Ultra-low jitter mode-locked fiber laser
Date of Issue2012
School of Electrical and Electronic Engineering
Lasers have been widely used in various fields since the invention of first laser in 1960. Low-noise passively mode-locked lasers, as a special kind of pulsed lasers, provide ultra low intensity fluctutation and timing jitter and therefore have significant applications in optical sampling, frequency metrology and high-precision clock distribution, etc. Therefore, it is important to investiage the methods to reduce the phase noise and timing jitter of the mode-locked lasers. Moreover, various noise conversion phenomena also affect the laser noise such as the noise conversion from the pump to the mode-locked lasers, or degrade the microwave signal quality synthesised from the mode-locked lasers such as the excess phase noise conversion in the photodetectors. It is also meaningful to understand and characterize these noise conversion phenomena. In this thesis, we focus on three main aspects. They are timing jitter reduction for passively mode-locked fiber lasers, noise conversion from the pump to the mode-locked lasers and excess phase noise conversion induced by the photodetection process. For the timing jitter reduction for passively mode-locked lasers, we first analyze the optimization of the cavity loss in order to suppress the indirect noise source coupled from the frequency jitter to the timing jitter. For the mode-locked laser in our experiment with a cavity net dispersion of -0.1 pS2, a timing jitter reduction of 24% is demonstrated. Then a feedback control loop based on Proportional-Integral-Derivative (PID) control and piezoelectric transducer (PZT) is set up to lock the repetition frequency of the laser to an external reference oscillator. The phase noise at low offset frequency is found to be suppressed when the feedback is turned on. We also investigate the effect of external incoherent addition structures. Two structures called Mach-Zehnder (MZ) and ring are studied. Both MZ and ring structures are found to be able to suppress the phase noise at specific spectrum positions (dependent on the design of the MZ or ring structures) for pulse trains with a background phase noise level above -130 dBc/Hz at high offset frequency range. This result can be applied to reduce the timing jitter for the pulse trains after amplification. For the noise conversion from the pump to the mode-locked lasers, we first experimentally investigate the linear noise conversion from the pump relative intensity noise (RIN) to the RIN and phase noise of mode-locked fiber lasers at 1.55 urn. Two mode locking mechanisms, nonlinear polarization rotation (NPR) and semiconductor saturable absorber mirror (SESAM) are compared. Pump RIN is found to be the dominant noise source for both lasers (NPR laser and SESAM laser) and thus their RIN and phase noise can be predicted with the measured noise conversion ratios and pump RIN. It is also found that the SESAM laser shows a higher phase noise than the NPR laser due to the additional intracavity RIN to phase noise conversion caused by the slow saturable absorber effect of the SESAM. Then we study the nonlinear RIN conversion from the pump to the mode-locked lasers. The nonlinear RIN conversion is found to generate additional noise power at various harmonics kfM with respect to the fundamental pump modulation frequency 1M. An exponential decay model is proposed to describe the behavior of the nonlinear RIN conversion. Physical explanation from the view of gain modulation is proposed and verifies the validity of this exponential decay model. For the excess phase noise conversion induced by the photodetectors, we present a method based on the power dependent impulse response of the photodetectors in order to characterize the excess noise conversion from optical RIN to electrical pulse width jitter (PWJ), relative amplitude noise (RAN) and phase noise in the photodetectors when detecting the optical pulse train from mode-locked lasers. Theoretical analysis is presented to describe the relations among these noise conversion ratios and to predict the measured electrical RAN and phase noise power spectral densities. The effect of the pulse width of incident optical pulses is also discussed. Moreover, a photodetector with higher impulse-response saturation power is found to allow a larger input optical power range while maintaining low RIN-tophase- noise conversion ratio. These results provide useful guidelines for low-noise microwave synthesis by photo-detecting the output of a low-noise mode-locked laser.
DRNTU::Engineering::Electrical and electronic engineering