CN101738216A - Single fiber multiplexing method for all-same low-reflectivity fiber bragg grating - Google Patents

Abstract

A fiber grating utilization method on a single optical fiber, in which the fiber grating is used for series connection and multiplexing, thereby forming a large single-fiber quasi-distribution system. A multiplexing method based on OTDR technology and using identical low reflectivity fiber grating is analyzed and studied theoretically. Analysis shows that multiple reflections between gratings are the main reason for limiting the number of multiplexing. Using this multiplexing method, the single fiber grating detection system can be used for the detection of a large distribution system consisting of hundreds of fiber gratings on the same optical fiber without adding too many complicated programs.

Description

Single Fiber Multiplexing Method of Identical Low Reflectivity Fiber Bragg Grating

technical field

The invention relates to the field of optical fiber distributed sensing, in particular to a multiplexing method of an identical low reflectivity fiber grating on a single optical fiber based on OTDR (Optical Time Domain Reflectometer) technology, which is useful for designing cost-effective distributed grating sensing Networks are instructive.

Background technique

Since KOHill ^[1] reported the world's first fiber grating in 1978, it has been widely used in the rapidly developing field of optical fiber communication due to its good reliability, strong anti-interference ability, and convenient use. develop rapidly. However, the central wavelength of the fiber grating is easily affected by the ambient temperature and strain. The shortcoming of the fiber grating, which needs to be overcome in the application of optical fiber communication, has quickly become a widely recognized and has great potential in the field of sensing and measurement. Sensing Technology. After more than 20 years of research and development, fiber grating sensors for various parameter measurements have emerged continuously. However, so far, this sensing technology has not been widely used in practical engineering. The reason is that the expensive wavelength demodulation part is the main limiting factor ^[2] . On the other hand, in many applications, such as smart structures, large-scale distributed measurement is required, so the effective multiplexing technology of fiber gratings has become an important direction of fiber grating sensing technology research. At the same time, it is also hoped that multiplexing technology will be used to reduce the cost of a single measurement point, so that the fiber grating sensing technology can be competitive.

At present, there have been reports of various multiplexing methods, such as: SDM (Space Division Multiplexing), TDM (Time Division Multiplexing), WDM (Wavelength Division Multiplexing), SCM (Subcarrier Multiplexing) and FMCW (Frequency Modulation Continuous wave multiplexing) and various combinations of these multiplexing methods ^[3-8] . Although these multiplexing methods have their own characteristics, none of them can well solve the relationship between factors such as multiplexing degree, cost, and connection difficulty. Multiple fiber gratings with the same characteristic wavelength are multiplexed. If this problem can be solved and combined with any of the above methods, a system with greater reusability can be obtained.

The present invention proposes a method based on OTDR technology and an identical low-reflection fiber grating to realize multiplexing of a large number of gratings on the same optical fiber.

[1] Hill K O, Fujii Y, Johnson D C, and Kawasaki B S, Photosensitivity in optical fiber waveguides: Application to reflection filter fabrication. Appl. Phys. Lett., 1978, 32: 647-649

[2] Jiang De-sheng, He Wei, Review of Applications for Fiber Bragg Grating Sensors, Journal of Optoelectronics Laser, 2002, 13(4): 420-430

[3] Kersey A D, Interrogation and multiplexing techniques for fiber grating strain sensors. SPIE 1993, 2071: 30-48

[4] Rao Y J, In-fiber Bragg grating sensors. Meas. Sci. Technol., 1997, 8: 355-375

[5] Idriss RL, Kodindouma M B, Kersey A D et al.. Multiplexing Bragg grating optical fiber sensors for damage evaluation in highway bridges, Smart Material Structure, 1998, 7: 209-216

[6] Davis MA, Bellemore D G, Putnam M A et al..A 60 element fiber Bragg grating sensor system, Proc.OFS12th, 1997: 100-103

[7] Weis R S, Kersey A D, Berkoff T A.A four-element fiber grating sensor array with phase-sensitive detection, Photon.Technol.Lett., 1994, 6:1469-1472

[8]Chan P K C, Jin W, Demokan M S.FMCW multiplexing of fiber Bragg grating sensors, J.LightwaveTechnol., 2000, 6(5): 756-763

[9] Liu Jiansheng, Application No.69902011, National Natural Science Foundation of China, 1999.

