Multistage Lyot filter module, manufacturing method, design method and optical system

Abstract

The invention provides a multi-stage Lyot filter module, a manufacturing method, a design method and an optical system. The filter module comprises an input unit, a plurality of cascaded Lyot filtering units and an output unit, wherein the input unit, the plurality of cascaded Lyot filtering units and the output unit all adopt polarization-maintaining passive optical fibers; the lengths of the polarization-maintaining passive optical fibers adopted by all the Lyot filtering units are sequentially decreased, and the two ends of the polarization-maintaining passive optical fibers in all the Lyot filtering units are provided with 45-degree angle deviation welding points respectively; the input unit is used for outputting linearly polarized light and is optically connected with the first Lyot filtering unit, so that a 45-degree angle is formed between the polarization direction of the linearly polarized light and the fast axis or the slow axis of the polarization-maintaining passive optical fiber in the first Lyot filtering unit; and the output unit is used for outputting the filtered linearly polarized light and is optically connected with the last Lyot filtering unit. The problems that an existing optical fiber type single-stage Lyot filter structure is low in sidelobe suppression ratio, insufficient in filtering selectivity, high in loss and insufficient in stability can be solved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of fiber lasers, and particularly relates to a multi-stage Lyot filter module, a manufacturing method, a design method and an optical system. BACKGROUND

[0002] A Lyot filter is an optical filter based on birefringence effect, has the characteristics of narrow-band filtering and wavelength tunability, and is widely used in laser technology. The traditional Lyot filter adopts a spatial optical structure, is composed of a series of birefringent crystals and polarizers, and has the disadvantages of large volume, low integration, poor vibration resistance and the like, and is difficult to meet the needs of modern fiber communication and fiber sensing systems. In order to realize the fiberization of the Lyot filter, some Lyot filter structures based on polarization maintaining optical fibers have been proposed in the prior art. However, these structures often have the problems of poor filtering performance, large insertion loss, poor temperature stability and the like, and therefore, there is an urgent need for a multi-stage Lyot filter module with full-fiber integration, high performance and high stability. SUMMARY

[0003] In view of this, the embodiments of the present application provide a multi-stage Lyot filter module, a manufacturing method, a design method and an optical system to eliminate or improve one or more defects in the prior art.

[0004] One aspect of the present application provides a multi-stage Lyot filter module, comprising: an input unit, a plurality of cascaded Lyot filter units and an output unit, all of which adopt polarization maintaining passive optical fibers; The lengths of the polarization maintaining passive optical fibers adopted by each of the Lyot filter units decrease in turn, and both ends of the polarization maintaining passive optical fiber in each of the Lyot filter units are respectively provided with a 45-degree angle offset fusion splice point; The input unit is used to output linearly polarized light, and is optically connected between a 45-degree angle offset fusion splice point and a first Lyot filter unit in the plurality of cascaded Lyot filter units, so that the polarization direction of the linearly polarized light forms a 45-degree angle with the fast axis or the slow axis of the polarization maintaining passive optical fiber in the first Lyot filter unit; The output unit is used to output filtered linearly polarized light, and is optically connected between a 45-degree angle offset fusion splice point and a last Lyot filter unit in the plurality of cascaded Lyot filter units.

[0005] In some embodiments of the present application, in the plurality of cascaded Lyot filter units, the length of the polarization maintaining passive optical fiber adopted by any one of the Lyot filter units is twice the length of the polarization maintaining passive optical fiber adopted by the next one of the Lyot filter units.

[0006] In some embodiments of the present application, the input unit employs a polarization maintaining passive optical fiber pigtail; The polarization maintaining passive optical fiber pigtail in the input unit is optically connected with an input port of the first Lyot filter unit, and the input port is connected with the first Lyot filter unit through a 45-degree angle offset splicing point.

[0007] In some embodiments of the present application, the output unit employs a polarization maintaining passive optical fiber pigtail; The polarization maintaining passive optical fiber pigtail in the output unit is optically connected with an output port of the last Lyot filter unit, and the output port is connected with the last Lyot filter unit through a 45-degree angle offset splicing point.

[0008] In some embodiments of the present application, the polarization maintaining passive optical fiber includes a panda polarization maintaining optical fiber, a bow-tie polarization maintaining optical fiber or an elliptical cladding polarization maintaining optical fiber.

[0009] In some embodiments of the present application, the deviation of the offset angle of the 45-degree angle offset splicing point is within the interval of [-0.5°, +0.5°].

[0010] In some embodiments of the present application, the multi-stage Lyot filter module further comprises a temperature control module; The temperature control module is thermally coupled with the Lyot filter units, and is configured to control the working temperature of the Lyot filter units so that the deviation between the working temperature of the Lyot filter units and a preset temperature threshold is within the interval of [-0.5℃, +0.5℃].

[0011] The second aspect of the present application provides a manufacturing method of the multi-stage Lyot filter module provided by the first aspect, comprising: According to the target number of cascaded Lyot filter units and the length of the polarization maintaining passive optical fiber of each Lyot filter unit, the polarization maintaining passive optical fiber employed by each Lyot filter unit is selected; The polarization maintaining passive optical fibers employed by each Lyot filter unit are sequentially cascaded and spliced, and in the splicing process, a target angle is formed between the slow axes of two adjacent polarization maintaining passive optical fibers, and the deviation between the target angle and 45 degrees is within the interval of [-0.5°, +0.5°]; The polarization maintaining passive optical fiber of the input unit is spliced with the polarization maintaining passive optical fiber in the first Lyot filter unit in the plurality of cascaded Lyot filter units through a 45-degree angle offset splicing point; and the polarization maintaining passive optical fiber of the output unit is fused with the polarization maintaining passive optical fiber in the last Lyot filter unit in the plurality of cascaded Lyot filter units through a 45-degree angle offset fusion point.

[0012] A third aspect of the present application provides a design method of the multi-stage Lyot filter module of the first aspect, comprising: obtaining target performance parameters, the target performance parameters including a target free spectral range and a target 3dB bandwidth; determining a shortest length of the polarization maintaining passive optical fiber in the last Lyot filter unit based on the target free spectral range, a preset operating center wavelength, and a birefringence of the polarization maintaining passive optical fiber; determining a minimum number of cascaded Lyot filter units required to meet the bandwidth requirement as the target number of cascaded Lyot filter units through a simulation by a transfer matrix method according to the target 3dB bandwidth and the shortest length; determining the length of the polarization maintaining passive optical fiber of each Lyot filter unit according to the target number of cascaded Lyot filter units and the shortest length; outputting the target number of cascaded Lyot filter units and the length of the polarization maintaining passive optical fiber of each Lyot filter unit as design parameters of the multi-stage Lyot filter module.

