Optical fiber transverse mode field regulation and control system and method

Abstract

The invention relates to an optical fiber transverse mode field regulation and control system and method. The system comprises a laser injection unit and an annular cavity unit, the annular cavity unit is bidirectionally and optically connected with the laser injection unit; a second coupler, a first optical regulation and control assembly, a multimode optical fiber, a polarization controller, a second optical regulation and control assembly and a third coupler are sequentially arranged in the annular cavity unit from the access position of the laser injection unit in the clockwise direction. The annular cavity unit is constructed into a two-way transmission light path supporting two transmission modes of clockwise transmission and anticlockwise transmission; by rotating the angle of the first optical regulation and control assembly or the second optical regulation and control assembly, the optical power and the polarization state of the optical signal in the clockwise or anticlockwise direction are adjusted. Compared with the prior art, the method has the advantages that on the premise that high stability and low cost are guaranteed, flexible regulation and control of transverse mode field distribution in the multimode optical fiber are achieved, and the like.

Description

Technical Field

[0001] This invention relates to the field of optical fiber communication and optical modulation technology, and in particular to an optical fiber transverse mode field modulation system and method. Background Technology

[0002] With the rapid development of fiber optic communication, fiber optic sensing, and high-power laser systems, the demand for precise control of optical field modes is increasing. Especially in multimode fiber applications, precise control of optical field modes is crucial for achieving specific transverse mode field distributions. However, traditional mode field control systems typically rely on spatial optical components or complex active feedback control systems. This not only results in large system size and poor stability but also high construction costs, making it difficult to meet the stringent requirements of compactness, low cost, and high reliability in practical applications.

[0003] In recent years, bidirectional transmission systems based on ring cavity structures have attracted widespread attention in fields such as fiber optic gyroscopes, non-reciprocal devices, and laser mode selection. However, most existing ring cavities employ unidirectional propagation designs, resulting in a lack of ability to independently control the optical field in both clockwise and counterclockwise directions. Especially in multimode fiber environments, the mode field distribution is extremely difficult to control precisely due to significant coupling effects between different modes. Furthermore, most systems still require polarization manipulation using free-space optical components, leading to low system integration and extreme sensitivity to environmental vibrations and temperature changes. Although some studies have attempted to adjust the polarization state of light to influence mode excitation through waveplates and polarization beam splitters, a simple, all-fiber system capable of simultaneous bidirectional independent control and applicable to various light source types has yet to be proposed for transverse mode field manipulation.

[0004] For example, invention patent CN117199980A discloses a bidirectional output single-longitudinal-mode thulium-doped fiber laser and its implementation method. This scheme achieves single-longitudinal-mode laser output in different directions through a beam combiner, thulium-doped fiber, polarizer, polarization controller, and cascaded three-ring cavity structure. Although this prior art achieves bidirectional output, it has the following significant drawbacks: First, the core objective of this scheme is to achieve frequency selection for single longitudinal modes, rather than morphological control of the transverse spatial mode field. It is primarily designed for single-mode fiber environments and cannot address the complex transverse mode competition and coupling issues in multimode fibers, thus failing to achieve the flexible transverse mode field distribution emphasized in this invention.

[0005] Secondly, the proposed solution is overly complex and costly. It relies on a complex "cascaded three-ring cavity" structure for mode selection and uses expensive fiber optic circulators and fiber gratings of specific wavelengths. This complex cascaded structure places extremely high demands on the splicing process and environmental stability, making it difficult to achieve low-cost, large-scale industrial applications.

[0006] Furthermore, this scheme lacks a bidirectional independent fine control mechanism; the polarization controller in this scheme is mainly used for wavelength switching, and the optical paths in the clockwise and counterclockwise directions are tightly coupled, making it impossible to independently trim the optical power and polarization state in the two directions without interference, as is the case in this invention, through independently set control components.

[0007] In summary, existing technologies suffer from drawbacks such as complex system structures that make low-cost mass production difficult, inability to effectively control the transverse mode field of multimode fibers, and lack of bidirectional independent control capabilities. There is an urgent need for a new system that can flexibly control the transverse mode field distribution in multimode fibers while ensuring high stability and low cost. Summary of the Invention

[0008] The purpose of this invention is to overcome the defects of the prior art and provide an optical fiber transverse mode field control system and method.

