CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. Patent Application No. 16/513,191, filed on July 16, 2019.

STATEMENT REGARDING

FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0002] This invention was made with Government support under contract SBIR Phase 2 Contract SC0015905 awarded by the Department of Energy. The Government has certain rights in this invention.

BACKGROUND

[0003] Fiber lasers are becoming increasingly deployed in industrial, scientific, medical, and defense applications due to their high efficiency, robust and reliable construction, and their relatively low system size and weight. Pulsed fiber lasers, such as ultrafast fiber lasers, are of interest. Furthermore, pulsed fiber lasers can create femtosecond (fs) level pulses in small, rugged and reliable packages. Fiber nonlinearities, such as stimulated Brillouin scattering (SBS), Raman scattering, and self-phase modulation (SPM), act as impediments to realizing higher peak powers and even shorter pulse widths.

SUMMARY

[0004] In an exemplary embodiment, the present disclosure provides a fiber optic assembly. The fiber optic assembly includes: a gain fiber configured to output signal light; a first taper corresponding to the gain fiber, wherein the first taper is configured to expand the signal light output by the gain fiber; and a reflector configured to receive counter-pumping light and direct the counter-pumping light into the first taper. The first taper is further configured to receive the counter-pumping light and focus the counter-pumping light as the counter-pumping light propagates towards the gain fiber.

[0005] In a further exemplary embodiment, the reflector is disposed relative to the first taper such that the reflector does not impede the expanded signal light output from the first taper. [0006] In a further exemplary embodiment, the fiber optic assembly further includes a pump fiber and a second taper corresponding to the pump fiber for carrying the counter pumping light to the reflector.

[0007] In a further exemplary embodiment, the fiber optic assembly further includes a support structure having a first groove or channel for holding the gain fiber and the first taper and a second groove or channel for holding the reflector.

[0008] In a further exemplary embodiment, the reflector is part of a dichroic beam splitter assembly and comprises a dichroic coating for reflecting the counter-pumping light and passing the signal light.

[0009] In a further exemplary embodiment, the gain fiber is spliced to the first taper.

[0010] In a further exemplary embodiment, the first taper is a tapered fiber or a tapered glass rod.

[0011] In a further exemplary embodiment, a first end of the first taper interfacing with the gain fiber has a first diameter corresponding to a cladding diameter of the gain fiber, and a second end of the first taper interfacing with the reflector has a second diameter larger than the first diameter.

[0012] In a further exemplary embodiment, the first taper comprises: a first section having a diameter which increases from a first value to a second value over the length of the first section; and a second section having a constant diameter.

[0013] In a further exemplary embodiment, the fiber optic assembly further includes an antireflective coating at an interface between the first taper and the reflector.

[0014] In a further exemplary embodiment, the gain fiber is a large mode area (LMA) fiber or a photonic crystal fiber (PCF).

[0015] In a further exemplary embodiment, the gain fiber is a composite fiber comprising a photonic crystal fiber (PCF) and a large mode area (LMA) fiber with a mode adapter interfacing the PCF fiber to the LMA fiber.

[0016] In another exemplary embodiment, the present disclosure provides a fiber amplifier system. The fiber amplifier system includes: an amplifier front end configured to pre-amplify light from a seed source and output the pre-amplified light to a first section of gain fiber; a first mode adapter configured to connect the first section of gain fiber to a second section of gain fiber; a laser diode pump for providing counter-pumping light through a pump fiber; and a reflector assembly for directing the counter-pumping light from the pump fiber towards the first and second sections of gain fiber. [0017] In a further exemplary embodiment, the reflector assembly comprises a first taper, a reflector, and a second taper.

[0018] In a further exemplary embodiment, the second section of gain fiber comprises a photonic crystal fiber (PCF).

[0019] In a further exemplary embodiment, the fiber amplifier system further includes a second mode adapter configured to connect the second section of gain fiber to a third section of gain fiber, wherein the third section of gain fiber comprises a photonic crystal fiber (PCF).

[0020] In a further exemplary embodiment, the fiber amplifier system further includes a third mode adapter configured to connect the third section of gain fiber to a fourth section of gain fiber.

[0021] In yet another exemplary embodiment, the present disclosure provides a fiber amplifier array. The fiber amplifier array includes: a plurality of sleeves, wherein each sleeve comprises a gain fiber and a first taper; a plurality of interstitial spaces disposed between respective sleeves of the plurality of sleeves; and a plurality of reflectors, wherein each reflector is configured to direct counter-pumping light from at least respective one interstitial space to at least one respective first taper.

