Objective Laser diodes in the 800 nm band hold significant application value in laser power transmission, laser lighting, and solid-state laser pumping. However, the poor beam quality and low brightness of laser diodes fail to meet the demand for high-power, high-brightness light sources required for long-distance laser power transmission. Grating external cavity spectral beam combining (SBC) technology has been demonstrated as an effective approach to enhance the output power and brightness of laser diodes. Nevertheless, achieving stable frequency locking across the full operating current range in external cavity SBC systems presents considerable challenges, particularly in multi-channel combining where "lose of lock" phenomena frequently occur. In this study, we design a transmission grating-based external cavity spectral tuning experiment for 800 nm-band laser diode single emitters. The spectral characteristics, frequency-locking tuning behavior, linewidth narrowing, and locking efficiency under various driving currents are systematically investigated. Additionally, we analyze the lockable spectral range, the maximum combinable channel count, and the efficiency variation of the SBC system under full operating current conditions. Methods Based on a transmissive diffraction grating, this study designs an external cavity spectral tuning experiment. First, the optical parameters of the laser in the frequency locking system are determined by leveraging grating diffraction and external cavity frequency locking principles. This includes the design of the transformation lens parameters (focal length of 700 mm), grating specifications (grating density of 1851 line/mm, Littrow angle theta littrow=50.08 degrees@828.781 nm), and cavity mirror configurations. Figure 6 illustrates the schematic diagram of the grating external cavity tunable laser diode. The designed structure enables convenient spectral tuning by rotating the output mirror to adjust the locked wavelength of the emitted laser, while simultaneously achieving significant linewidth narrowing. Subsequently, a single-emitter laser diode operating at 800 nm is employed for structural packaging and beam collimation. The divergence angle and beam width are measured to finalize the experimental device. Spectral tuning experiments are conducted to investigate fundamental spectral characteristics, tuning range, locking efficiency, and the numbers of beam-combining channels. Finally, a comprehensive analysis is performed to evaluate critical parameters of the grating external cavity laser diode system under full operating current conditions, including frequency locking range, bandwidth, and beam-combining channel count. Results and Discussions Through structural packaging, the laser diode single emitter demonstrates stable room-temperature operation with a threshold current of 0.65 A, a slope efficiency of 1.53 W /A, and an electro-optic conversion efficiency of 61.32%. Under 9.5 A continuous current driving, it achieves a 11.32 W output power [Fig. 3(a)], exhibiting strong gain in the 820 -840 nm spectral range [Fig. 3(b)], with feasibility for spectral frequency stabilization and tuning. Fast-axis and slow-axis collimating lenses respectively collimate the beam dimensions to 0.408 mm [Fig. 5(a)] and reduce the fast-axis divergence to 2.95 mrad [Fig. 5(b)]. Without frequency stabilization, the central wavelength shifts from 829.972 nm at 1.5 A to 831.794 nm at 9.5 A, showing a current-dependent wavelength drift coefficient of 0.227 nm/A [Fig. 7(a)]. Grating-stabilized operation reduces this drift to 1.75 pm/A while maintaining 0.083 nm spectral linewidth (full width at half-maximum, FWHM) [Fig. 7(b)], achieving a stable narrow-linewidth output across current variations. Frequency-locked tuning ranges span 821.974-833.853 nm at 1.5 A and 826.768-834.141 nm at 9.5 A (Fig. 8). High-current operation shows a reduced tuning range due to intensified competition between intracavity lasing modes and grating feedback modes. Full operational current conditions yield an 826.768 - 833.084 nm tuning range, with an 828.348 nm demonstrating peak output power [Fig. 12(a)] corresponding to the central wavelength of 828.781 nm of the gain spectrum, where mode loss minimization enhances power output. Maximum electro-optic efficiency of 54.82% occurs at 828.348 nm [Fig. 12(b)]. At 9.5 A driving current, the system achieves a 9.92 W output [Fig. 13(a)] with 828.348 nm stabilized wavelength and 0.083 nm linewidth [FWHM, Fig. 13(b)]. The 0.007 nm discrepancy between measured and theoretical linewidths originates from energy attenuation artifacts in charge coupled device (CCD)-based beam width measurements used for simulations. Conclusions This study focuses on investigating the spectral tuning characteristics and linewidth properties of a grating external cavity laser diode under varying driving currents, aiming to determine its full operational current-dependent tuning range and experimentally quantify its specific linewidth and tunable bandwidth parameters. Based on a single-emitter 800 nm-band laser diode, we design and conduct a series of frequency-locking and tuning experiments using the grating external cavity configuration. Experimental results demonstrate that the spectral beam combining system employing this grating-external-cavity laser diode achieves a frequency-lockable spectral range spanning 826.768 nm to 833.084 nm under comprehensive operating current conditions (1.5-9.5 A), corresponding to an effective bandwidth of 6.316 nm. The system currently supports up to 32 beam-combining channels through precise wavelength control. However, challenges persist regarding the limited tunable bandwidth and relatively low channel count in the 800 nm-band laser diode devices. Future researches will prioritize overcoming these limitations by optimizing the external cavity grating configuration, enhancing the thermal management strategies, and improving the wavelength stabilization mechanisms. Key objectives include expanding the spectral coverage while maintaining narrow linewidth characteristics, increasing the numbers of combinable channels without compromising output stability, and achieving robust full-current-range operation for the grating-based SBC laser source. These advancements aim to establish a high-power, spectrally stable laser system with extended tunability for applications demanding precise wavelength control and multi-channel beam combining in industrial and scientific domains.
