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08-11-2011, 04:40 PM #1
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The_LED_Museum
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Reading power output of a quasi-CW laser on thermal LPM?

Question here...how do you correctly read the power output (as an average power output value) of a quasi-CW laser on a laser power meter with a thermal sensor, such as the LaserBee 2.5W USB LPM?

The parameters of the laser in question are as follows:

Wavelength: 404.60nm
Duty cycle: 16.670%
Pulse rep rate: Lower AF range (approx. 265Hz)

And this is the measurement I took:

As you can see, it tops out at 12mW on this meter.

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08-11-2011, 05:11 PM #2
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Re: Reading power output of a quasi-CW laser on thermal LPM?

Hm... on 16.670% of the time. So off ( 100 - 16.670 )%.

Pretending for a moment that this is valid math, 12 / ( 16.670 / 100 ) = 71.98. I'd guess it's outputting close to that for its on-time.

To get a real reading you'd probably need an optical sensor.

-Trevor
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08-11-2011, 06:24 PM #3
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Re: Reading power output of a quasi-CW laser on thermal LPM?

Quote:
 Originally Posted by Trevor Hm... on 16.670% of the time. So off ( 100 - 16.670 )%. Pretending for a moment that this is valid math, 12 / ( 16.670 / 100 ) = 71.98. I'd guess it's outputting close to that for its on-time. To get a real reading you'd probably need an optical sensor. -Trevor

It's on for 16.67% of the time, and off for 83.33% of the time...I'm attempting to extrapolate the *AVERAGE* power as if it were a true CW laser; *NOT* the power output value in its "ON" time.

The average output power would be calculated by multiplying the optical LPM's displayed value by 0.1667 to find the true average output power/energy. If my optical sensor-based meter is recording a peak value like most inexpensive silicon meters do; the 12.00mW displayed on the LPM converts to 2.0004mW. I've used just such a meter to measure the output of this laser; I just wanted to know if the LaserBee (a thermal sensor-based meter) can be used to measure quasi-CW lasers or not.

Last edited by The_LED_Museum; 08-11-2011 at 06:25 PM.

08-11-2011, 06:33 PM #4
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Re: Reading power output of a quasi-CW laser on thermal LPM?

The LaserBee 2.5W Thermopile should theoretically average

I personally have not done any pulsed laser experiments to
date.

Jerry
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08-11-2011, 06:41 PM #5
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Re: Reading power output of a quasi-CW laser on thermal LPM?

This is pretty much what I was hoping to hear...I'll go ahead and publish that chart on the laser's eval. web page with a disclaimer stating that optical (silicon sensor-based) measurements may be better for deadly accurate measurements of the quasi-CW laser of the type seen in that evaluation.

08-12-2011, 03:44 PM #6
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Re: Reading power output of a quasi-CW laser on thermal LPM?

Just my 2-cents:
Optical sensors (current mode) are going to be impacted by the technology used to display the current/voltage... say a DC meter or an Oscilloscope. The LaserBee will provide a type of "integration" because of the thermal mass of the sensor and the large heatsink and will likely give an OK reading. From a physics point of view, the proper device for measurement is the bolometer : Bolometer - Wikipedia, the free encyclopedia

08-12-2011, 04:24 PM #7
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Re: Reading power output of a quasi-CW laser on thermal LPM?

^ also, remember the sensitivity curve ..... optical sensors are a lot of things, EXCEPT linear

for a 404 / 405nm emission, a standard silicon optical sensor for lasers must be compensated, multiplying the reading for it's correction factor for the wavelenght ..... Ofcourse the OP may have a special photodiode, but i don't know this, so i'm assuming theorically that he have a "standard" silicon photodiode, not a "blue enhanced" or any other special photodiode ..... their sensitivity at quasi-UV range is, normally, around 0.2 (assuming at 1 the usual peak of sensitivity that they have around 900/910nm), so this mean that for the given wavelenght, the reading need to be multiplied at least for 5 .....

Then there is the average versus effective power emitted, and the frequency response of the measuring apparate ..... 16.6% duty cycle at 265Hz, if i'm correct, means something like 626 microseconds "on-state" lenght pulses, right ? ..... what is used for the measure, a common LPM or an oscilloscope ? (the diode itself usually don't have any problem, at this frequency)

There are also some other things to keep in consideration, when a photodiode is used for take measures, like gain peaking of the couple diode-opamp, sensitivity shift due to biasing current and to temperature changes, and so on, but these ones usually are importants only for lab type precision.
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Last edited by HIMNL9; 08-12-2011 at 04:26 PM. Reason: typos ..... they're everywhere :p

08-12-2011, 08:11 PM #8
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Re: Reading power output of a quasi-CW laser on thermal LPM?

Quote:
 Originally Posted by HIMNL9 ^ also, remember the sensitivity curve ..... optical sensors are a lot of things, EXCEPT linear ...

I know this, but I'm going to bet that many of my website viewers don't...so on 08-14-11 (or "14 Aug 2011"), the following update will be added to that laser's eval. on my website (minus the strings of "-" signs):

--------------------------------------------------------------------------
UPDATE 08-14-11:

Output power measures 12mW on my LaserBee 2.5W USB Laser Power Meter w/Thermopile.
DISCLAIMER! This is a thermal sensor LPM, not a silicon photodetector-based one. Optical (silicon sensor-based) measurements may be better for deadly accurate measurements of quasi-CW lasers like the Sonar. However, I've been told by the manufacturer of the LPM that I used for this test that this value should be reasonably close to the actual average power produced by the Sonar.

