JP4429817B2 - Code converter - Google Patents

Description

  The present invention relates to a code conversion apparatus, and more particularly to a code conversion apparatus that is applied to an optical transmission system and converts an intensity modulation code into a Manchester modulation code in optical signal processing at the time of reception or relay reproduction of a high-speed packet signal. .

  In recent years, with the increase in speed and functionality of optical transmission systems, optical communication using ultra high-speed packet signals exceeding 40 Gb / s is expected. In such an optical transmission system, there are known functional devices such as an optical switch and an optical amplifier that process an optical signal without converting the optical signal into an electrical signal.

  FIG. 1 shows a conventional optical communication system using ultra high-speed packets. User terminals 506 to 508 are connected by optical fibers 504 and 505 via optical routers 501 to 503. It is assumed that the user terminal 506 and the user terminal 508 are at a short distance of, for example, 5 km, and the user terminal 507 and the user terminal 508 are at a long distance of, for example, 35 km.

  Here, ultra high-speed packets are transmitted from the user terminals 506 and 507 to the user terminal 508, respectively. FIG. 2 shows a reception waveform of an ultrafast optical packet at the user terminal 508. In FIG. 2A, a waveform 509 is a reception waveform of an ultrafast optical packet from the user terminal 506, and a waveform 510 is a reception waveform of an ultrafast optical packet from the user terminal 507. Since the optical fiber has a loss of about 0.1 to 0.2 dB per km (here, 0.15 dB as an example), the optical signal from the user terminal 506 and the optical signal from the user terminal 507 are compared. A difference in signal strength of 4.5 dB is caused by a distance difference of 30 km.

  FIG. 2B is an enlarged view of the waveform 509b. A threshold level 511 is set in the optical receiver in the user terminal 508, an optical signal having a threshold level of 511 or higher is determined as a logical code 1, and an optical signal having a threshold level 511 or lower is determined as a logical code 0. Therefore, if there is a change in the input level of the ultrafast optical packet received for each user terminal, the optimum threshold level always changes, making it difficult to determine the received waveform.

  Therefore, it is known that an optical receiver of the user terminal 508 is provided with an equalizer, an optical limiter amplifier, etc., and after performing waveform shaping, the received waveform is determined (for example, see Non-Patent Document 1). . FIG. 3 shows a reception waveform of an ultrafast optical packet at the user terminal 508. FIG. 3 (a) reproduces FIG. 2 (a) for comparison. FIG. 3B shows a waveform received between the user terminal 508 and the optical router 503 via an optical limiter amplifier. A waveform 601 is a reception waveform of an ultrafast optical packet from the user terminal 506, and a waveform 602 is a reception waveform of an ultrafast optical packet from the user terminal 507. The optical limiter amplifier has a function of suppressing an intensity difference between optical packets, and outputs a signal having a predetermined optical intensity regardless of the input level of the optical signal received for each user terminal. Accordingly, by setting an appropriate threshold level 603, it is possible to determine all the received waveforms from the user terminals with one threshold level 603.

  FIG. 4 shows the structure of a conventional optical limiter amplifier. The optical limiter amplifier is a low-saturation semiconductor optical amplifier, and is connected to both ends of the active layer 701 that amplifies the optical signal and the active layer 701, and spot size converters 702a and 702b for inputting and outputting the optical signal to and from the optical fiber. And electrodes 703 and 704. The active layer 701 has a buried structure with the same pn junction as that of the semiconductor laser, and has anti-reflective coating on both end faces. An optical signal input from one end of the active layer 701 is amplified by a current applied between the electrodes 703 and 704 and output from the other end of the active layer 701.

  FIG. 5 shows the input / output characteristics of the optical limiter amplifier. The output light intensity with respect to the input light intensity is illustrated. The optical limiter amplifier has high amplification when the input light intensity is weak, and low amplification when the input light intensity is strong. With this characteristic, the output light intensity can be set to a constant value with respect to the input light intensity exceeding a certain constant light intensity.

