Far-end measurement instrument for baseband frequency response, based on swept frequency method, is designed and constructed. In constituting the measurement instrument, it is important to stabilize modulation frequency characteristics of laser diode which is significantly affected by externally reflected light to the laser diode. It has been found that by coupling laser diode to a fiber with obliquely polished end face, the influence of externally reflected light can be negligibly small. Optimum coupling conditions, concerning tilt angle of fiber end and distance between fiber and laser facet are clarified. Moreover, automatic power control and temperature control circuit has been applied to the above laser to fiber coupler and far-end measurement instrument of baseband frequency response has been constructed. As a result, sufficient stabilization of modulation frequency characteristics to measure baseband frequency response of graded-index optical fiber has been attained.

A measurement technique of chromatic dispersion in single-mode fiber transmission lines is proposed. By using a local signal generator, an optical line to transmit a reference signal is unnecessary in the proposed measurement technique, even if the input and output end points of the fiber are separated by a large distance.

Simple formula has been proposed to estimate baseband bandwidth of an optical transmission line made of different fibers spliced together. Using the formula, cumulative bandwidth is predicted merely from the bandwidth for each individual fiber. The usefulness of the formula has been confirmed by experiments.

An optical time domain reflectometer for single-mode fibers has been designed and constructed at a 1.3 µm wavelength. A fast digital averager enables 44 dB signal-to-noise ratio improvement within about 40 seconds. Curve fitting by a least squares method reduces the variation in fiber loss measurements. By using an acoustooptical directional coupler, Fresnel reflection is removed. In addition, backscattered signal fluctuation caused by polarization fluctuation is suppressed to a negligible extent. Current drift in receiving circuits is decreased by using a drift compensation circuit. The optical time domain reflectometer that has been built can locate breaks in single-mode fibers with attenuations as high as 17.5 dB.

A short birefringent launching fiber is proposed to reduce the fluctuation of backscattering from single-mode fibers. The fluctuation is due to polarization-sensitive measurements. It is analyzed that the polarization direction of linearly polarized inputted light is required to make an angle of 27.4 with respect to the principal axes of the birefringent launching fiber. It is also clarified that degree of coherence should vanish at the group delay of τ and 2 τ where τ is the group delay difference of the orthogonal polarization modes in the birefringent launching fiber. Experiments reveal that the birefringent launching fiber of only 3-m length makes it possible to reduce the backscattered power fluctuation to less than 0.05 dB. Required properties of laser diodes are discussed by considering the coherency of the laser diodes with multi-longitudinal modes.

A heterodyne interferometer utilizing a light source with short coherence length has led to a significant improvement in distance resolution of an optical reflectometer. The resolution less than 1 cm along a fiber is easily achieved by a Fabry-Perot type diode as a short coherence length source.

This letter proposes a novel technique for evaluating the longitudinal distribution of the Raman gain characteristics in optical fibers connected to a reference optical fiber with a known Raman gain efficiency. This technique can evaluate the Raman gain efficiency in test fibers using a simplified experimental setup. We performed experiments on various test fibers and confirmed that their Raman gain efficiency can be obtained easily and accurately by employing a reference fiber.

We report on Brillouin optical-fiber time domain reflectometry (BOTDR) for distributed temperature or strain measurement along a single-mode optical fiber. BOTDR uses Brillouin scattering in optical fibers, whose Brillouin frequency shift increases in proportion to temperature or strain induced in the fiber. This method requires access to only one end of a fiber, as with conventional optical time domain reflectometry (OTDR) which uses Rayleigh scattering in optical fibers. In BOTDR, a coherent optical detection method is used as a backscattered light detection technique. This technique can achieve both high sensitivity and high frequency resolution and easily separate a weak Brillouin line from a strong Rayleigh scattering peak and Fresnel reflected light. Experimental results show the potential for measuring temperature and strain distribution with respective accuracies of 3 or 0.006%, and a spatial resolution of 100m in an 11.57km long fiber.

This paper proposes a surveillance system concept, which includes the analysis of fiber fault factors and monitored items, the architecture for diagnosing fiber degradation and the system configuration. Fiber faults are classified into two types. One is fiber failure caused by fiber axial tensile strain and the other is fiber loss increase caused by fiber bending and the absorption of hydrogen molecules. It was found that there is an urgent need for fiber axial strain monitoring, sensitive loss monitoring operating at longer wavelengths and water sensing, in order to detect the origin and early indications of these faults before the service is affected. Moreover, an algorithm for predicting and diagnosing fiber faults based on the detected results was investigated and systematized.