US 20020012163 A1
An optical fiber amplifier has an amplification stage that uses an optical fiber pumped with pump energy of a first wavelength that oscillates through the gain medium. In one embodiment, the pumping is essentially a cavity resonator that is coupled to either end of the optical fiber such that oscillating pump energy is directed into one end of the fiber and out the other, as it reflects back and forth between the ends of the cavity. Highly reflective gratings are used to maintain the oscillation of the pump energy, and a pump energy source, such as pumped doped optical fiber, is coupled to at least one of the gratings. In another embodiment, the pump source comprises multiple reflectors that are employed at each end of the fiber, and independent pump sources each having a slightly different wavelength within the absorption spectrum of the amplifier are coupled together, such that two pump wavelengths are simultaneously oscillated through the gain medium. In another embodiment of the invention, the reflective gratings are integrated directly into a portion of the pathway through which the signal to be amplified passes. This embodiment uses a pump source that causes amplification of the pump energy in the gain medium which, in turn, provides amplification of the optical signal. A two-pass embodiment is also shown in which the optical signal enters and exits from the same optical side of the amplifier fiber. An optical circulator may be used to provide the necessary unidirectional porting of the input and output signals.
1. An optical amplifier system for amplifying an optical signal at a signal wavelength, comprising:
at least one first pump source to provide first pump light at a first wavelength;
a first optical fiber having a first active material for generating second pump light at a second wavelength and including a lasing resonator between spatially disposed reflectors forming a second pump laser source, the second pump source optically coupled to receive the first pump light from the first pump source, the first optical fiber absorptive of the first pump light and providing gain to the second pump light; and
a second optical fiber having a second active material forming a fiber amplifier at the signal wavelength, the second optical fiber optically coupled within the lasing resonator of the second pump laser source to launch the second pump light into the second optical fiber, the second optical fiber absorptive of the second pump light and providing gain to the signal wavelength.
2. The optical amplifier system of
3. The optical amplifier system of
4. The optical fiber amplifier of
5. The optical fiber amplifier of
 This application is a continuation of patent application Ser. No. 08/970,493 filed Nov. 14, 1997.
 This invention relates to the field of optical fiber amplifiers and, more particularly, to means of applying pump energy to an optical fiber amplifier.
 As is known in the art, an optical amplifier is a device that increases the amplitude of an input optical signal fed thereto. If the optical signal at the input to such an amplifier is monochromatic, the output will also be monochromatic, with the same frequency. A conventional fiber amplifier comprises a gain medium, such as a single mode glass fiber having a core doped with a rare earth material, connected to a WDM coupler which provides low insertion loss at both the input signal and pump wavelengths. The input signal is provided, via the coupler, to the medium. Excitation occurs through optical pumping from the pumping source. Pump energy that is within the absorption band of the rare earth dopant is combined with the optical input signal within the coupler, and applied to the medium. The pump energy is absorbed by the gain medium, and the input signal is amplified by stimulated emission from the gain medium.
 Such amplifiers are typically used in a variety of applications including, but not limited to, amplification of weak optical pulses such as those that have traveled through a long length of optical fiber in communication systems. Optical amplification can take place in a variety of materials including those materials, such as silica, from which optical fibers are formed. Thus, a signal propagating on a silica-based optical fiber can be introduced to a silica-based optical fiber amplifier, and amplified by coupling pump energy into the amplifier gain medium.
 Fiber amplifiers are generally constructed by adding impurities to (i.e. “doping”) an optical fiber. For a silica-based fiber, such dopants include the elements erbium and ytterbium. For example, one type of fiber amplifier referred to as an erbium (Er) amplifier typically includes a silica fiber having a single-mode core doped with erbium ions (conventionally denoted as Er3+). It is well known that an erbium optical fiber amplifier operating in its standard so-called three level mode is capable, when pumped at a wavelength of 980 nanometers (nm), of amplifying optical signals having a wavelength of approximately 1550 nanometers (nm). Likewise, an amplifier having a silica-based fiber “co-doped” with erbium and ytterbium shows excellent amplification of a 1550 nm optical signal when pumped with a wavelength of approximately 1060 nm. Since 1550 nm is the lowest loss wavelength of conventional single-mode glass fibers, these amplifiers are well-suited for inclusion in fiber systems that propagate optical signals in the wavelength vicinity of 1550 nm.
