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根据表1中所示的结构,笔者进行了外延生长,流片和测试表征。首先利用金属有机化学气相沉积(MOCVD)进行了905 nm多有源区半导体激光器的外延生长,隧道结处的外延生长需要重掺杂的GaAs以减小电阻和作为多个有源区的隧穿路径[14]。
Number Gain region type P-WG thickness/μm Barrier thickness /μm Fundamental mode Г Bulk resistance/Ω Threshold gain ratio A1 a 0.75 0.01 1.69% 0.22 4.5 A2 a 0.45 0.01 1.31% 0.21 1.2 A3 a 0.35 0.01 1.04% 0.205 0.9 B b 0.35 0.03 1.59% 0.18 1.6 C c 0.25 0.07 1.58% 0.18 2 Table 1. Device preparation parameters
外延生长后进行激光器的波导结构刻蚀和电极制备。由于重掺杂的隧道结和有源区之间激光器的注入电流会产生严重的横向扩展。为了抑制电流扩展,采用深腐蚀沟槽结构,刻蚀深度超过接近衬底一侧的有源区量子阱,在三有源区和四有源区的隧道结级联激光器中,刻蚀深度均超过12 μm。沉积300 nm厚的SiO2作为绝缘层,然后光刻形成200 μm宽的电极窗口,蒸镀Ti-Pt-Au和电镀金作为P面电极,然后背面减薄至150 μm左右,并溅射N面电极金属。接着将晶圆解理为800 μm腔长的巴条,进行腔面钝化,后腔面镀99%以上的高反射膜,前腔面镀15%反射率的增透膜。最后解理为400 μm宽的单管芯片,P面朝上封装在TO管壳上用作后续的测试。
测试使用的条件都是100 ns脉冲宽度,1 kHz的重复频率,脉冲测试系统是基于LSP-40脉冲模块,这个模块脉冲宽度在50~1000 ns之间可调,输出电流在0~41.6 A之间。功率采集使用高灵敏度的功率计(型号为PowerMax PM150)采集平均功率,然后通过占空比计算峰值功率。远场的测试是用CCD采集在41.6 A的脉冲电流工作下的输出图像,然后用Matlab对采集到的图像进行垂直方向的扫描,得到光强分布随角度变化的远场发散角。
笔者对A1、A2、A3三个结构进行了PIV(图4(a))和垂直方向远场发散角(图4(b))的测试,结果如图4所示。随着P面波导厚度减小,体电阻减小,阈值增益比也随之减小。A1结构的激光器,在25 ℃、100 ns、1 kHz的条件,41.6 A的驱动电流下A1、A2、A3峰值功率分别为107 W、125 W和118 W。并且在低电流情况下,三种结构的斜率效率基本一致,在电流大于15 A以后呈现不同程度的功率饱和现象,并且峰值功率A2>A3>A1,表明不同的波导结构对载流子泄露引起的功率饱和有影响。
Figure 4. (a) PIV curves of triple-active regions semiconductor lasers with waveguide of A1 (red), A2 (green), A3 (blue); (b) Far field divergence angles of triple-active regions semiconductor lasers with waveguides of A1 (red), A2 (green), A3 (blue) in the vertical direction
可以看出,A1、A2、A3三个结构的阈值增益比分别为4.5,1.2和0.9。在阈值增益比为4.5时,激光器垂直方向单模激射,输出光斑为基模(文中只讨论垂直于PN结方向的激光模式,后文所述基模,单模和高阶模为垂直于PN结方向的基模,单模与高阶模式),其垂直方向远场发散角半高全宽(Full width at half maximum,FWHM)为26.8°。在阈值增益比等于1.2时,激光器对高阶模式的限制能力减弱,有高阶模的激射,远场输出光斑呈双峰,表现为基模与一阶模的叠加,其FWHM急剧上升,为57.5°。随着阈值增益比进一步减小,当阈值增益比等于0.9时,激光器高阶模式激射,输出光斑呈双峰,表现为完全一阶模,其FWHM继续增加至59.2°。随着阈值增益比减小,激光器不能维持基模工作,高阶模的成分逐渐增加。
笔者对增加势垒数量的B结构激光器进行表征,如图5所示,B波导结构的激光器具有和A3相同的P面波导厚度,在25 ℃、100 ns、1 kHz的条件下,41.6 A的峰值功率提高到132 W,且未出现明显的功率饱和。说明相比A型波导,B型波导结构增加了两个势垒结构,增加了限制载流子泄露的能力,提高了高脉冲电流下的峰值功率。但是其FWHM为30.1°,远场发散角比模拟值大很多,表明一阶模有部分激射,导致远场发散角增大。说明1.6的阈值增益比仍有高阶模部分激射的风险,需要进一步增大阈值增益比。
Figure 5. (a) PIV curve of triple-active regions semiconductor laser with B waveguide structure; (b) Divergence angle of far field in vertical direction of triple-active regions semiconductor laser with B waveguide structure
如图6所示,在B波导结构的基础上,为了进一步优化输出性能,使用了C波导结构,对N面势垒增厚至50 nm,提高了基模限制因子。 C波导结构的激光器,在25 ℃、100 ns、1 kHz的条件下,41.6 A的峰值功率三有源区和四有源区分别达到了133 W和177 W。C波导结构的激光器的阈值增益比等于2,三有源区和四有源区FWHM分别为24.1°和24.3°,与模拟值24.7°接近,说明是基模激射。三有源区的C结构达到了和B结构相当的峰值功率,且高阶模得到了完全抑制,实现了垂直于PN结方向的单模工作,远场发散角有较大的优化。
