Photorefractive semiconductors have the attractive features of fast response times and operation at near-infrared wavelengths. This has opened some new research opportunities in the field of photorefractive nonlinear optics which is significant for applications in real-time image processing and optical computing. This paper presents recent experimental demonstrations of several basic optical information processing techniques using photorefractive GaAs crystals. The results of these demonstrations illustrate that photorefractive compound semiconductors has a great potential as a new medium for light beam interaction based on the dynamic holographic principle.
We report the growth of organic crystal of DAN by Bridgman method, where both bulk (6 x 20 mm) and
cored-fiber (O.2x30 mm) single crystals were grown. Characterization of these crystals was performed for doubling
Nd:YAG laser operated at 1.064 pm and 1.32 jim, where favor orientation of the Bridgman-grown DAN
was demonstrated. Phase-matching loci were measured and compared with the calculated data. Conditions for
single-mode cored-fiber are analyzed.
Recently, energy transfer has been observed between two pulsed, degenerate laser beams
copropagating almost collinearly into isotropic Kerr media with temporal relaxation.
Theoretical calculations have been made which agree with the experimental data. This paper
will briefly describe the experiment, and develop the theory which simulates the lab situation.
The physical mechanism regulating this beam combination will also be discussed. Comparisons
between the numerical simulations and the experimental data will also be presented.
Phase-conjugate four-wave mixing in a waveguide has many advantages over that in a bulk medium due to the
waveguiding effect . It is also relatively easy to introduce an optical gain in the waveguide to amplify the weak signal.
For sufficient image resolution, a multimode waveguide with a large number of guided modes is needed. For high
fideIiy image replication, the nonlinear phase conjugate efficiency has to be mode-independent. These requirements
are examined for two types of multimode waveguides: the cylindrical fiber waveguide without carrier diffusion and
the rectangular semiconductor waveguide with carrier diffusion. In the fiber waveguide, the cylindrical geometry and
the tensorial nature of makes the nonlinear coupling coefficient quite mode-dependent. In the semiconductor
waveguide, carrier diffusion severely limits the efficiency of coupling the power in a particular pump mode to other
modes in the conjugate signal. The existence of a gain will further aggravate these problems. In order to avoid
these problems while maintaining the advantages of the waveguiding effect, we propose to use an array of single-mode
waveguides. In this case, the only requirements are that the elements are identical and equally pumped and that
there is no cross-talk between individual elements. A high-fidelity signal can be generated with an optical gain for
amplification. In addition, the signal beam can be steered by changing the incident direction of one of the pump
In this paper, we report on a novel cavity design for optical parametric
generators that removes the cavity mode constraints present in conventional
parametric oscillators. A conversion efficiency of 30% is obtained in a 532-nm
pumped BBO oscillator using this design.
Collinear microwave phase conjugation was observed in an artificial Kerr medium consisting
of short graphite fibers suspended in a binary liquid mixture. Using an 18 GHz pump beam
with up to 20 W continuous power, characterization of the changes in the 94 GHz refractive
index were made by interferometry. A nonperturbative method for describing the response of
the medium was used to analyze the phase-shift measurements for the static berefringence and
the time response as functions of microwave intensity.
The newly discovered nonlinear crystal of lithium triborate, LiB3O5 (LBO), has several advantages over
another borate family, 8-BaB2O4.13 These include nonhydroscopic, wide transparency (0.16-2.6 sm), high
damage threshold and may be temperature-tuned for noncritical phase-matching (NPCM). NCPM using LBO
(temperature-tuned) for the second harmonic generation (SHG) and sum frequency mixing (SFM) of N&YAG
laser and YAG-laser-pumped Raman cell in H2 gas was reported.4
In this paper we present the phase-matching curves of LBO for the generation of UV-sources (160-250
nm), where 160 nm is the absorption edge of LBO crystal (see Fig. 1). In Section 2, we analyze the phasematching
conditions for second harmonic generation (SHG), third harmonic generation (THG) and sum-frequency-
mixing (SFM) and provide the general guidance for achieving phase-matchable shortest wavelengths.
Section 3 shows the numerical results based on the Seilmeir equations of LBO, where UV and IR sources are
mixed for deep-UV sources. Schematics of proposed experimental setups are also shown.
Experimental results of a self-pumped phase conjugate mirrior with external ring cavity in Cu :SBN
crystal are presented. The phase conjugate reflectivity of the device can reach 50%. A light wave with aberration caused by a distorter is corrected by the device.
A general theory for coherent coupling between two co-propagating or counterpropagating
guided waves is formulated within the slowly varying envelope approximation. A pair of coupled
nonlinear equations is obtained for the evolution of envelope functions. A numerical result is
given for coherent coupling of two co-propagating pulses in a single mode fiber.
A long pulse one Joule excimer laser was used as a pumping source
for stimulated Raman scattering in lead vapour and hydrogen. High
efficiency Raman frequency conversion into blue-green region was
obtained . A decrement of Raman conversion efficiency at high
repetition rate was observed in a sealed Raman cell. A hydrogen
gas circulating system was designed with gas pressure up to 50
Atm,it can be operated at high repetition rate without decrement
of Raman conversion efficiency.
