Ultrahigh speed photonic components are necessary to exploit the wide bandwidth available when using lightwaves as the carrier frequency for telecommunication and instrumentation applications. On the transmitter end, a high speed modulator is needed to impress the electrical signal onto the lightwave carrier. Direct modulation of a semiconductor laser is most efficient but suffers from spectral broadening . Spectrally pure signal are necessary for long distance transmission in optical fibers neccesitating the development of external modulators. Over two decades ago, modulators were developed in both lithium niobate and GaAs -. However due to the poor material quality in GaAs resulting in high excessive insertion losses, the material of choice was lithium niobate. With numerous refinement, the modulator and switch technologies in lithium niobate have progressed to Mach Zhenders and directional couplers with bandwidths in excess of 40 GHz. With recent improvements in III-V epitaxial technologies, substrate qualities and superlattice techniques to pin defects, the III-V modulator is once again been studied   . Moreover, because III-V materials offer the obvious advantage of monolithic integration of active/passive photonic and electronic devices to form the foundation of optical electronic integrated circuits (OEIC), III-V may eventually replace lithium niobate. Travelling wave GaAs electro-optic waveguide modulators at a wavelength of 1.3 μm with bandwidth in excess of 20 GHz have been developed and characterized. The design and characteristics of both p-i-n modulators in microstrip configuration and Schottky barrier on n− – GaAs/semi-insulating (S.I.) GaAs in the coplanar strip configuration modulators will be discussed. It will be shown that microwave loss and slowing on n+ GaAs substrates will limit the bandwidth of the microstrip modulator to less than 10 GHz for a device 8 mm in length. Modulators with bandwidths in excess of 10 GHz are fabricated on S.I. GaAs substrates. The structures of the devices are shown in Fig.(l) and (2) .
© 1988 Optical Society of AmericaPDF Article
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