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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)

Figure A


2. Fiber Type
There are the mainstream fibers manufactured and marketted today : SM(singlemode) and MM(multimode).

fiber type MM SM
fob02_1 fob02_2
fiber size 50/125μm
62.5/125ƒÊm, 100/140μm
9/125μm
10/125μm
type Multimode Step-index fiber (SI)
fob02_3
fob02_4
Multimode Graded-index fiber (GI)
fob02_5
Application Short Distance
LAN
Long Distance
Telecoms,CATV,
Brioadcast, Data communication
Figure B


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)

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
Figure D


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   output   optical loss
0dBm - (-15dBm) = 15dB


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
Figure F


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.

  LED LD
Spectral Width ex.
Peak wavelength 850nm

FWHM=Full Width of Half Maximum
ex.
Emission Angle
Output Power Low
Ex.-25dBm (for 850nm)
High
Ex.-4dBm (for 1310nm)
Figure G



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