Question

What functions as a square law device and gives out current?

(a) Plastic fibre

(b) Photodetector

(c) He-Ne Laser

(d) CO2 Laser

The question was asked in final exam.

Enquiry is from PACS (Picture Archiving and Communication System) in section Telemedicine Technology of Biomedical Instrumentation

Answer 1

Correct answer is (b) Photodetector

The best I can explain: The photodetector functions as a square law device and gives out current, which is proportional to the intensity of the incident light and, therefore, to the frequency of beating of the shifted and unshifted signals. The light falling on the photodetector is an optically mixed signal involving a Doppler-shifted signal back scattered from the moving red blood cells with the ‘reference’ signal reflected from the non-moving skin surface.