Objective The laser diode(LD) coupled with the fiber is an effective and high-quality pump source of fiber and solid lasers. Although this technology is relatively mature, its efficiency, structure, and optical design need to be optimized. The optical alignment and adjustment directly influence the efficiency of the LD module, which has become a restriction in the development of high-efficiency and high-brightness LDs. To improve the efficiency of the LD module, without improving the LD chip itself and electric to optical (e-o) efficiency, reducing the optical loss by the maximum possible extent is a research key. In the optical assembly and adjustment engineering practice, beam collimation has an important impact on LD modules to achieve high brightness, high power, and pigtail output. Furthermore, to ensure smaller divergence, the directivity of the optical axis should also be improved, so as to realize precise beam coupling. In direct LD applications, controlling the direction of the laser beam based only on mechanical alignment is limited by high volume, high cost, and poor stability. Thus, the optical axis should be optically controlled.
Methods The pointing error of the fast axis is mainly influenced by the ultraviolet glum solidification technology; however, in practice, we cannot ensure the pointing error accuracy because there exists inherent shrinkage stress when the UV (ultraviolet) glue solidifies. Through an optical method based on the principle of laser refraction propagation in a medium, calibration of fast-axis directivity via a bizarre slow-axis collimation mirror was studied. When light is normally incident upon a medium, the directions of the transmitted light and source light are the same, but at oblique incidence, the directions of the two light beams differ. We employed this basic principle to design a slow-axis collimation mirror with a certain inclination angle according to the relationship, based on Eqs. (1)--(3), between the inclination angle and pointing error of the fast axis to correct the pointing error.
Results and Discussions We designed four kinds of slow-axis collimation mirrors with a tilt angle of 0.23°, a lens height of approximately 2 mm, and an edge-thickness difference of approximately 0.009 mm (Table 2). We also measured the central offset along the fast axis with and without the bizarre slow-axis collimation mirror (Fig. 8). The central point coordinate decreased from 1.07 mm to 0.15 mm when the focal length of the Fourier transformation lens was 510 mm. After measurement, the original average deviation of fast-axis directivity was approximately 2.1 mrad; after calibration by our designed slow axis collimation mirror matched with the pointing error of the fast axis, the directivity of the fast axis was maintained at 290 μrad. In this way, the power entering the fiber was maximized, and the efficiency of the coupled fiber was increased. We found that the average pointing error was reduced by an order of magnitude based on the seven-step LD module with pigtail output (Fig. 9). The laser spot was not completely filled with the coupled fiber when the pointing error was high,and some of the laser spot overflowed to the fiber outside (Fig. 10). The fiber would be filled with the laser spot only when the pointing error was lower. Thus, the efficiency of the 60-W fiber-output LD module increased from about 53% to 55%. The power and e-o efficiency of the LD module after calibration were measured (Fig. 11).
Conclusions In this research, we proposed a novel method to correct the pointing error of the fast axis by employing a bizarre slow-axis collimation mirror (SAC). This method has applications in high-brightness and high-efficiency LDs with pigtail outputs, and its advantages include not requiring extra devices to compensate for the pointing error of fast axis in existing conditions, so precise compensation can be achieved. The material and machining costs of the SAC are reduced because complex processing is not needed. Globally, domestic advantageous units have reported e-o efficiencies of approximately 51%--53%, whereas the efficiency is approximately 51% overseas. Our technical specifications contribute to the domestic leading efficiency. Our research provides a novel method for high-efficiency LDs with fiber output. In terms of the LD’s direct application, this study has numerous potential applications in laser fuse, radar, ranging, and illumination. We have achieved the goal of applying tens of thousands produced LD modules to a pump source.
