Fibre Optical Sensor has the characteristics such as fire-proof and explosion-proof, corrosion-resistant, electromagnetism interference, has the range of application more wide than other sensor.Along with grating inscribes the fast development of technology, by inscribing grating in fiber drawing process, to overcome in traditional handicraft the complex operations of the multiple grating of welding one by one on an optical fiber, avoid splice loss, splice attenuation to raster count quantitative limitation, achieve the grating inscribing any amount on an optical fiber, for the monitoring of vast capacity optical fiber grating sensing provides material base.
Common optical grating reflection rate is higher, and luminous power is fast by decay after multiple optical grating reflection in transmitting procedure, and the grating number that an optical fiber is connected generally can not exceed dozens of.In order to increase the quantity of series connection grating, the technology such as such as wavelength-division multiplex, frequency division multiplexing, code division multiplexing are introduced into, system complex and cost price is high, but the effect of dilatation is unsatisfactory, applies very limited.In recent years, be suggested along with grating inscribes technology online, the method increasing grating quantity by reducing optical grating reflection rate is subject to the favor of people, makes low light level grid inquiring technology become the focus of research.Adopt the low low light level grid (as 0.1% reflectivity) of reflectivity, in theory an optical fiber can be connected hundreds of gratings based on time-division multiplex technology; The reflectivity of further reduction grating, adopt pole low light level grid (as reflectivity 0.01%) to build sensor-based system, acquisition significantly promotes by its capacity.Extremely weak grating sensing system can not only realize multiplexing thousands of and even tens thousand of gratings on an optical fiber, and obtains by inscribing on ordinary optic fibre, simplifies grating scribing process greatly.But the power of extremely weak grating pair inquiry light source has higher requirement, current typical SLED is less than-35dBm in the output power of certain wavelength points, after low light level grid (reflective gratings as 0.1%) reflection, the power of its reflected signal is lower than-65dBm, suitable with the ground unrest of Rayleigh scattering in optical fiber, consider the loss in transmitting procedure again, signal will be flooded by noise completely.In addition, hundreds of weak gratings in series, adopt continuous light to irradiate or periodic pulse train irradiates, and the mutual crosstalk that many optical grating reflections produce and " shadow effect " are also very serious, and this will deteriorate signal to noise ratio (S/N ratio) greatly, limits the capacity of system.How to avoid entirely with the large technical barrier that the crosstalk between grating reflected signal is the inquiry of weak gratings in series.Adopt conventional Time-domain detection technique cannot avoid many optical grating reflections (patent No. 201010590361.8 " the sensing multiplex system based on unequal interval weak Bragg reflection optical fiber grating array " completely; The patent No. 201110373500.6 " adopts the weak reflection distributed sensor of FBG and the accurate positioning method of each FBG thereof ") crosstalk, signal handling equipment is complicated, and minimum reflectivity and the maximum multiplexing quantity of grating are severely limited; When optical grating reflection rate is higher, complete same many gratings in series, the reflection of leading portion grating can reduce luminous power corresponding to bragg wavelength faster, the peak power of follow-up optical grating reflection is obviously reduced, there is " flat-top " phenomenon in reflected impulse, i.e. so-called " shadow effect ", this will cause judgement difficulty, limit maximum multiplexing grating quantity.In addition, the complete same accurate time division multiplex of many gratings in series search request, ns level is reached to the real-time detection speed of reflected impulse, existing igh-speed wire-rod production line circuit or analog electronic switching circuit self time-delay obvious, accurately cannot detect and process the pulse signal reflected, the disclosed correlation technique (patent No. 201110326745.3 " the optical fiber grating sensing method and system based on wavelength-division multiplexing multichannel output time-domain address finding technology "; Patent No. 200610009939.X " time division multiplex optical fiber grating sensing testing system based on CPLD ") spacing distance between grating need be kept at tens of rice, there is obvious defect, the crosstalk between pulse is not isolated yet, system complex and poor performance, involve great expense, have no the report of related application.The reflectivity how reducing grating further carrys out capacity, overcomes the impact of crosstalk and " shadow effect " between many gratings, becomes the matter of utmost importance that optical fiber grating sensing network technology of future generation needs to solve.
Summary of the invention
Technical matters to be solved by this invention is the limitation for above-mentioned fiber grating sensing technology, proposes a kind of pole weak optical fiber Bragg grating sensor-based system and querying method thereof.Native system possesses the features such as power system capacity is large, fast response time, dirigibility is high, cost is low.
It is as follows that the present invention solves the problems of the technologies described above adopted scheme:
A kind of pole weak optical fiber Bragg grating sensor-based system, is made up of arrowband tuning source, modulation module, circulator, bragg grating array, sampling module, photodetector and data handling machine; Described arrowband tuning source connects modulation module, and the light signal that modulation module exports is coupled into bragg grating array by circulator; Another port of described circulator connects sampling module; Sampling module connects photodetector, and photodetector wire is connected with data handling machine.
