Abstract:
An optical amplifier that is configured to amplify multiple optical signals using time-multiplexed optical energy pulses. The time-multiplexed optical energy pulses are supplied to multiple gain blocks of the optical amplifier in an alternating manner and each of the gain blocks uses the optical energy pulses that it receives to amplify one of the multiple optical signals. An optical amplifier may be configured with an optical switch to perform a switching function to direct the time-multiplexed optical energy pulses received from the pump laser to the gain blocks in an alternating manner. The total optical energy contained in each optical energy pulse may be independently controlled by varying its duty cycle or amplitude.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    Embodiments of the present invention relate generally to optical amplifiers and more specifically to optical amplifiers used in fiber-optic communications networks. 
         [0003]    2. Description of the Related Art 
         [0004]    Optical amplifiers are devices that allow the amplification of an optical signal directly, without the need for conversion to an electrical signal, and are pumped by a laser source to produce the desired signal gain. The laser source for an optical amplifier is typically a laser diode, referred to as a pump laser, and is typically the most expensive component in an optical amplifier. For applications in optical networks, optical amplifiers are commonly used in conjunction with wavelength division multiplexing (WDM), in which multiple wavelength signals contained in a single optical signal, referred to as channels, are amplified with a single optical amplifier. Because the optical amplifier serves multiple channels in WDM applications, the cost of the optical amplifier is distributed over a plurality of channels, and the per-channel cost of the pump laser contained in the optical amplifier is not a significant issue. 
         [0005]    Single-channel applications for optical amplifiers include boosting optical transmission power (booster amplifier), amplifying signals for transmission over long distance (in-line amplifier), or amplifying signals at the receiver end (pre-amplifier). In single-channel applications, the per-channel cost of a conventional pump laser is a limiting factor in providing a cost-effective solution. This is because the pump laser cost does not scale downwardly in proportion to pump laser power. In addition, in single-channel applications, the pump laser cost is not distributed over multiple channels. Consequently, there is an on-going effort to reduce the per-channel cost of optical amplifiers in fiber-optic communications networks, particularly for single-channel applications. 
         [0006]    One approach known in the art relies on the use of an optical power splitter to divide the optical energy of a single pump laser between multiple gain blocks. A substantial drawback of this method is the disproportionate increase in power consumption that results relative to the magnitude of amplification provided. When a power splitter is used to allow a single pump laser to amplify multiple optical signals, power consumption relative to amplification increases for two reasons. First, a loss of approximately 10 dB occurs at the splitter itself. Second, an optical power splitter amplifies each channel equally. As a result, all channels are amplified by the same factor as determined by the channel requiring the most amplification. The high power consumption also results in an increase in heat sink size, and a larger heat sink can appreciably increase the size and cost of an optical amplifier. 
         [0007]    Therefore, there is a need in the art for methods and apparatus that provide optical amplification with a lower per-channel cost than prior art devices and that can do so without a power consumption penalty. 
       SUMMARY OF THE INVENTION 
       [0008]    Embodiments of the invention are directed to an optical amplifier that is configured to amplify multiple optical signals using time-multiplexed optical energy pulses. The time-multiplexed optical energy pulses are supplied to multiple gain blocks of the optical amplifier in an alternating manner and each of the gain blocks uses the optical energy pulses that it receives to amplify one of the multiple optical signals. An optical amplifier may be configured with an optical switch to perform a switching function to direct the time-multiplexed optical energy pulses received from the pump laser to the gain blocks in an alternating manner. 
         [0009]    An optical amplifier according to an embodiment of the invention includes a pump laser for generating a series of optical energy pulses, and multiple gain blocks. Each gain block has an optical signal input through which an optical signal is to be received and a pump laser input through which a subset of the optical energy pulses generated by the pump laser is to be received. Where M is the number of gain blocks, the first of the M gain blocks receives the first optical energy pulse and every M-th optical energy pulse thereafter and the second of the M gain blocks receives the second optical energy pulse and every M-th optical energy pulse thereafter. 
         [0010]    An optical amplifier according to another embodiment of the invention includes a pump laser, an optical switch coupled to the pump laser to receive optical energy pulses from the pump laser, and multiple gain blocks, each of which is coupled to the optical switch to receive a series of optical energy pulses from the optical switch for use in generating an amplified optical signal. The optical switch comprises a pump laser input and multiple pump laser outputs, each coupled to a different one of the gain blocks, and the optical switch directs the optical energy pulses received through the pump laser input to the multiple pump laser outputs in an alternating manner. 
