|
Victor Grubsky
Can sunlight be bounced/focused/ bent or in anyway be made into a laser beam?
Ad by Whole Tomato Software
Want to get more out of Visual Studio?
Check out Visual Assist by Whole Tomato!
Free Trial
4 Answers
Bill Otto
, former Principal Engineer, Lasers and Electro-optics at Boeing (1994-1997)
Answered August 9, 2019 · Upvoted by
Stephen Selipsky
, ex-particle theorist; Stanford Ph.D., research at CERN, BU, Yale · Author has 5.7K answers and 15.5M answer views
The popular press tries to make something magical out of laser beams. They call the light in a laser beam monochromatic and coherent.
The actual case is that no light is truly monochromatic or coherent. Most lasers are about as monochromatic as a good filtered gas lamp - probably the best known is a sodium lamp. This was used for interferometry before lasers were invented. But some lasers are multi-line. The first lasers I built and did research with had dozens of lines and covered a band from 2.6 microns to 3.2 microns. That is practically white light. In fact, another laser I worked with early on, a Argon-krypton ion laser can emit several lines in the visible band simultaneously to emit white light. Is there such thing as a white laser?
It is a myth that laser beams are collimated because of coherence. It is a myth that they don’t diverge. It is a myth that all the photons are in the same state. In actuality, a range of different frequencies comes out of the best lasers and this makes them partially incoherent. We measure how coherent a laser beam is in two ways: across the beam (spatial coherence) and along the beam (temporal coherence). Just to confuse people a little bit, we measure the temporal coherence length in meters. A white light laser might have a coherence length of a millimeter, while a high quality metrology laser may have a coherence length of tens of meters.
You are wondering what this has to do with your question. Well, what is it about laser light that defines a laser beam? Is it the fact that a gain medium provides light amplification? Unfortunately, light amplification by itself yields a light source like the old sodium lamp. In fact I can almost guarantee that light amplification is taking place in that lamp.
What most people consider a laser is a gain medium plus a linear resonator. They probably expect it to operate on a single atomic transition. That is gas laser thinking. Today, diode lasers operate over a wider range. Dye lasers are tunable, and most solid state lasers are operable over a range by tuning. In fact, we have to put filters into these lasers to make them closer to monochromatic.
So back to your question about sunlight being made into a laser beam. No, sunlight is emitted as blackbody radiation from a very thin plasma in the photosphere of the Sun. It can never be laser light by the definition I presented.
What if I change your question to “Can I get a high intensity beam of collimated light from sunlight using mirrors?” At first, it appears that the answer is “no”.
If I use imaging optics, I have a problem. Sunlight is from an extended object. Collimated light comes from a point source. If I try to image a very small piece of the Sun, say less than a milliradian, I can match the performance of a laser pointer. Let’s say I have a 6 inch telescope and I can place a spatial filter in the telescope somewhere. My first problem is that the spatial filter will burn up so I need to make it out of polished copper and water cool it.
So now I capture about 24 watts of broadband sunlight in the telescope and throw away about 23.6 watts of it, leaving about 0.4 watts. So I can get a 400 milliwatt beam of light that is as well collimated as a laser pointer.
Can I use it to set things on fire? Of course, but probably not very far away. I could even put a filter in my telescope to get narrow band green light. Now it resembles a laser pointer a lot. But it is not a laser.
Don’t want to throw away that much sunlight? Now you have to enter the world of non-imaging optics. It has been shown that you can get a brighter spot by using things like a light funnel. This diagram is just to indicate the principle of the idea:
The idea is to get to the bottom of the funnel with a very small hole. That approximates a point source, which you could then place at the focus of a high quality lens and get a collimated beam with 24 watts instead of just 0.4 watts.
Although it would act like a laser beam in many respects, it would not actually be a laser beam.
2.1K views
, Ph.D. Physics, University of Southern California
Answered August 11, 2019 · Author has 210 answers and 113.3K answer views
In contrast to laser beams, sunlight has a few important differences:
It is produced by an extended source, which results in a divergent beam. Technically speaking, it has poor spatial coherence. In contrast, laser beams have high spatial coherence and their divergence is usually only limited by diffraction, which is a much smaller effect.
Sunlight is composed of many different frequencies, from ultraviolet to infrared. This can pose problems in optical systems, as different frequencies tend to refract and reflect differently. In contrast, laser beams are essentially monochromatic and do not hav
Do'stlaringiz bilan baham: | 
