JP6042250B2 - Optical receiver - Google Patents

Description

  The present invention relates to a quality improvement technique in an optical communication system.

  Currently, optical communication systems are moving from conventional intensity modulation or on / off modulation (OOK) to more complex multilevel modulation schemes such as quadrature phase modulation (QAM). Furthermore, an optical OFDM system that intensity-modulates continuous light with an orthogonal frequency division multiplexing (OFDM) modulated baseband signal has also been proposed. Patent Document 1 discloses an optical modulator used in such multilevel modulation or optical OFDM modulation. Patent Document 2 discloses a coherent receiving apparatus for receiving light subjected to multilevel modulation or optical OFDM modulation.

JP-T-2004-516743 JP2011-199687A

  The optical modulator uses, as its basic constituent element, a Mach-Zehnder interferometer that takes continuous light as input and changes the intensity and phase of light output in accordance with an applied voltage applied to a terminal. FIG. 1 is a diagram illustrating the operating principle of a Maha-Zehnder interferometer. FIG. 1A is a diagram illustrating the relationship between the applied voltage and the intensity and phase of the output light. FIG. 1A shows that when the voltage applied to the terminal is A, the phase of the output light is 0 and the intensity is maximum. Further, when the applied voltage is −A, the phase of the output light is π, and the intensity is maximum. Therefore, for example, when the applied voltage is zero, the Maha-Zehnder interferometer is set so as to operate at the reference point indicated by the black square, and the applied voltage is determined according to whether the transmission data is 1 or 0. By inputting A or -A to, a PSK modulated optical signal can be output. FIG. 1B shows the positions on the IQ plane corresponding to the white circles, black circles, and squares as reference points in FIG. 1A with the same symbols. The reference point can be adjusted by a bias voltage input to the optical modulator.

  In the optical modulator, two Maha-Zehnder interferometers whose principles are shown in FIG. 1A are provided for the in-phase (I) component and the quadrature (Q) component, and the input continuous-image light is branched to each Mah-Zehnder. The signal is input to the interferometer and modulated, and the phase of the output light of the quadrature component Mach-Zehnder interferometer is shifted by π / 2, and is combined with the output light of the in-phase component Mach-Zehnder interferometer. Note that, from the characteristics shown in FIG. 1A, the intensity of the output light also changes depending on the applied voltage. Therefore, the optical modulator can be used for QAM modulation in which information is also placed on amplitude and optical OFDM modulation.

  Here, a baseband signal when performing optical OFDM modulation or multilevel modulation generally has a large peak-to-average power ratio (PAPR). For this reason, it is necessary to operate the optical modulator so that the maximum amplitude of the baseband signal becomes the maximum value of the light intensity in FIG. 1A. In this case, the intensity of many parts of the optical signal is small. turn into. That is, the average output power of the optical modulator is reduced. For this reason, the transmitted optical signal is likely to deteriorate.

The present invention is to provide a light receiving device that can improve the communication quality.

  According to an aspect of the present invention, the optical receiver includes a receiving unit that receives the modulated light and outputs an electrical signal having a change in amplitude corresponding to a change in the intensity of the modulated light, and the amplitude of the electrical signal is maximum. The electric signal reaches the maximum value by reversing the amplitude of the period from when the electric signal reaches the maximum value until the next maximum value is reached with respect to the maximum value. And converting means for outputting the converted electric signal by executing every other period until the next maximum value is reached.

  The resistance to noise is increased and the communication quality can be improved.

Explanatory drawing of operation | movement of an optical modulator. Explanatory drawing of operation | movement of the modulation | alteration part of the optical transmitter by one Embodiment. Explanatory drawing of the optical signal which the optical transmitter by one Embodiment transmits. 1 is a schematic configuration diagram of an optical transmitter and an optical receiver according to an embodiment. FIG.

  Hereinafter, exemplary embodiments of the present invention will be described with reference to the drawings. In the following drawings, components that are not necessary for the description of the embodiments are omitted from the drawings.

