220–325-GHz Horn-Type Adapter for Terahertz Microstructured Fiber Measurements

In this article, a novel and innovative approach to characterize THz Bragg fibers using a horn-type adapter is presented, enabling a two-tier calibration method for a direct, efficient, and reliable way to measure THz Bragg fibers, including return loss (RL) and insertion loss (IL). The proposed approach is robust, accurate, and repeatable, making it suitable for designing and optimizing THz Bragg fibers and systems and enabling continued research and development. This study employs a calibration approach, utilizing short-short-load-thru (SSLT) and thru-reflect-line (TRL) calibrations. The horn-type adapter connects a standard WR-3.4 rectangular waveguide and a THz Bragg fiber, allowing the mode conversion from the TE10 mode in the rectangular waveguide to the HE11 mode in the Bragg fiber, with a middle stage of the TE11 mode in the tapered horn region. Additionally, the highly accurate measurement quality presented advantages compared to the existing THz measurement setups, for example, setup complexity, coupling efficiency, impedance adjustability, and less sensitivity to measurement environments, etc. The present approach shows advantages in experimental setup complexity, coupling efficiency, impedance adjustability, measurement repeatability, operator experience required, and setup tool cost compared to other existing THz measurement techniques.


I. INTRODUCTION
R ECENTLY the most widely accepted definition of the THz band is that it extends from approximately 10 THz to 100 GHz (0.03-3.00 mm in wavelength) and is located between the microwave and infrared (IR) frequency bands [1], [2], [3].Terahertz technology advancements and growing interest in applications have increased the demand for developing new sources [4], [5], [6], [7], [8], detectors, waveguides, and other components for efficient terahertz wave control [9].However, high frequency, especially in the terahertz band, suffers from high metal ohmic loss and dielectric material absorption [10], which means that the traditional waveguides and fibers are not appropriate for low-loss long-distance waveguiding of the THz wave.Therefore, new concepts have to be developed for low-loss THz waveguides and fibers [11], [12], [13].
THz microstructured fibers [14], [15], i.e., the waveguides with artificially designed sub-wavelength microstructured transverse cross sections, have attracted the interest of researchers due to their exceptional optical properties, which provide a versatile platform for tailoring the effective mode area and dispersion characteristics to suit various linear and nonlinear applications over wide bandwidths.An efficient coupling structure supporting quasi-single-mode propagation is needed, as unwanted multimode interference causes higher insertion loss (IL) and measurement complications.Singlemode operation is also crucial for many applications [16], and the design of a Bragg fiber that can provide low-loss, medium-range, and flexible solutions for interconnecting different functional components in THz communication, imaging, and sensing systems while also allowing for robust and repeatable characterization, is necessary.The development of THz measurement and characterization techniques require measurement accuracy, cost-effectiveness, and ease of setup.These properties are frequently influenced by the physical interaction between the device-under-test (DUT) and THz probes, and the design and optimization of such interactions are critical for achieving accurate and consistent measurements.
Contactless or free-space measurements [17], [18] are commonly used to characterize THz components and employ a variety of antennas, such as horn antennas [19], [20], [21], parabolic reflectors, and planar antennas, to transmit and guide THz test signals onto the input terminal of DUT and receive THz responses from its output.Furthermore, the free-space measurement technique overcomes many limitations of contact probe measurements, such as limited probe lifetime, mechanical, and electrical degradation of the contact surface due to contact force, and fragility issues.On the other hand, contactless measurements are also sensitive to environmental conditions and alignment issues.Besides, contactless measurement setups require costly components such as parabolic mirrors, reflectors, and alignment units.THz microstructured fibers can also be an essential structure for designing functional devices, such as THz Bragg fiber filtering antennas, THz gas/liquid sensors [22], and THz endoscopy front-ends.
One method for THz-frequency measurements is waveguide-to-planar-probe measurements, which utilize rectangular waveguide flanges.These measurements can be performed using commercially available THz measurement extenders connected to a network analyzer, with options for different waveguide frequency bands up to 1.5 THz [23].However, waveguide-to-planar-probe measurements, especially in THz bands, present contact resistance and other parasitic effects at the physical interface locations between the measurement probes and the DUT, substantially affecting the measurement integrity, accuracy, and reproducibility.Moreover, the operational life cycle of probes is also limited due to equipment durability.Apart from that, contact measurements are commonly used because of uncomplicated installations, better in repeatability and inexpensive compared to contactless measurements, which require some costly components and experienced technicians to construct and set up.
This article proposes a robust, accurate, and repeatable characterization method of low-loss THz Bragg fibers [24] using a horn-type interface to connect to a standard rectangular waveguide and using a two-tier calibration technique [25].The measurement setup is compact and straightforward, as it uses a horn-type adapter that directly interfaces with the THz Bragg fiber, eliminating the need for additional optical components and intricate alignment procedures.Furthermore, the two-tier calibration technique simplifies the calibration process and reduces the number of required calibration standards, making the setup less complex and more user-friendly than other THz measurement techniques.Compared to conventional THz measurement setups, the measurement system presented in this study allows for cost-effective and efficient customization to meet the measurement requirements.The setup can be easily customed to the needs of users, and the low-cost nature of the design makes it an attractive alternative to existing measurement techniques.

