WO2018018666A1 - Few-mode fibre device - Google Patents

Abstract

A few-mode fibre device. A fibre core of the few-mode fibre device is composed of a main fibre core (1) and an auxiliary fibre core (2), wherein there is only one main fibre core (1), and the normalized frequency V _m meets the condition that V _m > 2.405, i.e. supporting high-order mode transmission; the number of auxiliary fibre cores (2) is M, wherein M ≥ 1; one end of the auxiliary fibre cores (2) intersects with a side face of the main fibre core (1), and the central axis of the main fibre core (1) is in the same plane as the central axis of the auxiliary fibre cores (2); the other end of the auxiliary fibre cores (2) is located on an end face of the fibre, and in the end face, the centre distance between any two fibre cores is greater than the sum of the radii of the two fibre cores; and a parameter between any auxiliary fibre core (2) and the main fibre core (1) meets: the main fibre core (1) at least has a transmission mode of which the effective refractive index is less than the effective refractive index of a fundamental mode of the auxiliary fibre cores (2). A mode in a few-mode fibre can be converted, combined, separated or filtered.

Description

A modeless fiber optic device Technical field

The invention relates to the field of optical fiber communication, in particular to a fiber optic device capable of realizing functions of converting, merging, separating or filtering out modes in a mode-less optical fiber.

Background technique

In recent years, due to the limitation of transmission capacity of single-mode optical fibers, communication technologies based on small-mode optical fibers have attracted widespread interest. The use of mode-less fiber to transmit different information in different modes, the so-called mode-multiplexing technology, can multiply the transmission capacity of the fiber, while the mode-less fiber can have a larger core area. To reduce the nonlinear effects of fiber caused by various multiplexing technologies. In addition to its application in analog-to-division multiplexing, small-mode fibers can also achieve large-mode single-mode transmission, dispersion compensation, ultrashort pulse transmission, and nonlinear applications by selectively exciting higher-order modes.

In a modular multiplexed system, a small-mode fiber device capable of inter-mode conversion, multiplexing of different modes to a mode-less fiber, and demultiplexing modes in a mode-less fiber is a key factor in determining system performance. One. Although the method of using the spatial optical element realizes the multiplexing and demultiplexing of the mode, there are disadvantages such as large additional loss, poor stability, and large device size, and the all-fiber device can well overcome these disadvantages. High-speed conversion between the fundamental mode and the higher-order mode of the mode-less fiber can be achieved by using a long-period fiber grating with a bandwidth of 34 nm [IEEE Photon. Technol. Lett., 2015, 27(9): 1006-1009]. The long-period fiber grating cascade method can theoretically achieve high-order mode conversion, but its 3dB bandwidth is only about 10nm [Opt.Express, 2014, 22(10): 11488-11497]. The separation of modes can be achieved based on a two-core fiber coupled with a single mode fiber and a small mode fiber, but it is difficult to avoid coupling between different modes [Opt. Fiber Technol., 2011, 17(5): 490-494]. Multi-core fiber can realize multiplexing and separation of multiple modes, and its mode field deformation is more serious [Opt. Express, 010, 18(5): 4709-4716]. Mode separation can also be achieved by using a waveguide structure, but the structure is relatively complicated [Opt. Express, 2013, 21(15): 17904-17911, Opt. Express, 2013, 21(17): 20220-20229]. Broadband mode conversion can also be achieved with a tapered photonic crystal fiber structure [Opt. Lett., 2007, 32(4): 328-330], but only between the fiber fundamental mode and the higher order mode can be achieved.

Generally, the conversion characteristics of a mode converter based on the mode coupling principle have strong wavelength dependence, a narrow operating bandwidth, and poor output spectral uniformity. Since the effective definite refractive index of the quadruple degenerate mode in the fiber is equal, there is also great difficulty in multiplexing and decomposing this mode. For the application of the small-mode fiber system, there is still a lack of Means to achieve selective filtering of one or some specific patterns. In summary, fiber optic devices that can flexibly convert between different modes in a small-mode fiber and implement multiplexing, merging, separating, and filtering of different modes remain to be developed.

Summary of the invention

In view of the deficiencies of the prior art, the present invention provides a mode-less optical fiber device capable of implementing various functions such as converting, combining, separating, or filtering out modes in a mode-less optical fiber.

The technical solution of the present invention is:

A Reduced-mode fiber devices, the core and the cladding layers, wherein: said core comprises a primary core and a secondary core M, M ≧ 1; a main core normalized frequency V _m satisfies V _m >2.405, which supports high-order mode transmission; one end of the auxiliary core intersects the side of the main core, and the central axis of the main core is in the same plane as the central axis of the auxiliary core; the other end of the auxiliary core extends to The multi-core end face of the fiber, the single-core end face of the fiber has only the main core; the end of the multi-core end face, the center distance of any two cores is greater than the sum of the core radii of the two; any auxiliary core and main core The parameter between the two is satisfied: the effective core of the main core has an effective refractive index lower than the effective refractive index of the auxiliary core mode.

