MX2007006571A - Method and apparatus for reducing crosstalk and nonlinear distortions induced by raman interactions in a wavelength division mulitplexed (wdm) optical communication system. - Google Patents

Abstract

A method and apparatus is provided for transmitting a WDM optical signal. The method begins by modulating a plurality of optical channels that are each located at a different wavelength from one another with (1) a respective one of a plurality of information-bearing electrical signals that all embody the same broadcast information and (2) a respective one of a plurality of RF signals having a common functional broadcast waveform, at least one of the RF signals being out of phase with respect to remaining ones of the plurality of RF signals. Each of the modulated optical channels are multiplexed to form a WDM optical signal. The WDM optical signal is forwarded onto an optical transmission path.

Description

METHOD AND APPARATUS TO REDUCE DINY AND NON-LINEAR DISTORTIONS INDUCED BY RAMAN INTERACTIONS IN A MULTIPLEXED OPTICAL COMMUNICATION SYSTEM BY LENGTH DIVISION OF WAVE (WDM) Field of the Invention The invention relates generally to the transmission of optical multiplexed signals by wavelength division (WDM), and more particularly to a method and apparatus for reducing crosstalk and nonlinear signal distortions induced by Raman interactions between the channels. optical Background of the Invention In recent years, optical transmission systems multiplexed by wavelength division (WDM) have been increasingly deployed in optical networks. These include coarse wavelength division (CWDM) multiplexed systems and multiplexed by dense wavelength division (DWDM). If a system that is CWDM or DWDM is considered, they simply depend on the optical frequency space of the channels used in the system. Although WDM optical transmission systems have increased the speed and capacity of optical networks, the performance of such systems is limited by several factors such as chromatic dispersion and nonlinearity of the fiber, which can cause a change in the shape of the impulse in the case of digital baseband signals and distortions in the case of analog signals. These deteriorations degrade the quality of the optically transmitted transmission. Non-linearities of the fiber, for example, can lead to crosstalk between optical signals operating at different wavelengths. When crosstalk occurs, the modulation components of one signal are superimposed on another signal at a different wavelength. If the level of crosstalk is large enough, it will degrade it. information that is transmitted by the optical signals impacted by this deterioration. A common cause of crosstalk, in a fiber optic communication system with multiple wavelengths, is Raman scattering. This type of crosstalk is caused by Raman stimulated scattering (SRS) in silica fibers (and other materials) when a pump wave propagates along with a signal wave through it. Raman stimulated dispersion is an inflexible dispersion process in which an incident pumping photon loses its energy to create another photon of reduced energy at a lower frequency. The remaining energy is absorbed by the fiber medium in the form of molecular vibrations (ie optical phonons). FIGURE (1) is a schematic diagram of the Raman stimulated scattering process. The image represents a scattering of pumping photons in the Raman medium. As a result of the scattering event, the pumping photon is destroyed and a new photon signal at the Stokes frequency is created along with an optical phonon at the Stokes shift frequency. Both energy and moment are conserved: ft pump = f > Ú) sigiial + ^? ??? ?????? ? ' ^ pump = ^ signal + ^ Op plwnon where ?? is the frequency of x and kx is the associated wave vector of x and ti is the Planck constant divided by 2n. The difference between the optical frequency of the pump wave (the highest frequency) and the wave that is amplified is referred to as the Stokes shift. The Raman gain window of a typical silica fiber is approximately 25 THz (terahertz) in amplitude. The Stokes change for maximum Raman power transfer in a typical silica fiber is approximately 13 THz. The Raman gain between two optical signals increases from zero when the frequency separation between the two signals increases until the peak gain is reached at a separation of 13 THz. Then it decreases back to zero when the separation increases beyond 25 THz. As a result of SRS, the energy of the pump wavelength can amplify a signal at a longer wavelength (lower optical frequency) as long as the optical frequency separation between the two signals falls within the gain window Raman of fiber. The pumping wave loses energy (annihilation of pumping photons) in wavelength of signals (creation of changed photons of Stocks) and also in fiber (creation of optical phonons). In this way, due to SRS, the pumping amplitude decreases as its population of photons is depleted while the wavelength of the signal is amplified as its population of photons increases. SRS amplification is a benign process if the pump wave is not modulated with some kind of information that may cause its amplitude to change over time. In this case, the Raman amplification is set at a constant level with time. This serves simply to reinforce the amplitude of the Stokes shift signal but does not disturb the information it may be carrying. Problems arise, however, when the pump and signal wavelengths are modulated. In this case, the amplitude of the pump wavelength is changed with time, and therefore, the SRS amplification level varies along with the pump modulation. This process imparts a scaled duplication of the pump modulation over the signal wavelength which is referred to as crosstalk. Crosstalk can interfere with and degrade the quality of the original information that is transmitted by the signal wavelength. The level of crosstalk modulation depends on the Raman gain value, which in turn depends on the optical frequency separation of the transmitted waves among other parameters. In addition, the crosstalk process is a shared experience between the pump and signal wavelengths when both are modulated. It is more likely that a pumping photon will be dispersed and annihilated in an SRS event where more signal photons are available to facilitate the process. Therefore, at points in the space-time where the signal wavelength is high, due to the modulation it carries, the pump will more easily lose the photons during SRS. The inverse is true, too, in points in space and a time where the signal wavelength is smaller due to the modulation of time variation it takes, the pump is less likely to lose a photon in an SRS event. Therefore, the pump loses photons along with the signal wavelength modulation. The result is that an inverted scaling duplication of the signal wavelength modulation is imparted over the pump wavelength. This crosstalk can interfere with and degrade the quality of the original information that is transmitted by the pump wavelength. FIGURE 2 - shows how this energy transfer leads to crosstalk. FIGURE 2 is a simplified illustration that is useful for facilitating an understanding of Raman crosstalk between two optical channels or signals Si and Sj, where Sj is at a wavelength longer than Si. FIGURE 2A shows the signal Si and FIGURE 2B shows the signal Sj. For simplicity of illustration, Sj is shown as a signal with constant amplitude (ie, a continuous string of zeros or ones). As indicated in FIGURE 2C, the pattern of the Si signal (shaded line) is recorded in the signal Sj by the Raman amplification process. In other words, the signal Sj now includes as one of its components, the signal pattern Si. Similarly, since the signals from Si are pumping the signal Sj, the pattern of the Sj signals (which has been modulated) can be recorded on the pump Si by the process of emptying the pump. In a multiple wavelength system (three or more wavelengths), the crosstalk process is similar to the one described above but the complexity increases since there are now multiple sources of pumping and multiple signals that generate crosstalk in each of optical waves. Raman crosstalk is a problem for analog and digital modulation schemes. In addition to the