Patent Publication Number: US-6665499-B2

Title: Parallel optical transmission/reception module

Description:
This application is a Divisional Application of application No. 09/129,407, filed Aug. 5, 1998, now pending. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to data transfer using parallel optical transmission, in particular, to a parallel optical reception module for receiving optical data signals through a plurality of parallel data channels, and/or a parallel optical transmission module for transmitting optical data signals through a plurality of parallel data channels. 
     2. Description of the Prior Art 
     In the field of computer systems, the improvement in calculation or processing capability is supposed to require faster and larger data transfer between computers. Faster data transfer has been realized by the establishment of data transfer using optical transmission. However, larger data transfer apparently has not yet been realized sufficiently. 
     The implementation of larger data transfer is supposed to require “parallel” optical transmission in data transfer. However, the parallel optical transmission has suffered from a problem such that skews or time lags could be generated between parallel data channels because of the difference in data transmission rate of optical fibers and in processing rate of photodetectors for converting optical signals to electric signals. The skew would make computers incapable of calculating or processing received data signals. The further the transferred data should reach, the larger the skew gets. Skews are obstacle to implementation of parallel optical transmission in data transfer. 
     SUMMARY OF THE INVENTION 
     It is accordingly an object of the present invention to provide a parallel optical transmission/reception module realizing a larger and longer data transfer using parallel optical transmission by detecting and/or correcting skews between parallel data channels. 
     According to a first aspect of the present invention, there is provided a parallel optical transmission module comprising a skew data transmission unit capable of transmitting parallel synchronous pattern signals for a plurality of parallel data channels. 
     According to a second aspect of the present invention there is provided a parallel optical reception module comprising a skew data reception unit capable of taking out parallel pattern signals synchronously transmitted from other side in a plurality of parallel data channels and detecting a skew between the data channels based on the parallel pattern signals. 
     With a combination of the above parallel optical transmission and reception modules, it is possible to easily detect a skew between the parallel data channels by transferring the parallel pattern signals in the respective parallel data channels. In this case, the parallel pattern signals preferably have wavelength different from that of parallel data signals transferred in the parallel data channels. Difference in wavelength between the pattern signal and data signal enables employment of a single optical fiber common to the transfer of the pattern signal and data signal. 
     The parallel pattern signals may be transferred with or without the transfer of the data signals. Accordingly, a skew can be detected prior to the transfer of the data signals, and even during the transfer of the data signals. When the parallel pattern signals are transferred along with the data signals, the pattern signals are multiplexed on the data signals. The parallel pattern signals having wavelength different from that of the data signals enables easy extraction of the parallel pattern signals from the multiplexed signals. If the transfer of the parallel pattern signals is kept with or without the transfer of the data signals, a skew can be detected immediately after the skew has varied. 
     The parallel optical reception module may further comprise a skew correction circuit capable of correcting the skew of the data signal based on the skew detected by the skew data reception unit. The skew correction circuit serves to output the parallel data signals without skews. Accordingly, the parallel data may simultaneously be subjected to processing or calculation. 
     On the other hand, a parallel optical transmission module may comprise a skew data reception unit capable of taking out parallel pattern signals synchronously transmitted from other side in a plurality of parallel data channels and detecting a skew between the data channels based on the parallel pattern signals. In addition, the parallel optical transmission module may further comprise a data transmission circuit capable of transmitting parallel data signals with the skew corrected based on the skew detected by the skew data reception unit into the parallel data channels. If the parallel data signals are transmitted with time lags corresponding to the length of the detected skews, a parallel optical reception module can simultaneously receive the parallel data signals under the effect of the skews. 
     When the skew data reception unit is assembled in a parallel optical transmission module, the parallel data signals may be transmitted after the reception of the parallel pattern signals are confirmed. The transfer after the confirmation serves to inspect the connection between the parallel optical transmission and reception modules by optical transmission paths such as optical fibers. In this case, it is possible to apply Class 1 regulation in the laser safety regulation to the transmission of optical pattern signals while applying Class 4 regulation to the transmission of optical data signals, so that larger power may be obtained according to Class 4 regulation for data transmission, and at the same time, reduced requirements can be applied to handling of the modules according to Class 1 regulation. It is possible to realize an open fiber control system. 
