Real world objects: capturing using in-line digital

Real world objects: capturing using in-line digital holography, Projecting using spatial light modulators James P. Ryle*a,b,c, David S. Monaghana,b,c, Susan McDonnelld and John T. Sheridana,b,c a

UCD Communications and Optoelectronics Research Centre, b SFI Strategic Research Centre in Solar Energy Conversion, c School of Electronic, Electrical and Mechanical Engineering, University College Dublin, Belfield, Dublin 4, Ireland. d

School of Chemical and Bioprocess Engineering, University College Dublin, Belfield, Dublin 4, Ireland.

ABSTRACT Digital holography is the process where an object’s phase and intensity information is retrieved from intensity images obtained using a digital camera (CCD or CMOS sensor). Unlike off-axis holography, object information is not modulated onto carrier fringes, thus in-line digital holography makes optimum use of the recording device’s sampling bandwidth resulting in higher resolution digital holograms. However, reconstructed images are obscured by the linear superposition of the unwanted out of focus twin images. In addition to this, speckle noise degrades overall quality of the reconstructed images. The speckle effect is an optical phenomenon arising when laser sources are used in digital holographic systems. Minimizing the effects due to speckle noise, removal of the twin image and using the full sampling bandwidth of the capture device, aids overall reconstructed image quality. Using interferometric techniques, it is possible to record whole field information about an object. Digital processing of the reconstructed holograms can remove or suppress the twin image while effects from speckle noise can also be reduced numerically. Machine vision techniques can then be applied to segment and distinguish objects of interest in the hologram. Coding the resulting phase information onto a spatial light modulator (SLM), real world, three dimensional images of objects can be reconstructed using the computer generated hologram. Keywords: In-line digital holography, Off-axis holography, Microlens arrays.

1. INTRODUCTION IN the late 1940’s while trying to overcome lens aberrations in electron microscopy, Denis Gabor serendipitously discovered the concept of holography1. This is the two step process of recording and replaying a hologram containing amplitude or intensity as well as phase information. Practical holography based on the interference of two coherent waves has only become feasible since the invention of the laser in the 1960’s. The interferometric concept requires two beams, and unscattered known reference beam and an unknown object beam to interfere at a surface2. The resulting interference pattern is the hologram. Conventional ‘wet’ holography photo-chemically records the interference pattern onto or within a holographic plate. Such wet holograms must be latentally processed before the hologram can be replayed. The processed holographic plate is illuminated by a laser resulting in a reconstructed hologram. Digital holography numerically records the interference pattern using a digital sensor such as a charge coupled device (CCD) or complementary oxide semiconductor (CMOS) camera. The digital intensity image of the hologram must undergo processing such as complex wave retrieval3 in off axis holography4.5. After such steps, the hologram is then numerically propagated by means of Fresnel approximation which described wave propagation in free space to the image plane. This results in a complex image containing both amplitude and phase information. By segmenting the in focus portions of multiple reconstructed amplitude images at different reconstruction depths. It is possible to render a volume image of the object6.

Optics and Photonics for Information Processing III, edited by Khan M. Iftekharuddin, Abdul Ahad Sami Awwal, Proc. of SPIE Vol. 7442, 74421B · © 2009 SPIE · CCC code: 0277-786X/09/$18 · doi: 10.1117/12.826885

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Digital holography has been implemented using many setups (Michelson and Mach-Zehnder interferometers) in both offaxis or in-line geometries. In this paper, we present results from an off-axis Mach-Zehnder type interferometer using the complex wave retrieval algorithm to obtain the profile of microlenses. We also present results from a lensless in-line setup, based on Gabor’s initial holographic principle.

2. PRINCIPLE OF OPERATION The use of interferometry to obtain whole field information is achieved with two waves, a known reference wave, Aref(x,y,0) and an unknown object wave which contains whole field information about the object, Aobj(x,y,0). The coherent superposition of the resulting diffraction patterns known as the hologram, H(x,y,0), is a result of the interference of these two beams and is recorded by the camera plane,

H (x, y,0 ) = Aref (x, y,0 ) + Aobj ( x, y,0 ) . 2

(1) The coherent superposition of these two waves gives rise to the interference terms known as the DC or zero-order term and the problematic out-of-focus twin image as well as the desired object image3 : * (x, y,0)Aref (x, y,0) + Aobj (x, y,0)Aref* (x, y,0) H (x, y,0 ) = Aref (x, y,0 ) + Aref ( x, y,0 ) + Aobj 14444 4244444 3 144424443 144424443 2

