Light Beam Length

Consult a cartoon physicist like Wile E. Coyote?

Like containing the light in a magnetic field, aka a star wars light sabre?

What's the application and distance involved? Can you not just put a light absorbing object in the way at the appropriate distance?

It depends, is the force with you?

Then there's virtual reality...

Well, that puts a new twist on storing sh*t in the cloud, instead, we now store it in a rock? Does that mean I can repurpose my cell phones I've "bricked" over the years??

And, what will happen to the hard drives in the computers? Will you carry around a bunch of rocks in your pocket for your mass storage?

Ha Ha!

The easiest ways to control the length of travel of a beam of light through free space are:

1. Choose the desired length of travel to be zero (whatever units) and control by not turning the light on.

2. Choose the desired length of travel to be a function of elapsed time (L=ct) and control by turning the light on.

Soon to be posted be agua_doc.....

My distance of travel is 2 feet. Please advise me a suitable optical switch and control circuit that has a suitable switching response and speed to achieve this.

ROFL.

Two thoughts:

You could use a large optical system (lenses and/or mirrors) to focus the light at a given distance. The light won't stop there, of course, but the beam will begin to expand beyond that point, and the intensity will drop.

Or, you could use 'light' at a near infrared wavelength (like around 2 microns) where the atmophere attenuates the light dramatically. By selectively fine-tuning the wavelength of the infrared 'light', you can use Beer's Law of Attenuation to determine how far the beam will travel until the intensity drops to whatever percentage you consider as 'stopped'. You would need specific details of the atmospheric extinction for the wavelength(s) you choose. Here's a graph showing the opacity of the atmosphere across the E-M spectrum. (Notice that there are a number of absorption bands in the IR.)

Sounds interesting, but where's the link?

That's odd. I posted an image showing transmission of E-M energy vs wavelength through the atmosphere, but it disappeared.

Here's the link.

http://www.everythingweather.com/atmospheric-radiation/absorption.shtml

I have to ask:

1) Is the "adaptive" something like a servo motor adding or removing stress from the back of a mirror?

2) What would be a typical bandwidth for the mechanical motion?

3) At any given instant are the "non-uniformities in the atmosphere" large enough that the full mirror is considered to be looking through one uniform index of refraction or are the "non-uniformities in the atmosphere" small enough that different parts of the mirror are looking through different indexes of refraction?

Inquiring minds want to know.
Thanks

There are different ways to do it. Typically a deformable mirror is placed in the focal path between the primary mirror and the focal point, with hundreds of micro-actuators attached to the rear of the deformable mirror. A small 'pick-off' mirror intercepts a portion of the light and sends it to an array for analysis, which then tells the actuators how to move so that the resulting surface corrects the distortions in the wavefront. Here's a link to more info:

https://en.wikipedia.org/wiki/Adaptive_optics

Quite often two AO mirrors are used: a 'tilt-tip' mirror that removes the majority of low-order abberations; and a thin, deformable mirror backed by numerous actuators. The deformable mirror at the Hobby-Eberly Telescope at McDonald Observatory has a bandwidth of around 500 Hz. Pretty brisk. Now the main mirror segments at HET are also backed by actuators on 'whiffle-trees'. These are not real-time AOs, but to compensate for the mirrors' and supporting structure's sag at different orientations due to gravity. This is a static compensation. The real AO action takes place at the secondary mirror.

Shine it at a black hole, perhaps?

A brick wall will do it too. (Less treacherous too... you can't trust those black holes, don't invite one round for tea...)
Del

You could produce two laser beams exactly out of phase and arrange for them to meet at the point you want the beam to disappear; of course it would reappear again after that point.

Perhaps you could combine this idea with PWSlack's. Pass the two beams between two massive bodies: so that they converge but never diverge.

To stop the light... draw the curtains.
Next!
Del

Yeah, light always has been a push over for pencil sketches of flowing material..

In circularly polarized light, the photons have a spin angular momentum.

https://en.wikipedia.org/wiki/Spin_angular_momentum_of_light

You just have to figure out how to put a backspin on them. (It works with the cue ball in billiards.)

Does it have to be a beam of photons?

If other particle beams might be utilized, some control over beam length is probably feasible yet almost surely impractical.

If you used a very rapidly decaying particle like muons or pions, then beam length could be controlled, though the edge would not be sharp.

.

Alternately if you used a combination of a beam of particles along with a beam of the corresponding antiparticle and controlled the spacing between particle antiparticle precisely, the length of the beam could be controlled .

