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modulateable laser diode drivers

Discussion in 'Electronic Design' started by Mike Noone, Aug 26, 2006.

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  1. Mike Noone

    Mike Noone Guest

    Hi - can anybody lend me any guidance as to how to build a laser diode
    driver that can be modulated? I would like to be able to modulate it at
    1-100Mhz or so. Most importantly, I need it's rising edge to be as
    sharp as possible. Are there any inexpensive ICs that can do this? If
    not, can anybody lend me any guidance as to how to go about creating
    such a device? My understanding is that the driver needs to be a
    current source with over-current limiting so as not to burn out the
    diode, but I'm unsure as to how to go about creating such a circuit.

  2. Ludwig Weise

    Ludwig Weise Guest

    It is standard task in CD and DVD drives to modulate laser diodes. When
    asked for something specific, you can look at Sam's laser faq:

    I have used the IC_Haus chips for low power applications and the IXYS
    MOSFETs and gate drivers if more power is required.

  3. Rich Grise

    Rich Grise Guest

    Well, at least this year they're asking their homework questions at the
    beginning of the semester, rather than at the beginning of finals week. ;-)

  4. Guest

    The rise time is a function of the bandwidth. The old rule of thumb is

    Are you going to pulse the laser, or do the sine method?

    Is this hacking, or a project for money? Hopefully it is hacking as I
    would like to see the results of what you come up with.
  5. Joerg

    Joerg Guest

    Hello Mike,

    Analog Devices offers laser diode driver chips. ADN2841 and things like
    that. Most are under $20 but if that's too much you'd have to roll your
    own. Basically a current source where current is steered into ground
    until you want a laser pulse. Now the steering transistor lets go for a
    few nanoseconds or whatever pulse length you'd like, then closes again.

    As you have recognized the current through the laser needs to be limited
    and regulated. There is a photodiode feedback on the laser diodes for
    that, usually. Stabilization is not a trivial tasks if you are dealing
    with occasional pulsing. Best to read up on that stuff before a few
    hundred Dollars go poof.

    Usually you'd also need a controller for the thermo-electric cooler. AD
    and others carry TEC chips for that, same price range. Unfortunetaly it
    all goes towards PWM which isn't so cool should phase noise be critical
    in your case (but can be done).
  6. Jamie

    Jamie Guest

    we have a system at work that uses an inline high frequency
    transformer to modulate the HV line going to the laser.
    its a simple basic laser and the HV supply is pasted through
    a round core a few turns, on the other side is a pulse amplifier
    we use to TX a signal.. it only modulates the laser at a small
    percentage due to the fact the simple laser unit wasn't designed
    for a large modulated signal like this in this manner.
    at the other end, it simply goes into a detector with slow AGC
    so that the AM signal can be detected.
    it wasn't designed by us but it works great.
    i don't think a 100 mhz could be done using the
    technique above, well not on the lasers we are using..
  7. Mike Noone

    Mike Noone Guest

    Well, at least in this post you're jumping to incorrect assumptions
    immediately, instead of asking questions first to find out the right

  8. Mike Noone wrote...
    You are talking about modulation, and not 100% on/off driving,
    right? The way I do that is quite simple. I use a standard
    moderate-speed light-sensor feedback laser-diode current-source
    controller, and add a modulation network at the laser diode.
    I operate the laser at about half current, as appropriate.

    The laser impedance is very low at these currents, a few ohms.
    The low impedance allows me to treat this node like a summing
    junction, so that adding a 50-ohm resistor in series with an
    ac-coupling cap from the coax cable carrying the RF modulation
    provides both the RF signal to the diode, and well terminates
    the 50-ohm coax. Using small dimensions, I have been able to
    modulate low-cost CD-ROM lasers up to 200MHz in this fashion,
    and done a bit differently in a quick lash up, up to 1.5GHz
    (I may have modulated the laser successfully well above that,
    but the photodiode I was using wimped out).

    From my Jan 10 2005 s.e.d. post,
    "The 664nm red DVD laser was an Hitachi HL6504FM and my detector
    was an Optek OPF480 PIN diode, biased at -100V, measured into a
    25-ohm load (double-end termination) with an HP network analyzer.

