multimode fiber and light source

[sarahr202]

Hi everybody.

Multimode fiber:

We use LED as a light source for multimode fiber.

Because when i look at say red led producing red light,which means it  is producing a wavelength 650 nm.but yet i find a few  link saying a LED produce a number of wavelengths

The LED light sources sometimes used with multi-mode fiber produce a range of wavelengths

I appreciate your help.

have a great weekend.

[Reza Sharifi · Accepted solution]

Hi Sarah,

Usually for multimode the wavelengths are at 850 and 1300 nm and for single mode the wavelengths are at 1310 and 1550.

Here is doc that explains the wavelength types more:

http://www.thefoa.org/tech/wavelength.htm

HTH

Reza

[Giuseppe Larosa · Accepted solution]

Hello Sarah,

modes and wavelengths are two different concepts.

The modes are the solutions of the Maxwell's equations for the fiber as a waveguide they are a function of the geometry of the fiber ( core size in micrometers) and of refraction index radial distribution (depending on material).

The wavelength is the inverse of the frequency of the electromagnetic wave.

Given a geometry of fiber it is single mode or multi mode depending on the ratio between fiber core diameter and the signal wavelength.

A LED  diode transmits a signal that has a large spectral width when compared with that of a laser diode, but this does not mean it is transmitting in multiple wavelengths.

The spectral characteristics can be seen with an optical sprectrum analyzer. (that is quite an expensive instrument by the way).

A laser provides so called coherent light beacuse it has a very narrow spectral width.

The spectral width is important because the electrical modulation of the optical transmitter (the bit stream)  has the effects to increase the spectral width roughly it is a product of original spectral width and of the bandwith of the modulating signal.

In simple words the wavelength is the central frequency of the optical signal and lasers emit a much more clean signal then leds.

Hope to help

Giuseppe

[sarahr202]

Hi Giuseppe.

Just want to confirm:

Modes are the possible paths a ray can travel inside a fiber. So if we have a emitter emitting a particular wavelength say lamda1,  this wavelength or signal is launched into a fiber in the form of rays which will take specific paths ( as  we have specific angles at which signal can be launched inside a fiber, for which these rays or modes will continuously go through total internal reflection.

This is what I understand.

spectral width of a light source.

http://www.fiber-optics.info/articles/light-emitting_diode_led

The following is a quote from the above link:

Ideally, all the light emitted from an LED would be at the peak  wavelength, but in practice the light is emitted in a range of  wavelengths centered at the peak wavelength. This range is called the  spectral width of the source

Say we have a red LED , red LED will emit a wavelength of 650 nm, lets name it lamda2.

But this is not the only wavelenth the LED will emit, there will be others wavelength. Let say we have lambda1 and lamda3 at -3dbm of max output  .Our central wavelength is lamda2 ( 650nm).

our spectral width=  lamda3 --- lamda1

Am I correct?

When we generate a signal using signal generator , we always end up with " Harmonics" as a side effect.

Is it not like spectral width because we want out red LED to emits only one particular wavelength i.e 650 nm  but we end up with spectral width because LED will also generate some other wavelengths

Thanks for your help and detailed logical answer you always provide.

[Giuseppe Larosa · Accepted solution]

Hello Sarah,

your understanding of spectral width is correct. From a practical point of view we refer to the central wavelength and we say there is a spectral width around it ( this is what we could see with an optical sprectrum analyzer)

Using optics terms yes the modes are the ways the signal can propagate into the fiber.

Just a little add: on the multi mode fiber  LED excites multiple modes on the fiber and the optical power is distributed over these multiple modes at the end of the fiber path a lens focuses all the beams over the photodiode receiver combining the power contributes of each mode.

Clearly the various modes travel with a different effective speed in the fiber so there is a form of interleaving (different delay between modes) between the various beam components if we think of a digital modulation.

This is the reason why single mode fiber can be used to send higher speed modulating signals as this recombination does not happen at photodiode receiver.

>>When we generate a signal using signal generator , we always end up with " Harmonics" as a side effect.

>>Is it not like spectral width because we want out red LED to emits only one particular wavelength i.e 650 nm  but we end up with spectral width because LED will also generate some other wavelength

Harmonics generation in an electrical signal generator are caused by non linear behaviour of active components in the generator circuit.

The spectral width is different, we could see it as a form of noise an unwanted modulation of a tone (the ideal source) otherwise ideal.

Hope to help

Giuseppe

[sarahr202]

Hi Giuseppe.

Just a little add: on the multi mode fiber  LED excites multiple modes  on the fiber and the optical power is distributed over these multiple  modes at the end of the fiber path a lens focuses all the beams over the  photodiode receiver combining the power contributes of each mode

So at transmitter end the power s distributed over modes. These modes represent different path rays can take as a result some rays at the receiver arrive sooner than others. This is what causes " Pulse broadening"

This is the reason why single mode fiber can be used to send higher  speed modulating signals as this recombination does not happen at  photodiode receiver.

In case of a single mode fiber, we have NA so small that allows only one mode. But question is how do we receive this signal if not using " photo diode receiver? In other words what kind of receiver is used at receiver end to recover the electrical signal from the light?

Have a great day.

[Joseph W. Doherty · Accepted solution]

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Posting

The LED light sources sometimes used with multi-mode fiber produce a range of wavelengths

Simply, LEDs don't generate just one wavelength like a laser, but colored LEDs emit a narrow range of wavelengths.

See the "red" frequency distribution in: http://upload.wikimedia.org/wikipedia/commons/f/f4/Red-YellowGreen-Blue_LED_spectra.png