WO2013058276A1 - 予等化光送信機および予等化光送信方法 - Google Patents
予等化光送信機および予等化光送信方法 Download PDFInfo
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- WO2013058276A1 WO2013058276A1 PCT/JP2012/076831 JP2012076831W WO2013058276A1 WO 2013058276 A1 WO2013058276 A1 WO 2013058276A1 JP 2012076831 W JP2012076831 W JP 2012076831W WO 2013058276 A1 WO2013058276 A1 WO 2013058276A1
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
- H04B10/50—Transmitters
- H04B10/516—Details of coding or modulation
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
- H04B10/25—Arrangements specific to fibre transmission
- H04B10/2507—Arrangements specific to fibre transmission for the reduction or elimination of distortion or dispersion
- H04B10/2513—Arrangements specific to fibre transmission for the reduction or elimination of distortion or dispersion due to chromatic dispersion
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
- H04B10/50—Transmitters
- H04B10/501—Structural aspects
- H04B10/503—Laser transmitters
- H04B10/505—Laser transmitters using external modulation
- H04B10/5057—Laser transmitters using external modulation using a feedback signal generated by analysing the optical output
- H04B10/50572—Laser transmitters using external modulation using a feedback signal generated by analysing the optical output to control the modulating signal amplitude including amplitude distortion
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
- H04B10/50—Transmitters
- H04B10/516—Details of coding or modulation
- H04B10/5161—Combination of different modulation schemes
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
- H04B10/50—Transmitters
- H04B10/516—Details of coding or modulation
- H04B10/5162—Return-to-zero modulation schemes
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B2210/00—Indexing scheme relating to optical transmission systems
- H04B2210/25—Distortion or dispersion compensation
- H04B2210/254—Distortion or dispersion compensation before the transmission line, i.e. pre-compensation
Definitions
- the present invention relates to a pre-equalized optical transmitter and a pre-equalized optical transmission method, and more particularly to a pre-equalized optical transmitter and a pre-equalized optical transmission method for use in optical communication.
- transmission / reception equalization in the electrical domain or the optical domain can be cited as a compensation technique for deterioration of transmission characteristics in a communication path.
- various compensation techniques such as transmission equalization (pre-equalization) and reception equalization are being studied.
- various compensation techniques such as dispersion compensating fibers are being studied.
- the transmission equalization technique in the electric domain is a system that can obtain good characteristics without causing noise enhancement.
- dispersion compensation using dispersion compensation fibers in the optical region the installation cost and installation location of the fiber are generally a problem.
- the advantage of reducing the installation cost and installation location of the dispersion compensation fiber by using pre-equalization There is.
- the pre-equalization transmission method includes an NRZ (Non-Return to Zero) method and an RZ (Return to Zero) method.
- the RZ system is more resistant to intersymbol interference than the NRZ system, and has the characteristics that the average power can be reduced when the maximum amplitude is made equal.
- the RZ method since the use band is increased as compared with the NRZ method, application according to use conditions is desired.
- WDM Widelength Division Multiplexing
- WDM transmission is known as a method for enabling large-capacity optical communication.
- different signals are multiplexed and transmitted using a plurality of wavelengths. Therefore, the transmission power to the optical fiber varies depending on the number of wavelength multiplexing, the modulation method, and the like. Due to this transmission power fluctuation, excess or deficiency in transmission power occurs, which greatly affects transmission quality.
- FIG. 6 shows a schematic diagram.
- the conventional optical communication device shown in FIG. 6 includes a transmission side device a, a reception side device b, and a transmission path c connecting them.
- the transmission side device a includes an optical amplifying unit 1a, an optical filter 2a that allows the output of the optical amplifying unit 1a to pass through, and a VOA 3a at the final stage connected to the optical filter 2a.
- the signal light is transmitted with its transmission power kept constant.
- the reception side device b includes an optical filter 2b that allows the signal light received from the transmission side device a to pass through the transmission path c, and an optical amplification unit 1b connected to the optical filter 2b.
- the reception-side device b passes the received signal light through the optical filter 2b, and adjusts the reception power by the optical amplification unit 1b.
- the conventional optical communication apparatus shown in FIG. 6 is a system that realizes stable communication with a constant transmission power by providing a VOA in the transmission side apparatus as described above.
- pre-equalization transmission it is possible to reduce the installation cost and installation location of the dispersion compensation fiber and to transmit a large capacity.
