JP3536688B2 - WDM optical transmission system and method, and optical transmitter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to a WDM (wavelength division multiplexing) optical transmission system and method, and an optical transmitter.

[0002]

2. Description of the Related Art In wavelength division multiplexing optical communication, a wavelength interval of 100 GHz is standardized by ITU recommendations. However, when a WDM signal having the same wavelength interval is transmitted near the zero dispersion wavelength of the optical fiber transmission line, crosstalk occurs due to four-photon mixing, and the transmission characteristics deteriorate. As a means for solving this, a WDM transmission system that makes the wavelength intervals non-uniform has been proposed (for example, F. Forghier).
i et al., IEEE Photonics Technology
oggy Letters, vol. 6, No. 6, p
p. 754-756, 1994). For example, if the adjacent wavelength intervals are 90 GHz, 120 GHz, 110 GHz,
The intervals are unequal, such as 80 GHz, 130 GHz and 70 GHz.

As a technique for improving the transmission characteristics of an optical pulse, a method of performing phase modulation (prechirping) on an optical pulse on the transmission side is known (for example, NS Ber.).
gano et al., OFC'98, Paper PD12,1
998).

[0004]

The prechirp technique is
There is a disadvantage that the signal spectral line width is widened. Experiments have shown that in the prechirp technique, the modulation degree of the phase modulation must be increased as the signal wavelength moves away from the zero dispersion wavelength of the transmission line. However, when the modulation degree of the phase modulation is increased, the signal spectral line width is expanded correspondingly, and it becomes easy to interfere with an adjacent wavelength.

FIG. 11 shows an example of a spectrum after multiplexing 16 wavelengths at intervals of 0.6 nm and transmitting 9000 km. Here, the signal light of all the channels CH1 to CH16 is phase-modulated at a phase modulation degree such that the transmission characteristic of the center CH8 is optimized and transmitted to the transmission line. Note that the zero dispersion wavelength is located between CH7 and CH8. Although the spectra of CH6 and CH7 somewhat overlap, the channels CH1 to CH16 are well separated as a whole.

FIG. 12 shows an example of a spectrum after multiplexing 16 wavelengths at intervals of 0.6 nm and transmitting 9000 km. However, here, all the channels CH1 to CH4 have a phase modulation degree such that the transmission characteristics of CH2 located at the end are optimized.
The signal light of CH16 was phase-modulated and transmitted to the transmission path. The zero dispersion wavelength is located between CH7 and CH8. As can be seen from FIG. 12, the spread of the spectral line width of each of the channels CH1 to CH16 is large and overlaps with the adjacent wavelength.

An object of the present invention is to provide an optical transmission system and method capable of transmitting light of many wavelengths over a longer distance while improving transmission characteristics by a prechirp technique, and an optical transmission apparatus therefor.

[0008]

An optical transmission system according to the present invention comprises: a signal light generating means for generating a plurality of signal lights of different wavelengths; and a phase modulation of each signal light output from the signal light generating means. Phase modulation means, multiplexing means for multiplexing each signal light output from the phase modulation means, optical fiber transmission line for transmitting the signal light multiplexed by the multiplexing means, and optical fiber transmission line And a receiving device that receives the signal light from the optical fiber transmission line, the wavelength interval of each signal light increases as the distance from the zero-dispersion wavelength of the optical fiber transmission line increases, and the phase modulation degree of each signal light in the phase modulation means increases. The larger the distance from the zero-dispersion wavelength, the larger the characteristic.

A WDM transmission method according to the present invention is a WDM optical transmission method in which signal lights of different wavelengths are wavelength-division multiplexed and transmitted through an optical fiber transmission line, and the distance from the zero dispersion wavelength of the optical fiber transmission line increases. A signal light generating step of generating each signal light at a wide wavelength interval, and each signal light generated in the signal light generating step is shifted at a larger phase modulation degree as the distance from the zero dispersion wavelength of the optical fiber transmission line increases. A phase modulation step of performing phase modulation, a multiplexing step of multiplexing each of the phase-modulated signal lights generated in the phase modulation step, and a signal light multiplexed by the multiplexing step on the optical fiber transmission line. It comprises a transmitting step of transmitting and a receiving step of receiving signal light from the optical fiber transmission line.

An optical transmitter according to the present invention comprises: signal light generating means for generating a plurality of signal lights having different wavelengths; and phase modulation means for individually phase modulating each signal light output from the signal light generating means. Multiplexing means for multiplexing each signal light output from the phase modulation means, and the wavelength interval of each signal light increases as the distance from the zero dispersion wavelength of the optical fiber transmission line increases, The phase modulation degree of each signal light increases as the distance from the zero dispersion wavelength increases.

