Method for coupling optical fiber and optical coupler

Abstract

A method of optically coupling a first optical fiber and a second optical fiber to each other is described. The method generally has the steps of directing a free end of a first optical fiber, a second optical fiber, and a liquid into close proximity with each other within a coupling region, the free end of the first optical fiber having a dimension below a critical dimension, the free end of the first optical fiber moving within the liquid to contact the second optical fiber along a given coupling length, the contact optically coupling the free end of the first optical fiber and the second optical fiber to each other.

Description

Technical Field

[0001] The present improvements relate generally to optical fibers and, more particularly, to optically coupling optical fibers to each other. Background Art

[0002] In some types of optical couplers, a smaller optical fiber (e.g., a fiber taper) will be optically coupled to a larger optical fiber, or vice versa, so that the optical signal propagating in one optical fiber is transmitted to the other optical fiber with the highest possible efficiency. In such cases, it may be challenging to bring the smaller optical fiber to the larger optical fiber in a manner that facilitates continuous optical coupling between the smaller optical fiber and the larger optical fiber. Existing methods of optically coupling a smaller optical fiber to a larger optical fiber typically include positioning the smaller optical fiber and the larger optical fiber side by side with each other under tension. While tension is maintained on the two optical fibers, the smaller optical fiber and the larger optical fiber are brought into contact with each other and then fused or otherwise bonded to each other, thereby fixing the optical coupling between the two optical fibers. Although existing methods of optically coupling optical fibers to each other are satisfactory to a certain extent, there is still room for improvement. Summary of the invention

[0003] It has been discovered that a need exists to optically couple a smaller optical fiber to a larger optical fiber (or vice versa) while reducing risks associated with straining, fusing and / or manipulating the smaller and larger optical fibers and / or improving the efficiency of transmitting optical signals from one optical fiber to another.

[0004] According to a first aspect of the present disclosure, a method for optically coupling a first optical fiber and a second optical fiber to each other is provided, the method comprising: guiding a free end of the first optical fiber, the second optical fiber, and a liquid to be in close proximity with each other in a coupling region, the free end of the first optical fiber having a size below a critical size, the free end of the first optical fiber moving within the liquid to contact the second optical fiber along a given coupling length, the contact optically coupling the free end of the first optical fiber and the second optical fiber to each other.

[0005] According further to the first aspect of the present disclosure, the second optical fiber may, for example, not be under tension during the guiding.

[0006] Still further according to the first aspect of the present disclosure, the guiding may, for example, include: guiding the free end and the second optical fiber to be in close proximity with each other in the coupling region, and then injecting the liquid into the coupling region.

[0007] Still further in accordance with the first aspect of the present disclosure, the method may, for example, further include: after the liquid has evaporated, bonding the free end of the first optical fiber and the second optical fiber to each other.

[0008] Further in accordance with the first aspect of the present disclosure, the method may, for example, also include: before the guiding, tapering the first optical fiber so that the waist extends between the descending tapered portion and the ascending tapered portion, and removing one of the descending tapered portion and the ascending tapered portion from the first optical fiber to make the waist free, thereby serving as the free end of the first optical fiber.

[0009] According to the first aspect of the present disclosure, the method may further include: during the guiding, tensioning the second optical fiber.

[0010] Further according to the first aspect of the present disclosure, the movement may, for example, also include: the free end of the first optical fiber is spirally wound around the second optical fiber.

[0011] Still further according to the first aspect of the present disclosure, the guiding may, for example, be performed gradually in a direction leading to a free tip of the free end of the first optical fiber.

[0012] According to still further the first aspect of the present disclosure, the liquid may be, for example, ethanol.

[0013] According further to the first aspect of the present disclosure, the first optical fiber and the second optical fiber may be made of dissimilar materials, for example.

[0014] According to the first aspect of the present disclosure, the first optical fiber may be made of, for example, silica, and the second optical fiber may be made of, for example, low phonon energy glass.

[0015] According further to the first aspect of the present disclosure, the size of the free end of the first optical fiber may be, for example, smaller than the size of the second optical fiber.

