A two-stage fiber amplifier based on commercially available technology can increase the strength of optical signals by 20 dB over the frequency band from 1526 to 1613 nm. Developed by researchers at Lucent Technologies (Holmdel, NJ), the device has the potential to carry more than 100 channels in a wavelength-division-multiplexing network; at per-channel data rates of 2.5 Gbit/s, this would correspond to 5 million phone calls.
The amplifier consists of commercially available erbium-doped silica fiber co-doped with aluminum. All wavelengths share a common first stage. The second stage consists of two separate fiber paths, one carrying the conventional, or C-band (1526-1563 nm), and the other carrying the long, or L-band (1569-1613). A broadband grating and an optical circulator after the first stage of the amplifier split the incoming wavelengths into the appropriate bands for the second stage; according to researcher Yan Sun, splitting the bands after the first stage yields an improved noise figure compared to other approaches.
Pump it up
The Lucent group end-pumped the first stage of the amplifier with 100 mW from an indium gallium arsenide (InGaAs) diode laser operating at 980 nm. The first stage of the system is configured for counter-pumping—a multiplexer launches the pump beam into the fiber carrying the signal beam, in the direction opposing signal propagation.
The gain spectrum of an EDFA depends in part on the inversion level of the system. To achieve the inversion necessary to provide gain over the L-band, the researchers combined two separate lengths of erbium-doped fiber, pumping the first segment with 100 mW from an InGaAs diode laser and pumping the second segment with 300 mW from an InGaAs-based master oscillator/power amplifier (SDL; San Jose, CA) operating at 980 nm. The cumulative length of the L-band fiber was several times longer than that of the C-band amplifier, which was pumped with a standard 100-mW InGaAs diode laser.
The amplifier provides a total gain of 20 dB across the bandwidth listed, with 10 dB of nonflatness. To flatten the gain on the C-band, the group used long-period fiber gratings (see Laser Focus World, June 1996, p. 243), about which project head John Zyskind was quite enthusiastic. "They're adaptable and can fit complex spectra, so the technology gives you the flexibility you need," he says. The 10 dB of nonflatness stemmed from the L-band, which was uncorrected in this experimental configuration. Future plans call for gain flattening of the L-band, a task Zyskind expects to be straightforward.
In addition, the group will work to extend the spectral range of the system by using alternative filters to narrow the 5-nm transition region between the C-band and the L-band to further increase useful bandwidth.
Zyskind stressed the capabilities of the amplifier design. "Wavelength division multiplexing is really constrained by the amount of bandwidth available. The amplifier has the potential to remove this limitation and to have tremendous impact on the way optical networks are designed," he says. The device could help realize the promise of video on demand--incorporating the current incarnation of the amplifier, a network operating at 10 Gbit/s per channel would be able to carry a half-million movies in real time.