That's not what I experienced. E.g. with "Frank's" (Mitsubishi) diode lasers I've never been able to get more than some extremely dim holograms - even when making contact copies. By the way, speaking of contact copies, filtered LEDs produced much brighter results.Colin Kaminski wrote:Even with most mode hops a laser can be used for contact copies. It really takes a pretty wide line to erase a contact copy.
Holographic gratings
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Martin
Holographic gratings
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Colin Kaminski
Holographic gratings
That is quite interesting. Once warmed up I found Frank's laser to be pretty stable, not stable enough for making a hologram but shifting in wavelength be less than .1nm. I only measured it with a DVD as a grating across the lab so I didn't end up with an exact measurement.
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Hans
Holographic gratings
You can make holograms with diodes that are not temperature stabilized. I have done it many times. Also with Frank's D&S laser. The thing to watch is that it does not already mode hop already before starting the exposure. When a diode is hopping, the hops are not incidental but very frequent. If I remember correctly the mode hops were usually about 0.1nm apart. They were easily detectable by reflecting the laser light off a dvd. When run at not too high power and finding a current where there was no hopping, the mode hops usually would stay away for a few minutes.
The problem was with wavelength drift between mode hops which prevented bright holograms a lot of the time. Though the hops were about 0.1nm, the diodes can slowly drift about 0.01nm between mode hops. This drift is continuous and due to small temperature variations.
The problem was with wavelength drift between mode hops which prevented bright holograms a lot of the time. Though the hops were about 0.1nm, the diodes can slowly drift about 0.01nm between mode hops. This drift is continuous and due to small temperature variations.
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DJ Mathson
Holographic gratings
Air currents can influence the behavior. There is an interesting paper from ILX about TE controlling laser diodes. They state this about air currents:
"Two important performance criteria of temperature controllers are stability and speed. Under the right conditions, all three types of control architectures described above can achieve stabilities of better than 0.001° C. In practice, air currents in the vicinity of the load are usually the limiting factor for stability. Figure 7 shows the temperature fluctuation of the mounting plate in the fixture depicted in Figure 4, with and without the rear cover plate installed. In this example, active temperature control was performed with a proportional-integral feedback loop. Further improvement could be obtained by using an insulating foam or glass wool inside the fixture, to further limit air currents."
It is interesting to see how big the effect is from air currents on the temperature fluctuation. See Figure 7 on Page 7.
Controlling Temperatures of Diode Lasers and Detectors Thermoelectrically
"Two important performance criteria of temperature controllers are stability and speed. Under the right conditions, all three types of control architectures described above can achieve stabilities of better than 0.001° C. In practice, air currents in the vicinity of the load are usually the limiting factor for stability. Figure 7 shows the temperature fluctuation of the mounting plate in the fixture depicted in Figure 4, with and without the rear cover plate installed. In this example, active temperature control was performed with a proportional-integral feedback loop. Further improvement could be obtained by using an insulating foam or glass wool inside the fixture, to further limit air currents."
It is interesting to see how big the effect is from air currents on the temperature fluctuation. See Figure 7 on Page 7.
Controlling Temperatures of Diode Lasers and Detectors Thermoelectrically
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Colin Kaminski
Holographic gratings
It should be noted that TomB makes some of the brightest holograms I have ever seen. With a diode and no temperature control. It is not that it can't be done but it does take work.
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Danny Bee
Holographic gratings
can you explain a bit more about this Hans, what type dvd (gold)?Hans wrote:They were easily detectable by reflecting the laser light off a dvd. When run at not too high power and finding a current where there was no hopping, the mode hops usually would stay away for a few minutes.
i also was reading that you can do trans copies in dcg improving the master a little
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DJ Mathson
Holographic gratings
Here is a thread with a description of the dvd test for mode hopping in laser diodes:
http://holographyforum.org/phpBB2/viewt ... f=2&t=4556
http://holographyforum.org/phpBB2/viewt ... f=2&t=4556
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Hans
Holographic gratings
Thank you DJ.Here is a thread with a description of the dvd test for mode hopping in laser diodes:
viewtopic.php?f=2&t=4556
For red and green lasers any DVD will do just fine. Even a normal CD works. I don't have a blue laser, so I was not able to try it. I guess with a BlueRay disk it would even be easier to see the hops. But at any rate, when you look at the reflection off the disk on the wall and vary the current to the diode, you will easily spot when the diode mode-hops and when it does not.can you explain a bit more about this Hans, what type dvd (gold)?
I am not sure I understand what you wrote here.i also was reading that you can do trans copies in dcg improving the master a little
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Colin Kaminski
Holographic gratings
The best way to detect mode hops I have found was given to me by Jonathan Head.
I know other people use it as well, take a microscope slide and use it as a beam splitter, aim the dim beam to a photodiode, hook up the photodiode to an amplifier and listen to the sound. If it crackles don't open the shutter. When it quiets down, go for it. I have also seen people take the small beam and run it though a interferometer and watch the fringes. One of the best versions of this was using a microscope slide as an Fabry Perot interferometer and making a set of ring fringes. It was quite simple. I have not tried that one so I am not sure how tricky the alignment it.
While learning about the diode, make a Michelson interferometer, until the fringes stay still you can not use the diode. I spent a summer with my bench set up in a Michelson and watching a diode every night. I was always convinced I was almost there with the diode.
Jonathan also came up with a very good way to temperature control a diode. He made a copper bar and inserted the diode in one end. The bar was placed in a 1 L jar of water and he would put a digital thermometer in it. He would then add water of a known temperature, Ice water or boiling water with a seringe and could calculate the resulting temperature. He made some very nice holograms with this set up.
I know other people use it as well, take a microscope slide and use it as a beam splitter, aim the dim beam to a photodiode, hook up the photodiode to an amplifier and listen to the sound. If it crackles don't open the shutter. When it quiets down, go for it. I have also seen people take the small beam and run it though a interferometer and watch the fringes. One of the best versions of this was using a microscope slide as an Fabry Perot interferometer and making a set of ring fringes. It was quite simple. I have not tried that one so I am not sure how tricky the alignment it.
While learning about the diode, make a Michelson interferometer, until the fringes stay still you can not use the diode. I spent a summer with my bench set up in a Michelson and watching a diode every night. I was always convinced I was almost there with the diode.
Jonathan also came up with a very good way to temperature control a diode. He made a copper bar and inserted the diode in one end. The bar was placed in a 1 L jar of water and he would put a digital thermometer in it. He would then add water of a known temperature, Ice water or boiling water with a seringe and could calculate the resulting temperature. He made some very nice holograms with this set up.