CN113810099B - Optical time domain reflectometer based on asymmetric double-sideband chirped pulse modulation - Google Patents
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Abstract
The invention provides an optical time domain reflectometer based on asymmetric double-sideband chirp pulse modulation, which comprises a narrow-linewidth laser, a frequency modulation module, an erbium-doped optical fiber amplifier, an optical filter, a sensing module, a data acquisition module and a signal demodulation module, and is characterized in that: the frequency modulation module is adopted to carry out asymmetric chirp modulation on the detection light in sequence, so that sideband light with different chirp bandwidths is generated; then, modulating the asymmetric two sideband chirped light into pulses, and performing time-sharing measurement; finally, the external disturbance quantity is demodulated in a signal demodulation module by using a cross-correlation algorithm. Compared with a symmetrical double-sideband chirped pulse optical time domain reflectometer, the invention provides that asymmetric chirped pulse light is used as detection light, two times of measurement are carried out in time, and the measurement resolution and the measurement range to be measured are adjusted by adjusting the bandwidth range of the chirped pulse light, so that the function of considering both a wide-range measurement range and a high measurement resolution is realized.
Description
Technical Field
The invention belongs to the technical field of optical fiber sensing, and particularly relates to an optical time domain reflectometer based on asymmetric double-sideband chirped pulse modulation.
Background
An Optical Time Domain Reflectometer (OTDR) is a distributed optical fiber sensing system for nondestructive measurement and analysis of loss of whole optical fiber, and its principle is that the optical pulse interacts with optical fiber medium in the process of optical fiber propagation to generate backward Rayleigh scattering light, the light pulse and the echo of the scattering pulse are received at the transmitting end of the light to form an attenuation curve of the backward scattering light power, and the position of the defect point on the optical fiber is determined according to the attenuation curve of the light power.
In 1988, dakin and Lamb et al propose a double-pulse method for the first time in patent (GB 2222247A), which uses a light-splitting and frequency-shifting device to modulate a probe pulse into double-pulse signals with different front and rear frequencies and separated in time, the probe light is double-frequency double-pulse light, i.e., a pulse pair with the same pulse width and a single and different frequency, and modulates phase information at different positions in an optical fiber into an interference signal, thereby realizing measurement of a disturbance phase by a direct detection OTDR.
In 2010, roger Ian Crickmore and David John Hill disclose a distributed optical fiber sensing system based on double pulses for the first time in a patent (US 07652245B 2), on the basis of OTDR, continuous light emitted by a laser is divided into two paths, two acousto-optic modulators are respectively used for modulating two pulses with different frequencies, a delay fiber is connected to one path of the two paths, so that double pulse signals with different front and rear frequencies and separated time are formed, detection light is still double-frequency double pulse light, but phase noise is lower compared with the double pulse light.
In 2019, european and chinese, et al, the university of electronic technology, in patent (CN 201811573333.8), disclose a double-pulse fiber vibration sensing method based on a delay fiber, where a pulse modulator modulates pulsed light and then divides the modulated pulsed light into two paths, one of the two paths is added to the delay fiber to form two detection light pulses with overlapping portions, and demodulation of phase change on the sensing fiber is realized from interference signals of rayleigh scattered light of the two pulses to obtain vibration information.
Martins et al in H.F. of Spain (WO 2017093588A 1) in 2016 discloses a system based on chirping pulses and direct detection
System of
The detection light in the system is modulated into chirp single pulse light, namely within the single pulse, the frequency changes linearly along with the time, the frequency compensation is carried out on the external disturbance, and finally the back Rayleigh scattering spectrum at the disturbance position is shown in timeThe time delay phenomenon in the time domain is processed through cross correlation with a reference map when no disturbance exists, the time delay can be calculated, the disturbance is calculated, and then the quantitative measurement of the disturbance intensity is realized; the large chirp range has high requirements on hardware and high cost.
In 2019, bang-bang et al of tianjin university disclose a differential COTDR distributed acoustic sensing device and method of heterogeneous double-sideband chirped pulses in a patent (CN 1108647974), the system is based on coherent detection OTDR, an IQ modulator is used to load two different electrical signals respectively, the detection light is synchronous double-sideband chirped pulse light, that is, double-sideband single pulse light with the same chirped spectrum content and opposite slopes, and the detection light utilizes reference light to improve the sensitivity of a signal to be measured, and simultaneously improves the signal-to-noise ratio, but still has the defect that the strain measurement range is limited.
