Abstract:
An optical time-division multiplexer branches input light into a plurality of optical paths with different optical lengths. At least one path includes a section of optical fiber with an optical length that is adjustable by physical stretching. The input light is separately modulated on each optical path, and the modulated light signals are recombined into a multiplexed output light signal. Adjustment of the optical length of the optical fiber can compensate for changes in input wavelength and adjust the phase of successive pulses in the multiplexed output signal to provide a π-radian shift that enables long-haul optical transmission.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an optical time-division multiplexer for use in optical transmission apparatus. 
   2. Description of the Related Art 
   An optical multiplexer module (OMUX module) having a plurality of optical paths with different optical path lengths provides a way to multiplex optical signals in a plurality of channels on the time axis (time-division multiplexing). Optical time-division multiplexer modules can be classified into two types, a fiber-delay type and a spatial-coupling type, according to the method by which the path length differences are created. 
   In the fiber-delay type of multiplexer module, the input light (consisting of short pulses occurring at fixed intervals) is generally branched in an optical coupler or splitter, and the branched pulses are modulated (by on/off data modulation) in an electro-absorption modulator. The modulated light pulses are launched into optical fibers of different lengths. After passing through the optical fibers, the light pulses are combined in a device such as a three-decibel (3-dB) coupler to obtain an optical signal in which a plurality of channels are multiplexed on the time axis. 
   The spatial-coupling type of multiplexer module employs optical elements such as half mirrors, rather than optical couplers or splitters and optical fibers, to create a plurality of optical paths on which light travels for different distances through space within the module. 
   The fiber-delay type of multiplexer module has a simple construction, but the refractive index of the cores of its optical fibers varies slightly with ambient temperature, stress, and torsion, shifting the phase of the light that has passed through the optical fibers. Moreover, a certain length of fiber must be allowed for interconnections between optical fibers, and between optical fibers and electro-absorption modulators, so the optical fibers in this type of module cannot be extremely short. As a result, the cumulative phase shift due to refractive index variation in the fiber cores is fairly large. Therefore, it has not been possible to maintain the optimal phase relationship, necessary for long-distance optical transmission, among the light pulses exiting the optical fibers; specifically, it has not been possible to reverse the phase of the multiplexed pulse light at each successive pulse (i.e., to shift the phase of the output light by π radians between successive pulses). 
   A spatial-coupling multiplexer module does not suffer from the problem of phase shift due to variation in the refractive index of the core of an optical fiber, but this type of multiplexer cannot compensate for changes in the wavelength of the input light. When the wavelength of the input light varies, the phase of the multiplexed output light also varies. Since the optical paths in a spatial-coupling multiplexer module are fixed, it has not been possible to change their lengths to compensate for these variations in the phase of the output light. A spatial-coupling multiplexer module employing waveguides (a planar lightwave circuit or PLC) has a similar disadvantage, and the further disadvantage of a large insertion loss. 
   SUMMARY OF THE INVENTION 
   An object of the present invention is to provide an optical multiplexer that overcomes the above problems: more specifically, one that enables long-distance optical transmission by enabling the phase of the multiplexed output light to be shifted by exactly π radians at each pulse, and that can compensate for phase deviation in the output signal light due to changes in the wavelength of the input light. 
   The invented optical time-division multiplexer branches input light into a plurality of optical paths having different path lengths. At least one of the optical paths includes a section of optical fiber functioning as an optical path length adjustment section. Means are provided for altering the optical length of the optical fiber in the optical path length adjustment section. The input light propagating on each optical path is modulated by a signal to be transmitted, and the modulated light signals that have traversed the optical paths are combined into an output light signal. 
   By adjusting the optical length of the optical fiber in the optical path length adjustment section, the invented optical time division multiplexer can arrange for the phase of the multiplexed output light to be shifted by exactly π radians at each successive pulse, and can compensate for deviations in this phase shift due to a change of the wavelength of the input light, thus making long-distance optical transmission possible. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the attached drawings: 
       FIG. 1  shows the layout of an optical time-division multiplexer according to a first embodiment of the invention; and 
       FIG. 2  shows the layout of an optical time-division multiplexer according to a second embodiment of the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Embodiments of the invention will be described with reference to the attached drawings. 
   Referring to  FIG. 1 , 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  FIG. 1 , 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. 
     FIG. 2  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.