Optical time domain reflectometer based on ring structure modulation and working method thereof

Abstract

The invention discloses an optical time domain reflectometer based on ring structure modulation and a working method thereof, and relates to the technical field of optics. The optical time domain reflectometer comprises a laser, an optical fiber coupler, a semiconductor optical amplifier, an acoustic optical modulator, an optical isolator, a delay optical fiber, an erbium-doped optical fiber amplifier, a dense wavelength division multiplexer, a circulator, a Raman laser, piezoelectric ceramics, an arbitrary function generator, an arbitrary waveform generator, a polarization controller, a photoelectric balance detector, a data acquisition card, a computer and the like. Multi-frequency modulation is carried out based on an annular structure, the multi-frequency modulation range is only limited by the bandwidth of a detector and the sampling rate of an acquisition card, a pulse sequence is generated through modulation of the optical annular structure, dependence on the bandwidth performance of a microwave signal source and a modulator is reduced, and the bandwidth is improved; and by injecting the Raman light, the sensing distance is also increased, the equipment cost is effectively controlled, and meanwhile, a new scheme is provided for time sequence pulse sequence modulation.

Description

Technical Field

[0001] The present invention relates to the field of optical technologies, and particularly to an optical time domain reflectometer based on ring structure modulation and a working method thereof. Background Art

[0002] The phase-sensitive optical time domain reflectometer has a high response speed in the order of milliseconds or even sub-milliseconds, and is widely used in fields such as perimeter intrusion monitoring, seismic wave monitoring, and structural health detection. In the phase-sensitive optical time domain reflectometer, in order to avoid mutual aliasing between detection optical pulses, the repetition period of the detection optical pulses should be greater than or equal to the time required for a single optical pulse signal to propagate back and forth in the sensing optical fiber once. According to the Nyquist sampling theorem, the maximum response bandwidth of the system is half of the repetition frequency of the detection optical pulses. The phase-sensitive optical time domain reflectometer generally has a problem that the response bandwidth and the sensing distance restrict each other. At present, the main method to solve the mutual restriction between the response bandwidth and the sensing distance of the system is time-division frequency multiplexing. People have been working hard to break the restriction relationship between the response bandwidth and the sensing distance of the phase-sensitive optical time domain reflectometer (Φ-OTDR) system, that is, to increase the distance-bandwidth product.

[0003] In reference [1], a Mach-Zehnder interferometer (MZI) is combined with Φ-OTDR to obtain a response bandwidth of 3 MHz on a 1064 m sensing optical fiber; in reference [2], a Φ-OTDR and MZI hybrid sensing system based on multi-core fiber spatial division multiplexing is proposed and demonstrated, which has a wide vibration frequency response range and a high spatial resolution. The remote sensing distance is 2.42 km, the spatial resolution is 1 m, and the vibration sensing is up to 12 kHz; in reference [3], by multiplexing four different frequencies in Φ-OTDR, the response bandwidth of the system is increased to 20 kHz on a 10 km sensing optical fiber; reference [4] demonstrates an effective new method to expand the frequency response range by increasing the time interval between discrete sampling points in one measurement. In the experiment, two pulses carried by different waves with a short time delay of length are injected into the sensing. The experimental results show the frequency-expanded response range. This method allows breaking this trade-off between the sensing optical fiber length and the frequency response range. Reference [5] proposes a phase-sensitive optical time domain reflectometer (Φ-OTDR) using a time-sequential multi-frequency (TSMF) source. This technology can improve the detection bandwidth of the system without reducing the sensing range. The experimental results show that within a range of 9.6 km, the detection bandwidth can reach 0.5 MHz.

[0004] However, the above traditional time-division frequency multiplexing scheme has the disadvantage that it needs to rely on the bandwidth performance of microwave signal sources and modulators, which will result in a relatively high equipment cost. Summary of the Invention

[0005] To this end, the present invention proposes an optical time domain reflectometer based on ring structure modulation and its working method to solve the problem of mutual restriction between the system response bandwidth and the sensing distance.

[0006] According to one aspect of the present invention, there is provided an optical time domain reflectometer based on ring structure modulation, which includes: a laser 1, a first optical fiber coupler 2, a semiconductor optical amplifier 3, a first acousto-optic modulator 4, a first optical isolator 5, a second optical fiber coupler 6, a delay optical fiber 7, a first erbium-doped optical fiber amplifier 8, a second acousto-optic modulator 9, a second optical isolator 10, a second erbium-doped optical fiber amplifier 11, a first dense wavelength division multiplexer 12, a circulator 13, a second dense wavelength division multiplexer 14, a Raman laser 15, a piezoelectric ceramic 16, an arbitrary function generator 17, a third erbium-doped optical fiber amplifier 18, a third dense wavelength division multiplexer 19, an arbitrary waveform generator 20, a polarization controller 21, a third optical fiber coupler 22, a photoelectric balanced detector 23, a data acquisition card 24, and a computer 25; wherein:

[0007] The optical signal output end of the laser 1 is connected to the optical signal input end of the first optical fiber coupler 2, and the two optical signal output ends of the first optical fiber coupler 2 are respectively connected to the input ends of the semiconductor optical amplifier 3 and the polarization controller 21;

