Fabrication of a large-aperture continuous phase plate in
two modes using atmospheric pressure plasma processing
Xing Su (苏 星)
1
, Longguang Xia (夏龙光)
1
,KanLiu(刘刊)
1
, Peng Zhang (张 鹏)
1
,
Ping Li (李 平)
2
, Runchang Zhao (赵润昌)
2
, and Bo Wang (王 波)
1,
*
1
Center for Precision Engineering, Harbin Institute of Technology, Harbin 150001, China
2
Research Center of Laser Fusion, China Academy of Engineering Physics, Mianyang 621900, China
*Corresponding author: bradywang@hit.edu.cn
Received May 26, 2018; accepted August 16, 2018; posted online August 31, 2018
In order to fabricate a large-aperture continuous phase plate (CPP) using atmospheric pressure plasma process-
ing (APPP) with high efficiency and precision, the position dwell mode and velocity mode were proposed and the
iterative calculation method was developed for the non-constant removal rate. Two 320 mm × 320 mm × 2 mm
CPPs were fabricated with two processing modes. The experiment results show that the velocity mode is capable
of significantly reducing the processing time and shape error. The total processing time is decreased from 13.2 h
to 9.3 h, and the surface shape error is decreased from 0.158λ to 0.119λ (λ = 632.8 nm) (root mean square).
OCIS codes: 220.4610, 220.5450.
doi: 10.3788/COL201816.102201.
Continuous phase plates (CPPs) are an important kind of
diffractive optical element
[1,2]
that plays an important role
in kilojoule- and megajoule-class laser systems of inertial
confinement fusion projects such as the National Ignition
Facility at Lawrence Livermore National Laboratory, La-
ser MegaJoule in France, and the SG-III laser facility in
China
[3–5]
. The function of the CPP is to control the shape,
wavefront profile, and energy distribution of the laser
beam precisely through different continuously varying
microstructures on its surface
[6]
. In recent years, atmos-
pheric pressure plasma processing (APPP) has been pro-
posed, which is a deterministic sub-aperture optical
manufacturing technique with a stable and controllable
Gaussian-shape influence function. These characteristics
make it possible to fabricate a complex topographical
CPP pattern with different spatial wavelengths and
peak-to-valley (PV) values
[7–10]
. The position dwell mode
in APPP, which refers to the plasma torch staying for
the solved time at every dwell point along the given tool
path to realize deterministic material removal, is generally
applied to optical fabrication. However, there exist several
obvious disadvantages in this mode. First, the movement
between the neighboring dwell points generates extra
processing time. Furthermore, the extra time increases lin-
early with the expansion of the machining region, espe-
cially for large-aperture CPP. Second, every dwell time
is only a few seconds and the worktable starts and stops
frequently, which seriously affects the processing stability.
In this Letter, the fabrication of CPP with two modes is
studied. First, the principle of APPP is introduced. Then
the position dwell mode and velocity mode are proposed.
Considering the non-constant removal rate caused by the
thermal effect, the iterative calculation method is devel-
oped. Finally, two 320 mm × 320 mm × 2 mm CPPs
are fabricated with the position dwell mode and veloci ty
mode. The machining result with the velocity mode
reduces the processing time and shape error, which dem-
onstrates the industrial application of APPP for CPP and
other free-form optics.
A schematic diagram of the APPP setup is shown in
Fig.
1. The APPP setup is comprised of a plasma gener-
ation device, a gas supply system, and a computer numeri-
cal control (CNC) machine tool. In order to improve the
stability of the plasma discharge, a double-inlet capacitive
coupling plasma (CCP) torch with shielding gas was de-
veloped to avoid the surrounding flow disturbance. The
inner mixed gas, including He, O
2
, and CF
4
, is excited
in a radio frequency (RF) electromagnetic field at the
end of the needle electrode. The outer shield gas is N
2
.
The flow rate of gas is managed precisely by the multi-
channel mass flow contro ller.
Compared with traditional optical manufacturing tech-
niques, the material removal mechanism of APPP is
chemical etching at atmospheric pressure. The reactive
fluorine species are generated by an RF electromagnetic
field, and these species react with the substrate material.
For a silicon-based material, the reaction product is gas-
eous at room temperature and the material removal is
realized, as shown in Eq. (
1).
SiO
2
þ F → SiF
4
↑ þ CO
2
↑;
SiC þ F þ O → SiF
4
↑ þ CO
2
↑: (1)
The chemical etching process brings several advantages
such as high efficiency, no sub surface damage, and low
cost. Moreover, the influence function is a Gaussian shape
that is suitable for fabricating free-form surfaces. Thus,
APPP shows a strong potential to fabricate a large-
aperture CPP.
A set of trench etching experiments were carried out
to study the removal characteristics and to obtain
the suitable parameters for fabricating CPP. Generally,
COL 16(10), 102201(2018) CHINESE OPTICS LETTERS October 10, 2018
1671-7694/2018/102201(5) 102201-1 © 2018 Chinese Optics Letters
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