Green Laser Question: Explaining the Double Pumping Method

[omega-centauri]

I took apart my green laser, and found a red laser diode inside! Even stranger, I found a little cylinder that fit over the laser that normally would have thought to be a filter, but when I put it on the red laser, green laser light came out! I was totally stumped as to how this worked, but then read something about a double pumping method and the filter is really a set of two crystals that turn the red wavelength into green, but I was wondering if anyone had an easy explanation for this that I can share with my students? I didn't know you could change the wavelength (like cut it in half) like this! It's totally amazing what we can do these days, especially for under $10!

Oh and I should add that when I turned the filter-looking cylinder around and put it in backwards, white light reflected back in the direction of the diode... !

 

[Quantum Defect]

I took apart my green laser, and found a red laser diode inside! Even stranger, I found a little cylinder that fit over the laser that normally would have thought to be a filter, but when I put it on the red laser, green laser light came out! I was totally stumped as to how this worked, but then read something about a double pumping method and the filter is really a set of two crystals that turn the red wavelength into green, but I was wondering if anyone had an easy explanation for this that I can share with my students? I didn't know you could change the wavelength (like cut it in half) like this! It's totally amazing what we can do these days, especially for under $10!

Oh and I should add that when I turned the filter-looking cylinder around and put it in backwards, white light reflected back in the direction of the diode... !

The thing in front is a non-linear optic. When you drive the atoms in this crystal very hard, they do not respond linearly to the oscillating electric field, and you get out frequencies that are 2x the driving frequency. With these kinds of crystals, the effect is observable in only certain directions.

With other crystals, you can get sum frequency mixing.

Google "nonlinear optics" to see more about this.