[0022] In a further exemplary embodiment, the plurality of sleeves are arranged to form a 1-D array, a 2-D square array, or a 2-D hexagonal array.

[0023] In a further exemplary embodiment, at least one reflector of the plurality of reflectors is configured to direct counter-pumping light from multiple interstitial spaces to multiple first tapers.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 illustrates a fiber optic assembly for counter pumping according to an embodiment of the disclosure.

[0025] Figure 2 illustrates an amplifier incorporating counter pumping according to an embodiment of the disclosure.

[0026] Figure 3 illustrates another amplifier incorporating counter pumping according to an embodiment of the disclosure.

[0027] Figure 4 illustrates yet another amplifier incorporating counter pumping according to an embodiment of the disclosure.

[0028] Figure 5 illustrates an end view of an array of fiber amplifiers according to an embodiment of the disclosure. [0029] Figure 6 illustrates a fiber optic assembly for counter pumping according to an embodiment of the disclosure.

[0030] Figure 7A illustrates beam patterns in the fiber optic assembly of Figure 6.

[0031] Figure 7B is a zoomed in view of a portion of Figure 7A.

[0032] Figure 8A illustrates a fiber optic assembly for counter pumping according to an embodiment of the disclosure.

[0033] Figure 8B is a zoomed in view of a portion of Figure 8A.

[0034] Figure 9 illustrates a fiber optic assembly for counter pumping according to an embodiment of the disclosure.

[0035] Figures 10A-10B illustrate a fiber optic assembly for counter pumping according to an embodiment of the disclosure.

[0036] Figure 11 illustrates a fiber optic assembly for counter pumping with a double core capillary and fiber support spacers according to an embodiment of the disclosure.

[0037] Figure 12 illustrates a fiber optic assembly for counter pumping according to an embodiment of the disclosure.

DETAILED DESCRIPTION

[0038] Ameliorating the effects of SBS, Raman scattering, and SPM generally involves utilizing rare-earth-doped gain fibers with large mode area (LMA) fiber cores where the light is guided and amplified. It can be desirable for these ultrafast fiber laser systems to employ all-fiber architectures whereby the only free space beam resides at the output of the fiber laser. Also, ultrafast fiber lasers and fiber lasers in general perform more efficiently and with higher non-linear thresholds when the pump light propagates in the opposite direction to the signal propagation (referred to as“counter pumping”).

[0039] Ultrafast fiber lasers are sensitive to SPM. This effect usually occurs when a signal pulse is at its highest intensity at the output end of a fiber laser. In monolithic or all fiber co-pumped configurations and in free space counter pumped arrangements, the amount of fiber past the amplifying gain fiber can be very short, e.g., under 10 mm. While having a short amount of fiber after the gain fiber is advantageous, co-pumped amplifiers suffer from lower efficiency and lower non-linearity thresholds compared to counter pumped amplifiers. Counter pumped configurations can achieve 3 dB more average power output or intensity before onset of non-linearity compared to co-pumped configurations. [0040] Conventional counter pumped ultrafast amplifiers have large fiber pump coupling optics and mounting hardware and require large, stable and heavy optical benches for holding the pump fibers and guiding the pump light into the end of the fiber amplifier. Alternatively, monolithic counter pumped fiber lasers can have up to 20 cm of fiber past the gain fiber for the fiber pump combiner, the output fiber, and any other transition fibers. An indicator commonly used for characterizing pulsed lasers is the“B Integral,” which corresponds to the integral of the intensity times the nonlinear index. Having 20 cm of non-gain fiber adds to the B Integral without adding any value from an amplification standpoint. Therefore, a monolithic counter pumped fiber laser designed as such has a disadvantage of increasing pulse width.

[0041] Embodiments of the disclosure provide counter pumped amplifier architectures having the compactness and reliability of monolithic designs. Embodiments of the disclosure also achieve counter pumping in an architecture where a large number of ultrafast fiber amplifiers are contained with their outputs in a closely packed 2-D array. Embodiments of the disclosure avoid having free space and therefore avoid the need for heavy optical benches for holding components together.

