CN102102999B - Sensing multiplexing system based on non-equidistant weak Bragg reflection fiber Bragg grating array - Google Patents

Sensing multiplexing system based on non-equidistant weak Bragg reflection fiber Bragg grating array Download PDF

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CN102102999B
CN102102999B CN201010590361A CN201010590361A CN102102999B CN 102102999 B CN102102999 B CN 102102999B CN 201010590361 A CN201010590361 A CN 201010590361A CN 201010590361 A CN201010590361 A CN 201010590361A CN 102102999 B CN102102999 B CN 102102999B
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fiber grating
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grating
fiber bragg
optical fiber
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孙琪真
刘德明
张满亮
李晓磊
王梓
沃江海
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Huazhong University of Science and Technology
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Abstract

The invention discloses a sensing multiplexing system based on a non-equidistant weak Bragg reflection fiber Bragg grating array. The system comprises a tunable laser (T), a modulator (E), an optical isolator (I), an optical circulator (C) and a wavelength demodulation device (D) which are connected in turn, wherein the optical circulator (C) is connected to a sensing optical fiber with a plurality of fiber Bragg gratings. The interference effect caused by multiple reflection among the fiber Bragg gratings can be avoided, the multiplexing capacity of the system can be greatly improved, and long-distance high-density high-capacity distributed detection is realized.

