WO2006090938A1

WO2006090938A1 - Wavelength division multiplxed metro optical network using negative dispersion fiber

Info

Publication number: WO2006090938A1
Application number: PCT/KR2005/000500
Authority: WO
Inventors: Yun Chur Chung; Hwan Seok Chung
Original assignee: Korea Advanced Institute Of Science And Technology
Priority date: 2005-02-23
Filing date: 2005-02-23
Publication date: 2006-08-31

Abstract

A wavelength division multiplexing (WDM) metro optical communication system using a negative dispersion fiber is disclosed. The WDM metro optical communication system includes a transmitting terminal, a receiving terminal and an optical fiber. The transmitting terminal includes transmitters employing direct modulation and a multiplexer. The receiving terminal includes a demultiplexer and receivers. The optical fiber connects the multiplexer and the demultiplexer, and has a range of negative dispersion of - 0.5ps/nm/km ~ -3.3ps/nm/km and a positive dispersion slope, at 1530~1560nm. Therefore, as the optical fiber employing the direct modulation and a properly adjusted negative dispersion is utilized, optical signal distortion can be reduced, errors can be prevented, transmission capacity can be increased, upgrade to the DWDM system can be easily performed, directly modulated signals can be rapidly transmitted over long distances, and the structure can be simply implemented, thereby implementing a cost-effective metro optical communication network.

Description

[DESCRIPTION] [Invention Title]

WAVELENGTH DIVISION MULTIPLXED METRO OPTICAL NETWORK USING NEGATIVE DISPERSION FIBER

[Technical Field]

The present invention relates to a wavelength division multiplexing-(WDM) metro optical communication network system using a negative dispersion fiber, and more particularly to a coarse wavelength division multiplexing (CWDM) metro optical communication network system, a dense wavelength division multiplexing (DWDM) metro optical communication network system, or a C WDM/DWDM combined metro optical communication network system, which utilize a negative dispersion fiber.

[Background Art]

As various data services are rapidly increased through the Internet, transmission networks are required to handle increasingly transmission capacity. In order to comply with this requirement, a wavelength division multiplexing (WDM) optical transmission system has been introduced in which the WDM optical transmission system serves to multiplex signals of lights having different wavelengths and transmits the multiplexed signals through an optical fiber. Such a WDM optical transmission system is widely used to increase the transmission capacity in a long distance communication network and in metro networks, such as a city communication network or a local communication network, etc.

When constructing such a metro network, economical efficiency must be firstly considered. It is important to select a transmitter and an optical fiber according to optical signal modulation to comply with the economical efficiency. Methods of modulating optical signals at a transmitting terminal are roughly divided into external modulation and direct modulation. External modulation serves to modulate light outputted from a laser into digital signals of '1 ' and '0' using a separately installed external modulator. Direct modulation serves to modulate driving current of a laser according to input signals. Since the external modulation is performed via such a separate modulator, chirping does not occur in the modulated optical signal such that the modulated optical signals can be transmitted over long distances. Here, "chirping" refers to a phenomenon indicative of instantaneous changing of optical signal's frequency depending on inputted electrical digital signals. However, modulators employing external modulation have disadvantages in that, because they must be driven at a high voltage, they require a separately installed amplifier for high voltage electric signals, thereby increasing the costs thereof. On the other hand, modulators employing direct modulation have advantages in that, since they do not require a separately installed modulator, their costs can be decreased, a relatively high power can be obtained, and they can be simply configured. Meanwhile, when a laser is directly modulated, the frequency of optical signals from the laser is changed according to change of carrier density in the laser, such that chirping is inherently generated in a front portion of pulses in which a relatively short wavelength component (blue shift) occurs and in a rear portion of pulses in which a long wavelength component (red shift) occurs.

Considering the costs for implementing the modulators, interest in coarse wavelength division multiplexing (CWDM) systems and dense wavelength division multiplexing (DWDM) systems, which utilize a directly modulated laser (DML), is increased to implement the metro network. For example, a method wherein CWDM signals are transmitted over a long distance such as above 120km using a semiconductor optical amplifier is tried. Also, a method wherein transmission capacity is increased by combining DWDM signals with CWDM signals is tried. The transmission capacity of such a CWDM/CWDM combined system can be easily upgraded as modulation rate of the DML per channel is increased above 10Gb/s or a part of CWDM channels is replaced with the DWDM channels.