[0013] A fourth aspect of the present application provides an optical system, comprising: the multi-stage Lyot filter module of the first aspect; the multi-stage Lyot filter module works as a wavelength selection element or a wavelength demodulation element in the optical system.

[0014] The multi-stage Lyot filter module provided in this application includes: an input unit, all using polarization-maintaining passive fiber; multiple cascaded Lyot filter units; and an output unit. The length of the polarization-maintaining passive fiber used in each Lyot filter unit decreases sequentially, and each Lyot filter unit has a 45-degree angle offset fusion splice at both ends. The input unit outputs linearly polarized light and is optically connected to the first Lyot filter unit in the cascaded Lyot filter units via a 45-degree angle offset fusion splice, so that the polarization direction of the linearly polarized light forms a 45-degree angle with the fast or slow axis of the polarization-maintaining passive fiber in the first Lyot filter unit. The output unit outputs filtered linearly polarized light and is connected to the last Lyot filter unit in the cascaded Lyot filter units via a 45-degree angle offset fusion splice. The optical connections between the Lyot filter units aim to address the problems of low sidelobe suppression ratio, insufficient filter selectivity, high loss, and insufficient stability in existing fiber-optic single-stage Lyot filter structures. Compared to single-stage structures, it has a narrower filter bandwidth and a steeper sideband roll-off characteristic. The multi-stage Lyot filter module provided in this application is more suitable for high-precision spectral analysis. The filter parameters can be flexibly designed by adjusting the length of the polarization-maintaining passive fiber to adapt to different wavelengths of fiber lasers, exhibiting strong scalability. The longer polarization-maintaining passive fiber determines the passband bandwidth, while the shorter segment dominates the free spectral range. Furthermore, by adjusting the number of cascaded Lyot filter units, different filter bandwidths and sidelobe suppression ratios can be achieved. It is mentioned that no spatial optical components and polarization controllers are required for mechanical adjustment, which can effectively reduce energy loss caused by polarization mismatch, realize an all-fiber design, and has the advantages of compact structure, high integration, and good vibration resistance.

[0015] Additional advantages, objectives, and features of this application will be set forth in part in the description which follows, and will in part become apparent to those skilled in the art upon review of the following description, or may be learned by practice of the application. The objectives and other advantages of this application can be realized and obtained by means of the structures specifically pointed out in the specification and drawings.

[0016] Those skilled in the art will understand that the purposes and advantages that can be achieved with this application are not limited to those specifically described above, and that the above and other purposes that this application can achieve will be more clearly understood from the following detailed description. Attached Figure Description

[0017] The accompanying drawings, which are included to provide a further understanding of this application and form part of this application, do not constitute a limitation thereof. The components in the drawings are not drawn to scale but are merely for illustrating the principles of this application. For ease of illustration and description of certain parts of this application, corresponding portions in the drawings may be enlarged, i.e., may appear larger relative to other components in an exemplary device actually manufactured according to this application. In the drawings: Figure 1 This is a schematic diagram of the first structure of a multi-stage Lyot filter module in one embodiment of this application.

[0018] Figure 2 This is a schematic diagram of a second structure of a multi-stage Lyot filter module in one embodiment of this application.

[0019] Figure 3 This is a schematic diagram of the third structure of a multi-stage Lyot filter module in one embodiment of this application.

[0020] Figure 4 This is a schematic flowchart illustrating the manufacturing method of a multi-stage Lyot filter module according to an embodiment of this application.

[0021] Figure 5 This is a flowchart illustrating the design method of a multi-stage Lyot filter module in one embodiment of this application.

[0022] Figure 6 This is a schematic diagram of a 45-degree angle offset welding in one example of this application.

[0023] Figure 7 This is a schematic diagram of the structure of the 3-stage Lyot filter module in the first application example of this application.

[0024] Figure 8(a) is a schematic diagram comparing the transmittance curves between single-stage and three-stage Lyot filter modules when the length of the first-stage Lyot filter unit is L=2m in the first application example of this application.

[0025] Figure 8(b) is a schematic diagram comparing the transmittance curves between a single-stage and a three-stage Lyot filter module when the length of the first-stage Lyot filter unit is L=4m in the first application example of this application.

[0026] Figure 9 This is a schematic diagram of the structure of the 4-stage Lyot filter module in the second application example of this application.

[0027] Figure 10(a) is a schematic diagram comparing the transmittance curves between a single-stage and a four-stage Lyot filter module when the length of the first-stage Lyot filter unit is L=2m in the second application example of this application.

[0028] Figure 10(b) is a schematic diagram comparing the transmittance curves between a single-stage and a four-stage Lyot filter module when the length of the first-stage Lyot filter unit is L=4m in the second application example of this application.

[0029] The attached icon numbers include: 1. Input unit; 2. Lyot filter unit; 3. Output unit; 4, 104, 305, Input ports; 5, 105, 306, Output ports; 6, 206, 407, Temperature control module; 61. Cooling element; 62. Placement platform; 63. Heat dissipation device; 64. Circulating water pump; 7. The fast axis of the polarization-maintaining passive fiber in the (i-1)th stage Lyot filter unit; 8. The slow axis of the polarization-maintaining passive fiber in the (i-1)th stage Lyot filter unit; 9. After the polarization-maintaining passive fiber in the (i-1)th stage Lyot filter unit is fused with the polarization-maintaining passive fiber in the i-th stage Lyot filter unit at a 45-degree angle, the axis on the polarization-maintaining passive fiber in the i-th stage Lyot filter unit is parallel to the slow axis of the polarization-maintaining passive fiber in the i-th stage Lyot filter unit. 10. The angle between the slow axis of the polarization-maintaining passive fiber and the polarization-maintaining passive fiber in the i-th stage Lyot filter unit during 45-degree fusion splicing; 101, 301, First-stage Lyot filter unit; 102, 302, second-stage Lyot filter unit; 103, 303, third-stage Lyot filter unit; 304. Fourth-stage Lyot filter unit; 204, 405, First weld point; 205, 406, Second welding point. Detailed Implementation

[0030] To make the objectives, technical solutions, and advantages of this application clearer, the application will be further described in detail below with reference to the embodiments and accompanying drawings. Here, the illustrative embodiments and their descriptions are used to explain this application, but are not intended to limit it.

