JP3371857B2 - Optical transmission equipment - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a transmission device and a transmission / reception device for optical communication. The present invention particularly relates to a technique for reducing the RZ optical signal band and the base band electric signal band of the transmitter.

[0002]

2. Description of the Related Art In recent optical fiber transmission systems, optical fiber amplifiers having a high output and a wide amplification band characteristic have been widely used, and the input optical power in the fiber in the optical fiber transmission line can easily be 10 dBm. Has come to exceed. As a result, the optical signal itself undergoes phase modulation due to the Kerr effect in which the refractive index in the optical fiber is modulated by the input optical signal intensity, the optical modulation spectrum is broadened, and waveform distortion occurs due to interaction with the dispersion of the optical fiber. . Further, in the wavelength multiplexing system, waveform distortion and S / N deterioration occur due to nonlinear crosstalk between channels.

It is known that this effect depends strongly on the signal format, and for example, RZ (Return-to-Zer).
Compared to NRZ (Non-Return-to-Zero) signal, the pulse width of o) signal is more uniform for each bit, so it is easy to deteriorate the waveform distortion due to non-linear effect during long distance transmission. It has been reported to have advantages.

For example, reference: D. Breuer et al., Compariso
n of NRZ and RZ-Modulation Format for 40-Gbit / s TD
M Standard-Fiber Systems, IEEE Photon. Technol. Le
tt.vol.9 No.3 pp.398-400, 1997 ", RZ signal is 40G compared to NRZ signal in a linear repeater system in which 1.3um zero dispersion fiber transmission line is dispersion-compensated for each repeater section.
It is predicted by simulation that the regenerative relay distance can be increased by about 3 times at bit / s. Also,
Reference: RMJopson et al., Evaluation of return-to-z
ero modulation for wavelength-division-multiplexed
transmissionover convention single-mode-fiber, R.
M. Jopson et al, in Tech. Digest of Optical Fiber Co
mm. Conf.'98 FE1, p.406-407, 1998 ", 10-Gb
In the it / s 8-wavelength WDM transmission system, the RZ signal is NR
It has been experimentally shown that the power per channel can be increased as compared with the Z signal. Reference: A. Sano et al. I
In EE Electronics Letters Vol.30, p.1694-1695 1994, by applying phase modulation synchronized with data, SB
The S scattering level can be improved and the incident power in the fiber can be increased.

Therefore, in a high-speed optical transmission system, it is preferable to transmit in the form of RZ signal.

The signal format is RZ (Return-to-Zer).
28 and 29 show the configuration of a conventional optical transmission device for transmitting the optical signal of o). In FIG. 28, the input NRZ electric signal is converted into an RZ electric signal in the NRZ / RZ conversion circuit 51, and the RZ electric intensity modulator 50 is directly driven by the RZ electric signal amplified by the RZ optical intensity modulator driving circuit 52. Then, the continuous (CW) light from the light source 5 is modulated to RZ
This is a configuration for generating an optical signal.

In FIG. 29, the input NRZ electric signal is first amplified by the NRZ light intensity modulator drive circuit 62, and the continuous (CW) light from the light source 5 is output from the first NRZ light intensity modulator 60. Modulate to generate an NRZ optical signal. Next, the input NRZ electric signal (transmission rate: B (bit
/ S)) input clock signal (frequency: B (H
z)) is used to drive the clock light intensity modulator 61 in the second stage with a sine wave by the clock light intensity modulator drive circuit 63, and the NRZ generated in the NRZ light intensity modulator 60 in the first stage An RZ optical signal is generated from the optical signal (for example, A. Sano et al. IEEE Electronics Letters v
ol.30, P.1694-1695 1994).

Further, Japanese Patent Laid-Open Publication No. 8-254673 (: Method and apparatus for generating data-encoded pulse in return-to-zero format) (corresponding USP 5,625, US Pat.
722) (Method and apparatus for generating data
In encoded pulses in return-to-zero format), the periodicity of the transmittance of an optical Mach-Zehnder modulator is used as a full-wave rectification characteristic to convert a binary NRZ electric signal into an RZ
A configuration for converting to an optical signal is shown. A binary NRZ electric signal is input, converted into a coded NRZ electric signal precoded by a precoding circuit called an encoder, and after branching, one logic is inverted. An RZ signal is generated by modulating a Mach-Zehnder type modulator with a coded NRZ electric signal having a differential configuration.

Further, a configuration for generating and modulating an optical clock pulse signal from a clock electric signal is described in the following 3
It is described in two documents. Reference (K. Iwatsuki et al.
“Generation of transform limited gain-swiched DFB
-LD pulses <6 ps with linear fiber compression an
d spectral window ”, Electronics Letters vol.27, pp
1981-1982, 1991), a method using a gain-switched semiconductor laser as the generating element, and reference (M. Suzuki, et al. “N.
ew application of sinusoidal driven InGaAsP electr
oabsorption modulator to in-line optical gate with
ASE noise resuction effect ”, J. Lighitwave Techno
L., 1992, vol.10 pp.1912-1918 ”), a method of modulating a semiconductor laser optical signal oscillating in CW by an electroabsorption type semiconductor modulator, K. Sato et al.“ Frequency Range Exte
nsion of actively mode-locked lasers integrated wi
th electroabsorpstion modulators using chirped gra
ting ”J. of selected topics in quantum electronics
vol.3 No.2, 1997, pp.250-255), a method using an integrated mode-locked semiconductor laser is shown. However, the modulation means is not mentioned above.

[0010]

In any of the above-mentioned conventional techniques, the optical modulation band of the RZ optical signal generated is 4 B or more (transmission rate: B (bit / s)), regardless of which method is used.
This is more than twice the NRZ optical signal band. Therefore, NRZ
There is a problem that waveform distortion is more likely to occur due to wavelength dispersion in the transmission line than in the case of optical signals. FIG. 30 is a diagram showing an NRZ optical signal spectrum by the conventional optical transmission device, and FIG. 31 is a diagram showing an RZ optical signal spectrum by the conventional optical transmission device. As can be seen from FIGS. 30 and 31, according to the conventional technique, the optical modulation band of the RZ optical signal is twice or more the NRZ optical signal band.

