Abstract:
Methods, systems, and apparatus for optical communications. In one aspect, an optical amplifier includes a feed-forward first amplification stage including a rare-earth doped fiber receiving an optical signal and an injected pump light; and a plurality of subsequent amplification stages each including a corresponding rare-earth doped fiber, wherein each of the subsequent amplification stages receives a separately injected portion of the remnant pump light from the first amplification stage, the remnant pump light being split into portions directed to each respective subsequent amplification stage.

Description:
BACKGROUND 
     This specification relates to optical communications. 
     Optical fiber amplifiers are commonly used in communication systems. Types of optical fiber amplifiers include Rare Earth Doped Fiber Amplifiers, for example, Erbium Doped Fiber Amplifiers (“EDFAs”). The optical fiber amplifiers are usually pumped by one or more light emitter diode (LEDs) or laser pump sources. 
     An erbium doped fiber (EDF) is a form of a single-mode fiber, having a core that is heavily doped with erbium. Conventional EDFA&#39;s include a pump laser that provides a pump light to the erbium doped fiber to provide amplification. For example, when pump light at 980 nm or 1480 nm is injected into an EDF, the erbium atoms absorb the pump light, which pushes the erbium atoms into excited states. When stimulated by light streams, for example an input optical signal having one or more wavelengths in a C-band (1528-1570 nm) or an L band (1570-1620 nm), the excited atoms return to a ground or lower state by stimulated emission. The stimulated emission has the same wavelength as that of the stimulating light. Therefore, the optical signal is amplified as it is propagating through the EDF. Furthermore, the EDF typically amplifies the entire optical signal regardless of wavelength. 
     SUMMARY 
     In general, one innovative aspect of the subject matter described in this specification can be embodied in optical amplifiers that include: a feed-forward first amplification stage including a rare-earth doped fiber receiving an optical signal and an injected pump light; and multiple subsequent amplification stages each including a corresponding rare-earth doped fiber, wherein each of the subsequent amplification stages receives a separately injected portion of the remnant pump light from the first amplification stage, the remnant pump light being split into portions directed to each respective subsequent amplification stage. 
     The foregoing and other embodiments can each optionally include one or more of the following features, alone or in combination. In particular, one embodiment includes all the following features in combination. The optical amplifier includes a pump source coupled to a first pump wavelength division multiplexer (WDM) for injecting the pump light to the first amplification stage. The optical amplifier includes a second pump WDM coupled to an output of the first amplification stage configured to separate the remnant pump light from the optical signal. The optical amplifier includes a tap coupled to the second pump WDM and configured to receive the remnant pump light and to separate the remnant pump light into multiple ports corresponding to respective paths. The optical amplifier includes a third pump WDM configured to combine a split portion of the remnant pump light with the optical signal from the first amplification stage prior to entering a first subsequent amplification stage. The optical amplifier includes a fourth pump WDM configured to combine a split portion of the remnant pump light with the optical signal from the first subsequent amplification stage prior to entering a second subsequent amplification stage. The optical amplifier further includes an input tap coupled to the first amplification stage configured to tap a portion of the optical signal input and to route the tapped portion to an input photo detector; and an output tap coupled to the one or more second amplification stages configured to tap a portion of an amplified optical signal output and to route the tapped portion to an output photo detector. The optical amplifier further includes a controller configured to measure the power of the input optical signal and the amplified optical signal and to control the pump source. The output tap is also coupled to an output port of the optical amplifier. The rare-earth doped fiber is an erbium doped fiber. The optical amplifier includes a gain flattening filter (GFF) positioned between a pair of the multiple second amplification stages. The GFF is positioned between a first subsequent amplification stage and a second subsequent amplification stage. 
     In general, one innovative aspect of the subject matter described in this specification can be embodied in optical amplifiers that include: a pump light source; an input coupled to a first amplification stage, the first amplification stage including a first rare-earth doped fiber and configured to receive an optical signal combined with pump light from the pump light source; a first pump wavelength division multiplexer (WDM) for separating remnant pump light from the light exiting the first amplification stage; an optical component that splits the separated remnant pump light into two or more ports; a second pump WDM for combining a first split portion of the remnant pump light with an optical signal from the first amplification stage; a second amplification stage including a second rare-earth doped fiber and configured to receive the combined first split portion of the remnant pump light combined with the optical signal from the first amplification stage; a third pump WDM for combining a second split portion of the remnant pump light with an optical signal from the second amplification stage; and a third amplification stage including a third rare-earth doped fiber and configured to receive the combined second split portion of the remnant pump light combined with the optical signal from the second amplification stage. 
