阅读说明：本技术 光传输特性补偿方法及光传输特性补偿系统 (Optical transmission characteristic compensation method and optical transmission characteristic compensation system ) 是由 山岸明洋 保城笃志 田中克也 土屋英祐 中村政则 松下明日香 于 2020-05-21 设计创作，主要内容包括：通过配置于光发送机(3)的前级的发送机补偿部(8)进行光发送机(3)的传输特性的补偿的一部分。通过配置于光接收机(5)的后级的接收机补偿部(12)进行光发送机(3)的传输特性的补偿的剩余部分以及光接收机(5)的传输特性的补偿。设定发送机补偿部(8)的发送机补偿特性,以使发送机补偿部(8)的输出信号的峰均功率比成为预定值以下。(Part of the compensation of the transmission characteristics of the optical transmitter (3) is performed by a transmitter compensation unit (8) disposed at a stage preceding the optical transmitter (3). The remaining part of the compensation of the transmission characteristic of the optical transmitter (3) and the compensation of the transmission characteristic of the optical receiver (5) are performed by a receiver compensation unit (12) disposed at the subsequent stage of the optical receiver (5). The transmitter compensation characteristic of the transmitter compensation unit (8) is set so that the peak-to-average power ratio of the output signal of the transmitter compensation unit (8) is equal to or less than a predetermined value.)

1. An optical transmission characteristic compensation method for compensating transmission characteristics of an optical transmitter and an optical receiver connected to each other via an optical transmission path, comprising:

a part of compensation of transmission characteristics of the optical transmitter by a transmitter compensation unit disposed at a preceding stage of the optical transmitter; and

a receiver compensation unit disposed at a subsequent stage of the optical receiver compensates the transmission characteristics of the optical receiver and the remaining part of the compensation of the transmission characteristics of the optical transmitter,

the transmitter compensation characteristic of the transmitter compensation unit is set so that the peak-to-average power ratio of the output signal of the transmitter compensation unit is equal to or less than a predetermined value.

2. An optical transmission characteristic compensation method for compensating transmission characteristics of an optical transmitter and an optical receiver connected to each other via an optical transmission path, comprising:

a part of compensation of transmission characteristics of the optical transmitter by a transmitter compensation unit disposed at a preceding stage of the optical transmitter;

a remaining part for performing compensation of transmission characteristics of the optical transmitter by an optical filter disposed at a subsequent stage of the optical transmitter; and

the transmission characteristics of the optical receiver are compensated by a receiver compensation section disposed at a subsequent stage of the optical receiver,

the transmitter compensation characteristic of the transmitter compensation unit is set so that the peak-to-average power ratio of the output signal of the transmitter compensation unit is equal to or less than a predetermined value.

3. The optical transmission characteristic compensation method according to claim 2,

the filter function of the wavelength selective switch is used as the filter.

4. The optical transmission characteristic compensation method according to any one of claims 1 to 3,

adjusting the transmitter compensation characteristic and the transmitter compensation characteristic to reduce a peak-to-average power ratio of the output signal of the transmitter compensation section when an optical signal-to-noise ratio at which a predetermined signal quality is obtained in the output signal of the receiver compensation section is greater than a predetermined value,

and adjusting the transmitter compensation characteristic and the transmitter compensation characteristic to improve the peak-to-average power ratio of the output signal of the transmitter compensation part when the optical signal-to-noise ratio for obtaining the predetermined signal quality is smaller than the predetermined value.

5. The optical transmission characteristic compensation method according to any one of claims 1 to 4,

the transmitter compensation characteristic is a nonlinear suppression characteristic obtained by superimposing any one of a gaussian characteristic, a super-gaussian characteristic, and an averaged characteristic of a transmission characteristic of the optical transmitter, or a combination thereof.

6. The optical transmission characteristic compensation method according to any one of claims 1 to 5,

generating a nonlinear suppression characteristic such that a peak-to-average power ratio of an output signal of the transmitter compensation unit is equal to or less than a predetermined value, superimposing the nonlinear suppression characteristic on a compensation characteristic of the transmitter compensation unit, and superimposing an inverse characteristic of the nonlinear suppression characteristic on a compensation characteristic of the receiver compensation unit, or

The nonlinear suppression compensation characteristic is directly generated so that the peak-to-average power ratio of the output signal of the transmitter compensation unit is equal to or less than a predetermined value, and the nonlinear suppression compensation characteristic is set as the transmitter compensation characteristic in the transmitter compensation unit.

7. The optical transmission characteristic compensation method according to claim 6,

when the nonlinear suppression compensation characteristic is directly generated and the transmitter compensation unit is set as the transmitter compensation characteristic,

the transmitter compensation unit generates nonlinear suppression inverse characteristics, which are inverse characteristics of nonlinear suppression characteristics such that a peak-to-average power ratio of an output signal of the transmitter compensation unit becomes a predetermined value or less, estimates a transfer function of the optical transmitter using characteristics obtained by superimposing the nonlinear suppression inverse characteristics on a transfer function of the optical receiver, and sets the estimated transfer function of the optical transmitter as the nonlinear suppression compensation characteristics.

8. The optical transmission characteristic compensation method according to any one of claims 1 to 7,

superimposing all or a part of the compensation characteristic of the optical transmission path into the transmitter compensation characteristic.

9. The optical transmission characteristic compensation method according to any one of claims 1 to 8,

the receiver compensation characteristic of the receiver compensation unit is added with a characteristic of an amplitude component of 1/2 frequency that improves the baud rate.

10. An optical transmission characteristic compensation system that compensates transmission characteristics of an optical transmitter and an optical receiver connected to each other via an optical transmission path, comprising:

a transmitter compensation unit disposed in a preceding stage of the optical transmitter and configured to partially compensate transmission characteristics of the optical transmitter;

a receiver compensation unit arranged at a subsequent stage of the optical receiver, the receiver compensation unit performing a remainder of the compensation of the transmission characteristic of the optical transmitter and a compensation of the transmission characteristic of the optical receiver;

a receiver compensation characteristic setting unit that adjusts the receiver compensation characteristic of the receiver compensation unit so that the transmission characteristic of the output signal of the receiver compensation unit becomes a predetermined characteristic; and

and a transmitter compensation characteristic setting unit that sets the transmitter compensation characteristic of the transmitter compensation unit so that a peak-to-average power ratio of an output signal of the transmitter compensation unit becomes equal to or lower than a predetermined value.

11. An optical transmission characteristic compensation system that compensates transmission characteristics of an optical transmitter and an optical receiver connected to each other via an optical transmission path, comprising:

a transmitter compensation unit disposed in a preceding stage of the optical transmitter and configured to partially compensate transmission characteristics of the optical transmitter;

an optical filter which is disposed at a subsequent stage of the optical transmitter and performs a remaining part of compensation of transmission characteristics of the optical transmitter;

a receiver compensation unit which is disposed at a subsequent stage of the optical receiver and compensates for transmission characteristics of the optical receiver;

a receiver compensation characteristic setting unit that adjusts the receiver compensation characteristic of the receiver compensation unit so that the transmission characteristic of the output signal of the receiver compensation unit becomes a predetermined characteristic; and

and a transmitter compensation characteristic setting unit that sets the transmitter compensation characteristic of the transmitter compensation unit so that a peak-to-average power ratio of an output signal of the transmitter compensation unit becomes equal to or lower than a predetermined value.

12. The optical transmission characteristic compensation system of claim 11,

the filter is a filter function of the wavelength selective switch.

13. The optical transmission characteristic compensation system according to any one of claims 10 to 12,

the transmitter compensation characteristic setting section adjusts the transmitter compensation characteristic so as to lower a peak-to-average power ratio of the output signal of the transmitter compensation section when an optical signal-to-noise ratio at which a predetermined signal quality is obtained in the output signal of the receiver compensation section is larger than a predetermined value,

the transmitter compensation characteristic setting unit adjusts the transmitter compensation characteristic and the transmitter compensation characteristic to improve a peak-to-average power ratio of an output signal of the transmitter compensation unit when the optical signal-to-noise ratio at which the predetermined signal quality is obtained is smaller than the predetermined value.

