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Thread: Green DPSS Modulation

  1. #1
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    Question Green DPSS Modulation

    Can any DPSS expert explain this?

    I have a cheap DPSS greenie outpitting 80mw. When given 5v for cw mode operation there seems to be a delay of 1 second before the output reaches its max. Is this normal or is the construction of the laser to blame i.e. Crystal temp and other complicated reasons that come with DPSS...?

    Thanks

  2. #2
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    Some lasers incorperate a safety delay on power-up, but after that a 5V signal to the TTL/ analog input(s) should provide full output after the warm up time. Even before the warm up time you should be getting >90% output.

  3. #3
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    Even after a warm up time, the response seems a bit slugish. With fast modulation, the output does not reach 100% due to this 'slow' response. There is a start up delay, but I don't think that it has to do anything with this.

    In the end I ended up disassembling the whole thing and opened up the head. Driver seems to be a linear tec driver and a low side diode driver.

    The head looks like this:
    Click image for larger version. 

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    Does the laser head construction have anything to do with the 'slow' response? From what I've read around here (PL), DPSS are quite a pain when it comes to setup/construction and temperature control. The TEC temp settings and laser power output are still original factory settings. Should I mess with these so as to 'optimize' this module?

    The laser module is one of the chinese ones having TTL mod. The module was rated at 80mW, but since I don't have a power meter I cannot exactly say what its output is.

    Click image for larger version. 

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    Last edited by Fenechlaser; 09-03-2011 at 04:06.

  4. #4
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    I tried modulating the laser diode using one of the solarfire's drivers so as to have analog modulation. The whole rig wokred Left the driver on for temperature control since crystals are very finnicy on temp control.

    One thing i noticed is that during a static frame, eg ilda test, there are periods where the output suddenly increases and then dips back down again. Any suggestions for this? Driver and DAC are outputting correctly.

    Also what is the optimum temp for the SHG/KTP crystals for analog mod?

    Thanks.

  5. #5
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    Problem is that with the single TEC laser designs is that the room temp SHG is less likely to stay at a given good temp. A dual TEC laser head will modulate better.....but only if the driver will hold the SHG temps and diode temps constant IMO. These are expensive...or home made/tweaked.
    You are the only one that can make your dreams come true....and the only one that can stop them...A.M. Dietrich

  6. #6
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    Quote Originally Posted by MechEng3 View Post
    Problem is that with the single TEC laser designs is that the room temp SHG is less likely to stay at a given good temp.
    So laser head construction is to blame Can this be remedied in some way by using a more advanced temp controller say with PI or PID control? I was thinking of adjusting the temp of the whole assembly so as to find a better temp. Are there any cons to this?

  7. #7
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    Yes and no. The design of the head is in such a way that the diode is kept cool and near peak gain regions for the SHG. Trouble is that when it powers up...heat is not evenly controlled as it travels about in the system. The majority of DPSS lasers will deploy the diode
    on it's own TEC.... and then the SHG crystal{s} has a totally independant TEC to keep them at the correct temps. You can't do this with this laser. It is designed to be compact....then again quality of the SHG is a big key to how this design will modulate as well. Better crystal cuts are more expensive.....that is somewhat of a problem with bulk lasers....one may perform way over spec and the very next laser in the production line may not be able to make spec power...so it falls into a lesser value and may be sold as such. You will need to take the temp reading from the thermistor while in circuit...it is probably glued right next to the diode deep inside the sandwich...not sure how that will effect the driver's ability to keep it cool though.
    Last edited by MechEng3; 09-04-2011 at 06:18.
    You are the only one that can make your dreams come true....and the only one that can stop them...A.M. Dietrich

  8. #8
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    Latest update.

    I tried the driver with a red diode and noticed that the same flicker happened. I the end the problem was that there were flux residues under the adjustment pots, which resulted in the sudden output increases. After setting up the driver again and adjusting the pots, the output was brilliant. I have no idea and apparatus to test linearity, but to some extent analog modulation worked. I will try to post details of the laser in another post. Thanks MechEng3 and steve-o for the help

  9. #9
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    I have just finished modding this laser to analog Output starts at around 0.8v mod input (need to change a resistor - will be fixed). As voltage goes up, so does the brightness, until at around 3.7v to 5v the output appears to remain the same in brightness. Is this how a normal Green DPSS is expected to modulate? I know that this was designed for TTL and quite frankly I didn't expect this much change in the output. Thanks

  10. #10
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    Good to hear of your success!
    Cheers :]

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