 It has been an ongoing pursuit in the field of optical fiber amplifiers to increase the power output of the amplifiers. Traditionally, pump energy is applied to the gain medium by coupling into the doped fiber either in the same propagation direction as the signal to be amplified (referred to as “co-pumping”), or by coupling it into the doped fiber in the opposite direction as the signal to be amplified (referred to as “counter-pumping). Each of these pumping methods has its own advantages, but also its own limitations. It is an object of this invention to go beyond these traditional pumping methods to provide a high power optical amplifier by providing a new means of pumping a doped optical fiber.
 In accordance with the present invention, an optical fiber amplifier is provided in which a doped optical fiber gain medium is pumped by pump energy that is oscillated through a substantial portion of the gain medium. That is, a resonant cavity for the pump energy is formed that includes the amplifier fiber, such that the pump energy is reflected back and forth through the gain medium. The output coupling for the resonant cavity is absorption by the doped fiber, which results in amplification of the optical signal by stimulated emission as it passes through the fiber.
 The optical pumping apparatus used to generate the oscillating pump energy may take a number of different forms. In general, reflectors that reflect optical energy at the pump wavelength are coupled to either side of the optical fiber, and reflect the pump energy back and forth through the gain medium. In one embodiment, each reflector is located in its own optical pathway separate from the optical fiber, a first of these pathways being coupled to a first side of the optical fiber while the second is coupled to the second side of the fiber. The coupling is preferably by wavelength selective couplers, such as WDMs, so that only the pump energy is diverted from the signal path and directed to the reflectors. In one variation of this embodiment, each reflector is coupled to a pump energy generator, preferably in the form of a pumped optical fiber, so that pump energy is generated on either side of the optical fiber. In another variation, the pump energy is generated at only one side, while the other side has only a reflector. In either case, the pump energy is oscillated in the pathway between the reflectors, providing the desired oscillation of pump energy through the fiber gain medium.
 When the pump energy is coupled into the optical fiber using wavelength selective couplers, another variation of the invention involves using a plurality of pump wavelengths, each of which is within the absorption band of the doped optical fiber. In such an embodiment, a plurality of reflectors may be used in each of the two pump energy pathways located, respectively, to either side of the optical fiber. The different pump wavelengths are preferably close in wavelength, and each set of reflectors (i.e. each group of reflectors located to one optical side of the doped fiber) may be coupled together using narrowband wavelength selective couplers, such as narrowband WDMs. Furthermore, some or all of the reflectors may be coupled to pump sources that generate optical energy at the desired pump wavelengths.
 In each of the above embodiments, the optical fiber may be doped with erbium/ytterbium (Er/Yb), which provides amplification of a 1550 nm optical signal when the fiber is pumped at a wavelength of 1064 nm. The highly reflective gratings of the fiber may then be selective to reflect the 1064 nm wavelength, and the pump sources may themselves be optical fibers doped, preferably with ytterbium (Yb), and pumped with optical energy at a wavelength of, for example, 915 nm.
 In another embodiment of the invention, the reflectors for providing oscillation of the pump energy through the gain medium are integrated into a portion of the signal pathway, and may be integrated into the amplifier fiber itself. These reflectors, preferably highly reflective Bragg gratings, are wavelength specific, and do not significantly interfere with the optical signal to be amplified. That is, the reflectors maintain oscillation of optical energy at the pumping wavelength through the gain medium, while the optical signal passes through them and through the doped optical fiber. To cause generation of energy at the pump wavelength, a pump source is coupled into the gain medium and causes amplification of optical energy at the pump wavelength within the gain medium. Thus, the output of the pump source is absorbed by the doped optical fiber, and amplifies the pump energy that oscillates between the two reflectors. The oscillating pump energy, in turn, is absorbed by the gain medium and amplifies the optical signal passing through the fiber. In such an embodiment, a ytterbium-doped fiber may be used. The signal wavelength could then be 1090 nm, the pump energy wavelength 1064 nm, and the pump source wavelength 915 nm.
 In one variation of the embodiment having reflectors integrated into the signal pathway, the amplifier is a two-pass amplifier. A signal reflector is provided at one end of the doped fiber that reflects optical energy at the wavelength of the optical signal. The optical signal is then coupled through an input port into the other end of the fiber. The optical signal is amplified as it passes through the fiber, which is pumped by oscillating pump energy. Upon reaching the end of the fiber, the optical signal encounters the signal reflector, and is directed back through the optical fiber, where it is further amplified. At the end of the fiber where it initially entered, the amplified optical signal is coupled out through an output port.
