In this paper we present free-space micro-optics coupling results for 10 Gb/s avalanche photodiode (APD) receivers. A free-space micro-optics approach provides compact cost-effective coupling with minimum insertion loss. Both DC and high frequency (RF) coupling characteristics are presented, and respective dimensions of the photosensitive areas are calculated. Effects of both axial and lateral misalignments on 10 Gb/s APD performance are presented. An analytical model for the photosensitive area of 10 Gb/s APDs is developed based on the RF coupling performance and can be used in optical design and opto-mechanical tolerance analysis. It is shown that free-space micro-optics coupling of APDs enlarges the effective size of the photosensitive area for achieving good RF device performance and provides robust packaging of 10 Gb/s APD receivers.

In this paper, an ultra compact highly reliable all solid state photopolarimeter is presented. This innovative polarimeter includes: (1) a set of digital Faraday rotators; (2) a photodetector and corresponding processor; (3) and a novel measurement technique so that the temperature and wavelength dependences of magneto-optic crystals are automatically compensated. The major advantages of this novel polarimeter are (1) highly reliable (no mechanical moving parts); (2) higly accurate; (3) fast speed (< 1 ms); and (4) ultra compact size (e.g., 25 mm x 10 mm x 10 mm). It is anticipated this unique polarimeter can have a variety of applications in the areas of polarization monitoring in optics networks, ellipsometry, non-invasive bio-optic sensors, etc.

In this paper, a unique polarization independent athermal electro-optic modulator using lithium niobate, LiNbO₃, and/or lithium tantalite, LiTaO₃ crystals is presented. The modulator uses a unique sandwich configuration, in which a pair of lithium niobate crystals are bonded together and an embedded electrode is sandwiched in between them. By applying a proper electric field, the output intensity of the device can be continuously adjusted independent of polarization state of incoming light beam and the ambient temperature.

Long-period fiber grating (LPFG) having grating periodicity in the hundreds of microns, functions as a band rejection filter. This device is useful for gain equalizing applications of fiber amplifiers.The transmission characteristics are mainly determined by the initial fabricating conditions such as grating periodicity and UV exposure time. Therefore, it was very difficult to tune the transmission characteristics with wide range after the LPFG was made. Many works have been reported to tune the resonant wavelength and depth of the dip point. But, tuning range of resonant wavelength was not so wide and resonant depth was not easy to control after the grating fabrication. In this paper, we have proposed, analyzed and evaluated the novel and simple method to tune the wavelength characteristics of LPFG after the grating fabrication. In this device, the grating is divided into two regions. One region is the conventional LPFG, and the other region is the modified LPFG surrounded outside the cladding with a medium having higher or lower refractive index than that of the cladding. In case of the higher refractive index, only resonant depth could be controlled over 20dB without resonant wavelength shift. In case of the lower refractive index, resonant wavelength could be moved around 40 nm while resonant depth change is small. The experimental results were compared with the theoretical analysis. Both results showed good agreements.

Fiber Bragg gratings (FBGs) are widely used in optical communication and sensing applications. The accuracy and stability of the center wavelength of the FBG is affected by the fluctuations of the ambient conditions, especially the temperature. The center wavelength shift can be reduced either by using a temperature compensating package or by keeping the FBG in an athermal environment. A novel coating design is proposed for achieving passive athermalisation of FBGs. The FBG is coated at different locations with materials having different coefficient of thermal expansion and stiffness. The differential thermal expansion gives rise to an effective strain at the FBG which can compensate the wavelength shift due to temperature change. Theoretical analysis of the proposed model has been carried out and the effect of coating length and thickness is analyzed. It is proved theoretically that almost zero wavelength shifts can be achieved by optimizing the design of the coating.

