JP6865676B2 - Optical transmitter - Google Patents

Description

The present invention relates to an optical transmitter of an optical communication system that uses a direct detection method.

Wavelength dispersion fading occurs in optical communication systems. Further, by wavelength dispersion fading, an intensity modulation (IM) optical signal (or an amplitude modulation (AM) optical signal) is converted into a phase modulation (PM) optical signal, and the PM optical signal is converted into an IM (or AM) optical signal. It is known that a phenomenon called IM (AM) -PM conversion, which is converted, occurs. For example, the IM optical signal transmitted by the optical transmitter gradually changes to a PM optical signal according to the transmission distance. This PM optical signal is further gradually changed to an IM optical signal according to the transmission distance. This IM-PM conversion has frequency characteristics. That is, when a plurality of IM optical signals are wavelength-multiplexed and transmitted from an optical transmitter, after transmitting a predetermined distance, a part thereof becomes a PM optical signal, and a part thereof remains as an IM optical signal. The rest is in the middle, that is, intensity-modulated and phase-modulated optical signals. Since the direct detection type optical receiver extracts the change in the intensity of the IM optical signal as an electric signal, it cannot detect the PM optical signal whose intensity does not change. Therefore, if the optical receiver receives light when the IM optical signal transmitted by the optical transmitter is converted into a PM optical signal, this optical communication system cannot operate. Further, when the IM optical signal transmitted by the optical transmitter is not converted into a complete PM optical signal and is received by the optical receiver at the place where the optical signal is intensity-modulated and phase-modulated, the optical receiver is used. Can extract changes in the intensity of the optical signal, but the level becomes smaller and the signal-to-noise ratio deteriorates.

Therefore, in Non-Patent Document 1, two modulators, an intensity modulator and a phase modulator, are provided in the optical transmitter, depending on the transmission distance (distance to the optical receiver) and the frequency band (signal band) of the signal. Therefore, a configuration using either an intensity modulator or a phase modulator is disclosed. 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 for the same transmission distance for each frequency. Specifically, when an IM optical signal whose signal band is the first frequency band is transmitted for only the 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 by the first distance, the PM optical signal becomes a complete IM optical signal. Utilizing this characteristic, Non-Patent Document 1 determines whether to apply intensity modulation or phase modulation 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.

Further, Non-Patent Document 2 uses a two-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 that compensates for wavelength dispersion.

S. Ishimura, et al. , "1.032-Tb / s CPRI-equiverbalent data transition communication IF-over-Fiber system for high capacity communication mobile mobile communication (in-Europe", in-Europe PDP. B. 6, 2017 R. I. Killey, et al. , "Electronic distribution compensation by signal predistatory processing digital processing and a dual-drive Mach-Zehnder model", 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 transmitter. Further, in the configuration of Non-Patent Document 1, depending on the frequency band and the transmission distance, whichever the IM optical signal or the PM optical signal is transmitted, the optical receiving device may have an intermediate optical modulation signal. .. In such a case, the level (signal detection level) of the electric signal directly detected and output by the optical receiver becomes small (deteriorated). On the other hand, in the configuration of Non-Patent Document 2, complete dispersion compensation is possible and the signal detection level is not deteriorated, but wideband signal processing is required, which causes a significant increase in the cost of the optical transmitter.

The present invention provides an optical transmission device having an inexpensive configuration and capable of suppressing deterioration of the signal detection level by the optical receiving device.

According to one aspect of the present invention, the optical transmission device phase-modulates the first continuous light based on the signal input to the first port to generate a first phase-modulated optical signal, and transfers the second continuous light to the second port. An optical modulation means that generates a second phase-modulated optical signal by phase-modulating based on the signal input to the signal, synthesizes the first phase-modulated optical signal and the second phase-modulated optical signal, and outputs the signal. The electric signal is branched into a first electric signal and a second electric signal, the first electric signal is input to the first port, and the third electric signal obtained by inverting the second electric signal is input to the second port. The first processing means to be input to the above, 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 the said. A second processing means for inputting the input electric signal to the second port, a third processing means for inputting the input electric signal to the first port, and a fourth processing means for inputting the input electric signal to the second port. It is characterized by having.

