JP5071020B2 - Optical code division multiplexing transmission / reception apparatus and optical code division multiplexing transmission / reception method - Google Patents

Description

  The present invention is suitable for use when the communication rate is high or when the number of channels to be multiplexed is large, and an optical code division multiplexing (OCDM) transmission / reception apparatus and OCDM transmission / reception that are less prone to receive errors. Regarding the method.

  In the field of optical communications, in order to be able to multiplex and transmit optical pulse signals of multiple channels on one optical fiber transmission line, until now, optical time division multiplexing (OTDM) Multiplexing of transmission signals by wavelength division multiplexing (WDM) has been studied. OTDM is a method for separating channels according to time slots occupied by optical pulses constituting an optical pulse signal, while WDM is a method for separating channels according to wavelengths of optical pulses constituting an optical pulse signal.

  Recently, in addition to these, OCDM communication has attracted attention. (For example, see Non-Patent Documents 1 and 2). OCDM is a method of separating channels by pattern matching of encoded optical pulse signals. That is, OCDM communication is communication using a method of assigning a different code (pattern) for each channel and extracting a signal by pattern matching. That is, OCDM communication is an optical pulse signal that is transmitted on the transmission side after being encoded and transmitted, and the encoded and transmitted signal is decoded on the reception side to reproduce and receive the original optical pulse signal. Multiple communication.

  The communication method using OCDM will be described more specifically as follows. First, on the transmission side, an optical pulse signal to be transmitted with a different optical code for each communication channel is encoded, and then the encoded transmission signals of each channel are combined to generate a code division multiplexed signal for transmission. . The receiving side receives this code division multiplexed signal, distributes it to each channel, decodes each channel using the same code as the transmitting side, and obtains the received signal by returning to the original optical pulse signal. To do.

  In OCDM communication, only the optical pulse signal with the correct code is extracted and processed by decoding, so multiple channels can be set in the same time slot on the time axis, or on the wavelength axis. Has a feature that a plurality of communication channels can be set to the same wavelength. In addition, OCDM communication has an operational flexibility that there is no restriction on the width on the time axis of an optical pulse signal allocated per bit.

  Even in OCDM communication, increasing the number of users that can be multiplexed, that is, increasing the number of channels is an essential technical problem.

  In general, in OCDM communication, the longer the code length of a code to be used, the greater the number of codes that can be used, so that the number of channels can be increased. However, if the coding is performed with a code having a long code length while keeping the bit rate of the optical pulse signal to be transmitted, the time interval allocated to one chip constituting the code is shortened.

  Here, for convenience of the following description, the terms signal period, communication rate, chip period, chip rate, code length, and code period are defined as follows.

  The signal period refers to a time interval assigned to each optical pulse constituting the optical pulse signal, and the communication rate refers to the bit rate of the optical pulse signal to be transmitted, that is, the reciprocal of the signal period. The chip period refers to the time interval allocated per chip constituting the code, and the chip rate refers to the reciprocal of the chip period. The code length refers to the total number of chips constituting the code. The code period refers to a time interval that a set of these chips occupies on the time axis by arranging all the chips constituting the code on the time axis. The chip period is obtained by dividing the code period by the code length.

  In addition, an area on the time axis allocated to one chip on the time axis may be referred to as a time slot for a chip pulse. A region on the time axis allocated to one optical pulse constituting the optical pulse signal on the time axis may be referred to as a time slot for the optical pulse.

  For example, when encoding is performed with a code having a code period of 800 ps (corresponding to 1.25 Gbit / s) and a code length of 128, the chip period of the encoded signal is 6.25 ps (= 800 ps / 128), and the chip rate is 160 GHz / chip (= 1 / 6.25 ps). The chip period is 1/128 of the code period, and the chip rate is 128 times the communication rate.

  The signal encoded in this way is decoded on the receiving side and reproduced as an autocorrelation signal. The optical pulses constituting the autocorrelation signal appear on the time axis so as to be within the time width allocated to one chip. Each of the optical pulses constituting this autocorrelation signal is recognized as a signal representing one bit of the received signal. That is, each of the optical pulses included in the autocorrelation signal is correctly recognized as a significant signal component having an intensity that can be clearly distinguished from the noise component, whereby reception in OCDM communication is completed. The Hereinafter, the optical pulse constituting the autocorrelation signal may be simply referred to as the optical pulse of the autocorrelation signal.

  As described above, in OCDM communication, in order to increase the number of channels to be multiplexed while fixing the communication rate, it is necessary to increase the code length of codes used for encoding and decoding. Narrow chip pulses will be handled.

  In order to increase the communication rate, it is necessary to narrow the time slot for the optical pulse. One optical pulse is spread on the time axis as a number of chip pulses equal to the code length by encoding. As will be described later, since the signal period is set to be twice or more the code period, the code period becomes shorter as the signal period becomes shorter. Therefore, if the code length is fixed, the time slot for the optical pulse is narrowed on the time axis, so that the time slot for the chip pulse is also narrowed. Corresponding to the narrow time slot for the chip pulse, the time width of the chip pulse is also narrowed.

  In response to the narrowing of the time slot for the chip pulse, the time width of the optical pulse of the autocorrelation signal that is decoded and generated from the chip pulse having a narrow time width is also narrowed. This is due to the following reason. That is, in the decoding process, the optical pulse of the autocorrelation signal is generated by selecting and adding some of the chip pulses on the time axis according to the code set in the decoder. Because it is done. Since the optical pulse of the autocorrelation signal is generated by matching the peak position of the chip pulse on the time axis and added, the generated optical pulse also has a time width as narrow as the time width of the chip pulse. It will be.

  For example, an optical pulse signal having a signal period of 1.6 ns (corresponding to a signal rate of 625 Mbit / s), a code period of 800 ps (corresponding to 1.25 Gbit / s), and a code length of 128. When encoded with a code, the chip period of the encoded signal is 6.25 ps (= 800 ps / 128), and the chip rate is 160 GHz, which is 128 times the code period as shown in the above calculation example. / chip (= 1 / 6.25 ps). Further, as described above, since the width of the optical pulse of the autocorrelation signal is about the same as the width of the chip pal, the duty ratio of the optical pulse of the autocorrelation signal, that is, [(time width of the optical pulse of the autocorrelation signal] ) / (Signal cycle)] × 100 is [(time width of optical pulse of autocorrelation signal) / (signal cycle)] × 100 = [(6.25) ps / (1.6) ns] × 100≈0.39% .

  In the above example, it is assumed that the signal period is 1.6 ns and the code period is 800 ps. That is, the signal period is set to twice the code period. In general, when an optical pulse signal is encoded and generated as a sequence of chip pulses arranged on the time axis, chip pulses generated from adjacent optical pulses on the time axis of the optical pulse signal overlap on the time axis. The signal period is set to at least twice the code period so that there is no signal.

  When chip pulses overlap on the time axis, the chip pulse that generates the autocorrelation signal interferes with the chip pulse that generates the cross-correlation signal, and the correlation is higher than when the chip pulses do not overlap on the time axis. The difference in peak intensity between the autocorrelation signal and the cross correlation signal constituting the signal is reduced. Here, when the difference in peak intensity between the autocorrelation signal and the cross correlation signal is reduced, the ratio between the peak intensity of the optical pulse of the autocorrelation signal and the intensity of the optical pulse constituting the cross correlation signal approaches 1. means. As a result, the peak intensity of the autocorrelation signal becomes relatively weak, and reception errors tend to occur.

  Therefore, in the above example, it is assumed that the duty ratio of the optical pulse of the autocorrelation signal is generally obtained as the maximum value set in the OCDM transceiver. That is, generally, the duty ratio of the optical pulse of the autocorrelation signal is further smaller than the above value.

  As described above, the autocorrelation generated by decoding the code division multiplexed signal on the receiving side, either by increasing the communication rate or increasing the number of channels to be multiplexed by increasing the code length. The time width of the optical pulse of the signal is narrowed.

  In order to photoelectrically convert a light pulse with a narrow time width into an electrical pulse, the photoelectric converter has sufficiently high time response characteristics (hereinafter sometimes referred to as an O / E converter). Necessary. In addition, it is difficult to expand a signal eye pattern to a certain extent for an optical pulse signal composed of an optical pulse having a very narrow time width compared to a time slot for an optical pulse. If the signal eye pattern is narrow, even if a slight time jitter is included in the optical pulse signal, it becomes an environment in which reception errors are likely to occur, such as difficulty in reception.

