CN100350219C - Method and device for multiplexing and demodulating sensor based on optical fiber grating in long cycle - Google Patents

Abstract

The invention relates to a method and equipment for multiplexing and demodulating sensors based on long-period fiber gratings. At present, no one has proposed an efficient and cheap LPGP multiplexing and demodulation method. In the present invention, the light intensity of the broadband light source is evenly distributed to a plurality of LPGPs with different optical path differences through the optical fiber coupler, and multiplexing is completed after the signal lights of each path are combined; the multiplexed signal light passes through an optical unidirectional The isolator enters the 3-dB fiber coupler, and the two beams of signal light are incident on the mirror through the fiber collimator and are reflected and coupled back to the fiber. By controlling the position movement of one of the mirrors, the phase introduced by LPGP with different optical path differences is compensated. difference; the compensated two-way optical signals are combined and interfered in the 3-dB coupler, converted into electrical signals by the photodiode, collected by the data acquisition card, and demodulated. The invention has the advantage of low cost and is suitable for integration and instrumentation.

Description

Based on multiplexing and demodulation method and the equipment thereof of long period fiber grating to sensor

Technical field

The invention belongs to technical field of optical fiber sensing, particularly a kind of long period fiber grating to the signal multiplexing and the demodulation method of sensor and the equipment of realizing this method.

Background technology

At sensory field of optic fibre, long period fiber grating is to (long-period grating pair, LPGP) because it is very responsive to little curved, temperature, stress, variations in refractive index, and become crucial senser element in the industry, be particularly useful for the high-acruracy survey of liquid refractivity.Utilize first long period fiber grating that the fiber core layer luminous energy is coupled half to fibre cladding, through the propagation of stretch journey again by second grating sandwich layer that is coupled back, converge with second half light through sandwich layer, long period fiber grating is Mach-Zehnder (M-Z) interferometer of two arms in fact having constituted one with fiber core layer and covering.Because the fibre cladding effective refractive index is subjected to the influence of extraneous refractive index, the optical path difference of these M-Z interferometer two arms variations in refractive index to external world is very responsive.Utilize the method for wavelength-modulated, the T.Allsop of UK is used for refractive index sensing with a LPGP, has obtained 1.8 * 10 ^-6Precision.Awl and lithographic technique are drawn in the utilization of domestic Zhejiang University, and that section optical fiber in the middle of the grating pair is handled, and improve the influence degree of extraneous refractive index to cladding index, and measurement sensitivity has been improved more than five times.

Fiber Bragg Grating FBG since its frequency domain reflectance spectrum very narrow (＜1nm), utilize wave length beam splitting device, asymmetric M-Z interferometer or tunable optical fiber filter can realize its multiplexing and demodulation on frequency domain.But long period fiber grating, its spectrum signature are a plurality of transmission losss peak and deposit, and the relative broad of the frequency domain bandwidth of each loss peak (tens of nm), can't realize multiplexing and demodulation on frequency domain.Although long period fiber grating itself has the sensing characteristics to little curved, temperature, stress, refractive index sensitivity, it seldom is used to distributed measurement.As a same reason, so far, also nobody proposes the multiplexed and demodulation method of a kind of LPGP of efficient cheapness.

Summary of the invention

The present invention is exactly at the deficiencies in the prior art, has proposed a kind of multiplexed and low-coherent light demodulation scheme based on the right sensor of long period fiber grating, and the equipment of realizing this method is provided simultaneously.