[0016] According to a second aspect of the present disclosure, an optical coupler is provided, comprising a first optical fiber and a second optical fiber optically coupled to the first optical fiber, wherein the first optical fiber has a free end having a size lower than a critical size, and the free end of the first optical fiber contacts the second optical fiber along a given coupling length, thereby providing optical coupling.

[0017] According to the second aspect of the present disclosure, the optical coupler may, for example, further include a protector on the free end and the second optical fiber along the given coupling length, the free end having a free tip with a cleaved end face, for example, below the protector. In some embodiments, the protector may partially extend above the free end, which may leave the tip of the free end uncovered by the protector.

[0018] According further to the second aspect of the present disclosure, the protector may be made of, for example, a polymer having a refractive index lower than that of the first optical fiber and the second optical fiber.

[0019] According to still further the second aspect of the present disclosure, the first optical fiber and the second optical fiber may be made of dissimilar materials, for example.

[0020] According to the second aspect of the present disclosure, the first optical fiber may be made of, for example, silica, and the second optical fiber may be made of, for example, low phonon energy glass.

[0021] According to still further the second aspect of the present disclosure, the free end and the second optical fiber may be fused to each other, for example, along the given coupling length, and the free end has a free tip with a circular end surface.

[0022] According to the first aspect of the present disclosure, the free end may be wound around the second optical fiber, for example. In some embodiments, the free end may be wound around the second optical fiber for a number of turns, the number of turns being at least 0.25, preferably at least 0.5, and most preferably at least 1.0.

[0023] According further to the second aspect of the present disclosure, the free end of the second optical fiber may, for example, have a length greater than a given length threshold.

[0024] According to still further the second aspect of the present disclosure, the size of the free end of the first optical fiber may be, for example, smaller than the size of the second optical fiber.

[0025] It has also been found that there is a need to optically couple a smaller optical fiber to a larger optical fiber without the use of a conventional glass processing platform. Typically, such glass processing platforms guide the optical fibers together and then fuse them to each other. Although such glass processing platforms are useful in some cases, they may be expensive, difficult to operate, and / or cumbersome for customers to carry for maintenance purposes. In addition, these glass processing platforms are generally not applicable in situations where optical fibers of different materials are to be optically coupled to each other, because the different melting points mean that when one of the optical fibers is heated to the desired temperature, the other optical fiber may have irreversibly melted and / or burned. Accordingly, once the smaller optical fiber is optically coupled to the larger optical fiber using the above-mentioned hinge method, and the liquid has evaporated from the coupling area, the optical fibers can be bonded to each other instead of being fused, and / or, bonded to each other in addition to fusing the optical fibers.

[0026] According to a third aspect of the present disclosure, there is provided a use of an optical coupler as defined above as a pump combiner for a fiber-based laser and / or amplifier. Such an optical coupler may allow a pump beam to be optically coupled into an active optical fiber used in a communication network or a fiber-based laser and / or amplifier. More specifically, the method proposed herein may allow a pump combiner with high coupling efficiency to be manufactured quickly and reliably without any complex mechanical or heating steps. The method proposed herein can reduce the amount of tricky manipulation of the optical fiber, which can reduce the risk of disconnecting any of the optical fibers while the optical fiber is being manipulated. Typical fused fiber combiners tend to interfere with the shape of the receiving optical fiber, which can reduce the performance of the optical fiber communication system, particularly when the core of the receiving optical fiber is close to the cladding surface. The method proposed herein can also achieve optical coupling between dissimilar optical fibers that have significantly different melting temperatures (e.g., silica-based glass with a melting temperature of about 2000°C and fluorine-based glass with a melting temperature of about 300°C) and therefore cannot be satisfactorily fused to each other using existing technologies. Therefore, the method proposed in this article enables the fabrication of all-fiber pump combiners using such special optical fibers (e.g., low phonon energy glass (e.g., fluorine glass) fibers) that are capable of transmitting from the ultraviolet to the mid-infrared region of the electromagnetic spectrum, and still avoids the use of complex, bulky, fragile, inefficient and expensive pump combining systems for these optical fibers.

