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how fast can cheap lasers be pulsed?

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Patrick

Tue Dec 11 2012, 06:14AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

for a optical project i need to pulse a cheap laser (one of those common pen lasers with the driver built in ). im assuming that the semiconductor lasers of all types can be pulsed fairly fast, but that the current and voltage regulators can impose a speed and other limitations on modulating. im hoping for (5mS on and 5mS off)

My question is... do any of you fellow 4hv'ers have experience with these cheap lasers? can they (generally speaking) be modulated by replacing the button with a transistor? and what is the speed limit? 1khz or 1MHz, or what? am i correct in presuming most of the limitations are due to the attached supporting circut?

i have read (sam goldwasser and others) that the drivers are hard to build and the laser diodes are super easy to kill, so id like to avoid building my own driver, and thus modulate the existing one.

i know this is kinda vague but i wont hold any of you to your answers if my idea and laser dont work.

Dr. ISOTOP

Tue Dec 11 2012, 08:55AM

Registered Member #2919 Joined: Fri Jun 11 2010, 06:30PM
Location: Cambridge, MA
Posts: 652

The cheapest red laser pointers just put a resistor in series with the diode. In this case, you're limited only by the rate at which the diode can turn on and off (which is usually on the order of microseconds at worst).

Shrad

Tue Dec 11 2012, 08:58AM

Registered Member #3215 Joined: Sun Sept 19 2010, 08:42PM
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you will be able to switch 5ms easily with any driver...

the pushbutton on a lsaer pen only switches the power on/off so it wont be anything performant

if you need something reliable, something in the first link will suit your needs, and the two other links are some ways to use current measurement techniques to adapt the circuits of the first link to practical situations

basically, you need a current source or current sink, with a voltage set point for modulation

PS: if you need nanosecond speed, some diodes self-modelock at a frequency between 500Mhz and 1Ghz

Steve Conner

Tue Dec 11 2012, 11:26AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

I'm pretty sure it would be easy to take a cheap laser pointer apart and get directly at the diode. (It certainly was in the last one I took apart

) The diode will probably modulate pretty fast. Because it's cheap, the wavelength and mode profile will vary horribly with current, but that may not be an issue in your application.

When I worked with telecoms lasers, I used an ordinary constant-current driver, and applied the modulation through a "Bias tee", which is just an inductor to stop high frequencies from being shorted by the bias circuit's output capacitance, and a capacitor to couple in the modulation.

This arrangement allows you to adjust the AC and DC drive levels separately. You want to feed the laser a little current just below its lasing threshold in the "off" state, so it'll turn on faster.

The driver I used was made of two AD811 op-amps paralleled up for extra current, and it ran happily at 40Mb/sec.

You can do the same with simpler hardware, like Ronja's driver made of a hex inverter chip, and the performance ought to be much the same.

Pinky's Brain

Tue Dec 11 2012, 11:35AM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
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I think just about every laser diode can manage ns range pulses. There are various circuits with current mirrors to make sure the current stays within limits ... you can also get relatively cheap pulse drivers from ichaus, the laser webshop I linked before has them.

Patrick

Tue Dec 11 2012, 07:51PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Dr. ISOTOP wrote ...

The cheapest red laser pointers just put a resistor in series with the diode. In this case, you're limited only by the rate at which the diode can turn on and off (which is usually on the order of microseconds at worst).

can it really be this simple?

Electra

Tue Dec 11 2012, 11:03PM

Registered Member #816 Joined: Sun Jun 03 2007, 07:29PM
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Posts: 156

If itâ€™s just on/off modulation your after, you could have a fairly fast response current source, and a high speed switching mosfet connected across the laser diode, therefore shunting the current to switch it on and off.

Presumably designing an equally fast low noise photo diode amplifier is going to be the more difficult task.

Pinky's Brain

Tue Dec 11 2012, 11:45PM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
Location:
Posts: 837

Patrick wrote ...

can it really be this simple?

For microsecond range, sure ... you can approximate the voltage drop over the diode as constant and use a resistor to set the current.

It's only when you want to go fast that things get harder.

The shunting approach works as well (and is used commercially) but is not very efficient for very low duty cycles ... on the other hand it's a lot more efficient for high duty cycles than say the resistor approach, because the current source can be a buck converter which just keeps circulating current through the shunt with low losses when the diode isn't on. You won't get single digit ns range rise/fall times though with saturated switching in the shunt.

Patrick

Wed Dec 12 2012, 01:49AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

im thinking 500uS is super fast, and if it rises to full intensity in 75-100nS even better. DC would be 50% to about 10% on-time.

Pinky's Brain

Wed Dec 12 2012, 08:21AM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
Location:
Posts: 837

Well then the choice seems between simplicity (resistor and mosfet) and efficiency (buck converter as current source and a mosfet shunt, you can probably use a led driver ic for the buck converter).

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