Showing papers by "Amnon Yariv published in 1993"

Observation of self-trapping of an optical beam due to the photorefractive effect.

Abstract: Recently, a new type of spatial soliton has been predicted to be observable in a photorefractive crystal.1,2 For example, consider an input laser beam that is diverging because of diffraction as it passes through a photorefractive material. Interference between the beam’s spatial-frequency components can be considered to write photorefractive gratings on the crystal. These gratings, or spatial variations in the index of refraction, have one component that is in phase and another that is 90° out of phase with their parent intensity pattern.

Wavelength stabilized laser sources using feedback from volume holograms

Abstract: A laser (11) utilizes feedback from a volume holographic (12) grating used as a wavelength standard to lock the laser output wavelength to its desired value. This feedback can be non-optical, wherein the holographic filter output is used to actively control the wavelength through an external control mechanism. This feedback can also be optical, wherein a volume hologram reflection grating is used to generate optical feedback into the laser gain.

Self-trapping of optical beams in photorefractive media

Abstract: We study the possibility of self-trapping of an optical beam in a photorefractive medium under the combined influence of diffraction and self-scattering (two-wave mixing) of its spatial frequency components. We investigate the spectrum of solutions for the resulting photorefractive spatial solitons and discuss their unique properties. Design considerations and material requirements for experimental realization of these solitons, together with specific examples, are given.

Method and apparatus for long-term multi-valued storage in dynamic analog memory

Abstract: A method and apparatus is described for refreshing the analog content of a analog memory having a volatile storage device. An analog value stored on a volatile storage device is iteratively adjusted to maintain the analog value in proximity to one of a set of predetermined discrete analog memory levels. Binary quantization of the stored value, yielding one bit of information corresponding to the analog value stored, determines whether to increase or decrease the stored value by a given small amount. In essence, the bit obtained by binary quantization encodes the direction toward the nearest discrete level. Memory retention achieved by periodic iteration of the method is robust to noise and random errors in the quantization, over a wide range of operating conditions. The apparatus includes as functional elements a binary quantizer and an increment/decrement refresh device, which may interface with the analog storage means in a variety of configurations, including configurations supporting multiplexed schemes for sharing quantizers and increment/decrement refresh devices among multiple storage cells.

Suppression of noise and distortion in fiber-optic systems

Abstract: Noise and distortion due to scattering and reflection in a fiber-optic communications system are suppressed by modulating the optical frequency of (chirping) a laser light source to broaden the spectrum of the laser light output. The broader spectrum spreads the noise produced by an unchirped source over a broader band. A noise and distortion suppression system includes a chirp signal generator coupled to the signal path of an RF input signal carrying information to modulate the laser optical output. In some cases, the frequency of the chirp generating signal may result in second-order intermodulation products falling within the information band. In such cases, the RF input signal is predistorted to offset the expected distortion products.

Temporal evolution of fanning in photorefractive materials.

Abstract: We present detailed calculations of the temporal and spatial evolution of beam fanning in photorefractive crystals that is initiated by scattering from noise. We show that fanning starts from beam coupling between the incident radiation and part of the incident radiation scattered by noise at or near the input plane. We show that scattering within the volume of the crystal has negligible effect on fanning, that absorption affects the time response but not the spatial pattern of the fanning, and that the difference between calculations including only phase-matched terms and those including non-phase-matched terms is negligible.

Temporal evolution of fanning in photorefractive materials

Abstract: We present detailed calculations of the temporal dependence of beam fanning obtained by numerical solutions of the paraxial wave equation for the optical beam, and the equation for the time dependence of the refractive index perturbation produced by interference between the incident beam and radiation scattered by imperfections in the crystal. Our calculations extend the results for steady- state fanning1 to temporal evolution and include the effects of absorption and scattering distributed throughout the crystal.

Electric-field multiplexing/demultiplexing of volume holograms in photorefractive media.

