Terahertz frequency doubling structure and method based on nonlinear transmission line

Abstract

The application discloses a terahertz frequency doubler structure and method based on a nonlinear transmission line. The terahertz frequency doubler structure comprises an input waveguide-microstrip transition structure, a low-pass filter, a nonlinear transmission line, a band-pass filter and an output microstrip-waveguide transition structure which are cascaded in sequence. The nonlinear transmission line comprises a coplanar waveguide transmission line and a plurality of nonlinear capacitor devices arranged along the transmission direction of the coplanar waveguide transmission line. The nonlinear capacitor devices are connected in parallel to corresponding loading nodes of the coplanar waveguide transmission line. The equivalent capacitance of the nonlinear capacitor devices under working bias and the spacing between adjacent loading nodes are configured to regulate the dispersion relation of the nonlinear transmission line, so that the fundamental wave and the second harmonic satisfy the phase matching condition. The application regulates the capacitance of the nonlinear capacitor devices and the spacing between adjacent loading nodes, so that the fundamental wave and the second harmonic satisfy the phase matching condition, and the output efficiency of the target second harmonic is improved.

Description

Technical Field

[0001] This invention relates in particular to a terahertz second harmonic structure and method based on a nonlinear transmission line, belonging to the field of terahertz signal generation technology. Background Technology

[0002] The terahertz spectrum lies between microwaves and far-infrared waves, and is defined as frequencies between 0.1 THz. Electromagnetic waves in the 10 THz range possess extremely rich spectral resources and, due to their high transmittance, low energy loss, and high bandwidth, show broad application prospects in fields such as spectral analysis, imaging technology, and wireless communication. A core component driving the application of terahertz technology is the terahertz light source, and the development of a high-response-bandwidth, high-power room-temperature terahertz light source is crucial for achieving breakthroughs in terahertz technology.

[0003] Currently, Schottky diode terahertz frequency multipliers are typically implemented using an integration method that combines external GaAs bulk diode pairs with the circuit structure. However, ensuring consistency and robustness during device integration is challenging.

[0004] In contrast, GaN technology offers advantages such as planarity and ease of integration. However, the cutoff frequency of existing GaN diodes still lags significantly behind that of GaAs devices, making it difficult to meet the high cutoff frequency requirements in terahertz frequency doubling, resulting in lower frequency conversion efficiency. Therefore, these factors limit the further application of GaN devices in the field of high-efficiency terahertz frequency doubling. Summary of the Invention

[0005] The main objective of this invention is to provide a terahertz second harmonic structure and method based on a nonlinear transmission line. Phase matching is achieved by utilizing the dispersion modulation of the nonlinear transmission line, thereby significantly improving the terahertz second harmonic efficiency while maintaining the advantages of planar integration. This structure can effectively improve the terahertz second harmonic efficiency while ensuring device consistency and integrability, and is suitable for high-frequency, highly integrated terahertz signal sources and related system applications.

[0006] To achieve the aforementioned objectives, the technical solution adopted by this invention includes: The first aspect of the present invention provides a terahertz frequency doubler structure based on a nonlinear transmission line, which includes an input waveguide-microstrip transition structure, a low-pass filter, a nonlinear transmission line, a bandpass filter, and an output microstrip-waveguide transition structure cascaded sequentially along the signal transmission direction. The nonlinear transmission line includes a coplanar waveguide transmission line and a plurality of nonlinear capacitors arranged along the transmission direction of the coplanar waveguide transmission line. The nonlinear capacitors are connected in parallel at the corresponding loading nodes of the coplanar waveguide transmission line, and the capacitance value of the nonlinear capacitors changes nonlinearly with the applied voltage. The distance between the equivalent capacitance of the nonlinear capacitor under operating bias and the adjacent loading node is configured to: regulate the dispersion relationship of the nonlinear transmission line so that the fundamental wave and the second harmonic satisfy the phase matching condition.

[0007] This invention replaces the traditional centralized single-tube frequency doubling structure by distributing multiple nonlinear capacitors onto a coplanar waveguide transmission line to form a nonlinear transmission line, thereby enabling the second harmonic to be distributed along the transmission line. Simultaneously, by coordinating the design of the equivalent capacitance of the nonlinear capacitors under operating bias and the spacing between adjacent loading nodes, the dispersion characteristics of the nonlinear transmission line are actively controlled, ensuring that the fundamental frequency and the second harmonic satisfy the phase matching condition. This solves the problem of low efficiency caused by insufficient cutoff frequency and dispersion in GaN devices, achieving efficient and integrable terahertz second harmonic conversion.

[0008] In one implementation, multiple nonlinear capacitors are periodically arranged along the transmission direction of the coplanar waveguide transmission line to form a periodically loaded nonlinear transmission line structure. By adjusting the equivalent capacitance and the spacing, the Bragg cutoff frequency of the nonlinear transmission line is controlled, thereby controlling the dispersion relation to meet the phase matching condition.

