Showing papers on "Phase conjugation published in 1994"

Acoustic pulse compression using passive phase‐conjugate processing

Abstract: Passive phase‐conjugate (PPC) processing consists of using the first arrival, from a stream of pulses that have traversed a complex refractive medium, as a filter for later pulse arrivals. Its connection with true ‘‘active’’ phase conjugation lies in invoking reciprocity for the backpropagation. When the intervening acoustic medium changes slowly, the PPC processor creates an unambiguous temporal peak even when a complex multipath environment separates the acoustic source and receiver. Temporal sidelobes in the PPC processor output can be suppressed by increasing the number of receivers and coherently summing their output. Recent deep‐ocean acoustic propagation measurements (IWAC ’90) are used to assess PPC processing in a fading multipath acoustic channel. For a range of 272.3 km at 460 Hz, the compressed peak persists on average for about half an hour. Fading of the PPC processor peak is found to be consistent with parabolic‐equation second‐moment theory for wave propagation in a random medium.

Nonlinearity-insensitive standard-fibre transmission based on optical-phase conjugation in a semiconductor-laser amplifier

Abstract: Experimental results are presented confirming that optical-phase conjugation in a semiconductor-laser amplifier has the potential to compensate for the interplay of chromatic dispersion and nonlinear self-phase modulation, thereby enabling high-capacity high-power transmission over standard singlemode fibres.

Cancellation of four-wave mixing in multichannel fibre transmission by midway optical phase conjugation

Abstract: The cancellation of waves generated by four-wave mixing (FWM) in multichannel optical fibre transmission is demonstrated using midway optical phase conjugation. An FWM reduction ratio of -10.6 dB is achieved in an experimental three-channel transmission experiment along a 40 km dispersion-shifted fibre.

Ultra-high capacity non-soliton optical transmission using optical phase conjugation

Abstract: The present invention provides an apparatus and method for achieving bit rate distance products on the order of 200 Tbits/s-km in non-soliton optical communication using optical phase conjugation. The apparatus and method utilize phase conjugation and adjustments of in-line amplifier number, spacing, and/or output power in order to compensate for the interaction between first order dispersion and fiber nonlinearity dispersion effects in an optical fiber span. The present invention provides additional techniques for adjusting system parameters, such as dispersion-length products of first and second portions of the fiber span, in order to compensate for changes in first order dispersion resulting from non-zero second order dispersion. The method and apparatus also provide an improved multi-channel optical phase conjugation system design.

Nonlinear optical interactions in dye-doped solids

Abstract: We present a brief review of non-linear optical investigations on dye-doped solids using low-power CW lasers. After a brief introduction to the photophysics of the dye molecules, we discuss specific nonlinear processes such as self-diffraction, optical phase conjugation, two-beam coupling and polarization gratings in these systems. The application potential of dye-doped solid devices is discussed.

Generation of optical phase-conjugate waves and compensation for pulse shape distortion in a single-mode fiber

Abstract: The generation of optical phase-conjugate waves and the application of optical phase conjugation (OPC) to optical communication systems is described. The method of pulse shape distortion compensation by OPC is outlined including distortion due to both fiber dispersion and the optical Kerr effect. The generation of a forward-going phase-conjugate wave in a third-order nonlinear medium is discussed and that by a nondegenerate forward four-wave mixing in a zero-dispersion single-mode fiber (SMF) is investigated. Suppressing the stimulated Brillouin scattering (SBS) of a pump wave in the fiber prevents saturation of the generation efficiency of the phase-conjugate wave even when the pump power exceeds the SBS threshold. In transmission experiments through a 200-km standard SMF with a 16-Gb/s intensity-modulated signal and a 5-Gb/s continuous-phase FSK (CPFSK) modulated signal, it is shown the applicability of OPC is modulation independent and that OPC effectively compensates for both chromatic dispersion and the optical Kerr effect. >

Phase conjugation of an optical near field

Abstract: Using degenerate four-wave mixing in an Fe:LiNbO(3) 0.04-wt. % crystal and an external-reflection near-field optical microscope, we have achieved phase conjugation of light emitted by a fiber tip. We observe that the phase-conjugated light at a wavelength of 633 nm can reach a power of ~0.1 nW and produce a 180-nm-wide spot image in the near-field microscope. This is the first direct demonstration, to our knowledge, of the phase conjugation of near-field components of optical fields.

