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# WTB: 10 Watt Laser Power Meter

##### Well-known member
LPF Site Supporter
I have not used a mirror that way, but I believe the mirror would need to be turned ~45 degrees (perhaps less?) so the 90% reflected goes to the right, or left, and away from the laser. Ask the seller if 90% is reflected, or 10%, not sure how the mirror works in regard to which is reflected and if the mirror is just as efficient at 45 degrees. I suspect the curves might have been taken at that angle and that is how it is normally used.

If you buy the mirror and set it up to measure the 10%, please take a photo and share with us in this thread

I have a LPM which can measure up to 300 watts, but this mirror trick seems so cool I want to do the same thing on one of my power power meters now. I think it was Cyparagon who first recommended this to a member a year or two ago as a way to measure higher power than a meter can read. For that matter, even a clear piece of very flat glass can be used if turned to 45 degrees, but not sure the amount reflected. Google is your friend when Cyparagon doesn't come to the rescue, but I am sure there are members who can tell us the answer to that too, if they come across this question and are inclined to respond.

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#### HydroSean

##### New member
But, what happened if the reflected beam accidentally goes back through the diode? Though i'm not really sure, it will most likely "burn" the diode's die right? (Assuming that the measured lasers are a diode laser)
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I'm not responsible for any of your actions, but i just want to share my knowledge here:
Measuring 10W laser using 5W LPM (for example Ophir 20C-A-1-Y) is quite easy if you can do a soldering and desoldering:
1. Open your 20C-A-1-Y without touching the trimmer/potentiometer
2. Desolder the 78L08 regulator, replace with 78L12 for max 10.8W max reading.
3. Keep your 78L08 as a means for backup.

The calibration will intact as long as the trimmer/pot is untouched.
This method is of course has a drawback, the initial offset will grow to 12/8 times from when you used 78L08.
But you can easily revert to 78L08 though.
You can also change 78L08 to 78L15 for 13.8W max reading with initial offset grows to 15/8 times from original.

But if you're in doubt with your soldering/desoldering ability please don't do these lol.
And please be wary of static electric as it will kill the IC inside.

The beamsplitter is set at a 45 degree angle most of the time for that reason as well as others. Here's how they work, I learned a lot from this page.
What are Beamsplitters?

##### Well-known member
LPF Site Supporter
Google was my friend again, well, actuall, Dr. Sam:

Extending the Range of a Laser Power Meter 2
So, you *finally* acquired that 10 W CW laser and have no way of measuring its output power because your laser power meter can only go to 100 mW.
Assuming your laser power meter can be used at the wavelength of your new BIG laser, it can easily be adapted to read high power as long as the polarization of the laser is fixed (see below). Send the laser beam through a pair of 45 degree plain glass beamsplitters (e.g., microscope slides) in series with the reflected beam from beamsplitter 1 going to beamsplitter 2 and sending only the reflected beam from beamsplitter 2 to your laser power meter's sensor. Each beamsplitter will reflect about 8 percent and pass 92 percent. So, after two reflections, you get about 0.64 percent. The reading on the laser power meter will then be about 0.64 percent of the true power or roughly 64 mW for a 10 W laser. It can be calibrated more accurately by using a laser of known power to test it. The laser doesn't need to be high power as long as 0.64 percent of its power can be measured with enough resolution on your laser power meter.

There are at least two advantages to this approach over that of using neutral density filters to cut down the beam intensity. The main one is that there is no problem with the beam passing through plain glass while a neutral density filter could easily be damaged by an intense beam. The other one is that the cost is negligible!

Where the polarization of the source isn't constant (e.g., it is from a randomly polarized ion laser or from a multimode fiber), it is essential that the beamsplitter be polarization insensitive. The plain glass at 45 degrees does not satisfy this requirement since its getting close to the Brewster angle. For example, using the plain glass beamsplitter with a high power laser diode fed through a multimode fiber may result in a power reading that varies by a factor of two or more by just moving the fiber as the polarizations of the various modes move and their polarizations change. Furthermore, since the distribution of power in the various modes tend to change with power, the reading may not be linear with respect to power even if the fiber isn't touched. For a random polarized HeNe laser, the power reading may change by 10:1 due to mode sweep as the tube warms up. If the angle of incidence is arranged to be close to 90 degrees (normal incidence) rather than 45 degrees, the error will be small, but this is generally difficult and may not be possible at all.

Commercial beamsplitters are also available which are relatively polarization insensitive. But many are far from perfect and a residual error of 5 to 10 percent is often present. This will depend on design and is generally somewhat wavelength dependent. The manufacturer will generally supply a plot of the S and P polarization versus wavelength.

However, a polarization-insensitive beam sampler can be made by using two identical beam reflecting plates in series oriented so that the orthogonal polarizations are reflected at the same two angles of incidence (but in opposite order). So, for example, orient the first plate at 45 degree incidence so the vertical polarization has a high reflectance while the horizontal polarization has a low reflectance. Orient the second plate so the opposite occurs for the beam reflected by the first plate. If done with care, the result will be a beam sampler that is totally independent of polarization.

The extension to even higher power or for a laser power meter with a lower maximum power rating should be obvious.

WARNING: Make sure that the non-reflected beams terminate in something that can take the power and not burst into flames!!! And don't forget the laser safety goggles!!!

Sam's Laser FAQ - Items of Interest

#### paul1598419

##### Well-known member
Yeah, Sam's Laser FAQs are a wealth of information about everything from laser optics to gas lasers and diodes, along with many other important, not easily found definitions and useful tidbits.

#### HydroSean

##### New member
Yeah, Sam's Laser FAQs are a wealth of information about everything from laser optics to gas lasers and diodes, along with many other important, not easily found definitions and useful tidbits.

Yeah I took one look at that database and about pooed myself. :thinking: