Abstract:
Presented is a method and system for phase locking a multi-stage parallel fiber amplifier. The method comprises receiving a signal beam from a first stage, in one path of the multi-stage fiber amplifier, onto a fiber that is pumped to produce a saturated signal beam that is then output to a second stage that outputs an amplified beam. A characteristic of the of the saturated signal beam is that its phase and amplitude do not substantially change based on the amplitude of the signal beam input onto the fiber. The method further detects a portion of the amplified beam to produce a phase indication of the amplified beam relative to amplified beams of the other paths of the multi-stage fiber amplifier. The method modulates the pump level of the first stage to control the phase of amplified beam, and further controls the phases of the other amplified beams of the other paths to phase lock the multi-stage parallel amplifier. The saturated signal beam reduces phase changes in the second stage that would be opposite to, and therefore counteract, the phase changes intentionally modulated in the first stage.

Description:
FIELD 
     Embodiments of the subject matter described herein relate generally to a method for combining fiber amplifiers in multiple stages to make a high power fiber amplifier. 
     BACKGROUND 
     Single-stage fiber amplifiers are limited in their ability to amplify a source signal. To further amplify a source signal, the outputs of multiple fiber amplifiers can be joined together to produce a higher power output signal. To achieve high amplification powers, the amplifiers are further arranged into multiple stages, with the output of a first stage fiber amplifier being the input to a higher power second stage fiber amplifier. Phase locking the fiber amplifiers prevents a loss in power output due to destructive interference between the various output signals. 
     One method of phase locking the multiple fiber amplifiers is described in U.S. Pat. No. 6,400,871 to Minden. Minden describes a method of coherently phase combining multiple fiber amplifiers by modulating the pump current of the multiple fiber amplifiers. Modulating the pump current of a fiber amplifier changes the gain of the fiber amplifier and induces a change in the phase of the output. A detector receives a portion of the output from each fiber amplifier and detects the change which is then used in a feedback loop to adjust the phases of each of the fiber amplifiers in order to phase lock the fiber amplifiers together. 
     The method described by Minden works so long as there is a detectable change that correlates to a corresponding change in phase. To achieve high power outputs, multiple stage fiber amplifiers are used. However, when the gain of the first stage is modulated in order to induce a change in phase, the modulating of the gain also modulates the output power that is being input into the second stage. The modulation in the output power changes the gain of the second stage, which causes an opposite change in phase in the second stage fiber amplifier. At high powers, the change in phase induced by modulating the pump current in the first stage is cancelled by the opposite change in phase induced by the changes to the gain of the second stage. 
     Also, particularly at high power amplification levels, system noise can cause amplified spontaneous emission (ASE), or unwanted lasing to spontaneously occur, especially when the source signal is a pulsed source signal. 
     SUMMARY 
     Presented is a method for coherently phase combining multiple fiber amplifiers in a multi-stage configuration to amplify a source signal to a higher power level than previously possible using either single-stage fiber amplifiers or multiple coherently phase combined fiber amplifiers, while reducing the occurrence of amplified spontaneous emission (ASE.) In various embodiments, the system and method improves output power characteristics of the amplifiers through the use of a saturated amplifier stage. 
     The features, functions, and advantages discussed can be achieved independently in various embodiments of the present invention or may be combined in yet other embodiments further details of which can be seen with reference to the following description and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying figures depict various embodiments of the system and method of phase-controlled high-gain multi-stage amplification. A brief description of each figure is provided below. Elements with the same reference number in each figure indicated identical or functionally similar elements. Additionally, the left-most digit(s) of a reference number indicate the drawing in which the reference number first appears. 
