Coarse wavelength division multiplexing is preferable to dense wavelength division multiplexing in many cases because the coarsely multiplexed channels obviate the need for precision wavelength control in sources and passive components. But the optical amplifier that works so well in dense multiplexing -- the erbium-doped fiber type -- lacks gain over the spectral widths that are required for coarse multiplexing. Figure 1. Either a 1050- (left) or a 1400-nm pump source (right), together with a 690-nm pump, can create gain at ~1470 nm in thulium. Images ©2005 IEEE.Many laboratories around the world have investigated thulium-doped fiber amplifiers because they provide gain in the S-band (1460 to 1530 nm), but the pumping schemes have been complex and inefficient. Recently, an international collaboration investigated what the researchers believe is a more efficient, dual-wavelength pumping scheme -- one that utilizes readily available and inexpensive diode lasers operating at 690 nm.Figure 2. Counterpropagating pump radiation at 690 nm and at either 1400 or 1050 nm created as much as 20 dB of gain in the thulium-doped fiber amplifier.The collaboration included researchers at Queen's University in Kingston, Ontario, Canada, at Sprint Advanced Technology Laboratories in Burlingame, Calif., at Central Glass Co. in Tokyo, at the University of Tokyo and at JDS Uniphase Corp. in Santa Rosa, Calif. Their dual-wavelength pumping schemes involved pump wavelengths that both populated the upper laser level (3H4) and depopulated the lower one (3F4) (Figure 1).Figure 3. The amplifier gain increased significantly with the addition of the 690-nm pump. The input signal was 235 dBm at 1482 nm.Although dual-wavelength pumping of thulium has been studied before, the scientists believe that theirs is the first to consider a 690-nm pump. Advantages of this pump include its higher ground-state absorption and its straightforward population (through 3F3) of the upper laser level. The other pump wavelength -- either 1050 or 1400 nm -- depopulates the lower laser level by excited-state absorption. Depopulation of this level is crucial because its spontaneous lifetime (9 ms) is long enough to terminate laser action if it were not depopulated.Pumping schemesThe researchers forward-pumped a 10-m length of thulium-doped (2000 ppm) fiber with 690-nm light and backward-pumped it with either 1050- or 1400-nm radiation (Figure 2). The 1050-nm source was a diode-pumped ytterbium laser, and the 1400-nm source was a laser diode. For the 690-nm pump, they used a Ti:sapphire laser, but they point out that a common red laser diode, the same inexpensive laser used in DVD players, could be employed.Figure 4. The amplifier produced more than 10 dB of gain across a significant portion of the S-band.The amplifier produced up to 20 dB of gain with either pumping scheme (Figure 3). Although positive gain existed for 1050-nm pumping alone, it increased markedly with the addition of the 690-nm pump. When the researchers injected eight −18-dBm channels into the amplifier, they observed gain in excess of 10 dB over a spectral range of more than 30 nm (Figure 4). They concluded that thulium-doped fiber amplifiers, pumped with either of the schemes, are candidates for use in future coarse wavelength division multiplexed optical networks.