Distributed optical fiber sensing system based on set frequency sequence pulse optical time domain reflection technology

Abstract

The embodiment of the application provides a kind of distributed optical fiber sensing system based on set sparse frequency sequence pulse light time domain reflection technology, it is related to optical fiber sensing technical field, including electro-optic modulator, laser frequency modulation module, line width compression module and optical fiber sensing module;The input end of electro-optic modulator is connected with light source, laser frequency modulation module includes fiber coupler and double parallel Mach-Zehnder modulator, the first input end of fiber coupler and the output end of electro-optic modulator are connected, the first output end of fiber coupler and the input end of double parallel Mach-Zehnder modulator are connected, the output end of double parallel Mach-Zehnder modulator and the second input end of fiber coupler are connected;The input end of line width compression module and the second output end of fiber coupler are connected;The output end of line width compression module is connected with the input end of acoustooptic modulator of optical fiber sensing module, to make that the spatial resolution, detection distance and frequency band width etc. Parameters of distributed optical fiber sensing system simultaneously have higher performance.

Description

Technical Field

[0001] This application relates to the field of fiber optic sensing technology, and in particular to a distributed fiber optic sensing system based on the time-domain reflectometry of frequency-sequential pulsed light. Background Technology

[0002] Distributed fiber optic sensing technology refers to a sensing technology that uses external signals along the fiber optic transmission path to modulate the light waves in the fiber, thereby achieving real-time measurement of the continuous space of the measured object. Distributed fiber optic sensing technology has advantages such as a large sensing range, simple structure, ease of use, low signal acquisition cost per unit length, and high cost-effectiveness. Therefore, this technology is widely used in many important military and civilian fields such as pipeline leak detection, seismic wave exploration, and hydrophone detection.

[0003] Among related technologies, distributed fiber optic sensing technology includes optical time domain reflectometry (OTDR) and optical frequency domain reflectometry (OFDR). In practice, OTDR can simultaneously achieve high performance in both detection range and bandwidth, while OFDR can simultaneously achieve high performance in both detection range and spatial resolution.

[0004] However, the spatial resolution of optical time-domain reflectometry (OTDR) is limited by factors such as pulse width and the detection bandwidth of the receiving system, making it difficult to achieve high spatial resolution. High spatial resolution of optical frequency-domain reflectometry (FDR) depends on a wide wavelength tuning range of the light source, but the bandwidth of the measured disturbance signal is limited by the wavelength tuning period of the light source. Therefore, to achieve high spatial resolution and long measurement distance, FDR struggles to measure wide-bandwidth disturbance signals. Consequently, the spatial resolution, detection distance, and bandwidth of existing distributed fiber optic sensing technologies are mutually restrictive. During the application of this technology, it is impossible to simultaneously achieve high performance in spatial distribution rate, detection distance, and bandwidth, thus limiting the detection accuracy of distributed fiber optic sensing. Summary of the Invention

[0005] This application provides a distributed optical fiber sensing system based on clustered-sparse frequency-sequenced pulsed light time-domain reflectometry technology to solve the technical problem that distributed optical fiber sensing systems cannot simultaneously achieve high performance in parameters such as spatial resolution, detection distance, and bandwidth.

[0006] This application provides a distributed optical fiber sensing system based on the time-domain reflectometry of frequency-sequential pulsed light, including an electro-optic modulator, a laser frequency modulation module, a linewidth compression module, and an optical fiber sensing module.

[0007] The input terminal of the electro-optic modulator is used to connect to the light source to receive and modulate the initial pulse width and initial frequency of the laser signal emitted by the light source;

[0008] The laser frequency modulation module includes a fiber coupler and a dual parallel Mach-Zehnder modulator. The first input end of the fiber coupler is connected to the output end of the electro-optic modulator, the first output end of the fiber coupler is connected to the input end of the dual parallel Mach-Zehnder modulator, and the output end of the dual parallel Mach-Zehnder modulator is connected to the second input end of the fiber coupler.

[0009] The dual parallel Mach-Zehnder modulator is configured to modulate the frequency of the laser signal and to make the laser signal circulate within the laser frequency modulation module to achieve an increase in the frequency of the laser signal within the pulse.

[0010] The fiber optic coupler is configured to receive the input of a laser signal and provide the output of a laser signal;

[0011] The input end of the linewidth compression module is connected to the second output end of the fiber optic coupler. The linewidth compression module forms a ring cavity. The linewidth compression module is configured to circulate the laser signal output from the second output end of the fiber optic coupler within the ring cavity to achieve a decrease in the wavelength of the linewidth of the laser signal within the pulse.

[0012] The output of the linewidth compression module is connected to the input of the acousto-optic modulator of the fiber optic sensing module. The acousto-optic modulator is configured to modulate the pulse width and the number of pulse frequencies of the laser signal output by the linewidth compression module.

[0013] In one feasible implementation, a radio frequency drive signal is loaded into a dual parallel Mach-Zehnder modulator;

[0014] The radio frequency drive signal is configured to adjust the frequency of the laser signal output from the first output of the fiber optic coupler;

[0015] The frequency value of the radio frequency drive signal is equal to the change in the laser signal each time it passes through the dual parallel Mach-Zehnder modulator.

[0016] In one feasible implementation, the laser frequency modulation module also includes a time-delay fiber, and the first output of the fiber coupler is connected to the input of the dual parallel Mach-Zehnder modulator via the time-delay fiber.

[0017] The delay fiber is configured to adjust the transmission duration of the laser signal within the laser frequency modulation module.

[0018] In one feasible implementation, the laser frequency modulation module further includes an optical fiber amplifier, the input of which is connected to the output of a dual parallel Mach-Zehnder modulator, and the output of which is connected to the second input of an optical fiber coupler.

[0019] The fiber amplifier is configured to compensate for the loss of the laser signal by the various components within the laser frequency modulation module.

[0020] In one feasible implementation, the laser signal is circulated j times within the laser frequency modulation module, and the pulse linewidth of the j-th laser signal increases linearly with the increase of the number of cycles j.

[0021] In one feasible implementation method

[0022] f j =f DFB-FL +jf ASE

[0023] Among them, f j f is the pulse linewidth of the laser signal after the j-th cycle; DFB-FL f is the linewidth before the laser signal is input to the laser frequency modulation module. ASE This is to broaden the linewidth caused by the spontaneous emission noise of the fiber optic amplifier.