[0031] It should also be noted that, in order to avoid obscuring this application with unnecessary details, only the structures and / or processing steps closely related to the solution according to this application are shown in the accompanying drawings, while other details that are not closely related to this application are omitted.

[0032] It should be emphasized that the term "including / comprises" as used herein refers to the presence of a feature, element, step, or component, but does not exclude the presence or addition of one or more other features, elements, steps, or components.

[0033] It should also be noted that the terms "center," "upper," "lower," "left," "right," "vertical," "horizontal," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, and are only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation on this application. Furthermore, the terms "first," "second," and "third" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0034] In the description of this application, it should be noted that, unless otherwise expressly specified and limited, the terms "installation," "connection," and "linking" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection between two components. Those skilled in the art can understand the specific meaning of the above terms in this application based on the specific circumstances.

[0035] In the following description, embodiments of the present application will be illustrated with reference to the accompanying drawings. In the drawings, the same reference numerals represent the same or similar parts, or the same or similar steps.

[0036] To address the issues of low sidelobe suppression ratio, insufficient filter selectivity, high loss, and inadequate stability in existing fiber-optic single-stage Lyot filter structures, embodiments of this application provide a multi-stage Lyot filter module, a manufacturing method for the multi-stage Lyot filter module, a design method for the multi-stage Lyot filter module, and an optical system (such as a fiber laser, fiber optic sensing system, etc.) incorporating the multi-stage Lyot filter. The multi-stage Lyot filter module provided in this application, based on polarization-maintaining fiber 45-degree offset fusion splicing technology, has a compact structure and excellent filtering performance.

[0037] The following examples will provide a detailed description.

[0038] Based on this, embodiments of this application provide a multi-stage Lyot filter, see [link to relevant documentation]. Figure 1The multi-stage Lyot filter specifically includes the following: Each unit employs a polarization-maintaining passive fiber input unit 1, multiple cascaded Lyot filter units 2, and an output unit 3. The number of cascaded Lyot filter units 2 can be denoted as m, where m ≥ 2.

[0039] In one or more embodiments of this application, the polarization-maintaining passive optical fiber is a type of glass fiber that does not generate or amplify optical signals and can maintain the stable polarization state of the optical signal. In this application, the use of polarization-maintaining passive optical fiber ensures that the input light energy excites the fast and slow axis modes at a precise 45-degree angle. When the light is transmitted in each segment of the optical fiber, the polarization state will not be disordered, and the 45-degree angle splicing between each stage is meaningful and effective; the final output filtered signal is stable and reliable.

[0040] It's important to note that multiple cascaded Lyot filter units (2) refer to multiple Lyot filter units connected end-to-end. The output of the first Lyot filter unit (also called the first-stage Lyot filter unit) is connected to the input of the second Lyot filter unit (also called the second-stage Lyot filter unit) via a 45-degree offset fusion splice. The output of the second Lyot filter unit is then connected to the input of the third Lyot filter unit (also called the third-stage Lyot filter unit) in the same way, and so on, until the last unit. Each Lyot filter unit is itself a filter, with a transmittance curve that has a broad peak and high sidelobes. When the optical signal passes through all the cascaded Lyot filter units sequentially, each stage filters the light. Only wavelengths that can be simultaneously transmitted by all the cascaded Lyot filter units can ultimately reach the output unit. This is equivalent to multiplying multiple transmittance curves. The result is that the overall transmittance peak becomes very sharp, and the 3dB bandwidth is significantly narrowed. For non-target wavelengths, if a wavelength is transmitted through a certain stage, it is likely to be suppressed in the next stage. This interstage effect greatly suppresses the sidelobes next to the transmission peak, resulting in a significant improvement in the stopband rejection ratio.

[0041] The length of the polarization-maintaining passive optical fiber used in each of the Lyot filter units 2 decreases sequentially, and each of the polarization-maintaining passive optical fibers in each Lyot filter unit 2 has a 45-degree angle offset fusion splice at both ends.

[0042] It is understood that the fusion splice point refers to the point where the end faces of two polarization-maintaining passive optical fibers are joined together by heating and melting. The 45-degree angle offset means that during the fusion splicing, the polarization axis (fast axis / slow axis) of the second polarization-maintaining passive optical fiber is rotated by 45 degrees relative to the first polarization-maintaining passive optical fiber. In other words, the 45-degree angle offset fusion splice point refers to a special fusion splice point where the "coordinate system" of one polarization-maintaining passive optical fiber is rotated by 45 degrees and then connected to another polarization-maintaining passive optical fiber.

[0043] Specifically, when a linearly polarized light beam enters a segment of polarization-maintaining passive fiber at a 45-degree angle (relative to the fast / slow axis), the light is equally decomposed into two orthogonal polarization modes propagating along the fast and slow axes. Due to the birefringence effect of the polarization-maintaining passive fiber, the light propagates at different speeds along the fast and slow axes. After propagation over a length L, a wavelength-dependent phase difference Δφ is generated between the two modes. When the light reaches the end of this fiber segment, the two light modes with a phase difference need to be re-merged (interfered). How they are merged determines the filter's effectiveness. If directly spliced ​​(0-degree fusion), the light from the fast axis will enter the fast axis of the next fiber segment, and the light from the slow axis will enter the slow axis. They will continue to propagate independently without interference, thus failing to form a filter. However, if a 45-degree offset fusion splice is used, the light from the fast axis of the first fiber segment will be equally projected onto the fast and slow axes of the second fiber segment at the splice point, and similarly, the light from the slow axis of the first fiber segment will be equally projected onto the fast and slow axes of the second fiber segment. In this way, at the beginning of the second fiber segment, the light on both the fast and slow axes contains information from both modes of the first fiber segment, and they are capable of interfering with each other. Only specific wavelengths whose phase differences satisfy the conditions for constructive interference can pass through, while other wavelengths are suppressed due to destructive interference.

[0044] In other words, the function of the 45-degree angle offset fusion splice is to act as a polarization mixer, forcing the two polarization modes from the previous fiber segment to interfere at the beginning of the next fiber segment, thereby achieving wavelength selection. It is the key mechanism for converting phase difference into filtering effect.

