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Decibel

Background

The requirement is to understand what is decibel and what it is used for

Initial questions

1. What is decibel?

    2. How to measure it?

    3. How to use it in optical fiber ?

Selected problem

All questions were selected to answer in detail.

My explanation

1. Decibel is a universal unit to measure sound level, but it is also used in electronics, signals and communications.
2. On the decibel scale, the smallest audible sound (near total silence) is 0 dB. A sound 10 times more powerful is 10 dB. A sound 100 times more powerful than near total silence is 20 dB.
3. In my opinion, decibel is used to measure signal loss in transmission in optical fiber.

Critical evaluation

More details and reading as well as reasoning that need to be provided.

Looking for more info about Decibel and also how to use it in fiber optic

Final theory:

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.

Figure 4 – How to Measure Optical Power

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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]
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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