WO2001047152A2 - Liaison de transmission optique sans perte - Google Patents

Liaison de transmission optique sans perte Download PDF

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Publication number
WO2001047152A2
WO2001047152A2 PCT/US2000/034928 US0034928W WO0147152A2 WO 2001047152 A2 WO2001047152 A2 WO 2001047152A2 US 0034928 W US0034928 W US 0034928W WO 0147152 A2 WO0147152 A2 WO 0147152A2
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WIPO (PCT)
Prior art keywords
pump
link
optical fiber
fiber
optical
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PCT/US2000/034928
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English (en)
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WO2001047152A3 (fr
Inventor
Vincent G. Dominic
David F. Welch
Robert G. Waarts
Stuart Maccormack
Mehrdad Ziari
Robert J. Lang
Donald R. Scifres
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Sdl, Inc.
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Publication of WO2001047152A2 publication Critical patent/WO2001047152A2/fr
Publication of WO2001047152A3 publication Critical patent/WO2001047152A3/fr

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    • 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
    • H04B10/29—Repeaters
    • H04B10/291—Repeaters in which processing or amplification is carried out without conversion of the main signal from optical form
    • H04B10/2912—Repeaters in which processing or amplification is carried out without conversion of the main signal from optical form characterised by the medium used for amplification or processing
    • H04B10/2916—Repeaters in which processing or amplification is carried out without conversion of the main signal from optical form characterised by the medium used for amplification or processing using Raman or Brillouin amplifiers
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/02—Optical fibres with cladding with or without a coating
    • G02B6/02042—Multicore optical fibres

