Hologram video from a mode hopping laserdiode

Kris Meerlo · Post by **Kris Meerlo** » Sat Sep 01, 2007 10:47 am

John you asked for it here is a video.

A video off a "mode hopping " hologram cause by a laserdiode.

I made the hologram a couple off years ago.

look at:

http://www.youtube.com/watch?v=hf07F9tpsWw

Joe Farina · Post by **Joe Farina** » Sat Sep 01, 2007 11:56 am

Wow, that really is a graphic illustration of countour-banding from a short coherence-length laser

JohnFP · Post by **JohnFP** » Wed Sep 05, 2007 9:27 am

Ok, are mode hopping and short coherence length the same thing? This is the second reference I have seen that seem to use the two interchangably. I thought a laser could have a very long coherence length but could mode hop, ie. jump to another very close wavelength which sometimes could be controlled by controlling the temperature of the cavity. Whereas a short coherence length laser may not mode hop at all.

Joe, please explain!

Thank you!

Joe Farina · Post by **Joe Farina** » Wed Sep 05, 2007 12:01 pm

John, as a holographer, I only care about coherence length. I define coherence length as the useable depth available for the image. (This is not the physicist's definition by the way.) The way I determine coherence length is by making a hologram. You can see a perfect illustration of coherence length in the video provided by Kris. As you move from the front of the image towards the back, a "multitude" of coherence lengths can be seen (the "slices" of sliced bread). It comes in, then goes out, over and over again. With Kris' laser diode, the coherence length is very short indeed. As to the question of why a laser diode does this, I will leave this to the physicists, as it is beyond me, and I have no desire to investigate it. But the resulting coherence length is graphically obvious.

You would see the same thing with a HeNe if you make a really deep hologram (and the deeper parts are well-illuminated). Instead of having alternating bright/dark bands that are 1/4 inch wide (typical for a laser diode), you will have alternating bright/dark bands that are (say) 6 inches wide. The transition among the light/dark bands is not sharp, of course, and this would be more obvious with the HeNe. The bright bands would be brighter at the center, and gradually darken until they go out, then the cycle will repeat. Or so I've been told.

JohnFP · Post by **JohnFP** » Wed Sep 05, 2007 12:08 pm

Ok, thanks Joe.

Colin Kaminski · Post by **Colin Kaminski** » Wed Sep 05, 2007 5:48 pm

John is should be noted that you can get finges in an interfermeter outside the coherence length. This is the cause of the sliced bread pattern. Double firing a pulsed laser also makes a sliced bread pattern unlice there is a injection seeding laser supplying the initial frequency.

JohnFP · Post by **JohnFP** » Thu Sep 06, 2007 1:31 pm

Well, it just looks like I will have to mount one of the legs of the interferometer on a z-axis translation stage. I think I have an old microscope I can disect or incorportate as one of the mirrors in the interferometer to give me a pretty nice z-axis movement.

Man, just goes to show how easy it was working with my Ion laser with etalon. I am so new to this mode hopping stuff.

Colin Kaminski · Post by **Colin Kaminski** » Thu Sep 06, 2007 2:15 pm

Yes, what you find as you move the mirror that you will lose the fringes. With a very short beat frequency it is a little hard to find. The laser pointer had a beat of about 2mm!

JohnFP · Post by **JohnFP** » Thu Sep 06, 2007 2:39 pm

Is an intra-cavity elton the only way to prevent this "dual line" in a DPSS laser?

Joe Farina · Post by **Joe Farina** » Thu Sep 06, 2007 3:13 pm

JohnFP wrote:Is an intra-cavity elton the only way to prevent this "dual line" in a DPSS laser?

John, I really don't know much about DPSS lasers, I just know that certain types have been reported to have good coherence properties for holography. Having said that, I would hazard a guess that none of the high-coherence DPSS lasers (like the 315, Uniphase Microlaser, Coherent 532, and Melles 58-BLD-605) use etalons. Perhaps someone can correct me if I'm wrong. I believe the 315, for example, has birefringent optic within the cavity, and the 532 series has a ring cavity.