Abstract:
A method and apparatus for providing broad band Raman amplification with reduced dependence on polarization state and wavelength. Pump beams of adjacent wavelength are perpendicularly polarized to each other. Further, the wavelength spacing between adjacent pump beams is chosen for the given fiber wavelength spectrum and distance between repeater stations such that adjacent pump beams experience the same polarization state evolution as they travel through the fiber.

Description:
FIELD OF THE INVENTION  
         [0001]    The invention pertains to Raman amplification. More particularly, the invention pertains to broad band fiber optic communication systems utilizing Raman amplification.  
         BACKGROUND OF THE INVENTION  
         [0002]    In transoceanic fiber optic communication systems, a signal may be transported via optical fiber several thousand kilometers between the transmitter and the receiver. Over such large distances, a light signal transported via fiber optic cable suffers significant degradation. Accordingly, repeater stations comprising amplifiers are placed intermittently in the optical fiber path to amplify the signal in order to compensate for the propagation degradation. Traditionally, in transoceanic transmission system, these repeater stations might be positioned approximately 50 kilometers apart and comprise electronic amplifiers. Accordingly, the light signal needed to be converted from light to electrical, then amplified, and then converted back to light for propagation via the fiber optic cable to the next repeater station or receiver.  
           [0003]    More recently, erbium-doped fiber amplification (EDFA) has been used to replace electronic amplifiers. EDFA is a light amplification system that does not require any conversion of the light signals to electrical signals.  
           [0004]    Even more recently, Raman amplification systems have been developed for amplifying signals in long-distance fiber optic transmission lines. Raman amplification is a well-known technique for amplifying signals in fiber optic transmission systems. Raman amplification is considered promising for trans-oceanic fiber optic communication systems such as are in use today for international telephonic and other communication applications.  
           [0005]    Raman amplification is a well-known phenomenon which will not be described in detail herein. However, in very general terms, one or more pump beams are introduced into a fiber carrying a signal beam. The pump beam(s) should be at a certain wavelength separation from the signal beam. Raman effect is a scattering of the incident photon of the pump beam by a molecule to a lower frequency photon. At the same time, the molecule makes a transition between vibrational states. The incident light acts as a pump to generate the lower frequency light of the signal beam, which is called Stokes wave.  
           [0006]    Unlike the aforementioned prior art amplification systems, Raman amplification is a distributed amplification system. That is, the amplification occurs throughout the entire length of the fiber and there is no particular discrete section in the data path to which amplification is limited.  
           [0007]    One of the many advantages of Raman amplification is that it can be used in connection with many types of inexpensive, transparent media because it is based on the molecular structure of non crystalline fibers, rather than any particular doping scheme. For instance, Raman amplification can be performed in silica fibers which are relatively low cost. Another advantage of Raman amplification is that Raman amplification can be achieved over a very broad bandwidth, stretching to almost 300 nanometers.  
           [0008]    In one common type of scheme, the pump beam(s) are launched to travel in the fiber in the opposite direction to the signal beam(s), i.e., the pump beam(s) are counter-propagating to the signal beam(s). In very long distance fiber optic cable lines, such as the aforementioned transoceanic application, pump beams may be introduced into the fiber at multiple locations between the transmitter and the receiver. However, as previously noted, while the laser sources for the pump beams and the apparatus for coupling the pump beams into the transmission fiber are discrete components, the amplification occurs throughout the entire length of the fiber.  
           [0009]    The Raman amplification provided by any given pump beam is not uniform over a range of signal beam wavelengths. FIG. 1 is a graph showing the Raman gain curve for an exemplary pump beam. FIG. 1 shows the gain curve over a frequency span of about 300 nm. By the use of multiple, wavelength-separated, equalized, pump beams, an overall gain curve that is relatively flat over a broad band of signal wavelengths can be provided. For example, FIG. 2A illustrates the gain curves for four closely spaced pump beams centered at about 1425 nm, 1439 nm, 1453 nm and 1467 nm, respectively. With equalization, these four pump beams such as illustrated in FIG. 2A combine to produce an overall amplification characteristic that is relatively flat over a broad range of wavelengths, such as illustrated in FIG. 2B.  