Preferably, both the main core and the auxiliary core have a circular cross section.

Preferably, the absolute value of the difference between the effective refractive index of the fundamental mode of the auxiliary core and the effective refractive index of any mode of the main core is greater than 0.0001.

Preferably, the auxiliary transmission core supports only a single mode, i.e., the normalized frequency required to satisfy V _f V _f <2.405.

Preferably, the auxiliary core satisfies: n _i-1 >n _f >n _i (I≥i>1) or n _f >n _i (i=1); wherein n _{i is the i} - _th core The effective refractive index of each mode, I is the total number of modes of the main core, and there is n _i-1 >n _i (I≥i>1), and n _f is the effective refractive index of the fundamental mode of the auxiliary core. The auxiliary core is the auxiliary core of the i-th mode of the main core, and can be used for inputting/outputting the i-th mode of the main core.

Preferably, a quadruple degenerate mold of the main core has two matching auxiliary cores, and an angle between a central axis of the two auxiliary cores and a plane defined by a central axis of the main core satisfies 90° /m; m is an integer greater than or equal to 1, and refers to the logarithm of the maximum value of the transverse electric field along the circumference of the quadruple degenerate mode of the main core.

Preferably, the relationship between the number M of auxiliary cores and the number of main core modes I and the number N of main core quadratic degeneracy modes is: M=I+N; wherein the central axis of the one auxiliary core and the main core The central axes are all in the same plane, and the auxiliary core is defined as the first type of auxiliary cores, which are respectively used for input/output one mode of the main core; and another N auxiliary fibers. The central axis of the core is not in this plane, defined as a second type of auxiliary core for the other of the N quadruple degenerate modes of the input/output main core.

Preferably, the projection length of the central axis of the auxiliary core on the central axis of the main core is such that the projection length of the central axis of any of the first type of auxiliary cores on the central axis of the main core is greater than any of the second type of auxiliary fibers. The projected length of the central axis of the core on the central axis of the main core.

Preferably, the first type of auxiliary core has one and only one matching main core mode; the second type of auxiliary core has one and only one quadruple mode of the main core.

Preferably, the effective refractive index of the fundamental mode of the first type of auxiliary core is greater than the effective refractive index n ₁ of the fundamental mode of the main core.

Preferably, the number of the auxiliary cores is 6, and the first type of auxiliary cores are four, and the distances of the four first-type auxiliary cores on the multi-core end faces are sequentially reduced according to the central axis and the central axis of the main core. Small arrangement; the second type of auxiliary core is two, the central axis of the two second type auxiliary cores and the plane of the central axis of the first type of auxiliary core and the plane of the central core of the main core are respectively 45 degrees And 90°.

The technical effect of the present invention is that the novel mode-less optical fiber device proposed by the present invention can realize mode field conversion between different modes in a mode-less optical fiber, and can convert different optical signals into different modes of a small-mode fiber through a bundle core. Mode, which realizes mode division multiplexing; the modes in the mode-less fiber can also be coupled to different bundle cores and output respectively, thereby realizing the separation of different mode signals in the mode-less fiber, that is, demultiplexing; Download and upload different mode signals, and selectively filter high-order modes in low-mode fibers and single-mode transmission in small-mode fibers to achieve separation of degenerate higher-order modes. The structure also has the advantages of being insensitive to parameters of the optical fiber, allowing for a large manufacturing tolerance, a wide operating wavelength range, and good output energy uniformity and polarization independence. The invention can be widely applied to systems such as small-mode optical fiber communication and sensing.

DRAWINGS

FIG. 1 is a schematic structural view of Embodiment 1 of the optical fiber according to the present invention.

2 is a mode energy curve output from a main core when light is input from the auxiliary core of Embodiment 1 of the optical fiber of the present invention, wherein the abscissa is a difference in refractive index between the auxiliary core and the cladding. In the figure, a, b, c, and d respectively indicate values at which the effective core refractive index of the auxiliary core is equal to the effective refractive index of the LP ₀₁ , LP ₁₁ , LP ₂₁ , and LP ₀₂ modes of the main core.

3 is a graph showing a relationship between a mode energy curve outputted from a main core and a wavelength when light is input from the auxiliary core of Embodiment 1 of the optical fiber of the present invention, wherein the effective refractive index of the auxiliary core is larger than that of the main core. The core's LP ₂₁ mode is smaller than the effective refractive index of the LP ₁₁ mode of the main core.

