This invention relates to Raman amplifiers, and more particularly to distributed Raman amplifiers with flat gain over a wide band of wavelengths.
Optical transmission systems employ Wavelength Division Multiplexing (WDM) to increase information handling of an optical fiber transmission line, typically a long haul transmission line. Early WDM systems operated with a relatively narrow wavelength bandwidth, centered around 1550 nanometers, e.g. 1530-1565 nanometers, often referred to as the C-band. This is the wavelength region where standard silica based optical fibers have optimally low absorption.
In most WDM systems there is a trade-off between the number of channels the system accommodates and the channel separation. Both goals favor a wide operating spectrum, i.e. a wide range of operating wavelengths.
Recently, systems have been designed that extend the effective operating wavelength range well above the C-band transmission band. In terms of wavelength, the new band, referred to as the L-band, is variously defined, but for the purpose of this description is 1570-1610 nanometers. Use of these added wavelengths substantially extends the capacity of WDM systems. There is an ongoing effort to further extend the effective operating wavelength window to above 1610 nm, for example to 1620 nm. Success of these efforts will depend on finding components, for example amplifiers, that provide effective operation over this broad wavelength range.
In WDM systems, it is important to have uniform gain over the entire WDM wavelength band. This objective becomes more difficult to reach as the operating wavelength range is extended to longer wavelengths. Recently, new types of optical fiber amplifiers have been developed that operate using Raman scattering. The most prominent of these is a distributed amplifier that operates over the normal transmission span as a traveling wave amplifier. Raman scattering is a process by which light incident on a medium is converted to light at a lower frequency than the incident light. The pump photons excite the molecule up to a virtual level (non-resonant state). The molecular state quickly decays to a lower energy level emitting a signal photon in the process. Because the pump photon is excited to a virtual level Raman gain can occur for a pump source at any wavelength. The difference in energy between the pump and signal photons is dissipated by the molecular vibrations of the host material. These vibrational levels determine the frequency shift and shape of the Raman gain curve. The frequency (or wavelength) difference between the pump and the signal photon is called the Stokes shift. In Ge-doped silica fibers, the Stokes shift at which the maximum gain is obtained is xcx9c13 THz. Due to the amorphous nature of silica the Raman gain curve is fairly broad in optical fibers.
Since Raman scattering can occur at any wavelength, this can be exploited to advantage in a telecommunication system that contains multiple signal wavelengths by using Raman pumps at several different wavelengths to amplify the signals. The gain seen by a given wavelength is the superposition of the gain provided by all the pumps, taking into account the transfer of energy between the pumps due to Raman scattering. By properly weighting the power provided at each of the Raman pump wavelengths it is possible to obtain a signal gain versus wavelength profile in which there is a small difference between the gain seen by different signal wavelengths (this difference is called the gain ripple or gain flatness).
A multiplicity of pumps has been used successfully in many different experiments. There is however one persistent problem with this approach. A deleterious nonlinear effect called four-wave mixing (FWM) can sometimes occur. In telecommunications systems, if FWM occurs in the signal band this may lead to transmission errors. As the number of pumps in a multi-pump wavelength Raman amplification scheme increases, the likelihood of FWM increases.
The harmful effects of four-wave mixing have been recognized. Recently one approach towards reducing these effects has been proposed [EP 1 148 666 A2]. In this approach the pump wavelengths are either time division multiplexed (TDM) together, or the frequency of the pump source is modulated (FM). Since the various pump wavelengths overlap for only a small distances along the fiber, FWM between the pump wavelengths should be eliminated or severely reduced.
While this approach would eliminate FWM, the nominal pump power requirements in this system are relatively high. Moreover, to TDM a relatively large number of pump wavelengths, some operating at relatively high power, adds significantly to the cost of the system. Reducing either of these requirements would significantly enhance the attractiveness of using multiplexed pump wavelengths to control deleterious FWM effects. In addition, a Raman amplifier that is effective in producing uniform and flat gain over the C+L-band would represent an important technological advance in DWDM system design.
The invention is based in part on an understanding that the FWM effect is not uniform for all pump wavelengths. We analyzed the pump wavelengths and powers required to provide a flat Raman gain to the C and L-band of a distributed Raman amplifier. From this analysis, certain pump wavelengths were identified where FWM is especially aggravated. Some wavelengths, the shortest wavelengths in the pump spectrum of the examples described below, produce little or no FWM. It was also observed that the power required at the longer wavelengths is significantly less than at the shorter wavelengths. This is because the shorter wavelengths pump the longer wavelengths in the transmission span. Following this understanding, the elimination of deleterious FWM can be realized by using TDM or FM only for the pump wavelengths that contribute to this process. The amount of power required by this TDM or FM scheme is reduced since it takes advantage of the pumping of the longer wavelengths by the shorter wavelengths in the transmission span. The longer wavelengths already have lower launch powers. Results include:
1) By TDM fewer pumps, the switching requirement on each pump is reduced, as well as the peak powers required.
2) The frequency range required of an FM source is reduced.
3) With fewer pumps to modulate the total cost of electronics decrease.
4) By narrowing the frequency range required for pump multiplexing, the demands on a swept wavelength source are reduced, making that option more attractive and feasible.