[0008] The output end of the semiconductor optical amplifier 3 is connected to the input end of the first acousto-optic modulator 4, the output end of the first acousto-optic modulator 4 is connected to the input end of the first optical isolator 5, the output end of the first optical isolator 5 is connected to one end of the ring structure, the other end of the ring structure is connected to the input end of the second erbium-doped optical fiber amplifier 11, the output end of the second erbium-doped optical fiber amplifier 11 is connected to the input end of the first dense wavelength division multiplexer 12, the output end of the first dense wavelength division multiplexer 12 is connected to the 1 port of the circulator 13, the 2 port of the circulator 13 is connected to the input end of the second dense wavelength division multiplexer 14, the other input end of the second dense wavelength division multiplexer 14 is connected to the Raman laser 15, and the output end of the second dense wavelength division multiplexer 14 is connected to the optical fiber under test; the output end of the optical fiber under test is connected to the input end of the piezoelectric ceramic 16, and the output end of the arbitrary function generator 17 is connected to the input end of the piezoelectric ceramic 16;

[0009] The 3 port of the circulator 13 is connected to the input end of the third erbium-doped optical fiber amplifier 18, the output end of the third erbium-doped optical fiber amplifier 18 is connected to the input end of the third dense wavelength division multiplexer 19, and the output end of the third dense wavelength division multiplexer 19 is connected to the 1 port of the third optical fiber coupler 22;

[0010] The output end of the polarization controller 21 is connected to port 2 of the third optical fiber coupler 22. The 3rd and 4th ports of the third optical fiber coupler 22 are connected to the input end of the opto-electric balanced detector 23. The output end of the opto-electric balanced detector 23 is connected to the input end of the data acquisition card 24. The output end of the data acquisition card 24 is connected to the computer 25;

[0011] The output ends of the arbitrary waveform generator 20 are respectively connected to the semiconductor optical amplifier 3, the first acousto-optic modulator 4, the second acousto-optic modulator 9, and the input end of the data acquisition card 24.

[0012] Further, the ring structure is composed of a second optical fiber coupler 6, a delay optical fiber 7, a first erbium-doped optical fiber amplifier 8, a second acousto-optic modulator 9, and a second optical isolator 10. Among them, the output end of the first optical isolator 5 is communicated with the 1st optical signal input end of the second optical fiber coupler 6. The 4th optical signal output end of the second optical fiber coupler 6 is connected to one end of the delay optical fiber 7. The other end of the delay optical fiber 7 is connected to the input end of the first erbium-doped optical fiber amplifier 8. The output end of the first erbium-doped optical fiber amplifier 8 is connected to the input end of the second acousto-optic modulator 9. The output end of the second acousto-optic modulator 9 is connected to the input end of the second optical isolator 10. The output end of the second optical isolator 10 is connected to the 2nd optical signal input end of the second optical fiber coupler 6. The 3rd optical signal output end of the second optical fiber coupler 6 is connected to the input end of the second erbium-doped optical fiber amplifier 11.

[0013] Further, the output power of the laser 1 is 15 mW, and the output wavelength is 1550 nm.

[0014] Further, the first optical fiber coupler 2 is a 90:10 1×2 coupler, the second optical fiber coupler 6 is a 50:50 2×2 coupler, and the third optical fiber coupler 22 is a 50:50 2×2 coupler.

[0015] Further, the extinction ratio of the semiconductor optical amplifier 3 is 52 dB; the output power of the Raman laser 15 is 15 mW, and the output wavelength is 1480 nm; the detection bandwidth of the opto-electric balanced detector 23 is 1 GHz.

[0016] Further, the carrier frequency of the first acousto-optic modulator 4 is 100 MHz, the frequency shift of the second acousto-optic modulator 9 is 40 MHz, and the extinction ratio is 48 dB; the working bands of the first dense wavelength division multiplexer 12, the second dense wavelength division multiplexer 14, and the third dense wavelength division multiplexer 19 are all 1480 nm and 1550 nm.

[0017] According to another aspect of the present invention, a working method of an optical time domain reflectometer based on ring structure modulation is proposed. The working method is implemented based on an optical time domain reflectometer with a ring structure modulation. The optical time domain reflectometer includes: a laser 1, a first optical fiber coupler 2, a semiconductor optical amplifier 3, a first acousto-optic modulator 4, a first optical isolator 5, a second optical fiber coupler 6, a delay optical fiber 7, a first erbium-doped fiber amplifier 8, a second acousto-optic modulator 9, a second optical isolator 10, a second erbium-doped fiber amplifier 11, a first dense wavelength division multiplexer 12, a circulator 13, a second dense wavelength division multiplexer 14, a Raman laser 15, a piezoelectric ceramic 16, an arbitrary function generator 17, a third erbium-doped fiber amplifier 18, a third dense wavelength division multiplexer 19, an arbitrary waveform generator 20, a polarization controller 21, a third optical fiber coupler 22, a photoelectric balanced detector 23, a data acquisition card 24, and a computer 25. The working method includes:

[0018] The continuous light output by the laser 1 is divided into two upper and lower branches by the first optical fiber coupler 2. The continuous light in the upper branch is modulated into pulsed light with a high extinction ratio by the semiconductor optical amplifier 3 and the first acousto-optic modulator 4. After entering the first optical isolator 5 and the second optical fiber coupler 6, it is divided into two paths again: the light in the upper branch directly enters the second erbium-doped fiber amplifier 11, and the light in the lower branch enters the ring structure for frequency modulation. The modulated light also enters the second erbium-doped fiber amplifier 11 for optical amplification, and then the spontaneous emission noise is filtered by the first dense wavelength division multiplexer 12. Then it is injected into the second dense wavelength division multiplexer 14 through the circulator 13. The Raman laser 15 injects Raman light into the second dense wavelength division multiplexer 14 as pump light for amplifying the power of the probe light. The Raman light and the probe light are injected into the fiber under test together, and the arbitrary function generator 17 continuously transmits vibrations by controlling the piezoelectric ceramic 16.

[0019] The backward Rayleigh scattering optical signal generated by the fiber under test is injected into the third erbium-doped fiber amplifier 18 through the second dense wavelength division multiplexer 14 and the circulator 13 for power amplification, and then enters the third dense wavelength division multiplexer 19 to filter out the spontaneous emission noise.

[0020] The continuous light in the lower branch separated by the first optical fiber coupler 2 is used as the eigen light after the polarization state is adjusted by the polarization controller 21. It enters the third optical fiber coupler 22 together with the backward Rayleigh scattering light filtered by the third dense wavelength division multiplexer 19 for coherent beat frequency, and is photoelectrically converted by the photoelectric balanced detector 23. Finally, the data acquisition card 24 samples and records the multi-frequency beat frequency optical signal, and inputs it into the computer 25 for data processing.

[0021] The arbitrary waveform generator 20 is used to control the pulse width of the semiconductor optical amplifier 3, the multi-frequency modulation of the first acousto-optic modulator 4, the chopping modulation and frequency shift of the second acousto-optic modulator 9, and the acquisition instruction input of the data acquisition card 24.

[0022] Further, the process of modulating the loop structure includes: the delay optical fiber 7 is used for time delay; the first erbium-doped fiber amplifier 8 is used to compensate for the splitting loss of the second fiber coupler 6 and the insertion loss of the second acousto-optic modulator 9. The second acousto-optic modulator 9 is used to perform frequency modulation on the pulsed light in the loop again. The second optical isolator 10 is used to block the light that is reversely transmitted in the loop. The pulsed light with a high extinction ratio is divided into two paths again by the second fiber coupler 6 every time it is transmitted in the loop structure. As the number of cycles increases, a pulsed sequence with different time delays and frequencies is formed.

[0023] Further, the number of pulses in the timing pulse sequence generated by modulating the loop structure is calculated in the following two ways:

[0024] When the period T of the pulsed light modulated by the first acousto-optic modulator 4 is used as the modulation reference, the calculation expression for the number of pulses M in the timing pulse sequence generated by modulating the loop structure is:

[0025]

[0026] In the formula, represents rounding down; c represents the speed, T represents the period of the pulsed light, n represents the number of times the pulsed light winds around the loop, and L R represents the length of the loop structure;

[0027] When the length L of the optical fiber to be measured is used as the modulation reference, the calculation expression for the number of pulses M in the timing pulse sequence generated by modulating the loop structure is:

[0028]

[0029] In the formula, represents rounding up; L represents the length of the optical fiber to be measured, and L R represents the length of the loop structure.

[0030] Further, the powers of the Raman light and the probe light are obtained by the following formula:

[0031]

[0032] where z represents the transmission distance of the optical fiber to be measured; P s represents the probe light power; P p represents the Raman pump light power; g eff represents the effective Raman gain coefficient; α s represents the attenuation coefficient of the probe light during transmission; ξ represents the transmission direction of the pump light and the probe light, 1 for the same direction and -1 for the opposite direction; α p represents the attenuation coefficient of the pump light during transmission; v p represents the transmission rate of the Raman light; vs Indicates the transmission rate of the probe light.

[0033] The beneficial technical effects of the present invention are as follows:

[0034] The present invention provides an optical time domain reflectometer based on ring structure modulation and its working method. Compared with modulation schemes based on multiple acousto-optic modulators, multi-light source multiplexing, or phase modulators, the present invention performs multi-frequency modulation based on a single ring structure. The multi-frequency modulation range is only limited by the detector bandwidth and the sampling rate of the acquisition card. Pulse sequences are generated by using optical ring structure modulation, which greatly reduces the dependence on the bandwidth performance of microwave signal sources and modulators. It not only improves the bandwidth but also increases the sensing distance; effectively controls the equipment cost, and at the same time provides a new scheme for the modulation of time-sequence pulse sequences. Description of the Drawings

[0035] By referring to the detailed description below with reference to the accompanying drawings, the above and other objects, features, and advantages of the exemplary embodiments of the present invention will become readily understood. In the drawings, several embodiments of the present invention are shown in an exemplary rather than restrictive manner, where:

[0036] Figure 1 is a schematic structural diagram of an optical time domain reflectometer based on ring structure modulation according to an embodiment of the present invention.

