Laser that doesn't have a resonator?

[girlinphysics]

Is it possible to have a laser without a resonator? My guess is you can as you would still get some proportion of light coming from the inverted medium. My question is how would this light be different (from that of a laser with a resonator) and are there any practical uses for such a device?

 

[DaveC49]

It would appear so. The following Wikipedia article has a reference for a single pass nitrogen laser (ref 9) which does not have a resonant cavity.
https://en.wikipedia.org/wiki/Laser i.e. "Nitrogen Laser". Light and Its Uses. Scientific American. June 1974. pp. 40–43. ISBN0-7167-1185-0.

 

[Daz]

I can’t see how that would work. The high Q of the resonator is what gives laser light its temporal coherence. Without the resonator all you would have is spontaneous emission plus a bit of amplified spontaneous emission (ASE).

 

[DaveC49]

Daz

Have a look at the Wikipedia article. I haven't yet tracked down the Sci. Am. article from 1974 to see whether it casts any light on the matter. I too originally thought that a cavity resonator was essential for laser operation,. The Wiki article does mention use of a mirror at one end but no half silvered mirror at the exit end. I guess the Light Amplification by Stimulated Emission is the critical part that can take place without the cavity as long as the population inversion is achieved by pumping. The efficiency and spatial and temporal coherence are no doubt improved by a high Q cavity. I'll ask a few colleagues who work on laser physics for their insights. I might try a search for papers on lasers without cavities and see what turns up.

 

[Daz]

Interesting - thanks Dave. I clicked through onto the Nitrogen laser page where it says they work on the superluminescence principle. Superluminescence is just ASE but the gain is so high you can get a useable beam without a cavity.

One of the references on that page was this, with lots of info on N2 lasers:
http://www.repairfaq.org/sam/lasercn2.htm

It seems that the lower lasing level in N2 is metastable so it wouldn’t work CW and the gas is pumped by an arc discharge which is too brief to establish resonance in a cavity anyway. So people just leave out the mirrors and get a bright beam of ASE with each discharge.

That page above claims it’s coherent, but I can’t get my head around that. Any ideas?

 

[Andy Resnick]

Is it possible to have a laser without a resonator? My guess is you can as you would still get some proportion of light coming from the inverted medium. My question is how would this light be different (from that of a laser with a resonator) and are there any practical uses for such a device?

As mentioned above, solid state lasers can be highly lossy because the gain is also high- they essentially function as single-pass amplifiers. There are also unstable resonator designs which can be used in high power applications.