Fiber loop ring-down spectroscopy with a long-period grating cavity

Cavity Ring-down spectroscopy is an absorption detection technique that makes the[sic]use of an optical cavity to realize a long effective optical path length through a sample and to render the measurement independent of intensity. These two features give the ring-down spectroscopy many advantages over traditional absorption techniques and allow it to measure chemicals present in trace amount. This thesis presents my study of fiber loop ring-down spectroscopy with a long-period grating (LPG) cavity as the sensing section. There are several advantages in this design: 1) It provides a relatively larger sensing area for the evanescent wave to interact with the environment when compared with a fiber taper and a D-shaped fiber; 2) The LPG cavity has a much lower insertion loss than the micro-cell; and 3) The tensile strength of the fiber is well preserved. Many issues such as the LPG mode order, LPG cavity loss, sample cell design, ring-down pulse detection and analysis have been studied. As an example, the technique was applied to measure the spectral absorption of 1-octyne in decane solution and a detection limit of 0.62% 1-octyne was determined with the exist[ing] experimental setup, which is a fact or 2 better than the result using fiber taper.