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 control the Raman gain based upon power measurements at one end of the transmission fiber.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]    This application claims priority to U.S. Provisional Application Serial No. 60/386,086, entitled “Method and Apparatus for Raman Gain Control”, by Eiselt, filed Jun. 4, 2002. 
     
    
     
       TECHNICAL 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 control the Raman gain based upon power measurements at one end of the transmission fiber.  
         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 equivalently wavelength, are 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 channels of 10 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 amplifiers include erbium doped fiber amplifiers (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]    When a plurality of high power Raman pump lasers are used it is desirable to control the gain. An example of Raman amplifier gain control in the current art is to measure the input signal power and output signal power and to compare these measurements. In a Raman amplifier that uses the fiber span, this control method is slow because of the large distance between the input and the output of the fiber span. Consequently there is a need for a fast Raman gain control method, and in particular a Raman gain control method that is based on power measurements on only one end of the transmission fiber span.  
           [0010]    In another Raman amplifier gain control method currently practiced, the gain was controlled with a probe wavelength. This method is not optimal, however, because extra hardware is required, and the additional optical signal limits the reach and capacity of the optical transport system. Consequently there is a need for a Raman gain control method that does not employ a probe wavelength.  
         SUMMARY OF THE INVENTION  
         [0011]    In the present invention, improvements to Raman gain control are taught based on power measurements on only one end of the transmission fiber span, and do not use a dedicated probe wavelength.  
           [0012]    In one aspect of the invention, a Raman gain control apparatus for counterpropagating pump and signal based on signal output power measurements is taught.  
           [0013]    In another aspect of the invention, a Raman gain control method for counterpropagating pump and signal based on signal output power measurements is taught.  
           [0014]    In one aspect of the invention, a Raman gain control apparatus for counterpropagating pump and signal based on backscattered pump power measurements is taught.  
           [0015]    In another aspect of the invention, a Raman gain control method for counterpropagating pump and signal based on backscattered pump power measurements is taught.  
           [0016]    In another aspect of the invention, a Raman gain control apparatus for co-propagating pump and signal based on signal input power measurements is taught.  
           [0017]    In another aspect of the invention, a Raman gain control method for co-propagating pump and signal based on signal input power measurements is taught.  
           [0018]    In one aspect of the invention, a Raman gain control apparatus for co-propagating pump and signal based on backscattered pump power measurements is taught.  
           [0019]    In another aspect of the invention, a Raman gain control method for co-propagating pump and signal based on backscattered pump power measurements is taught.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]    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:  
         [0021]    [0021]FIG. 1 is a schematic illustration of a Raman gain control apparatus for counterpropagating pump and signal based on signal output power measurements.  
         [0022]    [0022]FIG. 2 is a flow chart of a Raman gain control method for counterpropagating pump and signal based on signal output power measurements.  
         [0023]    [0023]FIG. 3 is a schematic illustration of a Raman gain control apparatus for counterpropagating pump and signal based on backscattered pump power measurements.  
         [0024]    [0024]FIG. 4 is a flow chart of a Raman gain control method for counterpropagating pump and signal based on backscattered pump power measurements.  
         [0025]    [0025]FIG. 5 is a block diagram of a Raman gain control apparatus for co-propagating pump and signal based on signal input power measurements.  
         [0026]    [0026]FIG. 6 is a flow chart illustrating a method of Raman gain control in accordance with one aspect of the invention.  
         [0027]    [0027]FIG. 7 is a block diagram of a Raman gain control apparatus for copropagating pump and signal based on backscattered pump power measurements.  
         [0028]    [0028]FIG. 8 is a flow chart illustrating a method of Raman gain control in accordance with another aspect of the invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0029]    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.  
         [0030]    In FIG. 1 is shown a block diagram of a Raman gain control apparatus for counterpropagating pump and signal based on signal output power measurements. The Raman gain control apparatus comprises a Raman pump laser  120  that is optically coupled to a wavelength selective optical coupler  124 . Wavelength selective coupler  124  is further optically coupled to fiber span  122  and optical tap  126 . The apparatus also comprises optical power meter  128 , and a Raman gain control unit  110 .  
