Abstract:
Doped fiber amplifiers (DFA) using rare-earth doping materials with linear and non-linear interactions between the optical signal to be amplified and the pump laser have become a standard element of optical telecommunications systems for multiple applications including for example extending the reach of optical links before opto-electronic conversion is required or support increased fanout. However, in many applications wherein multiple DFAs are employed the electrical power budget wherein the pump laser diode (LD) represents approximately 25% of the module power consumption directly, and closer to approximately 40-50% once the control and drive electronics for the thermoelectric cooler and LD are included. Accordingly, it would be beneficial to reduce the overall power consumption of a DFA by exploiting unused optical pump power such that multiple gain stages, within the same or different DFAs, may be driven from a single pump LD.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This patent application claims the benefit of U.S. Provisional Patent Application 61/727,193 filed Nov. 16, 2012 entitled “Methods and Devices for Efficient Optical Fiber Amplifiers”, the entire contents of this patent application being included by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to optical fiber amplifiers and more particularly to methods and architectures for efficient optical fiber amplifiers. 
       BACKGROUND OF THE INVENTION 
       [0003]    Optical fiber communications is seen as one of the most reliable telecommunication technologies to achieve consumers&#39; needs for present and future applications. It is reliable in handling and transmitting data through hundreds of kilometers with an acceptable bit error rate and today, optical fiber communication dominate as the physical medium for medium and long distance data transmission systems and telecommunications networks. At the same time optical fiber solutions appear in short-haul applications, local area networks, fiber-to-the-home/curb/cabinet, and digital cable systems. Fundamentally, optical transmission systems are based on the principle that light can carry more information over longer distances in a glass medium than electrical signals can carry information over copper or coaxial cable. 
         [0004]    Light is electromagnetic waves and optical fiber is a waveguide, and whilst very low loss in certain wavelength ranges, e.g. α&lt;0.22 dB/km for Corning SMF-28 single mode silica fiber, ultimately in order to compensate the loss of the waveguide, an optical amplifier is needed. Doped fiber amplifiers (DFA) are optical amplifiers which uses rare-earth doping materials including, Erbium (Er3+), Praseodymium (Pr3+), Europium (Eu3+), Neodymium (Nd3+), Terbium (Te3+), Lutetium (Lu3+), Ytterbium (Yb3+), Holmium (Ho3+), Dysprosium (Dy3+), Gadolinium (Gd3+), Samarium (Sm3+), Promethium (Pm3+), Cerium (Ce3+), Lanthanum (La3+) and Thulium (Tm3+) inside the optical fiber. Essentially, within a transmission line the DFA is connected to a pump laser and works on principle of stimulated emission wherein the pump laser is used to provide energy and excite ions to an upper energy level. These excited ions are then stimulated by photons of the information signal and brought down to lower levels of energy such that they emit photon energy exactly on the same wavelength of the input signal. In addition to optical amplification for medium and long haul telecommunications, particularly within optical fiber communication systems (OFCS), DFAs are also employed, for example, as non-linear optical devices and optical switches. 
         [0005]    In OFCS, the active medium of DFAs operating in the 1550 nm window is Erbium (Er3+) and significant research in the past 25 years has been addressed to their performance, optimization, and manufacturing resulting in thousands of publications on Erbium Doped Fiber Amplifiers (EDFAs) alone together with thousands of others to their use within systems and other optical elements of OFCS. Erbium doped silica based fibers which form the active medium within the EDFA are favoured as the emission of Er3+ ions is within a set of wavelength around 1550 nm where the silica fiber also exhibits the minimum attenuation on the information signal in its transmission via silica based fibers, such as Corning SMF-28 for example. EDFAs can gains as high as 40 dB, equivalent to 80 km of silica based singlemode fiber, with low noise. Important features of EDFAs include the ability to pump the devices at several different wavelengths, low coupling loss to the compatible fiber transmission medium and very low dependence of gain on light polarization due to the cylindrical shape of Erbium doped fiber. In addition, EDFAs are highly transparent to signal format and bit rate, since they exhibit slow gain dynamics, with carrier lifetimes of 0.1 to 10 ms, which result in the gain response of EDFAs being basically constant for signal modulations greater than a few kilohertz to tens of gigahertz. Consequently, they are immune from interference effects, such as crosstalk and inter-modulation distortion between different optical channels within a broad spectrum of wavelengths, typically a 30 nm spectral band referred to as the C-band ranging from 1530 to 1560 nm, that are injected simultaneously into the EDFA. 
         [0006]    Subsequently, L-band EDFAs with flat optical gain from 1574 nm to 1604 nm and S-band EDFAs with gain from 1490 nm to 1520 nm, were established allowing dense wavelength division multiplexing (DWDM) at up to 160 channels, each operating at 10 Gb/s and with 50 GHz channel spacing. Whilst, there have been thousands of papers in the literature for optimizing gain, noise figure, gain flatness, etc as well as the design and integration of inter-stage elements such as dispersion compensation fibers (DCF) or gain equalization filters (GEF), are commonly located within the stages in order to solve the tradeoff between noise figure degradation, output power decrease, and inter symbol interference. 
