Showing papers on "Phase conjugation published in 2000"

Time-reversed acoustics

Abstract: The objective of this paper is to show that time reversal invariance can be exploited in acoustics to create a variety of useful instruments as well as elegant experiments in pure physics. Section 1 is devoted to the description of time reversal cavities and mirrors together with a comparison between time reversal and phase conjugation. To illustrate these concepts, several experiments conducted in multiply scattering media, waveguides and chaotic cavities are presented in section 2. Applications of time reversal mirrors (TRMs) in hydrodynamics are then presented in section 3. Section 4 is devoted to the application of TRMs in pulse echo detection. A complete theory of the iterative time reversal mode is presented. It will be explained how this technique allows for focusing on different targets in a multi-target medium. Another application of pulse echo TRMs is presented in this section: how to achieve resonance in an elastic target? Section 5 explores the medical applications of TRMs in ultrasonic imaging, lithotripsy and hyperthermia and section 6 shows the promising applications of TRMs in nondestructive testing of solid samples.

Numerical simulation of an adaptive optics system with laser propagation in the atmosphere

Abstract: A comprehensive model of laser propagation in the atmosphere with a complete adaptive optics (AO) system for phase compensation is presented, and a corresponding computer program is compiled. A direct wave-front gradient control method is used to reconstruct the wave-front phase. With the long-exposure Strehl ratio as the evaluation parameter, a numerical simulation of an AO system in a stationary state with the atmospheric propagation of a laser beam was conducted. It was found that for certain conditions the phase screen that describes turbulence in the atmosphere might not be isotropic. Numerical experiments show that the computational results in imaging of lenses by means of the fast Fourier transform (FFT) method agree well with those computed by means of an integration method. However, the computer time required for the FFT method is 1 order of magnitude less than that of the integration method. Phase tailoring of the calculated phase is presented as a means to solve the problem that variance of the calculated residual phase does not correspond to the correction effectiveness of an AO system. It is found for the first time to our knowledge that for a constant delay time of an AO system, when the lateral wind speed exceeds a threshold, the compensation effectiveness of an AO system is better than that of complete phase conjugation. This finding indicates that the better compensation capability of an AO system does not mean better correction effectiveness.

Stimulated orientational and thermal scatterings and self-starting optical phase conjugation with nematic liquid crystals

Abstract: A quantitative theory and experimental results on self-starting optical phase conjugation, using stimulated orientational and thermal scattering in nematic liquid crystal films, are presented. The coupled wave-material equations for the laser-induced refractive index changes, grating formation, and coherent wave mixing effects are developed. Analytical solutions are obtained for the case of negligible pump depletion, and numerical solutions for various input and generated signals, taking losses into account, are obtained. Experimentally, we demonstrate the feasibility of realizing these stimulated scattering and phase conjugation processes in thin (200 \ensuremath{\mu}m) nematic liquid crystal with a milliwatt-power cw laser. Theoretical estimates for various gain constants and threshold intensities, and their dependence on various physical parameters, are found to be in good agreement with experimental observations.

160 Gbit/s wavelength shifting and phase conjugation using periodically poled LiNbO3 waveguide parametric converter

Abstract: High efficiency 160 Gbit/s wavelength conversion and optical phase conjugation using cascaded nonlinearities in periodically poled LiNbO3 waveguides is reported. The authors also report on use of this device as a dispersion compensator for transmission of 100 Gbit/s OTDM data through 160 km of singlemode fibre.

Time reversal of ultrafast waveforms by wave mixing of spectrally decomposed waves

Abstract: Two different realizations of time-reversal experiments of ultrafast waveforms are carried out in real time by use of four-wave mixing arrangements of spectrally decomposed waves. The first, conventional, method is based on phase conjugation of the waveform’s spectrum and achieves time reversal of real amplitude waveforms. The second arrangement of the spectrally decomposed waves spatially inverts the waveform’s spectrum with respect to the optical axis of the processor and achieves true time reversal for complex-amplitude ultrafast waveforms. We compare and contrast these two real-time techniques.

