Newbie on DCG holography

[MilanKarakas]

Right now made short time lapse video (25 seconds) from 262 photos during ~33 minutes period:

http://www.youtube.com/watch?v=HiDabG9tUv8

Note bright flash at the beginning! Very rare, but on occasion I can get up to 70 mW from this laser (If I get temperature of the laser module properly). After about 5 minutes it is stable, but with two additional frequencies, slightly lower than main (longer wavelengths). Tried cooling laser module with fresh ceramic tiles from refrigerator, and when lower temperature to 15C, then that two frequencies are not present, but then KTP doubling efficiency decreases, and the power remains the same, or even worst: begin to decrease over time.

Note that in the beginning two diffracted laser spots suddenly move to the right, and then back and stay still until end of the video. I don't know what causes that - KTP crystal or Nd:YVO4 crystal (or both). I have no any way of controlling optical path inside such small module. I wish that can add something inside to get better result.

One may ask why I bother so much with this laser and it's bad polarization: the reason is that every hologram has big interference fringes, looking like strange veil over the hologram. And, it is not due to laser light entering edge of the glass, since I covered it with red acrylic paint (very thick). Every hologram has it - much more visible than hologram itself.

I have to solve this problem(s) soon... did not made any hologram for too long time.

Best--
m--

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Edit: Forgot to mention method of observation. Used small diffraction grating, put close to the laser, and third order diffraction shine on the wall, some four meters away. It is pretty clear that there are more than one frequency - too bad for the holography. I checked my grating with HeNe laser and small red diode laser - no stray frequencies, except diode has slightly more broadband emission than 532 nm and 632.8 nm.

[Dinesh]

Milan
My impression is that you are concentrating too much on the science and the mechanics. Forget all this theory! Holography is a process, like cooking. When a cook puts salt in the food, he does not know the structure of sodium chloride, he just knows how much salt to put to make the food come out nice. When a photographer takes a picture, he does not understand the optics of lenses, he just know what kind of lighting and what exposure will produce a good picture. If a photographer came to you and said, "My lens has a focal length of <so many> metres, and the refractive index of my lens is 1.345 is this correct to get a good photograph?" I'm sure you would advise him to get a "feel" for the right exposure and lighting first. So with holography. Get a "feel" for the process, then worry about theory and cosmetics. First, polarisation is not necessary to make a hologram, many of us started with unpolarised lasers and got a good image. What is important is the correct exposure. Play with this, Cut your plates to small plates and make exposure tests. I believe that Ed Wesley has a way of making exposure masks on his website. Use this! Make test exposure masks and make several shots with different exposures. Choose an object that throws light in many different directions. One way to determine this is to light the object with an ordinary lamp in a dark room. Then take a white card and place it all around the object. If the white card only gets bright in one position, the object is not throwing light everywhere (this is called "diffuse lighting"). Choose an object that at least throws a lot of light in the direction of the plate. What the plate "sees', is what you will see. The more light that hits the plate, the better the plate will capture the image - like photography, you do not waste light. Place the object near the plate and expose with a mask. When you get the best exposure, change the development, one step at a time, slowly. Take many notes. Soon, you will get a feel for the right exposure and the right development. Then worry about chemistry and coating and physics and all that theory! Coating is not important now, in a display hologram, coating depth will only make the hologram colour very sharp if the coating is too deep. If the coating is too shallow, the image comes out a little white-ish. If all the plates from one batch are coated the same way, you can still say which process made the best. brightest hologram. Learn to cook eggs before you cook borscht!

As for the laser, power is not the same as wavelength. Make a Michaelson (there are many sites and there are many people here who will tell you how to do this). This will tell you how long of an exposure your table can stand. One "trick" is to make the plate holder with a small shelf. Place the object (glue it!) on the shelf. Then, both the plate and the object are on the same system - if one moves, they both move equally. Relative motion between plate and object is bad, if they both move together, then there are fewer motion problems.

[Ed Wesly]

"I believe that Ed Wesley has a way of making exposure masks on his website."

nlutie.com/ewesly

Explore and have fun!

[MilanKarakas]

Milan
My impression is that you are concentrating too much on the science and the mechanics. Forget all this theory! Holography is a process, like cooking. When a cook puts salt in the food, he does not know the structure of sodium chloride, he just knows how much salt to put to make the food come out nice. When a photographer takes a picture, he does not understand the optics of lenses, he just know what kind of lighting and what exposure will produce a good picture. If a photographer came to you and said, "My lens has a focal length of <so many> metres, and the refractive index of my lens is 1.345 is this correct to get a good photograph?" I'm sure you would advise him to get a "feel" for the right exposure and lighting first. So with holography. Get a "feel" for the process, then worry about theory and cosmetics.

