Abstract:
An optical network route and method are disclosed that mitigate distortion in a route having different types of fibers. For an optical network route that includes a plurality of fiber spans of a first type and a fiber span of a second type, assume that the optical network route is transporting optical signals having a plurality of original wavelengths where one or more of the original wavelengths is in a distortion wavelength region of the second type of fiber span. For optical signals entering the second type of fiber span, the original wavelength that is in the distortion wavelength region of the second type of fiber span is shifted to a temporary wavelength outside of the distortion wavelength region. The optical signals then travel over the second type of fiber span. For optical signals exiting the second type of fiber span, the temporary wavelength is shifted back to the original wavelength.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention is related to the field of optical networks, and in particular, to shifting wavelengths in optical network routes to mitigate distortion in different types of fiber. 
   2. Statement of the Problem 
   Many communication companies use fiber optic cabling as a media for transmitting data because of its high bandwidth capacity. The optical fibers of a fiber optic cable can reliably transport optical signals over long distances. Optical fibers inherently have nonlinearities which cause nonlinearity effects in optical signals as the optical signals travel over the optical fiber. Some common nonlinearity effects are cross-phase modulation (XPM), self-phase modulation (SPM), four-wave mixing (FWM), stimulated Raman scattering (SRS), and stimulated Brillouin scattering (SBS). As the optical signals travel over an optical fiber, the nonlinearity effects of the optical fiber may contribute to the optical signals distorting in the optical fiber. Excessive distortion of the optical signals can unfortunately result in a loss of data being carried by the optical signals. 
   A typical optical network route within a long haul network or an ultra-long haul network includes a transmitter node, a plurality of fiber spans, amplifiers, regenerator nodes, and a receiver node. An amplifier or regenerator node is positioned between one or more fiber spans to compensate for signal attenuation. Typically, optical network routes are comprised of fiber spans of the same type fiber. However, some optical network routes have one or more fiber spans that are a different type of transmission fiber than the rest of the fiber spans on the route, which can be problematic. 
   There are currently several types of transmission fiber, such as standard Single Mode Fiber (SMF), Dispersion Shifted Fiber (DSF), and Non-Zero Dispersion Shifted Fiber (NZ-DSF). Using different types of transmission fiber in the same route can cause problems because different fibers may have different nonlinearity effects on optical signals. The different types of fibers may cause signal distortion at different wavelengths making it difficult to select which wavelengths may be used over a particular route. 
   As an example, a Single Mode Fiber (SMF) has a zero dispersion wavelength at about 1310 nm. A Dispersion Shifted Fiber (DSF) has a zero dispersion wavelength at about 1550 nm and a small dispersion region between 1540 nm to 1560 nm. A Non-Zero Dispersion Shifted Fiber (NZ-DSF) has a zero dispersion region from 1550 nm to 1525 nm or from 1550 nm to 1575 nm (depending on the kind of NZ-DSF). Assume an optical network route includes multiple spans of single mode fiber and one span of dispersion shifted fiber. The single mode fiber spans have strong nonlinearity effects on wavelengths at about 1310 nm. Consequently, network administrators avoid using wavelengths around 1310 nm for optical signals traveling over single mode fiber spans. The dispersion shifted fiber span has strong nonlinearity effects on wavelengths between 1540 nm and 1560 nm. Thus, network administrators avoid using wavelengths between 1540 nm and 1560 nm for optical signals traveling over the dispersion shifted fiber span. 
   Unfortunately, usable wavelengths are being wasted on this optical network route because different types of fiber are being used. For this optical network route, network administrators avoid wavelengths in the region of 1540 nm to 1560 nm because of the dispersion shifted fiber span, but these wavelengths are wasted on the single mode fiber spans. Wavelengths in the 1540 nm to 1560 nm region could be used on the single mode fiber spans but are not because of the high nonlinearity effects of the dispersion shifted fiber span in this region. The 1540 nm to 1560 nm region comprises much of the C-band, which is used often for carrying data. Network administrators may desire to use certain wavelengths even though one or more types of fiber in an optical network route have high nonlinearity effects at those wavelengths. 