In the field of optical communication, an O / E specially designed to obtain high time response characteristics to convert optical pulse signals consisting of optical pulses with a small duty ratio into electrical pulses by photoelectric conversion. It is necessary to select and use a converter. Further, at present, a high level of technology is required to widen the signal eye pattern of an optical pulse signal composed of optical pulses with a small duty ratio so that it can be received. In any case, an OCDM transceiver that needs to handle an optical pulse signal consisting of an optical pulse with a small duty ratio uses an O / E converter near the upper limit of the design limit, and a wide signal eye pattern. The reception is performed in a state where the reception limit is. Therefore, in the above-described OCDM transmission / reception apparatus that needs to handle an optical pulse signal composed of an optical pulse with a small duty ratio, a reception error is likely to occur.
Naoya Wada, et al., “10Gbit / s optical code division multiplexing communication using time-gated detection”, IEICE General Conference Proceedings pp.684-685, SAB-1-7, (1999) Xu Wang, et al., "10-user truly-asynchronous OCDMA experiment with 511-chip SSFBG en / decoder and SC-based optical thresholder", OFC 2005, PDP33.

  Therefore, the present invention makes it possible to correctly recognize an autocorrelation signal as a received signal even when the optical pulse time width of the autocorrelation signal decoded and generated on the receiving side is narrow in OCDM communication. Another object of the present invention is to provide an OCDM transmission / reception apparatus and an OCDM transmission / reception method using the apparatus. That is, the present invention relates to an OCDM transmission / reception apparatus in which reception errors are less likely to occur by increasing the communication rate or increasing the number of channels to be multiplexed by increasing the code length, and a method of using the apparatus Is to provide.

  In order to achieve the above object, the OCDM transmission / reception apparatus of the present invention includes a transmission unit and a reception unit.

  The transmission unit assigns a different code to each channel, encodes the optical pulse signal of each channel with the assigned code, and generates an encoded optical pulse signal for each channel. The signal is multiplexed and a code division multiplexed signal is generated and transmitted.

  The receiving unit has a function of acquiring the received signal by decoding the code division multiplexed signal with the assigned code, a distributor, a decoder, an autocorrelation component extracting unit, and an optical pulse width expander And has.

  The distributor receives the code division multiplexed signal and distributes the code division multiplexed signal to each channel. The decoder performs correlation processing on the distributed code division multiplexed signal using a code assigned to each channel, and generates and outputs a correlation signal composed of an autocorrelation component and a cross-correlation component. The autocorrelation component extraction unit extracts an autocorrelation component from the correlation signal. The optical pulse width expander generates a reception signal by expanding the half-value width on the time axis of the optical pulse of the autocorrelation component.

  The reception signal output from the optical pulse width expander is an optical reception signal that exists as an optical pulse train. This optical reception signal is finally photoelectrically converted and received as an electric reception signal existing as an electric pulse train.

  It is preferable that the receiving unit further includes a clock signal extraction unit and a time gate processing unit for each channel. The clock signal extraction unit extracts a clock signal from the correlation signal, and the time gate processing unit extracts an autocorrelation component from the correlation signal by the clock signal.

  The optical pulse width expander is preferably configured using a chromatic dispersion medium. An optical fiber can be used as the wavelength dispersion medium.

  Also, the optical pulse width expander is preferably configured using a wavelength dispersion element. As the wavelength dispersion element, it is possible to use a chirped fiber grating having a configuration in which the refractive index modulation period of the core of the optical fiber is changed stepwise. An etalon can also be used as the wavelength dispersion element.

  The optical pulse width expander has a function of expanding the half-value width of the optical pulse of the autocorrelation component on the time axis to a width equal to the half-value width of the optical pulse constituting one bit of the optical pulse signal. It is suitable that it is.

  According to the OCDM transmission / reception apparatus of the present invention, the following OCDM transmission / reception method of the present invention can be realized. That is, the OCDM transmission / reception method of the present invention includes a transmission step and a reception step.

  The transmission step and the reception step are realized in the transmission unit and the reception unit of the OCDM transmission / reception apparatus of the present invention, respectively. In the transmission step, a different code is assigned to each channel, the optical pulse signal of each channel is encoded with the assigned code, and an encoded optical pulse signal for each channel is generated. This is a step of generating and transmitting a code division multiplexed signal by multiplexing.

  The reception step is a step of acquiring the reception signal by decoding the code division multiplexed signal with the assigned code, a multiple signal distribution step, a decoding step, an autocorrelation component extraction step, and an optical pulse width expansion step. And has. The multiple signal distribution step, the decoding step, the autocorrelation component extraction step, and the optical pulse width expansion step are each provided in the receiving unit of the OCDM transmission / reception device of the present invention, a distributor, a decoder, an autocorrelation component extraction unit, And an optical pulse width expander.

  The multiple signal distribution step is a step of receiving the code division multiplexed signal and distributing it to each channel. The decoding step is a step of performing correlation processing on the distributed code division multiplexed signal using a code assigned to each channel to generate and output a correlation signal composed of an autocorrelation component and a cross-correlation component. is there. The autocorrelation component extraction step is a step of extracting an autocorrelation component from the correlation signal. The optical pulse width expansion step is a step of generating a reception signal by expanding the half-value width on the time axis of the optical pulse constituting the autocorrelation component.

  Further, it is preferable that the above-described receiving unit further includes a clock signal extracting unit and a time gate processing unit. A clock signal extraction step, which is a step of extracting a clock signal from the correlation signal, is realized by the clock signal extraction unit. The time gate processing unit realizes a time gate processing step which is a step of extracting an autocorrelation component from the correlation signal by the clock signal.

  Further, in the optical pulse width expansion step, it is preferable to expand the half width on the time axis of the optical pulse constituting the autocorrelation component to a width equal to the half width of the optical pulse constituting one bit of the optical pulse signal. It is.

  The OCDM transmission / reception apparatus according to the present invention is characterized in that the receiving unit includes an optical pulse width expander that generates a reception signal by expanding the half-value width of the optical pulse of the autocorrelation component on the time axis.

  According to the OCDM transmission / reception apparatus of the present invention having the above-described features, it is possible to widen the time width of the optical pulse of the autocorrelation component generated by decoding in the reception unit by the optical pulse width expander. As a result, the time width of the optical pulse of the autocorrelation signal generated by decoding at the receiving unit is narrower than that required for photoelectric conversion and accurate conversion as an electrical pulse. However, by extending the time width of the optical pulse, it is possible to accurately perform photoelectric conversion and obtain it as a received signal.

  In addition, it is difficult to extend a signal eye pattern to a certain extent for an optical pulse signal composed of an optical pulse having a very narrow time width compared to a time slot for an optical pulse. By extending the time width of the optical pulse, the signal eye pattern can be received and expanded to a certain extent.

  That is, for an optical pulse signal composed of an optical pulse having a very narrow time width, if the optical pulse signal includes a time jitter larger than the time width of the optical pulse, the eye pattern is not opened. The optical pulse signal is not recognized as a signal. In an optical pulse signal composed of an optical pulse having a very narrow time width, a situation in which a time jitter larger than the time width of the optical pulse is included in the optical pulse signal can easily occur. On the other hand, if the time width of the optical pulse is expanded to the same extent as the time slot for the optical pulse, a situation in which a time jitter larger than the time width of the optical pulse is included in the optical pulse signal rarely occurs. Therefore, by expanding the time width of the optical pulse, the width on the time axis recognized as a signal is expanded from the existence range of the optical pulse having a very narrow time width to the same level as the time slot for the optical pulse. Therefore, the signal eye pattern can easily secure a receivable area.

  Thus, by expanding the half-value width on the time axis of the optical pulse of the autocorrelation component, the signal eye pattern in the OCDM transmission / reception method of the present invention is made the same as the signal eye pattern in the case of OTDM communication or WDM communication. can do.

  Therefore, according to the OCDM transmission / reception apparatus of the present invention, the same electronic element as that used for reception processing in the case of an optical multiplexing communication apparatus using OTDM or an optical multiplexing communication apparatus using WDM is used, and the same technique is used. The received signal can be acquired. In other words, the above-mentioned problems caused by the very narrow half-value width of the optical pulse constituting the received signal indicate that the half-width on the time axis of the optical pulse of the autocorrelation component is reduced to the optical pulse in the optical pulse width expansion step. The problem is solved by extending the signal to a width equal to the half-value width of the optical pulse constituting one bit of the signal.

