Patent Publication Number: US-2007115536-A1

Title: Raman amplifier and optical transmission system

Description:
The invention is based on a priority application EP 05 292 459.4 which is hereby incorporated by reference.  
     FIELD OF THE INVENTION  
      The invention relates to a Raman amplifier comprising a Raman active optical fiber and a Raman pump in operative connection with said optical fiber, the Raman pump comprising: a plurality of pairs of pump light sources emitting polarized pump light, each generating a pair of mutually orthogonal pump fields with respective central frequencies, and a pump field combiner in operative connection with the pump light sources and adapted to couple each of the plurality of pairs of pump fields at mutually orthogonal directions of polarization into said optical fiber.  
      The invention also relates to an optical transmission system, comprising: a transmitter unit for transmitting an optical signal, a receiver unit for receiving the optical signal from the transmitter unit, and an optical transmission link for propagating the optical signal between the transmitter unit and the receiver unit.  
      The invention further relates to a method for amplifying an optical signal in a Raman active optical fiber, comprising the steps of: (a) generating at least one pair of pump fields with mutually orthogonal directions of polarization, (b) propagating said at least one pair of pump fields through said Raman active optical fiber in a propagation direction of the optical signal to be amplified.  
     BACKGROUND OF THE INVENTION  
      For amplifying optical signals in optical transmission systems, widespread use has been made of stimulated Raman amplification, which is a non-linear optical process in which an intense pump wave is injected into a Raman active optical fiber propagating one or more optical signals to be amplified. In this context, a particular amplification technique is referred to as Raman co-pumping, wherein the optical signal to be amplified and the Raman pump wave(s) propagate in the same direction on an optical transmission link, e.g. an optical transmission fiber, connecting the transmitter unit and the receiver unit in an optical transmission system.  
      In order to achieve efficient Raman co-pumping, pump light sources—hereinafter also referred to as “pumps”—with low Relative Intensity Noise (RIN) have to be employed. Pump light sources with acceptable RIN are usually composed of semiconductor diodes. These diodes emit polarized light. In order to obviate the problem of polarization dependent gain (PDG), a widely used method includes depolarizing the Raman pump, which comprises individual pump light sources, by means of using a plurality of diodes, which operate at the same optical wavelength, and coupling their respective emissions with a polarization beam combiner (PBC). This approach, however, suffers from the following disadvantage: When the two orthogonal pump light emissions—hereinafter also referred to as “pump fields”—overlap in the spectral domain, intensity noise coming from the product of the orthogonal pump fields is induced on the Raman amplified optical signal. Experimentally, with Fiber Bragg Grating (FBG) stabilized diodes, a 10 dB degradation due to this overlap has been observed.  
      Prior art document U.S. Pat. No. 6,384,963 discloses a Raman amplifier for to achieve Raman co-pumping, in which said overlap between the orthogonal pump fields is avoided by separating the spectra of the pump diodes with respect to their emission wavelengths. As can be gathered from  FIG. 7  of said prior art document, the two diode spectra are separated by a corresponding frequency shift Δν=2.1 THz. As a consequence of this, one of the pumps produces Raman gain on some channels of a wavelength division multiplex (WDM) transmission system, while the other pump produces Raman gain on other channels of said system. Therefore, most of the channels in the system mainly receive their Raman gain from only one of the two pumps, which are mutually orthogonally polarized, such that the optical transmission system will present polarization dependent gain (PDG).  
      For instance, in an optical transmission system comprising a Raman amplifier with two orthogonal pumps having the same power, being separated by Δν=2.1 THz, and emitting light at 1467.8 and 1483 nm, respectively, an on/off Raman gain over a range of wavelengths from 1510 nm to 1620 nm is shown in  FIG. 6   c,  corresponding to an (extended) wavelength range of wavelength channels of a WDM system, the optical signals being transmitted between 1560 and 1600 nm. The upper (dotted) curve in  FIG. 6   c  shows the total gain imparted on the optical signals. The two lower curves in  FIG. 6   c  show the gain produced by two individual pumps, the curve having an on/off gain with a maximum at a lower/higher frequency corresponding to the pump with a center wavelength at 1467.8/1483 nm, respectively.  
