High-performance Brillouin spectroscopy using VIPA-etalon cascades

Brillouin spectroscopy noninvasively probes mechanical properties of biological materials and has become a valuable tool in various areas of biomedical research. However, the limited contrast of conventional single stage virtually imaged phased array (VIPA) spectrometers restricts many applications. Current contrast enhancement methods have limitations, including incompatibility with line-scanning collection, which is highly advantageous for fast biomechanical mapping.

The aim is to develop and characterize a high-throughput contrast enhancement technique using a VIPA-etalon cascade, designed for compatibility with both confocal and line-scanning geometries across a wide range of wavelengths.

One or more etalons with approximately matched thicknesses are integrated downstream of the VIPA to suppress its Lorentzian tails. A theoretical model and ray-tracing simulation were developed, complemented by experimental validation in confocal and line-scanning setups at 785 and 532 nm, respectively. Brillouin spectra were measured for reference materials and biological samples, including a porcine crystalline lens.

Experimentally, the integration of a single high-finesse cascade yielded contrast enhancements of 27 and $\sim 19\mathrm{dB}$ for the confocal and line-scanning geometries, respectively, with less than 50% reduction in peak transmission in both cases. Simulations demonstrated that substantially higher contrast can be achieved in principle with negligible impact on throughput by serializing multiple low-finesse etalons. High-quality Brillouin spectra were obtained in both collection geometries, demonstrating robust performance across diverse samples.

The VIPA-etalon cascade markedly improves the performance of Brillouin spectroscopy, offering a straightforward, cost-effective, and versatile solution for analyzing biological samples. It enables precise, high-resolution biomechanical investigations, advancing applications in biomedical research and clinical diagnostics.