   SUMMARY OF THE SOLUTION 
   The invention helps solve the above and other problems by shifting wavelengths for certain types of fiber in an optical network route to mitigate the distortion caused by the fiber. Advantageously, network administrators may use more wavelengths for data transmission in an optical network route that is comprised of different types of fiber. 
   One embodiment of the invention comprises a method of mitigating distortion in an optical network route having different types of fibers. The optical network route includes a plurality of fiber spans of a first type and a fiber span of a second type. The second type of fiber span has a distortion wavelength region that can highly distort wavelengths within that region. In this embodiment, the optical network route is transporting optical signals having a plurality of wavelengths where one or more of the wavelengths are in the distortion wavelength region of the second type of fiber span. For the method, one step includes shifting one or more of the original wavelengths that are in the distortion wavelength region of the second type of fiber span to temporary wavelengths outside of the distortion wavelength region. This shifting step occurs for optical signals entering the second type of fiber span. The optical signals then travel over the second type of fiber span. Another step of the method includes shifting the temporary wavelengths outside of the distortion wavelength region back to their original wavelengths. This shifting step occurs for the optical signals exiting the second type of fiber span. 
   By shifting the wavelengths in the distortion wavelength region for optical signals traveling over the second type of fiber span, the distortion wavelength region is avoided for the second type of fiber span. Consequently, the wavelengths in the distortion wavelength region may be used to carry data over the optical network route without concern over the effects of the distortion wavelength region of the second type of fiber span. 
   The invention may include other embodiments described below. 

   
     DESCRIPTION OF THE DRAWINGS 
     The same reference number represents the same element on all drawings. 
       FIG. 1  illustrates an optical network route in an embodiment of the invention. 
       FIG. 2A  illustrates wavelengths of optical signals traveling on the optical network route of  FIG. 1  in an embodiment of the invention. 
       FIG. 2B  illustrates a distortion wavelength region of a type-1 fiber span in an embodiment of the invention. 
       FIG. 2C  illustrates a distortion wavelength region of a type-2 fiber span in an embodiment of the invention. 
       FIG. 2D  illustrates optical signals with a wavelength shifted to another wavelength outside of a distortion wavelength region in an embodiment of the invention. 
       FIG. 2E  illustrates optical signals with the wavelength outside of the distortion wavelength region shifted back to its original wavelength in an embodiment of the invention. 
       FIG. 3  is a flow chart illustrating a method of mitigating distortion in the optical network route of  FIG. 1  in an embodiment of the invention. 
       FIG. 4  illustrates an optical network route in another embodiment of the invention. 
       FIG. 5A  illustrates wavelengths of optical signals traveling on the optical network route of  FIG. 4  in an embodiment of the invention. 
       FIG. 5B  illustrates a high nonlinearity wavelength region of Single Mode Fibers (SMF) in an embodiment of the invention. 
       FIG. 5C  illustrates a high nonlinearity wavelength region of a Dispersion Shifted Fiber (DSF) in an embodiment of the invention. 
       FIG. 5D  illustrates optical signals with wavelengths shifted to other wavelengths outside of a high nonlinearity wavelength region in an embodiment of the invention. 
       FIG. 5E  illustrates optical signals with wavelengths outside of a high nonlinearity region shifted back to their original wavelengths in an embodiment of the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1 ,  2 A- 2 E,  3 - 4 , and  5 A- 5 E and the following description depict specific embodiments of the invention to teach those skilled in the art how to make and use the best mode of the invention. For the purpose of teaching inventive principles, some conventional aspects of the invention have been simplified or omitted. Those skilled in the art will appreciate variations from these embodiments that fall within the scope of the invention. Those skilled in the art will appreciate that the features described below can be combined in various ways to form multiple variations of the invention. As a result, the invention is not limited to these specific embodiments described below, but only by the claims and their equivalents. 
