Fiber Laser Pump Reflectors

Abstract

A pump reflector (10) for efficiently recycling unabsorbed pump radiation in a diode-pumped fiber laser comprises a core (12) for guiding the laser beam, a pump cladding (14) and a tapered capillary (16). The pump radiation is adiabatically guided in the tapered capillary (16), which comprises a reflector (28) for reflecting the pump radiation. The pump reflector (10) can be packaged as a fiber component for co-propagating or counter-propagating fiber laser amplifiers and resonators.

Description

[0001] Priority claim

[0002] This application claims priority to U.S. patent application serial number 16 / 381,715 filed on April 11, 2019, the disclosure of which is incorporated herein in its entirety. Technical Field

[0003] The present invention relates generally to fiber laser resonators and amplifiers. In particular, the present invention relates to high power fiber laser resonators and amplifiers having cladding pumped gain fibers. Background Art

[0004] Laser radiation beams are increasingly used to cut, drill, mark and scribing workpieces made of a variety of materials; including metals, glass and polymers. Conventional mechanical machining produces unwanted defects such as micro-cracks that can propagate when the workpiece is subjected to forces, degrading and weakening the workpiece. Laser machining minimizes such unwanted defects, is generally cleaner, and results in a smaller heat-affected zone. Laser machining uses a focused laser beam to produce precise cuts and holes with high-quality edges and walls while minimizing the formation of unwanted defects. In laser welding, the focused laser beam precisely locates each weld point or weld seam and produces minimal incidental heating.

[0005] Fiber lasers are widely used in industrial laser processing applications that require high power and high beam quality. For example, laser cutting and laser welding of metals and metal alloys. In a fiber laser, the gain medium is an optical fiber with an optically active ion doped in the core, such as neodymium (Nd 3+ ), Ytterbium (Yb 3+ ), Thulium (Tm 3+ ) or Erbium 3+ ). A laser beam at the emission wavelength of the optically active ions is amplified and guided within the core. The core is typically powered by pump radiation provided by multiple diode lasers. The diode lasers efficiently convert electrical power into optical power that can be directed into the gain fiber. In a "cladding pumped" arrangement, the pump radiation is guided along the gain fiber in a pump cladding surrounding the core. An outer cladding surrounds the pump cladding.

[0006] Most of the pump radiation is absorbed by the core over a sufficient length of gain fiber, which is required to achieve the best overall efficiency. Too much unabsorbed pump radiation can lead to undesirable heating and even damage to optical components located in the path of the output laser beam. For example, for a cladding-pumped gain fiber that absorbs 1 decibel (dB) of pump radiation per meter (m) of fiber length, 20 m of gain fiber is required to absorb 20 dB of pump radiation.

[0007] For high-power operation, especially in gain fibers with small-diameter cores for single-mode operation, undesirable nonlinear processes are enhanced by confining the amplified laser beam within the core. For pulsed laser operation with high peak powers, nonlinear processes such as four-wave mixing and self-phase modulation can spectrally broaden the beam and reduce the overall efficiency. For laser beams with narrow spectral bandwidth and long coherence times, stimulated Brillouin scattering leads to back reflections that reduce the efficiency and ultimately limit the output power of the fiber laser. Such back reflections can permanently damage the fiber laser and render it inoperable. Nonlinear processes can be mitigated by reducing the length of the gain fiber. However, the concentration of optically active ions in the core must be increased to compensate and maintain the overall efficiency, which increases the thermal load of the gain fiber. For high-power operation, the high thermal load leads to transverse mode instabilities that significantly degrade the beam quality and mode stability.

[0008] For many optically active ions, notably thulium, self-absorption causes the wavelength of maximum net gain to shift to longer wavelengths as the gain fiber length increases. In a fiber laser resonator, resonator mirrors with relatively narrow spectral bandwidths are located at each end of the fiber laser resonator and are chosen to force operation at a desired wavelength within the gain fiber gain spectrum. However, self-absorption causes a mismatch between the desired wavelength and the wavelength of maximum net gain in the gain fiber. This mismatch becomes a loss that ultimately limits the power that can be extracted from the laser beam from the energized gain fiber. It also makes the operating wavelength of the fiber laser resonator sensitive to external back reflections. Again, wavelength mismatch and sensitivity to back reflections can be mitigated by reducing the length of the gain fiber.

