基于強度校正和稀疏自聚焦重建的太赫茲合成孔徑同軸全息

得益于太赫茲波對非極性材料的良好穿透性及非電離性等,太赫茲成像是一種極具潛力的前沿無損檢測技術。一方面,復振幅波前感知一直是光學領域主要研究方向之一,振幅信息表示物體的吸收特性,相位信息則揭示物體的折射率和厚度特性,振幅和相位分布共同表征了光場的主要信息。另一方面,受制于太赫茲波波長,較低的成像分辨率成為限制太赫茲成像應用于實際的主要因素之一。因此,獲得太赫茲波前的全場、高分辨率、復振幅分布對于從生物醫學診斷到材料分析等應用都具有重要意義。

為實現上述目標,中國工程物理研究院激光聚變研究中心太赫茲研究團隊開展了太赫茲合成孔徑同軸數字全息成像研究。相關結果發表在Photonics Research 2019年第7卷第12期上 (Zeyu Li, et al., Terahertz synthetic aperture in-line holography with intensity correction and sparsity autofocusing reconstruction)。

太赫茲合成孔徑同軸數字全息示意圖

該項研究基于其自主研發的太赫茲量子級聯激光器,構建了太赫茲無透鏡同軸全息成像裝置。為突破面陣探測器孔徑限制,提高成像分辨率,采用合成孔徑方法,提出一種全局最優化算法對子孔徑強度進行校正,實現子孔徑全息圖無縫融合,在4.3 THz(λ = 69.7 μm)實現70 μm(~λ)波長級橫向空間分辨率。為抑制同軸全息零級像與共軛像干擾,利用基于稀疏約束的相位復原算法,實現了高質量全息重建。為獲得最佳重建距離,首次在同軸數字全息中引入基于重建目標函數的自聚焦判據,將同軸全息自動聚焦與數字再現統一在同一優化框架下。基于此裝置,該團隊開展了生物和半導體樣品的太赫茲成像實驗,證明了其在生物醫學成像和材料分析領域的應用潛力。值得一提的是,該項研究提出的方法和算法并不局限于太赫茲波段,可直接應用于X射線和可見光波段。

研究團隊的周遜教授相信,該工作對于推動太赫茲成像技術的發展和應用具有重要意義;吳衛東教授認為,研究提出的稀疏自聚焦相位復原算法有效解決了共軛像干擾及自動聚焦的問題,為Gabor同軸數字全息再現提供了新的思路。

下一步工作將聚焦在太赫茲源的優化、太赫茲全息三維成像以及應用探索上。

Terahertz synthetic aperture in-line holography with intensity correction and sparsity autofocusing reconstruction

Terahertz (THz) imaging, benefiting from THz radiation’s capabilities of non-ionizing and penetration of non-conducting materials, serves as a cutting-edge non-destructive evaluation technology. One of the major challenges in photonics is complex amplitude wavefront sensing, the amplitude image indicates the absorption properties, while the phase image reveals the refractive and thickness information, thus simultaneously determining that the amplitude and phase distributions of the wavefront are highly desirable for applications ranging from bioimaging to material characterization. Due to the long wavelength of the THz wave, the imaging resolution is also one of the key considerations for THz applications.

To achieve THz full-field, high-resolution, and complex amplitude imaging, a THz research group from the Research Center of Laser Fusion at China Academy of Engineering Physics (CAEP) carried out THz synthetic aperture in-line holography. The research results are published in Photonics Research, Vol. 7, Issue 12, 2019 (Zeyu Li, et al., Terahertz synthetic aperture in-line holography with intensity correction and sparsity autofocusing reconstruction).

Schematic diagram of the terahertz synthetic aperture in-line digital holography.

In this work, a high-resolution and high-quality THz lensless in-line holographic setup was established based on a self-developed THz quantum cascade laser (THz-QCL). To enhance the spatial resolution limited by the numerical aperture (NA) of the array detector, the synthetic aperture method was adopted, a practical global optimization algorithm was proposed to correct the intensity differences among sub-holograms, and a lateral resolution better than 70 μm (~λ) at 4.3 THz was achieved. To overcome the twin-image problem for in-line holography, a sparsity-based iterative phase retrieval algorithm was used to give high-quality reconstructions. Moreover, to obtain the best in-focus reconstruction distance, a new autofocusing criterion based on the “reconstruction objective function” was introduced into in-line holography for the first time, so the autofocusing procedure and the reconstruction were unified within the same framework. They demonstrated the success of the THz synthetic aperture in-line holography on biological and semiconductor samples, showing its potential applications in bioimaging and materials analysis. Note that the proposed approaches can be applied to other wavebands as well, such as visible light and X-ray band.

Prof. Xun Zhou from the research group believes that this work is of great significance to promote the development and application of THz imaging technology; Prof. Weidong Wu believes that the proposed sparsity-based autofocusing phase retrieval algorithm effectively alleviates the twin-image problem and automatically obtains the optimal in-focus distance, providing new ideas for reconstruction in Gabor in-line digital holography.

Future work will focus on THz laser optimization, THz 3D holography, and THz imaging application.