  However, the conventional optical limiter amplifier has a problem called pattern effect. FIG. 6 is a diagram for explaining the pattern effect. FIG. 6A shows an input signal to the optical limiter amplifier, which is a signal obtained by sandwiching logical codes 10101010101111111111111 with 0 continuous codes. FIG. 6B shows an output signal of the optical limiter amplifier. Since the first logical code 1 after the logical code 0 continues, the average input light intensity input per unit time is low, so that the amplification factor of the optical limiter amplifier is relatively high and a waveform with high light intensity is output. The On the other hand, since the second and subsequent logical codes have a relatively low amplification factor of the optical limiter amplifier, a waveform with low light intensity is output. Further, since the operation of the optical limiter amplifier does not completely follow the input signal, the rising and falling of the waveform do not reach a predetermined light intensity level.

  On the other hand, when a continuous logical code 1 is input, the operation of the optical limiter amplifier follows the input signal, and thus reaches a predetermined light intensity level 1. On the other hand, when a continuous logical code 0 is input, a predetermined light intensity level 0 is reached. As a result, there is a problem that the amplitude of the output signal changes depending on the sign of the input signal, causing waveform deterioration.

  Therefore, a Manchester code representing “0” with 2 bits of “10” and “1” with 2 bits of “01” was used. When the Manchester code is applied to the logical code shown in FIG. 6A, “01100110011001100110010101010101010101010101” is obtained, and the continuation of the same code is a maximum of 2 bits. Even when the 0 code is continuous, since the “10” signal is continuous, there is an advantage that the pattern effect hardly occurs.

  Next, a wavelength conversion element that is inserted immediately before the optical limiter amplifier and converts the intensity modulation code into the Manchester modulation code in order to apply the Manchester code will be described (for example, see Non-Patent Document 2). FIG. 7 shows a cross section of a polarization maintaining fiber as an example of a transmission line having polarization dispersion. In the polarization maintaining fiber, two stress applying portions 902a and 902b are arranged around a core 901 through which an optical signal propagates. An optical signal propagating through the core 901 propagates slowly when it has a polarization direction along the slow axis 903 and propagates early when it has a polarization direction along the fast axis 904. The speed difference is about 0.8 to 1.0 ps per 1 m of fiber. On the other hand, the optical signals in the polarization directions 905 and 906 are divided into two components of a slow axis 903 and a fast axis 904 and propagate.

FIG. 8 shows a configuration of a UNI type wavelength conversion element. Here, a case where the input signal is a “0” code will be described. The wavelength conversion element includes a polarization maintaining fiber 911, a 2 × 1 wavelength coupler 912 connected to the signal light input unit 910, a polarization maintaining fiber 913, a semiconductor optical amplifier 915 that is a nonlinear medium, and a polarization maintaining fiber. and 919, a polarization child 921, and the wavelength filter 923 is cascade-connected in this order.

  The clock pulse 909 is an optical signal having a frequency of 40 GHz, a pulse width of 3 ps, and a center wavelength of 1545 nm, which is adjusted to an intermediate polarization state between the slow axis 907 and the fast axis 908. The signal light is a pulse signal having 40 Gb / s, a pulse width of 3 ps, and a center wavelength of 1555 nm. However, since the case of the “0” code is described here, there is no pulse signal. The polarization maintaining fibers 911 and 913 have a combined polarization dispersion of +5 ps. The polarization maintaining fiber 919 has a polarization dispersion of −5 ps.

  The clock pulse 909 is divided by the polarization maintaining fibers 911 and 913, and the fast axis component 916 propagates early and the slow axis component 914 propagates later. The difference between the two is 5 ps. The fast axis component 916 and the slow axis component 914 are input to the semiconductor optical amplifier 905. The clock pulse becomes a fast axis component 917 and a slow axis component 918 by adjusting the injection current of the semiconductor optical amplifier 905 so that both phases are preserved when the input signal is “0”.