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(2) To obtain as much of the object image as possible a number of methods have been devised to separate and remove the DC and out-of-focus twin image terms. In the off-axis geometry, the two waves do not interfere coincidentally, this means a small angle exists between the two waves, resulting in interference fringes. By spatially modulating the hologram onto carrier fringes, the DC, twin image and object image terms are separated. After a bandpass filtering operation, the hologram is digitally Fresnel propagated to the image plane resulting in a complex image. Unlike the off-axis geometry, in the in-line (or on-axis) geometry, the two waves are coincident making twin image removal more difficult. However, the influence of the DC term is minimized by constructing a normalized contrast hologram7,8,9, Hc(x,y,0), from the original in-line hologram, H(x,y,0), and using the reference illumination intensity, I0(x,y,0), of the light source originally used,

H c ( x, y,0 ) =

H (x, y,0 ) − I 0 (x, y,0 ) I 0 (x, y,0 )

. (3)

The twin image still remains, however this can be eliminated if a spherical wave is used as the illumination source. The hologram is a magnified diffraction pattern. A sample is placed at a distance ‘z’ away from the pinhole, with the digital recording device (e.g. CCD or CMOS camera) placed linearly immediately behind this a distance ‘L’ away from the pinhole. Magnification arises from the ratio of the of L/z with a longer effective reconstruction distance given by

Deff =

L (L − z ) . z (4)

The twin image has a minimum influence on the reconstructed amplitude images8,10; however, it is ‘smeared’ out over a greater area as a result of the effective distance used to reconstruct the whole field information9.

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3. EXPERIMENTAL SETUP In this Section, we present two recording geometries to record transmissive objects. The first uses a Mach-Zehnder interferometer to implement the off-axis geometry and thus to observe deformations in the fabrication of lenslet arrays during the curing section of the manufacturing process11. The second setup uses Gabor’s in-line lensless geometry where we image two fiber optic strands separated out of plane at different depths. 3.1. Off-axis Mach-Zehnder interferometer The schematic for the off-axis Mach-Zehnder interferometer used in these experiments is presented in Fig. 1. The beam from a laser is spatially filtered to remove higher-order variations in its intensity profile. This is achieved by focusing the beam using a microscopic objective (MO) onto a pinhole to spatially filter (SF) the beam. The resulting spherically diverging wavefield is collimated using a collimating lens (CL). Using a beam splitter (BS1), the wavefield is separated into an object beam and a reference beam. The object beam passes through the sample, while the reference beam is unaltered. Mirrors M1 and M2 steer the reference and object beams and are combined using a beam splitter (BS2). The resulting interference fringe pattern is captured using a CCD camera.

Figure 1: Mach-Zehnder interferometric setup to capture off-axis holograms.

The complex wave retrieval algorithm is applied to the captured fringe patters resulting in a complex hologram. This is then digitally Fresnel propagated from the camera plane to the sample plane resulting in a complex image containing the sample’s whole fields information, both amplitude and phase. 3.2. Digital In-line Holographic Microscope (DIHM) Fig. 2 presents the schematic of the lensless setup used to capture a hologram of two strands of optical fiber separated out of plane. An optical beam from the lasers is focused by a microscopic objective onto a pinhole which acts to spatially filter out the higher order intensity variations in the optical beam’s intensity profile. A spherically diverging wavefield emerges from the pinhole. Light which is scattered by the sample acts as the object wavefield, while the unscattered light is the reference wavefield. The CCD camera captures the hologram, afterwhich it is digital processed using the Kirchhoff-Helmholtz integral yielding a complex valued image at the sample plane.

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Figure 2: Lensless Digital In-line Holographic Microsope (DIHM) setup to capture in-line holograms.

Both wavefields originate from the same source and are coincident on one another, thus they are in-line as no angle exists between the reference and object wavefields. The microscopic objective lens used in the system focuses the light from the primary source (laser) onto the pinhole where the secondary source is emitted. Only light from the secondary source is used in forming the hologram, and thus the system is considered a lensless system in imaging terms.

4. EXPERIMENTAL RESULTS In this section, we present the results from the two recording geometries used to record transmissive objects. Firstly results are presented from the Mach-Zehnder interferometer used to capture holograms of microlens (lenslet) arrays. After this, results are presented produced using a Digital In-line Holographic Microscope (DIHM)12 used to capture holograms of optical fibers. The laser used in all the experiments is a Helium Neon (HeNe) with an illumination wavelength, λ = 632.2 nm. An Imperx 1M48 monochromatic digital camera (1000 x 1000 pixels) with pixel pitch of 7.4 μm was used to capture all the presented holograms. Digital processing was performed using the Mathworks Inc. MatLab 7.4.0 (R2007a) programming environment. Further details of the experimental equipment used is provided in the Sections 4.1. and 4.2. 4.1. Off-axis capture of microlens (lenselet) array Microlenses are used in a large variety of application, from research to entertainment, as components in Shack-Hartman arrays to 3D television integral imaging displays and even used in some digital cameras to enhance the effective fill factor of the sensor arrray. In this experiment, a number of droplets (~0.8 mg each) of an Ultra-Violet (UV) curable resin, Norland Optical Adhesive (NOA 61), were placed on a glass substrate. The profile of the wet resin was deformed by applying a large electric field produced by an electric potential of 15 kV across the slide from either side of the glass substrate. Upon deformation, the lenslet array was cured using a UV source. Holograms were obtained pre- and postdeformation as well as post-curing to examine what effects each stage had in altering the profile.