Abstract: We propose a new method of volume hologram multiplexing/demultiplexing in noncentrosymmetric media. Volume holograms may be multiplexed by tuning the material parameters of the recording medium (such as refractive index or lattice parameters) while keeping the external parameters (wavelength and angles) fixed. For example, an external dc electric field alters the index of refraction through the electro-optic effect, effectively changing the recording and reconstruction wavelengths in the storage medium. Then the storage of holograms at different fields, hence different indices of refraction, is closely related to wavelength multiplexing. We demonstrate this concept in a preliminary experiment by electrically multiplexing two volume holograms in a strontium barium niobate crystal.

Multiplexing and selective updatable fixing and erasing of volume holograms in photorefractive media and second harmonic generation in photorefractive media by optically induced periodic poling

Abstract: New methods are presented for multiplexing volume holograms in electrooptic materials. Multiple volume holograms can be superimposed in a medium and be individually addressed by tuning the underlying refractive index of the medium or the crystal parameters, while keeping the external parameters (wavelength, angles) fixed. According to the presented methods, the refractive index and crystal parameters of the material can be varied via the electrooptic, elastooptic or piezoelectric effects, alone or in combination with each other, by tuning the value of an applied electric field, or by tuning an applied mechanical stress, or by changing the material temperature. Volume holograms that have been multiplexed by either wavelength or angle or electric field or mechanical stress or temperature can be individually addressed by any one of the same five parameters. Additionally, selective, updatable fixing and erasing of volume holograms in photorefractive media is described. Each holographic page may be fixed individually and overwritten without destroying the other fixed pages. Further, Second Harmonic Generation and Parametric Amplification by optically induced periodic poling in ferroelectric photorefractive materials is described.

Very high modulation efficiency of ultralow threshold current single quantum well InGaAs lasers

Abstract: A record high current modulation efficiency of 5 GHz/[sqrt](mA) has been demonstrated in an ultralow threshold strained layer single quantum well InGaAs laser.

Selective page-addressable fixing of volume holograms in Sr 0.75 Ba 0.25 Nb 2 O 6 crystals

Abstract: We demonstrate selective fixing of volume holograms in photorefractive media. Each holographic page may be fixed individually and overwritten without destroying the other fixed pages. We present experimental results describing this process in Cr-doped Sr0.75Ba0.5Nb2O6 at room temperature, with hologram lifetimes exceeding 100 days during continuous readout with an intense beam (1 W/cm^2).

Interpage and interpixel cross talk in orthogonal (wavelength multiplexed) holograms.

Abstract: The problem of cross talk in image-bearing wavelength-multiplexed holograms was raised recently [A. Yariv, in Digest of Conference on Nonlinear Optics (Optical Society of America, Washington, D.C., 1992), postdeadline paper E-2]. In the limit of a large aperture (lens, crystal) it is shown that the cross talk is independent of the information content. The reduction of the hologram volume is shown to introduce interpixel as well as interpage cross talk.

Photorefractive Spatial Solitons

Abstract: Photorefractive crystals are electro-optic dielectrics that host a small amount of photosensitive impurities. Light propagation leads to the generation of outof-equilibrium mobile charge, that, in order to reach a stable electro-static configuration, redistributes throughout the crystal. The ensuing space-charge field, modifying electro-optically the crystal index of refraction, changes the trajectory of the ionizing light distribution, altering, in turn, the original charge-equilibrium conditions [1]. This feedback mechanism gives rise to a variety of nonlinear effects that go under the generic term of photorefractive nonlinear optics [2] [3] [4] [5]. For confined optical beams, nonlinear beam dynamics leads to two basic qualitatively different phenomena: beam fanning and self-lensing, connected, respectively, to the two basic charge transport mechanisms, diffusion and drift. Photorefractive spatial solitons emerge when beam self-focusing exactly balances diffraction, and are thus generally connected to regimes in which charge drift plays a fundamental role [6] [7] [8][9]. In a nonlinear beam perspective, spatial beams self-trap when the light-space-charge feedback mechanism finds its dynamic equilibrium point in a nondiffracting slab or needle of light, corresponding to an appropriate waveguide-like refractive index distribution. In what follows, we discuss such nonlinear phenomena, concentrating in particular on the basic theory and phenomenology.

