A wavelength interleaver structure with asymmetrical output spectra, an important yet seldom investigated device, is presented. The interleaver has become an important fiber optics communication component. Numerous interleaver designs have been proposed and studied. However, most interleavers are designed for symmetrical spectrum splicing, so they cannot be very useful in applications where only a few channels among multiples need to be added or dropped. As a result, the design capability of network engineers is severely limited by the nature of the symmetrical interleaver. In this work, an asymmetrical interleaver structure based on the modified Michelson interferometer is proposed and investigated. The asymmetrical interleaver implemented with the lattice filter approach is also investigated for comparison. The proposed structure has better performance compared to other asymmetrical interleaver structures such as the lattice filter approach, and it also has advantages of compact size and low material costs. With an asymmetrical interleaver, network engineers could enjoy great design flexibility they would not possess otherwise.