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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 P1 and P2 are represented as P1, 2 1 / 2 1 / 2 cos( 1, 2 sin( 2ft ) 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 4m, the length of parallel electrodes is L=10mm, and the gap between electrodes is G=24m. 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. References 1) F. Bien, M. Camac, H. J. Caulfield, and S. Ezekiel: Appl. Opt. 20 (1981) 400 2) B. Chen, X. Cheng, and D. Li: Appl. Opt. 41 (2002) 5933 3) C. E. Towers, D. P. Towers, D. T. Reid, W. N. MacPherson, R. R. J. Maier, and J. D. C. Jones: Optics Lett. 29 (2004) 2722 4) R. C. Twu, H. Y. Hong, and H. H. Lee: Opt. Express 16 (2008) 4366