Response Characterization of a Fiber Bragg Grating Sensor to Different Sound Tones

A new type of optical fiber sound sensor constructed with two fiber Bragg gratings (FBG) is proposed in this paper. Because of the photoelasticity with respect to the refractive index and the elasticity of the fiber, the sound tones modulate the Bragg reflection wavelength and thus the intensity of the laser light transmitted through the sensor FBG fiber varies. Theoretical analysis of sound tone measurements is described. We use the concept of wavelength discrimination which relies on amplitude modulation of the power output with wavelength shifts of the sensing FBG. It would be beneficial if one can determine the response characteristics of a uniform FBG to an extensive list of frequency components of sound. For example, in a system where an FBG is used as a sensing element for heart sounds, the overall response can be predicted with given individual frequency responses. That is, the FBG output can be analyzed down to its individual frequency response components. In this work, a series of experiments were performed to investigate the possibility and potential use of this sensor system for heart sound detection.