High gain, flattened, discrete Raman fiber amplifier and its transmission performance

The capacity of Erbium-doped amplifiers (EDFA) systems is rapidly being approached, and thus research is increasing in alternative technologies which can deliver broader gain spectrum than EDFAs to increase system capacity [1]. Fiber Raman Amplifiers (FRAs) provide gain in any spectral region provided that a suitable pump and fiber are available. Distributed amplifiers using the transmission fibre as a gain medium are used to improve noise performance but discrete Raman amplifiers also have the capability of extending the optical telecommunication bandwidth. Broadband gain can be achieved if multiple pumps are employed, however, gain flatness needs appropriate design of multi-pump configuration. In this experiment as shown in 1a, nonlinearity was used to spectrally broaden a 1455nm first order pump by propagating through a short length of TrueWave fiber with zero dispersion wavelength near the pump wavelength [2]. A very low power of the broadened pump was then used as a seed for the 1365nm second order pump, to counter-pump the gain fiber in this way a flattened Raman gain profile over 40 nm bandwidth can be achieved. The Raman gain fiber used was, 4.5km Inverse Dispersion Fiber (IDF), with attenuation 0.23dB/km, dispersion −44 ps/nm/km at 1550nm, dispersion slope −0.128 ps/nm
²
/km, and effective core area 31 μm
²
. The remnant pump was filtered via a second WDM coupler and dumped via an angled connector. The total loss for the Raman amplifier was ∼ 4dB and on-off gain can be as high as 25dB depending on the pump powers.