The Application Of High Order Mode Locking In Laser Frequency Stabilization System

At the Centre of Gravitational Experiments in Huazhong University of Science and Technology?HUST?,Aluminum ion(²⁷Al⁺)optical frequency standards are currently being carried out.The program is of great signification to precision measurement of time and frequency and to building unified time standards.It consists of ultra-stable laser system,²⁷Al⁺ion trap system and femtosecond frequency comb system.As clock laser,the ultra-stable laser system plays an important role in the whole optical frequency standard.In this thesis we introduce Pound-Drever-Hall?PDH?frequency stabilization technology,and discuss the major effects that can affect the frequency stability of the laser,such as length,temperature of ultra-stable cavities,size of laser beam and laser intensity jitter.In order to get higher frequency stability,we investigate the influence of high order modes on thermal noise limit of a reference cavity.And simulate phase masks which can generate high order Laguerre-Gause modes(LGM₃₃ and LGM₅₅ modes)with Matlab.Then we calculate the effects of electronic noise and thermal noise if we want to lock the laser to high order modes in PDH frequency stabilizing system.The results are compared.At last we draw a conclusion that in the same condition,laser frequency stability is still limited by thermal noise instead of electronic noise when locking the laser to high order cavity modes.During the experiment,I take participate in work to measure electronic noise in PDH frequency stabilization system,including the measurement of mixer response in a demodulating system.And the work plays an important role for measurement of electronic noise when the laser is locked to a high order cavity mode.After we verify the feasibility that higher order mode locking can reduce thermal noise limit,we first inject the laser to the cavity directly and lock the laser to a high order mode(TEM₀₂ mode)in the experiment.To improve the coupling efficiency,we also do mode match and improve the coupling efficiency to about 30%.Second,in order to measure linewidths of high order modes,we use both cavity ring-down and laser phase locking methods.The linewidths of high order modes are similar to that of the fundamental mode,which are all about 4 kHz.Third,we build a alser intensity stabilization system.The Allan deviation of the laser intensity is 5×10^-5 after locking.At last,with a 30 cm cavity ultra-stable laser system,we build a three-cornered hat system to measure the absolute laser frequency stabilities of three different laser systems.The frequency stability of high order mode locking system is 8×10^-16@1 s.