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Ghetto laser power meter

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Conundrum

Mon Feb 28 2011, 08:16PM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4061

Hi all.
As you know, I recently acquired some "300mW" IR laser diodes from an ebay seller with the intention of repairing my defunct green module.

Needless to say this didn't work. It appears that the crystal is "toast" due to excess input power and/or exposure to moisture.

However, I managed to kludge together a makeshift power meter using the dead module's sensor photodiode (3mm2) after carefully removing it from the turning mirror with a small screwdriver.
Diode 100% fine, looks perfect :)

I had a surplus homemade temperature sensor PCB (Vellemann kit) which uses a transistor and diode to measure absolute temperature by comparing the two voltages, so thought that if I replaced the diode with a photodiode in forward bias mode then the silicon offset with temperature should be swamped out by the photovoltaic voltage, but the two sensors should rise in temperature by the same amount antd therefore cancel.

Glued diode to transistor so the temperature rise was consistent, and did some tests.

I calibrated it roughly with the two trimpots until it read zero, then used my bluray test module to get an idea of the sensitivity.

So with 61.5mA into the BRD and focussed onto the sensor I got around -47.1 on the meter which rapidly dropped down to zero as the diode cooled to ambient.

Tried it with Mr IR diode and the readings are as follows.

61.5mA input = -6.5 (mv presumably)
69.2mA input = -9.8
82.4mA input = -47.8
104.8mA in = -118.1

From this I would estimate the lasing threshold for the diode used to be around 70mA as the sudden rise in output power takes place between 69.2 and 82.4 so extrapolating down the slope would put threshold around here.
This particular diode seems to peak in efficiency around 100mA which is to be expected.

1298927219 96 FT1630 Image014

[Edit: Image size]
-A

#include "FFSWEARGOGGLESOMGWTFBBQ.h"

haxor5354

Mon Feb 28 2011, 10:14PM

Registered Member #2063 Joined: Sat Apr 04 2009, 03:16PM
Location: Toronto
Posts: 352

you can toast a frequency doubler crystal?

Shrad

Tue Mar 01 2011, 09:37AM

Registered Member #3215 Joined: Sun Sept 19 2010, 08:42PM
Location:
Posts: 780

hi,

the best way to measure power from a laser diode is to use a very small peltier cooler with a layer of carbon powder on the measuring face and a heatsink on the other

the voltage developped will be directly proportionnal to the power

be aware that a bluray pointer will not give accurate comparison points, because usual black coatings (carbon, transistor body, etc...) will have a linear absorption curve only from 420nm to 1200nm, and it drops rapidly when you decrease in nanometers below that range

Bored Chemist

Tue Mar 01 2011, 08:04PM

Registered Member #193 Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022

Soot is near perfectly black at just about any wavelengths you will be looking at (Hard UV to far IR).

Conundrum

Wed Mar 02 2011, 09:00PM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4061

Tried a different diode and got a brief flash of green from the crystal as I changed power settings.

Evidently the alignment is more precise than I thought as both the X,Y and rotation (phase) are critical.
Shame that they don't make 1064nm diodes as this would make the doubling process more efficient.

-A

Shrad

Wed Mar 02 2011, 09:12PM

Registered Member #3215 Joined: Sun Sept 19 2010, 08:42PM
Location:
Posts: 780

they make them, but they cost more than a brand new pig pole

aligning the diode and the composite crystal can be a pita, but imagine aligning more optics like separate crystals, diode and output coupler in a more conventional cavity

I have already spent tens of hours on several projects only to get them aligned

Conundrum

Thu Mar 03 2011, 10:55PM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4061

Yay, I got stable green.

I did have an interesting idea though. This particular "core" has a right angle mirror inside which acts as the cavity so in theory I could attach a PS3 diode (via a suitable lens if needed) and a second turning mirror to make a very compact RGV laser.

Also use the existing diode inside the BRIR as the feedback
for the infrared.
Reckon this would work?

-A

Shrad

Sat Mar 05 2011, 01:33PM

Registered Member #3215 Joined: Sun Sept 19 2010, 08:42PM
Location:
Posts: 780

could you take a picture of this cavity?

it could be a beam pickoff for a light feedback to a photodiode, minus the photodiode (maybe a common base for the simple pointers and stabilized ones, I have already seen this)

what you could do with a PS3 sled is keep everything and use it as a RGV combiner in itself :

[Edit: Image size]

plazmatron

Sat Mar 05 2011, 05:38PM

Registered Member #1134 Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351

haxor5354 wrote ...

you can toast a frequency doubler crystal?

Kinda.
There are various failure modes.

Either you can cause the Nd:YV404 and KTP to de-laminate (in the case of optically contacted crystals)

In cheaper glued crystals the optical glue can fail.

The KTP can turn gray from excess power, (called gray tracking) leading to heating and ultimately shattering of the crystal.

Les

Conundrum

Sat Mar 05 2011, 08:43PM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4061

will do, i took it apart for the Nth time today but it appears that Epoxy didn't hold (the power kept jumping all over the place when I moved the module) so am trying to find a glue which actually holds without fuming.
(cyano + laser =BAD idea!)

Currently annoyed, not only does my uv glue not work properly but in the process of trying to glue a polariser to that myvu display the ribbon cable loosened. Blast!
Oh well. I still have the optics which make a nice laserdiode examination tool.

-A
"Bother!" said Pooh, as his insurance didn't cover solar flare damage...

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