134 CHINESE OPTICS LETTERS / Vol. 8, No. 2 / February 10, 2010
High quality beam shaping by square soft-edge diaphragm
combined with liquid crystal spatial light modulator
Yunfeng Ma (æææÂÂÂ)
1,2,3
, Zhongwei Fan (®®®)
1,3∗
, Jisi Qiu (¤¤¤ÄÄÄddd)
1,2,3
,
Chengyong Feng (¾¾¾«««]]])
1,2,3
, Tianzhuo Zhao (ëëëUUURRR)
1,2,3
, and Weiran Lin (,,,)
1
1
The Academy of Opto-Electronics, Chinese Academy of Sciences, Beijing 100080, China
2
Graduate University of Chinese Academy of Sciences, Beijing 100049, China
3
Beijing GK Laser Technology Co., Ltd, Beijing 100085, China
∗
E-mail: fanzw002@163.com
Received April 29, 2009
The square soft-edge diaphragm with round angle is designed by Matlab, and is sent to a liquid crystal
spatial light modulator by the computer. In order to obtain precompensation for the following laser system,
lo cal diaphragm transmission can be adjusted by feedback signals of surface-channel charge-coupled device
(SCCD). This method can reduce the diffraction effect and realize no modulation, high stability, high
homogeneity, and large scale laser beam. Several parameters of soft-edge diaphragms which affect the
laser beam quality are studied systematically, and the optimized values are obtained. The method can
avoid the serious modulation of hard edges and provide soft-edge diaphragms of different shapes in a fast
and convenient way for the large scale laser beam system.
OCIS co des: 050.1220, 140.3300, 230.3720.
doi: 10.3788/COL20100802.0134.
High power and high brightness lasers have wide ap-
plications in industry, military, and everyday life. The
methods to obtain large scale, high stability, high homo-
geneity, and high power lasers have attracted more and
more attention in recent years
[1−3]
.
Generally, in order to obtain the large scale laser beam
with high homogeneity, high intensity, and no ghost
point, formally, the laser is first extended by extender
lens, then, it is shaped by a diaphragm, and finally, it is
filtered by a spatial filter. So the large scale laser can
reach the demands of high quality
[4]
. There are several
diaphragms for shaping. For example, hard-edge di-
aphragm, saw-tooth diaphragm, etc.
[5−7]
However, these
diaphragms induce seriously high frequency comp onents
which affect the large scale laser beam quality. And the
conventional methods of making diaphragms include pho-
toetching, coating, machine processing, etc. Once these
diaphragms are made, their shapes cannot be changed
at will. Simultaneously, all methods do not make high-
quality soft-edge diaphragms and the transmission is not
adjustable.
Recently, the liquid crystal spatial light modulator
(LC-SLM) is used to shape the phase and amplitude
[8]
.
The advantage of the LC-SLM is that the shape of the
diaphragm is adjustable randomly by Matlab, and the
transmission and phase can be adjusted by gray scale
control of the image by feedback signals
[9]
. Chen et al.
proposed an idea of large scale laser shaping by the LC-
SLM
[10]
. Qin et al. reported square laser beam shaping,
by which the high frequency component is always easier
to be erased through a spatial filter
[11]
.
Nevertheless, the systemic square soft-edge diaphragm
with round angle has not yet been reported, which is
introduced in detail in this letter including mathemati-
cal model. The soft-edge diaphragm was designed at the
wavelength of 1064 nm and two key parameters of soft-
edge diaphragm were also discussed in our experiment.
It is well known that the sharp angle and hard edge
induce serious modulation inside the large scale laser
beam
[12−13]
. So the diaphragm is designed with a soft
edge and four round angles. The process of design is as
follows.
1) Set up Gamma curve
Since different gray scales have different transmissions
at 1064 nm, images of 0–255 red-green-blue (RGB) values
are imported into the computer through a video graphics
array (VGA) data line. The images are made for a step
of every 5 RGB values, by which the transmission values
are calculated and Gamma curve is plotted in Fig. 1.
The results show that the behavior strongly depends
on the p olarizer used. A better polarizer (1000:1) will
enhance the contrast (200:1) at 1064 nm. By virtue of
the Gamma datum, the soft-edge diaphragm is designed
by Matlab.
2) Design soft-edge diaphragm image
The LC-SLM shows serious diffraction effect and low
transmission of electrode plates. These effects pro duce
edge effect and energy loss which result in a duty ratio of
only 55% with a transmission of 20%. According to the
Gamma datum, the soft-edge diaphragm image is made
Fig. 1. Amplitude modulation curve which is called Gamma
curve.
1671-7694/2010/020134-04
c
° 2010 Chinese Optics Letters