Recently, a semiconductor laser team comprising the School of Interdisciplinary Sciences at the National University of Defense Technology and Suzhou Changguang Huaxin Optoelectronics Technology Co., Ltd. has made significant progress in dual-color semiconductor laser research. The findings, titled "Monolithic 960/1000nm bi-color semiconductor disk laser delivers a brightness of over 300MW/cm²sr," were published in ACS Photonics. Zhang Zhicheng, an assistant researcher at USTC, served as the first author, with Professors Wang Jun and Zhang Chao Fan as corresponding authors.
Semiconductor disk lasers (SDLs), also known as vertical-cavity surface-emitting lasers (VECSELs), have garnered significant attention in recent years. Combining the advantages of semiconductor gain and solid-state resonators, they effectively overcome the emission area limitations of conventional single-mode semiconductor lasers while offering flexible semiconductor bandgap design and high material gain characteristics. They find applications in numerous scenarios, including low-noise narrow-linewidth laser output, ultrafast high-repetition-rate pulse generation, high-harmonic generation, and sodium guide star technology. Advancing technology demands greater wavelength flexibility. Dual-wavelength coherent sources demonstrate immense potential in emerging fields like anti-jamming lidar, holographic interferometry, wavelength division multiplexing communications, mid-infrared or terahertz generation, and multicolor optical frequency combs. Achieving high-brightness dual-wavelength emission while suppressing gain competition between wavelengths remains a significant challenge in semiconductor disk lasers.
To address this challenge, the semiconductor laser team proposed an innovative chip design. Through in-depth numerical studies, they discovered that precisely controlling temperature-dependent quantum well gain filtering and semiconductor microcavity filtering effects could enable flexible dual-color gain regulation. Building on this, the team successfully designed a high-brightness gain chip operating at 960/1000 nm. This laser operates in near-diffraction-limited fundamental mode, achieving an output brightness of approximately 310 mW/cm²sr.
Research Innovations
Figure 1: High-brightness dual-wavelength semiconductor gain chip design
The semiconductor wafer gain layer is only a few micrometers thick, forming a Fabry-Perot microcavity between the semiconductor-air interface and the distributed Bragg reflector at the substrate. Treating the semiconductor microcavity as an integrated spectral filter modulates the quantum well gain. Simultaneously, the microcavity filtering effect and semiconductor gain exhibit distinct temperature drift rates. Combined with temperature control, this enables switching and regulation of the output wavelength. Leveraging these properties, the team computationally set the quantum well's gain peak at 950 nm at 300 K, with a gain wavelength temperature drift rate of approximately 0.37 nm/K. Subsequently, the team employed the transfer matrix method to design the chip's longitudinal confinement factors, achieving peak wavelengths of approximately 960 nm and 1000 nm. Simulations revealed a temperature drift rate of only 0.08 nm/K. Using metal-organic chemical vapor deposition (MOCVD) for epitaxial growth, the team successfully fabricated high-quality gain chips through continuous process optimization. Photoluminescence measurements fully matched the simulation results. To mitigate thermal load and enable high-power operation, a semiconductor-diamond chip packaging process was further developed.
Comprehensive Analysis of Semiconductor Gain Chip Output Characteristics
Following chip packaging, the team conducted a comprehensive evaluation of its laser performance. In continuous operation mode, the emission wavelength can be flexibly tuned between 960 nm and 1000 nm by controlling the pump power or heat sink temperature. Within a specific pump power range, the laser also achieved dual-wavelength operation with a wavelength spacing of 39.4 nm, reaching a maximum continuous-wave power of 3.8 W. Concurrently, the laser maintained near-diffraction-limited fundamental mode operation with a beam quality factor M² of only 1.1 and a brightness of approximately 310 MW/cm²sr. The team also investigated the laser's quasi-continuous wave performance. By inserting a LiB₃O₅ nonlinear optical crystal into the resonator cavity, they successfully observed sum-frequency signals, confirming the synchronization of both wavelengths.
This ingenious chip design achieves an organic integration of quantum well gain filtering and microcavity filtering, laying the design foundation for realizing dual-wavelength laser sources. In terms of performance metrics, this monolithic dual-wavelength laser achieves high brightness, high flexibility, and precise coaxial beam output. Its brightness ranks among the world's leading levels in the current field of monolithic dual-wavelength semiconductor lasers. For practical applications, this achievement holds promise in multi-color lidar systems. Leveraging its high brightness and dual-wavelength characteristics, it can effectively enhance radar detection accuracy and anti-interference capabilities in complex environments. In optical frequency comb applications, its stable dual-wavelength output provides critical support for precision spectral measurements and high-resolution optical sensing. Looking ahead, the team plans to deepen their research. On one front, they aim to develop electro-pumped devices by optimizing parameters such as electrode dimensions and doping to further increase single-mode power. On another, they will explore novel electro-pumped photonic crystal surface-emitting semiconductor lasers.
