CN101738216A - Single fiber multiplexing method of identical low-reflectivity fiber gratings - Google Patents

Abstract

The invention relates to a utilization method of fiber gratings on single fibers, which connects in series and multiplexes the fiber gratings so as to form a large single fiber distribution system. A multiplexing method which is based on an OTDR (Optical Time Domain Reflectometry) technology and uses identical low-reflectivity fiber gratings is analyzed and researched theoretically. Analysis shows that multiple reflections among the fiber gratings are a main reason of limiting multiplexing number. The multiplexing method can use a single fiber grating detection system in detecting the large distribution system formed by hundreds of fiber gratings on a same fiber on the basis without increasing complex programs.

Description

Single fiber multiplexing method of identical low-reflectivity fiber gratings

Technical field

The present invention relates to the optical fiber distributed type sensory field, be specifically related to a kind of complete in the multiplexing method of antiradar reflectivity fiber grating on simple optical fiber based on OTDR (Optical Time DomainReflectometer) technology, the distributed grating sensing network high to the designability price ratio has directive significance.

Background technology

Since K.O.Hill in 1978 ^[1]Reported in the world since first fiber grating that characteristic makes it that huge application be arranged because its good reliability, antijamming capability be strong, easy to use etc., and is developed by leaps and bounds in booming fiber optic communication field.And the centre wavelength of fiber grating is very easily with the influence of environment temperature and strain, and this shortcoming that needs to overcome in optical fiber communication is used of fiber grating but becomes a kind of sensing technology of extensively being approved and having great potential in sensing and fields of measurement soon.Through the research and development of two more than ten years, the fiber-optic grating sensor that different kinds of parameters is measured has constantly appearred being used for.Yet up to the present, this sensing technology also is not used widely in actual engineering.Trace it to its cause, wherein Ang Gui Wavelength demodulation partly is main limiting factor ^[2]On the other hand, in many application scenarios, need carry out large-scale distribution measuring, so effective multiplex technique of fiber grating also becomes an important directions of fiber grating sensing technology research as intelligence structure etc.Simultaneously, also place hope on and adopt multiplex technique to reduce the cost of single measurement point, thereby make fiber grating sensing technology have competitive power.

At present, seen the report that multiple multiplexing method is arranged, as: the various combinations of SDM (space division multiplexing), TDM (time division multiplex), WDM (wavelength-division multiplex), SCM (subcarrier multiplexing) and FMCW (frequency modulation continuous wave is multiplexing) etc. and these multiplexing methods ^[3-8]Though these multiplexing methods respectively have characteristics, but all fail to solve well the relation between the factors such as reusing degree, cost, connection complexity, also be do not have among them a kind of multiplexing method can be on same optical fiber multiplexing a plurality of fiber gratings with same characteristic features wavelength.If can address this problem, combine with above any method again, just can obtain to have the system of bigger reusing degree.

The present invention be exactly propose a kind of based on the OTDR technology be implemented in the fine grating of low light reflectivity entirely and realize the multiplexing method of a large amount of gratings on the same optical fiber.

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Summary of the invention

The present invention proposes a kind of multiplexing a large amount of entirely with the multiplexing methods of optical fiber grating on same optical fiber based on OTDR technology and antiradar reflectivity fiber grating.With the use of optical fiber grating, can realize using the online manufacturing technology of single masterplate by entirely, make the manufacturing cost of fiber grating lower, better reliability; The use of antiradar reflectivity fiber grating can improve the reusing degree of optical-mechanical system greatly, helps the application of fiber grating in multiplex system; The use of OTDR technology can realize the detection and localization of fiber grating sensing system.

