Fiber optic sensors

TLDR: In this paper, a dielectric filter is applied directly to the sensing end of an optical fiber detector which has the characteristic that it reflects back a reference light beam of one wavelength while passing a sensing signal of a different wavelength which is modulated in some predetermined fashion beyond the filter.

Abstract: Serveral improvements in fiber optic sensing systems are disclosed. One improvement incorporates a dielectric filter applied directly to the sensing end of an optical fiber detector which has the characteristic that it reflects back a reference light beam of one wavelength while passing a sensing signal of a different wavelength which is modulated in some predetermined fashion beyond the filter. Both light signals are reflected back through the fiber and are, accordingly, attenuated in the identical or nearly identical fashion in response to various noise sources. The ratio of the intensity of the two signals is proportional to the sensed parameter and renders the system essentially self-compensating. The dielectric filter is preferably directly coated onto the exit end of the fiber through vapor deposition techniques. Time division multiplexing is used to sequentially fire plural light sources and a single photodetector is used for measuring the intensity of the reflected back signals of the various wavelengths. Another improvement encompasses the detection unit in which the input and reflected signals are separated using an off-axis parabolic mirror which provides excellent separation efficiency. According to another principal feature of this invention, a concave surface is formed at the fiber end which increases the numerical aperture of the fiber which provides greater sensitivity to the deflection of a pressure sensitive diaphragm spaced from the confronting the fiber end.