Optical link has limit in range as there is always attenuation of propagating light in a normal fiber. One method to solve the problem is to detect the optical signal before the critical low value and convert it back to electrical domain and create new copy of optical signal. Another method is using optical amplifier. Without need of converting back optical signal to electrical domain, an amplifier can be used in different applications. It can be use as an inline amplifiers to compensate the attenuation of optical signals as well as pre-amplifiers in optical detectors to improve sensitivity of the receiver or as power boosters to raise output power of an optical transmitter.[1,149]

 There are two categories of amplifiers: semiconductor optical amplifiers (SOAs) and fiber amplifiers.  Optical gain in semiconductors is based on forward-biased junction.  Fiber amplifier contains optical fiber which is doped with a rare earth element such as neodymium (Nd) and praseodymium (pr) etc… The existence of dopants creates new energy bands within the fiber.

                                                  Figure1: Band diagram of a doped fiber amplifier. Copied from Azadeh, M. (2009). Fiber optics engineering.[1,150]

Figure 1 illustrates the dopants ions interact with the pump signal with frequency f_p and goes to higher energy state E_p . The ions have short lifetime ( T_p ) and quickly move to lower energy band, E_t , in which they have longer lifetime ( T_l ).  Because T_l >> T_p , population inversion is created. Inverted system interacts with main signal of frequency f , results in amplification of signal as stimulated emission happens.  

The erbium doped fiber amplifier (EDFA) is the most popular type as E_l – E_o exists in the wavelength of 1550 nm. In consequence, EDFA can amplify signal in lowest range of attenuation of silica fibers, results in the popularity of using in long distance communication. Typical EDFA amplifiers can supply gain in 30nm band (1530-1560 nm) with gain of 20-30 dB.

                                    Figure 2: Sample EDFA configuration. Copied from Azadeh, M. (2009). Fiber optics engineering.[1,151]

Figure 2 shows one example of co-directional pumping as the main and pump signals travel in same direction.  EDFAs have very important role in long-range optical fiber communication, and subjected to high interest of researching for increase of bandwidth and performance. 

The decibel (dB) is used to measure sound level but it is also used widely in communications, electronics and signals. In communications, the decibel is a logarithm way of describing a ratio between two signal power, such as power, sound pressure, voltage, or current levels The decibel is a common measurement used in the field of electronics to determine loss or gain in a system. 

Suppose we have 2 signals, signal 1 has a power of P1 watts, signal 2 has a power of P2 Watts, then the difference in decibels between 2 signals is defined to be:

                                       10log(P2/P1)dB   where the log is base 10

In order to measure optical loss, you can use two units, namely, dBm and dB. While dBm is the actual power level represented in milliwatts, dB (decibel) is the difference between the powers.

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Figure 1: How to measure optical power [1]

Light loss, L(dB), is a commonly used specification for fiber optic attenuation. For example, to determine the light loss of an optical fiber in a cable, a light source is connected to one end of the fiber cable (input). The light output power of the source is known to be 0.1 mW. When an optical power meter is connected to the opposite end of the fiber optic cable under test (output), the meter measures 0.05 mW. Using the decibel power loss formula, the optical fiber loss can be calculated as follows:

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Figure 2: How to measure fiber loss [2]
^{_{Image Modified}}
 

The light power loss of this optical fiber is 3 dB 

The dB unit is a logarithmic ratio of input and output levels and is therefore not absolute (i.e., has no units). An absolute measure of power in decibels can be made in the dBm form. The dBm unit is a logarithmic ratio of the measured power to 1 mW ^{_{of reference power}}

Reference

1. Introduction to optical fibers, dB, Attenuation and measurement: 
   http://www.cisco.com/c/en/us/support/docs/optical/synchronous-digital-hierarchy-sdh/29000-db-29000.html
2. Optical Power loss measurement in db- how to measure it fast and correct 
   http://www.ad-net.com.tw/?id=474

References

1. Culjat, Martin O.; Goldenberg, David; Tewari, Priyamvada; Singh, Rahul S. (2010). "A Review of Tissue Substitutes for Ultrasound Imaging". Ultrasound in Medicine & Biology 36 (6): 861–873.
2. Tole, Nimrod M. (2005). Basic physics of ultrasonographic imaging. 
   Chapter 3: http://www.isradiology.org/isr/docs_books/basic/Chapter3.pdf
3. Michael L. Oelzea; William D. O’Brien, Jr. (2002). Frequency-dependent attenuation-compensation functions for ultrasonic signals backscattered from random media.