Figure 4 is a graph showing the relationship between the mode energy curve and the wavelength of the output of the main core of the single-core end face, from the multi-core end main core input of the embodiment 1 of the optical fiber of the present invention, wherein the fundamental mode of the auxiliary core The effective refractive index is greater than the LP ₂₁ mode of the main core and less than the effective refractive index of the LP ₁₁ mode of the main core. Among them, (a) input LP ₀₁ mode, (b) input LP ₁₁ mode, (c) input LP ₂₁ mode, (d) input LP ₀₂ mode.

5 is a graph showing a relationship between a mode energy curve outputted from a multi-core end face and a wavelength when light is input from a single-core end main core of Embodiment 1 of the optical fiber of the present invention, wherein the fundamental mode of the auxiliary core is effectively refracted. The rate is greater than the LP ₁₁ mode of the main core and less than the effective refractive index of the LP ₀₁ mode of the main core. Among them, (a) input LP ₀₁ mode, (b) input LP ₁₁ mode, (c) input LP ₂₁ mode, (d) input LP ₀₂ mode.

Fig. 6 is a view showing the structure of a second embodiment of the optical fiber of the present invention.

Fig. 7 is a graph showing the relationship between the mode energy curve output from the auxiliary core and the wavelength when light is input from the single core end main core of the embodiment 2 of the optical fiber of the present invention. Among them, (a) input LP ₁₁ even mode, (b) input LP ₂₁ odd mode.

Figure 8 is a graph showing the relationship between the mode energy curve output from the main core and the wavelength when light is input from the multi-core end face auxiliary core of the embodiment 2 of the optical fiber of the present invention. Among them, (a) light is input from the auxiliary core 6, and (b) light is input from the auxiliary core 9.

In the picture,

1 is the main core, 2 is the auxiliary core, 3 is the cladding, 21, 22, 23, 24, 25 are the first type of auxiliary core, and 26 is the second type of auxiliary core.

detailed description

The present invention will be further described below in conjunction with the drawings and specific embodiments, but the scope of the present invention is not limited thereto.

The mode-less optical fiber device provided by the invention comprises a core and a cladding, the core comprises a main core and M auxiliary cores, M≥1; the main core and the auxiliary core are all round The shape and the central axis are straight lines. The normalized frequency V _{m of the} main core satisfies V _m >2.405, that is, supports high-order mode transmission; the absolute value of the difference between the effective refractive index of the fundamental mode of the auxiliary core and the effective refractive index of any mode of the main core is More than 0.0001. One end of the auxiliary core intersects the side of the main core, and the central axis of the main core is in the same plane as the central axis of the auxiliary core; the other end of the auxiliary core extends to the multi-core end of the optical fiber, and the single core of the optical fiber The end face has only the main core, and the end of the multi-core end face, the center distance of any two cores is greater than the sum of the core radii of the two; the parameters between any auxiliary core and the main core satisfy: the main core at least There is a mode in which the effective refractive index is lower than the effective refractive index of the auxiliary core mode.

Example 1

FIG. 1 is a schematic structural view of Embodiment 1 of an optical fiber according to the present invention. The number of the auxiliary cores is M=1, and satisfies: n _i-1 >n _f >n _i (I≥i>1) or n _f >n _i (i=1); wherein n _{i is the} main fiber The effective refractive index of the i-th mode of the core, I is the total number of modes of the main core, and has n _i-1 >n _i (I≥i>1), and n _f is the effective refractive index of the fundamental mode of the auxiliary core. The number of main cores and auxiliary cores is one. The main core and the auxiliary core are present on the multi-core end face, and only the main core exists at the end face of the single core.

At the working wavelength, the main core 1 is a small-mode core, that is, it can transmit a high-order mode. The present invention realizes the introduction, extraction, conversion, and the like of the mode in the main core 1 by introducing the auxiliary core 2.

1. Light from the auxiliary core input of the multi-core end face

It is assumed that the main core can support the transmission of the four modes of LP ₀₁ , LP ₁₁ , LP ₂₁ , and LP ₀₂ . If the effective refractive index of the fundamental mode (LP ₀₁ mode) of the auxiliary core is greater than the effective refractive index of the main core LP ₂₁ mode and less than the effective refractive index of the main core LP ₁₁ mode, when inputting the fundamental mode from the auxiliary core The LP ₂₁ mode will be excited in the main core and output from the single-core end face. If the effective refractive index of the fundamental mode (LP ₀₁ ) of the auxiliary core is greater than the effective refractive index of the main core LP ₀₂ mode and less than the effective refractive index of the main core LP ₂₁ mode, when inputting the fundamental mode from the auxiliary core, It will excite the LP ₀₂ mode in the main core and output it from the single core end face.

Light is input from the auxiliary core of the multi-core end face, and is output from the main core of the single-core end face, and satisfies: n _i-1 >n _f >n _i (I≥i>1) or n _f >n _i (i= 1); then the fundamental mode of the auxiliary core will be converted to the i-th mode of the main core. Here, the effective refractive index of the i-th mode of the main core is defined as ni, I is the total number of modes of the main core, and there are n _i-1 >n _i (I≥i>1), and n _f is the auxiliary core. The effective refractive index of the fundamental mode.

