JP2002094464A - Optical transmitter and optical transmission system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical transmitter which can generate a modulated optical signal following a RZ pattern by only one optical modulator, without requiring a wide-band optical modulator or a driver circuit. SOLUTION: An inputted data signal, following a NRZ pattern inputted from an input terminal 10, is converted by a signal conversion section 11 into a binary output signal, which makes a level transition from '0' level to '1' level or vice versa, each time it transits from '0' level to '1' level, and when '1' level continues extended to two units period. Thereafter, the binary signal is inputted into a low-pass filter 12 to be band-limited. By driving a Mach- Zehnder type optical converter 16, based on an output signal of the low-pass filter 12 via a driver circuit 13 such a modulated optical signal 17, is outputted whose light intensity becomes a maximum, when the output signal of the low- pass filter 12 is at '0' level or at '1' level and becomes a minimum, when the output signal of the low-pass filter 12 is in a process of gradual transition between '0' level and '1' level.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical transmission apparatus compatible with ultra-high-speed and long-distance transmission, and more particularly to an optical transmission apparatus capable of effectively compressing the band of an RZ optical signal as a transmission optical signal and a baseband electric signal. Related to the device. Furthermore, the present invention relates to an optical transmission system of an optical repeaterless transmission system using such an optical transmission device.

[0002]

2. Description of the Related Art In recent years, in high-speed, long-distance optical transmission, RZ
(Return-to-Zero) modulation schemes are receiving attention. The RZ modulation method has less bit pattern dependence of the waveform compared to the NRZ (Non-Return-to-Zero) modulation method, and in the wavelength division multiplexing transmission method, the overlap time with optical pulses of other wavelengths is short. Therefore, there is an advantage that nonlinear effects such as four-wave mixing and cross-phase modulation can be suppressed.

On the other hand, since the RZ signal requires a wider band than the NRZ signal, performing NRZ / RZ conversion at the electric signal stage requires a wide band characteristic for the optical modulator and its driver circuit in the optical transmitter. There is a problem that is.
In order to avoid this problem, the optical modulator has a two-stage configuration,
There is a method of generating an RZ optical signal by performing NRZ modulation with a data signal in a first-stage optical modulator and further performing modulation in accordance with a clock signal in a second-stage optical modulator. In this method, since the optical modulator has a two-stage configuration, the optical loss in the optical modulation stage increases, and the SNR (signal-to-noise ratio) of the transmission modulated optical signal deteriorates.
In addition, since two optical modulators are used, the cost is high and the size of the device is large.

[0004]

As described above, the conventional optical transmitter requires a wide-band optical modulator and a driver circuit for NRZ / RZ conversion, so that it is difficult to increase the speed and cost. In addition, in a system in which the optical modulator has a two-stage configuration, there is a problem that the SNR of the optical signal is deteriorated, and that the device scale is increased and the cost is increased.

It is an object of the present invention to provide an optical transmitter capable of generating a modulated optical signal of an RZ pattern with a single optical modulator without requiring a wideband optical modulator or driver circuit.

Another object of the present invention is to eliminate the need for a wide-band optical modulator or driver circuit, and to generate a modulated optical signal having an RZ pattern in which a carrier component is effectively suppressed by using a single optical modulator. An optical transmission device is provided.

Further, according to the present invention, there is provided an optical transmission system capable of performing optical non-repeated transmission in which nonlinear phenomena in an optical transmission line are effectively suppressed by using an RZ pattern modulated optical signal in which a carrier component is effectively suppressed. It is an object to provide a transmission system.

[0008]

In order to solve the above-mentioned problems, in an optical transmission apparatus according to the present invention, an input data signal of an NRZ pattern comprising a first level and a second level is input to a signal conversion means, The input data signal is converted into a binary output signal which causes a level transition between the first level and the second level every time the input data signal transitions from the first level to the second level and every time the second level continues for two unit periods. You.

The output signal of the signal conversion means is provided as an input signal to a band limitation means such as a low-pass filter.
A level transition between the first level and the second level gradually when a level transition occurs between the first level and the second level, and a signal that takes the first level or the second level according to the input signal otherwise. Is done.

The output signal of the band limiting means is input to the modulated optical signal generating means as a drive signal, and when the level of the output signal is the first level and the second level, the light intensity becomes minimum,
In addition, when the level of the output signal gradually changes between the first level and the second level, a modulated optical signal having the maximum light intensity is generated.

Since the frequency band of the output signal from the signal conversion means is the same as that of the input data signal composed of the NRZ pattern, the first level and the second level of the signal obtained by passing the signal through a low-pass filter. Is modulated so that the light intensity corresponding to the light intensity is the minimum and the light intensity is maximized in the process of the level transition between the first level and the second level gradually. A modulated optical signal having an RZ pattern is generated without expanding the bandwidth of the modulator and the driver circuit and without forming the optical modulator in a two-stage configuration.

[0012] Further, the frequency use efficiency is improved by the band limiting means, and the rise time and the fall time of the input signal to the optical modulator are adjusted by changing the frequency characteristic of the band limit means, so that the modulated light is improved. The duty ratio of the signal can be easily adjusted.

The output signal of the signal conversion means and the signal obtained by inverting the phase of the output signal are input to the modulated optical signal generating means via first and second low-pass filters, respectively, and the modulated optical signal generating means is pushed. A pull driving configuration may be adopted, whereby the voltage of the input signal to the optical modulator can be reduced, and the frequency chirp in the optical modulator can be suppressed to improve the transmission characteristics.

In another optical transmission device according to the present invention,
The output signal of the signal conversion means is input to the difference means, and the difference between the output signal of the signal conversion means and a signal obtained by delaying the output signal by a time corresponding to less than one bit of the input data signal is obtained, thereby obtaining the input data. Taking the third level and the fourth level alternately at each transition of the signal from the first level to the second level and at each successive two unit period of the second level;
In other periods, a ternary output signal having a fifth level intermediate between the third level and the fourth level is generated. The ternary output signal is input to the modulated optical signal generating means, and the light intensity corresponding to the fifth level period of the ternary output signal is minimum,
A modulated optical signal is generated in which the light intensity corresponding to the period of the third level and the fourth level of the value output signal is maximum and the optical phases are opposite to each other in the period of the third level and the fourth level. You.