Definitions

  • the present invention generally pertains to optical transmission systems and pertains more particularly to providing an effectively lossless optical fiber link.
  • Optical transmission losses in an optical fiber are inherent due to a number of factors including Rayleigh scattering and absorption.
  • An effectively lossless optical transmission link can be achieved by offsetting such loss with optical gain.
  • One way in which transmission losses may be offset is by interposing an optical amplifier, such as an erbium-doped fiber amplifier (EDFA) between spans or links of optical fiber or in discrete locations along the length of an optical fiber. If the gain of each amplifier is matched to the loss of an adjacent fiber link, the overall end-to-end effect of interconnected links and amplifiers can be a transmission system that is substantially lossless.
  • an optical amplifier such as an erbium-doped fiber amplifier (EDFA) between spans or links of optical fiber or in discrete locations along the length of an optical fiber.
  • an optical fiber is provided with optical pumping energy at a wavelength that is shorter than the wavelength of the signal to be amplified.
  • Raman scattering causes energy to be transferred from the pumping energy wavelength to the signal wavelength, thereby amplifying the signal and offsetting transmission losses.
  • This technique is attractive because the gain can be distributed along the length of the optical fiber, referred to as distributed Raman amplification, rather than concentrated in the discrete locations as is the case for localized optical amplifiers in place along the optical fiber.
  • co-propagation pumping For Raman amplification as well as EDFA amplification, pumping energy is provided in the same direction as signal propagation is referred to as co-propagation pumping or more simply as "co-pumping". Providing pumping energy in the direction opposite to signal propagation is referred to as counter-propagation pumping or more simply as “counter-pumping”. Co-pumping transfers noise on the optical pump beam to an optical signal more readily than counter-pumping because the relative walkoff velocity of the two beams is less for co-pumping than for counter- pumping. Also, even if the pump beam initially has no variation in its power level, it is possible for one signal channel to take energy away from the pump (via the gain mechanism) and thus affect the gain seen by the remaining signal channels.
  • Another object of this invention is the provision for Raman distributed amplification in an optical fiber link.
  • a further object of this invention is the provision of Raman distributed gain at an internal portion of an optical fiber link.
  • a still further object of this invention is provision of a peak Raman gain spectrum at a point along an optical fiber link.
  • an optical fiber link or transmission fiber in an optical transmission system comprises an optical fiber configured to produce Raman gain and provide for signal propagation in a signal wavelength range and to provide for Raman distributed gain along at least a portion of the fiber link.
  • a Raman pump source is coupled to the link to provide Raman distributed gain at a point where it is higher in an internal portion of the fiber than compared to either said of such a internal portion of the fiber link.
  • the distributed gain may include rare earth generated gain at the signal wavelengths as well as Raman generated gain.
  • one or more fiber Bragg gratings may be provided in the fiber link or in the coupling fiber or pigtail fiber between the Raman pump source and the fiber link to provide for gain distribution along the fiber link.
  • a plurality of gain cavities can be provided in the internal portion of the fiber link which are spatially separated or overlapping.
  • Such gain cavities can be Raman generated gain or rare earth generated gain or a combination of both.
  • the Raman pump source may be stabilized as to its wavelength or its wavelength spectrum by employing a stabilizing fiber Bragg grating at the pump source output. In such a case, the pump source may be driven to coherence collapse operation.
  • an optical fiber link comprising an optical fiber configured to produce Raman gain and to provide for Raman distributed gain for a plurality of optical signals propagating along the fiber link.
  • At least one Raman pump source is provided having a predetermined optical power level as provided via a control circuit for the pump source. Also, the control circuit may dynamically vary the wavelength output of the pump source.
  • a controller is employed to detect the number of optical signals propagating along the fiber and reduce or increase the pump source power as the number of optical signals propagating along the fiber is correspondingly reduced or increased.
  • the wavelength of its operation is at a first Raman order relative to the signal wavelength or bandwidth.
  • one pump operates at a first Raman order and the second pump operates at a second Raman order.
  • the pumps may pump the optical link from opposite ends of the link or can be both counter-pumping the fiber link from the downstream end, i.e., counter-pumping relative to the direction of propagation of the optical signals.
  • Another feature of this invention is the provision of one or more Raman pump sources for a fiber link configured to provide Raman gain and to provide for Raman distributed gain along the link where a controller for the pump source(s) control(s) the bandwidth of the source(s) to be within the Raman gain bandwidth of the fiber.
  • a further feature of this invention is the provision of a fiber link in an optical fiber transmission system utilizing a fiber that has optical transmission characteristics substantially maintaining the power of a optical signal propagating through the fiber link which substantially experiences a lossless condition such that, for example, the signal power along the fiber varies no more than about five dB over about thirty kilometers or more.
  • Reflectors such as fiber Bragg gratings, in the fiber or in the fiber and the pump coupling fiber are utilized to distribute the first Raman order power or second Raman order power throughout the fiber link.
  • the power level of the second Raman order pump which is maintained at a level higher than the power level of the first Raman order pump, is controlled to vary the point of the peak Raman gain spectrum along the fiber link.
  • Fig. 1 is a schematic block diagram of an optical transmission system.
  • Fig. 2 is a hypothetical graphical illustration of signal power as a function of distance along a conventional optical transmission system of the type shown in Fig. 1.
  • Fig. 3 is a schematic block diagram of two optical fiber links in an optical transmission system employing Raman amplification to reduce transmission losses.
  • Fig. 4 is a hypothetical graphical illustration of signal power as a function of distance along an optical transmission system of the type in Fig. 3 employing Raman amplification to reduce transmission losses.
