Abstract:
A fiber amplifier for amplifying an optical signal, comprising a pump energy source capable of emitting energy at a pump wavelength: optical apparatus for transmitting the optical signal, the optical apparatus having a plurality of discrete portions, each discrete portion comprising a length of optical fiber and first and second components disposed at first and second respective locations and configured to substantially prevent energy having an intermediate wavelength in the discrete portion from entering other discrete portions of the optical apparatus; and a plurality of waveguides, each waveguide coupled to the pump energy source and to one of the plurality of discrete portions, each waveguide for providing energy at the pump wavelength from the pump energy source to its corresponding discrete portion, thereby increasing an intensity of light at the discrete portion&#39;s intermediate wavelength.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation-in-part of International Application No. PCT/US02/24409, which has an international filing date of Aug. 1, 2002, and is entitled “Optical Fiber Amplifier”, and which in turn claims priority to U.S. Provisional Patent Application Ser. No. 60/310,195, which was filed Aug. 3, 2001 and is entitled “Multi-Pump Discrete Raman Amplifier”. The foregoing applications are incorporated herein by reference. 

   TECHNICAL FIELD 
   This invention relates to optical fibers (e.g., fiber lasers and fiber amplifiers), and systems containing such optical fibers. More particularly, the invention is directed toward fiber-based discrete optical amplifiers used in telecommunications, cable television and other fiber-optics applications. 
   BACKGROUND 
   In response to rising demand for information processing services, communications service providers have implemented optical communication systems, which have the capability to provide substantially larger information transmission capacities than traditional electrical communication systems. Information can be transported through optical systems in audio, video, data, or other signal format analogous to electrical systems. Likewise, optical systems can be used in telephone, cable television, LAN, WAN, and MAN systems, as well as other communication systems. 
   The development of the erbium doped fiber optical amplifier (EDFA) provided a cost effective means to optically amplify attenuated optical signal wavelengths in the 1550 nm range. EDFAs have been widely used in communication systems because their bandwidth coincides with the lowest loss window in optical fibers commonly employed in optical communication around 1550 nm. For wavelengths shorter than about 1525 nm, however, erbium atoms in typical glasses will absorb more than amplify. To broaden the gain spectra of EDFAs, various dopants have been added. For example, codoping of the silica core with aluminum or phosphorus can broaden the emission spectrum. Nevertheless, the absorption wavelength for various glasses is still around 1530 nm. 
   Raman fiber amplifiers offer an alternative to EDFAs. 
   Certain optical fibers can be used as fiber amplifiers or fiber lasers. 
   Fiber amplifiers are typically used to amplify an input signal. Often, the input signal and a pump signal are combined and passed through the fiber amplifier to amplify the signal at an input wavelength. The amplified signal at the input wavelength can then be isolated from the signal at undesired wavelengths. 
   Raman fiber lasers can be used, for example, as energy sources. In general, Raman fiber lasers include a pump source coupled to a fiber, such as an optical fiber, having a gain medium with a Raman active material. Energy emitted from the pump source at a certain wavelength λ p , commonly referred to as the pump energy, is coupled into the fiber. As the pump, energy interacts with the Raman active material in the gain medium of the fiber, one or more Raman Stokes transitions can occur within the fiber, resulting in the formation of energy within the fiber at wavelengths corresponding to the Raman Stokes shifts that occur (e.g., λ 1 , λ 2 , λ 3 , λ 4 , etc.). 
