阅读说明：本技术 一种基于空间三维波导的简并模式组的模式解复用器 (Mode demultiplexer based on degenerate mode group of spatial three-dimensional waveguide ) 是由 李巨浩 杨宇 陈章渊 何永琪 于 2019-07-18 设计创作，主要内容包括：本发明公开了一种基于空间三维波导的简并模式组的模式解复用器,其特征在于,包括包括一根少模波导和两根单模波导；第一单模波导的一端与该少模波导耦合,且耦合位置处第一单模波导的轴向与少模波导的轴向相同,另一端经一S型弯曲波导与一单模直波导一端连接；第二单模波导的一端与该少模波导耦合、另一端经一S型弯曲波导与一单模直波导一端连接；两单模直波导的另一端与一两模波导一端耦合,且圆心连线形成的夹角为90°；该少模波导的端口1接收混合模式的少模信号；该两模波导的另一端将输入少模波导的模式LP<Sub>pqa</Sub>和LP<Sub>pqb</Sub>对应的耦合至两模波导的LP<Sub>11a</Sub>和LP<Sub>11b</Sub>后输出；其中,p≠0。(The invention discloses a mode demultiplexer of a degenerate mode group based on a space three-dimensional waveguide, which is characterized by comprising a few-mode waveguide and two single-mode waveguides; one end of the first single mode waveguide is coupled with the few-mode waveguide, the axial direction of the first single mode waveguide at the coupling position is the same as the axial direction of the few-mode waveguide, and the other end of the first single mode waveguide is connected with one end of a single-mode straight waveguide through an S-shaped bent waveguide; one end of the second single-mode waveguide is coupled with the few-mode waveguide, and the other end of the second single-mode waveguide is connected with one end of a single-mode straight waveguide through an S-shaped bent waveguide; the other ends of the two single-mode straight waveguides are coupled with one end of one two-mode waveguide, and the included angle formed by the circle center connecting lines is 90 degrees; the port 1 of the few-mode waveguide receives a few-mode signal of a mixed mode; the other end of the two-mode waveguide correspondingly couples the modes LPpqa and LPpqb input into the few-mode waveguide to LP11a and LP11b of the two-mode waveguide and then outputs the modes; wherein p ≠ 0.)

1. A mode demultiplexer based on a degenerate mode group of a space three-dimensional waveguide is characterized by comprising a few-mode waveguide and two single-mode waveguides; one end of the first single mode waveguide is coupled with the few-mode waveguide, the axial direction of the first single mode waveguide at the coupling position is the same as the axial direction of the few-mode waveguide, and the other end of the first single mode waveguide is connected with one end of a single-mode straight waveguide through a first S-shaped bent waveguide; one end of the second single-mode waveguide is coupled with the few-mode waveguide, the axial direction of the second single-mode waveguide at the coupling position is the same as the axial direction of the few-mode waveguide, and the other end of the second single-mode waveguide is connected with one end of a single-mode straight waveguide through a second S-shaped bent waveguide; the other ends of the two single-mode straight waveguides are coupled with one end of one of the two single-mode waveguides, the axial directions of the coupling positions are the same, and the circle centers of the two single-mode straight waveguides and the circle center connecting line of the two single-mode straight waveguides form an included angle of 90 degrees; one end of the few-mode waveguide is marked as a port 1 and used for receiving a mixed mode few-mode signal, and the two single-mode waveguides are used for demultiplexing modes LPpqa and LPpqb contained in a degenerate mode group in the few-mode signal; the other end of the two-mode waveguide is marked as port 2, and is used for correspondingly coupling the modes LPpqa and LPpqb input into the few-mode waveguide to LP11a and LP11b of the two-mode waveguide and then outputting the modes; the other end of the few-mode waveguide is denoted as port 3, and is used for outputting other signals except for modes LPpqa and LPpqb in the few-mode signal, wherein p is not equal to 0.

2. The mode demultiplexer of claim 1, wherein the centers of the two single-mode waveguides at the coupling position form an angle β with a line connecting the centers of the few-mode waveguides; the value of β is determined by the spatial distribution of the patterns LPpqa and LPpqb.

