Water provides an important chemical contribution to the function and degradation of biological systems, plays a central role in regulating cell volume, nutrient transport, waste removal, and thermal regulation, and serves as a medium for biological reactions. Coherent anti-Stokes Raman scattering (CARS) imaging, as an important tool in biomedical applications, has the advantages of chemical specificity, free of label, high sensitivity, and so on, and it is widely used in brain tumor analysis, disease pathology analysis, and pharmacokinetics. Therefore, it is of great significance to study the CARS imaging light source for water molecules.

Two synchronous mode-locked fiber lasers are constructed using the mode-locking scheme of a nonlinear amplifying loop mirror (NALM). A portion of the pulse output of the erbium-doped fiber laser is output by pulse amplification module and frequency doubling module, which is called pump light, while another portion of the pulse is injected into the ytterbium-doped fiber laser to achieve pulse synchronization. The output pulse of the injected ytterbium-doped fiber laser is then amplified and emitted as Stokes light. An appropriate central wavelength fiber Bragg grating (FBG) is selected to control the central wavelength of the output pulse in the ytterbium-doped fiber laser, ensuring that the frequency difference between the two lasers and the vibration frequency of water meet the resonance condition. Finally, a dichroic mirror (DM) is used to spatially combine the pump light and Stokes light. The synchronous two-color pulses are injected into a commercial microscope (Olympus, FV1200) for CARS imaging.

In the experiment, a master-slave injection passive synchronous two-color fiber light source is built, consisting of pump light and Stokes light. The central wavelengths of the two outputs are 783 nm [Fig. 3(a)] and 1040.6 nm [Fig. 2(c)], respectively. The pulse widths are 146.0 fs [Fig. 3(a)] and 9.1 ps [Fig. 2(d)], with an output power of 146 mW and 2 W, respectively. The relationship between repetition frequency variation and cavity length mismatch with and without injection is studied. The maximum synchronous mismatch distance reaches 347 μm [Fig. 3(c)]. Finally, CARS imaging of fresh mouse ear subcutaneous adipose tissue is performed at 3156 cm^-1, as shown in the purple channel in Fig. 3(d). The distribution of intercellular water can be clearly observed.

In this paper, a CARS passive synchronous fiber laser for water is designed and constructed. The main pulse is injected into the slave laser cavity to achieve pulse synchronization. The relationship between frequency variation and cavity length mismatch with and without injection is studied. CARS imaging is performed on fresh mouse ear subcutaneous adipose tissue at 3156 cm^-1, and the imaging results are satisfactory. The passive synchronous two-color laser is expected to promote the application of CARS technology in the field of fast, real-time, and efficient pathology detection.