- pre-equalization transmission it is necessary to adjust the dispersion compensation amount according to the dispersion amount of the transmission path.
- the transmission power for pre-equalization varies greatly depending on the amount of dispersion compensation.
- the pre-equalization signal superimposed on a plurality of wavelengths adjusts the dispersion compensation amount asynchronously, it is conceivable that the combined WDM transmission signal light has a larger instantaneous power fluctuation.
- the WDM transmission method is applied to enable large-capacity optical communication.
- an optical compensation member such as a dispersion compensation fiber
- combining the WDM transmission method and the pre-equalization transmission method has been studied.
- the transmission quality deteriorates due to the transmission power fluctuation that occurs when adjusting the dispersion compensation amount in the pre-equalization transmission. Therefore, a high-performance and expensive optical amplifier and VOA are required to suppress this, and there is a problem that the manufacturing cost increases.
- the present invention was made to solve the problem, and suppresses fluctuations in transmission power by switching the transmission method of pre-equalization transmission, enabling high-quality and large-capacity optical communication.
- An object is to obtain a pre-equalized optical transmitter and a pre-equalized optical transmission method.
- the present invention relates to a pre-equalization optical transmitter that uses both the RZ and NRZ systems as a transmission system, wherein the tap coefficient for generating the RZ pre-equalization signal used in the RZ system and the NRZ used in the NRZ system are used.
- the tap coefficient for generating the pre-equalized signal is stored in advance, and the tap coefficient for generating the RZ pre-equalized signal or the NRZ pre-equalized signal is generated according to the value of the parameter that causes the variation in transmission power.
- a tap coefficient selection unit that selects and outputs one of the tap coefficients, and RZ pre-equalization when the tap coefficient output from the tap coefficient selection unit is a tap coefficient for generating the RZ pre-equalization signal.
- a pre-equalization signal generation unit that generates a signal and generates an NRZ pre-equalization signal when the tap coefficient output from the tap coefficient selection unit is a tap coefficient for generating the NRZ pre-equalization signal;
- a digital-analog converter for converting the RZ pre-equalization signal or the NRZ pre-equalization signal output from the pre-equalization signal generator to an analog signal; and the analog signal output from the digital-analog converter
- a pre-equalized optical transmission signal by modulating the optical signal from the light source based on the amplified analog signal output from the amplifying unit; And a modulation unit for generating the transmission, and the tap coefficient selection unit switches the tap coefficient according to the value of the parameter, thereby switching the transmission method between the RZ method and the NRZ method for transmission.
- a pre-equalized optical transmitter characterized in that it performs.
- the present invention relates to a pre-equalization optical transmitter that uses both the RZ and NRZ systems as a transmission system, wherein the tap coefficient for generating the RZ pre-equalization signal used in the RZ system and the NRZ used in the NRZ system are used.
- the tap coefficient for generating the pre-equalized signal is stored in advance, and the tap coefficient for generating the RZ pre-equalized signal or the NRZ pre-equalized signal is generated according to the value of the parameter that causes the variation in transmission power.
- a tap coefficient selection unit that selects and outputs one of the tap coefficients, and RZ pre-equalization when the tap coefficient output from the tap coefficient selection unit is a tap coefficient for generating the RZ pre-equalization signal.
- a pre-equalization signal generation unit that generates a signal and generates an NRZ pre-equalization signal when the tap coefficient output from the tap coefficient selection unit is a tap coefficient for generating the NRZ pre-equalization signal;
- a digital-analog converter for converting the RZ pre-equalization signal or the NRZ pre-equalization signal output from the pre-equalization signal generator to an analog signal; and the analog signal output from the digital-analog converter
- a pre-equalized optical transmission signal by modulating the optical signal from the light source based on the amplified analog signal output from the amplifying unit;
- a modulation unit for generating the transmission, and the tap coefficient selection unit switches the tap coefficient according to the value of the parameter, thereby switching the transmission method between the RZ method and the NRZ method for transmission.
- the transmission power fluctuation is suppressed by switching the transmission method of pre-equalization transmission, and the manufacturing cost While reducing the installation cost and installation location, to allow for optical communication the realization of large-capacity high-quality.