Method of the present invention may further comprise the steps:

1, selects the wideband light source at wavelength coverage covering long-period gratings transmission loss peak for use, strip of light wide region 20～60nm; With the LPGP sensor of light intensity mean allocation to the individual different grating centre distances of n (n 〉=2), the light signal in each LPGP sensor is introduced different optical path differences by fiber coupler; The flashlight that carries each road LPGP heat transfer agent converges to a single-mode fiber by fiber coupler, finishes the multiplexed of transducing signal.The optical path difference of the light signal in described each LPGP sensor is to utilize long period fiber grating to be coupled to this characteristic of covering to the luminous energy of fiber core layer, each LPGP sensor is coupled to fibre cladding to half luminous energy by first long period fiber grating, after the propagation through one section light path, by second long period fiber grating sandwich layer that is coupled back, converge with other half luminous energy by sandwich layer, the optical path difference of the light signal in the LPGP sensor is:

OPD＝Δn·L (1)

Wherein Δ n represents that the effective refractive index of fiber core layer and covering is poor, and L represents grating centre distance in the LPGP sensor.

2, the transducing signal after multiplexed enters a 3-dB fiber coupler through a light one-way isolator, and the 3-dB fiber coupler is divided into aplanatic two bundles with light signal; The aplanatic flashlight of two bundles incides on the catoptron by optical fiber collimator respectively and reflects coupled back into optical fibers; Move by the position of controlling one of them catoptron, make between the two paths of signals light and to introduce another and go into optical path difference, and produce scanning, thereby compensate the optical path difference that the LPGP sensor of different grating centre distances is introduced.The effect of one-way isolator mainly is to prevent the destruction of reflected light to light source.

3, the two ways of optical signals after the compensation is converged interference in the 3-dB coupling mechanism, and interference signal is converted into electric signal through an arm of 3-dB coupling mechanism by photodiode, by the data collecting card collection, finishes demodulation.

Can see from (1) formula, the optical path difference difference that the LPGP sensor of different grating centre distances is introduced, scan a certain position so work as an arm of Maxwell interferometer, the optical path difference of certain LPGP sensor is compensated for as zero, and then the low-coherent light interference signal of this LPGP sensor correspondence occurs.By being arranged in parallel the LPGP sensor of a series of different grating centre distances, and a scanning Maxwell interferometer arm wherein, can realize the multiplexing and demodulation of multichannel LPGP sensor.

The resolution of the demodulation scheme of this multiplexed LPGP sensor depends primarily on the coherent length of long-period gratings loss spectra:

$l_c=\frac{21\mathrm{<figure-callout\; id="2"\; label="n"\; filenames="C20051006138300101.png"\; state="\{\{state\}\}">n</figure-callout>}2}{\mathrm{\&pi;}}\frac{{\mathrm{\&lambda;}}^2}{\mathrm{\&Delta;\&lambda;}},---\left(2\right)$

Wherein, λ and Δ λ are respectively the centre wavelength and the bandwidth of long-period gratings loss peak.

The equipment of realizing such scheme is: the LPGP sensor input end of the individual different grating centre distances of n (n 〉=2) in parallel is connected with the wideband light source light signal by fiber coupler, and output terminal is connected with another fiber coupler light signal.Light one-way isolator input end is connected with another fiber coupler light signal, and the input end of its output terminal and photodiode is connected with an end light signal of 3-dB fiber coupler, and photodiode output is electrically connected with data collecting card.The other end of 3-dB fiber coupler is connected with two optical fiber collimator light signals respectively, and corresponding two optical fiber collimator positions are provided with two catoptrons, and one of them catoptron moves by its position of step motor control.

The present invention mainly is applicable to the demodulation of multiplexed LPGP sensor signal, spatial resolution is up to tens of microns, the maximum demodulation number of channel reaches hundreds of, can reach very high measuring accuracy by processing to interference signal, compare frequency domain measurement instruments such as those spectrometers simultaneously, have advantage with low cost, be fit to integrated and instrumentation.

Description of drawings

Fig. 1 is an one-piece construction synoptic diagram of the present invention.