By increasing the degree of phase modulation of each signal light as the distance from the zero-dispersion wavelength of the optical fiber transmission line increases, it is possible to optimize the transmission characteristics due to the phase modulation of each signal light.
Although the spectral line width is increased by increasing the phase modulation factor, interference between adjacent wavelengths can be prevented by increasing the wavelength interval of each signal light as the distance from the zero dispersion wavelength increases. As a result, in WDM transmission, transmission characteristics of each signal light can be individually improved by phase modulation, and inter-wavelength interference can be efficiently prevented.

The wavelength interval of the signal light is, for example, proportionally increased as the distance from the zero-dispersion wavelength increases, or increased stepwise, or sequentially increases up to a predetermined maximum wavelength interval. To be. Further, the phase modulation degree of each signal light may be increased stepwise as the distance from the zero dispersion wavelength increases.

[0013]

Embodiments of the present invention will be described below in detail with reference to the drawings.

FIG. 1 is a schematic block diagram of a first embodiment of the present invention. 10 is an optical transmission device, 12 is an optical fiber transmission line, and 14 is an optical receiving device. Optical transmission device 10
Is transmitted through the optical fiber transmission line 12 and reaches the optical receiver 14.

The optical transmitter 10 multiplexes n channels, ie, n signal lights of wavelengths λ1 to λn, and outputs the multiplexed signal light to the optical fiber transmission line 12. 20-1, 20-2, ..., 20-
n are mutually different wavelengths λ1, λ2,.
It is a laser light source that generates CW light of λn. The data modulator 22-i (where i = 1 to n, the same applies hereinafter) modulates the intensity of the output light of the corresponding laser light source 20-i with the data #i to be transmitted, and converts the data #i. NR to be transported
Form a Z light pulse, an RZ light pulse or a soliton pulse. The output light of the data modulator 22-i is the phase modulator 2
4-i.

The drive circuit 26-i makes one phase rotation in each optical pulse output from the data modulator 22-i according to the data #i and a clock serving as a reference clock for each of the data # 1 to #n. Further, a drive signal for driving the phase modulator 24-i is generated. It is known that it is preferable not to apply the phase modulation during the period in which no light pulse is present.
-I is an output of the data modulator 22-i from the input clock and data #i, and generates a drive signal that acts on the phase modulator 24-i only during a period in which an optical pulse exists.
Of course, regardless of the presence or absence of the light pulse, the data modulator 22-i
Phase modulation may be applied to the output of
Only the clock needs to be input to the drive circuit 26-1.

The output of the driving circuit 26-i is applied to an amplitude adjusting circuit 28-i. The amplitude adjustment circuit 26-i adjusts the amplitude of the drive signal from the drive circuit 26-i, and applies it to the phase modulator 24-i. The phase modulator 24-i modulates the phase of the output light from the data modulator 22-i according to the drive signal from the amplitude adjustment circuit 28-i. Amplitude adjustment circuit 2
By adjusting the amplitude of the drive signal to the phase modulator 24-i by 8-i, the modulation degree of the phase modulation by the phase modulator 24-i can be adjusted. Details of the phase modulation degree of each channel will be described later.

The multiplexing circuit 30 includes the phase modulators 24-1 to 24-1.
24-n output light is wavelength division multiplexed, and the optical amplifier 32
The output light of the multiplexing circuit 30 is optically amplified. The output light of the optical amplifier 32 is input to the optical fiber transmission line 12 as the output light of the optical transmission device 10.

From the optical transmitter 10 to the optical fiber transmission line 12
Is transmitted through the optical fiber transmission line 12 and reaches the optical receiver 14. The optical receiver 14 converts the signal light input from the optical fiber transmission line 12 into each of the wavelengths λ1 to λ1.
λn, and restore data # 1 to #n.

In this embodiment, the amplitude adjusting circuits 28-1 and 28-2
Since 8-n allows the phase modulation factor of each channel to be determined independently of the other channels, the following adjustment is made to optimize the transmission distance. That is, as the signal wavelength is closer to the zero dispersion wavelength λ0 of the optical fiber transmission line 12, the degree of phase modulation is reduced, and as the signal wavelength is farther from the zero dispersion wavelength λ0, the degree of phase modulation is increased.