- the pre-equalization optical transmitter concerning Embodiment 1 of this invention it is explanatory drawing which showed the transmission average power fluctuation
- the pre-equalization optical transmitter which concerns on Embodiment 1 of this invention it is explanatory drawing which showed the average power of the transmission at the time of selecting RZ pre-equalization transmission and NRZ pre-equalization transmission according to wavelength dispersion with the graph. is there.
- FIG. 1 is a configuration diagram showing a configuration of a pre-equalized optical transmitter according to Embodiment 1 of the present invention.
- RZ pre-equalization transmission and NRZ pre-equalization transmission can be selected according to the parameter value that causes the quality degradation of the transmission path.
- the pre-equalized optical transmitter according to the present embodiment can also be applied to WDM transmission, can reduce the installation cost and installation location of the dispersion compensating fiber, and can perform large-capacity optical communication.
- the pre-equalization transmission method can be selected and used from the RZ pre-equalization transmission method and the NRZ pre-equalization transmission method.
- the dispersion compensation amount of chromatic dispersion will be described as an example as a parameter that causes deterioration of the transmission path.
- the present invention is not limited to this case, and other parameters may be used as long as they cause deterioration of the transmission path.
- reference numeral 1 denotes a pre-equalization signal generation circuit (pre-equalization signal generation unit).
- the pre-equalization signal generation circuit 1 generates a RZ pre-equalization waveform or a NRZ pre-equalization waveform pre-equalization signal based on the tap coefficient input from the tap coefficient selection circuit 2, and outputs the pre-equalization signal.
- the output is divided into an I channel digital signal and a Q channel digital signal.
- Reference numeral 2 denotes a tap coefficient selection circuit (tap coefficient selection unit).
- the tap coefficient selection circuit 2 stores in advance a tap coefficient for generating an RZ pre-equalized signal and a tap coefficient for generating an NRZ pre-equalized signal, and sets the value of the dispersion compensation amount of chromatic dispersion that becomes a deterioration factor of the transmission path. In response, any one of those tap coefficients is selected, and the selected tap coefficient is set in the pre-equalization signal generation circuit 1.
- Reference numerals 3 and 4 denote digital-analog converters (digital-analog converters). The digital-analog converters 3 and 4 convert the I-channel digital signal and the Q-channel digital signal output from the pre-equalization signal generation circuit 1 from digital signals to analog signals, respectively.
- Reference numerals 5 and 6 denote drivers (amplifiers).
- the drivers 5 and 6 amplify the outputs from the digital-analog converters 3 and 4 to the input level of the modulator 8.
- Reference numeral 7 denotes a light source that generates light input to generate an optical signal by the modulator.
- Reference numeral 8 denotes a modulator (modulation unit). The modulator 8 generates a pre-equalized optical transmission signal by optically modulating the light from the light source 7 using the signals output from the drivers 5 and 6.
- the modulator 8 is provided with optical modulators 8a and 8b and an optical phase adjuster 8c as shown in FIG.
- the light modulators 8a and 8b are connected in parallel to the light source 7, and the demultiplexed light from the light source 7 is input to generate first and second optical electric fields.
- the optical phase adjuster 8c is connected to the optical modulator 8b and controls the phase difference between the first and second optical electric fields.
- the set phase (relative phase difference between the I channel and the Q channel) of the optical phase adjusting unit 8c is set to ⁇ / 2, for example.
- the demultiplexed light from the light source 7 is optically modulated by the optical modulators 8a and 8b, and the output from the optical modulator 8b is further optically adjusted.
- the phase adjustment is performed by the unit 8c, and the output from the optical modulation unit 8a and the output from the optical phase adjustment unit 8c are combined to generate and output a pre-equalized optical transmission signal.
- the tap coefficient selection circuit 2 uses a tap coefficient (that is, a tap coefficient for generating an RZ pre-equalized signal and a tap coefficient for generating an NRZ pre-equalized signal) as a parameter of a degradation factor of the transmission path.
- a tap coefficient that is, a tap coefficient for generating an RZ pre-equalized signal and a tap coefficient for generating an NRZ pre-equalized signal
- One of the tap coefficients is selected according to the value of the chromatic dispersion dispersion compensation, and the selected tap coefficient is delivered to the pre-equalization signal generation circuit 1.
- the tap coefficients held in advance by the tap coefficient selection circuit 2 are tap coefficients for generating the RZ pre-equalization signal and tap coefficients for generating the NRZ pre-equalization signal, and are adjusted so that the average power fluctuation is alleviated.