         [0031]    Raman pump laser  120  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, or purposefully at different wavelengths to provide uniform or otherwise tailored Raman gain across a broad spectrum. Optical fiber span  122  may be implemented using optical fiber, and in a preferred embodiment is single mode fiber such as SMF-28 or LEAF. Wavelength selective optical coupler  124  may be realized as a thin film optical coupler. Optical tap  126  may be a fused coupler, or a thin film coupler. Alternatively, wavelength selective coupler  124  may be a circulator. Optical power meter  128  may be a calibrated photodiode. Raman gain control unit  110  may be a microprocessor, or microcomputer, and fulfills the feedback loop between the optical power meter  128  and the Raman pump laser  120 .  
         [0032]    Also shown in FIG. 1 is the flow of optical energy in the apparatus, including Raman pump power  130  and optical signal  132 . Optical signal fraction  134  is also shown in FIG. 1. Raman pump power  130  is generated by Raman pump laser  120 . Optical signal  132  is, in a preferred embodiment, voice or data traffic that is being transmitted from one location to another. Raman pump power  130  is coupled into optical fiber span  122  via wavelength selective optical coupler  124 . In the arrangement, optical pump power  130  counterpropagates with optical signal  132 . Optical tap  126  samples a fraction of the optical signal and directs onto optical power meter  128 . It should be noted that the exact arrangement of the apparatus may be modified to achieve the same functionality.  
         [0033]    In the arrangement of FIG. 1, the gain of a Raman amplifier with counterpropagating pump is saturated as if the pump power were reduced by 1 to 1.5 times the signal power. Consequently, measurement of the power of optical signal  132  via optical tap  126  and optical power meter  128  allows Raman gain control through the Raman gain control unit  110 .  
         [0034]    At system turn-up, Raman pump power  130  is set to a value P pump,0  which yields the desired Raman gain when the amplifier is not saturated because the power of the optical signal  132  is zero. During operation, the signal output power  134 , P s,out , is measured. The Raman gain control unit is programmed to continually adjust the Raman pump power  130  to a value P pump=P   pump,0 +k*P s,out . The factor k depends on the unsaturated gain and on the fiber type of fiber span  122  (especially the signal loss and the pump loss) but is typically around 1.4. The factor k can be calculated approximately by the expression:  
       k   ≈         λ   s       λ   p              ln        (   G   )               α   p       α   s          ln                   (   G   )       -   1                               
 
         [0035]    where λ s  is the wavelength of optical signal  132 , λ p  is the wavelength of Raman pump power  130 , G is the net Raman gain, α p  is the fiber attenuation coefficient at the wavelength of Raman pump power  130 , and α s  is the attenuation coefficient at the wavelength of optical signal  132 .  
         [0036]    Any signal loss, L 2  between the output of fiber span  122  and optical power meter  128  needs to be taken into account. Likewise any pump loss, L p , between the pump power reference point and the input to fiber span  122  needs also to be considered. Losses L s , and L p  may be determined at system turn-up and programmed into Raman gain control unit  110 . Therefore, including these losses, the pump adjustment needs to yield: P pump =P pump,0 +k*P s,out /L s L p . 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 transmitting Raman pump power  130  in fiber span  122  at a value P pump,0  and recording the desired Raman gain when the amplifier is not saturated because the power of the optical signal  132  is zero. The method further comprises a second step  212  of counter-propagating optical signal  132  in fiber span  122 . Step  214  of the method entails measuring during operation, the signal output power  134 , P s,out . Step  216  of the method entails continually adjusting the Raman pump power  130  to a value P pump =P pump,0 +k*P s,out .  
