Showing papers by "Amnon Yariv published in 1992"

Spatial solitons in photorefractive media.

Abstract: We show that photorefractive media can support a new type of spatial soliton, in which the diffraction is balanced by the self-scattering (two-wave mixing) of the beam spatial frequency components. This photorefractive soliton possessses some unique properties, such as independence of the absolute light intensity, and can experience absorption (or gain) with no change in its transverse structure.

Optical data storage by using orthogonal wavelength-multiplexed volume holograms

Abstract: We propose a volume holographic data storage scheme that employs counterpropagating reference and image beams and wavelength multiplexing for page differentation. This method is compared with that based on angular multiplexing. A reduction in holographic cross talk is predicted. Further cross-talk reduction that is due to sidelobe suppression is observed in experiments by using photorefractive crystals and the proposed orthogonal data storage.

Photorefractive systems and methods

Abstract: A new type of holographic recording in photorefractive crystals is disclosed, termed orthogonal data storage, in which counterpropagating, reflection mode holograms are wavelength multiplexed to form gratings lying along a common axis. It is shown that this configuration provides substantial improvements in data storage capacity in comparison to prior art systems and that higher coupling coefficients are achieved. In consequence, not only storage of data but a wide range of other devices including wavelength selective filters, lenses and optical correlation systems are disclosed. Further, methods of writing and processing metastable images are disclosed which substantially improve the strength of the gratings that are permanently developed in the medium. These methods include advantageous use of electrical potentials and photovoltaic properties, together with different interrelationships during processing.

Spatial solitons in photorefractive media

Abstract: We present a new type of spatial soliton, generated by the photorefractive (PR) effect of the medium. The shape of the soliton modulates the refractive index via the PR effect, which results in an exact compensation for the effects of diffraction, and causes the light beam to propagate the unvarying profile. This index modulation is represented in the formalism as a distribution of index gratings, each one of them induced by the interference between two spatial (frequency) plane wave components of the light beam. Since the efficiency of this effect is independent of absolute light intensity, these new solitons can be generated even at very moderate light intensities. Moreover, a given soliton waveform can propagate unchanged in the medium, at very high or very low light intensities (and at all levels in between).

Characterization of a new photorefractive material: Kl-yLyT1-xNx

Abstract: We report the growth and characterization of a new photorefractive material, potassium lithium tantalate niobate (KLTN). A KLTN crystal doped with copper is demonstrated to yield high diffraction efficiency of photorefractive gratings in the paraelectric phase. Voltage-controllable index gratings with n, = 8.5 x 10^-5 were achieved, which yielded diffraction efficiencies of 75% in a 2.9-mm-thick sample. In addition, diffraction was observed in the paraelectric phase without an applied field. This effect is attributed to a growth-induced strain field.

Voltage‐controlled tunable GaAs/AlGaAs multistack quantum well infrared detector

Abstract: We describe a new type of intersubband GaAs/AlGaAs infrared detector consisting of three stacks of quantum wells; the quantum wells in a given stack are identical, but are different from stack to stack. Each stack is designed to yield an absorption and a photoresponse at a different peak wavelength. The resulting device is an infrared detector which can operate in a number of modes. Among the features of this device are a wide‐band detection domain, a tunable response and excellent responsivities and noise figures. The tunable operation includes a sharp peak‐switching response which follows the formation, expansion, and readjustment of electric field domains within the multiquantum well region.

A reduction of interferometric phase-to-intensity conversion noise in fiber links by large index phase modulation of the optical beam

Abstract: A noise reduction scheme for long haul fiber amplitude modulation (AM) systems is proposed and analyzed Such systems suffer from intensity noise which results from interference between the (twice) Rayleigh scattered light and the directly transmitted beam This interference converts the fundamental phase noise of the laser to intensity noise It is shown that a strong phase modulation of the output of the laser beam causes a large reduction of the detected signal noise in the vicinity of the detected signal components >

The extra differential gain enhancement in multiple-quantum-well lasers

Abstract: By accounting for the unavoidable thermal population of injected carriers in the optical confining layers it is found that the use of MQW's (multiple quantum wells) as active regions actually leads to an extra increase in differential gain. Specifically, the maximum differential gain increases with the number of wells in the QW structures. The transparency current density in the MQW structure does not scale as the number of QWs. These conclusions are at variance with presently accepted theory and have major implications for the design of high-speed, low-threshold semiconductor lasers. >

Observation of phase conjugation at 10.6 μm via intersubband third‐order nonlinearities in a GaAs/AlGaAs multi‐quantum‐well structure

Abstract: We describe the observation of phase conjugation at 10.6 μm in a GaAs/AlGaAs multi‐quantum‐well‐doped structure. The responsible nonlinear susceptibility χ(3) is due to a nearly resonant intersubband transition. The magnitude of χ(3) is 7×10−5 esu and the phase conjugate reflectivity is a few tenths of a percent.