14. The optical transmission characteristic compensation system according to any one of claims 10 to 13,

the transmitter compensation characteristic setting unit adds, as a nonlinear suppression characteristic, any one of a gaussian characteristic, a super-gaussian characteristic, and an averaging characteristic of a transmission characteristic of the optical transmitter, or a combination thereof to the transmitter compensation characteristic.

15. The optical transmission characteristic compensation system according to any one of claims 10 to 14,

the transmitter compensation characteristic setting unit includes:

a nonlinear suppression characteristic generation unit that generates a nonlinear suppression characteristic such that a peak-to-average power ratio of an output signal of the transmitter compensation unit becomes a predetermined value or less; and

a transmitter compensation characteristic generation section that superimposes the nonlinear suppression characteristic on a compensation characteristic of the transmitter compensation section,

the receiver compensation characteristic setting section superimposes an inverse characteristic of the nonlinear suppression characteristic on the compensation characteristic of the receiver compensation section, or

The transmitter compensation characteristic setting unit directly generates a nonlinear suppression compensation characteristic such that a peak-to-average power ratio of an output signal of the transmitter compensation unit is equal to or less than a predetermined value, and sets the nonlinear suppression compensation characteristic as the transmitter compensation characteristic.

16. The optical transmission characteristic compensation system of claim 15,

when the transmitter compensation characteristic setting unit directly generates the nonlinear suppression compensation characteristic and sets the transmitter compensation unit as the transmitter compensation characteristic,

the receiver compensation characteristic setting unit generates a nonlinear suppression inverse characteristic which is an inverse characteristic of a nonlinear suppression characteristic in which a peak-to-average power ratio of an output signal of the transmitter compensation unit is equal to or less than a predetermined value, estimates a transfer function of the optical transmitter using a characteristic obtained by superimposing the nonlinear suppression inverse characteristic on a transfer function of the optical receiver,

the transmitter compensation characteristic setting unit sets the estimated transfer function of the optical transmitter as the nonlinear suppression compensation characteristic.

17. The optical transmission characteristic compensation system according to any one of claims 10 to 16,

the transmitter compensation characteristic setting unit superimposes all or a part of the compensation characteristic of the optical transmission path on the transmitter compensation characteristic.

18. The optical transmission characteristic compensation system according to any one of claims 10 to 17,

the receiver compensation characteristic setting unit adds a characteristic of an amplitude component of 1/2 frequency that raises a baud rate to the receiver compensation characteristic of the receiver compensation unit.

Technical Field

The present invention relates to an optical transmission characteristic compensation method and an optical transmission characteristic compensation system in optical communication.

Background

In digital coherent optical communication, distortion of signals generated in an optical transmitter, an optical fiber transmission line, and an optical receiver is compensated by digital signal processing, and large-capacity transmission of several tens of Gbit/s or more is possible. This enables long-distance transmission with a reduced number of relays during transmission. Further, not only QPSK but also high-level multilevel modulation such as 16QAM or 256QAM can be applied to the modulation scheme of the signal, and therefore the transmission rate can be increased significantly.

With the increase and multivalue of transmission rates, an optical transceiver is required to have good transmission characteristics over a wide frequency band. The transmission characteristics of the transmission signal of the optical transceiver are expressed by a transfer function, and are compensated by respective compensation circuits of the transceiver. As the transmission rate increases, it is required to improve the compensation accuracy. Therefore, distortion in the optical transmitter is compensated for on the transmission side. Thus, the receiving side may compensate for distortion in the transmission line and the optical receiver.

Patent document 1 discloses a method of detecting waveform distortion of an optical transmitter from a training signal and correcting the waveform distortion in the optical transmitter. Patent document 2 discloses an apparatus and a method for compensating for distortion in an IQ modulator on the transmission side. Patent document 3 discloses a device for compensating nonlinear distortion in a wide band and with high accuracy in an optical transmitter for a wireless communication system of a linear modulation system. In this system, distortion is detected by an optical transmitter and compensation for the distortion is performed. The optical transmitter can form substantially flat transmission characteristics according to the compensation operation of the distortion.

Patent document 4 shows a structure as follows: that is, a configuration in which a nonlinear signal distortion compensation unit of an optical transmitter compensates in advance for nonlinear signal distortion generated in a transmission signal by a semiconductor optical amplifier of the optical transmitter, and a configuration in which a nonlinear distortion compensation unit of an optical receiver compensates (equalizes) only nonlinear signal distortion generated in the transmission signal on a receiving side. That is, in addition to a configuration in which a flat transmission characteristic is formed in the transmitter, a configuration in which all of them are equalized on the receiving side is shown.

Patent document 5 discloses a calibration method in configuring an optical communication system. In this correction method, a transfer function representing the transmission characteristics of the optical transmitter is estimated, and the inverse transfer function is set in the transmitter compensation unit. A transfer function representing the transmission characteristics of the optical receiver is estimated, and the receiver compensation unit is set with the inverse transfer function. Thus, the characteristics of the optical transmitter and the characteristics of the optical receiver are flat in principle.

Documents of the prior art

Patent document

Patent document 1: japanese patent laid-open publication No. 2016-072942

Patent document 2: japanese patent No. 4268760

Patent document 3: japanese laid-open patent publication No. 2001-060883

Patent document 4: japanese patent laid-open publication No. 2018-19255

Patent document 5: japanese patent No. 6319487

Disclosure of Invention

Problems to be solved by the invention

The optical transmitter on the transmission side generally has a low-pass type transmission characteristic. In order to compensate for this, a compensation characteristic that emphasizes a high frequency region is set in a compensation circuit in advance, and a transmission characteristic on a transmission side including the compensation circuit and the optical transmitter is set to a flat characteristic. When the transmission characteristics of the transmission side are flattened, the compensation of the reception side is performed on the transmission characteristics of the optical fiber transmission path and the optical receiver.

However, if the compensation characteristic of the compensation circuit on the transmission side is a characteristic that emphasizes a high frequency region, the high frequency increases, the waveform rises sharply, and as a result, an overshoot (overshoot) increases. Thus, there are problems as follows: the PAPR (Peak to Average Power Ratio) becomes large, and a nonlinear effect occurs in the transmitting optical transmitter, resulting in deterioration of transmission characteristics.

The present invention has been made in view of the above circumstances, and an object thereof is to obtain an optical transmission characteristic compensation method and an optical transmission characteristic compensation system that can prevent transmission characteristic degradation due to the occurrence of nonlinear effects in an optical transmitter.

Means for solving the problems

An optical transmission characteristic compensation method according to the present invention is a method of compensating transmission characteristics of an optical transmitter and an optical receiver connected to each other via an optical transmission path, wherein the method has the steps of: a part of compensation of transmission characteristics of the optical transmitter by a transmitter compensation unit disposed at a preceding stage of the optical transmitter; and a step of performing a remainder of the compensation of the transmission characteristic of the optical transmitter and a compensation of the transmission characteristic of the optical receiver by a receiver compensation unit disposed at a subsequent stage of the optical receiver, wherein the transmitter compensation characteristic of the transmitter compensation unit is set so that a peak-to-average power ratio of an output signal of the transmitter compensation unit becomes a predetermined value or less.

Effects of the invention

The invention can prevent the transmission characteristic from being degraded due to the nonlinear effect generated in the optical transmitter.

Drawings

Fig. 1 is a diagram illustrating an optical transmission characteristic compensation system according to embodiment 1.

Fig. 2 is a diagram showing an optical transmission characteristic compensation system according to a comparative example.

Fig. 3 is a diagram illustrating an optical transmission characteristic compensation system according to embodiment 2.

