EDF sigma laser using double-pass cascaded-chirped long-period fiber grating with birefringence compensation effect and its application for strain measurement

In this work, we have constructed an Erbium-doped fiber sigma laser (EDF sigma L) using a double-pass cascaded-chirped long-period fiber grating (C-CLPG) from the viewpoint of the sensor applications. The doublepass C-CLPG is composed of a C-CLPG and a Faraday rotator mirror (FRM) and works as a comb-like wavelength selection element as well as a sensor section in the sigma-branch of the laser cavity. Further, due to the FRM reflection scheme, the double-pass C-CLPG compensates for birefringence variation in the sigma-branch during the sensing. In the experiment, the sensor section near the C-CLPG is intentionally deformed to induce birefringence variation assuming the external disturbances; nevertheless, it is confirmed that the double-pass C-CLPG provides stable polarization states. Then, under the birefringence variation, it is demonstrated that the proposed EDF sigma L can achieve a stable oscillation and resultant precise measurements in contrast to the simple EDF ring laser using the single-pass C-CLPG. In addition, for the demonstration of the sensor with external force to the fiber, the oscillation wavelength of EDF sigma L in response to axial strain is investigated, and a linear response with sensitivity similar to 0.32 pm/ mu epsilon is obtained.