  • Fig. 5 is a schematic block diagram of two optical fiber links in an optical transmission system that employ counter- and co-pumped Raman amplification to provide more uniform optical gain along each link of the system.
  • Fig. 6 is a hypothetical graphical illustration of signal power as a function of distance along an optical transmission system of the type shown in Fig. 5 employing counter- and co-pumped Raman amplification.
  • Fig. 7 is a schematic block diagram of a single optical fiber link that uses counter- and co-pumped Raman amplification to reduce transmission losses so that a single link can provide reliable communication across greater lengths.
  • Figs. 8-10 are schematic block diagrams of optical fiber links including one or more pump sources coupled to locations near the middle of the link.
  • Fig. 11 is a schematic block diagram of an optical link that receives wavelength- multiplexed pumping energy from two pump sources.
  • Figs. 12-17 are schematic block diagrams of optical fiber links including one or more pump sources that deliver pump having wavelength, bandwidth and/or power that varies according to a controller.
  • Fig. 18 is a schematic block diagram of an optical fiber link that uses a reflective grating to stabilize a pump source.
  • Fig. 19 is a schematic block diagram of an optical fiber link that couples pump from a single pump source into multiple locations along the fiber link.
  • Figs. 20A-20D are cross-section schematic diagrams of optical fiber.
  • Fig. 21 is a schematic block diagram of an optical fiber link in which stress is applied to polarization-sensitive fiber to change the overlap between orthogonally- polarized pumping energy and the signal at multiple locations along the fiber link.
  • Fig. 22 is a schematic block diagram of an optical fiber link that uses reflective gratings to control the distribution of pumping energy along the length of the fiber link.
  • Fig. 23 is a hypothetical graphical illustration of pump power as a function of distance along the optical fiber link that is illustrated in Fig. 22.
  • Fig. 24 is a schematic block diagram another embodiment similar to Fig. 22 of an optical fiber link that uses reflective gratings to control the distribution of pumping energy along the length of the fiber link where one of the gratings is close to the counter-propagating pump source, i.e., in its pigtail fiber.
  • Fig. 25 is a schematic block diagram of an optical fiber link that uses reflective gratings to control the distribution of pumping energy along the length of the fiber link similar to Fig. 24 except the one grating close to the counter-propagating pump source is at the output end of the fiber link.
  • Fig. 26 is a combination schematic block diagram and graphic illustration of an optical fiber link illustrating the distributed Raman amplification profile along a fiber link in the case of second Raman order co-propagating and first Raman order counter- propagating pump sources.
  • Fig. 27 is a combination schematic block diagram and graphic illustration of an optical fiber link illustrating the distributed Raman amplification profile along a fiber link in the case of combined first and second Raman order counter-propagating pump sources.
  • Fig. 28 is a schematic block diagram of an optical fiber link that reduces transmission losses by providing Raman amplification in a fiber section that compensates for chromatic dispersion.
  • Fig. 29 is a hypothetical graphical illustration of signal power as a function of distance along an optical fiber link as a result of the gain provided by Raman amplification in a chromatic-dispersion compensating fiber segment.
  • Fig. 1 provides a schematic block diagram of an optical transmission system in which transmitter 10 launches into the "upstream" end of optical fiber link 30-1 an optical signal that represents an electronic input signal received from path 1.
  • the optical signal propagates along optical fiber link 30-1, sustaining losses in power or intensity due to several causes including Rayleigh scattering, optical couplers, splices, kinks and bends in the optical fiber, and various types of absorption, until it is received by optical amplifier 40-1.
  • Optical amplifier 40-1 receives the optical signal at the "downstream" end of link 30-1 and launches into optical fiber link 30-2 an amplified replica of the received optical signal.
  • the optical signal propagates along optical fiber links 30-2, 30-3 and 30-4 with amplification provided by optical amplifiers 40-2 and 40-3 until it reaches receiver 20.
  • Receiver 20 generates along path 9 an electronic signal that represents the optical signal received from link 30-4.
  • Each of the optical amplifiers may be a rare-earth doped fiber amplifier such as an erbium-doped fiber amplifier; however, no particular type or implementation of amplifier is critical.
  • Fig. 2 provides a hypothetical graphical illustration of optical signal power as a function of distance along the optical transmission system shown in Fig.l .
  • curve 42 optical signal power declines as the signal propagates along each optical fiber link and is boosted by each optical amplifier. If the gains of the optical amplifiers are carefully matched to the optical losses sustained in the fiber links, substantially “lossless" transmission can be provided between transmitter 10 and receiver 20.
  • the term “lossless” refers only to signal power or intensity. It does not refer to the loss of signal quality that occurs because the optical signal-to-noise ratio (OSNR) of the optical signal steadily degrades from transmitter to receiver.
  • OSNR optical signal-to-noise ratio
  • the span from transmitter 10 to receiver 20 may be used as a complete transmission system or it may be used as one segment of a larger transmission system in which receiver 20 of one segment is used to electronically regenerate a digital signal for transmitter 10 of a subsequent segment.
  • Fig. 3 provides a schematic block diagram of a portion of a transmission system like that shown in Fig. 1.
  • two optical fiber links 30-5 and 30-6 are coupled together by optical amplifier 40.
  • Raman amplification may be provided to offset some of the optical fiber transmission losses.
  • counter-pumping by pumping sources 51-5 and 51-6 provides for Raman amplification in links 30-5 and 30-6, respectively.
  • counter-pumping is generally preferred to co- pumping because counter-pumping is more resistant to noise in the pumping energy and to crosstalk between different amplified signals: however, co-pumping may be satisfactory in optical transmission systems that use a low-noise pumping source such as an InP semiconductor laser source. Furthermore, crosstalk may be reduced in co- pumped systems that convey a large number of optical signals due to an averaging effect of the signal patterns on the pump.
  • U.S. patent application Serial No. 09/430,394, filed October 22, 1999 and entitled, MULTIPLE WAVELENGTH OPTICAL SOURCES; U.S.
  • the pump sources may be a Raman resonator, cascaded Raman resonator, a cascaded Raman resonator powered by fiber laser, a semiconductor laser, a semiconductor optical amplifier (SOA) power by a fiber or semiconductor laser, or a semiconductor laser.
