Dual-loop Sagnac interferometer with a geometric phase shifter for quadrature phase bias locking

A stable quadrature phase bias is highly demanded in the balanced Sagnac interferometer to achieve sufficient detection sensitivity and unambiguity. We demonstrate a straightforward and effective quadrature phase locking technique in a free-space balanced Sagnac interferometer by using a dual-loop, one for sensing and the other for bias feedback. The sensing loop and the feedback loop operate on linearly polarized beams at two separate wavelengths and overlap for most of the area. A geometric phase shifter showing wavelength independence placed on the common path of the two loops introduces two almost identical nonreciprocal phase shifts between the counterpropagating beams for the two separate wavelengths, so that real-time compensation of the phase bias for the sensing loop can be implemented by the error signal of the feedback loop. Proof-of-concept experimental results demonstrated successful locking of the quadrature phase bias in the presence of signal fading due to the birefringence disturbance. The correction of the residual chromatism of the interferometer is discussed before the conclusion is made.