Fiber optic sensor arrangement having reflective gratings responsive to particular wavelengths

An arrangement for sensing changes in at least one monitored parameter exhibited by a body, comprising

• an optical fiber having two longitudinally spaced end portions and including
• at least one sensing fiber length disposed between said end portions, having two ends spaced by a predetermined distance from one another as considered longitudinally of said fiber, and including a sensing portion situated at a monitoring location of the body, and
• two periodic gratings of the same periodicity each situated in said fiber at one of said ends of said sensing fiber length and reflective to a predominant portion of any light that propagates in and longitudinally of said fiber and has a wavelength in a stopband range around a central wavelength corresponding to twice said periodicity while being substantially transparent to light of wavelengths outside said stopband range;
• means for launching broadband coherent light including sensing light with wavelengths within said stopband range into a first end of said optical fiber for propagation longitudinally of said fiber toward a first of said gratings for reflection of said predominant portion of said sensing light from said first grating and passage of the remainder of said sensing light into said sensing fiber length for resonant buildup therein of light at certain wavelengths that are located within said stopband range and depend on the length of said sensing portion as influenced by changes in the monitored parameter, and attendant rendering of said gratings substantially transparent to said sensing light at said plurality of wavelengths following said buildup;
• and
• means for detecting the effect of said gratings and of said sensing length on the wavelengths of the light emerging from one of said end portions of said optical fiber.