II. DESIGN OF THE HORN-TYPE ADAPTER
A horn-type adapter is a type of microwave connector with a flared shape, like a horn, which helps to reduce signal loss and improve the coupling efficiency between two interfaces.Fig. 1(a) illustrates the device under test, which is a hollow Bragg fiber prototype using Accura ClearVue obtained from 3-D Systems © .Fig. 1(b) illustrates the proposed horn-type adapter fabricated by CNC milling.The electronic-based THz measurement setup consists of the integration of a hollow metallic rectangular waveguide (HMRW)-horn-type adapter-Bragg fiber system, as depicted in Fig. 1(c).Two high-frequency extenders with an operating band from 220 to 325 GHz are connected to a network analyzer to produce and analyze the THz test signal (T x and R x ).The THz beam emitted from the horn antenna at the transmitting extender is fed into the input port of the DUT.It converts the dominant TE 11 mode in the connector into the quasi HE 11 mode in the Bragg fiber with low return loss (RL).Fig. 1(d) and (e) show the schematic and cross-sectional view of the copper horn-type adapter used in this measurement, and the designed geometric and optical parameters of the horn-type adapter are listed in Table I.The outermost layer has mechanical stability and resistance to environmental factors with a thickness of 4.6 and a width of support bridge of 0.65 mm, which are thick protective polymer layers and provide mechanical stability and resistance to environmental factors that absorb residual electromagnetic waves and isolate the fiber from outside interference.Fig. 2(b) compares the dispersion curves of the HE 11 mode of the Bragg fiber and the TE 11 mode of the hollow metallic circular waveguide (HMCW) using the CST Simulation.The output face of a horn-type adapter can be characterized by its waist radius, which is the distance from the center of the face to the point where the Gaussian beam has its minimum radius.Fig 2(c) and (d) show our measurement using terahertz timedomain spectroscopy, the refractive index, and absorption coefficient of the Accura ClearVue between 0.2 and 1 THz can be fit using n = −0.0123f 2 − 0.0335 f + 1.6262, and a(cm −1 t) = 6.4667 f 2 + 17.5066 f − 2.0294, respectively.Here, f is the frequency in THz unit.
This waist radius can be related to the core radius of a Bragg fiber through a parameter denoted as ρ.Different ratios of ρ can be used to optimize the coupling efficiency between the horn-type adapter and the Bragg fiber.Here, ρ = r 0 /r c , where r 0 is the core radius of the horn-type adapter at the output face, and r c is the core radius of the Bragg fiber.According to the momentum conservation principle, the phase velocity of the mode in the connector should be well matched to the phase velocity of the mode in the Bragg fiber to obtain the greatest transition between the horn-type adapter and the Bragg fiber.
As can be seen from Fig. 2(b), the dispersion curve of the HE 11 mode of the Bragg fiber overlaps with that of the TE 11 mode of HMCW at around 0.27 THz when ρ is 0.77, which means their phase velocities are well matched, and hence offers the highest coupling efficiency.The input mode in [26] is a free-space Gaussian beam, while in this article is a circular waveguide mode at the output aperture of the horn-type adapter.This mode is hybrid, with several competing modes present [27], but the main mode is the fundamental mode of HMCW, namely the TE 11 mode.The goal of phase matching is to minimize signal loss and reflection, ensuring that the maximum amount of electromagnetic energy is transferred from one waveguide to the other.
By achieving optimal phase matching, the electromagnetic energy can propagate efficiently from the horn-type adapter to the THz Bragg fiber, leading to more accurate and reliable measurements.The impedance values are measured experimentally using the vector network analyzer (VNA).The horn-type adapter is connected to a standard WR-3.4 waveguide and a Bragg fiber with an input aperture, which transfers the operating TE 10 mode from the rectangular waveguide into the fundamental HE 11 mode in the Bragg fiber, occurring as a linear polarization mode.The results reveal that the optimal value of ρ is 0.77.
In the case of a horn-type adapter, accurate measurement or simulation of the S-parameters is essential for achieving optimal impedance matching between the connectors and the waveguides.Impedance matching is critical for minimizing signal loss and reflection and ensuring the maximum transfer of electromagnetic energy between the two components [28].
In a WR-3.4 rectangular waveguide shown in Fig. 4(a), the TE 10 mode has the electric field polarized vertically.When this mode is delivered to a horn-type adapter, as shown in Fig. 4(b), it is converted to the TE 11 mode of the HMCW, which also has a vertical polarization, as shown in Fig. 4(c).The simulation results of the S-parameters of the connector are shown in Fig. 4(d).It can be seen from the results that the RLs at both ports, i.e., |S 11 | and |S 22 |, are lower than 25 dB over the whole frequency range of 220-325 GHz, which indicates that the horn-type adapter provides a good impedance matching between the waveguide and the Bragg fiber.In addition, the ILs in the two directions, i.e., |S 21 | and |S 12 |, are lower than 1 dB, as clearly seen in Fig. 4(e), which exhibits an efficient coupling provided by the horntype adapter.Fig. 4(e) also shows that |S 21 | and |S 12 | are identical, which demonstrates that the horn-type adapter has a reciprocal property.These characteristics of the horn-type adapter are achieved by properly designing the horn to have a specific geometry that provides a uniform field amplitude over the aperture, as well as the appropriate phase relationship of the field in the waveguide and the field at the output of the horn.

