Optical fiber type broadband frequency shift structure based on acousto-optic modulator and control method thereof

Abstract

The invention discloses an optical fiber type broadband frequency shift structure based on acousto-optic modulators and a control method thereof.The structure comprises a first acousto-optic modulator, a second acousto-optic modulator and a reflector plate, and the first acousto-optic modulator and the second acousto-optic modulator are arranged in a cascaded mode; light with different frequencies is output in parallel after frequency shift of the first acousto-optic modulator and the second acousto-optic modulator, the reflector plate is located on an output light path of the second acousto-optic modulator, and the reflector plate totally reflects the frequency shift light output by the second acousto-optic modulator and returns to the second acousto-optic modulator through the original path. According to the scheme, the problem that in the prior art, the bandwidth of an optical fiber type acousto-optic modulator is limited by the optical fiber coupling angle is solved, and the ultra-large bandwidth optical fiber type acousto-optic modulator can be achieved through cascade connection of the two bandwidth type acousto-optic modulators.

Description

Technical Field

[0001] The present invention relates to the field of optoelectronic technology, and in particular to an optical fiber type broadband frequency shift structure based on an acousto-optic modulator and a control method thereof. Background Art

[0002] The acousto-optic modulator (AOM) can shift the frequency of the laser. When an ultra-high frequency oscillation is applied, a series of frequency shifts will occur when the laser passes through the acousto-optic medium. The frequency-shifted light will produce the same frequency shift amount and corresponding direction change as the applied frequency. The bandwidth of the acousto-optic modulator (generally 3dB bandwidth) refers to the maximum frequency shift that can be achieved before the output optical power drops by 50%.

[0003] The frequency shift characteristics of the acousto-optic modulator are widely used in sensing fields such as lidar, frequency-modulated continuous wave generation, and distributed fiber optic sensing. The mapping of frequency and distance is achieved by modulating the light frequency and the beat frequency of the intrinsic light. Its spatial resolution is inversely proportional to the modulation bandwidth. The larger the bandwidth of the device, the more effectively the detection accuracy of the system can be improved. It is a key device for improving system performance.

[0004] Currently, a single fiber acousto-optic modulator is used for frequency shifting in fiber systems, as shown in the attached Figure 1 As shown in the figure, the laser input through the fiber collimator passes through the AOM and recouples the frequency-shifted light into the output fiber at the intermediate frequency. In addition to being affected by its own structure, the bandwidth is also affected by the fact that there is an angle between the frequency-shifted light and the incident light, and the angle is proportional to the frequency. When the frequency changes greatly, the direction of the frequency-shifted light changes greatly and is difficult to couple into the fiber, which greatly limits the bandwidth of the fiber AOM. The highest level that can be achieved is only 80MHz. Summary of the invention

[0005] In view of the above-mentioned deficiencies in the prior art, the technical problem to be solved by the present invention is: how to provide a fiber-type broadband frequency shifting structure based on an acousto-optic modulator and a control method thereof, which can greatly improve the bandwidth of the fiber acousto-optic modulator and enhance the system performance.

[0006] In order to solve the above technical problems, the present invention adopts the following technical solutions: An optical fiber type broadband frequency shift structure based on an acousto-optic modulator comprises a first acousto-optic modulator, a second acousto-optic modulator and a reflector. The first acousto-optic modulator and the second acousto-optic modulator are cascaded, and the control driving frequencies of the first acousto-optic modulator and the second acousto-optic modulator satisfy a set relationship, so that lights of different frequencies which are frequency-shifted by the first acousto-optic modulator and the second acousto-optic modulator in sequence are output in parallel with each other. The reflector is located on the output optical path of the second acousto-optic modulator, and the reflector totally reflects the frequency-shifted light output by the second acousto-optic modulator and returns it to the second acousto-optic modulator again through the original path.

[0007] Preferably, it also includes a polarization beam splitter prism and a quarter wave plate, wherein the polarization beam splitter prism is located on the input light path of the first acousto-optic modulator, and the polarization beam splitter prism transmits the collimated light in the horizontal polarization state to the first acousto-optic modulator, and the quarter wave plate is located between the second acousto-optic modulator and the reflector, and the quarter wave plate phase-delays the frequency-shifted light output by the second acousto-optic modulator by 90° to form vertically polarized light and then outputs it, and the reflector totally reflects the vertically polarized light output by the quarter wave plate and then inputs it into the second acousto-optic modulator again through the quarter wave plate.

