| Surface-emitting, 2nd-order distributed-feedback lasers have been studied for nearly three decades. It has been found, both theoretically and experimentally, that the devices fundamentally prefer to operate in an antisymmetric mode (i.e. two-lobed beam pattern). Attempts at obtaining a symmetric-mode-like pattern have been either impractical or have relied on a chirped grating, which causes operation in an off-normal, primarily single-lobed beam. Here we present the concept to fundamentally provide emission in a single-lobed, orthonormal beam at no penalty in device efficiency: the use of π-phaseshift grating in DFB/DBR devices. The fabrication of 2nd-order grating with central π-phaseshift using holographic systems via the image reversal method is discussed. The first demonstration of the proposed concept to get orthonormally emitted farfield pattern, which consists of a single, diffracted-limited beam (the theoretical beamwidth is 0.045°) is shown. The device spectrum is single-mode, instrument-limited to 0.8 Å FWHM. |
