modes in an optical fiber??

[karthy]

hi,
what does modes represent in an optical fiber? does that correspond to the orientation of electric and magnetic fields or to the different rays that propagate due to different angle of incidence?
-karthy

 

[golfbumb]

Both. And more.

Multimode fiber optic cables have concentric cylinders of glass with successively lower indexes of refraction from center to outside. There are some plastic cables that operate in a multimode, as well. They can use led transmitters which are much cheaper than the laser transmitters/receivers required of single mode fibers.

Intentional multimode fiber means that the light has multiple paths due to the broader spread of the transmitter (usually LEDs). Wave propogation is a function of frequency. Single mode fibers are laser-based and therefore transmit on a singular wavelength. Through quantum effects, all of the light travels a single path. But if a non-coherent light source is used on a single mode fiber, multiple modes would result from the different paths taken by different frequencies.

Unintentional multimodes can exist as a result of the internal reflections against the glass to sheath transition, but in a high quality single frequency matched fiber, these are minimized. In a low quality fiber, these are expected.

Polarization modes do exist as a result of waves crossing and superimposing each other. Constructive and destructive interference results.

 

[baluba]

Mathematically speaking, modes (in optical fiber or waveguide) are eigensolutions of the wave equation, and they have a certain polarization (in the general case they are hybrid modes). In a fiber, you cannot really describe a mode in a 'ray-picture', things are more complicated because you have to use cylindrical coordinates, there are no planar interfaces. The number of modes you get in a fiber depends on the geometrical dimension of the core. In practice, when you couple in the light from an external source, it can go into any of the available modes.

Hope this explanation helps.

 

[swilly]

Anybody knows how can I use fiber optic to sense temperature and pressure?