Significance Ultrastable space laser with an ultranarrow linewidth,ultralow noise,and high frequency-stability is widely used in important space programs such as space gravitational wave(GW)detection,space cold-atom physical experiments,and space laser remote sensing.For example,in space GW detection applications such as the Laser Interferometry Space Antenna(LISA),Taiji,Tianqin,and DECi-Hertz Interferometer Gravitational-wave Observatory(DECIGO),to obtain extremely weak GW signals from the several-million,long-distance,free-floating,test-mass-based optical interferometer,extremely stringent requirements must be imposed on the ultrastable laser specifications.The linewidth should be below 1 Hz,with the frequency stability below 10 1 and the frequency noise below 30 Hz/√Hz.Additionally,because the abovementioned interferometer is to be used in space,the laser must withstand the harsh space environment such as vacuum,vibrationsrough,temperatureextreme,and radiationshigh-dose.Furthermore,the laser should feature a small volume,low power consumption,and a low weight to reduce the flight budget.Because of these stringent requirements on ultrastable space laser,researchers worldwide are focusing on its technological development.In 2003,the Shanghai Institute of Optics and Fine Mechanics(SIOM)of Chinese Academy of Sciences(CAS)began developing space-grade laser.In 2007,China's first space-grade laser,i.e.,the ChangE-1 laser,which is a pulsed high-energy solid-state laser,was successfully launched.In 2008,SIOM began developing ultrastable,ultralow-noise,and high-stability continuous space wave lasers.In 2016,China's first space-grade ultrastable long-life laser was successfully launched simultaneously with the FY-4 weather satellite,which has been operated for 7 years and counting.This paper summarizes the technical operating principal,test results,and onboard performance of these ultrastable lasers.Progress The FY-4 satellite is a weather-forecast satellite that uses a Fourier-transform infrared spectrometer(FTIR)to measure the Earth's spectrum.To calibrate the frequency of the spectrometer,a laser with a stability level of 2×10-6and 5 years lifetime is required.To fulfill these requirements,two 852 nm DFB diode lasers with cold redundancy are used as the laser source.A caesium cell is used as the 852 nm frequency reference.Low-frequency dithering is applied to the current of the diode laser.Using the 852 nm frequency-absorption peak of caesium,the frequency of the laser is locked to the center of the peak and stabilized.After 7 years of onboard operation,the laser is still operating favorably,with less than 1％power degradation.The DQ1 satellite is an integrated path-differential absorption(IPDA)LiDAR system used to measure the global CO2 concentration both day and night for a global-warming project.It demands extremely stringent requirements on the frequency stability of the"on"and"off"laser source,i.e.,10-10 with 8 years lifetime.A 1572 nm DFB diode reference laser is frequency locked to the center absorption peak of CO2 at approximately 1572.018 nm via high-frequency modulation.Meanwhile,the"on"and"off"diode lasers are frequency locked to the reference laser using the optical phase-lock loop(OPLL)technique separately at frequency separations of 760 MHz and 8.08 GHz,respectively.Because of the low absorption strength of CO2 at approximately 1572 nm,a multipass cell was successfully constructed,which featured an absorption length of 10 m,a hermicity of 5×10-10 Pa·m3·s-1,and weight of 5.3 kg.The final frequency stability is approximately 1×10-11@10000 s.The DQ1 laser was successfully launched in April 6,2022 and has been operating favorably hitherto.The Cold-Atom Physics Rack(CAPR)is a science platform that has been successfully launched and deployed on the China's Space Station on October 31,2022.Researchers are planning to use rubidium and potassium in the CAPR to achieve the Bose Einstein condensation(BEC)and Fermi degeneration on the space station.The CAPR laser module provides a cooling,repumping,and detection laser with 10-11 frequency stability and agile frequency tenability for experiments.Three 1560 nm DFB diode lasers are amplified separately using a master oscillator power-amplifier(MOPA)and their separated frequencies are doubled using three PPLN crystals,which are used as the cooling,repumping,and detection laser with 900 mW output power.The frequency of the repumping laser is locked to the saturated absorption peaks of rubidium and potassium based on the modulation transfer spectrum(MTS).The cooling and detection lasers are locked to the repumping laser via the OPLL technique.The final frequency stability is approximately 2×10-12@1000 s.The Taiji program is a space GW-detection program proposed by CAS.Three phases has been proposed and included in the roadmap.The first is the Taiji-1 mission,which is a single satellite for testing key techniques for GW detection such as ultrastable lasers and interferometers.The key component of the Taiji-1 laser source is a nonplanar ring oscillator(NPRO)solid laser.The frequency and power noises of the laser source are improved significantly by applying precision-driving current control and temperature control.Frequency noise measuring 0.1 MHz/√Hz@0.1 Hz and power noise measuring 0.02％@0.1 Hz are obtained.In order to improve the frequency stability of the laser to meet the requirement for the ongoing Taiji-2 mission,a ultra stable fiber based on frequency stabilization technique is proposed and demonstrated successfully with 6 Hz/√Hz@10 mHz frequency noise,3×10-15@1 s,5×10-15@100 s frequency stability which is enough for the Taiji-2 mission.Conclusions and Prospects A series of ultranarrow linewidth,low-noise,and high frequency-stability ultrastable space lasers has been successively constructed at SIOM and used in important applications such as space GW detection,the FY weather satellite,and the Chinese Space Station.The onboard-laser frequency stability improved significantly from 10 7 to 1012,as well as to 10-15 with a linewidth of less than 1 Hz.The frequency stability of ultrastable space lasers shall be improved continuously to satisfy the major requirements of space programs.
space laserstable frequencyultra-narrow linewidthfrequency noise
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