      The channel at 1590 nm receives a total gain of 7.5 dB. Among this total gain, 2.3 dB comes from the pump at 1467.8 nm and 5.2 dB gain comes from the pump at 1483 nm. The 1483-nm pump brings 2.9 dB more gain than the 1467.8-nm pump. Consequently, this share of 2.9-dB gain is provided by the polarized pump field at 1483 nm alone. In co-pumped schemes, PDG can be as high as 60%, thus the channel at 1590 nm will suffer 1.8 dB PDG.  
     SUMMARY OF THE INVENTION  
      It is the object of the present invention to provide a Raman amplifier which can be employed in optical transmission systems using co-pumped Raman amplification, which obviates the above-mentioned disadvantages and which therefore does not induce polarization dependent gain (PDG) on channels of the optical transmission system.  
      It is further an object of the present invention to provide an optical transmission system, individual channels of which do not suffer polarization dependent gain (PDG).  
      It is another object of the present invention to provide a method for amplifying an optical signal in a Raman active optical fiber, comprising the steps of: (a) generating at least one pair of pump fields with mutually orthogonal directions of polarization, and (b) propagating said at least one pair of pump fields through said Raman active optical fiber in a propagation direction of the optical signal to be amplified.  
      According to the first aspect of the present invention the object is achieved by means of a Raman amplifier of the above-mentioned type, wherein the respective central frequencies of the pump fields of at least one, preferably each pair of mutually orthogonal pump fields are different from each other and exhibit a frequency shift below 1.8 THz.  
      According to the second aspect of the present invention the object is also achieved by providing an optical transmission system of the above-mentioned type, comprising a Raman amplifier according to said first aspect of the present invention, said Raman amplifier being in operative connection with the optical transmission link for amplifying the optical signal.  
      According to the third aspect of the present invention, the object is achieved by a method of the above-mentioned kind wherein the central frequencies of the pump fields of said at least one pair of pump fields are selected to be different from each other and to exhibit a frequency shift of below 1.8 THz.  
      As a general idea, the present invention proposes to specify a maximum frequency shift, i.e., Δν≦1.8 THz, between the central optical frequencies of the polarization combined pump fields for to minimize polarization dependent gain (PDG) in optical transmission systems. In a preferred embodiment of the Raman amplifier in accordance with the present invention the frequency shift Δν between the respective central frequencies is lower than 0.8 THz. The “pairs of pump fields” to which the invention relates may be defined as the couples of polarized pump fields that are mutually orthogonal and that are the closest to each other in the spectral domain (compared to all other possible pairs). The polarized pump fields of different pairs are not necessarily aligned along the same state of polarization.  
      Advantageously, in accordance with a further embodiment of the present invention, a spectral overlap between the orthogonal pump fields in the at least one, preferably each pair of pump fields is limited according to the relation: 
 
(ν C2 −Δν 2 ′/2)−(ν C1 +Δν 1 ′/2)&gt;0   (Eq. 1) 
 
 wherein Δν 1,2 ′(=Δν i ′) is a frequency bandwidth of the pump fields and wherein ν C1,C2  is the central frequency of the respective pump field such that:  
                 ∫       v   C     -     Δ   ⁢           ⁢       v   i   ′     /   2             v   C     +     Δ   ⁢           ⁢       v   i   ′     /   2           ⁢       P   ⁡     (   v   )       ⁢     ⅆ   v         =     A   ·       ∫     -   ∞       +   ∞       ⁢       P   ⁡     (   v   )       ⁢     ⅆ   v                   (     Eq   .           ⁢   2     )             
 
 for a given pump field, wherein P(ν) is the optical power spectrum of the pump field and A is a constant, 0.5&lt;A&lt;0.9, preferably A=0.8. In this way excessive overlap between pump fields is avoided which results in an effective reduction of intensity noise on the Raman amplified signal, as stated above. 
