强天光背景下基于钠导星的大气波前像差探测研究【增强内容出版】
Adaptive optics (AO) applied in compensation for atmospheric turbulence usually requires a sufficient guide star (GS) in the isoplanatic patch around the interesting object to provide accurate information of the wave-front distortion induced by atmospheric turbulence. However, as there are not enough bright and available natural guide stars (NGSs) in the sky, the concept of a sodium laser guide star (LGS) has been publicly proposed for overcoming the limitations due to the finite sky coverage of the observation telescope with AO, which is generated by resonance scattering from the sodium atoms in the mesospheric layer with a ground-based projected laser with a wavelength of 589 nm. Due to sufficient sampling of the atmospheric turbulence at high altitude, the concept of a sodium LGS has been receiving huge attention from the moment it was proposed and was first to be applied in the field of high-resolution astronomical observation through atmospheric turbulence with AO. However, due to the limitations of the excitation efficiency of LGS lasers and the sodium column density of the mesospheric layer, the actual brightness of the generated sodium laser guide star is limited. Therefore, so far, in the field of astronomical observation, almost all sodium LGS AO systems have to operate at night, so their operation hours have been greatly limited. Under daytime conditions, the effectiveness of sub-aperture segmentation wavefront centroid detection using a Hartmann-Shack (HS) sensor with weak photon returns from sodium LGS is challenging, due to the fact that the intensity of the skylight background can reach several thousand times that of the sodium LGS. The objective of this paper is to develop a reliable and practical atmospheric turbulence wave-front sensing technique for sodium LGS, which can provide a certain theoretical reference and engineering experience for daytime applications of sodium LGS AO systems in the future.
Based on the aforementioned purpose, by combining theoretical analysis, parameter design, component development, system integration and detection experiment, an atmospheric wave-front active sensing technique for sodium LGS during daytime has been investigated in this paper. Our guiding ideology is to use spectral filtering, spatial filtering, and temporal filtering to match and suppress strong stray light interference while making an effort to maintain the photon returns from the sodium LGS at the wavelength of 589 nm. The above mentioned strong stray light interference includes the skylight background and Rayleigh back scattering at 589 nm of the atmospheric molecules. Firstly, based on the optical spectrum distribution characteristics of the skylight background, the feasibility of spectral filtering for a high optical transmittance with a nanometer scale line-width and a 589.16 nm center wavelength is analyzed. Secondly, based on the field of view (FOV) distribution characteristics of the skylight background, the feasibility of spatial filtering for accurately matching the FOV of the sub-aperture for the HS sensor is analyzed. Finally, in combination with the universal dual telescope mode for pulsed sodium LGS laser projection/sodium LGS photon return detection, the mathematical expressions of important parameters such as the duration of the pulsed resonance sodium LGS scattered return-light ΔtNa(E), the suppression duration of the Rayleigh scattered light from low altitudes ΔtRayleigh-Stop(E) and so on are derived, which constitute the theoretical foundation of our temporal filtering. Based on the above analysis and combined with the construction of an experimental system, parameter design, detection ability estimation, and component development are carried out for our synthetic filtering and applied in traditional HS sensors. A daytime atmospheric wave-front detection experiment for sodium LGS is carried out.The experimental results are in good agreement with those of the theoretical analysis.
Due to the demand for atmospheric wave-front detection for sodium LGS AO during daytime, an active wavefront sensing technique with synthetic filtering (namely, spectral filtering, spatial filtering and temporal filtering) is proposed in this paper. The detection ability after synthetic filtering is estimated, it can achieve effective atmospheric wavefront detection of an equivalent of 7-magnitude brightness sodium LGS under typical 12 W/(m2·sr) skylight background conditions (Table 1). Compared with traditional sodium atom filtering (Table 2), it has advantages regarding the equivalent photon returns maintained and the signal to noise (SNR) ratio for sodium LGS in an HS sensor. Based on this technique, a real-time detection of atmospheric wave-front distortion with sodium LGS under 10 W/(m2·sr) skylight background conditions is achieved (Fig.11), which is a beneficial attempt for daytime atmospheric wave-front detection with sodium LGS.
In response to the demand for atmospheric wavefront distortion detection with sodium LGS under strong skylight background, an active wavefront detection technique with synthetic filtering is proposed and investigated in this paper. Synthetic filtering is used to match and suppress strong stray light interference while making an effort to maintain the photon returns from sodium LGS. The theoretical analysis, parameter design and detection capability estimation for this technique are discussed. Then, using this technique, atmospheric wave-front distortion detection for AO using sodium LGS is carried out experimentally. When the brightness of the sky background is 10 W/(m2·sr), the atmospheric wave-front distortion is effectively detected based on the pulsed sodium LGS in real-time. This work is helpful in expanding the working period of the sodium LGS AO systems.