generation of unwanted crosstalk, the SRS process can also lead to the generation of second order distortions induced by Raman (CSO: second compound order) and third order (CTB: triple compound pulse). These distortions occur as a result of the non-linear nature of the Raman amplification process that, in the non-exhausted regime, it is exponential in form. Suppose there are two optical waves in the wavelengths As (the signal wavelength)? (the wavelength of pumping) propagated through a fiber of length L with a corresponding Random gain coefficient Gsp. If at the transmitter site, the instantaneous optical power associated with ?? is Pp (t) and the instantaneous optical power associated with As is Ps (t) then in the unexploited power regime, the optical power at the wavelength As in the position L because the Raman scattering is given by: or Here Leg is the effective length in the fiber at the pump wavelength, a is the power attenuation factor in the fiber at the signal wavelength, pL is the average execution probability to find the two signals in the Same polarization state, ñ and ñs are the refractive indices at the respective wavelengths. When defining a simple function Hs, p: Then, (the) and (Ib) can be combined single equation: This will be practical when operating a multi-wavelength optical communication system. Expandable in exponential in (the) da: | AL P t) + Gsp pL Leff P) Pp. { t) + GspP "Lefff 'Ps { t) { PP { tf + ... (2) The second line of (2) provides the explanation sought for Raman-induced crosstalk, CSO distortions, and CTB in the almost zero dispersion optical communication system when the wave powers pumping and time dependent signal are represented by: = P0s + P0s > nsfs (t CSOs + CTBs (3a) Here, P0s, P0p are the average optical powers of the signal and pumping waves, Pms (t), Pmp (t) represent the time-dependent terms explicitly of the optical powers, and ms, mp are the optical modulation indices respective (IMO) for each laser. The third and fourth terms of the second lines in (3a) and (3b) represent the composite distortions of the second order (CSOs, CSOp) and triple composite pulsation (CTBS, CTBP) generated within the optical signal and pumping transmitters same. The distortions of CSOs and CSOp are natural to the transmitter and are independent of the Raman interactions that take place in the fiber. The time-dependent modulation functions fs. { t) and f (t) represented the information that is carried in each optical wave. After replacing (3a) and (3b) in (2) and retaining only the most dominant terms one gets: PÁ) = P, '", /, (') [! + G" pL Lr, P0p.}. E-aL + Gsp pL L, ffP " { Papmpfp (t)) e'aL + CTO, [l + Cv pL L, ffP0pYa 1 + Gv pL LtjrP0íCSOp e ~ aL + Gp pL Lp [p0l m, ft (t)] [ptpm "f" (/)] "" to L + C7¾, [l + GI (J ¾ LrjrP0liya, (')] [/' e_a The second line of (4) contains the RF subcarrier modulation of the undistorted signal transmitter multiplied by the Raman gain term (l + G pL LeffPüp) and an additional first order RF subcarrier term arising from the laser Modulated pumping. This additional first order term is RF subcarrier crosstalk, a direct transfer of the RF laser subcarrier modulation from pump (Popmpfp {t)) to the signal carrier scaled by the Raman factor G pL LefjP0s. When the RF subcarrier crosstalk is exactly in phase, the signals are added constructively (plus sign) while if they are exactly out of phase, they interfere destructively (minus sign), all other phase possibilities fall between these two extremes. The first terms of the third and fourth lines are the terms of CSOs, and CTBS generated by the signal laser each multiplied by the same Raman gain term as the RF subcarrier modulation of signal laser. The second terms of the third and fourth lines respectively are the direct transfer of CSOp and CTBP distortions of pumping laser (distortion crosstalk) to the signal wave selected by the same Raman factor as the RF subcarrier crosstalk that is transfers from the pump wave to the signal wave. The third terms of the third and fourth lines are the new generated distortions of Raman CSOR and CTBR that result from the product of the RF subcarrier modulations of the signal and pumping lasers. Collectively, the RF subcarrier crosstalk term, together with the second and third terms of the third and fourth lines constitute the degraded performance of the signal transmitter due to the Raman interactions between the modulated pump and signal lasers. These will be denoted collectively as the distortions induced by Raman. If the pump and signal laser modulations are not in phase with other interference it may also result between the various terms within each line of (4). It is the purpose of this invention to clarify the methods and apparatus that can be used to reduce the damaging effects of Raman-induced RF subcarrier crosstalk and the CSO distortion terms. The CTB induced by Raman is insignificant in magnitude and is not discussed here. When there are three or more lasers in the system modified to explain the multiple pumping, system with n transmitters (4) becomes: (5) Hp G, p pL ,,, V,, (') was 11 + L (f /? /,., or,., ¾ ,,, ", I + / ', / ,,?«, ,, G, .p¾ era, + / > "#», / , (/ > W2 The sums in (5) are about the parameters of the "n" transmitters in the system. It is important to note that equations (1) to (5) are to be interpreted as being in the optical domain. Therefore, the powers in these equations, which include the distortions (CSOs, CSOp, CTBS, and CTBp) are optical powers and not electric power (or RF) levels. Raman-induced crosstalk and non-linear directions are more pronounced when the wavelengths are located near the zero-dispersion wavelength of the optical transmission medium through which signals are propagated together (ie, optical fiber) ). In the case of a non-zero dispersion system, the optical pump and signal waves are propagating at almost identical group speeds through the medium. The zero dispersion wavelength of a transmission medium refers to the wavelength at which an optical signal will have no change in the group velocity (inverse) with respect to changes in its optical frequency. The zero dispersion wavelength differs from the different transmission medium. In this case, the relative positions of the waves with respect to another will remain almost fixed through the length of the transmission medium. Thus, if the signals Si and Sj are at or near the wavelength of zero dispersion, they will largely maintain their relative phase relative to each other. Therefore, with very little separation between the optical channels present, Raman-induced crosstalk and distortions can accumulate along the fiber in a constructive manner. The dispersion generally increases when the difference in wavelength between the optical signal and the zero dispersion wavelength increases. If the signals of Si and Sj are located at wavelengths very offset from the wavelength of zero dispersion, their relative phases will change as they propagate to the transmission path. The levels of Raman-induced crosstalk and distortions are much lower in the non-zero dispersion scenario because, when the signals move away from each other, it becomes more difficult for crosstalk and distortions to build constructively along the fiber length. Accordingly, it is desirable to have a method and apparatus for reducing the levels of Raman-induced crosstalk and distortions arising between individual channels comprising a WDM optical system. This is particularly true in the case of a system that uses optical channels that are located near the zero dispersion wavelength of the transmission medium.

SUMMARY OF THE INVENTION A method and apparatus are provided for transmitting an optical WDM signal. The method begins by modulating a plurality of optical channels that are each located at a different wavelength from each other with respect to a plurality of broadcast signals carrying information that all represent the same broadcast information, at least one of the Diffusion signals are out of phase with respect to the remainder of the plurality of broadcast signals. Each of the modulated optical channels is multiplexed to form a WDM optical signal. The WDM optical signal is sent over an optical transmission path.