[0045] The input unit 1 is used to output linearly polarized light and is optically connected to the first Lyot filter unit 2 in a plurality of cascaded Lyot filter units 2 through a 45-degree angle offset fusion splice, so that the polarization direction of the linearly polarized light is at a 45-degree angle to the fast axis or slow axis of the polarization-maintaining passive fiber in the first Lyot filter unit 2. The output unit 3 is used to output filtered linearly polarized light and is optically connected to the last Lyot filter unit 2 in a plurality of cascaded Lyot filter units 2 through a 45-degree angle offset fusion point.

[0046] As described above, the multi-stage Lyot filter provided in this application has a narrower filtering bandwidth and a steeper sideband roll-off characteristic compared to a single-stage structure. The multi-stage Lyot filter module provided in this application is more suitable for high-precision spectral analysis. The filtering parameters can be flexibly designed by adjusting the length of the polarization-maintaining passive fiber to adapt to fiber lasers of different wavelengths, with strong scalability. The longer polarization-maintaining passive fiber determines the passband bandwidth, while the shorter segment dominates the free spectral range. Furthermore, by adjusting the number of cascaded Lyot filter units, different filtering bandwidths and sidelobe suppression ratios can be achieved. It is mentioned that no spatial optical components and polarization controllers are required for mechanical adjustment, which can effectively reduce energy loss caused by polarization mismatch, realize an all-fiber design, and has the advantages of compact structure, high integration, and good vibration resistance.

[0047] In order to further narrow the filtering bandwidth to obtain a sharp main lobe and significantly suppress the side lobes, in one embodiment of the multi-stage Lyot filter module provided in this application, in the multiple cascaded Lyot filter units 2, the length of the polarization-maintaining passive fiber used in any one of the Lyot filter units 2 is twice the length of the polarization-maintaining passive fiber used in the next Lyot filter unit 2.

[0048] In other words, the length of the polarization-maintaining passive fiber in the i-th Lyot filter unit The length of the polarization-maintaining passive fiber in the (i-1)th stage Lyot filter unit The following conditions must be met: .

[0049] As described above, the multi-stage Lyot filter provided in this application, compared to a single-stage filter or a filter with randomly cascaded lengths, exhibits a broad and gentle cosine-shaped transmission peak. By employing binary length cascading, the transmission peak becomes needle-like, significantly reducing the 3dB bandwidth and greatly improving wavelength selectivity. Simultaneously, a single-stage filter exhibits a high number of sidelobes in its transmission spectrum, meaning that non-target wavelengths also have considerable transmittance, resulting in incomplete filtering and a poor signal-to-noise ratio. However, in this application, within the multi-stage binary cascaded system, the sidelobe position of a transmission peak in one stage may very well coincide with the valley of another stage's transmission peak. Therefore, the sidelobe signal is continuously suppressed and attenuated in subsequent stages.

[0050] To ensure a high-performance, low-loss, stable, and reliable connection between the filter and external systems, and to maintain the polarization state, in one embodiment of the multi-stage Lyot filter module provided in this application, see [reference needed]. Figure 2 The input unit 1 uses a polarization-maintaining passive fiber pigtail. It can be understood that a polarization-maintaining passive fiber pigtail is a short fiber in a polarization-maintaining passive fiber system, with a connector at one end and a bare fiber end face at the other.

[0051] The polarization-maintaining passive fiber optic pigtail in the input unit 1 is optically connected to the input port 4 of the first Lyot filter unit 2. The input port 4 is connected to the first Lyot filter unit 2 through a 45-degree angle offset fusion splice.

[0052] In other words, the input unit 1 outputs linearly polarized light, the polarization axis of which forms a 45-degree angle with the fast or slow axis of the polarization-maintaining passive fiber of the first Lyot filter unit 2, and is connected through the first 45-degree angle offset fusion splice. Specifically, the input unit 1 can be connected to the first Lyot filter unit 2 through the input port 4, that is, the input port 4 is connected to the first-stage Lyot filter unit through the 45-degree angle offset fusion splice.

[0053] To further ensure a high-performance, low-loss, stable, and reliable connection between the filter and external systems, while maintaining the polarization state, in one embodiment of the multi-stage Lyot filter module provided in this application, see [reference needed]. Figure 2 The output unit 3 uses a polarization-maintaining passive optical fiber pigtail; The polarization-maintaining passive fiber optic pigtail in the output unit 3 is optically connected to the output port 5 of the last Lyot filter unit 2. The output port 5 is connected to the last Lyot filter unit 2 through a 45-degree angle offset fusion splice.

[0054] In other words, the output unit 3 outputs filtered linearly polarized light, the polarization axis of which forms a 45-degree angle with the fast or slow axis of the polarization-maintaining passive fiber of the last Lyot filter unit 2, and is connected through the last 45-degree angle offset fusion splice. Specifically, the output unit 3 can be connected to the last Lyot filter unit 2 through the output port 5, that is, the output port 5 is connected to the last Lyot filter unit 2 through the 45-degree angle offset fusion splice.

[0055] To further clarify the available commercial polarization-maintaining fiber types and effectively enhance the practicality and feasibility of multi-stage Lyot filter modules, in one embodiment of the multi-stage Lyot filter module provided in this application, the polarization-maintaining passive fiber is one of panda-type, bow-tie-type, or elliptical-clad polarization-maintaining fiber. This makes the multi-stage Lyot filter module structure compatible with various mainstream polarization-maintaining fibers, effectively expanding the applicability and manufacturability of the multi-stage Lyot filter module, and reducing production costs and barriers to entry.

[0056] To further address the issues of increased insertion loss and deteriorated filtering performance caused by excessive weld angle deviation, in one embodiment of the multi-stage Lyot filter module provided in this application, the deviation of the offset angle of the 45-degree offset weld point is within the range of [-0.5°, +0.5°].

[0057] In other words, the angular deviation of the 45-degree offset welding is no greater than ±0.5°. By controlling the welding angle deviation within ±0.5°, the energy loss and polarization crosstalk at the welding point can be minimized, ensuring that the filter can achieve the designed high-performance indicators in actual manufacturing.

[0058] To further address the issues of wavelength drift and performance instability caused by environmental temperature variations in filters, in one embodiment of the multi-stage Lyot filter module provided in this application, see [link to relevant documentation]. Figure 2 The multi-stage Lyot filter module also includes a temperature control module 6.

[0059] The temperature control module is thermally coupled to the Lyot filter unit and is used to control the operating temperature of the Lyot filter unit so that the deviation between the operating temperature of the Lyot filter unit and the preset temperature threshold is within the range of [-0.5℃, +0.5℃].

