# A Spaceborne Mounting Method for Fixing a Cubic Fabry-Perot Cavity in Ultra-Stable Lasers
## 3 Experimental Results and Discussion  
To verify the feasibility of the developed FP cavity for use in space applications, aerospace environmental tests were carried out at Hangzhou Saibo Mechanics Environment Test Co., Ltd. (Gongshu District, Hangzhou, China), since the alignment requirements of the high-finesse FP cavity are extremely high. Additionally, since the ULE glass was fixed inside the vacuum hose, it was difficult to judge whether the FP cavity was offset or not. Therefore, optical systems were attached to the vacuum hose, both in front of and behind the FP cavity. Characterizing the optical parameters of the whole system (WS) can reveal information about the FP cavity. Figure 3 shows the optical systems in front of and behind the FP cavity. Figure 3a illustrates how the laser beam is aligned with the FP cavity in the optical system. A collimating lens (CL) first collimates the laser beam using a polarization-maintaining single-mode (PMSM) fiber (PM630-HP, Coherent). Since the laser radiates from the PMSM fiber, it possesses a mode distribution of TEM\({}_{00}\) and an \(M^{2}\) of less than 1.1 [22]. Besides collimation, the CL can also guarantee a good mode-match between laser beam and the FP cavity. After collimation, a 90:10 beam splitter (BS1) is used to split the laser beam. A 10% proportion of the laser beam is reflected into a photodiode detector (PD1) to monitor the laser power. The transmitted laser beam is then precisely aligned by four wedged prisms (WP1 and WP2 each hold two prisms). The four wedged prisms all have a wedge angle of 0.1\({}^{\circ}\). The rotation of each wedged prism leads to a fine adjustment of the laser-beam pointing. Subsequently, the laser is transmitted through a polarization beam splitter (PBS) and a quada-wave (QW) plate that is aligned perpendicularly to the center of the FP cavity. A photodiode detector (PD2) collects the reflected laser beam from the FP cavity to monitor error signals during laser frequency tuning. When the laser frequency is consistent with the resonance frequency of the FP cavity, the laser beam will be transmitted through the cavity. Figure 3b shows the optical system used to collect the transmitted laser beam. As shown in Figure 3b, the transmitted laser beam is split into two parts by a 50:50 beam splitter. The charge-coupled device (CCD) can monitor the mode distribution of the transmitted laser beam, whereas, the photodiode detector (PD3) can monitor the amplitude of the intensity of the transmitted laser beam. All of the optical elements in the optical systems, both in front of and behind the FP cavity, are glued to the titanium mirror mounts. Theses mounts are then screwed and glued to two titanium substrates (in front of and behind the FP cavity). Two substrates are screwed and glued to the vacuum hose. Thus, there is no spring-adjustment structure anywhere in the optical system. Two vibration tests, including sinusoidal and random tests, were carried out. The parameters of the above-mentioned tests are shown in Table 1. All of these parameters are based on the Long March 5 rocket platform [23]. It should be noted that the unit of the amplitude of the sinusoidal vibration is different in different frequency ranges. When the vibration frequency is in the range of 4-10 Hz, the unit \"mm\" is used to describe the peak-to-peak distance of the sinusoidal vibration, whereas the unit \"g\" is used to  
Figure 3: Optical systems (**a**) in front of the FP cavity and (**b**) behind the FP cavity. describe the acceleration of the sinusoidal vibration in other frequencies. The parameters shown in Table 1 are stricter than the actual conditions during a rocket launch so as to ensure the reliability of the payloads. Sinusoidal and random vibration tests were carried out sequentially. During each vibration test, the FP cavity was tested on three mutually perpendicular axes (X, Y, and Z). Figure 4 shows the WS (including the FP cavity, vacuum hose, ion pumps, and optical systems in front of and behind the cavity) during the vibration tests.