Evanescent field fiber optic probe for process analysis

Abstract

Interest in chemical sensor technology has skyrocketed in recent years due to the growing demand for continuous chemical monitoring. The ability to obtain a continuous stream of chemical information enhances feedback control of chemical processes and, if collected in-situ, circumvents labor-intensive sampling. Fiber optic-based chemical sensors have been investigated for these types of applications by many researchers. Diverse sensing environments encountered in industrial, environmental, and biomedical monitoring have resulted in a variety of probe designs. Many employ specific reagent chemistries and have been dubbed optrodes in the literature. However, in their simplest form fiber optics are used as light pipes, guiding light to and from the fiber endface, which is attached to a remote spectroscopic sampling cell. The result is a somewhat cumbersome combination of bulk and guide-wave optics in which difficulty is encountered when handling turbid or strongly absorbing samples. This requires diversion of a portion of the process stream to allow for sample conditioning. In some cases very short optical path lengths can be used; however, window fouling then becomes a problem. An alternative approach would be to use the fiber itself as an in situ absorption cell. Light can interact with an external absorber via the many reflections that take place as light propagates down the fiber. An all-fiber spectroscopic probe can be configured to widely vary its sensitivity. The optical design is simple, consisting of a single guided-wave element, which is very rugged and reliable. In addition, a polymeric coating on the fiber can be used to not only protect the fiber core from chemical attack and fouling but be selective to a specific analyte or group of analytes.