Further, in the conventional example shown in FIG. 28, an NRZ / RZ conversion circuit 51 for converting an NRZ electric signal into an RZ electric signal in an electric stage, and an RZ light intensity modulator driving circuit for amplifying the RZ electric signal. 52 and RZ light intensity modulator 5
0 and double the band required for NRZ electrical signals (DC
To 2B (Hz)) is required, and there is a problem that it becomes difficult to design a realistic circuit as the transmission speed increases.

In the conventional example shown in FIG. 29, the light intensity modulator has a two-stage configuration of the NRZ light intensity modulator 60 and the clock light intensity modulator 61, so the RZ output at the final stage.
In order to maintain the S / N ratio of the optical signal at the same level as that of the NRZ optical signal, it is necessary to increase the output of the light source 5 by about 6 to 9 dB in order to compensate for the loss and modulation loss of one stage of the optical intensity modulator. Therefore, realization of high output of the light source 5 is a problem. NRZ
There is complexity in that the phase control circuit 64 for controlling the modulation phase of the optical signal and the synchronization clock signal is required.

In the prior art, whichever device is used, the modulation spectrum of the generated RZ optical signal has a line spectrum of ± n · B (Hz) centered on the carrier frequency of continuous light. Therefore, when the signal power input to the transmission fiber exceeds about 7 dBm, in the dispersion shift fiber transmission line, the optical input power in the fiber is limited by the threshold due to stimulated Brillouin scattering (SBS). Therefore, it is necessary to widen the optical frequency line width of the continuous light source to relax the optical input power limitation due to SBS, and for this purpose, an external circuit such as the SBS suppressing line width modulation circuit 53 is required, which is complicated. .

Since the RZ optical signal has an optical carrier frequency component (fc) as shown in FIG.
If the Z power of the optical signal is high, and the maximum value of the RZ optical signal spectral density is higher than the RZ optical signal power at which the maximum value of the RZ optical signal spectral density is equal to the threshold density due to stimulated Brillouin scattering (SBS), the high density of the RZ optical signal causes the SBS to generate these high Spectral components undergo backscattering, causing waveform distortion (for example, H. Kawakami et al. “Overmodulation of intensi”).
ty modulated signal due to Stimulated Brillouin Sc
attering Electron. Lett. vol.30, No.18 pp.1507-150
8, 1994). In addition, these RZ optical signals are wavelength-multiplexed (WD
In the case of M), the four-wave mixing (FWM) is likely to occur in the portion where the optical signal spectral density is high, and the crosstalk due to the pump division is likely to occur.

Further, in the conventional method of amplifying the RZ electric signal as it is, when the capacitive coupling type driving circuit is used, the DC level fluctuation of the driving waveform occurs due to the fluctuation of the mark ratio of the signal, and the output dynamic range of the driving circuit is about 2. It is necessary to double or more, and a control circuit for compensating the bias point of the light intensity modulator, which varies depending on the mark rate, by the mark rate is required.

As described above, in the conventional configuration, the optical spectrum band required for generating the RZ optical signal is more than double that of the NRZ optical signal, and is affected by the signal deterioration due to the wavelength dispersion of the optical fiber transmission line. It is easy, the bandwidth required for the electric circuit handling the RZ electric signal is twice as fast as that at the time of transmitting the NRZ electric signal, and it is difficult to increase the speed. It is easy to be restricted by the optical input power in the fiber due to the SBS, and the optical frequency line width by the external circuit. The need for expansion of the four lightwave mixing (FW
The problems are that crosstalk due to M) is likely to occur and that the DC level of the drive waveform fluctuates due to fluctuations in the mark ratio of the signal.

The present invention has been made in view of such a background, and realizes an optical spectrum band required for generating an RZ optical signal with a half or less of a conventional one, and prevents signal deterioration due to wavelength dispersion of an optical fiber transmission line. An object of the present invention is to provide an optical transmission device that can be less susceptible to the influence. An object of the present invention is to provide an optical transmission device capable of generating an electric signal band required for an electric circuit and an optical intensity modulator at a transmission speed B. SUMMARY OF THE INVENTION It is an object of the present invention to provide an optical transmission device that can essentially eliminate the optical input power limitation in fiber due to SBS. An object of the present invention is to provide an optical transmission device capable of reducing the output power of a light source. An object of the present invention is to provide an optical transmission device capable of reducing the influence of crosstalk due to four-wave mixing (FWM). It is an object of the present invention to provide an optical transmission device that does not fluctuate in DC level due to a change in signal mark rate. It is an object of the present invention to provide an optical transmission device that hardly causes intersymbol interference that occurs in an optical fiber transmission line and an optical transmission / reception unit.

[0018]

The present invention is an optical transmission apparatus having an input terminal for inputting an NRZ electric signal and a means for converting the NRZ electric signal input from the input terminal into an RZ optical signal. is there. The feature of the present invention is that
The converting means includes precoding means for inputting an NRZ differential electric signal, and the precoding means delays the value of the NRZ differential electric signal and the code value of the output of the precoding means by 1 bit. A differential encoding means for outputting a value of an exclusive OR with a signal and generating differential electric signal pulses having different polarities at a rising point and a falling point of the value of the exclusive OR, and the differential electric signal pulse Centering on the second level of the ternary differential electrical signal according to
The differential electric pulse waveform is folded back and the first differential signal
And an optical intensity modulation means for performing optical RZ intensity modulation on continuous light so that the optical modulation signal phase corresponding to the third level differs by π. It is desirable that the light intensity modulation means includes a Mach-Zehnder intensity modulator.

Even when the input NRZ electric signal is a single end input, if the output of the precoding means is a differential output, the same function is realized.

In this way, by precoding the NRZ electric signal and then converting it into a differential signal, the input signal is a DC balanced code containing no DC component and is a ternary signal, so that the electric circuit, the optical signal The intensity modulator does not need the baseband amplification / modulation characteristics from DC, and can generate the required band at the transmission speed B. Furthermore, since the differential signal does not include a DC component, there is no DC level fluctuation due to the mark rate. Further, since the light intensity modulator having a one-stage structure can be used for the modulation, the output power required for the continuous light source can be reduced.