 
      In another embodiment in accordance with the present invention, the respective central frequencies of only one pair of pump fields with the lowest central frequencies of pump fields of said plurality of pairs of pump fields exhibit a frequency shift below 1.8 THz. The share of gain provided to the signal by one polarized pump field alone should be limited, for instance to 2 or 3 dB. Due to the specific shape of the Raman gain curve, the pump field (or the pair of orthogonal pump fields) with the highest wavelength (corresponding to the lowest central frequency) almost provide all the gain to the signals of higher wavelengths. It is thus a priority to specify the maximum frequency shift between the orthogonal pump fields having the highest wavelengths.  
      In a further preferred embodiment, the plurality of pairs of pump light sources are set up for emitting the polarized pump light through said Raman active optical fiber in a propagation direction of an optical signal to be amplified, thus performing Raman co-pumping.  
      In a preferred further embodiment of the Raman amplifier in accordance with the present invention, the pump field combiner comprises at least one of a polarization beam combiner, a wavelength multiplexer, and an optical coupler. In this way, various ways of technical realization of the Raman amplifier in accordance with the present invention are available for a person skilled in the art. In a general way, for pump depolarization, a plurality, i.e. at least two, polarized pump fields are coupled together, such that they enter the Raman active optical fiber (discrete or distributed Raman amplifier) with orthogonal states of polarization.  
      In order to efficiently limit a width of the pump fields, which is an important feature with respect to avoiding pump field overlap, in a preferred embodiment of the present invention at least one of the pump light sources is a Fiber Bragg Grating (FBG) diode, which display a spectral width of only 2 to 4 nm (250-500 GHz) and which are wavelength stabilized. Alternatively or additionally, at least one of the pump light sources can be devised as an inner-grating module, which have the additional advantage of presenting only low RIN.  
      When using FBG stabilized diodes, polarization maintaining fibers should not be used to depolarize the output field of the respective diodes, as the inventors have found out that such a depolarization of the diode field induces additional intensity noise on the Raman amplified signal.  
      Further advantages and characteristics of the present invention can be gathered from the following description of preferred embodiments with reference to the enclosed drawings. Features mentioned above as well as below can be used in accordance with the invention either individually or in conjunction. The embodiments mentioned are not to be understood as an exhaustive enumeration but rather as examples with regard to the underlying concept of the present invention.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a schematic block diagram of an optical transmission system in accordance with the present invention comprising a Raman amplifier in accordance with another aspect of the present invention with a pump field combiner;  
       FIG. 1   a, b  are schematic block diagrams embodiments of the pump field combiner of  FIG. 1 ,  
       FIG. 2  is a schematic illustration of pumping frequencies used in a Raman pump composed of six pumping light sources in accordance with the present invention;  
       FIG. 3  is a schematic illustration of pumping frequencies used in a Raman pump composed of four pumping light sources in accordance with the present invention;  
       FIG. 4  is an optical power spectrum of a Raman pump field;  
       FIGS. 5   a, b  are examples of profitable frequency shifts between pump fields in accordance with the present invention;  
       FIGS. 6   a, b  are on/off Raman gains produced by pump fields shifted in accordance with the present invention, and  
       FIG. 6   c  is an example of on/off Raman gains produced by pump fields shifted according to prior art.  
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      The following detailed description of the invention refers to the accompanying drawings. The same reference numerals may be used in different drawings to identify the same or similar elements.  
       FIG. 1  is a schematic block diagram of an optical transmission system  1  in accordance with the present invention. The optical transmission system  1  comprises a transmitter unit  2  and a receiver unit  3  which are connected by means of a transmission link  4 , e.g. an optical transmission fiber, for transmission of optical signals OS from the transmitter unit  2  to the receiver unit  3 . In the embodiment shown in  FIG. 1 , a section  4   a  of the optical transmission link fiber  4  is devised as a Raman active optical fiber.  
