JP2006054660A - Optical transmitter and optical transmission system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To increase an allowable chromatic dispersion amount of an RZ signal whose optical band is controlled and a modulation band is compressed.
SOLUTION: Carrier light output from a CW light source is incident on a double-sided drive Mach-Zehnder optical modulator having N electrodes, and an electrical signal of N-channel data is converted into RZ, and the polarity of the converted electrical signal can be switched. A delay of T / 2N × k (k: 1, 2, 3,..., N−1) is given to the electrical signals of channels 2 to N, and the double-sided drive Mach-Zehnder optical modulator is driven by this electrical signal. Thus, N-channel data multiplexing and RZ signal optical signal generation are performed using one double-sided drive Mach-Zehnder optical modulator. This optical signal is generated as an RZ signal as an optical signal in which the phase of light is inverted for each bit and the modulation band is compressed. By switching the polarity of the RZ-converted electrical signal that drives the optical modulator, the sign of the frequency chirp of the output optical signal is inverted.
[Selection] Figure 1

Description

  The present invention is used in an optical transmission apparatus and an optical transmission system that time-division-multiplexes a plurality of electrical signals, converts them into optical signals, and transmits them. In particular, the present invention relates to a chirp control technique for optical signals.

  In recent years, research on large-capacity optical transmission technology has been actively conducted, and speeding up per wavelength has been promoted. Increasing the speed per wavelength can easily improve the frequency utilization efficiency, and is advantageous in a large-capacity optical transmission system.

  In commercial systems, a system with a bit rate of 10 Gbit / s has been put into practical use, and currently a 40 Gbit / s system is being put into practical use. However, in ultra-high-speed optical transmission, transmission quality degradation due to various factors such as optical S / N degradation, nonlinear optical effect, group velocity dispersion, and polarization mode dispersion becomes a serious issue.

  As the bit rate increases, high input power to the optical transmission line is required to suppress optical S / N degradation. For this reason, an RZ signal that can tolerate a higher input power than the NRZ signal is attracting attention because the pulse width of the optical signal does not depend on the transmission signal pattern. Furthermore, wavelength multiplexing (WDM) is being densified for the purpose of increasing transmission capacity, and a modulation code with a small modulation bandwidth is desired.

  As a conventional technique for generating an optical signal of an RZ signal having a narrow modulation band, there is an optical transmitter that generates an optical signal of an RZ signal in which the modulation band is compressed by controlling the optical phase and the duty of a pulse (for example, Patent Document 1). reference).

  The configuration of a conventional optical transmitter is shown in FIG. A conventional optical transmitter includes a CW light source 1 that outputs carrier light, a Mach-Zehnder optical modulator 2 that has N (N ≧ 2) electrodes connected in series with the CW light source 1, and an electrical signal. RZ converting means 60-1 to 60-N for converting the NRZ signal of RZ into RZ, and channel ch. 2 to ch. It comprises delay means 61-1 to 61-N-1 that give delays to N electrical signals, respectively. The carrier light output from the CW light source 1 is input to the double-sided drive Mach-Zehnder optical modulator 2.

  Channel ch. At bit rate B (time slot T). 1-ch. NRZ signals as electrical signals of N are converted to RZ by RZ converting means 60-1 to 60-N, respectively, and channel ch. 2 to ch. Each of the N electrical signals is given a delay of T / 2N × k (k: 1, 2, 3,..., N−1) and then input to the N electrodes of the double-sided drive Mach-Zehnder type optical modulator 2. Is done. In this way, an optical signal having a bit rate of B × N can be generated by sequentially modulating the carrier light with the N-channel signal.

  The operation when there are two electrodes (N = 2) will be described with reference to FIG. The upper part of FIG. 19 shows the electrical signal of each channel that drives the double-sided drive Mach-Zehnder type optical modulator 2, the middle part shows the phase difference of light between both arms in the double-sided drive Mach-Zehnder type optical modulator 2, and the lower part is modulated light. Signals are shown. In the figure, the logical pattern of the signal and the relative phase of the light are also shown.

  When the adjacent bit is “0” in the multiplexed signal, the same optical signal as the electrical signal of one channel is output. When the multiplexed signal is “1” continuous, the rise and fall of the electrical signals of both channels overlap, so the light phase difference between the arms at the boundary between bits is faster than the rise and fall of the electricity. Change and extinguish at the boundary between bits.

  Whether or not the boundary portion between bits is extinguished depends on the logic of adjacent bits, so that a total of four types of pulses are generated. FIG. 20 shows an eye pattern under these conditions. Under this operating condition, a driving electric signal can be generated by an electric circuit in an operating band of a bit rate B × N. Therefore, the electric power of a conventional RZ signal optical transmission device in which two optical modulators are connected in series. Compared to a circuit, it is possible to configure an optical transmitter for RZ signals with a single optical modulator without increasing the operating band.

JP 2003-329989 A

  In an ultra high-speed optical transmission system, not only optical S / N but also chromatic dispersion becomes a big problem. Since the tolerance to chromatic dispersion decreases in inverse proportion to the square of the bit rate, a modulation code having a high chromatic dispersion tolerance is desired.

  A band-compressed RZ signal with controlled optical phase has a high chromatic dispersion tolerance due to a narrow modulation bandwidth compared to a normal RZ signal, but has a low chromatic dispersion tolerance compared to an NRZ signal widely used so far. Become.

  If the chromatic dispersion tolerance of the modulation code is low, the residual dispersion of the optical transmission system must be within the chromatic dispersion tolerance, so prepare chromatic dispersion guarantee products (such as chromatic dispersion compensation fiber) with various chromatic dispersion amounts. There is a need. As the number of products increases, the cost of the entire system increases. Therefore, it is desired that the modulation code applied to the ultrahigh-speed optical transmission system can tolerate large chromatic dispersion.

  An object of the present invention is to realize an optical transmission device and an optical transmission system that expand an allowable chromatic dispersion amount of an RZ signal whose optical band is controlled and a modulation band is compressed.

