Stimulated Raman scattering (SRS)^[1]has been regarded as an effective method of building tunable narrow-linewidth laser sources, especially in the ultraviolet and infrared spectral ranges^[2,3] since it was observed in 1963 for the first time. However, the use of traditional gas cells has been suffering from a limited light-gas interaction length, bulky size, and high sensitivity to environmental disturbances^[4,5]. The emergence of low-loss microstructured hollow-core fibers (M-HCFs) makes the gas-filled fiber cell possible, featuring excellent beam quality, excessively long effective length, compactness, and robustness^[6–11].

Low-loss anti-resonant hollow-core fibers (AR-HCFs) have been broadly adopted in the study of fiber gas Raman lasers (FGRLs) since the first demonstration of hydrogen SRS inside the AR-HCF by Wang et al. in 2014^[12]. The extremely low overlap between the light field and silica of the AR-HCF not only reduces material absorption losses in the mid-infrared spectral region but also increases the damage threshold of silica-based fibers^[13–15]. Moreover, the tunable transmission spectrum of the AR-HCF facilitates the efficient generation of targeted Raman lines at longer wavelengths. At present, FGRLs based on Raman active gases such as H2-, CH4-, and D2-filled AR-HCFs have achieved output wavelengths ranging from the near-infrared^[16–19] to the mid-infrared^[20–28], with the longest wavelength exceeding 4.4 µm^[20,23]. However, the output power was almost at the watt level.

Thanks to the mature development of solid-state/fiber laser sources at 1 µm as pumps, hydrogen-filled FGRLs at 1.9 µm wavelength based on AR-HCFs are broadly studied as listed in Table 1^{[12,17,18,29–33]}. In 2014, Wang et al. reported the first FGRL based on a H2-filled AR-HCF at 1.9 µm by pumping with a microchip laser at 1 µm. They achieved an average power output of 9.27 mW and a quantum conversion efficiency of 48%^[12]. Subsequently, Gladyshev et al. further improved the average output power (300 mW) and the quantum efficiency (87%)^[29]. In 2017, a 150-kW-peak-power, 2-GHz-linewidth, 1.9 µm laser source was achieved with a 1.4 m fiber length and only 3 bar H2 by Wang et al.^[32]. Similarly, Li et al. demonstrated an average output power of 570 mW and quantum efficiency of 51% with a 1 GHz linewidth^[18]. In 2024, Jiang et al. demonstrated a picosecond-pulsed SRS laser with a maximum output power of 7.3 W and a conversion efficiency of 34%^[31], the highest so far for the SRS conversion in the picosecond temporal domain.

It is noted that the SRS of gas would usually require a pulsed laser for the pump because of a relatively high threshold. The enhancement of light and gas interaction in the low-loss HCF of long lengths makes the continuous operation of FGRL feasible using a single-frequency continuous-wave laser source of high brightness as the pump. In 2007, Benabid et al. reported the first continuous-wave FGRL at 1135 nm using a 30 m M-HCF filled with 5 bar H2, pumped by a single-frequency fiber laser at 1064 nm^[10]. Subsequently, Couny et al. successfully increased the output power to 55 W^[11]. In 2022, Zhu et al. demonstrated a 1135 nm continuous-wave FGRL pumped by a single-frequency fiber laser at 1064 nm with a 10 kHz linewidth, achieving a maximum output power of 26 W^[19]. In 2023, Cui et al. reported a 110 W continuous-wave FGRL at 1153 nm in a H2-filled M-HCF^[34]. In 2021, Li et al. reported a 1693 nm continuous-wave FGRL pumped by a 1.54 µm narrow-linewidth fiber laser, achieving a Raman output of 1.8 W^[35]. However, the continuous operation of FGRLs at longer wavelengths has not been reported yet.

