1. Introduction

Rare-earth-doped high-power fiber lasers and amplifiers, well-known for their high beam quality and high efficiency properties have been rapid developed for the last decade [1–3], driven by the needs of a vast range of applications. Recently, the average power of the single-mode continuous-wave (CW) ytterbium-doped all-fiber master-oscillator power-amplifier (MOPA) has reached 20 kW. At the same time, the effort to extend the operating wavelength of fiber lasers and amplifiers [4–7] toward the longer mid-infrared wavelength region is also extremely active, driven by a large number of promising applications including LIDAR, gas sensing, and optical communication. In the most cases, thulium-doped fiber lasers and amplifiers typically operate efficiently between 1.8~2.1 µm wavelength regions [8–15] and have been demonstrated at output power exceeding 1 kW at 2045 nm [16]. However, thulium-doped fiber lasers and amplifiers are not able to high-efficiently access longer wavelengths (>2.1 µm) at high-power level due to the lower gain of thulium in silica fiber. For a range of applications including nonlinear optics, medicine, and sensing, a simple, efficient, and robust source of high-power >2.1 µm wavelength radiation is desirable. Recently, A number of holmium-doped fibre lasers and amplifiers have been demonstrated to enable the power scaling of laser emissions at >2.1 µm wavelength [17–19]. For instance, Hemming et al. demonstrated a single-mode, monolithic holmium-doped silica fibre laser resonantly pumped by thulium-doped fibre lasers through a fused fibre pump combiner, which produced 407 W of output power at 2.12 µm [18]. Simakov et al. demonstrated a tunable, high-power cladding-pumped holmium-doped fiber laser with a maximum output power of 18.2 W at 2.17 µm as well [19].

Another alternative method for the achievement of emission wavelength at >2.1 µm is to use Raman fiber lasers or amplifiers. The stimulated Raman scattering (SRS) has permitted the development of a wide variety of fiber lasers and amplifiers at wavelengths for which there are no rare-earth-doped gain media available by using diode pump sources based on ytterbium- [20–22], erbium- [23], and thulium-doped [24] fiber lasers. The principle is to wavelength convert the output of a rare-earth-doped fiber laser or amplifier to the required output wavelength by using the first order or cascaded Raman Stokes shifts [25]. Most notably, Fortin et al. have presented the first experimental demonstration of a Raman laser based on a fluoride glass optical fiber [26]. The maximum output power was 580 mW at the first Stokes order of 2.18 µm. And then, a multi-watt CW fluoride glass Raman fiber laser operating at 2.23 µm is demonstrated as well [27]. The maximum output power of 3.7 W was obtained from a nested cavity setup with a slope efficiency of 15% with respect to the 1.98 µm launched pump power. In recent work, Duhant et al. have demonstrated cascaded Raman wavelength shifting up to the fourth order ranging from 2.1 to 2.4 µm in a low-loss chalcogenide suspended-core fiber [28].

In this contribution, we demonstrated a 2147 nm silica-based Raman all-fiber amplifier with output power of 14.3 W directly pumped with a 1963 nm CW thulium-doped all-fiber MOPA. The conversion efficiency was 38.5% from 1963 nm to 2147 nm in the Raman fiber amplifier. The output power achievable was only currently limited by available 1963 nm input power and the architecture has significant scaling potential. To the best of our knowledge, this is the highest power operation of a Raman fiber amplifier at >2 µm wavelength region.

2. Experimental setup and results

The high-power silica-based Raman all-fiber amplifier system consists of a 2147 nm Raman Stokes seed laser, a 1963 nm Raman pump MOPA, and a piece of highly nonlinear Raman gain fiber. For improved long term reliability, all-fiber configuration was chosen for the Raman fiber amplifier system, the experimental setup is schematically shown in Fig. 1. The Raman Stokes seed laser is a 2147 nm CW thulium-doped all-fiber laser, which is constructed in a unidirectional ring cavity configuration depicted in Fig. 1.

 

Fig. 1 Schematic setup of the high-power silica-based Raman all-fiber amplifier at wavelength of 2147 nm, FBG: fiber Bragg grating, HNLF: highly nonlinear fiber.

