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在基于光学干涉测量的应用中，激光的频率噪声限制了测量灵敏度，往往需要对频率进行预稳。通常将激光频率锁定到超稳定的光学参考腔（超稳腔）上实现频率预稳。超稳腔的稳定性受到腔体、腔镜基底以及腔镜镀膜热噪声的限制。在室温下，可以通过选择合适的材料来优化腔体和腔镜基底的稳定性和热噪声。腔镜镀膜热噪声与高反射镜镀膜层的机械损耗以及厚度有关，可以采用多层多材料镀膜来抑制。本文综合温度稳定性和机械损耗，优化了室温下的超稳腔腔体和腔镜基底材料，构建了四种腔镜镀膜模型对超稳腔的热噪声进行对比和优化，与传统的电介质多层镀膜相比，超稳腔总体热噪声最大降低了26%。
Abstract：Objective Ultra-stable cavities are used to achieve highly stable and low-noise laser output, playing a crucial role in fields such as space-borne gravitational wave detection and optical frequency standards. However, thermal noise caused by molecular Brownian motion in ultra-stable cavities introduces random fluctuations in cavity length, limiting the precision of frequency stabilization. To achieve lasers with ultra-high stability and low noise, mitigating thermal noise in ultra-stable cavities is critical. A frequency-stabilized laser source is a fundamental requirement. Typically, this is achieved by locking the laser frequency to an ultra-stable frequency reference system, such as an optical cavity or an atomic or molecular transition. In the case of optical cavity reference systems, thermal noise arising from material Brownian motion is one of the dominant noise sources that restrict the accuracy of relative measurements. Several methods have been developed to mitigate thermal noise, including the use of cryogenic cooling, larger beam profiles, and low mechanical-loss materials. Recently, multi-material coating technology has been proposed, effectively reducing coating thermal noise in mirrors used in ground-based gravitational wave detection.Methods In this paper, we apply the multi-material coating technique to the ultra-stable cavity for frequency stabilization at room temperature. Compromising the temperature stability and mechanical loss, we select the ULE spacer and fused silica mirror substrate. For the mirror coatings, we design four models to compare their optical properties such as reflection spectra and mechanical properties including Young's modulus, Poisson ratio and mechanical loss. The four coating models are different in materials of double-layer and coating substrate, types of materials and number of layers, but with almost the same optical losses(～5×10-6).Results and Discussions Using a laboratory-based 10-cm-long plano-concave ultra-stable cavity, we performed simulations of thermal noise across various cavity parameters. The cavity spacer and mirror substrates were made of ULE glass and fused silica, respectively. Four different coating designs were applied to the ultra-stable cavity to investigate the influence of coating techniques on the overall thermal noise. The traditional coating method exhibited higher coating thermal noise compared to the multimaterial coating approach, and the extent of thermal noise reduction varied depending on the choice of coating substrate.Conclusions The results show that, by selecting low mechanical loss materials, the thermal noise of the spacer and mirror substrate are suppressed, while the coating noise becomes dominant. Two factors, the crystalline silicon(cSi) coating substrate instead of SiO2 and the multi-material adding amorphous silicon(aSi) instead of Ta_2O5 solely, suppress the coating thermal noise. The cSi's larger Young's modulus compared to SiO2 plays a key role in suppressing coating thermal noise. The aSi has a higher refractive index than Ta_2O5, making a thinner total coating layer and thus lower thermal noise. The combination of these two factors achieves a 26% reduction of the coating thermal noise compared to the original model with none of them.
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基本信息:
中图分类号:TN24
引用信息:
[1]刘静芳,刘佳敏,杜联联,等.基于室温的超稳腔热噪声优化[J].量子光学学报,2025,31(03):29-37.
基金信息:
国家重点研发计划（2021YFC2201802）; 山西省回国留学人员科研资助项目（2021-005）