Pump and signal power combiners for high - power fiber amplifier applications

In this work we present the results of our research on a pump power combiner fabrication process in configuration (N+1)×1. Our fabrication method is based on tapering a fiber bundle consisting of 6 multimode fibers and one signal fiber. We present the results on fabrication power combiners based on single mode fibers and large mode area fibers for the signal. Pump power combiners are one of the most important components in all-fiber laser and amplifiers construction technique [1]. In 1988 Eli Snitzer et al. presented the first cladding-pumped fiber laser – based on double-clad fiber [2]. It appeared that double clad (DC) fibers are a very beneficial waveguide for high-power laser and amplifier constructions [3]. Since that time, several pumping techniques have been proposed. Three well known approaches are based on bulk optics and mechanical processing of fibers. They are a side-pumping method [45], through the v-grove [6], and an end-pumping method, which is the most popular one with efficiency reaching 70-85% [7-8]. All these methods are very efficient, however, they are also very problematic. They all demand precise optical and mechanical adjustment, which makes the laser or amplifier setup construction very complicated and sensitive for external factors like the possibility of contamination and vibrations [9-10]. To avoid all this issues, a pump power combiner can be used. It is an all-fiber passive component which allows to effectively couple pump power and signal to an active double clad fiber. It consists of several fibers for the pump power at the input and one output passive double clad fiber. In the case of amplifier application, one of the input fibers is intended for signal transmission. This construction technique allows to make the laser or amplifier setup simple, robust, immune to external factors (e.g. contamination and vibration) and provide a very efficient coupling of pump power into an active doubleclad fiber [11-13]. A very important advantage in all-fiber construction is that the laser or amplified beam is not leaving the waveguide – core of the fiber. This allows to maintain high quality of the beam. * E-mail: [email protected] There were presented several approaches of power combiner fabrication techniques. One was presented by D. J. DiGiovanni et al. in 1999 [14]. The presented combiner was fabricated by bundling together multimode pump fibers with a signal fiber and tapering them down. The final diameter of the taper in the waist should be matched with the double clad fiber diameter. In this fabrication process a high tapering ratio (TR) can occur resulting in significant losses, because of mismatching mode field diameters. Therefore new fabrication techniques of pump power combiners are still being developed by scientific groups [10, 11, 15-19]. In this paper we present the results of our studies on pump power combiners fabrication in configuration (N+1)×1. For a fabrication process, a 3SAE LDS System (Large Diameter Splicer) was used [20]. The most important advantage of this splicer are three electrodes instead of conventional two. They make a so-called Ring Of Fire – ROF, so the tapered fiber or fiber bundle is evenly heated from each side. Additionally, the position of electrodes can be changed so the operation is possible on fibers with a diameter from 80μm to even 2mm. The splicer provides two methods of tapering – Fig. 1. Fig. 1. Single direction (A) and bi-directional (B) tapering method. Single directional tapering (Fig. 1A) is designed for capillary tubes, where the final shape of the taper has no influence on transmission. In this process the fiber is moving in one direction and the ROF remains in a central position. By increasing the velocity of one side of the fiber during heating by ROF, it is possible to fabricate a taper with a programmed length of the waist, down-slope, up-slope, and waist diameter. A bi-directional tapering process (Fig. 1B) is designed for tapering of fibers and fiber bundles, where the shape of the fabricated structure has crucial influence on transmission efficiency. Pump and signal power combiners for high-power fiber amplifier