
Adaptive modulation system for
liquid crystal phase modulator
Bin Zhang (张彬), Shiyu Liu (刘诗雨), Xianzhu Tang (唐先柱), and Jian'gang Lu (陆建钢)*
Department of Electronic Engineering and National Engineering Lab for TFT-LCD Materials and Technologies,
Shanghai Jiao Tong University, Shanghai 200240, China
*Corresponding author: lujg@sjtu.edu.cn
Received April 20, 2016; accepted July 19, 2016; posted online August 9, 2016
An adaptive modulation system for a liquid crystal (LC) phase modulator is demonstrated. The phase retar-
dation may be modulated by resetting the driving voltage automatically by matching the measured and ideal
transmittance of an LC cell sandwiched by crossed polarizers. By using this system, an LC phase modulator can
get a low error function of 0.25% in a short modulation time, which is less than the 10% obtained using a conven-
tional modulation method.
OCIS codes: 060.5060, 160.3710.
doi: 10.3788/COL201614.090604.
Phase modulators are playing a more and more important
role in modern optics. They can be widely applied in many
fields, such as optical tomography
[1]
, digital holography
[2]
,
spatial filtering
[3]
, optical communications
[4–7]
, optical
manipulation
[8,9]
, optical microfibers
[10]
, and optical fiber
sensing
[11]
. Due to the electric-control birefringence, liquid
crystal (LC) devices are becoming one of the most com-
petitive candidates for phase modulators. To improve
the performance of the LC phase modulator, multi-
electrode structures were introduced to match the ideal
refractive index profile, as it makes different LC molecular
orientation on different electrode
[12–15]
. Although the re-
sponse time of LC is usually a few milliseconds, the time
of conventional modulation process is mainly determined
by three steps: (1) estimate the phase retardation profile
to see if it is good or not, (2) decide how to revise a set of
driving voltages on electrodes, and (3) manually reset the
driving voltage on all electrodes. To reach its designed
performance, a multi-electrode LC device needs to repeat
the phase modulation process mentioned above at least
dozens of times. Therefore, the tedious and complex
modulation process takes at least several hours. The
refractive index profile of an LC phase modulator cannot
be illustrated directly in the modulation process, but
only evaluated indirectly in the application by its
performance.
In this Letter, an adaptive modulation system was pro-
posed to yield a refractive index profile that matches the
ideal one automatically. With this system, the refractive
index profile can be real-time evaluated, and the modula-
tion process can be efficiently shortened to 10% of that by
using a conventional modulation method.
Firstly, a typical LC cell sandwiched by a polarizer and
an analyzer is shown in Fig.
1. The relationship between
the incident light intensity (I
0
) and the emergent light
intensity (I ) can be expressed as Eq. (
1)
[16]
, when the
polarization directions of the polarizer and analyzer are
orthogonal,
T ¼
I
I
0
¼
1
2
sin
2
ð2βÞsin
2
πðn
eff
− n
o
Þd
λ
; (1)
where T is the transmittance, β is the polarization direc-
tions of the polarizer, d is the cell gap of the LC phase
modulator, and n
o
and n
eff
are the ordinary refractive
index and the effective refractive index of the LC, respec-
tively. If β is set to π∕4, which leads to the maximum T ,
Eq. (
1) can be given by
T ¼
I
I
0
¼
1
4
1 − cos
π ·
2ðn
eff
− n
o
Þd
λ
: (2)
Fig. 1. Typical LC phase modulator sandwiched by polarizer
and analyzer.
COL 14(9), 090604(2016) CHINESE OPTICS LETTERS September 10, 2016
1671-7694/2016/090604(4) 090604-1 © 2016 Chinese Optics Letters