Two-spherical-wave Self-interference Holographic Imaging

Exploring the science of life has always been a hot topic in scientific research. In life science research, the combination of fluorescent labeling techniques and optical imaging methods has enabled the observation of information on mesoscopic scale. This fills the gap between the macroscale and microscale information, and is of great importance for life science research. However, the generally used fluorescent optical imaging methods usually require a scanning process to obtain the three-dimensional information, and recording the fluorescent information from a whole three-dimensional volume at a single shot is not yet possible. Holographic imaging, on the other hand, can achieve non-scanning three-dimensional imaging, avoiding the time-consuming scanning process. The key to this is to record the complete information of the object wave based on interference. Fluorescent is a broadband light source, and the difficulty in fluorescent holography is to realize fluorescent interference. Besides, fluorescent signals are rather weak, and holography uses out-of-focus detection method, so making the most of the limited signal intensity is also crucial for the realization of fluorescent holography.Driven by the need for non-scanning fluorescent three-dimensional imaging and considering the low degree of coherence and weak signal intensity of fluorescence signals, we have systematically investigated into the holographic detection method for broadband weak signals as well as the optical path difference (OPD) theory in interference systems. Based on our study, we have proposed a two-spherical-wave self-interference holographic imaging method for the detection of broadband weak signals. The two-spherical-wave self-interference holographic detection system based on the proposed method overcomes the difficulty in current system of simultaneously achieving high signal-to-noise ratio (SNR) and high degree of coherence, and provides an effective way for holographic detection of broadband weak signals. This article is divided into three parts according to our studies, namely image quality, image resolution, and the Lagrange invariant.(1) We have studied the change of OPD in interference systems. It is found out that by employing the interference between two spherical waves, we can manipulate and reduce the OPD in the system, realizing interference with high degree of coherence on a small recording distance. The requirement of high SNR and high degree of coherence for the detection of broadband weak singles can be satisfied using the proposed method. Experimental results have confirmed that such method is effective in decoding wavefronts and improving SNR by3-20times (exact number varies with the recording parameters).(2) We have studied the resolution change in self-interference holography (SH) system, particularly in the spherical reference wave system. Factors that influence the resolution in our system have also been studied. These theoretical analyses were confirmed by our experiments. It is found that in SH system with spherical reference wave, the resolution can be improved by increasing the numerical aperture (NA) in the image space. Compared to the previous plane reference wave SH system that relies on lateral magnification for resolution improvement, our system has the advantage of improving resolution near the focal plane, where fluorescent signals are strong and of little OPD. This ensures not only the resolution of our system, but also the SNR and degree of coherence.(3) It is found that when a holographic system satisfies three requirements, namely the spatial incoherence, the inclusion of object information in both object wave and reference wave, and the wavefront of reconstructed image being the conjugated product of object wave and reference wave, the Lagrange invariant in a conventional optical system is no longer valid. We have found out and confirmed that the angular magnification and lateral magnification in SH system can be manipulated independently and no longer comply with the Lagrange invariant.