  In order to achieve the above object, in the present invention, carrier light output from a CW light source is incident on a double-sided drive Mach-Zehnder optical modulator having N electrodes, and an electrical signal of N-channel data is converted into RZ and converted into RZ. The polarity of the electrical signal can be switched, and a delay of T / 2N × k (k: 1, 2, 3,..., N−1) is given to the electrical signals of channels 2 to N, and the both-side drive Mach-Zehnder type with this electrical signal By driving the optical modulator, N-channel data multiplexing and RZ signal optical signal generation are performed using a single double-sided Mach-Zehnder optical modulator. This optical signal is generated as an RZ signal as an optical signal in which the phase of the light is inverted for each bit and the modulation band is compressed.

  By switching the polarity of the RZ-shaped electric signal that drives the double-sided drive Mach-Zehnder optical modulator, the relationship between whether the optical phase is advanced or delayed when light is emitted (extinction) at the output of the double-sided drive Mach-Zehnder optical modulator is switched. The sign of the frequency chirp (simply called chirp) of the output optical signal can be reversed.

  Thus, by switching the sign of the chirp of the optical signal, the optimum dispersion value can be changed, and the amount of chromatic dispersion that can be allowed within a certain penalty can be increased.

  That is, a first aspect of the present invention is an optical transmission apparatus that multiplexes N electrical signals, converts them into optical signals, and transmits them. A feature of the present invention is that CW outputs carrier light. A double-sided drive type Mach-Zehnder type modulator having N (N ≧ 2) electrodes connected in series with the CW light source, and electrical signals of channels 1 to N at a bit rate B (time slot T) N RZ conversion means for converting the NRZ signal to RZ, N polarity switching means for switching the polarity of the RZ signal as N electrical signals RZ converted by the RZ conversion means, and the polarity switching means The delays of T / 2N × k (k: 1, 2, 3,..., N−1) are given to the RZ signals as electrical signals of the channels 2 to N whose polarities are switched, respectively. RZ signal as electrical signal N-1 delay means for applying a voltage corresponding to the above-mentioned electrode to the electrodes of channel 1 other than the channels 2 to N to which a delay is given by the delay means, and the RZ signal as an electrical signal of this channel 1 And a bias circuit for applying a corresponding bias voltage.

  Further, as the RZ conversion means and the polarity switching means, the means for switching the polarity of the input NRZ signal, the DC voltage applying means, the output of the DC voltage applying means and the NRZ signal output from the means for switching the polarity And a means for generating an RZ signal (claim 2).

  Alternatively, the NRZ signal is a signal in which M NRZ signals are time-division multiplexed, and as the RZ conversion means and the polarity switching means, means for switching the polarity of the input M NRZ signals, respectively. The M NRZ signals output from the respective switching means are time-division multiplexed, DC voltage applying means, the output of the DC voltage applying means and the time-division output from the time-division multiplexing means And a means for generating an RZ signal by inputting the multiplexed NRZ signal.

  A second aspect of the present invention is an optical transmission system including the optical transmitter of the present invention, an optical transmission path for transmitting an optical signal, and an optical receiver for receiving an optical signal. A feature is that an optical signal output from the optical transmission line is distributed and one of the signals is input to the optical receiver, and the other optical signal distributed by the optical distribution unit is input. A chromatic dispersion measuring means for measuring the chromatic dispersion amount of the optical transmission line, and a sign of the chirp of the optical signal output from the optical transmitter based on the chromatic dispersion value obtained by the chromatic dispersion measuring means. And a control means for switching (claim 4).

  Alternatively, in the optical transmission system of the present invention, N optical transmitters and optical receivers are provided, and the N optical transmitters transmit optical signals having different wavelengths, and N optical transmitters The optical receiver receives optical signals having different wavelengths, wavelength-multiplexes optical signals transmitted from the N optical transmitters, and transmits the optical signals to the optical transmission line, and from the optical transmission line An optical distribution unit that distributes an incoming wavelength-multiplexed optical signal, and a wavelength that separates the optical signal for each wavelength from one of the optical signals distributed by the optical distribution unit and that is input to each of the N optical receivers A separating unit; a wavelength selecting unit that extracts an optical signal of a specific wavelength from the other of the optical signals distributed by the optical distributing unit; and a wavelength dispersion amount of the optical transmission line from the optical signal output from the wavelength selecting unit. Wavelength to measure Dispersion control means, and control means for switching the sign of the chirp of the optical signal output from the N optical transmission devices based on the value of the chromatic dispersion amount obtained by the chromatic dispersion measurement means. It is characterized (claim 5).

  Further, instead of the chromatic dispersion measuring means, photoelectric conversion means for converting an optical signal into an electrical signal, band selection means for extracting a specific band component from the electrical signal output from the photoelectric conversion means, and this band selection Intensity measuring means for measuring the intensity of the electrical signal output from the means, and in place of the control means, the sign of the chirp of the optical transmission device is changed to the one where the intensity obtained by the intensity measuring means is higher. Control means for switching may be provided (claim 6).

  The chromatic dispersion measuring means separates and extracts at least two different frequency components from the optical spectrum components of the optical signal, and electrically outputs the two optical signals output from the frequency component separating means. Two photoelectric conversion means for converting into signals, a phase comparison means for detecting a relative phase difference between two electric signals output from the two photoelectric conversion means, and a relative position obtained from the phase comparison means And chromatic dispersion calculating means for calculating an amount of chromatic dispersion that the optical signal has suffered in the optical transmission line based on the phase difference information.

  Alternatively, in the optical transmission system of the present invention, the optical receiver includes a means for detecting an error included in the optical signal, and the error rate is determined based on the error rate information obtained by the means for detecting the error. Control means is provided for switching the sign of the chirp of the optical signal output from the optical transmission device on the lower side (Claim 8).

  Alternatively, in the optical transmission system of the present invention, N optical transmitters and optical receivers are provided, and the N optical transmitters transmit optical signals having different wavelengths, and N optical transmitters The optical receiver receives optical signals having different wavelengths, wavelength-multiplexes optical signals transmitted from the N optical transmitters, and transmits the optical signals to the optical transmission line, and from the optical transmission line Wavelength separation means for separating an optical signal for each wavelength from an incoming wavelength-multiplexed optical signal and inputting it to each of the N optical receivers, and each of the N optical receivers receives the received optical signal. Means for detecting the included error, and on the basis of the error rate information obtained by the means for detecting this error, the chirp of the optical signal output from the N optical transmitters is reduced toward the error rate. The sign of each Characterized by comprising a control means for changing (claim 9).

