Semiconductor Optical Amplifier-Fiber Laser and Its Improvement for Sensor Application

We have developed a novel optical fiber ring laser using a semiconductor optical amplifier (SOA) as the gain medium, and taking advantage of polarization anisotropy of its gain. Since the frequency difference of the bi-directional laser is proportional to the introduced birefringence, the laser in the present study is applicable to a frequency-domain fiber sensor with the birefringent material. For the application the accuracy and stability of the beat signal frequency is important, but it was not sufficient because the polarization of the propagating light was not constant in the single-mode fiber (SMF) which was incorporated into the ring laser cavity. In order to reduce this ambiguity, we have improved the fiber ring laser by using polarization-maintaining fiber (PMF) and a PMF-coupler instead of SMF and a SMF-coupler. The linewidth of the beat signal has been narrowed and the accuracy of the beat signal frequency has been enhanced.