Stimulated Brillouin Scattering In Chalcogenide Microfiber Sensors and Random Fiber Lasers

Abstract

Stimulated Brillouin scattering (SBS), the interaction between two optical waves and an acoustic wave, has been extensively studied in optical fiber sensors and random fiber lasers over the previous three decades. Brillouin fiber sensors are essential for structural health monitoring. Brillouin fiber sensors provide high resolution, distributed detection, and absolute strain and temperature measurements over a wide dynamic range. Brillouin sensing has mostly been studied in conventional single-mode fibers (SMF). The low nonlinearity and large Young's modulus of SMFs have limits in many applications. Chalcogenide fibers with high nonlinearity and low Young's modulus are of particular interest for Brillouin scattering investigations and sensing applications. Coherent laser sources with small frequency drift and low intensity noise are essential for optical communication, sensing, and metrology. SBS with high gain and narrow linewidth in optical fibers contributes to single-mode Brillouin random fiber lasers (BRFL) working with a low threshold. Previous studies of BRFLs with weak scattering distributed feedback have failed to address frequency drift and intensity noise. Random fiber grating (RFG) distributed feedback with strong scattering can manipulate mode dynamics of BRFLs. In this thesis, we demonstrate that chalcogenide microfibers can improve Brillouin sensing, and that RFG distributed feedback can stabilize Brillouin random fiber lasing. Furthermore, we present the detection of acoustic waves using chalcogenide microfiber sensors. In the first part of this thesis, we explore SBS characterizations and Brillouin sensing in chalcogenide microfibers. Chapter 4 describes the fabrication and simulation of poly-methyl methacrylate (PMMA) coated chalcogenide (As₂Se₃) microfibers. Chalcogenide-PMMA (As₂Se₃-PMMA) microfibers are mechanically robust enough for regular handling, allowing for Brillouin strain and temperature sensing. We present a SBS model to calculate the Brillouin gain spectrum of chalcogenide microfibers. Chalcogenide microfibers with ultrahigh nonlinearity give rise to Brillouin sensing under low pump power. Chapter 5 investigates wide-range strain sensing in single-core chalcogenide microfibers based on Brillouin frequency shift and bandwidth. SBS sensing is characterized using the Brillouin optical time domain analysis (BOTDA) technique. Chalcogenide-PMMA microfibers with lower Young's modulus than SMFs lead to a larger strain measurement range. Chapter 6 explores SBS characterizations and Brillouin sensing in high-birefringence chalcogenide microfibers with dual-core and elliptical-core shapes. Different responses in two polarization axes of the high-birefringence microfibers are used to distinguish two parameter variations. Different pairs of pump-probe modes in elliptical-core fibers allow for both intermode SBS and intramode SBS to be experimentally measured. In the second part of this thesis, we show that the frequency drift of SBS random fiber lasers can be reduced by a random fiber grating array (RFGA), a random fiber grating ring (RFGR), and acoustic wave coupling. RFGs have strong scattering due to high refractive index modulations by an external femtosecond pulse laser. Chapter 7 demonstrates that the strong scattering RFGA-based BRFL has a long-lived lasing mode of 12 s and slight frequency drift of 51 kHz/s at high pump power. This is enabled by light localization in the RFGA resulting from wave interference and multiple scattering. Photon trapping in the same path over thousands of round trips offers coherent lasing without mode hopping. Chapter 8 presents the frequency-stabilized BRFL based on the RFGR distributed feedback. The RFGR consisting of a RFG and a high split ratio coupler exhibits narrow linewidth due to the circulating propagation of light within the ring. Taking the advantages of self-adjusting random states with slight frequency differences to small thermal and acoustic variations and self-injection locking through the high-Q RFGR, the BRFL has mode hopping free operation over 14.9 s with a small frequency drift of ∼340 kHz. Chapter 9 investigates the dynamics of the dual-wavelength orthogonal polarized BRFL based on polarization-maintaining (PM) fibers. Acoustic wave coupling is introduced in the all-PM BRFL when the pump light is aligned at 45 degrees from the slow axis of the PM fiber due to the changed polarization and overlapped gain spectrum in the birefringence fiber-based laser. Due to the higher peak in the overlapping gain region, the frequency drift in the BRFL with acoustic wave coupling is reduced by around 1 MHz compared to the BRFL without acoustic wave coupling. The multi-peaks of Pearson's correlation coefficient and replica symmetry breaking confirm acoustic wave coupling in the BRFL. Appendix A investigates acoustic wave sensing in dual-core chalcogenide microfibers. The acoustic wave is driven by the mechanical stress from a piezoelectric transducer, which leads to pressure on the dual-core microfiber. The phase change of the propagating light in the dual-core microfiber is detected by intensity variations. The chalcogenide fiber with low Young's modulus has large strain sensitivity, leading to a high-frequency ultrasound measurement. The ultrasound sensing range is increased in the microfiber with a small core diameter and close core separation because of the steep spectral slope of the interference spectrum. The fused dual-core microfiber achieves a broadband ultrasound range of 20 kHz to 80 MHz with an average signal-to-noise ratio of 31 dB.