[0024] In one feasible implementation, the linewidth compression module includes a Brillouin ring laser, and the linewidth of the laser signal output from the output terminal of the Brillouin ring laser is derived from the relationship between the linewidth of the laser signal input to the Brillouin ring laser and the parameters of the Brillouin ring laser.

[0025] The parameters of a Brillouin ring laser include the gain spectral width of the single-mode fiber inside the Brillouin ring laser, the fiber length inside the Brillouin ring laser, the propagation speed of the laser signal in the fiber inside the Brillouin ring laser, and the amplitude feedback coefficient of the Brillouin ring laser.

[0026] In one feasible implementation, the linewidth of the laser signal output by the Brillouin ring laser is calculated using the following formula:

[0027]

[0028] Where, Δυ S Δυ is the linewidth of the laser signal output from the Brillouin ring laser. P Δυ is the linewidth of the laser signal output from the second output terminal of the fiber coupler. B denoted as , where is the gain spectral width of the single-mode fiber within the Brillouin ring laser; L is the fiber length within the Brillouin ring laser; c / n is the propagation speed of the laser signal in the fiber of the Brillouin ring laser; and R is the amplitude feedback coefficient of the Brillouin ring laser.

[0029] In one feasible implementation, the laser frequency modulation module also includes an isolator, the input of which is connected to the first output of the fiber optic coupler, and the output of which isolator is connected to the input of a dual parallel Mach-Zehnder modulator via a time-delay fiber.

[0030] The isolator is configured to enable unidirectional output of the laser signal from the isolator to the dual parallel Mach-Zehnder modulator.

[0031] In one feasible implementation, the laser frequency modulation module further includes a filter, the input of which is connected to the output of the fiber amplifier, and the output of which is connected to the second input of the fiber coupler.

[0032] The filter is configured to filter the spontaneous emission noise of the fiber amplifier.

[0033] In one feasible implementation, the laser frequency modulation module further includes a first polarization state controller and a second polarization state controller. The input end of the first polarization state controller is connected to the first output end of the fiber coupler, and the output end of the first polarization state controller is connected to the input end of the dual parallel Mach-Zehnder modulator.

[0034] The input terminal of the second polarization state controller is connected to the output terminal of the filter, and the output terminal of the second polarization state controller is connected to the second input terminal of the fiber coupler.

[0035] Both the first polarization state controller and the second polarization state controller are configured to control the polarization state of the laser signal within the laser frequency modulation module.

[0036] In one feasible implementation, a first function generator and a second function generator are also included, the first function generator being connected to an electro-optic modulator and the second function generator being connected to a dual parallel Mach-Zehnder modulator.

[0037] The first function generator is configured to drive the electro-optic modulator to modulate the initial pulse width and initial frequency of the laser signal;

[0038] The second function generator is configured to drive a dual parallel Mach-Zehnder modulator to modulate the frequency of the laser signal.

[0039] The third function generator is configured to drive the acousto-optic modulator to modulate the pulse width and frequency of the laser signal between pulses.

[0040] This application provides a distributed fiber optic sensing system based on clustered-sparse frequency-sequential pulsed light time-domain reflectometry (TDR). This application utilizes an electro-optic modulator to adjust the initial pulse width and frequency of the laser signal. Within the laser frequency modulation module, a dual parallel Mach-Zehnder modulator can adjust the laser signal frequency. The laser signal can continuously cycle and adjust its frequency within the laser frequency modulation module, thereby achieving dense frequency-converted pulse widths within the laser signal pulses and improving the spatial resolution of the laser signal. This application also uses a linewidth compression module to circulate the laser signal output from the second output end of the fiber optic coupler within a ring cavity, compressing the linewidth of the laser signal and increasing its detection range. Furthermore, this application uses an acousto-optic modulator to adjust the pulse width between laser signal pulses, achieving staggered frequency-converted pulse widths between pulses, thus increasing the laser signal detection range. Additionally, the acousto-optic modulator can also adjust the number of frequencies between pulses, achieving staggered frequency conversion between pulses to broaden the frequency band of the laser signal. The embodiments of this application provide a distributed fiber optic sensing system that can simultaneously achieve high performance in parameters such as spatial resolution, detection distance, and bandwidth. Attached Figure Description

[0041] The accompanying drawings, which are included to provide a further understanding of the invention and form part of this invention, illustrate exemplary embodiments of the invention and, together with their description, serve to explain this application and do not constitute an undue limitation of the invention. In the drawings:

[0042] Figure 1 This is a schematic diagram of the basic structure of a distributed optical fiber sensing system based on the time-domain reflectometry of frequency-sequenced pulsed light, provided in an embodiment of this application.

[0043] Figure 2 yes Figure 1 A schematic diagram of the basic structure of a dual parallel Mach-Zehnder modulator;

[0044] Figure 3 This is a schematic diagram of a pulse after the laser signal is modulated sequentially by an electro-optic modulator and a laser frequency modulation module.

[0045] Figure 4 This is a schematic diagram of a laser signal pulsed sequentially through an electro-optic modulator, a laser frequency modulation module, a linewidth compression module, and then modulated by an acousto-optic modulator in an optical fiber sensing module.

[0046] Figure 5 yes Figure 1 A schematic diagram of the laser signal output from a distributed fiber optic sensing system and a diagram of the demodulated signal.

[0047] Explanation of reference numerals in the attached figures:

[0048] 100 - Light source; 200 - Electro-optic modulator; 300 - Laser frequency modulation module; 400 - Linewidth compression module; 500 - Fiber optic sensing module;

[0049] 210 - First function generator;

[0050] 310 - Fiber optic coupler; 311 - First input terminal; 312 - Second input terminal; 313 - First output terminal; 314 - Second output terminal; 320 - Dual parallel Mach-Zehnder modulator; 321 - Second function generator; 330 - First isolator; 340 - First delay fiber; 350 - First polarization state controller; 360 - First fiber amplifier; 370 - Filter; 380 - Second polarization state controller;

[0051] 410 - First circulator; 420 - Third polarization state controller; 430 - Narrowband filter; 440 - Output coupler; 450 - Second isolator; 460 - Second delay fiber; 470 - Wavelength division multiplexer; 471 - Pump source;

[0052] 510-Acousto-optic modulator; 511-Third function generator; 520-Second fiber optic amplifier; 530-Second circulator; 540-Detection unit; 550-Third fiber optic amplifier; 560-Third circulator; 570-Demodulation unit. Detailed Implementation

[0053] Distributed fiber optic sensing technology refers to a sensing technology that uses external signals along the fiber optic transmission path to modulate the light waves in the fiber, thereby achieving real-time measurement of the continuous space of the measured object. Distributed fiber optic sensing technology has advantages such as a large sensing range, simple structure, ease of use, low signal acquisition cost per unit length, and high cost-effectiveness. Therefore, this technology is widely used in many important military and civilian fields such as pipeline leak detection, seismic wave exploration, and hydrophone detection.

