Erbium-Doped Fiber Amplifiers (EDFAs)
Erbium-doped fiber amplifiers (EDFAs) make use of rare-earth elements such as erbium (Er3+) as amplification medium. It is doped into the fiber core during the manufacturing process. It consists of a short piece of fiber (typically 10 m or so) made of glass in which a small controlled amount of erbium is added as a dopant in the form of an ion (Er3+). Thus, the silica fiber acts as a host medium. It is the dopants (erbium) rather than silica fiber that determine the operating wavelength and the gain bandwidth. EDFAs generally operate in the 1550 nm wavelength region and can offer capacities exceeding 1 Tbps. So, they are widely used in WDM systems.
The principle of stimulated emission is applicable for the amplification mechanism of the EDFA. When the dopant (an erbium ion) is in a high-energy state, an incident photon of the input optical signal will stimulate it. It releases some of its energy to the dopant and returns to a lower-energy state (“stimulated emission”) that is more stable. The figure below shows the basic structure of an EDFA.
The pump laser diode normally produces an optical signal of wavelength (at either 980 nm or 1480 nm) at high power (~ 10–200 mW). This signal is coupled with the light input signal in the erbiumdoped section of the silica fiber through WDM coupler. The erbium ions will absorb this pump signal energy and jump to their excited state. A part of the output light signal is tapped and fed back at the input of pump laser through optical filter and detector. This serves as feedback power control mechanism so as to make EDFAs as self-regulating amplifiers. When all the metastable electrons are consumed then no further amplification occurs. Therefore, the system stabilizes automatically because the output optical power of the EDFA remains almost constant irrespective of the input power fluctuation, if any.
The above figure shows the simplified functional schematic of an EDFA in which a pump signal from the laser is added to an input optical signal (at 1480 nm or 980 nm) through a WDM coupler.
This diagram shows a very basic EDF amplifier. The wavelength of the pump signal (with pump power of about 50 mW) is 1480 nm or 980 nm. Some part of this pump signal is transferred to the input optical signal by stimulated emission within a short length of Erbium-doped fiber. It has typical optical gain of about 5–15 dB and less than 10 dB noise figure. For 1550 nm operation, it is possible to obtain 30–40 dB optical gain.
The above figure depicts a simplified operation of an EDFA with its practical structure when used in WDM application.
As shown, it includes the following major parts:
- An isolator at the input. This keeps the noise generated by an EDFA from propagating towards the transmitter end.
- A WDM coupler. It combines the low-power 1550 nm optical input data signal with high-power pumping optical signal (from pump source such as laser) at 980 nm wavelength.
- A small section of erbium-doped silica fiber. In fact, this serves as the active medium of the EDFA.
- An isolator at the output. It helps to prevent any back-reflected optical signal from entering the erbium-doped silica fiber.
The final output signal is an amplified 1550 nm wavelength optical data signal with a residual 980 nm wavelength pump signal.
Types of Erbium-Doped Fiber Amplifiers (EDFAs)
There are two types of structures of Erbium-Doped Fiber Amplifiers (EDFAs):
- EDFA with co-propagating pump
- EDFA with counter-propagating pump
The figure below shows counter-propagating pump and bidirectional pump arrangements that can be used in EDFA structures.
A co-propagating pump EDFA features lower output optical power with low noise; while a counter-propagating pump EDFA provides higher output optical power but produces greater noise too. In a typical commercial EDFA, a bi-directional pump with simultaneous co-propagating and counter-propagating pumping is used which results in a relatively uniform optical gain.
In a long-haul application of an optical fiber communication link, EDFAs can be used as a booster amplifier at the output of the optical transmitter, an in-line optical amplifier along with the optical fiber as well as a pre-amplifier just before the receiver, as shown in the above figure.
It may be noted that in-line EDFAs are placed at 20–100 km distance apart depending on the fiber loss. The optical input signal is at 1.55 μm wavelength, whereas the pump lasers operate at 1.48 μm or 980 nm wavelength. Typical length of Erbium-doped fiber is 10–50 m.
Amplification Mechanism in EDFAs
As stated earlier, the amplification mechanism in an EDFA is based on stimulated emission similar to that of in laser. High energy from the optical pump signal (produced by another laser) excites the dopant erbium ions (Er3+) in a silica fiber at the upper energy state. The input optical data signal stimulates the transition of the excited Erbium ions to the lower energy state and results in the radiation of photons with the same energy, i.e., the same wavelength as that of the input optical signal.
Energy-level Diagram: Free Erbium ions exhibit discrete levels of the energy band. When Erbium ions are doped into a silica fiber, each of their energy levels splits into a number of closely related levels so as to form an energy band.
To attain population inversion, Er3+ ions are pumped at the intermediate level 2. In indirect method (980-nm pumping), Er3+ ions are continuously moved from level 1 to level 3. It is followed by non-radiatively decay to level 2, from where they fall to level 1, radiating the optical signals in the desired wavelength of 1500–1600 nm. This is known as 3-level amplification mechanism.
Advantages Offered by EDFAs
- EDFAs can be used as power amplifier after optical transmitter, in-line optical amplifier along the fiber, and pre-amplifier before the optical receiver.
- The optical gain is almost insensitive to polarization.
- Inter-channel crosstalk does not occur in EDFAs due to large carrier lifetime (about 10-ms).
- Inter-channel crosstalk due to cross-gain saturation can be minimized in the non-saturated regime operation.
- EDFAs with a relatively high concentration of Erbium dopants (~ 500 parts per million) can be used as lumped optical amplifiers.
- Most EDFAs are capable of providing optical gain as high as 20–25 dB gain using Erbium-doped fiber length of approximately 10 m.
- The optical gain of EDFAs cannot be modulated at frequencies much larger than 10 kHz due to their relatively slow response.
- The bandwidth of EDFAs is large which make it an ideal choice for point-to-point WDM networks.
- EDFAs make WDM possible which increases the capacity of optical fiber communication links.
Limitations of EDFAs
- The optical gain spectrum is not uniform.
- EDFAs work only in the 1550-nm transparent window.
- EDFAs cannot find application in CATV industry which operates exclusively at 1300-nm.
- EDFAs are costlier as compared to other types of optical amplifiers.