CA1249030A - Optical fiber communications system comprising raman amplification means - Google Patents
Optical fiber communications system comprising raman amplification meansInfo
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- CA1249030A CA1249030A CA000521113A CA521113A CA1249030A CA 1249030 A CA1249030 A CA 1249030A CA 000521113 A CA000521113 A CA 000521113A CA 521113 A CA521113 A CA 521113A CA 1249030 A CA1249030 A CA 1249030A
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- radiation
- fiber
- pump
- optical fiber
- communications system
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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
Abstract
OPTICAL FIBER COMMUNICATIONS SYSTEM COMPRISING
RAMAN AMPLIFICATION MEANS
Abstract An optical fiber communications system with Raman amplification of the signal radiation comprises a broadband pump radiation source, or, preferably, a multiplicity of pump radiation sources. The sources are selected to result in a pump radiation spectrum such that pump radiation intensity in the fiber core is less than a critical intensity Ic. In particular, the average intensity of pump radiation in a first spectral interval, centered on any wavelength .lambda.p in the pump radiation spectrum and of width equal to the Brillouin line width of the fiber at .lambda.p, is to be less than that average intensity in the first spectral interval that results in conversion of 10% of the radiation in the first spectral interval to stimulated Brillouin radiation. Use of a multiplicity of pump sources not only can reduce pump noise and pump depletion due to stimulated Brillouin scattering, but typically also can result in enhanced system reliability and lower cost.
In a preferred embodiment, the invention is a soliton fiber communications system, with pump radiation injected at one or more intermediate fiber locations.
RAMAN AMPLIFICATION MEANS
Abstract An optical fiber communications system with Raman amplification of the signal radiation comprises a broadband pump radiation source, or, preferably, a multiplicity of pump radiation sources. The sources are selected to result in a pump radiation spectrum such that pump radiation intensity in the fiber core is less than a critical intensity Ic. In particular, the average intensity of pump radiation in a first spectral interval, centered on any wavelength .lambda.p in the pump radiation spectrum and of width equal to the Brillouin line width of the fiber at .lambda.p, is to be less than that average intensity in the first spectral interval that results in conversion of 10% of the radiation in the first spectral interval to stimulated Brillouin radiation. Use of a multiplicity of pump sources not only can reduce pump noise and pump depletion due to stimulated Brillouin scattering, but typically also can result in enhanced system reliability and lower cost.
In a preferred embodiment, the invention is a soliton fiber communications system, with pump radiation injected at one or more intermediate fiber locations.
Description
~ 3~31CI
OPTICAL EIBER COMMUNICATIONS SYSTEM COMPRISING
RA~AN AMPLIFICATION MEANS
Field of the Invention -Thi~ invention pertains to the field of S optical communications and involves optical fiber communications sy~tems comprising Raman amplification means.
Currently used long haul optical fiber co~munications syste~s typically require signal regenerators~ Such devices detect an optical signal, prod~ce a corresponding electronic signal which is amplified, reshaped and, typically, retimed, and then used to driv~ an appropriate radiation source, thereby 15 producing a fresh optical puls~ that is injected into the fiber. However, it has been known for some time that ;t is possible to amplify~ an~, under appropriate conditions even reshape, optical pulse~ without use of electronic regenerators of the type referred to above.
In particular, it ha~ been recognized that the Raman effect may be used to amplify optical signals. See, for instance~ R. H. Stolen, Proceedin~s of the IE~E, Vol. 6a, No. 10 ~1980~, pp. 1232-1~36~
Although Raman a~plification i~ possible in fibers other than silica-ba~ed ti~e., containing at leaxt 50~ by weight, typically > 80% by weight, SiO
optical fibers, for the sake of concreteness, the expo~ition below will frequently refer to, ana use material constants appropriate f~r, silica-based fiber.
Such fibers have two loss mini~a in the approximate range 1~2-1.6 ~m, and therefore ~om~unications systems that use silica-based fiber frequently use signal 3eùe~
radia~ion whose wavelength lies in that range.
Stimulated Raman Scattering (SRS~ is kno~n to produce sub~tantial gain in fused silica for frequency shift~ in the range from about 100 to about 600 cm l, with the maximum ~ain occurring for a frequency shift of about 450 cm l. This means that, in silica-based optical fiber J radiation of wavelengths ~O (to be termed the signal radiation) can be amplifie,d by means of pump radiation that is down-shifted in wavelength from ~O by amounts corresponding to shifts in wave number by abo~t lO0 to 600 cm lo For instance, for signal radiation of 1.56 ym, the appropriate pump radiation would have a wavelength between about 1.43 and 1.54 ~m, with peak amplification taking place for pump radiation of about 1.46 ~. It is also known that there is no inherent thrPshold power for amplification by SRS, althoughv in order for usable amplification to take place, a substantial amount of pump power, typically > 10 mW has to be lnjected into the fiber, due to the relative smallness of the Raman gain coefficient, which is of the order of lO ll cm/watt in fused silica~ For instance, in order to achieve a gain of 0.3 dB~km for 1.56 ~m signal radiation in a single mode silica-ba~ed iber of core area of 25 ~m)2, pump power of the order of 100 mW
is require~, if the pump wavelength is about 1.46 ~m.
It is also known that Stimulated Brillouin Scat~ering (SBSl can take place in optical fibers; and that such scattering can have a deleterious effect on systems performance, due principally to the fact that SBS can cause severe fluctuations in the pump intensity, which cause corresponding flu~tuations in the Raman gain and to the fact that SBS can result in pump depletion.
See, for in~tance, R. H. Stolenl op. cit. SBS can have a peak gain that is ~everal hundred times that for SRS, per unit frequency of pu~p radiation, but SBS linewidths are typically very narrow, 2~g., of the order of 20 MHz.
~29~3~
G. A. Koepf et all lectronics Letters, Vol. 18(22), 1982, pp. g42-9~3~ report on Raman amplification at 1.118 ~ in single mode fiber and its li~itation by SBS. They observed a deleterious efEect of SBS on the Raman gain, and s~ggest, inter alia, that an increase in the spectral width of the pump laser by modulation to values larger than the Brillouin linewidth would cause a decrease of the SBS gain and could be used for suppression of srillouin scattering. See also E. P. Ippen and R. H. Stolen, Applied Physics Letters, Vol. 21(11), pp. 539-541 (1972), which reports on the observation of SBS in optical fiber.