[0009] In one embodiment, the nonlinear capacitor is an AlGaN / GaN Schottky varactor diode.

[0010] As one implementation, the terahertz frequency multiplier structure based on the nonlinear transmission line further includes: a lead electrode structure, wherein the lead electrode structure is connected to an external driving circuit through leads to provide a DC operating bias voltage for the nonlinear capacitor in the nonlinear transmission line.

[0011] In one implementation, the input side of the input waveguide-microstrip transition structure is connected to a WR-10 standard rectangular waveguide, and the output side of the output microstrip-waveguide transition structure is connected to a WR-4 standard rectangular waveguide.

[0012] In one implementation, both the input waveguide-microstrip transition structure and the output microstrip-waveguide transition structure are microstrip E-plane probe coupling structures.

[0013] In one implementation, the low-pass filter is configured to allow the fundamental signal to pass through and be fed into the nonlinear transmission line while suppressing the reverse transmission of higher harmonic signals at the input. The bandpass filter is configured to: select the target second harmonic signal and output it to the output microstrip-waveguide transition structure, while reflecting the fundamental signal and the non-target higher harmonic signal back to the nonlinear transmission line, so that the reflected signal participates in the nonlinear frequency conversion again.

[0014] In one implementation, the passband of the low-pass filter covers the fundamental frequency band of 90 GHz to 110 GHz, and the passband of the band-pass filter covers the second harmonic frequency band of 180 GHz to 220 GHz.

[0015] A second aspect of this invention provides a terahertz frequency doubling method based on a nonlinear transmission line, applied to the aforementioned terahertz frequency multiplier structure based on a nonlinear transmission line, and comprising: The distance between the equivalent capacitance of the nonlinear capacitor under operating bias and the adjacent loading node is adjusted to regulate the dispersion relationship of the nonlinear transmission line so that the fundamental wave and the second harmonic satisfy the phase matching condition.

[0016] As one implementation method, the terahertz frequency doubling method based on nonlinear transmission lines includes: S1. The terahertz fundamental wave signal in the input waveguide is coupled to the microstrip transmission path through the input waveguide-microstrip transition structure, and after the stray harmonics are filtered out by the low-pass filter, it is fed into the nonlinear transmission line. S2. The nonlinear capacitance characteristics of the distributed parallel nonlinear capacitors on the nonlinear transmission line are used to generate a frequency comb signal of higher harmonics from the transmitted fundamental signal; by pre-configuring the equivalent capacitance of the nonlinear capacitors under the working bias and the spacing between adjacent loading nodes, the dispersion relationship of the nonlinear transmission line is adjusted so that the fundamental and second harmonics meet the phase matching condition. S3. Bandpass filtering is performed on the frequency comb signal output from the nonlinear transmission line to select the target second harmonic, while the non-target frequency components are reflected back to the nonlinear transmission line to participate in another nonlinear transformation. S4. The selected target second harmonic is coupled to the output waveguide through the output microstrip-waveguide transition structure to complete the output.

[0017] In one implementation, the frequency range of the terahertz fundamental wave signal is 90 GHz to 110 GHz, and the frequency range of the target second harmonic is 180 GHz to 220 GHz.

[0018] In one implementation, the phase matching condition is that the phase shift constant corresponding to the second harmonic is matched with twice the phase shift constant corresponding to the fundamental wave.

[0019] Compared with the prior art, the advantages of the present invention include: The terahertz frequency doubler structure and method based on nonlinear transmission lines provided by this invention have the following beneficial effects: 1) Overcoming the cutoff frequency limitation of GaN devices and improving frequency doubling efficiency: By constructing a periodically loaded nonlinear transmission line (NLTL) structure, distributed nonlinear capacitors are used to replace centralized single transistors, and the Bragg cutoff frequency and dispersion relationship are controlled by adjusting the equivalent capacitance and node spacing, so that the fundamental frequency and the second harmonic satisfy the phase matching condition (Δ). (≈0). This design effectively overcomes the conversion efficiency bottleneck of GaN diodes due to their relatively low cutoff frequency and dispersion, achieving efficient double-frequency output in the 200 GHz band.

[0020] 2) Combining the advantages of planar integration with high consistency: Utilizing AlGaN / GaN heterojunction Schottky varactor diodes as nonlinear components, combined with a coplanar waveguide transmission line design, this solution is entirely based on planar micro / nano manufacturing processes. Compared to traditional external GaAs bulk diodes or complex air-bridge structures, this approach avoids the consistency issues caused by discrete assembly of multiple devices, significantly improving device yield and making it more suitable for large-scale array integration and engineering applications in terahertz systems.

