Attenuation (F)

Authors:

Transparency (Thai) -> Minh

Decibel (Binh) -> completed

Amplifiers / optical (Minh) -> completed

frequency (Thai) -> completed

depending variables (Phat) -> completed

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Transparency, decibels, amplifiers, frequency and other variable dependencies

Minh's part: Amplifiers / optical

Background

The requirement is understand amplifier definition in optical fiber domain.

Initial questions

What is amplifier?

What is the role of amplifier in optical fiber study?

Selected problem

All questions were selected to answer in detail.

My explanation

Amplification is required to solve the problem of degradation of signal transmitting in optical fibers.

Critical evaluation

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

Finding more information

Reading more in Fiber optics engineering of Mohammad Azadeh.

Final theory

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_pand 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.

References: Azadeh, M. (2009). Fiber optics engineering.

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Decibel

Background

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

Initial questions

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

Decibel is a universal unit to measure sound level, but it is also used in electronics, signals and communications.
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.
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

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:

Figure 2: How to measure fiber loss [2]

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

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
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.

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.

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

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
B.G. Potter. Module 3 - Attenuation in optical fibers
http://opti500.cian-erc.org/opti500/pdf/sm/Module3%20Optical%20Attenuation.pdf

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Frequency

Background

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

Initial questions

How is attenuation dependent on frequency?

Selected problem

All questions were selected to answer in detail.

My explanation

Attenuation has relation in increasing with frequency, which means the higher the frequency the higher the attenuation.

Critical evaluation

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

Finding more information

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

Final theory

Attenuation is a loss of intensity in an energy beam as it passes through a substance or object or the energy loss of signal transmission through a given medium. Coefficient is a quantitative measure of either an effect or a property. It is the ratio by which a change in one property will change another property. The attenuation coefficient is thus a ratio comparing the loss of intensity to the distance that the energy beam passes through the material. The units used to express the intensity will depend on the precise energy beam.

The attenuation coefficient is also used in ultrasound. When ultrasound waves propagate in a medium, energy is removed from the ultrasound waves by two main processes, absorption and scattering.The mechanism that removes energy from the ultrasound waves is called “attenuation”. Ultrasound is absorbed by the medium if part of the wave energy is converted into other forms of energy, such as heat. The absorption is frequency dependence. When ultrasound waves propagate, they not only become smaller in amplitude but they also change shape. Absorption in the body has a major effect on the penetration depth. It would limit the detectable penetration of the ultrasound waves in the body or the maximum depth at which tissues can be imaged. The attenuation of ultrasound in a material could be described by the attenuation coefficient in the units of decibels per centimetre per megahertz (dB/cm/MHz).

Attenuation always serves as a measurement parameter that leads to the formation of theories to explain physical or chemical phenomenon, which decreases the ultrasonic intensity. Attenuation is generally proportional to the square of sound frequency. Quoted values of attenuation are often given for a single frequency, or an attenuation value averaged over many frequencies may be given. The attenuation coefficient (α) can be used to determine total attenuation in dB in the medium using the following formula:

α: attenuation coefficient

ℓ: medium length

ƒ: frequency of the incident ultrasound beam

The attenuation coefficients of common biological materials at a frequency of 1 MHz are listed below:

Material	α(dB/(MHz·cm))
Air	1.64 (20°C)
Blood	0.2
Bone, cortical	6.9
Bone, trabecular	9.94
Brain	0.6
Breast	0.75
Cardiac	0.52
Connective tissue	1.57
Dentin	80
Enamel	120
Fat	0.48
Liver	0.5
Marrow	0.5
Muscle	1.09
Tendon	4.7
Soft tissue (average)	0.54
Water	0.0022

Figure 1. Diffuse reflection. Copied from [1]

Reference

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.
Tole, Nimrod M. (2005). Basic physics of ultrasonographic imaging.
Chapter 3: http://www.isradiology.org/isr/docs_books/basic/Chapter3.pdf
Michael L. Oelzea; William D. O’Brien, Jr. (2002). Frequency-dependent attenuation-compensation functions for ultrasonic signals backscattered from random media.

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Decibel

Frequency

Background