What’s EDFA Amplifier?
EDFA (Erbium-doped Fiber Amplifier), firstly invented in 1987 for commercial use, is the most deployed optical amplifier in the DWDM system that uses the Erbium-doped fiber as optical amplification medium to directly enhance the signals. It enables instantaneous amplification for signals with multiple wavelengths, basically within two bands. One is the Conventional, or C-band, approximately from 1525 nm to 1565 nm, and the other is the Long, or L-band, approximately from 1570 nm to 1610 nm. Meanwhile, it has two commonly used pumping bands, 980 nm and 1480 nm. The 980nm band has a higher absorption cross-section usually used in low-noise application, while 1480nm band has a lower but broader absorption cross-section that is generally used for higher power amplifiers.
How does the EDFA amplifier enhance the signals?
Classification of EDFAs
Common power amplifiers and high power amplifiers (HPAs) based on the power
|Common power amplifier||The single-wavelength optical power is 1 dBm, and the total optical power is about 20 dBm.|
|High power amplifier||The single-wavelength optical power is 4–7 dBm, and the total optical power is generally greater than 23 dBm.|
Single-level amplifiers and multi-level amplifiers by the number of cascaded EDFAs
|Single-level amplifier||Single-level EDFA amplifiers are fixed gain amplifiers. The gain range of a single-level EDFA amplifier is small, usually a specific value in the range of 15–23 dB, for example, a single-level amplifier with a gain of 23 dB.|
|Multi-level amplifier||Multi-level EDFA amplifiers are generally formed by two levels of EDFAs connected in series. The gain range can cover a certain range of 16 dB to 40 dB. For example, if the gain range of the most commonly used multi-level amplifier is 20–31 dB, the 2-level EDFA structure is used.|
What’s Raman Amplifier?
As the limitations of EDFA amplifier working band and bandwidth became more and more obvious, Raman amplifier was put forward as an advanced optical amplifier that enhances the signals by stimulated Raman scattering. To meet the future-proof network needs, it can provide gain at any wavelength. At present, two kinds of Raman amplifiers are available on the market. One is lumped Raman amplifier that always uses the DCF (dispersion compensation fiber) or high nonlinear fiber as gain medium. Its gain fiber is relatively short, generally within 10 km. The other one is distributed Raman amplifier. Its gain medium is common fiber, which is much longer, generally dozens of kilometers.
Classification of Raman Amplifiers
Based on the position of the Raman amplifier on the fiber line, Raman amplifiers are classified into forward Raman amplifiers and backward Raman amplifiers.
|Forward Raman||Forward Raman amplifiers are placed at the transmit end of the line side and behind a high power EDFA.|
|Backward Raman||Backward Raman amplifiers are placed at the receive end of the line side and must be followed by an EDFA.|
According to the composition of Raman amplifiers, Raman amplifiers can be classified into independent Raman amplifiers and hybrid Raman amplifiers.
|Independent Raman amplifier||Early forward Raman amplifiers and backward Raman amplifiers are generally independent Raman amplifiers. Typically, an independent Raman amplifier installed in an independent subrack. The gain is about 10 dB to 20 dB.|
|Hybrid Raman amplifier||The Raman amplifier and the EDFA form a hybrid amplifier, which is integrated in a board. The gain range of the hybrid amplifier can cover each range from 20 dB to 50 dB.|
As technology develops, independent Raman amplifiers have been gradually replaced by the integrated hybrid Raman amplifiers.
Comparison Between EDFA and Raman Amplifiers
Both the EDFA and Raman amplifiers feature high gain, low noise figure, and high output power. The following table lists the differences between the two types.
|Gain wavelength||Fixed range. The operating wavelength is within the 1550 nm window.||Gain exists in any fiber regardless of the fiber type. The gain wavelength is determined by the pump wavelength. Theoretically, signals of any wavelength can be amplified.|
|Stability||Stable. The EDFA is insensitive to temperature. The gain has little correlation with polarization and is irrelevant to the system bit rate and data format.||Unstable and strong polarization-dependent gain. The orthogonal pump mode, transient gain, and backward pump mode are used.|
|Gain bandwidth||Unflatness||Flatness within the gain bandwidth range|
|Precautions||The optical surge problem of the EDFA is severe. The peak optical power may reach several watts, which may damage O/E converters and optical connectors. Therefore, it is not recommended EDFAs be cascaded.||The output optical power of the Raman amplifier is high. During maintenance, do not look into the optical port without eye protection. The laser of the Raman amplifier must be shut down before the board with fibers is removed. Otherwise, the maintenance personnel may be injured by strong light.|