Feedback-induced polarization-switching in the Nd-doped fibre-laser

Abstract

The use of coherent optical sources as pumps in solidstate lasers has led to the experimental feasibility of long-fiber lasers, permitting the study of coherent effects on increased time scales. Rare-earth-doped silica-fiber lasers reveal complex static as well as dynamical behaviors. For example, the single polarization Er-doped fiber laser spontaneously operates in a self-pulsing regime as a result of the presence of interacting ion pairs in the medium.' On the other hand, polarization effects are unavoidable for anisotropic2 as well as for quasi-isotropic cavities when no dichroic element is inserted in the experimental setup. These polarization effects were reported in the Nd3 -doped fiber laser for their pumpdependent static properties3 and also in the context of antiphase dynamics.4 In these examples the fiber output beam usually contains two linearly polarized components. As the pump input power is increased, the lower-threshold mode switches on first, followed by the second polarization eigenmode. The relative thresholds of these polarization modes can be adjusted with the orientation of the pump beam polarization.3 At high pump levels the intensity of both modes increases linearly with pump input power in such a way that the intensity of the lowerthreshold mode always exceeds that of the second mode, and we refer to these polarization states as strong and weak modes, respectively. On the other hand, optical feedback was proved to change drastically the static and dynamical properties of laser systems.5 In our study it is used to control the distribution of the total intensity among the eigenmodes. An interesting case is observed when the ratio of the strong-to-weak-mode thresholds is maximized. The purpose of this Letter is to demonstrate that this configuration gives clear evidence of alternate switching between the strong and weak modes when the external feedback loop is suitably adjusted. When the laser is operated between the two thresholds, the weak mode develops at the expense of the strong mode, which is completely washed out by the feedback signal. The experimental setup is schematically represented in Fig. 1. The pump is a laser diode with a collimated output beam at AP = 0.810 juam and an available output power as high as 150 mW. A half-wave plate at AP allows us to rotate the pump polarization. The laser diode optically pumps a 10-mlong silica fiber doped with 500 parts in 106 Nd3 ' by weight. The fiber core diameter is Ic = 2.7 snm, and the cutoff wavelength is A, = 0.86 j.m. The laser oscillation at A = 1.08 /-tm is obtained between mirrors Ml (R= 100% at 1.08 ,-m and T = 80% at Ap) and M 2 (R2 = 80% at 1.08 fum). Index-matching oil suppresses any parasite cavity between the fiber output end and mirror M2. The pump input current at the fiber laser threshold is typically -35-40 mA (corresponding approximately to 23-26-mW input power, measured before mirror Ml). The beam splitter inside the feedback setup is a 6-,tm-thick pellicle oriented at a 10° tilt with respect to the output axis so as to decrease its action as a polarization-selective element. A polarizer is inserted in the feedback loop and adjusted along the Y axis. A half-wave plate at 1.08,um permits calibrated analysis of the two orthogonal polarization components, which are separated by a Glan polarizing prism. The signals are analyzed with two fast-response (2-GHzbandwidth) Ge detectors, monitored with a digital oscilloscope, and then transferred to a microcomputer. We optimize the feedback efficiency by focusing the output beam onto the external mirror with a collimating lens. The effective feedback rate is estimated by the corresponding shift of the laser threshold.5 Before describing our experimental results we recall the following properties of the isolated laser (i.e., without optical feedback): (i) In spite of the highly longitudinal mode operation, in its transient regime the laser behaves as a two-supermode system. Each mode is polarized along an eigenaxis of the fiber (the strong mode along X and the weak mode along Y), and the propagation axes are fixed by the stress-induced birefringence of the medium. (ii)