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Screwy laser diode with monitor photodiode

I am working on an instrument that uses a VCSEL light source for some optical measurements. The current instrument has some problems with the light output drifting with temperature. In order to correct this I have been looking at using a VCSEL with a monitor photodiode to control the light output. Presumably if I keep the signal at the monitor photodiode constant, the output should be constant.

My problem is that as I change the current in the VCSEL the light output as seen by an external photodiode does not track the light output of the monitor photodiode packaged with the VCSEL. I have tried several different devices, and they all have this problem, though different devices have a different shaped relation between them, so this is not a fixed transfer function that I can calibrate out. Since the light out does not seem to track the monitor signal, if I control the VCSEL current to keep the monitor current constant, the light output is not nesicarily going to stay constant.

Does anybody have any suggestions as to what might be causing this? Is there some mechanism for the monitor diode to pick up energy from the VCSEL that is not from the light output?

My other thought is that it might be speckle on the two photodiodes causing the inconsistancy. However this seems unlikely, since I think the size of the photodiode is large relative to the size of a speck, (what is signular for speckle?) that this would average out to something insignificant.

Thanks for any suggestions.

Ethan Petersen

Reply to
Ethan
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VCSELs are a mess. They have mode jumps, instabilities, sensitivity to back reflections, all sorts of weirdness. What hits the monitor diode is only loosely related to what exits the package.

And if you find a good supplier, they're almost guaranteed to change the die or the process or discontinue the part in six months.

The old cleaved CD lasers were great, but you can't get them anymore.

Whose parts are you using? Lasermate never was very good; Optek was good for a while.

John

Reply to
John Larkin

The parts I have tried are: Advanced Optical Components HFE4083-322 Optec OPV310 and OPV314

I am just starting to set up an edge emmiting laser, US Lasers D850-5. We'll see how that goes. I also have a couple more VCSEL samples to try out, but don't have high hopes. One of which is from Lasermate. For some reason the case is connected to the monitor diode pin rather than the common, which just seems like a bad idea.

I am concerned about an an edge emmiting laser because of the oval shaped beam, and how that is going to effect the instrument calibration. I suppose this could be corrected with the right lenses, but that will complicate the physical/mechanical design.

We'll see how it goes.

Ethan

Reply to
Ethan

I have quart-size ziploc bags full of reject Lasermate vcsels... bad mode jumps, noise, rotten alignment. Our yield was under 50%.

And Optek keeps changing the chip - some are 8 pF, some are 35 - and doesn't like to answer phone calls. Lasers suck.

Let me know if you find anything good.

John

Reply to
John Larkin

I haven't used vcsel's, just edge emitters. But I have never trusted the internal photodiode on those. Any reflections that feed back to the diode will also affect the internal photodiode.

If your setup is such that there are no reflections coming back to the diode, then I don't know what else would cause this. I'm not sure if vcsel's would behave any better or worse than edge emitters.

Mark

Reply to
redbelly

redbelly wrote: (snip)

Almost all VCSELs have multiple transverse modes. I saw one that ran in an almost pure 7th order mode--12 off-axis peaks in the angular spectrum, and a nice null in the centre. Multiple modes will give you funny speckly patterns that are extremely noisy--often as much as 20 dB noisier than the total integrated output, due to competition between modes.

Getting the monitor PD to track the output accurately will be very difficult with a multimode device.

Cheers,

Phil Hobbs

Reply to
Phil Hobbs

So it sounds like using the monitor diode is not going to work for me. Does anybody have suggestions for how to get a tightly controlled output from a laser?

I am using this to measure optical density of a fluid. I shine a laser into the fluid and measure how much light comes out. I am modulating the laser at a couple kHz, just to make the detection easier, and to make it easier to cancel out external light sources.

This is pretty simple and for the most part this works pretty well. The problem is the light source is drifting with temperature.

I have a few bad ideas, they are all physically cumbersome, such as a beam splitter and an external monitor diode. Or a heater/thermister to maintain a constant temperature just above ambient.

Ideas anyone?

Thanks

Ethan

P.S. I am realizing I don't know as much about VCSELs and other lasers as I thought. Any suggestions on good books, or other references?

Reply to
Ethan

That sounds more like a muffin fan than a laser!

John

Reply to
John Larkin

It's fairly common to control the temperature of a diode laser. Usually a TEC (thermoelectric cooler) is used to cool it. A heater would be easier to set up, but you may be shortening the lifetime; if I did that I'd run the diode at reduced (half?) power (if your application can still work with less power). Of course, you'll need a thermistor or other temperature sensor, plus additional circuitry. PID or PI controllers are available from Wavelength Electronics,

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-- their site seems to be down right now or I'd hunt down the temp controller part # I used to use at my previous job.

HTH,

Mark

Reply to
redbelly

You can eliminate mode jumps using strong UHF modulation (300+ MHz).The circuit can be quite simple; e.g. I did it with a single transistor oscillator.

Could you use a LED instead? You can get quite narrow beam ones. You can also get them with internal monitor photodiodes.

--

John Devereux
Reply to
John Devereux

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