     FIG. 1  illustrates an optical network route  100  in an embodiment of the invention. Optical network route  100  is part of a larger optical communication network (not shown). Optical network route  100  is comprised of a plurality of fiber spans  111 - 115  and a plurality of network components  121 - 124 . Fiber span  111  connects to network component  121 . Fiber span  112  connects network component  121  to network component  122 . Fiber span  113  connects network component  122  to network component  123 . Fiber span  114  connects network component  123  to network component  124 . Fiber span  115  connects to network component  124 . The number of network components and fiber spans is just for illustration in  FIG. 1 , and optical network route  100  may include any number of network components, fiber spans, or other devices or components not shown. 
   Fiber spans  111 - 115  comprise transmission fibers, such as standard Single Mode Fiber (SMF), Dispersion Shifted Fiber (DSF), Non-Zero Dispersion Shifted Fiber (NZ-DSF), or any other type of transmission fiber. Fiber spans  111 ,  112 ,  114 , and  115  comprise type-1 transmission fibers and fiber span  113  comprises a type-2 transmission fiber. The terms “type-1” and “type-2” are not intended to indicate any industry standard type of fiber, but just to differentiate one type of fiber from another. A type-1 transmission fiber is different than a type-2 transmission fiber in that one or more properties of the type-1 transmission fiber distort or otherwise affect different wavelengths of optical signals than a type-2 transmission fiber. For instance, the nonlinearity effects of a type-1 transmission fiber may distort different wavelengths of optical signals than the nonlinearity effects of a type-2 transmission fiber. A “fiber span” may include one or more lengths of transmission fiber of the same type. 
   Network components  121 - 124  comprise any optical components or optical-to-electrical-to-optical components that connect fiber spans to one another. Examples of network components include wavelength converters, amplifiers (optical or otherwise), regenerator nodes, transmitter nodes, receiver nodes, or any combination thereof. Optical network route  100  comprises any route or path of an optical communication network. Optical network route  100  may comprise a route on a long haul network, an ultra-long haul network, a Wavelength Division Multiplexed (WDM) network, a Dense WDM (DWDM) network, a Course WDM (CWDM) network, or some other type of optical communication system. 
   When in operation, optical network route  100  receives optical signals to transport. The optical signals include one or more wavelengths that carry data.  FIGS. 2A-2E  are graphs stacked vertically to show wavelengths corresponding with one another.  FIG. 2A  illustrates the wavelengths (λ 1 -λ 4 ) of the optical signals in an embodiment of the invention. The optical signals could include more or less wavelengths than the ones shown in  FIG. 2A  in other embodiments. 
     FIG. 2B  illustrates a distortion wavelength region  201  of the type-1 fiber spans  111 ,  112 ,  114 , and  115  in an embodiment of the invention. The distortion wavelength region  201  is the range of wavelengths that is highly distorted by a type-1 fiber span. Optical signals in the highly distorted wavelength range result in more than 100 times the errors of other wavelengths ranges. The distortion wavelength region  201  may comprise a high nonlinearity wavelength region for the type-1 fiber spans  111 ,  112 ,  114 , and  115 . For instance, a zero dispersion wavelength region of a single mode fiber (e.g., around 1310 nm) may comprise the distortion wavelength region for a single mode fiber. Comparing the wavelengths of the optical signals shown in  FIG. 2A  to the distortion wavelength region  201  of the type-1 fiber spans  111 ,  112 ,  114 , and  115  shown in  FIG. 2B , the wavelengths of the optical signals do not fall within the distortion wavelength region  201  of the type-1 fiber spans  111 ,  112 ,  114 , and  115 . 
     FIG. 2C  illustrates a distortion wavelength region  202  of the type-2 fiber span  113  in an embodiment of the invention. The distortion wavelength region  202  is the range of wavelengths that is highly distorted by a type-2 fiber span. The distortion wavelength region  202  may comprise a high nonlinearity wavelength region for the type-2 fiber span  113 . Comparing the wavelengths of the optical signals in  FIG. 2A  to the distortion wavelength region  202  of the type-2 fiber span  113  in  FIG. 2C , one of the wavelengths (λ 3 ) falls within the distortion wavelength region  202  of the type-2 fiber span  113 . This can be problematic, as the high distortion caused by the type-2 fiber span on wavelength λ 3  can result in a loss of data on λ 3 . 
