Low-noise Single-frequency Fiber Laser Technique And Its Application Research

With the advantages of all-fiber compact structure,good beam quality,k Hz-order laser linewidth and effective thermal control,single-frequency fiber lasers have important application value in optical fiber communication,coherent beam combining,Doppler lidar and other fields.However,the noise characteristics of single-frequency fiber lasers limit their further development in precision laser applications such as high-precision spectroscopy,high-precision time-frequency transmission,optical pump magnetometers and gravitational wave detection.In order to improve the noise performance of single-frequency fiber lasers and meet the increasing technical indicators of light source noise of laser application systems,it is of great significance to carry out noise suppression research on single-frequency fiber lasers.In this paper,distributed Bragg reflection(DBR)single-frequency fiber laser is selected as the research object to carry out systematic research on the suppression of intensity noise and frequency noise.Additionly,applied research is carried out on the basis of the low-noise single-frequency fiber laser.The main research results are as follows:(1)Intensity noise suppression:a theoretical model of intensity noise suppression based on polarization-dependent gain saturation effect for semiconductor optical amplifier(SOA)is proposed,and the accuracy of the theoretical model is verified by experiments.Using the suppression technology combined with SOA and photoelectric feedback,coupled with an optimized high-saturation-power and low-noise photodetector,the relative intensity noise(RIN)of the output laser can reach-160 d B/Hz within a wide frequency range of 50 k Hz to50 MHz.The difference between it and the shot noise limit(-163.69 d B/Hz)is less than 3.7d B,which effectively improves the intensity noise performance of the single-frequency fiber laser.(2)Frequency noise suppression:A 2×2 coupler with coupled ratio of 50:50 is used to form a band-gap filter resonator,then a type-8 composite optical feedback loop is designed to introduce a vernier effect to enhance the feedback locking effect of the composite cavity.The frequency noise in the frequency band greater than 3 k Hz can reach the level of 0.3 Hz²/Hz,and the maximum suppression amplitude can reach 4 orders of magnitude.A Michelson interferometer with arm difference of 500 m combined with proportional integral derivative(PID)locking technology is used to achieve a maximum suppression amplitude of 5 orders of magnitude of frequency noise in the 30?100 Hz frequency band,and the minimum frequency noise can reach 0.1 Hz²/Hz.A comprehensive frequency noise suppression method combining type-8 optical feedback and an unbalanced fiber interferometer is carried out.The lowest level of frequency noise reaches 0.03 Hz²/Hz,and the maximum suppression amplitude is close to6 orders of magnitude.In the frequency range of 10 Hz?25 k Hz,the frequency noise of the output laser is less than 1 Hz²/Hz,and the single-frequency fiber laser with low frequency noise in a wide frequency band is realized.(3)Simultaneous suppression of intensity noise and frequency noise:a comprehensive noise suppression technology combining optical self-feedback and boost optical amplifier(BOA)is used to simultaneously suppress the intensity noise and frequency noise in the middle and high-frequency band.The maximum suppression amplitude of RIN exceeds 64 d B,and the RIN level is suppressed to-150 d B/in the frequency band of 75 k Hz to 50 MHz.In the 700 Hz?7 k Hz frequency band,the frequency noise is reduced by 2 orders of magnitude,and the minimum frequency noise is 0.15 Hz²/Hz.A combination of digital PID photoelectric feedback and Helium atom frequency modulation spectrum is used to achieve simultaneous suppression of low-frequency intensity noise and frequency noise.At 0.1 Hz,RIN achieves a maximum suppression amplitude of up to 32 d B,which is reduced to-102 d B/Hz.At 1 Hz,RIN also achieves an effective suppression of 20 d B.The frequency stability of 24 hours is improved by nearly 4 orders of magnitude,and the frequency instability is greatly reduced from 2.57×10^-6 to 4.22×10^-10.(4)Application of low-noise single-frequency fiber lasers:Using the polarization-dependent gain saturation effect of SOA,a laser encryption transmission scheme based on signal intensity control is designed.After applying intensity modulation to the laser signal,an intensity signal amplitude suppression of nearly 60 d B is achieved through signal encryption module.After 50km of optical fiber transmission,the signal-to-noise ratio obtained by direct measurement of light intensity is only 3 d B.While detecting with custom decryption module,the signal amplitude is increased by nearly 40 d B,and the basic state of the entire modulation signal is demodulated.A fiber strain precise measurement system via low-frequency-noise single-frequency fiber laser combined with fiber interferometer and phase generated carrier(PGC)modulation and demodulation technology is constructed.In the 30?200 Hz frequency band,the strain measurement sensitivity is improved by nearly 3 orders of magnitude from the original10 p?/Hz^1/2 magnitude to 10 f?/Hz^1/2 magnitude,which realizes a high-sensitivity optical fiber strain precision measurement.