Photonic Devices
1,000 Times Smaller
buscom.com
Scientists at Northwestern University (Evanston, IL 60208) have constructed a tiny, nanoscale photonic resonator that is a hundred times smaller than the cross-section of a human hair so small that it can be seen only with an electron microscope. When combined with an equally tiny semiconductor laser that the research group has already made, the components can form nanoscale photonic integrated circuits that are a thousand times smaller than those currently in use, and which should be cheaper to fabricate and more efficient to use.
The research group is headed by Seng-Tiong Ho (Tel: 847/491-7103, email: sth@ece.nwu.edu), associate professor of electrical and computer engineering at Northwestern. The discovery is said to be a great advancement toward very low threshold microcavity laser systems.
The first breakthrough for the Ho group came in 1993, with a theoretical paper showing that the spontaneous emission of photons from atoms is not unchangeablethat it could be modified by using nanoscale structures to channel the photons into tiny waveguides measuring in the ten thousandths of a millimeter. Waveguides in current photonic devices are a few hundredths of a millimeter wide.
In 1995 the group announced the construction of the first laser to take advantage of this effect, a photonic wire laser that was the smallest operating laser ever constructed (see OMEN June 1995). It had a cavity volume of 0.3 cu. microns. It also had an unprecedented level of efficiency, with 70% of the photons emitted in its microscopic quantum well entering into the lasing mode at threshold levels. In a typical laser, only one in 100,000 photons enters the lasing mode at the threshold level.
The announcement of the newly developed resonator, called a "nanoscale waveguide-coupled microcavity resonator," means that these tiny
lasers can now be used in photonic integrated circuits. The resonator can act as an on and off switch for the stream of photons, or as a
modulator to change their power levels. The resonator can be controlled electronically.
When the laser and the resonator are combined, a stream of photons moves down a nanoscale waveguide past the resonator. If the resonator is vibrating at the same frequency as the photons, the photons are channeled off into the circular resonator and into a second waveguide. The nanoscale waveguide is a few hundred nanometers wide, and the resonator measures only a few microns in diameter.
Results demonstrate that the photons had, in fact, been channeled into the second waveguide. The resonator has a cavity Q of about 10,000,
which means that the resonance levels were sharp and clearly distinguished. High cavity Q is difficult to achieve, but it is very important for frequency or wavelength selectivity. This means that the Northwestern group now has the main components to manufacture photonic
integrated circuits with nanoscale dimensions. The first nanoscale photonic integrated circuit facility in the world is under construction in the Northwestern University/Evanston Research Park.
Photonic integrated circuits are at the heart of fiber optic communications systems. They provide light sources modulated with high speed digital optical signals and detectors that can recognize those signals. The signals are transmitted down optical fibers and then channeled out to different geographical locations, again by photonic devices.
To transmit as many optical signals as possible, many different wavelengths of light can be transmitted at the same time down a single optical fiber using wavelength division multiplexing (WDM) techniques. In these WDM systems, as many as 30 different wavelengths are sent down a single optical fiber, and photonic devices are necessary to pick up each one of these wavelengths separately. The resonator proclaimed by the Ho group is capable of picking up each of these wavelengths with high precision because of the high cavity Q.
The first practical application of the nanoscale photonic integrated circuits will probably be for a WDM fiber optic communications network, which could be very useful in transmitting signals from fiber optic cables to individual homes, allowing for very high data rate fiber optic terminals that are needed to transmit high quality video images. |