           [0010]    The Raman amplification effect is polarization sensitive. Particularly, when a pump beam and a signal beam are parallelly polarized relative to each other, maximum Raman gain is obtained. On the other hand, when a pump beam and a signal beam are perpendicularly polarized relative to each other, Raman gain is close to zero. Accordingly, it is desirable when using Raman amplification to keep the pump and signal beams parallelly polarized to each other in order to achieve maximum gain. Another possibility is simply to use unpolarized pump and signal beams. This solution decreases gain from the maximum possible achievable gain, but causes all signal beams to experience approximately the same amplification.  
           [0011]    Semiconductor lasers are well known in the art. Generally, they are less expensive, smaller, and more reliable than standard laser light sources. Accordingly, it is generally desirable to use semiconductor lasers in most applications, including as pump and signal beams for Raman amplification systems. However, semiconductor lasers produce polarized light.  
           [0012]    Further, while polarization maintaining optical fibers are known, non-polarization maintaining fibers are much more common and are substantially lower in cost. It is believe that all existing transoceanic fiber optic cables are non-polarization maintaining.  
           [0013]    Accordingly, it is an object of the present invention to provide an improved Raman amplification method and apparatus.  
           [0014]    It is another object of the present invention to provide a Raman amplification method and apparatus in which polarization dependent gain is minimized.  
           [0015]    It is a further object of the present invention to provide a Raman amplification system in which signal beams of disparate wavelengths experience the same amplification.  
         SUMMARY OF THE INVENTION  
         [0016]    In accordance with the present invention, a Raman amplification method and apparatus is provided in which a plurality of wavelength division multiplexed signal beams are carried on an optical fiber. A plurality of pump beams are launched into the fiber in a counter propagating direction relative to the signal beams. The pump beams are spaced in wavelength relative to each other such that each pump beam has an adjacent pump beam(s) (i.e., the next lowest frequency and/or next highest frequency pump beam) that is within a wavelength range of it such that the two adjacent beams experience the same polarization evolution as they traverse the fiber. Further, pump beams of adjacent wavelengths are perpendicularly polarized relative to each other.  
           [0017]    In this manner, the gain experienced by all signal beams over a broad range of wavelengths is essentially the same. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    [0018]FIG. 1 is a graphical diagram illustrating a Raman amplification curve.  
         [0019]    [0019]FIG. 2A is a graphical diagram illustrating a plurality of Raman amplification curves produced by each of a plurality of pump beams wavelength displaced from each other.  
         [0020]    [0020]FIG. 2B is a graphical diagram illustrating the overall Raman amplification curve produced as a result of the plurality of pump beams illustrated in FIG. 2A.  
         [0021]    [0021]FIG. 3 is a pictorial diagram illustrating signal and pump beams in an optical fiber in accordance with the present invention.  
         [0022]    [0022]FIG. 4 is a block diagram illustrating an apparatus for producing a pump beam in accordance with a first embodiment of the present invention.  
         [0023]    [0023]FIG. 5 is a block illustrating an apparatus for producing a pump beam in accordance with a second embodiment of the present invention.  
         [0024]    [0024]FIG. 6 is a block diagram illustrating an apparatus for producing a pump beam in accordance with a third embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0025]    It is always desirable to maximize the amount of information that can be carried over any type of medium. However, it is particularly desirable with respect to transoceanic optical fiber cables due to the extremely high cost of laying such cables and associated equipment under the ocean. Accordingly, Wavelength Division Multiplexing (WDM) and particularly Dense Wavelength Division Multiplexing (WDM) fiber optic transmission systems have been developed for transoceanic and other long-distance transmission systems. In WDM and DWDM, multiple signals carried within different wavelength light are transmitted simultaneously on a single optical fiber. Accordingly, Raman amplification is particularly desirable for WDM and DWDM fiber optic systems because it can provide amplification over such a broad range of wavelengths.  
