Single Transverse-Mode Semiconductor Laser Arrays by Parity-Time Symmetry Breaking

Electrically pumped semiconductor laser array operating in the fundamental mode is of vital importance. In general, the competition of multiple transverse modes in the laser array leads to spatiotemporally unstable and degradation of beam quality. In this letter, we propose and experimentally demonstrate electrically pumped single transverse-mode semiconductor laser arrays by exploiting the concept of parity-time symmetry, which is insensitive to fabrication and frequency detuning. Laser arrays composed of four, six and ten-ridge gain lattice have been fabricated, and single transverse-mode operations are observed from the spectral response and far-field radiation. Insensitivity to the fabrication have also been experimentally demonstrated by varying the gap from $1.06 \mu \text{m}$ to $1.42 \mu \text{m}$ . The emission power of PT-symmetric laser arrays composed of a four-ridge gain lattice is about five times higher than that of the single-ridge laser with the same current density.

I. Introduction

Electrically pumped semiconductor lasers with high power output play a crucial role in a wide range of applications such as light detection and ranging systems. An integrated array of lasers can enlarge the active region and allow higher energies within the cavities, while avoiding the impact of optical nonlinearities [1]. Unfortunately, merely enlarging the active region inevitably leads to deterioration of beam quality. In general, a laser array supports multiple transverse modes (supermodes) of the same longitudinal order, and the spatial characteristic of the radiation degrades due to the non-linear modal competition [2]. A laser array design strategy based on supersymmetry (SUSY) is proposed to enforce fundamental mode operation in the inherently multi-mode laser array [3], [4], [5]. The propagation eigenvalues of the superpartner are carefully designed to match all higher-order supermodes except the fundamental mode associated with the main array. However, as the number of ridge waveguides in the array increases, more higher-order modes need to be carefully considered, and the design difficulty increases sharply. Fabrication error can also lead to propagation eigenvalues mismatch, resulting in deterioration of beam quality.