In 2021, yopenbo et al, university of guangdong industry, disclose an optical time domain reflectometer based on double-sideband chirped pulse modulation in patent (202110827977.0), which uses EOM to modulate symmetric double-sideband chirped pulses, uses two reflective FBGs to extract upper and lower sidebands respectively, adds an adjustable optical fiber delay unit in one path of the lower sideband to provide time delay, and the detected optical pulses are time-division symmetric double-sideband chirped pulse pairs, i.e., pulse pairs with the same pulse width, the same chirped spectrum content, and opposite slopes, and can double the strain measurement range by using opposite slopes, but the device structure is complicated and is not beneficial to instrumentation.
The device provides an optical time domain reflectometer based on asymmetric double-sideband chirped pulse modulation, wherein detection optical pulses are time-sharing asymmetric double-sideband chirped pulse pairs, namely pulse pairs with the same pulse width and different chirp spectrum contents. The IQ modulator is used for modulating sideband light with different chirp bandwidths in sequence, then the SOA is used for modulating two asymmetric sidebands into pulses, compared with the symmetric double-sideband chirped pulses, two times of measurement are carried out in sequence in time, and the bandwidth range of the chirped pulse light is adjusted to adjust the measurement resolution and the measurement range to be measured, so that the function of considering both a wide-range measurement range and a high measurement resolution is realized.
Disclosure of Invention
The invention aims to provide an optical time domain reflectometer based on asymmetric double-sideband chirped pulse modulation, which adopts time-sharing asymmetric double-sideband chirped pulses to measure twice in time, adjusts the measuring resolution and the measuring range to be measured by adjusting the bandwidth range of the chirped pulse light, and realizes the function of considering both a large-range measuring range and a high measuring resolution.
The purpose of the invention is realized by the following steps:
an optical time domain reflectometer based on asymmetric double-sideband chirp pulse modulation comprises seven parts, namely a narrow linewidth laser 1, a frequency modulation module 2, an erbium-doped fiber amplifier EDFA3, an optical filter 4, a sensing module 5, a data acquisition module 6 and a signal demodulation module 7; wherein, the frequency modulation module 2 comprises an IQ modulator 21, an MBC22, an AWG23, a 1: 99 coupler 24 and an SOA 27; the sensing module 5 comprises a third circulator 501 and a sensing optical fiber 502; the data acquisition module 6 comprises a photoelectric detector PD601 and a data acquisition card DAQ602; the signal demodulation module 7 is a digital signal processing system.
The single-frequency continuous light emitted by the narrow-linewidth laser 1 is modulated by a frequency modulation module 2, the IQ modulator is used for sequentially modulating sideband light with different chirp bandwidths, then the SOA is used for modulating two asymmetric sidebands into pulses to obtain time-sharing asymmetric double-sideband chirp pulses, the pulse width tau meets the condition that tau is not less than 20ns and not more than 200ns, the spatial resolution corresponding to the pulse width 200ns is 20m, if the pulse width is continuously increased, the spatial resolution of the system is gradually reduced, if the pulse width is lower than 20ns, the energy carried in one pulse is very weak, the phenomenon is not favorable for observation, and the requirement on the hardware of a signal source is very high; the sweep frequency ranges are unequal and satisfy that the absolute value delta v is more than or equal to 100MHz i (i =1, 2) | is less than or equal to 10GHz, and the larger the sweep frequency range is, the larger the detection range is; below 100M, the problem of too small a strain measurement range is likely to occur.
After the time-sharing asymmetric double-sideband chirped pulse light passes through a 1: 99 coupler, 1% of the light is injected into the MBC22 to perform feedback control on an IQ modulator to suppress carrier waves, 99% of the light is subjected to pulse modulation through an SOA27 and then is injected into an EDFA3 for amplification, then ASE noise is filtered by an optical filter 4, and then the detection light is injected into a sensing optical fiber 502 through a three-terminal circulator 501.
