A compact fiber optic sensor is described using Incoherent Broad-Band Cavity Enhanced Absorption Spectroscopy for
sensitive detection of nanoliter samples of aqueous chemicals and microorganisms in capillaries. Absorption was
measured in a 70 μm gap, comparable to the inside diameter of a capillary used for electrophoresis, between the ends of
two short segments of multimode fiber. The other ends of the fibers were optically contacted to dielectric mirrors to form
an 11-cm cavity resonator. Light from a superluminescent diode (λ=1054 nm, BW=35 nm FWHM) was coupled into one
end of the cavity, and transmission through the cavity was measured using a silicon photodiode. Dilute aqueous solutions
of near infrared dye were used to determine the minimum detectable absorption change of 4x10-6 for 10 second
integration and unity signal-to-noise ratio, which is approximately two orders of magnitude more sensitive than
previously published results for systems with comparable sample path lengths.
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