Hologram video from a mode hopping laserdiode

[JohnFP]

Sorry to be a pest. But in the Video that Kris made at the begining of the thread, is that a long coherence length laser with a mode hop, a short coherence length laser without a mode hop or a short coherence length laser with a mode hop?



I guess what I am striving for, is how to recognize from the fringes on a hologram if the laser hopped or the laser has repeating short coherence length. Or are the two indistinguishable from fringes on the hologram.



Thanks in advance.

[Joe Farina]

John, in my opinion this is a complex and difficult subject. When things get that way, I try to make the most basic assumption, and go from there. My assumption is that those lines, that banding or whatever you call it, is the result of a broader-than-optimum band of closely-spaced frequencies emanating from the laser. This is synonymous with my idea of coherence length. The thing that relates uniquely to holography in all this is that we are talking about coherence being maintained over considerable time periods, i.e., exposure times.



Now the complex part begins, and all of your questions relate to what follows. Mode-hops are highly irregular and unpredictable events, while a multi-mode laser (so I assume) is constantly putting out a broader range of closely-spaced wavelengths. But even then, I would tend to think that the mode behavior is chaotic and not stable (this is just a guess). Certain types of single-mode laser diodes are good for holography when they are not mode-hopping. I would guess that Kris' laser is of this type, but that it was mode-hopping for much, if not all, of the exposure time.



The time factor is crucial in the way I look at things. If Kris had a good single-mode diode and it did not mode hop during the length of the exposure, then he would have a long coherence-length laser, as proven by a particular holographic exposure. But as shown in the video, it did apparently hop, and it was proven to be a short coherence-length laser by that particular exposure. Diode lasers are notorious for changing their minds about having long or short coherence-lengths, and each exposure is a new "proof." That's why I don't like them.

[JohnFP]

Man, this is why I love holography believe it or not. Yet another avenue to explore and learn about.



So, to summarize.



The DPSS can actually be lasing at a single line and have a very nice coherence length. Then the temp starts to go up or whatever and another line starts to compete. At a certain point this competition starts to compromise the fringes and can shorten the coherence length of the laser, possibly without mode hopping. Then again, the the second line may go into a competition with the first line and completely take over and suppresss the first line so fast that we get a mode hop without a detrimental decrease in coherence length of the first line or the second line.



So, there may be a temp or set of variables that allows the laser to lase in a single line with most of a possible second line suppressed enough to not cause a problem. And there may be a temp or set of variables that simply keeps the two lines in close competion, possibly hoping back and forth or not hopping but both lasing enough to cause problems.



I see, possibly lots can happen in these lasers and much to think about.



I am glad your entertaining the questions I have as I would like to try to understand as much as I can (and believe me, I have been rereading all prior post on this subject as much as I can) before I do anything other then externally cooling and the real time monitoring. Hopefully, one or both of those will be enough.



If not, that skull with the fringes looks pretty cool and I may just have to pick subject that "NEED" those additional fringe lines on them. Then all can say....., yup, that's another hologram made by JohnFP, it has his trademark on it....LOL



So, does the mode hop and short coherence length fringes look the same on the final hologram or can they be distinguished?



Peace!

[Joe Farina]

So, does the mode hop and short coherence length fringes look the same on the final hologram or can they be distinguished?

I can't answer that from direct experience, but I assume it would look similar. If your argon laser is operational, maybe you could test this yourself by making a deep Denisyuk hologram without the etalon.



I have a paper (which I can't find) that describes holographic contouring (intentional) using a dye laser. The authors were able to vary the bandwith of the laser with some precision. The smaller the bandwidth, the bigger the contour fringes. Large bandwidths resulted in very small contour fringes. With diode lasers, I have seen both kinds, sometimes 1/4 inch, sometimes 1/16 inch, either faint or strong. I assume faint fringes show up when the mode-hopping only happens for a short time during exposure, while pronounced fringes appear when it happens for much of the exposure time.

[JohnFP]

Joe, the Argon is off line. Doubt I will be getting it back on line for quite some time. But I will keep you posted with developments with the DPSS I am playing with.



I do know that I can see 200 fringes with the interferometer and they are not blurry over that distance, so I can pretty confidently say I have almost 100um of coherence ....LOL