  That is, the problem that the optical pulse is not accurately converted into an electric pulse at the time of photoelectric conversion, and the problem that it is difficult to expand the signal eye pattern to the extent necessary for receiving the signal accurately. This is eliminated by extending the half-value width of the optical pulse of the autocorrelation component on the time axis.

  As described above, according to the OCDM transmission / reception apparatus of the present invention, even if the time width of the optical pulse of the autocorrelation signal decoded and generated in the reception unit is narrow, the autocorrelation signal is correctly recognized as the reception signal. Even if the communication rate is increased, it is possible to secure a sufficient number of channels to be multiplexed.

  The OCDM transmission / reception apparatus according to the present invention includes a clock signal extraction unit that extracts a clock signal from a correlation signal generated by a decoder, and a time gate processing unit that extracts an autocorrelation component from the correlation signal using the clock signal. Thus, the autocorrelation component is accurately extracted.

  As already described, in order to increase the number of multiplexed channels, it is necessary to increase the code length. If the code length is increased and the number of multiplexed channels is increased, the half width of the chip pulse becomes narrower, and the difference between the intensity of the autocorrelation component optical pulse and the intensity of the cross correlation component optical pulse is There is also the problem of becoming smaller. This problem will be described below on the assumption that a correlation signal is generated by decoding a code division multiplexed signal in which N channels (N is an integer of 2 or more) are multiplexed.

  In this case, the autocorrelation component and the cross-correlation component constituting the correlation signal are as follows. That is, 1 / N chip pulses constituting a code division multiplexed signal are used to generate an autocorrelation component, and the remaining (N-1) / N chip pulses generate a cross-correlation component. Used for. For this reason, as N increases, the number of chip pulses used to generate the cross-correlation component increases, so the peak intensity of the optical pulse of the autocorrelation component decreases relatively, and the intensity of the optical pulse of the autocorrelation component And the difference in the optical pulse intensity of the cross-correlation component becomes small.

  That is, it becomes difficult to separate the autocorrelation component and the cross-correlation component based on the difference in the intensity of the optical pulse that constitutes each. Therefore, it is difficult to accurately extract the autocorrelation component in the determination method in which an optical pulse having a large peak intensity among the optical pulses constituting the correlation signal is an autocorrelation component. In this case, the time correlation processing unit performs a shutter operation, that is, time gate processing so that the correlation signal can pass only in the time zone to which the autocorrelation component is assigned on the time axis, so that the autocorrelation component Can be extracted accurately.

  In the OCDM transmission / reception apparatus according to the present invention, since it is assumed that the number of multiplexed channels is increased, in order to separate the autocorrelation component and the cross-correlation component from the correlation signal, a time process is not used. It is effective to use a method by gate processing.

  According to the OCDM transmission / reception apparatus of the present invention, the OCDM transmission / reception method of the present invention is realized as described above. That is, in the OCDM transmission / reception method of the present invention, the reception step includes an optical pulse width expansion step for generating a reception signal by expanding the half-value width of the optical pulse of the autocorrelation component on the time axis. This feature is different from the same type of OCDM transmission / reception method.

  Therefore, according to the OCDM transmission / reception method of the present invention, it is possible to correctly recognize an autocorrelation signal as a reception signal even if the time width of the optical pulse of the autocorrelation component generated in the decoding step of the reception step is narrow. . That is, according to the OCDM transmission / reception method of the present invention, it is possible to secure a sufficient number of channels to be multiplexed even if the communication rate is increased.

  Hereinafter, an embodiment of the present invention will be described with reference to the drawings. Each drawing shows a configuration example according to the present invention, and only schematically shows the cross-sectional shape and arrangement relationship of each component to the extent that the present invention can be understood. It is not limited. In the following description, specific materials and conditions may be used. However, these materials and conditions are only one preferred example, and are not limited to these. Moreover, in each figure, the same component is shown with the same number, and the overlapping description may be omitted.

<Configuration and operation of OCDM transceiver>
With reference to FIG. 1, the configuration and operation of an OCDM transceiver apparatus according to an embodiment of the present invention will be described. FIG. 1 is a schematic block diagram of an OCDM transmission / reception apparatus according to an embodiment of the present invention. In FIG. 1, it is assumed that there are 3 channels to be multiplexed. However, the following explanation is not limited to the case of 3 channels, and it is clear that the same holds true for any number of channels. is there.

  The OCDM transmission / reception apparatus according to the embodiment includes a transmission unit 10 and a reception unit 40. The transmission unit 10 includes a transmission signal generation unit 12 for the first channel, a transmission signal generation unit 14 for the second channel, and a transmission signal generation unit 16 for the third channel. The configuration is the same. Therefore, here, the configuration and operation of the transmission signal generation unit 12 of the first channel will be described.

  The first channel transmission signal generator 12 includes an optical pulse signal generator 20 and an encoder 22. The optical pulse signal generation unit 20 generates and outputs an optical pulse train reflecting the binary digital signal that is the transmission signal of the first channel, that is, the optical pulse signal 21.

  Hereinafter, the optical pulse signal refers to the presence of the optical pulse reflecting the binary digital signal generated on the time axis by modulating the optical pulse train in which one optical pulse is aligned with respect to the time slot for the optical pulse. An optical pulse train in which optical pulses corresponding to non-existence are arranged. That is, the optical pulse signal generation unit 20 converts the transmission signal of the first channel into an optical pulse signal in the RZ (Return-to-Zero) format and outputs it.

  In the encoder 22, a code assigned to the first channel is set. In the encoder 22, each of the optical pulses constituting the optical pulse signal 21 is converted into a sequence of chip pulses based on the code set in the encoder 22. That is, the optical pulse signal 21 is input to the encoder 22, encoded, and generated and output as the encoded optical pulse signal 23.

  The encoder 22 can appropriately use a well-known Superstructure Fiber Bragg Grating (SSFBG) (for example, (1) Toru Mizunami “Optical Fiber Grating” Applied Physics Vol. 67 No. 9 pp. 1029-1034, (1998), (2) Akihiko Nishiki, Hideshi Iwamura, Hideyuki Kobayashi, Kyoko Serizawa, Koeda Oshiba "Development of Phase Encoder for OCDM Using SSFBG" Science Technique Technical Report of IEICE. (See OFT2002-66, (2002-11), or (3) Hideyuki Tonobayashi "Optical Code Division Multiplexing Network" Applied Physics Vol. 71, No. 7, pp. 853-859 (2002)). The SSFBG is composed of a plurality of unit fiber Bragg gratings (unit FBG: unit Fiber Bragg Grating) arranged in series along the light guiding direction, and the plurality of unit FBGs along the light guiding direction. A sign is set according to the relative positional relationship. In the following description, it is assumed that the encoder 22 uses SSFBG.

  The encoder 22 can appropriately use a transversal type optical filter (see, for example, JP-A-2006-319843). The transversal type optical filter is configured as a silica-based planar waveguide (PLC) and includes a delay line, a variable coupling rate optical coupler, a phase modulation unit, and a multiplexing unit as its main components. This is an optical filter. In the transversal optical filter, the code is set by appropriately adjusting the phase modulation amount set in the phase modulation unit.

  If a code having a code length of 128 is set in the encoder 22, each of the optical pulses constituting the optical pulse signal 21 is 1 / of the time slot for the optical pulse on the time axis. The time slot for 128 chip pulses is converted into an optical pulse train in which optical pulses corresponding to the presence and absence of the optical pulse are arranged reflecting the code. In other words, the encoder 22 converts one optical pulse constituting the optical pulse signal 21 into an optical pulse train in which an optical pulse is present or absent in a fine time slot of 1/128.

  The second channel transmission signal generation unit 14 and the third channel transmission signal generation unit 16 also have the same configuration as the first channel transmission signal generation unit 12. However, the codes set for the respective encoders are different. As described above, the transmission signal generator 12 of the first channel generates and outputs the encoded optical pulse signal 23. Similarly, the second-channel transmission signal generation unit 14 generates and outputs an encoded optical pulse signal 25, and the third-channel transmission signal generation unit 16 generates and outputs an encoded optical pulse signal 27. . The encoded optical pulse signal 23, the encoded optical pulse signal 25, and the encoded optical pulse signal 27 are input to the combiner 26, and are generated and output as a code division multiplexed signal 31. As the combiner 26, for example, a 3-input 1-output type optical coupler can be used.

