JP2000059304A - Device and method for reducing self-phase modulation/ group velocity diffusion in optical system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the non-linear distortion of an optical transmission system by rounding the edge part of an optical pulse to be received from the transmitter of the system so that a pulse to be used for modulation can be similar to a Gaussian pulse. SOLUTION: In order to promote the reduction of SPM/GVD distortion from an optical communication link, a transponder 43 is provided with an electric attenuator 44 placed between a photodiode 36 and an amplifier 38. This electric attenuator 44 smoothes the signal pulse of a sharp edge part by attenuating the said signal received by the amplifier 38. Especially, in order not to operate the amplifier 38 in deep saturated state, a predetermined attenuation level is applied to the electric signal received from the photodiode 36. When the electric attenuator 44 is placed in front of the amplifier 38, the electric signal to be inputted to the amplifier 38 is attenuated into power level lower than a power level for operating the amplifier 38 in the deep saturated state.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an apparatus and a method for reducing nonlinear distortion of a pulse in a high power optical communication system, and more particularly to a self phase modulation (SelfPhaseModul) in an optical system operating at a high power level.
ation: SPM) and group velocity dispersion (GroupVel)
The present invention relates to an apparatus and a method for reducing a distortion caused by an interrelationship between a plurality of types (OCDs).

[0002]

BACKGROUND OF THE INVENTION The availability of optical amplifiers with increasing output power capacity has increased the potential for high power communication systems. Prior to the availability of high-power optical amplifiers, optical transmission systems typically used relatively low-power light sources to generate signals within fiber optic systems, and regenerated and boosted the optical signal along its path. To rely on a series of repeaters or amplifiers. On the other hand, high power optical amplifiers allow for a reduction in the number of repeaters or amplifiers required along the fiber optic link.

However, optical signals traveling at high power levels in fiber optic systems are subject to significant distortion at low power levels. In conventional low-power systems, single-mode fiber is lossy,
Behave as a linear medium causing dispersion. Light pulses at low power levels attenuate as they travel along the fiber and become symmetrically spread due to first order group velocity dispersion (GVD) if the fiber is long enough, eg, 600 km. . At transmission rates approaching 100 Gb / s, secondary GVD also spreads data pulses asymmetrically. Nevertheless, typical optical communications at low power levels result in an overall linear response along standard transmission fibers.

For high bit rate systems, for example, with input powers in excess of 5 mW, single mode optical fibers begin to exhibit nonlinear distortion characteristics caused by self-phase modulation (SPM). When an optical pulse propagates through a transmission fiber at a high power level, the SPM produces new frequency components that cause sexual frequency chirp. SPM and GVD
The interaction between them causes non-linear distortion to the light pulse governed by several parameters. These include the optical peak power level emitted into the fiber, the sign and amount of dispersion in the transmission fiber, and the dispersion map of the entire link (ie, the state of the cumulative dispersion of the signal along the link).

[0005] Agrawal's "Nonlinear Fiber Optics"
(Academic Press, 2nd edition, 1989)
Publications, including SPM against Gaussian pulse
Theoretically describes the amount of positive chirp caused by. The power of such a pulse matches the following relationship:

[0006]

(Equation 1)

Where P ₀ is the peak power of the pulse, T ₀
Is the half width of the pulse at intensity point 1 / e. As is well known in the art, the value m corresponds to the magnitude of a Gaussian pulse. When m = 1, the pulse is a Gaussian pulse. Larger values of m represent super-Gaussian pulses, ie, sharp Gaussian pulses with relatively short rise and fall times. When the value of m becomes very high, such as m >> 1, the pulse approaches the shape of a square pulse. Regarding chirps where SPM occurs, Agra
Wal is defined mathematically as follows.

[0008]

(Equation 2)

However, m changes with the pulse shape, and the effective fiber length z _eff is z _eff = [1-exp (αz)]
/ Α and z are the fiber length, and the nonlinear length is L _NL = 1 /
(ΓP ₀ ) and γ are the nonlinear coefficients of the fiber.
The maximum spectral extension of the pulse is given by:

[0010]

(Equation 3)

However, similarly to φ _max = γP ₀ z _eff , GVD
Causes chirp in light pulses in high power systems. Agrawal has a GVD chirp (chir
p) is defined as follows:

[0012]

(Equation 4)

[0013] ^{_{However, L D = T 2 0 /}} | β 2 | distributed to the pulse length, beta ₂ is a group velocity dispersion parameter. Na
Ka et al., "In-line amplifier transmission distance determined by self-phase modulation and group velocity dispersion (In-line Am
plier Transmission Dist
ance Determined Byte Self-
PhaseModulation and Group
-Velocity Dispersion) "(Jo
urnal of Lightwave Techno
logy, Volume 12, Part 2, pages 280-287,
Feb. 1994) numerically analyzes the propagation of an intensity modulated signal in an optical fiber, accounting for self-phase modulation, group velocity dispersion, and secondary group velocity dispersion. It is shown that the transmission distance that produces the surprising and foreseeable penalty is associated with three characteristic lengths: dispersion length, quadratic dispersion length and non-linear length.

