1. Introduction

Owing to its advantageous mid-infrared operating wavelength at 2 µm, Tm³⁺-doped fiber lasers have attracted much attention in recent years. The combination of double-clad pumping with fully developed high-power laser diode (LD) at 793 nm has made it possible to scale the 2-µm fiber laser output to a new stage. Pumping the Tm³⁺ fiber laser with the 793-nm LD has the advantage of efficiently stimulating the cross relaxation (CR) process, which can effectively provide the ‘two-for-one’ excitation of the upper laser level, greatly improving the quantum efficiency of high power Tm³⁺-doped fiber lasers [1]. Since the first reported high-power double clad Tm³⁺-doped silica fiber laser [2], power scaling of the 2-µm fiber laser has achieved great progress [3–6]. Recently, more than 1 kW output power has been realized from the 2-µm Tm³⁺ fiber [7].

Many applications require not only high-power but also high stable, narrow spectral linewidth laser beams. High power narrow linewidth 2-µm lasers can find wide applications in such areas as ranging, Lidar, and spectral beam combining. Especially, spectral beam combination can provide high power levels unattainable from conventional single fiber laser sources.

Using fiber amplification of a well-controlled seed source has achieved several hundred watts of narrow bandwidth 2-µm laser output from Tm³⁺ fiber [8], but this system needs multiple amplification stages, having complicated structure. For single fiber lasers, it is very convenient to achieve narrow linewidth operation by using fiber Bragg grating (FBG) for mode selection. With FBG, it is also easy to realize all-fiber laser configuration, improving the robustness of the system. Zhang et al have shown tens of watts narrow linewidth 2-µm Tm³⁺ fiber lasers with FBG [9, 10]. However, the spectral linewidth increased greatly with the increase of output power, making it hard to maintain the narrow linewidth feature of the system at higher power levels. Recently, using volume Bragg grating (VBG) manufactured from opto-thermal refractive glass for selecting laser wavelengths in Tm³⁺ fiber lasers has attracted great attention, and more than 100-W 2-µm laser power and several-pm spectral bandwidth has been achieved [11–14]. However, it is impossible to realize an all-fiber configuration with such a kind of wavelength selective element.

In this paper, we report high power narrow-bandwidth 2-µm Tm³⁺ fiber lasers with an all-fiber configuration by using specially designed FBGs. The maximum output power is 137 W with a slope efficiency of 62%. At all output power levels, the laser wavelength can be stabilized at 2019 nm with spectral linewidth less than 3 nm. Influence of different laser propagating configurations and output couplings on the laser characteristics is also investigated. This kind of narrow linewidth fiber laser system has both high power and high robustness, and will find wide applications.

2. Experiment and results

The experimental setup for the narrow bandwidth Tm³⁺-doped fiber laser is shown in Fig. 1 . The double-clad Tm³⁺-doped silica fiber (Nufern Co.) had a ~25 µm diameter, 0.09 NA (numerical aperture) core doped with Tm³⁺ of ~4wt.% concentration. The pure-silica inner cladding, coated with a low-index polymer, had a 400 µm diameter and a NA of 0.46. The cladding absorption coefficient of the fiber at pump wavelength (~793 nm) was measured with the cut-back method to be ~3dB/m. The pump sources were six 793-nm high-power LDs with 200-µm (0.22 NA) pigtail fibers, which match to the pump fiber of the combiner. The total pump power was ~250 W. A (6 + 1) × 1 high-power fiber combiner (ITF Co. Canada) was used to couple the pump light into the gain fiber, with a coupling efficiency of ~95%. Specially designed FBGs (TeraXion Co., Canada) were used to complete the laser cavity and select laser wavelengths. The FBG was made with a matched passive fiber to the Tm³⁺ fiber for easy confusion splicing. A pair of FBGs was designed with the center wavelength of 2019.8 nm. One is high reflective (R = 98.7%) with a spectral FWHM (full width at half maximum) of 1.5 nm (19 dB side-mode suppression), and the other is partial reflective (R = 10%) with spectral FWHM of 0.5 nm (11 dB side-mode suppression).

