Null interferometric microscope for ICF-capsule
surface-defect detection
CONG WEI,
1
JUN MA,
1,
*LEI CHEN,
1
JIANXIN LI,
1
FAN CHEN,
1
RIHONG ZHU,
1,2,5
RENHUI GUO,
1
CAOJIN YUAN,
3
JIE MENG,
4
ZONGWEI WANG,
4
AND DANGZHONG GAO
4
1
School of Electronic and Optical Engineering, Nanjing University of Science & Technology, Nanjing 210094, China
2
Key Laboratory of Advanced Solid-State Laser Technology, Nanjing University of Science & Technology, Nanjing 210094, China
3
Jiangsu Key Laboratory on Optoelectronic Technology, School of Physics and Technology, Nanjing Normal University, Nanjing 210023, China
4
Research Center of Laser Fusion, China Academy of Engineering Physics, Mianyang 621900, China
5
e-mail: zhurihong@njust.edu.cn
*Corresponding author: majun@njust.edu.cn
Received 20 June 2018; revised 11 September 2018; accepted 17 September 2018; posted 17 September 2018 (Doc. ID 335644);
published 16 October 2018
Isolated defects on the surface of the inertial confinement
fusion (ICF) capsule reduce the probability of ignition.
Here, to the best of our knowledge, we present the first null
interferometric microscope (NIM) for direct and large-field
surface defects detection on ICF capsules. The planar refer-
ence mirror in conventional interferometric microscopes is
replaced by a spherical reference mirror to achieve null in-
terference in the full field of view. Further, via the use of a
short-coherence light source system, parasitic fringes are
avoided. The feasibility of the NIM is verified via experi-
ments on a 0.7 mm diameter capsule. A 1 mm diameter
ICF capsule is also t ested by the NIM to prove that the
NIM has the ability to measure capsules with different
diameters.
© 2018 Optical Society of America
OCIS codes: (120.0120) Instrumentation, measurement, and metrol-
ogy; (120.6650) Surface measurements, figure; (180.3170)
Interference microscopy.
https://doi.org/10.1364/OL.43.005174
In the inertial confinement fusion (ICF) experiment, the cap-
sule is the core element whose diameter is from a few hundred
micrometers to millimeters. However, isolated defects on ICF
capsules can seed Rayleigh–Taylor instability during implosion,
which leads to ablator material mixing with the hot core, cool-
ing the core and reducing the probability of ignition [1]. The
defects, called “bumps,” grow up during the coating in a cone
shape from a single point. To ensure stable capsule performance
during ignition shots at the National ignition Facility (NIF),
the capsule surface must be free of foreign particles of volumes
>30 μm
3
[2]. The stringent cleanliness requirement demands
a system that is capable of reliable three-dimensional measure-
ments of isolated defects.
Thus far, traditional point scanning methods, including
atomic force microscopy [3,4] and confocal microscopy
[5,6], have been used for ICF capsule surface profile
measurements. Although very high vertical resolution can be
obtained with these methods, drawbacks such as low efficiency
and low lateral resolution between scanning paths make them
unsuitable for isolated defect detection. As a more effe ctive
method, digital holographic microscopy (DHM) [7–9] has
been applied to characterize the ICF capsule. DHM records
only one hologram that is form ed by the interference of the
wave from the capsule with a reference wave. Subsequently,
the hologram is numerically processed to extract both ampli-
tude and phase information. However, hologram generation
requires a high-coherence light source, which can cause coher-
ence noise and, further, parasitic fringes formed by the inner
surface cannot be avoided. Moreover, the lateral resolution
of DHM is limited by the spatial carrier frequency.
Phase-shift interferometry (PSI) is an optical metrology
technique that is used for high-accuracy, noncontact analysis
of surfaces. In particular, phase-shifting diffraction interferom-
etry (PSDI), which has been used extensively to measure planar
diffraction-limited mirrors for extreme ultraviolet lithography,
has also been applied to capsule metrology [10–12]. PSDI uses
a pinhole to generate a perfect spherical wave that illuminates
the ICF capsule through an objective lens, but it does not image
the surface of the capsule directly; the phase is calculated by
propagating the wavefront from the detector surface to the
capsule surface numerically. However, the accuracy of diffrac-
tion calculation cannot be guaranteed due to uncertainty in the
positioning of the capsule and the complicated system error
model. Although PSDI accurately measures smooth, spherical
surface features, it is not reliable for the measurement of
isolated defects [2].
Against this backdrop, in this Letter, we present a null in-
terferometric microscope (NIM) for the direct and large-field
detection of ICF-capsule surface defects. An interferometric
microscope (IM) is the combination of an interferometer
and a microscope. Compared with other PSI methods such
as Fizeau interferometer, IM has higher magnification and res-
olution, and the information obtained is intuitive, which makes
5174
Vol. 43, No. 21 / 1 November 2018 / Optics Letters
Letter
0146-9592/18/215174-04 Journal © 2018 Optical Society of America