Abstract:
An optical amplifier includes a plurality of optical paths each carrying an optical signal and each including active optical fiber. A shared pump laser is coupled to the plurality of optical paths and provides pump power to the plurality of optical paths to individually amplify the optical signals. The plurality of optical paths includes input and output optical isolators and a coupler for coupling the pump power to the optical path. The active optical fiber is doped with an implant selected from the group of rare earth metals, erbium, ytterbium, and both ytterbium and erbium. A variable attenuator can be connected between the pump laser and the coupler of at least one of the plurality of optical paths or adjacent to the output isolator of one of the optical paths. Another optical amplifier serially couples optical signals to the optical path on a common gain media.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to fiber optics, and more particularly to rare earth doped fiber optic amplifiers.  
         BACKGROUND OF THE INVENTION  
         [0002]    Wavelength division multiplexing (WDM) or dense WDM (DWDM) communication systems increase the transmission capacity of an optical fiber in a communication system by combining several optical signals having different wavelengths onto the optical fiber. Generally these systems include an optical amplifier with a pump laser that provides pump power and rare earth doped optical fiber. The optical signals are coupled to the rare earth doped fiber and the pump power and rare earth doped fiber amplify the optical signals. These amplifiers may also include input and output isolators and gain flattening filters.  
           [0003]    To render these systems practical, the optical amplifiers must meet tight gain uniformity specifications over all channel wavelengths. Optical fibers often contain  80  channels at  80  wavelengths. The intrinsic gain spectrum of a rare earth doped fiber amplifier is highly non-uniform. Therefore, there are slight variations in the amount of amplification that is provided by the optical amplifier at the different wavelengths. The gain flattening filters flatten the gain profile of the channels after amplification.  
           [0004]    The gain profile of the fiber amplifier is complicated by other factors as well. The gain profile of a rare earth doped optical fiber amplifier (OFA), such as an erbium doped fiber amplifier (EDFA), is determined by the average inversion level of the erbium ions in the erbium doped optical fiber. This inversion level is a function of the power level of the signal or signals to be amplified and the applied power levels of the laser pump sources. If the signal power is sufficiently lower than the applied pump power, the fiber will maintain close to 100% inversion and the signal gain and the amplifier gain profile will not appreciably change with changes in input signal powers. However, as signal power increases, the signal gain in the amplifier becomes limited by the availability of pump power for the fiber amplifier. In other words, the output signal power is limited by the available pump power and the signal input power levels.  
           [0005]    As a result of these complex design considerations, rare earth doped fiber optical amplifiers are very expensive. The design and cost of optical switch fabrics is becoming more critical to optical system architectures. As the transparency of the optical network increases, the need for switching and variable optical add/drops (VOAD) will also increase. In other words, adding and dropping individual channels will become more common. The insertion loss from the switch fabric typically varies between 5 and 15 decibels (dB). The optical amplifiers that are associated with VOAD need to compensated for this insertion loss. However, the optical amplifiers described above will be too costly to implement or duplicate for individual channels. Therefore, optical amplifiers that can amplify individual channels and that are economical to implement are needed.  
         SUMMARY OF THE INVENTION  
         [0006]    An optical amplifier according to the invention includes a plurality of optical paths each carrying an optical signal and each including active optical fiber. A shared pump laser is coupled to the plurality of optical paths and provides pump power to the plurality of optical paths to individually amplify the optical signals.  
           [0007]    In other features of the invention, the plurality of optical paths include input and output optical isolators and a coupler for coupling the pump power to the optical path. The active optical fiber is doped with an implant selected from the group of rare earth metals, erbium, ytterbium, and both ytterbium and erbium. A variable attenuator is connected between the pump laser and the coupler of at least one of the plurality of optical paths or adjacent to the output isolator of one of the optical paths.  
           [0008]    In another aspect of the invention, an optical amplifier includes an optical path including a plurality of active optical fiber sections. A laser pump is coupled to the optical path and provides pump power on the optical path. A plurality of optical signals are serially coupled to the optical path at the active optical fiber sections, amplified, and decoupled from the optical path.  
