Semiconductor external modulators for 2.5 Gb/s optical transmission using selective growth by MOVPE

External modulators, which have smaller chirping characteristics than laser diode direct modulation, are desired for high-speed and long-distance optical fiber communication systems. We have developed InGaAsP/InP buried-hetero Franz-Keldysh type optical modulators suitable for practical use in 1.55 μm-2.5 Gb/s long-haul optical transmission systems. A novel fabrication method using the selective growth technique by metal-organic vapor phase epitaxy was implemented for the modulator fabrication. Since semiconductor etching is no longer required for narrow mesa stripe fabrication, the selective growth technique provides a simple fabrication process with highly accurate device dimension control. The fabricated modulators have been packaged, and fiber-pigtailed modulators with 8.1 dB fiber-to-fiber insertion loss, more than 13.4 dB extinction ratio at 3 V, and 3.7 GHz modulation bandwidth have been achieved. Normal fiber 120 km transmission at 2.5 Gb/s signal speed and 1.55 μm wavelength with power penalty less than 0.9 dB, using the packaged modulator, has been demonstrated. Moreover, excellent device reproducibility has been experimentally confirmed and a preliminary reliability check has been carried out.