[0042] Figure 1 illustrates a fiber optic assembly 100 for counter pumping according to an embodiment of the disclosure. The fiber optic assembly 100 can be used in a fiber laser (e.g., having fiber Bragg grating (FBG) mirrors on both ends of the gain fiber to create an oscillator) or a fiber amplifier application (e.g., having a signal on the input). In an embodiment, the fiber optic assembly 100 comprises a double-clad gain fiber 101 of an amplifier. The gain fiber 101 can be an LMA fiber with a core 109, a pump cladding 108 (also called an“inner cladding”), and a buffer 107 (also called an“outer cladding”). The core 109 of the gain fiber 101 may be rare-earth-doped. The buffer 107 may be made of glass or polymer, or the buffer 107 could be removed or be basically air (typically for practical purposes having air as the buffer 107 would only be feasible for a few cm due to concerns about contamination). The diameter of the core 109 of the gain fiber 101 can be classified as LMA when the diameter is substantially larger than the 6 pm to 9 pm core diameters found in single mode fibers (SMF). The substantially larger LMA diameters can be achieved through a number of ways, e.g., by precise control of index of refraction (index) between the core (e.g., the core 109) and the cladding (e.g., pump cladding 108) of the fiber, or through a microstmctured fiber, such as a photonic crystal fiber (PCF), a photonic bandgap fiber (PBG), or through other methods, such as a Chirally-Coupled Core (3C) fiber. These aforementioned modes of achieving LMA type fibers allow for large propagating modes in the fiber favorable to the lowest order or fundamental propagating mode of the fiber. The fiber optic assembly 100 in Figure 1 further includes a first taper 102 (e.g., a tapered fiber or a tapered glass rod). The first taper 102 has a diameter on its smaller end that matches the diameter of the pump cladding 108 of the gain fiber 101.

[0043] As depicted in Figure 1, the tapered design of the first taper 102 allows the amplified signal light out of the core 109 of the gain fiber 101 to expand naturally as determined by the numerical aperture (NA) of the gain fiber 101. The output beam 103 from the gain fiber 101 exits the core 109, grows within the first taper 102, and exits the first taper 102. The first taper 102 can be tapered either by thermal tapering or by acid etching such that pump light entering from its right is guided by the first taper 102 to the pump cladding 108 of the gain fiber 101. Both the pump cladding 108 and the buffer 107 may form a guiding structure for pump light with respect to the gain fiber 101.

[0044] Pump light is delivered to the gain fiber 101 in a counter pumping configuration from a pump diode by way of a pump fiber 106. A second taper 105 increases the diameter of the core of pump fiber 106 and reduces the NA in accordance with the law of conservation of brightness. The second taper 105 can also be a tapered fiber or a tapered glass rod. The expanded pump light out of the second taper 105 is directed through a reversing prism 104 to direct the pump light through the first taper 102 into the pump cladding 108 of the gain fiber 101. The reversing prism 104 may be a commercially available micro prism utilizing total internal reflection to both reverse and offset the pump light. The sizes and orientations of the second taper 105, the first taper 102, and the reversing prism 104 are configured such that the output beam 103 is not impeded by the reversing prism 104.

[0045] The fiber optic assembly 100 in Figure 1 directs pump light from the pump fiber 106 into the gain fiber 101. As discussed above, the gain fiber 101 is double-clad and may include a rare-earth-doped core where the signal propagates and a pump cladding 108 that contains and propagates pump light. The pump light internally reflects and distributes around in the pump cladding 108, passing periodically through the rare-earth-doped core 109 where it is absorbed. Most double-clad fibers have cladding absorptions anywhere between 1 and 10 db/m.

[0046] In conventional methods involving pulsed lasers with advanced large mode area gain fibers such as photonic crystal fibers, the pump light is coupled into the gain fiber using 2 lenses. One lens is positioned after the pump fiber to collimate the pump light, and the other lens is positioned near the gain fiber to focus the pump light into the gain fiber.

Through the law of conservation of brightness, when light in a fiber is tapered such that the diameter increases, the numerical aperture (NA) decreases. In the embodiment of Figure 1, the first and second tapers 102, 105 act as non-imaging lenses for collimating and focusing the light. As such, replacing conventional lenses with the first and second tapers 102, 105 provides a major difference because lenses image light while tapered fibers or tapered glass rods concentrate light.

[0047] In an exemplary implementation, the second taper 105 starts as a commercially available 400/440um .22NA multimode fiber that is then tapered to a 105um core diameter and spliced to a 105/125um .15NA pump fiber from the pump diode. Pump light from the diode is at 105um and .15NA. The second taper 105 changes this to 400um and 0.05NA.

The first taper 102 takes this to 200um and .55NA. The first taper 102 is a 1.5mm/1.6mm fiber tapered to 200um.

