How Does a Holographic Grating Work?

[dacruick]

Hi,

I'm just trying to learn about how holographic gratings work. I see a bunch of examples that make use of a beam splitter, but I don't understand physically what a holographic grating is.

Can anyone shed some light on this holographic grating? I'm hoping to have a more 3 dimensional understanding.

 

[Danger]

Welcome to PF, Ducky.
I'm not familiar with the term "holographic grating", so I am going to assume that you refer to the interference patterns that compose a hologram. If that is the case, and you're talking "Old School", then you need only to view the film when it is back-lit by a laser of the same frequency that was used as the reference beam when it was produced.

 

[Dr Lots-o'watts]

I can't go into the details myself, but here's a practical use for them:
http://www.ino.ca/en-ca/achievements/description/project-p/holographic-beam-sampler.html

 

[dacruick]

Hi Danger,

I've never been called Ducky before :).

So from what I understand, the difference between a hologram and an irradiance map is that an irradiance map doesn't contain information about the phase of the light.

Lets say I have an object which I want to produce a hologram of. If I shine a laser onto it and retrieve the irradiance map of the object I will have a 2 dimensional representation of it. If I use that same laser in a beam splitter, and I take light that hasn't hit the object into account on my Irradiance map, will that be a hologram?

I've also stumbled across a term called blazing. Are you familiar with this? It seems like it is a type of grating which uses regions of variable refractive indices to produce a certain interference pattern. Do you know how this ties in with holographics?

 

[Danger]

I've never been called Ducky before :).

Believe it or not, that was used as a term of affection. I'm both too tired and too drunk to try typing your user name. The first thing that crossed my mind was David McCullum's role in NCIS. His character name is Dr. Mallard, so everyone calls him "Ducky".
I must admit that your questions all just leave me puzzled. Are you new, or even older than me? (Holograms were invented when I was in public school.)
I really have no idea of how things are done now, which is one reason that your post confuses me. In my day, a laser was fired through a beam splitter first off. Part of it went straight to the target object, while the rest was redirected in order to illuminate it from a different angle (what I referred to earlier as a "reference beam".)
It appears that the technology has advanced greatly, in which case I am no longer qualified to comment.
Still, it's nice to have you on board. Stop by the "Welcome Tent" and get your official fish-slap welcome from the Sisterhood.

 

[dacruick]

I did not interpret it as a term of affection. The smiley face was simply amusement.

It is also possible that you don't understand my questions due to your state of fatigue and inebriation.

The part of the laser that reflects off of the target object is retrieved by a photo plate. Then the reference beam is shone onto the photo plate, giving us information about phase(depth) of the object. Does that sound right?

Also, does it matter at which angle the reference beam is fired at? And what happens when you move the reference beam in or out half a wavelength?

 

[Danger]

I did not interpret it as a term of affection.

Given that, I apologize profusely. I meant no offense.
As to my state of fatigue and inebriation, it's just business as usual. I have no expectation that you will either accept or respect my opinions, but I'm going to state them anyway.
At least when I was involved in the scene, it was not the reference beam impacting the plate that made a hologram; it was the combination of the primary beam and the reference beam that caused interference fringes which could then be reconstituted by using an illumination beam of like frequency.
Since I get the impression that you are hostile toward me for whatever reason, I am now leaving this thread.

 

[dacruick]

Hah well I guess it is my turn to apologize as I am not hostile towards you. I misread you as saying that it wasn't a term of affection, and I was trying to tell you that it was fine because I didn't interpret it that way.

Either way, thanks for your help.

 

[Danger]

Okay, I continued to monitor the thread although I didn't feel welcome as a contributor to it. Given your last post, I believe that we began with a misunderstanding that escalated. Can we put it down to history, and start fresh?

 

[Integral]

If you expose film to just the reference beam and the source beam with no object in place you will obtain a image of the interference pattern of two point sources. This is much like the pattern resulting form a dual slit, a series of uniformly spaced parallel lines. This pattern can be used as a diffraction grating. I carried one of these on old fashioned film in my wallet until the emulsion wore off. It was great fun to have a pretty good diffraction grating with me all the time.


I hope this is what you are talking about.

 

[Danger]

That approach never crossed my mind, Integral. Cool idea.

 

[dacruick]

Okay, I continued to monitor the thread although I didn't feel welcome as a contributor to it. Given your last post, I believe that we began with a misunderstanding that escalated. Can we put it down to history, and start fresh?

I would appreciate that.

If you expose film to just the reference beam and the source beam with no object in place you will obtain a image of the interference pattern of two point sources. This is much like the pattern resulting form a dual slit, a series of uniformly spaced parallel lines. This pattern can be used as a diffraction grating. I carried one of these on old fashioned film in my wallet until the emulsion wore off. It was great fun to have a pretty good diffraction grating with me all the time.


I hope this is what you are talking about.

I'm not sure that it is what I'm talking about. I have been researching the past few hours and I think I have a decent understanding of what is going on.


I still have some outstanding questions though.
1) If your reference beam is a different wavelength than the source, what are the consequences?
2) If your reference beam is moved in our out half of a phase, what are the consequences?
3) If I have a single object beam and a single reference beam, how am I able to see the hologram from multiple perspectives? Is that through changing the angle of the reference beam?

 

[Integral]

I would appreciate that.



I'm not sure that it is what I'm talking about. I have been researching the past few hours and I think I have a decent understanding of what is going on.


I still have some outstanding questions though.
1) If your reference beam is a different wavelength than the source, what are the consequences?

It won't work, in order to make a hologram there must be a constant phase difference between the source and reference beams. The reference beam is created by splitting the source beam. Since the reference beam is "part" of the source beam but it takes a different and longer optical path to arrive at the film.

2) If your reference beam is moved in our out half of a phase, what are the consequences?

You must have a phase shift to create a hologram. It will usually be multiple wavelengths plus some fraction of a wavelength. This is called the OPTICAL PATH LENGTH. It is critical that this difference be constant.

3) If I have a single object beam and a single reference beam, how am I able to see the hologram from multiple perspectives? Is that through changing the angle of the reference beam?

That is the magic of a hologram, remember that you are not capturing a 2d image but the diffraction pattern at the plane of the film. The diffraction pattern contains the information needed to create a 3d like image.

 

[Danger]

there must be a constant phase difference between the source and reference beams. The reference beam is created by splitting the source beam. Since the reference beam is "part" of the source beam but it takes a different and longer optical path to arrive at the film.

That's why I am constantly amazed that you can find the bloody things everywhere these days. When I was a kid, you needed an optical bench with sub-micron precision and total vibrational isolation in order to produce one. Nowadays, you can damned nearly do it with a cell phone.

 

[dacruick]

You must have a phase shift to create a hologram. It will usually be multiple wavelengths plus some fraction of a wavelength. This is called the OPTICAL PATH LENGTH. It is critical that this difference be constant.

It is critical that this distance be constant, but it doesn't necessarily matter what the distance is?

 

[Danger]

It is critical that this distance be constant, but it doesn't necessarily matter what the distance is?

Correct, although in practice the path has to be short enough to avoid beam degradation.

 

[Integral]

I believe the coherence length of the laser determines you maximum path length.

 

[dacruick]

coherence length is simply the distance at which the laser beam diverges too much?