JP6941633B2 - A device for determining the chirp parameters of an optical signal transmitted by an optical communication system - Google Patents

Description

The present invention relates to a technique for determining a chirp parameter of an optical signal transmitted by an optical communication system using a direct detection method.

Wavelength dispersion fading occurs in optical communication systems. Further, the wavelength dispersion fading converts the intensity modulation (IM) optical signal (or amplitude modulation (AM) optical signal) into a phase modulation (PM) optical signal, and the PM optical signal becomes an IM (or AM) optical signal. It is known that a phenomenon called IM (AM) -PM conversion, which is converted, occurs (hereinafter, IM-PM conversion). For example, the IM optical signal transmitted by the transmitting device gradually changes to a PM optical signal according to the transmission distance. This PM optical signal further gradually changes to an IM optical signal according to the transmission distance. This IM-PM conversion has frequency characteristics. For example, when a plurality of IM optical signals are frequency-multiplexed and transmitted from a transmission device, after transmitting a predetermined distance, a part of the IM optical signal becomes a PM optical signal, a part of the IM optical signal remains as an IM optical signal, and the rest. Is in the middle, that is, an optical signal that is intensity-modulated and phase-modulated. Since the direct detection type receiving device 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 receiving device receives light when the IM optical signal transmitted by the transmitting device is converted into a PM optical signal, this optical communication system cannot operate. Further, when the IM optical signal transmitted by the transmitting device is not converted into a complete PM optical signal and the receiving device receives light at a place where the optical signal is intensity-modulated and phase-modulated, the receiving device is optical. The change in signal strength can be extracted, but the level becomes smaller and the signal-to-noise ratio deteriorates.

FIG. 1 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. In FIG. 1, the maximum value of the reception level of the electric signal after photoelectric conversion is set to 0 dB. Here, the chirp parameter α = 0 corresponds to the IM optical signal, α = ∞ corresponds to the PM optical signal, and α = 1 and α = -1 are optical signals in which both intensity modulation and phase modulation are performed. Corresponds to. Note that the rotation direction of the phase differs depending on the data value between α = 1 and α = -1.

From FIG. 1, when the continuous light is modulated by an electric signal having a frequency band up to about 7 GHz, it is appropriate to set the chirp parameter α = 0, and when the continuous light is modulated by an electric signal from 7 GHz to 12 GHz. It is appropriate to set the chirp parameter α = 1, and when modulating continuous light with an electric signal from 12 GHz to 15 GHz, it is appropriate to set the chirp parameter α = ∞ to 15 GHz to 18 GHz. It can be seen that it is appropriate to set the chirp parameter α = -1 when the continuous light is modulated by the electric signal of.

Therefore, Non-Patent Document 1 has a configuration in which a chirp parameter α is controlled when the continuous light is modulated by each of a plurality of electric signals having different frequencies, and when the continuous light is modulated based on the frequency band of each of the plurality of electric signals. Is disclosed. The correspondence between each electric signal and the chirp parameter α is determined based on the frequency of the electric signal and the characteristic shown in FIG. 1 by examining the characteristics of FIG. 1 in advance.

S. Ishimura, et al. , "Direct-Electrode OFDM Transmission Usage Four-State Chirp Control With a Dual-Electrode MZM for Dispersion Compression", IEEE Physical. 10, no. 5, pp. 1-8, 2018

In the configuration of Non-Patent Document 1, in order to investigate the characteristics shown in FIG. 1, the test optical signal is transmitted in order for each of the four chirp parameters, and the test optical signal is photoelectrically converted at the position where the receiving device is installed to obtain electricity. It was necessary to measure the reception level (reception power) of each frequency of the signal.

The present invention provides a determination device capable of determining an appropriate chirp parameter by transmitting a test optical signal with one chirp parameter.

