Laser Power Meter

[monika]

Hi guys,

Doe a power meter differ in reading if the beam size is different or does it compensate for this? ie if I shine a 30mw laser at a power meter which has a 3mm dia beam will I get the same power reading if the beam was 1mm dia.

Thanks

 

[hwang21]

It will read the same output

 

[The Lightning Stalker]

It depends. If the beam is really small or really large it can affect the results. It should specify a minimum beam size on the datasheet. Of course if the beam is large enough that some of it goes outside the well then the reading will be off.

 

[monika]

From what I understand this is the energy per area so if a power meter provides the same power reading regardless of area, is there such a thing as a power density meter or is this only calculable?

There are circumstances where it is desirable to know the power density, so I guess there must be some way to measure it?

 

[ARG]

What LPM do you have?

 

[monika]

I don't have a LPM rather I'm just trying to understand a little bit more about them.

Some of them seem to be using thermopiles which convert heat to electricity. If this is the case if a beam is 3mm dia or 1mm dia they will have different power densities and I'm assuming different thermal characteristics hence the question regarding whether a the reading will vary.

 

[ARG]

Well, specifically which type of sensor do you want to know about?
The common ones in the hobby are optical sensors, radial thermopiles (such as ophir-20C heads), and TEC based sensors (used in most hobby LPM's)

 

[monika]

Well, specifically which type of sensor do you want to know about?
The common ones in the hobby are optical sensors, radial thermopiles (such as ophir-20C heads), and TEC based sensors (used in most hobby LPM's)

Well, all of them really. I'm trying to find out how each particular sensor works, whether it be light or temperature or however they work.
The pros and cons of each type of sensor; what makes them best suited fro a particular application etc....

I could have sworn at some point there was an abbreviations post in the forum somewhere. Am I going mad, I can't find it ?

 

[EpicHam]

Well if you have radial thermopiles like the REALLY expensive OPHIR heads.
Your beam diameter doesn't really matter since in the end .
From a hobbist's perspective , we are just calculating the final laser LPM by measuring the temperature difference between any two junctions as caused by the laser heating up "one side" of the thermopile.
This causes a voltage to be formed between the two junctions.
And with this potential difference coming from the thermocouple , we can obtain the laser power once we've calibrated the voltage with known laser outputs.
And VOLA!
You have yourself a LPM.

However, the beam diameter DOES have the potential to affect the measured output.
On the surface of the thermopile , it is coated with a surface absorber typically consists of an optically absorbing refractory material deposited on a heat conducting substrate of copper or aluminum.
If the beam diameter is TOO small , which in turns mean for a fixed laser power . The W/cm2 exceeds the damage threshold . This causes vaporization of the surface which ruins the absorber.
You have yourself a TOASTED LPM.

So ALWAYS ensure your beam diameter for a given power laser doesn't exceed the specification of the LPM's surface absorber coating.

 

[Livinloud]

If you are interested in getting a cheap ophir set you then you can buy a ophir head and connect it to a DMM. I have a power supply for sale in order to do this.

Ophir are the best LPMs available. The beam size should not be focused to infinity but shouod also not be larger than the sensor area. I have never measured my beam size, just eye ball it to make sure its not too tight and not too large

 

[EpicHam]

If you are interested in getting a cheap ophir set you then you can buy a ophir head and connect it to a DMM. I have a power supply for sale in order to do this.

Ophir are the best LPMs available. The beam size should not be focused to infinity but shouod also not be larger than the sensor area. I have never measured my beam size, just eye ball it to make sure its not too tight and not too large

Well theoretically you want the LARGEST beam diameter possible to avoid overstraining the coating, but then again its all just theory.

As for the OPHIR head.
Well with that setup you'll have yourself a LPM , but if you'll be missing out on the datalogging function if you go for other cheaper TEC based LPMs that comes stock with dataloggers.
You CAN add a datalogger to an OPHIR , but it'll be even more overhead cost

 

[Livinloud]

okay but the down fall with a TEC LPM, even with data logging, it has a 30 second max reading time. with almost every laser, this means you miss the peak power and your LPM is useless IMO

 

[EpicHam]

okay but the down fall with a TEC LPM, even with data logging, it has a 30 second max reading time. with almost every laser, this means you miss the peak power and your LPM is useless IMO

Well you got me there :shhh:
But I've always believed when it comes to hobbyist level .
Seeing DPSS peak for a fraction of a second doesn't really represent the practical output of the laser. You catch my drift?
Maybe if you are doing laser activations or SILEX refinery , but not when you are doing beamshows and burning stuff :beer:

 

[ARG]

Well with that setup you'll have yourself a LPM , but if you'll be missing out on the datalogging function if you go for other cheaper TEC based LPMs that comes stock with dataloggers.
You CAN add a datalogger to an OPHIR , but it'll be even more overhead cost

The Isis data logging addon is under 20$ to DIY, very cheap Paired with Peregrine that is open source, it's under 20$ for the whole setup.
http://laserpointerforums.com/f70/introducing-delta-isis-84435.html#post1221604

Also, the response times of the Ophir head make it far more accurate when measuring unstable DPSS lasers.
http://laserpointerforums.com/f70/lpm-response-time-who-needs-84428.html#post1221446

In total you can have yourself a home made LPM with data logging and a screen for under 250$. Amazing deal IMO, since the Ophir heads are $700 new or thereabouts, without a doubt the best sensor that hobbyists can get.

 

[hwang21]

~$250 on the Forum though^