III. MEASUREMENT RESULTS AND DISCUSSIONS
The multiple reflections of modes at different interfaces can lead to interferences and distortions of the signal.In the proposed measurement setup using a horn-type adapter, interferences and distortions happen at the interfaces among the HMCW, connector, and Bragg fiber, which require proper calibration to de-embed the DUT.To address these issues, a two-tier calibration [25] technique using short-short-loadthru (SSLT) [29] and thru-reflect-line (TRL) [30] methods is employed to improve the accuracy of the S-parameter measurements.It should be noted that the L in SSLT and TRL represent different standards.In SSLT, L means Load, whereas in TRL, L means Line.The first-tier full two-port SSLT calibration, which is also called offset short calibration, uses a commercial Ceyear©AV20302 mechanical calibration kit.Here, the SSLT calibration kits contain the following standards.
1) Short (S): A fixed flush short kit with a smooth metal reflection plane is used to terminate the Bragg fiber by reflecting the signal and hence defines the reference plane.
2) Offset-short (S): The offset-short is made up of a quarterwavelength straight section (shim) and an above-mentioned fixed flush short.
3) Load (L): A matched load kit with WR-3.4 rectangular waveguide aperture is used, and its RL is greater than 25 dB.
4) Thru (T): No additional kit is required for Thru measurement, which is performed by directly connecting the two WR-3.4rectangular waveguides of the two frequency extenders.
Following the standard calibration procedures and applying the built-in SSLT calibration algorithm in the VNA, the reference plane is moved to the end of the WR-3.4 rectangular waveguide thereafter.Fig. 5 shows the fabricated horn-type adapters and the Bragg fibers with different lengths used in the measurement system.The second-tier TRL calibration, which includes in situ Thru connection (T), reflection standard (R), and line standard (L), characterizes the performance of the component in a systematic and repeatable manner by moving the reference Authorized licensed use limited to the terms of the applicable license agreement with IEEE.Restrictions apply.plane away from the joint interfaces and to a place in the Bragg fiber, to eliminate the effect of test fixtures.Here, the custom-designed TRL calibration kits used to calibrate the Bragg fiber contain the following standards.
1) Thru (T): A 100 mm length Bragg fiber is provided for thru measurement, as the reference plane is again moved to 50 mm in the Bragg fiber, away from the interface between the horn-type adaptor and the Bragg fiber.
2) Reflect (R): The reflect standard is made up of a 50 mm Bragg fiber and a smooth metal mirror.They are tightly fixed and attached to the horn-type adaptor.The place of the metal mirror determines the reference plane.
3) Line (L): The line standard is similar to the Thru standard, but with an additional quarter-wavelength section in the middle.The additional quarter-wavelength section, located in the middle of the line standard, is 3-D printed and seamlessly integrated with the other sections (two 50 mm sections at the feed-in and feed-out ports).After measuring all the second-tier calibration standards and the DUT using the VNA based on the mounting configuration shown in Fig. 6, we post-processed the acquired data with MATLAB based on [30], thereby obtaining all the S-parameters of the DUT.
By combining these methods, the two-tier calibration technique can improve the accuracy and repeatability of measurements for Bragg fiber measurement by eliminating the effect of the horn-type connector and other text fixtures, but it requires careful execution and appropriate calibration standards to minimize environmental effects.
The IL is the ratio between the incident power to the transmitted power, expressed in dB, which is given by where |S 21 | represents the magnitude of the transmission coefficient [31].
The RL is the ratio of the incident power to the reflected power, expressed in dB, which is given by RL where |S 11 | represents the magnitude of the reflection coefficient [31].
To fabricate the Bragg fiber, the design was first created using CST simulation software.The design is then loaded into the 3-D Systems PolyJet 7000 HD 3-D printer, and the printer is set up to print using the stereolithography technique.The next step is to fabricate the horn-type adapter using CNC milling.The connector is made of metal and is flared to match the diameter of Bragg fiber.The CNC milling machine precisely shapes and sizes the connector according to the design specifications to allow for assembly with the Bragg fiber.Finally, the performance of the THz Bragg fiber is tested using the proposed VNA-based integrated measurement setup to measure its transmission and reflection characteristics.The data obtained are compared with the simulation results to verify the performance of the Bragg fiber.
Fig. 6 shows the measurement setup used in the study, with two horn-type adapters attached to both open-ended sides of the WR-3.4HMRW and the Bragg fiber fixed securely on the sample holders.By using the above-mentioned two-tier calibration technique described, the Bragg fiber can be easily investigated in terms of its uncomplicated setup and repeatability.Fig. 7 shows the simulated electric field propagation at 265 GHz in a back-to-back setup between the horn-type adapters and the Bragg fiber.The simulation exhibits the TE 10 mode, which is the fundamental mode of propagation in a WR-3.4 rectangular waveguide.However, this mode cannot efficiently couple to the Bragg fiber due to its confined electric and magnetic fields, which have a zero value at the waveguide walls.To overcome this limitation, a horn-type adapter was utilized to transform the TE 10 mode into the dominant TE 11 mode, which has a good beneficial field distribution for coupling.The shape of the horn gradually expands from the rectangular waveguide to a circular cross section, allowing the wave to spread and match the impedance of the subsequent Bragg fiber.
With similar mode profiles and phase velocity, the dominant TE 11 mode in the horn-type adapter can evolve into the quasi-single-HE 11 mode in the Bragg fiber.The Bragg fiber acts as a distributed reflector, causing multiple reflections of the electromagnetic wave, leading to the formation of a standing wave pattern.The proposed measurement setup is a VNA-based integrated setup.The key parameters of equipment used for experiments are listed in Table II.A Ceyear © VNA is used in this work, and the operating frequency range is extended to the range of 220-325 GHz using two frequency extenders, which are also from the Ceyear © .Standard WR-3.4 HMRWs are used in frequency extenders.The SSLT calibration technique was used to move the reference plane to the end of the WR-3.4HMRW.The SSLT calibration method was utilized to ensure precise measurements of THz signals traveling through the waveguide.
The S-parameters S 11 and S 21 of the Thru configuration were measured and corrected using the SSLT, as shown in Fig. 8.The S 11 and S 21 were found to have good performance, with S 11 less than −40 dB and S 21 approximately 0 dB across the frequency range of 220-325 GHz, demonstrating the first-tier calibration is valid.Additionally, the measured  Thru spectra exhibited S-parameter reciprocity [32], indicating that the calibration accurately characterized the behavior of the waveguide and removed any potential errors that could affect the measurement results.
To mitigate the effects of multiple reflections, it is important to carefully design the measurement setup and use appropriate calibration techniques.This includes the use of specialized calibration standards or techniques to account for the effects of reflections and other sources of measurement error.It is also important to carefully analyze the measurement data and perform appropriate data processing and filtering to remove any unwanted effects of multiple reflections and other sources of error.Therefore, in this study, the second-tier TRL calibration was performed to enhance the accuracy and reliability of the measurements.Fig. 9 shows the measured and simulated S-parameters and propagation loss of the Bragg fiber with a length of 124 mm.In Fig. 9(a) and (b), the de-embedded measured IL and propagation loss are less than 1.5 dB

