摘要
为了获得高功率、窄线宽和近衍射极限输出的半导体激光器,采用高阶光栅(high order Bragg gratings,HOBGs)和主控振荡功率放大器(Master Oscillator Power-Amplifier,MOPA)结构,成功研制出一种980 nm波段的HOBGs-MOPA半导体激光器。该激光器采用周期为11.37 μm的高阶光栅进行光模式选择,通过锥角为6°的锥形波导将单模激光功率放大,实现了输出功率2.8 W,3 dB光谱线宽31 pm,光束质量因子
半导体激光器具有体积小、质量轻、寿命长等特点,是光纤放大器、全固态激光器的主要泵浦光源,广泛应用于光通信、激光加工、国防安全等领
HOBGs-MOPA半导体激光器的器件结构如下图所示。如

图1 (a)HOBGs-MOPA激光器结构图,(b)外延层结构
Fig. 1 (a)Schematic diagram of HOBGs MOPA semiconductor laser, (b) epitaxial layer structure of HOBGs MOPA semiconductor laser
为了使激光器获得窄线宽发射,本文使用商业软件COMSOL Multiphysics对高阶光栅的参数进行了仿真设计。在式(
, | (1) |
, | (2) |
其中m和mg为整数,λ为增益光谱的中心波长,neff,g和neff,ave分别为沟槽处材料的有效折射率和激光器整体材料的平均折射率。利用Comsol偏微分方程
为了得到高功率激光输出,HOBGs-MOPA中的高阶光栅区长度不能太长,因为高阶光栅区的高阶散射损耗会导致出光功率下降,然而过短的光栅长度会降低光子寿命以及窄线宽输出特性,综合考虑我们将高阶光栅的长度设计为120 μm。HOBGs-MOPA激光器中的光学模式的有效折射率跟光栅的刻蚀深度密切相关:刻蚀越深,光学模式的有效折射率越小,其有效折射率跟波导中的有效折射率差越大,损耗越高,更容易得到窄线宽的器件,但是光栅沟槽的刻蚀深度也不可以太深,刻蚀深度过深会导致高阶散射增强, 使波导内光波的损耗增大,功率的损耗也将增大。计算了不同刻蚀深度下的反射、透射和损耗,如图

图2 不同刻蚀深度下HOBGs-MOPA激光器的反射光谱图
Fig. 2 Reflection spectrum of HOBGs MOPA laser at different etch depths

图3 不同刻蚀深度下HOBGs-MOPA激光器的透射光谱图
Fig. 3 Transmission spectrum of HOBGs MOPA laser at different etch depths

图4 不同刻蚀深度下HOBGs-MOPA激光器的损耗光谱图
Fig. 4 Loss spectrum of HOBGs MOPA laser at different etch depths
由
器件的测试工作均在连续波条件下进行,采用水冷散热方式,设定水冷温度为20℃。 首先,使用综合参数测试仪对器件的功率-电流-电压特性进行表征。在电光特性测试中,输出功率、电流、电压等数值都是直接从综合测试仪中读取出来的。在连续注入电流条件下,使用了横河AQ6370C光谱分析仪、积分球及一根纤芯直径的8 μm、分辨率为0.02 nm的单模光纤测试了HOBGs-MOPA激光器的光谱特性。采用快轴准直镜头、慢轴准直镜头和聚焦镜头准直后测量激光器输出光束的束腰宽度。在连续光输出下,高阶布拉格光栅MOPA器件的功率-电流-电压(P-I-V)特性如下图所示。最大输入电流设置为5 A。
如

图5 HOBGs-MOPA激光器的光功率-电流-电压特性曲线图
Fig. 5 Light power-current-voltage curve graph of HOBGs MOPA laser
如

图6 2.5 A电流条件下HOBGs MOPA激光器的光谱SMSR值
Fig. 6 Optical spectrum of HOBGs MOPA device at 2.5 A
在注入电流为2.5 A时HOBGs-MOPA激光器的远场图如

图7 HOBGs-MOPA的慢轴远场分布图
Fig. 7 The lateral far field patterns of HOBGs-MOPA laser. The far-field spot is shown in the figure.
本文利用高阶光栅和锥形波导的模式选择和光放大特性,研制出了一种高功率、窄线宽HOBGs-MOPA激光器。在注入电流为5 A时,其输出功率可达2.8 W,最大斜率效率大于0.62 W/A,电光转换效率最高可达31%。该器件从阈值到2.5 A保持单纵模输出,最大单纵模输出功率超过1.25 W,高于文献报道的单纵模激光
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