 

Fig. 1 Experimental setup of the Tm³⁺ fiber laser with the co-propagating configuration; LD: laser diode; FBG: fiber Bragg grating; DM: dichroic mirror.

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At the output end, either the 10% FBG or the fiber end facet (~3.5% Fresnel reflection) was employed to provide laser feedback and act as the output coupler. The free fiber ends of the grating were angle cleaved by ~10° to eliminate end reflections. The 5-m long Tm³⁺-fiber was wrapped on a water-cooled copper drum with a diameter of 10 cm for cooling, and both the pump and output fiber ends were clapped in copper heat sinks. Under the maximum output power levels, the temperature of the pump-end fiber was measured to be 45 °C, while that of the central part of the fiber was about 30 °C under stable operation. Due to the quasi-three-level nature of the laser, cooling the fiber was critical for achieving high slop efficiencies. At the output end, a double-apex lens was used to collimate the laser beam, and a dichroic mirror (R>99.9%@793nm, 45°) was used to filter the un-absorbed pump light. The laser output power was measured with a FieldMate power meter (Coherent Co.) and the laser spectrum was tested with a mid-infrared spectrometer (SandHouse Co.) with a spectral resolution of 0.22 nm.

In experiment, two kinds of laser propagating cavity configurations were employed to get a more efficient laser operation of the Tm³⁺-fiber laser. One is the co-propagating laser oscillation structure (as shown in Fig. 1), in which the laser output direction is the same as the pump beam propagation direction. The other is the counter-propagating laser oscillation structure, where the laser output direction is opposite to the pump beam propagation direction. Through exchanging the high-reflection FBG with the partial-reflection FBG in Fig. 1, the counter-propagating laser configuration could be realized. In each laser propagating cavity structure, both the partial-reflection FBG and the perpendicularly-cleaved fiber end facet (R~4%) were alternately used as the output coupler.

With the co-propagating laser configuration, the 2 µm laser output characteristics are shown in Fig. 2 . The laser threshold is nearly the same for the fiber-end facet coupler and the 10% FBG coupler, being ~7.6 W. With the fiber-end coupler, the maximum output power was 130.5 W, corresponding to a slope efficiency of 59% with respect to the absorbed 793-nm pump power. On the contrary, the 10% FBG coupler only produced a maximum power of 100.7 W with a slope efficiency of 46%, both of which are much lower than that obtained with the fiber-end facet coupler. We attribute the lower efficiency in the cavity with the FBG coupler to non-optimal output coupling at the grating wavelength.

 

Fig. 2 Output of the 5-m Tm³⁺ fiber laser with the co-propagating configuration.

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The 2 µm laser output characteristics with the counter-propagating laser oscillation configuration are shown in Fig. 3 . The laser threshold is similar to that of the co-propagating laser oscillation. The key point is that the counter-propagating laser oscillation can provide a higher output power and laser efficiency. Besides, the laser output power shows less dependence on the output coupling values. With the fiber-end facet coupler, the maximum output power was 137 W, corresponding to a slope efficiency of 62% versus the absorbed pump. The 10% FBG coupler also produced a maximum power of 127 W with a slope efficiency of 57.6%. The slope efficiency is somewhat lower than that achieved by Moulton et al. [6], which is mainly owing to the spectral filtering loss of the FBG in our fiber laser systems.

 

Fig. 3 Output of the 5-m Tm³⁺ fiber laser with the counter-propagating configuration.

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In both co- and counter-propagating laser configurations, the laser output power shows a linear increase with the pump power, indicating the possibility of further power scaling simply by enhancing pump power. From the comparison of the output characteristics of these two laser cavity configurations, we can conclude that it is more appropriate to output the laser beam from the pumping end for high-power 2-µm Tm³⁺ fiber lasers. Further improving the laser efficiency is possible by optimizing the laser cavity (fiber length, output coupling, etc.) and adopting more efficient cooling methods for the fiber.

