23 February 2018 Electronically tunable femtosecond all-fiber optical parametric oscillator for multi-photon microscopy
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We present a femtosecond fiber-based optical parametric oscillator (FOPO) for multiphoton microscopy with wavelength tuning by electronic repetition rate tuning in combination with a dispersive filter in the FOPO cavity. The all-spliced, all-fiber FOPO cavity is based on polarization-maintaining fibers and a broadband output coupler, allowing to get access to the resonant signal pulses as well as the idler pulses simultaneously.

The system was pumped by a gain-switched fiber-coupled laser diode emitting pulses at a central wavelength of 1030 nm and an electronically tunable repetition frequency of about 2 MHz. The pump pulses were amplified in an Ytterbium fiber amplifier system with a pulse duration after amplification of 13 ps. Tuning of the idler (1140 nm - 1300 nm) and signal wavelengths (850 nm - 940 nm) was achieved by changing the repetition frequency of the pump laser by about 4 kHz. The generated signal pulses reached a pulse energy of up to 9.2 nJ at 920 nm and were spectrally broadened to about 6 nm in the FOPO by a combination of self-phase and cross-phase modulation. We showed external compression of the idler pulses at 920 nm to about 430 fs and appleid them to two-photon excitation microscopy with green fluorescent dyes.

The presented system constitutes an important step towards a fully fiber-integrated all-electronically tunable and, thereby, programmable light source and already embodies a versatile and flexible light source for applications, e.g., for smart microscopy.
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Tim Hellwig, Tim Hellwig, Maximilian Brinkmann, Maximilian Brinkmann, Carsten Fallnich, Carsten Fallnich, "Electronically tunable femtosecond all-fiber optical parametric oscillator for multi-photon microscopy ", Proc. SPIE 10498, Multiphoton Microscopy in the Biomedical Sciences XVIII, 1049823 (23 February 2018); doi: 10.1117/12.2284380; https://doi.org/10.1117/12.2284380

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