Abstract:
An optical amplifier is disclosed. The amplifier includes an input and an optical splitter adapted to split the input into at least a first band signal portion and a second band signal portion. The first band signal portion includes a first reflector disposed optically downstream from the input, an amplifying gain medium disposed optically downstream from the first reflector, and a second reflector disposed optically downstream from the amplifying gain medium. A first amplifying power source is optically connected to the amplifying gain medium optically upstream from the amplifying gain medium and a second amplifying power source is optically connected to the amplifying gain medium optically downstream from the amplifying gain medium. The first reflector reflects a first light from the amplifying medium back into the amplifying medium and the second reflector reflects a second light from the amplifying medium back into the amplifying medium.

Description:
FIELD OF THE INVENTION  
         [0001]    The present application relates to optical amplifiers that amplify optical signals over the long optical wavelength band of approximately 1565-1620 nanometers.  
         BACKGROUND OF THE INVENTION  
         [0002]    Conventional erbium doped fiber amplifiers (EDFA) have been extensively used in optical telecommunications as means to amplify weak optical signals in the third telecommunication window (near 1550 nm) between telecommunication links. Much work has been done on the design of these amplifiers to provide efficient performance, such as high optical gain and low noise figure. However, with the recent enormous growth of data traffic in telecommunications, owing to the Internet, intranets, and e-commerce, new optical transmission bandwidths are required to provide increased transmission capacity in dense wavelength division multiplexing (DWDM) systems.  
           [0003]    There are a few solutions to this demand. One proposed solution is to utilize new materials compositions as a host for the fiber gain medium (instead of silica), such as telluride, which may provide broader amplification bandwidth (up to 80 nm). However, the non-uniform gain shape and poor mechanical properties of telluride glass make these amplifiers difficult to implement in telecommunication systems. Also, Raman amplifiers can be considered as an alternative solution to high bandwidth demand, since these amplifiers are capable of providing flexible amplification wavelength with a broad bandwidth. However, these amplifiers place restrictions on optical system architectures because of their required designs for efficient performance, such as long fiber length (&gt;1 km), high pump power (&gt;100 mW) and co-pumping configurations. On the other hand, relatively long erbium doped fibers (EDFS) may also provide amplification in the long wavelength range (1565-1620 nm) when they are used with high power pump sources. This range is commonly called “L band”, which can be further subdivided in a 1565-1605 nanometers range and a 1605 nanometers and greater range, which is referred to as “ultra-L band”. The conventional range, currently being used for most commercial applications, also known as “C band”, is in the wavelength range between 1520-1165 nm.  
           [0004]    With the need to increase transmission capacity to accommodate the rapid growth of optical telecommunications, the industry is looking to L band and possibly ultra-L band as solutions to this need. However, in order to amplify L band and ultra-L band signals, multiple amplifiers are currently required. It would be beneficial to provide a single optical amplifier that amplifies a light signal over a large bandwidth encompassing L band and ultra-L band light.  
         BRIEF SUMMARY OF THE INVENTION  
         [0005]    Briefly, the present invention provides an optical amplifier comprising an input and an optical splitter optically connected to the input, the optical splitter being adapted to split the input into at least a first band signal portion and a second band signal portion. The first band signal portion includes a first reflector disposed optically downstream from the input, a first amplifying gain medium disposed optically downstream from the first reflector, and a second reflector disposed optically downstream from the first amplifying gain medium. A first amplifying power source is optically connected to the first amplifying gain medium optically upstream from the first amplifying gain medium and a second amplifying power source is optically connected to the first amplifying gain medium optically downstream from the first amplifying gain medium. The first reflector reflects a first light from the first amplifying medium back into the first amplifying medium and the second reflector reflects a second light from the first amplifying medium back into the first amplifying medium. The amplifier further includes an optical combiner optically connected to the at least first and second band signal portions to form an output. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]    The accompanying drawings, which are incorporated herein and constitute part of this specification, illustrate the presently preferred embodiments of the invention, and, together with the general description given above and the detailed description given below, serve to explain the features of the invention. In the drawings:  
         [0007]    The Figure is a schematic view of an optical amplifier according to a first embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0008]    In the drawings, like numerals indicate like elements throughout. As used herein, when two or more elements are “optically connected”, light may be transmitted between the elements. Further, a second element is “optically downstream” of a first element when a light being transmitted through the first and second elements encounters the first element prior to encountering the second element. Also, “backward” is defined to mean a direction optically from a receiver toward a transmission source and “forward” is defined to mean a direction optically from the transmission source toward the receiver.  
         [0009]    An optical amplifier  100  according to a preferred embodiment of the present invention is shown schematically in the Figure. The optical amplifier  100  is preferably a planar waveguide, although those skilled in the art will recognize that the optical amplifier  100  may also be fiber based. The amplifier  100  includes an input  102  where a signal light λ S  enters the amplifier  100  from a transmission source (not shown). A first optical isolator  110  is optically connected to the input  102  optically downstream from the transmission source. The first optical isolator  110  prevents optical noise from traveling backwards from the amplifier  100  toward the transmission source. An optical splitter  120  is optically connected to the first optical isolator  110 . The optical splitter  120  may be an arrayed waveguide grating (AWG), a wavelength division multiplexer (WDM), or an optical circulator with reflectors, such as optical gratings.  