Park US Pat. No. 5,539,563
Discloses a system and method for simultaneously compensating for chromatic dispersion and self-phase modulation in optical fiber. At least one dispersion compensating (DCF) fiber is used to compensate for the chromatic dispersion of the externally modulated signal carried by at least one single-mode standard fiber optic cable. The signal power emitted into the DCF fiber is controlled to achieve accurate compensation for SPM effects in standard fiber.

[0015]

Other documents also discuss SPMs in optical communications regarding pulse compression devices and techniques.
And the impact of GVD. Peter et al., "Compression of a pulse whose spectrum has been spread by self-phase modulation using a fiber grating: A theoretical study of compression efficiency (Co
impression of Pulses Spect
rally Broadened by Self-P
Hase Modulation Using a F
iber-Grating: A Theoretica
l Study of the Compression
n Efficiency) "(Optics Com
munications, Vol. 112, pp. 59-66, November 1, 1994), a theoretical analysis of the possibility of using short fiber gratings with a constant grating period for the compression of spectrally broadened light pulses by SPM. Is discussed. For fiber gratings with constant grating period, this paper confirms by theory and simulation that the maximum achievable pulse compression factor is actually independent of the grating parameters and is typically on the order of two are doing.

Stern et al., "Self-phase modulation and dispersion (S) in high data rate fiber optic transmission systems.
elf-Phase Modulation and
Dispersion in High Data R
ate Fiber-Optical Transmission
ion Systems) ”(Journal ofT
technology, Volume 8, Part 7, 1009-10
16 (July 1990) describes the constraints imposed by the interaction of primary and secondary GVD with intensity-dependent SPM. This paper studies the theoretical transmission limitations imposed by such effects on the wavelength range around zero dispersion wavelength λ ₀ for fibers with negligible polarization dispersion. This paper states that operation at wavelengths longer than λ ₀ is 50G due to the cancellation of primary dispersion by SPM.
It has been found to improve the transmission distance for data rates greater than b / s. Above 100 Gb / s, higher order dispersion limits the transmission distance even at wavelengths equal to or greater than λ ₀ . The paper concludes that linear dispersion compensation using grating-telescope coupling can significantly improve system performance for wavelengths where primary dispersion is significant.

However, these documents concentrate on the performance of relatively flat Gaussian pulses in optical systems. Applicants have found that even at relatively high bit rates of 2.5 Gb / s, modulated light pulses in links with optical fibers shorter than 600 km are less likely to be affected by GVD pulse spreading as discussed in the literature for very long distances. I've observed that it doesn't confront the overlap. Applicants have further observed that the amount of frequency chirping is highly dependent on the shape of the pulse, especially on the pulse edges, and this is further dependent on the type of transmission device being shaped. Furthermore,
Applicants have noted that many conventional SDH and SONET based transmitters have sharp rising and falling edges that are quite different from the flat Gaussian pulses discussed by theoretical calculations, but rather similar to super Gaussian pulses. I discovered that. Applicants note that pulses with sharp rising and falling edges are usually preferred for optical communications in order to minimize the effects of phase jitter and improve detection. These pulses are subject to much more frequency chirping than theoretical Gaussian pulses, as observed by the applicant. Furthermore, Applicants have discovered that the pulse shape and the degree of sharpness of its rising and falling edges are not the same for different transmitters, but depend on the equipment used.

Furthermore, applicants note that due to the frequency chirping described above, the bit error rate (BER) at the receiver for such pulses is affected by the receiver characteristics, and in particular by the type of electrical filtering performed at the receiver. It was confirmed. This makes the characteristics of the optical system highly dependent on the choice of the transceiver or the degree of matching of the available transceiver.

[0019] US Patent No. 5, Tamburello et al.
No. 267,073 discloses an adapter for interconnecting fiber lines containing optical amplifiers, in which the transmitter and receiver have operating parameters different from those of the optical amplifier (eg, transmission speed, wavelength, and temperature with respect to temperature). Wavelength fluctuation). adapter·
The group includes a conversion unit that converts an optical signal into an electric signal,
A signal laser transmitter, a conditioning module connected to the output of the conversion means and including laser pilot means for controlling the signal transmitter by electrical signals, and an optical amplifier coupled to the output of the laser transmitter.

Alexander et al., US Pat.
No. 04,609 discloses an optical re-modulator for converting channel wavelengths and a wavelength division multiplexing system. No. 5,504,609 discloses a remodulator that includes an electro-optical converter that produces an electrical signal from a received optical signal having a wavelength λ _T1 . This electrical signal is amplified by a transimpedance amplifier, filtered to limit the noise bandwidth and waveform of the signal, and further amplified by a limiting amplifier. Optionally, the remodulator in U.S. Pat. No. 5,504,609 may include clock and data recovery circuits used for high data rate signals. A switch automatically selects the high data rate signals and passes these signals to a clock and recovery element. The re-modulator further includes a laser that produces the carrier signal λ _j and an external modulator.

