EDFA (Erbium-doped Fiber Amplifier) for WDM System
Optical amplifiers serve as an integrated part of long haul data transmission. The most prominent feature of optical amplifier is that the device can amplify optical signal directly without the need to convert the signal into an electrical one before amplifying. EDFA (Erbium-doped Fiber Amplifier) is the one type of many different optical amplifiers that can achieve signal amplification over long-hual optical communication.
Optical Amplifier Basics
When transmitting over long distance, the optical signal has to be amplified many times in between owing to the signal loss from fiber attenuation, connectivity losses, fiber splicing losses, etc. Before optical amplifier is invented, the optical signal has to be first converted into electrical signal, amplified, and then converted back to optical signal again. The process is very complicated and expensive. Optical amplifier has since been invented that can amplify signals directly, this process is significantly cheaper and started a fiber optic revolution. There are three fiber optic amplifier types: EDFA, Raman amplifier and semiconductor optical amplifier(SOA).
What Is EDFA Amplifier?
EDFA, or Erbium-doped Fiber Amplifier, is the most deployed optical amplifier in the WDM system that uses the Erbium-doped fiber as optical amplification medium to directly enhance the signals. The amplifying medium is a glass optical fiber doped with erbium ions. The erbium-doped fiber gain medium amplifies light at wavelengths that are in the neighborhood of 1550 nm—the optical wavelengths that suffer minimum attenuation in optical fibers. EDFA works best in the range 1530 to 1565 nm, and it possesses low noise and can amplify many wavelengths simultaneously, making it the fiber amplifier of choice for most applications in optical communications.
Three EDFA Amplifier Types for DWDM Connectivity
According to the functions, EDFA can be divided into three types: booster amplifier, in-line amplifier and pre-amplifier.
A booster amplifier operates at the transmission side of the link, designed to amplify the signal channels exiting the transmitter to the level required for launching into the fiber link. It’s not always required in single channel links, but is an essential part in DWDM link where the multiplexer attenuates the signal channels. It has high input power, high output power and medium optical gain. The common types are 20dBm Output C-band 40 Channels 26dB Gain Booster EDFA, 16dBm Output C-band 40 Channels 14dB Gain Booster EDFA and so on.
An in-line amplifier is generally set at intermediate points along the transmission link in a DWDM link to overcome fiber transmission and other distribution losses. Optical line amplifier is designed for optical amplification between two network nodes on the main optical link. In-line amplifiers are placed every 80-100 km to ensure that the optical signal level remains above the noise floor. It features medium to low input power, high output power, high optical gain, and a low noise figure.
A pre-amplifier operates at the receiving end of a DWDM link. Pre-amplifier is used to compensate for losses in a demultiplexer near the optical receiver. Placed before the receiver end of the DWDM link, pre-amplifier works to enhance the signal level before the photo detection takes place in an ultra-long haul system, hence improving the receive sensitivity. It has relatively low input power, medium output power and medium gain.
How Does EDFA Work?
Basically, EDFA consists of a length of EDF(Erbium doped fibre), a pump laser, and a WDM combiner. The WDM combiner is for combining the signal and pump wavelength, so that they can propagate simultaneously through the EDF. The lower picture shows a more detailed schematic diagram of EDFA.
working priciple of EDFA
The optical signal, such as a 1550nm signal, enters an EDFA amplifier from the input. The 1550nm signal is combined with a 980nm pump laser with a WDM device. The signal and the pump laser pass through a length of fiber doped with Erbium ions. The 1550nm signal is amplified through interaction with the doping Erbium ions. This action amplifies a weak optical signal to a higher power, effecting a boost in the signal strength.
Why EDFA Fiber Amplifier Is Essential for DWDM System?
Nowadays, EDFA optical amplifier rises as a preferable option for signal amplification method for DWDM systems, owing to its low-noise and insensitive to signal polarization. Besides, EDFA deployment is relatively easier to realize and more affordable compared with other signal amplification methods.
EDFAs have high pump power utilization (>50%)
Directly and simultaneously amplify a wide wavelength band (>80nm) in the 1550 nm region, with a relatively flat gain.
Flatness can be improved by gain-flattening optical filters.
Gain in excess of 50 dB.
Low noise figure suitable for long haul applications.
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