Based on Rayleigh, Brillouin, and Raman scattering, and weak reflection arrays in optical fiber, distributed optical fiber sensors (DOFSs) can achieve real-time monitoring with long range and high spatial resolution for multiple parameters such as optical fiber loss, temperature, strain, vibration, and sound. As a result, DOFSs catch more and more attention. The received signals from the sensing fiber of a DOFS are normally very weak and thus the received signal-to-noise ratio (SNR) is quite small. Although the SNR can be enhanced by increasing the optical pulse power launched into the sensing fiber, it is generally limited by fiber nonlinearity and then is upperbounded. A pulse with long duration can also be employed to improve the SNR but the spatial resolution is sacrificed. A better alternative to enhance the SNR without spatial resolution loss is to adopt an optical pulse sequence with some coding at a fairly low power to avoid fiber nonlinearity. Therefore, it has become an essential technique to enhance the performance of a DOFS by a long coded pulse train in DOFS. As technical characteristics of various DOFSs are different, applicable coding scheme has to be carefully designed for a particular DOFS. Design considerations may include several aspects such as code sequence, modulation format, detection scheme, and decoding methods. Hence, it is important and necessary to summarize the existing research on DOFS coding techniques for performance enhancement to guide the future development of this field.

In principle, the response of a DOFS with coding can be considered as the convolution of the coding sequence with the impulse response of the sensing fiber. The aperiodic autocorrelation of the coding sequence is utilized to construct an impulse function, and the impulse response of the sensing fiber can be recovered by correlating the DOFS response with the coding sequence itself at the receiver site. Therefore, the aperiodic autocorrelation characteristics of the coding sequence are critical, which can be evaluated from three aspects of coding gain, spatial resolution, and crosstalk suppression. A sequence with better aperiodic autocorrelation performance is always pursued. The sensing principle of a DOFS also exerts some effects on coding sequence selection. Unipolar sequences are frequently employed in DOFSs with intensity detection. A typical unipolar sequence is Simplex sequence. Bipolar sequences such as Golay complementary sequences can also be converted to unipolar sequences, and they are popular in DOFSs with phase detection and can be implemented by binary phase shift keying modulation via Mach-Zehnder modulator (MZM) (Fig. 3). A widely employed bipolar sequence is Golay complementary sequence. Polyphase unimodular sequences have also been proposed recently in DOFSs with phase detection for much better crosstalk suppression capability (Figs. 5-8). Such sequences have been realized via modulation by an acoustic-optical modulator (AOM) (Fig. 4).

Various DOFSs with coding techniques have been proposed. For Rayleigh scattering sensors with incoherent optical sources, Simplex sequence, Golay complementary sequences, CCPONS, and other unipolar sequences have been put forward to improve performance such as dynamic range, spatial resolution, and measurement speed. For Rayleigh scattering sensors with coherent optical sources, both unipolar and bipolar sequences have been proposed. Multi-input-multi-output (MIMO) technique has also been demonstrated in a Rayleigh scattering DOFS with polarization multiplexing and coding to increase measurement bandwidth. For Raman scattering sensors, Simplex sequences and other unipolar sequences are popular. The coding sequence performance is frequently degraded by the transient effect of erbium-doped fiber amplifier (EDFA). Many schemes have been proposed to demonstrate their anti-degradation capability. Coding techniques have also long been explored in Brillouin scattering sensors to improve the performance in measurement accuracy, spatial resolution, and measurement speed. In addition to conventional correlation based decoding schemes, deconvolution-based decoding techniques have also been presented. Weak fiber Bragg grating array (WFBGA) is an emerging DOFS, with coding techniques explored in such a DOFS. For WFBGA with interrogation based on intensity, Golay complementary sequences with return zero (RZ) code format have been discussed. For WFBGA with interrogation based on phase, MIMO techniques with Golay complementary sequence and polyphase unimodular sequence using polarization multiplexing have been demonstrated, with much better crosstalk suppression performance (Figs. 9-10).

After decades of development, DOFSs have been widely employed in various areas and a lot of applications have been developed based on DOFSs. Those applications have raised increasingly higher requirements for DOFS performance. Coding technique is an important technical method to enhance the performance. We analyze the underlying principle of coding technique and manifest the connection between sensing performance and characteristics of coding sequences. Meanwhile, features, performance, and implementation of some widely used sequences in DOFSs are summarized. We analyze the technical characteristics and applicable coding schemes of DOFSs based on Rayleigh scattering, Raman scattering, Brillouin scattering, and WFBGA, and summarize the improvement of SNR, spatial resolution, sensing bandwidth, and sensing range by employing the coding techniques. As the DOFS technology is advancing, coding techniques will be further developed, which calls for the research on higher-performance coding sequences and their implementation schemes. Additionally, coding techniques will also integrate other techniques such as frequency division multiplexing, wavelength division multiplexing, spatial division multiplexing, and channel equalization to explore the fiber characteristics of low loss and high bandwidth. Finally, the space for DOFS performance improvement will be expanded.