JP3257576B2 - Time division multiplex transmission method of optical solitons - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a time division multiplex transmission method of an optical soliton which realizes ultra-high speed and long distance optical communication by a time division multiplex method.

[0002]

2. Description of the Related Art In an anomalous dispersion wavelength region in which the wavelength derivative of the group velocity is negative, the optical soliton in the optical fiber balances its group velocity dispersion with the nonlinear optical effect. It has the characteristic that it propagates through an optical fiber without changing its waveform. Also, since a pulse of 10 ps or less can be propagated relatively easily, a transmission method using optical soliton has attracted attention as a promising transmission method for realizing long-distance, large-capacity optical communication that is not limited by group velocity dispersion. Have been.

Recently, the problem that the balance between nonlinearity and dispersion is lost after long-distance propagation due to the optical loss of the transmission optical fiber and the waveform is changed can be solved by the development of the erbium optical fiber amplifier. Ultra-high-speed and long-distance optical soliton transmission experiments using this technology have been reported (for details, see M. Nakazawa, K. Suzuki, E. Yamada, H. Kub).
ota, and Y.Kimura, "10Gbit / s-1200km error free solit
on data transmissionusing erbium-doped fiber ampli
fiers ", Electronics Letters, vol. 28, p. 817 (1992).).

[0004] By the way, at present, several tens Gb
It is difficult to realize signal processing exceeding it / s. Therefore, in order to realize transmission at a speed higher than several tens Gbit / s, optical time division multiplexing or wavelength multiplexing transmission technology is used. However, in wavelength division multiplexing transmission, it is necessary to prepare pulsed light sources for optical solitons for the wavelength used, and in dispersion-shifted fibers used for ordinary optical soliton transmission, the difference in group velocity dispersion due to the difference in signal light wavelength. Since the values are different, there is a disadvantage that it is difficult to control the amplitude of the optical soliton pulse when amplifying and relaying the multi-wavelength components collectively.

On the other hand, optical time division multiplexing uses n
This can be easily realized by multiplexing the Fbit / s signals with an optical multiplexing circuit while shifting the timing, whereby an n × Fbit / s signal can be obtained. For the multiplex division, a method using a LiNbO ₃ optical intensity modulator or an electroabsorption semiconductor modulator as a high-speed optical switch (for details, see RSTucker et al., "16 Gbit / s optical
time-division multiplexed transmission system exp
eriment ", Proceedings of Optical Fiber Communicati
on, ThB2 (1998).) or a method of selectively converting the signal of the channel to be separated into a different wavelength from the signal light by using stimulated four-photon mixing in an optical fiber or semiconductor (details) Describes PA Andrekson et a
l. , "16Gbit / s all = optical
damplexing using four
−wave mixing ”, Electronics
Letters, vol. 27, p. 922 (199
1). Non-linear loop mirror (NOLM: Non)
linear Loop Mirror or a nonlinear amplification loop mirror (NALM)
irror) as a high-speed optical switch (see KJBlow, NJDoran, and BPNelson, "Demonstrati
on of the nonlinear fiber loop mirror as an ultraf
ast all-optical demultiplexer ", Electronics Letter
s, vol. 26, p. 962 (1990)).

[0006]

In order to increase the transmission speed of the optical soliton, it is necessary to increase the multiplicity of the soliton pulse to shorten the interval between the optical solitons. It is necessary to reduce the ratio of the repetition period. However, if adjacent optical solitons have the same amplitude, the nonlinear interaction during the long-distance propagation of the optical solitons is required unless the ratio of the interval between the optical solitons and the pulse width of the optical solitons is 5 times or more. The optical soliton pulse is pulled or repelled by
There is a disadvantage that the pulse interval changes and a code error occurs during light reception.

[0007] In any of the multiplex division methods described above, a clock signal having a frequency F corresponding to the repetition frequency of the signal before multiplexing is required for switching. However, the n-channel optical soliton signal is
In the method of multiplexing so that the amplitudes of the respective pulses become equal, the frequency component of the frequency F is not included as a bright line spectrum in the multiplexed signal spectrum, and the clock component of the frequency F is efficiently converted. It was difficult to extract.