[0027] After reading this disclosure, many other features related to the present improvement and combinations of these features will be apparent to those skilled in the art. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In the accompanying drawings,

[0029] Figures 1A to 1E Depicted are steps of an example of a method of optically coupling a first optical fiber to a second optical fiber, wherein the first optical fiber has a smaller dimension than a dimension of the second optical fiber, in accordance with one or more embodiments;

[0030] Figure 2A is a schematic diagram of an example of a receiving optical fiber according to one or more embodiments;

[0031] Figure 2B is a schematic diagram of an example of a feeding optical fiber according to one or more embodiments;

[0032] Figure 3A is after tapering according to one or more embodiments Figure 2B A cross-sectional view of a feeder optical fiber of FIG. 1 showing a waist extending between a descending tapered portion and an ascending tapered portion;

[0033] Figure 3B FIG. 1 is a diagram of a 3D image ... Figure 3A A cross-sectional view of a tapered feeding optical fiber;

[0034] Figures 4A to 4F Depicts a method of applying a liquid to a coupling region according to one or more embodiments. Figure 2A The receiving fiber is optically coupled to Figure 3B Example steps of a method of tapering a free end of a feeding optical fiber;

[0035] Figure 5 is a schematic diagram of an example of an optical coupler in which free ends of a receiving optical fiber and a tapered feeding optical fiber have been bonded to each other within a coupling region according to one or more embodiments; and

[0036] Figure 6 is a schematic diagram of an example of an optical coupler in which free ends of a receiving fiber and a tapered feeding fiber have been fused to each other within a coupling region in accordance with one or more embodiments. DETAILED DESCRIPTION

[0037] Figures 1A to 1E The figure shows an example of a method of optically coupling a first optical fiber 10 to a second optical fiber 12 , wherein the first optical fiber 10 has a dimension D1 that is smaller than a dimension D2 of the second optical fiber 12 .

[0038] In some embodiments, the first optical fiber 10 and the second optical fiber 12 have similar nominal dimensions, such as the second dimension D2. In such embodiments, for the purpose of optical coupling, the first optical fiber 10 can be tapered downward to the first dimension D1. In some other embodiments, the first optical fiber 10 can have a smaller nominal dimension than the second dimension D2 of the second optical fiber 12. Note that in embodiments where the optical fiber is a circular cross-sectional shape, the dimension can be the diameter of the corresponding optical fiber. However, in some other embodiments, the optical fiber may not have a circular cross-sectional shape. For example, the optical fiber may have a polygonal cross-sectional shape, an irregular cross-sectional shape, or any other suitable cross-sectional shape.

[0039] In any case, the first dimension D1 is smaller than the critical dimension D c . Critical size D c The critical dimension D may vary from embodiment to embodiment and is generally defined as the dimension below which the free end 14 of the first optical fiber 10 will freely move and slide over the second optical fiber 12 when within the liquid 16. In some embodiments, the critical dimension D c Below 50 μm, preferably below 35 μm, most preferably below 20 μm. Critical dimension D c Depends on the first optical fiber 10. For example, the critical dimension D c It may depend on the shape and / or material of the first optical fiber 10 .

[0040] like Figure 1A As shown in FIG. 1 , the first optical fiber 10 and the second optical fiber 12 will be optically coupled to each other through the liquid 16 in the coupling region 18. More specifically, Figure 1B 1 shows that when the free end 14 of the first optical fiber 10, the second optical fiber 12 and the liquid 16 are guided to be in close proximity with each other in the coupling region 18, the free end 14 of the first optical fiber 10 moves in the liquid 16 to move along the Figure 1C The partial coupling length L shown in p land, and guide as Figure 1D The coupling length L shown in c Now, as the liquid 16 evaporates, the coupling length L c The contact formed optically couples the free end 14 of the first optical fiber 10 and the second optical fiber 12 to each other, as a result of which Figure 1E In some embodiments, the coupling length L c It can vary between 5 mm and 100 mm. In some other embodiments, the coupling length L c It may be at least 100 mm, preferably at least 200 mm or longer. In some embodiments, the contact is permanently fixed by fusing or bonding at least the free end 14 and the second optical fiber 12 to each other in the coupling region 18. Alternatively, the contact may be permanently fixed by fusing and bonding at least the free end 14 and the second optical fiber 12 to each other in the coupling region 18.

