In a Raman amplifier, the signal is intensified by Raman amplification, which is based on stimulated Raman scattering (SRS). This type of scattering occurs in a nonlinear medium when an incident pump photon at higher frequency ωp releases its energy to create another photon (signal) of reduced energy at lower frequency ωs (inelastic scattering); the remaining energy ωp-ωs is absorbed by the medium in the form of molecular vibrations (optical phonons). Raman amplification effect can be achieved by a nonlinear interaction between a signal and a pump laser within an optical fiber. Raman amplification is used in optical telecommunications to provide all-band wavelength coverage and in-line distributed signal amplification. In particular, optical transmission fiber can be used as a nonlinear medium for Raman amplification. Pumping the transmission fiber with radiation of the right frequency produces amplification of optical signals travelling in the transmission fiber.
The pump light may be coupled into the transmission fiber and travel in the same direction as the signal (co-directional pumping), in the opposite direction (contra-directional pumping) or both. Contra-directional pumping is more common as the transfer of noise from the pump to the signal is reduced.
The principal advantage of Raman amplification is its ability to provide distributed amplification within the transmission fiber, thereby increasing the length of spans between amplifier and regeneration sites. The amplification bandwidth of Raman amplifiers is defined by the pump wavelengths utilized and so amplification can be provided over wider, and different, regions than may be possible with other amplifier types which rely on dopants and device design to define the amplification ‘window’.
Raman amplifiers have some fundamental advantages. First, Raman gain exists in every fiber, which provides a cost-effective means of upgrading from the terminal ends. Second, the gain is non-resonant, which means that gain is available over the entire transparency region of the fiber ranging from approximately 0.3 to 2 μm. A third advantage of Raman amplifiers is that the gain spectrum can be tailored by adjusting the pump wavelengths. For instance, multiple pump lines can be used to increase the optical bandwidth, and the pump distribution determines the gain flatness. Another advantage of Raman amplification is that it is a relatively broad-band amplifier with a bandwidth >5 THz, and the gain is reasonably flat over a wide wavelength range
To produce Raman gain in the transmission fiber for signals in a particular wavelength band requires that the fiber be pumped at a relatively high-power level (hundreds of milliwatts) at a wavelength, or wavelengths, shifted down from the signal wavelength(s) by an amount corresponding to the characteristic Raman shift of the fiber. For typical silica fiber, the Raman gain spectrum consists of a relatively broad band centered at a shift of about 440 cm−1. Therefore, to provide gain for signals in the C-band (1530 to 1565 nm) for example, requires pump energy in the 1455-nm region.
In typical prior-art distributed Raman amplification embodiments, the output of a high-power laser source (e.g. a Raman fiber laser with a center wavelength of ˜1455 nm) or a group of multiplexed laser diodes with wavelengths in the 1455-nm region is launched from a receiving or repeater terminal to pump the fiber and provide gain for the incoming C-band signals. To extend the amplification bandwidth for high-capacity WDM systems, the launched pump spectrum is broadened by using multiple Raman lasers (each with a predetermined power and wavelength) or by multiplexing additional laser diodes of specific wavelength and power.
In a Raman laser the fundamental light-amplification mechanism is stimulated Raman scattering. In contrast, most “conventional” lasers rely on stimulated electronic transitions to amplify light. Raman lasers are optically pumped. However, this pumping does not produce a population inversion as in conventional lasers. Rather, pump photons are absorbed and “immediately” re-emitted as lower-frequency laser-light photons (“Stokes” photons) by stimulated Raman scattering. The difference between the two photon energies is fixed and corresponds to a vibrational frequency of the gain medium. This makes it possible, in principle, to produce arbitrary laser-output wavelengths by choosing the pump-laser wavelength appropriately. This is in contrast to conventional lasers, in which the possible laser output wavelengths are determined by the emission lines of the gain material.
In fiber-based Raman lasers, tight spatial confinement of the pump light can be maintained over relatively large distances. This significantly lowers threshold pump powers down to practical levels and furthermore enables continuous-wave operation. For optical telecommunications applications it is desirable to design Raman lasers with the highest possible launch power into the transmission fiber to stimulate the highest achievable Gain. A higher Raman Gain achieved into the transmission fiber enables longer spans reach between terminals and improves Optical Signal to Noise Ratio (OSNR) at the receiver. This improvement together with state of the art FEC and Digital Signal Processing, maximize robustness and distances covered by modern high bit rate digital optical systems.
It is within this context that embodiments of the present invention arise.