Distributed optical fiber detection light source system based on pulse dense comb frequency modulation

Abstract

The invention relates to the technical field of optical fiber sensing, and provides a distributed optical fiber detection light source system based on pulse dense comb frequency modulation, and the system comprises a light source assembly which is used for generating detection light; the modulation device is connected with the light source assembly and forms a frequency shift ring cavity; the modulation device comprises a first optical fiber coupler, a delay optical fiber assembly, a double-parallel Mach-Zehnder modulator and an erbium-doped optical fiber assembly which are arranged in sequence; the modulation device is configured to modulate the frequency of the detection light in the frequency shift ring cavity; and the output optical fiber is connected with the output end of the first optical fiber coupler and is used for outputting the frequency-shifted detection light. The distributed optical fiber detection light source system based on the pulse dense comb frequency modulation is simple in structure, the speed of cyclic frequency conversion of the detection light in the frequency shift ring can be effectively reduced, then output of the detection light after frequency conversion is accelerated, and therefore the detection efficiency of the detection light used for detection is improved, and the detection time is shortened.

Description

Technical Field

[0001] The present application relates to the field of optical fiber sensing technology, and in particular to a distributed optical fiber detection light source system based on pulse dense comb frequency modulation. Background Art

[0002] Distributed fiber optic sensing technology achieves continuous spatial measurement of external signals by modulating light waves in optical fibers. It is widely used in many fields such as pipeline leak detection, seismic exploration, and ranging.

[0003] Optical time-domain reflectometry and optical frequency-domain reflectometry are commonly used for measurement. However, optical time-domain reflectometry requires wide pulses to achieve long detection distances, which prolongs single measurement times and makes it difficult to quickly respond to dynamic signals. Optical frequency-domain reflectometry requires a wide wavelength tuning range to achieve high spatial resolution, but the long sweep cycle of the light source significantly increases detection time, making it inadequate for real-time monitoring.

[0004] Therefore, there is an urgent need for a detection technology that can effectively shorten the detection time and improve the detection efficiency. Summary of the Invention

[0005] The present application provides a distributed optical fiber detection light source system based on pulse dense comb frequency modulation to solve the technical problems of long detection time and low efficiency in existing technologies.

[0006] The distributed fiber optic detection light source system based on pulse dense comb frequency modulation provided in the present application includes: a light source assembly, configured to generate detection light; a modulation device, connected to the light source and forming a frequency-shifted ring cavity; the modulation device includes a first fiber coupler, a delay fiber assembly, a dual-parallel Mach-Zehnder modulator and an erbium-doped fiber assembly arranged in sequence; the modulation device is configured to modulate the frequency of the detection light in the frequency-shifted ring cavity; wherein the input end of the first fiber coupler is connected to the light source assembly and the erbium-doped fiber assembly respectively, and the output end of the first fiber coupler is connected to the delay fiber assembly; the delay fiber assembly is configured to delay the detection light; the dual-parallel Mach-Zehnder modulator is configured to modulate the frequency of the detection light; the erbium-doped fiber assembly is configured to receive the detection light and compensate for the detection light loss; an output fiber is connected to the output end of the first fiber coupler, and the output fiber is configured to output the frequency-shifted detection light.

[0007] In some feasible implementations, the frequency increase of the detection light provided in the present application is positively correlated with the number of cycles of the detection light in the frequency-shift ring cavity, and the time it takes for the detection light to circulate once in the frequency-shift ring cavity is less than or equal to the pulse width of the detection light.

[0008] In some feasible implementations, the delay fiber assembly includes an optical switch, a first delay fiber, and a second delay fiber; the input end of the optical switch is connected to the first fiber coupler; the output end of the optical switch is connected to the first delay fiber and the second delay fiber, respectively; the optical switch is configured to turn on the first delay fiber or the second delay fiber to control the detection light delay.

[0009] In some possible implementations, the first delay fiber and the second delay fiber have different lengths.

[0010] In some feasible implementations, the erbium-doped fiber assembly includes an erbium-doped fiber and an erbium-doped fiber amplifier; the distributed fiber optic detection light source system based on pulse dense comb frequency modulation also includes a first wavelength division multiplexer; the two input ends of the erbium-doped fiber are connected to the input end of the first wavelength division multiplexer, and detection light of the same frequency interferes in the erbium-doped fiber to form a refractive index grating; the erbium-doped fiber amplifier is connected to the output end of the first wavelength division multiplexer, and the erbium-doped fiber amplifier is configured to compensate for the detection light loss.