2 is a mode energy curve output from the main core when light is input from the auxiliary core of the structure of Embodiment 1, wherein the abscissa is the difference between the auxiliary core and the cladding refractive index. As the difference between the refractive index of the auxiliary core and the cladding increases (assuming that the refractive index of the cladding is constant, that is, the refractive index of the auxiliary core is actually increased), the fundamental mode of the auxiliary core is effectively refracted. The rate also increases, so that the effective refractive index relationship with the mode in the main core also changes. In the figure, a, b, c, and d respectively indicate auxiliary cores and packages corresponding to the effective refractive indices of the auxiliary core base modes respectively equal to the effective refractive indices of the LP ₀₁ , LP ₁₁ , LP ₂₁ , and LP ₀₂ modes of the main core. The position of the difference in layer refractive index. That is, when the effective refractive index of the fundamental mode of the auxiliary core sequentially exceeds the LP ₀₂ , LP ₂₁ , LP ₁₁ and LP ₀₁ modes of the main core, the light input by the auxiliary core is also sequentially converted into the LP _{02 of the} main core, LP ₂₁ , LP ₁₁ and LP ₀₁ modes. At the same time, in addition to the ad position and the small range nearby, the main core will excite two modes. In other positions, it can achieve high-efficiency conversion, and the conversion loss does not exceed -0.03dB. The conversion characteristic has the characteristic of being insensitive to the refractive index of the auxiliary core, that is, the change of the refractive index of the auxiliary core in a certain interval has little influence on the output energy. It can be seen from the figure that when the effective core refractive index of the auxiliary core mode is smaller than the LP ₀₂ mode of the main core, that is, the effective refractive index of the main core mode is larger than the effective refractive index of the auxiliary core mode, it cannot form an effective mode conversion. .

3 is a graph showing a relationship between a mode energy curve outputted from a main core and a wavelength when light is input from the auxiliary core of Embodiment 1, wherein the effective refractive index of the auxiliary mode of the auxiliary core is larger than the LP ₂₁ mode of the main core; The effective refractive index of the LP ₁₁ mode of the main core. It can be seen that the main core LP ₂₁ mode is excited in the ultra-wide wavelength range, and the other mode energies in the main core are less than -20 dB, that is, only the LP ₂₁ mode is effectively excited.

2. Light input from the core of the multi-core end

If light is input from the main core of the multi-core end face, the output from the single-core end face is different depending on the auxiliary core parameters. It is assumed that the main core can support the transmission of the four modes of LP ₀₁ , LP ₁₁ , LP ₂₁ , and LP ₀₂ , and the effective refractive index of the fundamental mode of the auxiliary core (LP ₀₁ ) is greater than the effective refractive index of the main core LP ₂₁ mode. It is smaller than the effective refractive index of the main core LP ₁₁ mode. Then, when the main cores of the multi-core end face are respectively input into the LP ₀₁ , LP ₁₁ , and LP ₂₁ modes, the modes output from the port B are: LP ₀₁ , LP ₁₁ , and LP ₀₂ modes, that is, the first two modes are unchanged. The LP ₂₁ mode will be converted to the LP ₀₂ mode; if the input is the LP ₀₂ mode, it will leak and will not be output from port B. Similarly, if the effective refractive index of the fundamental mode (LP ₀₁ ) mode of the auxiliary core is greater than the effective refractive index of the main core LP ₁₁ mode and less than the effective refractive index of the main core LP ₀₁ mode. Then, when the main cores of the multi-core end face are respectively input into the LP ₀₁ , LP ₁₁ , and LP ₂₁ modes, the modes output from the single-core end faces are: LP ₀₁ , LP ₂₁ , and LP ₀₂ modes, that is, the latter two modes are changed. If the input is LP ₀₂ mode, it will leak and will not be output from port B.

Light is input from the main core of the multi-core end face, and is output from the main core of the single-core end face; the parameters of the main core and the auxiliary core satisfy: n _i-1 >n _f >n _i (I≥i>1) or n _f >n _i (i=1); if there is a positive integer j that satisfies I>j≥i, the jth mode of the main core input from the multi-core end face will be converted to the j+1th of the main core The mode output, and the first mode of the main core input from the multi-core end face will be filtered out.

4 is a graph showing the relationship between the mode energy curve and the wavelength of the main core output of the single core end from the input of the main core of the multi-core end face of the embodiment 1. The effective refractive index of the auxiliary mode of the auxiliary core is greater than that of the main core. The LP ₂₁ mode is smaller than the effective refractive index of the LP ₁₁ mode of the main core. Among them, (a) input LP ₀₁ mode, (b) input LP ₁₁ mode, (c) input LP ₂₁ mode, (d) input LP ₀₂ mode. As can be seen from the figure, the input LP ₀₁ mode and LP ₁₁ mode still maintain low loss transmission; while the output modes of other modes are less than -20 dB, that is, they cannot be effectively excited. The input LP ₂₁ mode is converted to the LP ₀₂ mode, and the other modes are very low in energy. The input LP ₀₂ mode produces a leak that is attenuated and does not excite other modes.

