Temperature and laser??

Hi.
So I live indoors and right now though its summer, its about 60 degrees in my house due to A/C. I own a 31mw laser that has no melting capabilities. Im not sure what it peaks at. Anyhow today I noticed after I had left the case on top of my a/c it wasnt very bright. I checked my batterys with a multi-meter on my Li Mh battery's and they were putting out 1.36 volts. (thats normal) i then discovered after heating the laser in my hand it shined a little brighter than normal. I heated it again in my hands and tried to pop a baloon with black marker. It did not pop but it left a melt mark on it. So I had a hunch. I took my lasers batteries out charged them, held the diode in the toaster oven for a minute or two assembled it quickly and shined it at the same balloon in a different spot....

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it bursted almost instantly! was it that I charged my batteries or heated the diode? The batteries only put out 1.38 volts each after the charge. Either way this was still pretty interesting to watch

Most LED (light emmiting devices) responed poorly to heat, thats pretty cool.

Wish I could afford to set my AC below 80 degrees Maybe 76 degrees ---

Mike

Yeah its weird as hell. Im completely confused. Please someone I really want an explanation

I think it is quite common that lasers either like warm or cold. My wicked laser 35mw is like that too. It gets noticeably brighter after it has been warmed. Usually I first just warm it with my hands and then put it on for about 20 seconds continously which warms the diode.

If it is really cold outside then the beam gets very weak and changes mode, splits into two beams. But anyways I can't explain in detail why this happens.

Check out Sams laser faq for details on the temperature sensitivity of laser crystals
BTW, at 60 degrees have you ever wonder how large your carbon footprint is?

Diodes perform better with temperature. Look at thermal runaway, a diode is more than happy to burn itself to death, if you let it. The warmer the PN junction the more current will pass, more current more heat ect. ect...

Look at precision diode instruments they are all temperature controlled with a heater or temp compensated.

Next time hook up a meter to measure current. Then spray the laser with canned air to chill it. You should see the current drop.

Doesn't temp normally raise resistance which normally bottlenecks and causes the current to drop a bit. That's what I think it is I could be wrong I forget sometimes.

Tek465 said:

Diodes perform better with temperature. Look at thermal runaway, a diode is more than happy to burn itself to death, if you let it. The warmer the PN junction the more current will pass, more current more heat ect. ect...

Click to expand...

No, diodes perform 'better' and most efficiently at cool temperatures. They also live a lot longer. A diode kept at 20C might have a lifetime of 100,000 hours, while the same part operating at 40-50C will have its lifetime shortened by several orders of magnitude (< 1000 hours). Thermal runaway is often caused by the APC supply - as the diode warms it requires more current to lase at a given power level, so the power supply cooks the diode by increasing the amount of current to it as it warms, until it dies (usually this means the diode was not adequately heatsinked).

The primary reason greenies seem to like it warm is the KTP xtal which converts the 1064nm light to green (532 nm). It works best at warmer temps, and is the reason many greens will initially be weak when first started in cold temps. Basically there's a lot of temperature sensitive parts in a green DPSS laser, and unless they are all stabilized the output of most laser pointers will vary considerably with temperature.

I'll disagree with that. (Current does increase in a PN junction as temp increases.)

If I put a led on a potentiometer, It will get brighter as the resistance is lowered and the current increases. (Until it goes *poof*)

Same thing happens if I keep resistance and voltage the same but change the ambient temp of the led. The led is brighter at a higher temperature.

Sure the lifetime of the led is shortended exponentially as the current or temp is increased. But the original question was about why the laser is brighter at higher temperature. LD's have such a narrow gap between lasing and runaway that they should have a compensating circuit. (Which is the intent of APC)

You might be right about the KTP crystals. But until you take a greenie apart and freeze spray each section seperately. It's hard to tell what causes it, or if its a whole system thing.

But I have seen first hand the results between current, heat and bi-polar junctions. Working in electronics you can find alot of problems with a can of freeze spray.

Edit: One trick I've learned over the years is to get brighter Leds was to Pulse them with higher current. Normally this would kill them in CW operation, but if the pulse duration is short enough not much harm is done.

Try these sites: http://www.stockeryale.com/i/leds/lit/app001.htm

http://www.piclist.com/techref/io/led/pulse.htm

Tek465 said:

I'll disagree with that. (Current does increase in a PN junction as temp increases.)

If I put a led on a potentiometer, It will get brighter as the resistance is lowered and the current increases. (Until it goes *poof*)

Same thing happens if I keep resistance and voltage the same but change the ambient temp of the led. The led is brighter at a higher temperature.

Click to expand...

Laser diodes != LEDs. I'm assuming at the very least you're limiting current to the LD using a resistor. As the diode warms its output drops.