[0009] One way to maintain efficiency in shorter gain fibers without increasing the concentration of optically active ions in the core is to recycle any unabsorbed pump radiation back into the pump cladding. This pump recycling works best in gain fibers that are pumped from only one end, because the highest heat load is at the pump end of the gain fiber. The unabsorbed pump radiation is recycled at the unpumped end of the gain fiber. Various recycling schemes have been proposed and tried. For example, Lang US 6,477,295 has a groove around the circumference of the gain fiber. The surface of the groove is inclined at 45° and must extend through most of the thickness of the pump cladding to effectively recycle the pump radiation, which mechanically weakens the gain fiber. Lang also suggested applying a highly reflective coating to most of the end face of the fiber, fusion splicing another etched fiber to the center of the end face to guide the laser beam through the end face. Again, this structure is mechanically weak. Depositing a uniform coating on such an assembly is difficult due to shadowing. In addition, the coating with a relatively small area is susceptible to optical damage when exposed to unabsorbed pump radiation. Nikolajsen US6,700,697 proposes a highly reflective coating for pump and laser beams that completely covers a flat or hemispherical end face. Such coated end faces are also susceptible to optical damage and are only useful in more complex multi-pass amplifier arrangements.

[0010] Starodoumov US 7,286,283 describes a fiber amplifier having a pump coupler at each end of a gain fiber. Each pump coupler includes multiple pump fibers that are bundled together and taper toward the gain fiber. Pump fibers that are not used in other ways may include mirrors to reflect unabsorbed pump radiation back to the gain fiber. Alternatively, unabsorbed pump radiation can be routed from an unused pump fiber in one coupler to an unused pump fiber in another coupler. It should be noted that such rerouting increases the thermal load of the hotter pump end. Although these arrangements increase efficiency, only a small portion of the unabsorbed pump radiation is recovered, which limits the efficiency improvement. In addition, pump couplers with multiple bundled pump fibers are complex and difficult to manufacture.

[0011] A fiber laser architecture with short gain fibers that can efficiently recycle unabsorbed pump radiation and that can be scaled to high powers is needed. The architecture should be mechanically robust and resistant to optical damage. Preferably, the pump cycle will use relatively simple laser components that are inexpensive and easy to manufacture. SUMMARY OF THE INVENTION

[0013] In one aspect, a pump reflector for reflecting pump radiation according to the present invention comprises a core extending between a first end and a second end of the pump reflector. The core has a refractive index. A pump cladding having a refractive index lower than that of the core is provided. The pump cladding is concentric with the core and encapsulates the core. The pump cladding extends from the first end to the second end. A capillary is made of the same material as the pump cladding. The capillary is concentric with the pump cladding and surrounds the pump cladding. The capillary has a large end and a small end. The capillary tapers from a larger outer diameter at the large end to a smaller outer diameter at the small end. For pump radiation, the capillary is adiabatically tapered. The small end of the capillary is fused to the pump cladding at a first end. The large end reflects the pump radiation. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate schematically preferred embodiments of the invention and, together with the general description given above and the detailed description of the preferred embodiments given below, serve to explain the principles of the invention.

[0015] Figure 1A is a cross-sectional view schematically illustrating a preferred embodiment of a pump reflector according to the present invention, comprising a core, a pump cladding, a capillary and a reflector for reflecting unabsorbed pump radiation from an optical fiber back into the optical fiber, while a laser beam from the optical fiber propagates through the core.

[0016] Figure 1B yes Figure 1A Cross-section of the pump reflector and optical fiber, with shading indicating the materials guiding the pump radiation.

[0017] Figure 1C and 1D yes Figure 1A Different cross-sections of the pump reflector and optical fiber. Figure 1B The way is indicated by shading.

