Abstract:
An improved method is provided for reducing four-wave mixing induced degradation in an optical data signal traversing an optical transport network that uses Raman amplifiers. The method includes: introducing at least two Raman amplifiers into the optical transport network, each Raman amplifier having a laser source, and synchronously modulating the laser sources of the Raman amplifiers, thereby reducing four-wave mixing in the optical data signal. Modulation of the laser sources is performed at such a rate that crosstalk from the modulation to the optical data signals experiencing the Raman gain is minimized.

Description:
[0001]    This application claims priority under 35 U.S.C. §119(e) to U.S. Provisional Application No. 60/315,192 filed on Aug. 27, 2001 and entitled “FWM REDUCTION IN RAMAN AMPLIFIERS” the specification and drawings of which are hereby expressly incorporated by reference. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates generally to a method for reducing four-wave mixing induced degradation in an optical data signal traversing an optical transport network and, more particularly, to a method for reducing four-wave mixing caused by Raman amplifiers operating in long haul, wavelength division multiplexing optical transport networks.  
         BACKGROUND OF THE INVENTION  
         [0003]    Optical fibers are used to carry optical signals in an optical transport network. However, optical signals may suffer from attenuation (i.e., loss of power) as they travel over long distances. For long fiber spans, the optical signal may be so attenuated that it no longer can be accurately or reliably detected. If the optical signal is to travel further, it must be amplified. Optical amplifiers are employed at different points along the signal path in order to reconstitute the attenuated optical signal, thereby expanding the effective reach of an optical transport network.  
           [0004]    Of particular interest, Raman amplifiers may be used to amplify optical signals traversing through long haul optical transport networks. Here, pump sources are used to introduce intense pump light into the transmission fiber to achieve gain via the non-linear Raman effect. In order to achieve wide gain bandwidth, multiple pump laser sources operating at different wavelengths are typically used at a given amplification site.  
           [0005]    Four-wave mixing has been a persistent problem in the drive to develop dense wavelength division multiplexing (DWDM) optical transport networks. When multiple wavelengths pass through the same optical fiber, they may interact with each other to generate an additional lightwave. This is known as four-wave mixing. In the context of DWDM optical systems, the additional lightwave will likely interfere with an existing signal channel, thereby causing signal degradation within the system. The efficiency of the FWM product generation depends on many factors, including the fiber dispersion at wavelengths of the interacting light signals, and the polarization orientation of the interacting wavelengths.  
           [0006]    Raman amplifiers operating in such DWDM optical transport networks may contribute to the four-wave mixing problem. For instance, the pump lasers associated with a Raman amplifier designed to give gain in the L-band wavelength region (e.g., 1570-1610 nm) may produce four-wave mixing products in the C-band wavelength region (e.g., 1520-1565 nm). The four-wave mixing products in turn cause degradation of the optical data signals residing in the C-band wavelength region. Likewise, Raman amplifiers operating in both L-band and C-band wavelength regions may produce four-wave mixing products in both L-band and C-band wavelength data regions. In the past, this problem did not exist because C and L bands were not combined together in one optical fiber or, if combined, Raman amplifiers have not been used.  
           [0007]    Therefore, it is desirable to provide a method for reducing four-wave mixing caused by Raman amplifiers operating in long haul, wavelength division multiplexing optical transport networks.  
         SUMMARY OF THE INVENTION  
         [0008]    In accordance with the present invention, an improved method is provided for reducing four-wave mixing induced degradation in an optical data signal traversing an optical transport network that uses Raman amplifiers. The method includes: the presence of at least two Raman pump laser sources operating at different wavelengths that create four wave mixing products in an optical fiber span, and synchronously modulating the laser sources in such a way that at any time, in any section of the fiber span, all of the pump wavelengths required to produce the FWM products are not present at once, thereby preventing the four-wave mixing process. At the same time, pump modulation is performed at such a high rate that crosstalk from the modulation to the optical data signals experiencing the Raman gain is minimized.  
           [0009]    For a more complete understanding of the invention, its objects and advantages, reference may be had to the following specification and to the accompanying drawings.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    [0010]FIG. 1 is a block diagram depicting an exemplary Raman amplifier site integrated into an optical fiber of an optical transport network;  
         [0011]    [0011]FIG. 2 is a graph illustrating how to synchronously modulate two Raman laser sources in accordance with the present invention;  
         [0012]    FIGS.  3 A- 3 C are graphs illustrating different control schemes that may be employed to synchronously modulate three Raman laser sources in accordance with the present invention;  
         [0013]    [0013]FIG. 4 is a graph illustrating how polarization modulation may be used to in conjunction with synchronous modulation to further minimize operational overlap between two Raman laser sources in accordance with the present invention;  
         [0014]    [0014]FIG. 5 is graph illustrating how a tunable Raman laser source may be used to reduce four-wave mixing in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0015]    [0015]FIG. 1 illustrates an exemplary Raman amplification site  10  integrated into an optical fiber  12  of an optical transport network. In this example, two Raman amplifiers  14  are used to amplify an optical data signal traversing through the optical fiber  12 . Each Raman amplifier  14  may include one or more pump laser sources  16 . Each pump laser source  16  is operable to introduce intense pump light into the fiber as is well known in the art. The intense pump light excites vibrational modes in the atoms that form the fiber. The energy in the vibrational excitations amplifies the optical data signal in a process referred to as “stimulated Raman scattering”. It is understood that the pump laser sources may operate at the same or different wavelengths to achieve the desired Raman gain. Although two Raman amplifiers are shown in FIG. 1, it is readily understood that the amplification site  10  may include three or more Raman amplifiers. In addition, two or more Raman amplifiers may be combined to form a Raman amplifier module.  
