Depth-Resolved Imaging Using Photorefractive Holography

An example of depth-resolved photorefractive holography

We have demonstrated depth-resolved holography to be a valuable method of real-time high resolution 3-D imaging, applicable through turbid media, which provides rapid whole-field acquisition and high depth and transverse spatial resolution images. This technique has applications in 3-D profiling, biomedical imaging through tissue and imaging through the atmosphere and sea water.

Depth-resolved photorefractive holography

Photorefractive holography is advantageous over traditional film based techniques because it allows real-time hologram read-out and does not require developing. The equipment required for photorefractive holography is the same as for traditional holography except that the holographic film is replaced by a photorefractive medium that records the hologram (see Figure 1). The recording process occurs through the migration of charges within the crystal from the light regions to the dark regions of the hologram interference pattern. This results in a local space-charge field, which interacts with the electro-optics of the photorefractive crystal to form a change in refractive index which follows the interference pattern. The hologram is then reconstructed either by blocking the image beam, and letting the reference beam diffract off the refractive index pattern to record an image on the CCD camera, or by using a different wavelength source and Bragg-matching the read-out beam to the refractive index pattern. Any incoherent (e.g. scattered) light present in the object beam does not contribute to the hologram.

Figure 1    Schematic of photorefractive holography

Depth-resolved photorefractive holography, which was first demonstrated by our group, uses the different length paths travelled by light reflecting from different depths within an object to separate the different layers. When using a low coherence length source (e.g. a mode-locked laser), holograms are recorded only by photons reflected from the limited range of object depths which arrive at the photorefractive medium with an appropriate optical path length (within a coherence length of that of the reference beam). By scanning the reference beam path length it is therefore possible to image different layers in the object and depth resolutions better than 50 µm have been obtained. Figure 2(b)-(e) show depth-resolved images recorded of a 3-D "test object" (figure 2(a)) obtained using this technique with a photorefractive rhodium-doped barium titanate crystal, together with a reconstructed 3-D image of the object (Figure 2(f)). The image at the top of this page shows depth-resolved images (in 20 µm steps) of the embossed '5' on a 5p coin recorded in the same manner.

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Figure 2   (a) 3-D test object, (b) - (e) depth-resolved images of different object layers, (f) computer reconstruction of the object.

The speed of the recording process is determined by the photorefractive medium. Initially the photorefractive material we used was rhodium-doped barium titanate, a bulk ferroelectric crystal. While this has a high sensitivity, its response (recording) times were limited to ~ 1 second. Our recent research has concentrated on the use of photorefractive multiple quantum well (MQW) semiconductor devices, which are as sensitive but which have a response time <1 ms. These photorefractive MQW devices have been developed by the the Quantum Optoelectronics Research Group at Purdue University. A typical experimental holographic imaging set-up is shown in Figure 3. The source (a mode-locked Ti:sapphire laser) is split into a reference and object beam which are recombined on the photorefractive MQW device. Different depths in the object, recorded by matching the object and reference beam path lengths to within the (~50 µm) coherence length of the laser, are selected by adjusting the path length of the reference beam.

Figure 3    Experimental set-up used for holographic imaging with MQW devices

Figure 4 shows depth-resolved holographic images of a three-dimensional object obtained with the multiple-quantum-well photorefractive imaging system. The images have a depth and transverse spatial resolution better than 50 µm. The depth-resolved images can be viewed in real time and recorded directly onto a VCR with no signal processing. To our knowledge this represents the fastest 3-D imaging system reported. Real-time 3-D image aquistion direct to a standard VCR has been demonstrated.

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Figure 4    (a) video image of the original test object, (b) computer generated reconstruction from holographic images

3-D Imaging through turbid media

One of the main applications of photorefractive holography is for imaging through turbid media such as biological tissue or sea water. Usually, when an object to be imaged is embedded in a turbid medium, it can not be imaged because it is obscured by light scattered in the turbid medium.

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Figure 5     USAF test chart (a) directly imaged through 8 MFP of turbid media, (b) holographic reconstruction through 16 MFP

Photorefractive holography rejects this scattered light through two mechanisms. Firstly, when imaging through a moving turbid medium, the interference created by the scattered light at the photorefractive material quickly decorrelates and so does not write a persistent hologram. Secondly, since the scattered light will generally have taken a different length optical path compared to the image bearing light, most of it will arrive at the photorefractive recording device outside the coherence time of the laser source.

