Optical Chopper

SINGLE BEAM EXPERIMENT WITHEFFECTIVE CHOP FREQUENCIES TO 20KHZIn certain situations very high chop frequenciesare desirable. For example, you can infer thelifetime of a fluorescent decay by measuring thephase shift between the light which excites thematerial and the fluorescent decay output.τ = tanø / 2πƒwhere ø is the observed phase shift, ƒ is thechop frequency, and t is the lifetime. Accuracyis improved by using a chop frequency,ƒ ~ 1 / 2πτso that small errors in the phase measurementwill not drastically affect the lifetime.When a small diameter beam is chopped by ablade, the light intensity vs. time is a squarewave,I(t) = Io (sin wt + 1/3 sin 3wt + 1/5 sin 5wt +…).Providing that the optical media is not saturatedby the light beam, the media will respondlinearly to each Fourier component. In this case,we are interested in the response at the 5thharmonic, for which the chopper controller has areference signal output at 5ƒ.To use this feature, set the REFERENCEMODE switch to the center position and use theleft BNC reference output as the reference inputto the lock-in amplifier (figure 6). The lock-inwill now detect the response from theexperiment at the 5th harmonic of the chopfrequency (up to 20kHz). Note that the11