Parametric phase conjugation of a wide-band acoustic pulse in supercritical mode

Abstract: Parametric wave phase conjugation of wide-band ultrasonic pulses of 100% relative bandwidth is demonstrated. The giant amplification allowed by the supercritical mode of the magnetoacoustic technique used, leads to a relatively high efficiency of the conjugation process. In addition, the pulse response function of the magnetoacoustic conjugator in supercritical mode is derived by fitting the experimental results with the linear theory of phase conjugation. The latter provides the parameters that define the maximum value of acoustic power in the conjugate wave. The results are of interest for applications in nondestructive evaluation and acoustical imaging.

Measurement of small vibration amplitudes of a rough surface by an interferometer with a self-pumped phase-conjugate mirror

Abstract: The application of a Michelson interferometer with a self-pumped phase-conjugate mirror to measure small vibration amplitudes of a rough surface is described. The distorted wave front of the light that is diffusely reflected from the rough surface is restored by phase conjugation to provide an interference signal with a high signal-to-noise ratio. The vibration amplitudes of a stainless-steel sample are measured with a precision of ∼5 nm.

Shaping of photorefractive two-wave coupling by fast phase modulation

Abstract: Using the distinctive features of the photorefractive nonlinearity, we derive a general self-consistent set of equations to describe two-wave coupling in the presence of fast and arbitrary strong-phase modulation. By considering a number of important particular cases, we show that phase modulation is a powerful and useful tool for shaping the characteristics of two-wave coupling such as the value of the energy exchange, the diffraction efficiency of the recorded grating, and the structure of the grating fringes. Finally, we analyze the role of the phase modulation in the active stabilization of wave coupling by means of an electronically introduced phase feedback.

Digital communications using self-phased arrays

Abstract: A new technique for full duplex digital communications using adaptive phase conjugation is presented. The technique is based on mixing the RF signal to an intermediate frequency where it can be easily processed, and filtering the phase of the IF signal to separate the geometry phase and the message phase. A 6 GHz microstrip retrodirective antenna array was built, together with the signal IF processing needed for full duplex operation. The measured RCS values of a circular array are much flatter and are 0 to -5 dB up to /spl plusmn/80/spl deg/. Two way digital communications at a baud rate of 78 kbits/sec was also demonstrated with BER <10/sup -6/ for SNRs around 10 dB. The application areas are in high-performance digital mobile telecommunications for commercial and military applications.

Very high energy SBS phase conjugation and pulse compression in fluorocarbon liquids

Abstract: An optical phase conjugation and laser pulse compression of laser light via stimulated Brillouin scattering at high energy are investigated. SBS operation at high-energy and high average power in fluorocarbon liquids is clarified. Fundamental nonlinear and SBS optics data on particle-free Fluorinert liquids are presented. High capacity SBS cells filed with particle-free Fluorinert liquids were used to demonstrate a very high energy SBS mirror in recent experiments at GEKKO XII laser facility. The pulses of energy as high as 73 Joules were phase conjugated using a SBS phase conjugating mirror. High energy SBS laser pulse compressor was operated at the level 30 Joules. Laser pulse with duration 13 ns FWHM was compressed to 0.6 ns with a high SBS internal efficiency.

Compensation of pulse distortions by phase conjugation via difference-frequency generation

Abstract: We investigate theoretically the conditions under which optical phase conjugation via difference-frequency generation compensates for the effects of dispersion and self-phase modulation in the propagation of ultrashort pulses through fibers. We find that it is desirable to operate in the regime of large fiber dispersion and that nearly perfect pulse reconstruction can occur even with appreciable depletion of the pump wave in the nonlinear crystal.

