Wang, Zhennuo; Zhong, Li; Zhang, Deshuai; Liu, Suping; Pan, Zhipeng; Chang, Jinyuan; He, Tianjiang; Ma, Xiaoyu Source: Guangxue Xuebao/Acta Optica Sinica, v 44, n 8, April 2024;

Abstract:

Objective As the main pumping light source of solid state laser, fiber laser, and fiber amplifier, 976 nm diode laser has been widely used in industrial processing, medical treatment, communication, and other fields. As an important pumping light source of erbium-doped fiber amplifier, a 976 nm fundamental transverse mode diode laser can achieve high-efficiency coupling with fiber, improve the output performance of the fiber amplifier, and effectively reduce the cost of the fiber amplifier. It plays a very important role in improving the application of erbium-doped fiber amplifier in fiber communication and other fields. However, since the ridge waveguide in the ridge diode laser uses the weak refractive index guiding mechanism to suppress the higher-order transverse mode, it will be greatly affected by the lateral diffusion of carriers and the self-heating effect and eventually lead to the decline of the ridge waveguide mode guiding and the increase of the farfield angle. To further improve the coupling efficiency of diode lasers in fiber laser pumping applications and reduce the application cost of fiber lasers, it is still important to realize low far-field divergence angle and low power consumption of fundamental transverse mode ridge diode lasers. Methods Using InGaAs/GaAsP material as the strain-compensated quantum well structure, and GaAsP with high bandgap width as the barrier material can effectively reduce the carrier leakage effect of quantum well, provide strain compensation for InGaAs compressive strain quantum well, and improve the epitaxial growth quality. To achieve low loss, we optimize high output optical power and low far-field angle, the thickness of waveguide layers by using asymmetric large optical cavity epitaxial structure. The doping concentrations of the epitaxial layer materials are optimized to reduce the series resistance of the device, to achieve high power, high conversion efficiency, and low far-field output of the ridge diode laser. To achieve fundamental transverse mode output, we use the effective refractive index method to design and study the width and depth of the ridge waveguide and map the optical field distribution inside the device. Finally, according to the technological conditions, the ridge waveguide structure is selected with a width of 5 μm and a depth of 0.85 μm. Results and Discussions After the laser chip is designed and prepared, the output performance of the device is tested at 25 ℃ . The device threshold current is about 51.2 mA, and a maximum continuous output power of 422 mW can be obtained at 550 mA injection current, with a maximum electro-optic conversion efficiency of 53.6% (Fig. 3). The peak wavelength is 973.3 nm at 550 mA injection current, and the corresponding spectral line width (FWHM) is 1.4 nm. When the injection current is 500 mA, the vertical and horizontal far-field distribution diagrams of the device are drawn (Fig. 5), and the corresponding vertical and horizontal far-field divergence angles (FWHM) are 24.15° and 3.9°, respectively. This indicates that the prepared ridge diode laser has a good fundamental transverse model property, which is conducive to improving the coupling efficiency between the diode laser and the fiber. Subsequently, we analyze the temperature characteristics of the device at the operating temperature of 15‒35 ℃ and obtain a relatively high characteristic temperature of about 194 K. This is because GaAsP material with high band gap width is added to both sides of the InGaAs quantum well as the barrier layers, and there is a larger band level between the two materials, which can better suppress carrier leakage in the quantum well. The injection current utilization, the luminous efficiency, and the temperature stability of the laser device are improved. Similarly, the horizontal far-field changes little at 15 ℃, 25 ℃, and 35 ℃, and the corresponding horizontal far-field divergence angles are 3.45°, 3.90°, and 3.90°, respectively, which is conducive to increasing the coupling efficiency in optical pumping applications (Fig. 7). Conclusions We design and fabricate a 976 nm fundamental transverse mode ridge diode laser. To improve the conversion efficiency of the device, we improve high-bandgap GaAsP materials on both sides of the InGaAs compressive strain quantum well as a tensile strain barrier to improve the internal gain of the device, inhibit the carrier leakage in the quantum well, and improve the current utilization rate. In addition, we optimize the waveguide layer thickness and doping concentration in the device epitaxial structure to reduce the far-field divergence angle and achieve high-efficiency output by using an asymmetric large optical cavity epitaxial structure design. A 976 nm strain-compensated low far-field fundamental transverse mode ridge diode laser with a ridge width of 5 μm and a cavity length of 1500 μm is fabricated. At the operating temperature of 25 ℃, the maximum continuous output power of 422 mW can be obtained, the peak wavelength is 973.3 nm, and the spectral line width (FWHM) is 1.4 nm. When the injection current is 500 mA, the vertical and horizontal far-field divergence angles (FWHM) are 24.15° and 3.90°, respectively. In the operating temperature range of 15‒35 ℃, the far-field divergence angle of the ridge diode laser is tested and analyzed. It is found that the far-field distribution of the device changes little with the increase in the test temperature, and the far-field divergence angle can be kept small.

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