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Dennis Feucht

Case Study: Medical Laser System; Part 1: Stabilizing a Laser Feedback Control Loop

Dennis Feucht
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D Feucht
D Feucht
11/27/2017 3:59:06 PM
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Re: Laser feedback....

The loop bandwidth was less than 100 Hz, so a processor in the feedback loop could have made the adjustment. But this was in the early 1990s when the processors in use in the medical instrument were 8-bit and 5 MHz. I trust the analog circuit more than a uC and its software in this case. The square-root circuit was low-cost because an op-amp used in another package was used. (I try very hard to use all the multiple parts in such packages instead of wasting them. I didn't quite accomplish that with the BJT array.)

Your idea to use "2 tilted mirrors 90 deg to each other" is a good one, but even with one mirror, the amount of light that got through (for either polarization) was small and to decrease it by x 1.414 would not have been received favorably for design. However, if it solved the problem, it would have been worth considering.

The uC starting problem should have been solved by either the software or hardware engineers, but it wasn't. Actually, the uC comm links were RS-232 and one of the extra command lines indicating "acknowledged" should have been used to speed up the initialization. I was too busy trying to figure out the entire system than to bother the others with that solution!

11/8/2017 10:09:00 AM
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Laser feedback....
Interesting story, Dennis, thank you.  It would be interesting to know the control loop bandwidth.  Clever solution to the non-linear power input->laser output relationship, to use the square-root circuit.  Might have been easier to run the loop in a processor and deal with this digitally, but it is an analog column after all!

What I know about laser oscillation modes and polarization is nearly zero.  However, I was thinking that if the laser was simply switching between vertical and horizontal (polarization) modes, and the tilted partial reflection mirror was fixed and not able to follow the polarization (of course), that perhaps 2 tilted mirrors 90 deg to each other (in rotation about the long axis of the beam) each with their own detector might solve the feedback problem.  The vector sum of the outputs of 2 sensors would then be insensitive to the V/H polarization.  Of course of the beam patterns are more complex, this idea quickly falls apart.

To the beginner's luck part of your story, with the simple delay in the master's startup fixing the random startup problem, some thoughts:  1) a serial communication scheme, particularly in a medical device controlling a laser, one thinks there ought to have been timeout/retry algorithms in place.  2) I once consulted at a place where the microcontroller systems had what was called "Crazy Ivan" behaviors where the systems would periodically go stupid and need to be rebooted.  This was accepted as a normal course of events, even though the behavior would regularly kill 3-day long experiments.  One day my sidekick, another old guy with beginners luck, was working on the code and noticed that variables in the code that were messed with in interrupt routines were not bracketed with disable/enable interrupt instructions.  It is not how easy we make the hard problems but how hard we make the easy ones!


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