile. We present a top hat solution for individual fast axis and slow axis beam shaping using micro optical components for a standard high power laser diode bar with a wavelength of 810 nm. The compact design allows the individual beam shaping in a very small space. The quality of the beam shaping depends mainly on the precision of the micro optics and the advanced assembly process. These solutions enable the development of laser diode modules with individual and application specific beam shapes and small dimensions.
Microresonators: Novel Topologies, Materials, and Applications I
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Temporal dynamics of Kerr frequency combs in whispering-gallery mode resonators
Aurélien Coillet, Irina Balakireva, Rémi Henriet, et al.
Based on a modal description of the evolution of the mode's amplitude in a whispering-gallery mode resonator, we numerically study the generation of Kerr combs. We show that a stable primary comb appear for pump power slightly above threshold, enabling potential applications in metrology. For high input power however, chaotic signals are observed.
Coherent perfect absorption in nonlinear optics
Recently, a concept of time reversed lasing or coherent perfect absorber (CPA) has been proposed by A. D. Stone and co-workers, and was shortly experimentally demonstrated by them. The CPA system is illuminated coherently and monochromatically by the time reverse of the output of a lasing mode and the incident radiation is perfectly absorbed. Shortly afterwards, Stefano Longhi extended the idea to realize a CPA for colored incident light, and have theoretically shown that the time reversal of optical parametric oscillation (OPO) in a nonlinear medium could also realize a colored CPA for incident signal and idler fields which can be seemed as a kind of nonlinear CPA. Here we present the realization of such time-reversed processes in nonlinear optics regime, including time-reversed second harmonic generation (SHG) for coherent absorption at harmonic frequency of the pump and time-reversed optical parametric amplification (OPA) for coherent attenuation of colored travelling optical fields. Time reversed SHG is carried out at both phase matching and mismatching conditions, which shows parametric near perfect absorption at the harmonic frequency of the pump. The time reversal of OPA is demonstrated experimentally in a nonlinear medium to form a coherent absorber for perpendicularly polarized signal and idler travelling waves, realizing in the condition of OPA by a type II phase matching scheme. The absorption of signal/idler pair occurs at some specific phase difference. This is the first experimental demonstration of coherent absorption processes in nonlinear optics regime.
Fabrication and modelling of truncated oblate and prolate microresonators
Michalis N. Zervas
We have demonstrated softening and micromorphing techniques of amorphous/crystalline materials for fabricating high quality oblate or prolate microresonators. Firstly, “soften-and-compress” technique has been developed for fabricating bottle microresonators from standard fibers or capillaries using splicers. Also, a “soften-and-squash” technique has been demonstrated for fabricating microdiscus resonators by squashing microspheres between glassy-carbon plates. Finally, a softening process was developed for turning crystalline microstructures into predominantly oblate microresonators. We have also developed an efficient transfer-matrix method for calculating resonant frequencies and field distributions in truncated oblate and prolate structures and applied it to analyze resonant characteristics of the new microresonators.
Focused ion beam engineered disc resonators
D. C. Aveline, L. M. Baumgartel, B. Ahn, et al.
We report the demonstration of whispering gallery mode (WGM) resonators augmented by focused ion beam (FIB) microfabrication. We demonstrate that we can precisely mill features directly into the perimeter of crystalline disc optical resonators. By cutting a narrow opening completely through the disc, we can create full access to the internal modes. Applying FIB techniques, we can also precisely engrave a grating structure on the disc surface. The diffraction grating provides a simple and highly directional free-space coupling mechanism with superior stability to evanescent coupling techniques. These embedded gratings can also provide control of the resonance spectrum, significantly reducing the mode density. This FIB fabrication process does not introduce significant loss; Q~-108 has been demonstrated. The wavelength dependence of the diffraction angle was found to be in excellent agreement with grating theory. The versatility of mode accessibility, spectral control and far-field grating coupling will have significant impact in WGM resonator applications in lasers, sensors, and optoelectronics.
Microresonators: Novel Topologies, Materials, and Applications II
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Complex polarization states of off-axis uniaxial whispering gallery mode resonators
Florian Sedlmeir, Martin Hauer, Josef U. Fürst, et al.