[0018] Figure 2 It is schematically shown Figure 1A Further details of the pump reflector are shown in an enlarged cross-sectional view. Arrows indicate the propagation of the pump radiation and the laser beam.

[0019] Figure 3 A preferred embodiment of a fiber laser amplifier according to the invention in a co-propagation arrangement is schematically shown, comprising Figure 1A Optical active gain fiber, pump laser, seed laser, pump combiner and pump reflector.

[0020] Figure 4 Another preferred embodiment of a fiber laser amplifier according to the invention in a counter-propagating arrangement is schematically illustrated, similar to Figure 3 Fiber laser amplifier.

[0021] Figure 5 yes Figure 1A A cross-sectional view of the pump reflector and optical fiber, showing the dimensions of the pump reflector.

[0022] Figure 6 is a schematic diagram showing the manufacturing Figure 1A Flowchart of a method of pumping a reflector. DETAILED DESCRIPTION OF THE INVENTION

[0024] Reference is now made to the drawings, wherein like parts are represented by like numerals. Figure 1A is a cross-sectional view schematically showing a preferred embodiment 10 of a pump reflector according to the present invention. The pump reflector 10 includes a core 12, a pump cladding 14 covering the core 12, and a capillary 16 surrounding the pump cladding 14. The pump cladding 14 and the capillary 16 are concentrically arranged around the core 12. The core 12, the pump cladding 14 and the capillary 16 are preferably all made of glass to achieve high power operation. For example, the core, the pump cladding and the capillary are made of silica or fluoride glass. The pump cladding 14 and the capillary 16 are preferably made of the same glass. The core 12 has a higher refractive index than the pump cladding 14 and the capillary 16. The core 12 and the pump cladding 14 extend along the entire length of the pump reflector from the first end 18 to the second end 20.

[0025] The capillary 16 has a large section at a large end, a small section at an opposite small end, and a tapered section therebetween. This taper is formed by heating and stretching a uniform capillary, which generally maintains the aspect ratio of the outer diameter to the inner diameter along the entire length of the tapered capillary. The small section of the capillary 16 is fused to the pump cladding 14 at a first end 18. The fusion interface is indicated by the dashed line in the figure. However, during fusion, the melting eliminates any discontinuities, so that the pump cladding and the capillary become essentially one piece of glass.

[0026] The pump reflector 10 is depicted as being spliced ​​to an optical fiber 22, which may be, by way of example, a gain fiber in a fiber laser. The pump reflector and the optical fiber are joined at a first end 18 by a connector 24. Here, the optical fiber 22 has a core of the same diameter as the core 12 and a pump cladding of the same outer diameter as the pump reflector 10 at the first end 18, preferably for high power operation. Any difference in these dimensions results in power loss and undesirable heating around the connector 24. Therefore, the capillary 16 is tapered to match the outer diameter of the small section to the outer diameter of the pump cladding of the optical fiber 22. Similarly, any lateral misalignment between the pump reflector and the core and pump cladding of the optical fiber results in power loss and unwanted heating.

[0027] Here, the pump cladding 14 is the pump cladding of another optical fiber that has been etched at the first end 18 to have a uniform outer diameter that is slightly smaller than the inner diameter of the small section of the capillary 16 before fusion. The other end of the other optical fiber becomes the second end of the pump reflector 10. The transition between the etched and unetched portions of the optical fiber can be tapered or steep as shown. An outer coating 26 having a lower refractive index than the pump cladding 14 provides mechanical and chemical protection for the unetched portion. For the purpose of illustration, the unetched portion of the pump reflector wrapped in the outer coating 26 has been shortened. However, this portion will have a length determined by practical considerations. For example, a length short enough to allow splicing to another optical component or a length long enough to deliver the output laser beam to an application. It should be noted here that the pump cladding of such a "double-clad fiber" is essentially the core for pump radiation, sometimes referred to as a "pump core" by practitioners in the art.

