Abstract:
The invention pertains to optical fiber transmission systems, and is particularly relevant to optical transport systems employing Raman optical amplifiers. In particular the invention teaches an apparatus and method to provide initial tuning of a Raman pump module. In the present invention, improvements to Raman gain control are taught in order to provide for an advantageous Raman gain spectral profile.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims priority to Provisional Application Serial No. 60/385,921 entitled “Method of Initial Tuning of Raman Pump Module”, by Eiselt, filed Jun. 4, 2002. 
     
    
     
       TECHNIAL FIELD OF THE INVENTION  
         [0002]    The present invention relates, in general, to the field of optical communications, and in particular to, an optical transport system that uses Raman optical amplifiers. In particular the invention teaches an apparatus and method to provide initial tuning of a Raman pump module. In the present invention, improvements to Raman gain control are taught in order to provide for an advantageous Raman gain spectral profile.  
         BACKGROUND OF THE INVENTION  
         [0003]    A goal of many modem long haul optical transport systems is to provide for the efficient transmission of large volumes of voice traffic and data traffic over trans-continental distances at low costs. Various methods of achieving these goals include time division multiplexing (TDM) and wavelength division multiplexing (WDM). In time division multiplexed systems, data streams comprised of short pulses of light are interleaved in the time domain to achieve high spectral efficiency, high data rate transport. In wavelength division multiplexed systems, data streams comprised of short pulses of light of different carrier frequencies, or equivalent wavelength, co-propagate in the same fiber to achieve high spectral efficiency, high data rate transport.  
           [0004]    The transmission medium of these systems is typically optical fiber. In addition there is a transmitter and a receiver. The transmitter typically includes a semiconductor diode laser, and supporting electronics. The laser may be directly modulated with a data train with an advantage of low cost, and a disadvantage of low reach and capacity performance. After binary modulation, a high bit may be transmitted as an optical signal level with more power than the optical signal level in a low bit. Often, the optical signal level in a low bit is engineered to be equal to, or approximately equal to zero. In addition to binary modulation, the data can be transmitted with multiple levels, although in current optical transport systems, a two level binary modulation scheme is predominantly employed.  
           [0005]    Typical long haul optical transport dense wavelength division multiplexed (DWDM) systems transmit 40 to 80 10 channels at Gbps (gigabit per second) across distances of 3000 to 6000 km in a single 30 nm spectral band. A duplex optical transport system is one in which traffic is both transmitted and received between parties at opposite end of the link. In current DWDM long haul transport systems transmitters different channels operating at distinct carrier frequencies are multiplexed using a multiplexer. Such multiplexers may be implemented using array waveguide grating (AWG) technology or thin film technology, or a variety of other technologies. After multiplexing, the optical signals are coupled into the transport fiber for transmission to the receiving end of the link.  
           [0006]    At the receiving end of the link, the optical channels are de-multiplexed using a de-multiplexer. Such de-multiplexers may be implemented using AWG technology or thin film technology, or a variety of other technologies. Each channel is then optically coupled to separate optical receivers. The optical receiver is typically comprised of a semiconductor photodetector and accompanying electronics.  
           [0007]    The total link distance may in today&#39;s optical transport systems be two different cities separated by continental distances, from 1000 km to 6000 km, for example. To successfully bridge these distances with sufficient optical signal power relative to noise, the total fiber distance is separated into fiber spans, and the optical signal is periodically amplified using an in line optical amplifier after each fiber span. Typical fiber span distances between optical amplifiers are 50-100 km. Thus, for example, 30 100 km spans would be used to transmit optical signals between points 3000 km apart. Examples of inline optical amplifers include erbium doped fiber amplifers (EDFAs) and semiconductor optical amplifiers (SOAs).  
           [0008]    Alternatively, a Raman optical amplifier may be used to boost the optical signal power. Most Raman optical amplifiers comprise at least one high power pump laser that is launched into the fiber span. Through the nonlinear optical process of stimulated Raman scattering in the SiO 2  of the glass of the fiber span, this pump signal provides gain to the optical signal power. A Raman amplifier may be co-propagating or counter-propagating to the optical signal, and a common configuration is to counter-propagate the Raman pump. A Raman amplifier may be used alone, or in combination with an alternate example of an inline optical amplifier, such as an EDFA. For example, a Raman amplifier may be used in conjunction with an inline optical amplifier to accommodate high loss spans and to bring the net span loss within an allowable system dynamic range.  
