The C-band, between 1530 and 1565 nm, has been the spectral region of choice for dense wavelength division multiplexing telecommunications during the past decade because erbium-doped fiber amplifiers (EDFAs) provide efficient broadband gain in that region and because conventional silica glass fibers have minimum loss there. As the load demand on existing systems increases, however, telecommunications engineers have been eyeing the possibility of using the C-band and the O-band simultaneously to carry information. The O-band, between 1260 and 1360 nm, is attractive because conventional fibers have minimum dispersion in that region.Figure 1. To measure the optical gain of their amplifier, the researchers pumped it at 810 nm and probed it with the wavelengths indicated in the lower part of the figure. For any given gain measurement, only the 1308-nm probe from the top laser and a single wavelength from the bottom laser were simultaneously present. LD = laser diode; DFB = distributed feedback. Images reprinted with permission of IEEE Photonics Technology Letters.The problem with the O-band is the absence of an EDFA-like device that can efficiently amplify wavelengths across that entire region. Researchers have explored both praseodymium-doped fluoride fiber amplifiers and fiber Raman amplifiers, but although both provide amplification in the O-band, they suffer from narrow bandwidth and low efficiency. Moreover, praseodymium-doped fluoride fibers are brittle and cannot readily be fusion-spliced with conventional silica fibers.Recently, however, researchers at the Japan Science and Technology Agency in Saitama and at Osaka University, both in Japan, demonstrated a bismuth-doped silica glass fiber amplifier that appears to have none of these disadvantages. Oddly, though, the researchers cannot yet identify the exact physics that leads to light emission in their fibers.Figure 2. The gain at 1308 and 1323 nm was very nearly the same. Similar measurements showed similar results at 1272, 1298 and 1347 nm.They measured the optical gain in their fiber by pumping it at 810 nm with a diode laser from Unique M.O.D.E. AG in Jena, Germany (Figure 1). They probed the fiber’s gain with two simultaneous wavelengths, from diode lasers manufactured by Afonics Fibre Optics Ltd. in Witney, UK, and by Mitsubishi Electric in Japan. In the dual-wavelength measurements, one wavelength was always 1308 nm, and the other was one of the four indicated in the figure, between 1272 and 1347 nm. Data from the 1308- and 1323-nm measurement are shown in Figure 2. The researchers observed similar results for measurements taken at 1308 and 1272 nm, 1308 and 1298 nm and 1308 and 1347 nm.The gain in all four cases was relatively constant (Figure 3). And because the bismuth-doped fibers are based on silica, they can easily be fusion-spliced to conventional telecommunications fibers. These considerations led the researchers to conclude that their amplifiers are good candidates to serve as EDFA-like amplifiers in the O-band.Figure 3. The observed optical gain was relatively smooth across the O-band. The blue line shows the fluorescence spectrum of the bismuth-doped silica glass.But they are concerned that the exact nature of light emission from the bismuth-doped material is not well understood. Emission almost certainly arises from a transition of bismuth’s valence electron, but there is virtually no emission from bismuth-doped silica glass samples fabricated without aluminum. The researchers believe that aluminum plays a role in the formation of bismuth luminescent centers, and they are considering forms of coupling between bismuth and aluminum ions that could be responsible for the emission.IEEE Photonics Technology Letters, Sept. 15, 2006, pp. 1901-1903.