         [0007]    However, whilst research activities were focused to reducing noise figure and higher output powers, such as were achieved through combinations of increasing pump laser output power and multiple pump sources, one significant design parameter of the EDFA and in general DFAs received little emphasis and focus, this being the efficiency of the DFA in terms of the pump power converted into the output channel signal(s). This pump power conversion efficiency (PCE) became a focus when combined C+L band EDFAs were being developed as researchers exploited a variety of single, dual, multi-pump designs with single, double, triple and quadruple pass configurations such as discussed by Naji et al in “Review of Erbium-doped Fiber Amplifier” (Int. J. Phys. Sci., Vol. 6, pp. 4674-4689). However, here the primary focus was again increasing the L-band output power through these configurations as well as shifting the pumping wavelength from 980 nm or 1480 nm into the C-band, such as 1545 nm for example. In fact nearly twenty years after the first EDFA demonstrations fundamental analysis of PCE within erbium doped fiber (EDF) configurations began to define operating regimes and present alternatives to the prevalent use in high power applications of large mode area fiber with low numerical aperture (NA) to lower pump power intensity. Whilst this prevalent design approach reduces the nonlinear effects such as 980 nm pump excited state absorption it limits the power conversion efficiency at high power to approximately 30%. 
         [0008]    This analysis, such as by Wang et al entitled “Novel Erbium Doped Fiber for High Power Applications” (Proc. SPIE Passive Components and Fiber-Based Devices 2005) showed that whilst PCE varies with pump power for constant NA and peak PCE occurs at different pump powers for different NA fibers it still only reaches 50-53%. Discrete PCE results have been reported above these values using titanium-sapphire lasers, such as Mahdi et al in “Single-Mode Pumping Scheme for EDFA with High-Power Conversion Efficiency using a 980 nm Ti:S Laser” (Microwave and Optical Technology Letters, Vol. 48, pp 71-74), where the PCE reached 60%, representing a quantum efficiency of 95%, these have been achieved using large research lasers and laboratory optical arrangements rather than the technician assembled semiconductor laser pumped configurations suitable for widespread deployment in telecommunications. Accordingly, the dominant commercial EDFA designs using large mode area fibers, production optical sub-assemblies, and commercial semiconductor laser diode pump sources operate at only approximately 30% power conversion efficiency from their pump signal, typically 980 nm, to the optical signals being amplified. 
         [0009]    A high power EDFA operating at +23 dBm (200 mW) maximum output power requires approximately 600 mW of 980 nm pump power when operating at 30% efficiency. Within an EDFA module, such as for example a JDS Uniphase® WaveReady™ WRA-217 blade module, wherein the typical power consumption of the overall pump, coolers, control electronics etc is 18 W typically and 24 W maximum this “wasted” 400 mW of 980 nm optical power may not seem that significant. However, the power consumption of the 980 nm pump laser itself is approximately 2 W and the thermo-electric cooler (TEC) required to maintain the semiconductor laser diode operating temperature under varying ambient conditions typically consumes approximately between 2 W and 3 W at high ambient temperatures such as common within equipment cases and racks. The remaining power consumption is associated with network interfaces, power supplies etc which are only required where there are active electronic or electro-optical elements. 
         [0010]    Accordingly, the 980 nm pump laser diode (LD) represents approximately 25% of the module power consumption directly, which is actually closer to approximately 40-50% once the control and drive electronics for the TEC and LD respectively are included within the calculation. It would therefore be beneficial to reduce the overall power consumption of a DFA by exploiting the unused optical pump power such that a lower power LD may be exploited thereby similarly reducing the requirements for TEC, TEC drive circuit, and LD drive circuit. 
         [0011]    Accordingly, where multiple DFAs are to be employed in conjunction with one another such as for example at optical switches, optical cross-connects, and multi-channel reconfigurable optical add-drop multiplexers then every channel will exploit a similar DFA consuming, in the case of the WaveReady™ WRA-217, approximately 18 W. Accordingly, an 12×12 optical cross-connect, representing a cross-connect for example at the intersection of two links each comprising 6 optical fibers, 3 East and 3 West within a first ring and 3 North and 3 South in the second ring, with a DFA per channel therefore would consume 12×18 W=216 W of power. However, if the remaining optical pump power of the DFA can be re-used within another DFA then there is an opportunity to significantly reduce the power consumption of the DFAs associated with the optical cross-connect. For example, using the example above of DFAs consuming 200 mW maximum 980 nm pump power with a 600 mW 980 nm LD then potentially only a single LD may be employed to provide the optical pump power required across 3 DFAs. Accordingly, rather than the prior 12 DFAs with 12 pump LDs it would be beneficial to reuse the unused optical pump power such that 12 DFAs with only 4 pump LDs were required. Accordingly the DFAs would now consume only 4×18 W=72 W, representing a saving of 144 W. 
         [0012]    It would be further evident, from the prior art analysis and experiments such as taught by Wang that the PCE of an optical amplifier varies with optical pump power such that for example a NA=0.14 EDF varies from a PCE of below 0.4 to above 0.52 as the pump power varies from about 50 mW to 300 mW. Accordingly, it would be beneficial to maintain a DFA within a predetermined operating regime for increased performance overall of the amplifier node from a power consumption viewpoint. It would also be evident that where multiple amplifiers are utilizing the same pump laser within a serial coupling of the pump to the multiple amplifiers rather than a parallel configuration that the power supplied sequentially between each pair of DFAs should be within a predetermined range in order to ensure that each amplifier operates as intended. 
