An optical fiber filter and an optical fiber amplifier

Abstract

The application provides an optical fiber filter and an optical fiber amplifier, which can realize flat amplification of a few-mode optical signal, and can realize controllable adjustment of optical power of each optical signal mode in a plurality of optical signal modes transmitted by a fiber core of the optical fiber filter. The optical fiber filter comprises a fiber core, an inner cladding and an outer cladding, the refractive index of the fiber core, the refractive index of the inner cladding and the refractive index of the outer cladding are sequentially decreased, the fiber core is used for transmitting at least two different first optical signal modes, the inner cladding is used for transmitting at least two different second optical signal modes, and the fiber core is etched with at least one fiber grating; at least part of optical power of a target first optical signal mode is only coupled with a target second optical signal mode at the fiber grating, wherein the target first optical signal mode is one of the at least two first optical signal modes, and the target second optical signal mode is one of the at least two second optical signal modes.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of optical fiber communication, and in particular to an optical fiber filter and an optical fiber amplifier. BACKGROUND

[0002] The optical fiber filter is used for adjusting the optical power of an optical signal to achieve gain equalization. Figure 1 As shown in Figure 1 The structure of an optical fiber 100 is shown. The optical fiber filter is formed by cascading multiple optical fibers 100.

[0003] The optical fiber 100 includes a core 101 and a cladding 102. The optical power of a specific wavelength of optical signal transmitted by the core 101 is coupled to the cladding 102 based on phase matching to achieve adjustment of the optical power of the specific wavelength of optical signal. The optical fiber filter includes multiple optical fibers 100 cascaded, and different optical fibers 100 are used to adjust the optical power of optical signals of different wavelengths to achieve gain equalization between optical signals of different wavelengths.

[0004] However, if the core 101 transmits two or more optical signal modes, the same wavelength of the same optical signal mode will be coupled to multiple optical signal modes supported by the cladding 102 in the optical fiber 100 based on phase matching, so that the optical power of the optical signal is coupled to the multiple optical signal modes supported by the cladding 102, resulting in uncontrollable adjustment of the optical power of the two or more optical signal modes. It can be seen that the optical fiber filter shown in the prior art cannot achieve gain equalization of two or more optical signal modes. SUMMARY

[0005] The present application provides an optical fiber filter and an optical fiber amplifier, which can solve the problem that the existing optical fiber filter cannot achieve gain equalization of two or more optical signal modes.

[0006] The first aspect of the embodiment of the present application provides an optical fiber filter, which includes a core, an inner cladding, and an outer cladding, the inner cladding is wrapped around the outer circumferential surface of the core, the outer cladding is wrapped around the outer circumferential surface of the inner cladding, the refractive index of the core, the refractive index of the inner cladding, and the refractive index of the outer cladding decrease in turn, the core is used for transmitting at least two different first optical signal modes, the inner cladding is used for transmitting at least two different second optical signal modes, and the core is etched with at least one optical fiber grating; at least part of the optical power of a target first optical signal mode is only coupled with a target second optical signal mode at the optical fiber grating, wherein the target first optical signal mode is one of the at least two first optical signal modes, and the target second optical signal mode is one of the at least two second optical signal modes.

[0007] Using the fiber optic filter described in this aspect, at least a portion of the optical power of a first optical signal mode can only be coupled to one second optical signal mode, preventing the optical power of the same first optical signal mode from being coupled to multiple second optical signal modes. This effectively ensures the controllability of the optical power adjustment of the first optical signal mode and effectively achieves gain equalization between different first optical signal modes. Moreover, this fiber optic filter is based on fiber Bragg gratings for optical power coupling, which have advantages such as simple structure, low insertion loss, small size, and low cost.

[0008] Based on the first aspect of the embodiments of this application, in an optional implementation, the first optical signal mode of the target is coupled with the second optical signal mode of the target, satisfying the following formula:

[0009]

[0010] Among them, the The effective refractive index of one of the first optical signals included in the target first optical signal mode; The effective refractive index of one of the second optical signals included in the target second optical signal mode; λ is the wavelength of the first optical signal and the second optical signal; and Λ is the grating period of the fiber optic grating.

[0011] Based on the first aspect of the embodiments of this application, in an optional implementation, the first optical signal mode of the target is coupled with the second optical signal mode of the target, satisfying the following formula:

[0012]

[0013] Among them, the The effective refractive index of one of the first optical signals included in the target first optical signal mode; The effective refractive index of one of the second optical signals included in the target second optical signal mode; λ is the wavelength of the first optical signal and the second optical signal; and Λ is the grating period of the fiber optic grating.

[0014] As can be seen, based on the formula shown above, the effective refractive index of the first optical signal at wavelength λ was established. The effective refractive index of the second optical signal And the corresponding relationship between the fiber Bragg grating's grating period Λ. Based on this correspondence, the fiber Bragg grating is enabled to couple at least a portion of the optical power of the first optical signal with the wavelength λ to the second optical signal, thereby enabling the fiber filter to achieve gain equalization not only between different first optical signal modes, but also between different first optical signals. This allows for precise adjustment of the optical power of the target first optical signal mode, improving the gain equalization effect.

[0015] In an optional implementation manner of the first aspect of the present application, the core is etched with a plurality of fiber gratings, and grating periods of the fiber gratings are different from each other.

[0016] The fiber filter shown in the present aspect can effectively ensure that different fiber gratings can couple optical power of different first optical signal modes to the inner cladding, so as to realize independent adjustment of the optical power of the different first optical signal modes, avoid mutual interference, and effectively improve the accuracy of the adjustment of the optical power of the first optical signal mode.

[0017] In an optional implementation manner of the first aspect of the present application, each of the at least two first optical signal modes corresponds to a different second optical signal mode.

[0018] In an optional implementation manner of the first aspect of the present application, each of the at least two first optical signal modes corresponds to a second optical signal mode. That is, it is also possible that a plurality of different first optical signal modes correspond to the same second optical signal mode; even, it is also possible that all the first optical signal modes correspond to the same second optical signal mode.

[0019] In an optional implementation manner of the first aspect of the present application, in the at least two first optical signal modes, except for the first optical signal mode corresponding to the lowest optical power, each of the remaining first optical signal modes corresponds to a second optical signal mode.

[0020] The fiber filter shown in the present aspect can effectively ensure that the optical power of one first optical signal mode can be coupled to only one second optical signal mode, and the same first optical signal mode cannot be coupled to multiple second optical signal modes, so as to effectively ensure controllable adjustment of the optical power of the first optical signal mode, and effectively realize gain equalization between different first optical signal modes.

[0021] In an optional implementation manner of the first aspect of the present application, a length of the fiber grating along an axial direction of the core and a size of the optical power coupled to the target second optical signal mode are in a positive correlation relationship.

[0022] In order to realize accurate adjustment of the optical power of the target first optical signal mode to realize gain equalization, the size of the optical power coupled to the target second optical signal mode needs to be adjusted. The present aspect can adjust the size of the optical power coupled to the target second optical signal mode by adjusting the length of the fiber grating along the axial direction of the core.

[0023] In an optional implementation of the first aspect of the present application, the optical power of the target first optical signal mode coupled to the outer cladding is less than the optical power of the target first optical signal mode coupled to the target second optical signal mode.

