Explaining Why Lasers are Easy to See on Projector Screens

[doctortim]

Q) Why is it easy to see the output of a laser pointer on a screen illuminated by an overhead projector. Assume 1mW laser and 500W quartz halogen bulb. Explain quantitatively, and state all other assumptions.


Im not sure whether it involves the coherence length/time of the two waves, visibility or some variation of the two. I am really desperate for help on this one.

Im going to ask a double question (still lasers though) since this is my first post

Q) second things is simply what controls whether a energy level scheme is suitable or unsuitable for CW operation

I believe it depends on the difference between the two decy rates, or is that slightly wrong? or totally?

 

[Tide]

HINT: Compare the intensity of the "images" on the screen from each of the sources.

 

[Physics Monkey]

In the simplest 3 level laser system you have a ground state | 0 \rangle, and two excited states | 1 \rangle and | 2 \rangle. The ground state | 0 \rangle is pumped to the upper excited state | 2 \rangle by the driving field. The simplest useful systems have a very small rate of spontaneous decay from the upper excited state to the ground state, while the rate of spontaneous decay from the first excited state to the ground state is usually very large, much bigger than all the other rates in the problem. You can easily see that this leads to a nearly empty |1 \rangle state as compared with the | 2 \rangle state, thus achieving the needed population inversion on the lasing transition.

Just as a note, you can imagine other three level laser schemes which are slightly rearranged, but nothing is essentially different.

 

[doctortim]

Thanks to both of you, physics monkey I am going to assume that t2-1=20ns and t1-0=1000ns would give a large enough inversion for CW operation

May I also extend my question a bit more...If the resonator of a laser has one curved mirror and the other a flat grating will this make any difference to calculations of the beam parameters, or will it be the same as the curved-flat mirror geometry?

 

[mezarashi]

I understand that resonators are only there for logitudinal mode control purposes. If they support the same modes, I don't see why they would be different. They also introduce losses in the path unfortunately, but this normally has an effect on efficiency rather than the output beam itself.