Fig. 4 is a diagram illustrating an optical transmission characteristic compensation system according to embodiment 3.

Fig. 5 is a diagram illustrating an optical transmission characteristic compensation system according to embodiment 4.

Fig. 6 is a diagram illustrating an optical transmission characteristic compensation system according to embodiment 5.

Fig. 7 is a diagram illustrating an optical transmission characteristic compensation system according to embodiment 6.

Fig. 8 is a diagram illustrating an optical transmission characteristic compensation system according to embodiment 7.

Fig. 9 is a diagram illustrating an optical transmission characteristic compensation system according to embodiment 8.

Fig. 10 is a diagram illustrating an optical transmission characteristic compensation system according to embodiment 9.

Fig. 11 is a diagram illustrating an optical transmission characteristic compensation system according to embodiment 10.

Detailed Description

An optical transmission characteristic compensation method and an optical transmission characteristic compensation system according to an embodiment will be described with reference to the drawings. The same or corresponding components are denoted by the same reference numerals, and repeated description thereof is omitted.

Embodiment 1.

Fig. 1 is a diagram illustrating an optical transmission characteristic compensation system according to embodiment 1. The optical transmission characteristic compensation system compensates the transmission characteristics of the optical transmitter 3 of the transmission device 2 and the optical receiver 5 of the reception device 4 connected to each other via the optical fiber transmission line 1.

In the transmission device 2, the signal processing unit 6 performs error correction coding or the like on input data. The band limiting filter 7 limits the band of the output data of the signal processing unit 6. The transmitter compensation unit 8 arranged at the front stage of the optical transmitter 3 compensates the transmission characteristics of the optical transmitter 3 at the rear stage in advance. However, as described below, the transmitter compensation unit 8 only partially compensates the transmission characteristics of the optical transmitter 3.

The PAPR calculating unit 9 calculates the PAPR (peak to average power ratio) of the output signal of the transmitter compensating unit 8. If the inverse characteristic of the transmission characteristic of the optical transmitter 3 is set for the transmitter compensation unit 8, the output signal of the transmitter compensation unit 8 is determined by the waveform of the output signal of the known band limiting filter 7 and the inverse characteristic of the transmission characteristic of the optical transmitter 3. Therefore, the PAPR can be obtained from the transmission characteristics of the optical transmitter 3.

The transmitter compensation characteristic setting unit 10 calculates a transmitter compensation characteristic such that the PAPR of the output signal of the transmitter compensation unit 8 becomes equal to or less than a predetermined value, and sets the transmitter compensation unit 8 so as to reduce the influence of the nonlinear effect in the optical transmitter 3. Specifically, the transmitter compensation characteristic is obtained by superimposing a nonlinear suppression characteristic for suppressing the nonlinear effect on a normal compensation characteristic for compensating the transmission characteristic of the optical transmitter 3. As will be described later, the transmitter compensation characteristic setting unit 10 also obtains the transmitter compensation characteristic by adjusting the nonlinearity suppression characteristic with reference to the measurement result of the signal quality measurement unit 11 of the receiving apparatus 4.

The optical transmitter 3 includes an analog circuit such as a D/a converter, a quadrature modulator, and a buffer amplifier, converts an electric signal input from the transmitter compensation unit 8 into an optical signal, and supplies the optical signal to the optical fiber transmission line 1. When the PAPR of the output signal of the transmitter compensation unit 8 is large, a nonlinear effect occurs in an analog circuit such as a quadrature modulator and a buffer amplifier. When the amplitude of the analog signal to be formed by the D/a converter is equal to or larger than the amplitude that can be formed by the D/a converter, the signal having the amplitude or larger is clipped (clip). The clipped signal is supplied to a quadrature modulator, a buffer amplifier, and the like of the optical transmitter 3. In this case, the number of clipped samples in a certain interval of samples of the signal is referred to as a Clipping rate (Clipping rate). The higher the clipping rate, the lower the PAPR of the signal. Therefore, the PAPR calculating unit 9 may calculate the PAPR in consideration of the clipping rate of the D/a converter.

The optical fiber transmission path 1 transmits an optical signal output from the optical transmitter 3 of the transmission device 2 to the optical receiver 5 of the reception device 4. The optical receiver 5 includes an analog circuit such as a quadrature demodulator or a buffer amplifier, and converts an optical signal received from the optical fiber transmission line 1 into an electrical signal.

The receiver compensation unit 12 disposed at the subsequent stage of the optical receiver 5 mainly compensates the transmission characteristics of the optical receiver 5. As described below, the receiver compensation unit 12 also performs the remaining compensation of the transmission characteristic of the optical transmitter 3 that is not compensated by the transmitter compensation unit 8.

The receiver compensation characteristic setting unit 13 sets the receiver compensation characteristic of the receiver compensation unit 12. Band limiting filter 14 band-limits the output signal of receiver compensation unit 12. The signal processing unit 15 performs various signal processing such as compensation of the transmission characteristics of the optical fiber transmission line 1 and frequency offset compensation on the output signal of the band limiting filter 7, and reproduces the input data. The signal quality measuring unit 11 measures the error rate characteristic of the data reproduced by the signal processing unit 15.

In addition, in this specification, "flattening" or "flat characteristic" means that the passband characteristic is flat. The "band characteristics" and the "transmission characteristics" refer to frequency characteristics in the pass band. The "transfer function" means that "transfer characteristics" are expressed in a functional manner. The transmitter overall characteristic is a characteristic obtained by integrating the transmitter compensation unit 8 and the optical transmitter 3. The receiver overall characteristic is a characteristic obtained by integrating the optical receiver 5 and the receiver compensation unit 12. The transmitter/receiver characteristic is a characteristic obtained by integrating the transmitter characteristic and the receiver characteristic.

Next, a basic operation of the optical transmission characteristic compensation system according to the present embodiment will be described.

[ step 1]

The PAPR calculating unit 9 calculates the PAPR of the output signal of the transmitter compensating unit 8. The transmitter compensation characteristic setting unit 10 calculates a transmitter compensation characteristic such that the PAPR becomes equal to or less than a predetermined value, and sets the calculated transmitter compensation characteristic in the transmitter compensation unit 8. This makes it possible to set the PAPR of the output signal of the transmitter compensation unit 8 to a predetermined value or less, and suppress the nonlinear effect in the optical transmitter 3 of the next stage. Therefore, the transmitter compensation characteristic at this time is regarded as a nonlinear suppression compensation characteristic. In addition, even if the PAPR becomes equal to or higher than the predetermined value due to the BER adjustment in step 3, the nonlinear effect is suppressed if the PAPR is set to a value near the predetermined value. However, the overall transmitter characteristics of the transmitter compensation unit 8 and the optical transmitter 3 are not necessarily flat.

Such nonlinear suppression compensation characteristic can be directly calculated from the PAPR, but may be obtained by arranging a flat transmitter overall characteristic and then adding an additional characteristic (nonlinear suppression characteristic) for suppressing the nonlinear effect to the transmitter compensation characteristic at that time. The nonlinear suppression characteristic is basically any characteristic as long as it can suppress a high frequency, and for example, any one of a gaussian characteristic, a super-gaussian characteristic, and a characteristic obtained by excluding and averaging fluctuation amounts of the transmission characteristic of the optical transmitter 3, or a combination thereof may be used. In addition, the compensation characteristic of the optical fiber transmission path 1 may be entirely or partially superimposed on the transmitter compensation characteristic of the transmitter compensation unit 8 within a range that satisfies the setting condition of the PAPR (range in which desired nonlinearity suppression can be obtained).