         [0013]    Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures. 
       SUMMARY OF THE INVENTION 
       [0014]    It is an object of the present invention to address limitations within the prior art in respect of optical fiber amplifiers and more particularly to methods and architectures for efficient optical fiber amplifiers. 
         [0015]    In accordance with an embodiment of the invention there is provided a method comprising:
   providing a first optical amplifier comprising:
       a first optical gain block for receiving optical signals to be amplified within a first predetermined wavelength range;   an first optical pump signal coupled to the first optical gain block for establishing a population inversion within the first optical gain block to provide gain to optical signals within a second predetermined wavelength range; and   a first optical control signal coupled to the first optical gain block, the first control signal being within a third predetermined wavelength range;   
       providing a wavelength selective coupler disposed after the first optical gain block for separating the amplified optical signals from the first gain block from the residual optical pump signal;   providing a second optical amplifier comprising:
       a second optical gain block for receiving the amplified optical signals from the first optical gain block;   a second optical pump signal for establishing a population inversion within the second optical gain block to provide additional gain to optical signals within a third predetermined wavelength range; and   a second optical control signal coupled to the second optical gain block, the second control signal being within a fourth predetermined wavelength range; and   generating using residual first optical pump signal from the first optical amplifier at least one of the second optical control signal and second optical pump signal.   
       
 
         [0026]    In accordance with an embodiment of the invention there is provided a method comprising:
   providing a first optical amplifier comprising:
       a first optical gain block for receiving optical signals to be amplified within a first predetermined wavelength range;   an first optical pump signal coupled to the first optical gain block for establishing a population inversion within the first optical gain block to provide gain to optical signals within a second predetermined wavelength range; and   
       providing a wavelength selective coupler disposed after the first optical gain block for separating the amplified optical signals from the first gain block from the residual optical pump signal;   providing a second optical amplifier comprising:
       a second optical gain block for receiving the amplified optical signals from the first optical gain block;   a second optical pump signal for establishing a population inversion within the second optical gain block to provide additional gain to optical signals within a third predetermined wavelength range; and   generating using residual first optical pump signal from the first optical amplifier at least one of the second optical control signal and second optical pump signal.   
       
 
         [0035]    In accordance with another embodiment of the invention there is provided a method comprising:
   providing a first optical amplifier comprising:
       a first optical gain block for receiving first optical signals from a first optical fiber to be amplified within a first predetermined wavelength range;   an first optical pump signal within a second predetermined wavelength range coupled to the first optical gain block for establishing a population inversion within the first optical gain block to provide gain to the first optical signals; and   
       providing a wavelength selective coupler for separating the amplified first optical signals from the first optical amplifier from residual optical pump signal not absorbed by the first optical gain block;   providing a second optical amplifier comprising:
       a second optical gain block for receiving the second optical signals from a second optical fiber to be amplified within a second predetermined wavelength range;   using residual first optical pump signal from the first optical amplifier to provide a second optical pump signal for establishing a population inversion within the second optical gain block to provide gain to the second optical signals within the third predetermined wavelength range.   
       
 
         [0043]    Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0044]    Embodiments of the present invention will now be described, by way of example only, with reference to the attached Figures, wherein: 
           [0045]      FIG. 1  depicts a single stage DFA according to the prior art; 
           [0046]      FIG. 2  depicts a dual stage DFA according to the prior art; 
           [0047]      FIG. 3  depicts an optical amplifier according to an embodiment of the invention employing dual pump sources and in-band channel power management; 
           [0048]      FIG. 4A  depicts an optical amplifier according to an embodiment of the invention employing feed-forward optoelectronic and optoelectronic conversions; 
           [0049]      FIGS. 4B and 4C  depict optical amplifiers according to embodiments of the invention employing feed-forward optoelectronic and optoelectronic conversions; 
           [0050]      FIG. 5  depicts an optical amplifier according to an embodiment of the invention employing feed-forward optoelectronic and optoelectronic conversions in conjunction with dynamic in-band channel power management; 
           [0051]      FIG. 6A through 6C  depicts controller variants for optical amplifiers according to embodiments of the invention for an optical amplifier such as depicted in  FIG. 5  employing feed-forward optoelectronic and optoelectronic conversions in conjunction with dynamic in-band channel power management; 
           [0052]      FIG. 7  depicts parallel optical amplifiers according to an embodiment of the invention exploiting optical pump re-use; 
           [0053]      FIG. 8  depicts parallel optical amplifiers according to an embodiment of the invention exploiting optical pump re-use; and 
           [0054]      FIG. 9  depicts a micro-optic hybrid circuit providing compact low loss implementation of optical functional elements of a parallel optical amplifier configuration. 
       
    
    
     DETAILED DESCRIPTION 
       [0055]    The present invention is directed to optical fiber amplifiers and more particularly to methods and architectures for efficient optical fiber amplifiers. 
         [0056]    The ensuing description provides exemplary embodiment(s) only, and is not intended to limit the scope, applicability or configuration of the disclosure. Rather, the ensuing description of the exemplary embodiment(s) will provide those skilled in the art with an enabling description for implementing an exemplary embodiment. It being understood that various changes may be made in the function and arrangement of elements without departing from the spirit and scope as set forth in the appended claims. 
