JP2007036785A - Optical transmission system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical transmission system capable of compensating deterioration in the waveform of light and improving the receiving accuracy of an optical receiver. <P>SOLUTION: The optical transmission system 2 includes an optical transmitter 10 for transmitting signal light by emphasizing and modulating an optical carrier by a sub-carrier signal, and the optical receiver 20 for receiving the signal light transmitted from the optical transmitter 10 and demodulating the sub-carrier signal, wherein the optical transmitter 10 sets the optical carrier frequency of light for modulating the strength of the optical transmitter so that a value obtained by converting a delay time difference due to a propagation route difference of the signal light from the optical transmitter 10 up to the optical receiver 20 into a phase difference of the optical carrier is approximately fixed in a range of -π/2 to π/2. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an optical transmission system that transmits and receives communication signals by light.

  Conventionally, when performing information communication, there are a wireless communication method using electromagnetic waves and a wired communication method using cables. In particular, in the case of high-speed communication, there is optical communication using light. Here, FIG. 11 shows a schematic configuration diagram of a transmitter and a receiver when performing wireless communication. FIG. 12 shows a schematic configuration diagram of a transmitter and a receiver when optical communication is performed.

  The transmitter 120 illustrated in FIG. 11 includes an output circuit 121 that outputs a transmission signal, an oscillator 122 that outputs a carrier signal, a modulation circuit 123 that modulates the carrier signal from the oscillator 122 with the transmission signal, and modulation by the modulation circuit 123. And a transmitting antenna 124 for transmitting the signal wave. The receiver 125 receives the signal wave from the transmission antenna 124, the oscillator 129 that outputs a local oscillation signal for demodulating the transmission wave modulated by the modulation circuit 123, and the reception antenna 126. A demodulating circuit 127 that demodulates the signal wave with the local oscillation signal from the oscillator 129, and a detecting circuit 128 that detects the signal demodulated by the demodulating circuit 127.

  On the other hand, in the optical transmission system that transmits the subcarrier signal, the receiver 132 shown in FIG. 12 receives a signal light transmitted from the transmitter 131 via the optical fiber 135 and converts it into an electrical signal. And a detection circuit 134 for detecting the electrical signal from the photodiode 133 with the subcarrier signal of the transmission signal at the transmitter 131.

  Here, since light is not affected by electromagnetic waves during propagation, the communication method using optical communication is resistant to noise, and thus has many advantages over the communication method using wireless communication. However, when a multimode optical fiber is applied as an optical fiber, there is a problem that a demodulated signal waveform deteriorates on the reception side due to mode dispersion of light in the multimode optical fiber. This is a major issue in the future to realize optical communication at 10 Gbit / s or higher using a multimode optical fiber.

Therefore, in order to solve the above problems, a method has been developed that removes the influence of the propagation delay time difference by converting the light received on the receiving side into an electrical signal and passing it through a compensation filter such as a transversal filter (for example, non-patent) See literature 1 or 2.) In addition, since the transmission loss of the optical fiber is substantially constant at a high frequency, there is a method of setting the subcarrier frequency to a high frequency (see, for example, Non-Patent Document 3).
IEEE Transaction on Communications, vol. 38, no. 9, 1990, pp. 1439-1453 IEEE Photonic Technology Letters, vol14, no. 8, August 2002, pp. 1187-1189 Electronics Letters, vol. 34, no 7, 1998

  However, in optical communication, as shown in FIG. 11, the signal light is detected by the light receiving element such as the photodiode 133 in the receiver 132 as shown in FIG. 12 by detecting the light intensity, that is, the square detection of the electric field of the light. Unlike the case of wireless communication, light phase information is lost. That is, the receiver 132 shown in FIG. 12 detects only the intensity of light that has interfered due to the propagation delay time difference caused by the multimode optical fiber. Therefore, in the methods described in Non-Patent Documents 1 to 3, there is a limit to the waveform deterioration compensation of the communication signal caused by the light propagation delay time difference in the multimode optical fiber. Further, since the propagation delay time difference due to the multimode optical fiber changes every moment, the value to be compensated also changes every moment.

  Therefore, an object of the present invention is to provide an optical transmission system that can compensate for the waveform deterioration of light and improve the reception accuracy in the optical receiver.

  In order to achieve the above object, the inventor decided to set the optical carrier frequency of the light modulated by the optical transmitter so that the influence of the light interference in the optical receiver becomes constant. In the present specification, predetermined terms are used in the definitions described below.

  “Subcarrier frequency” refers to a method in which information is placed on carriers of different frequencies, each carrier signal is frequency-multiplexed in the electrical domain, and the frequency-multiplexed signal is converted into an optical signal for transmission. The carrier frequency. The “subcarrier signal” refers to a method in which information is carried on carriers having different frequencies, each carrier signal is frequency-multiplexed in the electrical domain, and the frequency-multiplexed signal is converted into an optical signal for transmission. A signal in which information is carried on a carrier in a region. The “optical carrier frequency” refers to the frequency of light in a state where the optical region is not modulated. In addition, the “optical subcarrier frequency” refers to a method in which information is placed on carriers having different frequencies, each carrier signal is frequency-multiplexed in the electrical domain, and the frequency-multiplexed signal is converted into an optical signal for transmission. A signal having a frequency newly generated by modulation when converted into an optical signal. The “angle modulation frequency” refers to a frequency when the optical carrier frequency is intentionally changed in order to stabilize the optical carrier frequency.