Embodiment

As shown in Figure 1, LPGP sensor 3 input ends of a plurality of different grating centre distances in parallel are connected with wideband light source 1 light signal by fiber coupler 2 respectively, and output terminal is connected with another fiber coupler 4 light signals.Light one-way isolator 5 input ends are connected with fiber coupler 4 light signals, its output terminal be connected with an end light signal of 3-dB fiber coupler 6 with the input end of photodiode 10, the output terminal of photodiode 10 is electrically connected with data collecting card 11.The other end of 3-dB fiber coupler 6 is connected with two optical fiber collimator 7 light signals respectively, and corresponding two optical fiber collimator 7 positions are provided with two catoptrons 8, and one of them catoptron moves by stepper motor 9 its positions of control.Above-mentioned a plurality of LPGP sensors 3 are embedded in the monitored buildings as microbend sensor, wideband light source 1 and signal demodulation part are all in monitoring center, broadband light enters a plurality of LPGP sensors 3 by single-mode fiber, passes flashlight back monitoring center by single-mode fiber again and carries out signal demodulation and processing.

Selecting wavelength coverage for use is 60nm (can cover the broadband at long-period gratings transmission loss peak) light source, by fiber coupler the light intensity mean allocation is arrived a plurality of LPGP sensors, the light signal in the LPGP sensor of each different grating centre distance has different optical path differences; The flashlight that carries each road LPGP heat transfer agent converges to a single-mode fiber by fiber coupler, finishes the multiplexed of transducing signal.The optical path difference of the light signal in each LPGP sensor is to utilize long period fiber grating to be coupled to this characteristic of covering to the luminous energy of fiber core layer, each LPGP sensor is coupled to fibre cladding to half luminous energy by first long period fiber grating, after the propagation through one section light path, by second long period fiber grating sandwich layer that is coupled back, converge with other half luminous energy by sandwich layer, the optical path difference of the light signal in each LPGP sensor is:

OPD＝Δn·L (1)

Wherein Δ n represents that the effective refractive index of fiber core layer and covering is poor, and L represents grating centre distance in the LPGP sensor.

Transducing signal after multiplexed enters a 3-dB fiber coupler through a light one-way isolator, and the 3-dB fiber coupler is divided into aplanatic two bundles with light signal; The aplanatic flashlight of two bundles incides on the catoptron by optical fiber collimator respectively and reflects coupled back into optical fibers; Control the position of one of them catoptron and move, make between the two paths of signals light and to introduce another and go into optical path difference, and produce scanning, thereby compensate the optical path difference that the LPGP sensor of different grating centre distances is introduced.

Two ways of optical signals after the compensation is converged interference in the 3-dB coupling mechanism, interference signal is converted into electric signal through an arm of 3-dB coupling mechanism by photodiode, by the data collecting card collection, finishes demodulation.

Can see from (1) formula, the phase differential difference that the LPGP sensor of different grating centre distances is introduced, scan a certain position so work as the Maxwell interferometer, the phase differential of certain LPGP sensor is compensated for as zero, and then the low-coherent light interference signal of this LPGP sensor correspondence occurs.By a LPGP sensor that is arranged in parallel a series of different grating centre distances and a wherein arm that scans the Maxwell interferometer, can realize the multiplexing and demodulation of multichannel LPGP sensor.

The resolution of the demodulation scheme of this multiplexed LPGP sensor depends primarily on the coherent length of long-period gratings loss spectra:

$l_c=\frac{21\mathrm{<figure-callout\; id="2"\; label="n"\; filenames="C20051006138300101.png"\; state="\{\{state\}\}">n</figure-callout>}2}{\mathrm{\&pi;}}\frac{{\mathrm{\&lambda;}}^2}{\mathrm{\&Delta;\&lambda;}},---\left(2\right)$

Wherein, λ and Δ λ are respectively centre wavelength and bandwidth such as long-period gratings loss peak, and λ gets 1550nm (optical communication typical case wave band) in the present embodiment, and Δ λ gets 20nm (long-period gratings loss peak normal bandwidth), and according to formula (2), interfering length is 53 μ m.Scanning step motor 1cm, maximum 377 LPGP sensor signals can be by demodulation.Between the LPGP interference signal for adjacent channel aliasing not taking place, according to formula (1) and formula (2), considers that again general single mode fiber sandwich layer cladding index difference Δ n is 10 ^-2About, adjacent LPGP length difference needs 5.3mm.