According to one aspect of the present invention, the determination device is a test optical signal in which continuous light is modulated by a plurality of first electric signals having different frequencies with the same charp parameter, and is transmitted from a transmission end of an optical communication system. A receiving means for receiving the test optical signal at the receiving end of the optical communication system, a converting means for photoelectrically converting the test optical signal received by the receiving means and outputting the plurality of first electric signals, and the above. The difference in frequency between the measuring means for measuring the reception level of each of the plurality of first electric signals output by the conversion means and the two adjacent first electric signals on the frequency axis of the plurality of first electric signals. A determination means for determining the relationship between the frequency and the inclination of the reception level based on the difference between the reception levels of the two first electric signals measured by the measuring means, and the frequency and the inclination of the reception level determined by the determination means. Based on the relationship of

According to the present invention, an appropriate chirp parameter can be determined by transmitting a test optical signal with one chirp parameter.

Explanatory drawing of disclosure contents of background technology. The block diagram of the optical communication system by one Embodiment. An explanatory diagram of a method of generating a test optical signal transmitted by a transmitting device in a determination process according to an embodiment. The figure which shows the signal received by the receiving apparatus by one Embodiment, and the ideal signal. An explanatory diagram of how to obtain the inclination according to one embodiment. Explanatory drawing of the chirp parameter determination method based on the inclination by one Embodiment.

Hereinafter, embodiments will be described in detail with reference to the accompanying drawings. 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 plurality of features described in the embodiments may be arbitrarily combined. In addition, the same or similar configuration will be given the same reference number, and duplicated explanations will be omitted.

FIG. 2 is a configuration diagram of an optical communication system according to the present embodiment. The optical communication system includes a transmitting device 1, a receiving device 2, and an optical transmission line connecting the transmitting device 1 and the receiving device 2. Continuous light is input from the light source 10 to the modulation unit 11 of the transmission device 1. Further, electric signals # 1 to # N (N is an integer of 2 or more) for carrying information are input to the modulation unit 11. The frequency bands of the electric signals # 1 to # N are different from each other. The modulation unit 11 outputs a modulated optical signal generated by modulating continuous light with each of the electric signals # 1 to #N to an optical transmission line. Here, any of the four chirp parameters α = 0, 1, -1 and ∞ is assigned to each of the electric signals # 1 to # N.

For example, when the characteristics of this optical communication system are as shown in FIG. 1, the chirp parameter α = 0 is assigned to the electric signals # 1 to #N whose frequency band is up to 7 GHz, and the chirp parameter α = 0 to 7 GHz to A chirp parameter α = 1 is assigned to those of 12 GHz, a chirp parameter α = ∞ is assigned to those of 12 GHz to 15 GHz, and a chirp parameter α = -1 is assigned to those of 15 GHz to 18 GHz. Then, the modulation unit 11 modulates the continuous light according to the chirp parameter α assigned to the electric signal #n (n is an integer from 1 to N). As the modulator 11 that modulates one continuous light with a plurality of electric signals # 1 to # N according to a plurality of different chirp parameters α, for example, the modulator described in Non-Patent Document 1 can be used. can. However, the present invention is not limited to those using the modulator described in Non-Patent Document 1. For example, an electric signal to which the same chirp parameter α is assigned is frequency-multiplexed, and in a modulator dedicated to the chirp parameter α, continuous light is modulated by the frequency-multiplexed electric signal, and a modulator dedicated to each chirp parameter α outputs. It is also possible to generate a modulated optical signal by combining the signals to be generated.

The photoelectric conversion unit 21 of the receiving device 2 receives the modulated optical signal transmitted by the transmitting device 1 via the optical transmission line, photoelectrically converts the received modulated optical signal, and obtains electric signals # 1 to electric signal # N. Output. As described above, the chirp parameters are assigned to the electric signals # 1 to the electric signals # N based on the characteristics of FIG. Therefore, at the position of the receiving device 2, since the intensity is modulated over the entire frequency band, the electric signal # 1 to the electric signal # N can be output by direct detection. After that, each of the electric signals # 1 to the electric signal # N is determined by a demodulation unit (not shown) to determine the data value of the information carried by each. The measuring unit 22 and the determining unit 23 are used in the determination process described later, respectively.

The chirp parameter α assigned to the electric signals # 1 to # N transmitted by the transmission device 1 needs to be determined in advance and set in the modulation unit 11 of the transmission device 1. For that purpose, it is necessary to investigate the frequency characteristics of the optical communication system in advance as shown in FIG. In the conventional method, the test optical signal is transmitted with all the electric signals # 1 to #N as the same chirp parameter α, and the reception level of the electric signals # 1 to #N after photoelectric conversion is measured in the receiving device 2. Had to be done for each of the four chirp parameters used.

