MadSci Network: Physics
Query:

Re: Can this procedure create a laser light image?

Date: Fri Feb 11 17:14:10 2000
Posted By: Steve Guch, Post-doc/Fellow, Physics (Electro-Optics/Lasers), Litton Systems, Inc., Laser Systems Division
Area of science: Physics
ID: 950292632.Ph
Message:

The question is pretty complex because of the multiple elements, but I'll 
try to talk to the underlying physical principle -- then answer the 
individual elements.

A laser beam is just that -- a bunch of photons of similar (but probably 
not identical wavelength) moving together along roughly the same line.  

If you direct them toward a mirror, part (but not all) of the beam will be 
reflected.  The reflected beam will be reduced in power because no mirror 
is perfectly reflecting.  The best-optimized reflectors aren't metallic, 
but are stacks of optically clear dielectric materials (like magnesium 
fluoride or silicon dioxide or hafnium dioxide or aluminum oxide) which 
are specially made up so that each layer provides partial reflection and 
that each partial reflection adds with all the others exactly in phase so 
that the result is exceptionally good reflection -- it can be 99.9%.

If you take a laser beam and put it onto a target with another laser beam, 
it's pretty likely that you won't see anything very remarkable.  If the 
beams are of significantly different wavelength or aren't made by very 
very very very very stable lasers, the beams don't really interact with 
each other in a meaningful way.  If the two beams are very close in 
wavelength, you may observe interference as the parts of the beams which 
are in phase with each other add to produce a bright area and the parts 
that are out of phase subtract to produce a dark area.  Depending on 
whether the beams are expanding or contracting or going in exactly the 
same direction, the area of interference may exhibit vertical or 
horizontal or circular or elliptical dark and light stripes... or other 
complex shapes depending on the details of the beam characteristics.  

Unfortunately, laser beams don't carry image information on themselves 
unless they have somehow been modulated -- either in position across the 
beam or in time:  they're just pure beams of light.  If you arrange to 
modulate them, you can recover the information, but that's generally 
pretty complex stuff and gets either into optical data transmission 
(usually high speed fiber optics) or holography -- both of which are 
beyond the scope of my answer here.

Image formation, on the other hand, occurs on a daily basis any time that 
light -- from a laser, or the sun, or a light -- falls on an object and is 
scattered by the object.  The scattered light is collected by a lens and 
focused -- not to a point, but to a series of points depending on the 
angle of every point on the object from the center of the lens.

So now, back to your questions:

1.  What is the maximum times a beam splitter can split a single laser 
beam?   A beam can be split an infinite number of times, but it just keeps 
getting weaker and weaker and, if the mirrors aren't perfectly flat, the 
beams start to look progressively more and more mottled in appearance.

2.  If two laser beams intersect will they provide constructive 
interference and will a point of light appear?  In this case, the waves 
will interfere if they come from a common source, but the exact form of 
the beams determines whether and where on the beam the interference is 
constructive and/or destructive.  I should also add here that for visible 
interference to occur, it's necessary to have the beams closely matched.  
This can be easily done by splitting a single beam into two beams, but 
it's also necessary in most cases to make sure the pathlengths followed by 
the beams before they hit a target are very nearly identical.  It turns 
out that lasers "burp" every now and then (in the trade, they are said to 
take mode hops which change the wavelength or phase), so that they won't 
even interfere with themselves unless they are interacting with a piece of 
the beam that was created at exactly the same time.  Generally, the 
interference area will be the full beam diameter, not a spot, but there 
are some pathological conditions that might produce a spot -- but never an 
image in the usual sense unless the beam is sent through a transparency or 
modulated or some such thing.

3.  I told (him or her?) that in order for the light to be 
visible it would need to be reflected to the eye from something, like a 
screen????  Yep, in general you're exactly right, for the reasons I 
indicated above.

Thanks for the question!




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