日冕仪微弱杂散光的相干外差检测

Coronal observations provide means to study the evolution and propagation of coronal mass ejection (mainly the CMEs), streamers and solar wind almost from the source region on the solar surface. It plays an important role on comprehensive understanding of solar activity and space weather forecasting. Coronagraph is a key scientific instrument to continuously observe the solar corona in long-term. Such an instrument has never been built in China before, and the key technical problems need to be tackled.. In order to detect the weak corona above dozen solar radius field of view, the stray-light of coronagraph should be suppressed as low as 1e-13 of solar disk brightness or less. This is the strictest requirement on instrument optical and mechanical design, and stray-light testing. Here we propose a coherent heterodyne method for the extremely weak stray-light detection based on the lasted progress of laser feedback technology. The frequency of micro-chip laser output is shifted by acousto-optic modulator, and illuminated coronagraph as a testing source. The stray-light of key elements such as external occulter, and the whole instrument, could be coupled back to laser resonator, which can be amplified with 1e6 gain factor. Then the returned coherent light superposes with oscillating laser to modulate the light intensity. The modulated signal of laser is detected and combined with the driving signal of acousto-optic modulator, which can be distinguished with heterodyne technology. So in this way the feedback laser ( i. e., coronagraph stray-light) could be tested with sensitivity as high as 1e-12. As a crossing subject, this research could prompt the coronagraph independent development and could widen application scope of laser technology combining with solar observation.

日冕观测是研究日冕物质抛射的演化和传播、早期太阳风演化等科学问题的主要手段，对于深入理解太阳活动及空间天气预报至关重要。日冕仪是实现长时间连续观测日冕的重要科学仪器。目前我国的日冕仪研制还处于起步阶段，需进行关键技术攻关。.为观测视场达到十几太阳半径以上的日冕弱信号，日冕仪的杂散光相对于太阳亮度应低于1e-11量级，这对仪器的光机设计和各级杂散光的检测提出了非常高的要求。本项目结合激光回馈技术最新进展，提出了微弱杂散光的相干外差检测方法。微片激光器输出光经声光调制移频后，作为测试光源照射日冕仪，可使外掩体等部件或整机产生的杂散光耦合返回谐振腔，获得1e6放大因子，并与腔内光场相干叠加形成回馈，调制激光强度。探测激光光强并与声光调制信号进行外差检测，可实现对回馈光（即杂散光）高达1e-12量级探测灵敏度。该方法有助于提升日冕仪自主研制能力，促进激光技术与太阳观测技术的学科交叉应用。

太阳外冕的观测对人们全面理解太阳活动是极为重要的，特别是作为日球层和磁层活动的驱动源如CME（日冕物质抛射）及其他爆发现象，有可能引起地球上的空间灾害性天气。研制日冕仪在更大范围内持续观测日冕和CME对于太阳物理研究和空间天气预报非常必要。目前我国空间日冕仪研制还处于起步阶段，需要进行关键技术攻关。. 本项目针对外掩式空间日冕仪研制需求，完成了基于微片激光移频回馈的高灵敏检测技术研究。设计微片Nd:YVO4激光器，调整其振荡模式处于高增益、低噪声状态，搭建自动补偿环境扰动的光纤光路，实现对移频回馈光10e6增益系数的再放大效应，结合相干外差信号的相敏检波技术，可对空间日冕仪遮光部件低至10e-11量级微弱杂散光的检测。进一步在回馈外腔中引入相位调制，测试对弱光信号的动态响应，通过相位测量对表面发黑处理的外掩体进行非接触测量，得到其受环境因素影响的位置变动，分辨率达到nm量级。完成对空间白光日冕仪总杂散光的建模分析，数据处理后评估外视场的杂散光水平达到7x10e-12。. 在此基础上优化空间日冕仪的系统方案，对数据校准处理、微弱日冕信号提取进行研究，正论证搭载风云卫星项目。相关技术应用于国家重大科研仪器研制项目和子午工程二期新建光谱成像日冕仪项目，提高自主研制空间日冕仪的技术能力，为获取自主可控的科学数据做好技术储备。