Abstract: The invention provides a planar lightwave circuit type variable optical attenuator in which a small PDL is maintained even when an attenuation is large. The variable optical attenuator is constituted by a Mach-Zehnder interferometer comprising a substrate, two optical waveguide arms buried in a clad formed on the substrate, and thin-film heaters disposed on the surface of the clad and adjusting optical lengths of the optical waveguide arms. The optical waveguide arms are each made of a silica-based glass material, and a difference &Dgr;L0 between arm lengths is set to be in the range of 0.38 &mgr;m to 0.52 &mgr;m.

Abstract: The invention provides an apparatus for and method of monitoring wavelength multiplexed signal light, which can obtain monitored data for multi-channel optical signals contained in the wavelength multiplexed signal light, which can avoid a large size and complicated structure and high cost of monitoring apparatus, and which can easily monitor the wavelength multiplexed signal light. The invention also provides an optical transmission system employing the monitoring apparatus and method. An optical filter being able to control a loss pattern is disposed on a monitoring waveguide for the wavelength multiplexed signal light, which is branched for monitoring from an input waveguide and an output waveguide constituting a main optical transmission path in a wavelength multiplexed signal light monitoring apparatus.

Abstract: A silica based optical waveguide is described, comprising a substrate, a core waveguide formed thereon, and an over cladding part comprising a silica based glass with a refractive index lowering dopant and a refractive index increasing dopant added, formed on the substrate so as to cover the core waveguide, wherein a segregation layer with a higher concentration of the refractive index increasing dopant is formed in a part of the over cladding part in contact with the substrate and the core waveguide such that at least a part of the refractive index increase in the segregation layer provided by the refractive index increasing dopant with respect to the part of the over cladding part other than the segregation layer is offset by decline of the refractive index by increasing the amount of the refractive index lowering dopant added in the segregation layer and/or adding another refractive index lowering dopant.

Abstract: A main optical path and an auxiliary optical path which are formed in a substrate are optically coupled to each other with a plurality of optical couplers, so as to constitute a Mach-Zehnder interferometer. By controlling the temperature regulation of main and auxiliary optical paths effected by heaters, a control unit regulates the transmissivity of light across an optical input end and an optical output end. Respective optical path lengths of the main and auxiliary optical paths are set so as to lower birefringence when optical loss becomes greater.

Abstract: Three Mach-Zehnder interferometers 141 to 143 are connected in a series between an optical input terminal and an optical output terminal in an optical filter. Each interferometer is composed of two optical paths optical coupled at both ends by optical couplers. Only one interferometer has same optical path lengths between optical couplers. Heaters are disposed on each optical paths to allow the slope of transmission loss to change through the adjustment of the phase shift between those respective optical paths.

Abstract: The invention provides a planar lightwave circuit type variable optical attenuator in which a small PDL is maintained even when an attenuation is large. The variable optical attenuator is constituted by a Mach-Zehnder interferometer comprising a substrate, two optical waveguide arms buried in a clad formed on the substrate, and thin-film heaters disposed on the surface of the clad and adjusting optical lengths of the optical waveguide arms. The optical waveguide arms are each made of a silica-based glass material, and a difference &Dgr;L0 between arm lengths is set to be in the range of 0.38 &mgr;m to 0.52 &mgr;m.

Abstract: The invention provides an apparatus for and method of monitoring wavelength multiplexed signal light, which can obtain monitored data for multi-channel optical signals contained in the wavelength multiplexed signal light, which can avoid a large size and complicated structure and high cost of monitoring apparatus, and which can easily monitor the wavelength multiplexed signal light. The invention also provides an optical transmission system employing the monitoring apparatus and method. An optical filter being able to control a loss pattern is disposed on a monitoring waveguide for the wavelength multiplexed signal light, which is branched for monitoring from an input waveguide and an output waveguide constituting a main optical transmission path in a wavelength multiplexed signal light monitoring apparatus.

Abstract: A main optical path and an auxiliary optical path which are formed in a substrate are optically coupled to each other with a plurality of optical couplers, so as to constitute a Mach-Zehnder interferometer. By controlling the temperature regulation of main and auxiliary optical paths effected by heaters, a control unit regulates the transmissivity of light across an optical input end and an optical output end. Respective optical path lengths of the main and auxiliary optical paths are set so as to lower birefringence when optical loss becomes greater.

Abstract: Three Mach-Zehnder interferometers 141 to 143 are connected in a series between an optical input terminal and an optical output terminal in an optical filter. Each interferometer is composed of two optical paths optical coupled at both ends by optical couplers. Only one interferometer has same optical path lengths between optical couplers. Heaters are disposed on each optical paths to allow the slope of transmission loss to change through the adjustment of the phase shift between those respective optical paths.

Abstract: A silica based optical waveguide is described, comprising a substrate, a core waveguide formed thereon, and an over cladding part comprising a silica based glass with a refractive index lowering dopant and a refractive index increasing dopant added, formed on the substrate so as to cover the core waveguide, wherein a segregation layer with a higher concentration of the refractive index increasing dopant is formed in a part of the over cladding part in contact with the substrate and the core waveguide such that at least a part of the refractive index increase in the segregation layer provided by the refractive index increasing dopant with respect to the part of the over cladding part other than the segregation layer is offset by decline of the refractive index by increasing the amount of the refractive index lowering dopant added in the segregation layer and/or adding another refractive index lowering dopant.

Abstract: This process for producing an optical waveguide comprises a step of sintering a glass fine-particle layer to form a upper cladding layer, and a step of controlling a refractive index of the upper cladding layer by changing a temperature decreasing rate when the glass layer is cooled down to the room temperature.

Abstract: A process for forming film structure using Flame Hydrolysis Deposition (FHD) in which (1) glass soot is deposited on a substrate via FHD to form a first porous vitreous layer having a first bulk density, (2) a second porous vitreous layer having a second bulk density that is larger than the first bulk density is formed from a portion of the first porous vitreous layer, and (3) a third porous vitreous layer having a third bulk density is formed by depositing glass soot containing a refractive index increasing dopant on the second porous vitreous layer by FHD. The first, second and third porous vitreous layers are then heated to form an undercladding layer and a core layer, the undercladding layer being formed from the first and second porous vitreous layers and the core layer being formed from the third porous vitreous layer.

Abstract: In the method for forming an optical waveguide according to this invention, an optical waveguide comprising a core of quartz as a main component, and a cladding layers surrounding the core is formed by deposition of glass fine particles by flame hydrolysis deposition and vitrifying the glass fine particle layers. This method includes a step of transiently increasing a feed amount of phosphorus to a flame burner in forming glass fine particle layers to be the cladding layers. Feeding phosphorus in this step for the first time after a glass fine particle layer is deposited without feeding phosphorus to the flame burner, whereby generation of foreign objects near the core dan be suppressed.

Abstract: In the first step, a fuel and raw material gases are fed to burner while flames from the burner scan a Si substrate. Synthesized fine glass particles are deposited on the substrate to form a first porous vitreous layer to be an under cladding layer. In the second step, the first porous vitreous layer is heated by the flames. A bulk density of an upper part of the first porous vitreous layer is raised to 0.3 g/cm.sup.3. Having a raised bulk density, this upper part functions as a shield layer against GeO.sub.2. In the third step, a second porous vitreous layer, to be a core layer, is deposited uniformly on the first porous vitreous layer. In the fourth step, the first and the second porous vitreous layers are sintered. In this case, the shield layer with a higher bulk density hinders the GeO.sub.2 component which has evaporated from the second porous vitreous layer from diffusing into the first porous vitreous layer.