According to one aspect of the invention, a 180 degree phase change can be applied to at least one of the plurality of broadcast signals relative to the remainder of the plurality of broadcast signals. According to another aspect of the invention, a phase change can be applied to those selected from the plurality of broadcast signals in such a way that the modulated optical channels consequently have contributions to the Raman crosstalk in the selected optical channels. that are diminished by contributions to Raman crosstalk from optical channels modulated by RF signals that do not undergo a phase change. According to another aspect of the invention, a phase change can be applied to the selected one of the plurality of diffusion signals in such a way that the first and second diffusion signals modulate optical channels in the first and second optical wavelengths, respectively, in such a way that the crosstalk and Raman-induced distortions are reduced to a third optical channel. According to another aspect of the invention, a transmission signal can be combined with each broadcast signal before modulation. According to another aspect of the invention, a difference in wavelength between any of the optical channels may be less than the change in Stokes of maximum power transfer in the optical transmission path. According to another aspect of the invention, the relative amplitudes of the first, second and third modulated optical channels can be adjusted to further reduce the Raman crosstalk. According to another aspect of the invention, the relative laser polarization of the first, second and third modulated optical channels can be adjusted to further reduce the Raman crosstalk. According to another aspect of the invention, the path can be an HFC network. According to another aspect of the invention, the optical transmission path can be located in a CATV transmission network. According to another aspect of the invention, the optical transmission path can be located in a PON. According to another aspect of the invention, the optical channels can be located at wavelengths at or near a zero dispersion wavelength of the transmission path. According to another aspect of the invention, the optical channels are located at wavelengths far from a wavelength of zero dispersion of the transmission path but the scattering impact is not important. In accordance with another aspect of the invention, WDM optical transmitters are provided. Each transmitter may include a plurality of optical sources to generate optical channels located at different wavelengths; a plurality of optical modulators each having an input for receiving the respective signal from the plurality of broadcast signals carrying information representing all the same broadcast information, each optical modulator being associated with the respective one of the plurality of optical sources to thereby provide a plurality of modulated optical channels; a phase switch for adjusting a phase of at least one of the plurality of broadcast signals so that it is out of phase relative to another of the plurality of broadcast signals; and a multiplexer coupled to the plurality of optical sources for receiving and combining the modulated optical channels to produce a multiplexed optical signal.

Brief Description of the Drawings FIGURE 1 is a schematic diagram illustrating the Raman scattering process. FIGURES 2A and 2B show signals Si and S, respectively, and FIGURE 2C shows the signal Si that pumps signal Sj, for the purpose of facilitating an understanding of Raman crosstalk. FIGURE 3 shows a simplified block diagram of a conventional WDM transmission arrangement. FIGURE 4 illustrates a WDM system for common broadcast and different broadcast transmissions in CATV transmission systems. FIGURE 5 shows optical signals of WDM, Si, S2, S3 and S4, with signals S2 and S3 selected to be 180 degrees out of phase with respect to the Si and Si signals. FIGURE 6 shows an example of a transmitter arrangement according to the present invention. FIGURE 7 is a flow chart showing an example of the method performed by the transmission arrangement shown in FIGURE 5. FIGURE 8 shows the architecture of a broadband passive optical network (BPON) in which the transmitter arrangement of FIGURE 5 can be used.

Detailed Description Equations (4) and (5) presented in the above are the starting points in the discussion about eliminating unwanted Raman-induced crosstalk and the distortions that occur in an almost null dispersion optical communication system. Assuming that a constant phase change can be applied to the composite RF subcarrier modulation applied to each transmitter. This can be achieved by means of a broadband phase switch such as an all-pass filter transformer having a constant phase change through the RF band of the subcarriers. The composite modulation signal that powers each transmitter is executed through a phase switch specifically designed for each laser with a specific phase. For the two wavelength system if a phase change of < ps is applied to the signal laser and the phase change of f? it is applied to the equation (4) of pumping laser then it is modified to the following in the frequency domain: P0s [\ + Gsl, pLLeffPBp] e-aL + P0s m, ei < P > 3 [/ s (t)] + [l + Gsp pL LeffP0p} e ~ a 1 + Gsp pL LefP0s I Ñ? ' + e'2 < P * Ñ '§ «that, (Pos m,? [L + C" ¾ / ¾ * ° ¿+ Ñ' v? '+ EÍ2 (p »' ^ Kcso, F Glp pL LcjrPosse ~ L + e'2 ^ 3Ñ, 'cneff + e'3 ^ Ñ'ZCTBeff) ^ Kcm (Pu¡ »is) 3 [\ + Gsf, pL LeffPüp] e aL + 2 < P, i3 < p - |? ' + T? E ?? J,? V'5T 'KCTBp. { PoP > "and? GsP PL Ps e a L + 3el < P * Ñ '+, 5' »s (» / >) 2 < 2 a / - + cc. (6) Where C.C means complex conjugate and is the positive frequency portion of the Fourier transform of the RF subcarrier modulation signals. Also, the parameters KCSOSÍ KCso r C BS / and KCTBp are given by: KcSOs / electric! ms Pos NCSOeffM rCSOs 2 (7b) K 'CSOp 1 electric! mp ^ Op ^ CSO ff ¡'CSOp 4 (c; K'c: i electric! (/ », / >",) 2 N; -racry'CTBs 4 K NcTBeff VrC. (7d) Where 'or rcsop'c' are the natural CSO ratios of the pumping signal transmitters in electric units, also rcf 'rical and rcfBcpMcal are the natural CTB ratios of the signal transmitters. and pumping in electric units NC'SOeff is the effective pulse count (after taking into account the random phase of the RF subcarriers) at a specific frequency of CSO NC'TBeff is the effective pulse count (after take into account the random phase of the RF subcarriers) at a specific frequency of CSO For a multiple wavelength system if a phase change of (ps) is applied to the laser under investigation and the phase changes of f? apply to the other lasers, equation (5) then modifies to the following in the frequency domain (ignoring the third order terms CTB that are insignificant): (8) -al 1 + V? ".., N S H ,, p G ,, p Pl P < , "Mpe" + N S H ,, p G, .P Pi, .p V / > There are n of these equations, one for each of the wavelengths transmitted in the system, so that (8) is also a matrix equation for each impairment. Note that the Raman gain is zero when the optical frequency separation between the signals goes to zero, so we have the terms s = p of (8) (the diagonal elements of the matrix equation:Pt (f.L) = P9, e-a 1 + P0í m, e * '3 [(/) G a ¿+? 