[0008] Preferably, it also includes a base plate, an input fiber collimator and an output fiber collimator, wherein the input fiber collimator, polarization beam splitter prism, first acousto-optic modulator, second acousto-optic modulator, quarter-wave plate, reflector and output fiber collimator are all arranged on the base plate; the input fiber collimator is used to shape the input polarized light into collimated light for output, the polarization beam splitter prism is located on the output light path of the input fiber collimator, the output fiber collimator is located on the reflected output light path of the polarization beam splitter prism, and the output fiber collimator shapes the vertically polarized light reflected by the polarization beam splitter prism for output.

[0009] Preferably, the frequency shift light directions of the first AOM and the second AOM are the same, and when the first AOM performs positive frequency shift, the second AOM performs negative frequency shift, and when the first AOM performs negative frequency shift, the second AOM performs positive frequency shift.

[0010] A control method for an optical fiber type broadband frequency shift structure based on an acousto-optic modulator is disclosed. The optical fiber type broadband frequency shift structure based on an acousto-optic modulator is used to perform frequency shift on the incident light in sequence through the first acousto-optic modulator and the second acousto-optic modulator. Lights of different frequencies after frequency shifting by the first acousto-optic modulator and the second acousto-optic modulator are output in parallel with each other. The frequency-shifted light output by the second acousto-optic modulator is totally reflected by a reflector and then sequentially passes through the second acousto-optic modulator and the first acousto-optic modulator through the original path to be frequency shifted before being output.

[0011] Preferably, the control driving frequency applied to the first AOM and the driving frequency applied to the second AOM satisfy the following setting relationship: The sum of the radio frequency frequency of the first AOM and the radio frequency frequency of the second AOM is equal to the sum of the intermediate frequency frequency of the first AOM and the intermediate frequency frequency of the second AOM.

[0012] Preferably, when the first acousto-optic modulator performs positive frequency shift f1 When the second AOM performs negative frequency shift -f 2 , when the first AOM performs negative frequency shift - f 1 When the second AOM performs positive frequency shift f 2 .

[0013] Preferably, the first acousto-optic modulator deflects light at an angle of i 1 , the deflection angle of the light by the second AOM is i 2 , the total deflection angle of the incident light after passing through the first AOM and the second AOM is θ, θ = i 1 +θ 2 .

[0014] Preferably, when the first AOM increases the frequency df When , the deflection angle of the light by the first AOM is i 1 +dθ , the frequency shift of the light by the first acousto-optic modulator is f 1 +df , at this time the second AOM reduces the frequency df , the deflection angle of the light by the second AOM is i 2 -dθ , the frequency shift of the light by the second AOM is -（f 2 -df) Then the total deflection angle of the incident light after passing through the first AOM and the second AOM is still θ, θ = i 1 +θ 2 ， The total frequency shift of the incident light after passing through the first AOM and the second AOM is f 1 -f 2 +2df; The frequency-shifted light output by the second acousto-optic modulator is delayed by 90° in phase by the quarter-wave plate and then reaches the reflector. The reflector returns all the light along the original path. The returned light passes through the second acousto-optic modulator and the first acousto-optic modulator in sequence and is frequency-shifted again. The returned light after frequency shifting again is reflected by the polarization beam splitter prism and then output and coupled into the optical fiber. The total frequency shift of the light coupled into the optical fiber is 2( f 1 -f 2 +2df ).

[0015] Preferably, the intermediate frequency of the light outputted by the output fiber collimator is twice the sum of the intermediate frequency of the first acousto-optic modulator and the intermediate frequency of the second acousto-optic modulator, and the bandwidth of the light outputted by the output fiber collimator is twice the sum of the bandwidth of the first acousto-optic modulator and the bandwidth of the second acousto-optic modulator.

[0016] Preferably, the frequency range is ( f 1 ，f 1 +df ) of the first acousto-optic modulator to perform negative frequency shift, using a frequency range of ( f 2 ，f 2 +df ) performs positive frequency shift, then the frequency range of the frequency shifted light output by the output fiber collimator is （2（f 2 -f 1 -df), 2 (f) 2 -f 1 +df)) .

[0017] Compared with the prior art, the present invention has the following advantages: 1. The present invention can realize large-bandwidth laser frequency shift under the condition of optical fiber input and output. The two bandwidth-type acousto-optic modulators of the present invention are placed in cascade, the center frequency and bandwidth are variable, and the control driving frequencies added by the two acousto-optic modulators always satisfy a fixed relationship, so that the output lights of different frequencies are parallel to each other. Since the output light angles of different frequencies are unchanged, the light is returned to the original path through the reflector and finally synthesized into a beam, thereby coupling into the optical fiber to realize the input and output of large-bandwidth optical fiber. The use of a suitable combination can realize frequency shift of any frequency and any bandwidth.