 The foregoing embodiment may be accomplished by using an optical circulator, which allows unidirectional coupling of an optical signal from one port of the circulator to another. If the optical signal is input to a first port of the circulator, the amplifier may be located in a branch coupled to a second port, which receives the optical signal from the first port. The amplified optical signal, after passing twice through the gain medium, returns to the second port of the circulator, where it is coupled to a third port. In one version of this embodiment, the third port is simply a system output port. However, a second amplifier, identical to that connected to the second port, may be located in a branch coupled to the third port, and a fourth port of the circulator could then serve as the system output port. Additional amplifier branches can also be added in a similar manner up to the maximum port capacity of the circulator.
FIG. 1 is a schematic view of a fiber amplifier according to a first embodiment of the present invention that couples pump sources into either side of a gain medium to oscillate pump energy across the gain medium.
FIG. 2 is a schematic view of a fiber amplifier according to a second embodiment of the present invention that couples a pump source into a first side of a gain medium, and reflects pump energy into an opposite side of the gain medium using a periodic grating that is highly reflective at the pump wavelength.
FIG. 3 is a schematic view of an alternative embodiment of the invention which is similar to the embodiment of FIG. 2, but which uses a plurality of coupled pump sources directed into each side of the amplifier gain medium.
FIG. 4 is a schematic view of another alternative embodiment of the invention in which gratings that reflect the pump signal are integrated directly into the path of the signal to be amplified.
FIG. 5 is an embodiment similar to that of FIG. 4 that uses an optical circulator to direct a signal to be amplified into and out of one or more arms of the circulator, each of which contains an oscillating pump signal in a gain medium.
 Depicted in FIG. 1 is a fiber amplifier having an optical pumping arrangement that uses two pump sources 30, 32 together to provide oscillation of the pump signal. In the preferred embodiment, an optical signal entering the amplifier via input port 10 has a wavelength λS in the wavelength range of 1550 nm, and is therefore in the peak transmission range of conventional silica-based optical fiber. From port 10, the input signal is directed to wavelength division multiplexer (WDM) 24, by which it is coupled into doped optical fiber 26. The signal is amplified in the fiber 26, and is coupled out of the fiber gain medium by WDM 28 and directed to output port 34, where the amplified signal may be used in any desired application.
 The optical fiber 26 is doped with a rare earth element, and is the heart of the fiber amplifier. In the preferred embodiment, the fiber is doped with erbium/ytterbium (Er/Yb) to create the desired gain medium. Given an Er/Yb doping, the optical fiber 26 may then be pumped with optical pump energy in the wavelength range of 1064 nm by the combination of pump sources 30 and 32, which are coupled into the fiber 26 via 1064/1550 WDMs 24 and 28, respectively. Pumping at this wavelength results in absorption of the pump energy by the doped fiber, and a corresponding amplification of the optical signal within the amplifier gain medium by stimulated emission at the signal wavelength. As discussed below, the arrangement of pump sources 30, 32 as shown provides pump energy that oscillates back and forth through the amplifier gain medium.
 In the preferred embodiment, each of the pump sources 30, 32 is of identical construction. Pump source 30 consists of double-clad optical fiber 36, laser diode 40 and highly reflective Bragg grating 42. Pump source 32 consists of double-clad optical fiber 38, laser diode 44 and highly reflective Bragg grating 46. Each of the Bragg gratings 42, 46 is highly reflective to the pumping wavelength of 1064 nm. The fibers 36, 38 are each doped with ytterbium and each has optical energy input to it by its respective laser diode source 40, 44 at a wavelength in a high absorption wavelength range for a Yb-doped fiber. This results in a population inversion in each of the pump fibers 36, 38 which, in the resonant cavity arrangement of the two pump sources, results in the development of pump signal energy at the desired 1064 nm wavelength that oscillates between the gratings 42, 46. For example, generating pump energy at a wavelength of 915 nm with diodes 40, 44 and injecting it into the double-clad fibers is sufficient to cause the stimulated emission of optical radiation at 1064 nm. The gratings 42, 46 then reflect the 1064 nm pump energy back and forth between them, and through the optical fiber 26.
 By establishing the pump energy reflection path through the optical fiber 26 itself, the pump energy at 1064 nm, necessary to continuously amplify the 1550 nm optical signal, can be replenished without the risk of destabilization that would exist if the outputs of two individual lasers were directed toward each other. For example, the direction of 1064 nm fiber lasers toward each other has been shown to destabilize both lasers, and cause both to lase at 1106 nm. In this embodiment, the two gratings 42, 46 combine to form the two ends between which the pump energy is reflected. The output coupler of the cavity is the absorption of pump energy by the fiber amplifier. Thus, by having the two sources 30, 32 act in concert, the risk of destabilization is removed.
