Two pumps in one fiber provide broadband amplification
By Sunny Bains
A new kind of continuous-wave parametric amplifier that balances three different types of fiber nonlinearity was reported in a postdeadline paper at OFC 2002 in March. Researchers at Lucent Technologies' Bell Laboratories (Holmdel, NJ).and OFS Fitel (Broendby, Denmark), presented a prototype of the new device that allows more than 40-dB gain over 22 nm with a flat gain curve. According to its inventors, the amplifier is capable of broader, higher-gain performance by more careful choice of fiber and pump beam wavelengths. Initial experiments with simultaneous amplification of different wavelengths suggest that the device should be particularly useful for WDM.
Parametric amplifiers usually work with two identical pump photons that are absorbed and then emitted to produce two photons of symmetrically higher and lower frequency. The nonlinear processes that allow this conversion to take place are known as modulation interaction (MI) and degenerate four-wave mixing (FWM). Unfortunately, the band of wavelengths that can be amplified using these processes is rather narrow, and the main mechanism for broadening it—using different pieces of fiber to address different areas of the spectrum—is lossy.
The new technique exploits pump beams of two different wavelengths to produce a much more complex interaction. The wavelengths of one of the pumps is chosen so that it falls in the anomalous dispersion region of the fiber, the other in the normal dispersion region. When these two combine, there are many more options available for nonlinear interaction: not only do MI and degenerate FWM take place, but also nondegenerate FWM (caused by phase conjugation) and other stable frequency exchanges caused by Bragg reflection.
Among the experiments the Lucent/OFS Fitel team performed was one to demonstrate how the broadening of the amplification band could not be attributed by the simple addition of an extra pump: interaction between the two is necessary (see figure). Another showed how the amplifier could be used to both amplify and wavelength-convert several incoming signals simultaneously.
1. S. Radic et al., OFC 2002 Postdeadline Paper FB5-1.