We propose an improved cascade all-optical quantizing scheme for the resolution enhancement of all-optical analog-to-digital converter. Employing an additional intensity-modulated label channel, remarkable quantization resolution over 8 bit is feasible for an ultra-wideband signal. Simulation results show that a 20 GHz radio frequency signal is digitalized using the proposed scheme with the spur-free dynamic range of 53.76 dBc and effective-number-of-bits of 8.18 bit.