Further, since the RZ optical signal generated by using the differential signal has no carrier frequency component of light regardless of the mark ratio, the RZ optical signal spectral density is lower than that of the conventional RZ optical signal spectral density. Become. Therefore, the RZ optical signal power at which the maximum value of the spectral density of the RZ optical signal of the present invention becomes equal to the threshold density due to stimulated Brillouin scattering (SBS) can be made higher than that of the RZ optical signal of the conventional configuration. In addition, due to the low spectral density, four-wave mixing (FW), which is a problem in conventional RZ and NRZ transmission, even when wavelength division multiplexing transmission is performed near the zero-dispersion wavelength of the transmission line.
The influence of crosstalk due to M) can be reduced.

Furthermore, since the phase of the pulse by pulse is inverted, even if fading occurs due to multipath due to polarization dispersion in the transmission line, the phase is inverted at the overlapping portion of the pulse edges, so that the intensity modulation is performed. In the generated signal, the intensity of the overlapping portion of the pulse edges is canceled by the interference, and it is difficult to cause intersymbol interference.

As another configuration of the present invention, the converting means is a means for generating a clock electric signal with which the NRZ electric signal is synchronized, and an optical signal synchronized with the clock electric signal with the clock electric signal as an input. A clock pulse light source for generating a clock pulse signal and a precoding means for inputting a single-ended NRZ electric input signal are provided, and the precoding means outputs the value of the NRZ electric signal and the code value of the output of the precoding means. An exclusive OR value with a signal delayed by 1 bit is differentially output, and the NRZ electrical input signal and the precoded differential NRZ signal are input, and the NRZ electrical input signal and the precoded differential NRZ are input.
Two differential AND code means for differentially outputting the logical product value of the signals and the optical clock optical signal as an input are provided, and the modulators of the respective arms are electrically insulated according to the differential AND code. Rather than modulating two modulators arranged in series,
It is also possible to provide an optical intensity modulating means for independently modulating the intensity and phase of the optical clock optical input signal and performing intensity modulation so that the phase of the mark bit of the output optical signal is different by π alternately.

As still another configuration of the present invention, the converting means is a means for generating a clock electric signal with which the NRZ electric signal is synchronized, and a means for generating the clock electric signal, and the clock electric signal is synchronized with the clock electric signal. It has a clock pulse light source for generating an optical clock pulse signal, and a precoding means for inputting a single-ended NRZ electric input signal, and the precoding means has a value of the NRZ electric signal and a code value of the output of the precoding means. The value of the exclusive OR with the signal delayed by 1 bit is differentially output, and the NR
The Z electrical input signal and the precoded differential NRZ signal are input, and the logical product value of the NRZ electrical input signal and the precoded differential NRZ signal is differentially output. 2
One differential logical product encoding means, and a power addition means for outputting a power addition code for performing power addition of two logical product coded NRZ signals having different logics from the two differential logical product means.
Using the optical clock optical signal as an input, the power addition code is folded around the second level of the ternary power addition code according to the power addition code, and corresponds to the first and third levels of the power addition code. It is also possible to adopt a configuration in which the optical clock optical signal is provided with intensity modulating means so that the optical modulation phase differs by π.

As another structure of the present invention, the converting means is a first light intensity modulating means for performing light intensity modulation of continuous light in accordance with a clock signal, and an output light signal of the first light intensity modulating means. Has a second light intensity modulating means for performing light intensity modulation according to the NRZ electric signal, and a precoding means for inputting the NRZ electric signal. The precoding means has a value of the NRZ electric signal and the precoding means. Precoding means for outputting a value of exclusive OR with a signal obtained by delaying the output code value by 1 bit, and 1 of the output optical signal of the second light intensity modulating means according to the value of the exclusive OR. It is also possible to adopt a configuration including a phase modulation unit that gives a phase change of π for each pulse.

[0026]

BEST MODE FOR CARRYING OUT THE INVENTION FIG. 1 and FIG.
0, FIG. 12, FIG. 20 and FIG. 26.
FIG. 1 is a block diagram of the essential parts of an optical transmission apparatus according to the first embodiment of the present invention. FIG. 10 is a block diagram of the essential parts of an optical transmission apparatus according to the second embodiment of the present invention.

FIG. 12 is a block diagram of the essential parts of an optical transmission apparatus according to the third embodiment of the present invention. FIG. 20 is a block diagram of the essential parts of an optical transmission apparatus according to the fourth embodiment of the present invention. FIG. 26 is a block diagram of the essential parts of the fifth optical transmission device of the present invention.

The first embodiment of the present invention, as shown in FIG. 1, has input terminals 14 and 1 for inputting an NRZ electric signal.
4'and a means for converting the NRZ electrical signal inputted from the input terminals 14 and 14 'into an RZ optical signal.

Here, a feature of the present invention is that the converting means includes precoding means for inputting the NRZ differential electric signals input from the input terminals 14 and 14 '. Is composed of precoding circuits 1 and 1'for outputting the value of the exclusive OR of the value of the NRZ differential electric signal and the signal obtained by delaying the code value of the output of the precoding means by 1 bit. Bandpass filters 2 and 2'which are differential encoding means for generating differential electric signal pulses having different polarities at the rising point and the falling point of the value of the logical OR, and the three-value differential electric signal according to the differential electric signal pulse. The differential electric pulse waveform is folded back around the second level of the signal, and the continuous light intensity is adjusted so that the phase of the optical modulation signal corresponding to the first and third levels of the differential signal is π different. There is to provided a light intensity modulator 4 is an optical intensity modulating means for modulating. The light intensity modulator 4 includes a Mach-Zehnder intensity modulator.

In the second embodiment of the present invention, as shown in FIG. 10, an input terminal 71 for inputting an NRZ electric signal and a means for converting the NRZ electric signal input from the input terminal 71 into an RZ optical signal. And an optical transmission device including.

Here, a feature of the present invention is that the converting means includes precoding means for inputting a single-ended NRZ electric signal input from the input terminal 71, and the precoding means is the NRZ. The differential output precoding means 72 differentially outputs the value of the exclusive OR of the value of the electric signal and the signal obtained by delaying the code value of the output of the precoding means by 1 bit. Bandpass filters 2 and 2'which are differential encoding means for generating differential electric pulses having different polarities at the rising point and the falling point of the value, and according to the differential electric pulse, the ternary differential electric signal A light intensity that folds the differential electric pulse waveform around the second level and intensity-modulates continuous light so that the light modulation phases corresponding to the first and third levels of the differential signal differ by π. There is to provided a light intensity modulator 4 is adjusting unit. The light intensity modulator 4 includes a Mach-Zehnder intensity modulator.