Long photon confinement and high optical fields require good optical resonators. Some of the best optical resonators with a small footprint are whispering gallery mode (WGM) resonators. Their principle is based on continuous total internal reflection at the interface of a round dielectric. Currently most WGM resonators are fabricated fully symmetric to their rotational axis. In WGM resonators fabricated from uniaxial crystals this symmetry axis then coincides with the optic axis, such that the modes are either parallel or perpendicular polarized. If the optic axis is however tilted with respect to the symmetry axis the polarization of the modes changes dramatically. We report on high Q resonances in a slightly birefringent MgF2 WGM resonator, cut at an angle of 20° with respect to the optic axis. A novel type of mode is observed that can be fully coupled (decoupled) with a right (left) hand circular polarized beam of light. Furthermore, the polarization properties at different outcoupling positions, determined via full Stokes measurements, are recorded and show a continuous complex change in ellipticity. We present the experimental results. Understanding the polarization behavior in an off-axis, birefringent WGM resonator may offer a new way for phase-matching in non-linear χ(2) materials.
Whispering gallery states of neutrons and anti-hydrogen atoms and their applications to fundamental and surface physics
Valery Nesvizhevsky
The ‘whispering gallery’ effect has been known since ancient times for sound waves in air, later in water and more recently for a broad range of electromagnetic waves: radio, optics, Roentgen and so on. It is intensively used and explored due to its numerous crucial applications. It consists of wave localization near a curved reflecting surface and is expected for waves of various natures, for instance, for neutrons and (anti)atoms. For (anti)matter waves, it includes a new feature: a massive particle is settled in quantum states, with parameters depending on its mass. In this talk, we present the first observation of the quantum whispering-gallery effect for matter particles (cold neutrons) 1-2. This phenomenon provides an example of an exactly solvable problem analogous to the ‘quantum bouncer’; it is complementary to recently discovered gravitational quantum states of neutrons3. These two phenomena provide a direct demonstration of the weak equivalence principle for a massive particle in a quantum state. Deeply bound long-living states are weakly sensitive to surface potential; highly excited short-living states are very sensitive to the wall nuclear potential shape. Therefore, they are a promising tool for studying fundamental neutron–matter interactions, quantum neutron optics and surface physics effects. Analogous phenomena could be measured with atoms and anti-atoms 4-5.
Microresonators in Photonic Lightwave Circuits
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On-chip ultrahigh-Q microcavities for highly unidirectional emission
Xue-Feng Jiang, Chang-Ling Zou, Lan Yang, et al.
Optical whispering gallery mode (WGM) microcavities are promising candidates for basic research and optoelectronic applications. Due to the isotropic emission property resulting from the rotational symmetry, traditional WGM microcavities have to rely on external couplers to excite the modes and collect their emission. One of the most possible solutions is to deform microcavities from rotational symmetry, which could provide directional emission instead of isotropic characteristic. Here we report the first experimental realization of on-chip microcavities which support both highly unidirectional emission and ultra-high Q factors. The demonstrated Q factor exceeds 100 million in near infrared. By doping erbium into the deformed microcavity, lasing action in 1550 nm band was observed under convenient freespace optical pumping, with the threshold as low as 2 μW. Remarkably, the lasing emission is along a single direction with a narrow divergence angle about 10 degrees.
Silicon-based monolithically integrated whispering-gallery mode resonators
Fernando Ramiro-Manzano, Mher Ghulinyan, Nikola Prtljaga, et al.
We report on the realization and characterization of a silicon-based integrated optical platform which implements a vertical coupling scheme between a Whispering-gallery type microresonator and a buried dielectric waveguide. The vertical coupling allows for the separation of the resonator and the waveguide into different planes, which enables the realization of the optical components in different materials/thicknesses. The high optical quality of this micro-optical system follows from the accurate planarization of the waveguide topography, which is achieved by multiple depositions-and-reflows of a borophosphosilicate glass over strip waveguides. Importantly, we demonstrate the feasibility of our approach for wafer-scale mass fabrication of freestanding planar resonators suspended in air and coupled to integrated bus waveguides. This opens the door for the realization of stable all-integrated resonator systems and it has the potential to substitute todays complicated fiber-taper coupling schemes.
Metal-graphene-metal photodetectors
Thomas Mueller, Marco Furchi, Alexander Urich, et al.
