JP6418973B2 - Dispersion compensation method, optical signal transmitter, and optical communication system - Google Patents

Description

  The present invention relates to a dispersion compensation method used in the field of optical fiber communication.

For the purpose of increasing the capacity of optical communication, an optical transmission system employing a multi-level modulation method has been popularized.
In particular, the Pulse Amplitude Modulation (PAM) transmission system, which transmits multi-level modulated optical signal amplitude and receives light by direct detection (square detection), is compared with a digital coherent system that combines coherent reception and digital signal processing. Since the frequency utilization efficiency can be improved with a simple transceiver configuration, it is promising as a method for realizing high-efficiency and large-capacity communication between metro areas and short-range data centers.

On the other hand, when the transmission distance exceeds several tens of kilometers, waveform distortion due to group-velocity dispersion (GVD) of the transmission fiber becomes large, which causes deterioration in error rate characteristics.
Optical dispersion compensation using a dispersion compensation fiber or dispersion compensation module that has been conventionally used is effective because it operates on a multi-wavelength signal with a single passive component, but increases the cost and size of the system, Further, since the signal-to-noise ratio (SNR) is deteriorated by a large insertion loss, it is not preferable from this viewpoint.

  Since the digital coherent method can receive the complex amplitude of the signal, it is possible to perform dispersion compensation by digital signal processing at the receiver. However, since the PAM transmission method uses direct detection, the same compensation operation is fundamental. Is impossible.

Therefore, a dispersion compensation method has been proposed in which digital signal processing is performed on the transmission side, and GVD of the transmission path is compensated in advance before transmitting the signal using a digital-analog converter (DAC) and an IQ modulator. (Non-Patent Document 1 to Non-Patent Document 4).
As a specific means of digital signal processing, a method using a finite impulse response (FIR) filter is generally used, but after performing Fourier transform on the signal data, various types of filtering are performed in the frequency domain. Sometimes it is done.

J. McNicol et al., “Electrical Domain Compensation of Optical Dispersion,” Proc. OFC2005, paper OThJ3. R.I.Killey et al., “Electronic Dispersion Compensation by Signal Predistortion Using Digital Processing and a Dual-Drive Mach-Zehnder Modulator,” Photon. Technol. Lett., Vol.17, p.714 (2005). P.M.Watts et al., “Performance of Electronic Predistortion Systems with 1 Sample / bit Processing using Optical Duobinary Format,” Proc. ECOC2007, paper 3.1.6. L.Dou et al., “Electronic Pre-Distortion Operating at 1Sample / symbol with Accurate Bias Control for CD Compensation,” Proc. OFC2010, paper OThT4.

In order to perform complete dispersion compensation before transmission by digital signal processing and DAC and IQ modulator, as reported in Non-Patent Document 1 and Non-Patent Document 2, at a sampling rate of 2 samples / symbol or more Digital signal processing and DAC are required.
On the other hand, if dispersion compensation can be implemented by digital signal processing of one sample / symbol and digital-analog conversion (DA conversion), the required sampling rate of hardware can be kept low, which is advantageous in terms of cost.

However, the method for determining the tap coefficient of the FIR filter for that purpose is not intuitive, and the inverse Fourier transform of the inverse function of the transfer function in the frequency domain of the transmission path is used as it is for the tap coefficient of the FIR filter. The method is not applicable.
The reason for this is that when digital signal processing of 1 sample / symbol and digital-analog conversion are used, if the symbol rate of the signal is B, dispersion compensation is performed for the signal spectrum component whose frequency range is | f | <B / 2. On the other hand, it is impossible to control spectral components for frequencies outside the range, and thus a waveform with correct dispersion compensation cannot be obtained as it is.

  Therefore, a method using a look-up table (Non-Patent Document 3) or a method in which a signal spectrum is limited in advance to a frequency range of | f | <B / 2 by a physical filter in the electrical domain (Non-Patent Document 3). 4) has been proposed, but none of them are practical, and a more flexible and practical method is required.