3. The pattern demultiplexer of claim 2, wherein a theoretical value of β is β ═ (2m "1) pi/(2 p), where m is a natural number.

4. A pattern demultiplexer as claimed in claim 3, wherein the value of β is not less than 90 °.

5. the mode demultiplexer of claim 3 or 4, wherein when p is 1, β is 90 °; p is 2, β is 135 °; p is 3 and β is 90 °.

6. The mode demultiplexer of claim 1, wherein a curvature of the first S-bend waveguide conforms to a curvature of the second S-bend waveguide, and a radius of curvature is greater than or equal to 3 cm.

7. The mode demultiplexer of claim 1, wherein the effective indices of LPpqa and LPpqb in the few-mode waveguide are equal to the effective index of LP01 in the single-mode waveguide.

8. The mode demultiplexer of claim 1, wherein the effective refractive index of the LP11a and LP11b modes in the two-mode waveguide is equal to the effective refractive index of the LP01 mode in the single-mode waveguide.

9. A cascaded mode demultiplexer based on the mode demultiplexer of claim 1, comprising a first mode coupler, a second mode coupler and a plurality of mode demultiplexers, wherein one end of the few-mode waveguide of the first mode coupler as a first stage is used as a few-mode signal input end, and the other end of the few-mode waveguide of the first mode coupler as a first stage is connected to the port 1 of the mode demultiplexer of a next stage, the port 3 of the mode demultiplexer is connected to the port 1 of the mode demultiplexer of the next stage, the second mode coupler as a last stage, and the few-mode waveguide signal input end of the second mode coupler is connected to the port 3 of the mode demultiplexer of the previous stage.

10. A mode demultiplexing method based on a degenerate mode group of a space three-dimensional waveguide comprises the following steps:

1) Injecting mixed mode few-mode signals into few-mode waveguides, and demultiplexing LPpqa and LPpqb in the few-mode waveguides by using two single-mode waveguides;

2) Two S-shaped bent waveguides are used for separating the outlets of the two single-mode waveguides from the outlet of the few-mode waveguide and outputting two single-mode signals;

3) And respectively inputting the two single-mode signals into a single-mode straight waveguide, and respectively coupling output signals of the two single-mode straight waveguides to LP11a and LP11b in the two-mode waveguide for output.

Technical Field

The invention relates to a demultiplexing structure of a degenerate mode group, in particular to a demultiplexer for demultiplexing and recombining paths respectively based on the degenerate mode group of a space three-dimensional waveguide, which can be applied to the technical field of new-generation information such as optical fiber communication, optical information processing and the like.

Background

In recent years, in order to meet the capacity demand of an optical fiber communication system, the mode division multiplexing technology has become one of the research hotspots in the optical fiber communication field, and a mode multiplexer/demultiplexer, which is one of the key technologies, has attracted attention. Since the cross talk between modes within each mode group is large, but the cross talk between mode groups is low. Therefore, in a long-distance analog-division multiplexing system, a multiple-input multiple-output digital signal processing technology is required to eliminate crosstalk between modes. For short-distance transmission scenes such as a super computer, a data center and the like, each mode group carries one path of information without using multiple-input multiple-output digital signal processing, and higher software and hardware costs are saved in a mode of sacrificing transmission capacity.

The modes in a generally circularly symmetric few-mode fiber can be characterized by Linear Polarization (LP) modes. In the LPpq (p is 0,1,2, …; q is 1,2,3, …) modes supported by the LPpq, the spatial distribution of all the p is 0 modes is circularly symmetric, and the spatial distribution is a mode group. The spatial distribution of all modes with p ≠ 0 is non-circularly symmetric, and comprises two degenerate modes LPpqa and LPpqb, the effective refractive index difference of the two degenerate modes is extremely small, the spatial distribution in the fiber core is only different in the angle direction and is often classified into a mode group called LPpq degenerate mode group. At the transmitting end of the few-mode fiber mode group multiplexing system, only one of LPpqa and LPpqb modes in the LPpq degenerate mode group is generally excited. Due to perturbations such as imperfections in the fiber geometry, bending of the fiber, and temperature variations, the mode rotates after transmission in the fiber and the rotation is gradual over time. For the receiving end, it is equivalent to receive two degenerate modes, i.e. LPpq degenerate mode sets, of LPpqa and LPpqb simultaneously. A common mode demultiplexer generally uses the multiplexer in reverse, i.e. the few-mode optical signal is injected from the few-mode fiber and converted into LP01 mode in the single-mode fiber. This approach has no problem for the circularly symmetric pattern LP0q pattern. However, for the non-circularly symmetric degenerate modes LPpqa and LPpqb, the normal mode demultiplexer can only demultiplex one of them, so that a degenerate mode group demultiplexer capable of simultaneously receiving two degenerate modes (i.e., LPpq degenerate mode group) of LPpqa and LPpqb at the receiving end is required.