- the tap coefficients are switched at the selected switching point. This adjustment will be described later.
- the tap coefficient is delivered from the tap coefficient selection circuit 2 to the pre-equalization signal generation circuit 1 and a data series (or symbol series) is input.
- the pre-equalization signal generation circuit 1 performs pre-equalization processing using the input tap coefficient and the data series (or symbol series), and generates pre-equalization digital signals (I channel digital signal and Q channel digital signal). Generate. Since the pre-equalization process may be a known one, description thereof is omitted here.
- the pre-equalized digital signal is converted into a pre-equalized analog signal (I-channel analog signal and Q-channel analog signal) by the digital-analog converters 3 and 4, and further, the input level of the modulator 8 by the drivers 5 and 6 Amplified until.
- the modulator 8 modulates the light from the light source 7 based on the amplified pre-equalized analog signals from the drivers 5 and 6, thereby generating a pre-equalized optical transmission signal.
- the pre-equalized optical transmission signal generated in this way is transmitted to a pre-equalized optical receiver (not shown) via a transmission line (not shown).
- FIG. 2 shows fluctuations in the average value of transmission power (hereinafter referred to as average power) after modulation by the modulator 8 according to each dispersion compensation amount during pre-equalization transmission.
- the horizontal axis represents the dispersion compensation amount
- the vertical axis represents the average power.
- the solid line 9 indicates the average power fluctuation of the pre-equalization signal in the NRZ system
- the alternate long and short dash line 10 indicates the average power fluctuation of the pre-equalization signal in the RZ system.
- the average power is large when the dispersion compensation amount (the absolute value thereof) of chromatic dispersion is small.
- the dispersion compensation amount is 0 ps / nm (no chromatic dispersion compensation)
- the average power becomes the maximum value.
- the dispersion compensation amount (the absolute value thereof) is large, the impulse response of chromatic dispersion spreads in the time direction, so that the average power of the convolved pre-equalization signal becomes small.
- an absolute value difference of about 9 dB is generated.
- the dispersion compensation amount of chromatic dispersion is described as an example of the parameter of the degradation factor of the transmission path.
- the present invention is not limited to this case, and other parameters may be used. Even when other parameters are used, the NRZ method and the RZ method have characteristics of average power corresponding thereto.
- FIG. 3 shows the wavelength of RZ pre-equalization transmission and NRZ pre-equalization transmission based on the average power characteristics shown in the graph of FIG. 2 in the pre-equalization optical transmitter according to Embodiment 1 of the present invention. It shows the average power of transmission when selected according to the value of dispersion compensation amount of dispersion.
- 11 indicates a transmission method (RZ pre-equalization transmission and NRZ pre-equalization transmission) selected according to the amount of compensation of chromatic dispersion
- a thick line 12 indicates RZ pre-equalization transmission and NRZ pre-equalization transmission. It shows the average power of transmission when switching.
- the dispersion compensation amount (absolute value) is used as the average power 12.
- RZ pre-equalization transmission with a small transmission power is selected in the region R1 with a small value), and NRZ pre-equalization transmission with a large transmission power in the regions R2 and R3 with a large dispersion compensation amount (absolute value thereof).
- an absolute value difference of about 3 dB which is an average power difference can be reduced.
- the region (region R2) from the left end of the graph having the largest absolute value of the dispersion compensation amount to the switching point ch1 is the NRZ method
- the region from the switching point ch1 to the switching point ch2 (region R1) is RZ.
- the NRZ method is selected for the region (region R3) from the switching point ch2 to the right end of the graph with the largest dispersion compensation amount.
- the switching is performed by the tap coefficient selection circuit 2. That is, while the tap coefficient selection circuit 2 outputs the tap coefficient for generating the NRZ pre-equalized signal, the NRZ method is selected. On the other hand, while the tap coefficient selection circuit 2 outputs the tap coefficient for generating the RZ pre-equalization signal, the RZ method is selected.
- the tap coefficient selection circuit 2 performs switching depending on which tap coefficient is output.
- the threshold value is set as ch1 and ch2 in advance for the dispersion compensation value, and when the dispersion compensation value becomes ch1 or ch2, the tap coefficient output is output.
- the dispersion compensation amount at the time when the average power value in the NRZ method becomes the maximum value of the average power in the RZ method may be set as ch1 and ch2.
