JPH11252013A - Optical transmission system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To minimize waveform degradation at the time of transmitting single wavelength light or wavelength multiplex signal light. SOLUTION: In an optical transmission means, signal light intensity-modulated by data signals and optical phase-modulated in synchronism with a clock frequency so as to make the optical frequency of an optical pulse first half part higher than an optical carrier frequency and to make the optical frequency of an optical pulse second half part lower than the optical carrier frequency is outputted. In the optical transmission means 10a, at the time of defining an optimum phase modulation degree for minimizing the waveform degradation when a signal light wavelength is the average zero dispersion wavelength λ0 of the entire transmission line as mopt , the phase modulation degree is set to be larger than mopt in the case that the signal light wavelength is shorter than λ0 and the phase modulation degree is set to be smaller than mopt in the case that the signal light wavelength is longer than λ0 .

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an optical transmission system for transmitting single wavelength light or wavelength multiplexed signal light. In particular, waveform degradation due to the combined effect of self-phase modulation and chromatic dispersion, which are non-linear effects in single-wavelength optical transmission, and the non-linear effects of optical fibers (four-wave mixing, cross-phase modulation) unique to WDM transmission. The present invention relates to a technique for improving waveform deterioration due to a combined effect with chromatic dispersion.

[0002]

2. Description of the Related Art In an optical transmission system, there is a problem of waveform deterioration due to a combined effect of chromatic dispersion and nonlinear effect of an optical fiber. Non-linear effects include self-phase modulation in single wavelength optical transmission, four-wave mixing and cross-phase modulation in optical wavelength multiplex transmission.

In self-phase modulation, phase modulation occurs in the signal light itself when the refractive index of the optical fiber changes according to the intensity of the signal light, and the optical frequency at the rising portion of the optical pulse is lower than the optical carrier frequency. This is a phenomenon in which the optical frequency of the falling part increases. Four-wave mixing and cross-phase modulation are caused by the interaction between signal lights when a plurality of signal lights having different wavelengths propagate in the transmission line optical fiber. The phenomenon in which signal light components corresponding to the respective wavelength differences are generated by this interaction is four-wave mixing, and the phenomenon in which signal light undergoes phase modulation is cross-phase modulation.

As means for improving the waveform deterioration due to the combined effect of the nonlinear effect and the chromatic dispersion of such an optical fiber,
While setting the dispersion value of the transmission line optical fiber large,
A method is known in which a dispersion compensator having a dispersion value opposite to that of a transmission line optical fiber is disposed at every fixed distance ("Study of 10 Gbit / s / ch WDM transmission system using dispersion management", IEICE Technical Report, OCS 96-57).

In order to suppress cross-phase modulation, a method is known in which the signal format is an RZ (Return-to-Zero) intensity modulation signal. For example, see the literature (“Optimization of signal waveform in long-distance optical amplification relay transmission system”, IEICE Technical Report, OCS97-4
4) shows an example in which the pulse occupancy of the RZ intensity modulation signal is changed and the waveform deterioration due to the cross phase modulation is numerically analyzed.

[0006]

Conventionally, the dispersion compensation interval and the pulse occupancy of the signal waveform have been optimized to improve the waveform deterioration. However, in long-distance transmission, self-phase modulation and cross-phase modulation have been performed. The influence of modulation became large, and there was a limit to the improvement of waveform deterioration. Also, in the short-distance transmission, when the signal light wavelength is different from the average zero dispersion wavelength of the entire transmission path, the accumulated dispersion caused by the dispersion slope becomes large, and the waveform is deteriorated.

An object of the present invention is to provide an optical transmission system capable of minimizing waveform deterioration when transmitting single-wavelength light or wavelength-multiplexed signal light.

[0008]

According to an optical transmission system of the present invention, in an optical transmitting means, intensity modulation is performed by a data signal, an optical frequency of a first half of an optical pulse is higher than an optical carrier frequency, and an optical frequency of a second half of the optical pulse is increased. A signal light that is optically phase-modulated in synchronization with a clock frequency so that an optical frequency is lower than an optical carrier frequency is output (claim 1).