  An optical transmission system according to the present invention includes an optical transmitter that modulates the intensity of an optical carrier with a subcarrier signal and transmits signal light, and receives the signal light transmitted from the optical transmitter and demodulates the subcarrier signal. An optical receiver that converts a delay time difference due to a difference in propagation path of the signal light from the optical transmitter to the optical receiver into a phase difference of an optical carrier. The optical carrier frequency of the light whose intensity is modulated by the optical transmitter is set so that the obtained value becomes a substantially constant value between −π / 2 and π / 2.

  In the optical transmitter, by setting the optical carrier frequency so that the phase difference of the optical carrier is a substantially constant value between −π / 2 and π / 2, even if light interferes due to the propagation path difference, The receiver can receive signal light having a substantially constant light intensity. Therefore, it is possible to improve the reception accuracy at the optical receiver by compensating for the waveform deterioration of the light. Here, the optical transmission system according to the present invention includes a multi-mode optical fiber and a system using a space as a transmission medium.

  In the optical transmission system, it is desirable that the phase difference is substantially zero.

  By setting the phase difference to approximately 0, the intensity of the optical carrier received by the optical receiver is maximized. Therefore, signal degradation can be minimized by compensating for waveform degradation of the subcarrier signal.

  The optical transmission system further includes delay time difference feedback means for acquiring the delay time difference and feeding back from the optical receiver to the optical transmitter, and the optical transmitter is fed back from the delay time difference feedback means. It is also desirable to set the optical carrier frequency of the light following the delay time difference.

  By having the delay time difference feedback means and following the delay time difference that changes every moment, the optical carrier frequency is set, thereby maintaining the optical waveform degradation compensation effect and improving the reception accuracy at the optical receiver. It becomes possible to keep.

  In the optical transmission system, it is preferable that the optical transmitter angle-modulates the optical carrier in advance.

  By using an angle-modulated optical carrier, the optical carrier frequency of the signal light is controlled by the optical transmitter even if the delay time difference due to the propagation path difference changes suddenly due to disturbance, and the optical carrier phase difference is maintained substantially constant. It becomes possible to do. Therefore, it is possible to stabilize the light intensity of the signal light received by the optical receiver.

  In the optical transmission system, it is desirable that the optical transmitter angle-modulates branched light that has been branched in advance from the light that is intensity-modulated and couples it to the signal light.

  The optical carrier frequency of the signal light can be changed even if the delay time difference due to the propagation path difference changes suddenly due to the angle modulation of the branched light that has been branched from the light whose intensity is modulated and coupled to the signal light. The phase difference of the optical carrier can be maintained substantially constant, and the influence of angle modulation on the intensity-modulated signal light can be reduced. Therefore, it is possible to stabilize the light intensity of the signal light received by the optical receiver.

  In the optical transmission system, it is preferable that the optical transmitter sets an optical carrier frequency so that a phase difference between optical carriers having a delay time difference received by the optical receiver is substantially zero.

  By setting the optical carrier frequency so that the phase difference becomes substantially zero, the subcarrier signal can be demodulated stably by the optical receiver.

  In the optical transmission system, the optical transmitter further includes phase difference feedback means for acquiring the phase difference and returning the phase difference from the optical receiver to the optical transmitter. It is desirable to set the optical carrier frequency following the phase difference fed back from the phase difference feedback means.

  By having the phase difference feedback means and setting the optical carrier frequency following the phase difference that changes every moment, it is possible to minimize the waveform deterioration after compensation in the optical receiver.

  In the optical transmission system, it is preferable that the optical receiver includes a compensation filter that equalizes the electric signal to be demodulated in advance based on the delay time difference.

  The compensation filter compensates for the waveform deterioration of the transmission signal caused by the difference in the propagation delay time of light, and can be demodulated with high accuracy in the optical receiver.

  In the optical transmission system of the present invention, it is possible to compensate for the waveform deterioration of the light and improve the reception accuracy at the optical receiver.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In addition, this invention is not limited to embodiment shown below. In the specification and the drawings, components having the same reference numerals indicate the same components.

  FIG. 1 is a schematic configuration diagram illustrating an optical transmission system according to the present embodiment.

  The optical transmission system 2 according to the present embodiment includes an optical transmitter 10 that modulates the intensity of an optical carrier with a subcarrier signal and transmits the signal light, and a signal light transmitted from the optical transmitter 10 and receives the subcarrier signal. And an optical receiver 20 to demodulate. Further, a first optical transmission path 41 and a second optical transmission path 42 for transmitting signal light between the optical transmitter 10 and the optical receiver 20 are provided.

  Here, the optical transmitter 10 includes a light source 11 that outputs light, an intensity modulation circuit 13 that modulates the intensity of an optical carrier from the light source 11 with a subcarrier signal, and transmits the signal light to the first optical transmission line 41; A frequency setting circuit 12 that sets an optical carrier frequency output from the light source 11.

