Abstract
A switchable and tunable six-wavelength single-longitudinal mode (SLM) thulium-doped fiber laser in the 2050 nm band is proposed and experimentally demonstrated. Two cascaded sampled fiber Bragg gratings (SFBGs), acted as an integrated six-channel reflector, are to define the lasing wavelength channels in a multi-wavelength switchable fiber laser for the first time. The switchability of the fiber laser was realized by polarization-dependent loss modulation. SLM lasing in each channel was guaranteed by a novel passive dual-ring compound cavity (DRCC), composed of two symmetric 3 × 3 optical couplers (OCs). Six single-wavelength operations are easily obtained and switched, with the high optical signal-to-noise ratios of >71 dB, >72 dB, >62 dB for <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">λ</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sub> – <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">λ</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">13</sub> and > 65 dB, >61 dB, >59 dB for <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">λ</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">21</sub> – <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">λ</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">23,</sub> respectively. The maximum power and wavelength fluctuations are as low as 0.56 dB and 0.01 nm, respectively. The measured laser linewidths ranged from 7.5 to 9.2 kHz, calculated by the β-separation line method based on the measured frequency noise spectra. Nine switchable dual-wavelength operations with different wavelength intervals and two triple-wavelength operations were also obtained. Our proposed fiber laser has great potential in free-space optical communication.
Highlights
SINGLE-longitudinal-mode (SLM) thulium-doped fiber lasers (TDFLs) have gained intense research interest due to their promising scientific, technical, and military application potential
This allows for the fabrication of 2 μm SLM fiber lasers that can be widely applied in high-resolution sensing, especially for coherent Doppler lidar wind detection [1,2,3]; this emission range covers the atmospheric transparency window, where the permissible power transmission for light in the free-space can be several orders of magnitude higher than other wavelength bands
The active main ring cavity (AMRC) composed of SFBG1 (AMRC1) was 12.5 m, and the AMRC composed of SFBG2 (AMRC2) was about 12.8 m
Summary
SINGLE-longitudinal-mode (SLM) thulium-doped fiber lasers (TDFLs) have gained intense research interest due to their promising scientific, technical, and military application potential. The broad emission range (1660–2200 nm) of TDFL radiation falls in the eye-safe spectral region and overlaps with the absorption lines of various atmospheric gases This allows for the fabrication of 2 μm SLM fiber lasers that can be widely applied in high-resolution sensing, especially for coherent Doppler lidar wind detection [1,2,3]; this emission range covers the atmospheric transparency window, where the permissible power transmission for light in the free-space can be several orders of magnitude higher than other wavelength bands. Fiber laser at 2050 nm with multi-wavelength channels, each being in the SLM state, are highly expected to be used in next-generation LiDARs that require a seed laser with wavelength tunability, wavelength stability, and narrow linewidth as well as in wavelength division-multiplexing (WDM) free-space optical communication. The preferable output performance enables the TDFL promisingly to be used in next-generation lidar systems as well as in WDM free-space optical communication
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