Manifestation of Berry's phase in image-bearing optical beams.

Abstract: We show that optical image-bearing beams manifest a generalized version of Berry's phase. This topological phase results in a rotation, equal to Berry's phase (with an opposite sign), of an arbitrary input optical image. Our method of measuring this phase applies to coherent as well as incoherent illumination, and thus is demonstrated experimentally using an incandescent light bulb. Furthermore, this is one of a few cases where a phase property of an ensemble of ``particles'' is unique, regardless of their energies, momenta, and spins.

Observation of third‐order intersubband dc Kerr effect at the midinfrared wavelengths in GaAs quantum wells

Abstract: We report on the first observation of third‐order intersubband nonlinearities in a quantum well structure. We have measured the dc Kerr effect in a symmetric quantum well and found that the Kerr coefficients due to intersubband transitions are six orders of magnitude larger than that of bulk GaAs. To our best knowledge this is the largest value ever measured for the third‐order susceptibility. By including dc screening effects and evaluating the internal electric field in the well, a good agreement between the calculated coefficients and the experimental ones was found.

Method of manufacturing a distributed light emitting diode flat-screen display for use in televisions

Abstract: A novel display screen structure and method of manufacturing such screens for use, for example, in large screen television displays. The process of the present invention is one which can be accomplished with no new materials, no critical geometric requirements such as critical separations and alignments and only low voltage drivers. The combination of these features results in a technology which can be easily scaled to large sizes to provide relatively low-cost large screens for televisions. An important step in a first embodiment of the present invention is the alignment of a large plurality of columnar-shaped light emitting diode slivers in an uncured optical epoxy by applying an electric field through a mixture of such slivers and epoxy and then curing the epoxy to effectively fix the light emitting diode slivers in that aligned configuration. In a second embodiment, the LED slivers are mixed with molten glass which is formed into elongated glass fibers. The fibers are sandwiched between conductive glass plates and heated while an aligning voltage is applied. The light being emitted by such diodes is thereafter controlled by orthogonally directed electrodes at least one of which is optically transparent and which are placed on opposing surfaces of the thin plate-like structure fabricated in accordance with the invention.

New photorefractive mechanism in centrosymmetric crystals: A strain-coordinated Jahn-Teller relaxation.

Abstract: We present observations and an explanation of a photorefractive effect in strained centrosymmetric KTN and KLTN crystals in the absence of an externally applied electric field. Centrosymmetric crystals are forbidden to display the classical photorefractive effect without application of an applied field. Nevertheless, in diffraction experiments, centrosymmetric KTN:Cu and KLTN:Cu crystals show index changes of up to 1.7×10 -5 and photorefractive response at more than 120 o C above the phase transition

Analysis and verification of an analog VLSI incremental outer-product learning system

Abstract: An architecture is described for the microelectronic implementation of arbitrary outer-product learning rules in analog floating-gate CMOS matrix-vector multiplier networks. The weights are stored permanently on floating gates and are updated under uniform UV illumination with a general incremental analog four-quadrant outer-product learning scheme, performed locally on-chip by a single transistor per matrix element on average. From the mechanism of floating gate relaxation under UV radiation, the authors derive the learning parameters and their dependence on the illumination level and circuit parameters. It is shown that the weight increments consists of two parts: one term contains the outer product of two externally applied learning vectors; the other part represents a uniform weight decay, with time constant originating from the floating gate relaxation. The authors address the implementation of supervised and unsupervised learning algorithms with emphasis on the delta rule. Experimental results from a simple implementation of the delta rule on an 8*7 linear network are included. >

The gain and carrier density in semiconductor lasers under steady-state and transient conditions

Abstract: The carrier distribution functions in a semiconductor crystal in the presence of a strong optical field are obtained. These are used to derive expressions for the gain dependence on the carrier density and on the optical intensity-the gain suppression effect. A general expression for high-order nonlinear gain coefficients is obtained. This formalism is used to describe the carrier and power dynamics in semiconductor lasers above and below threshold in the static and transient regimes. >

Broad-band wavelength tunable picosecond pulses from CW passively mode-locked two-section multiple quantum-well lasers

Abstract: Wavelength tunable CW (continuous-wave) passive mode-locking of a two-section quantum-well laser coupled to an external cavity is demonstrated. A tuning range of 26 nm is achieved with typical autocorrelation full widths at half maximum of 4.5 ps. The pulses are not transform limited, having a typical time-bandwidth product of 2.5. >

Demonstration of the optical Kerr effect at 10.6 μm via intersubband nonlinearities in a multi‐quantum well structure

Abstract: We demonstrate the optical Kerr effect at 10.6 μm in a GaAs/AlGaAs multi‐quantum well doped structure. The effect is due to the nonlinear susceptibility χ(3) near a resonant intersubband transition.