TABLE III COMPARISONS OF KEY PROPERTIES BETWEEN DIFFERENT THZ MEASUREMENT SETUPS
proposed setup to estimate the S-parameters and propagation loss of the Bragg fiber.The RL is less than 15 dB, and the IL is less than 1.5 dB from 220 to 305 GHz.The RLs of both the measured and simulated results are relatively low, indicating that most power has been coupled into the Bragg fiber.Fig. 9(d) shows the simulated propagation loss of the THz Bragg fiber, which is well consistent with the measured results shown in Fig. 9(b).There are several factors that can cause measurement results to differ slightly from simulation results.Some of the most important factors are the nonuniform dimensions of the Bragg fiber, surface roughness, and material impurities that may contribute to higher propagation losses.
1) The VNA-based quasi-optical setup is very complex and requires expensive setup tools.It offers high measurement repeatability but moderate coupling efficiency and impedance matching capability.It also requires a high level of operator experience.
2) The THz-TDS-based optical setup is very complex and requires high-cost setup tools.It provides high measurement repeatability but intermediate coupling efficiency and impedance matching capability.It also requires very experienced operators.
3) The VNA-based integrated setup is less complex and requires low-cost setup tools.It offers high measurement repeatability, coupling efficiency, and impedance-matching capability.In addition, it requires only a small amount of operator experience.From this comparison, the proposed VNA-based integrated setup using a horn-type adapter has more overall advantages compared to other existing measurement methods.