However, since a typical one of the prior art optical fibers, such as a single mode fiber (SMF) having dispersion of approximately 16ps/nm/km at a wavelength of 1550nm, has positive dispersion, likewise, chirping generated when directly modulating optical signals of a laser, the front and rear portions of the pulse are shifted to blue and red, respectively. Therefore, when optical signals directly modulated at above 1 OGb/s are transmitted through the SMF, due to chirping of the DML and positive dispersion of the optical fiber; a pulse-spreading phenomenon is accelerated such that a transmission distance can be strictly limited. In order to resolve such problems, there has been proposed a method of optical phase conjugation or mid-span spectral inversion for transforming phases of optical signals in the transmission system to limit pulse spreading. Also, there has been proposed a method of eliminating a part of wavelength components, which are generated by chirping, using an optical filter. However, these methods have disadvantages in that, since they greatly complicate the system and/or reduce available bandwidth of an optical fiber, they hardly improved performance of the system. On the other hand, there has been proposed another method for limiting pulse-spreading phenomena, generated in the optical fiber, using a dispersion compensation fiber (DCF). However, this method has drawbacks in that, since the DCF is expensive, costs for configuring the network are increased. Also, the method has disadvantages in that it requires an additional optical amplifier for compensating for loss generated in the DCF. There is a Low Water Peak Single Mode Fiber (LWP-SMF) according to the prior art, in which overhead (OH) is eliminated, in order to accommodate CWDM signals of a relatively wide bandwidth. However, since the LWP-SWF is manufactured on the basis of the prior art SMF in which loss for a bandwidth of 1400nm is reduced, it must require an external modulator and a DCF for accommodating rapidly modulated signals. As such, compensation of dispersion and use of an external modulator greatly complicate the system and reduce cost- effectiveness.

[Disclosure] [Technical Problem]

Therefore, the present invention has been made in view of the above problems, and it is an object of the present invention to provide a wavelength division multiplexing (WDM) metro optical communication network system capable of transmitting directly modulated fast CWDM or DWDM signals over long distances, without additional dispersion compensation, and increasing modulation rate of the CWDM signals such that the system can be easily upgraded later, or replacing a part of the CWDM signals with the DWDM signals.

[Technical Solution]

In accordance with an aspect of the present invention, the above and other objects can be accomplished by the provision of a wavelength division multiplexing (WDM) metro optical communication network system using negative dispersion fiber comprising: a transmitting terminal, a receiving terminal and an optical fiber. The transmitting terminal including coarse WDM (CWDM) transmitters for directly modulating light into optical signals to be outputted, in which the CWDM transmitters are used in a CWDM system, and a multiplexer for performing a multiplexing operation for each of the optical signals from the CWDM transmitters. The receiving terminal including a demultiplexer for receiving the multiplexed signals from the multiplexer and demultiplexing the multiplexed signals based on each wavelength, and receivers for receiving the demultiplexed optical signals, respectively, and converting the demultiplexed optical signals into electrical signals. The optical fiber connecting the multiplexer and the demultiplexer, the optical fiber having a range of negative dispersion of - 0.5ps/nm/km ~ -3.3ps/nm/km and a positive dispersion slope, in a wavelength range of 1530-1560nm.

In accordance with another aspect of the present invention, there is provided a wavelength division multiplexing (WDM) metro optical communication network system using negative dispersion fiber comprising: a transmitting terminal, a receiving terminal, and an optical fiber. The transmitting terminal including at least one coarse WDM (CWDM) transmitter for directly modulating light into optical signals to be outputted, in which the CWDM transmitter is used in a CWDM system, at least one dense WDM (DWDM) transmitter for directly modulating light into optical signals to be outputted, in which the DWDM transmitter is used in a DWDM system, and a multiplexer for performing a multiplexing operation for each of the optical signals from the CWDM transmitter and the DWDM transmitter. The receiving terminal including a demultiplexer for receiving the multiplexed signals from the multiplexer and demultiplexing the multiplexed signals based on each wavelength, and a plurality of receivers for receiving the demultiplexed optical signals, respectively, and converting the demultiplexed optical signals into electrical signals. The optical fiber connecting the multiplexer and the demultiplexer, the optical fiber having a range of negative dispersion of - 0.5ps/nm/km ~ -3.3ps/nm/km and a positive dispersion slope, in a wavelength range of l530~1560nm .