           [0009]    The gain profile of Raman gain in an optical fiber is not spectrally flat, and it would be desirable to achieve control over the Raman pump source in order to achieve a spectrally flat Raman gain. It is further desirable to be able to control the gain profile of the Raman gain in order to achieve a spectral dependence that may not necessarily be flat, but may be advantageous in other regards.  
           [0010]    The power of the Raman pumps can be designed (e.g. by simulations) to yield flat (or arbitrarily shaped) gain for a nominal (typical) fiber span. But two parameters of the real fiber are random and unknown: 1) the wavelength dependent coupling loss between pump laser and fiber input and 2) the wavelength dependent loss of the fiber. To compensate for these unknowns, the pump powers need to be adapted.  
           [0011]    One way to obtain the correct pump power values is to measure the spectral gain shape and adapt the power values for flat gain shape. But that requires expensive channel power monitors (measuring wavelength resolved power values) it also requires signals present at all wavelengths which may not be possible in some systems where all channels are not equiped. The present invention discloses a solution that is based on simple (overall) power measurements and only requires a single channel in the system to be active.  
         SUMMARY OF THE INVENTION  
         [0012]    In the present invention, improvements to Raman gain control are taught in order to provide for an advantageous Raman gain spectral profile.  
           [0013]    In one aspect of the invention, an apparatus to achieve a flat Raman gain profile is taught using a plurality of Raman pump lasers.  
           [0014]    In another aspect of the invention, an apparatus to achieve an advantageously shaped Raman gain profile is taught using a plurality of Raman pump lasers.  
           [0015]    In another aspect of the invention, a method to achieve a flat Raman gain profile is taught using a plurality of Raman pump lasers.  
           [0016]    In another aspect of the invention, a method to achieve an advantageously shaped Raman gain profile is taught using a plurality of Raman pump lasers.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    For a more complete understanding of the features and advantages of the present invention, reference is now made to the detailed description of the invention along with the accompanying figures in which corresponding numerals in the different figures refer to corresponding parts and in which:  
         [0018]    [0018]FIG. 1 is a schematic illustration of a Raman gain control apparatus to achieve an advantageously shaped Raman gain spectral profile.  
         [0019]    [0019]FIG. 2 is a flow chart of a Raman gain control method to achieve an advantageously shaped Raman gain spectral profile. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0020]    While the making and using of various embodiments of the present invention are discussed in detail below, it should be appreciated that the present invention provides many applicable inventive concepts which can be embodied in a wide variety of specific contexts. The specific embodiments described herein are merely illustrative of specific ways to make and use the invention and do not delimit the scope of the invention.  
         [0021]    In FIG. 1 is shown a block diagram of a Raman gain control apparatus to achieve an advantageously shaped Raman gain spectral profile. The Raman gain control apparatus comprises a plurality of Raman pump lasers. Shown in FIG. 1 are Raman pump laser  102  and Raman pump laser  104 . Raman pump laser  102  and Raman pump laser  104  are optically coupled to wavelength selective coupler  106 . Wavelength selective coupler  106  is further optically coupled to fiber span  108  and optical tap  110 . The apparatus also comprises optical power meter  112 , and a Raman gain control unit  114 . In FIG. 1 is shown optical tap  120  and optical power meter  122 . Power meter  122  is coupled to control unit  114  by communication link  140 . Optical tap  120  is coupled to the opposite end of fiber span  108  from wavelength selective coupler  106 . Also shown for reference in FIG. 1 is in-line optical amplifier  132  and in-line optical amplifier  134 .  
         [0022]    Raman pump laser  102  and Raman pump laser  104  may be implemented as a sufficiently powerful laser such as a high power semiconductor diode lasers, or a plurality of high power semiconductor lasers. The plurality of high power semiconductor lasers may be of the same wavelength. In the context of this invention, it will be understood that Raman pump laser  102  and Raman pump laser  104  will be purposefully at different wavelengths to provide uniform or otherwise tailored Raman gain across a broad spectrum. It should be noted that while FIG. 1. shows a preferred embodiment with two Raman pump lasers with respectively two emission wavelengths, this invention allows more than two Raman pump lasers with more than two emission wavelengths.  