     Further, according to a third aspect of the present invention, there is provided a determination circuit for a later data channel, capable of outputting a pattern signal in a first data channel at a point when a phase difference is generated between the pattern signal in the first data channel and a pattern signal in a second data channel, said channels having a skew. The determination circuit serves to detect the level of a signal in the first data channel at a point when the skew between the first and second data channel causes the phase difference. Accordingly, the later data channel which receives data later than the other data channel can be identified by the level (high or low) of a signal in the first data channel. 
     The determination circuit may be implemented by: an exclusive OR gate capable of detecting a difference in level between a pair of pattern signals, said difference being caused by a phase difference in first and second data channels having a skew; and a flip-flop circuit capable of receiving an output of the exclusive OR gate at a clock terminal and a pattern signal in the first data channel at a data terminal. This type of the determination circuit allows the exclusive OR gate to output a high level pulse signal corresponding to the skew generated between the first and second data channels. The duration or length of the pulse signal serve to represent the phase difference between the pattern signals, namely, the length of the skew. The flip-flop circuit serves to store the later data channel. Here, parallel synchronous pattern signals are preferably transferred in the first and second data channels. 
     The determination circuit may further comprise a further flip-flop circuit between the output of the exclusive OR gate and the clock terminal for switching over high level and low level signals at the clock terminal in response to rise in the output of the exclusive OR gate. The further flip-flop circuit serves to identify a later data channel even when the later data channel shifts from one data channel to the other. 
     The pattern signal in the first data channel may be supplied via a delay circuit to the data terminal in the aforementioned determination circuit. The delay circuit serves to reliably identify a later data channel irrespective of the transmission delay inevitably generated in the exclusive OR gate and the flip-flop circuit. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and advantages of the present invention will become apparent from the following description of the preferred embodiment in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is a block diagram illustrating an example of a computer system; 
     FIG. 2 is a block diagram illustrating the structure of a parallel optical transmission module according to a first embodiment of the present invention; 
     FIG. 3 is a block diagram illustrating the structure of a parallel optical reception module according to the first embodiment of the present invention; 
     FIG. 4 is a block diagram illustrating the structure of a skew detection circuit; 
     FIG. 5 is a block diagram illustrating the structure of a pair phase comparator; 
     FIG. 6 is a time chart illustrating the operation of the pair phase comparator in case where the first skew data reaches earlier than the second skew data; 
     FIG. 7 is a time chart illustrating the operation of the pair phase comparator in case where the second skew data reaches earlier than the first skew data; 
     FIG. 8 is a block diagram illustrating the structure of a skew pulse generating circuit; 
     FIG. 9 is a block diagram illustrating the structure of a skew length select circuit; 
     FIG. 10 is a block diagram illustrating the structure of a skew correction circuit; 
     FIG. 11 is a time chart illustrating the operation of the skew correction circuit; 
     FIG. 12 is a time chart illustrating the operation of the skew correction circuit; and 
     FIG. 13 is a block diagram illustrating the structure of parallel optical transmission and reception modules according to a second embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 illustrates an example of a computer system using parallel optical transmission in data transfer. The computer system  10  comprises a host computer  12  for calculating or processing data taken out from an input/output apparatus  11 . The host computer  12  may be operated by manual operation to a keyboard and/or a mouse, not shown, in the aid of visual display on a screen, not shown. The input/output apparatus  11  may, for example, be a magnetic tape control apparatus, a file control apparatus with a plurality of disk drive units, and the like. 
     The host computer  12  is connected to the input/output apparatus  11  through optical fiber ribbon or tape fibers  13 . Optical signals transmitted from parallel optical transmission modules  14  in the host computer  12  and the input/output apparatus  11  are received, via the optical fiber ribbons  13 , by corresponding parallel optical reception modules  15  in the host computer  12  and the input/output apparatus  11 . Parallel optical transmission in data transfer can be implemented by cooperation of the optical fiber ribbons  13 , the parallel optical transmission modules  14  and the parallel optical reception modules  15 . The parallel optical transmission and reception modules  14 ,  15  may be assembled in the host computer  12  and the input/output apparatus  11 , or be detachably attached to the host computer and the input/output apparatus  11 . 
