Waveguide coupled nonlinear transmission line terahertz heterodyne detector structure

Abstract

The invention discloses a waveguide coupled nonlinear transmission line terahertz heterodyne detector structure. The structure comprises a waveguide structure used for collecting terahertz input signals; the microstrip transmission line is used for coupling and transmitting a terahertz input signal into the frequency mixer; the terahertz frequency mixer is used for performing heterodyne frequency mixing to obtain a terahertz response signal; the filter structure is used for isolating the terahertz response signal; the lead electrode structure is used for providing power supply voltage and outputting a terahertz response signal; wherein the terahertz frequency mixer comprises a variable capacitance diode and a coplanar waveguide transmission line, and the variable capacitance diode and the coplanar waveguide transmission line are matched to form a nonlinear transmission line. According to the invention, the variable capacitance diode is used as the mixer, shot noise is fundamentally suppressed, so that the device has the advantage of high sensitivity, the technical scheme of the NLTL is adopted, the device has the broadband characteristic, in addition, the scheme of waveguide coupling is adopted, terahertz light is incident from a waveguide port, the stability is high, and the device has high coupling efficiency.

Description

Technical Field

[0001] The present invention relates to a terahertz detector, and more particularly to a waveguide-coupled nonlinear transmission line terahertz heterodyne detector structure. Background Art

[0002] The terahertz spectrum lies between microwaves and far-infrared waves, defined as electromagnetic waves with frequencies in the range of 0.1 - 10 THz. It has extremely rich spectral resources and, due to its high transmittance, low energy loss, and high bandwidth, shows broad application prospects in fields such as spectral analysis, imaging technology, and wireless communication. The core components for promoting the application of terahertz technology include terahertz detectors and terahertz light sources. Among them, developing room-temperature terahertz detectors with high sensitivity and room-temperature terahertz light sources with high response bandwidth and high power are the keys to achieving breakthroughs in terahertz technology.

[0003] Nonlinear transmission line (NLTL) technology has been widely used in the microwave field and, due to its low-noise and broadband characteristics, shows great potential in fields such as frequency multiplication and mixing. In recent years, NLTL technology has gradually expanded to the millimeter-wave band and preliminary research results have been achieved. Further increasing the Bragg cut-off frequency of NLTL and the cut-off frequency of varactor diodes can promote its application expansion to the terahertz band, thereby achieving efficient terahertz signal conversion.

[0004] Currently, the room-temperature sensitivity of AlGaN / GaN high electron mobility transistor (HEMT) high-speed and high-sensitivity terahertz detectors still has a gap of 2 - 3 orders of magnitude compared to GaAs Schottky diode (6%D) detectors. The main reason is that under the drive of a high local oscillator power, a milliampere-level self-mixing DC signal will be generated in the detector channel, and the shot noise generated by this DC signal limits the sensitivity of the heterodyne detector. Summary of the Invention

[0005] Aiming at the deficiencies of the prior art, the purpose of the present invention is to provide a waveguide-coupled nonlinear transmission line terahertz heterodyne detector structure.

[0006] To achieve the aforementioned invention purpose, the technical solutions adopted by the present invention include:

[0007] In a first aspect, the present invention provides a waveguide-coupled nonlinear transmission line terahertz heterodyne detector structure, which includes:

[0008] A waveguide structure, at least for collecting terahertz input signals;

[0009] A microstrip transmission line, connected to the waveguide structure, at least for coupling and transmitting the terahertz input signal output by the waveguide structure into a mixer;

[0010] A terahertz mixer, connected to the microstrip transmission line, is at least used for heterodyne mixing of the received terahertz input signal to obtain a terahertz response signal;

[0011] A filter structure, connected to the terahertz mixer, is at least used to isolate the terahertz response signal from the terahertz input signal;

[0012] A lead electrode structure, connected to the filter structure, is at least used to provide a power supply voltage and output the terahertz response signal;

[0013] Wherein, the terahertz mixer includes a varactor diode and a coplanar waveguide transmission line, and the varactor diode and the coplanar waveguide transmission line cooperate to form a nonlinear transmission line.

