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Frank-95

- 52

- 1

I would have two questions, related to laser and photodiodes spectra.

1) We know that lasers produce a very monochromatic radiation, even if they are low to moderatly expensive. That is because the emitted frequency light is dependend on

**E**which is the bandgap. So electrons are stimulated to "jump" from the higher level to the lower level, emitting a photon. Photodiodes works similarly; they absorb a photon and an electron jump from the lower level to the higher one.

_{c}- E_{v}= E_{g}The question is, if the bandgap is the same, why emitting spectrum of laser is much narrower than absorption spectrum of photodiodes?

2) Now suppose that you have a perfect monochromatic laser. They emit only at a certain frequency, so the light is a pure sine in time domain. The Fourier Transform is than a Dirac Delta (actually two deltas), and it's right because it emits only at one frequency.

Now suppose you don't turn the light on permanently but you emit a single short pulse. In the time graph we could model the function in many ways like a sine times a gaussian, or a sine times a raised-cosine. If we make the Fourier transform of the signal we would obtain not a delta but some broader curve. And this is mathematically right.

But what happens physically? If the laser is perfect and emits at only one wavelength, if I pulse it, it should still emits at that frequency, isn't it? But the spectrum mathematically broadens, so I cannot figure out what happens

Thank you