JPH08125635A - WDM optical transmission system - Google Patents

Abstract

(57) [Summary] [Objective] To provide a wavelength division multiplexing optical transmission system that realizes high transmission quality. [Configuration] Two signal lights having different wavelengths λa and λb are combined by a multiplexer D
The wavelength-multiplexed optical signal formed by multiplexing at 1
It is transmitted via the optical fiber transmission line F provided with An. At the end of the optical fiber transmission line F, pre-optical amplifiers Ba and Bb and receivers RXa and RXb are provided via an optical demultiplexer D2.
Are cascaded. Optical demultiplexer D2 is 1: 1
After branching the wavelength-division-multiplexed optical signal with the branching ratio of 1, the respective signals are demultiplexed by a filter having a predetermined pass band, so that the signal light of the wavelength λa is transmitted to the pre-optical amplifier Ba and the wavelength λb.
And outputs the signal light to the pre-optical amplifier Bb. Then, by individually adjusting the gains of the pre-optical amplifiers Ba and Bb, the signal lights having the optimum optical levels are received by the receivers RXa and RX.
can be supplied to b.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a wavelength division multiplexing optical transmission system for transmitting a plurality of signal lights having different wavelengths through the same optical transmission line.

[0002]

2. Description of the Related Art In the conventional optical transmission technology, in order to compensate for signal light attenuation in an optical fiber transmission line, OE / EO amplifiers are provided in the transmission line at intervals of about several tens of km, and each OE / EO amplifier is provided. The EO amplifier photoelectrically converts the signal light transmitted from the optical fiber transmission line on the input side to amplify the power,
The power-amplified electric signal is converted into signal light again and output to the optical fiber transmission line on the output side.

However, in such an OE / EO amplifier, it is necessary to generate a clock signal corresponding to the transmission rate of the signal light to be transmitted, so that the usable transmission rate is limited and the transmission capacity is limited. In order to increase the transmission speed in order to increase the
There was a problem that the EO amplifier had to be removed and replaced with a new OE / EO amplifier that complies with the new transmission standard. This has been one factor that makes it difficult to reduce the transmission cost. Further, in a backbone transmission system of a public communication line, in general, 622 Mbps or 2.488 Gbp
A transmission rate such as s is applied, and there is also a problem that the manufacturing cost of the OE / EO amplifier itself having a high frequency power amplification gain that can cope with such high-speed transmission becomes extremely high.

Further, in order to improve the transmission capacity per one optical fiber transmission line, one optical signal (hereinafter referred to as wavelength-multiplexed optical signal) formed by multiplexing a plurality of signal lights having different wavelengths is used. When applying the wavelength division multiplexing optical transmission system that multiplexes transmission using an optical fiber transmission line, each OE / EO
After demultiplexing into the signal light of each wavelength in the amplifier, the signal light of each wavelength is independently photoelectrically converted and power-amplified, converted again to the signal light of each wavelength, and then multiplexed to form a wavelength multiplexed optical signal. Need to output. Therefore, as the number of wavelengths applied to the signal light increases, the number of internal systems for power amplification increases, the OE / EO amplifier increases in size, power consumption increases, and future transmission capacity increases. There is a problem that it is difficult to cope with the increase in

In recent years, in order to solve the problems of the transmission system to which the conventional OE / EO amplifier is applied, an optical amplifier which directly amplifies signal light without converting it into an electric signal has been developed. It was If this optical amplifier is applied to a repeater to compensate the optical attenuation characteristic of the optical fiber transmission line, the optical amplification characteristic does not depend on the transmission rate, so that the internal system such as the OE / EO amplifier is increased. It is possible to solve the problem of an increase in transmission cost without causing a problem. Furthermore, since it is possible to collectively amplify a plurality of signal lights having different wavelengths, it is suitable for applying the wavelength division multiplexing optical transmission system, and can cope with future increase in transmission capacity.

[0006]

However, when the above optical amplifier is used in the transmission system to which the wavelength division multiplex transmission system is applied, the wavelength division multiplexed optical signal is formed because the amplification gain of the optical amplifier has wavelength dependence. It is difficult to set a uniform gain for a plurality of signal lights having different wavelengths, and there is a problem that the level of the signal light after amplification differs for each wavelength. Documents disclosing such problems include (1) “MNZervas, RILaming, JDMinelly and DN
Payne, in Optical Amplifiers and their Application
s, vol.14, pp.96-99, 1993 OSA TechnicalDigest Seri
es (Optical Society of America Washington DC, 199
(3) '', (2) `` M.Suyama, T.Terahara, S.Kinoshita, T.Chikama
and M. Takahashi, in Optical Amplifiers and their
Applications, vol.14, pp.126-129, 1993OSA Technica
l Digest Series (Optical Society of America Washin
gton DC, 1993) ”.

