1. Introduction

Tm³⁺-doped fiber laser output ranging from 1.7µm to 2.1µm provides excellent candidates as high-efficient laser sources [1–3]. Recently, IPG Photonics reported their 415 W single-mode cw thulium fiber laser in an all-fiber format with output linewidth <1 nm pumped by Er³⁺-doped fiber lasers at 1567 nm [4]. That is the highest cw output power ever reported for a single-mode Tm³⁺-doped fiber laser. This wavelength range is especially interesting for use in gas sensors, frequency mixing, and as a source for eye-safe lidar applications [5]. However, for detection and measurement applications such as lidar or countermeasure systems, there are elevated demands on precise matching for wavelength and ultra-narrow linewidth. It is feasible for a fiber laser to acquire the needed center wavelength, narrow linewidth, and even single-frequency laser output with a fiber Bragg grating (FBG) being the cavity coupler [6]. The Yb³⁺-doped single-frequency fiber-laser output linewidth reached 2 kHz, with a maximal output power of 177 mW [7]. The 2 kHz linewidth Er³⁺-doped fiber laser also realized an all-fiber circular cavity [8]. Under the same method, the Tm³⁺-doped fiber laser acquired a 1 mW single-frequency, 10 pm linewidth output with a FBG as the output coupler [9]. In order to acquire a high-power, narrow linewidth fiber laser source, the cladding-pumped high-power narrow linewidth fiber laser realized a 0.1 nm linewidth, 50 W output at 1.09 µm [10] and a 0.16 nm linewidth, 43 W output at 1.53 µm [11].

In this letter, we report that we produced a high-power, narrow linewidth, <50 pm cw Tm³⁺-doped silica fiber laser, which was cladding-pumped by a 793 nm laser diode (LD). A 1.9 µm region high-reflectivity dichroic and a low-reflectivity FBG as the output coupler composed the fiber laser cavity. A stable, narrow linewidth 1936.4 nm laser was generated, and the linewidth was <50pm. With the launched pump power of 13.9 W, the fiber laser generated maximal output power up to 2.4 W cw.

2. Experimental setup

The silica fiber used for this project had a core diameter of 25 µm and NA of 0.22, and an octagonal inner-cladding diameter of 250µm and NA of 0.46. The fiber core was codoped Tm³⁺:Al³⁺ (the doped ratio was 1:10), and the Tm³⁺-doped concentration was 2.0 wt. %. The fiber length was 1.1 m in the experiment. The 793 nm LD pump light had an attenuation of 6 dB/m in the inner cladding. Using a phase mask and a 248 nm laser, grating was written into the core of a photosensitive passive double-clad silica fiber in which the diameter was matched to the Tm³⁺-doped fiber. The photosensitive passive fiber had a V-value of 4.05 (core NA of 0.1) at 1.9 µm, so there were three transmission modes in the fiber core. The reflectivity ratio of the Bragg grating in the multimode fiber was difficult to get very high [12], and the passive fiber containing the grating was spliced to the Tm³⁺-doped fiber as the output coupler. A dichroic (1936nm HR >99%, 793nm HT <97%) was tightly butted to another end of a Tm³⁺-doped fiber as the cavity mirror. Figure 1 shows the experimental setup of the narrow linewidth Tm³⁺-doped fiber laser. The pump 793 nm LD was output with a pigtail fiber at 200µm, NA of 0.22. The pump light was refocused into the Tm³⁺-doped fiber inner cladding with a 1:1 multi-lens.

 

Fig. 1. Experimental setup of the linewidth Tm³⁺-doped fiber laser.

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3. Experimental results and discussions

Removing the dichroic using Tm³⁺-doped fiber fluorescence as the light source [13] and grating spectrum analyzer, we measured the FBG transmission spectrum with a launched pump power of 2.4 W. The spectrum was measured by a monochromator. The monochromator had an optical path length of 0.5 m and a spectral resolution of ~0.02 nm. The optical signal was detected by an InGaAs detector. The transmission spectrum of the FBG is shown in Fig. 2. According to the experimental and theoretical investigation results of Bragg grating in a multimode fiber [12,14], the measured FBG spectrum should have three high-reflective peaks and also exit some low-reflective peaks around the three main peaks. In our measured results, three main peaks were at 1936.4 nm, 1937.2 nm, and 1938.8 nm, and some low-reflective peaks appeared around the three main peaks.

 

Fig. 2. Measured transmission spectrum of the passive FBG fiber.

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Figure 3(a) shows the stable output laser spectrum when the dichroic was removed and the cavity was being lased by the fiber Fresnel reflection. The spectrum was measured from the FBG end by the EXFO WA-150 laser wavelength meter with a launched pump power of 13.9 W. The output center wavelength was located at 1936.4 nm, and the FWHM was only 45.7 pm. When the dichroic was tightly butted to the Tm³⁺-doped end, the output laser spectrum appeared with many output peaks, as shown in Fig. 3(b). The output laser spectrum was similar to the FBG transmission spectrum and composed of several main high-reflective peaks and many low-reflective peaks. The multi-wavelength output laser spectrum could explain that the high-reflectivity ratio dichroic excited most of the reflective peaks.

 

Fig. 3. Output laser spectrum measured from the FBG fiber end. (a) The dichroic was removed from the Tm³⁺-doped fiber end. (b) The dichroic was butted to the Tm³⁺-doped fiber end.

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In order to select a single FBG reflection peak, we made the dichroic reflector and the fiber end set up a spectrally selective Fabry Perot etalon. When we moved the dichroic and there was a space (>10 µm) between the dichroic and the fiber end facet, the output laser modes decreased a lot, as shown in Fig. 4(a). The weak laser center wavelength was located at 1936.56 nm and had a FWHM of 51.6 pm. In Fig. 2, a peak located at 1936.6 nm could be found, so it should be excited in Fig. 4(a). When we kept on enlarging the space, the output laser spectrum ultimately become the FWHM of a <50 pm single peak located at 1936.4 nm, as shown in Fig. 4(b), and the center wavelength was very consistent with Fig. 3(a).

 

Fig. 4. Output laser spectrum measured from FBG fiber end. (a) The dichroic moved a space of about >10 µm from the Tm³⁺-doped fiber end. (b) Increasing the space until the output spectrum was single peak.

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Figure 5 shows the fiber laser output power characteristics. When the dichroic was removed and with the increasing of launched pump power, both the Tm³⁺-doped fiber end and FBG fiber end had laser output, and the output power was measured respectively. With the dichroic as the cavity, when the space between the dichroic and the fiber end facet was enough long to be output in a single peak, the narrow linewidth output maximal power got to 2.4 W. The slope efficiency was 24.2%, but the saturated phenomenon was very obvious. The very low threshold was nearly 1.8 W.

 

Fig. 5. Output narrow linewidth laser power as a function of the launched pump power.

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4. Conclusion

A <50 pm narrow linewidth cladding-pumped Tm³⁺-doped silica fiber laser was realized. The maximal output power reached was 2.4 W. The fiber laser had low-slope efficiency and output-saturated phenomenon, and it was perhaps because the Tm³⁺-doped fiber and the FBG fiber were not mode-matched. In subsequent experimental investigations, we would rectify the specifications of the Tm³⁺-doped fiber to scale the maximal output power and increase the slope efficiency.