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Thread: Part 2 - Laser Power Meters

  1. #1
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    Default Part 2 - Laser Power Meters

    Part 2 of this series I will cover Laser Power Meters
    As with any of these series I come up with .
    I am sure to make a few spelling mistakes.
    if it doesnt make sense ...let me know.
    Later I will come up with useful circuits as soon as I figure a neat way to present them.
    For a few years now, I have been trying to come up with a good cheap solution to measuring high-power laser diodes.

    I have tried LED's , Photocells..Peltiers...be it Silicon or Selenium and Germanium.,
    Paper and glass and plastic attenuators..you name it..Ive probably tried it.

    My favorite is the Peltier device..but I'll get into that in a bit.
    The worst has got to be the LED trick everyone has been seeing on the net .
    While this is cheap and can measure... the problem is ..NOT one LED is the same as the next one.
    See this test performed by my friend Redlum: http://perso.wanadoo.fr/redlum.xohp/...greenLEDs.html

    So the correction factors will NOT work from one LED to the next..
    The average correction factor I have found seems to be around 4.8uA/mW
    and this is with the Rad Shack diodes...but after about 75mW they drift badly.
    So, they easily get saturated by too much light therefore making measurement non-linear and inaccurate....
    Also they deteriorate with use , especially if they are subjected to too much light.. POOOOFFF goes the die.
    Like a CCD or Cmos camera chips..they loose their sensitivty by burned surfaces..ruining junctions and compound elements that make them up.
    If tried for more than a few seconds at anything greater than 20 or 30mw they will die from cooking the die..
    Hey , the Heat has to go somewhere, And ...I dont see any heatsink on that LED for the laser power to get absorbed.
    And with each use this way .. the LED will be something different.

    AH well...
    ....
    No arguements here is nessesary.. We have bought thousands of LEDs
    for use with other projects and have tried hundreds just for laughs...
    They just vary too much from one batch and manu to another to be of any use.
    Period...end of story...You wont find any reference on Sams Laserfaq...So Dont waste your time.


    Photocell...Be it Selenium or Silicon or otherwise...most of these work great too..
    But they suffer from the same problem as the LED's they too get saturated by to much signal only good for about 20 to 50 mw.
    But they are more linear to almost the end...and are more sensitive to weaker
    photons...
    They are both a bit better about the burn up problem too..
    but they too can be destroyed... but it takes lots of power.
    But you will find more low-power power meters dependent on this type
    of sensor than any other...as most measurements are only up to a few hundred mW.
    You can find a few good circuits on Sams Laserfaq as well and they do work .
    To make them work at higher powers , you will need some way to attenuate the beam...
    Known ratio beam splitters and attenuators (ND filters) come into play here.
    But this can be messy.
    This would require you add an attenuator in the beam path everytime you
    wanted to make a measurement above the sensors max.
    You maybe will have several for different power scales..
    If you forget.."poof"

    They only real problem now is..To calibrate to a specific wavelength.
    Each one of these devices are sensitive to light but in a different way.
    Photocells such as Selenium, Silicone..yes that LED will put out a different voltage for a different wavelength for the same amount of power.
    So conversion charts or a wavelength selector to make up for this will be needed..as well as a range selector.
    Are we having fun yet...I didnt.

    My favourite here is Selenium..Its been around since the turn of the last century and still is in use today..
    You will mostly find them in Photographic Light Meters and other light measuring equipment.
    You have probably already seen a few of the GE light meters converted for laser use already..(on the cheap...Fleabay).
    They can be quite accurate I can attest...but because of age (up to 60+ years old) they wont be the same from one unit to another.
    All you would need to calibrate one for up to 70mw use ...is to choose
    the proper attenuator...and a calibrated meter to compare with.
    Pieces of white copy paper works fine but you will need several different types and thicknesses ..then followed up by a Nylon diffuser.
    (Most important) Dont forget to add a diffuser to your sensor...
    cause if you dont, you will get varying readings dependent on angle and spot size. making it useless..