This is being over thunk. It shouldn't be that hard to.control the distance of travel through free space of light beam.

It should be easier to control the distance of travel of a light beam than of a comparable heavy beam or standard beam.

Lots of great ideas posted so far. agua_doc it's time to post some specifics and an example!

A laser has a theoretical fresnel zone of 19 light years.

Huh?

Really more applicable for two-way wireless communication links than one way light transmissions.

Link

Yes, I'm well aware of Fresnel zones, what they are and how they work. I'm 'Huh'-ing at his magic number of '19 light years.'

https://books.google.com/books?id=qFQSbAR4fMcC&pg=PA104&lpg=PA104&dq=laser+fresnel+zone+length&source=bl&ots=aGTBCSUqk3&sig=E8_NhY-yE1ZroZv5OWV_AEQNtqE&hl=en&sa=X&ved=0ahUKEwjJibb205_UAhXGWCYKHYRdCrY4ChDoAQg4MAM#v=onepage&q=laser%20fresnel%20zone%20length&f=false

I hope i copied the link correctly. The inverse square law does not correctly identify a laser. The intensity of a dot of a laser does not follow (I1/I2=D2^2/D1^2) The reflected light does. If you had a perfect laser with a perfect lens(good luck with that) in a total vacuum , no dust or free floating hydrogen, it would travel in a beam without spreading for a given long distance without spreading. Just try it with a laser dot. Measure the diameter of the dot at a given distance and then double the distance. The dot does not quadruple in size.

The reflected light however does. We are working on a laser measurement system where we measure the reflected energy with a 650 nm laser reflected back to a photo diode thru a band pass filter and then to a trans impedance amplifier.

We are still in the development stage but have received good linear results. Reason for the investigation is that commercial laser triangulation systems are not rugged and have failed ,sometimes, on the test bench without even going out to the field at a cost of around 3400$ a pop. And this was from a well known German company.

Also the current methods of a PSD or CCD receiver are greatly affected by laser speckle which occurs when reflected off of a rough surface steel plate, such as a pipe wall.

Currently we are using ultrasonic at 240 Khz in a pitch catch mode but can only resolve around .012". Our goal is +/- .001".

Used on this unit.

Note the distances in your linked piece are in AU - Astronomical Units - not light-years. 1 AU is the mean Earth-to-Sun distance. One light year is approx 63241.1 AU.

Beam divergence is diffraction limited. The wavelength and exit aperture diameter determine the divergence. There is no getting around it because for any finite aperture what you've got is a single-hole interferometer. For a round beam the divergence is the angular diameter of the central Airy disk.

For a round exit aperture the divergence is:

θ = 1.22·λ / D

where

θ = divergence in radians

λ = wavelength in metres

D = aperture diameter in metres

1.22 is the first zero of the order-one Bessel function of the first kind J₁(x) divided by pi.

Your beam diameter is proportional to distance; it does not quadruple when you double the distance, its cross-sectional area does however, in agreement with the Inverse Square Law.

I got the 19 light years from a physicist that i was working with on the FEL instillation at Boeing. His statement was , when i asked, under total ideal conditions around 19 light years.

All i was doing on the FEL was overseeing the cooling piping system.

Do you recall what assumptions he was making, because that system would have to scale correspondingly. It would dwarf the setup in that linked piece. He's not going to get that from a 500 km Fresnel lens.

"....Just try it with a laser dot. Measure the diameter of the dot at a given distance and then double the distance. The dot does not quadruple in size.

The reflected light however does..."

.

Does this apply to all lasers, or are certain types excluded? Many types of lasers have been reflected numerous times in the process of forming the beam.

That may be the case but other factors come in to play which make this a moot point (especially for a highly directional light beam).

Not to mention coherence length. Even for the best single-mode fibre lasers it's well under 1000 km.

If you figure out how to arbitrarily stop photons in "free space", you can collect your Nobel Prize.

Is free space free of gravity? Otherwise PW's answer holds.

Free space is like free puppies, it's all wonderful until the bill arrives.

Just let the light spread out until you cannot any longer get a photon count above shot noise, and then S/N <1 and you are done, any way you look at it, get it?

Diffraction limited TEM₀₀ is the best case scenario, and beam energy profile (traversing the beam) changes with distance, i.e. - it gets wider, but not as badly as would incoherent light. There is one obvious difference at play here - coherence.

No, but you can control the intensity at a point along the length using output optics that help to focus or disperse the beam (as in non-parallel rays).