  Alternatively, the optical transmission system of the present invention is provided with variable loss means for reducing the intensity of the optical signal arriving from the optical transmission path, and the optical receiver includes the optical signal output from the variable loss means. Based on the error rate information obtained by the error detection unit, the sign of the chirp of the optical signal output from the optical transmission device is set to the direction where the error rate is reduced. Control means for switching is provided (claim 10).

  Alternatively, the optical transmission system of the present invention is provided with variable loss means for reducing the intensity of the optical signal transmitted from the optical transmission apparatus, and the optical reception apparatus is output from the variable loss means, and the optical transmission Means for detecting an error contained in the optical signal transmitted through the path, and is output from the optical transmission device in a manner of decreasing the error rate based on the error rate information obtained by the means for detecting the error. Control means for switching the sign of the chirp of the optical signal is provided (claim 11).

  Alternatively, in the optical transmission system of the present invention, N optical transmitters and optical receivers are provided, and the N optical transmitters transmit optical signals having different wavelengths, and N optical transmitters The optical receiver receives optical signals having different wavelengths, wavelength-multiplexes optical signals transmitted from the N optical transmitters, and transmits the optical signals to the optical transmission line, and from the optical transmission line Wavelength separation means for separating an optical signal for each wavelength from an incoming wavelength-multiplexed optical signal, and N variable loss means for inputting an optical signal for each wavelength output from the wavelength separation means and reducing its intensity And the N optical receivers include means for detecting errors included in the optical signals respectively output from the N variable loss means, and an error rate obtained by the means for detecting the errors. Based on information Wherein the error rate comprises a control means for switching each chirp positive or negative sign of the optical signal output from the N of the optical transmission device towards a decrease (claim 12).

  Alternatively, in the optical transmission system of the present invention, N optical transmitters and optical receivers are provided, and the N optical transmitters transmit optical signals having different wavelengths, and N optical transmitters The optical receiving device receives optical signals having different wavelengths, inputs N optical signals transmitted from the N optical transmitting devices, and reduces the intensity of the N variable loss means, Wavelength multiplexing means for wavelength-multiplexing the optical signals output from the variable loss means and sending them to the optical transmission line; and the optical signals separated for each wavelength from the wavelength-multiplexed optical signals coming from the optical transmission line; Wavelength separation means for inputting to each of the optical receivers, and the N optical receivers are provided with means for detecting errors included in the received optical signals, respectively. Mistake Based on the rate information, there is provided control means for switching the sign of the chirp of the optical signal output from each of the N optical transmission devices in the direction of decreasing the error rate (claim 13). .

  In addition, the optical receiving device may include error correction means, and error correction number information may be used as the error rate information.

  According to the present invention, it is possible to change the value of the optimum chromatic dispersion amount and to increase the allowable chromatic dispersion amount by switching the sign of the chirp of the RZ signal whose optical band is controlled and the modulation band is compressed. By increasing the allowable chromatic dispersion amount, it is possible to increase the dispersion value increments of chromatic dispersion compensation products prepared as a system, and it is possible to realize cost reduction by reducing the number of products. In addition, the system that automatically switches the sign of the chirp of the transmission signal optimally can reduce the amount of work for workers when the system is introduced.

(First Example)
This is an embodiment of the optical transmission apparatus of the first embodiment, and the optical transmission apparatus of the first embodiment will be described with reference to FIG. FIG. 1 is a block diagram showing an optical transmission apparatus according to the first embodiment. The optical transmitter of the first embodiment includes a CW light source 1 that outputs carrier light, a double-sided drive Mach-Zehnder type modulator 2 having N (N ≧ 2) electrodes connected in series with the CW light source 1, RZ conversion means 3-1 to 3-N for converting an NRZ signal as an electrical signal of channels 1 to N (Data # 1 to #N) to RZ at a bit rate B (time slot T), and an RZ converted electrical signal The polarity switching means 4-1 to 4-N for switching the polarity of the RZ signal and the RZ signal as the electrical signal of the channels 2 to N (Data # 2 to #N) are respectively T / 2N × k (k: 1, 2, 3,..., N−1) and applies a voltage corresponding to the RZ signal as an electrical signal of the channels 2 to N to delay circuits 5-1 to 5 -N−1. The channel 1 (Data # 1) electrode is connected to the channel 1 Composed of the bias circuit 6 for applying a bias voltage corresponding to the RZ signal as No. (claim 1).

  The carrier light output from the CW light source 1 is input to the double-sided drive Mach-Zehnder optical modulator 2. The NRZ signals as electrical signals of channels 1 to N at the bit rate B (time slot T) are converted to RZ by the RZ converting means 3-1 to 3 -N, respectively. The polarity is set by ˜4-N, and a delay of T / 2N × k (k: 1, 2, 3,..., N−1) is given to the electrical signals of channels 2 to N, respectively. Is input to N electrodes of the optical modulator 2.

  In this way, an optical signal having a bit rate of B × N can be generated by sequentially modulating the carrier light with the N-channel signal. FIG. 2 shows an operation example when the polarity of the RZ signal as an electric signal is inverted by the polarity switching means 4-1 and 4-2 in this embodiment (N = 2). It can be seen that the direction of change of the optical phase difference between the modulator arms is reversed compared to the operation example of the prior art (FIG. 19).

  In this way, by switching the polarity of the RZ signal as an electrical signal, the direction in which the optical phase changes when light is emitted (or extinguished) at the modulator output can be switched, and the sign of the chirp of the output optical signal is positive or negative Can be switched. FIG. 3 shows the relationship between the chromatic dispersion value and the eye opening deterioration when the sign of the chirp is switched. By switching the sign of the chirp sign, the value of the optimum chromatic dispersion amount can be changed, and the width of the allowable chromatic dispersion amount value below the penalty threshold value can be expanded.

