• Chinese Journal of Lasers
  • Vol. 50, Issue 11, 1101022 (2023)
Kun Zhu, Hui Li*, Yongqin Hao, Ran Qian, and Dongyue Wang
Author Affiliations
  • National Key Laboratory on High Power Semiconductor Lasers, Changchun University of Science and Technology, Changchun 130022, Jilin, China
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    DOI: 10.3788/CJL221297 Cite this Article Set citation alerts
    Kun Zhu, Hui Li, Yongqin Hao, Ran Qian, Dongyue Wang. Design of Grating Structure in Distributed Bragg Reflector Semiconductor Laser[J]. Chinese Journal of Lasers, 2023, 50(11): 1101022 Copy Citation Text show less

    Abstract

    Objective

    The existence of edge modes in the distributed Bragg reflector(DBR)semiconductor laser emission spectrum has a significant influence on the beam quality. The output multi-edge modes and high side lobes deteriorate the semiconductor laser beam and output power. The F-P effect formed on the sides of the DBR laser with a homogeneous duty cycle is one reason for this, resulting in side-mode resonance enhancement. Second, the energy coupled to the narrow-ridge waveguide through the grating decreases near the rim of the narrow ridge. This study proposes a modification scheme of the regular DBR structure to inhibit the edge mode on the DBR laser spectrum, ensuring maximized reflectance at the central wavelength while weakening the side lobe strength.

    Methods

    Based on coupled-mode theory, a rectangular grating model with a gradual duty cycle is established for the DBR laser. The influence of the duty cycle distribution on the edge mode suppression and the reflectivity maximization at the central wavelength is analyzed using the finite-difference time-domain (FDTD) method and changing the DBR duty cycle. The electro-optic model of the rectangular grating with gradient duty cycle is simulated, and the reflectance at the central wavelength and the side mode suppression ratio are obtained after device optimization. A tapered duty cycle model for the DBR is established, and the influence of the grating structure with tapered duty cycle on the output side lobe intensity of the DBR laser is analyzed.

    Results and Discussions

    In DBR lasers, the F-P effect exists in rectangular uniform duty cycle gratings, resulting in edge-mode oscillation enhancement (Fig. 5). From the relationship of coupling coefficient and duty cycle (Fig. 6) and the duty cycle under the maximum reflectivity at the central wavelength, the grating duty-cycle range can be determined. Within the gradient range, the simulation of grating when the duty cycle is truncated sinc function distribution is carried out. Within the gradient ranges of 0.545-0.580 and 0.580-0.545, the reflection intensity of the edge mode is suppressed, and the reflection peak value at the central wavelength reaches 0.8 (Fig.7). For a grating with length of 0.6 mm, comparing the reflection spectra of rectangular uniform grating and rectangular grating with gradient duty cycle, it can be observed that, although the edge mode is suppressed for the rectangular grating with gradient duty cycle, its reflection peak value at the central wavelength is lower than that of the rectangular uniform grating. Setting the duty cycle at 0.58 in the center of the grating, the edge mode suppressing effect is studied for the case when the duty cycle decreases from the center of the grating length to the two ends . The results show that the edge mode suppression under truncated sinc functions is enhanced, but the reflectivity at the central wavelength does not improve (Figs.9 and 10). When the grating length increases to 1 mm, the central reflection peak value of the grating with gradient duty cycle remains unchanged (Fig.11). When adopting the grating with gradient duty cycle, the coupling coefficient at the center of the grating length is small; thus, using the combination of duty cycle that presents truncated sinc function distribution and constant duty cycle can increase the coupling coefficient at the center of the grating. Although the reflection peak value at the central wavelength is nearly similar to that of the uniform grating, the edge mode suppression ratio reaches 48 dB (Fig.13). Finally, by observing the field intensity distribution of tapered grating with gradient duty cycle and rectangular uniform grating on the far-field lateral tangent, we find that tapered gratings with gradient duty cycle can reduce the sidelobe intensity of the DBR laser output.

    Conclusions

    The existence of the F-P effect in the rectangular uniform grating for conventional DBR lasers results in side-mode oscillation enhancement. In addition, more side modes in the reflectance spectra of semiconductor lasers not only deteriorate the beam quality but also reduce the output power at the central wavelength. It is found that changing the duty-ratio distribution in the DBR laser can destroy the grating effect, leading to a reduction in the side-mode reflection intensity. Furthermore, using a gradual duty cycle at both ends of the grating and a constant duty cycle in the center of the rectangular grating length, not only reduces the side mode in the reflection spectrum, but also ensures a larger reflectivity at the central wavelength. It is found that the optical field propagating at the edge of the narrow ridge waveguide, when passing through a tapered gratings with gradient duty cycle, can be better coupled into the narrow ridge waveguide, leading to an effective reduction in the intensity of the side lobe. Compared with previous schemes, this requires less complex process steps while enhancing the side mode suppressing ratio. Additionally, it can reduce the side lobe strength, providing a valuable reference for the structural design of high-power and high-beam-quality semiconductor lasers.

    Kun Zhu, Hui Li, Yongqin Hao, Ran Qian, Dongyue Wang. Design of Grating Structure in Distributed Bragg Reflector Semiconductor Laser[J]. Chinese Journal of Lasers, 2023, 50(11): 1101022
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