Optical Crosstalk and Performance in Time Division Multiplexed Derivative Sensing Techniques

In many time-domain multiplexing (TDM) schemes, the demodulation process can fail at relatively high modulation frequencies, often referred to as signal over-scaling. A novel frequency modulated (FM-TDM) derivative sensing scheme has been developed to tackle this problem. The derivative scheme produces simultaneously two types of measurements: (a) a principal phase measurement, corresponding to the phase of the sensor due to a change in optical path length; and (b) a differential measurement of the principal phase, this being the difference of the phase at two times, τ apart. If signal over-scaling occurs, the differential signal is integrated to recover the primary signal. In this way, over 180 dB of dynamic range can be achieved for frequencies less than 100 Hz. The behaviour of the derivative scheme can be understood in terms of a two-point difference operator - which is directly proportional to frequency and shown to be the basis of optical crosstalk in the measurement of the principal phase. A method of crosstalk reduction was demonstrated for the principal phase measurement. However, crosstalk is independent of frequency for the differential measurements. The benefits of the scheme are discussed, including the capacity to recover an over-scaled signal with a consequent increase in dynamic range.