Photonic Integrated Circuits

A semiconductor laser contains a light‐amplifying gain medium and a miniature waveguide that confines the light to the laser cavity. With today's laser‐fabrication technology, one can integrate lasers with other active optical elements—such as detectors, optical amplifiers, optical modulators, and switches—on one semiconductor chip. The interconnections are miniature, transparent, passive waveguides that pipe light from one device to the next, playing a role similar to the metal lines that carry electricity in conventional integrated circuits. An exploratory technology to create such photonic integrated circuits is emerging from advances in vapor‐phase crystal growth and large‐area wafer processing. Initial research efforts are focused on relatively simple devices such as laser‐modulator, laser‐detector, and laser‐amplifier photonic integrated circuits. These configurations enhance the functional capabilities of a single laser, promising advantages in cost, compactness, and ease of packaging. Although today's complex optical systems require large optical benches and many delicate optical alignments, one can also envision these systems transformed into miniature, inexpensive, and robust photonic integrated circuits. Recent research demonstrations of such single‐chip circuits include wavelength‐division‐multiplexing transmission sources and balanced heterodyne receivers.