In the present embodiment, only the transmitting device 1 transmits the test optical signal generated by any one of the four chirp parameters, and the receiving device 2 determines the frequency characteristic of the optical communication system, whereby the electric signal # Performs a determination process to determine the chirp parameter α to be assigned to 1 to #N. That is, in the past, it was necessary to transmit the test optical signal four times with four chirp parameters, but in the present embodiment, the test optical signal is transmitted only once with one chirp parameter. The receiving device 2 notifies the transmitting device 1 of the chirp parameters α assigned to the electric signals # 1 to # N determined by the determination process by a communication line from the receiving device 2 to the transmitting device 1 (not shown). The transmitting device 1 sets the modulator 11 according to the chirp parameters α of the electric signals # 1 to # N notified from the receiving device 2.

Hereinafter, the determination process according to the present embodiment will be described. In the following description, the chirp parameter α used in the determination process is set to 0, but it may be 1, -1 or ∞. In the determination process, the transmission device 1 of the present embodiment generates an electric signal (hereinafter, referred to as a test electric signal) shown in FIG. 3 and inputs it to the modulation unit 11. The test electrical signal is composed of a plurality of sinusoidal signals, each having a different frequency. The frequency of each sine wave signal is selected so that a plurality of sine wave signals are included in the frequency bands of the electric signals # 1 to # N. In the present embodiment, since the chirp parameter α is 0, the modulation unit 11 generates a test light signal by IM-modulating continuous light with the test electric signal, and the generated test light is generated via an optical transmission line. The signal is transmitted to the receiving device 2.

In the determination process, the photoelectric conversion unit 21 of the receiving device 2 receives the test optical signal transmitted by the transmitting device 1 via the optical transmission line, photoelectrically converts the received test optical signal, and outputs the test electric signal. .. The measuring unit 22 of the receiving device 2 measures the reception level (received power) of each sine wave signal included in the test electric signal after the photoelectric change. From the frequency of each sinusoidal signal and its reception level, the relationship between the frequency and the reception level can be obtained as shown in FIG. 4 (A). As shown in FIG. 4A, there is a frequency band in which the reception level in the receiving device 2 drops due to the influence of IM-PM conversion. Further, the higher the frequency, the larger the amount of attenuation. Therefore, the higher the frequency, the smaller the reception level.

On the other hand, FIG. 4B shows a case where there is no increase in the amount of attenuation due to the increase in frequency and the fluctuation of the reception level of each sinusoidal signal in the receiving device 2 is solely due to the influence of IM-PM conversion. It shows the relationship between frequency and reception level. As shown in FIG. 4 (B), assuming that there is no increase in the amount of attenuation due to the increase in frequency, the chirp parameter α used for each optical signal # 1 to # N is determined from the graph of FIG. 4 (B). can do. For example, the graph of FIG. 1 shows the fluctuation of the reception level only due to the influence of the IM-PM conversion. From FIG. 1, the reception level of the optical signal with the chirp parameter α = 0 is attenuated by about 3 dB to 10 dB, and the reception level is attenuated by about 3 dB to 10 dB. At frequencies of 7 GHz to 12 GHz, where the amount of attenuation increases as the frequency increases, it can be determined that it is appropriate to use the chirp parameter α = 1. Further, it can be determined that it is appropriate to use the chirp parameter α = ∞ at the frequency of 12 GHz to 15 GHz at which the reception level of the optical signal with the chirp parameter α = 0 is attenuated by 10 dB or more. Further, at a frequency of 15 GHz to 18 GHz in which the reception level of the optical signal with the chirp parameter α = 0 is attenuated by 3 dB to 10 dB and the amount of attenuation decreases as the frequency increases, the chirp parameter α = -1 should be used. Can be determined to be appropriate. Then, it can be determined that it is appropriate to use the chirp parameter α = 0 in the frequency band in which the attenuation of the reception level of the optical signal with the chirp parameter α = 0 is 3 dB or less.

