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Registered Member #580
Joined: Mon Mar 12 2007, 03:17PM
Location: Melbourne, Australia
Posts: 410
For the laser diode displayed here, what would be a suitable way to create a regulator for this laser such that it could cope with power supply (very simplistic ones) turn on and turn of voltage transients aswell as a way to turn on and off the laser at up to 30Mhz. I think it would be easiest not to use any photodiode feedback and have a 2.4v voltage source. I have tried an LM317 and a series resistor but it was not suitable and the laser died. I do not know of a 2.4v zener diode (or at least not sold here). Maybe 4 1n4148s in series and all in parallel with a reversed one to prevent negative voltages?
Registered Member #95
Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308
Laser diodes are best driven with a constant-current source. I've used op-amps and plain transistors to make a variable current-sources before. See c4r0's website for a transitor and LM317 design.
I used this circuit when experimenting with a CD-RW diode, no spikes or other problems.
As for 30MHz PWM, I don't think it's possible given the sensitivity of laser diodes and all the micro-scopic inductances your circuit will have. What's the point of such a high frequency anyway?
Registered Member #580
Joined: Mon Mar 12 2007, 03:17PM
Location: Melbourne, Australia
Posts: 410
the laser control can be after the regulator, it does not need to be before it, that would only make things very complicated. i assume normal logic chips and transistors should work here. I had a plan one day to try to measure the speed of light with it, that is why i can not use one of those cheap pre-regulated modules, they cant be oscillated fast.
A constant current source might go a bit crazy with the load suddenly 'appearing and disappearing'
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
To apply modulation, you want a thing called a Bias-T. It's a capacitor and inductor network that superimposes RF power from a separate source onto the constant current output of the laser driver.
This means the laser driver can be optimized for precision and low noise, without worrying about its speed, while the RF amp can be optimized for speed alone. The inductance issue as mentioned by Uzzors, can also be solved by putting the bias tee very close to the laser and connecting it to the RF amp by a matched 50 ohm line.
One product I helped to design used a Bias-T driven by two AD811 high-speed op-amps in parallel, to modulate a laser diode at up to 40MBit/sec. We did try measuring the speed of light in a 1km reel of optical fibre with it, and a suitably fast receiver.
Registered Member #580
Joined: Mon Mar 12 2007, 03:17PM
Location: Melbourne, Australia
Posts: 410
I do not want to use any components designed for any specific frequency, 1Hz to 30Mhz is the range i might use it at, or even DC. surly a fast voltage regulator followed by a transistor is sufficient? also what about the diode over-voltage idea, how does that sound, any obvious problems with that?
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Laser diode regulator
