Low-frequency-shift all-fiber frequency shifter and heterodyne detection system based on acoustic fiber Bragg grating

Abstract

This application discloses a low-frequency all-fiber frequency shifter and heterodyne detection system based on acoustic fiber grating. The frequency shifter includes a few-mode fiber, a polarization controller and a fiber acousto-optic unit. The few-mode fiber is used to transmit LP 01 Mode or LP 11 Mode beam; fiber optic acousto-optic unit is used to transmit the beam from LP 01 Mode conversion to LP 11 mode, or from LP 11 Mode conversion to LP 01 mode, and the resulting frequency shift; a polarization controller is used to adjust the polarization state of the light beam; and the coating of the few-mode fiber must be stripped when passing through the fiber acousto-optic unit. A heterodyne detection system includes this frequency shifter. This application can simultaneously achieve low-frequency shifting for lasers of different wavelengths. The technical solution is simple and effective, with high signal-to-noise ratio, easy integration and packaging, low cost, flexible use, low insertion loss, and low frequency shift, making it suitable for practical applications.

Description

Technical Field

[0001] The present application belongs to the field of laser coherent detection technology, and specifically relates to a low-frequency all-fiber frequency shifter and a heterodyne detection system based on acoustic fiber gratings. Background Art

[0002] The development of optical coherent (homodyne and heterodyne) detection technology has enabled optical communications and optical sensing systems to achieve higher sensitivity and interference resistance. Compared to direct detection, coherent detection technology can convert optical frequency signals, which photodetectors cannot directly respond to, into radio frequency signals with lower frequencies, resulting in better frequency selectivity. Heterodyne detection is simpler to receive and process than homodyne detection. Laser communications, lidar, fiber optic gyroscopes, and microscale measurement are rapidly developing based on optical heterodyne technology.

[0003] Frequency shifters are often used in heterodyne detection systems, particularly acousto-optic modulators (AOMs) based on Raman-Nath diffraction or Bragg diffraction. These are categorized as free-space AOMs and fiber-coupled AOMs, with typical frequency shifts ranging from 20 MHz to 350 MHz. Their operating mechanism is that a grating crystal diffracts light under the influence of a transducer driven at a specific frequency, and the diffracted light undergoes a frequency shift equal to the driving frequency. These AOMs suffer from drawbacks such as large size, high insertion loss, and a fixed frequency shift. Furthermore, with increasing experimental demands, existing AOMs are unable to directly generate the required lower frequency shifts. To achieve frequency shifts in the megahertz range or even lower, several AOMs must be cascaded to generate the difference frequency. This approach not only increases system cost, reduces system integration and reliability, but also significantly increases insertion loss.

[0004] In summary, although acousto-optic modulators are widely used in various fields, finding a compact all-fiber frequency shifter with low frequency shift and low loss has become a concern for researchers. Summary of the Invention

[0005] This application proposes a low-frequency-shift all-fiber frequency shifter and heterodyne detection system based on acoustic fiber Bragg gratings to solve the above problems and achieve the effects of diverse frequency shifting, high efficiency and low frequency shifting.

[0006] To achieve the above objectives, this application provides the following solutions:

[0007] A low-frequency all-fiber frequency shifter based on acoustic fiber Bragg gratings, comprising a few-mode fiber, a first polarization controller, and a fiber acousto-optic unit;

[0008] The few-mode fiber is used to transmit LP 01 Mode beam or LP 11 mode beam;

[0009] The optical fiber acousto-optic unit is used to 11 Mode beam conversion to LP 01 mode beam;

[0010] The first polarization controller is used to adjust the LP 11 The polarization state of the mode beam;

[0011] The coating layer of the few-mode optical fiber needs to be stripped when passing through the optical fiber acousto-optic unit.