Description

Sensing multiplex system based on Bragg reflection optical fiber grating array a little less than the unequal interval
Technical field
The present invention relates to field of sensing technologies, refer in particular to a kind of sensing multiplexing method based on Bragg reflection optical fiber grating array a little less than the unequal interval.
Background technology
FBG (Fiber Bragg Grating FBG) sensing shows as the centre wavelength coding, and through the variation that the monitoring that FBG centre wavelength is moved can be measured extraneous parameter, detectable signal does not receive the influence of light source power fluctuation.But a great problem based on the distributed sensing of FBG faces is the restriction that the system multiplexing capacity receives light source and filter bandwidht.
Seen at present the report that multiple multiplexing method is arranged; The combination of and these multiplexing methods multiplexing etc. like space division multiplexing, time division multiplex, wavelength-division multiplex, subcarrier multiplexing, frequency modulation continuous wave; These methods respectively have characteristics; But all can not fine solution reusing degree, factors such as cost, practicability, be far from being enough for the highdensity Distributed Detection of long distance.
The report that evenly spaced single fiber multiplexing method of identical low-reflectivity fiber gratings was once arranged; This method need be considered repeatedly to reflect the interference effect that causes, cause by the grating multiplexing number of the repeatedly reflection restriction between fiber grating be inversely proportional to the square root of the reflectivity of grating entirely.
Summary of the invention
Technical matters to be solved by this invention is: to the problem of existing distributed optical fiber sensing technology existence; A kind of sensing multiplex system based on Bragg reflection optical fiber grating array a little less than the unequal interval is provided; The multiplexing capacity that it improves sensor-based system greatly makes it in the intensive sensing of long distance of parameters such as temperature, stress and vibration, obtain widespread use.
The present invention solves its technical matters and adopts following technical scheme:
Sensing multiplex system based on Bragg reflection optical fiber grating array a little less than the unequal interval; Comprise the tunable laser, modulator, optoisolator, optical circulator and the Wavelength demodulation device that link to each other successively, optical circulator also is connected to the sensor fibre that has a plurality of Fiber Bragg Grating FBGs.
Interval between said each Fiber Bragg Grating FBG is unequal in twos.
All Fiber Bragg Grating FBG adopt complete with weak Bragg reflection optical fiber grating.
The maximum number j of said Fiber Bragg Grating FBG satisfies following formula:
Figure 2010105903618100002DEST_PATH_IMAGE001
，
Wherein t is the transmissivity of Fiber Bragg Grating FBG, and r is the reflectivity of Fiber Bragg Grating FBG.
The present invention compared with prior art mainly has the following advantages:
One of which, the present invention adopts weak Bragg reflection optical fiber grating as sensing unit, and luminance factor back scattering technology will exceed several orders of magnitude, surveys more easy and measuring accuracy is higher, and reflectivity can be according to the application requirements flexible design.
Its two, the present invention adopts weak Bragg reflection optical fiber grating as sensing unit, the retroreflectance of each position is extremely low, can satisfy long distance detecting and can not omit the metrical information of any position.Therefore, distance sensing will get a qualitative improvement with the detection closeness.
Its three, the present invention adopts optical time domain reflection technology (OTDR), realize to survey simultaneously and the location, need not Wavelength demodulation, real-time is high, cost is low.
They are four years old; The theoretical analysis surface; It is the principal element of the multiplexing capacity of restriction system that fiber grating directly repeatedly reflects, and the present invention adopts the weak Bragg reflection optical fiber grating array structure of unequal interval, through the multiplexing method of this unequal interval; Avoid the interference effect that repeatedly reflects, improved the reusing degree of fiber grating in the sensor-based system greatly.
The present invention can break through the predicament of existing optical fiber sensing technology; Increase substantially sensor-based system capacity and distance sensing, can be widely used in monitoring fields such as bridge security control, power transmission and transformation line monitoring, regional circumference Security alert, nuclear plant safety monitoring and seabed supervision.
Description of drawings
Fig. 1 is based on the sensing multiplexing method synoptic diagram of the weak Bragg reflection optical fiber grating array of unequal interval: wherein, I is an isolator; C is an optical circulator; F 1, F 2, F 3... F jBe Fiber Bragg Grating FBG, the interval L between the adjacent FBG 1≠ L 2≠ ... ≠ L (j-1)
Fig. 2 is based on the system multiplexing number and full analysis result figure with optical fiber grating reflection rate of the sensing multiplex system of the weak Bragg reflection optical fiber grating array of unequal interval: wherein; Horizontal ordinate is for entirely with optical fiber grating reflection rate (Reflective (0.01%-1%)); Ordinate is the reusing degree (Total FBG number) of system; Incident power-10dB, the photodetector limit is-70dB.
Fig. 3 be uniformly-spaced with unequal interval a little less than the comparison diagram of system multiplexing number of sensing multiplex system of Bragg reflection optical fiber grating array; Wherein, Horizontal ordinate is for entirely with optical fiber grating reflection rate (Reflective (0.01%-1%)); Ordinate is the reusing degree (Total FBG number) of system, incident power-10dB, and the photodetector limit is-70dB.
Fig. 4 is based on the formation synoptic diagram of the sensing multiplex system of the weak Bragg reflection optical fiber grating array of unequal interval: wherein, T is a tunable laser, and E is a modulator, and I is isolator (not having I among Fig. 4); C is an optical circulator, and D is the Wavelength demodulation device, F 1, F 2, F 3... F jBe Fiber Bragg Grating FBG, the interval L between the adjacent FBG 1≠ L 2≠ ... ≠ L (j-1)