[0021] 3) Harmonic Energy Recovery and Spectral Purification: Through the coordinated design of the input low-pass filter and the output band-pass filter, not only is precise gating of the target second harmonic (180 GHz~220 GHz) and effective feeding of the fundamental frequency (90 GHz~110 GHz) achieved, but non-target frequency components are also reflected back to the nonlinear transmission line to participate in a second nonlinear transformation. This mechanism, which combines frequency gating and energy recovery, significantly improves the energy utilization rate of the frequency comb generated by the NLTL, further enhancing the output power and spectral purity of the target harmonic. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of the discrete equivalent circuit model of the nonlinear transmission line according to an embodiment of the present invention; Figure 2 This is a structural layout diagram of the terahertz frequency doubler according to an embodiment of the present invention; Figure 3 This is a schematic diagram of the three-dimensional structure of the AlGaN / GaN heterojunction varactor diode according to an embodiment of the present invention; Figure 4 This is a diagram illustrating the system architecture and signal flow of a terahertz frequency doubler according to an embodiment of the present invention. Figure 5 This is a graph showing the frequency-phase relationship within the first passband of a nonlinear transmission line according to an embodiment of the present invention. Figure 6 This is a simulation diagram of the output spectrum of the terahertz frequency doubler according to an embodiment of the present invention. Detailed Implementation

[0023] In view of the shortcomings of the prior art, the inventors of this invention have proposed the technical solution of the present invention through long-term research and extensive practice.

[0024] Existing Schottky diode terahertz frequency multipliers typically employ an external bulk material diode pair integrated with the circuit structure. However, for large-scale array applications, ensuring consistency during device integration is difficult, hindering large-scale system integration and engineering applications. GaN technology offers advantages such as planarity and ease of integration, making it suitable for large-scale array structures. However, the cutoff frequency of existing GaN diodes is still significantly lower than that of GaAs devices. This makes it difficult to meet the high cutoff frequency requirements in terahertz frequency multiplication and mixing devices, resulting in lower frequency conversion efficiency. These shortcomings limit the frequency conversion efficiency of GaN-based terahertz frequency multipliers, thus restricting their further application in the field of high-efficiency terahertz frequency multiplication.

[0025] To address the problems existing in the prior art, this invention proposes a nonlinear transmission line (NLTL) terahertz frequency doubler based on AlGaN / GaN heterojunction. While ensuring good device consistency and integrability, the Bragg cutoff frequency of the NLTL can be adjusted by designing the equivalent capacitance parameters of the diodes loaded in the transmission line (i.e., the equivalent capacitance of the diodes under operating bias, which can also be understood as the capacitance under the optimal operating bias voltage) and their spatial distribution spacing (i.e., the spacing between the corresponding loaded nodes of the transistors connected in parallel to the coplanar waveguide transmission line). This allows the second harmonic to meet the phase matching condition with the fundamental frequency, thereby improving the terahertz frequency doubling efficiency and meeting the application requirements of high-frequency, highly integrated terahertz signal sources.

[0026] This invention introduces a nonlinear transmission line (NLTL) structure as a frequency multiplier. By designing the equivalent capacitance parameters of the diodes loaded in the transmission line and their spatial spacing, the Bragg cutoff frequency of the NLTL can be adjusted, thereby ensuring that the second harmonic and the fundamental frequency meet the phase matching condition. Theoretically, it can be deduced that the second harmonic output amplitude reaches its maximum when the phase of the second harmonic is twice the phase of the fundamental frequency, thus achieving efficient second harmonic conversion.

[0027] To make the objectives, technical solutions, and advantages of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Unless otherwise defined, all technical and scientific terms used in this invention have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0028] Example 1 like Figure 2 and Figure 4 As shown, a terahertz frequency doubler structure based on a nonlinear transmission line is disclosed. The structure includes an input waveguide-microstrip transition structure, a low-pass filter, a nonlinear transmission line, a bandpass filter, and an output microstrip-waveguide transition structure cascaded sequentially along the signal transmission direction.

[0029] Specifically, the input waveguide-microstrip transition structure uses a WR-10 standard rectangular waveguide as its input waveguide to receive terahertz fundamental signals in the 90 GHz to 110 GHz frequency band; the output microstrip-waveguide transition structure uses a WR-4 standard rectangular waveguide as its output waveguide to output target second harmonic signals in the 180 GHz to 220 GHz frequency band. In this embodiment, both the input waveguide-microstrip transition structure and the output microstrip-waveguide transition structure employ a microstrip E-plane probe coupling structure. This structure utilizes probe insertion into the waveguide E-plane to achieve field mode conversion, offering a wider bandwidth and lower insertion loss compared to traditional ridge waveguide transitions. This effectively ensures efficient feeding of the fundamental signal and low-loss output of the second harmonic signal, enhancing the compatibility of the device with external standard waveguide systems.

[0030] For details, please refer to Figure 2 The microstrip transmission line is inserted into the waveguide in the form of a microstrip E-plane probe to couple electromagnetic waves from the WR10 waveguide to the microstrip transmission line. The coupled signal first passes through a low-pass filter with a passband range of 0–110 GHz to suppress higher harmonic components and ensure that the fundamental signal operating in the 90 GHz–110 GHz band is smoothly fed into the nonlinear transmission line (NLTL), thereby achieving the frequency doubling function.