      The optical transmission system  1  further comprises a Raman amplifier  5  in operative connection with the optical transmission link  4 ,  4   a.  The Raman amplifier  5  comprises an additional optical fiber segment  6  in connection with the optical transmission link fiber  4 ,  4   a  of the optical transmission system  1 . The Raman amplifier  5  further comprises a Raman pump  7  depicted with dashed lines, which is connected with the optical fiber  6 . The Raman pump  7  includes three pairs of polarized pump light sources { 8 . 1   a,    8 . 2   a }, { 8 . 1   b,    8 . 2   b }, { 8 . 1   c,    8 . 2   c } which emit respective polarized pump light at different pumping optical frequencies ν 1a-c , ν 2a-c . In accordance with the present invention, said pump light sources  8 . 1   a - c,    8 . 2   a - c  are preferably devised as semiconductor diodes, e.g. Fiber Bragg Grating (FBG) stabilized diodes or inner-grating modules.  
      The pump light sources  8 . 1   a - c,    8 . 2   a - c  are operatively connected by means of a pump field combiner  9  adapted to couple the generally polarized pump light PL of pump light sources  8 . 1   a - c,    8 . 2   a - c,  i.e. of a plurality of pump fields, into the optical fiber  6 ,  4 ,  4   a  in order to achieve Raman amplification of the optical signals OS propagated on the optical transmission link  4 ,  4   a  between the transmitter unit  2  and the receiver unit  3 . In the following, two possible implementations of the pump field combiner will be described in greater detail with reference to  FIGS. 1   a, b.    
       FIG. 1   a  shows a pump field combiner  9  which comprises three polarization beam combiners (PBCs)  11 . 1  to  11 . 3 , each receiving the pump light from a respective pair of pump light sources { 8 . 1   a,    8 . 2   a }, { 8 . 1   b,    8 . 2   b }, { 8 . 1   c,    8 . 2   c } transmitted through polarization maintaining fibers (represented by thick lines). The pump light of the first pump light source  8 . 1   a - c  of a respective pair has a direction of polarization which is orthogonal to the direction of polarization of the second respective pump light source  8 . 2   a - c  of the same pair. The output light of the PBCs  11 . 1  to  11 . 3  is provided as an input to a wavelength division multiplexer  10  thus forming a common pump signal which is provided to the optical fiber segment  6  shown in  FIG. 1 . It is not necessary to control the direction of polarization of the light from different pairs of pump light sources { 8 . 1   a,    8 . 2   a }, { 8 . 1   b,    8 . 2   b }, { 8 . 1   c,    8 . 2   c } with respect to each other, such that no polarization maintaining fibers are necessary for connecting the PBCs  11 . 1  to  11 . 3  and the WDM  10 .  
      In an alternative implementation of the pump field combiner  9  shown in  FIG. 1   b,  two polarization maintaining wavelength division multiplexers (PM WDM)  10   a,    10   b  are provided. The first PM WDM  10   a  collects the polarized light of the first pump light sources  8 . 1   a - c  of each pair of pump light sources { 8 . 1   a,    8 . 2   a }, { 8 . 1   b,    8 . 2   b }, { 8 . 1   c,    8 . 2   c }, the second PM WDM  10   b  collects the polarized light of the second pump light sources  8 . 2   a - c  of each pair, having a polarization state that is orthogonal to the polarization state of the pump light of the first pump light sources  8 . 1   a - c.  In this case, the polarization directions of the first pump light sources  8 . 1   a - c  and the second pump light sources  8 . 2   a - c,  respectively, are aligned along the same state of polarization. A PBC  11 . 4  of the polarization field combiner  9  is connected to the outputs of the first and second PM WDM  10   a,    10   b  via a polarization maintaining fiber and adapted to combine the mutually orthogonal polarization states to form a common pump signal.  
      Since according to  FIG. 1 , the Raman pump light PL and the optical signals OS propagate in a common direction (as indicated by the arrows in  FIG. 1 ), this amplification technique is referred to as Raman co-pumping.  