         FIG. 1A  is a functional diagram of a prior art fiber amplification system; 
         FIG. 1B  is a graph of the optical phase response of a fiber amplification system at a low amplification level; 
         FIG. 1C  is a graph of the optical phase response of a fiber amplification system at a high amplification level; 
         FIG. 1D  is a graph of amplified spontaneous emission (ASE) backscattered from a fiber amplification system at various amplification levels; 
         FIG. 2  is a functional diagram of a multiple-stage fiber amplifier incorporating a saturation amplifier and an optical modulator in one embodiment of the phase-controlled high-gain multi-stage amplification system and method; 
         FIG. 3A  is a schematic diagram of a saturation amplifier in one embodiment of the phase-controlled high-gain multi-stage amplification system and method; 
         FIG. 3B  is a schematic diagram of an alternate embodiment of a saturation amplifier in one embodiment of the phase-controlled high-gain multi-stage amplification system and method; 
         FIG. 4  is functional diagram of a multiple-stage fiber amplifier utilizing a plurality of optical modulators in one embodiment of the phase-controlled high-gain multi-stage amplification system and method; 
         FIG. 5A  is a graph of the optical phase response of the multiple-stage fiber amplifier at high amplification levels in one embodiment of the phase-controlled high-gain multi-stage amplification system and method; and 
         FIG. 5B  is a graph showing reduced amplified spontaneous emission (ASE) backscattered from the multiple-stage fiber amplifier at high amplification levels in one embodiment of the phase-controlled high-gain multi-stage amplification system and method. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description is merely illustrative in nature and is not intended to limit the embodiments of the invention or the application and uses of such embodiments. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the preceding technical field, background, brief summary or the following detailed description. 
     Fiber amplifiers are limited in their ability to amplify a source signal. Typically, a fiber amplifier can amplify an input signal by approximately 20 db. To further amplify a source signal, multiple fiber amplifiers are joined together and synchronized to produce a higher power output signal. U.S. Pat. No. 6,400,871 to Minden describes a method of coherently phase combining multiple fiber amplifiers to produce a higher power output signal. 
     Referring now to  FIG. 1A , a phase coherent multiple fiber amplifier  100  comprises a laser source  102 , a pulse signal generator  104 , a fiberoptic splitter  106 , a plurality of fiber optic fibers  122 , a plurality of fiberoptic pre-amplifiers  108 , a plurality of power amplifiers  110 , a collimator  112 , a beam splitter  116 , a detector  118 , a phase controller  120 , and a monitoring port  124 . The phase coherent multiple fiber amplifier  100  produces a focused spot of illumination  114 . 
     The pulse signal from the pulse signal generator  104  is applied to the laser source  102  to produce a low-power coherent beam of pulsed light or electromagnetic radiation. In an embodiment, the laser source  102  is a fiberoptic amplifier. In other embodiments, the laser source  102  is a solid state or gas laser, for example a diode laser, a Nd:YAG laser, a Nd:YVO 4  laser, or Nd:GdVO 4  laser. In an embodiment, the laser source  102  is a 1064 nm laser. In an embodiment, the laser source  102  is a 940 nm to 1600 nm laser. In an embodiment, the pulse signal from the pulse signal generator  104  comprises 2-5 ns pulses at a frequency of between 10 khz to 1 Mhz. In the embodiment shown in  FIG. 1A , the low power pulsed light is split by fiberoptic splitter  106  into three fibers  122  corresponding to the three pre-amplifiers  108 , and three power amplifiers  110 . In an embodiment, each of the fibers  122  is an Nd-doped single mode fiber. The fibers  122  connect the fiberoptic splitter  106  to the pre-amplifiers  108 . In embodiments, there can be any number of fibers  122 , pre-amplifiers  108 , and power amplifiers  110 , however in practice the phase coherent multiple fiber amplifier  100  typically uses ten or fewer fibers  122 , pre-amplifiers  108 , and power amplifiers  110 . In embodiments, the pre-amplifiers  108  utilize laser diodes, and the power amplifiers  110  are Watt-level Er—Yb co-doped fiber amplifiers or Yb-doped fiber amplifiers. Each of the fibers  122 , pre-amplifiers  108 , and power amplifiers  110  defines a path through which the light is amplified. 
     In each path, the pre-amplifiers  108  increase the power of the low power pulse light. The pump inputs of the pre-amplifiers  108  are in communication with the detector  118  and phase controller  120  in a feedback loop to adjust the pump currents of the pre-amplifiers  108 . In embodiments, the communication between the detector  118  and phase controller  120  is an electrical signal, or an other communication signal. In operation, changes in the pumping level of the pre-amplifiers  108  changes the phase of signal input into the power amplifiers  110  and is detectable by the detector  118 . Therefore, varying the pumping levels in the pre-amplifiers  108  is used to phase control the power amplifiers  110 . 