[0011] In some feasible implementations, the system further includes: a second fiber coupler, a first polarization controller, and a second polarization controller; the input end of the second fiber coupler is connected to the ends of the first delay fiber and the second delay fiber facing away from the optical switch; the first polarization controller is arranged between the second fiber coupler and the dual parallel Mach-Zehnder modulator, and the first polarization controller is configured to control the polarization-maintaining form of the detection light input to the dual parallel Mach-Zehnder modulator; the second polarization controller is arranged between the erbium-doped fiber amplifier and the first fiber coupler, and the second polarization controller is configured to control the polarization-maintaining form of the detection light output by the dual parallel Mach-Zehnder modulator.

[0012] In some feasible implementations, the light source assembly includes a light source, a second wavelength division multiplexer, and an active phase-shifted fiber grating; the light source is configured to generate pump light of a first wavelength; the second wavelength division multiplexer is connected between the light source and the active phase-shifted fiber grating; the second wavelength division multiplexer is configured to transmit the pump light of the first wavelength to the active phase-shifted fiber grating and output laser light of a second wavelength; wherein the second wavelength is greater than the first wavelength.

[0013] In some feasible implementations, the system further includes: an isolator and a filter; the isolator is disposed between the first fiber coupler and the optical switch, and is configured to isolate the return light; the filter is disposed between the dual parallel Mach-Zehnder modulator and the first wavelength division multiplexer, and is configured to filter noise.

[0014] In some feasible implementations, it further includes: a third fiber coupler and an electro-optical modulator; the third fiber coupler is arranged between the second wavelength division multiplexer and the electro-optical modulator; the electro-optical modulator is connected to the input end of the first fiber coupler, and the electro-optical modulator is configured to modulate the detection light.

[0015] In some feasible implementations, it also includes: a first signal generator and a second signal generator; the first signal generator is connected to the electro-optical modulator, and the first signal generator is used to control the electro-optical modulator; the second signal generator is connected to the dual parallel Mach-Zehnder modulator, and the second signal generator is used to control the dual parallel Mach-Zehnder modulator.

[0016] The distributed fiber optic detection light source system based on pulse dense comb frequency modulation in this application has a relatively simple structure and can effectively reduce the speed of the detection light circulating in the frequency shift loop, thereby accelerating the output of the detection light after frequency conversion, thereby improving the detection efficiency of the detection light when used for detection and shortening the detection time. In addition, the light source system can simultaneously take into account the three major parameters of spatial resolution, detection distance, and bandwidth, effectively ensuring the performance of use. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] In order to more clearly illustrate the technical solution of the present application, the following is a brief introduction to the drawings required for use in the embodiments. Obviously, for ordinary technicians in this field, other drawings can be obtained based on these drawings without any creative work.

[0018] Figure 1 This is a schematic structural diagram of a distributed optical fiber detection light source system based on pulse dense comb frequency modulation provided by an embodiment of the present application; Figure 2 Schematic diagram of the structure of a dual-parallel Mach-Zehnder modulator provided in an embodiment of the present application.

[0019] 100-Distributed fiber optic detection light source system based on pulse dense frequency modulation; 10-light source assembly; 11-light source; 12-second wavelength division multiplexer; 13-active phase-shifted fiber Bragg grating; 20 - modulation device; 20a - frequency-shifting ring cavity; 21 - first fiber coupler; 22 - delay fiber assembly; 221 - optical switch; 222 - first delay fiber; 223 - second delay fiber; 23 - dual parallel Mach-Zehnder modulator; 24 - erbium-doped fiber assembly; 241 - erbium-doped fiber; 242 - erbium-doped fiber amplifier; 25 - second fiber coupler; 26 - first polarization controller; 27 - second polarization controller; 28 - isolator; 29 - filter; 30-output optical fiber; 40 - first wavelength division multiplexer; 50 - third optical fiber coupler; 60 - electro-optical modulator; 70 - first signal generator; 80 - second signal generator. DETAILED DESCRIPTION

[0020] The following will clearly describe the technical solutions in the embodiments of the present application in conjunction with the accompanying drawings in the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, not all of the embodiments. Based on the embodiments of the present application, other embodiments obtained by ordinary technicians in this field without making any creative work are all within the scope of protection of this application.

[0021] Hereinafter, the terms "first," "second," etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of the technical features being referred to. Thus, a feature identified with "first," "second," etc., may explicitly or implicitly include one or more of the features. In the description of this application, unless otherwise specified, "plurality" means two or more.

[0022] In addition, in this application, directional terms such as "upper", "lower", "inner" and "outer" are defined relative to the orientation of the components in the drawings. It should be understood that these directional terms are relative concepts. They are used for relative description and clarification, and they can change accordingly according to changes in the orientation of the components in the drawings.

[0023] In order to solve the technical problems of long detection time and low efficiency in the existing technology, this application proposes a new distributed optical fiber sensing technology, which optimizes the frequency shift speed, frequency shift range, pulse width, instantaneous linewidth, optical power, signal-to-noise ratio, etc. of the light source.

