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I would like to build a proper laser driver and use an audio output to modulate the laser output.

At the moment, I connected the audio output instead of the battery in a laser pointer. It works, but the laser signal is very weak and I would like to use laser diodes instead of laser pointers. The ready-built laser drivers often use TTL signals, MHz modulation or not enough current. The laser can handle currents up to 26mA. A circuit plan would be great!

Final Project: I would like to use the audio line output to modulate the laser and use the audio line input to read the photodiode signal, laser audio communication setup.

This previous question (link) is amazing, but I have an analogue signal and not a TTL signal.

Thanks a lot in advance.

Sarah
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    Olin's answer to the linked question will work for analog as well as digital inputs, you'll just need to adjust component values for your signal bandwidth, bias point, and gain. – The Photon Oct 23 '18 at 21:43
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    Welcome to EE.SE. As you might suspect laser diodes and lasers in general much prefer to be modulated with a digital signal, even PWM would work. Using analog signals puts the laser into 'inbetween' states where it waist much energy as heat and has a non-linear response. Your options are to feed it a digital or PWM signal. –  Oct 23 '18 at 21:44
  • @Sparky256, That's nonsense. Analog modulation of a laser diode is a common technique. – The Photon Oct 23 '18 at 21:46
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    @ThePhoton. Agreed, but the response is non-linear which limits bandwidth and dynamic range. –  Oct 23 '18 at 21:48
  • @Sarah, can you share a link to the datasheet for your laser? What kind of medium are you sending the signal through (glass fiber, plastic fiber, free space, ...)? – The Photon Oct 23 '18 at 21:51
  • @ThePhoton. Sarah can use analog signals but not with todays 120dB+ dynamic range. Log compression and bandwidth limiting will give better results. At the receiving end this can be reversed. Note that analog modulation could cause the MOSFET driver to heat up a by great amount. –  Oct 23 '18 at 22:18
  • @Sparky256, I planned to modulate the laser with a sinus and not a TTL because I prefere the sinus for the photodiode signal processing on the computer afterwards. I expected that the laser lifetime would benefit from the slow sinus changes compared to the sharp TTL changes. – Sarah Oct 24 '18 at 17:42
  • @The Photon: The laser I would like to use is the 5mW version https://de.aliexpress.com/item/1pcs-Focusable-5mw-10mW-405nm-Violet-Blue-Laser-Dot-Diode-Module-LD-3VDC-5VDC/32623747510.html?spm=a2g0s.9042311.0.0.27424c4dv2lajE – Sarah Oct 24 '18 at 17:46
  • @Sarah, This isn't just a diode laser, it's a module with an internal power supply circuit. I don't see any indication that this laser module is designed to have the output power modulated. So this is probably not a good choice for what you're trying to do. – The Photon Oct 24 '18 at 18:10
  • Und ich glaube ich habe die Deutsche Beschreibung mindestens 90% verstehen. – The Photon Oct 24 '18 at 18:11
  • @The Photon: You are perfectly right. I am sorry. I made a mistake. The idea is realy just to use the diode itself and therefore it also does not have 350mA but 26mA. I am very sorry! That is the right link also with a proper datasheet: www.laser66.com/html/4146-101S.pdf – Sarah Oct 24 '18 at 18:19
  • @The Photon: wow, you speak German :-D – Sarah Oct 24 '18 at 18:21

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You can analog modulate the laser diode as long as you do not saturate the MOSFET fully ON or OFF with too strong a signal, as this will disrupt the constant current loop and risk burning up the laser diode. Also the MOSFET will get hot due to being in a linear region during analog transmission.

You can use the circuit in the link but C2 should be 1uF with (+) side facing op-amp. Also change R5 to 20 ohms to get closer to your expected idle current for the laser diode. NOTE: If all resistors are 1% and capacitors are 5% quality Polypropylene or Mylar then results will be much closer to expectations. Do not increase C2 to get a deeper bass, as bass has a time constant longer than your servo loop can correct for. If it is unstable with C2 = 1uF then C2 can be no larger than 100nF.

Count on mounting the MOSFET to a medium heat-sink about 1"\$^{3}\$. If heat-sink gets too hot to touch add a small cooling fan about the same size as the heat-sink, or 2" sq at most, and make sure it can run on your local DC supply, plus or minus a couple of volts. It must blow cool air into the heat-sink.