[0031] The nonlinear transmission line is the core frequency multiplication unit in this embodiment. It includes a coplanar waveguide transmission line and multiple nonlinear capacitors arranged along the transmission direction of the coplanar waveguide transmission line. The nonlinear capacitors are connected in parallel at the corresponding loading nodes of the coplanar waveguide transmission line, and the capacitance value of the nonlinear capacitors changes nonlinearly with the applied voltage.

[0032] like Figure 1 , Figure 2 and Figure 4 As shown, multiple nonlinear capacitors are periodically arranged along the transmission direction of the coplanar waveguide transmission line to form a periodically loaded nonlinear transmission line structure. By adjusting the equivalent capacitance and the spacing between adjacent loading nodes, the Bragg cutoff frequency of the nonlinear transmission line is controlled, thereby controlling the dispersion relation to meet the phase matching condition.

[0033] Specifically, this periodic arrangement means that the dispersion characteristics of the nonlinear transmission line are no longer determined solely by the geometry of the transmission line itself, but rather depend on the loading period and the load capacitance. By precisely designing the spacing of the loading nodes and the equivalent capacitance of the nonlinear capacitors under operating bias, the Bragg cutoff frequency can be set at a position much higher than the target second harmonic frequency, while simultaneously establishing a specific phase shift constant relationship at the fundamental and second harmonic frequencies.

[0034] In this embodiment, the nonlinear capacitor is preferably an AlGaN / GaN Schottky varactor diode, such as... Figure 3 As shown, this device utilizes a two-dimensional electron gas channel formed by an AlGaN / GaN heterojunction to create a variable capacitor. The anode employs a Schottky contact, and the cathode uses an ohmic contact. Compared to the traditional approach using external GaAs bulk diode pairs, this avoids the consistency issues caused by the discreteness of multiple devices, and is more conducive to large-scale array integration and engineering implementation. Common Schottky diode terahertz frequency multipliers typically employ an air-bridge structure, which is complex and difficult to fabricate. The varactor diode in this invention is implemented using planar GaN technology, which is simpler and easier to integrate.

[0035] To achieve efficient frequency doubling, the equivalent capacitance of the nonlinear capacitor under operating bias and the spacing between adjacent loading nodes are configured to: regulate the dispersion relationship of the nonlinear transmission line so that the fundamental wave and the second harmonic satisfy the phase matching condition.

[0036] This embodiment modulates the phase relationship between the target harmonic and the fundamental wave through the aforementioned periodic loading structure, so that the fundamental wave and the second harmonic can satisfy a specific phase relationship in the 90 GHz~110 GHz fundamental wave band and the 180 GHz~220 GHz second harmonic wave band, thereby suppressing the problem of low efficiency caused by dispersion.

[0037] In the signal processing link, the low-pass filter is configured to allow the fundamental signal to pass through and be fed into the nonlinear transmission line, while suppressing the reverse transmission of high-order harmonic signals at the input end; the band-pass filter is configured to select the target second harmonic signal and output it to the output microstrip-waveguide transition structure, while reflecting the fundamental signal and the non-target high-order harmonic signal back to the nonlinear transmission line, so that the reflected signal participates in the nonlinear frequency conversion again.

[0038] Specifically, the low-pass filter's passband covers the fundamental frequency band from 90GHz to 110GHz, ensuring lossless fundamental transmission while preventing high-order harmonics generated by the nonlinear transmission line from returning and interfering with the source stage. The band-pass filter's passband covers the second harmonic frequency band from 180GHz to 220GHz. By utilizing the filter's high-reflection stopband characteristics, it reflects incompletely converted fundamental energy, as well as the generated third and higher harmonic energy, back into the nonlinear transmission line. These reflected signals are transmitted in reverse within the NLTL and interact with the forward signal, undergoing another nonlinear mixing process, where some energy is reconverted into useful second harmonics. This synergistic mechanism of filtering, frequency selection, and energy recovery significantly improves the energy utilization of the frequency comb generated by the NLTL, resulting in superior second harmonic power and spectral purity compared to traditional unidirectional transmission frequency multiplier architectures.

[0039] Specifically, please refer to Figure 1 , Figure 2 and Figure 4 , Figure 1 The equivalent circuit diagram of a nonlinear transmission line is shown. Let the first... The instantaneous voltage at each loading node is The equivalent series inductance of the transmission line segment between adjacent loading nodes is , No. The equivalent nonlinear capacitance at each loading node is: Then the voltage relationship between adjacent loading nodes can be expressed as: (1) in, , and The first , No. and the Instantaneous voltage at each loading node; This is the equivalent series inductance of the transmission line segment between adjacent loading nodes; For the first The equivalent nonlinear capacitance at each loading node varies with the node voltage. change; For time.