         [0057]    Referring to  FIG. 1  there is depicted a single stage DFA according to the prior art disposed between an input port  100 A and an output port  100 B. The optical signal or signals received are coupled to an input isolator  110  before being combined in WDM  130  with the pump laser signal from pump LD  120 . The combined pump laser signal and optical signals are then coupled to optical fiber block  140  wherein the pump laser signal generates the required inversion within the dopants of the optical fiber thereby amplifying the optical signals as is well known within the prior art. The output from the optical fiber block  140  is then coupled to output isolator  150  thereby isolating the single stage DFA from any reflections downstream. Accordingly, in operation the optical output power of the pump LD  120  is dynamically adjusted in response to the requirements of the optical amplifier in terms of optical output power per channel. A control loop, not shown for clarity, may interact with the controller, not shown for clarity, for the pump LD  120  to adjust the optical output power based upon either a determination made local to the optical amplifier (such as number of channels, power, etc) or information conveyed to the optical amplifier from a network management system. 
         [0058]    Now referring to  FIG. 2  there is depicted a dual stage DFA according to the prior art. As depicted an input port  100 A is coupled to a first DFA stage comprising input isolator  210 , tap coupler  220 , tap detector  230 , pump LD  240 , WDM  250 , doped optical fiber  260 , and stage isolator  270 . Accordingly the optical signals received at the dual stage DFA are isolated from the preceding optical network by input isolator  210  wherein the combined optical signals are tapped by tap coupler  220 , which may for example be a 1% power tap, wherein the tapped signal is coupled to the tap detector  230  wherein the output of the tap detector  230  is coupled to a control circuit, not shown for clarity. The optical signals are themselves coupled to WDM  250  wherein they are combined with the pump LD  240  and coupled to the doped optical fiber  260  wherein they are amplified in dependence upon the characteristics of the doped optical fiber  260 , the pump LD  240 , and the number of channels, their power distribution etc. The amplified optical signals are then isolated via stage isolator  270  before being coupled to first inter-stage port  200 B. First inter-stage port  200 B may be coupled directly to second inter-stage port  200 C or via an interim optical element such as a gain-flattening filter (GFF), chromatic dispersion compensator, amplified spontaneous emission (ASE) filter, or dynamic channel equalizer (DCE) for example. 
         [0059]    The second inter-stage port  200 C is coupled to second-stage DFA  280  and therein output port  200 D. As depicted second-stage DFA  280  is essentially a replica of the first-stage DFA comprising input isolator, tap coupler, tap detector, pump LD, WDM, doped optical fiber, and stage isolator in a configuration such as described supra. Accordingly, as is well documented within the prior art the first stage typically provides linear amplification of the optical signals whereas the second stage, second-stage DFA  280 , provides power. The first stage can be viewed as a low-noise preamplifier whilst the second stage can be viewed as a power amplifier. Generally, the first stage DFA, and accordingly the pump LD  240 , is dynamically adjusted to provide variable gain according to the input conditions whereas the second stage, second-stage DFA  280 , and its pump LD are controlled for constant gain. Accordingly, the pump LDs within the two stages are operated independently of one another. 
         [0060]    Within the descriptions below of optical amplifiers according to embodiments of the invention multiple elements have been omitted for clarity including, but not limited to, optical isolators, optical tap couplers, wavelength division multiplexers (WDMs), and monitor photodiodes. According to these embodiments some or all of these may be employed within an optical amplifier. 
         [0061]    Referring to  FIG. 3  there is depicted an optical amplifier according to an embodiment of the invention employing dual pump sources and in-band channel power management. As depicted optical signals received at an input port  300 A, λ signal , are coupled via to a first optical amplifier  320  which also receives a pump signal, λ C , from first pump LD  310  and provides a gain β. The output of the first optical amplifier  320  is coupled to a tunable splitter  330  wherein a proportion, α, of the signal λ C  is coupled to first output port  300 B such that a combined optical signal βλ signal +(1−α)λ C  is coupled to the second optical amplifier  340 . The second optical amplifier  340  also receives a pump signal, λ 980 , from a 980 nm pump LD coupled to second input port  300 C wherein the output of the second optical amplifier  340  is coupled to second output port  300 D via optical element  350 . Accordingly, signal λ C  acts a pump signal for the first optical amplifier  320  whilst mitigating transient variations in λ signal  wherein after amplification αλ C  is coupled out from the output of the first optical amplifier  320  prior to coupling the combined optical signal to the second optical amplifier  340  wherein the remaining portion (1−α)λ C  is combined with the pump signal, λ 980 , in order to pump the second optical amplifier  340 . As such this all-optical scheme mitigates transient variations in λ signal  without wasting pump laser power. Alternatively, in other embodiments of the invention the remaining portion (1−α)λ C  may be designed to be out of the band of the second optical amplifier for amplification but as a second optical signal to be amplified thereby competing with the λ signal . 
         [0062]    Such a situation for example arising wherein the first amplifier is a Raman amplifier and the residual optical signals at 14XX nm do not pump the second Erbium doped optical amplifier. 
