Rayleigh scattering usually occurs at sub-wavelength scaled particles in an imperfect optical waveguide. The localized and unique “finger-print” like Rayleigh scattering spectrum could be used for spatial positioning and recognition. While the Fresnel reflection arises at the interface with a refractive index gradient, carrying the optical waveguide performances within the entire fiber segment. Moreover, the signal-to-noise ratio of the Fresnel probe light is much higher than that of the Rayleigh light, as the intensity of Fresnel reflection is 3‒4 orders higher than that of the background Rayleigh scattering. Thus, the optical fiber can be measured and characterized with the Rayleigh scattering and Fresnel reflection using polarized coherent optical frequency domain reflectometry (OFDR).

OFDR, born from frequency modulated continuous wave (FMCW), was first utilized for fault positioning and diagnosis in all-fiber network. Thereafter, an auxiliary Michelson interferometer consisting of two Faraday rotation mirrors was used to calibrate scanning nonlinearity of the tunable laser source (TLS). Thus, its spatial resolution was improved to millimeter level within tens of meters. Besides, dual-polarization harvesting is also used for polarized intensity maintenance, where the spectral correlation along the fiber is free from the zero signals caused by orthogonal reference and measurement paths.

In recent years, the improved dual-polarization coherent OFDR system was applied in different scenarios including extremely high/low temperature monitoring, large range of strain measurement in structuring engineering, as well as the mode group recognition and waveguide characterization. Particularly, mode groups in a few-mode optical fiber was further discussed with their appearances one by another, in which their differential mode delay can be quantified by the frequency difference between the Fresnel reflection peaks. Moreover, birefringence was measured and calculated in a dual-air-hole microstructured optical fiber using OFDR as well, with the result close to that given by the manufacturer. Additionally, the birefringence can also be regulated and observed by OFDR where two polarized modes exchange their group velocity at the zero points in frequency domain. The OFDR promises a good prospect in high-resolution distributed optical fiber sensing and waveguide characterization.

A dual-polarization harvesting coherent OFDR was proposed and demonstrated as a sub-millimeter spatial resolution distributed optical fiber temperature and strain sensor. The 0.5 mm spatial resolution was not only calculated in theoretical analysis but also verified in experiment with deliberately introduced small scaled optical fiber segments with different axial strain distribution [Fig. 3(d)]. The high temperature monitoring was achieved from room temperature to 500 ℃ using a 23 m gold coated optical fiber [Fig. 4(a)]. The wind energy research group of the United States Department of Energy measured the strain distribution along the loaded CX-100 wind turbine blade, locating the fatigue area after repeated loading [Fig. 5(a)]. The OFDR distributed strain sensor was embedded in a reinforced concrete of a bridge over the Black River in Canada, where the loads were clearly recognized with the strain distribution [Fig. 5(b)]. Compared with S² method as well as the low coherent interferometry, OFDR stands out for its high contrast and mode separation in frequency domain. Thus, the mode excitation from LP₀₁ mode to LP₁₂ mode in turn was observed and characterized in a 6.6 m six-mode optical fiber (Fig. 7). The demonstration provides a feasible and flexible method for mode group identification and characterization of all kinds of fibers. Similarly, LP₀₁ mode to LP₁₁ mode were observed in a dual-mode fluorine-trench optical fiber, and their differential mode delay can be realized by the beat frequency difference between two Fresnel peaks (Fig. 8). A special hybrid mode was found just between the two Fresnel peaks, which may be regarded as the mode convention at the optical fiber end. The birefringence in a dual-hole microstructured optical fiber was numerically calculated and characterized with an OFDR method (Fig. 10). Due to the asymmetric dual air-holes in the cross section, the polarized LP01X and LP01Y modes propagate with different group velocities and time delays. The group birefringence of -9.68×10^-4 was obtained with a beat frequency difference of 50.03 Hz, which was in good agreement with that given by numerical analysis. Moreover, the birefringence could be further regulated with selective infiltration using a functional material with a high thermal-optic coefficient. Due to the co-effects of the filled and unfilled fiber segments, the twin critical zeros of the overall group birefringence were observed at 16.5 and 43.0 ℃, respectively (Fig. 11). The unique position exchange of LP01X and LP01Y presents at the twin critical zeros, which has been investigated via the OFDR technique as well.

The OFDR technique is briefly reviewed in this paper with improved processes using hardware and software methods. This technique with excellent performances is applied not only in high-resolution sensing, but in waveguide measurement and characterization as well. Rayleigh scattering and Fresnel reflection in optical fibers carry abundant information, which are capable of characterizing mode components, purity, group velocity, birefringence in time/spatial/frequency domain. With an induced Fresnel reflection surface at a hollow-core fiber end, the OFDR method is also suitable for hollow-core microstructured optical fibers. The OFDR method promises possibility for group velocity measurement in hollow-core fiber close to that in vacuum as a compact high-resolution optical instrument.