Crystal stack for coherent optical beam combination

Abstract: A stack of optically contacted birefringent crystals is configured to carry out coherent beam combination (multiplexing) of narrow-bandwidth phase-locked beams from multiple lasers or laser amplifiers (e.g., multimode Yb-doped fiber amplifiers). A stack of N crystals can multiplex the output of 2 N laser amplifiers into a single diffraction-limited beam. Phase control of the beams is maintained by an electronic servo which monitors the optical power emitted into certain undesired beams and minimizes this power by means of phase adjusters (e.g., piezoelectric fiber stretchers) on each amplifier. A configuration is described where a front-end laser master oscillator (FMO) (e.g., a Nd:YAG laser) is demultiplexed by the crystal stack, passes through multiple laser amplifiers, is reflected back through the amplifiers by phase-conjugating mirrors (e.g., passive multimode fibers generating stimulated Brillouin scattering), and is multiplexed on the return trip through the crystal stack. Phase conjugation removes beam distortion and depolarization produced on a single pass through the amplifiers, and the reflected beams are generated with the spatial profiles and alignment needed for beam recombination. An additional back-end laser master oscillator (BMO) (e.g., a second Nd:YAG laser) seeds the phase-conjugate field to phase-lock the different amplifiers.

Apparatus for phase conjugation conversion and wavelength conversion and system therefor

Abstract: PROBLEM TO BE SOLVED: To provide an apparatus for phase conjugation conversion and wavelength conversion usable as a converter which is wide in a conversion band and has no dependence on polarization. SOLUTION: Signal light and pumping light including first and second polarization components respectively having first and second polarization planes intersecting orthogonally with each other are supplied to the first port 10A of a polarization beam splitter 10. The first and second ports 10B are coupled to each other by the first polarization plane. The first and third ports 10C are coupled to each other by the second polarization plane. A polarization maintaining fiber(PMF) has a polarization mode to be maintained between its first end and second end. The conversion light generated by the four light wave mixing based on the signal light and pumping light in the PMF is outputted from the first port and may, therefore, be taken out by an optical circulator 8.

Fourth-Order Interference on Polarization Beats with Phase-Conjugation Geometry

Abstract: The effect of fourth-order coherence on ultrafast modulation spectroscopy (UMS) with phase-conjugation geometry (PCUMS) in a cascade three-level system is investigated using chaotic and phase-diffusion models. It has been found that the modulation terms of the beat signal depend on the second-order coherence function, and different stochastic models of the laser field affect only the fourth-order coherence function. The difference between the PCUMS and UMS is discussed from a physical viewpoint.

Nonlinear optical properties of GaSb and GaInAsSb and their application for phase conjugation in degenerate four-wave mixing

Abstract: The nonlinear optical properties of, and phase conjugation in, the bulk semiconductor GaSb were investigated at 2.1 µm by use of the Z-scan and the degenerate four-wave mixing techniques. Measurements were also carried out near the fundamental bandgap of the quaternary compound Ga0.87In0.13As0.11Sb0.89. Z-scan measurements as a function of sample temperature, in conjunction with theoretical modeling, identified the predominant sources of the medium nonlinearity to be nonequilibrium free carriers generated through two-photon absorption in GaSb, while it is saturation of real transitions near the fundamental band edge in Ga0.87In0.13As0.11Sb0.89 that was identified as the primary cause of this nonlinearity. Degenerate four-wave mixing phase-conjugate reflectivity of as much as 14% has been achieved in GaSb.

