JP2019121840A - Optical transmitter - Google Patents

Abstract

To provide a low-cost optical transmitter by suppressing the deterioration of a signal detection level in an optical receiver.SOLUTION: The optical transmitter includes: means for modulating first continuous light on the basis of a signal input to a first port to generate first phase modulated light, and for modulating second continuous light on the basis of a signal input to a second port to generate second phase modulated light, so as to synthesize the first phase modulated light with the second phase modulated light and output synthesized light; means for branching an input electric signal into two i.e. a first electric signal and a second electric signal, and inputting the first electric signal to the first port and inputting a third electric signal obtained by inverting the second electric signal to the second port; means for branching an input electric signal into two i.e. a fourth electric signal and a fifth electric signal, and inputting the fourth electric signal to the first port and inputting the fifth electric signal to the second port; means for inputting an input electric signal to the first port; and means for inputting an input electric signal to the second port.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an optical transmission apparatus of an optical communication system using a direct detection method.

  In an optical communication system, chromatic dispersion fading occurs. Also, due to wavelength dispersive fading, the intensity modulated (IM) optical signal (or amplitude modulated (AM) optical signal) is converted to a phase modulated (PM) optical signal, and the PM optical signal is converted to an IM (or AM) optical signal. It is known that a phenomenon called IM (AM) -PM conversion occurs, which is converted. For example, the IM optical signal transmitted by the optical transmission apparatus gradually changes to a PM optical signal according to the transmission distance. The PM light signal further changes gradually to an IM light signal according to the transmission distance. Note that this IM-PM conversion has frequency characteristics. That is, when a plurality of IM optical signals are wavelength-multiplexed and transmitted from the optical transmitter, after transmitting a predetermined distance, a part thereof becomes a PM optical signal and a part thereof remains the IM optical signal, The remainder is an intermediate, ie intensity-modulated and phase-modulated optical signal. The direct detection type optical receiving apparatus can not detect the PM optical signal whose intensity does not change because it picks up the change of the intensity of the IM optical signal as an electric signal. Therefore, this optical communication system can not operate when, for example, the light receiving device receives light at the point where the IM light signal transmitted by the light transmitting device is converted into the PM light signal. In addition, in the case where the optical receiving apparatus receives light such that the IM optical signal transmitted by the optical transmitting apparatus is not converted into a complete PM optical signal but becomes an optical signal which is intensity-modulated and phase-modulated, Can take out changes in the intensity of the optical signal, but the level becomes smaller and the signal to noise ratio is degraded.

  For this reason, Non-Patent Document 1 provides an optical transmitter with two modulators, an intensity modulator and a phase modulator, according to the transmission distance (distance to the optical receiver) and the frequency band (signal band) of the signal. Discloses a configuration using either an intensity modulator or a phase modulator. This is because the conversion from the IM optical signal to the PM optical signal and the conversion from the PM optical signal to the IM optical signal occur at the same transmission distance for each frequency. Specifically, when an IM optical signal whose signal band is the first frequency band is transmitted for a first distance, the IM optical signal is assumed to be a complete PM optical signal. In this case, when the PM optical signal whose signal band is the first frequency band is transmitted for the first distance, the PM optical signal becomes a complete IM optical signal. Based on this characteristic, Non-Patent Document 1 determines which of intensity modulation and phase modulation is to be applied to the optical signal based on the signal band of the optical signal to be transmitted and the transmission distance to the optical receiver. doing.

  In addition, Non-Patent Document 2 uses a dual electrode Mach-Zehnder modulator (DEMZM) and digital signal processing to generate a waveform having an inverse characteristic of dispersion generated in a transmission line. Discloses a configuration for compensating for wavelength dispersion.

S. Ishimura, et al. "1.032-Tb / s CPRI-equivalent data rate transmission using IF-over-fiber system for high capacity mobile fronthaul links", in European Conference on Optical Communication (ECOC 2017), Th. PDP. B. 6, 2017 R. I. Killey, et al. "Electronic dispersion compensation by signal predication using digital processing and a dual-drive Mach-Zehnder modulator", IEEE Photon. Technol. Lett. , Vol. 17, 714-16, 2005

  The configuration of Non-Patent Document 1 requires two modulators, an intensity modulator and a phase modulator, which increases the cost of the optical transmission apparatus. Further, in the configuration of Non-Patent Document 1, depending on the frequency band and the transmission distance, either the IM optical signal or the PM optical signal may be transmitted, and in the optical receiving apparatus, an intermediate optical modulation signal may occur. . In such a case, the level (signal detection level) of the electric signal directly detected and output by the light receiving apparatus becomes smaller (deteriorated). On the other hand, in the configuration of Non-Patent Document 2, complete dispersion compensation is possible, and deterioration of the signal detection level does not occur. However, wideband signal processing is required, which leads to a significant increase in the cost of the optical transmission apparatus.