1 引言
用于实时补偿大气湍流光学波前畸变的自适应光学(AO)通常需要信标来提供待补偿目标光路的波前畸变信息,然而,实际科学目标等晕区域内可用自然亮星的数量比较有限[1]。在以自然亮星为信标的AO望远镜天空覆盖率受限的情况下,人们提出了由可控天空指向受激产生的大气钠层原子D2线共振散射(589.15xx nm)钠导星,其具有海拔高(位于80~100 km)以及对大气湍流(集中于32 km以下)采样充分的优势,一经提出便受到了国内外的广泛关注[2-3],并率先在夜间天文观测成像校正中获得实际应用[4-5]。然而,受到钠导星激光器与大气钠层原子相互作用激发效率的限制,实际钠导星的亮度十分有限。就目前来看,受激产生的单颗共振散射钠导星亮度通常不超过7等星[6],从而在很大程度上限制了钠导星AO望远镜的有效工作时段(只能选择在夜晚)。如果能够将其工作时段扩展至晨昏甚至白天,在探测背景强且变化迅速、子孔径内强度分布不均匀的条件下,实现基于暗弱钠导星回光信号的大气湍流波前畸变探测补偿,对于钠导星AO的工程应用具有重要意义[7]。据笔者所知,目前国内外尚无钠导星AO应用时段扩展的公开研究报道。
国内外对强天光背景下的AO应用进行了大量探索。在国外,美国国家太阳观测站的Beckers[8]基于超窄光谱滤波技术对白天AO应用的波前探测方案进行了初步探索与分析,美国加州理工大学的Wilson等[9]也对白天AO应用的深空激光通信进行了初步尝试。然而,据笔者所知,没有公开的报道显示上述探索性工作在实际系统应用上取得了成果。在国内,中国科学院光电技术研究所一直致力于拓展AO应用领域、工作效率、工作时段等方面的理论及实践工作。如,李超宏[10]提出了两类视场偏移哈特曼波前探测技术,该技术可用于实现强天光背景下针对目标本体回光的被动式大气湍流波前探测。其中:基于分光棱镜的视场偏移方式会导致对目标进行有效探测的谱段发生能量分光,而基于扫描振镜的视场偏移方式会导致在天光背景测量条件下对目标的波前探测与补偿出现间歇性中断。面对极其有限的钠回光光子数,上述视场偏移波前探测技术并不能满足强天光背景下针对脉冲型钠导星的主动式大气湍流波前探测的应用需求。
基于上述应用需求,笔者开展了强天光背景下针对暗弱钠导星的大气湍流波前畸变探测研究:提出了一种采用谱域+空域+时域综合滤波抑制天光背景与同波长瑞利散射干扰的主动式波前探测技术,完成了对该技术的理论分析、参数设计、部件研制及其与传统哈特曼波前探测器的系统集成。基于该技术,在白天较强的天光背景条件下,实现了基于钠导星的大气湍流波前畸变像差的实时探测,并对钠导星AO应用的工作时段扩展进行了初步尝试。
2 理论分析
强天光背景光谱覆盖了暗弱钠导星回光的波长,而具有极窄线宽(35 Å)但中心波长能量利用率偏低(>20%)的钠原子滤光技术,在抑制天光背景的同时会导致钠导星回光光子数发生极大损失,无法满足实际应用需求。鉴于此,笔者提出了谱域+空域+时域综合滤波的技术思路,在抑制天光背景与瑞利散射的同时,极力保持钠导星波长的能量利用率。
首先,在谱域滤波方面适当降低线宽要求,以极力保持钠导星波长的能量利用率,通过匹配钠导星中心波长抑制天光背景。目前,接受商用定制的589 nm中心窄带干涉滤光片可以做到以下指标:中心波长为589.16 nm,中心波长峰值透射率为94%,透射线宽为0.5~1.0 nm(≥50%峰值透射率对应的谱线宽度)。该高透射线宽完全覆盖了钠层原子D2线荧光光谱(589.1570~589.1605 nm)的范围[6]。在实际应用中,为了兼顾对自然亮星信标的大气湍流波前探测能力,一般哈特曼波前探测光路的光谱会覆盖可见光波段。因此,理论上谱域滤波通过高效匹配钠导星中心波长来抑制其余天光背景波段对波前探测光路的影响,如
图 1. 匹配钠导星中心波长谱域滤波抑制天光背景的原理示意图
Fig. 1. Schematic diagram of sodium laser guide star (LGS) center wavelength matched spectrum filtering for suppressing skylight background
其次,常用AO系统对钠导星回光成像的子孔径视场很小,一般在100 μrad量级@望远镜[11]。但是,需要抑制的天光背景在AO系统外视场可视为无限大,可被当作朗伯体来看待,即使与钠导星回光一起进入AO系统后,其视场也会比后者大,进而波前探测子孔径内的天光背景影响必然存在交叠[10]。因此,理论上空域滤波就是在谱域滤波的基础之上,通过精确匹配波前探测器的子孔径视场来抑制天光背景,从而进一步避免其在子孔径内出现相互交叠的现象,如
图 2. 匹配子孔径视场空域滤波抑制天光背景的原理示意
Fig. 2. Schematic diagram of sub-aperture field matched spatial filtering for suppressing skylight background
匹配589.16 nm中心波长窄带谱域滤波后的天光背景亮度为SSpectrum-Filtering[单位为W/(m2·sr)],则匹配哈特曼波前探测器子孔径视场空域滤波后的单像素对应的天光背景功率为
式中:SHS-Pixel、FEffect、SHS-Subaperture分别为对应接收望远镜端的哈特曼波前探测器的等效像素面积、等效焦距、等效子孔径面积。
