JP2000183815A - Optical transmission system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To effectively use the band for wavelength-division multiplex transmission by effectively suppressing crosstalk from an adjacent channel and shortening wavelength intervals. SOLUTION: The optical transmission system multiplexes and transmits by a transmission part signal lights of N channels generated by modulating lights of wavelengths λ1 to λN (N: integer larger than 1, λ1<λ2<...<λN) with signals of channels 1 to N to an optical fiber transmission line, demultiplexes the signal lights of the N channels after wavelength-multiplex transmission through the optical fiber transmission line, and detects the signal lights of the respective channels; and the transmission part 10 is equipped with dispersion imparting parts 12-1 to 12-n which impart mutually different wavelength dispersion to the signal lines of at least adjacent channels (wavelengths) and a reception part 20 is equipped with dispersion compensators 22-1 to 22-n which compensate the wavelength dispersion imparted to the signal lights of the respective channels and the wavelength dispersion of the optical fiber transmission line.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to an optical transmission system using wavelength division multiplexing.

[0002]

2. Description of the Related Art FIGS. 9, 10 and 11 show examples of the configuration of a conventional optical transmission system using wavelength division multiplexing. In each figure, the transmission unit 50 and the reception unit 60 are connected to the optical fiber transmission line 1.
Connected via

In a transmitting section 50 shown in FIGS. 9 and 10, light having wavelengths λ _{1 to} λ N input to optical modulators 51-1 to 51- _N is modulated by signals of channels 1 to N, respectively, and optical multiplexing is performed. Vessel 5
The signal is multiplexed at 2 and transmitted to the optical fiber transmission line 1. 9, the wavelength division multiplexed signal light transmitted through the optical fiber transmission line 1 is input to a dispersion compensator 61, and the chromatic dispersion of the optical fiber transmission line 1 is compensated for by an optical demultiplexer 62. To enter. The signal light of each wavelength demultiplexed by the optical demultiplexer 62 is detected by the corresponding one of the optical receivers 63-1 to 63-N. The receiving unit 60 shown in FIG. 10 has a configuration in which dispersion compensators 61-1 to 61-N are arranged at the subsequent stage of the optical demultiplexer 62 to compensate for the chromatic dispersion of the optical fiber transmission line 1 for each channel.

[0004] The optical transmission system shown in FIG.
Instead of the configuration that compensates for the chromatic dispersion on the receiving unit 60 side as in the case of 0, the transmitting unit 50 is provided with a dispersion imparting unit 53 that provides the chromatic dispersion of the opposite sign to the chromatic dispersion of the optical fiber transmission line 1. The wavelength multiplexed signal light having been subjected to the chromatic dispersion compensation is received by the receiving unit 60 by giving wavelength dispersion to the wavelength multiplexed signal light multiplexed by the optical multiplexer 52 in advance.

[0005] The dispersion imparting unit 53 provided in the transmitting unit 50
And a dispersion compensator 61 provided in the receiving unit 60 in some cases.

[0006]

In the conventional configuration for performing chromatic dispersion compensation, there is a problem that crosstalk occurs when the wavelength intervals of the respective channels are close to each other. That is, the n-th channel (2 ≦
The condition under which the signal light of (n ≦ N−1) is dispersion-compensated is substantially equal to the condition under which the signal light of the adjacent (n−1) th channel and the (n + 1) th channel is dispersion-compensated. Therefore, FIG.
As shown in FIG. 13, in the dispersion compensator of channel n (dispersion compensation for signal light of channel n), even for leakage light from adjacent channels n−1 and n + 1,
Dispersion compensation is performed at the same time as the signal light of the channel n, and the leakage light level increases. Therefore, the signal light of the channel n cannot obtain a large eye opening, which is a factor of deteriorating the reception characteristics.

SUMMARY OF THE INVENTION It is an object of the present invention to provide an optical transmission system capable of effectively suppressing crosstalk from an adjacent channel in wavelength division multiplexing transmission, making wavelength intervals close to each other, and effectively using a band. I do.