D. Cotter, Electronics Letters, Vol. 18(15), 1982, pp. 638-640, discloses a technique for suppression o~ SBS during transmission of high power narrowband laser light in monomode fibers. The technique involves imposition of phase modulation on the optical field launched into the fiber so as to reduce the SBS gain.
This is achieved, for in~ance, by placlng between the laser and the fiber a periodically driven optical phase modulator, or by using a mode-beating e~fect produced when the radiation ~ield comprises two discrete but closely spaced optical frequencies. This, it is suggested, could be achieved by using two single-frequency lasers operating at slightly differentwavelengths, or perhaps more easily by using a single laser which is arranged to operate in two longitudinal modes. This principle was applied by 3. Hegarty et al, Electronics Letters, Vol. 21(7) 1985, pp. 290-~92, who used a laser operating in two modes separated by 2 GHz.
Although SRS can be used to amplify "linear"
pulses~ i.e., pulses in which no particular relationship between pulse peak power and pulse peak width is required, amplification by SRS can be advantageously used in soliton communications systems. ~. ~lasegawa et al have shown ~ ~ ~ Letters, Vol. 23(3), pp~ 142-14~ (1973)) that under certain conditions 9~3~D
shape-maintaining pulses can exist in single mode optical fiber. Such pulses are termed solitonsr and, in silica-based fiber, typically have center wavelengths in the range 1.45-1.60 l~m. The existence o~ solitons has been experimentally demonstrated (L. F. ~ollenauer et al, Physical Review Letters, Vol. 45(13), pp. l095-1098 ________ ______ _______ (1980)), and their utility for high capacity communications systems has been disclosed (U.S. Pa~ent 4,406,516, issued September 27, 1983 to A. Hasegawa, co-assigned with this). Furthermore, it has been found that solitons can be amplified nonelectronically without loss of soliton character (see A. Hasegawa, Optics Lett_rs, Vol. 8, pp.
650-652 (1983). Co-assigned U~S. Patent No. 4,558,921 issued December 17, 1985 ~o Akira Hasegawa et al discloses a soliton optical communications system comprising non-electronic means for increasing the pulse height and decreasing the pulse width of soliton pulses. See also A. Hasegawa, Applied Optics, Vol. 23(19), pp. 3302-3309 _______ ___ __ (1984). This coupling between pulse height and pulse width is an attribute of solitons, and its existence has been experimentally verified in single mode fiber, with loss compensated by Raman gain. ~ F~ Mollenauer et al, Optics Letters, Vol~ 10~ pp. 229-231 (1985)).
Since Raman amplification of signal pulses in fiber communications systems, especially in soliton systems, potentially has substantial advantages over pulse regeneeation as currently practiced, a Raman amplification scheme that, among other advantages, avoids the introduction of significant amounts of SBS-caused pump noise yet is easily and inexpensively implemented would be of considerable interest. This application discloses such a system.
Su~mary of the Invention ___________ __ _________ In accordance with an aspect of the invention there is provided an optical fiber communications system 3~3 - 4a .
with Raman ampli~ication, the system comprising a) first means for generating first electr~magnetic radiation of waveleng~h ~O, a length of optical fiber having a core and a cladding~ detector means for detecting the first radiation, and means for coupling the first radiation into the optical fiber at a irst fiber location, the coupled-in firs~ radiation guided in the fiber to a second fiber location that is spaced apart from the first fiber location, at least some of the coupled-in first radiation emitted from ~he fiber at the second fiber location and detected by the detector meansl the system further comprising b) second means for generating second electro-magnetic radiation, assoc;ated with the second radiation being a second radiation spectrum; and c) means for 15 coupling the second radiation into the optical fiber at a third fiber location that is intermediate the first and the second fiber locations; characterized in that d) the second means are selected to result in a second radiation spectrum having a wid~h greater than the Brillouin line-width of the optical fi~er, and further selected such ~hat the intensity of second radiation in the fiber at any wavelength ~p is less than a critical intensity ~c~
where by "intensity of second radia~ion in the fiber at ~p" is meant the average intensi~y of second radiation in the fiber core in a first spectral interval, the fir6t spectral interval being centered on ~p and having a wid~h equal to the Brillouin linewidth of the fiber at ~p, and where Ic is that average intensity of radiation in the fiber core in the first spectral interval that results in conversion of 10% of the radiation in the first spectral interval to stimulated Brillouin radiation.
Brief Descri~tion of ~he Drawings FIG. 1 schematically depicts a communications system according to the invention;
FIG. 2 is an exemplary spectrum of a semiconductor laser;
~I5S. 3-6 schematically indicate exemplary Raman amplification schemes; and FIGS. 7 and 8 schematically illustrate exemplary techniques for coupling pump radiation into ~n optical fiber.
The same reference numerals are used to identify analogous features in different figures.
The In~ention _.
A fiberguide communications system according to th~ invention comprises a broadband source of pump radiati3n, or, preferably~ a multiplicity of sources of pu:mp radiation, with source characteristics such as center wavelengths and spectral widths chosen such that the pump radiation intensity in the fiber core ~optical fiber comprises a core of relatively higher refractive index, and, contactingly surrounding the core, a cladding of relatively lower refractive index) at any given wavelength does not exceed a critical value, to be defined below. The multiplicity of pump radiation sources c~n comprise discrete sources, e.g., discrete semicond~ctor lasers, gas lasers r or other sources vf coherent or noncoherent radiationt or an array of devices integrated on a chip. In a currently preferred embodiment, the sources are discrete semiconductor lasers. It will, of course, be appreciated that a combination of discrete and integrated sources can also be used, or that more than one source-carrying chip may be used. Furthermore, it is to be understood that in long-haul communications systems, including soliton : systems, frequently pump radiation is injected into the fiber at a multiplicity of fiber locations. The appropriate spacing between adjacent injection points depends on the characteristics of the communications system and can be determined by known methods ~see, for instance, A. Hasegawa, ~ ed ~ , Vol. 23, ,, 3~
pp. 3302-3309).
