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Thread: Pulsed Laser Power meter

  1. #1
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    Default Pulsed Laser Power meter

    I need to tune/align a laser that produces 1-2pps at several joules and up. I can attenuate the output, but I need a way to determine how to maximize the energy as I align or change media or increase the input energy. I do not need anything like a relationship the actual output energy, just a relative measurement ie 100 units vs 105 units. The pulses are in the 10-100 usec range, visible wavelength and I would like to see either a repetitive trace of the pulse on an oscilloscope or a real time numerical readout as each pulse occurs. My Ophir meter is way too slow and reads the results of accumulated pulses like an unstable laser. I don't want to call Coherent and ask this question. They'll offer a really nice meter that I can't afford. I'm also looking to avoid a complex project to construct a DYI meter if there is a fairly simple solution available.

  2. #2
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    I would imagine a fast photodiode or forward biased phototransistor connected to your scope would do what you want (assuming your scope has vpp calculation, you should be able to tune the laser realtime).

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    Perhaps something in Sam's FAQ will offer a solution:

    http://www.repairfaq.org/sam/laserioi.htm#ioihlpm

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    Thanks for the suggestions. I suspect that the photodiode route will be the way to go. The amount of attenuation will have to be significant and adjustable. Because multimode is a real possibility, I will need to capture all the beam with a convergent lens and so I'm thinking of various ways to attenuate these non-polarized beams.

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    Quote Originally Posted by planters View Post
    Because multimode is a real possibility, I will need to capture all the beam with a convergent lens and so I'm thinking of various ways to attenuate these non-polarized beams.
    Does the attenuation have to be repeatable over a long period of time? Perhaps you could use a small fishtank filled with (water?) and add food coloring or some other dye to adjust the attenuation as desired. You could have several tanks with different degrees of attenuation. Reflections could be a problem, though.

    I suppose a stack of sunglass lenses from the local dollar store is insufficient.

    I ordered some dark acrylic from eBay; the attenuation is the same by color grade regardless of thickness, so you could get a thicker material to withstand the energy and stack up little squares of those all day. I.E., I got a 1/8" sheet and a 1/4" sheet of the same "color," and they both had the same light attenuation.

    Photographic neutral density filters? Not sure about power handling.

    Plate beamsplitters?

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    I'm thinking a factor of about 1000:1 would be about right. Maybe a pair (in series) of approx 99% first surface mirrors angled down to a dark absorbing surface would work. I think it has to involve a beam dump. The powers will probably be way too high for a thin absorbing medium. However, I like your dye in a cell (tank) idea. Pick an absorbing dye based on the current wavelength (585nm) and titrate in concentrate until it's just dense enough.

  7. #7
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    Eric,

    do it the "PRO" way.

    You order a uncoated wedge from Thorlabs, part # PS811 , This is your three point nine percent beam splitter. The wedging lets you separate the ghost beam from the measurement beam when using Fresnel reflection. You then use either a DET10A from Thorlabs or one of these from Ebay. If the face of the splitter is within 10' of the incident beam, the reflection is a pretty constant 3.9%. You can also order a calibrated quartz one from Altos Photonics that comes with a spreadsheet of actual reflection vs wavelength vs incident angle.

    http://www.ebay.com/itm/MOLECTRON-MO.../361008518926?

    http://www.ebay.com/itm/GENTEC-ED-20...item2c8a817ef1

    http://www.ebay.com/itm/GENTEC-JOULE...item2c8a83684f

    I've bought two of the used GENTECs from that Seller with GOOD results.

    Those go straight into a Oscope with a 50 Ohm termination for the photodiode or the standard 10 Meg 20 pF scope input for the Joulemeters.
    I'd prefer a 100 Mhz Oscope with capture, but slower scopes work.

    A Joulemeter sensor is a very thin Piezo mounted on a metal/carbon adsorber.

    There is also the "cuvette of tiny glass beads" attenuator for photodiodes. All Parts, including the plastic cuvette available at your local crafts outlet at the mall for less then 10$.
    More beads = more attenuation.

    I do my high power measurements with said WEDGE.

    Steve
    Last edited by mixedgas; 08-26-2014 at 12:37.
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    Thanks Steve. This is just what I needed. When I win the lottery I'll give you a call and you can run my lab.

  9. #9
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    Default One Tic-Tac of Attenuation!

    Eric, Pictured is One "Tic-Tac" of Attenuation!

    A brilliant Post-Doc named Eli showed me this trick. His Prof was in no hurry to buy him optics, so he improvised. The plastic box actually takes several tens of Joules if the beam is a few millimeters in Diameter. All parts, ie "tiny" clear necklace beads and bead container are housewife grade and readily available in craft stores. The holes in the beads are a great help in scattering the light. Eli grows experimental laser rods for a living, so if he says it can take Joules, it can take Joules.

    --------------------As for improvising a fast photodiode----------------------------

    Generally any Silicon Photodiode with a 1 mm^2 or less area and back-biased or reverse Biased with 5-9 volts can hit ~ 1 nanosecond rise-time if coupled properly into a 50 Ohm line. The battery or power source must be stiffened, filtered, and bypassed with a 10 uf cap and a .1 uF ceramic cap. The caps also affect the bandwidth, by allowing the pulse to return. Phototransistors are too slow for this. They will show the pulse, but not its envelope. Any PD lead wires should have "nonexistent" lengths.

    See Attached,

    Click image for larger version. 

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ID:	44541Click image for larger version. 

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    Steve
    Last edited by mixedgas; 08-26-2014 at 19:22.
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  10. #10
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    To be trite and nostalgic, I would have suggested Gillettes, but they wouldn't give you your 5% tuning variation - (unless you're popping a few dozen at a time!)
    - There is no such word as "can't" -
    - 60% of the time it works every time -

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