[10] Choi Han-Sun, Siems L.Characterization of FBG reflector arrays by wavelength tuning of a pulsed DFBlaser, SPIE 2000, 4185: 166-169

[11] Valente L C G, Braga A M B, Ribeiro A S. et al.. Time and wavelength multiplexing of fiber Bragg grating sensors using a commercial OTDR, Optical Fiber Sensors Conference Technical Digest, OFS 2002, 11: 151-154

[12] Derickson D. Fiber Optic Test and Measurement, 1998, ISBN 0-13-534330-5, Prentice-Hall Inc.

Contents of the invention

The invention proposes a multiplexing method for multiplexing a large number of identical fiber gratings on the same optical fiber based on OTDR technology and low reflectivity fiber gratings. Through the use of identical fiber gratings, the online manufacturing technology using a single template can be realized, making the manufacturing cost of fiber gratings lower and the reliability better; the use of low reflectivity fiber gratings can greatly improve the reusability of the grating system, It is beneficial to the application of fiber grating in the multiplexing system; the use of OTDR technology can realize the positioning detection of the fiber grating sensing system.

The invention adopts single pulse with adjustable wavelength to inject into the distributed multi-point system formed by the series connection of the same low-reflection fiber grating, and measures the reflected time-varying signal (OTDR), so as to demodulate the signal and wavelength. As shown in Figure 1, the single pulse with adjustable wavelength enters the single-fiber system composed of gratings connected in series through the coupler L, a part of the signal will be reflected at the grating _G1 , and the rest will be partially transmitted and reach the grating _G2 , where it will be reflected again and transmission, the rest of the transmitted light continues to reach the gratings G ₃ , G ₄ ...... Finally, the pulse trains reflected back by the gratings at all levels will enter the filter and detection unit through the coupler L. By detecting the intensity of the reflected signal, the position of the center wavelength of the grating is determined; by detecting the return time of the pulse, it is determined which fiber grating the pulse comes from.

Reducing the reflectivity of the identical grating can greatly increase the multiplexing number of the system, as shown in Figure 2. Among them, D is the dynamic range of the system receiver.

The reflectivity of the gratings used in the system involved in the present invention is less than 5%, and the lower the reflectivity, the higher the multiplexing degree of the system. But in practice, the reduction of reflectivity cannot be arbitrary, and it must be affected by factors such as Rayleigh scattering, multiple reflections between gratings, and fiber loss.

First consider the effect of Rayleigh scattering. Since the characteristic reflection spectrum of the fiber grating overlaps with the Rayleigh scattering spectrum in the optical fiber, in order to distinguish the reflected signal light from the Rayleigh scattering light, the reflectivity should be selected so that the optical power reflected by the grating is greater than that generated by the corresponding excitation light pulse. The power to scatter light towards Rayleigh. Considering the last raster (worst case), one should make

P _FBG (K) > P _Rayleigh

In fact, in order to ensure the effective extraction of signal light information, the

P _FBG (K)≥10P _Rayleigh

From the above, the number of grating multiplexing limited by Rayleigh scattered light is:

$\mathrm{<span\; class="notranslate">\; K</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&rho;</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; \&le;</span>\frac{\mathrm{<span\; class="notranslate">\; 10</span>}\mathrm{<span\; class="notranslate">\; lg</span>}\frac{\mathrm{<span\; class="notranslate">\; \&rho;</span>}}{\mathrm{<span\; class="notranslate">\; 10</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; S</span>}<span\; class="notranslate">\; \&CenterDot;</span>{\mathrm{<span\; class="notranslate">\; \&alpha;</span>}}_{\mathrm{<span\; class="notranslate">\; S</span>}}<span\; class="notranslate">\; \&CenterDot;</span>\mathrm{<span\; class="notranslate">\; \&Delta;l</span>}<span\; class="notranslate">\; )</span>}}{<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 20</span>}\mathrm{<span\; class="notranslate">\; lg</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; \&rho;</span>}<span\; class="notranslate">\; )</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span>$