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In the experiment, in order to achieve lasing at >2.1 µm wavelength region, a piece of 13 m thulium-doped double-clad single-mode fiber (Nufern, Inc. cladding-absorption of 3 dB/m at 793 nm) was chosen to strength the re-absorption. The core of the thulium-doped double-clad fiber has a diameter of 10 µm and a numerical aperture (NA) of 0.15, and its inner cladding has a diameter of 130 µm and a NA of 0.46. The pump source was two high-power fiber-pigtailed multimode diode lasers (BWT Beijing Ltd., China.) at 793 nm with fiber core of 105 µm (NA = 0.22), and the total maximum output power of 22.5 W. A (2 + 1) x1 pump combiner was used to deliver pump light to the thulium-doped double-clad fiber. A narrow bandwidth fiber Bragg grating (FBG) (99.5% reflectivity at 2147 nm wavelength, full-width at half-maximum (FWHM) of 0.08 nm) is fusion spliced to the second port of the three port circulator. The circulator has 30 dB isolation from port 3 to port 1, the circulator also acts as an isolator which ensures a unidirectional propagation and suppresses self-pulsing generation. The maximum insert loss of the circulator was 3 dB at 2.1 µm wavelength region, which is quite high. The 2147 nm laser is out of the 70% port of the 70/30 fused coupler while the 30% is spliced back into the laser cavity, and the insert loss of the fused coupler was 1.5 dB at 2.1 µm wavelength region.

Figure 2(a) shows the output power of 2147 nm seed laser as a function of the 793 nm incident pump power. Due to the large loss of the total cavity round-trip, the laser has a pump threshold of 5.5 W. The output power increased almost linearly with the increase of 793 nm incident pump power. At the incident pump power of 22.5 W, the maximum output power of the 2147 nm seed laser was 2.4 W. And the slope efficiency was 14% from 793 nm to 2147 nm, which is relatively lower compared with the thulium-doped fiber laser or amplifier operating at 1.9~2 µm wavelength regions [14, 15]. However, the pump threshold and slope efficiency should be improved significantly with low insertion loss optical components. The higher output power achieved was only limited by available pump power. Figure 2(b) shows the optical spectrum (in log and linear scale) of the 2147 nm seed laser at highest output power of 2.4 W, which was measured by an optical spectral analyzer (YOKOGAWA AQ 6375) with resolution of 0.05 nm. The central lasing wavelength of the seed laser was around 2147.6 nm, which is same to the resonant peak of the FBG; the spectral FWHM bandwidth was 0.1 nm.

 

Fig. 2 (a) Output power of the 2147 nm seed laser with the increase of 793 nm incident pump power. (b) Optical spectrum of the 2147 nm seed laser at output power of 2.4 W.

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The 1963 nm seed laser was a home-made CW single-mode thulium-doped all-fiber laser, which has a center wavelength of 1962.6 nm (3 dB spectral bandwidth is 0.09 nm) and the maximum output power of 200 mW. And then, the 2147 nm seed laser and the 1963 nm seed laser are combined to the fiber core by using a 90/10 high-power fused coupler, as shown in Fig. 1. A two-stage cladding-pumped thulium-doped all-fiber amplifier at wavelength of 1963 nm is seeded with the 2147 nm seed laser at the same time. A high-power broadband isolator is followed after the two seed lasers (1963 nm and 2147 nm) to ensure two lasers would not be affected by the backward light from the thulium-doped fiber preamplifier. The active fiber of the preamplifier was a piece of 3 m thulium-doped double-clad fiber, characterized by the same parameters as 2147 nm seed laser mentioned above. Two high-power fiber-pigtailed diode lasers at 793 nm are employed as the pump source, and the total maximum output power of 24 W. Figure 3(a) shows the total output power of the 1963 nm and 2147 nm laser from the thulium-doped fiber preamplifier with the increase of 793 nm incident pump power. The thulium-doped fiber preamplifier produced 4.2 W output power for 13 W incident pump power, which corresponds to a slope efficiency of 45%. Figure 3(b) shows the output spectrum (in log scale) of the 1963 nm and 2147 nm laser from the thulium-doped fiber preamplifier at output power of 4.2 W. The 3 dB spectral bandwidth of the 2147 nm laser is the same as that in the Raman Stokes seed laser. According to the emission spectrum, the 1963 nm laser power ratio is calculated to be ~40%, which corresponds to the 1963 nm laser is 1.68 W.

 

Fig. 3 (a) Total output power of the 1963 nm and 2147 nm laser from thulium-doped fiber preamplifier with the increase of 793 nm incident pump power. (b) Output spectrum of the 1963 nm and 2147 nm laser from thulium-doped fiber preamplifier at output power of 4.2 W.

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In the thulium-doped fiber power amplifier, a segment of 4 m thulium-doped double-clad fiber was used as the gain medium, characterized by the same parameters as thulium-doped double-clad fiber mentioned above. To improve the conversion efficiency in the fiber power amplifier, the thulium-doped double-clad fiber was cooled to 10°C in a water-cooled heatsink in the high-power amplification process. A (6 + 1) x1 pump combiner was used to deliver pump light to the thulium-doped double-clad fiber from six fiber-pigtailed diode lasers, which give the total output power of 90 W at wavelength of 793 nm in a 0.46 NA 10/125 µm double-clad passive fiber. A piece of 2 m passive single-clad fiber (SMF-28) spliced to the thulium-doped double-clad fiber for stripping the residual 793 nm pump laser. Figure 4(a) shows the total output power of the thulium-doped fiber power amplifier versus 793 nm incident pump power. The maximum output power was 40 W at 90 W incident pump power limited by available pump power, which corresponds to a slope efficiency of 44% and the output power increased almost linearly with the increase of 793 nm incident pump power. Due to the quasi-three level nature of the laser, cooling the thulium-doped fiber to the lower temperature was very critical for achieving high slope efficiency in the high-power fiber power amplifier [14, 15, 29]. Figure 4(b) shows the optical spectrum (in log scale) of the thulium-doped fiber power amplifier at output power of 40 W. According to the spectra, the gain at 1963 nm is much higher than 2147 nm and the 1963 nm signal laser get amplified greater than the 2147 nm signal laser, and the 1963 nm laser power ratio is calculated to be ~90%.

