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THz-to-Optical Conversion in Wireless Communications Using an Ultra-Broadband Plasmonic Modulator
Sandeep Ummethala, Tobias Harter, Kira Koehnle, Zheng Li, Sascha Muehlbrandt, Yasar Kutuvantavida, Juned Kemal, Jochen Schaefer, Axel Tessmann, Suresh Kumar Garlapati, Andreas Bacher, Lothar Hahn, Martin Walther, Thomas Zwick, Sebastian Randel, Wolfgang Freude, Christian Koos
TL;DR
The paper addresses seamless integration of THz wireless links with fiber-optic infrastructures through direct conversion between optical and THz signals. It uses global normalization to characterize a POH modulator and reports a flat response beyond 0.36 THz, alongside THz-amplifier and back-to-back measurements.
Problem
The supplementary material addresses how to characterize the POH modulator’s frequency response across multiple frequency bands despite frequency-dependent drive power and operating-point uncertainties.
Method
The POH modulator response is evaluated by globally normalizing the measured phase modulation index to the effective electrical drive voltage, with drive losses and impedance included.
Results
The POH MZM has a flat frequency response exceeding 0.36 THz with no sign of bandwidth limitation in the measured range.
Takeaways & Limitations
Global normalization relates measurements from different frequency bands to the actual electrical drive amplitude and permits inference of the overall modulator response.
Abstract
from arXiv · showhide
Future wireless communication networks have to handle data rates of tens or even hundreds of Gbit/s per link, requiring carrier frequencies in the unallocated terahertz (THz) spectrum. In this context, seamless integration of THz links into existing fiber-optic infrastructures is of great importance to complement the inherent portability and flexibility advantages of wireless networks by the reliable and virtually unlimited capacity of optical transmission systems. On the technological level, this requires novel device and signal processing concepts for direct conversion of data streams between the THz and the optical domains. Here, we report on the first demonstration of a THz link that is seamlessly integrated into a fiber-optic network using direct terahertz-to-optical (T/O) conversion at the wireless receiver. We exploit an ultra-broadband silicon-plasmonic modulator having a 3 dB bandwidth in excess of 0.36 THz for T/O conversion of a 50 Gbit/s data stream that is transmitted on a 0.2885 THz carrier over a 16 m-long wireless link. Optical-to-terahertz (O/T) conversion at the wireless transmitter relies on photomixing in a uni-travelling-carrier photodiode.
Supplementary Information
The supplementary information identifies the contributors and institutions, then organizes supporting material into five sections covering fabrication, characterization, transmission experiments, back-to-back measurements, and references.
- Contributors and affiliations: The paper lists contributors affiliated with the Karlsruhe Institute of Technology and the Fraunhofer Institute for Applied Solid State Physics.The affiliations include IPQ, IMT, INT, and IHE at KIT, plus Fraunhofer IAF.
- Supplementary sections: Section I covers the frequency response of the plasmonic-organic hybrid modulator.
- Supplementary sections: Sections II and III cover characterization of the uni-travelling-carrier photodiode and THz amplifiers, followed by the data-transmission setup.
- Supplementary sections: Section IV reports back-to-back data-transmission measurements, while Section V provides references.
I. Frequency Response of the Plasmonic-Organic Hybrid (POH) Modulator
The POH Mach-Zehnder modulator’s frequency response is measured across 2 GHz to 0.36 THz by calibrating sideband modulation against the effective electrical drive. A global normalization across measurement bands yields a flat response exceeding 0.36 THz, without visible bandwidth limitation.
- Measurement principle: The first-order sideband-to-carrier power ratio determines the phase modulation index under the small-signal condition η ≪ 1.The analysis uses the Bessel-function approximation J_0(η) = 1 and J_1(η) = η/2.
- Operating-point correction: The operating-point correction factor is unity at quadrature but is estimated from a fitted transmission spectrum because exact adjustment is difficult.Noise and distortions of the unbalanced MZM’s cos²-type transfer function complicate direct quadrature operation.
