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Thread: The Really Big Dye

  1. #31

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    It is a bit of a tricky question to answer, there have been plenty of experiments done where gas was ionized with a dye laser. The most notable example being laser isotope separation (for example, CERN's ISOLDE laser system with is pumped by copper vapor lasers or more recently dpss lasers) but this is on-resonance ionization which is a completely different regime that you are working in. There were also modelocked dye lasers used for early multiphoton/tunneling ionization experiments (for example, https://journals.aps.org/pra/abstrac...hysRevA.37.747 ) but these were MOPA system pumped by lamp-pumped DPSS lasers. They only had 3mJ pulse energy, but since they had a sub-ps pulse duration and ~10um focused spot size that was all they needed to ionize the heck out of their samples.

    When you limit your search to only flashlamp pumped lasers the records start to get sparse. There is a russian result by Baltakov et al where 400J in 10us was claimed using a R6G in a coaxial geometry (flashlamp surrounding dye), but the original paper is in russian and I cannot find it online anywhere. I am not sure if he ever tried to ionize air with that system, but he probably did accidentally at some point!
    Last edited by krazer; 04-20-2017 at 10:28. Reason: typo

  2. #32
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    I read that paper and there was no mention of air ionization. It is unlikely that they ever achieved it do to the terrible beam specs...35mrad. However, although I also suffer from a long, 20usec pulse length, the beam specs are much, much better.

    I do not think this has ever been done before, not with a flash lamp pumped dye

  3. #33
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    I have only seen air ionization once. It was in Toronto at the Science Center. They had a CO2 laser that had to be 10 feet long. It melted bricks like butter. They used a parabolic mirror and it would cause the air to make a snapping sound. I was only 8 but that stuck with me. I also go to taste water made from hydrogen and oxygen in a spark ignited lightshow. Was actually not very tasty.

  4. #34

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    Upon closer reading, you are probably correct that the Baltakov paper did not achieve air ionization. I had misread his result as 10ns instead of the correct 10us. With only 40MW pulse average power he would have had to focus quite tightly to achieve the ~10^13w/cm^2 needed to ionize air with a microsecond pulse, so if he was using a highly multimode cavity this was likely impossible.

    You mention that you have read his paper, does he mention the beam diameter that goes with his 35mrad beam divergence? I calculate that for a diffraction limited beam the BPP at 590nm is 0.19mm*mrad (1/e^2 radius and half angle divergence) so if he had a 1mm diameter beam that would be about 100 times the diffraction limit which, like you said, is really quite poor.

    There were a few other systems I found however...
    System 1: Very vague details, I was only able to find a reference which states 'A commercial laser has delivered more than 100J in 2us pulses using a coaxial flashlamp' and references 'R. Sierra: Laser Focus/Electro-Optics, 24 (1988)'. That system would have had a pulse average power of over 50MW putting it in a similar category as the Baltakov laser, but it is not clear what sort of beam this laser had.

    System 2:
    4J/2us (2MW pulse average power, comparable to your system) with a measured BPP of 0.47 (specifically that 63% of their beam energy was contained in an area which was 6.3 times the area containing 63% of the energy of a tem00 beam). They also mention that they were working on going both to shorter pulse durations and high pulse energies/reprates but I could not find any published results of these improvements. Note - they were using a C504 dye and were thus operating in the blue/green so it is not clear how their laser would have performed with r6g. http://proceedings.spiedigitallibrar...icleid=1241624

    There were also some talk in the above reference of a laser that I suspect is similar to the 'system 1' 100j laser, where they claim to get 10J in a beam with 0.12mRad divergence and a 14mm beam diameter (stated M^2 of 2.9 or I calculate a BPP of 0.54) in a commercial coaxial lamp dye laser, however they note the divergence suffers if they go to higher pump energy. But assuming they have the normal 10ns pulse duration you likely have that system beat.

    Comparing to your system, you mention you have 50J in 20ns, or about 2.5MW pulse average power and probably a bit more peak power depending on your pulse shape. You also mention a 0.3mRad divergence but not your beam diameter. But if you had a 3mm beam that puts you at a BBP of about 0.45mm*mrad so you should be able to achieve a focused spot size of about 1-2um which puts you well above the 10^13w/cm^2 air ionization threshold. So clearly what you need is a more expensive focusing lens...

    In any case it looks like have a decent shot at the crown for 'highest reported focused intensity of an unseeded flashlamp pumped dye laser'. It is frustrating doing background research for such a claim since the only results which are available are ones which have been stored for 50 years and then digitized, we may as well be comparing bonfire sizes of cave men tribes

  5. #35
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    The Baltakov laser was a 50KJ coaxial design with a relatively short 40cm cell, but the cavity length was not specified. The cell diameter was 50mm and their dye concentration was way too high for these dimensions at 1x 10-4M. They made several attempts to improve the beam quality including a unique (as far as I am aware) use of a two dimensional array of corner cube prisms.