(Second embodiment)
The second embodiment is an embodiment for realizing the RZ conversion means 3-1 to 3-N and the polarity switching means 4-1 to 4-N of the optical transmission apparatus of the first embodiment, see FIG. To explain. FIG. 4 is a diagram showing a block configuration and an operation example of the RZ conversion means 3-1 to 3-N and the polarity switching means 4-1 to 4-N of the second embodiment. In the second embodiment, the RZ conversion means 3-1 to 3-N and the polarity switching means 4-1 to 4-N are divided into a polarity switching means 7 for switching the polarity of the input NRZ signal, and a DC voltage applying means 8. The output of the DC voltage applying means 8 and the NRZ signal output from the polarity switching means 7 are inputted to form a 2: 1 selector means 9 for generating an RZ signal (claim 2).

  By simultaneously inverting the logic of the NRZ signal input to the 2: 1 selector means 9 and the DC voltage, the polarity of the RZ signal output from the 2: 1 selector means 9 can be inverted. Since the polarity inversion of the NRZ signal can be realized by an EXOR circuit, it is possible to easily switch the sign of the chirp by using a commercially available electric circuit by performing RZ conversion and polarity switching in this way.

(Third embodiment)
The third embodiment is an embodiment for realizing the RZ conversion means 3-1 to 3-N and the polarity switching means 4-1 to 4-N of the optical transmission apparatus of the first embodiment, see FIG. To explain. FIG. 5 is a block diagram of the RZ conversion means 3-1 to 3-N and the polarity switching means 4-1 to 4-N of the third embodiment. In the second embodiment, the NRZ signal input to the polarity switching means 7 is a signal in which M NRZ signals are time-division multiplexed. In the third embodiment, the RZ conversion means 3-1 to 3-N. The polarity switching means 4-1 to 4-N are output from the polarity switching means 7-1 to 7-M for switching the polarity of the input M NRZ signals and the polarity switching means 7-1 to 7-M. Time division multiplexing circuit 10 for time-division multiplexing the M NRZ signals, DC voltage application means 8, and time-division multiplexing output from DC voltage application means 8 and time-division multiplexing means 10. The NRZ signal is input to the 2: 1 selector means 11 for generating the RZ signal. The RZ signal is input to the bias circuit 6 or the delay circuits 5-1 to 5-N-1 shown in FIG.

  The difference from the second embodiment is that the polarity inversion of the signal is performed with a low-speed signal before time division multiplexing. By using a low-speed electric signal, a function of switching the polarity of the modulator drive signal can be realized at low cost.

(Fourth embodiment)
The fourth embodiment is an embodiment of an optical transmission system. The optical transmission system of the fourth embodiment will be described with reference to FIG. FIG. 6 is an overall configuration diagram of the optical transmission system according to the fourth embodiment. The fourth embodiment is an optical transmission system including the optical transmitter 20 described in the first embodiment, an optical transmission path 21 that transmits an optical signal, and an optical receiver 22 that receives the optical signal. The optical distribution unit 23 distributes the optical signal output from the optical transmission path 21 and inputs one of the optical signals to the optical receiver 22, and the other optical signal distributed by the optical distribution unit 23 is input to the optical transmission path. The chromatic dispersion measuring means 24 for measuring the chromatic dispersion amount 21 and the sign of the chirp of the optical signal output from the optical transmitter 20 are switched based on the chromatic dispersion value obtained by the chromatic dispersion measuring means 24. And a control circuit 25 (Claim 4).

  When the sign of the value of the chromatic dispersion amount measured by the chromatic dispersion measuring unit 24 is positive (negative), the control circuit 25 sets the sign of the chirp of the optical transmission device 20 to positive (negative), and higher transmission quality. Select the chirp code that yields. Thus, by automatically selecting the optimum sign of the chirp sign, the work of the system introduction worker and the maintenance person can be reduced.

(Fifth embodiment)
The fifth embodiment is an embodiment of an optical transmission system. The optical transmission system of the fifth embodiment will be described with reference to FIG. FIG. 7 is an overall configuration diagram of the optical transmission system according to the fifth embodiment. In the fifth embodiment, the optical transmitters 20-1 to 20-N described in the first embodiment, the optical transmission line 21 for transmitting the optical signal, and the optical receivers 22-1 to 22- for receiving the optical signal are used. N optical transmission devices 20-1 to 20-N and N optical receiving devices 22-1 to 22-N are provided, and N optical transmission devices 20-1 are provided. ˜20-N transmit optical signals having different wavelengths, and N optical receivers 22-1 to 22-N receive optical signals having different wavelengths, and N optical transmitters 20 Wavelength multiplexing means 26 for wavelength-multiplexing optical signals transmitted from -1 to 20-N and sending them to the optical transmission line 21, and optical distribution means 23 for distributing wavelength-multiplexed optical signals coming from the optical transmission line 21; An optical signal for each wavelength from one of the optical signals distributed by the optical distribution means 23 Wavelength separation means 27 that is separated and inputted to each of N optical receivers 22-1 to 22-N, and wavelength selection means that extracts an optical signal having a specific wavelength from the other of the optical signals distributed by the optical distribution means 23 28, a chromatic dispersion measuring unit 24 for measuring the chromatic dispersion amount of the optical transmission line 21 from the optical signal output from the wavelength selecting unit 28, and a value of the chromatic dispersion amount obtained by the chromatic dispersion measuring unit 24. And a control circuit 29 for switching the sign of the chirp of the optical signal output from the N optical transmitters 20-1 to 20-N.

  By branching the wavelength-multiplexed optical signal, extracting the optical signal of each wavelength and performing chromatic dispersion measurement, the chromatic dispersion measuring means 24 can be shared by all wavelengths, and the cost of the system can be reduced. .

(Sixth embodiment)
The sixth embodiment is an embodiment of an optical transmission system. The optical transmission system according to the sixth embodiment will be described with reference to FIG. FIG. 8 is an overall configuration diagram of the optical transmission system according to the sixth embodiment. In the optical transmission system of the sixth embodiment, instead of the chromatic dispersion measuring means 24 described in the fourth or fifth embodiment, a photoelectric conversion means 30 for converting an optical signal into an electrical signal, and an output from the photoelectric conversion means 30 Band selecting means 31 for extracting a specific band component from the electric signal to be output, and intensity measuring means 32 for measuring the intensity of the electric signal output from the band selecting means 31 in the fourth or fifth embodiment. Instead of the control circuit 25 or 29 described above, a control circuit 33 is provided for switching the sign of the chirp of the optical transmitter 20 so that the intensity obtained by the intensity measuring means 32 becomes higher.

