Spectroscopic Sensing Via Dual-CladOptical Fiber

Many optical sensors can be simplified and improved through the use of dual-clad fiber (DCF). Here, the authors highlight two applications related to combustion: H2O vapor temperature measurements in a gas-turbine combustor (by direct absorption spectroscopy) and piston temperature measurements in a reciprocating engine (by fiber-Bragg-grating thermometry). For both applications, the DCF conveniently provides a single-mode light “pitch” and a multimode light “catch” coaxially and within a single fiber. Both applications benefit from the ease of alignment and the access to “tight spaces,” which comes from sensing through a single fiber port. In addition, both applications are improved in “bi-directional” mode: the former with an increased absorption in a double pass through the engine, and the latter by monitoring a reflection feature instead of a relatively weak-transmission feature. The authors discuss potential designs for DCF-based sensors in detail, including options for connecting both a light source and detector to one end of the DCF.