Sensor and Actuator Center Work Could Find Use in Communications, Optics, Medicine and Environmental Sensing Systems
by Robert Sanders
Researchers at Berkeley have taken an important step toward miniaturizing optical systems that use movable beam-steering mirrors, like those found in supermarket scanners.
The researchers have built a microscopic version of these mirrors for use in fiber-optic systems, with mirror adjustments made by a micromotor not much larger than the cross section of a human hair.
The entire movable micromirror, motors and all, fits within an area four-hundredths of an inch square.
The gold-plated micromirror was designed to reflect laser beams into optic fibers, the typical coupling mechanism in communication systems. But the mirrors also are useful in other fiber-optic systems, such as those designed for environmental sensing, or for optical scanners where the mirrors are continuously vibrated.
The micromirors will be especially useful in systems that require precise alignment, a major problem in designing and building the components of an optical system.
"For systems on silicon, which are now seeing increasing applications, you have to place the optical components with very high precision," says postdoctoral researcher Norman C. Tien. "We have put a mirror in a system with a laser and an optic fiber.
"Without the tedious high-cost steps conventionally used for such systems, the new micromirror system has provided coupling efficiencies -- which are very important performance parameters -- that are comparable to the best commercially obtainable."
Tien and colleagues in the Berkeley Sensor and Actuator Center reported on the new motor-driven micromirror in December at the 1994 IEEE International Electron Devices Meeting in San Francisco.
The micromirror is built by a technique known as surface micromachining, which evolved from the processes and materials developed to make integrated circuits. This evolution has been heavily influenced by research carried out over the past decade by many contributors at the Berkeley Sensor and Actuator Center, which pioneered the field of micromachining and developed several devices and systems that now are entering the marketplace.
Among the important developments are microscopic motors that move a slider back and forth by repeatedly striking it with a battering ram of silicon, and miniature hinges that allow the mirror to tip at varying angles. The slider moves in incredibly small steps--about one hundred-thousandth of an inch, or 0.25 micron. By acting in concert with a twin, the motors can adjust the angle of the micromirror.
Traditional fiber-optic couplings are very labor intensive to build, Tien says, since the mirrors must be adjusted by hand before packaging. A coupling efficiency of 50 percent is considered good, which means that 50 percent of the laser light power actually is transmitted into the optical fiber. On silicon substrates efficiencies typically are lower than 10 percent.
If the optical component is heavily jarred or subjected to extremes of temperature variation, the coupling may be so drastically reduced that the coupler must be discarded.
The Berkeley group has shown that 45 percent coupling efficiencies can routinely be achieved with smaller and less labor-intensive systems. Owing to its very small mass, these micromirror systems can survive even a six-foot fall without going out of alignment.
Even if there were sufficient disruption to cause misalignment, the mirror could be moved to bring the system back into proper operation.
A further part of the Berkeley research involved an assembly including a semiconductor laser diode, a glass bead to serve as a lens to focus the laser, and an optic fiber. All of these components were placed on a chip with a mirror made using surface micromachining. The entire "micro-optical bench" measured less than 15 hundredths of an inch on a side.
"This micro-optical bench is much easier to align and adjust and therefore would markedly reduce the cost of systems like laser transmitters," Tien says. "With these new technologies, optical microsystems can be built with improved performance, lowered fabrication costs, and reduced system sizes and weights."
The mirrors described today have been demonstrated only as devices for initial alignment of the optical systems. Future research, Tien says, will lead to couplers that automatically reposition themselves to hold fixed a preset alignment.
The project began when post-doctoral researcher Olav Solgaard and Professor Kam Lau approached the Berkeley Sensor and Actuator Center regarding the application of micromachining to optical systems.
"Berkeley is the ideal place to do this type of interdisciplinary work," says Solgaard, "because of the strong programs in both quantum optics and micromachining."
The overall goal of the Berkeley program, which has been done with the support of the Hewlett-Packard Co., is to build complete micro-optical systems using micromachining technologies.
These systems may find use in communication systems, in components for optical research, for medical applications and for environmental sensing applications.
Project members include Richard Muller, professor of electrical engineering and computer sciences and co-director of the Berkeley Sensor and Actuator Center, and graduate students Mike Daneman and Meng-Hsiung Kiang.