However, the aforementioned US Pat. No. 5,504,60
No. 9 is not for communication link operation at high input power, nor for distortion from SPM and GVD interaction. Regarding the handling of high data rate signals, U.S. Pat. No. 5,504,609 shows the optical paths for the high and low data rate signals in FIG. For high data rate signals, a switch directs the electrical signal to a clock and data recovery circuit. This circuit does not include a device placed after the clock and data recovery circuit to smooth sharp edges in the pulses produced by the clock and data recovery circuit. No. 5,504,609 does not disclose other approaches to handling high data rate signals.

Applicants have determined that the nonlinear interaction of SPM-GVD depends on the parameter values in the above-mentioned theoretical equation:
It was noted that the transmitted signal could be improved or degraded.
To understand the effects of SPM and GVD, it is important to evaluate L _NL , L _D and z _eff from the above equations. These parameters identify the length scale at which the phenomena of nonlinearity, dispersion and attenuation become significant.

Applicants have determined that if the total length L _{T of the} optical link is greater than the length L _M given by: the distortion caused by pulses having sharp rising and falling edges is undesirable and significant.

[0024]

(Equation 5)

Where z is the span length of the fiber between adjacent amplifiers, or the span length for one span.

[0026]

SUMMARY OF THE INVENTION The present invention reduces non-linear distortion in optical transmission systems caused by the interaction of SPM and GVD which substantially reduces one or more of the limitations and disadvantages of the prior art devices described above. It is directed to methods and apparatus. The objects and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims. Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention.

In general, the invention includes techniques that make the shape of the light pulses modulating the optical carrier in an optical transmission system relatively independent of the shape of such same pulses received by an apparatus implementing the invention. In particular, transponders consistent with the present invention include circuitry to round the edges of optical pulses received from the transmitter of the system such that the pulses used for modulation resemble Gaussian shaped pulses.

To achieve the above and other objects and advantages, and in accordance with the objects of the present invention as embodied and broadly described herein, the present invention comprises, in a first aspect, a first aspect.
A transmitter that sends out light pulses at a wavelength of
A transponder coupled to receive an optical pulse containing the optoelectronic device and convert it to a second waveform; means for smoothing rising and falling transitions of the electrical pulse received from the optoelectronic device; an electrical amplifier; An electro-optic modulator, a plurality of spans linearly coupled to the transponder, each having an optical transmission fiber of length z and at least one optical amplifier, and a receiver coupled to the plurality of spans. An optical communication system for reducing nonlinear distortion caused by interaction between self-phase modulation and group velocity dispersion, wherein the total length of the plurality of spans is greater than ( _LNL / _zeff ) z, where _LNL is the nonlinearity of the fiber. Length, z
_eff is the effective span fiber length.

According to a second characteristic, the invention comprises a transmitter for transmitting a light pulse of a first wavelength, an optoelectronic device,
A transponder coupled to receive an optical pulse and convert it to a second wavelength, including means for smoothing rising and falling transitions of the electrical pulse received from the optoelectronic device, an electrical amplifier, a light source, and an electro-optic modulator; _Resulting from the interaction between self-phase modulation and group velocity dispersion, including an optical transmission fiber section having an effective length _zeff longer than the nonlinear length _LNL of the fiber, and a receiver coupled to the fiber section. This is an optical communication system that reduces nonlinear distortion.

In both the above first and second aspects of the invention, the means for smoothing the electrical pulses are arranged between the optoelectronic device and an electrical amplifier or a low-pass filter arranged after the electrical amplifier. Electrical attenuator may be included. In one embodiment, the electrical pulse smoothing means includes a clock and data recovery circuit located between the optoelectronic device and the electrical amplifier, and a low pass filter located after the electrical amplifier.

In another aspect, the invention is a method for reducing nonlinear optical distortion caused by the interaction of self-phase modulation and group velocity dispersion, comprising receiving an optical pulse from a transmitter and converting the optical pulse into an electrical pulse. Amplifies the electrical pulse, smoothes the edges of the rising and falling transitions of the electrical pulse, modulates the optical carrier signal with the electrical pulse, L _NL / z _eff (where L _NL is the nonlinear length of the fiber, z _eff comprises transmitting the modulated optical carrier signal to a plurality of transmission spans having a cumulative length greater than the span fiber length described above). The method desirably further comprises the step of compensating for the dispersion of the modulated optical carrier signal at a location along a plurality of transmission spans using, for example, a chirped fiber grating. In still another aspect, the present invention includes a method for receiving an optical pulse of a first wavelength generated by an optical transmitter, modulating an optical carrier with the optical pulse, and providing non-linear distortion caused by an interaction between self-phase modulation and group velocity dispersion. A photodiode that is optically coupled to receive a light pulse and convert it to an electrical pulse, and
An electrical amplifier operating in saturation and electrically coupled to receive and amplify the electrical pulse, and a low coupled electrically coupled to receive the electrical pulse from the electrical amplifier to increase the rise and fall times of the electrical pulse. A light source providing a second wavelength optical carrier; and an electro-optical modulator arranged to modulate the optical carrier with an electrical pulse from the low pass filter.