  In the present invention, when a given sample transmission line dispersion value is subjected to dispersion compensation using a digital signal processing of one sample / symbol and a DAC and an IQ modulator on the transmission side, tap coefficients of the FIR filter Proposes an apparatus and method that can be easily obtained by calculation.

(1)
An optical transmitter comprising at least a FIR filter, a DAC, a laser diode capable of outputting a single wavelength continuous light, and an IQ modulator,
A digital signal having an input signal amplitude of one sample per symbol is subjected to digital signal processing by an FIR filter, and then an IQ signal is converted from a DA signal by a DAC and a single wavelength continuous light output from the laser diode. It is configured to output the modulated optical signal,
The FIR filter uses the tap coefficient determined to compensate the accumulated group velocity dispersion amount (D) of the optical transmission line to and from the optical receiver that receives the modulated optical signal from the optical transmitter. An optical transmitter characterized by processing to compensate for a group velocity dispersion amount of an optical transmission line within the transmitter.
(2)
The optical transmitter as set forth in (1), wherein the tap coefficient c _n of the FIR filter is obtained by the following expressions (1) and (2).
(3)
An optical transmission system comprising the optical transmitter described in either (1) or (2).

(4)
A method for compensating for group velocity dispersion in an optical transmission line within a transmitter,
In the transmitter comprising at least a FIR filter, a DAC, a laser diode capable of outputting a single wavelength continuous light, and an IQ modulator,
Given the cumulative group velocity dispersion amount (D) of the optical transmission line, the tap coefficient is determined so as to compensate the cumulative group velocity dispersion amount,
The FIR filter is configured using the obtained tap coefficient,
A digital signal having an input signal amplitude of one sample per symbol is subjected to digital signal processing by the configured FIR filter,
Next, the IQ modulator sends out the modulated optical signal to the optical transmission line from the electric signal that is DA-converted by the DAC and the single-wavelength continuous light output from the laser diode. A method of compensating for velocity dispersion within a transmitter.
(5)
Wherein said tap coefficients c _n of the FIR filter to compensate for the group velocity dispersion of the optical transmission line described is characterized in that determined by the formula described below (1) and (2) (4) in the transmitter.
(6)
(4) A program for executing the method according to any one of (5) and a storage medium storing the program.

  Further, according to the present invention, an arbitrary transmission is performed when dispersion compensation is performed using a digital signal processing of one sample / symbol and a DAC and an IQ modulator on a given transmission path dispersion value. An apparatus and method for configuring a transfer function in a frequency domain equalizer (FDE) so as to compensate for a path dispersion value in advance are proposed.

(7)
An optical transmitter including at least an FDE, a DAC, a laser diode capable of outputting a single wavelength continuous light, and an IQ modulator, wherein a digital signal having an input signal amplitude of one sample per symbol is a digital signal by FDE in the frequency domain. The IQ modulator is configured to output the modulated optical signal from the electrical signal that has been processed and then DA-converted by the DAC and the single-wavelength continuous light output from the laser diode, and the optical transmitter The FDE performs digital signal processing using the transfer function obtained to compensate the accumulated group velocity dispersion amount (D) of the optical transmission line between the optical receivers receiving the modulated optical signal from An optical transmitter characterized in that the group velocity dispersion amount of an optical transmission line is compensated in the apparatus.
(8)
(7) The optical transmitter for compensating for the group velocity dispersion of the optical transmission line in the transmitter, characterized in that the transfer function is obtained as follows. From h (t) obtained by equation (1) described later, n is an integer, B is a symbol rate, sampling time interval is T = 1 / B, and h _n = h (nT) is an inverse of H _{n obtained} by Fourier transform. _Let H _n ⁻¹ be the transfer function.
(9)
An optical transmission system comprising the optical transmitter according to any one of (7) and (8).

