Showing papers on "Signal beam published in 1996"

Heterodyne interferometer with two‐wave mixing in photorefractive crystals for ultrasound detection on rough surfaces

Abstract: Heterodyne interferometers using two‐wave mixing in photorefractive cubic crystals for ultrasound detection on rough surfaces are demonstrated. The speckled scattered beam from a rough surface sample interferes with a planar coherent pump beam inside a photorefractive crystal. A third frequency‐shifted beam is used to read the grating. The diffracted readout beam and the transmitted signal beam are wavefront matched, resulting in an optimal heterodyne interference signal. The signal to noise ratio for the two commonly used crystallographic configurations with cubic crystals, G∥〈110〉∥ and G∥〈001〉, where G is the grating wave vector, are investigated. Very good sensitivity is demonstrated for the detection of small amplitude ultrasonic surface displacements.

Three-color coherent light system

Abstract: A three-color coherent light system includes a laser source for generating a laser beam in the 1000-1100 nm wavelength region; means, responsive to the laser source, for generating the second harmonic of the laser beam; an optical parametric oscillator for providing a signal beam and an idler beam; means for splitting the second harmonic of the laser beam into a first beam for providing a source of coherent green light and a second beam for pumping the optical parametric oscillator to produce the signal and idler beams; means for generating the second harmonic of the signal beam as a source of coherent blue light; and means for generating the second harmonic of the idler beam as a source of coherent red light.

Optical bus and signal processor

Abstract: To improve resistance to environmental changes and enhance system extensibility, the present invention provides an optical bus for transmitting an optical signal, which has diffusers distributed in a optical transmission layer to diffuse an input signal beam and propagate the diffused signal beam, and a signal processor for carrying out signal processing including transmission and reception of a signal beam using the optical bus.

Compact architecture for holographic systems

Abstract: The holographic memory system we describe here integrates the various system components very closely to the crystal by using a smart pixel array and making use of phase-conjugate read-out to eliminate the need for imaging optics. This significantly reduces the alignment requirements and the number of required components. Our angularly multiplexed integrated read/write memory system is shown. An optoelectronic integrated circuit (OEIC) performing the functions of a spatial light modulator and a detector array is attached to the face of a 900-geometry photorefractive crystal perpendicular to the axis of the signal beam, while liquid crystal beam steerers are attached to the two adjacent faces that intersect the reference beam (the beam steerer behind the crystal is not visible in the figure). These beam steerers address the angularly multiplexed memory. Holograms are recorded by signal beam interference.

Quantum images in double-slit experiments with spontaneous down-conversion light

Abstract: Summary form only given. This work analyzes patterns seen when a signal beam of spontaneous parametric downconverted light illuminates a double-slit (Young) and a coincidence pattern ("object") appears on scanning behind the double-slit arrangement and, conversely, when the conjugated idler beam is scanned spatially keeping the signal detector fixed and a "Young" coincidence pattern ("image") is revealed. Mandel's theory for downconversion is applied, in the (r)-space, to this analysis.

Rubidium titanyl arsenate difference-frequency generation and validation of new Sellmeier coefficients

Abstract: Rubidium titanyl arsenate (RTA), a crystallographic isomorph of potassium titanyl phosphate, shows promise for nonlinear-frequency generation throughout the 1–5-µm spectral region. Difference-frequency generation in an RTA crystal produced tunable output in the 3.2–4.2-µm wavelength range. A single 1.064-µm Nd:YAG laser pumped both a LiNbO3 optical parametric oscillator used to generate a tunable signal beam and the RTA crystal used for difference-frequency generation. Conversion efficiencies were limited to 4% primarily by the large beam divergence of the signal beam. Phase-matching measurements were in excellent agreement with new IR-corrected RTA Sellmeier equations based on refractive-index measurements.

Spatio-angular multiplexing geometry for volume holographic storage

Abstract: Partially overlapping holograms are stored in a cylindrical volume holographic storage medium capable of rotation about and translation along its longitudinal axis. The reference and signal beams are mutually perpendicular, and each is perpendicular to the longitudinal axis. An index-matched housing encloses the medium laterally. An optional helically-varying optical axis (c-axis) orientation allows recording at constant angular intervals over a full revolution. Signals from stored pages are used to dynamically adjust the positioning of the medium relative to the light beams as the medium continuously spins at high velocity, and to control the access of the signal beam to the readout camera.

Direct and ghost interference in double-slit experiments with coincidence measurements.

