Longwave IR lattice matched L-valley Ge/GeSiSn waveguide quantum cascade detector.

We propose a lattice-matched Ge/GeSiSn quantum cascade detector (QCD) capable of operating in the longwave infrared. The optical absorption and carrier transport based on intersubband transitions all occur within the L-valley of the conduction band of the group-IV material system using N-doped quantum wells (QWs). The waveguided lattice matched structure can be deposited strain free on top of a Ge buffer grown on Si substrate, and is end-coupled to low-loss on-chip Ge waveguides. We optimized the QCD structure through the analysis of the photoresponsivity and detectivity D*. The QCD operates in photovoltaic mode with narrow spectral response that is peaked anywhere in the 9 to 16 µm range, tunable by design. This work aims to push the optical response of the photodetectors made from the SiGeSn material system to longer wavelengths. The study suggests the QCD response can indeed significantly extend the spectral range beyond that of the photodiodes and photoconductors made from the same group-IV system for a wide variety of applications in imaging, sensing, lidar, and space-and-fiber communications.

High efficiency thin-film crystalline Si/Ge tandem solar cell.

We propose and simulate a photovoltaic solar cell comprised of Si and Ge pn junctions in tandem. With an anti-reflection film at the front surface, we have shown that optimal solar cells favor a thin Si layer and a thick Ge layer with a thin tunnel hetero-diode placed in between. We predict efficiency ranging from 19% to 28% for AM1.5G solar irradiance concentrated from 1 approximately 1000 Suns for a cell with a total thickness approximately 100 microm.

Design of a Si-based lattice-matched room-temperature GeSn/GeSiSn multi-quantum-well mid-infrared laser diode.

This paper presents modeling and simulation of a silicon-based group IV semiconductor injection laser diode in which the active region has a multiple quantum well structure formed with Ge(0.9)Sn(0.1) quantum wells separated by Ge(0.75)Si(0.1)Sn(0.15) barriers. These alloy compositions were chosen to satisfy three conditions simultaneously: a direct band gap for Ge(0.9)Sn(0.1), type-I band alignment between Ge(0.9)Sn(0.1) and Ge(0.75)Si(0.1)Sn(0.15,) and a lattice match between wells and barriers. This match ensures that the entire structure can be grown strain free upon a relaxed Ge(0.75)Si(0.1)Sn(0.15) buffer on a silicon substrate - a CMOS compatible process. Detailed analysis is performed for the type I band offsets, carrier lifetime, optical confinement, and modal gain. The carrier lifetime is found to be dominated by the spontaneous radiative process rather than the Auger process. The modal gain has a rather sensitive dependence on the number of quantum wells in the active region. The proposed laser is predicted to operate at 2.3 µm in the mid infrared at room temperature.

Low-cross-talk 2 x 2 optical switch.

An electro-optical 2 x 2 switch for fiber-optic applications has been developed. Optical cross talk is reduced significantly by inserting two 2 x 1 switches in each optical path. A compact structure is used to contain the four 2 x 1's that are required. We observed -27-dB cross talk and 2.5-dB insertion loss in both switching states. The device is controlled by a 5-V twisted-nematic liquid-crystal cell. Multimode, unpolarized light is switched.

Electrooptic 4x4 matrix switch for multimode fiber-optic systems.

A new fiber-optic switch for multimode fiber networks has been developed. This voltage-controlled device can switch light from any of four input fibers to any of four output fibers. Four input/output connections are made simultaneously, and the matrix has twenty-four such states. No moving parts are needed because the switching is accomplished with an electrooptic effect in nematic liquid crystals. Low levels of optical crosstalk, ~40 dB below the output signal level, are obtained with a unique two-stage architecture. The design, construction, and observed performance of this optical switch are described.

Liquid-crystal fiber-optic switch.

