narrow (<20ns) pulses from a 5$ red laser pointer

[verdet]

Greetings:

I was wondering if anyone had tried pulsing the cheapo 650nm red laser pointers ( 5 dollar variety).

Recently I took a hacksaw and cut a red laser pointer case near the spring (terminal). I hooked the case to ground and the spring to the + output of an agilent 33220A fcn generator. I used a TTI fast photodiode and HP 500 MHz scope and voila -- the laser output was beautiful looking pulses about 20 nansec wide (the narrowest pulse the function generator can muster).. So what I'd like to know is.. just how narrow (in time) can one pulse these lasers--less than 5 ns? - 1ns???

thanks,
bill

 

[timelord]

Re: narrow (<20ns) pulses from a 5$ red laser poin

Welcome to the forum Bill an if possible could you do a little video of your creation please??

 

[verdet]

Re: narrow (<20ns) pulses from a 5$ red laser poin

I can take some pics of the laser, fcn generator, and scope in action -- but give me a couple of days. I took apart the setup (we used those components to measurte the speed of light) and my students are presently using the function generator in another arrangement to learn about phase sensitive detection (just an Optics teaching lab),

Bill

 

[timelord]

Re: narrow (<20ns) pulses from a 5$ red laser poin

Ok Bill we'll look forward to that very much

 

[SenKat]

Re: narrow (<20ns) pulses from a 5$ red laser poin

I am not certain how narrow a pulse you can create. If it is a CW device, (unsure - so please do not quote me ) but cannot damage be done by forcing it to pulse ? Better bet might be to get a pulsed diode that is deisgned to take that kind of punnishment, I would think. Someone please correct me if I am incorrect ! Also - Thanks for joggin the brain, and posting ! Welcome to the forum

 

[Proctor]

Re: narrow (<20ns) pulses from a 5$ red laser poin

I would think pulsing it allows it to build up energy and give it that initial burst of power when the lasers start up. It allows for cooling too for higher outputs.

 

[Hemlock_Mike]

Re: narrow (<20ns) pulses from a 5$ red laser poin

I don't think it will damage the diode so long as the peak current doesn't just blow the hell out of the junction. I'd watch for voltage spikes during turn on or inductive kick back at shut off.

Mike

 

[Proctor]

Re: narrow (<20ns) pulses from a 5$ red laser poin

Isn't pulsing in hz? What's "ns"? Nanoseconds?

 

[Hemlock_Mike]

Re: narrow (<20ns) pulses from a 5$ red laser poin

Hertz generally implies a sin wave frequency. When getting into PWM and duty cycles, Time on vs Time off tells what's happening.
Simply stating 20 ns pulses is not complete info if, for instance, the off time is 1 minute.

Mike

 

[SenKat]

Re: narrow (<20ns) pulses from a 5$ red laser poin

Hertz generally implies a sin wave frequency.  When getting into PWM and duty cycles, Time on vs Time off tells what's happening.  
Simply stating 20 ns pulses is not complete info if, for instance, the off time is 1 minute.

Mike

I think what is being infered is that it is pulsing at 20ns intervals...

 

[verdet]

Re: narrow (<20ns) pulses from a 5$ red laser poin

Concerning the pulsed laser.

I will be setting it up again. I am curently working on some other stuff.

changes -- I am shopping the used market for pulse generators in the < 5ns range (so we'll see what those cheapo pointers will do). I was operating the function generator with 20 ns full width half max (FWHM) at 5 MHz rep rate. The duty cycle was minimized to about 10%. Part of the trick is using a good photodetector. I was using a TIA 525 detectector (terahz technologoies) and a 500 MHz HP digital scope (more scope than one needs)

Pictures will be arriving soon.

if I can get a cheapo pulse generator like the TEK PG502 I will be able to build a short length table top -speed of light apparatus - that I can "store on the shelf" and not have to disassemble time and time again. (that is a mirrors glued to a translating arm). This allows one to obtain pulse time differences over a range of distances and obtain "c" graphically with a linear fit of distance vs. time between pulses on scope (c is the slope). d=ct

Peace