         [0026]    In non-polarization maintaining optical fibers, the polarization state of light traversing the fiber evolves over the distance traveled. However, polarized light remains polarized as it propagates through the fiber. The evolution of the polarization state of a light beam as it travels through a non-polarization maintaining fiber is dependent on a complex relationship between a very large number of factors and, for purposes of this discussion, can generally be considered unpredictable. One of the factors upon which polarization state evolution is dependent is the wavelength of the light. Specifically, while the polarization evolution of a light beam of a particular wavelength in a non-polarization maintaining fiber is essentially unpredictable, it is known that two light beams having the same wavelength and that simultaneously travel through a given optical fiber segment will experience the same polarization evolution. In fact, two beams that are very close to each other, but not identical, in wavelength will experience the same polarization evolution in a fiber. E.g., S. G. Evangelidis Jr., L. F. Mollenauer, J. P. Gordon, and N. S. Bergano, “ Polarizatlion Multiplexing With Solitons ”, Journal of Lightwave Technology, Volume 10, Number 1, January, 1992.  
         [0027]    The equation below can be used to determine the maximum wavelength separation of two beams in order for them to experience essentially the same polarization evolution through an optical fiber. 
         
       {overscore (δf)}=( 
       2{square root}, {square root over (&lt;δ T   2 &gt;)}) 
       −1 
     
         [0028]    where  
         [0029]    {overscore (δf)}=the average frequency range within which two beams will experience the same polarization evolution=the average period with change of frequency; δT is the delay difference between two polarization states; and 
         {square root}{square root over ((δ T   2 ))}=(0.2 ps /{square root}{square root over (Km))} {square root}{square root over (Z)},   
         [0030]    where Z=the effective length of the fiber over which there is interaction between the pump and signal beams.  
         [0031]    As an example, if we assume light at about 1500 nm and a fiber loss factor, α p , of about 0.22 dB/Km, then the effective interaction length, Z, would be about 18 Km. Then {overscore (δf)} would be approximately 2.35 THz. Still assuming a beam having a wavelength of about 1500 nm, this yields a wavelength range of 18 nanometers. Thus, with the conditions set forth in this example, two beams that have wavelengths separated by less than 18 nm will experience essentially the same polarization evolution in the fiber segment.  
         [0032]    In WDM and DWDM transmission systems, it is, of course, desirable for all signal beams to experience the same amount of amplification. However, this goal can be very difficult to achieve because Raman amplification is polarization dependent and the polarization evolution of a beam in an optical fiber is wavelength dependent. Accordingly, signal beams at disparate wavelengths can experience disparate Raman amplification.  
         [0033]    Also, for any given pump beam/signal beam pair, the Raman gain experienced by the signal beam will be relatively consistent over time due to an averaging effect. For example, both the pump beam and the signal beam will experience essentially unpredictable polarization evolution through the fiber. Thus, at any given point in the fiber, the relative polarization of the pump beam and signal beam is different from other points in the fiber. For instance, at some points, the polarization states of the pump beam and the signal beam may be parallel to each other and at other points they may be perpendicular to each other (and every other possible relationship). However, over the total length of the fiber, the points at which a pump and signal beams were perpendicular to each other will average out with those points where they are parallel to each other as well as all other points to produce relatively consistent gain over time.  
         [0034]    When there are multiple simultaneous pump beams and multiple simultaneous signal beams spread out over a broad spectrum of wavelengths, different wavelength pump beams will experience different polarization state evolutions. Accordingly, the whole spectrum of signal beams will be unstable in that different wavelength signal beams will experience different gains over the same fiber segment.  
