Showing papers on "Optical modulator published in 1988"

Frequency chirping in external modulators

Abstract: A general formula is given that expresses frequency chirping in some types of external intensity modulators, such as the loss modulator, directional-coupler-type modulator, Mach-Zehnder interferometry-type modulator, and total-internal-reflection-type modulator. The chirping phenomenon treated is caused by the phase modulation due to an accompanied refractive index change. It is uniquely expressed in terms of an alpha -parameter that contributes to frequency chirping in the same manner as in the direct modulation of a semiconductor laser. In addition, the transmission bandwidth of a single-mode fiber system using an external modulator is discussed and compared with the results obtained utilizing direct laser modulation. >

Multiple quantum well (MQW) waveguide modulators

Abstract: Because the electroabsorption effect in semiconductor multiple quantum well material is approximately 50 times larger than in bulk semiconductors, significant interest has been generated in the use of MQW's in optical modulators Small high-speed devices have been made which show promise for external modulators in optical transmission systems, as well as for encoding and processing components in optical interconnect and signal processing systems The fact that these modulators are made from III-V semiconductors had led to interest in integration of these components with other active optoelectronic components Although most devices have operated with light of a wavelength of 085 mu m, recently much progress has been made in applying this technology to devices operating near 155 mu m The author reviews the work of the last few years in this field and indicates some future directions >

Phase-only modulation with twisted nematic liquid-crystal spatial light modulators.

Abstract: It is shown that twisted nematic liquid-crystal spatial light modulators behave as phase-only modulators when operated below the conventional optical threshold. Thus such devices, when operated in a reflection mode, behave as spatial amplitude modulators when used between crossed polarizers above the optical threshold; they behave as phase modulators when used between parallel polarizers and operated below that threshold.

Reduction of intermodulation distortion in interferometric optical modulators

Abstract: A dual-polarization technique for reducing intermodulation distortion in integrated-optic interferometric modulators is described. While this technique results in a decrease in modulator sensitivity, the intermodulation distortion can be significantly reduced for a given depth of optical modulation by adjusting the relative amounts of TE and TM optical power. Reductions in intermodulation distortion as high as 21 dB were demonstrated by means of this technique with a Ti:LiNbO(3) device.

Electron-beam-tunable interference filter spatial light modulator.

Abstract: A new type of electron-beam-addressed spatial light modulator has been demonstrated. It is based on the local tuning of an optothermal nonlinear interference filter by the scanned electron beam. Such an e-beam-tunable interference filter device should find applications in both display technology and optical data processing.

Analysis of depletion edge translation lightwave modulators

Abstract: Presents a complete analysis of waveguide phase modulators based on the depletion-edge-translation concept. The phenomena taking place inside the depletion region which contribute to changing the refractive index there are studied. It is shown that the behavior of these modulators can be understood in terms of two electric field-related and two carrier-related effects: linear electrooptic, electrorefractive, plasma, and band filling. The sum of the refractive index variations produced by each one of these effects, taking into account the waveguide geometry, accounts quantitatively for the experimental phase shifts measured in the devices. No fitting parameters are used and a very good agreement between theory and experiment is obtained. Based on this theory, an analysis of the device is made in terms of the optimum values for the doping in the waveguide, and also in terms of the wavelength dependence of the device phase modulation properties. >

Active pulse compression using an integrated electro-optic phase modulator

Abstract: We describe a pulse compression technique that uses integrated electro‐optic phase modulators to linearly chirp optical pulses for compression by a dispersive delay line. In contrast to passive chirp techniques such as self‐phase modulation, this approach of active pulse compression does not depend on the optical power and thus shows promise for application to low‐power solid‐state lasers as an alternative to mode locking for realizing compact picosecond sources. We have demonstrated this technique by compressing the pulses from a mode‐locked neodymium:yttrium aluminum garnet laser from 100 to 45 ps.

Long-range surface plasmon electrooptic modulator.

Abstract: An electrooptic modulator based on electrically varying the degree of coupling of a light beam to a long-range surface plasmon excitation is modeled. A specific embodiment of a modulator is shown to be capable of changing its reflectance from 0.00 to 0.84 on the application of 100 V across a 1-μm thick electrooptic film with a second-order susceptibility of 2 × 10−7 esu. The affect that the width of the long-range surface plasmon excitation resonance has on the performance of the modulator is considered. Plane-wave calculations show that decreasing the width of the resonance increases the amount of modulation possible for a given voltage change. More realistic calculations which take into account diffraction of a beam of finite width show that a decrease in resonance width can in some cases degrade device performance.

Blue shift of the absorption edge in AlGaInAs‐GaInAs superlattices: Proposal for an original electro‐optical modulator

Abstract: The modulation by an external electric field of the resonant tunneling coupling between adjacent wells in a thin layer superlattice results in a blue shift of the effective absorption edge. We present a study of the electroabsorption in Al0.24Ga0.24In0.52As‐Ga0.47 In0.53 As superlattices, which evidence directly this remarkable effect, and we discuss its application to the design of new electro‐optical modulators.