  In the polarization maintaining fiber 919, the fast axis and the slow axis are inverted with respect to the polarization maintaining fibers 911 and 913, and the length is equal to the sum of the polarization maintaining fibers 911 and 913. For this reason, the fast axis component 917 catches up with the slow axis component 918 and becomes a composite signal 920 at a position intermediate between the slow axis and the fast axis.

Polarization child 921 cuts out only the polarization component in the direction of the composite signal 920. That is, the signal is transmitted as it is to become a composite signal 922. Since the wavelength filter 923 transmits the clock pulse and blocks the pulse signal, the combined signal 922 is transmitted as it is to become the output signal 933. That is, since the output signal 933 exists with respect to the input pulse signal “0”, an inverted output of “1” is obtained.

  FIG. 9 shows a configuration of a UNI type wavelength conversion element. Here, a case where the input signal is a “1” code will be described. That is, the signal light 924 from the signal light input unit 910 is a “1” code. The polarization direction of the signal light 924 is arbitrary. The signal light 924 is input to the wavelength coupler 912.

  Next, the fast axis component 916 and the slow axis component 914 are synchronized so as to become the signal light component 925 and input to the semiconductor optical amplifier 915. Here, the fast axis component 916 that passes first passes while maintaining polarization, and becomes a slow axis component 928. On the other hand, the slow axis component 914 is inverted by setting the light intensity of the signal light component 925 to an optimum value, and is output as the fast axis component 927. The signal light component 925 also changes its polarization plane due to the fast axis component 916, but does not affect the system, so it is ignored here as the signal light component 926.

The fast axis component 927 catches up with the slow axis component 928 and becomes a composite signal 930. Combined signal 930, the combined signal 920 and forms a vertical, it is blocked by the polarization child 921. On the other hand, since the polarization direction of the signal light component 931 unknown, although some is output as the signal Mitsunari component 932 is blocked by the wavelength filter 923. As a result, with respect to the input pulse signal “1”, the output is an inverted output that is “0”.

  If the polarization direction of the clock pulse 909 is set at a position perpendicular to FIGS. 8 and 9, the output is “0” for the input “0” and the output “1” for the input “1”. The output can be obtained.

Y. Shibata, et al., “Semiconductor laser diode optical amplifiers / gates in photonic packet switching”, J. of Lightwave Technology, vol.16, No.12, pp.2228-2235, 1998 N.S.Patel, et al., “40-Gb / s Demultiplexing Using an Ultrafast Nonlinear Interferometer (UNI)” IEEE Photonics Technology Letters, Vol.8, No.12, pp.1695-1697, 1996

  However, since the Manchester code expresses 1 bit as 2 bits, there is a problem that the transmitter needs twice as much bandwidth and the load on the transmitter increases. Since the wavelength bandwidth is also required to be doubled, there has been a problem that the number of wavelengths is halved in the case of wavelength division multiplexing (WDM) transmission.

  The present invention has been made in view of such problems. The object of the present invention is to convert a transmitted intensity modulation code into a Manchester code and input it to an optical limiter amplifier to reduce waveform deterioration due to a pattern effect. An object of the present invention is to provide a wavelength conversion device that can be prevented from occurring.

