Fig. 1. The principle, experimental setup, analogous representation, and conceptual diagrams of the FOCUS method. (a) Schematic illustration of the FOCUS. (b) Spatial representation of frequency-to-time conversion in FOCUS. (c) Experimental setup of the FOCUS system based on FWM via fiber optics. (d) Conceptual diagram of the on-chip FOCUS system. MLL: mode-locked laser; SMF: single-mode fiber; OC: fiber optical coupler; AMP: amplifier; SS: spectral shaper; PC: polarization controller; BPF: band-pass filter; HNLF: highly nonlinear fiber; COMP: compressor; GVD: group velocity dispersion.

Fig. 2. The simulation analysis results for the temporal resolution and spectral characteristics of the FOCUS. (a) Output pulse duration with and without compression for different expected sub-pulse durations. The vertical line represents the threshold for rect, sech2, and Gauss shaping. (b) Temporal intensity distribution of a cat’s face profile with an expected sub-pulse duration of 1 ps and a record length exceeding 1.2 ns. (c) Temporal simulation result for an expected sub-pulse duration of 30 ps. (d) Spectral simulation result for an expected sub-pulse duration of 30 ps. (e) Temporal simulation result for an expected sub-pulse duration of 3 ps. The inset shows the duration of uncompressed single sub-pulse with rectangular shaping. (f) Spectral simulation result for an expected sub-pulse duration of 3 ps.

Fig. 3. Typical output spectrum of the FWM in the FOCUS method. The setup is the same as in the primary experimental configuration.

Fig. 4. Experimental results of the FOCUS system. (a) Signal spectrum in the primary configuration for testing system record length. (b) Signal spectrum in the second configuration for testing system resolution. (c) Idler spectrum in the primary configuration. (d) Idler spectrum in the second configuration. (e) Output temporal profile (blue line) in the primary configuration, measured by a 30 GHz oscilloscope with a 45 GHz photodetector. The red line shows alternative output sub-pulse intensity distribution obtained by adjusting the polarization state of the pulses before the FWM process. (f) Output temporal profile measured by an autocorrelator in the second configuration.

Fig. 5. Temporal resolution and record length characteristics of the FOCUS system. (a) Dependence of temporal resolution on spectral resolution (SR) and pump bandwidth (PB). (b) Dependence of record length on signal GDD and pump bandwidth.