Balanced Heterodyne Brillouin Spectroscopy Towards Tissue Characterization

Cooparation with the Experimental Ophthalmology Group of Prof. Dr. Oliver Stachs, Department of Ophthalmology, Rostock University Medical Center

This project was a very successful cooperation with the Experimental Ophthalmology Group of Prof. Dr. Oliver Stachs, Department of Ophthalmology, Rostock University Medical Center. Here, we applied the well understood balanced heterodyne detection in Brillouin spectroscopy of various materials, including porcine eye lenses. For details see DOI link.

Schematic of the experimentally realized setup. The two lasers (Innolight Diabolo and Coherent Mephisto S) are overlapped on a fast photodiode (beat note diode) to measure the beat frequency between the two lasers. The main laser (red) is sent through a chopper, periodically blocking the signal, and is afterwards focused into the sample. The Brillouin backscattered light (blue) is sent into the heterodyne detector, where it is overlapped on a 50:50 beam splitter with the local oscillator (green). The AC signals of the photodiodes are subtracted and sent to the computer, where the difference signal is further processed, along with the signal from the GHz diode and a trigger signal from the chopper.
Measured Brillouin spectrum in acetone (data in blue o’s, Lorentzian fit as blue solid line) obtained in a one-shot measurement of approximately 0.5 minute and in water (data in green x’s, Lorentzian fit as green dashed line) obtained by averaging over five single measurements of approximately 2.5 minutes and a total measurement time of five minutes. The Lorentzian fits gives a Brillouin shift of (2.9982 ± 0.0015) GHz and a Brillouin linewidth of (101 ± 5) MHz while an overall relative error of 6 % for the spectrum (see text) of acetone and a Brillouin shift of (3.702 ± 0.004) GHz and a Brillouin linewidth of (153 ± 12) MHz with an overall relative error of 10 % for the spectrum water.
Measured Brillouin peak in glass (SF10, Schott AG) by averaging over 180 single measurements of approximately 90 minutes and a total measurement time of 180 minutes. The unfiltered data is shown in light blue o’s and the data filtered with a moving mean over 12 neighboring data points is shown in dark blue x’s. The Lorentzian fit gives a Brillouin shift of (12.937 ± 0.027) GHz and a Brillouin linewidth of (45 ± 7) MHz while the overall relative error of the spectrum of SF10 is 20 %.
Measured Brillouin peak in a yellow gummy bear (Haribo) by averaging over 180 single measurements of approximately 90 minutes and a total measurement time of 180 minutes. The unfiltered data is shown in light blue o’s and the data filtered with a moving mean over 12 neighboring data points is shown in dark blue x’s. The Lorentzian fit gives a Brillouin shift of (9.16 ± 0.09) GHz and a Brillouin linewidth of (663 ± 200) MHz while the overall relative error of the spectrum of the gummy bear is 35 %.
Measured Brillouin peak in an ex vivo porcine lens obtained by averaging over ten single measurements of approximately 5 minutes and a total measurement time of ten minutes. The unfiltered data is shown in light blue o’s and the data filtered with a moving mean over 12 neighboring data points is shown in dark blue x’s. The Lorentzian fit gives a Brillouin shift of (4.70 ± 0.06) GHz and a Brillouin linewidth of (726 ± 230) MHz while the overall relative error of the spectrum of the porcine lens is 34 %.