Abstract:
An optical amplifier ( 200 ) splits an optical signal into two signals ( 210, 212 ). A first amplifier section ( 202 ) receives the first signal ( 210 ). The first amplifier section ( 202 ) includes a first optical fiber ( 220 ), having a first input, for generating a first output power ( 230 ), and a first pump source ( 222 ) is coupled to the first input, for supplying a first energy amount to the first optical fiber ( 220 ). The optical amplifier ( 200 ) also includes a second amplifier section ( 204 ) to receive the second signal ( 212 ), which is arranged in parallel to, and under common control with, the first amplifier section ( 202 ). The second amplifier section ( 204 ) includes a second optical fiber ( 240 ), having a second input, for generating a second output power ( 250 ), and a second pump source ( 232 ) is coupled to the second input, for supplying a second energy amount to the second optical fiber ( 240 ). A total power ( 280 ) of the first output power ( 230 ) and the second output power ( 250 ) is at least about 600 mill Watts.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field  
         [0002]     Aspects of this invention relate generally to optical fiber communication, and, more specifically, to an optical amplifier and to a method for amplifying an optical input signal.  
         [0003]     2. Description of Related Art  
         [0004]     Optical amplifiers, such as rare earth doped fiber amplifiers, are frequently found in fiber-optic communication systems and networks. The cable television industry, for example, provides communication of information (for example, audio, video, multimedia, and data) between a headend and a plurality of consumer devices at least in part via a fiber-optic network—the headend typically transmits information in an optical format, using one or more fiber optic links, and consumer devices may also generate information that may be converted into an optical format for transmission to the headend.  
         [0005]     Passive optical networks (“PONs”) are increasingly being used to deliver cable communication services to consumers at affordable prices. A PON architecture is one in which active components are located either centrally (for example, at the headend), or locally (for example, at consumer locations), while passive components are disposed in between. Single optical amplifiers capable of delivering power in a range of about  600  mill Watts (“mW”) to  3  Watts, which can each serve several-hundred customer locations, are desirable to overcome losses of passive components in a PON.  
         [0006]     Optical amplifiers that generate suitable powers, however, are often difficult to reliably achieve using conventional rare earth doped fiber technologies. A conventional erbium-doped fiber amplifier architecture  10  is shown in  FIG. 1 . Amplifier architecture  10  features two stages of erbium-doped fibers  12 ,  14 . To provide gain to erbium-doped fibers  12 ,  14 , fibers  12 ,  14  are pumped optically by pumps  16 . Pumps  16  are coupled to wave division multiplexers  18 . Erbium-doped fibers  12 ,  14  and wave division multiplexers  18 , however, are often unable to handle higher pump powers, restricting an efficient output power  20  of amplifier architecture  10  to about 500 mW. In addition, as output power  20  increases, the optical components of amplifier architecture  10  should be qualified for high power, increasing the cost and reliability of the amplifier. Further, failure of any particular pump in amplifier architecture  10  may lower output power  20 , which in turn may lower the power in an entire downstream network, affecting multiple consumers. Still further, having a number of pumps and fibers in a serial configuration may cause the amplified wavelength range to shift to longer wavelengths, for example, 1560 nanometers, which may be undesirable in many applications.  
         [0007]     The use of cladding pumped technology, in which an ytterbium fiber laser (915 or 975 nanometer pump) is used for pumping in an erbium-ytterbium amplifier, may be suitable for some applications. Cladding pumped technology alone, however, is not currently as developed or reliable as conventional erbium-doped fiber technology, and also requires the use of special pumps and components, increasing amplifier cost. Moreover, a single component having the serial architecture illustrated in  FIG. 1  may still not reliably generate up to  3  Watts of output power.  
         [0008]     There is therefore a need for a reliable, low-cost, easily configurable, single-component optical amplifier capable of producing at least about 600 mW—and in some variations up to 3 Watts—of output power.  
       SUMMARY  
       [0009]     According to an aspect of the present invention, an optical amplifier for amplifying an optical input signal includes a first optical coupler, for splitting the optical input signal into a first optical signal and a second optical signal. A first amplifier section is responsive to receive the first optical signal. The first amplifier section includes a first rare earth doped optical fiber for generating a first optical output power, a first pump source, and a second pump source. The first and second pump sources are for supplying a first energy amount, in a common wavelength band, to the first rare earth doped optical fiber. The optical amplifier also includes a second amplifier section responsive to receive the second optical signal, which is arranged in parallel to, and under common control with, the first amplifier section. The second amplifier section includes a second rare earth doped optical fiber for generating a second optical output power, a third pump source, and a fourth pump source. The third and fourth pump sources are for supplying a second energy amount, in the common wavelength band, to the second rare earth doped optical fiber. A total power of the first optical output power and the second optical output power is at least about 600 mill Watts.  
