JPH10242942A - Optical transmitter and parallel encoding transmission system using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To realizing encoding with narrow optical spectrum extension and to increase the number of wavelength multiplexing by independently executing duobinary modulation on the phase and the intensity of the respective wavelengths. SOLUTION: An error correction encoding circuit 10 inputs the data signals S1 -SN of an N system and encodes an error code. The parity bit signals P1 -PK of a K system are outputted to the data signals S1 -SN of the N system. Light- emitting elements 11-1 to 11-N, 12-1 to 12-K output the light beams of the mutually different wavelengths. The light beams of the respective wavelengths are inputted to corresponding modulation means 13-1 to 13-N and 14-1 to 14-K and they are modulated by the data signals S1 -SN and the parity bit signals P1 -PK. The signal light beams of the modulated wavelengths are multiplexed in an optical multiplexer 15 and are transmitted to an optical fiber transmission line 16. The data signals and the parity bit signals are not multiplexed in a time area but are wavelength-divided and multiplexed so as to transmit them. Thus, an error correction function can be added with the transmission rate of international standard as it is.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to an optical transmitter for wavelength-multiplexing and transmitting a plurality of series of data signals and a parallel coded transmission system using the same.

[0002]

2. Description of the Related Art In an optical fiber communication system, generally, the output light of a light emitting element is intensity-modulated by an NRZ modulation signal by an external modulator to transmit about several Gbit / s.
As a method for increasing the transmission capacity, a wavelength division multiplexing (WDM) transmission method is being studied. This WDM transmission system is configured such that optical signals modulated by relatively low-speed transmission signals are arranged in parallel on the optical frequency axis, and each optical signal is multiplexed and transmitted through one optical fiber transmission line. The apparent total transmission capacity can be increased.

The gain band of an optical amplifier arranged on an optical fiber transmission is, for example, 1.54 μm to 1.56 μm for an erbium-doped optical fiber amplifier, and the number of wavelength multiplexes is limited. Therefore, it is necessary to increase the transmission speed for each channel. However, simply performing high-speed modulation processing causes a broadening of the optical spectrum, which causes a problem of crosstalk when multiplexing and demultiplexing optical signals of each wavelength (Oda
Other, "A Study on ADM Ring Network Using Optical FDM Technology", IEICE Autumn Meeting 1992, SN-7-
3, pp. 296-297).

[0004] Therefore, adoption of a modulation system with a small optical spectrum spread even at the same signal speed is being studied. Such a modulation scheme is well known as "partial response scheme" or "correlation level coding" in the field of communication theory, and especially in the field of optical communication, "duobinary modulation scheme" has suppressed the optical spectrum spread. This is known as a modulation method (Y.Yano et al., "2.6 Terabit /
s WDM TransmissionExperiment using optical duob
inary coding ", 22nd European Conference onOptical
Communication (ECOC'96), postdeadline paper ThB.3.
1, pp.5.3-5.6).

FIG. 10 shows a configuration example of a conventional optical transmitter corresponding to the duobinary modulation method. In the figure, a signal generator 81 generates a binary NRZ signal a _k . This binary NRZ signal a _k is converted into a signal b by a precoder 82.
_k (= a _k + b _k−1 , + is the addition of modulo 2). This signal b _k is branched into two systems, one of which is input to a low-pass filter 83-1 to become a ternary code sequence _ck , and the other is sent to a low-pass filter 83-2 via a logical inversion circuit 84. It is input and becomes a ternary code sequence d _k . Here, the low-pass filters 83-1 and 83-2 have an appropriate cutoff frequency (for example, about 1/4 of the bit rate).
With. The code sequences c _k and d _k are input to a Mach-Zehnder modulator 85 of a push-pull type and modulate CW light output from a light source 86.