The problem will be described in more detail with reference to FIGS. 3 and 4. As a typical example, as shown in FIG. 3, the signal light of wavelength λa output from the first transmitter TXa and the signal light of wavelength λb output from the second transmitter TXb are combined by the multiplexer C1. Wavelength multiplexing optical signals composed of light having different wavelengths λa and λb are transmitted by wave propagation,
Optical amplification is performed by optical amplifiers A1 to An built in a plurality of repeaters provided in an optical fiber transmission line, and an optical splitter C2 on the receiver side splits into signal lights of respective wavelengths λa and λb. ,
Consider a wavelength division multiplexing optical transmission system that receives signals at specific receivers RXa and RXb. Further, as shown in FIG. 4A, the gain characteristic of the optical amplifier A1 will be considered as a representative, and the input level of the input signal light component of the wavelength λa is P1 and that of the input signal light component of the wavelength λb. Input level is P
2. The output level of the output signal light component of wavelength λa is P
3, the output level of the output signal light component of wavelength λb is P4
Suppose

When the wavelength-multiplexed optical signal composed of the signal light of two wavelengths λa and λb is amplified in this way, as shown in FIG. 4B, the gain for one wavelength λb is only the input level P2 of that wavelength λb. Of course, it also depends on the input level P1 of the other wavelength λa. That is, when the input level P1 of the other wavelength λa is P1 = P10, P1 = P11, and P1 = P12, the gain for one wavelength λb varies.

Further, by applying the multi-stage optical amplifiers A1 to An, noise components generated by the respective optical amplifiers are accumulated, and the gain variation is caused also by the input level and spectrum of the noise light. Furthermore, the optical amplifiers A1 to
When the gain characteristics with respect to each wavelength λa and λb for each An are non-uniform, the level difference between the wavelengths λa and λb of the signal light output from the final stage optical amplifier An becomes large, and in the worst case, , Interrupt the minimum reception level of the optical receiver,
It causes a state of communication failure.

When the input level of each signal light input to each optical amplifier A1 to An is known, the gain characteristics for each wavelength of each optical amplifier A1 to An are made uniform accordingly. It is possible to make an optimized design,
In the actual optical fiber transmission line, each optical amplifier A1 to
Since there are large variations in transmission loss characteristics between An,
Such a method can be said to be impractical.

Further, the wavelength band that can be amplified by the optical amplifier is limited. For example, in the Er-doped optical fiber amplifier which has been most studied at present, 1530 to 1570n.
It is limited to the wavelength range of about m. Furthermore, a plurality of Er
When the doped optical fiber amplifier is applied, the usable wavelength band is further limited in consideration of variations in gain characteristics for each wavelength of signal light. As a result, it is necessary to construct a wavelength division multiplexing optical transmission system using a plurality of signal lights having wavelengths extremely close to each other (for example, at intervals of several nm), which makes it extremely difficult for the receiver to separate each wavelength. Be invited. As described above, there is a problem to be solved even in the wavelength division multiplexing optical transmission system to which the conventional optical amplifier is applied.

The present invention has been made in view of the above problems of the conventional technique, and an object of the present invention is to provide a wavelength division multiplexing optical transmission system capable of realizing higher transmission quality.

[0013]

In order to achieve such an object, the present invention provides a wavelength division multiplexing for transmitting a wavelength division multiplexing optical signal formed by multiplexing a plurality of signal lights of different wavelengths through an optical fiber transmission line. An optical transmission system, wherein at least one optical amplifier installed in the middle of the optical fiber transmission line to optically amplify the wavelength multiplexed optical signal, and the wavelength multiplexed optical signal transmitted on the optical fiber transmission line An optical demultiplexer for demultiplexing into signal light of a plurality of wavelengths and outputting each signal light to different optical fibers respectively, a pre-optical amplifier installed for each optical fiber, and each pre-optical amplifier And a plurality of optical receivers that are connected in series with each other.

[0014]

According to such a system configuration, the wavelength division multiplexed optical signal transmitted through the optical fiber transmission line and optically amplified during the transmission is demultiplexed by the optical demultiplexer into the original signal lights having a plurality of wavelengths. The demultiplexed signal lights are output to different optical fibers. The signal light passing through each optical fiber is amplified by the preamplifier and supplied to a predetermined optical receiver.