- Electrical calibration: The effective drive voltage is calculated from measured source power, probe loss, transmission-line loss, and the frequency-dependent output impedance.The tapered line is 150 µm long, with approximately 50 Ω input impedance and simulated output impedance Z_TL,out.
- Global normalization: The response is measured in four frequency bands with slightly different operating points and then globally normalized to remove frequency-dependent source-power variations.The bands are 2 GHz–64 GHz, 0.07 THz–0.11 THz, 0.11 THz–0.17 THz, and 0.22 THz–0.36 THz.
- Result: The normalized POH MZM response is flat beyond 0.36 THz, with no sign of bandwidth limitation within the measured range.The measured range is limited by available equipment rather than an observed device roll-off.
II. Characterization of the Uni-Travelling-Carrier Photodiode and the THz Amplifiers
The experiment characterizes the UTC-PD and amplification chain across frequency, then identifies the received-power range supporting the THz data link.
- 16 m of free-space transmission is characterized using a UTC-PD, horn antennas, PTFE lenses, and cascaded LNA and MPA amplification.The receiver-side amplification compensates for free-space loss before measuring available THz power.
- The measured red-curve dips are attributed to reflections caused by imperfect waveguide connector flanges between the amplifiers.
- 0.2885 THz is the operating carrier frequency selected for the data-transmission experiments.The frequency is marked on the measured available-power curve.
- 2.5 dBm of THz input power is required at the POH MZM for a BER of 4.5×10^-3, the 7 % overhead FEC threshold.This requirement defines the horizontal power reference used in the characterization.
III. Experimental Setup Used for Data Transmission Experiments
The setup generates a QPSK THz signal by optical-to-THz conversion, transmits it over 16 m, and directly converts it back to the optical domain at the receiver.
- Transmitter: A DAC-generated QPSK signal and a continuous-wave optical local oscillator are combined and photomixed in a UTC-PD at 0.2885 THz.The carrier frequency is tuned by adjusting the optical local-oscillator frequency.
- Transmitter: The THz signal is radiated by a 26 dBi horn antenna, collimated by a PTFE lens, and transmitted over 16 m of free space.
- Receiver: The POH MZM modulates the THz signal onto an optical carrier, after which EDFAs and optical band-pass filters amplify the signal and isolate one sideband.The experiment isolates the lower sideband because of the optical filter’s limited tuning range.
- Receiver: At the receiver, a PTFE lens, horn antenna, and cascaded H-band LNA and MPA amplify the THz signal to drive the POH MZM.
- Receiver: The received optical signal is detected, recorded, equalized, carrier-phase estimated, and evaluated offline to compute BER.
IV. Back-to-Back Data Transmission Measurements
The back-to-back experiment directly connected the UTC-PD transmitter to the POH MZM through an H-band amplifier and evaluated QPSK BER across symbol rates. BER worsened with increasing symbol rate, reaching the 7% FEC threshold at 8 GBd and falling below the 20% FEC limit at 15 GBd, while the wireless transmission setup achieved lower BER because it provided higher THz drive power.
- Measurement setup: The back-to-back setup directly connected the UTC-PD to the POH MZM using an H-band medium-power amplifier.The QPSK data signal was evaluated after offline digital signal processing.
- BER results: 5.4×10^-5 BER was recorded at a 2 GBd QPSK symbol rate, corresponding to a 4 Gbit s^-1 line rate.
- BER results: 15 GBd QPSK produced a 30 Gbit s^-1 line rate, with signal quality slightly below the 20% FEC limit.
- Limiting factors: BER was mainly limited by EDFA amplified spontaneous emission noise and increased at higher symbol rates as THz amplifier output power dropped.
- Comparison caveat: The back-to-back and wireless setups cannot be directly compared because their amplifier configurations differed.The back-to-back test omitted the LNA/MPA cascade to avoid excessive MPA input power.
- Comparison caveat: At 0.2885 THz, the wireless transmission setup provided more THz power after the UTC-PD, free-space link, and LNA/MPA cascade than the back-to-back setup.The higher available drive power translated into lower transmission BER under EDFA-ASE-limited optical reception.