Patent Description:
High average power fiber lasers with diffraction-limited beams that are suitable for High Energy Laser (HEL) applications are currently primarily limited in output power by Stimulated Brillouin Scattering (SBS) and Modal Instability. Some known systems have been effective to scale power to greater than the 2kW level by mitigating SBS. However, Model Instability may still bottleneck power-scaling in regular large-mode area (LMA) fiber, e.g., non-photonic-crystal fiber (PCF) and/or photonic bandgap (PBG) fiber. Specifically, Modal Instability may limit diffraction limited output power to a threshold near 2kW, such as ~<NUM> kW for <NUM> core step-index dual-clad fiber laser. <CIT> discloses a fiber amplifier with an integrated fiber laser pump. <CIT> discloses a tandem pumped fiber amplifier including a plural core fiber. <CIT>, or <NPL>; or <NPL>, each discloses a tandem pumped fiber amplifier including a single core fiber.

According to a first aspect, there is provided a tandem pumped fiber amplifier as claimed in claim <NUM>.

The accompanying drawings, wherein like reference numerals represent like elements, are incorporated in and constitute a part of this specification and, together with the description, explain the advantages and principles of the presently disclosed technology.

Core tandem-pumping may provide relatively high absorption and/or efficiency. As a result, fiber length may be relatively short, which may provide a relatively large margin for deleterious nonlinear effects such as SBS, SRS (stimulated Raman Scattering), FWM (four wave mixing), SPM (self-phase modulation), or the like, or combinations thereof. One embodiment provides a greater than 4kW single narrowband fiber amplifier.

Any tandem pumped fiber amplifier described herein may be less bulky (e.g., smaller and/or not as heavy) and/or less costly than a system using a 10kW single mode fiber laser in a regular LMA fiber using numerous <NUM> single mode tandem fiber lasers as high brightness pump sources. The tandem pumped fiber amplifiers may have lower power requirements and/or lower thermal dissipation requirements than such systems as well.

Whereas a system using a 10kW single mode fiber laser in a regular LMA fiber using numerous <NUM> single mode tandem fiber lasers as high brightness pump sources may require a reduction in quantum-defect heating from the usual ~<NUM>% in the <NUM> pumped system down to ~<NUM>% in the tandem <NUM> pumped amplifiers to achieve 10kW, embodiments disclosed herein may not be subject to the same requirement in order to achieve 10kW or greater. A system employing a tandem pumped fiber amplifier may be compatible with regular LMA fiber technology but scalable from a few kilowatts to 10kW or more by scaling up the multimode diode laser pump power. As a result, embodiments described herein may simplify manufacturing in high energy laser applications. In a system employing a tandem pumped fiber amplifier, a final amplifier stage may be greater than 1kW to address Model Instability. A tandem pumped fiber amplifier may push the single channel output power to greater than known thresholds created by Modality Instability, such as 3kW, 5kW, 10kW, or more.

Several advantages of this approach have been identified. First, the quantum defect in the power amplifier may be only <NUM>% at <NUM> when pumped at <NUM>, in some embodiments. At a nominal wavelength of <NUM>, this quantum defect is about <NUM>%. Which is less than half compared to pumping at <NUM> (which may be associated with a <NUM>% quantum defect). Secondly, the signal injected into the power amplifier may be significantly greater than <NUM>. Both factors may provide a higher threshold condition for Modal Instability. This may provide greater than 10kW of spectral beam combining (SBC) and coherent beam combining (CBC) combinable power. The tandem pumped fiber amplifier may use regular LMA fiber technology without compromising the total efficiency of the system. All of this can be achieved by pumping with low SWAP (size, weight and power) and low-cost multimode diode pumps rather than using a multitude of expensive and bulky single mode fiber lasers.

As used in this application and in the claims, the singular forms "a," "an," and "the" include the plural forms unless the context clearly dictates otherwise. Additionally, the term "includes" means "comprises. " Further, the term "coupled" does not exclude the presence of intermediate elements between the coupled items. The systems, apparatus, and methods described herein should not be construed as limiting in any way. Instead, the present disclosure is directed toward all novel and non-obvious features and aspects of the various disclosed embodiments, alone and in various combinations and sub-combinations with one another.