As described above, in the ternary output signal generated by the difference, the intersymbol interference due to the exponential change of the level like the ternary output signal generated by the differentiation is suppressed, and the input data signal Transmission characteristics independent of bit patterns can be realized. In the modulated optical signal generated based on the ternary output signal, since the optical phase between adjacent mark bits is always inverted, the carrier component is suppressed regardless of the bit pattern of the input data signal. Also, by changing the delay time of less than one bit when taking the difference, it is possible to adjust the optical pulse width of the modulated optical signal of the RZ pattern, that is, the duty ratio.

Further, first and second two differentiating means are provided to generate a first ternary output signal and a second ternary output signal having a phase opposite to that of the first ternary output signal. The modulated optical signal generating means may be push-pull driven by inputting the modulated optical signal to the modulated optical signal generating means, thereby reducing the voltage of the input signal to the optical modulator and reducing the frequency chirp in the optical modulator. To improve the transmission characteristics and accurately invert the phase of adjacent mark bits.

The ternary output signal may be input to the modulated optical signal generating means via a band limiting means such as a low-pass filter, so that not only can the spectral band of the modulated optical signal be suppressed, Since any level transition of the value output signal can transition at a time determined by the low-pass filter, it is possible to suppress bit pattern dependence.

Further, the present invention provides an optical transmitter for outputting a modulated optical signal of at least one channel which is intensity-modulated according to a binary input data signal of an NRZ pattern composed of a first level and a second level. , A transmitting station having a first optical amplifier for amplifying the modulated optical signal, an optical transmission path for transmitting an optical signal from the transmitting station, and a second optical amplifier for amplifying the optical signal transmitted by the optical transmission path. An optical transmission system comprising: an optical amplifier; and a receiving station having an optical receiver for receiving an optical signal output from the second optical amplifier and reproducing the original data. Takes the binary input data signal alternately between the third level and the fourth level each time the binary input data signal transitions from the first level to the second level, and each time the second level continues for two unit periods of the second level.
Binary-ternary conversion means for converting into a ternary output signal having a fifth level intermediate between the third level and the fourth level in other periods, and corresponds to a period of the fifth level of the ternary output signal. The light intensity is the minimum, the light intensity corresponding to the period of the third level and the fourth level of the ternary output signal is the maximum, and the optical phase is corresponding to the period of the third level and the fourth level of the ternary output signal. A modulated optical signal generating means for outputting modulated optical signals having phases opposite to each other.

In optical repeaterless transmission, it is necessary to increase the input light power to the optical transmission line (optical fiber) in order to extend the transmission distance. However, as the input optical power increases, the nonlinearity in the optical transmission line increases. Since the transmission characteristics deteriorate due to the phenomenon, the transmission distance is limited.

In the optical transmission system of the present invention, the above-mentioned 3
By driving the modulated optical signal generating means with the value output signal, the optical carrier component can be suppressed by reversing the phase of the modulated optical signal corresponding to the second level adjacent to the input data signal. Non-linear phenomena in the path can be suppressed, and the input optical power to the optical transmission path can be increased, and the transmission distance in the optical non-repeater transmission can be greatly increased.

[0021]

Embodiments of the present invention will be described below with reference to the drawings. (First Embodiment) FIG. 1 shows a configuration of an optical transmission apparatus according to a first embodiment of the present invention. FIG. 2 schematically shows the signal waveform of each part.

In FIG. 1, for example, a data signal having a binary NRZ pattern shown in FIG. This input data signal is supplied to the signal converter 1
1 causes the input data signal to transition from the first level ("0" level) to the second level ("1" level), and the second level ("1" level) of the input data signal continues over a plurality of unit periods. Then, the signal is converted into a binary output signal as shown in FIG. 2 (b) which causes a level transition from the “0” level to the “1” level and the “1” level to the “0” level. Here, the “0” level (low level) is the first level, and the “1” level (high level) is the second level. Conversely, the “1” level is the first level, “0”.
The level may be the second level. This is the same in the following description.

FIG. 3 shows an example of the configuration of the signal conversion unit 11, and FIG. 4 shows the signal waveform of each unit. The input data signal (a) is input to one input terminal of an exclusive OR circuit (EXOR) 31. The output signal (c) of the EXOR circuit 21 is branched into two, one of which is provided with a delay corresponding to the time (unit period) T for one bit of the input data signal by the delay unit 32 as shown in (b). The signal is input to the other input terminal of the EXOR circuit 31. As is clear from FIG. 4, the output signal of this circuit
In (c), when the input data signal (a) transits from the “0” level to the “1” level, and when the “1” level changes for two unit periods (2
It can be seen that a level transition has occurred when continuous over T).

Next, the output signal of the signal conversion section 12 is input to the low-pass filter 12, where the output signal is subjected to band limitation, so that the rising and falling waveforms are reduced as shown in FIG. Becomes That is, the output signal of the low-pass filter 13 gradually rises from the “0” level to the “1” level when the output signal of the signal conversion unit 12 transitions from the “0” level to the “1” level, When the output signal of the signal conversion unit 12 transitions from the “1” level to the “0” level, the output signal gradually falls from the “1” level to the “0” level.

Driver circuit 13, semiconductor laser (LD)
14, a bias control circuit 15, and a Mach-Zehnder interferometer type optical modulator (hereinafter, referred to as an MZ type optical modulator) 16 constitute a modulated optical signal generating circuit, and are driven by an output signal of a low-pass filter 13 to generate RZ. A modulated optical signal 17 having a pattern is generated.