   Generally, the Raman active material in the gain medium of a Raman fiber laser may have a broad Raman gain spectrum. Usually, conversion efficiency varies for different frequencies within the Raman gain spectrum and many Raman active materials exhibit a peak in their gain spectrum, corresponding to the frequency with highest conversion efficiency. Additionally, the gain spectrum for different Raman active materials may be substantially different, partially overlapping, or of different conversion efficiency. 
   Typically, a Raman fiber Laser is designed so that the energy formed at one or more Raman Stokes shifts is substantially confined within the fiber. This can enhance the formation of energy within the fiber at one or more higher order Raman Stokes shifts. Often, the fiber is also designed so that at least a portion of the energy at wavelengths corresponding to predetermined, higher order Raman Stokes shifts (e.g., λ sx  where x is equal to or greater than one) is allowed to exit the fiber. 
   Raman fiber amplifiers can be adapted to amplify a broad range of wavelengths. 
   SUMMARY 
   In general, the invention relates to optical fibers (e.g., fiber lasers and fiber amplifiers), and systems containing such optical fibers. 
   In one aspect, the invention features a fiber amplifier for amplifying an optical signal having a signal wavelength. The fiber amplifier includes an optical fiber for transmitting the optical signal, a pump energy source and a plurality of waveguides. The optical fiber has a plurality of discrete portions. Each discrete portion includes first and second components disposed at first and second respective locations and configured to substantially prevent energy having an intermediate wavelength in the discrete portion from entering other discrete portions of the optical fiber. The pump energy source is capable of emitting energy at a pump wavelength. Each waveguide is coupled to the pump energy source and to one of the plurality of discrete portions of the optical fiber. Each waveguide is configured to direct energy at the pump wavelength from the pump energy source to its corresponding discrete portion, thereby increasing an intensity of light at the discrete portion&#39;s intermediate wavelength in the corresponding discrete portion of the optical fiber. In embodiments, the fiber amplifier can be included in a system that also includes a signal source configured to direct the optical signal into the optical fiber, and a signal receiver configured to detect an output optical signal in the optical fiber. The output signal can be, for example, an optical signal that has been amplified by the fiber amplifier. 
   In another aspect, the invention features a fiber amplifier for amplifying an optical signal having a signal wavelength. The fiber amplifier includes an optical fiber having a plurality of discrete portions. Each discrete portion includes first and second components positioned at first and second respective locations in the discrete portion and configured to substantially prevent light having an intermediate wavelength in the portion from entering other portions of the optical fiber. The fiber amplifier also includes a coupler configured to couple pump energy from a pump energy source into the discrete portion so that the pump energy interacts with the optical fiber to increase the intensity of the intermediate wavelength in each portion. 
   In a further aspect, the invention features a fiber amplifier that includes an optical fiber having first and second sections coupled to each other. The first section is a double clad fiber laser, and the second section is an optical amplifier having a gain medium including P 2 O 5 . In embodiments, the fiber amplifier can be in a system that includes an input waveguide coupled to the second section of the fiber amplifier, and an output waveguide connected to the first coupler. 
   In certain embodiments, the fibers can be used as amplifiers rather than lasers. 
   Features, objects and advantages of the invention are in the description, drawings and claims. 