    "Components were 1206 SMT hand-soldered with zero-distance
    spacing. Bias-tees were 12GHz-bandwidth Picosecond Pulse Labs.
    A first attempt at a PCB stripline replacement only goes to
    600MHz so far."

    .. bias-tee laser
    .. ________| |______,---------,
    .. ________|-||-+--|______(-50R-|>|-'
    .. 50-ohm |____X__| \\ mirror PD bias
    .. coax | \\| optics 100V
    .. 50R //| & etc |
    .. | // __|____
    .. laser // ________| X |___50-ohm
    .. current ,-|>|----+-)_______|--+-||-|____ coax
    .. supply +-||-50R-' | coax |_______| term.
    .. '----------' bias-tee

    Jamie Morken answered, "Cool! That looks like a neat circuit, I
    am buying that same photodiode from Digikey (part#: 365-1029-ND)"

    Comment: In the latter approach, the bias-T isolates the 50-ohm
    coax RF signal from the low-frequency current source, all on the
    50-ohm line. But, making a good bias-T can be tricky, and using
    the low-Z at the laser diode instead should be an easier approach,
    perhaps not good to 1.5GHz, but certainly to 200-400MHz.
  9. Mike Noone

    Mike Noone Guest

    Hi Mr. Hill - Ideally, I would like to be able to drive the diode with
    any waveform. For now, my plan is to pulse it with a square wave at
    about 20mhz, with approximately a 2 or 4% duty cycle (pulse width = 1
    or 2ns). This signal will be going into an Acam TDC (Time to Digital
    Converter) chip, probabaly the TDC-GPX
    ( The idea is
    to measure the time between the rising edge of this pulse and the
    risinge edge coming from a photodetector that has optics filtering all
    but the wavelength of the laser, to get a distance measurement. Thus
    response time of the circuit is unimportant, but I do think it is
    fairly important to get a very clean rising edge on the pulse going to
    the laser driver.
    Do you think your circuit would be well suited for my application? I
    must admit that I am not as good at analog circuit design as I would
    like to be. (strength is really in digital embedded systems)


  10. Mike Noone

    Mike Noone Guest

    I am currently planning on pulsing it with a 1-2ns pulse, every 50ns or
    so. These numbers can be changed if necessary, but the important thing
    is the rising edge time, as it will be starting a time to digital
    converter and then the returning pulse will be stopping it.

    The project is for a robot I've been working on for the last 15 months
    or so. I want to give it the ability to avoid obstacles and map out
    areas. If you're interested, there are some pictures, schematics
    (outdated and with some mistakes), and some really old videos of it


  11. Mike Noone

    Mike Noone Guest

    Analog's offerings look quite good. I especially like the ADN2525, with
    a 24ps rise time! Unfortunately, I can't find it for sale in single
    quantites, nor do they seem to sample it. I plan on contacting them on
    Monday - but being a student that probabaly won't go too far. Some of
    the others look pretty good too - I especially like the ADN2870, which
    is available for about $10 in single quantities which is completely
    I have been thinking about going the TEC route. I think initially I
    will skip that as it is added cost and complexity, and I don't think it
    will be terribly important as I don't particuarly care about having the
    same power. Does that sound like a logical approach?


  12. colin

    colin Guest

    For moderate speed on off switching a common way is to just use a transistor
    to drive it or possibly a few very fast cmos gates in parallel.

    You need to limit the current of course, a fixed resistor is simple but a
    bit limited, an easy way to do this at high speed is to provide a constant
    current source then divert this away from the Laser with a diode conected to
    the cmos gates or a transistor.

    If you have low duty rate im not sure how you would implement optical power

    For high speed sinewave modulation as Win has sugested its a good idea to
    use a DC bias source and ac couple the modulation signal.

    Colin =^.^=
  13. colin

    colin Guest

    Looks nice, did you ever measure the capacitance of the laser diode ?
    The data sheet I found for that diode didnt mention capacitance.