That is, the output signal of the low-pass filter 13 is input to the MZ type optical modulator 16 via the driver circuit 13. The MZ type optical modulator 16 has an optical input terminal P1, a drive signal input terminal P2, a bias input terminal P3, and an optical output terminal P4. The light input terminal P1 has a semiconductor laser (L
D) The outgoing light (CW light) from 14 is input, the drive signal is input from the driver circuit 13 to the drive signal input terminal P2, and the bias control unit 15 is input to the bias input terminal P3.
Is supplied with a bias voltage. The MZ-type optical modulator 16 modulates the light from the semiconductor LD 14 in accordance with a drive signal (data signal) from the driver circuit 13 to generate a modulated optical signal 17 having an RZ pattern as shown in FIG. appear. The modulated optical signal 17 of the RZ pattern generated in this manner is an output optical signal of the optical transmission device according to the present embodiment, and is transmitted to an optical transmission line (not shown) (not shown).

FIG. 5 shows the transmission characteristics of the MZ type optical modulator 16. As shown in the figure, the MZ optical modulator 16 has a characteristic that the light transmittance changes periodically, that is, sinusoidally with respect to the applied voltage. Here, in the present embodiment, the “0” level and the “1” level of the output signal of the low-pass filter 12 are at the point where the transmittance of the MZ type optical modulator 16 is minimum (for example, point B and point B in FIG. (Point D), the output signal of the low-pass filter 12 is amplified by the driver circuit 13 and then input to the drive signal input terminal P2 of the MZ type optical modulator 16. In this case, while the output signal of the low-pass filter 12 gradually transitions from the “0” level to the “1” level and from the “1” level to the “0” level, the transmittance of the MZ optical modulator 16 is changed. Is the largest. Further, a bias voltage at a point (for example, point C in FIG. 1) at which the transmittance of the MZ optical modulator 16 is maximized is supplied from the bias control unit 15 to the bias input terminal of the MZ optical modulator 16. .

As described above, the "0" level and "1" level of the output signal of the low-pass filter 12 are
By making it correspond to the point where the transmittance of 6 is minimum,
The modulated optical signal 17 output from the optical output terminal P4 of the MZ type optical modulator 16 has a maximum output during the level transition of the output signal of the low-pass filter 12 as shown in FIG. Becomes the minimum output again. That is, according to the present embodiment, only one MZ type optical modulator 1 is used as the optical modulator.
6, the RZ modulated optical signal 17 can be generated. Therefore, optical loss can be reduced as compared with the conventional configuration in which the optical modulator has two stages, and the device scale is reduced.
Cost reduction can be achieved.

Further, in the present embodiment, as is apparent from comparison of FIGS. 2A and 2B, the output signal of the signal converter 11 has the same band as the NRZ input data signal to the input terminal 10. Since the band is limited by the low-pass filter 12, the driver circuit 13 and the MZ type optical modulator 16 can generate an RZ modulated optical signal at a frequency lower than the band of the input data signal.

FIGS. 6A and 6B show a low-pass filter 1.
MZ type optical modulator 16 when the frequency characteristic of
2 shows the driving signal waveform and the state of the RZ modulated optical signal. As shown by the solid line 41 in FIG. 6A, the rising time of the output signal waveform (the drive signal waveform output from the driver circuit 13 is the same) as the low-pass filter 12 corresponds to one bit of the input data signal. When a filter having a frequency characteristic substantially equal to T is used, the duty ratio of the modulated optical signal pulse output from the MZ type optical modulator 16 is approximately 50 as indicated by the solid line 43 in FIG. %.

On the other hand, the alternate long and short dash line 42 in FIG.
When the band of the low-pass filter 12 is expanded as shown by,
As shown by the one-dot chain line 44 in FIG. 6B, a modulated optical signal pulse with a smaller duty ratio can be generated. Usually R
In order to reduce the duty ratio of the Z-modulated optical signal pulse, the driver circuit 13 and the optical modulator 16 require a band wider than the input data signal. According to the method of changing the duty ratio by changing the value of (1), there is an advantage that the operating frequency band of the driver circuit 13 and the optical modulator 16 may be lower than the input data signal.

(Second Embodiment) FIG. 7 shows a second embodiment of the present invention.
1 shows a configuration of an optical transmission device according to an embodiment. FIG.
The same parts as those of the first embodiment are denoted by the same reference numerals, and only differences from the first embodiment will be described.

In the present embodiment, the output signal from the signal converter 11 is split into two, and one of the split signals is passed through a first low-pass filter 12A to a first driver circuit 13A.
The other signal is input to the inverting circuit 18, respectively.
The inverting circuit 12 outputs the signal “0” from the signal converter 11.
The level and the “1” level are inverted, and a signal complementary to the output signal of the signal converter 11 is output. Inverting circuit 18
Is input to the second driver circuit 13B via the second low-pass filter 12B.

First and second driver circuits 13A, 13B
Then, the first and second low-pass filters 12A, 12B
Are amplified so that the data amplitude between the “0” level and the “1” level is constant, and then input to the two drive signal input terminals P2A and P2B of the MZ optical modulator 19, respectively. Is done.

The MZ type optical modulator 19 in the present embodiment
Is an optical input terminal P1, and two drive signal input terminals P2A and P2.
B, a bias input end P3, and an optical output end P4. The light input from the semiconductor laser 14 to the optical input terminal P1 is internally branched into two, and the phase of these lights is changed to the drive signal input end P.
An optical modulator that modulates the light intensity by changing the driving signals having the opposite phases input to 2A and P2B and outputs a modulated optical signal from the optical output terminal P4, and is called a push-pull optical modulator. .

This push-pull MZ type optical modulator 1
9 indicates that the output signals of the first and second low-pass filters 12A and 12B are “0”, similarly to the behavior of the MZ optical modulator 16 with respect to the output signal of the low-pass filter 12 in the first embodiment. At both the level and the “1” level, the transmittance is substantially minimum. Therefore, the MZ type optical modulator 1
The modulated optical signal 17 output from 9 becomes a modulated optical signal having an RZ pattern as shown in FIG. 3D, as in the first embodiment.

