BY JAMES J. REFI
Not all fiber non-linear effects are bad. In fact, next-generation transmission systems will rely on the non-linear light scattering ability of transmission fibers to amplify the optical information signals that they carry. The efficiency of a fiber in performing this amplification is determined by its Raman gain efficiency (see Lightwave, "Raman amplification key to solving capacity, system-reach demands," August 2000, p. 69).
When a fiber carries high optical intensities, the optical power can be scattered because of interactions with molecular vibrations in the fiber. For low power levels, the scattered power is a small fraction of the incident power. But as the incident power increases, the scattered power increases at a faster pace and is said to be stimulated. There are two forms of non-linear stimulated scattering: Brillouin and Raman.
Stimulated Raman scattering (SRS) is an interaction between light and the fiber's molecular vibrations, as adjacent atoms vibrate in opposite directions (an "optical phonon"). Some of the energy in an optical pump wave is transferred to the molecules, thereby further increasing the amplitude of their vibrations. If the vibrational amplitudes become large, a threshold is reached at which the local index of refraction changes. These local changes then scatter light in all directions-similar to Rayleigh scattering. But unlike Rayleigh scattering, the wavelength of the Raman scattered light is shifted to longer wavelengths by an amount that corresponds to the vibrational frequencies of the molecules. The Raman scattered light amplifies information signals with a gain efficiency that depends on pump wavelength, signal wavelength, fiber effective area (see Lightwave, "TIA addresses fiber non-linearities," May 2000, p. 26), fiber material composition, fiber attenuation coefficient, and fiber length.
At its June meeting, the Telecommunications Industry Association FO-6.6, chaired by Greg Smith of Corning Inc., approved ITM-22, "Continuous-wave method for measuring the Raman gain efficiency of singlemode fibers."
Authored by yours truly, the informative test memorandum describes a method to measure a fiber's efficiency at converting pump power to information signal power. The pump wave is held constant as the signal wave sweeps from the pump wavelength to at least 120 nm higher.
The information obtained from this measurement can be used with a model of various physical processes (such as pump-to-pump power transfer, signal-to-signal power transfer in DWDM, pump depletion, and double Rayleigh scattering) to design practical discrete and distributed Raman amplifiers.
Yours truly has also cast this ITM into the International Electrotechnical Commission format, where it was discussed at the October meeting of SC86A/WG1, chaired by Allen Cherin of Lucent Technologies.
James J. Refi, recently retired, was formerly with Lucent Technologies (Norcross, GA) and participated in TIA and IEC standards groups on optical fiber.