FIG. 11 shows the operation principle of a conventional optical transmitter. The binary NRZ signal a _k is “00101110”. (a) is a signal b output from the precoder 82.
_k (11001011). (b) shows an eye pattern of the signal b _k . T ₀ in the figure represents a time width occupied by a 1-bit pulse. (c) is a low-pass filter 83
-1, a ternary (0, 0.5, 1) code sequence c _k
Is shown. (d) shows an eye pattern of the code sequence _ck .
(e) shows a ternary code sequence d _k output from the low-pass filter 83-2. Such a code sequence c _k , d _k
When the push-pull type Mach-Zehnder modulator 85 is driven, the amplitude modulation takes the binary value of (1, 0) and the phase modulation takes the binary value of (+ π / 2, −π / 2). Are output at the same time. Table 1 shows the above situation.

[0007]

[Table 1]

In this optical duobinary signal, the effect of the intensity modulation and the effect of the phase modulation are synergistic, so that the optical spectrum spread becomes narrower than when the intensity is modulated by the binary NRZ signal as shown in FIG. When this optical transmitter is applied to a WDM transmission system, the number of channels can be increased within a limited band. When an optical duobinary signal is received, a code sequence in which the code a _k is inverted is obtained, so that it is necessary to perform a logical inversion process on the received signal.

It is known that when optical duobinary signals are multiplexed at high density on the frequency axis, a nonlinear phenomenon occurs in an optical fiber transmission line. Since this nonlinear phenomenon occurs in relation to the polarization state of each optical signal, the optical frequency of each channel, the dispersion value of the optical fiber, and the like, the code error rate fluctuates irregularly with time. for that reason,
A method of performing error correction code in each channel has been proposed (NS Bergano et al., "100 Gb / s Error Free Trans
mission over 9100 km using twenty 5 Gb / s WDM Channe
ls ", Optical Fiber Communication (OFC'96), postdea
dline paperPD23-2).

FIG. 13 shows an example of the configuration of a conventional error correction coding transmission system. In the figure, an input data signal of 2.488 Gbit / s, which is a standardized transmission rate unit, is input to error correction coding circuits 91-1 and 91-2, respectively. The two-system code sequence output from each error correction coding circuit has a transmission rate of 2.67 Gbit / s because a parity check code is newly inserted, and 5.33 Gbit when multiplexed by the multiplexing circuit 92. / s The output signal of the multiplexing circuit 92 is converted into an optical signal by an optical transmitter 93 and transmitted to an optical transmission line 94. From the optical transmission line 94 to the optical receiver 9
The optical signal received at 5 is converted into an electric signal having a transmission rate of 5.33 Gbit / s, input to a separation circuit 96, and separated into two code sequences. The transmission speed of this code sequence is 2.67 Gbit / s, and the error correction decoding circuit 97-
1, 97-2, and output as a code sequence having a transmission rate of 2.67 Gbit / s and error correction.

[0011]

A conventional optical transmitter supporting the duobinary modulation method requires a push-pull type Mach-Zehnder modulator, which is not suitable for multi-channel transmission. Also, in the conventional error correction coded transmission system, since the parity bit signal used for error correction is time-division multiplexed with the data signal, a signal having a transmission rate deviating from the international standard transmission rate system must be processed. Did not. Therefore, when connecting to another circuit, a special interface circuit is required. Further, when modulating each channel, the modulation band is substantially increased, so that the bandwidth of each transmission / reception circuit is required to be large, which is a factor of increasing the cost.

The present invention provides an optical transmitter capable of realizing error correction coding without departing from the international standard transmission rate system and realizing coding with a narrow optical spectrum to increase the number of wavelength multiplexing. It is an object of the present invention to provide a parallel coded transmission system using the same.

[0013]

SUMMARY OF THE INVENTION An optical transmitter according to the present invention comprises an encoding circuit for encoding and outputting a plurality of data signals in parallel, a light emitting element for outputting light of a plurality of wavelengths different from each other, and a light emitting element. Means for modulating light of each wavelength output from the encoder with a plurality of series of data signals output from the encoding circuit, and an optical multiplexer for multiplexing and outputting signal light of each wavelength modulated by the modulation means (Claim 1). Thus, a plurality of data signals can be wavelength-multiplexed and transmitted.