    Ive managed to modulate a cheap pointer type laser diode @ 1ghz with a
    simple single transistor driver, originaly designed for 10mhz, although it
    was quite easy to get it to work at 100mhz, it required an ever increasing
    pull up current to get it to 1ghz, one problem is the capacitance of the
    diode im using, rlt6305, wich measures at over 50pf wich is surprisingly
    high, coupled with the lead inductance even with the diode next to the
    driver transistor must be quite a few nh.

    When I finally got it to work at 1ghz I measured the RF voltage accros the
    diode at the ends of the lead as >3vp-p but at the base of the diode case it
    was less than 1vp-p.

    My next layout im cropping off the laser diode leads and soldering the case
    directly to the ground plane,
    Im now also using a dc bias and AC coupling the RF, using a chain of MMIC
    and a big Gaas Fet.

    I try and avoid transmision lines if at all possible as its often hard to
    wideband match the terminations without lossy pads etc. The oscillator is at
    the start of the MMIC chain and part of the voltage accross the laser is
    easily fed back to the PLL via transmision line.

    Im not sure if the limiting factor is the capacitance or the internal speed
    of the Laser wich makes it necessary to drive the laser with a larger p-p
    voltage at these speeds, wich then makes the capacitance more of a problem,
    maybe driving the diode off harder doesnt speed up the laser much and I just
    need a better laser.

    Anyone know of any faster lasers (5mw visible)?

    Colin =^.^=
  14. colin wrote...
    Well, the above 50-ohm setup went to 1.5GHz, so that works out to
    no more than 2pF. The laser dies are very small, and fragile, too.
    Hmm, I suppose that can work if the laser is mounted right there.
  15. Mike Noone wrote...
    If I had presented my circuit with a 0.5ns-risetime pulse, I imagine
    it would have responded just fine. But remember, I was prebiasing
    the laser, which is important. I think you can bias it at low light
    levels, say 10 or 20% of its nominal output level, but for fastest
    response I'd guess it has to be above the lasing threshold.
  16. colin

    colin Guest

    50R and 2pf = 1.5ghz but if you take into account the estimated dynamic
    resistance of the diode at 1R in parallel it comes out at about 100pf.

    Ive measured 3 different types of laser diode and they come out at 30-60pf,
    I measured a varactor diode too just to make sure my LCR meter could measure
    this accuratly and it was spot on.
    I found it quite surprising it was so high and still not sure if I beleive
    it as it can work to 1ghz.

    I found a data sheet for the diode you mention from wich
    shows the modulation response and its dropping very fast at 1.5ghz, but they
    dont specify the test set up.

    Im wondering if I should switch to a vcesl as ive heard people mention a
    capacitance as low as 2pf, and theyr specd at 2.5gb/s but there doesnt seem
    to be any visible ones readily available.
    It helped with reducing inteference too, wich is mainly why I did it in the
    first place as my received light level is very low.

    Colin =^.^=
  17. colin wrote...
    That's correct, I shouldn't have used 50 ohms, sorry. In fact
    the dynamic resistance could be considerably under 1 ohm.
    If you measured the capacitance at 0V or with a reverse bias,
    you probably got a lower value than it has when operating.
    My measurements were so much faster than the datasheet, about 3x,
    I suspect they in fact were suffering from slow detector response.
  18. colin

    colin Guest

    just found a few here

    some go upto 155mhz but some of the datasheets with internal circuits give
    you ideas on how to build your own.

    Colin =^.^=
  19. Guest

    You are going to need to sweep the laser, much like a bar code reader.
    My gut feeling is you are going to have difficulty measuring short
    distances with a pulse technique. Have you investigated clock jitter?
    Worse yet, from the avi, I wouldn't call that smooth motion, so the
    crystal will have microphonic tendancies. That is, vibrating the
    crystal will modulate it. You can average out the readings I suppose to
    get around the crystal vibration.
  20. Mike Noone

    Mike Noone Guest

    Videos are of the robot's first steps. There were countless bugs in the
    code at that point, and motion was sloppy. If you look carefully in the
    video you can actually see that one of the legs turns off during
    certain movements - just becomes a complete gimp leg. Was a major
    software bug that I became aware of after taking that video.

    Anyways - I was planning on having the robot stop, and then scan, then
    start again. What clock jitter are you referring to?


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