Side pumping scheme for all-fiber counter-pumping of high power single-frequency fiber amplifiers

Summary form only given. Reliable, rugged and optically efficient fiber combiners are required for monolithic single frequency amplifier systems [1]. The most common fiber combiner type, a tapered fused fiber bundle (TFB), combines highly efficiently pump light up to several hundred watts, but with the drawback of an interrupted signal core, since this approach is based on the fiber end face pumping scheme [2]. Hence, the beam quality and the signal transmission can be influenced using the counter-propagation pumping (CPP) scheme. We developed an all-fiber side-pumping approach based on a coreless tapered intermediate fiber (see. Fig. 1) [3]. The signal feedthrough of this combiner offers the possibility to pass a high power signal in forward and, particularly, in reverse direction. Moreover, several pump feeding fibers can be laterally fused around a passive or active double-clad fiber (target fiber) with pump combining efficiencies of about 90%. We will present details and further progress of the optical fiber combiner design with the focus on the impact of the fiber combiner on the single-frequency amplifier performance in the case of CPP. Fig. 2 shows the all-fiber CPP Ytterbium fiber amplifier setup seeded by a preamplified nonplanar ring-oscillator operating at 1064 nm (spectral linewidth 1 kHz). The 4+1x1 pump combiner, based on the setup depicted in Fig. 1, consists of a centered double clad fiber (target fiber) with a cladding diameter of 250 μm (NA 0.46) and 4 pump feeding fibers with a core diameter of 105μm (NA 0.22). Each pump feeding fiber was axially fusion spliced to a coreless intermediate fiber with a cladding diameter of 125 μm. The 2.75 m long active fiber (Nufern, LMA-YDF-25/250-VIII) as well as the passive target fiber (Nufern, LMA-GDF-25/250) had a core diameter of 25 μm (NA 0.06). A single-frequency output power of 300 W was achieved with CPP at a wavelength of 976 nm [4]. That means, the pump combiner had to handle the pump power of 440 W and the high power signal propagating in reverse direction through the fiber component. The pump diodes were sufficiently isolated against the amplified signal light with about 30 dB. The measured signal insertion loss was less than 3 %. At an output power of 285 W the onset of stimulated Brillouin scattering (SBS) was observed. A further fiber integration step was realized by direct lateral fusion of four pump feeding fibers (with intermediate fibers) to a 3 m long polarization maintaining active fiber (Nufern, PLMA-YDF-25/250-VIII). Direct coupling of the pump light into the active fiber avoids the fusion splice at the high power output-side of the active fiber as well as the additional passive target fiber, which can affect the SBS threshold. In our first results with this setup we obtained an output power of 120 W without any indication of SBS. For both CPP amplifier configurations a beam quality factor M2 of about 1.2 and a stable polarized output beam was determined. In summary, the presented results show the progress in the development of highly integrated CPP single frequency amplifier systems.