# Laser pulse: spectral width vs time

Hi guys,

I am learning for an optics examination and found a picture that is supposed to show the duration of a light pulse with respect to the spectral bandwidth (appended).

At this point in the lecture, we have not gone through modelocking yet, so I assume one should understand the picture without it.
Why does the sun's light not consist of laser pulses when it covers such a large spectrum?

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Simon Bridge
Homework Helper
Why does the sun's light not consist of laser pulses when it covers such a large spectrum?

You are asking why the Sun's light does not consist of laser pulses? Um ... because it is not made by lasers?
Maybe you are asking why the Suns lihght is not well modelled as a sum over carefully selected laser pulses?
What makes you think it cannot?

What does the width of the spectrum have to do with whether or not the Sun's light consists of laser pulses?

Perhaps you mean "consistent"... ie how come the suns light has the duration is does considering the bandwidth?
Could that have something to do with not being a pulse? For instance, what is the duration of a continuous laser beam?
I have a feeling this may be more what you are asking about... but correct me if I am wrong.

Your attached picture illustrates how this happens: see how none of the samples waves in sunlight (or other continuous examples) ever "line up" completely like with the laser example?

Maybe you are asking how it is that the waves never line up ... well that is how geometry works: it is possible to arrange the output of a light source so that the waves periodically have the same phase at some stage and not at others. This makes a pulse. However, this is generally not the case ... normally you make a pulse by switching the source on then off again.

Usually a randomly selection of more than 3 or 4 (iirc) wavelengths all at random initial phases and amplitudes will rarely have a situation where they all match tidily. What you get instead is an irratic assortment of peaks and troughs as some waves match up and then not.
This is ignoring the sun not being a point source.

I am being a bit pedantic above because you are advanced enough to start to think about how you ask questions.
Most of physics is about forming the right question... it's something of an artform though.

Did I manage to cover your concern?

SchroedingersLion
Why does the sun's light not consist of laser pulses when it covers such a large spectrum?

You are asking why the Sun's light does not consist of laser pulses? Um ... because it is not made by lasers?
Maybe you are asking why the Suns lihght is not well modelled as a sum over carefully selected laser pulses?
What makes you think it cannot?

What does the width of the spectrum have to do with whether or not the Sun's light consists of laser pulses?

Perhaps you mean "consistent"... ie how come the suns light has the duration is does considering the bandwidth?
Could that have something to do with not being a pulse? For instance, what is the duration of a continuous laser beam?
I have a feeling this may be more what you are asking about... but correct me if I am wrong.

Your attached picture illustrates how this happens: see how none of the samples waves in sunlight (or other continuous examples) ever "line up" completely like with the laser example?

Maybe you are asking how it is that the waves never line up ... well that is how geometry works: it is possible to arrange the output of a light source so that the waves periodically have the same phase at some stage and not at others. This makes a pulse. However, this is generally not the case ... normally you make a pulse by switching the source on then off again.

Usually a randomly selection of more than 3 or 4 (iirc) wavelengths all at random initial phases and amplitudes will rarely have a situation where they all match tidily. What you get instead is an irratic assortment of peaks and troughs as some waves match up and then not.
This is ignoring the sun not being a point source.

I am being a bit pedantic above because you are advanced enough to start to think about how you ask questions.
Most of physics is about forming the right question... it's something of an artform though.

Did I manage to cover your concern?

The bold part is exactly my question. You are right about my question being too vague, I apologize, I am just a bit in a hurry at the moment.
So if I have a large number of frequencies, I have to experimentally make sure that they all interfere constructively at some point (and thus periodically). However, with the sun, this is not a given. And only if all the waves of different frequencies interfere constructively, I will get a pulse whose duration decreases with increasing bandwidth?

vanhees71
Gold Member
Well to a good approximation the sun is radiating black-body (i.e., thermal) radiation with the spectrum given by the Planck spectrum. It's as incoherent as anything can get!

Cthugha