Microstructures of ffpi sensors
Free-space FP cavities have been universally employed for applications in lasers, spectroscopy, and filters, just to name a few. There are however different types of FP structures, where the stable optical resonator consisting of two concave mirrors is most often used. Both free-space mirrors and the cavity itself are bulky, centimeters in diameter and tens of centimeters in length, and it is necessary to use the stable resonator to minimize the propagation loss and to achieve a high Q factor, which contributes much to the continuous development of high-power lasers, as well as getting longer light-matter interaction length in cavity ring-down spectroscopy.
Compared with conventional free-space FP devices, the development of fiber FP interferometers (FFPIs) represents a big step, not only in terms of great miniaturization but also in terms of the enrichment of the various structures, thanks to the variety of optical fibers. Actually, it already forms one of the most important directions in the field of optical fiber sensors, which is reflected in the large number of publications and the commercial product for practical use. In this chapter, we introduce various microstructures of FFPIs and their corresponding advantages.
There are several ways to classify FFPI sensors. First, in terms of fiber types, it includes FFPIs based on singlemode and multimode, as well as microstructured fibers, tapered or microfibers, etc. Second, they can be classified into intrinsic, extrinsic, and hybrid FFPIs. Usually, the two reflective surfaces in intrinsic FFPIs are separated by a section of singlemode fiber (SMF), while it is an air gap which forms the cavity of the extrinsic FFPIs (EFFPIs). Third, FFPI sensors can be classified in terms of Qfactor or finesse.