Techniques for partitioning and/or combining at least a portion of an optical network tree including one or more array waveguide gratings (“AWGs”) and fibers in wavelength division multiplexing (“WDM”) passive optical networks (“PON”) with cascaded AWGs are disclosed. Example methods include deriving the optimal positions for the AWGs to minimize fiber cost and then determining the arrangement of cascaded AWGs to minimize the total cost of AWGs and fibers. Determining the arrangement of cascaded AWGs may include recursive partitioning followed by recursive combination. An example recursive partition-combination based algorithm for optimizing a tradeoff between the AWG cost and the fiber cost is disclosed.