| In 2016,American Laser Interferometer Gravitation Wave Observatory(LIGO)first directly detected the gravitational waves(GW)from mergering black holes,opening a new window to the universe.At present,the main detection range of Advanced LIGO(the second generation)is 10Hz-10k Hz and the detection sensitivity is 10-23/?Hz,which can only realize the observation of a fraction of events in the visible universe.In the future,gravitational wave detectors(GWDs)will be dedicated to detect the GW from the convergence of supermassive binary black holes,binary neutron star convergence,star collapse and supernova evolution.It is necessary to improve the detection sensitivity and detection range of GWDs.Cryogenic silicon has very high thermal conductivity,which can effectively reduce the thermal noise of the detection system.Therefore,the third generation of GWD,LIGO Voyager,plans to upgrade the existing silica mirror,with a switch to silicon mirror.Then the GWDs can achieve detection range of 1Hz to 10k Hz,and the strain ensitivity is improved to 10-24/?Hz.Compared with the laser source at 1.064?m and 1.5?m band,the absorption of cryogenic silicon at 2?m band is lower and the laser scattering loss is smaller.Therefore,higher in-cavity circulating power can be obtained and higher detection sensitivity can be achieved.The noise level of the laser is another key index related to the sensitivity of the whole detection device,which determines the overall noise level of the system.Then it is necessary to develop an ultra-low noise high power 2?m single-frequency laser.This thesis aims to research on ultra-low noise high power 2?m single-frequency fiber laser applied to the third generation of the GWDs.Based on the analysis of the laser's noise source,by suppressing the noise of 2?m single-frequency fiber laser,the high power 2?m single-frequency laser laser with ultra-low noise(@1)1)=40Hz?400Hz?1)1)>2.1MHz)can be achieved.After that we complete the opto-mechatronics package,and realize the relevant laser prototype development.By cooperating with the university of western Australia,the application of the prototype in the third generation GWDs is realized.The specific research work of this paper is mainly divided into the following aspects:1.Research on noise mechanism of single frequency fiber laserBased on the rate equation and the self-thermal noise model,the theoretical models of the 2?m single-frequency fiber laser's intensity noise and frequency noise of the pumped intensity noise coupled are established respectively,which without considering the influence of external noise,such as thermal noise,vibration noise,acoustic noise and electrical noise.The intensity noise and frequency noise of the 2?m single-frequency fiber laser are calculated by using the established theoretical model.The calculation results show that the pump intensity noise is the main noise source of 2?m single-frequency fiber laser without the external ambient noise.In addition,the characterization methods of single-frequency fiber laser's intensity noise and frequency noise are analyzed,and the noise measurement methods are further studied and verified by the corresponding experiments.2.Research on low noise 2?m single-frequency fiber lasers based on different pumping modesBased on the above analysis results of noise mechanism,the intensity noise characteristics of three different types of light sources,namely 786nm laser diode(LD),1550nm single frequency laser(SFL)and 1550nm amplified spontaneous emission(ASE),are analyzed.The 2?m DBR single frequency fiber laser is pumped as the three pump sources,and comparing the intensity noise and frequency noise of 2?m single frequency laser with the theoretical simulation results.Finally,it is proved that pump intensity noise is the main source of 2?m single-frequency laser noise without considering environmental disturbance,such as mechanical vibration,temperature fluctuation,circuit noise,etc.Therefore,selecting appropriate pump source can effectively suppress single-frequency laser noise level.Comparing the three light sources's noise level,and considering the 2?m single frequency fiber laser's long-time power stability,the 1550nm SFL is selected as pump source,the low noise 2?m single frequency laser output can be realized,and frequency noise is less than100Hz?Hz@1)1)>100Hz.3.Research of 2?m single-frequency fiber laser with ultra-low noise and high powerIn order to further obtain an ultra-low noise 2?m single frequency laser output,an acoustic optical modulator(AOM)laser power feedback loop is used to suppress the intensity noise of 1550nm SFL,and the intensity noise reduction of more than 5d B(@1)1)=1Hz?50k Hz)was obtained.At the same time,the intensity noise of the 2?m single-frequency fiber laser has achieved the maximum 15d B@1)1)=10Hz noise suppression,and the intensity noise level is close to the detector limit(@1)1)=40Hz?400Hz).Meanwhile,the intensity noise level of the 2?m single-frequency fiber laser near the shot noise limit is obtained(@1)1)>2.1MHz).And the frequency noise can be suppressed with maximum 8.4d B(@1)1)=4k Hz?5k Hz),which is less than100Hz/?Hz@1)1)>13Hz.Using this ultra-low noise laser as seed source,combining with low-noise polarization-maintaining fiber amplifier technology,under the condition that the laser frequency noise level remains unchanged,the output power can be greater than 5.2W,the power fluctuation is less than 0.4%@1h,and the linewidth is less than5k Hz.4.Frequency sweeping characteristics of 2?m single-frequency fiber laserThe experimental study of 2?m single-frequency fiber laser is carried out by using PZT modulation and pump-induced photorefractive index change.Firstly,the laser is swept by PZT modulation mode,and the single-frequency laser output with frequency sweeping range of>920MHz,sweeping speed of>20k Hz,frequency response of>45MHz/V and frequency sweeping band-width of 2k Hz is obtained.Secondly,the pump-induced photo-induced refractive index change is used to realize laser frequency sweeping,and the single-frequency laser output with frequency sweeping range>690MHz,sweeping speed>5k Hz,frequency response>10MHz/V and frequency sweeping band-width 100Hz is obtained.Both schemes can achieve laser frequency locking on the cavity.However,due to the low sweeping band-width and low frequency response of photorefractive index mode,this method is difficult to achieve long-term frequency locking.Finally,PZT mode is selected as the sweeping mode of2?m single-frequency fiber laser.5.Development of engineering prototype of 2?m single-frequency fiber laser and application of GWWe have completed the packaging and integration of the 2?m single-frequency fiber laser prototype with the self-designed packaging structure of sound insulation and vibration isolation,low noise pumping circuit and high precision temperature control system.The noise level of the prototype in low frequency band is obviously better than that of the solid-state laser scheme used by LIGO,and the intensity noise and frequency noise of the prototype are reduced by the maximum of 27d B@1)1)=10Hz and11d B@1)1)=20Hz,respectively.Moreover,the performance of the prototype is also significantly better than that of the same type of commercial products from Ad Value Photonics,and its frequency drift within one hour is 0.7%of that of the commercial products.In addition,in collaboration with the centre of excellence for gravitational wave detection from University of Western Australia,a 2?m single-frequency fiber laser prototype has been successfully locked on a cavity with high precision(Q=4×10~8)in the Pound-Drever-Hall frequency stabilization system,which proves the feasibility of the application of the ultra-low noise 2?m single-frequency fiber laser in the GWDs. |
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