Theoretical Investigation of the Black-body Zeeman Shift for Microwave Atomic Clocks

With the development of microwave atomic clocks, the Zeeman shifts for the spectral lines of black-body radiation need to be investigated carefully. In this Letter, the frequency shifts of hyperfine splittings of atomic ground states due to the magnetic field of black-body radiation are reported. The relative frequency shifts of different alkali atoms and alkali-like ions, which could be candidates of microwave atomic clocks, were calculated. The results vary from $-0.977\times10^{-17}[T(K)/300]^{2}$ to $-1.947\times10^{-17}[T(K)/300]^{2}$ for different atoms considered. These results are consistent with previous work but with greater precision, detailed derivations, and a clear physical picture

Ultimate spatial resolution realisation in optical frequency domain reflectometry with equal frequency resampling

A method based on equal frequency resampling is proposed to suppress laser nonlinear frequency sweeping for the ultimate spatial resolution in optical frequency domain reflectometry. Estimation inaccuracy of the sweeping frequency distribution caused by the finite sampling rate in the auxiliary interferometer can be efficiently compensated by the equal frequency resampling method. With the sweeping range of 130 nm, a 12.1 µm spatial resolution is experimentally obtained. In addition, the sampling limitation of the auxiliary interferometer-based correction is discussed. With a 200 m optical path delay in the auxiliary interferometer, a 21.3 µm spatial resolution is realised at the 191 m fibre end. By employing the proposed resampling and a drawing tower FBG array to enhance the Rayleigh backscattering, a distributed temperature sensing over a 105 m fibre with a sensing resolution of 1 cm is achieved. The measured temperature uncertainty is limited to ±0.15 °C

A review on various optical fibre sensing methods for batteries

Batteries have rapidly evolved and are widely applied in both stationary and transport applications. The safe and reliable operation is of vital importance to all types of batteries, herein an effective battery sensing system with high performance and easy implementation is critically needed. This also requires the sensing system to monitor the states of batteries in real time. Among the available methods, optical fibre sensors have shown a significant advantage due to their advanced capabilities of which include the fast measurement of multiple parameters with high sensitivity, working without interfering the battery performance, being able to be composited in multiplexed configurations and being robust to various harsh environment conditions. This paper mainly discusses the current optical fibre sensing methods for batteries in terms of the working principles and critical reviews the sensing performance corresponding to different sensing parameters. Moreover, the challenges and outlooks for future research on battery sensing are derived

High sensing accuracy realisation with millimetre/sub-millimetre resolution in optical frequency domain reflectometer

By effectively suppressing the nonlinear sweep noise and random range of wavelength sweep in the optical frequency domain reflectometer, the theoretical spatial resolution and uniform sweep distribution are delivered for high sensing accuracy. A strain accuracy of 0.51 is realised with a 5 mm sensing resolution, while the accuracy is 5.89 with a 1 mm sensing resolution. Theoretical limitation between the strain accuracy and sensing resolution is further studied for the sub-millimetre resolution sensing. It is found that signal to noise ratio and frequency bandwidth of the calculated cross-correlation are critical factors in measuring accuracy. Increasing the sweep range can provide a better spatial frequency step for a high signal to noise ratio in the cross-correlation. With a 130 nm sweep range, the measurement accuracy is limited to 19.31 with a 0.5 mm sensing resolution. Besides, for the long-distance sensing of 104m, the measurement accuracy is 8.72 with a 1 mm sensing resolution

High-resolution ϕ-OFDR using phase unwrap and nonlinearity suppression

Phase-sensitive optical frequency domain reflectometer (Φ-OFDR) is investigated to deliver an accurate distributed measurement with high spatial resolution. It is found that random phase noise and quadrant discrimination during phase calculation are the main reasons for the random hopping in Φ-OFDR. By efficiently eliminating random hopping in the phase unwrap and suppressing the laser-induced nonlinear sweep for the theoretical spatial resolution, the proposed Φ-OFDR is proved to be able to decouple the limitation between resolution and accuracy in coherent OFDR (C-OFDR). Distributed strain measurement with 20 mm spatial resolution in Φ-OFDR is obtained and analysed. Measurement with little deviation and uniform sensitivity between applied strain and phase change both validate the efficient noise suppression for extreme resolution measurement. Then the influence of the initial sweep frequency between two times measurements is studied. With a further reduced 800 µm spatial resolution, the proposed Φ-OFDR is able to retain accurate distributed measurement compared to conventional C-OFDR methods. Besides, the computation time of the Φ-OFDR is only 3.2% of the C-OFDR