Questions about two color holography

[Holomark]

I have a new laser arriving - a Melles Griot LHP 927 from Phil and Sam. This will be going in my lab with my C315. So now I have a myriad of questions about using the two lasers together to make two color holograms. Not really sure in what ways using two different wavelengths is different from using a single laser. For example for exposure do I add the exposure from each diverged beam or does each wavelength need the "appropriate" exposure. For Example if plate specification is 400 Uw and I have 100uW at 532 and 70 uW at 632 (at the plate) do I count this as a total of 170 for a 2.35 sec exposure or do I want 400uW at each wavelength so I would then need 4 sec of 532 and 5.7 sec at 632. Or is the sensitivity different at different wavelengths requiring additional computations? And does it matter if exposure with the two wavelengths is simultaneous or in series?
Any other tips for someone about to add a second laser to the lab?
Thanks in advance
Holomark

[Joe Farina]

With panchromatic silver halide, I would expect the manufacturer to list the exposure requirements for at least 3 common wavelengths. Are there any panchromatic emulsions out there? I suppose PFG-03C is still available. Use those figures as a starting point for your exposures.

The most difficult hurdle I experienced with two-color holograms was getting both beams through a spatial filter, for a fairly uniform yellow spread beam (532 and 633 combined). Your red light will be most precious, so only waste 532 light. The method I used for beam combination was a dichroic mirror (reflects red and transmits green). The beams intersect at 90 degrees, the green light goes straight through from behind, the red strikes the front surface of the mirror (where the green beam is going through). The dichroic is at 45 degrees to either beam. Thus, the output is two co-linear beams. (I posted some pictures of my layout in the General section, the green beam goes straight through to a hole in the wall to the other room, while the red beam is reflected by the dichroic at 45 degrees to become co-linear and go through the same hole in the wall.) Two tricks to help the alignment are 1) make sure the intersecting beams strike exactly the same point on the dichroic mirror, and 2) make sure the beams are still co-linear at a long distance away, in my case the wall of the lab is about 15 feet away from the dichroic.

The difficult part is getting the green beam to approximate the red beam in terms of diameter and divergence. The beams should not have identical diameter to get through the spatial filter nicely, as I recall the red beam needs to be trifle larger (hopefully I don't have this reversed). The divergence of both beams also needs to be similar. The green divergence from a C-315M will be larger than a HeNe, so the green beam should be "tamed" to match the superior beam quality of the HeNe. I did this with three lenses in a zoom telescope arrangement on the 315, following the helpful advice of Ed Wesly.

Then, the combined beam needs to go through a spatial filter. After this, two-color holography should be a piece of cake, assuming you have the right recording material and know how to use it. You can get yellows, browns, greens, reds, just about everything except a clean white or anything with blue.

[142laser]

Thanks Joe! Sam and I have the laser part under control but I am a novice at holography. I agree with your advice. Choose the spatial filter for the beam spread you want for the HeNe and condition the beam of the 315M to have the correct beam diameter to get good throughout and matched divergence to the HeNe; there is more green power to waste than red. The LHP 927 he has is outputting over 45 mW. I use the same method to combine beams for demonstrations when needed; having a long room helps with making them colinear .

[Joe Farina]

The LHP 927 he has is outputting over 45 mW.

That's an awesome amount of HeNe light to have available. I don't have nearly that much power (about 33mW). My SP-127 is getting older. Even when new, I wonder if the 127 ever got up to 45. I'm not familiar with the LHP 927.

[142laser]

Yes the SP 127 can also output 45-47 mW and I think 50 mW is possible, new hot tube, clean windows and optics, perfect alignment...

The Melles Griot LHP 927 and LHP 928 are internal mirror polarized lasers with cylindrical hard sealed tubes. The 927 is a heavy wall cylindrical case, the 928 is a thinner wall cylinder in a rectangular case. All of these lasers are hard sealed and last a long time.

[Joe Farina]

Thank you, Phil. The day may come when my 127 needs replacement.

[Holomark]

Finally got a chance to turn on the new laser (Thank you Phil!). Saw 41.57mw readout on my handheld Lasercheck power meter. I will check again tonight as I may have input the wrong wavelength when I was playing last night. Interestingly, after I measured my 315M, then measured the HeNe without changing the wavelength I got a reading of 81+mw. Is there a reason the HeNe would measure so high at the "wrong" wavelength (523)? BTW the 315M was putting out 111mw.

[Joe Farina]

That's just normal behavior for the LaserCheck, don't worry about it. HeNe's need a long time to warm up to reach maximum power, if the alignment is correct. With a SP-127, it probably takes around 2 hours, maybe longer. From what Phil said, I assume your mirrors are sealed to the tube (is that the same thing as "internal" mirrors?). I assume such lasers would be much less likely to go out of alignment during shipping. External mirror HeNe's such as the 127 are rather notorious for going out of alignment.

There have been times when I've wondered about the accuracy of my LaserCheck.

[Dinesh]

... just about everything except a clean white or anything with blue.

Like this?
Full colour H1 (laser transmission). You can just barely see two 'false' images of green and blue due to cross talk. This was shot on Colourholographics pan emulsion, now sadly no longer available

color teapot.jpg (31.92 KiB) Viewed 4381 times

[Joe Farina]

Looks very nice Dinesh, especially the white. I didn't even know that full color transmissions were possible or practical, due to the crosstalk problem.