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High-contrast, fast chemical imaging by coherent Raman scattering using a self-synchronized two-colour fibre laser
摘要： Coherent Raman scattering (CRS) microscopy is widely recognized as a powerful tool for tackling biomedical problems based on its chemically specific label-free contrast, high spatial and spectral resolution, and high sensitivity. However, the clinical translation of CRS imaging technologies has long been hindered by traditional solid-state lasers with environmentally sensitive operations and large footprints. Ultrafast fibre lasers can potentially overcome these shortcomings but have not yet been fully exploited for CRS imaging, as previous implementations have suffered from high intensity noise, a narrow tuning range and low power, resulting in low image qualities and slow imaging speeds. Here, we present a novel high-power self-synchronized two-colour pulsed fibre laser that achieves excellent performance in terms of intensity stability (improved by 50 dB), timing jitter (24.3 fs), average power fluctuation (<0.5%), modulation depth (>20 dB) and pulse width variation (<1.8%) over an extended wavenumber range (2700–3550 cm?1). The versatility of the laser source enables, for the first time, high-contrast, fast CRS imaging without complicated noise reduction via balanced detection schemes. These capabilities are demonstrated in this work by imaging a wide range of species such as living human cells and mouse arterial tissues and performing multimodal nonlinear imaging of mouse tail, kidney and brain tissue sections by utilizing second-harmonic generation and two-photon excited fluorescence, which provides multiple optical contrast mechanisms simultaneously and maximizes the gathered information content for biological visualization and medical diagnosis. This work also establishes a general scenario for remodelling existing lasers into synchronized two-colour lasers and thus promotes a wider popularization and application of CRS imaging technologies.
关键词： Coherent Raman scattering,CRS microscopy,biomedical imaging,fibre laser,nonlinear optical imaging
更新于2025-09-23 15:21:01
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[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Interferometric Fourier-Transform Stimulated Raman Scattering
摘要： Coherent Raman scattering (CRS) is a powerful label-free spectroscopic technique that allows fingerprinting of molecules based on their intrinsic vibrational response. The vibrational transitions can be described using Lorentzian functions, whose superposition gives rise to a complex third-order resonant nonlinear susceptibility. Its direct experimental access would allow a very rich spectroscopic information, especially in the presence of mixtures of molecular species. Unfortunately, standard CRS techniques do not allow to fully extract it: stimulated Raman scattering (SRS) only measures the imaginary part of the nonlinear susceptibility, while coherent anti-Stokes Raman scattering (CARS) retrieves a mixture of real and imaginary parts, as the signal is proportional to the square of the nonlinear susceptibility plus a real, frequency-independent non-resonant term. The complex susceptibility can in principle be retrieved using: (1) analytic techniques, but they require the full spectrum of the nonlinear susceptibility, often not available; (2) interferometric SRS, employing a pulse shaper to generate two replicas of the broadband Stokes pulse, but the use of a spectrometer limits the modulation frequency of the pump and thus the SRS detection sensitivity; (3) interferometric CARS, employing a local oscillator (LO) field collinearly combined with the anti-Stokes field and at the same frequency, but it is technically very demanding as it requires three synchronized and phase-coherent light beams. Recently, we introduced a novel technique based on Fourier-transform (FT) spectroscopy able to detect in the time domain both the Stokes and the SRS broadband spectra. It employs an interferometer, based on a passive birefringent delay line, and a single-channel lock-in amplifier. Here we introduce a simple modification, which we call interferometric FT-SRS (IFT-SRS), allowing the retrieval of the full complex nonlinear susceptibility of the sample. In IFT-SRS a birefringent plate placed before the sample generates a reference LO beam anticipated by a few picoseconds (so that it does not experience any nonlinear interaction) and with perpendicular polarization with respect to the pump/Stokes pulses. After the sample, a delay line, consisting of a pair of birefringent wedges with optical axis rotated by 90° with respect to the first birefringent plate, is used to vary the overall delay between the pump and the LO from zero to a few picoseconds. Their interference on a photodetector (after projection to a common polarization direction by a linear polarizer) is sent to a lock-in amplifier which records both the linear and differential interferograms, whose FT provides simultaneously real and imaginary parts of the nonlinear susceptibility, as shown for ethanol, acetone and isopropanol solvents. They display well-resolved spectra in the real part of the FT, which are in good agreement with the spontaneous Raman spectra of the solvents, as well as the expected dispersive lineshapes in their imaginary parts.
关键词： stimulated Raman scattering,Fourier-transform spectroscopy,Coherent Raman scattering,nonlinear susceptibility
更新于2025-09-11 14:15:04