[0060] Among them, the temperature control module 6 can use a semiconductor cooling chip and a temperature sensor to stabilize the operating temperature within ±0.5℃.

[0061] Specifically, see Figure 3 The temperature control module 6 includes the following components: (1) Temperature sensor (not in) Figure 3 As shown in the figure): The temperature of the placement platform or Lyot filter unit 2 is monitored in real time and the signal is fed back to the temperature control circuit.

[0062] (2) Temperature control circuit (not in) Figure 3 As shown in the figure: receive the signal from the temperature sensor and compare it with the preset temperature threshold set by the user (e.g., 25±0.1℃).

[0063] Based on the comparison results, the magnitude and direction of the current to be applied to the cooling chip are calculated using a PID (proportional-integral-derivative) control algorithm to accurately compensate for temperature deviations.

[0064] (3) Cooling element 61 (thermoelectric cooler, TEC): It cools or heats based on the current provided by the temperature control circuit. That is, it is based on the Peltier effect. When a direct current passes through it, one end of the cooling element will become cold (cold end), and the other end will become hot (hot end). The direction of the current determines which side is cooled. When the filter temperature is higher than the set value, the control circuit provides a positive current to make the cooling chip close to the side of the placement stage (cold end) cool and absorb heat. When the filter temperature is lower than the set value, the control circuit reverses the current direction, causing the cooling chip to heat up the side of the placement platform (cold end) and release heat.

[0065] (4) Placement platform 62: Usually made of a metal with good thermal conductivity (such as aluminum or copper). The placement platform is tightly attached to the cold end of the cooler, and the Lyot filter unit is fixed on the placement platform. The cold or heat generated by the cooler is efficiently and evenly transferred to the controlled Lyot filter unit through the placement platform, ensuring that its temperature is consistent and controllable.

[0066] (5) Heat dissipation device 63 (radiator and fan): used to handle the waste heat generated by the hot end of the cooling chip. When the cooling chip is working, a large amount of heat will accumulate at its hot end. The radiator (usually a metal fin structure) enhances heat dissipation by increasing the contact area with the air. The fan forces air to flow over the radiator fins, quickly removing the waste heat, preventing the hot end temperature from getting too high, and ensuring the efficiency and lifespan of the cooling chip.

[0067] (6) Circulating water pump 64: Drives the liquid cooling circuit to achieve more efficient and quieter heat dissipation. That is, the water pump circulates the coolant (usually water or a special coolant) in a closed pipeline. The coolant flows through a water cooling head installed at the hot end of the radiator or cooling fins to absorb waste heat, and is then pumped to a larger external heat sink far away from the system for heat dissipation, and then circulates back after cooling.

[0068] In other words, the temperature control module 6, through the coordinated operation of the temperature sensor, temperature control circuit, cooling element 61, placement platform 62, heat dissipation device 63, and circulating water pump 64, constitutes a high-precision constant temperature environment. Through closed-loop feedback, it dynamically cools or heats, ultimately maintaining the operating temperature of the Lyot filter unit within a very small fluctuation range of ±0.5℃ or even ±0.1℃. This ensures the long-term stability and reliability of the center wavelength of the multi-stage Lyot filter, solving the core technical challenge of its temperature sensitivity.

[0069] exist Figure 3 In the text, "#1", "#2" and "#m" respectively represent the polarization-maintaining passive fiber used in the first Lyot filter unit 2, the second Lyot filter unit 2, and the last (i.e. the mth) Lyot filter unit 2.

[0070] To achieve high-precision and high-repeatability manufacturing of the high-performance multi-stage Lyot filter module provided in the aforementioned embodiments, this application also provides an embodiment of a method for manufacturing a multi-stage Lyot filter module, see [link to embodiment]. Figure 4 The manufacturing method of the multi-stage Lyot filter module specifically includes the following: Step 100: Select the polarization-maintaining passive fiber used by each Lyot filter unit according to the preset target number of Lyot filter units and the length of the polarization-maintaining passive fiber of each Lyot filter unit.

[0071] Step 200: The polarization-maintaining passive optical fibers used in each Lyot filter unit are cascaded and spliced ​​sequentially. During the splicing process, the target angle is formed between the slow axes of two adjacent polarization-maintaining passive optical fibers, and the deviation between the target angle and the 45-degree angle is within the range of [-0.5°, +0.5°].

[0072] Step 300: The polarization-maintaining passive fiber of the input unit is fused with the polarization-maintaining passive fiber of the first Lyot filter unit in the multiple cascaded Lyot filter units through a 45-degree angle offset fusion splice.

[0073] And, step 400: the polarization-maintaining passive fiber of the output unit is fused with the polarization-maintaining passive fiber in the last Lyot filter unit among the multiple cascaded Lyot filter units through a 45-degree angle offset fusion splice.

[0074] As can be seen from the above description, the manufacturing method of the multi-level Lyot filter module provided in this application provides a clear and operable manufacturing process. By controlling the slow axis alignment angle and its accuracy, it ensures that each manufactured filter meets the expected performance indicators, realizing high-quality and large-scale production from design to product.

[0075] To address the challenge of quickly and accurately designing corresponding filter structure parameters (number of stages m and length sequence) based on user-defined performance metrics (FSR and bandwidth), this application also provides an embodiment of a design method for a multi-stage Lyot filter module, see [link to relevant documentation]. Figure 5 The design method of the multi-stage Lyot filter module specifically includes the following: Step 010: Obtain target performance parameters, including target free spectral range and target 3dB bandwidth.

[0076] Step 020: Based on the target free spectral range, the preset working center wavelength, and the birefringence of the polarization-maintaining passive fiber, determine the shortest length of the polarization-maintaining passive fiber in the last Lyot filter unit.

[0077] Step 030: Based on the target 3dB bandwidth and the shortest length, perform simulation using the transfer matrix method to determine the minimum number of cascaded units required to meet the bandwidth requirements, which will be used as the target number of Lyot filter units to cascade.

[0078] Step 040: Determine the length of the polarization-maintaining passive fiber of each Lyot filter unit according to the target number of cascades and the shortest length.

[0079] Step 050: The target number of Lyot filter units and the length of the polarization-maintaining passive fiber of each Lyot filter unit are used as the design parameters of the multi-stage Lyot filter module for output.