         [0011]    A method of amplifying multiple optical signals, according to an embodiment of the invention, includes the steps of generating first and second optical energy pulses with a single pump laser, directing the first optical energy pulse to a first gain block via an optical switch, directing the second optical energy pulse to a second gain block via the optical switch, and amplifying the optical signals received by the first and second gain blocks using the first and second optical energy pulses. The pulse width and/or the amplitude of the optical energy pulses may be varied so that the total optical energy that is supplied to the gain blocks can be controlled independently. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0012]    So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
           [0013]      FIG. 1  is a block diagram of an optical amplifier according to an embodiment of the invention. 
           [0014]      FIGS. 2A ,  2 B, and  2 C are graphs of pump laser output v. time as generated in the optical amplifier of  FIG. 1 . 
       
    
    
       [0015]    For clarity, identical reference numbers have been used, where applicable, to designate identical elements that are common between figures. It is contemplated that features of one embodiment may be incorporated in other embodiments without further recitation. 
       DETAILED DESCRIPTION  
       [0016]    Embodiments of the invention contemplate an optical amplifier configured to multiplex the output of a single pump laser in the time domain, so that an optical energy pulse output from the single pump laser is directed to multiple gain blocks sequentially. The time period between the deliveries of optical pulses to each of the multiple gain blocks is controlled to be less than the mean lifetime of excited Erbium ions in the gain block. Consequently, the time multiplexing of the output power of the pump laser has little influence on the excited ion population in any gain block. 
         [0017]      FIG. 1  is a block diagram of an optical amplifier  100  according to an embodiment of the invention. Optical amplifier  100  is configured to amplify four optical input signals. In  FIG. 1 , dashed arrows, e.g.,  101 ,  103  and  105 , represent pathways of electrical or electronic signals, and solid arrows, e.g.,  110 A-D,  114 A-D and  118 A-D, represent pathways of light or optical signals. Preferably, the optical pathways illustrated in  FIG. 1  are made up of optical fibers and any associated focusing, collimating, and/or other optics needed to insert light into and extract light out of the optical fibers. Alternatively, the various optical pathways illustrated in  FIG. 1  may be constructed using other optical components, such as free-space optics, e.g., mirrors, prisms and lenses, or by using planar waveguides. 
         [0018]    Optical amplifier  100  includes a plurality of optical gain blocks  112 A-D that receive a plurality of optical signals  110 A-D to be amplified, and generate a plurality of amplified optical signals  114 A-D using optical energy pulses  108 A-D generated by a pump laser diode  106  and switched through optical switch  107 . In this embodiment, optical gain blocks  112 A-D include an Erbium-doped fiber, and a heat sink  109  is positioned adjacent pump laser diode  106  to provide cooling as necessary. 
         [0019]    Optical switch  107  is an optical switching device capable of routing optical signals on the timescale required to maintain each of optical gain blocks  112 A-D in a state of stimulated emission between optical energy pulses. In the embodiment illustrated in  FIG. 1 , the switching time of optical switch  107  is at least about an order of magnitude less than the pulse period of the optical energy pulses. For optical energy pulses having a period of about 10 ms, optical switch  107  is configured to have a switching time of less than 1 ms. Such optical switch may be a semiconductor switch, such as a gallium arsenide or indium-phosphide switch, a magnetic switch, or a switch based on an electro-optic ceramic material, such as lead-lanthanum-zirconate-titanate (PLZT), among others. Electro-mechanical switches may also be used in some situations. 
         [0020]    Optical amplifier  100  includes a pump driver  104  for providing a pulsed drive current  105  to pump laser diode  106  and a pump controller  102  for providing a pulsed electric control signal  103  to pump driver  104 . Optical amplifier  100  further includes a photo-detector  116  that is optically coupled to amplified optical signals  114 A-D of optical gain blocks  112 A-D and is electrically coupled to pump controller  102 . 
         [0021]    In operation, optical amplifier  100  receives optical signals  110 A-D and amplifies each according to the desired amplifier set point information provided in a setpoint signal  101 , thereby producing amplified optical signals  114 A-D. To that end, pump controller  102  receives the desired amplifier set point information in setpoint signal  101 , and generates pulsed electric control signal  103 . Pulsed electric control signal  103  is based on the desired amplifier set point information provided in setpoint signal  101  and on information contained in feedback signals  120 A-D related to the optical power of amplified optical signals  114 A-D, respectively. Pump driver  104  receives pulsed electric control signal  103  and produces pulsed drive current  105 , which is sent to pump laser diode  106 . The form of pulsed drive current  105  is related to the information provided in pulsed electric control signal  103 . 