High brightness solid-state laser with zig-zag amplifier

Abstract: A solid-state laser architecture producing a beam of extremely high quality and brightness, including a master oscillator (10) operating in conjunction with a zig-zag amplifier (16,30), an image relaying telescope (17) and a phase conjugation cell (20). One embodiment of the laser architecture compensates for birefringence that is thermally induced in the amplifier (16), but injects linearly polarized light into the phase conjugation cell (20). Another embodiment (19) injects circularly polarized light into the phase conjugation cell (20) and includes optical components that eliminate birefringence effects arising in a first pass through the amplifier (16,30). Optional features (26) permit the use of a frequency doubler assembly to provide output at twice optical frequencies, and an electro-optical switch or Faraday rotator (71) to effect polarization angle rotation if the amplifier material (16,30) can only be operated at one polarization. The zig-zag amplifier (16,30) is cooled by flow of cooling liquid, preferably using longitudinal flow to minimize temperature gradients in a vertical direction, and has cooling channel seals (50,52) disposed in dead zones that receive no light, to minimize optical damage to the seals (50,52). Light is input to the amplifier (16) at a near normal angle of incidence, to minimize polarization by reflection and to permit a polarizer (14) to be used to extract an output beam from the amplifier (16,30). Antireflective coatings (60) on edges and on sides of the amplifier (16,30) eliminate parasitic oscillations, and wedge-shaped windows (36) provide uniform pumping by eliminating gaps between diode arrays (32).

Effect of thermal self‐defocusing of degenerate four wave mixing in absorbing media

Abstract: The distortion of a phase conjugate wave produced by degenerate four wave mixing as a consequence of thermal self‐defocusing is reported. The thermally induced refractive index changes cause modulation in the phase of the phase conjugate wave. The thermal self‐defocusing effect may impose a limiting factor in high repetition rate nonlinear devices.

Phase conjugation of BaTiO3:Ce by backward stimulated photorefractive scattering

Abstract: We present a self‐pumped phase conjugator (SPPC) based on backward stimulated photorefractive scattering in a BaTiO3:Ce crystal in air without any external seed beam. Stable reflectivities of as much as 70% and good phase conjugate fidelity have been demonstrated in the 0°‐cut crystals. Over 50% reflectivities at 515 nm have been obtained at an incident angle between 20° and 70°. In comparison with undoped 0°‐cut and 45°‐cut crystals, the buildup time of SPPC is much faster for the same intensity.

Conjugate solitons in amplified optical fibre transmission systems

Abstract: The authors demonstrate by numerical simulations that midsystem optical phase conjugation can compensate for propagation effects in periodically amplified soliton transmission systems Within the domain of validity of the average soliton model, soliton interactions are undone and RMS Gordon-Haus jitter is halved >

Generation of spatial subharmonic gratings in the absence of photorefractive beam coupling.

Abstract: Using a new experimental geometry, we have proved for the first time that the generation of spatial subharmonic gratings in photorefractive crystals is not dependent on optical nonlinearity. We present results which confirm that the subharmonic gratings result from a parametric excitation of ultra low-frequency eigenmodes of a crystal by a time modulated fundamental grating.

Cancellation of four-wave mixing in a single-mode fiber by midway optical phase conjugation.

Abstract: Cancellation of waves generated by four-wave mixing (FWM) in a single-mode fiber by use of midway optical phase conjugation is proposed and analyzed, and the possible power reduction is estimated. Effective cancellation of FWM is demonstrated with phase-conjugate waves generated by forward FWM in a zero-dispersion single-mode fiber, and a power reduction ratio of -13.9 dB is achieved in experimental three-channel transmission along a 40-km dispersion-shifted fiber.

Laser system with phase-conjugator-enhanced output

Abstract: A laser system includes a linear array of broad area lasers. Such a laser source produces a beam that has a polarization axis in the direction in which the array extends and a coherency axis orthogonal to that. The laser is injection locked by phase conjugating a portion of the array's light output. The phase conjugator has a conjugation axis. The coherency axis is aligned with this conjugation axis. External cavity optics include a polarization rotator to align the polarization axis with the coherency axis before the beam reaches the phase conjugator. Thus, the polarization, coherency, and conjugation axes are all aligned, optimizing the action of the phase conjugator. The approach works in both a single-beam and a dual-beam phase conjugation mode. The result is a laser system output of higher intensity, greater coherence, and a narrower frequency range.

Mechanism transformation with wavelength of self-pumped phase conjugation in BaTiO(3):Ce.