The encoding circuit may be an error correction encoding circuit that inputs a plurality of N-sequence data signals and outputs a plurality of N-sequence data signals and a plurality of K-sequence parity bit signals used for error correction. Accordingly, the parity bit signal for error correction can be transmitted in parallel (claim 5). The modulation means generates a phase modulation signal and an intensity modulation signal from a binary NRZ code data signal, and a phase modulation signal which outputs the phase and intensity of light of each wavelength output from the light emitting element. And a modulating unit that performs duobinary modulation independently by the intensity modulation signal (claim 2).

The light emitting element and the modulating section of the modulating means are composed of a semiconductor element having a laser light generating region, a phase modulating region, and an intensity modulating region having a structure in which the oscillation wavelength is single and stable, and a modulation signal generating device. The phase modulation signal output from the section is applied to the phase modulation area, and the intensity modulation signal is applied to the intensity modulation area. Further, the light emitting device includes means for injecting light whose frequency is stabilized from the outside into the light emitting element, and stabilizing the oscillation wavelength by injection locking.

With such a configuration, the phase and intensity of light can be independently duobinary modulated without using a push-pull type Mach-Zehnder modulator, and signal light with a narrow optical spectrum spread can be generated. it can. Further, the parallel coded transmission system of the present invention includes the above optical transmitter, an optical fiber transmission line for transmitting the wavelength multiplexed signal light output from the optical transmitter, and a wavelength multiplexed signal light from the optical fiber transmission line. An optical demultiplexer that demultiplexes for each wavelength, a receiving circuit that receives signal light of each wavelength demultiplexed by the optical demultiplexer, and a plurality of data signals output from the receiving circuit, And a decoding circuit that outputs the data signal of (1).

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION

(First Embodiment of Parallel Coding Transmission System) FIG.
1 shows a first embodiment of a parallel coded transmission system according to the present invention. In the figure, an optical transmitter comprises an error correction coding circuit 10.
And light emitting elements 11-1 to 11-N, 12-1 to 12-K
And modulation means 13-1 to 13-N, 14-1 to 14-N
And an optical multiplexer 15.

The error correction coding circuit 10 receives the N-sequence data signals S _{1 to} S _N and performs error correction coding, and the N-sequence data signals S _{1 to} S _N and the K-sequence parity bit signals P ₁ to P ₁ . and outputs the P _K. Light emitting elements 11-1 to 11-N,
12-1 to 12-K output light having different wavelengths from each other. The light of each wavelength is transmitted to the corresponding modulation means 13-1.
To 13-N, is input to 14-1 to 14-N, modulated by the data signal S ₁ to S _N and the parity bit signal P ₁ to P _K. The modulated signal light of each wavelength is multiplexed by the optical multiplexer 15 and transmitted to the optical fiber transmission line 16.

The optical receiver includes an optical demultiplexer 17 and a receiving circuit 1
8-1 to 18-N, 19-1 to 19-K, and an error correction decoding circuit 20. Optical fiber transmission line 1
The wavelength division multiplexed signal light from No. 6 is demultiplexed for each wavelength by the optical demultiplexer 17, and the corresponding receiving circuits 18-1 to 18-
N, 19-1 to 19-K. Receiver circuit 18-
1 to 18-N outputs the data signals S ₁ to S _N of the N sequence, the receiving circuit 19 - 1 through 19-K and outputs the parity bit signal P ₁ to P _K of K series. N-sequence data signal S
The parity bit signals P _{1 to} P _K of _{1 to} S _N and the K sequence are input to the error correction decoding circuit 20, and the data signals S _{1 to} S _{N of} the N sequence with error correction are output.

As described above, the parallel coded transmission system of the present invention is characterized in that the data signal and the parity bit signal are not multiplexed in the time domain but transmitted by wavelength division multiplexing. As a result, an error correction function can be added with the transmission rate of the international standard, and the design of the interface becomes easy. As an encoding method suitable for such error correction, simple hamming (SEC-DED: single error c
orrection-double error detection) code, Hsiao code (MYHsiao et al., "Application of errorcoding co
des in computer reliability studies ", IEEE Trans. O
n ReliabilityR-18, pp.108-118, Aug, 1969).