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

The block diagram of the optical transmission device by one Embodiment. The explanatory view of the operation of the optical modulator according to one Embodiment. The figure which shows the frequency response of each modulation signal after 20km transmission.

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

FIG. 1 is a configuration diagram of an optical transmission device according to the present embodiment. A plurality of electric signals are input to the frequency conversion unit 1. As will be described later, this electric signal is a signal for modulating continuous light, and is hereinafter referred to as a modulated signal. Each of the modulated signals carries information. The frequency conversion unit 1 performs frequency conversion of each modulated signal so that the frequency bands of the plurality of modulated signals do not overlap, and outputs each modulated signal after frequency conversion to the distribution unit 2. The distribution unit 2 distributes each modulated signal to any 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 modulated signal into two at the same amplitude level, outputs one to the combiner 8 and outputs the other to the inverting unit 7. The inverting unit 7 inverts the waveform of the input modulated signal and outputs the inverted modulated signal to the combiner 9.

The adjusting unit 4 multiplies the amplitude of the input modulated signal by √2 and outputs the modulated signal after adjusting the amplitude to the combining unit 8. The adjusting unit 5 multiplies the amplitude of the input modulated signal by √2 and outputs the modulated signal after adjusting the amplitude to the combining unit 9.

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

The combiner 8 combines each of the input modulation signals to generate a first combiner signal, and applies the first combiner signal to port A of the two-electrode Machzenda modulator (DEMZM) 10. The combiner 9 generates a second combined wave signal by combining each input modulated signal, and applies the second combined wave signal to the port B of the DEMZM10.

FIG. 2 is an explanatory diagram of an optical modulation method using DEMZM10. Continuous light is input to the optical port IN of the DEMZM10 from a light source (not shown). The continuous light input to the optical port IN of the DEMZM10 is bifurcated 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 that has passed through the first path 301 and the second continuous light that has passed through the second path 302 are combined again and output from the optical port OUT of the DEMZM10. Here, the DC bias voltage to the port A and the port B is applied so that the phase difference between the first continuous light and the second continuous light is 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 ports A and B. Is shown on the complex plane.

A modulation signal is applied to port A of the DEMZM10 in addition to the orthogonal bias voltage. That is, a voltage obtained by adding the orthogonal bias voltage and the modulation signal is applied. Due to this modulated signal, on the complex plane, the angle of the first continuous light 100 with respect to the I-axis changes in proportion to the amplitude of the modulated signal, starting from the state in which only the orthogonal bias voltage is applied. That is, the DEMZM 10 phase-modulates the first continuous light 100 based on the modulation signal input to the port A. Similarly, a modulation signal is applied to the port B in addition to the orthogonal bias voltage. Due to this modulated signal, on the complex plane, the second continuous light 200 starts from a state in which only the orthogonal bias voltage is applied, and the angle with the I axis changes in proportion to the amplitude of the modulated signal. That is, the DEMZM 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 having the same amplitude value and reversed positive and negative is applied to port A and port B of the DEMZM10, the first continuous light 100 and the second continuous light 200 are directed 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 combined light output from the optical port OUT of the DEMZM10 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.

Further, when the same modulated signal is applied to port A and port B of DEMZM10, the combined light of the first continuous light 100 and the second continuous light 200 has the same amplitude in proportion to the amplitude of the modulated signal and only the angle with the I axis. Changes. That is, in this case, the combined wave light output from the optical port OUT of the DEMZM10 becomes an optical signal (PM optical signal) phase-modulated by the applied modulation signal. This PM optical signal corresponds to the chirp parameter α = + ∞ or −∞ (hereinafter, α = + ∞ or −∞ is collectively referred to as ∞).