[0041] like Figure 1B and 1C As can be seen in FIG. 1 , the contact formed between the free end 14 and the second optical fiber 12 can be gradually formed along the direction 20 of the free tip 22 of the free end 14 of the first optical fiber 10. However, in some other embodiments, the contact can be formed along the opposite direction, or along the coupling length L. c The entire length is formed simultaneously.

[0042] The order in which the free end 14, the second optical fiber 12, and the liquid 16 are directed within the coupling region 18 may vary depending on the embodiment. For example, the free end 14 and the second optical fiber 12 may be first directed into close proximity with each other within the coupling region 18, and then the liquid 16 may be injected within the coupling region 18. The liquid 16 may be injected using a pipette (e.g., a mL pipette, a liquid injection device) or any other liquid injection device. In some other embodiments, the liquid 16 may be first injected onto the free end 14 and then directed into close proximity with the second optical fiber 12 within the coupling region 18. Alternatively, or in addition, the liquid 16 may be first injected onto the second optical fiber 12 and then directed into close proximity with the free end 14 within the coupling region 18.

[0043] The type of liquid and its volume may vary from embodiment to embodiment. For example, examples of liquid 16 may include ethanol (e.g., isopropyl alcohol, acetone, etc.), solvents (e.g., toluene, water, etc.), and any other cleaning liquid. In some embodiments, alcohol is preferred because it is a fast drying liquid, however, any other fast drying liquid may be used. The volume of liquid used may also be based on the first dimension D1 and the second dimension D2 of the first optical fiber 10 and the second optical fiber 12 and on the coupling length L. c The evaporation time of the liquid 16 may depend on the type of liquid used and its volume.

[0044] The first optical fiber 10 and the second optical fiber 12 may have different geometries. The first optical fiber 10 and the second optical fiber 12 may also be of different types, including but not limited to single-clad optical fiber, double-clad optical fiber, multi-clad optical fiber, coreless optical fiber, single-core optical fiber and multi-core optical fiber and / or any suitable special optical fiber. The first optical fiber 10 and the second optical fiber 12 may also have similar or dissimilar cross-sections of any suitable shape, including but not limited to d-shaped, circular, octagonal, etc. Depending on the embodiment, each of the first optical fiber 10 and the second optical fiber 12 may have one, zero or several cores. As can be appreciated, in some embodiments, the first optical fiber 10 and the second optical fiber 12 may be made of similar materials. For example, both the first optical fiber 10 and the second optical fiber 12 may be made of silica-based glass or low phonon energy glass. In some other embodiments, the first optical fiber 10 and the second optical fiber 12 may be made of different materials. For example, the first optical fiber 10 may be made of silica-based glass (melting temperature is about 2000° C.), and the second optical fiber 12 may be made of low phonon energy glass (melting temperature is about 300° C.), or vice versa.

[0045] In the present disclosure, the expression "low phonon energy glass" is intended to encompass glass having a maximum phonon energy lower than that of silicon-based glasses (i.e., lower than 800 cm -1). Optical fibers made of low phonon energy glass typically have a transmission window from the ultraviolet (e.g., from 10 nm to 400 nm) to the mid-infrared region (i.e., 2.5 μm to longer wavelengths), which is different from optical fibers made of high phonon energy glass, which (e.g., silica-based glass) instead exhibits a transmission window only in a given portion of the near-infrared region (e.g., up to 2.4 μm). It can be understood that the use of optical fibers made of low phonon energy glass allows photons at wavelengths in the mid-infrared region to propagate along a corresponding length of optical fiber, which is opposite to optical fibers of high phonon energy glass, which instead prevent the propagation of these photons. Accordingly, the methods described herein are particularly useful for optical devices intended to operate in the mid-infrared region of the electromagnetic spectrum. Examples of low phonon energy glasses include fluoride-based glasses, chalcogenide-based glasses, chalcogen halide-based glasses, or telluride-based glasses. In some cases, some oxide glasses (e.g., tellurate-based glasses) can be considered low phonon energy glasses. For example, in some embodiments, the low phonon energy glass is a zirconium fluoride glass having a composition including ZrF4, such as ZBLAN (ZrF4 / HfF4, BaF2, LaF3, NaF, and AlF3). In some other embodiments, the low phonon energy glass is an indium fluoride glass having a composition including InF3. In an alternative embodiment, the low phonon energy glass is an aluminum fluoride glass having a composition including AlF3. In a further embodiment, the low phonon energy glass is a chalcogenide glass having a composition including As2S3, As2Se3, AsTe, AsSSe, AsSTe, GaLaS, GeAsS, GeAsSe, etc. Any other suitable low phonon energy glass having a transmission window in the mid-infrared region may be used.

