WO1999013588A2 - Optimisation des points de lancement pour solitons a propagation en dispersion conduite - Google Patents

Optimisation des points de lancement pour solitons a propagation en dispersion conduite Download PDF

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Publication number
WO1999013588A2
WO1999013588A2 PCT/US1998/018923 US9818923W WO9913588A2 WO 1999013588 A2 WO1999013588 A2 WO 1999013588A2 US 9818923 W US9818923 W US 9818923W WO 9913588 A2 WO9913588 A2 WO 9913588A2
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WIPO (PCT)
Prior art keywords
dispersion
chirp
optimal
fiber
length
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Application number
PCT/US1998/018923
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English (en)
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WO1999013588A9 (fr
WO1999013588A3 (fr
Inventor
William L. Kath
Tian-Siang Yang
Sergei K. Turitsyn
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Northwestern University
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Application filed by Northwestern University filed Critical Northwestern University
Priority claimed from US09/151,387 external-priority patent/US6462849B1/en
Publication of WO1999013588A2 publication Critical patent/WO1999013588A2/fr
Publication of WO1999013588A3 publication Critical patent/WO1999013588A3/fr
Publication of WO1999013588A9 publication Critical patent/WO1999013588A9/fr

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Classifications

    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/25—Arrangements specific to fibre transmission
    • H04B10/2507—Arrangements specific to fibre transmission for the reduction or elimination of distortion or dispersion
    • H04B10/25077—Arrangements specific to fibre transmission for the reduction or elimination of distortion or dispersion using soliton propagation

