May 29, 2003 A new study by University of Toronto researchers offers the first-ever glimpse inside a laser while it's operating, a breakthrough that could lead to more powerful and efficient lasers for fibre-optic communication systems. "We've seen the inner workings of a laser in action," says investigator Ted Sargent, a professor in the Edward S. Rogers Sr. Department of Electrical and Computer Engineering. "We've produced a topographical map of the landscape that electrons see as they flow into these lasers to produce light." He says the findings could influence laser design, change the diagnosis of faulty lasers and potentially reduce manufacturing costs. The study, which will appear in the June 9 issue of the journal Applied Physics Letters, offers direct experimental insight into how lasers function, says Sargent, who holds the Nortel Networks-Canada Research Chair in Emerging Technologies.
Lasers are created by growing a complex and carefully designed series of nanometer-sized layers of crystals on a disk of semiconductor material known as a wafer, Sargent explains. Ridges are etched into the crystal surface to guide laser light, thin metal layers are added on top and bottom and the wafer is then cut into tiny cubes or chips. During the laser's operation, an electrical current flows into the chip, providing the energy to generate intense light at a specific wavelength used in fibre-optic communications.
In their study, researchers focused on the "beating heart" portion of the laser (called the active region), where electronic energy is converted into light. Using a technique called scanning voltage microscopy, they examined the surface of an operating laser, picking up differences in voltage. These differences translate to a topographical image of the laser's energy surface, allowing researchers to visualize the forces an electron experiences along its path into the active region, Sargent says.
The team used its newly acquired information about the inside operations of the laser to determine the fraction of electric current that contributed to producing light. The balance of electrons are undesirably diverted from the active region: such current leakage wastes electrons and heats the device up, degrading performance.
"We used direct imaging to resolve a contentious issue in the field: the effectiveness of electronic funnelling into the active region of a ridge-waveguide laser," says Dayan Ban, the U of T doctoral candidate who made the measurements. "Previously, uncorroborated models had fueled speculation by yielding divergent results. Now we know where the electrons go." Ban is now a researcher at the Institute for Microstructural Sciences of the National Research Council of Canada.
"Direct imaging of the functions that drive the action of a living laser could transform how we think about laser 'diagnosis and therapy,'" says Sargent, referring to the measurement and optimization of laser structures and their determination of the devices' inner workings. Currently, designers use a variety of computer simulations to model how lasers work, but the U of T research may determine which simulations are the most accurate design tools. "With accurate models," says Sargent, "the designs we can create are more likely to result in devices that meet design requirements."
Co-investigator St. John Dixon-Warren, a physical chemist from Bookham Technology, a U.K.-based optical components manufacturer located in Kanata, Ont., says their research could also help in diagnosing faulty lasers. "If a particular laser fails," says Dixon-Warren, "the kind of measurements that we are taking could provide some idea of why it failed and the design could then be modified."
Sargent says the findings could have larger implications for the creation of optical circuits for fibre-optic communication. "If we could fully develop these models and fully understand how lasers work, then we could start to build optical circuits with confidence and high probability of success," he says. "Optical chips akin to electronic integrated circuits in computers must be founded on a deep and broad understanding of the processes at work inside current and future generations of lasers."
The research was supported by Nortel Networks Optical Components (recently acquired by Bookham Technology), the Natural Sciences and Engineering Research Council of Canada, the Ontario Research and Development Challenge Fund, the Canada Foundation for Innovation, the Ontario Innovation Trust and the Canada Research Chairs Program.
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