Fiber four-wave mixing source for coherent anti-Stokes Raman scattering microscopy

Abstract

We present a fiber-format picosecond light source for coherent anti-Stokes Raman scattering microscopy. Pulses from a Yb-doped fiber amplifier are frequency converted by four-wave mixing (FWM) in normal-dispersion photonic crystal fiber to produce a synchronized two-color picosecond pulse train. We show that seeding the FWM process overcomes the deleterious effects of group-velocity mismatch and allows efficient conversion into narrow frequency bands. The source generates more than 160 mW of nearly transform-limited pulses tunable from 775 to 815 nm. High-quality coherent Raman images of animal tissues and cells acquired with this source are presented.

Fig. 1

(Color online) Phase-matched FWM gain for an endlessly single-mode PCF. The ZDW is 1051 nm, the nonlinear parameter γ = 9.6(W km)⁻¹, and the cw power is P₀ = 3.6 kW.

Fig. 2

(Color online) Simulated FWM. (a) Full spectrum without idler seeding after propagation through 56 cm (light solid curve) and 2 m (light dotted curve), and with idler seeded after 30 cm (black solid curve). GVM length is ~50 cm. (b) Seeded FWM: signal (solid curve), pump (dotted curve), and idler (dashed curve) pulses. (c) Signal spectrum. The input pulse is centered at 1036 nm with 7.5 ps duration and 3.6 kW peak power. Idler seed power is 5 mW at 1470 nm.

Fig. 3

(Color online) Experimental setup: HWP, half-wave plate; PBS, polarizing beam splitter; LP, long-pass dichroic mirror; SF, filters (Thorlabs DMSP1000, Chroma HQ735LP); SP, short-pass dichroic mirror.

Fig. 4

(Color online) Experimental FWM results after 30 cm of PCF: signal (a) spectrum and (b) autocorrelation, (c) idler and seed. (d) Picked-off pump spectrum.

Fig. 5

(Color online) Forward-CARS images at 2850 cm⁻¹ (a) stratum corneum and (b) sebaceous gland in mouse ear, (c) mouse brain section, (d) rat fibroblasts. 512 × 512 pixels at 4 s/frame, no averaging.