最后的关键是时域滤波。由于受激共振散射钠导星通常是利用百微秒级(脉宽为100~200 μs)脉冲型钠导星激光(波长为589.15xx nm)激发大气钠层(高度为80~100 km)中钠原子的共振荧光散射产生的[12],其共振散射回光的时域特性同钠导星激光发射望远镜与AO观测望远镜的相对距离[13]、钠导星激光器的输出脉宽、观测目标仰角、大气采样厚度、大气钠层采样中心高度等有关。因此,笔者以
图 3. 钠导星分孔径收发的双望远镜模式示意图
Fig. 3. Sketch of sodium LGS launching and receiving with separated telescopes
式中:
特别地,当S=0时,
在受激共振散射钠导星产生的同时必然伴随着相同波长(589.15xx nm)的大气分子近程(一般高度在45 km以下)瑞利散射的产生,且其亮度比钠导星亮度高3个量级[14]。为了对该近程瑞利散射进行有效抑制,笔者以
特别地,当S=0时,
依据式(
在钠导星激光发射望远镜与AO观测望远镜的相对距离S=6.0 m、观测目标仰角E=45°~90°、钠导星激光器输出脉宽
图 4. 脉冲型共振散射钠导星回光时域宽度 随观测目标仰角E的变化
Fig. 4. Duration of the pulsed resonance sodium LGS scattered return-light as a function of the observing elevation E
图 5. 钠导星激光脉冲前后沿近程瑞利散射回光对应的抑制高度 和 随观测目标仰角E的变化
Fig. 5. Suppression altitudes and of the Rayleigh scattered light generated from the front- or rear-edge of the sodium LGS laser pulse as a function of the observing elevation E
与
图 6. 近程瑞利散射时域抑制宽度 随观测目标仰角E的变化
Fig. 6. Suppression duration of the Rayleigh scattered light from low altitudes as a function of the elevation E
如
3 实验验证
3.1 实验系统搭建
在第2节理论分析的基础上,笔者开展了基于人造钠导星的大气湍流波前探测实验。实验系统的基本原理如
图 7. 基于钠导星的大气波前像差探测实验系统示意图
Fig. 7. Experimental set-up schematic of atmospheric wave-front distortion detection using sodium LGS
3.2 综合滤波参数设计
笔者将综合滤波技术引入基于传统哈特曼波前探测器的光学结构,以期实现在强天光背景下对暗弱钠导星所携带的大气湍流波前畸变信息的实时探测。该实验系统中的哈特曼波前探测器主要由光学变倍模块、高精度机械快门模块、高精度二维可调视场光阑、589 nm中心窄带干涉滤光片、阵列微透镜、光学匹配模块、电子倍增型电荷耦合器件(EMCCD)探测器组成,如
如前所述,笔者所提综合滤波技术的思想是通过综合滤波匹配(谱域+空域+时域)抑制天光背景与589 nm瑞利散射,同时尽量保持589 nm钠导星共振回光对应波长的能量利用率。
589 nm中心窄带干涉滤光片用于实现高光学传输效率、纳米级谱宽精确匹配谱域滤波(中心波长为589.16 nm,中心波长峰值透射率为94%,透射线宽为1.0 nm),高精度二维可调视场光阑用于实现哈特曼子孔径视场精确匹配空域滤波(调节精度为20 μm/grid),高精度机械快门模块用于实现钠导星共振回光宽度/高度精确匹配时域滤波。
高精度机械快门模块的基本工作原理如
机械快门盘片上单个通光凹槽对应的扇形张角φ≥
在此基础上,笔者结合长脉冲型共振散射钠导星回光时域特性进行同步探测时序优化设计,对钠导星激光器同步出光时延
图 10. 钠导星激光器同步发射时延 与HS波前探测器同步读出时延 的精确外同步控制原理示意图
Fig. 10. Schematic diagram of external synchronized control for sodium LGS laser projection delay and HS wave-front CCD readout delay
钠导星激光器同步出光时延
即同步控制波前探测器同步曝光读出时延
在针对实际长脉冲型钠导星共振回光所携带的大气波前畸变信息进行外同步探测时,机械快门转盘上加工通光凹槽数目M为2,转速N为125 r/s,转盘通光挡光比为1/7,入射光束中心与快门轴心间距R为50 mm,最低开关时间设置精度ΔtError约为12 μs。
在此基础上,根据Bessel[16]所提计算方法并结合实验系统参数,笔者完成了综合滤波方案的探测能力预估[@典型12 W/(m2·sr)[10,17]]。结果显示,在尽量保持589 nm能量利用率的同时,所提技术可以实现6400倍(谱域滤波抑制200倍,空域滤波抑制4倍,时域滤波抑制8倍)的天光背景抑制比以及对瑞利散射的完全抑制。相较于前述具有极窄线宽(35 Å)但中心波长能量利用率偏低(>20%)的钠原子滤光技术,所提技术在滤波后的钠导星等效光子数以及探测信噪比上均具有优势。
