A spectral economized scheme for optical quantization is proposed by using cascaded unbalanced Mach–Zehnder modulators (UMZMs) to introduce phase differences between two arms. Less spectra are needed in this scheme since the period of sampling pulse can be modulated to be larger. Simulation results show that the effective number of bits (ENOB) reaches 4 when only 2 additional wavelengths and 4 periods are employed, in contrast to the 15 additional wavelengths required by traditional frequency quantization schemes. Larger NOB is predicated when more wavelengths are used in the quantization system. The influence of noise on ENOB is also studied. It is revealed that the timing and intensity jitters do not influence ENOB. Instead, the ultimate ENOB is limited by the largest phase difference introduced by UMZMs. The proposed scheme can be used in both low- and high-resolution optical quantizers, indicating promising spectrum economized applications for future optical circuits.