The optical transmission means for that purpose includes, for example, an intensity modulator, a phase modulator, a clock signal source, and a phase shifter. The intensity modulator generates a signal light intensity-modulated by the data signal. The phase modulator phase-modulates the intensity-modulated signal light. The phase shifter controls the intensity modulation and phase in the intensity modulator so that the optical frequency of the first half of the optical pulse of the intensity modulated signal light is higher than the optical carrier frequency and the optical frequency of the second half of the optical pulse is lower than the optical carrier frequency. A relative phase difference between the modulator and the phase modulation (hereinafter, referred to as “relative phase difference between the intensity and the phase modulation”) is adjusted. The clock signal source supplies a clock signal for operating the intensity modulator and the phase modulator in synchronization.

In the optical transmission means, if the optimum phase modulation degree at which the waveform deterioration is minimized when the signal light wavelength is the average zero dispersion wavelength λ ₀ of the entire transmission line is m _opt , the signal light wavelength becomes λ _{When the} wavelength is shorter than ₀ , the phase modulation degree is set to be larger than m _opt , and when the signal light wavelength is longer than λ ₀ , the phase modulation degree is set to be smaller than m _opt ( Claim 3).

Further, the configuration of the optical transmission system described above can be applied to a system for transmitting a wavelength multiplexed signal light. That is, the optical transmission unit outputs a plurality of intensity-modulated signal lights having different wavelengths from each other, and performs optical phase modulation on each of the intensity-modulated signal lights at a predetermined phase modulation degree. Means for wavelength-multiplexing and transmitting the intensity-modulated signal light. The optical receiving means includes means for splitting the wavelength multiplexed signal light into intensity-modulated signal light of each wavelength, and receiving the intensity-modulated signal light of each wavelength with dispersion compensation (claim 4).

Further, if a transmission line optical fiber having a normal dispersion (anomalous dispersion) is used, the pulse width and the spectrum width of the signal light are widened, and the transmission distance is restricted due to waveform deterioration. For this reason, in the optical transmission system of the present invention, a transmission line optical fiber having an average chromatic dispersion value of negative dispersion is used, and the chromatic dispersion of the transmission line optical fiber is compensated at a predetermined position of the transmission line optical fiber. Dispersion compensating means is inserted (claim 5). The average chromatic dispersion value is -1.
By setting the ratio to ps / nm / km or less, waveform deterioration due to four-wave mixing can be improved. As dispersion compensation means,
For example, a dispersion compensating optical fiber or an optical fiber grating can be used.

Further, an optical amplifier for amplifying the signal light may be inserted at a predetermined position of the transmission line optical fiber.

[0014]

(First Embodiment) FIG. 1 shows a first embodiment of the optical transmission system of the present invention.

The system shown here is an optical transmitter 10a
, An optical transmission line 20, and an optical receiver 30a. In the optical transmitter 10a, the CW light (continuous light) having the wavelength λ output from the light source 11 is converted into a 10 Gbit / s NRZ (Non-
(Return-to-Zero) The intensity is modulated using the random pulse signal, and further the phase is modulated by the phase modulator 13. Here, the intensity modulator 12 and the phase modulator 14 operate in synchronization with the 10 GHz clock signal output from the clock signal source 15, and the relative phase difference between the intensity-phase modulation is
It is adjusted by.

In an actual optical transmission system, a data signal for intensity-modulating CW light is a digital signal containing audio and video information input from the outside. In this embodiment, a random pulse signal capable of simulating various signal patterns is experimentally used instead of the data signal.

The optical transmission line 20 includes a transmission line optical fiber 21 for transmitting the signal light, an optical amplifier 22 for amplifying the signal light transmitted by the transmission line optical fiber 21, and an optical amplifier 22 inserted into the transmission line optical fiber 21. And a dispersion compensating medium 23 for compensating the chromatic dispersion of the transmission line optical fiber 21. As the dispersion compensating medium 23, for example, a dispersion compensating optical fiber or an optical fiber grating is used.