[ step 2]

The receiver compensation characteristic setting unit 13 adjusts the receiver compensation characteristic of the receiver compensation unit 12 so that the transmission characteristic of the output signal of the receiver compensation unit 12 becomes a predetermined characteristic. Specifically, the receiver compensation characteristic in which the overall transceiver characteristic, which is the characteristic obtained by integrating the transmitter compensation unit 8, the optical transmitter 3, the optical receiver 5, and the receiver compensation unit 12, is a flat characteristic is calculated and set in the receiver compensation unit 12. The receiver compensation characteristic is obtained by, for example, superimposing an inverse characteristic of a difference from a normal characteristic of the transmitter compensation characteristic (corresponding to an additional characteristic for suppressing the nonlinear effect in step 1) on a normal receiver compensation characteristic in which the overall receiver characteristic is originally flat.

Further, the receiver compensation characteristic may be added with a characteristic of an amplitude component of 1/2 frequency for improving the baud rate. Thus, since the overall characteristics of the transceiver are the characteristics of the 1/2 frequency band in which the baud rate is increased, the signal component for clock reproduction increases, and the reception device 4 can perform the baud rate clock reproduction at high speed and with high accuracy.

[ step 3]

After setting the compensation characteristics of the transmitter compensation section 8 and the receiver compensation section 12, the signal quality measurement section 11 measures the error rate (BER) characteristic as the signal quality. The BER characteristic can be easily obtained by inputting a known random signal from the transmission device 2 and comparing the input signal with the output signal of the signal processing unit 15. By changing the output of the optical transmitter 3 or the input of the optical receiver 5, the BER characteristic for the optical signal-to-noise ratio (OSNR) can be obtained. The BER characteristic measurement is not limited to the above method, and various methods can be performed. The comparison of the signal quality is not limited to the BER characteristic, and an index indicating the signal quality such as OSNR itself, a ratio of an optical signal to (noise + distortion), or a Q value may be measured and compared with a predetermined value. The distortion includes distortion due to a nonlinear effect. The same applies to the following explanation of BER characteristics.

The measured BER characteristic is compared with a predetermined OSNR, and the transmitter compensation characteristic in the transmitter compensation characteristic setting section 10 is adjusted as described below. When the OSNR that obtains a predetermined error rate (predetermined signal quality) from the output signal of the receiver compensation unit 12 is greater than a predetermined OSNR, that is, when there is a margin in the OSNR, the transmitter compensation characteristic set in the transmitter compensation characteristic setting unit 10 is adjusted so as to reduce the PAPR of the output of the transmitter compensation unit 8. Next, in step 2, the receiver compensation characteristic of the receiver compensation unit 12 is obtained. This step is performed until the OSNR at which the predetermined error rate is obtained becomes the predetermined OSNR. This can reduce the PAPR within the margin range of the OSNR, and can further reduce the nonlinear effect of the optical transmitter 3. On the other hand, when the OSNR at which the predetermined error rate is obtained is smaller than the predetermined OSNR, that is, when the desired error rate is not satisfied, the transmitter compensation characteristic set in the transmitter compensation characteristic setting unit 10 is adjusted so as to increase the PAPR of the output signal of the transmitter compensation unit 8. Next, in step 2, the receiver compensation characteristic of the receiver compensation unit 12 is obtained. This step is performed until the OSNR at which the predetermined error rate is obtained becomes the predetermined OSNR. In this case, the PAPR may be larger than a predetermined value, but improvement of BER characteristics based on increase of the OSNR is prioritized. If the PAPR can be set to values around a predetermined value, some suppression of the nonlinear effect can be expected.

Next, the effects of the present embodiment will be described in comparison with comparative examples. Fig. 2 is a diagram showing an optical transmission characteristic compensation system according to a comparative example. In the comparative example, the compensation characteristic of the transmitter compensation unit 8 compensates for the band characteristic of the optical transmitter 3, and the overall transmitter characteristic of the transmission device 2 becomes a flat characteristic. The compensation characteristic of the receiver compensation unit 12 compensates for the band characteristic of the optical receiver 5, and the overall receiver characteristic of the receiving apparatus 4 becomes a flat characteristic. In this case, the compensation characteristic of the transmitter compensation unit 8 in which the overall transmitter characteristic is flat is referred to as "normal transmitter compensation characteristic (normal transmission compensation characteristics)", and the compensation characteristic of the receiver compensation unit 12 in which the overall receiver characteristic is flat is referred to as "normal receiver compensation characteristic (normal receiver compensation characteristics)". The same applies hereinafter.

In general, the transmission characteristic of the optical transmitter 3 is similar to that of an isosceles triangle in some cases, because the amplitude component is small in a region with a high frequency. In addition, a fluctuation component is added, which is generated by emphasizing or attenuating a specific frequency component due to high-frequency reflection generated by an impedance mismatching point existing on a signal line in the optical transmitter. Therefore, the transmitter compensation unit 8 is set with compensation characteristics for compensating for the reduction of the amplitude component and the ripple component in the high frequency. Therefore, as the pre-compensation of the optical transmitter 3, the amplitude component in the high frequency region of the output signal of the transmitter compensation unit 8 is enhanced (enhance). When the high frequency component is enhanced, a sharp change occurs in the signal, whereby the PAPR increases. When a signal having an increased PAPR is supplied to the optical transmitter 3, a nonlinear effect is generated due to a nonlinear region of a modulator or an amplifier within the optical transmitter 3, and a large distortion is generated in an output waveform of the optical transmitter 3. In addition, since the D/a converter also defines the resolution (resolution), the signal quality of a signal having a large PAPR deteriorates. The distortion caused by these non-linear effects is difficult to improve by compensation of the filter and increase of the OSNR. Therefore, it is necessary to suppress the nonlinear effect at the time of passing through the optical transmitter 3.

In contrast, in the present embodiment, in step 1, the PAPR calculating unit 9 calculates the PAPR of the output signal of the transmitter compensating unit 8 in which the normal transmitter compensation characteristic is set. In addition, usually, the transmitter compensation characteristic can be directly calculated from the transmission characteristic of the optical receiver 5. The transmitter compensation characteristic setting unit 10 superimposes an additional characteristic for suppressing enhancement of the high-frequency component on the normal transmitter compensation characteristic so as to obtain a predetermined PAPR. The additional characteristic is, for example, a gaussian characteristic. However, as a characteristic for further suppressing enhancement of the high frequency component, super-gaussian characteristics (including characteristics obtained by squaring gaussian characteristics) having a higher degree of freedom of band shape than gaussian characteristics may be used. For example, the super-gaussian characteristic is expressed by the following numerical expression.

f(x)＝exp{-(log2/2)*(x/Bw)^(2*order)}

Where x is a variable associated with frequency, Bw is a 3dB band, and order is order. In addition, the additional characteristic may be a characteristic obtained by averaging a fluctuation portion of the transmission characteristic of the optical transmitter 3.

By superimposing the additional characteristics, the transmitter compensation characteristic of the transmitter compensation unit 8 according to the present embodiment is lower in amplitude characteristic in a high frequency range than the normal transmitter compensation characteristic of the transmitter compensation unit 8 according to the comparative example. Further, the overall transmitter characteristic is lower than the flat characteristic of the comparative example in the amplitude characteristic in the high frequency range. This prevents the transmission characteristics from deteriorating due to the nonlinear effect generated in the optical transmitter 3. In addition, the compensation characteristic of the optical fiber transmission path 1 may be superimposed with all or a part thereof on the transmitter compensation characteristic within a range satisfying the setting condition of the PAPR.

As described above, since the additional characteristic (nonlinear suppression characteristic) is not compensated for in the transmitter compensation unit 8, it is necessary to compensate for this part by the receiver compensation unit 12. That is, the receiver compensation unit 12 of the receiving device 4 sets a characteristic obtained by superimposing the normal receiver compensation characteristic, which is obtained by flattening the overall receiver characteristic, on the inverse characteristic of the additional characteristic of the transmitting device 2. For example, when the additional characteristic of the transmission device 2 is gaussian characteristic, the inverse characteristic thereof is a characteristic in which the amplitude characteristic in the high frequency region is increased. In the case of the super-gaussian characteristic, the amplitude characteristic in the high frequency region becomes further large. Even if the compensation amount in the high frequency range of the receiver compensation characteristic increases, the post-stage of the receiver compensation unit 12 is a digital processing unit, and therefore there is no fear of a nonlinear effect due to a large PAPR as in the transmission device 2. By this receiver compensation, the overall performance of the transceiver as a whole can be flat.