In a further aspect, the present invention receives an optical pulse of a first wavelength generated by an optical transmitter, modulates an optical carrier with the optical pulse, and results from an interaction between self-phase modulation and group velocity dispersion. A transponder for mitigating nonlinear distortion, a photodiode optically coupled to receive an optical pulse and convert it to an electrical pulse, and an electrical attenuator electrically coupled to receive an electrical pulse from the photodiode; An electrical amplifier operating in saturation and electrically coupled to receive and amplify electrical pulses from the electrical attenuator, a light source providing a second wavelength optical carrier, and an optical pulse from the electrical amplifier And an electro-optic modulator arranged for modulation.

It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only, and are not restrictive of the invention as claimed. .

[0034]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Reference will first be made in detail to preferred embodiments of the present invention, as illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts.

According to the present invention, an apparatus for reducing distortion caused by the interaction of SPM and GVD includes a transponder having a particular configuration and coupled to a long-haul optical communication link. FIG. 1 shows a typical telecommunications link for the present invention. The overall communication link, generally indicated at 10, includes a transponder 12, optical fibers 14-17 of various lengths, and a series of amplifiers 18-21.
And As shown in FIG. 1, in an experimental setup, the link also includes the same series of attenuators 22-25. Various attenuators 22-25 are used to help simulate a fully integrated optical link. Transponder 12 receives an optical communication signal from a transmitter, such as 26, and attenuator 25 passes the optical signal to an end of a communication link, such as 28, to a receiver. Transmitter 26 and receiver 28 may be individual SDH terminals or SONEs for larger optical communication networks.
May be part of a T terminal. In an actual configuration, the transmitter 26 and the receiver 28 will, of course, be located in separate locations, as opposed to the diagram of FIG. 1 which shows a setup for experimental purposes.

As is known in the art, transponder 12 typically operates to convert the wavelength received from transmitter 26 to a new wavelength suitable for optical link 10 as a whole. The wavelength received by transponder 12 may not correspond to a wavelength band that is acceptable for amplification by amplifiers 18-21 on the link. In particular, amplifiers 18-21 include a rare earth doped fiber, preferably an erbium doped fiber, having a defined wavelength band for amplification. Desirable erbium-doped fibers are available, for example, from about 1525 nm to 1565, although other amplification bands are available with active fibers doped with other active dopants such as erbium and / or rare earth dopants.
It has a desirable gain range for wavelengths between nm. As a result, transponder 12 will convert the wavelength received from transmitter 26 via line 30 to a selected wavelength within the amplification band for fiber amplifiers 18-21.

Referring to FIG. 2, a conventional transponder 35 for converting a receiving wavelength includes a photodiode 36, an electric amplifier 38, a light source 40, and an electro-optical modulator 42. The photodiode 36 is connected to the transmitter 2
6 and converts these signals into electrical signals in a known manner. The electric amplifier 18 boosts the electric signal from the photodiode 36. Typically, electrical amplifier 38 operates in saturation. Such saturation results in a non-linear response from the amplifier with respect to the input signal which reshapes the electrical signal and clamps its high level. The output from electrical amplifier 38 is sent to electro-optic modulator 42 to modulate the optical carrier signal generated by light source 40. The light source, or laser 40, produces a constant carrier wavelength corresponding to the wavelength or channel within the amplification band of amplifiers 18-21, unlike the light wavelength received by photodiode 36. In the experiments described below, the amplifiers 18 to
Other wavelengths in the 21 amplification bands could be used, but a laser with a wavelength of about 1557 nm was used.

In the configuration of FIG. 2, photodiode 36 receives pulses having a high bit rate. A high bit rate means that the pulse bit rate occurs above 2.5 Gb / s. Also,
The device upstream from the transponder 35, the transmitter 26, tends to produce light pulses with short rise and fall times that result in pulses with sharp edges as compared to Gaussian pulses. Further, the light source 4 in combination with the power amplifier 18 of the transmitter in FIG.
An output power of zero will provide a relatively high power level to the optical signal exiting the transponder. The term high power refers to peak input power at the beginning of an optical link, for example, in excess of 14 dBm per channel. Applicants have discovered that the combination of high bit rate and high power of the post-transponder light pulse results in detrimental nonlinear effects from SPM and GVD interactions that exceed the expectations of the known literature.

Applicants have noted that a transponder similar to transponder 35 shown in FIG. 2 does not effectively change the super-Gaussian pulse shape of the input pulse received by photodiode 36 from multiple SDH and SONET transmitters. Result transponder 3
It has been observed that the shape of the pulse leaving 5 depends on the pulse shape entering the transponder 35. Applicants have noted that replacing transponder 35 with a transponder that outputs a pulse shape that is somewhat independent of the input pulse shape, ie, that outputs a Gaussian-shaped pulse instead of a super-Gaussian-shaped pulse, is a matter of SPM and GVD interaction. Have been found to help mitigate the non-linear effects caused by.