Abstract: The transverse spatial properties of the twin light beams of the down-conversion have been recently investigated in experiments using double-slit and coincidence measurements. Fringes are obtained, with a controlled degree of visibility, placing the slits in the signal beam and scanning the signal detector [direct interference---Phys. Rev. A 49, 4176 (1994)] and also, for the degenerated type II down-conversion, with slits in the signal beam and scanning the idler detector [ghost interference---Phys. Rev. Lett. 74, 3600 (1995)]. The idea of a ghost source is used to discuss the direct interference experiment. Using a nondegenerated type I down-conversion, experimental results are obtained showing that the visibility of the ghost interference patterns can be controlled by the ghost source size, analogously as it can be done for a real light source. \textcopyright{} 1996 The American Physical Society.

Amplification of optical signals in Bi(12)TiO(20) crystal by photorefractive surface waves.

Abstract: We have demonstrated experimentally beam amplification by coupling between the signal beam and the photorefractive surfaces wave in Bi12TiO20 crystal. A gain of 16,000 has been measured, with an output signal-to-noise ratio of >20 for weak input signals.

Multi-pass optical parametric generator

Abstract: A parametric generator for efficiently converting short duration pump pulses of a first frequency to a signal beam of a desired frequency is described. The parametric generator includes a second order nonlinear parametric medium, a retro-reflector, and an optical re-imaging system. Each optical sub-path through the parametric medium is substantially distinct and non-overlapping. At least two optical sub-paths have a direction within a phase matching acceptance angle of the parametric material which corresponds to the desired wavelength. The parametric generator can be configured to be compact because it utilizes multiple transmissions through the parametric medium and it employs only a single pump beam input.

Detection of ultrasonic vibrations on rough surfaces through the photorefractive effect

Abstract: We present and describe two techniques used for optical detection of ultrasonic signals based on the photorefractive effect. These techniques used the wavefront adaptation properties of the photorefractive effect. In the photorefractive beam combiner, a local oscillator matched to the signal wavefront is created, leading to an homodyne detection system having a large etendue. In the double phase conjugate heterodyne detection system the signal beam wavefront is cleaned by a double phase conjugate mirror and transformed in a plane wave that is sent on a classical heterodyne detection system. Both systems are characterized and used to detect ultrasound.

Parametric amplification of a polychromatic image

Abstract: The picosecond parametric amplification of a polychromatic image with a wavelength bandwidth of 140 nm and a gain of 15 dB has been obtained in a type I, LBO crystal. Thirty per 30 points were resolved. These results are in good agreement with a numerical study of the phase-matching conditions around the collinear degeneracy where phase-matching is non- critical for the signal beam in angle as well as in wavelength.

Absorption effects in photorefractive volume-holographic memory systems. I. Beam depletion

Abstract: We consider how absorption-induced beam depletion alters the index-modulation profile, the diffracted beam profile, and the Bragg selectivity of volume holograms. We derive theoretical equations that consider beam depletion (in the absence of two-beam coupling) and compare these with experimental observations. We also derive a smart scheduling technique whereby such absorption effects can be compensated for during the scheduled recording of multiple holograms. In our smart scheduling approach, during recording the profile of the input signal beam is modified such that during recall its output profile is uniform.

Dynamic holography with nonplane waves in sillenites

Abstract: A three-dimensional model describing two-wave mixing in photorefractive crystals with finite beams of arbitrary shape is presented. The well-known longitudinal and transverse geometries are generalized by allowing an arbitrary orientation of the local grating vector. The coupled equations for the wave amplitudes are solved numerically in the small-angle approximation by introducing the light paths as characteristics. The influence of amplitude distributions and phase-front curvatures as well as the influence of geometrical arrangements and crystal properties (i.e. optical activity, externally applied voltage) on the energy exchange and the evolution of the polarization states is investigated. It is demonstrated that scanning the cross-section of the finite signal beam behind the crystal allows analysis of the coupling process inside the crystal which cannot be studied directly. The results of experiments with finite beams carried out on a BSO crystal are in good agreement with the numerical calculations.

Single-shot high-speed signal detection by multiple-angle spectral interferometry

Abstract: A novel technique for detecting ultra-fast signals at speeds beyond those of conventional electrical circuits is described that uses the coherent correlation between the signal wave and that of a modulated local oscillator. The spectral components of the signal beam interfere with their counterparts of the light from the local oscillator in a multiple-angle scheme. It is found that this scheme allows the mutual convolution of the interfering lights to be spatially displayed on the observation plane as an interference pattern. Typical applications of this technique include serial-to-parallel conversion of the signal light and packet detection. In an experiment, single-event high-speed signals of several mW are successfully detected by using two independently oscillating light sources for the signal and local beams. Finally, we discuss the deployment of this technique in optical communication systems.