[0037] Figure 2 is an exemplary diagram of a time-sequence frequency division multiplexed microwave signal in an embodiment of the present invention. Detailed Embodiments

[0038] The principles and spirit of the present invention will be described below with reference to several exemplary embodiments. It should be understood that these embodiments are provided only to enable those skilled in the art to better understand and then implement the present invention, and do not limit the scope of the present invention in any way. On the contrary, these embodiments are provided to make the present disclosure more thorough and complete, and to be able to fully convey the scope of the present disclosure to those skilled in the art.

[0039] To solve the problem that the injection frequency of the pulsed probe light and the length of the sensing optical fiber restrict each other, resulting in the limited maximum detectable frequency response range and sensing distance of the system to vibration signals, an embodiment of the present invention proposes a high-bandwidth and long-distance optical time domain reflectometer based on ring structure modulation, as Figure 1As shown in the figure, the optical time domain reflectometer includes a laser 1, a first fiber coupler 2, a semiconductor optical amplifier 3, a first acousto-optic modulator 4, a first optical isolator 5, a second fiber coupler 6, a delay fiber 7, a first erbium-doped fiber amplifier 8, a second acousto-optic modulator 9, a second optical isolator 10, a second erbium-doped fiber amplifier 11, a first dense wavelength division multiplexer 12, a circulator 13, a second dense wavelength division multiplexer 14, a Raman laser 15, a piezoelectric ceramic 16, an arbitrary function generator 17, a third erbium-doped fiber amplifier 18, a third dense wavelength division multiplexer 19, an arbitrary waveform generator 20, a polarization controller 21, a third fiber coupler 22, a photoelectric balanced detector 23, a data acquisition card 24, and a computer 25.

[0040] Among them, the optical signal output end of the laser 1 is connected to the optical signal input end of the first fiber coupler 2. The two optical signal output ends of the first fiber coupler 2 are respectively connected to the input ends of the semiconductor optical amplifier 3 and the polarization controller 21. The output end of the semiconductor optical amplifier 3 is connected to the input end of the first acousto-optic modulator 4. The output end of the first acousto-optic modulator 4 is connected to the input end of the first optical isolator 5. The output end of the first optical isolator 5 is connected to one end of a ring structure, and the other end of the ring structure is connected to the input end of the second erbium-doped fiber amplifier 11. The ring structure is composed of a second fiber coupler 6, a delay fiber 7, a first erbium-doped fiber amplifier 8, a second acousto-optic modulator 9, and a second optical isolator 10. Among them, the output end of the first optical isolator 5 is connected to the No. 1 optical signal input end of the second fiber coupler 6. The No. 4 optical signal output end of the second fiber coupler 6 is connected to one end of the delay fiber 7. The other end of the delay fiber 7 is connected to the input end of the first erbium-doped fiber amplifier 8. The output end of the first erbium-doped fiber amplifier 8 is connected to the input end of the second acousto-optic modulator 9. The output end of the second acousto-optic modulator 9 is connected to the input end of the second optical isolator 10. The output end of the second optical isolator 10 is connected to the No. 2 optical signal input end of the second fiber coupler 6. The No. 3 optical signal output end of the second fiber coupler 6 is connected to the input end of the second erbium-doped fiber amplifier 11.

[0041] The output end of the second erbium-doped fiber amplifier 11 is connected to the input end of the first dense wavelength division multiplexer 12. The output end of the first dense wavelength division multiplexer 12 is connected to the 1 port of the circulator 13. The 2 port of the circulator 13 is connected to the input end of the second dense wavelength division multiplexer 14. The other input end of the second dense wavelength division multiplexer 14 is connected to the Raman laser 15. The output end of the second dense wavelength division multiplexer 14 is connected to the optical fiber to be measured for sensing. The output end of the optical fiber to be measured for sensing is connected to the input end of the piezoelectric ceramic 16. The 3 port of the circulator 13 is connected to the input end of the third erbium-doped fiber amplifier 18. The output end of the arbitrary function generator 17 is connected to the input end of the piezoelectric ceramic 16.

[0042] The output end of the third erbium-doped fiber amplifier 18 is connected to the input end of the third dense wavelength division multiplexer 19, and the output end of the third dense wavelength division multiplexer 19 is connected to port 1 of the third fiber coupler 22; the output end of the polarization controller 21 is connected to port 2 of the third fiber coupler 22, ports 3 and 4 of the third fiber coupler 22 are connected to the input end of the optoelectronic balanced detector 23, the output end of the optoelectronic balanced detector 23 is connected to the input end of the data acquisition card 24, and the output end of the data acquisition card 24 is connected to the computer 25;

[0043] The output end of the arbitrary waveform generator 20 is respectively connected to the input ends of the semiconductor optical amplifier 3, the first acousto-optic modulator 4, the second acousto-optic modulator 9, and the data acquisition card 24.

