Principles and Applications of Laser / Discussion

This is the discussion related to the wiki page Principles and Applications of Laser.

time scales

Gokhan Bakan 05 May 2009 14:17

Nice page,

I have read a few papers where they utilize excimer and YAG lasers, and one thing i have notice that these lasers are pulsed (~ns) instead of running them continuously (researchers were trying to melt silicon with them). My question is: are these small time scales limitation of the lasers or are they trying trying to avoid dumping too much power?

Thanks.

Re: time scales

Liang 05 May 2009 15:17

Dear Gokhan,

Power of pulses of several ns is very huge. But the total energy can still be very small since the pulse is really short. So these lasers would deposit the energy for a period of time and them emit it. So the input power is not large.

Ns is really a new time scale for ultrafast lasers. Previously, people can only achieve ps pulses. But I think people are still working on shorter pulses, because of the need for a strong output power in research and national defense. Maybe it is the limitation now. But not in the future.

Re: time scales

Liang 05 May 2009 17:55

Another point is the uncertainty principle says $\Delta E\Delta t\geqslant\hbar/2$. If the pulse is too short, then the time difference between the beginning and the end of the pulse is too small, then the energy difference would be very large. Or the spectrum of the laser would be wide, which is not good for a laser. So there should be a compromise.

Fangxiao 05 May 2009 17:16

The population inversion phenomena is very interesting! Does this increase the laser emitting energy? And is the gain medium an requirement of the population inversion? Can you explain in more detail about how to form the gain medium? Thank you!

Re:

Liang 05 May 2009 17:47

Hi, Fangxiao. Let me explain in this way. Assume that in the gain medium, there are more electrons in the low energy state, and this is true most of the time. Then we provide some illumination on the medium. Although stimulated emission will give out photons, more photons are absorbed by the lower energy electrons. So the total light intensity is decreased. That does not make sense because we need a large amplify of the incident light to get a laser. In other words, it is mandatory for a laser to achieve population inversion.

Gain medium is that the material which has a suitable energy level scheme for population inversion. Laser gets amplified within the material. So we call it "gain". So if any new material with such a scheme, and the upper level lifetime for electrons is comparable to the lower level lifetime, it may be suitable for laser. And usually, in semiconductor lasers, people just use several layers of different depositions to form the gain layer and resonant cavity layer.