Fig. 5: the one-level multiple reflection synoptic diagram between fiber grating self reflection and the fiber grating.
Embodiment
The present invention proposes a kind of sensing multiplexing method based on Bragg reflection optical fiber grating array a little less than the unequal interval; Inhomogeneous at interval between the fiber grating; Can effectively avoid repeatedly reflecting the interference effect that causes, the grating multiplexing number that is limited by the repeatedly reflection between fiber grating is inversely proportional to full reflectivity with grating.
What the present invention proposed has improved the multiplexing capacity of system based on the sensing multiplexing method of Bragg reflection optical fiber grating array a little less than the unequal interval with respect to evenly spaced single fiber multiplexing method of identical low-reflectivity fiber gratings greatly, can more effective realization length apart from the jumbo Distributed Detection of high density.
The characteristic of the optical fiber optical grating array that the present invention uses is: full fiber grating together, and the fiber grating of use has identical centre wavelength, reflectivity and reflectance spectrum, and said structure shows weak reflection characteristic to centre wavelength.
Inhomogeneous at interval between the fiber grating among the present invention in the optical fiber optical grating array, avoid repeatedly reflecting the interference effect that causes, can improve the system multiplexing capacity greatly.
The present invention provides a kind of distributing optical fiber sensing localization method, will consider fibre loss, Rayleigh scattering in the practical application, the repeatedly reflection and the factors such as detection limit of photodetector between the fiber grating.A little less than based on unequal interval in the sensing multiplex system of Bragg reflection optical fiber grating array, the optical fiber optical grating multiplexing number of the repeatedly reflection restriction between the fiber grating be inversely proportional to optical fiber optical grating reflection rate entirely.
The present invention adopts optical time domain reflection technology (OTDR), realizes the fiber grating location in the weak Bragg reflection optical fiber grating array structure of unequal interval.
The present invention provides a kind of sensing multiplexing method; It is characterized in that sensing multiplexing method based on Bragg reflection optical fiber grating array a little less than the unequal interval; Monopulse through adopting tunable wave length injects the sensing multiplex system based on Bragg reflection optical fiber grating array a little less than the unequal interval; As shown in Figure 1, the monopulse of wavelength-tunable incides the weak Bragg reflection optical fiber grating array of unequal interval through optoisolator and optical circulator, and the back scattering pulsed light that fiber grating produces gets into the Wavelength demodulation device through optical circulator; If the centre wavelength of some fiber gratings is because extraneous parameter function influence drifts about; Cause itself and laser wavelength of incidence inconsistent, then input optical pulse reduces in the back scattering meeting of this fiber grating position, and the reflected signal on the corresponding time domain can die down.Reflected signal through detecting on the time domain is strong and weak, confirms the position of fiber grating, through detecting the time order and function that pulse is returned, each fiber grating is realized accurately location.
Below the present invention further is detailed.
At first consider the influence of Rayleigh scattering,, should make the back power to Rayleigh scattering light of the luminous power of fiber grating reflection greater than corresponding exciting light pulse generation for distinguishing reflected signal light and Rayleigh scattering light.Consider last grating (worst condition), should make:
Wherein, P FBG (j)The reflective power of expression FBG (j), P RayleighRepresent the back power that corresponding exciting light pulse produces to Rayleigh scattering light.
But in the practical application,, can set in order to guarantee to extract accurately flashlight:
Figure 2010105903618100002DEST_PATH_IMAGE003
Draw by deriving, be by the optical fiber optical grating multiplexing number of Rayleigh scattering effects limit with co-relation:
Figure 746903DEST_PATH_IMAGE004
（1）
Wherein, R is the fiber grating reflectivity; T is the transmissivity of fiber grating; Δ L is the length of exciting light pulse in optical fiber,
Figure 2010105903618100002DEST_PATH_IMAGE005
represent that rayleigh scattering coefficient, S represent the ratio of backscattering power and Rayleigh scattering general power.
Secondly, consider the repeatedly reflection between the fiber grating, because the FBG reflectivity that adopts is lower, multiple reflection light intensities at different levels differ bigger, the reflectivity such as 1%, and multiple reflection light intensities at different levels differ 10 4Therefore magnitude only considers the influence of one-level multiple reflection situation.
Because fiber grating adopts unequal interval to distribute among the present invention; The one-level multiple reflection number of path that arrives (optical path difference in ± Δ L scope, think arrive simultaneously, influential to the reflective power of FBG (j)) photodetector with last fiber grating reflected light simultaneously satisfies:
Figure 2010105903618100002DEST_PATH_IMAGE007
, and
Figure 258973DEST_PATH_IMAGE008
(j>=3)
N be fiber grating when being spacedly distributed and last fiber grating reflected light arrive the one-level multiple reflection number of path of photodetector simultaneously.Therefore:
Figure DEST_PATH_IMAGE009
（j≥3） （2）
Use tunable laser as light source, the spectrum that reaches photodetector at last is very narrow, but since among the present invention fiber grating adopt unequal interval to distribute, can effectively prevent repeatedly to be reflected in the interference effect that fiber grating uniformly-spaced produces under the situation.