The disclosed systems, methods, and apparatus are not limited to any specific aspect or feature or combinations thereof, nor do the disclosed systems, methods, and apparatus require that any one or more specific advantages be present or problems be solved. Any theories of operation are to facilitate explanation, but the disclosed systems, methods, and apparatus are not limited to such theories of operation. Although the operations of some of the disclosed methods are described in a particular, sequential order for convenient presentation, it should be understood that this manner of description encompasses rearrangement, unless a particular ordering is required by specific language set forth below. Moreover, for the sake of simplicity, the attached figures may not show the various ways in which the disclosed systems, methods, and apparatus can be used in conjunction with other systems, methods, and apparatus.

Additionally, the description sometimes uses terms like "produce" and "provide" to describe the disclosed methods. These terms are high- level abstractions of the actual operations that are performed. The actual operations that correspond to these terms will vary depending on the particular implementation and are readily discernible by one of ordinary skill in the art. In some examples, values, procedures, or apparatus' are referred to as "lowest", "best", "minimum," or the like. It will be appreciated that such descriptions are intended to indicate that a selection among many used functional alternatives can be made, and such selections need not be better, smaller, or otherwise preferable to other selections.

Examples are described with reference to directions indicated as "above," "below," "upper," "lower," and the like. These terms are used for convenient description, but do not imply any particular spatial orientation.

<FIG> illustrates a tandem pumped fiber amplifier <NUM>. The tandem pumped fiber amplifier <NUM> may include a seed laser <NUM> with a selected center wavelength that may be pseudo random bit sequence (PRBS) phase-modulated to achieve necessary bandwidth to suppress SBS and keep coherence length long enough for SBC and CBC applications. The selected center wavelength may be in the range of <NUM>-<NUM> for Yb-doped fiber, in some embodiments. The seed laser <NUM> may include a preamplifier (not shown) to generate sufficient power, e.g., greater than 20W of seed power for amplification in the subsequent sections.

Fiber of the seed laser <NUM> may be coupled to (e.g., spliced) with fiber of a tandem oscillator pump and booster amplifier (TOP-booster amplifier) <NUM>, which may be coupled to (e.g., spliced) with fiber of a power amplifier <NUM>. The TOP-booster amplifier <NUM> may include a first section of a single or plural active core fiber (e.g., a dual active core fiber), and at least one set of one or more diode pumps optically coupled, e.g., end-coupled, side-coupled, or the like, or combinations thereof, to the diode pump set.

In a plural active core example, some the cores of the plural active core fiber (e.g., both of the cores of a dual active core fiber laser) may be arranged along a same axis or a different axis (e.g., coaxial with the second core symmetrically surrounding the first core or with the second core asymmetrically surrounding the first core). <FIG> illustrates a cross-section view of a coaxial-type dual active core fiber <NUM> that may be utilized in the tandem pumped fiber amplifier <NUM> of <FIG>. The resulting arrangement is described for explanatory purposes but is not in accordance with claim <NUM>. <FIG> illustrates a cross-section view of a single active core fiber <NUM> that may be utilized in the tandem pumped fiber amplifier <NUM> of <FIG>, in some embodiments in accordance with claim <NUM>.

Referring now to <FIG>, the coaxial-type dual active core fiber <NUM> includes a first core <NUM> surrounded by a second core <NUM>. The first core <NUM> may have a first diameter, and the second core <NUM> may have a second different diameter (e.g., a second larger diameter). The first diameter may be ~<NUM> and the second diameter may be ~<NUM>.

The first core <NUM> may be doped differently than the second core <NUM>. The first core <NUM> may include a first doping concentration and the second core <NUM> may include a second doping concentration associated with a higher absorption coefficient. The first core <NUM> may include Yb-<NUM> (Yb ~<NUM> × <NUM><NUM> m-<NUM>) and the second core <NUM> may include with Yb-<NUM> (Yb ~<NUM> × <NUM><NUM> m-<NUM>).

In some examples, the first core <NUM> may have a numerical aperture (NA) that is not greater than an NA of the second core <NUM>. The first core <NUM> may have an NA of about <NUM> or less and the second core <NUM> may have an NA no less than <NUM>, e.g., in the range of <NUM>-<NUM>.