         [0010]     The common wavelength band may be between about 1540-1570 nm. Both the first and second rare earth doped optical fibers may be doped with erbium, or a combination of erbium and ytterbium. The pump sources may be single-mode or multi-mode pumps, and may have switched temperature or optical power control. The first pump source may be arranged in such a manner to supply energy to the first erbium doped optical fiber in a forward direction relative to the first optical signal, and the second pump source may be arranged in such a manner to supply energy to the first erbium doped optical fiber in a reverse direction relative to the first optical signal. Likewise, the third pump source may be arranged in such a manner to supply energy to the second erbium doped optical fiber in a forward direction relative to the second optical signal, and the fourth pump source may be arranged in such a manner to supply energy to the second erbium doped optical fiber in a reverse direction relative to the second optical signal.  
         [0011]     The optical amplifier may further include at least one wave division multiplexer (“WDM”) disposed in the first amplifier section, responsive to supply the first energy amount to the first rare earth doped fiber; and at least one WDM disposed in the second amplifier section, responsive to supply the second energy amount to the second rare earth doped fiber. Also, a first optical coupler, for splitting the first optical output power into a first plurality of output signals (for example, four), and a second optical coupler, for splitting the second optical output power into a second plurality of output signals (for example, four), may be provided. In addition, to increase the total power to a range between about 600 mill Watts to 900 mill Watts (or to at least about 1 Watt, using erbium/ytterbium-doped fibers, multi-mode pumps, power combiners, and/or high-power isolators), the optical input may be generated by a third rare earth doped optical fiber, and a fifth pump source, may supply energy to the third rare earth doped optical fiber.  
         [0012]     In accordance with another aspect of the present invention, an optical amplifier for amplifying an optical input signal includes a first optical coupler, for splitting the optical input signal into a first optical signal and a second optical signal. A first amplifier section is responsive to receive the first optical signal. The first amplifier section includes a first rare earth doped optical fiber, having a first input, for generating a first optical output power, and a first pump source coupled to the first input, for supplying a first energy amount to the first rare earth doped optical fiber. The optical amplifier also includes a second amplifier section responsive to receive the second optical signal, which is arranged in parallel to, and under common control with, the first amplifier section. The second amplifier section includes a second rare earth doped optical fiber, having a second input, for generating a second optical output power, and a second pump source coupled to the second input, for supplying a second energy amount to the second rare earth doped optical fiber. A total power of the first optical output power and the second optical output power is at least about 600 mill Watts.  
         [0013]     In accordance with a further aspect of the present invention, a method for amplifying an optical input signal includes: splitting the optical input signal into a first optical signal and a second optical signal; receiving the first optical signal at a first amplifier section, the first amplifier section having a first rare earth doped optical fiber for generating a first optical output power; supplying a first energy amount to the first rare earth doped optical fiber via a first pump source and a second pump source, the first and second pump sources operating in a common wavelength band; receiving the second optical signal at a second amplifier section, the second amplifier section arranged in parallel to, and under common control with, the first amplifier section, the second amplifier having a second rare earth doped optical fiber for generating a second optical output power; and supplying a second energy amount to the second rare earth doped optical fiber via a third pump source and a fourth pump source operating in the common wavelength band. A total power of the first optical output power and the second optical output power is at least about 600 mill Watts.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]      FIG. 1  illustrates a conventional erbium-doped amplifier architecture.  
         [0015]      FIG. 2  illustrates an optical amplifier architecture in accordance with certain aspects of the present invention.  
         [0016]      FIG. 3  is a schematic view of pump control electronics usable in the optical amplifier architecture shown in  FIG. 2 .  
         [0017]      FIG. 4  graphically depicts experimental results of output power vs. wavelength for various optical signals input to the optical amplifier architecture shown in  FIG. 2 .  
         [0018]      FIG. 5  graphically depicts experimental results of noise figure vs. wavelength for various optical signals input to the optical amplifier architecture shown in  FIG. 2 .  
         [0019]      FIG. 6  illustrates an optical amplifier architecture in accordance with other aspects of the present invention.  
         [0020]      FIG. 7  is a block diagram of a MEMS switch control, usable in connection with the optical amplifier architecture shown in  FIG. 6 . 
