Stimulated Raman Scattering Microscopy with an All-Optical Modulator

Nonlinear phenomena are a frequent occurrence in nature and inherent to many optical systems. Nevertheless, nonlinearities are often avoided or suppressed in optical resonators. Here we demonstrate the application of nonlinear phenomena to pump-probe spectroscopy by using period doubling in a fiber-feedback optical parametric oscillator. On the basis of an analysis of the optical single-pulse spectra and successive second-harmonic spectra, we introduce a method to enhance the pulse-energy modulation depth to the level of conventional high-frequency modulators. We find that the modulator is suitable for high-speed, low-noise stimulated Raman scattering imaging with performance equal to that of acousto-optic modulation. The concept removes the need for electronic synchronization, reduces the detection effort and artifacts, and has great potential in terms of scalability of the modulation frequency.

Figure 1

The all-optical modulator is based on a fiber-feedback optical parametric oscillator with a nonlinear feedback medium and a second-harmonic generation stage (SHG). OC, variable output coupler.

Figure 2

Bifurcation diagrams of the FFOPO intracavity pulse energy for increasing cavity delay (a)–(d). The steady state (P1) is highlighted in yellow, P2 cycles are highlighted in red, P3 cycles are highlighted in green, and P4 cycles are highlighted in blue. The different hues of the colors are used to indicate the different branches of the PN cycles for better visibility. These measured cross sections can be combined to form a map of the parameter space (e). For clarity, limit cycles (LC) and chaos (Ch) are excluded. For this figure, the output coupling ratio and crystal temperature remain constant. The projection of (e) is shown in (f) and includes limit cycles (orange) and chaos (purple). The white regions are unstable or oscillating between different attractors. The black circle indicates a P2 cycle at a delay of $156\mu \mathrm{m}$ and a power of 0.775 W, where later SRS measurements are performed (see Figs. 3–5).

Figure 3

(a) Setup for DFT and second-harmonic generation. (b) Combined spectrum as obtained by adding up the single-pulse spectra of two consecutive pulses (green) in comparison with a reference spectrum recorded with an optical spectrum analyzer (dark blue). (c) Single-pulse spectra of the two fundamental FFOPO pulses (black, red) prepared in a P2 attractor, as measured with the DFT oscilloscope. The inset showing the pulse train shows the actual energy ratio. (d) SHG pulse energy for the two different P2 pulses [colors according to the spectra in (c)] on variation of the SHG center wavelength by our tuning the phase-matching wavelength of the SHG crystal. Exemplarily, four measured SHG spectra are shown in blue (i–iv) with an illustration of their corresponding pulse trains, indicating the experimental modulation depth at different SHG center wavelengths. DET, detector; SMF, single-mode fiber.

Figure 4

(a) Spectra of the SHG in the P2 state. (b) Modulation depth versus wavelength obtained with a single P2 attractor. (c) Stability of the modulation depth over 1 h, sampled at a rate of 10 Hz. (d) SRG spectrum of acetone compared with the spontaneous Raman signal from the literature.

Figure 5

Images (150 pixels by 150 pixels) of 6-$\mu \mathrm{m}$ PMMA and polystyrene beads in aqueous solution taken at Raman pump-Stokes frequency detunings of $2953{\mathrm{cm}}^{-1}$ (blue) and $3066{\mathrm{cm}}^{-1}$ (red) with a phase-locked AOM at $f_{\mathrm{rep}}/2$ (a) and with the P2-based modulator (b), as well as corresponding cross sections (c),(d). The lock-in time constant is set to $10\mu \mathrm{s}$. The same image is shown for different time constants for the P2-based modulator (e). A comparison of the achieved signal-to-noise ratio (SNR) between the AOM (red) and the P2-based modulator (black) shows equal performance of both devices (f). All images are acquired with 10-mW pump power and 5.5-mW Stokes power on the sample.