Enhanced depth resolution in optical scanning
holography using a configurable pupil
Haiyan Ou,
1,2
Ting-Chung Poon,
3
Kenneth K. Y. Wong,
1
and Edmund Y. Lam
1,
*
1
Department of Electrical and Electronic Engineering, University of Hong Kong, Pokfulam, Hong Kong, China
2
Institute of Applied Physics, University of Electronic Science and Technology of China, 610054, Chengdu, China
3
Bradley Department of Electrical and Computer Engineering, Virginia Tech, Blacksburg, Virginia 24061, USA
*Corresponding author: elam@eee.hku.hk
Received September 16, 2013; revised January 28, 2014; accepted January 29, 2014;
posted January 31, 2014 (Doc. ID 197685); published March 28, 2014
The optical scanning holography (OSH) technique can capture all the three-dimensional volume information of an
object in a hologram via a single raster scan. The digital hologram can then be processed to reconstruct individual
sectional images of the object. In this paper, we present a scheme to reconstruct sectional images in OSH with
enhanced depth resolution, where a spatial light modulator (SLM) is adopted as a configurable point pupil.
By switching the SLM between two states, different Fresnel zone plates (FZPs) are generated based on the
same optical system. With extra information provided by different FZPs, a depth resolution at 0.7 μm can be
achieved. © 2014 Chinese Laser Press
OCIS codes: (090.1995) Digital holography; (100.3190) Inverse problems; (100.3020) Image reconstruction-
restoration; (110.1758) Computational imaging.
http://dx.doi.org/10.1364/PRJ.2.000064
1. INTRODUCTION
Digital holography is a technique that seeks to capture the
three-dimensional (3D) volume information of an object
and to record it on a two-dimensional (2D) hologram elec-
tronically [
1,2]. As one of the digital holographic techniques,
optical scanning holography (OSH) can record the 2D holo-
graphic information of the 3D object by a single raster scan
[
3]. The idea of OSH was generated by Poon et al. when he
was dealing with bipolar point spread functions in incoherent
image processing [
4,5]. The theory of OSH was then devel-
oped, and the first experiment was carried out in 1990 [
6].
Since then, OSH has undergone great development and has
found various applications ranging from biological micros-
copy [
7,8] to remote sensing [9,10] to 3D cryptography
[
11], etc.
Unlike laser scanning [
12,13] or confocal microscopy tech-
niques [
14,15], where multiple 2D scans are needed to capture
all the 3D information, OSH can greatly save on data acquis-
ition time. While it is so powerful in capturing the 3D informa-
tion, there is also a need in OSH to view the individual 2D
planes of the object from the acquired digital hologram, which
is known as sectioning or sectional image reconstruction. In
the sectioning process, the challenge lies in the suppressing of
the defocus noise, which is the undesired residue signal from
neighboring sections. The conventional method of achieving
this involves computing the convolution of the conjugate im-
pulse response at the desired section with the hologram, but
this approach suffers from large defocus noise [
3]. To reduce
the defocus noise, many methods including the Wiener filter
[
16], Wigner distribution [17], and inverse imaging [18,19]
have been demonstrated. Further development related to
inverse imaging includes the development of a blind edge
detection technique to locate the sections [
20], compressed
sensing [
21], and edge-preserving regularization for sharper
sections [
22,23]. The use of a random-phase pupil is also
shown to achieve better sectioning resolution [
24], which
requires the averaging of several independent holograms of
the same object. Another method using a dual-wavelength
laser source has been developed, with the depth resolution
improved up to 2.5 μm[
25,26]. More recently, Ou et al. have
proposed a double-detection method, which has led to a depth
resolution of 1 μm by capturing holographic information of the
same object at two different depth locations [
27].
In this paper, we propose a sectioning method based on a
configurable point pupil for achieving enhanced depth resolu-
tion. A spatial light modulator (SLM) is used as the switchable
pupil, which generates different Fresnel zone plates (FZPs)
for the same OSH system. We can then make use of the extra
information provided by the different FZPs to achieve a better
depth resolution. The proposed method is easier to realize in
practice compared to some of the methods mentioned above.
2. PRINCIPLE
The OSH system setup is shown in Fig. 1. In the optical sys-
tem, the laser source centered at ω is divided into two parts by
a beam splitter (BS1), of which one passes through a pupil
function p
1
x; y to have a spherical wavefront on the object.
Meanwhile, the other path first has a frequency shift Ω via an
acousto-optic frequency shifter (AOFS) and then arrives at a
point pupil p
2
x; y to result in a planar wavefront on the ob-
ject. The two coherent beams of different frequencies ω and
ω Ω are then combined by a second beamsplitter (BS2) and
are used to scan the object located at a distance z away from
the scanning mirror. Lens 3 is used to collect the light from the
object. A photodetector collects the transmitted and scattered
light from the object and converts it into an electronic signal.
64 Photon. Res. / Vol. 2, No. 2 / April 2014 Ou et al.
2327-9125/14/020064-07 © 2014 Chinese Laser Press