Project S01 - High-Speed Beamforming Concepts for THz Frequencies

Principal Investigator: Prof. Dr. Andreas Czylwik, UDE

Achieved Results in the 1^st Phase

We have investigated both, active and passive, beam steering systems covering multiple octaves of carrier frequency. In alignment with the MARIE vision, we have focused our attention on extreme wideband systems that have the potential to be integrated in an ultra-compact and robust portable device. Since there are only very few electronic concepts of multi-octave THz signal generation and beamforming, we focussed on optoelectronic THz signal generation.

Photonic phase shift beam steering with active phase control Frequency-Domain Spectroscopy (FDS) based on heterodyne detection of two single-mode laser diode signals is a particularly interesting wideband measurement concept in the THz range. Integrating the optical components in a photonic circuit, it has the potential to be realized in an extremely compact system. The major drawback of FDS is its low measurement speed, as it operates in swept-frequency mode and measures one frequency at a time.

One way to enable beam steering on the transmit side of an FDS system is a phased array of antenna-integrated photodiodes. The relative phases between the coherent THz signals generated in the photodiodes can be adjusted in the optical domain either by applying suitable time delays to both optical carriers or phase shifts to one of the optical carriers. Variable phase shifts are easy to realize both in fiber optics and integrated optics and can be adjusted with extremely high speed. However, particularly fiber-optic implementations of photonic phase shift beam steering are severely affected by random fluctuations of the optical phase in the beam steering network. We have proposed a system for closed-loop phase control that does not require phase detection at THz frequencies [2]. A block diagram of the concept is depicted in Fig. 1. By difference frequency mixing between a second pair of single-mode lasers, trace tones at significantly lower frequencies where phase detection is easily accomplished, are generated. The phase differences between these trace tones accurately track the phase differences between the generated THz signals. We have shown a successful proof of concept at a frequency of 6 GHz for a four-element array [1].

Fig. 1: Block diagram of photonic phase shift beam steering with phase control.

Ring resonator-based quasi true time delay beam steering for THz time-domain spectroscopy Time-Domain Spectroscopy (TDS), based on the generation and detection of THz radiation by impinging ultra-short optical pulses on semiconductor photomixers, is currently the state-of-the-art in wideband THz systems due to its extremely wide bandwidth of up to several THz and fast measurement speed. However, conventional TDS systems are too bulky for the mobile scenarios envisioned within MARIE due to the need for a fiber laser and a long mechanical delay line. One way to overcome this issue is by replacing the fiber laser with a monolithic Mode-Locked Laser Diode (MLLD) as depicted in Fig. 2. We have demonstrated THz TDS with a bandwidth above 1 THz using a monolithic MLLD and state-of-the-art photomixers from Fraunhofer Heinrich Hertz Institute (HHI). Within project S01, extensive work has been done to fundamentally improve the understanding of such systems. We have developed an analytical model that enables us to predict the detected spectrum from the optical spectrum of the MLLD and the transfer function of the THz channel [23, 25]. These results have enabled us to vastly improve the systems’ performance in terms of measurement speed and bandwidth. Among other things, we have demonstrated first results in optimizing the THz bandwidth by spectrally shaping the optical output signal of the MLLD [21]. An example of optical spectra before and after shaping is depicted in Fig. 3. Basically, the optical bandwidth of MLLDs is not limited to about 2 THz as shown in Fig. 3. In Literature an optical bandwidth of about 3.7 THz has been reported.

Fig. 2: Simplified block diagram of THz TDS using a mode-locked laser diode as the light source.

Fig. 3: (a) Optical and (b) THz spectra without (green) and with spectral shaping (red).

One distinct property of THz TDS with MLLDs is that both, the radiated as well as the detected THz signal, consist of discrete spectral components at frequencies that are multiples of the MLLD’s free spectral range. We have developed a beam steering concept that takes advantage of this property and enables a highly integrated system. A fundamental requirement for beam steering in a wideband system is that the direction of the main beam is the same across the entire frequency range. This can be realized by True Time Delay (TTD) beam steering, i.e., changing the relative delay between the optical signals exciting each element in an array of antenna-integrated photodiodes.

