Check fringes laserdiode

On: Tue, Apr 16, 02 03:27:53 PM

Ton wrote:

My way to shoot holograms is now: close behind the outlet of the lasermodule I 'steal' about 10% of the laserbeam with a glasplate. This 10%-beam I use in a Michelsen interferometer and project the fringes on a white plate. The interferometer is carded off from the holographic setup. The 90%-beam is used to make the holograms. In this way you can perfectly time your exposures, since you can see when the fringes stand still and have maximum contrast. As I have observed a maximum exposure of 5 minutes should be no big deal anymore.


Frank - Wed, Apr 17, 02 02:19:21 PM

This is a very interesting idea -- similar to stealing part of the beam for an "active" fringe-locker, but passive in that it is visible to the eye (as an indicator of fringe stability periods on the white card) without any electronic correction/compensation. Not sure how you have this set up, but having the beam paths of the interferometer the length of the entire table would be valuable as well -- and especially useful right now (spring) in the N. Hemisphere -- as temps. and humidity begin to rise, and amateur/hobbyist tables begin to "bend and twist" as a result (especially wood-based tables). Overall, a great little way to catch those "moments" when everything comes together . . . and it's time to release that shutter. Frank

Sergio Oliveira - Fri, Apr 26, 02 07:27:53 PM

I use a different aproach, instead the Michelson interferometer, a simple glass etalon will have the same effect, used with the diverging bean withouth lens, the output is the same 10%, the advanced feature is the superior interferometer stability due the etalon itself, any fringe movement and noise can be monitored.

Ton - Fri, May 10, 02 08:38:26 AM

Sergio, please describe you glass's not really clear to me...

Sergio - Sun, May 12, 02 01:04:40 PM

As your glassplate, the etalon steal a 10% laser beam at an angle that refects the beam back. This is a plane parallel glass plate 2mm thick that faces act as a Fabry-Perot interferometer, at one position you will see the Newton rings projected on the wall by the interferometric faces, any wavelength sweep and noise could be monitorated pretty well, all those external "movements" are filtered out, the laser woth etalon itself became as one integral piece, with this, I have the possibility to monitorate the effect of air currents on wavelength stability of my 30mW laser!Now I know the parameters for a modereate exposure time, >10min, and the laser settle time.

Tom B. - Sun, Apr 14, 02 03:47:29 AM

Sergio - thanks. - I'd seen this effect before but hadn't thought to look at how useful it would be for stability checks. Some observations: Microscope slides (0.8 mm) and cover slips (0.13 mm) work pretty well. The circular bullseye pattern is reflected back toward the laser. A white card with a hole for the laser makes a nice projection surface. The fringe spacing is proportional to glass thickness (thinner = wider spacing) and distance from the laser. If the glass faces are not perfectly parallel, the bullseye is not centered on the laser source, and rotates around the source when you rotate the glass. Interestingly, tilting the glass does not affect the position or appearance of the pattern much. I'm just getting an LNCQO5 diode going, and noticed that above 30 mW output, fringes started to disappear with the 0.8 mm slide but were still visible up to 50 mW with the 0.13 mm cover slip, so as expected the thickness of the glass determines the coherence length that it is sensitive to. I tried a 5 mm piece of glass, and saw the same thing as with 0.8 mm. I'll have to set up an interferometer to see how coherence behaves over longer path differences. I'm hoping that the glass plate will be a useful diagnostic substitute for an interferometer.

Sergio - Tue, May 28, 02 10:44:15 PM

This is an interesting instrument, precisely the Fabry Perot interferometer..all the facts that you related are well studied in this instrument, according to one formula, the etalon thickness determine the laser longitudinal linewidth, you could determine not only the laser longitudinal mode quality but all noise effects that will affect the diode sensitivity. The experiment that you related is very well delined, I think with the etalon "physics" you could manipulate precisely this instrument to precisely stabilish the coherence for diode holography. The tilted etalon is common used in dye laser selectivity, if you are interested in the etalon mathematics I could indicate one book in laser that is useful.

Colin Kaminski - Wed, May 29, 02 11:54:45 AM

I have noticed the line width changes just before the diode goes in to an unstable zone. (mode hop) Looking through my books I do not have any Optics books with the math to sort out linewidth from a etalon. Can you post the title of the book you would recommend?

Tom B. - Wed, May 29, 02 03:36:59 PM

Do you mean spectral linewidth? How are you measuring this? Or are you maybe referring to fringe sharpness/contrast or fringe spacing?

Colin - Thu, May 30, 02 11:15:13 AM

I think I mean spectral line width. What I am measuring is coherence length. Am I mistaken to think they are really the same thing? Fringe spacing is related to frequency. With my temperature controller I can adjust the fringe spacing by changing temperature.

Tom B. - Thu, May 30, 02 10:04:22 PM

OK, yes, linewidth and coherence length are reciprocally related. I guess I was confused since the thread was discussing reflected glass plate fringes and it wasn't clear how you would get linewidth or coherence length beyond the thickness of the glass from observing these. Unless you just meant that the fringes start to fade before mode hopping sets in.

Colin - Thu, May 30, 02 11:23:24 PM

When I read Sergio's post I think he implies you can either measure or adjust the line width with a etalon. I would be very interested in figuring this out.

Sergio - Thu, Jun 06, 02 06:28:49 PM

Colin, for this external etalon configuration you could only monitorate (precisely) the linewidth and mode stability in respect to quantity and quality, the fringe contrast can display the general laser noise too, linewidth is "more" related to mode numbers, practically for holography only one mode is useful for good coherence length, but you could find two or tree in HeNe laser systems. I think that well defined circular fringes, 0,1,0,1,0.. will define the longitudinal single mode, the transversal is always TEM00, for a good beam quality,in HeNe lasers the fringes are not equaly spaced in multimode lasers. Fringes fades would be related to current noise, I suppose, anyway the etalon will display this noise at laser diode thermal building up. The book that is very dictating in this respect is: "Solid State Laser Engineering" by Walter Koechner, Springer Verlag ISBN: 0-387-53756-2 See specially chapter 5.2.

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