Some experiments with the blue LD's

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

Some experiments with the blue LD's

Post by Martin » Sat Jul 31, 2010 6:41 am

Thieu wrote:Because space is so limited where I live now, peaceful coexistence of holographic plates and dinner plates is the only option in the kitchen at the moment so dichromate is a bad idea. That's why I'm particularly interested in FEG again. The last thing I remember is you were using ferric ammonium oxalate and developed with H2O2, is that where you both settled on or did you explore other salts/processing after that?
Hi Thieu - good to hear you might tackle FEG soon.
It remains to be seen if the ferric salts show similar levels of light sensitivity as the dichromates at 445nm (in some cases speed at 405nm was considerably higher for FEG than DCG).

The specific salts I did try successfully, were ferric ammonium citrate (FAC), ferric ammonium oxalate (FAO) and ferric oxalate. FAO proved to produce the best results. I'm not sure if FAO could be considered a "kitchen chemical" (though definitely better than the dichromates).

To this date at least two issues remain ambiguous
1) the choice of gelatin. We never systematically tested what kind of gelatin performed best etc. As of now a simple (swine) food gelatin, produced by Gelita, has become kind of a standard.

2) layer thickness may be another important thing to consider. Thin layers seem to do nicely, whereas thicker layers become prone to uneven development.

Thieu

Some experiments with the blue LD's

Post by Thieu » Sat Jul 31, 2010 6:44 am

@wler: Ah, OK, now I think I see what you meant in your previous post. So if the laser is lasing in a single longitudinal mode it must also be showing the spatial pattern corresponding to a TEM00 beam? Then it would indeed be very easy to determine singlemodedness (withing the limits you mentioned). I'll see if I can set the spectrometer up to verify this. But it may take a while, I put some 30 plates in bleach yesterday and will be mold coating them probably this weekend. We all know what happens after that, it may take weeks before I reappear again in the world of the living... :wink:

wler

Some experiments with the blue LD's

Post by wler » Sat Jul 31, 2010 7:21 am

Thieu wrote:@wler: So if the laser is lasing in a single longitudinal mode it must also be showing the spatial pattern corresponding to a TEM00 beam? ...


Not TEM00 but a simpler structured pattern. According to the article, the various supermodes get phase locked with fixed relative phases and this translates into the fixed, characteristically banded far-field interference pattern. The pattern washes out or disappears with the modes cease to be coherent, or if several such modes with different frequencies appear.

Here a brief excerpt from the abstract:
We present experimental evidence that the single filaments tend to be phase-locked with defined phase offset and build up a so called supermode. Depending on driving current a coherent or incoherent superposition of different supermodes can be observed, which has a significant impact on the corresponding lateral far-field pattern.
Thieu wrote:...it may take weeks before I reappear again in the world of the living...
Lol... happened to me after the blue diodes came out...

Thieu

Some experiments with the blue LD's

Post by Thieu » Sat Jul 31, 2010 8:26 am

@Martin, FAC seems to be in stock at the place where I order my chemicals from, both the brown and the green variety. They may be able to order FAO, but after comparing the MSDS for both I'll stick with FAC... I'll make some plates with both gelatines I have (dr oetker and an unknown type). As for film thickness, how thin would thin be? The last time I made plates I used mold coating with 10 g/100ml gelatin and a single scotch tape spacer. With a reflectometer I measured them to be about 5-15 um thick. Would that be ok you think?

Martin

Some experiments with the blue LD's

Post by Martin » Sun Aug 01, 2010 3:37 am

Thieu wrote:@Martin, FAC seems to be in stock at the place where I order my chemicals from, both the brown and the green variety. They may be able to order FAO, but after comparing the MSDS for both I'll stick with FAC... I'll make some plates with both gelatines I have (dr oetker and an unknown type). As for film thickness, how thin would thin be? The last time I made plates I used mold coating with 10 g/100ml gelatin and a single scotch tape spacer. With a reflectometer I measured them to be about 5-15 um thick. Would that be ok you think?
I guess that's fine for initial tests. The brightest holograms with large bandwidths may be rather 5um or even below (though this statement is entirely speculative on my part - we were unable to measure layer thickness consistently).