     The foregoing and other embodiments can each optionally include one or more of the following features, alone or in combination. In particular, one embodiment includes all the following features in combination. The optical amplifier further includes a controller coupled to the pump light source and configured to control the power of the pump light emitted from the pump light source. The optical amplifier further includes a first photo detector coupled to an input of the optical amplifier and configured to measure an input power of the optical signal; and a second photo detector coupled to an output of the optical amplifier and configured to measure an output power of the amplified optical signal. The pump light power is modified by the controller based on the measured input power and the measured output power. The optical amplifier further includes a gain flattening filter coupled between the second amplification stage and the third amplification stage. Each rare-earth doped fiber is an erbium doped fiber. 
     Particular embodiments of the subject matter described in this specification can be implemented so as to realize one or more of the following advantages. An optical amplifier configuration is described that provides high inversion and noise response using a feed-forward first amplification stage and provides flexibility of using multiple subsequent amplifier stages to provide further amplification using separately injected remnant pump light from the first amplification stage. 
     The details of one or more embodiments of the subject matter described in this specification are set forth in the accompanying drawings and the description below. Other features, aspects, and advantages of the subject matter will become apparent from the description, the drawings, and the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram of an example conventional optical amplifier with a pump splitting configuration. 
         FIG. 2  is a diagram of an example conventional optical amplifier with a pump feed-forward configuration. 
         FIG. 3  is a diagram of an example optical amplifier with a combined pump feed-forward and pump splitting configuration. 
     
    
    
     Like reference numbers and designations in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
       FIG. 1  is a diagram of an example optical amplifier  100  with a pump splitting configuration. In particular, the optical amplifier  100  includes an input tap  101  at an input port that receives an optical signal, e.g., from an optical fiber. The input signal can include multiple wavelength channels. The input tap  101  directs a portion of the input light signal to an input photo detector  111 . The input photo detector  111  uses the tapped optical signal to determine a measure of the total input power. The input photo detector  111  can be, for example, a photo diode. The input tap  101  can be, for example, a fused fiber coupler. In some implementations, the input tap  101  directs substantially 1% to 5% of the input optical signal to the input photo detector  111 . 
     The majority of the optical signal passes through the input tap  101  and is joined with a first pump wavelength division multiplexer (WDM)  102  before being input to a first rare-earth doped fiber  103  providing a first amplification stage. 
     The first pump WDM  102  multiplexes the input light signal with at least a portion of laser light emitted by a pump source  108 . Light from the pump source  108  is split by a tap  109  and a portion of the pump light is directed to the first pump WDM  102  and a portion of the pump light is directed to a second pump WDM  105 . This allows for a single pump source  108  to transmit pump light to multiple amplification stages having rare-earth doped fibers, which provide for a multi-stage optical amplifier design. 
     In some implementation, the tap  109  evenly splits the pump light. In some other implementations, a specified proportion of the pump light is routed to each respective pump WDM. 
     The combined light signal and pump light from the first pump WDM  102  is input to the first rare-earth doped fiber  103 . The first rare-earth doped fiber  103  can be an erbium doped fiber. The first rare-earth doped fiber  103  absorbs the received energy of the pump light. The absorbed pump energy is used to amplify the light of the optical signal to provide an amplified optical signal. However, because the pump light has been split, the first rare-earth fiber  103  may only receive a portion of the maximum pump light it can absorb. This may result is a noise figure of the optical amplifier  100  that is worse than when a maximum pump light is received. The noise figure refers to a measure of degradation of signal-to-noise ratio, which can be used to measure performance of an optical amplifier. 
     Thus, the input signal is amplified by the first rare-earth doped fiber  103 . However, the gain provided by the first rare-earth doped fiber  103  is not uniform over the signal spectrum (i.e., across all wavelengths of the optical signal). To provide a flat spectral gain across all wavelengths, the amplified signal from the first rare-earth doped fiber  103  is subsequently filtered by a gain flattening filter (GFF)  104 . The GFF  104  attenuates one or more wavelengths by a particular amount. 
     After passing through the GFF  104 , a resulting filtered optical signal intersects with the second pump WDM  105 , which combines the optical signal output from the GFF  104  with the split portion of the pump light from the tap  109 . The combined light signal and pump light from the second pump WDM  105  is input to a second amplification stage including a second rare-earth doped fiber  106 . The second rare-earth doped fiber  106  absorbs the energy of the split pump light, as well as any residual pump light from the first rare-earth doped fiber  103 , and further amplifies the optical signal. 
     The amplified optical signal is tapped by output tap  107 , which samples a portion of the amplified optical signal while allowing most of the amplified optical signal to exit the optical amplifier  100 , e.g., though an optical fiber at an output port coupled to the output tap  107 . The output tap  107  directs the tapped portion of the amplified light signal to an output photo detector  112 . The output photo detector  112  uses the tapped optical signal to determine a measure of the total output power. The output photo detector  112  can be, for example, a photo diode. The output tap  107  can be, for example, a fused fiber coupler. In some implementations, the output tap  107  directs substantially 1% to 5% of the input optical signal to the output photo detector  112 . 
