# Monitoring Vehicles on Highway by Dual-Channel \(\varphi\)-OTDR  
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1 Guangxi Key Laboratory of Optoelectronic Information Processing, Guilin University of Electronic Technology, Guilin 541004, China; [EMAIL_ADDRESS] (S.X. ); [EMAIL_ADDRESS] (W.Z.) Guangxi Engineering technology research center for road intelligent transportation system, Guangxi Transportation Research & Consulting CO., LTD, Nanning 530007, China  
Guangxi Key Laboratory of Wireless Wideband Communication and Signal Processing, Guilin University of Electronic Technology, Guilin 541004, China  
Guangxi Key Laboratory of Automatic Detecting Technology and Instruments, Guilin University of Electronic Technology, Guilin 541004, China; [EMAIL_ADDRESS]  
Correspondence: [EMAIL_ADDRESS]; Tel. : +86-139-7738-1793  
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magnetic sensor, inductive loops) and the latter being installed at different places on the highways (e.g., radar sensor, cameras) [1; 2; 3; 4]. These sensor-based technologies of traffic management effectively improve road safety and reduce highway congestion. It should be noted, however, that these sensors can only sense in a discrete manner and should not be deployed in full scale. Luckily, significant progress has been made in developing distributed optical fiber sensing (DOFS) systems, making them a promising approach for the vibration-sensitive measurement of transportation, due to their fully distributed nature [5; 6; 7]. The DOFS can locate the external dynamic disturbances at any position of optical fiber with meter-scale spatial resolution over tens of kilometers [8]. Importantly, long distance coverage can be economically achieved with optical fiber as the equivalent sensing arrays. Generally, the DOFS can be divided into Brillouin-based DOFS and Rayleigh-based DOFS in terms of vibration detection. Brillouin-based DOFS is limited to the applications of low frequency perturbations over a relatively short sensing length due to its large amounts of time consumed in data acquiring and frequency sweeping [9]. In contrast, Rayleigh-based DOFS shows the capability of measuring a higher frequency vibration over a much longer range, and thus has becomes a powerful tool in many important fields, including railway monitoring [5], pipeline safety [10], and intrusion detection [11]. Nowadays, phase-sensitive optical time domain reflectometry (\(\varphi\)-OTDR) is the most commonly utilized Rayleigh-based DOFS owning to \(\varphi\)-OTDR offers a much higher sensitivity to external vibrations. In this technique, an ultra-narrow linewidth laser pulse is launched into the sensing fiber, any mechanical vibrations would change the coherent interaction over a great number of Rayleigh scattering units within a pulse duration. Therefore, the phase and intensity of the backscattered Rayleigh light are modulated by the vibrations. After digital signal processing (DSP), the vibrations can be determined by analyzing the changes of phase or intensity of the Rayleigh scattering traces. In \(\varphi\)-OTDR systems, phase demodulation has widely been used based on the linear relationship between the phase and vibration [12; 13; 14; 15; 16; 17]. However, phase demodulation is at the expense of system complexity and cost. Additionally, the traditional \(\varphi\)-OTDR operates in single channel (SC-\(\varphi\)-OTDR), for which there are two important factors influencing the further improvement of its performance. The first is the signal fading that may happen at one or more points over the sensing link due to destructive interference of the backward-scattered Rayleigh lights or polarization mismatching between the Rayleigh signal and reference light in the coherent detection [18]. The second is the noises from temporal intensity fluctuation of laser pulses, laser phase noise and photodetector thermal and shot noise. Both reduce the signal-to-noise (SNR) of the time-domain traces in SC-\(\varphi\)-OTDR, leading to a decreased sensitivity to vibration events and an increased probability of unreliable vibration measurements.