Pulsed 978 nm swept-source laser for high-speed multiphoton microscopy

Spectro-temporal laser imaging by diffractive excitation (SLIDE) enables two-photon microscopy (TPM) with 2D imaging rates in the kHz range and is capable of performing real-time volumetric imaging at video rates. However, the application of SLIDE microscopy is constrained by its available excitation wavelength. To date, imaging has only been demonstrated with central wavelengths of 1064 nm and 775 nm. The underlying cause is the spectral limitation of appropriate fiber-based active gain media, such as ytterbium and erbium. These media play a crucial role in SLIDE in augmenting laser pulse power for two-photon excitation. In the case of commonly used green fluorescent dyes such as Alexa Fluor 488 or green fluorescent protein (GFP), it is necessary to amplify light with a center wavelength in the range of 976 nm. Ytterbium appears to be a preferred medium for this task, but its quasi-three-level lasing behavior at this wavelength poses significant challenges in its application. This study presents a high-power swept-source laser light source around 978 nm. The approach utilizes an all-fiber Fourier domain mode locking master oscillator power amplifier (FDML-MOPA) with a central wavelength of 978 nm and a sweep bandwidth of 4 nm. The light source employs a Mach-Zehnder electro-optical modulator (EOM) to generate 30 ps pulses at 82 MHz (512 pulses per sweep). The pulses are amplified by 41 dB across three amplifier stages. This configuration enables the generation of pulse peak powers of up to 305 W. Initial experiments with classic raster scanning and SLIDE scanning demonstrate the potential of this laser for high-speed GFP multiphoton microscopy using a pulse-on-demand method.