[0009] The objective of this invention can be achieved through the following technical solutions: According to one aspect of the present invention, an optical fiber transverse mode field modulation system is provided, comprising a laser injection unit 1 and a ring cavity unit 2; The laser injection unit 1 includes a cascaded laser source 11, an isolator 12, and a first coupler 13; The annular cavity unit 2 and the laser injection unit 1 are connected by bidirectional optical connection; Inside the ring cavity unit 2, starting from the access point 3 of the laser injection unit 1, a second coupler 21, a first optical control component 22, a multimode fiber 23, a polarization controller 24, a second optical control component 25, and a third coupler 26 are arranged sequentially in a clockwise direction; and the ring cavity unit 2 is constructed as a bidirectional transmission optical path that supports both clockwise and counterclockwise transmission modes. The first optical control component 22 and the second optical control component 25 are respectively disposed on the first rotating bracket and the second rotating bracket, and are configured to adjust the optical power and polarization state of the optical signal in the clockwise or counterclockwise direction by rotating the angle of the first optical control component 22 or the second optical control component 25.

[0010] As a preferred technical solution, the first optical control component 22 includes a first collimator 221, a first linear polarizer 222, a first half-wave plate 223, a first quarter-wave plate 224, and a second collimator 225 that are sequentially and bidirectionally optically connected. The second optical control assembly 25 includes a third collimator 251, a second quarter-wave plate 252, a second half-wave plate 253, a second linear polarizer 254, and a fourth collimator 255, which are sequentially connected in a bidirectional optical manner.

[0011] As a preferred technical solution, the first optical control component 22 is packaged and integrated into an integrated component; the second optical control component 25 is packaged and integrated into an integrated component.

[0012] As a preferred technical solution, all optical components in the fiber optic transverse mode field control system are connected through single-mode fiber 4, and the connection method is fusion splicing or flange connection, thereby forming a closed all-fiber structure. The type of single-mode fiber 4 is matched with the operating band of the laser source 11, including: when the operating band is 1 micrometer band, HI 1060 fiber is selected; when the operating band is 1.55 micrometer band, SMF-28 fiber or SMF-28e fiber is selected; when the operating band is 2 micrometer band, SM1950 fiber is selected.

[0013] As a preferred technical solution, the multimode fiber 23 specifically includes a multi-core fiber, a step-index multimode fiber 23, or a graded-index multimode fiber 23. When the multimode fiber 23 is a multi-core fiber, each core in the multi-core fiber is configured with different geometric dimensions or refractive index ratios.

[0014] As a preferred technical solution, the first coupler 13 is a 3 dB coupler; the second coupler 21 and the third coupler 26 are 3 dB couplers, 2 / 8 couplers, 3 / 7 couplers, or couplers with a split power percentage adjustment range of 0% to 100%.

[0015] According to another aspect of the present invention, a method for controlling the transverse mode field of an optical fiber is provided, the method comprising the following steps: The laser light source 11 is controlled to emit an optical signal, which is transmitted to the first coupler 13 via the isolator 12. The optical signal is split into two paths using the first coupler 13 and injected into the second coupler 21 and the third coupler 26 respectively, forming clockwise and counterclockwise optical signals within the ring cavity. The clockwise optical signal enters through the second coupler 21, passes through the first optical control component 22, the multimode fiber 23, the polarization controller 24, and the second optical control component 25 in sequence, and is output from the third coupler 26. The counterclockwise optical signal enters through the third coupler 26, passes through the second optical control component 25, the polarization controller 24, the multimode fiber 23, and the first optical control component 22 in sequence, and is output from the second coupler 21. Perform the lateral field control steps, including: Rotate the angle of the first optical control component 22 to adjust the power and polarization state of the clockwise transmitted optical signal; Rotate the angle of the second optical control component 25 to adjust the power and polarization state of the counterclockwise transmitted optical signal; By utilizing the mode transmission characteristics of multimode fiber 23 and combining the adjustment function of polarization controller 24, the transverse mode field of the output optical signal can be controlled.

[0016] As a preferred technical solution, in the transverse mode field modulation step, the operation of adjusting the power and polarization state of the clockwise transmitted optical signal is independent of the operation of adjusting the power and polarization state of the counterclockwise transmitted optical signal, so as to realize the dynamic adjustment of the ratio of optical power in the two directions.

[0017] As a preferred technical solution, the specific process of utilizing the mode transmission characteristics of multimode fiber 23 includes: injecting optical signals into the multi-core fiber, and utilizing the differences in geometric dimensions or refractive index ratios between the cores in the multi-core fiber to achieve differentiated mode excitation.

[0018] As a preferred technical solution, the method further includes: selecting the corresponding single-mode fiber type 4 and the anti-reflection coating type of the collimator according to the target operating wavelength, establishing a closed-loop optical path with all-fiber connection, and maintaining the closedness of the all-fiber structure during optical signal transmission.