   
     DESCRIPTION OF DRAWINGS 
       FIG. 1  is a schematic representation of an embodiment of a fiber amplifier system; 
       FIG. 2  is a schematic representation of another embodiment of a fiber amplifier system; 
       FIG. 3  is a schematic representation of another embodiment of a fiber amplifier system; 
       FIG. 4  is a schematic representation of another embodiment of a fiber amplifier system; 
       FIG. 5  is a schematic representation of another embodiment of a fiber amplifier system; and 
       FIG. 6  is a schematic representation of a system including a fiber amplifier. 
   

   Like reference symbols in the various drawings indicate like elements. 
   DETAILED DESCRIPTION 
     FIG. 1  illustrates a single cascade discrete Raman amplifier  100  (ignoring I sig   in , I sig   out ) in accordance with the present invention. Raman amplifier  100  is formed by mirrors R 1 , and R 1 ′, centered at the Stokes wave (λ s ) and is pumped by energy at pump wavelength λ p . Without wishing to be bound by theory, it is believed that in general, the performance of Raman amplifier  100  can be described, at least in part, by the following system of nonlinear differential equations: 
   
     
       
         
           
             
               
                 
                   
                     
                       
                         
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   The indices + and − represent propagation in the fiber from left to right and from right to left, respectively. I p  and I s  represent the intensities of energy propagating the fiber at wavelengths λ p  and λ s , respectively. The Raman gain coefficient is g, and α p  and α s  are the loss coefficients of energy propagating in the fiber at wavelengths λ p  and λ s , respectively. 
   These equations can be solved analytically and the following formula obtained: 
                   (       I   s   +     +     I   s   -       )     =         λ   p       λ   s       [         I   p           α   s     ⁢   L     -       1   2     ⁢     ln   ⁡     (       R   1     ⁢     R   1   ′       )             -       α   p     g       ]             (   2   )               
Here, I p  is the power of the injected pump, and L is the length of the fiber. R 1  and R 1 ′ represent the reflectivities of the reflectors (e.g., fiber Bragg gratings) in  FIG. 1 . This formula can give us the magnitude of the total intensity of the Stokes wave in the cavity. Equation (2) is the basic equation that gives the total Stokes power at A, and contains all cavity parameters as well as pump power. As an example for amplification of a signal at 1550 nm, the wavelengths are 1366 nm and 1452 nm.
 
   We now consider that there is a signal wave introduced in the cavity (see  FIG. 1 ) with power I sig   in  and the wavelengths shifted versus λ s  by the Raman Stokes shift. During its propagation through the cavity, the signal wave will be amplified through the mechanism of stimulated Raman scattering, which can be described by the following expression: 
                             ⅆ       I   sig     ⁡     (   y   )           ⅆ   y       =       (         g   ~     ⁡     (       I   s   +     +     I   s   -       )       -     α   sig       )     ⁢       I   sig     ⁡     (   y   )           ,           y   =     -   z                   (   3   )               
Here, {tilde over (g)} is the Raman gain coefficient, and the system of coordinates is reversed (y=−z) for simplicity of calculation. The signal wave is considered weak enough not to deplete the Stokes wave. Equation (3) then has the following solution for the output signal:
 
                   I   sig   out     =       I   sig             ⁢     i   ⁢           ⁢   n         ⁢           ⁢     ⅇ       (         g   ~     ⁡     (       I   s   +     +     I   s   -       )       -     α   sig       )     ⁢   L                 (   4   )               
which gives us amplification in dB as follows:
 
                 K   =       ⁢       10   ⁢           ⁢     log   ⁡     (       I   sig   out       I   sig             ⁢     i   ⁢           ⁢   n           )         =       10   ⁢           ⁢     log   ⁡     (   ⅇ   )       ⁢     (         g   ~     ⁡     (       I   s   +     +     I   s   -       )       -     α   sig       )     ⁢   L     =     4.3   ⁢     (         g   ~     ⁡     (       I   s   +     +     I   s   -       )       -     α   sig       )     ⁢   L                 (   5   )               
We can then substitute Equation (2) in Equation (5) and obtain:
 
                 K   =     4.3   ⁢     (         g   ~     ⁢       λ   p       λ   s       ⁢     (         I   p           α   s     ⁢   L     -       1   2     ⁢     ln   ⁡     (       R   1     ⁢     R   1   ′       )             -       α   p     g       )       -     α   sig       )     ⁢   L             (   6   )               
If we consider a completely closed cavity (i.e. R 1 =R 1 ′=1), then
 
   
     
       
         
           
             
               
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   We can then roughly evaluate the pump power level required to achieve, for example, 10 dB gain in a 100 m cavity. The following values will be used:
         λ p =1345 nm   λ s =1430 nm   g=0.006 1/m/W (highly GeO 2  doped fiber)   {tilde over (g)}=0.005 1/m/W   α p =0.00032 1/m   α s =0.00026 1/m   α sig =0.00025 1/m   L=100 m   K=10 dB       

           10   =       4.3   ⁢           ⁢     1345   1430     ⁢     {         0.005   ·     I   p       0.00026     -       (       0.00032   ⁢           ⁢     0.005   0.006       +     0.00025   ⁢           ⁢     1430   1345         )     ⁢   100       }       =     4   ⁢     (       19.2   ·     I   p       -   0.053     )               
Finally,
 
