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High contrast structures with a sub-wavelength pitch, small enough to suppress diffraction, exhibit extraordinary optical properties: depending on the design they may behave as perfect mirrors, anti-reflective interfaces, homogenous materials with controllable refractive index, or strongly dispersive materials. Here we discuss on the design possibilities such structures offer in planar waveguide devices in silicon-on-insulator. We briefly review the application of sub-wavelength structures in a variety of waveguide devices. We then focus on some of the latest advances in the design ultra-compact and ultra-wideband multimode interference couplers based on dispersion engineered sub-wavelength structures., High Contrast Metastructures III, February 4-6, 2014, San Francisco, CA, USA, Series: Proceedings of SPIE; no. 8995

Grating couplers have shown promising performance in terms of coupling efficiency and alignment tolerances as an interface between optical fibers and nanometric silicon-on-insulator (SOI) waveguides. In this paper we review our previous work, where the implementation of a fiber to chip grating coupler in micrometric rib SOI waveguide was demonstrated for the first time, showing measured coupling efficiency of -2.2dB for transverse-electric (TE) polarization. We also propose a new grating design that achieves a calculated coupling efficiency of -2dB for transverse-magnetic (TM) light. © 2012 SPIE., Silicon Photonics and Photonic Integrated Circuits III, April 16-19, 2012, Brussels, Series: Proceedings of SPIE; no. 8431

Three examples of cavity-enhanced measurements of refractive index and optical absorption are discussed. Using microphotonic silicon-on-insulator ring-resonators we determine the concentration of cyclohexane and m-xylene at detection levels of 300-3000 ppm. The gases are first absorbed into a siloxane polymer and its refractive index change is detected by a characteristic wavelength shift of the cavity resonance. In a second device phase-shift cavity ring-down spectroscopy is applied to simultaneously measure the optical absorption at two wavelengths of either a dye, nucleic acids or a pharmaceutical component. Multiplexing the ring-down measurement permits dual wavelength absorption spectroscopy without the use of a dispersion element. Finally, a combination of resonance wavelength measurements and cavity ring-down spectroscopy is used to simultaneously determine the change in refractive index and the absorption induced by adsorption of ethylene diamine on a 300 μm silica sphere. A whispering gallery mode of the microsphere resonator is excited with intensity modulated light and the intensity and AM modulation phase of the Rayleigh backscattered light is measured. © 2012 Copyright Society of Photo-Optical Instrumentation Engineers (SPIE)., Photonics and Optoelectronics Meetings (POEM) 2011: Optoelectronic Sensing and Imaging, November 2-5, 2011, Wuhan, Series: Proceedings of SPIE; no. 8332

Multiple Aperture Transform Chip Heterodyne (MATCH) spectrometers have been developed for targeted remote sensing applications in harsh environments. These waveguide-based Fourier Transform Spectrometers (FTS) offer significant improvements in resource efficiency over monolithic glass implementations, but are relatively limited in terms of input coupling efficiency and fill factor of the input facet. Integrated optics spectrometers have significant resource advantages for space applications. Monolithic Spatial Heterodyne Spectrometers are insensitive to vibration and do not require frequent calibration. In addition, Fourier Transform Spectrometers are known to provide significant performance advantages for emission spectroscopy. Ongoing work will improve the MATCH spectrometer input coupling efficiency from free space. This paper discusses the signal to noise improvements expected by incorporation of surface gratings, or back-thinning and stacking of slabs. We show that the use of surface gratings can increase the throughput over coupling to bare waveguides alone (in a single polarization), and provide close to 100% fill factor, albeit with limited field. Étendue improvements associated with stacked slabs are limited only by the sensing area available, but the fill factor of the input facet is limited to ∼10%. The impact of these improvements is assessed in the context of two space-based applications: 1) Atmospheric remote sensing in the context of Spatial Heterodyne Observations of Water (solar occultation absorption spectroscopy) near 1.3 μm and 2) Point emission spectroscopy (LIBS/Raman/fluorescence) for mineral identification on a planetary rover. © 2011 Copyright Society of Photo-Optical Instrumentation Engineers (SPIE)., Reliability, Packaging, Testing, and Characterization of MEMS/MOEMS and Nanodevices X, January 24-25, 2011, San Francisco, CA, USA, Series: Proceedings of SPIE; no. 7928

