Abstract: A substantially planar substrate (10), typically constituting a part of a hybrid electro-optical device, has a portion (12) defining a mounting location for optoelectronic components (14) such as laser sources, photodetector diodes, LEDs, requiring local hermetic protection of the bare chips. A planar lightwave circuit (PLC) waveguide structure formed on the substrate (10) extends to the mounting location (12) to define an optical signal feed-through for the device. At least one electrode (20) is associated with the planar lightwave circuit waveguide structure (18) and extends said mounting location (12) to define an electrical signal feed-through for the device (14). A ring-like structure (26) continuously surrounds said mounting location (12). The ring-like structure (26) is hermetic with respect to the planar substrate (12), and a continuous cover member (28) is arranged to cover the mounting location (12) and having a peripheral rim portion (32) co-extensive with the ring-like structure (26).