JP5182154B2 - Optical communication system - Google Patents

Description

  The present invention relates to a technique for reducing fading caused by rotation of a polarization plane of an optical carrier wave in an optical communication system using a direct reception system.

  Orthogonal frequency division multiplexing (OFDM) technology is a method of transmitting transmission data in parallel using a plurality of subcarriers. Since the symbol rate of each subcarrier is relatively low, it is resistant to intersymbol interference, and digital terrestrial broadcasting In addition, it is already used in a wireless local area network (LAN) system, and its application to an optical communication system is also being studied (for example, see Non-Patent Document 1).

  Optical OFDM communication can be classified into a coherent system and a direct reception system by processing in the optical receiver. The coherent method is a method in which a local light source is provided in the optical receiving device, and a signal obtained by interfering the local light and the received optical signal is converted into an electric signal by a photo diode. The direct receiving method is a photodiode of the optical receiving device. In this method, the received optical signal is directly converted into an electrical signal, and more specifically, the optical carrier wave in the received optical signal and the interference signal of the optical signal corresponding to information are converted into an electrical signal.

  The coherent method is superior to the direct reception method in terms of signal characteristics such as signal-to-noise ratio, but it requires polarization control in the optical receiver in addition to components that cause interference between the local light and the received optical signal. Therefore, the device cost is increased. For this reason, in an optical communication system that does not require long-distance transmission, a direct reception method with low apparatus cost is advantageous.

  In the direct reception system, the optical transmission apparatus modulates the optical carrier 50 with the OFDM signal, so that the optical carrier 50 and the optical OFDM signal 51 that is a sideband corresponding to the information, as shown in FIG. The optical signal containing is output. Since the optical OFDM signal 51 is generated as a result of modulating the optical carrier wave 50, its polarization plane coincides with the polarization plane of the optical carrier wave 50. However, due to the influence of the polarization mode dispersion of the optical transmission line, the polarization plane of the optical carrier 50 and the polarization plane of the optical OFDM signal 51 included in the optical signal received by the optical receiver are as shown in FIG. Therefore, a deviation occurs. Furthermore, since the polarization mode dispersion of the optical transmission line changes with time, the deviation between the polarization plane of the optical carrier 50 and the polarization plane of the optical OFDM signal 51 also varies with time.

  Here, in the direct reception method, an interference signal between the optical OFDM signal 51 and the polarization direction component of the optical OFDM signal 51 of the optical carrier 50 is converted into an electric signal by the photodiode of the optical receiver. . That is, the deviation between the polarization plane of the optical carrier 50 and the polarization plane of the optical OFDM signal 51 varies with time. When the polarization plane of the optical OFDM signal 51 is used as a reference, the amplitude of the optical carrier 50 that is the interference partner is the time. In other words, this causes the fading of the electric signal to be obtained, resulting in a problem that stable signal characteristics cannot be obtained.

  For this reason, a configuration has been proposed in which the signal is split into two by a polarization separator in the optical receiver, each of which is individually converted into an electrical signal, and then the received signal is combined to avoid the influence of fading (for example, (See Non-Patent Document 2)

Arthur James Lowry, et al. , “Orthogonal-frequency-division multiplexing for dispersal compensation of long-haul optical systems”, 2006 Optical Society of America ETS E No. 14 6, March 2006 Markus Mayrock, et al. "PMD Tolerant Direct-Detection Optical OFDM System", ECOC 2007, 8.2.5, September 2007.

  However, in the prior art described in Non-Patent Document 2, etc., photoelectric conversion processing and demodulation processing have to be performed for each route after branching, resulting in a complicated configuration and high cost. Further, in the prior art of Non-Patent Document 2, it is necessary to control the polarization plane of the optical carrier wave of the input optical signal to the polarization separator. However, since the polarization plane changes with time, the polarization that follows the change. Control is required, and its realization is difficult.

  Therefore, the present invention compares fading caused by rotation of the polarization plane of an optical carrier with respect to the polarization plane of an optical signal carrying information, in an optical communication system using a direct reception method. An object of the present invention is to provide an optical communication system that can be suppressed easily and at low cost.