Use of a multiplicity of pump sources according to the invention not only can essentially eliminate SB5-caused pump noise but also rexults in enhanced system reliability and, possibly, lower capital cos~. Reliability is enhanced since failure of one, or even sev~ral, pump sources in a syste~ according to the invention need not result in impaired system performance. The remaining sources typically can simply be run at higher output ~o make up for the failed sources. Also, low power sources often have a longer lifetime than high power sources.
An exemplary communications sy~tem accorcling to the invention is schematically depicted in PIG. 1 r wherein 10 is an optical fiber, typieally single mode fiber, 11 is a source of electro~agnetic radiat-ion 12 tof wavelength ~O, the signal radiation), 13 refers to means for coupling 12 into the fiber, and 14 to means for detecting signal radiation, e.g., a photodetector.
Furthermore, 15 refers to the array of p~lmp radiation sources, 16 to the totality of pump radiation emitted by all active sources of 15, and 17 to means for coupling the pump radiation into the fiber. Su~h well-known parts of a communication~ system as drive electronîcs, detector electronics, splices, attenuators, output means, etc., are not shown in FIG. 1. Furthermore, in a soliton system according to the invention one typically provides means for monitorin~ and adjusting the pump power and/or the signal power such that the signal pul~es remain solitons throughout their transmission through the fiber link. Such means can be conventional.
~ s is Xnown ~o those skilled in ~he art, the pump radiation can be injected into the fiber s~ch that it is co~propagating or counterpropagating with the signal radiation, or it can be injectcd s~ch that a portion co-propagates whereas the remainder counter-propagates. Typically, the signal radiation is in pulse 3~
form, and the pump radiation can be either CW or pulsed.
Use of pulsed pump radiation i5 frequently not advantageou~ with co-p~opagating signal pulses.
~ major objective of the invention being amplification of the signal radiation by means o SRS
without introduction of significant noise power due to sss, in syste~s according to the invention the pump power i5 spread over a spectral regiom such that the intensity of pump radiation at any given wavelength ~p in the fiber core is less than Ic, the critical intensity for SBS at that wavelength.
For purposes of this application we define Ic to be that average radiation int~nsity in the core of an optical fiber ~in the spectral interval that is centered at a wavelength ~p and that is equal in width to the Brillouin linewidth in the fiber at ~p) that results in conversion of 10% of the radiation in the spectral interval to stimulated ~rillouin radiation9 The "Brillouin linewidth" associated with an optical fiber is the FWHM ~full width at half maximum) of the ~rillouin spontaneous ~cattering spectru~ in the fiber, as determined with a narrow line source of radiation. A line source is "narrow" if the source line width is much less th~n the srillouin linewidth.
As an example, in an optical fiber having a pure, or lightly germania-doped, fused ~ilica core, the Brillouin linewidth of 1.46 ~m pump radiation is about 18 MHz. If the fiber is single mode fiber with an 8 ~m core diameter and a loss of about 0.2 dB~km, 30 I5 ~ 0.04 mW/(~m)2. This implies that the pu~p power in any 18 MHz wide spectral region at about 1.46 ~m is not to exceed about 2 mW.
The radiation intensity I in a single mode iber i5 relate~ to the radiation power P as follows:
I = PAeff~ whers Aef~ is the efEective core area. A
method ~or calrulating ~eff can be found in Opti~al Fi~er Telecommunications, S. E. Miller and 3~
A. G. Chynowethr editorsr Academic Press, 1979, pp. 1~7-135~ especially page 130. Howeverg -the thus calculated value of Aef~ typically is sufficiently close to the core area size of a single mode! fiber such that for most purposes it is permissible to substitute the value of the core area A ~or Aef f .
Altho~gh light-emitting diocles and other sources o~ non-coherent radiation canr in principle, be used in the practice of the invention, we currently consider semiconductor lasers to be preferred source~ of pump radiation. As is well known, semiconductor lasers typically have a m~lti-line emission spectrum, as e~emplified in FIG. 2. Each relatively narrow peak 20 is associated with a longitudinal mode of the laser.
The mode spacing depends on the laser design, especially ~he resonator length and the refractiv~ index oE the active region, and frequently is of the order 0.1 nm.
FIGo 2 also shows the envelope 21 of the e~ission spectrum.
The inten~ity envelope of the output of a radiation source can bs used to characterize the source output. In particular, the center wavelength and spectral width of a source are herein defined a~ the wavelength corresponding to the maximum in the intensity envelope and as the full width at half maxi0um of the intensity envelope, respectively. Semiconductor laser~
without mode locking typically have spectral widths of the order of 5 cm~l, or equivalently, about 1 nm at ~ ~ 1.5 ymO Furthermore, in such lasers, the linewidth of a single radiation mode/ although narrow, i5 typically much greater than the Brillouin linewidth. In accordance with our teaching that the pump radiation is to have a 1nite spectral width such that the above stated intensity criterion is met at all wavelengths, it may be advantageous to use lasers having a large number of lasing modes and/or having relatively broad emission mode~.
~2~3~
In ~used 5ilicar the peak of the Raman gain coefficient is about 200 cm~l wide (with the r~gion of gain be;ng much wider, of the order of 500 cm~l). Thus, the p~lmp sources can be chosen such that their center frequencies are distributed over a spectral reqion including all or part of the peakwidth, possibly even including all or part of the region o:E slgniicant gain outsid~ of the peakwidth. If, for in~stance, ~O = 1.5 ~m, and iE ten pump radiation sources are to be usad, the sources could be selected such that the cen~er freq~encies are distributed m~re or less evenly over the wavelength region between about 1.44 ym and about 1.48 ~m. The center frequencies thus would differ by about 4 nm, and there would be substantially no overlap of the source spectra. However, it is not necessary that sources be spaced such as to avoid overlap since even if the envelopes of two or more sources overlap, the probability that some mode lines oYerlap is relatively small. And even if two or more mode lines overlap, the above-specified inten$ity criterion is typically ea~ily met, since in a system according to th~
invention, the intensity in a spectral range equal to a Brillouin linewidth in a ~ode line is typically only a small part of the critical intensity.
