Brillouin Random Fiber Laser with Orthogonal Polarization Clamping

Jinxing Ning; Chunhua Wang; Nian Fang; Xiang Gu; Keshuai Wu

doi:10.3788/CJL220839

Abstract

Objective

The Brillouin random fiber laser (BRFL) is a new type based on stimulated Brillouin scattering (SBS) and a randomly distributed feedback resonator. Because SBS enables low-intensity noise, low-phase noise, and narrow-linewidth lasing light, it has significant advantages in random fiber laser construction. However, research on BRFL has been limited to nonpolarization parameters, such as lasing power, intensity phase noise, and line width, rarely to polarization properties. In this study, a BRFL with a polarization-maintaining fiber (PMF) line-cavity (PMF-BRFL), in which lasering light with polarization clamped at either one of the two orthogonal principal axes of the PMF, is proposed and demonstrated based on the nonlinear axial polarization pulling effect of SBS in PMFs. A theoretical model of the PMF-BRFL is established, and the polarization properties of the lasing light related to the pump light and system parameters are analyzed, discussed, and compared with the experimental results, which were in good agreement with each other.

Methods

First, based on the simplified polarization vector-propagation equations of SBS in PMFs, which theoretically indicate the axial polarization-pulling behavior of SBS in PMFs, we derived the analytical expression of the SBS gain in the PMFs, which presents the SBS gain expression to the input SOPs of the pump and signal light and the input pump power. Second, we derived lasing-pump power thresholds for the two polarization modes. We then analyzed the working conditions of these polarization regions. Furthermore, the width of the depolarization range W was analyzed, and its relationship with the pump power and cavity length was discussed.

The PMF-BRFL used a tunable laser to output the pump light with a center wavelength of 1553.73 nm, a polarization state generator (PSG) to generate 100-input SOPs of pump light with a relatively uniform distribution on the Poincaré sphere, an erbium-doped fiber amplifier (EDFA) to vary the input pump power, and a polarization controller to adjust the relative position of ${\hat{p}}_{i n}$ to the principal axis of the PMF before the pump light entering a 3 km PMF fiber line-cavity via a normal single-mode fiber circulator (Cir). In a random cavity, the PMF acts as an SBS gain medium and provides the first and second Rayleigh scattering (RS) in opposite directions for random reflection. The SBS Stokes the light generated in the PMF emitted through Cir. At the PMF-BRFL output, the polarization behaviors, including Stokes parameters and degree of polarization, lasing power, optical spectrum, and linewidth of the lasing light, were measured using a polarization analyzer (PSA), an optical spectrum analyzer (OSA), and an electronic spectrum analyzer (ESA).

Results and Discussions

When the cavity length increases from 1 to 5 km, the lasing threshold of the pump power at zero depolarization decreases from 75 to 29 mW, and the dynamics of the pump power for W<0.1 requirement decreases from 7 to 2.5 mW. For more extended cavities (5-11 km), I_p0th,_W₌₀ decreases slightly, only from 29 to 21 mW, and the dynamics of the pump power for W<0.1 requirement remains almost unchanged around 2.5 mW (Fig. 2). Therefore, the cavity length of the established PMF-BRFL system can achieve an increased pumping efficiency by selecting a PMF of 3-5 km.

For different settings of power emitted from the EDFA, I_{p0_edfa}=40, 50, 55, 80, and 120 mW. As the input SOP of the pump light is scanned in the generated ${\hat{p}}_{i n}^{# 100}$ pattern, almost all the SOPs of the lasing light clamp at either of the $\pm {\hat{β}}_{l}$ positions and show a definite relationship of $s i g n ({\hat{p}}_{i n} ? {\hat{β}}_{l}) {\hat{β}}_{l}$ , except for the cases for W of ${\hat{p}}_{i n} ? {\hat{β}}_{l} \approx 0$ . For lasing lights clamped at $\pm {\hat{β}}_{l}$ polarization, the system works in the full polarization regions, with measured DOP=1. However, for lasing lights with DOP<1, the system operates in the depolarization region, and both $\pm {\hat{β}}_{l}$ modes oscillate simultaneously with different quantities, resulting in different DOPs observed inside the Poincaré sphere (Fig. 5).

W attains the lowest value at I_{p0_edfa}=55 mW, indicating I_p0th,_W₌₀ of the PMF-BRFL with 3 km PMF is near 55 mW. At I_{p0_edfa}=55 mW, depolarization interval W is the narrowest, and the polarization clamping range of the system is the broadest. At I_{p0_edfa}<55 mW, a spontaneous region exists around ${\hat{p}}_{i n} ? {\hat{β}}_{l} \approx 0$ , where PMF-BRFL emits weak and depolarized light. At I_{p0_edfa}>55 mW, the depolarization region also increases, and W increases with increasing pump power. The relationship between W and I_p0,_W₌₀ of the PMF-BRFL was measured, and assuming that the Cir insert loss is 1.87 dB, the measured curve is consistent with the theoretical curve for a cavity length of 3 km (Fig. 6).

Moreover, the variation in the lasing power with ${\hat{p}}_{i n} ? {\hat{β}}_{l}$ was determined, and the spectra of the lasing light were measured for different ${\hat{p}}_{i n} ? {\hat{β}}_{l}$ values. The lasing power decreases with decreasing $|{\hat{p}}_{i n} ? {\hat{β}}_{l}|$ , with a power difference of ~15 dBm between $|{\hat{p}}_{i n} ? {\hat{β}}_{l}|$ =1 and 0 at the working point of I_{p0_edfa}=55 mW. Finally, the linewidths of the lasing lights in the two polarization modes are measured for $|{\hat{p}}_{i n} ? {\hat{β}}_{l}|$ =1 and 0, and all lights have a narrow linewidth of ~0.75 kHz (Fig. 8).

Conclusions

In this study, a BRFL with bistable orthogonal polarization was proposed and achieved based on the axial polarization pulling effect of the SBS effect in PMFs. First, the polarization mode working regions of the PMF-BRFL system and the corresponding working conditions were analyzed and discussed. In addition, a PMF-BRFL system was experimentally established using a 3 km PMF fiber. The laser can emit narrow-linewidth lasing light with a polarization state stably clamped onto one of the principal axes of the PMF, and the experimental results are consistent with the theoretical analysis. Furthermore, the effects of the pump power and cavity length on the working regions of the system and the characteristics of the lasing power, spectrum, and linewidth were investigated experimentally.