We present photoconductivity studies of metal/graphene interfaces, discuss the origin of the photoconductive behavior, and present ultrafast photocurrent measurements. Conversion of surface plasmon polaritons into electrical current at metal/graphene interfaces will also be presented. Based on these findings we developed several concepts for graphenebased photodetectors. One of these concepts involves the deposition of inter-digitated metal electrodes on graphene to realize a metal-graphene-metal photodetector. We used this device to demonstrate the faithful detection of data streams at rates of 10 gigabits per second. Another concept relies on the monolithic integrating of graphene with a Fabry-Pérot microcavity. This device benefits from the large increase of the optical field inside a resonant cavity, giving rise to increased absorption. We demonstrate that the optical absorption can be 26-fold enhanced as compared to conventional devices.
Far-field pattern simulation and measurement for unidirectional-emission circular microlasers
Yong-Zhen Huang, Xiao-Meng Lv, Heng Long, et al.
Three-dimensional circular resonators connected with an output waveguide were simulated by the three-dimensional finite-difference time-domain (FDTD) technique. For the microcircular resonator with vertical waveguiding consisted of active layer confined by upper and lower cladding layers with the refractive indices of 3.4 and 3.17, the mode Q factors are greatly influenced by the thickness of the upper cladding layer. The numerical results of the near field and the farfield patterns indicate that the vertical waveguide with semiconductor materials does not provide enough optical confinement for the confined modes in the resonator. Furthermore, the lasing spectra and far-field patterns are measured for a circular microlaser with a radius of 15 μm and a 2-μm-width output waveguide. Single mode operation with the side mode suppression ratio up to 33 dB is realized at room temperature, and multiple peaks are observed in the vertical far-field pattern due to the vertical radiation of the mode field.
Microresonator Sensors I
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Hybrid organic/inorganic resonators for sensing and telecommunications applications
Andrea M. Armani, Nishita Deka, Audrey Harker, et al.
Historically, integrated photonic devices have been fabricated from inorganic material systems, such as silicon, silicon nitride, silica and gallium arsenide. As a result of their inherently low material loss and compatibility with nanofabrication tools, high performance waveguides and resonant cavities have been demonstrated. However, to achieve many of the desired performance metrics, it is necessary to implement active stabilization systems. For example, as a result of the thermo-optic effect, the resonant wavelength of a microcavity will change with temperature, resulting in an unpredictable resonant wavelength without temperature stabilization. Therefore, new materials and material systems are desired. One approach is to combine the inorganic materials conventionally used in telecommunications with organic polymeric materials. These hybrid systems offer the ability to tune the optical and mechanical properties of the inorganic materials, achieving athermal or temperature-independent performance. Additionally, given the wide range of polymeric material available, new material systems with previously unrealized behavior are possible; for example, materials which mechanically respond to UV, humidity and specific chemicals. Using silica toroidal whispering gallery mode resonant cavities as the device platform, a series of hybrid organic/inorganic resonators were fabricated. Several different types of organic layers were studied, varying both the specific polymeric material and the deposition method. For example, polyisobutylene was coated on the devices using either a spin-coating method or a surface initiated cationic polymerization process. With the wide range of possible organic materials, many different devices have been fabricated, including athermal devices, humidity and bio/chemical sensors, and microlasers.
Microresonator Sensors II
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Whispering gallery mode microresonators: results on aptasensors and on a new sensing approach
We present results on the implementation of Whispering Gallery Modes (WGM) biosensors and on the demonstration of a new detection method for WGM based sensors. We first present a functionalization procedure based on the DNA-aptamer sequence immobilization on WGM resonators, able to recognize specifically thrombin protein. The protein binding was optically characterized in terms of specificity in buffer solution and in 10% diluted human serum. When performing the above measurements, we have used the typical detection scheme for WGM resonator based sensors, which relies on tracking the resonance shift − by scanning with a tunable laser − when a change of the refractive index in the region probed by the WGM takes place. In the second part of the presentation we propose a new sensing approach based instead on monitoring the position of the laser line of a fiber ring laser having a WGM microsphere in its loop. We demonstrate that the induced shift is the same for the ring laser line and for the microsphere resonance. The proposed method requires simpler and cheaper equipment and may also improve the sensor resolution because the ring laser line is narrower than the microsphere WGM resonance.
