Jannis Holzer | Searcharxiv

arXiv2026

Enhancing broadband second harmonic generation in a thin film lithium niobate racetrack resonator with tunable-coupling

Second harmonic generation in thin film periodically poled lithium niobate (PPLN) is constrained by an efficiency-bandwidth trade-off and fabrication-sensitive scaling. We demonstrate a racetrack resonator incorporating a short PPLN section and a tunable Mach-Zehnder interferometer coupler that enables in situ control of the coupling condition, compensating fabrication tolerances and stabilizing operation near critical coupling. The telecom pump is resonantly enhanced, while the near-infrared second harmonic is generated in single pass, eliminating dual-resonance requirements. The device achieves a 35 times efficiency enhancement over a non-resonant structure while maintaining a 7 nm bandwidth. This architecture provides a robust platform for broadband integrated frequency doubling.

physics.optics↗

arXiv2026

Test of a 34 GHz EOM laser frequency comb at ESPRESSO

Laser frequency combs (LFCs) are a promising technology for wavelength calibration of astronomical high-resolution spectrographs requiring utmost accuracy and stability, since they directly translate the fundamental SI time standard from the radio frequency regime to optical frequencies. However, they have so far seen limited use in practice, due to their complexity, incomplete wavelength coverage, but also the challenges in the data analysis they imply. Here, we present a detailed test of a 34 GHz electro-optic modulation comb with the ESPRESSO spectrograph. Using thin-film lithum-niobate waveguides for broadening and harmonic generation, the setup provides partial coverage of the IR, visible, and near-UV spectral ranges. We focus on assessing the quality of the delivered spectra and their capability to facilitate accurate and stable wavelength calibration. We present a detailed analysis of the spectrally-diffuse background, the line width, and characterize the line-spread function over a broader width than possible with the ESPRESSO facility LFC. Comparing both combs, we find strong local discrepancies in the wavelength calibration accuracy up to 15m/s , which correlate with the echellogram structure. These do not originate from the lasers, but from misalignments in the ESPRESSO calibration unit, highlighting the strong need to make instrument fiber feeds more robust to light-injection effects. Nevertheless, we demonstrate excellent stability of the wavelength calibration, with a scatter of only 17cm/s . This, however, can only be achieved when accurately modeling the non-Gaussian line-spread function, showcasing the need for advanced data analysis techniques when dealing with LFC spectra.

astro-ph.IM↗