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Extend the LPM range with a filter.

Blord

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I have tried the reflection method from Cyparagon to reduce the power of the beam. I have mixed results of this method. My test laser was a mid power 445nm M140 diode.
Measuring directly on my LPM the output was 888mW. When I use a glass in 45 degree angle to the LPM the measurement depends on the axial rotation of the laser !
First I use both reflected dots on the sensor. The second dot came from the other side of the glass. This side creates a second dot.
When the dot are in the vertical position the output is only 16mW. This is the setup in the picture.
But I rotate the laser 90 degree axially the output is suddenly 134mW !!!
The differences make the calculation very hard. A slightly turned laser could yield a different output.

Somehow the coherent nature of laser interfere with glass surface.
I think this method create an unnecessary factor in the LPM measurement.


Don't mind the LPM reading but look at the position of the dot.
 



ryansoh3

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I have tried the reflection method from Cyparagon to reduce the power of the beam. I have mixed results of this method. My test laser was a mid power 445nm M140 diode.
Measuring directly on my LPM the output was 888mW. When I use a glass in 45 degree angle to the LPM the measurement depends on the axial rotation of the laser !
First I use both reflected dots on the sensor. The second dot came from the other side of the glass. This side creates a second dot.
When the dot are in the vertical position the output is only 16mW. This is the setup in the picture.

But I rotate the laser 90 degree axially the output is suddenly 134mW !!!
The differences make the calculation very hard. A slightly turned laser could yield a different output.

Somehow the coherent nature of laser interfere with glass surface.
I think this method create an unnecessary factor in the LPM measurement.



Don't mind the LPM reading but look at the position of the dot.
Hmm... I'm just a bit confused because you said that in the picture, the dots are vertical.

However, I'm only seeing two horizontal dots on the LPM sensor, side by side. :thinking:


Now, this doesn't make much sense. What the heck could be causing this effect?

Thanks for experimenting. Cheers! :beer:
 
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Blord

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Just look at the position of one dot. It is vertical.

I made another picture of the dot in horizontal position. I have turned down the exposure because the dot is already very bright. You can see that the output is a lot more.
 

ryansoh3

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Gotcha. Sorry, I didn't notice that at first glance.

This phenomenon is mind boggling though... :thinking:

Cheers!
 

ryansoh3

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When light strikes the surface of a dielectric (non-conducting) material, a portion of the
incident light is reflected and the remainder is transmitted into the material. The fraction
of the incident light that is reflected depends on both the angle of incidence and the
polarization state of the incident light.
The functions that describe the reflection of light
polarized parallel and perpendicular to the plane of incidence are called the Fresnel
Equations. (The plane of incidence is the plane that contains the incident and reflected
rays and the normal to the surface.) Figure 1 shows a graph of the Fresnel equations.
Notice that perpendicular () polarization is almost always reflected more strongly than
the parallel (||) state. The exception occurs at normal incidence and at near 90 o
incidence, where they are both reflected equally. Figure 1 also shows that for one angle
of incidence, called Brewster's Angle, none of the parallel polarization is reflected.
Sorry for the block of text, but I think this is it. I'm guessing that the polarization of the laser beam directly affects the percentage of the light reflected via the glass.

The document is here:

http://uotechnology.edu.iq/dep-laserandoptoelec-eng/laboratory/Laser principles lab/(9)Brewster’s Angle.pdf

Cheers! :beer:
 
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Cyparagon

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I just tried it and I got the same problem. Not as pronounced (18mW vs 57mW), but definitely making it useless for your standard pointer. I know for a fact many systems use this method, but the glass, beam, and photo-diode never move with respect to each other.

I've personally used the diffraction grating method in the past though. I've just tried it again at various angles and rotations, and it makes about zero difference. Try that one. I like these, if you don't already have one.
 

lasersbee

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How do they hold up to a 2Watt beam...??
At what power would the plastic diffraction gratings
start to damage ??


Jerry

You can contact us at any time on our Website: J.BAUER Electronics
 

LSRFAQ

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Unless you compensate for it, the law of Malus (polarization vs angle) will come back to bite you.

Schott NG-5
Schott BG-5
Schott BG-10

Steve
 

LSRFAQ

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Unless you compensate for it, the law of Malus (polarization vs angle) will come back to bite you.

Schott NG-5 is a typical glass used as a flat attenuator.

The Schott data sheet shows the normalized adsorption. You'll have to pick a thickness corresponding to the attenuation you really need.

Its not truely flat on its curve, but close.

This is one of the reasons why the lasercheck has a EEPROM.

Steve
 




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