[0024] Figure 1 This is a structural diagram of a distributed optical fiber detection light source system based on pulse dense comb frequency modulation provided in an embodiment of the present application.

[0025] See also Figure 1 The distributed fiber optic detection light source system 100 based on pulse dense comb frequency modulation includes a light source component 10, a modulation device 20 and an output optical fiber 30.

[0026] The light source assembly 10 is used to generate detection light, which can be an infrared laser with a wavelength of 1550nm to match the transmission of long-distance and large-capacity optical signals.

[0027] The modulation device 20 is connected to the light source assembly 10 and has a frequency shift ring cavity 20a formed therein. The modulation device 20 is used to receive the detection light emitted by the light source assembly 10 and modulate the frequency of the detection light in the frequency shift ring cavity 20a to produce a frequency change.

[0028] Specifically, the modulation device 20 includes a first fiber coupler 21 , a delay fiber assembly 22 , a dual parallel Mach-Zehnder modulator 23 and an erbium-doped fiber assembly 24 , which are arranged in sequence.

[0029] A first fiber coupler 21 is disposed at the entrance of the frequency-shifting ring cavity 20a. The detection light emitted by the light source assembly 10 directly enters the frequency-shifting ring cavity 20a through the first fiber coupler 21. The first fiber coupler 21 is used for receiving and outputting the detection light. The input end of the first fiber coupler 21 is connected to the light source assembly 10 and the erbium-doped fiber assembly 24, respectively. In this way, the detection light emitted by the light source assembly 10 can enter the frequency-shifting ring cavity 20a through the input end of the first fiber coupler 21, and finally return to the first fiber coupler 21 through the erbium-doped fiber assembly 24, completing a complete cycle of the detection light within the frequency-shifting ring cavity 20a.

[0030] The delay optical fiber assembly 22 is used to receive and control the delay of the detection light during its propagation process, thereby facilitating the control of different pulse widths of the detection light and achieving modulation of different pulse widths and frequencies.

[0031] The dual parallel Mach-Zehnder modulator 23 is disposed between the delay fiber assembly 22 and the erbium-doped fiber assembly 24 , and is configured to receive the detection light transmitted by the delay fiber assembly 22 and modulate the frequency of the detection light.

[0032] The erbium-doped fiber assembly 24 is used to receive the detection light transmitted by the dual parallel Mach-Zehnder modulator 23 and compensate for the loss of the detection light when propagating in the frequency-shifting ring cavity 20 a.

[0033] The output fiber 30 is located outside the frequency-shifting ring cavity 20a. It is connected to the output end of the first fiber coupler 21 and is used to output the frequency-shifted detection light to perform specific functions, such as pipeline leak detection, seismic exploration, and distance measurement.

[0034] In a specific implementation, the dual parallel Mach-Zehnder modulator 23 can realize frequency shifting of the detection light in the frequency-shift ring cavity 20a, with an initial frequency of f 0 The continuous single-frequency light is modulated to form a pulse width τ, and the part connected to the output fiber 30 through the first fiber coupler 21 is directly output, and the other part of the light enters the frequency-shifting ring cavity 20a and is modulated by the dual parallel Mach-Zehnder modulator 23 to form a pulse width τ. f 0 +Ω, after being amplified by the first optical fiber coupler 21 to compensate for the optical loss, a part of the detection light is output through the output optical fiber 30, and the other part of the detection light enters the frequency shift ring cavity 20a again and the frequency changes to f 0 +2Ω. Repeat this cycle to continuously obtain the frequency-shift detection light output.

[0035] The distributed fiber optic detection light source system 100 based on pulse dense comb frequency modulation provided in the embodiment of the present application, by providing a frequency-shifting ring cavity 20a, allows the detection light energy after passing through the dual parallel Mach-Zehnder modulator 23 to be continuously cyclically frequency-modulated within the frequency-shifting ring cavity 20a, thereby achieving dense frequency and pulse width conversion of the detection light within the pulse. Furthermore, the distributed fiber optic detection light source system 100 based on pulse dense comb frequency modulation in the present application has a relatively simple structure, which can effectively reduce the speed of the cyclic frequency conversion of the detection light within the frequency-shifting ring cavity 20a, thereby accelerating the output of the frequency-converted detection light, thereby improving the detection efficiency of the detection light when used for detection and shortening the detection time.

[0036] In some feasible implementations, the transmittance of the detection light through the first fiber coupler 21 can be T. That is, after passing through the first fiber coupler 21, the detection light at 1-T enters the frequency-shifting ring cavity 20a for frequency shifting and amplification. The detection light at T is then output through the output fiber 30.