We are developing a photonic wire evanescent field (PWEF) sensor chip using 260 nm x 450 nm cross-section silicon photonic wire waveguides. The waveguide mode is strongly localized near the silicon surface, so that light interacts strongly with molecules bound to the waveguide surface. The millimeter long sensor waveguides can be folded into tight spiral structures less than 200 micrometers in diameter, which can be arrayed at densities up to ten or more independent sensors per square millimeter. The long propagation length in each sensor element gives a response to molecular binding much better than currently available tools for label-free molecular sensing. Cost of instrumentation, cost per measurement, ease-of-use, and the number of sensors that can be simultaneously monitored on a sensor array chip are equally important in determining whether an instrument is practical for the end user and hence commercially viable. The objective of our recent work on PWEF sensor array chips and the associated instrumentation is to address all of these issues. This conference paper reviews our ongoing work on the photonic wire sensor chip design and layout, on-chip integrated fluidics, optical coupling, and chip interrogation using arrays of grating couplers formed using sub-wavelength patterned structures. © 2011 Copyright Society of Photo-Optical Instrumentation Engineers (SPIE)., SPIE BiOS, January 22-23, 2011, San Francisco, CA, USA, Series: Proceedings of SPIE; no. 7888

The gain media of the quantum dot lasers consist of InAs QDs in an InGaAsP matrix on an InP substrate. The quantum dot lasers have different free spacing ranges (FSRs) corresponding to Fabry-Perot (F-P) cavity lengths. A silicon ring resonator and a QD laser have been combined to form comb laser. The output characteristics of the combined comb laser were investigated. The measured FSR was about 2.8nm and the extinction ratio was about 10(dB) when the FSR of the QD laser was about 0.4nm and the FSR of the ring resonator was about 0.47nm. The experimental results show that the ring resonator had a strong control on the FSR and extinction ratio of the comb laser.

We calculate the π-power and rise time for several 1×1 y-branch MZI SOI thermo-optic switches as a function of device size. These switches consist of waveguide core thickness ranging from 10 to 0.22 μm. Upon scaling the core thickness, the best power and speed performance occurs at 0.7 μm. Further miniaturization results in an increased power and reduced speed, mainly due to the mode expansion from the core into the cladding. We show that varying the MZI arm separation is an effective approach to improve the performance of miniaturized thermo-optic switches.

We present an SOI biosensor microarray chip that allows multiple molecular binding reactions to be simultaneously monitored. The individual biosensors are formed using 0.26 × 0.45 μm 2 silicon photonic wire waveguides, which are arranged in compact Mach-Zehnder interferometer geometries with near temperature independent response. The sharp bend radius permitted by the photonic wires is exploited to form dense spiral waveguide structures that provide several millimeters of path length in a compact 130 μm diameter circular area. This design provides the high sensitivity of a long waveguide, while maintaining compatibility with commercial microarray spotting tools. For low volume analyte delivery the sensor array chip contains a monolithically integrated microfluidic channel formed in an SU-8 overlayer. Multiple antibody-antigen reactions are observed in real-time by using an infrared camera to monitor the optical powers emerging from the sensor array output waveguides.

We present development of a compact, robust, waveguide Fourier-transform microspectrometer for high-resolution and high-throughput spectroscopy in space-based applications. The prototype device is being developed to monitor water vapor in the atmosphere from a micro-satellite platform. The instrument is based on a unique slab waveguide spatial heterodyne spectrometer (SHS) chip fabricated at the National Research Council Canada in silicon-on-insulator (SOI) technology.

We report on new types of sub-wavelength grating gradient index structures for efficient mode coupling in high index contrast slab waveguides. Using sub-wavelength gratings (SWGs), an adiabatic transition can be formed at the interface between slab waveguides of different core thicknesses. The SWG transition region minimizes both mode mismatch loss and coupling to higher order modes. By creating the adiabatic gradient effective index region in the direction of propagation, we demonstrate that vertical mode size transformation is readily achieved in structures that can be fabricated using a single etch step. Using 3D finite-difference time-domain simulations we study the loss, polarization dependence and the higher order mode excitation for two types (triangular and triangular-transverse) of SWG transition regions between silicon-on-insulator slab waveguides of different core thicknesses. We also demonstrate two solutions to mitigate polarization dependent loss, namely using a partial transverse SWG and a SWG in the region between the triangular teeth. Our mode transformer designs are optimized for applications in polarization compensator in echelle grating and arrayed waveguide grating (AWG) multiplexers and a gradient index interface between the ridge waveguide and the slab combiner region in a curved waveguide grating demultiplexer.

