Embodiments of present invention provide an ultra-small-pitch optical filter assembly. The assembly includes a fiber collimator being able to receive an optical signal; a WDM filter module being able to de-multiplex the optical signal from the fiber collimator into multiple optical beams; and an optical lens assembly being able to receive the multiple optical beams from the WDM filter module and to reduce a physical spacing among the multiple optical beams from a first pitch D to a second pitch d, wherein D/d A method of fabricating the ultra-small-pitch optical filter assembly is also provided. A method of producing a set of optical beams with ultra-small-pitch of spacing is provided as well.