[0037] In some feasible implementations, the frequency increase of the detection light is positively correlated with the number of cycles of the detection light in the frequency-shift ring cavity 20a. For example, the initial frequency of the detection light is f 0 , circulates once in the frequency shift ring cavity 20a, and the frequency increases to f 0 +Ω. It can be seen that the frequency increase of the detection light after one cycle within the frequency-shifting ring is Ω. In other words, as the number of cycles increases, the frequency increase also increases. For example, if the number of cycles is a, the frequency increase of the detection light is a times Ω. In this way, the desired frequency can be achieved by controlling the number of cycles of the detection light within the frequency-shifting ring cavity 20a.

[0038] In some feasible implementations, the time it takes for the detection light to circulate once within the frequency-shifted ring cavity 20a is less than or equal to the pulse width of the detection light. This effectively prevents overlap of the detection light within the frequency-shifted ring cavity 20a, thereby ensuring the normal operation of the distributed fiber-optic detection light source system 100 based on pulse dense comb frequency modulation.

[0039] In a specific implementation, see Figure 1 As shown, the delay fiber assembly 22 may include an optical switch 221 , a first delay fiber 222 and a second delay fiber 223 ; wherein the first delay fiber 222 and the second delay fiber 223 have different lengths.

[0040] The input end of the optical switch 221 is connected to the first fiber coupler 21; the output end of the optical switch 221 is connected to the first delay fiber 222 and the second delay fiber 223 respectively; the optical switch 221 is configured to conduct the first delay fiber 222 or the second delay fiber 223 to control the detection light delay.

[0041] In other words, the optical path of the detection light can be selected by controlling the state of the optical switch 221. That is, the first delay optical fiber 222 is selected as the optical path of the detection light, or the second delay optical fiber 223 is selected as the optical path of the detection light.

[0042] In some possible implementations, see Figure 1 The erbium-doped fiber assembly 24 includes an erbium-doped fiber 241 and an erbium-doped fiber amplifier 242 ; the distributed fiber optic detection light source system 100 based on pulse dense comb frequency modulation further includes a first wavelength division multiplexer 40 .

[0043] The first wavelength division multiplexer 40 is used to receive the frequency-shifted detection light transmitted by the dual parallel Mach-Zehnder modulator 23 and input the detection light into the erbium-doped fiber 241 through two input ends of the erbium-doped fiber 241 for interference.

[0044] Specifically, two input ends of the erbium-doped fiber 241 are connected to the input end of the first wavelength division multiplexer 40 , and detection lights of the same frequency interfere in the erbium-doped fiber 241 to form a refractive index grating, which is output through the first wavelength division multiplexer 40 .

[0045] The detection light is divided into two paths and enters the erbium-doped fiber 241 at the same time. The detection light of the same frequency will form two waves in the erbium-doped fiber 241 to mix and interfere, forming a refractive index grating with the same interval wavelength. The refractive index grating has a filtering effect on the light passing through, which can effectively reduce the spectral signal-to-noise ratio and spectral linewidth broadening of the detection light, and better obtain pulsed detection light with high signal-to-noise ratio and narrow linewidth, thereby ensuring the detection accuracy.

[0046] The relationship between the two-beam interference intensity in the optical interference phenomenon is as follows: ; It is the instantaneous total light intensity after the superposition of two coherent light beams at a certain point in space; is the intensity of one of the beams; is the intensity of the other beam of light; is the phase of one of the beams; is the phase of the other beam.

[0047] Since the two light beams have the same frequency and a constant phase difference, interference fringes, namely refractive index gratings, are formed inside the erbium-doped optical fiber 241 . The interval between the refractive index gratings is the wavelength of the pulse detection light.

[0048] The erbium-doped fiber amplifier 242 is connected to the output of the first wavelength division multiplexer 40. During operation, the erbium-doped fiber amplifier 242 receives the detection light output by the first wavelength division multiplexer 40 and outputs the detection light to the first fiber coupler 21. Through the output of the first fiber coupler 21, a portion of the detection light enters the frequency-shifting ring cavity 20a, while the remaining portion is output for detection. The erbium-doped fiber amplifier 242 is used to compensate for the loss of the detection light caused by the various optical components within the frequency-shifting ring cavity 20a.

[0049] In some possible implementations, see Figure 1 The modulation device 20 further includes a second fiber coupler 25 , a first polarization controller 26 and a second polarization controller 27 .

[0050] The input end of the second fiber coupler 25 is connected to the ends of the first delay fiber 222 and the second delay fiber 223 away from the optical switch 221. In other words, the second fiber coupler 25 is connected to the output ends of the first delay fiber 222 and the second delay fiber 223.

[0051] The first polarization controller 26 is disposed between the second fiber coupler 25 and the dual-parallel Mach-Zehnder modulator 23. The first polarization controller 26 is used to control the polarization-maintaining state of the detection light input to the dual-parallel Mach-Zehnder modulator 23. In this way, the detection light entering the dual-parallel Mach-Zehnder modulator 23 is ensured to be polarization-maintaining light.

