Enhanced spectral resolution for broadband coherent anti-Stokes Raman spectroscopy

Abstract

The spectral resolution of broadband Fourier-transform coherent anti-Stokes Raman spectroscopy is limited by the maximum optical path length difference that can be scanned within a short time in an interferometer. However, alternatives to the Fourier-transform exist which can bypass this limitation with certain assumptions. We apply one such approach to broadband coherent Raman spectroscopy using interferometers with short delay line (low Fourier spectral resolution) and large delay line (high Fourier spectral resolution). With this method, we demonstrate broadband coherent Raman spectroscopy of closely spaced vibrational bands is possible using a short delay line interferometer, with superior spectral resolution to the longer delay line instrument. We discuss how this approach may be particularly useful for more complex Raman spectra, such as those measured from biological samples.

Low Frequency Collinear Pre-Resonant Impulsive Stimulated Raman Spectroscopy

“…We retrieve the vibrational spectrum by performing Fourier transformation of the measured output of the lock-in amplifier on a time window extending from about 500-600fs following zero delay to 4-5ps (this translates to about 8cm −1 spectral resolution), although we note that there are alternative methods which are possibly preferential in terms of SNR. 29 We mention that while the noise sources discussed in this section are not related to shotnoise, the same arguments are also valid for the latter as it also depends on the input power, and doesn't depend on the wavelength. In figure 4c we present a numerical simulation, that takes into account the induced dipole effect and uses the Lorentz model to calculate the spectrum of the probe pulse as a function of the frequency of vibration.…”

Multimodal laser-scanning nonlinear optical microscope with a rapid broadband Fourier-transform coherent Raman modality

“…While we were able to demonstrate multiphoton fluorescence bioimaging with low-power femtosecond excitation, the current sensitivity limit for FT-CARS is only suitable for molecules with uncommonly large Raman cross sections, such as carotenoids [22,38], which are not ubiquitous in biological samples like other bio-macromolecules such as proteins, lipids and nucleic acids. In order to extend FT-CARS towards sensitive bioimaging of such molecules, the photon budget should be allocated efficiently, which may require replacing the polygonal mirror scanner for rapid delay scanning with a resonant scanner and using alternative spectrum estimation approaches to obtain high spectral resolution [39].…”

Dual-Comb Coherent Raman Spectroscopy with near 100% Duty Cycle

“…While the spectral resolution was sacrificed as it is determined by the group-delay scan range, which is inversely proportional to the modulation frequency, the spectral features previously buried in noise, 16 including the Raman peaks at 532 and 1210 cm −1 , are recognizable by virtue of the high energy efficiency of the quasi-DC-CARS scheme. The spectral resolution can be improved without sacrificing the spectral acquisition rate by a recently proposed method 25 in which the spectral resolution of Raman spectra obtained from time-domain interferograms measured in a limited temporal region can be enhanced by assuming that the interferograms are composed of multiple exponentially decaying sinusoidal functions.…”