    You may also add a green filter to suppress other wavelengths in the mix for green only.
    I have choosen not to in a few of mine..because...
    I'd sure like to know if that Super-Duper Pointer that some enterprising individual had claimed the output was true green or had the added IR .
    It is all too easy to get IR leakage ..
    Most of the time I have seen that when we're tuning for max green sometimes the IR may or may not be there in great amounts and sometimes it is....
    Because it is all due to proper alignment and absorbtion of the crystals and filtering by the mirrors and filters and temperature of the diode and crystals.
    You will know quickly if you have IR , just wave a IR filter in front of the sensor and watch the meter..
    If there is a big difference in measurement between with and without the filter guess what?
    And if you just wanted green measurement..or any measurement that you dont want any other wavelength influencing it..
    Go grab an IR filter or a filter made for the wavelength you are working at
    and measure again...

    You dont see filters on commercial power meters..why. would you want to hinder measurements.






    EOT *^_^*
    "My signature has been taken, so Insert another here"
    http://repairfaq.ece.drexel.edu/sam/laserfaq.htm
    *^_^* aka PhiloUHF

  2. #2
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    Ok..so I mentioned using a Peltier device for measuring Laser power.

    You may or may not know.
    This is a pretty neat device..You will find them in A/C heat pumps ..auto air conditioners and portable coolers.,
    Lab equipment to heat or cool small items...And DPSS lasers.
    Because of the "Peltier effect" by applying a current to the device.. one surface gets hot the other will cool down..
    Some are more efficent then others depending how old and what the application was for. And..how many elements they are comprised of.
    You will find them in the better DPSS laser units to cool the diode and vanadate or another to heat/cool the KTP.
    Also, It may not be known...that they will generate electricity when heated is applied to one side and heatsinked on the other.. ( Seebeck Effect )
    This is the basis of my latest Laser power meter.
    I have taken a small 3/8 of an inch peltier, found on ebay..
    It is the type with the solder coated faces for mounting with the solder to heatsink and the other side to the device under Test.
    And mounted it to a small heatsink mounted on a lens holder..
    Painted the face of the device with a Black Sharpie.
    Setting my Digital Volt Meter to read low AMPS scale ..I can linearly measure
    power from any laser , (well, almost).
    Now I dont have to care about what wavelength ..much.
    well at least in the 488 to 1064 range I can...
    It doesnt care either ...we are measuring heat as power.
    It does not suffer from the wavelength sensitivity as the other sensors I mentioned before,...
    However...It is very slow in making the measurement.
    For example: My little 3/8 of an inch mounted I would hit it with 228 mW of red
    gave me a reading of 4.00ma and with a multiplication factor of 57
    I was able to obtain 228 as was measured on the lasercheck.
    I also tried this with Green at a power of 228mW and it also read 4.00.
    So...lowering the output to 22.8mw on each gave me .400ma ..
    using the multiplication factor again gave me 22.8 as was also read on my
    lasercheck.
    It seems It is also very linear as well and can measure my 2watt diodes at 808..no problem.
    I have been able to measure as low as a milliwatt and as high as 2watts without any problem.(I dont dare go higher until I add a better coating).
    I have made a conversion chart for now.
    The secret is to keep the heatsink always at the same temp.
    Isolating the device from stray air currents and changes will help a lot.
    For now , I have it mount to my Optical table lens mount with the peltier enclosed in a lens holder,
    However I still need to build a box for it.
    Altho I have not tried it yet,
    To go Higher in power would require some way of dissapating the heat.
    So experimentation with carbon is in order for me...


    It does take a few seconds to measure..We are heating up the front side of the peltier. and we need to let the head settle to room temp first.
    It is ceramic and thin metal.
    Recovery is slow, but hey what do you want for next to nothing.
    now..the next thing I plan to do is build a circuit to convert the measurement to
    read in volts and this will be equated as Power in Watts. And I can add a range selector too...
    Also..it is very sensitive at very low levels to temperatue and stray RF as well as my computer monitor..
    I can even wave my hand or soldering iron in front of it,.. and it will see that too!!!
    but the amount is very very small...I plan to mount the peltier on another temperature controlled mount..
    Stabilized to some degree below or above ambient room temp and enclose it in an aluminum box.
    I could also add a thin nichrome wire winding to the mess for calibration... we'll see how that goes...
    I did add some by-passing at the test amp as the stray RF is bad here..A local AM station 1 mile away plus my Ham activities makes the meter jump around like no-ones business..