Nonlinear optical phase conjugation, obtained by four-wave degenerate mixing is utilized to create a new form of optical
pointing error sensor-theActiveOptical Self-Referencing Component(AO-SRC). Thisdevicewas conceived and constructed
during a 1988 nonlinearoptic initiative thatwas designed to demonstrate the expanded degrees of freedom available for designs
that incorporate nonlinear optic elements.
The AO-SRC concept was successfully demonstrated and verified that an absolute, seff-referenced form of pointing sensor
couldbeachievedwith the use of nonlinear photorefractive design elements. The self-referencing capabiity is obtained from the
ideal and generalized phase-conjugate reflectance from nonlinear mirrors. The absolute capability is related to the optical
relationship between the phase-conjugate and the Fresnel (specular) reflected energy from a common nonlinear medium. It is
seen that the same physical mechanism responsible for the well known self-referencing capability of phase-conjugate
interferometers (-i.e. elimination ofthe reference mirror) can be used to constructa high sensitivity, self-referencing, pointing
Current nonlinear materials such as barium titanate are limited to maximum input apertures ofabout 12 millimeters diameter.
Since sensitivity (minimum resolvable angle) is inversely related to aperture alze, the maximum size of nonlinear media may
represent a severe limiting factor for some AO-SRC device forms. Two mechanisms associated with nonlinear optic materials
were used to ease this constraint, specifically:
1. Electric Field Enhancement: Application of an electric field seross the mdium was used to construct a phase-shifting,
phase-conjugate interferometer. This construction will yield an increase in sensitivity of 1-2 orders of magnitude.
2. Nonlinear Synthetic Aperture: Utilization of the self-coupling capability of discrete nonlinear materials was shown to yield
effective aperture sizes of arbitrary dimension.
It is shown that the AO-SRC concept can be applied to such critical tasks as the assessment of the optical coherence achieved in
phasing (pointing error and piston) segmented or synthetic aperture optical systems.
We present a real-time holographic associative memory implemented with photorefractive KNSBN : Co crystal
as memory element and liquid crystal electrooptical switches as reflective thresholding device. The experimental
results show that the system has real-time multiple-image storage and recall function.
This paper examines thermal (light-scattering) fluctuations as a dominant noise source in
nonlinear optical processes. Fundamental limits to conjugate wave fidelity and signal power
requirement are obtained through the statistical thermodynamic treatment of light-scattering
noise in four-wave mixing based on the fluctuation-dissipation theorem. Several types of
nonlinear media are examined including artificial Kerr suspensions, isotropic Kerr media, and
fluids near a critical point. Where measurements are available, excellent quantitative
agreement is obtained between theory and experiment.
Theoretical and experimental results are presented for degenerate four-wave mixing In photorefractive
KNSBN crystals. The angular dependence of transient- state and steady -state phase- conjugate reflectivities
R in undoped and Pr -doped KNSBN have been measured respectively. Applied field dependence of
phase - conjugate reflectivity in undoped KNSBN shows that transient-state R exceeds 260 and steady -
state R can reach 122% at applied field E0 =5 kV/cm. By comparison ,Pr as a new impurity to provide a variety
trapping centers in KNSBN crystals makes the response time about 3 times less. The probe intensity dependence
of the response time and the steady phase- conjugate reflectivity in degenerate four -wave mixing
within Cu-doped SBN is given.
Optical phase conjugation at microwave and millimeter wavelengths offers numerous
applications for communications, radar and navigation. Recently, we have examined the
feasability of achieving phase conjugation at typical radar wavelengths ulitizing magnetic thin
films as the active medium. Our calculations indicate that magnetic materials are unusally
promising for generating phase conjugate radiation at microwave frequencies.
This paper presents the advantages to be gained from the use of unwished
transitory peak appeared at the output of a Fabry-Perot optical amplifier when
it is switched frorr a low to high state. Simulations and numerical results
show the possibility of achieving reshaping and contrast improvement of
optical incident pulses. Potential applications of this bistable laser
amplifier as untimed regenerative repeater under certain conditions are
Fourth harmonic generation in BBO and KD*P was investigated using a phase-conjugated Nd:YAG amplifier as a
pump source. The 1.1J, 1.06i source was converted to 0.55J @ 0.53j.t as the pump source for the investigation.
Energy outputs of 250mJ at 0.27ji. were obtained. Control of the angular dephasing error was found to be critical for
good conversion efficiency, particularly in BBO. We measured internal conversion efficiencies of 70% with the
12.7X12.7X2Omm KJYP crystal, and 58% with the 1OX1OX4.6mm BBO. Analysis correlated well with the
experimental results in the case of KD*P, but implied a higher nonlinear coefficient for BBO than is reported in the
literature. An energy imbalance of 15% due to nonlinear absorption was observed for both crystals when they were
tuned for optimum 0.27p. generation. This was found to cause rapid thermal dephasing during 10-20Hz operation.
We conclude that, using either of these materials, careful thermal management is necessary in obtaining efficient
fourth harmonic conversion for higher average power operation.