Further, when the modulation signal is applied only to the port A of the DEMZM10, both the amplitude and the angle of the I-axis of the combined light of the first continuous light 100 and the second continuous light 200 change on the complex plane. That is, in this case, the combined wave light output from the optical port OUT of the DEMZM 10 becomes a signal that is intensity / phase light modulated by the applied modulation signal (hereinafter, referred to as a first IPM optical 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 DEMZM10, both the amplitude and the angle of the I-axis of the combined light of the first continuous light 100 and the second continuous light 200 change on the complex plane. That is, in this case, the combined wave light output from the optical port OUT of the DEMZM 10 becomes a signal that is intensity / phase light modulated by the applied modulation signal (hereinafter, referred to as a second IPM optical signal). The second IPM optical signal corresponds to the chirp parameter α = + 1.

In the configuration of FIG. 1, the modulated signal input to the branching portion 3 is branched into two, one of which is applied to the port A of the DEMZM10 via the combining portion 8 and the other is inverted by the inversion portion 7. After that, it is applied to the port B of the DEMZM 10 via the combiner 9. Therefore, the DEMZM 10 intensity-modulates the continuous light with the modulation signal input to the branch portion 3 and outputs the IM light signal. Similarly, the modulated signal input to the branch portion 6 is branched into two, one of which is applied to the port A of the DEMZM10 via the combiner 8 and the other is a port of the DEMZM10 via the combiner 9. It is applied to B. Therefore, the DEMZM 10 phase-modulates continuous light with the modulation signal input to the branch portion 9 and outputs a PM light signal.

Further, the modulated signal input to the adjusting unit 4 is applied only to the port A of the DEMZM 10 via the combining unit 8. Therefore, the DEMZM 10 intensifies and phase-modulates the continuous light with the modulation signal input to the adjusting unit 4, and outputs the first IPM optical signal. Similarly, the modulated signal input to the adjusting unit 5 is applied only to the port B of the DEMZM 10 via the combining unit 9. Therefore, the DEMZM 10 intensifies and phase-modulates the continuous light with the modulation signal input to the adjusting unit 5, and outputs the second IPM optical signal.

The DEMZM10 actually outputs an optical signal in which the above four modulated optical signals are mixed, but the frequency bands of the modulated signals are prevented from overlapping each other by frequency conversion by the frequency conversion unit 1. In the modulated optical signal output by DEMZM10, each modulated signal has no overlap on the frequency axis. As is clear from FIG. 2B, the amplitudes of the IM optical signal and the PM optical signal output by the DEMZM10 are √2 times that of the first continuous light 100 and the second continuous light 200, respectively. The reason why the input modulation signals in the adjusting units 4 and 5 are multiplied by √2 is to suppress the difference in power of each modulated light.

FIG. 3 shows the frequency response characteristics of an electric signal obtained by transmitting a modulated optical signal having a chirp parameter α of 0, ∞, 1, -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 IPM optical signal. Corresponds to 2IPM optical signals.

As shown in FIG. 3, the IM optical signal has a large drop at 14 GHz, the PM optical signal has a large drop at 20 GHz, the first IPM optical signal has a large drop at 10 GHz, and the second IPM optical signal has a large drop at 17 GHz. This is because each of the transmitted modulated optical signals is a PM optical signal at those 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. This well-detected frequency indicates that each transmitted modulated signal is an IM optical signal at those frequencies.

Further, as is clear from FIG. 3, when looking only at the portion having the highest frequency characteristic of each modulated optical signal, although there is some variation depending on the frequency, the amount of variation is the configuration of Non-Patent Document 1 (α). = Less than (use only 0 and α = ∞). In the present embodiment, the distribution unit 2 has the characteristics shown in FIG. 3, and is best detected by the optical receiver from the four modulation methods according to the frequency (signal band) of the input modulated signal. The modulation method to be performed is selected and output to any of the corresponding paths, that is, the path from the branching unit 3, the path from the adjusting unit 4, the path from the adjusting unit 5, and the path from the branching unit 6. With this configuration, one optical modulator can be used to transmit a signal obtained by synthesizing a plurality of types of optical modulation signals, and deterioration of the signal detection level by the optical receiver of each optical modulation signal can be suppressed.

