Study On Mode Hopping In Ultra-narrow Line-width Fiber Ring Laser

The phenomenon of random mode hopping debases the stability of ultra-narrow line-width erbium-doped fiber ring laser (UNLW-EDFRL), which restricts the practical application of the light source with advantages of high coherence, fast modulation and low noise. To match the demands of high reliability in military and space system, it is practically urgent to explore mode-hopping suppression scheme for high coherence light source. In this dissertation, the mode-hopping phenomenon of UNLW-EDFRL has been studied both theoretically and experimentally, and typical regularity and basic mechanism of mode hopping are obtained, which make a good foundation for further revealment of mode-hopping mechanism and promotion of mode-hopping suppression technology.The main results of this thesis are as follows:1. The dynamical mode-hopping model of dual-mode EDFRL is established to simulate the mode-hopping mechanisms induced by both accident and slow loss change, which are good interpretation of accident output power change and increase of relative intensity noise (RIN).2. The saturable absorber grating (SAG) ,which is the key element to suppress linewidth of laser, is modeled and the steady state characteristics are simulated. Numerical results show the SAG has ultra-narrow passband filter feature which can suppress mode hopping. However, the strong power of signal light will destroy the filter which is the main reason for the increase of mode-hopping frequency in high pump power.3. On the base of analysis for influence of mode hopping on the typical performances of laser, five methods are utilized to measure the mode hopping parameters, which are FP interferometer method, optical intensity measurement, coherence of interferometer method, PGC phase demodulation method and phase working point method. The measurement region of each method is pointed out, which provides technical supports for measurement of mode-hopping regularity and analysis of mode-hopping mechanism. 4. Three-channel synchronous mode-hopping measurement system are designed and built. The influence of pump power and laser cavity length on mode-hopping characteristics is deeply investigated experimentally. Mode-hopping maps of fiber laser which is labeled by pump power and modulation amplitude voltage of cavity length are first measured. The mode-hopping map can provide valuable information for intrinsic mode hopping mechanism.