[0054] Among related technologies, distributed fiber optic sensing technology includes optical time domain reflectometry (OTDR) and optical frequency domain reflectometry (OFDR). In practice, OTDR can simultaneously achieve high performance in both detection range and bandwidth, while OFDR can simultaneously achieve high performance in both detection range and spatial resolution.

[0055] However, the spatial resolution of optical time-domain reflectometry (OTDR) is limited by factors such as pulse width and the detection bandwidth of the receiving system, making it difficult to achieve high spatial resolution. High spatial resolution of optical frequency-domain reflectometry (FDR) depends on a wide wavelength tuning range of the light source, but the bandwidth of the measured disturbance signal is limited by the wavelength tuning period of the light source. Therefore, to achieve high spatial resolution and long measurement distance, FDR struggles to measure wide-bandwidth disturbance signals. Consequently, the spatial resolution, detection distance, and bandwidth of existing distributed fiber optic sensing technologies are mutually restrictive. During the application of this technology, it is impossible to simultaneously achieve high performance in spatial distribution rate, detection distance, and bandwidth, thus limiting the detection accuracy of distributed fiber optic sensing.

[0056] Therefore, this application provides a distributed optical fiber sensing system based on clustered-sparse frequency-sequenced pulsed light time-domain reflectometry technology to solve the technical problem that existing distributed optical fiber sensing systems cannot simultaneously achieve high performance in parameters such as spatial resolution, detection distance, and bandwidth.

[0057] Figure 1 This is a schematic diagram of the basic structure of a distributed optical fiber sensing system based on the time-domain reflectometry of frequency-sequential pulsed light, provided in an embodiment of this application.

[0058] This application provides a distributed fiber optic sensing system based on the sparse-sparse frequency-sequence pulsed light time-domain reflectometry technique, referring to... Figure 1 It includes: an electro-optic modulator 200, a laser frequency modulation module 300, a linewidth compression module 400, and an optical fiber sensing module 500.

[0059] The input terminal of the electro-optic modulator 200 is used to connect to the light source 100 to receive and modulate the initial pulse width and initial frequency of the laser signal emitted by the light source 100.

[0060] The laser frequency modulation module 300 includes an optical fiber coupler 310 and a dual parallel Mach-Zehnder modulator 320. The first input terminal 311 of the optical fiber coupler 310 is connected to the output terminal of the electro-optic modulator 200, the first output terminal 313 of the optical fiber coupler 310 is connected to the input terminal of the dual parallel Mach-Zehnder modulator 320, and the output terminal of the dual parallel Mach-Zehnder modulator 320 is connected to the second input terminal 312 of the optical fiber coupler 310. The dual parallel Mach-Zehnder modulator 320 is configured to modulate the frequency of the laser signal and make the laser signal circulate within the laser frequency modulation module 300 to achieve an increase in the frequency of the laser signal within a pulse. The optical fiber coupler 310 is configured to receive the input of the laser signal and provide the output of the laser signal.

[0061] The input end of the linewidth compression module 400 is connected to the second output end 314 of the fiber optic coupler 310. The linewidth compression module 400 forms a ring cavity. The linewidth compression module 400 is configured to circulate the laser signal output from the second output end 314 of the fiber optic coupler 310 within the ring cavity to achieve a decrease in the wavelength of the linewidth of the laser signal within the pulse. The output end of the linewidth compression module 400 is connected to the input end of the acousto-optic modulator 510 of the fiber optic sensing module 500. The acousto-optic modulator 510 is configured to modulate the pulse width and the number of pulse frequencies of the laser signal output by the linewidth compression module 400.

[0062] For example, the light source 100 can be configured as a laser, such as a single-frequency fiber laser based on an active phase-shifting grating.

[0063] For example, the light source 100 outputs a laser signal to the input of the electro-optic modulator 200. For instance, the center wavelength of the laser signal output by the laser can be set to 1550.12 nm, the linewidth to less than 3 kHz, and the phase noise to less than 110 dBrad² / Hz after noise suppression. The laser signal passes through the electro-optic modulator 200, and the initial pulse width and initial frequency of the laser signal are set within the electro-optic modulator 200. For example, after modulation by the electro-optic modulator 200, the laser signal forms a laser signal with an initial pulse width of τ and an initial frequency of f. A portion of the laser signal is output sequentially through the first input terminal 311 and the first output terminal 313 of the fiber optic coupler 310, and another portion of the laser signal is output sequentially through the first input terminal 311 and the second output terminal 314 of the fiber optic coupler 310 to the laser frequency modulation module 300. The laser signal first passes through a dual parallel Mach-Zehnder modulator. After modulation by 320, the frequency of the laser signal is modulated to f+Ω by the dual parallel Mach-Zehnder modulator 320. Part of the laser signal with a frequency of f+Ω is output sequentially through the second input terminal 312 and the first output terminal 313 of the fiber optic coupler 310, and the other part is output sequentially through the second input terminal 312 and the first output terminal 313 of the fiber optic coupler 310 to the laser frequency modulation module 300. After passing through the dual parallel Mach-Zehnder modulator 320 again, the frequency of the laser signal is modulated to f+2Ω. After multiple cycles, the frequency of the laser signal can be increased within the pulse.

[0064] After being modulated by a dual parallel Mach-Zehnder modulator 320, the laser signal is output sequentially through the first input terminal 311 and the first output terminal 313 of the fiber optic coupler 310 to the linewidth compression module 400. The laser signal circulates within the annular cavity of the linewidth compression module 400 to continuously compress the linewidth, and is finally output from the output terminal of the linewidth compression module 400 to the fiber optic sensing module 500. After the acousto-optic modulator 510 of the fiber optic sensing module 500 adjusts the pulse width and the number of pulses between the laser signal, the laser signal is emitted to the object to be detected.