An adjustable access coupler for multimode fiber-optic networks has been constructed, based on the voltage-tunable total-internal-reflection effect in nematic liquid crystals. Fibers are coupled via graded-index rod lenses at normal incidence to flint-glass prisms in contact with a 6-microm liquid-crystal layer. The achromatic four-port switch has a 1.6-dB optical insertion loss, a tap ratio controllable from -4.6 to -48 dB, a directionality of 44 dB, and an operating voltage of 5 to 20 V rms.

N x N and 1 x N switching with chiral nematic liquid crystals.

New designs are presented for 4 x 4, 1 x 4, and 1 x 16 electrooptical switches that utilize chiral nematics.

Electronically scanned analog liquid crystal displays.

A new analog display technique for liquid crystal display panels is demonstrated. The size, shape, and location of display patterns can be changed continuously using low power electronic control. The display consists of a thin liquid crystal layer sandwiched between high resistance transparent area electrodes. Transverse voltage gradients on the electrodes actuate the device. The display operates with either dynamic scattering liquids or quiescent scattering liquids. Experimental results are given for three prototype analog displays: a voltmeter, a flying spot scanner, and a null indicator.

Carrier refraction in quantum well waveguides.

Carrier-induced refractive index changes in forward-biased InGaAs-InAlAs quantum well waveguides are calculated using a Kramers-Kronig transformation of Bar-Joseph's experimental absorption spectra [Phys. Rev. Lett. 59, 1357 (1987)]. At the l.65-,microm wavelength where the material is nominally transparent, an index change of -0.06 is found for an injection of 6 x 10(17) electrons/cm(3) . A quantum well waveguide 2 x 2 reversed-Deltabeta directional coupler switch with an active length of 480 microm is proposed.

Optical design of wollaston-prism digital light deflectors.

The optical design of Wollaston-prism digital light-deflection systems is analyzed. It is found that the performance of the system is limited by (1) the diffraction spreading of the incident light beam, (2) the walk-off of the beam at the exit aperture, (3) the generation of spurious light, and (4) distortions in the angular deviation of each prism. The magnitude and functional dependence of these limitations are calculated, and criteria are supplied for the design of practical Wollaston-prism light-deflection systems. The design procedure is outlined, and numerical examples are given. The examples show that high-speed Wollaston-prism systems having a capacity of more than 10(6) bits are practical at the present time.

Total switching of unpolarized fiber light with a four-port electro-optic liquid-crystal device.

A liquid-crystal switch capable of switching nearly 100% of unpolarized light from one multimode fiber to another had been conceived and demonstrated experimentally. The switch uses a double interaction with a single layer of nematic liquid crystal in which the liquid alignment is initially in plane at 90 degrees to the light propagation. This four-fiber optical reversing switch has a simple structure, a low insertion loss, and a low operating voltage. It should find a wide range of applications in fiber-optic communication systems.

Electro-optic Fabry-Perot pixels for phase-dominant spatial light modulators.

A theoretical analysis of phase enhancement by resonant Fabry-Perot picture elements in III-V semiconductor spatial light modulators (SLM's) is presented. For 90% reflecting electrodes, a phase modulation of 0.7pi rad is found in transmission when the electro-optic input phase is 0.06pi rad. Implementation of this resonant phase-dominant SLM in a 1.5-microm-thick AlGaAs/GaAs multiple quantum well (MQW) structure is proposed. Field effects and carrier-induced electro-optic effects are suggested for the MQW's.

Electrooptic polarization switch for multimode fibers.

An extensive theoretical analysis of an electrooptic polarization switch has been carried out. The main objective of the study is to ascertain whether it is possible to design such a switch with acceptable levels of optical crosstalk and control voltage if the device is used in conjunction with a multirmode optical fiber transmission system. The results of the study show that about 30-dB signal-to-crosstalk ratio can be obtained when an isotropic cubic material such as CdTe is used, and the fibers do not have an excessively large numerical aperture. Less isolation (~7 dB) can be obtained when using an anisotropic material with a small birefringence such as LiTaO(3). Drive voltages in either case range from 10 V to 50 V.

Compound optical crosspoint with low crosstalk.

