Programmable Dual‐Wavelength Switchable All‐fiber Laser via Hybrid Optical Amplifier in the NIR‐III Region for Multi‐Contrast Photoacoustic Microscopy

Abstract Photoacoustic microscopy (PAM) is a bond‐selective imaging technique that provides optical resolution in deep tissues. Employing multi‐contrast PAM expands the potential for non‐destructive and label‐free analysis of various substances. The efficiency of multi‐contrast PAM largely depends on laser sources possessing high pulse switching rates. Recently, a dual‐wavelength switchable fiber amplifier operating in the near‐infrared‐II (NIR) window is reported for multi‐contrast PAM. Nevertheless, transitioning to the NIR‐III region, which has higher absorption peaks, presents challenges due to the fixed Raman frequency shift and the absence of suitable pumps, complicating the search for a switchable source within this window. Herein, a unique dual‐wavelength switchable all‐fiber laser functioning within the NIR‐III region is presented via a hybrid optical amplifier. Thanks to flexible optical parametric conversion, the laser produces 1.5 µJ pulses and realizes arbitrary pulse trains with 1725‐nm and 1930‐nm wavelengths. Simultaneously, utilizing programmable pulse shaping for two telecom lasers, the proposed source generates high power‐spectral‐density pulses with a flexible pulse width and a switching frequency of up to 100 kHz. Multi‐contrast PAM using the proposed source distinguishes between the two types of microplastics in water, presenting a promising methodology for efficient microplastic detection in aqueous environments.