S-band amplification with >30 dB peak gain at 1500 nm, >20 dB gain for wavelengths between 1475 nm and 1520 nm, and 5 dB noise figure is demonstrated in Erbium-doped Alumino-germanosilicate fiber. Using standard MCVD processing and solution doping, we combined a depressed-cladding fiber design with erbium doping to create a new type of gain fiber. A fundamental mode cutoff near 1530 nm provides distributed suppression of C-band amplified spontaneous emission, thereby enabling the high population inversion required for S-band gain. This type of S-band amplifier is compatible with standard fusion splicing techniques and is pumped by standard 980 nm pump lasers. In this talk, we will describe gain and noise characteristics for several amplifier architectures, gain saturation characteristics, and gain flattening.

Tunable laser diodes are now widely used in dense wavelength division multiplexing (DWDM) optical fiber networks. A wide tunability range and good frequency stability are generally required, without mode hopping. Special applications require fast frequency measurement. Unfortunately, commercially available classical spectral measurement systems are much too slow for that purpose. In this paper, we present a new and low-cost system dedicated to fast spectral measurements based on a Michelson interferometer. The free spectral range (FSR) of the interferometer must be carefully optimized. We have designed optimized and tested an experimental system that can quickly analyze with good accuracy the tunability range of tunable distributed Bragg reflector (DBR) laser diodes, and their possible mode hopping. Our set-up can also be used for other spectral measurements (linewidth, chirp, Henry's alpha enhancement factor, spectral and power monitoring). After theoretical analysis of our experimental set-up, we finally present the fast spectral measurements results obtained with a typical 1550 nm two sections DBR QW (quantum well) laser diode. Our measurements have been obtained within less than 500 ps, which is much faster than limits induced by switching thermal effects in laser diodes, and that speed could still be increased.

An advanced non-traditional method of single expression (MSE) is used to analyze uniform and phase-shifted amplifying fiber Bragg gratings (FBGs). In the MSE Helmholtz's equation solution is presented in the generalized form of a single expression, contary to the widely accepted counter-propagating waves approach. Brief description of the principal statements of the MSE is presented. Uniform FBGs and FBGs with π phase shift in the middle of the structure are considered. Spectral dependences of gainless and amplifying uniform and π phase-shifted FBGs are obtained. Spatial distributions of the electric field amplitude, real part of permittivity and power flow density along the structure of FBG and in outside media are presented at central point of transparency. Simulations carried out on amplifying π phase-shifted FBGs showed that they can be used as narrow-band pass-band filter-amplifiers for DWDM systems.

Superluminescent diodes with broad emission bandwidth characteristics and the mechanism of carrier distribution in the active layer are explored. Asymmetric active layer structure is used for the broadband purpose. Using InP substrate with five 60Å InGaAsP quantum wells and two 150Å InGaAs quantum wells, we get a very broad emission spectrum. The spectral width is nearly 400 nm, almost covering the range from 1250nm to 1650nm.

We normally apply a precise amount of electrical feedback into the laser injection current, to stabilize the oscillation frequency of a semiconductor laser. This feedback method usually needs a small direct modulation to the laser injection current, to obtain an error signal. This broadens the oscillation width of the laser diode, but certain applications, such as those related to coherent optical communications, benefit from, and in fact require narrower oscillation linewidth. We obtain the error signal, and stabilize the laser oscillation frequency in narrower oscillation linewidth, using the Faraday effect of the Rb absorption line. Our next task involves frequency-stabilization, which we accomplish, using a large frequency discrimination gain (G_d). By incorporating our "PEAK" circuit, which utilizes the envelope detection method to determine the switching points between two different absorption signals corresponding to the different magnetic fields and different polarization conditions, we increases the G_d in our stabilization system.

We propose and demonstrate a new technique for realizing wavelength tuning that promises rapid response over a wide optical bandwidth. Wavelength tuning is obtained by varying the delay of the RF pulse that time-gates a linearly chirped Super Continuum (SC). Preliminary experiments demonstrating static tuning range of 90nm are reported.