[0019] Compared with the prior art, the present invention has the following beneficial effects: 1. This invention is based on an architecture that couples a laser injection unit with a ring cavity unit. Multimode fiber is integrated in the ring cavity, and a first optical control component and a second optical control component are set along the optical path. The ring cavity unit is constructed as a bidirectional transmission optical path that supports both clockwise and counterclockwise transmission modes. By utilizing the precise adjustment capability of the first or second control component set on the rotating support for the polarization state and power of the optical signal, combined with the cyclic feedback characteristics of the ring cavity, fine control of the transverse mode field excited in the multimode fiber can be achieved. This design, which combines conventional optical components with a ring cavity structure, avoids dependence on expensive spatial light modulators, achieving low cost, and ensures system stability through bidirectional optical path design. Thus, it achieves flexible control of the transverse mode field distribution in multimode fiber 23 while ensuring high stability and low cost.

[0020] 2. In this invention, the ring cavity unit is sequentially arranged in a clockwise direction with a second coupler, a first optical control component, a multimode fiber, a polarization controller, a second optical control component, and a third coupler, forming a bidirectional transmission optical path supporting both clockwise and counterclockwise transmission modes. Adjusting the power and polarization state of the clockwise transmitted optical signal is independent of adjusting the power and polarization state of the counterclockwise transmitted optical signal. Utilizing the non-reciprocal transmission characteristics of the ring cavity, the optical power and polarization state in the clockwise and counterclockwise directions can be adjusted separately by rotating the linear polarizer and waveplate, achieving bidirectional independent control.

[0021] 3. In this invention, all optical components in the system are connected by single-mode optical fibers, and the connection method is fusion splicing or flange connection, so that the system forms a closed all-fiber structure, eliminating the alignment error caused by air flow, dust deposition or mechanical vibration in traditional space optical structures. Combined with the integrated architecture of laser injection unit and ring cavity unit, this system is an all-fiber structure with a compact structure and strong anti-vibration interference capability, further ensuring the stability of mode field control.

[0022] 4. This invention utilizes a first optical control component and a second optical control component arranged sequentially in a clockwise direction within a ring cavity unit to construct a bidirectional transmission optical path that supports both clockwise and counterclockwise transmission modes. By utilizing the non-reciprocal transmission characteristics of the ring cavity, the operation of adjusting the power and polarization state of the clockwise transmitted optical signal is independent of the operation of adjusting the power and polarization state of the counterclockwise transmitted optical signal, thereby achieving non-interference and bidirectional independent control of the clockwise and counterclockwise optical paths.

[0023] 5. This invention utilizes conventional discrete optical components such as a first collimator, a first linear polarizer, a first half-wave plate, a first quarter-wave plate, and a corresponding second set of components to construct a control optical path. These combinations based on mature and universal active and passive fiber optic devices reduce the manufacturing threshold and integration complexity, facilitate standardized packaging and mass production, and greatly promote the large-scale industrial application of the technology.

[0024] 6. This invention allows for flexible selection of fiber optic type based on the operating wavelength. HI 1060 fiber is selected for the 1-micron wavelength, SMF-28 fiber for the 1.55-micron wavelength, and SM1950 fiber for the 2-micron wavelength. It also allows for the use of 3 dB couplers, 2 / 8 couplers, or couplers with a split power percentage adjustment range of 0% to 100%, thus supporting various light source types and coupling ratios, adapting to different application scenarios, and exhibiting strong compatibility.

[0025] 7. The present invention employs specific multimode optical fibers in the ring cavity, including multi-core optical fibers, step-index multimode optical fibers, or graded-index multimode optical fibers. It utilizes the geometric size differences or refractive index ratio differences between the cores in the multi-core optical fiber to achieve differentiated mode excitation, thereby enabling the matching of various types of multimode optical fibers and precise mode field control, which is especially suitable for space division multiplexing systems.

[0026] 8. The present invention sets the first optical control component and the second optical control component on the first rotating bracket and the second rotating bracket respectively. The state of light is changed by adjusting the angle through rotation. This clear rotary mechanical interface design allows the waveplate to be directly driven by a stepper motor. Combined with detection feedback, a closed-loop control system can be constructed, which is easy to expand into automated control. Attached Figure Description

[0027] Figure 1 This is an overall structural diagram of an optical fiber transverse mode field modulation system according to the present invention; Figure 2 This is a schematic diagram of the specific structure of the first optical control component in this invention; Figure 3 This is a schematic diagram of the specific structure of the second optical control component in this invention; Figure 4 This is an overall structural diagram of an optical fiber transverse mode field modulation system in one embodiment; In the figure, 1 is the laser injection unit, 11 is the laser source, 12 is the isolator, and 13 is the first coupler; 2 is the ring cavity unit, 21 is the second coupler, 22 is the first optical control component, 221 is the first collimator, 222 is the first linear polarizer, 223 is the first half-wave plate, 224 is the first quarter-wave plate, 225 is the second collimator, 23 is the multimode fiber, 24 is the polarization controller, 25 is the second optical control component, 251 is the third collimator, 252 is the second quarter-wave plate, 253 is the second half-wave plate, 254 is the second linear polarizer, 255 is the fourth collimator, 26 is the third coupler, 3 is the access point, and 4 is the single-mode fiber. Detailed Implementation

[0028] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of the present invention.

