Simultaneous mode-by-mode impulse response measurement of multi-mode optical systems based on linear optical sampling.

Linear optical sampling is presented as a useful method to characterize mode-by-mode impulse responses or the spectral transfer matrix of multimode optical systems. In a two non-degenerate linear-polarization (LP) mode transmission line, which actually supported six independent spatial modes, 6 × 6 spectral transfer functions were simultaneously measured using multiplexed mode-by-mode impulse responses in series. According to the singular value analysis of the matrix, the mode-dependent loss spectrum was analyzed over a frequency bandwidth of several hundred GHz, which was determined by the spectra of the probe and reference pulses. The technique would be useful for characterizing various multi-spatial-mode fiber systems that are intensively being developed to overcome the limit in capacity of single-mode optical fibers.

Rayleigh-based OTDR with dynamic modal crosstalk suppression.

We propose and demonstrate a novel approach for measuring the modal attenuation of the splice loss of a purely LP11 mode group by using a Rayleigh-based OTDR with a dynamic modal crosstalk (XT) suppression technique for few-mode fibers (FMFs). With the proposed approach, the Brillouin loss interaction with a Brillouin Stokes beam co-propagating with the OTDR probe removes the modal XT caused at the modal conversion point and suppresses the accumulated modal XT that is detected. A preliminary experiment is demonstrated using spliced FMFs with a core-offset. Experiments revealed that the proposed technique can accurately measure the splice loss variations of a purely LP11 mode group.

Reduction of modal evolution fluctuation in 2-LP mode optical time domain reflectometry.

This paper describes theoretical and experimental investigations of an optical time domain reflectometry (OTDR) that conducts measurements in the 2-LP mode region of the fiber under test. Rayleigh backscattering amplitude coefficients from the HE11 mode to higher-order vector modes are analyzed to clarify the evolution in the state of polarization of the backscattered vector modes with respect to that of the probe HE11 mode. Based on the analysis, we then propose a technique for reducing amplitude fluctuations in the OTDR traces attributed to intensity distribution evolution of the LP11 mode. A proof-of-concept demonstration is performed on optical fibers in the laboratory and field environments, and the measured results confirm its usefulness.

Distributed spatial mode dispersion measurement along strongly coupled multicore fibers based on the correlation analysis of Rayleigh backscattering amplitudes.

We describe a method for measuring spatial mode dispersion (SMD) distribution along a strongly coupled multicore fiber (SC-MCF). The SMD has been defined for characterizing an SC-MCF, and it changes with respect to local fiber bending and twisting. However, conventional measurement methods characterize only the overall SMD, and cannot identify fiber portions where the environmental conditions affect the SMD. This paper demonstrates distributed SMD measurement along an SC-MCF by auto-correlating Rayleigh backscattering amplitudes obtained with coherent optical reflectometry. We confirm our method experimentally, and distinguish the difference between the SMD growth along twisted and non-twisted fiber sections in concatenated SC-MCFs.

Highly sensitive detection of microbending in single-mode fibers and its applications.

This paper investigates the applicability of 1-µm-band mode-detection optical time domain reflectometry (OTDR), which detects microbending in single-mode fibers with high sensitivity by conducting measurements in the two-mode region of the fibers under test and observing the second-order mode of the backscattered light. To demonstrate its feasibility, we evaluate microbending losses as determined by the technique, and comparisons are made with conventional OTDR operating at the wavelength of 1650 nm. In addition, we discuss two potential applications of the technique. One is microbend sensing of fibers and cables for detecting microbending losses that are too small to be found with conventional OTDR. The other is health monitoring of installed fiber cables. This enables us to detect the growth in microbending losses at an earlier stage. Proof of concept is demonstrated experimentally.

Long-range measurement of Rayleigh scatter signature beyond laser coherence length based on coherent optical frequency domain reflectometry.

Long-range C-OFDR measurement of fiber Rayleigh scatter signature is described. The Rayleigh scatter signature, which is an interference pattern of backscatters from the random refractive indices in fibers, is known to be applicable to fiber identification and temperature or strain sensing by measuring its repeatability and its spectral shift. However, these applications have not been realized at ranges beyond the laser coherence length since laser phase noise degrades its repeatability. This paper proposes and demonstrates a method for analyzing the optical power spectrum of local Rayleigh backscatter to overcome the limitation imposed by laser phase noise. The measurable range and spatial performance are also investigated experimentally with respect to the remaining phase noise and noise reduction by signal averaging with the proposed method. The feasibility of Rayleigh scatter signature measurement for long-range applications is confirmed.