  By monitoring the intensity of a specific band component (for example, 1/2 or 1/4 of the bit rate) of the received signal, waveform distortion due to chromatic dispersion can be detected. FIG. 9 and FIG. 10 show the relationship between the frequency component intensity of ½ and ¼ of the bit rate and the value of the chromatic dispersion amount, respectively. The sign of the chirp that increases the frequency component intensity is selected. By lowering the frequency component to be monitored, it is possible to expand the width of the value of the chromatic dispersion amount that can be normally detected as to whether the sign of the positive or negative chirp can improve transmission quality.

(Seventh embodiment)
The seventh embodiment is an embodiment of an optical transmission system. The optical transmission system according to the seventh embodiment will be described with reference to FIG. FIG. 11 is an overall configuration diagram of the optical transmission system according to the seventh embodiment. In the optical transmission system of the seventh embodiment, the chromatic dispersion measuring unit 24 includes a frequency component separating unit 40 that separates and extracts at least two different frequency components from the optical spectrum components of the optical signal, and the frequency component separating unit 40. Between two photoelectric conversion means 41-1 and 41-2 for converting two optical signals output from the electric signal into an electric signal, and between the two electric signals output from the two photoelectric conversion means 41-1 and 41-2 Phase comparison means 42 for detecting the relative phase difference of the optical signal, and chromatic dispersion calculation for calculating the amount of chromatic dispersion that the optical signal suffered in the optical transmission line 21 based on the relative phase difference information obtained from the phase comparison means 42. Means 43 (Claim 7).

  Signal components having different frequencies are subjected to different delays due to the chromatic dispersion of the optical transmission line 21. Therefore, the amount of chromatic dispersion can be measured by detecting the phase difference between the components, and the chromatic dispersion code can also be detected from the phase difference code.

  Further, the optical transmitter 20 superimposes a tone signal having a frequency lower than the bit rate on the signal light as intensity modulation, and expands the detection range of chromatic dispersion by comparing the phase of the frequency component of the tone signal and detecting the intensity. It becomes possible. When the sign of chromatic dispersion measured by the chromatic dispersion calculating means 43 is positive (negative), the control circuit 25 sets the sign of the chirp of the optical transmitter 20 to positive (negative), so that higher transmission quality can be obtained. Switch to chirp sign. Thus, by automatically selecting the optimum sign of the chirp sign, the work of the system introduction worker and the maintenance person can be reduced.

(Eighth Example)
The eighth embodiment is an embodiment of an optical transmission system. The optical transmission system of the eighth embodiment will be described with reference to FIG. FIG. 12 is an overall configuration diagram of the optical transmission system according to the eighth embodiment. The eighth embodiment is an optical transmission system including the optical transmitter 20 described in the first embodiment, an optical transmission path 21 for transmitting an optical signal, and an optical receiver 22 for receiving an optical signal. The optical receiver 22 includes an error detection means 50 for detecting an error included in the optical signal. Based on the error rate information obtained by the error detection means 50, the optical transmission is performed so that the error rate decreases. A control circuit 51 is provided for switching the sign of the chirp of the optical signal output from the apparatus (claim 8).

  By selecting the sign of the chirp code with a low error rate, it is possible to switch to a chirp code with less waveform deterioration due to wavelength dispersion. By using the error detection function of the optical receiver 22 in this way, it is not necessary to prepare an additional circuit such as the chromatic dispersion measuring means 24, and the system can be realized at low cost.

(Ninth Example)
The ninth embodiment is an embodiment of an optical transmission system. The optical transmission system according to the ninth embodiment will be described with reference to FIG. FIG. 13 is an overall configuration diagram of the optical transmission system according to the ninth embodiment. In the ninth embodiment, the optical transmitters 20-1 to 20-N described in the first embodiment, the optical transmission line 21 for transmitting the optical signal, and the optical receivers 22-1 to 22- for receiving the optical signal are used. N optical transmission devices 20-1 to 20-N and N optical receiving devices 22-1 to 22-N are provided, and N optical transmission devices 20-1 are provided. ˜20-N transmit optical signals having different wavelengths, and N optical receivers 22-1 to 22-N receive optical signals having different wavelengths, and N optical transmitters 20 Wavelength multiplexing means 26 for wavelength-multiplexing optical signals transmitted from -1 to 20-N and sending them to the optical transmission line 21, and separating the optical signals for each wavelength from the wavelength-multiplexed optical signals coming from the optical transmission line 21 Wavelength separating means for inputting to each of the N optical receivers 22-1 to 22-N 7 and the N optical receiving apparatuses 22-1 to 22-N each include an error detecting means 50 for detecting an error included in the received optical signal, and error rate information obtained by the error detecting means 50 And a control circuit 53 for switching the sign of the chirp of the optical signal output from each of the N optical transmitters 20-1 to 20-N in the direction in which the error rate decreases. (Claim 9).

  By selecting the sign of the chirp code with a low error rate, it is possible to switch to a chirp code with less waveform deterioration due to wavelength dispersion. Thus, by using the error detection function of the optical receivers 22-1 to 22-N, it is not necessary to prepare an additional circuit such as the chromatic dispersion measuring means 24, and a wavelength multiplexing system is realized at low cost. it can.

(Tenth embodiment)
The tenth embodiment is an embodiment of an optical transmission system. An optical transmission system according to the tenth embodiment will be described with reference to FIG. FIG. 14 is an overall configuration diagram of the optical transmission system according to the tenth embodiment. The tenth embodiment is an optical transmission system including the optical transmitter 20 described in the first embodiment, an optical transmission path 21 for transmitting an optical signal, and an optical receiver 22 for receiving an optical signal. The variable loss means 52 for reducing the intensity of the optical signal arriving from the optical transmission line 21 is provided, and the optical receiver 22 detects an error included in the optical signal output from the variable loss means 52. And a control circuit 51 for switching the sign of the chirp of the optical signal output from the optical transmission device 20 to a lower error rate based on the error rate information obtained by the error detection unit 50. (Claim 10).