     
    
     DETAILED DESCRIPTION  
       [0021]     Turning now to the drawings, wherein like numerals designate like components,  FIG. 2  illustrates an optical amplifier architecture  200  in accordance with certain aspects of the present invention. Amplifier architecture  200  includes an amplifier section  202  and an amplifier section  204 , which are under common control (not shown). An optical coupler  206 , which may be a 1×2 50/50 optical coupler, such couplers being well-known and widely available, is responsive to receive an optical input signal  208 , and to split optical input signal  208  into a signal component  210 , which is receivable by amplifier section  202 , and a signal component  212 , which is receivable by amplifier section  204 . Optical input signal  208  may be a signal that is received directly from a passive optical network (“PON”), or may optionally be received from the output of a booster  205 .  
         [0022]     As shown, booster  205  includes a rare earth doped optical fiber  214 , pumped optically by pump  16 , which is coupled to a wave division multiplexer (“WDM”)  18 . The use of booster  205  further increases an output power  280  (discussed further below) over a predetermined wavelength range, and improves the noise figure (also discussed further below), of amplifier architecture  200 .  
         [0023]     Optical fiber  214  may be doped with erbium ions—an erbium-doped optical fiber is conventionally referred to as an “ER +3 ” fiber, and an optical amplifier using an ER +3  fiber is conventionally referred to as an erbium-doped fiber amplifier (“EDFA”)—although optical fiber  214  may be doped with other rare earth ions, such as neodymium, praseodymium, ytterbium, or a combination thereof. Pump  216  provides additional gain to optical fiber  214 . Pump  216  may be, for example, a laser diode or another type of fiber laser or device for imparting optical gain, such devices being well-known and widely available. WDM  218  couples wavelengths within a predetermined wavelength range, such as a range between 980 nanometers (“nm”), and 1550 nm, reducing optical energy supplied by pump  216  in the predetermined wavelength range. WDM  218  may be any suitable coupling device, such devices being well known and widely available. Booster  205  may be an integral part of, or a separate component from, amplifier architecture  200 .  
         [0024]     During operation, amplifier section  202  produces an optical power  230 , by amplifying signal component  210  in rare earth doped optical fiber  220 , using optical energy supplied by pumps  222  and  224 , which are coupled to WDMs  226  and  228 , respectively. Similarly, amplifier section  204  produces an optical power  250 , by amplifying signal component  212  in rare earth doped optical fiber  240 , using optical energy supplied by pumps  232  and  234 , which are coupled to WDMs  236  and  238 , respectively.  
         [0025]     As shown, rare earth doped optical fibers  220  and  240  are ER +3  fibers, but may be fibers doped with other rare earth ions. Pumps  222  and  232  supply optical energy to input sides of optical fibers  220  and  240 , respectively, while pumps  224  and  234  supply optical energy to output sides of optical fibers  220  and  240 , respectively (in a reverse direction relative to optical signals  210  and  212 , respectively). Pumps  222 ,  224 ,  232 , and  234  may be, for example, single-mode laser diodes or other types of fiber lasers or devices for imparting optical gain, such devices being well known and widely available. WDMs  226 ,  230 ,  236 , and  238  couple optical energy from pumps  222 ,  224 ,  232 , and  234 , respectively, in a common wavelength range—as shown, a range between 1480 nm and 1550 nm. It will be appreciated, however, that other arrangements of pumps, WDMs, and wavelength ranges are possible.  
         [0026]     Power splitters  260  and  270 , which may be 1×4 power splitters or other types of power splitters, such components being well-known and widely available, receive optical powers  230  and  250 . A sum of powers  230  and  250  provides a total output power  280 .  
         [0027]     If it is desirable to protect certain components—such as pumps  216 ,  222 ,  224 ,  232 , and  234 —from transient currents that may develop at higher powers, and/or to limit an amount of signal power launched into a particular length of optical fiber, certain electronics for temperature and optical power control may be employed.  FIG. 3  is a schematic view of control electronics usable in conjunction with pumps  216 ,  222 ,  224 ,  232 , or  234  (shown in  FIG. 2 ) o adjust pump biases, resulting in the substantially uniform reduction of the various components of output power  280  and/or output ports thereof.  FIG. 4  graphically depicts experimental results of output power (mW) vs. wavelength (run) for various optical signals (dB) input to an optical amplifier having amplifier architecture  200 , shown in  FIG. 2 . More specifically, the vertical axis of  FIG. 3  depicts output power  230  or  250  of amplifier section  202  or  204 , respectively, with the use of built-in booster  205  with a pump coupled to a WDM having a wavelength range of between about 980 nm-1550 nm. The optical amplifier is an EDFA, with single-mode pumps coupled to WDMs having wavelength ranges of between about 1480 nm-1550 nm. It can be seen that at an operating wavelength window of between about 1540 nm-1570 nm, the output power is at least about 300 mW.  
