An optical interposer includes grooves (310) for optical fiber cables (104) coupled to a transducer (120). The grooves are formed by etching a cavity (410) in a substrate (130), filling the cavity with some layer (520), then etching the layer to form the grooves. The cavity has outwardly sloped sidewalls on which mirrors (144) are later formed. The groove etch is selective not to damage the sidewalls. The groove depth is uniform due to high etch selectivity of the layer, and also because of good control over the cavity etch due to the low aspect ratio of the cavity. Electrical circuitry for connection to the transducer is fabricated after the cavity filling but before the groove etch. The cavity filling leaves the wafer planar, facilitating fabrication of the electrical circuitry. Grooves can be provided on top and bottom of the interposer. Other features are also provided.