[0024] To achieve the accurate adjustment of the optical power of the target first optical signal mode, the accurate adjustment of the optical power of the target first optical signal mode is achieved by the outer cladding. Specifically, the optical power of the target first optical signal mode is coupled only to the target second optical signal mode by controlling the optical power of the target first optical signal mode coupled to the outer cladding to be less than the optical power of the target first optical signal mode coupled to the target second optical signal mode, so as to avoid the coupling of the optical power of the target first optical signal mode to the outer cladding as much as possible.

[0025] In an optional implementation of the first aspect of the present application, the difference between the refractive index of the inner cladding and the refractive index of the outer cladding is positively correlated with the number of types of the second optical signal mode.

[0026] To ensure the controllable adjustment of the optical power of the target first optical signal mode, the optical fiber grating couples at least part of the optical power of the target first optical signal mode only to the target second optical signal mode. It can be seen that the number of types of the second optical signal mode supported by the inner cladding needs to be limited. If the number of types of the second optical signal mode supported by the inner cladding is infinite, the target first optical signal mode will be coupled to multiple second optical signal modes, resulting in the loss of control of the adjustment of the optical power of the target first optical signal mode. The number of types of the second optical signal mode supported by the inner cladding can be adjusted by adjusting the difference between the refractive index of the inner cladding and the refractive index of the outer cladding, so as to achieve the controllable adjustment of the optical power of the first optical signal mode.

[0027] In an optional implementation of the first aspect of the present application, the radius of the inner cladding is positively correlated with the number of types of the second optical signal mode.

[0028] The number of types of the second optical signal mode supported by the inner cladding can be adjusted by adjusting the radius of the inner cladding, so as to achieve the controllable adjustment of the optical power of the first optical signal mode.

[0029] The second aspect of the embodiment of the present application provides a fiber amplifier, which comprises a pump laser, a wavelength division multiplexer, a gain fiber and a fiber filter, the wavelength division multiplexer is coupled with the pump laser and the gain fiber respectively, the gain fiber is coupled with the fiber filter; the wavelength division multiplexer is used for multiplexing pump light from the pump laser and an optical signal; the gain fiber is used for gain amplifying the optical signal from the wavelength division multiplexer to output at least two first optical signal modes, the fiber filter is as shown in the first aspect, and details are not repeated.

[0030] In the scenarios such as long-distance transmission, super-capacity mode division transmission, super-capacity wavelength division transmission and the like, the fiber amplifier shown in the aspect is used for gain equalization of at least two different optical signal modes after gain amplification by the gain fiber, the fiber amplifier effectively ensures gain equalization between different optical signal modes, and effectively reduces the probability of optical signal transmission interruption. BRIEF DESCRIPTION OF DRAWINGS

[0031] Figure 1 A structure diagram of a section of fiber included in the fiber filter shown in the prior art;

[0032] Figure 2 An embodiment structure example diagram of the fiber amplifier provided by the present application;

[0033] Figure 3 A first structure example diagram of the fiber filter provided by the present application;

[0034] Figure 4 A first structure example diagram of the fiber core provided by the present application;

[0035] Figure 5 A second structure example diagram of the fiber filter provided by the present application;

[0036] Figure 6 A third structure example diagram of the fiber filter provided by the present application;

[0037] Figure 7 A first example diagram of the transmission spectrum provided by the present application;

[0038] Figure 8 A second example diagram of the transmission spectrum provided by the present application;

[0039] Figure 9 A third example diagram of the transmission spectrum provided by the present application;

[0040] Figure 10 A fourth example diagram of the transmission spectrum provided by the present application;

[0041] Figure 11This is a first example of a spectrum provided in this application;

[0042] Figure 12 This is a second example of a spectrum provided in this application;

[0043] Figure 13 This is an example diagram of the fourth structure of the fiber optic filter provided in this application;

[0044] Figure 14 This is a third example of the spectrum provided in this application;

[0045] Figure 15 This is the fourth example of a spectrum provided in this application. Detailed Implementation

[0046] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0047] The following first combines Figure 2 The specific structure of the fiber optic amplifier provided in this application is illustrated below:

[0048] The fiber amplifier shown in this embodiment can support gain amplification for two or more optical signal modes. For example, this fiber amplifier is a few-mode erbium-doped fiber amplifier (FM-EDFA). Different optical signal modes have different intensity distributions.

[0049] The fiber amplifier 200 includes an isolator 201, a pump laser 203, a wavelength division multiplexer 202, a gain fiber 204, and a fiber filter 205. The isolator 201, the wavelength division multiplexer 202, the gain fiber 204, and the fiber filter 205 are coupled sequentially, and the wavelength division multiplexer 202 is also coupled to the pump laser 203.

[0050] The isolator 201 is used to suppress reflected light from entering the optical fiber 206 and to transmit the optical signal from the optical fiber 206 to the wavelength division multiplexer 202. The wavelength division multiplexer 202 is used to multiplex the optical signal from the isolator 201 and the pump light from the pump laser 203. The gain fiber 204 is used to amplify the gain of the optical signal from the wavelength division multiplexer 202 to output at least two different optical signal modes.

[0051] In the scenarios such as long-distance transmission, super-capacity mode division transmission, super-capacity wave division transmission and the like, the gain fiber 204 has different amplification gains for different optical signal modes, and it is inevitable that errors are generated in the process of optical signal mode transmission. In the embodiment, the fiber filter 205 is used to perform gain equalization on at least two different optical signal modes from the gain fiber 204, which effectively ensures the gain equalization between different optical signal modes and effectively reduces the probability of optical signal transmission interruption. In the embodiment, the fiber filter 205 is taken as a gain flattening filter (GFF) for example.

[0052] The specific structure of the fiber filter will be described below in combination with Figure 3 the radial direction of the fiber filter, and the cross-sectional view of the fiber filter. Figure 3 The radial direction of the fiber filter, and the cross-sectional view of the fiber filter.

[0053] The fiber filter includes a core 310, an inner cladding 320 and an outer cladding 330 arranged in sequence from inside to outside. Specifically, the inner cladding 320 is wrapped around the outer peripheral surface of the core 310, and the outer cladding 330 is wrapped around the outer peripheral surface of the inner cladding 320. In the radial direction of the fiber filter, the core 310, the inner cladding 320 and the outer cladding 330 are all circular in the embodiment, and in other examples, one or more of the core 310, the inner cladding 320 and the outer cladding 330 can also be square, oval, irregular shape, etc., which is not limited in the embodiment.

[0054] Specifically, the refractive index of the core 310 is n core , the refractive index of the inner cladding 320 is n inner_clad , and the refractive index of the outer cladding 330 is n outer_clad . Wherein, n core >n inner_clad >n outer_clad . It can be seen that the refractive indexes of the core 310, the inner cladding 320 and the outer cladding 330 decrease in turn. Taking the core 310 as an example, the refractive index n core Specifically, it refers to the ratio of the propagation speed of the optical signal in the vacuum to the propagation speed of the optical signal in the core 310.

[0055] The radius of the core 310 is r core , the radius of the inner cladding 320 is r inner_clad , and the radius of the outer cladding 330 is r outer_clad . Wherein, r outer_clad >r inner_clad >r core . It can be seen that the radii of the core 310, the inner cladding 320 and the outer cladding 330 increase in turn.

[0056] The fiber core 310 shown in this embodiment is used to transmit at least two different first optical signal modes, and the inner cladding 320 is used to transmit at least two different second optical signal modes. For a detailed description of the first and second optical signal modes, please refer to [link to relevant documentation]. Figure 2 The description of the optical signal modes shown is omitted.

