Terahertz multi-branch waveguide directional coupler and design method thereof

Abstract

The invention relates to the technical field of terahertz passive, in particular to a terahertz multi-branch waveguide directional coupler and a design method thereof, and the design method comprises the steps: determining the size of a main waveguide, the size of an auxiliary waveguide and the number of branch waveguides based on the working frequency of the directional coupler; the initial length a of each branch waveguide is set as the length d1 of the main waveguide or the length d2 of the auxiliary waveguide; based on the cut-off frequency, the initial length a of each branch waveguide is optimized to obtain the final length a'of each branch waveguide smaller than the length d1 of the main waveguide and the length d2 of the auxiliary waveguide; and based on the final length a'of each branch waveguide, optimizing the initial width b of the branch waveguides at the two ends and the initial width c of the middle branch waveguide to obtain the final width b'of the branch waveguides at the two ends and the final width c 'of the middle branch waveguide. The minimum size of the directional coupler provided by the invention, namely the width of the branch waveguide, is increased, the processing difficulty and the production cost are reduced, the use stability is improved, and the service life is prolonged.

Description

Technical Field

[0001] The present invention relates to the technical field of terahertz passive technologies, and particularly relates to a terahertz multi-branch waveguide directional coupler and a design method thereof. Background Art

[0002] Due to the special position of terahertz waves in the electromagnetic wave spectrum, between microwaves and infrared radiation, they exhibit many unique properties different from other types of electromagnetic waves and have broad application prospects in wireless communication, biomedicine, deep space exploration, national defense security, etc.

[0003] A directional coupler is a commonly used four-port passive device in microwave circuits and is often used for power synthesis and distribution. Studying directional couplers in the terahertz frequency band has important engineering application value.

[0004] The branch waveguide directional coupler is developed from the branch-line coupler and has advantages such as high isolation and low insertion loss. It is a commonly used form of directional coupler in the terahertz frequency band.

[0005] The circuit size of the branch waveguide directional coupler will shrink as the frequency increases. Due to the high frequency in the terahertz frequency band, it poses a high challenge to the processing difficulty and precision, and at the same time, it will also lead to an increase in the device cost. Generally speaking, the narrowest part of the existing branch waveguide directional coupler is the most difficult part to process. A common structure of the branch waveguide directional coupler is as Figure 1 shown, consisting of two parallel main and secondary waveguides and several branch waveguides therebetween. Among them, the side length a 1 of the branch waveguide is usually equal to the long side length of the main and secondary waveguides, and the widths b 1 of the two end branch waveguides and the width c 1 of the middle branch waveguide are related to the frequency and may be less than 0.1 mm. 0.1 mm approaches the processing limit of CNC (Computer Numerical Control Lathe) technology, with high processing difficulty and difficult to guarantee precision. The width of each branch waveguide is usually the smallest dimension of the entire structure and is also the place with the highest processing difficulty. If the width dimension is too small, it may be forced to use a more difficult and costly processing technology. And because this width is extremely sensitive to processing precision, if the processing precision is insufficient, the device performance will deteriorate significantly. Even if the processing is completed, the use stability and lifespan may be poor due to the structure being too fragile. Summary of the Invention

[0006] The purpose of the present invention is to overcome the problems of small width of the branch waveguide and high processing difficulty of the existing terahertz multi-branch waveguide directional coupler, and thus provide a terahertz multi-branch waveguide directional coupler and a design method thereof. This directional coupler increases the width of the branch waveguide, which is beneficial to reducing the processing difficulty and improving the reliability and service life of the device operation.