Optical modulators based on polymers

Abstract: This invention relates to a device for electrooptic modulation of an optical beam. The device is a laminate comprising a coherent film of a conjugated polymer having semi-conducting properties as the active component laminated with two or more layers of electrically conducting, insulating or semi-conducting materials. At least the active component in the laminate is capable of interacting with the optical beam when under the influence of an electric signal and thereby result in a modulated beam. Means are provided for (i) establishing electrical contact with one or more layers of the laminate so as to enable an electrical signal to be applied across the active component and (ii) detection of the extent of modulation of the modulated beam emerging from the active component layer.

Optical characteristics of a deformable-mirror spatial light modulator.

Abstract: Some of the optical characteristics of a recently developed solid-state deformable-mirror spatial light modulator have been investigated. The device is composed of an array of 128 × 128 pixels, with each pixel consisting of four hinged reflective rectangular surfaces. Modulation at video frame rates has been achieved, providing real-time displays in coherent light, which may be useful for a variety of optical processing and computing applications.

Reduction Of Intermodulation Distortion In Interferometric Optical Modulators

Abstract: A dual-polarization technique for reducing intermodulation distortion in integrated-optic interferometric modulators is described. While this technique results in a decrease in modulator sensitivity, the intermodulation distortion can be significantly reduced for a given depth of optical modulation by adjusting the relative amounts of TE and TM optical power. Reductions in intermodulation distortion as high as 21 dB were demonstrated by means of this technique with a Ti:LiNbO(3) device.

Synthesis of crafted optical pulses by time domain modulation in a fiber‐grating compressor

Abstract: We report a method for direct generation of voltage programmable, arbitrarily shaped laser pulses of subpicosecond duration and roughly 100 fs resolution. A waveguide intensity modulator was fabricated and incorporated into a fiber‐grating pulse compressor to shape temporally the chirped pulse before compression. Optical pulses with sin(x)/x and square envelopes are synthesized to demonstrate the technique.

Electrically tunable Fabry–Perot mirror using multiple quantum well index modulation

Abstract: We describe a novel surface normal Fabry–Perot multiple quantum well index reflection modulator which may be tuned electrically. The Fabry–Perot etalon, composed of two AlAs/Ga1−xAlxAs quarter‐wavelength grating mirrors separating a multiple quantum well GaAs/Ga1−xAlxAs active medium, is formed in a single growth by molecular beam epitaxy. Contrast ratios of up to 8:1 at 873 nm have been measured.

Electrical and optical interactions between integrated InGaAsP/InP DFB lasers and electroabsorption modulators

Abstract: Electrical and optical interactions occurring in InGaAsP/InP integrated light sources composed of DFB lasers and electroabsorption modulators have been studied. Static measurements indicated that an isolation resistance between the laser and the modulator should be as large as 100 k Omega or more in order to reduce a lasing wavelength shift associated with biasing the modulator. The useful guided light out of the modulator was about 1-2 percent of the laser output. At high frequency modulation, asymmetric sidebands were observed in dynamic spectra measured at the modulator facet, which were caused by additional modulation of the laser part. Such additional modulation was relatively large at the relaxation oscillation frequency of the laser, but it was reduced by connecting an RF bypass condenser in parallel to the laser diode and by depositing an antireflection coating at the modulator facet. After eliminating the additional modulation, an integrated device showed a high frequency response of 5.7-GHz 3-dB bandwidth and a small linewidth enhancement factor of alpha =0.8. >

Acousto-optic modulators for optical fibers using Hertzian contact with a grooved transducer substrate

Abstract: The authors describe acousto-optic modulators for optical fibers, fabricated by the deposition of thin zinc-oxide transducers onto specially lapped glass capillary tubes. By exposing the inner surface of the capillary and contacting it to an optical fiber of the same diameter, a good Hertzian contact can be easily attained, providing efficient throughput of acoustic energy at frequencies in excess of 1 GHz. A phase modulator for a single-mode fiber has been fabricated in such a manner, achieving a maximum phase retardation of 1.0 rad/W input electrical power at 416 MHz. Its frequency, power, and polarization characteristics are discussed. Also presented are data from a frequency shifter/modal coupler for a two-mode optical fiber. This device achieved 6% power coupling from the fundamental optical mode to the second-order mode with 500 mW input power at 1.03 GHz. Single sideband modulation was observed with >19-dB suppression of the unwanted sideband. The authors also describe the design of an optical tap using the same technology. >

Electrodispersive multiple quantum well modulator

Abstract: Quantum‐confined Stark effect is combined with a Fabry–Perot resonance to build a multiple quantum well electro‐optic modulator. The structure consists of GaAs/AlGaAs quantum wells between two epitaxial AlAs/AlGaAs dielectric multilayer mirrors, all grown by molecular beam epitaxy. The modulator uses refractive index changes induced by applied electric fields. In reflection mode of operation, the modulator demonstrates >5:1 contrast ratio and >50% absolute maximum reflectivity with 17 V applied.