     Here, data channels for transmission may comprise output ports  16  of the host computer  12 , optical fibers corresponding to the output ports  16  in the optical fiber ribbon  13 , and input ports  17  corresponding to the optical fibers in the input/output apparatus  11 . Data channels for reception may comprise output ports  16  of the input/output apparatus  11 , optical fibers corresponding to the output ports  16  in the optical fiber ribbon  13 , and input ports  17  corresponding to the optical fibers in the host computer  12 . Otherwise, a common single optical fiber ribbon may include data channels for both transmission and reception. 
     As shown in FIG. 2, the parallel optical transmission module  14  according to a first embodiment of the present invention comprises four data channels CH0-CH3 extending from the output ports  16  to the corresponding optical fibers  13   a - 13   d  in the optical fiber ribbon  13 . Binary electric data signals are synchronized by a buffer  21  and then supplied to a driver  22  in the respective data channels CH0-CH3. The driver  22  serves to blink laser diodes (LD) in a laser array  23  in accordance with binary representations “0” and “1.” The blink of the laser diodes (LD) forms optical data signals λ 1 . The flash of the laser diodes (LD) may correspond to either one of binary representation “0” or “1.” 
     A skew data transmission unit  26  is connected to the data channels CH0-CH3 for transmitting optical pattern signals of synchronous phase. The skew data transmission unit  26  comprises a pulse generating circuit  27  which keeps outputting binary electric pattern signals which varies periodically. The output binary electric pattern signals are supplied to a driver  28 . The driver  28  serves to blink a laser diode (LD)  29  in accordance with binary representations “0” and “1” so as to generate optical pattern signals λ 2 . 
     An optical divider  30  serves to distribute the optical pattern signals λ 2  for four branches corresponding to the respective data channels CH0-CH3. An optical mixer  31  then serves to introduce the distributed optical pattern signals λ 2  into the respective data channels CH0-CH3. The optical pattern signals λ 2  of the identical phase are transmitted into the data channels CH0-CH3, respectively. The optical pattern signals λ 2  may be multiplexed onto the data signals λ 1  by the aid of the optical mixer  31  so as to provide multiplexed optical signals λ 1 +λ 2 . In any event, it is preferable to provide optical pattern signals λ 2  of a wavelength different from that of the optical data signals λ 1 . 
     As shown in FIG. 3, the parallel optical reception module  15  according to the first embodiment of the present invention comprises four data channels CH0-CH3 extending from the optical fibers  13   a - 13   d  in the optical fiber ribbon  13  to the corresponding input ports  17 . Photodetectors (PD)  41  serve to convert the received optical data signals λ 1  into electric data signals in the respective data channels CH0-CH3. The electric data signals are amplified and compared with a predetermined threshold voltage value by an amplifier/comparator  42 . The comparison in the amplifier/comparator  42  is adapted to restore original binary electric data signals based on the blink of the optical data signals λ 1 . The restored electric data signals are supplied to a skew correction circuit  43 . 
     A skew data reception unit  45  is connected to the data channels CH0-CH3 for taking out the optical pattern signals λ 2 , which are supposed to include the identical phase if there is no skew, so as to detect skews between the data-channels CH0-CH3 on the basis of the optical pattern signals λ 2 . The skew data reception unit  45  comprises an optical separator  46  for separating the optical pattern signals λ 2  from the multiplexed optical signals λ 1 +λ 2  in the respective data channels CH0-CH3. Since the wavelength of the optical pattern signals λ 2  are different from that of the optical data signals λ 1 , the separator  46  may easily separate the signals λ 1 , λ 2  from each other. Photodetectors (PD)  47  serve to convert the obtained optical pattern signals λ 2  into electric signals, which are in turn restored into binary skew data signals by the aid of an amplifier/comparator  48 . The binary skew data signals are supplied to a skew detection circuit  49 . 
     The skew detection circuit  49  serves to determine a data channel CH0-CH3 which has received the pattern signal λ 2  latest and detect skews for the other data channels based on the determined data channel, as described later. The determined data channel is called “latest data channel” hereinafter. The skew correction circuit  43  serves to eliminate the skews in the electric data signals based on the detected skews, as described later. The electric data signals without skews are synchronized in a buffer  51  and supplied to the input ports  17 . 