However, as a result of the verification, as described above, another problem occurs in the configuration in which the receiver device 4 compensates for the uncompensated amount in the transmitter compensation unit 8. In the above configuration, since the additional characteristic is not compensated in the transmitter compensation unit 8, an uncompensated portion, which is the transmission characteristic of the optical transmitter 3, is transmitted to the receiving device 4. Specifically, in the spectrum of the output signal of the optical transmitter 3, the amplitude characteristic in the high frequency region is low, and the signal is transmitted to the receiving device 4. In the receiving apparatus 4, in order to compensate for the uncompensated amount of the transmitter compensation unit 8, the compensation amount of the high frequency region is increased in the receiver compensation unit 12. In this case, generally, noise generated by an amplifier or the like is added to the receiving device 4, and thus a component in a high frequency region becomes large with respect to the noise. Thereby, the OSNR in the output side of the receiver compensation section 12 deteriorates compared to the case where the waveform is completely compensated in the transmission device 2. Particularly, when the influence of noise is large as in the case of long-distance transmission, it is found through experimental verification that when a characteristic portion added to the receiver compensation characteristic is emphasized, the OSNR deteriorates, and the compensation effect may not be sufficiently obtained. In addition, when the compensation amount in the receiving apparatus 4 is increased, the compensation width becomes large, and in the case of the same quantization bit number, it is assumed that the resolution (ENOB) also deteriorates. Therefore, the transmitter compensation unit 8 is set with the nonlinear suppression compensation characteristic such that the PAPR is equal to or less than the predetermined value, and the receiver compensation characteristic is set only in the receiving device 4 so that the overall transceiver characteristic becomes flat, so that the optimal compensation characteristic may not be obtained.

Thus, in addition to step 1 and step 2 in which the receiver compensation unit 12 compensates for the amount of uncompensated in the transmitter compensation unit 8, it is necessary to measure the BER in the receiving apparatus 4 and obtain an optimal solution of the compensation characteristic of the transceiver when the BER is optimal in step 3. The specific method is as follows.

Generally, a high signal quality (ratio of signal to (noise + distortion)) is required in a high multilevel modulation scheme used for large capacity transmission at a short distance. In this case, the influence of the deterioration of the signal quality (ratio of signal to (noise + distortion)) due to the nonlinear effect or the like caused by the PAPR of the transmission device 2 is larger than the deterioration of the OSNR due to the ASE (Amplified Spontaneous Emission) of the transmission path. In contrast, in a relatively low multilevel modulation scheme suitable for long-distance transmission, the influence of degradation of the OSNR of the receiving apparatus 4 is larger than the nonlinear effect due to the PAPR of the transmitting apparatus 2. Therefore, when there is a margin for a predetermined OSNR, it is considered that the transmitter compensation characteristic is adjusted to further suppress the nonlinear effect caused by the PAPR in the near-range transmission. On the other hand, when the predetermined OSNR is not satisfied and there is no margin, it is considered to be long-distance transmission, and the transmitter compensation characteristic is adjusted to mitigate suppression of the nonlinear effect due to the PAPR. The PAPR may be higher than a predetermined value but the improvement of the OSNR is prioritized. Even in this case, if the PAPR can be set to values around the predetermined value, some suppression of the nonlinear effect can be expected. As a result, the optimum condition can be set to reduce the nonlinear effect as much as possible and prevent the OSNR from deteriorating as much as possible. In addition, the receiver compensation characteristics are adjusted in either case by the implementation of step 2.

As described above, in the present embodiment, the transmitter compensation characteristic of the transmitter compensation unit 8 is set so that the PAPR of the output signal of the transmitter compensation unit 8 becomes equal to or less than the predetermined value. This prevents the transmission characteristics from deteriorating due to the nonlinear effect generated in the optical transmitter 3.

When the OSNR at which the predetermined error rate is obtained from the output signal of the receiver compensation unit 12 is greater than a predetermined value, that is, when there is a margin in the OSNR, the transmitter compensation characteristic is adjusted so as to reduce the PAPR of the output signal of the transmitter compensation unit 8. On the other hand, when the OSNR at which the predetermined error rate is obtained is smaller than the predetermined value, that is, when the desired error rate is not satisfied, the transmitter compensation characteristic is adjusted so as to increase the PAPR of the output signal of the transmitter compensation unit 8. This makes it possible to optimize both the PAPR and the OSNR, and to obtain the optimum BER characteristics.

Further, as a method of setting the transmitter compensation characteristic and the receiver compensation characteristic, there is a method of initially optimizing the BER, but since the initial setting value is not fixed, a very large amount of adjustment and adjustment time are required. Here, the transmitter compensation characteristic and the receiver compensation characteristic are first set according to a predetermined PAPR, and then the compensation characteristic is optimized by BER. This can reduce the adjustment amount and adjustment time for optimization.

The transmitter compensation characteristic of the transmitter compensation unit 8 is set by the following method: a method of superimposing nonlinear suppression characteristics (for example, but not limited to, gaussian characteristics, super-gaussian characteristics, and averaging characteristics) on the normal transmitter compensation characteristics in the case where nonlinear suppression is not considered, and a method of directly obtaining the nonlinear suppression compensation characteristics.

The receiver compensation characteristic of the receiver compensation unit 12 is set as follows: a method of superimposing an inverse characteristic of the nonlinear suppression characteristic or the optical equalization residual characteristic on a normal receiver compensation characteristic in a case where the nonlinear suppression in the transmission device 2 is not considered, and a method of calculating the characteristic by using the transmitter/receiver transfer function estimation system described in patent document 5. With the latter method, when the transmitter compensation characteristic of the transmission device 2 is set, the receiver compensation characteristic in which the overall characteristic of the transmitter and the receiver is flat can be easily estimated. In this case, the transmitter compensation characteristic and the receiver compensation characteristic can be estimated so that the characteristics of the band of ± baud rate/2 are improved in addition to the flat characteristic as the overall characteristic of the transmitter and the receiver.

The method of setting the "predetermined value" of the PAPR includes the following experimental method and SIM method. In the experimental method, first, in a configuration in which a transceiver is directly connected, a signal obtained by superimposing each nonlinear suppression characteristic and transmission side pre-equalization with a desired modulation format is input, and the PAPR is changed by changing the nonlinear suppression characteristic, thereby obtaining signal quality. Next, in a configuration in which an optical transmitter and an optical receiver are connected via a transmission line, the reception osnr (receiving osnr) is calculated or actually measured from the fiber propagation loss, NF of the optical amplifier, the span number, and the like. The PAPR value when the allowable signal quality characteristics are determined from these pieces of information is set as the "predetermined value" of the PAPR. In the SIM method, first, a signal obtained by superimposing each nonlinear suppression characteristic and transmission-side pre-equalization with a desired modulation format is input to an analog model of DAC performance (resolution), and the PAPR is changed by changing the nonlinear suppression characteristic to obtain signal quality. Next, in a configuration in which an optical transmitter and an optical receiver are connected via a transmission line, the reception OSNR is calculated or actually measured from the optical fiber propagation loss, NF of the optical amplifier, the span number, and the like. The PAPR value when the allowable signal quality characteristics are determined from these pieces of information is set as the "predetermined value" of the PAPR.