[0012] Optionally, the optical fiber acousto-optic unit includes an ultrasonic concentrator, a vibration generating device, and a sound-absorbing substrate connected in sequence from top to bottom, and the vibration generating device is further connected to an arbitrary waveform generator;

[0013] The arbitrary waveform generator is used to generate a radio frequency signal, and the radio frequency signal is used to drive the vibration generating device to generate ultrasonic vibration in a vertical direction;

[0014] The sound-absorbing substrate is used to absorb the reverse vibration of the radio frequency signal, so that the ultrasonic vibration is transmitted unidirectionally along the ultrasonic concentrator;

[0015] The bottom of the ultrasonic concentrator is connected to the vibration generating device, and the top of the ultrasonic concentrator is connected to the few-mode optical fiber with the coating stripped off. The ultrasonic concentrator is used to concentrate the ultrasonic vibration at the connection between the ultrasonic concentrator and the few-mode optical fiber.

[0016] Optionally, the ultrasonic concentrator is an aluminum cone;

[0017] The few-mode optical fiber with the coating removed is connected to the top of the aluminum cone;

[0018] The frequency shifter also includes a long period fiber grating;

[0019] The long period fiber grating is used to convert the LP 01 Mode beam conversion to LP 11 mode beam;

[0020] The optical fiber acousto-optic unit is used to 11 Mode beam conversion to LP 01 mode beam;

[0021] The few-mode fiber is a carrier of the long-period fiber grating and the fiber acousto-optic unit, and passes through the first polarization controller.

[0022] Optionally, the ultrasonic concentrator is an aluminum cone;

[0023] The few-mode optical fiber with the coating removed is connected to the top of the aluminum cone;

[0024] The frequency shifter further includes a second fiber optic acousto-optic unit;

[0025] The second fiber optic acousto-optic unit has the same structure as the fiber optic acousto-optic unit;

[0026] The second optical fiber acousto-optic unit is used to 01 Mode beam conversion to LP 11 mode beam;

[0027] The optical fiber acousto-optic unit is used to 11 Mode beam conversion to LP 01 mode beam;

[0028] The few-mode fiber is a carrier of the second fiber acousto-optic unit and the fiber acousto-optic unit, and passes through the first polarization controller.

[0029] Optionally, the ultrasonic concentrator is an aluminum triangular prism;

[0030] The aluminum triangular prism is placed horizontally, one side of the aluminum triangular prism is connected to the vibration generating device, and the edge opposite to the side is connected to the few-mode optical fiber;

[0031] The few-mode optical fiber is folded back and forth to form a plurality of optical fiber acousto-optic units in series with the aluminum triangular prism, the vibration generating device, the sound absorbing substrate and the arbitrary waveform generator;

[0032] The few-mode fiber passes through the first polarization controller once between every two of the fiber acousto-optic units.

[0033] Optionally, the vibration generating device uses piezoelectric ceramics.

[0034] Optionally, the sound absorbing substrate is made of metal.

[0035] Optionally, the LP 01 The relationship between the wavelength of the mode beam and the frequency of the RF signal is:

[0036]

[0037] (β 01 =2πn 01 / λ and β 11 =2πn 11 / λ),

[0038] Where Λ is the period of periodic refractive index modulation formed by ultrasound in the optical fiber, L B For LP 01 Mode and LP 11 The beat length between modes, R is the fiber radius in the acousto-optic region, C extrepresents the propagation speed of ultrasound in the optical fiber medium, f represents the frequency of the radio frequency signal, β 01 and β 11 Represents LP 01 Mode and LP 11 The propagation constant of the mode, n and λ represent the refractive index and wavelength respectively.

[0039] On the other hand, the present application also discloses a heterodyne detection system of a low-frequency-shifted all-fiber frequency shifter based on an acoustic fiber Bragg grating. In addition to the above-mentioned frequency shifter, the heterodyne detection system also includes a single-mode optical fiber, a tunable narrow-linewidth laser, a beam splitter, a second polarization controller, an optical fiber circulator, a front-coated reflector, a vibration unit, a beam combiner, a photodetector, and an oscilloscope.