The optical transmitter 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 optical receiver, for example, the frequency response characteristic (FIG. 3) corresponding to the transmission distance may be provided in the distribution unit 2. Further, the optical transmission device 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 an optical signal to a plurality of optical receivers, and the transmission distance to each optical receiver differs for each optical receiver. 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 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 in the configuration. 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 portion 3 to the DEMZM10 (excluding the DEMZM10) constitutes the first processing portion. Further, the path from the branch portion 6 to the DEMZM10 (excluding the DEMZM10) constitutes the second processing portion. Further, the path from the adjusting unit 4 to the DEMZM10 (excluding the DEMZM10) constitutes the third processing unit. Further, the path from the adjusting unit 5 to the DEMZM10 (excluding the DEMZM10) constitutes the 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. The distribution unit 2 processes the input modulated signal in the first processing unit, the second processing unit, and the third processing according to the user setting or according to the frequency band of the input modulated signal and its transmission distance. Output to either the unit or the fourth processing unit.

Further, in the configuration of FIG. 1, the distribution unit 2, the branch unit 3, the adjustment unit 4, the adjustment unit 5, the branch unit 6, the reversing unit 7, the combiner unit 8, and the combiner unit 9 are drive units for driving the DEMZM 10. Consists of. Then, this drive unit outputs the DEMZM10 based on the modulated signal so as to output the optical modulated signal of the chirp parameter selected according to the frequency band of the input modulated signal and the transmission distance of the modulated signal. It is configured to drive. Alternatively, the drive unit holds the set values of the chirp parameters associated with each of the plurality of modulated signals having different bands, which are input from the frequency conversion unit 1, and for each of the plurality of modulated signals, the associated chirp. It is configured to drive the DEMZM10 so as to output a parameter optical modulation signal.

3, 6: Branch part, 4, 5: Adjustment part, 7: Inversion part, 8, 9: Combined part, 10: Two-electrode Mach Zenda modulator

Claims (9)

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. An optical modulation means that generates a phase-modulated optical signal, synthesizes the first phase-modulated optical signal and the second phase-modulated optical signal, and outputs the signal.
The input electric signal is branched into a first electric signal and a second electric signal, the first electric signal is input to the first port, and the third electric signal obtained by inverting the second electric signal is the first electric signal. The first processing means to input to port 2 and
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 and
A third processing means for inputting an input electric signal to the first port, and
A fourth processing means for inputting an input electric signal to the second port, and
An optical transmitter characterized by being equipped with. The optical transmission device according to claim 1, wherein the optical modulation means branches the input continuous light into two to generate the first continuous light and the second continuous light. The claim is characterized in that when the same signal is input to the first port and the second port, the phase difference between the first phase modulated optical signal and the second phase modulated optical signal is π / 2. The optical transmitter according to 1 or 2. The optical transmission device according to any one of claims 1 to 3, wherein the optical modulation means is a two-electrode Machzenda modulator. It is further provided with a sorting means for distributing the input electric signal as an input of any 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 are input to the sorting means.
A claim characterized in that 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 5. The optical transmitter according to item 5. The distribution means uses the first processing means, the second processing means, and the second processing means for processing the electric signal based on the band of the electric signal and the distance for transmitting the electric signal for each of the plurality of electric signals. The optical transmission device according to claim 6, wherein it is determined which of the third processing means and the fourth processing means is used as the input. The optical transmission device according to claim 6 or 7, further comprising a conversion means that performs frequency conversion so that the bands of the plurality of electric signals are different from each other and outputs the frequency conversion to the distribution means. The optical transmission according to any one of claims 1 to 8, wherein the third processing means and the fourth processing means each have an adjusting means for adjusting the amplitude of the input electric signal. apparatus.

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