Abstract: Nonlinear optical devices and techniques are used to provide heterodyne detection for coherent optical communications. Heterodyne detection is achieved by matching the wave front of a local oscillator beam with the wave front of a received optical signal. Precise wave front matching provides high heterodyne efficiency and a wide field of view. In one embodiment, the received signal and a reference beam of the same frequency are directed into a nonlinear medium to form a volume hologram that contains the spatial information of the received signal. The reference beam is alternated with a local oscillator beam that has the same wave front and is parallel to the reference beam but at a different frequency. The hologram matches the wave fronts of the signal beam and the local oscillator beam to produce a large heterodyne signal. In an alternative embodiment, the received signal beam and the local oscillator beam are directed into a mutually pumped phase conjugator (MPPC).

Abstract: A nonlinear optical receiver includes a source of coherent light of frequency .omega. for illuminating a spatial region. The portion of the source light which is reflected from objects within the spatial region constitutes a signal beam. A reference beam of coherent light of frequency .omega. is also provided. A photorefractive element receives the signal and reference beams and amplifies the signal beam by two-wave mixing.The receiver may include a first polarizer, having a first polarization direction, placed between a cubic photorefractive element and the signal and reference beams, and a second polarizer, having a second polarization direction perpendicular to the first polarization direction, placed so that the amplified signal beam must pass therethrough. A frequency shifting element, such as a Bragg cell modulator, is used to change the frequency of the reference beam.

Abstract: A mutually pumped phase conjugator (MPPC) is used to remove spatial distortions from a high power laser beam. The high power beam with its "dirty" spatial profile is directed into one side of the MPPC. A spatially clean beam at the same nominal frequency as the high power beam is output from a continuous wave (CW) laser and directed into the opposite side of the MPPC. As a result of mutually pumped phase conjugation, the MPPC returns a phase conjugate beam that retains the clean spatial profile of the CW beam but acquires the temporal characteristics of the high power beam. A Faraday isolation system may be used to separate the phase conjugate beam from the incident CW beam to provide an output beam having a clean spatial profile. If desired, the high power beam can be transformed into an image bearing output beam simply by modifying the incident CW beam with a transparency or spatial light modulator.

Abstract: An apparatus for subtracting the amplitude of a first optical image on a first beam from the amplitude of a second optical image on a second beam includes a first beam splitter to divide the first beam into a first transmitted beam and a first reflected beam and to divide the second beam into a second transmitted beam and a second reflected beam. The first transmitted beam and the second reflected beam combine as a first mixed beam, while the first reflected beam and the second transmitted beam combine as a second mixed beam. A pinhole removes spatial information from the second mixed beam. The first and second mixed beams interfere within a nonlinear medium to create a composite hologram. A coherent reference beam directed at the nonlinear medium reads the interference pattern, so that the difference between the first and second images is superimposed on the reference beam emerging from the nonlinear medium.

Abstract: An intensity inverter includes an optically nonlinear medium and a reference beam having a uniform transverse intensity distribution and having the same wavelength as the signal beam. The reference beam is directed into the nonlinear medium. A readout beam having the same wavelength as the signal beam and counterpropagating with respect to the reference beam is also directed into the nonlinear medium. When the signal beam is directed into the nonlinear medium, the signal beam and the reference beam interact to form a hologram within the nonlinear medium, thereby diffracting a portion of the readout beam, the undiffracted portion of the readout beam having imposed on it an inversion of the signal.

Abstract: An adaptive optical network is provided for the implementation of learning algorithms. The network comprises a double Mach-Zehnder interferometer in conjunction with a photorefractive crystal that functions as a holographic medium. Light from selectable sources on opposite sides of a beamsplitter is passed through the interferometer, at least one arm of which includes a spatial light modulator for imprinting a data pattern on the light. The light is directed into the holographic medium to develop a refractive index grating corresponding to the data pattern. Light from the hologram is sensed by a photodetector that provides a signal to a threshold device. The output of the threshold device is compared with a reference signal to produce an error signal that can be used to select the source of light directed through the network. The interconnections of the optical devices function to compute the inner product between the elements of the data pattern and their weight factors.

Abstract: An apparatus for steering a coherent input beam of electromagnetic energy includes an optically nonlinear medium for positioning in the path of the input beam. A source of acoustic energy generates acoustic waves in the nonlinear medium with a predetermined frequency and direction. The frequency and direction of the acoustic waves are selected to diffract a portion of the input beam as an output beam at a predetermined angle with respect to the input beam, interference between the input beam and the output beam initially causing an electrostrictive grating to form in the medium, with the grating subsequently causing additional energy to be transferred from the input beam to the output beam by stimulated Brillouin scattering.