[0040] When the spacing between adjacent loading nodes When the voltage is much smaller than the operating wavelength, the discrete node voltage can be approximated as a voltage that varies continuously along the transmission direction. .in, Let the coordinates be along the direction of propagation of the nonlinear transmission line. For position ,time The instantaneous voltage at that point, at this time: (2) right and exist Performing a Taylor expansion at this point, and ignoring higher-order terms, we obtain: (3) The equivalent inductance and equivalent capacitance per unit length of a nonlinear transmission line are defined as follows: (4) in, The equivalent inductance per unit length is expressed in H / m. The equivalent capacitance per unit length is expressed in F / m. This is the equivalent nonlinear capacitance at a single loading node, in F; Let m be the distance between adjacent loading nodes. Substituting equations (3) and (4) into equation (1), we can obtain that the instantaneous voltage on the nonlinear transmission line approximately satisfies: (5) in, This includes the equivalent capacitance of the coplanar waveguide transmission line itself and the voltage-dependent capacitance introduced by the loaded nonlinear capacitor. Equation (5) shows that the voltage-dependent capacitance... This will enter the transmission line voltage equation, thus providing a nonlinear source for harmonic generation.

[0041] Specifically, AlGaN / GaN Schottky varactor diodes exhibit voltage-dependent capacitance characteristics under DC operating bias. Nearby, its equivalent capacitance can be expanded in first order as follows: (6) in, For work bias Equivalent capacitance per unit length at a given location. The capacitance-voltage slope at this operating point. For normalized nonlinear coefficients, This refers to the high-frequency voltage component superimposed on the DC bias.

[0042] Substituting equation (6) into equation (5), we can obtain that the nonlinear terms in the transmission line equation include: (7) When the input fundamental voltage is: (8) Then we have: (9) Therefore, the nonlinear term includes an angular frequency of The second harmonic component indicates that the loaded AlGaN / GaN Schottky varactor diode can act as a nonlinear capacitor source, generating a second harmonic signal on the nonlinear transmission line.

[0043] To describe the phase relationship between the fundamental and second harmonic frequencies in a periodically loaded structure, a loading period can be equivalent to a two-port network. This periodic element is assumed to consist of a coplanar waveguide transmission line and a nonlinear capacitor loaded at the node, with a phase frequency of 60°C. The following is the transfer matrix: (10) in, , , and These are the transfer matrix elements of the loading unit in this cycle, used to describe the relationship between the input voltage and current and the output voltage and current of this cycle unit. This refers only to matrix elements in the transmission matrix, and not the aforementioned nonlinear capacitance. .

[0044] According to the Bloch-Floquet condition for the periodic structure, the complex propagation constant corresponding to this periodically loaded unit can be expressed as: (11) in, Angular frequency The complex propagation constant under the following conditions, This represents the spacing between adjacent loaded nodes. Its imaginary part is the phase shift constant. (12) From equations (10) to (12), it can be seen that the distance between the loaded capacitor and the adjacent loaded node is... This will affect the transfer matrix elements of the periodically loaded unit, and further affect the phase shift constant. The fundamental angular frequency is The second harmonic angular frequency is The corresponding phase shift constants are respectively and The phase mismatch between the fundamental wave and the second harmonic can be expressed as: (13) Ignoring higher-order harmonic feedback and considering the approximate accumulation of the second harmonic along the transmission direction, the magnitude of the second harmonic voltage can be expressed as being proportional to the following factor: (14) in, For the transmission distance of a nonlinear transmission line, Let be the attenuation constant of the second harmonic. From equation (14), it can be seen that when... hour, To achieve the maximum value, thereby improving the output amplitude and frequency doubling efficiency of the target second harmonic.

[0045] Therefore, this invention adjusts the equivalent capacitance of the nonlinear capacitor and the spacing between adjacent loading nodes. Adjusting the dispersion relation of the nonlinear transmission line to satisfy the approximate phase-matching condition: (15) In addition, the terahertz frequency doubler structure also includes a lead electrode structure, which is connected to an external drive circuit via leads to provide a DC operating bias voltage for the nonlinear capacitor in the nonlinear transmission line.

[0046] Specifically, since the equivalent capacitance of an AlGaN / GaN Schottky varactor diode is highly dependent on the applied bias voltage, and the phase matching condition is strictly dependent on this equivalent capacitance, a stable DC bias is a necessary condition for maintaining the device operating at its optimal doubling point. The lead electrode structure is connected to the external ceramic substrate circuitry via a chip PAD, applying a precisely controllable DC voltage to each nonlinear capacitor. It should be understood that although this embodiment demonstrates a method of power supply via a unified lead, in other embodiments, segmented biasing or independent biasing networks can be used to accommodate differences in the optimal bias point of diodes at different locations due to RF power attenuation, further optimizing the overall phase matching performance.