     The measured input power and output power from the input photo detector  111  and the output photo detector  112 , respectively, are used by a controller  110  to control the power of the pump light generated by the pump source  108 . For example, the controller  110  can be used to determine whether the optical amplifier  100  is providing a specified amount of gain to the optical signal. The average gain of the optical amplifier  100  can be calculated as a ratio between the total output power measured by the output photo detector  112  and the total input power measured by the input photo detector  111 . The controller can signal the pump source  108  to increase or decrease pump light power based on the measurements and one or more specified amplification parameters, e.g., a specified output gain range. 
       FIG. 2  is a diagram of an example optical amplifier  200  with a pump feed-forward configuration. In the pump feed forward configuration, all of the available pump power is provided to a first stage of the optical amplifier  200  for higher inversion and better noise figure. The remnant pump light after a first amplification stage, e.g., a first rare-earth fiber, is input to a second amplification stage and so on. However, the remnant light may not be sufficient to provide a desired amount of amplification to a light signal through multiple stages of the optical amplifier. 
     The optical amplifier  200  includes an input tap  201  that receives an optical signal at an input port, e.g., from an optical fiber. The input signal can include multiple wavelength channels. The input tap  201  directs a portion of the input light signal to an input photo detector  211 . The input photo detector  211  uses the tapped optical signal to determine a measure of the total input power. The input tap  201  can be, for example, a fused fiber coupler. In some implementations, the input tap  101  directs substantially 1% to 5% of the input optical signal to the input photo detector  211 . 
     The majority of the optical signal passes through the input tap  201  and is joined with a first pump wavelength division multiplexer (WDM)  202  before being input to a first rare earth doped fiber  203 , providing a first amplification stage. The first pump WDM  202  multiplexes the input light signal with at least a portion of laser light emitted by a pump source  209 . 
     The combined light signal and pump light from the first pump WDM  202  is input to the first rare-earth doped fiber  203  of a first amplification stage. The first rare-earth doped fiber  203  can be an erbium doped fiber. The first rare-earth doped fiber  203  absorbs the received energy of the pump light. The absorbed pump energy is used to amplify the light of the optical signal to provide an amplified optical signal. 
     A second pump WDM  204  separates the initially amplified optical signal from the remnant pump light of the first rear-earth doped fiber  203 . The initially amplified optical signal passes through a GFF  205 , similar to the GFF  104  of  FIG. 1 . The remnant pump light is routed around the GFF  205  to combine with the initially amplified optical signal at a third pump WDM  206 . This combination of the remnant pump light and initially amplified optical signal is input to a second rare-earth doped fiber  207 . The second rare-earth doped fiber  207  absorbs the energy of the remnant pump light and further amplifies the optical signal. 
     The amplified optical signal exiting the second rare-earth doped fiber  207  is tapped by output tap  208 , which samples a portion of the amplified optical signal while allowing most of the amplified optical signal to exit the optical amplifier  200 , e.g., through an optical fiber at an output port coupled to the output tap  208 . The output tap  208  directs the tapped portion of the amplified light signal to an output photo detector  212 . The output photo detector  212  uses the tapped optical signal to determine a measure of the total output power as described above with respect to output photo detector  112 . 
     The measured input power and output power from the input photo detector  211  and the output photo detector  212 , respectively, are used by a controller  210  to control the power of the pump light generated by the pump source  209 . For example, the controller  210  can be used to determine whether the optical amplifier  200  is providing a specified amount of gain to the optical signal. The average gain of the optical amplifier  200  can be calculated as a ratio between the total output power measured by the output photo detector  212  and the total input power measured by the input photo detector  211 . The controller can signal the pump source  209  to increase or decrease pump light power based on the measurements and one or more specified amplification parameters, e.g., a specified output gain range. 
       FIG. 3  is a diagram of an example optical amplifier  300  with a combined pump feed-forward and pump splitting configuration. In particular, the optical amplifier  300  is configured to split remnant pump power after a first feed forward amplification stage to two or more subsequent amplification stages using pump light splitting. Thus, the configuration has an advantage of supplying full pump light power to a first stage resulting in high first stage inversion while allowing pumping to multiple subsequent amplification stages. 
     The optical amplifier  300  includes an input tap  301  that receives an optical signal at an input port, e.g., from an optical fiber. The input signal can include multiple wavelength channels. The input tap  301  directs a portion of the input light signal to an input photo detector  314 . The input photo detector  314  uses the tapped optical signal to determine a measure of the total input power. The input tap  301  and input photo detector  314  can be similar to the input tap  101  and input photo detector  111  described above. 
     The majority of the optical signal passes through the input tap  301  and is joined with a first pump WDM  302  before being input to a first rare earth doped fiber  303 , providing a first amplification stage. The first pump WDM  302  multiplexes the input light signal with pump laser light emitted by a pump source  312 . 
