Improved phase-shifting diffraction interferometer for
microsphere topography measurements
Guodong Liu (刘国栋), Binghui Lu (卢丙辉), Heyi Sun (孙和义), Bingguo Liu (刘炳国)*,
Fengdong Chen (陈凤东), and Zhitao Zhuang (庄志涛)
Instrument Science and Technology, Harbin Institute of Technology, Harbin 150001, China
*Corresponding author: liu_bingguo@hit.edu.cn
Received April 25, 2016; accepted May 5, 2016; posted online June 13, 2016
In this study, an improved phase-shifting diffraction interferometer for measuring the surface topography of a
microsphere is developed. A common diode-pumped solid state laser is used as the light source to facilitate ap-
paratus realization, and a new polarized optical arrangement is designed to filter the bias light for phase-shifting
control. A pinhole diffraction self-calibration method is proposed to eliminate systematic errors introduced by
optical elements. The system has an adjustable signal contrast and is suitable for testing the surface with low
reflectivity. Finally, a spherical ruby probe of a coordinate measuring machine is used as an example tested by
the new phase-shifting diffraction interferometer system and the WYKO scanning white light interferometer for
experimental comparison. The measured region presents consistent overall topography features, and the result-
ing peak-to-valley value of 84.43 nm and RMS value of 18.41 nm are achieved. The average roughness coincides
with the manufacturer’s specification value.
OCIS codes: 120.5050, 120.3180, 120.6650, 220.4830, 260.5430.
doi: 10.3788/COL201614.071202.
Microspheres are common but important objects
[1,2]
that
are widely used in micro-mechanics and micro-op tics.
Higher-precision measurements of microsphere topogra-
phies are required to enable the development of micro-
and nanoscale fabrication technologies. For example,
small ignition-target shells are an important component
in inertial confinement fusion (ICF)
[3,4]
, and even minute
defects on the surface of the target may result in failure
to achieve ignition. Accurate measurements are necessary
in mass production to identify the products that meet the
specifications.
Atomic force microscopy (AFM), a traditional method
for measuring the topography of a microsphere, requires
rotating and scanning the test samples
[5]
. Although very
high vertical resolution can be obtained, drawbacks in-
clude low efficiency and low lateral resolution between
scanning path s, and isolated defects may be missed.
Interferometry possesses considerable advantages in-
cluding accuracy, efficiency. and noncontact measurement
capabilities
[6–8]
. However, most commercial interferome-
ters are not designed for measuring microscopic objects.
Lawrence Livermore National Laboratory (LLNL) devel-
oped a phase-shifting diffraction interferometer (PSDI)
for ICF capsule inspection
[9]
that uses a short-coherence-
length laser to control the interference through phase
shifting. However, the high level of background light low-
ers the SNR, particularly when measuring lo w-reflectance
surfaces. Therefore, in this study we present a new design
for a PSDI for measuring the topography of a microsphere.
This design has a higher SNR and is simpler to implement
because a common laser is used.
A diagram of the PSDI is shown in Fig.
1. The linearly
polarized output beam from a diode-pumped solid state
laser (DPSSL) passes through a half-wave plate (HWP
1
),
which is used to adjust the intensity ratio. The beam is then
reflected by a right-angle prism into a polarized beam split-
ter (PBS). The transmitted beam is used for measurements
and the reflected beam is used as a reference. The beams are
directed to two retroreflectors, RF
1
and RF
2
.RF
2
is sta-
tionary and RF
1
is mounted on a phase shifter contained
in a delay stage that provides optical path compensation.
The measurement and reference beams are recombined at
the PBS and then pass through a second half-wave plate
(HWP
2
) before being directed into a single-mode polariza-
tion-maintaining fiber (SMPMF), which is used for light
transmission with vibration isolation and primary filtering.
The output beam from the SMPMF is focused by an objec-
tive (L
1
) onto a pinhole, which produces a nearly perfect
spherical wave by diffraction. The output beam from the
Fig. 1. Diagram of the PSDI.
COL 14(7), 071202(2016) CHINESE OPTICS LETTERS July 10, 2016
1671-7694/2016/071202(4) 071202-1 © 2016 Chinese Optics Letters