The correct answer is (a) Plastic fibre
To explain I would say: In principle, light from a low power (5 mW) He–Ne laser is coupled into a quartz fibre and transmitted to the skin. The light is reflected from both the non-moving tissues (reference beam) and moving red blood cells (Doppler-shifted beam). The two beams are received by a plastic fibre and transmitted back to a photo-diode where optical heterodyning takes place.