Microcavity single virus detection and sizing with molecular sensitivity
V. R. Dantham, S. Holler, V. Kolchenko, et al.
We report the label-free detection and sizing of the smallest individual RNA virus, MS2 by a spherical microcavity. Mass of this virus is ~6 ag and produces a theoretical resonance shift ~0.25 fm upon adsorbing an individual virus at the equator of the bare microcavity, which is well below the r.m.s background noise of 2 fm. However, detection was accomplished with ease (S/N = 8, Q = 4x105) using a single dipole stimulated plasmonic-nanoshell as a microcavity wavelength shift enhancer. Analytical expressions based on the “reactive sensing principle” are developed to extract the radius of the virus from the measured signals. Estimated limit of detection for these experiments was ~0.4 ag or 240 kDa below the size of all known viruses, largest globular and elongated proteins [Phosphofructokinase (345 kDa) and Fibrinogen (390 kDa), respectively].
Flow cytometer system for single-shot biosensing based on whispering gallery modes of fluorescent microspheres
Reno Lessard, Olivier Rousseau-Cyr, Maxime Charlebois, et al.
We report an innovative label-free biosensor based on statistical analysis of several whispering gallery modes spectral shifts in polystyrene fluorescent microspheres using a custom microflow cytometer. Whispering gallery modes analysis enables detection of nanometer-sized analytes showing promising possibilities for virus, bacteria and molecular detection. To demonstrate this, fluorophore-doped microspheres of the appropriate size parameter are mixed in an aqueous solution. Then, a syringe pump pushes the solution through a fiber optic flow cell where a laser beam illuminates the analysis area to excite the microspheres and their fluorescence is collected. This device provides a low-cost and user friendly solution that could enhance spectrum acquisition rates up to 5 spectra per second thanks to the considerable amount of microspheres flowing through the excitation area per unit time. Finally, the fluorescence spectra are statistically investigated using an instantaneous measurement of apparent refractive index algorithm to determine a reliable value for the refractive index of the environment since the exact radius of the microsphere scanned is unknown. This refractive index becomes an effective value for the local perturbation caused by inhomogeneities on the microsphere surface and hence, determines whether or not inhomogeneities, such as bacteria, are adsorbed by comparing to a control sample. Combining a flow cell with our detection algorithm, we reduce the period of a 50 microspheres experiment from 161 minutes to 14 minutes when the flow rate is 2000 µl/h and the microsphere concentration is 5 µsphere/µl.
Beam-coupled microsphere resonators for high-resolution electric field sensing
Amir R. Ali, Tindaro Ioppolo, M. V. Ötügen
In this paper we present a beam-coupled micro-optical sensor for electric field detection. The main components of the sensor are a microsphere optical resonator and a dielectric micro-beam. The dielectric beam is coupled with the microsphere using two different approaches. In the first approach the beam compresses the sphere causing a shift in its whispering gallery mode (WGM). In the second approach the fine tipped beam perturbs the evanescent field of the sphere thereby causing a shift its WGM. In this approach, electrostriction force exerted by the electric field on the beam induces a change in the gap between sphere and beam perturbing the evanescent field of the sphere. This in turn induces a shift in the whispering gallery modes of the sphere resonator. The material for both the sphere and the beam in the first approach is polydimethylsiloxane (PDMS). The resonator and the beam are made of silica in the second approach. Different beam and sphere sizes are studied to optimize the proposed sensor’s electric field resolution. The optical modes of the sphere are exited using a tapered single mode optical fiber that is coupled to a distributed feedback laser. The transmission spectrum through the fiber is monitored to detect WGM shifts.
Optimization of resonator radial dimensions for quartz enhanced photoacoustic spectroscopy systems
Samara L. Firebaugh, Eugene A. Terray, Lei Dong
A finite element model for QEPAS systems has been developed that can apply to both on-axis and off-axis systems. The model includes the viscous and thermal loss on the acoustic resonator sidewalls, and these factors are found to significantly affect the signal to noise ratio. The model results are compared to experimental data and it is found that the model correctly predicts the optimal radial dimensions for resonator tubes of a given length. The model is applied to examine the dependence of signal-to-noise ratio on resonator diameter and sidewall thickness. The model is also applied to off-axis systems.
Microresonators: Cavity QED, Optomechanics, and Frequency Conversion II
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Photonic crystal split-beam nanocavities for torsional optomechanics
A novel type of photonic crystal nanocavity nanocavity tailored to sensitively measure torques is theoretically investigated. Suspended low-mass elements (< pg) in the nanomechanical resonator are sensitive to environmental stimuli, such as a magnetic field from external sources or from embedded nanomagnetic systems. The torsional mechanical motion of these elements directly influences the optical field concentrated inside the optical nanocavity, resulting in a strong cavity optomechanical coupling rate up to 90 GHz/nm. The actuation of the mechanical resonator is readout with high sensitivity using evanescent coupling between the photonic crystal nanocavity and an optical fiber taper. A sub-100nm physical air gap in the middle of the nanobeam cavity allows torsional mechanical degrees of freedom as well as strong optical field confinement in a small mode volume. Numerical simulations show that high-Q ~ 106 optical cavities with a gap are possible. Potential applications incorporating these devices include sensitive magnetometry and probing the quantum properties of nanomagnetic systems.