Because little curved meeting causes the leakage of cladding mode among the LPGP, thereby weakens the intensity of output terminal interference signal, utilizes the data acquisition and processing (DAP) system that interference signal is extracted envelope, can obtain the little curved signal of LPGP sensor.Scanning Maxwell interferometer has promptly been realized the multiplexing and demodulation of multichannel LPGP.

Utilization can scan optical path difference that the Maxwell interferometer introduces LPGP and compensate and obtain to comprise the low-coherent light interference fringe of transducing signal, thereby has realized the multiplexing and demodulation to the LPGP sensor.The present invention also has the resolution height, measures the advantage that bandwidth is big, demodulation accuracy is high and with low cost.

Claims (3)

1. A method for multiplexing and demodulating sensors based on long-period fiber gratings, characterized in that the method comprises the following steps: (1) Select a broadband light source whose wavelength range covers the transmission loss peak of the long-period fiber grating, and the bandwidth range of the light source is 20-60nm; through the fiber coupler, the light intensity is evenly distributed to n long-period fiber grating sensors with different grating center distances, among which n≥2, each long-period fiber grating pair introduces a different optical path difference to the optical signal in the sensor; the signal light carrying the sensing information of each long-period fiber grating pair is merged into a single-mode fiber by a fiber coupler to complete Multiplexing of sensing signals; (2) The multiplexed sensing signal enters a 3-dB fiber optic coupler through an optical one-way isolator, and the 3-dB fiber optic coupler divides the optical signal into two beams of equal light intensity; the two beams of equal light intensity The signal light is incident on the mirrors through the fiber collimator and is reflected and coupled back to the optical fiber; by controlling the position movement of one of the mirrors, another optical path difference is introduced between the two signal lights, and scanning is generated, thereby compensating The optical path difference introduced in the sensor of the LPFG pair with different grating center distances; (3) The two optical signals after compensation are combined and interfered in the 3-dB coupler, and the interference signal is converted into an electrical signal by the photodiode through one arm of the 3-dB coupler, collected by the data acquisition card, and demodulated. 2. The multiplexing and demodulation method based on long-period fiber grating pair sensors as claimed in claim 1, characterized in that the optical path difference of the optical signal in each long-period fiber grating pair sensor is made by using a long-period optical fiber The grating can couple the light energy of the fiber core to the cladding. Each long-period fiber grating pair sensor couples half of the light energy to the fiber cladding through the first long-period fiber grating. , is coupled back to the core layer by the second LPFG, and merges with the other half of the light energy passing through the core layer. The optical path difference between the LPFG and the optical signal in the sensor is: OPD＝Δn·L (1) Where Δn represents the effective refractive index difference between the fiber core and the cladding, and L represents the distance between the long-period fiber grating and the grating center in the sensor. 3, adopt the equipment that claim 1 method uses, it is characterized in that the long-period fiber grating pair sensor (3) input end of n different grating center distances in parallel is passed optical signal of optical fiber coupler (2) and broadband light source (1) Connection, where n≥2, the output end is connected to another optical fiber coupler (4) optical signal; the input end of the optical one-way isolator (5) is connected to another optical fiber coupler (4) optical signal, the optical one-way isolator The output end of (5) and the input end of photodiode (10) are respectively connected with one end optical signal of 3-dB fiber optic coupler (6), and the output end of photodiode (10) is electrically connected with data acquisition card (11); The other end of the 3-dB fiber optic coupler (6) is respectively connected to the optical signals of the two fiber collimators (7), and two reflectors (8) are arranged corresponding to the positions of the two fiber collimators (7), one of which The position of the reflector is controlled by a stepper motor (9) to move.

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