Study on the Repetition Rate Locking System of the Femtosecond Laser

The new technique known as "The femtosecond frequency comb technology" has dramatic impact on the diverse fields of precision measurement and nonlinear optical physics. In order to acquire high-precision and high-stability femtosecond comb, it's necessary to stabilize the repetition rate f(Rep) and the offset frequency f(0). This article presents the details of stabilizing and controlling the comb parameter f(Rep) and finally phase lock the repetition rate of femtosecond laser to a radio frequency reference, derived from an atomic clock. In practice, the narrower the bandwidth of lock system (close-loop system), the higher stability we can achieve, but it becomes easier to be unlocked for external disturb. We adopt a method in servo unit to avoid this problem in this paper. The control parameters P and I can be adjusted and optimized more flexibly. The lock steps depend on the special servo system make it easier to find the right parameters and the lock becomes more convenient and quickly. With this idea, the locked time of repetition rate can be as long as the mode-locking time of the laser. The stability of laser can be evaluated by allan deviation. In this research, the contrast of stability of f(Rep) between the locked laser and the unlocked is given. The new lock system is proved reasonable.