In accordance with yet another aspect of the present invention, there is provided a wavelength division multiplexing (WDM) metro optical communication network system using negative dispersion fiber comprising a transmitting terminal, a receiving terminal and an optical fiber. The transmitting terminal including dense WDM (DWDM) transmitters for directly modulating light into optical signals to be outputted, in which the DWDM transmitters are used in a DWDM system, and a multiplexer for performing a multiplexing operation for each of the optical signals from the DWDM transmitters. The receiving terminal including a demultiplexer for receiving the multiplexed signals from the multiplexer and demultiplexing the multiplexed signals based on each wavelength, and receivers for receiving the demultiplexed optical signals, respectively, and converting the demultiplexed optical signals into electrical signals. The optical fiber connecting the multiplexer and the demultiplexer, the optical fiber having a range of negative dispersion of - 0.5ps/nm/km ~ -3.3ps/nm/km and a positive dispersion slope, at a range of wavelength of 1530-1560nm.

Preferably, the WDM metro optical communication network system may include at least one optical amplifier installed between the multiplexer and the demultiplexer.

Preferably, the optical fiber may have a wavelength of zero dispersion of 1560~1595nm.

Preferably, the WDM metro optical communication network system may include at least one of the transmitters having a forward error correction (FEC) encoder, and at least one of the receivers includes a forward error correction (FEC) decoder.

[Advantageous Effects]

According to the wavelength division multiplexing (WDM) metro optical communication network system using negative dispersion fiber, as direct modulation and optical fibers in which negative dispersion is properly adjusted are used therein, distortion of optical signals can be reduced and errors of the optical signals can be prevented. Also, transmission capacity of CWDM signals can be easily increased, upgrade to a DWDM system can be easily performed, and directly modulated signals can be rapidly transmitted over long distances.

Furthermore, a WDM metro optical communication network system is simple in its configuration, such that a metro optical communication network can be cost- effectively implemented.

Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.

[Description of Drawings]

The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:

Fig. 1 is a schematic block diagram illustrating a WDM metro optical communication network system according to an embodiment of the present invention;

Fig. 2 is a schematic block diagram of an experimental configuration for determining characteristics of an optical communication network system according to the prior art and of an optical communication network system according to an embodiment of the present invention;

Fig. 3 is a view illustrating graphs of measuring CWDM channel performance at 2.5 Gb/s based on transmission distances using the experimental configuration of Fig. 2;

Fig. 4 is a view illustrating simulation graphs for CWDM channel performance, when CWDM channels are upgraded from 2.5 Gb/s to 10 Gb/s, based on channels using the experimental configuration of Fig. 2;

Fig. 5 is a view illustrating Q-value which is measured based on transmission distances for DWDM channels operated at 1550nm using the experimental configuration of Fig. 2;

Fig. 6 is graphs showing performance of DWDM signal with a bandwidth of 1550nm, which is measured after signals directly modulated at 10Gb/s are transmitted over 320km through an NDF according to the present invention; Fig. 7 is a graph illustrating a maximum transmission distance based on optical fiber dispersions, such that Q-value can be maintained above 18dB, after transmitting directly modulated signals thereto without dispersion compensation;

Fig. 8 is a graph illustrating FWM values generated when WDM signals multiplexed every 100GHz intervals are inputted to an optical fiber through channels, at OdBm, and transmitted over a distance of 320km;

Fig. 9 is views illustrating an experimental configuration for confirming transmission possibility using an NDF according to the present invention, and a graph showing a Q-value based on transmission distances, when a modulation rate of CWDM or DWDM signals is increased by 40Gb/s; Figs. 10a to 1Od are eye-diagrams comparing cases wherein the NDF of the present invention is used in a 40Gb/s optical transmission system adopting the direct modulation of Fig. 9 with cases wherein the prior art LWP-SMF is used in the same system; and

Fig. 11 is a graph illustrating performance when a general Reed-Solomon error correction code is added to the WDM metro optical communication network system shown in Fig. 1. [Best Mode]

With reference to the drawings, the preferred embodiments of the present invention are described in detail below.