FIG. 3 shows a transponder 43 which is consistent with the first embodiment of the present invention. The transponder 43 of FIG. 3 includes a photodiode 36, an electric amplifier 38, a laser 40, and an electro-optical modulator 42 as in the configuration shown in FIG. Photodiode 36 may be, for example, an avalanche photodiode or PIN-FET device that includes a PIN detector and a FET preamplifier. To facilitate reduction of SPM / GVD distortion from optical communication link 10, transponder 43 includes an electrical attenuator 44 located between photodiode 36 and electrical amplifier 38. As mentioned earlier, the shape of the pulse is important in affecting non-linear distortion from SPM / GVD interactions. The attenuator 44 includes the electric amplifier 38
Smoothes sharp edged signal pulses by attenuating the electrical signal received by R. In particular, the attenuator 44
A predetermined attenuation level is applied to the electrical signal received from photodiode 36 so that electrical amplifier 38 does not operate at typical deep saturation. As is well known in the art, deep saturation occurs when the output power of an amplifier is substantially independent of its input power. When the attenuator 44 is placed in front of the electrical amplifier 38, the electrical signal entering the electrical amplifier 38 is attenuated to a lower power level than that which causes the amplifier 38 to operate in deep saturation. Under these conditions due to the use of attenuator 44, electrical amplifier 3
The output power from 8 depends on the input power received from attenuator 44 and is primarily related to the input power level.

The exact amount of attenuation required to obtain an improved response to non-linear effects depends on the amount of rise and fall time for the pulses generated by the transmitter 26, the type of amplifier 38 used, the communication link 1
It will be appreciated that it depends on the overall characteristics of the communication link 10, such as the amount of amplification performed on the optical pulse as it travels through zero, the pulse rate, and the like. Model WBA3-4-0 manufactured by ERA to limit amplifier 38
In one example for the present invention using 6G20N, Applicants have observed that attenuator 44 must be set to provide an input voltage to amplifier 38 of less than 0.5V. Typically, such amplifiers are operated in saturation with an input voltage of about 1V. To obtain an input voltage of 0.5 V to the amplifier, a high-frequency resistance of 50Ω
An attenuator was used for attenuator 44 to provide 6 dB of attenuation. Such attenuation results in linear behavior as opposed to saturation for amplifier 38, and when a sharp edge pulse is received, a relatively smooth pulse as output from amplifier 38. The consequences of using the transponder 43 shown in FIG. 3 in the communication link 10 of FIG. 1 will be described below. Generally, the transponder 43
Reduces chirp caused by SPM.

FIG. 4 shows a transponder 45 consistent with the second embodiment of the present invention. The transponder 45 includes a photodiode 36, an electric amplifier 38,
A laser or light source 40 and an electro-optic modulator 4 similar to transponder 35 of FIG. 2 and transponder 43 of FIG.
2 is included. Like transponder 43, transponder 45 of the second preferred embodiment receives high bit rate pulses from an upstream device. However, the transponder 45 is not an electrical attenuator 44 located between the photodiode 36 and the amplifier 38, but a low pass filter (LPF) located after the electrical amplifier 38.
46 is included. LPF 46 reduces the bandwidth of the electrical signal received from amplifier 38. Desirably 2488M
For a transmission rate of b / s, the LPF 46 can
Fourth-order Bessel-Thompson filter with a bandwidth of 1.866 GHz, commercially available from Anritsu under the model MA1619.
1-Thompson filter). By reducing the bandwidth of the electrical signal received from amplifier 38, LPF 46 extends the rise and fall times of the edges of the signal pulses used by electro-optic modulator 42 to modulate the optical carrier from laser 40. Help to do. As in the embodiment of FIG. 3, the smoothing of the pulses in the configuration of FIG. 4 is accomplished by the chirp produced by the SPM over the communication link when the light pulses from the transponder are operated at high input power and operate at high bit rates. Help to reduce.

Alternatively, the LPF 46 can be placed before the amplifier 38 instead of after the amplifier 38 in order to reduce the pulse bandwidth. Such an arrangement causes the amplifier 38 to transition from a saturated mode of operation to a linear mode of operation. But,
So that amplifier 38 can maintain its operation in saturation
It is desirable to place LPF 46 after amplifier 38. Thus, the output signal from the amplifier 38 can maintain independence from the input signal.

FIG. 5 shows a transponder 47 according to a third embodiment of the present invention. The transponder 47 of FIG. 5 includes a photodiode 36, an electrical amplifier 38, a low-pass filter 46, a laser 40, and an electro-optic modulator 42, similar to the transponder 45 of FIG. 3 and 4 transponders 43 and 45
As in, this third embodiment receives a super-Gaussian pulse with a high bit rate from an upstream device, which is combined with high input power to the optical link from transponders and booster amplifiers. At times, the interaction of SPM and GVD can cause non-linear distortion. The transponder 47 of FIG. 5 includes a data and clock recovery circuit (DCR) 48 located between the photodiode 36 and the electrical amplifier 38. DCR4
8 smoothes the sharp edge input pulses by reducing the jitter of the light pulses received from the photodiode 36 and making the signal driving the amplifier 38 actually independent of the shape of the light signal coming from the transmitter 26. Function to be added. Instead, the signal driving the electrical amplifier 38 depends only on the pulse shape produced by the DCR 48. Preferably, DCR 48 is model number LG160
AT & T device with 0FXH operating at 2488 Mb / sec.

