Study On The Noise Of Narrow Linewidth Single-frequency Fiber Laser And Its Applications

Narrow-linewidth single-frequency fiber laser is widely used in optical atomic spectrum detection,optical coherent measurement and sensing,coherent combining,etc,because of its advantages,including compact all-fiber structure,low frequency noise,flexible wavelength selection.The noises of fiber laser have great impactions on its application systems,especially the frequency noise determines the measurement accuracy and distance in coherent detection apllications.This paper focuses on investigating the frequency nosise of fiber lasers and its appplications.First,the theory of frequency noise measurement is studied based on unbalanced Michelson interferometer.To this end,I build a broad bandwidth frequency noise mesasurement system.Then,I measure and analyze the characteristic of Phosphate distributed bragg reflector?DBR?fiber laser through the measurement system,and study the intensity noise influence on the frequency.The frequency noise suppression technology at the relaxation oscillation frequency and low frequecncy range are subsequently studied.Last,I study the frequency noise influence on optical coherent detection systems.In this thesis,I make the following achievements:1)Demonstrate a broadband frequency noise measurement method based on an unbalancedMichelson interferometer with a high-speed LiNbO₃ phase modulator,such that the maximum measuring bandwidth is improved by 2 orders of magnitude,which is espically useful for analyzing the fiber laser's relaxation oscillation frequency noise.From the theoretical analysis,I optimize the modulation frequency,detecting bandwidth and data acquisition bandwidth,and build a broadband frequency measurement system.The bandwidth is increased from 25 kHz to 2.5 MHz,and the minimum measurable frequency noise level is 33.9 Hz²/Hz@10 Hz,0.35 Hz²/Hz@10 kHz,and 0.56 Hz²/Hz@1 MHz.2)Demonstrate a method to improve broadband frequency noise measurement accuracy through suppressing the fiber laser's intensity noise based on SOA saturation effects.Its influence on frequency measurement system is largely suppressed.The frequency noise measurement accuracy at relaxation oscillation peak of Phosphate DBR fiber laser is improved from 64 Hz²/Hz to 31 Hz²/Hz,after suppressing its relative intensity noise to -155 dB/Hz.The measurement accuracy has also been doubled.3)First time demonstrate a method to precisely control the linewidth of fiber laser in a wide range by external noise injection.The linewidths of phosphate DBR fiber laser are availably controlled in a wide range from 0.8 to 353 kHz by using a low-pass filtered white Gaussian noise?WGN?signal with different signal amplitudes and different cutoff frequencies loaded to a fiber stretcher of an optical feedback loop.The linewidth controlling ratio?LCR?is defined to evaluate the ability of linewidth-controlling technique,such that LCR increases linearly with the enhancement of WGN signal amplitude?A_n?,while remaining the cutoff frequency?f_c?unchanged.LCR has the saturation effect referring to the cutoff frequency.4)Demonstrate a technology based on self-injection lock the DBR fiber laser to improve the SNR of Doppler velocity measurement.The velocity signal in fiber laser relaxation oscillation frequency noise could not be resolved and extracted.In the 10 km fiber delay velocity measurement system,the detection SNR is improved by 29 dB at relaxation oscillation frequency through sellf-injection lock technolony,which suppresses the DBR fiber laser frequency noise.The velocity signal submerged in relaxation oscillation frequency noise is extracted.The detection SNR is improved by 4 dB over the whole bandwidth.