    I am a little skeptical of the 0.12mrad output from a lamp pumped dye laser without the use of a high magnification, unstable cavity. This type of cavity does produce much higher beam quality, but usually restricts the output efficiency. One exception to this is a 140J, planar laser with a semi-unstable cavity built at the Avco Research Laboratory.

    The beam from my laser is 6mm in diameter and is expanded to 10 mm in the demonstration for three reasons. The beam expansion obviously reduces the divergence proportionally, but the optics are chosen to also correct for the convergent focusing inherent in the beam secondary to the thermal lensing as well as the spherical aberration do to the parabolic nature of this thermal lensing. These last two effects are significant.

    I would like to think Aristotle rather than Grawp, but...well.

  6. #36
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    The next installment in this series of videos is up.

    https://www.youtube.com/watch?v=TBe0zyv4PvI

  7. #37

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    Very nice gas/electrical handling design, it really looks like some of the commercial and semi-custom systems I have seen.

    You are right about the 'system 3' (10J 0.12mrad from a SLL-8000 dye laser), they note
    "The dye cell diameter was 2.5 cm. When fitted with a positive branch confocal unstable resonator it had a beam divergence approximately 2.9 X DL [diffraction limited] at the 1/e intensity points, figure 8, at an output of 10 J."
    They also mention that the beam divergence is time dependent, which was further studied by Baltakov http://iopscience.iop.org/article/10...4n04ABEH006807 where he observed the same kind of behavior. All I can say is that I am glad I am not dealing with that can of worms...

    They gave more detail about 'system 2' (custom system built by the air force), noting that they use a "positive branch, confocal unstable resonator of magnification 2.3" (also a birefringent filter to lock the wavelength). The also note that they get double the output energy but ~10x higher divergence with a 1.2m long plane-plane resonator (the dye cell was roughly 13mm diameter by 600mm long, pumped by 2x 7mm bore flashlamps driven at 40kv/27kA each). I really wish I could find some results of their future work, they mentioned construction of systems with more pump energy and amplifier stages but I couldn't find them published anywhere.

    That is interesting about Baltakov laser (maybe we can call that 'system 0'), with a 50mm beam diameter and 35mrad divergence (solid angle) I get a BPP of ~2000 so he was really going for energy. It makes sense that he didn't try anything funny with his resonator, there was mention that his lamps only lasted a few shots at the energies he was running.

    As far as your laser, you should double check my numbers but I tried calculating your beam parameters using 2 methods:
    Method 1: Simple near-field calculation
    Beam diameter 10mm (1/e^2)
    Beam divergence 0.3mRad (half angle)
    BPP = 3mm*mrad, which implies M^2 of about 15

    Method 2: Full beam propagation
    Assuming your optimized your telescope for minimum beam size at the target (beam focused at target)
    Beam diameter at telescope exit: 10mm (1/e^2)
    Beam diameter at target: 16mm (1/e^2)
    Distance from telescope to target = 10m
    In this case I calculate that case your diffraction limited beam diameter would be 0.74mm, so your M^2 would be 22

    Working forward to calculate your maximum achievable focused intensity:
    Assuming a Gaussian pulse in time:
    50J energy
    FWHM duration of 20us
    Multiply by 0.94 (ratio of area at FWHM of a gaussian to a square pulse) to get peak power of 2.3MW

    Use a F=15mm asphere lens (consider AFL12-15-S-U as a possible candidate)
    Starting diameter 10m diameter (1/e^2)
    Diffraction limited spot size 1.1um (1/e^2) or
    Multiply by M^2 to get an actual beam diameter of 24um (1/e^2)
    Assuming a gaussian distribution we multiply by 0.85 (ratio of FWHM to 1/e^2 for gaussian) to a FWHM diameter of 20um

    For a gaussian beam with FWHM diameter of 20um, we get the effective area (top-hat equivalent) by squaring and dividing by 0.94^2 (same ratio as before) to get 453um^2
    Then the peak focused intensity is just power/area (2.3MW/453um^2) = 5.1e10^11 w/cm^2

    This then matches your observation that it is not possible to achieve air ionization, since you are right on the cusp of the reported value for air ionization, so any deviation from these 'optimistic' calculations are going to prevent success.

  8. #38
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    Peter,

    Thanks.

    I think your analysis is quite good and I agree with your results as well. With the high damage optics, I have little doubt that I will be able to get the energy above 100J. And, I believe the divergence can be further reduced with an adjustment of the current, unstable cavity. The temporal nature of the divergence is almost certain to be present in all lamp pumped dye lasers and with this assumption I designed the cavity to counter this effect even though I haven't devised (a practical) method to analyze this property.

    I think that if both of these improvements can compensate for the inevitable "rounding up" that I'm sure I am guilty of in my energy density calculations then maybe we'll see air breakdown. It will be a close thing.

    The next two videos will cover the pumping arrangement and the cavity design...then I plan to shift gears and begin a series on multi rotor design and construction. Same concept though, go big or go home.

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