However, DCR 48 also produces pulses with relatively short rise and fall times, ie, pulses with sharp edges. An LPF 46 located downstream of the DCR 48 receives the sharp edge pulse from the DCR 48 and reduces its bandwidth, which effectively rounds the edge to the beginning and end of the pulse. As in transponder 45 in FIG. 4, LPF 46 in transponder 47 of FIG. 5 is preferably located after electrical amplifier 38 instead of before. Thus, the LPF 46 is
The edges of the electrical pulses at transponder 47 can be smoothed without operating amplifier 38 in a linear state as opposed to saturation. LPF46 is amplifier 3
8 and the electro-optical modulator 42, the DCR 48
Can regenerate the received pulses, and the amplifier 38 can operate in saturation, so that the pulses received by the LPF 46 are independent of the received pulses and are powered up. LPF 46 reduces the bandwidth and converts the pulse from a super Gaussian pulse shape to a Gaussian pulse shape. As in the transponders 43 and 45 described above, an electro-optic modulator 42, which may be a Mach-Zehnder modulator, an electro-absorption modulator or a similar device, converts the optical carrier signal from the laser diode 40 by the converted pulse. Is modulated.

In an optical communication system having a high bit rate and producing high power level modulated pulses to help reduce non-linear distortion caused by the interaction of SPM and GVD, transponders 43, 45 and 4
It should be understood that various combinations of the seven can be used. For example, a transponder having the electrical amplifier 38 located between the attenuator 44 and the LPF 46 can be used. Other combinations of the components described above will be apparent to those skilled in the art and are contemplated by the present invention.

Returning to FIG. 1, an apparatus for an experimental test using the transponders of the first three embodiments will be described below. As mentioned above, the optical communication system includes a series of long-haul communication fibers 14-17. Each of these fibers includes a single mode optical fiber with zero dispersion near 1300 nm. All communication fibers have a length of about 509 km, where fiber 14 is 129 km, fiber 15 is 128 km,
Fiber 16 is 125 km, fiber 17 is 127 km
Met. The carrier wavelength, that is, the wavelength of the laser 40 in the transponder 12 was 1557 nm. Amplifiers 18-21 included erbium-doped fiber amplifiers operating at a pump wavelength of 980 nm. Amplifier 18 is a model TP manufactured by the applicant.
Although A / E-SW and operated as a power amplifier for the transmitter, amplifiers 19-21 were also model OLA / EF manufactured by the applicant and functioned as optical line amplifiers. The average output power of the amplifier is 13.
5 dBm, corresponding to a peak power of about 16.5 dBm. Transmission fibers 14-17 also added attenuation to the transmission link. In particular, the added 14-17 is about 130
Zero dispersion wavelength of 0 nm, 25.4 dB and 2 respectively.
It had 6.7 dB, 27.1 dB and 24.9 dB attenuation. In addition, the attenuators 22 to 25 add adjustable attenuation to the optical link 10. These attenuators allowed for a change in span attenuation. The optical amplifiers each included a filter (not shown) to remove the amplified instantaneous spontaneous emission and minimize other noise in the amplifier. An optical preamplifier 67 can be placed in front of the receiver 28 to increase sensitivity. However, the optical preamplifier 67 did not exist in the embodiment tested by the applicant. The optical preamplifier 67 may be, for example, a model RPA / BF manufactured by the applicant.

For a standard optical fiber and a peak power for an optical pulse of P ₀ = 16 dBm, the nonlinear length L _NL is about 14 km. m = 3, 2.5 Gb
/ S bit rate and fiber loss α = 0.2d
For a standard fiber with B / km and β ₂ = −20 ps ² / km, the dispersion length L _D is 1750 km. For a standard fiber with a fiber loss α = 0.2 dB / km and an average span length z = 126.5 km (between optical amplifiers), z _eff is 21.6 km.

Based on the above values, equation (5) gives L _M = 82
km. In this case, the total length L _T of the link is 509k
because it is m, the condition L _T> L _M of pulse distortion is verified. FIG. 6 shows the relationship between the bit error rate (BER) and the received power for the optical link shown in FIG. 1 under various conditions. Line 50 illustrates the system performance when transmitter 26 and receiver 28 are looped together in a back-to-back configuration. Line 52
Shows the relationship between BER and received power when the span in FIG. 1 has an attenuation of 38 dB. Line 5
4 shows the result from the same setup with 41 dB of attenuation, and line 56 shows the result with 32 dB of attenuation. Each test shown for lines 52-56 is shown in FIG.
A standard transponder 35 was used.

FIG. 6 also shows the results of the BER and the received power when the transponder 47 of FIG. 5 is used in place of the transponder 35. Line 58 is 38 dB
The result using the transponder 47 having the span attenuation of FIG. Similarly, line 60 shows the relationship between BER and power received using transponder 47 with 41 dB span attenuation, and line 62 shows the performance of the same setup with 32 dB span attenuation. Comparison of these results shows that transponder 47 produces excellent sensitivity, ie, minimum receiver power to obtain a fixed BER. Transponders 43 and 45 can produce similar results.

As is well known in the art, about 600 k
It is possible to send an externally modulated signal without significant dispersion penalty along fiber lengths less than m. Nevertheless, at high input powers, SPM can result in pulse chirping that results in signal degradation despite relatively short fiber lengths.