In the third embodiment of the present invention, as shown in FIG. 12, an input terminal 81 for inputting an NRZ electric signal and a means for converting the NRZ electric signal input from the input terminal 81 into an RZ optical signal. And an optical transmission device including.

Here, the feature of the present invention is that the converting means inputs an electric clock generating circuit 86 which is a means for generating a clock electric signal to which the NRZ electric signal is synchronized, and the clock electric signal. , A clock pulse light source 83 for generating an optical clock pulse signal synchronized with the clock electric signal, and a single-ended NRZ
Precoding means for inputting an electric input signal is provided, and the precoding means calculates an exclusive OR value of the value of the NRZ electric signal and a signal obtained by delaying the code value of the output of the precoding means by 1 bit. A differential NRZ pre-coded with the NRZ electric input signal, which is composed of pre-coding means 72 for differential output.
Two differential logical product encoding means 73, 73 ', 82, 82' for differentially outputting the logical product value of the NRZ electrical input signal and the precoded differential NRZ signal using the signal as an input, and the optical clock. The intensity and phase of the optical clock optical input signal can be obtained by using the optical signal as an input and modulating the two modulators that are electrically insulated and serially arranged by the modulators of each arm according to the differential AND code. And an optical intensity modulating means 84 for independently modulating the intensity of the mark bit of the output optical signal so that the phase of the mark bit of the output optical signal is different by π.

In the fourth embodiment of the present invention, as shown in FIG. 20, an input terminal 91 for inputting an NRZ electric signal and an RZ electric signal for inputting the NRZ electric signal input from the input terminal 91 are RZ.
An optical transmission device including means for converting to an optical signal.

Here, a feature of the present invention is that the converting means inputs an electric clock generating circuit 86 which is a means for generating a clock electric signal with which the NRZ electric signal is synchronized, and the clock electric signal. , A clock pulse light source 83 for generating an optical clock pulse signal synchronized with the clock electric signal, and a single-ended NRZ
A precoding means for inputting an electric input signal is provided, and the precoding means is a value of an exclusive OR of the value of the NRZ electric signal and a signal obtained by delaying the code value of the output of the precoding means by 1 bit. A precoding means 72 for differentially outputting
Two differential logical product encoding means 82 for differentially outputting the logical product value of the NRZ electrical input signal and the precoded differential NRZ signal with the NRZ electrical input signal and the precoded differential NRZ signal as inputs. 82 ', power adding means 100 and 101 for outputting power addition codes for performing power addition of two logical product coded NRZ signals having different logics from the two differential logical product means, and the optical clock optical signal. As an input, according to the power addition code, the power addition code is folded back around the second level of the ternary power addition code, and the optical modulation phases corresponding to the first and third levels of the power addition code differ by π. As described above, the optical intensity modulation means 102 for intensity-modulating the optical clock optical signal is provided.

In the fifth embodiment of the present invention, as shown in FIG. 26, an input terminal 18 for inputting an NRZ electric signal and a means for converting the NRZ electric signal input from the input terminal 18 into an RZ optical signal. And an optical transmission device including.

Here, the feature of the present invention is that the converting means is the first light intensity modulating means for performing the light intensity modulation of the continuous light in accordance with the clock signal input from the input terminal 19. An intensity modulator 31, an optical intensity modulator 32 that is a second optical intensity modulator that performs optical intensity modulation of an output optical signal of the optical intensity modulator 31 according to an NRZ electrical signal, and the NRZ electrical signal are input. A precoding circuit 1 that has precoding means, and outputs the value of the exclusive OR of the value of the NRZ electric signal and the signal obtained by delaying the code value of the output of the precoding means by 1 bit. And a phase modulator 40, which is a phase modulator that gives a phase change of π for each mark pulse of the output optical signal of the optical intensity modulator 32 according to the value of the exclusive OR.
And is equipped with.

[0038]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS (First Embodiment) A first embodiment of the present invention will be described with reference to FIGS. As shown in FIG. 1, precoding circuits 1 and 1 ′ for inputting multiplexed NRZ differential electric signals from input terminals 14 and 14 ′ and outputting precoded coded NRZ electric signals, respectively,
Bandpass filters 2 and 2'as differential circuits which receive the encoded NRZ electric signal and generate differential signal pulses having the same amplitude and different polarities centered on the ground level.
And the capacitively coupled drive circuits 3 and 3 ′ for amplifying the differential signal up to the drive voltage of the light intensity modulator 4, and the differential differential signal amplified by the drive circuits 3 and 3 ′ as inputs,
A push-pull type Mac that generates a Z optical signal
It is composed of an h-Zehnder type light intensity modulator 4 and an optical amplifier 6 for amplifying the output of the light intensity modulator 4.
The precoding circuits 1 and 1'and the bandpass filters 2 and 2'may be configured as bipolar code converters having complementary outputs. 2 shows the configuration of the precoding circuits 2 and 2 '. Precode circuit 2 of the present invention
And 2 ′ are formed by the exclusive OR circuit 7 and the 1-bit delay circuit 8. FIG. 3 shows the waveform of each part of the optical transmission device according to the first embodiment of the present invention.

