How does a diode laser lase?

[usakicksass]

I looked on Wiki and didn't find it so I figured someone here knows the probably simple but hard won technology of diode lasers. Do they all work the same or does every color work differently? Thanks, Craig

 

[GooeyGus]

I THINK its just a semiconductor doped in a material that emits light when excited by electricity. I think violets use gallium nitride.

It's probably a lot more in-depth than that, but thats how I understand them to work. I'm sure someone else will come along with a much better answer

 

[pseudonomen137]

http://en.wikipedia.org/wiki/Laser_diode
^ There's the wiki article with the in depth explanation.

Very very simplfied idea: You have a basic light emitting semiconductor junction like an LED, and cleaved facets on each end form a laser cavity.

 

[pullbangdead]

usa, to answer one of your questions, all diode lasers work on the same basic principle, that electrons sit at a higher energy level and are stimulated to fall to a lower energy level by previously-emitted photons. In fact, that's the basic principle of all lasers. Diode lasers do all, however, use the same basic principles and designs to achieve that phenomenon of stimulated emission. The differences in colors come from the materials used and the bandgap of the material used. Gooey is correct, violet is based on the gallium nitride system. Technically, the material doing the emitting is indium gallium nitride, but it's just gallium nitride that is slightly doped with indium. The same material system is used for blue diode lasers, and will *hopefully* be used with green diode lasers in the very near future. To get blue, and later green, just add more indium. More indium reduces the bandgap energy, and changes the color emitted to a higher wavelength. Green is tough to make, in fact impossible so far, because it's very hard to put that much indium into the gallium nitride and keep it stable. Aluminum nitride is also used in the diodes, so you could just categorize all the violet/blue/green as nitride lasers, sicne they're based on Group III-nitride compounds. For red and IR lasers, the materials used are aluminum, indium, and gallium phosphide, another III-V semiconductor, but the phosphides used for light emitting layers have lower bandgaps, and therefore emit longer wavelength light.

As far as the functioning of the laser diode, as said above, facets are produced on each end of the laser diode to make a laser cavity. At low input currents, the laser diode acts just like an LED. Viewing it as a simple pn junction, the applied voltage allows electrons from the n side and holes from the p side to flow by lowering the potential barrier innate in the diode (this barrier is there because of the pn junction). Once electrons and holes are allowed to flow, they meet in the middle, and electrons will spontaneously fall into the holes. The electrons are at a higher energy than the state they fall into, and that energy is roughly equal to the bandgap energy. When the electron falls, that energy is given off in the form of a photon, with energy roughly equal to the bandgap, producing roughly the characteristic color of light (I keep saying roughly because LEDs aren't as precise in their behavior). LEDs function continuously in this way, with all the light coming from spontaneous emission. With spontaneous emission, there is always an amount of time before the electron falls, and that amount of time varies around an average time.

Now, we get to the laser diode. The light emitted in certain directions (towards the facts/mirrors, along the laser cavity) bounces back and forth in the cavity naturally, with always some escaping and some reflecting back. When above a certain voltage/current, the diode will start to lase. When this happens, there is enough light bouncing back and forth in the cavity to begin stimulated emission. Stimulated emission happens when a photon passes by an electron that is able to fall into a hole, but has not done so yet (remember, there is a finite time before the electron will spontaneously fall). The photon passing by, since it is an oscillating electric field, will cause the electron to oscillate, and therefore falls. When ti falls, the new photon it emitted will exactly match the photon that stimulated the electron in the first place (aka, they will be coherent). This is a continuously increasing effect, such that eventually there is enough light in the cavity that all the light being emitted is due to stimulated emission that happens before any electrons can spontaneously fall. Therefore, all light being emitted will be coherent, and you have a laser. The catch comes with making a device such that there is a never-ending supply of electron hole pairs ready to emit light whenever the coherent light already in the cavity comes by. Some of the light will natrually be escaping through the imperfect mirrors on the facets, and you have a laser diode.

To make diodes better and more efficient, there are lots of tricks to keeping all the electrons and holes in a small area, and in keeping the light in that same small area (in the cavity), all making the laser more efficient. One of the most popular is the quantum well (and multiple quantum well) design, which is another description for another day, but it just sounds too cool to pass up any chance to say it. Hehe, "quantum well".

 

[usakicksass]

DAMN! I asked for a beer and got champange! Thanks