Fig. 1 is a schematic block diagram illustrating a WDM metro optical communication network system according to an embodiment of the present invention.

Referring to Fig. 1, the WDM metro optical communication network system includes a transmitting terminal including optical transmitters and a multiplexer, a receiving terminal including a demultiplexer and receivers, an optical fiber connected between the multiplexer and the demultiplexer, and optical amplifiers installed at a predetermined distance between the multiplexer and the demultiplexer.

The transmitters use a transmission rate of 2.5Gb/s, 10Gb/s, or 40Gb/s for transmitting CWDM signals, change driving current of a laser according to inputted signals, such that lights from the laser can be directly modulated to digital optical signals whose wavelengths are different from one another, and output the optical signals thereto. Each of the optical signals outputted from the transmitters is inputted to the multiplexer, and the multiplexer multiplexes and outputs the optical signals.

The demultiplexer receives the multiplexed signals from the multiplexer, demultiplexes the multiplexed signals wavelength by wavelength, and outputs the demultiplexed signals thereto. The receivers receive each of the demultiplexed signals outputted from the demultiplexer, and convert the multiplexed signals into electrical signals to be outputted.

The optical fiber connecting the multiplexer to the demultiplexer has a range of negative dispersion of -0.5ps/nm/km — 3.3ps/nm/km and a positive dispersion slope, in the wavelength range of 1530~1560nm. Especially, the present invention uses a negative dispersion fiber (NDF) having a zero dispersion wavelength of

1560-1590nm. When directly modulated signals are transmitted thereto through a prior art optical fiber having positive dispersion, pulse spreading is accelerated. Also, when an optical fiber having negative dispersion has a relatively large dispersion value, optical signals passing therethrough are seriously distorted. On the other hand, when the dispersion value of the optical fiber having negative dispersion reaches zero, the distortion of the optical signals decreases, but new interference signals caused by combining optical signals having different wavelengths is generated, which is referred to as "four-wave mixing (FWM)." Therefore, to prevent such a problem, the preferred embodiment of the present invention employs the NDF as mentioned above.

The optical amplifiers are installed between the multiplexer and the demultiplexer to compensate for optical fiber loss. Here, the optical amplifier is implemented as an Erbium doped fiber amplifier (EDFA). Since the EDFA amplifies optical signals having a wavelength component in a range of 1530-1565nm, if a system transmits optical signals in the range of the amplifier wavelength thereto, intensity of the optical signals due to the loss of the optical fiber is prevented from attenuating, and thus the transmission distance is prevented from decreasing. The preferred embodiment of the present invention is designed such that the optical amplifiers have a distance of 10~80km therebetween.

On the other hand, in another embodiment of the present invention, some of the CWDM transmitters installed in the transmitting terminal can use a transmission rate of 10Gb/s~40Gb/s per channel for DWDM signals. Here, some of the CWDM transmitters can be replaced with DWDM transmitters in which optical signals are directly modulated prior to output. Here, the DWDM transmitter has a channel interval of 100GHz~400GHz.

In another embodiment of the present invention, all of the CWDM transmitters installed in the transmitting terminal can use a transmission rate of 10Gb/s ~ 40Gb/s per channel for DWDM signals. Here, all of the CWDM transmitters can be replaced with DWDM transmitters in which optical signals are directly modulated prior to output. Here, the DWDM transmitter has a channel interval of 100GHz ~ 400GHz.

Also, if necessary, an optical add-drop multiplexer (OADM), which is not shown in the drawing, may be further installed between the multiplexer and the demultiplexer.

In addition, the WDM metro optical communication network system according to a first embodiment of the present invention is implemented such that at least one of the transmitters can include a forward error correction (FEC) encoder, and at lease one of the receivers can include a forward error correction (FEC) decoder. Here, forward error correction (FEC) refers to a technology wherein a transmitting side adds overhead (OH) with a predetermined size to signals and then transmits the signals with the overheads to a receiving side, and then the receiving side retrieves received signals while correcting errors based on a predetermined protocol if noise or distortion is present in the received signals. In order to introduce the FEC to the embodiment of the present invention, an encoder is further included in the transmitter, and a decoder is also included in the receiver. Therefore, even if pulse-spreading caused by relatively large accumulating dispersion is relatively large, error correction of the received signals based on such FEC makes it possible to perform long distance transmission.

