Keywords: 
• 锁模激光器 /
• 双波长锁模运转 /
• 自启动锁模 /
• 自发辐射放大

Abstract: Recently, multi-wavelength pulsed lasers have become a research hotspot due to their versatile applications, such as precision spectroscopy, microwave/terahertz photonics, optical signal processing, and wavelength division multiplexed optical fiber communication systems. As a promising candidate, passively mode-locked fiber laser has the advantages of ultrashort pulse, ultrahigh peak power, compact structure and low-cost. In the existing multi-wavelength passively mode-locked fiber lasers, multi-wavelength mode-locked operation is achieved by adjusting the intracavity modulators to a proper state after laser has worked. It is inconvenient for practical use, so, its application scope is restricted. In this paper, a new method to achieve dual-wavelength mode-locked operation in an erbium-doped fiber laser is proposed. For an erbium-doped fiber, the peaks of both absorption and emission spectra overlap in the 1530 nm-region. So the emission light in the 1530 nm-region will be re-absorbed by the erbium-doped fiber with low pump power or long gain fiber. Utilizing the emission re-absorption effect, the gain spectrum can be modified by different lengths of gain fiber. In the experiment, an all-fiber ring cavity is adopted and a transmission-type semiconductor saturable absorber is used as a modelocker. The cavity consists of ～3.2-m-long single mode fiber and an erbium-doped fiber. Gain fibers with different lengths are used in the cavity to reveal the dependence of emission re-absorption on both gain spectrum and mode-locked output spectrum. According to the experimental results, there are two humps in the amplified spontaneous emission spectrum located in the 1530 nm-region and 1560 nm-region, respectively. With the gain fiber length increasing, gain spectrum in the 1530 nm-region is suppressed, and gain intensity in the 1560 nm-region gradually surpasses that in the 1530 nm-region. Based on the experimental results, self-starting dual-wavelength mode-locked operation is achieved with a 31-cm-long gain fiber. The two spectral peaks with close intensity are located at 1532.4 nm and 1552.3 nm, respectively. The maximum output power is 4.8 mW at a repetition rate of 58.01 MHz and a signal-to-noise ratio of 58.2 dB. This self-starting dual-wavelength mode-locked erbium-doped fiber laser is convenient for practical use and can meet the requirements for many potential applications.