An “integrated” Fabry-Perot interferometer, such as for use in a spectrophotometer, is fabricated by attaching two micro-machined semiconductor-on-insulator wafers to one another. One mirror is formed on each micro-machined wafer. One mirror is supported by a thermally insulated, suspended micro-platform. In some embodiments, interferometer cavity length is adjustable. Detectors are disposed at least partially within the micro-platform. In some embodiments, the interferometer, a light source, and other circuitry and components, such as wireless communications components, are contained in a sealed package that includes a sampling region, thereby providing an integrated spectrophotometer. The integrated spectrophotometer can be implanted, for example, in animal tissue environments, such as for analyzing various compounds in the blood.