The existing scheme I is as follows: the two degenerate modes LPpqa and LPpqb are respectively demultiplexed by using two single-mode waveguides in different directions of the few-mode waveguide (the orientation is determined by the spatial distribution of the two degenerate modes LPpqa and LPpqb), then two single-mode signals are received by using a photoelectric detector, and finally two electric signals are combined by using a digital signal processing technology. The disadvantage of this approach is the high hardware and software costs due to the need to use digital signal processing.

The existing scheme is as follows: a fused biconical taper-based low-mode fiber degenerate mode group demultiplexer is disclosed, which is used for biconical taper of a low-mode fiber and another different two-mode fiber, so that two degenerate modes LPpqa and LPpqb can be simultaneously demultiplexed to LP11a and LP11 b. The same principle can also be implemented in the form of a waveguide. However, because the directional coupling condition between the high-order modes is severe and the coupling of the two modes is considered at the same time, the device implementation difficulty is too high, the coupling region is longer, and the device efficiency is generally lower.

Disclosure of Invention

Aiming at the problems existing in the existing scheme, the invention aims to provide a mode demultiplexer for respectively demultiplexing and recombining paths based on degenerate mode groups of a spatial three-dimensional waveguide.

since the purpose of the present invention is to design for a degenerate set of modes, all p ≠ 0 hereinafter.

Once again, at the transmitting end in a short-range mode-division multiplexing system, the degenerate mode sets LPpqa and LPpqb are typically excited out of one of them, both LPpqa and LPpqb modes are generated at the receiving end due to the circular symmetry of the fiber and the effects of the transmission process, and the degenerate mode sets LPpqa and LPpqb carry the same signal.

The technical scheme of the invention is as follows:

A mode demultiplexer based on a degenerate mode group of a space three-dimensional waveguide is characterized by comprising a few-mode waveguide and two single-mode waveguides; one end of the first single mode waveguide is coupled with the few-mode waveguide, the axial direction of the first single mode waveguide at the coupling position is the same as the axial direction of the few-mode waveguide, and the other end of the first single mode waveguide is connected with one end of a single-mode straight waveguide through a first S-shaped bent waveguide; one end of the second single-mode waveguide is coupled with the few-mode waveguide, the axial direction of the second single-mode waveguide at the coupling position is the same as the axial direction of the few-mode waveguide, and the other end of the second single-mode waveguide is connected with one end of a single-mode straight waveguide through a second S-shaped bent waveguide; the other ends of the two single-mode straight waveguides are coupled with one end of one of the two single-mode waveguides, the axial directions of the coupling positions are the same, and the circle centers of the two single-mode straight waveguides and the circle center connecting line of the two single-mode straight waveguides form an included angle of 90 degrees; one end of the few-mode waveguide is marked as a port 1 and used for receiving a mixed mode few-mode signal, and the two single-mode waveguides are used for demultiplexing modes LPpqa and LPpqb contained in a degenerate mode group in the few-mode signal; the other end of the two-mode waveguide is marked as port 2, and is used for correspondingly coupling the modes LPpqa and LPpqb input into the few-mode waveguide to LP11a and LP11b of the two-mode waveguide and then outputting the modes; the other end of the few-mode waveguide is denoted as port 3, and is used for outputting other signals except for modes LPpqa and LPpqb in the few-mode signal, wherein p is not equal to 0.

Furthermore, the circle centers of the two single-mode waveguides at the coupling position respectively form an included angle beta with a connecting line of the circle centers of the few-mode waveguides; the value of β is determined by the spatial distribution of the patterns LPpqa and LPpqb.

further, the theoretical value of β is (2m-1) pi/(2 p), where m is a natural number.

