164 CHINESE OPTICS LETTERS / Vol. 5, No. 3 / March 10, 2007
Sinusoidal phase-modulating laser diode interferometer for
real-time surface profile measurement
Guotian He (
)
1,3
, Xiangzhao Wang (
)
1
, Aijun Zeng (
)
2
, and Feng Tang (
)
1
1
Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800
2
Shanghai Hengyi Optics & Fine Mechanics Co., Ltd., Shanghai 201800
3
Graduate School of the Chinese Academy of Sciences, Beijing 100039
Received October 25, 2006
A sinusoidal phase-modulating (SPM) laser diode (LD) interferometer for real-time surface profile mea-
surement is proposed and its principle is analyzed. The phase signal of the surface profile is detected from
the sinusoidal phase-modulating interference signal using a real-time phase detection circuit. For 60 × 60
measurement points of the surface profile, the measuring time is 10 ms. A root mean square (RMS) mea-
surement repeatability of 3.93 nm is realized, and the measurement resolution reaches 0.19 nm.
OCIS codes: 120.3180, 220.4840, 230.5750.
Many kinds of sinusoidal phase-modulating (SPM) laser
diode (LD) interferometers have been proposed in the
past years
[1−5]
. In the SPM interferometers, LD is used
as the light source. Because the wavelength of the LD can
be controlled by its injection current, the phase modula-
tion required by the interferometer is easily produced by
modulating the injection current of the LD. The SPM LD
interferometers have many merits such as compactness,
light weight, easy modulation, and high efficiency
[6−8]
.
The interferometers can be used to measure the surface
profile, displacement, vibration, and angle etc.
[9−15]
.
With the development of precise manufacturing, real-
time and precision surface profile measurement becomes
more and more important. There are mainly three types
of SPM LD interferometers used for the surface profile
measurement. One is Fourier transform metho d
[3]
,which
is a non-real-time surface profile measurement technique.
The other two are phase-lock method
[5]
and integrating-
bucket method
[11]
, which are both real-time measure-
ment techniques. For the phase-lock method, when the
amplitude of the interference signal fluctuates due to the
external disturbances, a reset control of the peak-hold
circuit is required. However, it is difficult to reset this
control exactly. The integrating-bucket metho d can ob-
tain a serial data acquisition rate of 10 μs/point and
a measurement repeatability of ∼ λ/60, but the dis-
crimination condition, i.e. the efficiency of sin α(x, y)
equals that of cos α(x, y), the sinusoidal phase mod-
ulation depth z =2.45, and the initial phase of the
sinusoidal phase modulation θ =56
◦[11]
, is difficult to
meet in practice. Furthermore, the phase-lock method
and the integrating-bucket method require complex elec-
tronic circuits for feedback control, the time delay of the
signal processing is relatively long.
In this paper, we propose a novel real-time surface
profile measurement system. In this system, the phase
signal can be obtained directly from the interference
signal using a real-time phase detection circuit. The
integrating-bucket discrimination condition is not re-
quired. The measurement accuracy is improved and the
measurement time is reduced with simple processing cir-
cuits.
The SPM LD interferometer for real-time surface
profile measurement is shown in Fig. 1. A LD is used
as the light source. After collimated and expanded by a
lens L1, the beam from the LD is split into two beams
by a beam splitter (BS). One beam is reflected by a
mirror (M) and used as the reference beam, the other
is reflected by the object to be measured and used as
the object beam. The interference is formed by the two
beams. Theinterferencesignal is detected by a charge-
coupled device (CCD) ima ge sensor, and is input to a
real-time phase detection circuit. The real-time phase
detection circuit is shown in Fig. 2. It consists of two
amplifiers , a calculator, a low-pass filter (LPF), and a
sequential control circuit. After amplified, calculated,
and filtered, the interference signal is transformed into
phase signal. With the rea l-time discrimination circuit,
Fig. 1. Diagram of the SPM LD interferometer for real-time
surface profile measurement. S/H: sample/hold signal.
Fig. 2. Block diagram of the real-time phase detection circuit.
1671-7694/2007/030164-04
c
2007 Chinese Optics Letters
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