Coherent oscillation by self-induced gratings in the photorefractive crystal BaTi03

The possibility of phase conjugate mirrors (PCM's) with reflectivities exceeding unity was predicted by Yariv and Pepper, 1 and subsequently observed by Bloom, Liao, and Economou, and by Pepper, Fekete, and Yariv. 3 The properties of resonators with PCM's have been studied experimentally and theoretically. Early work employed high intensity Q switched lasers as the pump beams. Because of formal similarities of four-wave mixing in third-order optically nonlinear media, and in real-time holographic media/' the considerations of these early works may be carried over to the analysis at PCM's based on real-time holography in photorefractive media. For example, in a recent experiment with BaTi03 as the holographic photorefractive medium, Feinberg and Hellwarth 7 demonstrated a PCM with reflectivity exceeding unity using a cw argon laser at 514.5 nm for the pump beams. They used this mirror in conjunction with an ordinary mirror to demonstrate a cw oscillator of the type shown in Fig. l(a) and to examine its behavior with intracavity spatial filters and phase distorters. In this letter, we report the demonstration of several new oscillator configurations involving BaTi03 : (i) an oscillator using two conventional mirrors in which the gain is provided by two pumping beams [Fig. l(b)]; (ii) an oscillator pumped by a single beam using two conventional mirrors [Fig. l(c)]; (iii) a unidirectional ring oscillator pumped by a single beam [Fig. 1(d)]; (iv) a ring oscillator pumped with two counter-propagating beams [Fig. l(e)]. We pumped these oscillators, as well as the one shown in Fig. l(a) with a low-power ( -1 mW) He-Ne laser operating at 632.8 nm. This is the first demonstration of cw BaTi03 oscillators with the He-Ne laser. This new oscillator technology has thus been brought within the reach of even modestly equipped laboratories. The above oscillators do not operate by the simulated emission of radiation. Small signals experience gain when one or both pumps are diverted in the proper direction by Bragg reflection from self-induced index gratings as explained below. In the above oscillators, the amplified signal is fed back by one or more mirrors in the form of a resonator. Our resonators were constructed with plane mirrors so that they were unstable to diffraction loss, except for any stabilization due to the phase-conjugating properties of the crystal. Experiments are currently in progress to examine the effect of resonator stabilization using spherical mirrors. We tried one resonator [Fig. 1(f)] in which the feedback was via a Bragg reflection inside the crystal, as in DFB lasers. No oscillation has been observed yet. The crystal ofBaTi03 measured 7 X 4.5 X 4 mm. KIt was poled into a single domain so that the c axis was parallel to the 4 mm side. In order to make use of the large electro-optic coefficient r42 = 820X 1012 mv\ 9 the crystal was oriented so that the wave vectors of the holographic gratings were not parallel to any crystal axis. The polarization of all beams was extraordinary, and they were made to travel predominantly in the direction of the c axis. The polarity of the c axis determines the direction in which power is transferred in two beam coupling experiments. 10 The four beams shown in Fig. 2 enter and leave the crystal through the two parallel crystal faces which measure 7X4.5 mm. Coupling is also possible when the beams 1 and 4 enter different faces, although the interaction region may be reduced. In each resonator pump A 1 is delivered directly from the laser and has a diameter of approximately 3 mm. Table I shows the steady-state intensities of the oscillation beams A.~ and A 4 as they left the cuvette, and the time taken to reach steady state. They were measured either directly or through 1% transmitting mirrors. In each case, the angle a was 168° and,Bwas 160°.