         [0022]    Pump laser diode  106  receives pulsed drive current  105  and produces optical energy pulses  108 A-D, where optical energy pulses  108 A-D are arranged sequentially in a repeating pulse period in a fashion substantially similar to the pump laser output  250  shown in  FIG. 2B . In order to provide the optical gain necessary to produce amplified optical signals  114 A-D that match a desired amplifier setpoint, pulsed drive current  105  is adapted to independently vary the total optical energy contained in each of optical energy pulses  108 A-D as required. In one embodiment, the pulse width of each of optical energy pulses  108 A-D is modulated to vary the total optical energy contained therein. In such embodiment, the maximum output of optical energy during each energy pulse is held constant. In another embodiment, the maximum output of optical energy during each of optical energy pulses  108 A-D is modulated to vary the total optical energy contained therein. In such embodiment, the pulse width of each energy pulse is held constant. In yet another embodiment, both the pulse width and the maximum output of optical energy during each energy pulse are modulated to vary the total optical energy contained therein. In the embodiments described herein, it is understood that the total optical energy contained in each of optical energy pulses  108 A-D is independently controlled, and therefore the gain applied to each of optical signals  110 A-D by optical amplifier  100  is independently controlled. Pulse-width and intensity modulation of optical energy pulses, such as optical energy pulses  108 A-D, are described in greater detail below in conjunction with  FIGS. 2B and 2C . 
         [0023]    After pump laser diode  106  produces optical energy pulses  108 A-D, optical switch  107  then directs optical energy pulses  108 A-D to optical gain blocks  112 A-D, respectively. Gain blocks  112 A-D receive optical signals  110 A-D, respectively, and optical energy pulses  108 A-D, respectively, to produce amplified optical signals  114 A-D, respectively. The photo-detector  116  receives a sample portion  118 A-D of each amplified optical signal  114 A-D and outputs a corresponding feedback signal  120 A-D to the pump controller  102 . As noted above, feedback signals  120 A-D provide information to pump controller  102  related to the optical power of amplified optical signals  114 A-D, respectively, for comparison to the desired amplifier set point for each of gain blocks  112 A-D, respectively. 
         [0024]    Advantages of the embodiment described herein over the prior art include improvements in cost, size, complexity of control software, and power consumption of an optical amplifier configured for single-channel applications. When an optical amplifier configured with a single pump laser is, according to embodiments of the invention, used to independently amplify multiple optical signals, the per-channel cost of the optical amplifier is substantially reduced. This is due to cost sharing of common components, such as the pump laser diode, heat sink, electronics, housing, etc. In addition, by consolidating a number of components into a single mechanical package, the size of an optical amplifier configured to amplify multiple single-channel signals is generally smaller than multiple prior art optical amplifiers configured to amplify the same single-channel signals individually. For example, only a single pump controller, pump driver, pump laser diode, and heat sink are required for an optical amplifier configured according to embodiments of the invention, whereas each of these components is generally required for every prior art, single-channel optical amplifier. Further, control software is simplified, since a single pump controller and pump driver are used to control the amplification of multiple optical signals. Lastly, because the amplification of each optical signal is independently controlled, the increased power consumption associated with the use of an optical splitter is avoided. 
         [0025]    As noted above, embodiments of the invention contemplate time-multiplexing of the pump laser output to multiple gain blocks by directing an optical energy pulse to each gain block sequentially.  FIG. 2A  is a graph of pump laser output  200  to a gain block vs. time, according to an embodiment of the invention. In this embodiment, pump laser output  200  is made up of a plurality of individual optical energy pulses  201 , each having a maximum intensity  202  and a pulse width  210 , and separated by a zero-output interval  230 . During zero-output interval  230 , essentially no optical energy is directed at the gain block by the pump laser. For clarity, only two optical energy pulses  201  are illustrated, although it is understood that pump laser output  200  is made up of a large number of optical energy pulses  201  occurring sequentially. As shown, one optical energy pulse  201  per pulse period  220  is directed to a particular gain block. The length of pulse period  220  is selected to be substantially less than the mean lifetime of excited ions in the gain medium of the gain block. For example, when pump laser output  200  is directed to an Erbium-doped fiber, pulse period  220  is no more than about 10 ms in duration, which is less than the mean lifetime of an excited ion in an Erbium-doped fiber. Consequently, photon generation in the gain block for amplifying an optical signal takes place by stimulated emission throughout pulse period  220 . Even though optical energy in the form of optical energy pulse  201  is directed to the gain block for only a portion of pulse period  220 , i.e., for pulse width  210 , the output power of the gain block is effectively the time-average of the total optical energy input into the gain block over pulse period  220 . 