As described above, according to the present embodiment, in addition to the same advantages as the first embodiment, in particular, the MZ type optical modulator 19
By using a push-pull type optical modulator, a new term can be obtained in which chirp generated in the optical modulator can be suppressed and transmission characteristics can be improved.

In the present embodiment, the MZ type optical modulator 19
In order to perform light intensity modulation by changing the phase of each of the two branched lights, the driver circuits 13A, 13A
The voltages of the individual drive signals respectively output from B are the ordinary MZ type optical modulators, for example, the MZ type optical modulator used in the first embodiment.
It may be about half as compared with the type light modulator 17. That is, the first
In the embodiment, since the drive signal voltage to be input to the MZ type optical modulator 17 is required to be about twice as large as the conventional one, the driver circuit 1
Although the load on 3 is large, the load on the driver circuits 13A and 13B can be reduced to the same level as in the related art by using the MZ type optical modulator 19 composed of a push-pull type optical modulator as in the present embodiment.

(Third Embodiment) Next, an optical transmission apparatus according to a third embodiment of the present invention will be described with reference to FIGS. FIG. 8 shows the configuration of the optical transmission device according to the present embodiment, and FIG. 9 shows the signal waveforms at various parts in FIG.

In FIG. 8, the input terminal 10 is
The data signal of the NRZ pattern as shown in (a) is input, and is converted by the binary-ternary conversion circuit 20 into a ternary output signal described later. The binary-ternary conversion circuit 20 is configured as follows.

In the binary / ternary conversion circuit 20, the input data signal is first supplied to the EXOR circuit 3 shown in FIG.
1 and the delay unit 32, when the input data signal transits from the “0” level (first level) to the “1” level as shown in FIG. When the “1” level is continuous over a plurality of unit periods (an integer multiple of T), the signal is converted into a binary output signal that causes a level transition between the “0” level and the “1” level. Next, the output signal of the signal conversion unit 11 is branched into two by the first branch circuit 21 and then further branched into two by the second and third branch circuits 22A and 22B.

As shown in FIG. 9C, one output signal of the branch circuit 22A is output by a delay unit 23A for a time of 0.5T as shown in FIG.
That is, the delay unit 23A
Is input to the non-inverting input terminal (+ input) of the first differential circuit 24A. The other output signal of the branch circuit 22A is the same as the output signal of the signal conversion unit 11 as shown in FIG. 9D, and is directly input to the inverting input terminal (-input) of the first differential circuit 24A. You.

One output signal of the branch circuit 22B is the same as the output signal of the signal converter 11 as shown in FIG. 9 (f), and is directly input to the non-inverting input terminal of the second differential circuit 24B. You. As shown in FIG. 9 (g), the other output signal of the branch circuit 22B is output from the delay unit 23B for a time period of 0.5
T, ie, １ ／ bit, the delay 2
The output signal of 3B is input to the inverting input terminal of the second differential circuit 24B.

In the first and second differential circuits 24A and 24B, the difference between the signal input to the non-inverting input terminal and the signal input to the inverting input terminal is obtained. Here, as described above, in the first differential circuit 24A, the output signal of the delay unit 23A is converted to the output signal of the signal conversion unit 11 (the branch circuit 22A).
Is subtracted, and in the second differential circuit 24B, the output signal of the signal conversion unit 11 (branching circuit 22B
Is subtracted from the output signal of the delay unit 23B. Thereby, the first and second differential circuits 22A,
As shown in FIGS. 9 (e) and 9 (h), complementary ternary output signals having mutually opposite phases are output from 22B.

That is, the first and second differential circuits 22
A and 22B have a third level (“−1” level), a fourth level (“1” level), and a fifth level (“0” level) intermediate therebetween, and the first level of the input data signal. The “−1” level and the “1” level at each transition from the level (“0” level) to the second level (“1” level) and at each successive time over two unit periods of the second level (“1” level). A first ternary output signal and a second ternary output signal having alternate levels and having mutually opposite phases are output.

The first and second differential circuits 22A, 22A
The 2B-to-ternary output signal is the first and second
After being band-limited by the low-pass filters 12A and 12B, the signals are input to two drive signal input terminals of an MZ optical modulator 19, which is a push-pull optical modulator, via driver circuits 13A and 13B.

The MZ type optical modulator 19 in the present embodiment
Has an optical input terminal P1, two drive signal input terminals P2A and P2B, a bias input terminal P3, and an optical output terminal P4 as in the second embodiment.
The light input to 1 is internally split into two lights, and the light intensities are modulated by changing the phases of these lights by driving signals having opposite phases input to the driving signal input terminals P2A and P2B, respectively. This is a push-pull optical modulator that outputs a modulated optical signal from the optical output terminal P4.

In this embodiment, unlike the first and second embodiments, the transmittance of the MZ optical modulator 19 from the bias control unit 15 is minimized at the bias input terminal P3 of the MZ optical modulator 19. A bias voltage at a point (for example, point B or point D in FIG. 1) is supplied. Accordingly, as shown in FIG. 9 (i), the first and second modulated optical signals 17 of the RZ pattern are output from the optical output terminal P4 of the MZ type optical modulator 19.
Is 0 when the ternary output signal of
When the value output signal is at "-1" level or "1" level, an optical signal having the maximum output is obtained.

Here, since the MZ type optical modulator 19 is a push-pull type optical modulator, the phase of the output modulated optical signal 17 is the phase of the drive signal input to the two drive signal input terminals P2A and P2B. For example, as shown in FIG. 9 (j), when the ternary output signal is at "-1" level and the ternary output signal is at "1" level, the phase is π, and the value output signal is at "1" level. And the ternary output signal is "-
When the level is 1 ", the phase is always inverted by an adjacent light pulse such as the phase 0.

As described above, the modulated optical signal obtained by the present embodiment has an R signal in which the phase of an adjacent optical pulse is always inverted.
Since the modulated optical signal 17 has the Z pattern, the carrier component can be suppressed regardless of the bit pattern of the NRZ pattern of the input data signal, and there is an effect that a large resistance to a nonlinear phenomenon can be obtained.

