1. Introduction

Thulium-doped fiber lasers with distributed feedback (DFB) or distributed Bragg reflector (DBR) configuration have been established as efficient methods of generating single-frequency laser output at the 2 µm eye-safe region, by virtue of their advantage on compactness and robustness. The lasers under single-longitudinal-mode operation are in great demands in various scientific applications such as coherent LIDAR, high-resolution spectroscopy, and free-space optical communication [1–5], wherein narrower spectral linewidth of the laser source is preferred to improve the system’s performance. The linewidth of the single-frequency DFB or DBR thulium-doped fiber laser is generally in the level of tens of kHz, which is broader than that of erbium- and ytterbium-doped fiber lasers due to the quantum defects [6–10]. Narrower spectral linewidths below 10 kHz in 2 µm have been demonstrated by using a ring cavity with a saturable absorber, in which non-commercially available multicomponent glass fibers were used [11].

Stimulated Brillouin scattering (SBS) effect has been developed as a mature method to realize the linewidth narrowing of lasers [12–14]. By coupling the single-frequency Brillouin pump into a ring cavity, where a long piece of fiber serves as a Brillouin gain medium, Stokes light with linewidth much narrower than that of the Brillouin pump can be generated due to the strongly reduced phase noise [15]. In 2001, Debut et al. experimentally and theoretically investigated the linewidth narrowing in Brillouin fiber ring lasers, which found that the magnitude of the linewidth narrowing effect depends on the acoustic damping rate and the cavity loss rate. Besides, the acoustic noise is responsible for the existence of a lower limit to the linewidth of a single-frequency Brillouin laser [16]. In the past two decades, narrow-linewidth Brillouin fiber lasers (BFLs) with strong linewidth suppression have been demonstrated in 1 µm, 1.5 µm and 2 µm region [17–22]. Different from the BFLs at 1 µm and 1.5 µm region, the BFLs at 2 µm region are limited by the low SBS gain coefficient (inversely proportional to the square of wavelength) and high propagation loss (22 dB/km) in silica fiber, which induced that Watt-level single-frequency Brillouin pump power is typically required to reach the SBS threshold [18]. The use of high SBS-gain suspended-core chalcogenide fiber significantly decreases the SBS threshold to 52 mW. However, because of the large losses, the Stokes power is limited to the microwatt level [23]. Moreover, the highly dependence on specially designed high SBS-gain medium limits its applications.

A solution to the abovementioned problems is to incorporate the gain from the population inversion of the active fiber inside the Brillouin Stokes cavity, as was first demonstrated in 1996 by Gregory et al. with a hybrid Brillouin/erbium fiber laser (BEFL) system [24]. The Stokes light could be generated with Brillouin pump of only 0.5 mW and the SBS threshold was about 10 mW in terms of the 980-nm laser diode (LD) power. Until now, many researches have been reported about BEFLs and hybrid Brillouin/ytterbium lasers (BYFLs) [25–30]. Guan et al. and Chen et al. have studied the power performance of BYFL and BEFL, respectively, in which the influences of Brillouin pump power, output coupling and length of active fiber on power performance have been investigated experimentally and theoretically and it was found that the output power of first-order Stokes is limited by higher-order SBS [25,26]. Besides, strong linewidth narrowing was also achieved based on hybrid Brillouin/active fiber laser. In 2014, a BEFL was demonstrated, in which 20-MHz linewidth of Brillouin pump was narrowed to 950 Hz, corresponding a linewidth reduction ratio of over 2×10⁴ [27]. In the 2 µm region, a BTFL was reported in 2017, in which the resulting threshold in terms of the Brillouin pump was lower to 200 mW and the maximum Stokes power of 205 mW was achieved. A linewidth of 4.6 kHz was obtained with a narrowed degree of 8 times, which was limited by the noise of multimode 793-nm pump source [29]. However, the power performance and linewidth narrowing effect of BTFLs at 2 µm have not been systematically investigated. Therefore, it is significative to study the influence of cavity parameters on linewidth narrowing effect and output power for enhancing the performance of BTFL.