Self-quenching of the semiconductor laser linewidth below the Schawlow-Townes limit by using optical feedback

Abstract: We demonstrate theoretically and experimentally self-quenching of the fundamental semiconductor laser frequency fluctuations to a level that is orders of magnitude below the Schawlow-Townes limit for a solitary laser. It is shown that the main operative mechanism is the combined action of a frequency-dependent internal loss and amplitude-to-phase coupling. The internal frequency-dependent loss is introduced by means of spectrally narrow external optical feedback, which provides a strong frequency-dependent dispersion. Linewidth reduction by a factor of 2 X 10^3 is demonstrated by using a narrow Doppler-free Faraday resonance in Cs vapor.

Subpicosecond (320 fs) pulses from CW passively mode-locked external cavity two-section multiquantum well lasers

Abstract: Pulses from a passively mode-locked two-section multi-quantum well laser coupled to an external cavity are compressed to subpicosecond pulse widths using an external grating telescope compressor. A minimum deconvolved pulse width of 0.32 ps is measured, close to the transform limit, with peak powers of 1.9 W.

Vibration detection using dynamic photorefractive gratings in KTN/KLTN crystals

Abstract: We demonstrate a sensitive, all‐optical, self‐aligning holographic microphone/vibration sensor utilizing the zero external electric field photorefractive (Zefpr) effect. The device relies on the unique phase relationship, φ=0, between a spatially periodic intensity standing wave and the resultant index grating created with the Zefpr effect. Under this zero phase condition, the transmitted intensity of interfering beams in a two beam coupling geometry varies linearly with displacement of either the index grating or one of the interfering beams. In this way, vibrations are sensed remotely without any electrical signals in the vicinity of the sensor. The sensitivity of the microphone was determined as a noise equivalent power of 15 dB sound pressure level relative to 0.0002 μbar across the range 1.6–15.5 kHz.

Method and apparatus for monotonic algorithmic digital-to-analog and analog-to-digital conversion

Abstract: The present invention provides a compact and robust architecture and a corresponding method to implement a monotonic algorithmic D/A converter that processes the bits of the digital input in the order from MSB to LSB, and a successive approximation A/D converter employing the intermediate conversion results of this D/A converter. The invention is aimed at applications requiring a dense integration in general VLSI technologies of multiple D/A and A/D converters, where individual trimming of components to compensate for component offsets and mismatches is virtually impossible. The architecture comprises four charge holding components, one switch for charge sharing, two bi-directional replication elements for charge storage and recall, and one comparator. Also described is an efficient way of performing pseudo-logarithmic compression of conversion values merely by adjusting the relative sizes of two of the charge holding components.

Pattern matching and parallel processing with CCD technology

Abstract: A fully parallel charge coupled device (CCD) memory chip of N address lines is presented. It detects, in just one clock cycle, a perfect matching between input pattern and any of the stored patterns. It detects in fewer than N cycles the best matching in case a perfect one does not exist. The charge packets, representing binary words, are generated by external pulses that are applied to the chip through data input lines and then are compared to the data applied to the address lines. The chip architecture is described. This chip is suitable for applications in pattern recognition, Kanerva memories, data decoders, and other systems that require peak detection or Hamming distance calculation. Typical results expected in these kinds of implementations based on similar devices are reported. >

Second quantized state oscillation and wavelength switching in strained‐layer multiquantum‐well lasers

Abstract: The dependence of lasing wavelength on cavity length has been systemically studied in strained‐layer single, double, and triple quantum‐well InGaAs lasers. Lasing from the second quantized state has been observed for the first time in double and triple quantum‐well lasers. A wide range (∼500 A) wavelength switching between the first and second quantized states has been demonstrated by controlling the injection current and/or the operation temperature.

