Abstract:
A bidirectional optical amplifier amplifies optical signals having signal wavelength and signal power input from two directions. The amplifier is arranged so that two counter-propagating signals pass through a first pumped rare earth doped pre-amplifier before passing through other amplifiers downstream. Optical circulators route the two counter propagating signals so that they both pass through in a counter-propagating manner through subsequent pumped rare earth doped amplifiers downstream.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present invention claims priority from U.S. Provisional Patent Application No. 62/308,301, filed Mar. 15, 2016, which is incorporated herein by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to optical amplifiers, and in particular to low noise-figure (NF) bidirectional optical amplifiers and methods of suppressing the NF of bidirectional optical amplifiers. 
       BACKGROUND OF THE INVENTION 
       [0003]    In a wavelength division multiplexing (WDM) optical transmission system, optical signals at a plurality of wavelengths are encoded with digital streams of information. These encoded optical signals, or optical channels, are combined together and transmitted through a series of spans of an optical fiber comprising a transmission link of a WDM fiber-optic network. At a receiver end of the transmission link, the optical channels are separated, whereby each optical channel can be detected by an optical receiver. 
         [0004]    While propagating through an optical fiber, light tends to lose power. Yet some minimal level of optical channel power is required at the receiver end to decode information that has been encoded in an optical channel at the transmitter end. To boost optical signals propagating in an optical fiber, optical amplifiers are deployed at multiple locations, known as nodes, along the transmission link. The optical amplifiers extend the maximum possible length of the link, in some instances, from a few hundred kilometers to several thousand kilometers, by amplifying optical signals to power levels close to the original levels of optical power at the transmitter end. 
         [0005]    An erbium-doped fiber amplifier (EDFA) is one of the most practical types of optical amplifiers employed in many modern fiber-optic networks. A single EDFA module can amplify up to about a hundred of optical channels at a time, thus providing significant cost savings. 
         [0006]    It is well known that a bidirectional fiber optical communication system will have about 50% of cost saving, so a bidirectional optical amplifier is highly desired. Bidirectional optical amplifiers are also useful in making an amplifier array commonly known as an arrayed amplifier since a bidirectional amplifiers effectively function as two amplifiers. 
         [0007]    Bidirectional optical fiber amplifiers are well known; they pass optical signals therethrough in two opposing or counter-propagating directions. In practice, what makes an EDFA not practicable in many DWDM systems is a very high input power dependent noise figure (NF). In instances where the input power from one direction in a bi-directional amplifier is much higher than the input power from the opposite direction, the NF of the signal with lower input power will be very high and often not acceptable. 
       SUMMARY OF THE INVENTION 
       [0008]    In accordance with the invention a bidirectional optical amplifier is provided for amplifying optical signals having signal wavelength and signal power input from two directions, the optical amplifier having amplifier gain for the two directions, comprising: 
         [0009]    optical gain mediums including a first optical gain medium doped with rare earth elements; 
         [0010]    one or more optical pumps for pumping the optical gain mediums with pump light, whereby optical gain at the signal wavelength is created in the optical gain medium when the optical signals counter-propagate therethrough in the presence of pump light; 
         [0011]    two optical circulators configured to direct two input beams through the first optical gain medium in opposite directions so that they counter-propagate through the first optical gain medium and configured to direct light amplified by the first optical gain medium to one or more other of the gain mediums from opposite directions after being amplified by the first optical gain medium. 
         [0012]    In accordance with the invention there is further provided, a bidirectional optical amplifier having at a plurality of bi-directional amplifying stages, wherein one of the plurality of bi-directional amplifying stages is a bi-directional pre-amplifying stage having two ports, each port for receiving and transmitting light received from the other port, configured such that light passing through the pre-amplifying stage from either direction is first amplified by the pre-amplifying stage and subsequently amplified by a subsequent optical amplifier, or a plurality of optical amplifiers arranged in series, optically coupled to both ports of the pre-amplifying stage. 