We review our work developing label-free molecular sensors using silicon nanophotonic waveguides. We show that the high index contrast of these waveguides provides high surface sensitivity and enables compact sensor designs to be realized. We also describe new waveguide circuit geometries that allow photonic wire waveguide sensors to be conveniently arrayed in two dimensions for compatibility with commercial spotting tools, while simultaneously providing long interaction length and improved molecular capture efficiency. These sensor designs are shown to provide a practical route to the development of label-free, microarrayed biochips for multiparameter analysis.

We present multiaperture stationary spectrometers in planar optical waveguides. The devices are based on the spatial heterodyning technique, do not require moving parts, and use Fourier transformation for spectra retrieval. The design is based on arrays of waveguide interferometers with linearly increasing optical path delay. The spectrometers have increased optical throughput due to multiple input waveguides. We discuss design, fabrication, and first experimental results for these multiaperture spectrometers implemented in silicon-on-insulator (SOI) ridge waveguides.

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.

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.

We demonstrate Mach-Zehnder interferometer and ring resonator evanescent field sensors fabricated on the silicon-on-insulator material platform. These devices exploit the strong evanescent field of the transverse magnetic mode of a high index contrast, submicron dimension waveguide to obtain strong interaction of the guided mode with biomolecules adsorbed to the waveguide surface. We utilize the extremely small bend radius achievable with silicon photonic wire waveguides to fabricate spiral and folded waveguide sensors that maintain the high sensitivity of a long waveguide, while providing compact footprint. These devices offer a suitable geometry for the development of sensor arrays and provide compatibility with commercial spotting functionalization systems. We demonstrate sensors containing waveguides of up to 5.5 mm length that are contained within a 150 μm × 150 μm area.

We propose an original diffraction grating demultiplexer (spectrometer) device with a very small footprint, designed for the silicon-on-insulator waveguide platform. The wavelength dispersive properties are provided by a second-order diffraction grating lithographically defined and etched in the sidewall of a curved Si waveguide. The grating is blazed to maximize the -1 st order diffraction efficiency. The diffracted light is coupled into the silicon slab waveguide via an impedance matching sub-wavelength grating (SWG) graded-index (GRIN) anti-reflective interface. The waveguide is curved in order to focus the light onto the Rowland circle, where different wavelengths are intercepted by different output waveguides. The phase errors were substantially reduced using an apodized design with a chirped grating pitch, which assures a constant effective index along the grating length. The simulated crosstalk is -30 dB. The device has 15 channels with a spacing of 25 nm, thus a broadband operational bandwidth of 375 nm. Its performance approaches the diffraction limit. The device layout size is 90 μm × 50 μm, which is the smallest footprint yet reported for a mux/dmux device of a similar performance.

Design and simulations of a Fourier-transform planar wavegui de spectrometer are presented in the context of space-born observations of water in the near infrared spectral region. Spatial heterodyning of an optical signal is realized by using arrayed waveguide structures which prod uce spectrally-dependent interference fri nges. The light spectrum is calculated using discrete spatial Fourier transforma tion of the fringes. The arrayed wavegu ides form a multi-aperture input which markedly increases the optical throughput (etendue) of the device compared to single-aperture spectrometers. Keywords: spectrometers, waveguides, Fourie r spectroscopy, optical circuits 1. INTRODUCTION High-resolution microspectrometers have important applications in optical telecommunications, genomics and health diagnostics, environmental monitoring, and space-born sensing. In applications where small size, robustness and simplicity of fabrication are important it is advantageous to employ devices based on planar optical waveguides. For instance, waveguide spectrometers

Waveguide birefringence almost always exists, except for cases where the two orthogonally polarized modes are incidentally degenerate. Birefringence is in general undesirable as it causes an unwanted polarization dependent phase shift and wavelength shift in all interferometric devices such as spectrometers, Mach-Zehnder interferometers, and ring resonators. The birefringence due to the waveguide core geometry in high-index-contrast material systems such as silicon-on-insulator can significantly increase with decreasing core dimensions. One solution is to separate the two orthogonally polarized TE and TM components of the optical signal and process them individually. Waveguide polarization splitters/filters are the key elements in this polarization diversity approach. In this work, novel passive polarization splitters/filters in the SOI platform using a geometrically balanced Mach- Zehnder interferometer configuration are demonstrated experimentally. Polarization splitting/filtering functions are achieved by modifying the birefringence in one arm of the interferometer using a stressed cladding film. Only one additional fabrication step is required for patterning a stressed cladding film on top of the silicon ridge waveguides. A broadband polarization splitting performance was observed with an average extinction ratio of 10 dB in each of the output ports over the entire C-Band from 1530 to 1565nm. The device size is 2.5 mm x 16 μm. This work represents the first reported use of stress engineering for making SOI polarization splitters/filters.

A monolithically integrated asymmetric graded index (GRIN) waveguide structure for coupling light into high index contrast waveguides is described. When analyzed in terms of its waveguide modes, the GRIN coupler is shown to be a multimode interference (MMI) device. The design parameters and tolerances are calculated for quadratic index profile and uniform index amorphous silicon (a-Si) GRIN couplers optimized for coupling light into silicon-on-insulator waveguides. Calculations of coupling efficiencies into 0.5 μm SOI waveguides show that asymmetric GRIN couplers operate over a very wide wavelength range with low polarization dependence, and fabrication requires lithographic resolution of only ±1 μm. Experimental results are presented for a 3 μm thick single layer a-Si coupler integrated with a 0.8 μm SOI waveguide. The measured variation of coupling efficiency with coupler length is in agreement with theory, with an optimal coupling length of 15 μm.

We review the use of the oxide cladding stress induced photoelastic effect to eliminate the modal birefringence in silicon-on-insulator (SOI) ridge waveguide components, and highlight characteristics particular to high index contrast (HIC) systems. The birefringence in planar waveguides has its origin in the electromagnetic boundary conditions at the waveguide boundaries, and can be further modified by the presence of stress in the materials. It is shown that geometrical constraints imposed by different design and fabrication considerations become increasingly difficult to satisfy with decreasing core sizes. On the other hand, with typical stress levels of -100 MPa to -400 MPa (compressive) in SiO2 used as the upper cladding, the effective indices are altered up to the order of 10-3 for ridges with heights ranging from 1 μm to 5 μm. We demonstrate that the stress can be effectively used to balance the geometrical birefringence. Birefringence-free operation is achieved for waveguides with otherwise large birefringence by using properly chosen thickness and stress of the upper cladding layer. This allows the waveguide cross-section profiles to be optimized for design criteria other than zero-birefringence. Since the index changes induced by the stress are orders of magnitude smaller than the waveguide core/cladding index contrast, changes in the mode profiles are insignificant and the associated mode mismatch loss is negligible. We study the stress-induced effects in two parallel waveguides of varying spacing, to emulate the condition in directional couplers and ring-resonators. In the arrayed waveguide grating (AWG) demultiplexers fabricated in the SOI platform, we demonstrated the reduction of the birefringence from 1.3x10-3 (without the upper cladding) to below 1x10-4 across the spectral band by using a 0.6 μm oxide upper cladding with a stress of -320 MPa (compressive). Design options for relaxed dimensional tolerance and improved coupler performance made available by using stress engineering are discussed.

Coupling light into and out of small high index contrast waveguides is a fundamental challenge to implementing practical microphotonic waveguide and photonic crystal devices. Previous approaches include three-dimensional tapers, inverse taper waveguides, and grating based couplers. We propose and describe a much simpler coupler based on a short length of graded index (GRIN) material deposited on top of a silicon-on-insulator (SOI) microphotonic waveguide. The GRIN coupler has a refractive index that decreases from the index of silicon at the waveguide-coupler interface, to an optimized value at the coupler surface. Beam propagation method calculations are used to evaluate the coupling efficiency from a 4 μm thick coupler section to the fundamental mode of a 0.5 μm thick SOI waveguide. Coupling efficiencies are compared for couplers with smoothly varying quadratic index profiles and with one, two and three index steps. Coupling efficiencies of 75% (1.3 dB) or better are predicted using a three step GRIN structure with indices ranging from n=3.30 to 3.41 (Si). This index range is easily accessed using a-Si layers deposited by PECVD at varying deposition conditions, or by using composite digital alloys of high and low index films. With this method, microphotonic waveguide couplers can be designed and fabricated using only PECVD deposition and one patterning etch step with very modest tolerances. Efficiency increases to 87% (0.6 dB) when the index range of the 3-step coupler is extended to 3.0.