Simple methods of measuring the net photorefractive phase shift and coupling constant.

Abstract: We report measurements of the photorefractive phase shift and coupling constant of several photorefractive materials. We solve the problem of beam coupling and diffraction in a material with a dynamically written grating for arbitrary input beams. These solutions are used to determine the beam coupling as a function of the photorefractive phase φ and coupling constant g when one beam is either sinusoidally phase modulated or ramped in phase. Experimental results are obtained for LiNbO3, BaTiO3, and for paraelectric potassium lithium tantalate niobate as a function of applied electric field.

Potassium lithium tantalate niobate photorefractive crystals

Abstract: Photorefractive crystals having the formula K 1-y Li y Ta 1-x Nb x O 3 wherein x is between 0 and 1 and y is between 0.0001 and 0.15. The crystals are useful as a photorefractive material for use in optical systems. The crystals may be doped with various first transition elements and lanthanides including copper, vanadium, chromium, iron, and manganese, nickel, europium and cerium.

Wavelength-multiplexed computer-generated volume holography.

Abstract: We demonstrate recording and reconstruction of multiple-computer-generated wavelength-multiplexed volume holograms in a holographic storage medium. The holograms display high selectivity, and their reconstruction process results in a convenient conversion of wavelength into angular multiplexing.

Linewidth reduction and frequency stabilization of a semiconductor laser with a combination of FM sideband locking and optical feedback.

Abstract: We describe a novel method for semiconductor laser noise reduction that uses a combination of optical and electronic feedback. A Doppler-free Faraday resonance in Cs vapor provided both optical feedback and discrimination for an electronic feedback scheme incorporating FM sideband spectroscopy. The introduction of electronic feedback further reduced the low-frequency fluctuation noise power by more than 2 orders of magnitude, resulting in a linewidth of 1.4 kHz.

Temporal evolution of photorefractive double phase-conjugate mirrors.

Abstract: We present wave-optics calculations of the temporal and spatial evolution from random noise of a double phase-conjugate mirror in photorefractive media that show its image exchange and phase-reversal properties. The calculations show that for values of coupling coefficient times length greater than two the process exhibits excellent conjugation fidelity, behaves as an oscillator, and continues to operate even when the noise required for starting it is set to zero. For values less than two, the double phase-conjugation process exhibits poor fidelity and disappears when the noise is set to zero.

Self‐enhanced diffraction from fixed photorefractive gratings during coherent reconstruction

Abstract: We explore, theoretically and experimentally, the effects of self‐enhancement (or self‐depletion) of the diffraction which occurs during coherent reconstruction from fixed photorefractive gratings. These effects are caused by interference between a secondary grating, which forms between the readout and the reconstructed beams, and the fixed grating.

A comparison of amplitude-phase coupling and linewidth enhancement in semiconductor quantum-well and bulk lasers

Abstract: The amplitude-phase coupling factor alpha (linewidth enhancement factor) is compared for typical semiconductor quantum-well and bulk double heterostructure lasers. As a direct consequence of the reduction of the differential gain, there is no reduction of alpha in single-quantum-well lasers compared to bulk lasers. The number of quantum wells strongly affects the amplitude-phase coupling in quantum-well lasers. It is shown that the interband transition induced amplitude-phase coupling dominates that induced by the plasma effect of carriers in typical quantum-well lasers. By considering the spontaneous emission factor in the spectral linewidth, the authors show that there is an optimal number of quantum wells for achieving the narrowest spectral linewidth. >

Linewidth reduction and frequency stabilization of semiconductor lasers using dispersive losses in an atomic vapor

Abstract: The effects of a dispersive loss on the linewidth and chirp of a semiconductor laser are discussed. A van der Pol analysis is used to obtain expressions for linewidth reduction and chirp reduction in an arbitrary optical feedback configuration. Specifically, the use of an atomic cesium vapor as a dispersive loss mechanism is considered. The dominant factor in reducing the linewidth and chirp is shown to be the frequency-dependent phase change from the atomic vapor. However, it is shown that under certain operating conditions the frequency-dependent amplitude changes from the atomic vapor also contribute to the reduction. The results are shown to be in agreement with a detailed rate equation calculation. >