FIG. 2 shows the intensity-
The result of numerical analysis of the relationship between the phase modulation degree m of the phase modulator 13 and the eye opening deterioration with respect to the relative phase difference 間 の between the phase modulations is shown. The parameters were an average chromatic dispersion value of the transmission line optical fiber 21, -1 ps / nm / km, a dispersion compensation interval of 500 km, an optical fiber loss of 0.21 dB / km, an optical amplifier relay interval of 50 km, and a receiving optical filter band of 1 nm. Further, the signal light wavelength is set to the average zero dispersion wavelength λ ₀ (1558 nm) of the entire transmission line, the optical amplifier output −3.
The evaluation was performed with dB / ch and a transmission distance of 6000 km.

Here, the eye opening deterioration is caused by the eye opening a of the eye pattern of the transmission signal (random signal) shown in FIG.
And the ratio (−20 log (b / a)) of the received signal of FIG. 3B to the eye opening b of the received signal.

The relative phase difference 間 の between the intensity-phase modulation is shown in FIG.
As shown in (a), when the optical frequency of the first half of the optical pulse of the intensity-modulated signal light is higher than the optical carrier frequency and the optical frequency of the second half of the optical pulse is lower than the optical carrier frequency, it is defined as 0 rad. 4 (b) shows the case where Ψ = π / 2, FIG. 4 (c) shows the case where Ψ = π, and FIG. 4 (d) shows the case where ／ = 3π / 2.

As shown in FIG. 2, the optimum relative phase difference が with small eye opening deterioration is 0ra regardless of the phase modulation degree m.
d. This is because when the relative phase difference Ψ = 0, the eye opening is maximized due to the effects of phase modulation and pulse compression due to chromatic dispersion of the transmission line optical fiber. Also,
When the phase modulation degree m = π / 4, the eye opening degradation is minimized, so the optimal phase modulation degree m _opt is π / 4. In addition,
The phase modulation degree m is set by the amplitude of the clock signal source 15.

FIG. 5 shows the results of numerical analysis of the relationship between the signal light wavelength and the eye opening deterioration with respect to the phase modulation degree m when the relative phase difference Ψ = 0. Note that the signal light wavelength is represented by a deviation (λ−λ ₀ ) from the average zero dispersion wavelength λ _{0 of the} entire transmission line. After passing through the reception optical filter, the received signal compensated for the accumulated dispersion for each signal light wavelength caused by the dispersion slope (0.07 ps / nm ² / km).

It can be seen that the eye opening deterioration at each signal light wavelength greatly depends on the phase modulation degree m. When the signal light wavelength λ is shorter than λ ₀ , when the optimum phase modulation degree m _opt = π / 4, which is understood from FIG.
π) has a remarkable improvement effect. This is because the eye opening deterioration occurs due to the negative cumulative dispersion of the transmission path, and the pulse compression occurs more effectively by performing excessive phase modulation than at the time of transmission at λ ₀ , and the waveform deterioration can be improved. It is.

Conversely, when the signal light wavelength λ is longer than λ ₀ , the improvement effect is obtained in a wide wavelength region when the optimum phase modulation degree m _opt = π / 4 (m = π / 8). was gotten. This is because pulse compression due to the positive cumulative dispersion of the transmission line occurs, and thus even if the degree of phase modulation is reduced, deterioration of the eye opening can be obtained.

(Second Embodiment) FIG. 6 shows a second embodiment of the optical transmission system of the present invention. The optical wavelength multiplexing transmission terminal 10b includes light sources 11a to 11d and an intensity modulator 12a.
To 12d, phase modulators 13a and 13b,
a to 17c. The random data transmitter 14, clock signal source 15, and phase shifter 16 shown in FIG.
Has been omitted.

The light sources 11a to 11d have wavelengths λ ₁ , λ ₂ ,
The CW light of λ ₃ and λ ₄ is output. Each wavelength is equally spaced around the average zero dispersion wavelength λ ₀ of the entire transmission line (wavelength interval 1n
m). Here, λ ₁ and λ ₂ have shorter wavelengths than λ ₀ , and λ ₃ and λ ₄ have longer wavelengths than λ ₀ . CW of each wavelength
The light is intensity-modulated by the intensity modulators 12a to 12d using a 10 Gbit / s NRZ random pulse signal. Wavelength λ ₁ ,
The intensity-modulated signal light of λ ₂ is multiplexed by the multiplexer 17a, and phase-modulated by the phase modulator 13a at π larger than the optimum phase modulation degree m _opt = π / 4. The intensity-modulated signal lights of the wavelengths λ ₃ and λ ₄ are multiplexed by the multiplexer 17b, and phase-modulated by the phase modulator 13b at π / 8 smaller than the optimum phase modulation degree m _opt = π / 4. The intensity-modulated signal light phase-modulated by each phase modulator is multiplexed by the multiplexer 17c.