[0018] 2. This solution solves the problem in the prior art that the bandwidth of the fiber-type acousto-optic modulator is limited by the fiber coupling angle. By cascading two bandwidth-type acousto-optic modulators, an ultra-large bandwidth fiber-type acousto-optic modulator can be realized.

[0019] 3. This scheme can be applied to two acousto-optic modulators with the same bandwidth and arbitrary center frequency. The output optical intermediate frequency is twice the sum of the intermediate frequencies of the two devices, and the bandwidth is twice the sum of the bandwidths of the two devices.

[0020] 4. This solution uses a suitable combination of acousto-optic modulators to achieve frequency shifting of any frequency and any bandwidth within -1000 to +1000 MHz. By changing the working wavelength of each component, the scheme can be applied to a band range including but not limited to 200-2000nm. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 It is a structural schematic diagram of a fiber-optic acousto-optic modulator in the prior art; Figure 2 It is a structural schematic diagram of the fiber-type broadband frequency shifting structure based on the acousto-optic modulator of the present invention; Figure 3 It is a principle diagram of the optical path of the fiber-type broadband frequency shifting structure based on the acousto-optic modulator of the present invention; Figure 4 The figure is a test curve of 400 MHz bandwidth efficiency achieved by using the fiber-type broadband frequency shifting structure based on the acousto-optic modulator of the present invention.

[0022] Explanation of the reference numerals: input fiber collimator 1, polarization beam splitter prism 2, first acousto-optic modulator 3, second acousto-optic modulator 4, quarter-wave plate 5, reflector 6, output fiber collimator 7, bottom plate 8. DETAILED DESCRIPTION

[0023] In order to make the purpose, technical solution and advantages of the embodiment of the present invention clearer, the technical solution of the embodiment of the present invention will be clearly and completely described below in conjunction with the drawings of the embodiment of the present invention. Obviously, the described embodiment is a part of the embodiment of the present invention, not all of the embodiments. Based on the described embodiment of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention. Unless otherwise defined, the technical terms or scientific terms used herein should be the common meaning understood by people with general skills in the field to which the present invention belongs.

[0024] The words "first", "second" and similar words used in the patent application specification and claims of the present invention do not indicate any order, quantity or importance, but are only used to distinguish different components. Similarly, unless the context clearly indicates otherwise, the singular forms of "a", "an" or "the" and other similar words do not indicate a quantitative limitation, but indicate the existence of at least one. Words such as "include" or "comprise" mean that the elements or objects appearing before "include" or "comprise" cover the features, wholes, steps, operations, elements and / or components listed after "include" or "comprise", and do not exclude the existence or addition of one or more other features, wholes, steps, operations, elements, components and / or their collections. "Up", "down", "left", "right" and the like are only used to indicate relative positional relationships. When the absolute position of the described object changes, the relative positional relationship may also change accordingly.

[0025] First, this scheme provides a fiber-type broadband frequency shift structure based on acousto-optic modulator, as shown in the attached Figure 2 As shown, it includes a base plate 8, an input fiber collimator 1, a polarization beam splitter prism 2, a first acousto-optic modulator 3, a second acousto-optic modulator 4, a quarter wave plate 5, a reflector 6 and an output fiber collimator 7, wherein the input fiber collimator 1, the polarization beam splitter prism 2, the first acousto-optic modulator 3, the second acousto-optic modulator 4, the quarter wave plate 5, the reflector 6 and the output fiber collimator 7 are all arranged on the base plate 8, the input fiber collimator 1 is used to shape the input polarized light into collimated light and then output it, the polarization beam splitter prism 2 is located on the output light path of the input fiber collimator 1, and the polarization beam splitter prism 2 transmits the collimated light in the horizontal polarization state to the first acousto-optic modulator 3. The first acousto-optic modulator 3 and the second acousto-optic modulator 4 are cascaded, and the center frequency and bandwidth are variable. The second acousto-optic modulator re-shifts the frequency-shifted light of the first acousto-optic modulator, and the control driving frequencies of the first acousto-optic modulator 3 and the second acousto-optic modulator 4 satisfy the set relationship. The frequency-shifted light directions of the first acousto-optic modulator 3 and the second acousto-optic modulator 4 are the same, and when the first acousto-optic modulator 3 performs positive frequency shifting, the second acousto-optic modulator 4 performs negative frequency shifting, and when the first acousto-optic modulator 3 performs negative frequency shifting, the second acousto-optic modulator 4 performs positive frequency shifting, so that the lights of different frequencies after being frequency-shifted by the first acousto-optic modulator 3 and the second acousto-optic modulator 4 in sequence are output in parallel with each other, as shown in the attached figure. Figure 3 As shown, the quarter wave plate 5 is located between the second AOM 4 and the reflector, the quarter wave plate 5 phase-delays the frequency-shifted light output by the second AOM 4 by 90° to form vertically polarized light and then outputs it, the reflector 6 totally reflects the vertically polarized light output by the quarter wave plate 5 and then inputs it to the second AOM 4 again through the quarter wave plate 5, the output fiber collimator 7 is located on the reflected output light path of the polarization beam splitter prism 2, and the output fiber collimator 7 shapes the vertically polarized light reflected by the polarization beam splitter prism 2 and then outputs it.