The optical add/drop multiplexer (OADM) is an important device in modern optical networks. Optical filters in OADMs often introduce group-velocity dispersion (GVD) and/or slope of GVD, the accumulation of which could distort the signals significantly. A computer model is built for commercial filters, accounting for the filtering gain and dispersion characteristics. When the model is incorporated into a network simulator, the filter dispersion is found to severely limit the number of OADMs that may be cascaded when transmitting 40Gb/s WDM signals with a channel spacing of 100GHz. As such high spectral efficiency difficult to achieve, the next considerations would be to transmit 40Gb/s over 200GHz channel spacing, or 10Gb/s over 50GHz channel spacing. The dispersion problem is mitigated, but still an un-negligible factor of limitation. For a large OADM network size, low-dispersion filters should be used, or a proper dispersion compensator is needed to offset the filter dispersion.

The single mode fibers with chromatic dispersion varying along the length are an attractive medium for nonlinear optics. For optical soliton, a small dispersion varying perturbs soliton in the same way as an amplification or loss. Such fibers allow to realize both the regime of effective amplification and the effective compression of optical solitons. The method to draw fibers varying along the length from standard preform had been developed. The length of fiber with varying dispersion may be in the range from several meters to several kilometers. The dispersion deviation from the prearranged value is less than 0.1ps/nm/km. In accordance with modern theoretical underground it is possible to develop the DDF (dispersion decreasing fiber) with advanced dispersion profile to reduce radically the timing jitter and extend the transmission distance. In addition, one is able to design the dispersion profile of dispersion varying fiber (DVF) in such way that it would be possible to apply one and the same DVF both in linear WDM and soliton communication systems.

Photodiode structures were integrated in a low cost 0.5 μm silicon BiCMOS process using standard process flow without any technology modification. Rise times of 1.3ns and 1.4ns were measured for wavelengths λ of 660nm and 780nm with a responsivity of 0.23 A/W and 0.14 A/W, respectively. Photodiodes with a high responsivity of 0.42 A/W are reaching a risetime of 4ns for λ = 780nm. Comparable low values for the risetime at 780nm are reported in the literature for integrated photodiodes in standard silicon technologies only with a modification of the epitaxial substrate material. So this photodiodes are suitable for a wide variety of low-cost high-speed optical sensor applications, for optical fiber communication and fiber in home applications.

Results are presented of an investigation into the performance of an optical subcarrier multiplexed (OSCM) system using a novel simulation approach. The OSCM system analyzed is based upon two systems which have been demonstrated experimentally previously. Intersymbol interference and crosstalk degradation effects for both the baseband and subcarrier signals are calculated.

We investigate theoretically and experimentally the performances of two cascaded Semiconductor Optical Amplifiers (SOAs) in a counter-propagating topology. This configuration accentuates the gain non-linearity due to the mutual modulation of the two SOAs. Experimental results are obtained by using two SOAs of 500 μm and 750 μm lengths. By using this configuration, we present the performances of two all-optical functions. Firstly we demonstrate experimentally the all-optical switching operation for 2.5 GHz data pulses. Secondly we show the principle and the experimental results of all-optical logic NOR gate. For these two functions, an extinction ratio higher than 12 dB can be obtained for a wide range of input data wavelength between 1490 nm and 1560 nm.

A novel concept of Photonic Bandgap Quasi-Crystal (PBQC) as a platform for planar integrated WDM optical devices is proposed. The PBQC can be lithographically fabricated in a planar waveguide as a computer-generated two-dimensional hologram. In this approach the spectral selectivity of Bragg gratings, focusing properties of elliptical mirrors, superposition properties of thick holograms, photonic bandgaps of periodic structures, and flexibility of lithography on planar waveguides are combined. In distinction to conventional combination of independent planar Bragg gratings, in PBQC we create multiple bandgaps by synthesizing a synergetic super-grating of a number of individual sub-gratings. The device spectral selectivity is determined by those of the sub-gratings. The super-grating comprises million(s) of dashes etched on an interface of a planar waveguide. Each dash is a binary feature placed by a computer program to serve simultaneously many channels. For realization of PBQC devices the software for generating super-gratings (GDS-II format) and 2-D simulation of its transfer function was developed. Direct e-beam writing and photolithography were used for manufacturing PBQC structures. For verification of the ideas behind the concept a number of multichannel MUX/DEMUX devices have been manufactured and experimentally tested. The results of detailed experimental study of 4- and 16-channel devices will be presented. Channel isolation ~30 dB was achieved in the 4-channel devices. The applications of PBQC platform for integrated light wave circuits are discussed.