In order to guarantee to extract accurately flashlight, can the setting signal light intensity be 10 times of one-level multiple reflection light intensity.Consider under the worst situation:
1. a multiple reflection number of path N '=N who arrives simultaneously with FBG (j)
2. fibre loss is according to reaching distance calculation simultaneously with FBG (j) self reflection):
According to qualifications, through deriving, in the time of can obtaining the distribution of fiber grating unequal interval, the maximum multiplexing number that can obtain will satisfy:
Figure 282555DEST_PATH_IMAGE001
（3）
Can draw to draw a conclusion by following formula:
When the fiber grating unequal interval distributes, by repeatedly reflecting the optical fiber optical grating multiplexing number of restriction between fiber grating and being inversely proportional to optical fiber grating reflection rate entirely.
Because very big based on the multiplexing number of the sensing multiplexing method of Bragg reflection optical fiber grating array a little less than the unequal interval, so the loss of optical fiber needs to consider, simultaneously, because there is detection limit in photodetector, so under the prerequisite of considering the one-level multiple reflection, satisfy:
Figure 480318DEST_PATH_IMAGE010
,
Figure DEST_PATH_IMAGE011
is the detection limit of photodetector
Fig. 4 is the formation synoptic diagram based on the sensing multiplex system of Bragg reflection optical fiber grating array a little less than the unequal interval provided by the invention.Tunable laser T is as system source; Produce the monopulse of tunable wave length; Electrooptic modulator E through being driven by impulse function generator becomes modulation signal, incides the weak Bragg reflection optical fiber grating array of unequal interval through optoisolator I and optical circulator C again, and the back scattering pulsed light that fiber grating produces gets into Wavelength demodulation device D through optical circulator C; Reflected signal through detecting on the time domain is strong and weak; Confirm the position of fiber grating,, each fiber grating is realized accurately location through detecting the time order and function that pulse is returned.
Suppose that this system adopts the fiber grating of reflectivity r=1%, the exciting light pulse width is Δ L=20m, and the coherence length of laser is 15cm; Spacing between the fiber grating increases progressively successively and is arithmetic progression; J-1 and j fiber grating spacing are 20m+7.5 (j-1) cm, and incident power is-10dB the photodetector limit-70dB; Should take all factors into consideration fibre loss, Rayleigh scattering, the repeatedly influence of reflection and the factors such as detection limit of photodetector between the fiber grating.
Can know by (1) formula, can reach more than 300, so Rayleigh scattering is very little to the influence of multiplexing number purpose by the optical fiber optical grating multiplexing number of Rayleigh scattering effects limit.
When reflectivity was 1%, rayleigh scattering coefficient was with 10 -7/ m calculates, and can be got by formula (3), and repeatedly the optical fiber optical grating multiplexing number of reflection restriction is 46.
In practical application, should take all factors into consideration fibre loss, Rayleigh scattering; The repeatedly influence of reflection and the factors such as detection limit of photodetector between the fiber grating; The result of optimization system gives full play to the advantage based on the sensing multiplexing method of Bragg reflection optical fiber grating array a little less than the unequal interval to the full extent.
(following additional literal is fully disclosing in order to ensure patent)
Weak Bragg reflection optical fiber grating of the present invention is based on the carrier optical fiber designs of coat to 355nm wavelength uv transparent, and certain at interval delay optical fiber, continuous distribution a series of physical arrangements and the identical weak Bragg reflection periodic structure of optical characteristics on it.
According to coupled wave theory and transmission matrix analytical approach; Centre wavelength reflectivity of Fiber Bragg Grating FBG
Figure 496816DEST_PATH_IMAGE012
and bandwidth
Figure DEST_PATH_IMAGE013
are by index modulation intensity and grating length decision, promptly
Figure 181744DEST_PATH_IMAGE014
（4）
Figure DEST_PATH_IMAGE015
（5）
Wherein,
Figure 891074DEST_PATH_IMAGE016
is coupling coefficient; is grid region length;
Figure 996040DEST_PATH_IMAGE018
is the index modulation amplitude of fibre core in the fiber grating; is the cycle of grating;
Figure 65496DEST_PATH_IMAGE020
is the effective refractive index of communication mode in the optical fiber,
Figure DEST_PATH_IMAGE021
be Bragg (Prague) centre wavelength.
According to formula (4), (5), can be through changing the weak Bragg reflection periodic structure of different centre wavelengths of index modulation intensity and reflectivity with the modulator zone Design of length.The Bragg reflection periodic structure that adopts weak photosensitivity optical fiber and appearance position template construct specific reflectance, bandwidth is fully feasible.The length that the intensity that can obtain ultraviolet lighting through theoretical analysis and calculation and scanning are inscribed, and produce reflectivity and be weaker than 1% weak Bragg reflection periodic structure.

Claims (2)

1. based on the sensing multiplex system of Bragg reflection optical fiber grating array a little less than the unequal interval; It is characterized in that: comprise the tunable laser (T), modulator (E), optoisolator (I), optical circulator (C) and the Wavelength demodulation device (D) that link to each other successively, optical circulator (C) also is connected to the sensor fibre that has a plurality of Fiber Bragg Grating FBGs; Interval between each Fiber Bragg Grating FBG is unequal in twos; All Fiber Bragg Grating FBGs adopt complete weak Bragg reflection optical fiber grating together.
2. system according to claim 1 is characterized in that: the maximum number j of Fiber Bragg Grating FBG satisfies following formula:
j ≤ t 5 × r + 2 ,
Wherein t is the transmissivity of Fiber Bragg Grating FBG, and r is the reflectivity of Fiber Bragg Grating FBG.
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