# Phase-Coded and Noise-Based Brillouin Optical Correlation-Domain Analysis
## 4 Conclusions  
B-OCDA was first proposed 20 years ago to provide random-access measurements of strain and temperature with high resolution that is not restricted by the acoustic lifetime [13]. This review addressed two variants of the basic concept, which are meant to increase the flexibility in the synthesis of the correlation function. The first approach is based on the joint coding of the pump and signal waves by high-rate phase sequences [18]. Careful choice of codes can lead to significant reduction of residual off-peak Brillouin interactions [21]. The suppression of correlation side-lobes would be advantageous in slope-assisted implementation of phase-coded B-OCDA, which was recently demonstrated [57]. Phase-coded B-OCDA is successfully combined with pulse modulation of the pump wave, reaching the largest sets of resolution points of all correlation-domain sensors [20,22]. Further extensions aimed to reduce the number of averaged traces [19,25], cover the entire fiber with fewer position scans [23], or eliminate frequency scanning [24]. In a different variant of B-OCDA, modulated laser light is replaced by incoherent ASE noise [31]. That concept was used in the first Brillouin sensing experiment to break the millimeter resolution barrier [33]. Despite these advances, the majority of commercial Brillouin sensor systems continue to employ time-domain protocols. B-OTDA is conceptually simpler than B-OCDA and offers faster acquisition. Modulation of the light source in B-OCDA increases complexity. Furthermore, structures that require high-resolution monitoring are often smaller in size, and might not be compatible with the size and cost of Brillouin analyzers. Other fiber-sensing concepts, such as optical frequency-domain reflectometry (OFDR) [58,59], are making ground faster. OFDR can provide very high spatial resolution and excellent sensitivity [58,59]. The main drawback of OFDR, like many sensors that are based on Rayleigh backscatter, is the indirect assessment of temperature and strain. Measurements of the BFS provide more direct reading of both quantities. Phase coding with standard telecommunication modulators might make B-OCDA more accessible. The technique begins to find foothold in the monitoring of comparatively small structures, where high resolution is significant [26, 27, 28]. More examples are likely to follow. Noise-based B-OCDA may provide resolution that time-domain protocols cannot reach. In a broader context, the study of SBS is undergoing a change in perspective in recent years. Coupling between light and sound waves draws increasing interest as a means for fundamental study of light-matter interactions, as a potential platform for quantum information technology, and as a basis for analog signal processing [53, 54, 56, 55, 60, 61, 62, 63, 64, 65, 66, 67]. SBS over fiber, which involves a single, longitudinal bulk-like acoustic mode and a single spatial optical mode, is regarded as a specific case of a much  
Figure 12: ASE B-OCDA map of Brillouin gain as a function of frequency offset between pump and signal waves and correlation peak position [33]. Measurements were performed over a 20-mm-long chalcogenide glass waveguide with a spatial resolution of 800 \(\upmu\)m [33]. Reproduced with permission from [33], American Institute of Physics, 2018.  
broader portfolio of possible configurations of light and sound interactions [62]. Research on SBS in micro-wire and integrated-photonic platforms is flourishing, with exciting results in silica [30; 48; 49; 63; 64], chalcogenide glasses [29; 33; 53; 54; 55; 56], silicon-nitride [65], and even silicon [66; 67]. In parallel with these advances, analysis techniques that were developed for the purpose of sensing over fibers are being adapted by the broader opto-mechanics community. The mapping of opto-mechanical interactions over integrated devices naturally calls for higher spatial resolution.