Backward rayleigh scattered light generated by the detection pulse light sequence in the sensing optical fiber 502 enters the photoelectric detector PD601 through the three-terminal circulator 501 and is converted into an electric signal.
The data acquisition card DAQ602 samples the electrical signal.
The signal demodulation module 7 is a signal processing system, acquires N rayleigh scattering patterns, and performs cross-correlation operation on the measured rayleigh scattering patterns and rayleigh scattering reference patterns according to a window with a certain length, the rayleigh scattering patterns at the vibration position are shifted, so that the related peak is shifted, namely, the vibration region, and the magnitude of the dependent variable is determined by the shift amount of the cross-correlation peak, namely, the magnitude of the dependent variable is determined by the shift amount of the cross-correlation peak
Wherein the content of the first and second substances,
for sweep rate, Δ t is the cross-correlation peak offset, v 0 The sideband center frequency.
The voltages loaded on the I terminal and the Q terminal of the IQ modulator 21 are respectively:
in the formula, V i (i =1,2) represents the amplitude of the electrical signal and V 1 ＞V 2 ，f 1 、f 2 Indicating the center frequencies, δ v, of the upper and lower sideband chirped pulses 1 Indicating the upper sideband swept range, δ v 2 Indicating lower sideband sweepFrequency range, τ denotes pulse width, Δ τ denotes time delay between two pairs of electrical signals, K i For sweep rate, i.e.
The sweep range of the upper sideband chirped pulse is f 0 +f 1 ，f 0 +f 1 +δv 1 ]，f 0 The sweep frequency range of the lower sideband chirp pulse is [ f ] as the carrier frequency 0 -f 2 ，f 0 -f 2 -δv 2 ]。
The rayleigh scattered electric field expressions E1 (t), E2 (t) of the upper sideband chirp 25 and the delayed lower sideband chirp 26 are respectively expressed as:
wherein, tau i Time delay required for receiving scattered light of ith scattering point, E 0 Is the incident optical electric field strength.
As shown in fig. 3, the upper sideband chirped pulse 25 and the lower sideband chirped pulse 26 with time delay form a backward rayleigh scattering signal at each scattering point in the optical fiber, rayleigh scattering electric field signals generated at each scattering point are accumulated and finally received by the detector, and since the lower sideband chirped pulse 26 has a certain time delay, the received backward rayleigh scattering E2 (t) is also larger than the scattered signal E of the upper sideband chirped pulse 1 (t) is delayed, so that the delayed backward rayleigh scattered signal 521 generated by the upper sideband chirped pulse and the delayed backward rayleigh scattered signal 522 generated by the lower sideband chirped pulse are time-shared collected at the PD 601.
The optical time domain reflectometer based on asymmetric double-sideband chirp pulse modulation has the following characteristics of backward Rayleigh scattering signals and signal demodulation:
when time-sharing asymmetric double-sideband chirped pulses are transmitted in a sensing optical fiber, each pulse forms a backward Rayleigh scattering signal at a scattering point, mutual interference is carried out between the scattering signals, interference optical signals are received as I (t) by a photoelectric detector PD601, then the I (t) is acquired by a data acquisition card DAQ602, and then cross-correlation demodulation strain is carried out;
rayleigh scattering electric field expression E of interference signal of upper sideband chirped pulse and lower sideband chirped pulse beat1，2 (t) is expressed as:
E beat1，2 (t)＝E 1 (t)+E 2 (t) (5)
is converted into I by a photoelectric detector beat1，2 (t), the expression of which is:
taking the first Rayleigh scattering pattern as a reference, performing cross-correlation calculation on the N Rayleigh scattering patterns, and determining the magnitude of strain through the shift of a correlation peak, namely
As shown in fig. 4, when there is no external disturbance on the optical fiber, the rayleigh scattering patterns are consistent, and when there is external disturbance, there is a time delay when the rayleigh scattering patterns generated by the chirp signal are relatively unstrained on the time axis, and by using the cross-correlation operation between the reference pattern and the actual pattern, the time delay offset of the correlation peak can be obtained, thereby determining the strain magnitude.
The invention has the beneficial effects that:
the detection light is time-sharing asymmetric double-sideband chirped pulse, two times of measurement are carried out successively in time, and the bandwidth range of the chirped pulse light is adjusted to adjust the measurement resolution and the measurement range to be measured, so that the function of considering both a wide-range measurement range and a high measurement resolution is realized.