This effect will be described in more detail. In high-speed and long-distance optical transmission, as the transmission speed increases, it is necessary to increase the input optical power to the optical fiber in order to maintain a sufficient SNR. The phenomenon deteriorates the transmission characteristics. An RZ modulation method having a high resistance to deterioration due to this non-linear phenomenon has been proposed (JP-A-2000-2000).
-106543). This method increases the resistance to nonlinear phenomena by suppressing the optical carrier component, and is called a CS-RZ (Carrier-Suppressed RZ) method.

In the conventional CS-RZ system, an input data signal of an NRZ pattern is encoded by a circuit called a precoding circuit so that a state transition occurs at the time of transition from a space to a mark and at the time of a mark continuation. The signals are converted into ternary differential electric signals having different polarities at the point and the falling point, respectively, and input to the push-pull optical modulator via the drive circuit, so that the output is at the intermediate level of the ternary differential electric signals. The output is maximum at "-1" level and "1" level of the electric signal, and "-1"
A modulated optical signal is generated in which the phases of the optical modulated signals respectively corresponding to the level and the “1” level are different by π. This modulated optical signal has the advantage that the carrier wave component is suppressed and the resistance to the nonlinear phenomenon in the optical fiber is increased because the phases of adjacent optical pulses are inverted.

However, in the conventional CS-RZ system, the input data signal of the NRZ pattern is divided into three by a differentiating circuit.
Since the conversion into the value code is performed, the fall time of the converted code attenuates exponentially depending on the time constant of the differentiating circuit. FIGS. 11A and 11B show a differential waveform and a light output waveform when the NRZ bit pattern as the input data signal is “10110”. Since the falling part of the mark bit affects the next bit, it can be seen that pattern dependence has occurred. In particular, as shown in a portion surrounded by a broken line in FIG. 11B, when a space comes next to a mark, the falling portion of the mark bit leaks into the space bit due to the pattern dependency, so that a large space bit is generated. This causes intersymbol interference.

On the other hand, in the present embodiment, in the binary-ternary conversion circuit 20, the output signal of the signal conversion unit 11 corresponding to the pre-encoding circuit is divided into the branch circuits 21 and 22A, 2A and 2B.
A three-valued output consisting of a square wave having a sharp rise and fall is provided by a differential device including a delay device 23A and a first differential circuit 24A and a differential device including a delay device 23B and a second differential circuit 24B via 2B. Since the signal is converted into a signal, a modulated optical signal having an RZ pattern having no pattern dependency with respect to the NRZ pattern of the input data signal is obtained.

FIGS. 10A, 10B and 10C show that the bit pattern of the NRZ pattern as the input data signal is "101".
10 "differential circuit output (output signal of first or second differential circuit 24A, 24B), low-pass filter output (output signal of first or second low-pass filter 12A, 12B) 10A and 10B show waveforms of an optical output (modulated optical signal), as shown in FIG.
That is, the transition time in the state transition of the drive signal waveform of the optical modulator 19 has a constant value determined by the time constant of the low-pass filter.

Here, if the time constants of the low-pass filters 12A and 12B are set so that the rise and fall times of the drive signal waveform of the optical modulator 19 fall within one bit, the modulated optical signal 17 Is always output as a constant waveform without depending on the bit pattern. That is, by suppressing the carrier wave component, it is possible to realize an optical transmission device that has resistance to nonlinear phenomena in an optical fiber and does not depend on a bit pattern.

In the present embodiment, when calculating the difference with respect to the output signal of the signal converter 11, the delay units 23A and 23B
, The delay amount to be added is set to 0.5 bit. However, if the delay amount is 1 bit or less, the same effect as described above can be obtained. When the delay amount is 0.5 bits, the optical output RZ
Although the duty ratio of the modulated optical signal 17 in the pattern is 50%, the duty ratio of the modulated optical signal 17 can be arbitrarily changed by changing the delay amount.

According to the present embodiment, the ternary output signal generated by the ternary / ternary conversion circuit 20 is the same as that of FIG.
Although the frequency band is slightly widened as compared with the input data signal which is an NRZ pattern as shown in FIG. 2, every transition between the “0” level and the “1” level of the input data signal and 2T of the “1” level Since it is a ternary signal that alternates between “−1” level and “1” level having different polarities for each successive period, the transition between the “0” level and “1” level of the input data signal and And "1" level 2
The frequency band is narrower than that of an RZ pattern signal which is a binary signal having a constant polarity “1” level for each successive period of T. Therefore, similarly to the first and second embodiments, the driver circuits 13A and 13B and the MZ optical modulator 19 can generate an RZ modulated optical signal at a frequency lower than the band of the input data signal.

In this embodiment, the case where the push-pull MZ type optical modulator 19 is used has been described.
Similarly to the first embodiment, the present invention can be applied to a case where the MZ type optical modulator 16 having one drive signal input terminal P2 is used. In that case, in the binary-ternary conversion circuit 20, for example, the branch circuits 21, 22B, the delay unit 23B, and the second
And the second low-pass filter 12B and the driver circuit 13B may be omitted.

(Fourth Embodiment) Next, as a fourth embodiment of the present invention, an embodiment in which the present invention is applied to an optical transmission system of an optical repeaterless transmission system will be described. The optical non-relay transmission method refers to a method of transmitting an optical signal by performing relay without supplying electric energy to a relay point.

FIG. 12 shows the configuration of an optical transmission system of the optical repeaterless transmission system according to this embodiment. This optical transmission system is a system for transmitting an optical signal from a transmitting station 100A to a receiving station 300A via an optical fiber 200 which is an optical transmission line. First, in the transmitting station 100A, the modulated optical signal output from the optical
After being amplified by 2, the light is sent to the optical fiber 200.

The modulated optical signal transmitted by the optical fiber 200 is received by the receiving station 300A. Receiving station 300
In A, the received modulated optical signal is an optical amplifier 301
, And then input to the optical receiving unit 302. In the optical receiving unit 302, the input modulated optical signal is converted into an electric signal, and the original data signal is reproduced.

