Studies On A Broadband Passive Cavity For Filtering The Noise Of A Femtosecond Laser

With the rapid development of atomic clock technology, the precision of frequency signal has entered the level of 10-18. The application of high-precision time and frequency is playing an increasingly important role in areas of fundamental physics and many other areas of highly advanced technology and engineering infrastructure. As the laser signal has a higher carrier frequency and bandwidth than radio radiations, the time transfer by laser link received extensive attention recently. With the appearance of femtosecond optical frequency comb, the time transfer accuracy has been improved revolutionarily which takes the time synchronization accuracy from picosecond into the femtosecond within 2 km distance.The latest research shows that a new time transfer method based on quantum theory is expected to increase the accuracy to the order of attosecond. The core idea of this technology is using balanced homodyne detection techniques to measure the phase delay between the signal optical frequency comb and the reference comb. Further study reveals that the sensitive of time measurement is limited by the carrier-envelope-offset frequency(CEO) phase noise of the optical frequency comb. A passive optical cavity not only can be used as a low-pass noise filter, but also can inter-convert the phase and amplitude fluctuations of a light beam after transmission or reflection.In this paper, a complete theoretical mode for analyzing the transfer function of the noise field quadratures by a passive optical cavity is built. According to the analysis, the phase noise properties of a field under test can be explored by measuring its transferred amplitude noise through transmission and reflection of a passive cavity. Furthermore, we report on an experimental implementation, in which a broadband passive cavity with a finesse of 1500 and a free spectral range of 75 MHz is utilized to analyze and filter both the amplitude and phase noise of a commercial Ti:Sapphire femtosecond laser. The transfer function of the passive cavity helps to evaluate the phase noise of the femtosecond laser. The result shows that, the passive cavity has an evident noise suppression effect of both the amplitude and phase noise of the femtosecond laser.