The invention utilizes the chirp characteristic of the detection signal, the received signal is the superposition spectrum of the backward Rayleigh scattering signal under each frequency, the coherent fading is inhibited, simultaneously, the cross-correlation algorithm is utilized to demodulate the strain, which is equivalent to multiple averaging, the noise is reduced, and the sensitivity is improved.
Drawings
FIG. 1 is a schematic diagram of an asymmetric double-sideband chirped pulse modulation-based optical time domain reflectometer;
FIG. 2 is a schematic diagram of time-sharing asymmetric double sideband chirped pulses;
FIG. 3 is a schematic diagram of a scattering point of a detection signal in an optical fiber for generating backward Rayleigh scattering;
FIG. 4 is a diagram of a back Rayleigh scattering pattern generated by a chirped pulse;
Detailed Description
The narrow linewidth laser 1 has a center wavelength of 1550nm.
The first output end 230 of AWG and the second output end 231 of AWG repeatedly output linear chirped pulse signals with a width of 100ns to drive the IQ modulator, the third output end 233 of AWG outputs a synchronization signal to drive the SOA driving end 273, and the fourth output end 232 of AWG outputs a signal as a synchronization acquisition signal of the DAQ 602.
A pair of signals with initial phases of 0 DEG and different amplitudes are applied to an I end 212 of an IQ modulator, a pair of signals with initial phases of 90 DEG and-90 DEG are input to a Q end 213 of the IQ modulator, the two pairs of signals have the same time delay of 100ns, and single-frequency continuous light emitted by the narrow-linewidth laser 1 is modulated into upper sideband chirped light with a frequency sweep range of [1GHz,10GHz ], and lower sideband chirped light with a frequency sweep range of [1GHz,500MHz ].
The working wavelength of the 1: 99 coupler 24 is 1550nm, the 1: 99 coupler divides the asymmetric double-sideband chirped light with time delay into two paths, 1% of light enters the MBC, and the MBC performs bias control on the IQ modulator 21 to enable the IQ modulator to work in a carrier suppression working area; 99% of light enters an SOA to be subjected to pulse modulation, two asymmetric sideband chirp light is modulated into pulses, the pulse width is 100ns, then the pulse light enters an EDFA3 to be amplified, and the power threshold of detection signal light is lower than that of PD 601.
The operating bandwidth of the optical filter 4 is 10GHz.
The three-terminal coupler 501 has an operating wavelength of 1550nm.
The sensing fiber 502 is a single mode fiber, with a length of 5km, corresponding to a light transmission time of 50us to and fro.
The detector 501 is a photodetector with a bandwidth of 15GHz.
The sampling rate of the acquisition card 502 is 20GSa/s.
The detection light pulse after passing through the optical filter 4 is injected into the sensing optical fiber 502 through the three-terminal coupler 501, the detection light pulse is transmitted in the sensing optical fiber 502 and generates backward rayleigh scattering light, the backward rayleigh scattering light is injected into the PD601 through the three-terminal circulator 501, the backward rayleigh scattering light generates beat frequency light signals at the PD601, the PD601 converts the beat frequency light signals into electrical signals, and the electrical signals are acquired by the DAQ 602.
N Rayleigh scattering patterns are obtained, cross correlation operation is carried out on the measured Rayleigh scattering patterns and Rayleigh scattering reference patterns according to a window with a certain length, the Rayleigh scattering patterns at the vibration position are shifted, therefore, the relative peak is shifted, namely the vibration area, the calculation is carried out according to the formula (7), and the magnitude of the dependent variable is determined by the shift amount of the cross correlation peak.
The central wavelength of the laser is 1550nm, the sampling rate is 20GSa/s, the content of the chirp spectrum of the upper sideband is 10GHz, the content of the chirp spectrum of the lower sideband is 500MHz, the resolution of each point is about 33.3n epsilon after the Rayleigh scattering patterns of the chirp pulse of the upper sideband are subjected to cross-correlation operation, and the resolution of the chirp pulse of the lower sideband is about 1.66n epsilon.