[0028] The large section of capillary 16 includes a mirror 28 that reflects the pump radiation. Here, the mirror 28 is depicted as a reflective coating on the end face of the large section. For example, the coating can be a metallic coating or a thin film dielectric coating. Alternatively, a Bragg grating can be incorporated into the large section. Thin film dielectric coatings are preferred for high power operation because these coatings can be designed and deposited to have a high optical damage threshold. An advantage of the pump reflector 10 with capillary 16 is that the mirror 28 has a large area compared to the cross-sectional area of ​​the pump cladding in a typical optical fiber, which reduces the intensity of the incident unabsorbed pump radiation. A larger capillary 16 can be selected to further reduce this intensity. For example, the pump reflector can tolerate very high incident powers caused by the pump radiation being intentionally or unintentionally tuned away from the absorption wavelength of the gain fiber.

[0029] Figure 1B is a cross-sectional view of the pump reflector 10 and the optical fiber 22, and Figure 1A The pump cladding 14, the capillary 16 and the pump cladding of the optical fiber 22 are all Figure 1B As mentioned above, the shielding material preferably has the same composition and is essentially a piece of glass after being fused to make the pump reflector 10 and spliced ​​to the optical fiber 22. The guided pump radiation can therefore propagate through this shielding material with minimal losses. Figure 1C and 1D yes Figure 1B In the different cross-sectional views in the planes shown, identical elements are shaded. Figure 1C and 1D The concentric structure of the pump reflector is shown. At the large end of the capillary, an annular space is defined between the outer diameter of the pump cladding and the inner diameter of the capillary. Conveniently, air will fill this annular space, but it can be filled with another material with a lower refractive index than the pump cladding and capillary.

[0030] Figures 1A-1D The pump reflector 10 described in the drawings has the same dimensions as the optical fiber 22 at the second end 20. While this fiber versatility may be useful in some applications, in other applications it may be beneficial to have a cladding with a larger or smaller diameter at the second end 20. The second end can be designed accordingly and the pump cladding 14 made of an optical fiber having the desired dimensions.

[0031] Figure 2 It is schematically shown Figure 1A FIG. 1 is an enlarged cross-sectional view of further detail near the first end 18 of the pump reflector 10 in FIG. The solid arrows represent the propagation of the laser beam through the core 12. The hollow arrows represent the propagation of the pump radiation. Unabsorbed pump radiation propagates out of the optical fiber 22, through the connector 24, and into the pump reflector 10. Most of the unabsorbed pump radiation is directed into the capillary 16, reflected by the mirror 28 (not shown), and then directed back through the connector 24. A larger portion of the unabsorbed pump radiation returns to the optical fiber 22. A smaller portion of the unabsorbed pump radiation is directed into the pump cladding 14 and cannot return to the optical fiber 22. This smaller portion is depicted as a dashed line and can be determined from the dimensions of the pump reflector, as described below.

[0032] Figure 3 A preferred embodiment 50 of a fiber laser amplifier according to the present invention is schematically shown. The fiber laser amplifier 50 includes an optically active gain fiber 52, a pump laser 54 providing pump radiation, a seed laser 56 providing a laser beam to be amplified, a pump combiner 58 for directing the pump radiation and the laser beam into the gain fiber 52, and a pump reflector 10. These elements are connected by optical fibers as shown. Again, the solid arrows indicate the propagation of the laser beam, while the hollow arrows indicate the propagation of the pump radiation. The fiber laser amplifier 50 is a co-propagating device, and the pump radiation and the laser beam initially propagate into the gain fiber 52 in the same direction. The amplified laser beam emerging from the pump reflector 10 can be further amplified or directed to an application. After a first pass through the gain fiber, the pump reflector returns the unabsorbed pump radiation to the gain fiber for a second pass. Incorporating a pump reflector into a fiber laser amplifier can improve its efficiency. It can also allow the gain fiber to be shorter, thereby enabling higher power operation before being limited by nonlinear processes, transverse mode instability or self-absorption.

