Abstract: The present invention provides a light generation method and a light source that are preferable in obtaining a single-mode light having high outputs, a small wavelength spectral bandwidth, and a low intensity noise. A white output from a white-light source and having wavelength components over a wide-band in a wavelength spectrum passes through an isolator and is then filtered by a disc-shaped optical filter. A transmitted light is input to a semiconductor optical amplifier having its polarization plane regulated by a polarization controller. An output from the amplifier passes through an isolator and is then filtered by a disc-shaped optical filter again to generate a single-mode light having a small spectral bandwidth and high outputs. Polarization control by the polarization controller corresponds to control of the polarization plane of a semiconductor laser in the semiconductor optical amplifier so as to obtain a maximum gain from the amplifier.

Abstract: An optical wavelength division multiplexing network which can carry out large-capacity access service with a simple constitution is provided. In an optical network comprising a plurality of layers, a highest level network comprises a ring network having a center node and remote nodes; an intermediate level network comprises a ring centered around a node belonging to the higher level network, and a lowest level network comprises a star network centered around an access node which multiplexes traffic from a plurality of ONU, the ONU and the access node being directly joined together by optical fibers; the center node belonging to the highest level network and the ONU establish a direct communication path by using lights of different wavelengths; at nodes provided therebetween, the optical signals are not electrically processed but are amplified, branched, and routed.

Abstract: An optical cross-connect system which can be constructed using the same kind of optical switches having a small extinction ratio and a small number of optical switch driving circuits is provided.