[0040] The tunable narrow linewidth laser, the beam splitter, the second polarization controller and the optical fiber circulator are connected in sequence;

[0041] The input end of the front-coated reflective mirror is connected to the second port of the optical fiber circulator, and the output end of the front-coated reflective mirror is connected to the vibration unit;

[0042] The three ports of the optical fiber circulator are connected to an input end of the beam combiner;

[0043] The other port of the beam splitter is also connected to the frequency shifter and the other input end of the beam combiner in sequence;

[0044] The output end of the beam combiner is connected to the photodetector;

[0045] The photodetector is connected to the oscilloscope;

[0046] The tunable narrow linewidth laser, the beam splitter, the second polarization controller, the optical fiber circulator, the beam combiner, the photodetector and the frequency shifter are all connected through the single-mode optical fiber;

[0047] The vibration unit includes a vibration generating device and a sound absorbing substrate that are in contact with each other in sequence, and the vibration generating device is further connected to an arbitrary waveform generator.

[0048] The beneficial effects of this application are:

[0049] This application discloses a low-frequency-shift all-fiber frequency shifter and heterodyne detection system based on acoustic fiber Bragg gratings (ABGs). By leveraging the principle of acousto-optic interaction, the optical and acoustic vectors are superimposed in a specific manner, resulting in an upward or downward frequency shift of the optical signal. This technical solution can simultaneously achieve low-frequency shifting for lasers of different wavelengths. This simple and effective technical solution offers high signal-to-noise ratio, ease of integration and packaging, low cost, flexibility, low insertion loss, and low frequency shift, making it suitable for practical applications. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] In order to more clearly illustrate the technical solution of the present application, the following briefly introduces the drawings required for use in the embodiments. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative labor.

[0051] Figure 1 This is a structural diagram of a low-frequency all-fiber frequency shifter based on acoustic fiber Bragg grating according to the first embodiment of the present application;

[0052] Figure 2 Schematic diagram of the evolution of the fk vector according to the first embodiment of the present application, wherein (a) is a schematic diagram of the evolution of the fk vector according to the first solution, (b) is a schematic diagram of the evolution of the fk vector according to the second solution, and (c) is a schematic diagram of the evolution of the fk vector according to the third solution;

[0053] Figure 3 This is a schematic diagram of the mode evolution of Example 1 of the present application;

[0054] Figure 4 This is the transmission spectrum of the frequency shifter structure of the first solution in Example 1 of the present application;

[0055] Figure 5 This is a structural diagram of a heterodyne detection system of a low-frequency-shift all-fiber frequency shifter based on acoustic fiber Bragg grating according to the second embodiment of the present application;

[0056] Figure 6 This is the spectrum and waveform diagram obtained by the oscilloscope when the third solution is adopted in Example 2 of the present application.

[0057] Reference numerals

[0058] 1. Long-period fiber Bragg grating (LPB); 2. Few-mode fiber; 3. First polarization controller; 4. Fiber acousto-optic unit (FAOU); 5. Aluminum cone; 6. Vibration generator; 7. Sound-absorbing substrate; 8. Arbitrary waveform generator; 9. Aluminum triangular prism; 10. Tunable narrow-linewidth laser; 11. Single-mode fiber; 12. Beam splitter; 13. Fiber circulator; 131. One port of the fiber circulator; 132. Two ports of the fiber circulator; 133. Three ports of the fiber circulator; 14. Front-coated reflector; 15. Beam combiner; 16. Photodetector; 17. Oscilloscope; 41. Second fiber acousto-optic unit; 33. Second polarization controller. DETAILED DESCRIPTION

[0059] The following will be combined with the drawings in the embodiments of this application to clearly and completely describe the technical solutions in the embodiments of this application. Obviously, the embodiments described are only part of the embodiments of this application, not all of the embodiments. Based on the embodiments in this application, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of this application.

[0060] In order to make the above-mentioned purposes, features and advantages of the present application more obvious and easy to understand, the present application is further described in detail below with reference to the accompanying drawings and specific implementation methods.