Perhaps I am so curious and want to know what cause which phenomena.

First, polarisation is not necessary to make a hologram, many of us started with unpolarised lasers and got a good image. What is important is the correct exposure. Play with this, Cut your plates to small plates and make exposure tests. I believe that Ed Wesley has a way of making exposure masks on his website. Use this! Make test exposure masks and make several shots with different exposures. Choose an object that throws light in many different directions. One way to determine this is to light the object with an ordinary lamp in a dark room. Then take a white card and place it all around the object. If the white card only gets bright in one position, the object is not throwing light everywhere (this is called "diffuse lighting"). Choose an object that at least throws a lot of light in the direction of the plate. What the plate "sees', is what you will see. The more light that hits the plate, the better the plate will capture the image - like photography, you do not waste light. Place the object near the plate and expose with a mask. When you get the best exposure, change the development, one step at a time, slowly. Take many notes. Soon, you will get a feel for the right exposure and the right development. Then worry about chemistry and coating and physics and all that theory! Coating is not important now, in a display hologram, coating depth will only make the hologram colour very sharp if the coating is too deep. If the coating is too shallow, the image comes out a little white-ish. If all the plates from one batch are coated the same way, you can still say which process made the best. brightest hologram. Learn to cook eggs before you cook borscht!

I will definitively change my plan. Will try make some hologram soon.

As for the laser, power is not the same as wavelength. Make a Michaelson (there are many sites and there are many people here who will tell you how to do this). This will tell you how long of an exposure your table can stand.

Okay, I found one picture and built similar:

http://en.wikipedia.org/wiki/File:Miche ... ometer.jpg

And mine looks ugly, made in rush. Fringes are stable even when holding interferometer in my hand. What I did wrong?

Simple, 'quick and dirty' approach. I have not 50:50% beamsplitter, so made it with piece of glass (4:4%).

And got various shapes, mostly diagonal stripes, and after glue set got it horizontally.

Arrow and lines are made to see whether it moves. After a while, glue set, and pull something - and fringes change from diagonal to horizontal.

Something I get wrong, but don't know what.

One "trick" is to make the plate holder with a small shelf. Place the object (glue it!) on the shelf. Then, both the plate and the object are on the same system - if one moves, they both move equally. Relative motion between plate and object is bad, if they both move together, then there are fewer motion problems.

Yes, I got similar result with my interferometer... everything is stable. I don't understand what is going on? It should reflect vibrations of the floor but it did not.

Best--
m--

[Dinesh]

Milan,
You can't use a piece of glass as the beamsplitter. The piece of glass will reflect light from the front and the back. The light reflecting from the back of the glass will interfere with the light from the front of the glass. What you're seeing is the fringes caused by internal reflections within the beamsplitter. This will, of course, be stable because the glass does not swell. What you're testing is the stability of the glass. The beamsplitter must pass the beam straight through, or reflect from only one surface, and not reflect back

Can you stretch a thin piece of clear plastic across a frame? This is known as a "pellicle beamsplitter" (http://www.thorlabs.com/newgrouppage9.c ... oup_id=898) They are expensive to buy, but may be easy to make. Another way is to look inside a piece of equipment like a copier. These have beamsplitters inside. If you have any old cameras, sometimes there is a beamsplitter inside to "feed off" some of the light into a light meter. The mirrors must also be front-surface mirrors, both in your Michaelson and in your holographic setup.

Perhaps I am so curious and want to know what cause which phenomena.

Curiosity is a good thing, but the path to curiosity must be simple. Overcomplicate it, and you'll go five different ways all at once! Here in the US, we have a phrase: the KISS pholiosophy. Keep It Sweet and Simple (sometimes: Keep It Simple, Stupid!).

[MilanKarakas]

Milan,
You can't use a piece of glass as the beamsplitter. The piece of glass will reflect light from the front and the back. The light reflecting from the back of the glass will interfere with the light from the front of the glass. What you're seeing is the fringes caused by internal reflections within the beamsplitter. This will, of course, be stable because the glass does not swell. What you're testing is the stability of the glass. The beamsplitter must pass the beam straight through, or reflect from only one surface, and not reflect back

I am aware of drawback of the glass as a beamsplitter. Interference on picture in my previous post are not due to double reflection of the glass surface, laser beam is feed directly to the interferometer, then magnified by lens. That two reflections are offset by about 1.5 mm at 45 degrees. just slightly interfering. Before fixing with glue, at one moment got pretty sensitive interference fringes - even detecting my voice. But, after gluing, something went wrong. Probably I did not put equal light path from the beam splitter to the mirrors.

