High-power fiber sources, especially those operating in the eye-safe wavelength regimes around ∼1.5–1.6 and 2μm, have numerous applications in areas such as remote sensing, range finding, and free-space and satellite communications. For many of these, the need for high output power and good beam quality is accompanied by a requirement for narrow spectral width and, sometimes, flexibility in operating wavelength. One popular method for spectrally narrowing a fiber laser's output is to use fiber-Bragg gratings (FBGs), which offer narrow linewidths and high reflectivity with low insertion losses and, above all, alignment-free operation, since use of all-spliced geometries is 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 modes, so they are limited to low power levels. In addition, FBG tuning is difficult for a wide range using either thermal tuning or mechanical stress. Another popular approach is to use an external feedback cavity with a replica diffraction grating. This allows a wide tuning range, usually only limited by the emission spectrum of the gain media. However, it is difficult to obtain bandwidths narrower than 0.5nm and the laser setup is cumbersome, with a relatively large collimated beam size.
Volume-Bragg gratings (VBGs) recorded in photothermal refractive glass combine the advantages of high diffraction efficiency (>99%), narrow spectral width (<20pm), low insertion losses, a high damage threshold, and good thermal stability. These properties make them very attractive for applications of high-power, narrow-linewidth, and tunable laser sources.
We have demonstrated narrow-linewidth and widely tunable operation of cladding-pumped, high-power, thulium (Tm)-doped fiber lasers employing a VBG in their external cavity. We achieved over 112W of diffraction-limited (beam quality: M2∼1.5, where M2=1is fully diffraction limited) output at a wavelength of 1988nm, with a spectral linewidth of ∼12pm for 279W of launched pump power. This corresponds to a slope efficiency with respect to launched pump power of 43.4%. No discernable difference was observed in terms of output power and slope efficiency when using a broadband, highly reflective mirror instead of the VBG, suggesting that VBG insertion losses are very low.1 The Bragg wavelength (λB) of a VBG depends on the (internal) incident angle (θ) as λB=λ0cos θ, where λ0 is the wavelength at normal incidence. Wavelength tuning can be accomplished by adjusting the incident angle. We have investigated the tunability of our fiber laser under 137W of launched pump power. The operating wavelength was continuously tunable from 1930 to 1821nm, with >52W output power over a tuning range of 104nm and a relatively narrow spectral width of <15pm.
We achieved simultaneous multiwavelength and very-narrow-linewidth operation based on novel use of VBGs (see Figure 1). By combining VBGs of different specifications ‘in parallel’—see Figure 1(a)—and using a simple resonator configuration for wavelength selection, we demonstrated high-power operation of a dual-wavelength Tm:fiber laser at ∼2μm. The wavelength-splitting range is continuously tunable from 1 to 50nm (0.1–3.8THz), with >115W of diffraction-limited total output power for wavelength separations of <40nm.2 We obtained a maximum output power of 118W for 279W of launched pump power at 792nm, corresponding to a slope efficiency of 45%. The laser's performance can be readily extended to multiwavelength operation by simply adding more VBGs in the external cavity. We believe that this high-power, dual-wavelength laser source with extreme flexibility in wavelength tuning may be useful for terahertz generation.
Figure 1. Schematic diagram of volume-Bragg grating (VBG) pairs in (a) parallel and (b) serial configuration. HR: Highly reflective.
We successfully demonstrated further spectral narrowing of high-power Tm:fiber lasers by serially pairing two VBGs: see Figure 1(b). Figure 2 shows the effective reflectivity of such a serial VBG pair. It exhibits both a significant reduction in spectral bandwidth compared with that of a single grating and a relatively high peak reflectivity at the crossing point. With this technique, we generated more than 113W of diffraction-limited output at 1990nm, with a relatively narrow linewidth of ∼2.2pm (full width at half maximum) for 279W of launched pump power (see Figure 3).3 The lasing slope efficiency at 972nm was ∼43%. Compared to the conventional FBG method, our proposed technique should be particularly interesting for generation of high-power, widely tunable, and narrow-linewidth/single-longitudinal-mode laser emission using proper resonator design and VBG angle tuning. This represents the direction of our ongoing research.
Figure 2. Effective reflectivity of two VBGs in serial configuration.
Figure 3. Output power versus incident pump power of a VBG-pair-locked, thulium-doped fiber laser with a linewidth of 2.2pm.
1. F. Wang, D. Y. Shen, D. Y. Fan, Q. S. Lu, Spectrum narrowing of high power Tm:fiber laser using a volume Bragg grating, Opt. Express 18, no. 9, pp. 8937-8941, 2010.
2. F. Wang, D. Y. Shen, D. Y. Fan, Q. S. Lu, Widely tunable dual-wavelength operation of a high-power Tm:fiber laser using volume Bragg gratings, Opt. Lett. 35, no. 14, pp. 2388-2390, 2010.
3. F. Wang, D. Y. Shen, D. Y. Fan, Q. S. Lu, Spectral narrowing of cladding-pumped high-power Tm-doped fiber laser using a volume Bragg grating-pair, Appl. Phys. Express 3, no. 11, pp. 112701, 2010.