Described arrowband tuning source exports continuous laser, and light source power is greater than 0dBm, by data handling machine control realization pm level length scanning.
Described modulation module is made up of a SOA high speed optoelectronic switch and signal generator, modulates and amplify light signal, and the light impulse length that modulation exports is less than 20ns.
Described sampling module is opened the light by the 2nd SOA high speed optoelectronic and signal generator is formed, and selects in time domain the light pulse of reflection, and to allowing the light signal passed through to amplify.
Described bragg grating array by multiple extremely weak reflectivity >0.0001% entirely with fiber grating or multi-wavelength optical fiber optical grating constitution.
Two output ports of described signal generator open the light with a SOA high speed optoelectronic respectively, the 2nd SOA high speed optoelectronic opens the light and is connected; Connect with wire between the control port of narrow spectrum tuning source and signal generator and data handling machine.
The course of work of native system is as follows: the continuous laser that arrowband tuning source exports is after modulation module modulation, form the laser pulse signal amplified, pulse center wavelength power is at about 0dBm, the bragg grating array that there is certain intervals distance is coupled into through circulator, each grating produces reflection successively, the light pulse signal reflected enters sampling module, and sampling module, according to the phase differential of setting, controls the 2nd SOA high speed optoelectronic switch opens and shutoff.When SOA opens, the reflection light pulse of process is allowed through and obtains amplification; Otherwise, reflected impulse is significantly decayed, stops light signal to pass through.Data handling machine determines whether to there is Bragg reflection according to the intensity of reflected light signal, determines after Bragg reflection again according to the position of the phase difference calculating fiber grating between pulse; Photodetector carries out high speed optoelectronic conversion to sampled signal, and whether each corresponding grating of threshold determination according to setting exists Bragg reflection.Mark reflective light intensity is less than the grating limiting threshold value, then mark grating is fixed to the phase differential of inquiry, utilize arrowband tuning source to carry out length scanning to mark grating, detect the wavelength shift of corresponding grating, the measurand of fast demodulation current raster point.
The principle that fiber grating position is detected is as follows:
The phase differential Φ of putative signal generator 2 road signal, now detects the Bragg reflection light signal met the demands, then the distance of corresponding reflection grating distance the one SOA high-speed optical switch port is:
Wherein k is phase constant, relevant to the cycle of pulse.
Analyze the mapping relations of phase place and time delay, existing digital drift-phase technology is improved, the phase differential Φ by regulating phase shifter to produce can be realized, realize any time delay between rising edge of a pulse, delay length flexible adjustment, precision is high, the requirement of high-speed optoelectronic systems can be met very well, break away from the constraint of traditional time delay thought, overcome because delay time error causes greatly the obstacle that system accuracy is low, really realize the Real-time Collection of optical time domain signal.
Spacing distance between grating is determined jointly by the width of modulating pulse and sampling pulse, and the width of pulse is wider, requires that the spacing distance between grating is larger, assuming that light velocity of propagation c, the refractive index n=1.5 of fiber core, modulating pulse width t1, sampling pulse width t2, then actual sample length:
Such as, adopt single laser pulses irradiate, modulating pulse width 20ns, sampling pulse width 20ns, then actual sample length is 4m, and the theoretical spacing distance namely between grating need be greater than 4m, otherwise the reflected light between two adjacent gratings will there will be crosstalk.Because monopulse is in traveling process, there is larger time delay in the reflection between non-adjacent grating, can not produce crosstalk between sampling date, therefore, as long as the grating spacings of design is greater than actual sample length, can avoid the crosstalk of many optical grating reflections completely.Because SOA has high response speed, can produce the light pulse of below 10ns, the theoretical spacing distance between grating only need be greater than 2m, and system just can realize inquiring about without crosstalk, the Distance positioning ability of this and current OTDR is suitable, can meet the requirement of various fiber grating sensing system.
When not considering to reflect crosstalk between grating, grating number that same optical fiber is connected is determined by reflectivity.Assuming that i complete same gratings in series, the reflectivity of each grating is R (λ), then. the light intensity of i-th optical grating reflection is:
Reflectance reduction, the complete same grating number that can connect will sharply rise.When after modulating, light pulse power is at 0dBm, for low light level grid, as R (λ)=0.1%, light intensity declines when original 0.001% (-50dBm), and the maximum number of gratings in series is 2302 in theory; As R (λ)=0.01%, light intensity declines when original 0.001% (-50dBm), and the maximum number of gratings in series is 11513 in theory; Because ground unrest is about below-65dBm, without under cross talk conditions, the technology detecting-60dBm signal is also very ripe, reduces the reflectivity of grating further, the maximum number of gratings in series will break through hundreds thousand of in theory, meet the demand of Internet of Things to sensing capacity completely.