[0014] Based on the above technical solutions, compared with the prior art, the beneficial effects of the present invention at least include:

[0015] The present invention uses a varactor diode with low-noise characteristics as a mixer. The varactor diode is in a negative bias operating state, and the transistor itself does not generate DC signals, fundamentally suppressing shot noise. Therefore, it has the advantage of high sensitivity. For the problem that the capacitance reactance of a single varactor diode is large and difficult to match, the present invention adopts the technical solution of NLTL. The terahertz heterodyne detector structure with NLTL nonlinear transmission line has broadband characteristics. In addition, the present invention adopts a waveguide coupling scheme, with terahertz light incident from the waveguide port, high stability, and high coupling efficiency.

[0016] The above description is only an overview of the technical solutions of the present invention. In order to enable those skilled in the art to more clearly understand the technical means of the present application and implement it in accordance with the content of the specification, the following is a detailed description with reference to the preferred embodiments of the present invention and the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 is an overall structural schematic diagram of a terahertz heterodyne detector structure with a nonlinear transmission line provided by a typical embodiment of the present invention;

[0018] Figure 2 is a schematic diagram of the varactor diode structure of a terahertz heterodyne detector structure with a nonlinear transmission line provided by a typical embodiment of the present invention;

[0019] Figure 3 is a schematic diagram of the simulation principle of a terahertz heterodyne detector structure with a nonlinear transmission line provided by a typical embodiment of the present invention;

[0020] Figure 4 is a comparison diagram of the simulation result and the theoretical calculation result of the conversion loss of a terahertz heterodyne detector structure with a nonlinear transmission line provided by a typical embodiment of the present invention. Detailed implementation manners

[0021] In view of the deficiencies in the prior art, the inventors of this case have proposed the technical solution of the present invention through long-term research and a large number of practices. The following will further explain the technical solution, its implementation process, principle, etc.

[0022] In the following description, many specific details are set forth in order to provide a thorough understanding of the present invention. However, the present invention may also be implemented in other ways different from those described herein. Therefore, the scope of protection of the present invention is not limited by the specific embodiments disclosed below.

[0023] Moreover, relational terms such as "first" and "second" are only used to distinguish one component or method step with the same name from another, and do not necessarily require or imply any actual relationship or order between these components or method steps.

[0024] Current Schottky diode terahertz detectors usually utilize the I-V nonlinear principle for mixing. The present invention proposes an NLTL terahertz heterodyne mixer based on the CV nonlinear characteristics of the AlGaN / GaN heterojunction, which has the advantages of low noise and broadband. By introducing the NLTL technology, a room-temperature high-sensitivity broadband terahertz detector design based on the GaN material system can be realized.

[0025] The main technical concept of the present invention is to use the CV nonlinearity of varactor diodes for mixing, adopt the technical solution of NLTL for impedance matching, and use a waveguide coupling scheme. Terahertz light enters from the waveguide port and is transmitted according to the main mode TE10 mode in the waveguide, improving the consistency and directivity, with high stability and high coupling efficiency.

[0026] Based on the above technical ideas, referring to Figure 1 As shown, an embodiment of the present invention provides a waveguide-coupled nonlinear transmission line terahertz heterodyne detector structure, which includes:

[0027] A waveguide structure, at least for collecting terahertz input signals;

[0028] A microstrip transmission line, connected to the waveguide structure, at least for coupling and transmitting the terahertz input signal output by the waveguide structure into the mixer;

[0029] A terahertz mixer, connected to the microstrip transmission line, at least for performing heterodyne mixing on the received terahertz input signal to obtain a terahertz response signal;

[0030] A filter structure, connected to the terahertz mixer, at least for isolating the terahertz response signal from the terahertz input signal;

[0031] A lead electrode structure, connected to the filter structure, at least for providing a power supply voltage and outputting the terahertz response signal;

[0032] Wherein, the terahertz mixer includes a varactor diode and a coplanar waveguide transmission line, and the varactor diode and the coplanar waveguide transmission line cooperate to form a nonlinear transmission line.

