Fig. 1. (a) Schematic diagram of the proposed all-optical ADC. MLLD, mode-locked laser diode; AWG, arrayed waveguide grating; PD, photodiode. (b) Illustrated four sampled pulses with different optical power levels. (c) Corresponding SPM-induced optical spectrum broadening and splitting. Red and green solid lines indicate the filtering wavelengths λ1 and λ2 of the two quantization channels. Red and green dashed lines indicate the decision thresholds.

Fig. 2. (a) TEM image of the Si-nc film before depositing the upper layer of high-index doped silica glass. Inset: an individual silicon nanocrystal of ∼3  nm in dimension. (b) Calculated dispersion for the waveguides for the quasi-TE polarization. (c) SEM image for the waveguide with a thin layer of 20 nm Si-nc in the core center. (d) Corresponding simulated electric field profile of the quasi-TE polarization.

Fig. 3. (a) OFDR trace from the ∼50  cm long spiral waveguide pigtailed to the fiber array. The in/through ports of the spiral waveguides can be clearly identified as the high peaks at the beginning and end of the recorded traces. The linear fit of the waveguide backscattering (the short-dotted line) is performed over a wavelength range of 1525–1567 nm, the half-slope of which gives the propagation loss. Moreover, the linear relationship reflects the neglectable bending loss. The extent of loss fluctuation of the waveguide with the Si-nc strip is almost the same as that without the strip case, indicating high quality of the nanocrystal layer in the core center. (b) Reciprocal transmission as a function of the coupled peak power. Solid lines represent the fitting results. (c) FWM spectrum recorded by OSA for the spiral waveguide without Si-nc. (d) Conversion efficiency as a function of pump power. Dots: experimental results; solid lines: linear fitting results.

Fig. 4. (a) Experimental setup (FFL, femtosecond fiber laser; VOA, variable optical attenuator; PM, power meter). (b) Measured spectral profiles at the output of the 50 cm Si-nc loaded waveguide (1.75  μm×1.75  μm) with different input pump peak powers. (c) Measured power transfer functions of the two quantization channels by filtering the spectrum at the wavelengths of 1557 and 1558 nm, respectively. (d),(e) Power transfer functions of the two channels with binary decision results.

Fig. 5. Simulated (a) spectral and (b) temporal profiles when pumping at 1550, 1558, and 1565 nm, respectively. The wavelength axis of the spectral profiles is normalized to the pumping wavelengths.

Fig. 6. Simulated spectrograms at the pump peak power of (a) 35 W, (b) 95.7 W, (c) 220 W, and (d) 380 W, respectively. d_i, i=1, 2, 3, indicates the number of dips on the half-part of the spectrum.