Optimization of reflectivity, rise time, and fidelity of a self-pumped phase conjugate BaTiO 3 :Rh ring mirror at 1.06 μm

Abstract

It is now well known that rhodium doped barium titanate (BaTiO3:Rh) exhibits a significant photorefractive response at near infrared wavelengths .We studied and characterized this crystal at 1 .06 jtm by two-wave mixing experiments. In a 45°-cut crystal with a low absorption (0.1 cm1), we measured a photorefractive F of 23 cm1 with cw illumination and 16.6 cm1 with nanosecond illumination. Using spectroscopic determinations of the photorefractive sites 2(Rh3+, Rh4+, RhS+), we demonstrated that the photorefractive properties of BaTiO3:Rh are well described by a three charge state model '. Internal parameters of the material were derived using these experimental characterizations which allowed to accurately predict its performances at 1 .06 tm. Comparative characterizations of several BaTiO3 :Rh samples proved that this material is now well reproducible. Reproducibility, high photorefractive gain, low absorption and accurate theoretical description make BaTiO3:Rh a good candidate for realization and optimization of non linear functions like optical phase conjugation. The application we are interested in, is the dynamic wavefront correction of nanosecond Nd:YAG master-oscillator power amplifier (MOPA) laser sources. We implemented a ring self-pumped phase conjugate mirror using a BaTiO3:Rh crystal . This geometry brings several advantages. The threshold in terms of gain X interaction product, is low (F1=2) 6 This phase conjugate mirror does not require a source of long coherence length 7and the gratings involved in the four wave mixing process can be well controled. Moreover the phase conjugate beam can be efficiently selected among the backscattered light by inserting optical elements in the ring 8,9• Such a self-pumped mirror is self-starting and the four-wave mixing process is initiated by the beam-fanning. To avoid spurious internal oscillations in total reflection on the crystal faces that initially developed in several of our experiments and prevented phase conjugation we optimized the geometry of the crystal. Roof-cut, 45° orientation of the c axis and antireflection coatings of the crystal suppressed these internal oscillations and efficient phase conjugation was demonstrated.