Eigen polarization states and related eigen values of a fiber laser are theoretically obtained from the resonance conditions of the laser cavity. Experimental investigations confirmed the theoretical predictions. Using the polarization mode beat signals which are sensitive to the cavity birefringence, novel polarimetric fiber laser sensors are realized. Major theoretical and experiments are summarized as the following.
Light circulating inside the laser cavity must return to its original phase and polarization states after a complete round trip. From the resonance condition, eigen polarization modes and related eigen resonance frequencies are determined in fiber lasers. There exists two such orthogonal polarization modes with different resonance frequencies when the fiber laser consists of a single-mode optical fiber. In a Fabry-Perot cavity without non-reciprocal components, the eigen polarization modes turn out to be linearly polarized at the mirror positions, regardless of the cavity birefringence.
Fiber lasers were constructed by glueing dichroic mirrors at each end of rare-earth doped fibers. $Ar^{+}$ laser (λ=514.5 nm) was used as a pumping source. Slope efficiency and threshold pump power were about 2.9% and 14 mW, respectively, for the Nd-doped fiber laser, and 1.6% and 7 mW, respectively, for the Er-doped fiber laser. Laser signals were detected by fast photodiode after passing through a polarizer and the signals were monitored with an oscilloscope or a rf-spectrum analyzer. Polarization mode beat(PMB) signals appeared between the longitudinal mode beat (LMB) signals. Bandwidth of the PMB signal ($δf_p$) was determined by the product of net-birefringence between the polarization modes and optical spectral linewidth. In a Nd-doped fiber laser, optical spectrum could be tunned by rotating the polarizer inserted in the cavity, due to polarization mode dispersive properties of the laser fiber(made by York corp.). Tunning range varied from nearly zero to a few tens of nanometers depending on the settings of the polarization controller inside the cavity.
Effects of the pump polarization states and bending applied to the elliptic core type polarization maintaining(PM) fiber laser were investigated. The light in a polarization mode parallel to the pump polarization direction achieved more gain than the orthogonal polarization component. Bending in a fiber laser cavity induces more loss for the light whose polarization direction was parallel to the minor axis of the core ellipse of the fiber. By introducing a bend with a small diameter, elliptic core fiber laser could be operated with single polarization whose polarization direction was parallel to the major axis of the core ellipse.
By using the fact that PMB frequencies are sensitive to the net-birefringence of the cavity, polarimetric fiber laser sensors were demonstrated. To achieve a stable and narrow bandwidth PMB signal from the fiber laser sensor, PM fiber laser having small net-birefringence between the eigen polarization modes and narrow optical spectrum linewidth must be used. To minimize the net-birefringence of the PM fiber laser, 90˚ rotated splice was introduced in the middle of the laser fiber. Lateral stress was measured with a slope coefficient of 4.8 kHz/gram using a non-PM Nd-doped fiber laser. Temperature and longitudinal strain were measured with slope coefficients of 30 kHz/℃ㆍcm, 43 kHz/㎛, respectively, using the PM Nd-doped fiber laser, and 124 kHz/℃ㆍcm, 137 kHz/㎛, respectively, using the PM Er-doped fiber laser.