Definitions

  • the present invention relates to a transmission system using optical fibers and, more particularly, to a long-distance, large-capacity optical transmission system employing return-to-zero lightwave pulses, such as soliton lightwave pulses, and optical amplifiers.
  • optical fiber communication technology has made rapid-paced progress toward ultra-long-distance communication, now allowing implementation of a long-range communication system without the need of using regenerative repeaters.
  • conventional transmission systems suffer serious degradation of their transmission characteristics caused by the wavelength dispersion and nonlinear effect of optical fibers, imposing severe limitations on the realization of a high-speed, high-capacity transmission system.
  • timing jitter brought about by various causes during transmission is a main factor determining the transmission characteristic, along with degradation of the signal to noise ratio by accumulated optical noises.
  • the Gordon-Haus jitter forms a main part of such timing jitter.
  • the optical soliton carrier frequency which randomly fluctuates due to optical noises produced by optical repeater-amplifiers, is converted mainly by the wavelength dispersion characteristic of the fiber optic transmission line into fluctuations in the system.
  • the Gordon-Haus jitter increases with distance and, hence, exerts a great influence on long-distance soliton transmission.
  • the pulse spacing reduced by the Gordon-Haus jitter also increases the interaction between adjacent optical soliton pulses, newly causing timing jitter.
  • the Gordon-Haus jitter is an important problem which must be solved in order to transmit long
  • Dispersion management has proven to be an effective technique to reduce both effects simultaneously.
  • the idea of dispersion management is to concatenate fibers of both normal and anomalous dispersion to form a transmission line having both a high local group- velocity dispersion (GVD) and a low path-averaged GVD.
  • VGD group- velocity dispersion
  • This is beneficial since high local dispersion significantly reduces the efficiency of four- wave mixing, decreasing both the modulational instability gain and bandwidth.
  • lowering the average dispersion also reduces the Gordon-Haus timing jitter of soliton transmission systems.
  • dispersion management has been found to enhance the soliton energy; this additionally reduces the timing jitter below the amount that would be obtained in a system with constant dispersion equal to the path-averaged value.
  • intersymbol interference produced by the generation of dispersive waves (or, continuum radiation) shed by propagating pulses is a major limitation. It is therefore important to diminish the shedding of energy from the input pulse into a dispersive pedestal, which can be achieved by launching properly shaped and chirped pulses with optimum power into the fiber.
  • typical optical sources generate unchirped pulses and input pulse chirping is realized by using an additional piece of fiber preceding the beginning of the transmission line. It is therefore convenient to identify points in each dispersion-map period where pulses are naturally unchirped, and use one such location as the launch point. The partial map period preceding the first complete map period then plays the role of the prechirping fiber.
  • an object of the subject invention is an efficient, fiber-optic communication system.
  • a further object of the subject invention is a manner of optimizing a dispersion map for use in decreasing dispersive radiation in a fiber-optic system.
  • the subject invention includes dispersion maps with zero-chirp point positions independent of the dispersion values of the types of fiber comprising the map.
  • a zero-chirp point can be used as a special launching point for optical pulses; the subsequent evolution of the pulses is much cleaner than if other launching points are used since there is less dispersive radiation shed by the pulse as it evolves. Because these optimized dispersion maps identify zero-chirp launching points which are independent of the dispersion values of the types of fiber comprising the map, the launching points are, therefore, also independent of the wavelength since the second-order dispersion varies with wavelength.
  • such a dispersion map allows minimal shedding of dispersive radiation at several frequencies simultaneously, a situation that is ideal for wavelength-division-multiplexing.
  • this optimized map one need only choose the appropriate length of the fiber segments, locate the zero chirp point, and cut the combined fiber segments at the zero-chirp point location to form an optimal prechirping fiber.
  • Optimum lengths are chosen by using a theoretical estimate for the imbalance between the effects of the group velocity dispersion and nonlinear index of refraction.
  • Fig. 1 is a dispersion profile of a lightwave system which uses dispersion management.
  • Fig. 2 is a pulse profile showing the reduction in the- amount of dispersive radiation shed when a zero-chirp launch point is used (solid and dashed lines) versus other launching points (dot-dashed lines and long dashed lines).
  • Fig. 3 is a chirp produced in a dispersion managed system showing the local imbalance between linear dispersion and the nonlinear refractive index.
  • Fig. 4 is a pulse profile showing that the reduction in the amount of dispersive radiation shed can be optimized at two different frequencies when a specially-chosen dispersion map is used (solid and dashed lines) versus other launching points (dot-dashed lines and long dashed lines).
  • Dispersion is the effect that occurs when the propagation speed is dependent upon frequency; this process tends to break up signals and can pose a severe limitation upon the information capacity of optical communication systems.
  • optical solitons exploit the intensity -dependent index of refraction to compensate the dispersion.
  • the intensity-dependent refractive index produces an intensity-dependent pulse phase velocity, which can compensate _fw the frequency-dependent velocity produced by the group velocity dispersion. In this way, it is possible in theory to produce an optical pulse which can propagate for thousands of kilometers without significant distortion in an optically amplified transmission system.
  • Dispersion management arises when fibers with different dispersion parameters are concatenated to form a transmission line that has both low path-averaged group velocity dispersion and high local group velocity dispersion, as shown in FIG. 1.
  • Optimizing dispersion managed systems is also desirable.
  • high bit-rate terrestrial soliton communication systems for example, the generation of dispersive radiation by non-ideal starting conditions imposes a limit on the amplifier spacing. It is beneficial, therefore, to minimize the amount of dispersive radiation by either pre-chirping the pulses or by launching the pulses at the zero-chirp point of the dispersion map as shown in FIG. 2.
  • the chirp can be explained as an imbalance between the phase advance produced by the nonlinear index of refraction and the linear dispersion. In a dispersion- managed soliton, these balance on average, but in one part of the fiber the dispersion will be too small and the nonlinear index will dominate, while in another part of the fiber the dispersion will be larger and will dominate. An example of this is shown in FIG. 3.
  • a zero-chirp point is a point at which the accumulations of the two effects are locally balanced.
  • WDM wavelength-division-multiplexed
  • WDM systems employ two or more channels operating at different wavelengths to increase the total system capacity. Since the group-velocity dispersion parameter of an optical fiber depends upon wavelength or frequency, this means that a dispersion map's parameters will be different for each channel. This can necessitate independently adjusting and modulating each channel's laser transmitter to pre-chirp each channel to compensate for the differences.
  • a dispersion map constructed according to the subject invention renders the zero-chirp locations independent of the dispersion values of the two types of fiber comprising the map, thus making the ideal launching points independent of frequency.
  • Such special dispersion maps allow the amount of dispersive radiation shed by solitons in a WDM system to be reduced simultaneously over a range of frequencies without independent external adjustment or modulation, as shown in FIG. 4.
  • dispersion-managed solitons in lossless fibers are unchirped at the midpoints of the fiber segments.
  • pulses need to be amplified repeatedly due to fiber loss.
  • the subject analytic approach is suitable for cases where there are a finite number of amplifiers in each dispersion-map period. The goal is to locate the optimal launch points for dispersion-managed solitons where no pulse prechirping is needed.
  • Pulse evolution in dispersion-managed optical fibers with loss and gain is governed by the nonlinear Schr ⁇ dinger equation:
  • a practical implementation exploiting the use of these optimal chirp-free points can be constructed merely by using a portion of one dispersion map period before the first amplifier to aid in the launching of the optical pulses.
  • the dispersion map period is 120 km (20 km of anomalous dispersion fiber and 100 km of normal dispersion fiber), and for the parameters chosen there are two optimal chirp-free points, one at 11.3 km after the amplifier (in the anomalous dispersion fiber) and the other at 61.1 km before the amplifier (in the normal dispersion fiber).
  • the partial dispersion map has total length 61.1 km and comprises just 61.1 km of normal dispersion fiber. Either of these fiber lengths or partial dispersion maps can be used as a passive device to be installed before the first amplifier to reduce the amount of dispersive radiation generated by the propagating pulses, as shown in FIG. 2.
  • a similar wavelength-independent chirp-free point results in the second fiber segment by requiring the numerator and denominator of Eq. (3) to be zero simultaneously.
  • By employing one of these specially-constructed dispersion maps it is, therefore, possible to optimize the transmission of several wavelengths simultaneously, as shown in FIG. 4.
  • Such special dispersion maps are particularly suited for use as passive performance-enhancing devices in systems employing wavelength division multiplexing.
  • the invention involves a method of minimizing dispersive radiation in an optical fiber transmission system utilizing solitons with at least a first and a second type of optical fibers and an amplifier.
  • the method comprises first, determining a first optimal length of said first type optical fiber to form a first length of optical fiber; then determining a second optimal length of said second type optical fiber to form a second length of optical fiber, and connecting the optimal lengths of said first and second type of optical fibers to form a first dispersion map.
  • the first and second optimal lengths are those lengths where the net amount of chirp accumulated before and the net amount of chirp accumulated after said zero chirp point balance one another for all wavelengths and may be determined by the formula set forth above.
  • An amplifier is then located after every said dispersion map. Points in the dispersion map connected to the amplifier are then located where the pulse chip is zero.
  • a plurality of dispersion maps and amplifiers are connected to form in a transmission line; the first dispersion map in said transmission line is then cut at the zero chirp point to create an optimal launch point for all wavelengths in the transmission line.
  • a prechirping fiber may be prepared by first determining a first optimal length of a first type of optical fiber and a second optimal length of a second type optical fiber; the first and second optimal lengths are, as before, those lengths where the net amount of chirp accumulated before and the net amount of chirp accumulated after the zero chirp point balance one another for all wavelengths.
  • the first length is connected to the second length to form a dispersion map.
  • An amplifier is located after the dispersion map, and the pulse chirp zero points in the dispersion map connected to the amplifier is determined.
  • the dispersion map is cut at a zero chirp point to create an optimal prechirping fiber for all wavelengths.