      In the embodiment of  FIG. 1 , the Raman amplifier  5  is devised as a distributed Raman amplifier. However, as will be appreciated by a person skilled in the art, in accordance with the present invention the Raman active fiber section could be provided e.g. by a specific fiber of high Raman efficiency or a dispersion compensating fiber, thus forming a Raman amplifier of the discrete type.  
      In order to achieve efficient Raman co-pumping, a Raman pump  7  with low Relative Intensity Noise (RIN) has to be used. As stated before, pump units with an acceptable RIN generally comprise semiconductor diodes. These diodes emit polarized light. In order to avoid polarization dependent gain (PDG) and its negative effects on system performance, a widely used approach for effectively depolarizing the Raman pump has been to use diodes operating at a common wavelength and to couple the respective emissions with a polarization beam combiner, as shown at reference numerals  7 ,  9  in the above-described  FIG. 1 . As already stated above, this approach suffers from the disadvantage that due to the overlap of the orthogonal pump fields intensity noise coming from the product of the orthogonal field is induced on the Raman amplified signal, i.e. the optical signal OS in  FIG. 1 . When avoiding overlap between the orthogonal pump fields, however, a large number of channels in an optical transmission system  1  will effectively receive their respective Raman gain mainly from only one polarized Raman pump light source (e.g., pump light sources  8 . 1   a,    8 . 2   a  in  FIG. 1 ). This will result in an undesirable polarization dependent gain (PDG).  
      In this context, the present invention proposes a judicious choice of the overlap and/or the frequency shift between the combined pump fields of the Raman amplifier  5  in accordance with  FIG. 1 . This will now be explained with reference to appended FIGS.  2  to  6   b.    
       FIG. 2  is a schematic diagram of the optical frequency distribution of pump light sources of a Raman pump  7  ( FIG. 1 ) composed of six diodes (pump light sources). The horizontal axis in  FIG. 2  denotes an optical frequency, ν, whereas the upwards and downwards pointed arrows E 1   a - c,  E 2   a - c  denote the emissions of the six individual pump light sources of  FIG. 1 . Physically speaking, the arrows E 1   a - c,  E 2   a - c  represent a field vector of the individual pump light sources (cf. reference numerals  8 . 1 ,  8 . 2  in  FIG. 1 ). Thus, the corresponding emissions of the individual pump light sources can also be referred to as pump fields E 1   a - c,  E 2   a - c,  i.e. a plurality of six pump fields. As can be seen by the directions of the arrows E 1   a - c,  E 2   a - c  in  FIG. 2 , the pump fields of different pairs are separated into two groups E 1   a - c  and E 2   a - c,  respectively, the pump fields in each group being mutually orthogonal in polarization to the pump fields in the other group when coupled into the optical fiber  6 ,  4 ,  4   a  in  FIG. 1 . The frequency distribution of  FIG. 2  may preferably be produced at the output of the Raman pump  7  using the design of the pump field combiner  9  shown in  FIG. 1   b    
      Alternatively, when using the implementation of the pump field combiner  9  shown in  FIG. 1   a,  the angle between the directions (field vectors) of the pump fields of different pairs of pump fields E 1   a - c,  E 2   a - c  are chosen arbitrarily, as it is sufficient that the pump fields in each individual pair are mutually orthogonal.  
      In addition to their orthogonality, the individual optical frequencies ν 1a , ν b , ν 1c  of the first group of pump fields E 1 - c  are shifted in frequency with respect to optical frequencies μ 2a , ν 2b , ν 2c  in the other (orthogonal) group of pump fields E 2   a - c  by an amount (frequency shift) Δν, as depicted in  FIG. 2 . In this context, the frequency shift Δν is defined as a difference in frequency which separates the emissions of pump light sources from one group E 1   a - c  with respect to the emissions of neighboring (in terms of frequency) pump light sources from the other group E 2   a - c,  thus effectively defining pairs of pump light sources or pump fields, e.g. E 1   a /E 2   a  or E 1   b /E 2   b.  Since the emission of an individual pump light source does generally not form a single narrow emission peak at the pumping wavelength or frequency, respectively, the individual arrows E 1   a - c,  E 2   a - c  in  FIG. 2  actually denote individual central optical frequencies of the individual emission spectra of every pump light source, as will become apparent in the following discussion of  FIG. 4 .  