Next, comparison of characteristics between the WDM metro optical communication system using negative dispersion optical fiber according to the present invention and the prior art optical communication system is described in detail.

Fig. 2 is a schematic block diagram of an experimental configuration for determining characteristics of an optical communication network system according to the prior art and of an optical communication network system according to an embodiment of the present invention.

According to ITU-T G.694.2, a wavelength of CWDM system is distributed with channel intervals of 20nm at a bandwidth of 1270-16 IOnm. However, due to available lasers and loss of optical fibers, etc., substantially, the system utilizes eight channels operated in the range of 1470-16 IOnm. Referring to Fig. 2, in the experimental configuration, a DML directly modulated at 2.5Gb/s is used, its operation wavelength is 1470-1616nm, and its measured output power and attenuation ratio are 0.5dB and 9.8dB, respectively. Also, modulated CWDM channels have insertion loss of 2dB and are multiplexed by a CWDM passive filter having a bandwidth of 16nm. In order to confirm a possibility of whether upgrades are performed, such as increase of system capacity as a part of CWDM channels is replaced with the DWDM channel, DWDM channels used in a bandwidth of 1550nm are multiplexed based on a 100GHz channel interval, and 19 DWDM signals distributed in a wavelength range of 1542.94-1557.36nm are used. Therefore, the bandwidths of 1550nm of a CWDM multiplexer are all used by DWDM signals. In the experiment according to the present invention, an optical signal of channel number 11 is directly modulated at 10Gb/s and optical signals of the remaining channels are externally modulated by a LiNbO3 modulator. Because a laser for performing direct modulation is limited to satisfy the experiment characteristic of the present invention, even though only the optical signal of channel number 11 is directly modulated, optical signals of all channels may be directly modulated. When the DML is biased by a bias current of 6OmA, output power and 3dB bandwidth are measured by 6dB and 20GHz, respectively, modulation is performed to output signals with a length of 2³¹-1 bits, and an attenuation ratio is measured with 5.IdB. The multiplexed C WDM/DWDM signals are transmitted through the NDF of the present invention and the prior art LWP-SMF, respectively. CWDM signals are maximally transmitted up to 120km and then inputted to a receiver. Here, when the NDF is used, an optical link without dispersion compensation is formed. Also, when the LWP-SMF is used, an optical link compensating dispersion by using the DCF is formed. The multiplexer and the demultiplexer for the DWDM signals are implemented with a periodic AWG having a periodic transmission characteristic. Loss of the NDF is under 0.2dB at 1550nm, dispersion value is -2.5ps/nm/km, and a zero dispersion wavelength is 1585nm. The dispersion value of the LWP-SMF is +16ps/nm/km at 1550nm, a DCF module used for compensating dispersion in an LWP-SMF has a dispersion value of approximately -80ps/nm per kilometer to compensate for dispersion of the LWP-SMF, and optical loss is increased above 0.5dB such that an additional optical amplifier for compensating for the loss can be used, for example, a 2-stage amplifier.

Fig. 3 is a view illustrating graphs of measuring CWDM channel performance based on transmission distances using the experimental configuration of Fig. 2. As shown in Fig. 3, when an NDF according to the present invention as a transmission optical fiber is used, even if signals directly modulated at 2.5Gb/s are transmitted over a distance of 120km, power penalty of the signals is negative. On the other hand, if an LWP-SMF is used for transmission, the power penalty become larger as the transmission distance is increased. As mentioned above, when the optical signal of the laser is directly modulated, a short wavelength component is generated at the front portion of the pulse (blue shift), and a long wavelength component is generated at the rear portion of the pulse (red shift), which accelerate pulse broadening as the directly modulated optical signals pass through the LWP-SMF (positive dispersion fiber). On the other hand, when the NDF according to the present invention is employed, the shifts generated in the NDF are opposite to those of the LWP-SMF such that the pulses are compressed. Therefore, performance of an optical fiber having negative dispersion is better than that of an optical fiber having positive dispersion. Transmission capacity of CWDM channels can be easily upgraded as transmission rate per channel is increased.