Asymmetric underwater acoustic communication using a time-reversal mirror

Abstract: Phase conjugation using a time-reversal mirror is an array processing technique that allows accurate focusing of waves in poorly characterized media When used for underwater communication, phase conjugation requires sending a probe signal from the desired focal point to the mirror, providing local templates for subsequent waveform generation that creates signals with low intersymbol interference at the focus In spite of the simplicity of this method, and the potential reduction of receiver complexity that it can afford, frequent transmission of probes is undesirable because it forces the channel to remain almost idle during relatively long periods In this paper, pulse shapes are estimated from incoming data blocks received at the mirror, thus eliminating the need for explicit channel probes If a phase-conjugate array is used for bidirectional communication, pulse estimation can be an integral part of multichannel equalization algorithms, and its use in transmit mode entails virtually no increase in computational requirements Simulation results illustrate significant performance improvements at low signal to noise ratios relative to direct pulse measurement, as averaging in estimation algorithms greatly limits the impact of noise Formatting techniques that allow the mirror to change the data rate and pulse shape of the transmitted signal are also considered

Analysis of a large-mode neodymium laser passively Q switched with a saturable absorber and a stimulated-Brillouin-scattering mirror

Abstract: A neodymium laser passively Q switched with a saturable absorber and a stimulated-Brillouin-scattering mirror is numerically studied. An explanation is given for the laser beam spot-size widening. It is shown that this phenomenon is, most probably, due to nonperfect phase conjugation that occurs during the stimulated-Brillouin-scattering process through switching by intracavity radiation. The results of numerical simulation of the laser that account for this phenomenon are compared with experiment. It is demonstrated that the calculated output energy and pulse duration are in good agreement with those measured experimentally.

Phase conjugation at SBS of pulse laser radiation using kinoform optics

Abstract: Using the most complete in literature physical model of the non-steady-state stimulated Brillouin scattering (SBS), the numerical study is carried out of phase conjugation (PC) in the SBS-mirror that consists of an angular selector of Stokes radiation, an ordered raster of small lenses, a main focusing lens, and an SBS-cell. The ordered raster with controlled varying of its parameters allows to perform the effective angular filtering of non-conjugated Stokes components, to reduce the local light loads, and to avoid the competitive nonlinear effects. An optimal configuration of such SBS-mirror has been determined. It has the unique properties as compared with the current SBS-mirrors. It fixedly yields the PC quality that is near to an ideal (the PC coefficient is about of or more than 95%) at the selector transmittance 50 - 70% and any level of SBS saturation, i.e. any reflection coefficient. In the SBS-mirrors of different types the high PC quality in the focused beams takes place at the high reflection coefficients only that is difficult to realize as a rule. The first experimental data obtained at a Nd laser facility show the validity of the simulation results. The developed concept of SBS-mirror with unique properties can be applied for the improvement of wide class of industrial lasers.

High-quality phase conjugation even in a highly transient regime of stimulated Brillouin scattering

Abstract: A high-quality phase conjugation of the stimulated Brillouin scattering has been achieved even in a highly transient regime, as proved by the mask restoration in the backscattered signal. A special two-lens system of laser beam insertion into a scattering medium was used. A reliable onset of the phase conjugation has been reached using a splitted part of the oscillator Q-switched pulse as a prepulse in the Brillouin cell. Results with and without the prepulse are compared. A possibility of the creation of the phase conjugated mirrors even in high-power gaseous lasers with short pulses is thus proved.

Hologram recording and reproducing device

Abstract: PROBLEM TO BE SOLVED: To provide a hologram recording and reproducing device which can be downsized SOLUTION: The hologram recording and reproducing device has a supporting means which freely mountably holds and rotates a recording medium consisting of a photorefractive crystal having a parallel flat plate shape, a reference light means which makes a coherent recording reference light beam incident on the main surface of the recording medium, a signal light means which makes the coherent signal light beam modulated according to image data incident on the recording medium and intersects this beam with the recording reference light beam within the recording medium to form the refractive index grating of the light interference patterns of the signal light beam and the recording reference light beam, a means which makes the coherent reproducing reference light beam propagated in an opposite direction coaxially with the recording reference light beam incident on the recording medium and generate phase conjugation waves from the refractive index grating of the light interference patterns, a detecting means which separates the phase conjugation waves from the optical path of the signal light beam and a detecting means which detects the image data formed by the phase conjugation waves COPYRIGHT: (C)2002,JPO

Tunable phase conjugation by intracavity degenerate four-wave mixing in an injection-seeded solid dye laser

Abstract: We demonstrate efficient degenerate four-wave mixing in a laser injection-seeded solid dye laser. Phase-conjugate energy of as much as 140 times the total energy injected externally into the phase conjugator was extracted. Moreover, efficient phase-conjugate energy was obtained in the broad wavelength region from 558 to 567 nm.

