# Current modulator for laser driver and analog audio signal modulation

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.

• 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. Oct 23 '18 at 21:43
• 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.
– user105652
Oct 23 '18 at 21:44
• @Sparky256, That's nonsense. Analog modulation of a laser diode is a common technique. Oct 23 '18 at 21:46
• @ThePhoton. Agreed, but the response is non-linear which limits bandwidth and dynamic range.
– user105652
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, ...)? Oct 23 '18 at 21:51

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.