Optical Amplifiers

An optical amplifier is a device that directly amplifies an optical signal without the need for O/E conversion. Most optical amplifiers are similar to lasers in that the amplification is achieved through stimulated emission within the device. The main difference between them is that optical amplifiers do not have mirrors forming an optical cavity. Optical amplifiers amplify all of the wavelengths in the gain spectrum of the gain medium (see Figure 3-1(A)), so they are particularly useful in WDM systems. Optical amplifiers can be built in three basic technologies;

semiconductor laser technology, planar waveguide technology and optical fiber technology.

Most use the principle of the laser in their operation (stimulated emission) but some use other principles, such as the Raman effect.

3.4.1 Erbium Doped Fiber Amplifier (EDFA)

Erbium doped fiber amplifiers are a specific case in the class of amplifiers known as rare earth doped fiber amplifiers (REDFAs). The most common dopants are erbium and

praseodymium but other rare earth dopants, such as neodymium, can be used. Erbium atoms are added to the glass in a section of fiber. The fiber can be as long as 10 m and an EDFA can have up to 30 dB of gain. The EDFA requires a pump laser and also a coupler to combine the signal and the pump light as shown in Figure 3-11. A coupler at the output side can be used with a filter and detector to determine the output power level to provide feedback to the pump laser to control the gain and thus the output power level. The final piece in an EDFA is an isolator to prevent reflections from coupling back into the EDFA.

Input Signal (1530 – 1570 nm)

Pump Laser (980 or 1480 nm)

Erbium Doped Section of Fiber

Feedback Power Control Filter and Detector

Output Signal Coupler

Coupler

Isolator

10 – 200 mW

Figure 3-11: Schematic Diagram of An EDFA. (40540)

EDFAs use the same principle of operation as lasers. Electrons from the ground state are excited (pumped) to higher energy states in the erbium dopants in the fiber as shown in Figure 3-12. The lasers previously discussed in this reference guide are all electrically pumped.

However, the EDFAs are optically pumped. So a light source, typically a laser, with a wavelength shorter than the signal to be amplified is used to excite electrons to higher energy

states decay to a metastable state without emitting photons. The electron energy given up in these transitions is transferred to the fiber in the form of phonons (lattice vibrations). From the metastable state, electrons either decay back to the ground state through spontaneous emission or through stimulated emission of photons. A population inversion (more electrons in the

metastable state than in the ground state) is needed so that the light from the signal causes stimulated emission rather than being absorbed and exciting electrons from the ground state to the metastable state. The light from the stimulated emission has the same phase and direction as the light causing the emission, so the signal is amplified.

Energy

Pump Laser Wavelength and Emission Wavelength, nm

Unstable States

514

Ground State

532 667 800 980 1480 1530

Phonon-Emitting Transitions (Non-Radiative)

Photon-Emitting Transition Metastable State

Figure 3-12: Energy States of An Erbium Doped Fiber. (40541)

Erbium doped amplifiers can also be fabricated in planar waveguide technology. These devices are referred to as erbium doped waveguide amplifiers (EDWAs). The majority of the literature on EDWAs is geared towards components for the metro and enterprise network

applications. These devices have optical power gain in the range of 10 to 30 dB and waveguides lengths of approximately 7.5 to10 cm. The devices integrate the entire amplifier, including the pump laser and isolators, on a single chip. The next level of integration is to combine the EDWA with other components such as array waveguide gratings (AWGs). We have not been able to find any further information on EDWA development.

3.4.2 Semiconductor Optical Amplifier (SOA)

Semiconductor optical amplifiers (SOAs) are essentially FP lasers without an optical cavity. This is achieved by using low reflectivity mirrors, or no mirrors at all, at the ends of the device. Typically they are edge emitting devices with end facets made from the bare cleaved edges of the chip or with anti-reflections coatings on the cleaved edges. The gain can be increased by increasing the length of the device or by using higher reflectivity mirrors so the light passes through the gain medium several times before exiting the device. They can also be made with vertical cavity structures but the gain medium is so thin there is very little gain in a single pass through the device. So vertical cavity SOAs (VCSOAs) are made with higher reflectivity mirrors. In SOAs with mirrors on the ends, care must be taken to prevent the device from lasing.

SOAs are generally 400 µm – 2 mm in length with gain up to 30 dB for the longer devices. However, their output power is relatively low (several mW) and they have higher noise than EDFAs. As individual components, their power is further reduced by losses in the fiber pigtails. The main advantage of SOAs over EDFAs is that they are more easily integrated with other components in planar waveguide technologies. Figure 3-9 shows an example of an SOA integrated with a DBR laser and an EAM. For DWDM, an AWG with arrays of SOAs, lasers and modulators can be integrated on a single chip and an SOA and AWG with an array of photodiodes (PDs) can be integrated on a second chip, as shown in Figure 3-13, to fabricate a transmitter and receiver.

Tunable DFB Laser

OPM SOA EAM SOA PD

AWG MUX

AWG DEMUX

High-Speed Electrical Output

High-Speed Electrical Input

Figure 3-13: Diagrams of Possible DWDM Transmitter and Receiver Chips With Arrays of Devices. (40542)

3.5 References

[3-1] Dutton, H. J. R.: Understanding Optical Communications, IBM Redbook 1998.

http://www.redbooks.ibm.com/redbooks/pdfs/sg245230.pdf

[3-2] Li, H., and K. Iga (Editors): Vertical-Cavity Surface-Emitting Laser Devices. Springer Verlag, Berlin, 2002.