[0008]

In the optical transmission system according to the present invention, the wavelengths λ _{1 to} λ _N (N is an integer of 2 or more, λ ₁ <
[lambda] ₂ <... <[lambda] _N ), the N-channel signal light obtained by modulating the light of each of channels 1 to N is multiplexed and transmitted to an optical fiber transmission line. In the optical transmission system for demultiplexing the N-channel signal light and detecting the signal light of each channel, a dispersion imparting unit that gives a transmitting unit at least different chromatic dispersion to at least signal light of an adjacent channel (wavelength). And a dispersion compensator for compensating chromatic dispersion given to signal light of each channel by the transmission unit and chromatic dispersion of the optical fiber transmission line.

FIG. 2 shows the principle of suppressing light leaked from an adjacent channel in the optical transmission system of the present invention. In the dispersion imparting unit of the transmitting unit, adjacent channels k-1, k, k
The chromatic dispersions B _k−1 , B _k , and B _{k + 1} are respectively added to the +1 signal light.
And transmit it to the optical fiber transmission line. In the optical fiber transmission line, chromatic dispersion D is further provided, and D + B
_k−1 , D + B _k , and D + B _{k + 1} are provided. The dispersion compensator of the channel k of the receiver, and the signal light of channels k of wavelength dispersion D + B _k is applied, the leakage light and the wavelength dispersion from channel k-1 chromatic dispersion D + B _k-1 is assigned D + B k ₊ channel _{k, 1} has been granted +
Light leaked from 1 is input.

[0010] Here, when the wavelength dispersion value of the dispersion compensator of the channel k and -D-B _k, the chromatic dispersion imparted to the signal light of the channel k _{is, D + B k -D-B} k = 0 ... (1 ). On the other hand, channel k-1 and channel k
Wavelength dispersion given to the optical signal of +1, respectively _{D + B k-1 -D-} B k = B k-1 -B k ... (2) D + B k + 1 -D-B k = B k + 1 -B _k ... (3) remains.

Therefore, generally, the value of the chromatic dispersion B _k [ps / nm] given to the signal light of the adjacent channels k and k + 1
, B _{k + 1} [ps / nm] are set such that for a certain non-negative value Δ [ps / nm], | B _{k + 1} −B _k |> Δ (4) In the k dispersion compensators, the peak intensity of light leaked from channels k-1 and k + 1 can be significantly reduced. Here, it is assumed that the signal light of the adjacent channel receives the same amount of chromatic dispersion D from the optical fiber transmission line.

Thus, it is possible to effectively reduce the leakage light from the adjacent channel with respect to the signal light of the channel k and suppress the eye opening deterioration. Moreover, since the signal intensity of each channel transmitted from the transmission unit is pre-chirped to reduce the peak intensity, it is possible to suppress the effect of the nonlinear optical effect in the optical fiber transmission line.

Note that a certain non-negative value Δ [ps / nm] is Δ> 2πCT ₀ ² / λ _k when the light speed is C [nm / ps] and the duty width of the signal light is T ₀ [ps]. Given by ² . This is because Dλ ² / (2πCT ₀ ² ) = 1 in the equation T ₁ = T ₀ [1+ (Dλ ² / (2πCT ₀ ² )) ² ] ^1/2 which gives the degree of signal light distortion due to the dispersive medium. Is obtained as the chromatic dispersion amount D. Here, T ₀ and T ₁ represent the time width of the incident signal light and the output signal light to the dispersive medium, and D represents the total amount of chromatic dispersion of the dispersive medium (reference: GPAgrawal,
“Nonlinear FiberOptics (2nd edition)”, p. 65, equation (3.2.
Ten)) .

[0014]

(First Embodiment) FIG. 1 shows a first embodiment of the optical transmission system of the present invention. In the figure, the transmitting unit 10 has wavelengths λ _{1 to} λ _N (λ ₁ <λ ₂ <... <
λ _N ), optical modulators 11-1 to 11 -N for modulating light of channels 1 to N, respectively, dispersion providing units 12-1 to 12 -N for providing wavelength dispersion to signal light of each channel, It is composed of an optical multiplexer 13 that multiplexes the signal light that has passed through each dispersion imparting unit and sends the multiplexed signal light to the optical fiber transmission line 1.

