Response of fiber lasers to an axial magnetic field.

An axial magnetic field induces circular birefringence in optical fibers, resulting in the rotation of the polarization angle for a linearly polarized input (Faraday effect). The rotation angle is linearly proportional to the magnitude of the applied field. Accurate measurements of the polarization angle variation provide a means for monitoring the magnetic field. The Faraday effect in optical fibers has been successfully utilized for the measurement of the electric current that generates the magnetic field. – 5 For some other optical fiber sensors, such as fiber gyroscopes, the Faraday effect should be suppressed because it introduces nonreciprocal phase shift between the counterpropagating optical waves in the fiber. This nonreciprocity is the major feature that distinguishes the Faraday effect from the effect of fiber twist, which also induces circular birefringence. The recent development of fiber laser sensors has opened up new possibilities of active fiber sensors with frequency readout, eliminating complicated electronic signal processing. The polarization characteristics of fiber lasers play key roles for the sensors, and a thorough understanding of them is necessary for further development of such sensors. Previously we analyzed relatively complicated polarization behavior from a fiber laser cavity having a uniform elliptical birefringence induced by twisting a fiber cavity with intrinsic linear birefringence. In this Letter we describe the effects of another important parameter, nonreciprocal birefringence induced by an external magnetic field, on the polarization properties of fiber lasers. Unlike for the case of twist-induced circular birefringence, the Faraday effect makes the eigen states of polarization (SOP’s) elliptical polarizations at the location of the mirrors in the existence of an intrinsic linear birefringence in the fiber. The polarization mode beat (PMB) frequencies vary as a function of the magnetic field strength and can be utilized for magnetic field or current sensing with frequency readout. A theoretical analysis predicts the experimental observation with reasonable agreement. The SOP of the laser output has to satisfy a laser resonance condition in that it has to come back to the same SOP after a complete round trip inside the