POLYMER ELECTRO-OPTICS: Chromophores bulk up for subvolt modulators
Although the telecommunications industry has long been interested in the use of electro-optic (EO) polymer modulators in fiberoptic communications links, the required radio-frequency drive voltages have remained relatively high—with 5 V (half-wave) typical for polymeric and lithium niobate modulators. The solution may lie with polymer-based EO modulators developed at the University of Washington (UW; Seattle, WA) and fabricated at the University of Southern California (USC; Los Angeles, CA). The experimental devices convert electrical signals to optical signals at up to 100 Gbit/s using half-wave voltages of 0.8 V (at a telecommunications wavelength of 1318 nm).
The problem with some EO polymer modulators is related to a Mach-Zehnder interferometer architecture, which has only one arm modulated with a microstripline electrode. For some chromophores, intermolecular electrostatic interactions become a problem when there is high chromophore loading.
The new subvolt modulator design uses organic chromophores with their shape modified in the polymer matrix. According to project head Larry Dalton, a professor affiliated with both universities, controlling chromophore shape by bulking up the size of molecules eliminates problems from interactions between their electrostatic fields. Optical push-pull poling and driving also help reduce the half-wave voltage.1
In addition to two interferometric arms, the subvolt modulator has an electrode implementation that achieves optical push-pull while reducing the processing steps and preventing air dielectric breakdown. The electrode design also gives scientists the capability to define the molecular alignment orientation arbitrarily using the applied electric field.
During electric-field poling, the chromophores in the two arms align in opposite directions. The modulation drive signal is applied to the top electrode. The two bottom electrodes are grounded. The driving field is along the poling direction in one arm, the opposite direction in the other. The index modulations in the arms are always of opposite signs. One benefit, says Dalton, is that the total phase difference is thus twice as large as in the single-arm modulation architecture, and the half-wave voltage is reduced by a factor of two.
To demonstrate the use of chromophores and optical push-pull in optical intensity modulators, the USC and UW scientists fabricated several modulator chips using a CLD-1/ polymethylmethacrylate (PMMA) guest-host system as an active waveguide material. Each chip had a 3.2-µm-high polyurethane lower cladding layer, a 1.4-µm-high CLD-1/PMMA guiding layer, and a 2.9-µm-high UV-light-curable upper cladding layer. The chips were tested at Tacan Corp. (Carlsbad, CA).
The researchers believe the devices could some day expand applications for polymeric materials to include functions such as direct integration with high-speed electronic circuits and realization of lossless microwave links.
REFERENCE
- Y. Shi et al., Science 288, 119 (April 7, 2000).
Paula Noaker Powell | Senior Editor, Laser Focus World
Paula Noaker Powell was a senior editor for Laser Focus World.