The advantages of the present invention will now be described. The advantages of the present invention are the following four points. 1) Since the NRZ electric signal is precoded and then converted into a differential signal by a passive microwave component or the like, the input signal is a DC balanced code containing no DC component and is a ternary signal. For 3'and 3 ', the baseband amplification characteristic from direct current is not necessary and the 3 dB band is half the conventional B.
Amplification is possible at (Hz). 2) In the conventional method of amplifying the RZ electric signal as it is, if a capacitive coupling type drive circuit is used, the DC level of the drive waveform fluctuates due to fluctuation of the mark ratio of the signal, and the output dynamic range of the drive circuit is about twice or more. In addition, a control circuit for compensating for the bias point of the light intensity modulator 4 which varies depending on the mark rate by the mark rate is required. According to the present invention, since the differential signal does not include a DC component, the DC depending on the mark ratio
The above problem is solved without level fluctuation. 3) Also
FIG. 4 is a diagram for explaining the operation of the light intensity modulator 4 of the present invention, in which the horizontal axis represents the drive voltage and the vertical axis represents the light intensity. By using the periodic electro-optical response characteristics of the push-pull type Mach-Zehnder type optical intensity modulator 4 and selecting the operating point as shown in FIG. 4, pulses having different differential signals are The direct current level (second level) of A is folded back, and as a result, the RZ optical signal pulse is generated from the one-stage optical intensity modulator 4. At this time, the phases of the light at points B (first level) and C (third level) in FIG. 4 differ by π, and correspond to the rising edge and the rising edge of the phase of the encoded NRZ electric signal, respectively. Therefore, adjacent RZ optical signal pulse phases are always inverted.
Further, by using the 1/4 wavelength short stub line (at the clock frequency B (Hz) of the transmission speed), the pulse duty cycle becomes approximately 1/2. As a result, the generated RZ optical signal band becomes 2B and can be modulated by the one-stage optical intensity modulator 4, so that the output power required for the light source 5 can be made equal to that at the time of NRZ modulation. 4) Figure 5
FIG. 6 is a diagram showing an optical spectrum of an RZ optical signal by the optical transmission device of the present invention. Since differential signals are used, the existence probabilities of points B and C in FIG. As shown in FIG. 5, the RZ optical signal has no optical carrier frequency component regardless of the mark ratio, and the RZ optical signal spectral density is lower than that of the conventional example. Therefore, the RZ optical signal power at which the maximum value of the spectral density of the RZ optical signal of the present invention becomes equal to the threshold density by stimulated Brillouin scattering (SBS) can be made higher than that of the conventional example. In addition, due to the low spectral density, the effect of crosstalk due to four-wave mixing (FWM), which has been a problem in conventional RZ and NRZ transmission, can be reduced even when wavelength division multiplexing transmission is performed near the zero-dispersion wavelength of the transmission line. You can

FIG. 6 shows an optical transmission / reception device using the optical transmission device of the first embodiment of the present invention. In the example of FIG. 6, the transmission speed is 10
It is an example of composition of a phase inversion RZ signal transmission experiment in Gbit / s. The NRZ differential electric signals output from the data source 10 are input to the precoding circuits 1 and 1 ′, respectively. The outputs of the precoding circuits 1 and 1'are converted into the above-mentioned differential signals by the bandpass filters 2 and 2 '. This differential signal is amplified by the drive circuits 3 and 3 ', and the continuous light from the light source 5 is intensity-modulated by the output signals of the drive circuits 3 and 3'in the light intensity modulator 4. The output of the light intensity modulator 4 is amplified by the optical amplifier 6 and transmitted to the receiving side through the transmission line 12. On the receiving side, first, the gain of the received signal is limited by the attenuator 9 and input to the receiving device 13. In the receiving device 13,
The NRZ electric signal and the clock signal are reproduced and input to the receiving terminal 11. Note that the configuration and operation of the receiving side are known techniques and have no direct relation to the present invention, and therefore a description thereof will be omitted.

Bandpass filter 2 for generating a differential signal
And 2'are realized by using a 1/4 wavelength short stub line (at the clock frequency B (Hz) of the transmission speed). It can also be constructed using a suitable bandpass filter. In this configuration, the band of the light intensity modulator 4 is about 8
GHz and 10 Gbit / in approximately half the band of the conventional configuration
An s-phase inverted RZ optical signal was generated.

FIG. 7 is a diagram showing an RZ optical signal waveform by the optical transmission device of the present invention. As shown in FIG. 7, even if the mark ratio of the signal is changed to 1/2, 1/4, and 1/8, a stable RZ optical signal can be generated without using the DC level fluctuation control circuit.

FIG. 8 shows the optical spectrum of an RZ optical signal by the optical transmission apparatus of the present invention and the RZ by the conventional optical transmission apparatus.
It is a figure which shows the optical spectrum of an optical signal. The horizontal axis shows the relative frequency, and the vertical axis shows the relative output. As shown in FIG.
It can be seen that the optical signal band is about half that of the conventional example. Further, FIG. 9 is a diagram showing the chromatic dispersion tolerance of the present invention and the conventional example, in which the horizontal axis represents the dispersion range and the vertical axis represents the receiving sensitivity. FIG. 9 compares the wavelength dispersion characteristics at 10 Gbit / s between the conventional example and the present invention. Comparing the dispersion ranges in which the reception sensitivity deteriorates by 1 dB from FIG.
The Z optical signal is 1000 ps / nm, whereas the present invention is 1700 ps / nm, which is 1.7 times.
From the above, it is clear that the proof stress of deterioration due to wavelength dispersion is expanded.

(Second Embodiment) A second embodiment of the present invention is shown in FIG.
0 and FIG. 11. FIG. 10 is a block diagram of the essential parts of an optical transmission apparatus according to the second embodiment of the present invention. FIG. 11 is a diagram showing the precoding means of the optical transmission apparatus according to the second embodiment of the present invention. The second embodiment of the present invention is an example of outputting a differential precode signal output from a single-ended input NRZ electric signal as shown in FIG.

A differential precoding circuit 72 which inputs a multiplexed single-ended NRZ electric signal from an input terminal 71 and outputs a precoded coded NRZ electric signal, and a coded NRZ electric signal which are input and grounded Bandpass filter 2 as a differentiating circuit that generates a differential electric pulse having the same amplitude and different polarities centered on the level
And 2'and the differential signal amplified by the capacitive coupling type drive circuits 3 and 3'which amplifies the differential signal up to the drive voltage of the light intensity modulator 4, and the push-pull which generates the RZ optical signal. The Mach-Zehnder type optical intensity modulator 4 and an optical amplifier 6 for amplifying the output of the optical intensity modulator 4. The differential precoding circuit 72 of the present invention is
As shown in FIG. 11, an exclusive OR 7 and a 1-bit delay circuit 8 and a differential conversion circuit 73 are provided in the output stage so that the precoded output can output a complementary encoded NRZ signal. .

(Third Embodiment) A third embodiment of the present invention is shown in FIG.
This will be described using 2 to 19. FIG. 12 is a block diagram of the essential parts of an optical transmission apparatus according to the third embodiment of the present invention. FIG.
FIG. 8 is a diagram showing a precoding means of an optical transmission device according to a third exemplary embodiment of the present invention. FIG. 14 is a diagram showing the waveform of each part of the optical transmission apparatus according to the third embodiment of the present invention. FIG. 15 is a diagram for explaining the bias state of the binary electric signal applied to the optical modulator of the third embodiment of the present invention. 16 and 17 are diagrams for explaining a bias applying method for the optical intensity modulator of the optical transmission apparatus according to the third embodiment of the present invention.