     The skew detection circuit  49  will be described more in detail referring to FIG.  4 . The skew detection circuit  49  comprises a determination circuit  52  for determining the latest data channel among the data channels CH0-CH3 based on the received skew data signals. The determination circuit  52  specifies the latest data channel by binary signals such as A and B signals. When the first data channel CH0 is determined as the latest data channel, the A and B signals of low level are output. When the second data channel CH1 is determined as the latest data channel, the A signal of high level and the B signal of low level are output. Likewise, the A signal of low level and the B signal of high level are output when the third data channel CH2 is determined as the latest data channel, and the A and B signals of high level are output when the fourth data channel CH3 is determined as the latest data channel. 
     A trigger circuit  53  serves to convert the combination of the A and B signals into four latest channel signals MDCS1-MDCS4. Specifically, any one of the latest channel signals MDCS 1 -MDCS 4  is raised up to a high level for the latest data channel specified by the combination of the A and B signals. An AND gate  54   a  takes high level as a result of two inverted inputs only if the A and B signals both take low level. An AND gate  54   b  takes high level as a result of the inverted B signal only if the A signal of high level and the B signal of low level are supplied. Likewise, an AND gate  54   c  takes high level only if the A signal of low level and the B signal of high level are supplied. An AND gate  54   d  takes high level only if the A and B signals both take high level. 
     A skew pulse generating circuit  55  serves to output skew pulse signals SP 1 -SP 4  for the respective data channels CH0-CH3 based on the skew data signals for the data channels CH0-CH3 and the A and B signals, as described later. The skew pulse signals SP 1 -SP 4  correspond to the magnitude of the skews for the respective data channels CH0-CH3. 
     Here, the determination circuit  52  will be described more in detail referring to FIG.  4 . The determination circuit  52  comprises first to third pair phase comparators  56   a ,  56   b ,  56   c  for determining one of a pair of the data channels which has received the skew data signal later. The determined data channel is called “later data channel” hereinafter. When the first pair phase comparator  56   a  determines the later data channel, a first select circuit  57   a  supplies the skew data signal for the determined data channel to the third pair phase comparator  56   c . When the second pair phase comparator  56   b  determines the later data channel, a second select circuit  57   b  supplies the skew data signal for the determined data channel to the third pair phase comparator  56   c . When the third pair phase comparator  56   c  determines the later data channel, a third select circuit  57   c  serves to select one of the outputs from the first and second pair phase comparators  56   a ,  56   b.    
     For example, the first pair phase comparator  56   a  outputs an a signal of low level when it determines the first data channel CH0 as the later data channel, while it outputs the a signal of high level when it determines the second data channel CH1 as the later data channel. The second pair phase comparator  56   b  outputs a b signal of low level when it determines the third data channel CH2 as the later data channel, while it outputs the b signal of high level when it determines the fourth data channel CH3 as the later data channel. 
     When the first select circuit  57   a  receives the a signal of low level, an AND gate  58  maintains an activated status with the inverted input, so that the skew data signal of the first data channel CH0 is output through an OR gate  59 . When the a signal of high level is supplied to the first select circuit  57   a , an AND gate  60  maintains an activated status so that the skew data signal of the second data channel CH1 is output through the OR gate  59 . Likewise, the reception of the b signal at low level allows the skew data signal of the third data channel CH2 to be output from the second select circuit  57   b , while the reception of the b signal at high level allows the skew data signal of the fourth data channel CH3 to be output. 
     The third pair phase comparator  56   c  serves to compare the phases of the skew data signals from the first and second select circuits  57   a ,  57   b . If any of the first and second data channels CH0, CH1 is determined as the later data channel, the B signal of low level is output. If any of the third and fourth data channels CH2, CH3 is determined as the later data channel, the B signal of high level is output. 
     When the third select circuit  57   c  receives the B signal of low level, an AND gate  58  maintains an activated status with the inverted input, so that the a signal is output as the A signal. When the third select circuit  57   c  receives the B signal of high level, an AND gate  60  maintains an activated status, so that the b signal is output as the A signal. In this way, four sets of combination for the level of the A and B signals serve to specify the latest data channel. 