[0047] Example 2 A terahertz frequency doubling method based on a nonlinear transmission line is provided. This method is applied to the terahertz frequency multiplier structure based on a nonlinear transmission line described in Example 1. The method includes: adjusting the equivalent capacitance of the nonlinear capacitor under the working bias and the spacing between adjacent loading nodes to control the dispersion relationship of the nonlinear transmission line so that the fundamental wave and the second harmonic satisfy the phase matching condition.

[0048] Specifically, the present invention changes the equivalent capacitance of the nonlinear capacitor by precisely setting or fine-tuning the DC bias voltage, and in conjunction with a fixed node spacing parameter, actively controls the dispersion characteristics of the transmission line.

[0049] Please see Figure 2 and Figure 4 The specific execution process of this terahertz frequency second harmonic method includes the following steps: Step S1: The terahertz fundamental wave signal in the input waveguide is coupled to the microstrip transmission path through the input waveguide-microstrip transition structure, and then fed into the nonlinear transmission line after the stray harmonics are filtered out by a low-pass filter.

[0050] In one specific application scenario of this method, the frequency range of the terahertz fundamental signal is 90 GHz to 110 GHz. Specifically, this step achieves efficient conversion from a three-dimensional waveguide mode to a planar microstrip mode and pre-cleans the input signal's spectrum using a low-pass filter. This not only ensures low-loss transmission in the target fundamental frequency band but also effectively blocks potential parasitic harmonics from entering the nonlinear transmission line, avoiding interference from unintended mixing products in subsequent frequency doubling processes, thus laying a clean signal foundation for high-quality frequency conversion.

[0051] Step S2: Utilize the nonlinear capacitance characteristics of the distributed parallel nonlinear capacitors on the nonlinear transmission line to generate a frequency comb signal of higher harmonics from the transmitted fundamental signal; By pre-configuring the equivalent capacitance of the nonlinear capacitors under operating bias and the spacing between adjacent loading nodes, the dispersion relationship of the nonlinear transmission line is controlled to ensure that the fundamental and second harmonics meet the phase matching condition, thereby maximizing the amplitude of the output target harmonic.

[0052] Step S3: Bandpass filtering is performed on the frequency comb signal output by the nonlinear transmission line to select the target second harmonic, while the non-target frequency components are reflected back to the nonlinear transmission line to participate in another nonlinear transformation.

[0053] Step S4 involves coupling the selected target second harmonic to the output waveguide via the output microstrip-waveguide transition structure to complete the output. In this specific implementation scenario, the target second harmonic's frequency range is 180 GHz to 220 GHz. Specifically, the 200 GHz band signal, after selection and energy enhancement in step S3, is efficiently converted back to the WR-4 standard rectangular waveguide mode via the output microstrip-waveguide transition structure. This step ensures that the final output terahertz signal not only has high frequency doubling efficiency but also good spectral purity and standard interface compatibility, meeting the actual requirements of terahertz systems for high-quality light sources. It should be understood that although this embodiment uses 90 GHz~110 GHz to 180 GHz~220 GHz as an example for detailed explanation, the phase matching criterion and energy recovery mechanism established by this method are also applicable to frequency doubling applications in other terahertz bands, requiring only corresponding adjustments to the period parameters of the nonlinear transmission line and the filter passband.

[0054] Specifically, after NLTL processing, the output signal will simultaneously contain the fundamental frequency and multiple higher harmonic components. To ensure the spectral purity of the output signal and improve the target second harmonic (2... fThe frequency doubling efficiency is 0.180 GHz to 220 GHz. The frequency-doubled signal is then frequency-selected by a bandpass filter. The bandpass filter is used to select the second harmonic component and suppress the fundamental and other higher harmonic components, while reflecting non-target frequency components back into the NLTL to participate in the nonlinear action again, thereby improving the overall frequency doubling efficiency.

[0055] Combination Figure 1 The equivalent circuit diagram shows that when the fundamental signal propagates on the coplanar waveguide transmission line, the nonlinear capacitor at each loading node generates a nonlinear current due to the change in capacitance value with the instantaneous voltage, which in turn excites spectral components including the second harmonic in the frequency domain.

[0056] This method establishes a strict phase-matching condition through the aforementioned dispersion modulation, namely, the phase shift constant corresponding to the second harmonic is matched with twice the phase shift constant corresponding to the fundamental wave: (See Equation 15), combined with Figure 5 The frequency-phase curve shown represents the frequency-phase relationship within the first passband of the nonlinear transmission line. This condition corresponds to a specific operating point on the dispersion curve, as illustrated in the figure. 2=2 1 and 2 =2 1 The location.

[0057] Application Examples To further verify the performance of the terahertz frequency doubler structure and method based on nonlinear transmission lines provided by the present invention in practical applications, this application embodiment conducted joint simulation verification based on the technical solutions described in Embodiments 1 and 2 above.