         [0016]    A controller  20  is connected to each of the pump laser sources  16  residing in the amplification site  10 . A laser diode preferably serves as a pump laser source. The laser diode is driven by a current source as is well known in the art. However, it is envisioned that other types of pump laser sources are also with the scope of the present invention. As will be more fully described below, the controller  16  is operable to modulate the operation of two or more of the pump laser sources  16 , thereby reducing four-wave mixing in the optical data signal.  
         [0017]    In accordance with the present invention, at least two pump lasers sources are synchronously modulated in order to reduce four-wave mixing in the optical data signal. Rather than introduce a continuous wave from each of the Raman pump sources into the optical fiber, the light intensity output by the two pump laser sources is varied over time. In a preferred embodiment, the pump laser sources are “pulsed” on and off, such that the light pulses generated by each of the Raman pump laser sources are separated in time as shown in FIG. 2. The pump light introduced into the fiber by a first Raman pump laser source is shown at  22 ; whereas the pump light introduced into the fiber by a second Raman pump laser source is shown at  24 . In order to “pulse” the pump laser sources, the controller may preferably turn on and off the current source driving the laser diode. Although this technique is presently preferred, it is envisioned that other techniques may be suitable used to vary the light intensity output by the pump laser sources. For instance, the light intensity from a pump laser source may be modulated external to the pump laser source through the use of a variable optical attenuator.  
         [0018]    Separating light pulses generated by each of the Raman pump laser sources in time and space reduces and in some instance may eliminate the four-wave mixing products degrading the optical data signal. In the preferred embodiment, four-wave mixing is controlled by modulating the intensity of the light “pulses” generated by the Raman amplifiers. As will be apparent to one skilled in the art, the modulation rate and duty cycle of the light “pulses” required to minimize crosstalk from pump modulation to the optical data signals depends on various parameters, including the relative propagation directions of the data and pump lasers (i.e., co- or counter-propagating Raman pumps). It is to be understood that other techniques may be employed to minimize the overlap between the light pulses generated by each of the Raman pump laser sources.  
         [0019]    Different control schemes may be employed for amplification sites having three or more Raman pump laser sources. FIGS. 3A, 3B and  3 C illustrate some exemplary control schemes that may be employed by such amplification sites. In FIG. 3A, each of the three Raman pump laser sources are pulsed on and off, such that such that the light pulses generated by each of the Raman pump laser sources are separated in time. In FIG. 3B, four-wave mixing products are caused by only two of three Raman pump laser sources. In this case, the two Raman pump laser sources causing the four-wave mixing product may be synchronously modulated as described above, while a third Raman pump laser source operates in continuous wave mode. In the case of degenerate four-wave mixing products, two Raman pump laser sources are synchronously modulated in phase, while a third Raman pump laser source is modulated in antiphase as shown in FIG. 3C. It is to be understood that other control schemes designed to reduce four-wave mixing by synchronously modulating two or more Raman pump laser sources are also within the scope of the present invention.  
         [0020]    Although intensity modulation is a presently preferred technique for minimizing the operational overlap between Raman pump laser sources, this is not intended as a limitation on the broader aspects of the present invention. For example, polarization modulation may also be used to minimize the efficiency of four-wave mixing process between Raman pump laser sources. Polarization modulation involves changing the polarization of the light “pulses” generated by the Raman amplifiers. One skilled in the art will readily recognize that various techniques may be employed to vary the polarization of the light “pulse” generated by the Raman amplifiers, for example, by using a Faraday glass polarization rotator with modulated magnetic field.  
         [0021]    Polarization modulation is preferably used in conjunction with intensity modulation to further reduce and/or eliminate four-wave mixing degrading an optical data signal as shown in FIG. 4. In this example, at least some of the light “pulses” are polarized in a different orientation, thereby further minimizing the operational overlap between the Raman pump laser sources. However, it is to be understood that polarization modulation may also be employed independently from intensity modulation in order to reduce four-wave mixing products that may be caused by the interaction of two or more Raman pump laser sources.  
         [0022]    Tunable pump laser sources enable another technique for reducing four-wave mixing caused by the interaction of two or more Raman pump laser sources. Tunable pump laser sources allow the operational wavelength of the pump laser to be offset from the dispersion zero wavelength of the optical fiber, thereby reducing the efficiency of the formation of four-wave mixing products. In an alternative control scheme illustrated in FIG. 5, the operational wavelength of one tunable pump laser is modulated to quickly sweep across the range of wavelengths (e.g. Δλ1, Δλ2 and Δλ3) normally occupied by the multiple pump wavelengths required to achieve the broadband Raman gain. By using only one Raman pump laser source in this way the formation of four-wave mixing products is eliminated. Once again, the modulation rate required to minimize crosstalk from pump modulation to the optical data signals depends on various parameters, including the relative propagation directions of the data and pump lasers  
         [0023]    Likewise, the probability and/or level of four-wave mixing is also reduced when two or more Raman pump laser sources are quickly swept across the operating region. For a given Raman amplifier module, the number of tunable lasers required will depend on the tuning range available and the wavelength range to be amplified by the amplifier. In the case two or more tunable lasers are employed by a given Raman amplifier module, each laser instantaneous frequency could be synchronized in order to avoid the generation of four-wave mixing products in wavelength ranges that carry network traffic. Tunable lasers embodied in different Raman amplifier modules may be synchronized in a similar manner in order to reduce four-wave mixing products.  
         [0024]    While this invention has been described in its presently preferred form, it will be understood that the invention is capable of modification without departing from the spirit of the invention as set forth in the appended claims.