         [0037]    In FIG. 3 is a block diagram of an alternate embodiment of this invention in which the Rayleigh backscatter  336  portion of the Raman pump power  130  is used to control the Raman gain. The alternate embodiment of FIG. 3 is comprised of Raman pump laser  120  that is optically coupled to a wavelength selective optical coupler  124 . Wavelength selective coupler  124  is further optically coupled to fiber span  122 . The apparatus also comprises optical tap  126 , optical power meter  128 , and a Raman gain control unit  110 .  
         [0038]    Also shown in FIG. 3 is the flow of optical energy in the apparatus, including Raman pump power  130  and optical signal  132 . Rayleigh backscatter  336  is generated in the fiber span, and may be used as a control signal for Raman gain control. Rayleigh backscatter signal fraction  338  is also shown in FIG. 3. Raman pump power  130  is generated by Raman pump laser  120 . Optical signal  132  is, in a preferred embodiment, voice or data traffic that is being transmitted from one location to another. Raman pump power  130  is coupled into optical fiber span  122  via wavelength selective optical coupler  124 . In the arrangement, Raman pump power  130  counter-propagates with optical signal  132 . Optical tap  126  samples a fraction of the Rayleigh backscatter and directs Rayleigh backscatter signal fraction  338  onto optical power meter  128 . It should be noted that the exact arrangement of the apparatus may be modified to achieve the same functionality.  
         [0039]    In reference to operation of FIG. 3, an optical signal  132  of high power saturates and attenuates Raman pump power  130 , which is injected from the signal output fiber end. Thus, less Raman pump power penetrates into the fiber and less Rayleigh backscatter  336  is produced. Thus the power level of Rayleigh backscatter is a good measure of Raman gain saturation. The relationship between the power level of Rayleigh backscatter  336  and net Raman gain is nearly independent of gain saturation. Hence, in this invention, the Raman gain is controlled by adjusting the level of Raman pump power  130  such that the level of Rayleigh backscatter  336  is kept constant, or, measured Rayleigh backscatter signal fraction  338  is kept constant. The difference in signal level between Rayleigh backscatter  336  of the Raman pump and Rayleigh backscatter signal fraction  338  does not effect gain control accuracy. Further, no information on fiber loss parameters or scattering parameters is required by Raman gain control unit  110 .  
         [0040]    In some embodiments there will be deleterious reflections of the Raman pump power  130  at the input to fiber span  122 , or in the first few meters of fiber span  122 . If this return loss for Raman pump power  130  is known to be r pump , the measured Rayleigh backscatter  336  of the Raman pump is corrected by a term r pump *P pump  such that the power P back −r pump *P pump  is kept constant. In these expressions P pump  is the power level of Raman pump power  130 , and P back  is the power level of Rayleigh backscatter  336 .  
         [0041]    Further gain control accuracy will be achieved if a loop-back factor k is determined upon calibration such that Raman gain control unit  110  works to keep the power P back +k*P pump constant. In a preferred embodiment, P pump  is approximately 500 mW, P back  is approximately 0.5 mW and K is approximately 0.0002.  
         [0042]    In FIG. 4 is a flow chart illustrating a method of Raman gain control in accordance with another aspect of the invention. The method comprises a first step  410  of transmitting Raman pump power  130  in fiber span  122  and recording a value P pump . The method further comprises a second step  412  of counter-propagating optical signal  132  in fiber span  122 . Step  414  of the method entails measuring during operation, the Rayleigh backscatter signal power, P back . Step  416  of the method entails continually adjusting the Raman pump power  130  to keep the power P back +k*P pump  constant.  
         [0043]    In FIG. 5 is shown a block diagram of a Raman gain control apparatus for co-propagating pump and signal based on signal input power measurements. The Raman gain control apparatus comprises a Raman pump laser  520  that is optically coupled to a wavelength selective optical coupler  524 . Wavelength selective coupler  524  is further optically coupled to fiber span  522  and optical tap  526 . The apparatus also comprises optical power meter  528 , and a Raman gain control unit  510 .  
