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Thread: Improving Red (Maybe ALL) Beams

  1. #81
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    So in theory one should strive to get as big FL collimator as possible within the required aperture beam width. Generally that's what all (or most) chinese suppliers do: they put long FL lens as a collimator, so long that even FA divergence becomes... 'acceptable', and then don't even bothers correcting it. Nice/cheap approach, however, in the cost of wide beam at aperture (and FA/SA divergence difference)
    This is not true. Consider that the SA and FA are distinct. The different dimensions and divergences can not be optimally corrected by a single radially symmetric lens. No matter how long the cheep Chinese modules make the single collimator lenses they are forgoing the opportunity to more accurately correct both axises of the beam. The shame is that full correction and even spatial correction is not difficult and the results are remarkably better. For a given laser source such as a diode or a gas laser there is an inherent quality to the beam (related to the M^2 parameter) when it is generated and no combination of lenses or filters is going to create a silk purse out of a sow's ear, but the right combination can maximize the quality that is present.

  2. #82
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    Long FL lenses (over 8mm) will usually clip the FA unless the clear aperture is over 6mm or so.

  3. #83
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    Quote Originally Posted by planters View Post
    This is not true. Consider that the SA and FA are distinct. The different dimensions and divergences can not be optimally corrected by a single radially symmetric lens. No matter how long the cheep Chinese modules make the single collimator lenses they are forgoing the opportunity to more accurately correct both axises of the beam. The shame is that full correction and even spatial correction is not difficult and the results are remarkably better. For a given laser source such as a diode or a gas laser there is an inherent quality to the beam (related to the M^2 parameter) when it is generated and no combination of lenses or filters is going to create a silk purse out of a sow's ear, but the right combination can maximize the quality that is present.
    with the 'acceptable' FA divergence I'm not saying that they make it similar to SA divergence, just that they make it small enough to use for fat beams. The profile is still horribly line-shape and it's far from wanted square-shape, let alone TEM00. I fully agree that for any multimode LD separate axis correction is a must if we want to get something nice out of that laser module, however I've seen some (not so many, though I'm not sure if I want to see more) cheaper modules with the exactly thing done: long FL collimator with no correction optics. We probably see the same situation in red module resunltd has (and where this thread started), since the profile of that module beam gives the same shape as uncorrected mitsu or oclaro, just with the lower divergence.

  4. #84
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    I just had a wild and crazy idea… Is it possible to use a masked mirror at the focal point of the lenses to form a spatial filter instead of an aperture? This would allow the introduction of spatial filters in compact optical arrangements that use steering mirrors before the scanners but don't have any long straight stretches into which one might insert the usual spatial filter arrangement.

    Perhaps a standard mirror could be etched, just leaving just a small reflective spot equivalent to an aperture, instead of mask. If you had a CNC mill and a very small bit, perhaps you could make a variety of different sized reflective spots out of a single mirror.

  5. #85
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    There have been apodizing filters around for a long time. In astronomy, these are used to reduce high frequency information and can make visualization more pleasant. This is why may people prefer to observe through a smaller, lower resolution refactor as opposed to a reflector telescope that, despite it's higher resolution, has aberrations that are enhanced by the high frequency component of the image. These filters are also used (dark center variety) to block the light of a bright object (star) to allow imaging of a dim, off axis companion (planet, ie Arrakis).

    http://www.reynardcorp.com/products/...ilter-vis.html

    The challenge with modifying a reflective surface includes the diffraction effect at the transition. Machining the glass is likely to lead to chips and coating defects right where the light needs to transition from pass to block. Also, the intensity of light at the focus is very great and the coating needs to be in great shape and very clean.

    If the space required by a spatial filter was too great, I would recommend two very short FL lenses (10-20mm) and between them, a pin hole (you might need to trial a few sizes) that was placed on a true X/Y/Z stage. This is more like the classic spatial filter. They work well, pin holes are not very expensive and readily available, but precision is the trade off for compactness.

  6. #86
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    Interesting, thanks for the info!

  7. #87
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    What vendor should I use to order some of these telescopes and lenses for using to do the special filtering. I have built a few rigs with the 638 and daves mounts/lens. But really want to special filter.

  8. #88
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    In order to learn the technique, I would set the diode with its collimator on a bench, carefully adjust the lens in Dave's mount to produce the most symmetrical far field spot. Spatial filtering should only be used on a beam that is already well aligned and focused. Get some long FL, PCX lenses such as a 100mm and a 50mm and then you can appreciate not only the spatial filtering aspect, but also the effect of a 2 power and a 1/2 power telescope on beam size and divergence. I get most of my optics from Edmond Optics and some from Thorlabs. In these long focal ratios you do not need anything other than simple spherical lenses.

  9. #89
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    Quote Originally Posted by planters View Post
    In order to learn the technique, I would set the diode with its collimator on a bench, carefully adjust the lens in Dave's mount to produce the most symmetrical far field spot. Spatial filtering should only be used on a beam that is already well aligned and focused. Get some long FL, PCX lenses such as a 100mm and a 50mm and then you can appreciate not only the spatial filtering aspect, but also the effect of a 2 power and a 1/2 power telescope on beam size and divergence. I get most of my optics from Edmond Optics and some from Thorlabs. In these long focal ratios you do not need anything other than simple spherical lenses.
    Thanks. I think I have asked the question before. Now it makes a bit more sense.

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