Minimizing the effects of unmodulated light and uneven
intensity profile on the holographic images reconstructed
by pixelated spatial light modulators
Jhen-Si Chen (陈政锡)
†
, Jia Jia (贾 甲)
†
, and Daping Chu (初大平)*
Centre for Photonic Devices and Sensors, University of Cambridge, Cambridge CB3 0FA, UK
*Corresponding author: dcp31@cam.ac.uk
Received March 28, 2017; accepted June 16, 2017; posted online July 6, 2017
A simple and effective approach is proposed to minimize the effect of unmodulated light and uneven intensity
caused by the pixelated structure of the spatial light modulator in a holographic display. A more uniform image is
produced by purposely shifting the holographic images of multiple reconstructed lights with different incident
angles from the zero-diffraction-order and overlapping those selected different orders. The simulation and optical
experimental results show that the influence of the zero-diffraction-order can be reduced, while keeping the good
uniformity of the target images by this new approach.
OCIS codes: 090.1995, 070.7345, 100.6890.
doi: 10.3788/COL201715.100901.
Reconstructing 2D images from a pixelated spatial light
modulator (SLM), which can be a liquid crystal display
(LCD), a liquid crystal on silicon (LCoS) device, or a dig-
ital micro-mirror device (DMD), has been wid ely used to
project holographic (2D) images
[1–3]
. However, the non-
ideal device features cause the degradation of the quality
of the reconstructed images. Two common problems are
unmodulated light and the pixelation effect.
The unmodula ted light comes mainly from two sources:
(1) light reflected from the surfa ce glass and large size
(larger than light wavelength) pixels; (2) light reflected
from or passing through (depending on reflective or trans-
missive SLMs) the dead area between pixels. Both of them
contribute to the noises in the reconstructed images.
The type 1 unmodulated light usually generates a
bright spot on the reconstruction plane for a Fourier holo-
gram or a bright background on the reconstruction plane
for a Fresnel hologram. It can be reduced significantly by
using an anti-reflection (AR) layer on the glass surface
[4]
.
However, it is not only expensive, but also difficult to re-
move glass reflection completely, especially for a range of
wavelengths and different incident angles. A commercial
AR coating layer can reduce the reflection at a given angle
up to 0.5%
[5,6]
, which still produces a bright enough spot
after lens focusing in the Fourier hologram reconstruction.
Note that the intensity magnification after a lens from a
plane wave to a single point is large, considering the ratio
of the sizes between the SLM pan el and the focus point.
The type 2 unmodulated light caused by the dead area
can be minimized by the existing manufacture technology
with a fill factor of above 90% for the pixel pitch of
5–10 μm
[5,7]
. However, it still produces a noticeable grating
diffraction effect. Light passing through or reflected from
the dead area has a similar effect as the result of type 1
unmodulated light. It causes a zero-diffraction-order light
spot at the optical axis and the higher-order replicas on
the reconstruction plane for a Fourier hologram or multi-
ple bright backgrounds on the reconstruction plane of a
Fresnel hologram. The difference between the type 1
and the type 2 unmodulated lights is that dead areas
are located periodically, the same as pixel cells, so it works
as a 2D grating (matrix) and produces a diffraction
pattern.
Note that the dead area effect can be improved in theory
by coating the dead area with an AR layer in the reflec-
tive-type SLMs or a mask in the transmissive-type SLMs
[4]
without the property of the active area. However, this is
practically difficult, and the AR coating does not work for
the whole visible range light anyway.
The relation between the diffraction angle of different
orders, θ, and the angle of the illumination light, Φ,can
be written as sin θ þ sin Φ ¼ mλ∕d,wherem is the dif-
fraction order number, λ is the light wavelength, and d is
the grating pitch. It can be simplified to d sin θ ¼ mλ ,
assuming Φ is zero (perpendicular illumination)
[8]
.
One thing in need of clarification is that a “zero-
diffraction-order” in this Letter means the central diffrac-
tion light (m ¼ 0) among multiple diffraction orders
[d sin θ ¼ mλ, where m ∈ ð−n; nÞ]. In the holographic
image reconstruction by using SLMs, zero-diffraction-
order normally means the central part of type 2 unmodu-
lated light, which accidentally has the same position of the
bright spot caused by the type 1 unmodulated light, since
the glass surface and the back panel are parallel.
When using a pixelated SLM to generate a holographic
image, there are multiple replicative images of high dif-
fraction orders due to the pixelation effect, as labeled
by the blue arrows in Figs.
1(a) and 1(b). The diffraction
angle between two orders is the same for the holographic
image reconstruction because the grating diffraction of
the dead area shares the same pixel pitch on the same
device. Each order of the reconstructed image comes with
COL 15(10), 100901(2017) CHINESE OPTICS LETTERS October 10, 2017
1671-7694/2017/100901(4) 100901-1 © 2017 Chinese Optics Letters