Photorefractive materials are extremely non linear and exhibit time constants down to the millisecond. They are mainly used for recording of dynamic holograms, two wave mixing, beam amplification ...
We demonstrate that photorefraction may apply to natural non coherent light beams, for instance light emitted by thermal sources, and can be used to modify or restore the beam phase. We suppose that the phase distribution of the incoming incoherent beam can be measured by a wave-front analyser, and therefore is known. The demonstration is based on the Kukhtarev set of equations and uses the Fresnel ellipsoïd method.
The task is to monitor the photorefractive index and restore the desired optical path distribution in order to correct the phase map. This is done using an auxilliary laser beam used as a writing source. The two beams must have strictly the same path in the cristal. We first demonstrate, using the Kukhtarev set of equations, that it is possible to record the desired phase field within the crystal by properly controlling the intensity of the laser beam. The second condition is due to the propagation in the crystal. It is well known that light can propagate in a cristal for only two orthogonally polarised modes. On the other hand, a natural non polarised light beam can be projected on any couple of orthogonal polarisations. Using the Fresnel ellipsoïd method, we demonstrate that there exists a crystal orientation that allows the same optical path distribution for the two polarised modes.
Instrumentation draft: Due to the nature of photorefraction itself, the diameter of the beams in the photorefactive crystal must be as small as possible. A high aperture number optical system must be used to obtain the desired aperture diameters. The result is an extremly compact optical system. The writing beam intensity is monitored by a spatial attenuation modulator. The Kukhtarev equations show that a writing beam with a strong mean intensity is needed for optimal effect: ie intensity modulation must be small. The result is that very simple intensity modulator as a liquid crystal display can be used.
Applications: A photorefractive phase reconstructor can replace conventionnal Adaptive Optics using deformable mirrors. The gains expected from the photorefractive effect, compared to the discrete actuators of conventional adaptive mirrors, are: system simplification, compactness of the optical system, and costs reduction.
Moreover the use of LCD modulators greatly simplifies the control system, compared to the high voltage amplifiers needed by conventionnal adaptive mirrors. This allows a huge number of sampling points to be controlled (we can imagine up to 1 million samples in the aperture).
Domains of application can be: replacement of conventionnal adaptive optics, especially in applications for astronomy, residual aberrations correction for multiple lens optical systems: application in microlithography.
Keywords Photorefraction, adaptive optics, multiconjugate mirrors, beam control, real time image restoration.