[0065] From the above description, it can be seen that this solution achieves the following technical effects:

[0066] This application provides a distributed fiber optic sensing system based on clustered-sparse frequency-sequential pulsed light time-domain reflectometry (CTPR). In this embodiment, an electro-optic modulator 200 adjusts the initial pulse width and initial frequency of the laser signal. Within the laser frequency modulation module 300, a dual parallel Mach-Zehnder modulator 320 adjusts the frequency of the laser signal. The laser signal can continuously cycle and adjust its frequency within the laser frequency modulation module 300, thereby achieving dense frequency-converted pulse widths within the laser signal pulses and improving the spatial resolution of the laser signal. The linewidth compression module 400 allows the laser signal output from the second output terminal 314 of the fiber optic coupler 310 to circulate within a ring cavity, compressing the linewidth of the laser signal and increasing its detection range. The acousto-optic modulator 510 adjusts the pulse width between laser signal pulses, achieving staggered frequency-converted pulse widths between pulses, further increasing the detection range of the laser signal. Additionally, the acousto-optic modulator 510 can adjust the number of frequencies between pulses, achieving staggered frequency-converted pulse counts between pulses to broaden the frequency band of the laser signal. The embodiments of this application provide a distributed fiber optic sensing system that can simultaneously achieve high performance in parameters such as spatial resolution, detection distance, and bandwidth.

[0067] In some examples, a radio frequency (RF) drive signal is loaded within the dual parallel Mach-Zehnder modulator 320. The RF drive signal is configured to adjust the frequency of the laser signal output from the first output 313 of the fiber optic coupler 310. The frequency value of the RF drive signal is equal to the change in the laser signal each time it passes through the dual parallel Mach-Zehnder modulator 320.

[0068] Figure 2 yes Figure 1 A schematic diagram of the basic structure of the 320 dual parallel Mach-Zehnder modulator.

[0069] For example, refer to Figure 2The dual-parallel Mach-Zehnder modulator 320 uses a Ti-diffused LiNbO3 waveguide to form the master Mach-Zehnder interferometer (MZI) structure. The laser signal input to the dual-parallel Mach-Zehnder modulator 320 is split into four paths. Figure 2 The middle arrow "Input" indicates that the laser signal is input to the dual parallel Mach-Zehnder modulator 320, forming two sub-MZI structures (such as...). Figure 2 As shown, the two MZI structures are MZI1 and MZI2, and these two sub-MZIs serve as two arms, forming a main MZI structure. Each arm of MZI1 and MZI2 contains two LN modulators (e.g., ...). Figure 2 As shown, the four LN modulators (LN1, LN2, LN3, and LN4) are controlled by an external RF drive signal. The RF drive signal applied to MZI2 is phase-delayed by π / 2 with the RF drive signal on MZI1 through a 90° phase shifter. Each of the MZI1, MZI2, and one of the sub-arms of the main MZI structure has a phase shifter. and Controlled by DC bias voltages V1, V2, and V3, the two arms can be respectively used to achieve... and Phase difference tuning.

[0070] The input signal e of the dual parallel Mach-Zehnder modulator 320 input The formula for calculating (t) is as follows:

[0071]

[0072] In formula (1), A0 is the amplitude of the laser signal input to the dual parallel Mach-Zehnder modulator 320, and ω0 is the angular frequency of the laser signal input to the dual parallel Mach-Zehnder modulator 320. The initial phase of the laser signal input to the dual parallel Mach-Zehnder modulator 320.

[0073] Let the radio frequency signal V output by the RF source be... RF The calculation formula is:

[0074]

[0075] In formula (2), ω k These represent the amplitude and center frequency of the RF signal, respectively.

[0076] e input(t) The laser signal entering LN1 is jointly modulated by the RF signal and the DC bias voltage V1 to generate a phase change. The phase shift generated after being modulated by the RF signal is mcos(ω c t), where m is the RF modulation coefficient m = πV A / V π , and the phase shift generated by the modulation of V1 is

[0077] The laser signals entering LN2, LN3, and LN4 can be obtained in the same way.

[0078] Among them, the output signal e ouput (t) of the dual parallel Mach-Zehnder modulator 320 is calculated by the formula:

[0079]

[0080] Among them, in formula (3), J j (m) is the Bessel function of the first kind, and w c refers to the angular frequency of the RF signal.

[0081] It can be seen from formula (3) that after the optical signal is modulated by the dual parallel Mach-Zehnder modulator 320, the initial single-frequency laser signal will generate a series of sideband signals after being modulated by the dual parallel Mach-Zehnder modulator 320. The amplitude of each order of sideband component is jointly determined by the modulation coefficient of the RF signal and the phase shift caused by the three DC bias voltages. By adjusting the magnitudes of m, V1, V2, and V3, the amplitude of each order of sideband component can be controlled.

[0082] If the dual parallel Mach-Zehnder modulator 320 realizes frequency shift of the input laser signal, the magnitude of the frequency shift is exactly equal to the frequency of the RF drive signal, and the laser signal output after being modulated by the dual parallel Mach-Zehnder modulator 320 only needs to contain +1 order or -1 order sidebands.

[0083] When the modulation coefficient 0 < m < 4, it should be noted here that the values of Bessel functions above the 3rd order are relatively low, and the sidebands above the 3rd order are ignored at this time:

[0084]

[0085] In order to minimize the magnitude of the 3rd order component and increase the value of the 1st order component, the magnitude of the modulation coefficient m can be appropriately adjusted to make J1(m) >> J3(m), and at the same time, it is also necessary to satisfy that J1(m) has a relatively large value to obtain a better frequency shift effect. The modulation coefficient m is related to the energy of the RF signal loaded on the dual parallel Mach-Zehnder modulator 320.

[0086] In this embodiment, by loading a radio frequency drive signal onto the dual parallel Mach-Zehnder modulator 320, the frequency of the laser signal can be adjusted by regulating the change value of the laser signal each time it passes through the dual parallel Mach-Zehnder modulator 320.

[0087] In some examples, the laser frequency modulation module 300 also includes a first delay fiber 340, through which the first output 313 of the fiber coupler 310 is connected to the input of the dual parallel Mach-Zehnder modulator 320. The first delay fiber 340 is configured to adjust the transmission duration of the laser signal within the laser frequency modulation module 300.

[0088] For example, the length of the first delay fiber 340 is adjustable, thereby adjusting the transmission duration of the laser signal within the laser frequency modulation module 300. The longer the transmission duration of the laser signal within the laser frequency modulation module 300, the more times the laser signal cycles within the laser frequency modulation module 300, and the larger the sweep frequency range of the laser signal output to the linewidth compression module 400.

