Dispersion-based stimulated Raman scattering spectroscopy, holography, and optical coherence tomography

Abstract

Stimulated Raman scattering (SRS) enables fast, high resolution imaging of chemical constituents important to biological structures and functional processes, both in a label-free manner and using exogenous biomarkers. While this technology has shown remarkable potential, it is currently limited to point scanning and can only probe a few Raman bands at a time (most often, only one). In this work we take a fundamentally different approach to detecting the small nonlinear signals based on dispersion effects that accompany the loss/gain processes in SRS. In this proof of concept, we demonstrate that the dispersive measurements are more robust to noise compared to amplitude-based measurements, which then permit spectral or spatial multiplexing (potentially both, simultaneously). Finally, we illustrate how this method may enable different strategies for biochemical imaging using phase microscopy and optical coherence tomography.

Stimulated Raman scattering spectroscopic optical coherence tomography

“…These results are in good agreement with the known Raman amplitude response of olive oil, with a peak around 2900 cm −1 [21]. The phase response is in good agreement with our previous phase measurements [17]. …”

“…The spectral dependence of the nonlinear phase responses [Fig. 4(f)] at both selected points in the sample exhibits the characteristic behavior of lipids [17]. These spectra are also in good agreement with the transmission experiment (Fig.…”

“…Spatial multiplexing, particularly in the axial dimension, has also not been achieved using other types of coherent Raman scattering methods [16]. Recently, we developed an interferometric approach to assess both the SRS amplitude and phase changes (i.e., the imaginary and real parts of χ (3) , respectively) that was well suited for integration with OCT imaging, thereby enabling spatial multiplexing [17]. …”

“…2), which allows us to obtain a baseline/calibration SRS amplitude and phase response of lipids for comparison with the depthresolved spectra of SRS-SOCT. As previously described [17–19], the system uses a regenerative amplifier laser source (RegA, Coherent) with a repetition rate of 20 kHz ( λ 0 = 808 nm, Δ λ = 25 nm). Two time-delayed replicas of the output light (separated by time T ) serve as the reference and Stokes beams.…”

“…4(b) is normalized by the standard deviation of the phase at modulation frequencies ranging from 1.5 to 2.5 kHz. The normalization suppresses the large, noisy background produced from random phase fluctuations in regions with no signal (the phase noise is inversely proportional to the signal-to-noise ratio (SNR) of the OCT image [17]). We will revisit this point later.…”