  The light source 11 preferably outputs light having a set single frequency. The intensity modulation circuit 13 intensity-modulates the optical carrier from the light source 11 with the subcarrier signal. As the intensity modulation circuit 13, for example, a Mach-Zehnder optical modulator that couples light propagating through a bifurcated optical waveguide with a relative optical path difference can be applied. When the Mach-Zehnder type optical modulator is applied, the modulation response speed with respect to the modulation signal is high, so that deterioration when the modulation signal is converted into light can be reduced. In the present embodiment, the intensity modulation circuit 13 is provided separately from the light source 11, but the intensity of the light source 11 may be directly modulated instead of the intensity modulation circuit 13.

  As shown in FIG. 1, the subcarrier signals are output from the oscillators 16 a, 16 b and 16 c, modulated by the modulation circuits 15 a, 15 b and 15 c, and added by the adder circuit 14. Here, any of frequency modulation, phase modulation, or intensity modulation may be applied to the modulation of the subcarrier signal. Further, there can be any number of channels.

  Here, FIGS. 2 and 3 show other forms of the optical transmission system.

  The optical transmitter 10 shown in FIG. 1 is an angle modulator that modulates the angle of the optical carrier output from the light source 11 with the modulation signal output from the oscillator 17, like the optical transmitter 60 of the optical transmission system 3 shown in FIG. It is desirable to include a circuit 18. The angle modulation in the angle modulation circuit 18 may modulate either the phase or the frequency of the light from the light source 11. This angle modulation frequency is set to a frequency sufficiently lower than the frequency of the subcarrier signal. As described above, by using the optical carrier that has been angle-modulated in advance in the angle modulation circuit 18 shown in FIG. 2, even if the delay time difference due to the propagation path difference suddenly changes due to disturbance, the light of the signal light from the optical transmitter 60 The carrier frequency is controlled by the optical transmitter 10, and the phase difference of the optical carrier can be maintained substantially constant. Therefore, it is possible to stabilize the light intensity of the signal light received by the optical receiver 70. In the present embodiment, the angle modulation circuit 18 angle-modulates the optical carrier from the light source 11. However, as described above, the optical carrier frequency can be varied by varying the drive current of the light source 11. Alternatively, direct modulation may be performed by superimposing a modulation signal on the drive current of the light source 11.

  Further, like the optical transmitter 80 of the optical transmission system 4 shown in FIG. 3, the optical transmitter 10 shown in FIG. 1 branches the light output from the light source 11 and outputs the optical carrier of the branched light from the oscillator 17. It is desirable to provide an angle modulation circuit 18 that performs angle modulation with the modulated signal to be coupled to the signal light. The modulation method of the angle modulation circuit 18 can be the same as that described in FIG. Moreover, in order to branch the light from the light source 11, for example, an optical coupler or a directional coupler can be applied. In the case where the light from the angle modulation circuit 18 is coupled to the signal light, an optical coupler or a directional coupler can be similarly applied. As described above, when the branched light obtained by branching the light from the light source 11 shown in FIG. 3 is angle-modulated by the angle modulation circuit 18 and coupled to the signal light, the delay time difference due to the propagation path difference may change suddenly due to disturbance. Even in such a case, the optical carrier frequency of the signal light from the optical transmitter 80 can be controlled to maintain the phase difference of the optical carrier substantially constant, and the influence of angle modulation on the light intensity modulated signal light can be reduced. can do. Therefore, it is possible to stabilize the light intensity of the signal light received by the optical receiver 70.

  Further, the optical transmitter 60 shown in FIG. 2 desirably sets the optical carrier frequency so that the phase difference between optical carriers having a delay time difference received by the optical receiver 70 is substantially zero. Similarly, in the optical transmitter 80 shown in FIG. 3, it is desirable to set the optical carrier frequency so that the phase difference between optical carriers having a delay time difference received by the optical receiver 70 is substantially zero. The phase difference between the optical carriers having the delay time difference can be detected by synchronous detection in the optical receiver 70. Then, while monitoring the phase difference between optical carriers having a delay time difference in the optical receiver 70, the optical carrier frequency is changed in the optical transmitters 60 and 80, and the phase difference is set to approximately zero. In the present embodiment, the frequency difference is fed back from the optical receiver 70 to the optical transmitters 60 and 80 to control the optical carrier frequency so as to follow the phase difference. This will be described later. In this way, by setting the optical carrier frequency so that the phase difference becomes substantially zero, the optical receiver 70 can demodulate the subcarrier signal stably.

  The frequency setting circuit 12 shown in FIG. 1 outputs a frequency setting signal for setting a frequency. And the frequency setting signal sets the optical carrier frequency of the light which the light source 11 outputs by controlling the drive current of the light source 11, for example.

  Here, the setting of the optical carrier frequency of the frequency setting circuit 12 will be described with reference to FIG. FIG. 4 is a conceptual diagram of signal light propagating through the first optical transmission line 41 shown in FIG. In FIG. 4, the horizontal axis represents time, and the vertical axis represents the angular frequency of the signal light.

