Study on Bragg reflection waveguide diode laser
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摘要: 高功率半导体激光器在固体或光纤激光器泵浦、材料加工、医疗、传感、空间通讯和国防上有着极其重要的应用,但传统半导体激光器面临垂直发散角大、椭圆光斑的难题,限制了其直接应用。为了降低激光器的垂直发散角,本项目采用布拉格反射波导结构,利用光子带隙导引替代传统的全反射进行光场限制,优化设计了多种布拉格反射波导激光器结构,并制备了高性能的激光器器件。首先,采用传输矩阵理论和布洛赫波近似的方法计算了布拉格反射波导的模式色散关系,发现通过控制腔模光场分布,可实现不同远场的激光输出。接着,针对布拉格波导光子带隙导引机制,深入研究了四分之一波长布拉格反射波导激光器、单边布拉格反射波导激光器的光场特性,弄清了影响此类激光器远场的本质因素,最终设计并验证了一种布拉格反射波导双光束激光器,激光器在垂直方向可输出两个对称的、近圆形光束,单光束垂直和侧向发散角半高全宽分别低至7.2°和5.4°。另外,通过调控光缺陷层,使激光器工作在受抑隧穿光子带隙导引机制下,实现了超窄的单光束激光输出,激光器单管连续输出功率超过4.6 W,垂直发散角最低降至4.9°(半高全宽)和9.8°(95%功率)。这种高功率、窄的圆形光束输出可以大幅降低半导体激光器的应用成本,提高泵浦或光纤耦合效率,具有广阔的应用前景。Abstract: High power diode lasers are widely used for pumping of solid state lasers and fiber lasers, material processing, medical treatment, sensors, free-space optical communication, security and defense. However, the conventional diode lasers usually suffer from a large far-field divergence and strongly elliptical beam, which limit the direct applications. To improve the divergence, diode lasers based on Bragg reflection waveguide(BRW) are studied in this project, which utilizes the photonic bandgap(PBG) effect rather than the total internal reflection(TIR) to provide optical confinement. Several kinds of BRW lasers(BRLs) with different structures are designed and fabricated. First, the mode dispersion equation of the BRW is solved by the transfer matrix method and Bloch theory. The further analysis shows that the far-field distribution of BRW is determined by the mode shape in the cavity. In the case of Bragg form of PBG guidance, the optical field characteristics of a quarter-wave BRL and a single-sided BRL are studied. The essential reason affecting the far-field distribution is investigated. Finally, a twin-beam laser based on BRW is designed and demonstrated. Almost all the emission power of this laser is concentrated in two near-circular lobes in the vertical direction. The full-width at half maximum(FWHM) divergence angles of one beam are as narrow as 7.2° and 5.4° respectively in the vertical and lateral direction. Furthermore, the high brightness BRL with a ultra-narrow circular output beam is demonstrated by controlling the defect layer. The ultra-low vertical divergence of 9.8° with 95% power content and 4.9° with the FWHM definition is realized. The maximum output power exceeds 4.6 W under continuous-wave operation at room temperature. The narrow circular beam emission from the BRL can greatly improve the pumping efficiency and optical fiber coupling efficiency without expensive beam shaping. It is believed that the BRLs have a promising application prospect.