[0033] The present invention discloses a novel terahertz heterodyne mixer structure with a unique mixing method and mixer structure. Different from the conventional diode mixer using the I-V nonlinear principle, this mixer uses C-V nonlinearity for mixing and adopts the NLTL method to achieve impedance matching; the mixer structure mainly includes: a rectangular waveguide, a microstrip transmission line, an NLTL terahertz mixer, a filter, and a lead electrode structure. The terahertz mixer is connected to the microstrip transmission line and the filter structure, the filter is connected to the lead electrode structure, the rectangular waveguide is used to collect the terahertz input signal, the microstrip transmission line couples and transmits the terahertz input signal in the waveguide to the mixer, the mixer obtains the terahertz response signal through heterodyne mixing, the filter is used to isolate the terahertz input signal, and the lead electrode is used to provide the power supply voltage and output the terahertz response signal.

[0034] Regarding the specific structural features, in some embodiments, the terahertz NLTL heterodyne mixer includes a plurality of units connected in series in sequence, and each unit includes two sections of coplanar microstrip lines and a pair of varactor diodes;

[0035] Anodes of a pair of the varactor diodes are connected to the transmission line in the middle of the coplanar microstrip line, and cathodes are both connected to the grounding electrodes at both ends of the coplanar microstrip line for grounding.

[0036] In some embodiments, the physical length of each section of the coplanar microstrip line is L / 2, where L represents the spacing between multiple pairs of the varactor diodes.

[0037] In some embodiments, the varactor diode adopts a Schottky diode, and the anode and cathode thereof form a Schottky contact and an ohmic contact with the semiconductor layer respectively, and the cathode and the anode are interconnected through a two-dimensional electron gas channel in the semiconductor layer.

[0038] In some embodiments, the anode and the cathode are arranged coplanarly. The common zero-bias Schottky diode and the positive-bias Schottky diode terahertz heterodyne detectors usually adopt an air-bridge structure, with complex manufacturing processes and great difficulty. The varactor diode in the present invention is realized by a planarization process, with simple processes and easy integration.

[0039] In some embodiments, the semiconductor layer includes a channel layer and a barrier layer arranged in a stacked manner, and the semiconductor layer includes an AlGaN / GaN heterojunction.

[0040] In some embodiments, the varactor diode has a capacitance-voltage non-linearity and is expressed as:

[0041] C(V) = C s [1 + tanh(b(V Anode - V th ))]

[0042] where C(V) represents the capacitance of the varactor diode, C s represents the capacitance value of the varactor diode under zero bias voltage, b is the non-linearity coefficient, V Anode represents the DC operating point voltage of the varactor diode, and V th represents the threshold voltage of the varactor diode;

[0043] Also, the conversion loss of the varactor diode is:

[0044]

[0045] where CL represents the conversion loss, P LO represents the local oscillator power, Z WG is the waveguide port impedance, ω IF is the angular frequency of the intermediate frequency, Z0 = 50 ohms, dC / dV represents the derivative of the capacitance of the varactor diode with respect to voltage, which is used to describe the CV non-linearity of the varactor diode, f represents the radio frequency, and f T represents the cut-off frequency of the varactor diode.

[0046] In some embodiments, the terahertz NLTL heterodyne mixer includes more than 9 of the above units.

[0047] In some embodiments, the waveguide structure includes a rectangular waveguide.

[0048] In some embodiments, the microstrip transmission line is inserted into the waveguide structure in the form of a microstrip E-plane probe for coupling the terahertz input signal in the waveguide structure to the microstrip transmission line.

[0049] As some typical application examples of the above technical solutions, the mixing principle of a waveguide-coupled non-linear transmission line terahertz heterodyne detector structure provided by an embodiment of the present invention is based on the CV non-linearity principle for mixing, and the detection principle can be simply described as: when a terahertz wave is fed into the NLTL by a waveguide probe for mixing, the voltage signal loaded on the diode consists of three parts:

[0050] V(t) = V Anode + V LO cosω LO t + VRF cos(ω RF t + φ)

[0051] where V Anode , V LO and V RF are the DC operating point voltage of the diode, the local oscillator voltage signal, and the RF voltage signal, respectively.

