Fiber Laser Pump Reflector Based on Volume Bragg Grating

Double-clad rare-earth doped fiber lasers have been the subject of numerous studies due to their high-quality single-mode beam and overall simplicity partly arising from their cladding-pumped scheme. This approach reduces, however, greatly the pump absorption, which leads to longer laser cavity lengths which are inherently more lossy and costly. There is therefore a strong incentive in increasing the pump absorption while reducing laser cavity length. It was shown that a Long-Period Fiber Grating (LPG) could be used to couple the pump beam propagating in the cladding of the doped fiber to its core which increased the pump absorption by 35% [1]. Another approach consisted in having an all-fiber pump reflector. A reflectivity of 55% of the unabsorbed pump was reported by giving a right-angled conical shape to the fiber end [2]. Also, a pump reflector with a 46% reflectivity was obtained by writing a FBG with UV radiation directly in the photosensitive germanosilicate hydrogen-loaded cladding of a specialty fiber [3]. In this contribution, we report the writing of a volume Bragg grating (VBG) written directly in the pure-silica cladding of an erbium-doped fiber acting as a pump reflector for an all-fiber laser cavity. Such VBG was written using femtosecond pulses at 400 nm and the phase-mask writing technique and yields a peak reflectivity of 71% of the unabsorbed pump power of a 976-nm diode propagating in the cladding.