Pickup suppression in sagnac-based fiber-optic acoustic sensor array

Large-scale acoustic fiber sensor arrays consisting of hundreds of hydrophones distributed along kilometers-long fiber buses are required for applications such as undersea oil exploration. Sagnac-based sensor arrays (SSAs) exhibit attractive performance; however, the acoustic wave incident on the buses generates a pickup signal that can swamp the signals from the hydrophones. A simple technique for reducing this unwanted pickup is proposed, modeled, and demonstrated by periodically inserting deaf hydrophones in the buses instead of hydrophones so the pickup signal is measured at different locations along the buses. The first method directly subtracts the deaf hydrophone signal from the signal seen by the adjacent hydrophone (true signal and pickup) to recover the true hydrophone signal. This method is limited to small signal amplitudes by the nonlinearity of the Sagnac interferometer. The second method corrects the nonlinearity before pickup subtraction and allows in principle a full suppression of the pickup. When applied to an experimental two-rung SSA, this technique produced a -18.6-dB pickup suppression for pickup amplitudes as large as 0.44 rad and signal amplitudes of up to 0.44 rad and -15 dB for a signal as large as 0.88 rad. These values are limited by the accuracy of the 8-bit data acquisition and/or electronic noise. With a low-noise 12-bit data acquisition, the pickup suppression for small signal amplitudes is predicted to be -35 dB. This paper makes headway toward practical SSAs.