Since the noise of pump source can be transferred to the generated laser [31], which influences the laser linewidth eventually, the employment of pump laser with lower noise can be taken an effective method of optimizing the spectral linewidth of the BTFLs. Besides, the phase noise introduced by spontaneous emission [16] can be affected by the interaction of circulating Stokes field and phonon field in the Brillouin fiber laser. The output coupling and the length of the fiber can influence the linewidth narrowing of Brillouin fiber laser as well [18,32]. In this study, the influences of the output coupling, pump source and fiber length on the spectral linewidth and power behavior of the BTFL were experimentally investigated. A Stokes output of 344 mW with a narrow linewidth of 0.93 kHz was obtained by optimizing the abovementioned parameters. The results revealed that the BTFL is a promising routine to achieve sub-kHz single-frequency source at 2 µm, and that the trade-off between the output power and the spectral linewidth can be optimized with a suitable output coupling.

2. Experimental arrangement

Figure 1 shows the experimental arrangement of the narrow-linewidth BTFL, along with the power amplifier. A linear-polarized single-frequency thulium-doped fiber MOPA at 1956.49 nm, which has a maximum output power of 300 mW, and a spectral linewidth of 40 kHz, was served as the primary Brillouin pump to couple into the ring Stokes cavity through a circulator. A piece of 0.5-m long double-clad thulium-doped active fiber (Nufern PM-TDF-10/130-2000-HE) was inserted into the cavity to provide gain for both the clockwise propagating Brillouin pump and the anticlockwise propagating Brillouin Stokes. A piece of polarization-maintaining (PM) single-mode fiber (Corning PM1550) was spliced into the cavity for extra Brillouin gain. Part of the anticlockwise propagating Stokes was coupled out of the cavity through a coupler. Different output couplings of 10%, 30%, 50%, 70%, and 90%, and different PM1550 fiber lengths of 5 m and 8 m (total cavity length of 11 m and 14 m) were used in the experiment. Considering the energy level of thulium and the SBS threshold, 793-nm cladding pump and 1570-nm core pump are generally employed to demonstrate thulium-doped fiber laser at 2 µm. In this experiment, both of the two pump schemes were used to discriminate the performance of the BTFL, wherein the pump source (multimode 793-nm LD and single-mode 1570-nm fiber laser) was connected with the active fiber through a combiner and wavelength-division multiplexing (WDM), respectively. A cladding mode striper (CMS) was employed to strip the residual 793-nm pump power and it was cooled by water cooled heat sink to alleviate the linewidth broadening induced by thermal load. The active fiber has a core pump absorption coefficient at 1570 nm of 250 dB/m and a cladding pump absorption coefficient at 793 nm of 4.7 dB/m, respectively [33,34]. All the fiber and devices employed in the experiment were polarization-maintaining.

 

Fig. 1. Schematic of hybrid Brillouin/thulium fiber laser.

