A method of making a circuitized substrate (e.g., PCB) including at least one and possibly several internal optical pathways as part thereof such that the resulting substrate will be capable of transmitting and/or receiving both electrical and optical signals. The method involves forming at least one opening between a side of the optical core and an adjacent upstanding member such that the opening is defined by at least one angular sidewall. Light passing through the optical core material (or into the core from above) is reflected off this angular sidewall. The medium (e.g., air) within the opening thus also serves as a reflecting medium due to its own reflective index in comparison to that of the adjacent optical core material. The method utilizes many processes used in conventional PCB manufacturing, thereby keeping costs to a minimum. The formed substrate is capable of being both optically and electrically coupled to one or more other substrates possessing similar capabilities, thereby forming an electro-optical assembly of such substrates.