Anisotropic edge enhancement in optical scanning
holography with spiral phase filtering
Kelly K. Dobson
1
, Wei Jia (贾 伟)
2,
*, and Ting-Chung Poon
1
1
Bradley Department of Electrical and Computer Engineering, Virginia Tech, Blacksburg, VA 24061, USA
2
Lab of Information Optics and Optoelectronic Technology, Shanghai Institute of Optics and Fine Mechanics,
Chinese Academy of Sciences, Shanghai 201800, China
*Corresponding author: jw81@163.com
Received September 9, 2015; accepted October 27, 2015; posted online December 16, 2015
Anisotropic edge enhancement is simulated using a spiral phase plate (SPP) in optical scanning holography
(OSH). We propose to use a delta function and an SPP as the pupil functions to realize anisotropic edge enhance-
ment. The interference of these two pupils is used to two-dimensionally scan an object to record its edge-only
information. This is done in three ways: first, by shifting the SPP, second, by using two offset SPPs of same
charge, and finally, by using two oppositely charged SPPs. Our computer simulations show the capability of
selectively enhancing the edges of a given object.
OCIS codes: 100.2980, 090.1995, 070.6110.
doi: 10.3788/COL201614.010006.
A common device to generate helical wavefronts or vortex
beams is the spiral phase plate (SPP), which has an
azimuthal structure exp½jnφ, where n is the topological
charge, which is generally a nonzero integer, and 0 ≤ φ <
2π is the azimuthal coordinate
[1,2]
. The simplest SPP
realized with n ¼ 1 has been demonstrated as a spatial
filter in a 4f system to achieve edge enhancement
[3]
,
and more recently, it was used as a pupil function for iso-
tropic edge extraction in optical scanning holography
(OSH)
[4]
.
Edge enhancement has found utility in a variety of
image processing applications where detecting the edge or
shape is of interest, such as in industrial inspections
[5,6]
,
and fingerprint identification
[7,8]
. Generally, the enhance-
ment effect is isotropic or symmetrical such that regardless
of orientation, each edge of the input pattern is enhanced
uniformly. However, in some cases, the feature informa-
tion around certain orientations and edges is of greater
interest and therefore requires anisotropic edge enhance-
ment to emphasize these edges. The methods reported for
selective edge enhancement include changing the integer
power and offset angle of the sine function in the vortex
distribution
[9]
, fractional
[10]
or shifted vortex filters
[11,12]
,a
superposed vortex amplitude filter
[13]
, and the introduction
of astigmatism in the vortex filter
[14]
.
The use of SPPs for enhancement effects in digital
incoherent holography was first attempted using the
Fresnel incoherent correlation holography technique
[15]
,
and then by OSH
[4]
. The forme r approach is based on
self-interference and requires the capture of three phase-
shifted recordings sequentially, which could introduce bias
build-up when using complex objects
[16]
. The latter over-
comes this by heterodyne scanning the object such that
the holographic information is carried by the heterodyne
frequency, which can be extracted using electronic filter-
ing. With the scanning property , OSH can be applied in a
variety of fields, ranging from microscopes to remote
sensing
[17,18]
.
In this Letter, we demonstrate the use of OSH to record
selective edge-only information of an object holographi-
cally. We introduce three methods to do this: (1) shifting
the position of the SPP to position ðr
0
; φ
o
Þ in the pupil
plane, (2) using two symmetrically offset vortices of the
same charge, and (3) using two oppositely charged offset
vortices.
OSH is based on a two-pupil optical heterodyne image
processor
[19,20]
that captures the entire holographic infor-
mation of an object with a two-dimensional (2D) scan
while preserving the phase information in the recording
process. The two pupils in the standard OSH system
are such that one pupil is a uniform function or a plane
wave and the other is a delta function or point source.
The interference of these two pupils is used to 2D scan
an object to extract its holographic information.
The two pupil functions, p
1
ðx; yÞ and p
2
ðx; yÞ, in the
two-pupil heterodyne scanning image processor presented
in Fig.
1 are lo cated in the front focal plane of lens L
1
with
focal length f . The pupils are illuminated by collimated
laser beams with temporal frequencies ω
0
and ω
0
þ Ω, re-
spectively, where ω
0
≫ Ω and the frequency shift Ω results
from an acousto-optic modulator. The laser beams are
Fig. 1. Two-pupil heterodyne scanning image processor.
COL 14(1), 010006(2016) CHINESE OPTICS LETTERS January 10, 2016
1671-7694/2016/010006(5) 010006-1 © 2016 Chinese Optics Letters
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