  Therefore, the transmitter 10 is assigned a different code for each channel, encodes the optical pulse signal of each channel with the assigned code, and generates an encoded optical pulse signal for each channel. It has a function of generating and transmitting a code division multiplexed signal 31 by combining the encoded optical pulse signals.

  The code division multiplexed signal 31 propagates through the optical fiber transmission line 30 and is transmitted to the receiving unit 40. That is, encoded optical pulse signals obtained by encoding optical pulse signals of a plurality of channels are multiplexed and transmitted as a code division multiplexed signal 31 by one optical fiber transmission line 30.

  The receiving unit 40 includes a distributor 50, a first channel received signal generating unit 42, a second channel received signal generating unit 44, and a third channel received signal generating unit 46, each having a decoder. Has been placed. The codes set in the decoders arranged in the first to third channel reception signal generation units are set in the encoders arranged in the transmission signal generation units of the first to third channels, respectively. Is equal to the sign.

  That is, the configurations of the transmission signal generation units of the first to third channels are the same except that the codes set in the decoders arranged in the reception signal generation units of the first to third channels are different. Therefore, in FIG. 1, the configurations of the second channel received signal generation unit 44 and the third channel received signal generation unit 46 are the following: The overlapping components are not shown.

  The code division multiplexed signal 31 propagates through the optical fiber transmission line 30 and is input to the distributor 50 provided in the receiving unit 40, and is distributed to each of the first to third channel received signal generating units. That is, the distributor 50 receives the code division multiplexed signal 31, and receives the code division multiplexed signal 51-1, the code division multiplexed signal 51-2, and the code division multiplexed signal 51- for the first to third channels, respectively. Supply three. Since the code division multiplexed signals 51-1 to 51-3 can be generated by intensity dividing the code division multiplexed signal 31, the distributor 50 can appropriately use a 1-input 3-output type optical coupler.

  The function of receiving the received signal by decoding the code division multiplexed signal with the code assigned to the receiving unit 40 is a function that each of the received signal generating units of the first to third channels has in common. . Therefore, as a description of the function of the receiving unit 40, it is sufficient to describe the configuration and the operation of the received signal generating unit 42 of the first channel. That is, since the configurations of the reception signal generation units for the first to third channels are the same, the description of the functions of the reception unit 40 excluding the function of distributing the code division multiplexed signal 31 to each channel will be described here. Now, the configuration and operation of the reception signal generation unit 42 of the first channel will be described.

  The first channel received signal generator 42 includes a decoder 52, an autocorrelation component extractor 54, an optical pulse width expander 64, and a received signal processor 66.

  The decoder 52 receives the code division multiplexed signal 51-1 and performs correlation processing using the code assigned to the first channel to generate a correlation signal 53 composed of an autocorrelation component and a cross-correlation component. Output. In order to realize such a function in the decoder 52, if the SSFBG having the same structure as the SSFBG used in the encoder 22 installed in the first channel in the transmission unit 10 is also used in the decoder 52, Good.

  The autocorrelation component extraction unit 54 extracts an autocorrelation component from the correlation signal 53 including the autocorrelation component and the cross-correlation component. If the number of channels to be multiplexed is small, the peak intensity of the optical pulse of the autocorrelation component is larger than the peak intensity of the optical pulse of the cross correlation component. Therefore, it is possible to extract the autocorrelation component by threshold processing. . However, as the number of multiplexed channels increases, as described above, the difference between the intensity of the optical pulse of the autocorrelation component and the intensity of the optical pulse of the cross-correlation component decreases, so that the autocorrelation component is extracted by threshold processing. It becomes difficult.

  Therefore, a method of extracting an autocorrelation component by time gate processing is employed. Time gate processing means that a time gate window is opened only for a time slot in which an optical pulse of a cross-correlation component exists on the time axis, and a time gate window is closed for other time slots. Is used to extract the autocorrelation component from the correlation signal 53 by passing only the optical pulse of the autocorrelation component. In order to open and close the window of the time gate, a clock signal extracted from the correlation signal 53 is required.

  Correlation signal 53 is branched by branching device 58 into autocorrelation signal extraction correlation signal 59-1 and clock signal extraction correlation signal 59-2. The correlation signal 59-2 is input to the clock signal extraction unit 60, and the clock signal 61 is extracted and output. This clock signal 61 is input to the time gate processing unit 62 and used to open and close the window of the time gate.

  Although not shown in FIGS. 1 and 2, in order to open and close the window of the time gate by the clock signal 61, the clock signal 61 is an electric signal required for driving the time gate. It needs to be converted to form. That is, a time gate driving circuit for driving the time gate is used. The time gate drive circuit needs to be prepared according to the type of optical switch element used for the time gate.

  As an optical switching element used for the time gate processing unit 62, an optical modulator such as a Sagnac interferometer using an optical fiber loop or an electro-absorption modulator can be used as appropriate. .

  The configuration and operation of the clock signal extraction unit 60 will be described with reference to FIG. FIG. 2 is a schematic block configuration diagram showing a specific configuration of the reception signal generation unit 42 of the first channel of the reception unit 40 of the OCDM transmission / reception apparatus.

  The clock signal extraction unit 60 includes an O / E converter 70, a band-pass filter (BPF) 72, and a phase-locked loop circuit (PLL circuit) 74. The O / E converter 70 receives the correlation signal 59-2 for clock signal extraction, converts it into an electrical correlation signal 71, and outputs it. The BPF 72 receives the electrical correlation signal 71, selects and transmits the frequency component of the communication rate, and generates and outputs a PLL circuit input signal 73. The PLL circuit 74 receives the PLL circuit input signal 73, generates a clock signal 61, and outputs it.

  The clock signal extraction unit 60 shown in FIG. 2 symbolically shows an O / E converter 70, a BPF 72, and a PLL circuit 74 that a device for extracting a clock signal typically includes, It does not show a specific structure of an actual clock signal extraction device. In order to generate and output the clock signal 61 from the correlation signal 59-2 for clock signal extraction, a method appropriately selected from known methods may be used. Specifically, for example, it can be performed as follows.

  First, the correlation signal 59-2 for clock signal extraction is modulated by modulating the modulated electrical signal obtained by mixing the electrical signal with the bit rate frequency f of the optical pulse signal and the electrical signal with a frequency Δf smaller than f. An optical pulse signal is generated. It is possible to extract a clock signal having a frequency f from this modulated optical pulse signal (see, for example, Japanese Patent No. 3904567).

  As shown in FIG. 1, the autocorrelation signal 63 generated by extracting the autocorrelation component from the correlation signal 59-1 output from the autocorrelation component extraction unit 54 is input to the optical pulse width expander 64. The In the optical pulse width expander 64, the width of the optical pulse constituting the autocorrelation signal 63 is expanded. Hereinafter, the term “light pulse width” simply refers to the half-value width of the time waveform of the light pulse, that is, the half-value width of the light pulse on the time axis. The optical pulse width expander means an element having a function of expanding the half width of the optical pulse on the time axis.

  The optical pulse width expander 64 preferably expands the width of the optical pulse constituting the autocorrelation signal 63 to a width equal to the half-value width of the optical pulse constituting one bit of the optical pulse signal. The wider the width of the optical pulse constituting the autocorrelation signal 63, the more advantageous is the process for converting the optical reception signal executed in the reception signal processing unit 66 into an electric reception signal. However, when the optical pulse is expanded to a width exceeding the half-value width of the optical pulse constituting one bit of the optical pulse signal, a portion where the electric pulses that are adjacent bits of the electric reception signal overlap with each other is generated, causing a reception error. Therefore, the width of the optical pulse constituting the autocorrelation signal 63 is ideally expanded to a width equal to the half-value width of the optical pulse constituting one bit of the optical pulse signal.

  The optical pulse width expander 64 is preferably configured using a wavelength dispersion medium. Since the time width of the optical pulse constituting the autocorrelation signal 63 extracted from the correlation signal 59-1 is narrow, its wavelength spectrum is wide. That is, there is a relationship that the product of the half width on the time axis of the optical pulse constituting the autocorrelation signal 63 and the half width in the frequency spectrum is constant. This is because the optical pulse constituting the autocorrelation signal 63 and the optical pulse in the frequency spectrum space of this optical pulse are in a Fourier transform relationship with each other. Therefore, the optical pulse constituting the autocorrelation signal 63 is an optical pulse including a larger number of wavelength components because the half-value width in the frequency spectrum space is wider as the half-value width on the time axis is narrower.

