An all-fiber Fabry-Perot interferometer using coherence demultiplexing and rugate mirror reflectors grown with electron cyclotron resonance, plasma-enhanced chemical vapor deposition (ECR–PECVD) on the fiber ends is proposed. The interferometer consists of a laser diode source, two optical fiber cavities, three rugate mirrors, an optical phase modulator, and a quadrature phase detection system. Coherence demultiplexing allowed the use of fiber cavities of several meters in length with a partially coherent illuminating light source. Truly inhomogeneous refractive index mirrors were designed, taking into account dispersion and losses of SiOxNy at the required wavelength of 1.3 µm. The reflectance/transmittance ratios during growth were adjustable from 0.05/0.95 to 0.95/0.05 and mirrors used in the experimental interferometer had nominal reflectances of 40, 68, and 40%, respectively. Design and fabrication procedures permitted implementation of window functions and matching layers for flexible control over transmittance and reflectance characteristics. The long-cavity Fabry-Perot sensor was demonstrated for use as a strain gauge. Good agreement was shown with the results obtained by a conventional strain gauge.