In a Raman amplifier having a broadband pump beam which is not exactly collinear with the
Stokes seed beam, the Stokes output spectrum will be "fanned", i.e., angularly dispersed, because the
direction of maximum gain differs for growth of each component of the Stokes spectrum. The
spectral fanning produces transverse phase and intensity variations which degrade the Stokes output.
Approximate analytic expressions are given for beam quality degradation in cases where the Stokes
seed is narrowband, broadband and temporally correlated to the pump beam, -or broadband and
uncorrelated. The effect is predicted to be usually negligible, but can be significant for cases of
very large beam aperture sizes, bandwidths and/or angles. Use of a diffraction grating in the output
Stokes beam or a stepped mirror in the input pump beam are proposed for reducing the effect.
Optical switching properties of a photorefractive BaTiO3 crystal are studied. The data
shows a small but symmetric switching for ordinary polarized input beams and a larger but
nonsymmetric switching for extraordinary polarized beams. Slow drifts were also observed in the
outputs after switching with extraordinary beams. The extreme nonsymmetry and drifts are
believed to be caused by coupling that is due to gratings produced by internal reflections. We
suggest that these effects may make the photorefractive BaTiO3 crystal unsuitable for optical
switching applications but do not affect its ability to be used as a memory storage device with
The fluid equations governing the medium response to stimulated Brillouin scattering (SBS)
are solved to second order in the quasi steady-state approximation, and coupled with Maxwell's
equations to obtain the scattered radiation fluence profiles. Large scale fluid motion, or acoustical
streaming, is predicted as a second-order, time-dependent effect. Since this streaming motion
causes phase shifts in the optical grating, the gain of the scattered signal is reduced. 'The differential
equations governing the large-scale motion is integrated numerically to obtain the streaming phase
shifts. These phase shifts along with the acoustical amplitudes (obtained in the quasi steady-state
approximation) are then inserted into Maxwell's equations in the slowly varying envelope approximation
to obtain the intensity profile of the scattered radiation, assuming a speckle.inhomogeneous
field configuration and a Gaussian pump wave profile. The intensity profile of the scattered pulse
is then integrated over the pulse time to obtain the scattered pulse fluence.
Initially, scattering is the most intense near the center of the beam, and therefore streaming
phase shifts grow most rapidly in this region. As the pulse evolves, this grewth in phase shift causes
more reduction in gain near the center of the beam than in the wings. Therefore, it is predicted
that the intensity of the scattered wave profile is suppressed near the center. The calculations and
plots we obtained showing this effect are qualitatively consistent with some experimental results
obtained by Dolgopolov et. al.
Several years ago, Shiren, Arnold and Kazaka achieved an acousto-optical analog of optical
phase conjugation via degenerate four-wave mixing in glasses. These experiments, which were
conducted at 9 GHz, involved the generation of a time-reversed acoustic pulse from a pulsed
acoustic probe wave utilizing microwave beams as the pump waves. Here, we examine the
reverse situation; i.e., acoustic beams serve as the pump waves and the probe is electromagnetic
in nature. It is shown that the medium generates a phase conjugate electromagnetic wave
and the physics of this effect is examined.
Degenerate-four-wave mixing (DFWM) is an important technique for the generation of
phase-conjugate wave, to which much attention has been paid. In this paper. a tunalile CW
CO2 1.aser is used as the pump sourse and gaseous SF6 as the nonlinear medium, the
dependence of reflectivity of phase-conjugate wave generated by DFWM on the pump
intensity, the pressure in SF6, the angle between the pump and probe beams as welt
as the potarization of the optical fields etc, is investigated experimentatty and
exptained theoretically. Peak reftectivity of 2.1x101 is abtained at P=4 Torr in the
gaseous SF6. In the Doppler-free phase-conjugate spectrum of SF6 five absorption dips
are observed and the spectral resolving power is measured to be about 3 MHz.
In this communication, one looks at the influence of the saturation
process over the statistical fluctuations (signal to noise ratio) of the
optical power at the output of a travelling wave semiconductor optical
amplifier (TWOA) . The statistical behaviour of this device is described
starting from the photon density matrix equation. The obtained theoretical
results, both for the noise figure and for the signal to noise ratio, allow to
take out some conclusions about noise behaviour in the non-linear light
Nonlinear optics (NLO) is increasingly important for a variety of military and commercial needs
ranging from new types of laser weapons to fiber optic communications networks, to optical computing
applications. Rapid advances in optical phase conjugation, optical switching, and logic have
demonstrated the enormous potential for practical applications with concurrent fast-paced materials
development. Organic polymers such as rigid-rod polymers (also known as ordered polymers) and
polyimides have the excellent intrinsic properties and tailorability to meet the requirements of a wide
range of nonlinear optical devices. The research described in this manuscript is a multidisciplinary
effort to target key applications and material and processing requirements for these polymers. This
manuscript provides the framework for the applications development required for the rapidly growing
field of NLO devices from ordered polymeric materials. It discusses the characterization of the linear
and nonlinear optical properties of three materials, experimental approaches to observing optical
bistability, and the demonstration of dispersive optical bistability.