[0046] Figures 2A to 4F Another example of a method of coupling a first optical fiber to a second optical fiber is shown. In this example, the first optical fiber is a feeding optical fiber 110, i.e., an optical fiber intended to feed an optical signal (e.g., a pump signal) into the second optical fiber. Still in this example, the second optical fiber is a receiving optical fiber 112, i.e., an optical fiber intended to receive an optical signal from the feeding optical fiber 110. As shown in Figure 2A and 2B As shown in the specific example shown in , each of the feeding fiber 110 and the receiving fiber 112 has a core 130 surrounded by a cladding 132, which is in turn surrounded by one or more coatings 134, typically made of a polymer. As shown in these figures, the feeding fiber 110 and the receiving fiber 112 have similar nominal dimensions D0. Although the feeding fiber 110 has a core 130 in this embodiment, the feeding fiber 110 can be coreless in some other embodiments. Although the receiving fiber 112 has a single core in this embodiment, the receiving fiber 112 can have multiple cores.

[0047] Figure 3A The feeding fiber 110 is shown to be decoated along a given area 136 and then tapered so that the waist 138 extends between the descending tapered portion 140 and the ascending tapered portion 142. The tapering can be performed by heating the feeding fiber 110 and pulling both ends. To obtain the free end 114 of the feeding fiber 110, one of the descending tapered portion 140 and the ascending tapered portion 142 can be removed from the feeding fiber 110. For example, Figure 3B , the feeding fiber 110 is simply cut or otherwise cleaved near the junction of the waist 138 and the rising cone 142. Preferably, the feeding fiber 110 is cut to remove a large portion of the rising cone 142, or most preferably to remove the entire rising cone 142. It has been found that leaving a portion of the rising cone 142 at the end of the free end 114 may result in undesirable weight, thereby hindering the free movement of the free end 114 of the feeding fiber 110 when in a liquid. However, in some embodiments, as long as the undesirable weight is below a given undesirable weight threshold, this portion of the rising cone 142 can be left at the end of the waist 138 in a free-standing manner. Preferably, the tip of the free end 114 is located in the descending cone 140 or in the waist 138 to avoid undesirable weight at the tip. It may also be desirable to cut the feeding fiber 110 somewhere along the waist 138. In some embodiments, the waist 138 is longer than a given length threshold.

[0048] 4A to 4F Depicted is the use of a liquid 116 to couple the Figure 2A The receiving optical fiber 112 is optically coupled to Figure 3B Example steps of a method of tapering the free end 114 of the feeding optical fiber 110.

[0049] like Figure 4A , the coating 134 of the receiving optical fiber 112 is removed along a given portion of the receiving optical fiber, hereinafter referred to as the "uncoated portion 150". In some embodiments, the uncoated portion 150 can have a length extending up to 15 cm. As shown, the receiving optical fiber 112 is tensioned along its axial direction. More specifically, in this embodiment, the still coated end of the receiving optical fiber 112 is fixed to a separated surface 152, for example, using tape 154. Applying tension within the receiving optical fiber 112 can simplify other operations because the receiving optical fiber 112 can remain stable.

[0050] like Figure 4B, the still coated portion of the feeding fiber 110 is fixed to one of the surfaces 152 beside the receiving fiber, for example, using another adhesive tape 154'. In this position, the non-tapered portion 156, the descending tapered portion 140 and the waist 138 together form the free end 114 of the feeding fiber 110, which is partially free-standing beside the receiving fiber 112. The feeding fiber 110 and the receiving fiber 112 should remain close to each other. As shown in the figure, since the free end 114 of the feeding fiber 110 is not fixed to anywhere at this time, the free end 114 is suspended.

