New tunable source is based on Sagnac loop grid filter

March 1, 2002

A discrete wavelength-selectable source that could be the precursor to an all-fiber device has been demonstrated at the Center for High Technology Materials in Albuquerque, NM. The experimental 1.5-µm device has 25 channel outputs (more than 100 should be possible) with precise 50-GHz spacing that conforms to the International Telecommunications Union WDM grid. Though the performance of the initial implementation is insufficient for real applications—problems include fluctuations of the channel wavelength and the fact that the fiber-source itself has not yet been optimized—the team has already identified a number of components whose replacement could bring the device up to commercial standards.

The original idea to use the Sagnac interferometer for this application came from researchers at Virginia Polytechnic Institute and the State University in Blacksburg, VA.¹ They showed theoretically the complex interaction that takes place when two beams counter-propagate inside a loop of birefringent optical fiber. They proved that the intensity of the output of such a loop would vary with wavelength. By setting the parameters up correctly, they suggested, an entirely fiber-based laser system with this intensity variation could be used to produce a regular set of frequencies that could be tuned to optimize for WDM.

Then, last year, the New Mexico team demonstrated a device that not only included an integrated fiber laser, but exploited temperature and strain-dependence of the interferometer to allow spectral tuning.² Their system uses a Peltier cooler to control the temperature of the loop filter made of high-birefringence fiber of a bowtie design (see figure). Although they were able to conform to the ITU-WDM grid, the time delay caused by the nature of the cooler and its distance from the fiber lead to a fluctuation of 0.2 GHz, 20 times higher than the standard would allow. The researchers say that using elliptical-core birefringent fiber in the loop would make it less sensitive to temperature.

Other planned improvements include using a fiber-based tunable filter to select from the grid channels, reducing insertion loss, and the inclusion of elements that can restrict linewidth and control polarization.

For more information, contact Nathaniel Libatique at [email protected].

Sunny Bains

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