The receiving section 20 performs an optical demultiplexer 21 for demultiplexing the wavelength-division multiplexed signal light transmitted through the optical fiber transmission line 1 into signal light of each channel, and performs dispersion compensation of the signal light of each channel. Dispersion compensators 22-1 to 22-N and optical receivers 23-1 to 23-2 for detecting the signal light of each channel whose dispersion has been compensated.
3-N.

Here, assuming that the chromatic dispersion values of the dispersion imparting units 12-1 to 12-N are + B ₁ to + B _N and the chromatic dispersion value of the optical fiber transmission line 1 is + D, the dispersion compensators 22-1 to 12-N.
Wavelength dispersion value of 22-N becomes _{-D-B 1 ~-D-} B N. It is not necessary that all of + B ₁ to + B _N have different values, and it is sufficient that at least the chromatic dispersion values given to the signal lights of the adjacent channels are different from each other. That is,
Chromatic dispersion values B _k [ps / nm] and B _{k + 1} [ps / nm] given to the signal light of channel k and the signal light of channel k + 1
Satisfies the relationship | B _{k + 1} −B _k |> Δ for a certain non-negative value Δ [ps / nm].

For example, if the dispersion imparting unit 12 of the transmitting unit 10 sets the sign of the chromatic dispersion given to the signal light of the adjacent channel to be different, as shown in FIG. 3, the signal light of the channel k and the channel k− 1 and channel k + 1
The chirping inclination of the signal light is reversed. Although FIG. 3 shows the case _where the code of the chromatic dispersion D of the optical fiber transmission line 1 and the code of the chromatic dispersion B _k given to the signal light of the channel k are the same, both codes may be different. Receiver 2
When the dispersion compensator 22-k of the channel k of 0 compensates for the chromatic dispersion given by the transmission unit 10 and the chromatic dispersion of the optical fiber transmission line 1, the peak intensity of the signal light of the channel k becomes the state at the time of transmission. . On the other hand, dispersion compensator 2 for channel k
2-k acts in the direction of giving chromatic dispersion to light leaked from the channels k-1 and k + 1, and the peak intensity of the light leaks greatly.

The signs of the chromatic dispersion given to the signal light of the adjacent channels need not be different from each other as in the example shown in FIG. For example, as shown in FIG. 4, a chromatic dispersion value of 0 and a predetermined chromatic dispersion value may be alternately given to the signal light of each channel. In this case, the receiving unit 20
In the dispersion compensator 22-k of the channel k of the
When the chromatic dispersion given by 0 and the chromatic dispersion of the optical fiber transmission line 1 are compensated, the peak intensity of the signal light of the channel k is increased, but the chromatic dispersion is reduced for the light leaked from the channels k-1 and k + 1. Does not become 0, and the peak intensity is greatly reduced as compared with the conventional configuration.

In general, the k−1, k and k +
The value of chromatic dispersion B _k-1 [ps / n] given to the signal light of one channel
m], B _k [ps / nm] and B _{k + 1} [ps / nm] are, for a given non-negative value Δ [ps / nm], B _k−1 −B _k > Δ and B _{k + 1} −B _k > Δ or B _k−1 −B _k <Δ and B _{k + 1} −B _k <Δ.

(Second Embodiment) FIG. 5 shows a second embodiment of the optical transmission system of the present invention. The feature of this embodiment is that the value of chromatic dispersion given to signal light of an odd channel is + B
₁ and the value of the chromatic dispersion given to the signal light of the even-numbered channel is + B ₂ (≠ + B ₁ ). This is the first
As described in the embodiment, the present invention corresponds to a configuration in which different chromatic dispersion values are alternately applied to signal light of adjacent channels.

The signal light of the odd channels is supplied to the optical multiplexer 13-.
1 and input to the dispersion imparting unit 12-1 to obtain the chromatic dispersion +
Give the B _1. The signal light of the even-numbered channel is
Multiplexes -2 input to dispersion providing unit 12-2 gives the chromatic dispersion + B _2. The signal light that has passed through each dispersion imparting unit is multiplexed by the optical multiplexer 13-3 and transmitted to the optical fiber transmission line 1. In such a configuration, different kinds of wavelength dispersion can be given to the signal lights of the adjacent channels by the two kinds of dispersion giving sections. The configuration on the receiving unit 20 side is the same as in the first embodiment.