[0080] Specifically, only when the wavelength of light satisfies the phase synchronization condition, i.e., the phase difference is 1 / 2... Constructive interference and transmission through the filter only occur when the signal is an integer multiple of the signal. The final output spectrum consists of a series of transmission bands. Using the transfer matrix method, the m-stage Lyot filter module can be described as follows: (1) Where M represents the transfer matrix; This represents the phase difference of the first Lyot filter unit; This represents the phase difference of the second Lyot filter unit; This represents the phase difference of the m-th Lyot filter unit; This represents a complex exponential function that describes the phase delay of a light wave during its propagation. The state of the arc, while keeping the amplitude of the wave constant; Let represent a complex exponential function that describes the phase delay of a light wave during its propagation. The state of the arc, while keeping the amplitude of the wave constant; Let represent a complex exponential function that describes the phase delay of a light wave during its propagation. The state of the arc, while keeping the amplitude of the wave constant.

[0081] Linearly polarized light output from input unit 1 passes through input port 4 and is connected to the first-stage Lyot filter unit via a 45-degree angle offset fusion splice. It is incident at a 45-degree angle relative to the fast or slow axis of the polarization-maintaining passive fiber in the first-stage Lyot filter unit and propagates along the new fast and slow axes of the polarization-maintaining passive fiber, respectively. Due to the birefringence of the polarization-maintaining passive fiber, the propagation speeds of the fast and slow axes differ, resulting in a phase difference accumulating during transmission. : (2) in, It is the operating wavelength. To preserve the refractive index difference between the fast and slow axes of a polarization-preserving passive optical fiber, It is the length of the polarization-maintaining passive optical fiber.

[0082] Similarly, the phase difference generated by the m-th birefringent polarization-maintaining passive fiber for: (3) in, It is the length of the m-th birefringent polarization-maintaining passive fiber.

[0083] The normalized transmittance of the m-level Lyot filter module can be expressed as: (4) If the accumulated phase difference within the polarization-maintaining passive fiber segment is If the wavelength is an integer multiple of the laser wavelength, the boundary conditions applied to the laser cavity remain unchanged. Two adjacent wavelengths satisfying this condition are... and , It is an integer. Assume... The free spectral range (FSR) of the Lyot filter: (5) in, It is the shortest length of the polarization-maintaining passive fiber segment in the m cascaded Lyot filter units, that is, the length of the polarization-maintaining passive fiber in the m-th Lyot filter unit. Therefore, in a multi-stage Lyot filter module, the longest polarization-maintaining passive fiber segment determines the filter's operating bandwidth, while the shortest segment dominates the free spectral range.

[0084] Based on the multi-stage Lyot filter module provided in the foregoing embodiments, this application also provides an embodiment of an optical system in which the multi-stage Lyot filter module functions as a wavelength selection element or a wavelength demodulation element. That is, when the optical system is a fiber laser, the multi-stage Lyot filter module functions as a wavelength selection element; when the optical system is a fiber optic sensing system, the multi-stage Lyot filter module functions as a wavelength demodulation element.

[0085] In one or more embodiments of this application, see Figure 6 "#i-1" represents the polarization-maintaining passive fiber in the (i-1)th stage Lyot filter unit, "#i" represents the polarization-maintaining passive fiber in the ith stage Lyot filter unit, "#i+1" represents the polarization-maintaining passive fiber in the (i+1)th stage Lyot filter unit, 7 represents the fast axis of the polarization-maintaining passive fiber in the (i-1)th stage Lyot filter unit, 8 represents the slow axis of the polarization-maintaining passive fiber in the (i-1)th stage Lyot filter unit, 9 represents the axis parallel to the slow axis of the polarization-maintaining passive fiber in the ith stage Lyot filter unit after 45-degree fusion splicing of the polarization-maintaining passive fiber in the (i-1)th stage Lyot filter unit and the polarization-maintaining passive fiber in the ith stage Lyot filter unit, and 10 represents the angle between the slow axes of the two polarization-maintaining passive fiber segments during 45-degree fusion splicing.Figure 6 In this context, a 45-degree angle offset weld point can be simply referred to as a 45-degree weld point.

[0086] To further illustrate the above embodiments, this application also provides a first application example of a 3-stage Lyot filter module, see [link to relevant documentation]. Figure 7 In this first application example, the multi-stage Lyot filter module includes three Lyot filter units, namely a first-stage Lyot filter unit 101, a second-stage Lyot filter unit 102, and a third-stage Lyot filter unit 103 cascaded together. It also includes an input port 104 and an output port 105. The input port 104 is connected to the first-stage Lyot filter unit 101 via a 45-degree offset fusion splice, and the output port 105 is connected to the third-stage Lyot filter unit 103 via a 45-degree offset fusion splice.

[0087] In the first application example, the first-stage Lyot filter unit 101 includes a polarization-maintaining passive fiber and a first fusion splice 204 and a second fusion splice 205 located at both ends of the polarization-maintaining passive fiber. Both the first fusion splice 204 and the second fusion splice 205 are 45-degree angle offset fusion splices. Figure 7 In the diagram, "#201" represents the polarization-maintaining passive fiber in the first-stage Lyot filter unit 101; "#202" represents the polarization-maintaining passive fiber in the second-stage Lyot filter unit 102; and "#203" represents the polarization-maintaining passive fiber in the third-stage Lyot filter unit 103.

[0088] In the first application example, the length of the polarization-maintaining passive fiber in the first-stage Lyot filter unit 101 is L, the length of the polarization-maintaining passive fiber in the second-stage Lyot filter unit 102 is L / 2, and the length of the polarization-maintaining passive fiber in the third-stage Lyot filter unit 103 is L / 4.

[0089] In the first application example, panda-type polarization-maintaining fiber is used as the polarization-maintaining passive fiber, and the angle deviation of the 45-degree offset fusion splice is controlled within ±1°. The temperature control module 206 uses a semiconductor cooling chip with a temperature control range of 25±0.1℃.

[0090] In the first application example, the working wavelength is set to λ=1550nm, and the refractive index difference between the fast and slow axes of the polarization-maintaining passive fiber is Δn=1e-3.

[0091] The transmittance curves of the single-stage Lyot filter module and the three-stage Lyot filter module provided in this application example are compared in Figures 8(a) and 8(b). The horizontal axis represents wavelength / nm, and the vertical axis represents single-stage / total transmittance. T1 (black line) represents the transmittance curve of the first-stage Lyot filter; T2 (green line) represents the transmittance curve of the second-stage Lyot filter; T3 (blue line) represents the transmittance curve of the third-stage Lyot filter; and T (red line) represents the total transmittance curve of the three-stage Lyot filter.

