A laser pointer does not produce coherent light, contrary to popular belief. A laser pointer is not a true laser. It is an LED with polished and mirrored ends and has a lot of characteristics of a laser.
A HeNe laser at 0.1mw will do temporary damage to the eye, assuming a normal blink reaction. But a HeNe laser is a true laser and its light is one wavelength, and the waves are all in phase with each other. Because the photons are all in phase, the beam does not spread, it remains a pinpoint off into infinity, in theory anyway. Most commercial HeNe lasers use an ever so slightly curved mirror on the backside to make the beam spread a little.
A laser pointer puts out a beam that is with in a narrow bandwidth and the photons are not in phase with each other. It spreads out, but more importantly, it does not have all its energy at one single wavelength. If you look at the frequency spectrum, the laser pointer looks like a brick over a 50 nanometer band. A HeNe laser looks like a knife point.
BMW is using the a laser diode similar to a laser pointer, but it is actually closer to the laser in a blu ray player. Because it is not as coherent as a true laser like an HeNe or YAG laser, it is a little safer, but alone it is also not very useful. It’s advantage is efficiency. It converts more of its input energy to output energy than any other light source. But it is being used to activate a phosphoric coating to emit the actual useable light. The light emitted from the coatings on the bulb will be a broad spectrum light.
Because the photons are all in phase, the beam does not spread, it remains a pinpoint off into infinity, in theory anyway.
No, not even in theory. In theory, all laser beams diverge. The half-angle of divergence is given by
θ ≈ λ / π w0 radians,
where λ is the laser wavelength and w0 is the beam half-waist. (See the first equation under the heading Beam divergence in <font color=Red">Wikipedia: Gaussian beam.
For a 633 nm red He-Ne laser with a beam waist of 0.4 mm, from the above equation θ ≈ 0.000504 radians.
If such a laser were pointed at the moon (384,000 km from earth), its beam would diverge to a diameter of almost 400 km at the moon’s surface.
As for LED Daytime Running Lights (DRL) on European cars, they are a result of European legislation adopted in 2008 requiring low energy running lights on all new types of passenger cars and small delivery vans starting in February 2011. Trucks and buses will follow from August 2012.
Mechaniker, You miss understood your own reference. You need to read it again. If you can make a laser operate in the TEMoo mode, it will not diverge. The problem is with mirror alignment in the laser. It can be done in a lab under very stringent conditions, but to make a mobile laser, that is one that can be moved and set up in a different location, one or both of the mirrors gets a very slight curve.
The example you show is using a converging beam or focused beam. That means that at one point, the diameter of the beam is smaller than it was when it left the optics of the laser. The beam was focused to concentrate on a smaller spot. After the focused spot (the waist), the beam diverges faster than it would have if the beam left the optics as a divergent beam. That was the point of that one part of the article you took out of context.
You have to be careful when you use wikipedia as a reference. This particular article was well vetted and subjected to peer review. That is not the case with all articles in wikipedia. In fact they have some articles that are full of misinformation that have not yet been reviewed and either rejected or corrected.
The “in theory part” is because the port in a commercial HeNe is usually about 1mm in diameter. This is far too large to allow a TEMoo mode for the beam. The port would have to be exactly 317nm wide to operate in this mode, so the commercial HeNe laser is actually putting out a bunch of beams running parallel to each other (aka higher order mode). When the beams exit the port they start to converge or diverge according to the design of the optics.
edit, changed 633nm to 317 as the TEMoo mode is one half wavelength wide.
keith
Mechaniker, You miss understood your own reference. You need to read it again. If you can make a laser operate in the TEMoo mode, it will not diverge …
You may be right, but I would have to see a link/source that says a laser operating in TEM mode will not diverge before I could believe you. A number of articles discussing laser beam divergence can be found by googling laser divergence TEM mode. Two articles chosen at random from googling the previous words are Spatial Characteristics of Lasers and Fundamentals of Lasers. Both discuss laser beam divergence. Can you point out to me, in any article found on the internet, a reference to the claim that lasers operating in TEM mode do not diverge? Thanks.
One other point. You said:keith
If you look at the frequency spectrum, the laser pointer looks like a brick over a 50 nanometer band. A HeNe laser looks like a knife point. ...I am not sure of your definition of "knife point." Do you mean something like a mathematical δ-function? I ask this because no frequency spectrum can consist of a single line of zero bandwidth; this would imply that the energy and momentum of the photons were known precisely, which would violate Heisenberg's Uncertainty Principle. Every physical frequency spectrum must have a measurable, finite bandwidth.Interesting discussion. I'm enjoying it.
First I am going to cop out on divergence. I said “In theory”. Para 35a of your first reference is a short explanation of why it doesn’t happen in practice. Most of the rest of the article does not apply to this because it is basically how to calculate designed in divergence.
To explain the “knife point” comment, you are right about every frequency spectrum having a measurable and finite bandwidth. Every knife has some thickness to it as well. There is an illustration in wikipedia showing this, but I have to say that this particular article is not vetted and has not been subjected to peer review. It also appears that the diagram, the third down on the right hand side of the page did not come from any of the listed references, but it still illustrates my contention of the “knife point” of the HeNe laser spectrum compared to a Gallium Arsenide (diode) laser.
Oops, a little problem here. You have to copy the link and paste it into your browser or it goes to another wikipedia article. The -neon-laser fell out of the link.
keith
Para 35a of your first reference is a short explanation of why it doesn’t happen in practice. Most of the rest of the article does not apply to this because it is basically how to calculate designed in divergence.
The article(s) explain how to calculate laser beam divergence. A laser can be designed to have any divergence desired. The point I was making is that a laser beam cannot be designed to have zero divergence. Laser beam divergence is not something that designed in. Rather, it is something that cannot be eliminated.
You are right except for the part about not being designed in. While in practice, it cannot be eliminated, it can be controlled to some extent and depending on the application, it may be designed in. An example would be an industrial YAG cutting laser.
Note, I said MAY be designed in. Now that I look at my statement above, I see that it is close, but not exactly what I meant to say. Knowing the factors that affect divergence allows designers to have some control over it. This gives them the ability to design in a divergence, within limits, if desired.
The comments of some sound as confused and lacking in information as the article which told the reader nothing about how BMWs laser lights are supposed to work. I don’t know what the “visual spectrum” is, but , I have read about the visible spectrum. For laser light to be dangerous to anything it has to have not only great intensity but needs to be focused by way of a lens and the focal point adjusted for target distance. I would think if the laser lighting is going to be effective the beam would have to be white to help the driver. But, then, white light is all the wave-lengths of light - something completely opposite of laser light. So, what’s the point BMW - low energy usage to extend, say, battery life?