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Recently, Integrated Microwave Photonics (IMWP) on Silicon-on-Insulator (SOI) platform has attracted much attention. SOI as a platform for passive devices has been studied for operation at a wavelength of 1.55 μm. It has been acknowledged that Si suffers from Two-Photon Absorption (TPA) at this wavelength, which potentially limits the dynamic range of microwave photonic links. The TPA effect diminishes at longer wavelengths, and completely vanishes at a wavelength of 2.2 μm. So far, the detailed effects of TPA on the performance of the SOI platform in the context of IMWP have not been well-explored. In this work, a systemic simulation has been performed in order to investigate the effects of nonlinear TPA on the performance of SOI microwave photonic links at 1.55 μm, particularly the dynamic range of the system was studied in-depth. Furthermore, system performance at wavelengths from 1.55 to 2.2 μm was investigated by scaling the device design, where the parasitic effects of TPA are avoided. Based on theoretical analysis, the result showing microwave photonic links at 2.2 μm outperform that at 1.55 μm was obtained as expected. Strip and rib waveguides at 1.55 μm show P1dB points of 3.75 mW and 69.54 mW, respectively, and at 2.2 μm, these waveguides performed linearly throughout the simulated range (up to 1 W of input power) which is due to the dramatically reduced TPA effect. It is noted that the wavelengths at which SOI avoids parasitic TPA match well with the working wavelengths of emerging GeSn lasers and photodetectors.

The development of low-loss optical phase change materials (O-PCMs) promises to enable a plethora of nonvolatile integrated photonic applications. However, the relatively large optical constants change between different states of calls for a set of new design rationales. Here we report a non-perturbative design that enables low-loss device operation beyond the traditional figure-of-merit limit. The basic design rationale is to engineer the light propagation path through the OPCMs when it is in the low-loss amorphous state, and divert light away from the lossy crystalline state leveraging the large mode modification induced by the O-PCM phase transition. Following this approach, we demonstrate broadband photonic switches with significantly enhanced performances compared to current state-of-the-art.

Silicon-based optoelectronic devices have long been desired owing to the possibility of monolithic integration of photonics with high-speed Si electronics and the aspiration of broadening the reach of Si technology by expanding its functionalities well beyond electronics. To overcome the intrinsic problem of bandgap indirectness in the group-IV semiconductors such as Si and Ge, a new group-IV based material, GeSn alloy, has attracted increasing interests. The group-IV GeSn alloy has been demonstrated to become direct bandgap material with more than 8% Sn incorporation, which opens a new opportunity towards a Si-based light source with fully complementary metal-oxide-semiconductor (CMOS) compatibility. The GeSn laser contributes strongly to the progress of optoelectronic integration towards next-generation photonic integrated circuit on the Si platform, as it fills the deficiency of the efficient group-IV band-to-band lasers. Moreover, due to the tunable bandgap of GeSn, the lasing operation wavelength covers broad near- and mid-infrared range. Recently, the GeSn optically pumped lasers based on direct bandgap GeSn alloys have been demonstrated. In this work, the following aspects have been investigated: i) the novel growth strategy to obtain high Sn compositions based on spontaneous-relaxation-enhanced (SRE) Sn incorporation and the GeSn virtual substrate (VS) approaches. The maximum Sn composition of 22.3% was achieved; ii) the demonstration of GeSn optically pumped heterostructure lasers. The operation wavelength covers from 2 to 3 µm and the maximum lasing temperature is 265 K; iii) the demonstration of GeSn quantum well laser. The significantly reduced lasing threshold compared to heterostructure laser was achieved.

The dramatic optical property change of optical phase change materials (O-PCMs) between their amorphous and crystalline states potentially allows the realization of reconfigurable photonics devices with low power consumption, such as optical switches and routers, reconfigurable meta-optics, displays, and photonic memories. However, conventional O-PCMs, such as VO2 and Ge2Sb2Te5, are inherently plagued by their excessive optical losses even in dielectric states, limiting their optical performance and hence application space. In this talk, we present the development of a new group of O-PCMs and their implementations in novel photonic devices. Ge-Sb-Se-Te (GSST), obtained by partially substituting Te with Se in traditional GST alloys, feature unprecedented broadband optical transparency covering the telecommunication bands to LWIR. Capitalizing on the dramatically-enhanced optical performance, novel non-volatile, reconfigurable on-chip photonics devices and architectures are demonstrated. GSST-integrated Si photonics based on the material innovation and novel “non-perturbative” designs exhibit significantly improved switching performance over state-of-the-art GST-based approaches. The technology is further scalable to realize non-blocking matrix switches with arbitrary network complexity, paving the path towards high performance reconfigurable photonics chips.

