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How does a laser power meter actually work?

cosmonaut

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Hi LPF,

I've been wanting to gauge the output performance of my new diode with a LPM. I have access to a power meter in the optics lab of the physics department here at school, however it is only sensitive to HeNe wavelengths and this is a green diode. Instead, I figured a fun project would be to construct one, though it would probably be difficult. My question is, how is a LPM constructed? I envisioned a simple one using a phototransistor that would pass a current proportional to the power output of the light, and through a little math I could calculate the laser's power.
 



diachi

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A photodiode can be used, but the response on those is wavelength dependent so you need to apply a correction to any readings on it based on wavelength and a known response curve for that particular photodiode. That's probably what the optics lab is using. Power range for optical meters like that is usually low, an ND filter is usually required for higher powers. Optical meters are great if you need a quick response time or need to measure very low powers.

The other option is a thermal LPM which uses a thermocouple/TEC to measure the power. The surface absorbs the laser beam, converting it to heat which is then measured to determine the power output. These can typically tolerate much higher powers but are slower to respond and have trouble reading low powers.

There are threads here about DIY LPMs, use the search function. Problem is, without another LPM to calibrate against you can only really get a relative power reading and not an absolute one. You can do some math and get somewhere close but don't expect the accuracy to be all that good.
 

RedCowboy

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Hi LPF,

I've been wanting to gauge the output performance of my new diode with a LPM. I have access to a power meter in the optics lab of the physics department here at school, however it is only sensitive to HeNe wavelengths and this is a green diode. Instead, I figured a fun project would be to construct one, though it would probably be difficult. My question is, how is a LPM constructed? I envisioned a simple one using a phototransistor that would pass a current proportional to the power output of the light, and through a little math I could calculate the laser's power.

Here's an old thread that you may want to read through.

 

Lifetime17

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Hi
Save yourself the time and get a real LPM from Laserbee J. Bauer LPM's . They are of proffesional quality for the laser hobby we are in.
Many to choose from and great prices. If you intend to continue with this hobby you need the correct tools to succeed .

Rich:)
 

RedCowboy

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Wow.........I didn't know laserbee had a 10 watt unit, and it has 4 times the sensor area of my hyperion .........hindsight

 

lasersbee

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Wow.........I didn't know laserbee had a 10 watt unit, and it has 4 times the sensor area of my hyperion .........hindsight

What size of sensor is used on your hyperion RedC ??

Thanks for stepping in Rich...:)

Jerry
 

RedCowboy

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It's looks like about 6.5 x 6.5mm or maybe 6 x 7mm so it's actually 42.25 or 42 and yours is 15 x 15 so 225 so yours is 5 1/2 times the area 550%

64522
 

lasersbee

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I thought you had a bigger sensor. It is about the same
as the one on my 2W Astralist LPM (6mm X 6mm as
quoted by Astralist).

Is that the 20W version ?? How fast is the 100% response
time ??

Jerry
 

RedCowboy

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It takes about 12 seconds to hit 100%

Yes it's the 20w version, seems to be accurate based on comparing what some known diodes at known current should be as tested by DTR in his videos.

I am planning on getting a new meter with a larger sensor area.

I want to measure up 100w of ir 808-1050nm and 15w+ of 450nm and I know that could mean two different heads if I wanted to also measure less than 1w pointers.

I will probably end up with a collection of meters :)
 
Last edited:

Lifetime17

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Hi,
No problem Jerry the A10/RTA is awesome glad I purchased this a while ago with all the other LPM's from Lasserbee products.

Rich:)
 

lasersbee

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It takes about 12 seconds to hit 100%

Yes it's the 20w version, seems to be accurate based on comparing what some known diodes at known current should be as tested by DTR in his videos.

I am planning on getting a new meter with a larger sensor area.

I want to measure up 100w of ir 808-1050nm and 15w+ of 450nm and I know that could mean two different heads if I wanted to also measure less than 1w pointers.

I will probably end up with a collection of meters :)
Yeah one of the factors is.. the smaller the sensor the faster the
response. If you want to go to 100W I'd get a used commercial
Sensor Head designed for that much power and build a Meter
using it.
Remember that a 100W head has much less V/W output than a
15W head like the OPHIR 20C.

@Lifetime17...
Thanks for your continued support...;)


Jerry
 




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