Progress in optical switching technology currently faces several major obstacles. One of these is high power consumption, which quickly multiplies in cascaded switch configurations. As well, many implementations have long switching speeds and large footprints. An improved compact 1x2 digital optical switch (DOS) in InGaAsP/InP is presented, with experimental results compared to numerical modeling. The Y-junction waveguide switch operation is based on reconfiguration of output waveguide arms by carrier injection at the electrodes. We present experimental results of DOS fabricated with InGaAsP cores having bandgaps of 1.2um, 1.3um, and 1.4um. The results are compared with calculations of refractive index change versus carrier concentration in the different InGaAsP alloys. Additionally, wafer layer structure and waveguide parameters were redesigned to decrease, respectively, power dissipation and optical confinement. Switching current is significantly reduced from greater than 100 mA to about 20 mA, which not only provides power savings, but also results in less thermal overshoot in the switched optical pulse. The DOS has a measured switching contrast ratio of better than 12.5 dB, with a transition time of less than 5ns. Polarization dependence of switching contrast is also explored.

A compact Y-junction waveguide switch with electrically reconfigurable output waveguide arms is demonstrated in InGaAsP/InP. Simulations indicate that the plasma effect or the thermo-optic effect can be used as the active switching mechanism, as corroborated by experimental tests. For the plasma effect the induced index change under the electrode, Δn, is negative. The Y-junction device has a measured switch contrast ratio ~ 20 dB, with a response time of ~ 5 ns. Using the thermo-optic effect Δn is positive and the observed contrast ratio is better than 10 dB. The highly localized nature of the thermal gradient in these devices yields thermo-optic switching into the hundred of nanoseconds range, several orders of magnitude faster than the overall thermal response time. This is the fastest thermo-optic switch reported to date. Fabrication of these switches, and in particular the use of O+-ion implantation to provide electrical isolation of the waveguide branches, is described.© (2004) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

We propose a modified arrayed waveguide grating (AWG) demultiplexer formed by an array of parallel ridge waveguides of equal length fabricated in silicon-on-insulator (SOI). Each waveguide contains two sections with different widths. The lenghts of the two sections are modified from a waveguide to the adjacent waveguide, resulting in a wavelength dispersive phase array. With an appropiate demultiplexer output geometry, the light is focussed in the output slab waveguide combiner. The focal position is wavelength, dependent in the transverse direction, so that the optical signal is demultiplexed into separated output channels corresponding to different spectral bands. This device does not require bending waveguide sections which are essential in a conventional AWG demultiplexer, yielding very compact devices with potentially low loss. Numerical simulations have, been carried out to evaluate the influence of waveguide parameters such as lengths and widths of different waveguide sections on demultiplexer performance.

We show that stress engineering can be used to adjust the SOI waveguide birefringence to the stringent polarization tolerances expected of commercial devices, using only standard silicon processes. With decreasing device dimensions and high index contrast the waveguide birefringence becomes increasingly sensitive to device geometry. As a result, it is almost impossible to eliminate waveguide birefringence by adjusting waveguide profile alone. This paper presents, for the first time, a systematic study of the stress-induced birefringence of SOI waveguides. Through full-vectorial finite-element simulations, we investigate the variation of stress-induced birefringence with waveguide core and cladding geometries. It is found that the stress-induced birefringence is determined by the waveguide cross-section, the upper cladding layer thickness, and film stress levels. We develop a waveguide model that predicts the total waveguide birefringence and guides the post-fabrication processing steps. An experimental demonstration of a polarization insensitive SOI arrayed waveguide grating (AWG) demultiplexer is presented. The polarization dependent wavelength shifts measured experimentally agree well with the simulations.

