Polymers improve modulator performance

Over the past year, advances in polymer modulator technology have improved modulator performance to the point of competing with lithium niobate modulators. During a recent meeting on Integrated Photonics, Hernan Erlig and others at Pacific Wave Industries (Los Angeles, CA), University of California Los Angeles, and University of Southern California (Los Angeles) described the applications of improved polymer modulators.¹

In general, the modulators in question are based on Mach-Zehnder interferometers. A microwave frequency electrical signal introduced into the nonlinear optical material changes the refractive index of the material courtesy of the electro-optic effect. The higher the electro-optic coefficient of the material, the less voltage is required to modulate the optical signal (assuming all else is unchanged). The electro-optic coefficient of lithium niobate is a property of the material. The electro-optic coefficients of chromophore-doped polymers, however, can be tailored when the material is made.

Why polymersOver the past decade, the electro-optic coefficient from polymers has increased dramatically. From 1991 to 2000, says Erlig, the electro-optic coefficients available from polymers increased by almost a factor of 10 because of increased understanding of material engineering both of the chromophores and of how to align and incorporate the chromophores in the polymer.

Polymers also have an advantage over lithium niobate in velocity mismatch. Ideally, the speed of propagation of the electric signal is matched to the speed of the optical signal in the medium. Otherwise, the mismatch can limit the modulator's bandwidth. Lithium niobate has large velocity mismatch, which can be partly compensated for, but at the expense of additional complication to the design. In polymers, however, the electric signal and the optical beam travel at about the same speed in the set of organic compounds used by this group of researchers.

A third potential advantage of polymers may be the cost to produce a modulator. "We anticipate that manufacturing of the modulator chip is cheaper than lithium niobate," said Erlig. "We're investigating whether that will hold true through packaging."

The power-handling abilities of polymer modulators have been a concern because of the large intensities in the optical waveguides and the potential for chromophore damage. In the past year, the researchers have measured up to 20 mW of light launched into the polymer modulator without damage. The group has not had a chance to test at higher powers yet. Commercially available lithium niobate modulators have maximum ratings in the tens to hundreds of milliwatts.

For more information, contact Hernan Erlig at [email protected].

Yvonne Carts-Powell

REFERENCE

1. H. Erlig et al., Photonics in Switching Technical Meeting, OSA and IEEE LEOS, ItuH3-1 (June 13-15).