Hi all!(adsbygoogle = window.adsbygoogle || []).push({});

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 onEwhich 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

**Physics Forums - The Fusion of Science and Community**

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

# I Laser vs photodiodes and spectra

Have something to add?

Draft saved
Draft deleted

Loading...

Similar Threads - Laser photodiodes spectra | Date |
---|---|

I How to calculate the photons/pulse of Laser diode? | Jan 22, 2018 |

I Light amplification with open-ended laser tube | Oct 17, 2017 |

I Quantum Cascade Lasers | Sep 5, 2017 |

B Anti-atom spectra confirmed? | Jan 15, 2017 |

**Physics Forums - The Fusion of Science and Community**