7- + ei2 < P! '2 -al + C.C. (9) This is just the original signal that includes the natural term CSOs not influenced by Raman interactions. To understand how to eliminate impairments, each particular impairment must equal zero by extracting the appropriate terms from either (6) or (8). For crosstalk in the two-length system, this leads to the following equation: GvpLL0POsPOpm, eiV '+ GvpLL0Po, POpmpel (p' '= 0 (10) Which leads to the following conclusions: Raman-induced crosstalk in a system non-zero dispersion, of two wavelengths, can only be eliminated if the transmitters have identical (mp = ms) of OMI and the modulations are out of phase by the radians po if the polarization overlap averaged by length pL is zero. Polarization condition can be satisfied only or approximately by throwing the pumping and signal transmitters in orthogonal polarization states.In this case, pL can be set out from zero and gradually change to the value of ½ when the signals propagate together to the length of The larger the nearest fiber the pL value will be at ½ at the end of the link, therefore, the polarization method is only less effective at fiber links. s Considering the terms CSO in the system of two lengths, equation (6) leads to two equations one for the difference pulses and one for the sum pulses. Denote the total power in the CSO due to the difference pulses when and the total power of CSO due to addition pulses such as P, ZCSOioial then the terms CSO in (6) give: &CSOtolaI (eleven) . = A faith aL ei2 (PsKCSOs { PQs &nt; ns) 2 + e''2 < P > KCSOs. { P0s ms) 2Gsp pL LeffPQp + e mpf Gsp pL LeJfP0s + e [< ??]? pL LeffP0s msP0pn, /; | + C.C (12) These equations, when they are equal to zero, lead to the following, the phase requirements to be able to reduce the CSO levels in a two-wavelength system: yf? -p where n is an integer number (13) With these satisfied interface requirements (11) and (12) both are reduced to the same equation. Solve the optimal value of the product pLP0p gives: To summarize, in order to reduce the effects of CSO distortions on two identically modulated lasers in a nearly zero dispersion system, the modulation applied to the pumping and signal transmitters must be out of phase by the radians n and the product of the probability of polarization overlap averaged by length pL and the pumping throw power P0 must satisfy (14). If (14) indicates an optimum value pL of 0.5, this can be achieved by throwing an optical wave in a state of linear polarization and the other in a state of circular polarization. Equation (14) is only valid if (13) is also satisfied for the modulation phases applied to the transmitters. Note that the phase conditions for the reduction of CSO and the elimination of crosstalk are identical. The elimination of crosstalk also imposes the requirement that mp = ms. Then examine the methods to eliminate or reduce crosstalk in a multiple wavelength system. Equation (8) leads to the following matrix equation for crosstalk at all wavelengths in the system: x, = P0, m, ei (p 'f {') e ~ aLLeff? Hsp G! p pLsp P0p + (15) In (15) xs is a placeholder to remind us that there are n crosstalk equations that they must be solved simultaneously (all of xs will be set equal to zero in order to cancel Raman crosstalk at each wavelength): This also reduces X > = 0"E H ', P Gs, P PL s.p PC 0 p /> =' which can be put in the form of: < .- »-? Hs, p Gs, p pLS, p PoP (17b) This is also expressed as a matrix equation: ^ where the elements of the diaphony interaction matrix nxn are given by: Note that the transposition elements are related by such that if mp = ms and for? 3 > ?? so and 's.p |? '-?,?, ¾ ,, (20c! V In most typical WDM optical communication systems the ratio of refractive indices and wavelengths is approximately equal to unity: nrX. (20e; "A The only way that (18) can be satisfied in a non-trivial way, under all conditions, is if each of the matrix elements is zero.This can happen if: ys¡p = 0 if f? -f? = p and also mp = ms or pLsp = 0 (21) Because the polarization is a two-dimensional Hilbert space, for any agreed basis, exactly two orthogonal polarization states will exist. All other polarization states can be expressed as a linear combination of the base states chosen. Furthermore, once any of the modulation phases is set, the modulation phase of any other signal in the same polarization state must be moved by the radians n. Therefore, for any of the two states with the same polarization, there are only two phases that will satisfy the first line of (21) (f, f + p).

Essentially, then, there are only two orthogonal polarization states and two phases to choose from, the direct product of the modulation phase states and the polarization then form an effective vector space 2x2 in which there are four orthogonal states. If the two phase states of orthogonal modulation are + r) and if the polarization base states are denoted by | //) and then the base states of the newly formed 2x2 polarization space and modulation phase are given by: In other words, (18) can only be satisfied exactly for a system consisting of at most 4 independent optical transmitters (each with identical OMI values such that (21) is satisfied). These base states will make all matrix elements in the 4x4 crosstalk interaction matrix zero. In (22) the subscripts a, b, c, d were added to help identify each of the base vectors. Within any of the set of four base vectors, the matrix elements? all are zero because one of the following must be true: if (23a (f? f?) = 0 if f5 = f? + p (23b) Pol i Pol. p = 0 if the two polarization states are orthogonal (23c) Where Pols and Polp are only polarization states of sth and pth optical waves Here, it is implied that Pol) \ is averaged over fiber length in such a way that P l. Polp \ Crosstalk is zero within the group of four orthogonal states. This method completely eliminates Raman-induced crosstalk along with the static Raman gain imparted over the signal term. Now it is assumed that there is a system with more than 4 transmitters, it is impossible to use the previous method to completely eliminate the crosstalk generated by Raman in the system. However, by simplifying the procedure in such a way that only Raman-induced crosstalk is explicitly addressed and not the Raman gain in the signal under consideration, the level of Raman crosstalk can be significantly reduced. Taking this procedure, the first term in (16) is ignored in such a way that the following matrix elements are now available: Iy s, p = m "'p ec ^ > p ~ (^ s ^ 1 H1 s, p G w s, p ois.p (A \ The transposition elements are related by Ys.p = mp Hs, p Gs p PLS.P (25a) Ys.p If it is allowed that Then for < ? 2 ... < ?? _? < ?? under these simplifications (18) it is reduced to: If the system is further restricted to having a non-zero number of wavelengths (n non) then each row of the Raman crosstalk interaction matrix n x n contains an even number of nonzero elements. The system can be arranged in such a way that the terms in the last row are eliminated in pairs by assigning alternative phases as follows at the n wavelengths of the system: The simplest scheme is to allow < P \ = 0 then the phases assigned to the n wavelengths alternate between 0 and n (180 °) as: 0i, 1802, 03, I8O4, 05, 1806, ..., 0n (30b) Where the subscripts refer to the wavelength assigned to the particular phase in the alternative series. The phases assigned to the corresponding wavelengths must alternate in this way in order to ensure maximum cancellation of Raman-induced crosstalk signals. This can easily be deduced in the case where the coefficients e all are set equal to unity (£ «1) and the optical throwing powers are arranged so that each term in each line of (29) has the same magnitude. In this case, the phases alternate by 180 degrees and because there is an even number of non-zero terms in each line, a total cancellation of the Raman crosstalk is achieved. In the current case the coefficients "not all will be equal to the unit but will be very close to one.