[0007] To solve the above technical problems, a design method of a terahertz multi-branch waveguide directional coupler provided by the technical solution of the present invention includes:

[0008] Based on the operating frequency of the directional coupler, determine the dimensions of the main waveguide, the dimensions of the secondary waveguide, and the number of branch waveguides; and set the initial length a of each branch waveguide to be the length d of the main waveguide 1 or the length d of the secondary waveguide 2 ;

[0009] Based on the cut-off frequency, optimize the initial length a of each branch waveguide to obtain the final length a' of each branch waveguide that is less than the length d of the main waveguide 1 and the length d of the secondary waveguide 2 ;

[0010] Based on the final length a' of each branch waveguide, optimize the initial width b of the two end branch waveguides and the initial width c of the middle branch waveguide to obtain the final width b' of the two end branch waveguides and the final width c' of the middle branch waveguide.

[0011] As an improvement of the above method, the middle branch waveguide includes one or more branch waveguides, and the plurality of branch waveguides have the same or different initial widths c of the middle branch waveguide; the two end branch waveguides have the same or different initial widths b of the two end branch waveguides.

[0012] As an improvement of the above method, the initial width c of the middle branch waveguide and the initial width b of the two end branch waveguides are obtained by the odd-even mode analysis method.

[0013] As an improvement of the above method, the dimensions of the main waveguide and the secondary waveguide are the same.

[0014] As an improvement of the above method, the final length a' of each branch waveguide is obtained by the following method:

[0015] Using the cut-off frequency f c and the minimum value e of the initial width b of the two end branch waveguides and the initial width c of the middle branch waveguide, calculate the minimum length a of each branch waveguide min :

[0016]

[0017] where c 0 is the speed of light, m = 1, p = 0;

[0018] Set the length value range R of each branch waveguide, where a min ≤R<a; where a is the initial length of each branch waveguide;

[0019] Select the final length a' of each branch waveguide within the range R of the length values of each branch waveguide.

[0020] As an improvement to the above method, the final length a' of each branch waveguide is selected by the following method:

[0021] Using three-dimensional electromagnetic simulation software, simulate the length data within the range R of the length values of each branch waveguide, and use the length data corresponding to the maximum isolation, minimum insertion loss, maximum directivity, or widest operating bandwidth as the final length a' of each branch waveguide.

[0022] As an improvement to the above method, the final width b' of the two end branch waveguides and the final width c' of the middle branch waveguide are obtained by the following method:

[0023] Input the final length a' of each branch waveguide into the three-dimensional electromagnetic simulation software, and use the width of the two end branch waveguides corresponding to the maximum isolation, minimum insertion loss, maximum directivity, or widest operating bandwidth as the final width b' of the two end branch waveguides, and use the width of the middle branch waveguide corresponding to the maximum isolation, minimum insertion loss, maximum directivity, or widest operating bandwidth as the final width c' of the middle branch waveguide.

[0024] As an improvement to the above method, the method further includes: calculating and obtaining the final height h' of each branch waveguide and the final spacing w' of each branch waveguide by using the final length a' of each branch waveguide, the final width b' of the two end branch waveguides, the final width c' of the middle branch waveguide, the dimensions of the main waveguide, the dimensions of the sub-waveguide, and the number of branch waveguides.

[0025] To achieve another object of the present invention, the present invention also provides a terahertz multi-branch waveguide directional coupler, which is made by the above design method of the terahertz multi-branch waveguide directional coupler.

[0026] The advantages of the present invention are that the present invention proposes a terahertz multi-branch waveguide directional coupler and its design method. Compared with the traditional structure, the minimum size of the directional coupler, that is, the width of the branch waveguide, is increased, reducing the processing difficulty and the tolerance for processing accuracy, reducing the production cost, and being beneficial to improving the use stability and service life of the device. Description of the Drawings

[0027] Figure 1 Schematic diagram of the structure of a three-branch waveguide coupler in the prior art;

[0028] Figure 2 Schematic diagram of the structure of a three-branch waveguide coupler with narrowed branches provided according to an embodiment of the present invention;

[0029] Figure 3 For Figure 2 Front view and dimension marking of the embodiment;

[0030] Figure 4 For Figure 2 Side view and dimension marking of the embodiment;

[0031] Figure 5 For Figure 2 S-parameter simulation of the embodiment. Detailed implementation manners

[0032] The technical solutions provided by the present invention will be further described below in conjunction with embodiments.

