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, 8 (2), 153-159

Stimulated Raman Scattering Microscopy With a Robust Fibre Laser Source

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Stimulated Raman Scattering Microscopy With a Robust Fibre Laser Source

Christian W Freudiger et al. Nat Photonics.

Abstract

Stimulated Raman Scattering microscopy allows label-free chemical imaging and has enabled exciting applications in biology, material science, and medicine. It provides a major advantage in imaging speed over spontaneous Raman scattering and has improved image contrast and spectral fidelity compared to coherent anti-Stokes Raman. Wider adoption of the technique has, however, been hindered by the need for a costly and environmentally sensitive tunable ultra-fast dual-wavelength source. We present the development of an optimized all-fibre laser system based on the optical synchronization of two picosecond power amplifiers. To circumvent the high-frequency laser noise intrinsic to amplified fibre lasers, we have further developed a high-speed noise cancellation system based on voltage-subtraction autobalanced detection. We demonstrate uncompromised imaging performance of our fibre-laser based stimulated Raman scattering microscope with shot-noise limited sensitivity and an imaging speed up to 1 frame/s.

Figures

Figure 1
Figure 1. Schematic of the fibre-laser system and SRS microscope
a, Energy diagram of SRS. When the difference in photon energy between the pump and the Stokes beam matches the energy of a vibrational state of the target molecule, ħΩ, molecules are efficiently excited from the ground to the corresponding excited state and a pump photon is absorbed (stimulated Raman loss, SRL) and a Stokes photon is generated (stimulated Raman gain, SRG). b, Schematic of the fibre laser. The laser system starts with an Er-doped fibre oscillator, which is mode-locked with a carbon nanotube (CNT) saturable absorber. The output is split into two arms to generate the pump (upper arm) and Stokes (lower arm) beams. The Stokes beam is modulated at 10MHz with an electro optic modulator (EOM), temporally and spatially combined with the pump beam, and aligned into to a beam-scanning microscope. The transmitted beams are collected with a condenser. The pump beam is detected by the autobalanced detector after the Stokes beam is blocked with an optical filter. The reference beam is sampled in front of the microscope with a polarizing beam splitter (BS).
Figure 2
Figure 2. Characterization of the fibre laser source
a, b,c, Tuning ranges of the Er- and Yb-doped fibre power amplifiers. The Er-arm is frequency doubled to provide the pump beam. d, e, Optical auto-correlations of the pump and Stokes pulses. f, Measurement of the timing jitter. Intensity noise at the peak (red) and half maximum (blue) of the optical cross-correlation (inset) over 5 minutes.
Figure 3
Figure 3. Autobalanced detection
a, Principle of balanced detection. b Noise suppression of the home-built auto-balanced detector optimized for 10MHz signal. c Noise spectrum of the fibre laser with (red) and without (blue) autobalanced detection. The theoretical shot-noise is indicated in grey. d,e, SRS images of 1.1 polystyrene beads with (d) and without (e) auto-balanced detection. Scale bars, 5.
Figure 4
Figure 4. SRS spectral imaging with the fibre-laser source
a, Two color image of a sebaceous gland in mouse skin acquired at 2850cm−1 (mainly lipids, green), 2950cm−1 (mainly proteins, red) and the composite of the two colours. b, z-stack acquired at 1 frame/s and cross sections from different directions. Scale bars, 50µm.

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