The optical loss SNR of the signal light received by the variable loss means 52 is reduced, the error rate is increased, and the optimum sign of the chirp of the optical transmitter 20 is selected. When the optical SNR of the received signal is high, the error rate may be very low even if there is degradation due to chromatic dispersion. In this case, enormous measurement time is required to select the optimum sign of the chirp code based on the error rate information. When the bit rate is 40 Gbit / s and the error rate is 10 ⁻¹⁵ , several tens of hours of measurement time is required. By reducing the optical SNR of the signal light received by the variable loss means 52, it is possible to switch the sign of the optimum chirp between positive and negative in a short time.

(Eleventh Example)
The eleventh embodiment is an embodiment of an optical transmission system. The optical transmission system according to the eleventh embodiment will be described with reference to FIG. FIG. 15 is an overall configuration diagram of the optical transmission system according to the eleventh embodiment. The eleventh embodiment is an optical transmission system including the optical transmitter 20 described in the first embodiment, an optical transmission path 21 for transmitting an optical signal, and an optical receiver 22 for receiving an optical signal. Thus, variable loss means 52 for reducing the intensity of the optical signal transmitted from the optical transmitter 20 is provided, and the optical receiver 22 outputs the light output from the variable loss means 52 and transmitted through the optical transmission line 21. An error detection means 50 for detecting an error contained in the signal is provided, and based on the error rate information obtained by the error detection means 50, the chirp of the optical signal output from the optical transmission device 20 in the direction in which the error rate decreases The control circuit 51 is provided for switching the sign of (5).

  The difference between the optical transmission system of the eleventh embodiment and the optical transmission system of the tenth embodiment is that the variable loss means 52 is arranged on the transmission side, not on the reception side. Usually, in the optical transmission system, the optical SNR is limited in the optical transmission line 21, and the optical power of the received signal and the optical SNR are reduced by increasing the number of parts having loss on the receiving side. By arranging the variable loss means 52 on the transmission side in front of the optical transmission line 21 that mainly limits the optical SNR, it becomes possible to reduce the influence of the optical SNR reduction due to the addition of the variable loss means 52.

(Twelfth embodiment)
The twelfth embodiment is an embodiment of an optical transmission system. An optical transmission system according to the twelfth embodiment will be described with reference to FIG. FIG. 16 is an overall configuration diagram of the optical transmission system according to the twelfth embodiment. In the twelfth embodiment, the optical transmitters 20-1 to 20-N described in the first embodiment, the optical transmission line 21 for transmitting the optical signal, and the optical receivers 22-1 to 22 for receiving the optical signal are used. -N, an optical transmission system 20-1 to 20-N and N optical receivers 22-1 to 22-N are provided, and N optical transmitters 20- 1 to 20-N transmit optical signals having different wavelengths, and N optical receivers 22-1 to 22-N receive optical signals having different wavelengths, and N optical transmitters Wavelength multiplexing means 26 for wavelength-multiplexing optical signals transmitted from 20-1 to 20-N and sending them to the optical transmission line 21, and an optical signal for each wavelength from the wavelength-multiplexed optical signal coming from the optical transmission line 21 Wavelength separating means 27 for separating, and for each wavelength output from the wavelength separating means 27 N variable loss means 52-1 to 52-N that respectively input signals and reduce the strength thereof. The N optical receiving apparatuses 22-1 to 22-N include N variable loss means 52. -1 to 52-N are provided with error detection means 50 for detecting errors included in the optical signals respectively output, and the error rate is reduced based on the error rate information obtained by the error detection means 50. A control circuit 53 for switching the sign of the chirp of the optical signal output from each of the N optical transmitters 20-1 to 20-N is provided (claim 12).

  The optical loss ratio 52-1 to 52-N is used to lower the optical SNR of the received optical signal, increase the error rate, and select the sign of the optimum chirp code of the optical transmitters 20-1 to 20-N. . By reducing the optical SNR of the optical signal received by the variable loss means 52-1 to 52-N, it is possible to select the optimum sign of the chirp sign in a short time.

(Thirteenth embodiment)
The thirteenth embodiment is an embodiment of an optical transmission system. An optical transmission system according to the thirteenth embodiment will be described with reference to FIG. FIG. 17 is an overall configuration diagram of an optical transmission system according to the thirteenth embodiment. In the thirteenth embodiment, the optical transmitters 20-1 to 20-N described in the first embodiment, the optical transmission line 21 for transmitting the optical signal, and the optical receivers 22-1 to 22 for receiving the optical signal are used. -N, an optical transmission system 20-1 to 20-N and N optical receivers 22-1 to 22-N are provided, and N optical transmitters 20- 1 to 20-N transmit optical signals having different wavelengths, and N optical receivers 22-1 to 22-N receive optical signals having different wavelengths, and N optical transmitters N variable loss means 52-1 to 52-N which input optical signals transmitted from 20-1 to 20-N and reduce the intensity thereof, and these N variable loss means 52-1 to 52- Wavelength multiplexed signals for wavelength multiplexing the optical signals output from N and sending them to the optical transmission line 21 26 and wavelength separation means 27 for separating the optical signal for each wavelength from the wavelength-multiplexed optical signal coming from the optical transmission line 21 and inputting the separated optical signals to the N optical receivers 22-1 to 22-N, respectively. , N optical receiving apparatuses 22-1 to 22-N each include an error detecting means 50 for detecting an error included in the received optical signal, and based on the error rate information obtained by the error detecting means 50, A control circuit 53 is provided for switching the sign of the chirp of the optical signal output from each of the N optical transmitters 20-1 to 20-N in the direction in which the error rate decreases (claim 13). ).

