1. Field of the Invention
The present invention relates to an optical fiber temperature distribution measurement device configured to measure a temperature distribution of an optical fiber along a longitudinal direction of the optical fiber, and a method of measuring an optical fiber temperature distribution.
Priority is claimed on Japanese Patent Application No. 2013-038696, filed Feb. 28, 2013, the content of which is incorporated herein by reference.
2. Description of Related Art
There have been widely carried out research and developments regarding distribution measurement devices configured to use optical fiber as sensors and to measure the distribution of a physical quantity of optical fiber, wherein the distribution is defined in the longitudinal direction of the optical fiber. One of the distribution measurement devices is an optical fiber temperature distribution measurement device which is configured to measure a temperature distribution along the longitudinal direction of the optical fiber by measuring Raman back scattering light (Stokes light and anti-Stokes light) generated in the optical fiber. The optical fiber temperature distribution measurement device is also referred to as R-OTDR (Raman Optical Time Domain Reflectometry).
Specifically, the optical fiber temperature distribution measurement device described above is configured to repeatedly supplying a pulse of laser via a side of the optical fiber into the optical fiber and sequentially receiving Raman back scattering lights (Stokes light and anti-Stokes light) from the side of the optical fiber, where the Raman back scattering light is generated by propagating the pulse of laser in the optical fiber. The temperature distribution along the longitudinal direction of the optical fiber is obtained by calculating the intensity ratio of the Stokes light to the anti-Stokes light at each measuring point along the longitudinal direction of the optical fiber (to be exact, by calculating the ratio of the average value of the intensity of the Stokes light to the average value of the intensity of the anti-Stokes light).
Japanese Unexamined Patent Application, First Publication No. 2012-27001 discloses an example of an optical fiber temperature distribution measurement device in the related art. Japanese Unexamined Patent Application, First Publication No. 2002-278585 and Japanese Unexamined Patent Application, First Publication No. H11-174267 disclose a technique for eliminating noises in the related art. Specifically, Japanese Unexamined Patent Application, First Publication No. 2002-278585 discloses a noise elimination device configured to eliminate small-amplitude random noises (specifically, noises included in a sound signal such as a human voice and a music). Japanese Unexamined Patent Application, First Publication No. H11-174267 discloses a non-linear digital filter for an optical pulse tester configured to reduce the amount of noises overlapped with an OTDR waveform.
Indexes indicating a performance of an optical fiber temperature distribution measurement device include a “temperature resolution” and a “spatial resolution”. The “temperature resolution” is an index indicating the minimum measurable temperature difference, while the “spatial resolution” is an index indicating the minimum fiber length needed to measure a certain temperature change. For example, SEAFOM (Subsea Fiber Optic Monitoring Group) defines the average of 2σ as the “temperature resolution”, where a is the standard deviation of measured values obtained by repeatedly measuring an optical fiber having a constant temperature twenty times (measurement results at measuring points more than 51 points), and defines the minimum fiber length needed to measure a hot spot (a spot where a temperature difference is over 20° C.) as the “spatial resolution”.
Recently, improvements of the temperature resolution and the spatial resolution have made it necessary for optical fiber temperature distribution measurement devices to measure a temperature distribution more precisely. Especially, requirements for a correct measurement of a temperature distribution at a distant position from the optical fiber temperature distribution measurement device (for example, at a position which is closer to a first end than to a second end opposite to the first end, a laser is incident into the second end) are increased. Thus, it is necessary to improve the temperature resolution at the position near one side of the fiber.
Since the temperature resolution relates to an S/N ratio which is the ratio of a signal component (a received signal of Raman back scattering light) to a noise component, it is necessary to increase the level of the signal component or to decrease the level of the noise component in order to improve the temperature resolution. Methods of increasing the level of the signal component include a method of increasing the intensity of a laser input into the optical fiber. Methods of decreasing the level of the noise component include a method of narrowing a band of an optical filter used to separate the Raman back scattering light (Stokes light and anti-Stokes light) (a first method), and a method of performing a filtering process in the related art for the received signal of the received Raman back scattering light (a second method).
In the method of increasing the level of the signal component, when the intensity of the laser input into the optical fiber is larger than a certain value, stimulated Raman scattering is generated in the optical fiber and the intensity of the Stokes light increases rapidly. Thus, the measurement error of the temperature increases. In the first method of decreasing the level of the noise component, since there are limitations to the technique of narrowing the band of the optical filter, the improvements of the temperature resolution is not expected much. In addition, the optical filter is more expensive than an electrical filter.
In the second method of decreasing the level of the noise component, the noise component can be decreased using a simple filter of the related art, but the waveform of the signal component may be degraded. Thus, it is not easy to improve the temperature resolution. The degradation of the waveform of the signal component is caused by the facts that the waveform of the signal component obtained by measuring the optical fiber varies in accordance with the measuring condition, and that a noise overlapped with the signal component is a white noise (a noise including various frequency components).
An aspect of the present invention provides an optical fiber temperature distribution measurement device which enables the temperature resolution to be improved.