The GeSn alloy with Sn composition of 11% has been grown using an industry standard reduced-pressure chemical vapor deposition reactor in a single run epitaxy. Low-cost commercially available GeH4 and SnCl4 were used as Ge and Sn precursors, respectively. The material characterization showed that the threading dislocations were trapped in the Ge/GeSn interface and do not propagate to the GeSn layer, resulting in high quality material. The temperature-dependent photoluminescence study revealed that the direct bandgap GeSn alloy was achieved, as the emission intensity significantly increased at low temperature. The sample was than fabricated into photoconductive detectors and waveguide lasers. For the photodetector, the spectral response wavelength cutoff at 3.0 μm was observed. The specific detectivity of 3.5×1010 cm•Hz1/2W-1 was achieved, which is close to that of market dominating InGaAs photodetectors that are operating in the same wavelength range; For the waveguide laser, the lasing threshold pumping power density of 86.5 kW/cm2 at 10 K and the highest operating temperature of 110 K were obtained. Furthermore, the characteristic temperature was evaluated as 65 K.

This paper explores the use of honeycomb lattice waveguide meshes for universal linear operations and photonic integrated circuit synthesis by programming a common hardware. We discuss the main photonic processor architecture, the non-ideal effects to be considered and its application to different signal processing functionalities.

In this work, high performance GeSn photoconductor and light emitting diodes (LED) have been demonstrated. For the photoconductor, the high responsivity was achieved due to high photoconductive gain, which is attributed to the novel optical and electrical design. The longwave cutoff at 2.4 μm was also observed at room temperature. For LED, temperature-dependent study was conducted. The electroluminescence (EL) spectra at different temperatures were obtained and EL peak shift was observed. Moreover, the emission power at different temperatures was measured. High power emission at 2.1 μm was achieved.

Ge 1-x Sn x /Ge thin films and Ge/Ge 1-x Sn x /Ge n-i-p double heterostructure (DHS) have been grown using commercially available reduced pressure chemical vapor deposition (RPCVD) reactor. The Sn compositional material and optical characteristics have been investigated. A direct bandgap GeSn material has been identified with Sn composition of 10%. The GeSn DHS samples were fabricated into LED devices. Room temperature electroluminescence spectra were studied. A maximum emission power of 28mW was obtained with 10% Sn LED under the injection current density of 800 A/cm 2 .

Si based Ge1-xSnx photoconductors, with Sn incorporation of 0.9, 3.2, and 7%, were fabricated using a CMOS-compatible process. Temperature dependent study was conducted from 300 to 77 K. The first generation device (standard photoconductor, PD) shows long wavelength cut-off beyond 2.1 μm for 7%-Sn devices at room temperature. The peak responsivity and D* of the 7% Sn device at 1.55 μm were obtained at 77K as 0.08 A/W and 1×109 cm*Hz1/2*W-1, respectively. Improved responsivity and specific detectivity (D*) were observed on second generation devices by a newly designed electrode structure (photoconductor with interdigitated electrodes, IEPD). The enhancement factor of responsivity was up to 15 at 77 K.