Can you stretch a thin piece of clear plastic across a frame? This is known as a "pellicle beamsplitter" (http://www.thorlabs.com/newgrouppage9.c ... oup_id=898) They are expensive to buy, but may be easy to make. Another way is to look inside a piece of equipment like a copier. These have beamsplitters inside. If you have any old cameras, sometimes there is a beamsplitter inside to "feed off" some of the light into a light meter. The mirrors must also be front-surface mirrors, both in your Michaelson and in your holographic setup.

I will try this once when find something very thin. Now, found one half of the beamsplitter, cut long time ago from 10x10 mm piece. It is 30/70% beam splitter (30 % T, 70% R). Not the best option, but at least it is broadband, and has AR coating on the back. Right now I am not finishing yet second setup, but will post the results here. Need some more sleep - this morning collapsed down and unable to work much at all (few days not much sleeping at all).

Perhaps I am so curious and want to know what cause which phenomena.

Curiosity is a good thing, but the path to curiosity must be simple. Overcomplicate it, and you'll go five different ways all at once! Here in the US, we have a phrase: the KISS pholiosophy. Keep It Sweet and Simple (sometimes: Keep It Simple, Stupid!).

Good philosophy, but here where I live - things are not so simple. Lack of proper building materials, lack of hi-tech stuff, lack of everything, and mostly important; lack of money. For that reason, I ma forced to DIY approach all the time.

Best--
m--

[Dinesh]

Probably I did not put equal light path from the beam splitter to the mirrors.

This is an important parameter. The difference in distance tells you how far behind the plate your object can be - half the difference in distance between the mirrors on a Michaelson. So, it's important not to glue down both mirrors. One must be free. You start with both mirrors at equal distances from the splitter. If you get stable fringes for the time of your exposure (200 seconds , I believe you said), then the table and the floor is stable. Now slowly move one mirror back a little, stop and look at the fringes. As you move one mirror back, the fringes will get dimmer and dimmer, until they disappear completely. Now, when you make a hologram, you are actually taking a photograph of these fringes on a very small scale. So, as a photographer, judge how much the contrast between fringe and not-fringe must be to get a good photograph. The brightness of the hologram is directly affected by the contrast in the fringes.

[MilanKarakas]

Probably I did not put equal light path from the beam splitter to the mirrors.

This is an important parameter. The difference in distance tells you how far behind the plate your object can be - half the difference in distance between the mirrors on a Michaelson. So, it's important not to glue down both mirrors. One must be free. You start with both mirrors at equal distances from the splitter. If you get stable fringes for the time of your exposure (200 seconds , I believe you said), then the table and the floor is stable.

Actually, this was 200 minutes! Now I know that this time is not right. At first, my laser operated on batteries, and as it drains - power output ceases, and causing mode hopping. Now built 3V voltage stabilizer and hope it will improve performance and no need for such long exposure.

Now slowly move one mirror back a little, stop and look at the fringes. As you move one mirror back, the fringes will get dimmer and dimmer, until they disappear completely. Now, when you make a hologram, you are actually taking a photograph of these fringes on a very small scale. So, as a photographer, judge how much the contrast between fringe and not-fringe must be to get a good photograph. The brightness of the hologram is directly affected by the contrast in the fringes.

I moved maximum allowed by my construction (movable arm: 21-260 mm, fixed arm: 21 mm), and fringes are visible at all distances (good contrast between constructive and destructive interferences).

Here is one 7 minutes video:

http://www.youtube.com/watch?v=slEvdqRv ... e=youtu.be

(spent two days transferring video to my computer. first day almost finished, but near end, power outage,... and repeat everything again).

Laser seems table, but I need more observation. Movable arm is not fixed, just sitting on the glass plate. As far as I can tell, it is stable after five or six minutes of laser running. When switching on, briefly fringes move then stabilize virtually - slightly moving during first five or so minutes, then stop. After few hours found the same pattern on the screen. Only problem with this observation is that if I turn on electric heater, fringes begins to move. Not much, but slowly moves, until 'freezes' again, but this time longer time required.

On video is evident that laser on the wooden desk is bad idea (still working on laser, soon will move it into some box). I will fix it on one corner on the room (wall, near ceiling - shinning down at Brewster angle, cca. 4.5 m distance), and object holder will be at opposite wall, also near corner. Found that walls are more stable than floor, especially near corners of the room.

More later,
m--