- a threshold value is set in advance for the average power of the NRZ method, and when the average power value reaches the threshold value, the tap coefficient selection circuit 2 outputs the tap coefficient output to the NRZ pre-equalization signal. It may be switched from the generation to the RZ pre-equalization signal generation.
- the threshold for example, the maximum value of the average power in the RZ method is set as the threshold.
- the transmission method may be switched from NRZ to RZ when the average power value of the NRZ method becomes the same value as the threshold value.
- the attenuation amount of the average power can be adjusted over the entire dispersion compensation amount range.
- the absolute value difference between the minimum value and the maximum value of the average power 12 shown in FIG. 3 is compared with the absolute value difference between the minimum value and the maximum value of the average powers 9 and 10 of the NRZ method and the RZ method shown in FIG. It can be seen that the absolute value difference of the average power 12 is significantly smaller than the absolute value difference of the average powers 9 and 10.
- the tap coefficient selection circuit 2 performs the NRZ pre-equalization signal generation and the RZ pre-equalization signal generation according to the parameter value that causes the quality degradation of the transmission path. Since any one of the tap coefficients is switched and output, the transmission system can be switched between the NRZ system and the RZ system, and the variation of the transmission average power can be reduced as a whole. Specifically, the dispersion compensation amount of chromatic dispersion is used as a parameter that causes degradation of the transmission path, and RZ pre-equalization is performed in a region where the dispersion compensation amount (absolute value thereof) is smaller than a predetermined threshold.
- NRZ pre-equalization transmission with high transmission power is selected, and NRZ pre-equalization transmission with high transmission power is selected in a region where the dispersion compensation amount (absolute value thereof) is a predetermined threshold value or more.
- transmission power fluctuation is reduced by switching and mixing RZ pre-equalization transmission and NRZ pre-equalization transmission according to transmission degradation factors. Therefore, pre-equalization transmission with low load is possible for the optical amplifier and the VOA.
- transmission quality can be kept high by suppressing transmission power fluctuations.
- FIG. FIG. 4 is a block diagram showing the configuration of the pre-equalized optical transmitter according to Embodiment 2 of the present invention.
- the tap coefficient fine adjustment unit 10 is provided outside the pre-equalized optical transmitter.
- the tap coefficient selection circuit 2 stores in advance the tap coefficient for generating the RZ pre-equalization signal and the tap coefficient for generating the NRZ pre-equalization signal, and sets the transmission method to the transmission degradation factor.
- the embodiment has been described in which a change in the average power of transmission is suppressed by switching between the NRZ method and the RZ method according to the parameter value of and using a combination of both transmission methods.
- an external tap coefficient fine adjustment unit is provided before the tap coefficient selection circuit 2 stores the tap coefficient for the RZ pre-equalization waveform and the tap coefficient for the NRZ pre-equalization waveform. These tap coefficients are finely adjusted by 10 and the finely adjusted tap coefficients are transmitted (or downloaded) to the tap coefficient selection circuit 2 and stored.
- the tap coefficient fine adjustment unit 10 performs fine adjustment of the tap coefficient in the area before and after the transmission system switching point.
- the tap coefficient adjustment described in the first embodiment that is, the tap coefficient is used for NRZ pre-equalization wavelength generation. And switching between RZ pre-equalization wavelength generation are referred to as “coarse adjustment” of tap coefficients.
- the “rough adjustment” is intended to alleviate the overall average power fluctuation by the dispersion compensation amount
- the “fine adjustment” is intended to alleviate the average power fluctuation at the switching point between the RZ method and the NRZ method. Since other configurations are the same as those in the first embodiment, description thereof is omitted here.
- the switching point between the RZ pre-equalization transmission and the NRZ pre-equalization transmission (ch1, ch1) In ch2), a power fluctuation with an average power difference of about 3 dB occurs instantaneously. Therefore, as described above, the attenuation amount of the average power can be adjusted only by the “coarse adjustment” of the tap coefficient only for switching between the RZ method and the NRZ method as described in the first embodiment.
- FIG. 5 shows the average transmission power when “fine adjustment” is performed in order to suppress the occurrence of the power fluctuation in the pre-equalized optical transmitter according to the second embodiment of the present invention.
- the average power difference at the switching point between the RZ pre-equalization transmission and the NRZ pre-equalization transmission is finely adjusted by further applying pre-equalization in consideration of the degradation factors of the transmission line other than the dispersion compensation amount. is doing.
