Changxi Yang

HIGH-EFFICIENCY SELF-PUMPED PHASE-CONJUGATION AT 633 NM IN CERIUM-DOPED BARIUM-TITANATE CRYSTALS

Abstract Photorefractive self-pumped phase conjugation (SPPC) behavoir is examined at 633 nm in cerium-doped barium titane crystals. A reflectivity as high as 86.8% is reported. The buildup times are an order of magnitude faster than the corresponding values in Co: BaTiO3 and Cu: KNSBN crystals with input powers. A possible mechanism responsible for the high self-pumped phase conjugation reflectivities in the negative incident angle range is discussed.

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.

Studies on formation mechanisms of self-pumped phase conjugation in BaTiO 3 :Ce crystals at wavelengths from 570 to 680 nm

Formation mechanisms of self-pumped phase conjugation (SPPC) in two BaTiO3:Ce crystals with different doping concentrations have been experimentally studied. The pump beam was from dye lasers, with an overall tunable wavelength region lying between 570 and 680 nm. Variations of the optical beam path patterns as well as reflectivity of the phase conjugators with respect to the incident angle, the incident position, and the wavelength of the pump were investigated in two configurations. One configuration corresponds to that in which the pump enters an a face of the crystal; the other to that in which the pump enters the +c face of the crystal. It was found that SPPC is sometimes established in a way similar to that of a cat phase conjugator, but usually without the formation of a clear beam path loop. Sometimes SPPC is established by a stimulated photorefractive backscattering and four-wave mixing mechanism. In these cases, however, the crystal corners as well as the scattering centers on the crystal faces usually play an important role by seeding the stimulated photorefractive backscattering processes. In other cases SPPC is established by both the cat and the stimulated photorefractive backscattering and four-wave mixing mechanisms.

Propagation and self-pumped phase conjugation of femtosecond laser pulses in BaTiO 3

We investigate the properties of propagation and self-pumped phase conjugation (SPPC) of 90-fs laser pulses at 450 nm in BaTiO3. Femtosecond SPPC in BaTiO3, both in air and in immersion oil, is explored; it is demonstrated that femtosecond self-pumped phase-conjugate pulses are generated from all crystallographic faces of BaTiO3 in immersion oil. We measure the duration of the pulses after propagation and SPPC by using electric field cross correlation. The duration of SPPC increases as the incident angle increases. Because of the partial compensation of the negative angular dispersion, the self-pumped phase-conjugate pulse undergoes a total dispersion smaller than that of the transmitted pulse. By managing the angular dispersion of the femtosecond phase conjugator, we can control the total dispersion and thus the duration of the self-pumped phase-conjugate pulses. The experimental results agree with the theoretical simulations.

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.

PHOTOREFRACTIVE PROPERTIES OF POTASSIUM LITHIUM NIOBATE CRYSTALS

We report the photorefractive properties of K3Li2−xNb5+xO15+2x crystals grown by the edge-defined, film-fed technique. The crystals with lithium/niobate ratio from 18.5/51.5 to 26/44 are grown and investigated by two-beam coupling technique. The crystal of composition K3Li1.783Nb5.687O16.61 exhibits an intensity coupling constant of 8 cm−1 and an effective trap density of 1.8×1022 m−3. The photorefractive majority carriers are electrons in as-grown crystals. The grating recording time and erasure time are 530 and 260 ms, respectively, at an intensity of 2 W/cm2.

Stabilization of two diode laser wavelengths using photorefractive gratings

We propose to demonstrate a method of stabilizing the wavelengths of two diode lasers through the use of two photorefractive volume gratings. The two photorefractive gratings recorded at a visible line from an argon ion laser in one LiNbO3 crystal are designed to be Bragg matched for the counterpropagating reflections at infrared wavelengths. The two diode lasers are stabilized at the center wavelengths of the counterpropagating reflections of the two gratings. The stabilized wavelengths and the synthetic wavelength exhibit good short- and long-term stabilities. By using photorefractive crystal, we can effectively control the separation of the stabilized wavelengths. The thermal stability of the photorefractive gratings and the thermal stability of the synthetic wavelength are discussed.

Dispersion compensation for a femtosecond self-pumped phase conjugator.

Dispersion compensation for a photorefractive self-pumped phase conjugator with femtosecond pulses is analyzed. The self-pumped phase conjugator consists of a pair of dynamic gratings coupled by total internal reflections at the crystal surfaces (cat conjugator). The negative angular dispersions of refraction at the air-crystal interface and the gratings inside the crystal compensate for the positive dispersion of the finite crystal path. The experimental results show that with partial dispersion compensation the width of the self-pumped phase conjugation at 450 nm of femtosecond pulses is narrower than that of the transmitted pulses.

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.

HIGHLY EFFICIENT SELF-PUMPED PHASE-CONJUGATION IN BARIUM-TITANATE CRYSTALS

We report a highly efficient self-pumped phase conjugation (SPPC) at near-infrared wavelengths with an undoped BaTiO3. A 80% SPPC reflectivity at 741 nm was achieved. We made a detailed investigation on the buildup process of the phase-conjugate signal. A possible mechanism for this highly efficient SPPC mirror is also discussed.