In order to achieve the above object, according to the present invention, the first clock pulse (103) at the intermediate position between the fast axis and the slow axis of the polarization dispersion is converted into the first polarization. By the dispersion, the second clock pulse (108) on the fast axis and the third clock pulse (109) on the slow axis are divided into fourth clock pulses (orthogonal to the plane of polarization of the first clock pulse (103) ( 104), the polarization maintaining fiber (105, 112) for dividing the first clock dispersion into the fifth clock pulse (110) on the fast axis and the sixth clock pulse (111) on the slow axis by the first polarization dispersion; The second clock pulse (108) and the fifth clock pulse (110) are intermediate between the third clock pulse (109) and the sixth clock pulse (111). The phase of the wavelength coupler (107) that receives the pulse signal (134) and the third clock pulse (109) and the sixth clock pulse (111) output from the wavelength coupler (107) A semiconductor optical amplifier (113) that is inverted by the sign of the pulse signal (134) (to 116 or 127, 118 or 129), and connected to the output of the semiconductor optical amplifier (113). The second clock pulse (115 or 126) and the third clock pulse (116 or 127) are combined into a seventh clock pulse (120 or 130) having a polarization direction orthogonal to the wave direction, The clock pulse (117 or 128) and the sixth clock pulse (118 or 129) are converted into the eighth clock pulse (121 or 31) the polarization maintaining fiber (119) to be combined, and any one of the seventh clock pulse (120 or 130) and the eighth clock pulse (121 or 131), which coincides with the polarization plane of the first clock pulse. polarizing element to prevent (122) and passes through the seventh clock pulse (120 or 130) and the eighth clock pulse (121 or 131), a wavelength filter for blocking said pulse signal (135) (124) When connected to an output of the wavelength filter, and a said seventh optical fiber (123) to Ru gives a wavelength dispersion value with respect to the clock pulse and the eighth clock pulse corresponds to 1/2 bit of the pulse signal , The difference between the wavelength of the first clock pulse and the wavelength of the fourth clock pulse is 1 / of the pulse signal in the optical fiber. It is a wavelength difference that causes a delay difference corresponding to 2 bits, and the wavelength of the pulse signal is different from both the wavelength of the first clock pulse and the wavelength of the fourth clock pulse .

  According to a second aspect of the present invention, in the code conversion device according to the first aspect, the wavelength of the seventh clock pulse (132) and the eighth clock pulse (125) output from the optical fiber (123). A wavelength conversion element (126) for converting wavelengths into the same wavelength (138 and 137) is further provided.

  As described above, according to the present invention, since the intensity modulation code can be converted into the Manchester code and input to the optical limiter amplifier, the waveform deterioration due to the pattern effect can be prevented from occurring. .

FIG. 10 shows the configuration of the wavelength conversion element according to the first embodiment of the present invention. Here, a case where the input signal is a “0” code will be described. The wavelength conversion element includes a polarization maintaining fiber 105 that divides a clock pulse, a 2 × 1 wavelength coupler 107 connected to a signal light input unit 106 that inputs a pulse signal, a polarization maintaining fiber 112, and a sign of the pulse signal. a semiconductor optical amplifier 113 that converts the phase of the divided clock pulse by a polarization maintaining fiber 119 for synthesizing the divided clock pulse, the polarization child 122 and wavelength filter 124 to retrieve the desired clock pulses are combined The optical fibers 123 that give a desired delay to the clock pulses are connected in cascade.

  The clock pulse 103 is an optical signal having a frequency of 40 GHz, a pulse width of 3 ps, and a center wavelength of 1535 nm, adjusted to an intermediate polarization state between the fast axis 101 and the slow axis 102. The clock pulse 104 has a polarization plane orthogonal to that of the clock pulse 103 and has a center wavelength of 1545 nm. The signal light is a pulse signal having 40 Gb / s, a pulse width of 3 ps, and a center wavelength of 1555 nm. However, since the case of the “0” code is described here, there is no pulse signal. The polarization maintaining fibers 105 and 112 have +5 ps of polarization dispersion in total. The polarization maintaining fiber 119 has a polarization dispersion of −5 ps. The optical fiber 123 has a wavelength dispersion of 12.5 ps / 10 nm.

  The clock pulse 103 is divided by the polarization maintaining fibers 105 and 112, and the fast axis component 108 propagates early and the slow axis component 109 propagates slowly. The clock pulse 104 is divided into a fast axis component 110 and a slow axis component 111. The difference between the two is 5 ps. Each component is input to the semiconductor optical amplifier 113. By adjusting the injection current of the semiconductor optical amplifier 113 so that both phases are preserved when the input signal is “0”, the clock pulse becomes the fast axis components 116 and 118 and the slow axis components 115 and 117.

