embodiments of the invention will be described with reference to the attached drawings . referring to fig1 the optical time - division multiplexer according to the first embodiment comprises a pair of substrates 10 and 11 for attachment of optical fibers , a pair of electro - absorption modulators 12 - 1 and 12 - 2 , a pair of 3 - db couplers 13 - 1 and 13 - 2 , and optical fibers 14 - 1 , 14 - 2 , 14 - 3 , and 14 - 4 . the length difference between the optical fibers 14 - 1 and 14 - 2 , and likewise between the optical fibers 14 - 3 and 14 - 4 , is set to an appropriate value according to the frequency of the optical pulses input to the 3 - db coupler 13 - 1 and output from the 3 - db coupler 13 - 2 . for example , if the input optical pulse rate is ten gigahertz ( 10 ghz ), and the multiplexed output optical pulse rate is 20 ghz , then the optical path length difference is set to approximately one centimeter ( 1 cm ). the data to be transmitted ( binary w signals , not shown ) are supplied to the electro - absorption modulators 12 - 1 and 12 - 2 , the extinction ratios of which vary according to the data values , so that the input optical pulses are modulated in an on / off manner . a feature of the first embodiment is that the optical fibers disposed between the 3 - db couplers 13 - 1 and 13 - 2 are fastened to the substrates 10 and 11 everywhere except in optical path length adjustment section . as shown in fig1 the optical fibers 14 - 1 , 14 - 3 on the input side are everywhere attached to the substrate 10 , and the optical fibers 14 - 2 and 14 - 4 on the output side are attached to the other substrate 11 everywhere except in a short section between the substrate 11 and the electro - absorption modulators 12 - 1 and 12 - 2 , where they are left free . the free sections of the optical fibers 14 - 2 and 14 - 4 are kept under tension between the substrate 11 and the electro - absorption modulators 12 - 1 and 12 - 2 by being pressed laterally , in a direction perpendicular to their length direction , by mechanical members not shown in the drawing . these free sections are used for optical path length adjustment . in this arrangement , optical pulses entering the 3 - db coupler 13 - 1 from the input side ( in ) are split to create two signal channels , which are separately modulated in the electro - absorption modulators 12 - 1 , 12 - 2 . the two channels are combined in the 3 - db coupler 13 - 2 to obtain an optical pulse signal having twice the bit rate of each channel , for transmission through an output fiber ( out ). since the optical fibers are fastened to the substrates at substantially all points , they do not expand or contract in length , and the phase of the multiplexed output light is highly stable . as noted above , long - distance transmission through an optical fiber requires a reversal of phase at each successive pulse of light propagating through the fiber ; more specifically , the phase must shift by π radians at each successive pulse . in this embodiment , if the wavelength of the input light is about one micrometer ( 1 μm ), then the length of the free sections of the optical fibers 14 - 2 and 14 - 4 disposed between the substrate 11 and the electro - absorption modulators can be set to about one millimeter ( 1 mm ). the force applied in these sections can be adjusted by any appropriate means so as to stretch the optical fibers by adjustable amounts , thereby slightly altering their optical length , while keeping them under tension . the phase difference between adjacent pulses ( in this embodiment , the difference between the phase of the light output from optical fiber 14 - 2 and the phase of the light output from optical fiber 14 - 4 ) can thereby be set to precisely π radians in an easy way . as described above , in the first embodiment of the invention , substantially all parts of the optical fibers , except for short sections left free , are fastened to substrates in the fiber - delay time division multiplexer module , so that the phase of the output light is highly stable . also , since it is possible to fine - tune the phase of the output light by altering the optical length of the free sections , the phase of the output light can be reversed at each successive pulse ; more specifically , it can be shifted by exactly π radians at each successive pulse , enabling long - distance optical transmission . this optical length adjustment can also be used to compensate for refractive index variations in the optical fiber cores , and for changes in the wavelength of the input light . [ 0022 ] fig2 shows the structure of an optical time division multiplexer according to the second embodiment of the invention . the structure in this drawing includes a substrate 21 , electro - absorption modulators 22 - 1 and 22 - 2 , half mirrors 23 - 1 and 23 - 2 , mirrors 24 - 1 and 24 - 2 , an optical fiber 25 , optical fiber fasteners 25 a and 25 b , and an adjustable fiber bending member 25 c . a light pulse incident on half mirror 23 - 1 is branched into two pulses , one of which is modulated in electro - absorption modulator 22 - 1 and reflected by mirror 24 - 1 , then exits to the outside through half mirror 23 - 2 . the other branched pulse is modulated in electro - absorption modulator 22 - 2 and reflected by mirror 24 - 2 , passes through the optical fiber 25 and is reflected by half mirror 23 - 2 , then exits to the outside . thus a multiplexed optical pulse signal with twice the bit rate of the signal supplied to each electro - absorption modulator 22 - 1 , 22 - 2 can be obtained for transmission . since the refractive index of the optical fiber 25 ( about 1 . 5 ) differs from that of the ambient space ( about 1 . 0 ), the length of optical fiber 25 generates a difference in optical length between the two optical paths from half mirror 23 - 1 to half - mirror 23 - 2 . the length of the optical fiber 25 should be set to a value that gives an appropriate optical path length difference , depending on the pulse frequency of the input light and that of the multiplexed output light . the adjustable fiber bending member 25 c is rotatably pivoted on a shaft perpendicular to the substrate surface , and has a varying radius ; thus the distance from the shaft to the point touching the optical fiber 25 varies according to the rotational position of the adjustable fiber bending member 25 c . therefore , when the adjustable fiber bending member 25 c is turned , the optical fiber 25 is deformed ( bent ) laterally and stretched , slightly altering its optical length . if the wavelength of the input light varies , the difference between the phase of the light modulated in electro - absorption modulator 22 - 2 , which passes through the optical fiber 25 and is reflected by half mirror 23 - 2 to the outside , and the phase of the light modulated in electro - absorption modulator 22 - 1 , which passes through half mirror 23 - 2 to the outside , may deviate from its optimal value , i . e ., π radians , but the adjustable fiber bending member 25 c can be rotated to alter the length of the optical fiber 25 slightly , to compensate for the deviation and restore the optimal value . in the second embodiment , the required difference between the optical path lengths is obtained by the insertion of an optical fiber in one optical path , and the length of the optical fiber is altered slightly by the adjustable fiber bending member to compensate for any deviation from the optimal phase difference ( π radians ) between adjacent pulses of the output light caused by a change in the wavelength of the input light . the use of an optical fiber to produce the optical path length difference also enables the second embodiment to obtain a given optical path length difference in less space than required by a conventional spatial - coupling time division multiplexer module , thereby reducing the size of the module . the invention is not limited to the two embodiments described above . for example , to adjust the optical path length difference , the above embodiments use a member that applies mechanical force to deform an optical fiber , thus changing its length , but the adjustment may also be performed by utilizing thermal expansion of the optical fiber , or the slight variation of the refractive index associated with changes in temperature . also , the above embodiments describe the multiplexing of two signals , but the invention can be used to multiplex three signals , four signals , or more . those skilled in the art will recognize that further variations are possible within the scope claimed below .