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Regarding the laser diode(LM8-650) beam offset after 50 metres

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Advanced Member level 1
Jun 1, 2011
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Dear Sir,

I am using the Laser Diode LM8-650 for my application. i am fixing it ridigly. after 50 metres there is an offset of 20mm in the laser beam spot. what could be the cause for it?

24 hrs endurance test had been taken by marking the laser spot:
after 4 hrs --> 5mm offset
after 8 hrs ---> 10mm offset
after 12 hrs ---> 20mm offset
and after 24 hrs --> it come backs to the original position.

what could be reason for the laser offset after 50metres?
Please let me know

V. Prakash

Temperature drift or mode hopping would be my guess, maybe thermal lensing.

Laser diodes are prone to this sort of thing, look at the pointing angle stability spec on the better sort of datasheet.
Temperature control would probably help some, but at the end of the day this is a cheap module, not something from Coherent...

Regards, Dan.
At 50m, a 20mm offset means the error angle is 0.00039 radians (or 0.022918 degrees). Quite hard to keep long time on spot, without using a lot of kind of compensations.
laser diodes are fabricated (packaged) using various UV cure epoxies. All epoxies outgas. I would guess that the epoxy starts out solid, but then heats up, outgasses and there is a slight variation of the dielectric constant as it outgasses, and at some point the outgassing becomes minimal so the lens/adhesive system returns to its original shape?

One trick we used to use is to heat cure all epoxies WELL ABOVE the usage temperature. i.e. if I knew the Laser chip was going to run at 100 deg C....I would make sure I cured the epoxy at 130 deg C for a couple hours. then let it all cool and start using the electronics

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