Semiconductor lasers with compact structure, long life, high electro-optical conversion efficiency, and direct modulation are ideal for many traditional applications. The rapid development of intelligent sensing technology and the replacement of core devices have resulted in higher performance requirements of narrower spectral linewidth and higher output power for such lasers. The typical solution for realizing narrow-linewidth semiconductor lasers is the implementation of buried gratings. However, the necessary secondary epitaxial growth in the fabrication is difficult and time-consuming; it also leads to increased fabrication cycle time and cost of the device. The use of surface gratings in the devices has been proposed to address this issue. Prior studies indicate that distributed feedback (DFB) lasers based on surface gratings exhibit excellent operating characteristics [e.g., high power, narrow linewidth, large side-mode suppression ratio (SMSR), and small temperature drift]. In general, increasing the ridge width is a relatively straightforward method of increasing the output power of the device. However, for wide-ridge DFB lasers, additional high-order transverse mode suppression mechanisms must be introduced to ensure single-mode high-power operation of the device. An unstable cavity laser is the combination of unstable resonator and wide stripe semiconductor laser, which renders the device as exhibiting high lateral mode selectivity. Consequently, it can provide good linewidth in broad area lasers. However, the structures of unstable cavity semiconductor lasers are diverse and complex, with most using electron beam lithography, holographic lithography, multi-step etching, secondary epitaxy, and other processes. This results in high manufacturing costs and difficulties; thus, its practical implementation and engineering are challenging. The structural design and characteristic optimization of novel unstable cavity laser are vital and of practical significance for the performance improvement and application expansion of semiconductor lasers. This study examined an unstable cavity semiconductor laser with high-order curved gratings. The use of curved grating with a wide current injection region increases the loss difference between different oscillation modes and improves the mode discrimination such that it can obtain greater gain and power than the conventional narrow-ridge fundamental transverse mode lasers.

There are four main aspects to the methodology employed. 1) The device adopts a wide ridge structure for high power output of the device. The broad-area ridge structure enables the fundamental mode in the cavity to exhibit a larger mode volume, which facilitates the extraction of increased gain and power than conventional narrow-ridge fundamental transverse mode lasers. Moreover, the broad-area ridge reduces the power density and thermal load of the device and improves the stability of the laser. 2) Through the setting of the gain and non-gain regions on the ridge, the high-order side mode is suppressed. The parameters of current injection/non-injection regions in the ridge are set to suppress high order lateral modes. Further, the energy of the fundamental mode distributed in the gain region is greater than those of the first- and second-order modes distributed in the gain region. Consequently, the fundamental mode obtains a larger gain than the high-order mode, which is beneficial to the stable operation of the device under the fundamental mode. 3) The curved grating and the high reflective rear facet form an unstable cavity, which enhances the mode discrimination of the resonator and realizes the single-mode stable output of the device. The unstable cavity for mode selection is formed owing to the highly reflective rear facet and curved gratings that are defined by standard ultraviolet lithography. The structural design of the curved grating exhibits remarkable flexibility, which considerably reduces the difficulty and complexity of device design and fabrication. In addition, lateral modes regulated by curved gratings in the broad area device strengthen the mode discrimination and enhance the device’s ability to suppress spatial gain hole burning and filamentation. 4) High-order curved gratings are used to achieve selection of wavelengths in the cavity. A high-order grating is used to realize the distributed feedback of light in the cavity. Compared with the low-order grating, the designing and manufacturing are less challenging, and the cost is lower.

The experimental results show that the threshold current, continuous output power, and slope efficiency of the curved grating semiconductor laser device are 220 mA, 1.48 W, and 0.63 W/A, respectively (Fig. 7). The single longitudinal mode lasing spectra of the devices under the injection current of 450, 500, and 550 mA all exhibit good performance. With the increase of the current, the spectral linewidths are obtained as 0.117, 0.145, and 0.132 nm, respectively, and the side mode suppression ratio are 23.5 dB, 28.0 dB, and 28.4 dB, respectively. Further, the laser wavelengths of the device are 979.76, 980.07, and 980.4 nm, respectively. With increase in the injection current, the output wavelength of the device drifts to the long wavelength direction at a drift rate of 6.4 nm/A (Fig. 8). Through comparisons of the spectra of Fabry-Perot, linear grating DFB, and curved grating DFB lasers, the critical role of curved gratings in mode selection of semiconductor lasers is demonstrated. It is therefore beneficial to realize the narrow-linewidth single-mode operation of high-power DFB lasers with a lasing wavelength of 980.55 nm, a spectral linewidth of 0.121 nm, and a side-mode suppression ratio of 32 dB at an injection current of 1 A (Fig. 9).

This study proposes the fabrication of a novel broad-area distributed feedback laser with high order surface curved gratings based on standard ultraviolet lithography. The high-order curved grating is used to form an unstable cavity structure, which increases the loss difference between different oscillation modes, improves the mode discrimination, and obtains uniform gain over a broad area. The DFB laser emitting at wavelength of approximately 980.55 nm achieves a spectral width of 0.121 nm and maximum output power of 1.48 W. The threshold current and sidemode suppression ratio of the device are 220 mA and 32 dB, respectively. Further, through comparisons of the power-current-voltage curves and spectra of Fabry-Perot, linear grating DFB, and curved grating DFB lasers, it is demonstrated that curved gratings play a key role in power output and mode selection in semiconductors lasers; this finding is beneficial for the realization of narrow-linewidth single-mode operation of high-power DFB lasers.