Study On High-stability Single-frequency Fiber Laser

Narrow-linewidth single-frequency laser sources have been found many important applications in the field of optical communication system, fiber sensing, gravitational wave detection and LIDAR. The short-linear-cavity fiber laser that benefits from its compact structure, narrow spectral linewidth in the order of kilohertz and stable single-frequency operation, has attracted much attention. However, there is a relatively strong relaxation oscillation(RO) phenomenon in fiber laser in general. In addition, laser’s low-frequency intensity noise is influenced easily by the fluctuations of external environment and pump power, and the long-term stability of laser output power is reduced by it. Moreover, frequency fluctuation of a free-run laser is more than 10-7. However, for many practical situations, such as coherent communication and hydrophone, it should optimize the stability of present laser further. Therefore, it is significant to improve the stability of the laser. Focusing on the intensity and frequency stability of the laser, which is a homemade narrow-linewidth single-frequency DBR phosphate fiber laser at 1.5 μm, the main contents of this dissertation are as follows.(1). The laser’s intensity noise is simultaneously suppressed effectively at low-frequency domain and around the RO frequency, utilized a custom two-path feedback system. Moreover, the long-term power stability is improved. The design scheme, expected result and stability of the feedback system are analyzed theoretically. The design and debugging of three main parts, which include the power-modulation laser diode driver, temperature controller and feedback circuit, are discussed experimentally in detail. At low-frequency domain, the relative intensity noise(RIN) has been decreased by about 20 d B from 0 to 5 k Hz and over 10 d B from 5 to 10 k Hz. Especially, the RIN is approaching-150 d B/Hz at the frequency range from 0.2 to 5 k Hz. In addition, the intensity noise peak at the RO frequency is indeed reduced by 22 d B compared with the open-loop system. Moreover, the laser power long-term instability has been improved from ±0.5% to less than ±0.05%(24 hours).(2). Through analyzing the hydrogen cyanide(H13C14N) absorption line, the frequency modulation spectroscopy(FMS) technique is adopted. The theoretical analysis and experiment about stabilizing laser output frequency are performed. The laser is a homemade frequency modulated narrow-linewidth single-frequency fiber laser. The progress of designing and debugging all parts is analyzed. Depending on the mathematical simulation of the frequency stabilized system and the characteristics of gas cell, the parameters are determined in the practical system. The expected results and stability of the whole closed-loop system are analyzed theoretically. Through controlling the temperature of broadband FBG and gain fiber according the PZT voltage of narrowband FBG, more than 3.5 GHz tuning range without mode-hopping is achieved. Utilized this frequency-stabilized system, the output frequency fluctuation of the stabilized laser is less than ±600 k Hz. A FM-eliminated fiber laser of high frequency stability is achieved.(3). Based on the before-mentioned works, the experiment about stabilizing laser output frequency and power is performed. The completed experimental setup is described, and the progress of designing and debugging power stabilized parts is analyzed in detail. Through this feedback system, the output frequency fluctuation of the stabilized laser is less than ±500 k Hz, and the locked time is more than 24 hours. In addition, the power instability of less than ±0.2%(24 hours) is obtained. A high-stability FM-eliminated fiber laser is achieved by the customized feedback system.