Poster Session
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Enhancing the radiation efficiency of dye doped microresonators using coupling effects with a suspended core microstructured optical fiber
A. Francois, S. Afshar, K. J. Rowland, et al.
Recently we demonstrated that a dye-doped microresonator positioned onto the tip of a suspended core Microstructured Optical Fiber can be used as a dip sensor. In this architecture, the resonator is located on an air hole next to the fiber core, enabling a significant portion of the sphere to overlap with the guided light emerging from the fiber tip. When the resonator is excited through the suspended core fiber it exhibits an unusually high radiative efficiency, which was initially attributed to the higher excitation efficiency enabled by this architecture. Here we demonstrate that it is possible to enhance the radiative emission of a microresonator attached to the suspended core fiber tip by changing the size of the resonator and how it is positioned on the fiber tip. In particular, we have found that the way in which the sphere interacts with the air hole cavity of the suspended core fiber significantly changes its emission characteristics. We found that the enhancement was dependent upon the interaction between the modes of the resonator with the confined geometry of the suspended core fiber rather than a higher excitation efficiency alone. We also evaluate the impact of the radiative enhancement on the WGM lasing threshold in different configurations.
Microtaper fiber excitation effects in bottle microresonators
M. Narizee Mohd Nasir, Ming Ding, G. Senthil Murugan, et al.
We have carried out a systematic study of the effect of microtaper diameter on the spectral characteristics of bottle microresonators. By increasing the microtaper-diameter (Dt) from 2μm to 10μm results in progressively cleaner spectra. The transmission depth at resonance varies from ~15dB (@Dt=2μm) to >3dB (@Dt=10μm). The loaded Q factors were measured to be >10+6 in all cases. However, with microtaper Dt=10μm clearly-resolved single resonance peaks could be observed and free-spectral ranges could be easily identified. At some transmission resonances, we have observed LP01→LP11 mode transformation at the excitation microtaper waist, for the first time, as the resonance is scanned.
Fabrication and characterization of gain guided, index anti-guided planar waveguide laser resonator
Adam Dittli, Tsing-Hua Her, Lee Casperson
Gain-guided and index anti-guided waveguides hold great promise to increase laser output power while maintaining single mode operation. Although lasing in GG-IAG fibers was demonstrated, slope efficiency is poor and output power is low. This work presents the first, to the best of our knowledge, lasing characteristics of a diode-pumped GG-IAG planar waveguide in a plano-plano resonator configuration. The laser waveguide is fabricated using 1% doped Nd:YAG core with diffusion bonded TGG claddings. With continuous-wave pumping, 8.5% slope efficiency is demonstrated and an output power of 1.5W achieved, the highest power reported so far from a GG-IAG waveguide laser. Under such pumping condition, however, thermal lensing is shown to override gain guiding and mode narrowing is observed.
Process optimized beam profiles for laser micromachining
Christian Bischoff, Ulrich Rädel, Udo Umhofer, et al.
The Gaussian laser beam profile is for many applications in laser micromachining not optimally adapted. Therefore process optimized beam profiles with e.g. Top-Hat or torus shape are required to improve process quality. Other applications require multiple beam-lets for parallel processing to increase process efficiency. TOPAG’s new diffractive FBS (Fundamental Beam-Mode-Shaper) concept allows the generation of square, round or line Top-Hat profiles with near diffraction limited size for smallest possible patterning and spot sizes with just a few micrometers. FBS elements can be placed at nearly any position within the beam path and do not substitute the focusing system (objective) but can be integrated in existing optics. Furthermore the FBS beam shapers feature very homogeneous beam profiles (+/- 2.5%), a high efficiency (> 95%) and simplified handling. In combination with diffractive beam splitters the quality and throughput of the laser process can be improved as a result of several optimized beams from just one beam source. TOPAG presents also application results using FBS shapers and diffractive beam splitters for OLED scribing.