         [0044]    Raman pump laser  520  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, or purposefully at different wavelengths to provide uniform or otherwise tailored Raman gain across a broad spectrum. Optical fiber span  522  may be implemented using optical fiber, and in a preferred embodiment is single mode fiber such as SMF-28 or LEAF. Wavelength selective optical coupler  524  may be realized as a thin film optical coupler. Optical tap  526  may be a fused coupler, or a thin film coupler. Optical power meter  528  may be a calibrated photodiode. Raman gain control unit  510  may be a microprocessor, or microcomputer, and fulfills the feedback loop between the optical power meter  528  and the Raman pump laser  520 .  
         [0045]    Also shown in FIG. 5 is the flow of optical energy in the apparatus, including Raman pump power  530  and optical signal  532 . Optical signal fraction  534  is also shown in FIG. 5. Raman pump power  530  is generated by Raman pump laser  520 . Optical signal  532  is, in a preferred embodiment, voice or data traffic that is being transmitted from one location to another. Raman pump power  530  is coupled into optical fiber span  522  via wavelength selective optical coupler  524 . In the arrangement, optical pump power  530  co-propagates with optical signal  532 . Optical tap  526  samples a fraction of the optical signal  532  and directs onto optical power meter  528 . It should be noted that the exact arrangement of the apparatus may be modified to achieve the same functionality.  
         [0046]    The typical power levels of Raman pump laser  520  are lower in the co-propagating case than for the counter-propagating case in order to reduce pump saturation mitigated cross-talk between the channels of a DWDM system. Typically the net gain is around 10 dB. Larger saturation may be expected, however, because of the higher power of optical signal  532 .  
         [0047]    The gain saturation of Raman pump power  530  in the co-propagating case is equivalent to a reduction of the pump power by a multiple of the signal input power, as given by:  
         G   sat     =     exp        {         g   R       α   p            [       P        (   0   )       -       S        (   0   )              λ   s       λ   p            (           G   0     -   1       ln                   (     G   0     )         -   1     )         ]       }                             
 
         [0048]    where G sat  is the saturated net Raman gain, g R  is the Raman gain coefficient, α p  is the fiber attenuation coefficient at the Raman pump wavelength, P(0) is the Raman pump power  530  at the entrance to fiber span  522 , S(0) is the power of optical signal  532  at the entrance to fiber span  522 , λ s  is the wavelength of optical signal  532 , λ p  is the wavelength of Raman pump power  530 , and G 0  is the unsaturated Raman gain in linear units. The ratio, λ s /λ p  is typically approximately 1.07, and the maximum spectral gain,  
         (           G   0     -   1       ln                   (     G   0     )         -   1     )     ,                         
 
         [0049]    which comprises the second proportionality constant depends strongly on G 0 , the unsaturated Raman gain in linear units. Consequently, in the embodiment of the invention illustrated in FIG. 5, Raman gain control is implemented based on measured optical signal fraction  534 . At system turn-up, Raman pump power  530  is set to a value P pump,0  which yields the desired Raman gain. P pump,0  is the value of Raman pump power  530  when the amplifier is unsaturated by optical signal  532 . During operation, the power of optical signal  532  at the input of fiber span  522  P s,in  is measured by optical power meter  528 . The Raman gain control unit continually adjusts Raman pump laser  520  so that Raman pump power  530  satisfies P pump =P pump,0 +kP s,in , where  
       k   =         λ   s       λ   p              (           G   0     -   1       ln                   (     G   0     )         -   1     )     .                             
 
         [0050]    The power ratio, L s , between the input of fiber span  522  and the power at the optical power meter  528  needs to be taken into account. Likewise any pump loss, L p , between the pump power reference point and the input to fiber span  522  needs also to be considered. L s  and L p  may be determined at system turn-up and programmed into Raman gain control unit  510 . Therefore, including these losses, the pump adjustment needs to yield: P pump =P pump,0 +k*P s,in /L s L p .  