[0052] The capacitive current of the varactor diode is written as the reciprocal of the charge with respect to time:

[0053]

[0054] Ignoring the terms above the second order of the Taylor expansion and only considering the intermediate frequency AC signal obtained by heterodyne mixing, the following is derived:

[0055]

[0056] where dC / dV represents the first-order differential of the capacitance with respect to the voltage, the negative sign represents the direction, and the impedance of the capacitance at high frequency is 1 / (j(ω LO - ω RF ))C). The capacitive current is written in voltage form:

[0057]

[0058] The capacitance-voltage nonlinearity of the AlGaN / GaN heterojunction can be described by the hyperbolic tangent function:

[0059] C(V) = C s [1 + tanh(b(V Anode - V th ))]

[0060] where C s is the capacitance of the diode at zero bias voltage, b is the nonlinear coefficient used to describe the magnitude of the nonlinearity, and V Anode - V th is the effective voltage. Based on the intermediate frequency voltage and RF voltage in the above equation, converting them into power form, the conversion loss of the mixer can be obtained as:

[0061]

[0062] where the signals applied to the capacitance of the varactor diode are the effective local oscillator voltage signal and the effective RF voltage signal, and both of these effective signals are related to the cut-off frequency of the diode:

[0063]

[0064] where ω LO is the angular frequency of the local oscillator signal, f LOis the local oscillator frequency, f T is the zero-bias cut-off frequency of the varactor diode.

[0065] The effective RF signal similarly applied to the diode is written as:

[0066]

[0067] Therefore, the variable loss can be further simplified as:

[0068]

[0069] where P LO is the local oscillator power, Z WG is the waveguide port impedance, ω IF is the angular frequency of the intermediate frequency. In a typical embodiment, Z0 = 50 Ω.

[0070] The technical solution of the present invention will be further described in detail below through several embodiments in combination with the accompanying drawings. However, the selected embodiments are only used to illustrate the present invention and do not limit the scope of the present invention.

[0071] Embodiment 1

[0072] This embodiment provides a waveguide-coupled non-linear transmission line terahertz heterodyne detector. Please refer to Figure 1 , where the waveguide is a standard WR-10 rectangular waveguide. The microstrip transmission line is inserted into the waveguide in the form of a microstrip E-plane probe to couple the electromagnetic wave in the waveguide to the microstrip transmission line, and then fed into the NLTL mixer. The impedance matching of the varactor diode is completed by the NLTL. The specific implementation scheme is to use a coplanar waveguide transmission line with a characteristic impedance of 77 ohms, and periodically load varactor diodes to form an NLTL with a characteristic impedance of about 50 ohms. After mixing, the intermediate frequency signal is read out through a low-pass filter, and the PAD is used to connect to the peripheral circuit to realize the reading of the intermediate frequency signal and the supply of the operating voltage of the varactor diode.

[0073] The terahertz NLTL heterodyne mixer is composed of 9 units, and each unit is implemented by a pair of AlGaN / GaN SBDs. Please refer to Figure 2 , which is a three-dimensional structure diagram of an AlGaN / GaN varactor diode. Among them, the anode of the varactor diode is prepared by Schottky contact, the cathode of the varactor diode is prepared by ohmic contact, the cathode and the anode are connected through a two-dimensional electron gas channel, the anode length of the diode is 0.2 μm, the anode width is 20 μm, and the cut-off frequency is 265 GHz.

[0074] Referring to the simulation principle model diagram shown in the figure, the NLTL modeling is completed in ADS. The NLTL has 9 units, and each unit is implemented by a pair of AlGaN / GaN SBDs. In the model, the WR10 probe and the low-pass filter are modeled in the high-frequency electromagnetic simulation software. Specifically, the S-parameter file (S2P file) of the WR10 probe and the low-pass filter can be imported into ADS for joint simulation. Refer to Figure 4 , which is the relationship between the conversion loss (CL) of the NLTL terahertz detector and the frequency. Among them, at the optimal operating point, in the frequency range of 80 - 120 GHz, effective detection of terahertz signals is achieved, where CL reaches the minimum value at 80 GHz and 100 GHz, and the minimum conversion loss obtained by ADS simulation is 13 dB.

[0075] It should be noted here that the terahertz detector provided in this embodiment is mainly applied to heterodyne detection. Therefore, the above response signal is the intermediate-frequency signal formed after the measured terahertz wave is heterodyne mixed by the terahertz mixer, and then the response signal can be read out through an external circuit.