Therefore, it is concluded that under these conditions, crosstalk will nevertheless be minimized if not eliminated completely. We now change to treat the CSO induced by Raman in the case of a multiple wavelength system. The terms CSO are the last three lines of (8). Following the procedure of the previous paragraphs and establishing the terms of pulsation of sum and pulsation of difference equal to zero independently giving: 3 ACSOlolals = Ñ &'CSOcff - 1P0sm "' s 1 + ^ //? ^, - G ,, ^ S., C? / '/ Clearic" CSOeffM' CSOs . (31a) There are n of these equations, one for each of the wavelengths transmitted in the system, p ZCSOlolal s -ß''2f > ? CSOeff 71 | aL "m 2R Op N? 'CSO * ff - P0, mse? = Hs, p Gs, p PLs.pe say Lff + 7i m.N l C'SOeff CSOp p_ p -aL Ñ? H, .P r PLS.P ?? ™ e? Le (faith, (ps + CC p =] (31b) There are n of these equations, one for each of the wavelengths transmitted in the system. In (31a) and (31b), equations (7a) and (7b) were used to express the constants of KCsos and Csop in terms of the natural CSO ratios of the signal and pumping transmitters. Equations (31) are also each equations of matrix n x n. Set each of these equations equal to zero gives: (32a; (32b) Where 0 ?? and 0? s are simply placeholders to remind us that there are n equations to direct. Each of these equations can be expressed in the matrix form as: (33) Where the various elements of the matrix are given For the difference pulse equation (32a) the TSP matrix elements are: G3 ,? = HS, pGs, pPl.s.p? T? electric! V CSOeff ^ rCSOs (35a) While for the summation pulse equation (131b) the elements of matrix G are: 1 m "1 rs, p = Ha s, p PLS, electrical,?, Rr electrical 1vcso /" V'CSOÍ s 1vcso /; ~ vrcsoP (35b) The column vector of the elements? S arises from the CSO ' The naturalness of each transmitter in the system The crosstalk equation (18) had no column vector because there is no "natural crosstalk." All the purely real terms in (35a), (35b) as well as (34) are of positive value. If there is any possibility of eliminating the positive terms, in order to satisfy (33), the last term of (35a) and) 35b) must be real and negative, therefore the first restriction on the phase angles can be imposed: f? -f5 = p (36) Similar to the case of crosstalk elimination, once any of the modulation phases are fixed, the modulation phase of any other signal must be displaced by radians n. Therefore, for either of the two states, there are only two phases that will be fulfilled (36) (f, f + p). Essentially, then, there are only two modulation phases to choose from. This can be linked back to a two-dimensional Hilbert space with the modulation phase states \ f) and | < p +; r). Unlike the case of crosstalk, here we do not want the probabilities of polarization overlap averaged by length to be zero in all cases, otherwise the matrix elements G all would be zero and there would be no opportunity to satisfy (33). Since it is desired to keep these terms that are out of phase with the sth state by the p radians, orthogonal polarization states are not generally required. However, for those assigned states, the same modulation phase as the state sth all the terms of the matrix element G will be positive and therefore these states can not satisfy (33). These states must be in orthogonal polarization state with respect to the sth state under consideration in such a way that tJJ) * 0 and the matrix elements will be zero. If the states with modulation phase that are displaced by the radians p are required to be in the parallel polarization states pLsp ~ \ such that these matrix elements dominate, then a two-dimensional polarization base has been defined once again. Just as in the crosstalk discussion, the direct product of the modulation phase states and the polarization states then form an effective 2 x 2 vector space in which there are four orthogonal states. If the two phase states of orthogonal modulation are + p and if the polarization base states are denoted by | //) and then the base states of the newly formed 2-phase polarization space and modulation phase are exactly the same (22) ). It is also observed that for these states in which (36) is fulfilled (35a) and (35b) both reduce to the same equation: rs, p = ss ,, pp Gss ,, pp P r Ls, f (37a) For those states in which f -f? (35a) and (35b) both are reduced to the same equation: (37b) It will be required that LS.P ¾1 (parallel polarizations) for (37a) and p0 0 (orthogonal polarizations) for (37b). Since the Hilbert space is only of 4 dimensions (of 2x2 vector) only the phase and polarization conditions can be met exactly for the four orthogonal base states given by (22). It is assumed that the four states are: Then (33) is reduced to the following: 02 (- i) (39) 03: 04 The four equations are decoupled and can be easily solved. The solutions are identical to (14) with the probability of polarization superposition established equal to one (since we have pL «1). (40) In (14) there was some flexibility to adjust the transmitted power because the value of p ~ L could be adjusted to compensate. In the case of four wavelengths, the pLsp each has a fixed value. Compared to the case of two wavelengths in which it was considered desirable to establish the polarization superposition probability equal to ½ here in the nonzero terms of (39) they all have the same probabilities of polarization superposition set equal to 1. Therefore, the launch powers for the four-wavelength system will need to be halved in such a way that the two-wavelength system will release the power values for similar circumstances. In (39), the diagonal terms are zero because G - 0, the Raman gain is zero when the wavelengths are identical. Obviously, it will be quite difficult to eliminate the distortions of CSO in the case of five or more wavelengths. For example, suppose that there is a system of eight wavelengths with the following states: Then (28) it becomes (37) ?? (37), the superscripts (-) or (+) simply indicate whether the term mp / 2 in the matrix elements is negative or positive. Also note that only even subscripts are in pairs and only non-subscripts are in pairs and that there is no pair of non-pair numbers. Therefore (38) can be reduced to two equations of matrix 4 x 4: (38b) Therefore, the requirement release powers are given by: (39a; In (38) and (39), when calculating the matrix elements using (32a) (32b), all probabilities of polarization superposition PLs are equal to 1. Equations (39a) and (39b) can be easily solved using a computer-based mathematical analysis program The algorithm can be incorporated into a system controller that continuously adjusts the modulation-phase and polarization states of the transmitted optical waves to correct changing environmental conditions that can alter the dedicated balance of the parameters that In order to summarize, it is possible to completely eliminate Raman-induced crosstalk and CSO distortions by up to four identically modulated transmitters in an almost zero chromatic dispersion optical communication system.For systems consisting of five or more optical channels can not completely eliminate any deterioration although the deteriorations can be reduced by judiciously assigning the modulation-phase and polarization states to the transmitted waves. Now it is assumed that there is a system with more than four transmitters, it is impossible to use the previous method to completely eliminate the crosstalk generated by Raman in the system. However, by simplifying the procedure in such a way that only the Raman-induced CSO and not the natural CSO or Raman distortion crosstalk terms in the signal under consideration are explicitly addressed, the level of CSO induced by Raman can be significantly reduced. if it is not completely eliminated. This can be achieved quite easily by simply ignoring those terms in (32a) and (32b) that contain the natural factors of CSO 'that' '' 'rcsop "ca' · Taking this procedure we now have the following matrix equations: The matrix equation (33) is reduced to: (41) Where the matrix elements are, for the difference pulse equation (40a): And for the summation pulsation equations (40b) Note that (41) for the Raman-induced CSO has the exact same form as (18) for the Raman-induced crosstalk. In addition, the matrix elements (42a), for the differential CSO pulsation equation, are identical to the simplified Raman crosstalk matrix elements (24). The matrix elements for the sum pulse equation simply refer to those of the difference-beat equation by the complex conjugation: Therefore, equations (41) and (18) are identical to each other when considering only the explicit Raman crosstalk or Raman-induced CSO (the sum-pulse equation is simply the complex conjugate of the difference-beat equation). Therefore, equations (24) to (30b), together with all the discussions and conclusions that surround them, apply equally well to the CSO induced by Raman. In fact, you can simply replace the term "crosstalk" with "CSO" within those paragraphs. In this way, the same criteria apply to effectively reduce Raman-induced CSO and Raman-induced crosstalk. Particularly when establishing the phase criteria given by (30a) or (30b) and adjusting the IMO and / or powers to take each term in each line of (41) (and therefore (18)) there are almost the same, then both deteriorations of Raman-induced CSO and Raman-induced crosstalk will be substantially reduced. How closely the magnitudes can be adjusted to be equal to each other depends largely on how close each of the ss coefficients given by (28) enters the unit. FIGURE 3 shows a simplified block diagram of a conventional WDM transmission arrangement 200 in which the data or other information carrier signals Si, S2, S3 and S4 respectively apply to the inputs of modulators 210i, 2102, 2IO3, and 2IO4. The modulators 210i, 2102, 2103, and 2104 in turn drive the lasers 212x, 2122, 2123, and 2124, respectively. The 212ir 2122, 2123, and 2124 lasers generate optical channels modulated by data at wavelengths?,? 