Phase-conjugation using a photorefractive medium makes it possible to build optical resonator configurations. The medium is working as a light amplifier (via TWM) and in the case described below as a back-coupling element too.

We arranged a BaTiO₃-crystal in the setup shown in figure 1.

Figure 1: Arrangement realizing the back coupling of a wave.

The medium used shows a very strong beam fanning effect, that means that an important part of the pump-wave is scattered into a special angular range out of the direction of the pump-wave.

The creation of a loop-shaped beam course (via the two mirrors) makes it possible that the wave fanned in the crystal (fanout) and the pump-wave rotate clockwise and anticlock-wise, respectively, and thereby the waves travel the same distance. Therefore, they can interfere at their re-entry into the crystal and induce phase-gratings. The pump-wave is diffracted at these gratings.

The two-wave mixing process proceeding as a result and the back coupling by the loop cause a discrimination process that leads to a fanning that is directed into a smaller and smaller angular range.

A wave is created that is phase-conjugated with respect to the pump-wave after it has traveled the loop once. This wave can be verified using a semi-permeable mirror like shown in the figure.

In fact we could demonstrate this phase-conjugated wave for a wavelength of 514 nm as well as for one of 633 nm. The photorefractive medium was a BaTiO₃-crystal with 45°-cut.

Figure 2 shows the increase with time of the intensity of the phase-conjugated wave for both wavelengths. One can see that the increase is much faster for l=514 nm, but both curves have the same form.

This form of the measured curves reminds of functions describing the onset dynamic of a self-pumped phase conjugating mirror like the cat-SPPCM or the ESPPCM [1]. This is an indication of the fact that the discrimination process aforementioned is a self-organizing process. Such processes are investigated with growing interest for some years.

Figure 2 : Intensity of the phase-conjugated wave (output) as a function of time, with an Ar⁺-Laser (l = 514 nm) and a HeNe-Laser (l = 633 nm). The intensity for both wavelengths is

10 mW/mm².

However, the setup as a whole corresponds to a Sagnac-interferometer and represents a ring-resonator. Such resonators with BaTiO₃-crystals are known from literature [2,3], but, to our knowledge, have not been realized using a HeNe-Laser up to now.

Acknowledgement

This research has been partially supported by the Deutsche Forschungsgemeinschaft (DFG) within the Innovationskolleg "Optische Informationstechnik" (INK 1/A1) at the Friedrich-Schiller-Universität Jena.

References

[1] Esselbach, M., diploma thesis, FSU Jena, 1995

[2] Anderson, D. Z., and Erie, M. C., 1987,

Opt. Eng., 26, 434

[3] Klumb, H., Herden, A., Kobialka, T., Laeri, F.,

Tschudi, T., and Albers, J., 1988,

J. opt. Soc. Am. B, 5, 2379