Objective The fibre laser working in the eye-safe 2.0-μm band is an important pump source for producing the ~3--5 μm mid-infrared laser, and the single-longitudinal-mode fibre laser has the characteristics of large gain, good coherence, and stable spectrum. This laser can be applied to laser optical radio, coherent optical communication, optical fibre sensing, optical fibre remote sensing, and other fields having large light source requirements. The wavelength-switchable fibre laser has a flexible output laser wavelength, which has great application value in the wavelength division multiplexing and multi-parameter sensing systems. The single-longitudinal-mode thulium-doped fibre laser operating in the 2.0-μm band has a wide range of applications, and it is necessary to further study and optimize its performance. Single-longitudinal-mode fibre lasers have excellent and better power stability than the traditional semiconductor lasers, and their modulation amplitude does not change with the modulation frequency. However, there are very few reports on the 2.0-μm-band spatial optical network. The 2.0-μm-band wavelength division multiplexing system requires the single-longitudinal-mode and multi-wavelength thulium-doped fibre laser. In our study, we reported a three-wavelength-switchable single-longitudinal-mode thulium-doped fibre laser based on a Fabry-Perot fibre Bragg grating filter. Under the normal room temperature condition, this laser can obtain the single-longitudinal-mode, high stability, ultra-high optical signal-to-noise ratio, and high power output. Therefore, this study has an important application value in space optical communications.
Methods The experimental structure of the proposed laser is shown in Fig. 6. A 793-nm laser diode was used as the pump source, pumping a 2-m thulium-doped fibre. The circulator ensured the one-way operation of the optical path in the cavity; a three-channel narrow-band Fabry-Perot fibre Bragg grating filter and a fibre Bragg grating were fabricated using the phase-mask method. The fibre Bragg grating was placed on a translation stage and its reflection wavelength was changed using stress to make it overlap with each channel of the Fabry-Perot fibre Bragg grating. The optical signals passing through the filters can be selected and most of the longitudinal modes can be suppressed to vary the laser wavelength. The polarization controller balanced the gain and loss of the intracavity signal to stabilise the output with the highest optical signal-to-noise ratios. Two 50∶50 couplers formed an 8-shaped sub-cavity, which effectively increased the longitudinal mode interval in the composite cavity, so that each channel of the Fabry-Perot fibre Bragg grating filter obtained single-longitudinal-mode lasing. The 10% port of the 90∶10 coupler was used to output the laser, and the 90% port was connected to the main cavity. By reasonably adjusting the length of each resonant cavity, the single-longitudinal-mode output of the proposed laser was realized.
Results and Discussions The transmission spectrum of the self-made Fabry-Perot fibre Bragg grating filter had three narrow-band filtering channels with the centre wavelengths of 1941.48, 1941.57, and 1941.65 nm; the corresponding 3-dB bandwidths were 0.060, 0.054, and 0.066 nm, respectively. The 3-dB bandwidth of fibre Bragg grating was 0.11 nm and the reflectivity was ~97%. The centre wavelength of the fibre Bragg grating was matched with the centre wavelength of the three transmission channels of Fabry-Perot fibre Bragg grating through a translation stage (Fig.5). The designed 8-shaped sub-cavity expanded the longitudinal mode interval in the cavity to 0.3 nm, which was greater than the 3-dB bandwidth of each channel of the Fabry-Perot fibre Bragg grating filter, ensuring the single-longitudinal-mode lasing in each channel (Fig. 7). Switching between different laser wavelengths was achieved by changing the reflection wavelength of the fibre Bragg grating under the normal room temperature condition and the output lasers with three different wavelengths (Fig. 8). The wavelength fluctuations were less than the minimum resolution of optical spectrum analyser, which was 0.05 nm. Each lasing’s power fluctuations were less than 0.39 dB, 0.61 dB, and 0.55 dB (Figs. 9 and 10). The experimental results indicated that the laser could work with a good stability in 50 min. Using a spectrum analyser to observe the frequency spectra of the output laser, it was seen that the laser worked at the single-longitudinal-mode operation stably (Fig. 11).
Conclusions In the present study, first, the Fabry-Perot fibre Bragg grating filter is analyzed theoretically. Based on the filter, a three-wavelength-switchable single-longitudinal-mode thulium-doped fibre laser is verified. Then, the narrow-band Fabry-Perot fibre Bragg grating and fibre Bragg grating are fabricated. The laser wavelength can be switched among three wavelengths by stretching the fibre Bragg grating in the cavity through regulating the stress adjustment frame. The laser works stably at the single-longitudinal-mode operation by adjusting the polarisation controller. An 8-shaped passive sub-cavity is employed to expand the longitudinal mode spacing. At 24 ℃, the laser wavelengths were 1941.48, 1941.57, and 1941.65 nm and the corresponding optical signal-to-noise ratios were 61 dB, 61 dB, and 60 dB, respectively. The stability of lasing was measured in 50 min. The output power fluctuation of each lasing was less than 0.39, 0.61, and 0.55 dB, respectively. The wavelength fluctuations were less than 0.01 nm, which was less than the optical spectrum analyzer’s minimum resolution of 0.05 nm. Therefore, the three-wavelength switchable thulium-doped fibre laser has stable single-longitudinal-mode output characteristics and can be applied to the fields of optical communication and optical fibre sensing in the 2.0-μm band.