[0029] Although existing studies have attempted to influence mode excitation by adjusting the polarization state of light through waveplates and polarization beam splitters, no transverse mode field control system with a simple structure, all-fiber configuration, simultaneous bidirectional independent control, and applicability to various light source types has yet been proposed.

[0030] Therefore, there is an urgent need for a new type of fiber optic transverse mode field control system that can flexibly control the transverse mode field distribution in multimode fibers while ensuring high stability and low cost.

[0031] Example 1 In this embodiment, an optical fiber transverse mode field modulation system is employed, which is as follows: Figure 1 As shown, the specific scheme is as follows: the system includes a laser injection unit 1 and a ring cavity unit 2; The laser injection unit 1 includes a cascaded laser source 11, an isolator 12, and a first coupler 13; The annular cavity unit 2 and the laser injection unit 1 are connected by bidirectional optical connection; Inside the ring cavity unit 2, starting from the access point 3 of the laser injection unit 1, a second coupler 21, a first optical control component 22, a multimode fiber 23, a polarization controller 24, a second optical control component 25, and a third coupler 26 are arranged sequentially in a clockwise direction; and the ring cavity unit 2 is constructed as a bidirectional transmission optical path that supports both clockwise and counterclockwise transmission modes. The first optical control component 22 and the second optical control component 25 are respectively disposed on the first rotating bracket and the second rotating bracket, and are configured to adjust the optical power and polarization state of the optical signal in the clockwise or counterclockwise direction by rotating the angle of the first optical control component 22 or the second optical control component 25.

[0032] First optical control component 22, such as Figure 2 As shown, it includes a first collimator 221, a first linear polarizer 222, a first half-wave plate 223, a first quarter-wave plate 224, and a second collimator 225 connected in sequence in both directions; the second optical control assembly 25 is as follows: Figure 3 As shown, it includes a third collimator 251, a second quarter-wave plate 252, a second half-wave plate 253, a second linear polarizer 254, and a fourth collimator 255 connected in a bidirectional optical manner.

[0033] The first optical control component 22 is packaged and integrated into a single unit; the second optical control component 25 is packaged and integrated into a single unit.

[0034] All optical components in the system are connected via single-mode fiber 4, and the connection method is fusion splicing or flange connection, thus forming a closed all-fiber structure. The type of single-mode fiber 4 is matched with the working band of the laser source 11, including: when the working band is 1 micrometer band, HI 1060 fiber is selected; when the working band is 1.55 micrometer band, SMF-28 fiber or SMF-28e fiber is selected; when the working band is 2 micrometer band, SM1950 fiber is selected.

[0035] The multimode fiber 23 specifically includes a multi-core fiber, a step-index multimode fiber 23, or a graded-index multimode fiber 23; when the multimode fiber 23 is a multi-core fiber, each core in the multi-core fiber may be configured with different geometric dimensions or refractive index ratios.

[0036] The first coupler 13 is a 3 dB coupler; the second coupler 21 and the third coupler 26 are 3 dB couplers, 2 / 8 couplers, 3 / 7 couplers, or couplers with a split power percentage adjustment range of 0% to 100%.

[0037] This system can be widely used in fields such as space division multiplexing communication, fiber optic sensing, laser mode selection, and non-reciprocal devices.

[0038] The working steps of this system include: The laser light source 11 is controlled to emit an optical signal, which is transmitted to the first coupler 13 via the isolator 12. The optical signal is split into two paths using the first coupler 13 and injected into the second coupler 21 and the third coupler 26 respectively, forming clockwise and counterclockwise optical signals within the ring cavity. The clockwise optical signal enters through the second coupler 21, passes through the first optical control component 22, the multimode fiber 23, the polarization controller 24, and the second optical control component 25 in sequence, and is output from the third coupler 26. The counterclockwise optical signal enters through the third coupler 26, passes through the second optical control component 25, the polarization controller 24, the multimode fiber 23, and the first optical control component 22 in sequence, and is output from the second coupler 21. Perform the lateral field control steps, including: Rotate the angle of the first optical control component 22 to adjust the power and polarization state of the clockwise transmitted optical signal; Rotate the angle of the second optical control component 25 to adjust the power and polarization state of the counterclockwise transmitted optical signal; By utilizing the mode transmission characteristics of multimode fiber 23 and combining the adjustment function of polarization controller 24, the transverse mode field of the output optical signal can be controlled.

