Sagnac interferometer including a recirculating ring with an erbium-doped fibre amplifier

Abstract

The use of a Sagnac interferometer as an acoustic sensor has recently been demonstrated [1]. One reason for using a Sagnac interferometer is that the path imbalance is zero, such that no interferometric conversion of the source phase noise to intensity noise occurs. The low frequency response of a Sagnac interferometer increases with increasing loop length, and a very long length of fibre is needed to achieve high responsivity for acoustical frequencies. In this paper we present a scheme where we enhance the low frequency response using a recirculating ring within the Sagnac loop, as shown in fig. 1, thereby increasing the effective length of the Sagnac loop. To increase the number of recirculations an erbium-doped fibre amplifier is incorporated in the ring to compensate for both the coupling and the intrinsic losses of the ring. We also present the results of a noise analysis of a recirculating ring including a fibre amplifier, and show that the dominating noise in our sensor is the beat noise between the signal and the spontaneous emission produced by the fibre amplifier.

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