We have found a way to realize the required delays in an integrated system, using optical ring resonators. A sketch of the proposed system is depicted in Fig. 4. Ring resonators have a periodic attenuation and group delay spectrum whose periodicity is given by the round-trip time of the ring. The group delay between the input and output of the device can be tuned by adjusting the coupling coefficient between the through bus and the ring. We have investigated an approach using a Mach-Zehnder interferometer to adjust the coupling coefficient. If the free spectral range f_FSR of the ring matches that of the laser and the group delay spectrum is aligned to the optical spectrum such that its maxima overlap with the laser modes, the group delay is identical for all laser modes. To verify the possibility of realization of our concept, we have thoroughly investigated the effect of a mismatch between the free spectral ranges of the ring and the MLLD as well as the effect of laser frequency fluctuations. The free spectral ranges need to be matched to about 0.1% for the signal degradation to be negligible.

An example for the radiation pattern for a wide (a) and a narrow (b) spacing of a 10-element uniform linear antenna array is shown in Fig. 5.

Fig. 4: Array concept for THz beam steering with ring resonator delay elements.

High-efficiency passive THz beam steering based on a reconfigurable MEMS reflection grating As a passive beam steering system, MEMS-based reflection gratings have previously been successfully used for THz beam steering. However, the diffraction efficiency of current MEMS-based passive THz beam steering systems is relatively low due to the limited vertical displacement of the cantilevers. We have proposed the design of a reconfigurable MEMS-based reflection grating aiming to realize beam steering at frequencies from 0.3 THz to 1 THz with maximum diffraction grating efficiency [24]. The proposed diffraction grating (see Fig. 6) consists of subwavelength MEMS reflectors driven by 5-bit electrostatic actuators with a total throw of 600 µm. By varying the number of reflectors per grating period and changing the throw of individual reflectors to shape a blazed grating, we can steer the THz beam to discrete angles with maximum diffraction grating efficiency. Furthermore, we have proposed a mathematical model based on the Huygens-Fresnel principle for calculating the far-field radiation pattern of the THz beam reflected by general diffraction gratings. We compared the calculated radiation patterns with simulations using a Finite-Difference Time Domain (FTTD) ElectroMagnetic (EM) solver and found very good agreement with respect to main lobe and sidelobes.

Fig. 5: Frequency-dependent radiation patterns with uniform linear arrays.

Key components for wideband THz beam steering systems Finally, we have considered several key components of wideband THz systems. Among others, we have designed, fabricated, and characterized 3D-printed polymer THz waveguides and multimode interference couplers [6, 13, 22].

Fig. 6: Structure of a blazed grating, (a) 3D view and (b) side view

Selected Project-related publications

For all project-related publications please click here and scroll to the S01 section.

[1] X. Liu, K. Kolpatzeck, L. Häring, and A. Czylwik, "Experimental Validation of a Phase Control Concept for Photonically Steered Terahertz Phased Array Transmitters at Microwave Frequencies," in 2018 First International Workshop on Mobile Terahertz Systems (IWMTS), Velen, Germany, July 2018.

[2] K. Kolpatzeck, X. Liu, L. Häring, and A. Czylwik, "System-Theoretical Modeling and Analysis of Phase Control in a Photonically Steered Terahertz Phased Array Transmitter," in 2018 First International Workshop on Mobile Terahertz Systems (IWMTS), Velen, Germany, July 2018.

[3] P. Lu, M. Steeg, K. Kolpatzeck, S. Dülme, B. Khani, A. Czylwik, and A. Stöhr, "Photonic Assisted Beam Steering for Millimeter-Wave and THz Antennas," in 2018 IEEE Conference on Antenna Measurements & Applications (CAMA), Västerås, Sweden, September 2018.

[4] B. Friederich, K. Kolpatzeck, X. Liu, T. Schultze, J. C. Balzer, A. Czylwik, and I. Willms, "Preprocessing for Robust Estimation of Material Parameters by Continuous Wave THz Spectroscopy," in 2018 43rd International Conference on Infrared, Millimeter, and Terahertz Waves (IRMMW-THz), Nagoya, Japan, September 2018.