The present invention adopts the monopulse of wavelength-tunable to inject the distribution multipoint system that is made of with antiradar reflectivity fiber grating serial connection entirely above, and measures the time-varying signal (OTDR) of reflection, thereby carries out signal and Wavelength demodulation.As shown in Figure 1, the monopulse of wavelength-tunable enters the single fiber system that the grating serial connection constitutes through coupling mechanism L, and a part of signal will be at grating G ₁The place is reflected, and residue the part transmission takes place and arrives grating G ₂, reflection and transmission take place once more, remaining transmitted light part continues to arrive grating G again ₃, G ₄... after the train of impulses that optical grating reflection at different levels returns will enter filtering and detecting unit through coupling mechanism L.By the power of detected reflectance signal, determine the position of grating centre wavelength; By detecting the priority of pulse time of return, determine pulse is from which fiber grating.

Reduce to increase greatly with the reflectivity of grating full the multiplexing number of system, as shown in Figure 2.Wherein, D is the dynamic range of system receiver.

The grating reflection rate that system involved in the present invention adopts is all less than 5%, and reflectivity is low more, and the reusing degree of system is high more.But in practice, reducing of reflectivity can not be arbitrarily, be subjected to the repeatedly influence of factor such as reflection and fibre loss between Rayleigh scattering, grating.

At first consider the influence of Rayleigh scattering.Because the feature reflectance spectrum and the Rayleigh scattering spectrum in the optical fiber of fiber grating are overlapping, therefore, for distinguishing reflected signal light and Rayleigh scattered light, the back power that the luminous power that should make optical grating reflection selecting of reflectivity produces greater than corresponding exciting light pulse to the Rayleigh scattered light.Consider last grating (worst condition), then should make

P _FBG(K)＞P _Rayleigh

In fact, for guaranteeing to extract flashlight information effectively, can make

P _FBG(K)≥10P _Rayleigh

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

$K\left(\mathrm{\ρ}\right)\≤\frac{10\lg \frac{\mathrm{\ρ}}{10(S\·{\mathrm{\α}}_S\·\mathrm{\Δl})}}{-20\lg (1-\mathrm{\ρ})}+1---\left(1\right)$

Wherein, ρ is the optical grating reflection rate, and S is the rear orientation light light harvesting coefficient of optical fiber, α _SBe the Rayleigh scattering coefficient of optical fiber, Δ l is the length of exciting light pulse in optical fiber.

Secondly, should consider the repeatedly influence of reflection between grating.Owing to adopt full grating together, so when certain when constantly by chance the temperature at all grating places or stress are all identical, all grating reflection are composed can be overlapping fully, produce these from the light signal diverse location grating, that have an identical spectral line with the measured point grating after reflection repeatedly and measured point optical grating reflection light signal arrive the phenomenon of receiver simultaneously, thereby cause the measurement mistake, as shown in Figure 4.

For estimating this influence of repeatedly reflecting, use extreme case here to the grating reusing degree---the situation of thinking promptly that the centre wavelength of all gratings in the system is all identical by chance and will measuring last grating is considered.Because the grating reflection rate that this type systematic adopts is less than 5%, multiple reflection light intensities at different levels all differ 10 ⁴Therefore magnitude, only considers the influence of one-level multiple reflection phenomenon here.

When grating uniformly-spaced evenly distributes, can be by arranging, making up for K complete system of serial connection with grating, the path number N that arrives the one-level multiple reflection of receiver with last optical grating reflection light simultaneously is:

$N=\frac{(K-2)(K-1)}{2}---\left(2\right)$

Detect no matter be to use swept light source or wide range source to add wave filter, the spectrum of light that finally arrives detector is very narrow, needs to consider interference effect; And, therefore interfere for addition owing to arrive simultaneously.Simultaneously, in order to guarantee effective identification, establish signal light intensity and be repeatedly 10 times of reflective light intensity of one-level.Abovely can get through deriving, when grating uniformly-spaced evenly distributed, the maximum multiplexing number Kmax that can obtain was:

$K_{\max }\≈\frac{2}{\sqrt[4]{10{\mathrm{\ρ}}^2}}+2---\left(3\right)$

As can be seen, be inversely proportional to by the grating multiplexing number of the repeatedly reflection restriction between grating and the square root with the reflectivity of grating entirely, as shown in Figure 5.

