Low noise single frequency all-phosphate fiber laser

The noise power spectrum of solid-state lasers - including fiber lasers - exhibits a characteristic peak at the relaxation oscillation frequency. The tails associated with this peak extend to neighboring spectral ranges and may increase the noise level above acceptable limits in applications using weak signals. One of the key factors to reduce the relative intensity noise (RIN) amplitude is a low loss laser resonator. We describe a method to ultimately reduce the intensity noise in single frequency phosphate fiber lasers by minimizing intra-cavity losses caused by fusion splices between fibers made of different materials. Conventional fiber Bragg gratings written in silica fibers have been replaced with gratings written in phosphate glass fibers. The quality of the intra-cavity fusion splice has been improved due to material similarity. All-phosphate fiber laser devices have been built and tested utilizing the new gratings. The results show relative intensity noise amplitudes that are very similar to those of conventionally fabricated devices. Challenges in the grating writing process are currently preventing the new devices from surpassing their commercial counterparts in terms of performance. However, this type of all phosphate glass fiber lasers may ultimately lead to a new generation of commercial single frequency fiber lasers with improved intensity noise performance.