     The combined light signal and pump light from the first pump WDM  302  is input to the first rare-earth doped fiber  303  of the first amplification stage. The first rare-earth doped fiber  303  can be an erbium doped fiber. The first rare-earth doped fiber  303  absorbs the received energy of the pump light. The absorbed pump energy is used to amplify the light of the optical signal to provide an amplified optical signal. 
     A second pump WDM  304  separates the initially amplified optical signal from the remnant pump light of the first rare-earth doped fiber  303 . The remnant pump light is directed to a tap  305  that splits the remnant pump light into two ports corresponding to distinct paths. In some implementation, the tap  305  evenly splits the pump light. In some other implementations, a specified proportion of the pump light is routed to each respective port. 
     The initially amplified optical signal is routed to a third pump WDM  306 , which combines the initially amplified optical signal with remnant pump light from a first path from the tap  305 . This combination of remnant pump light and initially amplified optical signal is input to a second rare-earth doped fiber  307  forming a second amplification stage. The second rare-earth doped fiber  307  absorbs the energy of the remnant pump light and further amplifies the optical signal. 
     The amplified signal from the second rare-earth doped fiber  307  is subsequently filtered by a gain flattening filter (GFF)  308 . The GFF  308  attenuates one or more wavelengths by a particular amount. In some other implementations, there can be more than one GFF, the GFF can be positioned between different amplification stages including between the first amplification stage and the second amplification stage. 
     After passing through the GFF  308 , a resulting optical signal intersects with a fourth pump WDM  309 , which combines the optical signal with the split portion of the remnant pump light from the tap  305 . The combined optical signal is input to a third rare-earth doped fiber  310  forming a third amplification stage. The third rare-earth doped fiber  310  absorbs the energy of the remnant pump light and further amplifies the optical signal. 
     The amplified optical signal exiting the third rare-earth doped fiber  310  is tapped by output tap  311 , which samples a portion of the amplified optical signal while allowing most of the amplified optical signal to exit the optical amplifier  300 , e.g., though an optical fiber at an output port coupled to the output tap  311 . The output tap  311  directs the tapped portion of the amplified light signal to an output photo detector  315 . The output photo detector  315  uses the tapped optical signal to determine a measure of the total output power as described above with respect to output photo detector  315 . The output tap  311  and output photo detector  315  can be similar to the output tap  107  and output photo detector  112  described above. 
     The measured input power and output power from the input photo detector  314  and the output photo detector  315 , respectively, are used by a controller  313  to control the power of the pump light generated by the pump source  312 . For example, the controller  313  can be used to determine whether the optical amplifier  300  is providing a specified amount of gain to the optical signal. In some implementations, the average gain of the optical amplifier  300  is calculated as a ratio between the total output power measured by the output photo detector  315  and the total input power measured by the input photo detector  314 . The controller can signal the pump source  312  to increase or decrease pump light power based on the measurements and one or more specified amplification parameters, e.g., a specified output gain range. 
     In the optical amplifier  300 , as described above, all of the pump light power is applied to the first amplification stage. As a result, the optical signal is amplified with all of the available pump power for a better noise figure. The remnant pump of the first rare-earth doped fiber  303  is extracted by the second pump WDM  304  and split into two ports by tap  305 . Each port of pump power is injected back into the following amplification stages. 
     Although example in  FIG. 3  shows three amplification stages, more are possible. For example, the tap  305  can be configured to separate the remnant pump light into additional paths or one or more additional taps can be used to further split the remnant pump light. The resulting paths can be coupled to additional amplification stages of rare earth doped fibers using corresponding pump WDM&#39;s. 
     While this specification contains many specific implementation details, these should not be construed as limitations on the scope of any inventions or of what may be claimed, but rather as descriptions of features specific to particular embodiments of particular inventions. Certain features that are described in this specification in the context of separate embodiments can also be implemented in combination in a single embodiment. Conversely, various features that are described in the context of a single embodiment can also be implemented in multiple embodiments separately or in any suitable subcombination. Moreover, although features may be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination may be directed to a subcombination or variation of a subcombination. 
     Similarly, while operations are depicted in the drawings in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results. In certain circumstances, multitasking and parallel processing may be advantageous. Moreover, the separation of various system components in the embodiments described above should not be understood as requiring such separation in all embodiments, and it should be understood that the described program components and systems can generally be integrated together in a single software product or packaged into multiple software products. 
     Thus, particular embodiments of the subject matter have been described. Other embodiments are within the scope of the following claims. In some cases, the actions recited in the claims can be performed in a different order and still achieve desirable results. In addition, the processes depicted in the accompanying figures do not necessarily require the particular order shown, or sequential order, to achieve desirable results. In certain implementations, multitasking and parallel processing may be advantageous.