The parallel coded transmission system of the present invention is not limited to the parallel transmission of a data signal and a parity bit signal. For example, in a WDM network, the parallel coded transmission of a data signal and a supervisory signal (path information) is performed. And so on. The details will be described later as a second embodiment and a third embodiment of the parallel encoding transmission system.

(First Embodiment of Optical Transmitter) FIG. 2 shows a first embodiment of the optical transmitter. Here, a configuration example of a light emitting element and a modulating unit that constitute the optical transmitter is shown. In the figure, the light emitting element and the modulating unit of the modulating means are composed of a DFB (distribution period type) laser region 21, a phase modulating region 22, and an intensity modulating region 23 which oscillate in a single mode. Such a light emitting / modulating element may be integrated as a semiconductor element or may be a combination of individual elements.
Note that a DBR (distribution reflection type) laser may be used as the laser light generation region. These have a function of emitting light at a predetermined center wavelength and a predetermined output light intensity by setting a temperature and an injection current.

Next, the configuration of the modulation signal generator of the modulation means will be described. Data signal S which is a binary NRZ code
₁ to S _N and the parity bit signal P ₁ to P _K is here represented as an input signal a _K. The input signal a _K is split into two, one of which is input to the precoder 24 and the signal b
_k (= a _k + b _k−1 , + is the addition of modulo 2). This signal b _k is branched into two systems, one of which is input to the low-pass filter 25-1 to be a ternary code sequence _ck , and the other is passed through the logic inversion circuit 26-1 to the low-pass filter 25-. 2 and becomes a ternary code sequence d _k .
Here, the low-pass filters 25-1 and 25-2 have a cutoff frequency of about 1/4 of the bit rate. The ternary code sequences c _k and d _k are input to a subtraction circuit 27 to perform an operation of (c _k −d _k ), the output of which is amplified to a predetermined level by an amplifier 28, and used as a signal for phase modulation. It is applied to the phase modulation area 22.

Generally, when the injection current in the phase modulation region 22 is increased, the carrier density increases and the refractive index decreases, and when the injection current decreases, the carrier density decreases and the refractive index increases. Therefore, by applying an appropriate injection current according to the sign of c _k -d _k, the phase state as shown in Table 2 can be obtained.

[0025]

[Table 2]

[0026] 2 other branched input signal a _K, the level shift circuit 29, a logic inversion circuit 26-2, bit rate and inverted through the low-pass filter 30 having the same degree of cut-off frequency input signal a _k is applied to the intensity modulation area 23 as an intensity modulation signal. Generally, an electro-absorption modulator (EA modulator) is used as a semiconductor intensity modulation element. The EA modulator, as shown in FIG.
As the applied voltage level is lowered to the negative side, the light transmittance decreases. In order to perform intensity modulation using this characteristic, the range of the applied voltage must be set to 0 V or less. Therefore, the input signal a _k having a positive voltage value is shifted to a negative voltage value by using the level shift circuit 29. At this time, when the data is “1” and the applied voltage is near 0 V,
When the data is “0”, the applied voltage becomes a negative voltage (for example, −2 V). In this case, when the data is “1”, the light level is “light emission”, when the data is “0”, the light level is “non-light emission”, and when the phase in Table 2 is ± π / 2, the light level is “light emission”. Strength 0
Become. The information that the phase is ± π / 2 is an important parameter of the duobinary code, so that if the light intensity of that portion becomes 0, the information of the duobinary code cannot be transmitted. Therefore, the output of the level shift circuit 29 is inverted by the logic inversion circuit 26-2, and applied to the intensity modulation area 23 as an inverted input signal a _k .