 In order to maximize the efficiency of the amplifier, the relationship between the absorption by the amplifier and the pumping by the sources 30, 32 may be exploited. The optimum output transmission for a laser cavity is given by:
T opt =−L i+%(g o L i)
 where Topt is the optimum output transmission, Li is the internal cavity losses, and go is the unsaturated gain of the laser cavity. Since the fiber amplifier can be considered to be the output coupler for the resonant cavity formed by the pump sources 30, 32, and since the transmission through the fiber amplifier is T=e−αR (where R is the length of the fiber amplifier), the optimum length for optical fiber 26 is:
R opt=(−1/α)log[−L i+%(g o L i)].
 Shown in FIG. 2 is an alternative embodiment of the present invention. Unlike the embodiment of FIG. 1, which uses two pumped optical sources 30, 32 as the ends of the pump energy pathway, in FIG. 2, one of these sources is replaced with a periodic grating that is highly reflective at the pump energy wavelength. For example, grating 42 may be used to replace pump source 30. As in FIG. 1, the input signal enters port 10, is coupled into the gain medium of doped optical fiber 26 via WDM 24, and is coupled out of the gain medium via WDM 28 and directed toward output port 34. However, while the embodiment of FIG. 1 uses two individual pump sources 30, 32, the FIG. 2 embodiment uses only one. The high reflectivity of grating 42 at the desired wavelength of the pump energy (e.g., 1064 nm) allows it to function as one end of an oscillation path, while pump source 32 acts as the other end. The pump energy developed in the fiber 38 at 1064 nm is thus reflected back and forth between grating 46 and grating 42, passing through amplifier fiber 26 in the process and providing the desired pumping to the amplifier.
 Shown in FIG. 3 is another alternative embodiment of the present invention in which a plurality of fiber laser sources is used for each of the pump sources 30, 32. In the preferred version of this embodiment, each of the fiber lasers is similar to those used in the FIG. 1 embodiment, consisting of a double-clad fiber with a highly reflective Bragg grating and a laser diode pump. However, the fiber lasers for a given pump source 30, 32 each have different wavelengths, close to each other, in the range of 1064 nm. In pump source 30, double-clad fiber 68 is Yb-doped, and has a laser diode 70 as a source which provides pump energy in the range of, e.g., 915 nm. The grating 72 is selected to be highly reflective at a first wavelength in the 1064 nm range, such as 1060 nm. The double-clad fiber 74 and diode 76 can be identical to the fiber 68 and diode 70, respectively, except that Bragg grating 78 is highly reflective at a wavelength close to, but different from, the wavelength of grating 72. For example, grating 78 may be selected to be highly reflective at a wavelength of 1070 nm. The two different wavelengths (e.g. 1060 nm and 1070 nm) of pump source 30 are both in the wavelength absorption range of the Er/Yb doped amplifier fiber 26, and are therefore both adequate pump wavelengths for pumping the amplifier. These wavelengths are coupled together into the fiber 26 after being combined using narrowband WDM coupler 80, a 1060/1070 WDM.
 In the preferred version of this embodiment, the construction of pump source 32 is identical to that of pump source 30. Yb-doped, double-clad optical fiber 82 is pumped by diode 84 at a wavelength of, e.g., 915 nm, and is stabilized by highly reflective grating 86 to an output wavelength of 1060 nm. Yb-doped, double-clad optical fiber 88 is pumped by diode 90 at a wavelength of, e.g., 915 nm, and is stabilized by highly reflective grating 92 to an output wavelength of 1070 nm. The 1060 nm and 1070 nm wavelengths of the two fiber lasers are combined by narrowband WDM 94, which is coupled to the opposite end of the amplifier fiber 26.
 In the embodiment of FIG. 3, two overlapping oscillation paths are established, one for pump energy at 1060 nm and one for pump energy at 1070 nm. WDMs 80 and 94 allow these pump energies to be coupled for propagation through the optical fiber 26 of the amplifier, and segregated at the different fibers of each pump source 30, 32. It will be understood by those skilled in the art that, while the embodiment of FIG. 5 shows two fiber lasers per pump source, more than two fiber lasers per pump source could also be used. This would require the coupling of the additional pump energy wavelengths into the fiber amplifier using additional WDMs, but would function according to the same principles as the construction shown in FIG. 3.