Generally speaking, because the loss of optical fiber is very little, the error of bringing is very little, is negligible from the influence of fibre loss.

The present invention has the following advantages:

(1) single fiber multiplexing method of identical low-reflectivity fiber gratings involved in the present invention has improved the reusing degree of system greatly, helps the application of fiber grating in multiplex system.

(2) single fiber multiplexing method of identical low-reflectivity fiber gratings involved in the present invention can use the manufacturing technology of single mode plate, makes the manufacturing cost of fiber grating lower, better reliability.

(3) single fiber multiplexing method of identical low-reflectivity fiber gratings involved in the present invention can make fiber grating more cheap in working service, more convenient.

(4) multiplex system among the present invention only need use single cover light source and detuner, has improved the cost performance of system.

Description of drawings

Fig. 1: entirely with antiradar reflectivity optical fiber optical grating multiplexing method synoptic diagram; Wherein, L is a coupling mechanism; G ₁, G ₂, G ₃, G ₄Be optical fiber Bouguer grating.

Fig. 2: the system multiplexing number with entirely with the analysis result figure of the reflectivity of grating; Wherein, transverse axis is full reflectivity (Reflectivity) with optical fiber, and the longitudinal axis is system multiplexing degree (Numbers of Multiplexing).

Fig. 3: the synoptic diagram of the single fiber time division multiplex system constituted mode of antiradar reflectivity fiber grating, wherein: A is directly modulation; B ₁, B ₂, B ₃Be detector; C ₁, C ₂Be the superradiance device; D ₁, D ₂Be wave filter; E ₁, E ₂Be external modulator; F is a Wavelength tunable laser; G ₁, G ₂, G ₃Be Fiber Bragg Grating FBG; L ₁, L ₂, L ₃Be coupling mechanism.

Fig. 4: the repeatedly reflection synoptic diagram between grating; Wherein, light 3 repeatedly reflects through secondary, and light 2 repeatedly reflects through one-level, and light 1 returns receiving end simultaneously with light 2, and concerning signal light 1, light 1 is for disturbing.

Fig. 5: the analysis result figure of equidistant serial-connection system reflectivity of grating and reusing degree; Wherein, transverse axis is the equidistant serial-connection system reflectivity of grating (Reflectivity), and the longitudinal axis is system multiplexing degree (Multiplexibility).

Embodiment

Fig. 3 is the formation exemplary plot of the single fiber time division multiplex system of antiradar reflectivity fiber grating involved in the present invention.To scheme (c) is example, produce the monopulse of wavelength-tunable by the tunable wave length LASER Light Source, this pulse will be injected the distribution multipoint system that is made of the Fiber Bragg Grating FBG serial connection through the modulation signal of external modulator gained, again with gained reflected signal Access Probe device, thus the single fiber time-division that has constituted a complete antiradar reflectivity fiber grating utilize system.

Suppose that this system adopts the grating of reflectivity ρ=0.01%, grating is at a distance of 1m, and exciting light pulse width W=1m.Should take all factors into consideration between Rayleigh scattering, grating repeatedly factor such as reflection to the influence of system multiplexing degree.

By (1) formula as can be known, the grating multiplexing number that is limited by the Rayleigh scattering can reach thousands of, and therefore, the Rayleigh scattering is very little to the influence of reusing degree.

(the Rayleigh scattering coefficient is 10 when reflectivity is 0.01% ^-7/ m ^[12]), can get by formula (3), repeatedly the multiplexing number of the grating of reflection restriction is about 120.

Therefore in actual applications, should take all factors into consideration the repeatedly influence of factor such as reflection and fibre loss between Rayleigh scattering, grating, bring into play the advantage of single fiber multiplexing method of identical low-reflectivity fiber gratings to the full extent.

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.

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