[0061] Example 1

[0062] The low-frequency-shifted all-fiber frequency shifter based on acoustic fiber Bragg grating of the first embodiment includes a few-mode fiber 2, a first polarization controller 3 and a fiber acousto-optic unit 4. The few-mode fiber 2 is used to transmit LP 01 Mode beam or LP 11 mode beam, the fiber optic acousto-optic unit 4 is used to transmit the beam from the LP 11 Mode conversion to LP 01 mode, the first polarization controller 3 is used to adjust the polarization state of the light beam to obtain higher conversion efficiency and reduce insertion loss.

[0063] In this first embodiment, the fiber acousto-optic unit 4 is the key to achieving frequency-shifted mode conversion of the optical beam. It is essentially a long-period grating (LPFG), known as an acoustic fiber grating (AFBB), which can also be used independently as a mode conversion device. Therefore, the fiber acousto-optic unit 4 will be introduced first.

[0064] In the first embodiment, the fiber acousto-optic unit 4 includes an ultrasonic concentrator, a vibration generating device 6 and a sound absorbing substrate 7 connected in sequence from top to bottom. The vibration generating device 6 is also connected to an arbitrary waveform generator 8 .

[0065] Specifically, the arbitrary waveform generator 8 is used to generate a radio frequency signal, and the radio frequency signal is used to drive the vibration generating device 6 to generate ultrasonic vibration in the vertical direction. The sound-absorbing substrate 7 is used to absorb the reverse vibration of the radio frequency signal, so that the ultrasonic vibration is transmitted unidirectionally along the ultrasonic concentrator. The bottom of the ultrasonic concentrator is connected to the vibration generating device 6, and the top of the ultrasonic concentrator is in contact with the few-mode optical fiber 2 with the coating stripped off. The ultrasonic concentrator is used to converge the ultrasonic vibration at the connection between the ultrasonic concentrator and the few-mode optical fiber 2. The few-mode optical fiber 2 needs to have its coating stripped off when passing through the optical fiber acousto-optic unit 4 to ensure that the ultrasonic vibration propagates unimpeded in the acousto-optic action area.

[0066] In the first embodiment, the vibration generating device 6 uses piezoelectric ceramics as the piezoelectric material for generating ultrasonic vibrations, the sound absorbing substrate 7 should be a thick metal material, usually a square copper plate, and the ultrasonic concentrator uses an aluminum cone 5 or an aluminum triangular prism 9.

[0067] In this embodiment 1, LP 01 The wavelength of the mode beam is 1550nm, but not limited to 1550nm. There is a corresponding relationship between the wavelength and the frequency of the RF signal, as long as the phase matching conditions are met:

[0068]

[0069] (β 01 =2πn 01 / λ and β 11 =2πn 11 / λ),

[0070] The frequency shift can be realized at the corresponding wavelength. Where Λ is the period of periodic refractive index modulation formed by ultrasound in the optical fiber, L B For LP 11 Mode and LP 01 The beat length between modes, R is the fiber radius in the acousto-optic region, C ext represents the propagation speed of ultrasound in the optical fiber medium, f represents the frequency of the radio frequency signal, and in this embodiment, 0.916 MHz is used, corresponding to a working wavelength of 1550 nm, and β 01 and β 11 Represents LP 01 Mode and LP 11 The propagation constant of the mode, n and λ represent the refractive index and wavelength respectively.

[0071] Based on the structural characteristics of the optical fiber acousto-optic unit 4, the frequency shifter in the first embodiment has three combination schemes, such as Figure 1 As shown below.

[0072] like Figure 1 In the first solution, the ultrasonic concentrator in the fiber acousto-optic unit 4 adopts an aluminum cone 5, and a long-period fiber grating 1 is made on the few-mode fiber 2. The grating can be written by a carbon dioxide laser on a section of the few-mode fiber 2 with the coating stripped off in the middle. The input end of the long-period fiber grating 1 is connected to the light beam to be frequency-shifted, and the output end is connected to the fiber acousto-optic unit 4 through the first polarization controller 3. The fiber acousto-optic unit 4 serves as the acousto-optic action area and has a length of about 15 cm. The few-mode fiber 2 with the coating removed is connected to the top of the aluminum cone 5.

