445nm beam correction optic

This is a forum to share experiences and ideas about holography.
Thieu

445nm beam correction optic

Post by Thieu » Tue Aug 31, 2010 9:58 am

The problem with interferometric methods is that they can give false readings if the path length difference is close to a multiple of the diode cavity's optical length. You can compare that to watching the fringes in a bright region of the breadslices. The fringe contrast looks OK there, but the final holograms obviously don't. I never tried wler's method, but he seems to get good results with it. It requires some electronics though.

The method I'd recommend would be to build a simple spectrograph. It'll give you 100% accurate readings. It's much much easier and quicker to build than you might think and requires no electronics or expensive parts. You only need a simple lens (the one that came with the diode is probably OK) and a grating. I haven't tested this myself, but a thin radial slice of a DVD will probably do as well, since it works as a grating with about 1200 lines/mm. You may have to keep it flat by glueing it to a glass plate. Use the lens to create a sharp image of the diode's output at a large distance. 10 meters will be ok. Of course you can use mirrors to fold the path if space is limited. Make sure the diode is oriented in such a way that its highest divergence before the lens is in the horizontal plane. This will give you the smallest spot size in the horizontal direction in the projected image. Now bounce the beam of the grating/DVD, placed just after the lens and take a look at the first order. Make sure the lines of the grating are in the vertical plane, so the wavelength seperation is in the horizontal. Because the mode spacing for this diode is around 0.1 nm, the spacing of the modes in the image will be around 0.26 mm per meter distance. So at a distance of 10 meters, it will be 2.6 mm, not that hard to see. Because the optical system is less than perfect, instead of clear lines, you'll probably see a couple of dots seperated in the horizontal plane. There may even be some overlap, but that's no problem. If the dots come and go with changing the current, you spectrograph is working OK. The laser will be working single mode when only one dot is visible. It requires some fine tuning with the current to get it like this and over time it may slowly drift to multimode. Adjust the current a tiny little bit to get it single mode again and keep on doing this until it settles. You'll find that just waving your hand around the diode+heatsink or current source will thermally influence it, so try to avoid this, especially during an exposure.
Joe Farina wrote:Hi Thieu, yes thermal compound is part of the plan, I will be sure to keep it off the facet or window. By the way, I was wondering what you think of noise-detection methods for the 445nm diode. Do you think sampling the light output with a photodiode (and having an audible indicator, like with Wolfgang's noise-detector) is good, or would it be better to use something which generates fringes? Or maybe both? I have the feeling that the key to success with this diode will be in knowing when it's single-mode or not.

dave battin

445nm beam correction optic

Post by dave battin » Tue Aug 31, 2010 10:23 am

Thieu wrote:The problem with interferometric methods is that they can give false readings if the path length difference is close to a multiple of the diode cavity's optical length. You can compare that to watching the fringes in a bright region of the breadslices. The fringe contrast looks OK there, but the final holograms obviously don't.
Agreed! but an experienced holographer is used to the workings necessary to test a laser's coherent length, via leg differentials. If this condition is checked first (or “tuned in“) a good hologram can be made*.


If the laser is in multi-mode or shall we say not in "supermode" constant tight fringes will be seen pretty much anywhere you set one leg of the interferometer(not good), tune the laser to act like an hene laser with 12 inches of coherents and it should work!*





*thermal stabilizing is necessary as well!

JohnFP

445nm beam correction optic

Post by JohnFP » Tue Aug 31, 2010 11:03 am

John have you set the diode up in a Michelson interferometer yet ? By watching the fringes and and adjusting the power, you will quickly see where to set it to get nice steady fringes! watch them to drift for a good long while, give it a try please.
Sure Dave, I do still have my interferometer rail set up and I can easily do what you state. I was just anxious and had just an hour or so, so I threw something together just to try it.

Jeffrey Weil

445nm beam correction optic

Post by Jeffrey Weil » Tue Aug 31, 2010 1:36 pm

Hello Everyone,

I've posted this before but not with an image. This is the way I measure coherence. Not with math, speculation, educated guesses or anything but real holography. I find it more exact to the real world we shoot in.

Put some film in a divergent beam, place a ruler on the table in the same expanded beam. Make a holo and look at the ruler. Where it fades out, that's the actual usable coherence length for holography.

I've tried all the other methods listed here other than the spectrograph. I use gas lasers so tweaking the power levels to get long coherence doesn't exist.

Some of you might point out that since the master is tipped relative to the ruler the measurements won't be exact. Thats true but its pretty close and in a real world shooting situation a similar effect exists. Since the ref is hitting the plate at a non normal incidence the coherence length is being "used up" as it travels across the plate.