  FIG. 4A is a schematic configuration diagram of the optical transmission device 500 according to the present embodiment. The modulation unit 51 includes the optical modulator described above, and modulates continuous light with an electric signal corresponding to information to be transmitted, which is a baseband signal, for example, and outputs modulated light. In the case of orthogonal modulation, continuous light is branched, each continuous light is modulated with two electrical signals, and the phase of one modulated light is shifted by π / 2 and combined with the other modulated light. In the following description, only one electric signal is used to simplify the description. However, the number of electrical signals is not a problem in the present invention.

  FIG. 2 is an explanatory diagram of the operation of the modulation unit 51 of the optical transmission device 500 according to the present embodiment. As shown in FIG. 2, in the present embodiment, the bias voltage of the modulation unit 51 is set so that the point where the intensity of the output light of the modulation unit 51 is maximum becomes the reference point. That is, the intensity of the output light from the modulation unit 51 is set to the maximum when the amplitude of the electric signal is zero. In FIG. 2, the position having the maximum intensity in the phase 0 region is used as the reference point, but the point having the maximum intensity in the phase π region may be used as the reference point. In the present embodiment, the range where the light intensity does not become zero with the reference point as the center is set as the operation range of the modulation unit 51.

  FIG. 3 is an explanatory diagram of the modulated light output from the modulation unit 51 according to the present embodiment. FIG. 3A shows an electrical signal input to the modulation unit 51. As described above, the modulation unit 51 is set so that light of maximum intensity is output when the amplitude of the electrical signal shown in FIG. Therefore, according to the characteristics shown in FIG. 2, the intensity of the modulated light decreases regardless of whether the amplitude of the electric signal increases from zero or decreases from zero. FIG. 3B shows the intensity waveform of the modulated light obtained by modulating the continuous light with the electric signal of FIG. Note that the positions indicated by reference numerals 31, 32, 33, 34, 35, and 36 in FIG. 3B correspond to the positions indicated by reference numerals 21, 22, 23, 24, 25, and 26 in FIG. 3A, respectively. doing.

  As shown in FIG. 3B, the intensity waveform of the modulated light is obtained by folding a portion of the electric signal waveform whose amplitude, which is a reference value, is zero or more with respect to the reference value. Therefore, it can be seen that the intensity of the modulated light decreases as the absolute value of the amplitude of the electrical signal increases. Here, a large peak-to-average power ratio indicates that the absolute value of the amplitude of the electrical signal is small on average and the absolute value of the amplitude of the electrical signal is rarely large. Therefore, when the peak-to-average power ratio of the electrical signal is large, the intensity of the modulated light output from the modulation unit 51 becomes high on average and rarely low, contrary to the normal modulation method. Therefore, the average output power of the modulated light is also increased. This means that resistance to noise and the like is increased, and thus communication quality is improved.

  FIG. 4B shows a schematic configuration diagram of the optical receiver 600 according to the present embodiment. For example, the receiving unit 61 performs coherent reception, and outputs an electric signal whose amplitude changes according to a change in the intensity of the received light to the converting unit 62. For example, when the optical signal illustrated in FIG. 3B is received, the receiving unit 61 outputs the electrical signal having the waveform illustrated in FIG. The electric signal having the waveform shown in FIG. 3 (B) is a signal obtained by turning back a portion where the amplitude, which is the reference value of the original waveform shown in FIG. 3 (A), is zero or more so as to be zero or less with respect to the reference value. It is. The converter 62 loops back every other period from the maximum value of the amplitude of the electrical signal corresponding to the waveform shown in FIG. 3B to the next maximum value, and restores the waveform shown in FIG. .