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Figure 3: Lenslet array (a) hologram, (b) reconstructed in-focus amplitude, (c) corresponding wrapped phase in radians and (d) unwrapped phase in radians (pixel width = 7.4 μm and illumination wavelength, λ = 632.2 nm) .

The results of the cured lenslet array is shown in Fig. 4. Applying the complex wave retrieval algorithm3 to the data in Fig. 4 (a) and digitally Fresnel propagating the resulting hologram yielded the reconstructed amplitude and wrapped phase shown in Fig. 4 (b) and (c) respectively. Employing a fast unwrapping algorithm13 and applying it to the phase wrapped data in Fig. 4 (c) results in the unwrapped phase map shown in Fig. 4 (d). 4.2. DIHM imaging communication optical fiber A DIHM system was designed, developed and purposely built to image MCF-7 and MDA-MB-231 mammalian cancer cell cultures12,14. This was originally done because of its compact footprint compared to the Mach-Zehnder or Michelson interferometers. Laser light is focused by a 20X microscopic objective onto a 5 μm diameter pinhole. The fiber used is an optical communications single mode fiber, for use with wavelengths of 1330 nm. Using digital calipers, the fiber strands diameters were measured as 210 μm.

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Figure 4: (a) reference illumination intensity, (b) reconstructed amplitude at Plane 1, z1 = 1.5 mm, (c) reconstructed wrapped phase at plane 1, (d) single contrast hologram used to reconstruct whole field information of the fiber at different out-of-plane depths, (e) reconstructed amplitude at plane 2 and (f) reconstructed wrapped phase at Plane 2, z2 =2.5 mm .

The results presented in Fig. 5 are for a source-camera distance, L = 55 mm and a source-sample distance to the first infocus fiber, z1 = 1.5 mm with the second fiber located ~1 mm behind the first. The normalized contrast hologram, defined in Eqn. 3 and presented in Fig. 5 (d), is constructed using the original capture and reference illumination intensity, Fig. 5(a). From this single contrast hologram, it is possible to reconstruct whole field object information at multiple depths. Fig. 5 (b) shows the in-focus opaque core of the upper fiber, while Fig. 5 (e) shows the in-focus opaque core of the lower fiber place ~1 mm behind the first fiber. The core is also discernable in the respective corresponding wrapped phase images in Fig. 5 (c) and (f).

5. DISCUSSION AND CONCLUSION Holography is a two step process involving the recording of a hologram followed by processing and reconstruction of the valuable in-focus information embedded within the original hologram. Off-axis and in-line interferometric techniques exist which allow digital holographic capture of information by means of discrete capture devices such as CCD or CMOS cameras. Processing is performed digitally without the need to chemically process holograms, thus minimizing exposure to potentially harmful chemicals. Owing to the limited capabilities of two dimensional display devices, it is only possible to visualise intensities. Thus 2D intensity images representing the amplitude or phase can be displayed. It is possible to generate a volume image from a stack of multiple reconstructions at different depths; however this depth altered visualization technique is still displayed as a 2D rotated projections intensity image. In this paper we have presented results from two methods where holograms of real life objects have been captured. The first spatially modulated the object onto carrier fringes. These were transparent microlenses made from a UV curable resin. The second hologram was captured using a Digital In-line Holographic Microscpope (DIHM). The results presented here are of two strands of optical fiber at different out-of-plane depths. We have verified that it is possible to capture and display digital hologram using these techniques, however much work needs to be done to display the reconstructed image in 3D, visible from a multitude of perspectives.

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Towards this end, it would be advantageous to use diffractive optical elements such as Spatial Light Modulator (SLM) based on Liquid Crystal on Silicon (LCOS) technologies to project true 3D information15. However, for this to be realized, the amplitude and phase should be easily controlled by the SLM’s. In addition to this, the interface where the projected light forms the 3D image needs consideration. Visualisation of projected holograms can be achieved projecting into a smoke like filled chamber, or using a screen which moves though the projection area faster than the eye’s natural refresh rate.

ACKNOWLEDGEMENTS The authors wish to acknowledge the support of Enterprise Ireland and Science Foundation Ireland. James Ryle wishes to thank SPIE for supporting conference attendance through the Student Chapter Officer Travel Grant programme. He also acknowledges financial support from UCD’s Seed Funding Scheme. Karen Molony from NUIM is gratefully acknowledged for helpful discussion regarding DIHM.

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