title: Terahertz photonic spectrum analyser
Creators: name: Krause, Benedikt
Publisher: name: UNSPECIFIED
Year: 2025

description: The volume of data transmitted via wireless communication will further increase in the upcoming years, eventually surpassing the bandwidth capabilities provided by existing technologies. Increasing the carrier frequency used for the data transmission also increases the available bandwidth and makes new, yet unused bands available. Current technologies use frequencies below 100 GHz but are said to expand into the terahertz (THz) frequencies (100 GHz - 10 THz) with 6G technologies and beyond. THz radiation is non-ionising and, therefore, interesting for medical applications on living organisms like cancer detection. THz fingerprinting detects materials based on their absorption which security applications use to identify substances like drugs or explosives. Most of the THz applications require a THz source in their measurement equipment. Current sources are still either bulky, expensive or low in output power requiring further research for improvements in all categories. For the spectral signal output characterisation, accurate and reliable spectrum analysers are required. The only currently commercially available spectrum analysers for the THz frequency range are electronic spectrum analysers with extender modules. Each of these covers a bandwidth of approximately 42 % of their centre frequency limited by the typically employed rectangular metallic hollow waveguides. Measuring harmonics of a signal already requires several extender modules. Covering the full frequency range from 100 GHz up to 1.5 THz, which is simultaneously the highest yet commercially reachable frequency, necessitates at least seven extender modules. The investment costs for the full frequency range easily exceed half a million Euros.
An alternative to reaching the THz frequencies is photonic technology. Tuning a telecom-wavelength (1550 nm) laser by 1 THz equals a tuning by only 0.5 % and is easily accessible with commercial equipment. This thesis introduces several variants of a THz photonic spectrum analyser based on the difference frequency of two continuous-wave optical telecom-wavelength signals. A photoconductive mixer generates the optical difference frequency acting as a local oscillator and mixes it with the signal of a THz source. The mixing process transfers the spectral information of the source into the intermediate frequency, typically kHz or MHz, where an analog-digital converter acquires the data. The thesis introduces four measurement variants for the optical difference frequency generation and the data acquisition: 1.) The frequency sweep uses a continuously increasing difference frequency and measures all components falling within the bandwidth of a low-pass filter in the intermediate frequency chain. 2.) The offset frequency sweep employs a band-pass filter instead of the low-pass filter in order to reduce 1/f-noise. Each frequency component of the source is displayed twice. A deconvolution regains the original spectrum of the source. 3.) The Fourier transformation mode measures a time trace of the downconverted signal and transforms it to the frequency domain with a Fourier transformation. The resulting frequency information equals the spectrum surrounding the optical difference frequency. 4.) The final measurement mode is based on in-phase and quadrature demodulation. The signal is split into two paths and each downconverted within its respective photoconductive mixer. Both paths use the same optical difference frequency, yet with a relative phase difference of 90°. Experimentally, this thesis demonstrates three different implementations of the photonic THz spectrum analyser, each with its own way of generating the difference frequency. The first variant uses two free-running, temperature controlled distributed-feedback laser diodes and ErAs:InGaAs or low-temperature grown InGaAs:Be photoconductive mixers. We demonstrate operation between 100 GHz and 1.05 THz, yet may cover frequencies beyond 2.7 THz with a minimum resolution bandwidth of 1.2 MHz. The system reaches displayed average noise levels of -111.8 dBm/Hz at a frequency of 100 GHz and -98.0 dBm/Hz at a frequency of 1 THz. Using photoconductive mixers with planar, end-fire Vivaldi antennas, this photonic spectrum analyser variant extends to signals in dielectric waveguides and rectangular metallic hollow waveguides. The second photonic spectrum analyser implementation uses an electro-optical THz comb that generates the two required optical frequency components form a single laser with an electro-optical modulator. The electro-optical THz comb shows a phase noise of -108.6 dBc/Hz at an offset frequency of 1 MHz at a centre frequency of 40 GHz and a linewidth of 1.8 Hz at a frequency of 100 GHz. We used it for frequencies up to 110 GHz while frequencies up to 1 THz are currently possible. The third photonic spectrum analyser examined in this thesis utilises two continuous-wave lasers, both locked to the same frequency-comb and further to a global positioning system (GPS) signal, in combination with an InGaAs:Rh photoconductive mixer. This variant covers frequencies up to at least 6.5 THz with linewidths below 1 Hz at a frequency of 100 GHz and below 20 Hz at a frequency of 1 THz. Simultaneously, it offers a displayed average noise level of -145.6 dBm/Hz at a frequency of 100 GHz, -133.7 dBm/Hz at 1 THz and -111.5 dBm/Hz at a frequency of 4.5 THz.