Among them, ρ is the reflectivity of the grating, S is the collection coefficient of the backscattered light of the fiber, α _S is the Rayleigh scattering coefficient of the fiber, and Δl is the length of the excitation light pulse in the fiber.

Second, the effect of multiple reflections between gratings should be considered. Due to the use of identical gratings, when the temperature or stress at all gratings is the same at a certain moment, the reflection spectra of all gratings will completely overlap, resulting in these gratings from different positions that have the same spectral lines as the grating at the measured point The optical signal of the measured point reaches the receiver at the same time after multiple reflections and the optical signal reflected by the grating, which causes measurement errors, as shown in Figure 4.

In order to evaluate the impact of this multiple reflection on the grating multiplexing degree, the extreme case is considered here—that is, the center wavelength of all the gratings in the system happens to be the same, and the last grating is to be measured. Since the reflectivity of the grating used in this type of system is less than 5%, the intensity of multi-reflection light at each level differs by an order of 10 ⁴ , so here only the influence of the multi-reflection phenomenon of the first order is considered.

When the gratings are uniformly distributed at equal intervals, through arrangement and combination, for a system connected in series with K identical gratings, the number N of first-level multi-reflection paths that reach the receiver at the same time as the reflected light of the last grating is:

$\mathrm{<span\; class="notranslate">\; N</span>}<span\; class="notranslate">\; =</span>\frac{<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; K</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; K</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span>}{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; )</span>$

Regardless of whether a swept-frequency light source or a wide-spectrum source plus a filter is used for detection, the spectrum of the light that finally reaches the detector is very narrow, and interference effects need to be considered; and because it arrives at the same time, it is additive interference. At the same time, in order to ensure effective identification, the signal light intensity is set to be 10 times the light intensity of multiple reflections at the first level. The above derivation shows that when the gratings are uniformly distributed at equal intervals, the maximum multiplexing number Kmax that can be obtained is:

${\mathrm{<span\; class="notranslate">\; K</span>}}_{\mathrm{<span\; class="notranslate">\; max</span>}}<span\; class="notranslate">\; \&ap;</span>\frac{\mathrm{<span\; class="notranslate">\; 2</span>}}{\sqrt[\mathrm{<span\; class="notranslate">\; 4</span>}]{\mathrm{<span\; class="notranslate">\; 10</span>}{\mathrm{<span\; class="notranslate">\; \&rho;</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 3</span>}<span\; class="notranslate">\; )</span>$

It can be seen that the number of grating multiplexes limited by multiple reflections between gratings is inversely proportional to the square root of the reflectivity of the identical grating, as shown in Figure 5.

Generally, since the loss of the optical fiber is very small, the error caused is very small, and the influence from the loss of the optical fiber can be ignored.

The present invention has the following advantages:

(1) The single-fiber multiplexing method of the identical low-reflection fiber Bragg grating involved in the present invention greatly improves the multiplexing degree of the system, and is beneficial to the application of the fiber Bragg grating in the multiplexing system.

(2) The single-fiber multiplexing method of an identical low-reflectivity fiber grating involved in the present invention can use a single-template manufacturing technology, so that the manufacturing cost of the fiber grating is lower and the reliability is better.

(3) The single-fiber multiplexing method of the identical low-reflection fiber Bragg grating involved in the present invention can make the fiber Bragg grating cheaper and more convenient to use and maintain.

(4) The multiplexing system in the present invention only needs to use a single set of light source and demodulator, which improves the cost performance of the system.