         [0023]    Wavelength selective coupler  106  may be realized as a thin film optical coupler or other technology so long as the optical coupler acts to couple the Raman pump laser signals into to fiber span  108 , while allowing the optical data signal to proceed from fiber span  108  towards in-line amplifier  134 . Optical fiber span  108  may be implemented using optical fiber, and in a preferred embodiment is single mode fiber such as SMF-28 or LEAF. Typical distances for fiber span  108  are 75-125 km. In a preferred embodiment of the invention, a Raman amplifier can be used on every link in the transmission system to reduce the amplifier noise figure and enable more spans for the longer fiber spans with higher losses.  
         [0024]    Optical tap  110  and optical tap  120  may be fused couplers, or thin film couplers. Alternatively, wavelength selective coupler  106  may be a circulator. Optical power meter  112  and optical power meter  122  may be a calibrated photodiode. Raman gain control unit  114  may be a microprocessor, or microcomputer, and fulfills the feedback loop between the optical power meter  112 , optical power meter  122  and Raman pump laser  120 . In particular, feedback loop  140  connects optical power meter  122  and Raman gain unit  114 . In a preferred embodiment, feedback loop  140  may be implemented though the optical service channel of the optical transport system. Examples of inline optical amplifier  132  and optical amplifier  134  include erbium doped fiber amplifiers (EDFAs) and semiconductor optical amplifiers (SOAs). Potentially a discrete Raman amplifier may also be used for in-line optical amplifier  132  and in-line optical amplifier  134 .  
         [0025]    [0025]FIG. 1 may now be used to understand the operation of the invention to control the spectral dependence of the Raman gain. For example, to achieve a spectrally flat gain, two pump wavelengths may be used with the correct relative power ratio between Raman pump laser  102  and Raman pump laser  104 . However, power losses in the pump coupling components, for example wavelength selective coupler  106  and fiber losses, can be wavelength dependent, causing the signal from Raman pump laser  102  to be attenuated differently than the signal from Raman pump laser  104 . If the relative pump launch powers are not adjusted to take into account these loss variations, this can lead to a non-flat Raman gain spectrum.  
         [0026]    Consider first the case where both Raman pump laser  102  and Raman pump laser  104  are turned off. For an optical signal, at λ signal traveling from optical tap  120  to optical tap  110 , the inherent loss in the fiber span is equal to L 0 =P 1 /P 2  where P 2  is the power as measured in optical power meter  112  and P 1  is the power as measured in optical power meter  122 . Raman gain control unit  114  is programmed to calculate L 0  based on power measurements from optical power meter  112  and optical power meter  122  when Raman pump laser  102  and  104  are off.  
         [0027]    The second case is where Raman pump laser  102 , operating at wavelength λ 1  is turned on with power P L (λ 1 ). The power of the optical signal is again measured with optical power meter  112  and optical power meter  122 . The loss in the fiber span is again calculated as L 1 =P 1 /P 2 . Due to the gain from the Raman pump laser  102 , L 1  is smaller than L 0 . After coupling losses L c (λ 1 ), the Raman pump power coupled into fiber span  108  is L c (λ 1 )P L (λ 1 ). Theoretically, the Raman gain due to the presence of Raman pump laser  102  is given by the expression: 
           G   1 =exp└ g   fiber   r (λ signal −λ 1 ) L   C (λ 1 ) P   L (λ 1 ) L   eff (λ 1 )┘ 
         [0028]    where g fiber  is the power normalized peak Raman gain coefficient of fiber span  108 , and r(λ signal −λ 1 ) is the relative gain coefficient at the wavelength separation λ signal −λ 1 . In practice the value of G 1 =L 0 /L 1 . The power meter measurements provide a value G 1  to Raman gain control unit  114 . Leff(λ 1 ) is the effective fiber length at wavelength λ 1 , which is calculated as L eff (λ 1 )=(1−exp(−α(λ 1 )*L fiber ))/α(λ 1 ), where L fiber  is the length of the fiber span and α(λ 1 ) is the fiber attenuation coefficient at wavelength λ 1 .  