IV. CONCLUSION
In this article, a novel VNA-based measurement approach for the characterization of Bragg fibers in the THz frequency range is presented.It offers high repeatability, coupling efficiency, and impedance matching capability while requiring simple setup, little operator experience, and inexpensive setup tools.The proposed method uses a horn-type adapter to efficiently couple electromagnetic waves between the test instrument and the Bragg fibers for accurate characterization.The connector was designed for the operating frequency range of 220-325 GHz and fabricated by CNC milling technology.The suitable taper profile of the horn structure provides smooth guidance of electromagnetic waves, resulting in minimal signal loss and improved transmission efficiency.Mode matching from TE11 in a horn-type adapter to HE11 in a THz Bragg fiber is achieved by selecting a proper aperture size for the horn.The S-parameters were evaluated using the two-tier calibrations, SSLT and TRL, to eliminate sources of error and ensure reliable, consistent, and robust measurements.The performance of the horn-type-connector shows that the RL is less than 25 dB, and the IL is nearly zero, indicating good signal transmission efficiency and minimal signal loss and reflection.Moreover, the de-embedded simulation and measurement results of the DUT using the proposed setup also show the accuracy of the S-parameters and propagation loss of the Bragg fiber.The measured RL of the DUT is about 15 dB, and the propagation loss is about 12 dB/m, which agrees well with the simulated results.

Fig. 1 .
Fig. 1.Design and fabrication of a Bragg fiber and a horn-type adapter used in this article including the schematic of the measurement setup.(a) Fabricated the designed Bragg fiber prototype using the Accura ClearVue used as DUT.(b) Horn-type adapter fabricated by CNC milling.(c) Illustration of electronic-based THz measurement setup using the HMRW-connector-Bragg fiber integrated system: the dimension of WR-3.4 is 0.864 × 0.432 mm and the parameters of the Bragg fiber and the horn-type adapter are described in Table I.(d) Schematic of the horn-type adapter including the dimension parameters.(e) Cross-sectional view of the horn-type adapter with the dimension parameters.

Fig. 2 .
Fig. 2. Asymptotically single-mode hollow THz Bragg fiber used as DUT in this work, the relationship of impedance matching condition between the horn-type adapter and the Bragg fiber, and dielectric property of THz Bragg fiber using Accura ClearVue between 0.2 and 1 THz.(a) Geometrical structure and the cross-sectional view of THz hollow Bragg fiber, including its parameters.(b) Comparison of the dispersion curves of the effective refractive index profile of the HE 11 mode of the Bragg fiber and the TE 11 mode of the HMCW with the different ratios of ρ using CST simulation.(c) Refractive index of Accura ClearVue.(d) Absorption coefficient of Accura ClearVue.