     In the embodiment of  FIG. 1A , the outputs of the pre-amplifiers  108  are applied to the power amplifiers  110 . The outputs of the power amplifiers  112  are optically collimated by collimator  112  that recombines the outputs to produce a focused spot of illumination  114 . In embodiments, the focused spot of illumination  114  is focused in the far field, for example at a distance of meters to thousands of meters from the phase coherent multiple fiber amplifier  100 . In one embodiment, the detector  118  is also in the far field. In one embodiment, the detector  118  detects illumination returned from the focused spot of illumination  114  in the far field. In one embodiment, a portion of the output is directed to the detector  118  by means of a beam splitter  116 . In embodiments, the beam splitter  116  is a semi-silvered mirror, a birefringent crystal, a holographic element, or any other known beam splitting means as would be commonly understood by one of skill in the art. 
     The output of the detector  118  is input to the phase controller  120  which varies the power fed to the pumps of each of the plurality of preamplifiers  108  to phase lock the phase coherent multiple fiber amplifier  100 . In one embodiment, the phase controller  120  phase locks the outputs of each of the power amplifiers  110  to the same phase. In one embodiment, the phase controller  120  phase locks the outputs of each of the power amplifiers  110  to the slightly different phases, for example to perform beam steering. In one embodiment the detector  118  comprises a photodiode. In one embodiment, the detector  118  is an array of sensors, for example a CCD or array of photodiodes. In one embodiment, the detection is performed at a monitoring port  124 , for example using returned or backscattered radiation from one or more of the amplifier stages  108 ,  110 . 
     Power amplifiers  110  that are out of phase create beams that interfere destructively, thereby reducing power levels at the focused spot of illumination  114 . The power amplifiers  110  are phase locked to prevent destructive interference, resulting in more power at the focused spot of illumination  114 . Varying the pumping levels in the pre-amplifiers  108  produces a detectable change in the optical phase response of the beam created by the phase coherent multiple fiber amplifier  100 . The detectable change is detected by the detector  118 , transmitted to the phase controller  120 , and used as feedback to modify the pumping of the pre-amplifiers  108 . Without feedback, the output of the phase coherent multiple fiber amplifier  100  would vary from maximum to minimum due to ubiquitous phase perturbations such as from air perturbations, temperature variations, fiber vibrations, the wavelength drift of pump currents, and other sources. Ubiquitous phase perturbations are generally relatively slow changes and tend to occur between 10 ms and a few hundred milliseconds, allowing the detector  118  and phase controller  120  to gradually adjust the pumping of the pre-amplifiers  108  to negate the perturbations. 
     Although suitable for watt level applications, there is a limit to the amount of amplification in the pre-amplifiers  108 . For example, as the power output from the preamplifier  108  is increased from 0.1 Watt to 10 Watts, problems controlling the phase of the beams of the phase coherent multiple fiber amplifier  100  occurs because changes in the gain not only affects the phase shift in the pre-amplifier  108 , but also affects the gain and therefore the phase shift in the next stage, the power amplifier  110 . Also, as amplification is increased, system noise increasingly causes spontaneous lasing in the power amplifier  110  stage. 
     As the pump current from the pre-amplifier  108  is increased, it produces a change in phase and power level input to the power amplifier  110 . However, the additional power received by the power amplifier  110  produces an opposite phase change in the power amplifier  110 , cancelling to a large degree the phase change achieved by the pre-amplifier  108 . Referring now to  FIGS. 1B and 1C , graphs of phase changes  134 ,  136  are illustrated for the phase coherent multiple fiber amplifier  100  at 100 mW levels  130  in  FIG. 1B , and the phase coherent multiple fiber amplifier  100  at 5 Watt levels  140  in  FIG. 1C . The experimental results of using current pulses  132  applied to the pumps of the pre-amplifiers  108  to phase control the phase coherent multiple fiber amplifier  100  shows that for the 100 mW level  130 , there is an identifiable change in phase change  134 . However, at the 5 Watt level  140 , the higher power input into the power amplifier  110  from the preamplifier  108  changes the gain of the power amplifier  110  and creates a phase change that cancels the phase change of the pre-amplifier  108 . This results in only an overall slight phase change  134  that is output from the phase coherent multiple fiber amplifier  100 . Therefore, at the 5 Watt amplification level, pumping the pre-amplifier  108  does not produce a phase change  134  that the detector  118  could consistently and accurately use to phase lock the phase coherent multiple fiber amplifier  100 . 
