Background free CARS imaging by phase sensitive heterodyne CARS

In this article we show that heterodyne CARS, based on a controlled and stable phase-preserving chain, can be used to measure amplitude and phase information of molecular vibration modes. The technique is validated by a comparison of the imaginary part of the heterodyne CARS spectrum to the spontaneous Raman spectrum of polyethylene. The detection of the phase allows for rejection of the non-resonant background from the data. The resulting improvement of the signal to noise ratio is shown by measurements on a sample containing lipid

Shot noise limited heterodyne detection of CARS signals

We demonstrate heterodyne detection of CARS signals using a cascaded phase-preserving chain to generate the\ud CARS input wavelengths and a coherent local oscillator. The heterodyne ampli¯cation by the local oscillator re-\ud veals a window for shot noise limited detection before the signal-to-noise is limited by amplitude °uctuations. We\ud demonstrate an improvement in sensitivity by more than 3 orders of magnitude for detection using a photodiode.\ud This will enable CARS microscopy to reveal concentrations below the current mMolar range

Shot noise limited heterodyne detection of CARS signals

We demonstrate heterodyne detection of CARS signals using a cascaded phase-preserving chain to generate the CARS input wavelengths and a coherent local oscillator. The heterodyne amplification by the local oscillator reveals a window for shot noise limited detection before the signal-to-noise is limited by amplitude fluctuations. We demonstrate an improvement in sensitivity by more than 3 orders of magnitude for detection using a photodiode. This will enable CARS microscopy to reveal concentrations below the current mMolar range