An electric clock generation circuit 86 for generating a clock electric signal synchronized with the NRZ electric signal, a clock pulse light source 83 for generating an optical clock pulse signal synchronized with the clock electric signal, and a single-ended NRZ electric signal as an input terminal. A differential precoding circuit 72 which inputs from 81 and outputs a precoded encoded NRZ electric signal, an input NRZ electric signal and a precode NRZ
It is composed of differential logical product (AND) circuits 82 and 82 ′ that perform logical product of electric signals, a differential conversion circuit 73 that differentially outputs the logical product output, and a light intensity modulator 84.

In this configuration, all the signals in the electric circuit are binary NRZ signals, and the processing of the ternary signal in the electric stage is unnecessary. Further, since the optical pulse train with less pattern jitter is modulated, it is possible to reduce the pattern jitter of the modulated optical RZ signal. The input NRZ electric signal is input to the precode circuit 72, and differential precode encoded NRZ outputs B1 and B2 whose polarities are inverted each time a mark bit in the input NRZ electric signal is input are obtained.
In the AND circuits 82 and 82 ', the logical product (AND) of the input NRZ electric signal A and the precode coded NRZ electric signals B1 and B2 is performed, so that the mark bit is input in the input NRZ electric signal. The mark bit and the mark bit are alternately output to the logical product signal output ports C2 and C3.
FIG. 12 shows the case where an MZ type intensity modulator is used as the optical intensity modulator 84, and has a push-pull configuration in which the first optical modulator and the second modulator are connected in series. AND signals C2 and C3 applied to the two optical modulators
Are differential outputs D1 / D2 and D by a differential conversion circuit.
It is converted into 3 / D4, biased as shown in FIG. 15, and then applied to the two modulators in a push-pull configuration. Each time a mark bit is input to the input NRZ electric signal, the AND circuits 82 and 82 'are alternately opened, the first modulator and the second modulator are alternately modulated, and as a result, the first modulator. The phase of the RZ optical signal bit modulated by and the phase of the RZ optical signal bit modulated by the second modulator are modulated so as to be shifted by π.

FIG. 16 shows a configuration for setting the operating bias of the light intensity modulator 84. Based on the average power monitored by the photoelectric conversion means, a bias is applied to either one of the two optical waveguides through a bias port 85 that is electrically insulated from the first and second modulators.

FIG. 17 shows another configuration for setting the operation bias of the light intensity modulator 84. FIG. 18 shows a specific configuration of the bias control circuit. Further, FIG. 19 shows a low-pass filter circuit (LPF) 9 for detecting mark rate fluctuations.
4 is shown. 17, the amplitudes of the drive circuits 21 ′ and 21 ″ of the first modulator and the second modulator are weakly modulated at different frequencies, and then the intensity modulator 84 is modulated. The modulated RZ optical modulation signal is partially branched and then converted into an electrical signal by the opto-electric conversion means.In the band pass filters BPF1 and BPF2, the frequency components modulated by the respective bias control circuits are removed, and then the bias control is performed. Circuits 1, 2 (88, 88 ')
Are input respectively. In the bias control circuit shown in FIG. 18, the mixer 92 detects the phase difference between the original modulation phase of the modulation frequency signal source 90 and the phase of the detection signal 91 to detect the direction of the change in the bias signal. Control to minimize the amplitude of. At this time, the mark ratio detection circuit detects the mark ratio and measures d beforehand.
The bias applied voltage in FIG. 18 is corrected by the adder 93 in consideration of the value of the duty cycle and output.

(Fourth Embodiment) FIG. 2 shows a fourth embodiment of the present invention.
This will be described using 0 to 25. FIG. 20 is a block diagram of the essential parts of an optical transmission apparatus according to the fourth embodiment of the present invention. Figure 21
FIG. 8 is a diagram illustrating an example of power addition means of an optical transmission device according to a fourth exemplary embodiment of the present invention. FIG. 22 is a diagram showing the precoding means of the optical transmission device according to the fourth embodiment of the present invention. FIG. 23 is a diagram showing the waveform of each part of the optical transmission device according to the fourth embodiment of the present invention. FIG. 24 is a diagram for explaining the bias state of the ternary power addition electric signal applied to the optical modulator of the fourth embodiment of the present invention. FIG. 25 is a diagram illustrating a bias applying method for the optical intensity modulator of the optical transmission device according to the fourth embodiment of the present invention.

In this configuration, since the optical pulse train with less pattern jitter is modulated, it is possible to reduce the pattern jitter of the modulated optical RZ signal. The input NRZ electric signal is input to the precode circuit 72, and differential precode encoded NRZ outputs B1 and B2 whose polarities are inverted each time a mark bit in the input NRZ electric signal is input are obtained. AN
In the D circuits 82 and 82 ', the logical product (AND) of the input NRZ electric signal A and the precode-encoded NRZ electric signals B1 and B2 is performed, so that the mark bit is input in the input NRZ electric signal. The mark bit is alternately output to the logical product signal output ports C2 and C3. However, the logic of the C2 output signal is inverted with respect to the NRZ electric signal input. FIG. 20 shows a case where an MZ type intensity modulator is used as the light intensity modulator 102, which has a push-pull configuration. AND signal C
1, C2, C3, and C4 are power addition circuits 100 and 101.
Is converted into a three-valued differential power addition output signal D1'D2 ', biased as shown in FIG. 24, and then applied to the modulator in a push-pull configuration. MZ type light intensity modulator 1
In 02, the differential power addition output signal electric pulse waveform is folded back around the second level of the differential power addition output signal, and the optical modulation phases corresponding to the first and third levels of the differential power addition output signal are changed. R from the clock pulse light source so that π differs
The Z optical clock pulse is modulated.

FIG. 21 shows an example in which the power addition circuits 100 and 101 are composed of passive components. In the 50 ohm system, 6 dB power addition can be performed by selecting the resistance value R to be 50 ohm. FIG. 25 is an example of a configuration for giving an operation bias to the light intensity modulator 102. A bias is applied to either one of the two optical waveguides through a bias port 104 that is electrically isolated from the modulator.