[0057] The following combination Figure 4 As shown, the specific structure of the fiber core 310 is explained, in which... Figure 4 This is a cross-sectional view of the fiber optic filter along its axial direction.

[0058] Along the axial direction of the fiber filter, a fiber grating is etched onto the fiber core using a laser. This embodiment uses a few-mode fiber (FMF) as an example for illustration. In other examples, the fiber core can also be a multimode fiber (MMF). It should be noted that this embodiment does not limit the method of forming the fiber grating, as long as it can be formed on the fiber core. Figure 4 The fiber grating shown is sufficient.

[0059] Specifically, the fiber core 310 includes N fiber gratings, namely G1, G2, ... G N In this embodiment, the specific value of N is not limited, as long as N is a positive integer greater than or equal to 1. The grating periods of each fiber grating are different. The length of fiber grating G1 along the axial direction of the fiber core 310 is L1, and so on. N The length along the axial direction of the fiber core 310 is L N .

[0060] The following uses fiber Bragg grating G1 as an example to illustrate the specific structure of the fiber Bragg grating:

[0061] The fiber Bragg grating G1 comprises multiple sub-gratings. This embodiment does not limit the specific number of sub-gratings included in each fiber Bragg grating. Figure 4 For example, fiber optic grating G1 specifically includes sub-grating G 1-1 G 1-2 and G 1-3 .

[0062] Different sub-gratings can have different grating periods; for example, sub-grating G 1-1 The grating period is Λ 1_1 Subgrating G 1-2 The grating period is Λ 1_2 Subgrating G 1-3 The grating period is Λ1_3 And the grating period Λ 1_1 , grating period Λ 1-2 and grating period Λ 1_3 They are not the same.

[0063] It should be noted that this embodiment uses the example of different sub-gratings having different grating periods in a fiber optic grating for illustrative purposes. In other examples, different sub-gratings may also have the same grating period, and there is no specific limitation.

[0064] The following uses sub-grating G 1-1 For example, let's explain the grating period of a sub-grating:

[0065] Specifically, sub-grating G 1-1 With M grating periods Λ 1_1 And sub-grating G 1-1 It has M grating periods Λ 1_1 All are equal. Each grating period Λ 1_1 It includes a refractive index modulated portion 401 and a refractive index unmodulated portion 402. The refractive index modulated portion 401 refers to the region on the fiber core where the refractive index changes, etched by a laser, making this modulated portion 401 a refractive index change point. The refractive index unmodulated portion 402 refers to the region adjacent to the refractive index modulated portion 401 where the refractive index does not change. In this embodiment, the specific value of M is not limited.

[0066] The following combination Figure 5 The specific structure of the inner cladding is illustrated below. Figure 5 This is a cross-sectional view of the fiber optic filter along its axial direction.

[0067] The inner cladding 320 shown in this embodiment includes at least one sub-cladding 51. This embodiment uses multiple sub-cladding 51 segments as an example for illustrative purposes. Multiple coaxially arranged and sequentially connected sub-cladding 51 segments constitute the inner cladding 320. The number of sub-claddings shown in this embodiment is equal to the number of fiber Bragg gratings shown above. When there are multiple fiber Bragg gratings, the multiple sub-claddings respectively wrap different fiber Bragg gratings.

[0068] It needs to be clarified that, Figure 5 The illustration uses the example of sub-cladding layers with different radii used to wrap different fiber Bragg gratings as an example. For instance, the radii of sub-cladding layer 511 wrapping fiber Bragg grating G1 and sub-cladding layer 512 wrapping fiber Bragg grating G2 are different, and the radius of sub-cladding layer 511 is smaller than the radius of sub-cladding layer 512. In other examples, the radii of sub-cladding layers used to wrap different fiber Bragg gratings may also be the same.

[0069] The embodiment adjusts the number of the second optical signal modes supported by the inner cladding by adjusting the radius of the inner cladding. For example, the number of the second optical signal modes supported by the inner cladding can be reduced by reducing the radius of the inner cladding. Figure 5 For example, the radius of the sub-cladding 511 is smaller than the radius of the sub-cladding 512, and thus the number of the second optical signal modes supported by the sub-cladding 511 is smaller than the number of the second optical signal modes supported by the sub-cladding 512.

[0070] The fiber filter shown in the embodiment has the core 310 transmitting at least two different first optical signal modes. To achieve gain equalization, at least part of the optical power of the first optical signal modes needs to be coupled into the inner cladding. The following describes several optional coupling modes for achieving gain equalization.

[0071] Coupling mode 1

[0072] Each of the at least two first optical signal modes corresponds to a different second optical signal mode, so that at least part of the optical power of each first optical signal mode is coupled into the corresponding second optical signal mode, and the types of the second optical signal modes coupled by different first optical signal modes are different.

[0073] For example, the at least two first optical signal modes are LP01 and LP11, the at least two second optical signal modes are LP03 and LP12, LP01 corresponds to LP03, and LP11 corresponds to LP12. At least part of the optical power of LP01 is coupled into LP03, and at least part of the optical power of LP11 is coupled into LP12, to achieve gain equalization between LP01 and LP11.

[0074] Coupling mode 2

[0075] The at least two first optical signal modes correspond to the same second optical signal mode, so that at least part of the optical power of each first optical signal mode is coupled into the corresponding second optical signal mode, and the types of the second optical signal modes coupled by different first optical signal modes are the same.

[0076] For example, the at least two first optical signal modes are LP01 and LP11, the at least two second optical signal modes are LP03 and LP12, and LP01 and LP11 both correspond to LP03. At least part of the optical power of LP01 and at least part of the optical power of LP11 are both coupled into LP03, to achieve gain equalization between LP01 and LP11.

[0077] Coupling mode 3

[0078] In the at least two first optical signal modes, each of the rest of the first optical signal modes, except the one corresponding to the lowest optical power, corresponds to one of the second optical signal modes.

[0079] For example, the at least two first optical signal modes are LP01 and LP11, the at least two second optical signal modes are LP03 and LP12, and the optical power of LP01 is less than that of LP11. In order to achieve gain equalization, only the optical power of LP11 can be adjusted. In the case that LP11 corresponds to LP12, only at least part of the optical power of LP11 can be coupled to LP12 to achieve gain equalization between LP01 and LP11.

[0080] The process of coupling at least part of the optical power of the first optical signal mode to the inner cladding is described as follows:

[0081] If the optical power of one first optical signal mode is coupled to multiple second optical signal modes, the size of the optical power of the first optical signal mode coupled to each second optical signal mode cannot be adjusted, resulting in a loss of control over the adjustment of the optical power of the first optical signal mode and failure to achieve gain equalization between multiple first optical signal modes.

[0082] The optical fiber filter shown in the embodiment can only couple the optical power of one first optical signal mode to one second optical signal mode, and the same first optical signal mode cannot be coupled to multiple second optical signal modes, thereby effectively ensuring controllable adjustment of the optical power of the first optical signal mode and effectively achieving gain equalization between different first optical signal modes. The specific implementation process is described as follows:

[0083] In the embodiment, at least part of the optical power of the target first optical signal mode is coupled to the target second optical signal mode at the target fiber grating. The target first optical signal mode is one of the at least two first optical signal modes transmitted by the fiber core, the target second optical signal mode is only one of the at least two second optical signal modes transmitted by the inner cladding, and the target fiber grating is one of the fiber gratings included in the fiber core. It can be seen that the target fiber grating can only couple at least part of the optical power of the target first optical signal mode to the target second optical signal mode, effectively avoiding the coupling of the target first optical signal mode to multiple second optical signal modes, and further avoiding the loss of control over the adjustment of the optical power of the target first optical signal mode.