         [0063]    Now referring to  FIG. 4A  there is depicted a hybrid optoelectronic scheme for an optical amplifier according to an embodiment of the invention employing feed-forward optoelectronic and optoelectronic conversions. As depicted optical signals received at an input port  400 A, λ signal , are coupled via to a first optical amplifier  420  which also receives a pump signal, λ A:980 , from first pump LD  410  and provides a gain β to the optical signals received. The output of the first optical amplifier  420  is coupled to a WDM  430  wherein the remaining pump signal, δλ A:980 , is coupled to a detector  460  forming part of OEO controller  480 . The amplified optical signals, βλ signal , are coupled via optional delay  435  to a second optical amplifier  440 , the output of which is coupled via optical element  450  to output port  400 B. The optional delay  435  providing appropriate delay for the amplified optical signals to accommodate the signal conversion and/or signal processing within the OEO controller  480 . 
         [0064]    The converted residual pump signal, δλ A:980 , from the detector  460  in OEO controller  480  is coupled to signal source  470  operating at λ C  wherein the output of this signal source  470  is coupled to the second optical amplifier  440  together with second pump signal, λ B:980 , from second pump LD  490 . Accordingly, the first optical amplifier  420  provides a low noise linear amplifier but now the residual optical pump signal from the first optical amplifier  420  is converted through an OEO interface, OEO controller  480 , to a feed-forward signal λ C  which is used in conjunction with the second pump signal λ B:980  to provide the optical pump required by the second optical amplifier  440 . Accordingly, the design approach depicted in respect of  FIG. 4  provides for reduced pump power requirements, and corresponding reduced pump power consumption, together with constant gain operation and transient suppression. 
         [0065]    Optionally, OEO controller  480  includes dual emitters, one operating to provide a first feed-forward signal contributing to the pumping of the second optical amplifier  440  and the other to maintain the total input power to the second optical amplifier  440  constant wherein the second emitter emits therefore a signal out of band to the optical signals being amplified but within the gain bandwidth of the second optical amplifier  440 . Alternatively the converted residual pump signal, δλ A:980 , the detector  460  in OEO controller  480  is coupled to the second pump LD  490  to generate the second pump signal, λ B:980 . 
         [0066]    Now referring to  FIG. 4B  there is depicted a hybrid optoelectronic scheme for an optical amplifier according to an embodiment of the invention employing feed-forward optoelectronic and optoelectronic conversions. As depicted the overall structure comprises essentially the same optoelectronic elements and functions in the same manner as the hybrid optoelectronic scheme for an optical amplifier described supra and depicted in respect of  FIG. 4A . However, the first pump LD  410  is now replaced with Erbium/Raman pump source  4100  wherein the source provides optical signals at 980 nm for pumping the erbium doped optical fiber and 1480 nm signals for Raman amplification within the optical fiber within dual mode amplifier  4200 . Residual pump signals at 980 nm and 1480 nm are filtered from the amplified optical signals in edge filter  4300  wherein the filtered residual pump signals are then coupled to wavelength converter  4800  such that the residual pump signal at 980 nm and/or the residual pump signal at 1480 nm are converted to provide the signal λ C  to the second optical amplifier  440  together with the amplified signals of interest, λ SIGNAL , and second pump signal λ B:980 . It would be evident to one skilled in the art that the Erbium/Raman pump source  4100 , first optical amplifier  4200 , and wavelength converter  4800  may operate at one wavelength, i.e. 980 nm or 1480 nm, or both wavelengths. In the case that both 980 nm and 1480 nm wavelengths are active then the wavelength converter  4800  may convert both wavelengths to the same signal wavelength, λ C , or it may two signal wavelengths, λ C1  and λ C2  as depicted in  FIG. 4C . 
         [0067]    It is also evident in  FIG. 4C  that the configuration of the initial optical amplifier, co-counter Raman amplifier  4200 , exploits counter-propagating 14XX pumping which is depicted by first and second pump sources  4100 A and  4100 B respectively wherein first pump source  4100 A provides 1465 nm and 1470 nm forward propagating pump signals together with a 980 nm pump signal whilst second pump source  4100 B provides 1475 nm and 1480 nm counter propagating pump signals. The output of the co-counter Raman amplifier  4200  is coupled to filter  4300  that separates the residual 980 nm pump signal and residual 14XX nm forward propagating pump signals from the amplified optical signal, βλ SIGNAL . As depicted the residual 980 nm pump signal is coupled to a first wavelength converter block  4800 A therein generating first control signal λ C1  whilst the residual 14XX nm forward propagating pump signals are coupled to a second wavelength converter block  4800 B therein generating second control signal λ C2 . Each of the first control signal λ C1  and second control signal λ C2  are then coupled to the second optical amplifier  440  together with second 980 nm pump signal λ B:980  thereby amplifying the amplified optical signal, βλ SIGNAL  further. Optionally the residual counter propagating signals may also be coupled to a further wavelength converter or second wavelength converter  4800 B. 
