Optical amplifier is the device which increases the strength of the optical signal. Typical fiber cable has loss of about 0.2dB per km for 1.5 micro-meter light signal. Hence if the signal occurs loss of about 20dB if it travels a distance of 100km. The signal needs to be amplified in order to compensate for the losses at the regular intervals and to maintain SNR (or BER). Usually optical amplification is carried out by converting signal from optical to electrical and later signal is converted back to optical form. Erbium Doped Fiber Amplifier (EDFA) has made it possible to amplify the signal in the optical form without being converted to the electrical form avoiding costly high speed electronic devices needed for such conversions at frequency of more than 10GHz.
An optical amplifier is usually characterized by parameters such as gain, gain efficiency, gain bandwidth, gain saturation etc. The same is described below.
• Gain = ratio of output power by input power. It is measured in Decibel(dB).
• Gain taken as function of the input power is referred as gain efficiency .
• Bandwidth refers to range of wavelengths over which amplifier functions effectively and provides maximum gain.
• Gain saturation refers to the max. output optical amplifier can deliver. Above this limit amplification is not possible.
Types of Optical Amplifier
The main types of amplifiers are EDFA(Earth Doped Fiber Amplifier),Semiconductor Optical Amplifiers (SOA) and Raman Amplifier. Rare Earth Doped Fiber Amplifiers are of two types EDFA (Erbium Doped) and PDFA (Praseodymium Doped). EDFA works in 1500-1600nm band. PDFA works in 1300nm band. SOA works in 400-2000nm band.
EDFA (Erbium Doped Fiber Amplifier)
EDFA has been commercially available for use since about 1990. It performs best in the band from 1530 nm to 1565 nm. It provides gain of up to 30dB. As shown in the figure-1, Erbium Doped Fiber is pumped using semiconductor laser either at 980 nm or at 1480 nm.
When the stimulated emission gets dominated over spontaneous emission, efficient amplification can be obtained. As we know that light gets absorbed when it propagates through the medium. If the population at higher state of energy is more than that at lower state, light will be amplified when it travels from one end of fiber amplifier to the other end. Usually 980 nm pump is preferred due to its low noise amplification characteristics over 1480nm pump. But at 1480 nm wavelength silica fiber incur low loss and hence pump can also propagate along with the input signal to be amplified. Pump can also be placed remotely.
EDFAs are available in L band and C band. In L band EDFAs pump powers are higher than the L band EDFAs. Gain equalizing filter is employed if required at the output of the EDFA to equalize the gain over the bandwidth of interest. Following are the typical specifications of the EDFA type of optical amplifier.
• Gain of about 40dB or greater
• Wavelength in the range from 1530 nm to 1560 nm
• Maximum saturation of about 22 dBm
• Noise figure in the order of 5dB
• Pump power of about about 25 dBm
• bandwidth (3dB) - 30 to 60
• Polarization sensitivity not available with EDFA
EDFAs will provide high pump utilization of power and is used along with optical filters to equalize the gain response. There are so many merits of EDFA but as they are not small, they can not be seamlessly integrated with other semiconductor devices in the chain.
SOA (semiconductor optical amplifier)
Photons travelling from active region can cause electrons to lose energy in the form of released photons and if wavelength of these photons matches the initial photons, amplification can be achieved. This can be achieved by passing electric current through the device. Due to this principal optical signal which is passing from active region will be amplified with gain of certain order.
• Gain of about 30dB or greater
• Wavelength in the range from 1280 nm to 1650 nm
• Maximum saturation of about 18 dBm
• Noise figure in the order of 8 dB
• Pump power of about less than 400 mA
• bandwidth (3dB) - 60
• Polarization sensitivity available with SOA
Figure-3 depicts the application of EDFA in CATV transmission and reception. As shown in the figure, CATV signal is being amplified by EDFA with the gain of 10dB. Later optical splitter is used to split the signal so that individual connections are provided to the users.