The development of fiber-optic chemical sensors (FOCS) has been actively pursued over the last decade, and an exhaustive summary of this work has recently appeared in a two-volume monograph by Wolfbeis. Many FOCSs have utilized fluorescence to sensitively detect analytes of interest. In some fluorescence sensor applications, chromochemical reactions yielding fluorescent products have been slaved to the sensor to circumvent the exclusion of analytes which do not fluoresce. However, there have also been several fiber-optic absorbance sensors reported. In one general type of sensor optical design, the absorbance or fluorescence measurement depends on the ability of a single multimode fiber optic to conduct light to, as well as to transport light from, the sensor tip. In this sensor configuration a coupler/ splitter is required for the separation of the entry and emergent sensor light. Hirschfeld and co-workers first described an effective geometric coupler/splitter to be used with single-fiber-optic sensors. Their coupler/splitter incorporated an angled plane mirror with a centrally bored hole for entry light passage. The Hirschfeld configuration, and variants of it, have subsequently been used extensively with laser sources which are easily coupled with the fiber optic of the sensor. However, when extended (incoherent) sources are used, it becomes difficult to efficiently couple to fiber-optic sensors with the use of the Hirschfeld configuration. In our experience, this difficulty is mainly due to the need to image an extended source near the surface of the small-diameter (typically 100-200 μm) sensor fiber optic with the use of a long-focal-length lens.
David A. Landis and Carl J. Seliskar, "Alternative Coupler/Splitter for Single-Fiber-Optic Chemical Sensors," Appl. Spectrosc. 48, 414-416 (1994)