SLED has good wide spectral characteristics, be suitable for wavelength-division multiplex technique, but output power is low, the output power of current typical SLED Single wavelength is less than-35dBm, through pole low light level grid (as 0.1% reflectivity, about-30dBm) reflection after, the power of its reflected signal, lower than-65dBm, is flooded by noise completely.LD has higher output power, pole low light level grid generation can be made far above the reflected light signal of ground unrest, but spectrum width is very narrow.Select the LD with length scanning function, both overcome the problem that pole low light level grid intensity of reflected light is low, also may be used for determined wavelength drift, thus cancel traditional wavelength demodulation device, simplify system.
When light signal is propagated within the system, maximum ground unrest is entirely with the gain noise of the reflection crosstalk of grating, Rayleigh scattering (scattered power is about 1,000,000/) and SOA.Because the 2nd SOA high-speed optical switch is burst pulse sampling, to single pulse signal incident after a SOA high-speed optical switch, sampling time puts the position of space point on strict optical fiber, the light signal of other point reflection is because the propagation delay time of space length, not within the scope of sampling pulse, do not form crosstalk, thus it is completely isolated really to realize crosstalk between grating.Rayleigh scattering can produce interference at sampled point, when optical grating reflection rate is greater than more than Rayleigh scattering several times, can ignore its impact.In addition, SOA possesses good mode competition characteristic when amplifying signal, this is highly beneficial to the grating with wavelength chooses sexual reflex, as long as the reflectivity of Bragg grating is higher than Rayleigh scattering in theory, 2nd SOA high-speed optical switch can to Bragg reflection signal actual gain, improve signal to noise ratio (S/N ratio), facilitate follow-up photodetection, therefore this invention is not high to the reflectivity requirements of Bragg grating.The present invention can replace traditional fiber grating sensing system completely, carries out the quasi-distributed measurement of vast capacity in the temperature of oil tank, oil pipeline, high voltage substation and the stress, strained situation, health status etc. of highway, bridge, heavy construction etc.
The beneficial effect of pole of the present invention weak optical fiber Bragg grating sensor-based system and querying method thereof is as follows:
1, adopt accurately time-division multiplex technology, select extremely weak reflectivity fiber grating, only need optical grating reflection rate higher than Rayleigh scattering (being about 0.0001%), conventional detection means detected reflectance signal can be adopted.Because optical grating reflection rate is extremely low, the complete same grating of more than tens thousand of and even hundreds thousand of can be inscribed on an optical fiber, build the fiber grating sensing system of vast capacity, meet the demand of emerging Internet of Things to capacity.
2, by reading pulse delay amount, can stop position being obtained through simple computation, realizing the real-time location to detecting grating and measurand.Adopt the inquiry at a high speed of first fixed wave length, then mark grating carried out to the ad hoc inquiry method of length scanning, reduce scanning light source to the impact of inquiry velocity, realize fast query.
3, Time Domain Reflectometry location technology is ripe, without mutual crosstalk between grating, and the stable performance of high speed SOA optoelectronic switch, system stability is good; For the fiber lengths change caused because of variation of ambient temperature, cause the change of stop position, system can detect automatically, not by the interference of environmental factor.
4, the inscription of extremely weak reflectivity fiber grating can obtain by directly inscribing ordinary optic fibre, carrying the special process process such as hydrogen, greatly reducing grating and inscribing cost without the need to carrying out optical fiber.SOA is both as optoelectronic switch, and carry out gain to light signal again, cost performance is high, and systems versus software algorithm requires low, and structure is simple.
Below in conjunction with drawings and Examples, the present invention is further described:
A kind of pole of the present invention weak optical fiber Bragg grating sensor-based system as shown in Figure 1, is made up of arrowband tuning source 1, modulation module 2, circulator 3, bragg grating array 4, sampling module 5, photodetector 6 and data handling machine 7; Arrowband tuning source 1 connects modulation module 2, and the light signal that modulation module 2 exports is coupled into bragg grating array 4 by circulator 3; Another port of circulator 3 connects sampling module 5; Sampling module 5 connects photodetector 6, and photodetector 6 wire is connected with data handling machine 7.
Wherein arrowband tuning source 1 exports continuous laser, and light source power is greater than 0dBm, by data handling machine 7 control realization pm level length scanning.
Modulation module is made up of a SOA high speed optoelectronic switch 8 and signal generator 9, modulates and amplify light signal, and the light impulse length that modulation exports is less than 20ns.
Sampling module by the 2nd SOA high speed optoelectronic open the light 10 and signal generator 9 form, the light pulse of reflection to be selected in time domain, and to allowing the light signal that passes through to amplify.