Phase conjugation in Cr(4+):YAG at 1.06 microm.

Abstract: Phase conjugation in a Cr4+:YAG saturable absorber by degenerate four-wave mixing with nanosecond pulses at λ = 1.06 μm is demonstrated for the first time to our knowledge. An experimental phase-conjugate energy reflectivity of ~2% and an overall efficiency of ~0.4% are achieved. Because of the presence of absorbing dipoles oriented along specific directions, it is shown that the phase-conjugated beam can be made orthogonally polarized relative to the signal beam or to both pump beams with almost no reduction of the energy reflectivity.

Method of choosing an optimum angle between a reference beam and an object beam to record a hologram

Abstract: A method of choosing an angle between a reference beam and a signal beam in a holographic storage apparatus is presented. The angle between the reference and signal beams can be optimized in light of crosstalk, scattering and wavelength seperation considerations.

Spatial coherence methods in undersea image formation and detection

Abstract: Methods for the formation of images in scattering media generally rely upon temporal or spatial methodologies. Holography is a common example of image formation exploiting the mutual coherence between a reference beam and a signal beam. The coherence allows the formation of an interference pattern that carries the signal information on a "spatial carrier". In order for the method to be of use, the medium in which the beams are carried must preserve the coherence or phase spatially across the beams and in relation to the reference beam. In water, the distance over which the phase may be preserved is dependent upon many factors, including turbulence induced refractive index variations, thermal gradient structure, and relative motion. If pathlength differences exceed the temporal coherence length of the beam, interference is not obtained and the method breaks down. Holographic imaging has been demonstrated underwater and coherence measurement techniques have been suggested. Generally, the demands of maintaining a spatially coherent bean at optical frequency is difficult over long range thereby limiting the usefulness of the technique for image formation in turbid media. The authors describe a variation of the spatial interferometric technique that relies upon projected spatial gratings with subsequent detection against a quasi-coherent return signal. The method is advantageous in not requiring temporal coherence between reference and signal beams. Coherency of the spatial beam allow detection of the direction return, while scattered light appears as a noncoherent noise term. The theoretical foundation of the method and initial results for turbid media are developed.

Optically controlled phased array antenna

Abstract: PROBLEM TO BE SOLVED: To provide the optically controlled phased array antenna with a simple configuration in which a plurality of radio signal beams emitted from the array antenna are scanned in 2-dimension. SOLUTION: The antenna is provided with light emission sections 4a, 5a that changes an emission position of a plurality of beam lights in a prescribed direction to emit a plurality of signal beam lights modulated by each radio signal and a light emission section 6a that emits a reference beam light. Each signal beam light modulated by each radio signal is emitted respectively from a plurality of emission positions corresponding a plurality of radio signal uniquely in a desired beam forming direction, and a reference beam light is emitted to process the signal beam optically so as to generate a plurality (M) of radio signals with a prescribed phase gradient, each radio signal is emitted from each corresponding antenna element to form a plurality of beams in a desired direction thereby emitting a plurality of radio signals. The radiation direction of each beam of a plurality of radio signals is independently controlled based on each emission position. COPYRIGHT: (C)1998,JPO

Experimental study of the angular selectivity of volume phase holograms stored in LiNbO3

Abstract: Changes in the angular Bragg selectivity of volume holograms stored in photorefractive LiNbO3 are analysed for different experimental conditions in the transmission geometry. The effects of the refractive-index-change amplitude, the grating period, the modulation depth, and the wave-front modulation of the signal beam are considered. A very good agreement of the results with the off-Bragg diffraction theory is observed.

Ultrasound detection on rough surfaces using heterodyne photorefractive interferometer: applications to NDE

Abstract: Heterodyne interferometers using two-wave mixing in photorefractive cubic crystals for ultrasound detection on rough surfaces is demonstrated. The speckled scattered beam from a rough surface sample interferes with a planar coherent pump beam inside a photorefractive crystal. A third frequency-shifted beam is used to read the grating. The diffracted readout beam and the transmitted signal beam are wavefront matched, resulting in an optimal heterodyne interference signal. We show that the interferometer sensitivity is independent of the surface quality and an optical fiber in the path of the probe beam can be implemented. The interferometer is characterized and results are compared with a classical heterodyne interferometer.