     FIG. 3  is a flow chart illustrating a method  300  of mitigating distortion in optical network route  100  in an embodiment of the invention. First, method  300  includes shifting the wavelength λ 3  in the distortion wavelength region  202  to a temporary wavelength (λ 3′ ) (see  FIG. 2D ) outside of the distortion wavelength region  202  in step  302 . The shifting in step  302  takes place for optical signals entering fiber span  113 . The shifting step  302  may take place as the optical signals enter fiber span  113 , before the optical signals enter fiber span  113 , or immediately after the optical signals enter fiber span  113 . For instance, network component  122  may include a wavelength converter installed at the ingress point of the type-2 fiber span  113  to shift wavelength λ 3 . Also, network component  122  may include an amplifier or other device capable of shifting wavelength λ 3 , such as an amplifier  122 A shown in an optical network route  100 A depicted in  FIG. 1 .  FIG. 2D  illustrates the optical signals with wavelength λ 3  shifted to temporary wavelength λ 3′  in an embodiment of the invention. In this embodiment, wavelength λ 3  was shifted to a shorter wavelength λ 3′ , but wavelength λ 3  may be shifted to a longer wavelength in other embodiments. 
   With wavelength λ 3  shifted to temporary wavelength λ 3′  as shown in  FIG. 2D , the optical signals travel over fiber span  113  (see  FIG. 1 ). After the optical signals travel over fiber span  113 , method  300  includes shifting temporary wavelength λ 3  back to its original wavelength λ 3  (see  FIG. 3 ) in step  304 . The shifting in step  304  takes place for optical signals exiting fiber span  113 . The shifting step  304  takes place immediately before the optical signals exit fiber span  113 , as the optical signals exit fiber span  113 , or after the optical signals exit fiber span  113  but before the optical signals are amplified or regenerated in network component  123 . For instance, network component  123  may include a wavelength converter installed at the egress point of fiber span  113  to shift temporary wavelength λ 3′  back to original wavelength λ 3 . Also, network component  123  may include an amplifier or similar device capable of shifting temporary wavelength λ 3′  back to original wavelength λ 3 , such as an amplifier  123 A shown in the optical network route  100 A depicted in  FIG. 1 .  FIG. 2E  illustrates the optical signals with temporary wavelength λ 3′  shifted back to original wavelength λ 3  in an embodiment of the invention. 
   By using method  300 , wavelength λ 3  may be used in optical network route  100  even though wavelength λ 3  falls in the distortion wavelength region of fiber span  113 . In the prior art, network administrators would have had to avoid using wavelength λ 3  because of the distortion imparted by fiber span  113 . Advantageously, method  300  allows network administrators to use wavelengths, such as wavelength λ 3 , in certain routes that they could not previously use. Further, the shifting may advantageously be done with less expensive and less complex components, such as an all-optical wavelength converter, an all-optical amplifier, or a similar optical or optical-to-electrical-to-optical device. Network administrators may add or utilize these less expensive components to provide the wavelength shifting instead of using expensive and complex devices such as a complex Optical Add-Drop Multiplexer (O-ADM). 
     FIG. 4  illustrates an optical network route  400  in another embodiment of the invention. Optical network route  400  is part of a larger optical communication network (not shown). Optical network route  400  is comprised of a transmitter node  401 , a regenerator node  402 , a plurality of fiber spans  411 - 415 , a plurality of amplifiers  421 - 424 , and wavelength converters  431 - 432 . Fiber span  411  connects transmitter node  401  to amplifier  421 . Fiber span  412  connects amplifier  421  to amplifier  422 . Fiber span  413  connects amplifier  422  to amplifier  423 . Fiber span  414  connects amplifier  423  to amplifier  424 . Fiber span  415  connects amplifier  424  to regenerator node  402 . Wavelength converter  431  is coupled to fiber span  413  at an ingress point of fiber span  413 . Wavelength converter  432  is coupled to fiber span  413  at an egress point of fiber span  413 . The number of amplifiers and fiber spans is just for illustration in  FIG. 4 , and optical network route  400  may include any number of amplifiers, fiber spans, or other devices or components. 