Laser spectra recorded with the co-propagating configuration and the counter-propagating configuration were very similar. At lower output power levels (<2 W), the laser spectral linewidth was determined by the 10% FBG to less than 0.5 nm. With increase of output power, the 10% FBG could not completely confine the spectral bandwidth due to its comparatively low side-mode suppression (11 dB). At higher power levels, the spectral width was mainly determined by the high-reflection FBG, but the spectral filtering effect by the 10% FBG could still be observed in experiment. Here, we only discuss the spectra obtained in the counter-propagating configuration with the fiber-end coupler and the 10% FBG coupler.

Laser spectra measured after the fiber-end coupler (the ~3.5% reflection) under different power levels are shown in Fig. 4 . Under all measurements, the center laser wavelength was always confined at around 2019 nm, showing the highly efficient wavelength-stabilizing effect of FBGs for high-power fiber lasers. With increasing laser output power, the laser spectral bandwidth (FWHM) was broadened from 1.2 nm to 3 nm. Such a spectral width is by far narrower than that obtained with no wavelength-selecting elements [15, 16]. At higher power levels, the spectral broadening implies that more longitudinal laser modes oscillated due to increased gain. At high power levels, the laser spectral bandwidth was maintained at ~3 nm, indicating the efficient spectral confining effect of the FBG.

 

Fig. 4 Laser spectra of the Tm³⁺ fiber laser after the fiber-end coupler.

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Laser spectra detected after the 10% reflection FBG at different output power levels are shown in Fig. 5 . Compared with that from the fiber-end coupler, the spectral width is somewhat narrower at lower power levels (6, 40 and 80 W). This can be attributed to the spectral filtering effect of the 10% FBG coupler. However, the low-reflection FBG nearly had no spectral filtering effect at especially high-power levels (e.g. 120 W) due to its somewhat low side-mode suppression ratio. In this case, the spectral characteristics were mainly determined by the opposite-side high-reflection FBG. Therefore, it is indispensible to improve the performance of the high-reflection FBG and increase the side-more suppression of the low-reflection FBG for further narrowing the spectral width of high-power fiber lasers with an all-fiber configuration.

 

Fig. 5 Laser spectra of the Tm³⁺ fiber laser after the 10% FBG coupler.

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The output beam propagation was characterized through measuring the 86.5% beam waist with the knife-edge technique at various positions by a lens (f = 75 mm), as shown in Fig. 6 , where near-field and far-field laser beam profiles are also indicated as the inset. The laser beam quality factor (M²) at the 100-W level was fitted to be 1.21 ± 0.02, confirming the fundamental mode operation of the fiber laser. This can be attributed to the mode-filtering effect by coiling the fiber to a small diameter [17]. The output power stability was monitored with the power meter during a period of 30 min (1 record per minute), showing a RMS (root mean square) power instability of <5%.

 

Fig. 6 M² measurement of the Tm³⁺ fiber laser at the 100-W level. Inset shows the near-field (left) and far-field (right) laser spot profiles obtained through paper burning.

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3. Conclusion

In this work, over 130-W 2-µm laser output power with narrow bandwidth (~3 nm) has been achieved in double-clad Tm³⁺-doped fibers. The laser efficiency is 62% with respect to the absorbed 793-nm pump power. This laser system has a monolithic all-fiber structure formed by a pair of FBGs. The counter-propagating configuration can achieve a higher output than the co-propagating configuration. Therefore, for high-power Tm³⁺-doped fiber lasers, it is more appropriate to output the laser beam from the pump end. The laser wavelength is stabilized at ~2019 nm by the FBG. However, the spectral linewidth is a bit broader than the designed value (1.5 nm) owing to the comparatively moderate side-mode suppression (19 dB). Further narrowing the spectral bandwidth of the high-power all-fiber Tm³⁺ fiber laser requires decreasing the reflection bandwidth and improving the side-mode suppression ratio of the FBGs. This kind of all-fiber and narrow-linewidth 2-µm fiber laser systems are especially useful in potential power scaling through spectral beam combination, and can also find extensive applications in other areas.