2,? 3 and? 4, respectively, where? 4 > ? 3 > ? 2 > ?? . A wavelength division multiplexer 214 (WDM) receives the optical channels and combines them to form a WDM optical signal which is then sent on a simple optical transmission path 240. While the WDM transmission arrangement shown in FIGURE 3 multiplexes four optical channels on a single path, those of ordinary skill in the art will recognize that any number of optical channels can be multiplexed in this manner. In the context of a CATV network, optical channels?,? 2,? 3 and? 4 can be broadcast signals that contain all the same video signals, plus unicast signals that are different for optical channels other than ??,? 2,? 3 and? 4. The unicast signals that are multiplexed by RF frequency in broadcast channels are usually much lower in amplitude than the video or broadcast signals. The provision of sending the same broadcast signal and the different monodiffusion signals over multiple wavelengths (WDM) is a means to provide more segmentation in CATV networks. This is demonstrated in FIGURE 4, which shows an RF splitter 216 that divides the broadcast signal between the laser 212i, 2122, 2123, and 2124. As shown, the laser 212 can each receive a different unicast signal . In FIGURES 2 and 3, similar reference numbers denote similar elements. As previously observed, Raman crosstalk can occur between the optical channels ??,? 2,? 3 and? 4. In the case of the CATV application scenarios as described in the previous, Raman crosstalk not only causes interference (crosstalk) between the optical channels but signal distortions as well as when the broadcast signals are sent over different optical WDM channels. Due to the amplitude of the broadcast video signals is much higher than that of the unicast digital signal, Raman crosstalk has more impacts on the analog video signals between the optical channels than on the unicast channels. The present invention has recognized that this problem can be solved by adjusting the phases of broadcast video channels in optical channels with respect to each other. Specifically, Raman crosstalk and induced signal distortions can be reduced by changing the phase of some of the signals by 180 degrees with respect to other signals. The particular channels that undergo this phase change and the manner in which the phase change is achieved will depend on a number of factors such as the optical wavelengths employed, the channel spacing and the like. The following guidelines and examples that are discussed along with the lengths ?? -? 4 shown in the above are presented by means of illustration only and should not be taken as a limitation of the invention. FIGURE 5 shows an illustrative example where the signals Si, S2, S3 and S4, with the signals S2 and S3 selected to be 180 degrees out of phase with respect to the signals Si and S4. In this example, a digital signal is used to represent the analog broadcast signal for simplicity. It should be noted that since the individual broadcast signals are identical, those patterns will be the same before the phase change. As a result of the Raman specification, the Si signal will generate crosstalk on the S4 signal. However, this crosstalk can be reduced or even eliminated by the crosstalk generated by signals S2 and S3 in S4. This can be achieved if the amplitude of the crosstalk regenerated by Si is equal to the amplitude of the crosstalk generated by S2 and S3. The relative amplitudes of Si, S2, and S3 can be selected to ensure that this ratio between the three crosstalk components generated in S4 is satisfied. Noting the fact that when the optical wavelengths are close to the zero dispersion lengths or when the optical wavelengths are very close to each other, the phase relationship between the optical channels is maintained for a certain length of the transmission link , and therefore the reduction of Raman crosstalk is effectively achieved together with the transmission link. In a WDM signal, the wavelength spacing between the adjacent channels ??,? 2,? 3 and? 4 is generally much smaller than the Stokes change of maximum power transfer. Therefore, in general, the larger the space between any of the two channels, the greater the Raman crosstalk between them. That is, the Raman crosstalk between the Si and S4 signals will generally be greater than the crosstalk between S2 and S4, which in turn will generally be greater than the crosstalk between S3 and S4. This explains why in the previous example both signals S2 and S3 were selected to be 180 degrees out of phase with Si: the two smaller components of the crosstalk in S4 generated by S2 and S3 more easily eliminates the largest component of the crosstalk generated by Si in S4, thus requesting smaller adjustments in their relative amplitudes. Of course, those of ordinary skill in the art will recognize that the signals that are selected to be out of phase of the other signals may be selected in any desired manner based on the level of Raman generated crosstalk and relative scatter ratio etc. For example, in one example, the signals can be placed in out-of-phase pairs (ie, the even signals are in phase and the non-phase signals are out of phase). In the case of signals ??,? 2,? 3 and? 4 shown in the above, for example, signals Si and S3 can be selected to be out of phase with signals S2 and S4. Of course, if the crosstalk is to be completely eliminated, this selection will typically require a greater adjustment in its relative amplitudes than the selection shown in FIGURE 5. It should be noted that the reduction of Raman crosstalk between the technique described in the previous one reduces signal distortions induced by Raman also simultaneously. It should also be noted that the technique as explained in the above can be used in CWDM and DWDM and therefore, generally in any WDW. As mentioned previously, Raman crosstalk can be particularly acute when the channels are located at wavelengths close to the zero dispersion wavelength of the transmission path because the optical channels fully maintain their relative phases in those lengths cool. For the same reason, the prior art in which some of the channels are arranged to be out of phase with respect to other channels will be more effective when the channels are located near the zero dispersion wavelength of the transmission path. For example, for channels operating in the 1310 nm window (typically defined as the waveband between approximately 1280 nm and 1330 nm), a commonly used simple fiber optic is the SMF-28 ™ fiber, available from Corning, Incorporated. The SMF-28 fiber has a wavelength of zero dispersion at or near 1310 nm. Therefore, if this transmission fiber is used, Raman crosstalk can be more effectively reduced for optical channels having wavelengths in the vicinity of 1310 nm. Similarly, for optical wavelengths operating in the C-band (wavelengths between approximately 1525 to 1625 nm), a commonly available optical fiber is the Corning Leaf ™ fiber, which has a wavelength of zero dispersion close to 1500 nm. For Leaf ™ fiber, Raman crosstalk can be more effectively reduced for channels that have wavelengths in the vicinity of 1500 nm than channels in the vicinity of 1525 nm or 1565 nm. If, on the other hand, the optical wavelengths are operated in the L-band (wavelengths between approximately 1565 to 1625 nm), a commonly available optical fiber is Corning Leaf.R ™ fiber which has a wavelength null dispersion close to 1590 nm. For Leaf.R ™ fiber, Raman crosstalk can be more effectively reduced for channels that have wavelengths in the vicinity of 1590 nm than for channels in the vicinity of 1565 nm or 1625 nm. In the case where optical wavelengths in a WDM system are far from the zero dispersion wavelength, the narrower wavelength spacing between the WDM channels may be required or the link length may be limited to maintain the phase relative between the channels and therefore the effectiveness of this technique.

