PDF of this tutorial- Low noise buildup
- Simple design, as direct signal amplification is achieved in the optical fiber, and no special transmission medium is needed.
- Flexible assignment of signal frequencies, as Raman gain depends on the pump wavelength and not on a wavelength-sensitive material parameter of the medium, such as the emission cross-section of dopant in the erbium-doped fiber (EDF).
- Broad gain bandwidth is achievable by combining the Raman amplification effect of several pump waves that are placed carefully in the wavelength domain.
However, despite the many advantages of Raman amplification, there can be some degradation effects. For example, not only the specially launched pump waves but also some of the WDM channels may provide power to amplify the other channels. This would result in power exchange between WDM channels and thus cross-talk leading to signal degradation.
These negative effects occur in unidirectional and bidirectional WDM transmission. So for accurate analysis of advanced WDM systems, it is crucial to model all Raman interactions. Additionally, degrading effects like spontaneous Raman scattering and backward Rayleigh scattering have to be considered.
Table 1 gives an overview of important characteristics of Raman and EDF amplifiers. Note that hybrid amplification schemes, using Raman and EDF amplification in concatenation, can be designed to take advantage of both types.
| Characteristic | Doped-Fiber Amplifier | Raman Amplifier |
| Amplification Band | depends on dopant | depends on availability of pump wavelengths |
| Amplification Bandwidth | 20 nm, more for multiple dopants/fibers | 48 nm, more for multiple pump waves |
| Gain | 20 dB or more, depending on ion concentration, fiber length, and pump configuration | 4–11 dB, proportional to pump intensity and effective fiber length |
| Saturation Power | depends on gain and material constants | equals about power of pump waves |
| Pump Wavelength | 980 nm or 1480 nm for EDFAs | 100 nm lower then signal wavelength at peak gain |
Table 1. Comparison of Raman and Doped-Fiber Amplifier Characteristics
Raman amplifiers are topologically simpler to design than doped-fiber amplifiers, as the existing transmission fiber can be used as a medium if properly pumped. However, the selection of pump powers and wavelengths, as well as the number and separation of pumps, strongly determines the wavelength behavior of Raman gain and noise.
When building distributed Raman amplifiers, designers face the question of using forward or backward pumping (or even both) with respect to signal propagation. The backward pumping scheme is most commonly used as it offers several advantages. Pump noise strongly affects the WDM signals to be amplified if forward pumping is applied, as the Raman process is nearly instantaneous. When the Raman pump wave has slight random power fluctuations in time, which is almost always the case, individual bits might be amplified differentially, which leads to amplitude fluctuations or jitter. If backward pumping is applied, power fluctuations of the Raman pump will be averaged out, as each individual bit will see several milliseconds of the Raman pump wave. Figure 4 shows the general setup of a backward pumped DRA and the counter-propagation of signal and pump.
Figure 4. Backward-Pumped Raman Amplifier Showing Counter-Propagation of Pump Wave and Signal
Hybrid (EDF and Raman) amplification has been used successfully in recent designs to obtain the necessary optical signal-to-noise ratio (OSNR) for high-capacity dense wavelength division multiplexing systems (DWDM) or to achieve very large amplifier spacing in, for example, festoon applications. Figure 5 shows a possible design of a hybrid EDF/Raman amplifier. The doped fiber is pumped remotely via the transmission fiber where Raman amplification occurs.
Figure 5. Hybrid EDF/Raman Amplifier
The transversal power distribution of the signal over an amplified fiber span is strongly dependent on the applied amplification scheme and can be controlled by the Raman pump power and pump direction. Figure 6 shows the transversal span power profile employing different hybrid EDF/Raman amplification schemes.
Figure 6. Span Power Profile for EDFA–Based Systems (1),
System Using Hybrid Schemes with Backward Raman Amplification Only (2),
and Bidirectional Raman Amplification (3)
By properly selecting pump laser wavelengths, transmission fiber lengths, and types, many optimization targets can be reached—flattening of the EDFA gain through an optimized design of the frequency-dependent Raman gain, for example. Optimization can be achieved using numerical simulation.