According to the optical communication system of the present invention,
An optical communication system comprising an optical transmission device and an optical reception device that converts a received optical signal received from the optical transmission device into an electrical signal by a direct reception method, wherein the optical transmission device is a first optical carrier wave. An optical signal including a second optical carrier wave having the same frequency as that of the first optical carrier wave and a sideband signal having the same polarization plane as that of the first optical carrier wave is transmitted.

According to another aspect of the optical communication system of the present invention,
The optical transmission device includes a demultiplexing unit that branches the continuous light into a first continuous light and a second continuous light, a modulation unit that modulates the first continuous light, an output optical signal of the modulation unit, and a second continuous light. It is also preferable to include polarization combining means for receiving light, rotating one polarization plane by 90 degrees, and combining and outputting the other optical signal.

According to another aspect of the optical communication system of the present invention,
An optical transmission device includes a light source that generates continuous light, and a modulation unit that modulates a component in a first direction of polarization of the continuous light and does not modulate a component in a second direction orthogonal to the first direction. The polarization direction of the continuous light is preferably set to be 45 degrees with respect to the first direction.

  By generating and transmitting an optical signal including two optical carriers whose polarization planes are orthogonal at the same frequency, fading can be suppressed without performing special processing on the optical receiver side.

It is a block diagram of the optical transmitter by this invention. It is a figure which shows the outline of the optical spectrum of the optical signal which each part of the optical transmitter by this invention outputs. It is a figure explaining the optical communication system by this invention. It is a figure explaining other embodiment of the optical communication system by this invention. It is a figure explaining the problem in a direct reception system.

  EMBODIMENT OF THE INVENTION The form for implementing this invention is demonstrated in detail below using drawing.

  FIG. 1 is a configuration diagram of an optical transmission apparatus according to the present invention. According to FIG. 1, the optical transmission apparatus includes a light source 1, a duplexer 2, an optical modulator 3, and a polarization beam combiner 4. The light source 1 generates and outputs continuous light, and the duplexer 2 divides the continuous light into two and outputs it from the respective output terminals. The optical modulator 3 is a modulator that modulates one output signal of the duplexer 2 with an electrical signal that has been subjected to OFDM modulation. As shown in FIG. 1, the other output signal of the duplexer 2 and the signal modulated by the modulator 3 are respectively input to a polarization beam combiner 4, and the polarization beam combiner 4 receives one input signal. Is rotated by 90 degrees, and then combined with the other input signal that does not change the polarization plane and output.

  FIG. 2 schematically shows the optical spectrum of the optical signal output from the duplexer 2, the optical modulator 3 and the polarization beam combiner 4 in FIG. FIG. 2A shows an optical spectrum of the output signal of the optical modulator 3. Reference numeral 60 is an optical carrier wave, and reference numeral 62 is an optical OFDM signal. As shown in FIG. 2A, in the present embodiment, the optical OFDM signal 62 corresponding to the transmission data exists only on either side of the optical carrier wave 60, in this embodiment, on the higher frequency side. A predetermined unused band is provided between the optical carrier 60 and the optical carrier 60. That is, the optical OFDM signal 62 uses only subcarriers having a frequency higher than the frequency equal to the unused band.

  FIG. 2B shows a spectrum of an optical signal directly input from the duplexer 2 to the polarization beam combiner 4 and includes only the optical carrier 61 corresponding to continuous light. Since the optical carriers 60 and 61 are obtained by branching continuous light from the same light source 1, the polarization planes of the optical carriers 60 and 61 and the optical OFDM signal 62 are one in the stage before the polarization beam combiner 4. I'm doing it.

  Since the polarization beam combiner 4 rotates the polarization plane of one input signal by 90 degrees and combines it with the other input signal, the optical spectrum of the output signal of the polarization beam combiner 4 is shown in FIG. Similarly, the optical carrier 60 and the optical OFDM signal 62 whose polarization planes coincide with each other, and the optical carrier 61 having a polarization plane orthogonal to the polarization planes of these optical signals are included.