 Shown in FIG. 4 is another alternative embodiment of the invention, in which the oscillating pump energy used to pump a fiber amplifier is achieved by integrating two pump gratings directly into a fiber pathway through which the input optical signal passes. This pathway may or may not be part of the doped region of the amplifier fiber, but the embodiment removes the need for coupling the pump signals into the doped fiber, as is done using WDMs in the foregoing embodiments. As shown, one of the pump reflection gratings 42, 46 is positioned to either side of doped amplifier fiber 26. The gratings are highly reflective at the desired pumping wavelength, e.g. 1064 nm. Certain amplifier fibers (e.g., a double-clad, Yb-doped fiber), can serve as the gain medium for generating both the pump energy at 1064 nm, as well as for a desired signal wavelength λS such as 1090 nm.
 In the embodiment of FIG. 4, initial pumping of the fiber 26 is provided by optical source 41, which may be a laser diode with an output wavelength of 915 nm. This pumping energy is coupled into the gain medium via 915/1090 WDM 27. Absorption of the energy at 915 nm results in the development of oscillating pump energy between gratings 42 and 46 at the 1064 nm wavelength. That is, an oscillation path for the 1064 nm pump energy is maintained between the two gratings 42, 46. This further pumps the fiber gain medium, and allows the input optical signal, at the 1090 nm wavelength, to be amplified by stimulated emission as it passes through the amplifier fiber. The amplifier optical signal is thereafter directed to output port 34 via WDM 27.
FIG. 5 depicts a variation of the embodiment shown in FIG. 4. As in FIG. 4, gratings 42, 46 are integrated into a signal pathway to either side of a fiber amplifier 26, doped with, e.g., ytterbium. The gratings 42, 46 are highly reflective at a desired pumping wavelength, such as 1064 nm, and define the desired oscillation path for the pump energy. In the FIG. 5 embodiment, the amplifier and gratings 42, 46 are arranged as a first branch of an optical circulator 35. An optical circulator is a commercially available optical coupler that allows unidirectional one-to-one optical coupling between a set of optical ports. That is, optical energy input to one of the circulator ports is directed to only one other port, and may only be coupled between those two ports in one propagation direction.
 Also in the signal path with the fiber amplifier is signal grating 47, which is highly reflective at the wavelength of the desired optical signal (e.g., 1090 nm) and is positioned to the side of the amplifier fiber 26 and gratings 42, 46 away from circulator 35. A pump energy source 41 is coupled into the optical fiber amplifier. This source may be a laser diode having an output wavelength of, e.g., 915 nm. The output wavelength of the pump source 41 is absorbed by the doped fiber 26, and results in the development of an oscillating pump signal between gratings 42 and 46 at the 1064 nm wavelength. The 1064 nm wavelength signal, in turn, further pumps the amplifier fiber, allowing it to provide amplification to a 1090 nm signal passing through it by stimulated emission.
 In FIG. 5, the signal to be amplified is directed from input port 10 to a first port 43 of the circulator 35. This results in the signal being output at a second port 45 of the circulator, where it is directed into the pumped amplifier arrangement 51. The optical signal is amplified as it passes through the fiber 26, and is thereafter directed to grating 47. Grating 47, being highly reflected at the signal wavelength, redirects the amplified signal back through amplifier fiber 26, where it is further amplified. When the amplified signal returns to circulator port 45, it is directed to circulator port 49.
FIG. 5 shows a second amplifier stage 51 in the branch connected to circulator port 49. The second amplifier stage 51 may be identical to that located in the branch connected to port 45, and provides additional amplification of the optical signal in the same manner. After the signal passes through the amplification stage 51 of the second branch, it returns to circulator port 49, from which it is directed to port 53, and thereafter to signal output port 34. While two amplifier stages 51 are shown in FIG. 5, those skilled in the art will recognize that the number of stages used is optional. For example, port 49 of the circulator could as easily lead to signal output port 34, if only one stage of amplification was desired. Likewise, a circulator with more than four ports could be used, and additional amplification stages 51 beyond the two shown in FIG. 5 could be used.
 In the embodiments of the present invention, pump energy is oscillated through the gain medium comprising a fiber amplifier. The pump energy is thereby directed into both ends of the fiber amplifier without the destabilization risk associated with directing two independent pump energy sources toward each other. Oscillation of the pump energy through the gain medium also provides recycling of the pump energy (as compared with simply passing the pump energy once or twice through the gain medium), and therefore helps to improve the overall power conversion of the amplifier.
 While the invention has been shown and described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. For example, the pumping arrangements of the invention may be applied to amplifiers having different doping configurations and desired pump wavelengths.