For this reason, several methods for obtaining a clock signal from a multiplexed signal have been proposed. Here, one of the methods for obtaining a clock signal from a multiplexed signal (see S.Ka
wanishi, T.Morioka, O.kamatani, H.Takara, and M.Saruwa
tari, "Time-division-multiplexed 100 Gbit / s, 200km
optical transmission experiment using PLL timingex
traction and all-optical demultiplexing based on p
olarization insensitive four-wave mixing ", Proceedi
ngs of Optical Fiber Communication, PD23 (1994).) will be described with reference to FIG.

In FIG. 8, reference numeral 20 denotes an optical multiplexer, reference numeral 21 denotes a semiconductor laser amplifier, reference numeral 22 denotes a bandpass filter, reference numeral 23 denotes a light receiver, reference numeral 24 denotes a phase comparator, reference numeral 25 denotes a low-frequency oscillator, and reference numeral 26 denotes a low-frequency oscillator.
Is a multiplier, 27 is a voltage controlled oscillator, 28 is a mixer,
9 is an optical clock pulse generator. In this configuration, the sine wave of frequency F from the voltage controlled oscillator 27 and the signal of frequency Δf from the low frequency
8 to prepare a signal of frequency F + Δf.

From this signal, an optical clock pulse generator 29
Generates an optical pulse having a frequency of F + Δf, and multiplexes this pulse with a multiplexed signal pulse using an optical multiplexer 20. The signal obtained by the multiplexing is coupled to the semiconductor laser amplifier 21 to generate a correlation signal between the multiplexed signal and the optical clock pulse by using stimulated four-photon mixing in the amplifier. The correlation signal component is separated by the band pass filter 22 and converted into an electric signal by the light receiver 23. Since this electric signal contains a frequency component of nΔf, the electric signal is compared with the phase of a signal obtained by multiplying the low-frequency oscillator 25 by n by the frequency multiplier 26 by the phase comparator 24, and the error signal is compared. By feeding back to the voltage controlled oscillator 27, nFbit / s
And a clock signal of FHz synchronized in phase with the multiplexed signal can be obtained.

However, the above method has a disadvantage that the circuit configuration becomes very complicated and is not suitable for an economical optical transmission system configuration. SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and facilitates extraction and demultiplexing of a demultiplexing clock signal in a light receiving unit and economical time-division multiplex transmission of an optical soliton over a long time. The aim is to provide a method.

[0012]

According to the present invention, a plurality of optical soliton light sources are periodically generated in a time-division manner from a plurality of optical soliton light sources, and light generated by the plurality of optical soliton light sources by an optical modulator. An optical soliton for receiving a light emitted from the optical fiber after digitally modulating the soliton to generate a soliton pulse, entering the soliton pulse into a single mode optical fiber for optical soliton transmission, and repeating propagation as an optical soliton. In the time-division multiplexing transmission method, optical solitons are generated in a time-division manner from the plurality of optical soliton light sources in a time-division manner, and the plurality of time-sequential optical solitons are multiplexed by a multiplexing device. At the fiber input end, the signals of each channel are multiplexed
Are equal time intervals, and solitons of each channel
The amplitude of the pulse has a deviation from the average amplitude, and extracts the clock signal of the pulse repetition frequency of the optical soliton light source before multiplexing from a part of the optical soliton after propagation at the light receiving end of the optical fiber. The demultiplexing optical switch is operated in synchronization with the clock signal to separate into the same number of channels as before the multiplexing, and each of the separated channels is received. In addition, the present invention
Optical solitons in time-sharing form from several optical soliton sources
While periodically generated, the plurality of optical modulators
Digital modulation of optical soliton generated by optical soliton light source
To create a soliton pulse, and apply the soliton pulse to an optical soliton.
Light enters the single mode optical fiber for riton transmission and is
Out of the optical fiber after repeated propagation
Time Division Multiplexing Transmission Method of Optical Soliton that Receives Light
The optical soliton from the plurality of optical soliton light sources.
Time-sequentially generated in the form of division
A plurality of sequential optical solitons are multiplexed by a multiplexing device, and
At the input end to the optical fiber, the multiplexed signal
Adjacent optical solitons have the same pulse amplitude and
The time intervals of the channels are equal, but every (n + 1)
Adjust the amplitude so that there is "0" data in the
Combined from a part of the optical soliton after propagation at the receiving end of the bar
The pulse repetition frequency of the optical soliton light source
The clock signal is extracted and synchronized with the clock signal.
To operate the demultiplexing optical switch, and
Separate into channels and receive each channel after separation
It is characterized by doing.

