Test of digital in-line holography

Introduction

In 1948, Gabor invented a new microscope principle whose original goal is to avoid the spherical aberration of electron lenses and at first the optical model was just to verify the feasibility of this novel principle while applying to electron microscopy. The simple and magic principle then received enormous attention not only in electron field but also in optical microscopy, because of lenses-free setup and three-dimensional information recording.

At Gabor’s time, the recorded film or the optical interference pattern was reconstructed using optical reconstruction method. Later in the late 1960s, the photographic hologram of classical holography was first replaced by digitally recording the hologram and numerically reconstructing the image on a computer. However, computers were too slow and recording capabilities were too poor for digital holography to be useful in practice. Recent years, along with rapid development of GPUs, the limitation of computation capabilities of the computer was surpassed again and again. Digital inline Holography revived and become a really hot topic.

Digital inline holography has following advantages comparing with normal or traditional microscopies. First, it preserves the 3-Dimensional information. Because holography only recording one hologram while traditional microscopies record focused image at different depths. Holography can numerically reconstruct the focus image at different depths.

Second, digital autofocus can be conducted while conventional autofocus achieved by vertically changing the focal distance until the image is focused. This speeds up the image grabbing procedure because digital autofocus is much quicker than mechanical autofocus procedure. What’s more, we can also create extend focus image using only one hologram. This is an impossible mission for traditional microscopies.

Third, digital inline holographic system do not need an image forming lens which enable it avoid optical aberrations of the lenses. What’s more, the needed components for a digital inline holographic system are inexpensive optics and semiconductor components, such as a laser diode and an image sensor. The low component cost in combination with the auto focusing capabilities of digital inline holography, make it possible to manufacture digital inline holographic systems with a very low cost.

Every coin have two sides. Gabor inline holography also have drawbacks. First, the reconstructions of digital inline holograms comes with an inherent noise called twin image noise. The image quality of digital inline holography drops dramatically due to this kind of noise. Also, twin image noise ruined the reconstructed phase profile, which is a very valuable information of the sample. For tens of years, researchers tried all kinds of method to remove twin image noise but little was achieved.

Second, the resolution of the Gabor inline system is limited by the size of precision pinhole. The smaller the pinholes is, the higher resolution could be retrieve. However, smaller pinhole may also bring other problems such as low light intensity and difficulty of the alignment.

In order to deeply understand advantages and disadvantages and develop improvement of digital inline holography, we first built up two holographic microscope with Gabor inline setup.

System Set-up

Fig. 1 demonstrate the system diagram and experimental setup of the Gabor inline system. We employeda 405nm laser diode as the light source. The light source of the system is a point source filtered by an aperture (National Aperture, size: 5µm). The object is placed a few millimeter away from the aperture. The imaging sensor is place several times further. The laser beam from the aperture illuminates on the object and create a magnified diffraction pattern on the imaging sensor. The pattern then is captured by the high speed camera designed by our team.

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Figure 1. (a) System diagram and (b) experimental setup of digital inline system

Experimental results

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Figure 2. Experimental results of the digital inline system. (a) Hologram. (b) Reconstructed 2D image.

We adopted the angular spectrum method to reconstruct the 2D image of the sample. Fig. 2(a) illustrates the captured hologram of an amplitude resolution target (USAF 1951). Unlike traditional pictures, the capture hologram here is not a focused image. Instead, the hologram records the unfocused diffraction pattern of the resolution target. After 2D reconstruction, focused image as shown in Fig. 2(b) could be revealed. The elements 4 of group 7 of the USAF 1951 could be resolved which means the resolution of this system is better than 2.76µm.