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3. Results and discussion

3.1 Power performance

First, we characterized the power performance of the BTFL with the 5-m-long PM1550 fiber when using the 1570-nm core pump scheme. With a fixed incident Brillouin pump power of 250 mW (measured at port 2 of the circulator), the SBS threshold gradually increased from 0.35 W to 1.0 W in terms of pump power coupled into the thulium-doped fiber as the output coupling increased from 10% to 70%, as shown in Fig. 2(a). For the highest output coupling of 90% in the experiment, the SBS was not observed under the maximum available pump power of 1.43 W. Although the lower output coupling helped to reduce the SBS threshold to only 350 mW, the maximum single frequency output power was only 26 mW before high-order Stokes generation under a 1570-nm pump power of 620 mW in accompany with a sharp drop of output power. Figure 3 shows the BTFL output spectrum when the high-order SBS occurred (1570-nm pump with a power of 700 mW and 10% output coupling). This was recorded by an optical spectrum analyzer (OSA, Yokogawa 6375) with a resolution of 0.05 nm. As shown in Fig. 3, peaks at 1956.62 nm and 1956.74 nm can be observed as the pump power increased, which corresponds to the first and second Stokes light compared to the Brillouin pump wavelength of 1956.49 nm. It coincides with the 8.34 GHz (approximately 0.12 nm @ 1956nm) Brillouin shift in the silica fiber. However, considering that the first Stokes was propagating anticlockwise, and that the coupler only coupled the anticlockwise propagating light out of the ring cavity, there should not have been a second Stokes at the coupler’s output port. This matter will be further investigated in future work. For the output coupling of 30% and 50%, the maximum Stokes output power under a fixed 1570-nm pump power of 1.43 W and Brillouin pump power of 250 mW increased to 316 mW and 400 mW, respectively, with a corresponding slope efficiency of 43% and 57.6%. The increase of slope efficiency and maximum output power substantially benefited from the higher output coupling compared with the slope efficiency of 9.6% with an output coupling of 10%. The upper limit of output power was restricted by high-order SBS whose threshold was low when the output coupling of 10% was employed. With the increase of output coupling, the power density of first-order Stokes light reduced effectively, which increases the threshold of high-order Stokes light. Besides, the high output coupling was benefit for the output of first-order Stokes light, and the reduction of Stokes power intensity in the ring cavity induced that more gain in the thulium-doped fiber was used to amplify the Brillouin pump, which promoted the slope efficiency of Stokes light stimulated by Brillouin pump. With an output coupling of 70%, although the slope efficiency of 64.4% was higher than that with lower output couplings, the relatively high SBS threshold of 1 W limited the maximum Stokes output power to 277 mW. Therefore, a trade-off between the SBS threshold, slope efficiency, and output power should be considered when optimizing the power performance of the BTFL.

 

Fig. 2. SBS threshold and Stokes output power with different output coupling under (a) fixed Brillouin pump of 250 mW and (b) power transfer with output coupling for maximum output power under different BTFL pump schemes. The dashed lines serve as a visual guide.

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Fig. 3. Spectra of 1956.49 nm Brillouin pump, 1956.62 nm first Stokes, and 1956.74 nm second Stokes recorded using OSA.

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The laser performance under the 793-nm cladding pump was also investigated by using a combiner to couple the LD pump into the thulium-doped fiber. The SBS threshold and Stokes output power under the maximum pump are plotted in Fig. 2(a). With the incident Brillouin pump of 250 mW, the maximum Stokes output of 370 mW under a pump power of 6.2 W LD was obtained with an output coupling of 50%, in which the corresponding SBS threshold is 3.5 W and slope efficiency is 12.3%. The insufficient pump absorption of the 0.5-m thulium-doped fiber limited the efficiency under the cladding pump scheme. As will be described below, we used the cladding pump scheme to investigate the linewidth under a different pump scheme. Therefore, the fiber length was not optimized for output power. With the output couplings of 30% and 70%, the maximum output power was 280 mW and 240 mW, respectively, while the SBS threshold was 3.2 W and 3.9 W, respectively. Considering the high-order Stokes and high SBS threshold, the output couplings of 10% and 90% were not used with the 793-nm cladding pump scheme. Figure 2(b) shows the power transfer of the BTFL under a 1570-nm core pump and 793-nm cladding with an output coupling of 50%, with which the maximum output power was obtained.

3.2 Spectral linewidth

The spectral linewidth of the Stokes output was measured using a delayed self-heterodyne system with 10-km delay line, and is plotted in Fig. 4. Although two similar trends were observed with 793-nm cladding pump and 1570-nm core pump, respectively, the linewidths with 793-nm cladding pump were much broadened than that with the 1570-nm core pump, which is attributed to the inferior noise performance of multimode 793-nm LD and relatively less quantum defect of 1570-nm core pump. The intrinsic noise of multimode 793-nm LD is much larger than that of single-mode 1570-nm fiber laser, which will be introduced into the ring cavity through pumping process and impose adverse effects on linewidth narrowing effect [29,30]. In addition, quantum defect is the dominant factor of thermal noise contributing to the linewidth broadening, therefore, linewidth can be further narrowed with 1570-nm core pump resulting from lower thermal noise. When the 1570-nm core pump was employed, the spectrum linewidth gradually broadened from 1.7 kHz to 3.5 kHz as the output coupling increased from 30% to 70%. With a lower output coupling, more power was reserved in the ring cavity for the Stokes light. In this case, the lower pump power threshold can be achieved to generate the Stokes light, which was verified as shown in Fig. 2(a). Moreover, because the linewidth narrowing effect of the Brillouin laser was caused by the combined influence of the acoustic damping and cavity feedback, the linewidth narrowing ratio between the Brillouin pump and the Stokes signal can be increased with lower output coupling. In other words, the narrower Stokes linewidth could be derived by decreasing the output coupling [16]. Because the hybrid gain scheme was adopted in our BTFL, the system noise resulting from the spontaneous emission of the thulium-doped fiber is considered as another issue related to the narrowing of the laser linewidth. Enhanced cavity feedback with a lower output coupling could alleviate the effect of spontaneous emission noise and increase the monochromaticity degree of the Stokes light, respectively. Moreover, the output power of the linewidth-narrowed Stokes light significantly decreased as the output coupling reduced. As demonstrated by the experiment, the Stokes power decreased from 400 to 26 mW when the output coupling changed from 50% to 10%. Therefore, for an optimized BTFL, the output coupling should be carefully chosen to ensure performance balance between the linewidth narrowing and laser power. Because all the devices employed in the system were polarization-maintaining, a polarization extinction ratio of output Stokes laser was measured to be 22.3 dB using a Glan prism.