[0013]

According to the above-mentioned method, when the optical soliton signal is time-division multiplexed, the amplitude of the adjacent soliton pulse is changed to add the clock component of each channel before multiplexing to the spectrum of the multiplexed optical soliton signal. Include. For this reason, the demultiplexing clock signal is easily extracted in the light receiving unit. In addition, since the nonlinear interaction between adjacent optical solitons is reduced, the transmission capacity is increased as compared with normal optical soliton multiplexing.

[0014]

Embodiments of the present invention will be described below with reference to the drawings. [First Embodiment] First, a first embodiment will be described. FIG. 1 is a diagram for explaining a time-division multiplex transmission method of an optical soliton according to a first embodiment of the present invention. In FIG. 1, A ₁ ,..., An _denote digital data signal input terminals (n Is a natural number). L is an optical soliton light source, for example, a light source that performs gain switching and wavelength filtering, a mode-locked semiconductor laser, a mode-locked optical fiber laser, or the like is used. In this embodiment, the optical soliton light source L has a wavelength of 1.552.
μm, repetition frequency 10 GHz, pulse width 3 ps
It is assumed that a mode-locked erbium optical fiber laser is used. Further, reference numeral 4 denotes a 1: n optical branching circuit, which uses elements such as a planar optical waveguide and an optical fiber coupler.

[0015] _{M 1, ..., M n,} D 1, ..., D n, ATT
_{1, ..., ATT n, PS} 1, ..., PS n , respectively,
An optical intensity modulator, an optical modulator driving circuit, an optical attenuator, and an optical phase shifter. Reference numeral 5 denotes an optical multiplexer, and reference numeral 6 denotes an optical amplifier. For example, an erbium-doped optical fiber amplifier is used.
Reference numeral 7 denotes an optical soliton transmission fiber 7, for example, a dispersion-shifted fiber that causes anomalous dispersion at a signal light wavelength (1.552 μm in this embodiment). Further, 8 is an optical branching circuit, 9 is a clock regenerator, 1
0 is a 1: n optical branching circuit, and 11 is a clock distribution circuit. Also, SW ₁ ,..., SW _n are high-speed optical switches for demultiplexing, 13 is a light receiver, and 14 is an identification circuit.

The time division multiplex transmission method of the optical soliton realized by the above configuration will be described below. First, an optical pulse output from the optical soliton light source L is branched by the optical branching circuit 4 into n channels. Next, for each channel i (i is a natural number equal to or less than n), the optical modulator driving circuit D _i
But drives the optical intensity modulator M _i according to the digital data input to the data input terminal A _i. Thus, the n-channel optical pulse branched by the optical branching circuit 4 is modulated.

[0017] Here, a pulse repetition frequency F of the optical soliton light source L, and the digital data signal speed input to the data input terminal A _i, is set in advance so as to be identical. Here, as digital data, 10
A Gbit / s NRZ signal is used. The pulse width of the optical soliton light source L is set to be smaller than 1/3 of the repetition period of the multiplexed signal given by 1 / nF in order to prevent non-linear interaction between the solitons. .

Next, the signal of each channel after modulation, the amplitude using the optical attenuator ATT _i, adjusted to to produce a deviation of about 10 to 20 percent relative to the average amplitude.
Thereafter, using the optical phase shifter PS _i , the timing is adjusted so that each channel has a time interval of 1 / n of the repetition period of the optical soliton light source L and the channel numbers i are arranged in order from 1 to n. Input to the device 5.

FIG. 2 shows the state of the time division multiplexing described here. In this figure, L is an output pulse of the soliton light source L, A ₁ ,..., _An are data input terminals A ₁ ,.
NRZ data signal input to the _{n, B 1, ..., B} n is
Modulated by the optical modulators M ₁ ,..., M _n and the optical attenuator ATT
_1, ..., multiplexed prior signal of each channel of the amplitude and phase are adjusted by the ATT _n and optical phase shifter PS _i, C
Shows the signal waveform after multiplexing. As is clear from this figure, in the multiplexed signal, the pulse repetition cycle is at equal intervals, but the amplitudes of adjacent pulses are different.