Vario~s exemplary schemes for practicing the invention are schematically indicated in FIGS. 3-6.
FIG. 3 shows an optical fiber 10 carrying optical ~ignals in one direction, and pump radiation in the opposite direction, whereas otherwise identical FIG. 4 shows co~propagating ~ignal and pump radiation. FIGS. 5 and 6, on ~he other hand, show optical fiber carrying pump radiation in both directions, with FIG. 5 showing dividers 50 for splitting the pump radiatio~, and FIG. 6 illustrating the use of separate pump sources. FIG~ 5 also indicate~ a unidirectional signal stream, whereas FIG. 6 shows bidirectional signal streams. It will be appreciated that the illustrated systems are exe~plary ., 3~
only, and that other schemes are al~o possibleO In FIGS. 3~6, reference numeral 15 refers to an aggregate of pump sources, and ~0 to optical fiber serving to guide the pump radiation to a coupler 31, (and, in FIG. 5, to a splitter 50). Coupler 31 serves to couple pu~p radiation onto the transmission fiber without coupling out significant amounts of signal radiation.
~ system according to the inven~ion typically also requires ~eans for coupling the outputs of the battery of pump sources onto fiber 30. Exemplary means for achieving this are schematically depicted in FIG. 7, in which 151, 152, ... 15n indicate n sources of pump radiation 171, 172, ... 17n, respectively. The n beams of pump radiation are directed onto the surface of optical grating 70. The grating serves to combine the n individual beams into single beam 16 which is co~pled into fiber 30 by appropriate coupling means 71.
Other ways for coupling the output of two or more pump sources onto a single fiber, without causing interaction between the sources, are known. For instance, long taper fused fiber couplers can be used~
Another exemplary scheme is sche~atically depicted in FIGo 8~ in which 151-154 refers to four ~out of a battery of n) individual pump sources that emit polarlzed radiation~ with, for instance, sources 151 and 153 emittin~ radiation oE cent~r wavelength ~pl and ~p3, respectively, that is polarized perpendicular to some reference direction, and 15~ and 154 radiation of wavelen~th ~p2 and ~p4, respectiYely, that is polarized parallel to the reference direction. Fibers 82 are o~
the polarization preserving type, couplers 81 are polarization selective couplers~ and coupler 80 is of the previGusly referred to waveleng~h dependent type.
Those skilled in the art will appreciate that the coupling of pump radiation onto transmission fiber can be accomplished in a variety of ways, and ~hat still other ways to achieve this will undoubtedly be 3~
dlscovered in the futur2. All possible ways for coupling the pump radiation from the multiplicity of sources according to the invention are contemplated to be within the scope of the invention.
~lthough the use of multiple co-located pump sources is currently preferred by us, it will be appreciated that the invention can als,o be practiced with a broadband source whose output meets the intensity criterion. For instance, a solid state diode could be used as such a broadband source, provided that means for efficiently coupling its output to a single mode optical fiber can be devised.
Example: The optical fiber transmission channel consists of single mode dispersion shifted silica-based fiber having a 10s5 of 0.18 dB/km at 1.56 ~m and of 0.29 dB/km at 1.46 ~m, a dispersion of 2ps/nm km, an effective core area o~ 25 (~m)2, and has a length of 2200 km. A mode-locked laser produces bandwidth limited pulses of ~ - 1.56 ~m. The pulses are coupled into the fiber, the laser being adjusted such that the coupled-in pulses have a peak power of 27 mW, are substantially of sech-shape, and have a pulse width of 7.5 ps. The pulses thus are ~undamental ~N=l) solitons in the fiber. At intervals of about 40 km along the fiber are located pump radiation injection pointa. At each of these points is located a battery of ten semiconductor laser pump radiation sources, with center wavelengths substantially regularly spaced throughout the wavelength interval 1.44 to 1.48 ~m. The sources have an average halfwidth of 20 nm,~and typically emit in about 10 modes. The total pump power coupled into the fiber at each injection point is ~40 mW, with the pump power in the fiber core everywhere being substantially below l.0 mW (with the intensity everywhere being substantially below 0.04 mW/I~lm)2) for - any wavelength region in the pump radiation spectrum that i5 equal to the ~rillouin linewidth in this fiber, , approximately equal to 18 MH2. The cw pump radiation is bidirectionally coupled into the fiber by means of a grating and a wavelength-dependen~ coupler. The ~aman gain of the signal pulses over the 40 km amplifier spacing essentially equals -the signal loss over this distance, resulting in stable trans~ission of the soliton pulses, with an error rate < 10-9/bit achievable for bit rates up to 13 Gbit/sec. At the receiving end of khe system, the pulses are detected by conventional means.
OPTICAL EIBER COMMUNICATIONS SYSTEM COMPRISING
RA~AN AMPLIFICATION MEANS
Field of the Invention -Thi~ invention pertains to the field of S optical communications and involves optical fiber communications sy~tems comprising Raman amplification means.
Currently used long haul optical fiber co~munications syste~s typically require signal regenerators~ Such devices detect an optical signal, prod~ce a corresponding electronic signal which is amplified, reshaped and, typically, retimed, and then used to driv~ an appropriate radiation source, thereby 15 producing a fresh optical puls~ that is injected into the fiber. However, it has been known for some time that ;t is possible to amplify~ an~, under appropriate conditions even reshape, optical pulse~ without use of electronic regenerators of the type referred to above.
In particular, it ha~ been recognized that the Raman effect may be used to amplify optical signals. See, for instance~ R. H. Stolen, Proceedin~s of the IE~E, Vol. 6a, No. 10 ~1980~, pp. 1232-1~36~
Although Raman a~plification i~ possible in fibers other than silica-ba~ed ti~e., containing at leaxt 50~ by weight, typically > 80% by weight, SiO
optical fibers, for the sake of concreteness, the expo~ition below will frequently refer to, ana use material constants appropriate f~r, silica-based fiber.
Such fibers have two loss mini~a in the approximate range 1~2-1.6 ~m, and therefore ~om~unications systems that use silica-based fiber frequently use signal 3eùe~
radia~ion whose wavelength lies in that range.