  Therefore, it is preferable to use a chromatic dispersion medium, which is a light transmission medium whose refractive index depends on the wavelength, for the optical pulse width expander 64. Alternatively, it is preferable to use a wavelength dispersion element having a function of extending the time width of the optical pulse.

  Optical pulses having a wide half-value width in the frequency spectrum space pass through the chromatic dispersion medium, thereby causing different time delays corresponding to the wavelengths. As a result, by separating the short wavelength component and the long wavelength component on the time axis, the half width on the time axis is widened.

  An optical fiber can be most easily used as the chromatic dispersion medium. Since an optical fiber is used as a transmission line in optical communication, the use of an optical fiber as a wavelength dispersion medium has an advantage that it can be easily combined with other components constituting the OCDM transceiver. . However, since an optical fiber used for a normal optical transmission line does not have a large chromatic dispersion, a considerable length is required to use it as a chromatic dispersion medium.

As a wavelength dispersion element, a chirped fiber grating manufactured by changing the refractive index modulation period of a core of an optical fiber stepwise is known. (For example, URL: http://www.mitsubishi-cable.co.jp/jihou/pdf/94
/04.pdf [See July 24, 2007 search]). The chirped fiber grating has sufficiently large chromatic dispersion, so that it can perform the same function as an optical fiber with a short length. Therefore, if the chirped fiber grating is used as a wavelength dispersion element, a compact optical pulse width expander can be configured. However, since the chirped fiber grating is expensive, the cost for manufacturing the OCDM transmitter / receiver increases.

  It is also possible to use an etalon as a wavelength dispersion element (for example, URL: http://www.business-i.jp/sentan/jusyou/2007/suda.pdf [searched July 24, 2007]) See). In any case, what is used as the wavelength dispersion medium or the wavelength dispersion element belongs to a design matter that should be determined in consideration of the cost and the like when manufacturing the OCDM transceiver.

  In the optical pulse width expander 64, the width of the optical pulse constituting the autocorrelation signal 63 is expanded, and an optical reception signal 65 is generated and output. As shown in FIG. 2, the optical reception signal 65 is input to the reception signal processing unit 66, and is generated and output as an electric reception signal 83. This electrical reception signal 83 is a reception signal finally acquired by the reception unit 40.

The received signal processing unit 66 includes an O / E converter 76, an amplifier 78, a low-pass filter (LPF) 80, and a determination circuit 82. The O / E converter 76 receives the optical reception signal 65, performs photoelectric conversion, generates an electrical signal 77 , and outputs it. The amplifier 78 is necessary for the accurate processing operation of the LPF 80 and the determination circuit 82, and the finally obtained reception signal is used for using the reception signal installed at the subsequent stage of the reception signal processing unit 66. The electric signal 77 is amplified and output as an electric signal 79 so that the intensity required by the apparatus (not shown) is obtained.

The electrical signal 79 output from the amplifier 78 is transmitted by selecting a low frequency component including the bit rate of the optical pulse signal and transmitting it by the LPF 80, thereby removing the low frequency noise component. The signal 81 is output. The cutoff frequency of the LPF 80 is equal to the maximum value of the frequency components included in the electric signal 79 cut off by the LPF 80.

  The electrical reception signal 81 is input to the determination circuit 82, converted into a digital signal, and output as a digital electrical reception signal 83. The determination circuit can be formed, for example, as a known circuit configuration configured by combining a comparator and a D flip-flop circuit. The comparator can use the clock signal extracted by the clock signal extraction unit (not shown). The electric reception signal 81 input to the determination circuit as an analog signal is subjected to threshold processing by a comparator, and is generated as a digital reception signal by a D flip-flop circuit.

  If the final goal is to obtain the received signal as an analog signal, no decision circuit is required. In any case, the optical reception signal 65 is acquired as an electric reception signal by the reception signal processing unit 66. The receiving unit 40 is a component that performs a function of decoding a received code division multiplexed signal with an assigned code to obtain a received signal. Here, when the optical reception signal 65, the analog electrical reception signal 81, and the digital electrical reception signal 83 are collectively referred to, the term reception signal is simply used.

  The feature of the present invention is to extend the time width of the optical pulse of the autocorrelation signal decoded and generated on the reception side to a certain extent that it can be correctly recognized as the reception signal. As a function to be fulfilled, it is an indispensable condition that an optical reception signal 65 composed of an optical pulse train having a wide time width can be generated. That is, if the optical reception signal 65 composed of an optical pulse train having a wide time width can be generated, it can be said that the object of the present invention is achieved. Therefore, whether the reception signal finally received by the reception unit is in the form of an optical signal, an analog electric signal, or a digital electric signal is a secondary problem.

  Therefore, the received signal is a general term for the optical received signal 65, the analog electric received signal 81, and the digital electric received signal 83, and the function required for the receiving unit 40 is "assigned the received code division multiplexed signal. The received signal is obtained by decoding with the code. "

<OCDM transmission / reception method>
As described above, according to the OCDM transmission / reception apparatus of the embodiment of the present invention, the OCDM transmission / reception method of the present invention including the transmission step and the reception step can be realized.

  The transmission step and the reception step are realized in the transmission unit 10 and the reception unit 40 of the OCDM transmission / reception apparatus, respectively. Multiplex signal distribution step, decoding step, autocorrelation component extraction step, and optical pulse width expansion step constituting the reception unit step are respectively a distributor 50, a decoder 52, an autocorrelation component extraction unit 54, and an optical pulse. Performed by the width expander 64. The clock signal extraction step and the time gate processing step included in the autocorrelation component extraction step are executed by the clock signal extraction unit 60 and the time gate processing unit 62, respectively.

  The OCDM transmission / reception method of the present invention will be described with reference to FIGS. 1, 2 and 3A to 3E. FIGS. 3A to 3E are diagrams showing time waveforms of signals at various points in the OCDM transmission / reception apparatus of the present invention. The horizontal axis is the time axis, and is scaled on an arbitrary scale. Although the vertical axis is omitted, the light intensity is graduated in an arbitrary scale in the vertical axis direction.

  FIG. 3 (A) is a diagram showing a time waveform of the optical pulse signal 21. FIG. The half width on the time axis of the optical pulse constituting the optical pulse signal 21 is Δt. FIG. 3B shows an encoded optical pulse signal 23 output from the first channel transmission signal generation unit 12, an encoded optical pulse signal 25 output from the second channel transmission signal generation unit 14, and a third 3 is a diagram illustrating a time waveform of a code division multiplexed signal 31 generated by multiplexing an encoded optical pulse signal 27 output from a channel transmission signal generation unit 16 by a combiner 26. FIG. That is, FIG. 3 (B) is a diagram showing a time waveform of the code division multiplexed signal 31 generated and output by the transmission step.

  Also, the time waveform shown in FIG. 3 (B) is the time waveform of the code division multiplexed signal 51-1, the code division multiplexed signal 51-2, and the code division multiplexed signal 51-3 distributed to each channel by the distributor 50. It also shows. That is, the time waveform shown in FIG. 3B also shows the time waveform of the signal generated by the multiple signal distribution step.

  The time waveform of the code division multiplexed signal 31 and the time waveforms of the code division multiplexed signal 51-1, the code division multiplexed signal 51-2, and the code division multiplexed signal 51-3 are the optical pulses forming these signals. Only the intensity is different, and the waveform shape is exactly the same. When configured as an actual device, an optical amplifier (not shown) is arranged between the distributor 50 and the decoder of each channel, and the code division multiplexing distributed by the distributor 50 is performed. In some cases, the signal is amplified and supplied to each channel as a code division multiplexed signal 51-1, a code division multiplexed signal 51-2, and a code division multiplexed signal 51-3, respectively.

  The time waveform shown in FIG. 3B is a code generated by superimposing the chip pulses constituting the encoded optical pulse signal 23, the encoded optical pulse signal 25, and the encoded optical pulse signal 27 on the time axis. Since it is a time waveform of the division multiplexed signal, the half width of the chip pulse forming this time waveform is Δt ′ narrower than the half width Δt of the optical pulse. Specifically, Δt ′ is equal to a value obtained by dividing Δt by the code length. For example, when the code length is 128, Δt ′ = Δt / 128.

