Jaime Frejlich

Adjustable phase control in stabilized interferometry.

We report an optoelectronic feedback loop that permits the active stabilization of an interferometric setup for any chosen value of the phase between the interfering beams. This method is based on phase modulation and homodyne detection techniques. The phase can be stabilized with a precision of better than 1 deg for our experimental conditions.

Analysis of an active stabilization system for a holographic setup

An active stabilization system for a holographic setup based on detection of phase shift between the interference pattern and a reference hologram is described. Its basic feature is the possibility of operating for 0, pi or +/-pi/2 at will, always in a null-detection mode. The reference hologram may be a previously recorded permanent hologram or a real-time (even reversible) one. The use of the open loop response of the stabilization system is developed for analyzing its performance, which allows closer insight into parameters limiting Its behavior. The effects of different noise sources are analyzed in detail. The real-time effect in a positive resist is successfully employed for operating the stabilization setup for recording an improved grating in this material.

Interference-term real-time measurement for self-stabilized two-wave mixing in photorefractive crystals

We report the real-time direct interference-term measurement for a two-wave-mixing experiment in photorefractive crystals. Knowledge of the interference term may provide information concerning diffraction efficiency, interference pattern-to-recorded hologram phase shift, and optical activity and anisotropic diffraction properties of these materials. This method comprises phase modulation of one of the interfering beams and synchronous detection of the first and second harmonics in the resulting output irradiance modulation. Simultaneous detection of both harmonics enables the measurement to be made even in strongly perturbed conditions, since one harmonic is used for measuring and the other is used for operating Bi(12)TiO(20) are reported.

Photorefractive crystals for the stabilization of the holographic setup.

A photorefractive crystal of the sillenite family in a two-wave mixing experiment is used as a fundamental component in a negative feedback system for stabilizing a holographic setup. The behavior of such a stabilization system is theoretically analyzed, and the advantages and limitations of using a real-time nonpermanent recording material (photorefractive crystal) are discussed. We present experimental results using a Bi12TiO20 crystal, compare actual and predicted performances, and discuss the optimization of relevant parameters for better performance of the whole stabilizing system. Theoretical analysis and experimental results show this system to have interesting perspectives for further development.

Phase‐shift measurement in photorefractive holographic recording

A simple direct method for measuring the interference pattern of a light‐to‐recorded hologram phase shift in photorefractive crystals is reported. The method is used for studying the recording in Bi12SiO20 under an externally applied electric field. The results are then compared with theory and used for computing the density of photoelectron traps.

Dark conductivity, photoconductivity, and light-induced absorption in photorefractive sillenite crystals

We measure the bulk and light-induced absorption as well as the dark- and photoconductivity in doped and undoped photorefractive Bi12TiO20 crystals and compare the results obtained using different techniques. From these data we compute the quantum efficiency for the photogeneration of charge carriers and the carriers mobility–lifetime product, and characterize the suitability of these crystals for holographic recording. A two-center model theory is shown to adequately describe absorption and photoconduction in these materials.

Self-stabilized holographic recording in LiNbO3:Fe crystals

Abstract We describe the self-stabilized holographic recording in Fe-doped LiNbO 3 crystals reaching almost 100% diffraction efficiency using weak interfering beams of 514.5 nm wavelength, even for relatively low light pattern modulation values. The evolution of the recording process is shown to be adequately described by a simple mathematical formulation based on Kogelniks coupled-wave theory in spite of the dynamic nature of the recording process. Crystal parameters computed from this experiment are compared to the available data from literature.

Stabilized photorefractive modulation recording beyond 100% diffraction efficiency in LiNbO 3 :Fe crystals

A small photochromic grating is selectively and continuously detected from a much larger photorefractive modulation in Fe-doped LiNbO3 crystals in a two-wave mixing experiment. The recording setup is actively fixed to either the photochromic or the photorefractive grating. In the former case a photorefractive modulation largely exceeding the value required for 100% diffraction efficiency is achieved. In the latter case this modulation is limited to 100%. Experimental results are in good agreement with the theoretical model.

Transport length, quantum efficiency, and trap density measurement in Bi12SiO20

We report the measurement of diffusion transport length LD, quantum efficiency φ for photoelectron generation, and the Debye screening length and associated trap density NA in an undoped Bi12SiO20 (BSO) photorefractive crystal. The method is based on the optical erasing of holographically recorded gratings in the crystal. The measurement of the diffraction efficiency evolution is considerably facilitated profitting from polarization properties of anisotropic diffraction in BSO‐type crystals in a self‐diffraction experiment. Data processing is carried out using simple linear regression techniques for experimental conditions in two limiting situations: far from and well into trap saturation conditions. We prove that optical erasure closely verifies the decay law predicted by theory even if a highly contrasted pattern of light is used for recording. We measured NA=6.3×1016 cm−3, agreeing in order of magnitude, with already reported values, but found out LD=0.1 μm, which is too much lower than data in literat...

Selective two-wave mixing in photorefractive crystals.

A new technique that permits the selective steady-state continuous detection of phase (photorefractive) or amplitude (photochromic) effects in two-wave mixing experiments is described. We show the capabilities of this method for the selective stabilized recording of any one of both types of grating that is chosen. Experimental results from holographic recording in Bi(12)TiO(20) are reported.