When the modulation characteristic of the EA modulator is opposite to that shown in FIG. 3, that is, when the applied voltage level is increased to the positive side, the light transmittance is reduced, the level shift circuit 29 is used. In the output of (1), when the data is "1", the applied voltage becomes a positive voltage (for example, + 2V), and when the data is "0", the applied voltage becomes 0V. In this case, when the data is “1”, the light level is “non-light emitting”, when the data is “0”, the light level is “light emitting”, and when the phase in Table 2 is ± π / 2, the light level is “light emitting”. The intensity becomes zero. Therefore, even in this case, the information of the duobinary code cannot be transmitted, so that the output of the level shift circuit 29 is similarly inverted by the logic inversion circuit 26-2 and applied to the intensity modulation area 23 as an inverted input signal a _k. There is a need.

In the intensity modulation area 23, phase modulation may be applied according to the modulation code. In the case of the EA modulator, however, it is known that the phase change with respect to the intensity change can be made almost zero (chirp parameter: α ~ 0), no problem. Further, even if a slight phase change occurs, the direction in which the phase change occurs is constant in a direction in which the refractive index becomes “large” when the applied voltage is at “1” level. Therefore, by weighting the output levels of c _k and d _k , the problem of intermodulation of the phase can be solved.

(Second Embodiment of Optical Transmitter) FIG. 4 shows a second embodiment of the optical transmitter. In the present embodiment, a light emitting element and a modulating unit (semiconductor element) of a modulating unit are arrayed to further increase the parallel transmission capacity by WDM. In this case, the number of channels that can be integrated per semiconductor element is at most about 10 due to the limitation of the element size. By treating the plurality of channels as one group and multiplexing them using an optical group multiplexer / demultiplexer, multiplexing of about several hundred channels is performed. In addition, since it is difficult to stabilize the frequency of each channel individually, frequency stabilizing reference light is injected into each light emitting element from a light source whose frequency has already been stabilized, and frequency stabilization is realized by injection locking. . Further, by transmitting the parity bit signal for error correction separately from the data signal, a change in the transmission speed due to the addition of the error correction code is avoided.

The light emitting / modulating elements are provided in the laser regions 41-1 to 41-1.
41-M, the phase modulation areas 42-1 to 42-M, and the intensity modulation areas 43-1 to 43-M. The frequency-stabilizing reference light 44 includes an optical group splitter 45 and an optical group splitter 46-1.
-46-M and split into laser regions 41-1 to 41-M
Is injected into. The optical duobinary signals output from the intensity modulation areas 43-1 to 43-M are combined with the optical group multiplexer 47-1.
４７47-M are multiplexed by an optical group multiplexer 48 and output as a wavelength multiplexed optical duobinary signal 49.

The error correction encoding circuit 10 receives the N-sequence data signals S _{1 to} S _N and performs error-correction encoding, and the N-sequence data signals S _{1 to} S _N and the K-sequence parity bit signals P ₁ to P ₁ . and outputs the P _K. Data signals S ₁ to S _N and the parity bit signal P ₁ to P _K are inputted in a predetermined modulation signal generating unit 50-1 to 50-M. The phase modulation signal and the intensity modulation signal output from the modulation signal generation units 50-1 to 50-M are respectively in the phase modulation area 42-M.
1 to 42-M and the intensity modulation regions 43-1 to 43-M.

Hereinafter, the operation of the optical transmitter according to the present embodiment will be described. In the laser regions 41-1 to 41-M, a bias current is injected to cause laser oscillation. At this time, the frequency stabilizing reference light 44 is transmitted to the light group demultiplexers 45 and 46-1 to 4-4.
The laser area 4 is demultiplexed in channel units through 6-M
It is injected into 1-1 to 41-M, and the frequency of the oscillation light is stabilized.