         [0068]    Referring to  FIG. 5  there is depicted an optical amplifier according to an embodiment of the invention employing feed-forward optoelectronic and optoelectronic conversions in conjunction with dynamic in-band channel power management. As depicted optical signals received at an input port  500 A, λ signal , are coupled via to a first optical amplifier  530  which also receives a pump signal, λ A:980 , from first pump LD  510  as well as a first control signal, λ C1 , from first control source  520  and provides a gain β to the optical signals received. The output of the first optical amplifier  530  is coupled to a first optical element  540  wherein the remaining pump signal, δλ A:980 , is coupled to a first detector  560  forming part of OEO controller  595  as well as the first control signal, λ C1 , which is coupled to a second detector  570 . The amplified optical signals, βλ signal , are coupled via optional delay  545  to a second optical amplifier  550 , the output of which is coupled via optical element  565  to output port  500 B. The optional delay  545  providing appropriate delay for the amplified optical signals to accommodate the signal conversion and/or signal processing within the OEO controller  595 . 
         [0069]    The converted residual pump signal, δλ A:980 , and converted first control signal, λ C1 , from the first and second detectors  560  and  570  respectively in OEO controller  595  are coupled to control source  580  operating at λ C2  wherein the output of this control source  580  is coupled to the second optical amplifier  550  together with second pump signal, λ B:980 , from second pump LD  590 . Accordingly, the first optical amplifier  530  provides a low noise linear amplifier but now the residual optical pump signal λ A:980  from the first optical amplifier  530  is converted through an OEO interface, OEO controller  595 , to a feed-forward signal λ C2  which is used in conjunction with the second pump signal λ B:980  to control the operation of second optical amplifier  550 . Feed-forward signal λ C2  may for example be within the absorption band of the dopant to provide optical pumping in combination with the second pump signal λ B:980  or it may be employed within the gain bandwidth of the second optical amplify  550  to control the overall optical power coupled to the second optical amplifier  550 . 
         [0070]    Now referring to  FIG. 6A  there are depicted a controller variant for optical amplifiers according to embodiments of the invention employing feed-forward optoelectronic and optoelectronic conversions in conjunction with dynamic in-band channel power management of a similar configuration as that described above in respect of  FIG. 5 . However, the configuration of the OEO controller  595  has been varied as depicted by first controller variant  695 A wherein the first control signal, λ C1 , from first detector  660  is not only coupled to the control source  680  operating at λ C2  but also to pump LD  690  which generates the second pump signal, λ B:980 . Second detector  670  is also coupled to both the control source  680  and pump LD  690 . Accordingly, control for the control source  680  and pump LD  690  are derived from at least the first control signal but also electrical power for the control source  680  is derived from the residual optical pump. 
         [0071]    Now referring to second controller variant  695 B in  FIG. 6B  the first control signal, λ C1 , is not only coupled to the control source  680  operating at λ C2  but also to pump LD  690  which generates the second pump signal, λ B:980 . Similarly the residual optical pump signal λ A:980  from the first optical amplifier is coupled to both the control source  680  and the pump LD  690 . Accordingly, control for the control source  680  and pump LD  690  is now derived from both the first control signal λ C1  and the residual optical pump signal δλ A:980 . However, now electrical power pump LD  690  is derived from the residual optical pump signal δλ A:980  thereby reducing the power requirements of the optical amplifier. 
         [0072]    Now referring to third controller variant the first control signal, λ C1 , received by the controller detector  670  is not only coupled to the control source  680  operating at λ C2  but also to attenuator  695  which adjusts the power level of the residual first pump signal, δλ A:980 , to provide an output second pump signal χδλ A:980 . Similarly the residual optical pump signal δλ A:980  from the first optical amplifier is coupled to the control source  680  via tap-detector  665  which taps a predetermined portion of the residual optical pump signal δλ A:980  and provides the electrical signal to control source  680  whilst leaving the remainder of the residual optical pump signal δλ A:980  in the optical domain and coupling it forward to the attenuator  695 . Accordingly, control for the control source  680  and attenuator  695  are now derived from both the first control signal λ C1  and the residual optical pump signal δλ A:980 . However, now rather than a second pump LD direct adjustment of the residual optical pump signal δλ A:980  is provided thereby reducing the power requirements of the optical amplifier. 
         [0073]    Within embodiments of the invention described above first and second control signals λ C1  and λ C2  respectively may be at different wavelengths or alternatively the same out of band wavelength. First and second control signals λ C1  and λ C2  may overlap with a network control signal propagated along the optical link within which the optical amplifier according to an embodiment of the invention is deployed wherein this network control signal is extracted, processed, and re-inserted without passing through the optical amplifier. Where the network control signal is coupled through the optical amplifier as well as the optical signals carried by the optical link then the first and second control signals λ C1  and λ C2  respectively may be selected according to the gain profile of the optical amplifier and the filter characteristics of the add/drop elements within the optical amplifier for these signals. 
         [0074]    According to other embodiments of the invention first detector  660  and pump LD  690  which provide OEO conversion of the 980 nm pump signal may be replaced with a direct optical feed-forward path with a tap coupler, monitoring photodetector, and variable optical attenuator wherein the WDM blocks 980 nm pump signal directly between the stages. Optionally, a direct optical feed-forward path may replace the OEO conversion between the first control signal at λ C1  and the second control signal at λ C2  provided by second detector  670  and control source  680  which may or may not also provide wavelength conversion by virtue of the control source  680  emitting at a different wavelength to the first control source coupled to the first stage of the optical amplifier. It would be apparent to one skilled in the art that the direct optical feed-forward path may similarly comprise a tap coupler, monitoring photodetector, and variable optical attenuator as with the pump laser feed-forward path. It would also be apparent that all-optical wavelength conversion may be included, such as for example may be achieved with semiconductor optical amplifiers (SOAs), see for example Durhuus et al in “All-Optical Wavelength Conversion by Semiconductor Optical Amplifiers” (J. Lightwave Tech., Vol. 14, pp. 942-954). 