Stimulated Raman Scattering (SRS~ is kno~n to produce sub~tantial gain in fused silica for frequency shift~ in the range from about 100 to about 600 cm l, with the maximum ~ain occurring for a frequency shift of about 450 cm l. This means that, in silica-based optical fiber J radiation of wavelengths ~O (to be termed the signal radiation) can be amplifie,d by means of pump radiation that is down-shifted in wavelength from ~O by amounts corresponding to shifts in wave number by abo~t lO0 to 600 cm lo For instance, for signal radiation of 1.56 ym, the appropriate pump radiation would have a wavelength between about 1.43 and 1.54 ~m, with peak amplification taking place for pump radiation of about 1.46 ~. It is also known that there is no inherent thrPshold power for amplification by SRS, althoughv in order for usable amplification to take place, a substantial amount of pump power, typically > 10 mW has to be lnjected into the fiber, due to the relative smallness of the Raman gain coefficient, which is of the order of lO ll cm/watt in fused silica~ For instance, in order to achieve a gain of 0.3 dB~km for 1.56 ~m signal radiation in a single mode silica-ba~ed iber of core area of 25 ~m)2, pump power of the order of 100 mW
is require~, if the pump wavelength is about 1.46 ~m.
It is also known that Stimulated Brillouin Scat~ering (SBSl can take place in optical fibers; and that such scattering can have a deleterious effect on systems performance, due principally to the fact that SBS can cause severe fluctuations in the pump intensity, which cause corresponding flu~tuations in the Raman gain and to the fact that SBS can result in pump depletion.
See, for in~tance, R. H. Stolenl op. cit. SBS can have a peak gain that is ~everal hundred times that for SRS, per unit frequency of pu~p radiation, but SBS linewidths are typically very narrow, 2~g., of the order of 20 MHz.
~29~3~
G. A. Koepf et all lectronics Letters, Vol. 18(22), 1982, pp. g42-9~3~ report on Raman amplification at 1.118 ~ in single mode fiber and its li~itation by SBS. They observed a deleterious efEect of SBS on the Raman gain, and s~ggest, inter alia, that an increase in the spectral width of the pump laser by modulation to values larger than the Brillouin linewidth would cause a decrease of the SBS gain and could be used for suppression of srillouin scattering. See also E. P. Ippen and R. H. Stolen, Applied Physics Letters, Vol. 21(11), pp. 539-541 (1972), which reports on the observation of SBS in optical fiber.
D. Cotter, Electronics Letters, Vol. 18(15), 1982, pp. 638-640, discloses a technique for suppression o~ SBS during transmission of high power narrowband laser light in monomode fibers. The technique involves imposition of phase modulation on the optical field launched into the fiber so as to reduce the SBS gain.
This is achieved, for in~ance, by placlng between the laser and the fiber a periodically driven optical phase modulator, or by using a mode-beating e~fect produced when the radiation ~ield comprises two discrete but closely spaced optical frequencies. This, it is suggested, could be achieved by using two single-frequency lasers operating at slightly differentwavelengths, or perhaps more easily by using a single laser which is arranged to operate in two longitudinal modes. This principle was applied by 3. Hegarty et al, Electronics Letters, Vol. 21(7) 1985, pp. 290-~92, who used a laser operating in two modes separated by 2 GHz.
Although SRS can be used to amplify "linear"
pulses~ i.e., pulses in which no particular relationship between pulse peak power and pulse peak width is required, amplification by SRS can be advantageously used in soliton communications systems. ~. ~lasegawa et al have shown ~ ~ ~ Letters, Vol. 23(3), pp~ 142-14~ (1973)) that under certain conditions 9~3~D
shape-maintaining pulses can exist in single mode optical fiber. Such pulses are termed solitonsr and, in silica-based fiber, typically have center wavelengths in the range 1.45-1.60 l~m. The existence o~ solitons has been experimentally demonstrated (L. F. ~ollenauer et al, Physical Review Letters, Vol. 45(13), pp. l095-1098 ________ ______ _______ (1980)), and their utility for high capacity communications systems has been disclosed (U.S. Pa~ent 4,406,516, issued September 27, 1983 to A. Hasegawa, co-assigned with this). Furthermore, it has been found that solitons can be amplified nonelectronically without loss of soliton character (see A. Hasegawa, Optics Lett_rs, Vol. 8, pp.
650-652 (1983). Co-assigned U~S. Patent No. 4,558,921 issued December 17, 1985 ~o Akira Hasegawa et al discloses a soliton optical communications system comprising non-electronic means for increasing the pulse height and decreasing the pulse width of soliton pulses. See also A. Hasegawa, Applied Optics, Vol. 23(19), pp. 3302-3309 _______ ___ __ (1984). This coupling between pulse height and pulse width is an attribute of solitons, and its existence has been experimentally verified in single mode fiber, with loss compensated by Raman gain. ~ F~ Mollenauer et al, Optics Letters, Vol~ 10~ pp. 229-231 (1985)).
Since Raman amplification of signal pulses in fiber communications systems, especially in soliton systems, potentially has substantial advantages over pulse regeneeation as currently practiced, a Raman amplification scheme that, among other advantages, avoids the introduction of significant amounts of SBS-caused pump noise yet is easily and inexpensively implemented would be of considerable interest. This application discloses such a system.
Su~mary of the Invention ___________ __ _________ In accordance with an aspect of the invention there is provided an optical fiber communications system 3~3 - 4a .
with Raman ampli~ication, the system comprising a) first means for generating first electr~magnetic radiation of waveleng~h ~O, a length of optical fiber having a core and a cladding~ detector means for detecting the first radiation, and means for coupling the first radiation into the optical fiber at a irst fiber location, the coupled-in firs~ radiation guided in the fiber to a second fiber location that is spaced apart from the first fiber location, at least some of the coupled-in first radiation emitted from ~he fiber at the second fiber location and detected by the detector meansl the system further comprising b) second means for generating second electro-magnetic radiation, assoc;ated with the second radiation being a second radiation spectrum; and c) means for 15 coupling the second radiation into the optical fiber at a third fiber location that is intermediate the first and the second fiber locations; characterized in that d) the second means are selected to result in a second radiation spectrum having a wid~h greater than the Brillouin line-width of the optical fi~er, and further selected such ~hat the intensity of second radiation in the fiber at any wavelength ~p is less than a critical intensity ~c~
where by "intensity of second radia~ion in the fiber at ~p" is meant the average intensi~y of second radiation in the fiber core in a first spectral interval, the fir6t spectral interval being centered on ~p and having a wid~h equal to the Brillouin linewidth of the fiber at ~p, and where Ic is that average intensity of radiation in the fiber core in the first spectral interval that results in conversion of 10% of the radiation in the first spectral interval to stimulated Brillouin radiation.