           [0009]    In other features of the invention, the optical paths include input and output optical isolators and a coupler for coupling the pump power to the optical path. The active optical fiber is doped with an implant selected from the group of rare earth metals, erbium, ytterbium, and both erbium and ytterbium.  
           [0010]    Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein:  
         [0012]    [0012]FIG. 1 illustrates an optical amplifier that amplifies optical signals in parallel according to the prior art;  
         [0013]    [0013]FIG. 2 illustrates the optical amplifier of FIG. 1 in further detail;  
         [0014]    [0014]FIG. 3 illustrates a shared pump optical amplifier according to the present invention;  
         [0015]    [0015]FIGS. 4A and 4B illustrate first and second alternate shared pump optical amplifiers according to the present invention that amplify optical signals in series on a common gain media;  
         [0016]    [0016]FIG. 5 illustrates a third shared pump optical amplifier;  
         [0017]    [0017]FIG. 6 illustrates the shared pump optical amplifier of FIG. 3 with a variable attenuator located between the pump laser and the coupler; and  
         [0018]    [0018]FIG. 7 illustrates the shared pump optical amplifier of FIG. 3 with a variable attenuator located between the output optical isolator and the output connector. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0019]    The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses.  
         [0020]    Referring now to FIG. 1, an optical fiber amplifier  10  according to the prior art is shown. The optical fiber amplifier  10  amplifies multiple optical signals having different wavelengths in parallel. The optical fiber amplifier  10  includes an input connector  12 , such as an APC connector, that is coupled to one end of an optical fiber  14 . The optical fiber  14  can be active or passive optical fiber. As used herein, active optical fiber refers to rare earth doped optical fiber or other optical fiber that provides optical signal amplification when used with a pump laser. Passive optical fiber, on the other hand, does not provide amplification when used with a pump laser. The optical fiber  14  is connected to a coupler  16 . A pump laser  18  is connected to the coupler  16 . An output of the coupler  16  is connected to an active optical fiber  20  that preferably has an output connector  24 , such as an APC connector. The active optical fiber  20  is preferably an optical fiber that is implanted with a rare earth dopant such as erbium (Er) or ytterbium (Yb) or co-doped with Er and Yb. The coupler  16  is preferably a WDM coupler, a fused tapered coupler or any other suitable optical coupler. The input optical signal contains multiple optical signals having different wavelengths (represented by λ 1-n ).  
         [0021]    Referring now to FIG. 2, a multi-wavelength optical fiber amplifier  10 ′ according to the prior art is shown with additional components that provide increased functionality. For purposes of clarity, reference numerals from FIG. 1 will be used where appropriate to identify similar elements. An input tap  30  is connected to the input connector  12  using a passive or active optical fiber  32 . The input tap  30  is connected to a diode  36  such as a photodiode. The input tap  30  reflects a small portion of the input optical signal to the diode  36  and passes the remaining optical signal to the optical fiber  42 . The input tap  30  and the diode  36  are used to quickly verify that the input optical signal exists.  
         [0022]    An input optical isolator  40  is connected to an output of the input tap  30  by an active or passive optical fiber  42 . An output of the optical isolator  40  is connected to an input of the coupler  16  by an active or passive optical fiber  44 . The active optical fiber  20  is connected to an output optical isolator  46 . The output optical isolator  46  is connected by an active or passive optical fiber  48  to a gain-flattening filter  50 . The gain-flattening filter  50  provides compensation for the variable gain of the active optical fiber  20  as a function of wavelength, optical signal power, and pump power. An output of the gain-flattening filter  50  is connected by an active or passive optical fiber  50  to an output tap  52 . The output tap  52  is connected to a diode  54  such as a photodiode and the output connector  24 . The output tap  52  reflects a small portion of the output optical signal to the diode  54  to verify the existence of the output optical signal.  
