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Thread: focusing lenses / reflecting problem

  1. #1
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    Default focusing lenses / reflecting problem

    Hi,

    I would like to focus the beam using one lens.
    I know that glass without any coating reflects something about 5% of light.

    This is 3W 808nm TO-3 laser diode. 5% of 3000mw is a lot, 150mw, and I heard that infrared laser diodes doesn't like when reflected light come back to the diode. It is not working well. Is that true?

    I have appropriate lens, it is made of BK-7, but it is without any coating.
    In TO-3 output is a small glass, with coating I think. If I put this BK-7 lens 2-3 mm in front of the diode, will this small glass protect the diode from reflected from lens light?

    Click image for larger version. 

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    Maybe someone has done it already, or just know how it will react? Thanks in advande

  2. #2
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    Default

    You really need a AR coated lens. The number is 4% per surface from BK7 to air. Because the back focal length of your lens is short, the classic trick of tilting the lens to steer the back reflection away from the die is going to be difficult.


    www.surplusshed.com is a good source for small, inexpensive, coated lenses.

    A few minutes with their lensfinder program, and I think you will find a coated candidate for a few bucks.

    Steve

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    I just wanted to be sure. Very thank you.

    I have already the lens (I have it for free) and I will try to put coating only in one of the factory in my city. Maybe they will add my lens to the group of lenses they will be coating with AR. If it is expensive I will buy new one with coating.

    Coating must be on the flat part of lens or on the both sides?

  4. #4
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    Quote Originally Posted by 0th3r View Post
    I just wanted to be sure. Very thank you.

    I have already the lens (I have it for free) and I will try to put coating only in one of the factory in my city. Maybe they will add my lens to the group of lenses they will be coating with AR. If it is expensive I will buy new one with coating.

    Coating must be on the flat part of lens or on the both sides?
    Traditionally both sides. If you can only coat one side, coat the flat. Then face the flat to the diode. You are trying to avoid forming a 3 mirror resonant cavity, which is what happens with the reflection, a resonant build up of energy. Because S2, the curved side, is such a steep radius, it is unlikely to resonate if S2 is uncoated.

    Some will argue that turning the curved side of the lens toward the diode will result in a slightly better shaped focal spot, but in your case the difference is so small as to be insignificant. With a much larger diameter beam, you would face the curve inwards.
    In this case, go with the flat towards the laser for simplicity.

    Steve

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    You not only help me with laser but also teach me language^^ Ten minutes, the time I spent on understanding your post. My technical language sucks

    I don't need very good shape of focus spot. Maybe this is an insult for the experts, but I'm making a laser depilator, not like other members DPSS or burning laser. This is how my adventure with laser started

  6. #6
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    My technical language sucks
    end quote.

    Your English is not bad.

    Where is home?

    Steve

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    Quote Originally Posted by mixedgas View Post
    Your English is not bad.

    Where is home?
    I'm from Poland. My girlfriend is sitting next to me right now and she is laughing at me. She studies English filology.

    I try not to make many mistakes.

    I will have problem with positioning the lens in front of diode. If it was TO-9 I would just buy the housing. And now I have to make it myself... somehow. Maybe you know if it is possible to buy something like diode housing but for TO-3?

  8. #8
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    Quote Originally Posted by 0th3r View Post
    I'm from Poland. My girlfriend is sitting next to me right now and she is laughing at me. She studies English filology.

    I try not to make many mistakes.

    I will have problem with positioning the lens in front of diode. If it was TO-9 I would just buy the housing. And now I have to make it myself... somehow. Maybe you know if it is possible to buy something like diode housing but for TO-3?
    Only from a expensive place.
    Go here, enter CO4 in the search toolbar


    http://www.thorlabs.com/NewGroupPage...ID=179&pn=CP04

    Good but Expensive.

    Here is a good drawing of TO3 holes:

    http://www.cirrus.com/en/pubs/misc/A...agram_HS05.pdf

    Ebay:

    http://cgi.ebay.com/Heatsink-Power-O...item5641482dbf

    http://cgi.ebay.com/Heatsink-Power-O...item519698ad31



    Steve

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