Achieved results and methods

In the 1^st and 2^nd phases of C06, UDE and RUB developed an insight into the influence of scattering surfaces on the spectral fingerprint of different materials. This has been achieved by cutting-edge photonic THz transmitter, receiver, and multi-octave THz packaging technologies [1-4] as well as spectroscopy systems with record-breaking performance for characterizing and localizing materials and biologic tissue in static scenarios [5-10]. Highlights include THz bandwidth photonic transmitters providing mW-level output power per die at THz frequencies which were also provided to other projects. PI Kolpatzeck showcased ultra-high repetition rate THz-TDS (UHRR-THz-TDS) systems with 133 dB peak dynamic range and 1.5 THz bandwidth [6]. Moreover, he demonstrated novel photonic ranging approaches, that make optoelectronic radar competitive with electronic radar in the THz range.

Transmitters

C06 has developed and experimentally characterized high output power broadband InP-based 1.55 µm THz modified uni-traveling-carrier photodiodes (MUTC-PDs) for the cw THz transmitter. Typically, the saturation output power of fabricated MUTC-PDs with an average responsivity of 0.2 A/W is reached at a photocurrent level of ~30 mA. The measured saturation output power levels at 30 GHz, 60 GHz, 150 GHz, and 320 GHz are +11.2 dBm, +9.1 dBm, +6.1 dBm, and −4.6 dBm, respectively. The output power level at 500 GHz is found to be −24.5 dBm at only 10 mA photocurrent. The saturation output power at 500 GHz is expected to be −16.5 dBm at 30 mA photocurrent [11].

In addition to the original work plan and in preparation for the 3^rd phase, UDE innovated a monolithic integrated solution for a coherent photonic THz transceiver chip. For this, we have fabricated and successfully demonstrated the first, to our knowledge, monolithically integrated optically pumped Fermi-level managed barrier (FMBD) THz mixer with an InP-based MUTC-PD [12]. In this solution, the MUTC-PD provides the required ultra-broadband LO-signal to pump the THz FMBD-mixer. Fig. 2(a) shows the measured conversion loss of the optically pumped THz mixer from 5-300 GHz. A low conversion loss of 16.5 dB and a small NEP of 3.8 pW/√Hz are obtained at 30 GHz with an LO power of −12 dBm using in the direct detection mode [12]. Also, in preparation for the 3^rd phase, UDE successfully integrated a MUTC-PD into an ultra-broadband dielectric waveguide via a CPW-to-CPS and tapered slot transition. Fig. 2(b) shows the simulated and measured frequency response of the MUTC-PD coupled to a dielectric waveguide up to 1.1 THz [3]. At this moment, measurements were carried out up to 0.6 THz.

Material classification

We manufactured samples of different material classes with defined surface roughness and measured the TDS reflection distribution for these samples. Training of the classifier with flat surfaces results in mostly wrong classification of rough samples, while the prediction works when using training data from rough samples. Consequently, surface roughness must be considered in material mapping settings, where the surface structures are not known a priori.

 THz spectroscopy systems

Supplementing the work carried out in C06 in the 1^st and 2^nd phase, PI Kolpatzeck has developed ultra-high repetition rate THz time-domain spectroscopy (UHRR-THz-TDS) systems based on mode-locked laser diodes (MLLDs). In the 3^rd phase, C06 will build on these concepts to advance its research objective of developing a compact optoelectronic THz spectroscopy system. Using state-of-the-art monolithic MLLDs with chirp-compensated pulse durations of around 600 fs at 1550 nm, these novel UHRR-THz-TDS systems typically exhibit repetition rates of several 10 GHz. Although measured signals are acquired in the time-domain using an optical delay unit (ODU), the comparatively low pulse energy allows these systems to be accurately described using a linear frequency-domain model and makes it possible to employ a broadband PD as the THz transmitter [13]. The use of a system model facilitates the modification of the laser spectrum for optimizing spectral performance [14]. Using a novel interferometric approach for reconstructing the delay axis with fs-accuracy, it was possible to demonstrate a bandwidth of up to 1.5 THz and a record-high peak dynamic range of 133 dB at 100 GHz [6] as well as thickness and distance measurements with micrometer-resolution [7,15].