Bragg grating array 4 is complete in fiber grating or multi-wavelength optical fiber optical grating constitution by multiple extremely weak reflectivity (>0.0001%).
Two output ports of signal generator 9 the 8, the 2nd SOA high speed optoelectronic that opens the light with a SOA high speed optoelectronic respectively opens the light and 10 to be connected; Connect with wire between the control port of narrow spectrum tuning source 1, signal generator 9 and data handling machine 7.
The course of work of system is as follows: the continuous laser that arrowband tuning source 1 exports is after modulation module 2 is modulated, form the laser pulse signal amplified, pulse center wavelength power is at about 0dBm, the bragg grating array 4 that there is certain intervals distance is coupled into through circulator 3, each grating produces reflection successively, the light pulse signal reflected enters sampling module 5, and sampling module, according to the phase differential of setting, controls the 2nd SOA high speed optoelectronic switch opens and shutoff.When SOA opens, the reflection light pulse of process is allowed through and obtains amplification; Otherwise, reflected impulse is significantly decayed, stops light signal to pass through.Data handling machine 7 determines whether to there is Bragg reflection according to the intensity of reflected light signal, determines after Bragg reflection again according to the position of the phase difference calculating fiber grating between pulse; Photodetector 6 pairs of sampled signals carry out high speed optoelectronic conversion, and whether each corresponding grating of threshold determination according to setting exists Bragg reflection.Mark reflective light intensity is less than the grating limiting threshold value, then mark grating is fixed to the phase differential of inquiry, utilize arrowband tuning source 1 to carry out length scanning to mark grating, detect the wavelength shift of corresponding grating, the measurand of fast demodulation current raster point.
Detecting temperature below, is that example is described in detail with the time division multiplex sensor-based system that N number of complete same bragg grating (assuming that N=500, bragg wavelength 1303.3nm, reflectivity 0.01%, grating is interval 5m each other) is formed.
Time-division optical fiber grating sensing monitoring method in the present invention adopts " quasi-distributed optical fiber grating sensing " technology to carry out long-term, stable monitoring to the temperature of 500 monitoring points.
According to the structural representation of system in Fig. 2, its concrete implementation step is:
1, determine position and the distribution of temperature monitoring point: according to the concrete condition of monitoring field, determine the position of 500 monitoring points, the trend of temperature value and change thereof according to a preliminary estimate, calculate the overview that whole scene temperature distributes.
2, the 0.01% reflectivity fiber grating selecting Wuhan University of Technology's fiber optic hub to inscribe online, grating space 5m, Prague centre wavelength 1303.3nm, lay successively in selected monitoring point.
3, determine modulating pulse and sampling pulse width, sweep velocity, select the wavelength demodulation device of coupling.Co-wavelength grating space 5m, according to computing formula above, modulating pulse 20ns, sampling pulse width is set as 20ns, and grating spacings is greater than actual samples interval, can realize inquiring about without crosstalk; Calculate sensor fibre length 5*500=2500m, sampling arteries and veins light rushes width and is set smaller than 20ns, the grating required time of inquiry least significant end is 2500*4*2=20000ns, and namely single grating query time is 20us to the maximum, and the maximum query time of 500 gratings is 10ms.The speed of current arrowband tuning laser light source is about 100Hz/s, and visual inquiry speed is far above the sweep velocity of tuning source, therefore the speed of whole sensor-based system depends on the operating rate of scanning light source.Select the narrow band scanning laser instrument (1303.3+/-1nm) of sweep velocity 100Hz/s, first adopt static wavelength 1303.3nm to scan 500 gratings, to find and the time marking wavelength variations grating is about 10ms.
4, the COEFFICIENT K of optical fiber grating sensing temperature is determined.According to fixed form (stickups), the distribution mode (external, embedded) of fiber grating monitoring point at the scene, select values of factor K, and arrange in systems in which, to ensure the size conversing each point temperature value in data handling.
5, the determination of on-the-spot integrality: system initialization calibration, is scanned by 0.1 °, phase differential interval by computer control signal source after powering on, and the phase value that inquiry " study " 500 grating pairs are answered also is preserved; The operation wavelength of inquiry system default scan laser instrument is 1303.3nm, during specific works: first according to the phase information that " study " obtains, adopt static wavelength 1303.3nm to all grating periodic scannings, and sample detecting reflective light intensity signal, grating reflective light intensity being less than to threshold value marks.Then the fixing detected phase having marked grating, length scanning is carried out within the scope of 1302.3nm ~ 1304.3nm, the size of demodulation wavelength shift, thus the size of monitoring point temperature variation, data handling system carries out the computing of program, determine the state of on-the-spot overall distribution, and alerting signal is produced to ultimate limit state and automatically adjusts control signal.