Disclosed is a novel "hermetic" optical assembly. A radiation-­transmissive member (10) seals an aperture (22) in the assembly housing. The member comprises two major surfaces and a recess (12) that extends from one surface towards the other, with a "septum" (14) between the bottom of the recess and the other surface. An optical or opto-electronic device (23) typically is mounted on the inside surface of the member, and the end of an optical fiber is maintained outside of the assembly in coupling relationship with the device. In a currently preferred embodiment the recess extends from the outward-facing surface of the member and serves to position the end of the optical fiber with respect to a device mounted in a "well" (11) in the inward-facing surface of the member. In another embodiment, the outward facing surface of the member comprises geometrical features (e.g., recesses) that mate with corresponding features (e.g., protrusions) on a guide plate that serves to position the ends of a multiplicity of fibers with respect to devices mounted on the bottom of a recess extending from the inward-facing surface of the radiation-transmissive member. In all embodiments radiation is transmitted through the septum. In preferred embodiments the radiation-transmissive member is (100) Si, with the recess (and other features) produced by means comprising photolithography and anisotropic etching.