3. Light from the single core end of the main core input

5 is a graph showing a relationship between a mode energy curve outputted from a multi-core end face and a wavelength when light is input from a single-core end main core of Embodiment 1, and also assumes that the main core can support LP ₀₁ , LP ₁₁ , LP ₂₁ , The LP ₀₂ is transmitted in four modes and the effective refractive index of the fundamental mode of the auxiliary core is larger than the LP ₁₁ mode of the main core and smaller than the effective refractive index of the LP ₀₁ mode of the main core. Among them, (a) input LP ₀₁ mode, (b) input LP ₁₁ mode, (c) input LP ₂₁ mode, (d) input LP ₀₂ mode. That is, when the LP ₀₁ , LP ₂₁ , and LP ₀₂ modes are respectively input from the main core of the single-core end face, the modes output from the main core of the multi-core end face are: LP ₀₁ , LP ₁₁ , and LP ₂₁ modes, that is, the LP ₀₁ mode is not The LP ₂₁ and LP ₀₂ modes are converted to LP ₁₁ and LP ₂₁ modes, respectively. If the LP ₁₁ mode is input from the main core of the single core end face, it will be coupled to the auxiliary core to be converted into the fundamental mode output of the auxiliary core.

That is, the light is input from the main core of the single-core end face, and the parameters of the main core and the auxiliary core satisfy: n _i-1 >n _f >n _i (I≥i>1) or n _f >n _i (i=1) Then, the i-th mode of the input main core will be converted into the auxiliary core fundamental mode, and if there is a positive integer j, satisfying I≥j>i, the j-th mode of the main core input from the single-core end face It will be converted to the j-1th mode output of the main core.

In the structure of the present invention, the diameters of the auxiliary core and the main core do not change with the transmission distance, that is, the mode coupling between different cores is not realized by a taper or the like.

Based on the above structure and function, various complex functional devices are combined by increasing the number of auxiliary cores.

In the case where the difference in refractive index between the core and the cladding is small, the optical fiber device of the present invention has polarization independence, that is, two polarization states of the same mode have the same transmission characteristics.

Since the auxiliary core is close to the main core, if the effective refractive index of the auxiliary core and the main core is equal, the two modes can be strongly coupled to avoid such coupling between modes. It is required that at the working wavelength, the effective refractive index of the auxiliary core mode and the mode in the main core are not equal. As shown in Fig. 2, the effective refractive index of the fundamental mode of the auxiliary core and the effective refraction of any mode of the main core The absolute value of the difference between the rates should be greater than 0.0001 to avoid the effects of simultaneous conversion with the two main core modes.

Both the main core and the auxiliary core are round cores, i.e., have a circular cross section, and in other cases, it is often difficult to achieve low crosstalk mode switching and operation.

It can be seen from the foregoing analysis that the main core has at least one mode whose effective refractive index is lower than the effective refractive index of the auxiliary core mode, otherwise the function such as mode conversion cannot be realized.

The performance of the structure of the present invention is related to the characteristics of the main core mode. If a mode of the main core is a quadruple degenerate mode, the intersection area of an auxiliary core and the main core, the amplitude of the main core mode field is In the case where there is a maximum value at the intersection point, a process such as mode conversion may occur, and according to the orthogonality, another degenerate mode of the mode is at a minimum value position at the intersection position, so that no change occurs; Another auxiliary core, which intersects with the main core, is in the extreme position of the degenerate mode, thereby achieving effects such as mode switching. At the same time, the intersection of the auxiliary core and the main core should overlap with the maximum of the transverse electric field in the circumferential direction to form the strongest mode field transition. Therefore, the corresponding two auxiliary cores, The angle between the central axis and the two planes defined by the central axis of the main core should satisfy: 90°/m; where m is an integer greater than or equal to 1, referring to the transverse direction of the quadruple degenerate mode of the main core. The logarithm of the maximum value of the electric field along the circumference.

Since the LP ₁₁ mode is a quadruple degenerate mode, its two degenerate modes should be input or output from different auxiliary cores, respectively. Since the two degenerate modes have a 180 degree rotational symmetry relationship, the angle between the central axes of the corresponding two auxiliary cores and the plane formed by the central axis of the main core should be at an angle of 90 degrees. The two degenerate modes of the LP ₂₁ mode have a 45 degree rotational symmetry relationship, and therefore the angle between the central axis of the corresponding auxiliary core and the plane formed by the central axis of the main core should be 45 degrees.