       FIG. 3  schematically illustrates another exemplary embodiment of the Raman amplifier  5  ( FIG. 1 ) in accordance with the present invention, wherein the Raman pump  7  comprises four individual pump light sources (diodes) at optical frequencies ν 1a , ν 1b , ν 2a , ν 2b , which generate pump fields E 1   a,b;  E 2   a,b  with respective orthogonal polarization when coupled into the optical fiber  6 ,  4 ,  4   a  ( FIG. 1 ). A major difference between the embodiments of  FIG. 2  and  FIG. 3  resides in the fact that in accordance with the embodiment of  FIG. 3  pump field E 1   a  is shifted with respect to orthogonal pump field E 2   a  by an amount Δν a  which differs from the frequency shift between pump field E 1   b  and pump field E 2   b,  which are shifted by an amount Δν b.    
      The inventive choice of the respective frequency shifts Δν, Δν a , Δν b  in  FIGS. 2, 3  will now be explained with reference to  FIGS. 4, 5   a,    5   b.    
       FIG. 4  shows an optical power spectrum (dB scale) of a Raman pump field, e.g. pump field E 1   a  ( FIGS. 2, 3 ), from a single individual Raman pump light source, e.g. pump light source  8 . 1   a  in  FIG. 1 . Depicted is the optical power spectrum P(ν) versus an optical frequency ν measured in Gigahertz (GHz). From the pump field E 1   a  depicted in  FIG. 4 , a frequency bandwidth Δν′ may be selected which corresponds to 80% of the total optical power of the pump light, thus satisfying the following equation:  
                   ∫       v   c     -     Δ   ⁢           ⁢       v   ′     /   2             v   c     +     Δ   ⁢           ⁢       v   ′     /   2           ⁢       P   ⁡     (   v   )       ⁢     ⅆ   v         =     0.8   ·       ∫     -   ∞       +   ∞       ⁢       P   ⁡     (   v   )       ⁢     ⅆ   v             ,           (     Eq   .           ⁢   3     )               
 wherein P(ν) is the optical power spectrum of the pump field and ν C  is the center frequency of the Raman pump field. 
 
      In the simple case that the frequency spectrum of the pump light source has a symmetrical shape, the central frequency (i.e. pumping frequency) corresponds to the frequency of symmetry. However, in the case of a non-symmetrical shape of the frequency spectrum, the value of the central frequency may not be easy to find. In this case, Eq. 3 may be used as a definition for the central frequency: the frequency “around which” there is the most optical power density, thus solving Eq. 3 for the couple of values {Δν′, ν c } simultaneously, as will be described in the following.  
      Numerically, one may use a band-pass filter of width Δν′ and find the best central frequency ν c  for this filter to select as much power as possible. For each width Δν′ of this filter, the best central frequency ν c  is determined. Finally, one chooses the couple {Δν′, ν c } that respects Eq. 3. In this sense, Δν′ is the minimum frequency bandwidth such that there exists a central optical frequency ν c  that satisfies Eq. 3. As a remark, ν c  may not coincide with the frequency of the power peak, and also, it may depend on the choice of coefficient A.  
      As stated above, the central optical frequency ν c  can thus be determined as the center frequency of the frequency bandwidth Δν′ which constitutes the minimum bandwidth which satisfies Eq. 3. With a power spectrum of the pump light source which is almost symmetrical with respect to the pumping frequency as shown in  FIG. 4 , the central frequency ν c  essentially coincides with the pumping frequency, such that both frequencies are considered as equal in the following discussion.  