Brief Descri~tion of ~he Drawings FIG. 1 schematically depicts a communications system according to the invention;
FIG. 2 is an exemplary spectrum of a semiconductor laser;
~I5S. 3-6 schematically indicate exemplary Raman amplification schemes; and FIGS. 7 and 8 schematically illustrate exemplary techniques for coupling pump radiation into ~n optical fiber.
The same reference numerals are used to identify analogous features in different figures.
The In~ention _.
A fiberguide communications system according to th~ invention comprises a broadband source of pump radiati3n, or, preferably~ a multiplicity of sources of pu:mp radiation, with source characteristics such as center wavelengths and spectral widths chosen such that the pump radiation intensity in the fiber core ~optical fiber comprises a core of relatively higher refractive index, and, contactingly surrounding the core, a cladding of relatively lower refractive index) at any given wavelength does not exceed a critical value, to be defined below. The multiplicity of pump radiation sources c~n comprise discrete sources, e.g., discrete semicond~ctor lasers, gas lasers r or other sources vf coherent or noncoherent radiationt or an array of devices integrated on a chip. In a currently preferred embodiment, the sources are discrete semiconductor lasers. It will, of course, be appreciated that a combination of discrete and integrated sources can also be used, or that more than one source-carrying chip may be used. Furthermore, it is to be understood that in long-haul communications systems, including soliton : systems, frequently pump radiation is injected into the fiber at a multiplicity of fiber locations. The appropriate spacing between adjacent injection points depends on the characteristics of the communications system and can be determined by known methods ~see, for instance, A. Hasegawa, ~ ed ~ , Vol. 23, ,, 3~
pp. 3302-3309).
Use of a multiplicity of pump sources according to the invention not only can essentially eliminate SB5-caused pump noise but also rexults in enhanced system reliability and, possibly, lower capital cos~. Reliability is enhanced since failure of one, or even sev~ral, pump sources in a syste~ according to the invention need not result in impaired system performance. The remaining sources typically can simply be run at higher output ~o make up for the failed sources. Also, low power sources often have a longer lifetime than high power sources.
An exemplary communications sy~tem accorcling to the invention is schematically depicted in PIG. 1 r wherein 10 is an optical fiber, typieally single mode fiber, 11 is a source of electro~agnetic radiat-ion 12 tof wavelength ~O, the signal radiation), 13 refers to means for coupling 12 into the fiber, and 14 to means for detecting signal radiation, e.g., a photodetector.
Furthermore, 15 refers to the array of p~lmp radiation sources, 16 to the totality of pump radiation emitted by all active sources of 15, and 17 to means for coupling the pump radiation into the fiber. Su~h well-known parts of a communication~ system as drive electronîcs, detector electronics, splices, attenuators, output means, etc., are not shown in FIG. 1. Furthermore, in a soliton system according to the invention one typically provides means for monitorin~ and adjusting the pump power and/or the signal power such that the signal pul~es remain solitons throughout their transmission through the fiber link. Such means can be conventional.
~ s is Xnown ~o those skilled in ~he art, the pump radiation can be injected into the fiber s~ch that it is co~propagating or counterpropagating with the signal radiation, or it can be injectcd s~ch that a portion co-propagates whereas the remainder counter-propagates. Typically, the signal radiation is in pulse 3~
form, and the pump radiation can be either CW or pulsed.
Use of pulsed pump radiation i5 frequently not advantageou~ with co-p~opagating signal pulses.
~ major objective of the invention being amplification of the signal radiation by means o SRS
without introduction of significant noise power due to sss, in syste~s according to the invention the pump power i5 spread over a spectral regiom such that the intensity of pump radiation at any given wavelength ~p in the fiber core is less than Ic, the critical intensity for SBS at that wavelength.
For purposes of this application we define Ic to be that average radiation int~nsity in the core of an optical fiber ~in the spectral interval that is centered at a wavelength ~p and that is equal in width to the Brillouin linewidth in the fiber at ~p) that results in conversion of 10% of the radiation in the spectral interval to stimulated ~rillouin radiation9 The "Brillouin linewidth" associated with an optical fiber is the FWHM ~full width at half maximum) of the ~rillouin spontaneous ~cattering spectru~ in the fiber, as determined with a narrow line source of radiation. A line source is "narrow" if the source line width is much less th~n the srillouin linewidth.
As an example, in an optical fiber having a pure, or lightly germania-doped, fused ~ilica core, the Brillouin linewidth of 1.46 ~m pump radiation is about 18 MHz. If the fiber is single mode fiber with an 8 ~m core diameter and a loss of about 0.2 dB~km, 30 I5 ~ 0.04 mW/(~m)2. This implies that the pu~p power in any 18 MHz wide spectral region at about 1.46 ~m is not to exceed about 2 mW.
The radiation intensity I in a single mode iber i5 relate~ to the radiation power P as follows:
I = PAeff~ whers Aef~ is the efEective core area. A
method ~or calrulating ~eff can be found in Opti~al Fi~er Telecommunications, S. E. Miller and 3~
A. G. Chynowethr editorsr Academic Press, 1979, pp. 1~7-135~ especially page 130. Howeverg -the thus calculated value of Aef~ typically is sufficiently close to the core area size of a single mode! fiber such that for most purposes it is permissible to substitute the value of the core area A ~or Aef f .
Altho~gh light-emitting diocles and other sources o~ non-coherent radiation canr in principle, be used in the practice of the invention, we currently consider semiconductor lasers to be preferred source~ of pump radiation. As is well known, semiconductor lasers typically have a m~lti-line emission spectrum, as e~emplified in FIG. 2. Each relatively narrow peak 20 is associated with a longitudinal mode of the laser.
