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Laser driver optical feedback

Discussion in 'Electronic Design' started by colin, Sep 24, 2006.

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  1. colin

    colin Guest

    Hi,
    Im having problems with optical feedback with a pointer type 5mw visible
    laser diode,
    its modultaed with 1ghz wich is fed via AC coupling and a dc circuit bias
    circuit controls the diode current with feedback from the internal
    photodiode.

    I have to drive the diode quite hard to get it over 1ghz and RF seems to be
    interfering
    with the optical feedback, increasing the brightness substantualy, the laser
    current is increasing.

    It also tends to latch on brightly very occasionaly and wont turn off untill
    the RF modulation is reduced.

    Im not sure if this is due to RF pickup in the 2 transistor circuit as this
    would tend to reduce the brightness,
    or if the rf picked up on the photodiode is modulating it. The diode has 3v
    reverse bias accross it so it shouldnt be rectifing the RF unless its is
    seeing more than 3v.

    --o---o----------o---------o------- 0v
    | | | |
    | - .-. |
    | ^ pd | | |
    | | | | |
    | | '-' |
    | | | V laser
    | .-. .-----o -
    --- | | | | |
    --- | | V | |
    | '-' - | |
    | | | | |
    | | | | | ||
    | | | |< o----||- AC drive
    '---o----)---| | ||
    | | |\ |
    | | | |/
    .-. | o---o---|
    | | | | | |>
    | | | | | |
    '-' | | | |
    | .-. .-. | .-.
    '-->| | | | --- | |
    | | | | --- | |
    '-' '-' | '-'
    | | | |
    -----------o-----o---o-----o------- -5v
    (created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)


    Colin =^.^=
     
  2. Mark

    Mark Guest

    check the RF bypassing from -5V to ground...

    my ___guess___ is RF is blasting into your circuit via the supply
    lines...

    Mark
     
  3. the tradional way of doing this is to run a injected current and use a
    DC block in the microwave feed. Thorlabs sells a medium cost module for
    this. You can still do a light loop around it with care. The module is
    commanly called a "Bias-T" or "Bias Tee" googling this should find you
    circuit examples. It should let the RF see 50 ohm impedance and keep it
    out of the DC loop.

    good luck.

    Steve roberts

    Steve Roberts
     
  4. colin

    colin Guest

    Thanks for the replies, I re aranged it and it seems to be less sensitive,
    also I had used a p channel mosfet for the first rransistor, (fdv302)
    the trouble was it has a parasitic transistor, as the gate went positive it
    became the emiter of a pnp transistor !
    this cuased the latch up problem, and took a while to work out why.

    The emiter resistor of the driver transistor is now in the collector
    instead,
    unfortunatly the laser diode itself is hardly a 50R impedance, more like 1R,
    the ac drive is from a phemt right close to it so there is no transmision
    line to match or cuase reflections.

    The thing now is that as the modulation is turned up the laser apears
    brighter,
    yet the circuit reduces the laser current,
    does the eye react to 1ghz modulation ?

    Colin =^.^=
     
  5. search for the term "Bias T" or "BIAS TEE"
     
  6. Hi Steve,

    I did a bit of googling and found some interesting links. However, all
    of them seem to be designed for low power laser diodes and all seem to
    have a 50 Ohm input and output.

    I've been thinking about the following problem for quite a while now:
    I want to provide fast modulation for a high power laser diode module,
    something like a Jenoptik 75 W fibre-coupled package
    ( http://www.jold.com/sites_produkte/produkte-typen_fm_en.htm ). We
    want to have a modulation of a few A on top of the DC current of up to
    60 A. Modulation speed would be up to a few MHz. I thought already
    about something like the bias-t without knowing its name. However, it
    seems to be difficult to implement at this type of power and current
    levels. I was thinking about a matching transformer for the modulation
    path, but don't know enough about transformers to judge whether that
    makes sense.

    Thanks a lot for any advice,

    Christoph

    ------------------------------------------------------------------------
    CSIR - National Laser Centre
    Pretoria, South Africa
    Phone and email can be looked up on the staff list under
    "contact us" on the CSIR web pages:
    http://www.csir.co.za/
     
  7. I think that would work quite nicely but you might have to do it as a
    closed loop control scheme. 1 or 2 turn secondary, multiturn primary.

    --- sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/
    Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
    +Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
    | Mirror Sites: http://www.repairfaq.org/REPAIR/F_mirror.html

    Important: Anything sent to the email address in the message header above is
    ignored unless my full name AND either lasers or electronics is included in the
    subject line. Or, you can contact me via the Feedback Form in the FAQs.
     
  8. Hi Sam,

    Thanks for the advice. What do you mean by "closed loop control
    scheme"? Sense the current at the diode to control the voltage into
    the transformer? I was hoping this is not necessary.

    Christoph

    ------------------------------------------------------------------------
    CSIR - National Laser Centre
    Pretoria, South Africa
    Phone and email can be looked up on the staff list under
    "contact us" on the CSIR web pages:
    http://www.csir.co.za/
     
  9. That's sort of what I had in mind. It may not be necessary. Some careful
    experiments with a dying diode you don't care about may be in order. :)

    --- sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/
    Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
    +Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
    | Mirror Sites: http://www.repairfaq.org/REPAIR/F_mirror.html

    Important: Anything sent to the email address in the message header above is
    ignored unless my full name AND either lasers or electronics is included in the
    subject line. Or, you can contact me via the Feedback Form in the FAQs.
     
  10. colin

    colin Guest

     
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