  • SOA semiconductor optical amplifier
  • only one type of source can be employed in lieu of another, e.g., a semiconductor laser source can only be employed in cases of resonator distributed amplification, as will be evident from later discussion, because the use of a fiber laser source may result in feedback at different .
  • Fig. 4 provides a hypothetical graphical illustration of signal power as a function of distance along optical fiber links 30-5 and 30-6 as a result of the gain provided by optical amplifier 40 and Raman amplification distributed within the links.
  • Curve 42 represents the signal power that results from transmission losses in optical fiber links 30-5, 30-6 and the optical gain of optical amplifier 40 without the benefit of Raman amplification.
  • Curves 44 and 45 provide comparative illustrations of the signal power that can be achieved by adding Raman amplification.
  • the power of the optical signal that is launched into link 30-5 is kept the same as that for the example shown by curve 42, and the gain of optical amplifier 40 is reduced according to the gain provided by Raman amplification so that the same optical power is launched into link 30-6.
  • This implementation maintains a higher OSNR as compared to curve 42.
  • the rate of accumulation of ASE noise along a cascade of amplifiers is reduced, thus maintaining a high OSNR after each span compared to the case represented by curve 42.
  • the power of the optical signal that is launched into link 30-5 is reduced as compared to the example for curve 42, and the gain of optical amplifier 40 is reduced so that this same optical power is launched into link 30-6.
  • the level of launched power and the gain of optical amplifier 40 are chosen so that this implementation achieves the same OSNR as that for curve 42. This is the same OSNR at lower launch power because we have the same gain between the input and the output of the span but less ASE injected into the following span because of the reduced localized or discrete gain and the fact that the distributed Raman gain produces distributed ASE rather than lumping its ASE production at the span output.
  • the launched power can be set to any level but it is useful to point out that the power level can be set to balance a number of competing interests.
  • higher levels of launched power facilitate reliable transmission of higher data-rate signals and/or increased numbers of data channels, can be used to compensate for imperfections in electronic receiving and signal-regenerating circuitry, and permit the use of longer links between optical amplifiers.
  • lower levels of launched power reduce the power requirements on the amplifiers and also reduce various non-linear impairments such as those caused by four-wave mixing, self-phase and cross-phase modulation, and Raman signal-to-signal interactions.
  • Fig. 5 provides a schematic block diagram of two optical fiber links and an optical amplifier similar to that shown in Fig. 4.
  • pumping source 51-6 provides counter-pumping to optical fiber link 30-6,
  • pumping source 52-5 provides co-pumping to optical fiber link 30-5, and
  • pumping source 53-5 provides counter- pumping and co-pumping to links 30-5 and 30-6, respectively.
  • pumping sources 52-5, 53-5 and 51-6 provide pumping energy at the same or substantially the same wavelength, which differs from the wavelength of the signal to be amplified by one Stokes shift. For example, if the signal has a wavelength in a range from about 1530 nm to about 1560 nm, the wavelength of the pumping energy could be in a range from about 1430 nm to about 1460 nm.
  • wavelengths such as 1550 nm, 1450 nm and 1360 nm. These references should generally be understood to represent a range of wavelengths.
  • the nominal wavelength of 1550 nm is intended to represent a range of wavelengths such as, for example, from about
  • counter-pumping by pumping sources 53-5 and 51-6 provides pumping energy at the same or substantially the same first wavelength, which differs from the signal wavelength by one Stokes shift
  • co-pumping by pumping sources 52-5 and 53-5 provides pumping energy at the same or substantially the same second wavelength, which differs from the signal wavelength by two Stokes shifts.
  • the second pumping wavelength differs from the first pumping wavelength by one Stokes shift.
  • counter-pumping provides Raman amplification for the signal
  • co-pumping provides Raman amplification for the counter-pumped pumping energy.
  • Raman amplification provided by co-pumping partially offsets the transmission losses sustained by the counter-pumping energy.
  • the co-pumping energy amplifies the counter-pump energy thus providing substantial signal gain at both the output and the input end of the link.
  • pumping source 53-5 provides pumping energy at two different wavelengths.
  • Fig. 6 provides a hypothetical graphical illustration of signal power as a function of distance along an optical transmission system in which curve 46 represents optical signal power obtained from Raman amplification provided by counter- and co- pumping.
  • curve 46 represents optical signal power obtained from Raman amplification provided by counter- and co- pumping.
  • transmission losses of the optical fiber links are more closely offset along the entire length of each link and the gain of optical amplifier 40 may be reduced to zero or essentially zero. Note that in such a case the transmission system uses all Raman gain and the need for Er-doped amplifiers might be obviated.
  • the improved match between fiber transmission losses and distributed Raman amplification gain may be exploited in a number of ways including the use of longer links or lowered signal launch power which avoids nonlinear impairments.
  • Fig. 6 provides a hypothetical graphical illustration of signal power as a function of distance along an optical transmission system in which curve 46 represents optical signal power obtained from Raman amplification provided by counter- and co- pumping.
  • FIG. 7 provides a schematic block diagram of this situation where a single optical fiber link 30 uses counter- and co-pumped Raman amplification to provide reliable communication across the same distance that is spanned by the two links shown in Fig. 3, for example, about 1360 nm co-propagating and about 1455 nm counter- propagating.
  • Figs. 8-10 provide schematic block diagrams of several examples for providing pumping energy to an optical fiber link.
  • pumping source 51 -2 provides pumping energy at or near the downstream end of optical fiber link 30 and pumping source 51-1 provides pumping energy at or near the middle of link 30.
  • pumping energy is provided only by pumping source 51 at or near the middle of link 30.
  • Pump source 51 in Fig. 9 can be coupled to provide pump energy both upstream and downstream of link 30.
  • multiple pumping sources 51 -1 through 51-3 provide pumping energy at locations that are distributed along a middle portion of link 30. Fewer counter-pumping sources may be needed, or counter-pumping sources may be separated from one another more widely if one or more co-pumping sources are also used.