- a thick line 13 represents an average power of transmission when the RZ pre-equalization transmission and the NRZ pre-equalization transmission are switched with fine adjustment.
- a region between the left end of the graph with the largest absolute value of the dispersion compensation amount and the switching point ch1 is R2
- a region between the switching point ch2 and the right end of the graph with the largest dispersion compensation amount is R3.
- the fine adjustment switching point ch3 is appropriately set in the region R2
- the fine adjustment switching point ch4 is appropriately set in the region R3, so that the NRZ method at the time of the fine adjustment switching point ch3 is set.
- the tap coefficient is finely adjusted so that the average power changes gently up to the average power value P3 of the NRZ method at the time of the adjustment switching point ch3. That is, the tap coefficient fine adjustment unit 10 performs fine adjustment of the tap coefficient in the area before and after each of the switching points ch1 and ch2 for switching the transmission method, thereby including the switching points ch1 and ch2 as a whole. The average power is adjusted to change gently.
- the amount of change in the dispersion compensation amount from ch3 to ch1 is set to be the same as the amount of change from the time point ch1 to the time when the dispersion compensation amount is zero. Good.
- the change amount of the dispersion compensation amount from ch4 to ch2 is set to be the same as the change amount from ch2 to the time point when the dispersion compensation amount is 0. That's fine.
- Examples of the pre-equalization method that can be applied to the “fine adjustment” of the pre-equalization amount include (1) high-pass filter component (band-limit compensation by optical filter in transmission path, band-limit compensation by transmitter / receiver, narrowing of optical filter) Penalty compensation), (2) IQ constellation rotation, (3) Mitigation of spread of tap coefficient when dispersion compensation amount is large by frequency domain filtering, (4) Tap coefficient by time domain filtering to tap coefficient It is conceivable to reduce the spread, and (5) generate an intermediate waveform between RZ pre-equalization transmission and NRZ pre-equalization transmission.
- the tap coefficient selection circuit 2 may perform only “rough adjustment” by switching the transmission method as shown in FIG. 3, but the pre-equalization by the pre-equalization methods (1) to (5).
- the pre-equalization amount can be adjusted as long as it is a compensation object that can be expressed as a frequency characteristic. It is not limited to the method.
- Fine adjustment of the transmission power is performed based on the high-pass filter component that compensates for the band limitation by the optical filter of the transmission path and / or the band limitation of the transceiver.
- the fine adjustment of the transmission power by the high-pass filter component has a characteristic that a transmission penalty hardly occurs unless an excessive high frequency region is emphasized.
- the amount of pre-equalization increases due to the high-frequency filter component being emphasized by the high-pass filter component. Therefore, a high-pass filter component is added during NRZ pre-equalization transmission and a low-efficiency high-pass filter component is not added or added during RZ pre-equalization transmission. Therefore, the average power fluctuation can be reduced.
- the transmission power is finely adjusted by rotating the IQ constellation. Since pre-equalization transmission involves digital signal processing on the transmission side, a digital-analog converter such as a DAC (Digital-to-Analog-Converter) is required on the transmission side. Therefore, since the maximum amplitude that can be transmitted as an electric signal is limited by the maximum value of the DAC, the average power can be adjusted by rotating the IQ constellation.
- a digital-analog converter such as a DAC (Digital-to-Analog-Converter) is required on the transmission side. Therefore, since the maximum amplitude that can be transmitted as an electric signal is limited by the maximum value of the DAC, the average power can be adjusted by rotating the IQ constellation.
- the transmission power is finely adjusted by providing frequency filtering for limiting the frequency range to be compensated used as the tap coefficient.
- frequency filtering so as to limit the frequency range to be compensated, by removing components other than the components that pass through frequency filtering, it is possible to suppress the expansion of the time domain of the impulse response.
- the transmission average power of the pre-equalized signal after convolution can be adjusted.
- the average power by switching about 3 dB The difference can be mitigated.
- the intermediate waveform for example, a tap coefficient obtained by mixing a tap coefficient for an RZ pre-equalization waveform and a tap coefficient for an NRZ pre-equalization waveform is used.
- the transmission power can be finely adjusted by weighted averaging of tap coefficients for the RZ pre-equalization waveform and tap coefficients for the NRZ pre-equalization waveform.