Feel free to change the angles to anything that makes the measurement easier if you want. Almost anything will give you an image.

While its not perfect, it is "real" as opposed to being inferred.

And before someone else points out that since the recording beam and the playback beam are both divergent the size of the ruler will change. True again but its a small distortion. I also know I didn't draw the beams coming out of the lens correctly. Don't be picky.
Attachments
coherence2.jpg
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Thieu

445nm beam correction optic

Post by Thieu » Tue Aug 31, 2010 2:06 pm

I assumed the something that generates fringes Joe was referring to was a fixed path difference interferometer, like the reflection from a glass slide that is used by some. With that it's easy to get false readings.

I agree that setting up a real Michelson that can scan over some distance is ultimately the most direct option. You're checking at which path difference fringes would be recorded, so which parts of the hologram would be visible if it were made at that moment in time. It can show things that won't show up on the spectrograph, like drift and coherence loss due to the linewidth of the single mode. However, not everyone's an experienced holographer and able or willing to set up a Michelson.

An additional problem with a Michelson is that quite a lot of light is reflected back into the diode. This diode is very sensitive to backreflections. Wler has some dramatic examples of this on his page. So it may become single mode when guided to the michelson and multimode without it. Using a modified michelson is actually one of the techniques in forcing lasers into single mode operation. Apart from the small reflection at the lens, the spectrograph doesn't have this problem.
dave battin wrote: Agreed! but an experienced holographer is used to the workings necessary to test a laser's coherent length, via leg differentials. If this condition is checked first (or “tuned in“) a good hologram can be made*.If the laser is in multi-mode or shall we say not in "supermode" constant tight fringes will be seen pretty much anywhere you set one leg of the interferometer(not good), tune the laser to act like an hene laser with 12 inches of coherents and it should work!*
*thermal stabilizing is necessary as well!

dave battin

445nm beam correction optic

Post by dave battin » Tue Aug 31, 2010 2:23 pm

simply using good practices these feedback issues can be avoided, by using a larger distance to laser and slightly tilting lenses to steer back reflections away ............

Jeffery i never said tune up a hene to get good coherents........... when checking the coherent length of a proper tuned diode laser you will find a fringe zone VERY much LIKE A HE-NE Laser, if your diode acts like this, then there is a good chance your diode will now make a hologram. If you want to try the crap shoot and see all day long ,if it works or not, that’s ok with me. :lol: The interferometer is to tune/check you laser so it WILL make a hologram. not as the definitive answer all .....


this diode will easly shoot a hologram 10 inches deep! i know cause ive seen it! :shock:

Jeffrey Weil

445nm beam correction optic

Post by Jeffrey Weil » Tue Aug 31, 2010 3:26 pm

Hello Dave,

I knew what you meant. I didn't think you were saying to tweak my gas lasers power. Sorry if that came out wrong. I was just saying I don't have to set up an interferometer to get to a nice interference pattern.

I agree about the interferometer or the spectrograph to get to the best single line the diode can produce. Thats the quickest way to find the sweet spot on the power levels. But after that is found, I would go with my setup to find the real world coherence length. Moving mirrors in an interferometer is not a real recording. Also shooting a 3d job is great to see if it can do it but if you just want an actual measurement of real life coherence length nothing is better than the ruler.

I should be getting my 445 diode setup pretty soon. A friend of mine that I do laser shows with sometimes is making up proper driver/tec boards. The boards have been produced so its almost done. I don't even want to make holo's with it, just tweak my table without firing up the big he/cd. The metal vapor lasers are not to be fooled with. You just can't fire it up, right away mess with your optics and stop after a half hour of playing around. They take 5 or 10 min just to get a beam. Then you shouldn't turn them off for at least an hour or they will have a hard time starting up again. It also costs about 10 bucks an hour to run. The diode is pretty much free to run.

I'll still use the he/cd for real work though. With embossed stuff the final master has to be perfect. The diode just doesn't have the beam quality for that. Plus, embossed stuff is so bright things that would never be seen with other materials will show up really well. Everything has to be perfect.

JohnFP

445nm beam correction optic

Post by JohnFP » Tue Aug 31, 2010 6:12 pm

Boy Jeffery, I remember those days. You're bringing back memories.

Hae a good day!