Embodiment mode 2

Fig. 3 is a diagram illustrating an optical transmission characteristic compensation system according to embodiment 2. In the present embodiment, the transmitter compensation characteristic setting unit 10 includes a nonlinear suppression characteristic generation unit 16 and a transmitter compensation characteristic generation unit 17. The receiver compensation characteristic setting unit 13 includes a nonlinear suppression inverse characteristic generating unit 18 and a receiver compensation characteristic generating unit 19. The other structure is the same as embodiment 1.

The nonlinear suppression characteristic generation unit 16 of the transmission device 2 generates a nonlinear suppression characteristic such that the PAPR of the output signal of the transmitter compensation unit 8 is lower than a predetermined value. The transmitter compensation characteristic generation unit 17 superimposes the nonlinear suppression characteristic on the normal transmitter compensation characteristic, and sets the transmitter compensation unit 8.

The nonlinear suppression inverse characteristic generation unit 18 of the reception device 4 generates a nonlinear suppression inverse characteristic that is an inverse characteristic of the nonlinear suppression characteristic generated by the transmission device 2. The receiver compensation characteristic generation unit 19 superimposes the nonlinear suppression inverse characteristic on the normal receiver compensation characteristic, and sets the receiver compensation unit 12.

Next, the operation of the system according to the present embodiment will be described. First, a normal transmitter compensation characteristic for compensating for the transmission characteristic of the optical transmitter 3 is obtained by measuring the transmission characteristic. Next, the PAPR calculating unit 9 obtains the PAPR of the output signal of the transmitter compensating unit 8. The PAPR may be calculated from the measured transmission characteristic of the optical transmitter 3 or the normal transmitter compensation characteristic, in addition to being directly obtained from the output signal of the transmitter compensation unit 8. In this case, the amplitude limit ratio of the D/a converter in the optical transmitter 3 may be considered.

Next, the nonlinear suppression characteristic generation unit 16 generates a nonlinear suppression characteristic such that the PAPR becomes a predetermined value or less. For example, a gaussian characteristic can be used as the nonlinear suppression characteristic. At this time, the average value and standard deviation constituting the gaussian characteristic are determined so that the PAPR becomes a predetermined value or less. The nonlinear suppression characteristic is not necessarily limited to gaussian characteristics, and may be any characteristic that can suppress characteristics in a high frequency range. For example, super-gaussian characteristics having sharper characteristics or characteristics obtained by averaging fluctuations in the transmission characteristics of the optical transmitter 3 may be applied.

Next, the transmitter compensation characteristic generation unit 17 superimposes the nonlinear suppression characteristic on the normal transmitter compensation characteristic set in the transmitter compensation unit 8 to generate the nonlinear suppression compensation characteristic, and sets the nonlinear suppression compensation characteristic in the transmitter compensation unit 8. This makes it possible to set the PAPR of the output signal of the transmitter compensation unit 8 to a predetermined value or less, and to suppress the nonlinear effect in the optical transmitter 3. However, the transmission characteristics of the optical transmitter 3 are not completely compensated, and the characteristics that are not compensated by the transmitter compensation unit 8 are transmitted to the receiving device 4. The characteristic not compensated by the transmitter compensation unit 8 corresponds to a nonlinear suppression characteristic.

Next, the receiver compensation unit 12 compensates for the characteristics not compensated in the transmitter compensation unit 8 and the band characteristics of the optical receiver 5 together. At this time, in order to compensate for the characteristics that are not compensated for in the transmitter compensation unit 8, a nonlinear suppression inverse characteristic that is an inverse characteristic of the nonlinear suppression characteristic of the transmission device 2 is generated. The receiver compensation characteristic generation unit 19 superimposes the nonlinear suppression inverse characteristic on the normal receiver compensation characteristic (compensation characteristic of the band characteristic of the optical fiber transmission line 1 and the band characteristic of the optical receiver 5) to generate a receiver compensation characteristic, and sets the receiver compensation unit 12.

After the compensation characteristics of the transmitter compensation unit 8 and the receiver compensation unit 12 are set, the BER is measured, and the nonlinear suppression characteristics are adjusted based on the current set value, thereby determining the nonlinear suppression characteristics that are the optimal BER characteristics. This can provide the same effects as those of embodiment 1.

Embodiment 3

Fig. 4 is a diagram illustrating an optical transmission characteristic compensation system according to embodiment 3. In the present embodiment, the transmitter compensation characteristic setting unit 10 includes a nonlinear suppression compensation characteristic generation unit 20, as compared with embodiment 2. The nonlinear suppression compensation characteristic generation unit 20 directly generates a nonlinear suppression compensation characteristic such that the PAPR of the output signal of the transmitter compensation unit 8 is lower than a predetermined value, and sets the nonlinear suppression compensation characteristic in the transmitter compensation unit 8. This nonlinear suppression compensation characteristic corresponds to a superposition of the nonlinear suppression characteristic of embodiment 2 and the normal transmitter compensation characteristic. The other configurations and operations are the same as those of embodiment 2, and the same effects as those of embodiments 1 and 2 can be obtained.

Embodiment 4

Fig. 5 is a diagram illustrating an optical transmission characteristic compensation system according to embodiment 4. In the present embodiment, compared to embodiment 2, the receiver compensation characteristic setting unit 13 includes a transmitter transfer function estimating unit 21 and 1 st and 2 nd receiver transfer function estimating units 22 and 23.

As the transmitter transfer function estimating unit 21, the 1 st and 2 nd receiver transfer function estimating units 22 and 23, the transmitter transfer function estimating unit and the 1 st and 2 nd receiver transfer function estimating units of the optical transmission characteristic estimating system of japanese patent No. 6319487 can be used, respectively. That is, the 1 st receiver transfer function estimating unit 22 estimates a temporary transfer function of the optical receiver 5 by performing fourier transform on data output from the optical receiver 5 when a test signal having a known frequency spectrum is input to the input terminal of the optical receiver 5, and calculates the inverse of the temporary transfer function of the optical receiver 5 to obtain a temporary inverse transfer function of the optical receiver 5. When the 1 st known signal is transmitted from the transmission device 2 to the reception device 4, the transmitter transfer function estimating unit 21 compensates the transmission path characteristics and the transmission characteristics of the optical receiver 5 in the reception device 4, and the influence of the transfer function of the optical transmitter 3 remains in the 1 st known signal. The 1 st known signal is input to a digital filter, and the transfer function or the inverse transfer function of the optical transmitter 3 is estimated as a filter coefficient of the digital filter when the error with the 1 st known signal is minimized. The compensation of the transfer characteristic of the optical receiver 5 for the 1 st known signal is performed using the temporary inverse transfer function of the optical receiver 5. The 2 nd receiver transfer function estimating unit 23 inputs the 2 nd known signal transmitted from the transmitting apparatus 2 to the receiving apparatus 4 to the digital filter, and estimates the transfer function or inverse transfer function of the optical receiver 5 as a filter coefficient of the digital filter when an error between the output of the digital filter and a signal obtained by adding the estimated transfer function or inverse transfer function of the optical transmitter 3 and the transmission path characteristic to the original 2 nd known signal is minimized.

Next, the operation of the system according to the present embodiment will be described. First, the 1 st receiver transfer function estimating unit 22 estimates a temporary transfer function of the optical receiver 5. The transmitter transfer function estimating unit 21 estimates the transfer function or the inverse transfer function of the optical transmitter 3 using the temporary transfer function of the optical receiver 5, and obtains the normal compensation characteristic of the optical transmitter 3 based on the estimated transfer function or the inverse transfer function. The PAPR calculating unit 9 obtains the PAPR of the output signal of the transmitter compensating unit 8 from the normal transmitter compensation characteristic. As in embodiment 2, the nonlinear suppression characteristic generation unit 16 generates a nonlinear suppression characteristic such that the PAPR is lower than a predetermined value, and the transmitter compensation characteristic generation unit 17 superimposes the nonlinear suppression characteristic on the normal transmitter compensation characteristic and sets the resultant to the transmitter compensation unit 8.