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  • Physics & Mathematics (AREA)
  • Electromagnetism (AREA)
  • Engineering & Computer Science (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • Signal Processing (AREA)
  • Optical Communication System (AREA)

Abstract

L'invention concerne une théorie asymptotique permettant de prévoir l'évolution de solitons à propagation en dispersion conduite, en fonction des valeurs d'affaiblissement et de gain. On peut alors localiser facilement les points qui offrent un site de lancement optimal pour ces solitons, lesdits points correspondant à des emplacements où une précompression des impulsions n'est pas nécessaire. Les résultats numériques montrent que le lancement d'impulsions ayant le niveau adéquat de compression de phase et de puissance permet de réduire considérablement l'amplitude des impulsions et les oscillations en largeur ainsi que la quantité de rayonnement dispersif, indépendamment de la longueur d'onde.
PCT/US1998/018923 1997-09-10 1998-09-10 Optimisation des points de lancement pour solitons a propagation en dispersion conduite WO1999013588A2 (fr)

Applications Claiming Priority (4)

Application Number Priority Date Filing Date Title
US5836797P 1997-09-10 1997-09-10
US60/058,367 1997-09-10
US09/151,387 1998-09-10
US09/151,387 US6462849B1 (en) 1998-09-10 1998-09-10 Optimizing launch points for dispersion-managed solitons

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WO1999013588A2 true WO1999013588A2 (fr) 1999-03-18
WO1999013588A3 WO1999013588A3 (fr) 1999-07-29
WO1999013588A9 WO1999013588A9 (fr) 1999-09-02

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Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US6462849B1 (en) * 1998-09-10 2002-10-08 Northwestern University Optimizing launch points for dispersion-managed solitons
WO2003030411A1 (fr) * 2001-09-28 2003-04-10 Pirelli & C.S.P.A. Transmission de solitons sur des fibres optiques a dispersion de signe alterne et conjugaison de phase

Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US5504829A (en) * 1993-12-27 1996-04-02 Corning Incorporated Optical fiber for soliton transmission and method of making
US5559920A (en) * 1995-03-01 1996-09-24 Lucent Technologies Inc. Dispersion compensation in optical fiber communications
US5737460A (en) * 1995-12-29 1998-04-07 Lucent Technologies Inc. Applications of solitons in transmission systems employing high launch powers

Patent Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US5504829A (en) * 1993-12-27 1996-04-02 Corning Incorporated Optical fiber for soliton transmission and method of making
US5559920A (en) * 1995-03-01 1996-09-24 Lucent Technologies Inc. Dispersion compensation in optical fiber communications
US5737460A (en) * 1995-12-29 1998-04-07 Lucent Technologies Inc. Applications of solitons in transmission systems employing high launch powers

Non-Patent Citations (2)

* Cited by examiner, † Cited by third party
Title
CONFERENCE ON OPTICAL FIBER COMMUNICATIONS, IEE, Vol. 6, February 1997, pages 306-307, XP000901694. *
OPTICS LETTERS, Vol. 23, No. 8, 15 April 1998, YANG et al., pages 597-599, XP000885098. *

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US6462849B1 (en) * 1998-09-10 2002-10-08 Northwestern University Optimizing launch points for dispersion-managed solitons
WO2003030411A1 (fr) * 2001-09-28 2003-04-10 Pirelli & C.S.P.A. Transmission de solitons sur des fibres optiques a dispersion de signe alterne et conjugaison de phase

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WO1999013588A9 (fr) 1999-09-02
WO1999013588A3 (fr) 1999-07-29

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