[0089] Figure 3 This is a schematic diagram of the pulse after the laser signal is modulated sequentially by the electro-optic modulator 200 and the laser frequency modulation module 300.

[0090] Reference Figure 3 As shown, where, Figure 3 The midline segment E represents the pulse width within the pulse, from f1 to f... n Δf refers to the repetition frequency within a pulse, Δt refers to the maximum change in the repetition frequency within a pulse, and τ refers to the pulse width within a pulse. Figure 3 As can be seen, the laser signal is modulated by the electro-optic modulator 200 and the laser frequency modulation module 300 to achieve dense frequency conversion pulse width within the pulse, thereby improving the spatial resolution of the distributed fiber optic sensing system.

[0091] The embodiments of this application, by setting the first delay fiber 340, can adjust the transmission duration of the laser signal within the laser frequency modulation module 300, thereby adjusting the change in pulse width within the pulse and realizing dense frequency conversion pulse width within the pulse to improve the spatial resolution of the distributed fiber optic sensing system.

[0092] In some examples, the laser frequency modulation module 300 also includes an optical fiber amplifier, the input of which is connected to the output of a dual parallel Mach-Zehnder modulator 320, and the output of which is connected to the second input 312 of an optical fiber coupler 310. The optical fiber amplifier is configured to compensate for losses in the laser signal caused by the various devices within the laser frequency modulation module 300.

[0093] It should be noted that the fiber amplifier of the laser frequency modulation module 300 is the first fiber amplifier 360.

[0094] For example, the first fiber amplifier 360 can be configured as an erbium-doped fiber amplifier.

[0095] It is feasible that the laser signal is frequency-shifted in the dual parallel Mach-Zehnder modulator 320 and then input to the input end of the first fiber amplifier 360. It is amplified in the first fiber amplifier 360 and then output to the linewidth compression module 400 through the first input end 311 and the first output end 313 of the fiber coupler 310.

[0096] In this embodiment of the application, the laser signal output by the dual parallel Mach-Zehnder modulator 320 can be amplified by setting the first fiber amplifier 360, thereby realizing the amplification of the laser signal.

[0097] For example, while the first fiber amplifier 360 amplifies the laser signal, it will generate spontaneous radiation effects in the laser frequency modulation module 300. Noise will also pass through the first fiber amplifier 360 and be continuously amplified and accumulated. As the number of cycles of the laser signal in the laser frequency modulation module 300 increases, a large number of non-homogeneous optical field phases accumulate on the spectral components after multiple frequency shifts.

[0098] For example, the laser signal is circulated j times within the laser frequency modulation module 300, and j spectral lines are output. The frequency of the laser signal is f. k Then, considering the time-varying phase of the j-th spectral line, it is:

[0099]

[0100] In formula (5), The initial phase of the laser signal. The phase change is introduced by the spontaneous noise accumulated multiple times by the first fiber amplifier 360, where k is the phase change from the kth cycle to the kth cycle; τ L This is the time it takes for the laser signal to cycle through the frequency modulation module once.

[0101] For example, the relationship between the pulse linewidth of the laser signal after the j-th cycle, the linewidth of the laser signal before it is input to the laser frequency modulation module 300, and the linewidth broadening caused by the spontaneous emission noise of the first fiber amplifier 360 can be expressed by the following formula:

[0102] f j =f DFB-FL +jf ASE (6)

[0103] In formula (6), f jf is the pulse linewidth of the laser signal after the j-th cycle; DFB-FL f is the linewidth before the laser signal is input to the laser frequency modulation module 300; ASE The linewidth is broadened to compensate for the spontaneous emission noise of the first fiber amplifier 360.

[0104] As can be seen from formula (6), the linewidth of the j-th order spectral line increases linearly with the increase of the number of cycles j. By optimizing the number of cycles, a balance between the sweep frequency range and the laser linewidth parameter can be achieved.

[0105] In some examples, the linewidth compression module 400 includes a Brillouin ring laser, and the linewidth of the laser signal output from the output of the Brillouin ring laser is derived from the relationship between the linewidth of the laser signal input to the Brillouin ring laser and the parameters of the Brillouin ring laser.

[0106] The parameters of a Brillouin ring laser include the gain spectral width of the single-mode fiber inside the Brillouin ring laser, the fiber length inside the Brillouin ring laser, the propagation speed of the laser signal in the fiber inside the Brillouin ring laser, and the amplitude feedback coefficient of the Brillouin ring laser.

[0107] For example, refer to Figure 1 The Brillouin ring laser includes a first circulator 410, a third polarization state controller 420, a narrowband filter 430, a wavelength division multiplexer 470, a pump source 471, a second isolator 450, and an output coupler 440.

[0108] The first end of the first circulator 410 is connected to the output end of the laser frequency modulation module 300, the second end of the first circulator 410 is connected to the input end of the third polarization state controller 420, and the output end of the third polarization state controller 420 is connected to the narrowband filter 430.

[0109] The third terminal of the first circulator 410 is connected to the input terminal of the wavelength division multiplexer 470, and the input terminal of the wavelength division multiplexer 470 is also connected to the pump source 471. The output terminal of the wavelength division multiplexer 470 is connected to the input terminal of the second delay fiber 460 and the second isolator 450 through the second delay fiber 460. The output terminal of the second isolator 450 is connected to the input terminal of the output coupler 440. The output terminal of the output coupler 440 and the output terminal of the narrowband filter 430 are connected through an optical fiber and an optical fiber sensing system.

[0110] The laser signal is output from the output of the laser frequency modulation module 300 to the first circulator 410, and then enters the Brillouin ring cavity clockwise through the first circulator 410. After passing through the third polarization state controller 420, it passes through the narrowband filter 430. In the narrowband filter 430, the laser signal with a relatively wide linewidth can be converted into a first-order Stokes light with a narrower linewidth. At the same time, the pump source 471 emits a reverse Stokes light to the wavelength division multiplexer 470, which is transmitted counterclockwise in the ring cavity to form a resonant amplification. The second delay fiber 460 provides linear optical amplification. Part of the first-order Stokes light is output through the output of the Brillouin ring laser, and the other part interacts with the reverse Stokes light and passes through the narrowband filter 430 again to compress the linewidth, thereby making the Brillouin ring laser output a narrower laser signal.