         [0010]    Preferably, the optical splitter  120  splits the input  102  into two lines, a first signal line  122  and a second signal line  124 . The first signal line  122  is optically connected to a first amplifying gain medium  130 . Preferably, the first amplifying gain medium  130  is a rare earth doped medium, such as a fiber or a planar waveguide. Also preferably, the first amplifying gain medium  130  is approximately fifteen meters long. A first optical multiplexer  132  optically connects an amplifying power source, preferably a first pump laser  134 , to the first amplifying gain medium  130  via a pump line  136 . Preferably, the first pump laser  134  is a 980 nanometer pump laser and has a power of approximately 90 mW, although those skilled in the art will recognize that the first pump laser  134  may be other than 980 nanometers, such as 1480 nanometers, and have a power of other than 90 mW. A reflector  138  is optically disposed in the first signal line  122  between the first optical multiplexer  132  and the first amplifying gain medium  130 . The reflector  138  may be a fiber grating or other reflector designed to reflect predetermined wavelengths. While a single reflector  138  is shown, those skilled in the art will recognize that the reflector  138  may include a plurality of reflectors  138 . Preferably, the reflector  138  is tuned to reflect light having a wavelength of between approximately 1535 to 1560 nm.  
         [0011]    The second signal line  124  is optically connected to a second amplifying gain medium  140 . Preferably, the second amplifying gain medium  140  is a rare earth doped medium, such as a fiber or a planar waveguide. Also preferably, the second amplifying gain medium  140  is approximately sixty meters long. A second optical multiplexer  142  optically connects an amplifying power source, preferably a second pump laser  144 , to the second amplifying gain medium  140  via a pump line  146 . Preferably, the second pump laser  144  is a 980 nanometer pump laser and has a power of approximately 180 mW, although those skilled in the art will recognize that the second pump laser  144  may be other than 980 nanometers, such as 1480 nanometers, and have a power of other than 180 mW. Also preferably, a reflector  149  is optically disposed in the third signal line  124 . The reflector  149  may be a fiber grating or other reflector designed to reflect predetermined wavelengths. While a single reflector  149  is shown, those skilled in the art will recognize that the reflector  149  may include a plurality of reflectors  149 . Preferably, the reflector  149  is tuned to reflect light having a wavelength of approximately 1558 nm.  
         [0012]    A downstream end of the second amplifying gain medium  140  is optically connected to a third amplifying gain medium  160  through a second optical isolator  150 . Preferably, the third amplifying gain medium  160  is a rare earth doped medium, such as a fiber or a planar waveguide. Also preferably, the third amplifying gain medium  160  is approximately one hundred and twenty meters long. A third optical multiplexer  162  optically connects an amplifying power source, preferably a third pump laser  164 , to the third amplifying gain medium  160  via a pump line  166 . Preferably, the third pump laser  164  is a 980 nanometer pump laser and has a power of approximately 200 mW, although those skilled in the art will recognize that the third pump laser  164  may be other than 980 nanometers, such as 1480 nanometers, and have a power of other than 200 mW. Also preferably, a reflector  168  is optically disposed between the second amplifying gain medium  160  and the third optical multiplexer  162 . The reflector  168  may be a fiber grating or other reflector designed to reflect predetermined wavelengths. While a single reflector  168  is shown, those skilled in the art will recognize that the reflector  168  may include a plurality of reflectors  168 . Preferably, the reflector  168  is tuned to reflect light having a wavelength of approximately 1560 nm.  
         [0013]    An auxiliary power source in the form of a fourth pump laser  174  is optically connected to a fourth optical multiplexer  172  optically downstream of the third amplifying gain medium  160  via a pump line  176 . Preferably, fourth pump laser  174  is a 980 nanometer pump laser and has a power of approximately 200 mW, and is disposed to provide counter-pumping for the amplifying gain medium  160 , although those skilled in the art will recognize that the fourth pump laser  174  may be other than 980 nanometers, such as 1480 nanometers, and have a power of other than 200 mW. Also preferably, a reflector  178  is optically disposed between the second amplifying gain medium  160  and the fourth optical multiplexer  172 . The reflector  178  may be a fiber grating or other reflector designed to reflect predetermined wavelengths. While a single reflector  178  is shown, those skilled in the art will recognize that the reflector  178  may include a plurality of reflectors  178 . Preferably, the reflector  178  is tuned to reflect light having a wavelength of approximately 1555 nm.  
         [0014]    Although the Figure shows the third amplifying gain medium  160  to be disposed optically downstream from the second amplifying gain medium  140 , those skilled in the art will recognize that the second amplifying gain medium  140  may be disposed optically downstream from the third amplifying gain medium  160 , instead.  
