Distributed Bragg reflector laser diodes (DBR-LDs) are widely used in pump sources, detectors, sensors, solar cells, and other applications because of their small size, long operating life, and high photoelectric conversion efficiency. With the development of modern technology and the demand for laser sources, higher requirements have been proposed for lateral modes of semiconductor lasers. The output of the fundamental lateral mode can be achieved by etching a narrow-ridge waveguide structure as this can limit the formation of higher-order lateral modes; however, it is difficult to further improve the maximum output power owing to the limitation of the narrow-ridge structure. Lasers, integrated by connecting a narrow-ridge waveguide to an optical amplifier, can obtain higher output power in the fundamental lateral mode. However, integrated devices are large, and the manufacturing process is complex. The method of etching microstructures on wide-ridge waveguide devices proposed in recent years ensures that the device overcomes lateral mode limitations and achieves excellent output performance. In addition, research on DBR devices has primarily focused on the spectral study of Bragg gratings. There has been less analysis of the influence of the Bragg grating on lateral mode distribution. In this study, a wide-ridge waveguide-based distributed Bragg reflector semiconductor laser with a combination grating structure (CDBR-LD) is designed and fabricated, and the influence of the combined grating structure on the modulation of lateral modes is investigated. The combination grating can modulate the spectral characteristics of the device and overcome higher-order lateral mode limitations.

The internal action of a semiconductor laser resonator with a combined grating structure is analyzed and calculated using a finite-difference time-domain method. Owing to the complex internal actions of the device, the internal process is divided into two parts, which are analyzed separately: the incident light and feedback light . The combined grating consists of hybrid and Bragg grating areas. Herein, the incident light refers to the light from the direction of the ridge waveguide to the hybrid grating area (Fig.2). The feedback light refers to the light from the Bragg grating area after the incident light is acted upon by the hybrid grating area (Fig.3). According to the distribution law of lateral modes, the energy of the fundamental lateral modes is concentrated in the central region, whereas that of the higher-order lateral mode is dispersed. The loss mechanism of each order of the lateral modes in the incident light and feedback light in the hybrid grating area is analyzed. The value of the narrowest width of the mixed grating region is W_G; the effect of W_G on the energy transmittance of each order lateral mode is compared (Fig.4). The ideal energy transmittance difference between the fundamental and the higher-order lateral modes is obtained with W_G of 15 μm. Therefore, the hybrid grating area in the combined grating structure can suppress the higher-order lateral modes of the device.

According to the analysis of the far-field spots of the device (Fig.5), spectra(Fig.6), and the output power characteristics (Fig.7), the far-field spot of the DBR-LD has significant spot-splitting as the injection current increases from 0.7 A to 1.0 A because of the strong mode competition caused by the higher-order lateral modes. The far-field spot-splitting effect of the CDBR-LD is significantly eliminated as the injection current increases from 0.7 A to 1.0 A because the loss of the higher-order lateral modes caused by the hybrid grating area reduces mode competition. This indicates that the combined grating structure can play a role in modulating the lateral modes of the DBR device. The DBR-LD has a red shift from 1031.87 nm to 1036.1 nm, and the full width at half maximum (FWHM) of the spectrum increases from 1.17 nm to 1.44 nm as the injection current varies from 0.35 A to 0.95 A. The CDBR-LD can maintain good spectral characteristics, which shows a red shift from 1031.25 nm to 1037.15 nm, and the FWHM of the spectrum increases from 0.5 nm to 0.61 nm. Moreover, the FWHM of the CDBR-LD spectrum is narrower than that of DBR-LD because CDBR-LD has a larger grating area. Finally, the DBR-LD exhibits a saturation output power of 406 mW at an injection current of 1.2 A with a slope efficiency of 0.333 mW/A. Additionally, the CDBR-LD exhibits a saturation output power of 433 mW at an injection current of 1.25 A with a slope efficiency of 0.337 mW/A.

A DBR semiconductor laser with a combined grating structure is proposed in this study. By etching a hybrid grating area on the front side of the Bragg grating area, the loss of higher-order lateral modes increases and weakens the mode competition, eliminating the far-field spot-splitting phenomenon in wide-ridge waveguide DBR semiconductor lasers. Subsequently, a CDBR-LD is fabricated and tested. The experimental results show that the far-field spot splitting of CDBR-LD is significantly reduced as the injection current increases from 0.7 A to 1.0 A. The FWHM of the CDBR-LD spectrum is narrower than that of the DBR-LD as the injection current increases from 0.35 A to 0.95 A. A minimal difference is observed between the output powers of the DBR-LD and the CDBR-LD at an injection current of 1.2 A. In addition, the waveguide and grating structure of the CDBR-LD are etched in one step using the ultraviolet lithography, which has the advantages of being a simple process with a low cost. Based on these results, it is expected that a DBR semiconductor laser with good lateral-mode characteristics can be obtained by optimizing the structure.