Difference between solid-state, semiconductor and gas lasers

[says]

What is the difference between solid-state (SS), semiconductor and gas lasers?

I know SS lasers are optically pumped (generally), while semiconductor and gas lasers are electrically pumped (laser diode). The active mediums are also unique.

Beyond this though, I'm not entirely sure about any other differences. I've read about efficiencies for example, but usually it says, 'this laser is more efficient than this laser, or this laser is more efficient than that laser.' Nothing that gives general information, i.e. 'SS lasers are less efficient than semiconductor lasers'.

 

[jambaugh]

You as a very broad question. The fundamental differences are in how the amplification is achieved. All the lasers use some form of energy state inversion... where in a quantized energy system there are more elements in an excited state than in a lower energy state. There also needs to be sufficient lifetime of the inverted system for stimulated emission to yield amplification and of course coupling between the systems in the form of the light transmitted.

For semiconductors it is a matter of electrons and holes existing the semiconductor crystal in sufficient population. The existence of the free electrons and holes vs the ground state of electrons filling the holes is the inversion state. This is typically achieved by passing current through a P-N junction, the holes in the P type and electrons in the N type semiconductor will flow toward the junction. The junction itself is the intrinsic material so both holes and free electrons require extra energy to exist. The voltage of the current flow across the junction interfaces provide that energy. You also need the intrinsic layer to be optically clear in the frequencies corresponding to the pair annihilation energy.

For gas lasers you simply use excitation of the gas molecules electron orbitals. Its the same mechanism as your gas discharge lamps, such as neon lights or mecury florescent lamps. The difficulty is in achieving the inversion. Simply pumping current through will typically do it since the pumping is too random. You can achieve pulsed lasing this way but the gas then needs to cool down before subsequent excitation. The HeNe lasers take advantage of selective interaction between the helium and neon atoms. The electrons pump up the helium into a high excited density (though not an inversion) but then the helium can transfer energy to the neon in a way that maintains continuous inversion. (like a pump pushing water up to maintain a water fall.)

Solid state and dye lasers work on the same mechanism except they must be pumped optically (and at a higher frequency than the lasing freq.)
Now you can pump gas, semiconductor (in principle), and solid state laser using light. Typically optical pumping is less efficient than electrical excitation. There's one other class I know of and that's chemically pumped lasers. I believe one of the SDI developed military weapons lasers was based on reacting hydrogen and fluorine producing excited HFl gas in an excited state which then can lase.