[0048] The expanding and concentrating process is governed through the etendue of the light, which corresponds to the NA times the diameter for an optical system that is symmetric cylindrically about the propagation axis. In an exemplary implementation, the pump light travels in a 105 pm fiber diameter with an NA of 0.15. The etendue is defined as the radius times the solid angle of the light emission, but in a cylindrically symmetric geometry, the etendue can be determined as the diameter times the NA. The pump diameter-NA product for the 105 pm fiber with 0.15 NA is 15.75. A double-clad gain fiber with a diameter of 200 pm with a NA of 0.55 provides a product of 110. Since 15.75 is less than 110, the pump light should be able to be focused into the gain fiber pump cladding with 100% efficiency. The tapers serve to exchange diameter for NA while preserving the pump diameter-NA product.

In an embodiment, the second taper 105 takes the 105 pm / 0.15 NA pump light from the pump fiber 106 to 400um / 0.04NA, and the first taper 102 takes the 200 pm / 0.55 NA signal light from the gain fiber 101 to 1.6 mm / 0.07 NA. Therefore, the output of the second taper 105 can be abutted against the first taper 102, and all the pump light will go into the pump cladding 108 of the gain fiber 101. The reversing prism 104 provides convenience of placing the two fibers parallel to each other.

[0049] An advantage provided over conventional lens implementations is that lenses have to be positioned accurately and are susceptible to motion, but by using the geometry of the tapers, which may be tapered fibers or tapered glass rods, embodiments of the disclosure can very accurately and stably direct the pump light into the gain fiber. The fiber optic assembly 100 of Figure 1 is amenable to a monolithic structure where the pump light completely or mostly does not leave the fiber, and all the fibers are spliced together. Although not strictly monolithic, embodiments of the disclosure have properties associated with monolithic structures.

[0050] The fiber optic assembly 100 in Figure 1, according to some embodiments of the disclosure, provides several advantages. For example, the first taper 102 positioned after the gain fiber 101 can be designed to be very short, e.g., about 5 mm to 10 mm. Very short glass lengths can minimize the amount of SPM experienced and can increase the non-linearity thresholds. Non-linearity thresholds are increased because compared to light guided by fiber, the light entering the first taper 102 expands through diffraction instead of being confined by the structure of the gain fiber 101. To give another example, embodiments of the disclosure provide a compact design that is amenable to placing fiber amplifiers in densely packed 2-D arrays for coherent or wavelength combining. Furthermore, embodiments of the disclosure utilize an output taper, e.g., the first taper 102, that does not have light guiding properties. As such, the fiber optic assembly 100 can work with a number of different fibers without having to create specialty matching fibers.

[0051] Figure 1 illustrates counter pumping with a single fiber, but it will be appreciated that the fiber optic assembly 100 in Figure 1 can be adapted to accommodate multiple fiber pumps and reversing prisms surrounding a central beam. For example, in another exemplary embodiment, additional pump fiber(s), taper(s), and reversing prism(s) (similar to elements 106, 105 and 104 of Figure 1, which are below output beam 103) can be added above and/or on the sides of output beam 103.

[0052] In an embodiment, the reversing prism 104 can be eliminated and the pump laser can be pointed directly at the output end of the first taper 102. With respect to Figure 1, the reversing prism 104 can be removed and the second taper 105 and the pump fiber 106 are rotated 180 degrees and positioned at the bottom of the first taper 102.

[0053] In another exemplary embodiment, a second pump turning prism opposing the first one may be added, so as to double the available pumping power.

[0054] Multiple methods can be adopted to reduce or eliminate back reflections. For example, optically contacting the elements identified in Figure 1 can eliminate back reflections. In another example, the elements may be fused together by heating the elements in an optical fiber splicing machine or equivalent device. Light is in“free space” between the second taper 105 and the reversing prism 104 (which may be on the order of microns apart), and between the reversing prism 104 and the first taper 102. Thus, methods for reducing back reflections at these two interfaces can involve putting anti-reflective (AR) coatings on these surfaces or putting them in optical contact (basically touching each other). Connections between the gain fiber 101 and the first taper 102 and between the second taper 105 and the pump fiber 106 are spliced and do not require elimination of back reflections.

