To modulate the laser less than 100% (on/off), you can use a transistor across a resistor in the power feed to reduce the voltage from six volts to five volts. That reduced change should not turn the laser off, but only reduce the intensity enough to be sensed by the receiver.
An interesting alternative will be to try modulating a high intensity LED, which will not have an internal driver that limits the response time. If you are using a fiber-optic connection that should not be a problem.

Thank you! That's interesting, I'll definitely try that one.
Currently, I'm using a generic red 5mW Laser Diode (I guess this has some internal circuitry). I hope to use infrared laser next time so I have less interference with ambient light. No, no fiber optic, just want to use free-space.

 

At a recent maker space meeting, a laser audio link was demonstrated. It doesn't get much simpler than this.

In a TOSLINK sender, the LED was replaced by a diode laser.

On the receiving end, a boom box, the TOSLINK receiver connector was disassembled to expose the receiving element.

The link worked well over a distance of about 15 feet, hand-holding the laser end. Alignment is critical and difficult to maintain but the resulting audio was good.

A lens to defocus the laser might be helpful, as might be a diffuser (like from a PIR detector) in front of the receiver.

Just reporting what I observed at a demonstration. It's partially relevant here.

Thank you! That's interesting. Definitely, for longer distances I'd need lenses. Oh, a diffuser, I never thought of that. In my current prototype, I printed some 3D structures to hold laser and receiver. Adding a diffuser to receiver makes it much more easier