Abstract: A beam of coherent light is coupled into an array of light detectors using a beam splitter for dividing the beam into a probe beam and a pump beam and a spatial light modulator for varying the intensity of the probe beam in a first direction perpendicular to the direction of propagation of the beam. A photorefractive element is positioned to receive the modulated probe beam and the pump beam, the beans being oriented with respect to the photorefractive element and with respect to each other such that photorefractive two-wave mixing within the photorefractive element nonreciprocally transfers energy from the pump beam to the modulated probe beam, which is then detected by the detector array.

Abstract: A nonlinear optical matrix multiplier includes a nonlinear optical medium with a first optical input beam impinging on the medium, the transverse spatial intensity of the first input beam being modulated by a first two dimensional matrix. A second optical input beam impinges on the medium, with the transverse spatial intensity of the second input beam being modulated by a second two dimensional matrix. An optical probe beam impinges on the medium, the first input beam, the second input beam, and the probe beam being oriented with respect to one another and with respect to the medium such that the beams interact within the medium by means of four-wave mixing. A diffracted output beam emerges from the medium, with the transverse spatial intensity of the output beam modulated by the product of the first and second matrices.

Abstract: A coupled laser system includes a plurality of substantially parallel beams of coherent radiation with an output beam of coherent radiation at an oblique angle with respect to the plurality of beams. A photorefractive element is positioned to receive the plurality of beams and the output beams, the beams being frequency locked to within the photorefractive bandwidth of the photorefractive element such that laser energy is diffracted from the plurality of beams to the output beam by means of two-wave mixing. A lens may be used for focussing the parallel beams in the photorefractive element, while a plurality of mirrors may be supplied for adjusting the orientation of the output beam within the photorefractive element.

Abstract: A phase conjugate interferometer includes a source of coherent light and a first beam splitter for dividing the coherent light into a transmitted portion and a reflected portion. A second beam splitter is positioned to divide the transmitted portion into an outgoing transmitted beam and an outgoing reflected beam, with the outgoing transmitted beam oriented to pass through the first image and the outgoing reflected beam oriented to pass through the second image. A phase-conjugate reflector is positioned to reflect the outgoing transmitted beam as a first incoming beam and the outgoing reflected beam as a second incoming beam. The incoming beams are directed toward the second beam splitter, which divides the incoming beams into a first combined beam and a second combined beam. The second combined beam is directed toward the first beam splitter, which divides the second combined beam into a third combined beam and a transmitted beam.

Abstract: A system and method are described for establishing a common operating frequency and phase for the lasers in a two-dimensional array, each of which has an actual frequency which deviates from a common nominal frequency. A waveguide structure is placed in the path of the beams emitted from the various lasers, and a plurality of optical gratings are provided in the waveguide in alignment with the laser beams to deflect portions of the beams into propagation along the waveguide and into cross-coupling with the other lasers in the array. The periodic gratings are characterized by grating periods substantially equal to .lambda./n.sub.1, and the thickness of the waveguide core is at least equal to approximately .lambda./(4.sqroot.n.sub.2.sup.2 -n.sub.3.sup.2), where .lambda. is the wavelength of the guided mode, n.sub.1 is the effective refractive index for the guided mode, and n.sub.2 and n.sub.3 are the refractive indices for the waveguide core and substrate, respectively.

Abstract: An apparatus for compressing the spectral distribution of a coherent beam of light includes a narrowband filter for separating the coherent beam into a transmitted portion containing optical energy substantially limited to a single wavelength .lambda. and a reflected portion containing the remainder of the coherent beam. A photorefractive crystal is positioned to receive the transmitted and reflected portions. A mirror deflects the reflected portion toward the photorefractive crystal. The photorefractive crystal is oriented such that two-wave mixing within the crystal causes a substantial portion of the energy in the reflected portion to be converted into optical energy at the wavelength .lambda. and to emerge from the crystal in the direction of the transmitted portion. The spectral bandwidth of the coherent beam is less than 1/.tau., where .tau. is the holographic time constant of the photorefractive element.