(Third Embodiment) FIG. 6 shows a third embodiment of the optical transmission system of the present invention. The feature of the present embodiment is that, in the configuration of the second embodiment, the receiving unit 20 also performs chromatic dispersion compensation on the odd-numbered channel signal light and the even-numbered channel signal light using a common dispersion compensator. The configuration of the transmission unit 10 is the same as in the first embodiment.

The wavelength-division multiplexed signal light input from the optical fiber transmission line 1 to the receiving unit 20 is split by an optical splitter 21-1 into signal light of odd channels and signal light of even channels. Such an optical demultiplexer can be realized by an optical filter having a periodic demultiplexing characteristic, for example, a Mach-Zehnder filter. The signal light of the odd channel demultiplexed by the optical demultiplexer 21-1 is input to the dispersion compensator 22-1 and is subjected to chromatic dispersion-D-.
Give the B _1. At this time, even if the signal light of the even-numbered channel is mixed as leakage light, the chromatic dispersion given to the leakage light is B
_2- B ₁ (≠ 0) remains, and the peak intensity is reduced. The signal light of the even-numbered channel demultiplexed by the optical demultiplexer 21-1 is input to the dispersion compensator 22-2 and is subjected to chromatic dispersion-D-.
Give the B _2. At this time, even if the signal light of the odd-numbered channel is mixed as leak light, the peak intensity is similarly reduced.

The signal lights of the odd-numbered channels and the even-numbered channels that have passed through the respective dispersion compensators are respectively divided into optical demultiplexers 21-.
The signals are demultiplexed into the signal lights of the respective channels at 2 and 21-3 and input to the optical receivers 23-1 to 23-N.

In the first to third embodiments, the chromatic dispersion of the optical fiber transmission line has been described as the fixed value D.
However, in general, the chromatic dispersion of the optical fiber transmission line 1 is a function of the wavelength and is represented as D (λ). The first in this case
A configuration example corresponding to the third and third embodiments will be described below as a fourth embodiment and a fifth embodiment. Although an example corresponding to the second embodiment is omitted,
The configuration is the same.

(Fourth Embodiment) FIG. 7 shows a fourth embodiment of the optical transmission system of the present invention. In this embodiment,
In the configuration of the first embodiment, the dispersion compensation values of the dispersion compensators 22-1 to 22 -N of the receiving unit 20 are represented by −D (λ ₁ ) −B ₁ to
−D (λ _N ) −B _N

Here, the dispersion compensator 22-k of the channel k
If the chromatic dispersion value and -D (λ _k) -B _k, wavelength dispersion given to the optical signal of channel k _{is, D (λ k) + B} k -D (λ k) -B k = 0 ... ( 1 '). On the other hand, channel k-1 and channel k
The chromatic dispersion given to the +1 signal light is D (λ _k−1 ) + B _k−1 −D (λ _k ) −B _k (2 ′) D (λ _{k + 1} ) + B _{k + 1} − D (λ _k ) −B _k (3 ′) remains.

Therefore, generally, the value of the chromatic dispersion B _k [ps / nm] given to the signal light of the adjacent channels k and k + 1
, B _{k + 1} [ps / nm] are, for a non-negative value Δ [ps / nm], | B _{k + 1} −B _k + D (λ _{k + 1} ) −D (λ _k ) |> Δ .. (4 ′), the peak intensity of the light leaked from the channels k−1 and k + 1 can be significantly reduced in the dispersion compensator of the channel k.

Alternatively, the chromatic dispersion values B _k-1 [ps / nm], B _k [ps / nm], and B _{k +} given to the signal light of the (k−1) th channel, the kth channel, and the (k + 1) th channel adjacent to each other. ₁
[ps / nm] is B _k−1 −B _k + D (λ _k−1 ) −D (λ _k )> Δ and B _{k + 1} −B for a certain non-negative value Δ [ps / nm]. _{k + D (λ k + 1} ) -D (λ k)> Δ _{or, B k-1 -B k +} D (λ k-1) -D (λ k) <Δ cutlet _{B k + 1 -B k + D} ( λ _{k + 1} ) −D (λ _k ) <Δ. This specific example is shown in the following fifth embodiment.