 

Fig. 4. Stokes BTFL linewidths measured with different coupling and pump schemes. The dashed lines serve as a visual guide.

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3.3 Further linewidth narrowing

For further optimizing the spectral linewidth and power performance of the BTFL, we enlarged the length of the PM1550 fiber, since the linewidth narrowing ratio K² between Brillouin pump and Stokes light increases with the length of ring cavity L according to the equation in [35]:

The influence of primary Brillouin pump power on the power performance of the BTFL was also investigated. Figure 5(a) plots the BTFL output power with different primary Brillouin pump power, under the maximum 1570-nm pump power of 1.43 W. The minimum Brillouin pump power required for stable Stokes output was 210 mW. Though the power of the Stokes light increased when the Brillouin pump power increased from 210 mW to 250 mW, it declined when Brillouin pump power exceeded 250 mW, which resulted in a maximum output power of 344 mW under 1570-nm pump power of 1.43 W. When the power of Brillouin pump increased gradually from 210 mW to 250 mW, Stokes light was amplified by both Brillouin gain and thulium-doped fiber, nevertheless, most of the upper-level population of thulium-doped fiber is extracted by Brillouin pump as the power of Brillouin pump increased over 250 mW, and there is no enough gain to amplify the Stokes light, which leads to the decrease of output power. Figure 5(b) shows the power transfer of Stokes light versus the 1570-nm pump power with a fixed 250-mW primary Brillouin pump power, the SBS threshold was 700 mW and the slope efficiency was 49.5%, which is better than those with 5-m-long PM1500 fiber (total cavity length of 11 m) and the same output coupling of 30%. The inset of Fig. 5(b) shows the spectral linewidth recorded using the delayed self-heterodyne system. The 20-dB linewidth of 18.6 kHz indicates a 3-dB linewidth of 0.93 kHz, which corresponds to a 43-fold linewidth reduction. There is no sinusoidal amplitude modulation observed in the inset of Fig. 5(b), which illustrated that coherence elimination is achieved [19,27,36–38]. Considering the parameters in our demonstration, the Brillouin fiber laser has a theoretical linewidth reduction ratio of approximately 48 [16], a bit larger than the 43 achieved in our experiment, which ascribes to the influence of the ASE noise, mechanical vibration and environmental disturbance.

 

Fig. 5. (a) Output power with different Brillouin pump. (b) Power transfer of BTFL with 250-mW Brillouin pump power and 14-m-long cavity; inset: Stokes linewidth was measured at maximum output power of 344 mW. The dashed line serves as a visual guide.

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4. Summary

This paper experimentally investigated the influence of output coupling on the power and linewidth behavior of a narrow-linewidth BTFL. The experimental results revealed that there exists a trade-off between the SBS threshold, output power, and spectral linewidth of the BTFL. Thus, the laser performance can be optimized by choosing an appropriate output coupling. A Stokes output of 344 mW at 1956.62 nm with a narrow-linewidth of 0.93 kHz was obtained by optimizing the output coupling and cavity length of the BTFL.