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| 1. Optical fiber structure |
Optical fiber is information-carrying medium made with silica-based glass.
It consists of two regions : the core and the cladding. The center part of the fiber is called "core" with refractive index N1 and the part which surrounds the core with lower refractive index N2 is called "cladding".
When the light is launched into the fiber, the cladding confines the lights into the fiber core and the light travels down the fiber by internal reflection between the boundaries between the core and the cladding.(Figure A)
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| Figure A |
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| 2. Fiber Type |
There are the mainstream fibers manufactured and marketted today : SM(singlemode) and MM(multimode).
| fiber type |
MM |
SM |
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| fiber size |
50/125μm
62.5/125ƒÊm, 100/140μm |
9/125μm
10/125μm |
| type |
Multimode Step-index fiber (SI)
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Multimode Graded-index fiber (GI)
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| Application |
Short Distance
LAN |
Long Distance
Telecoms,CATV,
Brioadcast, Data communication |
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Figure B
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| 3. Wavelength |
The light injected into the fiber is a very small portion, which falls into visible wavelength from 400nm to 700nm and the near infrared wavelength from 700nm to 1700nm in the electromagnetic spectrum shown in the figureC. In these regions, there are four special wavelengths to be used for fiber optics transmission for the low loss characteristics as follows.(Figure D)
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| Figure C |
| windows |
wavelength |
loss |
| 1 st wavelength |
850nm |
3dB/km |
| 2nd wavelength |
1310nm |
0.5dB/km |
| 3rd wavelength |
1550nm (C band) |
0.2dB/km |
| 4th wavelength |
1625nm (L band) |
0.2dB/km |
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Figure D
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| 4. Optical Power |
To measure optical loss, two units such as dBm and dB are used. dBm is the actual power level referred to1 milliwat and dB(decibel) is the difference between the powers.
Please see the figure E.
Figure F
When the optical input power is P1(dBm) and the optical output power is P2(dBm), the power loss will be "P1-P2"dB.
As you see this dB value in the power conversion table, you will find how much power is lost between input and output.
(See the figure F)
For example, when LD optical input into the fiber is 0dBm and output power is -15dBm, optical loss for the fiber will be calculated as follows.
| input |
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output |
|
optical loss |
| 0dBm |
- |
(-15dBm) |
= |
15dB |
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In the power conversion table, 15dB for optical loss equals 96.8% of lost optical power.
Therefore, only 3.2% of optical power remains by travelling through the fiber.
Equation for calculation of dB(decibel).
dB = P1(dBm)-P2(dBm) or
dB = 10Å~log (P1(watt))/(P2(watt))
Equation for calculation of convertion for mW/dBm
dBm=10Å~log(mW)
mW=10(dBm/10)
| dB |
Power Out as a % of Power In |
% of Power lost |
Remarks |
| 1 |
79% |
21% |
- |
| 2 |
63% |
37% |
- |
| 3 |
50% |
50% |
1/2 the power |
| 4 |
40% |
60% |
- |
| 5 |
32% |
68% |
- |
| 6 |
25% |
75% |
1/4 the power |
| 7 |
20% |
80% |
1/5 the power |
| 8 |
16% |
84% |
1/6 the power |
| 9 |
12% |
88% |
1/8 the power |
| 10 |
10% |
90% |
1/10 the power |
| 11 |
8.0% |
92% |
1/12 the power |
| 12 |
6.3% |
93.7% |
1/16 the power |
| 13 |
5.0% |
95% |
1/20 the power |
| 14 |
4.0% |
96.0% |
1/25 the power |
| 15 |
3.2% |
96.8% |
1/30 the power |
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Figure F
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| 5. Light Sources |
There are two types of light sources : Light Emitting Diode(LED) and Laser Diode(LD).
Please see the table .(Figure G)
LED has the broad emission angle with low output power, it is difficult to launch the total light output power into the fiber core while LD has a narrow emission angle with high output power, therefore, it can be easily focused into the fiber core and has the advantage for long distance application.
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LED |
LD |
| Spectral Width |
ex.
Peak wavelength 850nm
FWHM=Full Width of Half Maximum |
ex.
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| Emission Angle |
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| Output Power |
Low
Ex.-25dBm (for 850nm) |
High
Ex.-4dBm (for 1310nm) |
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Figure G
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Copyright 2006, Hakuto Co.,LTD. All rights reserved.
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