Hiroshige Ohno

Dependence of the Brillouin gain spectrum on linear strain distribution for optical time-domain reflectometer-type strain sensors

We theoretically derive the shape of the Brillouin gain spectrum, that is, the Briilouin backscattered-light power spectrum produced in an optical fiber under conditions of a strain distribution that changes linearly with a constant slope. The modeled measurement system is an optical time-domain reflectometer-type strain sensor system. The linear strain distribution is one of the fundamental distributions and is produced in, for example, a beam to which a concentrated load is applied. By analyzing a function that expresses the shape of the derived Brillouin gain spectrum, we show that the strain calculated from the frequency at which the spectrum has a peek value coincide. with that at the center of the effective pulsed light. In addition, the peak value and the full width at half maximum of the Brillouin gain spectrum are both influenced by the strain difference between the two ends of the effective pulse. We investigate this influence in detail and obtain the relationship between strain difference and strain measurement error.

Development of highly stable BOTDR strain sensor employing microwave heterodyne detection and tunable electric oscillator

We successfully developed a new Brillouin-OTDR (BOTDR) that is highly stable in the face of surrounding temperature fluctuations it by employing a microwave heterodyne receiver and a tunable electric oscillator. We were able to realize good frequency and power stability because the BOTDR only pulse-modulates the probe light launched into the sensing fiber. The design is optimized to detect cracks that occur in concrete structures by measuring the strain in a sensing fiber fixed to the concrete structure at a certain level. We demonstrated that the BOTDR could measure the strain in a 10 km long sensing fiber to an accuracy of 10 (mu) (epsilon) (corresponding to 0.5 K in temperature) with an optimum spatial resolution of 20 ns. This accuracy is sufficient given the required strain of 50 (mu) (epsilon) for crack detection. In addition, the BOTDR strain deviation caused by temperature fluctuation was less than several (mu) (epsilon) . The simple design of the BOTDR meant that the measurement time could be reduced to 170 sec.

Strain and temperature characteristics of Brillouin spectra in optical fibers for distributed sensing techniques

Brillouin spectra along optical fibers were measured as a function of strain and temperature using an optical coherent detection method. The characteristics are promising as regards improving distributed strain and temperature measurement using Brillouin spectroscopy. We have presented the possibility of simultaneous distributed strain and temperature measurement along optical fibers, by evaluating the dependence of the spontaneous Brillouin frequency shift and Brillouin power on strain and temperature, which we measured using the optical coherent detection method. This means we will be able to develop a high performance OTDR for measuring strain, temperature and optical loss distribution along optical fibers.

Reduction of the effect of temperature in a fiber optic distributed sensor used for strain measurements in civil structures

We report on an approach for reducing the effects of temperature in a fiber optic distributed sensor. This technique employs a sensing fiber and a Brillouin optical time domain reflectometer (BOTDR). The BOTDR has been proposed for measuring both strain and optical loss distribution along optical fibers by accessing only one end of the fiber. The BOTDR analyzes changes in the Brillouin frequency shift caused by strain. This device can measure distributed strain with an accuracy of better than plus or minus 60 X 10-6 and a high spatial resolution of up to 1 m over a 10 km long fiber. However, temperature fluctuations have an adverse effect on the accuracy with which the Brillouin frequency shift can be measured because the shift changes with temperature as well as with strain. This has meant that both spatial and temporal fluctuations in temperature must be compensated for when a fiber optic distributed sensor is used for continuous strain measurements in massive civil structures. We describe a method for the simultaneous determination of distributed strain and temperature which separates strain and temperature in a fiber optic sensor.

Linear strain distribution dependence of the Brillouin gain spectrum

We undertook a theoretical study of the Brillouin gain spectrum dependence on the strain distribution in an optical fiber, which changes linearly with a constant slope. We also investigated the strain measurement error in BOTDR induced by the linear strain distribution.