Ultra-low jitter mode-locked fiber laser

Abstract

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.