This paper outlines the challenges and benefits of applying silicon-based photonic techniques in the 2 to 5 μm midinfrared (MIR) wavelength range for chem.-bio-physical sensing, medical diagnostics, industrial process control, environmental monitoring, secure communications, Ladar, active imaging, and high-speed communications at 2 μm. Onchip passive and active components, mostly waveguided, will enable opto-electronic CMOS or BiCMOS integrated “circuits” for system-on-a-chip applications such as spectroscopy and lab-on-a-chip. Volume manufacture in a silicon foundry is expected to yield low-cost (or even disposable) chips with benefits in size-weight-power and ruggedness. This is “long-wavelength optoelectronic integration on silicon” which we call LIOS. Room temperature operation appears feasible, albeit with performance compromises at 4 to 5 μm. In addition to the electronics layer (which may include RF wireless), a 3-D LIOS chip can include several inter-communicating layers utilizing the photonic, plasmonic, photoniccrystal and opto-electro-mechanical technologies. The LIOS challenge can be met by (1) discovering new physics, (2) employing “new” IV and III-V alloys, (3) scaling-up and modifying telecom components, and (4) applying nonlinearoptical wavelength conversion in some cases. This paper presents proposals for MIR chip spectrometers employing frequency-comb and Ge blackbody sources. Active heterostructures employing Si, Ge, SiGe, GeSn and SiGeSn are key for laser diodes, photodetectors, LEDs, switches, amplifiers, and modulators that provide totally monolithic foundry integration, while numerous III-V semiconductor MIR devices within the InGaAsSb and InGaAsP families offer practical hybrid integration on Si PICs. Interband cascade and quantum cascade lasers on Ge waveguides are important in this context.

Group IV mid-infrared photonics is attracting more research interest lately. The main reason is a host of potential applications ranging from sensing, to medicine, to free space communications and infrared countermeasures. The field is, however, in its infancy and there are several serious challenges to be overcome before we see progress similar to that in the near-infrared silicon photonics. The first is to find suitable material platforms for the mid-infrared. In this paper we present experimental results for passive mid-infrared photonic devices realised in silicon-on-insulator, silicon-on-sapphire, and silicon on porous silicon. We also present relationships for the free-carrier induced electro-refraction and electro-absorption in silicon and germanium in the mid-infrared wavelength range. Electro-absorption modulation is calculated from impurity-doping spectra taken from the literature, and a Kramers-Kronig analysis of these spectra is used to predict electro-refraction modulation. We examine the wavelength dependence of electro-refraction and electro-absorption, finding that the predictions suggest longer-wave modulator designs will in many cases be different than those used in the telecom range.

Mid-infrared group IV photonics is emerging as a field with possible applications ranging from gas sensing to free-space communications. Free-carrier induced electro-absorption and electro-refraction have become the most widely used modulation mechanisms in active near-infrared silicon photonic devices. This work examines the magnitude of this effect in group IV materials at mid-infrared wavelengths. In silicon electro-absorption effects are calculated from experimental absorption coefficient data, and electro-refraction is calculated through numerical Kramers-Kronig analysis of absorption spectra. In germanium the Drude-Lorentz equations are used to estimate both change in absorption and change in refractive index.

We propose integrated waveguides for terahertz (THz) and mid-infrared (MIR) applications on wafer platform. Based on the prototype of spoof plasmonic waveguides consisting of textured metallic surface, we explore the possibility of coating periodic metallic pattern with silicon (at 0.6 THz) or germanium (at MIR region of 30 THz) to further shrink the relative mode size of propagation spoof plasmonic waves. Numerical modeling via 3D finite-difference time-domain (FDTD) has shown deep sub-wavelength mode confinement in transverse directions to smaller than λ/50 by λ/50, with an estimated propagation loss of less than 0.1 dB for each repetitive unit.

We introduce a compact, low-power electro-optical switch on a silicon-on-insulator circuit through heterogeneous integration. A 10µm diameter III-V microdisk cavity is employed as the switching element. Switching of a 10Gbps optical signal is demonstrated by sweeping the bias between −1.1V and +0.9V with 15dB extinction ratio and 1.2ns switching speed.

The vision of longwave silicon photonics articulated in the Journal of Optics A, vol. 8, pp 840-848, 2006 has now come into sharper focus. There is evidence that newly designed silicon-based optoelectronic circuits will operate at any wavelength within the wide 1.6 to 200 μm range. Approaches to that LWIR operation are reviewed here. A long-range goal is to manufacture LWIR OEIC chips in a silicon foundry by integrating photonics on-chip with CMOS, bipolar, or BiCMOS micro-electronics. A principal LWIR application now emerging is the sensing of chemical and biological agents with an OE laboratory-on-a-chip. Regarding on-chip IR sources, the hybrid evanescent-wave integration of III-V interband-cascade lasers and quantum-cascade lasers on silicon (or Ge/Si) waveguides is a promising technique, although an alternative all-group-IV solution is presently taking shape in the form of silicon-based Ge/SiGeSn band-to-band and inter-subband lasers. There is plenty of room for creativity in developing a complete suite of LWIR components. Materials modification, device innovation, and scaling of waveguide dimensions are needed to implement microphotonic, plasmonic and photonic-crystal LWIR devices, both active and passive. Such innovation will likely lead to significant LIO applications.

