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Development of a Dual-Wavelength Optical Phase Measurement Instrument
Ruey-Ching Twu*, Hong-Yao Hou, and Yi-Huan Lee
Department of Electro-Optical Engineering, Southern Taiwan University, Tainan 710, Taiwan
Phone: +886-6-2533131 ext-3628, Fax: +886-6-2432912, Email*:[email protected]
Abstract: A novel homodyne metrology is demonstrated to simultaneously measure the dual-wavelength optical
phase variations. The homodyne light sources are produced by launching the different incident lights into a same
lithium niobate Zn-indiffused phase modulator. The LabVIEW-based instrument provides flexible signal processing
and real-time data display for the measured results.
© 2010 OSJ
Keywords: phase modulator, lithium niobate, homodyne metrology
1. Introduction
Optical interferometry has numerous industrial
applications in absolute-distance and step-height
measurements [1,2]. To extend the measurement range
and avoid phase ambiguity in a single-wavelength
interferometry, various methods for realizing a dualwavelength interferometer have been introduced in
recent years [1-3]. Therefore, simultaneous and precise
phase measurements of both different wavelengths are
essential in the interferometer. In this study, a commonpath optical homodyne polarization interferometer is
proposed to demonstrate the dual-wavelength phase
measurements performed in a LabVIEW-based
instrument. A Zn-indiffused phase modulator (ZIPM)
fabricated in an x-cut/z-propagation lithium niobate
(LN) substrate [4] that is used for phase modulations of
both
wavelengths.
The
simultaneous
phase
measurements can be achieved by utilizing the parallel
processing and multiplexed capability in the LabVIEW
platform. The experimental results show that the
simulated phases from the ZIPM can be extracted and
real-time display shown on the designed LabVIEW
front panel.
2. Measurement setup and principle
The schematic diagram of the measurement setup is
shown in Fig. 1.
a beam splitter (BS). In an ideal case, the magnitudes of
both orthogonal TE and TM polarizations are equal.
There the TM polarization is parallel with the x-cut of
waveguide substrate. The input lights are coupled into
the ZIPM through an objective lens (L1). The output
lights from the channel waveguide are focused through
another objective lens (L2). The scattering lights can be
blocked after passing through a pinhole (PH). An
optical grating (OG) is used to spatially separate the
different wavelength lights. The refracted angles of the
first-order diffracted light beams are 57 and 40 for the
wavelengths 1 and 2, respectively. The distance
between the grating and photo-detector is 33cm.
Therefore, it is enough to spatially separate two beams
of different wavelengths for two photo-detectors. After
passing through the analyzer (AL) also at the azimuth
angle of 45, the interferometric signals are detected
by the photo-detectors (PD1 and PD2). Then the
voltage signals were received through a dataacquisition module (USB-6251) and sent to a PC-based
LabVIEW environment for data analysis and real-time
display of related parameters. The normalized
interferometric intensities of both wavelengths P1 and
P2 are represented as
P1, 2  1 / 2  1 / 2 cos(  1, 2 sin( 2ft )  1, 2 (t )) .
where 1 and 2 are the modulation depths of applied
ac voltages for the wavelengths 1 and 2,
respectively. f is a modulation frequency of 100 Hz.
1(t) and 2(t) are the time-varying phase variations
for different wavelengths. The modulation depth is
defined as
 
Fig. 1. The measurement setup.
Two incident laser lights of 1=632.8nm (He-Ne laser)
and 2=532nm (Green laser) were used for the
measurement. By using two polarizers PL1 and PL2,
both linear polarizations at +45 respecting to the x-cut
of the waveguide substrate are collimated after passing
(1)
Vac ,
V
(2)
where Vac is the applied ac peak voltage. Vac is 5V for
this experiemnt. V is the voltage for a  phase-shift
between orthogonal polarizations. V is dependent on
the input wavelengths at the same channel waveguide
due to wavelength-dependent refractive indices and
modal profiles. Therefore,  is different under the same
Vac for different wavelengths. The received intensity
signals were further analyzed by a Fast Fourier
Transform (FFT) method performed in the LabVIEW
environment. To extract the phase variations based on
the homodyne technique, the relations between
different harmonic intensities (I1 and I2) and Bessel
functions (J1 and J2) are expressed by
 I1  J 2 ( ) 

 I 2  J1 ( ) 
 (t )  tan 1 
(3)
To verify the capabilities of simultaneous phase
measurements for both wavelengths, a slow voltage Vdc
of 0.01Hz was applied accompany with the ac voltage
of 100Hz. The simulated phase variations are
represented by
 (t )  (2  )no3 r22 Vdc ( L / G ) ,
The dynamic ranges of phase variations are 0.82 and
1.25 rad for wavelengths 1 and 2, respectively. The
ratio 1  2 is around 0.65. The phase curves are
repeatable and the biases are stable for the 632.8nm
wavelength. However, the repeatable phase curves with
gradually shifting biases are observed for the 532nm
wavelength due to more sensitivity photorefractive
effect in the LN crystals. In comparison with the phase
variations of 632.8nm wavelength, the higher dynamic
value of 532 nm wavelength is due to a shorter
wavelength, larger EO coefficient, and higher overlap
integral.
(4)
where  is the wavelength of the incident light, and no
is the ordinary refractive index of the LN substrate.  is
the overlap integral between electric field and guided
mode profile. r22 is the EO coefficient. In the ZIPM: the
width of channel waveguide is 4m, the length of
parallel electrodes is L＝10mm, and the gap between
electrodes is G＝24m. Because the EO coefficient and
overlap integral are wavelength dependent, the phase
variations are different for both wavelengths.
3. Results and discussions
Figure 2 shows the designed LabVIEW front panel for
real-time display the measurement results. All the
analyzed signals and calculated results including
interferometric intensities (P), FFT spectrum, harmonic
intensities (I), ratio of different harmonic intensities,
and phase variations ( ) can be monitored with the
flexible design of LabVIEW front panel. Especially, the
dual-wavelength phase variation can be exported to a
external text files for further comparison.
(3)
(4)
Fig. 3. Phase variations as a function of time
under the simulated applied voltages: (a)
Vdc=2V and (b) Vdc=4V.
4. Conclusions
We proposed and demonstrated a novel dualwavelength phase measurement instrument by
employing an optical homodyne technique. The
received signal and data process is performed in the
LabVIEW platform. Future work will demonstrate this
instrument for a variety of industrial and scientific
applications.
Fig. 2. The designed LabVIEW front panel with a
real-time display on the measured data.
To evaluate the real-time monitoring capabilities of
dynamic phase measurements, the slow sinusoidal
voltages of Vdc=2 and 4V (peak-to-peak) were applied
with the modulation frequency of 0.01Hz in the ZIPM.
The measured corresponding phase variations were
shown in the Fig. 3. In the case of Vdc=2Vas shown in
Fig. 3(a), the dynamic ranges of phase variations are
0.41 and 0.65 rad for wavelengths 1 and 2,
respectively. The ratio 1  2 is around 0.63. To
increase the applied voltage with Vdc=4V, the
increased dynamic ranges were shown in the Fig. 3(b).
Acknowledgement
This work was supported in part by the National
Science Council, Taiwan, R.O.C., under Grant NSC 982221-E-218-006.
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