[5] D. Damyanov, B. Friederich, J. Kohl, X. Liu, K. Kolpatzeck, T. Schultze, A. Czylwik, J. C. Balzer, and I. Willms, " Super-Resolution Restoration of Low-Resolution THz Camera Images," in 2019 Second International Workshop on Mobile Terahertz Systems (IWMTS), Bad Neuenahr, Germany, July 2019.

2018

[6] X. Liu, K. Kolpatzeck, A. Öztürk, B. Friederich, D. Damyanov, L. Häring, T. Schultze, J. C. Balzer, and A. Czylwik, "Wideband Characterization of 3D Printed THz Rectangular Dielectric Waveguides by THz Frequency-Domain Spectroscopy," in 2019 Second International Workshop on Mobile Terahertz Systems (IWMTS), Bad Neuenahr, Germany, July 2019.

[7] K. Kolpatzeck, X. Liu, B. Friederich, D. Damyanov, L. Häring, T. Schultze, J. C. Balzer, A. Czylwik, "Wideband Radiation Pattern Measurement of Terahertz Antenna-Integrated Photodiodes by Frequency-Domain Spectroscopy," in 2019 Second International Workshop on Mobile Terahertz Systems (IWMTS), Bad Neuenahr, Germany, July 2019.

[8] S. C. Tonder, K. Kolpatzeck, X. Liu, S. Rumpza, A. Czylwik, and J. C. Balzer, "A Compact THz Quasi TDS System for Mobile Scenarios," in 2019 Second International Workshop on Mobile Terahertz Systems (IWMTS), Bad Neuenahr, Germany, July 2019.

[9] K. Kolpatzeck, S. Tonder, X. Liu, A. Czylwik, and J. C. Balzer, "Characterization and Application of a Commercially Available Laser Diode in a THz System," in 2019 International Microwave Workshop Series on Advanced Materials and Processes 2019 conference (IMWS-AMP 2019), Bochum, Germany, July 2019.

[10] M. Alissa, B. Friederich, K. Kolpatzeck, A. Czylwik, and T. Kaiser, "Experimental Investigation of Terahertz Wave Scattering by Statistically Controlled Rough Surfaces," in 2019 International Microwave Workshop Series on Advanced Materials and Processes 2019 conference (IMWS-AMP 2019), Bochum, Germany, July 2019.

[11] D. Damyanov, B. Friederich, K. Kolpatzeck, X. Liu, M. Yahyapour, N. Vieweg, A. Deninger, T. Schultze, I. Willms, and J. C. Balzer, "High Resolution Lensless THz Imaging With An Ultrafast TDS System," in 2019 44th International Conference on Infrared, Millimeter, and Terahertz Waves (IRMMW-THz), Paris, France, September 2019.

[12] B. Friederich, D. Damyanov, J. Kohl, K. Kolpatzeck, X. Liu, T. Schultze, A. Czylwik, J. C. Balzer, and I. Willms, "High Resolution Image Processing Technique for The Detection Of Metal Entrapments Based On A THz Camera," in 2019 44th International Conference on Infrared, Millimeter, and Terahertz Waves (IRMMW-THz), Paris, France, September 2019.

[13] X. Liu, K. Kolpatzeck, B. Friederich, D. Damyanov, L. Häring, T. Schultze, J. C. Balzer, and A. Czylwik, "Spectroscopic Characterization Of 3D Printed THz Rectangular Polymer Waveguides," in 2019 44th International Conference on Infrared, Millimeter, and Terahertz Waves (IRMMW-THz), Paris, France, September 2019.

[14] K. Kolpatzeck, X. Liu, S. Nellen, B. Friederich, D. Damyanov, L. Häring, T. Schultze, B. Globisch, J. C. Balzer, and A. Czylwik, "Wideband Radiation Pattern Simulation and Measurement of a Photodiode-Based Continuous-Wave THz Emitter," in 2019 44th International Conference on Infrared, Millimeter, and Terahertz Waves (IRMMW-THz), Paris, France, September 2019.

