Exploring Fibre Optic Bandwidth & Single Mode

[ajayguhan]

It's given, "fibre optics are thin to accommodate single electromagnetic mode" what do they mean by single electromagnetic mode...?


And also mentioned it has high bandwidth, i know that bandwidth is difference of highest and lowest frequencies of the signal.

So if fibre optics has high bandwidth, shouldn't it accommodate electromagnetic waves of different frequencies rather than single electromagnetic wave...? And why did they gave as single electromagnetic mode...?

 

[Crazymechanic]

But it has electromagnetic waves of different frequencies , how else could it transmit information.
Ofcourse the frequencies are limited , like you can't send gamma or microwave or x ray down a fiber optic but the ones you can send do have a range from and to.
How long that range is I'm sure some other people will comment.

 

[CWatters]

Single mode does not mean only a single data frequency.

Single mode essentially means the diameter of the fibre is so small that light can only take one path straight down the fibre instead of reflecting around taking multiple paths. Multi mode fiber is largen in diameter and has a wider acceptance angle making it easier to work with (easier to join etc).

Normally only a single wavelength of light is used but that doesn't mean the data sent can't have a wide bandwidth. You can turn that single frequency of light on an off very fast.

 

[CWatters]

More here..
http://www.ad-net.com.tw/index.php?id=46

 

[ajayguhan]

So here mode is like the number of paths that are allowable to a electromangnetic wave..and bandwidth is high since we transmit high data by switching on and off the light very fast.

 

[Bobbywhy]

We can also send several different wavelengths (colors) through a single mode fiber at the same time. That alone multiplies the bandwidth. (the capacity to carry information).

 

[ajayguhan]

If we send lights of different wavelength together in a same path won't they interfere or overlap ...?

 

[Bobbywhy]

"In fiber-optic communications, wavelength-division multiplexing (WDM) is a technology which multiplexes a number of optical carrier signals onto a single optical fiber by using different wavelengths (i.e. colors) of laser light. This technique enables bidirectional communications over one strand of fiber, as well as multiplication of capacity."
http://en.wikipedia.org/wiki/Wavelength-division_multiplexing

 

[Andy Resnick]

There seems to be lingering confusion/obscuration between spatial modes and temporal frequencies. Optical fiber, like any waveguide, can propagate only certain discrete spatial modes of the electromagnetic field, and whether a fiber is 'single mode', multimode', or for birefringent fiber 'polarization preserving' depends on the diameter of the core and refractive indices of core and cladding. For some reason, introductory texts typically use the ray optics model to describe this, but the wave optics model is much more appropriate.

As for bit rate and/or bandwidth, this depends on the fiber dispersion- how the refractive index varies with wavelength. There are numerous techniques to maximize data bandwidth but is essential that different data channels do not interfere. In linear optics, this is trivially satisfied by ensuring the different channels are mutually incoherent (for example, by using different laser sources for the different channels). Nonlinear fibers (photonic bandgap fibers/holey fibers, etc.) actually *require* interference between various frequencies in order to achieve proper function- for example, supercontinuum output.

 

[ajayguhan]

There seems to be lingering confusion/obscuration between spatial modes and temporal frequencies. Optical fiber, like any waveguide, can propagate only certain discrete spatial modes of the electromagnetic field

What is spatial mode..?