[0058] Specifically, this application example sets up a terahertz signal generation scenario in the 200 GHz band, where the frequency range of the input terahertz fundamental signal covers 90 GHz to 110 GHz, and the target output second harmonic frequency range covers 180 GHz to 220 GHz. In this application scenario, the preferred structural parameters described in Example 1 are adopted, namely, the nonlinear transmission line is a coplanar waveguide transmission line with a characteristic impedance of 77 ohms, and nine AlGaN / GaN Schottky varactor diodes are periodically loaded as nonlinear capacitors. At the same time, the device bias and node spacing are configured strictly according to the phase matching conditions described in Example 2. The diodes designed in the simulation of this invention have an anode length of 0.2 μm, an anode width of 20 μm, and a cutoff frequency of 265 GHz.

[0059] like Figure 6The simulation results of the terahertz frequency doubler output spectrum shown indicate that, under the optimal DC operating bias, the terahertz frequency doubler achieves a frequency doubling efficiency of approximately 15% at a center frequency of 200 GHz.

[0060] Specifically, it can be clearly observed from the spectrum that the target second harmonic component exhibits a significant main peak at 200 GHz, while the fundamental component and other non-target higher harmonic components are effectively suppressed, resulting in high purity of the output spectrum. This simulation result directly proves that the technical solution of this invention, which achieves phase matching by controlling the dispersion relation of the nonlinear transmission line, is indeed effective.

[0061] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the scope of protection of the present invention. For example, regarding the specific type of nonlinear capacitor in a nonlinear transmission line, the geometric parameters of a coplanar waveguide transmission line, the topology of a filter, and the specific values ​​of the operating frequency band, corresponding equivalent substitutions or adaptive adjustments can be made without departing from the core technical concept of the present invention of achieving phase matching by adjusting the dispersion relationship through adjusting the equivalent capacitance and node spacing. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

[0062] This invention provides a novel 200 GHz terahertz frequency doubler structure. The nonlinear transmission line (NLTL) is constructed by periodically loading nonlinear capacitors for frequency double conversion in the terahertz band. The Bragg cutoff frequency of the NLTL is adjusted by the equivalent capacitance and cell spacing to ensure phase matching during the frequency doubling process, thereby achieving high frequency doubling efficiency. From a system-level perspective, the NLTL generates a frequency comb, and a frequency gating filter selects the target harmonic. Non-target harmonics are reflected back to the NLTL for reuse, further improving the frequency doubling efficiency.

[0063] This invention proposes a terahertz frequency doubling scheme based on nonlinear capacitor devices (NLTLs). Harmonic phase matching is achieved through Bragg cutoff frequency modulation. This scheme achieves efficient terahertz frequency doubling while taking into account the high consistency and array-scalable advantages of GaN technology. At the same time, the energy utilization rate of the NLTL frequency comb and the target harmonic output efficiency are improved by using a filter circuit design that combines frequency gating and harmonic energy recovery.

[0064] Compared to traditional centralized Schottky diode frequency multipliers, this invention uses a periodically loaded nonlinear transmission line to achieve second-harmonic conversion. Traditional frequency multipliers typically rely on one or a few nonlinear diodes to complete frequency conversion, with the target harmonics mainly generated in the local junction region. Their output efficiency is easily limited by diode cutoff frequency, parasitic parameters, local impedance matching, and assembly consistency.

[0065] In this invention, a nonlinear transmission line periodically loads multiple nonlinear capacitors along a coplanar waveguide transmission line, so that the second harmonic is no longer generated by a single centralized nonlinear device, but is distributed at multiple loading nodes. By adjusting the equivalent capacitance of the nonlinear capacitors and the spacing between adjacent loading nodes, the dispersion relation of the nonlinear transmission line can be controlled, so that the second harmonic components generated at each loading node satisfy the approximate phase matching condition, thereby improving the output amplitude of the target second harmonic.

[0066] Furthermore, traditional nonlinear transmission lines are mostly used for pulse sharpening, broadband harmonic generation, or frequency comb output, with energy distributed across multiple frequency components, including the fundamental, second, third, and higher harmonics. This invention does not aim for broadband frequency comb output, but rather focuses on a targeted design for the 180 GHz–220 GHz second harmonic band. An input low-pass filter ensures effective feeding of the 90 GHz–110 GHz fundamental frequency, while an output band-pass filter selects the target second harmonic and suppresses non-target frequency components, allowing the harmonic energy generated by the nonlinear transmission line to be more concentrated for the target second harmonic output.

[0067] Therefore, this invention has the advantages of distributed nonlinear generation, phase matching accumulation, target harmonic gating and planar integration, which is beneficial to improving the second harmonic output efficiency in a specific terahertz frequency band and is suitable for high-integration terahertz signal source applications.

[0068] It should be understood that the above embodiments are merely illustrative of the technical concept and features of the present invention, and are intended to enable those skilled in the art to understand the content of the present invention and implement it accordingly. They should not be construed as limiting the scope of protection of the present invention. All equivalent changes or modifications made in accordance with the spirit and essence of the present invention should be covered within the scope of protection of the present invention.