[0055] In an embodiment, the gain fiber 101 is PCF rod fiber with polarization- maintaining characteristics having an 85 pm core diameter and a 250 pm pump cladding diameter. The first taper 102 is a 1.5 mm to 2 mm fused silica rod tapered to 250 pm and spliced to the PCF rod fiber. Adiabatic or near adiabatic taper of the first taper 102 transforms the 250 pm .55 NA pump cladding to a 1.5 mm 0.09 NA cladding. The pump fiber 106 is spliced to the second taper 105 that increases from the 105 pm 0.15 NA core diameter of the pump fiber 106 to a 400 pm 0.04 NA output. The reversing prism 104 has anti-reflective (AR) coating on its long face and is aligned such that the light from the pump fiber 106 goes into the gain fiber 101 with high efficiency.

[0056] Figure 2 illustrates an amplifier incorporating counter pumping according to an embodiment of the disclosure. In the amplifier, light from a seed source is pre-amplified in an amplifier front end 205 through a fiber, e.g., a single mode fiber. A mode adapter 203 transforms the core light to match a larger core PCF fiber 201. The larger core PCF fiber 201 can be, for example, a 40 pm Yb-doped polarizing fiber. The core light travels through the larger core PCF fiber 201 and then out of a tapered end cap with a reflector assembly 202 (e.g., a reversing prism assembly). The reflector assembly 202 includes a first taper, a reflector (e.g., a reversing prism), and a second taper as previously described in Figure 1.

The laser pump light is pumped in a counter propagating fashion by a laser diode pump with drivers and control 204. Examples of mode adapters for matching two types of fibers can be found in U.S. Patent Application No. 15/192,389, which is hereby incorporated by reference in its entirety.

[0057] PCF-type fibers are generally a factor of 5 to 10 times higher in cost than normal large mode area gain fibers. In order to reduce the cost of building the amplifier of Figure 2, a composite-type fiber like the one shown in Figure 3 can be created. In Figure 3, the length of PCF fiber 201 is reduced considerably, e.g., from 1.5 m to around 30 cm. An LMA fiber 301, e.g., a 20 pm core 125 pm double clad Yb doped fiber, acts as a pre amplifier to the shortened PCF fiber 201.

[0058] The more costly PCF fiber 201 can be further shortened by adding a third fiber as shown in Figure 4. Comparing Figure 4 to Figure 3, the output end of the amplifier has an even larger core fiber 401. The core fiber 401 is added using double mode adapters 203 to allow counter pumped pump light to first pump the largest core fiber 401. The pump light that is unabsorbed then passes through the double mode adapter 203 and pumps the PCF fiber 201. The pump light that is unabsorbed from this stage then pumps the LMA fiber 301.

[0059] As the core signal light grows in intensity from the input to the output, the core diameter grows with the signal in a step-wise fashion such that the amount of nonlinearities that the amplifier sees is kept low while the cost of the system is greatly reduced. Although Figure 4 is described with various fiber types, e.g., PCF fiber and LMA fiber, the fibers identified by 301, 201, and 401 can be replaced by a fiber that is continually tapered and that grows in an adiabatic manner from input to output to achieve the same effect.

[0060] Smaller core fibers are cheaper and have higher gain for a given pump power, but they exhibit higher non-linearities for a given power level. Larger core fibers on the other hand are the opposite. In some embodiments, it is advantageous to begin the amplifier with a smaller core gain fiber and end the amplifier with a larger core gain fiber. Pump power is better utilized when injected at the output end of the amplifier (counter pumped

configuration) rather than the input end (co-pumped configuration). Some of the non- linearities such as self-phase modulation are governed by the B integral, which is the intensity of the light times the non-linear index of refraction with this product integrated over the length of the fiber. Since the non-linear coefficient is a constant, intensity is the variable to adjust. Therefore, embodiments of the disclosure have the intensity of the signal output light grow quickly right at the output of the amplifier.

[0061] Tapered end cap counter pumping according to embodiments of the disclosure are amenable to compact configurations that enable arrangement of precise arrays of fiber amplifiers. Figure 5 illustrates an end view of such an array of fiber amplifiers according to an embodiment of the disclosure. The circles in Figure 5 can represent fused Zirconia sleeves, such as those used in the telecom industry. The Zirconia sleeves can be 3.2 mm in outer diameter and 2.5 mm in inner diameter. These sleeves can be produced via a molding and fusing process in very large quantities and can be expensive. High tolerances can be selected for both the outer diameter and inner diameter by selecting sleeves with the proper tolerance. As such, sleeves with an interior diameter tolerance of +/-0.1 pm and an outer diameter tolerance of +/-0.5 pm are readily available. Within the sleeve is provided a fused Zirconia ferrule with a 2.5 mm outer diameter and an interior diameter that matches the signal gain fiber outer diameter, thus providing for a precise placement tolerance of the core of the fiber. [0062] Figure 5 also illustrates how these precise sleeves can be stacked to obtain a 1-D array, a 2-D square or 2-D hexagonal array such that the output cores of each fiber amplifier in the array can be aligned to a very high precision.