The invention realizes the mode processing in the mode-less optical fiber, and therefore, the main core must be a non-single mode fiber, and the optical fiber theory can support the high-order mode transmission when the normalized frequency is greater than 2.405 for the step structure fiber. The normalized frequency of the main core is required to be at least greater than 2.405. The main core has at least one transfer mode whose effective refractive index is lower than the effective refractive index of the auxiliary core mode, otherwise the effect of the present invention cannot be achieved. The invention combines the main core and the auxiliary core to form a composite structure, and at the intersection of the two cores and the vicinity, a core region with an increased cross-sectional dimension is formed, and the mode of input from the main core or the auxiliary core is in the fiber. The transition and change of the core region formation mode ultimately forms the effect of the present invention.

For the case where light is input from the main core to realize mode conversion and filter mode, the mode conversion and the filter mode are related to the effective refractive index of the mode in the auxiliary core, and the auxiliary core is converted into a single mode fiber for the effective control mode conversion. The core can more effectively control the switching mode and bandwidth, ie requires that its normalized frequency _Vf meets _Vf < 2.405. In the case of the auxiliary core input mode to excite the main core mode or the higher order mode, regardless of whether the auxiliary core is a single mode core, as long as the auxiliary core actually transmits only the fundamental mode, the same effect can be achieved. In this case the auxiliary core can be a non-single mode core.

Increasing the number of auxiliary cores allows for more complex functions. In this case, the function of each of the auxiliary cores and the main core is independent, that is, the auxiliary core and the main core are only mode-switched at the intersection and the vicinity of the two, and the auxiliary cores are interposed. Maintain relatively independent light transmission. Therefore, although all of the auxiliary cores are at the same end face of the fiber, the transmission is relatively independent, and thus, the structure of the plurality of auxiliary cores corresponds to a cascade structure of a single auxiliary core structure.

Example 2

The present invention can achieve multiplexing and demultiplexing of fiber modes. The relationship between the number M of auxiliary cores and the number of main core modes I and the number N of main core degeneracy modes is: M=I+N; the central axes of one auxiliary cores are in the same plane, and the auxiliary core is defined as The first type of auxiliary cores are respectively used for inputting one mode of the main core; the center axes of the other N auxiliary cores are not in this plane, and are defined as the second type of auxiliary cores for inputting the main cores respectively. N quadruple And another mode in the mode.

Figure 6 shows one of the schemes, the auxiliary core is six, of which four are the first type of auxiliary core, and the central axes and main cores of the four first-type auxiliary cores 21, 22, 23, and 24 The central axes are all in the same plane, and the four first-type auxiliary cores 21, 22, 23, 24 on the multi-core end face are sequentially arranged in accordance with the distance between the central axis and the central axis of the main core. The two auxiliary cores 25, 26 are the second type of auxiliary cores, the central axes of the two second types of auxiliary cores 25, 26 and the plane of the central axis of the first type of auxiliary cores and the central axis of the main core The angles are 45° and 90°, respectively.

It is still assumed that the main core can support the transmission of the four modes of LP ₀₁ , LP ₁₁ , LP ₂₁ , and LP _02. From the fiber mode theory, the LP ₁₁ and LP ₂₁ modes are quadruple degenerate modes, and the others are double degenerate modes. . For the effective refractive index interval (n _i-1 , n _i ) of the first type of auxiliary core for any of the main core modes, the effective refractive index of the fundamental mode of one and only one auxiliary core of the first type is in this interval, I ≥i>1. That is, the effective refractive indices of the fundamental modes of the first type of auxiliary cores 21, 22, 23, 24 and the second type of auxiliary cores 25, 26 are greater than the LP ₀₂ , LP ₂₁ , LP ₁₁ and LP ₀₁ , LP _{11 of the} main core, respectively. And the effective refractive index of the LP ₂₁ mode, and the fundamental mode effective refractive index of the first type of auxiliary cores 21, 22, 23 and the second type of auxiliary cores 25, 26 is smaller than the LP ₂₁ , LP ₁₁ , LP of the main core ₀₁ , LP ₀₁ , LP ₁₁ effective refractive index. When the basic modes are input from the auxiliary cores 21, 22, 23, 24, ₂₅ , ₂₆ , they respectively excite the LP ₀₂ mode, the LP ₂₁ odd mode, the LP ₁₁ odd mode, the LP ₀₁ mode, and the LP ₁₁ couple of the main core. Mode, LP ₂₁ even mode. Here, the excitation of the main core mode must satisfy a certain order, that is, since the auxiliary core excites the main core mode, the main core mode may still pass through the intersection area of the other auxiliary core and the main core, thereby The situation is equivalent to the case of the aforementioned single auxiliary core structure, and the light is input from the main core of the multi-core end face. That is, the excited mode may change when passing through the intersection of other auxiliary cores and the main core. To this end, we first excite the main core mode of the lower order (ie, the mode effective refractive index is higher), so that when this mode passes through the auxiliary core with a higher effective refractive index of the fundamental mode, it does not undergo mode switching. For the quadruple degenerate mode, it is also necessary to avoid the interference of the auxiliary core to other modes of the main core. For this reason, it is required to first excite the quadratic degenerate mode of the lower order mode, and then excite the higher order quadruple degenerate mode. Finally, another mode of quadruple degenerate mode and other double degenerate modes are excited. The auxiliary cores 22 and 26 are respectively used to excite the LP ₂₁ odd mode, LP ₂₁ even mode, and therefore the central axis of the two is required to be at an angle of 45 degrees to the determined two planes of the central axis of the main core, respectively. The auxiliary cores 23 and 29 are respectively used to excite the LP ₁₁ odd mode and the LP ₁₁ even mode, so that the angle between the central axes of the two cores and the determined two planes of the central axis of the main core is required to be 90 degrees. .

