October 10, 2006 / Vol. 4, No. 10 / CHINESE OPTICS LETTERS 601
Design and application of a laser beam alignment system
based on the imaging properties of a multi-pass amplifier
Daizhong Liu (
)
1
, Fengnian L¨u (
)
1
, Jinzhou Cao (
)
1
, Renfang Xu (
DZDZDZ
)
1
,
Jianqiang Zhu (
)
1
, Dianyuan Fan (
)
1
, Jianbo Xiao (
)
2
, and Xiaojun Zhou (
)
3
1
National Laboratory on High Power Laser and Physics, Shanghai Institute of Optics
and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800
2
Guangdong Ronghui Information Engineering Co.,Ltd, Guangzhou 510665
3
State Intellectual Property Of f ice of China, Beijing 100088
Received November 21, 2005
Image relaying is presented as a technique for aligning beams onto mm-sized target of high power laser. On
the basis of summarizing the preceeding w ork on the near-field image relaying of multiple spatial filters,
the far-filed image relaying is suggested firstly. The near-field and far-field image relaying properties of
multiple spatial filters in laser beams automatic alignment system are analyzed. A geometrical optics
approach and an ABCD ray matrix theory are used throughout. The reasonable and optimum scheme for
automatically aligning multi-pass beam paths is presented and demonstrated on the multi-pass amplifier
system of the SG-III prototype.
OCIS codes: 220.1140, 110.2970, 330.6110.
The inertial confinement fusion facility is the largest
and most complex one of high power lasers, such a s the
Nova laser, the National Ignition Facility, the GEKKO-
XII laser of Japan, the LMJ project of France, and the
SG-III facility of China. These lasers, propagating from
the master oscillator driver to the target, interact with
more than 100 near-field (NF) optical elements and pass
through several spatial filter pinholes over a distance
exceeding 100 m. To ensure the accuracy of these laser
systems, the beam automatic alignment system has been
installed on all of them. It has become an important and
absolute necessary part of this kind of laser.
It is well known that the multiple spatial filters can
sharply minimize nonlinear effects in high power laser
amplifiers
[1,2]
, which is accomplished by employing both
the normal filtering properties and the imaging proper-
ties of a spatial filter. This phenomenon may be called as
NF image relaying. O n basis of summarizing the preceed-
ing work on the NF image relaying of multiple spatial
filters, the far-field (FF) image relaying is suggested
firstly. The application of NF and FF image relaying
properties of multiple spatial filters in laser beam auto-
matic alignment system is analyzed. A geometrical op-
tics approach and an ABCD ray matrix theory are used
throughout. An automated, closed-loop, image relaying,
Fig. 1. Typical optical system for NF image relaying.
laser b eam alignment system is describ ed here. Its func-
tion is to sense beam alignment errors in the laser beam
system and automa tically steer mirrors to maintain beam
alignment. A reasonable and optimized scheme for au-
tomatically aligning multi-pass beam paths is presented.
It is demonstrated experimently on the multi-pass am-
plifier system of the SG-III prototype
[3]
.
A typical NF image relaying optical system is depicted
in Fig. 1, in which confocal lens pairs are used to meet
these imaging properties. The optical transfer matrix for
the kth pairs element of this system is
t
NF
k
=
AB
CD
=
−m
k
−m
k
d
1
k
− (m
k
)
−1
d
2
k
+ f
1
k
+ f
2
k
0 −1/m
k
, (1)
where m
k
=(f
2
k
)/(f
1
k
) is the magnification of the lens
pair. For a system requiring N lens pairs, the system
transfer matrix is
T
NF
N
=
N
k=1
t
NF
k
. (2)
In terms of geometrical optics, if
B = m
k
d
1
k
+(m
k
)
−1
d
2
k
− f
1
k
− f
2
k
=0, (3)
the beam profile in the plane d
1
k
will be imaged on the
plane d
1
k+1
with magnification m
k
.WhenEq.(3)sat-
isfies all k, the beam will be repeatedly imaged or relayed
throughout the optical train
[1,2,4,5]
.
Based on the research of NF image relaying, we find
that multiple spatial filters have properties of FF image
relaying as well. This means that the transverse intensity
profile in one spatial filter’s focal plane would be reim-
aged onto the next spatial filter’s focal plane. Figure 2
1671-7694/2006/100601-04 http://www.col.org.cn