[0073] Incident light (LP 01 mode) is incident from the input end of the long-period fiber grating 1 and is converted into LP mode by the action of the long-period fiber grating 1. 11 mode, and then enters the above-mentioned fiber acousto-optic unit 4, in the acousto-optic action area LP 11 The mode is then switched back to LP 01 mode, accompanied by an upward frequency shift, the amount of which is equal to the frequency of the applied RF signal.

[0074] like Figure 1 In the second scheme, a second fiber acousto-optic unit 41 is used to replace the long-period fiber grating 1 in the first scheme. The second fiber acousto-optic unit 41 has the same structure as the fiber acousto-optic unit 4. The ultrasonic concentrators in the two fiber acousto-optic units both use an aluminum cone 5. Two sections of the coating of the few-mode fiber 2 are stripped, which serve as part of the second fiber acousto-optic unit 41 and the fiber acousto-optic unit 4 respectively. The few-mode fibers 2 with the coating removed are connected to the top of the aluminum cone 5; the coating is retained in the middle part as a buffer area, and the polarization state of the light beam in the optical fiber is adjusted by the first polarization controller 3.

[0075] The scheme uses two fiber optic acousto-optic units, that is, there are two sections of acousto-optic action areas: in the first section of the acousto-optic action area, the incident light (LP 01 mode) propagation direction is opposite to the direction of acoustic wave propagation, and the light beam is transmitted from LP 01 Mode conversion to LP 11 Mode, the incident light is shifted upward in frequency by the first section of the acousto-optic action zone, and the frequency shift is equal to the frequency of the added RF signal; the light beam in the second section of the acousto-optic action zone is shifted upward from the LP 11 The mode is then converted to LP 01 In this mode, the propagation direction of the light beam is the same as that of the sound wave, and the frequency is shifted up again before being emitted. Compared with the incident light, the emitted light produces an upward frequency shift of twice the frequency of the RF signal.

[0076] like Figure 1 The third solution uses an aluminum triangular prism 9 as an ultrasonic concentrator. The aluminum triangular prism 9 is placed horizontally, with one side of the prism in contact with the vibration generator 6, and the edge opposite the side is connected to the few-mode fiber 2. The few-mode fiber 2 is folded back and forth to continuously form multiple fiber acousto-optic units. In this first embodiment, the few-mode fiber 2 is stripped of four equal-length coating sections, which are sequentially adhered to the same edge of the triangular prism. The few-mode fiber 2 passes through the first polarization controller 3 once between each two fiber acousto-optic units 4. As a result, the few-mode fiber 2 contacts the few-mode fiber 2 multiple times on the edge of the same aluminum triangular prism, allowing ultrasonic vibrations to be applied to multiple optical fibers simultaneously. The radio frequency signal drives the vibration generator 6, causing the four acousto-optic action areas to act simultaneously, and the outgoing light beam produces an upward frequency shift of four times the frequency of the radio frequency signal relative to the incident light beam.

[0077] Figure 2 (a) (b) (c) represent the evolution of frequency and wave vector in the first, second and third schemes respectively. Figure 2 The second solution shown in (b) is used as an example to illustrate the fundamental mode (LP 01 mode) light beam is coupled into LP through the first section of the acousto-optic action area 11 mode, accompanied by frequency shift, the direction of the acoustic wave vector is opposite to that of the light wave, in LP11 The light beam of the mode is then coupled back to the LP through the second acousto-optic region. 01 The frequency of the light beam is shifted upwards by twice the acoustic frequency. The other two schemes evolve in a similar way.