The 2 nd receiver transfer function estimating unit 23 obtains a receiver compensation characteristic in which the overall transceiver characteristic becomes a flat characteristic, and sets the receiver compensation characteristic in the receiver compensating unit 12. However, the characteristic (nonlinear suppression characteristic) that is not compensated by the transmitter compensation unit 8 is transmitted to the receiving device 4, and the 2 nd receiver transfer function estimation unit 23 also obtains the receiver compensation characteristic with reference to the characteristic. Other structures and operations are the same as those in embodiment 2.

In the present embodiment, since the normal transmitter compensation characteristic of the transmitter compensation unit 8 and the receiver compensation characteristic of the receiver compensation unit 12 are obtained by the existing system, it is not necessary to construct a new system as in embodiments 2 and 3, and the adjustment of the nonlinearity suppression characteristic can be performed more easily. Except for this, the same effects as in embodiments 1 and 2 can be obtained.

Embodiment 5

Fig. 6 is a diagram illustrating an optical transmission characteristic compensation system according to embodiment 5. In the present embodiment, the transmitter compensation characteristic setting unit 10 includes a nonlinear suppression compensation characteristic generation unit 20, as compared with embodiment 4. The receiver compensation characteristic setting unit 13 further includes a nonlinear suppression inverse characteristic generating unit 18. The other structure is the same as embodiment 4.

As in embodiment 4, the transmitter transfer function estimating unit 21 obtains the normal compensation characteristic of the optical transmitter 3. The PAPR calculating unit 9 obtains the PAPR of the output signal of the transmitter compensating unit 8 from the normal transmitter compensation characteristic. The nonlinear suppression inverse characteristic generating unit 18 generates a nonlinear suppression characteristic such that the PAPR is lower than a predetermined value, and generates a nonlinear suppression inverse characteristic which is an inverse characteristic of the nonlinear suppression characteristic. The transmitter transfer function estimating unit 21 estimates the transmitter transfer function of the optical transmitter 3 using the content obtained by superimposing the nonlinear suppression inverse characteristic on the temporary transfer function of the optical receiver 5. The transmitter transfer function is obtained by superimposing a nonlinear suppression characteristic on a transfer function obtained by using only a temporary transfer function. The nonlinear suppression compensation characteristic generation unit 20 sets the estimated optical transmitter transfer function as a nonlinear suppression compensation characteristic to the transmitter compensation unit 8.

For example, instead of superimposing the nonlinear suppression inverse characteristic on the temporary transfer function of the optical receiver 5, the optical transmitter transfer function may be obtained by superimposing the known signal input from the transmission device 2 or the received data acquired from the optical receiver 5. By superimposing the nonlinear suppression inverse characteristic on a predetermined parameter of the optical transmission characteristic estimation system, the transmitter transfer function on which the nonlinear suppression characteristic is superimposed can be easily obtained.

In the present embodiment, since the normal transmitter compensation characteristic of the transmitter compensation unit 8 and the receiver compensation characteristic of the receiver compensation unit 12 are obtained by the existing system, it is not necessary to construct a new system as in embodiments 2 and 3, and the adjustment of the nonlinearity suppression characteristic can be performed more easily. Except for this, the same effects as in embodiments 1 and 2 can be obtained.

Embodiment 6

Fig. 7 is a diagram illustrating an optical transmission characteristic compensation system according to embodiment 6. In the present embodiment, an optical equalizer 24 is provided at a stage subsequent to the optical transmitter 3, as compared with embodiment 1. As in embodiment 1, the nonlinear suppression characteristic is superimposed on the normal transmitter compensation characteristic, and the transmitter compensation unit 8 is set as the nonlinear suppression compensation characteristic so that the PAPR of the output of the transmitter compensation unit 8 becomes a predetermined value. That is, the transmitter compensation unit 8 only partially compensates the transmission characteristic of the optical transmitter 3, and the nonlinearity suppression characteristic is removed from the compensation of the optical transmitter 3 and becomes an uncompensated amount in the transmitter compensation unit 8.

The optical equalizer 24 includes, for example, an optical filter, and the transmission characteristic of the optical filter is designed so that the transmission characteristic of the output signal of the optical equalizer 24 becomes flat. The optical equalizer 24 is a device (optical filter) integrated with the modulator in the optical transmitter 3. Alternatively, the filter function of the Wavelength Selective Switch (WSS) may be used as long as the transmission path passes through the WSS.

The compensation characteristic (transmission characteristic of the optical filter) of the optical equalizer 24 is designed to be the inverse of the nonlinear suppression characteristic calculated by the transmitter compensation characteristic setting section 10. The optical equalizer 24 performs the remaining part of the compensation of the transmission characteristic of the optical transmitter 3. The filter can be designed to have a gaussian shape, a super gaussian shape, or a frequency band having inverse characteristics of each. The optical fiber transmission path 1 transmits the optical signal output from the optical equalizer 24 of the transmission device 2 to the optical receiver 5 of the reception device 4.

The overall transmitter characteristic of embodiment 6 is obtained by integrating not only the transmitter compensation unit 8 and the optical transmitter 3 but also the optical equalizer 24. The amplitude characteristic in the high frequency region is low for the transmitter overall characteristic of embodiment 1 (the band characteristic of the output signal of the optical transmitter 3), but the transmitter overall characteristic of embodiment 6 is a flat characteristic by the optical equalizer 24. That is, the filter of the optical equalizer 24 improves the amplitude characteristic in the high frequency region. For example, when the nonlinearity suppression characteristic is a super-gaussian characteristic, an inverse characteristic of the super-gaussian characteristic is set for the filter of the optical equalizer 24. In general, although an optical filter can be manufactured using a dielectric multilayer film, for example, super-gaussian characteristics or inverse characteristics thereof can achieve relatively close characteristics.

As described above, since the overall transmitter characteristics of the transmission device 2 can be flattened, the OSNR on the reception side can be improved as compared with embodiment 1 in which the OSNR on the reception side is deteriorated due to the uncompensated amount in the transmitter compensation unit 8. Therefore, in step 2, the receiver compensation characteristic of the receiver compensation unit 12 can be obtained only by the normal receiver compensation characteristic in which the overall receiver characteristic is a flat characteristic.

However, since the optical equalizer 24 is an analog circuit, it may be difficult to completely compensate the nonlinear suppression characteristic, and a characteristic (hereinafter referred to as an optical equalization residual characteristic) that the optical equalizer 24 cannot compensate may be transmitted to the receiving apparatus 4. However, the optical balance residual characteristic is smaller than the nonlinear suppression characteristic. The receiver compensation unit 12 compensates for both the optical balance residual characteristic and the band characteristic of the optical receiver 5. At this time, the optical equalization residual characteristic is obtained as a difference between the band characteristic and the flat characteristic of the output signal of the optical equalizer 24. Then, the receiver compensation characteristic generating unit 19 generates an optical equalization residual inverse characteristic, superimposes the optical equalization residual inverse characteristic on the normal receiver compensation characteristic (compensation characteristic of the band characteristic of the optical fiber transmission line 1 and the band characteristic of the optical receiver 5), and sets the optical equalization residual inverse characteristic to the receiver compensation unit 12.

When the optical equalization residual characteristic affects the OSNR of the receiving apparatus 4, the transmitter compensation characteristic can be adjusted based on the result of the signal quality measurement unit 11, as in embodiment 1, and as a result, optimal transmitter compensation and receiver compensation can be performed. However, in the present embodiment, since the flat characteristic is realized in the transmission device 2, the optical balance residual characteristic can be extremely smaller than the characteristic that is not compensated in the transmitter compensation unit 8 in embodiment 1, and therefore, the compensation characteristic of the transceiver can be set more favorably than that in embodiment 1. Except for this, the same effects as those of embodiment 1 can be obtained.

