COHERENT LIGHT AMPLIFICATION AND OPTICAL PHASE CONJUGATION WITH PHOTOREFRACTIVE MATERIALS

The electric field and fringe spacing dependence of the stationary energy transfer between two writing beams in volume phase -hologram formation with photoconductive electro -optic materials (KNb03:Fe2+ Bi12(Si,Ge)020,. ) has been studied. Experimental results are compared with the theoretical expressions describing the influence of the different photoinduced space charge fields in photorefractive media. A peak exponential gain of r= 15 cm -1 has been reached in KNb03:Fe2+ for the proper choice of experimental parameters such as recording wavelength, fringe spacing, applied electric field etc.). Results of the electric field dependence of the wavefront reflectivity p in degenerate four -wave mixing experiments are reported and discussed in terms of a similar theoretical treatment as the two -wave mixing experiments. It is shown, that peak reflectivities of P= 25 percent have been reached for KNb03:Fe2+, that conjugate complex wavefronts can be generated and that distorted optical wave -fronts can be effectively corrected by four -wave mixing experiments in KNb03:Fe2 +. Introduction Photoinduced refractive index changes in electro -optic materials can be used for recording phase holograms in the volume of the photorefractive crystals 1. The recording of such thick volume holograms permits the interference of an incident light beam with its own diffracted beam inside the recording material. This effect causes the continuous recording of a new grating that may add to or substract from the initial grating,that is not uniform through the thickness of the material, and which can be phase shifted to the initial grating 2. The energy redistribution between writing beams primarily depends on the phase shift between the fringe pattern and the recorded grating. Stationary energy transfer has been shown to be forbidden if there is a phase shift of zero or 7 between the interference pattern and the recorded hologram, due to destructive interference caused by the 5/2 phase shift of the Bragg diffracted beam with respect to the recording beam travelling in the same direction. A maximum energy transfer is therefore obtained, if there is an additional phase shift of IT/2 between the undiffracted signal beam and the diffracted reference beam. This is the case for a stationary grating phase shift of 0/2 with respect to the intensity distribution 2. Such a grating phase shift can be achieved, if diffusion is the dominant recording mechanism or if the photoconductive transport length LE is comparable or larger than the fringe spacing A (violation of quasineutrality) . The energy transfer between interacting beams within the photorefractive material can be used for the amplification of coherent light beams, including amplification of beams carrying optical information and for other applications in optical information processing. The first part of this paper deals with the light amplification in materials where quasineutrality can be violated at moderate applied electric field strength. Several interesting applications of nonlinear optical materials ford the restoration of phase distorted images (adaptive optics) have been proposed recently . It will be shown in this paper, that a series of these applications can be performed better by degenerate four -wave mixing (FWM) with photorefractive materials as the nonlinear element. Whereas the optical nonlinearity in nonlinear optical materials is due to the "instantaneous" electronic response, in electrooptic +materials it is due to both the electronic and lattice polarizability due to photoinduced spacecharge fields, the latter being non -instantaneous. The advantage of the electro -optic technique is that it needs much less optical power than in materials with electronic response only,and thus it can be used efficiently with continuous lasers too. For the experiments described in this paper we used a reduced KNb03:Fe crystal. The optimum crystal orientation as well as other relevant materials parameters and procedures for reducing the Fe -doped crystals have been described in a series of earlier papers 3,9,10 Reduced KNb03 as a ferroelectric material with photoconductivity comparable with or larger than that in Bi12Si020 and Bi12GeO2U 9' 10 offers many advantages compared with the other more commonly used photoconductors Bi12Si020. First, the rather large value of the spontaneous polarization gives large electrooptical effects leading to large steady -state photoinduced refractive -index changes. Second, the photoconductivity is high enough that even at wavelengths near 600 nm these steady -state refractive index changes are reached in less than a second at moderate optical power densities, i.e. 2 orders of magnitude less than in Bi12Ge020. Whereas beam coupling in Bil2Ge020 and Bi12Si020 is substantially reduced because of the large optical activity of these materials, the optical polarization remains 230 / SPIE Vol. 369 Max Born i a tical pha j ti t refr ti i l . (li> i , i r l I tit t f c ology H onggerberg, -8093 Zurich, Switzerl st energy transfer between i volu phase photoconduc tive electro-optic mat( C>3: 2 , ± ( s ' ) ° 2 ' * ^ s res are compared expres describing the influence of the di ferent photoinduced spacege ds . T 15 cm" 3:Fe r as cording wavelength, ge cing, . ) . f d endence of the P -wave mixing experiments re and discussed s treat as the two-wave g s. It is shown, t ak ectivities of 25 percent have been reached for C^r * and that distorted optical wave can be effectively -wave mixing C^rFe . inde changes in electro-optic for recording e ms ls . e ms it own e . us recording g at ay d o r a . hat is not uniform 9 , t ini grating . sh betw the sho to f ere s ase IT ttern and the m, by the TT/ di same direction. e i se shift TT / '2 between ted al . s case TT/ t i nsity distribution . t n e ved, hanism or ive t the fringe ality ) . i the o light beams, including s ions in ing. lification in ic field strength. i optical materials for e of hase ges ve ics) y i t th by degenerate -wave (FWM e as the optical i t t "in " electronic response, optic ̂ materials it is due to both the electronic and lattice polar izability space-charge lds, non-instantaneous. The advantage -optic technique is that it needs uch s power than in materials onic nse ly, with continuous a uced KNb0 3 :Fe o al materials parameters and -doped crystals have been described n a s of earlier papers ' ' onductivity comparable n s ° l? ^2f c wi th r e s ^ ^Ot ' ttie rather t lar electro-optical ects leading to steady index changes. cond, e ty is high n n the steady-state ctive i s n , i.e. le th Q i B ]_ G 0 a B 1 s °2 is substantially o t th optical r