[0033] Embodiment: Taking the 510 GHz coupler shown in Figures 2 - 4 as an example:

[0034] The main and auxiliary waveguides adopt WR-1.5 waveguides, that is, d 1 = d 2 = 0.38 mm, g = 0.19 mm, and the long side length a of the traditional branch waveguide 1 = d 1 = d 2 = 0.38 mm.

[0035] Use the design method of the traditional branch waveguide coupler to determine the initial values. Assume a three-branch line directional coupler, the width of the central branch line is c, the widths of the left and right branch lines are b, and the center spacing and height of the branch lines are both 1 / 4 of the working wavelength. Then the even-mode matrix of this network is as follows:

[0036]

[0037] Among them, the 1st, 3rd, and 5th matrices are the transmission matrices of the one-eighth wavelength open circuit lines, and the 2nd and 4th matrices are the transmission matrices of the quarter wavelength transmission lines. Just replace b in the above formula with -b to obtain the odd-mode matrix of this network:

[0038]

[0039] For an ideal 3 dB directional coupler, there should be no reflection in both even and odd modes. For a determined c, there are two possible values for b. Usually, the smaller value is taken to reduce the junction effect, that is:

[0040]

[0041] In addition, the output amplitude |S 21 | of port 2 and the output amplitude |S 31 | of port 3 should be equal, that is, |S 21 | = |S 31|, and the ideal values ​​of b and c are obtained by combining them. Substituting the center frequency into the initial values ​​of the branch line sizes, b = 0.1mm. c = 0.17mm.

[0042] Using the cutoff frequency f c The minimum value e of the initial width b of the branch waveguides at both ends and the initial width c of the middle branch waveguide is used to calculate the minimum length a of each branch waveguide. min :

[0043]

[0044] Among them, c 0 is the speed of light, m ​​= 1, p = 0;

[0045] Set the length range of each branch waveguide to R, where a min ≤R<a; where a is the initial length of each branch waveguide.

[0046] The final length a′ of each branch waveguide is selected within the length value range R of each branch waveguide.

[0047] Specifically, three-dimensional electromagnetic simulation software, such as Ansys HFSS, CST MICROWAVE STUDIO, or COMSOL, is used to simulate the length data within the length value range R of each branch waveguide, and the length data corresponding to the maximum isolation value, the minimum insertion loss value, the maximum directivity value, or the widest working bandwidth value is used as the final length a′ of each branch waveguide.

[0048] Specifically for the branch waveguide, according to the initial value calculation, b is taken as 0.1mm. To ensure at least 10% working bandwidth of the branch waveguide coupler, that is, to ensure that the cutoff frequency of the main mode TE10 mode is higher than 460GHz, that is, m=1, n=0, it is calculated that the length of the long side of the branch waveguide to be shortened a′ should not be less than 0.34mm.

[0049] Extended to the waveguide directional coupler with n+2 branches, assuming that the width of the branches on the left and right sides is b, the width of the remaining branches is c, and the center spacing w and the branch height h of each branch are both 1 / 4 of the working wavelength, the corresponding network even-mode matrix can be obtained

[0050] In this way, the initial values ​​of the widths b and c of each branch waveguide of the multi-branch waveguide directional coupler can be obtained.

[0051] The final width b' of the branch waveguides at both ends and the final width c' of the middle branch waveguide are obtained by the following method:

[0052] Input the final length a′ of each branch waveguide into the three-dimensional electromagnetic simulation software, and take the width of the two end branch waveguides corresponding to the maximum isolation, minimum insertion loss, maximum directivity or widest operating bandwidth as the final width b′ of the two end branch waveguides, and take the width of the middle branch waveguide corresponding to the maximum isolation, minimum insertion loss, maximum directivity or widest operating bandwidth as the final width c′ of the middle branch waveguide.