  • Raman amplification of the signal is desired most.
  • little or no Raman amplification is provided at or near the upstream end of an optical fiber link in transmission systems that use optical amplifiers to boost signal power between links.
  • pumping energy may be provided at one or more wavelengths.
  • the wavelengths may be exactly the same, or substantially the same in the sense that they differ from the signal wavelength by the same number of Stokes shifts, or they may differ significantly in the sense that they differ from the signal wavelength by a different number of Stokes shifts.
  • Each pumping source may be a wavelength-multiplexed and/or polarization- multiplexed combination of multiple sources as shown by the example illustrated in
  • 51-8 is multiplexed together and launched to counter-propagate into optical fiber link
  • Figs. 12-17 provide schematic block diagrams of several examples in which various characteristics of pumping energy are varied in response to a pump controller.
  • Fig. 18 provides a schematic block diagram of an example in which output power of a pumping source is stabilized.
  • controller 61 is used to vary wavelength, bandwidth and/or power of pumping energy by controlling the operation of a single pumping source 51.
  • This control may be used to compensate for changes in operating conditions such as variations in the number or intensity of optical signals or changes in operating characteristics of optical fiber link 30. such as changing signal traffic on the link, or other components like optical amplifier 40 that are caused by change in signal power due to channel loading, aging of the fiber link or variations in the operating environment like temperature.
  • a tunable Bragg grating 51TG can be employed in coupling fiber 51C to control the bandwidth of the wavelength spectrum output of pumps source 51 so as, for example, to be in the Raman gain bandwidth of fiber link 30.
  • the bandwidth of grating 51TG is changed through the tuning function, as is now known in the art such as, for example, by strain inducement, heat application of heat, piezo-electric induced vibrations and other such techniques to vary the grating bandwidth and its peak wavelength.
  • pumping sources 54-56 may each provide pumping energy at a different wavelength, bandwidth or power level and, in response to controller 62, these pumping sources may be selected to operate individually or in any combination.
  • the combined bandwidth output of sources 54, 55 and 56 can be adjusted through power cutoff or power adjustment of the sources so that their combined wavelength output is within the Raman gain bandwidth of the fiber link 30.
  • each of these single pumping sources may be replaced by a combination of multiple sources operating under a common controller.
  • optical fiber link 30 is used to transmit one or more distinct optical signals, perhaps differing from one another in wavelength.
  • Detector 63 at the upstream end of optical fiber link 30 is used to detect the number of distinct signals that are being transmitted at any give time and, in response, controller 65 causes counter-pumping source 51 to provide higher levels of pumping energy when larger numbers of signals are being transmitted and lower levels of pumping energy when smaller numbers of signals are being transmitted. Also, controller 65 can also change the wavelength spectrum of pump source 51 in response to the wavelength spectrum of signal channels currently loaded on the link.
  • controller 64 may cause counter-pumping source 51 to vary pumping energy wavelength in response to various characteristics such as the spectral content of the signals being transmitted. This may be accomplished in a variety of ways. One way varies the output level of multiple pumping sources that provide different wavelengths of pumping energy. Another way varies the operating temperature of a semiconductor diode laser pumping source. Yet another way varies the strain of reflective gratings used to tune the wavelength of a pumping source. Essentially any technique for varying the wavelength of a laser source may be used including known techniques for providing wavelength-tunable lasers.
  • FIG. 15 differs in that detector 63 is located at the downstream end of optical fiber link 30 and detector 63 is used to control, via controller 65, counter-pumping source 51 in the middle portion of optical fiber link 30.
  • the arrangement shown in Fig. 16 differs from the example shown in Fig. 14 in that detector 63 is used to control the operation of co-pumping source 52.
  • This implementation provides a faster response as compared to the other implementations discussed above because there are no propagation delays for either the optical signal, as in the case of the embodiment in Fig. 15, or in the case of the control signal in Figs. 14 and 17.
  • detector 63 is located in the upstream end of fiber link 30 and is used to control, via controller 67, counter-pumping sources 51 -1 to 51-3 spatially distributed along the middle portion of optical fiber link 30.
  • the bandwidth of these sources can be controlled so that their combined wavelength output are within the Raman gain bandwidth of fiber link 30.
  • Fig. 18 represents a different type of pumping source control.
  • the output of pumping source 51 is stabilized by forcing the source to operate in coherence collapse.
  • This is disclosed in U.S. patents 5,485,481 and 5.715,263, which are incorporated herein by reference. This may be achieved by placing reflective grating 77 in the optical path of the pumping energy at an optical distance from the source that exceeds its so-called coherence length.
  • operation in coherence collapse can be facilitated by driving pumping source 51 with time-varying drive current 59. Additional details on achieving coherence collapse may be obtained from U.S. patent application Serial No. 08/621,555, filed March 25, 1996, and Serial No. 09/197,062, filed November 20, 1998, both of which are incorporated herein by reference.
  • pumping source 52 emits pumping energy into optical fiber 31 , which is coupled to optical fiber link 30 at one or more locations.
  • core region 92 is surrounded by outer region 91.
  • the choice of materials, dopant if any, and geometry for these two regions preferably is selected to optimize the transmission of the optical signals to be amplified.
  • the mode field diameter of core region 92 may be reduced to reduce signal dispersion.
  • optical fiber 31 includes core region 102 surrounded by outer region 101; however, the choice of materials, dopant if any, and geometry for these two regions preferably is chosen to optimize the transmission of the pumping energy.
  • the mode field diameter of core region 102 may be increased to reduce pumping energy transmission losses while decreasing the numerical aperture of fiber 31.
  • Core regions 92 and 102 may be fused occasionally to couple the two fibers at distributed locations.
  • signal and pumping energy may be combined using a single optical fiber.