   Fiber spans  411 ,  412 ,  414 , and  415  comprise standard Single Mode Fibers (SMF). Fiber span  413  comprises a Dispersion Shifted Fiber (DSF). The nonlinearity effects of a SMF are different that the nonlinearity effects of a DSF. For instance, a SMF has high nonlinearity effects at about 1310 nm, whereas a DSF has high nonlinearity effects between about 1540 nm and 1560 nm. 
   When in operation, transmitter node  401  transmits optical signals over optical network route  400 . The optical signals include one or more original wavelengths that carry data.  FIGS. 5A-5E  are graphs stacked vertically to show wavelengths corresponding with one another.  FIG. 5A  illustrates the original wavelengths of the optical signals in an embodiment of the invention. The optical signals include four wavelengths (1535 nm, 1545 nm, 1555 nm, and 1565 nm) in this embodiment. The optical signals could include more or less wavelengths in other embodiments. 
     FIG. 5B  illustrates a high nonlinearity wavelength region  501  of the SMFs  411 ,  412 ,  414 , and  415  in an embodiment of the invention. SMFs  411 ,  412 ,  414 , and  415  have a high nonlinearity wavelength region  501  at about 1310 nm. Comparing the wavelengths of the optical signals in  FIG. 5A  to the high nonlinearity wavelength region  501  of the SMFs  411 ,  412 ,  414 , and  415  in  FIG. 5B , the wavelengths of the optical signals do not fall within the high nonlinearity wavelength region  501  of the SMFs  411 ,  412 ,  414 , and  415 . 
     FIG. 5C  illustrates a high nonlinearity wavelength region  502  of DSF  413  in an embodiment of the invention. DSF  413  has a high nonlinearity wavelength region  502  between 1540 nm to 1560 nm. Comparing the wavelengths of the optical signals in  FIG. 5A  to the high nonlinearity wavelength region  502  of DSF  413  in  FIG. 5C , two of the wavelengths (1545 nm and 1555 nm) fall within the high nonlinearity wavelength region  502  of DSF  113 . This can be problematic, as the high distortion caused by DSF  413  on the 1545 nm and 1555 nm wavelengths can result in a loss of data. 
   To mitigate distortion in DSF  413 , wavelength converter  431  receives the optical signals as they enter DSF  413  (see  FIG. 4 ). Wavelength converter  431  shifts the 1545 nm wavelength to a temporary 1515 nm wavelength outside of the high nonlinearity wavelength region  502  of DSF  413 . Also, wavelength converter  431  shifts the 1555 nm wavelength to a temporary 1525 nm wavelength outside of the high nonlinearity wavelength region  502  of DSF  413 .  FIG. 5D  illustrates the optical signals with the 1545 nm wavelength shifted to the temporary 1515 nm wavelength and the 1555 nm wavelength shifted to the temporary 1525 nm wavelength in an embodiment of the invention. The temporary 1515 nm and 1525 nm wavelengths were selected as an illustration for this embodiment. A network administrator may use other desired wavelengths outside of the high nonlinearity wavelength region  502 . 
   With the wavelengths shifted as shown in  FIG. 5D , the optical signals travel over DSF  413  (see  FIG. 4 ). After the optical signals travel over DSF  413 , wavelength converter  432  receives the optical signals as they exit DSF  413 . Wavelength converter  432  shifts the optical signals back to their original wavelengths. Wavelength converter  432  shifts the temporary 1515 nm wavelength back to the 1545 nm wavelength and shifts the temporary 1525 nm wavelength back to the 1555 nm wavelength.  FIG. 5E  illustrates the optical signals with the wavelengths shifted back in an embodiment of the invention. 
   Wavelength converters  431 - 432  advantageously shift the wavelengths in the high nonlinearity wavelength region  502  of DSF  413  to shorter wavelengths that are not as significantly affected by the nonlinearities of DSF  413 . Consequently, the 1545 nm wavelength and the 1555 nm wavelength may still be used in optical network route  400  to carry data.