[0078] A bunch of fundamental modes (LP 01 mode) of the light beam, after the first and second scheme modes in LP 01 and LP 11 After two switches, it returns to LP 01 The difference is that the first scheme has only one section of the acousto-optic action area, and the second scheme has two sections of the acousto-optic action area, so the frequency shifts of the two are f RF and 2f RF , f RF Corresponding to the RF signal frequency, the third solution has four mode switches, and the frequency shift is 4f RF The specific model evolution process is as follows Figure 3 shown.

[0079] Figure 4 The solid line in the middle shows the transmission spectrum for the first solution at a central wavelength of 1550 nm. Its insertion loss is 3.9 dB, and its 3-dB bandwidth is 15 nm. This structure can also be viewed as a bandpass filter with a low-frequency shift, with a sidelobe suppression ratio of 9.2 dB. Adjusting the first polarization controller 3 adjusts both the insertion loss and sidelobe suppression ratio. Furthermore, this structure can change the central wavelength, as shown by the dashed line in the figure, with a wavelength tuning range exceeding 100 nm.

[0080] Example 2

[0081] like Figure 5 As shown in the figure, the heterodyne detection system of the low-frequency all-fiber frequency shifter based on the acoustic fiber Bragg grating of the second embodiment of the present application is shown. Figure 1 The device also includes a single-mode optical fiber 11, a tunable narrow-linewidth laser 10, a beam splitter 12, a second polarization controller 33, an optical fiber circulator 13, a front-coated reflector 14, a vibration unit, a beam combiner 15, a photodetector 16, and an oscilloscope 17.

[0082] Specifically, the tunable narrow linewidth laser 10, the beam splitter 12, the second polarization controller 33 and the first port 131 of the optical fiber circulator 13 are connected in sequence; the input end of the front coated reflector 14 is connected to the second port 132 of the optical fiber circulator 13, and the output end of the front coated reflector 14 is connected to the vibration unit; the third port 133 of the optical fiber circulator 13 is connected to an input end of the combiner 15; the other port of the beam splitter 12 is also connected to the frequency shifter and the other input end of the combiner 15; the output end of the combiner 15 is connected to the photodetector 16; the photodetector 16 is connected to the oscilloscope 17; the tunable narrow linewidth laser 10, the beam splitter 12, the second polarization controller 33, the optical fiber circulator 13, the combiner 15, the photodetector 16 and the frequency shifter are all connected through a single-mode optical fiber 11; the vibration unit includes a vibration generating device 6 and an acoustic absorption substrate 7 connected in sequence, and the vibration generating device 6 is also connected to an arbitrary waveform generator 8.

[0083] In the second embodiment, the wavelength of the output light from the tunable narrow-linewidth laser 10 is consistent with the resonant wavelength (1550 nm) of the fiber acousto-optic unit 4; the beam splitter 12 is a 50:50 splitting coupler, or an appropriate splitting ratio can be selected as needed; the fiber circulator 13 includes three ports: port 131 of the fiber circulator, port 2 132 of the fiber circulator, and port 3 133 of the fiber circulator; the reflection band of the front-coated reflector 14 should include a wavelength of 1550 nm, and the coating material can be gold, silver, or other highly reflective materials. The front-coated reflector 14 is connected to the vibration unit, or it can be directly attached to the surface of the vibration source to be measured.

[0084] The tunable narrow linewidth laser 10 outputs a fundamental mode (LP) with a wavelength of 1550 nm. 01 The optical beam (mode) is divided into two parts by the beam splitter 12. The first part enters from the first port 131 of the optical fiber circulator, and then passes through the second port 132 of the optical fiber circulator and is perpendicularly emitted to the front coated reflector 14. When the reflector vibrates with the object it is attached to, the reflected light beam can carry the vibration information and return to the optical fiber circulator 13 from the second port 132 of the optical fiber circulator, and then enter the combiner 15 from the third port 133 of the optical fiber circulator; the other part directly enters Figure 1 In one of the three frequency shifting schemes, the beam pattern after the up-shift is unchanged compared to the pre-shift pattern. This beam is then combined with the first beam portion via a beam combiner 15. The output beam undergoes photoelectric conversion via a photodetector 16 and is connected to an oscilloscope 17 for signal recording.

