Showing papers on "Signal beam published in 1987"

Motor vehicle remote control system

Abstract: Motor vehicle control system for enabling persons in an authorized vehicle to control the movements of one or several general vehicles. The system includes a signal beam transmitter at the authorized vehicle, which can be directed in any direction by beam directing apparatus toward a general vehicle having a beam signal receiver attached thereto. Upon receiving the beam, the general vehicle may be stopped, slowed down or stopped after a certain delay. The beam energy may be laser light, microwave, sound energy or any other suitable energy. The beam transmitter may be combined with a video receiver and a video monitor so that the person in the authorized vehicle can view the general vehicle. The beam may be modulated and encoded with messages that can be decoded in the general vehicle.

Photorefractive spatial light modulation by anisotropic self-diffraction in KNbO(3) crystals.

Abstract: Photoinduced anisotropic self-diffraction in photorefractive KNbO3 has been studied It has been shown that this type of diffraction is ideally suited for spatial light modulation or incoherent-to-coherent conversion A phase grating that is photoinduced by two coherent light beams causes the diffraction of one of the beams By spatially modulating the amplitude of the phase grating with an incoherent signal beam it is possible to transfer an incoherent image onto the coherent self-diffracted beam The performance parameters of this modulator are described with a simple model

High-speed all-optical gate switching experiment in a Fabry-Perot semiconductor laser amplifier

Abstract: Optical gate switching for one signal beam using another control beam is presented in a Fabry-Perot semiconductor laser amplifier. High-speed switching, where the signal output is on without the control beam and off with it, is demonstrated employing modulation of an 800 Mbit/s pulse pattern.

Doppler-Free Spectroscopy Based on Phase Conjugation by Degenerate Four-Wave Mixing in Hollow Cathode Discharge:

Abstract: Optical phase conjugation in the hollow cathode lamp by degnerate four-wave mixing is a novel analytical laser spectroscopic method in which a laser beam is generated as the signal beam. With the use of a demountable hollow cathode discharge cell and a continuous-wave dye laser, the signal beam is clearly visible to the naked eye when 1 μg of sodium is placed inside the cathode cavity. Since the signal beam is a time-reversed replica of the probe laser beam, highly efficient optical signal detection can be performed with minimum optical background noise. Since the signal is Doppler-free with negligible Lorentzian broadening (30 Torr discharge pressure), it provides excellent spectral resolution. Sodium D2 hyperfine lines of 3s 2S1/2 (F' = 2) → 3p 2P3/2 (F = 3) and 3s 2S1/2 (F' = 1) → 3p 2P3/2 (F = 0) were resolved. In addition to excellent spectral resolution and ease of signal detection, the technique also offers unique properties of optical phase conjugation.

Generation of Twin Photon Beams by a Nondegenerate Optical Parametric Oscillator

Abstract: The photon statistics at the output of a nondegenerate optical parametric oscillator is studied. The noise spectrum SI(ω) of the difference I between the two signal beam intensities is evaluated theoretically. A simple "corpuscular" method, in which photons are considered as classical particles, and a full quantum treatment lead to the same conclusion: photon noise on SI(ω) is suppressed at the frequencies which are multiple of the free spectral range and it is reduced inside the Airy peaks.

Amplification of high bandwidth signals through two‐wave mixing in photorefractive Bi12SiO20 crystals

Abstract: We report a new nearly degenerate two‐wave mixing interaction in photorefractive Bi12SiO20 crystals in which the signal beam is time modulated at high frequency. The beam coupling phenomena and the time integration properties of the crystal allow us to achieve a differential gain γdiff on the transmitted signal beam. The dependence of γdiff versus the duty cycle of the modulated signal is analyzed. Amplification and regeneration of periodic, or random sequences of information bits are experimentally demonstrated.