  • a first core region 92 for the signal and a second core region 93 for the pumping energy are essentially parallel to one another and are both surrounded by outer region 91.
  • first core region 92 for the signal and second core region 93 are coaxial.
  • a single core region 94 that is surrounded by outer region 91 supports two optical modes, a first mode 121 for the signal and a second mode 122 for the pumping energy.
  • the materials, dopants if any, and geometry of the various regions may be established to transmit and couple the pumping energy and signal in whatever way is desired.
  • the overlap of the pumping energy with the optical signal path is increased where more Raman amplification is desired.
  • the distribution of Raman amplification or gain can be controlled to achieve a desired distribution of gain. If all of the gain that is realized by Raman amplification is confined to an interval or limited distance (for example, 5 km) at or near the downstream end of an optical fiber link, very little benefit in OSNR can be realized over what can be achieved using only conventional optical amplifiers between links.
  • the gain realized by Raman amplification is distributed uniformly along the entire length of the optical fiber. Unfortunately, this is difficult to achieve in practical implementations. Perfectly uniform amplification or gain, either Raman gain or from rare-earth provided gain, in the transmission fiber suffers from the accumulation of noise in the signal from multiple Rayleigh reflection events. Thus, the optimum gain distribution is necessarily not uniform.
  • the gain that is achieved by Raman amplification depends on both the intensity of the pumping energy and the degree to which the polarization orientations of the pumping energy and the signal overlap.
  • Raman amplification gain for orthogonally- polarized signal and pumping energy is very small.
  • the birefringent properties of the fiber cause the polarization orientations of the signal and the pumping energy to fluctuate. These fluctuations and the resulting Raman amplification gain are effectively averaged over the length of the optical fiber.
  • optical fiber link 30 is a polarization-sensitive or polarization maintaining (PM) fiber and pumping source 51 provides pumping energy that has a polarization orientation that is substantially orthogonal to the polarization of the signal to be amplified.
  • PM polarization-sensitive or polarization maintaining
  • the orthogonal polarization orientation of the pumping energy may be preserved, allowing the pumping energy to propagate along the optical fiber until it reaches a location where Raman amplification is desired.
  • the PM properties of the optical fiber can be disrupted at that location to allow the polarization orientation of the pumping energy to completely overlap with the polarization orientation of the signal.
  • the distribution of pumping energy within an optical fiber link can be controlled by reflectors that are designed to pass signal wavelengths but reflect certain pumping wavelengths.
  • Fiber Bragg gratings are one practical way to implement such reflectors; however, in principle, no particular type of reflector is critical to the present invention.
  • Fig. 22 provides a schematic block diagram of an optical fiber link that uses reflectors 71 and 72 to control the distribution of counter- and co-pumping energy provided by pumping sources 51 and 52, respectively.
  • the distribution is controlled to increase the amount of Raman gain in the middle portions of the optical fiber link and to avoid or limit increases in Raman gain at or near the upstream end of the fiber link where signal power levels approach the limits of linear or substantially linear operating characteristics of the fiber.
  • the signal wavelength is 1550 nm
  • the co-pumping wavelength of pumping source 52 is 1360 nm
  • the counter- pumping wavelength of pumping source 51 is 1450 nm.
  • Reflector 71 has a reflectivity level of essentially zero at 1360 nm and a reflectivity level of essentially 100% at 1450 nm.
  • Reflector 72 has a reflectivity level of essentially 100% at 1360 nm and a reflectivity level of essentially zero at 1450 nm.
  • counter-pumping energy from source 51 is substantially confined to the portion of optical fiber link 30 between reflector 71 and pumping source 51.
  • co- pumping energy from source 52 is substantially confined to the portion of optical fiber link 30 between pumping source 52 and reflector 72.
  • the 1360 nm co-pumping energy is distributed to increase Raman amplification of the 1450 nm counter- pumping energy in a middle portion of optical fiber link 30.
  • any portion of the link as well as the link itself may be provided with distributed gain continuous along the length of optical fiber link 30.
  • Fig. 23 provides a hypothetical graphical illustration of pump power as a function of distance along the optical fiber link as a result of the gain provided by Raman amplification.
  • Curve 81 represents the power level of 1360 nm pumping energy.
  • Curve 83 represents the power level of 1450 nm pumping energy without the benefit of the Raman gain provided by the 1360 nm pumping energy, and curve 84 represents the power level of 1450 nm pumping energy with the benefit of the Raman gain provided within interval 87 of the optical fiber link.
  • the net result of pump power along the fiber link is represented by dotted line 85 which, as can be seen in Fig. 23, is fairly spatially uniform and continuous along its length.
  • One or more reflectors may be used to achieve a wide variety of distributions of pumping energy along the fiber link.
  • Fig. 24 discloses another embodiment for distributed Raman amplification in fiber link 30 using a reflector pair or pairs to spatially distribute first Raman order pump power.
  • a semiconductor laser source such as, for example, an InP/InGaAs laser, is employed since it will not provide any feedback at a different stokes shift as in the case of a fiber laser source.
  • pump 51 operates at 1363 nm, i.e., at a second Raman order relative to a signal wavelength around 1550 nm, and has, in its coupling fiber or pigtail fiber 79, a fiber Bragg grating with a reflective bandwidth with a peak around 1455 nm, or the first Raman order relative to signal wavelength around 1550 nm.
  • Fiber link 30 includes, well upstream, a fiber Bragg grating 71 that is 100% reflective of the first Raman order wavelength at 1455 nm and is transparent to the signal wavelength at 1550 nm, propagating from left to right along fiber link 30.
  • WDM coupler 73 is, for example, a fused biconical coupler fabricated or drawn to permit the coupling between link 30 and fiber 79 of the first Raman order wavelength but not the signal wavelength, which remains on fiber link 30.
  • pump source 51 provides second Raman order pump power counter- propagating in fiber link 30 that is stokes shifted to the first Raman order or 1455 nm which provides distributed gain to the 1550 nm signal as first Raman order propagates along link 30 toward fiber Bragg grating 71.