[0044] In this embodiment, preferably, the laser 1 uses a single-frequency narrow-linewidth fiber laser, with an output power of 15 mW and an output wavelength of 1550 nm. The first fiber coupler 2 is a 90:10 1×2 coupler, the second fiber coupler 6 is a 50:50 1×2 coupler, and the third fiber coupler 23 is a 50:50 2×2 coupler. The carrier frequency of the first acousto-optic modulator 4 is 100 MHz; the frequency shift of the second acousto-optic modulator 9 is 40 MHz, and the extinction ratio is 52 dB. The working bands of the first dense wavelength division multiplexer 12 and the second dense wavelength division multiplexer 14 are 1480 / 1550 nm, and the detection bandwidth of the optoelectronic balanced detector 24 is 1 GHz.

[0045] Another embodiment of the present invention proposes a working method of an optical time domain reflectometer based on ring structure modulation. The working method is implemented based on the above-mentioned optical time domain reflectometer, and the working method includes:

[0046] The continuous light output by the laser 1 is divided into upper and lower branches by the first fiber coupler 2. The continuous light in the upper branch is modulated into pulsed light with a high extinction ratio by the semiconductor optical amplifier 3 and the first acousto-optic modulator 4, and then enters the first optical isolator 5. The first optical isolator 5 is used to prevent the scattered light in front from returning to damage the optical devices, playing the role of optical isolation. Subsequently, it enters the second fiber coupler 6 and is divided into two paths again. The light in the upper branch (port 3) is optically amplified by the second erbium-doped fiber amplifier 11 and then passes through the first dense wavelength division multiplexer 12 to filter out the spontaneous emission noise introduced by the second erbium-doped fiber amplifier 11. The light in the lower branch (port 4) enters the loop structure composed of the delay fiber 7, the first erbium-doped fiber amplifier 8, the second acousto-optic modulator 9, the second optical isolator 10 and the second fiber coupler 6. Among them, the delay fiber 7 is used as a fiber delay line to introduce an accurate time delay. The first erbium-doped fiber amplifier 8 is used to compensate for the splitting loss of the second fiber coupler 6 and the insertion loss of the second acousto-optic modulator 9. The second acousto-optic modulator 9 is used to perform frequency modulation on the pulsed light in the loop again. The second optical isolator 10 is used to prevent the light from transmitting in the reverse direction in the loop. The pulsed light with a high extinction ratio is divided into two paths again by the second fiber coupler 6 every time it transmits in the loop. As the number of cycles increases, a pulsed sequence with different time delays and frequencies is formed. Then it enters the second erbium-doped fiber amplifier 11, and finally enters the first dense wavelength division multiplexer 12 to filter out the spontaneous emission noise introduced by the second erbium-doped fiber amplifier 11.

[0047] It is injected into the second dense wavelength division multiplexer 14 through the circulator 13, and the Raman laser 15 injects Raman light at 1480 nm into the second dense wavelength division multiplexer 14. The Raman light at 1480 nm and the probe light at 1550 nm are injected into the fiber optic sensor to be measured together. The Raman light at 1480 nm transfers energy to the probe light at 1550 nm, achieving the purpose of amplifying the probe light and making the sensing distance longer.

[0048] The arbitrary function generator 17 continuously transmits vibrations by controlling the piezoelectric ceramic 16. The backward Rayleigh scattered light signal generated by the non-uniform distribution of the refractive index of the medium in the fiber optic sensor to be measured is injected into the third erbium-doped fiber amplifier 18 for power amplification through the second dense wavelength division multiplexer 14 and the circulator 13, and then enters the third dense wavelength division multiplexer 19 to filter out the spontaneous emission noise.

[0049] The continuous light in the lower branch split by the first fiber coupler 2 is adjusted in polarization state by the polarization controller 21 and used as the eigenlight. It undergoes coherent beat frequency with the backward Rayleigh scattered light filtered by the third dense wavelength division multiplexer 19 through the third fiber coupler 22, and is then photoelectrically converted by the photoelectric balanced detector 23. Finally, the data acquisition card 24 samples and records the multi-frequency beat frequency optical signal. The computer 25 is used for data processing and extracts the phase information through frequency division multiplexing.

[0050] The pulse width of the semiconductor optical amplifier 3, the multi-frequency modulation of the first acousto-optic modulator 4, and the chopping modulation and frequency shift of the second acousto-optic modulator 9, as well as the input of the acquisition command of the data acquisition card 24, are all synchronously controlled by the arbitrary waveform generator 20.

[0051] In this embodiment, preferably, the number of pulses in the timing pulse sequence generated by the above annular structure modulation can be calculated in the following two ways:

[0052] 1) When taking the pulse light period T with high extinction ratio modulated by the first acousto-optic modulator 4 as the modulation reference, the calculation formula for the number of pulses M in the timing pulse sequence generated by the annular structure modulation is:

[0053]

[0054] In the formula, denotes rounding down; c represents the speed, T represents the pulse light period, n represents the number of times the pulse light winds around the ring, and L R represents the length of the annular structure.

[0055] At this time, the time interval between the first M - 1 pulse lights is all The time interval between the (M - 1)-th and the M-th pulse lights is The frequency modulation period of the second acousto-optic modulator 9 is synchronized with the first acousto-optic modulator 4 as T, and the light passing time At this time, the timing frequency division multiplexing Φ-OTDR system conducts non-uniform dense detection on the vibration events along the sensing optical fiber.