         [0035]    The present invention minimizes polarization dependent gain and thus wavelength dependent gain in a Raman amplification system. FIG. 3 is a pictorial diagram conceptually illustrating the present invention. An optical fiber segment  12  of length Z is shown. A multiplicity of wavelength division multiplexed signal beams  14 - 1 ,  14 - 2 ,  14 - 3 , . . . ,  14 -n are introduced into a first end  12   a  to propagate in a first direction through the fiber segment  12 . A plurality of counter propagating pump beams  16 - 1 ,  16 - 2 ,  16 - 3 , . . . ,  16   n  are introduced into the fiber section at the opposite end  12   b . The pump beams  16 - 1 ,  16 - 2 ,  16 - 3  . . . ,  16 -n are arranged in order of decreasing wavelength (i.e., increasing frequency). In accordance with the invention, the adjacent pump beams have wavelengths that are within a range of each other, as calculated in accordance with equations (1) and (2) above, such that they experience the same polarization evolution as they travel through the fiber. In a preferred embodiment, the wavelength spread between adjacent wavelength pump beams is no greater than 65% of the value calculated by equations (1) and (2). In a more preferred embodiment, the wavelength spacing may be about 50% of that given by equations 1 and 2.  
         [0036]    The signal beams,  14 - 1  through  14 -n, experience Raman amplification in the fiber section  12  due to the presence of the pump beams,  16 - 1  through  16 -n. The amplification is over a broad range of wavelengths and is relatively flat over that entire range of wavelengths due to the multiplicity of wavelength-spaced, counter propagating, pump beams. All of the pump beams,  16 - 1  through  16 -n, are polarized. The odd numbered pump beams,  16 - 1 ,  16 - 3 ,  16 - 5 , . . . ,  16 -(n-1), are vertically polarized. The even numbered pump beams,  16 - 2 ,  16 - 4 ,  16 - 6 , . . . ,  16 -n, are horizontally polarized. Accordingly, every pump beam is polarized perpendicularly to its adjacent pump beam(s). For example, pump beam  16 - 3  is perpendicularly polarized to the adjacent lower frequency pump beam  16 - 2  and is also perpendicularly polarized to the adjacent next higher frequency pump beam  16 - 4 .  
         [0037]    With these two conditions, namely, (1) that adjacent wavelength pump beams are perpendicularly polarized relative to each other and (2) adjacent wavelength pump beams are within the above-noted range of each other, dependence of Raman amplification gain on wavelength and polarization is minimized. Specifically, each signal beam will primarily experience gain due to the two pump beams that are closest to it in wavelength. Since these two pump beams are perpendicularly polarized to each other, the signal beam will experience the same gain throughout the length of the fiber segment. That is, at any given point in the fiber segment, the signal beam may be closer to parallel to one of the two pump beams (so as to experience more gain from that pump beam) and, thus closer to perpendicular to the other pump beam (so as to experience less gain from that pump beam). At other points in the fiber segment, the polarization of the signal beam will have different relationships to these two pump beams. However, since these two pump beams experience the same polarization evolution in the fiber, they are always perpendicular to each other. Thus, the sum of the relative polarization displacement of the signal beam from each of these two pump beams will always be the same. Accordingly, the cumulative gain experienced by the signal due to both pump beams is the same over the entire fiber span. The above-described effect apples to all signal beams relative to all pump beams.  
         [0038]    Those of skill in the art of fiber optic communication systems will realize that there are many ways to introduce a plurality of perpendicularly polarized, wavelength-spaced, pump beams into a fiber. Three such ways are disclosed in FIGS. 4, 5 and  6 .  
