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Thread: Making 488 from 976 using nonlinear crystals

  1. #1
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    Default Making 488 from 976 using nonlinear crystals

    Does anybody have any experience building a frequency doubling system that would accomplish this? Or is it beyond the scope of the hobbyist..
    10W 976nm pump diodes aren't hard to find, and .5-1W of 488 cyan would be very nice in a projector imho

    http://www.alphalas.com/products/las...-crystals.html

    Click image for larger version. 

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  2. #2
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    It will be nearly impossible to frequency double a multimode diode like that.

    The crux of nonlinear optics is that they require extremely high intensities to get reasonable conversion efficiencies. For example, the intracavity power of a normal 532nm laser pointer can easily be 100w, with a beam waist of only a few tens of micrometers in the doubling crystal.

    In general, diode lasers are not capable of generating high enough intensities, so directly doubles lasers use some sort of cavity enhancement to make it work. The Coherent D3 (http://www.repairfaq.org/sam/laserdio.htm#diocod3) uses an external fabery-perot cavity locked to the laser diode, creating a sort of 'fake laser' which has an intracavity power high enough for the nonlinear process work. The novalux diodes use an extended cavity diode, so that the crystal can be put inside the diode laser cavity, which they were able to get working by using a fancy large mode area VSCEL which can handle very high intracavity powers (http://www.repairfaq.org/sam/laserdio.htm#diopr488). The advancement of PP-KTP waveguides (similar to the crystal you linked) have drastically reduced the necessary power to efficiently generate second harmonic, such that it is possible to start with a single mode diode of with powers as low as 50mw (or as high as you can find a suitable diode for) and couple into the waveguide and get reasonable conversion efficiency, such as is achieved with the melles griot BDD series (https://marketplace.idexop.com/store...ils?pvId=35354 and http://laserpointerforums.com/f40/48...tem-85658.html for a picture of the insides). Of course with all of these techniques there are extremely stringent demands on the frequency purity of the diode (the D3 requires a truly stable SLM diode with many meter coherent length, the BDD can theoretically work with any diode which you can couple into a single mode waveguide, although practical limitations mean that you need a frequency stabilized SLM diode as well), the novalux is the least sensitive but still requires temperature stabilization down to a few hundredths of a degree kelvin for stable operation.

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    Wow thanks for the very detailed and concise explanation Krazer. I Guess we'll be waiting for the 488nm DI diode then. Thanks
    and Cheers !

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    Pardon my ignorance, but if it is so hard, why are the 532 DPSS modules a dime a dozen from china now? Seriously, you can get 532 laser pointers for less than $10 on eBay. I thought that was also a frequency doubling system?

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    Diode beams are typically broad band and even single mode and focused produce relatively low intensities. The 532 is a Nd crystal laser that is PUMPED by a diode. The fundamental bandwidth (wavelength spread) is very narrow and when the nonlinear crystal is then inserted into the cavity, between the cavity mirrors, the high intensities and narrow bandwidth make SHG efficient. I agree with krazer that the periodically pooled non linear crystals are pretty amazing. I switched over from bulk KTP to PP lithium niobiate and a crystal 1/3 as long produces the same conversion efficiency and avoids the low damage threshold of the KTP.

    I'm waiting for the gap between 532 and 638 to be filled. This is a huge spread and if addressed the whole 532 vs 520 debate becomes moot.

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    Didn't you build a high power yellow laser Eric? I remember a video. Was that crystal or dye? I don't remember. I do remember it was pretty amazing ..

    tribble wrote:
    Pardon my ignorance, but if it is so hard, why are the 532 DPSS modules a dime a dozen from china now? Seriously, you can get 532 laser pointers for less than $10 on eBay. I thought that was also a frequency doubling system?
    Yes they are freq doubled. The Chinese have 532 dpss down to a mass production science. It's a lot harder to build a 532 from scratch at home (I've tried) .
    And with the advent of the d.i. 520nm diodes, the complexity of building the 532 dpss will be non profitable and out-dated so my assumption is that they are now having to take a loss instead of a profit in order to clear excess 532 stock from their shelves.
    Last edited by steve-o; 10-04-2014 at 10:14.

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    I built two "yellow" lasers. One is simply a LS pumped dye laser and the other is a Raman shifted vanadate, similar to the DPSS I demonstrated in the video. Yellow-orange is the problem. The lasers I built are simply too complex or expensive for practical projectors. A Pr+ laser pumped with a blue diode would be as simple as a DPSS for up to about 200mw. Losses in the gain crystal produce problematic heating above this level. But a yellow diode (not submerged in LN2) would be the answer. There was a recent thread that discussed the fact that common AlGaAs lasers generate lines and emission all the way down to the green. Could a junction, especially a long one without cavity optics, be induced to lase in the yellow-orange with the appropriate external optics? Someone here on the forum works for DILAS and maybe other experts could address the potential of this set up. Has it been tried? I know that external cavity optics have been used with IR diodes to facilitate wavelength stepping/shifting for extreme brightness fiber launching.

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    I am curious why we haven't seen more of the 594nm frequency summation DPSS lasers as well as the raman shifted lasers. Are these designs, the raman shifted laser and the frequency sumation laser difficult to produce?
    Those who fail to grasp art are the ones who criticize it.

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    Fundamentally, these are only one step beyond a 532nm DPSS laser. In fact, Nd has a number of weak lines that can be doubled just like 532, but these lines are weak enough that special coatings are needed to allow these lines to lase and it is necessary to inhibit the stronger 1064 line from lasing by reducing the reflectivity of the cavity coating at this line. Ramen shifting can occur in the gain crystal itself and so an additional element is not necessary, but the Ramen process red shifts by converting the energy difference between the fundamental and the shifted wavelength into heat. This increases the thermal problems that already are a significant problem with Nd lasers. An alternative to self Ramen is a separate CVD diamond in the beam path. This material has become so cheap that it is competitive with the gain crystal as far as cost and produces a better beam especially at higher powers. So, this can be doe (I did it), but it is just too complicated to be competitive with a mixed 532 and 638 beam for these wavelengths. Summation or wavelength mixing can be done and is becoming more feasible with periodically pooled non-linear crystals, but the wavelength and temperature stability requirements are difficult to meet and so this is restricted to a laboratory technique.

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    Thank you for the information, Eric. I would imagine that the thermal requirements would make modulation of the pump diode difficult, if possible at all. I think we'd be left with AOM as a modulation option. It sounds like a more realistic expectation, outside of beam mixing, would be a yellow diode.

    I would love to see some video of your Yellow DPSS, if you ever get around to it. Your fiber fed vanadate was a very interesting video to watch not to mention all of your mitsu red videos!
    Those who fail to grasp art are the ones who criticize it.

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