Raman Amplifier

Technology Basics

The Raman optical amplifier is based on an entirely different principle than EDFA’s or conventional lasers. In EDFA’s and conventional lasers, atoms are pumped to a high energy state and then drop to a lower state, releasing their energy, when a suitable wavelength photon passes nearby. Raman optical amplifiers utilize Raman scattering (the Raman effect, often called SRS - Stimulated Raman Scattering) to create optical gain. Initially SRS was considered to be a detriment to high channel count DWDM systems. In these systems, as light traveled down the fiber, energy would be "robbed" from the shorter wavelength channels, boosting the amplitude of the longer wavelength channels.

Figure 1 shows the typical transmit spectrum or a six-channel DWDM system. Note that all of the six wavelengths have identical amplitudes. These signals are all in the 1550 nm window.	Figure 1 - Dual-Pump EDFA with Mid-Stage Access (Click to Enlarge Image)
Now refer to Figure 2. Several things can be noticed to be different. First, the noise background is a bit higher. The main change is that the amplitudes of the six wavelengths are now very different. The lower wavelengths now have smaller amplitude than the upper wavelengths. This is because the SRS effect has robbed energy from the lower wavelengths and fed that energy to the upper wavelengths.	Figure 2 - Optical Gain Spectrum of a Hybrid Optical Amplifier (Click to Enlarge Image)
A Raman optical amplifier is not a "black box" like an EDFA. It consists of little more than a high-power pump laser, usually called a Raman laser, and a WDM or directional coupler. The optical amplification occurs in the transmission fiber itself. The optical amplification is distributed along the transmission line. Optical signals are amplified up to 10 dB in the network optical-fiber. The Raman optical amplifiers have wide gain bandwidth (up to 100 nm). They can use any installed transmission optical-fiber (single-mode optical fiber, TrueWave, etc.). In effect, they reduce the effective span loss and improve noise performance of the transmission line by boosting the optical signal in transit. They can be combined with erbium-doped fiber amplifiers (EDFA) to achieve very wide optical gain flattened bandwidth. Figure 3 shows the topology of a typical Raman optical amplifier. The two key elements of the Raman optical amplifier are the pump laser and the directional coupler. The pump laser, in this case, has a wavelength of 1535 nm. The circulator provides a convenient means of injecting light backwards in to the transmission path with minimal optical loss.	Figure 3 - Wideband EDFA/Raman Amplifier Examples (Click to Enlarge Image)
Figure 4 shows the optical spectrum of a forward-pumped Raman optical amplifier. In this case, the pump laser is injected at the transmit end rather than the receive end as shown in Figure 3. The pump laser has a wavelength of 1535 nm. As is usually the case, the amplitude of the pump laser is much greater than the data signals. Figure 5 shows the received signal after the same length of fiber used in the SRS example above. While the amplitude of the pump laser is significantly decreased, the amplitude of the six data signals is now much stronger and they all have roughly equal amplitudes. In this case, a great deal of energy was robbed from the 1535 nm pump laser signal and redistributed to the six data signals.	Figure 4 - Raman Amplifier Optical Spectrum (Click to Enlarge Image)