Further, the value of β is not less than 90 °.

Further, when p is 1, β is 90 °; p is 2, β is 135 °; p is 3 and β is 90 °.

Further, the curvature of the first S-shaped curved waveguide is consistent with that of the second S-shaped curved waveguide, and the curvature radius is larger than or equal to 3 cm.

Further, the effective refractive indices of LPpqa and LPpqb in the few-mode waveguide are equal to the effective refractive index of LP01 in the single-mode waveguide.

Further, the effective refractive index of the LP11a and LP11b modes in the two-mode waveguide is equal to the effective refractive index of the LP01 mode in the single-mode waveguide.

A cascade mode demultiplexer is characterized by comprising a first mode coupler, a second mode coupler and a plurality of mode demultiplexers, wherein one end of a few-mode waveguide of the first mode coupler as a first stage is used as a few-mode signal input end, the other end of the few-mode waveguide of the first mode coupler as the first stage is connected with a port 1 of the mode demultiplexer of the next stage, a port 3 of the mode demultiplexer is connected with the port 1 of the mode demultiplexer of the next stage, the second mode coupler as the last stage, and the few-mode waveguide signal input end of the second mode coupler is connected with the port 3 of the mode demultiplexer of the previous stage.

A mode demultiplexing method based on a degenerate mode group of a space three-dimensional waveguide comprises the following steps:

1) injecting mixed mode few-mode signals into few-mode waveguides, and demultiplexing LPpqa and LPpqb in the few-mode waveguides by using two single-mode waveguides;

2) Two S-shaped bent waveguides are used for separating the outlets of the two single-mode waveguides from the outlet of the few-mode waveguide and outputting two single-mode signals;

3) and respectively inputting the two single-mode signals into a single-mode straight waveguide, and respectively coupling output signals of the two single-mode straight waveguides to LP11a and LP11b in the two-mode waveguide for output.

The scheme of the present invention is as shown in fig. 1, a mixed mode few-mode signal is injected from a port 1 into a few-mode waveguide, two single-mode waveguides (phase matching conditions need to be satisfied, and the following section will be elaborated) are used to demultiplex LPpqa and LPpqb in the few-mode waveguide respectively, three waveguides are axially the same, the included angle between the centers of the three waveguides is β, the size of β is determined by the spatial distribution of the two modes LPpqa and LPpqb (the value of β is not unique, theoretically β ═ 1 pi/(2 p), where m ═ 1,2, and 3 … …, but to prevent two single-mode waveguides from being coupled with each other due to too close distance, the value of β cannot be lower than 90 °, when several sets of common values are given by the above two limiting conditions, β ═ 90 °,/2 ═ 135 °,/3, β ═ 90 °,/so that the wavelength of the two single-mode waveguides can not be coupled with each other waveguides, and the wavelength of the two waveguides can not be, this step is referred to as the degenerate mode set demultiplexing separately, as shown by the dashed box (1) in fig. 1. Secondly, the S-shaped bent waveguide is usually used for separating the outlet position of the waveguide, two S-shaped bent waveguides are used for separating the outlets of the two single-mode waveguides from the outlet of the few-mode waveguide, the two single-mode signals are led to the position of the port 2 to be processed in the next step, the optical path difference of the two single-mode signals is required to be ensured to be as small as possible, the optical path difference is required to be smaller than hundreds of micrometers, and the two S-shaped bent waveguides can be realized only by basically consistent curves; on the other hand, the radius of curvature of the S-bend waveguide needs to be on the order of centimeters (generally, 3cm or more), so as to reduce the power loss of the S-bend waveguide. The last step is called combining, two single-mode signals respectively enter a single-mode straight waveguide after passing through the S-shaped curved waveguide, the two single-mode straight waveguides and the two single-mode straight waveguides are arranged in parallel, the axial directions are the same, the included angle between the circle centers is 90 °, and under the condition of meeting the phase matching, the signals of the two single-mode straight waveguides are respectively coupled to LP11a and LP11b in the two single-mode waveguides and output from the port 2, as shown by a dashed box (2) in fig. 1. The other modes injected from port 1 are output from port 3.

