Switching Fiber-Optic Circuits with
Advanced computational fluid dynamics modeling software plays a critical role in the development of a revolutionary photonic switching platform.
John Uebbing, Agilent Technologies
Fiber-optic cable has provided dramatic increases in data communications throughput, and there’s a growing demand for technology that will make it possible to switch large volumes of data without converting the switching signal from optical to electronic and back. In the mid-1990s, Agilent Laboratories (when it was part of Hewlett-Packard Labs) realized the importance of an all-optical circuit switch and started a research program to develop a compact, scalable switch fabric that had minimal impact on the optical signal. The research team capitalized on two established technologies—inkjets and planar light wave circuits—to build a switch that routes optical beams from one path to another without having to convert the switching signal.
To work, the photonic switching platform is placed at the intersection of three fiber-optic ribbons. When a light signal comes in through a fiber, it can cross the planar light-wave circuit unimpeded via the straight-through waveguide. But if the signal must be
The switch operates by blowing bubbles in a fluid that is index matched to the array of crossed optical waveguides. The bubbles are formed by evaporation induced by electrical heaters on the device substrate. The fluid fills an array of microtrenches located at the cross points of the waveguides. Total internal reflection from the bubble wall causes light to be switched from one waveguide to another.
The problem is that the acceptance angle, or numerical aperture, of the optical waveguides is low. If the vertical reflecting wall of the bubble is not perpendicular to the axis of the waveguide, the light will not be properly reflected into the output waveguide, and there will be signal loss.
Dimples Impact Performance
When the computer simulations showed that there were dimples forming on each side of the bubble (see Figure 2), it dawned on the researchers that the dimples might be the cause of the humps on the power curve and the reason the reflected signal was so unstable.
The team’s ability to take physical measurements with sensors did not extend to the scale of the MEMS device. The most they could do was use special optics to take photomicrographs (see Figure 1). The pictures couldn’t show the dimple directly because it was thin on a wavelength scale.
Simulating the Bubble
In the meantime, Agilent’s researchers began looking for commercial software that could handle the complicated physics of the problem. Unfortunately, most of the packages they looked at didn’t have a bubble model that would solve the problem without extensive modification. However, a company by the name of Flow Science said its software, Flow-3D, could handle the problem (see the "Flow-3D" sidebar).
Flow Science’s new homogeneous bubble model assumes uniform bubble pressure and temperature, which is a good approximation of reality. One of the key issues is the modeling of the contact line, where the liquid, vapor, and solid come together. The new model balances all of these forces and fluxes in the computational cell.
Simulations using Flow-3D showed the dimples that proved so important in explaining the experiments. The simulation and experiment also showed that the bubbles oscillated at 35 kHz, but researchers had no idea why. Simple calculations showed that it was just a simple spring mass or a trenched version of the classic bubble radial oscillation. This unexpected correlation with reality gave the team confidence in the results of the simulation.
The simulation results went beyond what Agilent could measure in the tests by showing the flow velocity, pressure, and temperature at every point in the problem domain. With these results, the researchers were able to determine what was happening: The dimple was caused by capillarity. The condensing fluid piled up on the wall of the bubble and tried to escape through the thin film of liquid on the wall of the trench. To push liquid through such a layer required significant pressure difference. The high pressure in the center of the bubble wall caused the bubble to form a dimple.
Solving the Problem
The second method—also verified by Flow-3D—is to make a static bubble in the microtrench. These bubbles exist if the device temperature is hotter than the pressure-setting reservoir temperature. The device temperature creates enough pressure to push the bubble into the corners of the trench, but not enough to blow the bubble out through the gap between the waveguide array and the heater substrate. Static bubbles can be turned off with a nearby “crusher” bubble, which temporarily generates enough overpressure to cause the static bubble to collapse. The crusher bubble itself is in a smaller trench, so surface tension forces are enough to make it collapse after it’s done its work. Flow-3D simulations were also used to show the switch operation in this mode.
John Uebbing is Senior Scientist, Agilent Technologies, Santa Clara, CA; 408-970-2862, firstname.lastname@example.org.