For the above fiber structure, if the light is input from the main core of the single-core end face, the input LP ₀₂ mode, LP ₂₁ odd mode, LP ₁₁ odd mode, LP ₀₁ mode, LP ₁₁ even mode, LP ₂₁ even mode will be respectively The auxiliary cores 21, 22, 23, 24, 25, 26 are output. That is, this structure can implement the demultiplexing function of the mode.

Fig. 7 is a graph showing the relationship between the mode energy curve output from the auxiliary core and the wavelength when light is input from the single core end main core of the structure shown in Fig. 6. Among them, (a) input LP ₁₁ even mode, (b) input LP ₂₁ odd mode. As can be seen from the figure, when the LP _{11 is} input from the main core, it will be output from the auxiliary core 25, and its output energy is greater than -0.1 dB in a wide wavelength range of light wavelengths greater than 1.36 μm, while other auxiliary fibers The core output energy is below -35dB, ie its crosstalk is very low. When the main core mode is switched to the auxiliary core output, the energy of the output mode varies little with wavelength, reflecting the ultra-wide operating bandwidth of this structure. Similarly, when the LP ₂₁ odd mode is input from the main core, it will be output from the auxiliary core 22, and its output energy is greater than -0.05 dB in a wide wavelength range of light wavelengths less than 1.66 μm, while other auxiliary cores The output energy is below -25dB, ie its crosstalk is very low. That is, in the ultra-wide wavelength range of 300 nm, both modes can achieve mode conversion and demultiplexing with low loss and low crosstalk.

As another solution, the fiber structure of Fig. 6 is still adopted, and it is still assumed that the main core can support the transmission of the four modes of LP ₀₁ , LP ₁₁ , LP ₂₁ , and LP ₀₂ . Here, the effective refractive index of the fundamental modes of the second type of auxiliary cores 25, 26 is greater than the LP ₁₁ and LP ₂₁ modes of the main core, respectively, and smaller than the effective refractive index of the LP ₀₁ and LP ₁₁ modes of the main core. The effective refractive index of the fundamental modes of the first type of auxiliary cores 21, 22, 23, 24 is greater than the effective refractive index of the LP ₀₁ mode of the main core. That is, the basic modes of the second and second auxiliary cores 25 and 26 enter the main core, respectively exciting the LP ₁₁ even mode and the LP ₂₁ even mode of the main core, and the two modes pass through other auxiliary cores and main fibers. Mode transitions do not occur in the core intersection area. The base mode of the auxiliary core 24 enters the main core, and the LP ₀₁ mode of the main core is excited. When it passes through the intersection area of the auxiliary core 23 and the main core, it is converted into an LP ₁₁ odd mode, and then assisted. When the core 22 intersects with the main core, it is converted into an LP ₂₁ odd mode. Finally, when it passes through the intersection of the auxiliary core 21 and the main core, it is converted into an LP ₀₂ mode, that is, the auxiliary core 21 The fundamental mode is ultimately converted to the LP ₀₂ mode output of the main core. Similarly, the fundamental modes input from the auxiliary cores 21, 22, and 23 are finally converted into the LP ₀₁ mode, the LP ₁₁ odd mode, and the LP ₂₁ odd mode output of the main core, respectively. Therefore, when the fundamental mode is input from the auxiliary core 21, 22, 23, 24, ₂₅ , ₂₆ , it will respectively excite the LP ₀₁ mode, the LP ₁₁ odd mode, the LP ₂₁ odd mode, the LP ₀₂ mode of the main core, LP ₁₁ even mode, LP ₂₁ even mode.

For the above scheme, if the light is input from the main core of the single-core end face, the input LP ₀₁ mode, LP ₁₁ odd mode, LP ₂₁ odd mode, LP ₀₂ mode, LP ₁₁ even mode, LP ₂₁ even mode will be respectively assisted. Core 21, 22, 23, 24, 25, 26 output. That is, this structure can implement the demultiplexing function of the mode.