         [0028]      FIG. 5  graphically depicts experimental results of noise figure (CNR or NF) vs. wavelength (nm) for various optical signals (dB) input to the optical amplifier configured as set forth in connection with  FIG. 3 . It can be seen that, for a particular optical signal, the noise figure is substantially constant over a wavelength range of between about 1535 nm to over 1560 nm. To further improve noise figures (for example, to address bleed-through of certain wavelengths, such as 1480 nm, from pumps such as pumps  228  and/or  238 , shown in  FIG. 2 ), filter-based WDMs may be used.  
         [0029]     Referring again to  FIG. 2 , it can be seen that using amplifier architecture  200 , which features all optics within a single optical tree/component, the optional use of booster  205 , and/or certain electronic pump controls or WDM filters, an overall output power  280  of at least about 600 mW, and up to about 900 m, is reliably achievable, and a wavelength band from approximately 1540 nm to 1560 nm can be amplified with a substantially similar amount of gain Because output power  280  does not all pass through a single path, special components with high power tolerances are not necessary. In addition, the number of output ports is configurable with minimum effort, and bifurcated power paths may operate independently, minimizing effects of component failures in any one path, and allowing individual sections of a network to be serviced/maintained independently.  
         [0030]      FIG. 6  illustrates an optical amplifier architecture  600 , in accordance with further aspects of the present invention, which is capable of producing an increased total output power  680  (discussed further below). Like optical amplifier architecture  200 , amplifier architecture  600  includes an amplifier section  602  and an amplifier section  604 , which are under common control (not shown). An optical coupler  606 , which may be a 1×2 50/50 optical coupler, such couplers being well-known and widely available, is responsive to receive an optical input signal  608 , and to split optical input signal  608  into a signal component  610 , which is receivable by first section  602 , and a signal component  612 , which is receivable by second section  604 .  
         [0031]     Optical input signal  608  may be a signal that is received directly from a PON, or may optionally be received from the output of one or more boosters  615 , which would include elements such as rare earth doped optical fiber  214 , pump  216  and WDM device (not shown, shown in, described in connection with,  FIG. 2 ). Booster  615  further increases output power  680  over a predetermined wavelength range, and improves the noise figure of amplifier architecture  600 .  
         [0032]     During operation, amplifier section  602  produces an optical power  630 , by amplifying signal component  610  in rare earth doped optical fiber  620 , using optical energy supplied by pumps  622  and  624 , which are coupled to a multimode power combiner  626 . Similarly, amplifier section  604  produces an optical power  650 , by amplifying signal component  612  in rare earth doped optical fiber  640 , using optical energy supplied by pumps  632  and  634 , which are coupled to a multimode power combiner  636 .  
         [0033]     As shown, rare earth doped optical fibers  620  and  640  are double-clad erbium-ytterbium fiber spools, but may be fibers doped with other rare earth ions. Pumps  622  and  624  supply optical energy to the input side of optical fiber  620 , while pumps  632  and  634  supply optical energy to the input side of optical fiber  640 . Pumps  622 ,  624 ,  632 , and  634  may be high-power multimode pumps, such pumps being well known and widely available.  
         [0034]     To improve the noise figure(s) of amplifier architecture  600 , optical powers  630  and  650  produced by optical fibers  620  and  640 , respectively, are fed into high-power isolators  642  and  644 , respectively. High-power isolators are well-known and widely available components (for example, high-power WDMs). Power splitters  660  and  670  may be utilized to configure a number of output ports (eight are shown), and total output power  680  is at least about 1 W, and up to about 3 W. To further improve serviceability and safety, MEMS switches  690  may be located on the output ports, allowing the output ports to be switched off independently. A block diagram of a sample switch control for a MEMS switch  690  is shown in  FIG. 7 .  
         [0035]     Although specific functional elements and arrangements thereof have been described herein, it is contemplated that the embodiments described herein may be implemented in a variety of ways. For example, functional elements may be packaged together or individually, or may be implemented by fewer, more, or different devices, and may be either integrated within other products, or be adapted to work with other products externally. When one element is indicated as being responsive to another element, the elements may directly or indirectly coupled. Connections depicted herein may be logical or physical in practice, to achieve a coupling or communicative interface between elements.  
         [0036]     It will furthermore be apparent that other and further forms of the invention, and embodiments other than the specific embodiments described above, may be devised without departing from the spirit and scope of the appended claims.