FIG. 2 shows a change in the degree of phase modulation when the zero dispersion wavelength λ0 is located at the center of the signal wavelength band. The horizontal axis indicates the wavelength, and the vertical axis indicates the degree of phase modulation of the phase modulator 24-i. The transmission characteristic of each signal light can be individually optimized by increasing the degree of phase modulation as the signal wavelength departs from the zero dispersion wavelength λ0.

Further, in this embodiment, the wavelengths λ1 to λn of the n channels are densely arranged in the vicinity of the zero dispersion wavelength λ0, and are coarsely arranged away from the zero dispersion wavelength λ0. FIG.
7 shows an example of a change in the wavelength interval between channels when the zero dispersion wavelength λ0 is located at the center of the signal band. In FIG. 3, the horizontal axis represents the wavelength, and the vertical axis represents the wavelength interval. The wavelength interval increases proportionately as one moves away from the zero dispersion wavelength λ0.

Although the spectral line width is increased by increasing the phase modulation factor, as shown in FIG. 3, by increasing the wavelength interval of each channel as the distance from the zero dispersion wavelength λ0 increases, the wavelength between adjacent wavelengths increases. Interference can be prevented.

It is not necessary that the zero dispersion wavelength be located at the center of the signal band. FIG. 4 shows an example of a change in the wavelength interval between channels when the zero dispersion wavelength λ0 is located at a longer wavelength, and FIG. 5 shows a wavelength interval between channels when the zero dispersion wavelength λ0 is located at a shorter wavelength. The following shows an example of the change. 4 and 5
The horizontal axis indicates the wavelength and the vertical axis indicates the wavelength interval. In each case, the wavelength spacing increases proportionately as one moves away from the zero dispersion wavelength λ0.

FIGS. 6, 7 and 8 show examples in which the wavelength interval between channels increases stepwise as the wavelength interval departs from the zero dispersion wavelength λ0. In FIG. 6, the zero dispersion wavelength λ0 is located at the center of the signal band, in FIG. 7, the zero dispersion wavelength λ0 is located at a position longer than the long wavelength, and in FIG.
0 is located at shorter wavelengths. In each of the figures, the horizontal axis indicates the wavelength, and the vertical axis indicates the wavelength interval.

FIG. 9 shows an example in which the zero-dispersion wavelength λ0 is located at the center of the signal band, and the wavelength interval increases as the distance from the zero-dispersion wavelength increases, but is limited to a predetermined maximum wavelength interval or less. The horizontal axis indicates the wavelength, and the vertical axis indicates the wavelength interval.
If the maximum wavelength interval is specified, the wavelength interval is limited within that value.

In the above embodiment, each signal light is individually phase-modulated. However, a plurality of signal lights are divided into a plurality of bands composed of one or more signal lights, and each band is collectively subjected to the same phase modulation. Phase modulation may be performed. In this case, the phase modulation factor is
As the wavelength of the signal light departs from the zero-dispersion wavelength, the wavelength is increased stepwise.

FIG. 10 is a block diagram showing a schematic configuration of an embodiment of an optical transmitting apparatus for performing phase modulation in band units composed of one or more signal lights. In this embodiment, n signal lights having wavelengths λ1 to λn are divided into m (m <n) bands, and phase modulation is performed for each band. In FIG. 10, the first band has a wavelength λ.
The m-th band exemplifies a configuration in the case of signal light of 1 to λj, and the m-th band includes signal light of λk to λn. Also, the first
Only the configuration for the band and the m-th band is shown,
The configuration for the intermediate band is the same as the configuration for the first band and the m-th band.

The laser light sources 50-1 to 50-n generate CW lights having different wavelengths λ1 to λn, respectively, and the output light is applied to the data modulators 52-1 to 52-n. The phase adjusters 54-1 to 54-n respectively adjust the phases of the data # 1 to #n to be transmitted in accordance with the clock in band units, and apply the data to the data modulators 52-1 to 52-n. This is because the phase modulation is performed in band units so that the optical pulse phases of the signal light within the same band are matched.

The data modulators 52-1 to 52-n control the laser light sources 50-1 to 50-n in accordance with the data # 1 to #n phase-adjusted in band units by the phase adjusters 54-1 to 54-n.
The CW light from 50-n is intensity-modulated and data # 1 to #n
Is output (NRZ optical pulse, RZ optical pulse or soliton pulse).