Ultra-small silicon-based photonic-crystal ring resonators (PCRRs), both passive and active, will be key contributors to the emerging low-power nanophotonic technology. We have modeled and simulated the diameter-dependent loss, Q, and FSR of such PCRRs and find a 0.02 dB "intrinsic" loss that is independent of diameter, unlike the ~1/D loss of micro-strip resonators. Close to 100% drop efficiency at the drop channel of 1557.5nm was obtained by design with a high spectral selectivity of Q greater than 1319 in the single-ring PCRR-based add-drop filters with ring radius of 1.2 μm. Ultra-compact polymer modulators were proposed and simulated, based on the hybrid integration of functional polymer materials with Si based PCRRs, which can lead to high speed modulators, suitable for photonic integration and RF photonics.

The GaN/AlGaN multilayer structure for the active regions of terahertz quantum cascade lasers (QCLs) was grown by metal organic chemical vapor deposition (MOCVD). The surface morphology of the grown sample showed good surface quality with an average roughness of less than 1 nm. The x-ray diffraction pattern and transmission electron microscopy images showed that the well-controlled quantum cascade GaN/AlGaN layers were grown. The Fourier transform infrared spectrometer measurement showed a distinct A 1 (LO) phonon frequency at 822 cm -1 that is red-shifted with respect to the single Al 0.2 Ga 0.8 N layer due to the good periodicity of the grown quantum cascade GaN/Al 0.2 Ga 0.8 N structure. MOCVD growth should be a viable technique for fabrication of AlGaN/GaN quantum cascade laser and phonon frequency shift should be a key indicator for the good periodicity of the grown QCL structure.

A recently proposed THz laser concept in homoepitaxially grown p-Ge with layered doping is reviewed. Prospects for realizing a similar design in Si or GaAs are considered.

We present design and simulation results on two types of inplane, resonant 2 x 2 optical switches in the strip-waveguided silicon-on-insulator (SOI) system. Both switches are intended for cascading into N x N matrices, and the intersecting strips in the second floating ring switch facilitate construction of an N x N crossbar cross-connect. We examine complex index perturbations of both rings due to electrical injection of free carriers into the rings or to the generation of electron-hole pairs induced by above-bandgap control light focused upon the rings. Switching behavior produced by free-carrier-plasma dispersion, Franz-Keldysh, quantum-confined Stark, Kerr, and thermooptic effects are investigated and compared at the 1.33, 1.55, and 10.6 mm wavelengths. A study of design tolerances and their consequences shows that the 10.6 mm wavelength is preferred from the standpoint of photolithographic feasibility and the stronger carrier-optic effect. The SOI high index contrast requires small coupling gaps for the 1.33 and 1.55 mm wavelengths, which in turn necessitates a high photolithographic precision.

Due to recent investments by government and industry, silicon-based photonics has a chance of becoming “the” mainstream photonics technology. This paper presents a survey of recent results found in journal articles and conference proceedings. Emerging trends in silicon-based photonic components (waveguides, ultrafast modulators, switches, light sources, detectors, direct bandgap SiGeSn/GeSn devices, photonic-crystal and plasmonic devices) are identified and discussed. In principle, Si PICs and OEICs can operate anywhere within the 0.3 to 100 μm wavelength range -- enabling transceivers, on-chip processing, and interfacing with fibers or free-space light beams. Thus, in addition to the very important 1.55 μm telecomm applications, there are significant Si photonic opportunities throughout the infrared-and-visible spectrum. The push towards smarter, ever-denser on-chip photonic networks, drives a “convergence” of micro-, nano- and plasmo-photonic techniques for progressively smaller devices (Moore’s law for photonics) and for improved functionality of modulators, switches, emitters, detectors, waveguides, resonators, tapers and filters. This convergence includes composite components: monolithic integration of microstrip waveguides, 2D and 3D photonic-crystal elements and metal/Si plasmon-optics that ultilize buried or surface-mounted 2D arrays of metal stripes or nanodots.