The signal injected into the demultiplexer is generally a mixed signal light of a plurality of modes, and generally the effective refractive indices of the degenerate mode groups LPpqa and LPpqb are almost equal. The phase matching condition during demultiplexing means that the effective refractive index of LPpqa and LPpqb in the few-mode waveguide is equal to the effective refractive index of LP01 in the single-mode waveguide (the effective refractive index of waveguides with different thicknesses can be calculated by using commercial simulation software), so that LPpqa and LPpqb can be coupled into the single-mode waveguide as much as possible, the thickness of the few-mode waveguide is usually adjusted to realize phase matching (the thickness of the waveguide is usually changed by a section of tapered waveguide meeting adiabatic approximation conditions), the phase matching condition is a relatively severe condition, the requirement on the thickness of the waveguide is high, and the difference cannot be larger than 0.2 μm in weak waveguide (the refractive index difference between a waveguide core and a cladding is far smaller than 1); meanwhile, when in combination, proper parameters of the two-mode waveguides need to be designed, generally speaking, given refractive index difference is given, and effective refractive indexes of LP11a and LP11b modes in the two-mode waveguides with different thicknesses are calculated to be equal to the effective refractive index of the LP01 mode in the single-mode waveguide.

Because the optical signals in the two single-mode waveguides carry the same signal, the directions of the two single-mode waveguides need to be as similar as possible to ensure that the optical path difference experienced by the two single-mode signals is as small as possible, otherwise, a larger time delay is introduced, which is not beneficial to correctly receiving the signal, the rate of the signal is set to be 25Gbaud/s, and the rate of the light in the waveguide is about 2 × 108m/s, so the requirement on the optical path difference is not high, and the difference of hundreds of micrometers can be correctly received.

When in combination, the two single-mode signals are coupled into LP11a and LP11b of a two-mode waveguide, instead of coupling one signal into LP01 and the other signal into LP11a, or higher-order modes (such as LP21a and LP21 b). The reason is as follows: first, LP11a and LP11b are two orthogonal modes, have the same effective refractive index, can coexist in the waveguide, and are different from the single-mode waveguide position in coupling, but the simultaneous coupling is also facilitated; secondly, the coupling is carried out in a mode with a lower order, so that the tolerance requirement on the structure is low, and the processing is convenient; thirdly, the area of the high-speed photoelectric detector for receiving light is small, the optical fiber coupling efficiency with the small core diameter is higher, and the receiving of electric signals is more convenient.

Further, for mixed mode signals at the receiving end, the invention needs to divide the signals into two types. Firstly, for a mode LPpq (p ≠ 0), the mode LPpq is received by respectively demultiplexing and recombining the degenerate modes; second, for the mode LPpq (p ═ 0), the mode can be demultiplexed using a common mode coupler, i.e., one single mode waveguide. Finally, the invention can cascade the demultiplexers of each mode, and the cascade order can be arranged in turn according to the effective refractive indexes of the modes from small to large. As shown in fig. 2, the present invention and a common coupler can receive four signals after being cascaded: one signal is in LP02 mode, one signal is in LP21 mode group (the transmitting end only excites LP21a mode, but due to factors such as non-ideal circular symmetry of the optical fiber and rotation of the mode in the optical fiber, the receiving end contains LP21a and LP21b, and LP21a and LP21b carry the same signal), one signal is in LP11 mode group (LP11a and LP11b carry the same signal), and the other signal is in LP 01.

Compared with the prior art, the invention has the following positive effects:

According to the scheme provided by the invention, the degenerate mode groups are respectively demultiplexed and then combined in the waveguide, so that the digital signal processing during the combination of the electric signals is avoided, the implementation cost is greatly reduced, and meanwhile, the two orthogonal modes are still formed after the combination, and the influence of interference is avoided; the degenerate mode group is demultiplexed to a single-mode signal and then combined to the LP11a and LP11b, so that the severe condition required by the interconversion of high-order modes is avoided, and the coupling efficiency can be improved.

Drawings

FIG. 1 is a schematic diagram of a degenerate mode set demultiplexer.

Fig. 2 is a schematic diagram of a receive-side demultiplexer cascade.

fig. 3 is a diagram of a demultiplexer waveguide structure of degenerate mode set LP 21.