[0026] In addition, the present invention also provides a control method for an optical fiber-type broadband frequency shift structure based on an acousto-optic modulator. The optical fiber-type broadband frequency shift structure based on an acousto-optic modulator is used to perform frequency shift on the incident light in sequence through the first acousto-optic modulator 3 and the second acousto-optic modulator 4. Lights of different frequencies after frequency shifting by the first acousto-optic modulator 3 and the second acousto-optic modulator 4 are output in parallel with each other. The frequency-shifted light output by the second acousto-optic modulator 4 is totally reflected by a reflector 6 and then passes through the second acousto-optic modulator 4 and the first acousto-optic modulator 3 in sequence through the original path to be frequency shifted and then output.

[0027] Specifically, the control driving frequency applied to the first AOM 3 and the driving frequency applied to the second AOM 4 satisfy the following setting relationship: The sum of the RF frequency of the first AOM 3 and the RF frequency of the second AOM 4 is equal to the sum of the IF frequency of the first AOM 3 and the IF frequency of the second AOM 4. If one increases the frequency, the other needs to decrease the same frequency. f 1 When the second acousto-optic modulator 4 performs negative frequency shift -f 2 , when the first AOM 3 performs negative frequency shift - f 1 When the second acousto-optic modulator 4 performs positive frequency shift f 2 .

[0028] Here, the first acousto-optic modulator 3 is used for positive frequency shift f 1 The second acousto-optic modulator 4 performs negative frequency shift -f 2 Take this as an example to illustrate, the total frequency shift is f 1 -f 2 According to the diffraction angle formula of the acousto-optic deflector: θ=𝜆f / V Where: λ is the wavelength of light, V is the speed of sound, and the deflection angle of light by the first AOM 3 is i 1 , the deflection angle of the light by the second AOM 4 is i 2 , the total deflection angle of the incident light after passing through the first AOM 3 and the second AOM 4 is θ, θ = i 1 +θ 2 .

[0029] When the first AOM 3 increases the frequency df When , the deflection angle of the light by the first AOM 3 is i 1 +dθ , the frequency shift of the light by the first acousto-optic modulator 3 is f 1 +df At this time, the second AOM 4 reduces the frequency df , the deflection angle of the light by the second AOM 4 i 2 -dθ , the frequency shift of the light by the second AOM 4 is -（f 2 -df) Then the total deflection angle of the incident light after passing through the first AOM 3 and the second AOM 4 is still θ, θ = i 1 +θ 2 ， The total frequency shift of the incident light after passing through the first AOM 3 and the second AOM 4 is f 1 -f 2 +2df; Since the final output light of different frequencies does not change its angle and is parallel to each other, all the light is returned to the original path through the reflector 6, and frequency shifts again through the two acousto-optic modulators, and finally a beam of light is reversibly synthesized according to the optical path. Specifically, the frequency-shifted light output by the second acousto-optic modulator 4 is delayed by 90° in phase by the quarter-wave plate 5 and reaches the reflector 6. The reflector 6 returns all the light to the original path, and the returned light passes through the second acousto-optic modulator 4 and the first acousto-optic modulator 3 in sequence and frequency shifts again. The returned light after frequency shifting again is reflected by the polarization beam splitter prism 2 and then output and coupled into the optical fiber. The total frequency shift of the light coupled into the optical fiber is 2 ( f 1 -f 2 +2df ).