  The optical transmission system of the thirteenth embodiment is different from the optical transmission system of the twelfth embodiment in that the variable loss means 52-1 to 52-N are arranged on the transmission side, not on the reception side. . Usually, in the optical transmission system, the optical SNR is limited in the optical transmission line 21, and the optical power of the received signal and the optical SNR are reduced by increasing the number of parts having loss on the receiving side. By disposing the variable loss means 52-1 to 52-N on the transmission side in front of the optical transmission line 21 that mainly restricts the optical SNR, the optical SNR is reduced by adding the variable loss means 52-1 to 52-N. It is possible to reduce the influence of.

  In the eighth to thirteenth embodiments, the optical receivers 22, 22-1 to 22-N include error correction means, and error correction number information can be used as the error rate information. ).

  According to the present invention, it is possible to increase the increment of the dispersion value of a chromatic dispersion compensation product prepared as a system, and it is possible to realize cost reduction by reducing the number of products. In addition, the system that automatically selects the sign of the optimum chirp of the transmission signal can reduce the amount of work performed by workers when the system is introduced.

The block block diagram of the optical transmission apparatus of a 1st Example. The figure which shows the operation example at the time of inverting the polarity of the RZ electrical signal by the polarity switching means in the first embodiment (N = 2). The figure which shows the relationship between the eye opening degradation of the optical transmitter of a 1st Example, and the value of chromatic dispersion amount. The block block diagram of the polarity switching means and RZ conversion means of 2nd Example. The block block diagram of the polarity switching means and RZ conversion means of 3rd Example. The whole block diagram of the optical transmission system of 4th Example. The whole block diagram of the optical transmission system of 5th Example. The whole block diagram of the optical transmission system of 6th Example. The figure which shows the relationship between the signal strength of the frequency B / 2 zone | band and chromatic dispersion in the optical transmission system of 6th Example. The figure which shows the relationship between the signal strength of the frequency B / 4 zone | band and chromatic dispersion in the optical transmission system of 6th Example. The whole block diagram of the optical transmission system of 7th Example. The whole optical transmission system block diagram of 8th Example. The whole block diagram of the optical transmission system of 9th Example. The whole optical transmission system block diagram of a 10th Example. The whole block diagram of the optical transmission system of 11th Example. The whole block diagram of the optical transmission system of 12th Example. The whole block diagram of the optical transmission system of 13th Example. The block block diagram of the conventional optical transmitter. The figure for demonstrating the operation example of the conventional optical transmitter. The figure which shows the eye pattern in the operation example of the conventional optical transmitter.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 CW light source 2 Both-side drive Mach-Zehnder type optical modulator 3-1 to 3-N RZ conversion means 4-1 to 4-N, 7, 7-1 to 7-N Polarity switching means 5-1 to 5-N-1 Delay circuit 6 Bias circuit 8 DC voltage application means 9, 11 2: 1 selector means 10 Time division multiplexing circuit 20, 20-1 to 20 -N Optical transmission device 21 Optical transmission line 22, 22-1 to 22-N light Receiver 23 Optical distribution means 24 Chromatic dispersion measurement means 25, 29, 33, 51, 53 Control circuit 26 Wavelength multiplexing means 27 Wavelength separation means 28 Wavelength selection means 30, 41-1, 41-2 Photoelectric conversion means 31 Band selection means 32 intensity measuring means 40 frequency component separating means 42 phase comparing means 43 chromatic dispersion calculating means 50 error detecting means 52, 52-1 to 52-N variable loss means 60-1 to 60-N RZ converting means 61-1 to 61- N-1 Slow Extension means

Claims (14)

