Traditionally, homodyne and heterodyne detection is the mixing of two signals with different frequencies, followed by a low-pass filter. Mixing two signals of frequencies f_1 and f_2, generates signals of frequencies f_1 + f_2 and f_1-f_2 and their integer multiples. Both multiplicative heterodyne and phase sensitive detection has been demonstrated optically, by using photorefractive four-wave mixing (FWM). The multiplicative characteristic of FWM is used for mixing, and the response time of the photorefractive medium is used for low-pass filtering. If one of the input beams is both spatially and temporally modulated using a controlled oscillating membrane, depending on which mode is heterodyned, one can generates orthogonal sets of Bessel band pass filters. This scheme can be integrated part within parallel data acquisition systems for applications involved nondestructive testing.
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