As for FAC I never tried the brown stuff, since green FAC was said to provide better speed. However, brown FAC might be worth a try anyway, assuming that it's likely to absorb longer wavelengths.

wler

Some experiments with the blue LD's

Post by wler » Tue Aug 03, 2010 6:55 pm

I ran a first test with a (slightly wedged) glass plate approx 1.2cm from the collimated diode, and can confirm wide spread single mode operation beyond 350mA, or 150mW. This is quite similar to an ECDL, both setup have here and there some, however mostly weak, side modes whose effect or suppression must be investigated furhter. It seems that multi-mode operation can be quite reliably detected by noise in the output. Stay tuned ;-)

Thieu

Some experiments with the blue LD's

Post by Thieu » Wed Aug 04, 2010 7:24 am

That's great news! More power is always welcome.

In the meantime I've checked the near field pattern at various currents, it indeed looks filamented like in the article. There is however one big difference. In the article the pattern is highly asymmetric, while in the diode I tested it's very symmetric. It was very hard to make a video of it, because of the huge contrast and intensity differences. But this is what I saw:

- at low currents <300 mA, the pattern is symmetrical and looks gaussian. It spreads out over the two central quarters of the stripe. Like this: __/**\__
- at higher currents two extra spots appear on both sides: *_/**\_* This nicely explains the pattern in the far field. It's again another gaussian with a double slit pattern (i.e. a sine) superimposed. And this is exactly what you see.
- at even higher currents more spots appear, but I couldn't discern the fine structure.

What's also interesting is that I measured the stripe width to be only about 10 um instead of 100um.

I made the first holograms yesterday and after settling down after about an hour the laser was stable for the rest of the evening. During the warm-up I did have to adjust the current a mA or two once in a while to stay in the stable region. The spectrograph is a great tool for checking if everything is alright.

The holograms though were bad as usual :-( There must be something I'm doing wrong cause the holograms come out either dim or milky, depending on my processing temperature. But that's more a DCG problem, I started another thread about it to hopefully get some help.

Also working on the fast axis correction. If I can get that to work, it will mean a big improvement in the effective power on the plates because the beam is so elongated now.

wler

Some experiments with the blue LD's

Post by wler » Wed Aug 04, 2010 7:37 am

Thieu wrote:What's also interesting is that I measured the stripe width to be only about 10 um instead of 100um.
Indeed very interesting and good to know. Some rough estimates based on the opening angles pointed in this direction, but there were so many contradicting statements/claims about this on the web that a clear picture didn't yet emerge. Thanks for clarification.

dave battin

Some experiments with the blue LD's

Post by dave battin » Wed Aug 04, 2010 11:11 am

Thieu wrote: During the warm-up I did have to adjust the current a mA or two once in a while to stay in the stable region.
i too had this same problem, which i have solved (temporarily),by simply leaving it on all the time! :doh:

These diodes should have a life of 20,000 hours! That’s like letting it run for 3 years straight!

Paulos

Some experiments with the blue LD's

Post by Paulos » Wed Sep 08, 2010 10:12 am

Last weekend I put together a temperature stabilized 445nm laser diode.

I tried a very a simple setup to check out if the beam is single mode and it seems to work out:
I used two glass plates of different thickness to reflect the expanded beam separately and watched the interference lines.
Image
If both glass plates reflect solid interference fringes, there is a good chance that the laser is single (longitudinal) mode.

http://holographyforum.org/phpBB2/viewt ... 042#p57925
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.<<

For the above setup, If the path length difference of the reflections of one plate is a multiple of the cavity's optical length, it is most unlikely (but not impossible) that this is also valid for the second glass plate at the same time.

My test results:
The fringes disappear or are of low contrast at most mA levels either. The beam is not single mode there, or the coherence length is two short.

There are regions where both sets of fringes are of high contrast though.
For my laser diode I found out two potentially single mode regions:
225 mA, T=15ºC (12mW) and
325 mA T= 15ºC, (85 mW).
(The measured output power is indicative only, my laser power meter is calibrated for 633nm.)

Image

The question was if the laser is indeed single longitudinal mode in those regions and what is the coherence length.

So, I made a denisyuk at 225mA. The hologram came out without any breadslices,
I increased the current to 335mA and made a second denisyuk. It also came out slice free.

Then I put an object 10 cm and another object 20cm behind the film, (path difference = 20 and 40cm correspondingly) and a made a third hologram at 335mA.
There is a faint - but not sliced image of the first object, no image at all of the far object.
That means that the coherence length was slightly above 20cm but less than 40cm.

Image
This is a photo of the second hologram at 325mA.
Silver halide film.
Note that the beam is not spatial filtered, parts of the hologram are milky due to overexposure.

Locked