[0053] Then, using the final length a′ of each branch waveguide, the final width b′ of the two end branch waveguides, the final width c′ of the middle branch waveguide, the dimensions of the main waveguide, the dimensions of the secondary waveguide, and the number of branch waveguides, calculate and obtain the final height h′ of each branch waveguide and the final spacing w′ of each branch waveguide.

[0054] Basic design process:

[0055] First, select the dimensions of the main and secondary waveguides according to the operating frequency, and then determine the number of branches. When the coupling degree is determined, when the number of branches increases, there will usually be a wider operating bandwidth and better amplitude flatness, but the width of each branch line will become narrower. Considering the processing difficulty and performance requirements comprehensively, select an appropriate number of branch lines.

[0056] After determining the number of branch lines of the coupler, calculate the initial value of the width of each branch line through the odd-even mode analysis method. On the premise of ensuring no cut-off occurs within the operating frequency band, appropriately narrow the length of the branch waveguide in exchange for a wider width of the branch line.

[0057] Through optimization, obtain the final width b′ of the two end branch waveguides, the final width c′ of the middle branch waveguide, the final height h′ of each branch waveguide, and the final spacing w′ of each branch waveguide to obtain the optimal structure.

[0058] Therefore, the final length a′ of the long side of the shortened branch waveguide in the present invention has a value range of 0.34 - 0.38 mm. The preferred value of this design is 0.35 mm, and further optimize each value according to specific requirements and simulation results. Finally, the other values are: h′ = 0.22 mm, w 1 ′ = 0.12 mm, w 2 ′ = 0.14 mm, b 1 ′ = 0.11 mm, c′ = 0.14 mm, b 2 ′ = 0.12 mm; where h′ is the final height of each branch waveguide; w 1 ′ is the final spacing between the first end branch waveguide and its adjacent branch waveguide; w 2 ′ is the final spacing between the second end branch waveguide and its adjacent branch waveguide; b 1 ′ is the final width of the first end branch waveguide, c′ is the final width of the middle branch waveguide, b2 ′ is the final width of the first-end branch waveguide.

[0059] The simulated S-parameter diagram is as Figure 5 shown. At the center operating frequency, the isolation is 33.6 dB. A high isolation of better than 30 dB can be maintained over a bandwidth of more than 10 GHz, and an isolation of better than 20 dB can be maintained over a bandwidth of 30 GHz. At the same time, the return loss is better than 20 dB, and it has good operating performance and can be used for signal isolation, distribution, and synthesis in terahertz systems, etc.

[0060] To achieve similar performance using a traditional five-branch waveguide directional coupler, the narrowest dimension of the entire structure is 0.08 mm, approaching the processing limit of the CNC process from the cost perspective. The processing is difficult and the accuracy is difficult to guarantee. By narrowing the branch design, the narrowest dimension of the entire structure is increased to 0.11 mm, and it can be more easily processed at low cost using the CNC process. Moreover, the structural stability and service life of the device are also improved.

[0061] As can be seen from the above specific description of the present invention, the present invention modifies the long-side length of the branch waveguide from being the same as that of the main and secondary waveguides in the traditional branch waveguide coupler to being slightly less than the long-side lengths of the main and secondary waveguides, thereby widening the width of the branch waveguide at the narrowest part and reducing the processing difficulty. Since in the branch waveguide directional coupler, the branch waveguide serves as the energy path for electromagnetic waves to be coupled from the main waveguide to the secondary waveguide, when the length of the branch waveguide is narrowed, widening the width can keep the coupling degree roughly unchanged, so that the width of the branch waveguide is increased compared with the traditional structure. It should be noted that the length of the branch waveguide cannot be infinitely narrowed, otherwise the electromagnetic waves in this frequency band will be cut off and unable to propagate. Compared with the traditional branch waveguide directional coupler, the present invention can reduce the processing difficulty and the requirements for processing accuracy under the condition of similar performance, thereby reducing the production cost of the device.