In this embodiment, the multiplexed signal is amplified by the optical amplifier 6 and propagated through the optical soliton transmission fiber 7. Note that the peak amplitude of the pulse coupled to the optical soliton transmission fiber 7 is equal to one relay distance (2
0-50 km) is set under the condition that the pulse width does not change after propagation. For example, when the average optical loss of the transmission fiber 7 is 0.22 dB / km, the peak amplitude is set to about 1.2 to 1.7 times the amplitude of the basic soliton.

As described above, after optical amplification and propagation are repeated in multiple stages, the optical soliton signal is demultiplexed and received. The waveform of the optical soliton after propagation becomes the waveform indicated by D in FIG. 2, and the deterioration from the transmitted optical waveform C is extremely small. Then, in the signal receiving section, first, the optical branching circuit 8
, A part of the signal is extracted and input to the clock regenerator 9. The clock regenerator 9 extracts the frequency component of the basic repetition period before multiplexing from the multiplexed signal after propagation. As shown by C in FIG. 2, the signal used for transmission changes the amplitude of each channel in advance at the time of multiplexing, so that its frequency spectrum includes a component of the basic repetition frequency F × n ( In addition to the bit rate in the transmission path, a component of the basic repetition frequency F is always included.)

Here, FIGS. 3 (a), 3 (b), 4
With reference to (a) and FIG. 4 (b), the calculation result of the frequency spectrum of the optical soliton signal modulated by the pseudo random pattern will be discussed. FIG. 3A shows a channel i before multiplexing.
In the signal spectrum, a bright line spectrum corresponding to the repetition frequency F and the harmonic component of the signal is seen. In addition,
The frequency on the horizontal axis indicates a value normalized by F. Also,
3 (b), 4 (a), and 4 (b) show spectra when four signals having the waveform shown in FIG. 3 (a) are multiplexed. The multiplexing was performed using optical waveguides composed of Mach-Zehnder interferometers having different arm lengths, and was set so as to give a time difference of 3/2 times and 3/4 times the basic repetition period.

FIG. 3B shows a signal spectrum when multiplexing is performed so that the amplitude of each channel becomes equal.
FIG. 4A shows a spectrum when the intensity of one of the four channels to be multiplexed is set to be 1.1 times that of the other channels, and FIG. 4B shows that the intensity ratio is 1.2 times. FIG. As shown in FIG. 3B, when multiplexing is performed by the conventional method, only a component of the basic repetition frequency F × n and a broad frequency component (by a random data signal) can be seen in the signal spectrum. It is. In this case, it is difficult to reproduce a clock signal for demultiplexing from the spectrum shown in FIG.

However, in the case of FIGS. 4A and 4B, there is a basic repetitive component as a bright line spectrum. Therefore, the clock regenerator 9 includes the optical branching circuit 8
After receiving and amplifying the optical signal from the optical receiver with a high-speed light receiver, a desired frequency component is extracted by interposing a narrow-band (high Q value) band-pass filter that passes only the basic repetition frequency component F. It is possible. Q value is 1000
Since it is sufficiently possible to use a band pass filter of about 10 to about 10 to 20%, it is sufficient to change the intensity of each channel at the time of multiplexing.

The remainder of the light signal of the optical branching circuit 8, 1: n optical branching circuit 10 is branched into n channels, each channel using the demultiplexing high-speed optical switch SW _i, separated into each channel component Is done. Demultiplexing is
The clock signal obtained frequency F in the clock regenerator 9, carried out by supplying to each demultiplexing for high-speed optical switch SW _i. Demultiplexing clock signal is supplied to the demultiplexing for high-speed optical switch SW _i in a timing that can sample the signal of the desired channel.

