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LASERS - just a bit about them

Just a couple ideas for those who are set on experimenting with lasers.

FIRST and most important: Lasers are dangerous. I am not espousing that you do anything with lasers, as you can be blinded BEFORE YOU CAN BLINK, if you use lasers stronger than the common "keychain" laser pointers.

SECOND: If you DO work with lasers the next IMPORTANT thing is to get yourself laser glasses or goggles. The OD rating is the optical density, which means how well these glasses will protect you from eye damage.  OD of 2 means laser light is cut to 1/100th its prior level. If you use OD2 glasses with a 1 watt laser, 1/100th of a watt or 10 milliwatts still gets through and that is dangerous to the eyes. OD3 means laser light is cut to 1/1000th the original. For lasers in the watt range, OD3 is the least protection you should accept.

THIRD: Do NOT use common sunglasses or glasses or goggles that say they are for lasers, but do not show their OD rating at the frequency of your laser. Ask yourself this, "How much is my eyesight worth to me?"

FOURTH: Lastly, if you do screw up, don't say I didn't warn you how dangerous lasers are.


Now what I was thinking of posting here are a couple schematics to give you an idea how to build a laser pulser circuit. These can also be used for pulsing LEDs as well, especially the high current/high brightness LEDs. These circuits are only guides and need to be adjusted to work within the parameters of the LEDs or LASERs you are working with.

 

LED high-brightness pulser circuit

This transistor drew about 22.5 mA from the picaxe output, and sent 250 mA through the diode. This varies slightly with each individual transistor. NOTE that the picaxe outputs are not supposed to go over 20 mA each, and I was drawing a little more than that. Consequently, remember that you MUST let the circuits cool down after each pulse. I indicated not repeating the pulses more often than 1000 times per second (1 kHz.)

I tested this with the components shown and was getting ~250 mAmp pulses through the LED as indicated. Repeated at 1kHz. you should definately see brighter light from the LEDs. And, YES, this can shorten the life of your LEDs. I am only showing how it might be done, not advising you to do it. <g>

 

In the above circuit, I used 2N2222s in a darlington configuration. I have some actual darlingtons here, but did not use them because they did not respond quickly enough for proper pulse shaping. The 2N2222s are borderline and results can vary from transistor to transistor, (so I would advise using an oscilloscope when building your driver circuit). Higher frequency transistors should work better, but I chose 2N2222s because they are common. You might also use 3 or 4 series LEDs in the circuit to test it before trying it with a more expensive Laser diode.  This latter circuit draws much less current from the picaxe chip than the earlier one.

Good Luck and **Be Careful**.

@ Tinhead - You mentioned your laser diode is in the 50 to 200 mW range. The laser diode I used here is a bit stronger, but I limited the current to be in the same ballpark. At 3.7 volts it passes 200 mA, with an average power out of 53 mW.

The minimum voltage is supposed to be 3.8 volts, but this one still lased at 3.7 volts. Take a look at the pictures and ask yourself if your laser will do the same.  If not, you might try pulsing it.

Not exactly wood, this is only paper, but how quickly it starts to burn shows that it would go through wood as well, just slower.

...and you also mentioned plastic. Here I took an old junk CD (One of those we used to get in the mail free from AOL and other places.) And because dark colors will absorb heat quicker, I tried this on the shiny side of the CD -- being careful about where the laser beam would bounce to if it did reflect ! --. Each letter took between 5 and 10 seconds to burn in. As above this was 3.7 volts supply giving an average 53 mW PULSED output, not CW.

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If I got things straight from the pulsing circuit you posted you use that the logic chip to create a nanosecond wide pulse form the input pulse, so because I have to try it my way to really understand how it works I build the below. Hopefully I'm not totally off in my understandig of this.

This is the current circuit I'm playing with:

The ATMega is setup to PWM pin PB3 at the highest fast PWM speed possible using timer2 (yes in the schematics there is an atmega8 but it is actually a 328), i.e. 16 Mhz with a 1% pulse width giving 62.5 nano seconds per pulse.

I used a resistor of 100 ohms to limit the current in this test but when I first attempted this I had no resistor in there.

Ignore the missing XTAL and reference stuff missing they are there in reality. 

So what I think I did wrong and burned the laserdiodes diodes is that I have tried to increase the pulse width instead of juggling with the input voltage/current at the input marked +5V in the schematics. Correct?

As we discussed on ShoutBox, that sounds like the case. Increasing the pulse width at a certain current/voltage level will increase the average power output. The average must stay below whatever the diodes are rated at even though all the power is concentrated into short blasts (pulses).

 

I have to admit I need to grubble on this for a while to get an understanding of the pulsing circuit. As far as I understand so far you feed it a square wave which then gets divided by the hex inverter to get a 198 nano second pulse?

That would make sense since the the pulsewidth is so narrow ... 

Correct.

Hmmm, this was supposed to be in answer to the comment just below. Somehow I posted it as a separate comment.

____________________________________________

This question is hard to answer. Here is why:

Firstly, you indicate that you are not sure the exact parameters of the laser diode you are using, and

Secondly, laser diodes can vary from one to the next, even within the same run.

If you do not have a spec sheet for your laser diode, search the internet for it.  If it was removed from a DVD, then search for what diode is used in that equipment. Once you find the specs for your diode, it tells you the minimum parameters that the diode is good for. What it does not tell you is whether the diode came out better than that, allowing you to increase the wattage output you are shooting for.  Regrettably, without sophisticated equipment, the only way to determine this is trial and error. In the case of lasers, you can ruin your laser diode by running too much current through it, as that will cause it to overheat and burn out.

Here is the best way I can suggest for you to seek out the upper power limits on your laser diode. Hook your pulsing circuit to a variable voltage, regulated power supply, preferably calibrated down to hundredths of a volt (or less) start by driving the diode at a power level you are sure it will handle (the CW values). You will also need a way to read out the temperature. (Remember that the laser junction will be hotter than the heat sink.)

Now, SLOWLY increase the voltage to the diode (preferably by hundredths of a volt at a time) and wait several seconds between each increase. This long wait is to give the heat sink time to heat up at that power level, so you get a more realistic temperature reading. Stop increasing when you are approaching the allowed maximum temperature found on the spec sheet.

During this slow process, you have left the frequency of the pulses alone.  Next, try giving the laser more time to cool down, by decreasing the pulse repetition frequency. For instance, if you were pulsing it at 10 kHz, try cutting the frequency to 1 kHz. If you were pulsing it at 1 kHz, try cutting the frequency to 100 Hz. Theoretically, the pulses through the laser diode could be made 10 times as great with ten times as long to cool in between, but since this relates to your heat-sink arrangement and other factors, you probably can't realise that great of an increase.

 

 

 

I've been playing sometime ago with CD-RW and DVD diodes, probably in the 50 to 200mW range, eye protection is deffinetly something to have in mind with those especially for IR which cannot be seen.

My question related to pulse driving is how much power can you actually get? Enough to cut stuff like wood or maybe plastics?

With 200mW I know you can barely cut black insulation tape, no way other stuff.

see above (sorry)

 

OH, and since pulse power hits with a wavefront like a higher powered laser,

be SURE your safety glasses have as high an optical density rating

as you would need for the higher power laser.

 

The ones I have for my 2W blue laser are over OD5  ---which cuts the light down to 1/100000th   (OD5 = 5 zeros)

That allows me to pulse it as though it were a very high power laser without danger to my eyes. (I got the glasses for $13 USD which included shipping.)