Noise in stimulated Raman scattering measurement: From basics to practice

X Audier,P Volz,S Heuke,H Rigneault,I Rimke

doi:10.1063/1.5129212

Abstract

We revisit laser intensity noise in the context of stimulated Raman scattering (SRS), which has recently proved to be a key technique to provide label free images of chemical bonds in biological and medical samples. Contrary to most microscopy techniques, which detect a weak photon flux resulting from light matter interactions, SRS is a pump-probe scheme that works in the high flux regime and happens as a weak modulation (10−4–10−6) in a strong laser field. As a result, laser noise is a key issue in SRS detection. This practical tutorial provides the experimentalists with the tools required to assess the amount of noise and the ultimate SRS detection limit in a conventional lock-in-based SRS system. We first define the quantities that are relevant when discussing intensity noise and illustrate them through a conventional model of light detection by a photodiode. SRS is then introduced in its lock-in-based implementation, and the model presented is adapted in this particular case. The power spectral density, relative intensity noise (RIN), signal to noise ratio, and sensitivity of the system are derived and discussed. Two complementary methods are presented that allow measurement of the RIN and assessment of the performance of a SRS system. Such measurements are illustrated on two commercial laser systems. Finally, the consequences of noise in SRS are discussed, and future developments are suggested. The presentation is made simple enough for undergraduate students, graduate students, and newcomers in the field of stimulated Raman and more generally in pump-probe based schemes.

Highlights

For an average photocurrent of Iavg = 5 mA, the recovered laser relative intensity noise (RIN) is plotted in Fig. 3(a) for devices 1 and 2
This corresponds to a signal to noise ratio (SNR) for the system—at such modulation frequencies and photocurrent—that is only a factor of 3 below the shot noise limit, meaning that a measurement requires a bandwidth 3 times smaller than that of a shot noise limited system to receive the same SNR [Eq (15)]
Complementary to the frequency analysis, one needs to measure the RIN for different photodiode currents to characterize the optimal setup working parameters. This analysis was performed by recording the RIN as a function of frequency for photocurrents ranging from 0.5 mA to 7 mA, after which the detector saturates

Summary

Introduction

Over the last two decades, coherent Raman imaging has evolved as a mature, label-free, imaging technique with numerous applications in biology and medicine. The seminal work of Zumbusch in 1999 revived coherent anti-Stokes Raman scattering (CARS) as a vibrational microscopic imaging modality. Since the coherent Raman imaging field experienced a second revolution in 2008 when stimulated Raman scattering (SRS) was demonstrated as a powerful vibrational imaging scheme. Contrary to CARS, SRS is free of nonresonant background and scales linearly with the molecular concentration. These key features initiated the development of SRS imaging technologies and facilitated their successful applications in biology, chemistry, and medicine as a quantitative and label free chemical imaging modality.Contrary to CARS, which detects faint generated photons at specific wavelengths, SRS is a pump-probe scheme that works in the high photon flux regime. Contrary to CARS, SRS is free of nonresonant background and scales linearly with the molecular concentration.. Contrary to CARS, SRS is free of nonresonant background and scales linearly with the molecular concentration.7 These key features initiated the development of SRS imaging technologies and facilitated their successful applications in biology, chemistry, and medicine as a quantitative and label free chemical imaging modality. Contrary to CARS, which detects faint generated photons at specific wavelengths, SRS is a pump-probe scheme that works in the high photon flux regime. It manifests itself as a weak modulation (10−4–10−6) that is transferred from an amplitude modulated (AM) laser on an unmodulated (probe) laser beam.

Results

Discussion

Conclusion