FIGURE 6 shows an example of a transmitter arrangement in which the selected individual signals can be arranged to be out of phase with respect to other signals. In FIGURES 3 and 5 similar elements are denoted by similar reference elements. As shown, an RF signal g (t) is applied to the input of a divider 220. The divider 220 has an output for each of the wavelengths to be multiplexed together by the transmitter arrangement. In the example shown in FIGURE 5, the splitter has four outputs 222i, 2222, 2223 and 2224. Each output is directed to an input of one of the modulators. Specifically, the output 222i is directed to an input of the modulator 2124, the output 2222 is directed to an input of the modulator 2123, the output 2223 is directed to an input of the modulator 2122, and the output 2224 is directed to an input of the modulator 212i . The modulators 210i, 2102, 2103, and 2104, in turn, drive the lasers 212i, 2122, 2123, and 2124, respectively. The divider 220 is configured in such a way that a selected one of its outputs changes the phase of the input signal by 180 degrees. Such phase change splitters are well known components and need not be discussed in detail. The phase change can also be achieved separately. If the particular modulator pattern shown in FIGURE 5 is employed, the output 2222 and 2223 of the splitter 220 change the RF signal g (t) by 180 degrees. An amplitude adjuster 230 can be provided to adjust the relative amplitudes of the RF signals modulating the lasers, i.e., the modulation index, or the optical signal output level generated by each of the lasers 212lt 2122, 2123, and 212. Additionally, the bias adjusters 213i, 2132, 2133, and 2134 (under the control of the bias controller 231) can be used to provide bias control of the optical wavelengths that can be used to achieve maximum reduction in crosstalk and distortion as it is represented in FIGURE 6. It should be noted that the data signals added to the modulator 210i, 2102, 2103, and 2104, generally (but not always) include different monodiffusion signals. In addition, while FIGURE 6 shows direct modulators that are employed, the techniques described herein can be applied to external modulators as well. FIGURE 7 is a flowchart showing an example of the method performed by the transmitter arrangement shown in FIGURE 6. The method begins at step 505 by receiving multiple electrical bearer information signals representing all the same broadcast information . Electrical signals, for example, can represent audio and / or video broadcast programming. Then, in step 510, multiple optical channels that are each located at a different wavelength from one another are modulated with the respective one of the electrical signals. In addition, in step 515, each optical channel is modulated with an RF signal. The RF signals will have a common functional diffusion waveform. At least one of the RF signals is out of phase with respect to the remaining ones of the plurality of RF signals. The modulated optical channels are multiplexed in step 520 to form a WDM optical signal. Finally, in step 525, the WDM optical signal is sent on an optical transmission path. In addition to reducing crosstalk arising from Raman interactions, the methods and techniques described herein can also mitigate and even eliminate the effects of distortion that arise from Raman interactions, particularly second order distortion, which is knows that it is especially serious for analog signals. While analogous channels are more vulnerable to distortion, digital channels are also impacted and thus the methods and techniques described herein can reduce the Raman distortion that arises in both analog and digital signals. The transmitter arrangement described above can be advantageously used in any optics in which a broadcast signal is multiplexed over multiple optical wavelengths or channels. Such networks include, without limitation, several all optical networks, hybrid coaxial fiber networks (HFC) and networks that use a passive architecture, which are often referred to as Passive Optical Networks (PON). In typical HFC architectures, the broadcast signal is divided into optical connections and then sent to different nodes along with unicast signals. In the fiber node, the optical signal is converted into an electronic signal, and carried over multiple multiple coaxial buses for distribution through a neighborhood. On the other hand, in a PON architecture, the fibers carry signals from the optical line terminator (OLT) to the optical splitters, and also to optical network units (ONU), where the conversion from optical to electronic takes place. In the case of PON architecture, broadcast and unicast signals are sent in a similar way. Both HFC and PON generally carry the same downstream signals to multiple customers. In both networks, multiple paths typically use beyond the first node. The use of DM in a fiber for a node allows more and more node segmentations. A main advantage of a PON is its reliability, ease of maintenance and the fact that the network deployed in the field does not need to be energized. Accordingly, PONs are often used as access networks by cable television and telecommunication providers for the purpose of distributing their services from their facility to the customer's premises (for example, a house or a business). An example of PON is sometimes referred to as broadband PON (BPON), which is a combination of PON with wavelength division multiplexing (DM) for downstream and upstream signals. The WDM techniques can be used for downstream signals and allow different optical wavelengths to support diffusion and multiple unicast transmissions (dedicated for each wavelength) in the fiber used in the BPON. WDM can also be applied to different PON standards. FIGURE 8 shows the architecture of a BPON in its most generalized form. The BPON includes a connection 102, remote node 104 that is deployed in the field, and network interface units 106 (NIU). The connection 102, the remote nodes 104 and the NIU 106 are in communication with each other over fiber optic links. If the BPON 100 is a telecommunication network, the connection 102 is a central office or called OLT. The NIU 106 may be an equipped terminal located at the customer's premises or may serve multiple clients, in which case the NIU 106 simply provides another level in the network hierarchy under the remote nodes. A method and apparatus has been described to reduce Raman-induced crosstalk and the distortion that arises between individual channels and an optical WDM signal, which are particularly severe between channels that are located near the zero dispersion wavelength of the medium of transmission. The method and apparatus are particularly suitable when the individual channels support broadcast signals carrying the same information, which are sometimes transmitted over a transmission network such as an HFC or PON network.