Assuming that the size of the strain applied to the vibration region is 0.1 mu epsilon, the frequency shift amount brought by the strain is about 15MHz calculated by a formula (7), the cross-correlation time offset delta t of the large chirp spectrum content 10GHz is about 0.15ns, the time of each point corresponding to the sampling rate 20GSa/s is 0.05ns, and 3 points of offset exist after cross-correlation and can be correctly identified; the cross-correlation time offset delta t of the small chirp spectrum content 500MHz is approximately equal to 3ns, and 60 point offsets exist after cross-correlation, so that the small chirp spectrum can be correctly identified.
Assuming that the magnitude of the strain applied to the vibration region is 2 μ ∈, as calculated by formula (7), it can be known that the frequency shift Δ v =300Mhz due to the strain is far over 3% of 500Mhz, the pulse with small chirp spectrum content cannot be compensated, the cross-correlation time offset Δ t with large chirp spectrum content of 10GHz is approximately 3ns, and the time corresponding to each point at the sampling rate of 20GSa/s is 0.05ns, then 60 point offsets exist after cross-correlation, and can be correctly identified. The strain applied to the vibration region is assumed to be 10n epsilon, the strain resolution of the large chirp pulse is 33.3n epsilon and is far larger than the applied strain, a real strain value cannot be calculated through cross-correlation, and the cross-correlation time offset delta t of the small chirp spectrum content 500MHz is approximately equal to 0.3ns, so that the offset of 6 points exists after the cross-correlation, and the offset can be correctly identified.
Due to the limitation of a cross-correlation algorithm, for large strain, a rayleigh scattering pattern detected by a pulse with a small chirp spectrum content is subjected to cross-correlation, time shift is too large, cross-correlation results are wrong due to existence of interference peaks, and a wrong strain value is demodulated, so that only the pulse with the large chirp content can be correctly detected; for small strain, if the applied strain is smaller than the strain resolution, the true strain value cannot be identified, and the higher the resolution of the pulse with small chirp spectrum content is, the higher the detection precision is. The detection light of the device is a time-sharing asymmetric double-sideband chirped pulse pair, time-sharing measurement is carried out, two Rayleigh scattering patterns can be separated from a time domain, a cross-correlation curve with two strain resolutions exists, and the function of considering both a wide-range measurement range and a high measurement resolution is realized by adjusting the bandwidth range of the chirped pulse light.
Claims (4)
1. The utility model provides an optical time domain reflectometer based on asymmetric double sideband chirp pulse modulation, includes narrow linewidth laser (1), frequency modulation module (2), mixes bait fiber amplifier EDFA (3), optical filter (4), sensing module (5), data acquisition module (6) and signal demodulation module (7), its characterized in that:
1) The narrow linewidth laser (1) is connected with the input end of the frequency modulation module (2), the output end of the frequency modulation module (2) is connected with the input end (30) of the erbium-doped fiber amplifier EDFA, the output end (31) of the erbium-doped fiber amplifier EDFA is connected with the input end (40) of the optical filter, the output end (41) of the optical filter is connected with the input end of the sensing module (5), the output end of the sensing module (5) is connected with the input end of the data acquisition module (6), and the output end of the data acquisition module (6) is connected with the signal demodulation module (7);
2) The frequency modulation module (2) consists of an IQ modulator (21), a modulator bias controller MBC (22), an arbitrary signal generator AWG (23), a 1: 99 coupler (24) and a semiconductor optical amplifier SOA (27); the narrow-linewidth laser output end (10) is connected with a first input end (210) of an IQ modulator, the output end (211) of the IQ modulator is connected with an input end (240) of a 1: 99 coupler, a second output end (242) of the 1: 99 coupler is connected with an input end (270) of a SOA of a semiconductor optical amplifier, meanwhile, a first output end (230) of the AWG is connected with an I end (212) of the IQ modulator, a second output end (231) of the AWG is connected with a Q end (213) of the IQ modulator, a third output end (233) of the AWG is connected with a SOA drive end (273) of the semiconductor optical amplifier, a first output end (241) of the 1: 99 coupler is connected with an MBC input end (220), a first output end (221) of the MBC is connected with a second input end (214) of the IQ modulator, and an output