The N-sequence data signals S _{1 to} S _N are encoded by the error correction encoding circuit 10 and
Parity bit signals P _{1 to} P _{K of 1 to} S _N and K sequence are generated. The data signals S _{1 to} S _N and the parity bit signals P _{1 to} P _K are supplied to predetermined modulation signal generation units 50-1 to 50-5.
The signals are input to 0-M and converted into a signal for phase modulation and a signal for intensity modulation for generating an optical duobinary signal. The phase modulation signal and the intensity modulation signal are applied to the phase modulation areas 42-1 to 42-M and the intensity modulation areas 43-1 to 43-M, respectively, to output an optical duobinary signal. In general, the parity bit signal P ₁
Since ＰP _K is a low speed, it may be configured not to be transmitted as an optical duobinary signal.

The optical duobinary signals output from the light emitting / modulating elements are multiplexed by the optical group multiplexers 47-1 to 47-M, respectively, and the multiplexed outputs are multiplexed by the optical group multiplexer 48. It is waved and output as a wavelength multiplexed optical duobinary signal 49. The wavelength multiplexed optical duobinary signal 49 includes an error correction signal channel, and can achieve an increase in transmission capacity and high reliability while maintaining an international standard transmission speed system.

In the second embodiment, the configuration in which the frequency stabilizing reference light 44 has a plurality of wavelength components, and is demultiplexed into each wavelength to perform injection locking. In contrast, FIG.
And a plurality of frequency stabilizing reference beams 44-1 to 44-M are prepared as in the third embodiment shown in FIG.
Through the laser region 41-M.
You may make it inject | pour into 1-41-M.

(Method of Generating Reference Light for Frequency Stabilization) FIG.
Shows an example of a method for generating the frequency stabilizing reference light. In the figure, a laser beam having a frequency f ₀ output from a frequency stabilizing laser 61 is injected into a mode-locked laser 63,
The center value of the oscillation frequency of the mode-locked laser 63 is set to f ₀ by injection locking. The mode-locked laser 6
Reference numeral 3 outputs a pulse train having a repetitive optical frequency equal to the driving frequency B of the oscillator 62. At this time, if the value of B is set to an integral multiple of the resonance frequency interval of the resonator of the mode-locked laser 63, the side mode occurs at a frequency interval equal to the drive frequency B of the oscillator 62, so that a comb-shaped spectrum is formed on the frequency axis. Carriers are generated. This is amplified to a predetermined level by an optical amplifier 64, and has an zero dispersion wavelength value very close to the oscillation frequency of the laser, an extremely small dispersion fluctuation value in the longitudinal direction, or an optical fiber 65 having a small wavelength dependence of the dispersion value. Incident on. At this time, due to a synergistic effect of nonlinear phenomena such as the four-wave mixing effect and the self-phase modulation effect in the optical fiber 65, comb-shaped carriers having a frequency interval B are generated over a band of about 10 THz. One of them
The two carrier frequencies are equal to the oscillation frequency f ₀ of the frequency stabilized laser 61. As a result, the frequency-stabilized reference light 44 is generated.

The frequency-stabilized laser 61 includes:
By stabilizing using the absorption line of the HCN gas molecule, a light emitting element having a frequency accuracy of about 10 MHz can be realized. FIG. 7 shows an example of the configuration of an optical group multiplexer / demultiplexer.

FIG. 7A shows an example of an optical multiplexer / demultiplexer 71 having a periodic transmittance characteristic. The transmittance characteristics of the optical multiplexer / demultiplexer 71 are, for example, as shown in (b), when the optical frequencies f1 to f4 of the frequency interval Δf are demultiplexed, they are divided into two groups of optical frequencies f1 and f3 and optical frequencies f2 and f4. It is split. Similarly, by connecting optical multiplexer / demultiplexers having different periods in multiple stages, the number of groups that can be multiplexed / demultiplexed can be increased. Such an optical multiplexer / demultiplexer includes, for example, a Mach-Zehnder optical demultiplexer, a Fabry-Perot optical demultiplexer, and an arrayed waveguide grating optical demultiplexer.

