Shot-Noise Limited Time-Encoded Raman Spectroscopy

Figure 2

Raman spectra extraction by two-stage balanced detection, leading to an SRG spectrum of benzene. Left: The first analog balancing step employs a balanced, differentially amplified photoreceiver to remove the probe light offset. Only power changes in the sample arm remain (cf. Figure 1). Middle: The analog-to-digital converter (ADC) card is used to synchronously sample the FDML probe laser sweeps. The 800 mV range is sampled at 12-bit depth resolution. The lower part shows a zoom-in on a part where SRG signals occur. The vertical lines represent the positions of the pump pulses following the TICO-Raman excitation technique [17]. The SRG signals are clearly visible and fully recorded as ensured by the synchronized detection (see text). The second digital balancing step subtracts consecutive sweeps to further lower the noise level to the shot-noise limit. Only pump-induced intensity changes remain. This step makes use of TICO-Raman pattern, where pulse positions increase from sweep to sweep. Thus, through subtraction, any artifacts due to chromatic imperfections are removed. Right: The last step assigns wavenumber values to the Raman spectrum. First, the spectrum of the FDML laser (inset) is recorded on an optical spectrum analyzer (OSA). Then, the wavelength-to-time mapping of the FDML laser is calculated and from that the energy difference to the pump laser is determined. The result is accurate Raman shift energies for each spectral point. Shown in blue is the final shot-noise limited TICO-Raman spectrum of benzene, acquired in 9 ms.