(10)
A method for compensating for group velocity dispersion in an optical transmission line within a transmitter,
In the transmitter comprising a laser diode and an IQ modulator capable of outputting at least FDE, DAC, and single-wavelength continuous light,
Given the accumulated group velocity dispersion amount (D) of the optical transmission line, a transfer function is obtained to compensate for the accumulated group velocity dispersion amount,
The FDE is configured using the obtained transfer function, and a digital signal having an input signal amplitude of one sample per symbol is subjected to digital signal processing in the frequency domain by the configured FDE, and then DA-converted by the DAC. The IQ modulator compensates the group velocity dispersion of the optical transmission line in the transmitter, characterized in that the IQ modulator sends the modulated optical signal to the optical transmission line from the electrical signal and the single wavelength continuous light output from the laser diode. how to.
(11)
(10) The method for compensating for group velocity dispersion of an optical transmission line in the transmitter according to (10), wherein the transfer function is obtained as follows. From h (t) obtained by equation (1) described later, n is an integer, B is a symbol rate, sampling time interval is T = 1 / B, and h _n = h (nT) is an inverse of H _{n obtained} by Fourier transform. _Let H _n ⁻¹ be the transfer function.
(12)
(10) A program characterized by implementing the method according to any one of (11) and a storage medium storing the program.

  When digital signal processing of 1 sample / symbol and DAC / IQ modulator are used to compensate the transmission line dispersion value of any value, the tap coefficient of the FIR filter can be easily obtained, and the transfer function can be configured in the FDE. .

It is a figure showing the whole transmission system of this invention. FIG. 2 shows an amplitude time waveform obtained by coherently receiving an impulse train generated by a transmitter and reproducing its waveform data by digital signal processing. An amplitude time waveform representing the result of calculating the waveform h (t) obtained by adding the accumulated dispersion value of D = −1700 ps ² of the transmission line to the impulse response g (t) shown in FIG. Yes (after transmission). The numerical value of the real and imaginary parts of the tap coefficients c _n of the FIR filter are those shown in FIG. 4 is a time amplitude waveform when the tap coefficient FIR filter obtained in FIG. 4 is applied to the impulse response g (t) (before transmission). 6 is a time amplitude waveform when a cumulative dispersion value of D = −1700 ps ² is applied to the waveform of FIG. 5 (after transmission). It is the transmission system whole figure used at the time of the experiment of Example 2. FIG. It is a figure showing the eye pattern of the received signal in case the length of a transmission fiber is (a) 0 km (Back to back), (b) 40 km, (c) 80 km, and (d) 120 km. It is a figure showing the measurement result of the bit error rate (BER) of the received signal obtained when the optical signal to noise ratio (OSNR) of the received signal is changed. It is a figure showing the measurement system which acquires the impulse response time waveform g (t) of the whole transmitter. It is a process schematic diagram which acquires the impulse response time waveform g (t) of the whole transmitter. It is the figure which showed the procedure which calculates | requires the FIR filter tap coefficient for implementing transmission side dispersion compensation by calculation. It is a figure showing the whole transmission system of this invention when implementing transmission side dispersion compensation using FDE of Example 4. FIG. It is the figure which showed the procedure when implementing transmission side dispersion compensation in FDE. It is a diagram illustrating a procedure for acquiring a transfer function H _n ^-1 used in FDE.

A mode for carrying out the present invention in a PAM transmission system will be described.
FIG. 1 shows the entire transmission system.
The application of the present invention is not limited to the PAM transmission method.
In FIG. 1, some of the components and devices that are normally used in the transmission system, such as an optical amplifier and an optical filter, are omitted. However, there is no problem in explaining the present invention.

In the transmitter of FIG. 1, an information source consisting of a bit string is input, and mapping to PAM amplitude is performed.
Digital signal processing by the FIR filter is performed on the PAM signal amplitude.

  The FIR filter used here can execute processing for compensating for distortion of transmitter hardware such as a DAC or IQ modulator in addition to the processing for dispersion compensation of the present invention described later.

The tap coefficient of the FIR filter is a complex number, and a complex output is obtained with respect to an input of a PAM signal amplitude that is a real number.
Digital-to-analog conversion for each of the real part and the imaginary part of the complex number is performed by the DAC and input to the input port of the in-phase component (Quadrature) of the IQ modulator.