[0063] Due to the precise alignment of the shown hexagonal array depicted in Figure 5, the pumping configuration of Figure 1 can be implemented by placing long corner reflectors 502 along the array of fibers to serve as reversing prisms 104. The pump fiber 106 and second taper 105 of Figure 1 can be positioned precisely by running these fibers into the interstitial spaces in the array, positioning the pump fiber 106 and the second taper 105 precisely relative to the reversing prism 104 and the first taper 102.

[0064] Figure 6 illustrates a fiber optic assembly for counter pumping according to an embodiment of the disclosure. The fiber optic assembly includes a pump fiber spliced unit 602 which includes a 105/125 pm fiber spliced to tapered 400/440 pm which has an effective output NA of 0.045, a reversing prism 608, a 200/40 pm PCF 606 and a 15 mm taper 604 that ranges from 0.200 mm to 1.6 mm.

[0065] Figure 7A illustrates beam patterns in the fiber optic assembly of Figure 6.

Element 704 corresponds to beams from the pump, and element 702 corresponds to output beams. Figure 7B is a zoomed in view of a portion of Figure 7A, showing beams 704 and output beam 702. The beams 704 correspond to pump light from the pump fiber spliced unit 602 which is guided by reversing prism 608 through the taper 604 and to the PCF 606. The output beams 702 corresponds to the signal output from the PCF 606 which expands through the taper 604.

[0066] Figure 8A illustrates a fiber optic assembly for counter pumping according to an embodiment of the disclosure. Figure 8B is a zoomed in view of a portion of Figure 8A. As can be seen from these figures, a simple and stable support structure can be created with 2 simple“V” grooves holding a very precise optical alignment. In an example, the first“V” groove holds the pump fiber spliced unit 602 on the bottom and the PCF 606 and the taper 604 on top. The second“V” groove holds a reversing prism 608.

[0067] Figure 9 illustrates a fiber optic assembly for counter pumping according to an embodiment of the disclosure. The fiber optic assembly of Figure 9 includes a doped fiber amplifier gain fiber 902, a tapered end cap assembly 904 which may be passive or active, an antireflective coating 906 for both pump and signal light, a dichroic beam splitter assembly 908, an antireflective coating 910 for signal light, a dichroic coating 912 for reflecting pump light but passing signal light 911, a pump redirecting prism 914, a tapered pump fiber 916, and a pump fiber 918. [0068] In Figure 1, the pump reversing prism 104 is placed as not to intersect the signal light exiting the first taper 102. This arrangement can be lower loss due to the fewer surfaces the signal will cross, but higher brightness requirements are allocated to the pump assembly. In contrast, Figure 9 uses a dichroic beam splitting / pump redirecting assembly (the dichroic beam splitter assembly 908) to direct the pump light directly down the fiber axis instead of being offset to the side. The fiber optic assembly in Figure 9 reduces the brightness requirement for the pump, allowing for higher pump powers to be utilized. In addition, the fiber optic assembly in Figure 9 allows for the use of an active ion doped tapered end cap assembly as the tapered end cap assembly 904. The active ion doped tapered end cap assembly can provide additional amplifying energy to the overall laser assembly.

[0069] Exemplary embodiments of the disclosure have been tested and demonstrated to achieve high efficiency. In an exemplary implementation as depicted in Figure 10A, a 105/125um .15NA pump fiber was up-tapered to a 400um core fiber with a reduction of NA to 0.05. The length of the pump taper 1106 is configured to be long enough to satisfy the adiabatic condition (i.e., such that the expansion of the fiber is gradual enough to avoid light leakage, which may occur if a taper is too short/steep and does not satisfy the adiabatic condition). The pump taper 1106 may be, for example, around 20mm in length, and the pump taper 1106 does not have a cladding. The gain fiber 1101 may be a PCF fiber having a 200um/0.55NA core, and the tapered endcap 1102 corresponding to gain fiber 1101 may include an up-taper from a 195um core to a lOOOum core, with no clad (the smaller end of the up-taper may be made slightly smaller than the diameter of the PCF fiber to provide some tolerance for alignment purposes). The length of the taper in the tapered end cap is configured to be long enough to satisfy the adiabatic condition but not too long (being too long would cause clipping of the signal beam)— for example, 15mm in this exemplary implementation. A mirror, such as a dichroic mirror 1104 at lOOOnm having two angled surfaces (e.g., at a 90 degree angle relative to each other) reflects counter-pumped light which exits the pump taper 1106 back towards the gain fiber 1101.