Description of drawings

Figure 1: Schematic diagram of the multiplexing method of identical low-reflectivity fiber gratings; where, L is a coupler; G ₁ , G ₂ , G ₃ , and G ₄ are fiber Bourget gratings.

Figure 2: Analysis results of the system multiplexing number and the reflectivity of the identical grating; where the horizontal axis is the reflectivity of the identical optical fiber (Reflectivity), and the vertical axis is the system multiplexing (Numbers of Multiplexing).

Figure 3: Schematic diagram of the composition of a single-fiber time-division multiplexing system with low-reflectivity fiber gratings, where: A is direct modulation; B ₁ , B ₂ , and B ₃ are detectors; C ₁ , C ₂ are superradiative devices; D ₁ and D ₂ are filters; E ₁ and E ₂ are external modulators; F is wavelength tunable lasers; G ₁ , G ₂ and G ₃ are fiber Bragg gratings; L ₁ , L ₂ and L ₃ are couplers.

Figure 4: Schematic diagram of multiple reflections between gratings; among them, light 3 undergoes two-stage multiple reflection, light 2 undergoes one-stage multiple reflection, and light 1 and light 2 return to the receiving end at the same time. For signal light 1, light 1 for interference.

Figure 5: Analysis results of reflectivity and multiplex of grating equidistant series connection system; where the horizontal axis is reflectivity of grating equidistant series connection system (Reflectivity), and the vertical axis is system multiplexability.

Detailed ways

Fig. 3 is a diagram showing an example of the composition of a single-fiber time-division multiplexing system of a low-reflectivity fiber grating according to the present invention. Taking Figure (c) as an example, a wavelength-tunable single pulse is generated by a wavelength-tunable laser source. The modulated signal obtained by the pulse through an external modulator will be injected into a distributed multi-point system composed of fiber Bragg gratings connected in series, and then the obtained The reflected signal is connected to the detector, thereby forming a complete single-fiber time-division utilization system of low-reflectivity fiber grating.

Assume that the system adopts a grating with reflectivity ρ=0.01%, the distance between the gratings is 1m, and the excitation light pulse width W=1m. The influence of factors such as Rayleigh scattering and multiple reflections between gratings on the system multiplexing degree should be considered comprehensively.

It can be seen from formula (1) that the multiplexing number of gratings limited by Rayleigh scattering can be up to thousands, therefore, Rayleigh scattering has little influence on the multiplexing degree.

When the reflectivity is 0.01% (Rayleigh scattering coefficient is 10 ^-7 /m ^[12] ), it can be obtained from the formula (3), the number of multiplexing of the multiple reflection limited grating is about 120.

Therefore, in practical applications, the influence of factors such as Rayleigh scattering, multiple reflections between gratings, and fiber loss should be considered comprehensively, and the advantages of the single-fiber multiplexing method of identical low-reflectivity FBGs should be maximized.

Claims (6)

1. optical fiber optical grating multiplexing method of carrying out big quantity on simple optical fiber.

2. in the multiplexing method described in the claim 1, the fiber grating that uses as parameters such as centre wavelength, reflectivity and reflectance spectrum all identical entirely with the optical fiber grating.

3. in the multiplexing method described in the claim 1, that uses is the antiradar reflectivity fiber grating with the optical fiber grating entirely, and the reflectivity of fiber grating is all less than 5%, and reflectivity is low more, and reusing degree is high more.

4. in the multiplexing method described in the claim 1, the reflectivity of fiber grating can not reduce arbitrarily, be subjected between Rayleigh scattering, grating the influence of factor such as repeatedly reflection, between grating repeatedly the grating multiplexing number of reflection restriction be inversely proportional to the square root with the grating reflectivity entirely.

5. in the multiplexing method described in the claim 1, use the OTDR technology, realize the grating location of simple optical fiber optical fiber optical grating multiplexing structure.

6. based on any optical fiber optical grating multiplexing system of claim 1-5.

Images