[0084] Specifically, the embodiment is exemplarily described by taking the coupling of the target first optical signal mode to the target second optical signal mode based on the phase matching condition as an example:

[0085] Phase matching condition 1

[0086] The corresponding relationship created based on the phase matching condition is shown in the following formula:

[0087]

[0088] The parameters in the formula are described as follows:

[0089] First, the effective refractive index is described: specifically, the target first optical signal mode includes one or more first optical signals with different wavelengths, and the effective refractive index is the effective refractive index of a first optical signal included in the first optical signal mode.

[0090] wherein, The β1 is a propagation constant, used to represent the phase change per unit distance of the first optical signal propagating in the core, and the wavelength λ is the wavelength of the first optical signal.

[0091] Second, the effective refractive index is described: specifically, the target second optical signal mode includes one or more second optical signals with different wavelengths, and the effective refractive index is the effective refractive index of a second optical signal included in the second optical signal mode.

[0092] wherein, The β2 is a propagation constant, used to represent the phase change per unit distance of the second optical signal propagating in the inner cladding, and the wavelength λ is the wavelength of the second optical signal.

[0093] To achieve the adjustment of the optical power of the first optical signal mode, the optical power of each first optical signal included in the first optical signal mode needs to be adjusted. Specifically, the optical power of the first optical signal can be adjusted by coupling at least part of the optical power of the first optical signal to the second optical signal. To achieve the coupling of the first optical signal to the second optical signal, the wavelength of the first optical signal and the wavelength of the second optical signal are equal, and both are equal to the wavelength λ.

[0094] Third, the grating period Λ is described: as shown in Figure 4 It can be seen that the core of the embodiment includes a plurality of fiber gratings, and different fiber gratings are used to adjust the optical power of different optical signal modes, thereby achieving independent adjustment of the optical power of different optical signal modes. A target fiber grating includes a plurality of sub-gratings, and different sub-gratings are used to adjust the optical power of optical signals with different wavelengths, thereby achieving independent adjustment of the optical power of different optical signals.

[0095] When a target fiber grating is used to adjust the optical power of a first optical signal having a wavelength λ, the grating period of the target fiber grating can be the grating period of a target sub-grating, where the target sub-grating is a segment of a sub-grating included in the target fiber grating. Continuing as... Figure 4 As shown, the fiber grating G1 includes sub-gratings G. 1-1 grating period Λ 1_1 If the correspondence shown in the above formula is satisfied, then the sub-grating G 1-1 The target sub-grating, G 1-1 Used to adjust the optical power of the first optical signal.

[0096] Phase matching condition 2

[0097]

[0098] For a detailed explanation of the parameters shown in phase matching condition 2, please refer to phase matching condition 1; further explanation will not be provided here. In this example, in When the gain equalization is less than 110% and greater than 90%, it has been verified that precise adjustment of the optical power of the first optical signal mode of the target can be achieved, improving the gain equalization effect. This embodiment addresses this... The description of the numerical range is an optional example and is not limited, as long as it can achieve precise adjustment of the optical signal of the first optical signal mode of the target.

[0099] As can be seen, based on the formula shown in phase matching condition 1 or phase matching condition 2, the effective refractive index of the first optical signal at wavelength λ is established. The effective refractive index of the second optical signal And the correspondence between the grating period Λ of the target fiber grating. Based on this correspondence, the target fiber grating is enabled to couple at least a portion of the optical power of the first optical signal with the wavelength λ to the second optical signal, thereby enabling the fiber filter to achieve gain equalization not only between different first optical signal modes, but also between different first optical signals.

[0100] As described above, to ensure controllable adjustment of the optical power of the target first optical signal mode, the target fiber grating couples at least a portion of the optical power of the target first optical signal mode only to the target second optical signal mode. Therefore, the number of types of second optical signal modes supported by the inner cladding needs to be finite. If the inner cladding supports an infinite number of second optical signal modes, the target first optical signal mode would couple to multiple second optical signal modes, leading to uncontrolled adjustment of the optical power of the target first optical signal mode. The following explains how to control the number of types of second optical signal modes supported by the inner cladding:

[0101] It should be noted that the following provides several ways to control the number of second optical signal modes supported by the inner cladding, in actual applications, one or more of the following ways can be used to control the number of second optical signal modes supported by the inner cladding.

[0102] Method 1

[0103] The number of second optical signal modes supported by the inner cladding is controlled by controlling the refractive index of the inner cladding and the refractive index of the outer cladding. The difference between the refractive index of the inner cladding and the refractive index of the outer cladding is positively correlated with the number of second optical signal modes supported by the inner cladding.

[0104] It can be seen that if the number of second optical signal modes needs to be increased, the difference between the refractive index of the inner cladding and the refractive index of the outer cladding is increased, and if the number of second optical signal modes needs to be reduced, the difference between the refractive index of the inner cladding and the refractive index of the outer cladding is reduced.

[0105] Method 2

[0106] The number of second optical signal modes supported by the inner cladding is controlled by controlling the radius of the inner cladding. The radius of the inner cladding is positively correlated with the number of second optical signal modes supported by the inner cladding.

[0107] It can be seen that if the number of second optical signal modes needs to be increased, the radius of the inner cladding is increased, and if the number of second optical signal modes needs to be reduced, the radius of the inner cladding is reduced.

[0108] In order to realize accurate adjustment of the optical power of the target first optical signal mode to realize gain equalization, the size of the optical power coupled to the target second optical signal mode needs to be adjusted. The specific adjustment method is as follows:

[0109] It should be noted that the following provides several ways to adjust the optical power, in actual applications, one or more of the following ways can be used to adjust the optical power.

[0110] Adjustment method 1

[0111] The optical power of the target first optical signal mode is adjusted by adjusting the length of the target fiber grating along the axial direction of the fiber core, wherein the length of the target fiber grating along the axial direction of the fiber core is positively correlated with the size of the optical power coupled to the target second optical signal mode.

[0112] Specifically, the adjustment process of one first optical signal of the first optical signal mode of the target is illustrated as an example: that is, the optical power of the first optical signal is adjusted by adjusting the length of the target sub-grating along the axial direction of the fiber core. For a detailed explanation of the target sub-grating, please refer to [link to relevant documentation]. Figure 4 As shown, further details are omitted. The length of the target sub-grating is positively correlated with the optical power coupled to the second optical signal mode of the target. Continuing as... Figure 4 As shown, the target sub-grating G 1-1 The length L along the axial direction of the fiber core 1-1 The larger the value, the larger the target sub-grating G. 1-1 The greater the optical power coupled to the target's second optical signal mode, the greater the length L. 1-1 The smaller the value, the better the target sub-grating G. 1-1 The smaller the optical power coupled to the second optical signal mode of the target, the better. Therefore, the target sub-grating G can be determined by adjusting the optical power of the first optical signal mode of the target as needed. 1-1 Length L 1-1 .

[0113] Adjustment method 2

[0114] Precise adjustment of the optical power of the target first optical signal mode is achieved through an outer cladding layer. Specifically, to achieve controllable adjustment of the optical power of the target first optical signal mode, the optical power of the target first optical signal mode needs to be coupled only to the target second optical signal mode, and coupling of the optical power of the target first optical signal mode to the outer cladding layer should be avoided as much as possible. This is achieved by ensuring that the first coupling coefficient is less than the second coupling coefficient.