         [0075]    Referring to  FIG. 7  there is depicted a parallel optical amplifier configuration according to an embodiment of the invention exploiting optical pump re-use comprising first to third optical amplifiers (OAs)  7000 A to  7000 C respectively of a chain of N amplifiers terminating with Nth optical amplifier  7000 N. First OA  7000 A is depicted as amplifying optical signals received at first input port  700 A and coupling these to first output port  700 B. As depicted a pump source  710  provides the required population inversion of the first doped fiber  720  via pump signal λ 980  in order to amplify the received signals received at first input port  700 A. Also coupled to first doped fiber  720  is first control signal, λ C1 , from first control source  780 A. At the output of the first doped fiber  720  a first WDM  730  separates the first control signal, λ C1 , and the first residual pump signal δλ 980  from the amplified optical signals which are then coupled to the first output port  700 B. The first control signal, λ C1 , being coupled to first controller  790 A. 
         [0076]    Accordingly, the first residual pump signal δλ 980  is coupled to second doped fiber  740  within second OA  7000 B together with second control signal λ C2  from second control source  780 B. Second WDM  750  now separates second control signal, λ C2 , and the second residual pump signal γλ 980  from the amplified optical signals coupled to second doped fiber  740  from second input port  700 C which are then coupled to the second output port  700 D. This residual pump signal γλ 980  is then coupled to third doped fiber  760  within third OA  7000 C together with third control signal λ C3  from third control source  780 C. Third WDM  770  now separates the third control signal, λ C3 , and the third residual pump signal φλ 980  from the amplified optical signals coupled to third doped fiber  760  from third input port  700 E which are then coupled to the third output port  700 F. Accordingly this sequence repeats until the Nth OA  7000 N and within each of the second and third OAs  7000 B and  7000 C respectively the second and third control signals, λ C2  and λ C3  respectively, are coupled to second and third controllers  790 B and  790 C respectively. 
         [0077]    Using the design example supra wherein pump source  710  provides an output optical pump power of 600 mW then the residual pump powers are δλ 980 =420 mW, γλ 980 =294 mW and φλ 980 =206 mW for a constant PCE of 30% each of the first to third doped fibers  720 ,  740 , and  760  respectively. Accordingly the absorbed powers in first to third OAs  700 A through  700 C respectively would be 180 mW, 126 mW, and 88 mW respectively. However, if the first to third doped fibers are provided with PCE&#39;s of 25%, 34%, and 50% then the power absorbed within the first to third OAs  7000 A to  700 C respectively would be 150 mW, 153 mW, and 148 mW respectively. In each instance losses between sequential amplifiers and as a result of the WDM elements etc have been ignored but it would be apparent to one skilled in the art how such amplifier chains may be designed. Optionally, after the third OA  7000 C the residual pump power is φλ 980 =148 mW for the PCE sequence of 25%, 34%, 50% with initial pump power 600 mW. Accordingly, it would be evident that this residual pump power may be coupled to an OEO converter such as described above in respect of  FIGS. 4 through 6  respectively wherein the OEO converter in this instance may provide the third control signal λ C3 . 
         [0078]    Referring to  FIG. 8  there is depicted a parallel optical amplifier configuration according to an embodiment of the invention exploiting optical pump re-use. In essence the parallel optical amplifier configuration is similar to that in  FIG. 7  described supra. However, in this instance each of the depicted first to third OAs  8000 A to  8000 C respectively comprises first and second doped fiber sections  720  and  830  respectively. First OA  8000 A comprises first content source  780 A, pump source  710 , WDM  730 , and such that the first doped fiber section  720  is actively pumped and fed with the first control signal, λ C1 , in addition to the optical signals to be amplified as with the first OA  7000 A described above in respect of  FIG. 7 . However, now the output from WDM  730  is a first channel comprising the residual pump signal, γλ 980 , which couples via tap coupler  810  to the second OA  7000 B thereby providing the pump signal for that and subsequent amplifiers, and a second channel comprising the first control signal, λ C1 , which is coupled to first controller  790 A. The tapped residual pump signal, γλ 980 , is also coupled to first controller  790 A such that the first control signal, λ C1 , and tapped residual pump signal, γλ 980 , are employed in establishing the pump power for the second doped fiber section  830 . 
         [0079]    It would be apparent to one skilled in the art that variations of the embodiment described above in respect of  FIG. 8  may be employed including, but not limited to, those described above in respect of  FIGS. 4 through 7 . Optionally, a variable attenuator or fixed attenuator may be disposed between sequential amplifier stages to adjust the optical power level of the residual pump signal propagating from one optical amplifier to another. 