     Also, as the gain of the preamplifier  108  is increased, noise is also amplified. This amplified noise received by the power amplifier  110  causes the phase coherent multiple fiber amplifier  100  to become less stable. Referring now to  FIG. 1D , a spectral graph of backscattered radiation  150  received from the phase coherent multiple fiber amplifier  100  at the monitoring port  124  and analyzed by a spectrometer is presented. The increased power from the preamplifier  108  received by the power amplifier  110  makes a phase coherent multiple fiber amplifier  100  more prone to amplified spontaneous emission (ASE)  152 , especially at higher power levels. 
     Although suitable for watt level applications, pre-amplifiers  108  are limited in the overall amount of gain to around 20 db and in most cases less than 40 db. To achieve higher gains, for example 40 db to create fiber amplifiers that produce hundred Watt levels or more, multiple stages are required. An intermediate amplifier can be added between the pre-amplifier  108  and power amplifier  110 . Although this increases the possible gain, merely adding an intermediate amplifier between the pre-amplifier  108  and power amplifier  110  only increases the unwanted gain and phase interactions. 
     To increase the amount of overall system gain, and to isolate the effects of the difference in phase change by the preamplifier  108  and power amplifier  110  and to stabilize the power input to the power amplifier  110 , a saturation amplifier, shown as  300  in  FIGS. 2 ,  3 B and  4 , and as  208  in  FIG. 3A , is utilized. Referring now to  FIGS. 2 ,  3 A,  3 B, and  4 , a saturation amplifier  208  is used as an intermediate amplification stage between the preamplifier  108  and power amplifier  110  in each fiberoptic amplifier path. 
     Referring now to  FIG. 2 , a multi-stage fiber amplifier  200  comprises a continuous wave laser source  102 , a pulse signal generator  104 , an electro-optic modulator  204 , a regenerator amplifier  206 , a fiberoptic splitter  106 , a plurality of fiber optic fibers  122 , a plurality of fiberoptic pre-amplifiers  108 , a plurality of saturation amplifiers  300  and isolators  210 , a plurality of power amplifiers  110 , a collimator  112 , a beam splitter  116 , a detector  118 , a phase controller  120 , and a monitoring port  124 . The multi-stage fiber amplifier  200  produces a focused spot of illumination  114 . The operation of the multi-stage fiber amplifier  200  in  FIG. 2  is similar to the phase coherent multiple fiber amplifier  100  of  FIG. 1 , with differences described below. 
     In one embodiment, the signal beam fed into the pre-amplifier  108  stage is created using a continuous wave source  202  that is modulated using an electro-optical modulator  204 . In one embodiment, the continuous wave source  202  is a solid state laser, such as a 10 mW Lightwave  122  laser diode operating at 1064 nm. In embodiments, the continuous wave source  202  produces a single frequency, single mode beam at a power of one to several milliwatts. In other embodiments, the continuous wave source  202  is a gas or other laser. In one embodiment, the electro-optical modulator  204  is a lithium niobate crystal, for example an electro-optic intensity modulator from Alenia Marconi Systems. In embodiments, the electro-optical modulator interrupts or modulates the intensity of the beam from the continuous wave source  202 . 
