Dual-core fiber flattens amplifier gain
Flat gains in erbium-doped fiber amplifiers (EDFAs) are necessary for WDM systems to avoid uneven amplification and channel crosstalk. Researchers at the University of New South Wales (Sydney, Australia) recently developed a gain-flattening method that is relatively simple and economical.
Professor Pak Lim Chu and researcher Margaret Yi Bin Lu at the university used a dual-core fiber to provide a relatively flat gain from 1525 to 1555 nm.1 "There are many approaches to achieve a flat gain in EDFA," said Chu. "All these methods are complicated compared to ours."
The researchers' amplifier is much like typical EDFAs, except that, instead of a single doped erbium core, it uses a fiber with two cores, one of which is undoped. The input-fiber coupler combined the 1550-nm signal and the 980-nm pump power into the erbium-doped core. At the end of the double-core fiber, the output-fiber coupler, connected to the erbium-doped core, separated the amplified 1550-nm signal from the residual pump power.
Coupling efficiency from the doped core to the undoped core is highest at the wavelength (1533 nm) where the gain is highest. The reciprocal coupling is low, however, because of the difference in core dopants.
Gain spectra for different pump powers (see figure) is flatter for higher powers—at 50 mW the gain variation is less than 0.7 dB. Although this variation grows at lower pump powers, it is still only 1 dB at 28 mW. The researchers also noted that the flatness of the gain depends on the fiber length: shorter fibers result in more variation. The noise performance of the amplifier is a function of pump power, with powers producing less noise, and of fiber length, with longer fibers producing more noise. The noise curves tend to approach 4 dB, which is low compared to EDFAs gain-flattened by other methods.
The fiber created in the researchers' lab has one core doped with erbium and the other core undoped. The first core is doped with erbium at a concentration of 280 ppm and its diameter is 3.2 µm, linear refractive index 1.478, numerical aperture is 0.23, and attenuation 4.8 dB/m at 1533 nm. The second core is undoped and has a diameter of 3.2 µm, linear refractive index of 1.477, and numerical aperture of 0.22. The core-to-core spacing is 11.25 µm.
Although the fiber is not now produced commercially, Chu still believes it is economical. "Once a dual-core preform is manufactured, it can be drawn into hundreds of meters and only 11 m are needed for one EDFA," said Chu.
The researchers have obtained a patent protecting this invention and intend to develop it into a commercial product. "We are also working on using dual-core fibers for other WDM devices," said Chu. For more information, contact Professor Pak Lim Chu at firstname.lastname@example.org.
- Y. B. Lu, P. L. Chu, IEEE Phot. Tech. Lett. 12, 1616 (December 2000).
Yvonne Carts-Powell is a science and technology writer based in Belmont, MA