A new 2D PBG directional coupler switch is proposed and analyzed by the FDTD method. A 1.55-micrometers device is designed in a square lattice of silicon posts in air. The switching mechanism is a change in the conductance of posts located in the waveguide coupling region, Conductance is induced electrically by carrier injection or is induced optically by electron-hole pair generation. Low insertion loss and optical crosstalk below -23 dB in both the cross and bar switching states are predicted.

Terahertz (far-infrared) intersubband electroluminescence is reported in p-type Si/SiGe quantum wells and quantum cascade structures. Surface-normal emission (without the aid of a surface grating) from light hole - heavy hole intersubband transitions has been observed for the first time in a quantum cascade device. Edge-emission measurements have also been performed, which show emission from both heavy hole - heavy hole and light hole - heavy hole transitions, and have allowed demonstration of the polarisation dependence of the emitted power, according to the selection rules for the intersubband interactions. The electroluminescence is visible up to temperatures of ~150K, in the multiple quantum well structures, and >=77K in the quantum cascade structure.

This paper presents modeling and simulation results on Si- based quantum-well intersubband THz detectors and THz lasers (tasers) in the 3 to 10 THz range where the intersubband transition energy is 12 to 41 meV. The incoherent cryogenically cooled (4 K to 20 K) quantum well terahertz detector (QWTD) consists of p-type Si 0.9 Ge 0.1 QWs with Si barriers on an Si substrate, or of p-Si 0.85 Ge 0.15 /Si on a relaxed Si 0.97 Ge 0.03 buffer on Si. The QWTD senses THz radiation at normal incidence (the XY polarization on the HH1 to LH1 transition) or at edge- illumination (the Z polarization on the HH1 to HH2 transition). Resonant-cavity enhancement, coupling to Si THz waveguides, and integration with SiGe transistor preamplifiers appear feasible for QWTDs. The quantum staircase taser is a simplified far-infrared version of the quantum cascade laser in which each superlattice transfer region is replaced by a thin tunnel-barrier layer. We have adapted to group IV the III-V idea of Sun, Lu, and Khurgin; the `inverted mass taser'. On a Si 0.81 Ge 0.19 substrate, we find that an inverted effective mass exists in LH1 at k g equals 0.013 angstroms -1 in 9-nm single- wells of Si 0.7 Ge 0.3 with 5-nm Si barriers. Selective electrical injection of holes into LH1 at T equals 77 K is postulated. This offers local-in-k-space LH1-HH1 population inversion and tasing at 7.2 THz. Since the taser emission is XY-polarized, the active MQW staircase (a set of identical square QWs) is suitable for insertion into a vertical cavity surface-emitting taser. The VCSET would have resonator thickness of (lambda) /2n equals 6 micrometers , and Bragg mirrors constructed from SiO 2 /Si multilayers.

We have performed quantum-mechanical analyses of strain- symmetrized Ge/Si QWIPs grown upon a realized buffer layer of Si0.4Ge0.6 on Si. The multi-quantum-well (MQW) QWIP has 50 angstrom-thick compressively strained p-doped Ge quantum wells and 200 angstrom-thick tensile-strained Si- rich SiGe barriers. This MQW allows shorter-wavelength IR sensing than prior-art unbuffered Si0.64Ge0.36/Si asymmetrically strained QWIPs because the valence band offsets are approximately 3x larger. We predict normal- incidence higher-temperature operation over the 1.7-to-3.8- micrometers wavelength band using the bound-to-bound and bound-to- continuum transitions HH1-SO1 and HH1-SO-C respectively. We expect that the p-i-p Ge/Si MQW pixels analyzed here can be fashioned into 2D imaging arrays, and that the arrays can be integrated monolithically with Si readout circuity. We also anticipate that multi-spectral IR imaging will be feasible by the technique of vertical epitaxial stacking of 'sub- QWIPs' within each pixel - where each sub-QWIP has a narrowband spectral response that differs from its neighbor's, and each sub-QWIP's electrical readout current is independent of its neighbor's.© (1999) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Several relatively new photonic components have recently come to the forefront in applications related to the photonic processing of broadband microwave signals. Included in this set of photonic elements are the wavelength tunable laser, high dispersion optical fiber, and the Bragg reflection grating. Because of the efficiency and potential cost effectiveness offered by these components they hold the promise of revolutionizing the field of broadband photonic processing. Practical commercial photonic processors are likely to become viable in the near term as a result of these new components. This paper presents a brief overview of the performance characteristics of these components and shows how they can be used for two broadband phased array beamforming applications. A technical overview is presented for each system with experimental data provided to specifically demonstrate the performance of a discrete true time delay beamformer. The second configuration introduced has the capability of providing continuous beamsteering; an essential characteristic for null steering applications. Both systems shown take maximum advantage of component reuse and fully integrate the transmit and receive modes in one efficient hardware compressive topology.

