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Richard Gaughan

Photonics Spectra
Mar 2001
Michael D. Wheeler

As watchers of the telecommunications industry know, when it comes to bandwidth, too much is never enough. Wavelength division multiplexing (WDM) has been revolutionary, multiplying the potential capacity of each fiber with not just one, but many wavelengths of light. A new technique that stabilizes the relative output of diode lasers could dramatically improve the next generation of WDM systems.

To date, systems with 40 separate wavelength channels are available, and 100-channel systems are in development. Researchers are investigating ways to lock the relative wavelengths of the lasers used in the systems, with hopes of minimizing crosstalk and lowering the bit-error rate. They have employed interference filters, fiber gratings and Fabry-Perot interferometers to this end, but each approach has limitations. Interference filters, for example, require thermal compensation, and fiber gratings effectively stabilize only one laser at a time.

Min Xiao and Changxi Yang of the University of Arkansas have taken a different approach, based on the photorefractive effects of crystals. Crystals have long attracted the attention of researchers, because material such as LiNbO3 is easy to grow and polish, and has superior optical uniformity. It also has large photorefractive effects and a long storage time. Xiao and Yang knew that fixed gratings within a crystal would be immune to fluctuations in ambient temperature, a cause of drift in diode lasers. Such a fixed grating would lock each diode wavelength at its peak wavelength.

Using angular and wavelength multiplexing, the team created more than 10,000 holographic gratings in a LiNbO3 crystal with a 488-nm argon-ion laser. The period of the grating, the researchers believed, should precisely determine the readout wavelength.

Wavelength Stability

The group tested its theory by stabilizing two infrared laser diodes to a high degree of precision. It also demonstrated that the method could be scaled to a large number of channels with a low bit-error rate.

"Using the high information storage capacity of photorefractive crystal, our method may have an impact on the high-capacity transmission systems," Xiao said.

The researchers, who published the results of the study in the Oct. 9, 2000, issue of Applied Physics Letters, are investigating ways to integrate the crystal with telecommunications systems. Xiao said he also is exploring a method to write the multiple gratings in different structures to enhance the bandwidth.

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