Explain the two three-wave interactions which give rise to the stimulated Brillouin scattering phenomenon in optical fibres, including the part played by electrostriction. If the backscattered optical frequency is shifted by 11.75 G Hz from the forward, pump wave at 1550 nm what is the acoustic velocity in the fibre medium if its refractive index is 1.46 at the pump wavelength? Devise a purely electronic means for measuring the amplitude of the frequency-shifted signal, using only a PIN photo detector.