  In addition to the electrical signals corresponding to the in-phase and quadrature components of the modulation signal, the IQ modulator receives a single-wavelength continuous light output from a laser diode (LD) and finally outputs a modulated optical signal. The

The optical signal output from the transmitter is transmitted through a transmission fiber.
Here, the dispersion effect of the transmission fiber is accumulated.

  In the receiver, the input optical signal is square-detected by a photodetector (photodetector; PD), converted into an electric signal, and restored to a bit string that is an original information source transmitted by a decoder. .

  Next, a method for obtaining a tap coefficient for compensating the cumulative dispersion effect of the transmission path by the FIR filter in the transmitter will be described.

  First, using the measurement system shown in FIG. 10, the impulse response time waveform g (t) of the entire transmitter is physically measured by the procedure of FIG.

Specifically, (S101) an impulse signal is output from the impulse generation source, (S102) the impulse signal is added to the DAC corresponding to either the in-phase / quadrature component, and an analog electric signal is output.
At this time, when the DAC or IQ modulator has distortion, an FIR filter (a dotted line portion in the figure) for performing digital signal processing for compensating for this may be applied.

  (S103) Next, the analog electric signal is applied to the IQ modulator to which continuous light from the LD is input, and an optical impulse waveform corresponding to either the in-phase or quadrature component is output.

  (S104) This optical waveform is received by a coherent receiver including a 90-degree hybrid, a local oscillation light source (Local Oscillator; LO), a balance photodetector (BPD), and an oscilloscope (OSC). (S105) The obtained digital signal is subjected to demodulation processing such as carrier wave regeneration by offline digital signal processing, and the optical impulse response waveform g (t) is digitized in the form of complex amplitude.

  Once g (t) is measured and digitized, after that, even if the transmission path is switched and the total amount of dispersion is changed, dispersion compensation is performed on the transmission side by performing the calculation procedure described later again. It is easy to derive again the tap coefficients of the FIR filter to perform.

Next, a method of obtaining FIR filter tap coefficients for performing transmission-side dispersion compensation by calculation when the total amount of dispersion values to be compensated (cumulative dispersion value) is D [s ² ] is shown in the procedure of FIG. It explains according to.

If the dispersion value per unit length of the transmission path is β ₂ [s ² / m] and the total length is L [m], the accumulated dispersion value is given as D = β ₂ L [s ² ].
(S201) With respect to the optical waveform having the impulse response g (t) measured previously, (S202) the waveform h () that would be obtained when transmitting through the transmission line whose accumulated dispersion value is D [s ² ]. t) is calculated by the following formula.

Where G (ω) is the Fourier transform of g (t), and F ⁻¹ represents the inverse Fourier transform.
Although h (t) obtained here is a temporally continuous waveform in concept, it is actually discrete data sampled at a finite time interval, and the sampling rate is sufficiently higher than the symbol rate. It is assumed that the value is large.
As for the implementation of the Fourier forward transform and the inverse transform, it is practical to use a fast Fourier transform (FFT / IFFT) algorithm for the sampled data.

(S203) Next, h (t) is sampled at a sampling rate equal to the symbol rate B, and the obtained waveform is set to h _n .
However, n is an integer, and is obtained as h _n = h (nT) with a sampling time interval T = 1 / B.
The range of _n is determined as a range of values until the absolute value of the amplitude of h _n becomes sufficiently small.
(S204) and a material obtained by Fourier transform of h _n and H _n, is obtained by inverse Fourier transform of H _n ^-1 which is the inverse (S205).

  Are used as tap coefficients of the FIR filter.

As a result, 1 even when a DAC sample / symbol, the FIR filter having the c _n as a tap coefficient, accumulated dispersion D of the transmission path [s ^2] is possible to obtain a pre-compensated waveform Become.

  Below, the Example and effect of this invention based on a principle confirmation experiment are demonstrated concretely.

A quaternary PAM signal modulated by a pseudo random bit sequence having a symbol rate of 12 Gbaud and a length of 2 ¹⁵ -1 is used as a transmission signal.

