Ghz Traveling-Wave Optical Modulator For Precision Distance Measurement

Abstract

GHz Traveling-wave Optical Modulator for Precision Distance Measurement Ichiro FUJIMA, Katuo SETA, Hirokazu MATSUMOTO and Tadanao OH'ISHIDepartment of Quantum Metrology, National Research Laboratory of Metrology 1-1-4, Umezono, Tsukuba, Ibaraki 30$, JAPANABSTRACTTraveling-wave type optical modulators using LiTaO^ crystals have been designed and fabricated for high resolution distance measurement. The modulation property of the modulator is characterized at GHz frequencies. The distance measuring-system is constructed using two of these optical modulators and is preliminarily tested. The modulators were driven by slightly different frequencies in the GHz region in order to measure the phase of the modulated signal using the heterodyne method. The displacement of a target was measured with resolution of 1 5um and with a linearity of 0.2%.1. IntroductionRecently, long distance measurement technique with a high accuracy is required in many fields with development of science and industry; such as ranging to earth orbiting satellites, geodetic survey and so on. Especially, it is useful for earthquake prediction, because observation of small strains of earth's crust is one of the most efficient methods. However, in order to measure long distance accurately, it is required to correct the refractive index of air along the optical path.The optical crystal modulation method is one of best choices because very high resolution can be obtained by increasing the modulation frequency in the distance meter. For the refractive index correction of air by the multi-color method, very high frequency modulation is necessary because the distance-measuring resolution is reduced by several ten times.''' In this paper, optical modulators using crystals with traveling-wave electrode structure have been designed, fabricated and tested. And also preliminary distance measurement was done using these modulators.2. Requirement on optical modulators2.1 Optical powerThe intensity of laser beam is reduced by a factor of about 10~5~1o~^^ when the beam propagates along several tens of kilometers in the atmosphere. In Japan, the effect will be serious since the transparency of the atmosphere is poor. In general, the optical output power of several milliwatts is required at least for distance measurement using the optical modulation method. Therefore, it is difficult to use a LiNbO^ waveguide optical modulator which has a possibility of high frequency modulation, because the optical damage threshold is low and is less than output power of several milliwatts. On the other hand, bulk type crystal is able to obtain big power output because no optical damage exists.2.2 Modulation frequencyThe resolution of distance measurement is improved by increasing modulation frequency. For distance measurement of several tens of kilometers, however, the modulation frequency of a few GHz seems to be proper because higher modulation frequency is no use due to atmosphere turbulence.2) The GHz optical modulators using bulk crystals are classified into the resonator type and the traveling-wave type. The traveling-wave type electro-optic modulator (TEOM) can be operated with low voltage microwave supply while the resonator type needs high power supply. The resonator type has another demerit due to its narrow bandwidth, the large temperature effect in the modulation efficiency. On the other hand, the TEOM has broad bandwidth. Therefore, the TEOM was chosen, and designed so that it may have 3dB-bandwidth of 3GHz.3. Fabrication of optical modulatorsFigure 1 (a) shows the outline of the electro-optic modulator using a LiTa03 crystal (transverse field type). Electric field is parallel to Z-axis of the crystal and light beam runs parallel to Y-axis. Incident laser beam is linearly polarized light and the polarization direction makes 45 degrees to X-axis and Z-axis of the crystal. Figure 1(b) shows the construction of. the crystal used in our TEOM. Its dimension is 0.5mm wide, 0.65mm high and 10mm long. Both of the end planes of the crystal are anti-reflection coated, and the crystal has Au electrodes on upside and on down planes. The crystal is