Citation: Sun, T.; Su, X.; Zhang, Y.; Zhang, H.; Xu, T.; Zheng, Y. All-Fiber Wavelength-Tunable Narrow- Linewidth Polarization-Maintaining Tm-Doped Fiber MOPA System. Photonics 2022, 9, 778. https:// doi.org/10.3390/photonics9100778 Received: 25 September 2022 Accepted: 14 October 2022 Published: 19 October 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). photonics hv Communication All-Fiber Wavelength-Tunable Narrow-Linewidth Polarization-Maintaining Tm-Doped Fiber MOPA System Tianran Sun 1,2 , Xinyang Su 1,2, * , Yunhong Zhang 1,2 , Huaiwei Zhang 1,2 , Tianjia Xu 3 and Yi Zheng 1,2 1 School of Physical Science and Engineering, Beijing Jiaotong University, Beijing 100044, China 2 Key Laboratory of Luminescence and Optical Information, Ministry of Education, Beijing Jiaotong University, Beijing 100044, China 3 School of Optics and Photonics, Beijing Institute of Technology, Beijing 100081, China * Correspondence: xysu@bjtu.edu.cn Abstract: An all-fiber wavelength-tunable narrow-linewidth polarization-maintaining (PM) Tm- doped fiber master oscillator power amplifier (MOPA) system is presented. We demonstrate an all-fiber ring cavity Tm-doped fiber master-oscillator (MO) with a tuning range of 110 nm (from 1925 to 2034 nm). The maximum output power of 459 mW is obtained at 1992.9 nm for 16 W of launched pump power at 793 nm, corresponding to a slope efficiency of 5.6% concerning launched pump power. By using a one-stage Tm-doped fiber amplifier combined with a high gain of >10 dB, the maximum slope efficiency is 12.6% and the output power is 5.65 W at 1993 nm. The 3 dB linewidth is less than 0.5 nm, M 2 ≈ 1.25, and the polarization extinction ratio (PER) reaches 21.4 dB. The influence of different active fiber lengths on laser amplification is also studied. Keywords: tunable; all-fiber; polarization-maintaining; Tm-doped fiber; master oscillator power amplifier (MOPA); optimal fiber length 1. Introduction In recent years, cladding-pumped Tm-doped fiber laser sources operating in the eye-safe 2 μm spectral region have attracted increasing interest due to their numerous applications in medical, remote sensing, and eye-safety lidar [1–3]. With these applica- tions in demand, broadband tunability is crucial, as it allows controlling the depth of laser penetration in the tissue [4] and obtaining the absorption spectrum of atmospheric gases [5]. The Tm-doped fiber owns an emission spectrum of more than 300 nm in the 2 μm band. At the same time, thanks to the geometry of thulium-doped fiber with thermal management and the quantum efficiencies approaching two brought by the fortuitous “two-for-one” cross-relaxation process when a 790 nm high-power diode laser is used as a pump source [6,7], it provides the potential for power scaling while maintaining good beam quality [8]. To meet the above conditions, a tunable thulium-doped fiber laser system with a ring cavity structure is essential. Its advantage is that it does not need bulk-optic components, so its structure is compact, reliable, and flexible [9]. However, as the emission cross-section area of Tm ions decreases sharply in the long wavelength part, the gain competition of the emission spectrum in this region is strengthened. In comparison with the shorter wavelength, Tm-doped fiber laser (TDFL) operating in long-wavelength will produce significantly amplified spontaneous emission (ASE), which will reduce the output signal- to-noise ratio (SNR) and increase the sensitivity to the parasitic laser. Highly wavelength- selective elements, such as fiber Bragg grating (FBG) [9,10], diffraction gratings [11–13], and volume Bragg grating (VBG) [14], need to be placed in the laser cavity to eliminate parasitic laser so that Tm-doped fiber laser can produce stable laser output at long wavelength [12,15]. For large mode area (LMA) fiber, FBG is not very effective for wavelength selection, because LMA fiber maintains both fundamental mode and higher transverse mode. In addition, it is Photonics 2022, 9, 778. https://doi.org/10.3390/photonics9100778 https://www.mdpi.com/journal/photonics