The fiber <NUM> may include a cladding <NUM> surrounding the cores <NUM> and <NUM>, and a jacket <NUM>. The cladding <NUM> may be a glass-clad multimode pump guiding octagonal core with a third diameter that is greater than the second diameter of the second core, e.g., ~<NUM>.

Referring now to <FIG>, the single active core fiber <NUM> may include a first section (<NUM>, <FIG>) and a second section (<NUM>, <FIG>). The first section <NUM> may include a core <NUM> having a diameter of ~<NUM>-<NUM>. The core <NUM> of this section <NUM> may be doped with Yb-<NUM> (Yb ~<NUM> × <NUM><NUM> m-<NUM>). The core <NUM> of this section <NUM> may have an NA of about <NUM> (in one embodiment, <NUM>).

The fiber <NUM> in the first section <NUM> may include a cladding <NUM> surrounding the core <NUM>, and a jacket <NUM>. The cladding <NUM> and the jacket <NUM> may be similar to any cladding or jacket described herein. The cladding <NUM> may be a glass-clad multimode pump guiding octagonal core with a diameter that is greater than the diameter of the core <NUM>, e.g., ~<NUM>.

The second section <NUM> may include a core <NUM> having a diameter of ~<NUM>-<NUM>. The core <NUM> of this section <NUM> may be doped with Yb-<NUM> (Yb ~<NUM> × <NUM><NUM> m-<NUM>) and/or with a doping profile of <NUM>-<NUM>% confined doping. The core <NUM> of this section <NUM> may have an NA of about <NUM> (in one embodiment, <NUM>). The fiber <NUM> in the second section <NUM> may have a similar cladding <NUM> and jacket <NUM> as in the first section <NUM>.

Referring again to <FIG>, the TOP booster-amplifier <NUM> may include the first section of the single or plural active core fiber. The first section may include a first fiber Bragg grating (FBG) and a second FBG (e.g., an HR (highly reflective) FBG and a PR (partially reflecting) FBG, respectively), which may include diameters corresponding to core structure, e.g., the second core <NUM> (<FIG>) or the core <NUM> (<FIG>). The first and second FBGs may be centered at a selected wavelength to form a multi-mode oscillator which builds up necessary power for the tandem pump to be used by the power amplifier <NUM>.

The selected wavelength may be less than a center wavelength of the seed laser. A difference may be <NUM>% or less (e.g., <NUM>%) in some examples and/or in a range of <NUM>-<NUM>%. In this range, with a core structure having sufficient dimensions to suppress Modal Instability, and SBS. The selected wavelength may be in the range of <NUM>-<NUM> (e.g., <NUM>), in one embodiment.

In some examples, the doping of the first core <NUM> (<FIG>) or the core <NUM> (<FIG>) may be selected to generate only sufficient single mode seed power while it is bi-chromatically pumped by both the multimode <NUM> pump as well as the <NUM> tandem pump (which may be generated within the core structure). The oscillator of the first section of the single or plural active core fiber may use the doped core structure to convert most of the <NUM> multi-mode pump power into <NUM> wavelength within the core structure. Residual unabsorbed <NUM> pump (e.g., several percent) may enter the cladding of the second section of the single or plural active core fiber (e.g., the power amplifier <NUM>) and be utilized, which may optimize overall efficiency (e.g., overall electrical-to-optical power conversion efficiency). An o-o efficiency corresponding to the first section may be <NUM>% more efficient due to lower quantum defect when generating <NUM> wavelength compared to <NUM> amplifiers.

<FIG> (dual active core example) illustrate, respectively, a graph <NUM> (<FIG>) of calculated signal power and tandem pump power inside a final power amplifier along with upper state population for a tandem pump fiber amplifier <NUM> (<FIG>), a block diagram of the tandem pump fiber amplifier <NUM>, and a cross-section view of a dual-core and all-glass-fiber <NUM> (<FIG>) of the tandem pump fiber amplifier <NUM>. <FIG> (single active core embodiment) illustrate, respectively, a graph <NUM> (<FIG>) of calculated signal power and tandem pump power inside a final power amplifier along with upper state population for a tandem pump fiber amplifier <NUM> (<FIG>), a block diagram of the tandem pump fiber amplifier <NUM>, and a cross-section view of a single active core and all-glass-fiber <NUM> (<FIG>) of the tandem pump fiber amplifier <NUM>.