However, in reality, in addition to the influence of IM-PM conversion, the amount of attenuation increases as the frequency increases, so as described above, it is applied to each optical signal #n within the range of the measured reception level of the sinusoidal signal. It is not possible to determine the chirp parameter α to be used. For example, at the frequency position of reference numeral 50 in FIG. 4 (A), the amount of attenuation of the reception level of the chirp parameter α = 0 is large. obtain. However, in reality, the frequency position of the reference numeral 50 is the position where the reception level of the chirp parameter α = 0 peaks, and it is appropriate to use the chirp parameter α.

Therefore, in the present embodiment, the determination unit 23 differentiates the graph showing the relationship between the frequency and the reception level shown in FIG. 4A based on the reception level of each sine wave signal measured by the measurement unit 22. Ask for. Specifically, the gradient is obtained based on the frequency difference between two adjacent sine wave signals on the frequency axis of the test electric signal and the difference in the reception level of the two sine wave signals. For example, as shown in FIG. 5, the frequency of one of the two adjacent sine wave signals is f1, the frequency of the other sine wave signal is f2 (f2> f1), and the reception of the frequency f1. Assuming that the level is p1 and the reception level of the frequency f2 is p2, the determination unit 23 obtains the inclination (differential value) at the frequency (f2 + f1) / 2 as (p2-p1) / (f2-f1). As a result, the determination unit 23 obtains information indicating the relationship between the frequency and the inclination, as shown in FIG. In the present embodiment, the slope obtained based on the reception level of the sine wave signal of frequency f1 and the sine wave signal of frequency f2 is the slope at the intermediate frequency (f2 + f1) / 2, but the slope of frequency f1. Or the slope of the frequency f2. That is, the gradient obtained based on the reception level of the sine wave signal of the frequency f1 and the sine wave signal of the frequency f2 can be an arbitrary frequency gradient between the frequency f1 and the frequency f2.

In the present embodiment, as shown in FIG. 6, the relationship between the tilt range and the charp parameter α to be used is determined in advance as determination information, and is set in advance in the determination unit 23. Then, the determination unit 23 determines the chirp parameter α of the electric signal #n based on the average value of the slopes of the electric signal #n in the frequency band. For example, when the average value of the slopes of the electric signal #n in the frequency band is within the range of Th1 to Th2, the determination unit 23 determines the chirp parameter α of the electric signal #n to be -1. Further, when the average value of the slopes of the electric signal #n in the frequency band is within the range of Th2 to Th3, the determination unit 23 determines the chirp parameter α of the electric signal #n to be 0. Further, when the average value of the slopes of the electric signal #n in the frequency band is within the range of Th3 to Th4, the determination unit 23 determines the chirp parameter α of the electric signal #n to 1. Then, the determination unit 23 determines the chirp parameter α of the electric signal #n to ∞ in other cases. Then, the determination unit 23 notifies the transmission device 1 of the chirp parameter α of each determined electric signal #n.

As described above, the slope of the reception level of the sinusoidal signal at each frequency, that is, the differential value of the relationship between the frequency and the reception level is obtained, and the chirp parameter α of each electric signal #n is determined based on the differential value. As a result, the influence of the increase in the amount of attenuation due to the increase in frequency can be eliminated, and the chirp parameter α to be assigned to each electric signal #n can be determined by transmitting the test optical signal with one chirp parameter α only once. Can be done.

In this embodiment, the determination unit 23 is provided in the receiving device 22. However, the determination unit 23 may be provided in the transmission device 1. In this case, the measuring unit 22 of the receiving device 2 notifies the transmitting device 1 of the receiving level information indicating the receiving level of each sine wave signal. Further, the determination unit 23 may be provided, for example, in a control device (not shown) that controls the transmission device 1 and the reception device 2. In this case, the measuring unit 22 of the receiving device 2 notifies the control device of the receiving level information. Then, the determination unit 23 of the control device notifies the transmission device 1 of the chirp parameter α of each determined electric signal #n.