Here, the optical transmitter 101 is, for example, an optical transmitter having the configuration shown in FIG. 8 described in the third embodiment.
The input data signal of the NRZ pattern is converted into the above-described ternary output signal by the binary-ternary conversion circuit 20, and the low-pass filters 12A and 12B and the driver circuits 13A and 1
3B, the optical carrier component is suppressed by being input to the push-pull MZ optical modulator 19.
A modulated optical signal of a Z pattern is generated. Therefore, as described above, a large resistance to the non-linear phenomenon in the optical fiber 200 is obtained, so that a large optical power can be incident on the optical fiber 200 as a modulated optical signal, and the transmission distance without optical repeater can be reduced. Can be greatly expanded.

Further, as described in the third embodiment, if the output stage of the binary-ternary conversion circuit 20 is constituted by a differentiator, the bit pattern dependency of the modulated optical signal is eliminated, so that the intersymbol interference is eliminated. Long-distance optical repeaterless transmission with less transmission.

(Fifth Embodiment) FIG. 13 shows a configuration of an optical transmission system of an optical repeaterless transmission system according to a fifth embodiment of the present invention. In this embodiment, a plurality of optical transmission units 101-1, 101-2,...
Modulated optical signals having different optical frequencies modulated by input data signals of NRZ patterns of -N to N channels are output respectively. After these modulated optical signals are multiplexed by the optical multiplexer 103, they are collectively amplified by the optical amplifier 102 and transmitted to the optical fiber 200.

An optical signal obtained by multiplexing modulated optical signals of a plurality of different optical frequencies transmitted by the optical fiber 200 is
Received at receiving station 300B. In the receiving station 300B, after the received optical signal is amplified by the optical amplifier 301 and further demultiplexed by the optical demultiplexer 303 into a modulated optical signal of each optical frequency, the plurality of optical receiving units 302-1 and 302-1. 302
,..., 302-N. In the optical receiving units 302-1, 302-2, ..., 302-N,
After the input modulated optical signal is converted into an electric signal, the original N-channel data signal is reproduced.

Also in this embodiment, the optical transmission unit 101-
, 101-N, for example, using the optical transmission device having the configuration shown in FIG.
A Z-pattern input data signal is converted to a binary / ternary conversion circuit 20
After the conversion into the ternary output signal described above, the low-pass filters 12A and 12B and the driver circuits 13A and 13B
By inputting to the push-pull configuration MZ optical modulator 19 via B, a modulated optical signal having an RZ pattern in which an optical carrier component is suppressed is output. Since the optical fiber 200 has resistance and can input a large optical power as a modulated optical signal into the optical fiber 200, the transmission distance without optical repeater can be greatly increased, and furthermore, a binary-ternary conversion circuit. 2
By configuring the output stage of 0 with a differentiator, the bit pattern dependency of the modulated optical signal is eliminated, and long-distance optical repeaterless transmission with little intersymbol interference becomes possible.

(Sixth Embodiment) FIG. 14 shows the configuration of an optical transmission system of an optical repeaterless transmission system according to a sixth embodiment of the present invention. In the present embodiment, the transmitting station 100C
A remote optical amplifier 400 is inserted in the middle of the optical fiber 200 between the optical fiber 200A and the receiving station 300C, that is, between the optical fibers 200A and 200B. The configuration of the optical transmission unit 101 provided in the transmission station 100C may be the same as in the fourth and fifth embodiments.

In the remote optical amplifier 400, the optical coupler 401 in the transmitting station 100C is operated by the optical coupler 401.
2 and an excitation light source 104 provided in the transmitting station 100C.
After the pump light transmitted through the optical fiber 210 through the optical fiber 210 is multiplexed, the optical signal is amplified by being input to the optical fiber 402. In this case, the transmitting station 10
Light energy is supplied as energy for amplification from 0C to the remote optical amplifier 400, which is a relay point, and optical non-repeated transmission is performed.

On the other hand, the excitation light source 304 is
And an optical coupler 305 is newly provided, and the excitation light output from the excitation light source 304 is supplied to the optical fiber 200B. The optical fiber 200 into which the pump light has been injected.
In B, due to the nonlinear phenomenon in the optical fiber, the transmitting station 10
The optical signal transmitted from OC through the remote optical amplifier 400 is amplified. The optical signal thus optically amplified by the remote optical amplifier 400 and the optical fiber 200B is input to the receiving station 300C, and the optical coupler 305
After being separated from the pump light by the optical amplifier 301, the light is input to the optical receiver 302 via the optical amplifier 301, converted into an electric signal, and the original data signal is reproduced.

As described above, in the present embodiment, the transmitting station 100C
Optical fiber 2 which is an optical transmission path from the optical fiber to the receiving station 300C
00 to the remote optical amplifier 400, the transmitting station 100
C to supply excitation light via the optical fiber 201,
By injecting the pumping light from the receiving station 300C into the optical fiber 200B and amplifying the optical signal without optical relay, the transmission distance can be further expanded.

In this embodiment, both the transmitting station 100C and the receiving station 300C send out the pumping light for amplifying the optical signal. However, the same effect can be obtained by using only one of them to send out the pumping light. Needless to say, this is obtained.

[0073]

As described above, according to the optical transmitter of the present invention, a modulated optical signal of an RZ pattern is generated with a single optical modulator without the need for a wideband optical modulator or driver circuit. it can. It is another object of the present invention to provide an optical transmitter capable of generating a modulated optical signal having an RZ pattern in which a carrier component is effectively suppressed by using a single optical modulator. Further, according to the optical transmission system of the present invention, optical non-repeated transmission is performed in which nonlinear phenomena in an optical transmission line are effectively suppressed using a modulated optical signal having an RZ pattern in which a carrier component is effectively suppressed. It becomes possible.

[Brief description of the drawings]

FIG. 1 is a diagram showing a configuration of an optical transmission device according to a first embodiment of the present invention.

FIG. 2 is a signal waveform diagram of each unit for explaining the operation of the first embodiment.