Modeling of the tandem pump fiber amplifier <NUM> (<FIG>) has shown an absorption of greater than <NUM>. 51dB (of <NUM> and <NUM>) and greater than 15dB (of <NUM>) in the second core. When the longest signal wavelength of <NUM> at 20W is launched into a first section of the fiber <NUM>, greater than <NUM> kW of signal may be generated which can be amplified in the second section of the fiber <NUM>. Modeling of the plural active core example of <FIG> predicts greater than <NUM>% conversion efficiency for the <NUM> multimode pump into a <NUM> tandem pump, and power levels of greater than <NUM>. 5kW may be generated in the second core of the fiber <NUM> given a <NUM>-meter-long oscillator. Results of this simulation are shown in the graph <NUM>.

Modeling of the tandem pump fiber amplifier <NUM> (<FIG>) has shown similar results. In a ~<NUM> meter "effective" fiber length, a core with Yb-<NUM> may achieve absorption of greater than <NUM>. 51dB (of <NUM> and <NUM>) and greater than 15dB (of <NUM>) in the core. However, when the longest signal wavelength of <NUM> at 20W is launched into a first section of the fiber <NUM>, greater than 10kW of signal may be generated which can be amplified in the second section of the fiber <NUM>. Modeling of the single active core embodiment of <FIG> predicts greater than <NUM>% conversion efficiency for the <NUM> multimode pump into a <NUM> tandem pump, and power levels of greater than <NUM>. 5kW may be generated in the core of the fiber <NUM> given a <NUM>-meter-long oscillator and Yb-<NUM> doping. Results of this simulation are shown in the graph <NUM>.

Referring now to <FIG>, <FIG> shows a refractive index of the first core, the second core, the inner glass clad, as well as relative doping concentration in the cores. In this example, the values <NUM>-<NUM> may be, respectively, <NUM>, <NUM>, <NUM>, and <NUM>, and the core region <NUM> may include a first doping profile of Yb-<NUM> ~<NUM> × <NUM><NUM> m-<NUM> and the core region <NUM> may include a second different profile of Yb-<NUM> ~<NUM> × <NUM><NUM> m-<NUM>. In other examples, the values <NUM>-<NUM> may be, respectively, <NUM>, <NUM>, <NUM>, and <NUM>. <FIG> shows a refractive index of the core, the inner glass clad, as well as doping concentration in the core. In this embodiment, the values <NUM>, <NUM>, and <NUM> may be, respectively, <NUM>-<NUM>, <NUM>-<NUM>, and <NUM>-<NUM> (oscillator fiber) / <NUM>-<NUM> (power amplifier fiber), and the core region <NUM> may include a doping profile of Yb-<NUM> ~<NUM> × <NUM><NUM> m-<NUM> (oscillator fiber and/or power amplifier fiber) or <NUM>-<NUM>% confined doping in the core of the power amplifier fiber.

The seed laser of any tandem pump fiber amplifier described herein may be, for SBC or CBC, a single-mode semiconductor laser such as a distributed feedback (DFB) laser or a non-planar ring oscillator (NPRO) and phase modulated to suppress SBS (for other applications any appropriate single mode seed, e.g., any 20W single mode seed, may be used). The seed laser may be <NUM>.

A TOP boost amplifier of the tandem pump fiber amplifier <NUM> (<FIG>) may include a dual active core and a plural clad, e.g., dual or triple clad. The inner core may be doped to generate sufficient power at seed wavelength need to enter the power amplifier stage. The outer multimode core may be doped to convert a portion (e.g., most) of the <NUM> diode pump into the <NUM> tandem pump. The tandem oscillator pump may use HR and PR FBG's to generate a ~<NUM> bandwidth multi-mode oscillator using a combination of the inner and outer cores, which may be doped at the same level or differently. HR and PR FBG's may have the same core diameter as the outer multimode core of the TOP-booster gain amplifier. The seed wavelength that is injected into this stage may be amplified by both the <NUM> multimode pump as well as the <NUM> tandem pump that is generated inside the oscillator cavity, e.g., with different absorption coefficient determined by the rare earth dopant absorption cross section at these wavelengths and the core-to-clad area ratios. The power scaling of this laser may be done by adding more diode pumps as needed to reach target power values, e.g., 3kW, 5kW, 10kW, etc. In some examples, the "effective multimode HR-FBG" may be one of the following types:.

