# The Influence of Laser Linewidth on the Brillouin Shift Frequency Accuracy of BOTDR  
[PERSON]  
1 Key Laboratory of Advanced Transducers and Intelligent Control Systems (Ministry of Education and Shanxi Province), Taiyuan University of Technology, Taiyuan 030024, China; [EMAIL_ADDRESS] (Q.B. ); [EMAIL_ADDRESS] (M.Y. ); [EMAIL_ADDRESS] (B.X. ); [EMAIL_ADDRESS] (Y.G. ); [EMAIL_ADDRESS] (D.W.); [EMAIL_ADDRESS] (Y.W. ); [EMAIL_ADDRESS] (M.Z. ); [EMAIL_ADDRESS] (H.Z.) [PERSON]  
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1 Key Laboratory of Advanced Transducers and Intelligent Control Systems (Ministry of Education and Shanxi Province), Taiyuan University of Technology, Taiyuan 030024, China; [EMAIL_ADDRESS] (Q.B. ); [EMAIL_ADDRESS] (M.Y. ); [EMAIL_ADDRESS] (B.X. ); [EMAIL_ADDRESS] (Y.G. ); [EMAIL_ADDRESS] (D.W.); [EMAIL_ADDRESS] (Y.W. ); zhangmingjiang@tyut.edu.cnand underground mines [7; 8; 9; 10]. Consequently, the measurement accuracy of Brillouin frequency shift (BFS), which determines the temperature and strain accuracy, has inevitably been a focused issue for performance enhancement of BOTDR. The measurement accuracy of BFS can be affected by numerous factors, such as the fitting algorithm for Brillouin gain spectrum (BGS), the features of probe pulses, and even the characteristics of the laser source. Hence, a large number of papers have focused on above factors to discuss and improve the accuracy of BOTDR. Originally, a simple Levenberg-Marquart algorithm was used for BGS fitting [11]. Then, a series of time-frequency analysis methods called Cohen's class were proposed for the signal processing of BOTDR, reducing the BFS fluctuation by three times [12]. For improving both the measurement accuracy and data processing speed, a similarity matching method was proposed, making the standard derivation of BFS results three times better [13]. An iterative quadratic fitting method can also be utilized to extract BFS from noisy signals for improvement of the BFS accuracy [14]. Besides, the features of probe light pulses also have a significant influence on the accuracy of BFS, including the pulse shape and pulse extinction ratio. [PERSON] et al. analyzed the effects of different modulated pulse on the backscattered Brillouin power spectra of BOTDR, including the pulse shapes of Lorentzian, Gaussian, hyperbolic-secant, super Gaussian, triangular, and rectangular pulses [15; 16]. The pulse sequences were complementarily coded in a BOTDR scheme to achieve a high measurement accuracy and fast measurement speed [17]. The simplex pulse codes were also testified to be beneficial for improving the signal-to-noise ratio ratio (SNR) of the BOTDR signal and the measurement accuracy of BFS [18]. The higher extinction ratio of the probe pulses was capable of enhancing the SNR as well as reducing the measurement uncertainty of BFS [19; 20]. Moreover, it is noted that the wavelength and linewidth of the laser source likewise impact BFS accuracy. [PERSON] et al. used a wavelength diversity technique with a Brillouin ring laser in a conventional BOTDR system to improve SNR [21]. A multi-wavelength heterodyne-detection technique was utilized to provide 4.2 dB SNR enhancement, enabling the measurement accuracy to be increased by two times [22]. As an important parameter of the laser, the wide linewidth imposes a broadening effect on BGS, which affects the BFS accuracy partly, but the broadening effect is indistinctive when the laser linewidth is less than 1 MHz [23]. Meanwhile, it has been proven that the BFS accuracy is determined not only by the width of BGS, but also by the SNR of the BOTDR system [24]. The system SNR of BOTDR is inherently related with the laser linewidth closely, because the coherent heterodyne detection, which is sensitive to the laser linewidth, is commonly used in BOTDR [25]. Hence, the influence of the laser linewidth on the BFS accuracy of BOTDR needs to be further discussed in detail, considering both the BGS width and the system SNR.