From HIMNL9 on Laser Pointer Forums comes this (no changes to grammar, capitalisation, or syntax were made):

"also, remember the sensitivity curve ..... optical sensors are a lot of things, EXCEPT linear

for a 404 / 405nm emission, a standard silicon optical sensor for lasers must be compensated, multiplying the reading for it's correction factor for the wavelenght ..... Ofcourse the OP may have a special photodiode, but i don't know this, so i'm assuming theorically that he have a "standard" silicon photodiode, not a "blue enhanced" or any other special photodiode ..... their sensitivity at quasi-UV range is, normally, around 0.2 (assuming at 1 the usual peak of sensitivity that they have around 900/910nm), so this mean that for the given wavelenght, the reading need to be multiplied at least for 5 .....

Then there is the average versus effective power emitted, and the frequency response of the measuring apparate ..... 16.6% duty cycle at 265Hz, if i'm correct, means something like 626 microseconds "on-state" lenght pulses, right ? ..... what is used for the measure, a common LPM or an oscilloscope ? (the diode itself usually don't have any problem, at this frequency)

There are also some other things to keep in consideration, when a photodiode is used for take measures, like gain peaking of the couple diode-opamp, sensitivity shift due to biasing current and to temperature changes, and so on, but these ones usually are importants only for lab type precision."

--------------------------------------------------------------------------

If for whatever reason, this is unacceptable to you, just pipe up and this info. (the text in yellow here) will *NOT* be published.

08-12-2011, 08:23 PM #9
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Re: Reading power output of a quasi-CW laser on thermal LPM?

Quote:
 Originally Posted by The_LED_Museum I know this, but I'm going to bet that many of my website viewers don't...so on 08-14-11 (or "14 Aug 2011"), the following update will be added to that laser's eval. on my website (minus the strings of "-" signs)
Why removing the "strings"?

j/k your way to write is very nicety .beer. (the dots next to the "beer" weren´t a typing error.
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405nm >> Wickedlasers SONAR by lazeerer peak@968mW
445nm >> Kryton Smooth by lazeerer peak@2075mW
453nm >> Lipstick by rhd peak@158mW
473nm >> Optotronics RPL-B peak@112mW
515nm >> Skylasers HL peak@41mW
532nm >> Novalasers Alpha HP peak@284mW
589nm >> CNI PGL-III-A peak@89mW
594nm >> Laserglow Rigel peak@4mW
635nm >> MXDL by DTR peak@38mW
642nm >> Saillaser peak@23mW
660nm >> CR2 by lazeerer peak@524mW
685nm >> MXDL by lazereer peak@42mW

Above lasers measured with an Ophir nospin LPM

- - - - - - - - -
- - - - - - - - -

Note that Imageshack is automatically resizing some of my older
pictures back to original full-size after a while.

08-12-2011, 11:24 PM #10
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Re: Reading power output of a quasi-CW laser on thermal LPM?

Measurements on virtually all thermal lpms should be fine at this frequency. Response time depends on the sensor ofcourse, but its rarely faster than 0.1 second for a twofold change in reading. Since you are modulating over 10 times that speed, i would not expect any serious fluctuations or misreadings.

Optical sensors (e.g. phototransistors/diodes etc) can pose a problem as they are much faster than your modulation frequency and could read either your peak power, zero power, or anything randomly in between depending on how the sensor is polled.

08-13-2011, 07:09 AM #11
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Re: Reading power output of a quasi-CW laser on thermal LPM?

@ The_LED_Museum : i have nothing against that, only, please check the text for typos, before post it ..... you know that i'm not English, and that my English is self-learned, so sometimes my grammar is bad (it's called "grammatical massacre", i think ) ..... you can use it as is, or modify it, if you prefer ..... but if you use it as is, maybe is better to state that the writer is not English native (and that the eventual errors are a free gift )
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08-13-2011, 11:56 AM #12
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Re: Reading power output of a quasi-CW laser on thermal LPM?

^^^^ Now that's funny....

Jerry
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08-13-2011, 12:39 PM #13
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Re: Reading power output of a quasi-CW laser on thermal LPM?

Quote:
 Originally Posted by HIMNL9 @ The_LED_Museum : i have nothing against that, only, please check the text for typos, before post it ..... you know that i'm not English, and that my English is self-learned, so sometimes my grammar is bad (it's called "grammatical massacre", i think ) ..... you can use it as is, or modify it, if you prefer ..... but if you use it as is, maybe is better to state that the writer is not English native (and that the eventual errors are a free gift )
I've made the appropriate corrections, thank you.

I also changed the "disclaimer" so that it reads as follows:

"From HIMNL9 on Laser Pointer Forums comes this (corrections to grammar, capitalisation, punctuation, and syntax were made with the writer's permission as the writer's native tongue is not English):"

I trust that this is now satisfactory.

BTW, you did ask...I used a Sper Scientific Pocket Laser Power Meter # 840011 for the first battery of power output measurements. It is a silicon-based LPM, so I applied the wavelength/sensitivity multiplier as found on the inner surface of the proudct's lid (in addition to the duty cycle multiplier) to arrive at the average power output values shown in my eval. of that laser.

Last edited by The_LED_Museum; 08-13-2011 at 03:00 PM.

08-13-2011, 02:02 PM #14
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Re: Reading power output of a quasi-CW laser on thermal LPM?

Quote:
 Originally Posted by lasersbee ^^^^ Now that's funny.... Jerry
Why ? ..... it's true, after all (well, "funny" can be defined some of my grammatical massacres, i have to admit that ..... maybe not funny as some automated translations from east languages to English, but still "funny" enough ..... in a some weird way )

Quote:
 Originally Posted by The_LED_Museum I've made the appropriate corrections, thank you. .....
Thanks to you too
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