Joe Farina

445nm beam correction optic

Post by Joe Farina » Tue Aug 31, 2010 10:15 pm

Thieu wrote:The method I'd recommend would be to build a simple spectrograph. It'll give you 100% accurate readings. It's much much easier and quicker to build than you might think and requires no electronics or expensive parts. You only need a simple lens (the one that came with the diode is probably OK) and a grating. I haven't tested this myself, but a thin radial slice of a DVD will probably do as well, since it works as a grating with about 1200 lines/mm. You may have to keep it flat by glueing it to a glass plate. Use the lens to create a sharp image of the diode's output at a large distance. 10 meters will be ok. Of course you can use mirrors to fold the path if space is limited. Make sure the diode is oriented in such a way that its highest divergence before the lens is in the horizontal plane. This will give you the smallest spot size in the horizontal direction in the projected image. Now bounce the beam of the grating/DVD, placed just after the lens and take a look at the first order. Make sure the lines of the grating are in the vertical plane, so the wavelength seperation is in the horizontal. Because the mode spacing for this diode is around 0.1 nm, the spacing of the modes in the image will be around 0.26 mm per meter distance. So at a distance of 10 meters, it will be 2.6 mm, not that hard to see. Because the optical system is less than perfect, instead of clear lines, you'll probably see a couple of dots seperated in the horizontal plane. There may even be some overlap, but that's no problem. If the dots come and go with changing the current, you spectrograph is working OK. The laser will be working single mode when only one dot is visible. It requires some fine tuning with the current to get it like this and over time it may slowly drift to multimode. Adjust the current a tiny little bit to get it single mode again and keep on doing this until it settles. You'll find that just waving your hand around the diode+heatsink or current source will thermally influence it, so try to avoid this, especially during an exposure.
Thanks a lot for sharing that method in detail, Thieu.

I have not personally tried the grating method to show how the mode behavior changes with laser diodes. I was watching the videos on your webpages. I was wondering if that's the same kind of effect that will be observed when using a grating as you describe above. Kind of a back-and-forth blinking effect.

As for Wler's electronic noise-detection system, Ahmet showed me how to pare-down that circuit to only a few components, if the LED portion of the circuit is eliminated. Actually, the only part that I consider to be necessary is the audible portion, and not the LED. An audible indicator is better in my opinion, becuase it's like an alarm going off, and you don't need to be actually looking at the mode-patterns on the wall to know it's mode-hopping.

I think perhaps the grating method and the electronic "noise detection" method are possibly detecting the same thing. When a laser diode is mode-hopping, a weak AC signal is generated when the beam is incident on a photodiode. With the grating method, the "blinking" seems to be a kind of AC signal, shown in visual terms. I guess if you placed some kind of large photo-detector on the wall (where the grating projects the beams) you could convert the "visual" grating method into the "audible" noise-detection method. Does that make any sense? Maybe I'm making an erroneous analogy.

wler

445nm beam correction optic

Post by wler » Wed Sep 01, 2010 6:12 am

Well unfortunately things are not simple - it can happen that multimode operation is accomagnied with no noise whatsoever. Noise occurs when modes jump discretely (mostly in an ECDL) or when there is some chaotic instability due to mode competition. But there can be a few strong modes simultaneously without any accompagnying noise! This is a typcial behavior at currents just below the chaotic regime. For the 445nm diodes, this is around 250mA, give or take.

That's the main problem in this game and trying to solve it keeps me busy already since a long time (solving means to somehow ensure/detect single mode operation without external machinery). In fact a grating spectrometer is quite, but not 100% reliable in detecting multimode because it cannot resolve very close lines. A scanning interferometer is better in this respect, but more diffucult to use and it tends to produce backreflections.

To make things worse, things are not even that clearcut, because when increasing power there can be more and more initialy weak side modes - a spectrum will never be entirely clean, so at what point should one draw the line and tell, this spectrum is good and that one is bad? Typically in an ECDL there are always side modes beyond 100mW or so. I have no good idea how spectral purity is related to hologram quality - can a side mode at 10% level be tolerated, does it lead to a noticeable reduction in contrast or not?

My findings so far show that no matter what, things tend to become messy above 60-80mW - irrespective of a grating or glass plate feedback. The feedback makes things much more stable and manageable below this power, it reduces the threshold current as a function of the strength of the feedback, but it also reduces the power at which things become unstable. Somehow these diodes don't want to run stable SLM beyond this level by all means!

Thieu wrote:You'll find that just waving your hand around the diode+heatsink or current source will thermally influence it, so try to avoid this, especially during an exposure.
Yes I think the diode must be very well thermally shielded (closed box) otherwise this won’t really be reliable; I just wonder how poeple get away without active temperature stabilization ;-) Even a few 1000ths of degree of change can induce a mode jump.

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