[0085] In order to verify the frequency shifting effect and time stability of the frequency shifter, the heterodyne detection system of the second embodiment obtains the beat frequency spectrum and waveform diagram as shown in the following figure: Figure 6 As shown, the frequency shift structure in the third solution is only used as a reference.

[0086] Specific steps: Turn on the tunable narrow linewidth laser 10 and adjust it to 1550nm wavelength. Do not turn on the vibration unit connected to the front coated reflector 14 and fix it on the optical vibration isolation platform. Turn on the arbitrary waveform generator 8 in the third solution and output the RF signal as a 0.916MHz sinusoidal signal. Turn on the oscilloscope 17 and adjust the first polarization controller 3 in the two beams until a stable sinusoidal signal is displayed on the oscilloscope 17. Using the spectrum analysis function of the oscilloscope 17, the following can be obtained: Figure 6 The spectrum signal is shown.

[0087] Figure 6 The frequency shift amount shown corresponds to four times the RF signal frequency, i.e., 3.66 MHz. In addition, by adjusting the two first polarization controllers 3 in the third scheme, a frequency shift amount of one, two or three times that of the RF signal can be obtained, i.e., a frequency shift amount of 0.916 MHz, 1.83 MHz or 2.75 MHz. This flexibility is also not available in the other two schemes.

[0088] The above verification demonstrates that the low-frequency-shift all-fiber frequency shifter based on acoustic fiber Bragg gratings (AFBGs) in Example 1 utilizes the principle of acousto-optic interaction. The optical and acoustic vectors are superimposed in a specific manner, causing the optical signal to undergo an up- or down-shift. The resulting up- or down-shift depends on the direction of inter-mode coupling and the direction of acoustic wave propagation relative to the light. Both Examples 1 and 2 involve up-shifting. The first solution, combined with a long-period fiber Bragg grating (LPFBG), achieves a 3-dB bandwidth of 15 nm for a 1550 nm center wavelength and can simultaneously achieve low-frequency shifts for lasers of different wavelengths, expanding the solution's applicability. The outgoing light beam does not change relative to the incident light mode, and the frequency shift is the same as the frequency of the added RF signal (acoustic signal). The scheme is simple and effective, with a high signal-to-noise ratio. The second scheme has two acousto-optic action areas, and the frequency shift is twice the frequency of the RF signal. The two acousto-optic action areas are integrated on the same optical fiber without connection damage, which greatly reduces the insertion loss of the scheme. The third scheme uses a set of signal generating devices to realize four-stage acousto-optic effects, which doubles the frequency shift again without increasing the complexity of the device. In actual operation, by adjusting the two first polarization controllers 3 in the third scheme, frequency shifts of one, two, and three times that of the RF signal can be achieved, which increases the flexibility of device use.

[0089] The embodiments described above are merely descriptions of the preferred embodiments of the present application and do not limit the scope of the present application. Without departing from the design spirit of the present application, various modifications and improvements made to the technical solutions of the present application by ordinary technicians in this field should fall within the scope of protection determined by the claims of the present application.