     Referring to FIG. 5, the pair phase comparators  56   a ,  56   b ,  56   c , namely, determination circuits for the later data channel will be described more in detail. The pair phase comparators  56   a ,  56   b ,  56   c  comprise an exclusive OR gate  62  receiving the first and second skew data signals SK 1 , SK 2 . The output from the exclusive OR gate  62  is directed to the clock terminal C of a first flip-flop circuit  63 . A second flip-flop circuit  64  is interposed between the output of the exclusive OR gate  62  and the clock terminal C for alternately switching the output signal between low and high levels in response to the rise in the output of the exclusive OR gate  62 . The data terminal D of the first flip-flop circuit  63  receives the skew data signals SK 1  delayed by a delay circuit  65 . 
     Assume that the first pair phase comparator  56   a  receives the pattern signal of 400 ps pulse width, for example as shown in FIG.  6 . When a high level pulse appears in the second skew data signal SK 2  150 ps later than the first skew data signal SK 1 , the exclusive OR gate  62  outputs a high level signal having a duration corresponding to a skew of 150 ps. The delay of 50 ps is inevitably brought to the rise in the output of the exclusive OR gate  62  with respect to the rise in the first skew data signal SK 1  because of the transmission rate of the exclusive OR gate  62 . The output from the exclusive OR gate  62  serves to raise the output of the second flip-flop circuit  64 . The second flip-flop circuit  64  keeps the output at high level until it receives another high level signal from the exclusive OR gate  62 . Accordingly, the second flip-flop circuit  64  keeps outputting the high level signal until the first skew data signal SK 1  turns to low level from high level. The high level signal is supplied to the clock terminal C of the first flip-flop circuit  63 . 
     At a point when the clock terminal C of the first flip-flop circuit  63  receives the high level signal, the data-terminal D is supposed to have received the first skew data signal SK 1  from the delay circuit  65 , so that the high level signal included in the first skew data signal SK 1  is transmitted through the first flip-flop circuit  63  as an output signal. In this way, a high level signal is output from the first flip-flop circuit  63  if delay is found in the second skew data signal SK 2 . 
     To the contrary, as shown in FIG. 7, when a high level pulse appears in the first skew data signal SK 1  150 ps later than the second skew data signal SK 2 , the exclusive OR gate  62  outputs a high level signal having a duration corresponding to a skew of 150 ps. In the same manner as described above, the high level signal from the second flip-flop circuit  64  is supplied to the clock terminal C of the first flip-flop circuit  63 . However, at a point when the rise appears in the second skew data signal SK 2 , the first skew data signal SK 1  is supposed to have kept low level. Accordingly, the low level signal included in the first skew data signal SK 1  is transmitted through the first flip-flop circuit  63  as an output signal. In this way, a low level signal is output from the first flip-flop circuit  63  if delay is found in the first skew data signal SK 1 . Here, the delay circuit  65  is provided in view of the transmission rate of the exclusive OR gate  62  and the second flip-flop circuit  64 , so that a low level signal in the first skew data signal SK 1  is supplied to the data terminal D of the first flip-flop circuit  63  even if the first skew data signal SK 1  rises immediately after the rise in the second skew data signal SK 2 . 
     Next, the skew pulse generating circuit  55  will be described more in detail referring to FIG.  8 . The skew pulse generating circuit  55  comprises a skew length detecting circuit  67  for detecting the length of a skew for every combination of a pair of the data channels CH0-CH3. Ten combinations may be considered in this embodiment. The voltage of “0” is set to the length of “0” for any combination of the identical data channels CH0-CH3 which exhibits no skew. Each exclusive OR gate  69  finds out the length of a skew for the remaining combinations of the data channels CH0-CH3. These exclusive OR gates  68  serve to output high level pulses having a duration corresponding to the length of skews in the same manner as described referring to EOR output in FIGS. 6 and 7. 
