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Old 12-15-2016, 08:01 PM #1
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Default [Physics Question] Is the illumination pattern of a laser diode constant?

Hi Everyone!

Let's say I expand a laser diode (single mode or multi-mode) using a set of cylindrical lenses, and measure the resulting illumination pattern using a camera. Will the illumination pattern that I image vary:

1) From diode to diode?
2) Depending on the temperature of the diode?
3) Depending on the output power of the diode? (Obviously the absolute intensity will increase, but will the pattern change?)
4) Depending on the age of the diode (e.g. wear and tear)?
5) Depending on the time that the diode is on?
6) Any other factors that might affect the illumination pattern?

Looking for either experimental data or a physics explanation. Thanks!


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Old 12-15-2016, 11:54 PM #2
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Default Re: [Physics Question] Is the illumination pattern of a laser diode constant?

Quote:
Originally Posted by ArKi View Post
Hi Everyone!

Let's say I expand a laser diode (single mode or multi-mode) using a set of cylindrical lenses, and measure the resulting illumination pattern using a camera. Will the illumination pattern that I image vary:

1) From diode to diode?
2) Depending on the temperature of the diode?
3) Depending on the output power of the diode? (Obviously the absolute intensity will increase, but will the pattern change?)
4) Depending on the age of the diode (e.g. wear and tear)?
5) Depending on the time that the diode is on?
6) Any other factors that might affect the illumination pattern?

Looking for either experimental data or a physics explanation. Thanks!
Laboratory lasers will perform extremely well within designed manufacturing parameters.
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Old 12-16-2016, 08:29 AM #3
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Default Re: [Physics Question] Is the illumination pattern of a laser diode constant?

Quote:
Originally Posted by ArKi View Post
Hi Everyone!

Let's say I expand a laser diode (single mode or multi-mode) using a set of cylindrical lenses, and measure the resulting illumination pattern using a camera. Will the illumination pattern that I image vary:

1) From diode to diode?
2) Depending on the temperature of the diode?
3) Depending on the output power of the diode? (Obviously the absolute intensity will increase, but will the pattern change?)
4) Depending on the age of the diode (e.g. wear and tear)?
5) Depending on the time that the diode is on?
6) Any other factors that might affect the illumination pattern?

Looking for either experimental data or a physics explanation. Thanks!
Just quick answers without going too much in the depth:

1) Yes, it is depending on optical cavity of diode - different diode might have differnet cavity.
2) As far as I know the temperature influences output power and also it might change optical cavity due to thermal expansion -> I would say yes.
3) I've noticed my diode lasers shift modes when changing power - the pattern changes than according to mode.
4) Not sure about that. I guess it might change due to changes in the diode itself - the cavity might change.
5) I think for a good lab lasers after warmup it does not (they are well cooled and with feedback to keep the beam stabilized - anyway it depends how stabilized the beam is). In general in the time laser is on, it increases its temperature - so there are some duty cycles to prevent damage.

6) There are many - the illumination pattern is in general depending on the optical cavity of laser, so what influences cavity influences the pattern.

You might use some form of spatial filtering to get just the central part of the beam to get desired pattern.

BTW: If more experienced members of LPF find some errors in my post sorry for that. Please let me know, I will be happy to learn.
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Last edited by Radim; 12-16-2016 at 08:37 AM.
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Old 12-16-2016, 03:47 PM #4
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Default Re: [Physics Question] Is the illumination pattern of a laser diode constant?

Radim is correct. There can be many variations in the optical performace of laser diodes due to he factors you mention. Power, temperature, and mechanical changes are all major and related factors. They all can cause changes in the lasing medium and optical cavity.

The build quality of the diodes is a HUGE factor though. These sorts of variations are the reason lab lasers exist and can be so much more expensive than consumer lasers.
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Old 12-18-2016, 03:31 AM #5
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Default Re: [Physics Question] Is the illumination pattern of a laser diode constant?

I've seen a IR muiltmode change as it got hotter and when i'd click it off for a few seconds and back on it would change shape. no lens was on
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Old 12-18-2016, 01:35 PM #6
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Default Re: [Physics Question] Is the illumination pattern of a laser diode constant?

Have you ever seen the bar "twitch" at distance?
I have at about 75 feet with the laser clamped down the bar shaped output was twitching up and down at a random rate.
I have also read that they introduce impurities at the edges of the substrate to define the edges, many MM diodes that we see are hotter in the center, but that varies too from diode to diode.
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Old 12-18-2016, 03:02 PM #7
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Default Re: [Physics Question] Is the illumination pattern of a laser diode constant?

...of course the lasing medium also has influence. Lasers - even diode lasers - are quite complex electromagnetic radiation sources.

To add more about the illumination patterns there are also variations in terms of speckle patterns. I've noticed them as a kid when playing with red laser pointer and they were fascinating. Also when I'm photographing objects iluminated by lasers it is quite difficult to get sharp image at near distance due to these patterns. So, when considering the setup of the questioner - there is not only laser diode, but also the surface iluminated, camera lens and whole apparatus setup involved in measurment under certain detail. More is here (well it's wiki - so just informative): https://en.wikipedia.org/wiki/Speckle_pattern
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Last edited by Radim; 12-18-2016 at 05:00 PM.
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