[0030] This solution can be applied to two AOMs with the same bandwidth and arbitrary center frequencies. The intermediate frequency of the light outputted by the output fiber collimator 7 is twice the sum of the intermediate frequency of the first AOM 3 and the intermediate frequency of the second AOM 4. The bandwidth of the light outputted by the output fiber collimator 7 is twice the sum of the bandwidth of the first AOM 3 and the bandwidth of the second AOM 4. For example, the frequency range is ( f 1 ，f 1 +df) of the first acousto-optic modulator 3 to perform negative frequency shift, using a frequency range of ( f 2 ，f 2 +df ) performs positive frequency shifting, then the frequency range of the frequency shifted light output by the output fiber collimator 7 is （2（f 2 -f 1 -df), 2 （f 2 -f 1 +df)) .

[0031] For example, the arbitrary frequency shift output within the bandwidth of 0-400MHz can be realized by using two acousto-optic modulators (130, 230) and (230, 330), as shown in the attached figure. Figure 4 As shown, by switching the positive and negative frequency shift modes of the device, -400-0MHz frequency shift output can also be achieved. The use of a suitable combination of acousto-optic modulators can achieve frequency shifts of any frequency and bandwidth within -1000 to +1000MHz. By changing the working wavelength of each component, the band range in which this solution can be applied includes but is not limited to 200-2000nm.

[0032] Compared with the prior art, the present invention can realize large-bandwidth laser frequency shift under the condition of optical fiber input and output. The two bandwidth-type acousto-optic modulators of the present invention are placed in cascade, and the center frequency and bandwidth are variable. The control driving frequency added by the two acousto-optic modulators always satisfies a fixed relationship, so that the output lights of different frequencies are parallel to each other. Since the output light angles of different frequencies are unchanged, the light is returned to the original path through the reflector 6, and finally synthesized into a beam, so as to couple into the optical fiber, realize the input and output of large-bandwidth optical fiber, and adopt a suitable combination to realize frequency shift of any frequency and any bandwidth. This scheme solves the problem that the bandwidth of the optical fiber acousto-optic modulator in the prior art is limited by the optical fiber coupling angle. By cascading two bandwidth-type acousto-optic modulators, an ultra-large bandwidth optical fiber acousto-optic modulator can be realized. This scheme can be applied to two acousto-optic modulators with the same bandwidth and arbitrary center frequency. The intermediate frequency of the output light is twice the sum of the intermediate frequencies of the two devices, and the bandwidth is twice the sum of the bandwidths of the two devices. This solution uses a suitable combination of acousto-optic modulators to achieve frequency shifting of any frequency and any bandwidth within -1000 to +1000 MHz. By changing the working wavelength of each component, the solution can be applied to a band range including but not limited to 200-2000nm.

[0033] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit the technical solution. Those skilled in the art should understand that those modifications or equivalent substitutions of the technical solution of the present invention that do not depart from the purpose and scope of the technical solution should be included in the scope of the claims of the present invention.

Claims

1. An optical fiber broadband frequency shifting structure based on an acousto-optic modulator, characterized in that: The invention comprises a first acousto-optic modulator, a second acousto-optic modulator and a reflector, wherein the first acousto-optic modulator and the second acousto-optic modulator are cascaded, and the control driving frequencies of the first acousto-optic modulator and the second acousto-optic modulator satisfy a set relationship, so that lights of different frequencies which are frequency-shifted by the first acousto-optic modulator and the second acousto-optic modulator are output in parallel with each other, and the reflector is located on the output optical path of the second acousto-optic modulator, and the reflector totally reflects the frequency-shifted light output by the second acousto-optic modulator and returns the light to the second acousto-optic modulator through the original path.

2. The fiber-type broadband frequency shifting structure based on an acousto-optic modulator according to claim 1, characterized in that: The invention also includes a polarization beam splitter prism and a quarter wave plate. The polarization beam splitter prism is located on the input light path of the first acousto-optic modulator. The polarization beam splitter prism transmits collimated light in a horizontal polarization state to the first acousto-optic modulator. The quarter wave plate is located between the second acousto-optic modulator and the reflector. The quarter wave plate phase-delays the frequency-shifted light output by the second acousto-optic modulator by 90° to form vertically polarized light and then outputs the light. The reflector totally reflects the vertically polarized light output by the quarter wave plate and then inputs the light to the second acousto-optic modulator through the quarter wave plate again.

