Research On Ultru-Narrow Linewidth Multi-Wavelength Fiber Laser

Multi-wavelength laser (MWL) source is one of the key instruments in wavelength division multiplexing (WDM) system of large scale fiber-optic hydrophone array, especially in the system with phase generated carrier (PGC) demodulation and unbalanced interferometer which not only requires multi-wavelength source but also demands rigid standards in the laser source characteristics such as ultra-narrow linewidth, high stability and rapid tunability. To match the demands of large scale fiber-optic hydrophone array,this thesis mainly explored the implement method of ultra-narrow linewidth dual-wavelength fiber ring laser. The research includes theoretical analyses, numerical simulations and experiments. The main work and results are as follows:1. The analytic researches on characteristics of Erbium-doped fiber (EDF) and Erbium-doped fiber laser (EDFL) were carried out based on Giles model in different approximate level. The simulation model of Erbium-doped fiber ring laser was derived from the spectrum model of Erbium-doped fiber, and simulations on single wavelength and dual wavelength laser oscillation with forward and backward ASE noise were implemented. The balanced condition for dual-wavelength oscillation in ordinary ring cavity was deduced from analytic model and was validated by simulations and experiments. Simulation results showed that the gain equilibrium method could be applied to achieve dual-wavelength fiber laser, however, even tiny changes in loss spectrum had great impact on the balance of dual-laser because of the homogeneous characteristics of EDF. The balance could be broken by a loss bias larger than 0.05dB.2. The laser oscillation of three types of dual-wavelength fiber ring laser structures based on gain equilibrium technology was experimentally validated. Based on the parallel FBG structure, a novel experiment setup was built to measure the influence of pump power, mode losses and wavelength interval on the output power. Experiment results showed that the condition for balanced oscillation was valid when pump power was far away from the threshold point. The stability of variable optical attenuator (VOA) had so great impact on the balance of dual-wavelength that the balanced dual-wavelength oscillation could be destroyed by a loss bias larger than 0.3dB, which resulted in one mode between them vanished. By carefully reducing the loss perturbation of laser system, the power fluctuation of 1546 nm and 1556 nm lasers over 30 minutes was less than 0.5dB. 3. To study the ultra-narrow linewidth fiber laser, the saturable characteristic of unpump EDF and the loss spectrum resulting from spatial hole burning (SHB) induced by stand wave in it were measured. Two types of ultra-linewidth dual-wavelength fiber ring laser structures which were cascaded FBGs and parallel FBGs were designed to carry out experiments. Results showed that the linewidth suppress effect didn't existed in cascaded structure, whereas the parallel structure could emit dual-wavelength ultra-narrow linewidth lasers but the power difference achieved 30dB. However, it possessed the bistability speciality controlled by attenuation, which could be applied to build wavelength switchable ultra-linewidth dual-wavelength fiber laser.The experiment results of dual-wavelength laser and ultra-narrow linewidth dual-wavelength laser indicate that narrow linewidth multi-wavelength laser is easy to obtain while the ultra-narrow linewidth multi-wavelength laser is difficult to achieve because of the mode competition mechanism. Thus, the deeper research will focus on the stable operation of ultra-narrow linewidth dual-wavelength EDFL.