In this paper, we report, for the first time, to the best of our knowledge, the experimental demonstration of a continuous-wave FGRL at 1.9 µm wavelength based on high-pressure hydrogen filled in a customized five-tube nested low-loss AR-HCF. A linearly polarized single-frequency fiber laser with a 8.5 kHz linewidth at 1064 nm was used as a pump source. A forward Stokes laser over 25 W is measured at the output of FGRL, corresponding to a power conversion efficiency of 40% and a quantum efficiency of 72.5%. The backward Stokes power is found increasing as the pressure rises, exceeding 5 W at 30 bar. The backward SRS competition could limit the further increase in forward Stokes output power at high pressure.

The customized nested type of quasi-single-mode AR-HCF was fabricated using Heraeus F300 fused silica tubes by a stack-and-draw method. Figure 1(a) shows its scanning electron microscope (SEM) image. The AR-HCF consists of five nested capillaries in the cladding and a core with a diameter of approximately 31 µm. The average diameter of the outer/inner nested capillaries is approximately 32/18 µm and the outer/inner core wall thickness is 702/603 nm. The fiber attenuations were measured about 2 and 21 dB/km at 1064 and 1907 nm by two cut-backs from 455 to 45 m and from 200 to 20 m, respectively. As shown in Fig. 1(b), the molecular absorption feature in the spectral range from 1.7 to 1.94 µm is attributed to the water vapor and CO2 in the air and AR-HCF^[36].

The experimental setup of FGRL is illustrated in Fig. 2(a). A single-pass configuration was adopted using a 47-m-long nested AR-HCF. Both ends of the AR-HCF were sealed into customized gas cells to vacuum and fill with H2. A 4f lens system was employed to achieve around 86% coupling efficiency at the incident end of AR-HCF. The coupling efficiency decreases slightly as the pump power increases, reaching approximately 76% at the maximum output power. A dichroic mirror (DM1), with a transmission efficiency of 99% at 1064 nm and a reflection efficiency of 98.5% at 1907 nm, was used to monitor the backward Stokes power. At the output end, the output beam passed a second dichroic mirror DM2 to separate the residual pump and the forward Stokes laser.

A customized single-frequency continuous-wave fiber laser at 1064 nm was used as the pump source. It has a 3 dB linewidth of 8.5 kHz, which remains stable even at the maximum output power of 100 W. The laser output is linearly polarized and nearly diffraction-limited with an M2 of 1.2 at the maximum output power. The output spectrum was measured by an optical spectral analyzer (YOKOGAWA-AQ6374 in a wavelength range from 350 to 1700 nm), as shown in Fig. 2(b). The linewidth was characterized by self-heterodyne measurement, shown in Fig. 2(c).

In the experiment, no rotational SRS was observed in either forward or backward direction in the condition of H2 pressure up to 30 bar. Figure 3 shows the typical measured Stokes light and anti-Stokes light in the forward direction, at 10 bar H2 pressure. Two optical fiber spectrometers were used (Ideaoptics-NIR2500 in a wavelength range from 900 to 2500 nm, and OceanInsight-Maya2000Pro in a wavelength range from 165 to 1100 nm). The threshold of forward Stokes light at 1907 nm wavelength was measured at 31 W. As the pump power was raised to approximately 50 W, the first vibrational anti-Stokes light appeared at 737 nm in the forward direction. Notably, the measured loss at 737 nm was excessively high. The suppression of the first anti-Stokes light conversion could partly contribute to the high quantum efficiency in our laser system. The absence of rotational SRS was attributed to the suppression of rotational SRS gain where the linear polarization of the pump is regarded as favorable for vibrational SRS^[7,37].

Figure 4 shows the measurement of the output of FGRL in dual directions. The SRS threshold power decreases as the gas pressure increases, and a minimum of 27.3 W at 30 bar was measured as shown in Fig. 4(a). It is attributed to a higher Raman gain coefficient at higher pressures, as demonstrated in our previous work^[19]. In Figs. 4(b) and 4(c), it is noted that the forward output power and quantum efficiency decrease at higher pressure above 10 bar. In contrast, the backward quantum efficiency increases with the rise in pressure, surpassing 15% at 30 bar as depicted in Fig. 4(d). Different from previous studies^[12,17], the conversion of backward SRS observed in our experiment was significantly more efficient. Such enhancement could be originated from the use of a narrow-linewidth pump source, which significantly raises the backward SRS gain^[38,39]. Furthermore, the SRS gain in the backward direction rises with pressure^[40], and the competition would contribute to the degradation of conversion efficiency of forward SRS as shown in Fig. 4(c). Notably, no vibrational anti-Stokes light was measured in the backward direction in our experiment.