         [0013]    In accordance with another aspect of the invention there is provided, method of amplifying a first and a second optical signal so as to lessen a difference in noise-figure that would otherwise be associated with counter-propagating signals having a significantly different power levels through multiple series amplifying stages, comprising: 
         [0014]    launching the first optical signal into a first end of an optical fiber having a first actively pumped span of rare earth doped optical fiber; 
         [0015]    launching the second optical signal into a second end of the optical fiber and allowing the two signals to pass through the actively pumped span of rare earth doped optical fiber; 
         [0016]    only after the first and second signal have been amplified by counter-propagating through the actively pumped span of erbium doped optical fiber, so as to be pre-amplified, counter-propagating the first and second pre-amplified signals through a second amplifying stage. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    Exemplary embodiments will now be described in conjunction with the drawings in which: 
           [0018]      FIG. 1  is an optical schematic of the prior-art bidirectional amplifier; 
           [0019]      FIG. 2A  Gain distribution along the fiber length. 
           [0020]      FIG. 2B  is the noise figure (NF) of prior-art bidirectional amplifier in the case that power of input signal from left end is higher than power from right end; 
           [0021]      FIG. 3  is a block diagram of a bidirectional twin optical amplifier according to the present invention; 
           [0022]      FIG. 4  is the NF of a bidirectional twin optical amplifier according to the present invention; 
           [0023]      FIG. 5  is another block diagram of a bidirectional twin optical amplifier according to the present invention; 
           [0024]      FIG. 6  is another block diagram of a bidirectional twin optical amplifier according to the present invention; 
           [0025]      FIG. 7  is another block diagram of a bidirectional twin optical amplifier according to the present invention; 
           [0026]      FIG. 8  is another block diagram of a bidirectional twin optical amplifier according to the present invention; Gain for the two directions can be different; and, 
           [0027]      FIG. 9  is another block diagram of a bidirectional twin optical amplifier according to the present invention; Gain for the two directions can be different. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0028]    While the present teachings are described in conjunction with various embodiments and examples, it is not intended that the present teachings be limited to such embodiments. On the contrary, the present teachings encompass various alternatives, modifications and equivalents, as will be appreciated by those of skill in the art. 
         [0029]    Referring now to  FIG. 1  a conventional bidirectional amplifier is shown having two optical circulators  102   a  and  102   b  having input ports  101   a  and  101   b , and having output ports  103   b  and  103   a  respectively. Two rare earth doped spans of optical fiber  108  and  110 , preferably erbium doped, are shown between the optical circulators and pumps  104   a  and  104   b  and WDM filters  106   a  and  106   b  allow pump light to be combined with signal light input into input ports  101   a  and  101   b . A gain flattening filter  109  is disposed between the erbium-doped spans of optical fiber  108  and  110 . The gain distribution along the erbium-doped fiber is highly dependent on the input power level of light launched into ports  101   a  and  101   b . If the input power from the two sides of the optical circuit is essentially the same and the pump power is the same, the gain distribution along the fiber will be substantially symmetric. However, in the instance where input power from one end is significantly higher than input power from the other end, gain distribution along the fiber is mainly determined by the higher power signal and becomes similar to a unidirectional amplifier. This can be readily understood when referencing  FIG. 2A , which illustrates the gain distribution where the input power from left end entering port  101   a  is higher than the input power from the right end entering port  101   b . It is well known a signal having higher gain launched into an optical amplifier, results in lower noise figure (NF) at the output of the amplifier and conversely a signal having low gain launched into the optical amplifier results in an amplified signal with higher NF. This can be readily understood in  FIG. 2B . Of course it is undesirable to have a high NF for signal with low power. 
         [0030]    It is well known that a signal launched into an optical amplifier from the end having higher gain, will have lower noise figure (NF) at the output of the amplifier and conversely a signal launched into the optical amplifier from the end having low gain will have higher NF. This can be readily understood in  FIGS. 2A and 2B . Of course it is undesirable to have a high NF for signal with low power. 
