Limitations in acousto-optic FM demodulators

The ability of an acousto-optic cell to deflect a light beam through an angle proportional to the frequency applied to its transducer has been exploited to construct a simple yet effective FM demodulator. By measuring the position of the deflected beam with some form of detector assembly, the deviation from a given centre frequency can be obtained. The performance of the system is analysed and expressions for the system linearity, bandwidth and minimum frequency resolution are obtained. Two different types of detector assembly are considered: a single detector with a knife-edge spatial filter and a bicell detector. The performance of the bicell system is found to be superior by virtue of its ability to reduce/cancel the effects of amplitude modulation on the signal and variations in laser intensity. It is shown that systems can be designed to operate at centre frequencies from tens of MHz to a few GHz, with maximum frequency deviations and modulation frequencies from a few kHz to tens of MHz and with the ability to detect minimum frequency deviations down to tens of Hz. Experimental results are presented to demonstrate the validity of these expressions. It is concluded that an acousto-optic system may be used to measure the FM characteristics of a signal. There is also potential for applications in other areas where fine measurement of frequency deviation is required, such as Doppler analysis