  The portion corresponding to the transmitter in the overall transmission system shown in FIG. 1 includes an arbitrary waveform generator, an IQ modulator based on lithium niobate (LN), and a laser that outputs continuous light with a wavelength of 1552.52 nm. Consists of a light source.

  The signal generation and digital signal processing by the FIR filter are performed off-line, and the result is input as digital data to an arbitrary waveform generator corresponding to a DAC.

  The arbitrary waveform generator has a sampling rate of 12 GSample / s and has an analog waveform output of 2 channels, which is used as two DACs corresponding to a real part and an imaginary part of a complex number.

Fig. 2 shows that when an impulse sequence of in-phase components is generated at a repetition frequency of 1 GHz from the above transmitter, this is coherently received and waveform data is captured by a real-time oscilloscope, and phase reproduction is performed by offline digital signal processing. It is the amplitude time waveform acquired in this way.
The horizontal axis of the figure is normalized by the time interval when sampling at 12 GSample / s, and 1 corresponds to 83.3 ps.

The waveform in FIG. 2 is a real number, but the imaginary part is ignored because the value is small.
Since a similar waveform is obtained for the quadrature component of the transmitter, the waveform shown in FIG. 2 is assumed to be a real impulse response g (t) common to the in-phase and quadrature components.
Note that g (t) is a complex number when the impulse response includes chirp or the like, but even in this case, the following method can be applied.

As an example of the fiber constituting the transmission line, a standard single mode fiber (SSMF) having a dispersion value of β ₂ = −21.25 ps ² and a length of L = 80 km is considered.
At this time, the accumulated dispersion value of the transmission line is D = −1700 ps ^2, and the waveform h (t) when the accumulated dispersion value is added to the impulse response g (t) shown in FIG. The result calculated by (1) is shown in FIG.

In FIG. 3, the solid line and the broken line represent the real part and the imaginary part of h (t), respectively, and the point represents the amplitude value h _n when sampling at 12 GSample / s, which is the DAC sampling rate.
When dispersion compensation is not performed on the transmission side, an impulse response as shown in FIG. 3 is obtained on the reception side, and the waveform of the reception signal is greatly distorted due to intersymbol interference.

FIG. 4 shows a calculation result of the tap coefficient c _n obtained by the equation (2) using H _n obtained by Fourier transform of h _n indicated by each point in FIG.
Figure 4 is the fact represents the real and imaginary parts of the tap coefficients c _n of the FIR filter, outside of the range tap number is -10 ≦ n ≦ 10 where the value of the coefficient is determined to be sufficiently small, the tap The number was determined as 21.

FIG. 5 shows the result of calculating the waveform when the tap coefficient FIR filter obtained in FIG. 4 is applied to the impulse response g (t) of the transmitter shown in FIG.
In this way, if the dispersion value of the transmission path is compensated in advance by the FIR filter, the impulse response of the signal output from the transmitter is greatly distorted, resulting in intersymbol interference.

On the other hand, FIG. 6 shows a calculation result of a waveform when a cumulative dispersion value of D = −1700 ps ² is applied to the waveform of FIG.
In FIG. 6, a solid line, a broken line, and an alternate long and short dash line indicate the real part, the imaginary part, and the strength, respectively.

  As shown in the figure, since the intersymbol interference becomes zero at each sampling point, it is expected that a waveform in which the accumulated dispersion of the transmission path is compensated is obtained on the receiving side.

  In addition, although the amplitude is large at the transition between each sampling point, since this uses digital signal processing of 1 sample / symbol and DAC, spectral components outside the range of | f | <B / 2 become uncontrollable. This is because a large amplitude transition occurs as in the case of using the Nyquist filter (roll-off filter having a roll-off coefficient close to zero).

In the following, when the SSMF distance as the transmission fiber is changed to 0km, 40km, 80km, 120km, 12Gbaud 4PAM signal is transmitted after dispersion compensation on the transmitting side using the proposed method and received by the receiver The result of the experiment which evaluated signal quality is shown.
FIG. 7 shows the experimental system.