[0056] 2) When taking the length L of the sensing optical fiber to be measured as the modulation reference, the calculation formula for the number of pulses M in the timing pulse sequence generated by the annular structure modulation is:

[0057]

[0058] In the formula, denotes rounding up; L represents the optical fiber length, and L R represents the length of the annular structure.

[0059] At this time, the period of the high extinction ratio pulse light modulated by the first acousto-optic modulator 4 is nML R / c, the frequency modulation period of the second acousto-optic modulator 9 is synchronized with the first acousto-optic modulator 4 as nML R / c, and the light passing time τ R ∈(n(L - L R ) / c, nL / c); at this time, the timing frequency division multiplexing Φ-OTDR system conducts uniform dense detection on the vibration events along the sensing optical fiber.

[0060] In summary, as Figure 2As shown, the timing pulse sequence modulated by the ring structure can increase the distance-bandwidth product of the Φ-OTDR system by M times, and the value of M depends on L R ; The initial frequency of the continuous light is f0, and every time it passes through the first acousto-optic modulator 4 (AOM1 in the figure), the frequency increases by Δf AOM1 (Δf AOM1 is the carrier frequency of the first acousto-optic modulator 4); Every time it passes through the second acousto-optic modulator 9 (AOM2 in the figure), the frequency increases by Δf AOM2 (Δf AOM2 is the carrier frequency of the second acousto-optic modulator 9), so the frequency modulation range is:

[0061] [f0 + Δf AOM1 + Δf AOM2 , f0 + Δf AOM1 + MΔf AOM2

[0062] It can be seen that it depends on the carrier frequencies of the first acousto-optic modulator 4 and the second acousto-optic modulator 9, rather than depending on the microwave signal source and the modulator. The use of the ring structure reduces the dependence on the bandwidth performance of the microwave signal source and the modulator, increases the bandwidth, and effectively controls the experimental cost. Further, Raman light is injected in front of the fiber to be measured, and the power of the Raman light is transferred to the probe light in the early stage, improving the sensing distance.

[0063] In this embodiment, preferably, injecting Raman light to extend the fiber sensing distance. In the case of a single pump wavelength and with the amplification of the Raman amplifier, the powers of the probe light and the pump light of the signal can be obtained from the following coupling equation:

[0064]

[0065] Where P s is the power of the probe light, P p is the power of the Raman light pump; α s is the attenuation coefficient of the probe light during transmission, α p is the attenuation coefficient of the pump light during transmission, g eff is the effective Raman gain coefficient, and its typical value can be obtained by looking up the table; ξ is the transmission direction of the pump light and the probe light, 1 for the same direction and -1 for the opposite direction; z is the transmission distance of the fiber; v p represents the transmission rate of the Raman light; v s represents the transmission rate of the probe light.

[0066] ​The above coupling equations generally cannot obtain analytical solutions, and numerical solutions of the signals need to be obtained through calculations. However, considering that fiber Raman amplifiers generally operate under small-signal conditions, in the small-signal approximation, the pump consumption caused by signal amplification can be ignored, that is, the first term in the following formula above. At this time, the analytical solution of this coupling equation can be obtained, and this analytical solution can also be used to preliminarily analyze the amplification effect of the Raman amplifier in simulation to guide the setting of the amplification parameters of the system. The analytical solution obtained under the small-signal approximation condition can be expressed by the following formula:

[0067]

[0068] where P0 is the total Raman pump power, and r f is the ratio of the forward Raman pump power to the total Raman pump power; P s (0) represents the initial power of the probe light.

[0069] The present invention proposes a high-bandwidth and long-distance optical time-domain reflectometer based on ring structure modulation. A pulse sequence with different frequencies and time delays is generated by modulating an optical ring modulation structure and injected into the sensing fiber, which greatly reduces the dependence on the bandwidth performance of the microwave signal source and the modulator. By injecting Raman light, the sensing distance is also increased, effectively controlling the experimental cost, and at the same time providing a new scheme for modulating the time-sequence pulse sequence.

[0070] Although the spirit and principles of the present invention have been described with reference to several specific embodiments, it should be understood that the present invention is not limited to the specific embodiments disclosed, and the division of each aspect does not mean that the features in these aspects cannot be combined for benefit. This division is only for the convenience of expression. The present invention aims to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.