[0033] Figure 4Another preferred embodiment 70 of a fiber laser amplifier according to the present invention is schematically illustrated. The fiber laser amplifier 70 has the same components as the fiber laser amplifier 50, but adopts a counter-propagation arrangement. The pump radiation is guided into the gain fiber 52 via the pump combiner 58. The laser beam to be amplified is guided into the gain fiber 52 via the pump reflector 10. The pump radiation and the laser beam thereby initially propagate into the gain fiber in opposite directions. The amplified laser beam is emitted from the pump combiner 58. The pump reflector returns the unabsorbed pump radiation again, after a first pass through the gain fiber, back to the gain fiber for a second pass. Incorporating a pump reflector into the fiber laser amplifier 70 provides the same advantages as incorporating it into the fiber laser amplifier 50.

[0034] Although Figure 3 and 4 Embodiments 50 and 70 are fiber laser amplifiers, but the pump reflectors of the present invention can be incorporated into fiber laser resonators in the same manner. An equivalent laser resonator would omit the seed laser 56, but include a high reflector and an output coupling mirror at opposite ends of the gain fiber 52. These mirrors define a fiber laser resonator. Typically, these resonator reflectors are fiber Bragg gratings.

[0035] Figure 5 is a cross-sectional view of the pump reflector 10 and the optical fiber 22, and Figure 1A The same represents the size of the pump reflector. D1 is the diameter of the small section of the capillary 16, D2 is the outer diameter of the etched portion of the pump cladding 14, D3 is the inner diameter of the large section of the capillary 16, D4 is the outer diameter of the large section of the capillary 16, D5 is the diameter of the unetched portion of the pump cladding 14, and D6 is the diameter of the core 12. L1 is the length of the small section of the capillary 16, L2 is the length of the tapered section of the capillary 16, L3 is the length of the large section of the capillary 16, L4 is the distance between the capillary and the taper of the pump cladding 14, L5 is the length of the taper between the etched and unetched portions of the pump cladding 14, and T is the thickness of the capillary wall.

[0036] An exemplary pump reflector for a fiber laser with an ytterbium-doped gain fiber is energized by pump radiation having a wavelength of about 976 nanometers (nm) and produces amplified laser radiation having a wavelength of about 1070 nm, and has the following dimensions: diameter D1 about 250 micrometers (μm), diameter D2 about 80 μm, diameter D3 about 250 μm, diameter D4 about 750 μm, diameter D5 about 250 μm, diameter D6 about 25 μm, length L2 about 20 millimeters (mm), and thickness T about 250 μm. Other dimensions are less critical to the operation of the pump reflector, but some practical ranges are: length L1 about 0.1 mm to about 50 mm, length L3 about 0 mm to about 50 mm, and length L5 about 0 mm to about 10 mm. It should be noted that the pump cladding 14 can be fabricated without a taper between the etched and unetched portions, if desired.

[0037] The etched portion of the pump cladding 14 is depicted as protruding from the capillary 16. A practical range for distance L4 is from about 0 to about 50 mm. However, if the diameter D3 is large enough, the etched portion can be recessed within the capillary. This is a design option for the example pump reflector. By making the diameter D5 slightly smaller than the diameter D3 and by partially recessing the unetched portion of the pump cladding into the capillary, the capillary can provide mechanical support for the pump cladding.

[0038] Those skilled in the art will recognize that the length L2 can be any length that provides adiabatic propagation of pump radiation between the small end and the large end of the capillary 16. A practical range for the length L2 is from about 2 mm to about 50 mm. A practical range for the diameter D1 is from about 100 μm to about 600 μm. The lower limit of about 100 μm can be used for splicing to small single-mode optical fibers. The upper limit is near the maximum diameter of flexible silica optical fibers. Those skilled in the art will recognize that the pump reflector can be designed for use with larger "rod fibers" without departing from the spirit and scope of the present invention. As described above, the diameter D5 can be selected according to the requirements of the application, but other considerations are the same as for the selection of D1.