3. The fiber-type broadband frequency shifting structure based on an acousto-optic modulator according to claim 2, characterized in that: It also includes a bottom plate, an input fiber collimator and an output fiber collimator, wherein the input fiber collimator, the polarization beam splitter prism, the first acousto-optic modulator, the second acousto-optic modulator, a quarter wave plate, a reflector and the output fiber collimator are all arranged on the bottom plate; The input fiber collimator is used to shape the input polarized light into collimated light and then output it. The polarization beam splitter prism is located on the output light path of the input fiber collimator. The output fiber collimator is located on the reflected output light path of the polarization beam splitter prism. The output fiber collimator shapes the vertically polarized light reflected by the polarization beam splitter prism and then outputs it.

4. A control method for a fiber-type broadband frequency shift structure based on an acousto-optic modulator, characterized in that: By using the optical fiber type broadband frequency shifting structure based on the acousto-optic modulator as claimed in claim 1, the incident light is frequency shifted by the first acousto-optic modulator and the second acousto-optic modulator in sequence, the lights of different frequencies after frequency shifting by the first acousto-optic modulator and the second acousto-optic modulator are output in parallel with each other, the frequency-shifted light output by the second acousto-optic modulator is totally reflected by a reflector, and is frequency shifted and output after passing through the second acousto-optic modulator and the first acousto-optic modulator in sequence along the original path.

5. The control method of the fiber-type broadband frequency shift structure based on an acousto-optic modulator according to claim 5, characterized in that: The control driving frequency applied to the first AOM and the driving frequency applied to the second AOM satisfy the following setting relationship: The sum of the radio frequency frequency of the first AOM and the radio frequency frequency of the second AOM is equal to the sum of the intermediate frequency frequency of the first AOM and the intermediate frequency frequency of the second AOM.

6. The control method of the fiber-type broadband frequency shift structure based on an acousto-optic modulator according to claim 5, characterized in that: When the first AOM performs positive frequency shift f 1 When the second AOM performs negative frequency shift -f 2 , when the first AOM performs negative frequency shift - f 1 When the second AOM performs positive frequency shift f 2 .

7. The control method of the fiber-type broadband frequency shift structure based on an acousto-optic modulator according to claim 6, characterized in that: The deflection angle of the light by the first acousto-optic modulator is θ 1 , the deflection angle of the light by the second AOM is θ 2 , the total deflection angle of the incident light after passing through the first AOM and the second AOM is θ, θ = θ 1 +θ 2 .

8. The control method of the fiber-type broadband frequency shift structure based on an acousto-optic modulator according to claim 7, characterized in that: When the first AOM increases the frequency df When , the deflection angle of the light by the first AOM is θ 1 + dθ , the frequency shift of the light by the first acousto-optic modulator is f 1 +df , at this time the second AOM reduces the frequency df , the deflection angle of the light by the second AOM is θ 2 -dθ , the frequency shift of the light by the second AOM is -（f 2 -df), Then the total deflection angle of the incident light after passing through the first AOM and the second AOM is still θ, θ = θ 1 + θ 2 ， The total frequency shift of the incident light after passing through the first AOM and the second AOM is f 1 -f 2 +2df; The frequency-shifted light output by the second acousto-optic modulator is delayed by 90° in phase by the quarter-wave plate and then reaches the reflector. The reflector returns all the light along the original path. The returned light passes through the second acousto-optic modulator and the first acousto-optic modulator in sequence and is frequency-shifted again. The returned light after frequency shifting again is reflected by the polarization beam splitter prism and then output and coupled into the optical fiber. The total frequency shift of the light coupled into the optical fiber is 2( f 1 -f 2 +2df ).

9. The control method of the fiber-type broadband frequency shift structure based on an acousto-optic modulator according to claim 5, characterized in that: The intermediate frequency of light outputted by the output fiber collimator is twice the sum of the intermediate frequency of the first acousto-optic modulator and the intermediate frequency of the second acousto-optic modulator, and the bandwidth of light outputted by the output fiber collimator is twice the sum of the bandwidth of the first acousto-optic modulator and the bandwidth of the second acousto-optic modulator.

10. The control method of the fiber-type broadband frequency shift structure based on an acousto-optic modulator according to claim 6, characterized in that: The frequency range is ( f 1 ，f 1 +df ) of the first acousto-optic modulator to perform negative frequency shift, using a frequency range of ( f 2 ，f 2 +df ) performs positive frequency shift, then the frequency range of the frequency shifted light output by the output fiber collimator is （2 （f 2 -f 1 -df),2(f 2 -f 1 +df)) .

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