Applicants have disclosed a dispersion compensating fiber segment or chirped fiber, such as a Bragg grating, along an optical fiber link operating at high input power and high bit rate having a fiber shorter than about 600 km. It has been discovered that inserting a compensator helps reduce nonlinear distortion caused by the interaction of SPM and GVD.

FIG. 7 therefore shows an SPM consistent with the present invention.
And another optical communication link 64 for reducing the effects from GVD interactions. The optical link 64 is shown in FIG.
The optical link 10 includes substantially the same components.
That is, the link 64 includes the transmitter 26 that supplies an optical pulse signal to the transponder 12, and the optical amplifiers 18 to 21.
, Transmission optical fibers 14 to 17 and attenuators 22 to 25 for transmitting an optical pulse to the receiver 28 over a certain distance. Transponder 12 may be a conventional transponder 35 as shown in FIG. 2 or may be an enhanced transponder such as 43, 45 or 47 shown in FIGS. However, the optical link 64 includes a dispersion compensator, preferably a grating 66, which is preferably placed in front of the receiver 28. This grating includes a Bragg grating filter or similar device with chirp and passband set according to the characteristics of the light pulses used in the link. Although the grating 66 is located at several locations on the optical link 64, the grating 66 is
8 is desirable. Although not used in the embodiment tested by the applicant, the optical preamplifier 6
7 can be placed before the grating 66 to increase sensitivity. The optical preamplifier 67 may be, for example, a model RPA / BF manufactured by the applicant. According to another embodiment, not shown, the grating 66 can be integrated with the optical preamplifier 67 and can serve as a bandpass filter. According to yet another embodiment, a dispersion compensating fiber can be used in place of the grating 66.

FIG. 8 shows the bit error rate (BE) for the optical link shown in FIG. 7 under various conditions.
R) and the received power are shown. Again, as in the test for optical link 10, line 68 shows the system performance when transmitter 26 and receiver 28 of optical link 64 are looped together in a back-to-back configuration. Line 70 has a span of 38 in FIG.
10 shows the relationship between BER and received power when dB attenuation is used but dispersion compensation is not used. In such a situation, the transponder 12 has a 6 dB electrical attenuator 44 in front of the electrical amplifier 38 driving the electro-optical modulator 42, so that its configuration corresponds to that of the transponder 43 of FIG. there were. Line 70 is
BE for received power values greater than about -31 dBm
The bottom value (floor) of R is shown. This corresponds to a BER where the received power does not decrease below a value between 10 ^-10 and 10 ^-9 . Line 72 indicates that fiber grating 66 is connected to transponder 12 of optical link 64 and booster amplifier 1.
8 shows the results from the same configuration placed between FIG.
This grating has a bandwidth of 7 nm and 1700 ps / n
m (corresponding to the dispersion of a standard fiber of about 100 km). Line 72 is line 7
Although not as noticeable as the bottom value of 0, it shows the bottom value in BER. Line 74 indicates that fiber grating 66 is
Shows the result from the same configuration as line 70 except that it is placed before the receiver 28 of FIG. It can be seen that improved noise performance is achieved without being affected by the bottom in the BER curve.

Although the above description and experiments relate to an optical system having a plurality of spans, Applicants have determined that the effective length of the fiber span, z _eff, exceeds the nonlinear length, L _NL , thereby causing pulse distortion. If so, it has been determined that high power single span optical systems, such as those used on repeaterless submarine links, can also benefit from the present invention. The effects of SPM / GVD can be countered in such systems as well by the use of transponders as described above with respect to FIGS. 3-5 and / or by dispersion compensation as described with respect to FIG. It is possible.

The embodiments described so far relate to single-channel transmission. Multiple channels or WDM
Transmission also uses SPM to determine the peak power of each channel.
If a high power optical amplifier is available to boost beyond a critical value that can cause pulse distortion due to / GVD interaction, the invention can benefit. Such a critical value depends on the characteristics of the optical link described above.

Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. The scope and spirit of the invention is indicated by the appended claims, and the specification and examples are to be regarded only as examples.

[Brief description of the drawings]

FIG. 1 is a block diagram of an optical communication system used in the present invention.

FIG. 2 is a block diagram of a conventional transponder used in the optical communication system of FIG.

FIG. 3 is a block diagram of a transponder used in the optical communication system of FIG. 1 according to the first embodiment of the present invention;

FIG. 4 is a block diagram of a transponder consistent with a second embodiment of the present invention.

FIG. 5 is a block diagram of a transponder consistent with the third embodiment of the present invention.

FIG. 6 is a chart of test results showing the relationship between bit error rate and received power for some configurations of the present invention in the optical communication system of FIG. 1;

FIG. 7 is a block diagram of an optical communication system according to a fourth embodiment of the present invention.