FIG. 3C shows an example of a band-pass type optical multiplexer / demultiplexer 72. The optical multiplexer / demultiplexer 72 allows the optical frequencies f1 and f2 to pass therethrough, as shown in FIG.
f4 is output from another port. As such an optical multiplexer / demultiplexer, there are an arrayed waveguide grating type optical demultiplexer having a flat pass band, a multilayer optical demultiplexer, and the like. (e) shows an example of a band reflection type optical multiplexer / demultiplexer 73. This optical multiplexer / demultiplexer 73
Pass the optical frequencies f1 and f2 and the optical frequencies f3 and f
4 is reflected and taken out to another port via the optical circulator 74. Such an optical multiplexer / demultiplexer includes a multilayer optical demultiplexer and a fiber grating having a chirped period.

(Second embodiment of the parallel coded transmission system of the present invention)
FIG. 8 shows a second embodiment of the parallel coded transmission system of the present invention. This embodiment is applied to a multi-hop type optical wavelength division multiplexing network. In the figure, nodes 75-1 to 75-9 are connected via an optical fiber transmission line in a matrix. A multi-hop optical wavelength division multiplexing network is a network that transmits a signal from a transmitting node to a receiving node while passing through another node. At this time, the wavelength used for communication between the nodes does not cause a wavelength collision. Is appropriately set as described above. Such a network can be configured by arranging the optical transmitter and the optical receiver of the present invention at each node and connecting them in cascade. At this time, in order to monitor and operate the network, it is necessary to know which path (path) the transmitted signal has traveled. At this time, as shown in the figure, by transmitting the path information in parallel to another channel separately from the main signal and transmitting the same, transmission becomes possible without increasing the transmission speed of the main signal.

(Third Embodiment of Parallel Coding Transmission System of the Present Invention)
FIG. 9 shows a third embodiment of the parallel coded transmission system according to the present invention. In the figure, an optical cross connect node (OXC) 76 and an optical add / drop multiplex node (OA)
DM) 77 is connected via an optical fiber transmission line. The optical transmitter and the optical receiver of the present invention are used as a transmission unit and a reception unit of each node. Optical cross connect node 76
Has a function of distributing the input wavelength multiplexed signal light to the optical fiber transmission line to which the destination node is connected.
The optical add / drop multiplex node 77 has a function of receiving only a signal light having a predetermined wavelength from the input wavelength multiplexed signal light, and multiplexing the signal light carrying the transmission signal with another signal light and transmitting the multiplexed signal light. With. [lambda] _i , [lambda] _j , [lambda] _k , [lambda] _l , [lambda] _m indicate the wavelength for transmitting the main signal, and [lambda] _SV indicates the wavelength for transmitting the monitoring information.

As shown in the figure, in an optical wavelength division multiplexing network, since various wavelengths are communicated via each node, it is necessary to manage path information. Conventionally, this path monitoring information has been superimposed on the main signal by using the subcarrier multiplexing technique, or embedded in the code string of the main signal by the time division multiplexing technique and transmitted. In the former case, there is a problem that the quality of the monitoring signal cannot be sufficiently ensured due to the interference between the subcarrier signal and the main signal. In the latter case, it is necessary to modify the conventional signal format or increase the transmission speed of the main signal in order to embed it in the overhead portion of the time division multiplexed signal. On the other hand, the conventional problem is solved by performing parallel coding according to the present invention and transmitting monitoring information using a channel other than the main signal.

[0043]

As described above, the optical transmitter according to the present invention can generate signal light with a narrow optical spectrum spread by the configuration in which the phase and intensity of light are independently duobinary modulated, and can perform wavelength multiplexing. You can increase the number. Further, in the parallel coded transmission system of the present invention, for example, a data signal and a parity bit signal used for error correction thereof are assigned to each wavelength and transmitted as a wavelength-division multiplexed signal light without departing from an international standard transmission rate system. Can have an error correction function. In an optical wavelength division multiplexing network, the same applies to the transmission of a data signal and its monitoring information.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a first embodiment of a parallel coded transmission system according to the present invention.

FIG. 2 is a diagram showing a first embodiment of the optical transmitter.

FIG. 3 is a diagram showing modulation characteristics of an electro-absorption (EA) modulator.

FIG. 4 is a diagram showing a second embodiment of the optical transmitter.

