Lens-free in-line optical isolators.

We constructed a lens-free in-line optical isolator by embedding an isolator chip in a thermally expanded core (TEC) fiber without complicated optical alignment. The chip consists of two pairs of rutile wedges and garnet plates. We fabricated the TEC fibers by heating a single-mode fiber with a 200-mum outer diameter; low loss and a spot diameter as large as 49 mum were successfully obtained. The fabricated isolator has excellent optical properties, an insertion loss of 0.45 dB, and an isolation of more than 50 dB at a 1.55-mum wavelength, which confirms the usefulness of the integration techniques.