Optical-Guided-Wave Modulators
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MRS BULLETIN/AUGUST 1988
1000°C. Both planar waveguides (confined only in the depth direction) and Channel waveguides (confined in the width and depth directions) can be accomplished through appropriate pattering of the titanium. The Steps involved in waveguide fabrication are outlined in Figure 1. While Figure 1 depicts the formation of a Single waveguide it also applies to any other complicated waveguide circuit. Refractive index changes achieved by this process are approximately 1%. LiNb0 3 is a fascinating material exhibiting electro-optic, acousto-optic, piezoelectric, and pyroelectric effects. (Reference 10 gives an excellent summary of the various physical constants.) Using waveguides in the material can confine the light spatially to achieve very efficient electrooptic effects. Efficiencies can be improved greater than 1,000-fold over those obtained with bulk devices. To apply the electric fields which interact with the optical fields, metal electrodes are deposited on the surface as shown in Figure 2.
modulation at \ = 1.3 p and with modulation rates of 3 GHz. Such devices can be used in a variety of applications including coherent Communications, phased array Systems, and sensors. The Mach-Zehnder (MZ) intensity modulator shown in Figure 3b allows the optical intensity to be modulated as a function of applied voltage. The Operation of this device is similar to that of a classical Mach-Zehnder interferometer where phase change differentials in the interferömeter's arms are converted into an intensity change at the Output (via a spatial shift in the interference fringe pattern). In the MZ modulator shown in Figure 3b, the input light is split into the arms of the waveguide interferometer at the Y-splitter. At the second Y-splitter, if the light in the two arms is in phase, then it constructively interferes exciting the lowest order mode of the Output waveguide. If instead it is 180° out of phase (because of an applied 180° phase differential due to a voltage applied to the electrodes), then it destructively recombines, exciting the secpnd-order mode. Since the Output waveguide will only support the lowest order mode, the second mode radiates into the Substrate and thus no light exits from the waveguide's Output. An expression for the Output light intensity, 7(r) as a function of driving voltage, V(t) is
where V„ = TT/KL, the voltage required to change I(t) from its maximum value to its minimum value, K and L are defined as they were for the phase A variety of high-performance devices modulator, and is any static phase can be built using simple waveguide imbalance that may be present due to and electrode configurations. Three slight asymmetry in the interferometer such devices are shown in Figure 3. Figarms. It has been shown that the I(t) of ure 3a schematically depicts a phase an actual MZ intensity modulator corremodulator comprised of a straight sponds to 2 to within 1%, even with waveguide and a pair of electrodes. A applied voltage V* greater than 50 V„. voltage applied to the electrodes results Appli
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