The transmitter has the same configuration as the transmitter of FIG.
It is assumed that the transmission path has one of (a) 0 km, (b) 40 km, (c) 80 km, and (d) 120 km depending on the fiber length.

Further, the accumulated dispersion generated in each length transmission fiber obtained by calculation of the tap coefficients c _n for compensating at the transmitter was performed dispersion compensation applied to the transmitter of the FIR filter.

When the length of the SSMF is 40, 80, 120 km, the number of taps of the tap coefficients c _n were respectively 19, 21, 23.

The average power of the optical signal output from the transmitter is amplified to 1 mW by an optical amplifier before entering the transmission fiber.
After transmitting the fiber, the optical power is changed by a variable optical attenuator (VOA), and then optical amplification is performed to adjust the optical signal-to-noise ratio (OSNR) of the received signal.

OSNR was measured with an optical spectrum analyzer (OSA), and the optical noise bandwidth in defining OSNR was set to 0.1 nm.
On the other hand, optical noise outside the signal band is removed by a band pass filter (BPF), and light reception by direct detection is performed by a photodetector (PD).

  The electrical signal output from the PD is captured by a real-time oscilloscope (OSC), antialiasing filtering is performed by off-line digital signal processing, resampling at 24 GSample / s, and clock recovery are performed, and finally the signal quality is evaluated.

FIG. 8 shows the eye pattern of the received signal when the length of the transmission fiber is (a) 0 km (Back to back), (b) 40 km, (c) 80 km, and (d) 120 km.
The received OSNR values are 34, 34, 35, and 35 dB, respectively.

  From the result of FIG. 8, it can be seen that the 4PAM signal amplitude is obtained at a specific sampling time for the received signal waveform after transmission, and the accumulated dispersion value of the transmission fiber at each distance is compensated.

  On the other hand, it can be seen that as the transmission fiber length increases, the accumulated dispersion value also increases, and as a result, the fluctuation range when the amplitude undergoes time transition increases.

FIG. 9 is a diagram illustrating a measurement result of the bit error rate (BER) of the received signal obtained when the received OSNR is changed.
A BER of 10 ⁻³ or less was obtained for all transmission fiber lengths, indicating that a practical received signal quality that is error-free when an error correction code is applied was obtained.

  When measuring the impulse response g (t) of the transmitter using the measurement system of FIG. 10, when the impulse response waveform is received and analog-to-digital converted by the receiver, and demodulation such as phase reproduction is performed by digital signal processing Instead of performing offline digital signal processing after data acquisition and analog-to-digital conversion using a real-time oscilloscope, data acquisition and signal analysis may be performed using an integrated real-time digital signal processor .

  When performing the digital signal processing of the present invention, it may be performed in the frequency domain without using the FIR filter.

  That is, (S301) subject signal data is subjected to Fourier transform using (S302) FFT, (S303) multiplied by a transfer function for performing transmission side dispersion compensation in the frequency domain, and ( S304) A frequency domain equalizer (FDE) that can obtain complex amplitude digital data of a dispersion-compensated signal waveform by performing inverse Fourier transform by IFFT may be applied.

FIG. 13 shows a transmission system implemented using FDE in the present invention, FIG. 14 shows a procedure for performing transmission side dispersion compensation in FDE, and FIG. 15 shows a transfer function H _n ⁻¹ used in FDE. The procedure to do was shown.

  By using the proposed method in the transmitter in the field of optical fiber communication, dispersion compensation can be implemented by digital signal processing and analog-to-digital conversion with a low sampling rate, especially when PAM signals are transmitted over long distances. Waveform deterioration caused by GVD can be suppressed, and the transmittable distance can be extended.

