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  • Chip-Sized Device Converts Beam to Pulses
Oct 2011
WEST LAFAYETTE, Ind., Oct. 19, 2011 — A device that can convert continuous laser light into numerous ultrashort pulses and that is small enough to fit on a computer chip has been created by researchers at Purdue University and the National Institute of Standards and Technology (NIST), who said the breakthrough technology may have applications in more advanced sensors, communications systems and laboratory instruments.

Dubbed a “microring resonator,” the device creates ultrashort pulses that repeat at rates corresponding to hundreds of billions per second.

The microring is about 80 µm in diameter (the width of a human hair) and is fabricated from silicon nitride, which is compatible with silicon materials widely used for electronics. Infrared light from a laser enters the chip through a single optical fiber and is directed into the microring by a waveguide.

Researchers have created a tiny "microring resonator" small enough to fit on a computer chip (at left). The device converts continuous laser light into numerous ultrashort pulses, a technology that might have applications in more advanced sensors, communications systems and laboratory instruments. At right is a grooved structure that holds an optical fiber leading into the device. (Image: Birck Nanotechnology Center, Purdue University)

These pulses have many segments that correspond to different frequencies, or frequency combs. By precisely controlling the frequency combs, the team hopes to create advanced optical sensors that detect and measure hazardous materials or pollutants. The comb technology also has potential for a generation of high-bandwidth electrical signals with possible applications in wireless communications and radar.

The light originates from a continuous-wave laser, also called a single-frequency laser, which is a very common type of laser. “The intensity of this type of laser is constant, not pulsed, but in the microring the light is converted into a comb consisting of many frequencies with very nice equal spacing,” said Purdue’s Andrew Weiner. “The microring comb generator may serve as a competing technology to a special type of laser called a mode-locked laser, which generates many frequencies and short pulses. One advantage of the microring is that they can be very small.”

According to the team, the laser light undergoes nonlinear interaction while inside the microring, generating a comb of new frequencies that is emitted out of the device through another optical fiber.

“The nonlinearity is critical to the generation of the comb,” said Purdue doctoral student Fahmida Ferdous. “With the nonlinearity, we obtain a comb of many frequencies, including the original one, and the rest are new ones generated in the microring.”

The research is published online in the journal Nature Photonics.

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mode-locked laser
A laser that functions by inducing a fixed phase relationship between all of the modes present in the laser cavity. Once all modes are in phase, the modes are then said to be locked. The interference of these modes with one another inside the cavity then allows the laser to produce high energy, peak-powered short pulses of light in time durations that range from femtoseconds to picoseconds.
optical fiber
A thin filament of drawn or extruded glass or plastic having a central core and a cladding of lower index material to promote total internal reflection (TIR). It may be used singly to transmit pulsed optical signals (communications fiber) or in bundles to transmit light or images.  
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