Tunable Dual-Wavelength Bismuth Fiber Laser With 37.8-GHz Frequency Spacing

A tunable dual-wavelength Brillouin-bismuth fiber laser with a triple-Brillouin frequency spacing of ∼37.8 GHz was demonstrated by employing modular structures. The modular designs can be divided into two main cavities, whereby cavity 1 consisted of a 2-km single-mode fiber (SMF) and a loop mirror that was able to generate a single-spaced Brillouin shift, while cavity 2 comprised of 10-km dispersion compensating fiber (DCF) that was able to produce a double-spaced Brillouin shift. Based on this design, a stable dual-wavelength lasing was established with a low peak power fluctuation of 0.13 dB and 0.6 dB for the Brillouin pump (BP) and third Brillouin Stokes line (BS₃), respectively. By varying the center wavelength of the Brillouin pump, the dual-wavelength laser can be tuned across a 40 nm span, ranging from 1300 nm to 1340 nm. The frequency spacing of the Brillouin-bismuth fiber laser can also be easily switched between ∼12.6 GHz, ∼25.2 GHz, and ∼37.8 GHz by utilizing these modular structures. The proposed dual-wavelength laser has a promising potential for application as a low phase noise source for microwave generation.

I. Introduction

Multiwavelength fiber lasers, capable of producing multiple discrete laser emission lines, have seized lots of attention in recent years. These lasers can be used in the wavelength-division multiplexed (WDM) network [1]–[4] to enhance further or support greater bandwidth capacity. Moreover, multiwavelength fiber lasers have also emerged as pivotal light sources for optical sensing systems [5]–[7], optical instruments testing, and microwave photonics technology [8], [9]. Until now, their unique laser performances have been gradually explored in the aspects of structure, output power, beam quality, stability, and multiwavelength generation. Various method has been demonstrated to realize multiwavelength fiber lasers. These include nonlinear polarization rotation [10]–[12], nonlinear optical loop mirror [13]–[16], multi-channel filters based on fiber Bragg grating (FBG) [17], [18], and cascaded stimulated Brillouin scattering (SBS) [19]–[24]. Of numerous multiwavelength fiber laser configurations that have been investigated, the requirements for dual-wavelength laser output are very rigorous. They are well-known to be useful for application in microwave photonics generation [25], [26]. To date, numerous filters have been designed to generate dual-wavelength outputs in fiber laser systems, such as the use of arrayed waveguide grating (AWG) [27], few-mode fiber Bragg gratings [28], [29], phase-shift fiber Bragg gratings [30]–[32], photonics crystal fiber (PCF) [33], Sagnac loop mirror filter [34], [35] and Mach-Zehnder interferometer filter [36], [37]. Nevertheless, the fabrication of FBG-based narrowband filters is often complex and not cost-effective. In contrast, PCF, Sagnac and Mach-Zehnder filter are sensitive to environmental perturbation, further degrading the optical performance of the fiber laser. Recently, various novel Brillouin fiber lasers have been actively reported due to their outstanding characteristics, such as narrow linewidth, a stable multiwavelength operation at room temperature, low noise intensity, and a broad tuning range [38]–[45].