The Study On Phase Identify And Control Based On Mode-Locked Fiber Laser

The development of high-speed and capacity optical communication isthe main of the research of optical communication at present in the world.WDM and OTDM can overcome optical components and electroniccomponents bottleneck effect. And they can improve the capacity of opticalcommunication. Ultra-short optical pulse source is very important in OTDMcommunication system. The actively mode-locked fiber laser has wideadjustable range, small chirp of output pulse, high repetition rate and it easy tosynchronization. So it is deemed to perfect ultra-short optical pulse source.The problem of the stability of the actively mode-locked fiber laser can't besolved, which restrict the actively mode-locked fiber laser's application. Theprecondition of the actively mode-locked fiber laser output is that thefrequency of modulate signal is multiple of vertical mode interval in cavity.There is longer fiber in the actively mode-locked fiber laser's cavity. Moreover,the length of fiber will occur excursion because influence of outer condition.So modulate signal and the length of cavity is hard to match. We can takespecial measure to stabilize the actively mode-locked fiber laser's cavity. Sothe actively mode-locked fiber laser can stably operate long term.The phase-locked loop (PLL) is a phase error control system. It compareinput signal with output signal of VCO. The phase error controls voltage inorder to change the frequency of VCO when it is the same with the frequencyof input signal. In this way, we can add PLL circuit in the activelymode-locked fiber laser. The output of PLL will control the length of fiber inthe actively mode-locked fiber laser. This measure can improve the long-termstability of the actively mode-locked fiber laser.This paper introduces the principle of mode-locked about the activelymode-locked fiber laser, and chiefly analyzes two instabilities of the activelymode-locked fiber laser which include the short-term instability and the long-term instability. The main reasons of the short-term instability are relaxationoscillation and super-mode competition. There are many factors can makerelaxation oscillation, such as the change of cavity length and so on. Therelaxation oscillation of non-stable state and the non-uniformity of gainmedium can make super-mode competition. The long-term instability showsthat when length of cavity or modulation frequency changes, laser will notgenerate stable pulse. To solve short-term instability, the method is utilizingthe nonlinear polarization rotation. To solve long-term instability, the methodis utilizing feedback principle. The methods are phase-locked frequencymodulation feedback control and feedback error signal modulate the length ofcavity. This paper presents a kind of ameliorative measure based on the studyon the feedback error signal modulate the length of cavity. The design of PLLcircuit is according to existing principle and design method to improve theperformance of PLL. And it presents the method of improving the acquisitionperformance by analyzing the parameters of PLL. The loop filter is moreimportant in PLL. So this paper gives the design formula of loop filter. Theameliorative PLL circuit is simulated by Matlab software.