- the fluctuation of the instantaneous average power is provided by having an area for selecting an intermediate waveform between the RZ pre-equalization waveform and the NRZ pre-equalization waveform before and after the switching points ch1 and ch2 between the RZ method and the NRZ method. Suppress.
- the pre-equalization method as shown in the above (1) to (5) the switching point by “rough adjustment”.
- the tap coefficient is adjusted so as to make the instantaneous fluctuation of the average power at the “fine adjustment” gentle.
- the pre-equalization methods (1) to (5) described above are performed.
- the tap coefficient selection circuit 2 performs the stored RZ pre-equalization waveform tap coefficient and NRZ pre-equalization waveform according to the value of the dispersion compensation amount. One of the tap coefficients is selected and output to the pre-equalization signal generation circuit 1.
- tap coefficients are generated by combining “coarse adjustment” and one or more “fine adjustments” as described above, and are output from the tap coefficient selection circuit 2 to the pre-equalization signal generation circuit 1.
- Which of the above pre-equalization methods (1) to (5) the tap coefficient fine adjustment unit 10 uses may be set at the design stage or stored in the tap coefficient selection circuit 2 When generating the tap coefficient to be performed, the operator (user) may select the configuration appropriately.
- the transmission power becomes larger than that during the pre-equalization transmission.
- the transmission power is large when the chromatic dispersion compensation amount is 0 ps / nm (without chromatic dispersion compensation).
- the transmission waveform fluctuates with almost two values. Therefore, the transmission power fluctuation can be further reduced by applying a transmission power reduction due to a simple amplitude reduction.
- the same effect as in the first embodiment can be obtained, and the instantaneous average power at the tap coefficient switching point can be obtained. Since the tap coefficient fine adjustment unit 10 that finely adjusts the tap coefficient is provided in order to suppress fluctuations in the average frequency, fluctuations in average power are moderated even at the tap coefficient switching point, so that transmission quality can be further stabilized. it can.
- 1 Pre-equalization signal generation circuit 2 tap coefficient selection circuit, 3, 4 digital-analog converter, 5, 6 driver, 7 light source, 8 modulator, 10 tap coefficient fine adjustment section.
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Abstract
Description
