Time-division multiplexed pump wavelengths resulting in ultra broad band, flat, backward pumped Raman gain

#This invention relates generally to the field of optical communications and in particular to a method and apparatus for producing a flat Raman gain over very long bands.

#Early proposals for all-optical transmission (see, e.g., A. Hasegawa, “Numerical study of optical soliton transmission amplified periodically by the stimulated Raman process,” Appl. Opt., 23,1095 (1984); L. F. Mollenauer, J. P. Gordon, and M. N. Islam, “Soliton propagation in long fibers with periodically compensated loss,” IEEE J. Quantum Electronics QE-22, 157 (1986)) were based on the use of gain from the Raman effect to turn spans of transmission fiber into their own amplifiers. The scheme afforded many fundamental advantages, and indeed, it was used successfully for the first demonstration of an all-optical, long-distance transmission and subsequently reported by L. F. Mollenauer and K. Smith, in an article entitled “Demonstration of soliton transmission over more than 4000 km in fiber with loss periodically compensated by Raman gain,” which appeared in Opt. Lett. 13, 675 (1988).

#Nevertheless, with the advent of the erbium fiber amplifier in the late 1980's, Raman amplification temporarily fell out of vogue, largely due to the pump powers required. That is, within the context of single-channel transmission, where the signal powers are rarely more than one or two milliwatts, the several hundreds of milliwatts threshold power required for net positive Raman gain seemed excessive and, at the time, impractical. With the recent ascendance of dense WDM (where net signal levels can easily reach one hundred mW or more), and with the simultaneous commercial availability of several-hundred mW output semiconductor pump lasers, however, opinion has changed. Now Raman gain is highly prized for its ability to overcome noise/non-linear penalties, and for the fact that the position and extent of the gain band depend only on the available pump wavelengths. Another extremely important advantage of Raman amplification for dense WDM lies in the fact that, in stark contrast to erbium amplifiers, the shape of the Raman gain band is essentially independent of pump and signal levels.

#FIG. 1 is a schematic drawing illustrating the Raman effect and a graph depicting the Raman gain;

#FIG. 2 is a graph showing the Raman gain for dispersion shifted optical fiber;

#FIG. 3 is a graph showing pump wavelength vs. distance for a Raman pump;

#By way of background and with reference to FIG. 1, there is shown in schematic form a diagram depicting the Raman effect. Briefly, and with reference to that Fig, the Raman effect in silica-glass fibers begins with a pump-induced transition from a ground state 101 to a virtual state 102, followed by an emission 103 from that virtual state 102, where the emission terminates on an excited state 104 of the lattice. Emission of an optical phonon 105 (which typically takes place within a few femtoseconds) then completes return to the ground state 101.

#The optical transitions are really highly non-resonant ones to the excited electronic levels, which in silica glass, lie some 5-6 eV above the ground state. Hence, the optical transitions get their strength primarily from the matrix elements of the very strong, allowed transition to the first electronic state.

#Because of the extremely fast relaxation, the population of the terminal state of the optical emission tends to be determined by equilibrium with the surrounding phonon bath, and hence is almost independent of the rates of optical pumping and emission. This fact is the source of the independence, cited above, of the shape of the Raman gain band from the optical pumping dynamics.