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Old 03-05-2011, 03:26 PM #1
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Exclamation Laser Point Thermal Effects

Hi,

I was just wondering what the effect of temperature variations would be on a typical laser pointer. I haven't noticed any major variations using my device in hot environments, although I don't have the equipment to measure this accurately.

Obviously, I'd expect increased carrier energies at raised temperatures and therefore, possibly increased leakage currents across the pn junction (thus reducing the available carriers for radiative recombination). I tried thinking about this in terms of the ideal shockley equation, concluding that a greater threshold current would be required at raised temperatures too. From reading around on the web, 'mode hopping' was mentioned as well.

I was just wondering if anyone with more experience in this could comment if the above seems plausible / suggest any other effects please?

Many Thanks,


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Old 03-09-2011, 09:51 PM #2
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Default Re: Laser Point Thermal Effects

Why would you expect increased carrier energy? And why would this be related to the leakage current? Laser diodes aren't simple PN junctions but heterojunctions.
I'd ahve to look up how it works, I'm currently following the course on solid state physics and we just started with semiconductor. What I do know is that laser diode have a shorter wavelength a lower temperatures. The threshold of a laser diode is a purely optical effect, the temperature probably won't change the gain much.
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Old 03-09-2011, 10:56 PM #3
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Exclamation Re: Laser Point Thermal Effects

Quote:
Originally Posted by Bluefan View Post
Why would you expect increased carrier energy? And why would this be related to the leakage current? Laser diodes aren't simple PN junctions but heterojunctions.
I'd ahve to look up how it works, I'm currently following the course on solid state physics and we just started with semiconductor. What I do know is that laser diode have a shorter wavelength a lower temperatures. The threshold of a laser diode is a purely optical effect, the temperature probably won't change the gain much.
Agreed, but there is a shift in frequency with 445nm diodes, and LOC-815 660-650nm depending on temperature as you are sort of explaining.

I am a bit out of my legue here on this.
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Old 03-09-2011, 11:44 PM #4
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Default Re: Laser Point Thermal Effects

With direct diode lasers (405, 445, reds), there is little practical implication. Its certainly true that wavelength and forward voltage vary with temperature, but within a reasonable range (say 10-40 celcius) these effects go mostly unnoticed.

Looking at green DPSS lasers, the effects are very pronounced. These rely on a 808 nm pump diode to provide energy to the solid state laser, which has a relatively small absorption bandwidth. Heating or cooling the 808 nm laser diode will alter its wavelength enough for the absorption to drop drastically.

Professional quality green lasers have mechanisms to control the diode temperature (usually active TEC cooling) to mitigate this problem, but most laser pointers do not. Its quite common for green laser pointers to drop power by a factor of 10 when under about 10 centigrade or over about 35 - the optimum varies with make, model and even individual unit.
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Old 03-11-2011, 12:14 PM #5
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Default Re: Laser Point Thermal Effects

The lattice constant of a semiconductor changes with temperature because of expansion which affects the bandgap, this changes the wavelength of a laser diode.
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