Therefore, even if the attenuation characteristic of the optical fiber transmission line and the gain of the optical amplifier provided therein are different for each signal light having a different wavelength, the pre-amplifier provided at the input side of the optical receiver By adjusting the gain, it is possible to obtain the optimum level signal light for the operation of the optical receiver.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a wavelength division multiplexing optical transmission system according to the present invention will be described below with reference to the drawings. Incidentally, as a typical example,
The case of transmitting a wavelength division multiplexed optical signal composed of signal lights having mutually different wavelengths λa and λb will be described as a representative.

A schematic configuration will be described with reference to FIG. The signal light of wavelength λa output from the first transmitter TXa and the signal light of wavelength λb output from the second transmitter TXb are multiplexed by the multiplexer D1, and the wavelengths λa and λb are different from each other. A wavelength division multiplexed optical signal composed of signal light is transmitted by the optical fiber transmission line F. A plurality of repeaters are installed at appropriate distances in the optical fiber transmission line F, and the wavelength-multiplexed optical signal is optically amplified by the optical amplifiers A1 to An built in each repeater.

The optical demultiplexer D2 connected to the output end of the optical fiber transmission line F demultiplexes the transmitted wavelength-multiplexed optical signal into signal lights of wavelengths λa and λb, and also signals of wavelength λa. The light and the signal light of the wavelength λb are branched and output to the fiber cables Fa and Fb.

A pre-optical amplifier Ba for optically amplifying signal light of wavelength λa and a receiver RXa are cascade-connected to the fiber cable Fa, and the fiber cable Fa has wavelength λ.
Pre-optical amplifier Bb for optically amplifying the signal light of b and receiver RX
b is cascade-connected.

The optical demultiplexer D2 has the structure shown in FIG. 2, for example. That is, one input end of the 2-input / 2-output optical branch D3 is connected to the output end of the optical fiber transmission line F, and the other input end is terminated by the non-reflection termination element.
Further, the branching ratio of the optical branch D3 is set to 1: 1 and one output end is connected to the input end of the pre-optical amplifier Ba via a bandpass filter BPFa that transmits only the signal light of the wavelength λa. The other output end is connected to the input end of the pre-optical amplifier Bb via a bandpass filter BPFb that transmits only the signal light of the wavelength λb.

In the wavelength division multiplexing optical transmission system having such a system configuration, the first and second transmitters TXa, T
The signal lights of the wavelengths λa and λb output from Xb are combined by the multiplexer D
The wavelength-division-multiplexed optical signal formed by multiplexing by 1
Are optically amplified by the optical amplifiers A1 to An provided on the way to compensate the optical attenuation characteristic of the transmission line F.

The optical branching device D3 in the optical demultiplexer D2 branches the WDM optical signal at a branching ratio of 1: 1 and the WDM optical signals of the same level are passed through the bandpass filters BPFa and BP.
Supply to Fb. Here, since the optical branching device D3 only branches the transmitted wavelength-multiplexed optical signal, the wavelength λ is applied to both the bandpass filters BPFa and BPFb.
Signal lights of a and λb are input. Then, due to the unique transmission wavelength bands set in the bandpass filters BPFa and BPFb, respectively, the bandpass filter BPFa supplies the signal light of the wavelength λa to the preamplifier Ba, and the bandpass filter BPFb supplies the preamplifier light. Signal light of wavelength λb is supplied to the amplifier Bb.

The pre-optical amplifiers Ba and Bb optically amplify the input signal lights and supply them to the receivers RXa and RXb. The gains of the pre-optical amplifiers Ba and Bb are set so as to supply the optimum level of signal light to the corresponding receivers RXa and RXb. That is, the level of each signal light of each wavelength λa and λb of the wavelength-multiplexed optical signal input to the bandpass filters BPFa and BPFb varies due to the wavelength dependence of the transmission line F and the optical amplifiers A1 to An. The respective signal lights output from the bandpass filters BPFa and BPFb are demultiplexed into their own wavelengths λa and λb, respectively. Therefore, the gains of the pre-optical amplifiers Ba and Bb are set independently, and each receiver RXa is set. , RXb can be set to an optimum signal level. Also, in general, the optical receiver R
In Xa and RXb, there are upper and lower limits of the allowable optical level, but the optimum level can be easily adjusted by individually setting the gains by the pre-optical amplifiers Ba and Bb.

Further, the branching device D3 and the bandpass filter B
The optical demultiplexer D2 composed of PFa and BPFb has about 4 dB.
Although some transmission loss occurs, such loss can be easily compensated by adjusting the gains of the pre-optical amplifiers Ba and Bb.

As the optical branching device D3, a so-called optical fiber coupler obtained by melting and extending two fibers or a type using an interference film filter type beam splitter can be applied. In addition, the bandpass filter BPFa,
For the BPFb, an interference film type bandpass filter having a characteristic of transmitting only a specific wavelength can be applied.

