Volume holographic MACH correlator

Volume holographic correlators offer the ability to encode and compare thousands of templates in one operation. Angle multiplexing of each individual template means the position of the correlation spot in the output plane corresponds to the matching template. To be useful as a correlator the shift invariance must be restored by scanning the input image. This can be achieved by implementing the input signal modulation on a high speed SLM such as a MQW or DLP that is capable speeds in excess of 30kHz. The output correlation peak is read out using a high-speed linear CCD camera. The Bragg angle affects the number of templates that can be held on the hologram. However, this is not the same in both directions and this changes the correlator's shift invariance ability in different scan directions. In this paper we investigate this and how it affects the correlator's performance. This arrangement allows thousands of templates to be searched at video rate. The scanning nature allows space domain correlation to be implemented. The system we describe offers the ability to pre-filter the signal. We report on the results of a MACH filter implemented in a volume holographic correlator. The scanning window allows some interesting pre-filtering to be performed, such normalisation and non-linear optimisation.