         [0039]    [0039]FIG. 4, for instance, illustrates a convenient manner of coupling N individual pump beams into N transmission fibers, where N is a integer. Laser light sources  40 ,  41 ,  42  and  43  are, for instance, semiconductor laser diodes generating polarized light. Laser diode  41  generates light which is vertically polarized and at a particular wavelength. Laser diode  42  generates light that is horizontally polarized and at a next longer wavelength within the range discussed above. Laser diode  43  generates light that is vertically polarized and at a next longer wavelength than the wavelength of laser diode  42 . Laser diode  44  generates light that is horizontally polarized and at a next longer wavelength than that of laser diode  43 . The output beams  45  and  46  of diodes  41  and  42 , respectively, are input to a 3 dB polarization maintaining coupler  47 . Likewise, output beams  48  and  49  of laser diodes  43  and  44 , respectively, are input to a second 3 dB polarization maintaining coupler  50 . In accordance with the well known operation of 3 dB polarization maintaining couplers, each coupler  49  and  50  has two outputs each of which comprises half the power (minus losses) of each of the input beams. Accordingly, output beam  51  combines half the power of beam  45  of diode  41  and half the power of beam  46  of diode  42 . The same is true for output  52 . Likewise, output  53  comprises half the power of beam  48  of diode  43  and half the power of beam  49  of diode  44 . The same is true for output  54 . The particular wavelengths that might be used would depend on the amplifier design, the particular transmission fiber, saturation, input power and several other factors that are well known to those of skill in the art of Raman amplifier design.  
         [0040]    Output  51  and  54  are combined again in another 3 DB polarization maintaining coupler  55  while outputs  52  and  53  are combined by an even further 3 DB polarization maintaining coupler  56 . Accordingly, four output beams  57 ,  58 ,  59  and  60  are produced each of which contains ¼ of the power of each of the four pump beams  45 ,  46 ,  48  and  49 . Each output signal  57 ,  58 ,  59  and  56  can be coupled to a different transmission fiber (Raman amplifier).  
         [0041]    As another example, when the number of pump wavelengths is greater than the number of transmission fibers, a system such as shown in FIG. 5 can be employed. The particular system shown in FIG. 5 produces N/2 output beams for coupling to N/2 transmission fibers, where N is the number of pump beam sources. The four pump beam sources  41 ,  42 ,  43 ,  44  are the same as described above in connection with FIG. 4. The output of laser diodes  41  and  42  are combined in a polarization beam coupler  65 . The output of polarization beam coupler  65  has the total combined power of pump beams  45  and  46  (minus losses) and thus comprises light of two adjacent wavelengths. Beams  48  and  49  from diodes  43  and  44  are combined in the same manner by a second polarization beam coupler  66 . The output of polarization beam coupler  66  comprises the combined power and wavelengths of pump beams  48  and  49  (minus losses). Outputs  67  and  68  of polarization maintaining beam couplers  65  and  66  are combined in a 3 dB polarization maintaining coupler  69 . Accordingly, outputs  70  and  71  of the coupler  68  each comprise half the total power of its two input beams  67  and  68 . Thus, each output  70 ,  71  includes ½ of the total power of each of the input beams  45 ,  46 ,  48  and  49 . Accordingly, each output beam  70  and  71  includes all four of the wavelengths of the four laser diodes  41 ,  42 ,  43  and  44 . Outputs  70  and  71  can be coupled to two transmission fibers (Raman amplifiers).  
         [0042]    [0042]FIG. 6 shows an embodiment for producing one output from N input pump beams. In this embodiment, all four pump beams  45 ,  46 ,  48  and  49  are inputs to a polarization maintaining wavelength multiplexer  80  which produces a single output  81  combining all four beams. High density, reliable, reasonable cost, polarization maintaining, wavelength multiplexers are presently in the developmental stage. However, it is expected that they will be available in the market within a short time.  
         [0043]    Having thus described a few particular embodiments of the invention, various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications and improvements as are made obvious by this disclosure are intended to be part of this description though not expressly stated herein, and are intended to be within the spirit and scope of the invention. Accordingly, the foregoing description is by way of example only, and not limiting. The invention is limited only as defined in the following claims and equivalents thereto.