[0052] The second polarization controller 27 is disposed between the erbium-doped fiber amplifier 242 and the first fiber coupler 21 . The second polarization controller 27 is configured to control the polarization-maintaining shape of the detection light output by the dual parallel Mach-Zehnder modulator 23 .

[0053] By providing the first polarization controller 26 and the second polarization controller 27, it is possible to effectively ensure that the detection light in the frequency-shift ring cavity 20a is in a polarization state, thereby avoiding problems such as signal distortion and power fluctuation caused by polarization effects.

[0054] In some possible implementations, see Figure 1 The light source assembly 10 may include a light source 11, a second wavelength division multiplexer 12, and an active phase-shifted fiber Bragg grating 13. The light source 11 is configured to generate pump light of a first wavelength. The second wavelength division multiplexer 12 is connected between the light source 11 and the active phase-shifted fiber Bragg grating 13. The second wavelength division multiplexer 12 is configured to transmit the pump light of the first wavelength to the active phase-shifted fiber Bragg grating 13 and output laser light of a second wavelength. The second wavelength is greater than the first wavelength. In other words, the detection light referred to in this application is laser light of the second wavelength.

[0055] Specifically, the light source 11 can use a pump light of a first wavelength, and the pump light enters the active phase-shifted fiber grating 13 of a second wavelength after passing through the second wavelength division multiplexer 12. The first wavelength of the example can be 980nm, and the second wavelength can be 1550nm. Among them, the active phase-shifted fiber grating 13 can be a single-frequency fiber detection optical device based on the active phase-shifted fiber grating 13, which combines the distributed feedback detection light polarization line with the polarization-maintaining fiber slow axis-to-axis coupling technology to achieve a narrow linewidth detection light polarization-maintaining laser output with a polarization extinction ratio greater than 30dB, with a central wavelength of 1550.12nm, a linewidth less than 3kHz, and a phase noise less than 110dB rad2 / Hz after noise suppression.

[0056] In one specific implementation, the frequency spacing of the light source 11 is determined by the frequency shift of the dual-parallel Mach-Zehnder modulator 23, and the frequency shift range of the output detection light is determined by the number of cycles. A greater frequency shift range increases the frequency shift range, and the spontaneous emission light is amplified with each cycle. As the number of cycles increases, the spontaneous emission light accumulates noise, reducing the spectral signal-to-noise ratio and spectral linewidth of the detection light, thereby shortening the system's detection range. The dual-parallel Mach-Zehnder modulator 23 in the frequency-shifting ring cavity 20a can determine important parameters such as the frequency shift speed, frequency shift range, pulse width, and instantaneous linewidth of the scanning detection light through control parameter optimization and noise suppression.

[0057] In some possible implementations, see Figure 1 The modulation device 20 may further include: an isolator 28 and a filter 29 .

[0058] Isolator 28 is disposed between first fiber coupler 21 and optical switch 221. Its input is connected to the output of first fiber coupler 21, while its output is connected to optical switch 221. Isolator 28 isolates the return light, thereby ensuring a single-phase output of the detection light within frequency-shifted ring cavity 20a.

[0059] The filter 29 is provided between the dual parallel Mach-Zehnder modulator 23 and the first wavelength division multiplexer 40. The filter 29 is used to filter out the noise emitted by the amplifier spontaneous emission, thereby improving the signal quality. The parameter selection of the filter 29 can filter out the portion of the detection light with an increased frequency, or can also filter out the portion of the detection light with a decreased frequency, so as to obtain pulsed light with different frequency intervals according to different requirements. For example, when the detection light passes through the dual parallel Mach-Zehnder modulator 23, two frequencies can be generated, including a frequency increase of f 0 The detection light and frequency of +Ω are reduced to f 0 -Ω detection light, filter 29 can filter out f 0 -Ω this part.

[0060] In some possible implementations, see Figure 1 The distributed fiber optic detection light source system 100 based on pulse dense comb frequency modulation may further include: a third fiber optic coupler 50 and an electro-optic modulator 60 .

[0061] The third fiber coupler 50 is disposed between the second wavelength division multiplexer 12 and the electro-optical modulator 60 .

[0062] Specifically, the input end of the third optical fiber coupler 50 is connected to the output end of the second wavelength division multiplexer 12 , and the output end of the third optical fiber coupler 50 is connected to the input end of the electro-optic modulator 60 .

[0063] The electro-optical modulator 60 is connected to the input end of the first optical fiber coupler 21 , and is used to receive and modulate the on time and repetition frequency of the detection light.

[0064] The distributed fiber optic detection light source system 100 based on pulse dense comb frequency modulation may further include: a first signal generator 70 and a second signal generator 80 .

[0065] The first signal generator 70 is connected to the electro-optical modulator 60 and is used to control the electro-optical modulator 60 . The first signal generator 70 can be used to control the on-time and repetition frequency of the electro-optical modulator 60 for modulating the detection light.