The signals of each channel separated by the demultiplexing for high-speed optical switch SW _i is shown in FIG. 2 E _1, ..., represented by E _n. As apparent from FIG, E _1, ..., the signal waveform represented by E _n, B _1, equal to the optical pulse waveform of each channel of the transmitter shown in ... B _n. Each of the separated channels is converted into an electric signal by the photodetector 13 and the original NRZ signal is reproduced by the identification circuit 14. Here, the waveform demultiplexing for high-speed optical switch SW _i mentioned, for example, non-linear loop mirror (NOLM: Nonlinear OpticalLoop Mirr
or a nonlinear amplification loop mirror (NAL)
M: Nonlinear Amplifing Loop Mirror)
It is also possible to use a device using stimulated four-photon mixing in an optical fiber or LD amplifier, a Mach-Zehnder type light intensity modulator such as LiNbO _3, or an electroabsorption type light modulator.

[Second Embodiment] Next, a second embodiment of the present invention will be described. FIG. 5 is a diagram for explaining a time-division multiplexing transmission method of an optical soliton according to the second embodiment. In this figure, the same reference numerals are given to parts common to the respective parts in FIG. Configuration shown in FIG. 5 differs from that shown in FIG. 1, the optical attenuator ATT _1, ..., to remove the ATT _n, the optical intensity modulator M _1, ..., each output of the M _n, directly corresponding optical phase shifter PS ₁ of, ..., a point that is configured to enter into PS _n.

The time division multiplex transmission method of the optical soliton realized by the above configuration will be described below. First, similarly to the first embodiment, the optical pulse output from the optical soliton light source L is branched into n channels by the optical branching circuit 4, and for each channel, the optical modulator driving circuit Di _connects the data input terminal A modulating the pulse by driving the light intensity modulator M _i according to the digital data input from the _i. Next, using the optical phase PS _i , the timing is adjusted so that the channel numbers are arranged in the order of 1 to n at 1 / (n + 1) time intervals of the repetition period of the optical soliton light source L,
Input to the optical multiplexing device 5.

FIG. 6 is a diagram showing the waveform of each part (eye _{pattern), L, A 1, ...} , E n , similarly to FIG. 2 represents a portion corresponding waveform is obtained. As can be seen from this figure, the multiplexed signal has the same pulse amplitude and the repetition period is equally spaced, but there is "0" data for every (n + 1) signals.

The multiplexed signal is propagated through the optical soliton transmission fiber 7 in multiple stages while amplifying the signal using the optical amplifier 6 so that the pulse width of the propagating optical soliton does not change. The signal is demultiplexed and received. As shown by D in FIG. 6, the waveform of the optical soliton after propagation has extremely small deterioration from the transmitted optical waveform C. Then, in the light receiving section of the signal, a part of the signal is first extracted by the optical branching circuit 8 and input to the clock regenerator 9 to extract the frequency component of the basic repetition before multiplexing.

As shown by C in FIG. 6, since the signal used for transmission has "0" data every (n + 1) bits, its frequency spectrum has a basic repetition frequency F × (n + 1). Component (bit rate in the transmission path)
In addition, the component of the basic repetition frequency F is always included. FIGS. 7A and 7B show spectra of a multiplexed signal calculated using a pseudo random pattern. FIG.
(A) and FIG. 7 (b) show the results in the case of 4 multiplexing and 8 multiplexing, respectively. In addition to the components of 5F and 9F corresponding to the bit rate in the transmission path, there is a component of the frequency F of the basic repetition. You can see that. Therefore, a clock signal for demultiplexing can be easily extracted.

In the present embodiment, demultiplexing is performed in the same manner as in the first embodiment. In Figure 6 E ₁ signals of each channel separated by the demultiplexing for high-speed optical switch SW _i, ..., shown by E _n. The signal waveform B _1, ..., it is found equal to the optical pulse waveform of each channel of the transmitter shown in B _n. Each of the separated channels is converted into an electric signal by the optical receiver 13 and the original NRZ signal is reproduced by the identification circuit 14.

As described above, according to the first and second embodiments of the present invention, by changing the amplitude of the time-division multiplexed optical soliton, the nonlinear interaction of the soliton can be reduced as compared with the conventional one. Can be reduced. Also, if the pulse amplitude ratio is set to 1.2 and the ratio between pulse interval and pulse width is set to 4 or more, soliton transmission with almost no change in pulse interval and almost constant pulse interval is realized. can do.