[0039] A practical lower limit for diameter D2 is about 35 μm. Diameter D3 must be greater than D2. Diameter D4 is determined by the selection of diameter D2 and thickness T. An advantage of pump reflector 10 is that thickness T can be increased as needed to reduce the intensity of unabsorbed pump radiation on mirror 28 to below the optical damage threshold of the mirror coating. A practical range for thickness T is from about 100 μm to about 1000 μm. However, a pump reflector having a capillary thicker than about 1000 μm can be made using the method described below.

[0040] The core diameter D6 is selected to match the core diameter of the optical fiber 22. Typical ranges for industrial fiber lasers are about 6 μm to about 40 μm. The pump reflector 10 can have a core 12 containing optically active ions or can be a passive component in the laser. For high power applications, it may be preferred to have an up-doped core 12 to reduce the thermal load on the pump reflector itself.

[0041] The total reflectivity R of the pump reflector 10 for the unabsorbed pump radiation propagating out of the optical fiber 22 can be calculated. As described above, most of the unabsorbed pump radiation returns to the optical fiber 22 after being reflected by the reflector 28, which has an R at the wavelength of the pump radiation. M Using thin film coating technology, the reflectivity R M It is not difficult to approach a 100% reflectivity of the mirror 28. A smaller portion directed into the pump cladding 14 is lost and cannot return. These losses are the largest contribution to the overall reflectivity R and are therefore approximately:

[0042]

[0043] For the exemplary pump reflector described above, the reflectivity R is about 95%. M This will result in a total reflectivity R of approximately 85%. For a pump reflector 10 constructed as described above and manufactured as follows, it should be straightforward to achieve a total reflectivity of at least 80%. A reflectivity within this range means that the pump reflector of the present invention effectively halves the length of gain fiber required to substantially absorb the pump radiation provided by the pump laser. For example, the length of gain fiber required to absorb 10 dB of pump radiation.

[0044] The coating of the reflector 28 can also be designed to have a low reflectivity at the wavelength of the laser beam. Any reflection of the laser beam is generally undesirable in a fiber laser. In a fiber laser resonator, such reflections can cause unstable operation. In a fiber laser amplifier, the reflected beam can be amplified and cause permanent optical damage.

[0045] Figure 6 A preferred embodiment 80 of a method for manufacturing a pump reflector 10 according to the present invention is schematically shown. A capillary is selected and cleaved or polished to form an end face that will be coated later. The capillary is heated to above the softening point in the region between its two ends and then gradually tapered by pulling the two ends while moving the heat source. The tapered capillary has an hourglass shape with the waist located in the heated region. Between the waist and the end face there are (in order) a small section, a tapered section, and a large section. A mirror coating is deposited on the end face of the large section. For example, a thin film dielectric coating is deposited that is reflective at the pump wavelength and transmissive at the laser wavelength. The coating can be deposited on multiple tapered capillaries in a batch process.

[0046] A double-clad fiber is selected. One end is stripped of the outer cladding and then partially etched through the pump cladding. The pump cladding is partially etched to a diameter just smaller than the inner diameter of the tapered capillary segment. The partially etched end is inserted and held through the tapered capillary segment while at least a portion of the segment is heated, collapsed, and fused to the partially etched pump cladding. The fused portion of the segment and the partially etched end are cleaved to form an end ready for splicing. The completed pump reflector is spliced ​​to an optical fiber in a fiber laser resonator or amplifier.

[0047] The glass surface of the pump reflector can be encased in a protective coating that can be the same material as the outer coating. The pump reflector can be fixedly attached to a base plate or other support structure to provide it with mechanical strength. The pump reflector can be packaged as a fiber component. Unpackaged or packaged pump reflectors can be attached or mounted to a cooling plate to provide additional thermal resilience.

[0048] The present invention has been described above according to preferred embodiments and other embodiments. However, the present invention is not limited to the embodiments described and depicted herein. Instead, the present invention is limited only by the appended claims.