  The present invention provides an optical transmission apparatus that can suppress deterioration of the signal detection level by the optical reception apparatus with an inexpensive configuration.

  According to one aspect of the present invention, the optical transmitter phase-modulates the first continuous light based on the signal input to the first port to generate a first phase-modulated optical signal, and generates the second continuous light at the second port. Optical modulation means for generating a second phase modulation optical signal by phase modulation based on the signal input to the input signal, and combining and outputting the first phase modulation optical signal and the second phase modulation optical signal; Electrical signal is branched into a first electrical signal and a second electrical signal, the first electrical signal is input to the first port, and a third electrical signal obtained by inverting the second electrical signal is the second port. Processing means for input to the first and second electric signals are branched into a fourth electric signal and a fifth electric signal, the fourth electric signal is inputted to the first port, and the fifth electric signal is A second processing means for inputting to a second port, and an electric signal to be inputted is Wherein the third processing means for inputting a fourth processing means for inputting an electrical signal to said second port to be input, in that it comprises a.

  According to the present invention, it is possible to suppress the deterioration of the signal detection level by the optical receiver with an inexpensive configuration.

BRIEF DESCRIPTION OF THE DRAWINGS The block diagram of the optical transmission apparatus by one Embodiment. Explanatory drawing of the operation | movement of the optical modulator by one Embodiment. The figure which shows the frequency response of each modulation signal after 20 km transmission.

  Hereinafter, exemplary embodiments of the present invention will be described with reference to the drawings. The following embodiment is an exemplification, and the present invention is not limited to the contents of the embodiment. Further, in each of the following drawings, components that are not necessary for the description of the embodiment will be omitted from the drawings.

  FIG. 1 is a block diagram of an optical transmitter according to the present embodiment. A plurality of electrical signals are input to the frequency converter 1. This electric signal is a signal for modulating continuous light as will be described later, and is hereinafter referred to as a modulation signal. Each modulated signal carries information. The frequency conversion unit 1 performs frequency conversion of each modulation signal so that the frequency bands of the plurality of modulation signals do not overlap, and outputs each modulation signal after frequency conversion to the distribution unit 2. The distribution unit 2 distributes each modulated signal to any one of the branch unit 3, the adjustment unit 4, the adjustment unit 5, and the branch unit 6.

  The branching unit 3 branches the input modulation signal into two at the same amplitude level, outputs one to the multiplexing unit 8, and outputs the other to the inverting unit 7. The inverting unit 7 inverts the waveform of the input modulation signal, and outputs the inverted modulation signal to the multiplexing unit 9.

  The adjustment unit 4 multiplies the amplitude of the input modulation signal by √ 2 and outputs the modulation signal after the amplitude adjustment to the multiplexing unit 8. The adjustment unit 5 multiplies the amplitude of the input modulation signal by √ 2 and outputs the modulation signal after the amplitude adjustment to the multiplexing unit 9.

  The branching unit 6 branches the input modulation signal into two at the same amplitude level, outputs one to the multiplexing unit 8, and outputs the other to the multiplexing unit 9.

  The combining unit 8 combines the input modulation signals to generate a first combined signal, and applies the first combined signal to the port A of the two-electrode Mach-Zehnder modulator (DEMZM) 10. The multiplexing unit 9 multiplexes each of the input modulation signals to generate a second multiplexed signal, and applies the second multiplexed signal to the port B of the DEM ZM 10.

  FIG. 2 is an explanatory view of a light modulation method by the DEM ZM 10. Continuous light is input to the light port IN of the DEM ZM 10 from a light source (not shown). The continuous light input to the optical port IN of the DEM ZM 10 is branched into a first continuous light passing through the first path 301 and a second continuous light passing through the second path 302. The first continuous light passing through the first path 301 and the second continuous light passing through the second path 302 are combined again and output from the optical port OUT of the DEM ZM 10. Here, DC bias voltages to the port A and the port B are applied such that the phase difference between the first continuous light and the second continuous light is multiplexed at π / 2. (Hereinafter, this DC bias voltage is referred to as an orthogonal bias voltage.) FIG. 2B shows the first continuous light 100 and the second continuous light 200 when the orthogonal bias voltage is applied to the port A and the port B. Is shown on the complex plane.

