Glossary
J 7 DEFINITIONS TO UNDERSTAND EVERYTHING ON LIGHT
“dB”definition : Expression of an optical power in log base. The 3dB Rule states that for every added 3dB of attenuation,one looses 50% of the power.
dB
Transmitted light (%)
0.01
99.8 97.7 79.4 63.1
0.1
1 2 3
50
Insertion loss : Power loss created by an additional element on a link. Under log base,it is the ratio between the output power Pout and the input Pin. The lower the number is, the better the insertion loss (generally measured in dB) Return loss (single mode applications) : The return loss is the ratio of the reflected optical power Pback to the incident power Pin in dB unit. Hence,it is the back reflection power from a device on a link. The lower the negative value in dB is, the better (ie –50dB is better than –30dB) Wavelength : Considering monochromatic light as a wave,it is the distance between two peaks of signal (ie 850 and 1300nm for multimode applications; 1300 and 1550nm for single mode ones)
P in
P out
n2
n1
n2
P back
n1
P in
Wavelength
5
Intensity
merged pulses
Bandwidth : Difference between the high and low frequency of a transmission band. The higher the number, the better.
MHz
0
900
Mode : The light can be described as a wave going upon a specific path. This path is called a mode. In a multimode fiber,the light goes upon several paths in the fiber core. A single mode fiber allows the light to go only upon one way.
n2
Mode 1
n1
Mode 2
Filter type
Cross section
Index
Ray propagation
Fiber types : Waveguides are classified,on one hand by the core material index profile and on the other hand by the mode propagation ability. Single mode fibers have a constant index profile on the core cross section. Most multimode fibers are gradient index fibers. As opposed to traveling in a straight line in single mode fibers,the rays travel in a spiral form around the optical axis for the multimode ones.
n(r)
Single mode Step Index
Multi mode Gradient Index
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