Laser interferometry is a well-developed and useful technique having advantages of high accuracy and non-contact measuring method. Many industrial instruments require small sensors having high accuracy and high-speed response. If the laser interferometer can be integrated in a small package meeting the requirements, there are large application areas. The interference signal from the new PD is rather generated by a convolution of two different resonant fields, not by the simple standing wave detection. The incoming light will be in resonance with Fabry-Perot cavity and large optical intensity is stored inside the Si sensing layer. The transmitted light is again back reflected from the reflection mirror, and also generates the resonance field in the Si layer. These two resonance fields interfere with each other resulting in the intensity modulation depending on the outer reflection mirror position.