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Laser Diode driver - kHz modulation & 1% duty cycles
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mikerj:
Just to make sure I understand your intentions, you want to provide a constant average power output independent of PWM duty cycle, so low duty cycles require much higher peak currents than high duty cycles?  I don't see any current limiting in the circuit, does the 2.5v laser supply guarantee you can't blow it up if the drive transistor is saturated?
smoothVTer:

--- Quote from: janoc on April 23, 2019, 09:07:32 pm ---
--- Quote from: smoothVTer on April 23, 2019, 08:39:34 pm ---I'll try building up this circuit with a discreet single-supply OPA, with a GBWP somewhat higher than 20MHz.  I'll also use a potentiometer rather than a DAC.   Try to test out the concept with higher bandwidth components and work from there. Maybe it is something weird to do with the PIC's integrated OPA.

--- End quote ---

GBWP is not what matters so much, do make sure your opamp has sufficient slew rate.


--- Quote from: smoothVTer on April 23, 2019, 08:39:34 pm ---Using discreets, I can't toggle the OPA between driving and high-z because thus far I haven't found any OPA that advertises this. 

--- End quote ---

What's wrong with using an extra transistor there?  :-//

--- End quote ---


You're right ... I could have back-to-back NFETs at the OPA output pin to force the high-z state on any OPA I choose ... however ... at least in my preliminary experiments with this, I find large negative and positive going spikes upon the switch on/off state.  This might have to do with charge injection of the series mosfets, in which case I'd be worried about momentarily overdriving Q1's base and blowing the LD.  For example:



Here's switching the gate of the back-to-back NFETs and the signal transmission of a 1MHz sine wave:


Zoomed into the rising switching edge, I get some nasty 1V spikes positive on the ON edge and -1V spikes on the OFF edge:


This may or may not work ... I could choose much higher Ron NFETs to minimize the gate capacitance and thus the charge injection.
smoothVTer:

--- Quote from: mikerj on April 23, 2019, 10:07:51 pm ---Just to make sure I understand your intentions, you want to provide a constant average power output independent of PWM duty cycle, so low duty cycles require much higher peak currents than high duty cycles?  I don't see any current limiting in the circuit, does the 2.5v laser supply guarantee you can't blow it up if the drive transistor is saturated?

--- End quote ---

The idea is to be able to set the peak LD current to a constant, known level, independent of the LD's own Ipd characteristic.  Once the LD is "peaked", so to speak, the idea is to modulate at several kHz and vary the duty cycle to scale the output power.    The complication arises because the same part number laser diode ( red or IR, in this case )  can have an Ipd anywhere from 50uA to 250uA for an output power Po = 10mW     The LD is most efficient at this "peaked" output level, so to achieve highest efficiency, you want to always reach that peak power during the PWM "on" time.

I guess another way to word it is that I want to achieve a constant peak power, that I can then PWM/vary the duty cycle to scale the perceived average power.  To achieve this constant peak power, I run this system as a fast linear regulator, using the DAC as my peak power control signal.

ajb:
There's a fundamental problem with switching the output of the opamp as you're doing: you're breaking the feedback loop.  An op amp without feedback is a comparator, and not a very good one, often exhibiting problematic behavior when changing state.  The fast rising edge, resultant ringing, and slow ramp up to the set point is entirely consistent with an op amp coming out of saturation and getting a hold of its feedback again.  You will almost certainly achieve better results by switching the setpoint on and off, and allowing the op amp to stay closed loop at all times. 
duak:
I last worked with laser diodes in the early 90's.  I don't think things have changed much though: https://www.newport.com/medias/sys_master/images/images/he9/hd7/8797049520158/AN05-Laser-Diode-Characteristics-Overview.pdf  It explains some aspects but for what it's worth, here's my spin on it too:

- it's normal to have a variation in monitor current.  The photodiode sits at the other end of the laser diode chip and measures how much light leaks out of the cavity back facet or mirror. I can't remember the exact values for the leakage but you can see that a difference between 99.9% and 99.99% reflectivity is an order of magnitude variation in monitor current.
- a laser emits non-coherent radiation as an LED below lasing threshold.  The amount and wavelength is not usually specified.
- the LD_supply voltage seems a little low.  It should work but Q1 is running close to saturation ie., VBE > VCE
- you've got to be really careful not to exceed either the forward current limit or the power limit spec or you will blow the facet off the die and it will no longer lase.  This may have changed over the years, but back then the manufacturer wasn't kidding about exceeding limits.  We joked that you could kill a $100 device in 100 ns for a burn rate of $1B per second.
- back when I worked with these things, if the laser was modulated, the monitor was sampled and used to determine the drive current.  This was then switched or steered so as to not cause any current spikes that could damage the laser diode.
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