| The expectation rate of optical fiber communications in the next generation is 10GHz and even 40GHz. At present, the frequency of these high-speed devices is constantly developing, and some devices have achieved commercialization. 10GHz/40GHz ultrafast actively mode-locked fiber laser research is developing from basic theory to gradual application stage. Ultrafast actively mode-locked fiber lasers can produce ultra-high-speed optical soliton pulse, considered to be the ideal light source of the next long-range fiber communication system without relay station. However, the stability technical problems of the ultrafast actively mode-locked fiber laser still remain unresolved, which makes ultrafast actively mode-locked fiber laser be unpractical for application. It mainly manifested in the poor stability of the soliton pulse after mode locked, and the solution pulse is very sensitive to temperature changes. The ulstafast actively mode-locked fiber laser cannot work stably for long time, just only a few hours.Stability problems must be solved for the actively mode-locking fiber lasers'application. The stability work of the actively mode-locked fiber laser concentrated mainly in the following three aspects : 1. eliminate the fluctuation of polarization state in the laser cavity; 2. Reduce supermode noise impact bonded with the harmonic mode-locking technique;3.Concentrate the cavity length's turbulence caused by the external environment disturbance .The most effective means to eliminate the fluctuation of polarization state is to use polarization-maintaining (PM) erbium-doped single-mode fiber, PM fiber, PM isolator, PM couplers and polarization controller. These devices can be used to increase the stability of the output mode-locked pulse effectively.Supermode noise is an important factor which affects the stability of actively mode-locked fiber laser. It is a kind of phase noise, and it is an inherent problem in harmonic mode-locking technique. PZT drum technique and narrow-band Fabry-Perot filter inserted in the cavity can effectively reduce the supermode noise.As the changes of the cavity length or refractive index optical fiber affected by environmental disturbulence and microwave modulation frequency drift will cause the cavity length and frequency modulation dismatching, the pulse will gradually lead to collapse. In order to achieve stable operatioin of actively mode-locked fiber laser, normally error signal is taken from the cavity length and frequency modulation disorders to control the cavity length, which makes it unchanged and strictly match the modulation frequency. To solve this problem, regeneratively mode-locking technique is the most common method. However there is an serious shortcomings in regeneratively mode-locked fiber laser, that is: when the laser operates in free conditions, repetition frequency of output pulse is difficult to stabilize at a fixed frequency for it automatically tracking changes of the cavity length. Therefore, phase-locked loop (PLL) technique is used based on using piezoelectric ceramics (PZT) to control and compensate the cavity length. In this technique, the harmonic components extracted from output pulses are no longer feedbacked to the modulator. It is taken as driving voltage to drive PZT, which compensate the varity of the cavity length and keep it unvariable. So the repetition rate of the output pulse can remained unchanged. Now this technolique seems very valuable for it may make the actively mode-locked fiber laser merchandized in the future.In PLL technique, whether PZT can compensate the varity of the cavity length real-time lies in two points: Firstly, whether the PZT driving circuit has good dynamic response to driving signals; Secondly, whether the fiber wounded on PZT can respond immediately to signals added on PZT.PZT is inseparable from the corresponding driving circuit。Normally, effective work of PZT depends on the response characteristics especially dynamic characteristics of the driving circuit. PZT is approximately equivalent to capacitive load electrically. Then the load (PZT) and the output impedance R of the driving circuit forms RC circuit which will greatly affects the dynamic properties of PZT. In the ultrafast actively mode-locked fiber laser system, in order to achieve real-time compensation for laser cavity's length change, PZT driving circuit must possess the following characteristics: (1) small output resistance.By reducing the output resistance R of driving circuit, RC time constant will be lower thereby dynamic performance of PZT driving circuit will be improved. (2) can amplify the input signal linearly. Then the output signal can be amplified linearly, with little distortion and small fluctuations. ( 3 ) the range of output signal can be adjusted continuously. . We designed and achieved a PZT driving circuit with good dynamic response characteristics: broad response frequency bandwidth (10KHz); fast speed (the rise time and fall time is only 4 .6μs and 18 .4μswhen the input signal is rectangular waveform); amplifying times is about 26. So it is suitable to drive PZT applied in the compensation for cavity length of ultrafast actively mode-locked fiber laser.In order to study response characteristics of the fiber wounded on PZT to the driving signals added to PZT, We set up a Mach-Zehnder fiber interferometer structure and differential Sagnac fiber interferometer structure to do experiments.We suggested a novel method to measure the phase difference between the two fiber arms of Mach-Zehnder fiber interferometer by measuring the photocurrents from the two fiber arms and the interferometing photocurrent when the devices are put in a temperature controller. However,for the system must operate in quadrature it is not easy to operate. Therefore, we choosed an alternative structure—differential Sagnac fiber interferometer structure to measure the response characteristics of the fiber wounded on PZT to the driving signals added to PZT.In differential Sagnac interferometer structure, the system need not work in quadrature, there is no need to use polarization controller. As the two interferent light beams propagate in the same path, the system can work stablely with the temperature, stress and other environmental factors'disturbulence. From the theory analysis and experiment results we can know that the variation of the fiber length wounded on PZT can response successfully to the driving signals added to PZT when the frequency of the driving signals is no more than 2000Hz. For the cavity variation of ultrafast mode-locked fiber laser is usually induced by the slow environment fluctuation such as temperature's turbulence, stress etc, so it is efficient to use PZT to compensate the cavity variation of the fiber laser.
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