Figure 4(e) shows the calculated total quantum efficiency. Notably, when the gas pressure exceeds 10 bar, the total quantum efficiency decreases with increasing pressure and coupled pump power, indicating the presence of certain factors affecting Raman conversion under higher pressures and higher pump power. This phenomenon has been explored in previous studies^{[7,16,41–44]}. Some studies attribute it to the absorption of gas molecules^[41,44]. Other studies have proposed explanations such as a Raman-enhancement-induced self-focusing effect^[7,16] and higher-order Raman shifts^[43]. However, based on our experimental results, we believe this phenomenon can be primarily attributed to the thermal effect of the gas. As previously reported^[42], the quantum defect in the SRS conversion process generates significant heat. Therefore, the temperature gradient and refractive index distribution of the gas inside the HCF may be changed, causing the pump light and Stokes light to diverge, thereby reducing the power densities of both and leading to a decrease in SRS gain. This phenomenon becomes particularly pronounced under higher gas pressure and higher pump power. In our experiment, the residual pump power gradually increased with rising gas pressure at higher pump power, as shown in Fig. 4(f). The decline in pump utilization suggests that Raman conversion is hindered, supporting the above interpretation.

Figures 5(a) and 5(b) show the characterization of linewidths of the pump and forward vibrational Stokes laser using scanning Fabry-Perot (FP) interferometers (Thorlabs, SA200-8B for 820–1275 nm and SA200-12B for 1275–2000 nm, both with a free spectral range of 1.5 GHz and a resolution of 7.5 MHz). The calculation of linewidth by FP followsΔν=FSRΔT·Δt,(1)where FSR is the free spectral range, Δt is the full width at half-maximum (FWHM) of the pulse signal, and ΔT is the time interval between two adjacent pulse signals. The calculated linewidths were approximately 9.5 and 10.5 MHz for the pump and forward vibrational Stokes laser, respectively, implying FP was hardly capable of resolving the linewidth of the vibrational Stokes laser. It is noteworthy that the expected Raman gain linewidth is approximately 300 MHz^[45], which is an order of magnitude larger than the measured spectral width of the Stokes line, indicating a significant gain spectrum narrowing effect^[46].

Additionally, we compare the performance of FGRL with the latest reported single-frequency Tm3+-doped fiber lasers at 1.9 µm wavelength in Table 2. FGRL in this work demonstrates promising potential as a new high-power narrow-linewidth mid-IR laser source compared with the cutting-edge technology of Tm3+-doped fiber lasers.

The temporal stability of FGRL output in the forward direction was recorded as shown in Fig. 6. At 10 bar and 20 bar pressures, average output powers were measured at 15.39 and 8.72 W, respectively, with the root-mean-square deviations of 1.4% and 2.9%. The temporal output power fluctuations could be attributed to the mechanical and thermal-induced drift of incident coupling. It is noted that no water cooling was applied in the gas cell, and at the incident end, the temperature of the gas cell could reach above 80°C when the coupled pump power of 60 W exceeded 5 min.

For the first time, to the best of our knowledge, we successfully demonstrated an efficient 1.9 µm narrow-linewidth continuous-wave FGRL based on hydrogen-filled nested AR-HCFs. The rotational SRS was effectively suppressed by employing a linearly polarized pump beam. For coupled pump power of 65 W, the forward vibrational Stokes power reached over 25 W at 10 bar, corresponding to a power conversion efficiency of 40% and a quantum efficiency of 72.5%. In the future, a continuous-wave high-power FGRL at 4 µm could be built by a pump at 1.55 µm wavelength. Meanwhile, more efforts will be necessary to focus on the suppression of backward SRS to maximize the output of FGRL.