The mode spacing depends on the laser design, especially ~he resonator length and the refractiv~ index oE the active region, and frequently is of the order 0.1 nm.
FIGo 2 also shows the envelope 21 of the e~ission spectrum.
The inten~ity envelope of the output of a radiation source can bs used to characterize the source output. In particular, the center wavelength and spectral width of a source are herein defined a~ the wavelength corresponding to the maximum in the intensity envelope and as the full width at half maxi0um of the intensity envelope, respectively. Semiconductor laser~
without mode locking typically have spectral widths of the order of 5 cm~l, or equivalently, about 1 nm at ~ ~ 1.5 ymO Furthermore, in such lasers, the linewidth of a single radiation mode/ although narrow, i5 typically much greater than the Brillouin linewidth. In accordance with our teaching that the pump radiation is to have a 1nite spectral width such that the above stated intensity criterion is met at all wavelengths, it may be advantageous to use lasers having a large number of lasing modes and/or having relatively broad emission mode~.
~2~3~
In ~used 5ilicar the peak of the Raman gain coefficient is about 200 cm~l wide (with the r~gion of gain be;ng much wider, of the order of 500 cm~l). Thus, the p~lmp sources can be chosen such that their center frequencies are distributed over a spectral reqion including all or part of the peakwidth, possibly even including all or part of the region o:E slgniicant gain outsid~ of the peakwidth. If, for in~stance, ~O = 1.5 ~m, and iE ten pump radiation sources are to be usad, the sources could be selected such that the cen~er freq~encies are distributed m~re or less evenly over the wavelength region between about 1.44 ym and about 1.48 ~m. The center frequencies thus would differ by about 4 nm, and there would be substantially no overlap of the source spectra. However, it is not necessary that sources be spaced such as to avoid overlap since even if the envelopes of two or more sources overlap, the probability that some mode lines oYerlap is relatively small. And even if two or more mode lines overlap, the above-specified inten$ity criterion is typically ea~ily met, since in a system according to th~
invention, the intensity in a spectral range equal to a Brillouin linewidth in a ~ode line is typically only a small part of the critical intensity.
Vario~s exemplary schemes for practicing the invention are schematically indicated in FIGS. 3-6.
FIG. 3 shows an optical fiber 10 carrying optical ~ignals in one direction, and pump radiation in the opposite direction, whereas otherwise identical FIG. 4 shows co~propagating ~ignal and pump radiation. FIGS. 5 and 6, on ~he other hand, show optical fiber carrying pump radiation in both directions, with FIG. 5 showing dividers 50 for splitting the pump radiatio~, and FIG. 6 illustrating the use of separate pump sources. FIG~ 5 also indicate~ a unidirectional signal stream, whereas FIG. 6 shows bidirectional signal streams. It will be appreciated that the illustrated systems are exe~plary ., 3~
only, and that other schemes are al~o possibleO In FIGS. 3~6, reference numeral 15 refers to an aggregate of pump sources, and ~0 to optical fiber serving to guide the pump radiation to a coupler 31, (and, in FIG. 5, to a splitter 50). Coupler 31 serves to couple pu~p radiation onto the transmission fiber without coupling out significant amounts of signal radiation.
~ system according to the inven~ion typically also requires ~eans for coupling the outputs of the battery of pump sources onto fiber 30. Exemplary means for achieving this are schematically depicted in FIG. 7, in which 151, 152, ... 15n indicate n sources of pump radiation 171, 172, ... 17n, respectively. The n beams of pump radiation are directed onto the surface of optical grating 70. The grating serves to combine the n individual beams into single beam 16 which is co~pled into fiber 30 by appropriate coupling means 71.
Other ways for coupling the output of two or more pump sources onto a single fiber, without causing interaction between the sources, are known. For instance, long taper fused fiber couplers can be used~
Another exemplary scheme is sche~atically depicted in FIGo 8~ in which 151-154 refers to four ~out of a battery of n) individual pump sources that emit polarlzed radiation~ with, for instance, sources 151 and 153 emittin~ radiation oE cent~r wavelength ~pl and ~p3, respectively, that is polarized perpendicular to some reference direction, and 15~ and 154 radiation of wavelen~th ~p2 and ~p4, respectiYely, that is polarized parallel to the reference direction. Fibers 82 are o~
the polarization preserving type, couplers 81 are polarization selective couplers~ and coupler 80 is of the previGusly referred to waveleng~h dependent type.
Those skilled in the art will appreciate that the coupling of pump radiation onto transmission fiber can be accomplished in a variety of ways, and ~hat still other ways to achieve this will undoubtedly be 3~
dlscovered in the futur2. All possible ways for coupling the pump radiation from the multiplicity of sources according to the invention are contemplated to be within the scope of the invention.
~lthough the use of multiple co-located pump sources is currently preferred by us, it will be appreciated that the invention can als,o be practiced with a broadband source whose output meets the intensity criterion. For instance, a solid state diode could be used as such a broadband source, provided that means for efficiently coupling its output to a single mode optical fiber can be devised.
Example: The optical fiber transmission channel consists of single mode dispersion shifted silica-based fiber having a 10s5 of 0.18 dB/km at 1.56 ~m and of 0.29 dB/km at 1.46 ~m, a dispersion of 2ps/nm km, an effective core area o~ 25 (~m)2, and has a length of 2200 km. A mode-locked laser produces bandwidth limited pulses of ~ - 1.56 ~m. The pulses are coupled into the fiber, the laser being adjusted such that the coupled-in pulses have a peak power of 27 mW, are substantially of sech-shape, and have a pulse width of 7.5 ps. The pulses thus are ~undamental ~N=l) solitons in the fiber. At intervals of about 40 km along the fiber are located pump radiation injection pointa. At each of these points is located a battery of ten semiconductor laser pump radiation sources, with center wavelengths substantially regularly spaced throughout the wavelength interval 1.44 to 1.48 ~m. The sources have an average halfwidth of 20 nm,~and typically emit in about 10 modes. The total pump power coupled into the fiber at each injection point is ~40 mW, with the pump power in the fiber core everywhere being substantially below l.0 mW (with the intensity everywhere being substantially below 0.04 mW/I~lm)2) for - any wavelength region in the pump radiation spectrum that i5 equal to the ~rillouin linewidth in this fiber, , approximately equal to 18 MH2. The cw pump radiation is bidirectionally coupled into the fiber by means of a grating and a wavelength-dependen~ coupler. The ~aman gain of the signal pulses over the 40 km amplifier spacing essentially equals -the signal loss over this distance, resulting in stable trans~ission of the soliton pulses, with an error rate < 10-9/bit achievable for bit rates up to 13 Gbit/sec. At the receiving end of khe system, the pulses are detected by conventional means.