図1は、この発明の実施の形態1に係る予等化光送信機の構成を示した構成図である。図1に示した予等化光送信機においては、伝送路の品質劣化要因となるパラメータの値に応じて、RZ予等化伝送とNRZ予等化伝送とを選択することができる。なお、本実施の形態に係る予等化光送信機においても、WDM伝送に適用でき、分散補償ファイバの設置コストおよび設置場所を削減し、かつ、大容量の光通信を行なうことができる。本実施の形態においては、予等化伝送方式を、RZ予等化伝送方式とNRZ予等化伝送方式とから選択して用いることができる。そのため、伝送路の劣化要因に応じて、それらの方式を組み合わせて用いることにより、全体として送信パワーの変動を低く抑えることができる。なお、ここでは、伝送路の劣化要因となるパラメータとして、波長分散の分散補償量を例に挙げて説明する。しかしながら、その場合に限定されるものではなく、伝送路の劣化要因となるものであれば、他のパラメータを用いるようにしてもよい。
図4は、この発明の実施の形態2に係る予等化光送信機の構成を示した構成図である。図4においては、予等化光送信機の外部に、タップ係数微調整部10が設けられている。
Claims (9)
- 伝送方式としてRZ方式とNRZ方式の両方を切り替えて用いる予等化光送信機であって、
前記RZ方式で用いるRZ予等化信号生成用のタップ係数と前記NRZ方式で用いるNRZ予等化信号生成用のタップ係数とを予め保存し、送信パワーの変動の要因となるパラメータの値に応じて、前記RZ予等化信号生成用のタップ係数または前記NRZ予等化信号生成用のタップ係数のいずれか一方を選択して出力するタップ係数選択部と、
前記タップ係数選択部から出力される前記タップ係数が前記RZ予等化信号生成用のタップ係数の場合はRZ予等化信号を生成し、前記タップ係数選択部から出力される前記タップ係数が前記NRZ予等化信号生成用のタップ係数の場合はNRZ予等化信号を生成する予等化信号生成部と、
前記予等化信号生成部から出力された前記RZ予等化信号または前記NRZ予等化信号をアナログ信号に変換するデジタル-アナログ変換部と、
前記デジタル-アナログ変換部から出力された前記アナログ信号を増幅する増幅部と、
光信号を出力する光源と、
前記増幅部から出力される増幅された前記アナログ信号に基づいて前記光源からの前記光信号を変調することにより、予等化光送信信号を生成する変調部と
を備え、
前記タップ係数選択部により、前記パラメータの値に応じて、前記タップ係数の切り替えを行なうことにより、伝送方式をRZ方式とNRZ方式との間で切り替えて送信を行なう
ことを特徴とする予等化光送信機。 - 前記タップ係数選択部で前記タップ係数の選択に用いる前記パラメータは、波長分散の分散補償量であり、
前記タップ係数選択部は、前記分散補償量の値が所定の閾値よりも小さい場合には前記RZ予等化信号生成用のタップ係数を選択し、前記分散補償量が所定の閾値以上の場合には前記NRZ予等化信号生成用のタップ係数を選択する
ことを特徴とする請求項1に記載の予等化光送信機。 - 前記タップ係数の切り替えを行なう切替点の送信パワーの変動を抑制するために、前記切替点の前後の領域において前記タップ係数の微調整を行なうためのタップ係数微調整部をさらに備え、
前記タップ係数選択部は、前記タップ係数に対して前記タップ係数微調整部による微調整を行なったタップ係数を予め保存する
ことを特徴とする請求項1または2に記載の予等化光送信機。 - 前記タップ係数微調整部は、前記伝送路の光フィルタによる帯域制限、自機の帯域制限、及び/又は、自機が送信した信号を受信する受信機の帯域制限を予等化により補償することにより、前記タップ係数の微調整を行なう
ことを特徴とする請求項3に記載の予等化光送信機。 - 前記タップ係数微調整部は、IQのコンスタレーションを回転させることにより送信パワーの調整を行なうことにより、前記タップ係数の微調整を行なう
ことを特徴とする請求項3または4に記載の予等化光送信機。 - 前記タップ係数微調整部は、周波数範囲を制限する周波数フィルタリングを実施することにより、前記タップ係数の微調整を行なう
ことを特徴とする請求項3ないし5のいずれか1項に記載の予等化光送信機。 - 前記タップ係数微調整部は、前記タップ係数として用いる周波数特性のインパルス応答を時間軸方向に制限する時間領域フィルタリングを実施することにより、前記タップ係数の微調整を行なう
ことを特徴とする請求項3ないし6のいずれか1項に記載の予等化光送信機。 - 前記タップ係数微調整部は、前記RZ予等化信号の波形と前記NRZ予等化信号の波形との間の中間波形を有する予等化信号を生成するためのタップ係数を生成することにより、前記タップ係数の微調整を行なう
ことを特徴とする請求項3ないし7のいずれか1項に記載の予等化光送信機。 - 伝送方式としてRZ方式とNRZ方式の両方を切り替えて用いる予等化光送信方法であって、
前記RZ方式で用いるRZ予等化信号生成用のタップ係数と前記NRZ方式で用いるNRZ予等化信号生成用のタップ係数とを予め保存し、送信パワーの変動の要因となるパラメータの値に応じて、前記RZ予等化信号生成用のタップ係数または前記NRZ予等化信号生成用のタップ係数のいずれか一方を選択して出力するタップ係数選択ステップと、
前記タップ係数選択ステップにより出力される前記タップ係数が前記RZ予等化信号生成用のタップ係数の場合はRZ予等化信号を生成し、前記タップ係数選択ステップにより出力される前記タップ係数が前記NRZ予等化信号生成用のタップ係数の場合はNRZ予等化信号を生成する予等化信号生成ステップと、
前記予等化信号生成ステップにより出力された前記RZ予等化信号または前記NRZ予等化信号をアナログ信号に変換するデジタル-アナログ変換ステップと、
前記デジタル-アナログ変換ステップにより出力された前記アナログ信号を増幅する増幅ステップと、
光信号を出力する光信号出力ステップと、
前記増幅ステップにより出力された増幅アナログ信号に基づいて前記光信号出力ステップにより出力された前記光信号を変調することにより、予等化光送信信号を生成する送信信号生成ステップと
を備え、
前記タップ係数選択ステップにおいて、前記パラメータの値に応じて、前記タップ係数の切り替えを行なうことにより、伝送方式をRZ方式とNRZ方式との間で切り替えて送信を行なう
ことを特徴とする予等化光送信方法。
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