Claims

1. A pump reflector for reflecting pump radiation, comprising: a core extending between the first end and the second end of the pump reflector, the core having a refractive index; A pump cladding, wherein the refractive index of the pump cladding is lower than the refractive index of the core; the pump cladding is concentric with the core and encapsulates the core, and the pump cladding extends from the first end to the second end; as well as a capillary made of the same material as the pump cladding, the capillary being concentric with the pump cladding and surrounding the pump cladding, the capillary having a large end and a small end, the capillary tapering from a larger outer diameter at the large end to a smaller outer diameter at the small end, the small end of the capillary being fused to the pump cladding at the first end, and the large end reflecting pump radiation; Wherein, an annular space is defined between the outer diameter of the pump cladding at the large end of the capillary and the inner diameter of the capillary.

2. The pump reflector according to claim 1, wherein: Pump radiation propagating into the pump reflector and into the capillary at the first end is reflected at the large end of the capillary and directed to propagate back toward the pump reflector and exit the pump reflector at the first end.

3. The pump reflector according to claim 2, wherein: The pump radiation propagates into the pump reflector at the first end and propagates out of the pump reflector at the first end with an efficiency of at least 80%.

4. A pump reflector according to any preceding claim, wherein: The laser beam propagated into the core at the first end propagates within the core from the first end to the second end.

5. A pump reflector according to any preceding claim, wherein: The end face of the large end of the capillary comprises a thin film dielectric coating for reflecting the pump radiation.

6. The pump reflector according to any one of claims 1 to 4, wherein: The end face of the large end of the capillary comprises a metal coating for reflecting the pump radiation.

7. The pump reflector according to any one of claims 1 to 4, wherein: The large end of the capillary comprises a Bragg grating for reflecting the pump radiation.

8. A pump reflector according to any one of the preceding claims, wherein: The mirror in the large end is reflective at the pump wavelength and transmissive at the laser wavelength.

9. The pump reflector according to claim 8, wherein: The pump radiation at the pump wavelength is the unabsorbed pump radiation from the fiber laser.

10. A pump reflector according to any one of the preceding claims, wherein: The core, the pump cladding and the capillary are made of glass, and the pump cladding and the capillary are made of the same glass.

11. A pump reflector according to any one of the preceding claims, wherein: The capillary is adiabatically tapered for the pump radiation.

12. The pump reflector according to claim 1, wherein: The annular space is filled with air.

13. A method for manufacturing a pump reflector for reflecting pump radiation, comprising the following steps: Cleaving or polishing the capillary to form the end faces; heating the capillary in a region between its ends to above its softening point; The end of the heated capillary is gradually tapered by pulling to form a small section, a tapered section, and a large section thereof; depositing a mirror coating on the end faces of the large segments; Partially etching one end of the double-clad optical fiber through the pump cladding of the double-clad optical fiber, wherein the outer diameter of the partially etched end is just smaller than the inner diameter of the small segment of the tapered capillary; Passing the partially etched end of the optical fiber through a small segment of a tapered capillary tube; as well as heating at least a portion of the tapered capillary segment to cause a portion of the segment to collapse and fuse to the partially etched end of the optical fiber; Therein, an annular space is defined between the outer diameter of the partially etched pump cladding and the inner diameter of the large section of the capillary.

14. The method for manufacturing a pump reflector according to claim 13, wherein: The end faces of the large sections are ground flat.

15. The method for manufacturing a pump reflector according to claim 13 or claim 14, wherein: The mirror coating is a thin film dielectric coating for reflecting the pump radiation.

16. The method for manufacturing a pump reflector according to claim 13 or claim 14, wherein: The mirror coating is a metallic coating which serves to reflect the pump radiation.

17. The method for manufacturing a pump reflector according to any one of claims 13 to 16, wherein: The mirror coating is reflective at the pump wavelength and transmissive at the laser wavelength.

18. The method for manufacturing a pump reflector according to any one of claims 13 to 17, wherein: The pump cladding and the capillary are made of the same glass.

19. The method for manufacturing a pump reflector according to any one of claims 13 to 18, further comprising the following steps: The fused and partially etched ends of the segments are cleaved to produce the ends of the pump reflectors ready for splicing.

20. The method for manufacturing a pump reflector according to any one of claims 13 to 19, further comprising the step of encapsulating a glass surface of the pump reflector in a protective coating.

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