Fig. 4 is a view illustrating simulation graphs for CWDM channel performance, when CWDM channels are upgraded from 2.5Gb/s to 10Gb/s, based on channels using the experimental configuration of Fig. 2

The simulations are performed based on measurement of chirp characteristics of a directly modulated 10Gb/s DML. Here, line- width enhancement factor is 3.26. With reference to Fig. 4, in case of the LWP-SMF, although signals are transmitted by only approximately 15km at a minimum dispersion value of 1470nm, power penalty is generated above 6dB. However, in case of the NDF, even if signals are transmitted 80km, performance reduction for all the channels is minimized. Therefore, it is expected that the NDF according to the present invention can exhibit higher performance in a metro network to be generally used in the future than the LWP-SMF can. Also, the NDF makes it possible to easily upgrade capacity of the prior art CWDM system without additional dispersion compensation.

Fig. 5 is a view illustrating Q-value which is measured based on transmission distances for DWDM channels operated at 1550nm using the experimental configuration of Fig. 2 in which transmission rate per channel is 10Gb/s. Q-value denotes a ratio of optical signal to noise in a receiving terminal.

Using the Q-value, performance of an optical transmission system can be estimated. Generally, Q-value of an optical transmission system must be maintained at above 18dB (BER<10~15). Since the more the Q-value increases the less the BER decreases, errors are generated relatively small.

Referring to Fig. 5, when a transmission line is employed by the LWP-SMF, denoted by 'A₅' the maximum distance wherein Q-value is maintained at 18dB is less than 20km. On the other hand, despite no additional dispersion compensation, when a transmission distance is above 320km, Q-value is above 20.2dB in an optical communication network employing the NDF according to the present invention, denoted by 'C. In addition, the transmission performance of the case (C) is superior to a case (B) wherein dispersion of LWP-SMF is compensated by DCF. Because, as mentioned above, an additional optical amplifier is used in order to compensate relatively much optical loss generated in the DCF, and a ratio of optical signal to noise is decreased.

Fig. 6 is graphs showing performance of DWDM signal at 1550 nm band, which is measured after signals directly modulated at 10Gb/s are transmitted over 320km through an NDF according to the present invention.

Referring to Fig. 6, even if an additional dispersion compensation module is not used, after a directly modulated 10Gb/s signal is transmitted over 320km, as Q- value of the signal is measured by 19.8dB (BER < 10^"15 ), and Q-value of an externally modulated signal is measured by 23.5~25.5dB. Here, entire capacity of the DWDM signal of a bandwidth of 1550nm is 190Gb/s (19 x 10Gb/s). Therefore, the NDF according to the present invention is designed to rapidly transmit the directly modulated signal over long distances without any additional dispersion compensation. However, although a part or all of the DWDM channels are externally modulated, they can be transmitted over long distances without any additional dispersion compensation.

Also, from the above result, in order to effectively use chirp characteristics of the directly modulated laser, similar to the optical communication network according to the present invention, the absolute value of optical fiber dispersion must be relatively small and negative in its sign.

Fig. 7 is a graph illustrating a maximum transmission distance based on optical fiber dispersions, such that Q-value can be maintained above 18dB, after transmitting directly modulated signals thereto without dispersion compensation, in which transmission rate per channel is assumed to be 10Gb/s.

Referring to Fig. 7, when using the NDF according to the present invention, under the conditions wherein dispersion value is -2.5ps/nm/km, and Q-value is 18dB, the maximum transmission distance is 400km. Therefore, the maximum accumulated dispersion value (which is acquired by obtaining a distance of a point wherein Q-value is 18dB and then by multiplying the obtained distance by optical fiber dispersion value) is -1000ps/nm. Therefore, when performing direct modulation, in order to transmit signals to above 300km without any dispersion compensation, dispersion value must be greater than -3.3ps/nm/km, which is acquired as -1000ps/nm, which is the maximally allowable accumulation dispersion value, is divided by the transmission distance of 300km. Namely, a range of dispersion of the optical fiber must be -3.3ps/nm/km~Ops/nm/km. However, only if dispersion value of the optical fiber must be above a predetermined value, a WDM optical transmission system in which several channels are multiplexed to be transmitted does not generate FWM.