The multiplexing circuits 56-1 to 56-m wavelength-division multiplex the output lights of the data modulators 52-1 to 52-n belonging to the same band. For example, the multiplexing circuit 56-1 wavelength-division multiplexes the output lights of the data modulators 52-1 to 52-j, and the multiplexing circuit 56-m outputs the data modulators 52-k to 52-j.
-N wavelength division multiplexed output light. Multiplexers 56-1
The output lights of 56-m are phase modulators 58-1 to 58-1 respectively.
58-m.

The driving circuits 60-1 to 60-m respectively control the phase modulators 58-1 to 58-m in accordance with the clocks # 1 to #m of each band.
Generate a drive signal to drive −m. Drive circuit 60-1
To 60-m are output from the amplitude adjustment circuits 62-1 to 62-m.
Is applied. The amplitude adjustment circuits 62-1 to 62-m adjust the amplitudes of the drive signals from the drive circuits 60-1 to 60-m, and apply the adjusted signals to the phase modulators 58-1 to 58-m. The phase modulators 58-1 to 58-m include amplitude adjustment circuits 62-1 to 62-1.
The phase of the output light of the multiplexing circuits 56-1 to 56-m is modulated according to the output of 62-m. Amplitude adjusting circuits 62-1 to 62
By adjusting the amplitude of the drive signal for the phase modulators 58-1 to 58-m with −m, the phase modulator 58-1 is adjusted.
The modulation degree of the phase modulation by -58-m can be adjusted. The change in the phase modulation degree with respect to the wavelength is the same as that shown in FIG. Specifically, the degree of phase modulation increases as the band moves away from the zero dispersion wavelength λ0. Thus, the phase modulation degree changes stepwise with respect to the wavelength.

The multiplexing circuit 64 includes phase modulators 58-1 to 58-1.
The output light of 58-m is wavelength division multiplexed, and the optical amplifier 66
The output light of the multiplexing circuit 64 is amplified and output to an optical fiber transmission line.

As described above, by providing the phase modulator for each band, the number of the phase modulators can be reduced.
As a result, manufacturing costs can be reduced.

[0035]

As can be easily understood from the above description, according to the present invention, each signal subjected to wavelength division multiplexing can be transmitted with good transmission characteristics. More specifically, good transmission characteristics can be realized at all wavelengths by increasing the degree of phase modulation as the wavelength departs from the zero dispersion wavelength. As the distance from the zero-dispersion wavelength increases, the wavelength interval between channels increases, so that interference between adjacent wavelengths can be reduced even if the spectral line width increases due to a large phase modulation factor.

[Brief description of the drawings]

FIG. 1 is a schematic block diagram of a first embodiment of the present invention.

FIG. 2 is a diagram illustrating a change in a phase modulation degree according to the present embodiment.

FIG. 3 is a diagram illustrating a first example of a wavelength interval between channels according to the present embodiment.

FIG. 4 is a diagram illustrating a second example of a wavelength interval between channels according to the present embodiment.

FIG. 5 is a diagram illustrating a third example of the wavelength interval between channels according to the present embodiment.

FIG. 6 is a diagram illustrating a fourth example of the wavelength interval between channels according to the present embodiment.

FIG. 7 is a diagram illustrating a fifth example of the wavelength interval between channels of the present embodiment.

FIG. 8 is a diagram illustrating a sixth example of the inter-channel wavelength interval of the present embodiment.

FIG. 9 is a diagram illustrating a seventh example of an inter-channel wavelength interval according to the present embodiment.

FIG. 10 is a schematic configuration block diagram of a second embodiment of the present invention.

FIG. 11 is a spectrum distribution after transmission of 9000 km when all wavelengths are phase-modulated at a phase modulation factor optimum for the transmission characteristic of CH8 close to zero dispersion wavelength in WDM transmission at equal wavelength intervals.

FIG. 12 is a spectrum distribution after transmission of 9000 km when all wavelengths are phase-modulated with a phase modulation degree optimum for the transmission characteristic of CH2 shorter than the zero dispersion wavelength in WDM transmission at equal wavelength intervals.

