Random fiber lasers, a new type of fiber laser that uses disordered medium to provide distributed feedback, have drawn considerable interest in the photonics community over the past ten years. Stimulated Brillouin scattering (SBS), with a typical narrow spectral width of ~100 MHz, provides an important gain mechanism for random fiber lasers. Brillouin random fiber laser (BRFL) has shown excellent advantages in generating highly coherent photons and in ultrasound sensing. However, the accompanied large intensity noise in BRFLs hinders its further performance improvement and practical applications. In order to design a low noise BRFL, it is important to explore the fundamental physics behind BRFL and study its output characteristics. This thesis focuses on the study of random lasing mechanism in BRFL, which lays the foundation for the demonstration of a low noise BRFL. The main research results and contributions are as follows: (1) In order to understand the dynamic noise properties of BRFLs, the properties of the acoustic wave generated by BRFL, including its intrinsic spectral width, intensity dynamics, distributed spectrum and distributed intensity statistics are characterized for the first time. The characterization method is based on the SBS enhanced polarization decoupled four wave mixing process, where the pump wave, Stokes wave, probe wave and reflected probe wave are coupled through the fiber density variation induced by the acoustic wave. It is demonstrated that the intrinsic spectral width of the acoustic wave in the Brillouin gain fiber depends on the spectral convolution of pump light and Stokes light. Stochastic behaviour is introduced to the intensity dynamics of the acoustic wave when the linewidth of the pump light (or the Stokes light) is larger than several MHz. The distributed spectra of the dynamic grating are determined by the birefringence of the Brillouin gain fiber, which have maximum change on the order of 10-7 to 10-6 when the BRFL is on operation. Different proportion of optical rogue waves are detected at high gain position and low gain position near the lasing threshold, proving the nonlinear amplification of the SBS process. (2) In order to study the mode selection mechanism of the distributed random feedback and explore new physics phenomenon in BRFLs, the conventional Rayleigh scattering fiber in BRFL is replaced by the artificially controlled random scattering medium. First, weak FBG array with random spacing offers distributed feedback with varied length, which demonstrate the longitudinal mode filter function of the distributed random feedback. Single longitudinal mode operation of BRFL is realized by using appropriate length of the FBG array. Then, scattering from random fiber grating (RFG) with varied grating period is used to provide feedback for BRFL. The enhanced backscattering strength from RFG improves the slope efficiency of BRFL to 29.3% and reduces the lasing threshold to 10.2 mW. By calculating the correlation of the intensity fluctuation spectra from trace to trace, the correlation of two traces is found to be dependent on the specific two chosen traces, demonstrating the replica symmetry breaking phenomenon in photonics. (3) RFG with relatively large refractive index modulation shows potentials in improving the performance of the BRFL. In order to investigate the working mechanism of the RFG, optical frequency domain reflectometry (OFDR) with spatial resolution of 8 μm is employed to characterize the property of RFG. The backscattering strength and spectral response of RFG is highly related to the degree of randomness of RFG. Theoretically, entropy is introduced to build a quantitative relationship between the degree of randomness and backscattering strength of the RFG based on the transfer matrix method. A linear relationship between the average reflectivity of the RFG in dB scale and sub-grating’s entropy is found. Further, based on a polarization maintaining RFG, a low noise BRFL is proposed and demonstrated. Compared to Rayleigh scattering, the polarization maintaining RFG can tolerate environmental perturbation, leading to a 20 dB intensity noise suppression of the BRFL in the low frequency domain from 10 Hz to 1 kHz. (4) The dynamic properties of the slowly varying frequency drift of a dual-wavelength BRFL in polarization maintaining fiber are characterized. Two principal lasing peaks in each polarization are enabled by the combined distributed Rayleigh scattering and the Brillouin gain provided by the polarization maintaining fiber with large birefringence. Polarization dependent and polarization independent spectral variations are studied in the dual-wavelength BRFL due to the environmental perturbation and gain competition. The probability distribution of the lasing frequency exhibits a dip near the mean frequency that is caused by the spectral hole burning. By calculating the matrix of the Pearson correlation coefficient, the internal correlations between different part of random fiber laser spectra are found, which enhances the understanding of the fundamental physics of random lasing process.

[pt] Diferentes tipos de lasers podem ser usados para gerar pulsos de luz com uma ampla faixa de durações de pulso, energias e potências de pico. As técnicas de Q-switching e mode-locked são relatadas há anos por vários autores e pesquisadores e são frequentemente utilizadas na geração de lasers de pulso ultracurto com duração de pulso no domínio do tempo na faixa de nanossegundos até femtossegundos. Uma configuração, com ganho fornecido por um amplificador óptico semicondutor (SOA) e amplificador de fibra dopada com érbio (EDFA) é proposta e emprega a técnica de gerenciamento de dispersão para gerar um trem de pulsos ópticos exibindo alta potência de pico, taxa de repetição ultra-baixa e largura temporal curta, habilitando que este laser seja usado como uma fonte para aplicações de alta resolução em reflectometria óptica no domínio do tempo (OTDR). A operação mode-locked é conhecida por ocorrer apenas em lasers ordenados padrão por um longo tempo e até recentemente foi encontrado também em lasers de fibra aleatórios desordenados (RFL). Embora tenha havido progresso no sentido de travar modos espaciais e longitudinais em lasers aleatórios, a literatura carece de relatos sobre geração de pulsos limitada por transformada de Fourier, apesar das muitas décadas de campo. O autor demonstra experimentalmente um mode-locked random fiber laser (MLRFL) operando como um refletômetro óptico de domínio do tempo sensível à fase. Aqui, a saída total do laser fornece o sinal de detecção, em contraste com o pequeno sinal retroespalhado medido em um OTDR convencional. O laser opera como um sensor acústico distribuído (DAS) e sensor de temperatura distribuído (DTS). / [en] Different types of lasers can be used to generate light pulses with a wide range of pulse durations, energies and peak powers. Q-switching and mode-locked techniques have been reported for years by several authors and researchers and are frequently used in the generation of ultra-short-pulse lasers with time-domain pulse durations from the nanosecond to femtosecond range. A configuration, with gain provided by a semiconductor optical amplifier (SOA) and erbium-doped fiber amplifier (EDFA) is proposed and employ the dispersion management technique to generate a train of optical pulses exhibiting high-peak-power, ultralow repetition rate, and fast temporal width, enabling this laser to be used as a source for high-resolution optical time domain reflectometer (OTDR) applications. The mode-locking operation has been known to occur only in standard ordered lasers for a long time and until recently it was found to also occur in disordered random fiber lasers (RFL). Although progress has been made towards locking spatial and longitudinal modes in random lasers, the literature lacks reports on Fourier transform-limited pulse generation despite the many decades of the field. The author experimentally demonstrates a mode-locked random fiber laser (MLRFL) operating as a lasing phase-sensitive optical time domain reflectometer based on random feedback from a sensing fiber. Here, the full output of the laser provides the sensing signal, in contrast to the small backscattered signal measured in a conventional OTDR. The laser operates as a distributed acoustic sensor (DAS) and distributed temperature sensor (DTS).