[0039] In the transverse mode field modulation step, the operation of adjusting the power and polarization state of the clockwise transmitted optical signal is independent of the operation of adjusting the power and polarization state of the counterclockwise transmitted optical signal, and is used to achieve dynamic adjustment of the ratio of optical power in the two directions.

[0040] The specific process of utilizing the mode transmission characteristics of multimode fiber 23 includes: injecting optical signals into the multi-core fiber, and using the differences in geometric dimensions or refractive index ratios between the cores in the multi-core fiber to achieve differentiated mode excitation.

[0041] Select the corresponding single-mode fiber type 4 and collimator antireflection coating type according to the target operating wavelength, establish a closed-loop optical path with all-fiber connection, and maintain the sealing of the all-fiber structure during optical signal transmission.

[0042] The optical signal power emitted by the laser source 11 is evenly divided by the 3 dB coupler after passing through the isolator 12, and the initial power of each is 50% of the input optical signal power; The optical signal entering clockwise through the first coupler 13 passes sequentially through the second coupler 21, the first collimator 221, the first linear polarizer 222, the first half-wave plate 223, the first quarter-wave plate 224, the second collimator 225, the multimode fiber 23, the polarization controller 24, the third collimator 251, the second quarter-wave plate 252, the second half-wave plate 253, the second linear polarizer 254, the fourth collimator 255, and the third coupler 26 before being output. The optical signal entering counterclockwise through the first coupler 13 passes sequentially through the third coupler 26, the fourth collimator 255, the second linear polarizer 254, the second half-wave plate 253, the second quarter-wave plate 252, the third collimator 251, the polarization controller 24, the multimode fiber 23, the second collimator 225, the first quarter-wave plate 224, the first half-wave plate 223, the first linear polarizer 222, the first collimator 221, and the second coupler 21 before being output. Optical signals in clockwise and counterclockwise directions are transmitted in opposite directions within the ring cavity, forming a complete bidirectional closed-loop optical path.

[0043] The laser source 11, isolator 12, coupler, collimator, linear polarizer, half-wave plate, quarter-wave plate, multimode fiber 23, and polarization controller 24 are connected by fusion splicing or flange connection of single-mode fiber 4 to achieve all-fiber connection and form a stable all-fiber structure.

[0044] By rotating the first linear polarizer 222, the first half-wave plate 223, and the first quarter-wave plate 224 in the clockwise direction, the optical power and polarization state in the clockwise direction can be changed. Similarly, by rotating the second linear polarizer 254, the second half-wave plate 253, and the second quarter-wave plate 252 in the counterclockwise direction, the optical power and polarization state in the counterclockwise direction can be independently adjusted, thereby realizing the dynamic adjustment of the ratio of optical power in the two directions.

[0045] The laser source 11 can output pulsed light or continuous light, and the working wavelengths include 1064 nm, 1550 nm and 2 μm.

[0046] The coupler can be selected from 3 dB couplers, 2 / 8 couplers or 3 / 7 couplers, etc., and the percentage of split power can be adjusted in the range of 0% to 100%, that is, the output power of the optical signal in the clockwise and counterclockwise directions can be different.

[0047] The first collimator 221, the first linear polarizer 222, the first half-wave plate 223, the first quarter-wave plate 224, and the second collimator 225 in the clockwise direction are integrated into a single module; the corresponding components in the counterclockwise direction are also integrated into a single module, which facilitates installation and debugging.

[0048] Multimode fiber 23 includes, but is not limited to, multi-core fiber, step-index multimode fiber 23, and graded-index multimode fiber 23. When using multi-core fiber, a spatial light modulator can be added to the system to effectively change the percentage of power in different cores, but this will disrupt the all-fiber structure of the system. When using multi-core fiber, the geometric dimensions or refractive index ratios of each core can be different to achieve differentiated mode excitation.

[0049] Collimators require appropriate antireflective coatings depending on the light source's wavelength, and half-wave and quarter-wave plates also need to be selected accordingly. Furthermore, the single-mode fiber used must be selected based on the laser's operating wavelength. For example, HI 1060 fiber is chosen for 1-μm wavelength lasers; SMF-28 or SMF-28e fiber is chosen for 1.55-μm wavelength lasers; and SM1950 fiber is chosen for 2-μm wavelength lasers, and so on.

[0050] In this embodiment, a full-fiber mode field modulation system based on a 1064 nm pulsed fiber laser is specifically adopted; The system is based on the above-mentioned fiber optic transverse mode field control system scheme, and its components are as follows: laser source 11, isolator 12, 3 dB coupler, coupler, collimator, linear polarizer, half-wave plate, quarter-wave plate, collimator, multimode fiber 23, polarization controller 24, collimator, quarter-wave plate, half-wave plate, linear polarizer, collimator, coupler.