  • any residual first Raman order pump light is reflected by reflector 71 downstream in the link where it may be reflected again by reflector 74 back into fiber link 30.
  • reflectors 71 and 74 primarily confine substantially all of the 1455 nm first Raman order energy within an optical cavity formed between these two reflectors.
  • FIG. 25 Another version of the embodiment shown in Fig. 24 is illustrated in Fig. 25.
  • fiber Bragg grating reflector 74 is positioned in fiber link 30 beyond and downstream of WDM coupler 75 rather being placed in pigtail fiber 79. Further, WDM coupler 75 is fabricated so that first Raman order wavelength at 1455 nm and the propagating signal to be amplified at 1550 nm remain in link 30 and the second Raman order wavelength at 1363 nm from pump source 51 is coupled through WDM coupler 75 to counter-propagate in fiber link 30. As is the case of the version in Fig.
  • an optical cavity is established in an optical cavity formed between reflectors 71 and 74 so that the first Raman order, 1455 nm pump energy is spatially confined between reflectors 71 and 74 in link 30 to provide gain to the propagating signal at 1550 nm.
  • Fig. 26 illustrating co-propagating pump energy at a second Raman order, for example, at 1363 nm, and counter-propagating pump energy at a first Raman order, for example, at 1455 nm, in fiber link 200.
  • Raman gain is achieved in fiber link 200 for optical signals, such as around 1550 nm via first
  • Raman order pump light As illustrated in Fig. 26, the second Raman order extends the penetration of first Raman order pump energy from the counter pump source 51 , which extension is diagrammatically illustrated at 204, further upstream in fiber link
  • Fig. 27 illustrating the employment of combined first Raman order and second Raman order pump energy at 301 and 303 coupled at the downstream end of fiber link 300 for counter-propagating in the link.
  • the first Raman order pump wavelength may be 1455 nm and the second Raman order wavelength may be 1363 nm.
  • the second Raman order provides gain to the first Raman order which provides signal amplification. With greater gain provided to the first Raman order, its capacity for distributed amplification or gain along fiber link 300 is extended further upstream in the link and its peak Raman gain spectrum 304 can be toward the internal portion of fiber link 300.
  • These pump sources may be coupled to fiber link 300 in the manner as previously illustrated and discussed in connection with Fig. 1 1.
  • the pump energy of the second Raman pump source is made stronger or higher than that of the first Raman order pump source so that the peak spectrum of Raman distributed amplification is extended further upstream in fiber link 300 as illustrated at 302 in fiber link 300 in Fig. 27 as well as the peak Raman gain spectrum 304 as indicated in the Raman distributed amplification profile 306 in the graphic representation of Fig. 27.
  • the Raman pump power is indicated by the descending portion 308 of profile 306.
  • the point of the peak Raman gain spectrum 304 can be changed along fiber link 300, i.e., the higher the second order power, the greater the distance upstream of the point of the peak Raman gain spectrum 304 into the fiber link internal portion, and the lower the second order power, the less the distance upstream of the point of the peak Raman gain spectrum 304 into the fiber link internal portion.
  • optical fiber manifest a characteristic known as chromatic dispersion, which is caused by different wavelengths traveling at different velocities in the fibers. Chromatic dispersion is undesirable because it causes temporal packets of light, often used to represent binary bits of information, to spread and overlap with other packets of light. Techniques are known to produce optical fibers that have low chromatic dispersion. Unfortunately, these fibers manifest other deficiencies including degraded signal quality caused by four-wave mixing, self- and cross-phase modulation impairments. Therefore, to overcome both problems simultaneously, it is necessary to form optical fiber links that simultaneously have high amounts of "local" chromatic dispersion and low amounts of "global" chromatic dispersion.
  • the chromatic-dispersion characteristics of one fiber type is used to offset or cancel the dispersion sustained in the other fiber type.
  • the lengths of each type of fiber are chosen to yield an overall effect of essentially no chromatic dispersion.
  • DC fibers can be placed in many locations.
  • DC fiber can be placed at any point of an optical fiber link including the middle, the end, or between stages of a dual-stage inline optical amplifier.
  • One or more segments of DC fiber may be used.
  • DC fiber has higher concentrations of germanium and a smaller mode- field diameter than does typical transmission fiber. Both of these features provide for a higher Raman amplification gain. These characteristics can be used to improve gain uniformity in an optical fiber link.
  • An example of an optical fiber link that uses a segment of DC fiber as a Raman amplifier is shown in Fig. 28.
  • segment 31 is a DC fiber that is placed between segments 30-1 and 30-2 of optical fiber.
  • Figure 29 provides a hypothetical graphical illustration of signal power as a function of distance along the optical fiber link as a result of the gain provided by Raman amplification in DC fiber segment 31.
  • Curve 42 represents the signal power that results from transmission losses of the optical fiber link without benefit of Raman amplification.
  • Curve 44 represents the signal power achieved using Raman amplification that results from pumping energy provided at the downstream end of the link without any additional gain provided by the DC fiber segment.
  • Curve 48 represents the signal power achieved using the additional Raman amplification provided within interval 47 of DC fiber segment 31. A more uniform signal power can be achieved by using multiple segments of DC fiber in a similar manner.
  • Raman amplification One fundamental factor that affects the gain achieved by Raman amplification is the intensity of the pumping energy. If this pumping energy is coupled into an optical fiber link at one of its ends, then the Raman gain distribution is largely determined by the pump energy transmission characteristics of the optical fiber link.
  • This distribution can be modified by altering one or more characteristics of the optical fiber link along its length.
  • these characteristics include the germanium concentration, the mode-field diameter of the pumping energy, and the spatial separation of signal and pumping energy.
  • Another example is the glass or "host" composition of the optical fiber.
  • One way to achieve a more uniform gain distribution is to augment Raman amplification with other types of amplification.
  • One type of amplification can be provided by a rare-earth dopant such as erbium and a suitable pump source.
  • the concentration of the rare-earth dopant varies along the length of the optical fiber link to provide a varying amount of gain that complements the varying gain provided by Raman amplification.