      In a more general way, Eq. 3 can be written as  
                   ∫       v   C     -     Δ   ⁢           ⁢       v   ′     /   2             v   C     +     Δ   ⁢           ⁢       v   ′     /   2           ⁢       P   ⁡     (   v   )       ⁢     ⅆ   v         =     A   ·       ∫     -   ∞       +   ∞       ⁢       P   ⁡     (   v   )       ⁢     ⅆ   v             ,           (     Eq   .           ⁢   4     )             
 
 wherein A is a positive constant with 0.5&lt;A&lt;0.9. 
 
      As stated before, when two orthogonal pump fields overlap, intensity noise coming from the product of these orthogonal fields is induced on the Raman amplified signal. Therefore, in accordance with an embodiment of the present invention, the spectral overlap of the pump fields is limited in accordance with the following relation: 
 
(ν C2 −Δν 2 ′/2)−(ν C1 +Δν 1 ′/2)&gt;0, 
 
 wherein the definitions of Eq. 1 have been used for two pump fields (e.g., pump fields E 1   a,  E 2   a  in  FIGS. 2, 3 ) with respective central frequencies ν C1  and ν C2  (with ν C2 &gt;ν C1 ) and with respective frequency bandwidths Δν 1 ′ and Δν 2 ′. This is illustrated in the following  FIGS. 5   a, b.  
 
       FIGS. 5   a, b  show the overlap of two pump fields, e.g. pump fields E 1   a,  E 2   a  in  FIGS. 2, 3 , by displaying a relative power measured in decibels (dB) versus frequency shift measured in Gigahertz (GHz). In accordance with  FIG. 5   a,  the pump fields E 1   a,  E 2   a  are shifted by an amount of Δν=0.8 THz with respect to their central optical frequencies ν C1 =ν 1a , ν C2 =ν 2a , respectively, such that there is substantially no overlap between the two pump fields E 1   a,  E 2   a.  This is in accordance with the relation in Eq. 1 above.  
      In the embodiment of  FIG. 5   b  the frequency shift Δν amounts to only Δν=0,42 THz, which is essentially the lower limit for the frequency shift Δν before the two pump fields E 1   a,  E 2   a  excessively overlap in violation of the relation of Eq. 1. In this way, in accordance with the present invention the overlap of orthogonal pump fields is effectively limited, thus reducing intensity noise on the Raman amplified optical signal OS ( FIG. 1 ). It has been experimentally verified that without the inventive feature of limiting pump field overlap a degradation of up to 10 dB can be observed.  
      Furthermore, and in accordance with the general idea of the present invention, a maximum wavelength/frequency shift between pairs of pump fields (E 1   a ,/E 2   a  and E 1   b /E 2   b,  respectively, in  FIG. 3 ) is specified in order to avoid polarization dependent gain (PDG). Therefore, in accordance with the present invention optical frequency shifts Δν i  in pairs of pump fields are restricted to values Δν i ≦1.8 THz, preferably Δν i &lt;0.8 THz. However, in order to avoid excessive pump field overlap, as stated above, a minimum frequency shift in pairs of pump fields is specified according to Δν i &gt;0.4 THz. In this context, the term “pair of pump fields” denotes pump fields, i.e. emissions from two individual pump light sources ( FIG. 1 ), which are sufficiently close in term of their respective optical emission frequencies ν i  such that they will effectively interact with respect to their influence on the amplified optical signal OS ( FIG. 1 ). Referring again to  FIG. 3 , only pump fields E 1   a,  E 2   a  and pump fields E 1   b,  E 2   b,  respectively, are sufficiently close in terms of optical frequency ν i  to be regarded as pairs of pump fields to which the above Eq. 1 and the inventive specification with respect to the maximum wavelength/frequency shift apply.  
