1.Key Laboratory of Quantum Optics and Center of Cold Atom Physics, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China 2.University of Chinese Academy of Sciences, Beijing 100049, China
Fund Project:Project supported by the National Natural Science Foundation of China (Grant Nos. 11604353, 11704391) and the Key Research Program of the Chinese Academy of Sciences (Grant No. KJZD-EW-W02).
Received Date:11 October 2018
Accepted Date:11 November 2018
Available Online:01 March 2019
Published Online:05 March 2019
Abstract:Intrinsic thermal noise in optical fibers is an ultimate factor limiting the performances of fiber-based sensors and measurement systems. Therefore, it is important to have a thorough understanding of this kind of noise. However, the mechanism of the intrinsic thermal noise which has a 1/f spectral density remains unclear so far. There are two theoretical models to explain the mechanism of this kind of noise: thermoconductive noise model and thermomechanical noise model. The thermoconductive noise model states that the noise is caused by diffusion of local entropy fluctuations associated with random spontaneous emission events, while the thermomechanical noise model says that the noise is caused by spontaneous fluctuations of fiber length induced by mechanical dissipation. Which theoretical model is correct is still an open question. In this paper, we experimentally investigate the intrinsic thermal noise in optical fibers by using a balanced fiber Michelson interferometer with heterodyne detection technique. When a fiber-stabilized laser with ultralow frequency noise is used as a laser source and other noise sources are carefully controlled, the 1/f spectral intrinsic thermal noise can be observed down to infrasonic frequency. According to these measurements, in order to verify which theoretical model is the mechanism of generating the intrinsic thermal noise with 1/f spectral density, we study the relationship between the level of the intrinsic thermal noise and the concentration of the dopant in fibers and the applied tension of fibers. We observe that the level of the 1/f spectral intrinsic thermal noise is independent of the concentration of the dopant in fibers. This means that the thermoconductive noise model is not suitable to this case. We also observe that the level of the 1/f spectral intrinsic thermal noise can be reduced by increasing the tension exerted on the optical fibers. Because the mechanical loss of a fiber can be lower than the loss of the material which the fiber is made of when the fiber is subjected to a certain tension, this observation proves the fact that the 1/f spectral intrinsic thermal noise in optical fibers originates from the mechanical dissipation process inside optical fibers. This is consistent with the predictions of the thermomechanical noise model. Finally, the inconsistency between the experimental data and the theoretical results for thermomechanical noise is discussed. Keywords:optical fibers/ thermal noise/ interferometers
图 1 测量光纤热噪声的实验装置(OFI, 光纤隔离器; OFC, 光纤耦合器; FM, 法拉第反射镜; PD, 光电管; FFT, 快速傅里叶变换分析仪; AOM, 声光调制器)
图 2 测量系统的噪声来源
图 3 SMF-28光纤与纯硅芯光纤热噪声测量结果
图 4 不同反应时间下的光纤本征热噪声测量结果
图 5 无张力与受张力0.1 N状态下的光纤本征热噪声测量结果
图 6 不同张力情况下SMF-28光纤本征热噪声测量结果
