When a normal optical fiber core is doped with trivalent ‘erbium’ ions, erbium doped fiber is formed. This erbium doped fiber act as a gain medium that amplifies an optical signal. Hence, it is named as EDFA (Erbium Doped Fiber Amplifier). The erbium doped fiber is pumped with a laser, at a wavelength of 980 nm or 1480 nm and produce optical gain in the 1550 nm region.
We can’t directly send laser light into EDFA. Before that, laser needs to be multiplexed into the erbium doped fiber. For that purpose, we use WDM (Wavelength Division Multiplexing) coupler to multiplex laser into EDFA. After that EDFA is pumped with laser, to achieve optical gain.
EDFA uses population inversion technique to achieve optical amplification. Before we jump into EDFA, first let’s see how the actual laser works. LASER stands for Light Amplification and Stimulated Emission of Radiation. As the name suggests, laser amplifies the light by using a process called stimulated emission of radiation.
Shall we go deeper into the concept!
Every object in the universe is made up of tiny particles called atoms. However, atoms are not the smallest particles in the universe. There are some particles which are much smaller than atoms. These particles are electrons, protons and neutrons. Combining these particles make an atom. Each atom has a set of electrons, protons and neutrons. Electrons have negative charge, protons have positive charge and neutrons have no charge. Protons and neutrons always stick together because of the strong nuclear force between them.
The protons and neutrons which are stick together are known as nucleus. The overall charge of the nucleus is positive because of the positive protons (neutrons does not have charge). On the other hand, electrons have negative charge (opposite charge to protons). As we know that, there exists an attractive force between the opposite charges. So the electrons always rotate around the nucleus at different distances because of the electrostatic force of attraction between electrons and nucleus.
The electrons which are revolving at different distances from the nucleus have different energy levels associated with it. The electrons which are revolving at a very close distance from the nucleus have the lowest energy level whereas; the electrons which are revolving at a larger distance from the nucleus have the highest energy level.
The electrons at a larger distance from the nucleus have highest energy level because they reached that level by gaining additional energy from the external energy sources like light, heat and voltage. This process of gaining additional energy from the external energy sources to jump into higher energy level is called absorption of radiation.
The electrons in the higher energy level will not stay for a long time. After a short period of time, they fall back to the lower energy level by releasing energy in the form of heat or light. This process is called spontaneous emission of radiation. Laser works based on this concept.
Although, the electrons release energy in the form of light, there is no light amplification in this process. So the spontaneous emission process is not used to build a laser.
The laser works based on a special process called stimulated emission of radiation. To achieve light amplification, the number of electrons in the higher energy level (E2) must be greater than the number of electrons in the lower energy level (E1). In a two level energy system, the population of electrons in the higher energy level is always lesser than the population of electrons in the lower energy level. In some cases, the population of electrons in higher energy level will becomes equal to the population of electrons in the lower energy level. So the two level energy system is not useful for light amplification.
The light amplification is achieved by using a 3 or more energy level system. The greater the energy levels are, the greater will be the light amplification. For example, a 4 level energy system will produce more optical gain than the 3 level energy system.
The EDFA is pumped using two laser diodes (bidirectional pumping) or a single laser diode (unidirectional pumping). The EDFA pumped with a single laser diode is most commonly used. In this tutorial, EDFA pumped with a single laser diode is discussed.
EDFA is pumped with two different wavelengths of photons: 980 nm or 1480 nm.