  Specifically, the electrical signal having the waveform shown in FIG. 3B is sampled, for example, and the sampling value is stored in the memory. Subsequently, the time when the sampling value reaches the maximum value is determined in order of time. In order to determine whether the maximum value has been reached, a value obtained by subtracting a predetermined value from the theoretical maximum value is used as a threshold value. When the sampling value exceeds this threshold value, it is determined that the sampling value has reached the maximum value. Note that the reason why the predetermined value is reduced is to cope with an error.

  In the case of the waveform shown in FIG. 3B, it is determined that the maximum value is reached at positions 31, 32, 33, 34, 35 and 36. The conversion unit 62 inverts the sampling value between the code 31 where the sampling value reaches the maximum value first and the code 32 where the sampling value reaches the maximum value second, with respect to the maximum value. On the other hand, the sampling value between the code 32 where the sampling value reaches the maximum value second and the code 33 where the sampling value reaches the maximum value third is left as it is. Similarly, the sampling value between the code 33 at which the sampling value reaches the maximum value third and the code 34 at which the sampling value reaches the maximum value fourth is inverted with respect to the maximum value, and the sampling value is fourth. The sampling value between the reference numeral 34 that reaches the maximum value and the reference numeral 35 that reaches the fifth maximum sampling value is left as it is. Further, the sampling value between the code 35 where the sampling value reaches the maximum value fifth and the code 36 where the sampling value reaches the maximum value sixth is reversed with respect to the maximum value. Then, at the maximum value before inversion, the sample value after inversion is shifted so that the amplitude of the baseband signal becomes zero, which is the reference value. Finally, the converter 62 outputs the electrical signal shown in FIG. 3A to the demodulator 63 based on the shifted sampling value, and the demodulator 63 demodulates the received electrical signal. In addition, it can also be set as the structure reversed after shifting previously.

  In the above-described embodiment, the period from when the maximum value is reached oddly to when the maximum value is reached is reversed. In this case, the conversion unit 62 may output an electric signal having a waveform that is obtained by completely inverting the waveform shown in FIG. However, since the demodulation process in the demodulation unit 63 is performed by detecting the reference phase, this does not affect the demodulation process in the demodulation unit 63. Therefore, it may be a form in which the time between reaching the maximum value evenly and then reaching the maximum value is reversed.

  In the above embodiment, the modulation unit 51 is set so that the reference value of the amplitude of the electric signal is zero and the intensity of the modulated light is maximized at this time. However, for example, an intermediate value between the maximum value and the minimum value of the amplitude of the electric signal can be used as the reference value. Further, it can be set so that the intensity of light output from the modulation unit 51 is maximized when no electrical signal is input to the modulation unit 51. Furthermore, any amplitude other than the maximum absolute value of the amplitude of the electric signal can be used as the reference value. In any case, when the amplitude of the electrical signal is the reference value, the modulation unit 51 outputs an optical signal having the maximum intensity, and as the difference between the amplitude of the electrical signal and the reference value increases, the modulation unit 51 increases its strength. Is reduced to output an optical signal. With this configuration, the average power of the modulated light can be increased and the signal quality can be improved as compared with the conventional modulation method.

Claims (4)

Receiving means for receiving modulated light and outputting an electrical signal having a change in amplitude corresponding to a change in intensity of the modulated light;
Determining whether the amplitude of the electrical signal has reached a maximum value, and reversing the amplitude of the period from when the electrical signal reaches the maximum value until the maximum value is reached next to the maximum value, Conversion means for executing every other period from the time when the electric signal reaches the maximum value until the time when the electric signal reaches the maximum value, and outputting the converted electric signal;
An optical receiver characterized by comprising: The optical receiving apparatus according to claim 1 , wherein the converting unit converts the maximum value to be a predetermined value of the amplitude of the converted electric signal. The optical receiver according to claim 2 , wherein the predetermined value is zero. And the converting means, the amplitude of the electrical signal, when the threshold is exceeded by subtracting the predetermined value from the maximum value, any one of claims 1 to 3, characterized in that determined to have reached the maximum value The optical receiver according to item 1.

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