    Now , I'm Having fun...

    EOT *^_^*
    "My signature has been taken, so Insert another here"
    http://repairfaq.ece.drexel.edu/sam/laserfaq.htm
    *^_^* aka PhiloUHF

  3. #3
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    Somehow double posted

    For more information on Calorimeter or Pyroelectric powermeters or Joulemeters .. Please look here:
    http://www.scientech-inc.com/laserpowernotes.phtml
    This will give you an idea where to start like I did...

    And dont forget to read here:
    http://repairfaq.ece.drexel.edu/sam/...oi.htm#ioiscil

    *^_^*
    "My signature has been taken, so Insert another here"
    http://repairfaq.ece.drexel.edu/sam/laserfaq.htm
    *^_^* aka PhiloUHF

  4. #4
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    For more information on Calorimeter or Pyroelectric powermeters or Joulemeters .. Please look here:
    http://www.scientech-inc.com/laserpowernotes.phtml
    This will give you an idea where to start like I did...

    Since the last writing I have acquired TWO Scientech Power/Joulemeters
    both included calibrated heads...
    One an analog unit :

    This Unit is easier to use for tuning and it is also battery or AC operated
    altho slower reacting than the digital for some reason. Max power 30 watts

    The other has digital read-out (No chicken-bones about it -my Fav ).

    Reaction time for measurements was fairly quick.. in the order of seconds
    Max power 20 Watts

    The Calibration was simple and fairly accurate and reads down to a fraction of a milliwatt.
    Altho this type of sensor is meant for Pulsed operation.. they will work for CW measurements without a chopper
    as I have found.. They are very close in agreement with my Coherent Lasercheck
    and My Liconix power meters. At least for measurements up to a few hundred milliwatts.
    I have NOT tested them in higher powers just as yet.
    But it does look like I will be buying a chopper wheel next (ask me, I will tell you what thats for.)

    Also, I have purchased a few Bare sensors to experiment with as well..
    More on this later. They are the ( thermo-couple ) type.
    Picked on ebay for 30USD each:
    The seller has over 400 of these units so I expect you will see more of these listed from time to time..

    I suggest picking up one of these meters on ebay hopefully with the matching head .
    Or, you will need to calibrate it against a known source if you dont.
    You to can also find the heads and match one of these up to just about any Scientech
    or similar meters going cheap on eBay with or without the heads.
    Calibration is very simple.. Just dont forget to "SPREAD THE BEAM" a little as
    too small of a spot size will damage the head sensor (black absorbing material). The Idea is to cover the area of the sensor as best as possible..
    But for low power < 200mw , And if the beam is not focused at near distances...
    I dont see any problem with this...Yet.
    Since most of these "heads" have a calibration resistor inside., it makes
    calibrating the meter a snap.. But you need to know the correction factor for it too..
    I was able to determine the correction factors for the heads I have here by
    comparison to know calibrated meters.
    Ohms law is used to calibrate the meter ,
    A known voltage and current ( 1w in my case) times the correction factor is applied to this built-in resistor..
    Then , the box ( digital or analog ) meter is adjusted to read 1w ... Thats it.
    I was also able to calibrate one for the peltier sensor I had made earlier, It worked just fine.

    My idea is to come up with a few "cheap" solutions for measuring higher laser power..

    edit:
    Its been some time now since I've had these meters..
    I find myself always using them as the are always on , on my bench.
    I still, I am always comparing meter to meter..
    And find that these always read correctly and easy to read ( the digital ) display.
    And most of all.. can be viewed from across the room with ease.
    I think I can sell one of my Laserchecks now, Im finished with it.
    Oh, And speaking of Laserchecks..
    I've owned three of them ...all read +/- 10 percent of one another.
    They are also temperature sensitive too...
    The last one I bought directly from the factory..
    I requested +/- 3 percent accuracy from it.. Im keeping that one.