A novel MEMS-based tunable optical filter, an essential component for monitoring and reconfiguration of optical wavelength-division multiplexing networks, is presented. The device is based on a Fabry-Perot interferometer employing multiple solid-state silicon cavities and silicon-based dielectric Bragg mirrors. Tuning is achieved through thermal modulation of the resonator's optical thickness. It is fabricated as a free-standing membrane using silicon MEMS technology. The filter membrane is fixed by micromachined suspension arms, which thermally isolates it against the substrate. The present concept features low power consumption and fast thermal modulation. Light coupling to the filter array is realized by positioning of fibers and the filter chip in a micro-optical bench setup.

We demonstrate a new all-fiber spectrometer based on acousto-optic tunable filter (AOTF) on cladding etched single-mode (SM) fiber. The spectrometer has a free spectral range (FSR) of 200 nm, a wavelength resolution of 2 nm, a dynamic range of 20 dB and a potential scanning rate of ~125 μs/sample. The spectrometer has the potential to be developed as a low-cost spectral monitoring device for dynamic gain equalizer (DGE) in Er-doped fiber amplifier modules.

Thin film interference coatings (TFIC) are the most widely used optical technology for telecom filtering, but until recently no tunable versions have been known except for mechanically rotated filters. We describe a new approach to broadly tunable TFIC components based on the thermo-optic properties of semiconductor thin films with large thermo-optic coefficients 3.6X10[-4]/K. The technology is based on amorphous silicon thin films deposited by plasma-enhanced chemical vapor deposition (PECVD), a process adapted for telecom applications from its origins in the flat-panel display and solar cell industries. Unlike MEMS devices, tunable TFIC can be designed as sophisticated multi-cavity, multi-layer optical designs. Applications include flat-top passband filters for add-drop multiplexing, tunable dispersion compensators, tunable gain equalizers and variable optical attenuators. Extremely compact tunable devices may be integrated into modules such as optical channel monitors, tunable lasers, gain-equalized amplifiers, and tunable detectors.

A demultiplexer utilizing multiple grating to increase the performance of current demultiplexer scheme is presented. Two similar approaches employing two different grating configurations are used to determine the maximum DWDM (Dense Wavelength Division Multiplex) signal separation.

We demonstrate an optical switch with a holographic Mach-Zehnder interferometer in a bulk photorefractive crystal. We perform theoretical studies of the grating recording conditions, required crystal orientation, and applied voltage for this photorefractive optical switch. We experimentally demonstrated the switch in a LiNbO₃:Fe crystal cut in the shape of an elongated hexagon using the 514 nm line from an argon laser. A modulation depth of 75% was obtained experimentally.

Direct bandgap of most III-V semiconductors (AlGaAs, InGaAs, InGaP, InAs) increases with hydrostatic pressure at the rate of about 10 meV per kbar. Thus the emission wavelength of semiconductor lasers shifts to the blue under the application of high pressure. We demonstrate that this effect can be used for wavelength tuning of laser diodes in a very wide spectral range. Using the specially designed liquid pressure cell working up to 20 kbar the 1550 nm laser was tuned down to 1270 nm, the 1300 nm laser was tuned down to 1100 nm, and the 980 nm laser was tuned down to 840 nm. The emitted light passes through the sapphire window or through the fiber directly coupled to the laser. The threshold current and the quantum efficiency for the 980 nm laser remained constant with pressure, for the two other lasers the thresholds decreased with pressure. Thus we obtained the constant emission power in the full tuning range. We hope that this compact device will find applications as a tool for characterization of some optical network devices or parts of optical transmission lines.