Why is laser light monochromatic?

[mohdfasieh]

Hello



Can any genius answer my question



Q:I know that LASER is monochromatic.But i want to know WHY?


Q:I also know that metastable state has greater life time .But i want to know why?

 

[Gonzolo]

1. Lasers are built around one specific transition = one pair of energy levels = one wavelength, thus the monochromaticity.

2. You can see a metastable state as a local minimum. A relativily large bandgap has to be crossed for a subsequent transition to the next or fundamental state. Surely, one wouldn't expect every state to have the same lifetime. Some are extraordinarily long.

Gonzolo, apparent genius-hood pending :zzz:

 

[Laser Jock]

1. Lasers are built around one specific transition = one pair of energy levels = one wavelength, thus the monochromaticity.

Actually, if you want to get technical, no laser is truly monochromatic. Some are closer than others and can be treated as such to a very good approximation, but the laser would have to be emitting light for all eternity to be considered purely mononchromatic.

Another factor in creating a narrow-line laser is cavity design. Many solid-state laser media (ie. Ti:Sapphire) can lase over 100 nm (very broad band). If you place a Fabry-Perot Etalon inside the laser cavity, you can get just one wavelength to lase and get a narrow-line laser.

2. You can see a metastable state as a local minimum. A relativily large bandgap has to be crossed for a subsequent transition to the next or fundamental state. Surely, one wouldn't expect every state to have the same lifetime. Some are extraordinarily long.

One cause of extremely long-lived states is that the transition is technically forbidden by quantum mechanics. A good example of this is the ruby laser transition. How can the transition happen at all? Thermal effects nudge the electrons to a lower state. As you cool the ruby, the life time increases.

Gonzolo, apparent genius-hood pending :zzz:

Hmm..., where do you apply for that sort of thing?

 

[Gonzolo]

Yup. Femtosecond pulses are very wide. I think your explanation for metastable states is an alternative to my bandgap wording (only one cause). I tend to be vulgar.

Usually, physicists don't call each other geniuses, so I didn't think it necessary to be too technical. The term "Genius" reminds me of Wile E. Coyote's business card in one of the classic cartoons :

"Wile E. Coyote, Genius"

Well, since I have a respectable corroboration now...

Gonzolo, Apparent Genius