A multimode electrooptical switch that has high isolation between channels has been demonstrated experimentally in a 70-microm-thick plate of Z-cut LiTaO(3). The fiber-compatible four-port switch is intended for integration into an optical matrix. The crosspoint consists of an in-plane crossover, two simple optical switches, and a curved connecting waveguide. For convergent input light with a 0.07 numerical aperture, 10% of the initially channeled light was transferred into the crosschannel, and the optical switching ratio of that channel was 26 dB using 450-V dc for control.

Multimode optical multiplexing with low-voltage drivers.

A series-resonant electrical-drive circuit has been used in conjunction with a fiber-coupled multimode electro-optic switch to achieve 2:1 optical time-division multiplexing from a 5-V sine-wave source at a 1.1-MHz rate. The LiTaO(3) mux-demux device is a unique fiber-optic switch that uses a curved prism electrode to deflect a trapped fiber-light cone while preserving the focusing obtained from an internal mirror.

Calcite 2 x 2 optical bypass switch controlled by liquid-crystal cells.

Three blocks of birefringent calcite and two 6-V twisted nematic liquid-crystal cells have been used to make a 2 x 2 optical bypass switch for series-type fiber-optic networks. Optical cross-talk levels of -32 dB have been obtained in both states. The insertion loss was 3 dB in a nonoptimized switch.

Hybrid fiber-optic sensors using liquid-crystal light modulators and piezoceramic transducers.

A new technique is described for linking remote sensors by multimode optical fibers. It combines two well-known technologies and is compatible with multimode optical multiplexing. At the sensing location, the output of a transducer (such as a piezoceramic hydrophone) is connected directly to a low-voltage liquid-crystal light modulator that is coupled to fiber-optic transmission lines by graded-index rod lenses. Transducer voltages as low as 10 microV produce intensity modulation of the fiber light. An optical hydrophone based on the technique demonstrated a minimum detectable pressure of 39 dB re 1 [equiation] at 500 Hz. The device performance was evaluated over the 8-45 degrees C temperature range. Sensitivity was found to decrease linearly by 0.2 dB/ degrees C over that range. The technique provides a means by which rugged nonmechanical sensors can be constructed that are suitable for incorporation into practical fiber-optic sensor systems.

Silicon antiresonant reflecting optical waveguides.

A new silicon waveguide called ARROW-C is proposed and analyzed at the 1.3-microm wavelength. The silicon structure uses two buried layers of SiO(2) or beta-SiC with a thickness of 14-23 nm to permit optical tunneling for leakymode propagation. A propagation loss of ~0.5 dB/cm is predicted for the TE(0) or the TM(0) mode in a 5-microm-thick Sicore layer. Novel silicon ARROW-A and ARROW-B guides are also discussed. The former uses a pair of buried GeSi layers; the latter employs buried SiO(2) and GeSi films.

Fail-safe fiber-optics data bus using active multimode mirror terminals.

A prototype fail-safe optical data bus utilizing active LiTaO(3) electrooptic mirror terminals has been constructed and tested. Features of the system include (1) a single optical source; (2) an optical insertion loss of less than 6 dB and a tapoff ratio of 13 dB for the mirror terminals in the fail-safe mode; (3) compatibility with commercially available LED sources, P-I-N photodiode detectors, and step-index multimode monofibers; (4) remote terminal modulation depth approaching 50% for 100 V applied; and (5) the use of a pulse transformer technique which allows the required electrooptic modulation voltages to be obtained from a 5-V electrical supply. The construction of a working prototype data bus using mirror terminals demonstrates the feasibility of such systems for use in optical communications at the present state of the art.

Optical waveguiding in a single-crystal layer of germanium silicon grown on silicon.

Low-loss waveguiding at lambda = 1.3 microm has been observed in a partially strained, 10-microm-thick, single-crystal layer of Ge(0.1)Si(0.9) grown by chemical-vapor deposition upon an intrinsic (100) silicon substrate. The TM-mode propagation loss in the multimode planar guide was 1.9 dB/cm.