Claims

1. A terahertz frequency doubler structure based on a nonlinear transmission line, characterized in that, It includes an input waveguide-microstrip transition structure, a low-pass filter, a nonlinear transmission line, a bandpass filter, and an output microstrip-waveguide transition structure, which are cascaded sequentially along the signal transmission direction. The nonlinear transmission line includes a coplanar waveguide transmission line and a plurality of nonlinear capacitors arranged along the transmission direction of the coplanar waveguide transmission line. The nonlinear capacitors are connected in parallel at the corresponding loading nodes of the coplanar waveguide transmission line, and the capacitance value of the nonlinear capacitors changes nonlinearly with the applied voltage. The distance between the equivalent capacitance of the nonlinear capacitor under operating bias and the adjacent loading node is configured to: regulate the dispersion relationship of the nonlinear transmission line so that the fundamental wave and the second harmonic satisfy the phase matching condition.

2. The terahertz frequency multiplier structure based on a nonlinear transmission line according to claim 1, characterized in that: Multiple nonlinear capacitors are periodically arranged along the transmission direction of the coplanar waveguide transmission line to form a periodically loaded nonlinear transmission line structure. By adjusting the equivalent capacitance and the spacing, the Bragg cutoff frequency of the nonlinear transmission line is controlled, thereby controlling the dispersion relation to meet the phase matching condition. Preferably, the nonlinear capacitor is an AlGaN / GaN Schottky varactor diode.

3. The terahertz frequency multiplier structure based on a nonlinear transmission line according to claim 1 or 2, characterized in that, Also includes: The lead electrode structure is connected to an external driving circuit via leads to provide a DC operating bias voltage for the nonlinear capacitor in the nonlinear transmission line.

4. The terahertz frequency multiplier structure based on a nonlinear transmission line according to claim 1, characterized in that: The input side of the input waveguide-microstrip transition structure is connected to the WR-10 standard rectangular waveguide, and the output side of the output microstrip-waveguide transition structure is connected to the WR-4 standard rectangular waveguide. Preferably, both the input waveguide-microstrip transition structure and the output microstrip-waveguide transition structure are microstrip E-plane probe coupling structures.

5. The terahertz frequency multiplier structure based on a nonlinear transmission line according to claim 1, characterized in that: The low-pass filter is configured to allow the fundamental signal to pass through and be fed into the nonlinear transmission line, while suppressing the reverse transmission of high-order harmonic signals at the input. The bandpass filter is configured to: select the target second harmonic signal and output it to the output microstrip-waveguide transition structure, while reflecting the fundamental signal and the non-target higher harmonic signal back to the nonlinear transmission line, so that the reflected signal participates in the nonlinear frequency conversion again.

6. The terahertz frequency multiplier structure based on a nonlinear transmission line according to claim 5, characterized in that: The low-pass filter has a passband covering the fundamental frequency band of 90 GHz to 110 GHz, and the band-pass filter has a passband covering the second harmonic frequency band of 180 GHz to 220 GHz.

7. A terahertz second harmonic method based on a nonlinear transmission line, characterized in that, The terahertz frequency multiplier structure based on a nonlinear transmission line, as described in any one of claims 1-6, comprises: The distance between the equivalent capacitance of the nonlinear capacitor under operating bias and the adjacent loading node is adjusted to regulate the dispersion relationship of the nonlinear transmission line so that the fundamental wave and the second harmonic satisfy the phase matching condition.

8. The terahertz frequency second harmonic method based on a nonlinear transmission line according to claim 7, characterized in that, include: S1. The terahertz fundamental wave signal in the input waveguide is coupled to the microstrip transmission path through the input waveguide-microstrip transition structure, and after the stray harmonics are filtered out by the low-pass filter, it is fed into the nonlinear transmission line. S2. The nonlinear capacitance characteristics of the distributed parallel nonlinear capacitors on the nonlinear transmission line are used to generate a frequency comb signal of higher harmonics from the transmitted fundamental signal; by pre-configuring the equivalent capacitance of the nonlinear capacitors under the working bias and the spacing between adjacent loading nodes, the dispersion relationship of the nonlinear transmission line is adjusted so that the fundamental and second harmonics meet the phase matching condition. S3. Bandpass filtering is performed on the frequency comb signal output from the nonlinear transmission line to select the target second harmonic, while the non-target frequency components are reflected back to the nonlinear transmission line to participate in another nonlinear transformation. S4. The selected target second harmonic is coupled to the output waveguide through the output microstrip-waveguide transition structure to complete the output.

9. The terahertz frequency second harmonic method based on nonlinear transmission lines according to claim 8, characterized in that: The frequency range of the terahertz fundamental wave signal is 90 GHz to 110 GHz, and the frequency range of the target second harmonic is 180 GHz to 220 GHz.

10. The terahertz frequency doubling method based on nonlinear transmission lines according to claim 8, characterized in that: The phase matching condition is that the phase shift constant corresponding to the second harmonic is matched with twice the phase shift constant corresponding to the fundamental wave.

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