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Thread: Why Are Blue DPSS Lasers So Expensive?

  1. #1
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    Default Why Are Blue DPSS Lasers So Expensive?

    I know very little about 473nm DPSS lasers due to the fact there is not much info avalable on the internet. According to Wikipedia, and the details seem fuzzy to me. A blue DPSS uses the same components as a green module except for the pump diode..... At least that's how I read it. The KTP recieves 946nm or so to produce a 473nm output. So, do they use a 940nm or so pump diode directly to the KTP or do they use a variation of YAG or YVO4 to create this wavelength to pump the KTP? Anyway, What's the expensive part in this package? I'd imagine getting a stablized LD to pump out 946nm at a few watts isn't that hard.
    Thanks
    Adam

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    Hi Adam,

    In a nutshell, though the principle of operation is similar, the optical crystals used for the IR-to-blue frequency conversion are more complicated to synthesise, currently produced in lower volumes, and also significantly less efficient than their IR-to-green counterparts. All of these factors contribute to their increased cost. Perhaps somebody else can provide more detailed information if you're interested.

    Kind regards,

    sonaluma

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    Cool

    Quote Originally Posted by 300EVIL View Post
    A blue DPSS uses the same components as a green module except for the pump diode.....
    Not quite; you've got it backwards. The pump diode is actually the same, as is the lasing crystal. It's the doubling crystal that is different. (Well, that and the optical coatings, see below.)

    Both blue and green DPSS lasers use a pump diode that lases at 808 nm. That near IR radiation is used to pump the ND:YAG or ND:YVO4 crystal, which then lases in the far IR. How the crystal lases (or, more to the point, at what wavelength it lases) is where the difference starts.

    Green DPSS lasers have optical coatings on the crystal that favor the 1064 nm line. So even though the crystal will lase at several different wavelengths, by applying the proper coatings they force it to lase on just the 1064 nm line. Fortunately, the 1064 nm line has a high gain, so the process is fairly efficient. The 1064 nm output then passes through the KTP crystal, which doubles it to 532 nm.

    Blue DPSS lasers, on the other hand, have optical coatings on the crystal that favor the 946 nm line. This line does not have nearly the gain of the 1064 nm line, which is why the pump diode on a 100 mw DPSS blue laser will be larger than the pump diode on a 100 mw DPSS green laser. (Bigger pump diode = more $$$, plus a larger TEC, which means still more $$$.)

    The 946 nm output of the crystal still needs to be frequency-doubled in order to get the 473 nm output, but in this case you use a different doubling crystal, either KNBO3, LBO or BBO. (LBO and BBO being most common) These crystals are less common (and thus more expensive) than KTP.
    What's the expensive part in this package?
    Well, it's the whole package, really. First the pump diode has to be larger. Second, the doubling crystal is more exotic. Third, if you use KNBO3 you need a very accurate temperature control circuit because it's extremely temperature sensitive. (which is why LBO and BBO are used even though they're less efficient) Fourth, blue DPSS lasers have fewer applications (outside light shows) that aren't already being serviced by green DPSS lasers, so companies sell fewer blue DPSS lasers. All of this works to make blue lasers more expensive.

    Note that even if you had a direct injection diode that lased at 946 nm and fired it directly into a doubling crystal, you'd still have problems 2 through 4 above. (Not to mention a direct injection diode at 946 nm is going to be a rare beast, which also drives up the cost.)

    Sonoluma said it best: the parts are less common, more complex, and less efficient for a blue DPSS laser. That's three major factors working against a low price. And while blue lasers *are* getting more affordable, I don't think they'll ever be as cheap as DPSS green lasers are.

    On the other hand, direct injection blue lasers *do* have the potential to become very affordable, both because they're less complex and thus are easier to manufacture, and also because they are being used in a lot more products. I'd say the most promising one is the 445 nm diode, though the human eye isn't nearly as sensitive to that wavelength as it is to the 473 line.

    Adam

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    Default

    AH! okay, I get it now. So YAG or YVO also produces a line at 946nm when pumped with 808nm, just way less output than at 1064nm. Right? Next it is pumped into a LBO or BBO crystal versus a KTP. Obviously the HR and OC coatings have to be different for this wavelength.

    Bottom line is,,,,,, higher pump power, rare non-linear crystals and limited production....
    Thanks Guys!
    Adam

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    Default

    Quote Originally Posted by Buffo View Post
    [edit]
    The 946 nm output of the crystal still needs to be frequency-doubled in order to get the 473 nm output, but in this case you use a different doubling crystal, either KNBO3, LBO or BBO. (LBO and BBO being most common) These crystals are less common (and thus more expensive) than KTP.
    [edit]
    Adam
    What is KNBO3? There seems to be something wrong with that formula, as N is nitrogen, and despite it being a long time since my chem. degree I don't think it's possible to have such a compound.

    Do you perhaps mean potassium niobate which would be something like

    KNbO3

    Oh yeah, you must be referring to that. Had a look at:
    http://en.wikipedia.org/wiki/Nonlinear_optics

    which shows KNbO3 to be a typical material.

    Gotta watch those upper and lower cases in chem formulae.
    Have a good day!

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    Quote Originally Posted by Buffo View Post
    [edit]
    Note that even if you had a direct injection diode that lased at 946 nm and fired it directly into a doubling crystal, you'd still have problems 2 through 4 above. (Not to mention a direct injection diode at 946 nm is going to be a rare beast, which also drives up the cost.)[edit]
    Great explanation.

    Aren't some of the recent 488nm solid state lasers based on directly doubled 976nm diode lasers?
    Have a good day!

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    Red face

    Hi OPO226;

    Yeah, you're right. In my haste to try to get the subscript to look correct I inadverently dropped the lowercase b from the KNbO3. (Once I got it looking right in the preview window I copied it and pasted it throughout the post, so the mistake was replicated everywhere!)

    And for what it's worth, the subscript never did come out right. It looked good in the preview window but after I posted the message it reverted to normal script. Sigh. That's what I get for trying to be clever.

    As for the 488 nm diode lasers, you may be correct. I only just read about them a short while ago and I haven't done a lot of reseach on how they work yet. I know they use KTP for the doubling stage, but I'm not sure what they use for a source of 976 nm input...

    Adam

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