[15] A. Stöhr, P. Lu, T. Haddad, M. Steeg, J. Tebart, B. Sievert, A. Rennings, M. Hofmann, S. Dülme, K. Kolpatzeck, and A. Czylwik, " Microwave Photonic mm-Wave & THz Beam Steering for Imaging, Radar and Communications," in Asia Communications and Photonics Conference (ACPC) 2019, Chengdu, China, November 2019.

[16] D. Damyanov, B. Friederich, M. Yahyapour, N. Vieweg, A. Deninger, K. Kolpatzeck, X. Liu, A. Czylwik, T. Schultze, I. Willms, and J. C. Balzer, "High Resolution Lensless Terahertz Imaging and Ranging," in IEEE Access, vol. 7, pp. 147704-147712, 2019.

[17] D. Damyanov, B. Friederich, K. Kolpatzeck, X. Liu, A. Czylwik, T. Schultze, I. Willms, and J. C. Balzer, "A Novel Approach for Lensless High-Resolution Terahertz Imaging," in SPIE Photonics West 2020 (online only), Paris, France, March 2020.

[18] L. Samfaß, P. Schmitt, X. Liu, A. Czylwik, and M. Hoffmann, “Micromechanical Reflect-Array for THz Radar Beam Steering based on a Mechanical D/A Converter and a Mechanical Amplifier,” 2020 Third International Workshop on Mobile Terahertz Systems (IWMTS), Essen, Germany, July 2020.

[19] D. Damyanov, A. Batra, B. Friederich, K. Kolpatzeck, X. Liu, T. Kaiser, T. Schultze, I. Willms, and J. C. Balzer, "High Resolution VNA THz Imaging For Large Distances," in 45th International Conference on Infrared, Millimeter and Terahertz Waves (IRMMW-THz), Buffalo, USA, November 2020 (accepted for publication).

[20] K. Kolpatzeck, X. Liu, K.-H. Tybussek, L. Häring, M. Zander, W. Rehbein, M. Möhrle, A. Czylwik, and J. C. Balzer, "Analytical Modeling Of Terahertz Time-Domain Spectroscopy With Monolithic Mode-Locked Laser Diodes," in 45th International Conference on Infrared, Millimeter and Terahertz Waves (IRMMW-THz), Buffalo, USA, November 2020 (accepted for publication).

[21] K. Kolpatzeck, M. Dedic, P. Krämer, X. Liu, V. Cherniak, K.-H. Tybussek, M. Zander, L. Häring, J. C. Balzer, and A. Czylwik, "Enhancement Of Terahertz Spectra By Model-Driven Spectral Shaping Of A Mode-Locked Laser Diode In A Terahertz Time-Domain Spectroscopy System," in 45th International Conference on Infrared, Millimeter and Terahertz Waves (IRMMW-THz), Buffalo, USA, November 2020 (accepted for publication).

[22] X. Liu, C. Geng, X. Guo, K. Kolpatzeck, L. Häring, J. C. Balzer, and A. Czylwik, "Design And Characterization Of 3D Printed Polymer Terahertz Multi-Mode Interference Couplers," in 45th International Conference on Infrared, Millimeter and Terahertz Waves (IRMMW-THz), Buffalo, USA, November 2020 (accepted for publication).

[23] K. Kolpatzeck and J. C. Balzer, "Towards ultra-compact broadband photonic terahertz systems enabled by monolithic laser diodes" (Invited Paper), in SPIE Photonics West 2020, San Francisco, USA, February 2020.

[24] X. Liu, L. Samfaß, K. Kolpatzeck, L. Häring, J. C. Balzer, M. Hoffmann, and A. Czylwik, “Terahertz Beam Steering Concept Based on a MEMS Reconfigurable Reflection Grating,” in Sensors 2020, 20, 2874.

[25] K. Kolpatzeck, X. Liu, K.-H. Tybussek, L. Häring, M. Zander, W. Rehbein, M. Möhrle, A. Czylwik, and J. C. Balzer, “System-theoretical Modeling of Terahertz Time-Domain Spectroscopy with Ultra-High Repetition Rate Mode-Locked Lasers,” in Optics Express 28, 16935-16950 (2020).