In order to improve transmission efficiency of optical-fiber communication system for hybrid 10 Gbit/s and 40 Gbit/s system, a novel device configuration for realizing an asymmetrical optical interleaver based on planar optical waveguide is proposed, which is a clever combination of optical micro-ring and Mach-Zehnder interferometers. Theoretical results show that optical interleaver with arbitrary bandwidth ratio and with the proposed structure can be obtained by controlling some key parameters. As an example, an asymmetrical interleaver with 12 interleaving ratio is presented, which can apportion 32.85 GHz and 67.15 GHz passband bandwidth for the 10 Gbit/s and 40 Gbit/s channels over 100 GHz channel spacing respectively. Therefore, the bandwidth efficiency requirement of the 40 Gbit/s channel is relieved. Furthermore, by analyzing the effect of optical power splitting ratio of the three two-mode interferometers on the output spectrum bandwidth ratio, the design instances of asymmetrical interleaver with different bandwidth ratios at the same channel space are given.