Fig. 8 is a graph illustrating FWM value generated when WDM signals multiplexed every 100GHz intervals are inputted to an optical fiber through channels, at OdBm, and transmitted to a distance of 320km.

Generally, since power penalty of IdB is generated if four- wave mixing (FWM) value is smaller by approximately 24dB than signal, the absolute value of an optical fiber must be greater than approximately 0.5ps/nm/km.

Referring to Fig. 8, dispersion value of an optical fiber must be -3.3ps/nm/km ~ -0.5ps/nm/km, such that signals directly modulated at 10Gb/s without performance decrease are transmitted over long distances, which is caused by FWM in C band (1530 ~ 1560nm) of a general optical amplifier. On the other hand, when a system using the NDF according to the present invention employs L band (1570 ~ 1610nm) for increasing its capacity in the future, since dispersion value becomes positive by dispersion slope of the optical fiber, such a band must be used in the external modulation fashion. When the optical communication system according to the present invention employs an optical fiber, in which dispersion value is under -2.5ps/nm/km at a wavelength of 1550nm, and a zero dispersion wavelength is 1585nm, optical signal distortion can be reduced, errors can be prevented, transmission capacity of CWDM demultiplexed signals can be easily increased, and upgrade towards DWDM systems can be easily performed such that the structure is simply configured, thereby implementing a cost-effective metro optical communication network.

Fig. 9 is views illustrating an experimental configuration for confirming transmission possibility using an NDF according to the present invention, and a graph showing a Q-value according to transmission distances, when a modulation rate of CWDM or DWDM signal is increased by 40Gb/s. Here, a bias current and a modulation current of a laser are increased by 103mA and lOOmApp, respectively. Also, output power and attenuation ratio of the laser are measured by 10.6dBm and 3.4dB, respectively. With reference to Fig. 9, 40Gb/s signals used in the experiment are electrically multiplexed into four 1 OGb/s signals, which are directly inputted to a laser. After transmitting, 40Gb/s signals are optically divided into 20Gb/s signals using an electro-absorption modulator (EA) to be inputted to a receiver. The 20Gb/s signals inputted to the receiver are electrically divided into two 10Gb/s signals. After that, a bit error ratio is measured on the basis of the two 10Gb/s signals. 40Gb/s optical time division multiplexing (OTDM) receiver, which has been used in the experiment, can be replaced with a 40Gb/s electrical time division multiplexing (ETDM) receiver whose size is relatively small. After performing transmission of 40km, Q-value is measured as above 18dB without additional dispersion compensation, and an accumulation dispersion value becomes -100ps/nm (=2.5ps/nm/km X 40km) which is greater than +/-62.5ps/nm, which is necessary to transmit signals externally modulated at 40Gb/s without chirping. The reason why the accumulation dispersion value is large is because pulse spreading is prevented by NDF dispersion and laser interaction.

Figs. 10a to 1Od are eye-diagrams comparing cases wherein the NDF of the present invention are used in a 40Gb/s optical transmission system adopting the direct modulation of Fig. 9 with cases wherein the prior art LWP-SMF are used in the same system. More specifically, Fig. 10a is an eye diagram for signals outputted from a laser, Figs. 10b and 10c are eye diagrams after the signals outputted from the laser are transmitted 20km and 40km, respectively, through the NDF, and Fig. 1Od is an eye diagram after the signal outputted from the laser is transmitted to lkm through an LWP-SMF.

Here, the eye diagram is a scale indicative of degree of distortion of optical signal. When a system is controlled such that degree of eye opening in the eye diagram is maximized, the system enters a state wherein distortion of an optical signal is reduced. Referring to Fig. 10, in the case of use of the NDF according to the present invention, the eye openings are still relatively large opened even after signals directly modulated at 40Gb/s are transmitted over 40km. On the other hand, in the case of use of the LWP-SMF, the eye opening is nearly closed only after the signals are transmitted over lkm. Fig. 1 1 is a graph illustrating performance when a general Reed-Solomon error correction code is added to the WDM metro optical communication network system shown in Fig. 1.

As mentioned above, the FEC serves to be additionally inserted to data at a transmitter, and then detected to correct errors which may occur in received data at a receiver, thereby enhancing performance for signals in the communication system.

