光纤离轴抛物面反射准直器的准直误差特性分析

Analysis of Collimation Errors in Off-axis Parabolic Reflective Fiber Collimators

为明确输入光纤及其定位误差对光纤离轴抛物面反射准直器出射光准直特性的影响,采用光线追迹法,针对不同光纤的实测输出,进行仿真研究。结果表明,同样的准直器参数下,输入光纤纤径越粗,对旋转定位误差越敏感,输入光纤纤径越细,对位移定位误差越敏感。相同的反射焦距下,大离轴角、短母焦距准直器下出射光束指向更稳定;小离轴角、长母焦距准直器下出射光束的角度能量分布更稳定。研究结果为高精度法布里-珀罗定标源等角度能量分布敏感光学系统中的准直器参数设计和光机结构稳定性要求提供了参考。

In this paper,the influence of mirror parameters,fiber type and positioning error on the collimation quality of an off-axis parabolic reflective fiber collimator is simulated and analyzed.Compact off-axis parabolic reflective collimators with achromatic performance are widely deployed in optical systems requiring polychromatic illumination.For optical systems demanding a high level of long-term stability,fiber feeding allows for rigidification of the optical pathway,repeatable plug-in performance,and tight environmental control of the optomechanical assembly.However,light launched from the fiber end surface cannot be viewed as a perfect idealistic point source when high collimation quality is required.Different fiber core sizes,changes in far and near field energy distributions,and varying positioning errors may all affect the quality of the exit beam.From a geometric point of view,compared with axially symmetric transmissive fiber collimators,the response characteristics of the reflective off-axis parabolic collimator are more complex.Certain applications,with Fabry-Perot calibration sources for high-precision radial velocity observation and astronomical spectral calibration as a good example,are highly sensitive to changes within the angular energy distribution of the illumination condition.The radial velocity method is an important method for exoplanet detection.It measures the wavelength position changes over time of characteristic spectral lines in the star spectrum,possibly due to gravitational motions from accompany bodies.To enable detection of exoplanets residing in the habitable zone,it is necessary to achieve radial velocity measurement precision at m/s level or higher by means of simultaneous comparison with a high-precision calibration source.Fabry-Perot calibration sources have the advantages of broadband spectral coverage,uniform spectral line distribution,and robust performance for long-term continuous operation.When used in cooperation with traditional calibration sources such as thorium-argon hollow cathode lamps,it can significantly improve the wavelength calibration accuracy of high-resolution astronomical spectrographs.However,the Fabry-Perot calibration source requires the output spectrum peak to be maintained at an extremely high stability level(1 m/s change in peak position corresponds to 3.3×10−9 fractional drift in spectral line center).Faced with the challenges of wide operating band and extremely high spectral stability for high-precision radial velocity measurement,it is necessary to perform thermal-vacuum environmental control on the core Fabry-Perot etalon.Reflective off-axis parabolic fiber collimators are thus an ideal structure for coupling light into the Fabry-Perot etalon through the vacuum vessel.Yet,to the best of our knowledge,there is still a lack of relevant literature reports on the characteristics of collimation quality degradation caused by fiber positioning errors in those collimators.This paper navigates around the application requirements of the newly emerged Fabry-Perot astronomical spectral calibration system and jointly analyzes the experimentally measured fiber output results and the mathematical model of the off-axis parabolic mirror,to assess the response in collimation quality in regard to different system characteristics.Raytracing simulation studies are carried out in order to determine the influence of different input fibers and fiber positioning errors on the collimation characteristics of off-axis parabolic reflective collimators.It is derived that with the same collimator parameters,the larger the input fiber core diameter,the more sensitive it is to rotation positioning errors,and the smaller the input fiber core diameter,the more sensitive it is to shift positioning errors.Under the same reflective focal length,pointing of the collimated beam is less sensitive to fiber positioning errors when the collimator is of larger off-axis angle and shorter parent focal length,while the angular energy distribution of the collimated beam is less sensitive to errors when the collimator is of smaller off-axis angle and longer parent focal length.For applications that require higher parallelism and pointing stability of the outgoing light,an off-axis parabolic collimator with a large off-axis angle should be used under the condition of a certain reflective focal length.For applications that require higher stability of the angular energy distribution of the outgoing light,such as the Fabry-Perot spectral calibration system,off-axis parabolic collimators with small off-axis angles and long parent focal lengths should be selected within the allowable size constraints.For practical cases,system size,clear aperture limitations,machining and assembling tolerances,and other application-specific requirements also need to be considered.