Figure 6. (a) PIV curves of semiconductor lasers in the triple-active regions (black)and quadruple-active regions (red) with C waveguide structure; (b) Far field divergence angles of semiconductor lasers in the triple -active regions (black) and quadruple-active regions (red) with C waveguide structure
综合以上个三结构,笔者发现激光器的发散角与阈值增益比有强相关性。如图7所示,将不同阈值增益比的激光器41.6 A的实测发散角和模拟的基模发散角进行对比。可以看出,当阈值增益比大于等于2 时,激光器的实测发散角与模拟的基模发散角很好地吻合。但当阈值增益比小于等于1.6时,实测的发散角大于模拟值,说明高阶模式已经开始工作,且随阈值增益比的降低,高阶模式的成分逐渐增加。根据实验数据的分析,笔者总结存在支持多个模式的大光腔波导结构中,阈值增益比大于等于2的非对称大光腔波导结构可以实现基模工作。
Figure 7. Variation trend of far-field divergence angle of semiconductor lasers with riple-active regions under different threshold gain ratios; measured (black), simulated (red)
不同的增益区类型、不同波导结构的器件输出性能表明:相对折射率高的势垒层和更窄的P面波导有利于限制载流子泄露及降低载流子泄露导致的功率饱和现象,使得在高脉冲电流工作的情况下,器件有更高的峰值功率;同时更窄的P面波导能有效降低体电阻,这两点共同提高了器件的输出效率。但是,P面波导厚度减小时限制因子和阈值增益比也同时减小,有高阶模式激射的风险。因此,在波导结构的设计中应该在保证垂直于PN结方向单模工作时(阈值增益比大于等于2)尽量降低P面波导厚度。
Effect of waveguide structure on beam quality and power of 905 nm cascade semiconductor lasers with tunnel junctions
doi: 10.3788/IRLA20210979
- Received Date: 2021-12-17
- Rev Recd Date: 2022-02-22
- Accepted Date: 2022-03-03
- Publish Date: 2022-06-08
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Key words:
- waveguide structure /
- semiconductor laser /
- tunnel junction cascade /
- beam quality /
- power optimization
Abstract: In recent years, lidar applications had put forward higher requirements for detection distance and sensitivity. As an ideal light source, 905 nm semiconductor lasers also urgently needed to improve the peak power and beam quality. In this context, the effects of different gain region types and waveguide structures on beam quality and power efficiency of 905 nm tunnel-junction pulsed semiconductor lasers were investigated based on asymmetric large optical cavity structures. By optimizing the gain region type and waveguide structure, the bulk resistance and internal loss were reduced. The ability to limit carrier leakage was enhanced, and the peak power and electro-optical efficiency of the device working at high currents were improved. By increasing the threshold gain ratio of the multimode to the fundamental mode, the high-order mode lasing was suppressed, and the far-field divergence angle was reduced. On this basis, the developed quadruple-active regions semiconductor laser with 800 μm cavity length and 200 μm electrode achieved a peak power output of 177 W at a pulse current intensity of 41.6 A in pulse power test with a pulse width of 100 ns and a repetition rate of 1 kHz; fundamental mode emitting in the vertical direction, the full width at half maximμm far-field divergence angle was 24.3°.