       FIGS. 6   a, b  show the effects of the invention with respect to the Raman gain on the optical signals OS propagated from the transmitting unit  2  to the receiver unit  3  in the optical transmission system  1  of  FIG. 1  in a wavelength range between 1560 nm and 1600 nm. In both Figures, the on/off gain in decibel (dB) is depicted versus the optical wavelength measured in nanometers (nm). The upper (dotted) curve in  FIGS. 6   a, b  shows the total gain imparted on the optical signals OS by means of the inventive Raman amplifier  5  ( FIG. 1 ).  
      The six lower curves in  FIG. 6   a  represent the gain produced by six individual pumps (pump fields), corresponding to the gain spectrum produced by pump fields chosen in accordance with  FIG. 2 . The pump light of the individual pumps is emitted at the center wavelengths of: 1446.3 nm, 1451.5 nm, 1457.9 nm, 1463.2 nm, 1486 nm, and 1491.3 nm, respectively. It can be gathered from  FIG. 6   a  that the difference in on/off gain of the respective polarized pump fields over the critical wavelength range from 1560 nm to 1600 nm does not exceed 0.5 dB. Moreover, an overall flat total gain is obtained for this wavelength band.  
      In  FIG. 6   b,  the on/off gain produced by four individual pump fields corresponding to the gain spectrum chosen in accordance with  FIG. 3  is shown. The pump wave-lengths are chosen in this case to be: 1451.5 nm, 1463.2 nm, 1486 nm, and 1491.3 nm for signals in a range between 1560 and 1600 nm. Though the pump fields at 1451.5 nm and 1463.2 nm are separated by 1.8 THz, the gain coming from the polarized pump field at 1463.2 nm alone is only 1.2 dB in the worst case (i.e. on the signal channel at 1575 nm).  
      As can be gathered from  FIG. 6   a, b,  when a Raman pump comprises a plurality of pump light sources, the frequency shifts between the pump fields are usually chosen so as to achieve flat Raman gain over the signal transmission band. For this purpose, and due to the specific shape of the Raman gain curve, the pump fields of lower wavelengths (i.e. of higher optical frequencies) are separated by less than 1.8 THz, as separating them by more than 1.8 THz yields unwanted ripple on the total Raman gain spectrum.  
      Again, due to the specific shape of the Raman gain curve again, the pump field (or the pair of orthogonal pump fields) of highest wavelengths almost provide all the gain to the signals of higher wavelengths of the transmission band. (By contrast, the signal channels of lower wavelengths get their gain from low and high-wavelength pump fields together). Consequently, it is a priority to specify the limit of the maximum frequency shift between the orthogonal pump fields of higher wavelengths, leading to a frequency shift of 0.8 THz between the pump fields of higher wavelengths and a frequency shift of 1.8 THz between the pump fields with lower wavelength, as shown in  FIG. 6   b,  still having a sufficiently flat gain profile and reduced PMD. In general, when using several pump fields, the frequency shift between the pump fields with lower wavelengths can therefore be chosen higher than that of the pump fields with higher wavelengths. In particular, the pair of pump fields having the highest wavelengths may have a frequency shift of 0.8 THz and all the other pairs of pump fields may have a greater frequency shift, preferably at or above 0.8 THz and below 1.8 THz.  
      In this way the proposed solution does not induce polarization dependent gain (PDG) in the co-pumped Raman amplifier. While being non-obvious, the proposed solution is essentially straightforward, as Raman pumps with polarization-combined pump light sources (diodes) already exist as such. On the market, however, a common wavelength is usually specified for the individual pump light sources combined in polarization. In accordance with the present invention, a different approach is proposed in order to devise a Raman amplifier which does neither induce intensity noise nor polarization dependent gain on optical signals to be amplified. Furthermore, the range of possible frequency shifts in accordance with the present invention, i.e., about 0.4 THz to 0.8 THz for FBG diodes, is wide enough to relax commercial constraints. In general, a lower limit of the shift of the center frequencies of the respective pump fields of each pair may be defined by 0.4 THz, thus sufficiently reducing the overlap between the pump fields.  
      It should be noted that although the proposed solution is described above with respect to a co-propagating geometry (cf.  FIG. 1 ), it is equally applicable to counter-propagating geometries.