         [0023]    As can be appreciated by skilled artisans, the optical amplifiers  10  and  10 ′ are used to amplify multiple optical signals having different wavelengths in parallel. As the number of wavelengths increases, the power of the pump laser  18  must also be increased and the increased pump laser specifications significantly increase the cost of the pump laser  18 . Furthermore, the amplifiers  10  and  10 ′ are not suitable from a cost standpoint for VOAD applications.  
         [0024]    Referring now to FIG. 3, a shared pump optical amplifier  100  is shown. In FIGS.  3 - 7 , solid lines indicate active optical fiber and dotted lines indicate passive optical fiber. As can be appreciated, active optical fiber can be substituted in place of passive optical fiber in FIGS.  3 - 7  if desired. A first optical signal having a first wavelength is carried by an active or a passive fiber  102 - 1  and is input to an input optical isolator  104 - 1 . The active or passive optical fiber  102 - 1  is preferably attached to an input connector (not shown). An active or passive optical fiber  108 - 1  connects the input optical isolator  104 - 1  to a coupler  110 - 1 . An active optical fiber  112 - 1  connects the coupler  110 - 1  to an output optical isolator  114 - 1 . An active or passive optical fiber  116 - 1  is preferably connected to an output connector (not shown). The input and output connectors can be APC connectors.  
         [0025]    Similarly, additional optical signals having different wavelengths are carried by active or passive optical fibers  102 -,  102 - 3 , . . . ,  102 -n and are input to input optical isolators  104 - 2 ,  104 - 3 , . . . ,  104 -n. Active or passive optical fibers  108 - 2 ,  108 - 3 , . . . ,  108 -n connect an output of the input optical isolators  104 - 2 ,  104 - 3 , . . . ,  104 -n to couplers  110 - 2 ,  110 - 3 , . . .  110 -n. Active optical fibers  112 - 2 ,  112 - 3 , . . . ,  112 -n connect the couplers  110 - 2 ,  110 - 3 , . . . ,  110 -n to output optical isolators  114 - 2 ,  114 - 3 , . . .  114 -n. Active or passive optical fibers  116 - 2 ,  116 - 3 , . . . ,  116 -n are preferably connected to output connectors (not shown).  
         [0026]    Inputs of the couplers  110  are connected to a single pump laser  120 . Advantageously, the gain specifications of the pump laser  120  are relaxed as compared with the amplifiers  10  and  10 ′. There is no requirement for the gain-flattening filter or multi-stage access that are generally required in parallel optical amplifiers. The output power requirements are less than 0 dB/m. Preferably, the pump laser  120  is a 980 nm pump laser.  
         [0027]    The cost per channel is typically dictated by the amount of optical fiber. Optical fiber lengths can be made as short as 0.5 m. The absorption of Er/Yb optical fiber is 3-5 times greater than standard Er optical fiber and can be made to be even greater as the output power requirements are less than standard optical fiber. Because the required output power of the amplifier is 0 dB/m and the conversion efficiency is greater than 30%, the required input power can be as low as 3-4 mW or {fraction (1/50)} of the total output power. Two pump laser modules can be used to power a full 80 channel array of optical signals.  
         [0028]    Referring now to FIG. 4A, a shared pump optical amplifier  150  that serially amplifies individual optical signals on a single optical path is shown. A plurality of optical signals having different wavelengths are serially amplified along an optical path  152 . A pump laser  153  provides pump power on the optical path  152 . A first optical signal having a first wavelength is input to an optical isolator  156 - 1 . An output of the optical isolator is connected to a first input coupler  160 - 1  that is also connected to the optical path  152 . An active optical fiber  164 - 1  connects the first input coupler  160 - 1  to a first output coupler  168 - 1 . The first output coupler  168 - 1  is connected to the optical path  152  and an output isolator  170 - 1 . The first optical signal is coupled to the optical path  152 , amplified and then de-coupled from the optical path  152 .  