Wavelength tunable source of subpicosecond pulses from CW passively mode-locked two-section multiple-quantum-well laser

Abstract: A wavelength-tunable passively-mode-locked semiconductor laser source of subpicosecond pulses is demonstrated. The system includes a two-section multiple-quantum-well laser which is coupled to an external grating for tuning and is followed by an external grating pair for pulse compression. A tuning range of 16 nm round 846 nm is obtained, resulting in compressed pulse widths as short as 260 fs and pulse widths shorter than 600 fs for all wavelength values within this tuning range. Time-bandwidth products are one to two times the transform limit. >

Observation of birefringence induced by intersubband transitions in quantum wells

Abstract: We present a direct measurement of the birefringence induced by the intersubband transitions in quantum wells Phase delays of up to 40° were observed in our samples, corresponding to a 007 difference in the index of refraction between the polarizations parallel and perpendicular to the plane of the wells in a GaAs/AlGaAs silicon‐doped structure The measurement was conducted at several wavelengths—enabling us to deduce the total linear birefringence near the absorption resonance The observed birefringence is in close agreement with the Kramers–Kronig transform of the absorption spectra

Increased photorefractive sensitivity in double-doped KTa(1-x)Nb(x)O(3):Fe,Ti.

Abstract: Double-doped KTN:Fe,Ti exhibits increased photorefractive sensitivity compared with single-doped KTN:Fe and KTN:Ti. Absorption and photoconductivity measurements correlate the increase with an increased concentration of Fe in the reduced Fe2+ valence state and to an increased fraction of Fe incorporated into the crystal from the flux.

Adiabatic nonperturbative derivation of electric-field-induced optical nonlinearities in quantum wells.

Abstract: We show that, for dc electric fields easily applicable to quantum wells, and for optical fields close to a transition frequency, the nonlinear susceptibilities derived from perturbation theory diverge. Alternatively, a general expression for the susceptibility based on solving the adiabatic quantum-well problem is presented, leading to a radically different behavior at large fields. Our approach is valid at low modulating rates for any system which is soluble under an applied electric field

Controlled, Periodic Domain Formation in Strontium Barium Niobate Induced by Volume Holograms

Abstract: We demonstrate an all-optical technique of controlled, periodic ferroelectric domain reversal induced by electronic space charge fields associated with photorefractive volume holograms. This technique may be used to selectively fix and update individual holograms in photorefractive crystals.

Effects of nonuniform current injection on the spectral dynamics of multisection distributed feedback lasers

Abstract: The effects of nonuniform current injection on the linewidth enhancement factor (α) and the adiabatic chirp in a multielectrode distributed feedback laser are measured through a FM/AM modulation technique This technique measures the adiabatic chirp due solely to carrier density effects and excludes the contributions of current induced temperature changes The effects are shown to be a function of the pumping ratio between two active sections in the laser The adiabatic chirp is enhanced by as much as a factor of 3 and the α parameter is reduced from a value of approximately 4 to 2

Parallel CCD memory chip and method of matching therewith

Abstract: A fully parallel CCD memory chip of N address lines which detects in just one clock cycle, a perfect match between an input pattern and any of a plurality of stored patterns and also detects in less than (N+1)-comparison cycles and still just one XOR operation, the best matching in case a perfect one does not exist. The chip disclosed herein has a fully parallel architecture in which an input word is compared to all stored words at one time. A preferred embodiment of the invention uses a four phase CCD, wherein each stored word occupies one row of the CCD and each such bit of each such word occupies two cells. Where perfect matches exist, only one comparison clock cycle is needed to compare the input word with all stored words and where there is no perfect match, the best match will be detected on a subsequent comparison pulse. Charge packets represent binary words 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 sensing is done directly on the cells in a non-destructive sensing process in parallel, rather than at the end of each row.