The optical performance of green lasers is greatly improved




Laser is considered to be one of the greatest inventions of mankind in the twentieth century, and its appearance has strongly promoted the progress of detection, communication, processing, display and other fields. Semiconductor lasers are a class of lasers that mature earlier and progress faster. They have the characteristics of small size, high efficiency, low cost, and long life, so they are widely used. In the early years, infrared lasers based on GaAsInP systems laid the cornerstone of the information revolution. . Gallium nitride laser (LD) is a new type of optoelectronic device developed in recent years. The laser based on GaN material system can expand the working wavelength from the original infrared to the entire visible spectrum and ultraviolet spectrum. Processing, national defense, quantum communication and other fields have shown great application prospects.
The principle of laser generation is that the light in the optical gain material is amplified by oscillation in the optical cavity to form light with highly consistent phase, frequency and propagation direction. For edge-emitting ridge-type semiconductor lasers, the optical cavity can confine light in all three spatial dimensions. The confinement along the laser output direction is mainly achieved by cleaving and coating the resonant cavity. In the horizontal direction The optical confinement in the vertical direction is mainly realized by using the equivalent refractive index difference formed by the ridge shape, while the optical confinement in the vertical direction is realized by the refractive index difference between different materials. For example, the gain region of the 808 nm infrared laser is a GaAs quantum well, and the optical confinement layer is AlGaAs with a low refractive index. Since the lattice constants of GaAs and AlGaAs materials are almost the same, this structure does not achieve optical confinement at the same time. Material quality issues due to lattice mismatch can arise.
In GaN-based lasers, AlGaN with low refractive index is usually used as the optical confinement layer, and (In)GaN with high refractive index is used as the waveguide layer. However, as the emission wavelength increases, the refractive index difference between the optical confinement layer and the waveguide layer decreases continuously, so that the confinement effect of the optical confinement layer on the light field decreases continuously. Especially in green lasers, such structures have been unable to confine the light field, so that the light will leak into the underlying substrate layer. Due to the existence of the additional waveguide structure of the air/substrate/optical confinement layer, the light leaked into the substrate can be A stable mode (substrate mode) is formed. The existence of the substrate mode will cause the optical field distribution in the vertical direction to be no longer a Gaussian distribution, but a "calyx lobe", and the degradation of the beam quality will undoubtedly affect the use of the device.

Recently, based on the results of previous optical simulation research (DOI: 10.1364/OE.389880), the research group of Liu Jianping from Suzhou Institute of Nanotechnology, Chinese Academy of Sciences proposed to use AlInGaN quaternary material whose lattice constant and refractive index can be adjusted at the same time as the optical confinement layer. The emergence of the substrate mold, the related results were published in the Fundamental Research journal, which is directed and sponsored by the National Natural Science Foundation of China. In the research, the experimenters firstly optimized the epitaxial growth process parameters to heteroepitaxially grow high-quality AlInGaN thin layers with step flow morphology on the GaN/Sapphire template. Subsequently, the homoepitaxial time-lapse of AlInGaN thick layer on the GaN self-supporting substrate shows that the surface will appear disordered ridge morphology, which will lead to the increase of surface roughness, thus affecting the epitaxial growth of other laser structures. By analyzing the relationship between stress and morphology of epitaxial growth, the researchers proposed that the compressive stress accumulated in the AlInGaN thick layer is the main reason for such morphology, and confirmed the conjecture by growing AlInGaN thick layers in different stress states. Finally, by applying the optimized AlInGaN thick layer in the optical confinement layer of the green laser, the occurrence of the substrate mode was successfully suppressed (Fig. 1).


Figure 1. Green laser with no leakage mode, (α) far-field distribution of light field in vertical direction, (b) spot diagram.

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