Optical systems for sensing chemicals. An example system includes a light
source, a light sensor, a processing device in signal communication with
the light source and the light sensor, and a fiber optic cable that
receives light from the light source and delivers light to the light
sensor. The fiber optic cable includes a cladding material that is
permeable to a predefined substance and an optical fiber core surrounded
by the cladding material. The optical fiber core is a single mode optical
fiber having a diameter greater than 30 .mu.m. The optical fiber core
includes a hollow center having a diameter between 1-50 .mu.m. The
optical fiber core includes a plurality of lengthwise holes positioned to
provide single mode light propagation properties. The plurality of
lengthwise holes have a diameter between 0.2-4 .mu.m.