[0071] The documents cited in the present invention are as follows:

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[0076] [5]Wang Z,Pan Z,Fang Z,et al.Ultra-broadband phase-sensitive opticaltime-domainreflectometry with a temporally sequenced multi-frequency source[J].Optics letters,2015,40(22):5192-5195。

Claims

1. An optical time domain reflectometer based on ring structure modulation, characterized in that Including: Laser (1), first optical fiber coupler (2), semiconductor optical amplifier (3), first acousto-optic modulator (4), first optical isolator (5), second optical fiber coupler (6), delay optical fiber (7), first erbium-doped fiber amplifier (8), second acousto-optic modulator (9), second optical isolator (10), second erbium-doped fiber amplifier (11), first dense wavelength division multiplexer (12), circulator (13), second dense wavelength division multiplexer (14), Raman laser (15), piezoelectric ceramic (16), arbitrary function generator (17), third erbium-doped fiber amplifier (18), third dense wavelength division multiplexer (19), arbitrary waveform generator (20), polarization controller (21), third optical fiber coupler (22), optoelectronic balanced detector (23), data acquisition card (24), computer (25); wherein: The optical signal output end of the laser (1) is connected to the optical signal input end of the first optical fiber coupler (2), and the two optical signal output ends of the first optical fiber coupler (2) are respectively connected to the input ends of the semiconductor optical amplifier (3) and the polarization controller (21); The output end of the semiconductor optical amplifier (3) is connected to the input end of the first acousto-optic modulator (4), the output end of the first acousto-optic modulator (4) is connected to the input end of the first optical isolator (5), the output end of the first optical isolator (5) is connected to one end of a ring structure, the other end of the ring structure is connected to the input end of the second erbium-doped fiber amplifier (11), the output end of the second erbium-doped fiber amplifier (11) is connected to the input end of the first dense wavelength division multiplexer (12), the output end of the first dense wavelength division multiplexer (12) is connected to port 1 of the circulator (13), port 2 of the circulator (13) is connected to the input end of the second dense wavelength division multiplexer (14), the other input end of the second dense wavelength division multiplexer (14) is connected to the Raman laser (15), and the output end of the second dense wavelength division multiplexer (14) is connected to the optical fiber under test; the output end of the optical fiber under test is connected to the input end of the piezoelectric ceramic (16), and the output end of the arbitrary function generator (17) is connected to the input end of the piezoelectric ceramic (16); Port 3 of the circulator (13) is connected to the input end of the third erbium-doped fiber amplifier (18), the output end of the third erbium-doped fiber amplifier (18) is connected to the input end of the third dense wavelength division multiplexer (19), and the output end of the third dense wavelength division multiplexer (19) is connected to port 1 of the third optical fiber coupler (22); The output end of the polarization controller (21) is connected to port 2 of the third optical fiber coupler (22), ports 3 and 4 of the third optical fiber coupler (22) are connected to the input end of the optoelectronic balanced detector (23), the output end of the optoelectronic balanced detector (23) is connected to the input end of the data acquisition card (24), and the output end of the data acquisition card (24) is connected to the computer (25); The output end of the arbitrary waveform generator (20) is respectively connected to the input ends of the semiconductor optical amplifier (3), the first acousto-optic modulator (4), the second acousto-optic modulator (9), and the data acquisition card (24).

2. The optical time domain reflectometer based on ring structure modulation according to claim 1, characterized in that, The annular structure is composed of a second optical fiber coupler (6), a delay optical fiber (7), a first erbium-doped optical fiber amplifier (8), a second acousto-optic modulator (9), and a second optical isolator (10). Among them, the output end of the first optical isolator (5) is connected to the No. 1 optical signal input end of the second optical fiber coupler (6), the No. 4 optical signal output end of the second optical fiber coupler (6) is connected to one end of the delay optical fiber (7), the other end of the delay optical fiber (7) is connected to the input end of the first erbium-doped optical fiber amplifier (8), the output end of the first erbium-doped optical fiber amplifier (8) is connected to the input end of the second acousto-optic modulator (9), the output end of the second acousto-optic modulator (9) is connected to the input end of the second optical isolator (10), the output end of the second optical isolator (10) is connected to the No. 2 optical signal input end of the second optical fiber coupler (6), and the No. 3 optical signal output end of the second optical fiber coupler (6) is connected to the input end of the second erbium-doped optical fiber amplifier (11).

3. The optical time domain reflectometer based on ring structure modulation according to claim 2, wherein, The output power of the laser (1) is 15 mW, and the output wavelength is 1550 nm.

4. The optical time domain reflectometer based on ring structure modulation according to claim 1, wherein, The first optical fiber coupler (2) is a 90:10 1×2 coupler, the second optical fiber coupler (6) is a 50:50 2×2 coupler, and the third optical fiber coupler (22) is a 50:50 2×2 coupler.

5. The optical time domain reflectometer based on ring structure modulation according to claim 1, characterized in that The extinction ratio of the semiconductor optical amplifier (3) is 52 dB; the output power of the Raman laser (15) is 15 mW, and the output wavelength is 1480 nm; the detection bandwidth of the optoelectronic balanced detector (23) is 1 GHz.

6. The optical time domain reflectometer based on ring structure modulation according to claim 1, characterized in that The carrier frequency of the first acousto-optic modulator (4) is 100 MHz, the frequency shift of the second acousto-optic modulator (9) is 40 MHz, and the extinction ratio is 48 dB; the working bands of the first dense wavelength division multiplexer (12), the second dense wavelength division multiplexer (14), and the third dense wavelength division multiplexer (19) are all 1480 nm and 1550 nm.