[0115] The first coupling coefficient is the ratio of the first optical power to the second optical power, and the second coupling coefficient is the ratio of the third optical power to the second optical power.

[0116] The first optical power is the optical power of the target first optical signal mode coupled to the outer cladding layer, the second optical power is the optical power of the target first optical signal mode in the uncoupled state, and the third optical power is the optical power of the target first optical signal mode coupled to the target second optical signal mode.

[0117] The greater the first coupling coefficient is smaller than the second coupling coefficient, the smaller the optical power of the target first optical signal mode coupled to the outer cladding, thereby effectively ensuring the accuracy of the optical power adjustment of the target first optical signal mode. For example, if the second coupling coefficient is about 10 times the first coupling coefficient, it means that the first coupling coefficient is much smaller than the second coupling coefficient, and at this time the optical power of the target first optical signal mode coupled to the outer cladding is extremely weak, and at this time the coupling between the target first optical signal mode and the outer cladding can be ignored, effectively ensuring that the optical power of the target first optical signal mode is only coupled to the target inner cladding, achieving accurate adjustment of the optical power of the target first optical signal mode, and achieving gain equalization.

[0118] In order to better understand the optical fiber filter provided in the present application, the optical fiber filter will be specifically described below in combination with specific application scenarios:

[0119] Application scenario one

[0120] The present application scenario is described in combination with Figure 6 The present application scenario takes an example in which the number of types of optical signal modes transmitted by the fiber core and the number of types of second optical signal modes transmitted by the inner cladding are equal. Specifically, the fiber core transmits four first optical signal modes, namely LP01, LP11, LP21 and LP02. The second optical signal modes transmitted by the inner cladding are LP31, LP12, LP03 and LP22.

[0121] The present embodiment takes an example of realizing optical power adjustment of different optical signal modes based on the same structural parameters, wherein the structural parameters are: the radius of the fiber core is 6.6 microns (μm), the radius of the inner cladding is 12.5 μm, and the radius of the outer cladding is 62.5 μm. The structural parameters are also: the refractive index of the fiber core is 1.45952, the refractive index of the inner cladding is 1.44782, and the refractive index of the outer cladding is 1.44402.

[0122] Among them, the first optical signal mode LP01 transmitted by the fiber core is coupled to the second optical signal mode LP31 based on phase matching, the first optical signal mode LP11 transmitted by the fiber core is coupled to the second optical signal mode LP12 based on phase matching, the first optical signal mode LP21 transmitted by the fiber core is coupled to the second optical signal mode LP03 based on phase matching, and the first optical signal mode LP02 transmitted by the fiber core is coupled to the second optical signal mode LP22 based on phase matching. The specific coupling mode is described as follows:

[0123] Coupling mode 1

[0124] In combination with Figure 6 and Table 1, the coupling of the first optical signal mode LP01 transmitted by the fiber core to the second optical signal mode LP31 is specifically described as follows:

[0125] Table 1

[0126]

[0127] The first optical signal mode LP01 is coupled through the target fiber optic grating 601. The first optical signal mode LP01 includes six first optical signals, and the center wavelengths of the six first optical signals are 1530nm, 1535nm, 1540nm, 1545nm, 1555nm and 1555nm, respectively.

[0128] The target fiber grating 601 includes six grating segments, namely G 1-1 G 1-2 G 1-3 G 1-4 G 1-5 and G 1-6 Among them, the sub-grating G 1-1 The grating period Λ is 0.01209 cm, and the sub-grating G 1-1 It comprises 285 grating periods Λ, and the sub-grating G 1-1 The length along the axial direction of the fiber core is 3.45 cm. This sub-grating, based on phase matching, can couple at least a portion of the optical power of a first optical signal with a center wavelength of 1530 nm to a second optical signal mode LP31. For a detailed explanation of phase matching, please refer to the above description; it will not be repeated here. Based on Table 1, and so on, this G... 1-6 It is capable of coupling at least a portion of the optical power of a first optical signal with a center wavelength of 1555nm to a second optical signal mode LP31.

[0129] Coupling method 2

[0130] Combination Figure 6 The coupling from the first optical signal mode LP11 to the second optical signal mode LP12 transmitted in the fiber core, as shown in Table 2, will be explained in detail below:

[0131] Table 2

[0132]

[0133] The first optical signal mode LP11 is coupled through the target fiber optic grating 602. The first optical signal mode LP11 includes three first optical signals, and the center wavelengths of the three first optical signals are 1532nm, 1545nm and 1550nm, respectively.

[0134] The target fiber grating 602 includes three sub-gratings, namely G 2-1 G 2-2 and G 2-3 For G 2-1 G2-2 and G 2-3 The grating periods and the number of grating period types of G 2-1 , G 2-1 are shown in Table 2, which will not be repeated. Take G 2-3 for example, G 3-1 is used to couple at least part of the optical power of the first optical signal with a center wavelength of 1532 nm to the second optical signal mode LP12, and by analogy, G 3-2 is used to couple at least part of the optical power of the first optical signal with a center wavelength of 1545 nm to the second optical signal mode LP12.

[0135] Coupling mode 3

[0136] The coupling of the first optical signal mode LP21 transmitted by the core to the second optical signal mode LP03 is specifically described in combination with Table 3: Figure 6 Table 3

[0137]

[0138] The coupling of the first optical signal mode LP21 is realized by the target fiber grating 603, the first optical signal mode LP21 includes three first optical signals, and the center wavelengths of the three first optical signals are 1532 nm, 1545 nm and 1555 nm respectively.

[0139] The target fiber grating 603 includes three sub-gratings, namely G 3-1 , G 3-2 and G 3-3 The grating periods and the number of grating period types of G 3-1 , G 3-2 and G 3-3 are shown in Table 3, which will not be repeated. Take G 3-1 for example, G 3-1 is used to couple at least part of the optical power of the first optical signal with a center wavelength of 1532 nm to the second optical signal mode LP03, and by analogy, G 3-3 is used to couple at least part of the optical power of the first optical signal with a center wavelength of 1555 nm to the second optical signal mode LP03.

[0140] Coupling mode 4

[0141] The coupling of the first optical signal mode LP02 transmitted by the core to the second optical signal mode LP22 is specifically described in combination with Table 4:

[0142] Figure 6 Table 4

[0143]

[0144]

[0145] The coupling of the first optical signal mode LP02 is realized by the target fiber grating 604, the first optical signal mode LP02 includes three first optical signals, and the center wavelengths of the three first optical signals are 1531nm, 1550nm and 1557nm respectively.

[0146] The target fiber grating 604 includes three sub-gratings, i.e. 4-1 , G 4-2 and G 4-3 . For the grating periods and the number of grating periods of G 4-1 , G 4-2 and G 4-3 , please refer to Table 4, which will not be repeated. Taking the sub-grating G 4-1 as an example, the sub-grating G 4-1 is used to couple at least part of the optical power of the first optical signal with a center wavelength of 1531nm to the second optical signal mode LP22, and the like. The sub-grating G 4-3 is used to couple at least part of the optical power of the first optical signal mode with a center wavelength of 1557nm to the second optical signal mode LP22.

