Optical Chopper

DUAL BEAM EXPERIMENTSIn this arrangement, the output from a singlesource is split in two and chopped at twodifferent frequencies by the same chopper wheel(figure 8). One of the beams passes through theexperiment, while the other beam, a referencebeam, passes through a control arm. The beamsare recombined and sent to the same detector.Two lock-in's are used to detect the two signalswhich are at different frequencies. The signal atƒ corresponds to the control arm, the signal atƒ inner is the response from the experimental arm.If the detected signal in the experimental arm isratioed to the detected signal in the control arm,then effects due to changing source intensity anddetector efficiency are removed.Also note that each beam passes through onebeam splitter, reflects off one beam splitter, andreflects off one mirror, so that effects due tothese components are cancelled in the ratiooutput.13