Semiconductor lasers have been widely used in industrial manufacturing, medical diagnosis, lidar, and other fields because of their small size, high electro-optical conversion efficiency, long life, and direct-current drive. With the development of technology, higher requirements have been placed on laser light sources for different applications, such as high output power, narrow spectral linewidth, stable wavelength, and near-fundamental mode output. Researchers have performed a great deal of work in this area, making a series of breakthroughs ranging from broad-area semiconductor lasers to narrow-ridge semiconductor lasers and then to grating coupling. Distributed feedback semiconductor lasers using buried gratings can obtain high spectral purity; however, there are preparation difficulties in their re-growth. Researchers have found that surface gratings for coupling optical fields exhibit good working characteristics. To improve the power, a distributed Bragg reflector laser diode with tapered gratings combined with a master oscillator power amplifier is produced. Increasing the ridge width is a more direct method, which is commonly used; however, additional transverse mode suppression mechanisms need to be introduced, such as transverse coupled gratings and lateral microstructures. The exploration of single-mode stable-output semiconductor lasers has been a popular topic in related fields worldwide. In this study, a wide-ridge waveguide-distributed feedback semiconductor laser based on high-order curved surface gratings is prepared. Curved gratings and current-limited injection structures can suppress the high-order transverse mode in a wide-ridge waveguide and improve the power and spectral purity of the device. In addition, the use of ultraviolet lithography significantly reduces the difficulty of fabrication.
The transverse mode of the device is investigated using curved gratings and a current-limited injection structure, and the experimental results are analyzed. The effect of the ridge waveguide on the transverse mode is analyzed. It has been pointed out that a wide-ridge waveguide requires an additional transverse-mode suppression mechanism. Subsequently, two methods, curved grating and current-limited injection structure, are proposed. First, the gratings in the center of the curved gratings are regarded as linear gratings, which are used to narrow the linewidth. The gratings in the edge area combined with the cavity facet of the resonator form an unstable resonator, which leads to the beam propagation of the high-order transverse mode in the cavity and increases the feedback loss. The formula for calculating the curvature of the curved grating is given. Second, the current-limited injection structure is set such that the high-order transverse mode lasing threshold is greater than the basic mode threshold, whereas the gain is lower than that of the fundamental mode. Subsequently, the grating order is given, the period is determined by the Bragg condition, and the structural parameters of the gratings are optimized by software simulation to determine the duty cycle and etching depth suitable for device fabrication. Subsequently, the designed device structure is prepared experimentally. An electron microscope scan of the experiment is performed, and a device that meets the expected requirements is packaged and tested. Finally, the transverse mode of the curved grating device is analyzed using the spectrum, spot, and far-field divergence angle, which proves the validity of the structure and provides the optimization direction.
The prepared curved grating device exhibits the expected single-mode output characteristics. Experiments show that the far-field slow axis divergence angle of the device is 5.3° at 0.5 A [Fig.10(a)], the optical spot presents a single lobe [Fig.9(a)], the 3 dB spectral linewidth is 0.173 nm, and the side-mode suppression ratio is 22.6 dB (Fig.8). The results show that the curved grating structure plays a key role in the suppression of the high-order transverse mode in the cavity, and the center is regarded as a high-order linear grating that narrows the linewidth. This provides a new concept for a single-mode stable output device.
A distributed feedback semiconductor laser with high-order curved gratings is fabricated. The high-order transverse mode is suppressed using curved gratings and a current-limited injection structure. At room temperature, the measured threshold current of the device is 0.49 A, the optical spot presents a single lobe, the far-field slow axis divergence angle is 5.3°, the fast axis divergence angle is 29.2°, the measured emission wavelength is 1051.93 nm, the 3 dB spectral linewidth is 0.173 nm, and the side mode suppression ratio is about 22.6 dB at 1 A. The output power can reach 939.8 mW at 2.2 A, and the device can achieve the expected single-mode output effect. In addition, the device adopts the ultraviolet-lithography preparation process, which greatly reduces manufacturing difficulty and provides a simpler and more effective solution for semiconductor laser devices with a stable output of a single mode. However, the performance of the device must be improved further because of the high threshold current. In later stages, the sidewall morphology, structure, and curvature parameters of the curved gratings are fully optimized to obtain better performance.