7. The working method of an optical time domain reflectometer based on ring structure modulation, characterized in that, The working method is realized based on an optical time domain reflectometer with annular structure modulation. The optical time domain reflectometer includes: a laser (1), a first optical fiber coupler (2), a semiconductor optical amplifier (3), a first acousto-optic modulator (4), a first optical isolator (5), a second optical fiber coupler (6), a delay optical fiber (7), a first erbium-doped optical fiber amplifier (8), a second acousto-optic modulator (9), a second optical isolator (10), a second erbium-doped optical fiber amplifier (11), a first dense wavelength division multiplexer (12), a circulator (13), a second dense wavelength division multiplexer (14), a Raman laser (15), a piezoelectric ceramic (16), an arbitrary function generator (17), a third erbium-doped optical fiber amplifier (18), a third dense wavelength division multiplexer (19), an arbitrary waveform generator (20), a polarization controller (21), a third optical fiber coupler (22), an optoelectronic balanced detector (23), a data acquisition card (24), and a computer (25); the working method includes: The continuous light output by the laser (1) is divided into two upper and lower branches by the first fiber coupler (2). The continuous light in the upper branch is modulated into pulsed light with a high extinction ratio by the semiconductor optical amplifier (3) and the first acousto-optic modulator (4). After entering the first optical isolator (5) and the second fiber coupler (6), it is divided into two paths again: the light in the upper branch directly enters the second erbium-doped fiber amplifier (11), and the light in the lower branch enters the ring structure for frequency modulation. The modulated light also enters the second erbium-doped fiber amplifier (11) for optical amplification, and then the spontaneous emission noise is filtered by the first dense wavelength division multiplexer (12). Then, it is injected into the second dense wavelength division multiplexer (14) through the circulator (13). The Raman laser (15) injects Raman light into the second dense wavelength division multiplexer (14) as pump light for amplifying the power of the probe light. The Raman light and the probe light are injected into the fiber under test together. The arbitrary function generator (17) continuously transmits vibrations by controlling the piezoelectric ceramic (16); The backward Rayleigh scattering optical signal generated by the fiber under test is injected into the third erbium-doped fiber amplifier (18) through the second dense wavelength division multiplexer (14) and the circulator (13) for power amplification, and then enters the third dense wavelength division multiplexer (19) to filter out the spontaneous emission noise; The continuous light in the lower branch split by the first fiber coupler (2) is used as the eigenlight after the polarization state is adjusted by the polarization controller (21). It enters the third fiber coupler (22) together with the backward Rayleigh scattering light filtered by the third dense wavelength division multiplexer (19) for coherent beat frequency, and is photoelectrically converted by the photoelectric balanced detector (23). Finally, the multi-frequency beat frequency optical signal is sampled and recorded by the data acquisition card (24) and input into the computer (25) for data processing; The arbitrary waveform generator (20) is used to control the pulse width of the semiconductor optical amplifier (3), the multi-frequency modulation of the first acousto-optic modulator (4), the chopping modulation and frequency shift of the second acousto-optic modulator (9), and the acquisition instruction input of the data acquisition card (24).

8. The working method of the optical time domain reflectometer based on ring structure modulation according to claim 7, characterized in that, The modulation process of the ring structure includes: the delay optical fiber (7) is used for time delay; the first erbium-doped fiber amplifier (8) is used to compensate for the splitting loss of the second fiber coupler (6) and the insertion loss of the second acousto-optic modulator (9). The second acousto-optic modulator (9) is used to perform frequency modulation on the pulsed light in the loop again. The second optical isolator (10) is used to block the light transmitted in the reverse direction in the loop; the pulsed light with a high extinction ratio is divided into two paths again by the second fiber coupler (6) every time it transmits in the ring structure. As the number of cycles increases, a pulsed sequence with different time delays and frequencies is formed.

9. The working method of the optical time domain reflectometer based on ring structure modulation according to claim 8, characterized in that, The number of pulses in the timing pulse sequence generated by the modulation of the ring structure is calculated in the following two ways: When the period T of the pulsed light modulated by the first acousto-optic modulator (4) is used as the modulation reference, the calculation expression for the number of pulses M in the timing pulse sequence generated by the modulation of the ring structure is: In the formula, represents rounding down; c represents the speed, T represents the pulsed light period, n represents the number of times the pulsed light winds around the ring, and L R represents the length of the ring structure; When the length L of the fiber under test is used as the modulation reference, the calculation expression for the number of pulses M in the timing pulse sequence generated by the modulation of the ring structure is: In the formula, represents rounding up; L represents the length of the optical fiber to be measured, and L R represents the length of the ring structure.

10. The working method of the optical time domain reflectometer based on ring structure modulation according to claim 8, characterized in that the powers of the Raman light and the detection light are obtained by the following formula: Among them, z represents the transmission distance of the optical fiber to be measured; P s represents the detected optical power; P p represents the Raman pump optical power; g eff represents the effective Raman gain coefficient; α s represents the attenuation coefficient of the detected optical signal during transmission; ξ represents the transmission direction of the pump optical signal and the detected optical signal, 1 for the same direction and -1 for the opposite direction; α p represents the attenuation coefficient of the pump optical signal during transmission; v p represents the transmission rate of the Raman optical signal; v s represents the transmission rate of the detected optical signal.

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