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Amplifiers / optical (Minh) -> completed

frequency (PhatThai) -> Thai ?

depending variables (Phat) -> Thai ?

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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.

 

Finding more information

 

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

 

db_290008.gif

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:

4.jpg

Figure 2: How to measure fiber loss [2]
2(1).jpg
 

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
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Other Variable Dependencies

Background

 

The requirement is to understand what factors and dependencies cause attenuation in fiber optic.

 

Initial questions

 

How does attenuation happen?

 

What kind of factors cause the attenuation ?

 

Selected problem

 

All questions were selected to answer in detail.

 

My explanation

 

Attenuation affects the quality of the signal transmission. Attenuation can be caused by long transmission line and the signal loses its energy while transmitting.

 

Critical evaluation

 

My explanation is not detailed enough and more investigations need to be done to understand more deeply.

 

Finding more information

 

More information is obtained by reading books and searching for online information.

 

Final theory

In fiber optic, attenuation is the loss of signal energy or intensity when signal is transmitted in long distance. There are many factors that cause attenuation. In general, attenuation is caused by the medium components such as, cables, connectors. Below are factors that degrade the signal strength in the fiber.

The first phenomenon is optical absorption. When light travel through the optical fiber, photons can be observed by the material structure which result in the higher energy state of the material. Because of photon absorption, light loses its intensity and hence signal is degraded. The travel of light can be described in the following formula:
         n* = n(ω) + iκ(ω)
n*: complex refractive index
n(ω): real portion of the refractive index
κ(ω): extinction coefficient
So, the material structure has a effect on the signal strength through optical absorption.

The next factor is light scattering. If the surface of the material is rough and uneven, propagation of light in the fiber can be reflected in random direction. This kind of reflection is also called as diffuse reflection.

Image Added
Figure 1. Diffuse reflection. Copied from [1]

As we can see, the blue lights hit the surface of the core. If the surface is rough, the reflected red lights will go in random directions following the low of reflection. This results in the loss of the light power.

Next one is connection loss. It is important to align two fibers correctly because it will minimize the lateral offset of the core, tilt, angular mismatch... Fiber misalignment can have large impact on the signal loss because the light is not reflected correctly.

Image Added
Figure 2. Two spliced fibers. Copied from [2]

The above image shows the misalignment between two fibers that affect the fiber coupling efficiency. Moreover, air between fiber connections may exist and has impact on the medium. Therefor, air should be minimized as much as possible to produce an optimal medium for propagation of light.

Reference

  1. Reflection and the Ray Model of Light - Lesson 1 - Reflection and its Importance
    http://www.physicsclassroom.com/class/refln/Lesson-1/Specular-vs-Diffuse-Reflection
  2. B.G. Potter. Module 3 - Attenuation in optical fibers
    http://opti500.cian-erc.org/opti500/pdf/sm/Module3%20Optical%20Attenuation.pdf