If it is limited to two-wave multiplex transmission,
"CMRAGDALE et al," Narrowband Fiber Grating Filt
ers ”IEEE J. Communications., vol.8, No.6, pp1146-1
The grating fiber disclosed in the document of "150, August 1990" can be applied. That is, the branch device D
It is possible to apply a “grating fiber” in which a grating is formed in the core of the optical fiber connected to each output terminal of 3 and has a characteristic of reflecting only light in a specific wavelength range. In this case, the bandpass filter BPFa has a characteristic of reflecting the wavelength λb, and the bandpass filter BPFa
b has a characteristic of reflecting the wavelength λa. In this case,
In order to prevent the reflected light from the bandpass filters BPFa and BPFb from adversely affecting the transmission characteristics, it is preferable to dispose an optical isolator on the input side of the optical branching device D3.

Furthermore, the wavelengths λa and λ of the wavelength-division-multiplexed light which has passed through the optical fiber transmission line including the plurality of optical amplifiers A1 to An.
When the optical level difference of b is known, the optical branching device D in FIG.
Each pre-optical amplifier Ba is adjusted by adjusting the branching ratio of 3 to increase the loss of the light of the high level wavelength to the output port and reduce the loss of the light of the low level wavelength to the output port. , Bb may be equalized in optical input level to limit the operation range.

In this embodiment, the case of transmitting a wavelength-multiplexed optical signal composed of signal lights of two wavelengths λa and λb has been described, but the present invention is a wavelength-multiplexed light composed of signal lights of two or more wavelength components. Can be applied to the case of transmitting. That is, the optical branching device D3 in the optical demultiplexer D2 is configured to branch into wavelength-multiplexed optical signals corresponding to the number of wavelengths (N), and only the signal light of a specific wavelength is passed from each wavelength-multiplexed optical signal. The N band pass filters may be provided, and the signal light output from each band pass filter may be individually and individually amplified by N pre-optical amplifiers.

[0029]

As described above, according to the present invention, even if the attenuation characteristics of the optical fiber transmission line and the gain of the optical amplifier provided therein are different for each signal light having a different wavelength, an optical receiver is provided. By adjusting the gain with the pre-amplifier provided on the input side of, it is possible to obtain the optimum level signal light for the operation of the optical receiver.

Therefore, the restrictions on the design of the repeater optical amplifier and the system design and the like in consideration of the variations (loss or length) between the repeater amplifiers of the optical fiber transmission line are greatly relaxed, and the optical amplifier is reduced. This greatly contributes to the development of the wavelength division multiplexing optical transmission system used.

Further, even when the signal lights of a plurality of wavelengths that are close to each other are multiplexed and transmitted, the signal lights of the respective wavelengths can be demultiplexed by the filter, so that such an effect can be obtained.

[Brief description of drawings]

FIG. 1 is a system configuration diagram showing a configuration of an embodiment of a wavelength division multiplexing optical transmission system according to the present invention.

FIG. 2 is a block diagram showing a detailed configuration of the optical demultiplexer in FIG.

FIG. 3 is a system configuration diagram showing a configuration of a conventional wavelength division multiplexing optical transmission system.

FIG. 4 is an explanatory diagram for explaining a problem of the conventional wavelength division multiplexing optical transmission system.

[Explanation of symbols]

TXa, TXb ... Transmitter, D1 ... Multiplexer, D2 ... Optical demultiplexer, D3 ... Optical brancher, F ... Optical fiber transmission line, A1-A
n ... Optical amplifier, Ba, Bb ... Pre-optical amplifier, RXa, R
Xb ... receiver, BPFa, BPFb ... bandpass filter.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI technical display location H04B 10/18

Claims (2)

[Claims] 1. A wavelength division multiplexing optical transmission system for transmitting a wavelength division multiplexing optical signal formed by multiplexing a plurality of signal lights of different wavelengths through an optical fiber transmission line, which is installed in the middle of the optical fiber transmission line, At least one optical amplifier that optically amplifies the wavelength-multiplexed optical signal, and the wavelength-multiplexed optical signal transmitted through the optical fiber transmission path is demultiplexed into signal lights of the plurality of wavelengths, and the signal lights are respectively different lights An optical demultiplexer for outputting to a fiber, a pre-optical amplifier installed for each optical fiber, and a plurality of optical receivers cascade-connected for each pre-optical amplifier, Characteristic WDM optical transmission system. 2. The optical demultiplexer includes a filter that gives a loss to light of a wavelength other than the signal light of each predetermined wavelength that should be received by each optical receiver, so that The WDM optical transmission system according to claim 1, wherein the WDM optical transmission system demultiplexes the signal light.