# 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. – The Photon 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. – Sparky256 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
• @ThePhoton. Agreed, but the response is non-linear which limits bandwidth and dynamic range. – Sparky256 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

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.