With reference to Fig. 11, if Q-value of inputted signals is 11.5dB, performance of signals using an error correction code, indicated by D, is more enhanced than that without using the error correction code. Namely, the signal performance using an error correction code is 18dB increased by 6.5dB comparing with the performance of signal without using the error correction code. As such, when the error correction code is applied to the system employing the direct modulation, the transmission distance of the metro optical transmission network system using the NDF can be more increased.

Claims

[CLAIMS]

[Claim 1 ] A wavelength division multiplexing (WDM) metro optical communication network system using negative dispersion fiber comprising: a transmitting terminal including coarse WDM (CWDM) transmitters for directly modulating light into optical signals to be outputted, in which the CWDM transmitters are used in a CWDM system, and a multiplexer for performing a multiplexing operation for each of the optical signals from the CWDM transmitters; a receiving terminal including a demultiplexer for receiving the multiplexed signals from the multiplexer and demultiplexing the multiplexed signals based on each wavelength, and receivers for receiving the demultiplexed optical signals, respectively, and converting the demultiplexed optical signals into electrical signals; and an optical fiber connecting the multiplexer and the demultiplexer, the optical fiber having a range of negative dispersion of - 0.5ps/nm/km ~ -3.3ps/nm/km and a positive dispersion slope, in a wavelength range of 1530-1560nm.

[Claim 2] A wavelength division multiplexing (WDM) metro optical communication network system using negative dispersion fiber comprising: a transmitting terminal including at least one coarse WDM (CWDM) transmitter for directly modulating light into optical signals to be outputted, in which the CWDM transmitter is used in a CWDM system, at least one dense WDM (DWDM) transmitter for directly modulating light into optical signals to be outputted, in which the DWDM transmitter is used in a DWDM system, and a multiplexer for performing a multiplexing operation for each of the optical signals from the CWDM transmitter and the DWDM transmitter; a receiving terminal including a demultiplexer for receiving the multiplexed signals from the multiplexer and demultiplexing the multiplexed signals based on each wavelength, and a plurality of receivers for receiving the demultiplexed optical signals, respectively, and converting the demultiplexed optical signals into electrical signals; and an optical fiber connecting the multiplexer and the demultiplexer, the optical fiber having a range of negative dispersion of - 0.5ps/nm/km ~ -3.3ps/nm/km and a positive dispersion slope, in a wavelength range of 1530-1560nm.

[Claim 3] A wavelength division multiplexing (WDM) metro optical communication network system using negative dispersion fiber comprising: a transmitting terminal including dense WDM (DWDM) transmitters for directly modulating light into optical signals to be outputted, in which the DWDM transmitters are used in a DWDM system, and a multiplexer for performing a multiplexing operation for each of the optical signals from the DWDM transmitters; a receiving terminal including a demultiplexer for receiving the multiplexed signals from the multiplexer and demultiplexing the multiplexed signals based on each wavelength, and receivers for receiving the demultiplexed optical signals, respectively, and converting the demultiplexed optical signals into electrical signals; and an optical fiber connecting the multiplexer and the demultiplexer, the optical fiber having a range of negative dispersion of - 0.5ps/nm/km ~ -3.3ps/nm/km and a positive dispersion slope, at a range of wavelength of 1530~1560nm.

[Claim 4] The system as set forth in any one of claims 1 to 3, further comprising at least one optical amplifier installed between the multiplexer and the demultiplexer.

[Claim 5] The system as set forth in claim 4, wherein the optical amplifier is spaced apart from another optical amplifier at a distance of 10~80km.

[Claim 6] The system as set forth in any one of claims 1 to 3, wherein the optical fiber has a wavelength of zero dispersion of 1560-1595nm.

[Claim 7] The system as set forth in any one of claims 1 to 3, wherein at least one of the transmitters includes a forward error correction (FEC) encoder, and at least one of the receivers includes a forward error correction (FEC) decoder.

[Claim 8] The system as set forth in claim 1 or 2, wherein the CWDM transmitter has a transmission rate of 2.5Gb/s, 10Gb/s or 40Gb/s per channel.

[Claim 9] The system as set forth in claim 2 or 3, wherein the DWDM transmitter has a range of transmission rate of 10Gb/s~40Gb/s.

[Claim 10] The system as set forth in claim 2 or 3, wherein the DWDM transmitter has a channel interval of 100GHz~400GHz.