(Fifth Embodiment) FIG. 8 shows a fifth embodiment of the optical transmission system of the present invention. In this embodiment,
In the configuration of the third embodiment, the dispersion compensation values of the dispersion compensators 22-1 and 22-2 of the receiving unit 20 are set to −B ₁ and −B ₂ , and the transmitting unit 10 first sets the odd-numbered signal light and the even-numbered signal. The chromatic dispersion given to the signal light of the channel is compensated. Next, the chromatic dispersion D (λ) given by the optical fiber transmission line 1 for each wavelength is compensated for the signal light of each channel demultiplexed by the optical demultiplexers 21-2 and 21-3. .

That is, to the signal light of channel 1 (wavelength λ ₁ ) demultiplexed by the optical demultiplexer 21-2, chromatic dispersion −D (λ ₁ ) is given by the dispersion compensator 24-1. compensating for the chromatic dispersion with respect to wavelength lambda ₁ in the optical fiber transmission line 1.
Similarly, the dispersion compensators 24-2 to 24-N compensate for the chromatic dispersion corresponding to the demultiplexed wavelengths.

[0032]

As described above, according to the optical transmission system of the present invention, the signal light of the adjacent channel (wavelength) is given different chromatic dispersion to the signal light of the adjacent channel (wavelength) by the transmission unit, so that the signal light of channel k is added to the signal light of the reception unit. When dispersion compensation is performed for
It is possible to completely prevent dispersion compensation from leaked light from the adjacent channels k−1 and k + 1. In the conventional configuration, the dispersion is almost completely compensated for the light leaked from the adjacent channel. However, in the configuration of the present invention, the predetermined wavelength dispersion remains for the light leaked from the adjacent channel. It can be significantly reduced compared to the configuration.

In particular, by alternately giving different chromatic dispersions to signal light of adjacent channels (wavelengths), the number of dispersion imparting units required for the transmitting unit can be reduced and the configuration can be simplified.

As described above, by providing different wavelength dispersion to the signal light of the adjacent channel (wavelength) in the transmitting unit, the dispersion compensator of the receiving unit imperfectly compensates for the leakage light from the adjacent channel. This can be performed in such a manner as to be performed in a simple manner, or can be made to function in a direction in which dispersion is imparted, so that deterioration of the eye opening of the signal light of each channel can be suppressed. Therefore, it is possible to effectively use the band by making the wavelength intervals close to each other.

Further, since the signal light of each channel transmitted from the transmission section is pre-chirped to reduce the peak intensity, the influence of the nonlinear optical effect in the optical fiber transmission line can be suppressed.

[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 the principle of suppressing light leaked from an adjacent channel in the optical transmission system of the present invention.

FIG. 3 is a diagram illustrating a case where signs of chromatic dispersion given to signal light of adjacent channels are different.

FIG. 4 is a diagram showing a case where values of chromatic dispersion given to signal light of adjacent channels are different.

FIG. 5 is a block diagram showing a second embodiment of the optical transmission system of the present invention.

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

FIG. 7 is a block diagram showing a fourth embodiment of the optical transmission system of the present invention.

FIG. 8 is a block diagram showing a fifth embodiment of the optical transmission system of the present invention.

FIG. 9 is a block diagram showing a configuration example of a conventional optical transmission system using wavelength division multiplexing.

FIG. 10 is a block diagram showing a configuration example of a conventional optical transmission system using wavelength division multiplexing.

FIG. 11 is a block diagram showing a configuration example of a conventional optical transmission system using wavelength division multiplexing.

FIG. 12 is a diagram showing light leakage from an adjacent channel in a conventional optical transmission system.