     Skew length select circuits  68  serve to select combinations, required in skew correction, among the full combinations of the data channels CH0 32  CH3, and to supply the selected combinations to the skew correction circuit  43 . For example, when the first data channel CH0 is determined as the latest data channel, the skew length select circuits  69  serve to output the voltage of “0” or the output from the exclusive OR gates  68  all input to [ 00 ], [ 10 ], ( 20 ] and [ 30 ] terminals as skew pulse signals SP 1 -SP 4 . AND gates  70   a - 70   d  in the skew length select circuit  69  receive the A and B signals as shown in FIG.  9 . If the A and B signals both take low level, the first AND gate  70   a  assume an activated status so as to allow a signal at the ( 00 ] terminal to be transmitted through an OR gate  71 . Likewise, when the second data channel CH1 is determined as the latest data channel, signals at [ 01 ], ( 11 ], [ 21 ] and [ 31 ] terminals are output. Otherwise, signals at [ 02 ], [ 12 ], [ 22 ] and [ 32 ] terminals are output when the third data channel CH2 is determined as the latest data channel, and signals at [ 03 ], [ 13 ], [ 23 ] and [ 33 ] terminals are output when the fourth data channel CH4 is determined as the latest data channel. 
     FIG. 10 illustrates the structure of the skew correction circuit  43  provided in the respective data channels CH0-CH3. The skew correction circuit  43  comprises a delay generation circuit  74  formed by a plurality of AND gates  73   a - 73   f  (D 1 -D 6 ) for sequentially transferring the supplied electric data signal. Each AND gate  73   a - 73   f  allows delay of 50 ps, for example, for processing. Accordingly, six AND gates  73   a - 73   f  are supposed to allow delay of maximum 300 ps in total. The outputs from the AND gates  73   a - 73   f  are supplied to a choice circuit  75 . 
     The choice circuit  75  comprises an AND gate  76   a  (C 1 ) for receiving the electric data signal prior to the reception by the first AND gate  73   a , and six AND gates  76   b - 76   g  (C 2 -C 7 ) for receiving the outputs from the AND gates  73   a - 73   f , respectively. The AND gate  76   a  serves to output the electric data signal in response to a high level signal included in the latest channel signals MDCS1-MDCS4 from the trigger circuit  53 . The remaining AND circuits  76   b - 76   g  serve to output the electric data signal in response to a high level signal included in a select signal supplied from a select signal generation circuit  78 . The electric data signal from the AND gates  76   a - 76   g  is output as a skew corrected data signal DATA through an OR gate  79 . 
     The select signal generation circuit  78  comprises a set of delay gates  80   a - 80   g  (A 1 -A 7 ) for applying delay of, for example, 50 ps to the skew pulse signal SP 1  passing through the delay gates  80   a - 80   g . The delay gates  80   a - 80   g  serve to sequentially output high level signals with the interval of 50 ps after the skew pulse signal SP 1  reaches high level. The duration of the skew pulse signal SP 1 , namely, the duration corresponding to the length of the skew allows the corresponding number of delay gates  80   a - 80   g  to take an activated status so as to output a high level signal. 
     The outputs from the delay gates  80   a - 80   g  are supplied to data terminals D of flip-flop circuits  81   a - 81   g . A high level signals are output only from the flip-flop circuits  81   a - 81   g  which receive the high level signal at the data terminal D at a point when the skew pulse signal SP 1  is brought down to low level. 
     Exclusive OR gates  82   a - 82   f  serve to compare the outputs from a pair of adjacent flip-flops  81   a - 81   g . Only one of the exclusive OR gates  82   a - 82   f  is capable of outputting a high level signal in accordance with the duration of skew pulse signal SP 1 . The sole output from one of the exclusive OR gates  82   a - 82   f  is supplied to the choice circuit  75  via AND gates  83   a - 83   f  (B 1 -B 6 ). 
     Assume that the skew correction circuit  43  receives a skew pulse signal SP 1  having the duration of 250 ps, as shown in FIG.  11 . When the skew pulse signal SP 1  reaches high level, every delay gate  80   a - 80   g  receives a high level signal at one of the input terminals. The transmission rate of gate circuits allows the delay gate  80   a  to output a high level signal 50 ps after the skew pulse signal SP 1  is raised to high level. The delay gates  80   a - 80   g  which receive the high level signal from the skew pulse signal SP 1  sequentially output a high level signal in this manner. In this case, the delay gates  80   a - 80   f  at positions determined by the transmission rate of gate circuits output a high level signal, respectively, for the delay of 250 ps. 
