Electronics > Projects, Designs, and Technical Stuff

Laser Diode driver - kHz modulation & 1% duty cycles

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jmelson:

--- Quote from: smoothVTer on April 23, 2019, 08:43:09 pm ---
0.374V is about 1/3 of the headroom of the DAC's possible output ( Vref feeding the DAC is 1.024V )    So I think this resistor is more or less in the correct range, no?   Some room for adjustment down, more room for adjustment up. 

--- End quote ---
Yes, in a strictly DC sense, but your problem is poor AC bandwidth.  High impedances wreck your bandwidth.

--- Quote ---Also: you mean a transimpedance amplifier ( not transconductance ) ?   

If you meant transimpedance amplifier, there is a single reason I cannot use one: This is a single supply system, and as such, I cannot use a TIA because using a TIA with an N-type laser diode would generate a negative output voltage in proportion to Ipd * Rf      Since I don't have a negative supply, I cannot use a TIA in this fashion.

--- End quote ---
Yes, right, incorrect term, I did mean transimpedance.  Well, if the output is at the wrong DC resting point, then you have to shift it.  Since you don't have a negative rail to bias the input, you need to shift the non-inverting input more positive.  When set right, that will cause the output to idle at a more positive value, and then move in the minus direction with increasing laser output.  But, this presents a low impedance to the photodiode, and will therefore improve the AC response.

Jon

chris_leyson:
Hi smoothVTer, it's been a few years since I last worked with laser diodes and back then we were driving them CW.
I probably had a mini rant about fixing some of the legacy laser drivers that we used because 50% current overshoot was not uncommon, all down to putting too many 100n feedback caps around op-amps and then not testing the transient response  :palm: Any new laser drivers, ~50mW power, just used a single op-amp driving a mosfet but needed to have a select on test resistor to set the correct PD feedback voltage for each laser diode. I used just enough capacitance to roll off the high frequency gain and managed to get a good step response with 5% overshhot. The only problem with a single op-amp solution was that if you have a set point of zero then the laser diode is always driven at it's threshold current.
I was looking at fast(ish) laser drivers for a personal project but the company I worked went belly up so that never got finished, I had more important things to think about.

Anyway, something you might find useful is a little circuit to mimic or emulate a small laser diode with an N style pinout.
The laser diode drive transistor or mosfet is represented by the current source I1 and diodes D1 and D2 together with Q1 Vbe mimic the LD forward voltage somewhat. R1 sets the threshold current for the emulator, 30 ohms gives about 20mA, Q1 and R2 drive an LED at about 10mA for a max LD drive current of 50mA in this example and Q2 and R3 mimic the PD feedback current set to 330uA at 50mA drive.  It's a reasonable approximation to a small laser diode and a lot cheaper to fix if you let the magic smoke out.

StillTrying:
I don't think any of these solution work very well, so here's yet another. >:D

mikerj:

--- Quote from: smoothVTer on April 24, 2019, 05:16:27 pm ---Thanks mikerj,

I found this app note from Maxim about doing pretty much what you describe in the first half of your reply, I think.  Yet I do not understand how this circuit could possibly work ( regulating average power ) because then there is no limit to how much the current can increase at low duty cycles ... this circuit below would push the current very high and blow the laser diode at low duty cycles.

Taken from https://www.maximintegrated.com/en/app-notes/index.mvp/id/1811


--- End quote ---

When using a laser for transmitting data the data signal is always DC balanced, i.e. the average duty cycle (over some number of bits) is maintained at 50% i.e. an equal number of ones and zeros for an NRZ signal.  Not only does this maintain a fixed average power output, it makes the receivers job far easier as the slicing level can be set based on average power.

That app note does raise another issue that you might run into; biasing the laser.  For the fastest response you don't want to turn the laser fully off as this slows the response considerably.  Typically the laser will be driven so that it always remains above it's threshold current, though at kHz speeds this may not be an issue.

awallin:
not sure if it's been said/linked already but the usual constant-current circuit used for sciency-stuff is:
Libbrecth-Hall http://www.submm.caltech.edu/kids_html/DesignLog/DesignLog179/MillerMUSICReadoutDocs/HEMT%20Power%20Supply/Libbrecht%20and%20Hall,%20A%20Low%20Noise%20High%20Speed%20Diode%20Laser%20Current%20Controller.pdf
Erickson https://arxiv.org/abs/0805.0015
Seck https://arxiv.org/abs/1604.00374

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