Optical phase conjugation by backscattering in barium titanate

Generation of self-pumped phase conjugation from the -c face of BaTiO3 with femtosecond pulses

We demonstrate a new configuration of self-pumped phase conjugation in BaTiO(3) with 450-nm femtosecond pulses. The pump beam enters the -c face of the crystal at an acute angle with respect to the +c axis. This geometry is different from all known geometries of self-pumped phase conjugations in BaTiO(3). Phase-conjugate reflectivity of 1.7% and a rise time of approximately 24 s are obtained at an average pump power of 10 mW. The ability of the phase conjugator to serve phase distortions correctly is demonstrated. We propose that the femtosecond phase conjugator operate with the hologram mechanism sharing as the double phase conjugator. The self-pumped phase-conjugate pulses are analyzed by femtosecond electric-field cross correlation.

Spatial-Frequency Response of Photorefractive Phase Conjugators with Ce:BaTiO(3)

We propose and demonstrate an interferometric method to measure thespatial-frequency response of photorefractive phase conjugators withCe:BaTiO(3). Two coherent beams are incident on a crystaland form an interference pattern inside the crystal. The two beamsundergo stimulated photorefractive backscattering, which creates theircorresponding phase conjugations. Then the four waves interactwithin the crystal. The spatial-frequency resolution of the phaseconjugators is measured to be as high as 3750 line pairs/mm by use ofthe interferometric method. There are several factors that limitthe measured spatial resolution when using a U.S. Air Force ResolutionChart. The output modulation deviates from the input modulation forhigh spatial frequencies. In the presence of a strong additionalpump beam, the output modulation of the phase conjugators is almost thesame as the input modulation for a wide range of input spatialfrequencies. The phase conjugator exhibits a large dynamic range ofintensity. We analyze theoretically the modulation transferfunction of photorefractive phase conjugators withCe:BaTiO(3) for two mutually coherent beams. Thetheoretical analysis is in agreement with the experimental resultswithin a small incident-angle region.

Investigation into self-pumped and mutually pumped phase conjugation with beams entering the negative c face of doped (K(0.5)Na(0.5))(0.2)(Sr(0.75)Ba(0.25))(0.9)Nb(2)O(6)crystals

We investigated some novel geometries of self-pumped phase conjugation (SPPC) and mutually pumped phase conjugation (MPPC), relying on total internal reflection both from the a face and from corners, with beams entering the negative c face of doped (K(0.5)Na(0.5))(0.2)(Sr(0.75)Ba(0.25))(0.9) Nb(2)O(6) crystals. The different situations for internal light paths and their direct transformation at the same incident wavelength were observed. Similarities and differences between SPPC and MPPC are discussed. The dynamic features of SPPC and MPPC in different situations were also observed. Three or more four-wave-mixing interaction regions were clearly observed inside the phase conjugators. The multistep interaction and its influence on the response rate and conjugation fidelity are analyzed. The large size of the samples seems necessary to ensure an optical path that is long enough for multistep bifurcation. The stability of the light channels is discussed based on the fanning effect.

Self-pumped phase conjugation with femtosecond pulses by use of BaTiO(3)

Self-pumped phase conjugation is generally believed to be attributed to degenerate four-wave mixing. Accordingly, the phenomenon requires the existence of three waves at certain interaction regions for the excitation of the fourth one. Although this condition is not satisfied by a train of femtosecond pulses with a repetition rate of 110 MHz, we have observed self-pumped conjugate pulses retroref lected by a BaTiO(3) crystal. The conjugate character of the pulses is demonstrated with the distorter method.

Stochastic photorefractive backscattering from LiNbO(3) crystals

Our experiment shows that stimulated backscattering from photorefractive LiNbO(3):Fe crystals occurs not only for an orientation of the polar axis that is parallel to the input beam but also for the opposite orientation, in which the spatial amplification of seed scattering through pi/2-shifted gratings is impossible. In the latter case scattering exhibits pronounced noise properties. We show analytically and numerically that the observed regularities are explained by a model including a source of low-frequency temporal noise for the space-charge field and a strong photovoltaic effect.

Self-pumped phase-conjugation properties of cerium-doped BaTiO(3) crystals in the near infrared

Studies of a new Ce-doped BaTiO(3) crystal as an efficient self-pumped phase conjugator in the near infrared are presented. The internal beam production during the establishment of self-pumped phase conjugation (SPPC) is closely observed, and the corresponding mechanisms of SPPC are identified accordingly. When the incident beam is in an a-face-incidence geometric arrangement, it is observed for the first time that SPPC mechanism has a transition from stimulated photorefractive backscattering and four-wave mixing to total internal reflection when λ is varied from 706 to 733 nm. Variations of the SPPC reflectivity with the λ, incident angle, and position of the input beam have been investigated, and a high reflectivity of approximately 80% has been obtained. In addition, SPPC reflectivity of approximately 40% is obtained when the incident beam enters the crystal by the +c face, and a variation with λ of optical beam patterns in the crystal has also been observed. Qualitative explanations of the phenomenon of SPP mechanism transition with λ that will be very useful in practical applications are given.

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

We report what is to our knowledge the first observation of mechanism transformation with wavelength of self-pumped phase conjugation (SPPC) in BaTiO(3):Ce crystals in the near infrared. 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.

Self-pumped phase conjugation and four-wave mixing in 0 degrees - and 45 degrees -cut n-type BaTiO(3):Co

Relatively fast self-pumped phase-conjugate and four-wave-mixing rise times are reported in n-type cobalt-doped barium titanate. With the crystal oriented in a 45 degrees cut as compared with the same crystal in a 0 degrees cut we find a factor of 3 decrease in the 0-90% rise time to 800 ms with 25-mW input power at 514.5 nm. Also, the self-pumped phase-conjugate reflectivity increases from 20% to 40%. We deduce that the phase conjugation is from internally seeded stimulated photorefractive backscattering. The four-wave-mixing rise time of the 45 degrees -cut crystal is 4 ms with a reflectivity of 48% when the pumping beams are derived from self-pumped phase conjugation that has an input power of 25 mW.

Optimal incident conditions for a Cat-type self-pumped phase-conjugate mirror

We present a simplified method, based on experimental observations, for predicting the best incident conditions for obtaining the highest reflectivity of a Cat-type self-pumped phase conjugator with given crystal dimensions.

Phase conjugation involving incoherent counterpropagating beams in photorefractive media

We propose and analyze a new type of phase conjugation between counterpropagating beams in photorefractive materials based on diffraction from transmission gratings only. This effect does not require temporal coherence between the counterpropagating beams and may occur spontaneously, with no external seeding, even with wide (frequency) band light sources.

Dynamic holographic method of imaging phase objects

We describe a new method of imaging phase objects which uses the self-phase-conjugating phenomenon in barium titanate and a technique of incomplete aberration removal.

Double phase-conjugate mirror: analysis, demonstration, and applications

We report on the operation of the double phase-conjugate mirror (DPCM). Two inputs to opposite sides of a photorefractive barium titanate crystal, which may carry different spatial images, are shown to pump the same four-wave mixing process mutually and are self-refracted without any external or internal crystal surface. This results in the phase-conjugate reproduction of the two images simultaneously. This device is analyzed theoretically, and applications in image processing, interferometry, and rotation sensing are discussed. We also demonstrate the operation of a ring laser, using the DPCM, as well as a photorefractive resonator with two facing DPCM's that can support spatial information in its oscillations.