DISTRIBUTED DYNAMIC STRAIN SENSING VIA PERFECT PERIODIC COHERENT CODES AND A POLARIZATION DIVERSITY RECEIVER
Rayleigh scattering-based dynamic strain sensing with high spatial resolution, fast update rate and high sensitivity is highly desired for applications such as structural health monitoring and shape sensing. A key issue in dynamic strain sensing is the trade-off between spatial resolution and the Signal to Noise Ratio (SNR). This trade-off can be greatly relaxed with the use of coding. A sequence of optical pulses is injected into the fiber and the detected backscattered signal is cross-correlated with the original signal. With the use of coding, SNR is indeed improved, but if the sequence is not well chosen, the resulting peak to sidelobe ratio (PSR) can be rather low. An excellent choice of codes are biphase Legendre sequences which offer near perfect periodic autocorrelation (PPA). Other common issues in Rayleigh scattering-based sensing techniques are signal fading and dynamic range. The former issue can occur due to destructive interference between lightwaves that are scattered from the same spatial resolution cell and, in coherent detection schemes, when the polarization states of the backscattered light and the reference light are mismatched. The latter issue is a concern in phase sensitive schemes which require signal jumps not to exceed 2 pi. In this paper, a biphase Legendre sequence with 6211 pulses is used in conjunction with polarization diversity scheme and a PM fiber. The setup provides two independent measurements of the sensing fiber complex profile and achieves highly sensitive, distributed dynamic strain sensing with very low probability of fading. In addition, the system can handle both very large perturbation signals and very small perturbation signals. The system operated at a scan rate of ~ 107kHz and achieved spatial resolution of ~10cm and sensitivity of ~1.1 mrad= p Hz. The ratio between the powers of the maximum and minimum excitations that can be measured by the system is 136 dB.