end (271) of the SOA of the semiconductor optical amplifier is connected with an EDFA input end (30) of the erbium-doped optical fiber amplifier; 3) The single-frequency continuous light output by the narrow-linewidth laser (1) is modulated into time-sharing asymmetric double-sideband chirped pulse light through the frequency modulation module (2), namely the upper sideband chirped pulse spectral content delta v 1 Chirp pulse spectrum content delta v with lower sideband 2 The two-dimensional chirp pulse length is not equal, a delay delta tau is arranged between the upper sideband chirp pulse (25) and the lower sideband chirp pulse (26), and the delay width delta tau is equal to the pulse width tau; the IQ modulator (21) is a MZM type modulator, two groups of electric signals are loaded to the IQ modulator (21) respectively by the time delay of the interval delta tau, the first group of electric signals is that the initial phase of the I end (212) of the IQ modulator is input to be 0 DEG, and the amplitude of the I end is input to be V 1 The signal of (2), an initial phase of 90 DEG and an amplitude of V are inputted to a Q terminal (213) of the IQ modulator 1 A first group of signals loaded on the I terminal (212) with a phase of 0 DEG and an amplitude of V 1 Phase 90 DEG and amplitude V on signal sum Q terminal (213) 1 The signals are synchronized, and the second group of electric signals are input with an initial phase of 0 DEG and an amplitude of V at an I end (212) of the IQ modulator 2 The signal of (2) is inputted with an initial phase of-90 DEG and an amplitude V at a Q terminal (213) of the IQ modulator 2 A second set of signals loaded on the I terminal (212) with a phase of 0 DEG and an amplitude of V 2 Phase-90 DEG amplitude V at signal sum Q terminal (213) 2 The signals are synchronous, the second group of electric signals have time delay delta tau relative to the first group of electric signals, the IQ modulator (21) suppresses the carrier wave through the modulator bias controller MBC (22), the frequency sweep ranges are unequal, and the absolute value delta v of 100MHz is more than or equal to i (i =1, 2) | 10GHz or less of upper sideband chirped light and lower sideband chirped light, and a time delay delta tau exists between the upper sideband chirped light and the lower sideband chirped light; the semiconductor optical amplifier SOA (27) modulates two asymmetric sideband chirped lights into an upper sideband chirped pulse (25) and a lower sideband chirped pulse (26), and the pulse width is more than or equal to 20ns and less than or equal to tau and less than or equal to 200ns.
2. An asymmetric double sideband chirped pulse modulation based optical time domain reflectometry according to claim 1, characterized in that the sensing module (5) further comprises:
the sensing module (5) consists of a three-terminal circulator (501) and a sensing optical fiber (502); the output end (41) of the optical filter is connected with a first end (501 a) of the three-end circulator, a second end (501 b) of the three-end circulator is connected with a sensing optical fiber (502), the detection light generates a backward Rayleigh scattering signal in the sensing optical fiber (502), and the backward Rayleigh scattering signal is injected into a photoelectric detector PD (601) through a third end (501 c) of the third circulator to carry out signal interference and photoelectric conversion.
3. An asymmetric double-sideband chirped pulse modulation-based optical time domain reflectometer according to claim 1, characterized in that the data acquisition module (6) further comprises:
the data acquisition module (6) comprises a photoelectric detector PD (601) and a data acquisition card DAQ (602); the PD output end (601 b) of the photoelectric detector is connected with the DAQ input end (602 b) of the data acquisition card, meanwhile, the AWG fourth output end (232) in the frequency modulation module (2) is connected with the DAQ drive end (602 a) of the data acquisition card, and the DAQ output end (602 c) of the data acquisition card transmits data to an upper computer and enters the signal demodulation module (7).
4. An asymmetric double sideband chirped pulse modulation based optical time domain reflectometry according to claim 1, characterized by signal demodulation module (7), further comprising:
the signal demodulation module (7) is a digital signal processing system, obtains N Rayleigh scattering patterns from the system, takes the length of a cross-correlation window as tau, selects a first Rayleigh scattering pattern from a reference pattern, performs cross-correlation operation on the subsequent Rayleigh scattering patterns and the reference pattern to obtain the time offset of the vibration position of the reference pattern, and calculates the strain according to the time offset.
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