Laser, DFB module: TEC reaction speed

Joerg · 2006-12-23T19:19:28+00:00

Hello Folks,We need to lock onto one side of a sharp but moving resonance similar to a Fabry-Perot dip. Sharp meaning around 10pm. Long story short, when scooting the TEC via a ADN8831 chip it's tough to achieve regulation without overshoot. IOW tuning the PID loop is a bear and the locking mechanism overshoots the target which shall not happen in this case. It's a DFB module (JDS and one from Sumitomo).What is the fastest TEC tuning speed that the laser expert here have achieved with those modules without "ringing"?Of course it's possible to move 10C in a couple of seconds but when checking how professional equipment such as the ILX Lightwave controller is the lab does that it overshoots by a lot more than 1C and takes several seconds to stabilize. They seem to really be stepping on it for the derivative part which is ok in telco apps where you usually set the wavelength once and then leave it there. In our case we must remain agile. The total range we need is around 10C (hopefully...). A wee overshoot of 0.1-0.2C would be ok but when tuning the PID loop towards that goal the whole thing becomes like molasses.-- Regards, Joerg

Dec 23, 2006 63 Replies

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Joerg 19 years ago

That response is what I'll still have to measure but from a coarse test today it looks quite linear. The TEC doesn't seem to follow any easy formula though and all the delays in there are a bit unorthodox. But at least the PID is basically running. Not fast enough though, about

3sec/degreeC. I'd like to spiff that up a bit.

The crux is that I have to drag the TEC until a rather sharp valley is hit, then bring it to a screeching halt. We typically won't know where that lone resonance is going to be and it's rather abrupt, without the usual LC type slope. Just flat and then off the cliff. And we have to cling to the face of that cliff.

The finish of the last PID tuning round was greeted by one of the DIP switches breaking off. Oh man, those things are so cheesy these days.

That's where wave digital filters come in handy if there aren't enough MIPS. They behave quite gracefully on transitions and drop-outs.

Maybe. But its a DFB module where you can't get to all that. Got to live with what's in that can :-(

Regards, Joerg http://www.analogconsultants.com

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bill.sloman 19 years ago

Check out my paper in Measurement Science and Technology, volume 7, pages 1653-1664, (1996). Appendix A gives an expression for the heat transferred by a Peltier junction as a function of the temperature difference across the junction and the current through it in terms of the maximum heat transferred at zeo temperaturedifference and the maximum temperature difference at zero heat transfer - both available from the manufacturer's data.

If you can estimate thermal resistances accurately enough, this can be reworked to give heat transferred per unit current from the controlled region to ambient - also discussed in Appendix A.

The expression isn't particularly precise - it looks as if some of the coefficients are temperature dependent - but it is good enough to keep a PID loop pretty close to critically damped.

I've seen the expression published elsewhere - in EDN a few years later

- and I got it into a comment published in Rev.Sci. Instrum. in 2004

-volume 75 pages 788-89.

It ought to be in Pelter junction application notes, but I've yet to see it anywhere useful.

Bill Sloman, Nijmegen (but in Melbourne at the moment).

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Joerg 19 years ago

Ok, folks, just in case anyone else runs into this issue: I got it down to a little under 3sec/degreeC for modest overshoot (under 0.1sec). I can push it down to below 2sec but then the overshoot will stray out too much in some regions. We need a range of up to 20C and the TEC behavior is very non-constant over that wide range. My feeling is that the only way to push it faster would be PID with adaptive coefficients, IOW where the PID coefficients can be changed depending on the temperature the TEC is sitting right now. Some brute force digital algorithm might work as well but currently we don't have a micro controller on there.

Regards, Joerg http://www.analogconsultants.com

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