[0051] As discussed above, and in Figure 4C As shown in , the liquid 116 can be injected into the coupling region 118, which in this case is located near the junction between the descending tapered portion 140 and the waist 138, so that the waist 138 of the feeding optical fiber 110 moves toward the receiving optical fiber 112. It has been found that in some embodiments, when the liquid 116 is gradually injected along the first direction 120 and directed toward the free tip 122 of the free end 114 of the feeding optical fiber 110, the free end 114 of the feeding optical fiber 110 will spirally wrap itself around the second optical fiber 112. In order to facilitate this winding, it has been found that it is convenient to inject the liquid 116 in an advance and retreat sequence, which includes injecting the liquid 116 along the first direction 120 and then along the first direction 120 as shown in FIG. Figure 4D 160, and so on, until the free end 114 has spirally wound itself around the second optical fiber 112 for a desired number of turns. In some embodiments, the desired number of turns can be at least 0.25, preferably at least 0.5, and most preferably at least 1.0. For example, in some preferred embodiments, the desired number of turns varies from 2 to 4 turns. This lasso-type winding facilitates satisfactory contact between the free end 114 and the receiving optical fiber 112. Figure 4FIt is shown that once the liquid 116 has evaporated, the feeding fiber 110 and the receiving fiber 112 are optically coupled to each other according to the methods described herein, which may take several minutes. It is assumed that the free end 114 will be spirally wound around the receiving fiber 112 and thereafter remain so due to friction and / or intermolecular forces acting between the free end 114 of the feeding fiber 110 and the receiving fiber 112. The manner in which the free end 114 is wound around the receiving fiber 112 is advantageous in at least some circumstances. For example, in embodiments where the feeding fiber 110 and / or the receiving fiber 112 have multiple cores (e.g., multiple cores distributed circumferentially within the cladding), the winding can bring each of these cores closer to another optical fiber, which in turn can improve the coupling efficiency of the resulting optical coupler. In some embodiments, when the feeding fiber 110 is guided into the coupling region 118, the receiving fiber 112 is also tension-free, rather than being in a tensioned state or otherwise loose. In some other embodiments, the free end 114 of the feeding optical fiber 110 may not be wrapped around the receiving optical fiber 112. For example, the free end 114 may be parallel to the receiving optical fiber 112 at least along a given portion of the receiving optical fiber 112.

[0052] To verify that the optical coupling is satisfactory, a coupling efficiency test may be performed. If the coupling efficiency is not satisfactory, the feed fiber 110 and the receive fiber 112 may be separated from each other, and the method may be repeated until the coupling efficiency is deemed satisfactory.

[0053] Figure 5A portion of an optical coupler 200 is shown in which the contact between the free end 214 of the feeding optical fiber 210 and the receiving optical fiber 212 is secured by applying a protector 262, such as an adhesive, a polymer (e.g., Cytop CTX-AE, Defensa EXOP-255), glass, or any other protective material, along the coupling region 118. In such an embodiment, the free tip 222 of the free end 214 is left with its cleaved end face 264. Depending on the embodiment, the cleaved end face 264 may be a regular cut or an irregular cut. In the embodiment shown, the cleaved end face 264 is located below the protector 262. However, in some other embodiments, since the protector 262 may only partially cover the free end 214, the cleaved end face 264 may not be below the protector 262. In some embodiments, the adhesive and / or polymer may have a refractive index lower than the refractive index of the feeding optical fiber 210 and the receiving optical fiber 212 to guide the light propagating therein to be confined within the optical fiber and improve the coupling efficiency. However, in some other embodiments, the refractive index of the adhesive and / or polymer can be greater than or equal to the refractive index of the feeding optical fiber 210 and the receiving optical fiber 212. In some embodiments, the polymer can be UV curable. In some other embodiments, the protector can be provided in the form of two glass slides, and the two glass slides sandwich the feeding optical fiber 210 and the receiving optical fiber 212 between each other. In some embodiments, in addition to using two glass slides, a polymer can also be used. As shown in this specific embodiment, the protector 262 can be in physical contact with the heat sink 263.