  A plurality of signal lights propagating through the first optical transmission line 41 shown in FIG. 1, from a low-order mode having a large reflection angle to a high-order mode having a small reflection angle, due to multiple reflection of the signal light in the first optical transmission line 41. Propagated in a distributed mode. Therefore, the signal light transmitted from the optical transmitter 10 is received by the optical receiver 20 as a plurality of dispersed signals, resulting in a delay time difference τ.

  When the subcarrier transmission method is employed as in this embodiment, the signal wave transmitted from the optical transmitter 10 shown in FIG. 1 propagates as a signal obtained by adding the optical carrier signal and the optical subcarrier signal. For this reason, the signal light is in a state where the optical carrier signal and the optical subcarrier signal interfere with each other to generate a beat. In FIG. 4, the light that reaches the optical receiver 20 first in the signal light transmitted from the optical transmitter 10 shown in FIG. 1 is referred to as a preceding wave 61 (FIG. 4), and the light that reaches the optical receiver 20 later. The delay wave 62 (FIG. 4) is assumed. Considering the delay time difference τ between the preceding wave 61 and the delayed wave 62 shown in FIG. 4, the leading wave 61 and the delayed wave 62 are, as shown in the shaded portion of FIG. 4, an optical carrier signal and an optical subcarrier signal. Are overlapping in time. Here, in FIG. 4, each frequency of the optical subcarrier signals 64 and 66 is ωm, and each frequency of the optical carrier signals 63 and 65 (that is, light output from the light source) is ωc. In consideration of the fact that the optical subcarrier signals 64 and 66 are intensity-modulated by the subcarrier signal, ωm has a certain width.

  When the signal light from the optical transmitter 10 shown in FIG. 1 propagates with a delay time difference τ, the optical receiver 20 converts the signal light into a preceding wave optical carrier signal 63, a preceding wave optical subcarrier signal 64, and a delayed wave. The four signals of the optical carrier signal 65 and the delayed optical subcarrier signal 66 are multiplied with each other to be received as a combined signal of the following six signals. The following (1) to (6) correspond to (1) to (6) shown in FIG. FIG. 5 shows a frequency spectrum of signal light as power detected by the optical receiver 20 shown in FIG. In FIG. 5, the horizontal axis indicates the frequency, and the vertical axis indicates the received power. The dotted line in the figure indicates the characteristics of the bandpass filter. Further, (1) to (6) in the figure correspond to the following signals (1) to (6).

(1) A preceding wave optical carrier signal 63 and a preceding wave optical subcarrier signal 64
(2) Delayed-wave optical carrier signal 65 and delayed-wave optical subcarrier signal 66
(3) An optical carrier signal 63 of a preceding wave and an optical carrier signal 65 of a delayed wave
(4) Optical carrier signal 65 of delayed wave and optical subcarrier signal 64 of preceding wave
(5) Leading-wave optical carrier signal 63 and delayed-wave optical subcarrier signal 66
(6) Leading-wave optical subcarrier signal 64 and delayed-wave optical subcarrier signal 66

  Of the six signals, the signals (1) and (2) are the original signals that should be received by the optical receiver 20 shown in FIG. The signal (3) is a signal obtained by multiplying optical carrier signals having the same frequency. In the optical receiver 20 shown in FIG. 1, since the signal light is square-detected by a light receiving element such as a photodiode, when the signal (3) is received by the optical receiver 20 shown in FIG. 1, it is detected as a direct current component. Further, the signal (6) is a signal obtained by multiplying the optical subcarrier signals, and is detected as a low-frequency signal in the optical receiver 20 shown in FIG. Therefore, it can be removed by a band pass filter as shown in FIG. The signals (4) and (5) are signals in the same frequency band as the signals (1) and (2), but the waveform deteriorates due to interference between the preceding wave 61 and the delayed wave 62 shown in FIG. It is a signal to make. That is, since the delay time difference τ can change from time to time as described with reference to FIG. 4, the amplitude values of the signals in (4) and (5) change from time to time, and the signal from the optical transmitter 10 shown in FIG. As a result, the transmission signal cannot be detected accurately. Therefore, the signals (4) and (5) are signals to be removed from the signal light from the optical transmitter 10 shown in FIG.

  However, since the signals (4) and (5) are in the same frequency band as the signals (1) and (2), they cannot be removed in the electrical domain of the optical receiver 20. Therefore, in the present embodiment, the frequency setting circuit 12 shown in FIG. 1 has a value obtained by converting the delay time difference τ due to the propagation path difference of the optical carrier signal from the optical transmitter 10 to the optical receiver 20 into a phase difference of −π /. The optical carrier frequency of the light output from the light source 11 is set so as to be a substantially constant value between 2 and π / 2. By maintaining the delay time τ at a constant value, waveform deterioration can be eliminated by compensating the received signal even when the signals (4) and (5) are superimposed. In the present embodiment, the optical carrier frequency is set so that the value obtained by converting the delay time difference τ into the phase difference is constant from the periodicity of the wave. In this embodiment, the delay time difference is fed back from the optical receiver 20 to the optical transmitter 10 to control the optical carrier frequency so as to follow the delay time difference. This will be described later. Thus, in the optical transmitter 10, by setting the optical carrier frequency, even if the light interferes due to the propagation path difference, the optical receiver 20 can receive the signal light having a substantially constant light intensity. It is possible to improve the reception accuracy at the optical receiver 20 by compensating for the waveform deterioration of the light. Note that the delay time difference τ may be measured in accordance with a measurement method defined in TIA / EIA-492AAAC.