  FIG. 3C shows a time waveform of correlation signal 53 output from decoder 52. That is, FIG. 3C shows a time waveform of the correlation signal 53 that is a signal generated by the decoding step. As shown in FIG. 3C, the correlation signal 53 includes a chip pulse constituting an autocorrelation component (indicated by “s” in FIG. 3C) and a chip pulse constituting a cross-correlation component (FIG. 3). (In (C), it is indicated by c.)). The half-value width of the chip pulse constituting the autocorrelation component is equal to Δt ′ as already described.

  FIG. 3 (D) is a diagram showing a time waveform of the autocorrelation signal 63 output from the autocorrelation component extraction unit 54. That is, the time waveform shown in FIG. 3D is a time waveform of the autocorrelation signal 63 that is the signal extracted by the autocorrelation component extraction step. Since the cross-correlation component is removed by the autocorrelation component extraction unit 54, the chip pulse constituting the cross-correlation component indicated by c in FIG. 3C does not appear. In the autocorrelation component extraction step, only the cross-correlation component is removed, and the half width of the chip pulse constituting the autocorrelation component is not affected, so the time waveform of the autocorrelation signal 63 shown in FIG. The half width of the chip pulse appearing at is Δt ′.

  FIG. 3 (E) is a diagram illustrating a time waveform of the optical reception signal 65 output from the optical pulse width expander 64. That is, the time waveform shown in FIG. 3E is a time waveform of the optical reception signal 65 that is a signal generated in the optical pulse width expansion step. The chip pulse shown in FIG. 3 (E) is expanded until it becomes equal to Δt, which is the half-value width of the optical pulse signal 21 shown in FIG. 3 (A), in the optical pulse width expansion step. In addition, the relative positional relationship on the time axis between the peak positions of the plurality of chip pulses shown in FIG. 3 (E) is the peak position of the plurality of optical pulses of the optical pulse signal 21 shown in FIG. It is consistent with the relative positional relationship on the time axis. That is, the time waveform of the optical reception signal 65 is similar to the time waveform of the optical pulse signal 21 shown in FIG. 3 (A), and when the optical pulse width expansion step ends, the optical pulse signal 21 Indicates that it has been played back.

<Effect of optical pulse width expander>
The width of the pulse of the electric reception signal 81 generated as a result of photoelectric conversion or filtering by the optical reception signal 65 being subjected to the time response speed of the O / E converter 76 or the LPF 80, that is, the frequency response band. Spread. On the other hand, the frequency component not included in the frequency response band of the O / E converter 76 or the LPF 80 among the frequency components included in the optical reception signal 65 is blocked due to the restriction due to the frequency response band, The power loss of the generated electric reception signal 81 occurs, and the quality of the signal is lowered.

  For example, if the frequency response band of the O / E converter 76 does not have a sufficient width, the photoelectric conversion efficiency is lowered, and the gain of the amplifier 78 arranged in the subsequent stage must be increased. As a result, in the amplifier 78, a lot of noise is mixed into the electric reception signal 81. Further, if the frequency response band of the LPF 80 does not have a sufficient width, the filter transmission loss increases, the power loss of the electric reception signal 81 also occurs, and the signal quality is lowered.

  Referring to FIGS. 4 to 6, the autocorrelation signal 63 output directly from the time gate 62 without performing the optical pulse width expansion step is obtained from the LPF 80 when photoelectrically converted by the O / E converter 76. The result of examining the duty ratio and power loss of the pulse of the output signal will be described. FIG. 4 is a diagram extracted from the schematic block diagram of the OCDM transmitting / receiving apparatus according to the present invention, showing the configuration from the time gate processing unit 62 of the first channel of the receiving unit 40 to the LPF 80. In FIG. 4, the optical pulse width expander 64 is not disposed between the time gate processing unit 62 and the O / E converter 76.

  In the pulse duty ratio and power loss calculation of the signal output from the LPF 80 described above, an optical pulse signal in the RZ format is assumed as a signal corresponding to the correlation signal 63 output from the autocorrelation component extraction unit 54. That is, as shown in FIG. 4, the optical pulse signal 59-1 ′ in the RZ format as a signal corresponding to the correlation signal 59-1 passes through the time gate processing unit 62 as it is, and O / E as the optical pulse signal 63 ′. The calculation was performed assuming that the signal was input to the converter 76.

  The optical pulse signal 63 ′ input to the O / E converter 76 has a signal bit rate of 625 Mbit / s (that is, a signal period of 1.6 ns), an optical pulse width of 6.3 ps, and an optical pulse shape of a Gaussian curve. Assumed. The frequency response band of the O / E converter 76 is set to 160 GHz corresponding to the chip rate, and the generated electric pulse signal 77 ′ is amplified by the amplifier 78 and filtered by the LPF 80 as the electric pulse signal 79 ′ to thereby change the frequency band. The duty ratio of the pulse of the electric signal 81 ′ obtained by limiting and the power loss were calculated. This duty ratio was calculated as [(time width of optical pulse of electric signal 81 ′) / (signal period)] × 100.

  FIG. 5 is a diagram showing the relationship between the cutoff frequency of the LPF 80 and the duty ratio of the electric signal 81 ′ output from the LPF 80 and the passing loss of the electric pulse signal 79 ′ passing through the LPF 80. is there. The passage loss of the electric pulse signal 79 ′ that passes through the LPF 80 may be rephrased as the power loss of the electric pulse signal 79 ′ that passes through the LPF 80.

  The horizontal axis shows the cutoff frequency of LPF 80 scaled in GHz. The left vertical axis indicates the duty ratio of the electric signal 81 ′ output from the LPF 80 in%. Further, the vertical axis on the right side shows the passage loss of the electric pulse signal 79 ′ passing through the LPF 80 in a unit of dB.

  In FIG. 5, the duty ratio of the electric signal 81 ′ output from the LPF 80 with respect to the cut-off frequency of the LPF 80 is indicated by a black triangle mark, and the passing loss of the electric pulse signal 79 ′ passing through the LPF 80 is indicated by a cross mark. It is shown. The calculation of the relationship with the duty ratio of the electric signal 81 ′ output from the LPF 80 and the power loss in the LPF 80 is based on the ratio of the intensity of the electric signal 81 ′ to the intensity of the electric pulse signal 79 ′ passing through the LPF 80. , As equivalent to power loss in LPF 80.

  An electric pulse that passes through LPF 80 at 160 GHz corresponding to the chip rate, where the cutoff frequency of LPF 80 corresponds to the rightmost observation point in FIG. 5 (the position of the downward arrow indicated by P in FIG. 5). It can be read that the passing loss of the signal 79 ′ is almost 0, but the duty ratio of the electric signal 81 ′ at this time is 0.4%, which is equal to the duty ratio in the state of the optical pulse signal. As the cut-off frequency of LPF 80 becomes lower, the duty ratio of electric signal 81 ′ increases, but at the same time, the passage loss of LPF 80 also increases.

  For example, assuming that the width of the optical pulse constituting the optical pulse signal 63 ′ is 6.3 ps, the width of the electrical pulse constituting the electrical signal 81 ′ output from the LPF 80 is used in normal communication. It can be seen that the LPF 80 cut-off frequency must be 2.5 GHz in order to expand to 150 ps, which corresponds to an electric pulse signal in the RZ format, with a duty ratio of 10% (indicated by Q in FIG. 5). See the location of the down arrow). It can be seen that the LPF 80 passing loss at this time is 14 dB. In order to expand the width of the electric pulse until the duty ratio reaches 50%, which corresponds to an electric pulse signal of NRZ (Non-Return-to-Zero) format used in normal communication, LPF 80 It can be seen that the cut-off frequency of 469 has to be 469 MHz (see the position of the downward arrow indicated by R in FIG. 5). It can be seen that the LPF 80 passing loss at this time is 21 dB.

  FIG. 6 is a diagram showing the relationship of the duty ratio of the electric signal 81 ′ output from the LPF 80 and the relationship of the passage loss of the LPF 80 with respect to the optical pulse width constituting the optical pulse signal 63 ′ output from the time gate. It is. The calculation was performed when the cutoff frequency of LPF 80 was 469 MHz and when it was 938 MHz. The horizontal axis shows the optical pulse width constituting the optical pulse signal 63 ′ output from the time gate in a scale in ps. The left vertical axis indicates the duty ratio of the electric signal 81 ′ output from the LPF 80 in%. Further, the vertical axis on the right side shows the passage loss of the electric pulse signal 79 ′ passing through the LPF 80 in a unit of dB. In FIG. 6, the calculation results when the cutoff frequency of the LPF 80 is 469 MHz and 938 MHz are indicated by a black triangle mark and a black square mark, respectively.