Further, in the present embodiment, during the determination process, the transmission device 1 generates and transmits a test optical signal, and the reception device 2 determines the chirp parameter α of the electric signal #n. The transmitting device 1 and the receiving device 2 are actually used in an optical communication system. However, the function described as the receiving device 2 in the present embodiment can also be realized as a dedicated determining device that determines the chirp parameter α of each electric signal #n. Before using the optical communication system, this determination device receives a test optical signal from the transmission device 1 at the position where the receiving device 2 is installed, that is, at the receiving end of the optical communication system to be measured, and receives each electric signal #. The chirp parameter α of n is determined, and the information indicating the chirp parameter α of each determined electric signal #n is notified to the transmission device 1 or output to the user. Then, when the optical communication system is used for providing the communication service, the determination device is removed and the receiving device 2 is installed. The determination device can present the determined electric signal #n to the user by displaying the information indicating the chirp parameter α on the display, printing the information, or storing the information in the memory. Similarly, in the present embodiment, the transmitting device 1 actually used in the optical communication system transmits the test optical signal in the determination process, but the dedicated test optical signal transmitting device, not the transmitting device 1, measures. The test optical signal may be transmitted from the transmission end of the target optical communication system.

21: Photoelectric conversion unit, 22: Measurement unit, 23: Determining unit

Claims (9)

A test optical signal in which continuous light is modulated by a plurality of first electric signals having different frequencies with the same chirp parameters, and the test optical signal transmitted from a transmission end of the optical communication system is received by the optical communication system. The receiving means to receive at the end and
A conversion means that photoelectrically converts the test optical signal received by the receiving means and outputs the plurality of first electric signals.
A measuring means for measuring the reception level of each of the plurality of first electric signals output by the converting means, and a measuring means.
Based on the difference in frequency between two adjacent first electric signals on the frequency axis of the plurality of first electric signals and the difference in reception level of the two first electric signals measured by the measuring means, the frequency and Judgment means for determining the relationship with the slope of the reception level,
Based on the relationship between the frequency determined by the determination means and the slope of the reception level, the determination means for determining the chirp parameter when the continuous light is modulated by the second electric signal transmitted by the optical communication system, and the determination means.
A determination device characterized by being equipped with. The determination device according to claim 1, wherein the determination device is a receiving device of the optical communication system. A test optical signal in which continuous light is modulated by a plurality of first electric signals having different frequencies with the same charp parameter, and the test optical signal transmitted from a transmission end of the optical communication system is received by the optical communication system. An acquisition means for acquiring reception level information indicating the reception level of each of the plurality of first electric signals received at the end and output by photoelectric conversion of the received test optical signal.
Based on the difference in frequency between two adjacent first electric signals on the frequency axis of the plurality of first electric signals and the difference in reception levels of the two first electric signals indicated by the reception level information, the frequency and Judgment means for determining the relationship with the slope of the reception level,
Based on the relationship between the frequency determined by the determination means and the slope of the reception level, the determination means for determining the chirp parameter when the continuous light is modulated by the second electric signal transmitted by the optical communication system, and the determination means.
A determination device characterized by being equipped with. The determination device according to claim 3, wherein the determination device is a transmission device of the optical communication system. The determination device according to claim 3 or 4, further comprising a generation means for generating the test optical signal. The determination means is a value obtained by subtracting the reception level of the low frequency electric signal among the two first electric signals from the reception level of the high frequency electric signal having a high frequency among the two first electric signals. , The value divided by the frequency difference between the high-frequency electric signal and the low-frequency electric signal is obtained as the value of the slope of a predetermined frequency between the frequency of the low-frequency electric signal and the frequency of the high-frequency electric signal. The determination device according to any one of claims 1 to 5, which is characterized. The determination means obtains the inclination of the second electric signal in the frequency band based on the relationship between the frequency determined by the determination means and the inclination of the reception level, and based on the inclination of the second electric signal in the frequency band, the determination means. The determination device according to any one of claims 1 to 6, wherein a chirp parameter for modulating continuous light by a second electric signal is determined. The determination means has determination information indicating a correspondence relationship between the inclination and the chirp parameter, and the chirp parameter corresponding to the inclination in the frequency band of the second electric signal indicated by the determination information is obtained by the second electric signal. The determination device according to claim 7, wherein the chirp parameter for modulating continuous light is determined. 7. Determination device.

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