FIG. 5 is a diagram showing a third embodiment of the optical transmitter.

FIG. 6 is a diagram illustrating a method of generating a frequency stabilizing reference light.

FIG. 7 is a diagram showing a configuration example of a light group multiplexer / demultiplexer.

FIG. 8 is a block diagram showing a second embodiment of the parallel coded transmission system of the present invention.

FIG. 9 is a block diagram showing a third embodiment of the parallel coded transmission system of the present invention.

FIG. 10 is a diagram showing a configuration example of a conventional optical transmitter supporting a duobinary modulation method.

FIG. 11 is a diagram illustrating the operation principle of a conventional optical transmitter.

FIG. 12 is a view for explaining a compression effect of an optical spectrum spread by an optical duobinary signal.

FIG. 13 is a block diagram showing a configuration example of a conventional error correction encoding transmission system.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 Error correction encoding circuit 11, 12 Light emitting element 13, 14 Modulation means 15 Optical multiplexer 16 Optical fiber transmission line 17 Optical demultiplexer 18, 19 Receiving circuit 20 Error correction decoding circuit 21 DFB laser area 22 Phase modulation area 23 Intensity modulation area 24 Precoder 25, 30 Low pass filter 26 Logical inversion circuit 27 Subtraction circuit 28 Amplifier 29 Level shift circuit 41 Laser area 42 Phase modulation area 43 Intensity modulation area 44 Frequency stabilization reference light 45, 46 Light group demultiplexing Devices 47, 48 optical group multiplexer 49 wavelength multiplexed optical duobinary signal 50 modulation signal generator 51 optical branching device 61 frequency stabilizing laser 62 oscillator 63 mode-locked laser 64 optical amplifier 65 optical fiber 71, 72, 73 optical multiplexer / demultiplexer Container 74 optical circulator 75 node 76 optical cross connect De (OXC) 77 optical add drop multiplexer nodes (OADM)

Claims (6)

[Claims] An encoding circuit that encodes a plurality of data signals in parallel and outputs the encoded data signals; a light emitting element that outputs light of a plurality of wavelengths different from each other; Modulating means for modulating each signal with a plurality of series of data signals output from the multiplexing circuit; and an optical multiplexer for multiplexing and outputting signal light of each wavelength modulated by the modulating means. Optical transmitter. A modulating means for generating a phase modulation signal and an intensity modulation signal from a binary NRZ code data signal; a phase and intensity of light of each wavelength output from the light emitting element; The optical transmitter according to claim 1, further comprising: a modulation unit that independently performs duobinary modulation using the phase modulation signal and the intensity modulation signal. 3. The light emitting element and the modulating section of the modulating means are composed of a semiconductor element having a laser light generating region, a phase modulating region, and an intensity modulating region having a structure in which the oscillation wavelength is single and stable, and a modulating signal is generated. 3. The optical transmitter according to claim 2, wherein a signal for phase modulation output from the section is applied to a phase modulation area, and a signal for intensity modulation is applied to the intensity modulation area. 4. A light emitting device comprising means for injecting light whose frequency is stabilized from the outside into the light emitting element and stabilizing an oscillation wavelength by injection locking.
An optical transmitter according to any one of the above. 5. The encoding circuit is an error correction encoding circuit that inputs a plurality of N-sequence data signals and outputs a plurality of N-sequence data signals and a plurality of K-sequence parity bit signals used for error correction thereof. The optical transmitter according to any one of claims 1 to 4, wherein: 6. The optical transmitter according to claim 1, wherein: an optical fiber transmission line for transmitting wavelength-division multiplexed signal light output from the optical transmitter; and an optical fiber transmission line. An optical demultiplexer that demultiplexes the wavelength-division multiplexed signal light for each wavelength, a receiving circuit that receives signal light of each wavelength demultiplexed by the optical demultiplexer, and a plurality of series of signals output from the receiving circuit. An optical receiver comprising: a decoding circuit that inputs a data signal and outputs a plurality of series of data signals;