  In the polarization maintaining fiber 119, the fast axis and the slow axis are inverted with respect to the polarization maintaining fibers 105 and 112, and the length is equal to the sum of the polarization maintaining fibers 105 and 112. For this reason, the fast axis components 116 and 118 catch up with the slow axis components 115 and 117, and the combined signals 120 and 121 are located at intermediate positions between the slow axis and the fast axis.

Polarization child 122 cuts out only the polarization component in the direction of the composite signal 121. That is, the synthesized signal 121 is transmitted as it is, but the synthesized signal 120 is blocked. Since the wavelength filter 124 transmits the clock pulse and blocks the pulse signal, the synthesized signal 121 is transmitted as it is.

  FIG. 11 shows the configuration of the wavelength conversion element according to the first embodiment of the present invention. Here, a case where the input signal is a “1” code will be described. That is, the signal light 133 from the signal light input unit 106 is a “1” code. The polarization direction of the signal light 133 is arbitrary. The signal light 133 is input to the wavelength coupler 107.

  Next, the fast axis components 108 and 110 and the slow axis components 109 and 111 are synchronized so as to become the signal light component 134 and input to the semiconductor optical amplifier 113. Here, the fast axis components 108 and 110 that pass first pass while maintaining polarization, and become slow axis components 126 and 128. On the other hand, the slow axis components 109 and 111 are inverted by setting the light intensity of the signal light component 134 to an optimum value, and are output as fast axis components 127 and 129. The signal light component 134 also changes its polarization plane under the influence of the fast axis components 108 and 110, but does not affect the system, so it is ignored here as the signal light component 135.

The fast axis components 127 and 129 catch up with the slow axis components 126 and 128 to become the combined signals 130 and 131. Combined signal 130 and 131, the combined signal 120, 121 and form a vertical, combined signal 131 by the polarization child 122 is blocked, the combined signal 130 is transmitted. On the other hand, since the polarization direction unknown signal light components 135, some is transmitted through the polarization child 122 is blocked by the wavelength filter 124.

  Here, since the optical fiber 123 has chromatic dispersion of 12.5 ps / 10 nm, comparing FIG. 10 and FIG. 11, the synthesized signal 121 having a wavelength of 10 nm long wave is 12.5 ps compared to the synthesized signal 130. Proceed quickly and become the output signal 125. In other words, the synthesized signal 130 having a wavelength of 10 nm shortwave advances 12.5 ps later than the synthesized signal 121 and becomes the output signal 132. Since the input signal is 40 Gb / s, considering a time slot 12.5 ps that is half of 1 bit 25 ps, the output signal is “10” (1 bit 12 ps) with respect to the “0” code (1 bit 25 ps) of the input signal. .5 ps), and the output signal is “01” (1 bit 12.5 ps) with respect to the “1” code (1 bit 25 ps) of the input signal. In this way, a normal intensity modulation code is converted into a Manchester code.

  Although the wavelength of the signal light has been described as 1555 nm, the wavelength of the signal light may change according to time. By reversing the positions of the clock pulses 103 and 104, or by using an optical fiber having a minus sign in the optical fiber 123 and having a chromatic dispersion of half a bit (-12.5 ps / nm), "0" Can be converted into inverted Manchester codes of “01” and “1” to “10”.

  12 and 13 show the configuration of the wavelength conversion element according to the second embodiment of the present invention. FIG. 12 shows a case where the input signal has a “0” code, and FIG. 13 shows a case where the input signal has a “1” code. In the wavelength conversion element shown in FIGS. 10 and 11, since the wavelengths of the output signal 125 and the output signal 132 are different, when the long-distance transmission is performed using an optical fiber, the interval between the output signal 125 and the output signal 132 is widened by chromatic dispersion. End up. That is, the interval between Manchester codes “01” and “01” changes.

