A flexible and fast synthesis method for designing asymmetric interleavers with flat-top, low dispersion, adjustable isolation, and bandwidths at both outputs is proposed. Interleavers are designed on birefringent technology allowing linear-phase designs. Z-transform properties and digital filter design techniques are used for designing interleavers under a set of specifications. At a second level, a genetic algorithm optimization approach allows improving the interleaver design adjusting the minimum isolation, at both outputs, below -43 dB and the stop bandwidths to specific normalized values of ±0.15 and ±0.3, with zero dispersion at one output. Improvement of the interleaver order and dispersion below ±50 ps/nm at both outputs are also reported.