Numerical investigation on influence of the saturable absorber in Tm-doped fiber laser

Compact and environmentally passively mode-locking fiber lasers, which can generate ultrashort pulses with high beam quality and high energy, have played an increasingly crucial role in various fields including optical communication, optical spectroscopy, industry shipbuilding, military defense security, high-resolution astronomy observation and so on [1], [2], [3], [4], [5], [6]. Meanwhile, passive mode-locking fiber laser can provide an ideal platform for studying the nonlinear dynamic process of soliton interaction. All kinds of dynamic behavior of soliton pulse in fiber laser are the research hot spots, such as soliton collision [7], harmonic mode-locking [8], [9], multi-wavelength mode-locking [10], soliton molecule [11], [12], Mamyshev oscillator [13], [14], [15] and vector soliton [16]. To achieve passive mode-locking in fiber lasers, the figure-of-eight technique [17], nonlinear polarization rotation [18], semiconductor saturable absorption mirrors (SESAMs) [19], and novel 2D materials [20], [21], [22] have been widely employed as the saturable absorber (SA) [23]. The SA, a basic passive optical device, may be inserted into a fiber cavity to create high-quality ultrashort pulses, which is mostly dependent on the nonlinear absorption behavior of the SA [24]. Many promising nonlinear optical materials have been used as SAs due to their excellent optical properties, such as carbon nanotubes [25], [26], graphene [27], topological insulators [28], and MXenes [29]. The SAs fabricated by these materials with appropriate recovery time could be classified as “slow” or “fast” saturable absorbers [30], [31], [32]. The most important transmission characteristics for the fast SA are modulation depth, saturation energy, and nonsaturable loss. These factors are crucial to the performance of output pulses in mode-locking fiber lasers. The recovery time may be regarded as an additional significant factor for the slow SA.

To date, several investigations have been conducted to reveal the influence of the SA parameters on the output performance of pulse. For example, Hönninger et al. investigated the transition between the regimes of continue wave mode locking and Q-switched mode locking of a solid-state laser considering SA parameters [33]. Lecaplain et al. revealed the high-contrast passive modulation is very promising for the pulse self-starting and energy scaling [34]. Tokurakawa et al. numerically analyzed the relationship between the available shortest pulse duration and the modulation depth of fast SA mode-locked Yb-doped solid-state lasers [35]. Ma et al. showed the impact of SA parameters including the modulation depth, nonsaturable absorption and saturation intensity on the dissipative solitons’ physical properties in passively mode-locked fiber laser [36]. In addition, Jinho Lee et al. investigated the impact of the recovery time and modulation depth variation of a SA on the mode-locking performance of a fiber laser [37], [38]. Indeed, these investigations have provided a helpful guide for selecting the appropriate SA for Yb or Er doped mode-locked lasers. However, to the best of our knowledge, there still lacks the concrete study about the impact of the parameters of SA on the Tm-doped fiber lasers in previous reports. As a result, it is important to conduct a comprehensive study on SA in Tm-doped fiber lasers in order to enhance the optimization of fiber laser research.

Thus, in this work, we theoretically investigate the impact of SA parameters including modulation depth, nonsaturable loss, and recovery time in a Tm-doped mode-locked fiber lasers operating in the anomalous dispersion regime. The fast and slow saturable absorption effect are considered in the simulation model. The influence of SA parameters on the temporal and spectral shape, pulse width, 3-dB spectrum bandwidth of the output pulses have been numerically investigated. In addition, we demonstrate that the SA’s modulation depth and cavity dispersion both have impacts on the recovery time for stable conventional soliton pulse generation. Our work will promote the development of fiber lasers and serve as a guide for the practical application of SA.

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