Junaid Ul Haq Mohmad, Rosenberger A T
Department of Physics, Oklahoma State University, Stillwater, OK 74078-3072, USA.
Sensors (Basel). 2025 Sep 3;25(17):5458. doi: 10.3390/s25175458.
Dissipative sensing in a whispering-gallery-mode (WGM) microresonator entails monitoring changes in WGM throughput dip depth or linewidth due to analyte absorption. In our earlier work, we showed that dip depth sensitivity can be two orders of magnitude greater than linewidth sensitivity for sensing the broadband absorption of a dye in methanol. Here we experimentally demonstrate enhancement of absorption sensing of methane. Its narrowband absorption lines (a few GHz linewidth) necessitate strain tuning of the WGM of our hollow bottle resonator (HBR) to bring the WGM into resonance with the absorption line. Three asymmetric tapered fibers with different nonadiabaticities were designed to excite multiple fiber modes that couple into the WGM to interact with methane inside the HBR via the internal evanescent field. Measurements were carried out for both pure and trace (in 1 atm of air) methane at 1654 and 1651 nm. Enhancement factors as large as 141 were found; the experimental results agree with theoretical calculations and with the predictions of a limiting-case model. Effective absorption path lengths as large as 273 cm, more than ten thousand times the HBR diameter, were achieved for trace methane sensing, with detection limits estimated to be in the hundreds of ppm.
回音壁模式（WGM）微谐振器中的耗散传感需要监测由于分析物吸收导致的WGM吞吐量凹陷深度或线宽的变化。在我们早期的工作中，我们表明，对于检测甲醇中染料的宽带吸收，凹陷深度灵敏度可比线宽灵敏度高两个数量级。在此，我们通过实验证明了甲烷吸收传感的增强。甲烷的窄带吸收线（线宽为几吉赫兹）需要对我们的空心瓶谐振器（HBR）的WGM进行应变调谐，以使WGM与吸收线共振。设计了三根具有不同非绝热性的非对称锥形光纤，以激发多种光纤模式，这些模式耦合到WGM中，通过内部倏逝场与HBR内的甲烷相互作用。在1654和1651nm处对纯甲烷和痕量甲烷（在1个大气压的空气中）进行了测量。发现增强因子高达141；实验结果与理论计算以及极限情况模型的预测一致。对于痕量甲烷传感，有效吸收路径长度高达273cm，比HBR直径大一万多倍，检测限估计在数百ppm。
