# Controlling light diffraction angle with a holographic "lens"

Mike Gaffer
I'm a physics student, and working on a class project that requires coming up with a method to control
the exit angle (diffraction angle) of a monochromatic light source.

For example, taking a laser (monochromatic, coherent light source), spreading the beam, and directing the light at a piece of holographic film. Then, depending on how the original "hologram" or diffraction grating was recorded or encoded, determining the direction of the transmitted light. Basically, I'm trying to create a "holographic lens" of sorts.

Is this possible? and what resources would you recommend in figuring out how to expose or record the diffraction pattern?

Thanks for any info in advance!

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Mike Gaffer
I guess the question is, how could I make a diffractive optical element (DOE), and is it possible to do this with holographic film?

I guess the question is, how could I make a diffractive optical element (DOE), and is it possible to do this with holographic film?

I suspect you could do this with a spatial light modulator.

Mike Gaffer
Gold Member
When you say 'control', do you mean variable control or just deflect by a given angle?
Your two diagrams seem to suggest that a diffraction grating could do the job. You can make a basic diffraction grating photographically using Young's slits (just one half of the pattern will have equal spaced maxima). You can work out the slit spacing for a given grating pitch and the slit spacing will need to be wider than for a normal Young's demo because the grating needs to be finer than you can easily see.
I don't know the present state of the art with home-made holography but I do know that the optics and the supports need to be held pretty steady and the film needs to be high res. (Those comments apply whatever your final solution is.)
PS the slits will need to be very narrow (as near omnidirectional patterns as possible) if you want the grating to have many maxima of 'equal' amplitude.

Mike Gaffer
Gold Member
Ah yes - the Zone Plate works like a convex lens. I was well impressed with that when I first came across it. (But I had forgotten the name so I couldn't look it up )

Nik_2213
How rapidly do you need to steer the beam ? Plane or volume ??

D'uh, how about a mirror plus $10 servo-motor per DOF ? Provided the angle is modest, internal reflection via an optical-grade prism may be easier than keeping a small, front-surface mirror clean. You'll find ample Arduino etc source-code on-line. Trouble-shoot / demonstrate your rig with a budget 'servo tester'... Don't laugh-- One of my friends built such a 'micro-disco FX' as a cat-toy... Science Advisor Gold Member Ah yes - the Zone Plate works like a convex lens. I was well impressed with that when I first came across it. (But I had forgotten the name so I couldn't look it up ) But I think the zone plate loses half the power. Mike Gaffer I suspect you could do this with a spatial light modulator. https://www.meadowlark.com/spatial_light_modulators.php I'm not familiar with spatial light modulators, other than having seen the term before... is this the type of module that is being used to generate true holograms in real time with massive amounts of computer data being fed into them? Would you know any place to start on _how_ I would use one? Mike Gaffer When you say 'control', do you mean variable control or just deflect by a given angle? Your two diagrams seem to suggest that a diffraction grating could do the job. You can make a basic diffraction grating photographically using Young's slits (just one half of the pattern will have equal spaced maxima). You can work out the slit spacing for a given grating pitch and the slit spacing will need to be wider than for a normal Young's demo because the grating needs to be finer than you can easily see. I don't know the present state of the art with home-made holography but I do know that the optics and the supports need to be held pretty steady and the film needs to be high res. (Those comments apply whatever your final solution is.) PS the slits will need to be very narrow (as near omnidirectional patterns as possible) if you want the grating to have many maxima of 'equal' amplitude. By _control_, I was looking at dividing an optical element, whether it be holographic film, or something else, into a grid; having each segment of the grid diffract a beam that is directed at it, at a unique output angle, and be able to calculate the output angle predictably. Then, be able to make this optical element so that I can have for example, square grid on the element (col 1, row 1) redirect light at 15 degrees, col 1, row 2 at 30degrees, col 2, row 1 at -10 degrees, etc... almost like a complex arrangement of little mirrors. Mike Gaffer But I think the zone plate loses half the power. Very interesting! Will look into this further. Mike Gaffer How rapidly do you need to steer the beam ? Plane or volume ?? D'uh, how about a mirror plus$10 servo-motor per DOF ? Provided the angle is modest, internal reflection via an optical-grade prism may be easier than keeping a small, front-surface mirror clean.

You'll find ample Arduino etc source-code on-line. Trouble-shoot / demonstrate your rig with a budget 'servo tester'...

Don't laugh-- One of my friends built such a 'micro-disco FX' as a cat-toy...
The angle, once determined, won't need to change. But I need to have potentially hundreds of different angle "quadrants" or areas on the element that would each need to be "steered" uniquely. Also weight, power, and potentially production type quantities would make the motor not applicable for this application - but otherwise good idea! Thanks

Mike Gaffer
When you say 'control', do you mean variable control or just deflect by a given angle?
Your two diagrams seem to suggest that a diffraction grating could do the job. You can make a basic diffraction grating photographically using Young's slits (just one half of the pattern will have equal spaced maxima). You can work out the slit spacing for a given grating pitch and the slit spacing will need to be wider than for a normal Young's demo because the grating needs to be finer than you can easily see.
I don't know the present state of the art with home-made holography but I do know that the optics and the supports need to be held pretty steady and the film needs to be high res. (Those comments apply whatever your final solution is.)
PS the slits will need to be very narrow (as near omnidirectional patterns as possible) if you want the grating to have many maxima of 'equal' amplitude.
By "control" I mean, divide a piece of film into a grid of for example, 100 columns, and 100 rows, and have each square of the grid cause a beam impacting it to be "steered" at a different output angle... almost like a piece of engineered bumpy glass.

I'm not familiar with spatial light modulators, other than having seen the term before... is this the type of module that is being used to generate true holograms in real time with massive amounts of computer data being fed into them? Would you know any place to start on _how_ I would use one?

I don't know your level of expertise, but:

https://www.hamamatsu.com/resources/pdf/ssd/e12_handbook_lcos_slm.pdf
http://spie.org/Publications/Book/2281295?SSO=1
https://arxiv.org/pdf/1708.02485.pdf

Mike Gaffer