Spatial light modulator and optical dynamic memory using a 6×6 array of self-electro-optic-effect devices

Abstract: Using a 6 × 6 array of integrated quantum-well self-electro-optic-effect devices, we demonstrate an optically addressed spatial light modulator able to convert a visible, incoherent image into coherent infrared (IR) light Depending on the IR wavelength used, the output is either a positive, binary-thresholded version of the input (bistable mode) or its linear, negative (self-linearized) mode This device can also function as a dynamic bistable memory that can retain its internal state without power for times as long as 30 sec

Self‐electro‐optic effect device and modulation convertor with InGaAs/InP multiple quantum wells

Abstract: We report the first observation of the self‐electro‐optic effect in InGaAs/InP multiple quantum wells, grown by organometallic vapor phase epitaxy. Clear bistability and switching are observed over a range of 40 nm around 1.61 μm with 20–30 V bias. We demonstrate the operation of a modulation convertor, which converts a modulation from a carrier at 1.6 μm onto a carrier at 0.85 μm and vice versa.

Low-voltage phase modulation in GaAs/AlGaAs quantum well optical waveguides

Abstract: We report GaAs/AlGaAs quantum well waveguide phase modulators with high phase shift coefficients, as large as 520 degrees per V mm. By operating at wavelengths far below the bandedge and applying DC bias we achieve large electrooptic modulation with low absorption loss in device lengths on the order of 100μm and drive voltages on the order of 1 V.

Guided-wave intensity modulators using amplitude-and-phase perturbations

Abstract: A theoretical analysis of intensity modulation in coupled waveguides and Mach-Zehnder interferometers is reported. Simultaneous phase and amplitude perturbations Delta n+ iota Delta k are considered. Predictions are made about the performance of electrooptic GaAs and InP modulators controlled by the free-carrier effect ( Delta N) or by the Franz-Keldysh effect ( Delta E). The phase-dominant condition Delta n>5 Delta k is optimal. The predicted depth of modulation is greater than that of conventional loss-modulators over a range of Delta N or Delta E. >

Bidirectional optical data communications system

Abstract: A system and method are disclosed for optically communicating data between two data handling units where one of the data units supplies all of the optical power needed for the optical communications between the two data units. In a preferred embodiment a first data unit comprises a first data source, first and second optical sources of optical energy, and a first optical detector, while a second data unit comprises a second data source, a second optical detector and an optical modulator. In a first mode of operation, a first stream of digital data from the first data source pulse modulates the first optical source on and off causing the first optical source to transmit optical pulses to the second optical detector. These optical pulses are converted by the second optical detector back into a representation of the first stream of digital data for use by the second data unit. In a second mode of operation unmodulated optical energy is transmitted from the second optical source in the first data unit to the optical modulator in the second data unit. A second stream of digital data from the second data source is applied to the optical modulator to accordingly modulate the received unmodulated optical energy. Modulated optical pulses from the optical modulator are therefore reflected to the first optical detector. These reflected modulated optical pulses are converted by the first optical detector back into a representation of the second stream of digital data for use by the first data unit.

Theoretical and experimental studies of optical absorption in strained quantum-well structures for optical modulators.

Abstract: Theoretical and experimental results on excitonic and band-to-band absorption spectra in strained multiple-quantum-well structures are presented and discussed. The study is particularly motivated by the need to enhance optical absorption in devices such as optical modulators and high-speed photodiodes. We show that it is possible to merge the heavy- and light-hole excitonic resonances through use of biaxial tensile strain in the well regions. Calculations and experiments show a strong increase in optical absorption, by almost a factor of 2, due to this merger. The calculations include the heavy-hole--light-hole coupling which is essential to understand this phenomenon. The electric field dependence of optical absorption in strained quantum wells is also presented.

Silicon double‐injection electro‐optic modulator with junction gate control

Abstract: Novel structures for carrier‐induced electro‐optical phase modulation in crystalline silicon are examined. A new dual‐injection field‐effect transistor structure for guided‐wave light modulation at 1.3 μm is proposed and analyzed. It consists of an elongated cathode‐anode‐gate structure integrated in a rib waveguide. Dual‐gate and single‐gate control are considered. The overlap between the plasma charge density and the optical guided mode is computed. For a cathode‐anode voltage of 0.32 V, the effective refractive index of the waveguide mode changes by ΔN=1×10−3 when the gate voltage is altered by 12 V. Numerical estimates of the bias current, pinchoff voltage, interaction length, and modulator speed are given.