[0066] The second signal generator 80 is connected to the dual parallel Mach-Zehnder modulator 23 and is used to control the dual parallel Mach-Zehnder modulator 23. The second signal generator 80 can be used to control the frequency shift of the detection light by the dual parallel Mach-Zehnder modulator 23.

[0067] Specifically, the first signal generator 70 can realize frequency modulation of the detection light at different pulses by controlling the electro-optical modulator 60, thereby obtaining pulse detection light with equally spaced steps or a certain proportional relationship in frequency increase or decrease; the dual parallel Mach-Zehnder modulator 23 can perform single-sideband frequency shift on the pulse detection light, thereby obtaining an equally spaced frequency shift or a certain regular output spectrum.

[0068] In this application, the pulse width and detection distance Positively correlated with the detection frequency band f H Inversely correlated with spatial resolution A z Inversely correlated.

[0069] Among them, the detection distance of the distributed optical fiber sensing system is By detecting pulse width and light source line width The specific relationship is reflected by the following formula.

[0070] ; is the attenuation coefficient; is the Brillouin scattering power; is the propagation speed of pulse light in the medium; is the Rayleigh scattering coefficient; is the scattering loss coefficient; is the wave number; is the reduced Planck constant; is the light frequency; is the coherence bandwidth.

[0071] Detection band f H and detection distance L Inversely proportional to the detection pulse width The specific relationship is reflected by the following formula.

[0072] ; in, is the pulse frequency; is the speed of light in a vacuum; is the refractive index of the optical fiber; is the input optical power.

[0073] Spatial resolution refers to the minimum distance between two disturbance events that the system can effectively distinguish, and is related to the positioning accuracy of the sensing system. When the distance is less than the spatial resolution, the two disturbance events cannot be distinguished. The system monitors the interference of backscattered light within the pulse time. In theory, spatial resolution is mainly related to the pulse width of the detection pulse light. Ideally, the spatial resolution Az The expression is: .

[0074] That is, the spatial resolution Az By pulse width Decide.

[0075] The single-frequency pulse light source, continuous scanning light source and intra-pulse continuous scanning light source in the distributed system detection light source are single or continuous frequency scanning modulation within the pulse, which can achieve high spatial resolution measurement, but it is difficult to achieve long-distance and high-frequency synchronous measurement; the multi-frequency pulse light source is a single modulation of equal time intervals of multiple frequency pulses within the scanning period, which can achieve higher frequency band measurement, but it is difficult to achieve high spatial resolution and high-frequency synchronous measurement; the above-mentioned light sources are single modulation within the pulse (single or continuous) and between pulse sequences (multi-frequency), and do not involve research on intra-pulse (equally spaced continuous multi-frequency) and inter-pulse joint modulation. Therefore, they cannot essentially solve the problem of the restrictive relationship between long detection distance, wide detection frequency band and high spatial resolution of traditional distributed optical fiber sensing technology.

[0076] The distributed fiber optic detection light source system 100 based on pulse dense frequency modulation provided in the present application can simultaneously jointly modulate the intra-pulse signal and the inter-pulse signal. According to the relationship between the detection distance, the detection bandwidth and the spatial resolution, the pulse light can be densely modulated to achieve a distributed detection light source that meets the requirements of long detection distance, wide detection bandwidth and high spatial resolution.

[0077] The expression of multi-frequency detection optical pulse sequence is: ; in, is a time-varying function, which indicates that the laser electric field t The instantaneous value of i Indicates the i Pulse train period, T represents the pulse train period, M represents the number of pulse trains, r Indicates the r A light pulse, represents the negative exponential phase term, A r Indicates the r The light pulse amplitude, t represents the detection light emission time, f 0 represents the fundamental frequency of the multi-frequency detection optical pulse sequence, N represents the number of multi-frequency detection optical pulse sequences, Ω represents the frequency increase step of the multi-frequency detection optical pulse sequence, u r Indicates the r The pulse occurrence time, W r Indicates the r The pulse width of a light pulse.

[0078] In order to better understand the distributed optical fiber detection light source system 100 based on pulse dense comb frequency modulation provided by the present application, the following Figure 1Provide a brief introduction to the work process.

