The ripple index is one of the most crucial parameters of superluminescent diodes (SLDs). A low ripple index is necessary for the application of SLDs in sensing areas such as fiber-optic gyroscopes. Therefore, reduced reflectivity of the facet is required, and the main strategy involves coating with antireflective (AR) films. The reflectivity of the AR films is generally less than 0.1%, which is a strict requirement. Although the AR film designed using the plane wave method (PWM) is widely used, its performance in SLDs is not ideal, and the actual reflectivity deviates from the design value. Therefore, the purpose of this study is to design and fabricate AR films for SLDs that can effectively reduce the ripple index.

The first part is the simulation method. The finite difference time domain (FDTD) method is used to analyze the optical properties of the films, and a perfect matching layer (PML) is used as the boundary condition. In addition, to reduce the resource requirements of the FDTD method, a simplified simulation model is used, which highlights the main influencing factors. To optimize the films, parameters such as film thickness and refractive index are scanned by FDTD method to determine the parameter range of low reflectivity, and the particle swarm algorithm is used to obtain the optimal parameters within this range. For the coating process, the optical film is manufactured by reactive magnetron sputtering, and an AR ion beam assists the coating process. In addition, the film thickness is monitored and controlled online using a crystal oscillator control system. Reflectivity measurement of thin films is important, including direct measurement of the accompanying films and indirect measurement of the facet of the SLD. The SLD is indirectly measured by spectral ripple, which is also the mainstream reflectivity measurement method for other semiconductor optoelectronic devices.

The simulation results show that the design of the PWM film faces many problems. For the single-layer and double-layer AR films, the reflection curve becomes blue-shifted, and the shift exceeds 150 nm. In addition, the reflectivity is more than one order of magnitude higher than the designed value, and the deviation is further enlarged at a large angle. Therefore, the AR film designed by PWM does not meet the requirements and cannot be used for SLD. To solve these issues, the optical film is optimized. For single-layer AR films, the average reflectivity is less than 0.11% and the lowest reflectivity is 0.04%. The optimized design of the double-layer film provides better results: the average reflectivity is less than 0.05% and the lowest reflectivity is only 0.01%. The optimized design effectively reduces the reflectivity, particularly for the optimized double-layer AR film, which has evident advantages over the film designed by PWM.

The double-layer AR films are prepared by reactive magnetron sputtering. After optimization, the reflectivity of the AR film on the companion substrate is 0.12%, and the low-reflectivity bandwidth is greater than 200 nm, which verifies the coating process. The design and measurement curves do not fit because the size and structure of the companion substrate are different from those of the SLD. The reflectivity of the AR film on the SLD is indirectly measured. After optimization, the average reflectivity of the AR film decreases by 50%, indicating that the optimized design effectively reduces the reflectivity. The spectrum of the SLD shows that the intensity and number of ripples are clearly reduced, and the average ripple index is only 0.019, which is 44.5% of that before optimization. The average modulation index decreases from 4.79×10^-3 to 2.30×10^-3, a decrease of more than 50%. In addition, under a driving current of 100 mA, the output power of the SLD chips remains above 10 mW, maintaining a high output power and high efficiency.

In this study, the AR film designed by the PWM is analyzed, particularly its poor performance in inclined-cavity SLD. The FDTD method is used to analyze and determine the deviation of the reflection curve and high reflectivity. Therefore, the AR film design is optimized and the film is coated by reactive magnetron sputtering; this is verified by reflectivity and spectrum measurements. After optimization, the average reflectivity of the double-layer AR film is controlled within 0.1% and the lowest value is only 0.05%. The AR film effectively suppresses the ripple, and the ripple and modulation index are only 0.019 dB and 2.30×10^-3, respectively, with a decrease of more than 50% compared with those of the traditional PWM film. The prepared SLD chips still maintain a 10-mW output power under a current of 100 mA. The AR film developed in this study can effectively reduce the reflectivity of the facet, and the fabricated weak-ripple SLD is prepared by using the film. The research results provide a reference for the development of optical films of SLD and other semiconductor optoelectronic devices.