Abstract-A fiber-optic low-coherent reflectometer was developed to accurately monitor fiber length variation. A large lengthcoverage range was obtained by using a fiber Bragg grating array in a wavelength-division-multiplexing configuration. The polarization mismatch-induced signal fading was eliminated by applying a polarization spreading technique at the optical receiver and, therefore, no active polarization adjustment was necessary for long-term measurement.Index Terms-Optical delay lines, optical distance measurement, optical fiber measurement, optical interferometry, optical polarization. PRECISE MEASUREMENT of the length of an optical fiber is useful in optic communications as well as in fiber-optic sensors. A popular way to measure fiber length is to use a time-domain reflectometer (OTDR). By evaluating the propagation delay of optical pulses, an OTDR can be used to measure fibers as long as several hundred kilometers. However, the length resolution of a typical OTDR is on the order of meters due to pulsewidth limitations. On the other hand, for high precision measurements, optical low-coherence reflectometers (OLCRs) have been used extensively to measure the distribution of reflections in optical components [1], [2]. The OLCR concept is illustrated in Fig. 1, where a wide-band low-coherent light source is used. Coherent interference happens only when the length difference between the two interferometer arms is shorter than the coherence length of the light source. The length resolution of an OLCR is related to the bandwidth of the light source by [1] ( 1) where is the center wavelength and is the refractive index. A resolution as fine as 10 m can be achieved [2]. The maximum measurement range of an OLCR is usually limited to a few centimeters, which is mainly determined by the length coverage of the scanning optical delay line. Range extension has been proposed in [3] using a pair of retroreflectors in the optical delay line. By letting the light bounce back and forth between the two retroreflectors for times, the length coverage range of the delay line is increased by times. In addition to extra insertion loss in the delay line, another disadvantage of this method is that both motor step size and mechanical errors in the translation stage (which controls the position of the retroreflectors in the scanning delay line) will be amplified by times, degrading the length resolution of the measurement. Furthermore, since an OLCR is based on coherent interference, it obviously requires the matching of polarization states of the signals reflected from both interferometer arms. Due to the random nature of polarization-mode coupling in an optical fiber, polarization-state mismatch may occur, which causes temporary fading of the coherent beating signal at optical receiver.A polarization-independent OLCR has been demonstrated [4], where the low coherent light source was first polarized and then launched into an interferometer composed of polarization maintaining (PM) fibers, thus to eliminate the polarization sensi...

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