The distributed Brillouin sensing technique has been developed rapidly since its first demonstration three decades ago. Numerous investigations on the performance enhancement of Brillouin sensors in respect to spatial resolution, sensing range, and measurement time have paved the way to its industrial and commercial applications. This chapter provides an overview of different Brillouin sensing techniques and mainly focuses on the most widely used one, the Brillouin optical time domain analysis (BOTDA). The history and the development of Brillouin sensing regarding the performance enhancement in various methods and their records will be reviewed, commented, and compared with each other. As well, related sensing errors and limitations will be discussed, together with the corresponding strategies to avoid them.

Real-time spectrum sensing is essential to enable dynamic and rapid spectrum sharing of unused frequencies to cater the substantial demands of new wireless services deploying the existing RF bands. In this paper, we present a novel, real-time spectrum sensing approach for widely used RF signals based on Brillouin-scattering-induced transparency (BIT). A temporal discrimination of multi-channel input frequencies is achieved through the group delay tuning by BIT. By tuning the pump power and frequency, the proposed technique is fully reconfigurable and viable for a broad range of spectrum sensing. Several experimental illustrations of the time domain sensing are presented for two-tone channels with 0.9, 1.8, and 5 GHz frequencies to detect the unused spectrum within 3G, 4G, and 5G signals.

In this article, we demonstrate the noise reduction and signal to noise ratio (SNR) enhancement in Brillouin optical time-domain analyzers (BOTDA). The results show that, although the main noise contribution comes from the Brillouin interaction itself, a simple low pass filtering on the detected radio frequency (RF) signal reduces remarkably the noise level of the BOTDA traces. The corresponding SNR enhancement depends on the employed cut-off frequency of the low pass filter. Due to the enhancement of the SNR, a mitigation of the standard deviation error of the Brillouin frequency shift (BFS) has been demonstrated. However, RF filters with low cut-off frequency could lead to distortions on the trace signals and therefore detection errors on a non-uniform BFS. The trade-off between the noise reduction and the signal distortion as well as an optimal cut-off frequency are discussed in detail.

The excess noise due to stimulated Brillouin scattering (SBS) in gain and loss-based Brillouin optical time-domain analyzers (BOTDA) has been investigated theoretically and experimentally for the first time to the best of our knowledge. This investigation provides a full insight to the SBS-induced noise distribution, which mainly comes from phase-to-intensity conversion noise and the beating noise between the probe wave and spontaneous Brillouin scattering. A complete theoretical model, which is in good agreement with the experimental results, is presented to describe the noise. The results show that a loss-based BOTDA setup gives a better noise performance than a gain-based one in both time and frequency domain. The SBS-induced noise has been characterized in dependence on the pump and probe power and the spatial resolution.

Exploiting the outstanding performance of optical atomic clocks for improved timekeeping, relativistic geodesy, and fundamental physics beyond the standard model demands comparing distant state-of-the-art optical clocks. Interferometric optical fiber links have been demonstrated as an eminent method for such frequency comparisons over distances up to thousands of kilometers. However, for such distances, the optical fiber attenuation mandates signal amplification. Fiber Brillouin amplification (FBA) has been proven as an efficient amplification technique for coherent frequency transfer. Demonstrated FBA schemes have been designed based on costly narrow-linewidth pump lasers and analog pump-to-signal phase locking schemes. Furthermore, the high pump power requirement of these FBAs hinders the integration of FBA-based frequency dissemination on fiber connections for shared telecommunication signals in the C-band. In this paper, we propose and experimentally demonstrate a novel FBA module (FBAM) employing cost-effective distributed feedback (DFB) pump lasers assisted by a digital phase locking scheme based on a field programmable gated array. The new FBAM is compact, cost-effective, and directly applicable to different bands, which opens up new opportunities to establish a frequency metrology infrastructure within existing telecommunication fiber networks. Additionally, the small-footprint of the DFB-FBAM allows for frequent amplification stages with lower pump power to reach continental scale optical metrology links with an optimized signal-to-noise ratio. We characterized the DFB-FBAM’s frequency transfer uncertainty using a two-way layout over an in-lab 100 km long optical fiber link and reach a fractional frequency instability of 9.3 × 10⁻²² at a 10 ks integration time. The DFB-FBAM characterizations show uncertainty contributions of (−2.1 ± 3.3) × 10⁻²² and below for averaging times >100 ks. For the first time, we assess the temporal Brillouin frequency shift variations in an underground fiber link and implement a scheme to track these changes in a remote FBAM.

High-speed communication systems mandate instantaneous microwave frequency measurement for an efficient utilization of the available spectrum. We propose and experimentally demonstrate a novel, real-time microwave frequency measurement approach based on the time-frequency coherence representation of each signal. The temporal discrimination between the spectral components of the incoming microwave signal is realized through the harnessing of stimulated Brillouin scattering-based quasi light storage (QLS). The QLS is achieved by equidistant sampling of a signal spectrum. Consequently, several copies of that signal will be produced in the time domain and can be discriminated from other spectral components. By tuning the central frequency of the comb, the proposed method is fully reconfigurable and feasible for centimeter, millimeter and even THz signals. Here, we present simultaneous discrimination of single and two frequency components of multi-frequency microwave signals. Since the method can be integrated and supports real-time processing, it might become an integral part for efficient bandwidth usage in future wireless communication systems.