Quality modulation parameter for a quasi-static optical modulator based on Frustrated Total Internal Reflection applied to infrared spectroscopy

An optical modulator based on the Frustrated Total Internal Reflection represents an alternative to the optical modulation based on the classic chopping wheel. This alternative permits to eliminate the presence of moving parts within infrared spectroscopy equipment. The proposed design of the modulator has two elements of dielectric material. These elements are separated by an air gap. The radiation passes through the air gap from the first element to the second one. The transmission coefficient presents a non-linear dependence on the separation distance. Furthermore, the micro-movement has a non-sinusoidal waveform. Both effects result on a non-sinusoidal waveform of the intensity modulation and make the energy of the modulated radiation to be distributed among several harmonics. The quality of the modulation has been studied by means of the optical efficiency and the total harmonic distortion index. The modulator based on the Frustrated Total Internal Reflection, FTIR, and an ideal square chopper can be compared using the optical efficiency coefficient. The total harmonic distortion index shows the ratio of chopper energy that is not hold by the main harmonic does.