  As shown in FIG. 6, when the cutoff frequency of the LPF 80 is 469 MHz, the duty ratio of the electric signal 81 ′ can be 50%, and when it is 938 MHz, the electric signal 81 ′ It can be seen that the duty ratio can be reduced to 25%. However, as the optical pulse width constituting the optical pulse signal 63 ′ output from the time gate becomes narrower, that is, as the duty ratio of the optical pulse signal 63 ′ becomes smaller, the electric pulse signal 79 ′ passing through the LPF 80 It can be seen that the passage loss increases.

  Next, referring to FIG. 7 to FIG. 9, the autocorrelation signal 63 output from the time gate for the case where the optical pulse width expansion step, which is a feature of the OCDM transceiver of the present invention, is performed, The result of the same examination regarding the duty ratio and power loss of the pulse of the signal output from the LPF 80 when photoelectric conversion is performed by the converter 76 will be described. FIG. 7 is a schematic block diagram of the OCDM transmission / reception apparatus according to the present invention, showing the configuration from the time gate processing unit 62 of the first channel of the receiving unit 40 to the LPF 80 including the optical pulse width expander 64. It is the figure extracted and shown. FIG. 7 differs from FIG. 4 described above in that an optical pulse width expander 64 is disposed between the time gate processing unit 62 and the O / E converter 76, as in the OCDM transmission / reception apparatus of the present invention. Is a point.

  Here again, as in the case of the examination in the case where the optical pulse width expander 64 is not disposed, the pulse ratio of the signal output from the LPF 80 and the power loss calculation are output from the autocorrelation component extraction unit 54. As a signal corresponding to the correlation signal 63, an optical pulse signal in the RZ format was assumed. That is, as shown in FIG. 7, the optical pulse signal 59-1 ′ in the RZ format as a signal corresponding to the correlation signal 59-1 passes through the time gate processing unit 62 as it is, and the optical pulse signal 63 ′ is O / E. The calculation was performed assuming that the signal was input to the converter 76.

  The optical pulse signal 63 ′ output from the time gate processing unit 62 and input to the optical pulse width expander 64 has a signal bit rate of 625 Mbit / s (ie, a signal period of 1.6 ns), and the optical pulse shape is A Gaussian curve shape was assumed.

  FIG. 8 shows how the duty ratio of the optical pulse signal 65 ′ that is generated and output when the optical pulse signal 63 ′ is input to the optical pulse width spreader 64 and the optical pulse width is expanded is calculated. It is a figure which shows the result. That is, FIG. 8 is a diagram showing the relationship between the duty ratio of the optical pulse signal 65 ′ output from the optical pulse width expander 64 and the chromatic dispersion amount added by the optical pulse width expander 64.

  FIG. 8 shows the case where the optical pulse width of the optical pulse signal 63 ′ input to the optical pulse width expander 64 is 6.3 ps, 13 ps, 25 ps, and 50 ps, respectively. The duty ratio of the pulse signal 65 ′ is shown. The horizontal axis indicates the chromatic dispersion amount added by the optical pulse width expander 64 in a scale in units of ps / nm. The vertical axis shows the duty ratio of the optical pulse signal 65 ′ output from the optical pulse width expander 64 as a scale in%.

  According to FIG. 8, when the optical pulse width of the optical pulse signal 63 ′ is 6.3 ps, 1400 ps / nm chromatic dispersion is added by the optical pulse width expander 64 in order to set the duty ratio to 50%. I understand that I have to. Adding this degree of chromatic dispersion is easily realized by using an optical fiber that is a chromatic dispersion medium, a chirped fiber grating that is a chromatic dispersion element, or an etalon, which is suitable for the optical pulse width expander 64 described above. Is done. The passage loss of these chromatic dispersion media and chromatic dispersion elements is about 2 to 3 dB.

  By passing through the wavelength dispersion medium or wavelength dispersion element, a power loss of about 2 to 3 dB occurs, but the duty ratio of the optical pulse signal 65 ′ input to the O / E converter 76 is 50%. The O / E converter 76 can convert the electric pulse signal 77 ′ without being restricted by the frequency band. That is, since it is converted into the electric pulse signal 77 ′ with almost no power loss in the O / E converter 76, it is necessary to set a large gain of the amplifier 78 installed at the subsequent stage of the O / E converter 76. There is no.

  Therefore, the noise component mixed in the electric pulse signal 77 ′ in the amplifier 78 is small. That is, an electric pulse signal 79 ′ having a small noise component is generated and output by the amplifier 78 and input to the LPF 80. Also in the LPF 80, since the duty ratio of the electric pulse signal 79 ′ is 50%, it is filtered without being restricted by the frequency band, so that no power loss occurs here.

  As described above, in order to increase the duty ratio in the O / E converter 76 or the LPF 80, the power loss received by the electric pulse signal 81 ′ output from the LPF 80 is 14 to 21 dB compared with It can be seen that the power loss caused by passing through the chromatic dispersion medium or the chromatic dispersion element is very low, about 2-3 dB.

  FIG. 9 shows a case where the optical pulse width of the optical pulse signal is expanded using the optical pulse width expander 64 and then converted into an electric pulse signal. It is a figure which shows how much the power loss of the signal by the frequency band restriction | limiting produced | generated by LPF80 is reduced compared with the case where the pulse width of a pulse signal is expanded. That is, FIG. 9 is a diagram illustrating the difference in power loss in the LPF 80 when the optical pulse width expander 64 is not used and when it is used as an LPF passage loss improvement amount.

  In FIG. 9, the LPF passage loss improvement amount is shown with respect to the chromatic dispersion amount added by the optical pulse width expander 64. FIG. 9 shows LPF 80 passing loss improvement amounts when the optical pulse width of the optical pulse signal 63 ′ output from the time gate processing unit 62 is 6.3 ps, 13 ps, and 25 ps, respectively. The horizontal axis shows the chromatic dispersion amount added by the optical pulse width expander 64 in a scale in units of ps / nm. The vertical axis shows the LPF passage loss improvement amount in dB. Here, the calculation is performed on the assumption that the cut-off frequency of LPF 80 is 469 MHz, and the duty ratio of electric pulse signal 81 ′ output from LPF 80 is 50%.

  According to FIG. 9, for example, an optical pulse signal 63 ′ having an optical pulse width of 6.3 ps is added with chromatic dispersion of about 350 ps / nm to extend the optical pulse width to 200 ps, and then O / E conversion is performed. When the duty ratio is increased up to 50% by limiting the frequency band of LPF 80 by converting the electrical pulse signal 77 'with the converter 76, the optical pulse signal 63' with an optical pulse width of 6.3 ps is directly converted to an O / E converter. It can be seen that the LPF passage loss is improved by 15 dB as compared with the case where the duty ratio is increased to 50% by the frequency band limitation of the LPF 80 by converting into the electric pulse signal 77 ′ at 76.

  When the duty ratio is increased after the optical pulse width is expanded by the optical pulse width expander 64, the optical pulse width expander 64 that is not present when the duty ratio is increased without using the optical pulse width expander 64 is used. Passing loss of 2 to 3 dB exists. However, even if the loss amount of 2 to 3 dB, which is the passage loss of the optical pulse width expander 64, is considered, the final result is 12 dB compared to the case where the optical pulse width expander 64 is not used. It can be seen that the above loss improvement is expected.

  That is, as described above, by taking the means of adding the chromatic dispersion and extending the optical pulse width, the improvement amount of LPF pass loss can be obtained by 15 dB. Therefore, the pass loss of the optical pulse width expander 64 is 2 Even considering a loss of ~ 3 dB, 15 dB – 3 dB = 12 dB, so a loss improvement of 12 dB or more is expected.

  As discussed above, when the optical pulse signal or electrical pulse signal is limited by the frequency band of an electronic element such as an O / E converter or LPF, the pulse width is expanded according to the frequency band. However, since the signal component out of the frequency band of the electronic element among the signal components is cut off, the signal power is reduced. That is, as the frequency band of the electronic element is set lower, the signal pulse width can be increased and the signal duty ratio can be increased, but the signal power loss also increases accordingly.

