https://hdl.handle.net/20.500.11811/695 Fri, 17 Apr 2026 17:34:50 GMT 2026-04-17T17:34:50Z https://hdl.handle.net/20.500.11811/10748 Spectroscopic Gas Sensor Based on a Fiber Fabry-Perot Cavity Saavedra, Carlos; Pandey, Deepak; Alt, Wolfgang; Meschede, Dieter; Pfeifer, Hannes Optical spectroscopic sensors are powerful tools for analyzing gas mixtures in industrial and scientific applications. Whilst highly sensitive spectrometers tend to have a large footprint, miniaturized optical devices usually lack sensitivity or wideband spectroscopic coverage. By employing a widely tunable, passively stable fiber Fabry-Perot cavity (FFPC), we demonstrate an absorption spectroscopic device that continuously samples over several tens of terahertz. Both broadband scans using cavity mode width spectroscopy to identify the spectral fingerprints of analytes and a fast, low-noise scan method for single absorption features to determine concentrations are exemplary demonstrated for the oxygen A-band. The introduced scan method uses an injected modulation signal in a Pound-Drever-Hall feedback loop together with a lock-in measurement to reject noise at other frequencies. The FFPC-based approach provides a directly fiber-coupled, extremely miniaturized, light-weight, and robust platform for analyzing small analyte volumes that can straightforwardly be extended to sensing at different wavelength ranges, liquid analytes and other spectroscopic techniques with only little adjustments of the device platform. Mon, 17 Oct 2022 00:00:00 GMT https://hdl.handle.net/20.500.11811/10748 2022-10-17T00:00:00Z https://hdl.handle.net/20.500.11811/10743 Fluctuation-Dissipation Relation for a Bose-Einstein Condensate of Photons Öztürk, Fahri Emre; Vewinger, Frank; Weitz, Martin; Schmitt, Julian For equilibrium systems, the magnitude of thermal fluctuations is closely linked to the dissipative response to external perturbations. This fluctuation-dissipation relation has been described for material particles in a wide range of fields. Here, we experimentally probe the relation between the number fluctuations and the response function for a Bose-Einstein condensate of photons coupled to a dye reservoir, demonstrating the fluctuation-dissipation relation for a quantum gas of light. The observed agreement of the scale factor with the environment temperature both directly confirms the thermal nature of the optical condensate and demonstrates the validity of the fluctuation-dissipation theorem for a Bose-Einstein condensate. Fri, 20 Jan 2023 00:00:00 GMT https://hdl.handle.net/20.500.11811/10743 2023-01-20T00:00:00Z