Polarization properties of a fiber laser with a Faraday rotating mirror (FRM) were investigated theoretically and experimentally. The laser consisted of a FRM, a planar mirror, and erbium-doped fibers.
The eigen states of polarization (SOP) and their eigen values which represent the polarization properties and lasing frequency of the fiber laser were determinied theoretically by applying the laser resonance condition that light should have the same polarization and phase after one round trip in a laser cavity. There exist two orthogonal eigen SOP's with different frequencies at every points of the cavity. The eigen SOP's at the planar mirror position were always circular polarizations independent of intrinsic and extrinsic birefringence of the cavity fiber. However, the eigen SOP's at the FRM position were elliptical in general, depending on the birefringence of the cavity fiber. The frequency spacing between the longitudinal modes for both eigen SOPs were equal, but the frequencies of one eigen SOP were halfway between the frequencies of the other eigen SOP regardless of the reciprocal birefringence in the cavity fiber. However, when a non-reciprocal birefringence was introduced, for example, by applying a magnetic field, the frequency difference between the two eigen SOP's changed as a function of the amount of the induced non-reciprocal birefringence. The frequency difference could be measured by reading polarization mode beat (PMB) frequency resulting from the interference between the two eigen SOPs through a polarizer.
In order to improve the signal-to-noise ratio of the PMB signal, the number of oscillating modes was reduced by using a saturable absorber as a narrowband frequency filter. At the same time, spatial hole burning in gain medium was eliminated by choosing the optimal position for the gain medium. The gain medium was located near the FRM and the eigen SOP's at the FRM were made to be circular by adjusting a polarization controller in the laser cavity. In this case the intensity patterns of the standing waves for the two eigen SOP's were out of phase in the gain medium, so that the spatial hole burning was eliminated.
By using the mode selection technique, we demonstrated a fiber laser oscillating in single longitudinal mode for each eigen SOP. The laser was applied to the fiber laser current or magnetic field sensor. The 10 m-long fiber laser consisted of an Er-doped fiber amplifier, a saturable absorber which was an unpumped Er-doped fiber, a conventional single mode fiber, a Faraday rotating mirror, and a planar mirror. A solenoid was used to produce a uniform axial magnetic field and was placed near the planar mirror. The PMB frequency was measured as a function of the current applied to the solenoid. A linear relationship with a scale factor of 10.24 kHz/A was obtained, which agreed well with the theoretical value of 10.54 kHz/A. To configure the simple fiber laser current sensor, a simple electronic signal processing method using a frequency demodulation technique was applied. In this method, the change of PMB frequency was converted to electrical voltage. The input waveform of sinusoidal current could be reproduced and the linear relationship between input current and the processed output signal was successfully demonstrated.