Self-pumped phase conjugation is generally believed to be attributed to degenerate four-wave mixing. Accordingly, the phenomenon requires the existence of three waves at certain interaction regions for the excitation of the fourth one. Although this condition is not satisfied by a train of femtosecond pulses with a repetition rate of 110 MHz, we have observed self-pumped conjugate pulses retroref lected by a BaTiO(3) crystal. The conjugate character of the pulses is demonstrated with the distorter method.

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Using the beam Lanning effect in a BaTi03 crystal, we propose two simple set-ups to perform the function of an optical image combiner and an optical AND gate respectively. The optical image combiner combines two mutually coherent as well as mutually incoherent patterns transmitted to it separately into a single coherent pattern. This set.up can also be used as an optical OR gate. For the AND gate, the two input signals may also be mutually coherent or incoherent. Experimental results showed the response time of these two devices to range from 0.01 second to 2.5 seconds.Key words : optical image combiner, optical OR gate, beam fanning effect, photorefractive crystal IntroductronSince the discovery of photorefractive effect, several 10gic operations have been accomplished by its use. Photorefractive materials are used for making these devices because the effect responsible for the desired Lunctions is fast and it does not require large incident power.Image combiners have also been proposed by several groups.1'2) However, if the input patterns to be combined are mutually incoherent, the resultant combined pattern would not be coherent in general, and thus it is often not possible to treat such cases by the well-developed coherent optical techniques in the subsequent stages of the system.Making use of the fanning phenomenon in BaTi03 crys' tals, we propose here a simple set-up which combines two mutually coherent or incoherent patterns into a single coherent pattern. The device can also function as an optical OR gate. By making use of the sarne phenomenon, color converter and optical AND gate can be fabricated. The former has been discussed elsewhere.3,4) A simple set-up performing the latter function is proposed in this paper. The two input signal beams in this set-up can also be mutually coherent as well as incoherent. Principle of OperationWhen a beam of coherent light penetrates into a piece of photoreLractive crystal, some portion will be scattered by the defects and impurities contained in the crystal, thus producing scattered waves. These scattered waves interfere with the original incident beam locally and lead to the formation of gratings, usually called noise gratings. These gratings can transfer the energy in the original incident waves to the scattered waves, thus amplifying the latter and depleting the energy in the incident beam if the process of writing and reading these noise gratings continues. Since the propagation directions of the scattered waves are in general not too different from the original incident wave and since the two kinds of waves cannot be differentiated from each other, the incident beam phenomenologically fans out. This phenomenon is called "beam fanning" in photorefractive optics. The energy of the incident beam can be transferred to the scattered waves almost completeley if the incident beam is an extraordinary ray and the grating vector of the noise grating is about 45 degrees from the c axis of a high-gain BaTi03 crystal.5,6) Under these conditions, strong beam fanning results. ...