We discuss the challenges encountered when scaling down photonic waveguide devices, and demonstrate possible solutions in silicon-on-insulator (SOI) platform. First, sources of waveguide birefringence such as waveguide geometry and stress in the waveguiding layer are discussed. Birefringence sensitivity to inaccuracy of waveguide dimensions is compared for different waveguide geometries, including trapezoidal and rectangular cross-sections. Results show that trapezoidal waveguides are more robust, which makes fabrication tolerances less stringent. Methods for minimizing the waveguide birefringence using stress induced by an over-cladding dielectric film, and by inducing form birefringence through deposition of thin layers of high and low refractive index materials, are discussed. Compact arrayed waveguide grating (AWG) devices are presented, with internal loss of -5.9 dB, crosstalk better than -20 dB, and polarization dependent wavelength shift of

As the AWG size is reduced our experimental and theoretical work demonstrates that it becomes increasingly difficult to suppress higher order modes and birefringence using ridge dimension alone. In part, it simply becomes difficult to meet the required fabrication tolerances when the ridge dimension approaches the order of a micron. We show that a novel polarization compensator scheme similar to that previously reported for a grating based demultiplexer in InP and consisting of simple shallow etched regions in the combiner sections of an SOI AWG, can eliminate the polarization sensitivity of the device by reducing the initial polarization dispersion of 2.22 nm to 0.04 nm. By combining the polarization compensator with mode filtering using appropriate array waveguide curvature, the shape of the array waveguides is no longer constrained. This allows the size of an AWG device to be scaled down to very small dimensions (e.g. less than a millimeter) and also permits the use of simple fabrication techniques such as wet etching. Our results were obtained on AWG devices based on 1.5 micrometers thick Si-on-insulator waveguides with a typical waveguide array area of a few square millimeters.

Novel Si waveguide MSM photodetector suitable for high speed/high quantum efficiency applications is proposed and demonstrated. Silicides are formed on a silicon-on-insulator (SOI) substrate through metal/Si reaction under heat treatment, in two areas separated by a narrow gap. The silicide sidewalls on the two sides of the narrow gap provide lateral waveguide confinement, and also serve as electrodes. The silicide/Si interface forms a Schottky junction, making the structure a MSM diode. The waveguide structure provides a long optical path length to increase the quantum efficiency at near infrared wavelengths. The distance between electrodes can be changed easily through photolithography, and can be made very small to reduce the transit time between electrodes for high-speed operation. Since the devices are made on SOI substrates, the drift component of the photocurrent can be eliminated, further facilitating high-speed operation. First set of photodetectors was made using PtSi on commercially available SOI substrates with 0.34micrometers Si layer. Initial experiments have demonstrated a responsivity of near 200mA/W at (lambda) equals980 nm for a detector with 486micrometers long electrodes and 2 micrometers gap size. The dark current was on the order of 0.1 nA/micrometers 2 at 5V bias.

Quasi phase-matched second-harmonic generation in the co-propagating geometry is demonstrated in an asymmetric quantum well waveguide. Modulation of the nonlinear susceptibility along the waveguide was achieved using a patterned quantum well intermixing process. Photoluminescence measurements ofthe quanturn well band edges indicated that a grating of alternating regions of intermixed and as-grown asymmetric quantum wells was produced for periods between 2 jtm and 12 jim . The variation of the second harmonic intensity generated by guided incident light between A=1480 nm and 1600 nm was measured. The resulting second-harmonic spectra show sharp quasi-phase matching resonances for grating periods near 3 pm , demonstrating that a periodic modulation of the quantum well nonlinear susceptibility was produced.

This paper presents theoretical and experimental results detailing the design and performance of arrayed waveguide grating (AWG) demultiplexers fabricated in silicon-on- insulator (SOI). The SOI waveguide is inherently multimode because of the high refractive index difference between Si and SiO2, although appropriate tailoring of the rib width to height ratio can be used to make single mode rib waveguides. This single mode condition cannot be met in the input and output combiner sections, which can therefore support many higher order modes. Modeling results demonstrate that coupling from a single mode ridge waveguide to the fundamental slab mode is typically two orders of magnitude larger than the coupling to higher modes. Hence the effect of multimode combiners on performance should be minimal. We also present calculations of bending losses which indicate that with a Si thickness of 1.5 micrometers , single mode rib waveguides can be made with radii of curvature as low as 200 micrometers . Such waveguides can also be made with zero birefringence. AWG devices were fabricated with 8 channels centered around (lambda) equals 1550 nm, and chip sizes less than 5 X 5 mm. The performance of these devices is compared with our modeling results.© (2000) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