[0054] For example, Figure 4F The optical coupler shown in FIG. 1 (without adhesive or polymer) in which the feed fiber 210 is optically coupled to the receive fiber 212 has achieved a coupling efficiency of about 92%, while Figure 5 The optical coupler 200 has achieved a coupling efficiency of about 95%.

[0055] In addition to being less time consuming, such fusion-free optical couplers have been found to be useful in applications where the shape of a receiving optical fiber is to be preserved (e.g., in optical communications applications) and in applications where light is to be optically coupled between two optical fibers of different materials that cannot fuse with each other due to their different melting temperatures.

[0056] Figure 6An alternative optical coupler 300 is shown in which the contact between the free end 314 of the feeding optical fiber 310 and the receiving optical fiber 312 is secured by fusing the free end 314 and the receiving optical fiber 312 to each other. In such an embodiment, the free tip 322 of the free end 314 is slightly melted, thereby having a melt-like appearance, such as having a rounded end face 366. In some embodiments, only a portion of the free end 314 may be fused to the receiving optical fiber 312, thereby leaving the free tip 322 with a cleaved end face.

[0057] Such optical couplers find application in cladding-pumped fiber lasers and fiber amplifiers. In these lasers, the fiber core contains active ions that must be energized to enter an excited state where they will provide gain for the laser or amplifier. The energy can be provided by a lower wavelength pump laser. In the case of high power, multi-core or multi-material fiber lasers or amplifiers, it may be beneficial to inject the pump into the fiber cladding. Such optical couplers provide a means of injecting pump into the cladding that has very good efficiency, is simple, easy to use, and is compatible with multi-core fibers and coupling between different fiber materials (e.g., from a silica feed fiber to a fluoride receiving fiber).

[0058] In some embodiments, the length of the descending cone can be at least 4.0 cm, preferably at least 5.0 cm, and most preferably at least 7.0 cm. The length of the waist can be at least 1.0 cm, preferably at least 2.0 cm, and most preferably at least 4.0 cm. In some embodiments, the diameter of the waist can be between 15 μm and 44 μm. These exemplary values ​​are provided as examples only, and as with other dimensions for the length of the descending cone, the length of the waist and its dimensions can be used in some other embodiments. In view of some experiments, it has been found that by increasing the length of the descending cone and by reducing the diameter of the waist, improved coupling efficiency may result.

[0059] Optocoupler - Example 1

[0060] An optical coupler was formed by optically coupling a coreless optical fiber made of silica-based glass with a length of 75 mm and a descending tapered portion with a length of 2.0 cm to a double-clad optical fiber made of silica-based glass with a waist diameter of 22 μm. A low-refractive-index polymer was used to fix the contact. A coupling efficiency of about 92% was achieved using this coupler.

[0061] Optocoupler - Example 2

[0062] An optical coupler was formed by optically coupling a coreless optical fiber made of silica-based glass and a tapered optical fiber with a length of 75 mm to a double-clad optical fiber made of phonon energy glass (more specifically ZBLAN). A low-refractive-index polymer was used to fix the contact. A coupling efficiency of about 90% was achieved with this coupler.

[0063] Optocoupler - Example 3

[0064] An all-fiber silica-fluorine glass fiber combiner was formed by optically coupling a coreless fiber made of silica-based glass fusion-spliced ​​with a coreless fiber taper having a length of 90 mm to a double-clad fiber made of 7% erbium-doped ZBLAN. A low-refractive-index polymer was used to fix the contacts. A coupling efficiency of about 90% was achieved with this coupler.

[0065] It is understood that the examples described and shown above are intended to be exemplary only. For example, it is understood that in some embodiments, the feeding fiber will be optically coupled to the cladding of the receiving fiber. In some embodiments, it is not necessary to evaporate the liquid before bonding and / or fusing the receiving fiber to the feeding fiber, because the liquid can be simply removed using a cloth. In addition, in some embodiments, the feeding fiber can be cut at a certain point along the descending tapered portion, thereby removing a portion of the descending tapered portion, the waist, and the ascending tapered portion. In such an embodiment, as long as the size of the free end of the feeding fiber is below the critical size, the free end of the feeding fiber can move toward the receiving fiber in the liquid. It is desirable that the first optical fiber and the second optical fiber can be guided toward each other along a transverse orientation. More specifically, in some other embodiments, the first optical fiber and the second optical fiber can be guided toward each other in an oblique manner. The scope is indicated by the attached claims.