Optical bistability using a Fabry-Perot semiconductor-laser amplifier with two holding beams

Abstract: Bistable switching of an optical signal beam by a control beam with a different wavelength, using the nonlinearity of a Fabry-Perot semiconductor-laser amplifier, is reported for the first time to the authors' knowledge. Depending on the initial detuning of the optical signal beam, both normal (counterclockwise) and reverse (clockwise) hysteresis are theoretically and experimentally obtained in the signal output power as a function of the control input power.

A robust adaptive array processor

Abstract: A post-beam-former interference canceller (PIC) processes the signals derived from an antenna array by forming two beams using fixed beam-forming weights. The weighted output of the one beam, referred to as the interference beam, is subtracted from the other beam, referred to as the signal beam, which has a specified response in the signal direction. This paper presents an algorithm to adjust the complex weight of the interference channel iteratively, studies its convergence, presents an exact expression for the misadjustment, and compares the performance of this processor with that of an element space processor. The paper shows that in the absence of errors the performance of the two processors is identical but in the presence of the signal direction error the PIC processor performs better than the element space processor. Furthermore, the adaptive algorithm of the PIC needs to adjust only one complex weight per iteration, whereas in the element space processor the number of complex weights required to be updated at each iteration is equal to the number of elements in the array.

Device for optical heterodyne detection of an optical signal beam and optical transmission system provided with such a device

Abstract: An optical heterodyne detection device is described in which with the aid of three controllable elements (A, B and C), to influence the state of polarisation and having a limited control range, such as Faraday rotators or birefringent electro-optical crystals, the state of polarisation of radiation originating from a local oscillator (30) is made to correspond to the signal beam transmitted through a long-distance transmission fibre (10).

Saturation spectroscopy for optically thick atomic samples

Abstract: Doppler-free saturation spectroscopy in the regime of strong pumping intensities and optically thick atomic samples is investigated experimentally and theoretically. It is shown that a very high signal-to-background ratio can be obtained and, at the same time, subnatural linewidths can be reached. Further contrast and linewidth improvements can be attained by supplementing the primary depleted pumping beam with a second pumping beam. By using the signal beam from a first setup as the pumping beam for a second identical arrangement, extreme values for contrast and linewidth should be attainable.

Bistable optical photorefractive crystal device

Abstract: An optical bistable photorefractive crystal device is provided in which a photorefractive crystal disposed between two mirrors is uniformly illuminated by a light beam, a signal beam being focused on a given portion of the crystal to cause bistable switching of this portion inside the crystal.

Four-wave mixing reflectivity of silicon at 1.06 μm: influence of free-carrier absorption

Abstract: The phase-conjugate reflectivity obtainable by degenerate four-wave mixing in silicon at a 1.06 μm is calculated including free-carrier absorption. A maximum reflectivity of more than 100% is possible. The dependence of the reflectivity on the signal and pump energy densities up to 70 mJ/cm2 is measured and found to agree with theory. The wave-front-correction property of DFWM is demonstrated with a lens in the signal beam.

Waveguide type photo-displacement sensor

Abstract: PURPOSE:To make a sensor of longer distance of displacement measurement and available for identification of direction of the displacement, by installing two inclined reflecting surface on the specimen, mans for converting a signal light into parallel beams and phase-shifting means of wave guide light respectively. CONSTITUTION:Asymmetric X dividing type photo-waveguide paths 10, 12 are formed, a beam of light of waveguide paths 14, 24 are reflected as the reference light by reflecting films 5, 6, beams of light of waveguide paths 13, 23 are reflected as a signal beam by a movable mirror 4 provided with two inclined reflecting. Surfaces, and an output beam of intensity depending on a phase difference of the reflecting beam is fed out from waveguide paths 12, 22. As phase-shifting grooves 25 are constructed in the waveguide 24, a phase deviation is created between the output beams of the waveguide paths 12 and 22 enabling identification of the direction of displacement of the mirror 4 and by deviation of the incidence angle theta onto the mirror 4 enables identification of difference of their light paths. A rod lens 3 conducts conversion into parallel orientation of the beams and intensity of light can be maintained constant regardless the displacement of the mirror 4 and the distance of displacement measurement can be increased and the direction of displacement can be identified also.

