Some experiments with the blue LD's

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

Some experiments with the blue LD's

Post by dave battin » Wed Sep 08, 2010 1:11 pm

Great work Paulos! Its nice to see the casio diodes are beginning to take a hold of holography now!

BobH

Some experiments with the blue LD's

Post by BobH » Wed Sep 08, 2010 1:27 pm

What a great technique for monitoring for single mode operation! :clap: :clap: :clap:

Thieu

Some experiments with the blue LD's

Post by Thieu » Wed Sep 08, 2010 4:21 pm

That's a very clever solution Paulos, thanks a lot for sharing. Looks like wler's sleepless nights are over. ;-)

JohnFP

Some experiments with the blue LD's

Post by JohnFP » Wed Sep 08, 2010 8:47 pm

Paulos, that is simply a fantastic way of checking. I got my diode mounted the opposite way in the heat sink and let it run for about 2 hours. Then set up the two pieces of glass and wow, I could dial it right in to get max power with both sets of fringes still showing. Can't wait to try a hologram.

DJ Mathson

Some experiments with the blue LD's

Post by DJ Mathson » Wed Sep 08, 2010 9:26 pm

What kind of drivers are you guys using for the diode?

Thieu

Some experiments with the blue LD's

Post by Thieu » Thu Sep 09, 2010 6:13 am

At the moment I use a very simple current source based on a cheap lm317. It's a three pin device that only needs one extra resistor to set the current. Instead of using a pot to set the current I use a switchable bank of metal foil resistors. Their temperature stability is better than that of a pot. It's very basic but it works.

The circuit is far from optimal though. Its efficiency and stability have a lot to be desired. It's only because the stability of the diode+heatsink is just as bad that you can get away with it. If the diode and heatsink need an hour to stabilize then the circuit has plenty of time to do so as well. Waving your hand around the circuit will probably cause mode hops, just like with the diode+heatsink.

If you're going to build something new and are willing to spend a bit more money, another type of current source is a much better alternative. Ahmet and I are working on an ECDL and Ahmet has designed a good current source. Wler has another new design as well. Of course I can't speak for them, but they may be willing to share/sell their designs/boards/kits.

Paulos

Some experiments with the blue LD's

Post by Paulos » Wed Sep 22, 2010 6:05 am

The coherence length, hence the line width seems to strongly depend on the current level.
I set up an interferometer with a path length difference of 80cm.
Although the beam is single mode at both 225 and 335mA, the fringes at the lower current level (225mA) were of very high contrast,
whereas there aren't almost any fringes at 335mA.

I use a PID1500 temperature controller from "Wavelength Electronics" and a self made current controller based on a LM317.
As Thieu said, this is not optimal but it works. I am using a combination of switches and a pot, for coarse and fine current adjustment.

wler

Some experiments with the blue LD's

Post by wler » Wed Sep 22, 2010 6:46 am

Paulos wrote:Although the beam is single mode at both 225 and 335mA, the fringes at the lower current level (225mA) were of very high contrast,
whereas there aren't almost any fringes at 335mA.
Which means that there is no single mode operation there, otherwise there would be good contrast!

It seems that there is a magic barrier at about 230-260mA above which several supermodes lase, and this is true also for all sorts of ECDLs. This cannot be easily seen by sampling the spectrum at a single spot., rather different spatial, transverse mode components of the beam can correspond to different, mutually incoherent modes. I made a brief summary of my findings here.

As for my driver board, I am about completing the final stability measurements of the first production sample and all looks fine so far.

Thieu

Some experiments with the blue LD's

Post by Thieu » Wed Sep 22, 2010 5:19 pm

Great vids on your site wler, unfortunately it looks like the diode behaves as expected from earlier results. I made quite good progress on collimating the beam and removing the circularity and astigmatism using cylindrical lenses. Do you think it's worth the effort to test an ecdl setup with this improved beam?

Paulos

Some experiments with the blue LD's

Post by Paulos » Wed Sep 22, 2010 5:35 pm

wler wrote:Which means that there is no single mode operation there, otherwise there would be good contrast!
But the path length difference of the interferometer was 80cm,
and the hologram at 335mA came out with limited depth (coherence length less than 40 cm) but not banded.
I would rather say that the laser can run single mode at some higher currents, but the linewidth (FWHM) becomes broader.
It is not surprisingly for such a short cavity.
As for my driver board, I am about completing the final stability measurements of the first production sample and all looks fine so far.

Wler, if you have some drivers for sale, I am interested in having one or two.

I am more than satisfied with your laser light meter,
http://argonholo.webhop.net/electronics/lightmeter.html
it is perfect for holography, absolutely reliable, with a very high sensitivity.

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