         [0029]    The third case occurs as Raman pump laser  102  is turned off and Raman pump laser  104  is turned on, operating at wavelength λ 2  with power P L (λ 2 ). The power of the optical signal is again measured with optical power meter  110  and optical power meter  122 . The loss in the fiber span is now calculated as L 2 =P 1 /P 2 . Due to the gain from the Raman pump laser  104 , L 2  is smaller than L 0 . After coupling losses L c (λ 2 ), the Raman pump power coupled into fiber span  108  is L c (λ 2 )P L (λ 2 ). Theoretically, the Raman gain due to the presence of Raman pump laser  104  is given by the expression: 
           G   2 =exp└ g   fiber   r (λ signal −λ 2 ) L   C (λ 2 ) P   L (λ 2 ) L   eff (λ 2 )┘ 
         [0030]    where g fiber  is the power normalized peak Raman gain coefficient of fiber span  108 , and r(λ signal −λ 2 ) is the relative gain coefficient at the wavelength separation λ signal −λ 2 . L eff (λ 2 ) is the effective fiber length at wavelength λ 2 , which is calculated as L eff (λ 2 )=(1−exp(−α(λ 2 )*L fiber ))/α(λ 2 ), where L fiber  is the length of the fiber span and α(λ 2 ) is the fiber attenuation coefficient at wavelength λ 2 . In practice the value of G 2 =L 0 /L 2 . The power meter measurements provide a value G 2  to Raman gain control unit  114 .  
         [0031]    From G 1  and G 2 , Raman gain control unit  114  will now calculate the ratio between the coupling and fiber losses for the two pump wavelengths:  
               L   eff          (     λ   1     )              L   C          (     λ   1     )               L   eff          (     λ   2     )              L   C          (     λ   2     )           =           ln        (     G   1     )            r        (       λ   signal     -     λ   1       )             ln        (     G   2     )            r        (       λ   signal     -     λ   2       )                    P   L          (     λ   1     )           P   L          (     λ   2     )                                 
 
         [0032]    “r” is the Raman coefficient and is taken as a known value which is independent of fiber type. Raman gain control unit  114  will use this loss ratio to adjust the relative power of Raman pump laser  102  to Raman pump laser  104  to yield a correct power ratio in fiber span  108  to achieve a flat Raman gain spectrum. The optimum ratio of the pump powers is determined based on simulations. These ratios depend on the fiber type, the wavelength range, span lengths and other parameters. The method described is used to ensure that these power ratios are true at the input to the fiber and also takes into account varying wavelength dependent span losses.  
         [0033]    As will be clear to one skilled in the art, if it is advantageous produce a tilted Raman gain spectrum, with higher gain at either λ 1  or λ 2  then Raman gain control unit  114  can be programmed to adjust the relative powers to provide a tilted Raman gain spectrum. Additional Raman pump lasers and additional G measurements provide additional data to Raman control unit  114  and may be used to provide more complicated Raman spectral gain profiles.  
         [0034]    In FIG. 2 is a flow chart illustrating a method of Raman gain control in accordance with one aspect of the invention. The method comprises a first step  210  of measuring the inherent loss of a fiber span  108  at a signal wavelength. For an optical signal traveling from optical tap  120  to optical tap  110 , the inherent loss in the fiber span is equal to L 0 =P 1 /P 2  where P 2  is the power as measured in optical power meter  112  and P 1  is the power as measured in optical power meter  122 . Raman gain control unit  114  is programmed to calculate L 0  based on power measurements from optical power meter  112  and optical power meter  122 .  