[0076] It should be understood that the above embodiments are only used to illustrate the technical concept and characteristics of the present invention, and their purpose is to enable those skilled in the art to understand the content of the present invention and implement it accordingly, and cannot be used to limit the protection scope of the present invention. Any equivalent changes or modifications made according to the spirit and essence of the present invention should be covered within the protection scope of the present invention.

Claims

1. A waveguide-coupled nonlinear transmission line terahertz heterodyne detector structure, characterized in that, Comprising: A waveguide structure, at least for collecting a terahertz input signal; A microstrip transmission line, connected to the waveguide structure, at least for coupling and transmitting the terahertz input signal output from the waveguide structure into a mixer; A terahertz mixer, connected to the microstrip transmission line, at least for performing heterodyne mixing on the received terahertz input signal to obtain a terahertz response signal; A filter structure, connected to the terahertz mixer, at least for isolating the terahertz response signal from the terahertz input signal; A lead electrode structure, connected to the filter structure, at least for providing a power supply voltage and outputting the terahertz response signal; Wherein, the terahertz mixer includes a varactor diode and a coplanar waveguide transmission line, and the varactor diode and the coplanar waveguide transmission line cooperate to form a nonlinear transmission line.

2. The waveguide-coupled nonlinear transmission line terahertz heterodyne detector structure according to claim 1, characterized in that: The terahertz NLTL heterodyne mixer includes a plurality of units connected in series in sequence, and each unit includes two sections of coplanar microstrip lines and a pair of varactor diodes; The anodes of a pair of the varactor diodes are connected to the transmission line in the middle of the coplanar microstrip line, and the cathodes are both connected to the grounding electrodes at both ends of the coplanar microstrip line for grounding.

3. The waveguide-coupled nonlinear transmission line terahertz heterodyne detector structure according to claim 2, characterized in that: The physical length of each section of the coplanar microstrip line is L / 2, and L represents the distance between multiple pairs of the varactor diodes.

4. The waveguide-coupled nonlinear transmission line terahertz heterodyne detector structure according to claim 2, wherein: The varactor diode uses a Schottky diode, in which the anode and the cathode form a Schottky contact and an ohmic contact with the semiconductor layer respectively, and the cathode and the anode are interconnected through a two-dimensional electron gas channel in the semiconductor layer.

5. The waveguide-coupled nonlinear transmission line terahertz heterodyne detector structure according to claim 4, characterized in that: The anode and the cathode are arranged coplanarly.

6. The waveguide-coupled nonlinear transmission line terahertz heterodyne detector structure according to claim 4, characterized in that: The semiconductor layer includes a channel layer and a barrier layer arranged in a stacked manner, and the semiconductor layer includes an AlGaN / GaN heterojunction.

7. The waveguide-coupled nonlinear transmission line terahertz heterodyne detector structure according to any one of claims 1-6, characterized in that, The varactor diode has capacitance-voltage nonlinearity, and is expressed as: C(V) = C s [1 + tanh(b(V Anode - V th ))] where C(V) represents the capacitance of the varactor diode, C s represents the capacitance value of the varactor diode under zero bias voltage, b is the non-linear coefficient, V Anode represents the DC operating point voltage of the varactor diode, V th represents the threshold voltage of the varactor diode; And, the conversion loss of the varactor diode is: Among them, CL represents the conversion loss, P LO represents the local oscillator power, Z WG is the waveguide port impedance, ω IF is the angular frequency of the intermediate frequency, Z0 = 50 ohms, dC / dV represents the derivative of the capacitance of the varactor diode with respect to voltage, used to describe the CV nonlinearity of the varactor diode, f represents the radio frequency, f T represents the cut-off frequency of the varactor diode.

8. The waveguide-coupled nonlinear transmission line terahertz heterodyne detector structure according to claim 2, wherein: The terahertz NLTL heterodyne mixer includes more than 9 of the above units.

9. The waveguide-coupled nonlinear transmission line terahertz heterodyne detector structure according to claim 1, characterized in that: The waveguide structure includes a rectangular waveguide.

10. The waveguide-coupled nonlinear transmission line terahertz heterodyne detector structure according to claim 1, characterized in that: The microstrip transmission line is inserted into the waveguide structure in the form of a microstrip E-plane probe for coupling the terahertz input signal in the waveguide structure onto the microstrip transmission line.

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