Correlation radar

Complementary to the aforementioned broadband THz spectroscopy systems, PI Kolpatzeck contributes concepts for seamlessly integrating highly sensitive correlation radar in an optoelectronic system. For example, an optoelectronic m-sequence radar was demonstrated by modulating the intensity of the cw infrared signals driving the photodiode in the THz transmitter and the photoconductive THz receiver with delayed maximum-length sequences. The low-pass behavior of the receiver is exploited to determine the cross correction of the transmitted and the received waveforms in the analog domain with minimal conversion loss. This approach made it possible to perform distance measurements at a frequency of 100 GHz with an unambiguous range of several meters using off-the-shelf components [8]. Preliminary results show that phase modulation leads to significantly more pronounced correlation peaks and allows the radar to detect more targets than intensity modulation.

Selected project-related publications

[1] S. Makhlouf, O. Cojocari, M. Hofmann, T. Nagatsuma, S. Preu, N. Weimann, H.-W. Hübers and A. Stöhr, “Terahertz Sources and Receivers: From the Past to the Future,” IEEE Journal of Microwaves, July 2023. [DOI:10.1109/JMW.2023.3282875]

[2] M. Grzeslo, S. Dülme, S. Clochiatti, T. Neerfeld, T. Haddad, P. Lu, J. Tebart, S. Makhlouf, C. Biurrun-Quel, J. L. Fernández Estévez, J. Lackmann, N. Weimann, and A. Stöhr, “High saturation photocurrent THz waveguide-type MUTC-photodiodes reaching mW output power within the WR3.4 band,” Optics Express, Feb. 2023. [DOI: 10.1364/OE.475987]

[3] S. Iwamatsu, M. Ali, J.L.F. Fernández Estévez, M. Grzeslo, S. Makhlouf, A. Rivera, G. Carpintero, A. Stöhr, “Terahertz photodiode integration with multi-octave-bandwidth dielectric rod waveguide probe,” Optics Letters, Nov. 2023. [DOI: 10.1364/OL.504354]

[4] C. Brenner, N. Surkamp, M. R. Hofmann, “Y-shaped tunable monolithic dual colour lasers for THz technology”, Advances in Radio Science, Dez. 2023. [DOI: 10.5194/ars-21-1-2023]

[5] S. Dülme, M. Steeg, I. Mohammad, N. Schrinski, J. Tebart, A. Stöhr, “Ultra-Low Phase-Noise Photonic Terahertz Imaging System based on Two-Tone Square-Law Detection,” Optics Express, Sep. 2020. [DOI: 10.1364/OE.400405]

[6] V. Cherniak, T. Kubiczek, K. Kolpatzeck, and J. C. Balzer, “Laser diode based THz-TDS system with 133 dB peak signal-to-noise ratio at 100 GHz,” Scientific Reports, Aug. 2023. [DOI: 10.1038/s41598-023-40634-3]

[7] K. Kolpatzeck, X. Liu, L. Häring, J. C. Balzer, and A. Czylwik, „Ultra-high repetition rate terahertz time-domain spectroscopy for micrometer layer thickness measurement,” Sensors, Aug. 2021. [DOI: 10.3390/s21165389]

[8] K. Kolpatzeck, S. Akdas, J. C. Balzer, and A. Czylwik, “An Optoelectronic M-Sequence Radar for the Terahertz Range,” 48^th International Conference on Infrared, Millimeter, and Terahertz Waves, Sep. 2023. [DOI: 10.1109/IRMMW-THz57677.2023.10299055]

[9] N. Surkamp, A. Gerling, J. O’Gorman, M. Honsberg, S. Schmidtmann, U. Nandi, S. Preu, J. Sacher, C. Brenner, and M. R. Hofmann, “Current tuned slotted Y-branch laser for wafer thickness measurements with THz radiation,” Electronics Letters , Nov. 2021. [DOI: 10.1049/ell2.12314]

[10] I. Mohammad, T. Haddad, S. Makhlouf, and A. Stöhr, “Multi-Spectral Photonic THz Imaging using MUTC-PDs and Dielectric Rod Waveguide Antennas,” 48^th International Conference on Infrared, Millimeter, and Terahertz Waves, Sep. 17-22, 2023. [DOI: 10.1109/IRMMW-THz57677.2023.10299194]