[0147] The application scenario is exemplarily illustrated by coupling the optical power of different first optical signal modes to different second optical signal modes. It should be noted that this is not limited, for example, the optical power of multiple first optical signal modes can also be coupled into the same second optical signal mode, as long as the optical power of any first optical signal in any first optical signal mode is only coupled to one second optical signal mode. It can be seen that one second optical signal mode can receive the optical power of multiple first optical signal modes, and different second optical signal modes cannot receive the optical power from the same first optical signal mode.

[0148] It can be seen that the fiber core including the cascaded target fiber gratings 601, 602, 603 and 604 can realize gain equalization of the entire C band (1530-1565nm). The fiber core shown in the application scenario is exemplarily illustrated by realizing gain equalization of the entire C band. In other examples, the fiber core can also realize gain equalization of other bands, for example, gain equalization of the L band.

[0149] The same wavelength can also be equalized in different optical signal modes by using the application scenario. Please refer to Table 5 as follows:

[0150] Table 5

[0151]

[0152] For example, as shown in Table 5, the first optical signal modes LP01, LP11, LP21 and LP02 each include one optical signal with a center wavelength of 1545 nm, and the optical power of the optical signal with the center wavelength of 1545 nm is adjusted by the fiber grating G1, the fiber grating G2, the fiber grating G3 and the fiber grating G4 respectively to achieve gain equalization of the optical signal with the center wavelength of 1545 nm in the first optical signal modes LP01, LP11, LP21 and LP02.

[0153] The target fiber grating shown in the application scenario can adjust the optical power of one first optical signal mode, and different target fiber gratings can adjust different first optical signal modes. It can be seen that different target fiber gratings can adjust the optical power of different first optical signal modes based on different phase matching. That is, each target fiber grating can independently adjust the optical power of each first optical signal mode, effectively avoiding mutual interference. Among them, avoiding mutual interference means that in the process of transmitting the first optical signal modes LP01, LP11, LP21 and LP02, the target fiber grating will not interfere with the optical power of other first optical signal modes transmitted by the target fiber grating during the coupling process of one first optical signal mode. For better explanation of the fiber filter shown in the application scenario which can effectively avoid mutual interference, please see the simulation example as follows:

[0154] Simulation Example 1

[0155] This simulation example is based on the coupling mode 1 shown above, that is, the first optical signal mode LP01 transmitted by the fiber core is coupled to the second optical signal mode LP31. Specifically, in this simulation example, different values of the number N of grating periods possessed by the target fiber grating 601 are simulated. For the explanation of the number N of grating periods, please see the coupling mode 1, which will not be repeated here. The simulation example does not limit the sub-grating including the number N of grating periods. For example, the sub-grating including the number N of grating periods can be one or more of G 1-1 , G 1-2 , G 1-3 , G 1-4 , G 1-5 and G 1-6 .

[0156] Optionally, the number N of grating periods takes values in the interval 40 to 400 in this simulation example, for example, the number N of grating periods takes values in the interval in a periodic manner with a period of 40, that is, the number N of grating periods takes values of 40, 80, 120, 160, 200, 240, 280, 320, 360 and 400.

[0157] The first optical signal mode LP01 is coupled to the target fiber grating 601 with the number N of grating periods to obtain a transmission spectrum 701 as shown in the figure, the abscissa is wavelength, unit is nanometer (nm), the ordinate is relative power, unit is decibel (dB), which is the size of the optical power of the first optical signal mode LP01 coupled into the second optical signal mode LP31 and the size of the optical power of the first optical signal mode LP01 which has not been coupled. Figure 7

[0158] According to the transmission spectrum 701, when the number N of grating periods has different values, the first optical signal mode LP01 has different attenuation curves under the coupling effect of the target fiber grating. For example, when the number N of grating periods has the above-mentioned 10 values, ten attenuation curves as shown in the transmission spectrum 701 can be obtained. It can be seen that the target fiber grating 601 with different number N of grating periods can couple the optical power of the first optical signal mode LP01.

[0159] In the case that the target fiber grating 601 transmits the first optical signal modes LP11, LP21 and LP02, the transmission spectra of the first optical signal modes LP11, LP21 and LP02 are respectively shown in the transmission spectra 702, 703 and 704 as shown in the figure. It can be seen that the attenuation curves of the first optical signal modes LP11, LP21 and LP02 hardly change with the different values of the number N of grating periods of the target fiber grating 601. That is, when the number N of grating periods has the above-mentioned 10 values, ten almost coincident attenuation curves as shown in the transmission spectra 702, 703 and 704 can be obtained. Figure 7

[0160] The maximum relative power of the first optical signal modes LP11, LP21 and LP02 is respectively 0.027, 0.25 and 0.35. It can be seen that when the target fiber grating 601 couples the first optical signal mode LP01, it hardly causes interference to the optical power of the first optical signal modes LP11, LP21 and LP02.

[0161] Simulation Example 2

[0162] This simulation example is combined with the coupling mode 2 shown in the above-mentioned figure, that is, the first optical signal mode LP11 transmitted by the fiber core is coupled to the second optical signal mode LP12. Specifically, in this simulation example, different values of the number N of grating periods of the target fiber grating 602 are simulated, and the number N of grating periods is described in the above-mentioned simulation example 1, which is not repeated here.

[0163] ​​Couple the first optical signal mode LP11 to the target fiber grating 602 having the number N of the aforementioned grating periods to obtain, as shown above. Figure 8 The transmission spectrum 802 shown above, and the coordinates of this transmission spectrum 802, are explained in the simulation example 1 above, and will not be repeated here.

[0164] According to the transmission spectrum 802, when the number of grating periods N is different, the first optical signal mode LP11 exhibits different attenuation curves under the coupling effect of the target fiber optic grating 602. For example, when N has the aforementioned 10 values, ten attenuation curves as shown in the transmission spectrum 802 can be obtained. It is evident that the target fiber optic grating 602 with different numbers of grating periods N can couple the optical power of the first optical signal mode LP11.

[0165] When the target fiber optic grating 602 transmits the first optical signal modes LP01, LP21, and LP02, the transmission spectra of the first optical signal modes LP01, LP21, and LP02 are respectively as follows: Figure 8 The transmission spectra 801, 803, and 804 are shown. It can be seen that the attenuation curves of the first optical signal modes LP01, LP21, and LP02 hardly change with the different values ​​of the number of grating periods N of the target fiber optic grating 602. That is, when N has the above 10 values, the ten almost overlapping attenuation curves obtained are shown in the transmission spectra 801, 803, and 804. The maximum relative powers of the first optical signal modes LP01, LP21, and LP02 are 0.0028, 0.06, and 0.07, respectively. It is evident that when the target fiber optic grating 602 couples to the first optical signal mode LP11, it does not cause interference to the first optical signal modes LP01, LP21, and LP02.

[0166] Simulation Example 3

[0167] This simulation example combines the coupling method 3 shown above, where the first optical signal mode LP21 transmitted in the fiber core is coupled to the second optical signal mode LP03. Specifically, in this simulation example, the simulation is performed based on different values ​​of the number of grating periods N of the target fiber grating 603. For an explanation of the value of the number of grating periods N, please refer to the simulation example 1 above, which will not be repeated here.

[0168] Couple the first optical signal mode LP21 to the target fiber grating 603 having the number N of the aforementioned grating periods to obtain, as shown above. Figure 9 The transmission spectrum 903 shown is described in detail in Simulation Example 1 above, and will not be repeated here.