  A modulation signal is applied to the port A of the DEM ZM 10 in addition to the quadrature bias voltage. That is, a voltage obtained by adding the quadrature bias voltage and the modulation signal is applied. Due to this modulation signal, the first continuous light 100 starts from the state where only the orthogonal bias voltage is applied on the complex plane, and the angle with the I axis changes in proportion to the amplitude of the modulation signal. That is, the DEM ZM 10 phase modulates the first continuous light 100 based on the modulation signal input to the port A. Similarly, the modulation signal is applied to the port B in addition to the orthogonal bias voltage. Due to this modulation signal, the second continuous light 200 starts from the state where only the orthogonal bias voltage is applied on the complex plane, and the angle with the I axis changes in proportion to the amplitude of the modulation signal. That is, the DEM ZM 10 phase modulates the second continuous light 200 based on the modulation signal input to the port B. When the modulation signals applied to the port A and the port B are the same, the amount of change in the angle with the I axis is the same.

  Therefore, when a modulation signal with the same amplitude value and opposite polarity is applied to the port A and port B of the DEMZM 10, the first continuous light 100 and the second continuous light 200 with respect to the I axis on the complex plane It changes symmetrically, so that the combined light travels on the I axis. That is, in this case, the multiplexed light output from the optical port OUT of the DEMZM 10 becomes an optical signal (IM optical signal) whose intensity is modulated by the applied modulation signal. This IM optical signal corresponds to the chirp parameter α = 0.

  When the same modulation signal is applied to port A and port B of DEMZM 10, the combined light of the first continuous light 100 and the second continuous light 200 has an equal amplitude in proportion to the amplitude of the modulation signal and only an angle with the I axis. Changes. That is, in this case, the multiplexed light output from the optical port OUT of the DEMZM 10 becomes an optical signal (PM optical signal) phase-modulated by the applied modulation signal. The PM light signal corresponds to a chirp parameter α = + ∞ or −∞ (Hereinafter, α = + ∞ or −∞ is collectively referred to as ∞).

  In addition, when the modulation signal is applied only to the port A of the DEM ZM 10, the combined light of the first continuous light 100 and the second continuous light 200 changes both the amplitude and the angle with the I axis on the complex plane. That is, in this case, the multiplexed light output from the optical port OUT of the DEMZM 10 becomes a signal (hereinafter, referred to as a first IPM optical signal) that is intensity and phase light modulated by the applied modulation signal. The first IPM optical signal corresponds to the chirp parameter α = −1.

  Further, when the modulation signal is applied only to the port B of the DEM ZM 10, the combined light of the first continuous light 100 and the second continuous light 200 changes both the amplitude and the angle with the I axis on the complex plane. That is, in this case, the multiplexed light output from the optical port OUT of the DEMZM 10 becomes a signal (hereinafter, referred to as a second IPM optical signal) which is intensity and phase light modulated by the applied modulation signal. The second IPM optical signal corresponds to the chirp parameter α = + 1.

  In the configuration of FIG. 1, the modulation signal input to the branching unit 3 is branched into two, one of which is applied to the port A of the DEM ZM 10 through the multiplexing unit 8, and the other is inverted by the inverting unit 7. After that, it is applied to the port B of the DEM ZM 10 through the multiplexing unit 9. Therefore, the DEM ZM 10 intensity-modulates the continuous light with the modulation signal input to the branching unit 3 and outputs an IM optical signal. Similarly, the modulation signal input to the branching unit 6 is branched into two, one of which is applied to the port A of the DEMZM 10 through the multiplexing unit 8 and the other is the port of the DEMZM 10 through the multiplexing unit 9 Applied to B. Therefore, the DEM ZM 10 phase modulates the continuous light with the modulation signal input to the branching unit 9 and outputs a PM optical signal.

  Further, the modulation signal input to the adjustment unit 4 is applied only to the port A of the DEM ZM 10 through the multiplexing unit 8. Therefore, the DEM ZM 10 intensity-phase modulates the continuous light with the modulation signal input to the adjustment unit 4 and outputs a first IPM optical signal. Similarly, the modulation signal input to the adjustment unit 5 is applied only to the port B of the DEM ZM 10 through the multiplexing unit 9. Therefore, the DEM ZM 10 intensity-phase modulates the continuous light with the modulation signal input to the adjustment unit 5, and outputs a second IPM optical signal.

