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Thread: How to create super bright laser beams

  1. #11
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    Very clever. (Reminds me of the album cover of Dark Side of the Moon.) Why limit it to just red wavelengths? Why not a full spectrum of LD's, violet to red, all combined in a single prism? Hasn't this also been done with a diffraction grating? I guess dichro beam combining is easier and more efficient than a prism when there is a big difference in wavelengths? Great topic.

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    Quote Originally Posted by andy_con View Post
    Eric is bang on the money for this one.



    Put a single white beam into a prism and out the other side you get the full colour spectrum. So turn that around and put different frequencies of light into a prism and it acts as a dichro and you get a single white beam out, simple. Telecommunications companies use this method i believe with RGB light and some company has patented and copy righted etc...

    Were not dealing with a laser company here were dealing with a big multi billion pound company, just bare that in mind.

    So using the above principle you could add different frequencies of red light into a prism or other unnamed optic and get a single red beam out. The prism would act as a dichro so each beam would be perfectly over lapped even at aperture, its the holy grail of diode combining and would produce a more powerful and better beamed red than an opsl 5w red.

    Think of it like this, if you could get 10 red diodes @ 170mw each with a 1nm difference then you could combine them using the above method into a 1mm by 1.5mm beam @ 0.5mR ( or whatever spec a 2mm fl lens gives on a single mode red – cant remember off the top of my head).

    So thats 1.7w in a 1mm beam
    Now stack 6 sets together so you have 3 wide and 2 high that would give a beam of 3mm by 3mm @ 0.5mR at a power of 10W in a tiny beam. Then cube another 6 in and you have 20w of red in a 3mm by 3mm beam at 0.5mR, beat that coherent!

    It all sounds so easy and perfect ... yeah sadly its a right cunt to actually get to work.

    Each diode would need to be temperature controlled to probably 0.1 degree

    You can only set it up using a spectrometer, from testing every diode gives a slightly different frequency. At the same current and same temp diodes can be as far as 3nm different. you have to adjust the temp of each diode until the required frequency is reached, the spectrometer would need to be accurate to around 0.2nm to make it work properly.

    The most nm difference i managed to get out of a diode with a temp range of 0-40 degrees was 8nm, so that means you could only combine 8 diodes into the prism.

    The further the distance between the diodes and the prism the easier it is to get it to work. Im sure someone could do some sums and work out the distance and angle each diode would need to be. But the closer to the prism the smaller the angle, the further away the bigger the angle.

    If you have 8 diodes on an aluminium base plate, at one end a diode is cooled to 0 degrees then 150mm away the furthest diode is heated to 40 degrees, with some more diodes in the middle at different temps. Managing the temp of the base plate is tricky. Not looked into this in the slightest.

    When you first turn the unit on each diode is at the same temp, room temp, so out the prism would be lots of little dots. You would need a way to turn the diodes on once each has reached is pre set temp. So you couldn’t turn the a projector on and start using it straight away.


    Ok now for the fun part some real world testing

    3 diodes, 2x 110mw 683nm diodes tec’ed and a 660nm diode un tec’ed – all single mode. One 638 tec’ed to 0 degrees to other to about 20 degrees, no idea on what wavelength they were giving out as i don’t have a spectrometer.
    All 3 diodes had 2mm fl lens on them



    As you can see beams all tight together



    The combining optic isn’t important at the moment, but i was able to get all 3 beams perfectly over lapped into a 1mm beam




    The really fun part is dicking around with the temp of one diode and watching it slowly drift across the wall, kept me amused for hours.


    Interesting shit, i just need a really really accurate spectrometer to complete the project now 
    I don't know of anyone doing dwm with rgb, most optical communications happens 1500-2100nm

  3. #13
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    Quote Originally Posted by Photonbeam View Post
    Why limit it to just red wavelengths? Why not a full spectrum of LD's, violet to red, all combined in a single prism?
    Beat me to it! Seems to me that getting this approach going for RGB would require less knifeedging tolerances and make a nice proof of concept. Although, it looks like andy_con has already proven the concept with red only!

  4. #14
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    It gets really fun trying to do it with RGB. The usual method is two prisms in series and lens pair in between for reverse combining. This is so you have enough room on the baseplate by increasing the acceptance angle at the cost of transmitted power. If you can tolerate the loss of the second prism. This is not new in the show world, it has been done in a select few old abstract projectors to allow for independent modulation of eight colors prior to PCAOM.

    Remember you need at least 2 pairs of XY adjustments to correctly overlap a beam. It gets easier if the prism is on a type of optics mount called a "prism table".


    It would help if the diodes were binned for wavelength at purchase.

