Jamal Muhammad T, Jakobsen Michael L, Hanson Steen G, Hansen Anders K, Jensen Ole B
Appl Opt. 2022 Feb 1;61(4):989-994. doi: 10.1364/AO.445383.
A speckle-displacement-based wavemeter is combined with a spatial-fundamental-mode-pass filter to eliminate the influence of multimode operation on the directionality of the resulting output from a distributed Bragg reflector (DBR) tapered laser. The proposed setup is characterized theoretically and experimentally, and detections of mode hops and side-mode suppression ratios (SMSRs) in the optical output are demonstrated. The laser illuminates a rough surface at an oblique angle, and a camera observes the corresponding speckle pattern from an almost identical back-scattering direction. As the wavelength of the laser shifts, the speckle pattern responds with a corresponding displacement, which is approximately linear with respect to the shift within the detection area. The wavemeter tracks continuously the shifts of the speckles pattern by tracking the peak of the covariance function of sequentially acquired images. In this way, the speckle-displacement-based wavemeter achieves a spectral resolution of 10.4 MHz. Mode hops in the laser do not cause any impeding decorrelation of the speckle patterns. Interestingly, the actual SMSR is related to the peak height and width of the absolute covariance function. A wavemeter, which is capable of measuring wavelengths, mode hops, and SMSRs, is highly useful for spectroscopy, quantum optics, nonlinear frequency conversion, and other applications requiring stable single-frequency laser light, especially when using diode lasers.
一种基于散斑位移的波长计与空间基模通过滤波器相结合，以消除多模操作对分布式布拉格反射器（DBR）锥形激光器输出方向性的影响。对所提出的装置进行了理论和实验表征，并展示了对光输出中的模式跳变和边模抑制比（SMSR）的检测。激光器以倾斜角度照射粗糙表面，相机从几乎相同的后向散射方向观察相应的散斑图案。随着激光波长的移动，散斑图案会以相应的位移做出响应，该位移在检测区域内相对于波长移动近似呈线性关系。波长计通过跟踪顺序采集图像的协方差函数的峰值来连续跟踪散斑图案的移动。通过这种方式，基于散斑位移的波长计实现了10.4 MHz的光谱分辨率。激光器中的模式跳变不会导致散斑图案出现任何妨碍性的去相关。有趣地是，实际的SMSR与绝对协方差函数的峰值高度和宽度有关。一种能够测量波长、模式跳变和SMSR的波长计对于光谱学、量子光学、非线性频率转换以及其他需要稳定单频激光的应用非常有用，特别是在使用二极管激光器时。