FIG. 3 is a circuit diagram showing a configuration example of a signal conversion unit according to the first embodiment;

FIG. 4 is a signal waveform diagram of each unit for explaining the operation of the signal conversion unit of FIG. 3;

FIG. 5 is a diagram showing transmittance characteristics with respect to an applied voltage of an MZ optical modulator used in the present invention.

FIG. 6 is a diagram showing an optical modulator driving waveform and a modulated optical signal for explaining an effect when the frequency characteristic of the low-pass filter in the first embodiment is changed.

FIG. 7 is a diagram illustrating a configuration of an optical transmission device according to a second embodiment of the present invention.

FIG. 8 is a diagram showing a configuration of an optical transmission device according to a third embodiment of the present invention.

FIG. 9 is a view showing signal waveforms of respective units for explaining the operation of the third embodiment.

FIG. 10 is a diagram more specifically showing a signal waveform of a main part in the third embodiment.

FIG. 11 is a waveform chart for explaining a problem of the conventional CS-RZ system.

FIG. 12 is a diagram showing a configuration of an optical transmission system according to a fourth embodiment of the present invention.

FIG. 13 is a diagram showing a configuration of an optical transmission system according to a fifth embodiment of the present invention.

FIG. 14 is a diagram showing a configuration of an optical transmission system according to a sixth embodiment of the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 10 ... Data signal input terminal 11 ... Signal conversion part 12, 12A, 12B ... Low-pass filter 13, 13A, 13B ... Driver circuit (optical modulator drive circuit part) 14 ... Semiconductor laser (light source) 15 ... Bias control part 16 ... MZ-type optical modulator 17... Modulated optical signal 18... Inverting circuit 20... Binary-ternary conversion circuit 21, 22 A, 22 B… branch circuit 23 A, 23 B… delay device 24 A, 24 B. ... Transmission stations 101, 101-1, ... 101-N ... Optical transmission unit 102 ... Optical amplifier 103 ... Optical multiplexer 104 ... Excitation light source 200, 200A, 200B ... Optical transmission line 201 ... Optical fiber 300A, 300B, 300C ... Reception Station 301 optical amplifier 302 optical receiver 303 optical demultiplexer 304 excitation light source 305 optical coupler 400 remote optical amplifier Vessel 401 ... optical coupler 402 ... rare-earth doped optical fiber

Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat II (reference) G02F 2/00 H04B 9/00 B H04B 10/28 10/26 10/14 10/00 H04L 25/49 F term (reference 2H079 AA02 AA12 BA01 CA04 EA05 FA03 FA04 HA11 KA18 2K002 AA02 AB18 5K002 AA02 AA03 CA01 CA13 CA14 CA16 DA05 FA01 5K029 AA11 CC04 FF03 LL01

Claims (14)