Fig. 2 (
Fig.2(a) illustrates the cross-sectional view of THz hollow Bragg fiber and its geometrical parameters.The Bragg fiber consists of an air core with the refractive index (n c = 1) surrounded by periodic concentric dielectric layers alternating between high (n a ) and low (n b ) refractive index materials with thicknesses of 0.64 and 3.88 mm, respectively.The outermost layer has mechanical stability and resistance to environmental factors with a thickness of 4.6 and a width of support bridge of 0.65 mm, which are thick protective polymer layers and provide mechanical stability and resistance to environmental factors that absorb residual electromagnetic waves and isolate the fiber from outside interference.Fig.2(b)compares the dispersion curves of the HE 11 mode of the Bragg fiber and the TE 11 mode of the hollow metallic circular waveguide (HMCW) using the CST Simulation.The output face of a horn-type adapter can be characterized by its waist radius, which is the distance from the center of the face to the point where the Gaussian beam has its minimum radius.Fig2(c) and (d) show our measurement using terahertz timedomain spectroscopy, the refractive index, and absorption coefficient of the Accura ClearVue between 0.2 and 1 THz can be fit using n = −0.0123f 2 − 0.0335 f + 1.6262, and a(cm −1 t) = 6.4667 f 2 + 17.5066 f − 2.0294, respectively.Here, f is the frequency in THz unit.This waist radius can be related to the core radius of a Bragg fiber through a parameter denoted as ρ.Different ratios of ρ can be used to optimize the coupling efficiency between the horn-type adapter and the Bragg fiber.Here, ρ = r 0 /r c , where r 0 is the core radius of the horn-type adapter at the output face, and r c is the core radius of the Bragg fiber.According to the momentum conservation principle, the phase velocity of the

Fig. 3 .
Fig. 3. CST simulation of the horn-type adapter using a back-to-back full simulation to compare the S 11 and S 21 with different taper lengths L and ρ.(a) S 21 by adjusting the taper length (L) between 25 and 65 mm and ρ = 0.77.(b) S 11 by adjusting the L between 25 and 65 mm and ρ = 0.77.(c) S 21 by adjusting the ρ between 0.6 and 1 and L = 45 mm.(d) S 11 by adjusting the ρ between 0.6 and 1 and L = 45 mm.

Fig. 3
shows the CST simulation results of a parametric study for the design of a horn-type adapter.Two design parameters which are the taper length L and the ratio ρ of the core radius of the horn-type adapter at the output face to the core radius of the Bragg fiber were investigated.The scattering parameters, S 11 and S 21 , were computed and compared for different values of L and ρ by varying one parameter at a time.Fig. 3(a) and (b) show |S 21 | and |S 11 |, respectively, for

Fig. 4 .
Fig. 4. Simulation and analysis of TE 10 to TE 11 mode conversion in WR-3.4 rectangular waveguide using horn-type adapter: S-parameters and reciprocity analysis.(a) Vertical polarization of TE 10 mode at port 1 in the WR-3.4 rectangular waveguide.(b) Schematic of a horn-type adapter to convert TE 10 mode from a WR-3.4 rectangular waveguide to TE 11 mode in a horn-type adapter.(c) Vertical polarization of TE 11 mode at port 2 from the horn-type adapter.(d) CST simulation result of S-parameters at port 1 and port 2. (e) CST simulation result of reciprocity of RL between port 1 and port 2.

Fig. 5 .
Fig. 5. Fabricated horn-type adapter and different lengths of the Bragg fibers for the second-tier TRL calibration technique and fully assembled the Bragg fiber with the horn-type adapter used in this article.

Fig. 6 .
Fig. 6.Proposed measurement setup.The two horn-type adapters were attached to both open-ended sides of the WR-3.4HMRW, and the Bragg fiber was fixed stability on the sample holders.

Fig. 7 .
Fig. 7. Simulated electric field propagation at 265 GHz between the horn-type adapters and the Bragg fiber using a back-to-back setup.

Fig. 8 .Fig. 9 .
Fig. 8. Measured thru spectra with the WR-3.4 rectangular waveguides of the frequency extenders directly connected after applying the first-tier SSLT calibration.

TABLE II KEY
PARAMETERS OF THE EQUIPMENT USED FOR EXPERIMENTS