         [0015]    An optical combiner  180  is disposed optically downstream from the first amplifying gain medium  130  and the third amplifying gain medium  160  and combines the first signal line  122  and the second signal line  124  to form the amplifier output  192 , disposed optically downstream of the optical combiner  180 . Similar to the optical splitter  120 , the optical combiner  180  may be an AWG, a WDM or an optical circulator with optical gratings. A third optical isolator  190  is optically disposed along the amplifier output  192 . The third optical isolator  190  prevents optical noise from traveling backwards to the amplifier  100  from a receiver (not shown) disposed optically downstream of the amplifier  100 .  
         [0016]    Operation of the amplifier  100  is as follows. The broadband signal light λ S , having a spectrum of approximately between approximately 1565 and 1620 nm, is provided to the input  102  from the transmission source (not shown). The signal light λ S  travels through the optical isolator  110  and to the optical splitter  120 . The optical splitter  120  splits the signal light λ S  into the L band signal light λ L , having wavelengths of approximately between 1565 and 1605 nanometers, and the ultra-L band signal light λ LL , having wavelengths of approximately between 1605 and 1620 nanometers. The L band signal light λ L  is transmitted along the signal line  122  to the first optical multiplexer  132 , where first pump light λ P1 , generated by the first pump laser  134  and transmitted along the pump line  136 , joins the L band signal light λ L .  
         [0017]    The combined L band signal light λ L  and first pump light pl are transmitted along the first amplifying gain medium  130  where the L band signal light λ L  is amplified, as is well known to those skilled in the art. Backward ASE, generated during amplification of the L band signal light λ L , is transmitted from the first amplifying gain medium  130  along the signal line  122  optically toward the optical splitter  120 . ASE having a wavelength of approximately between approximately 1535 to 1560 nanometers is reflected by the reflector  138  back into the first amplifying gain medium  130 . The ASE acts as a seed to supplement the pump power of the first pump laser  134 , increasing the amplification of the L band signal light λ L .  
         [0018]    The ultra-L band signal light λ LL  is transmitted along the signal line  124  to the second optical multiplexer  142 , where second pump light λ P2 , generated by the second pump laser  144  and transmitted along the pump line  146 , joins the ultra-L band signal light λ LL .  
         [0019]    The combined ultra-L band signal light λ LL  and second pump light λ P2  are transmitted along the second amplifying gain medium  140  where the ultra-L band signal light λ LL  is amplified. Backward ASE, generated during amplification of the ultra-L band signal light λ LL , is transmitted from the second amplifying gain medium  140  along the signal line  124  optically toward the optical splitter  120 . ASE having a wavelength of approximately 1558 nanometers is reflected by the reflector  146  back into the second amplifying gain medium  140 . The ASE acts as a seed to supplement the pump power of the second pump laser  144 , increasing the amplification of the ultra-L band signal light λ LL .  
         [0020]    The ultra-L band signal light λ LL  is further transmitted along the signal line  124 , through the second optical isolator  150 , to the third optical multiplexer  162 , where third pump light λ P3 , generated by the third pump laser  164  and transmitted along the pump line  166 , joins the ultra-L band signal light λ LL .  
         [0021]    The combined ultra-L band signal light λ LL  and third pump light λ P3  are transmitted along the third amplifying gain medium  160  where the ultra-L band signal light λ LL  is further amplified. Backward ASE, generated during amplification of the ultra-L band signal light λ LL , is transmitted from the third amplifying gain medium  160  along the signal line  124  optically toward the second amplifying gain medium  140 . ASE having a wavelength of approximately 1560 nanometers is reflected by the reflector  168  back into the third amplifying gain medium  160 . The ASE acts as a seed to supplement the pump power of the third pump laser  164 , increasing the amplification of the ultra-L band signal light λ LL .  
         [0022]    Generally simultaneously, the fourth pump laser  174  provides a fourth pump light λ P4  to counter-pump the third amplifying gain medium  160 . The fourth pump light λ P4  is counter-pumped through the third amplifying gain medium  160  toward the optical splitter  120 , where the ultra-L band signal light λ LL  is further amplified. Forward ASE, generated during amplification of the ultra-L band signal light λ LL  by the counter-pumping, is transmitted from the third amplifying gain medium  160  along the signal line  124  optically toward the optical combiner  180 . ASE having a wavelength of approximately 1555 nanometers is reflected by the reflector  178  back into the third amplifying gain medium  160 . The ASE acts as a seed to supplement the pump power of the fourth pump laser  174 , increasing the amplification of the ultra-L band signal light λ LL . The remaining ASE is absorbed by the second optical isolator  150 .  
         [0023]    The ultra-L band signal light λ LL , now amplified, combines with the L band signal light λ L , also now amplified, at the combiner  180  to reform the signal light λ S , now amplified, which is transmitted along the amplifier output  192 , the third optical isolator  190 , and out of the amplifier  100 . Those skilled in the art will recognize that a gain flattening filter, not shown, may be installed in the first and second signal lines  122 ,  124 , optically downstream from the first and third amplifying gain media  130 ,  140 , to flatten the gain of the ultra-L band signal light λ LL  and the L band signal light λ L .  
         [0024]    It will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular embodiments disclosed, but it is intended to cover modifications within the spirit and scope of the present invention as defined by the appended claims.