[0051] The laser source 11 employs a pulsed fiber laser (Q-switched, mode-locked, etc.) with a center wavelength of 1064 nm to provide the light source for the system. The isolator 12, with an isolation >60 dB, supports 1064 nm and prevents reflected light from returning to the source, protecting the laser. The 3 dB first coupler 13 is a fused biconical taper type coupler with an operating wavelength of 1064 nm, a splitting ratio of 50:50, and a directivity >50 dB. It splits the optical signal from the source into two beams, one clockwise and one counterclockwise, each receiving approximately 50% of the optical power. It forms the starting point for beam splitting in the ring cavity and is a key component for achieving bidirectional transmission.

[0052] The second coupler 21 and the third coupler 26 are general-purpose fiber optic couplers, which can be set with different splitting ratios (such as 3 dB, 2 / 8, 3 / 7, etc.) according to design requirements. In this system, they are used to construct clockwise and counterclockwise optical paths respectively. The second coupler 21 is used to output counterclockwise optical signals; the third coupler 26 is used to output clockwise optical signals.

[0053] The first collimator 221, the second collimator 225, the fourth collimator 255, and the third collimator 251 require optimized coating and focal length for a 1064 nm wavelength to convert the divergent light signal into parallel light. In this system, the collimators used in pairs (such as the first collimator 221 and the second collimator 225) form a "collimation-modulation-refocusing" link. Among them, the second collimator 225 and the third collimator 251 recouple the optical signal into the single-mode fiber 4.

[0054] The extinction ratio of the first linear polarizer 222 and the second linear polarizer 254 is >30 dB, which can be used as polarizer and polarizer analyzer, that is, to extract linearly polarized light in a single direction from incident light of any polarization state.

[0055] The first half-wave plate 223, the second half-wave plate 253, the first quarter-wave plate 224, and the second quarter-wave plate 252 have a nominal wavelength of 1064 nm. The anti-reflection coating is optimized and, in conjunction with the linear polarizer, is used to adjust the polarization azimuth angle of the optical signal injected into the multimode fiber 23.

[0056] Multimode fiber 23 includes, but is not limited to, multi-core fiber, step-index multimode fiber 23, and graded-index multimode fiber 23, and is the core unit for realizing "lateral mode field modulation".

[0057] The polarization controller 24 is a three-ring fiber winding structure that actively adjusts the polarization input conditions of light in the multimode fiber 23, thereby enhancing the stability of mode control.

[0058] Example 2 In this embodiment, an all-fiber mode field modulation system based on a 1550 nm continuous light source is used; the general structure of the system is as follows. Figure 4 As shown; The system is based on a fiber optic transverse mode field control system scheme in Embodiment 1, and its components are as follows: laser source 11, isolator 12, first coupler 13, second coupler 21, first collimator 221, first linear polarizer 222, first half-wave plate 223, first quarter-wave plate 224, second collimator 225, multimode fiber 23, polarization controller 24, through third collimator 251, second quarter-wave plate 252, second half-wave plate 253, second linear polarizer 254, fourth collimator 255 and third coupler 26.

[0059] The laser source 11 uses a continuous semiconductor laser with a center wavelength of 1550 nm to provide the light source for the system. The isolator 12 uses an isolation degree of >60 dB to support the 1550 nm wavelength, preventing reflected light from returning to the source and protecting the laser. The first coupler 13 uses a 3 dB fused tapered coupler to split the optical signal from the source into two beams, one clockwise and one counterclockwise. Each beam receives approximately 50% of the optical power and is the starting point for beam splitting in the ring cavity, as well as a key component for achieving bidirectional transmission.

[0060] The second coupler 21 and the third coupler 26 are general-purpose fiber optic couplers, which can be set with different splitting ratios (such as 3 dB couplers, 2 / 8 couplers, 3 / 7 couplers, etc.) according to design requirements. In this system, they are used to branch and connect optical paths in the clockwise and counterclockwise directions. The second coupler 21 is used to output optical signals in the counterclockwise direction; the third coupler 26 is used to output optical signals in the clockwise direction.

[0061] The first collimator 221, the second collimator 225, the fourth collimator 255, and the third collimator 251 require optimized coatings and focal lengths for a 1550 nm wavelength to convert divergent light signals into parallel light. In this system, the collimators used in pairs (such as the first collimator 221 and the second collimator 225) form a "collimation-modulation-refocusing" link. The second collimator 225 and the third collimator 251 recouple the optical signal into the single-mode fiber 4.

[0062] The first linear polarizer 222 and the second linear polarizer 254 are fiber-optic polarizers with an extinction ratio >30 dB. They are used for polarization initiation and polarization analysis to extract linearly polarized light in a single direction from incident light of any polarization state.

