Lasing in a gain-guided index antiguided fiber

fiber with a cladding index of refraction larger than that f the core is an index antiguiding (IAG) fiber. In such a ber total internal reflection at the core–cladding interace does not take place. Light generated in the core of the ber can leak out into the cladding of an IAG fiber, though t experiences some guiding by grazing angle reflection. here is some loss on each reflection, but if there is gain n the core this reflection loss can be compensated and a hreshold condition for gain guiding achieved. In 2003 iegman proposed such gain guiding in fibers with a very mall but negative difference between the index of refracion of the core and the cladding of a fiber [1]. In that ork the suggested amplitude of the normalized index of efraction difference was less than 5, corresponding to a ifference in index of refraction was less than 10−4. In an rticle in this issue Siegman points out that for large amlitudes of IAG the field is confined in the core by antiguding rather than the weaker gain guiding [2]. Gain guidng then plays a vital role in reshaping the mode profile ithin the core region, resulting in a confined and rapped profile when the gain exceeds that required for hreshold. Additional gain is necessary to achieve laser scillation. The threshold for gain guiding of the lowestrder mode, LP01, is shown in Ref. [2] to be 0.39 times hat of the next-higher-order mode, LP11. If the fiber is hen pumped further to achieve oscillation, gain saturaion by the LP01 mode depletes the gain and, as a conseuence, the output of such a laser is expected to be conned to the lowest-order mode of the fiber. In this paper e present experimental evidence for this type of lowestrder mode, lasing in a gain-guided fiber with IAG proprties. The experiments were performed using flashlamp umping to expedite our work. Although this approach oes not allow evaluation of the potential efficiency of uch fiber lasers we were able to demonstrate several ery important properties of the laser. That is, spectral arrowing, relaxation oscillations, threshold, and lowestrder-mode oscillation at pump inputs well above threshld were clearly evidenced. Discussion is presented that utlines the importance of thermal management to obtain