Claims (7)
1. An optical fiber communications system with Raman amplification, the system comprising a) first means for generating first electromagnetic radiation of wavelength .lambda.o, a length of optical fiber having a core and a cladding, detector means for detecting the first radiation, and means for coupling the first radiation into the optical fiber at a first fiber location, the coupled-in first radiation guided in the fiber to a second fiber location that is spaced apart from the first fiber location, at least some of the coupled-in first radiation emitted from the fiber at the second fiber location and detected by the detector means, the system further comprising b) second means for generating second electromagnetic radiation, associated with the second radiation being a second radiation spectrum; and c) means for coupling the second radiation into the optical fiber at a third fiber location that is intermediate the first and the second fiber locations;
CHARACTERIZED IN THAT
d) the second means are selected to result in a second radiation spectrum having a width greater than the Brillouin linewidth of the optical fiber, and further selected such that the intensity of second radiation in the fiber at any wavelength .lambda.p is less than a critical intensity Ic, where by "intensity of second radiation in the fiber at .lambda.p" is meant the average intensity of second radiation in the fiber core in a first spectral interval, the first spectral interval being centered on .lambda.p and having a width equal to the Brillouin linewidth of the fiber at .lambda.p, and where Ic is that average intensity of radiation in the fiber core in the first spectral interval that results in conversion of 10% of the radiation in the first spectral interval to stimulated Brillouin radiation.
CHARACTERIZED IN THAT
d) the second means are selected to result in a second radiation spectrum having a width greater than the Brillouin linewidth of the optical fiber, and further selected such that the intensity of second radiation in the fiber at any wavelength .lambda.p is less than a critical intensity Ic, where by "intensity of second radiation in the fiber at .lambda.p" is meant the average intensity of second radiation in the fiber core in a first spectral interval, the first spectral interval being centered on .lambda.p and having a width equal to the Brillouin linewidth of the fiber at .lambda.p, and where Ic is that average intensity of radiation in the fiber core in the first spectral interval that results in conversion of 10% of the radiation in the first spectral interval to stimulated Brillouin radiation.
2. Communications system of claim 1, wherein the second means comprise a multiplicity of second radiation sources.
3. Communications system of claim 2, wherein the second means comprise at least an i'th and a j'th second radiation source, associated with each second radiation source being a center wavelength and a spectral width, the i'th and j'th second radiation sources selected such that the center wavelengths of the i'th and the j'th second radiation sources differ by at least about the spectral width of the i'th second radiation source.
4. Communications system of claim 3, wherein at least the i'th and the j'th second radiation sources are semiconductor lasers.
5. Communications system of claim 3, wherein the first radiation is pulsed radiation, and the first radiation coupled into the optical fiber forms soliton pulses in the fiber.
6. Communications system of claim 3, wherein the optical fiber is silica-based optical fiber, .lambda.o is in the range 1.2-1.6 µm, the second radiation spectrum contains a wavelength .lambda.p that is about 0.1 µm shorter than .lambda.o, and the intensity of second radiation in the fiber at any wavelength in the second radiation spectrum is less than 0.04 mW/(µm)2.
7. Communications system of claim 1, wherein the first radiation is pulsed radiation, and the second radiation is cw radiation.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US06/791,915 US4699452A (en) | 1985-10-28 | 1985-10-28 | Optical communications system comprising Raman amplification means |
| US791,915 | 1985-10-28 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1249030A true CA1249030A (en) | 1989-01-17 |
Family
ID=25155204
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000521113A Expired CA1249030A (en) | 1985-10-28 | 1986-10-22 | Optical fiber communications system comprising raman amplification means |
Country Status (4)
| Country | Link |
|---|---|
| US (1) | US4699452A (en) |
| EP (1) | EP0221711A3 (en) |
| JP (1) | JPS62102585A (en) |
| CA (1) | CA1249030A (en) |
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| US4616898A (en) * | 1980-03-31 | 1986-10-14 | Polaroid Corporation | Optical communication systems using raman repeaters and components therefor |
| JPS56165437A (en) * | 1980-05-26 | 1981-12-19 | Kokusai Denshin Denwa Co Ltd <Kdd> | Optical repeating system for optical communication |
| US4406516A (en) * | 1981-04-27 | 1983-09-27 | Bell Telephone Laboratories, Incorporated | Monomode fiberguide telecommunication system utilizing soliton pulses |
| US4558921A (en) * | 1982-02-25 | 1985-12-17 | At&T Bell Laboratories | Soliton fiber telecommunication systems |
| FI74371C (en) * | 1982-06-04 | 1988-01-11 | British Telecomm | Optical transmission. |
| US4546476A (en) * | 1982-12-10 | 1985-10-08 | The Board Of Trustees Of The Leland Stanford Junior University | Fiber optic amplifier |
-
1985
- 1985-10-28 US US06/791,915 patent/US4699452A/en not_active Expired - Fee Related
-
1986
- 1986-10-20 EP EP86308101A patent/EP0221711A3/en not_active Withdrawn
- 1986-10-22 CA CA000521113A patent/CA1249030A/en not_active Expired
- 1986-10-28 JP JP61254838A patent/JPS62102585A/en active Pending
Also Published As
| Publication number | Publication date |
|---|---|
| EP0221711A3 (en) | 1989-11-15 |
| EP0221711A2 (en) | 1987-05-13 |
| JPS62102585A (en) | 1987-05-13 |
| US4699452A (en) | 1987-10-13 |
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| MKEX | Expiry |