976 nm Swept-Source FDML-MOPA Laser for Multiphoton Microscopy

Genetically-expressed calcium indicators based on green fluorescence have become an invaluable tool to study cellular activity in vivo. Multiphoton microscopy (MPM) is particularly suited for investigating processes in living animals or in thicker tissue due to its superior penetration depth. To achieve the required photon density for MPM, we use an ytterbium-doped fiber amplifier (YDFA) to provide excitation at 976 nm. Here, efficient amplification is challenging due to strong three-level behavior [1]. This work demonstrates a swept-source Fourier Domain Mode Locked Master Oscillator Power Amplifier (FDML-MOPA) laser to enable ultra-fast MPM [2]. The basis for this is an FDML laser, which provides a sweep frequency of 160 kHz at a sweep range of 4 nm around a center wavelength of 977.8 nm. This seed laser is pulse-modulated using an electro-optical modulator (EOM) with a pulse repetition rate of 80 MHz and a pulse width of approx. 30 ps FWHM. The pulses are then amplified with two core-pumped YDFAs and finally brought to a peak power of up to 305 W using a double-clad YDFA. The laser achieves performance parameters suitable for high-speed MPM, as presented in the full characterization in Fig. 1.