The behaviour of self-pumped phase conjugating mirrors (SPPCM) under the influence of external light is studied. The normal cat-PCM as well as the external self-pumped phase conjugating mirror (ESPPCM) [1] are considered.

The result of the influence of external light consists in the erasing of the SPPCM. Under the erasing of a SPPCM the destruction of the refractive index grating structures inside the crystal, which are responsible for the phase conjugating reflection, shall be understood [2]. As a result the reflectivity of the PCM may go down to zero. Especially for the ESPPCM the erasing by interruption of the loop that builds up between the photorefractive crystal and the external mirror is investigated too.

First, the erasing by white light is investigated. Therefore, the crystal is illuminated by white light perpendiculary to the plane of incidence of the signal wave. The development in time of the reflectivity is recorded. The results are shown in Fig.1.

The reflectivity of the cat-PCM goes down to zero within some seconds, whereas the ESPPCM shows only an unimportant decrease of its reflectivity, approximately 10...20 %. This is an important advantage of an ESPPCM for applications, because, it allows using it in not darkened rooms and even in daylight, what is not usual for optical arrangements.

Next, the erasing by external Laser light is investigated. Fig.2a shows the setup and Fig.2b the course in time of the reflectivity of an ESPPCM during erasing.

There are no principal differences between the behaviour of a cat-PCM and an ESPPCM, except the erase times t, that means the times within which the reflectivity goes down to 5 % of ist initial value, that are much smaller for the cat-PCM. This is an indication of the stability of ESPPCM too.

Now the influence of the intensity of the erase beam on the erase time t is investigated. Fig.3 shows the result. The erase time decreases with increasing intensity of the erase beam. The curves seem to follow a law like t~1/I , at least in a rough approximation, as it could be expected of photorefractive media [3]. The influence of the polarisation of the erase beam on the erase behaviour is investigated too, but there is no effect provable.

As mentioned above, the self-erasing of an ESPPCM shall be studied. Self-erasing means that the loop of an ESPPCM that builds up between the crystal and the external curved mirror is interrupted, and so the signal beam acts as erase beam itself.

The destruction of the grating structures inside the crystal that are responsible for the loop gets obvious owing to the fact that the fanout, which was concentrated in a small angle range before, takes up a larger range again. This results in an abate behaviour of the PCM as shown in Fig. 4.

It is remarkable that the reflectivity does not go down instantaneously but a continual exponential decrease occurs. In order to verify this, a function of the form R~exp(-a×t) (dashed line in Fig.4) is fitted to the measured curve (solid line). A very good correspondence is obviously.

This behaviour requires a new model to ex-plain the phase conjugating reflection process of an ESPPCM.

One possibility would be the following. The fanout builds a grating by interfering with the light reflected at the external mirror. The fanout following later is reflected at this grating (and not at the mirror) and represents the read-out wave for the hologram written by signal wave and fanout (real time holography). In this case the phase conjugating reflection would occur also if the loop is interrupted until the gratings reflecting the fanout fade away by erasing by the signal wave [2].

A second possibility is that the behaviour is based on the retro-reflection grating [1] built by the signal wave and the phase conjugated wave. Such a process can be called "stimulated photorefractive backward scattering" [4] and could explain the behaviour of the ESPPCM as well.

An interesting stability behaviour of the ESPPCM against self-erasing could be noticed at l=633 nm and low intensities of the signal wave (» 1 mW/mm²). If the loop is blocked until the reflectivity reaches zero and then it is opened again, there occurs no normal onset process, as it could be expected, but the reflectivity jumps instantaneously on a certain value, see Fig.5 left, and then it increases slowly to its final value. From this fact it is possible to conclude that there are gratings inside the crystal which can be read out directly by the light reflected at the external mirror and which thus do their share to the entire reflectivity of the PCM. These gratings seem to be more resistant to erasing. For quantitative investigations the rest-reflectivity (in terms of percentage of the maximum reflectivity) in dependence on the loop interruption time is measured. Fig.5 right shows the result.

Because the process is an erase or abate process it is fair to consider that an exponential relation exists. Therefore, a function of the form R_rest/R_max~exp(-a×t) (dashed line in Fig.5) is fitted to the measured values. But, the bad correspondence is obviously. The relation is described better by the function R_rest/R_max~1/t (solid line in Fig.5), but there is no explanation for such a relation.

This long-time memory effect could be used in order to realize a double exposing technique in the hologram-interferometry. An advantage is that the described effect appeares at low signal intensities only, because thus it is possible to use Laser diodes.

However, the influence of external light and the erase behaviour of SPPCM has been studied and some interesting new effects could be demonstrated.

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] H. Rehn and R. Kowarschik: Experimental Investigation of

the External Self-Pumped Phase Conjugate Mirror,

Opt. Commun. 109, (1994)

[2] M. Esselbach: Experimentelle Untersuchungen zum Zeitver-

halten selbstgepumpter phasenkonjugierender Spiegel,

diploma thesis, FSU Jena, 1995

[4] J.E. Ford, Y. Fainman, and S.H. Lee: Time-integrating interferometry using photorefractive fanout, Opt. Lett. 13, No. 10 (1988)

[3] P. Yeh: Introduction to Photorefractive Nonlinear Optics,

John Wiley & Sons, New York (1993)