[0169] According to the transmission spectrum 903, when the value of the number N of grating periods is different, the first optical signal mode LP21 has different attenuation curves under the coupling effect of the target fiber grating 603. For example, when the value of N has the above-mentioned 10 values, ten attenuation curves as shown in the transmission spectrum 903 can be obtained. It can be seen that the target fiber grating 603 with different numbers N of grating periods can couple the optical power of the first optical signal mode LP21.

[0170] When the target fiber grating 603 transmits the first optical signal modes LP01, LP11 and LP02, the transmission spectra of the first optical signal modes LP01, LP11 and LP02 are shown in the transmission spectra 901, 902 and 904, respectively. Figure 9 It can be seen that the attenuation curves of the first optical signal modes LP01, LP11 and LP02 hardly change with the different values of the number N of grating periods of the target fiber grating 603. That is, when the value of N has the above-mentioned 10 values, ten almost coincident attenuation curves as shown in the transmission spectra 901, 902 and 904 are obtained. The maximum relative power of the first optical signal modes LP01, LP11 and LP02 is 0.002, 0.005 and 0.055, respectively. It can be seen that when the target fiber grating 603 couples the first optical signal mode LP21, it will not interfere with the first optical signal modes LP01, LP11 and LP02.

[0171] Simulation Example 4

[0172] This simulation example is combined with the coupling mode 4 shown in the above-mentioned coupling mode 4, that is, the first optical signal mode LP02 transmitted by the fiber core is coupled to the second optical signal mode LP22. Specifically, in this simulation example, according to different values of the number N of grating periods of the target fiber grating 604, the value of the number N of grating periods is explained, please refer to the above-mentioned simulation example 1, and will not be repeated here.

[0173] The target fiber grating 604 with the above-mentioned number N of grating periods couples the first optical signal mode LP02 to obtain the transmission spectrum 1004 as shown in Figure 10 The coordinates of the transmission spectrum 1004 are explained in detail in the above-mentioned simulation example 1, and will not be repeated here.

[0174] According to the transmission spectrum 1004, when the number of grating periods N is different, the first optical signal mode LP02 exhibits different attenuation curves under the coupling effect of the target fiber grating 604. For example, when N has the aforementioned 10 values, ten attenuation curves as shown in the transmission spectrum 1004 can be obtained. It is evident that the target fiber grating 604 with different numbers of grating periods N can couple the optical power of the first optical signal mode LP02.

[0175] When the target fiber grating 604 transmits the first optical signal modes LP01, LP11, and LP21, the transmission spectra of the first optical signal modes LP01, LP11, and LP21 are as follows: Figure 10 The transmission spectra 1001, 1002, and 1004 are shown. It can be seen that the attenuation curves of the first optical signal modes LP01, LP11, and LP21 hardly change with the different values ​​of the number of grating periods N of the target fiber grating 604. That is, when N has the above 10 values, the ten almost overlapping attenuation curves obtained are shown in the transmission spectra 1001, 1002, and 1004. The maximum relative powers of the first optical signal modes LP01, LP11, and LP21 are 0.001, 0.023, and 0.07, respectively. It is evident that when the target fiber grating 604 couples to the first optical signal mode LP02, it does not interfere with the first optical signal modes LP01, LP11, and LP21.

[0176] The following simulation verifies the gain equalization effect of the fiber optic filter shown in this application scenario:

[0177] First see Figure 11 As shown, where, Figure 11 The image shows the spectra of the four first optical signal modes LP01, LP11, LP21, and LP02 output from the gain fiber described above. The horizontal axis of the spectra represents wavelength, and the vertical axis represents power. Since the first optical signal modes LP01, LP11, LP21, and LP02 are not filtered by the fiber optic filter, the power of each optical signal mode exhibits significant fluctuations in the spectra, and the power differences between the optical signal modes are also relatively large, making it impossible to achieve inter-mode equalization.

[0178] Combination Figure 12 As shown, where, Figure 12 The spectra of the four optical signal modes output from the gain fiber are shown after filtering by the fiber filter. The first optical signal modes LP01, LP11, LP21, and LP02 have been filtered by the fiber filter; each optical signal mode exhibits relatively small power fluctuations in the spectrum. Figure 12As shown, the ratio of the power maximum value to the power minimum value of each optical signal mode is less than 0.47, and the power difference between the optical signal modes is very small, and is basically balanced.

[0179] As can be seen, based on the optical fiber filter shown in the application scenario, the optical powers of the four first optical signal modes LP01, LP11, LP21 and LP02 output by the gain optical fiber can be adjusted to realize gain balancing between different first optical signal modes.

[0180] Application scenario two

[0181] The application scenario combines Figure 13 As shown, the application scenario takes the number of types of optical signal modes transmitted by the core and the number of types of second optical signal modes transmitted by the inner cladding as an example. Specifically, the core transmits two optical signal modes, i.e., LP01 and LP11. The second optical signal modes transmitted by the inner cladding are LP21, LP02, LP12 and LP31. Among them, the first optical signal mode LP01 transmitted by the core is coupled to the second optical signal mode LP21 based on phase matching, and the first optical signal mode LP11 transmitted by the core is coupled to the second optical signal mode LP02 based on phase matching.

[0182] This embodiment takes the power adjustment of different optical signal modes based on different structural parameters as an example, wherein the first structural parameter is used to couple the first optical signal mode LP01, and the second structural parameter is used to couple the first optical signal mode LP11. Among them, the first structural parameter is: the radius of the core is 6.8 μm, the radius of the inner cladding is 9.0 μm, and the radius of the outer cladding is 62.5 μm. The refractive index of the core is 1.4536, the refractive index of the inner cladding is 1.4473, and the refractive index of the outer cladding is 1.44402. The second structural parameter is: the radius of the core is 6.8 μm, the radius of the inner cladding is 11.0 μm, and the radius of the outer cladding is 62.5 μm. The refractive index of the core is 1.4536, the refractive index of the inner cladding is 1.4473, and the refractive index of the outer cladding is 1.44402. The specific coupling mode is described as follows:

[0183] Coupling mode 1

[0184] Combined Figure 13 The coupling of the first optical signal mode LP01 transmitted by the core to the second optical signal mode LP21 is specifically described as shown in Table 6 and Table 6:

[0185] Table 6

[0186]

[0187] The target fiber grating 1301 realizes coupling of the first optical signal mode LP01, which includes three first optical signals with central wavelengths of 1530 nm, 1545 nm and 1555 nm shown in Table 6.

[0188] The target fiber grating 1301 includes three sub-gratings, i.e., G 1-1 , G 1-2 and G 1-3 . The grating period, the number and length of the grating period of each sub-grating are shown in Table 6.

[0189] Specifically, the sub-grating G 1-1 can couple at least part of the optical power of the first optical signal with a central wavelength of 1530 nm to the second optical signal mode LP21 based on phase matching. The specific description of the phase matching is shown above and will not be repeated. By analogy, the sub-grating G 1-3 can couple at least part of the optical power of the first optical signal with a central wavelength of 1555 nm to the second optical signal mode LP21.

[0190] Coupling mode 2

[0191] In combination with Figure 13 and Table 7, the coupling of the first optical signal mode LP11 transmitted by the core to the second optical signal mode LP02 is specifically described:

[0192] Table 7

[0193]

[0194] The target fiber grating 1302 realizes coupling of the first optical signal mode LP11, which includes three first optical signals with central wavelengths of 1530 nm, 1545 nm and 1555 nm shown in Table 7.

[0195] The target fiber grating 1302 includes three sub-gratings, i.e., G 2-1 , G 2-2 and G 2-3 . The grating period, the number, length and central wavelength of the grating period of each sub-grating are shown in Table 7.