    I don't mean to steal Eric's fire. This is one way of doing it.


    Steve
    Last edited by mixedgas; 12-19-2014 at 08:34.
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    I have been thinking about this recently as well. The "stop point" in my thinking came when I thought I couldn't get enough difference in the refraction angle over the wavelengths to be combined with a prism(in a small footprint) I found through google that a Bragg grating could be used to combine them due to higher diffraction over the given wavelengths. Reading about Bragg gratings put me out of my comfort zone of comprehension.

    Practical control of individual diode temps might be easy if.... All the diodes where the same or sorted to be linear in their difference from one to the next. By placing them equally spaced on say a brass bar and heating one end and cooling the other a linear temp gradient would be setup across the bar. The wavelength deviation would depend on the difference in temps at each end.

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    When I looked into this and I suspect that is why the blacked out portion of Andy's pic, I think the combiner is a significant design component. Free space gratings (diffraction) are very likely not the way to go due to the low efficiency. This is due to the energy that is deposited in the diffraction orders beyond the first and so 70% in the first order would be very hard to achieve. Fiber Bragg would require very skinny, single mode fiber and I am not sure how the beams could be launched with the long FL lenses mandated by the clearance requirements. I am pretty sure the optimal solution would be a single Littrow prism operating @ Brewster's angle for the middle of the wavelength range.

    All the diodes where the same or sorted to be linear in their difference from one to the next. By placing them equally spaced on say a brass bar and heating one end and cooling the other a linear temp gradient would be setup across the bar. The wavelength deviation would depend on the difference in temps at each end.
    Despite this "macro" approach, I agree that this might actually be the best especially when compared with the extra complexity of the multichannel loops. Also, it might not be necessary to adjust each diode to a precise temperature. The interval between each diode likewise would not have to be the same, just not wildly different. The variations can be adjusted out with each knife edge, so if a particular set of neighboring diodes had a larger temperature step then they would be adjusted to converge at a proportionally larger angle. Finally, the brass bar could be "adjusted" to give a reasonably nice gradient by machining notches between each diode to control the heat flow.

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    Finally, the brass bar could be "adjusted" to give a reasonably nice gradient by machining notches between each diode to control the heat flow.
    Simpler yet would be to have the diode mounts adjustable along the bar. See crude paint diagram.
    Click image for larger version. 

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    Heat flow from each diode *should* be the same. So it should add in a linear fashion along the bar. Best thing about the mechanical temp regulation, as opposed to electrical, is any change it end temp would result in an even change to all diodes. Nice thing about electrical would be "tuning" of each diode via a pot, not actually moving the hardware.

    This discussion has really peaked my interest in this. How much was your spectrometer?

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    from the testing ive done I can tell you getting each diode at the right wavelength is very very difficult.

    I don't think any suggestion above will work, but that's just my view.
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    The modules I've seen use a really specialized Holo based grating and lots of compensation optics or a Bragg optic.
    The diodes all have some form of wavelength feedback, ie fiber Bragg grating or Fabry Perot.

    Really better communications stuff uses eight or nine lasers into a lossy combiner and then a fiber amplifier. No fiber amps for red that I am aware of.

    Incidentally I looked at FBGs for stabilizing a seed laser at 1064, a few months ago. Best accuracy yet Affordable stuff was all +/- .3 nm. Good enough for our purposes, except 2 nm steps between red wavelengths are NOT a stock product.

    Littman-Metcalf feedback into the diode can burn a lot of power.... If your tossing 30-40% to stabilize the wavelength its not worth doing.


    Still watching this thread....

    Steve
    Last edited by mixedgas; 12-19-2014 at 12:27.
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  10. #20
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    Quote Originally Posted by logsquared View Post
    Simpler yet would be to have the diode mounts adjustable along the bar. See crude paint diagram.
    Click image for larger version. 

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Views:	27 
Size:	8.6 KB 
ID:	45732

    Heat flow from each diode *should* be the same. So it should add in a linear fashion along the bar. Best thing about the mechanical temp regulation, as opposed to electrical, is any change it end temp would result in an even change to all diodes. Nice thing about electrical would be "tuning" of each diode via a pot, not actually moving the hardware.

    This discussion has really peaked my interest in this. How much was your spectrometer?
    Hi log

    I think your diagram works well if only materials ( heathink and baseplate ) are machined by extrusion on bar block Al/Cu. If is hot laminated materials, the temperature varation have an inpact on form memory material like ) or ( if heat or cold.
    Just my experience on the metal plates, and the longer it takes the more it moves.
    (Thanks google to help me for translation )

    Regards, Christophe.

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