  According to the OCDM transmission / reception apparatus of the present invention, the autocorrelation signal, which is the form of the optical pulse signal output from the time gate, is configured to increase the duty ratio by the optical pulse width expander. As described above, the power loss of the signal generated when passing through the optical pulse width expander is sufficiently smaller than the signal power loss in the frequency band limitation of the electronic element.

  As described above, the OCDM transmission / reception apparatus according to the present invention is capable of reducing the power of the autocorrelation signal with little power loss even if the time width of the optical pulse of the autocorrelation signal decoded and generated on the reception side is narrow. It is possible to extend the time width of the optical pulse of the autocorrelation signal in a state where the loss is extremely small. Then, by extending the time width of the optical pulse of the autocorrelation signal, the autocorrelation signal can be correctly recognized as a received signal.

1 is a schematic block diagram showing an embodiment of an OCDM transmission / reception apparatus of the present invention. FIG. 3 is a schematic block configuration diagram showing a specific configuration of a reception signal generation unit of a first channel of a reception unit of the OCDM transmission / reception apparatus according to the embodiment of the present invention. It is a diagram showing a time waveform of a signal in various places of the OCDM transceiver of the embodiment of the present invention, (A) is an optical pulse signal, (B) is a code division multiplexed signal, (C) is a correlation signal, (D) is a diagram showing an autocorrelation signal and (E) is a diagram showing a time waveform of an optical reception signal. It is the figure which extracted and showed the structure from a time gate to LPF from the schematic block block diagram of the OCDM transmission / reception apparatus of embodiment of this invention. It is a figure which shows the relationship with the duty ratio of the optical pulse output from LPF with respect to the cut-off frequency of LPF, and the relationship with the passage loss of the optical pulse which passes LPF. It is a figure which shows the relationship of the duty ratio of the electric signal output from LPF, and the relationship of the passage loss of LPF with respect to the optical pulse width which comprises the optical pulse signal output from a time gate. It is the figure which extracted and showed the structure from time gate to LPF including an optical pulse width expander from the schematic block block diagram of the OCDM transmission / reception apparatus of embodiment of this invention. It is a figure which shows the relationship with respect to the chromatic dispersion amount added by the optical pulse width expander of the duty ratio of the optical pulse signal output from an optical pulse width expander. It is a figure which shows the difference of each power loss in LPF when not using an optical pulse width expander, and when using as LPF passage loss improvement amount.

Explanation of symbols

10: Transmitter
12: Transmission signal generator for the first channel
14: Second channel transmission signal generator
16: Third channel transmission signal generator
20: Optical pulse signal generator
22: Encoder
26: Synthesizer
30: Optical fiber transmission line
40: Receiver
42: Received signal generator for the first channel
44: Second channel received signal generator
46: Third channel received signal generator
50: Distributor
52: Decoder
54: Autocorrelation component extraction unit
58: Turnout
60: Clock signal extractor
62: Time gate
64: Optical pulse width expander
66: Received signal processor
70, 76: O / E converter
72: Bandpass filter (BPF)
74: Phase synchronization circuit (PLL circuit)
78: Amplifier
80: Low-pass filter (LPF)
82: Judgment circuit

Claims (9)

A different code is assigned to each channel, and an optical pulse signal of each channel is encoded with the assigned code to generate an encoded optical pulse signal for each channel, and these encoded optical pulse signals are combined. A transmitter for generating and transmitting a code division multiplexed signal;
A reception unit that obtains a received signal by decoding the code division multiplexed signal with the assigned code;
The receiving unit
A distributor that receives the code division multiplexed signal and distributes the signal to each channel;
The distributed code division multiplexed signal is correlated for each channel using the assigned code to generate and output a correlation signal composed of an autocorrelation component and a cross-correlation component for each channel. A decoder to be
An autocorrelation component extracting unit arranged for each channel for extracting the autocorrelation component from the correlation signal;
An optical pulse width expander arranged for each channel that generates the received signal by expanding a half-value width on the time axis of the optical pulse constituting the autocorrelation component;
The autocorrelation component is used as an optical reception signal, the optical reception signal is converted into an electric reception signal and output, and a reception signal processing unit arranged for each channel,
The received signal processor is
A photoelectric converter that photoelectrically converts the received optical signal to generate and output an electrical signal;
An amplifier for amplifying the electrical signal;
A low-pass filter that receives the amplified electrical signal output from the amplifier and selects and transmits a frequency component that includes a bit rate of the optical pulse signal lower than that to generate the electrical reception signal;
With
The optical pulse width expander has a function of expanding the half-value width on the time axis of the optical pulse constituting the autocorrelation component to a width equal to the half-value width of the optical pulse constituting one bit of the optical pulse signal. Have
When the optical pulse width of the optical pulse signal is expanded using the optical pulse width expander and converted into an electric pulse signal, the power loss of the signal due to the frequency band limitation generated by the low-pass filter and The power of the signal due to the frequency band limitation generated by the low-pass filter when the sum of the transmission loss of the optical pulse width expander is converted into an electric pulse signal without using the optical pulse width expander An optical code division multiplexing transmitter / receiver characterized by being reduced by loss . The receiving unit further extracts a clock signal from the correlation signal; a clock signal extracting unit arranged for each channel;
2. The optical code division multiplexing transmission / reception apparatus according to claim 1, further comprising a time gate processing unit arranged for each channel for extracting only the autocorrelation component from the correlation signal by the clock signal.   3. The optical code division multiplexing transmitter / receiver according to claim 1, wherein the optical pulse width expander is configured using a wavelength dispersion medium.   4. The optical code division multiplexing transmitter / receiver according to claim 3, wherein the wavelength dispersion medium is an optical fiber.   3. The optical code division multiplexing transmitter / receiver according to claim 1, wherein the optical pulse width expander is configured using a wavelength dispersion element.   6. The optical code division multiplexing transmitter / receiver according to claim 5, wherein the wavelength dispersion element is a chirped fiber grating having a configuration in which a refractive index modulation period of an optical fiber core changes stepwise.   6. The optical code division multiplexing transmission / reception apparatus according to claim 5, wherein the wavelength dispersion element is an etalon. In the transmission unit, a different code is assigned to each channel, the optical pulse signal of each channel is encoded with the assigned code, and the encoded optical pulse signal for each channel is generated. A transmission step of generating and transmitting a code division multiplexed signal by combining
In the receiving unit, the code division multiplexing signal comprises a receiving step of decoding the assigned code with the assigned code to obtain a received signal,
The receiving step includes
Multiplex signal distribution step of receiving the code division multiplexed signal and distributing to each channel;
A decoding step of performing correlation processing on the distributed code division multiplexed signal using the assigned code for each channel to generate and output a correlation signal composed of an autocorrelation component and a cross-correlation component;
An autocorrelation component extraction step for extracting the autocorrelation component from the correlation signal;
An optical pulse width expansion step for generating a reception signal by expanding a half-value width on a time axis of an optical pulse constituting the autocorrelation component;
A reception signal processing step for each channel, wherein the autocorrelation component is an optical reception signal, the optical reception signal is converted into an electric reception signal and output;
The received signal processing step includes:
A photoelectric conversion step of photoelectrically converting the optical reception signal to generate and output an electrical signal;
An amplification step for amplifying the electrical signal;
Filtering the amplified electrical signal to select and transmit a frequency component that includes and lower than the bit rate of the optical pulse signal to generate the electrical received signal;
Contains
In the optical pulse width expansion step, the half-value width on the time axis of the optical pulse constituting the autocorrelation component is expanded to a width equal to the half-value width of the optical pulse constituting one bit of the optical pulse signal,
When the optical pulse width of the optical pulse signal is expanded by the optical pulse width expansion step and converted into an electric pulse signal, the power loss of the signal due to the frequency band limitation generated by the filtering step and the light The filtering step when the sum of the loss generated in the pulse width expansion step is converted into an electric pulse signal without expanding the optical pulse width of the optical pulse signal in the optical pulse width expansion step An optical code division multiplexing transmission / reception method characterized by being reduced by a power loss of a signal due to a frequency band limitation occurring in FIG . The receiving step further includes, for each channel,
A clock signal extraction step of extracting a clock signal from the correlation signal;
9. The optical code division multiplexing transmission / reception method according to claim 8 , further comprising a time gate processing step of extracting the autocorrelation component from the correlation signal by the clock signal.

Images