FIG. 4 is a schematic diagram of a degenerate mode group LP21 demultiplexing and recombining;

(a) The mode conversion process of LP21a, and (b) the mode conversion process of LP21 b.

Fig. 5 is the insertion loss of the degenerate mode set LP21 demultiplexer.

Detailed Description

in order to make the objects, technical solutions and advantages of the present invention more apparent, the following is a detailed description of the present invention by taking a method for manufacturing a degenerate mode set LP21 demultiplexer as an example and combining with the accompanying drawings. It should be understood that the specific examples described herein are intended to be illustrative only and are not intended to be limiting.

FIG. 3 is an example of demultiplexing a degenerate mode group LP21, the few-mode waveguide of this example supporting four modes LP01, LP11, LP21, and LP 02. The whole device design is that in borosilicate glass with background refractive index of 1.4877, the few-mode waveguide has refractive index of 1.499 and diameter of 12.18 μm; the refractive index of each of the single-mode waveguides a and b is 1.4928, and the diameter of each of the single-mode waveguides is 8.2 mu m; the two-mode waveguide has a refractive index of 1.499 and a diameter of 8.18 μm. Under the parameters, the effective refractive indexes of the five modes of the few-mode waveguide LP21a and LP21b, the single-mode waveguide LP01 and the two-mode waveguide LP11a and LP11b are equal, namely, the phase matching condition of directional coupling is satisfied. The waveguide pitch (here waveguide edge-to-edge distance) is 5 μm in this example.

Mixed mode signals in the few-mode waveguide are injected from a port 1, and pass through a dotted line frame (1), LP21a and LP21b in the few-mode waveguide are coupled into LP01 in single modes a and b respectively, the length of a coupling region is 2000 mu m, and the step is called as degenerate mode group demultiplexing respectively; two paths of single-mode signals respectively pass through different S-shaped bent waveguides, the spatial positions of the waveguides are adjusted, preparation is made for the next step, the S-shaped bent waveguides are designed only to ensure that the bending loss is small, generally, the curvature radius is larger than 3cm, and meanwhile, because the spatial paths of the single-mode waveguides are similar, the lengths of the two paths of waveguides are naturally approximately equal; the two single-mode signals are respectively coupled into the LP11a and the LP11b of the same two-mode waveguide in a dotted line frame (2), the length of a coupling region is 3045 microns, and the step is called combining. The combined light comes out from port 2 and the other modes in the few-mode waveguide that cannot be demultiplexed come out from port 3.

Because the mode field distributions of LP21a and LP21b are shown in fig. 4, LP21a can only be coupled into LP01 of a single-mode waveguide in the horizontal or vertical direction, and LP21b can only be coupled into LP01 of a single-mode waveguide in the 45 ° or 135 ° direction, and in order to reduce the coupling between two single-mode waveguides, the distance between the two single-mode waveguides is as far as possible, so that the connecting line between the centers of the three waveguides can only be 135 °, and the spatial distribution of the waveguides is shown by the dashed box (1) in fig. 3. And due to mode field limitations of LP11a and LP11b, the design is needed when the two are combined, as shown by the dashed box (2) in fig. 3.

The device design in this example is about 17810 μm long in the y-direction, about 127 μm long in the x-direction, and about 30 μm long in the z-direction, and since the length in the y-direction is much greater than the other two directions, the S-bend waveguide design hardly affects the length difference of the two single mode waveguides.

the demultiplexer insertion loss for the LP21 mode group as a function of wavelength is shown in fig. 5. At a wavelength of 1.55 μm, the insertion loss has a minimum value of 0.07 dB; at a wavelength of 1.565 μm, the insertion loss is 1.89 dB; at a wavelength of 1.53 μm, the insertion loss is 3.84 dB. In general, the insertion loss of the C wave band commonly used in optical fiber communication is not higher than 3.84dB, which indicates that the device has very good demultiplexing efficiency.

the present invention is not limited to the above embodiments, and those skilled in the art can implement the present invention in other various embodiments according to the disclosure of the present invention, so that all designs and concepts of the present invention can be changed or modified without departing from the scope of the present invention.