Wavelength-switchable synchronously pumped Raman fiber laser near 1.7 µm for multispectral photoacoustic microscopy

We present a wavelength-switchable, synchronously pumped Raman fiber laser based on a dual-cavity design operating in the 1.7 µm region, specifically developed for multispectral photoacoustic microscopy (PAM). By precisely engineering the lengths of the pump laser cavity and the Raman laser cavity and meticulously controlling the modulation frequency, we achieved synchronous pumping by synchronizing the Stokes light within the Raman laser cavity to have the identical round-trip time as the pump light. Adjusting the electrical modulation frequency of the pump laser to 126.398 kHz, 128.136 kHz, and 130.808 kHz enabled rapid switching of the Raman laser output to final wavelengths of 1700.2 nm, 1710.4 nm, and 1720.3 nm, respectively, with a pulse duration of 4 ns and a pulse energy exceeding 500 nJ. Integrating this advanced laser into a near-infrared PAM system facilitated the acquisition of spectroscopic photoacoustic signals and images from polymer films with distinct C─H compositions: polyethylene, polypropylene, and polyimide. Our results demonstrate the efficacy of the proposed laser for wavelength switching in the 1700 nm region and its significant potential for differentiating molecules with various C─H groups in PAM applications.