This paper investigates the application of single, low-moded, fiber optic sensors to the detection of structural vibration. Several laboratory vibration systems which demonstrate a range of component frequencies and dynamic range are analyzed in an effort to characterize the sensor's behavior. We compare frequency spectra of the sensor output with calculated frequency components of the structures to determine if the sensor is responding to the various structural harmonics. Specifically, we wish to demonstrate the dynamic range and frequency response of the sensing technique. Experimental results and observations are preceded by a brief review of sensor mechanisms in optical fibers and presentation of a theory describing the operation of the modal domain sensor.