On-the-Bubble Design

Modeling solves design of optical switcher

—edited by Richard Mandel

While fiber-optic cable has provided dramatic increases in data communications throughput, there continues a strong desire to switch large volumes of fiber optic data without the inefficiency of turning optical signals into electrical signals and then back to optical signals. In the mid-1990’s, Agilent Laboratories (when it was part of Hewlett-Packard Labs) recognized the importance of an all-optical circuit switch and started a research program to develop such a technology. A team of engineers and scientists formed within what is now Agilent Labs’ Communications and Optical Research Laboratory (CORL), Palo Alto, CA, to develop a compact, scalable switch fabric that has minimal impact on the optical signal. The team capitalized on two established technologies — inkjets and planar lightwave circuits — to build a switch that routes an optical beam from one path to another without having to convert the switching signal from optical to electronic and back.

Bubble trouble

In its operating environment, the Agilent Photonic Switching Platform is placed at the intersection of two fiber-optic networks. When a light signal comes in through a fiber, it can cross the planar lightwave circuit unimpeded via the straight-through wave-guide. However, if the signal must be redirected to a different optical fiber, a bubble is inserted into the cross point of the two wave-guide paths, altering its optical properties and reflecting the signal down the path to the output fiber. Formed by evaporation induced by electrical heaters on the device substrate, the bubbles are blown into a fluid that is index-matched to the array of crossed optical wave-guides. The fluid fills an array of micro-trenches located at the cross points of the waveguides. The total internal reflection from the bubble wall is what causes light to be switched from one waveguide to another. The bubbles can be formed and removed in less than five milliseconds, all without the use of mirrors or mechanical moving parts. 

Early prototypes of the Agilent Photonic Switching Platform showed performance problems that indicated the something was unstable with the bubble reflection. The problem was that the acceptance angle, or numerical aperture, of the optical waveguides was fairly 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 wave-guide and there will be signal loss. However, the small size of the bubbles made it impossible to make comprehensive physical measurements that would be needed to diagnose and solve the problem.

Agilent senior scientist John Uebbing used computational fluid dynamics software to simulate a bubble. From this model, the Agilent team discovered that condensation on the walls of the trench caused a fluid buildup that, in turn, determines much of the switch’s behavior. Further simulations helped the researchers validate two different methods of altering the device to give stable signals.

Dimples impact performance

Extensive experimental testing, performed on the early prototypes, showed the effects of heater power and ambient pressure on the optical reflection characteristics and on the bubble shape and size. These tests showed that the reflected optical signal vs. heater power curve not did not meet the tight requirements needed for effective optical switching and that there were instabilities in the reflected light signals. 

When the computer simulations showed that there were dimples forming on each side of the bubble, it dawned on the Agilent research team that the dimples might be what caused the humps on the power curve and why 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. These pictures could not show the dimple directly, because the dimple was very thin on a wavelength scale. 

Simulating the Bubble

Initially, a number of alternatives for simulating the operation of the bubble were considered. The team had been using various analytical models to investigate the bubble formation, but these models predicted that the current prototypes should produce good bubbles, so they were clearly too simple to capture the problem. They hired a college professor to write custom software, but this project was going to take a considerable amount of time to complete. In the meantime, Uebbing began searching for a commercial software package that could handle the complicated physics of the problem. “I talked to several CFD software developers but from what I could determine none of them had a bubble model that would solve the problem without extensive modification,” Uebbing said. “Flow Science, on the other hand, said they were working on a model that they thought could handle the problem and that it would be ready soon.”

The homogeneous bubble model, created in FLOW-3D by Flow Science, Sante Fe, NM, assumes a uniform bubble pressure and temperature. Uebbing started out using the older version of the software, but as soon as an upgrade was released, Uebbing applied it to the problem at hand. The sofware revision provided a close approximation to reality. 

One of the key issues is the modeling of the contact line, where the liquid, vapor and solid all come together. The homogenous bubble model balances the forces and fluxes in the computational cell at this point. “The simulation results showed the dimples that were eventually to prove so important in explaining the experiments,” Uebbing said. “Just as interestingly, the simulation showed that the bubble oscillated at 35 kHz. We had taken experimental data that showed it really did oscillate at that frequency, but we had no idea why. The simulation showed that it was just a simple spring mass or a trenched version of the classic bubble radial oscillation. 

“This rather unexpected correlation with reality gave the team confidence in the results of the simulation,” continues Uebbing. “The simulation results went far beyond what we were able to measure in the tests by showing us the flow velocity, pressure and temperature at every point in the problem domain. With these results, we were able to figure out what was happening. The dimple is caused by capillarity. What happens is the condensing fluid piles up on the wall of the bubble. It tries to escape through the thin film of the liquid on the wall of the trench. To push liquid through such a thin layer requires a significant pressure difference. The high pressure in the center of the bubble wall causes the bubble to form a dimple.”

Solving the problem

Understanding how the dimples were formed suggested two methods of correcting the shape of the bubbles in order to provide stable signals. The first was to extend the bubble heater under the glass sidewall of the trench. Heat would then flow up the wall of the micro-trench and dry out its surface. Basic physics suggests that if the bubble temperature is less than the wall temperature the wall will be dry. This expectation was confirmed with FLOW-3D simulations, which showed that dry wall bubbles give very stable switched signals.

The second method, also verified with FLOW-3D, was to make a so-called “static” bubble in the micro-trench. Static bubbles exist if the device temperature is somewhat hotter than the pressure setting reservoir temperature. This device temperature creates enough pressure to push the bubble into the corners of the trench, but not enough for the bubble to blow out through the gap between the waveguide array and the heater substrate. These static bubbles can be turned off with a nearby “crusher” bubble. This bubble temporarily generates enough overpressure to cause the static bubble to collapse. The crusher bubble itself is in a smaller trench so that surface tension forces are enough to make it collapse after it has done its work. FLOW-3D simulations were also used to show the switch operation in this mode.

“Simulation helped us determine exactly what was causing the dimple and helped us to identify and evaluate several alternative solutions,” Uebbing said. “This progress in bubble switch engineering would not have been possible without advanced modeling features available in the FLOW-3D software. Flow Science engaged technical staff with the expertise to understand exactly what we wanted to accomplish. At several stages of the process, they provided critical help that allowed us overcome significant obstacles.”