基于增益光纤长度优化的双波长运转掺铒光纤锁模激光器

Dual-wavelength mode-locked Er-doped fiber laser based on optimizing gain fiber length

构建了可自启动的双波长运转掺铒光纤锁模激光器.通过优化增益光纤长度,利用掺铒光纤在1530nm附近的再吸收效应调节激光器的增益谱,使激光器在1530nm和1560nm附近具有相同的增益强度.实验中采用31cm掺铒光纤作为增益光纤,以透射式半导体可饱和吸收体作为锁模器件,实现了自启动双波长锁模运转.激光器锁模输出重复频率为58.01MHz,信噪比为58.2dB,最高输出功率为4.8mW.锁模输出的光谱在1532.4nm和1552.3nm处具有两个强度接近的谱峰,谱峰间距约为20nm.该激光器无需手动调节即可实现双波长运转,更便于实际使用.

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 of58.2 d B. 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.