Direct diode laser has many applications, such as pump source of fiber laser, laser illumination, laser fuze, beacon laser, and wireless energy transmission. However, it is subject to low beam quality and broad spectrum, leading to limited applications at high polymer absorption, coherent light source, spectrum analysis, and industrial processing. High beam quality can be achieved by improving the chip brightness or beam combing. Single-wavelength or narrow linewidth can be achieved through volume Bragg grating (VBG) locking, distributed Bragg reflector (DBR), or distributed feedback (DFB) grating structure on a chip. Unfortunately, under low power conditions, the wavelength cannot be locked because of weak feedback. Therefore, a novel method for wavelength locking was developed using an electric switch and a fiber beam combiner to solve the problem. We developed an LD (laser diode) source comprising a single LD module, electric function module, thermoelectric (TEC) heat dissipation module, fiber beam combiner, and collimation lens barrel. An LD with power exceeding 120 W, center wavelength of 809±1 nm, beam divergence smaller than 1°, spot inhomogeneity lower than 10% (root mean square) was realized. In addition, the relative environment experimentc were conducted.
The gain spectrum of LD changes with current and temperature. Therefore, we kept the work current and temperature constant and controlled the number of LDs to adjust power through the electric function module and fiber beam combiner. We conducted electric, optics, and thermal designs. Compared to the traditional approach, we utilized a separate power supply for each single LD to prevent extinguishing with the LD overload voltage. For the optics, a 19×1 fiber beam combiner was used to combine several LDs to achieve power combing and high uniformity. In addition, the TEC module was responsible for the heat dissipation of multiple LDs; in this manner, the maintainability of LDs which were distributed was improved. After estimation, we selected 12 pieces of 55 mm×55 mm TEC coolers for refrigeration. The total cooling capacity was approximately 200 W. We designed the structure of the entire LD source. The size was approximately 420 mm×400 mm×200 mm, and the interface included an electric component, communication module, and fiber optic flange.
As mentioned above, an integrated design of optics, mechanics, thermotics, and electricity was performed. Next, we demonstrated the feasibility and correctness of the design. The power was measured at -55-50 ℃. The power value can reach a maximum of 124.7 W, and the e-o efficiency was approximately 45%-48%. The temperature of the LDs ranged from approximately 28 to 38 ℃, and the wavelength drifted by approximately 2 nm. Correspondingly, it increased at 0.2 nm/℃ on average (Fig. 14). The beam uniformity was tested. The inhomogeneity is 7% (RMS) lower than 10% based on the scattering image (Fig. 15). The temperature of the fiber flange was monitored in real time to guarantee usability. The temperature of the flange was lower than 80 ℃, which ensured the safety of the LD source (Fig. 16). The environmental experiments of high- and low-temperature, vibration, and electromagnetic compatibility were developed. Under these conditions, the main index of the LD exhibited no abnormality, and the results demonstrated the correctness of the proposed design and product.
In this study, we developed a novel method to stabilize the wavelength of the LD source with 12 single-LD modules of 12 W, whose spectrum was constant. The power, wavelength, beam divergence, and uniformity were measured. The power was 120 W, wavelength stability was ±1 nm, divergence was less than 1°, and inhomogeneity was lower than 10%. The environment experiments, including high- and low-temperature, vibration, and electromagnetic compatibility, were conducted successfully. The proposed design and demonstration provide support for advancements in military-grade laser sources. Finally, more than 10 sets of the LD sources using this method have been applied in relative programs.
