Experimental Study On The Effect Of Spectral Density Of Frequency Noise On The Coherence Of Semiconductor Lasers

Semiconductor lasers have been used at the forefront of many scientific researches due to their wide wavelength coverage,compact structure and portability.In particular,gravitational wave detection,satellite communication,coherent manipulation of monatomic molecules and preparation of entangled atoms,semiconductor lasers are the most important detection and manipulation tools.Coherence,as an important index of semiconductor laser,directly determines the application performance of the system.It includes the spectral resolution and spectral measurement precision of precision spectral experiment,the purity of atomic state preparation in atomic and molecular physics experiment and the measurement sensitivity in precision measurement field.For the semiconductor laser used in actual experiments,the coherence of the coherent light field is mainly determined by the frequency noise of the light source itself.Therefore,it is of great significance to quantitatively study the relationship between the spectral density characteristics of frequency noise of semiconductor lasers and their coherence,so as to effectively suppress the frequency noise of the light source and enhance its coherence.Generally,it is difficult to obtain a semiconductor laser with line width of Hz magnitude and to accurately measure the line width of a laser below k Hz magnitude,and it is difficult to precisely and quantitatively control the frequency noise of a semiconductor laser.Therefore,it is difficult to carry out experimental research on the effect of spectral density of frequency noise on the coherence of a semiconductor laser.In view of the above problems,we carried out the following research:1)The line width of semiconductor laser is compressed to Hz by suppressing the frequency noise.For precise control of semiconductor laser frequency noise,the design of ultra-high bandwidth frequency servo loop of semiconductor laser frequency noise,based on the Pound-Drever-Hall（PDH）technology,the combination of ultra-low thermal expansion materials high precision method in Fabry-Perot cavity（FP cavity）,electrooptic modulator as transducer to realize the feedback servo loop bandwidth of 3.5 MHz ultra-high bandwidth frequency frequency noise suppression.The line width of semiconductor laser reaches Hz level.;2)A new method of beat frequency signal processing based on time resolution is proposed.To solve the problem of accuracy of laser line width below k Hz in traditional methods.The period of beat frequency signal is acquired in real time based on time interval analyzer and the spectrum information of beat frequency signal is obtained by discrete Fourier transform.Compared with the usual method of analyzing the beat frequency signal with the scanning type spectrum analyzer,we realize the spectrum measurement of the Fourier limit resolution,especially for the measurement of narrow line width（<k Hz order of magnitude）,more accurate results can be obtained;3)Based on the line width of Hz,the frequency stabilized semiconductor laser system is phase-modulated by an electro-optical modulator driven by a pseudo-random white Gaussian noise function whose parameters are controlled.The frequency characteristics of the modulation sideband are the same as the Gaussian white noise function.The first order sideband is filtered by optical filtering,and the linewidth of the first order sideband is measured accurately by time resolution method.It is found that when the amplitude H₀ of the frequency noise spectral density of coherent optical field is less than the noise bandwidth,the relationship between the line width of laser and the frequency noise spectral density satisfies the linear relationship.When H₀ is greater than the noise bandwidth,the laser line width is proportional to the square root. （?）.