Blue frenzy hits the holographer too…

There has been for the last 10 days or so an incredible frenzy over the 1W 445nm Nichia diodes that can be harvested from certain projectors; see the turmoil at the various light show and ballon popper forums. Well the holographer, being not particularly interested in using his lasers as lamps in discotheques, or in wielding 1W laser pointers in public, is more interested in the mode stucture of these little beasties. So I had to get a few of those, and today, within the hour of receiving it, I set it up and checked the mode structure with my CCD spectrum analyzer (due to lack of a 1 Amp capable driver I had first to mod my ol’ proven SDL800 such as to allow it to run blue diodes with higher forward voltage). And voila – without any systematic search, I found that the diode runs single longitudinal mode up to 55mW (with Lens-27 at 221mA and 15.4C) ! This is truly exciting, not the least because the diodes are transverse multimode and apparently have many emitters. Here is a quick sample overview:

Nichia-spec

From top to bottom the current was 221mA, 225mA, 300mA, and 400mA. I didn’t turn the diode up to more than 500mA where it was yielding 430mW (with non-optimal Lens-27), because single mode cannot be expected at high powers anyway. Note the beautiful single mode at 221mA!

PS1: I now was completing the first systematic landscape scan and the result is shown here. A sweet spot at 233mA and 14.9C yielded 64mW! I tested also an ECDL setup but so far the power wasn’t significantly higher, but this can probably be improved as well.

PS2: A first first test of an ECDL didn’t pan out quite well, there are many parameters into this game indeed and perhaps things can be improved. Indeed a test with better collimator, different grating and careful adjustment, more than 160200mW single longitudinal mode was achieved, but this was quite unstable. Stay tuned for more results.

In fact beam quality appears the main downside of these diodes and may be the deciding factor of their suitability for holography. Firsts tests of shooting through a spatial filter were the opposite of exciting…. probably, for a holography application, best would be to knife-edge split up the beam and use the pieces as object and reference beams.

PS3: Right now I was spending time in designing a higher power version of my super stable driver boards, which would be optimized for ECDLs using the 445nm diodes. Since I was running out of the HY5600 TEC controllers I was forced to cook up a completely new circuit, this time however PWM based and with readily available parts. It runs at least as well as the old circuit.

PS4: I now verified reports here and here that even a simple glass plate can give sufficient feedback in order to provoke stable single mode operation; indeed the results look promising, see here for details.

Below a pic of a test setup with glass plate feedback:

PS5: After a couple of weeks where I have spend quite some effort to try various different configurations for glass plate feedback, the following scheme seems to emerge. Within limits the amount of feedback does not matter much, what changes is how much the threshold is current is lowered. But with a reduced threshold also the maximal current for stable or single mode operation is reduced. It turns out that no matter what I tried, the maximal power for stable single mode operation stays always at about 60-80mW. This is not much different from the free running diode (except that the proportion of single mode zone modes is higher and the general stability is much better).

Also with grating feedback the generic situation is not much different, but there are lucky expections. But even there higher power operation is quite a gamble.

So here a brief summary: single mode operation with the blue Nichia diode is generically possible up to 60-80mW, quite independently of the setup. With sacrifices on stability and mode purity, more than 200mW are possible, also relatively independent of the feedback. However, there is then a tendency for several supermodes to lase simultaneusly. For an extended summary see here.

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9 Responses to “Blue frenzy hits the holographer too…”

  1. 200mw laser Says:

    I once tried to make a laser pointer from a DVD player, I messed up 😦
    I am following your project here, it is complicated but I am trying my best to follow it and learn something!

  2. rxlaser Says:

    Hi, for making laser pointers, you may want to have a look to the dedicated forum: http://laserpointerforums.com/f65/

  3. Holofan Says:

    Hello,

    will you be selling ECDL lasers and you much would you charge?

    SIncerely,
    Peter

  4. rxlaser Says:

    Hi Peter, yes I am preparing a couple of red and blue ECDL’s for sale, but I am not yet sure what to charge, I still need see until all is done. That will be mostly for getting rid off components I have acquired and recover self-costs, after that I’ll stop as it is going to much work for me, esp I don’t enjoy mechanical work at all.

  5. Johan Reinink Says:

    I’ve read the article on your website, looks very proffesional! I found it while looking for a way to use my Coherent DPY315M without the proper power supply (I only have the head). You really have an impressive lab, I’m hoping mine will become just like it, I’m making good progress.

    About this 445nm diode, the graphs on the ECDL page only go up to 400mA. You say you get over 200mW out. Do you know how high these diodes go in power with the added feedback? Without feedback they go up to 1.6W, some guy even reached 2W (with the the A140 diode).

    Second point, if the diode is operating with a single longitudional mode, does is also operate on a single transversal mode? Is the transversal mode behaviour linked to the longitudinal mode behaviour

  6. rxlaser Says:

    Hi Johan,

    I am not sure how much the 445nm diodes can be pushed with feedback, the slope is somewhat smaller than for the free running diode. I would think that one could simply extrapolate this to the order of 1A without much problems. Howeber, things become very unstable beyond 100mW or so, 200mW can be reached in single modes with effort but can be maintained only for a short time, and things get much worse beyond. So I don’t think it makes much sense to go much beyond 300-400mA.

    As for transverse modes.. I understand from the literature, and this is consistent with what I oberserved, the following. At low currents when there is single longitudinal mode, there is a single coherent, so-called “supermode” running which extends over the whole chip, which has some kind of inhomogenous “filaments” which create interference patterns in the far field, and these look like multiple transverse mode.

    When the current is increased, typically above like 250-300mA, then several such supermodes start to lase, which have different frequencies and are not coherent with respect to each other. Each of them produces a different interference pattern in the far field. One can see this by naked eye because the pattern starts to blur. And one can check by a spectrum analyzer that different portions of the pattern have different frequencies. So this mode of operation must be avoided. I didn’t succeed to improve this beyond approx 100mW, no matter what feedback I used. I guess the reason is in the diode, eg that internal temperature gradients distort the wave front, or something like that.

    Perhaps with a precision optics setup, incl mode slit and careful optimizing the wavefront hitting the grating, things can be improved, but it seems a lot of work!

    Best,
    W.

  7. Johan Reinink Says:

    Hi W,

    If you could estimate a M^2 value for both axis of the diode, how bad would it be at higher powers? Would some spatial filtering be able to improve the beam quality significantly, or is this diode pretty hopeless beyond 100mW?

    Regards,
    Johan

  8. rxlaser Says:

    Hi Johan,

    no I can’t give an educated estimate fir M^2, perhaps have a look at the various posted pictures at PL and laserfreak.net. Also have a look at the pictures at the bottom of my page:
    http://redlum.xohp.pagesperso-orange.fr/laser/ECDL-test.html#NDB7352
    and neglect the vertical interferometric fringes; it is the horizontal bands that matter. The left pic refers to SLM and the right mixed SLM/multi mode, the spectra can be seen above.

    Yes I was playing with spatial filtering, one can clean the beam but at the expense of great power loss due to the axes ratio. I didn’t yet shape the beam so this can be improved. Also knife-edge splitting the beam to create obect and ref beams may be a way to go. See also this page with however preliminary, not very useful info:
    http://redlum.xohp.pagesperso-orange.fr/laser/blue-beamshaping.html

    All the best,
    W

  9. Some intermezzo in repairs and metrology | W's Laserblog Says:

    […] references. This created a group buy frenzy which reminded me of the one of a few years back about the blue 1W laser diodes – a gold rush type excitement where people buy dozens if not hundreds. Obviously I […]

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