         [0035]    The method further comprises a second step  212  of measuring the gain, G 1  of a fiber span due to a first Raman pump laser  102 . Raman pump laser  102 , operating at wavelength λ 1  is turned on with power P L (λ 1 ). The power of the optical signal is again measured with optical power meter  112  and optical power meter  122 . The loss in the fiber span is now calculated as L 1 =P 1 /P 2 . Due to the gain from the Raman pump laser  102 , L 1  is smaller than L 0 . After coupling losses L c (λ 1 ), the Raman pump power coupled into fiber span  108  is L c (λ 1 )P L (λ 1 ). Theoretically, the Raman gain due to the presence of Raman pump laser  102  is given by the expression: 
           G   1 =exp└ g   fiber   r (λ signal −λ 1 ) L   C (λ 1 ) P   L (λ 1 ) L   eff (λ 1 )┘ 
         [0036]    where g fiber  is the power normalized peak Raman gain coefficient of fiber span  108 , and r(λ signal −λ 1 ) is the relative gain coefficient at the wavelength separation λ signal −λ 1 . L eff (λ 1 ) is the effective fiber length at wavelength λ 1 , which is calculated as L eff (λ 1 )=(1−exp(−α(λ 1 )*L fiber ))/α(λ 1 ), where L fiber  is the length of the fiber span and α(λ 1 ) is the fiber attenuation coefficient at wavelength λ 1 . In practice the value of G 1 =L 0 /L 1 . The power meter measurements provide a value G 1  to Raman gain control unit  114 .  
         [0037]    Step  214  of the method entails measuring the gain, G 2  of a fiber span due to a second Raman pump laser  104 . Raman pump laser  102  is now turned off, and Raman pump laser  104 , operating at wavelength λ 2  is turned on with power P L (λ 2 ). The power of the optical signal is again measured with optical power meter  112  and optical power meter  122 . The loss in the fiber span is now calculated as L 1 =P 1 /P 2 . Due to the gain from the Raman pump laser  102 , L 1  is smaller than L 0 . After coupling losses L c (λ 2 ), the Raman pump power coupled into fiber span  108  is L c (λ 2 )P L (λ 2 ). Theoretically, the Raman gain due to the presence of Raman pump laser  104  is given by the expression: 
           G   2 =exp└ g   fiber   r (λ signal −λ 2 ) L   C (λ 2 ) P   L (λ 2 ) L   eff (λ 2 )┘ 
         [0038]    where g fiber  is the power normalized peak Raman gain coefficient of fiber span  108 , and r(λ signal −λ 2 ) is the relative gain coefficient at the wavelength separation λ signal −λ 2 . L eff (λ 2 ) is the effective fiber length at wavelength λ 2 , which is calculated as L eff (λ 2 )=(1−exp(−α(λ 2 )*L fiber ))/α(λ 2 ), where L fiber  is the length of the fiber span and α(λ 2 ) is the fiber attenuation coefficient at wavelength λ 2 . In practice, G 2 =L 0 /L 1′ . The power meter measurements provide a value G 2  to Raman gain control unit  114 .  
         [0039]    Step  216  of the method entails Calculating relative coupling losses of first Raman pump laser and second Raman pump laser from G 1  and G 2 . From G 1  and G 2 , Raman gain control unit  114  will now calculate the ratio between the coupling and fiber losses for the two pump wavelengths:  
               L   eff          (     λ   1     )              L   C          (     λ   1     )               L   eff          (     λ   2     )              L   C          (     λ   2     )           =           ln        (     G   1     )            r        (       λ   signal     -     λ   1       )             ln        (     G   2     )            r        (       λ   signal     -     λ   2       )                    P   L          (     λ   1     )           P   L          (     λ   2     )                                 
 
         [0040]    “r” is the Raman coefficient and is taken as a known value which is independent of fiber type. Step  218  of the method entails Adjusting relative power in first and second Raman pump lasers for an advantageous gain spectral profile. Raman gain control unit  114  will use this loss ratio to adjust the relative power of Raman pump laser  102  to Raman pump laser  104  to yield a correct power ratio in fiber span  108  to achieve a flat Raman gain spectrum. The optimum ratio of the pump powers is determined based on simulations. These ratios depend on the fiber type, the wavelength range, span lengths and other parameters. The method described in the application is used to ensure that these power ratios are true at the input to the fiber and also takes into account varying wavelength dependent span losses.  
         [0041]    As will be clear to one skilled in the art, if it is advantageous produce a tilted Raman gain spectrum, with higher gain at either λ 1  or λ 2  then Raman gain control unit  114  can be programmed to adjust the relative powers to provide a tilted Raman gain spectrum. Additional Raman pump lasers, and additional G measurements provide additional data to Raman control unit  114  and may be used to provide more complicated Raman spectral gain profiles.  
         [0042]    While this invention has been described in reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. It is therefore intended that the appended claims encompass any such modifications or embodiments.