Fiber sensing system based on a bragg grating and optical time domain reflectometry

Abstract

Optic fiber sensor characterized in that the sensing fiber is provided with a continuous Bragg grating covering the entire fiber length which is dedicated to sensing and along which spatially resolved measurements are performed.

Claims

1. An optical fiber sensor comprising a sensing fiber provided with a single and continuous Bragg grating covering an entire length of the sensing fiber; a tunable laser source; an optical pulse shaper receiving a signal from the tunable laser source and launching a shaped pulse signal into one end of the sensing fiber; and reflection measuring means in connection with the one end of the sensing fiber, wherein the reflection measuring means is configured to spatially resolve measurements of a parameter along the entire length of the sensing fiber by Optical Time Domain Reflectometry (OTDR).

2. The optical fiber sensor according to claim 1 wherein said grating is uniform along said fiber length.

3. The optical fiber sensor according to claim 1 wherein said grating is chirped or step-wised along said fiber length.

4. The optical fiber sensor according to claim 1 wherein said reflection measuring means is Optical Time Domain Reflectometry.

5. A method of sensing using the optical fiber sensor of claim 1 and based on OTDR.

6. Use of an optical fiber sensor according to claim 1 for sensing temperature and/or strain along the fiber.

7. The optical fiber sensor according to claim 1 wherein the Bragg grating has less than 20% overall reflectivity.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1: Schematic diagram representing a preferred embodiment of the invention

(2) FIG. 2A: Photo of the optic fiber sensor

(3) FIG. 2B: Photo of the measured reflection in absence of an external hot spot

(4) FIG. 2C Photo of the measured reflection in presence of an external hot spot

DETAILED DESCRIPTION

(5) Referring now to the drawings, FIG. 1 depicts a preferred schematic diagram of the proposed distributed sensing system to interrogate distributed temperature and/or strain along the fiber using a long and weak fiber Bragg grating.

(6) A weak and long fiber Bragg grating is used as a sensing fiber and the temperature dependence of the peak frequency of the FBG is measured to be 1.23 GHz/K. A FWHM 50 ps signal pulse was generated after passing through a pulse shaping device that can be an electro-optic modulator (EOM) and then launched into the FBG. In turn, the central frequency of the pulse was incremented by steps of several MHz in the vicinity of the FBG reflection spectral window, simply by changing the frequency of the tunable laser source (TLS), and detected by the reflection measuring means, for example the Optical Time Domain Reflectometer (OTDR).

(7) In FIG. 2(a) the FBG being used as a distributed sensor is shown. A hot spot is generated and can be precisely located using a distributed FBG according to the invention.

(8) FIG. 2(b) shows a measured distributed reflection spectrum of a FBG sensor according to the invention as a function of the fiber distance, in absence of any external hot spot and strain applied to the FBG sensor. The information delivered to the Optical Time-Domain Reflectometer (OTDR) corresponds to the return signal based on the original Bragg frequency shift. So, the measured distribution of FBG reflection spectrum is used as reference, so that any change in local Bragg frequency in consequent measurements can indicate qualitatively a change in temperature and strain at that position. However, it must be pointed out that FIG. 2(b) provides another important information about the uniformity of fabricated long FBG and frequency chirp information along the FBG based on the distribution of Bragg frequency in distance.

(9) FIG. 2(c) shows a measured distributed reflection spectrum of a FBG sensor according to the invention as a function of the fiber distance in presence of an external hot spot (FIG. 2 c) along the long FBG. It is clearly observed that the presence of an hot spot leads to a measurable shift in the Bragg frequency. The amount of temperature change can be simply estimated as a result of the linear relationship of 1.23 GHz/K.

(10) In this embodiment, the FBG was uniform, which means that the Bragg frequency over the whole length of the grating is nearly constant. However, it is not a necessary condition for this invention. For instance, the distribution of Bragg frequency along the grating can be linearly varied with respect to the distance or step-wised over the distance.

(11) It must be also specified that an absolute continuity of the FBG along the covered sensing range is not strictly required and short segments of fiber without imprinted FBG may be present, since in most fabrication processes FBGs can be imprinted only along a finite length. It may therefore be required to append many gratings to extend the sensing length and a gap between gratings can be intentionally or accidentally be present. It is sufficient to require that this distance gap is smaller than the spatial resolution of the interrogating system to implement the invention. This case will be indistinctively identified as a continuous FBG in the description of this invention.

(12) It is also not strictly required to shape the interrogating light signal as a single pulse, but other coding techniques can be implemented, such as multiple pulse coding or radio-frequency modulation scanning of the input light signal to retrieve the time-domain information via a Fourier transform.

(13) Various modifications and variations to the described embodiments of the invention will be apparent to those skilled in the art without departing from the scope of the invention as defined in the appended claims. Although the invention has been described in connection with specific preferred embodiments, it should be understood that the invention as claimed should not be unduly limited to such specific embodiment.

REFERENCES

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