Research On A Novel Silica Fiber Distributed Bragg Reflective Single-frequency Laser For Advanced Applications

As an important witness of the progress of human science and technology,laser has been involved in the frontier application from the very beginning.In particular,a family of high performance lasers,single-frequency lasers play an important role in many modern cutting-edge technologies,from the world's most sophisticated interferometer "LIGO" to the world's most accurate clock "strontium atomic light lattice clock".As the lasers with the narrowest line width and the lowest noise,single frequency fiber lasers are very important in many fields,such as laser radar,atomic molecular physics,terahertz generation,microwave photonics,quantum optics,coherent communication,and so on.The requirements on the performances of single frequency fiber lasers vary with the applications.For example,in the field of LiDAR,a single frequency fiber laser with pulsed output is required and the line-width is not a concern.In the field of gravitational wave detection and atomic and molecular physics,a single frequency fiber laser with narrow linewidth and low noise is normally demanded and pulsed output is not necessary.In the field of terahertz and microwave photonics,the phase relation between the single-frequency laser output becomes the major concern.The current mainstream commercial single frequency fiber lasers are composed of distributed Bragg Reflection(distributed Bragg Reflector,DBR)structure or distributed feedback(distributed feedback,DFB)structure,and therefore the laser is compact and provides stable output with kHz-level linewidth.But these commercial products also provide less comprehensive functions and parameters,and thus cannot satisfy the variety of requirements from many applications.This thesis aims to study universal and reliable single frequency fiber lasers for meeting the needs of various frontier applications.DBR structure is chosen for constructing the single frequency fiber laser,due to the advantages of simple structure,good versatility and high energy conversion efficiency.The commercial DBR single frequency fiber lasers are typically based on low-melting-temperature soft glass with high rare-earth doping concentration.Instead,in this work commercial rare-earth doped silica fibers are selected as the gain medium.Although problems such as self pulse and gain insufficiency might occur in the case of the doped silica fibers,doped silica fiber will not suffer from the problem from the large temperature difference between doped soft glass fiber and passive silica fiber when splicing them together.Thus the configuration of all-silica DBR single-frequency fiber laser provides excellent mechanical robustness and good thermal stability,decent environmental adaptability,and clear convenience to be realized.What is more,doped silica fibers are widely commercially available.On the basis of the single-frequency all-silica fiber laser,I have investigated three types of single frequency fiber lasers,i.e.,(i)low-noise,(ii)pulsed,and(iii)dual-wavelength single frequency fiber lasers,in order to meet the needs of different frontier fields.The specific research work of this paper includes the following parts:1.Theoretical analysis of DBR Single frequency fiber laser and the fabrication of the key devicesFirstly,by studying the spectral characteristics of different doped ions in silica fibers,the optimization of the gain fiber and pumping scheme can be decided for achieving the highest gain.It is known that the relaxation oscillation process plays the important role in the origin of the self pulsing.The rate equation is established to solve the time-dependent photon number in the relaxation oscillation process.It is known that the strength of relaxation oscillation can be suppressed by increasing the photon lifetime in the cavity and the pump power,and consequently the self-pulsing can be suppressed fundamentally.Utilizing this conclusion,simulation model of the fiber Bragg grating Reflection spectrum shows when the highest reflectivity of the fiber Bragg grating for output coupling is 85%~90%,together with the single-longitudinal fiber grating with high reflectivity of 99%,self pulsing can be restrained and stable continuous-wave(CW)single frequency fiber laser can be realized2.All-silica fiber CW DBR single frequency laser for gravitational wave detectionBy using the fiber Bragg gratings with the optimized parameters at different wavelengths,1-?m,1.5-?m and 2-?m stable CW single frequency fiber lasers have been realized respectively.None self pulsing has been observed in these lasers,indicating the validness of the above theoretical analysis.Although the gain fibers used here are ordinary commercial doped silica fibers with relatively low rare-earth doping concentrations and low gain coefficients,laser oscillation has still be obtained by FBG made in house.The line-widths of these fiber lasers are all less than 10 kHz.Cooperating with teams overseas,we have obtained the output of 160 W experimentally in the high-power 2-?m single-frequency fiber laser for next-generation of gravity-wave detection.A 5W 2-?m single frequency fiber laser has been constructed overseas for early-stage tests.3.High-power low-noise single-frequency fiber lasers for high-precision detectionIn this part,I have analyzed the origins and characteristics the intensity noise and the frequency noise of single-frequency fiber laser.The intensity noise and the frequency noiseBy using passive optical feedback loop,the intensity noise and frequency noise of the existing single frequency fiber lasers have been constrained and the laser linewidth is compressed.The relative intensity noise of the relaxation oscillation intensity has been reduced from-99.9db/hz @ 993 kHz to-119.4db/Hz @ 192 kHz,the frequency noise intensity decreases by 30 dB in the range of 10 kHz and 100 kHz,and the laser linewidth is compressed from 3.96 kHz to 540 Hz.When the signal is amplified to 10 W,no obvious increase has been seen on the intensity noise and the frequency,while the laser linewidth is maintained less than 1kHz.4.Integrated gain-switched single frequency fiber lasers for LidarAn integrated pulsed single frequency fiber laser with tunable repetition rate is realized by using gain-switching technology.The 975 nm single-mode semiconductor laser with electrical modulation is used as the pump source.The pump is composed of the pulsed component and the DC component simultaneously.The DC component is set slightly below the threshold,in order to minimize the requirement on the pulsed pump energy.Under low pump power,pulsed single-frequency laser output has been achieved with the pulse width of 150 ns,and linewidth of 14 MHz.The repetition rate can be tuned from 10 kHz to 400 kHz,through external trigger signals.5.Polarization-maintaining dual-wavelength single-frequency fiber lasers for high-frequency microwave generationIn this part,dual wavelength single frequency laser is demonstrated by using a polarization-maintaining fiber based overprinted Bragg grating as a dual wavelength selecting component.The two longitudinal modes are at the same polarization state.The extinction ratio of the polarization states is greater than 20 dB for both longitudinal modes,and the spectral signal-to-noise ratio is greater than 60 dB.Since longitudinal modes share the same optical path in the cavity,the two longitudinal modes have high coherence and the same noise components.In the process of microwave generation by frequency beating,the noise components from each longitudinal mode are largely offset,resulting into low phase noise and high stability microwave signal.The central frequency of the microwave signal is located at 28.4474 GHz.The spectral signal-tonoise ratio is seen higher than 65 dB.The spectral signal line is measured to be 500 Hz.And the standard deviation of frequency fluctuation within 1-hour operation is observed to be 58.592 kHz.