See SAM's FAQ:
http://www.repairfaq.org/sam/laserdio.htm#diotoc
(especially this section on diode life)
http://www.repairfaq.org/sam/laserdio.htm#dioldl

FloggedSynapse said:

Laser diodes != LEDs. I'm assuming at the very least you're limiting current to the LD using a resistor. As the diode warms its output drops.

See SAM's FAQ:
http://www.repairfaq.org/sam/laserdio.htm#diotoc
(especially this section on diode life)
http://www.repairfaq.org/sam/laserdio.htm#dioldl

Click to expand...

I have limited experience with red laser diodes. It has been my experience that as the diode warms up, the output drops considerably, and the current does NOT change. Why do so many manufactures make laser modules and include TEC? I have found them in all colors. From my experience, the cooler the diode, the happier it is.

LD = specialized LED

An LD is just an LED with intergrated lasing cavity. LD's can be pushed past maximum CW current, that's why they give you specs for Pulsed and CW current operation in Datasheets.

The laser (simplistically speaking) is two parallel mirrors at some wavelength apart so photon emission is amplified and coherent. LD's get the mirror surface by cleaving the led crystal so a cavity is formed. Heat can cause that cavity to permently deform so you get an expensive led or it simply destroys the PN junction of the diode itself.

Edit: What can also happen with heat is the lasing material can change emission wavelengths that is no longer optimal for the physical lasing cavity. That's why you might see a decrease in output of the laser at different temps.

But you can pulse a LD to output more energy at a certain temp, you just have to know where the limit is and not cross it. CW operation will certainly get you over that limit in a hurry.

Hi Folks,

I must reiterate the known fact that semiconductor junction devices of all types will continue to draw increasing amounts of current as the junction temperature increases, given a constant applied voltage. This is well known and is the fact responsible for the phrase "thermal runaway". The job of the non-APC driver circuit is to keep the laser diode voltage and current constant despite increasing junction temperature.

The charcteristic described in this thread is most certainly attributable to poor alignment and heat-sinking of the crystal assembly. Depending upon the manufacturer, some crystals are mounted so poorly that they have virtually no heat-sink at all.

I have seen many lasers with positive temperature coefficients. It is almost always due to a pretty coincidence: First, the MCA or crystal assembly is not mounted in the proper orientation to produce maximum power at ambient or near ambient temperatures, normally taken to be 68 degrees Fahrenheit. Next, the MCA is not properly heat-sinked. Then, as the temperature increases, the crystals move around in the optical cavity due to thermal expansion. Then, by sheer luck, the MCA moves into an orientation that is more efficient with regards to the incident pump energy, and power increases up to a point, beyond which power will begin to decrease as temperature continues to climb and the crystals move around some more.

It is sheer coincidence that you have a laser with these properties, accidentally born of poor manufacturing and design techniques. If a DPSS crystal assembly is mounted properly, heat-sinked adequately, and aligned with the LD for maximum power at ambient (room) temperature, (and if the driver/regulator circuits are working properly), the result will always be a laser that exhibits a negative coefficient of output power vs. temperature ( non-APC ).

What this tells me is that you could probably fix the problem by re-aligning the LD to MCA orientation in both the X-Y and polarity (rotational) axes, and squirting a little heat paste in around the edges of the MCA where it touches the cavity barrel....not an easy thing to do, but not impossible, either. One must perform the alignment carefully using a power meter, allowing the assembly to cool to ambient temperature each time an adjustment is made to observe the effect from a cold start. After the assembly is hot, it makes no sense to continue to try to perform the alignment. Adjust it, measure it, let it cool, and try again. And yes, check to make sure that current is held constant over the entire duty cycle regardless of temperature to be sure the regulator circuit is working correctly. The goal is to produce maximum output in the time period between a cold start and approximately 10 to 20 seconds of sustained operation and heating. If you peak it while it is hot, then you will inevitably end up with a laser that only peaks when hot! ( makes sense, eh?)

Cheers,

Lew

As far as I know, every electronic and mechanical components have its own working temperature range specification. In this case with laser diode is around 30C. If it gets colder, the output drops. If it gets hotter, the output will drops as well. You can only attain its peak optical output at its working temperature range as specified. Read up on its data sheet to be sure.

Cheers.

Here's what I believe may be happening, however, I haven't put my laser pointer under the scope yet to verify it.

Many of these laser pointers use a LM431 shunt regulator to set an internal 2.5V reference voltage. The feedback from the photo diode (PD) is compared to a fraction of the reference voltage.

NiMH batteries, under load, will have a very hard time sustaining the reference voltage (2x series NiMH -> approx 2.4V). This can lead to the feedback from the PD being compared to a lower voltage and the output will be lower. As Lew has mentioned above, heating up the Laser Diode (LD) will lower its forward voltage (and output) at a set current. This works in favor of NiMH batteries due to their lower voltage output.

I am considering getting 3x 2/3AAA NiMH cells to try in my green laser.

Paul