  The optical transmission apparatus according to the present invention transmits an optical signal having the optical spectrum shown in FIG. 2C to the optical transmission line. Since this optical signal is affected by the polarization mode dispersion in the optical transmission line, the optical reception is performed. At the position of the apparatus, a shift that varies with time occurs in the polarization planes of the optical carrier 60 and the optical OFDM signal 62. However, in the present invention, since transmission is performed by adding an optical carrier 61 orthogonal to the optical carrier 60 on the transmission side, the polarization of the optical OFDM signal 62 of each optical carrier generated when the polarization plane rotates. The fluctuation of the direction component is smaller than when only the optical carrier wave 61 is used, and therefore fading caused by fluctuation of the rotation amount of the polarization plane can be suppressed.

  FIG. 3 is a diagram for explaining that fading can be suppressed according to the present invention. In FIG. 3, reference numeral 70 denotes the direction of the polarization plane of the optical OFDM signal 62. Due to polarization mode dispersion, an angle difference of θ occurs between the polarization planes of the optical carrier 60 and the optical OFDM signal 62 at the position of the receiving apparatus. ing. Since the transmission losses in the optical transmission lines of the optical carriers 60 and 61 are almost the same, if both the amplitudes of the optical carriers 60 and 61 are A, the optical carrier 60 interferes with the optical OFDM signal 62 and becomes an electric signal. The component that contributes to the conversion is | A cos θ |, and the component that interferes with the optical OFDM signal 62 of the optical carrier 61 and contributes to the conversion to the electrical signal is | A sin θ |. | A cos θ | and | A sin θ | are complementary to each other, that is, when one is small, the other is large, and when one is small, the other is large, so that fading is suppressed.

  As described above, fading can be suppressed without performing any special processing on the optical receiving device side by generating and transmitting an optical signal including two optical carriers whose polarization planes are orthogonal at the same frequency on the transmitting side. It becomes possible. The optical components added to the optical transmitter according to the present invention are only the duplexer 2 and the polarization synthesizer 4, so that it is not necessary to control the complicated polarization plane as in the prior art, thus increasing the cost. Without fading.

Next, another embodiment of the present invention will be described with reference to FIG. A LiNbO ₃ optical modulator mainly used as an optical modulator has a reference polarization plane 71, and the LiNbO ₃ optical modulator has only a component that matches the reference polarization plane 71 in the input continuous light 73. And the component in the direction of polarization plane 72 orthogonal to the reference polarization plane is output without modulation. As shown in FIG. 4, by setting the polarization plane of the continuous light 73 to 45 degrees with respect to the reference polarization plane of the optical modulator, an optical signal having the same optical spectrum as in FIG. As described above, fading can be suppressed.

DESCRIPTION OF SYMBOLS 1 Light source 2 Demultiplexer 3 Optical modulator 4 Polarization combiner 50, 60, 61 Optical carrier 51, 62 Optical OFDM signal 70 Polarization plane of optical OFDM signal 71 Reference polarization plane of modulator 72 Reference polarization plane of modulator Orthogonal polarization plane 73 Polarization plane of continuous light

Claims (3)

An optical transmitter;
An optical receiver that converts a received optical signal received from the optical transmitter into an electrical signal by a direct reception method;
An optical communication system comprising:
The optical transmitter
Transmits the optical signal including a first optical carrier, the first second optical carrier and the first sideband signal of the optical carrier of the same polarization plane of polarization in the optical carrier of the same frequency are orthogonal,
The optical receiver includes one receiver and does not perform conversion from phase modulation to intensity modulation.
Optical communication system. The optical transmitter
Branching means for branching continuous light into first continuous light and second continuous light;
Modulation means for modulating the first continuous light;
A polarization beam combining means that receives the output optical signal of the modulation means and the second continuous light as input, rotates one polarization plane by 90 degrees, and combines and outputs the other optical signal;
The optical communication system according to claim 1, comprising: The optical transmitter
A light source that generates continuous light;
Modulation means for modulating the component in the first direction of the polarization of the continuous light and not modulating the component in the second direction orthogonal to the first direction;
With
The direction of polarization of the continuous light is set to be 45 degrees with respect to the first direction.
The optical communication system according to claim 1.

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