  Although the DEM ZM 10 actually outputs an optical signal in which the four modulated optical signals are superimposed, the frequency bands of the respective modulated signals are made to not overlap each other by frequency conversion in the frequency conversion unit 1. In the modulated optical signals output from the DEM ZM 10, the respective modulated signals have no overlap on the frequency axis. As is apparent from FIG. 2B, the amplitudes of the IM optical signal and the PM optical signal output from the DEM ZM 10 are √2 times that of the first continuous light 100 and the second continuous light 200, respectively. The reason why the input modulation signals are multiplied by {square root over (2)} in the adjustment units 4 and 5 is to suppress the power difference between the respective modulated lights.

  FIG. 3 shows the frequency response characteristics of an electrical signal obtained by transmitting a modulated optical signal having a chirp parameter α of 0, 、, 1 or 1 for 20 km and performing direct detection, that is, photoelectric conversion. As described above, the chirp parameter α = 0 corresponds to the IM optical signal, α = ∞ corresponds to the PM optical signal, α = −1 corresponds to the first IPM optical signal, and α = 1 corresponds to the first 2 correspond to the IPM optical signal.

  As shown in FIG. 3, the IM optical signal drops significantly at 14 GHz, the PM optical signal drops significantly at 20 GHz, the first IPM optical signal drops significantly at 10 GHz, and the second IPM optical signal drops significantly at 17 GHz. This is because each modulated optical signal transmitted is a PM optical signal at these frequencies. On the other hand, the IM optical signal is well detected at 20 GHz, the PM optical signal is well detected at 14 GHz, the first IPM optical signal is well detected at 17 GHz, and the second IPM optical signal is well detected at 10 GHz. The well detected frequencies indicate that at each of these frequencies, each transmitted modulated signal is an IM optical signal.

  Further, as apparent from FIG. 3, when only the highest part of the frequency characteristic of each modulated optical signal is seen, although there is a slight fluctuation depending on the frequency, the fluctuation amount is the configuration of the non-patent document 1 (α = 0 less than 0 and α = よ り 小 さ い) In the present embodiment, the distribution unit 2 has the characteristics shown in FIG. 3 and is most suitably detected in the optical receiving apparatus from four modulation methods according to the frequency (signal band) of the input modulation signal. The selected modulation method is selected and output to the corresponding path, that is, the path from the branch unit 3, the path from the adjustment unit 4, the path from the adjustment unit 5, or the path from the branch unit 6. According to this configuration, it is possible to transmit a signal obtained by combining a plurality of types of light modulation signals by using one light modulator, and to suppress deterioration of the signal detection level of the light modulation signal of each light modulation signal.

  The optical transmission apparatus of the present invention can be applied to, for example, a PP type optical communication system. In this case, since the transmission distance is determined according to the installation position of the light receiving apparatus, for example, a frequency response characteristic (FIG. 3) corresponding to the transmission distance may be provided in the distribution unit 2. Further, the optical transmission apparatus of the present invention can be applied to a P-MP type optical communication system such as PON. In this case, the optical transmitter transmits optical signals to the plurality of optical receivers, and the transmission distance to each optical receiver differs depending on the optical receivers. In this case, the frequency response characteristic (FIG. 3) corresponding to each transmission distance may be provided in the distribution unit 2. Further, instead of providing the frequency response characteristic as shown in FIG. 3 in the distribution unit 2, the distribution unit 2 may be configured to calculate the frequency response characteristic as shown in FIG. 3 based on the transmission distance. Further, the operator determines the frequency of each modulation signal and which modulation method to use based on the frequency response characteristics shown in FIG. 3 and sets the frequency conversion unit 1 and the distribution unit 2 You can also. In this case, the frequency conversion unit 1 performs frequency conversion according to the setting, and the distribution unit 2 performs distribution according to the setting.

  In the configuration of FIG. 1, the path from the branch unit 3 to the DEMZM 10 (except for the DEMZM 10) constitutes a first processing unit. Further, a path from the branching unit 6 to the DEMZM 10 (except for the DEMZM 10) constitutes a second processing unit. Furthermore, the path from the adjustment unit 4 to the DEMZM 10 (except for the DEMZM 10) constitutes a third processing unit. Furthermore, the path from the adjustment unit 5 to the DEMZM 10 (except for the DEMZM 10) constitutes a fourth processing unit. Then, the distribution unit 2 outputs the input modulation signal to any one of the first processing unit, the second processing unit, the third processing unit, and the fourth processing unit. Note that the distribution unit 2 performs the first processing unit, the second processing unit, and the third processing on the modulation signal to be input according to the user setting or according to the frequency band of the modulation signal to be input and the transmission distance thereof. Output to either the unit or the fourth processing unit.