Claims

1. A low-frequency all-fiber frequency shifter based on acoustic fiber Bragg grating, characterized in that: It comprises a few-mode optical fiber (2), a first polarization controller (3) and an optical fiber acousto-optic unit (4); The few-mode optical fiber (2) is used to transmit LP 01 Mode beam or LP 11 mode beam; The optical fiber acousto-optic unit (4) is used to 11 Mode beam conversion to LP 01 mode beam; The first polarization controller (3) is used to adjust the LP 11 The polarization state of the mode beam; The coating layer of the few-mode optical fiber (2) needs to be stripped when passing through the optical fiber acousto-optic unit (4); The optical fiber acousto-optic unit (4) comprises an ultrasonic concentrator, a vibration generating device (6), and a sound-absorbing substrate (7) connected in sequence from top to bottom, and the vibration generating device (6) is further connected to an arbitrary waveform generator (8); The arbitrary waveform generator (8) is used to generate a radio frequency signal, and the radio frequency signal is used to drive the vibration generating device (6) to generate ultrasonic vibration in a vertical direction; The sound-absorbing substrate (7) is used to absorb the reverse vibration of the radio frequency signal, so that the ultrasonic vibration is transmitted unidirectionally along the ultrasonic concentrator; The bottom of the ultrasonic concentrator is connected to the vibration generating device (6), the top of the ultrasonic concentrator is connected to the few-mode optical fiber (2) with the coating layer stripped off, and the ultrasonic concentrator is used to converge the ultrasonic vibration at the connection between the ultrasonic concentrator and the few-mode optical fiber (2); The ultrasonic concentrator is an aluminum triangular prism (9); the aluminum triangular prism (9) is placed horizontally, one side of the aluminum triangular prism is connected to the vibration generating device (6), and the edge opposite to the side is connected to the few-mode optical fiber (2); The few-mode optical fiber (2) is folded back and forth to form a plurality of optical fiber acousto-optic units (4) in succession with the aluminum triangular prism (9), the vibration generating device (6), the sound absorbing substrate (7) and the arbitrary waveform generator (8); Between every two of the optical fiber acousto-optic units (4), the few-mode optical fiber (2) passes through the first polarization controller (3) once.

2. The low-frequency-shift all-fiber frequency shifter based on acoustic fiber Bragg grating according to claim 1, characterized in that: The vibration generating device (6) adopts piezoelectric ceramics.

3. The low-frequency-shift all-fiber frequency shifter based on acoustic fiber Bragg grating according to claim 1, characterized in that: The sound absorbing substrate (7) is made of metal.

4. The low-frequency-shift all-fiber frequency shifter based on acoustic fiber Bragg grating according to claim 1, characterized in that: The LP 01 The relationship between the wavelength of the mode beam and the frequency of the RF signal is: ， and , in, The period of periodic refractive index modulation formed by ultrasound in the optical fiber, For LP 01 Mode and LP 11 The beat length between patterns, is the fiber radius in the acousto-optic action area, Indicates the propagation speed of ultrasonic waves in optical fiber media. Indicates the frequency of the RF signal, and Represents LP 01 Mode and LP 11 The propagation constant of the mode, and represent the refractive index and wavelength respectively.

5. A heterodyne detection system of a low-frequency-shift all-fiber frequency shifter based on an acoustic fiber Bragg grating, comprising the frequency shifter according to any one of claims 1 to 4, characterized in that: The heterodyne detection system further includes a single-mode optical fiber (11), a tunable narrow linewidth laser (10), a beam splitter (12), a second polarization controller (33), an optical fiber circulator (13), a front-coated reflector (14), a vibration unit, a beam combiner (15), a photodetector (16), and an oscilloscope (17); The tunable narrow linewidth laser (10), the beam splitter (12), the second polarization controller (33), and the optical fiber circulator (13) are connected in sequence; The input end of the front-coated reflective mirror (14) is connected to the second port (132) of the optical fiber circulator (13), and the output end of the front-coated reflective mirror (14) is connected to the vibration unit; The three ports (133) of the optical fiber circulator (13) are connected to an input end of the beam combiner (15); The other port of the beam splitter (12) is also connected in sequence to the frequency shifter and the other input end of the beam combiner (15); The output end of the beam combiner (15) is connected to the photodetector (16); The photodetector (16) is connected to the oscilloscope (17); The tunable narrow linewidth laser (10), the beam splitter (12), the second polarization controller (33), the optical fiber circulator (13), the beam combiner (15), the photodetector (16), and the frequency shifter are all connected via the single-mode optical fiber (11); The vibration unit comprises a vibration generating device (6) and a sound absorbing substrate (7) that are in contact with each other in sequence, and the vibration generating device (6) is further connected to an arbitrary waveform generator (8).

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