| Taiji space gravitational wave detection plans to detect gravitational wave signals with frequencies ranging from 0.0001 Hz to 0.1Hz by building a constellation of space satellites.an interferometer with the arm length up to three million kilometers.Each satellite consists mainly of the telescope,an optical interferometer platform and test mass.The distance change of picometers between test masses is monitored by laser interference signals,so as to reflect the gravitational wave signal.Among them,the space gravitational wave telescope transmits the local laser signal to the distant satellite while receiving the laser of the distant satellite.Therefore,space gravitational wave telescopes need to meet the beam expansion and beam shrinkage of laser signals at the same time.After the remote laser signal is transmitted over millions of kilometers,the power attenuated to the order of 100 picowatts.Since the telescope directly faces the interferometer,the backward scattered light generates when the laser is transmitted through the telescope and will be recoupled to the interferometer platform while receiving the laser signal and forms interference noise.On the other hand,in the complex space environment,spacecraft will inevitably be subject to jitter,temperature gradients,and stress release.The optical path between the telescope and the optical interferometer stage needs to meet stability requirements on the order of picometers.This is extremely challenging both engineering and scientifically.The generation of stray light by telescopes is a dynamic process and results in coherent noise which simultaneously exist amplitude and phase modulation of the spacecraft’s local interference signal.Therefore,the stray light generated by the telescope must be reasonably controlled.Otherwise,the signal laser will be obliterated in the stray light generated by the telescope.This paper studies stray light produced by space gravitational-wave telescopes.Firstly,in order to understand the mechanism of stray light noise and quantify the influence of stray light generated by space gravitational wave telescopes,this paper studies the coupling mechanism of stray light in space gravitational wave telescopes based on the current research status.The influence of backward coherent stray light on the sensitivity of local interference signals and gravitational wave signals is derived.The currently existing calibration and measurement methods of the main coherent stray light are introduced and compared.According to the requirements of the project,the evaluation of forward incoherent stray light and backward coherent stray light of the telescope is realized by modeling the ray tracing of the telescope.During the fabrication of the telescope mirror,the optical surface inevitably has roughness,scratches or different degrees of particle pollution.Therefore,the optical mirror cannot perfectly reflect the incident light,and the optical system inevitably produces scattered light.Generally,for optical mirrors used in general optical systems, the effect of scattering effects is less than that of low-order-path stray light,which is negligible.However,due to the particularity of stray light requirements for space gravitational wave detection,the scattered light generated by the optical surface must be strictly controlled.Therefore,this paper conducts a surface study of the sample of the telescope principle.The sample studied was a glass-ceramic substrate,which has good thermodynamic stability.To increase reflectivity near the operating wavelength,the substrate material is coated with enhanced reflection and protected with a silica film.Topography and scattering measurements were carried out,respectively.The results of the various measurements are ultimately linked by the power spectral density.In order to evaluate the effect of the amplitude of backward coherent stray light on the amplitude and phase of the interference signal,the scattered light produced by each optics in the field of view is evaluated.On the other hand,the mechanism of optical scattering transport is studied in detail,and the scattering transport equation for optical system considering single and multiple scattering events is established.Aiming at the problems of low scattering calculation efficiency,long time consumption and high failure rate of complex optical systems,this paper proposes a stray light calculation method based on the evaluation of the transmission performance of optical systems on the basis of studying the relationship between ray transfer efficiency,optical transfer efficiency and optical throughput in optical systems,and analyzes the transmission behavior of scattered light in optical systems.Based on this method,stray light caused by roughness and freefalling particles on the optical surface of the telescope is evaluated.The results show that under the current level of processing and pollution control,the flux of backward coherent stray light generated by the space gravitational wave telescope is slightly greater than the index requirements.Based on the telescope performance of stray light,optimization strategy is given from the perspective of surface processing and optical layout.Through this paper,the influence of stray light of space gravitational waves on interferometry can be understood.The characterization method of superpolished surface and the generation mechanism of scattered light are demonstrated.Taking the current state of processing and pollution control as an example,the complete process of surface morphology to light scattering to ray tracing is established,and the characterization of stray light level of space gravitational wave telescope is completed,which provides technical and theoretical basis for the control of stray light in space gravitational wave telescope.Scientifically,this paper shows the transport mechanism of scattered light and shows how to evaluate the scattered light contribution of known paths at very low light transmission efficiency. |
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