The final power amplifier of the tandem pump fiber amplifier <NUM> may use all of the <NUM> tandem pump in the inner cladding and a portion (e.g., all) any residual <NUM> light not absorbed in the oscillator but guided in the outer clad to amplify seed wavelength and mode-field-adaptor may be used to match the single mode beam in the TOP-booster fiber and the final power amplifier. Some examples may use a cladding light stripper (CLS) to strip off the residual <NUM> pump before the output endcap.

Some examples include a plural active core fiber in a ~<NUM> meter "effective" fiber length including a Yb-doped core region, an undoped inner clad region, and an outer clad region (e.g., an outer glass-clad). In one example, the dimensions of the doped core region, the undoped inner clad region, and the outer clad region may be <NUM>-<NUM>, <NUM>, and <NUM>-<NUM>, respectively (pump guide may be <NUM>). The plural active core fiber may include a final power amplifier including a mode-field adaptor to match the mode to a power amplifier section of the plural active core fiber. A <NUM> tandem pump may be guided in the inner clad region and may pump the signal in the innermost core of the plural active core fiber.

<FIG> illustrates a refractive index <NUM> of a fiber that is similar to the fiber <NUM> of <FIG> and includes a dual active core triple fiber clad. In this example, the values <NUM>-<NUM> are, respectively, <NUM>-<NUM>, <NUM>-<NUM> /. <NUM> NA or higher, ~<NUM> (Yb-<NUM> doped annulus / <NUM> NA), ~<NUM> (Yb-<NUM> doped / <NUM> NA).

Referring now to <FIG>, a tandem oscillator pump and booster amplifier in accordance with the invention (TOP-booster amplifier) may include a core (<NUM>-<NUM>) and a plural clad (e.g., DCF or TCF). For a seed wavelength of <NUM>-<NUM>, the pump wavelength may be <NUM>-<NUM>. The TOP-booster amplifier may be coupled to (e.g., spliced) with fiber of a power amplifier. The power amplifier may include a core (<NUM>-<NUM>) and a plural clad (e.g., DCF or TCF). An outer clad may be <NUM>-<NUM>. In some embodiments, the power amplifier comprises an end-core & side-clad-pumped tandem power amplifier with a wavelength of <NUM> to <NUM>. The power scaling of this laser may be done by adding more diode pumps as needed to reach target powervalues of greater than 10kW.

The final power amplifier of the tandem pump fiber amplifier <NUM> may use all of the <NUM> tandem pump in the inner cladding and a portion (e.g., all) any residual <NUM> light not absorbed in the oscillator but guided in the outer clad to amplify seed wavelength and mode-field-adaptor may be used to match the single mode beam in the TOP-booster fiber and the final power amplifier. Some embodiments may use a cladding light stripper (CLS) to strip off the residual <NUM> pump before the output endcap.

Claim 1:
A tandem pumped fiber amplifier for High Energy Laser (HEL) applications (<NUM>; <NUM>), the apparatus comprising:
an optical fiber (<NUM>; <NUM>) including a single active core (<NUM>), wherein a seed laser (<NUM>) is optically coupled to the single active core;
the optical fiber (<NUM>; <NUM>) including a first section (<NUM>) to operate as a single active core oscillator and a second different section (<NUM>) to operate as a single active core power amplifier; and
wherein the seed laser (<NUM>) and one or more diode pumps are optically coupled to the first section of the optical fiber,
wherein the single active core (<NUM>) of the first section is doped to convert the one or more diode pumps into a tandem pump, wherein the one or more diode pumps and the tandem pump bi-chromatically pump the single active core power amplifier; and wherein the first section is configured to also act as a booster amplifier for the seed laser;
wherein a selected wavelength associated with the single active core oscillator is less than a center wavelength of the seed laser, wherein the single active core (<NUM>) is configured to suppress modal instability when a difference between the selected wavelength and the center wavelength is in a range of <NUM>-<NUM>% and a greater than <NUM> kW final amplifier stage is employed as the single active core power amplifier.