[0111] The power of the laser signal output by the Brillouin ring laser is affected by multiple parameters such as pump, gain length, and splitting ratio of output coupler 440. The relationship between these parameters is established in order to obtain an optimal value through parameter optimization.

[0112] For example, the linewidth of the laser signal output by a Brillouin ring laser is calculated using the following formula:

[0113]

[0114] Where, Δυ S Δυ is the linewidth of the laser signal output from the Brillouin ring laser. P Δυ is the linewidth of the laser signal output from the second output terminal 314 of the fiber optic coupler 310. B denoted as , where is the gain spectral width of the single-mode fiber within the Brillouin ring laser; L is the fiber length within the Brillouin ring laser; c / n is the propagation speed of the laser signal in the fiber of the Brillouin ring laser; and R is the amplitude feedback coefficient of the Brillouin ring laser.

[0115] The mathematical model for the power of a Brillouin ring cavity laser is similar to that of stimulated Brillouin scattering (SBS) in erbium-doped fiber, the difference being that cavity feedback leads to different boundary conditions. The specific model for the output power of a Brillouin ring laser is shown below:

[0116]

[0117]

[0118]

[0119]

[0120] In formula (11), P p0It is the pump power of 980nm, P B η is the Brillouin pump power, K is the output coupling ratio of the fiber coupler 310, and η is the additional loss of the resonant cavity.

[0121] p, 0, and 1 represent 980nm pump light, Brillouin pump light, and first-order Stokes light, respectively; P p P0 represents the power of the 980mm pump light in the erbium-doped fiber; P1 represents the power of the Brillouin pump light in the erbium-doped fiber; and P2 represents the power of the first-order Stokes light in the erbium-doped fiber.

[0122] Forward 980nm pump light and Brillouin pump light are injected into the fiber from the beginning of the fiber at Z=0, while backward Stokes light and backward 980nm pump light are generated and transmitted from the end of the fiber at z=L.

[0123] a i and g i (i = p, 0 and 1) represent the corresponding light loss and gain; l i (i = p, 0 and 1) are the additional losses of each beam; N2 is the population inversion; A eff It refers to the effective area within the Brillouin annular cavity.

[0124] The relaxation method can be used to simulate and calculate the laser output based on formula (11), and the basic relationship between the laser output and each parameter can be obtained according to different fiber parameters, device structure parameters, etc. Through the study of Brillouin ring cavity structure and the study of Brillouin laser single-mode operation mechanism and conditions, the linewidth compression of time-gain-switched ultra-high-speed pulsed laser can be realized.

[0125] For example, the laser frequency modulation module 300 also includes a first isolator 330, the input end of the first isolator 330 is connected to the first output end 313 of the fiber optic coupler 310, and the output end of the first isolator 330 is connected to the input end of the dual parallel Mach-Zehnder modulator 320 through the first delay fiber 340.

[0126] The first isolator 330 is configured to enable unidirectional output of the laser signal from the first isolator 330 to the dual parallel Mach-Zehnder modulator 320.

[0127] In this embodiment, the first isolator 330 ensures that the laser signal is output unidirectionally from the first output end 313 of the fiber coupler 310 to the dual parallel Mach-Zehnder modulator 320, thus preventing the laser signal from being reversed from the input end of the dual parallel Mach-Zehnder modulator 320 back into the first output end 313 of the fiber isolator, which would affect the laser frequency modulation module 300's modulation effect on the laser signal.

[0128] For example, the laser frequency modulation module 300 also includes a filter 370, the input of which is connected to the output of the fiber amplifier, and the output of which is connected to the second input 312 of the fiber coupler 310. The filter 370 is configured to filter spontaneous emission noise from the fiber amplifier.

[0129] This embodiment of the application, by setting the filter 370, can filter the spontaneous emission noise of the fiber amplifier in the laser frequency modulation module 300, thereby improving the anti-interference ability and signal-to-noise ratio of the laser signal, and thus improving the detection accuracy of the distributed fiber optic sensing system.

[0130] For example, the laser frequency modulation module 300 further includes a first polarization state controller 350 and a second polarization state controller 380. The input terminal of the first polarization state controller 350 is connected to the first output terminal 313 of the fiber optic coupler 310, and the output terminal of the first polarization state controller 350 is connected to the input terminal of the dual parallel Mach-Zehnder modulator 320. The input terminal of the second polarization state controller 380 is connected to the output terminal of the filter 370, and the output terminal of the second polarization state controller 380 is connected to the second input terminal 312 of the fiber optic coupler 310. Both the first polarization state controller 350 and the second polarization state controller 380 are configured to control the polarization state of the laser signal within the laser frequency modulation module 300.

[0131] This embodiment of the application, through the setting of the first polarization state controller 350 and the second polarization state controller 380, can adjust the polarization state of the laser signal entering the laser frequency modulation module 300, improve the signal-to-noise ratio of the laser signal, and further improve the detection accuracy of the distributed optical fiber sensing system.

[0132] In some examples, a first function generator 201 and a second function generator 321 are also included, the first function generator 201 being connected to the electro-optic modulator 200 and the second function generator 321 being connected to the dual parallel Mach-Zehnder modulator 320;

[0133] The first function generator 201 is configured to drive the electro-optic modulator 200 to modulate the initial pulse width and initial frequency of the laser signal;

[0134] The second function generator 321 is configured to drive the dual parallel Mach-Zehnder modulator 320 to modulate the frequency of the laser signal.

[0135] The third function generator 511 is configured to drive the acousto-optic modulator 510 to modulate the pulse width and the number of pulse frequencies of the laser signal.

[0136] In some examples, the fiber optic sensing module 500 includes an acousto-optic modulator 510, a second fiber optic amplifier 520, a second circulator 530, a detection unit 540, a third fiber optic amplifier 550, a third circulator 560, and a demodulation unit 570.

[0137] The input terminal of the acousto-optic modulator 510 is connected to the output terminal of the linewidth compression module 400. The output terminal of the acousto-optic modulator 510 is connected to the input terminal of the second fiber amplifier 520. The input terminal of the second fiber amplifier 520 is connected to the first terminal of the second circulator 530. The second terminal of the second circulator 530 is connected to the input terminal of the detection unit 540. The third terminal of the second circulator 530 is connected to the input terminal of the third fiber amplifier 550. The output terminal of the third fiber amplifier 550 is connected to the first terminal of the third circulator 560. The second and third terminals of the third circulator 560 are connected to the input terminal of the demodulation unit 570.