[0063] The center wavelengths of the first half-wave plate 223, the second half-wave plate 253, the first quarter-wave plate 224, and the second quarter-wave plate 252 are calibrated to 1550 nm. They are used in conjunction with linear polarizers to adjust the polarization state of the optical signal injected into the multimode fiber 23.

[0064] Multimode fiber 23 includes, but is not limited to, multi-core fiber, step-index multimode fiber 23, and graded-index multimode fiber 23, and is the core unit for realizing "lateral mode field modulation".

[0065] The polarization controller 24 is a three-loop fiber winding structure, which can flexibly adjust the polarization state of the optical signal transmitted in the fiber, enhancing mode control stability. It is worth mentioning that this polarization controller 24 can be replaced with an electrically controlled polarization controller 24 (EPC), which uses intelligent algorithms to precisely control the intracavity fiber birefringence.

[0066] Example 3 In this embodiment, an all-fiber mode field manipulation system based on 2-μm pulsed fiber laser is used; the general structure of the system is as follows. Figure 4 As shown; The system is based on a fiber optic transverse mode field control system scheme in Embodiment 1, and its components are as follows: laser source 11, isolator 12, 3 dB first coupler 13, second coupler 21, first collimator 221, first linear polarizer 222, first half-wave plate 223, first quarter-wave plate 224, second collimator 225, multimode fiber 23, polarization controller 24, through third collimator 251, second quarter-wave plate 252, second half-wave plate 253, second linear polarizer 254, fourth collimator 255 and third coupler 26.

[0067] The laser source 11 uses a pulsed fiber laser with a center wavelength of 2-μm to provide the light source for the system. The isolator 12 is a fiber optic isolator optimized for the 2-μm band with an isolation degree of >60 dB, which can effectively block reflected light in the optical path from returning to the laser source 11 and avoid power fluctuations or damage to the source due to backlight.

[0068] The 3 dB first coupler 13 is a fused tapered type coupler with a working wavelength of 2-μm, a splitting ratio of 50:50, and a directivity of >50dB. It splits the optical signal from the light source into two beams, one clockwise and one counterclockwise, with each beam receiving about 50% of the optical power. It is the starting point for beam splitting in the ring cavity and a key component for achieving bidirectional transmission.

[0069] The second coupler 21 and the third coupler 26 are 2-μm universal fiber optic couplers, which can be set with different splitting ratios (such as 3 dB, 2 / 8, 3 / 7, etc.) according to design requirements. In this system, they are used to connect the clockwise and counterclockwise optical paths, respectively. The second coupler 21 is used to output the counterclockwise optical signal; the third coupler 26 is used to output the clockwise optical signal.

[0070] The first collimator 221, the second collimator 225, the fourth collimator 255, and the third collimator 251 require optimized coating and focal length for a 2-μm wavelength to convert divergent light signals into parallel light. In this system, the collimators used in pairs (such as the first collimator 221 and the second collimator 225) form a "collimation-modulation-refocusing" link. Among them, the second collimator 225 and the third collimator 251 recouple the optical signal into the multimode fiber 23.

[0071] The first linear polarizer 222 and the second linear polarizer 254 are fiber-optic polarizers with an extinction ratio >30 dB. They are used for polarization initiation and polarization analysis to extract linearly polarized light in a single direction from incident light signals of arbitrary polarization states.

[0072] The first half-wave plate 223, the second half-wave plate 253, the first quarter-wave plate 224, and the second quarter-wave plate 252 have a nominal wavelength of 2-μm. The anti-reflection coating is optimized and, in conjunction with the linear polarizer, is used to adjust the polarization state of the optical signal injected into the multimode fiber 23.

[0073] Multimode fiber 23 includes, but is not limited to, multi-core fiber, step-index multimode fiber 23, and graded-index multimode fiber 23, and is the core unit for realizing "lateral mode field modulation". Polarization controller 24 is a three-loop fiber winding structure that actively adjusts the polarization input conditions of the optical signal in multimode fiber 23 to enhance mode control stability.

[0074] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any person skilled in the art can easily conceive of various equivalent modifications or substitutions within the technical scope disclosed in the present invention, and these modifications or substitutions should all be covered within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A fiber optic transverse mode field modulation system, characterized in that, It includes a laser injection unit (1) and a ring cavity unit (2); The laser injection unit (1) includes a cascaded laser source (11), an isolator (12), and a first coupler (13). The annular cavity unit (2) is bidirectionally optically connected to the laser injection unit (1); Within the ring cavity unit (2), starting from the access point (3) of the laser injection unit (1), a second coupler (21), a first optical control component (22), a multimode fiber (23), a polarization controller (24), a second optical control component (25), and a third coupler (26) are arranged sequentially in a clockwise direction; and the ring cavity unit (2) is constructed as a bidirectional transmission optical path that supports both clockwise and counterclockwise transmission modes; The first optical control component (22) and the second optical control component (25) are respectively disposed on the first rotating bracket and the second rotating bracket, and are configured to adjust the optical power and polarization state of the optical signal in the clockwise or counterclockwise direction by rotating the angle of the first optical control component (22) or the second optical control component (25).