The intensity modulators 12a to 12d and the phase modulators 13a and 13b operate in synchronization with the 10 GHz clock signal as in FIG. 1, and the relative phase difference 強度 between the intensity-phase modulation is 0 rad.

The optical transmission line 20 includes a transmission line optical fiber 21 for transmitting a wavelength multiplexed signal light, an optical amplifier 22 for amplifying the wavelength multiplexed signal light, and a transmission line optical fiber 21 inserted into the transmission line optical fiber 21 at regular intervals. And a dispersion compensating medium 23 for compensating the chromatic dispersion of the optical path fiber 21.

The optical wavelength division multiplex receiving terminal 30b is a demultiplexer 31 for demultiplexing the transmitted wavelength division multiplex signal light into signal light of each wavelength.
And dispersion compensating media 32a to 32d for dispersion-compensating the signal light of each wavelength, and optical receivers 33a to 33d for receiving the dispersion-compensated signal light of each wavelength.

FIG. 7 shows the result of numerical analysis of the relationship between the signal light wavelength and the eye opening deterioration with respect to the phase modulation m during four-wave multiplex (wavelength interval 1 nm) transmission. Various parameters are the same as in the first embodiment. (2) shows a case where two wavelengths shorter than λ ₀ are phase-modulated with a phase modulation factor π, and two wavelengths longer than λ ₀ are phase-modulated with a phase modulation factor π / 8. , △ are the cases where m = π / 8 is fixed. As a result, even in optical wavelength multiplexing transmission where there is concern about four-wave mixing and cross-phase modulation, the present optical transmission system reduces eye opening deterioration by 1.5 dB or more compared to the case where phase modulation is performed at the same phase modulation factor for each channel. Thus, an eye opening improvement effect almost similar to that at the time of single wavelength transmission was obtained.

FIG. 8 shows the results of numerical analysis of the relationship between the signal light wavelength and the eye opening deterioration with respect to the phase modulation m during eight-wave multiplex (wavelength interval 1 nm) transmission. Various parameters are the same as in the first embodiment. (2) shows a case where four wavelengths shorter than λ ₀ are phase-modulated with a phase modulation factor of π, and four wavelengths longer than λ ₀ are phase-modulated with a phase modulation factor of π / 8. , △ are the cases where m = π / 8 is fixed. As a result, an eye opening improvement effect of 0.5 dB or more was confirmed as compared with the case where the phase modulation was performed at the same phase modulation degree for each channel.

In the above-described embodiment, the description has been made on the condition that the data signal is the NRZ signal. However, even if the pulse occupancy of the transmission signal changes, the optical frequency of the first half of the optical pulse is higher than the optical carrier frequency, and The same effect can be obtained by setting the relative phase difference between intensity and phase modulation (Ψ = 0) so that the optical frequency of the latter half of the pulse is lower than the optical carrier frequency, and performing optical phase modulation.

[0033]

As described above, in the optical transmission system of the present invention, the optical frequency of the first half of the optical pulse is higher than the optical carrier frequency, and the optical frequency of the second half of the optical pulse is lower than the optical carrier frequency. By performing optical phase modulation on the waveform, waveform deterioration can be improved.

When the wavelength of the signal light is shorter than the average zero dispersion wavelength λ _{0 of the} entire transmission line, the phase modulation degree is determined by the optimum phase modulation degree m _opt (= π / 4) at the time of transmission of the average zero dispersion wavelength. Is set to be large, and in the case of a long wavelength, it is set to be small, so that waveform deterioration can be improved.

The present optical transmission system has a great effect of improving waveform deterioration in optical wavelength multiplex transmission in which each signal light wavelength is different from the average zero dispersion wavelength of the entire transmission line.

