Diode laser

On: Thu, Nov 29, 01 06:18:36 AM

Stefano Sica wrote:

RESPONSES

Colin Kaminski - Thu, Nov 29, 01 12:50:30 PM

I was told by the owner of Laser-Max that the 635nm diodes mode hop with a smaller change in temperature than the 660nm diodes. If you can keep their frequencies stable I think the lower the wavelength the better. In reality I know of no one who has tried. He tried the 635 mw lasers for interferometry. Also I am sure the diodes will evolve. He wrote me that in June of 2001. Please let us know what you find. 64.167.149.132

Jonathan - Fri, Nov 30, 01 02:59:01 PM
Colin is right - if your diode is prone to mode hopping it will play havoc with the quality of your holograms, and drive you crazy. And if shorter wavelength diodes are harder to keep stable then I would avoid them, unless you have a very good heat sink, and a quality drive circuit.

I had a problem which I believe is due to mode hopping. Has anyone seen this?

My transmission holograms looked a bit like interferograms. The virtual image would display the "contour" lines that you see in double-exposure images. At first I thought the problem was movement, but this proved not to be the case. And the real image side of the hologram would show an arcing secondary interference pattern, seen clearly when the film is held up to a room ceiling light. Worst of all the problem was intermittent. Sometimes it was severe, and sometimes the hologram was perfectly fine. Two shots in a row, with nothing changed, could produce one good one and one that looked like an interferogram. Very frustrating.

It all points to a problem of mode hopping, I believe. When it occurs the jumps are in discrete amounts - which would explain the "two exposure" effect I describe above. Has anyone seen this problem? Is this hypothesis correct? 209.90.160.126

Tom B. - Sat, Dec 01, 01 04:39:10 AM

Jonathan - Your guess sounds reasonable, but I don't know. Something I stumbled across when playing with an interferometer setup - I connected a photodiode to an audio amplifier with headphone output and positioned the photodiode to intercept some of the spread beam. I'll be darned if I didn't hear occasional clicks as the diode warmed up, accompanied by jerks in the fringe pattern. Sometimes I heard waves of raspy noise come and go, during which time the fringes were washed out. If (and this is just conjecture) you can hear mode hopping, you might be able to get a better handle on the stability problem, or at least time your exposures to take place during quiet periods. Anyway, if anyone can confirm (or debunk) this observation, I'd be interested in their reports. You don't need a fancy photodiode - I also tried a cheapo solar cell and it worked OK, though it was quite sensitive to vibration (microphonics). 24.67.253.203

Tom B. - Sat, Dec 01, 01 02:26:28 PM

Re the last bit - the reason I mentioned the vibration sensitivity of the solar cell was not that it is a problem in a quiet stable setup, but that I nearly blew out my eardrums when I dropped it with headphones on and the gain turned way up. Ouch! 24.67.253.203

Joe Farina - Sat, Dec 01, 01 02:44:40 PM

Jonathan, Based on the literature I've read, and upon my own experience, I think you are right, those contour lines are due to mode-hopping. I used a 35mW Hitachi diode to do some long exposures (5 to 10 minutes) on red-sensitive DCG in Denisyuk format. Some bright holograms were produced, but they had contour bands across the image. As I recall, the bands were about 3/8 inch wide. In another case, I used a less powerful diode to do shorter exposures on Slavich material. These holograms didn't show those pronounced contour bands as before, but they were still weakly visible if you looked carefully, and thinner this time, maybe around 1/16 inch wide. I don't recall the expsoure times in these latter tests, but I was using a pointer-type diode to expose PFG-03M plates that were 2.5 X 2.5 inches, so it was a lot longer than just a few seconds. What I believe is happening is that the diode is changing its wavelength very slightly during the exposure (I guess that's basically the same thing as "mode- hopping"). Actually, two-wavelength contour holography is quite common in the literature, and it was stated (as I recall) that the smaller the wavelength change, the bigger the contour bands become. I've also read that it's possible to detect mode-hopping by allowing the beam (or part of the beam) to intercept a photodiode which is then connected to a sensitive AC voltmeter. (The sensitivity of this voltmeter has to be very high, beyond that of commonly- available meters.) When the laser is mode-hopping, a weak AC singal will be produced via the photodiode, since there will be a rapid back-and-forth switching effect as the diode changes from one wavelength to another. I tried to monitor my diode in this way, but I was not certain (to say the least) that I was actually detecting mode-hopping. This might have been due to the meter I was using. It's been quite a while since I did these experiments, and maybe someone else can shed some light on the subject. I would really like to use one of the new Hitachi 35mW diodes which emit at ~635nm (and which are said to cost ~$152) but I wonder if they will be prone to mode-hopping, even with active temperature control. 216.65.173.82

Jonathan - Sun, Dec 02, 01 01:06:32 PM
Thanks for the input, guys. Your information confirms my suspicions about this type of banding problem. If a way could be found to work around it, if not completely solve it with active heat control, then at least holographers could use the shorter wavelength red diodes like the 635 nm Hitachi. I'm wondering if this mode-hopping problem is even more prevalent in the blue diodes. (Power Technology sells a 25 mW which emits at 405 nm, for example).

Tom, your idea about trying to "hear" fluctuations in the beam using a solar cell is worth a try I think. If there's a correlation with the effect on the fringes it would at least be possible to time the exposures to avoid it as you say, whether it's mode-hopping or some other phenomenon.

I'm thinking of using a radio circuit as the audio amplifier, and connecting it to a solar cell placed in the beam path (partially of course, so as to not block it, and to avoid flooding the solar cell). The antenna input could then be used as the solar cell input. A suitable resistor circuit in between would presumably be needed to prevent overloading the radio circuit. Do you think this would work?

209.90.160.125

Tom B. - Mon, Dec 03, 01 03:03:10 AM

Jonathan - I don't think the antenna input would work for audio. If all you have is a radio for audio amplification, you MIGHT try connecting to one of the volume control leads through a small capacitor to block DC from the solar cell. And you want to connect another lead to ground on the radio. Test it by pointing the cell at an incandescent light - the hum should be very noticeable. regards, tom 24.67.253.203

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