Distributed Dynamic Strain Sensing via Perfect Periodic Coherent Codes and a Polarization Diversity Receiver

Abstract

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 2π. 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/ √ Hz. The ratio between the powers of the maximum and minimum excitations that can be measured by the system is 136 dB.

Polarization fading mitigation in distributed acoustic sensors based on a high-speed polarization rotator

“…For dual-pulse DAS systems, the phase fading has been eliminated by shifting the phase difference between subsequent pulse-pairs by 2π/3, but at the cost of reduced measurement bandwidth [19]. For coherent DAS systems, a commonly used technique is based on a 90°hybrid [20][21][22].…”

“…However, for applications that are based on wound fibers such as distributed optical fiber hydrophones [26] or helically-wound fiber optic cables used in vertical seismic profiling [27], polarization fading might result in a substantial signal fading. So far, only coherent DAS systems have addressed polarization fading through polarization diversity detection [21,28]. In this technique, a polarization beam splitter is used to split the backscattered light into two orthogonal polarizations and the phase information at each State of Polarization (SoP) is obtained through coherent detection.…”

Review of a Specialty Fiber for Distributed Acoustic Sensing Technology

“…In addition, since a laser pulse with a narrow linewidth and high coherence is injected into the fiber, the interference between the Rayleigh scattering points in one pulse will produce interference cancellation, resulting in coherent fading of the backscattered light, forming a "dead zone" on the sensing fiber [11]. In recent years, in order to solve the defects of the poor SNR values and the sensitivity of phase demodulation in SMF-DAS systems, many suppression technologies for noise have been studied, such as coherent fading, while at the same time the complexity and cost of the systems has increased, leading to sacrifices in terms of the response frequency band and sensing distance [12][13][14][15][16][17][18]. Furthermore, it is also important to enhance the performance of the sensing fiber itself.…”

Adaptable Pulse Compression in ϕ-OTDR With Direct Digital Synthesis of Probe Waveforms and Rigorously Defined Nonlinear Chirping

“…Another approach used for the suppression of sidelobes is the use of perfect periodic binary phase codes [22]. More specifically, the use of a specific family of perfect periodic codes, called bi-phase Legendre sequence, of 6211 pulses coupled with a polarization diversity scheme has been shown to provide low probability of fading with high scan rate, enhanced dynamic range and sub-meter resolution, with reported sensing distance of 144 m [23]. The choice of such bi-phase sequences is driven to a large part by the simplicity of their generation and subsequent piggybacking on an optical carrier and, in general, longer codes are needed to achieve higher energy efficiency and compression ratio [24].…”