对于较强天光背景条件下的钠导星大气波前探测,综合滤波技术与钠原子滤波技术的探测能力分别如
表 1. 综合滤波的探测能力
Table 1. Detection capability evaluation for the synthetic filtering technique
|
表 2. 钠原子滤波的探测能力
Table 2. Detection capability evaluation for the sodium atom filtering technique
|
3.3 典型实验结果
基于上述综合滤波技术,开展了强天光背景下基于钠导星的大气波前像差探测研究,典型结果如
图 11. 综合滤波技术在较强天光背景下实现对暗弱钠导星大气波前像差探测的典型实验结果。(a)强天光背景对暗弱钠导星大气波前像差探测的影响;(b)谱域滤波+空域滤波对HS波前探测器子孔径内天光背景的抑制作用;(c)时域滤波对HS波前探测器子孔径内天光背景的抑制作用;(d)综合滤波后HS波前探测器子孔径内的自适应阈值处理
Fig. 11. Typical experimental results of the atmospheric wave-front aberration detection with sodium LGS under strong skylight background by our proposed synthetic filtering technique. (a) Influence of strong skylight background on the atmospheric wave-front aberration detection with weak sodium LGS; (b) inhibitory effect on skylight background in sub-apertures of HS wave-front sensor by the spectrum filtering plus spatial filtering; (c) further inhibitory effect on skylight background in sub-apertures of HS wave-front sensor by the temporal filtering; (d) adaptive threshold processing in sub-apertures of HS wave-front sensor after the proposed synthetic filtering technique
图 12. 综合滤波后HS波前探测器子孔径内的钠导星探测信噪比
Fig. 12. Experimental results of SNR of the sodium LGS in sub-apertures of HS wave-front sensor after the proposed synthetic filtering technique
图 13. 重构钠导星采样大气湍流波前的第3~35阶Zernike模式统计方差结果
Fig. 13. The 3rd‒35th Zernike variances of the atmospheric wave-front aberration reconstruction with sodium LGS
4 结论
结合较强天光背景下对暗弱钠导星携带的大气波前畸变像差信息实时探测的AO应用需求,本文提出了一种谱域+空域+时域综合滤波的主动式波前探测技术,该技术可在匹配抑制强天光背景与589 nm强瑞利散射干扰的同时,尽量保持589 nm钠导星波长的能量利用率。笔者完成了对该技术的理论分析,并结合实验系统完成了该技术的参数设计和探测能力预估。然后以此分析为基础,开展了对该技术的部件研制、系统集成,以及白天钠导星的大气波前探测实验。实验结果显示,在约10 W/(m2·sr)的天光背景下,所提技术实现了基于钠导星的大气波前畸变像差的实时探测。理论分析与实验结果表明,该技术可为钠导星AO系统在白天条件下的应用提供一定的理论参考与工程经验。
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Article Outline
罗曦, 李新阳, 王彩霞, 王晓云, 胡诗杰. 强天光背景下基于钠导星的大气波前像差探测研究[J]. 中国激光, 2024, 51(8): 0804002. Xi Luo, Xinyang Li, Caixia Wang, Xiaoyun Wang, Shijie Hu. Atmospheric Wave‑Front Aberration Detection Using Sodium Laser Guide Star Under Strong Skylight Background[J]. Chinese Journal of Lasers, 2024, 51(8): 0804002.