         [0026]    When a gain block is pumped using pump laser output  200 , the output power of the gain block can, within limits, be increased or decreased by using pulse-width modulation, i.e., modulating the duration of pulse width  210 . This is because the total optical energy contained in an optical energy pulse  201  is equal to the area under the curve that defines optical energy pulse  201  in  FIG. 2A , i.e., the region bounded by rise time  204 , maximum output  205 , and fall time  206 . For example, the output power of the gain block can be increased by lengthening the duration of pulse width  210  and, conversely, the output power of the gain block can be decreased by shortening the duration of pulse width  210 . The maximum duration of pulse width  210  is limited by the number of gain blocks sequentially pumped by the pump laser and by the duration of pulse period  220 . For example, when a pump laser sequentially provides pump laser output  200  to two gain blocks and the duration of pulse period  220  is 10 ms, the maximum duration of pulse width  210  provided to each gain block is approximately 5 ms, which is a 50% duty cycle. “Duty cycle,” as used herein, is defined as the percentage of pulse period  220  during which optical output  200  is non-zero. In another example, when a pump laser sequentially provides pump laser output  200  to four gain blocks, the maximum duty cycle for each optical energy pulse  201  is about 25%. 
         [0027]      FIG. 2B  is a graph of pump laser output  250  vs. time, where pump laser output  250  is used to amplify four gain blocks, according to an embodiment of the invention. In this embodiment, pump laser output  250  is made up of a plurality of individual optical energy pulses  201 A-D, each having a maximum intensity  202  and a pulse width  210 A-D. As shown, optical energy pulses  201 A-D are arranged sequentially in each pulse period  220 , and then are repeated in subsequent pulse periods. For clarity, only one pulse period  220  is illustrated in  FIG. 2B . Each of optical energy pulses  201 A-D is directed to a different respective gain block for amplifying an optical signal, as described above in conjunction with  FIG. 1 , and is separated from adjacent optical energy pulses by a switching time  251 . Hence, optical energy pulse  201 A is directed to a first gain block during pulse width  210 A. Then, for the duration of zero-output interval  230 A, no further optical energy is directed to the gain block. Instead, during zero-output interval  230 A optical energy is sequentially directed to a second, a third, and a fourth gain block via optical energy pulses  201 B-D, respectively. As shown in  FIG. 2B , each of optical energy pulses  201 A-D has a duty cycle of approximately 25%. However, it is contemplated that the total optical energy contained in each of optical energy pulses  201 A-D may be modulated by varying the duty cycle of each energy pulse as required to independently control the optical amplification of each corresponding gain block. 
         [0028]    Switching time  251  represents the switching time of optical switch  107  in  FIG. 1 . As noted above, it is advantageous for the duration of switching time  251  to be less than about an order of magnitude less than the duration of pulse period  220  in  FIG. 2B . Referring to  FIG. 2B , it can be seen that when switching time  251  is on the order of 10% of pulse period  220  or more, the pulse widths  210 A-D of optical energy pulses  201 A-D may be substantially reduced, thereby limiting the maximum optical gain that can be provided to multiple optical signals by an optical amplifier according to embodiments of the invention. 
         [0029]    It is also contemplated that the total optical energy contained in each pulse  201  can be varied by modulating the maximum intensity  202  of each pulse. Intensity modulation is particularly beneficial when pulse-width modulation may be limited, e.g., when four or more gain blocks are pumped by a pump laser, and the maximum allowable duty cycle of each pulse  201  is 25% or less.  FIG. 2C  is a graph of pump laser output  260  vs. time, where pump laser output  260  is used to amplify four gain blocks, according to an embodiment of the invention. In this embodiment, pump laser output  260  is made up of a plurality of individual optical energy pulses  261 A-D, each having a pulse width  210  and a maximum intensity  262 A-D, respectively. As shown, the total optical energy provided to each gain block, i.e., the area under optical energy pulses  261 A-D, is varied based on maximum intensities  262 A-D, respectively. Hence, total optical energy content of each pulse may be varied by pulse-width modulation, intensity modulation, or a combination of both. In this way, the amplification of multiple gain blocks is independently controlled even though only a single pump laser is used to provide optical energy to each of the gain blocks. 
         [0030]    It is contemplated that embodiments of the invention may be used to amplify multiple optical inputs in other applications, as well. For example, high-power applications, such as laser welding, may also benefit from the time-multiplexing of pump laser output to amplify multiple optical inputs. In this case, a gain block may contain different dopants and have a substantially different mean lifetime than the embodiments described herein. Hence, the associated pulse width and pulse period of this embodiment may differ with respect to other embodiments described herein. 
         [0031]    While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.