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  • Physics & Mathematics (AREA)
  • Electromagnetism (AREA)
  • Engineering & Computer Science (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • Signal Processing (AREA)
  • Optical Modulation, Optical Deflection, Nonlinear Optics, Optical Demodulation, Optical Logic Elements (AREA)
  • Optical Communication System (AREA)

Abstract

Selon l"invention, une liaison optique sans perte dans un système de transmission optique comprend une fibre optique configurée pour produire un gain de Raman et fournir un gain de Raman réparti, par l"intermédiaire d"une ou de plusieurs sources de pompage, le long de la fibre afin que, au bout du compte, le gain réalisé par un ou plusieurs signaux de propagation dans la liaison optique soit suffisamment uniformisé le long de la liaison optique ou au moins sur une partie de ladite liaison, pour ne pas subir un écart, par exemple, supérieur à cinq dB sur la longueur de la fibre optique. Les différentes formes de réalisation décrites mettent en oeuvre différentes architectures de pompage/élément optique permettant d"atteindre le résultat escompté.
PCT/US2000/034928 1999-12-23 2000-12-22 Liaison de transmission optique sans perte WO2001047152A2 (fr)

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Cited By (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP1271813A2 (fr) * 2001-06-29 2003-01-02 Sumitomo Electric Industries, Ltd. Unité de sources de lumière de pompage, amplificateur Raman et système de transmission optique
EP1309113A2 (fr) * 2001-09-19 2003-05-07 JDS Uniphase Corporation Liaison de transmission optique utilisant l'amplification Raman
DE10256215A1 (de) * 2002-12-02 2004-06-24 Siemens Ag Pumpmodul eines breitbandigen Raman-Verstärkers

Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0421675A2 (fr) * 1989-10-06 1991-04-10 AT&T Corp. Amplification distribuée pour système de transmission sur fibres optiques
EP0938172A2 (fr) * 1998-02-23 1999-08-25 Lucent Technologies Inc. Article comprenant un laser Raman à fibre en cascade amélioré
US5966236A (en) * 1996-10-07 1999-10-12 Nec Corporation Optical signal channel counter and optical amplification device using the same

Patent Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0421675A2 (fr) * 1989-10-06 1991-04-10 AT&T Corp. Amplification distribuée pour système de transmission sur fibres optiques
US5966236A (en) * 1996-10-07 1999-10-12 Nec Corporation Optical signal channel counter and optical amplification device using the same
EP0938172A2 (fr) * 1998-02-23 1999-08-25 Lucent Technologies Inc. Article comprenant un laser Raman à fibre en cascade amélioré

Non-Patent Citations (1)

* Cited by examiner, † Cited by third party
Title
KAO M S ET AL: "EXTINCTION RATIO DEGRADATION IN RAMAN AMPLIFIED OPTICAL COMMUNICATION SYSTEM" ELECTRONICS LETTERS, IEE STEVENAGE, GB, vol. 27, no. 14, 4 July 1991 (1991-07-04), pages 1235-1237, XP000240646 ISSN: 0013-5194 *

Cited By (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP1271813A2 (fr) * 2001-06-29 2003-01-02 Sumitomo Electric Industries, Ltd. Unité de sources de lumière de pompage, amplificateur Raman et système de transmission optique
EP1271813A3 (fr) * 2001-06-29 2005-08-24 Sumitomo Electric Industries, Ltd. Unité de sources de lumière de pompage, amplificateur Raman et système de transmission optique
EP1309113A2 (fr) * 2001-09-19 2003-05-07 JDS Uniphase Corporation Liaison de transmission optique utilisant l'amplification Raman
EP1309113A3 (fr) * 2001-09-19 2003-11-05 JDS Uniphase Corporation Liaison de transmission optique utilisant l'amplification Raman
DE10256215A1 (de) * 2002-12-02 2004-06-24 Siemens Ag Pumpmodul eines breitbandigen Raman-Verstärkers

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