[0079] After the pump light source 11 is turned on, it generates 980nm pump light. After passing through the second wavelength division multiplexer 12 and inputting into the active phase-shifted fiber grating 13, it generates a 1550nm laser. The laser then passes through the third fiber coupler 50 and enters the electro-optical modulator 60. A first signal generator 70 controls the electro-optical modulator 60 to adjust the on-time and repetition rate of the detection light. After modulation, the detection light enters the frequency-shifting ring cavity 20a through the first fiber coupler 21. It then passes through the isolator 28, the optical switch 221, the first delay fiber 222, the second delay fiber 223, the second fiber coupler 25, the first polarization controller 26, the dual parallel Mach-Zehnder modulator 23, the filter 29, the first wavelength division multiplexer 40, the erbium-doped fiber 241, the erbium-doped fiber amplifier 242, and finally returns to the first fiber coupler 21 through the second polarization controller 27, completing a cycle of the detection light within the frequency-shifting ring cavity 20a. After one cycle, the frequency of the detection light increases. A portion of the detection light can be directly output through the output optical fiber 30, and the other portion of the detection light passes through the isolator 28 and re-enters the frequency-shifting ring cavity 20a for a second cycle. By this cycle, the detection light of the required frequency can be obtained.

[0080] Figure 2 Schematic diagram of the structure of a dual-parallel Mach-Zehnder modulator provided in an embodiment of the present application.

[0081] See also Figure 2 As shown, the dual parallel Mach-Zehnder modulator 23 can be an IQ modulator, which is a key device for realizing the single-sideband modulation mode. It mainly uses a Ti-diffused LiNbO3 (lithium niobate) waveguide to form a main Mach-Zehnder interferometer (MZI) structure, such as Figure 2 As shown, the input light is divided into four paths, forming two sub-MZI structures. These two sub-MZIs serve as two arms to form a main MZI structure. Each arm of the MZI1 structure and the MZI2 structure has an LN modulator, namely the first modulator LN1, the second modulator LN2, the third modulator LN3, and the fourth modulator LN4, and are controlled by an external RF drive signal. The RF drive signal loaded on MZI2 is delayed by 90° with the RF drive signal on MZI1 through a 90° phase shifter. There is a phase shifter on each of the MZI1, MZI2 and one of the sub-arms in the main MZI structure, which is controlled by a DC bias voltage. V 1 、 V 2 and V 3 Control can be achieved between the two arms Δφ 1 、 Δφ 2 and Δφ 3 The phase difference is tuned. The interference relationship satisfies the following formula.

[0082] ; is the total light intensity; 、 The intensity of two independent detection beams of light; is the initial phase difference; is the phase modulation depth; is the modulation angular frequency; is the time variable; is the DC component of the total light intensity; is the interference term amplitude.

[0083] After Bessel function expansion, it becomes: ; is a Bessel function of the first kind, with an order of k, where k can be 0, 2n, or 2n+1 in the above formula.

[0084] is the carrier angular frequency; is the output signal; is the constant amplitude factor; is the carrier angular frequency; is the initial phase.

[0085] In order to minimize the size of the third-order component and increase the value of the first-order component, the modulation coefficient can be adjusted appropriately. m The size of J 1 (m) >> J 3 (m) , but also need to meet A larger value is used to obtain a better frequency shift effect. m It is related to the energy of the radio frequency signal loaded on the dual parallel Mach-Zehnder modulator 23.

[0086] The distributed fiber optic detection light source system 100 based on pulse dense frequency modulation provided by the present application can simultaneously meet the relationship between the detection distance, detection bandwidth and spatial resolution and the pulse width and pulse bandwidth. It can be modulated by an electro-optical modulator 60 and a dual parallel Mach-Zehnder modulator 23, and the pulse can be modulated intra-pulse and inter-pulse. Intra-pulse frequency modulation can use a dual parallel Mach-Zehnder modulator 23 to modulate the frequency within the pulse, and the first signal generator 70 and the electro-optical modulator 60 can be used to complete the inter-pulse. By establishing a parameter balancing mechanism of dense frequency conversion pulse width within the pulse, sparse frequency conversion frequency number between pulses, pulse width and equal interval time, a long detection distance, high detection bandwidth and high spatial resolution are achieved, dense frequency modulation within the pulse and sparse frequency modulation between pulses are achieved, and adjustable pulse output detection light with multiple frequency changes within and between pulses is obtained. In addition, the distributed fiber optic detection light source system 100 based on pulse dense frequency modulation provided by the present application is relatively simple in structure. Under the premise of meeting the detection accuracy, it can effectively shorten the detection time and improve the detection accuracy.

[0087] It should be noted that those skilled in the art will readily conceive of other embodiments of the present application after considering the specification and practicing the application disclosed herein. This application is intended to cover any variations, uses, or adaptations of the present application that follow the general principles of this application and include common knowledge or customary techniques in the art that are not disclosed in this application.

[0088] It will be understood that the present application is not limited to the precise construction that has been described above and shown in the accompanying drawings, and that various modifications and changes may be made without departing from the scope thereof, the true scope being indicated by the present application.