FIG. 13 is a diagram showing a state in which leakage light from an adjacent channel is dispersion-compensated.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Optical fiber transmission line 10, 50 Transmission part 11, 51 Optical modulator 12 Dispersion provision part 13, 52 Optical multiplexer 20, 60 Reception part 21, 62 Optical demultiplexer 22, 24, 61 Dispersion compensator 23, 63 Light Receiver 53 dispersion providing unit

 ────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Yoshiaki Yamabayashi F-term (reference) in Japan Telegraph and Telephone Corporation 3-19-2 Nishishinjuku, Shinjuku-ku, Tokyo 5K002 CA01 DA02 FA01

Claims (8)

[Claims] _1. An _N- channel signal obtained by modulating light of wavelengths λ _{1 to} λ _N (N is an integer of 2 or more, λ ₁ <λ ₂ <... <Λ _N ) with signals of channels 1 to N at a transmission unit. In an optical transmission system in which light is multiplexed and transmitted to an optical fiber transmission line, the N-channel signal light wavelength-division multiplexed and transmitted through the optical fiber transmission line is demultiplexed, and the signal light of each channel is detected individually. The transmitting unit includes a dispersion imparting unit that imparts different chromatic dispersion to at least signal light of an adjacent channel (wavelength); and the receiving unit includes a chromatic dispersion provided to the signal light of each channel by the transmitting unit. An optical transmission system comprising a dispersion compensator for compensating chromatic dispersion of the optical fiber transmission line. 2. A chromatic dispersion value B given by a dispersion imparting unit corresponding to each of signal light (k is an integer of 1 or more and N-1 or less) of an adjacent k-th channel and k + 1-th channel.
_k [ps / nm] and B _{k + 1} [ps / nm] are some non-negative values Δ [p
The optical transmission system according to claim 1, wherein the following relationship is satisfied with respect to [s / nm]: | B _{k + 1} −B _k |> Δ. 3. The (k−1) -th, (k) -th and (k + 1) -th adjacent
Channel signal light (k is an integer of 2 or more and N-1 or less)
The chromatic dispersion values B _k-1 [ps / nm], B _k [ps / nm], and B _{k + 1} [ps / nm] given by the corresponding dispersion imparting units.
For a given non-negative value Δ [ps / nm], B _k−1 −B _k > Δ and B _{k + 1} −B _k > Δ or B _k−1 −B _k <Δ and B _{k +} The optical transmission system according to claim 1, wherein a relationship of ₁₋ B _k <Δ is satisfied. 4. The chromatic dispersion value of the optical fiber transmission line is represented by D
(λ) [ps / nm], the signal light of the adjacent k-th channel and the signal light of the (k + 1) th channel (k is 1 or more and N−
(An integer of 1 or less), the chromatic dispersion values B _k [ps / nm] and B _{k + 1} [ps / nm] given by the corresponding dispersion imparting units.
Satisfies the relationship of | B _{k + 1} −B _k + D (λ _{k + 1} ) −D (λ _k ) |> Δ for a certain non-negative value Δ [ps / nm]. Item 2. The optical transmission system according to item 1. 5. The chromatic dispersion value of the optical fiber transmission line is represented by D
(λ) [ps / nm], the signal light of the adjacent (k−1) th channel, kth channel and (k + 1) th channel (k
Is an integer of 2 or more and N-1 or less), and the chromatic dispersion values B _k-1 [ps / nm] and B _k given by the corresponding dispersion imparting units.
[ps / nm] and B _{k + 1} [ps / nm] are some non-negative values Δ [ps /
nm], B _k−1 −B _k + D (λ _k−1 ) −D (λ _k )> Δ and B _{k + 1} −B _k + D (λ _{k + 1} ) −D (λ _k )> Δ or B _k−1 −B _k + D (λ _k−1 ) −D (λ _k ) <Δ and B _{k + 1} −B _k + D (λ _{k + 1} ) −D (λ _k ) <Δ The optical transmission system according to claim 1, wherein 6. The non-negative value Δ [ps / nm] when the light speed is C [nm / ps] and the duty width of the signal light is T ₀ [ps].
Is, Δ> 2πCT ₀ optical transmission system according to any one of claims 2 to 5 be given to said at ² / ^λ _k ^2. 7. The transmission unit according to claim 1, wherein each of the odd-numbered channel signal light and its part and the even-numbered channel signal light and its part is provided with chromatic dispersion using a common dispersion imparting unit. The optical transmission system according to claim 1, wherein: 8. The receiving section shares the chromatic dispersion given by the dispersion imparting section of the transmitting section with the odd-numbered channel signal light or a part thereof and the even-numbered channel signal light or a part thereof. The optical transmission system according to claim 7, wherein the dispersion compensator is configured to perform dispersion compensation using the dispersion compensator.