Method of manufacturing an optical module for wavelength division multiplex optical transmission with regulation of the coupling ratio and coupling length

A method of manufacturing an optical module for wavelength-division multiplex optical transmission, said module including a first optical fiber having a first port at one end thereof and a second port at the other end thereof, and second and third optical fibers fused to said first optical fiber at the side face thereof such that said first optical fiber is positioned between center portions of said second and third optical fibers, said second optical fiber having a third port, said third optical fiber having a fourth port, said third and fourth ports being located on a same side as that of said second port of said first optical fiber, said method comprising the steps of:

• fusing said second and third optical fibers to said first optical fiber;
• regulating an aspect ratio at a fused portion while heating said fused portion and elongating said fused portion at zero speed or at a low rate substantially near zero speed, wherein said aspect ratio is a ratio (b/a) of a longer length (b) of a cross-section of said fused portion to a shorter length (a) of said cross-section of said fused portion, wherein said step of regulating said aspect ratio includes a step of directing at least a light source to said first port of said first optical fiber;
• and
• regulating a coupling length of said fused portion while elongating said fused portion at a higher speed than in said aspect ratio regulating step under conditions such that a heating temperature of the fused portion is lowered and said aspect ratio is kept unchanged, wherein said coupling length is a length of said fused portion which has been elongated, wherein said step of regulating said coupling length includes a step of directing at least said light source to said first port of said first optical fiber,
• wherein an intensity of a light beam outputted from said second port, having a first wavelength (.lambda.1), is dependent on said coupling length,
• wherein a second wavelength (.lambda.2), at which intensities of light beam outputted from said third and fourth ports become equal, is subtracted from said first wavelength (.lambda.1) to provide a difference wavelength (.lambda.1-.lambda.2), said difference wavelength (.lambda.1-.lambda.2) being determined based on said aspect ratio, and
• wherein the step of regulating said aspect ratio is performed so that the difference between the first wavelength and the second wavelength (.lambda.1-.lambda.2)is 0.25 .mu.m, and the step of regulating said coupling length is performed so that said first wavelength (.lambda.1) is equal to 1.55 .mu.m.