  Here, the phase difference obtained by converting the delay time difference τ is preferably approximately zero. By setting the phase difference to approximately 0, the intensity of the optical carrier received by the optical receiver 20 is maximized. Therefore, signal degradation demodulated by the optical receiver 20 can be minimized.

  Next, for example, a multimode optical fiber can be applied to the first optical transmission line 41. A multimode optical fiber is an optical fiber that is frequently used as an optical transmission line. In this embodiment, even when a multimode optical fiber is applied, it is possible to reduce waveform degradation of mode dispersion due to the multimode optical fiber. Here, as the multimode optical fiber, for example, a graded index optical fiber or a step index optical fiber can be applied. In the present embodiment, a mode in which signal light is transmitted / received via the first optical transmission line 41 is shown, but signal light is propagated in space without applying the first optical transmission line 41 and transmitted / received. It is good to do. That is, in this embodiment, the delay time difference due to mode dispersion in the first optical transmission line 41 is made constant. However, by making the delay time difference due to the propagation path difference of the signal light propagating in space constant, It is also possible to compensate for the waveform deterioration.

  Next, the optical receiver 20 shown in FIG. 1 receives a transmission signal from the optical transmitter 10 and outputs an electric signal, and a first light based on the electric signal output from the light receiving circuit 21. A delay time difference detection circuit 25 that detects a propagation delay time difference of signal light in the transmission line 41, a bandpass filter 22 that removes unnecessary frequency components from the electrical signal output from the light receiving circuit 21, and an output from the bandpass filter 22 From the optical transmitter 10, the compensation filter 23 that equalizes the electrical signal to be compensated to compensate for the propagation delay time difference and outputs the compensation signal, the frequency division circuit 24 that frequency-divides the compensation signal output from the compensation filter 23, and the optical transmitter 10. Detection circuit 29a, 29b, 29c for detecting the transmission signal of the signal, and a low pass for removing high frequency components from the electrical signals output from the detection circuits 29a, 29b, 29c. Filter with 30a, 30b, and 30c, the. Further, an optical transmission circuit that transmits information signals as the delay time difference τ and the phase difference to the optical transmitter 10 in order to feed back the information of the delay time difference τ and the phase difference acquired by the optical receiver 20 to the optical transmitter 10. 27.

  Here, the light receiving circuit 21 can be exemplified by a photodiode, for example. The delay time difference detection circuit 25 detects the delay time difference τ. Then, the optical receiver 20 transmits information on the detected delay time difference τ. In the optical transmitter 10, an optical carrier frequency setting signal is generated by the frequency setting circuit 12 based on the frequency control signal corresponding to the delay time difference transmitted from the optical receiver 20, output toward the light source 11, and output to the light source 11. The optical carrier frequency of the light output from is varied. As described above, the delay time difference detection circuit 25 has a function as a delay time difference feedback means, and the frequency setting circuit 12 follows the feedback delay time difference to determine the optical carrier frequency of the light output from the light source 11. Change. By having a delay time difference feedback means and setting the optical carrier frequency of the light output from the light source 11 following the delay time difference τ that changes from time to time in the optical transmitter 10, the effect of compensating the waveform deterioration of the signal light can be obtained. It is possible to maintain the reception accuracy at the optical receiver 20 with high accuracy.

  Further, in the optical receiver 70 in the optical transmission system of another form shown in FIG. 2 and FIG. 3, optical signals having a delay time difference received by the optical receiver 70 in addition to the configuration of the optical receiver 20 shown in FIG. Has a mixer 43 for detecting the phase difference of. The mixer 43 passes the modulation signal having the same frequency as the modulation frequency in the angle modulation circuit 18 output from the oscillator 44 in the optical transmitters 60 and 80 through the low-pass filter 26 that passes the modulation frequency in the light receiving circuit 21. Synchronous detection is performed by multiplying the detected signal. The optical receiver 70 detects a phase difference between optical carriers having a delay time difference by synchronous detection, modulates it as a phase difference signal by the optical transmission circuit 27, and transmits the optical transmitter 60 (see FIG. 2) Transmit to 80 (FIG. 3). In the optical transmitters 60 and 80, the frequency setting circuit 12 generates a frequency setting signal so that the phase difference transmitted from the optical receiver 70 becomes zero. Then, the optical carrier frequency of the light output from the light source 11 is varied. In this way, by setting the optical carrier frequency following the delay time difference that changes from time to time, it is possible to minimize waveform deterioration after compensation in the optical receiver 70.

  The compensation filter 23 shown in FIG. 1 is preferably a transversal filter. Here, FIG. 6 shows a schematic configuration diagram of the transversal filter. FIG. 7 is a schematic diagram of the intensity of signal light at each point (A) to (C) of the transversal filter shown in FIG.