[Explanation of symbols]

10: Optical transmitter 12: Optical fiber transmission line 14: Optical receiver 20-i (20-1 to 20-n): laser light source 22-i (22-1 to 22-n): data modulator 24-i (24-1 to 24-n): phase modulator 26-i (26-1 to 26-n): drive circuit 28-i (26-1 to 26-n): amplitude adjustment circuit 30: Multiplex circuit 32: Optical amplifier 50-1 to 50-n: laser light source 52-1 to 52-n: Data modulator 54-1 to 54-n: phase adjuster 56-1 to 56-m: Multiplex circuit 58-1 to 58-m: phase modulator 60-1 to 60-m: drive circuit 62-1 to 62-m: amplitude adjustment circuit 64: Multiplex circuit 66: Optical amplifier

Continuation of the front page (56) References US Patent 6,048,842 (US, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H04B 10/02 H04B 10/18 H04J 14/00 H04J 14/02

Claims (16)

(57) [Claims] 1. A signal light generating means for generating a plurality of signal lights of different wavelengths, a phase modulating means for phase modulating each signal light outputted from the signal light generating means, and a signal outputted from the phase modulating means. Multiplexing means for multiplexing each signal light, an optical fiber transmission line for transmitting the signal light multiplexed by the multiplexing means, and a receiving device for receiving the signal light from the optical fiber transmission line, The wavelength interval of each signal light increases as the distance from the zero dispersion wavelength of the optical fiber transmission line increases, and the phase modulation degree of each signal light in the phase modulation means increases as the distance from the zero dispersion wavelength increases. WDM
Optical transmission system. 2. The WDM optical transmission system according to claim 1, wherein the wavelength interval of the signal light increases proportionally as the distance from the zero dispersion wavelength increases. 3. The wavelength interval of the signal light increases stepwise as the distance from the zero-dispersion wavelength increases.
2. The WDM optical transmission system according to 1. 4. The WDM optical transmission system according to claim 1, wherein as the distance from the zero-dispersion wavelength increases, the wavelength interval of the signal light increases up to a predetermined maximum wavelength interval, and thereafter becomes constant at the maximum wavelength interval. 5. The WDM optical transmission system according to claim 1, wherein the phase modulation degree of each signal light in the phase modulation means increases stepwise as the distance from the zero dispersion wavelength increases. 6. A WDM optical transmission method for wavelength division multiplexing signal light having different wavelengths and transmitting the resulting signal through an optical fiber transmission line, wherein each of the signals has a wider wavelength interval as the distance from the zero dispersion wavelength of the optical fiber transmission line increases. A signal light generating step of generating light, and a phase modulation step of phase-modulating each signal light generated in the signal light generating step with a phase modulation degree that increases as the distance from the zero dispersion wavelength of the optical fiber transmission line increases. A multiplexing step of multiplexing each phase-modulated signal light generated in the phase modulation step, and a transmission step of transmitting the signal light multiplexed by the multiplexing step on the optical fiber transmission line, A receiving step of receiving the signal light from the optical fiber transmission line. 7. The WDM optical transmission method according to claim 6, wherein a wavelength interval of the signal light increases proportionally as the distance from the zero dispersion wavelength increases. 8. The wavelength interval of the signal light increases stepwise as the distance from the zero dispersion wavelength increases.
2. The WDM optical transmission method according to 1. 9. The WDM optical transmission method according to claim 6, wherein as the distance from the zero dispersion wavelength increases, the wavelength interval of the signal light increases up to a predetermined maximum wavelength interval, and thereafter becomes constant at the maximum wavelength interval. 10. The WDM optical transmission method according to claim 6, wherein the phase modulation degree of each signal light in the phase modulation step increases stepwise as the distance from the zero dispersion wavelength increases. 11. A signal light generating means for generating a plurality of signal lights of different wavelengths, a phase modulating means for phase modulating each signal light outputted from the signal light generating means, and a signal outputted from the phase modulating means. Multiplexing means for multiplexing each signal light, the wavelength interval of each signal light increases as the distance from the zero dispersion wavelength of the optical fiber transmission line increases, and the degree of phase modulation of each signal light in the phase modulation means is An optical transmission device characterized in that the distance from the zero-dispersion wavelength increases. 12. As the distance from the zero dispersion wavelength increases,
The wavelength interval of the signal light is proportionally increased.
The optical transmission device according to item 1. 13. The distance from the zero-dispersion wavelength,
The optical transmission device according to claim 11, wherein the wavelength interval of the signal light increases stepwise. 14. The distance from the zero dispersion wavelength,
The optical transmission device according to claim 11, wherein the wavelength interval of the signal light increases up to a predetermined maximum wavelength interval, and thereafter becomes constant at the maximum wavelength interval. 15. The signal light generating means according to claim 11, wherein said signal light generating means comprises a light source for generating light of each wavelength, and data modulating means for modulating light of each wavelength from said light source with each data to be transmitted. The optical transmission device as described in the above. 16. The optical transmission apparatus according to claim 11, wherein the phase modulation degree of each signal light in the phase modulation means increases stepwise as the distance from the zero dispersion wavelength increases.