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- Thread starter semc
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I would therefore like to ask what happens if I pass my monochromatic light through a optical chopper. Is Fourier transform not applicable in this case since ultra short pulse generation does not require many frequencies?

Ultra short pulses DO require many frequencies. Fourier transform theory applies

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marcusl

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Ultra short pulses DO require many frequencies. Fourier transform theory applies

I am aware that you need many frequencies to get short pulse. That's what Fourier transform tells us. In my example, I specifically said I only have single wavelength as source.

A single frequency like a pure sine wave is only a single frequency if it starts at the beginning of time and goes on forever.

So if I turn on and off my laser I introduce higher harmonics as well?

Ain't this just the math? I am confused how higher frequencies are generated.

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marcusl

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Don't really understand how this comes about. Is there any reference to explain in detail about this?

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I am confused how higher frequencies are generated.

That's an application of the uncertainty relation. When you chop down the beam you are making a measurement of the photon location. By the uncertainty principle the photon momentum becomes uncertain creating a spread on the wavelength. In other words, the uncertainty principle is the physical principle that expresses the spread in wavelength required by the mathematics of waves which can be studied and understood through Fourier transforms.

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marcusl

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"Quote by dauto

Ultra short pulses DO require many frequencies. Fourier transform theory applies

I am aware that you need many frequencies to get short pulse. That's what Fourier transform tells us. In my example, I specifically said I only have single wavelength as source."

Once you modulate the carrier (pure sine) with an envelope, you no longer have a pure sine of single wavelength. You have a time-varying envelope that may have a rich spectrum, as I indicated earlier.

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Fourier decomposition of a (periodic) signal will give you a long string of harmonics along with their amplitudes that when added together will give you your original signal back. If the wave is square then you will only get odd harmonics. In practice you may well get low amplitude even harmonics as well.

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You might start here http://en.wikipedia.org/wiki/Gibbs_phenomenonDon't really understand how this comes about. Is there any reference to explain in detail about this?

A square wave has an infinite spread of frequencies. Gibbs ringing is what happens when you try to approximate a square wave with a finite spread of frequencies.

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Claude Bile

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Claude.

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