A new tunable source of infrared radiation based on intersubband electron Raman scattering in semiconductor quantum wells is theoretically studied. The structure consisting of three levels in two coupled GaAs/AlGaAs quantum wells is optimized for a maximum Raman gain at zero bias. Raman gain as large as 400/cm can be achieved according to our calculations. Lasing wavelengths are tuned by applying external dc bias field along the growth direction. An infrared tuning range of 8 to 12 micrometers , with moderate Raman gain, is predicted as the electric field is varied from -40 to 40 kV/cm.

Because of the absence of polar optical scattering, the lifetime difference of the upper and lower lasing levels, to which population inversion and laser gain are proportional, is an order of magnitude larger in silicon-based structures than in the III-Vs. Further enhancements are due to phonon confinement. For lasing wavelengths 10 micrometers and longer, Ge x Si 1-x /Si is used. For operation at near infrared wavelengths, high barrier materials are needed. To avoid large operating voltages, designs are considered which rely on parallel rather than sequential operation.

A microwave variable delay line has been demonstrated based on the dispersive properties of optical fibers. This processor offers high bandwidth and tunability with the stability that fiber optics offers. A two-tap processor has been demonstrated with a 1.3 ns spacing.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

A new, fast, intersubband 1.55 micrometers electro-optic modulator in the SiGe/Si/CaF 2 -on- Si stepped-quantum-well system is proposed and analyzed. At applied electric fields of +/- 8 V/micrometers , resonant 1 - 3 conduction-intersubband absorption is predicted to given 18 dB of optical extinction for narrow-linewidth transitions. The procedure for incorporating conduction band nonparobolicity for higher lying subbands is described, and the issue of narrow linewidths is discussed.

We have built and tested new multiple-layer silicon optical waveguide structures that have applications in wafer-scale optical interconnects. A repeated sequence of oxygen implantation annealing and Si epitaxy was used to make 5-layer or 6-layer structures containing a pair of 2- jim-thick Si waveguide cores separated by 1200A or 3700A of Si02. Coupled or uncoupled dual waveguiding at 1. 3 xm was observed. Inter-guide coupling for this stacked 3-D structure is analyzed here. Applications to all-silicon guided-wave optical interconnects are discussed.© (1991) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Low-loss waveguiding at X = 1.3 µm has been observed in a partially strained, 10-µm thick, single-crystal layer of Ge0,1Si0.9 grown by chemical vapor deposition on a high-resistivity (100) silicon substrate. The TM-mode propagation loss in the multimode planar guide was less than 1.9 dB/cm. Device applications of GeSi waveguides are discussed.

The use of active components for data distribution in multi-terminal fiber networks is reviewed in this paper. Our approach is to place electro-optic switches at nodes of the network. These signal-routing devices are interconnected by individual strands of multimode fiber. We shall present recent results on multimode switching in the areas of optical data busing, multiplexing, and circuit switching. A new "mirror terminal" has been developed as a low-loss means of coupling information on and off a serial bus, while time-division multi-plexing has been implemented with multipole LiTaO3 switches. The "optical crossbar" or matrix is a first step toward telecommunication switching. The matrix provides communica-tion between two groups of N terminals. This can also be done with a passive star coupler, but the matrix does not suffer a 1/N power-division loss, it sets up N independent "con-versations" simultaneously, and the circuits can be reconfigured rapidly. Multimode crossbars 3 x 2 in size are now operating in our laboratory. Fiber guides are butt-coupled to the crossbar crystal at an angle in order to collimate the 0.15-NA fiber light in the crossbar.

