Array-mode selection in nonabsorbing-mirror diode laser arrays

Abstract

Nonabsorbing mirror (NAM) GaAs/AlGaAs arrays have been shown to deliver thermally limited cw output power as high as 2.4 W from a 100-μm aperture.1 Typically, these devices operate in high-order array modes with a double-lobe far field. In this paper, we show that with careful design, the NAM section can be utilized to favour single far-field lobe operation. We treat the NAM array as a composite cavity consisting of a waveguide region in which parallel single-mode waveguides are coupled through evanescent fields of guided modes and a uniform NAM section that provides further coupling between individual waveguide modes via diffraction. The eigenmodes of the waveguide section (supermodes) are calculated using the improved coupled-mode theory.2 The supermode mixing coefficients, determining the eigenmodes of the composite cavity, are obtained by evaluating the reflected image at the interface between the waveguiding and uniform sections of the device using the 3-D diffraction integral. If the phase difference between light returning from the NAM section and injected back into any particular waveguide and its nearest neighbor is an integer multiple of 2π, the in-phase supermode is reinforced. On the other hand, a small variation in the NAM-section length and/or the waveguide spacing is sufficient to dramatically change the farfield. The performance of NAM arrays, is therefore, very sensitive to details of the waveguide structure.