Experimental Investigation On Spectral Transfer Of 1.5?m Optical Frequency Comb

Femtosecond optical frequency comb connects baseband frequency signal and optical frequency signal,making precise measurement of laser signal possible and playing an important role in many research fields,such as time frequency measurement,high-precision time frequency transmission,superstable microwave generation,precise spectroscopy and measurement of basic physical constants.At present,the application scenarios of optical frequency comb are still increasing,and some applications have higher requirements on the reliability and main technical indexes of femtosecond optical comb.On the one hand,the early system based on solid state femtosecond laser has poor adaptability to the environment,which can not meet the application needs of a variety of special environments well.On the other hand,broadband frequency control technique has made up high noise for the fiber optic frequency comb,the stability of frequency control in the optical fiber comb ring is equal to that of solid state optical comb.Therefore,more and more application systems choose the system based on fiber laser.The erbium-doped fiber optical comb laser has a central wavelength of 1.5 microns.Because the dispersion of waveguide material in this band is close to zero,it is convenient to construct a highly reliable laser light source.Therefore,it has become the first choice for spatial application and outdoor application of optical comb.The direct output spectrum range of erbium-doped fiber optical frequency comb cannot cover the target wavelength required by many applications,it is necessary to apply spectrum conversion technique to extend its working wavelength.In this paper,femtosecond laser source,nonlinear amplification and supercontinuum generation were studied for the spectrum conversion application requirements of erbium-doped fiber optical frequency comb,in order to provide reference for the spectrum transfer design of erbium-doped fiber optical frequency comb,especially for the parameter selection and usage of high nonlinear fiber.The main research contents and innovative results of this paper are as follows: 1.Designed and accomplished a highly reliable 1.5 micron laser source.The laser source is based on two mode-locking mechanisms,nonlinear polarization rotation(NPR)and nonlinear amplifying loop mirror(NALM).It inherits the common advantages of the two mode-locking mechanisms and has the characteristics of self-locking,high conversion efficiency,large mode-locking margin and low noise level.The repetition frequency of the laser is 168 MHz,the output pulse width is 47 fs,and the amplitude noise level is-135 dbc /100Hz,which is better than the general fiber mode locked laser.2.By emoloying chirped pulse amplification technique,an erbium-doped fiber laser amplifier was built.The pulses with 50 m W average power,53 fs pulse width and 0.24 n J single pulse energy are amplified by forward and backward pumping direction.After amplification,the peak power of the output pulse reaches 35 k W,corresponding to the single pulse energy reaches 2 n J and the pulse width is 60 fs,which meets the input conditions of the subsequent spectral broadening laser.3.Experimental studies on spectral broadening characteristics of 5 kinds of highly nonlinear optical fibers are carried out.In the experiment,,the output spectral spectrum was compared and studied under two dimensions of different pump power and fiber length,and the out spectrum covering 950 nm to 2350 nm was generated.The results show that the key parameters of the optical fibers are similar to the existing photonic crystal fibers and can be divided into three categories according to dispersion.Under the anomalous dispersion pumping mechanism,spectral energy is efficiently transferred to the short wavelength far away from the incident light(900 nm to 1200 nm);under the normal dispersion pumping mechanism,the spectral energy is transferred symmetrically to the direction of long and short wavelength;under the pumping mechanism of anomalous dispersion and normal dispersion,most of the spectral energy is evenly transferred to the side of the incident light.4.With the above experimental results as reference,the spectral broadening design and experiment for two applications of direct optical frequency measurement are realized.Firstly,by means of first spectral expansion,the working wavelength of erbium-doped optical comb is extended to twice the working wavelength of strontium optical clock for the frequency measurement of strontium optical clock,and then the periodic polarized lithium niobate crystal is used for frequency doubling.Finally,the single-mode energy of the comb teeth at 698 nm reached 1.65 ?W,and the beat frequency SNR of the 698 nm single-frequency laser(0.2 m W)reached 35 d B(resolution 300 k Hz).Secondly,by means of first spectrum expansion and later frequency doubling,the D1 line frequency of Li atom at 671 nm was measured.The SNR of laser beat frequency reached 60 d B(resolution 1 Hz).Both designs met the requirements of optical frequency testing.