         [0029]    Other optical signals having second, third, . . . , nth wavelengths are input to input optical isolators  156 - 2 ,  156 - 3 , . . . ,  156 -n. An output of the input optical isolator  156  is connected to input couplers  160 - 2 ,  160 - 3 , . . . ,  160 -n that are also connected to the optical path  152 . Active optical fibers  164 - 2 ,  164 - 3 , . . . ,  164 -n connect the input couplers  160  to output couplers  168 - 2 ,  168 - 3 , . . . ,  168 -n. The output couplers  168  are connected to the optical path  152  and output isolators  170 - 2 ,  170 - 3 , . . .  170 - 4 . The optical signals are serially coupled to the optical path  152 , amplified and then de-coupled from the optical path  152 .  
         [0030]    In FIG. 4A, the optical path  152  includes active and inactive optical fiber. The pump laser  153  provides pump power along the optical path  152 . At a minimum, the active optical fiber is located between the input and output couplers. The remaining portions of the optical path  152  can include passive optical fiber. Alternately, the optical path  152  can include active optical fiber between adjacent pairs of couplers as is illustrated in FIG. 4B.  
         [0031]    Referring now to FIG. 5, an alternate shared pump optical amplifier  200  is illustrated. For purposes of clarity, reference numbers from FIG. 3 are used to identify similar elements. A first optical signal carried by active or passive optical fiber  102 - 1  is input to an input optical isolator  104 - 1 . The active or passive optical fiber  102 - 1  can be connected to an input connector (not shown). An active or passive optical fiber  108 - 1  connects an output of the input optical isolator  104 - 1  to an input coupler  110 - 1 . An active optical fiber  112 - 1  connects the input coupler  110 - 1  to an output coupler  113 - 1 . The output coupler  113 - 1  is connected to an optical isolator  114 - 1 . An active or passive optical fiber  116 - 1  is preferably connected to an output connector (not shown).  
         [0032]    Similarly, additional optical signals having different wavelengths are carried by active or passive optical fibers  102 - 2 ,  102 - 3 , . . . ,  102 -n are input to input optical isolators  104 - 2 ,  104 - 3 , . . . ,  104 -n. Active or passive optical fibers  108 - 2 ,  108 - 3 , . . . ,  108 -n connect an output of the input optical isolators  104 - 2 ,  104 - 3 , . . . ,  104 -n to input couplers  110 - 2 ,  110 - 3 , . . .  110 -n. Active optical fibers  112 - 2 ,  112 - 3 , . . . ,  112 -n connect the input coupler  110 - 1  to output couplers  113 - 2 ,  113 - 3 , . . . ,  113 -n. The output couplers  113  are coupled to output optical isolators  114 - 2 ,  114 - 3 , . . .  114 -n. Active or passive optical fibers  116 - 2 ,  116 - 3 , . . . ,  116 -n are preferably connected to output connectors (not shown).  
         [0033]    The output coupler  113 - 1  is connected to one of the input coupler  110 - 2  (connection not shown) or the output coupler  113 - 2  (connection shown). The other of the input coupler  110 - 2  (shown) or the output coupler  113 - 2  (not shown) is connected to one of the input coupler  110 - 3  (shown) or the output coupler  113 - 3  (not shown).  
         [0034]    Variable attenuators can be added into any of the foregoing optical amplifiers. The variable attenuators can be incorporated into each channel in a traditional manner through the signal source path or incorporated into the pump laser path. For example in FIG. 6, variable attenuators  220  are added between the pump laser  120  and the couplers  110  to perform gain flattening if needed. Alternately, the variable attenuators  220  are added before or after the optical isolators as is illustrated in FIG. 7. Skilled artisans will appreciate that the variable attenuators can be positioned in other locations while providing similar functionality.  
         [0035]    Those skilled in the art can now appreciate from the foregoing description that the broad teachings of the present invention can be implemented in a variety of forms. Therefore, while this invention has been described in connection with particular examples thereof, the true scope of the invention should not be so limited since other modifications will become apparent to the skilled practitioner upon a study of the drawings, the specification and the following claims.