  A transversal filter 50 shown in FIG. 6 includes a delay circuit 51 that delays one of the branched input signals, and an amplitude adjustment circuit that adjusts the amplitude of one of the branched output signals output from the delay circuit 51. 53, a delay circuit 52 that delays the other, an amplitude adjustment circuit 54 that adjusts the amplitude of the output signal from the delay circuit 52, and the other of the branched input signals and the output from the amplitude adjustment circuits 53 and 54 And an adder circuit 55 for adding the signals. Here, when the preceding wave and the delayed wave are input to the transversal filter 50, when the output signal of (A) and the output signal of (B) are added by the adder circuit 55, as shown in FIG. The delayed wave 72 of (A) between the preceding wave 71 of A) and the delayed wave 74 of (B) can be canceled by the preceding wave 73 of (B). In the case of this embodiment, the signal light transmitted from the optical transmitter 10 shown in FIG. 1 generates a preceding wave and a delayed wave due to a propagation delay time difference generated in a multimode optical fiber and a time difference generated by reflection and interference in spatial propagation. If the delay time difference is τ, the detection signal in the light receiving circuit 21 shown in FIG. 1 is a composite signal of the transmission signal delayed by the delay time difference τ. Therefore, in the delay circuit 51 shown in FIG. 6, the input signal is delayed by τ. Then, the amplitude adjustment circuit 54 determines the attenuation rate so that the amplitude value of the preceding wave 73 in FIG. 7B matches the amplitude value of the delayed wave 72 in FIG. Further, when the output signal of (B) in FIG. 6 and the output signal of (C) are added by the adder circuit 55, as shown in FIG. 7, the preceding wave 73 in (B) and the delayed wave 76 in (C) The (B) delayed wave 74 can be canceled by the preceding wave 75 of (C). In this way, the equalized signal shown in FIG. 7D can be output in FIG. In the present embodiment, the configuration in which the number of delay stages including the amplitude adjustment circuit and the delay circuit is two has been described. However, if the number of stages is increased, the amount of equalization can be increased. As described above, when a transversal filter is applied as the compensation filter 23 shown in FIG. 1, the transversal filter compensates for the waveform deterioration of the transmission signal due to the propagation delay time difference of the light, and the optical receiver 20 highly accurately transmits the transmission signal. Can be demodulated. Instead of the transversal filter, a lattice filter that equalizes based on the delay time difference may be applied.

  Here, a signal transmission / reception operation in the optical transmission system according to the present embodiment will be described with reference to FIGS.

  First, when a transmission signal to be transmitted is input, the optical transmitter 10 modulates subcarrier signals from the oscillators 16a, 16b, and 16c in the modulation circuits 15a, 15b, and 15c for the respective transmission signals. Here, the modulation may be intensity modulation, frequency modulation, or phase modulation. Thereafter, the subcarrier signals modulated by the modulation circuits 15a, 15b, and 15c are added by the adder circuit 14 to obtain a modulated signal. Then, the optical transmitter 10 outputs an optical carrier whose frequency is set in advance by the frequency setting signal from the frequency setting circuit 12 from the light source 11, and intensity-modulates the modulation signal from the addition circuit 14 in the intensity modulation circuit 13. Send.

  Here, as shown in FIG. 2, the optical carrier output from the light source 11 may be angle-modulated in advance by the angle modulation circuit 18 at the front stage of the intensity modulation circuit 13. The optical carrier frequency of the signal light from the optical transmitter 60 is changed by the frequency setting circuit 12 even if the delay time difference due to the propagation path difference is suddenly changed by disturbance by using the optical carrier that has been angle-modulated in advance in the angle modulation circuit 18. It is possible to control and maintain the phase difference of the optical carrier substantially constant. Therefore, it is possible to stabilize the light intensity of the signal light received by the optical receiver 70. As shown in FIG. 3, the light output from the light source 11 may be branched at the front stage of the intensity modulation circuit 13 and angle-modulated by the angle modulation circuit 18 and coupled to the signal light. Even if the delay time difference due to the propagation path difference suddenly changes due to disturbance, the branched light obtained by branching the light from the light source 11 is angle-modulated by the angle modulation circuit 18 and coupled to the signal light. By controlling the optical carrier frequency of the signal light, the phase difference of the optical carrier can be maintained substantially constant, and the influence of angle modulation on the light intensity-modulated signal light can be reduced. Therefore, it is possible to stabilize the light intensity of the signal light received by the optical receiver 70.