Optical wdm information distributing network

Abstract: PURPOSE: To provide the optical WDM information distributing network with which the fluctuation or the like of communication demand can be flexibly dealt with. CONSTITUTION: Optical signals outputted from plural optical transmitters 1 are synthesized at a star coupler 5 and distributed out to the reception side. A multichannel wavelength selecting means 93 selects the plural signal beams of a prescribed wavelength out of the distributed out wavelength multiplexed beams, and the selected signal beams are simultaneously amplified by an optical amplifying means 94. This amplified output is distributed to plural outputs by an optical distributing means 95, outputted and supplied to an optical WDM receiver. At each optical WDM receiver, only the signal beam of the wavelength of a receiving object is passed through a tunable optical filter 98 and demodulated to an electric signal at an optical receiver 99.

Passive acousto-optic modulator

Abstract: A passive acousto-optic modulator for the modulation of a light beam as a function of a variation in air pressure comprising a beam splitter arranged relative to the beam so as to produce a signal beam and reference beam, the beam splitter ideally comprising a flat interface located between two optically transparent mediums such as the end of an optic fibre located within air, the interface being arranged relative to the light modulator such that when the light beam travels through the interface, some of the light beam is able to pass through the interface to form the signal beam, the rest of the light beam being reflected back off the interface to form the reflected reference beam; a light modulator which modulates the signal beam, the light modulator comprising an approximately flat optically reflective surface capable of movement, the reflective surface being located in a position which, at rest is a predetermined distance from the beam splitter and is approximately parallel to and facing the beam splitter and a piezoelectric element, the piezoelectric element being connected to the reflective surface in such a way that when an electric potential is applied across the piezoelectric element, it causes the reflective surface to move; and conversion means to convert variations in air pressure into corresponding variations in a signal for driving the light modulator; the arrangement being such that the modulated signal beam is combined with the reference beam to produce constructive or destructive interference.

Four-wave-mixing interferometer.

Abstract: We have developed a new method of determining in time both the real and the imaginary parts of third-order nonlinear-optical coefficients. In this method the phase-conjugated four-wave-mixing signal beam interferes with one of the transmitted pump beams. By comparing the interference pattern with that of a known reference, we are able to determine the real and the imaginary parts of the third-order optical nonlinearity for an unknown sample. The real and the imaginary parts can also be separated when one incoming beam is delayed. We apply this method to a bis-thienyl polyene molecule containing five conjugate double bonds and find that the hyperpolarizability is γ1111 = (−2.5 − 0.17i) × 10−30 esu.

Photorefractive two‐wave mixing bistability in Fe: KNbO3 without external feedback: Increasing gain bistability

Abstract: The observation of photorefractive two‐wave mixing bistability without external feedback is reported. Hysteresis with an on/off contrast ratio of 30 was obtained through the interaction of two 514.5 nm laser beams within a Fe: KNbO3 crystal. A signal beam modulation rate less than 25 mHz was necessary to obtain the hysteresis when the total optical irradiance of the pump and signal beams was ∼1 mW/mm2. The data are shown to correspond well with a two‐state increasing gain model.

A rotary electric coupling for machines having a rotatable turret

Abstract: A rotating electrical coupling for machines comprising a fixed part and a rotatable part comprises two electronic cards (51, 52) for modulation, demodulation and management of these signals, of which one is fixed to the fixed part and the other is rotatable with the movable part. Two cards (51, 52) communicate between one another by means of an optical signal beam so as to be uninfluenced by the relative angle between the fixed part and the rotatable part.

Optical packet address detector

Abstract: An improved optical packet address detector which is capable of enabling a high speed packet switching network of 100 Gbps, which includes a glass substrate having a mirror disposed at the back side thereof; a channel optical waveguide formed on the glass substrate and extended in a predetermined direction; a plurality of grating couplers spaced-apart on the channel optical waveguide for dispersing a packet signal beam passing through the channel optical waveguide into the inside of the glass substrate; an optical absorber attached to one end of the channel optical waveguide for absorbing part of the packet signal beam which is not dispersed by the grating coupler; a reflective lens spaced-apart from the optical absorber by a predetermined distance and formed on the glass substrate for focusing the signal beam dispersed by the grating coupler and reflected by the mirror; and a detector spaced-apart from the reflective lens and formed on the glass substrate for detecting the signal beam passing through the reflective lens and the mirror.