When the optical equalizer 24 is inserted to the output side of the optical transmitter 3, a predetermined OSNR is easily obtained as compared with the case where it is not inserted. However, when a predetermined OSNR is not obtained, although the PAPR may be slightly larger than the predetermined value, the BER characteristic is preferentially improved by the increase in the OSNR. The PAPR can be set to values around a predetermined value, and suppression of a certain degree of nonlinear effect can be expected.

Embodiment 7

Fig. 8 is a diagram illustrating an optical transmission characteristic compensation system according to embodiment 7. In the present embodiment, the transmitter compensation characteristic setting unit 10 includes a nonlinear suppression characteristic generation unit 16 and a transmitter compensation characteristic generation unit 17. The receiver compensation characteristic setting section 13 includes an optical equalization residual inverse characteristic generating section 25 and a receiver compensation characteristic generating section 19. The other structure is the same as embodiment 6.

The nonlinear suppression characteristic generation unit 16 of the transmission device 2 generates a nonlinear suppression characteristic such that the PAPR of the output signal of the transmitter compensation unit 8 is lower than a predetermined value. The transmitter compensation characteristic generation unit 17 superimposes the nonlinear suppression characteristic on the normal transmitter compensation characteristic, and sets the resultant to the transmitter compensation unit 8.

The nonlinear suppression characteristic includes an uncompensated amount that is not compensated for in the transmitter compensation section 8. The optical equalizer 24 compensates for the uncompensated amount. If this compensation is performed completely, the band characteristic of the output signal of the optical equalizer 24 becomes flat. However, when the compensation by the optical equalizer 24 is incomplete, the compensation cannot be completed, and the remaining amount becomes an optical equalization residual characteristic and is transmitted to the receiving apparatus 4. The optical equalization residual characteristic can be detected as a difference between the band characteristic and the flat characteristic of the output signal from the optical equalizer 24, but includes a compensation error in the transmitter compensation unit 8 in addition to the incompleteness of the optical equalizer 24.

The optical balance residual inverse characteristic generating section 25 of the receiving device 4 generates the optical balance residual inverse characteristic from the above-described optical balance residual characteristic. The receiver compensation characteristic generation unit 19 superimposes the optical equalization residual inverse characteristic on the normal receiver compensation characteristic, and sets the receiver compensation unit 12. This can provide the same effects as those of embodiment 6.

Embodiment 8

Fig. 9 is a diagram illustrating an optical transmission characteristic compensation system according to embodiment 8. In the present embodiment, the transmitter compensation characteristic setting unit 10 includes a nonlinear suppression compensation characteristic generation unit 20, as compared with embodiment 7. The nonlinear suppression compensation characteristic generation unit 20 of the transmission device 2 directly generates a nonlinear suppression compensation characteristic such that the PRPA of the output signal of the transmitter compensation unit 8 is lower than a predetermined value, and sets the nonlinear suppression compensation characteristic to the transmitter compensation unit 8. This nonlinear suppression compensation characteristic corresponds to a characteristic obtained by superimposing the nonlinear suppression characteristic of embodiment 7 and the normal transmitter compensation characteristic. The band characteristic of the optical filter of the optical equalizer 24 is preferably designed as an inverse characteristic of a difference (corresponding to the nonlinear suppression characteristic) between the nonlinear suppression compensation characteristic of the nonlinear suppression compensation characteristic generation unit 20 and the normal transmitter compensation characteristic. The other structure is the same as embodiment 7, and the same effects as embodiments 6 and 7 can be obtained.

Embodiment 9

Fig. 10 is a diagram illustrating an optical transmission characteristic compensation system according to embodiment 9. In the present embodiment, compared to embodiment 7, the receiver compensation characteristic setting unit 13 includes a transmitter transfer function estimating unit 21 and 1 st and 2 nd receiver transfer function estimating units 22 and 23. The transmitter transfer function estimating unit 21 and the 1 st and 2 nd receiver transfer function estimating units 22 and 23 have the same configurations and functions as those of embodiment 4. The other structure is the same as embodiment 7.

As in embodiment 4, the 1 st receiver transfer function estimating unit 22 estimates a temporary transfer function of the optical receiver 5. The transmitter transfer function estimating unit 21 estimates the transfer function or the inverse transfer function of the optical transmitter 3 using the temporary transfer function of the optical receiver 5. At this time, the optical equalizer 24 is bypassed, and thereby the normal transmitter compensation characteristic is obtained. The operations of the nonlinear suppression characteristic generation unit 16, the transmitter compensation characteristic generation unit 17, and the optical equalizer 24 are the same as those in embodiment 7. The 2 nd receiver transfer function estimating section 23 estimates a transfer function which is a transfer characteristic of the optical receiver 5. The estimated transfer function of the optical receiver 5 also contains the optical equalization residual characteristics that are not compensated by the transmitting means 2. The receiver compensation unit 12 compensates the optical balance residual characteristic and the band characteristic of the optical receiver 5 together.

In the present embodiment, since the normal transmitter compensation characteristic of the transmitter compensation unit 8 and the receiver compensation characteristic of the receiver compensation unit 12 are obtained by the existing system, it is not necessary to construct a new system as in embodiments 7 and 8, and the adjustment of the nonlinearity suppression characteristic can be performed more easily. Except for this, the same effects as in embodiments 6 and 7 can be obtained.

Embodiment 10

Fig. 11 is a diagram illustrating an optical transmission characteristic compensation system according to embodiment 10. In the present embodiment, the transmitter compensation characteristic setting unit 10 includes a nonlinear suppression compensation characteristic generation unit 20, as compared with embodiment 9. The receiver compensation characteristic setting unit 13 further includes a nonlinear suppression inverse characteristic generating unit 18. The other structure is the same as embodiment 9. The functions and operations of the receiver compensation characteristic setting unit 13 and the nonlinear suppression compensation characteristic generating unit 20 are the same as those of embodiment 4. This can provide the same effects as those of embodiment 4.

In the present embodiment, since the normal transmitter compensation characteristic of the transmitter compensation unit 8 and the receiver compensation characteristic of the receiver compensation unit 12 are obtained by the existing system, it is not necessary to construct a new system as in embodiments 7 and 8, and the adjustment of the nonlinearity suppression characteristic can be performed more easily. Except for this, the same effects as in embodiments 6 and 7 can be obtained.

Further, the phase compensation may be performed by recording a program for realizing the functions of the optical transmission characteristic compensation method and the optical transmission characteristic compensation system according to embodiments 1 to 10 in a computer-readable recording medium, and causing a computer system or a programmable logic device to read and execute the program recorded in the recording medium. The "computer system" herein includes hardware such as an OS and peripheral devices. In addition, the "computer system" also includes a WWW system having a homepage providing environment (or a display environment). The term "computer-readable recording medium" refers to a storage device such as a flexible disk, a magneto-optical disk, a removable medium such as a ROM or a CD-ROM, or a hard disk incorporated in a computer system. The "computer-readable recording medium" also includes a medium that holds a program for a certain period of time, such as a volatile memory (RAM) in a computer system constituting a server or a client when the program is transmitted via a network such as the internet or a communication line such as a telephone line. The program may be transmitted from a computer system storing the program in a storage device or the like to another computer system via a transmission medium or by a transmission wave through the transmission medium. Here, the "transmission medium" for transmitting the program refers to a medium having a function of transmitting information, such as a network (communication network) such as the internet, or a communication line (communication line) such as a telephone line. The program may be used to implement a part of the functions described above. In addition, the above-described functions may be realized by a combination with a program already recorded in the computer system, so-called differential file (differential program).

Description of reference numerals:

1 optical fiber transmission line (optical transmission line), 3 optical transmitter, 5 optical receiver, 8 transmitter compensation unit, 12 receiver compensation unit, 13 receiver compensation characteristic setting unit, 10 transmitter compensation characteristic setting unit, 24 optical equalizer (optical filter), 16 nonlinear suppression characteristic generation unit, and 17 transmitter compensation characteristic generation unit.