Visual sensor using laser beam

Abstract: PURPOSE:To make it possible to obtain accurate measured value by avoiding measuring obstruction caused by secondary reflected beam, by confirming an imaginary measured value due to the secondary reflected beam of signal laser beam and an actually measured value due to primary reflected beam and discarding the former while taking the latter. CONSTITUTION:A beam emitter L and a beam receiver R are respectively provided at two points OL, OR on a visual sensor main body G. The beam emitter L emits signal laser beam l performing angular vibration around the point OL to project the same to an object and a bright spot A is generated at the projection point to scan the surface of the object. A signal beam control circuit MthetaL imparting angular vibration to the beam l is provided at the side of the beam emitter L and sends out a signal (j) relating to the direction OL of the beam l to a microprocessor mu-PR. A signal component detection circuit MthetaR is provided at the side of the beam receiver R and detects a light receiving element outputting a signal beam component and sends out a signal (i) showing the element number (i) thereof to the microprocessor mu-PR. The microprocessor mu-PR writes the signals (i), (j) corresponding to each other in a memory MEMO and further processes the written data and discards an imaginary measured value while picking up an actually measured value to calculate the surface shape of the matter.

Moving Photorefractive Gratings in Bi12SiO20 and Applications to Phase Conjugation and Optical Signal Processing

Abstract: We review the conditions for high gain in wave-mixing experiments, with photorefractive Bi12SiO20 crystals. These interactions are based on recording a moving interference pattern in the volume of the crystal (“drift” recording mode with an externally applied electric field). This technique has recently resulted in large values of the exponential gain coefficient г = 8 to 12 cm-1 for an optimized grating velocity and grating spacing. Applications to image amplification, laser beam steering, amplified phase conjugation, and self-induced optical cavities are presented.

Bistable optical device with a photorefractive crystal

Abstract: Dispositif bistable a cristal photoreactif dans lequel un cristal photoreactif (1) compris entre deux miroirs (2 et 3) est eclaire uniformement par un faisceau lumineux (FE). Photoreactive bistable crystal device wherein a photoreactive crystal (1) between two mirrors (2, 3) is uniformly illuminated by a light beam (FE). Un faisceau signal (FS) est focalise sur une portion determinee du cristal (1) et effectue la commutation bistable de cette portion a l'interieur du cristal. A signal beam (FS) is focused on a certain portion of the crystal (1) and performs the bistable switching of this portion inside the crystal.

Misadjustment in adaptive postbeamformer interference canceller

Abstract: As shown in Figure 1, a postbeamformer interference canceller [1,2], PIC, processes the signals derived from an antenna array by forming two beams using fixed beamforming weights. One beam, referred to as the target or signal beam, is steered in the signal direction or constrained to have a fixed response in the signal direction. The second beam, referred to as the interference beam, is steered in the interference direction or constrained to have a fixed response in the interference direction. The output of the interference beam is processed to estimate the interference. The processing of the interference beam output requires the adjustment of the weights of a taped delay line filter for the broadband processor and the adjustment of a complex weight for the narrowband processor.

Squeezing Intensity Noise on Laser-like Beams

Abstract: Parametric down-conversion generates beams having strong quantum correlations. This property can be easily understood by reasoning in terms of photons : let us consider a non-linear crystal irradiated by light at frequency ω0. For each pump photon absorbed at frequency ω0, it simultaneously emits two “twin” signal photons with frequencies ω1 and ω2 (such that ω0 = ω1 + ω2). Such non-classical correlations have been observed by photon coincidence techniques |1|. However, with the available cw lasers and non-linear crystals, the parametric process generates only very weak correlated beams (typically a few photons/second), because the pump power is spread into an infinity of twin modes. To concentrate the output energy into a few pair of modes only, the non-linear crystal can be inserted in an optical cavity having mirrors with a high reflectivity for the signal frequencies ω1 and ω2. Above some pump power threshold, the system can oscillate and yields a pair of intense, laser-like, “twin” beams.