Fiber optic gyro stabilized by harmonic components of detected signal

Abstract: Light waves are allowed to pass clockwise and counterclockwise through a circuit optical path of at least one loop, and the light waves having passed through the optical path are interfered with each other. Phase modulator is inserted in one end of the circular optical path, by which the light waves are phase modulated. The intensity of the interference light is detected, as an electric signal, by a photodetector. The fundamental wave component of the modulation frequency of the phase modulator, contained in the electric signal converted by the photodetector, is synchronously detected by a first synchronous detector, from which is output a signal corresponding to an input angular velocity. Second and fourth harmonic components of the modulation frequency of the phase modulator, contained in the electric signal, are synchronously detected by second and third synchronous detectors. The phase modulator is controlled so that the ratio between the outputs from the second and third synchronous detectors is always equal to a preset reference value.

Linear 1 × 2 directional coupler for electromagnetic field detection

Abstract: A 1×2 directional coupler which can be used as an integrated optical electromagnetic field detector is described. Modulators have been formed from titanium‐indiffused lithium niobate waveguides operating at the 1.3 μm wavelength. Linear dynamic ranges of 74.3 dB with 190 μW light and 76.3 dB with 760 μW light were observed for separate devices. Voltage sensitivity measurements are in good agreement with theoretical predictions. A 3.1 μV rms sensitivity was obtained for a device with a 10 mm electrode length.

Visualization of electrical domains in semi‐insulating GaAs:Cr and potential use for variable grating mode operation

Abstract: The results of an experimental optical technique for imaging the electrical domain repartition in semi‐insulating GaAs:Cr are reported. The technique is based on the use of the crystal as the active component of a transverse electro‐optic two‐dimensional light modulator. Under dc applied voltage, the electrical domains are traveling from the cathode to the anode at a velocity that increases with the applied voltage and with the incident illumination (v≂10–100 mm/s). Results for ac applied voltages are also presented. In particular, the observation of stationary and periodically distributed high‐field domains in GaAs:Cr is reported for sawtooth applied voltages (1 kV, 50–250 Hz). These high‐field domains induce a phase structure whose period is shown to be electrically controllable. This is the first reported demonstration of the possibility of a variable grating mode operation in semiconductors.

Materials for Photonic Switching and Information Processing

Abstract: In electronic processors, heat dissipation and interconnection delay are serious design and performance limiting factors. Why then consider photonic components where both the energy and size of the photon are large (˜1 eV and ˜1 μm, respectively) and the required nonlinear interactions between electric or magnetic fields and photons for switching or modulation are small? There are several answers to this question. First, the wide bandwidth of optical communications systems is taxing the current capabilities of electronic switching technologies. Even a slow optical switch can switch a very wide bandwidth optical signal from one fiber to another. External optical modulators will likely be required in ultrawide bandwidth communications because of basic limitations on direct modulation of lasers. Because of the weak electromagnetic interaction and low dispersion, optical interconnection of electronic circuits offers considerable advantages in high speed computer architectures. Some of these applications would appear to be relatively near term since they build on current capabilities of optical communication.Longer term and more speculative are applications of photonics to computation and image processing — areas where electronics technology is already mature. Current research can be divided into two groups — ultrafast processing and parallel processing. The first group concentrates on processing with ultra-fast optical pulses. Optical pulses as short as 6 fs — orders of magnitude shorter than any electronic pulses — have been generated in the research laboratory. High processing rates are achievable by serial processing of high repetition rate ultrashort pulses. This approach requires ultrafast switches, which in turn requires materials with ultrafast nonlinear optical response time. Indeed, the shortest electrical signals are now measured by optical sampling techniques.

A silicon light modulator

Abstract: There are a number of potentially powerful applications where light modulators can be useful, depending on the ability to produce miniature, inexpensive, high accuracy, high reliability modulators integrated with electronic circuitry. The authors discuss a silicon-based micromechanical light modulator that has been developed. Micromechanical fabrication technology in silicon, compatible with silicon circuit technology, has been used. A comparison of the modelled and the experimentally measured characteristics of the micromechanical light modulator is presented, and potential applications are given.

AlGaAs/GaAs multiple quantum well reflection modulators grown on Si substrates

Abstract: We report for the first time large excitonic absorption at room temperature in AlGaAs/GaAs multiple quantum well structures grown on Si substrates in a p‐i‐n configuration, using photocurrent measurements. We demonstrate an optical reflection modulator which is based on the quantum‐confined Stark effect and exciton broadening with a reverse bias voltage applied across the p‐i‐n structure. A 7.7% change in the reflectivity of the device with 6 V reverse bias voltage was observed. These results demonstrate clearly that optical device quality AlGaAs/GaAs is obtainable directly on Si substrates which has great implications with regard to the monolithic integration of optical III‐V and electronic Si technology.