In an optical transmission apparatus that multiplexes N electrical signals, converts them into optical signals, and transmits them.
A CW light source that outputs carrier light;
A double-sided drive type Mach-Zehnder type modulator having N (N ≧ 2) electrodes connected in series with the CW light source;
N RZ conversion means for converting the NRZ signals as electrical signals of channels 1 to N to RZ at a bit rate B (time slot T);
N polarity switching means for switching the polarity of the RZ signal as N electrical signals converted into RZ by the RZ conversion means,
A delay of T / 2N × k (k: 1, 2, 3,..., N−1) is given to the RZ signals as the electrical signals of the channels 2 to N whose polarities are switched by the polarity switching means. N-1 delay means for applying a voltage corresponding to the RZ signal as an electrical signal of channels 2 to N to the electrodes;
And a bias circuit for applying a bias voltage corresponding to the RZ signal as an electrical signal of the channel 1 to the electrodes of the channel 1 other than the channels 2 to N delayed by the delay means. Transmitter device.   As the RZ conversion means and the polarity switching means, a means for switching the polarity of the input NRZ signal, a DC voltage application means, an output of the DC voltage application means, and an NRZ signal output from the means for switching the polarity. 2. The optical transmission device according to claim 1, further comprising means for inputting and generating an RZ signal. The NRZ signal is a signal obtained by time-division multiplexing M NRZ signals,
As the RZ conversion means and the polarity switching means, means for switching the polarities of the input M NRZ signals, and means for time-division multiplexing the M NRZ signals output from the switching means, respectively. 2. A DC voltage applying means; and means for generating an RZ signal by inputting an output of the DC voltage applying means and a time division multiplexed NRZ signal output from the time division multiplexing means. The optical transmission device described. An optical transmission system comprising the optical transmission device according to any one of claims 1 to 3, an optical transmission path for transmitting an optical signal, and an optical reception device for receiving the optical signal.
An optical distribution means for distributing an optical signal output from the optical transmission line and inputting one of the optical signals to the optical receiver;
Chromatic dispersion measuring means for measuring the chromatic dispersion amount of the optical transmission line by inputting the other of the optical signals distributed by the optical distributing means;
An optical transmission system comprising: control means for switching the sign of the chirp of the optical signal output from the optical transmission device based on the value of the chromatic dispersion amount obtained by the chromatic dispersion measuring means. An optical transmission system comprising the optical transmission device according to any one of claims 1 to 3, an optical transmission path for transmitting an optical signal, and an optical reception device for receiving the optical signal.
Each of the optical transmitter and the optical receiver is provided with N, the N optical transmitters transmit optical signals having different wavelengths, and the N optical receivers have different wavelengths. Receive optical signal,
Wavelength multiplexing means for wavelength-multiplexing optical signals transmitted from the N optical transmitters and sending them to the optical transmission line;
Optical distribution means for distributing wavelength-multiplexed optical signals coming from the optical transmission line;
Wavelength separation means for separating an optical signal for each wavelength from one of the optical signals distributed by the optical distribution means and inputting the signals to the N optical receivers;
Wavelength selection means for extracting an optical signal of a specific wavelength from the other of the optical signals distributed by the optical distribution means;
Chromatic dispersion measuring means for measuring the chromatic dispersion amount of the optical transmission line from the optical signal output from the wavelength selecting means;
Control means for switching the sign of the chirp of the optical signal output from the N optical transmitters based on the value of the chromatic dispersion obtained by the chromatic dispersion measuring means. system. Instead of the chromatic dispersion measuring means,
Photoelectric conversion means for converting an optical signal into an electrical signal;
Band selection means for extracting a specific band component from the electrical signal output from the photoelectric conversion means,
An intensity measuring means for measuring the intensity of the electric signal output from the band selecting means,
The optical transmission system according to claim 4, further comprising a control unit that switches the sign of the chirp of the optical transmission device so that the intensity obtained by the intensity measurement unit becomes higher, instead of the control unit. The chromatic dispersion measuring means includes
Frequency component separation means for separating and extracting at least two different frequency components from the optical spectrum components of the optical signal;
Two photoelectric conversion means for converting two optical signals output from the frequency component separation means into electrical signals;
Phase comparison means for detecting a relative phase difference between the two electrical signals output from the two photoelectric conversion means;
6. An optical transmission system according to claim 4, further comprising: chromatic dispersion calculating means for calculating an amount of chromatic dispersion that the optical signal suffers in the optical transmission line based on relative phase difference information obtained from the phase comparing means. . An optical transmission system comprising the optical transmission device according to any one of claims 1 to 3, an optical transmission path for transmitting an optical signal, and an optical reception device for receiving the optical signal.
The optical receiver includes means for detecting an error included in an optical signal,
Control means for switching the sign of the chirp of the optical signal output from the optical transmission device to the lower error rate based on the error rate information obtained by the error detecting means. And optical transmission system. An optical transmission system comprising the optical transmission device according to any one of claims 1 to 3, an optical transmission path for transmitting an optical signal, and an optical reception device for receiving the optical signal.
Each of the optical transmitter and the optical receiver is provided with N, the N optical transmitters transmit optical signals having different wavelengths, and the N optical receivers have different wavelengths. Receive optical signal,
Wavelength multiplexing means for wavelength-multiplexing optical signals transmitted from the N optical transmitters and sending them to the optical transmission line;
Wavelength separation means for separating the optical signal for each wavelength from the wavelength-multiplexed optical signal coming from the optical transmission line and inputting the optical signal to each of the N optical receivers,
Each of the N optical receivers includes means for detecting an error included in the received optical signal,
Control means for switching the sign of the chirp of the optical signal output from each of the N optical transmitters in accordance with the error rate information obtained by the error detecting means, in accordance with the error rate decreasing. An optical transmission system characterized by this. An optical transmission system comprising the optical transmission device according to any one of claims 1 to 3, an optical transmission path for transmitting an optical signal, and an optical reception device for receiving the optical signal.
Variable loss means for reducing the intensity of the optical signal coming from the optical transmission path is provided;
The optical receiver includes means for detecting an error included in the optical signal output from the variable loss means,
Control means for switching the sign of the chirp of the optical signal output from the optical transmission device to the lower error rate based on the error rate information obtained by the error detecting means. And optical transmission system. An optical transmission system comprising the optical transmission device according to any one of claims 1 to 3, an optical transmission path for transmitting an optical signal, and an optical reception device for receiving the optical signal.
Variable loss means for reducing the intensity of the optical signal transmitted from the optical transmitter is provided,
The optical receiver includes a means for detecting an error included in the optical signal output from the variable loss means and transmitted through the optical transmission path,
Control means for switching the sign of the chirp of the optical signal output from the optical transmission device to the lower error rate based on the error rate information obtained by the error detecting means. And optical transmission system. An optical transmission system comprising the optical transmission device according to any one of claims 1 to 3, an optical transmission path for transmitting an optical signal, and an optical reception device for receiving the optical signal.
Each of the optical transmitter and the optical receiver is provided with N, the N optical transmitters transmit optical signals having different wavelengths, and the N optical receivers have different wavelengths. Receive optical signal,
Wavelength multiplexing means for wavelength-multiplexing optical signals transmitted from the N optical transmitters and sending them to the optical transmission line;
Wavelength separation means for separating the optical signal for each wavelength from the wavelength-multiplexed optical signal coming from the optical transmission line;
N variable loss means for inputting optical signals for each wavelength output from the wavelength separation means and reducing the intensity thereof, and
The N optical receiving apparatuses include means for detecting errors included in the optical signals respectively output from the N variable loss means,
Control means for switching the sign of the chirp of the optical signal output from each of the N optical transmitters in accordance with the error rate information obtained by the error detecting means, in accordance with the error rate decreasing. An optical transmission system characterized by this. An optical transmission system comprising the optical transmission device according to any one of claims 1 to 3, an optical transmission path for transmitting an optical signal, and an optical reception device for receiving the optical signal.
Each of the optical transmitter and the optical receiver is provided with N, the N optical transmitters transmit optical signals having different wavelengths, and the N optical receivers have different wavelengths. Receive optical signal,
N variable loss means for respectively inputting optical signals transmitted from the N optical transmission devices and reducing the intensity thereof;
Wavelength multiplexing means for wavelength-multiplexing the optical signals respectively output from the N variable loss means and sending them to the optical transmission line;
Wavelength separation means for separating the optical signal for each wavelength from the wavelength-multiplexed optical signal coming from the optical transmission line and inputting the optical signal to each of the N optical receivers,
Each of the N optical receivers includes means for detecting an error included in the received optical signal,
Control means for switching the sign of the chirp of the optical signal output from each of the N optical transmitters in accordance with the error rate information obtained by the error detecting means, in accordance with the error rate decreasing. An optical transmission system characterized by this.   The optical transmission system according to any one of claims 8 to 13, wherein the optical receiver includes error correction means and uses error correction number information as the error rate information.

Images