Abstract: An optical fiber rotation sensor includes an optical fiber coil and a laser for providing a source of coherent light. A beam splitter divides the light source into a first beam and a second beam. The first beam is coupled into a first end of the coil, so that the first beam propagates through the coil from the first end to a second end of the coil. A reflector, such as a mirror or a phase conjugate reflector, reflects the second beam back toward the beam splitter. A scalar phase conjugate reflector reflects the first beam at the second end of the coil so that the reflected first beam propagates back through the coil to the first end and is recombined with the reflected second beam in the beam splitter. A detector measures the phase difference between the reflected first beam component and the reflected second beam component in the recombined beam.

Abstract: Laser energy is combined using dynamic beam splitters that can automatically accommodate changes in the laser cavity mode structure. Nonlinear optical methods and real time holography are employed to achieve phase locking among multiple lasers. A single laser output beam can be produced from a multitude of laser cavities which collectively contribute to the output power. No outside monitoring and servo mechanisms are required, since nonlinear optical processes automatically perform the functions of both monitoring and control. A coupled laser system includes two or more lasers, each laser having a resonant cavity, a laser gain medium in the resonant cavity, and a nonlinear optical element. Each nonlinear optical element is positioned in its respective resonant cavity to diffract laser energy from the cavity to a coupling beam by means of four-wave mixing (phase conjugation).

Abstract: A relative position sensor is suitable for inertial navigation situations in which deviations from a predetermined position schedule are detected. A standing wave is established between a pair of opposed phase conjugate reflectors in a resonating cavity, and relative movement between the standing wave and the cavity is sensed. Since the standing wave continues to travel at its initial velocity regardless of deviations from that velocity by the resonator, deviations in the position of the resonator relative to its projected position based on the initial velocity can be measured. In a preferred embodiment, four-wave mixers are used as the phase conjugate reflectors, the phase conjugate medium within the mixers is made coextensive with the wave propagation medium between the mixers, and relative movement is sensed by monitoring the intensity of the standing wave at at least one location which is fixed relative to the resonator.

Abstract: An optical filter includes a first polarizer for transmitting light having a first polarization direction and a first birefringent element for receiving the light transmitted by the first polarizer. A second birefringent element, of a different material having a different birefringence, receives the light transmitted by the first element, while a second polarizer receives the light transmitted by the second element and transmits the portion having a second polarization direction. The birefringences and the thicknesses of the elements are arranged to effect a predetermined amount of net phase retardation, preferably zero, for light of a wavelength .lambda. in traversing the first and second elements.

Abstract: Disclosed is an optical bandpass filter for transmitting light in a narrow band around the wavelength .lambda..sub.c, including a first series of layers, each of thickness t, having a refractive index n.sub.1 and a second series of layers, each of thickness t, having a refractive index n.sub.2 and alternating with the first series. The thickness t is selected so that t=.lambda..sub.c /4n.sub.c and .vertline..DELTA..vertline.>.pi..vertline.D-1/.lambda..sub.c .vertline., where ##EQU1## and n.sub.c is the common value of n.sub.1 and n.sub.2 at the wavelength .lambda..sub.c. Also disclosed is a filter with each succeeding layer in the first and second series thicker than the preceding layer. The thickness of the thinnest layer is .lambda..sub.1 /4.sub.n and the thickness of the thickest layer is .lambda..sub.2 /4n Cos .theta., where .lambda..sub.1 is the minimum wavelength to be reflected by the filter, .lambda..sub.2 is the maximum wavelength, n=(n.sub.1 +n.sub.2)/2, and .theta.

Abstract: Disclosed is an optical filter for selectively transmitting light in a passband at the wavelength .lambda., including an initial polarizer with a polarization direction perpendicular to a beam path and a final polarizer spaced from the initial polarizer along the path and having the same polarization direction. A birefringent element is positioned between the polarizers with an optic axis perpendicular to the path and rotated with respect to the polarization direction, the element having a birefringence which is zero at the wavelength .lambda.. Lyot-Ohman and Solc embodiments of the filter are also described.

Abstract: A multiple cavity electro-optic tunable filter includes an ordered row of n electro-optically tunable plates (n=2, 3, 4 . . . ), each plate having an incident surface and an emergent surface, an incident reflective coating on the incident surface of the first tunable plate in the row, an emergent reflective coating on the emergent surface of the nth tunable plate in the row, and a series of (n-1) absorbent films, one of the films being disposed between each pair of adjacent plates in the row in mating contact with the adjacent surfaces of the adjacent plates.