Continuously pumped all-solid-state laser system with fiber phase conjugation

Abstract: Summary form only given. For industrial applications like materials processing continuously pumped, repetitively Q-switched solid-state laser systems are attractive due to their and compactness and reduced financial efforts. Conventional laser systems suffer from thermally induced phase distortion, which reduces the beam quality significantly. Phase conjugation can overcome this problem by compensating phase distortion in double pass arrangements. Stimulated Brillouin scattering (SBS) facilitates self-pumped phase conjugation in multimode glass fibers. These solid-state devices are preferable because they are easy to handle and the SBS peak power threshold can be determined by the appropriate fiber core diameter. Due to the long interaction length inside the fiber the SBS threshold can be reduced down to the 200-W level. Continuously pumped, Q-switched all-solid-state MOPA systems with fiber phase conjugator are presented and different amplifier arrangements are discussed.

The build up of stimulated Brillouin scattering excited by pulsed pump radiation in a long optical fibre

Abstract: Summary form only. This work investigates the build-up of phase conjugation SBS in a fiber in the case of excitation by a long pulse. The source of pump radiation was a single longitudinal and transverse mode CW Nd:YAG laser. A mechanical chopper produced pump pulses with a rectangular waveform of 750 /spl mu/s duration and rise-time 2 ps.

Spectral filtering of gain gratings and spectral evolution of holographic laser oscillators

Abstract: We present results of the spectral filtering properties of volume gain gratings and their role as diffractive optical elements in the spectral evolution of holographic laser oscillators. We show that the gain grating acts as a spectral filter with a filter bandwidth that is approximately inversely proportional to the medium length. We investigate holographic laser oscillators and find that, when either the gain grating is written by monochromatic radiation or when it is initiated from broad-band radiation, the output of such oscillators can evolve to single longitudinal mode in correspondence to experimental observations.

Optical isolator using stimulated brillouin scattering phase conjugation mirrors and its application to an optical amplifier system

Abstract: An optical isolator with perfect rejection ratio and its application to an optical amplifier system. Stimulated Brillouin scattering phase conjugation mirrors are symmetrically disposed to form a cross-type optical amplifier system, which makes the optical system insensitive to its optical alignment. If the optical amplifier system is used for laser amplification, the output power of the laser amplifier system can be increased as desired with same repetition rate.

Optical spatial shock waves in photorefractive media

Abstract: We show that the evolution equations describing the two-wave mixing interaction between two codirectional optical beams in photorefractive media can allow spatial shock-wave solutions. Our analysis indicates that these kink-type wavefronts move together at an angle that falls outside the initial +/-straight theta sector of propagation. The apparent direction of propagation and the spatial widths of these optical shock-wave beams are directly related to their relative intensity.

Low- and high-frequency ocean acoustic phase conjugation experiments

Abstract: We have performed a series of acoustic phase conjugation (PC0 experiments in shallow water. A time reversal mirror (TRM) was implemented for center frequencies of 450 Hz and 3500 Hz. Very sharp focal regions out to ranges of 30 km were obtained. Analysis, including attenuation effects, indicates that the diffraction limit on the size of the focal region was reached. This has implications for defining the maximum resolution of a matched field processor because the latter is simply a hybrid computational implementation of PC. The fact that a TRM both spatially and temporally refocuses energy with the aid of a probe suggests that ocean self-equalization with respect to communication processing is possible. Indeed, a preliminary demonstration of this self-equalization process was experimentally achieved.