light back into the laser cavity 20 after reflection from an external surface this is detected using a photo detector 10 by analyzing the intensity or electric modulation induced by the optical feed back in the laser displacements e g vibrations of the surface or the velocity of the surface or the acceleration of the surface can be quantified this same principle can be applied to detect the surface displacements or velocities or accelerations caused due to the photo acoustic waves generated by illuminating the skin with pulsed electromagnetic radiation the problem of needing laboratory size apparatus for non invasive glucose measurements can be addressed since ldsm can be implemented with much more miniaturization the ldsm sensor has a laser diode and operates as an interferometer the light reflected from many objects such as the skin is diffusive due to a rough surface therefore only a fraction of light is available for the interference with the reference beam which is the laser diode cavity hence the intensity variation is hard to measure due to displacements which are of the order of half the wavelength of the probe beam small displacements can be measured by looking at the phase changes which are proportional to the displacement these small displacements can be determined by modulating the frequency and or intensity of the laser diode the light from the skin surface mixes with the frequency and or intensity modulated beam of the laser diode due to the displacement of the skin the.