We report on the development of building blocks of Si/Si 1- x Ge x /Si planar waveguide WDM circuits -- an optical splitter/combiner and the first demonstration of an Si/Si 1- x Ge x /Si arrayed waveguide grating (AWG) demultiplexer. The prototype AWG is a four channel device with a 400 GHz ((Delta) (lambda) equals 2.3 nm) channel spacing, and operates in the 1300 nm wavelength range. Strain and refractive index in the Si 1-x Ge x epilayers play a critical role in device performance. The AWG waveguide heterostructure has total Si 1-x Ge x layer thickness well beyond the measured critical thickness for lattice relaxation, but is stabilized against dislocation formation by the insertion of Si capping layers during growth. Single mode curved ridge waveguides formed from this material show no obvious bend losses for radii of curvature as small as 4 mm.

For Si-based photonic integrated circuits (PICs), photodiodes with good responsivity at 1.3 micrometer and 1.55 micrometer wavelengths made of Si-based materials are highly desirable. Previously, work has been reported using epitaxial SiGe planar multiple quantum wells (MQWs) on Si substrates. Since the high lattice mismatch limits the maximum Ge concentration and SiGe layer thickness, responsivity at 1.55 micrometer was limited. Under appropriate growth conditions, strained SiGe QW's grow with periodic thickness variations along the surface plane. Ge tends to migrate towards the thickness maxima. This increase in local Ge concentration and the reduced quantum confinement at the coherent wave crest produces strained QW's with significantly lower band-gaps compared to planar QW's with the same nominal composition. In this paper, we report the first MSM SiGe waveguide photodetectors fabricated using coherent wave growth mode with a band gap below 800 meV. The heterostructures were grown on a SOI substrate by an ultra- high vacuum chemical vapor deposition (UHVCVD) system. The 2 micrometer thick Si/SiGe/Si on oxide structure provides waveguiding for the detector structures and permits effective fiber coupling. Preliminary measurements have demonstrated internal responsivities of approximately 1 A/W at 1.3 micrometer wavelength and 0.1 A/W at 1.55 micrometer wavelength for a 240 micrometer long device.

The design of manufacturable Si/Si 1-x Ge x waveguide WDM components involves several unique materials and fabrication issues which must be confronted and resolved. Accurate data for the refractive indices of the waveguide materials are essential. Furthermore, the waveguide design is tightly constrained by the requirement that Si/Si 1-x Ge x layer thickness is within the pseudomorphic growth limit. By combining refractive indices determined from mode profile measurements of MBE and CVD grown waveguides, and epilayer thickness constraints set by the pseudomorphic growth limits, we have determined a set of design criteria for Si/Si 1-x Ge x waveguides for WDM and optical signal routing applications. Optically smooth and vertical Si/Si 1-x Ge x waveguide facets are critical in permitting highly efficient coupling between the fiber and the Si chip. Since Si has an equal probability of cleaving alone either the or planes, producing such high quality facets consistently is extremely challenging. We have demonstrated that high quality facets can be obtained consistently by cleaving, with and without a dielectric layer on Si substrates.

Optical second-harmonic (SH) generation in nonlinear semiconductors can be used to convert infrared radiation from semiconductor laser sources to visible light. The high incident light intensities and spatial modulation of the linear and nonlinear optical properties occurring in AlxGa1-xAs vertical cavity heterostructures can lead to SH generation efficiencies many orders of magnitude larger than possible homogeneous GaAs. SH generation in vertical cavity heterostructures is analyzed using a model incorporating multilayer reflections and spatially varying nonlinear sources.© (1993) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Ten years ago Normandin and Stegeman demonstrated a novel surface harmonic generation geometry but the cross sections were too low for applications. Using periodic multilayers in the waveguide, cross sections were increased by over a million. Theory refinements and new results are presented for large angle light deflectors, integrated intracavity doubling visible lasers, and high resolution (< 10 GHz) optical spectrometers. Applications in display, telecommunication, interconnect, and coding are also discussed.© (1992) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.