         [0080]    Now referring to  FIG. 9  there is depicted first to third micro-optic hybrid circuits  9000 A through  9000 C providing compact low loss implementations of the optical functional elements of parallel optical amplifier configurations described above in respect of  FIGS. 7 and 8 . As depicted first micro-optic hybrid circuit  9000 A comprises first to third micro-optic devices  910  through  930  respectively. First micro-optic device  910  comprises a combiner, isolator, and WDM such that the first control signal, λ C1 , is initially combined with the signals to be amplified, λ SIGNAL , which are then coupled via an isolator to an output port of the first micro-optic device  910 . Also coupled to a port on the output side of the first micro-optic device  910  is pump source, λ 980 , which is reflectively combined with the first control signal, λ C1 , and signals to be amplified, λ SIGNAL , which are then coupled via first doped fiber  940  and thence to second micro-optic device  920 . Second micro-optic device  920  separates the residual pump signal, δλ 980 , which is then coupled to the second micro-optic circuit  900 B. The amplified optical signal and first control signal are coupled via an isolator to the output of the second micro-optic circuit  920  and thence to third micro-optic circuit  930  comprising a WDM that separates the amplified first control signal, βλ C1 , and amplified signal, βλ SIGNAL . 
         [0081]    Accordingly, each of second and third micro-optic circuits  9000 B and  9000 C comprises internally a similar configuration of first to third micro-optic devices  910  through  930  allowing in each case a control signal to be combined with signals to be amplified, these to be amplified and then separated wherein the optical link is also isolated on either side of the amplifying doped optical fiber. Each of the first to third micro-optic circuits  910  through  930  exploit graded refractive index lenses (GRIN lenses) to collimate/focus the optical signals through one or more optical elements including, but not limited to, transmissive thin-film filters, reflective thin-film filters, polarizers, Faraday rotators, attenuators, broadband partially reflective filters, etc in order to implement a variety of optical functions including, but not limited to, narrowband WDMs, broadband WDMs, band filters, single-stage optical isolators, multi-stage optical isolators, optical circulators, and optical taps allowing the required optical functionality of the first to third micro-optic devices  910  through  930  respectively to be implemented. 
         [0082]    It would be evident to one skilled in the art that according to the optical designs implemented for the micro-optic devices that the required optical functions may be partitioned into one, two, three or more micro-optic devices which may be manufactured as multiple discrete elements that are then fusion spliced together for example. Examples of such micro-optic devices may for example be found within U.S. Pat. Nos. 6,347,170; 7,113,672; and 7,440,652 as well as U.S. Patent Applications 61/657,937; 61/657,943; 61/659,047; and U.S. Pat. No. 7,440,172 by the inventors which relate to low cost small diameter micro-optic devices. Existing commercial devices may for example be 35 mm long with diameter 5.5 mm although developments supported by the patent applications of the inventor would allow reduced diameters down to 2 mm, 1.5 mm, and potentially lower. Accordingly, very compact micro-optic circuits and hybrid circuits may be implemented with high performance including, but not limited to, low loss, high isolation, high rejection, low crosstalk, low polarization dependent loss (PDL), chromatic dispersion, and polarization mode dispersion (PMD). 
         [0083]    Within the embodiments of the invention described above in respect of  FIGS. 3 through 9  respectively emphasis has been placed upon λ=980 nm pump sources operating in conjunction with EDFA modules, gain blocks and/or amplifiers. However, it would be evident to one skilled in the art that alternatively the EDFA configurations may exploit λ=1480 nm. Optionally, within embodiments of the invention as described in respect of  FIGS. 4 through 6  wherein opto-electronic conversion or all-optical conversion is employed one stage may exploit one wavelength whilst the second stage may exploit another wavelength. It would also be evident that even wherein the pump source wavelengths are nominally the same, such as λ=980 nm, that slightly different wavelengths may be employed such as for example λ A:980 =973.5 nm and λ B:980 =975 nm. In alternate embodiments of the invention multiple pump sources may be combined such as in the instance of so-called 14xx pump sources wherein 1420 nm≦λ A:1480 ≦1456 nm, 1466 nm≦λ A:1480 ≦1495 nm, and 1496 nm≦λ A:1480 ≦1510 nm. In addition to EDFA amplifiers embodiments of the invention may exploit other dopants other than erbium including, but not limited to, Praseodymium (Pr3+), Europium (Eu3+), Neodymium (Nd3+), Terbium (Te3+), Lutetium (Lu3+), Ytterbium (Yb3+), Holmium (Ho3+), Dysprosium (Dy3+), Gadolinium (Gd3+), Samarium (Sm3+), Promethium (Pm3+), Cerium (Ce3+), Lanthanum (La3+) and Thulium (Tm3+) as well other amplification topologies such as Raman amplification for example. 
         [0084]    Specific details are given in the above description to provide a thorough understanding of the embodiments. However, it is understood that the embodiments may be practiced without these specific details. For example, circuits may be shown in block diagrams in order not to obscure the embodiments in unnecessary detail. In other instances, well-known circuits, processes, algorithms, structures, and techniques may be shown without unnecessary detail in order to avoid obscuring the embodiments. 
         [0085]    The foregoing disclosure of the exemplary embodiments of the present invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many variations and modifications of the embodiments described herein will be apparent to one of ordinary skill in the art in light of the above disclosure. The scope of the invention is to be defined only by the claims appended hereto, and by their equivalents.