Slow Wave Electrooptic Light Modulators

Texas A&M Engineering Experiment Station, College Station TX

Investigators

Abstract

The goal of this project is to demonstrate extremely efficient and highly linear microwave-to-optical conversion using new modulator design principles. The theoretical basis for this proposal was developed recently, while the planned experiments will build upon over a decade of research at Texas A&M in ferroelectric materials for guided wave optics. The proposed electrooptic modulators utilize a traveling wave (TW) Mach-Zehnder interferometer configuration. Integrated grating reflectors in each interferometer arm form an etalon which reduces the average optical propagation speed in the forward direction. The 'slow" waveguide structures provide two features which lead to improved modulator performance over conventional "fast" TW designs: (1) optical/microwave velocity matching in substrates with high electrooptic coefficients and dielectric constants, and (2) enhancement of electrooptic interaction strength due to the "dwell time" of the light in the modulation region. For devices fabricated in the conventional lithium niobate (LN) substrate material, these two factors lead to a potential improvement of an order of magnitude in electrical power dissipation over conventional velocity-matched designs. Additional orders-of-magnitude reduction in driving power is anticipated from the use of tungsten bronze substrates such as strontium barium niobate (SBN), which have much higher electrooptic coefficients than LN. Better response linearity in interferometric modulators is also possible using slow wave structures in both LN and SBN. Etalons with N equally spaced reflectors (N _ 3) which exhibit high transmittance over a wide spectral range have been designed for use in the slow-wave modulators. Since the transmittance of such a structure is periodic in optical frequency, a single modulator with appropriate reflector spacing could be used on any channel in a dense wavelength-division- multiplexed (WDM) communication system. Modulators designed to operate at a wavelength near 1.5 gm will be fabricated in LN and SBN substrates. Conventional lithography, etching, and diffusion techniques will be used to produce waveguide and electrode patterns. Corrugated gratings will be produced on the surface of the substrate by reactive ion etching or ion milling using a holographic phase mask with an argon laser as the light source to define the 0.35 gm-period patterns. Measured electrical power dissipation, pi-voltage (Va), and linearity of response will be compared with results reported for "fast-wave" devices in LNto bandwidths> 10 GHz. These modulators are expected to find application in digital and analog fiber optic communication systems, where order-of-magnitude reductions in electrical power requirements would have a major impact on the size and cost of optical transmission equipment. Furthermore, since the need for very thick (~ 15-30 ~tm) electrodes is eliminated, the cost of the integrated optic chip can be reduced considerably. Analog fiber optic links operating in the GIfl regime would also benefit from enhanced dynamic range, which presently is limited by the maximum drive power available from microwave amplifiers and the linearity of response of integrated optic modulators.