  Further, in the configuration of FIG. 1, the distributing unit 2, the branching unit 3, the adjusting unit 4, the adjusting unit 5, the branching unit 6, the inverting unit 7, the combining unit 8 and the combining unit 9 drive the DEMZM 10. Are configured. Then, the drive unit outputs the DEM ZM 10 based on the modulation signal to output an optical modulation signal of the chirp parameter selected according to the frequency band of the input modulation signal, the distance for transmitting the modulation signal, and the like. It is configured to drive. Alternatively, the drive unit holds setting values of chirp parameters, which are input from the frequency conversion unit 1 and are respectively associated with a plurality of modulation signals different in band from one another, and the chirps associated with each of the plurality of modulation signals The DEM ZM 10 is configured to drive an optical modulation signal of a parameter.

  3, 6: Branching part 4, 5: Adjustment part, 7: Inversion part, 8, 9: Coupling part, 10: Two-electrode type Mach-Zehnder modulator

Claims (13)

The first continuous light is phase-modulated based on the signal input to the first port to generate a first phase-modulated optical signal, and the second continuous light is phase-modulated based on the signal input to the second port to An optical modulation unit that generates a phase modulation optical signal and combines and outputs the first phase modulation optical signal and the second phase modulation optical signal;
The input electrical signal is branched into a first electrical signal and a second electrical signal, the first electrical signal is input to the first port, and a third electrical signal obtained by inverting the second electrical signal is the third electrical signal. First processing means for inputting to 2 ports;
A second process in which the input electric signal is branched into a fourth electric signal and a fifth electric signal, the fourth electric signal is input to the first port, and the fifth electric signal is input to the second port Means,
Third processing means for inputting an input electric signal to the first port;
Fourth processing means for inputting an input electric signal into the second port;
An optical transmitter characterized by comprising:   The light transmitting apparatus according to claim 1, wherein the light modulation unit splits the input continuous light into two to generate the first continuous light and the second continuous light.   When the same signal is input to the first port and the second port, a phase difference between the first phase modulated optical signal and the second phase modulated optical signal is π / 2. The optical transmitter as described in 1 or 2.   The optical transmission apparatus according to any one of claims 1 to 3, wherein the light modulation unit is a two-electrode Mach-Zehnder modulator.   The apparatus further comprises distribution means for distributing an input electric signal as an input of any one of the first processing means, the second processing means, the third processing means and the fourth processing means. The optical transmitter according to any one of claims 1 to 4. A plurality of electric signals having different bands from each other are input to the distribution means,
The distribution means distributes each of the plurality of electric signals as an input of any one of the first processing means, the second processing means, the third processing means, and the fourth processing means. Item 6. An optical transmitter according to Item 5.   The distribution unit is configured to generate the first processing unit, the second processing unit, and the electric signal based on a band of the electric signal and a distance of transmitting the electric signal for each of the plurality of electric signals. 7. The optical transmission apparatus according to claim 6, wherein it is determined which of the third processing means and the fourth processing means is to be input.   8. The optical transmission apparatus according to claim 6, further comprising conversion means for performing frequency conversion so that bands of a plurality of electric signals are different from each other and outputting the same to the distribution means.   The optical transmission according to any one of claims 1 to 8, wherein each of the third processing means and the fourth processing means has adjusting means for adjusting the amplitude of the input electric signal. apparatus. A two-electrode Mach-Zehnder modulator,
Drive for driving the two-electrode Mach-Zehnder modulator based on the electrical signal so as to select a chirp parameter based on a band of the electrical signal and a distance for transmitting the electrical signal and output an optical modulation signal of the selected chirp parameter And an optical transmitter. A plurality of electrical signals having different bands from each other are input to the driving means;
The driving means selects a chirp parameter for each of the plurality of electrical signals, and the two-electrode Mach-Zehnder modulator outputs an optical modulation signal of the selected chirp parameter for each of the plurality of electrical signals. The optical transmitter according to claim 10, wherein the two-electrode type Mach-Zehnder modulator is driven.   The optical transmission apparatus according to claim 10, wherein the driving means selects one chirp parameter from four chirp parameters. A two-electrode Mach-Zehnder modulator,
Holding means for holding chirp parameters associated with each of a plurality of electrical signals in different bands that are input;
And d) driving means for driving the two-electrode type Mach-Zehnder modulator so as to output an optical modulation signal of the associated chirp parameter for each of the plurality of electrical signals.

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