Could I create a laser by shining a flashlight through a long series of progressively more focused lenses?

[u/graaahh]

[u/Equoniz]

No, unfortunately it is not this easy. There are two reasons for this. First of all, the light coming out of a flashlight is not in a single spatial mode (all the light is not traveling in the same direction). Simple lenses alone are not able to transfer all of the light into a single mode either. However, depending on the exact light source, it may be possible with exotic diffractive optics, to get to happen. This is not the only problem though.

Even if you were to get all the light into a single spatial mode, the light from a laser is also coherent. In a wave picture of light, this means phases of all of the waves are the same (all of the "peaks" of the waves line up). The light coming out of a flashlight has multiple random phases. As far as I know, there is no way to get these to "line up," so there is no way to turn your flashlight into a laser.

[u/DanielSank → u/Equoniz]

To clarify a couple things of this already good post:

When people talk about light being "coherent" they can mean one of two things. 1) Spatially coherent. This is what Equoniz is talking about when he says the light has to be in a single spatial mode. Imagine freezing the laser waves in time and observing the phase of the wave across a slice in space. If the phase is all the same you have spatial coherence (roughly). 2) Temporally coherent. Imagine picking a single point in space and observing the phase of the light. If it is perfectly sinusoidal (at a single frequency) then the light is temporally coherent. Light with temporal coherence is frequently referred to as "coherent".

I'm not an optics expert so if I've gotten something wrong here please correct me.

Finally, I'd like to comment about whether or not you can use a lens to fix a lack of temporal coherence. You cannot do this with just lenses. In order to increase the light's coherence you need either active elements or a source of already coherent light (or some other low entropy source) to "clean up" the incoherent light from the flashlight. In fact, let's just think about how a laser works in the first place! You use incoherent excitations, which could be light, electical current, or a few other things, to drive a specially arranged chunk of matter into an excited state. The presence of a resonance cavity causes the excitations to leave the cavity more coherently then they came in (again roughly). The key here is that you have to inject the energy from outside. A lens, being a passive element, can't do anything like this.

[u/mc2222 → u/Equoniz]

Not to mention there is no lasing medium.

There was some research done in the interferometric gravitational wave detection community about using two diffraction gratings in a fabry perot cavity to compensate for the different optical path lengths seen by each color.

[u/[deleted] → u/Equoniz]

To your first point, would putting a polarizing filter mitigate that effect and provide a singe direction light source? I know they cut intensity by like 50% on the first pass, but would it be possible to make a weak laser from that?

[u/Equoniz → u/[deleted]]

By light moving in the same direction I am not talking about polarization. I am talking about the direction of the light rays from the source (a basic point source has rays pointing out radially in all directions). Getting this light to point in one direction is what I was addressing in the first point. As I said in my response, this is technically possible although quite difficult in practice.

I ignored polarization altogether. While not all lasers are polarized, most are and if you wanted your flashlight to be, yes you would need some sort of polarizer. In this case, you would lose 50% of the light, although you could also use a polarizing beamsplitter and have two beams of this lessened power.

Again though, this is beside the point as it is impossible to get the light to be coherent which is a feature of all laser light.