Schemes for referencing of intensity-modulated optical sensor systems

Abstract

For over a decade, intensity modulation has remained one of the most extensively investigated forms of optical signal modulation for sensing applications [1-10]. The simple reason for the extensive and diversified usage of this modulation scheme is a multitude of potential benefits that include the inherent simplicity, reliability, flexibility and relatively low costs. Although intensity-modulated optical fiber sensors have been fabricated in many different designs and with varying degrees of complexity, the essential building blocks of a simple optical fiber sensor system are depicted in Fig. 12.1. Light from an optical source, such as a light-emitting diode (LED), is coupled into an optical fiber for transmission to the optical sensor where it can be modulated in accordance with the state of a measurand. When using reflection-mode sensing the modulated optical signal can be retroreflected into the same optical fiber for transmission to the photo-detector [9]. However, in transmission-mode sensing a second optical fiber is normally used for the transmission of the modulated signal to the photodetector.