445nm beam correction optic

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dave battin

445nm beam correction optic

Post by dave battin »

wler wrote:
Thieu wrote:You'll find that just waving your hand around the diode+heatsink or current source will thermally influence it, so try to avoid this, especially during an exposure.
I just wonder how poeple get away without active temperature stabilization ;-) Even a few 1000ths of degree of change can induce a mode jump.
Realizing the diode was running stone cold at low outputs, I checking the operating parameters online and figured it might be easier to stabilize it if i warmed it a bit, so locating the diode in close proximity to the voltage regulator seemed like an easy solution....... this set up should work for any of these diodes being run at low output settings with an insulating layer on it's exterior.
Colin Kaminski

445nm beam correction optic

Post by Colin Kaminski »

Jeffrey Weil wrote:Hello Everyone,

I've posted this before but not with an image. This is the way I measure coherence. Not with math, speculation, educated guesses or anything but real holography. I find it more exact to the real world we shoot in.

Put some film in a divergent beam, place a ruler on the table in the same expanded beam. Make a holo and look at the ruler. Where it fades out, that's the actual usable coherence length for holography.

I've tried all the other methods listed here other than the spectrograph. I use gas lasers so tweaking the power levels to get long coherence doesn't exist.

Some of you might point out that since the master is tipped relative to the ruler the measurements won't be exact. Thats true but its pretty close and in a real world shooting situation a similar effect exists. Since the ref is hitting the plate at a non normal incidence the coherence length is being "used up" as it travels across the plate.

Feel free to change the angles to anything that makes the measurement easier if you want. Almost anything will give you an image.

While its not perfect, it is "real" as opposed to being inferred.

And before someone else points out that since the recording beam and the playback beam are both divergent the size of the ruler will change. True again but its a small distortion. I also know I didn't draw the beams coming out of the lens correctly. Don't be picky.
I have done this in reflection mode. It works pretty well. I will have to try it in transmission mode.
Tom B.

445nm beam correction optic

Post by Tom B. »

Re the spectrogram method, an easy and fun demo is to take a cheapo laser pointer, point at a diffraction grating so that the first order spot is on a wall a good distance away, 5-10m or more is good, and touch the laser pointer with an ice cube to chill it down and then watch for mode hops as it warms up. Depending on how well focused the spot is, the mode hops look like chaotic elliptical blurs lasting at least a few seconds until the new mode stabilizes. I tried it with a real diffraction grating but don't see why a CD or DVD (DVD is better, with more dispersion) wouldn't work.
Jeffrey Weil

445nm beam correction optic

Post by Jeffrey Weil »

Hello Colin,

It works much better in transmission.

445nm beam correction optic

Post by Thieu »

Hi Tom B. I can confirm that it works with a DVD in littrow configuration (reflecting the first order back in the direction of the source). Somehow it works best if you have a dvd with some blank regions on the text side and use those.

445nm beam correction optic

Post by wler »

Actually checking the fringes with a glass plate seems a pretty reliable method to verify SLM operation for ECDLs. Mode hops are more pronounded with ECDL as compared to free running diodes and are easy to detect with the naked eye (and ear). For red diodes this works perfectly, but for blue ones one must look more carefully because of the bad beam quality. That makes an automatic detection (via CCD sensor) more problematic for those.