         [0051]    [0051]FIG. 6 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  610  of transmitting Raman pump power  530  in fiber span  522  at a value P pump,0  which yields the desired Raman gain when the amplifier is not saturated because the power of the optical signal  532  is zero. The method further comprises a second step  612  of counter-propagating optical signal  532  in fiber span  522 . Step  614  of the method entails measuring during operation, the signal input power  534 , P s,in . Step  616  of the method entails continually adjusting the Raman pump power  530  to a value  
         P   pump     =         P     pump   ,   0       +     k   *     P     s   ,   in                       wher      e                   k       =         λ   s       λ   p              (           G   0     -   1       ln                   (     G   0     )         -   1     )     .                               
 
         [0052]    In FIG. 7 is a block diagram of an alternate embodiment of this invention in which the Rayleigh backscatter  736  portion of the Raman pump power  530  is used to control the Raman gain. The alternate embodiment of FIG. 7 is comprised of Raman pump laser  520  that is optically coupled to a wavelength selective optical coupler  524 . Wavelength selective coupler  524  is further optically coupled to fiber span  522 . The apparatus also comprises optical tap  526 , optical power meter  528 , and a Raman gain control unit  510 .  
         [0053]    Also shown in FIG. 7 is the flow of optical energy in the apparatus, including Raman pump power  530  and optical signal  532 . Rayleigh backscatter  736  is generated in the fiber span, and may be used as a control signal for Raman gain control. Rayleigh backscatter signal fraction  738  is also shown in FIG. 7. Raman pump power  530  is generated by Raman pump laser  520 . Optical signal  532  is, in a preferred embodiment, voice or data traffic that is being transmitted from one location to another. Raman pump power  530  is coupled into optical fiber span  522  via wavelength selective optical coupler  524 . In the arrangement, Raman pump power  530  copropagates with optical signal  532 . Optical tap  526  samples a fraction of the Rayleigh backscatter and directs Rayleigh backscatter signal fraction  738  onto optical power meter  528 . It should be noted that the exact arrangement of the apparatus may be modified to achieve the same functionality.  
         [0054]    In reference to operation of FIG. 7, an optical signal  532  of high power saturates and attenuates Raman pump power  530 , which is injected from the signal output fiber end. Thus, less Raman pump power penetrates into the fiber and less Rayleigh backscatter  736  is produced. Thus the power level of Rayleigh backscatter is a good measure of Raman gain saturation. The relationship between the power level of Rayleigh backscatter  736  and net Raman gain is nearly independent of gain saturation. Hence, in this invention, the Raman gain is controlled by adjusting the level of Raman pump power  530  such that the level of Rayleigh backscatter  736  is kept constant, or, measured Rayleigh backscatter signal fraction  738  is kept constant. The difference in signal level between Rayleigh backscatter  736  of the Raman pump and Rayleigh backscatter signal fraction  738  does not effect gain control accuracy. Further, no information on fiber loss parameters or scattering parameters is required by Raman gain control unit  510 .  
         [0055]    In some embodiments there will be deleterious reflections of the Raman pump power  530  at the input to fiber span  522 , or in the first few meters of fiber span  522 . If this return loss for Raman pump power  530  is known to be r pump , the measured Rayleigh backscatter  736  is corrected by a term r pump *P pump  such that the power P back −r pump *P pump  is kept constant. In these expressions P pump  is the power level of Raman pump power  530 , and P back  is the power level of Rayleigh backscatter  736 .  
         [0056]    Further gain control accuracy will be achieved if a loop-back factor k is determined upon calibration such that Raman gain control unit  510  works to keep the power P back +k*P pump  constant. In a preferred embodiment, K is approximately −0.00002.  
         [0057]    In FIG. 8 is a flow chart illustrating a method of Raman gain control in accordance with another aspect of the invention. The method comprises a first step  810  of transmitting Raman pump power  530  in fiber span  522  at a value P pump . The method further comprises a second step  812  of counter-propagating optical signal  532  in fiber span  522 . Step  814  of the method entails measuring during operation, the Rayleigh backscatter signal power, P back . Step  816  of the method entails continually adjusting the Raman pump power  530  to keep the power P back +k*P pump  constant.  
         [0058]    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.