[0034]

As described above, according to the present invention,
Since the amplitude of the soliton pulse after multiplexing changes with the repetition period of the signal before time-division multiplexing, the clock signal extraction processing required for demultiplexing at the light receiving unit is compared with the conventional method. There is an effect that an ultrahigh-speed optical soliton transmission system can be realized with a simple and relatively simple configuration. Further, since the interaction between the pulses is smaller than that of the conventional method using optical solitons, even if optical pulses having the same pulse width are used, the interval between the soliton pulses can be reduced. Therefore, there is an effect that optical transmission with a high bit rate can be realized.

[Brief description of the drawings]

FIG. 1 is a diagram for explaining a time division multiplex transmission method of an optical soliton according to a first embodiment of the present invention.

FIG. 2 is a diagram showing waveforms at respective parts shown in FIG.

FIG. 3 is a diagram illustrating a frequency spectrum of a signal when the time-division multiplexing transmission method of the optical soliton according to the first embodiment of the present invention is applied.

FIG. 4 is a diagram showing the same frequency spectrum.

FIG. 5 is a diagram for explaining a time division multiplex transmission method of an optical soliton according to a second embodiment of the present invention.

FIG. 6 is a diagram showing waveforms at respective parts shown in FIG.

FIG. 7 is a diagram illustrating a frequency spectrum of a signal when a time division multiplex transmission method of an optical soliton according to a second embodiment of the present invention is applied.

FIG. 8 is a block diagram showing a configuration of a clock extraction device using a conventional PLL circuit.

[Explanation of symbols]

4, 8, 10 Optical branching circuit 5 Optical multiplexer 6 Optical amplifier 7 Optical fiber for optical soliton transmission 9 Clock regenerator 11 Clock distribution circuit 13 Optical receiver 14 Identification circuit A ₁ ,..., _An data signal input terminal ATT ₁ , , ATT _n optical attenuator D ₁ ,..., D _n optical modulator driving circuit L optical soliton light source M ₁ ,..., M _n optical intensity modulator PS ₁ ,..., PS _n optical phase shifter SW ₁ ,. , SW _n High-speed optical switch for demultiplexing

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int. Cl. ⁷ , DB name) H04B 10/00-10/28 H04J 14/00-14/08 H04J 3/00 JICST file (JOIS)

Claims (2)

(57) [Claims] An optical soliton is periodically generated in a time-sharing manner from a plurality of optical soliton light sources, and an optical modulator digitally modulates the optical soliton generated by the plurality of optical soliton light sources, soliton pulses. In the time-division multiplexing transmission method of the optical soliton, which receives the light emitted from the optical fiber after repeating the propagation as the optical soliton, the soliton pulse is incident on the optical soliton transmission single mode optical fiber, While generating the optical solitons from the plurality of optical soliton light sources in a time-sequential manner in a time-division manner, the time-sequential optical solitons are multiplexed by a multiplexing device, and each channel is formed at an input end of the optical fiber. Signal after multiplexing
Are arranged at equal time intervals, and
The amplitude of the impulse has a deviation from the average amplitude, and extracts a clock signal of the pulse repetition frequency of the optical soliton light source before multiplexing from a part of the optical soliton after propagation at the light receiving end of the optical fiber. Time division multiplex transmission of an optical soliton, wherein an optical switch for demultiplexing is operated in synchronization with the clock signal to separate the same number of channels as before the multiplexing and to receive each of the separated channels. Method. (2) Time-sharing form from multiple optical soliton sources
Periodically generates optical solitons in the
Optical soliton generated by the plurality of optical soliton light sources in a vessel
Is digitally modulated to create a soliton pulse,
Ton pulse into single mode optical fiber for optical soliton transmission
After being incident and repeating propagation as an optical soliton, the light
Time and minute of optical soliton receiving light emitted from fiber
In the split multiplex transmission method, The optical solitons are time-divided from the plurality of optical soliton light sources.
Generated in a time-sequential manner, and
A number of optical solitons are multiplexed by a multiplexing device,
At the input end to the bar, the multiplexed signals are temporally adjacent
Optical solitons have equal pulse amplitudes,
The time intervals are equal, but “0”
Adjust the amplitude so that the data
From the part of the optical soliton after propagation to the light before multiplexing
Clock signal of pulse repetition frequency of soliton light source
Extract and demultiplex in synchronization with the clock signal
for Activate the optical switch to set the same number of channels as before
And receive each channel after separation.
Characteristic time-division multiplex transmission method for optical solitons.