(Fifth Embodiment) FIG. 2 shows a fifth embodiment of the present invention.
6 and FIG. 27. FIG. 26 is a block diagram of the essential parts of an optical transmission apparatus according to the fifth embodiment of the present invention.
FIG. 27 is a diagram showing signal waveforms of respective parts of the optical transmission apparatus according to the fifth embodiment of the present invention. In the fifth embodiment of the present invention, as shown in FIG. 26, the light intensity modulation and the light phase modulation are performed by the light intensity modulators 31, 32 and the phase modulator 40, respectively.

The continuous light output from the light source 5 is modulated by a light intensity modulator 31 with a clock signal synchronized with the transmission rate to generate a clock pulse train signal light, which is a light intensity modulator 32. The intensity is modulated by the NRZ electric signal which is data, and a normal RZ optical signal is generated. A part of the data signal is branched, and code conversion (point D) is performed in the precoding circuit 1 as shown in FIG. 27. According to the encoded NRZ electric signal, as shown in the optical phase change at point E in FIG. By performing the phase modulation of (0, π) by the phase modulator 40, RZ optical signals in which the phases of the individual pulses are different from each other by π are generated.

[0056]

As described above, according to the present invention,
Since the electric signal band required for the drive circuit / light intensity modulator can be halved as compared with the conventional device, the speed of the optical transmission device can be increased. Further, since the optical signal band can be halved as compared with the conventional device, it is possible to realize the optical transmission device capable of reducing the deterioration of the transmission quality due to the wavelength dispersion of the optical fiber transmission line. In the present invention, the optical carrier is suppressed, so that the optical signal spectrum does not include the line spectrum frequency component.
Therefore, it is advantageous for limiting the input power in the fiber due to SBS and waveform distortion due to four-wave mixing. Also pulse b
Since the phase of the y pulse is inverted, even if fading occurs due to multipath due to polarization dispersion or the like in the transmission line, the phase is inverted at the overlapping portion of the pulse edges. The strength of the overlapping portion of the edges is canceled by the interference, and it is difficult to cause intersymbol interference.

That is, the optical spectrum band required for generating the RZ optical signal can be realized with a half or less of the conventional one.
Further, the band required for the electric circuit and the light intensity modulator can be generated at the transmission speed B. Furthermore, the SBS in-fiber optical input power limitation can be essentially eliminated. Moreover, the output power of the light source can be reduced. Further, there is no DC level fluctuation due to the change of the mark ratio of the signal. Further, it is possible to reduce the influence of crosstalk due to four-wave mixing (FWM). Furthermore, it is difficult to cause intersymbol interference.

[Brief description of drawings]

FIG. 1 is a block diagram of a main part of an optical transmission device according to a first embodiment of the present invention.

FIG. 2 is a diagram showing a configuration of a precoding circuit.

FIG. 3 is a diagram showing a waveform of each part of the optical transmission device according to the first embodiment of the present invention.

FIG. 4 is a diagram for explaining the operation of the light intensity modulator of the present invention.

FIG. 5 is a diagram showing an optical spectrum of an RZ optical signal by the optical transmission device of the present invention.

FIG. 6 is a diagram showing a configuration of an optical transmission / reception device using the optical transmission device according to the first embodiment of the present invention.

FIG. 7 is a diagram showing an RZ optical signal waveform by the optical transmission device of the present invention.

FIG. 8 is a diagram showing an optical spectrum of an RZ optical signal by the optical transmission device of the present invention and an optical spectrum of an RZ optical signal by the conventional optical transmission device.

FIG. 9 is a diagram showing chromatic dispersion proof stress of the present invention and a conventional example.

FIG. 10 is a block diagram of a main part of an optical transmission device according to a second embodiment of the present invention.

FIG. 11 is a diagram showing the configuration of a differential precoding circuit according to a second embodiment of the present invention.

FIG. 12 is a block diagram of essential parts of an optical transmission device according to a third embodiment of the present invention.

FIG. 13 is a diagram showing the configuration of a differential precoding circuit according to a third embodiment of the present invention.

FIG. 14 is a diagram showing the waveform of each part of the optical transmission device according to the third embodiment of the present invention.

FIG. 15 is a diagram for explaining the operation of the light intensity modulator of the third embodiment of the present invention.

FIG. 16 is a diagram showing a configuration example of bias control of a light intensity modulator according to a third embodiment of the present invention.

FIG. 17 is a diagram showing a configuration example of another bias control of the light intensity modulator of the third embodiment of the present invention.

FIG. 18 is a diagram showing an example of a bias control circuit of a light intensity modulator according to a third example of the present invention.

FIG. 19 is a diagram showing a configuration of a mark ratio variation detection circuit according to a third embodiment of the present invention.

FIG. 20 is a block diagram of a main part of an optical transmission device according to a fourth embodiment of the present invention.

FIG. 21 is a diagram showing the configuration of a power adder circuit according to a fourth embodiment of the present invention.

FIG. 22 is a diagram showing the configuration of a differential precoding circuit according to the fourth embodiment of the present invention.

FIG. 23 is a diagram showing the waveform of each part of the optical transmission device according to the fourth embodiment of the present invention.

FIG. 24 is a diagram for explaining the operation of the light intensity modulator of the fourth embodiment of the present invention.

FIG. 25 is a diagram showing a configuration example of bias control of a light intensity modulator of a fourth embodiment of the present invention.

FIG. 26 is a block diagram of the essential parts of an optical transmission device according to a fifth embodiment of the present invention.

FIG. 27 is a diagram showing signal waveforms of respective parts of the optical transmission device according to the fifth embodiment of the present invention.

FIG. 28 is a diagram showing a configuration of a conventional optical transmission device.

FIG. 29 is a diagram showing a configuration of a conventional optical transmission device.

FIG. 30 is a diagram showing an NRZ optical signal spectrum by a conventional optical transmission device.

FIG. 31 is a diagram showing an RZ optical signal spectrum by a conventional optical transmission device.