[0138] For example, the output of the linewidth compression module 400 outputs the laser signal to the input of the acousto-optic modulator 510. After the acousto-optic modulator 510 modulates the pulse width and the number of laser frequencies between laser signal pulses, it outputs the laser signal to the second fiber amplifier 520. The laser signal is amplified in the second fiber amplifier 520 and then output to the detection unit 540 through the first end of the circulator. The detection unit 540 outputs the laser signal into the environment to be detected. The laser signal excites Rayleigh scattering signal in the detection unit 540. The Rayleigh scattering signal is output to the demodulation unit 570 through the first end of the third circulator 560. The demodulation of the sound wave is completed in the demodulation unit 570.

[0139] Figure 4 This is a schematic diagram of a pulse after the laser signal passes through an electro-optic modulator 200, a laser frequency modulation module 300, a linewidth compression module 400, and is modulated by an acousto-optic modulator 510 of an optical fiber sensing module 500.

[0140] Reference Figure 4 As shown, Figure 4 The line segment F1 represents the pulse width within a pulse, and the line segment F2 represents the width between pulses. (f1 to f...) n This refers to the repetition frequency within the pulse, f. n to f m This refers to the repetition frequency between pulses, U refers to the pulse width within a pulse, W refers to the pulse width between pulses, T refers to one pulse period, and f refers to the repetition frequency between pulses. N It refers to the pulse width within one pulse cycle. From Figure 4It can be seen that after the laser signal passes through the electro-optic modulator 200, the laser frequency modulation module 300, the linewidth compression module 400, and the acousto-optic modulator 510, it achieves dense frequency conversion of the pulse width within the pulse, thereby improving the spatial resolution; diffuse frequency conversion of the pulse width between pulses, thereby increasing the detection distance; and adjusting the number of diffuse frequency conversion frequencies between pulses to achieve a widening of the frequency band.

[0141] It should be noted that in the prior art, the Rayleigh scattering light of the fiber optic distributed sensor is as follows: at t=0, a narrow linewidth pulse light with frequency f and pulse width W is introduced into the fiber, and the system Rayleigh scattering signal is:

[0142]

[0143] In formula (12), i represents the i-th scattering point, τ i a represents the time it takes for the Rayleigh scattering signal to return to the incident end. i The amplitude of the scattered light is represented by N, the number of scattering points on the fiber is represented by c, and the speed of light is represented by n. f α represents the refractive index of the optical fiber, and α represents the attenuation coefficient of the optical fiber.

[0144]

[0145] The timing gain-switching pulsed laser source 100 signal of the frequency-sequential pulsed light time-domain reflectometry system can be expressed as:

[0146]

[0147] The signal injected into the fiber optic sensing module 500 is as follows Figure 4 As shown, within a long pulse period T, the acousto-optic modulator 510 controls the number of dense frequency conversions within the pulse and the pulse width U within the pulse. The acousto-optic modulator 510 also controls the number of sparse frequency conversions between pulses, the pulse width W between pulses, and the equal interval time t. By simultaneously optimizing the signal modulation of the electro-optic modulator 200 and the acousto-optic modulator 510, a pulsed laser sequence with multiple frequency change modes within and between pulses is modulated, generating a periodic cluster-sparse frequency conversion pulse sequence.

[0148] In this embodiment, at t=0, a periodic frequency-converted pulse sequence is introduced into the optical fiber, and the Rayleigh scattering signal is transformed by formula (12):

[0149]

[0150] As can be seen from formula (15): N frequency light pulses sequentially excite backscattered Rayleigh light in a certain time sequence. The backscattered Rayleigh light detected in the same period contains Rayleigh scattering signals of N frequency light pulses at different positions in succession.

[0151] To address the competitive constraints among high spatial resolution, long detection distance, and wide bandwidth in traditional distributed fiber optic sensing systems, a mechanism is established to balance parameters such as the number of intra-pulse dense frequency conversions, pulse width, the number of inter-pulse sparse frequency conversions, pulse width, and equal interval time.

[0152] Figure 5 yes Figure 1 A pulse diagram and a demodulated signal spectrum of the laser signal output by the distributed fiber optic sensing system.

[0153] like Figure 5 As shown, coordinate A is the pulse diagram output by the distributed optical fiber sensing system based on the time-domain reflectometry of frequency-sequential pulse light provided in this embodiment of the application. The laser signal pulses in the distributed optical fiber sensor have relatively dense frequency-converted pulse widths within the pulse, thereby improving the spatial resolution of the distributed optical fiber sensor. The reference curve a is the curve of the amplitude of the laser signal changing with the detection distance. It can be concluded that within the pulse width of one pulse, the corresponding detection distance is 0.5m, thus improving the spatial resolution.

[0154] Curve b is the curve of the amplitude of the laser signal changing with the detection distance. It can be seen that in coordinate A, the number of frequency conversions between pulses is sparsely distributed, thereby increasing the detection distance.

[0155] Curve e represents the amplitude of a 0.1 Hz laser signal over time; curve f represents the amplitude of a 1 Hz laser signal over time; curve g represents the amplitude of a 100 Hz laser signal over time; curve h represents the amplitude of a 1 kHz laser signal over time; curve m represents the amplitude of a 10 kHz laser signal over time; and curve n represents the amplitude of a 20 kHz laser signal over time. It can be seen that the laser signal output by the distributed fiber optic sensor in this embodiment has multiple different frequencies between pulses, thereby widening the bandwidth.

[0156] Curves c and d are both signals to be measured. Curve c1 is the demodulation result curve output by the distributed optical fiber sensing system based on the time-domain reflectometry of frequency-sequential pulsed light provided in this embodiment of the application, and curve d1 is the demodulation result curve output by the distributed optical fiber sensing system in the prior art. A comparison of curves c1 and d1 shows that the fluctuation trend of curve c1 is quite similar to that of the signal to be measured. Therefore, the distributed optical fiber sensor provided in this embodiment of the application can detect the object to be measured more accurately.

