Designs of optical devices particularly useful for separating channels signals that are originally multiplexed are disclosed. In accordance with one design, an optical filter configured to reflect a particular channel signal is positioned to receive a multiplexed signal at a small incident angle so as to minimize residuals of other signals in the channel signal. To further increase isolation from the other channel signals, another filter is introduced to transmit only the reflected channel signal, the another filter has a frequency response reciprocal to that of the original optical filter. One of the key advantages, benefits and objects of the designs is to increase channel signals separation efficiency by increasing isolation among separated channel signals.