Raman amplifier extends transmission window

At Imperial College, University of London (England), researchers have demonstrated the use of a dual-wavelength pump intended to allow transmission of L- and U-band data communications (1565 to 1625 nm and 1625 to 1675 nm respectively). Though amplification of the L-band is incomplete, amplification is successful over most of the remainder of the two bands. Experiments have shown a peak gain of more than 30 dBm in the 1600- to 1670-nm region, with a low 6-dB noise figure.

The purpose of the work is to extend the range of possible communications wavelengths beyond the C-band (1530 to 1565 nm) for dense WDM.¹ Stimulated Raman scattering, researchers said, is a useful option because it can supply gain at any wavelength as long as the appropriate pump is used. In the case of U-band transmission, for instance, this is relatively easy: the required wavelength is about 1540 nm, which can be supplied by relatively inexpensive erbium devices. For the L-band, the wavelength required is around 1500 nm. Though this is not in the range that can be supplied by erbium sources, it is available using a commercial cascaded Raman laser.

Researchers set up an experimental system using 9 km of dispersion-shifted fiber to supply the Raman gain (see figure). To pump in the L-band they used 370 to 410 mW at wavelengths from 1505 to 1525 nm, with a commercial erbium-doped fiber amplifier supplying the gain for the U-band (0.8 to 1.66 W, broadband from 1535 to 1570 nm). After experimenting with the two pumps individually and noting the changes in the gain characteristics upon changing their wavelength and/or power, the two were applied together. The on:off Raman gain started at 8 dB around 1600 nm, and reached over 11 dB at other wavelengths up 1670 nm, providing more than 33-dBm peak gain.

Though four-wave mixing between the pumps produced a Stokes line at 1600 nm (which would make transmission at this wavelength difficult), the Imperial team said that the use of a fiber with higher dispersion at pump wavelengths should mitigate the problem. Improvement in the gain could also be produced using a Raman pump unit optimized for 1515 nm.

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