In recent years, mode-locked holmium-doped fiber laser operating in 2.1μm region has become one of the research hotspots due to their potential applications in free-space optical communication, remote sensing, and mid-infrared laser generation [1], [2], [3]. Different types of mode-locking techniques have been used to obtain stable pulse operation in 2.1μm band. Based on the saturable absorption properties, different materials such as graphene, semiconductor saturable absorber mirrors (SESAM) and metallic carbon nanotube have been used [4], [5], [6], [7]. In addition, mode-locking can also be achieved by inducing artificial saturable absorption effects based on fiber nonlinearity such as nonlinear polarization rotation (NPR) and nonlinear amplifying loop mirror (NALM) [8], [9]. Compared with NPR, NALM has less polarization dependence, which can improve the stability and robustness of mode-locked fiber laser. Consequently, NALM based mode-locked fiber lasers have been sufficiently researched and developed.

Dissipative soliton resonance (DSR) has attracted a great amount of interest due to the advantage of high pulse energy [10], [11]. The formation of the DSR was first theoretically predicted by N. Akhmediev et al. [12]. In theory, the width of DSR pulse increases with the enhancement of pump power while the amplitude remains constant. Therefore, DSR pulse can break the energy limitation of conventional soliton and obtain the higher pulse energy [13]. In normal dispersion and anomalous regimes, DSR with high pulse energy has been demonstrated [14], [15]. In addition to focus on the realization of high energy pulse operation, DSR-fiber laser with wavelength-tunable capability expands the potential applications of fiber lasers in coherent Raman scattering imaging, gas analysis and so on [16], [17]. Different spectral filtering techniques have been employed to achieve wavelength tunability in fiber lasers [18], [19], [20]. Wang et al. demonstrated a wavelength tuned DSR from 1029.8 to 1033.9 nm operating in ytterbium-doped fiber laser with the utilization of Lyot filter [19]. For the erbium-doped fiber laser, Li et al. achieved a wavelength-tunable DSR of 37.8 nm based on birefringence-induced spectral filtering effect [21]. Xu et al. achieved a wavelength tunable range of 28.95 nm for DSR operation in a thulium-doped mode-locked fiber laser with the utilization of birefringence-induced filter [22]. For holmium-doped fiber laser, Li et al. demonstrated the tunable wavelength DSR operation by utilizing a fiber birefringence-induced filter [23]. The center wavelength is shifted from 2054.4 to 2076 nm. Unfortunately, the employment of high nonlinear fiber in the reported fiber laser increases the intracavity loss, which leads to low pulse energy and restricts the availability of DSR at longer wavelengths. To date, there are no reports of wavelength-tunable DSR fiber lasers exceeding 2.1μm. The emitting wavelength range of holmium-doped fiber is poorly utilized. More importantly, enabling wavelength tunable DSR operation beyond 2.1μm can enlarge the versatility and application range of holmium-doped fiber lasers in the fields of industrial processing, gas detection and nonlinear frequency conversion. However, the low gain and high loss in the fiber bring challenges for wavelength-tunable DSR operation above 2.1μm, which necessarily requires careful design of the fiber laser construction.

In this paper, we report a wavelength-tunable DSR mode-locked fiber laser with operation wavelength exceeding 2.1μm for the first time. A long holmium-doped fiber (HDF) is applied to ensure adequate gain at 2.1μm. Employing a piece of ultra-high numerical aperture fiber (UHNAF) with normal dispersion allows the fiber laser to operate in the normal dispersion regime. The high nonlinearity of the UHNAF increases the intracavity nonlinearity to favor DSR formation and reduces the pump power required for mode-locking. With the optimized lengths of 9.8 m HDF and 55 m UHNAF, stable DSR operation with central wavelength of 2109 nm and 3-dB spectrum width of 0.46 nm can be achieved. The maximum pulse energy reaches 29.7 nJ. By using the birefringence-induced spectral filter, the center wavelength of the DSR is capable of tuning from 2102.7 to 2120.5 nm, corresponding to a spectral tuning range of 17.8 nm.