     The triggering of the electro-optical modulator  204  is controlled by the pulse signal generator  104 , for example a pulse generator from Avtech Electrosystems Ltd. In an embodiment the pulse signal from the pulse signal generator  104  is a 1 to approximately 10 ns pulse repeating at a frequency of between approximately 10 khz to approximately 1 Mhz. In another embodiment the pulse signal is application dependent, and therefore is pulsed appropriate for the application. In non-limiting examples of applications, the pulses are encoded for communications with a distant receiver or transceiver; the pulses are modulated at a power and frequency that can be easily received by a sensor, such as the detector  118 , and filtered to retrieve information about the returned signal; the pulses are pulsed and/or encoded to identify a target illuminated by the focused spot of illumination  114 ; the pulses are modulated into a beat frequency to disrupt or alter a signal perceived by a remote sensor; the pulses are modulated to effect a specific frequency or power to disrupt or injure the retinal function of a target subject. 
     Continuing to refer to  FIG. 2 , the electro-optical modulator  204  reduces the power level of the signal beam by several db. A regenerator amplifier  206  amplifies the modulated signal beam, boosting or regenerating the modulated signal beam before it is split by the fiberoptic splitter  106  and routed to the pre-amplifiers  108  in each path using fibers  122 . In an embodiment where the signal beam from the continuous wave source is not modulated, the electro-optical modulator  204  and regenerator  206  are not required. 
     Referring now to  FIG. 4 , in an embodiment a first electro-optical modulator  402  and a second electro-optical modulator  404  modulate the signal beam. In further embodiments, multiple similar and/or different modulators are utilized to produce narrow pulses or shaped pulses. 
     Referring again to  FIG. 2 , an isolator  210  such as a IO-F-1064 from OFR reduces the amount of electromagnetic radiation that escapes from the power amplifier  110  and returns to the pre-amplifier  108 . Light or electromagnetic radiation from the power amplifier  110  that returns to the pre-amplifier  108  can damage or cause the pre-amplifier  108  to function incorrectly. 
     Continuing to refer to  FIGS. 2 and 4 , in embodiments, the saturation level of intermediate saturation amplifier  300  is adjusted to meet desired criteria for stable operation of the multi-stage fiber amplifier  200 . In one embodiment, the intermediate saturation amplifier  300  has an output power sufficient to saturate the power amplifier  110 . In one embodiment, the intermediate saturation amplifier  300  has an input characteristic such that small changes to the input power from the preamplifier  108  do not substantially alter the output power of the intermediate saturation amplifier  300 . In one embodiment, the intermediate saturation amplifier  300  does not substantially alter the phase of the beam input from the preamplifier  108 . In one embodiment, the intermediate saturation amplifier  300  has a phase modulation characteristic that is essentially invariant with respect to the power level input from the signal from the preamplifier  108 . In one embodiment, when being pulsed the intermediate saturation amplifier  300  has an output power that is less than the self-phase-modulating threshold, and therefore does not induce or trigger the self-phase-modulating effect. 
     Referring now to  FIG. 3A , a schematic diagram of an exemplary saturation amplifier  208  is presented. In one embodiment, the exemplary saturation amplifier  208  comprises a wave division multiplexor  302  that is connected to another wave division multiplexor  302  through a meter long Nd doped single mode fiber  304 . In embodiments, the fiber  304  is any suitable length and is submeter or multiple meters in length. The fiber  304  is pumped at both ends through the wave division multiplexors  302  by pump diodes  306 . The pumping process of the saturation amplifier  208  amplifies the modulated signal beam from the pre-amplifier  108  to produce a saturated signal beam if any portion of the modulated signal beam from the pre-amplifier is at or above a specific threshold. In an embodiment, the saturation amplifier  208  amplifies the modulated signal beam to the level necessary for stable operation of the multi-stage fiber amplifier  200 . 
     The pump diodes  306  are connected to the wave division multiplexors  302  through pigtailed single mode fibers  308 . In one embodiment, the signal beam from the pre-amplifier  108  is a 1064 nm signal beam, and the pump diodes  306  emit electromagnetic radiation at 818 nm. In embodiments, the fiber  304  is a Yb doped fiber, a Er doped fiber, an Er—Yb doped fiber, or other fiber as would be understood in the art. In embodiments, the wave division multiplexors  302  are for example Gould Electronics Inc. wave division multiplexors. In embodiments, the pump diode  306  is pump diode between 700 nm and 1500 nm, for example a 915 nm, 976 nm or other wavelength pump diode, a JDSU pump diode, or other pump sources as would be understood in the art. In embodiments, the fiber  304  is an LMA fiber, for example an LMA fiber having a 10000 nm or greater core, or physical core diameter, a 20000 nm core LMA fiber, or other fiber as would be understood in the art. 