Claims (28)

1. NOVELTY OF THE INVENTION Having described the present invention it is considered as novelty and therefore claimed as property -as described in the following claims. CLAIMS 1. A method for transmitting a WDM optical signal, characterized in that it comprises: modulating a plurality of optical channels that are each located at a wavelength different from each other with the respective one of a plurality of broadcast signals carrying information that they all represent the same broadcast information, at least one of the broadcast signals is out of phase with respect to the remainder of the plurality of broadcast signals; multiplexing each of the modulated optical channels to form an optical WDM signal; and sending the optical WDM signal over an optical transmission path.
2. 2. The method of compliance with the claim 1, further characterized in that it comprises applying a phase shift of at least 180 degrees for one of the plurality of broadcast signals relative to the remainder of the plurality of broadcast signals.
3. 3. The method according to claim 1, further characterized in that it comprises applying a phase change to the selected one of the plurality of diffusion signals in such a way that the modulated optical channels consequently have contributions for the Raman crosstalk in the selected one. of the optical channels that is diminished by the contributions to the Raman crosstalk of the optical channels that do not undergo a phase change.
4. The method according to claim 1, characterized in that the step of modulating a plurality of optical channels further includes combining a unicast signal with each broadcast signal before modulation.
5. The method according to claim 1, characterized in that the modulation further comprises: changing a phase of a first broadcast signal with respect to a second broadcast signal, wherein the first and second broadcast signals modulate optical channels in a first and second optical wavelengths, respectively, such that the Raman crosstalk and the distortions are reduced to a third optical channel that is located at a third optical wavelength.
6. The method according to claim 1, characterized in that the modulation further comprises: phase change of at least one of the plurality of broadcast signals representing all the same information, where the diffusion signals changed by phase and the Replaced signals with no remaining phase modulate optical channels at different optical wavelengths, respectively, such that Raman crosstalk and distortions are reduced in an optical channel that is predetermined at an optical wavelength.
7. The method according to claim 6, characterized in that a difference in a wavelength between any of the optical channels is less than the change of Stokes of maximum power transfer in the optical transmission path.
8. 8. The method of compliance with the claim 3, further characterized in that it comprises adjusting the relative amplitudes of the first, second and third modulated optical channels to further reduce the Raman crosstalk.
9. 9. The method of compliance with the claim 4, further characterized in that it comprises adjusting relative amplitudes of the first, second and third modulated optical channels to further reduce the Raman crosstalk.
10. The method according to claim 5, further characterized in that it comprises adjusting relative amplitudes of the first, second and third modulated optical channels to further reduce the Raman crosstalk.
11. 11. The method according to claim 3, further characterized in that it comprises adjusting the relative laser polarization of the first, second and third modulated optical channels to further reduce the Raman crosstalk.
12. 12. The method according to claim 4, further characterized in that it comprises adjusting the relative link polarization of the first, second and third modulated optical channels to further reduce the Raman crosstalk.
13. The method according to claim 5, further characterized in that it comprises adjusting the relative laser polarization of the first, second and third. modulated optical channels to further reduce Raman crosstalk.
14. The method according to claim 1, characterized in that the optical transmission path is located in an HFC network.
15. 15. The method according to claim 1, characterized in that the optical transmission path is located in a CATV transmission network.
16. 16. The method according to claim 1, characterized in that the optical transmission path is located in a PON.
17. 17. The method according to claim 1, characterized in that the optical channels are located at wavelengths at or near a zero dispersion wavelength of the transmission path.
18. 18. The method according to claim 1, characterized in that the optical channels are located at wavelengths that are far away at a zero dispersion wavelength of the transmission path, but the dispersion impact is not important.
19. 19. An optical WDM transmitter, characterized in that it comprises: a plurality of optical sources for generating optical channels located at different wavelengths; a plurality of optical modulators having an input to receive a respective one of the plurality of information-carrying broadcast signals that all represent the same broadcast information, each optical modulator is associated with the respective one of the plurality of optical sources to provide accordingly a plurality of modulated optical channels; a phase switch for adjusting a phase of at least one of the plurality of broadcast signals such that it is out of phase relative to another of the plurality of broadcast signals; and a multiplexer coupled to the plurality of optical sources for receiving and combining the modulated optical channels to produce a multiplexed optical signal.
20. The WDM transmitter according to claim 19, characterized in that the phase switch is configured to apply a phase shift of 180 degrees for at least one of the plurality of broadcast signals relative to the remainder of the plurality of broadcast signals.
21. 21. The WDM optical transmitter according to claim 19, characterized in that the phase switch is configured to apply a phase change to the selected one of the plurality of broadcast signals in such a way that the optical channels modulated accordingly have for Raman crosstalk in the selected optical channels that are diminished by contributions to Raman crosstalk from optical channels that do not undergo a phase change.
22. 22. The WDM optical transmitter according to claim 21, characterized in that the phase switch is configured to change a phase of a first broadcast signal with respect to a second broadcast signal, where the first and second broadcast signals modulate optical channels in the first and second optical wavelengths, respectively, such that the Raman crosstalk and the induced distortions are reduced in a third optical channel.
23. 23. The WDM optical transmitter according to claim 22, characterized in that the plurality of optical sources are configured in such a way that a difference in the wavelength between any of the first, second and third optical channels is less than the change of Stokes of maximum power transfer in an optical transmission path in which the optical signal will be transmitted.
24. 24. The WDM optical transmitter according to claim 22, further characterized in that it comprises an amplitude adjuster for adjusting the relative amplitudes of the first, second and third modulated optical channels to further reduce the Raman crosstalk.
25. 25. The WDM optical transmitter according to claim 22, further characterized in that it comprises a light bias adjuster for adjusting the relative light polarization of the first, second and third modulated optical channels to further reduce the Raman crosstalk and the distortions. induced.
26. 26. The WDM optical transmitter according to claim 19, characterized in that the optical channels are located at wavelengths at or near a zero dispersion wavelength of an optical transmission path in which the signal is to be transmitted. optics
27. 27. The WDM optical transmitter according to claim 19, characterized in that the optical channels are located at wavelengths far from a zero dispersion wavelength of the transmission path in which the optical signal is to be transmitted.
28. 28. The WDM optical transmitter according to claim 19, characterized in that the plurality of optical modulators are configured to receive unicast signals that are combined with each broadcast signal before modulation.