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Thread: Active Q-Switched Lasers

  1. #1
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    Question Active Q-Switched Lasers

    Hi, I need some advice.

    I'm working on my engineering physics capstone project. We've been asked by our sponsor to build a high-speed camera to take pictures of ultrasonic pulses travelling through water. At the frame rates and resolutions required for this application, there is no high-speed camera available on the open market for any price, so we're going to make a composite video from pictures of thousands of individual pulses.

    The trick is that we need/want sub-pixel blur as the wave moves across the image plane. For a 5cmx5cm image plane, with speed of sound in water 1500m/s, and using a 4Megapixel camera, the wave moves 1 pixel about every 15ns. This means that we need to use a Q-switched laser to illuminate the pulse, and that the pulse duration needs to be < 15ns.

    I have a few considerations:
    • We want to trigger the pulses, or at least time the pulse repetition frequency to be exactly the same as the camera's framerate
    • I want to generate enough light to illuminate the CMOS camera chip without saturating or cooking it.
    • I have a 15k CAD budget for the whole project, but I can't blow it all on a laser. I need camera, optics, water tanks, laptop computer, etc...
    • The response of the camera that I think we'll be using is best in the 450-700nm range, and infared lasers will probably not through the glass lenses that we'll be using.


    Can anyone advise me on how to go about getting a controllable (Active) Q-switch laser with pulse duration < 15ns? I've found some stuff on ebay, but I'm hesitant to blow half my budget.

    Thanks.

  2. #2
    mixedgas's Avatar
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    I'm studying for an exam for my Masters tonight. I can perhaps help you tomorrow after jury duty, if I'm not sequestered///... Stay tuned.

    Finding used Q-Switched lasers is easy.. Finding a used digital delay generator with nanosecond accuracy, sort of easy...

    Steve
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  3. #3
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    Hey Steve, I eagerly await your elaborated response, and good luck with your exam.

    As far as digital relay goes, I plan to use a 200MHz clocked PIC32 dev board to control the whole experiment. I should be able to use the timers to generate square waves with precision on the order of 5 or 10ns, depending on the architecture of the timer circuits (which I haven't delved into yet). I prefer to use this to a built-in driver, because I can control it over a serial port with a quick & dirty bit of Python. I think I'll use some fast-switching MOSFETs as relays.

    I also have an FPGA board that I can use to drive the experiment with ~1ns accuracy, depending on the rise time and repeatability of the relay itself, but my VHDL is weak (and I've been told that FPGA development is hard), so I'll try to avoid doing that.

    Alex

  4. #4
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    If you go the surplus route, I can give you an example to look at...

    My university has two of what you need setting in disposal storage. Why, they weigh 340 pounds for the PSU/Heat Exchanger, and 100 pounds for the laser head. Then they need 208-240V 3 phase power and a frequency doubler module installed. I'll pull and scan the manual so you can start to get an idea of what your asking. How large a field of view do you need?

    I'm a former field service engineer on YAG lasers, so this is right up my ally. I'm not so sure you'll like generating the timing in a microprocessor.


    Steve
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  5. #5
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    I'm not convinced that I need that much power, but I could conceivably get access to three phase if I could get the laser cheaply enough.

    If you look at the ebay link in my first post, you'll see that the specs are close, but pulse width is (just barely) too long. If you'll look at crystalaser's selection of Q-switched lasers, you'll see that the power supply runs on 90-209V AC and draws 40W, so I don't believe that I need something quite as powerful as what you mentioned.

    The image plane is 5-12cm wide depending on the mode of operation.

  6. #6
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    ... I have an old (1976) N2-TAE-laser, that generates pulses with 200ps duration and up to 400kW peak power at 30Hz max. repetition rate - it was used as pumping source for dye-lasers.

    There are several DIY-receipes to build a pulsing TAE-laser on the desk, but there should be some comercial ones out there in the wild too ...

    Viktor

  7. #7
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    One of the reasons for buying a hotter laser then you need is then you can place a mode limiting aperture or spatial filter in the beam path and get a cleaner beam for photography. A single pulse from a small DPSS head is not all that energetic.
    -

    I'm working on the manual scans... I just checked, our surplus ones might have too long a pulsewidth to meet your specs..

    To get a short pulse you either need a very short cavity, have a EO modulator intracavity, or to use pulse compression on master oscillator..

    ---
    That Explorer is actually not a bad deal... I'm not so sure I'd let it slip by... Except for one thing. 20 Microjoules per pulse is almost nothing... So its math time:
    .
    5*5 cm = 25 cm^2, 20 uJ /25 cm^2 = .8 uJ cm^2. Take the .8 uJ per cm^2 and figure you have worse case 40% optical losses entering and leaving the tank... Now your down to .48 uJ cm^2.
    ---
    Treat the tank as a limited case of a Bragg cell, ie Acousto-Optic Modulator, or Schlieren Imaging, or as a Twyman Green interferometer. None of these processes is known for their optical diffraction efficiency. They all suffer from high losses. All of these processes are well documented in the literature as well. So is PIV. So from digging in some papers, you should be able to compute how much light you need... I'm not sure that Explorer is bright enough, and the peak power is low...

    .
    My bet is you end up needing something from Continuum, Ekspla etc with much more energy per pulse... There are some used laser vendors out there...
    .
    None of the Ekspla pulse compressed lasers are on Ebay right now...
    .
    You really do not want to have to tangle with building an intensified camera. :-)
    -

    Try this: http://www.ebay.com/itm/Continuum-Su...item3f543cb0c2
    -
    He might just negotiate or rent you that one... On a good day, a doubling crystal in a pulsed laser gets close to its 56% theoretical conversion efficiency, so that Ebay spec is not bad...
    -
    1064 makes it thru just fine on most glasses...

    Steve
    Last edited by mixedgas; 09-28-2015 at 09:01.
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  8. #8
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    Have you decided what frequency you want to run the system at? If you are acquiring frames at 4MPixel I am guessing it will be on the order of 10hz max.
    Then you can go and investigate the amount of energy you need per pulse. I would venture to say that even 1mJ is enough for most photography applications, but depending on the objects you are are looking at you might need upwards of 100mJ to 1J to get a good image. For reference, even the flash on a point and shoot camera is about a joule of visible light.

    As far as picking a wavelength, running at 1um will be easier on the laser design, but if you double to 530nm the camera will be much more sensitive (easily making up for the losses due to the doubling), and you will be able to align the experiment by eye.

    That surelight Steve posted might be a decent choice if you want to have plenty of laser power available. The vendor is asking way more for it that I would be willing to pay (I bought a Quanta Ray DRC-11 which is roughly equivalent to that continuum for $250 about a year ago).

    If you can get away with only a few mJ I have a DPSS system that is currently operating at 1um (but I have the parts to double it if necessary). I will need to check the energy but it should be easy enough to get 1mj of green out of it. In any case it would run off 110v just fine, and won't require freight shipping like a lamp pumped laser. PM for details.

  9. #9
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    What about using a pockels cell to rapidly switch a polarized light source to illuminate the subject. These can switch as fast as 100 psec, but 10 nsec should be pretty easy. In any case, if your source is a little slow, say several pixel widths of blur, then you can post process this consistent blur out of the final image with an image deconvolution program.

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