Experimental amplifier boosts wider bandwidth
By STEPHEN HARDY
Lucent Technologies, Murray Hill, NJ, through its Bell Laboratories and Optical Networking/Synchronous Optical Network (Sonet) Units, has demonstrated an ultra-wideband optical amplifier that could significantly enhance networks using wavelength-division multiplexing (wdm). The experimental amplifier has the potential to boost signals carried simultaneously over more than 100 channels, according to the company.
The amplifier spans 80 nm--nearly seven times the bandwidth used today in wdm systems. This bandwidth is three times that of erbium-doped fiber amplifiers and twice that of experimental erbium-doped silica fiber amplifiers now under study at Bell Labs. Lucent says this figure is 50% higher than that reported for experimental dual amplifiers.
The key to providing such wide bandwidth lies in a two-stage design that uses only conventional erbium-doped fiber, according to John Zyskind, a distinguished member of the technical staff at Bell Labs, and head of the team that developed the experimental system (see photo). Light enters the amplifier and passes through a wdm component. A combination of a circulator and a broadband Bragg grating then splits the light into two bandwidths--a shorter-wavelength component of a conventional 1525- to 1565-nm range (which Zyskind calls the C-band) and a longer-wavelength component, dubbed the L-band, that extends from 1570 to 1600 nm. These components travel along different paths for amplification.
Because the gain spectrum of an erbium-doped fiber amplifier depends on the level of inversion at which it is operated, according to Zyskind, splitting the light into two bandwidths enables the amplifier to apply different inversion levels that are optimized for the two wavelengths. The C-band branch features what Zyskind terms "fairly high" inversion; a gain equalization filter provides additional flattening, primarily at the 1530-nm peak. The L-band runs at a lower inversion, which is better-suited for longer wavelengths.
The result, according to a paper delivered at the recent 1997 Topical Meeting on Optical Amplifiers and Their Applications in British Columbia, Canada, is a spectral range for which gain exceeds 20 dB from 1525.6 to 1562.5 nm in the C-band and an L-band range from 1569.4 and 1612.8 nm. The C-band gain bandwidth is 36.9 nm; when combined with the L-band`s 43.4-nm figure, a total gain bandwidth of 80.3 nm results.
Zyskind reports that his team is working to improve the performance of the amplifier through the use of better gratings. "That will probably take us to 85 nm," he says. "And then we may be able to flatten things further and get a little more."
The use of conventional erbium-doped silica should ease the amplifier`s evolution into a commercially available product, according to Zyskind "We think it will keep down the cost, and we think it will limit the risks as well."
The amplifier could significantly expand the capacity of wdm networks. If the 100-GHz spacing recommendation now before the International Telecommunication Union is used, Zyskind says the unit would enable the transmission of more than 100 channels. Present systems use 8 or 16 channels, although 32-channel systems are considered in some quarters to be nearly ready for implementation.
Besides Zyskind, the team responsible for the breakthrough includes Yan Sun, Atul K. Srivastava, James W. Sulhoff, and Chuck Wolf of Bell Labs, Crawford Hill, NJ; Jianhui Zhou of Lucent`s Optical Networking/Sonet Business Unit; and Thomas A. Strasser, J.R. Pedrazzani, Justin B. Judkins, Rolando P. Espindola, and Ashish M. Vengsarka of Bell Labs in Murray Hill. The Multiwavelength Optical Networking program provided partial support to the effort. o