         [0031]    Referring now to  FIG. 3 , a block diagram of an optical amplifier of the present invention is shown. The optical amplifier is a cascading of two single-stage bidirectional amplifiers where the length of the erbium doped fiber between the two input ends is much shorter than shown in  FIG. 1 ; it is easy to find from  FIG. 2A  that gain at beginning for both ends is high enough to achieve a low noise figure. In  FIG. 3  light  301  is launched in to optical circulator  302  on a first port and simultaneously light  310  is launched into a first port of optical circulator  311  at an opposite end of the amplifier. In operation light circulates from port  1  of circulator  302  to port  2  shown as  303  and is directed to a short span of pumped erbium doped fiber  307  passing through WDM  305  and  308  on route to optical circulator  311 . Co-propagating light  310  launched into optical circulator  311  is directed out of port  2  denoted as  309  and passes through WDM  308 , EDFA  307  and WDM  305  entering port  303  of circulator  302 . LD  306  provides pump light to amplify light passing through EDFA  307 . Port  2  denoted as  304  of circulator  302  carries light  310  that was launched into circulator  311  and pre-amplified by EDFA  307  and this light traverses the lower branch consisting of WDM  317 , EDFA  316 , GFF  315 , EDFA  314 , WDM  313  and entering port  3  of optical circulator  311 . Counter-propagating light traverses the same path arriving from the opposite direction. Pump light is generated at LD  319  is split at power-splitter (PS)  318  and is directed to WDM  317  and  313 . Optical circulators  302  and  311  ensure that input signals at opposite ends of the amplifier are first pre-amplified by EDFA  307 . This results in a similar noise figure for both signals regardless of their difference in power at input ends of the amplifier; then the pre-amplified signals are further amplified by  316  and  314 . The length of the short span of doped ED fiber should preferably be less than 4 meters relative to an EDF doped density of about 7 dB/m absorption. Alternatively, the length can be described as short enough to make the gain of the pre-amplifying stage less than 10 dB even with a saturation pump. 
         [0032]      FIG. 4  is a table illustrating the Gain (dB) and NF (dB) of the input signals. 
         [0033]    Referring now to  FIG. 5 , an amplifier similar to that of  FIG. 3  is shown having a more economical pumping scheme. WDM  308  provides a means to tap off excess unabsorbed 980 nm pump light and provides this light to a 50:50 power splitter  318 . WDMs  317  and  313  receive the pump light from the power splitter  318  and multiplex the pump light with incoming signal light to excite the two EDFAs  314  and  316  so that the signal light is amplified. Circulators  310  and  302  work in a similar manner as  310  and  302  described above. 
         [0034]      FIG. 6  shows alternative embodiment wherein the lower branch, which includes EDFAs  313 , and  314  are absent pumps shown in  FIG. 3 . In  FIG. 6 , the EDFs in lower branch are not pumped. Typically there is almost always some un-pumped region at output end of a pumped erbium doped fiber. However in the very short length of EDF  307 , this may not be the case, as is shown in  FIG. 5 . However, in  FIG. 6  the un-pumped erbium doped fiber lengths serve to balance the gain spectrum, thereby making the gain of higher than the lower wavelengths being amplified. Because the un-pumped EDFs  316  and  314  absorb shorter wavelengths and amplify longer wavelengths the gain is substantially balanced. 
         [0035]    Referring now to  FIG. 7  a bidirectional EDFA similar to that of  FIG. 3  is shown. The difference however is that the two 4-port circulators  302  and  311  shown in  FIG. 3  have been replaced with four, 3-port circulators  702   a ,  702   b ,  711   a  and  711   b.    
         [0036]      FIG. 8  shows a modification to  FIG. 7  wherein an attenuator  801  is disposed between the 3-port circulators  711   a  and  711   b . In operation only the signal entering the amplifier at the input port of  702   a  becomes attenuated. This would be used in a situation where there was pre-knowledge that that input signal required attenuation. 
         [0037]      FIG. 9  is an illustration of an embodiment, which is similar to  FIG. 8 , however a 4-port circulator  302  is used with two 2-port circulators  711   a  and  711   b.    
         [0038]    The foregoing description of one or more embodiments of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto.