JP7306652B2 - Optical transmitter and optical communication system - Google Patents

Description

The present invention relates to an optical communication system that performs coherent optical detection and an optical transmission device for the optical communication system.

Coherent optical detection technology and polarization multiplexing technology are used to cope with the increase in communication capacity. Normally, four balanced photodetectors, that is, eight photodiodes (PD) are used for coherent photodetection of polarization-multiplexed signal light (hereinafter, polarization-multiplexed light). Non-Patent Document 1 discloses a configuration for reducing the number of PDs used for coherent optical detection of polarization multiplexed light to four. Non-Patent Documents 2 and 3 disclose a configuration in which noise caused by beats between signal lights is removed by digital processing.

SebJ. Savory, "Digital filters for coherent optical receivers", Opt. Express 16, 804-817, 2008 Wei-Ren Peng, et al. , "Spectrally Efficient Direct-Detected OFDM Transmission Incorporating a Tunable Frequency Gap and an Iterative Detection Techniques", J. Am. Lightwave Technol. 27, 5723-5735, 2009 Zhe Li, et al. , "SSBI Mitigation and the Kramers-Kronig Scheme in Single-Sideband Direct-Detection Transmission With Receiver-Based Electronic Dispersion Compensation", J. Am. Lightwave Technol. 35, 1887-1893, 2017

The balanced photodetector uses two PDs to cancel out the noise caused by the beat between the signal lights, and the configuration of Non-Patent Document 1 uses one PD instead of the balanced photodetector. Noise due to beats between signal lights is output. Non-Patent Document 1 states that by making the amplitude (intensity) of the local light stronger than the amplitude of the signal light, noise due to beats between signal lights can be relatively ignored. has an upper limit, and the amplitude of the signal light may be increased by pre-amplification, so there may be cases where noise due to beats between signal lights cannot be ignored.

It is also conceivable to apply the techniques described in Non-Patent Documents 2 and 3 in order to suppress noise due to beats between signal lights, but the processing load of digital signal processing is heavy, and it is not realistic for high-speed transmission. . In addition, a digital signal processing circuit is additionally required to suppress noise due to beats between signal lights.

SUMMARY OF THE INVENTION The present invention provides an optical transmitter and an optical communication system that can eliminate beats between signal lights in an optical receiver.

According to one aspect of the present invention, an optical transmission device includes: first phase modulation means for phase-modulating first continuous light with a first electrical signal to output first modulated light; second phase modulation means for outputting a second modulated light by phase-modulating with a signal; output means for outputting polarization multiplexed light in which the first modulated light and the second modulated light are polarization multiplexed; wherein the first phase modulation means is a Mach-Zehnder modulator that outputs the first modulated light by changing only the phase of the first continuous light according to the amplitude of the first electrical signal; The two-phase modulation means is a Mach-Zehnder modulator that changes only the phase of the second continuous light according to the amplitude of the second electrical signal and outputs the second modulated light, and the first modulated light and the second modulated light are output. The amplitude of the 2-modulated light is characterized by being constant even during transitions between symbols.

According to the present invention, it is possible to remove beats between optical signals in an optical receiver.

FIG. 2 is a configuration diagram of an optical receiving device; 1 is a configuration diagram of an optical transmission device according to an embodiment; FIG. FIG. 4 is an explanatory diagram of optical phase modulation according to one embodiment;

Hereinafter, embodiments will be described in detail with reference to the accompanying drawings. It should be noted that the following embodiments do not limit the invention according to the claims, and not all combinations of features described in the embodiments are essential to the invention. Two or more of the features described in the embodiments may be combined arbitrarily. Also, the same or similar configurations are denoted by the same reference numerals, and redundant explanations are omitted.

FIG. 1 is a configuration diagram of an optical receiver described in Non-Patent Document 1. As shown in FIG. The optical receiver receives polarization multiplexed light generated by polarization multiplexing signal light of a first polarization and signal light of a second polarization orthogonal to the first polarization on the transmission side. A polarization beam splitter (PBS) 20 separates the X-polarized component and the Y-polarized component of the received polarization multiplexed light into X-polarized signal light (hereinafter referred to as signal light X) and X-polarized light, respectively. Y-polarized signal light (hereinafter referred to as signal light Y) orthogonal to the wave is output. The directions of the X-polarized wave and the Y-polarized wave on the receiving side are independent of the directions of the first polarized wave and the second polarized wave on the transmitting side. That is, each of the signal lights X and Y can contain both components of the signal light of the first polarization and the signal light of the second polarization.

The splitter 21 splits the signal light X into two and outputs the signal light X to the multiplexers 23 and 24 . The splitter 22 splits the signal light Y into two and outputs the signal light Y to the multiplexers 25 and 26 . The light source 40 generates continuous light (local light) and outputs it to the splitter 41 . Note that the plane of polarization of the local light is adjusted to be different from that of the X-polarized wave and the Y-polarized wave. That is, the plane of polarization of the local light is adjusted so that the local light has both an X-polarized component and a Y-polarized component. For example, in order to equalize the amplitudes of the X-polarized component and the Y-polarized component of the local light, the plane of polarization of the local light is adjusted to 45 degrees with respect to each of the X-polarized wave and the Y-polarized wave. . The splitter 41 splits the local light into four and outputs two of them to the phase shifters 42 and 43 . Phase shifters 42 and 43 each shift the phase of the local light by π/2 and output the phase-shifted local light.

The multiplexer 23 causes the local light and the signal light X to interfere with each other, and outputs the interference light to the PD 27 . The multiplexer 24 causes the phase-shifted local light and the signal light X to interfere with each other, and outputs the interference light to the PD 28 . The multiplexer 25 causes the local light and the signal light Y to interfere with each other, and outputs the interference light to the PD 29 . The multiplexer 26 causes the phase-shifted local light and the signal light Y to interfere with each other, and outputs the interference light to the PD 30 .

Each of the PDs 27 to 30 photoelectrically converts the input interference light and outputs an electric signal. Since the phases of the local light contained in the interference light input to the PD 27 and the local light contained in the interference light input to the PD 28 differ by π/2, the electric signal output from the PD 27 is the real component of the signal light X ( XI), the electrical signal output from the PD 28 is the imaginary number component (XQ) of the signal light X. Similarly, if the electrical signal output by the PD 29 is the real component (YI) of the signal light Y, the electrical signal output by the PD 30 is the imaginary component (YQ) of the signal light Y. By processing the electrical signals output from each of the PD27 to PD30 in the subsequent MIMO processing unit, the optical receiver demodulates the information carried by the signal light of the first polarization and the signal light of the second polarization. It can be performed.

In an optical receiver using a conventional balanced photodetector, the PDs 27 to 30 in FIG. 1 are each replaced with a balanced photodetector having two PDs. Each PD of the balanced photodetector has a square component of the signal light (a beat component between the signal lights), a square component of the local light (a beat component between the local lights), and a product component of the signal light and the local light. (Beat component of local light and signal light) and is output, but since the difference between the electrical signals output by the two PDs is taken, the beat component between signal light and the beat component between local light are canceled. Only beat components of signal light and local light necessary for demodulation are output. However, in the optical receiver shown in FIG. 1, the balanced photodetector is replaced with one PD, so the electrical signals output from the PDs 27 to 30 are the signal light and the beat component of the local light necessary for demodulation. , contain beat components between signal lights and beat components between local lights that become noise.

Here, the intensity of the local light can be controlled to be constant in the optical receiver. In this case, the beat component between the local lights becomes a DC component, and the optical receiver can easily remove the beat component between the local lights. However, when the amplitude of the signal light changes with time, the beat component between the signal lights does not become a DC component and may affect demodulation. For this reason, in this embodiment, the amplitude of the signal light is kept constant in the optical transmitter, so that the beat component between the signal lights is converted to a DC component, so that the beat component between the signal lights can be removed in the optical receiver. to

FIG. 2 is a configuration diagram of the optical transmission device according to this embodiment. The light source 10 generates continuous light and outputs it to the PBS 11 . The PBS 11 depolarizes the continuous light, inputs X-polarized continuous light to the optical phase modulator 12 , and inputs Y-polarized continuous light orthogonal to the X-polarized wave to the optical phase modulator 13 . In order to equalize the amplitude of the X-polarized continuous light and the Y-polarized continuous light, the plane of polarization of the continuous light generated by the light source 10 is set at 45 degrees with respect to each of the X-polarized wave and the Y-polarized wave. adjusted to

The optical phase modulator 12 phase-modulates X-polarized continuous light with an electrical signal corresponding to information to be transmitted, and outputs X-polarized modulated light. Similarly, the optical phase modulator 13 phase-modulates Y-polarized continuous light with an electrical signal corresponding to information to be transmitted, and outputs Y-polarized modulated light. A polarization beam combiner (PBC) 14 polarization-multiplexes the X-polarized modulated light and the Y-polarized modulated light, and outputs the polarization-multiplexed light to an optical transmission line.

The optical phase modulators 12 and 13 are, for example, Mach-Zehnder modulators, and adjust the phase of passing continuous light according to the amplitude of the electrical signal. FIG. 3 is an explanatory diagram of phase modulation by the optical phase modulators 12 and 13. FIG. As shown in FIG. 3, the optical phase modulators 12 and 13 change only the phase of the continuous light 50 while keeping the amplitude constant. That is, when the modulated light output from the optical phase modulators 12 and 13 is displayed on the complex plane, it moves only on the dotted circle in FIG. When phase modulation is performed using an IQ modulator as in the conventional art, the amplitude changes at the transition between symbols. In order to keep the amplitude of the modulated light constant, the beat component between the signal lights contained in the electrical signals output from the PDs 27 to 30 of the optical receiver becomes a DC component, and the optical receiver adjusts the beat between the local lights and between the signal lights. Components can be easily removed to demodulate polarization multiplexed light.

In the optical transmission device of this embodiment, the local light generated by the light source 10 is polarization-separated to generate X-polarized and Y-polarized continuous light, but the method of polarization multiplexing is not limited to this. . For example, the local light generated by the light source 10 is split into two with equal amplitude, each of which is phase-modulated by an optical phase modulator, and the modulated light output from one of the two phase modulators is a half-wave plate. For example, the plane of polarization may be rotated by 90 degrees and combined. Further, by splitting the local light generated by the light source 10 into two with equal amplitude and rotating the plane of polarization of one of the local lights by 90 degrees with a half-wave plate or the like, two local lights whose planes of polarization are orthogonal to each other are generated. The configuration may be such that

Further, according to the present invention, an optical communication system including the optical transmitter shown in FIG. 2 and the optical receiver shown in FIG. 1 is provided. As described above, in order to keep the amplitude of the signal light constant, the optical transmitter uses the optical receiver that uses only four PDs for coherent optical detection, as shown in FIG. can be easily removed and demodulated with high accuracy.

The invention is not limited to the above embodiments, and various modifications and changes are possible within the scope of the invention.

10: light source, 11: optical beam splitter, 12, 13: optical phase modulator, 14: optical beam combiner

Claims (3)

a first phase modulation means for outputting a first modulated light by phase-modulating the first continuous light with a first electrical signal;
a second phase modulation means for outputting a second modulated light by phase-modulating the second continuous light with a second electrical signal;
output means for outputting polarization multiplexed light obtained by polarization multiplexing the first modulated light and the second modulated light,
The first phase modulating means is a Mach-Zehnder modulator that changes only the phase of the first continuous light according to the amplitude of the first electrical signal and outputs the first modulated light,
The second phase modulating means is a Mach-Zehnder modulator that changes only the phase of the second continuous light according to the amplitude of the second electrical signal and outputs the second modulated light,
An optical transmission apparatus, wherein the amplitudes of the first modulated light and the second modulated light are constant even during transitions between symbols. 2. The optical transmitter according to claim 1, wherein the planes of polarization of said first continuous light and said second continuous light are orthogonal to each other. An optical communication system comprising the optical transmitting device according to claim 1 or 2 and an optical receiving device,
The optical receiver is
demultiplexing means for polarization demultiplexing the polarization multiplexed light from the optical transmission device into a first signal light of a first polarization and a second signal light of a second polarization orthogonal to the first polarization;
generating means for generating a first local light having a component of the first polarization and a second local light having a component of the second polarization;
a first interfering means for outputting a first interference light resulting from interference between the first signal light and the first local light;
a second interfering means for outputting a second interference light resulting from interference between the first signal light and light obtained by shifting the phase of the first local light by π/2;
a third interference means for outputting a third interference light obtained by causing the second signal light and the second local light to interfere with each other;
a fourth interfering means for outputting fourth interference light obtained by interfering the second signal light with the light obtained by shifting the phase of the second local light by π/2;
a first photoelectric conversion means for performing photoelectric conversion of the first interference light;
a second photoelectric conversion means for performing photoelectric conversion of the second interference light;
a third photoelectric conversion means for performing photoelectric conversion of the third interference light;
a fourth photoelectric conversion means for photoelectrically converting the fourth interference light;
An optical communication system comprising :

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