Henrik Volkens

@inproceedings{10.1117/12.3080934,
author = {Henrik Volkens and Christin Grill and Florian Denk and Philipp Lamminger and Sebastian Freidank and Norbert Linz and Hendrik Husstedt and Robert Huber and Ralf Brinkmann},
title = {{A home-built flexible fiber laser to investigate optimal parameters for stimulating the tympanic membrane}},
volume = {13849},
booktitle = {Optical Interactions with Tissue and Cells XXXVII},
editor = {Joel N. Bixler and Alex J. Walsh and Norbert Linz},
organization = {International Society for Optics and Photonics},
publisher = {SPIE},
pages = {1384904},
abstract = {This work investigates optimizing optoacoustic stimulation of tympanic membrane models as a non-occlusive alternative to conventional acoustic drivers. We used a home-built, ytterbium-based master oscillator power amplifier (MOPA) operating at 1064 nm to stimulate an artificial tympanic membrane within a simplified middle ear model. The MOPA system can generate single laser pulses with 200 ps minimum pulse duration as well as concatenating multiple single pulses to MHz-bursts with burst durations up to 100 ns. Burst durations and burst energies were systematically varied between 30 and 100 ns and from 10 to 40 μJ. The laser-induced displacement of the membrane model was measured using phase-sensitive optical coherence tomography. Simultaneously the sound pressure level within a 0.4 ccm volume that mimics the middle ear cavity was measured. The results indicate that the membrane displacement and sound pressure increases both with higher burst energies at the same burst duration and longer burst durations at the same burst energy. Specifically, at a low burst repetition rate of 16 Hz, 100-ns pulse bursts yielded the most efficient stimulation. Furthermore, we demonstrated the system's capability for sound transmission up to 5 kHz by operating the MOPA at a repetition rate of 10 kHz. Using an acousto-optic modulator (AOM) for pulse amplitude modulation, we transmitted a speech signal onto the artificial membrane. The resulting acoustic signal was clearly audible and measurable within the middle ear model. These findings validate the feasibility of using tailored infrared laser pulses for middle ear stimulation. The ability to modulate complex audio signals via flexible, fiber-based laser architecture is a promising approach for developing next-generation hearing restoration technologies that avoid the occlusion effects and discomfort associated with traditional hearing aids.},
keywords = {Master oscillator fiber amplifier, Tympanic membrane, Temporal pulse shaping, Flexible fiber laser, Thermoelastic bending, Parameter optimization, Optical tissue stimulation},
year = {2026},
doi = {10.1117/12.3080934},
URL = {https://doi.org/10.1117/12.3080934}
}

@inproceedings{10.1117/12.3080401,
author = {Henrik Volkens and Sebastian Freidank and Philipp Lamminger and Alfred Vogel and Robert Huber and Ralf Brinkmann and Norbert Linz},
title = {{Dynamic shockwave photography using a home-built MOFA laser system with flexible repetition rate up to 5 GHz}},
volume = {PC13849},
booktitle = {Optical Interactions with Tissue and Cells XXXVII},
editor = {Joel N. Bixler and Alex J. Walsh and Norbert Linz},
organization = {International Society for Optics and Photonics},
publisher = {SPIE},
pages = {PC1384903},
abstract = {Laser-induced ablation in liquids (LAL) is widely used for nanoparticle generation, yet its underlying mechanisms are not completely understood. We investigate interactions between shockwave, cavitation bubble and target material by multi exposure imaging with high temporal and spatial resolution. Our home-built Yb-based master oscillator fiber amplifier system delivers 170 ps pulses at 2 µJ and tunable burst rates up to 5 GHz, ideal for capturing transient events. Speckle-free imaging is achieved using a fiber-based rapid optical mode mixing approach combining spectral broadening with optical delay and spatial mode mixing of frequency-doubled 532 nm pulses.},
keywords = {Laser Ablation in Liquids (LAL), Shockwave Photography, High-Speed Imaging, Multi-Exposure Illumination, Master Oscillator Fiber Amplifier (MOFA), Speckle-Free Imaging, Cavitation Bubble, Nanoparticle Generation},
year = {2026},
doi = {10.1117/12.3080401},
URL = {https://doi.org/10.1117/12.3080401}
}

@inproceedings{10.1117/12.3080520,
author = {Henrik Volkens and Philipp Lamminger and Norbert Linz and Sebastian Freidank and Robert Huber and Ralf Brinkmann},
title = {{Towards optoacoustic transient shaping using a flexible fiber laser system}},
volume = {13851},
booktitle = {Photons Plus Ultrasound: Imaging and Sensing 2026},
editor = {Alexander A. Oraevsky and Lihong V. Wang},
organization = {International Society for Optics and Photonics},
publisher = {SPIE},
pages = {138511F},
abstract = {We aim to increase the efficiency of optoacoustic signal generation for precise, in vivo, real-time tissue temperature monitoring during thermal retinal interventions, by matching the timing of multiple laser excitation events to the acoustic response of the examined specimen. To achieve this goal, we utilized a home-built Ytterbium-based master oscillator power amplifier (MOPA) fiber laser system that provides unprecedented control over the temporal pulse structure, allowing for pulse-burst durations from picoseconds to nanoseconds and arbitrary repetition rates for investigating the influence of the excitation duration on the amplitude of the resulting optoacoustic transients. Methodologically, experiments were performed on ex vivo explants of porcine retinal pigment epithelium (RPE) consisting of the RPE, choroid, and sclera embedded in a cuvette filled with saline solution. Optoacoustic transients were detected using a piezoelectric ring transducer (fres = 1 MHz, Medical Laser Center Lübeck, Germany) integrated into a standard ophthalmic contact glass with a distance of 24 mm to the specimen. We systematically investigated the influence of pulse-burst durations between 10 and 100-ns with the total burst energy of 3 μJ matching a typical probe pulse energy. Each burst was produced with a repetition rate of 500 MHz. Results demonstrate that, at typical pulse energies of 3 μJ, shorter pulse-burst durations down to 30 ns significantly increase the amplitude of the generated acoustic transients compared to longer pulse-bursts. While higher burst energy consistently results in stronger signals, signal generation efficiency is highly dependent on the temporal burst width. With decreasing burst durations, the amplitude of the resulting transients decreases lower than that of the 30-ns burst. We hypothesize that shorter excitation bursts result in a signal consisting of higher-frequency components that are stronger attenuated in water. These findings highlight that tailoring the temporal excitation profile is essential for maximizing signal-to-noise ratio. The compact and scalable fiber-based MOPA architecture offers a versatile alternative to traditional bulk lasers, providing the necessary degrees of freedom for optimized optoacoustic tissue characterization and in future real-time monitoring.},
keywords = {Master Oscillator Fiber Amplifier (MOFA), Optoacoustics, Transient shaping, Temperature measurement, Tailored optoacoustic excitation, Flexible fiber laser, Retinal laser treatment, Multi-GHz repetition rate},
year = {2026},
doi = {10.1117/12.3080520},
URL = {https://doi.org/10.1117/12.3080520}
}