   
     
       
         
           
             
               
                 
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   Thus, 133 mW power at 1345 nm pump will provide amplification of 10 dB for a signal wave at about 1526 nm wavelength in a 100 m long cavity. 
   In a closed cavity with high finesse, the intensity of the Stokes wave builds up to a very high magnitude, which allows one to obtain very efficient amplification of a signal wave. 
   The in-cavity intensity of the Stokes wave for the same parameters (see, e.g., Equation (2)) is: 
   
     
       
         
           
             
               
                 
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   In a cavity having the parameters defined in (8) and pumped by I p =133 mW, the intensity of the Stokes wave is:
 
( I   s   +   +I   s   − )=4.8 W  (11)
 
   This result allows for the use of a single low power laser diode to obtain a high gain amplifier as shown in  FIG. 1 . 
   The current invention provides a highly efficient Raman amplifier suitable for a variety of applications. This invention further allows for a very simple, truly multiple wavelength, Raman amplifier because in this design one can isolate pieces of fiber for generation of individual Stokes waves λ si , where i=1, 2, 3, . . . , using closed cavities, and generate a large number of these wavelengths using a relatively low pump power (1-2 W) at 13xx nm by sharing it between cavities. In this case, the intensities of individual Stokes waves can be easily and independently controlled by a power splitter. One example of such an amplifier  200  is shown in  FIG. 2 . 
   As shown in  FIG. 2 , instead of keeping all wavelengths (λ si ) together in the same lengths of fiber, they have been isolated from each other, thus reducing effects associated with their interaction. Further, the use of closed cavities allows the intensities of these waves to be kept constant along the lengths of the cavities. A further feature of the embodiment shown in  FIG. 2  is that it works well with short cavities. For example, Equation (7) shows that there is no L dependence scaled with I p , while losses decrease with the shortening of L. 
     FIG. 3  shows another embodiment of an amplifier  300  in accordance with the present invention. The embodiment shown in  FIG. 3  includes couplers  320 ,  322  and  324  (e.g., WDM couplers, circulators, etc.) that form ring cavities for generation of λ si  (i=1, . . . , n) Stokes waves in the 14yy nm wavelength domain. All reflectors R p  are highly reflective at the wavelength(s) of the master pump source (13xx nm). WDM couplers and/or circulators placed in the length of principle fiber that guides the amplified signal are selected so that they are completely “transparent” for an amplified WDM signal, but able to keep waves λ i  in the ring cavities. Amplification happens along the fiber lengths L 1 , L 2 , . . . , L n . The counter-propagation configuration of the presented amplifier reduces noise transfer from the pump to the amplified signal. 
     FIG. 4  shows a further embodiment of an amplifier  400  in accordance with the present invention. Amplifier  400  includes fiber laser  410  pumped by pump source  10 . Pump source  10  can be one or more multi mode laser diodes. Fiber laser  410  is preferably doped with Yb. The output of fiber laser  410  is preferably at approximately 1116 nm. The output of fiber laser  410  is used to pump a length of optical fiber  405 . Optical fiber  405  is preferably about 1-2 km in length and doped with phosphorous (P 2 O 5 ). Optical fiber  405  has two couplers  420  and  422 , to provide separation of the pump and signal waves. Couplers  420  and  422  are preferably WDM couplers at 1116 and 1310 nm respectively. As shown in  FIG. 4 , signal  415  enters amplifier  400  from the right side, while pump wave  425  enters amplifier  400  from the left side, resulting in a counter-propagating amplification scheme. Other propagation schemes may be used (e.g. co-propagating amplification, etc.). 
   In the embodiment shown in  FIG. 4 , amplification occurs in optical fiber  405  according to the principle of stimulated Raman amplification. If optical fiber  405  is doped with P 2 O 5  rather than GeO 2 , a larger Stokes shift can be obtained (e.g., approximately 1330 cm −1  (as compared with 420-440 cm −1 ). This large Stokes shift allows for the use of the output from fiber laser  410  to directly pump optical fiber  405  to produce a simple, low cost optical amplifier at 1310 nm. 