[0070] The configuration of the tapers provided an acceptance NA which produces very high efficiency. For example, when tested at low power (input 1W at 0.15NA to pump taper 1106), the output counter-pumped light of the pump taper 1106 was also 1W at 0.05NA (100% efficiency), and the counter-pumped light output by the tapered endcap 1102 entering the gain fiber 1101 was .9996W at .55NA (-100%). Further, the output of the counter- pumped light as detected at the other end of the gain fiber 1101 (after traversing the gain fiber 1101) was .9968W at .55NA (-100%). [0071] In another exemplary implementation, where the arrangement shown in Fig. 10A was fabricated and tested at 257mW, the efficiency corresponding to the counter-pumped light at the output of the tapered endcap 1102 was -96%, with further optimizations with respect to coupling, AR coatings on the fiber, and positioning being implementable to further increase the efficiency.

[0072] It will be appreciated that certain exemplary embodiments of this disclosure depict the use of a reversing prism or a dichroic beam splitter assembly as a reflector, and that other types of reflectors, such as dichroic mirror(s), may be used to achieve a similar result. The dichroic mirror(s) 1004 may be implemented, for example, as a single bent dichroic mirror 1004 (e.g., having a 90 degree bend), or as two dichroic mirrors 1004 joined together. The dichroic mirror(s) 1004 may have a dichroic coating on the inner surfaces for reflecting counter-pumped light and allowing signal light to pass through, and an AR coating on an outer surface for avoiding signal light reflection.

[0073] Figure 10B illustrates signal light 1003 exiting the gain fiber 1001 and expanding as it propagates through the tapered endcap 1002. Due to adiabatic expansion of the signal light 1003 in the tapered endcap 1002, there may be deleterious effects on the output signal beam. Thus, at the end of the tapered endcap 1002, a flat surface may be used to minimize back reflections.

[0074] Figure 11 illustrates an exemplary practical implementation in which the tapers were constructed and spliced to PCF1 gain fibers, and with the fiber optic assembly mounted in a glass capillary. In this exemplary embodiment, gain fiber 1101 and pump fiber 1107 are inserted into respective channels of a glass capillary tube (double core capillary 1110) through respective openings in a first surface of the glass capillary tube. The core diameters for the two“cores” (or“channels”) of the may be, for example, 1mm in diameter. Fiber support spacers 1111 (which may, for example, may be made of glass and contain smaller capillaries through which the fibers are inserted) are used to hold each of the fibers in place, and to achieve a precise alignment relative to each other and relative to a dichroic turning mirror 1104 (which may, for example, be a right angle mirror). Signal light from the gain fiber 1101 (which, for example, may have a wavelength of lum) propagates through tapered end cap 1106 and is passed through dichroic turning mirror 1104. Counter-pumped light from pump fiber 1107 (which may, for example, have a wavelength of 976nm) propagates through pump taper 1102 and is reflected into tapered end cap 1106 by dichroic turning mirror 1104. [0075] It will be appreciated that although the embodiments disclosed above depict an advantageously compact double-backed configuration (in which the gain fiber and pump fibers are approximately parallel and in which the counter-pumped light is reflected back 180 degrees to enter the gain fiber in a reverse direction relative to its previous direction of propagation), exemplary embodiments of the present disclosure also include other

arrangements of such fiber optic assemblies. For example, in Figure 12, gain fiber 1201 and pump fiber 1207 are arranged in a perpendicular arrangement. It will be appreciated that other arrangements and other relative angles between the gain fiber 1201 and pump fiber 1207 may also be used.

[0076] In an exemplary implementation of the exemplary embodiment depicted in Figure 12, the gain fiber 1201 is a 40/200 PCF fiber or an 85/250 PCF rod, with a 0.03 core NA and a .055 clad NA. The tapered endcap 1202 has a maximum diameter of 800um, and is approximately 25 mm long and has a 0.14NA. The reflector 1204 is a 1030nm pass/976 reflect dichroic mirror. The pump fiber 1207 is a 105/125um 0.15NA pump fiber that provides over 200W. The pump taper 1206 is a 400/440um up-taper with a 0.05NA.