  Next, the optical receiver 20 shown in FIG. 1 receives the signal light propagated through the first optical transmission line 41 by the light receiving circuit 21, converts it into an electrical signal, and outputs it. The optical receiver 20 detects the phase difference in the optical carrier between the preceding wave and the delayed wave by the delay time difference detection circuit 25, modulates the detected phase difference as a transmission signal in the optical transmission circuit 27, and transmits it to the optical transmitter 10. Send to. The optical transmitter 10 sets the frequency of light output from the light source 11 by the frequency setting circuit 12 based on the phase difference transmitted from the optical receiver 20. Specifically, the optical carrier frequency of the light output from the light source 11 is set so that the value obtained by converting the delay time difference into the phase difference of the optical carrier becomes a substantially constant value between −π / 2 and π / 2. By setting the optical carrier frequency in the optical transmitter 10, signal light having a substantially constant light intensity can be received by the optical receiver 20 even if light interferes due to a transmission path difference. Thus, it is possible to improve the reception accuracy at the optical receiver 20. Further, the optical carrier phase difference caused by the delay time difference τ is fed back to the optical transmitter 10 and the optical carrier frequency is set following the delay time difference τ that changes from time to time, thereby maintaining the optical waveform deterioration compensation effect. As a result, the receiving accuracy at the optical receiver 20 can be kept high. Further, it is desirable that the phase difference is substantially zero. By setting the phase difference to approximately 0, the intensity of the optical carrier received by the optical receiver 20 is maximized. Therefore, signal degradation can be minimized by compensating for waveform degradation of the subcarrier signal.

  Next, in the optical receiver 20, an unnecessary frequency component is removed from the electrical signal output from the light receiving circuit 21 by the band pass filter 22, and then equalized by the compensation filter 23 and generated in the first optical transmission line 41. Remove τ. Then, the electrical signal from which the delay time difference is removed by the compensation filter 23 is frequency-divided by the frequency dividing circuit 24, and then detected by the detection circuits 29a, 29b, 29c by the demodulated signals from the oscillators 28a, 28b, 28c, and the low-pass filter 30a, High frequency components are removed and output at 30b and 30c. On the other hand, as shown in FIGS. 2 and 3, when frequency modulation is performed in the angle modulation circuit 18 together with intensity modulation in the optical carrier in the optical transmitters 60 and 80, the electrical signal output from the light receiving circuit 21 in the optical receiver 70. And the phase difference between the optical signals having a delay time difference received by the optical receiver 70 is detected. Then, the optical receiver 70 modulates the detected phase difference as a transmission signal by the optical transmission circuit 27 and transmits it to the optical transmitters 60 and 80. The optical transmitters 60 and 80 set the optical carrier frequency of the light output from the light source 11 by the frequency setting circuit 12 based on the phase difference transmitted from the optical receiver 70. Specifically, in the optical transmitters 60 and 80, the optical carrier frequency is set where the phase difference is substantially zero. By setting the optical carrier frequency so that the phase difference becomes substantially zero, the optical receiver 70 can stably demodulate the subcarrier signal. In addition, the phase difference detected by the optical receiver 70 is fed back to the optical transmitters 60 and 80 so that the optical carrier frequency is set following the delay time difference that changes from time to time. 23, it is possible to minimize the waveform deterioration after compensation.

  Next, verification of the compensation effect of signal light in the optical transmission system according to the present embodiment will be described.

  In FIG. 1, the optical transmitter 10 uses 128-bit PRBS (Pseudo Random Bit Stream) as four transmission signals. The PRBS transmission speed was set to 1 Gbps. Also, four different subcarrier signals were modulated by PSK (Pulse Shift Keying) by the PRBS. The PSK modulation signal was subjected to FDM (Frequency Division Multiplex) by the adder circuit 14. On the other hand, a photodiode is used as the light receiving circuit 21 on the optical receiver 20 side. A transversal filter is used as the compensation filter 23. As signals received by the optical receiver 20, output signals output from the low-pass filters 30a, 30b, and 30c after being synchronously detected by the detection circuits 29a, 29b, and 29c are detected. Based on the above specifications, the optical transmitter 10 performs two intensity modulations of DSB (Double Side Band) and SSB (Single Side Band). Further, the optical receiver 20 detects the output signal for each of the two intensity modulations by changing the number of delay stages of the transversal filter as the compensation filter 23 from 1 to 5. Further, when verifying the compensation effect, the light propagating through the first optical transmission line 41 has only two modes, and the delay time difference between the preceding wave and the delayed wave is 1.5 ns (1.5 time slots). .

  FIG. 8 shows a signal waveform before modulation by the optical transmitter 10 shown in FIG. 1 for each of DSB modulation and SSB modulation, and shows a signal waveform before passing through a transversal filter by the optical receiver 20. Show. In FIG. 8, (a) shows a signal waveform before modulation by the optical transmitter 10, (b) shows a signal waveform before passing through a transversal filter in the case of DSB modulation, and (c) shows The signal waveform before passing through the transversal filter in the case of SSB modulation is shown. FIG. 9 shows the waveform of the output signal when the number of delay stages of the transversal filter is changed. In FIG. 9, (a) shows the case where the number of delay stages of the transversal filter is 0, (b) shows the case where the number of delay stages of the transversal filter is 1, and (c) shows the transversal filter. The case where the number of delay stages is three is shown, and (d) shows the case where the number of delay stages of the transversal filter is five. FIG. 10 shows eye opening deterioration of the output signal when the number of delay stages of the transversal filter is changed. 9 and 10 show the case of DSB modulation.

