1. Introduction

Cladding-pumped high power fiber lasers have attracted considerable attention in recent years due to their high efficiency, compactness and good beam quality. Fiber-based sources benefit from a geometry that offers simple thermal management and a high degree of immunity from effects of heat loading. Moreover, the output beam quality is generally determined only by the core parameters of the fiber and is independent of the power level [1]. An additional attraction of rare earth doped fibers is the broad emission bandwidth that is typical in glass hosts [2], which allows for considerable flexibility in operating wavelength selection. For many applications the need for high output power and good beam quality is also accompanied by the requirement for narrow spectral width and, sometimes, flexibility in operating wavelength. One popular method for spectrally narrowing a fiber laser is to use fiber Bragg gratings (FBGs), which offer a narrow linewidth and high reflectivity with low insertion loss and above all, alignment-free operation as all-spliced geometries are possible. However, FBGs have the disadvantage that they are not very effective for wavelength selection and spectrum narrowing in large mode area fibers that sustain higher transverse mode [3]. Furthermore, FBGs are difficult to be tuned for wide range by either thermal tuning or mechanical stress. Another popular approach is to use an external feedback cavity with a replica diffraction grating, which has the virtue of wide tuning range usually only limited by the emission spectrum of the gain medium. However, it is difficult to obtain bandwidth narrower than 0.5 nm and the laser setup is cumbersome with a relatively large collimated beam size.

An alternative strategy for wavelength selection is to use a volume Bragg grating (VBG). VBGs recorded in photo-thermal refractive (PTR) glass can be made with diffraction efficiency exceeding 99% while have a spectral and angular selectivity (FWHM) narrower than 20pm and 0.1 mrad, respectively. In addition, VBGs allow thermal stability of up to 400°C, and have laser damage threshold of 40 J/cm² for 8 ns pulses, and tolerance to CW laser radiation in the near IR region at least up to several tens of kilowatts per square centimeter [4]. These elements have low losses and have been used for locking the wavelength and narrowing spectrum width of diode lasers [5], optical parametric oscillators [6,7] and solid-state lasers [8–12]. More recently, VBGs have been successfully used as wavelength selective and spectral narrowing element of cladding-pumped high power Yb fiber laser at 1.066 μm [13] and Er,Yb [2] fiber laser at 1.553 μm, yielding 138 W and 103 W of cw output with spectra linewidths of 0.2 and 0.4 nm (FWHM), respectively. Thulium doped fiber lasers operating in the eye-safe 2 μm wavelength region are not only of interest for laser radar (lidar) and medical applications, but also provide an ideal platform for the generation of mid-infrared (3~5μm) radiation through nonlinear frequency conversion. Wavelength-locking and spectral narrowing of thulium fiber lasers using VBGs has very recently been investigated and reported [3,14], up to 17 W of continuous-wave output power at 2054nm with a narrow linewidth of 50 pm has been achieved.

In this paper we report a cladding-pumped high power Tm-doped silica fiber laser using a VBG for wavelength-selection and spectral narrowing. Over 112 W of nearly diffraction-limited continuous-wave output at 1988 nm was generated with a narrow linewidth of 12 pm for 279 W of launched pump power corresponding to a slope efficiency of 43.4%. Performance characteristics of the laser have been compared to that using a broadband dielectric mirror, instead of VBG, as external reflector, and the two laser set-ups performed very similarly in terms of threshold, slope efficiency and output power.

2. Experiments and results

The experimental setup is shown in Fig. 1 . The fiber used in our experiments had a 25 μm diameter (0.17 NA) Tm-doped alumino-silicate core, surrounded by a 300 μm diameter D-shaped pure silica inner-cladding with a nominal NA of approximately 0.46. The effective absorption coefficient for pump light launched into the inner-cladding at 792 nm was estimated, via a cut-back measurement, to be ~4 dB/m and hence a fiber length of ~4.8 m was used for efficient pump absorption. Pump light at 792 nm was provided by a diode laser which could deliver a maximum cw output power of ~376 W with beam quality factors, M_x ² ~80 and M_y ² ~100 in orthogonal planes. The pump beam from the diode laser was split into two beams of roughly equal power and then launched into opposite ends of the fiber with the aid of anti-reflection coated lenses of 20 mm focal length and dichroic mirrors with high transmission (>96%) at 780-803 nm and high reflectivity (>99.5%) at 1800-2050 nm at 45° degrees. The total pump power available in front of the two fiber ends was 349 W, and the launch efficiency into the fiber inner cladding was estimated to be ~80%.

 

Fig. 1 Schematic diagram of the laser setup.

