Noise and Index Decomposition of Taiji-2 Interferometer System
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摘要: 空间引力波探测太极计划是中国科学院发起的mHz频段引力波探测任务, 由围绕太阳旋转的三颗卫星组成等边三角形, 星间距3×106 km, 通过激光差分干涉方法实现卫星间pm级位移波动测量. 作为太极计划“三步走”战略的承上启下的关键环节, 太极二号将全面验证太极计划的各项关键技术. 太极二号卫星构型与太极三号相同, 激光干涉系统作为核心测量手段, 需达到30 pm·Hz–1/2的噪声水平. 本文从顶层指标出发, 逐步分解各子系统噪声指标及关键参数, 包括激光器、干涉仪光学平台、相位计、望远镜、呼吸角补偿机构等. 研究结果可为太极二号后续工程任务的指标划分奠定理论基础.Abstract: The Taiji program for space-based gravitational wave detection is a mission initiated by the Chinese Academy of Sciences to explore gravitational waves in the mHz frequency band. It consists of three satellites orbiting the Sun in an equilateral triangle formation with an arm length of 3×106 km, employing laser interferometry to measure picometer-level displacement fluctuations between satellites. As a pivotal link in Taiji's "three-step" development strategy, Taiji-2 will comprehensively validate all key technologies of the program. The satellite configuration of the Taiji-2 satellite shares the same configuration as Taiji-3, with its laser interferometry system requiring a noise level of 30 pm·Hz–1/2 as the core measurement technique. This paper systematically decomposes the top-level requirements into subsystem noise budgets and key parameters, including the laser system, interferometer optical bench, phasemeter, telescope, and breathing angle compensation mechanism. The research findings establish a theoretical foundation for subsequent engineering task allocation in the Taiji-2 mission.
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表 1 太极计划顶层指标参数
Table 1. Top-level indicator parameters of the Taiji program
卫星 臂长
/m光功率/W 望远镜口径/cm 系统测距噪声/(pm·Hz–1/2) 加速度噪声/(m·s–2·Hz–1/2) Taiji-3 3×109 2~3 40 8 3×10–15 Taiji-2 3×109 2~3 40 30 3×10–14 表 2 Taiji-2干涉仪系统各功能指标参数
Table 2. Indicator parameters of Taiji-2 interferometer function
功能 参数 需求 星间测距测角 本地位移测量噪声/(pm·Hz–1/2) ≤10 本地角度测量噪声/(nrad·Hz–1/2) ≤10 星间通信
与绝对距离测量通信速率/(kbit·s–1) ≥20 距离测量精度/m ≤10 星间时钟噪声传递 噪声/(pm·Hz–1/2) ≤10 弱光锁相 噪声/(pm·Hz–1/2) ≤6 锁臂 噪声/(Hz·Hz–1/2) ≤10–4 表 3 Taiji-2测距噪声指标分解
Table 3. Indicator decomposition for ranging noise in Taiji-2
噪声类型 指标/
(pm·Hz–1/2)噪声模型 相关参数 数值 散粒噪声 6 $ {\tilde{x}}_{\mathrm{S}\mathrm{N}}=\dfrac{2\sqrt{hc}}{{\mathrm{\pi }}^{2}}\dfrac{{\lambda }^{\tfrac{3}{2}}L}{\varepsilon {D}^{2}}{{P}_{\mathrm{o}}}^{-\tfrac{1}{2}} $ Po激光输出功率/W 1.2 D望远镜口径/m 0.4 ε总光学效率 0.3 激光频率噪声
(TDI)10 ― 频率稳定度/(Hz·Hz–1/2) 30 TDI/(pm·Hz–1/2) 10 激光功率噪声 2 $ {N}_{1\mathrm{f}-\mathrm{R}\mathrm{I}\mathrm{N}}=\dfrac{\sqrt{2\left({P}_{\mathrm{m}}+{P}_{\mathrm{r}}\right)}}{\sqrt{{\eta }_{\mathrm{h}\mathrm{e}\mathrm{t}}{P}_{m}{P}_{\mathrm{r}}}}\tilde{r}\left(1{\mathrm{f}}_{\mathrm{h}\mathrm{e}\mathrm{t}}\right)\left|\mathrm{s}\mathrm{i}\mathrm{n}\left(\dfrac{\varphi -{\varphi }_{\mathrm{R}}}{2}\right)\right| $
$ {N}_{2\mathrm{f}-\mathrm{R}\mathrm{I}\mathrm{N}}=\sqrt{2}\tilde{r}\left(2{\mathrm{f}}_{\mathrm{h}\mathrm{e}\mathrm{t}}\right)\left|\mathrm{s}\mathrm{i}\mathrm{n}\left(\varphi -{\varphi }_{\mathrm{R}}\right)\right| $MHz-RIN/Hz–1/2 10–8 $ F=\dfrac{P}{c}\left(1+\rho \right) $ mHz-RIN/Hz–1/2 10–4 光程噪声 10 ― 光学平台光程噪声/(pm·Hz–1/2) 10 TTL噪声 15 $ {\alpha }_{\mathrm{T}\mathrm{T}\mathrm{L}}=\dfrac{{{x}}_{\mathrm{T}\mathrm{T}\mathrm{L}}}{{\theta }_{\mathrm{p}}} $ 光束抖动/(rad·Hz–1/2) 3 望远镜放大倍数 100 耦合系数/(m·rad–1) 5 超前指向噪声 5 ― 机构光程噪声/(pm·Hz–1/2) 5 相位测量噪声 5 ― 探测器等效光功率噪声、相位计测相
噪声、相位计模拟前端噪声/(pm·Hz–1/2)5 时钟噪声 10 $ {\tilde{\phi }}_{\mathrm{U}\mathrm{S}\mathrm{O}}=2\mathrm{\pi }\cdot {\tilde{t}}_{\mathrm{U}\mathrm{S}\mathrm{O}}\cdot {f}_{\mathrm{h}} $ 计时误差/(s·Hz–1/2) 4×10–13 差分频率/MHz 5~25 望远镜稳定性噪声 10 ― ― ― 激光指向噪声 10 $ \tilde{x}=\dfrac{\mathrm{\pi }d{D}^{2}}{2{\lambda }^{2}}{\theta }_{\mathrm{d}\mathrm{c}}{\tilde{\theta }}_{\mathrm{p}} $ 望远镜镜面平整度@1064 nm λ/20 指向静态偏差/nrad 30 指向抖动/(nrad·Hz–1/2) 30 杂散光 5 $ \Delta \varphi =-\sqrt{\dfrac{{P}_{\mathrm{s}}}{{P}_{\mathrm{o}}}}\Delta {\varphi }_{\mathrm{s}} $ 杂散光功率/nW 1 参考光功率/μW 100 结构稳定性/(nm·Hz–1/2) 1 呼吸角补偿光程噪声 5 ― ― ― 其他噪声 5 ― ― ― 总噪声/(pm·Hz–1/2) 29.8 -
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