Observations concerning the reply to our comments on 'Fiber optic ultrasonic sensor using Raman-Nath light Diffraction.'(response to Y.Wu, P.M. Lewin, and D.P. Koller reply, IEEE Transactions, vol. 42, pp. 956-957, October 1995.)

The authors Wu et al. failed to resolve the problems associated with the position-dependent changes in sensor sensitivity for fiber optic ultrasonic sensors. According to them, the effects of the ultrasound on the light beam ranges from light deflection, through Raman-Nath regime, to the transition regime approaching Bragg diffraction for ranges of 3-15 MHz. The reason of its fallout is discussed.