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The laser resonator was formed on one side by Fresnel reflection provided by the perpendicularly cleaved fiber end and on the other side by an antireflection-coated plano–convex collimating lens followed by a reflective VBG which was mounted in a copper heat sink with a layer indium foil (0.1 mm in thickness) in between to ensure good thermal contact. The VBG (OptiGrate Corp.) used in our experiments was 10.89 mm thick with a clear aperture of 10 × 6 mm and anti-reflection coating (R<0.2%) at 1800-2100 nm. It was designed to have reflectivity greater than 99% at a center wavelength of 1989.7 nm with a spectral width (FWHM) of 0.65 nm. The focal length of the collimating lens was chosen to be 15 mm to make full use of the VBG aperture. The fiber end nearest the grating was angle cleaved at ~6° to suppress broadband feedback from the uncoated facet. The fiber laser output was extracted by means of a dichroic mirror near the perpendicularly cleaved fiber end and collimated by an anti-reflection coated 30 mm focal length lens into a power meter (Gentec-EO). Both end sections of the fiber were carefully mounted in water-cooled V-groove heat sinks to prevent thermal damage to the fiber coating due to unlaunched pump power and by heat generated in the core due to quantum defect heating. The central section of the fiber was coiled into a water cooled 20 cm diameter Al heat sink to provide thermal management and hence improve laser efficiency, since the performance of Tm³⁺ ions are highly dependent on temperature as a quasi-three-level system. The spectrum of the output was analyzed using a 0.55 m monochromator containing a 300 lines/mm grating blazed at 1800 nm and a TE-cooled InGaAs detector (0.8-2.2 μm): the resolution of the monochromator is specified to be 50 pm at 435.8 nm, and is estimated to be ~0.9 nm at around 1998 nm. A plane/plane scanning Fabry-Perot (FP) interferometer with a free spectral range (FSR) of 25 GHz was used to determine the actual spectral linewidth. The plane mirrors used in the FP interferometer were coated to provide 98% of reflectivity at 1850-2100 nm, and were wedge-shaped to avoid Fresnel reflection from the two uncoated outside facets.

The laser reached threshold at a launched pump power of about 7 W and generated a maximum output power of 112 W at 1988 nm for 279 W of launched pump power (see Fig. 2 ), corresponding to an average slope efficiency with respect to launched pump power of 42.4%. For pump powers more than about four times above threshold, the slope efficiency increased to ~43.4% with respect to launched pump power due to the saturation of the reabsorption loss. The beam quality parameter (M²) of the laser output was estimated with laser beam profiler (NanoScan, Photon Inc.) to be ~1.5. To compare the performance characteristics of the wavelength-locked narrowband Tm:fiber laser with that of a free-running one (i.e., without wavelength selection), the VBG was replaced by a broadband dielectric mirror with high reflectivity at 1800–2200 nm. Output power as a function of launched pump power is shown in Fig. 2. It can be seen that the two setups have almost identical performance in terms of slope efficiency, threshold pump power and output power, suggesting that the insertion loss of the VBG is very low.

 

Fig. 2 Output power versus launched pump power (inset: F-P spectrum of the VBG locked laser output).

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Figure 3 shows the measured spectrum of the fixed-wavelength and free-running Tm doped fiber laser (TDFL). The free-running TDFL has a relatively broad emission bandwidth of ~20 nm centered at 1985 nm. In contrast, the laser with VBG for wavelength selection has a much narrower lasing spectrum centered at 1988 nm. The actual linewidth was determined using a scanning Fabry-Perot interferometer instead of monochroamtor to be ~12 pm as shown in the inset of Fig. 2, indicating a spectral brightness increase of >10³.

 

Fig. 3 Typical spectral output from free-running laser compared to VBG-locked laser.

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The 12 pm spectral FWHM achieved here is much narrower than its specified parameter 0.65 nm, which could be explained by mode competition due to the unflat-top reflectivity curve of the VBG, as a result of which, only the wavelengths with lowest loss survived. It should be noted that the measured central wavelength 1988 nm is about 2 nm offset the wavelength 1989.7 nm specified by OpiGrate most likely due to different calibrations of the monochromators used.

We can see from Fig. 2 that the output power increases more or less linearly with pump power, and no degradation of the VBG performance had been seen even at the maximum pump power. Thus, it appears that there is scope for further scaling in output power by simply increasing the pump power. Possessing combined advantages of very low insertion loss, high damage threshold and good thermal stability, VBGs offer an excellent approach for high power narrow linewidth light source generation from cladding pumped large mode area fiber lasers. Future work for power scaling should include more efficient cooling of the VBG to avoid possible detrimental thermal effects that may arise at high operation power levels. The output of the VBG-locked laser was monitored throughout the experiment with a high speed InGaAs detector (25 ns rise time) and a 100 MHz digital oscilloscope. No self-pulsation was observed at all power levels and the short-term stability was measured to be < 0.91% (rms).

3. Conclusion

In this paper we present a cladding-pumped high power Tm fiber laser using a volume Bragg grating as wavelength selective and spectral narrowing element. Up to 112 W of narrow linewidth diffraction limited (M² ~1.5) output power at 1988 nm was obtained with a spectral linewidth of 12 pm for 279 W of launched pump power, corresponding to a slope efficiency with respect to launched pump power of 43.4%. The VBG-locked laser has almost identical performance in terms of slope efficiency, threshold and output power with that of a free running laser with a broadband dielectric mirror, showing that the use of volume Bragg grating in combination with high power fiber laser technology is an attractive approach to generate high power narrow linewidth laser sources.