2. The fiber optic transverse mode field modulation system according to claim 1, characterized in that, The first optical control component (22) includes a first collimator (221), a first linear polarizer (222), a first half-wave plate (223), a first quarter-wave plate (224), and a second collimator (225) that are sequentially connected in bidirectional optical connection. The second optical control component (25) includes a third collimator (251), a second quarter-wave plate (252), a second half-wave plate (253), a second linear polarizer (254), and a fourth collimator (255) that are connected in a bidirectional optical connection.

3. The fiber optic transverse mode field modulation system according to claim 2, characterized in that, The first optical control component (22) is packaged and integrated into an integrated component; The second optical control component (25) is packaged and integrated into an integrated component.

4. The fiber optic transverse mode field modulation system according to claim 2, characterized in that, In the optical fiber transverse mode field control system, all optical components are connected by single-mode optical fiber (4), and the connection method is fusion splicing or flange connection, so that the system forms a closed all-fiber structure. The type of the single-mode fiber (4) is matched with the working band of the laser source (11), including: when the working band is 1 micrometer band, HI 1060 fiber is selected; when the working band is 1.55 micrometer band, SMF-28 fiber or SMF-28e fiber is selected; when the working band is 2 micrometer band, SM1950 fiber is selected.

5. The fiber optic transverse mode field modulation system according to claim 2, characterized in that, The multimode fiber (23) specifically includes multi-core fiber, step-index multimode fiber (23) or graded-index multimode fiber (23). When the multimode fiber (23) is a multi-core fiber, each core in the multi-core fiber is configured with different geometric dimensions or refractive index ratios.

6. The fiber optic transverse mode field modulation system according to claim 2, characterized in that, The first coupler (13) is a 3 dB coupler; the second coupler (21) and the third coupler (26) are 3 dB couplers, 2 / 8 couplers, 3 / 7 couplers or couplers with a split power percentage adjustment range of 0% to 100%.

7. A method for controlling the transverse mode field of an optical fiber, characterized in that, This method operates using an optical fiber transverse mode field modulation system as described in any one of claims 1-6, and the method steps include: The laser light source (11) is controlled to emit light signals, which are transmitted to the first coupler (13) via the isolator (12). The optical signal is split into two paths using the first coupler (13) and injected into the second coupler (21) and the third coupler (26) respectively, forming clockwise and counterclockwise optical signals in the ring cavity. The clockwise optical signal enters through the second coupler (21), passes through the first optical control component (22), multimode fiber (23), polarization controller (24) and the second optical control component (25) in sequence, and is output by the third coupler (26). The counterclockwise optical signal enters through the third coupler (26), passes through the second optical control component (25), polarization controller (24), multimode fiber (23) and the first optical control component (22) in sequence, and is output by the second coupler (21). Perform the lateral field control steps, including: Rotate the angle of the first optical control component (22) to adjust the power and polarization state of the clockwise transmitted optical signal; Rotate the angle of the second optical control component (25) to adjust the power and polarization state of the counterclockwise transmitted optical signal; By utilizing the mode transmission characteristics of multimode fiber (23) and combining the adjustment function of polarization controller (24), the transverse mode field of the output optical signal can be controlled.

8. The optical fiber transverse mode field modulation method according to claim 7, characterized in that, In the transverse mode field modulation step, the operation of adjusting the power and polarization state of the clockwise transmitted optical signal is independent of the operation of adjusting the power and polarization state of the counterclockwise transmitted optical signal, which is used to achieve dynamic adjustment of the ratio of optical power in the two directions.

9. The optical fiber transverse mode field modulation method according to claim 7, characterized in that, The specific process of utilizing the mode transmission characteristics of multimode fiber (23) includes: injecting optical signals into the multi-core fiber, and using the differences in geometric dimensions or refractive index ratios between the cores in the multi-core fiber to achieve differentiated mode excitation.

10. The optical fiber transverse mode field modulation method according to claim 7, characterized in that, The method further includes: selecting the corresponding single-mode fiber (4) type and the anti-reflection coating type of the collimator according to the target working wavelength, establishing a closed-loop optical path with all-fiber connection, and maintaining the closedness of the all-fiber structure during optical signal transmission.

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