Double cladding fiber amplifiers with lens-less side-pumping

Summary form only given. Compared with end-pumping of double cladding fiber (DCF) amplifiers, side-pumping offers the advantages of simpler construction, no signal loss, no requirement for dichroic components, free access to the fiber ends for splicing, and the possibility of power scaling using multiple pump entry points along the length of the gain fiber. The two most common side pumping methods are the multimode fused fiber coupler and the v-groove pump side-coupling. The v-groove method is attractive for some applications since it is simple to implement, does not add to the total amplifier length, and does not require separate pigtailing of the broad stripe pump diode. Previously, a v-groove coupling efficiency of approximately 85% was demonstrated by using an anamorphic micro-lens to capture the light from the diode and focus it onto a v-groove facet. We report a simplified v-groove coupling approach where no lens is used, and the pump light is directly captured and coupled into the fiber. When compared with the technique which relies on a microlens, the direct method reduces the number of critical alignments required for amplifier packaging from two (lens alignment and v-groove alignment) to one (v-groove alignment), decreases the sensitivity of coupling efficiency to changes in fiber position by approximately tenfold, and results in only a small reduction in the coupling efficiency.