The first silicon-on-sapphire optical waveguides have been demonstrated. Waveguiding at λ=1.3 microns has been observed in silicon films 0.95 and 1.5 microns thick and in raised strip structures 1.5 microns thick.

Detailed calculations have been made of the change in refractive index and absorption of InP, GaAs, GaSb, InAs, and InSb produced by electrical injection of electrons or holes. Bandfilling, band-gap shrinkage, and the plasma effect were taken into account. A wide range of carrier concentrations and optical wavelengths was considered. The total electro-optic effect is relatively large at high levels of injection (or depletion). Index changes of 10-2, or more, are predicted and suggest that low-loss phase modulators and switches using carrier injection are feasible in these materials.© (1989) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Adjustable access couplers for multimode fiber -optic networks have been constructed. The4 -port fiber -coupled devices make use of voltage -controlled total internal reflection by athin film of nematic liquid crystal. The devices can serve as tee -couplers in unidirec-tional single- strand data buses or in duplexed two -strand buses. This active -couplerapproach gives low loss between any pair of addressed terminals on the bus. In the devices,multimode fibers are coupled to rod lenses, and those lenses are, in turn, coupled toflint glass prisms that confine a 6 -pm liquid- crystal layer. The achromatic switches havea 1.6 dB optical insertion loss, a tap -ratio that is controllable from -48 dB to -4.6 dB, adirectionality of 44 dB, and an operating voltage of 5 to 20 V rms at 1 kHz. The structureis extendable to matrix switching arrays and to higher -order switching functions.IntroductionThis paper describes a first step toward a practical "marriage" of fiber optics andliquid crystals. A number of fiber switching devices have been built and tested in ourlaboratory, and we shall describe how they work and how they can be used in fiber -opticcommunication networks. The early experimental results are encouraging.We chose to investigate liquid crystal switching because this technique held out thehope of lower control voltage and lower optical insertion losses than our previous LiTa03electro -optic approach. These expectations have been borne out, as discussed below.

Crystalline silicon (c-Si) exhibits two electro-optical effects that are suitable for device-application in guided-wave modulators and switches. The effects are electrorefraction and free-carrier dispersion. The dispersion is controlled by electrical injection or electrical depletion of charge carriers. The magnitude of these effects has been calculated in this paper. The results for c-Si are compared to those for GaAs and InP. Low-loss guided-wave electro-optical phase modulation structures in c-Si are proposed here.

Effective mass theory and zero-field absorption data are used to calculate electric-field-induced changes in optical refraction and absorption (the Franz-Keldysh effect) for five direct gap III-V semiconductors: InP, GaAs, GaSb, InAs, and InSb. Results are given for fields of 10 4 V/cm to the estimated breakdown strength and for photon energies of 40 and 80 meV below the band gap for each semiconductor.

A new optical switching component for multimode fiber-optic networks has been developed. This voltage-controlled switch is an 8-port device coupled by GRINrod lenses to four input fibers and to four output fibers (graded index, 50-μm core diameter). The matrix switches light from any input to any output, and four simultaneous input-output connections are made. There are 24 such states. Switching is done via controlled reflection from two layers of nematic liquid crystal. This switch is the first multimode, electro-ontic, fiber-optic matrix. Construction is simple and compact, using only four glass prisms and two liquid crystal layers. The matrix architecture is based on a new dual-switching-array concept that provides very low levels of optical crosstalk, more than 53 dB below the optical input level. The 5-ms on/off response time is faster than that of electro--mechanical matrices, although the 40 Vrms addressing voltage is higher. The optical insertion loss had a minimum value of 6 dB and an average value of 15 dB. The latter is due to misalignment of fibers during attachment to the switch, which can be remedied. Because the present switch does not use moving parts, it is quite reliable.© (1982) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

A strain-induced ordered phase has been observed recently in the alloy layers fo a GeSi/ Si strained-layer superlattice, in which the ordered unit cell is non-centrosymmetric. Using the bond-charge model of Jha and Bloembergen, the second-order susceptibility and the linear electro-optic (Pockels) coefficient of the GeSi layers are calculated and are found to be comparable in magnitude to those of GaAs. The calculation has also been extended to artificial crystals consisting of alternating bilayers of Si and Ge on (001) Si substrates recently reported. For the indicated structure, the Pockels coefficient vanishes in this case for E [001].