1 Transmitter 2 Signal Source 3 PAM Signal Amplitude Mapping 4 FIR Filter 5 DAC
6 DAC
7 Laser diode 8 IQ modulator 9 Transmission line 10 Receiver
11 Photodetector 12 Decoder 13 Impulse source 14 Oscilloscope 15 DAC
16 DAC
17 Laser Diode 18 IQ Modulator 19 Coherent Receiver 20 Local Oscillation Light Source 21 90 Degree Hybrid 22 Balanced Photodetector 23 Balanced Photodetector 24 Optical Amplifier 25 Variable Optical Attenuator 26 Optical Spectrum Analyzer 27 Bandpass Filter 28 Photodetector 29 Oscilloscope 30 DSP
31 FIR filter 32 Frequency domain equalizer

Claims (10)

An optical transmitter comprising at least a FIR filter, a DAC, a laser diode capable of outputting a single wavelength continuous light, and an IQ modulator,
One sample digital signal of the input signal amplitude per symbol is digital signal processed by the FIR filter, single-wavelength continuous light which the digital output of the FIR filter is the DAC in DA converted electric signal to the laser diode output the IQ modulator is configured to output a modulated optical signal and a,
Said FIR filter is a digital signal processing using tap coefficients for compensating for accumulated group velocity dispersion amount of an optical transmission path (D) between the light receiver receiving the modulated optical signal from the optical transmitter to compensate for the accumulated group velocity dispersion amount of an optical transmission path in the optical transmitter and,
The impulse response obtained by measuring the modulated optical signal by inputting the impulse signal to the DAC without using the FIR filter of the optical transmitter is g (t), and G (ω) is the Fourier transform of g (t). , F ^-1 represents the inverse Fourier transform, n represents an integer, B is the symbol rate, the h _n = h sampling time interval as T = 1 / B a (nT) obtained by Fourier transform and H _n Then
The tap coefficient c _n of the FIR filter is
h (t) = F ⁻¹ [G (ω) exp (iω ² D / 2)]
c _n = F ⁻¹ [H _n ⁻¹ ]
Optical transmitter calculated by: Optical transmission system characterized by having an optical transmitter according to claim 1. A method for compensating for group velocity dispersion in an optical transmission line within a transmitter,
In the transmitter comprising at least FIR filter and DAC and the laser diode and the IQ modulator capable of outputting a single wavelength continuous light,
When the accumulated group velocity dispersion amount of the optical transmission line (D) is given, determine the tap coefficients for compensating for the accumulated group velocity dispersion amount,
Using the tap coefficients obtained by forming the FIR filter,
Single the digital signal of the input signal amplitude of one sample per symbol, and digital signal processing in the configured FIR filter, the digital output of the FIR filter output is the DAC in DA converted electric signal to the laser diode the IQ modulator transmits the modulated optical signal to the optical transmission line from the wave continuous light,
The impulse response obtained by inputting an impulse signal to the DAC without using the FIR filter of the transmitter and measuring a modulated optical signal is g (t), and G (ω) is a Fourier transform of g (t). F ^-1 represents the inverse Fourier transform, n represents an integer, B is the symbol rate, when those of the sampling time interval obtained by Fourier transform of h _n = h (nT) as T = 1 / B and H _n ,
The tap coefficient c _n of the FIR filter is
h (t) = F ⁻¹ [G (ω) exp (iω ² D / 2)]
c _n = F ⁻¹ [H _n ⁻¹ ]
Method for compensating for group velocity dispersion in the optical transmission path in the transmitter, characterized in that determined by. An optical transmitter comprising a laser diode and an IQ modulator capable of outputting at least FDE, DAC, and single-wavelength continuous light,
1 sample of the input signal amplitude of the digital signal per one symbol is digital signal processed in the FDE in the frequency domain, single-wavelength continuous optical digital output of the FDE is output from the DAC by the DA converted electric signal to the laser diode the IQ modulator is configured to output a modulated optical signal and a,
The FDE is a digital signal processing using the transfer function to compensate for the accumulated group velocity dispersion amount of an optical transmission path between the light receiver receiving the modulated optical signal from the optical transmitter (D) to compensate for the accumulated group velocity dispersion amount of an optical transmission path in the optical transmitter Te,