[Brief description of the drawings]

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

FIG. 2 is a diagram illustrating phase modulation degree dependency of eye opening deterioration in single wavelength transmission.

FIG. 3 is a diagram illustrating the definition of eye opening deterioration.

FIG. 4 is a view for explaining the definition of a relative phase difference 間 の between intensity-phase modulation.

FIG. 5 is a diagram illustrating signal light wavelength dependence of eye opening deterioration in single wavelength transmission.

FIG. 6 is a block diagram showing a second embodiment of the optical transmission system of the present invention.

FIG. 7 is a diagram illustrating signal light wavelength dependence of eye opening deterioration in four-wave multiplex transmission.

FIG. 8 is a diagram showing signal light wavelength dependence of eye opening deterioration in eight-wave multiplex transmission.

[Explanation of symbols]

 Reference Signs List 10a Optical transmitter 10b Optical wavelength division multiplexing transmitting terminal station 11, 11a-11d Light source 12, 12a-12d Intensity modulator 13, 13a-13d Phase modulator 14 Random data transmitter 15 Clock signal source 16 Phaser 17a-17c Device 20 Optical transmission line 21 Transmission line optical fiber 22 Optical amplifier 23 Dispersion compensating medium 30a, 33a to 33d Optical receiver 30b Optical wavelength division multiplex receiving terminal 31 Demultiplexer 32, 32a to 32d Dispersion compensating medium

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁶ Identification code FI H04J 14/02 H04B 10/02 10/18

Claims (6)

[Claims] 1. An optical transmission system for transmitting a signal light output from an optical transmitting means to an optical receiving means via a transmission line optical fiber, wherein the optical transmitting means modulates the intensity by a data signal and outputs the first half of an optical pulse. The optical frequency of the optical pulse is higher than the optical carrier frequency, and the optical phase modulated signal light is output in synchronization with the clock frequency so that the optical frequency of the latter half of the optical pulse is lower than the optical carrier frequency. Optical transmission system. 2. An optical transmitter, comprising: an intensity modulator for generating signal light intensity-modulated by a data signal; a phase modulator for phase-modulating the intensity-modulated signal light; and a first half of an optical pulse of the intensity-modulated signal light. The relative position between the intensity modulation in the intensity modulator and the phase modulation in the phase modulator so that the optical frequency of the optical modulator is higher than the optical carrier frequency and the optical frequency of the latter half of the optical pulse is lower than the optical carrier frequency. The optical transmission system according to claim 1, further comprising: a phase shifter that adjusts a phase difference; and a clock signal source that supplies a clock signal for operating the intensity modulator and the phase modulator in synchronization. 3. When the optimal phase modulation degree that minimizes waveform degradation when the signal light wavelength is the average zero dispersion wavelength λ ₀ of the entire transmission line is m _opt , the signal light wavelength is shorter than λ ₀ . In this case, the phase modulation degree is set to be larger than m _opt , and when the signal light wavelength is longer than λ ₀ , the phase modulation degree is set to m _opt
The optical transmission system according to claim 1, wherein the optical transmission system is set to be smaller than the optical transmission system. 4. An optical transmission means comprising: means for outputting a plurality of intensity-modulated signal lights having different wavelengths; and optical phase modulation for each of the intensity-modulated signal lights at a predetermined phase modulation factor. Means for wavelength-multiplexing and transmitting each of the intensity-modulated signal lights obtained, wherein the optical receiving means demultiplexes the wavelength-multiplexed signal light into intensity-modulated signal lights of each wavelength, and separates the intensity-modulated signal light of each wavelength. 2. The apparatus according to claim 1, further comprising means for receiving dispersion-compensated data.
4. The optical transmission system according to any one of claims 1 to 3. 5. A configuration in which the average chromatic dispersion value of the transmission line optical fiber is negative dispersion, and dispersion compensation means for compensating for the chromatic dispersion of the transmission line optical fiber is inserted at a predetermined position of the transmission line optical fiber. The optical transmission system according to claim 1, wherein: 6. The optical transmission system according to claim 1, wherein an optical amplifier for amplifying signal light is inserted at a predetermined position of the transmission line optical fiber.