  As shown in FIG. 8, the waveform of the signal received by the optical receiver 20 shown in FIG. 1 is degraded due to mode dispersion in the first optical transmission line 41 in the case of DSB modulation and SSB modulation. I understand that. The deteriorated signal approaches the signal before intensity modulation shown in FIG. 8A as the number of stages of the transversal filter is increased as shown in FIG. That is, it can be seen that the deterioration of the waveform is improved. As shown in FIG. 10, this deterioration has an improvement of about 7 dB when the number of delay stages of the transversal filter as the compensation filter 23 shown in FIG. It can be seen that the transmission signal from the optical transmitter 10 can be completely reproduced in the optical receiver 20 shown in FIG.

  The optical transmission system according to the present embodiment can be applied not only to a local area network and an access network, but also to a trunk network such as a metropolitan network that needs to communicate large information at a time.

1 is a schematic configuration diagram illustrating an optical transmission system according to an embodiment. It is the schematic block diagram which showed the other form of the optical transmission system. It is the schematic block diagram which showed the other form of the optical transmission system. It is a conceptual diagram of the signal light which propagates an optical transmission line. It is the figure which showed the frequency spectrum of the signal light as electric power detected with an optical receiver. It is a schematic block diagram of a transversal filter. It is the schematic of the intensity | strength of the signal light in each point of (A) to (C) of a transversal filter. It is the figure which showed the signal waveform before modulating with an optical transmitter, and the signal waveform before passing a transversal filter with an optical receiver about each in the case of performing DSB modulation and SSB modulation. It is the figure which showed the waveform of the output signal when changing the delay stage number of a transversal filter. It is the figure which showed eye opening degradation of the output signal when changing the delay stage number of a transversal filter. It is a schematic block diagram of a transmitter and a receiver when performing wireless communication. It is a schematic block diagram of the transmitter and receiver in the case of performing optical communication.

Explanation of symbols

2: optical transmission system 3: optical transmission system 4: optical transmission system 10: optical transmitter 11: light source 12: frequency setting circuit 13: intensity modulation circuit 14: addition circuit 15a: modulation circuit 15b: modulation circuit 15c: modulation circuit 16a : Oscillator 16b: Oscillator 16c: Oscillator 17: Oscillator 18: Angle modulation circuit 20: Optical receiver 21: Light receiving circuit 22: Band pass filter 23: Compensation filter 24: Frequency division circuit 25: Delay time difference detection circuit 26: Low pass filter 27 : Optical transmission circuit 28a: oscillator 28b: oscillator 28c: oscillator 29a: detection circuit 29b: detection circuit 29c: detection circuit 30a: low pass filter 30b: low pass filter 30c: low pass filter 41: first optical transmission line 42: second optical transmission Path 43: Mixer 44: Oscillator 50: Transversal filter 51: Delay times 52: delay circuit 53: amplitude adjustment circuit 54: amplitude adjustment circuit 55: addition circuit 60: optical transmitter 61: preceding wave 62: delay wave 63: preceding wave optical carrier signal 64: preceding wave optical subcarrier signal 65: Delayed wave optical carrier signal 66: Delayed wave optical subcarrier signal 70: Optical receiver 71: Leading wave 72: Delayed wave 73: Leading wave 74: Delayed wave 75: Leading wave 76: Delayed wave 77: Leading wave 78: Delay wave 80: optical transmitter 120: transmitter 121: output circuit 122: oscillator 123: modulation circuit 124: transmission antenna 125: receiver 126: reception antenna 127: demodulation circuit 128: detection circuit 129: oscillator 131: transmitter 132 : Receiver 133: Photodiode 134: Detection circuit 135: Optical fiber

Claims (8)

  An optical transmitter that transmits signal light by modulating the intensity of an optical carrier with a subcarrier signal, and an optical receiver that receives the signal light transmitted from the optical transmitter and demodulates the subcarrier signal. In the transmission system, the optical transmitter has a value obtained by converting a delay time difference due to a propagation path difference of the signal light from the optical transmitter to the optical receiver into a phase difference of an optical carrier, from −π / 2 to π. An optical transmission system, wherein the optical carrier frequency of the light for intensity modulation of the optical transmitter is set so as to be a substantially constant value between / 2.   The optical transmission system according to claim 1, wherein the phase difference is substantially zero.   It further comprises delay time difference feedback means for acquiring the delay time difference and feeding back from the optical receiver to the optical transmitter, and the optical transmitter follows the delay time difference fed back from the delay time difference feedback means. The optical transmission system according to claim 1, wherein an optical carrier frequency of the light is set.   The optical transmission system according to claim 1, wherein the optical transmitter modulates an angle of an optical carrier in advance.   4. The optical transmission system according to claim 1, wherein the optical transmitter angle-modulates the branched light that has been branched in advance from the light whose intensity is to be modulated and couples it to the signal light. 5.   The optical transmitter according to claim 4 or 5, wherein the optical transmitter sets an optical carrier frequency so that a phase difference between optical carriers having a delay time difference received by the optical receiver is substantially zero. Transmission system.   The optical transmitter further includes phase difference feedback means for acquiring the phase difference and feeding back from the optical receiver to the optical transmitter, and the optical transmitter is fed back from the phase difference feedback means. The optical transmission system according to claim 6, wherein the optical carrier frequency is set following a phase difference.   The optical transmission system according to claim 1, wherein the optical receiver includes a compensation filter that equalizes the electric signal to be demodulated in advance based on the delay time difference.

Images