The impulse response obtained by inputting the impulse signal to the DAC without using the FDE of the optical transmitter and measuring the modulated optical signal is g (t), and G (ω) is the Fourier transform of g (t), F ⁻¹ represents the inverse Fourier transform, n is an integer, B is the symbol rate, and the sampling time interval is T = 1 / B.
h (t) = F ⁻¹ [G (ω) exp (iω ² D / 2)]
From an optical transmitter and calculates the inverse H _n ^-1 of h _n = h a (nT) obtained by Fourier transform H _n as the transfer function. An optical transmission system comprising the optical transmitter according to claim 4 . A method for compensating for group velocity dispersion in an optical transmission line within a transmitter,
In the transmitter comprising a laser diode capable of outputting at least FDE, DAC, and single wavelength continuous light, and an IQ modulator,
When the accumulated group velocity dispersion amount of the optical transmission line (D) is given, determine the transfer function to compensate for the accumulated group velocity dispersion amount,
Using the transfer function obtained the constituting said FDE,
One sample of the digital signal of the input signal amplitude per symbol, and digital signal processing in the frequency domain in the configured FDE, the digital output of the FDE is output from DA converted electrical signal to the laser diode by the DAC the IQ modulator transmits the modulated optical signal to said optical transmission path and a single-wavelength continuous light,
An impulse response obtained by inputting an impulse signal to the DAC without using the FDE of the transmitter and measuring a modulated optical signal is g (t), G (ω) is a Fourier transform of g (t), and F ⁻¹ represents an inverse Fourier transform, n is an integer, B is a symbol rate, and a sampling time interval is T = 1 / B.
h (t) = F ⁻¹ [G (ω) exp (iω ² D / 2)]
From, h _n = h method for compensating for group velocity dispersion in the optical transmission path in the transmitter which the reciprocal H _n ^-1 of H _n of the Fourier transform of (nT) and obtains as the transfer function. Digital signal processing using a tap coefficient is performed on a digital signal having an input signal amplitude of one sample per symbol to compensate for the accumulated group velocity dispersion amount (D) of the optical transmission line with the optical receiver. An FIR filter;
A DAC that DA-converts the digital output of the FIR filter and outputs an electrical signal;
A laser diode capable of outputting a single wavelength continuous light;
An IQ modulator that outputs a modulated optical signal from the electrical signal output from the DAC and the single-wavelength continuous light output from the laser diode;
For optical transmitters with
The impulse response obtained by measuring the modulated optical signal by inputting the impulse signal to the DAC without using the FIR filter of the optical transmitter is g (t), and G (ω) is the Fourier transform of g (t). , F ^-1 represents the inverse Fourier transform, n represents an integer, B is the symbol rate, the h _n = h sampling time interval as T = 1 / B a (nT) obtained by Fourier transform and H _n Then
The tap coefficient c _n of the FIR filter is
h (t) = F ⁻¹ [G (ω) exp (iω ² D / 2)]
c _n = F ⁻¹ [H _n ⁻¹ ]
The tap coefficient calculation method calculated by A digital signal processing in the frequency domain is performed on a digital signal having an input signal amplitude of one sample per symbol using a transfer function, and the accumulated group velocity dispersion amount (D FDE that compensates for
A DAC that DA-converts the digital output of the FDE and outputs an electrical signal;
A laser diode capable of outputting a single wavelength continuous light;
An IQ modulator that outputs a modulated optical signal from the electrical signal output from the DAC and the single-wavelength continuous light output from the laser diode;
For optical transmitters with
The impulse response obtained by inputting the impulse signal to the DAC without using the FDE of the optical transmitter and measuring the modulated optical signal is g (t), and G (ω) is the Fourier transform of g (t), F ⁻¹ represents the inverse Fourier transform, n is an integer, B is the symbol rate, and the sampling time interval is T = 1 / B.
h (t) = F ⁻¹ [G (ω) exp (iω ² D / 2)]
From the transfer function calculating method for calculating a reciprocal H _n ^-1 of H _n where h _n = h a (nT) and Fourier transform as the transfer function.   A program for causing a processor to execute the tap coefficient calculation method according to claim 7.   A program for causing a processor to execute the transfer function calculation method according to claim 8.

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