[0077] It will be appreciated that fiber tapering is utilized to increase or decrease pump fiber diameter to adjust the“jump” from the pump fiber to the amplifier over a distance of less than 5mm. For example, a high brightness 105/125um 0.15NA pump fiber may be up tapered to a 400/440 um fiber with a resulting NA reduction of under 0.05NA. This low divergence pump beam“jumps” a < 5mm gap to a taper that goes from approximately 1mm to a 200 to 250 um core diameter of an amplifier gain fiber. On the signal side, the amplified output signal grows adiabatically unguided to the output. Further, a dichroic mirror may be used to separate the two beams.

[0078] It will be appreciated that“tapered endcap” and“pump taper” as referred to in various embodiments described above are both tapers. The pump taper is spliced to a pump fiber and acts as a collimating lens which expands the counter-pumped light and reduces the NA (without requiring an actual lens). The tapered endcap is spliced to a gain fiber, receives the counter-pumped light reflected by a reflector (e.g., a reversing prism or a dichroic mirror), and acts as a focusing lens which focuses the counter-pumped light and increases the NA (again without requiring an actual lens). The tapered endcap also expands the signal light beam as it propagates through the tapered endcap and thus also acts as a collimating lens for the signal light beam.

[0079] Although splicing is typically preferred for attaching the tapered endcap to the gain fiber and attaching the pump taper to the pump fiber, it may be possible in alternative embodiments to simply position the fibers directly adjacent to one another, or to form the taper and the gain fiber monolithically from a single original fiber.

[0080] It will also be appreciated that certain of the figures herein depict the tapered endcap and the pump taper as including a constant-diameter portion. In exemplary implementations, due to the way the tapers are formed and cleaved, such constant-diameter portions may be included for ease of manufacture. However, it is not required to include a constant-diameter portion, and certain exemplary embodiments depicted herein omit the constant-diameter portion of the taper element.

[0081] In view of the foregoing discussion, it will be appreciated that embodiments of the present disclosure provide fiber optic assemblies for counter-pumping a PCF or other LMA pulsed fiber amplifier using fiber tapers (also referred to as tapered endcaps), which may be useful in pulse-based and ultrafast applications (such as with respect to laser-based radar, scientific material processing, and in certain medical devices). These architectures provide a way to achieve the advantages of free space counter pumping without the drawbacks of having to use lenses, precision mountings, and long distances as is required with free space counter pumping, and without using a combiner for counter pumping which adds extra fiber length. Instead, the architectures disclosed herein utilize fiber tapers and compact mounting distances.

[0082] Exemplary advantages achieved by embodiments of the present disclosure include, but are not limited to the following:

• Being completely agnostic as to the type of gain fiber dimensions utilized in the

amplifier, so new counter pumped combiner transition fibers do not need to be developed and fabricated.

• The core signal output of the fiber amplifier expands adiabatically and in an unguided manner, so there are no intermediate guiding sections to affect the efficiency or quality of the signal output beam.

• The matching of the pump brightness to the gain fiber brightness allows for pump coupling that is greater than 95% efficient.

• The coupled tapers create an architecture that is highly alignment insensitive,

allowing for very small and very rugged mountings.

• The tapered end cap may be about 15mm long so as to achieve virtually no

contribution to the B integral, which is particularly advantageous for ultrafast systems. • Both the pump input and the laser output may be AR coated, and the length of the tapered endcap will produce very little in the way of backward coupled light.

[0083] All references, including publications, patent applications, and patents, cited herein are hereby incorporated by reference to the same extent as if each reference were individually and specifically indicated to be incorporated by reference and were set forth in its entirety herein.

[0084] The use of the terms“a” and“an” and“the” and“at least one” and similar referents in the context of describing the invention (especially in the context of the following claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The use of the term“at least one” followed by a list of one or more items (for example,“at least one of A and B”) is to be construed to mean one item selected from the listed items (A or B) or any combination of two or more of the listed items (A and B), unless otherwise indicated herein or clearly

contradicted by context. The terms“comprising,”“having,”“including,” and“containing” are to be construed as open-ended terms (i.e., meaning“including, but not limited to,”) unless otherwise noted. Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g.,“such as”) provided herein, is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention.

[0085] Preferred embodiments of this invention are described herein, including the best mode known to the inventors for carrying out the invention. Variations of those preferred embodiments may become apparent to those of ordinary skill in the art upon reading the foregoing description. The inventors expect skilled artisans to employ such variations as appropriate, and the inventors intend for the invention to be practiced otherwise than as specifically described herein. Accordingly, this invention includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the invention unless otherwise indicated herein or otherwise clearly contradicted by context.