  Therefore, the wavelength converter 126 for wavelength conversion is connected to the wavelength conversion element according to the first embodiment. As the wavelength converter 126, the UNI type wavelength conversion element shown in FIGS. At this time, the input port 136 corresponds to the polarization maintaining fiber 911, and the optical fiber 123 corresponds to the signal light input unit 910. The wavelength converter 126 converts the output signal 125 and the output signal 132 into, for example, one wavelength of 1555 nm that is the same as the signal light. In this way, a change in the Manchester code interval due to wavelength dispersion can be prevented.

It is a block diagram which shows the optical communication system using the conventional super-high-speed packet. It is a figure which shows the reception waveform of the ultra high-speed optical packet in a user terminal. It is a figure which shows the receiving waveform of the ultra high-speed optical packet received via the optical limiter amplifier. It is a perspective view which shows the structure of the conventional optical limiter amplifier. It is a figure which shows the input-output characteristic of the conventional optical limiter amplifier. It is a figure for demonstrating the pattern effect of an optical limiter amplifier. It is sectional drawing which shows the conventional polarization maintaining fiber. It is a figure which shows the structure of the conventional UNI type wavelength conversion element. It is a figure which shows the structure of the conventional UNI type wavelength conversion element. It is a figure which shows the structure of the wavelength conversion element concerning the 1st Embodiment of this invention. It is a figure which shows the structure of the wavelength conversion element concerning the 1st Embodiment of this invention. It is a figure which shows the structure of the wavelength conversion element concerning the 2nd Embodiment of this invention. It is a figure which shows the structure of the wavelength conversion element concerning the 2nd Embodiment of this invention.

Explanation of symbols

105,112,119 polarization maintaining fiber 107 the wavelength coupler 113 the semiconductor optical amplifier 122 polarization child 124 wavelength filter 126 wavelength converter

Claims (2)

Dividing the first clock pulse at the intermediate position between the fast axis and the slow axis of polarization dispersion into a second clock pulse on the fast axis and a third clock pulse on the slow axis by the first polarization dispersion; A polarization maintaining fiber that divides a fourth clock pulse orthogonal to a polarization plane of the first clock pulse into a fifth clock pulse on the fast axis and a sixth clock pulse on the slow axis by the first polarization dispersion; ,
A wavelength coupler for inputting a pulse signal in synchronization with the middle of the second clock pulse and the fifth clock pulse and the third clock pulse and the sixth clock pulse;
A semiconductor optical amplifier that inverts the phases of the third clock pulse and the sixth clock pulse output from the wavelength coupler according to the sign of the pulse signal;
Connected to the output of the semiconductor optical amplifier, having a polarization direction orthogonal to the polarization direction of the first polarization dispersion, and synthesizing the second clock pulse and the third clock pulse into a seventh clock pulse; A polarization maintaining fiber that synthesizes the fifth clock pulse and the sixth clock pulse into an eighth clock pulse;
It coincides with the plane of polarization of the first clock pulse, a polarization child to prevent any of the seventh clock pulse and the eighth clock pulse,
A wavelength filter that transmits the seventh clock pulse and the eighth clock pulse and blocks the pulse signal;
It is connected to the output of the wavelength filter, and an optical fiber Ru gives a wavelength dispersion value corresponding to 1/2 bit of the pulse signal to the seventh clock pulse and the eighth clock pulse,
The difference between the wavelength of the first clock pulse and the wavelength of the fourth clock pulse is a wavelength difference that causes a delay difference corresponding to ½ bit of the pulse signal in the optical fiber,
The code converter according to claim 1, wherein the pulse signal has a wavelength different from both the wavelength of the first clock pulse and the wavelength of the fourth clock pulse .   The code conversion according to claim 1, further comprising a wavelength conversion element that converts the wavelength of the seventh clock pulse and the wavelength of the eighth clock pulse output from the optical fiber into the same wavelength. apparatus.

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