[Explanation of symbols] 1, 1'precode circuit 2,2 'bandpass filter 3, 3 ', 21, 22, 23 drive circuit 4, 31, 32 Light intensity modulator 5 light sources 6 Optical amplifier 7 Exclusive OR circuit 8 1-bit delay circuit 9 Athene evening 10 data sources 11 Receiving terminal 12 transmission lines 13 Receiver 14, 14 ', 18, 19 Input terminals 40 Phase modulator 50 RZ light intensity modulator 51 NRZ / RZ conversion circuit 52 RZ light intensity modulator drive circuit 53 SBS suppression line width modulation circuit 60 NRZ light intensity modulator 61 Clock Light intensity modulator 62 NRZ light intensity modulator drive circuit 63 clock light intensity modulator drive circuit 64 phase control circuit 71, 81 Single-ended NRZ input terminal 72 Differential precoding circuit 82 AND circuit 83 clock pulse light source 84 Light intensity modulator 85 Bias port 90 Modulation frequency signal source 92 Phase comparator 93 adder 94 Low Pass Filter (LPF) 100, 101 power adder 102 Light intensity modulator 104 Bias application terminal

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁷ Identification code FI H04L 25/493 (56) References JP-A-9-247087 (JP, A) JP-A-9-236781 (JP, A) Special Kaihei 8-139681 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) H04B 10/00-10/28 H04J 14/00-14/08 G02F 1/01 H04L 25/493 JISC file (JOIS)

Claims (7)

(57) [Claims] 1. An input terminal for inputting an NRZ (Non-Return-to-Zero) electric signal, and means for converting the NRZ electric signal input from the input terminal into an RZ (Return-to-Zero) optical signal. In the optical transmission device including the above, the converting means includes precoding means for inputting the NRZ differential electric signal, and the precoding means outputs the value of the NRZ differential electric signal and the output of the precoding means. And a differential encoding means for outputting a value of an exclusive OR with a signal delayed by 1 bit and generating differential electric signal pulses having different polarities at a rising point and a falling point of the value of the exclusive OR. , 3 according to this differential electric signal pulse
A light intensity that folds the differential electric pulse waveform around the second level of the differential electric signal and modulates the intensity of continuous light so that the optical modulation signal phases corresponding to the first and third levels of the differential signal differ by π. An optical transmission device comprising a modulator. 2. A terminal for inputting an NRZ (Non-Return-to-Zero) electric signal, and the NR input from this input terminal.
An optical transmission device comprising means for converting a Z electric signal into an RZ (Return-to-Zero) optical signal, wherein the converting means comprises precoding means for inputting a single-ended NRZ electric input signal, The coding means differentially outputs the value of the exclusive OR of the value of the NRZ electric signal and the signal obtained by delaying the output of the precoding means by 1 bit, and the rising point of the value of the exclusive OR and
At the falling point, differential encoding means for generating differential electric pulses having different polarities, and according to this differential electric pulse, the differential electric pulse waveform is folded back around the second level of the ternary differential electric signal, and differentiated. Signal first and third
And an optical intensity modulation means for intensity-modulating continuous light so that the optical modulation phase corresponding to the level is different by π. 3. A terminal for inputting an NRZ (Non-Return-to-Zero) electric signal, and the NR input from this input terminal.
In an optical transmission device comprising means for converting a Z electric signal into an RZ (Return-to-Zero) optical signal, the converting means includes means for generating a clock electric signal synchronized with the NRZ electric signal, and the clock. It has a clock pulse light source for generating an optical clock pulse signal synchronized with the clock electric signal as an input, and a precoding means for inputting a single-ended NRZ electric input signal, and the precoding means is provided for the NRZ electric signal. The value of the exclusive OR of the signal value and the signal obtained by delaying the code value of the output of the precoding means by 1 bit is differentially output, and NR
Two differential AND code means for differentially outputting a logical product value of the NRZ electrical input signal and the precoded differential NRZ signal with the Z electrical input signal and the precoded differential NRZ signal as inputs; With the optical clock optical signal as an input, the modulators of the respective arms are modulated by two modulators that are electrically insulated and arranged in series according to the differential logical product code. An optical transmission device comprising: an optical intensity modulator that independently modulates the intensity and the phase, and performs the intensity modulation so that the phase of the mark bit of the output optical signal is different from each other by π. 4. The optical transmission device according to claim 3, wherein the operating point bias voltage is applied to a terminal electrically separated from the differential AND code. 5. A terminal for inputting an NRZ (Non-Return-to-Zero) electric signal, and the NR input from this input terminal.
In an optical transmission device comprising means for converting a Z electric signal into an RZ (Return-to-Zero) optical signal, the converting means includes means for generating a clock electric signal synchronized with the NRZ electric signal, and the clock. It has a clock pulse light source for generating an optical clock pulse signal synchronized with the clock electric signal as an input, and a precoding means for inputting a single-ended NRZ electric input signal, and the precoding means is provided for the NRZ electric signal. The value of the exclusive OR of the signal value and the signal obtained by delaying the code value of the output of the precoding means by 1 bit is differentially output, and NR
Two differential AND code means for differentially outputting a logical product value of the NRZ electrical input signal and the precoded differential NRZ signal with the Z electrical input signal and the precoded differential NRZ signal as inputs; Power addition means for outputting a power addition code for performing power addition of two logical product coded NRZ signals having different logics from the two differential AND means, and the power addition code with the optical clock optical signal as an input. In accordance with the above, the power addition code is folded back around the second level of the ternary power addition code, and the optical clock optical signal is adjusted so that the optical modulation phases corresponding to the first and third levels of the power addition code differ by π. And an optical intensity modulation means for intensity modulating 6. The optical transmission device according to claim 1, wherein the light intensity modulator includes a Mach-Zehnder intensity modulator. 7. An input terminal for inputting an NRZ (Non-Return-to-Zero) electric signal, and means for converting the NRZ electric signal input from this input terminal into an RZ (Return-to-Zero) optical signal. In the optical transmission device including :, the converting means includes first light intensity modulating means for performing light intensity modulation of continuous light in accordance with a clock signal, and the first light intensity modulating means.
Second optical intensity modulating means for performing optical intensity modulation of the output optical signal of the optical intensity modulating means in accordance with the NRZ electrical signal, and precoding means for inputting the NRZ electrical signal. A value of exclusive OR of the value of the NRZ electric signal and the signal obtained by delaying the code value of the output of the precoding means by 1 bit is output, and the second light intensity modulation is performed according to the value of the exclusive OR. An optical transmission device, comprising: a phase modulation unit that applies a phase change of π for each pulse of the output optical signal of the unit.