Electrooptic Polymer Mach—Zehnder Modulators

Abstract

Electro-Optic polymer Mach-Zehnder modulators are described with regard to design, fabrication, and testing at high operating speeds. The expected frequency response is discussed in terms of an electrical signal propagating co-linearly with an optical signal, as well as the conductivities of the electrodes. The effects of polymer conductivity and poling efficiency on modulator switching voltage are described. Modulators fabricated to have 5 V switching levels were tested using a bit error rate test system, and the results show excellent response times at a clock rate of 900 Mb/s. The response was limited by test equipment, and the actual performance is calculated to be significantly above the equipment limitations.Electro-optic (EO) modulators based on certain polymer materials have significant advantages over inorganic devices, such as LiNt>03, in four major areas: 1) higher operating speeds using simple electrode designs, 2) lower manufacturing costs using integrated circuit (IC) fabrication techniques, 3) ease of integration with underlying substrate circuitry, and 4) simpler optical fiber attachment using inherent characteristics of Si substrates. The high speed device operation using simple electrode designs, such as a true microstrip electrode, is due to a low effective dielectric constant, on the order of 2.5. This permits the electrical phase front of a microstrip electrode, which is directly over an optical waveguide, to travel at essentially the same velocity as the optical phase front in the waveguide. Thus, small changes in the polymer index of refraction caused by the electrode field will add cumulatively as the electrical and optical fronts travel together. Also significant to very high speed operation is the fact that the EO response in these polymers results from small changes in the system of electrons responsible for polarization, rather than a displacement of nuclear coordinates as in inorganic materials. 1 The low cost of making polymer devices results from using small quantities of polymer materials fabricated on top of substrates such as Si or GaAs, while employing the same equipment that is used to build ICs. In addition, polymer waveguide structures can be easily fabricated direcdy on top of circuitry previously built into these substrates. This is much more difficult, or impossible, for inorganic materials such as LiNb03. Attachment of optical fibers to polymer devices can be assisted by using Si wafers with etched V-grooves that align precisely with the 1 Current address: Applied Materials, Santa Clara, CA 95050 0097-6156/95/0601-0456$12.00/0