Claims

1. A distributed optical fiber detection light source system based on pulse dense frequency modulation, characterized in that: include: a light source assembly configured to generate detection light; A modulation device connected to the light source assembly, the modulation device forming a frequency-shift ring cavity; The modulation device includes a first fiber coupler, a delay fiber assembly, a dual parallel Mach-Zehnder modulator, and an erbium-doped fiber assembly arranged in sequence; the modulation device is configured to modulate the frequency of the detection light in the frequency-shifting ring cavity; The input end of the first fiber coupler is connected to the light source assembly and the erbium-doped fiber assembly respectively, and the output end of the first fiber coupler is connected to the delay fiber assembly; the delay fiber assembly is configured to delay the detection light; the dual parallel Mach-Zehnder modulator is configured to modulate the frequency of the detection light; the erbium-doped fiber assembly is configured to receive the detection light and compensate for the detection light loss; An output optical fiber is connected to an output end of the first optical fiber coupler, and the output optical fiber is configured to output the frequency-shifted detection light.

2. The distributed optical fiber detection light source system based on pulse dense frequency modulation according to claim 1, characterized in that: The frequency increase of the detection light is positively correlated with the number of cycles of the detection light in the frequency-shift ring cavity, and the time for the detection light to circulate once in the frequency-shift ring cavity is less than or equal to the pulse width of the detection light.

3. The distributed optical fiber detection light source system based on pulse dense comb frequency modulation according to claim 2, characterized in that: The delay fiber assembly includes an optical switch, a first delay fiber, and a second delay fiber; The input end of the optical switch is connected to the first fiber coupler; the output end of the optical switch is connected to the first delay fiber and the second delay fiber respectively; the optical switch is configured to conduct the first delay fiber or the second delay fiber to control the detection light delay.

4. The distributed optical fiber detection light source system based on pulse dense frequency modulation according to claim 3, characterized in that: The first delay fiber and the second delay fiber have different lengths.

5. The distributed optical fiber detection light source system based on pulse dense comb frequency modulation according to claim 3, characterized in that: The erbium-doped fiber assembly includes an erbium-doped fiber and an erbium-doped fiber amplifier; the distributed fiber optic detection light source system based on pulse dense comb frequency modulation also includes a first wavelength division multiplexer; The two input ends of the erbium-doped optical fiber are connected to the input end of the first wavelength division multiplexer, and the detection lights of the same frequency generate interference in the erbium-doped optical fiber to form a refractive index grating; The erbium-doped fiber amplifier is connected to the output end of the first wavelength division multiplexer, and the erbium-doped fiber amplifier is configured to compensate for the detection light loss.

6. The distributed optical fiber detection light source system based on pulse dense frequency modulation according to claim 5, characterized in that: The modulation device further includes: a second optical fiber coupler, a first polarization controller and a second polarization controller; The input end of the second optical fiber coupler is connected to the first delay optical fiber and the end of the second delay optical fiber away from the optical switch; The first polarization controller is disposed between the second fiber coupler and the dual parallel Mach-Zehnder modulator, and is configured to control the polarization-maintaining shape of the detection light input into the dual parallel Mach-Zehnder modulator; The second polarization controller is disposed between the erbium-doped fiber amplifier and the first fiber coupler, and is configured to control the polarization-maintaining shape of the detection light output by the dual-parallel Mach-Zehnder modulator.

7. The distributed optical fiber detection light source system based on pulse dense frequency modulation according to claim 6, characterized in that: The light source assembly includes a light source, a second wavelength division multiplexer and an active phase-shifted fiber grating; The light source is configured to generate pump light of a first wavelength; The second wavelength division multiplexer is connected between the light source and the active phase-shifted fiber grating; the second wavelength division multiplexer is configured to transmit the pump light of the first wavelength to the active phase-shifted fiber grating and output laser light of the second wavelength; The second wavelength is greater than the first wavelength.

8. The distributed optical fiber detection light source system based on pulse dense comb frequency modulation according to claim 7, characterized in that: The modulation device further includes: an isolator and a filter; The isolator is disposed between the first fiber coupler and the optical switch, and is configured to isolate the return light; The filter is disposed between the dual parallel Mach-Zehnder modulator and the first wavelength division multiplexer, and is configured to filter noise.

9. The distributed optical fiber detection light source system based on pulse dense comb frequency modulation according to claim 8, characterized in that: Also included: a third fiber coupler and an electro-optic modulator; The third optical fiber coupler is arranged between the second wavelength division multiplexer and the electro-optical modulator; The electro-optical modulator is connected to the input end of the first optical fiber coupler, and is configured to modulate the detection light.

10. The distributed optical fiber detection light source system based on pulse dense frequency modulation according to claim 9, characterized in that: Also included: a first signal generator and a second signal generator; The first signal generator is connected to the electro-optical modulator, and the first signal generator is used to control the electro-optical modulator; The second signal generator is connected to the dual-parallel Mach-Zehnder modulator, and the second signal generator is used to control the dual-parallel Mach-Zehnder modulator.

Images