[Claims] 1. An NRZ comprising a first level and a second level.
A level transition occurs between the first level and the second level in the pattern of the input data signal every time the input data signal transitions from the first level to the second level and every time the second level continues for two unit periods. A signal converting means for converting the output signal into a value output signal; and performing a band limitation using the output signal of the signal converting means as an input signal, and when the input signal causes a level transition between the first level and the second level. A band-limiting means for gradually producing a level transition between the first level and the second level, and otherwise outputting a signal having a first level or a second level according to the input signal; Driven by the output signal of the limiting means, the light intensity is minimized when the level of the output signal is the first level and the second level, and the level of the output signal gradually changes between the first level and the second level. To Rebe Transition optical transmitter and a light intensity and a modulated optical signal generating means for outputting a modulated optical signal having a maximum when the process of. 2. A light source for emitting light of a predetermined wavelength, a light input / output terminal, a drive signal input terminal, and a bias input terminal. And a Mach-Zehnder interferometer-type optical modulator to which the first and second levels of the output signal of the band-limiting unit substantially correspond to the minimum transmittance of the optical modulator. An optical modulator driving means for supplying a drive signal to a drive signal input terminal of the optical modulator as a drive signal; and a bias control means for supplying a bias signal substantially corresponding to the maximum transmittance of the optical modulator to a bias input end of the optical modulator. 2. The optical transmission device according to claim 1, further comprising: outputting, as the modulated optical signal, light obtained by modulating light emitted from the light source according to the drive signal from an optical output end of the optical modulator. 3. An NRZ comprising a first level and a second level.
A level transition occurs between the first level and the second level in the pattern of the input data signal every time the input data signal transitions from the first level to the second level and every time the second level continues for two unit periods. Signal conversion means for converting the output signal into a value output signal, and performing band limiting using the output signal of the signal conversion means and a signal obtained by inverting the phase of the output signal as respective input signals, wherein the input signal is a first level and a second level. A signal that gradually transitions between a first level and a second level when a level transition occurs between the first and second levels, and that takes a first level or a second level according to the input signal otherwise. And push-pull drive by the output signals of the first and second band limiting means, and when the output signal is at the first level and at the second level, the light intensity Is the best And a modulating optical signal generating means for outputting a modulating optical signal having a maximum light intensity when the level of the output signal gradually transitions between the first level and the second level. An optical transmission device characterized by the above-mentioned. 4. The modulated light signal generating means has a light source for emitting light of a predetermined wavelength, a light input / output terminal, two drive signal input terminals, and a bias input terminal. A Mach-Zehnder interferometer type optical modulator to which the outgoing light is input, and wherein the first level and the second level of the output signal of the first and second band limiting means substantially correspond to the minimum transmittance of the optical modulator. Optical modulator driving means for amplifying the output signal and supplying it as drive signals to two drive signal input terminals of the optical modulator, respectively; and a maximum transmittance of the optical modulator at a bias input terminal of the optical modulator. And a bias control unit for providing a bias signal substantially corresponding to the drive signal, wherein the light output from the light source is modulated according to the drive signal, and the light is output from the light output end of the light modulator as the modulated light signal. Claim 3 The transmission device. 5. An NRZ comprising a first level and a second level.
A level transition occurs between the first level and the second level in the pattern of the input data signal every time the input data signal transitions from the first level to the second level and every time the second level continues for two unit periods. A signal conversion unit for converting the output signal into a value output signal; and obtaining the difference between the output signal of the signal conversion unit and a signal obtained by delaying the output signal by a time corresponding to less than one bit of the input data signal. The first level and the second level are alternately taken every time the data signal transitions from the first level to the second level and every time the second level continues for two unit periods, and the first level and the second level during the other periods. And a difference means for generating a ternary output signal having a third level intermediate between the first level and the first level of the ternary output signal. No. Modulated light signal generating means for outputting a modulated light signal in which the light intensity corresponding to the level period is maximum and the optical phases are opposite to each other in the first level and the second level period. An optical transmission device characterized by the above-mentioned. 6. The optical transmitting apparatus according to claim 5, further comprising a band limiting unit that limits the band of the ternary output signal from the difference unit and supplies the band to the modulated optical signal generating unit. 7. The modulated light signal generating means has a light source for emitting light of a predetermined wavelength, a light input / output terminal, a drive signal input terminal, and a bias input terminal.
A Mach-Zehnder interferometer type optical modulator in which the light emitted from the light source is input to an optical input end; a fifth level of the ternary output signal corresponds to a minimum transmittance of the optical modulator; Third level and fourth
Optical modulator driving means for amplifying the ternary output signal so that the level substantially corresponds to the maximum transmittance of the optical modulator and supplying the ternary output signal as a drive signal to a drive signal input terminal of the optical modulator; A bias control unit for providing a bias signal substantially corresponding to the minimum transmittance of the optical modulator to a bias input terminal of the optical modulator, and modulating the light emitted from the light source according to the drive signal from the optical output terminal of the optical modulator. The optical transmitter according to claim 6, wherein the modulated light is output as the modulated optical signal. 8. An NRZ comprising a first level and a second level.
A level transition occurs between the first level and the second level in the pattern of the input data signal every time the input data signal transitions from the first level to the second level and every time the second level continues for two unit periods. A signal conversion unit for converting the output signal into a value output signal; and obtaining the difference between the output signal of the signal conversion unit and a signal obtained by delaying the output signal by a time corresponding to less than one bit of the input data signal. The third level and the fourth level are alternately taken every time the data signal transitions from the first level to the second level and every time the second level continues for two unit periods, and the first level and the second level during other periods. A first ternary output signal having a fifth level intermediate between the first ternary output signal and a second ternary output signal having a phase opposite to that of the first ternary output signal. And the first And the light intensity corresponding to the fifth level period of the second ternary output signal is the minimum, and the light intensity corresponding to the third level and fourth level periods of the first and second ternary output signals is The third of the maximum and the first and second ternary output signals
An optical transmission device comprising: a modulated optical signal generating unit that outputs modulated optical signals whose optical phases are opposite to each other in a period corresponding to the level and the fourth level. 9. A first and second band limiting means for band limiting the first and second ternary output signals from the first and second difference means and supplying the signal to the modulated optical signal generating means. The optical transmission device according to claim 8, further comprising: 10. The modulated light signal generating means has a light source for emitting light of a predetermined wavelength, a light input / output terminal, two drive signal input terminals, and a bias input terminal. A Mach-Zehnder interferometer type optical modulator to which the outgoing light is input; a fifth level of the first and second ternary output signals substantially corresponds to a minimum transmittance of the optical modulator; Amplifying the first and second ternary output signals so that the third level and the fourth level of the ternary output signal substantially correspond to the maximum transmittance of the optical modulator, and driving the optical modulator. First and second signals supplied to the input terminal as first and second drive signals
An optical modulator driving means, and a bias control unit for applying a bias signal to a bias input terminal of the optical modulator, the bias signal substantially corresponding to a minimum transmittance of the optical modulator, and an optical output terminal of the optical modulator. 9. The optical transmission device according to claim 8, wherein a light obtained by modulating light emitted from the light source according to the drive signal is output as the modulated light signal. 11. An optical transmitter for outputting a modulated optical signal of at least one channel intensity-modulated according to a binary input data signal of an NRZ pattern composed of first and second levels, and a modulated light from the optical transmitter. A transmitting station having a first optical amplifier for amplifying a signal; an optical transmission path for transmitting an optical signal from the transmitting station; a second optical amplifier for amplifying an optical signal transmitted by the optical transmission path; A receiving station having an optical receiving unit that receives an optical signal output from the second optical amplifier and reproduces the original data, wherein the optical transmitting unit converts the binary input data signal into the binary input data. First of data signal
The third level and the fourth level are alternately taken at every transition from the level to the second level, and at every continuation over the two unit periods of the second level, and in the other periods, the third level and the fourth level are intermediate. Convert to ternary output signal taking 5 levels 2
Value-to-ternary conversion means, wherein the light intensity corresponding to the fifth level period of the ternary output signal is minimum, and the light intensity corresponding to the third level and fourth level periods of the ternary output signal is maximum And a modulated optical signal generating means for outputting modulated optical signals whose optical phases are opposite to each other in a period corresponding to a third level and a fourth level of the ternary output signal. system. 12. An optical amplifying means for amplifying a modulated optical signal transmitted through the optical transmission line based on pump light supplied from at least one of the transmitting station and the receiving station. 2. The method according to claim 1, further comprising:
2. The optical transmission system according to 1. 13. The binary to ternary conversion means converts an input data signal of an NRZ pattern comprising a first level and a second level from a first level of the input data signal to a second level.
A signal conversion unit that converts the output signal into a binary output signal that causes a level transition between the first level and the second level for each transition to the level and for each continuation over the two unit periods of the second level; A difference means for generating the ternary output signal by calculating a difference between an output signal and a signal obtained by delaying the output signal by a time corresponding to less than one bit of the input data signal. Item 12. The optical transmission system according to item 11. 14. The optical transmission system according to claim 11, further comprising a band limiting unit between said binary-ternary conversion unit and said modulated optical signal generating unit.