8 is a chart of test results showing the relationship between bit error rate and received power for some configurations of the present invention in the optical communication system of FIG. 7;

[Explanation of symbols]

 Reference Signs List 10 optical communication link 12 transponder 14-17 long-distance communication fiber 18-21 electric amplifier 22-25 electric attenuator 26 transmitter 28 receiver 35 transponder 36 photodiode 38 amplifier 40 laser 42 electro-optical modulator 43 transponder 44 electric attenuator 45 Transponder 46 low pass filter (LPF) 47 transponder 48 data and clock recovery circuit (DCR) 64 optical communication link 66 grating 67 optical preamplifier

 ──────────────────────────────────────────────────の Continued on the front page (71) Applicant 591011856 Pirelli Cavies System e. p. A (72) Inventor Mauro Macchi Varese, Italy, 21050 Gourla Maggiore, Via Don Zelvi 14 (72) Inventor Paolo Ottreng F-92330 France France Saw, Ryu Udan 43

Claims (15)

[Claims] 1. An optical communication system for reducing non-linear distortion caused by an interaction between self-phase modulation and group velocity dispersion, comprising: a transmitter for transmitting an optical pulse of a first wavelength; An optoelectronic device coupled to convert to a second wavelength, means for smoothing rising and falling transitions of electrical pulses received from the optoelectronic device, an electric amplifier, a light source, and an electro-optic modulator. And a transponder that is primarily coupled to the transponder, each having a length z
Wherein the total length of the plurality of spans is greater than (L _NL / z _eff ) z, where L _NL is the nonlinear length of the fiber. optical communication system comprising z _eff is the span of the effective span fiber length) plurality of, and a receiver coupled to the plurality of spans, the. 2. The optical communication system according to claim 1, wherein said means for smoothing electrical pulses includes an electrical attenuator disposed between the optoelectronic device and the electrical amplifier. 3. The optical communication system according to claim 1, wherein said means for smoothing an electric pulse includes a low-pass filter disposed between an electric amplifier and an electro-optical modulator. 4. The optical communication system according to claim 3, wherein said means for smoothing said electrical pulses further comprises a data and clock recovery circuit located between the optoelectronic device and the electrical amplifier. 5. The optical communication system according to claim 1, wherein each of the plurality of spans approximately exhibits the same amount of optical attenuation. 6. The method according to claim 6, wherein said transmitter is at least 2.5
The optical communication system according to claim 1, wherein the pulse is transmitted at a rate of Gb / sec. 7. The optical communication system according to claim 1, further comprising a dispersion compensator disposed between the plurality of spans and the receiver, wherein the total length of the plurality of optical transmission fibers does not exceed 600 km. 8. An optical communication system for reducing non-linear distortion caused by the interaction between self-phase modulation and group velocity dispersion, comprising: a transmitter for transmitting an optical pulse of a first wavelength; A transponder coupled to convert to a wavelength of light, comprising: an optoelectronic device; means for smoothing rising and falling transitions of electrical pulses received from the optoelectronic device; an electrical amplifier; a light source; and an electro-optic modulator. An optical communication system comprising: a section of an optical transmission fiber having an effective length z _eff greater than a non-linear length L _{NL of the} fiber; and a receiver coupled to the section of the fiber. 9. A method for reducing non-linear optical distortion caused by the interaction of self-phase modulation and group velocity dispersion, comprising: receiving an optical pulse from a transmitter and converting the optical pulse into an electrical pulse; Amplifying; smoothing the rising and falling transition edges of the electrical pulse; modulating an optical carrier signal with the electrical pulse; L _NL / z _eff (where L _NL is the nonlinear length of the fiber) Transmitting the modulated optical carrier signal over a plurality of transition spans having a cumulative length greater than _zeff is the effective fiber span length. 10. The method of claim 9, further comprising the step of reducing the bandwidth of the electrical pulse after the amplification of the electrical pulse. 11. The method of claim 9, further comprising compensating for dispersion of the modulated optical carrier signal at locations along a plurality of transmission spans. 12. The method of claim 11, wherein the compensating step occurs after the transmitting step. 13. A transponder for receiving an optical pulse of a first wavelength generated by an optical transmitter, modulating an optical carrier signal with the optical pulse, and reducing nonlinear distortion caused by an interaction between self-phase modulation and group velocity dispersion. And operating in a saturated state, wherein the photodiode is optically coupled to receive the light pulse and convert the light pulse into an electric pulse, and electrically coupled to receive and amplify the electric pulse. An electrical amplifier; a low pass filter electrically coupled to receive the electrical pulse from the electrical amplifier, extending a rise and fall time of the electrical pulse; and a light source for providing a second wavelength optical carrier signal; An electro-optical modulator arranged to modulate the optical carrier signal with electric pulses from the low-pass filter. Transponder. 14. The transponder of claim 13, further comprising a data and clock recovery circuit disposed between said photodiode and said electrical amplifier. 15. A method for receiving an optical pulse of a first wavelength generated by an optical transmitter and modulating an optical carrier signal with the optical pulse to reduce nonlinear distortion caused by an interaction between self-phase modulation and group velocity dispersion. A transponder, a photodiode optically coupled to receive the light pulse and convert the light pulse to an electrical pulse; and an electrical attenuator electrically coupled to receive the electric pulse from the photodiode. An electrical amplifier operating in saturation, electrically coupled to receive an electrical pulse from the electrical attenuator and amplify the electrical pulse; a light source providing a second wavelength optical carrier signal; An electro-optic modulator arranged to modulate with an electrical pulse from the electrical amplifier.