[0062] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not restrictive. Although the present invention has been described in detail with reference to the embodiments, those of ordinary skill in the art should understand that any modification or equivalent replacement of the technical solutions of the present invention does not depart from the spirit and scope of the technical solutions of the present invention, and they should all be covered within the scope of the claims of the present invention.

Claims

1. A design method for a terahertz multi-branch waveguide directional coupler, comprising: Based on the operating frequency of the directional coupler, determining the dimensions of the main waveguide, the dimensions of the secondary waveguide, and the number of branch waveguides; And set the initial length a of each branch waveguide to be the main waveguide length d 1 or the secondary waveguide length d 2 ; Based on the cut-off frequency, optimize the initial length a of each branch waveguide to obtain a final length a' of each branch waveguide that is less than the length d of the main waveguide 1 and the length d of the secondary waveguide 2 ; Based on the final length a' of each branch waveguide, optimizing the initial width b of the two end branch waveguides and the initial width c of the middle branch waveguide to obtain the final width b' of the two end branch waveguides and the final width c' of the middle branch waveguide.

2. The design method for a terahertz multi-branch waveguide directional coupler according to claim 1, wherein, The middle branch waveguide includes one or more branch waveguides, and the plurality of branch waveguides have the same or different initial widths c of the middle branch waveguide; the two end branch waveguides have the same or different initial widths b of the two end branch waveguides.

3. The design method for a terahertz multi-branch waveguide directional coupler according to claim 2, wherein, The initial width c of the middle branch waveguide and the initial width b of the two end branch waveguides are obtained by the odd-even mode analysis method.

4. The design method for a terahertz multi-branch waveguide directional coupler according to claim 1, wherein, The dimensions of the main waveguide and the secondary waveguide are the same.

5. The design method for a terahertz multi-branch waveguide directional coupler according to claim 1, wherein, The final length a' of each branch waveguide is obtained by the following method: Using the cut-off frequency f c and the minimum value e of the initial width b of the two end branch waveguides and the initial width c of the middle branch waveguide, the minimum length a of each branch waveguide is calculated and obtained min : where c 0 is the speed of light, m is the first mode index, p is the second mode index, where m = 1 and p = 0; Set the value range R of the length of each branch waveguide, where a min ≤R < a; where a is the initial length of each branch waveguide; Selecting the final length a' of each branch waveguide within the length value range R of each branch waveguide.

6. The design method for a terahertz multi-branch waveguide directional coupler according to claim 5, wherein, The final length a' of each branch waveguide is selected by the following method: Using three-dimensional electromagnetic simulation software, simulating the length data within the length value range R of each branch waveguide, and taking the length data corresponding to the maximum isolation, the minimum insertion loss, the maximum directivity, or the widest operating bandwidth as the final length a' of each branch waveguide.

7. The design method for a terahertz multi-branch waveguide directional coupler according to claim 1, wherein, The final width b' of the two end branch waveguides and the final width c' of the middle branch waveguide are obtained by the following method: Inputting the final length a' of each branch waveguide into three-dimensional electromagnetic simulation software, taking the width of the two end branch waveguides corresponding to the maximum isolation, the minimum insertion loss, the maximum directivity, or the widest operating bandwidth as the final width b' of the two end branch waveguides, and taking the width of the middle branch waveguide corresponding to the maximum isolation, the minimum insertion loss, the maximum directivity, or the widest operating bandwidth as the final width c' of the middle branch waveguide.

8. The design method for a terahertz multi-branch waveguide directional coupler according to claim 1, wherein, The method further includes: using the final length a' of each branch waveguide, the final width b' of the two end branch waveguides, the final width c' of the middle branch waveguide, the dimensions of the main waveguide, the dimensions of the secondary waveguide, and the number of branch waveguides to calculate and obtain the final height h' of each branch waveguide and the final spacing w' of each branch waveguide.

9. A terahertz multi-branch waveguide directional coupler, characterized in that, the directional coupler is fabricated by using the method according to any one of claims 1-8.

Images