Experimental Study On Generation Of A High-repetition-rate Dichromatic Femtosecond Laser Based On A Frequency-doubled External Cavity

Femtosecond laser refers to the laser pulse lasting 10^-12?10^-15s.Femtosecond laser pulse has extremely high peak power,extremely wide spectrum width and extremely short emission time.The optical comb has equal frequency intervals in the frequency domain and is characterized as a periodic pulse sequence in the time domain.It is the most important optical frequency measurement tool nowadays,and has extensive and important application value in precise spectrum,astrophysics,quantum control and other scientific fields.repetition frequency(reciprocal of pulse period,abbreviated as repetition frequency)is an important parameter to describe the characteristics of the optical frequency comb.The higher the repetition frequency is,the greater the frequency mode interval of the laser is,and the smaller the corresponding pulse time interval is.For high-repetition optical frequency combs with repetition frequencies up to the order of GHz,it has important application value in high-precision astronomical spectroscopy,optical sampling systems,and pump-detection systems.Optical frequency combs with multi-mode quantum characteristics have huge application prospects in the fields of high-precision time-frequency transmission,quantum networks,and quantum communications.Synchronous pumped optical parametric oscillator(SPOPO)is an important method for the preparation of quantum optical frequency combs.At present,the repetition frequency of the laser used in the SPOPO system is generally around 70-80MHz,and the corresponding SPOPO cavity length is about 4m,so the experimental optical path is relatively complex.In the experiment,the pumping light is usually produced by the seed light passing through the octave crystal for a single time.When the seed light and the pump light are injected into the SPOPO cavity,it is necessary to meet the synchronization in the time domain and the spatial mode is fully matched,so as to realize the parametric action.Therefore,it is usually necessary to add an additional optical delay line device to ensure the synchronization of the seed light and the pump light in the time domain,which increases the difficulty of the experiment.Based on the above discussion,we will increase the femtosecond pulse repetition frequency by shortening the external cavity frequency doubling cavity length,and at the same time output the fundamental frequency light and the frequency doubling light from the same output mirror,saving the construction of optical delay and achieving synchronization between the two Inject into the SPOPO cavity.The main work of this paper:1.Theoretically introduced the basic principle of using a short resonant cavity to realize the repetition frequency conversion of femtosecond laser pulses,and through theoretical research shows that the power amplification in the frequency doubling resonator will increase with the increase of the repetition frequency.Decrease,thereby limiting the increase in frequency multiplication efficiency.2.In the experiment,a six-mirror bow-tie ring frequency-doubling resonator with a total cavity length of 1m and a free spectral region of 300MHz was designed and built.The input fundamental frequency light is a central wavelength of 815nm and a repetition rate of 75MHz.At the same time,fundamental frequency light and the frequency-doubling optical femtosecond pulse are obtained,and the repetition rate of both outputs is increased from 75MHz to 300MHz.The optimal frequency doubling efficiency is 4.7%.3.The H-C locking technology is used to realize the dynamic locking between the frequency doubling cavity and the laser to ensure the stable output of the fundamental and frequency doubling light.The uninterrupted locking time is 50 minutes,and the root mean square of the optical power fluctuation is 0.8%.4.The side mode rejection ratio and beam spatial profile of the output fundamental and doubler frequencies are measured.The basic frequency edge mode rejection ratio reaches 30d B,and the double frequency edge mode rejection ratio reaches 20d B.The ellipticity of the output baseband light is 95%,and that of the output frequency doubling light is 83%,which is significantly higher than that of the input baseband light(57%).