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One thing I'm finding in my research is that LDs don't like to be switched fully on/off when modulated, and so it's better to switch the drive current between max and a lower value that brings the diode below the lasing threshold, but not actually off. Is this true to what you know?
Yes, keeping the diode powered, but just under threshold improves response time, and therefore linearity, when going from dark to powered.
Thanks for the confirmation... I'll add that into my design, unless Daedal comes through and I get a much better schematic that's just as simple! :-)
I have modified the original schematic I posted after breadboarding the circuit and adding another resistor to provide a minimum current so the diode is never fully off. I have tested it with a 1.0 ohm resistor in place of the LD to measure current, and the calculations hold for various values of R. I will add a reverse bias diode for LD protection, but this circuit SEEMS pretty good so far.
It appears the calculation for Max I ( I = 0.7/R ) is dependent of the transistors Vbe. The calculations seem to suggest that for the 2N2222A, that 0.7 value should actually be 0.66 - but then again it could be due to the resistor values being off slightly... not sure which yet.
I'll post my results again when I have a fully functioning LD with modulation applied at the desired current (or after I cook my LD, whichever comes first... :-) )
Dave
I changed the location of the minimum current setting resistor and added the reverse spike protection diode, and I'm PWM'ing my diode with a propeller (a microcontroller if you aren't familiar with what a propeller is). I've run it to 10,000 Hz and have a nice square waveform going in, and a fairly square response as read on a photodiode.
Am I missing anything here - does anyone see any demons lurking in my schematic waiting to bite my faithful diode? :-)
Thanks for your thoughts.
Dave