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Key words:
- diode laser /
- Bragg reflection waveguide /
- photonic bandgap /
- low divergence
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图 2 不同光子带隙导引机制对应的近场分布
Figure 2. Mode shapes of near field distribution for different photonic bandgap guiding mechanisms
图 3 Qtw-BRW激光器的折射率分布和能带分布示意图
Figure 3. Refractive index profile and energy band profiles of the Qtw-BRW laser
图 4 15~40 ℃下激光器的连续功率-电流特性曲线，左上角为不同温度下的阈值电流，右下角为刻蚀脊形的SEM图[
Figure 4. Measured CW L-I curves from 15 ℃ to 40 ℃,the top inset shows the temperature dependent threshold current,bottom inset shows the SEM image of the etched ridge
图 5 Qtw-BRW激光器的垂直远场分布
Figure 5. Far-field profile of the Qtw-BRW laser in the vertical direction
图 7 单边布拉格反射波导激光器的折射率分布、基横模近场强度分布及能带分布图
Figure 7. Distribution profiles of refractive index profile,calculated vertical near-field intensity of designed single-sided BRL structure and energy band profiles
图 8 连续条件下测得的单边BRL的L-I-V特性曲线
Figure 8. CW L-I-V characteristics of the single-sided BRL
图 9 (a)试验测量和(b)理论计算的单边BRL的垂直远场分布
Figure 9. (a)Measured and (b)claculated far field distribution of the single-sided BRL in the vertical direction
图 11 布拉格反射波导双光束激光器的折射率分布，基横模的近场电场幅度分布和远场分布
Figure 11. Refractive index profile of the BRW twin-beam laser,calculated vertical NF electric field distribution and calculated FF curves of the fundamental mode
图 12 脊型BRL在连续和准连续工作下的L-I-V特性
Figure 12. L-I-V characteristics of the ridge BRL under CW and QCW operation
图 13 实验测得的脊型BRL的近场光斑和远场光斑
Figure 13. Measured near-field and far-field patterns of the ridge BRL
图 14 不同电流下测得的BRL的垂直和侧向远场分布
Figure 14. Measured vertical FF and lateral FF distribution of the BRL at different currents
图 15 不同电流下脊形BRL的激射光谱
Figure 15. Color-scale mapping of the optical spectrum versus injected current for the ridge BRL
图 16 260 mA电流下测得的脊形BRL的激射光谱
Figure 16. Measured optical spectrum of the ridge BRL at 260 mA
图 17 (a)宽条BRL的连续和脉冲L-I-V特性，不同电流下测得的(b)2D远场光斑和(c)垂直远场分布；(d) 不同电流下两束激光的垂直和水平方向的半高全宽发散角对比
Figure 17. (a)CW and pulsed L-I-V characteristics of the broad area BRL; (b)measured 2D FF patterns and (c)vertical FF profiles at different currents; (d)measured FWHM of the upper and bottom lobes in the vertical(hollow squares) and lateral(solid squares) direction as a function of injected current
图 20 锥形BRL的锥形和脊形部分输出的远场光斑
Figure 20. Measured 2D FF patterns at different current from the tapered and ridge facet of tapered BRL
图 22 808 nm BRL的折射率分布，计算的垂直方向、近场分布和远场分布
Figure 22. Refractive index profile of the 808 nm BRL,calculated near-field and far-field distribution in the vertical direction
图 23 电极条形和深脊波导宽区BRL的L-I-V特性，插图为不同温度下的激射谱
Figure 23. L-I-V characteristics of the broad area BRLs with shallow mesa and deep mesa,the top inset shows the measured lasing spectra at different temperatures
图 24 电极条形BRL(a)在不同电流下的远场光斑；(b)3 A电流下的垂直和侧向远场分布
Figure 24. (a)Measured far-field spot at various currents of the shallow mesa BRL; (b)measured vertical far-field and lateral far-field distribution of the laser at 3.0 A
图 25 深刻蚀脊形BRL在不同电流下的远场光斑
Figure 25. Measured far-field spot at various currents of the deep mesa BRL
图 26 10 μm条宽脊形BRL单管的L-I-V特性曲线和不同电流下的远场光斑
Figure 26. L-I-V characteristics of the ridge BRL and measured far-field spot at different currents
图 27 SQW-BRL、DQW-BRL和TQW-BRL的近场光场分布，黑色实线：基模，虚线：一阶高阶模，右上角插图为有源区折射率分布示意图
Figure 27. Calculated vertical NF distribution of the fundamental(solid line) and high-order(dot line) modes for the (a)SQW-BRL,(b)DQW-BRL and (c)TQW-BRL,the inset shows the refractive index distribution of the defect layer
图 28 未封装DQW-BRL和TQW-BRL在准连续工作下的L-I-V特性曲线
Figure 28. L-I-V characteristics of the unpackaged BRLs with DQWs and TQWs under QCW operation
图 29 DQW-BRL和TQW-BRL在不同电流下的远场光斑
Figure 29. Measured FF spot patterns at various currents of DQW-BRL and TQW-BRL
图 30 TQW-BRL在2.25A电流下的垂直远场分布
Figure 30. Measured vertical FF distribution of the TQW-BRL
图 32 1.5和4 mm腔长DQW-BRL的L-I-V特性曲线，插图为4 mm腔长器件激射波长随热功率的变化关系
Figure 32. L-I-V characteristics of the DQW-BRLwith 1.5 mm and 4 mm cavity length,the inset shows the lasing wavelength of versus wasted power under 10 ℃ and 15 ℃
图 33 (a)采用直接光纤耦合和单透镜准直方式测得的DQW-BRL和传统半导体激光器的光纤耦合效率；(b)单透镜准直的光纤耦合装置图
Figure 33. (a)Fiber coupling efficiency of the DQW-BRL for the methods of direct coupling and single lens collimation compared with conventional diode laser; (b)the optical system for the method of single lens collimation
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