Abstract
A brief review in the cavity ring-down technique (CRD) is presented. In this review, there will only be considered the conventional fiber CRD configuration, i.e., there will only be presented researches involving cavities with two couplers with 99:1 ratios, due to the large amount of publications involving this spectroscopy method. The presented survey is divided in different topics related to the measurement of physical parameters, such as strain and temperature, curvature, pressure, refractive index, gas and biochemical sensing.

Abstract
A new microstructured optical fiber is presented as a sensor of acetone evaporation. Sensing is performed by observing the time response of the reflected signal at 1550 nm when the device is dipped in acetone or a 50% acetone-50% water mixture. The sensor consists on a caterpillar-like microstructured fiber spliced to a single-mode fiber, where the spliced end of the sensor has a transversal microfluidic channel etched using focused ion beam. Upon heating, different behaviors are visible between the dipping and the evaporation of acetone. The sensor is able to track the evaporation of acetone and to distinguish between pure acetone and a 50% acetone-50% water mixture. The sensor is also able to detect when the acetone in a mixture with water is fully evaporated. The detection of water vapor with no particular orientation of the sensor is achieved due to the presence of the microfluidic channels. (c) 2016 Wiley Periodicals, Inc.

Abstract
This work demonstrates the viability of using a cavity ring-down technique (CRD) for remote sensing. A conventional CRD configuration is used where and optical circulator is added inside the fibre loop to couple 20 km of optical fibre with a gold mirror at its end with the purpose of remote sensing. As a proof-of-concept, an intensity sensor based on an eight-figure configuration is used at the end of the 20 km of fibre for displacement sensing. In this case, a commercial OTDR is used as modulated light source to send impulses down to the fibre ring.

Abstract
Fiber probe structures composed of two physical microcavities were created using focused ion beam technology. These structures have a tip-like shape as they were milled in preciously etched tapered fiber tips. The microprobes are then characterized for temperature and refractive index sensing using a signal filtering technique to discriminate signals from distinct microcavities. Using fast Fourier transforms combined with band-pass filters, it is possible to reconstruct the spectra of each cavity independently and thus measure their individual spectral shifts.

Abstract
In this work, it is demonstrated a fiber ring resonator connected to an interrogation system usually implemented on a cavity ring-down technique. A long-period grating (LPG) was inserted in the resonant cavity, being operated as a curvature sensor. The experimental results demonstrate that using this setup, different sensitivities can be achieved in terms of analyzing decay time over the radius of the curvature and over curvature. The LPG sensor presented higher sensitivity with larger radius of curvature, namely 6.91 s/m. (c) 2016 Wiley Periodicals, Inc. Microwave Opt Technol Lett 58:267-270, 2016

Abstract
A Mach-Zehnder sensor based on a large knot fiber resonator with a diameter of a few millimeters is designed using a single long taper. The long taper of some centimeters is fabricated with a CO2 laser technique. In air, light cannot couple between adjacent sections in the knot, and no signal is observed. However, in liquid, light is less confined and there is coupling between adjacent sections of the knot, resulting in a phase difference and consequent interference. The Mach-Zehnder is formed by the two contact points in the knot. The refractive index sensing of liquid compounds is achieved by monitoring the wavelength shift of the spectra. A sensitivity of 642 +/- 29 nm/refractive index unit (RIU) is achieved for refractive index sensing in the range of 1.3735-1.428 with a resolution of 0.009 RIU. For temperature sensing, a sensitivity of -42 +/- 9 pm/degrees C is observed. A low influence of temperature in the refractive index change is observed: 6.5 x 10(-5) RIU/degrees C.