     The flip-flop circuits  81   a - 81   e  which receive the high level signal at the data terminal D from the corresponding delay gates  80   a - 80   f  at a point when the skew pulse signal SP 1  goes down to low level output a high level signal. The high level signals are kept until the duration of skew pulse signal SP 1  changes. Accordingly, only the exclusive OR gate  82   f  outputs a high level signal until the skew varies. The remaining exclusive OR gates  82   a - 82   e ,  82   g  compare a pair of high level signals or a pair of low level signals, so that the OR gates  82   a - 82   e ,  82   g  fail to output a high level signal. 
     When the AND gate  76   f  receives the high level signal from the corresponding exclusive OR gate  82   e  via the corresponding AND gate  83   e , it allows the received electric data signal to transmit to the OR gate  79 . The skew corrected data signal DATA is obtained in this manner. The aforementioned process is conducted in all of the data channels other than the data channel which are determined as the latest data channel. On the other hand, the electric data signal without any delay is output from the OR gate  79  in the data channel determined as the latest data channel since the corresponding latest channel signal MDCS 1 -MDCS 4  reaches high level. 
     In the above-described manner, data signals DATA without skews are obtained in all of the data channels CH0-CH3 in response to the skew data signal as shown in FIG.  12 . Maximum duration of a skew to be corrected may be determined by duration of a pulse and intervals between pulses in a pattern signal, delay time of the AND gates  73   a - 73   f  and the delay gates  80   a - 80   g , and the number of the AND gates  73   a - 73   f  and the delay gates  80   a - 80   g . Constant transmission of pattern signals serves to accept the variation in skews. 
     The operation of the skew data transmission unit  26  and the skew data reception unit  45  may be started when the host computer  12  and the input/output apparatus  11  are turned on. In addition, transfer of pattern signals between the skew data transmission unit  26  and the skew data reception unit  45  may be kept irrespective of no transfer of data signals. 
     FIG. 13 illustrates a parallel optical transmission module  101  and a parallel optical reception module  102  according to a second embodiment of the present invention. In this embodiment, an optical pattern signal λ 2  is transmitted from the parallel optical reception module  102  to the parallel optical transmission module  101  in transferring optical data signals λ 1  between the parallel optical transmission and reception modules  101 ,  102 . The common reference numerals or characters are attached to structures identical to those of the aforementioned first embodiment. 
     Specifically, an optical pattern signal λ 2  generated at the laser diode (LD)  29  is first distributed to four branches corresponding to respective data channels CH0-CH3 by the optical divider  30 , and then introduced into the respective optical fibers  13   a - 13   d  in the optical fiber ribbon  13  by an optical directional coupler  103 . The transmitted optical pattern signals λ 2  are supplied to the photodetectors (PD)  47  by the operation of an optical directional coupler  104  in the parallel optical transmission module  101 . The optical directional couplers  103 ,  104  serve to guide lights along specific passages in response to the direction of lights. 
     The skew detection circuit  49  in the parallel optical transmission module  101  serves to detect the length of skews based on the received optical pattern signals λ 2  in the same manner as described above. The skew correction circuit  43  outputs data signals in accordance with the detected length of the skews. The data signal is immediately output from the AND gate  76   a , as shown in FIG. 10, in the latest data channel. The longer the length of the skew gets with respect to the latest data channel, the later the data signals are output in the remaining data channels. As a result, the parallel optical reception module  102  simultaneously receives the skew corrected data signals in the respective data channels CH0-CH3. 
     With the above structure, for example, the host computer  12  is capable of transmitting optical data signals λ 1  after it confirms the connection between the host computer  12  and the input/output apparatus  11  by the optical fiber ribbon  13 . In this case, it is possible to apply Class 1 regulation in the laser safety regulation to the transmission of optical pattern signals λ 2  while applying Class 4 regulation to the transmission of optical data signals λ 1 , so that larger power may be obtained according to Class 4 regulation for data transmission, and at the same time, reduced requirements can be applied to handling of the modules according to Class 1 regulation. It is possible to realize an open fiber control system.