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Figure 6    Computer reconstructions from the depth-resolved holograms of the 3-D object (a) without an obscuring scattering layer, (b) through 12 mfp of turbid medium, (c) through 14 mfp of turbid medium

Figure 5 shows a United States Air Force test-chart imaged through a turbid medium: (a) when directly imaged through only 8 scattering mean free paths (mfp's), showing that the test-chart is totally obscured, (b) when imaged with the photorefractive holography system through 16 mfp of turbid medium, showing that the 100 µm bars of the test-chart can now be resolved. Figure 6 shows the 3-D test object imaged through 0, 12 and 14 mfp of turbid medium respectively.

Remote depth-resolved imaging using holography

Remote sensing is of importance in many fields. Being able to image at a distance in three dimensions and to a high resolution, and to be able to image through turbid media could have a number of uses. Possible applications include long distance imaging, through seawater or through clouds, and studying surface relief at a distance. Three-dimensional imaging could provide accurate shape detection or surface profiling with higher resolution than radar or sonar because of the shorter optical wavelengths. In all the photorefractive imaging experiments so far reported, however, the objects to be imaged have all been close to the imaging system. It is not trivial to coherently detect distant objects: a holographic imaging system obviously needs a coherent reference pulse and the complications associated with optically delaying the reference pulse mechanically on appropriate timescales are considerable. We have, however, recently devised a scheme to achieve this and have implemented the concept in our 3-D photorefractive holographic imaging system.

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Figure 7    Photorefractive imaging at a distance of 11.2m: (a) video image of test object, (b) object viewed end-on through imaging system, (c)-(e) holographic images of depth-resolved layers

An experimental set-up, similar to that shown in Figure 2, was constructed except that the object was now located approximately 11.2 m from the photorefractive crystal. The object in this case was a 3-D aluminium object consisting of three cylinders of 8, 16, and 25 mm diameter, separated in depth by 500 µ m. The results are shown in Figure 7. Note that the transverse resolution of the images was limited by the aperture of the collection optics to ~ 0.5 mm. The depth resolution in this experiment was demonstrated to be better than 200 µm. The achievable transverse spatial resolution of this remote imaging system is limited only by the aperture (i.e. f/number) of the collection optics.

Conclusions and future work

We have demonstrated photorefractive holography as a method of rapid whole-field 3-D imaging of near and distance objects. Previous experiments have shown that photorefractive holography may be used to image through turbid media with sub-100 µm resolution and, by using multiple-quantum-well photorefractive devices, real-time image recording to a standard VCR has been demonstrated. Using rhodium-doped barium titanate, we have also demonstrated remote depth-resolved imaging of a 3-D test object ~ 11 m from the detection system.

In the near future we wish to demonstrate the ability of the MQW devices to record depth-resolved holograms using multiple wavelengths i.e. colour 3-D imaging in real-time. We are also working to develop cheaper portable sources of high average power short coherence length light in order to make this technology more widely applicable. We are currently examining applications in surface profiling and biological imaging. We also intend to apply the rapid 3-D imaging capability to farfield microscopy with a view to imaging living organisms. In future remote imaging experiments, we would like to investigate using photorefractive multiple-quantum-well devices as the holographic recording material and to increase the distance to the remote object. We would also like to develop a system with electronically controllable imaging distance.

This work has been funded by grants from the DERA, EPSRC and by Kodak UK Ltd.

Relevant publications

S. C. W. Hyde, N. P. Barry, R. Jones, J. C. Dainty, P. M. W. French, M. B. Klein, and B. A. Wechsler, "Depth-resolved holographic imaging through scattering media using photorefraction", Optics Letters 20, 1331 (1995)

S. C. W. Hyde, N. P. Barry, R. Jones, J. C. Dainty, and P. M. W. French, "High resolution depth resolved imaging through scattering media using time resolved holography", Optics Communications 122, 111 (1995)

R. Jones, S. C. W. Hyde, M. J. Lynn, N. P. Barry, J. C. Dainty, P. M. W. French, K. M. Kwolek, D. D. Nolte, and M. R. Melloch, "Holographic storage and high background imaging using photorefractive multiple quantum wells", Applied Physics Letters 69, 1837 (1996)

S. C. W. Hyde, N. P. Barry, R. Jones, J. C. Dainty, P. M. W. French, K. M. Kwolek, D. D. Nolte and M. R. Melloch, "Real-time 3-D imaging through turbid media with ballistic light using time-gated holography", Invited paper, IEEE JSTQE Special Issue on Lasers in Medicine and Biology 2, 965 (1996)

R. Jones, M. Tziraki, P. M. W. French, K. M. Kwolek, D. D. Nolte and M. R. Melloch, "Direct-to-video holographic 3-D imaging using photorefractive multiple quantum well devices", Optics Express 2, 431-453 (1998)