[0092] As can be seen, compared to a single-stage Lyot filter, the three-stage Lyot filter provided in this application example has the characteristics of a narrower main lobe, significantly reduced side lobes, a sharper transmittance curve, and stronger selectivity, resulting in a significant improvement in filtering performance. This is mainly due to the cumulative phase difference enhancement effect brought about by the multi-stage cascaded structure, which effectively suppresses the transmission of non-target wavelengths. Furthermore, as the length L of the polarization-maintaining passive fiber in the first-stage Lyot filter 101 increases from 2m to 4m, correspondingly, L... min Increasing the fiber length from 0.5m to 1m reduces the FSR of the 3-stage Lyot filter from 4.79nm to 2.4nm, and the 3dB bandwidth from 0.39nm to 0.19nm. The main lobe transmittance becomes more concentrated, sidelobe suppression is stronger, and wavelength selectivity is significantly improved. This demonstrates that increasing the fiber length can further enhance the filtering accuracy and out-of-band suppression of the Lyot filter, which is beneficial for achieving more precise wavelength selection and signal separation in practical applications.

[0093] To further illustrate the above embodiments, this application also provides a second application example of a 4-stage Lyot filter module, see [link to relevant documentation]. Figure 9 In this second application example, the multi-stage Lyot filter module includes four Lyot filter units: a first-stage Lyot filter unit 301, a second-stage Lyot filter unit 302, a third-stage Lyot filter unit 303, and a fourth-stage Lyot filter unit 304 cascaded together. It also includes an input port 305 and an output port 306. The input port 305 is connected to the first-stage Lyot filter unit 301 via a 45-degree offset fusion splice, and the output port 306 is connected to the fourth-stage Lyot filter unit 304 via a 45-degree offset fusion splice. Figure 9 In the diagram, "#401" represents the polarization-maintaining passive fiber in the first-stage Lyot filter unit 301; "#402" represents the polarization-maintaining passive fiber in the second-stage Lyot filter unit 302; "#403" represents the polarization-maintaining passive fiber in the third-stage Lyot filter unit 303; and "#404" represents the polarization-maintaining passive fiber in the fourth-stage Lyot filter unit 304.

[0094] In the second application example, the first-stage Lyot filter unit 301 includes a polarization-maintaining passive optical fiber, and a first fusion splice 405 and a second fusion splice 406 located at both ends of the polarization-maintaining passive optical fiber. Both the first fusion splice 405 and the second fusion splice 406 are 45-degree angle offset fusion splices.

[0095] In the second application example, the length of the polarization-maintaining passive fiber in the first-stage Lyot filter unit 301 is L, the length of the polarization-maintaining passive fiber in the second-stage Lyot filter unit 302 is L / 2, the length of the polarization-maintaining passive fiber in the third-stage Lyot filter unit 303 is L / 4, and the length of the polarization-maintaining passive fiber in the fourth-stage Lyot filter unit 304 is L / 8.

[0096] In this application example, panda-type polarization-maintaining fiber is used as the polarization-maintaining passive fiber, and the angle deviation of the 45-degree offset fusion splice is controlled within ±1°. The temperature control module 407 uses a semiconductor cooling chip with a temperature control range of 25±0.1℃.

[0097] In this application example, the working wavelength is set to λ=1550nm, and the refractive index difference between the fast and slow axes of the polarization-maintaining passive fiber is Δn=1e-3.

[0098] The transmittance curves of the single-stage Lyot filter module and the four-stage Lyot filter module provided in this application example are compared in Figures 10(a) and 10(b). The horizontal axis represents wavelength / nm, and the vertical axis represents single-stage / total transmittance. T1 (black line) represents the transmittance curve of the first-stage Lyot filter; T2 (green line) represents the transmittance curve of the second-stage Lyot filter; T3 (blue line) represents the transmittance curve of the third-stage Lyot filter; T4 (purple line) represents the transmittance curve of the fourth-stage Lyot filter; and T (red line) represents the total transmittance curve of the four-stage Lyot filter.

[0099] As can be seen, compared to a 3-stage Lyot filter, the 4-stage Lyot filter provided in this application example has a wider FSR, a steeper filter edge, higher wavelength selection accuracy, and better out-of-band suppression capability. When the polarization-maintaining passive fiber length L = 2m in the first-stage Lyot filter unit, comparative analysis shows that the FSR of the 4-stage Lyot filter is 9.64nm, significantly wider than the 3-stage structure. This phenomenon is mainly because as the number of cascaded stages increases from 3 to 4, the minimum segment length (L) of the polarization-maintaining passive fiber segment in the Lyot filter unit decreases. minThe further reduction in the number of cascaded elements leads to a more concentrated cumulative phase difference change between stages, resulting in a larger frequency interval (FSR) between adjacent transmission peaks. The increased number of cascaded elements makes the periodic transmittance characteristics of the filter more compact, with a wider transmission peak interval, thereby expanding the free spectral range and improving the filter's resolution over a wide wavelength range. Furthermore, as the length L of the polarization-maintaining passive fiber in the first-stage Lyot filter unit increases from 2m to 4m, the corresponding L... min Increasing the fiber length L from 0.25m to 0.5m, the FSR of the 4-stage Lyot filter decreased from 9.64nm to 4.82nm, and the 3dB bandwidth decreased from 0.39nm to 0.19nm. This shows that as the polarization-maintaining passive fiber length L of the first-stage Lyot filter unit increases with the minimum segment length L... min With the increase in wavelength, the spacing between adjacent transmission peaks in the 4th-order Lyot filter decreases, but the main lobe transmittance becomes more concentrated, and the side lobe suppression is stronger, thus achieving a narrower 3dB bandwidth. This indicates that the filter's wavelength selectivity and filtering steepness are significantly improved, making it suitable for applications requiring high-precision wavelength selection.

[0100] In summary, the multi-stage Lyot filter module (also referred to as a multi-stage Lyot filter) provided in this application has the following beneficial effects: 1. Compared with single-stage structures, multi-stage Lyot filters have narrower filtering bandwidth and steeper sideband roll-off characteristics, making them suitable for high-precision spectral analysis.

[0101] 2. Filtering parameters can be flexibly designed by adjusting the length of the polarization-maintaining passive fiber to adapt to different wavelengths of fiber lasers. It has strong scalability. The longer polarization-maintaining passive fiber determines the passband bandwidth, while the shorter segment dominates the free spectrum range.

[0102] 3. By adjusting the number of cascaded Lyot filter units, different filter bandwidths and sidelobe suppression ratios can be achieved.