Figure 8 is a graph showing the relationship between the mode energy curve output from the main core and the wavelength when the light is input from the multi-core end face auxiliary core as shown in Figure 6. Among them, (a) light is input from the auxiliary core 22, and (b) light is input from the auxiliary core 25. As can be seen from the figure, when light is input from the auxiliary core 22, it excites the LP ₁₁ odd mode, and in a wide wavelength range of light wavelength less than 1.635 μm, the LP ₁₁ odd mode output energy is greater than -0.05 dB, while other mode outputs The energy is below -20dB. When light is input from the auxiliary core 25, it excites the LP ₁₁ even mode, and in a wide wavelength range of light wavelengths greater than 1.36 μm, the LP ₁₁ even mode output energy is greater than -0.05 dB in a wide wavelength range, and other mode outputs The energy is below -20dB. That is, in the ultra-wide wavelength range of 275 nm, both modes can achieve mode conversion and multiplexing with low loss and low crosstalk.

By combining the auxiliary cores with different parameters and their positions with the main core, it is also possible to implement functions such as mode division multiplexing and mode selective filtering.

The embodiments are a preferred embodiment of the invention, but the invention is not limited to the embodiments described above, and any obvious improvements, substitutions or alternatives that can be made by those skilled in the art without departing from the spirit of the invention. Variations are within the scope of the invention.

Claims (10)

1. A Reduced-mode fiber devices, the core and the cladding layers, wherein: said core comprises a primary core and a secondary core M, M ≧ 1; a main core normalized frequency V _m satisfies V _m >2.405, which supports high-order mode transmission; one end of the auxiliary core intersects the side of the main core, and the central axis of the main core is in the same plane as the central axis of the auxiliary core; the other end of the auxiliary core extends to The multi-core end face of the fiber, the single-core end face of the fiber has only the main core; the end of the multi-core end face, the center distance of any two cores is greater than the sum of the core radii of the two; any auxiliary core and main core The parameter between the two is satisfied: the effective core of the main core has an effective refractive index lower than the effective refractive index of the auxiliary core mode.
2. The mode-less optical fiber device according to claim 1, wherein the main core and the auxiliary core have a circular cross section.
3. The mode-less optical fiber device according to claim 1, wherein an absolute value of a difference between an effective refractive index of the fundamental mode of the auxiliary core and an effective refractive index of any mode of the main core is greater than 0.0001.
4. The mode-less optical fiber device according to claim 1, wherein the auxiliary core supports only single mode transmission, that is, the normalized frequency V _f is required to satisfy V _f < 2.405.
5. The mode-less optical fiber device according to claim 1, wherein said auxiliary core satisfies: n _i-1 >n _f >n _i (I≥i>1) or n _f >n _i (i= 1); where n _{i is} the effective refractive index of the i-th mode of the main core, I is the total number of modes of the main core, and has n _i-1 >n _i (I≥i>1), n _f is The effective refractive index of the fundamental mode of the auxiliary core. The auxiliary core is the auxiliary core of the i-th mode of the main core, and can be used for inputting/outputting the i-th mode of the main core.
6. A mode-less optical fiber device according to claim 1, wherein a quadruple degenerate mode of the main core has two supporting auxiliary cores, a central axis of the two auxiliary cores and a main core The angle between the planes determined by the central axis satisfies 90°/m; m is an integer greater than or equal to 1, and refers to the logarithm of the maximum value of the transverse electric field along the circumference of the quadruple degenerate mode of the main core.
7. The mode-less optical fiber device according to claim 1, wherein the relationship between the number M of the auxiliary cores and the number I of the main core modes and the number N of the main core quadruple degeneracy modules is: M=I+N Wherein the central axis of one of the auxiliary cores is in the same plane as the central axis of the main core, and the auxiliary core is defined as the first type of auxiliary core for respectively inputting and outputting the I mode of the main core; The central axes of the N auxiliary cores are not in this plane, defined as the second type of auxiliary cores, which are used to input/output the other of the N quadruple degenerate modes of the main core.
8. A mode-less optical fiber device according to claim 7, wherein the center axis of the auxiliary core is at the main The projection length on the central axis of the core is such that the projection length of the central axis of any of the first type of auxiliary cores on the central axis of the main core is greater than the central axis of any of the second type of auxiliary cores on the central axis of the main core The projection length.
9. The mode-less optical fiber device according to claim 7, wherein the first type of auxiliary core has one and only one matching main core mode; and the second type of auxiliary core has one and only one matched main fiber. The quadruple degenerate mode of the core.
10. The mode-less optical fiber device according to claim 7, wherein the effective refractive index of the fundamental mode of the first type of auxiliary core is greater than the effective refractive index n ₁ of the fundamental mode of the main core.

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