Apparatus for all-optical self-aligning holographic phase modulation and motion sensing and method sensing and method for sensing such phase modulation

Abstract: An all-optical, self-aligning, holographic phase modulation and motion sensing apparatus includes a crystal exhibiting a zero electric field photorefractive effect, a phase modulation or vibration source or mechanism, a source of coherent optical radiation, beam splitting and directing optics, and at least one optical radiation detector. The output from the optical radiation source is split into separate beams, one or all of which are phase modulated. The beams are directed through the crystal exhibiting a zero electric field photorefractive effect, and the resultant transmitted beams are detected by an optical radiation detector to provide a measurement of the phase modulation of the beams. The sensor functions in the absence of an electric field because of the unique characteristic of the crystal exhibiting the zero electric field photorefractive effect whereby if either the phase of the crystal's index grating or of the interfering beams is modulated with a phase change very much less than π/2, the intensity of the beam transmitted through the crystal varies linearly with the modulation. Such linear modulation allows crystals exhibiting the zero electric field photorefractive effect to remotely sense phase modulations or vibrations of any type in the absence of electrical signals in the vicinity of the sensor.

High Resolution Volume Holography using Orthogonal Data Storage

Abstract: : Holographic volume data storage has long been an intriguing subject of research interest. The main reason for this interest is the promise of enormous data storage capacities. In a 1 sq.cm. volume, storage 10 to the 12th bits or more is possible using visible light. However, to date, all attempts at attaining and demonstrating a volume holographic data storage system of this density have fallen far short of this theoretical limit. Although materials limitations contributed to this shortcoming, of more fundamental importance, however, were the problems associated with the angular multiplexing schemes used to holographically access the volume of the storage medium.

Fundamental Limits in Quantum Well Intersubband Detection

Abstract: Since the concept of infrared (IR) photodetection by intersubband absorption in superlattices was first proposed and demonstrated by Smith et al.(1), the field of intersubband physics and applications has grown and attracted an increasing interest. Along with the demonstration and the analysis of intersubband absorption(2), came the possibility of studying a very basic quantum system in the laboratory. These efforts revealed the richness of intertweened basic physical effects that have to be understood or taken into account for the full mastering of intersubband physics and applications: Stark(3), depolarization(4,5) and other energy shifts, exchange interactions in doped quantum wells(6), effects of real material systems, polarization(7) and parity selection rules - just to name a few. On the application side another important channel of interest was opened with the realization that very large optical nonlinearities could be achieved through intersubband transitions in superlattices(8-10).

Method and apparatus for making highly accurate potential well adjustments in CCD's

Abstract: An adjustable CCD gate structure utilizing ultra-violet light activated floating gates, wherein a floating polysilicon gate is used between a CCD electrode and the underlying substrate to provide a fixed voltage bias to the CCD gate during the manufacturing process thereof The floating gate is programmed with a desired voltage bias during the application of ultra-violet light and is thereafter fixed at that adjusted level, upon the removal of the ultra-violet light. Thus, the method of the present invention comprises the steps of providing a CCD gate structure in which there is such a floating polysilicon gate between the CCD electrode and the underlying substrate; applying an ultra-violet light activation to the floating polysilicon gate; applying a voltage to the conventional CCD electrode which is resistively coupled to the floating electrode for adjusting the bias on the floating electrode to a desired level; and then removing the ultra-violet light to fix the voltage bias at the floating polysilicon gate at a permanant level.

Interferometric electro-optical signal processors with partially coherent illumination

Abstract: Fourier- and Hartley-related transforms are realized in a family of interferometers. The implementation of these interferometers as image correlators is investigated theoretically and experimentally with both coherent and spatially incoherent illumination. Several correlators that can be used for pattern recognition are studied and demonstrated experimentally as special cases.

Modulation Bandwidth Enhancement in Single Quantum Well GaAs/AlGaAs Lasers

Abstract: The state filling effect in semiconductor quantum well lasers significantly affects the modulation dynamics The state filling effect strongly depends on the optical confining layer structure As a direct consequence of the reduction of the state filling effect in a properly designed graded index separate confinement heterostructure, a record 3dB bandwidth in excess of 9 GHz has been obtained in uniformly pumped single quantum well GaAs/AlGaAs lasers by a careful tailoring of the device parameters

Resonant tunneling through low dimensional quantum structures

Abstract: Tunneling mechanism in zero‐ and one‐dimensional quantum structures is studied. Several new results, peculiar to low dimensions, are predicted. We find that subband mixing and multichannel tunneling induce the appearance of new tunneling channels with unusual interference patterns, and allow for longer lifetime of the resonances at higher energies in various channels. It is shown that in low dimensions, there exists a critical size of the structure below which the resonance nature of the tunneling process is diminished. In zero and one dimensions, there exists a critical magnitude of the confinement potential, below which there are no resonances in the transmission function for any size of the well. Negative differential resistance and other phenomena related to the resonance characters of the tunneling will not appear in this case. We also develop a generalized transfer matrix method that takes into account subband mixing; this formalism can be used to describe any transport problem in low dimensions.