Claims

1. A distributed optical fiber sensing system based on clustered-sparse frequency-sequenced pulsed light time-domain reflectometry, characterized in that, include: Electro-optic modulator, laser frequency modulation module, linewidth compression module, and fiber optic sensing module; The input terminal of the electro-optic modulator is used to connect to the light source to receive and modulate the initial pulse width and initial frequency of the laser signal emitted by the light source; The laser frequency modulation module includes an optical fiber coupler, a time-delay fiber, and a dual parallel Mach-Zehnder modulator. The first input end of the optical fiber coupler is connected to the output end of the electro-optic modulator. The first output end of the optical fiber coupler is connected to the input end of the dual parallel Mach-Zehnder modulator through the time-delay fiber. The output end of the dual parallel Mach-Zehnder modulator is connected to the second input end of the optical fiber coupler. The time-delay fiber is configured to adjust the transmission duration of the laser signal within the laser frequency modulation module. The dual parallel Mach-Zehnder modulator is configured to modulate the frequency of the laser signal and to circulate the laser signal within the laser frequency modulation module to achieve an increase in the frequency of the laser signal within a pulse. The fiber optic coupler is configured to receive the input of the laser signal and provide the output of the laser signal; The input end of the linewidth compression module is connected to the second output end of the fiber optic coupler. The linewidth compression module forms a ring cavity. The linewidth compression module is configured to circulate the laser signal output from the second output end of the fiber optic coupler, whose linewidth is broadened by circulating within the laser frequency modulation module, within the ring cavity, so as to achieve a wavelength decrease in the linewidth of the laser signal within the pulse. The output of the linewidth compression module is connected to the input of the acousto-optic modulator of the fiber optic sensing module. The acousto-optic modulator is configured to modulate the pulse width and the number of pulse frequencies of the laser signal output by the linewidth compression module. The dual parallel Mach-Zehnder modulator is loaded with a radio frequency drive signal. The radio frequency drive signal is configured to adjust the frequency of the laser signal output from the first output terminal of the fiber coupler; The frequency value of the radio frequency drive signal is equal to the change in the laser signal each time it passes through the dual parallel Mach-Zehnder modulator.

2. The distributed optical fiber sensing system based on the sparse-frequency-sequence pulsed light time-domain reflectometry technology according to claim 1, characterized in that, The laser frequency modulation module also includes an optical fiber amplifier, the input end of which is connected to the output end of the dual parallel Mach-Zehnder modulator, and the output end of which is connected to the second input end of the optical fiber coupler. The fiber amplifier is configured to compensate for the loss of the laser signal by the various devices within the laser frequency modulation module.

3. The distributed optical fiber sensing system based on the sparse-sparse frequency-sequence pulse light time-domain reflectometry technology according to claim 2, characterized in that, The laser signal is circulated j times within the laser frequency modulation module. The pulse linewidth of the laser signal in the jth cycle increases linearly with the increase of the number of cycles j.

4. The distributed optical fiber sensing system based on the sparse-sparse frequency-sequence pulse light time-domain reflectometry technology according to claim 3, characterized in that, in, The pulse linewidth of the laser signal after the j-th cycle; The linewidth before the laser signal is input to the laser frequency modulation module; The linewidth is broadened to compensate for the spontaneous emission noise of the fiber amplifier.

5. The distributed optical fiber sensing system based on the sparse-frequency-sequence pulse time-domain reflectometry technology according to claim 1, characterized in that, The linewidth compression module includes a Brillouin ring laser, and the linewidth of the laser signal output from the output terminal of the Brillouin ring laser is derived from the relationship between the linewidth of the laser signal input to the Brillouin ring laser and the parameters of the Brillouin ring laser. The parameters of the Brillouin ring laser include the gain spectral width of the single-mode fiber within the Brillouin ring laser, the fiber length within the Brillouin ring laser, the propagation speed of the laser signal in the fiber within the Brillouin ring laser, and the amplitude feedback coefficient of the Brillouin ring laser.

6. The distributed optical fiber sensing system based on the sparse-sparse frequency-sequence pulse light time-domain reflectometry technology according to claim 5, characterized in that, The linewidth of the laser signal output by the Brillouin ring laser is calculated using the following formula: in, The linewidth of the laser signal output by the Brillouin ring laser; The linewidth of the laser signal output from the second output terminal of the fiber coupler; denoted as , where L is the gain spectral width of the single-mode fiber within the Brillouin ring laser; L is the fiber length within the Brillouin ring laser; c / n is the propagation speed of the laser signal in the fiber of the Brillouin ring laser; and R is the amplitude feedback coefficient of the Brillouin ring laser.

7. The distributed optical fiber sensing system based on the sparse-frequency-sequence pulse time-domain reflectometry technology according to claim 1, characterized in that, The laser frequency modulation module also includes an isolator, the input end of which is connected to the first output end of the fiber coupler, and the output end of the isolator is connected to the input end of the dual parallel Mach-Zehnder modulator through the delay fiber. The isolator is configured to enable unidirectional output of the laser signal from the isolator to the dual parallel Mach-Zehnder modulator.

8. The distributed optical fiber sensing system based on the sparse-frequency-sequence pulse time-domain reflectometry technology according to claim 2, characterized in that, The laser frequency modulation module also includes a filter, the input end of which is connected to the output end of the fiber amplifier, and the output end of which is connected to the second input end of the fiber coupler; The filter is configured to filter out the spontaneous emission noise of the fiber amplifier.

9. The distributed optical fiber sensing system based on the sparse-frequency-sequence pulse time-domain reflectometry technology according to claim 8, characterized in that, The laser frequency modulation module further includes a first polarization state controller and a second polarization state controller. The input terminal of the first polarization state controller is connected to the first output terminal of the fiber coupler, and the output terminal of the first polarization state controller is connected to the input terminal of the dual parallel Mach-Zehnder modulator. The input terminal of the second polarization state controller is connected to the output terminal of the filter, and the output terminal of the second polarization state controller is connected to the second input terminal of the fiber coupler; Both the first polarization state controller and the second polarization state controller are configured to control the polarization state of the laser signal within the laser frequency modulation module.

10. The distributed optical fiber sensing system based on the sparse-frequency-sequence pulsed light time-domain reflectometry technology according to claim 1, characterized in that, It also includes a first function generator, a second function generator, and a third function generator, wherein the first function generator is connected to the electro-optic modulator, and the second function generator is connected to the dual parallel Mach-Zehnder modulator; The first function generator is configured to drive the electro-optic modulator to modulate the initial pulse width and initial frequency of the laser signal; The second function generator is configured to drive the dual parallel Mach-Zehnder modulator to modulate the frequency of the laser signal; The third function generator is configured to drive the acousto-optic modulator to modulate the pulse width and the number of pulse frequencies of the laser signal.

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