[0103] 4. No need for mechanical adjustment of spatial optical components and polarization controllers, reducing energy loss caused by polarization mismatch, realizing all-fiber design, and having advantages such as compact structure, high integration and good vibration resistance.

[0104] Those skilled in the art will understand that the exemplary components, systems, and methods described in conjunction with the embodiments disclosed herein can be implemented in hardware, software, or a combination of both. Whether implemented in hardware or software depends on the specific application and design constraints of the technical solution. Those skilled in the art can use different methods to implement the described functions for each specific application, but such implementation should not be considered beyond the scope of this application. When implemented in hardware, it can be, for example, electronic circuits, application-specific integrated circuits (ASICs), appropriate firmware, plug-ins, function cards, etc. When implemented in software, the elements of this application are programs or code segments used to perform the required tasks. The programs or code segments can be stored on a machine-readable medium or transmitted over a transmission medium or communication link via data signals carried on a carrier wave.

[0105] It should be clarified that this application is not limited to the specific configurations and processes described above and shown in the figures. For the sake of brevity, detailed descriptions of known methods are omitted here. In the above embodiments, several specific steps are described and shown as examples. However, the method process of this application is not limited to the specific steps described and shown. Those skilled in the art can make various changes, modifications, and additions, or change the order of steps, after understanding the spirit of this application.

[0106] In this application, features described and / or illustrated for one embodiment may be used in the same or similar manner in one or more other embodiments, and / or combined with or in place of features of other embodiments.

[0107] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Various modifications and variations can be made to the embodiments of this application by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A multi-stage Lyot filter module characterized by, The application relates to a polarization maintaining fiber Lyot filter, which comprises an input unit, a plurality of cascaded Lyot filter units and an output unit, wherein the input unit, the output unit and the plurality of cascaded Lyot filter units are all made of polarization maintaining passive optical fibers. The lengths of the polarization maintaining passive optical fibers used by the respective Lyot filter units are sequentially decreased, and the two ends of the polarization maintaining passive optical fiber in each Lyot filter unit are respectively provided with a 45-degree angle offset splicing point. The input unit is used for outputting linearly polarized light and is optically connected between a 45-degree angle offset splicing point and the first Lyot filter unit in the plurality of cascaded Lyot filter units, so that the polarization direction of the linearly polarized light forms a 45-degree angle with the fast axis or the slow axis of the polarization maintaining passive optical fiber in the first Lyot filter unit. The output unit is used for outputting filtered linearly polarized light and is optically connected between a 45-degree angle offset splicing point and the last Lyot filter unit in the plurality of cascaded Lyot filter units. In the plurality of cascaded Lyot filter units, the length of the polarization maintaining passive optical fiber used by any one of the Lyot filter units is twice the length of the polarization maintaining passive optical fiber used by the next one of the Lyot filter units.

2. The multi-stage Lyot filter module of claim 1, wherein, The input unit uses a polarization maintaining passive optical fiber tail fiber.

3. The multi-stage Lyot filter module of claim 1, wherein, The polarization maintaining passive optical fiber tail fiber in the input unit is optically connected between the input port of the first Lyot filter unit and the input unit, and the input port is connected between the first Lyot filter unit through a 45-degree angle offset splicing point. The output unit uses a polarization maintaining passive optical fiber tail fiber.

4. The multi-stage Lyot filter module of claim 1, wherein, The polarization maintaining passive optical fiber tail fiber in the output unit is optically connected between the output port of the last Lyot filter unit and the output unit, and the output port is connected between the last Lyot filter unit through a 45-degree angle offset splicing point. The polarization maintaining passive optical fiber comprises a panda polarization maintaining optical fiber, a bow-tie polarization maintaining optical fiber or an elliptical cladding polarization maintaining optical fiber.

5. The multi-stage Lyot filter module of claim 1, wherein, The deviation of the offset angle of the 45-degree angle offset splicing point is within the interval of [-0.5°, +0.5°].

6. The multi-stage Lyot filter module of claim 1, wherein, The application further comprises a temperature control module.

7. The multi-stage Lyot filter module of claim 1, wherein, The temperature control module is thermally coupled with the Lyot filter units and is used for controlling the working temperature of the Lyot filter units so that the deviation between the working temperature of the Lyot filter units and a preset temperature threshold is within the interval of [-0.5℃, +0.5℃]. The application comprises the following steps: According to the target number of cascaded Lyot filter units and the lengths of the polarization maintaining passive optical fibers of the respective Lyot filter units, the polarization maintaining passive optical fibers used by the respective Lyot filter units are selected.

8. A method of manufacturing a multi-stage Lyot filter module as claimed in any one of claims 1 to 7, characterized in that, The polarization maintaining passive optical fibers used by the respective Lyot filter units are sequentially cascaded and spliced, and in the splicing process, the slow axes of the two adjacent polarization maintaining passive optical fibers form a target angle, and the deviation between the target angle and the 45-degree angle is within the interval of [-0.5°, +0.5°]. The polarization maintaining passive optical fiber of the input unit is spliced with the polarization maintaining passive optical fiber in the first Lyot filter unit in the plurality of cascaded Lyot filter units through a 45-degree angle offset splicing point. ​ ​ And the output unit is fused with the polarization maintaining passive optical fiber in the last Lyot filter unit of the plurality of cascaded Lyot filter units through a 45-degree angle offset fusion point.

9. A method of designing a multi-stage Lyot filter module as claimed in any one of claims 1 to 7, characterized in that, The application relates to a multi-stage Lyot filter module. The target performance parameters include a target free spectral range and a target 3dB bandwidth. The target free spectral range, a preset working center wavelength and the birefringence of the polarization maintaining passive optical fiber are used to determine the shortest length of the polarization maintaining passive optical fiber in the last Lyot filter unit. The target 3dB bandwidth and the shortest length are used to determine the minimum number of cascaded Lyot filter units required to meet the bandwidth requirement through simulation by using the transfer matrix method, and the minimum number of cascaded Lyot filter units is used as the target number of cascaded Lyot filter units. The target number of cascaded Lyot filter units and the lengths of the polarization maintaining passive optical fibers in the respective Lyot filter units are used as the design parameters of the multi-stage Lyot filter module. The application relates to a multi-stage Lyot filter module.

10. An optical system characterized by comprising: The multi-stage Lyot filter module is used as a wavelength selection element or a wavelength demodulation element in the optical system. ​ ​

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