region in other words the width of the narrow section is preferably sufficiently small such that oxidation of the mode control region results in the confinement of the eigen mode in the passive region inside the narrow section and a width of the wide section is sufficiently large such that an effective refractive index of the wide section is negligibly influenced by oxidation of the mode control region and therefore the eigen mode is confined in the active region inside the wide section the width of the wide section should not be too large in order to avoid very deep oxidation and large electrical capacitance the width of the narrow section should not be too small in order to avoid a considerable overlap of the optical mode with the active region and bottom cladding layer which would cause additional internal and bending loss in preferred embodiments the width of the wide section ranges from approximately 0 5 u03bcm to approximately 5 u03bcm and the width of the narrow section ranges from approximately 0 3 u03bcm to approximately 3 u03bcm mode 9 shown in fig1 is the same mode in a one dimensional profile as mode 90 shown in fig3 in a two dimensional profile vertical cross sections of the tapered ridge waveguide 100 of fig2 are shown in fig3 a 3 c and 3 e taken along section surfaces 3 a 3 c and 3 e respectively as indicated in fig2 a corresponding optical field distributions are shown in fig3 b 3 d and 3.