Fig. 1. (a) Schematic and (b) photograph of our deep-red femtosecond MLFL. (c) Transmission spectrum of visible dichroic mirrors. DM, dichroic mirror; PBS, polarization beam splitter; TGs, transmission gratings; HWP, half-wave plate; QWP, quarter-wave plate; PD-ISO, polarization-dependent isolator; DSM, D-shaped mirror; HRPM, hollow roof prism mirror.

Fig. 2. Characteristics of our sub-100-fs deep-red MLFL under the net dispersion of $+0.005{\mathrm{ps}}^2$. (a) Spectra of mode-locked and continuous-wave output pulses. (b) Oscilloscope trace of the pulse train (inset: a screenshot of the oscilloscope trace). (c) Autocorrelation trace of the output pulses (inset: autocorrelation trace of the pulses extracted after PBS via a mirror). (d) RF spectrum at the fundamental frequency (inset: RF spectrum in the range of 3.6 GHz span).

Fig. 3. Characteristics of our sub-100-fs deep-red MLFL under the net dispersion of $+0.005{\mathrm{ps}}^2$. (a) Spectra at different times under the pump power of 1.96 W. (b) Power stability curve of deep-red mode-locked laser under 2.10-W pump power. (c) Average power and pulse energy with different pump powers. (d) Pulse duration and peak power with different pump powers.

Fig. 4. Characteristics of deep-red MLFL under different dispersions. (a) Spectra and (b) autocorrelation traces of the pulses extracted after PBS via a mirror. (c) Autocorrelation traces of the dechirped pulses outputted from the DSM.

Fig. 5. Simulation performance of our deep-red MLFL with the net dispersion of $+0.005{\mathrm{ps}}^2$. (a) Pulse and (b) spectrum evolution with the roundtrip number. (c) Output pulse temporal profile (solid) and frequency chirp (dashed). (d) Output spectra on a logarithmic (solid) or linear (dashed) scale. (e) Intracavity spectrum. (f) Pulse evolution. (g) Spectral bandwidth and pulse duration with the intracavity position.

Fig. 6. Application of the 717-nm MLFL as the excitation laser source in 2PM. (a) Composite 2PM image of DAPI-, AF488-, and AF568-labeled structures in the mouse kidney section. (b) Emission spectra of DAPI, AF488, and AF568, obtained from Ref. 34. The shadowed regions indicate the working bandwidth of the optical filters. The emission filters are carefully chosen to minimize the channel crosstalk and maximize the detection efficiency. (c) Separated images for the three dyes. Scale bar: $20\mu \mathrm{m}$. (d) PSF of the 717-nm 2PM. Inset: the intensity distribution of the beam focus after the objective lens. (e) Two-photon excitation spectra of the mCherry, tdTomato, and mStrawberry, obtained from Ref. 35. The shadowed region indicates the excitation region of the 717-nm laser. (f) and (g) 2PM images of the mCherry-labeled C. elegans.