Claims

1. A method for optically coupling a first optical fiber and a second optical fiber to each other, the method comprising: The free end of the first optical fiber, the second optical fiber, and a liquid are guided into close proximity with each other within a coupling region, the free end of the first optical fiber having a dimension below a critical dimension, the free end of the first optical fiber moves within the liquid to contact the second optical fiber along a given coupling length, the movement comprising: the free end at least partially wrapping around the second optical fiber along the given coupling length, the contact optically coupling the free end of the first optical fiber and the second optical fiber to each other.

2. The method of claim 1, wherein: During said guiding, said second optical fiber is not tensioned.

3. The method according to claim 1, wherein: The guiding includes guiding the free end and the second optical fiber to be in close proximity with each other in the coupling region, and then injecting the liquid in the coupling region.

4. The method according to claim 1, further comprising: The liquid is evaporated, and the free end of the first optical fiber and the second optical fiber are bonded to each other.

5. The method according to claim 1, further comprising: Prior to the guiding, the first optical fiber is tapered with a waist extending between a descending tapered portion and an ascending tapered portion, and one of the descending tapered portion and the ascending tapered portion is removed from the first optical fiber to free the waist for use as the free end of the first optical fiber.

6. The method of claim 5, further removing the waist portion so that the other of the descending tapered portion and the ascending tapered portion serves as the free end of the first optical fiber.

7. The method of claim 1, further comprising: During the guiding, the second optical fiber is tensioned.

8. The method according to claim 1, wherein: The winding includes winding the free end around the second optical fiber for at least one full turn.

9. The method according to claim 1, wherein: The guiding is performed gradually in the direction of a free tip of the free end of the first optical fiber.

10. The method according to claim 1, wherein: The liquid is ethanol.

11. The method according to claim 1, wherein: The first optical fiber and the second optical fiber are made of different materials.

12. The method according to claim 11, wherein: The first optical fiber is made of silica, and the second optical fiber is made of low phonon energy glass.

13. The method according to claim 1, wherein: The second optical fiber is a multi-core optical fiber.

14. The method according to claim 1, wherein: The dimension of the free end of the first optical fiber is smaller than the dimension of the second optical fiber.

15. An optical coupler comprising a first optical fiber; a second optical fiber optically coupled to the first optical fiber, the first optical fiber having a free end, the free end having a size below a critical size, the free end of the first optical fiber being wrapped around the second optical fiber along a given coupling length to provide optical coupling; and a protector on the free end and the second optical fiber along the given coupling length, the free end having a free tip with a cleaved end face.

16. The optical coupler according to claim 15, wherein The protector is made of at least one of a polymer and glass having a refractive index lower than that of the first optical fiber and the second optical fiber.

17. The optical coupler according to claim 15, wherein: The protector is in physical contact with the heat sink.

18. The optical coupler according to claim 15, wherein: The first optical fiber and the second optical fiber are made of different materials.

19. The optical coupler according to claim 15, wherein: The first optical fiber is made of silica, and the second optical fiber is made of low phonon energy glass.

20. The optical coupler of claim 15, wherein: The free end and the second optical fiber are fused to each other along the given coupling length, and the free end has a free tip with a circular end surface.

21. The optical coupler of claim 15, wherein: The free end is wrapped around the second optical fiber for at least one complete turn.

22. The optical coupler of claim 15, wherein: The free end of the first optical fiber has a length that is greater than a given length threshold.

23. The optical coupler of claim 15, wherein: The dimension of the free end of the first optical fiber is smaller than the dimension of the second optical fiber.

24. Use of the optical coupler of any one of claims 15 to 23 as a pump combiner for a fiber-based cladding pumped laser and / or amplifier.

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