Dark-Field Confocal Brillouin Spectrum Detection System
NING Ying1, WU Han-xu1, XU Meng2, QIU Li-rong1, ZHAO Wei-qian1, NI He1*
1. Beijing Key Lab for Precision Optoelectronic Measurement Instrument and Technology, School of Optics and Photonics, Beijing Institute of Technology, Beijing 100081, China
2. Institute of Instrument Science and Photoelectric Engineering, Beijing Information Science and Technology University, Beijing 100192, China
Abstract：Confocal Brillouin spectroscopy is widely used in physical chemistry, materials science, and mineralogy due to its advantages of non-invasive, label-free, high spatial resolution. Since the spontaneous Brillouin scattering intensity is weak, the signal spectrum easily overlaps with the elastic background or be obliterated when the extinction ratio of the system is insufficient, so accurately measuring the brillouin frequency shift is unachievable. The increasing demand for viscoelastic detection of turbid media in biomedicine also put forward higher requirements on the extinction ratio of the Brillouin detection system. In order to improve the extinction ratio of the confocal Brillouin system, a dark-field confocal Brillouin detection system is constructed in this paper. The non-intersecting optical path configuration avoids the collection of reflected light so it weakens the elastic background light to improve the extinction ratio while ensuring the excitation intensity. Experiments show that compared to the traditional bright field configuration, the extinction ratio of the dark field system is increased by 20 dB; The background light of an intralipid solution at a concentration of 0.001% is obviously suppressed in the dark illumination configuration, the signal spectrum is revealed, thereby attains the accurate measurement of the turbid medium’s Brillouin frequency shift data; Three standard samples of distilled water, PMMA, and SiO2 glass were selected to verify the non-strict backscatter angle. The experimental result is consistent with the theoretical analysis, which ensures that subsequent calculations of axial sound velocity and longitudinal elastic modulus are accurate and effective. The dark field confocal Brillouin spectrum detection system combines the advantages of dark field illumination and confocal detection. It not only has a high resolution of confocal detection, but also improves the system’s anti-elastic background light performance and achieves high extinction detection. The dark field confocal Brillouin spectrum detection system provides a new idea for real-time non-destructive detection of the mechanical properties of substances in biomedicine and materials science.
Key words：Confocal Brillouin；Dark field illumination；Extinction ratio
