Confined propagation and near single-mode laser oscillation in a gain-guided, index antiguided optical fiber

Abstract

The authors report laser oscillation in what appears to be a single transverse mode with very large mode area in optical fibers having heavily Nd-doped 100μm diameter cores with refractive index significantly lower in the core than in the surrounding cladding. Since fibers of this type cannot support conventional index-guided modes, their results appear to confirm a recent analysis which predicts gain-guided single-mode propagation in index antiguided fibers, provided the gain coefficient in the core exceeds a threshold value. Fibers of this type may be of significant interest for amplifiers and oscillators having large power outputs and/or small nonlinear pulse distortion.

Beam-quality factor of single-mode gain-guided fiber lasers

“…The large value of M 2 in the GG+IAG region is the result of the long tail of the beam extending all the way into the cladding region, the truncation of which can lower M 2 substantially. The reported values of M 2 in several experiments on GG+IAG fiber lasers are in the range of 1.05-2.0 [2,3,4,5]. For example, Ref.…”

“…Several experiments have demonstrated GG+IAG in various fiber laser configurations [2,3,4,5]. The reported values of the beam quality factor M 2 in these experiments are notably larger than unity, even in fibers that are designed to operate as single-mode.…”

Large area single-mode parity–time-symmetric laser amplifiers

“…Selective excitation of the fundamental mode using ultrashort pulses was demonstrated in multimode fibers [9], while spatial doping has been used to achieve gain filtering among different modes [10]. Gain guiding with index antiguiding provides yet another technique for single-mode large area lasers [11]. Given that none of the aforementioned methods can single-handedly address all the underlying problems in this area, of interest will be to explore alternative routes to achieve this goal for both one-and two-dimensional structures in different geometries.…”

Response to “Comment on ‘Confined propagation and near single-mode laser oscillation in a gain-guided, index antiguided optical fiber’” [Appl. Phys. Lett. 102, 026101 (2013)]

“…Cao et al point out that if the cladding is not infinitely thick, the cladding-air interface may have to be taken into account when interpreting experimental results such as those we presented in Ref. 3. Though they are correct in suggesting that index guided modes involving the cladding-air interface can potentially oscillate, we insist the evidence for gain guided index anti-guided (GG IAG) lasing conducted by ourselves and published in collaboration with Professor Siegman is indisputable.…”

“…3. That photo shows that the cladding circumference was nicked in several places as a result of the manner in which we cleaved and polished the (soft phosphate glass) fiber end.…”