     FIG. 5  shows a further embodiment of an amplifier  500  in accordance with the present invention. Amplifier  500  includes fiber laser  510  pumped by pump source  10 . Pump source  10  can be one or more multimode laser diodes. Fiber laser  510  is preferably doped with Yb. The output of fiber laser  510  is preferably downshifted to approximately 1286 nm. This output can be obtained through wavelength conversion (e.g., by using a multistage GeO 2 /SiO 2  based Raman laser (shifter), single stage P 2 O 5  based Raman laser (shifter), etc.). Shifters are described, for example, in commonly owned U.S. Provisional Patent Application Ser. 60/302,603, filed on Jul. 2, 2001, and entitled “Multi-Wavelength Optical Fiber”, which is hereby incorporated by reference. 
   Referring again to  FIG. 5 , the output of shifter  530  is used to pump a length of optical fiber  505 . Optical fiber  505  is preferably about 1-2 km in length and doped with phosphorous (P 2 O 5 ). Optical fiber  505  has two couplers  520  and  522 , to provide separation of the pump and signal waves. Couplers  520  and  522  are preferably WDM couplers at 1286 and 1550 nm respectively. As shown in  FIG. 5 , signal  515  enters amplifier  500  from the right side, while pump wave  525  enters amplifier  500  from the left side, resulting in a counter-propagating amplification scheme. Other propagation schemes may be used (e.g. co-propagating amplification, etc.). 
   In the embodiment shown in  FIG. 5 , amplification occurs in optical fiber  505  according to the well-known principle of stimulated Raman amplification. If optical fiber  505  is doped with P 2 O 5  rather than GeO 2 , a larger Stokes shift can be obtained (e.g., approximately 1330 cm −1  as compared with 420-440 cm −1 ). 
   While the foregoing description has been made for a system in which the reflectance of a reflector is fixed. In some embodiments, the reflectance of a reflector can be variable. Various combinations of tunable reflectors are contemplated. Furthermore, these systems can include, for example, appropriate electronics to form a feedback loop so that the systems can monitor the intensity of energy output at one or more wavelengths and vary the reflectance of one or more reflectors (e.g., vary the reflectance of one or more reflectors in real time) to obtain one or more desired output intensities at one or more wavelengths. In certain embodiments, a reflector can be formed of a variable output coupler. Such couplers are described, for example, in commonly owned U.S. Provisional Patent Application Ser. 60/300,298, filed on Jun. 22, 2001, and entitled “Variable Spectrally Selective Output Coupler For Fiber Laser”, which is hereby incorporated by reference. 
   While certain embodiments have been described, the invention is not limited to these embodiments. For example, the reflectors need not be in the form of fiber Bragg gratings. One or more of the reflectors can be a loop mirror, or one or more reflectors can be in the form of a coated mirror (e.g., a coated mirror at one or both ends of a section of optical fiber), etc. As an additional example, the type of laser used for pumping can be varied. Examples of lasers that can be used include semiconductor diode lasers (e.g., high power semiconductor diode lasers), double clad doped fiber lasers, conventional free space coupled lasers, and the like. As another example, various types of optical fibers can be used, including, for example, double clad optical fibers and polarization maintaining optical fibers. Furthermore, the optical fibers can be formed of, for example, silica based materials (e.g., fused silica based) or fluoride based materials. As yet another example, the relative and/or absolute lengths of one or more of the sections of the optical fiber can be varied based upon the intended use of the Raman fiber amplifier. 
   The foregoing fiber amplifiers can be used in a variety of situations.  FIG. 6  is a schematic representation of a system  700  including a transmitter  710 , an amplifier (e.g., one of the above-described amplifiers)  720  and a detector  730 . Transmitter  710  and amplifier  720  are in optical communication via optical conduit (e.g., optical fiber)  740 , and amplifier  720  and detector  730  are in optical communication via optical conduit (e.g., optical fiber)  750 . 
   Other embodiments are in the claims.