[0196] Specifically, the sub-grating G 2-1 can couple at least part of the optical power of the first optical signal with a central wavelength of 1530 nm to the second optical signal mode LP02 based on phase matching. The specific description of the phase matching is shown above and will not be repeated. By analogy, the sub-grating G 2-3at least part of optical power of the first optical signal with a center wavelength of 1555 nm can be coupled to a second optical signal mode LP02.

[0197] It can be seen that the target fiber gratings 1301 and 1302 in the core can realize gain equalization of the entire C-band (1530-1565 nm). Moreover, the target fiber gratings can independently adjust the optical power of each first optical signal mode, effectively avoiding mutual interference. For specific description of related interference, please refer to the application scenario one shown above, which will not be repeated. The effect of the fiber filter shown in the application scenario on realizing gain equalization is verified by simulation as follows.

[0198] Firstly, refer to Figure 14 , wherein, Figure 14 is a spectrum diagram of two first optical signal modes LP01 and LP11 output by the gain fiber shown above. For description of the coordinates of the spectrum diagram, please refer to Figure 11 , which will not be repeated. The first optical signal modes LP01 and LP11 are not filtered by the fiber filter, and the power of each first optical signal mode in the spectrum diagram has relatively large fluctuations.

[0199] In combination with Figure 15 , wherein, Figure 15 is a spectrum diagram of two first optical signal modes LP01 and LP11 output by the gain fiber. The first optical signal modes LP01 and LP11 have been filtered by the fiber filter, and the power of each first optical signal mode in the spectrum diagram has relatively small fluctuations, as Figure 15 , the ratio of the maximum power and the minimum power of each first optical signal mode is less than 0.5.

[0200] Based on the above, the effective effect of the fiber filter shown in the application is described as follows:

[0201] The fiber filter shown in the embodiment can independently adjust the optical power of two or more first optical signal modes, thereby effectively realizing gain equalization between different optical signal modes and gain equalization between first optical signals of different wavelengths in different optical signal modes, and effectively reducing the probability of interruption of optical signal transmission.

[0202] Based on the target fiber grating adjusting the optical power of the target first optical signal mode, the optical power of other first optical signals is not interfered, and interference is effectively avoided.

[0203] The target fiber grating of the fiber filter can couple at least part of the optical power of the target first optical signal mode into a target second optical signal mode, and the target second optical signal mode is only one optical signal mode supported by the inner cladding, so the optical power of the same target first optical signal mode is not coupled into multiple second optical signal modes, thereby realizing controllable adjustment of the optical power of the target first optical signal mode, and effectively improving the accuracy of the optical power adjustment of the target first optical signal mode.

[0204] The fiber filter is based on the fiber grating in the fiber core to couple the optical power, and the fiber grating has the advantages of simple structure, small insertion loss, small size and low cost.

[0205] The terms "first", "second", etc. in the specification and claims of the present application and the above-described drawings are used to distinguish similar objects, and do not necessarily describe a specific order or sequence. It should be understood that the data used in this way can be interchanged under appropriate circumstances, so that the embodiments described herein can be implemented in an order other than that illustrated or described herein. In addition, the terms "include" and "have" and any variations thereof are intended to cover non-exclusive inclusion, for example, a process, method, system, product or device including a series of steps or modules does not necessarily have to be limited to those steps or modules clearly listed, but can include other steps or modules that are not clearly listed or inherent to the process, method, product or device.

[0206] The above-described embodiments are only used to illustrate the technical solutions of the present application, and not to limit them; although the present application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that they can still modify the technical solutions recorded in the foregoing embodiments, or make equivalent replacement for part of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present application.

Claims

1. An optical fiber filter, characterized by, The fiber includes a fiber core, an inner cladding and an outer cladding, the inner cladding is wrapped around the outer peripheral surface of the fiber core, the outer cladding is wrapped around the outer peripheral surface of the inner cladding, the refractive index of the fiber core, the refractive index of the inner cladding and the refractive index of the outer cladding decrease in turn, the fiber core is used for transmitting at least two different first optical signal modes, the inner cladding is used for transmitting at least two different second optical signal modes, and the fiber core is etched with at least one fiber grating; At least part of the optical power of the target first optical signal mode is only coupled with the target second optical signal mode at the fiber grating, wherein the target first optical signal mode is one of the at least two first optical signal modes, and the target second optical signal mode is one of the at least two second optical signal modes; The length of the fiber grating along the axial direction of the fiber core is in a positive correlation with the size of the optical power coupled into the target second optical signal mode.

2. The optical fiber filter according to claim 1, characterized in that, The coupling of the target first optical signal mode and the target second optical signal mode satisfies the following formula: Wherein, the is the effective refractive index of a first optical signal included in the target first optical signal mode; the is the effective refractive index of a second optical signal included in the target second optical signal mode; the λ is the wavelength possessed by the first optical signal and the second optical signal, and the Λ is the grating period of the fiber grating.

3. The optical fiber filter of claim 1, wherein, The coupling of the target first optical signal mode and the target second optical signal mode satisfies the following formula: Wherein, the target first optical signal mode includes a first optical signal, and the target second optical signal mode includes a second optical signal. The effective refractive index of the first optical signal included in the target first optical signal mode; the effective refractive index of the second optical signal included in the target second optical signal mode; λ is the wavelength of the first optical signal and the second optical signal, and Λ is the grating period of the fiber grating. The effective refractive index of the first optical signal included in the target first optical signal mode; the effective refractive index of the second optical signal included in the target second optical signal mode; λ is the wavelength of the first optical signal and the second optical signal, and Λ is the grating period of the fiber grating.

4. The optical fiber filter according to any one of claims 1 to 3, characterized in that, The fiber core is etched with a plurality of fiber gratings, and the grating periods of the fiber gratings are different from each other.

5. The optical fiber filter according to any one of claims 1 to 4, characterized in that, Each of the at least two first optical signal modes corresponds to a different second optical signal mode.

6. The optical fiber filter according to any one of claims 1 to 4, characterized in that, Each of the at least two first optical signal modes corresponds to a second optical signal mode.

7. The optical fiber filter according to claim 5 or 6, characterized in that, In the at least two first optical signal modes, each of the first optical signal modes corresponds to a second optical signal mode except for the first optical signal mode with the lowest corresponding optical power.

8. The optical fiber filter according to any one of claims 1 to 7, characterized in that, The optical power of the target first optical signal mode coupled into the outer cladding is less than the optical power of the target first optical signal mode coupled into the target second optical signal mode.

9. The optical fiber filter according to any one of claims 1 to 8, characterized in that, The difference between the refractive index of the inner cladding and the refractive index of the outer cladding is in a positive correlation with the number of the second optical signal modes.

10. The optical fiber filter according to any one of claims 1 to 9, characterized in that, The radius of the inner cladding is in a positive correlation with the number of the second optical signal modes.

11. An optical fiber amplifier, characterized by, The fiber amplifier includes a pump laser, a wavelength division multiplexer, a gain fiber and a fiber filter, the wavelength division multiplexer is coupled with the pump laser and the gain fiber respectively, and the gain fiber is coupled with the fiber filter; The wavelength division multiplexer is used for multiplexing pump light from the pump laser and optical signals; The gain fiber is used for gain amplifying the optical signals from the wavelength division multiplexer to output at least two first optical signal modes, and the fiber filter is as shown in any one of claims 1 to 10.

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