As we know, the longer the optical transmission distance is, the weaker the optical signals will be. For a long DWDM system, this phenomenon easily causes transmission error or even failure. Under this case, what can we do for a smooth, long DWDM system? The answer is optical signal enhancement. Only by enhancing the optical signals, can the DWDM transmission distance be extended. In this post, we are going to learn two effective solutions, optical amplifier (OA) and dispersion compensation module (DCM) to enhance the signals, for making a smooth, long DWDM system.
Optical Amplifier Solution
We used to utilize repeater to enhance the signals in fiber optics, which should firstly convert the optical signals into an electrical one, amplify the electrical signals, and then convert the electrical signals into an optical one again. Finally, you can get the enhanced optical signals. However, this method of enhancing signals can not only cause more signal loss, but also add unwanted noises in the actual signal. Taking these issues into account, the optical amplifier is more recommendable.
An optical amplifier is a device that enables direct optical signal enhancement or amplification. Its working principle is not so complicated as that of the repeater, while its performance is much higher. From the following figure, we can learn that the original reach of the DWDM system is limited to 80 km due to the signal loss. But with the optical amplifier, the signals are enhanced and the reach can be extended to 160 km. It is really an ideal option to enhance the signals for a long DWDM system.
At present, there are mainly three major kinds of optical amplifiers, Semiconductor Optical Amplifier (SOA), Doper Fiber Amplifier (DFA), and Raman Amplifier (RA).
Semiconductor Optical Amplifier: as its name implies, the semiconductor in a SOA is used to offer the gain medium. This kind of optical amplifier has a similar structure to the FP laser diode. However, it is designed with anti-reflection elements at the end face that can greatly reduce the end face reflection. Meanwhile, the SOA features small package and low cost that suits for most users to enhance the optical signals.
Doper Fiber Amplifier: in a DFA, the doped optical fiber acts as the gain medium for signal amplification. When the DFA works, the signal to be amplified and a pump laser are multiplexed into the doped fiber. And then the signal is amplified through interaction with the doping ions. The most common DFA is the Erbium Doped Fiber Amplifier (EDFA). Its gain medium is a optical fiber doped with trivalent erbium ions that always enhances the signals near 1550nm wavelength. Undoubtedly, the EDFA is a great choice to enhance the optical signals.
Raman Amplifier: different from the SOA and DFA, the signal in a RA is amplified through the nonlinear interaction between the signal and a pump laser within an optical fiber. In details, two kinds, distributed and lumped Raman amplifier (DRA and LRA) are available on the market. The distributed one multiplexes the pump wavelength with signal wavelength through the transmission fiber to enhance the signals, while the amplification of the lumped one is provided by a dedicated, shorter length of fiber.
(Note: if you want to know more information about these three kinds of optical amplifier, you can take the previous post Optical Amplifier Overview as reference.)
Dispersion Compensation Solution
Apart from signal amplification, we can also use dispersion compensation to enhance the optical signals. Once the dispersion occurs, the signal will be tended to skew due to the different frequencies, which has a negative effect on the quality of signal transmission. At that moment, we use the dispersion compensation module to enhance the skew signal, for achieving a longer transmission distance. As shown in the figure below, the DWDM system is extended to longer than 80 km with the use of 80km passive dispersion compensation module.
The dispersion compensation module is an important component for a long fiber optical link. It typically connects to the mid-stage of an OA like EDFA, in the long haul transmission system. Except for the 80km DCM mentioned above, FS.COM also provides other DCM modules that allow long transmission distance extension. The compensation distances can range from 10km to 140 km, as shown in the following table.
|10KM Passive Dispersion Compensation Module, Plug-in Type, LC/UPC
|20KM Passive Dispersion Compensation Module, Plug-in Type, LC/UPC
|40KM Passive Dispersion Compensation Module, Plug-in Type, LC/UPC
|60KM Passive Dispersion Compensation Module, Plug-in Type, LC/UPC
|80KM Passive Dispersion Compensation Module, Plug-in Type, LC/UPC
|100KM Passive Dispersion Compensation Module, Plug-in Type, LC/UPC
|140KM Passive Dispersion Compensation Module, Plug-in Type, LC/UPC
The optical amplifier has the ability to directly boost the weak signal, while the dispersion compensation module can reshape the deformed signal and offer a long compensation distance. Considering that the signal strength would become weak as the transmission distance increases, using the optical amplifier and dispersion compensation module to enhance the signals is very necessary when building a long DWDM system.