Polymer microring tackles WDM

Polymer microring tackles WDM
Dense wave-division multiplexing systems require many lasers and modulators to generate the required optical signals to be transmitted. Previously, microring resonators were considered as filters and lasers in WDM systems. In an effort to integrate and minimize the cost of such modulators, we have demonstrated a novel wavelength-sensitive modulator based on a microring resonator.

Conventional electro-optic modulators are based on a Mach-Zender interferometer. Recently high-performance Mach-Zender modulators using polymer electro-optic materials have been demonstrated. We have used the same polymers for high-speed wavelength-selective modulation using a microring structure. In this type of device, light is coupled to the input waveguide and an applied voltage changes the refractive index of the micro ring. This will change the resonant frequency, and hence the light coupled to the drop port and throughput port will be modulated. The modulation in the through port is particularly interesting since only the wavelength close to resonance will be modulated and other wavelengths will be transmitted without distortion. To demonstrate this idea we have fabricated a microring using polymer materials.

This material is based on a special structural design to reduce the large dipole-dipole interactions between chromophores, which prevent alignment during polling and thereby increase the electro-optic coefficient. The chromophores are a ring-locked phenyltetraene, which is labeled CLD1.

The device fabrication starts with a gold-coated silicon substrate. A 5-micron UV-curable epoxy, UV15, is used as the lower cladding. The 5-micron thickness is required to ensure negligible plasmon loss due to the bottom gold electrode. A 1-micron CLD1/APC layer is coated and etched using reactive ion etching in oxygen to form a channel waveguide in the form of a microring. The width of the ring waveguide is 5 microns.

Next, a middle cladding of UFC-170 is spin-coated on the device. The thickness of this layer, which determines the distance between the ring and channel waveguides, is 4.5 microns. The UFC-170 layer is patterned in the form of a waveguide. Next, the required depth is etched into the UFC-170. At the next step, SU-8 is used as the material for the channel waveguide.

Efficient coupling

The effective refractive index of the SU-8 waveguide matches the effective refractive index of the whispering-gallery mode of the CLD1/APC microring. Hence efficient coupling is achieved. After an upper gold cladding is patterned on the device to cover the microring, the device is cut using a dicing saw.

The microring was tested using cleaved fiber as the input and output waveguide. For 1,300 nanometers, the full-width half-maximum (FWHM) bandwidth of the device is approximately 4 GHz for TE, defined as the electrical field perpendicular to the device surface, and 3 GHz for TM, or the magnetic field at a right angle to the TE.

Hence the quality factor of the device is 6.2 x 104 and 7.6 x 104 for TE and TM, respectively. The calculated loss based on the measured quality factor for this device is 5.2 for TE and 4.2 dB/cm for TM.

Normally high-speed polymer electro-optic modulators are limited by the microstrip RF waveguide loss. However, since the microring modulator is much smaller than the microwave wavelength (even up to 100 GHz), the device's high-speed behavior is mainly capacitive. Hence one does not need to worry about the microwave loss provided that the electrode capacitance is small enough to not limit the modulation. High-speed modulation is limited due to the optical signal having a limited time to build up or decay in the resonance tank. High-speed measurement of the device confirms this. A relatively clear eye diagram up to 1.5 Gbytes/second is obtained for this device.

The complete paper is being presented at the Optical Fiber Communications Conference.

See related chart