An input device encodes motion using a laser self-mixing velocimeter. The laser beam is directed at a disk or other moving member. As light from that beam is reflected back into the emitting cavity of the laser, the beam power output fluctuates because of the self-mixing effect and generates a beat signal. The beat signal frequency is used to determine the speed of the member's motion. The direction of motion is also determined based on the beat signal. A difference frequency analog phase locked loop filters the beat signal. A zero-point control disables motion detection when the average beat signal amplitude falls below a threshold. A triangle modulator generates a triangle wave for the laser bias current, with the triangle wave frequency being a submultiple of the beat signal frequency.