Mid-infrared (3?5 μm) lasers play a pivotal role in gas sensing and free-space communication due to their alignment with molecular absorption lines and low atmospheric scattering. However, conventional second-order distributed feedback (DFB) surface-emitting lasers suffer from poor far-field symmetry, characterized by a dual-lobe emission pattern, which severely compromises the beam quality. This work addresses this critical challenge by proposing a hybrid second- and fourth-order DFB grating design for interband cascade lasers (ICLs). The innovation lies in the fundamental optimization of the optical mode distribution to achieve single-lobe far-field emission, thereby enhancing beam collimation and radiation efficiency. This advancement is essential for practical applications requiring a high beam quality, such as in portable gas sensors and long-range optical communication systems.

The ICL structure is epitaxially grown on an n-type GaSb substrate using molecular beam epitaxy. A hybrid grating combining the second-order (period of 0.95 μm) and fourth-order components is fabricated via electron-beam lithography and dry etching techniques (Fig. 4). COMSOL Multiphysics simulations are conducted to analyze the electric field distributions and the coupling coefficients under symmetric and antisymmetric modes (Figs. 1 and 3). Key structural parameters, such as a grating duty cycle of 0.3, are optimized to balance coupling strength and fabrication feasibility. The devices are processed into 4.5-μm-wide ridge waveguides, with Si₃N₄/SiO₂ passivation and Ti/Pt/Au metallization. Performance metrics, including spectral purity, power-current-voltage (P-I-V) characteristics, and far-field patterns, are systematically evaluated under continuous-wave (CW) operation at 25 °C.

The hybrid grating ICL demonstrates stable single-mode lasing with a side-mode suppression ratio (SMSR) of 30 dB, a significant improvement over the 20 dB SMSR of conventional second-order DFB devices (Fig. 5). The enhanced spectral purity stems from the higher coupling coefficient of hybrid grating, which suppresses mode hopping.

The hybrid grating enables the surface-emitting power comparable to the edge-emitting power, achieving a surface radiation efficiency exceeding 50% (Fig. 6). In contrast, the uniform second-order gratings exhibit only a 25% efficiency. Despite a slight threshold current increase due to additional waveguide losses, the hybrid design maintains a competitive output power.

One of the most significant achievements of this work is the elimination of the dual-lobe far-field pattern characteristic of conventional devices. The hybrid grating ICL produces a symmetric single-lobe profile with a divergence angle of 0.7° (Fig. 7), representing a remarkable 61% reduction compared to the 1.8° divergence angle of the second-order DFB lasers. Simulations confirm that the hybrid grating redistributes the electric field maxima, reducing antisymmetric mode losses and enabling single-lobe emission (Fig. 3).

This work demonstrates a notable advancement in mid-infrared laser technology by integrating hybrid second- and fourth-order DFB gratings into ICLs. The successful achievement of single-lobe far-field emission with a low divergence angle of 0.7°, enhanced spectral purity (30 dB SMSR), and a high surface emission radiation efficiency (>50%) paves the way for high-beam-quality mid-infrared sources. The hybrid grating design overcomes the inherent limitations of traditional DFB lasers, offering a robust solution for applications in gas sensing, lidar, and free-space communication, where precise beam control is essential. Future research will focus on scaling output power while maintaining beam quality, further enhancing the industrial applicability of the proposed technique.