I'm trying to make a laser detection circuit, and I need to modulate a laser (I have a 5mw laser diode module) at a certain frequency so that the detector can bandpass filter to that frequency.

my question is about the circuitry involved to drive the laser. Am I basically just looking for a square wave generator at a certain frequency?
I've been looking for ICs made to do what I need, but have not had any luck, somewhere in a post someone said Maxim may have something for laser modulation, but I've had no luck at their site.
does anyone know of any ICs that would work?

Thanks

__________________
---------------------------------
Rogue5
University of New Hampshire
Electrical Engineering
- - - - - - - - - - - - - - - - - - -
SETI@Home:
TeAm AnandTech
----------------------------------

 

" Am I basically just looking for a square wave generator at a certain frequency? "

Right. You need a square wave of sufficient voltage to drive the diode and a series resistor to limit the current to a safe value.

__________________
see my website: www.geocities.com/russlk

 

As i know, all laser diode also contain a photodiode to monitoring the intensity, so You need some opamp for precise current setup.
If the module contain this circuit, forget this.

 

I was hoping my laser diode module would have the current limiting circuitry...
I've read that laser diodes are very sensitive to too much current, and it's kind of tricky to use them without wrecking a couple to get it right. This led to me getting the module because now I don't need to worry about current limiting. I read that with a ld module or the laser out of a laser pointer the current limiting would be taken care of.

I got most of my info here:
http://www.eio.com/repairfaq/sam/laserdps.htm

__________________
---------------------------------
Rogue5
University of New Hampshire
Electrical Engineering
- - - - - - - - - - - - - - - - - - -
SETI@Home:
TeAm AnandTech
----------------------------------

 

Do you need a fixed modulation or a variable so you are able to transmit information over the beam?

The trick is to keep the Laser diode at the lowest threshold for lasing and to add the modulation on top of that so the condition of the output swings between the two extremes.
Use an overcompensating heat sink on the diode , let me repeat that , Use an overcompensating heat sink on the diode. The biggest enemy of the diode is the prospect of a thermal avalanche- heat with a sharp knee leading to death.
Here's a look at technical paper with regards to Laser diode communication.
http://captain.haddock.8m.com/laser/laser1.html

A simple modulation scheme would be far less difficult......... a few gates making a clock that ramps up the current thru a resistor ladder.

 

I don't need to transmit any information over the beam, I just want it to be amplitude modulated so that I can have a photodiode array that is filtered to the modulation frequency to so i can effectively "see" the laser dot with the photodiodes.
would the frequency matter? I figured I'd just pick a frequency out of the air like 30K, are there any real factor contributing to the decision of what modulation frequency? I believe I read that a basic laser diode like mine can handle up to 100MHZ, so I can't go too high.
I think the filter I'll be using can handle 150K, and I obviously don't want to go too low or I'll get some output thanks to lights other than my laser.

__________________
---------------------------------
Rogue5
University of New Hampshire
Electrical Engineering
- - - - - - - - - - - - - - - - - - -
SETI@Home:
TeAm AnandTech
----------------------------------