Desgin and Characterization of an Optoelectronic Laser Synchronization System for Spectrum and Network Analysis

As global reliance on high-speed data communication and advanced sensing technologies intensifies, the need for precise, high-performance measurement tools has become a strategic necessity for developing the next-generation devices and systems. Especially, test platforms for the characterization of signals and components in the upper microwave to terahertz frequency range are gaining critical relevance, as these spectral regions are expected to unlock new capabilities in bandwidth, resolution, and reliability for upcoming scientific and industrial applications. Nowadays, electronic spectrum and network analyzers are well-established instruments to perform these measurement tasks. Such platforms operate by sweeping the frequency across the desired measurement bandwidth to analyze signal parameters like amplitude, phase, impedance, and noise characteristics. In the radio, microwave, and millimeter-wave domains - ranging up to several tens of gigahertz - testing is typically carried out using commercially available, standalone systems that integrate signal generation, processing, and analysis in a single unit. However, due to inherent limitations in scaling to higher frequencies, extending the frequency coverage into the terahertz band usually requires the addition of electronic extension modules. These modules upconvert lower-frequency signals into the terahertz range using nonlinear mixing processes and dedicated multiplier chains, while the received signals are similarly downconverted for analysis within the native bandwidth of the base instrument. Since each module is constrained in both bandwidth and output power, multiple units are often needed to cover a broad spectral range - significantly increasing the complexity and cost of such test solutions. Moreover, even in today’s most advanced systems, the maximum achievable frequency remains limited to around 1.5 THz. In this work, a novel approach to high-resolution spectrum and network analysis in the terahertz domain is presented, combining electronic and photonic technologies to enable high-frequency operation and precise signal characterization. The developed measurement systems are based on an optoelectronic - also called hybrid - concept for the frequency synchronization of laser beat signals with electronic narrow-band emitters. By phase-locking the laser difference frequency to the highly stable output of the electronic multiplier chains, the fundamental linewidth limitation of the individual lasers - typically in the megahertz range - can be effectively suppressed, resulting in a beat signal whose spectral purity is defined by the electronic reference. Sending these phase-locked signals to a biased photoconductive antenna (PCA), the narrow linewidth directly translates into the generation of highly coherent continuous-wave terahertz radiation. Similarly, at the receiver side, illuminating a second PCA with the phase-locked optical tones enables coherent detection of terahertz radiation, where the induced photocurrent carries both amplitude and phase information of the incoming terahertz wave. This optically driven heterodyne approach shifts generation and detection into the photonic domain, offering a reliable and accurate measurement platform for terahertz analysis with exceptional spectral resolution and phase stability, surpassing traditional photonic analyzer systems in terms of precision and versatility. Unfortunately, the bandwidth and upper frequency limit of the system remain bound to the specifications of the electronic reference source. To overcome this constraint and extend the frequency range while preserving high spectral purity and phase coherence, additional lasers can be integrated in a cascaded configuration. Incorporating a third laser allows for the generation and detection of terahertz signals at frequencies up to twice the range defined by the electronic multiplier chain. Furthermore, by introducing a second reference source and accordingly tuning the laser frequencies relative to one another, seamless spectral coverage can be achieved, effectively closing gaps in the measurement bandwidth. Leveraging this hybrid concept, it is demonstrated in this thesis that nearly arbitrary terahertz signals can be generated and detected with hertz precision, extending up to 1 THz. Building on this synchronization approach, a high-resolution photonic spectrum analyzer (PSA) and a photonic vector network analyzer (PVNA) were developed. While the PSA is designed as a standalone system capable of analyzing signals up to 1 THz, the PVNA is realized as an extension module for electronic network analyzers, serving as the backend. Owing to the PVNA configuration, one laser pair must be allocated to generate the stable intermediate frequency expected by the electronic backend, which, given the availability of only three lasers, restricts the measurement range to 70 GHz to 520 GHz. To evaluate the performance of the developed photonic systems, spurious harmonics from different multiplier chains are analyzed with the PSA, while the PVNA extender is used to characterize various bandpass filters and waveguides. The resulting measurements demonstrate strong agreement with comparison data from simulations and commercial extension modules, confirming the accuracy and reliability of the concepts. Lastly, four-wave mixing (FWM) is explored as an alternative to laser cascading for extending the measurement range, where the beat signal from a laser pair is fed into an optical amplifier that generates conjugate frequencies through the nonlinear FWM process. By mixing the two conjugate frequencies, bandwidth tripling is achieved, significantly increasing the spectral coverage of the photonic systems. However, since the conjugate frequencies are significantly weaker than the original pump signals, effective filtering of the pump is required to use the signal for terahertz generation and detection. To address this, an experimental filtering setup is presented, utilizing two transmission gratings to spatially isolate and filter the respective frequency components. As a result, phase-locked beat signals with frequencies of more than 1 THz can be achieved using just a single laser pair, demonstrating potential for reaching even higher measurement bandwidths in future photonic concepts.