Abstract: We report what is to our knowledge the first observation of mechanism transformation with wavelength of self-pumped phase conjugation (SPPC) BaTiO3:Ce crystals in the near infrared. in It was found that the SPPC mechanism changed from a backscattering and four-wave-mixing mechanism to a cat mechanism when the pump wavelength was varied from 706 to 770 nm. In addition, SPPC was obtained when the pump beam entered the crystal by the +c face, and a variation with wavelength of optical beam paths in the crystal was also observed.

Four-Wave Mixing in a 3D Optical Lattice

Abstract: We have observed the reflection of a probe laser beam by a three-dimensional optical lattice of rubidium atoms. The reflection spectrum shows different kinds of narrow resonances which we attribute to degenerate (phase conjugation) and non-degenerate multiwave mixing processes. In order to explain the extremely narrow linewidth (300 Hz) of the degenerate four-wave mixing resonance and its dependence on the angle of incidence of the probe, we propose a novel physical mechanism which includes the formation of a long-range atomic-density grating acting as a hologram.

Resonant cascaded surface-emitting second-harmonic generation: a strong third-order nonlinear process.

Abstract: A novel practical scheme for implementation of the cascaded nonlinearity with surface-emitting second-harmonic generation in the Fabry–Perot cavity is presented. We show that such a scheme can be efficiently used for optical limiting and optical phase conjugation at a pump power of less than 100 mW.

Recovery of the soliton self-frequency shift by optical-phase conjugation

Abstract: The recovery of the self-frequency shift effect on a soliton by optical phase conjugation is investigated. It is proved that, without third-order dispersion and fiber loss, the soliton self-frequency shift can be completely recovered. The recoveries for a fundamental soliton and a second-order soliton in the presence of third-order dispersion and of fiber loss, which is compensated by the distributed erbium-doped fiber amplifiers, are numerically studied.

Doppler-free ultrafast modulation spectroscopy with phase-conjugation geometry.

Abstract: We have studied theoretically a phase-conjugate ultrafast modulation spectroscopy (PCUMS) in a Doppler-broadened system. The pump beams consist of two frequency components which drive two different transitions. It is found that the four-wave-mixing (FWM) signal exhibits damping oscillation as the relative time delay between two pump beams is varied, from which the energy-level splitting can be deduced. We have considered the cases that pump beams have either narrow band or broadband linewidth and found that for both cases a Doppler-free precision in the measurement of the energy-level splitting can be achieved. The advantage of PCUMS is that the energy-level splitting between states can be widely separated. Finally, the relationship between PCUMS and other Doppler-free techniques in frequency and time domains has also been discussed.

High-quality self-pumped phase conjugation of nanosecond pulses at 532 nm using photorefractive BaTiO(3).

Abstract: We implement a feedback loop oscillator, using a BaTiO3 crystal that provides stable high-quality phase conjugation for nanosecond pulses, with an efficiency close to the theoretical maximum.

Model of vortices nucleation in a photorefractive phase-conjugate resonator

Abstract: The nucleation of a pair of vortices in the field of a boundary dominated confocal photorefractive phase-conjugate resonator is studied in detail. A phase instability is seen to precede the nucleation which occurs when the phase step reaches a threshold of π. The validity of a scenario based on the competition of a few modes is verified experimentally using a multichannel optical correlator equipped with matched filters designed to identify the modes presumed by the model.

Observation of optical limiting and backscattering of nanosecond laser pulses in liquid-crystal fibers.

Abstract: We report the recent observation of nonlinear-optical phenomena occurring in isotropic liquid-crystal-cored optical including passive all-optical self-limiting effects, stimulated backscattering, and phase conjugation. These fibers, processes occur at relatively low thresholds and short fiber lengths.

Double phase conjugation

Abstract: We model the double phase-conjugate mirror (DPCM) as a function of time, the average direction of propagation of the two beams forming the DPCM, and one transverse coordinate. Calculations show that the conjugation fidelity and reflectivity have different dependencies on the photorefractive coupling coefficient times length; the fidelity turns on abruptly with a threshold, whereas the reflectivity increases smoothly. The DPCM behaves as an oscillator at and above threshold: the time required for the reflectivity to reach the steady state dramatically slows down near threshold (like critical slowing down in lasers); above threshold the DPCM is self-sustaining even if the random noise terms used to start the process are set to zero. A decrease in the noise level improves the fidelity but increases the response time. The use of unbalanced input beam ratios results in asymmetric conjugation such that the fidelity obtained on the side of the weaker input beam is significantly reduced. The slowing down diminishes with increasing noise level or unbalanced input intensities.