 

Fig. 4 (a) Total output power of the 1963 nm and 2147 nm laser from thulium-doped fiber power amplifier with the increase of 793 nm incident pump power. (b) Output spectrum of the 1963 nm and 2147 nm laser from thulium-doped fiber power amplifier at output power of 40 W.

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In the Raman fiber amplifier, a piece of 50 m silica-based HNLF is used as Raman gain medium, which is directly spliced to the single-mode passive SMF-28 by using an arc-fusion splicer, which resulted in splice losses of ~1 dB. The HNLF had a mode field diameter of ~2.5 µm, and a NA of 0.35 to reduce bend-induced losses at wavelength >2.1 µm. The HNLF has a cutoff wavelength of 1.48 µm. The output power of the 1963 nm and 2147 nm laser from the Raman fiber amplifier versus the 1963 nm incident pump power is plotted in Fig. 5(a). When the output power of thulium-doped fiber power amplifier reaches 40 W, corresponding to a 1963 nm incident pump power of ~28.8 W (splice efficiency is ~80% between the SMF-28 and HNLF, and the 1963 nm laser power ratio is ~90%). The total output power of 1963 nm residual laser and 2147 nm Raman laser from the HNLF is up to 20 W.

 

Fig. 5 (a) Output power of the 1963 nm residual laser and 2147 nm Raman laser from Raman fiber amplifier versus the 1963 nm incident pump power. (b) Output spectrum of the 1963 nm residual laser and 2147 nm Raman laser from Raman fiber amplifier at different output power.

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The output spectrum (in log scale) of the 1963 nm residual laser and 2147 nm Raman laser from Raman fiber amplifier at different output power is depicted in Fig. 5(b). According to the spectra, when the total output power was 20 W, the power ratios of 1963 nm and 2147 nm are ~28.4% and 71.6%, respectively. The power of the 1963 nm residual laser and 2147 nm Raman laser are calculated to be ~5.7 W and 14.3 W, respectively. The conversion efficiency was 38.5% from 1963 nm to 2147 nm in the Raman fiber amplifier. However, the 2147 nm Raman laser power ratio from the final output can be improved by increasing the length of HNLF. The 3 dB spectral bandwidth of the 2147 nm Raman laser in the final output and it broadened from 0.1 nm (2147 nm Raman Stokes seed laser) to 2.3 nm at the maximal output power of 14.3 W, which is attributed to the nonlinear phenomena, especially four-wave mixing (FWM) between numerous longitudinal modes associated with a piece of 50 m HNLF fiber [21, 22]. The 2147 nm Raman laser was also monitored by a 25 GHz realtime oscilloscope (Agilent DSO-X92504A) and a 12.5 GHz InGaAs photodetector (EOT, ET-5000F) and no sign of self-pulsing is observed. In addition, we also measured the output power fluctuation of the 2147 nm Raman laser at maximum output power. An output power fluctuation of ± 0.5% was measured within 30 minutes, which indicates a good stability for the high-power Raman fiber amplifier system. Figure 6 shows the 1963 nm residual laser and 2147 nm Raman laser power ratio as a function of the total output power from the Raman fiber amplifier. When the total output power was above ~12.5 W, the first order Raman Stokes shift from 1963 nm to 2147 nm becomes significant already. Future work will involve further power scaling efforts and simulation studies to better understand the relations between various laser parameters and to obtain the higher power operation.

 

Fig. 6 1963 nm residual laser and 2147 nm Raman laser output power ratio as a function of the total output power from Raman fiber amplifier.

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3. Conclusion

In summary, we have demonstrated a silica-based Raman all-fiber amplifier system delivering as much as 14.3 W of output power at a wavelength of 2147 nm. The conversion efficiency for the Raman fiber amplifier was 38.5% from 1963 nm to 2147 nm. The higher output power achieved was only limited by available 1963 nm input power. To the best of our knowledge, this is the highest power operation of a Raman fiber laser or amplifier at >2 µm wavelength region. Power scaling of the Raman fiber amplifier into hundred-watt-levels can be done with the proposed architecture [22].