     As shown in  FIG. 3B , a saturation amplifier  300  comprises only one wave division multiplexor  302  and one pump diode  306 . In the embodiment, fiber  304  is connected to a fiber coupler  308 , for example an unpumped wave division multiplexor. In another embodiment knot shown), fiber  304  connects directly to the power amplifier  110  without the coupler  308 . 
     In an embodiment, the choice of fiber  304 , pump diode  306  wavelength, power of the pumped diode  304 , and choice of wave division multiplexor  302 , are selected to adjust the output power and characteristics of the saturation amplifier  300 . In one embodiment, the selection results in a saturation amplifier  300  that has an output power sufficient to saturate the power amplifier  110 . In one embodiment, the selection results in a saturation amplifier  300  that has an input characteristic such that small changes to the power from the pre-amplifier  108  does not substantially alter the output power of the saturation amplifier  300 . In one embodiment, the selection results in a saturation amplifier  300  that has an output power that is less than the self-phase-modulating threshold when amplifying a pulsed signal beam. In one embodiment, the selection results in a saturation amplifier  300  that has an output power that does not substantially alter the phase of the signal beam. In one embodiment, the selection results in a saturation amplifier  300  that has an output with a phase shift, but that phase shift is substantially invariant over a range of signal beams. By invariant, it is meant that any minor dynamic changes in phase of the saturation amplifier do not produce an opposite change in phase that would cancel, or substantially interfere with, the intentional phase change induced in the pre-amplifier  108 , such that a phase change will normally be detectable by the detector  118 . 
     Changes to the phase of the signal beam are proportional to changes in the refractive index of the fiber. The equation that relates the change of the refractive index to the change of the population of the upper states of the doped ions inside the pumped fiber is:
 
Δ n (ν)=2 πF   2   Δp (ν)Δ N/n   0   [1]
 
Where Δn(ν) is the change of the refractive index; F is the Lorentz factor; Δp(ν) is the difference of the ion polarizabilities in the excited and the ground states; ΔN is the difference of the population of the upper states of the doped ions inside the pumped fiber; and no is the refractive index of the fiber  304  without pumping. [1] Antipov et al., J. Opt. Soc. Am. B, Vol. 16, No. 7, 1073, 1999.
 
     Referring now to  FIG. 5A , a graph of power output is illustrated for the multi-stage fiber amplifier  200  at 22 W levels  500 . The experimental results of forcing 16 Amp current pulses  502  into the pumps of the pre-amplifiers  108  of the multi-stage fiber amplifier  200  shows that for the 22 W level  500 , there is an identifiable phase change  504  of approximately π/5 sufficient to use as feedback to phase lock the multi-stage fiber amplifier  200 . Referring now to  FIG. 5B , a spectral graph of backscatter  510  from the multi-stage fiber amplifier  200  and picked up by the interferometer  224  is presented. When compared to  FIG. 1D ,  FIG. 5B  illustrates that the stable power output from the intermediate saturation amplifier  208  into the power amplifier  110  reduces system instability and makes the multi-stage fiber amplifier  200  less prone to amplified spontaneous emission (ASE)  512 , even at high power levels. 
     Although for purposes of illustration and simplicity of explanation the preceding figures and description describe a two stage fiber amplifier, the system and methods described herein are equally applicable to fiber amplifiers having two, three, or multiple stages. No limitation to a fiber amplifier is implied or intended. 
     The embodiments of the invention shown in the drawings and described above are exemplary of numerous embodiments that may be made within the scope of the appended claims. It is contemplated that numerous other configurations of the multi-stage fiber amplifier may be created taking advantage of the disclosed approach. It is the applicant&#39;s intention that the scope of the patent issuing herefrom will be limited only by the scope of the appended claims.