Low-coherent WDM reflectometry for accurate fiber length monitoring

Abstract

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...

Frequency-Shifted Interferometry — A Versatile Fiber-Optic Sensing Technique

“…As group delay can be determined by scanning the input wavelength and recording the optical lengths, chromatic dispersion of the test fiber can also be calculated [30]. The setup of a frequency-shifted Sagnac interferometer is simpler than common fiber optical length measurement techniques such as the optical time-domain reflectometry (OTDR) [41], optical coherence-domain reflectometry (OCDR) [42,43], or optical frequency-domain reflectometry (OFDR) [44,45]. The wide measurement range demonstrated by FSI is superior to that of OCDR (∼10 cm) [42,43] and OFDR (∼km) [44,45].…”

Comparative signal-to-noise analysis of fibre-optic based optical coherence tomography systems

“…When an OCT system is required to make clinical measurements including internal investigations, control over ambient conditions is much more difficult and polarization mismatches can become more troublesome. Several approaches to Another alternative is to construct the interferometer using polarizationpreserving fibres and fibre components [18]. This is a satisfactory solution when the sample is non-birefringent.…”

High-resolution, large dynamic range fiber length measurement based on a frequency-shifted asymmetric Sagnac interferometer

“…Examples include in-service fiber line identification in a complex fiber network 1 , and fiber chromatic dispersion measurement 2 , etc. The most common optical length measurement techniques are the optical time domain reflectometer (OTDR) 3 , optical coherent domain reflectometer (OCDR) [4][5] , and the optical frequency domain reflectometer (OFDR) 1,6 . These techniques are complicated to implement, and they suffer from either a small dynamic range or a low resolution.…”