A chopper was constructed to modulate the output of an infrared Helium-Neon laser at 3.39 microns. The chopper was constructed to fit onto a previously constructed optical bench which houses the laser. The chopper also encompasses a number of inexpensive electrical components including an LED (the source) and a phototransistor (the detector) and the accompanying electronics to power the components and provides a reference wave of the characteristics required for the reference input of the PAR model 5101 Lock-In Amplifier.