    Altho this may seem academic to some.. I feel this is a learning experience for
    me.. Just want to share my experience with you all.. 8)


    And dont forget to read here:
    http://repairfaq.ece.drexel.edu/sam/...oi.htm#ioiscil
    *^_^*
    "My signature has been taken, so Insert another here"
    http://repairfaq.ece.drexel.edu/sam/laserfaq.htm
    *^_^* aka PhiloUHF

  5. #5
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    Nice!

    I've been so swamped I haven't had time to get on here much this last month...
    (All laser related so it'll be very exciting when I have some free time to post...)

  6. #6
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    Yeah Yadda ,, We've noticed..

    Keep up the good work dude !!!

    Come by when you can.. I'm still on MSN
    "My signature has been taken, so Insert another here"
    http://repairfaq.ece.drexel.edu/sam/laserfaq.htm
    *^_^* aka PhiloUHF

  7. #7

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    Marconi, I heard of this Peltier idea before, but never tried it.. One thing I remember is that if you build a differentiator circuit you can get a faster reading based on the rate of change (should also speed recovery time after removing input light), but I never thought this through so I don't know if it's difficult to match that to the longer term reading when heat difference stabilises. Another thing that bugs me is can it be long term stable? Surely that could only work if you controlled the cold side temperature and kept it constant.

    If I knew a bit more I might be tempted to try, I have a couple of small Peltiers around here somewhere.

  8. #8
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    Hey there fellow Whovian... Welcome aboard!!!

    I heard of this Peltier idea before, but never tried it.. One thing I remember is that if you build a differentiator circuit you can get a faster reading based on the rate of change (should also speed recovery time after removing input light), but I never thought this through so I don't know if it's difficult to match that to the longer term reading when heat difference stabilises..
    Thats something I should try sometime..However, Im pretty content the way its operating now!!!.. But thanks for throwing that in the mix.. good idea.

    Another thing that bugs me is can it be long term stable? Surely that could only work if you controlled the cold side temperature and kept it constant
    Yes, This has been extremely stable..Altho. I do keep my lab the same temp +/- a few degrees as best as possible.. I do like to be comfortable. hi.

    On the 200mw and 2W scales Ive only noticed a couple of mW change.
    But that is easily takin care of by pushing a button to reset to zero.

    Glad you could join us..Maxy says Hi!!
    "My signature has been taken, so Insert another here"
    http://repairfaq.ece.drexel.edu/sam/laserfaq.htm
    *^_^* aka PhiloUHF

  9. #9

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    Hello. Both you and Maxy. I'm pleased to find you here.

    Good to find this forum, several people I've seen on eBay and alt.lasers, I think this is where things really get good, bridging the gap between the unreachable strata of high optics and the people (like me) who used to only be able to wish for them.

    Let me know if you get any mileage out the differentiator. I'll maybe have a go too, I've started dreaming of a two-peltier device that could be both stable and fast. But knowing me it would be overengineered to the point of stupidity, and I'd do better taking your advice on eBay heads and meters. The thing that put me off was getting access to a good calibration, but that might not be as hard as it was. The net is most useful. I'm wary of blowing on a Lasercheck, and surely there is a better way if I can use work to save money.

    Would a thin aluminium shim work well, if it was black anodised? I imagine this being stuck on the first peltier with a thin circle of white heatsink paste, and the corners being dabbed with superglue. Twisting it briefly against the peltier to squeeze out air and get good contact, being made to fit it well, it seems could quickly make a very good heat sensitive surface for a standard ceramic surfaced Peltier.

  10. #10
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    The old big heat based ones are really quick and accurate! I borrowed an
    Coherent 210 analog meter for a few months from Holospectra (now those
    are some interesting guys!) and the thing reacts instantly!



    It completely blows our "high end" digital power meter out of the water... You
    can tell how stable a laser is with that thing...

    Is anyone else interested in making a "cheap" modern revamp of the classic
    power meter? I've been shopping for a 210 ever since I gave that thing back...
    The detectors are pretty common and cheap and I peeked inside the unit and
    the circuit seemed very simple...

    I wonder what the complications you have to consider are...

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