Properties And Applications Of Photorefractive Crystals

Abstract: PROPERTIES AND APPLICATIONS OF PHOTOREFRACTIVE CRYSTALSGerald ROOSEN and J.P. HUIGNARD*Institut d'Optique, U.A. CNRS, Bât. 503, Centre Universitaire d'Orsay, B.P. 4391406 ORSAY Cedex, FRANCELab. Central de Recherche, Thomson CSF, D. de Corbeville, B.P. 10, 91401 ORSAY Cedex, FRANCECoherent optics offers very unique advantages for information processing and transmission with a highparallelism and a strong data rate. The implementation of these idea requires non linear materialspermitting efficient interactions between optical beams.Electro-optic and photoconductive crystals are of considerable importance as they allow such non linearinteractions with low power laser beams through the photorefractive effect. This effect has been foundin a large variety of crystals (LiNb03, BaTiO3, KNb03, Bi12Si(Ge)020, Ga As, InP, ...)1 -4 resulting ina broad range of spectral sentivities, spatial resolutions, efficiencies and speeds. Therefore numerousoperations for optical signal processing have been demonstrated using these photorefractive crystals596.However no material displays all the desired features for the realisation of truly operating devices.Extensive research work is still required in order to determine the limits of the effect, to understandhow structural changes and material compositions affect the photorefractive properties and to optimizethe performances according to the considered application.As most of the basic photorefractive properties, optical methods of characterisation and developpedapplications have similar features for any photorefractive material, we will illustrate these pointsconsidering crystals of the sillenite family, i.e. Bi12Si020 (BSO) or Bi12Ge020 (BGO). However theimportant specificities of other classes of crystals will be underlined.The photorefractive effect results from charge excitation by the light, followed by their migration andretrapping in another location.This charge redistribution induces a strong electrostatic field inside the crystalline material. Throughthe linear electro -optic effect (Pockels' effect) this spatially varying space charge field is transfor-med into a refractive index modulation, thus a phase grating. The effect is mainly sensitive to theabsorbed optical energy. Under low power continuous excitations, the time constant T of thephotorefractive effect is controlled by Idi, the dielectric relaxation time, which is much longer thanTR and Td, respectively the recombination and diffusion times of the excited charges. I also depends ontwo other parameters, L the diffusion length of the excited carriers and N the density of trap sites7.Thus, studying the grating decay rate under uniform illumination permits to determine Idi, L and N. Thiswas performed for various BSO and BGO samples by either varying the grating period$ or the magnitude ofan externally applied electric field9. Strong differences are evidenced pointing out the importance ofsuch basic researches. Indeed the speed and magnitude of the space charge grating induced by quasicontinuous illumination is governed by these only three material parameters that gives the effectivenessof the photorefractive effect for device applications.Physical mechanisms in photorefractive crystals lead to the recording of an index grating spatiallyshifted relatively to the incident light pattern1,5. This may result in an energy redistribution betweenthe interfering beams which direction is directly related to the sign of the charge carriers. Eitherhole or electron conduction can be dominant according to samples,10 the origin of which being not fullyunderstood yet.In spite of the little reliability of the materials,many interesting applications have been developpedbased on either two or four wave mixing configurations.Two wave mixing permits either the amplification of a low intensity signal beam containing spatial infor-mationt1 or the realisation of self starting optical resonators when an optical feedback is added 12-14.In both cases one takes advantage of the energy transfer phenomenon that can exhibit very high gains. Insillenite crystals such gains are obtained in the drift mode (applied electric field) by moving thefringe pattern at a constant velocityl5 -17. The speed is adjusted so that at any time the recorded indexmodulation has a 7/2 spatial phase shift relatively to the incident fringe pattern.