STRUCTURED DOUBLE SHELL FIBER

Description

[0001] The invention relates to an optical double-clad fiber with a core region and a cladding region, wherein the cladding region has an inner region and an outer region with a lower refractive index compared to the inner region and the core region, wherein the outer region surrounds the inner region, wherein the inner region has an internal structure that causes a spatial overlap of higher-order modes with the core region that is less than the spatial overlap of a fundamental mode with the core region, wherein the internal structure is formed by channels extending substantially in the longitudinal direction of the fiber, wherein the ratio of the diameter d of the channels to the center-to-center spacing A of adjacent channels is less than 0.5, preferably less than 0.3, and wherein the center-to-center spacing A is greater than 20 times the wavelength of an electromagnetic radiation propagating in the fiber.

[0002] Double-clad fibers of this type are used in laser systems, particularly fiber laser systems. Here, the pump light, with its lower beam quality, can be coupled into the cladding and / or core of the fiber, while the signal light, with its significantly higher beam quality, propagates primarily within the core.

[0003] The quality of any fiber laser system depends significantly on the fiber used. Therefore, continuous development of the fibers employed is pursued to overcome the existing limitations of fiber laser systems known in the prior art. The introduction of double-clad technology made it possible for the first time to utilize the high power output of pump diodes with low brilliance (beam quality). Commercially available CW laser systems now achieve output powers of 10 kW.

[0004] By amplifying both the pump light and the signal light over long fiber lengths, active optical fibers can achieve very high efficiencies. Furthermore, they possess excellent thermal properties and emit high-quality, diffraction-limited radiation regardless of the output power.

[0005] Propagating high-intensity signal light in a relatively small fiber core over long distances can lead to problems caused by nonlinear effects, which can significantly impair the quality of the output beam. Some of these nonlinear effects are self-phase modulation (SPM), Raman scattering, Brillouin scattering, and four-wave mixing (FWM). These nonlinear effects substantially hinder further increases in output power.

[0006] To avoid the occurrence of nonlinear effects, various strategies known from the prior art have been developed. The most effective method is undoubtedly increasing the core diameter. This reduces the light intensity across the fiber cross-section, directly counteracting the occurrence of nonlinear effects. Furthermore, a larger core diameter allows for the introduction of more active ions, thereby increasing the energy stored in the fiber. This reduces the length of the necessary absorption path within the fiber, resulting in a shorter fiber. However, a problem with this approach is that a larger core diameter simultaneously allows a multitude of modes to propagate within the fiber core. This reduces the beam quality of the laser system.Simultaneously, laser instabilities can occur, which significantly affect the laser's operating characteristics. Therefore, when increasing the core diameter, it is always necessary to ensure single-mode operation within the fiber.

[0007] Large Mode Area (LMA) fibers with core diameters greater than 10 µm and excellent beam quality are known from the prior art. LMA fibers can be implemented as step-index fibers, where the numerical aperture (NA) is technically limited to a value of approximately 0.06. This corresponds to a maximum single-mode core diameter of approximately 13 µm. LMA step-index fibers with larger core diameters therefore inherently carry multiple modes (multimode operation). Single-mode operation can be achieved by selectively exciting the fundamental mode of a multimode fiber. This approach is disclosed, for example, in US patent 5,818,630 C1. A problem with this approach is that the signal light must be coupled very carefully into the fiber to excite only the fundamental mode. In practice, however, this is very complex and becomes difficult to achieve at high power levels.

[0008] Another well-known approach involves the use of photonic crystal fibers (PCFs). These fibers have an actively doped core and a cladding of periodically arranged air channels around this core. Such a fiber design is disclosed, for example, in WO 2006 / 082348 A9. Provided the air channels are sufficiently narrow and extend at a large radial distance from the core, guidance based on the effective refractive index difference can be assumed. The advantage of these fibers is that the channel diameter d and the center-to-center spacing A between two channels influence the numerical aperture of the fiber. This allows the numerical aperture to be reduced to very low values ​​of approximately 0.01. In the prior art, mode field diameters of over 50 µm have been achieved, enabling effective single-mode operation. However, since photonic crystal fibers do not have a clear boundary between the core and cladding, they always operate in multimode mode.This in turn leads to the well-known problems of modal instabilities and power fluctuations at high output power. Effectively, these fibers are therefore limited to a mode field diameter of approximately 80 µm.

[0009] The patent US 7,787,729 B2 discloses so-called leakage channel fibers, in which the air channels within the fibers function like a mode filter. Higher-order modes, due to their smaller structure size, can pass into the cladding and experience increased propagation losses, while the fundamental mode remains within the fiber core. This enables effective single-mode operation. However, since this concept relies on energy loss from the core to a cladding of infinite extent, it is only applicable to passive or core-pumped single-cladding fibers.

[0010] Poli, F. et al. ("Single-Mode Regime in Large-Mode-Area Rare-Earth-Doped Rod-Type PCFs", IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, Vol. 15, No. 1, pages 54 - 60) describe a structured double-jacketed fiber in which, due to the structuring, the spatial overlap of higher-order modes with the core region is less than the spatial overlap of the fundamental modes with the core region.

[0011] US 2008 / 199135 A1 and WO 01 / 31376 A1 show further designs of structured double-jacketed fibers.

[0012] Against this background, the object of the invention is to create a double-jacketed fiber of the type mentioned above, which operates effectively in single-mode operation and, due to its scalability, avoids the emergence of non-linear effects even at high intensities of the guided laser light.

[0013] The invention solves this problem by selecting the diameter of the outer region such that the fundamental mode has a spatial overlap with the core region that is at least 25% greater than all other modes of the electromagnetic radiation propagating in the double-clad fiber, wherein the inner structure consists of two concentric rings of hexagonally arranged channels, the core region being formed by the absence of one channel.

[0014] The double-clad fiber according to the invention operates in multimode mode because it has a double-clad structure and, unlike leakage-channel fibers, does not cause different propagation losses for different modes. The effectively single-mode operation in an active laser system can be explained by two properties. Even during the coupling of the signal beam, the excitation of higher-order modes is made more difficult due to their delocalization. This eliminates the need for targeted coupling and adjustment to excite only the fundamental mode, contrary to the prior art. Additionally, in active fibers, the reduced overlap of the higher-order modes with the doped core region results in a significantly lower gain compared to the fundamental mode guided in the core region.

[0015] According to the invention, the delocalization of higher-order modes can be limited to the axial fiber core and extend to other regions of the fiber. These selected regions, which may also exhibit less overlap with the higher-order modes, are likewise considered core regions within the meaning of the invention.

[0016] The structured double-clad fiber according to the invention typically supports several hundred modes. In a double-clad fiber, all modes propagate without significant losses, since, in the theoretical ideal case, no mode coupling can occur. Because there is no closed boundary between the core and cladding regions, the modes of both regions cannot be clearly separated. Therefore, the modes of the core region are also directly modes of the entire fiber, although most of their energy is concentrated in the core region of the fiber. In contrast to step-index fibers, the fundamental mode within the structured double-clad fiber does not always have the highest effective refractive index. The effective refractive indices of higher-order core modes can also be mixed with the cladding modes.Therefore, terms like "core modes" and "mantle modes" must be defined based on their localization, since these primarily propagate within the core and mantle regions, respectively. This mixing of the effective indices of core and mantle modes can lead to avoided crossing between modes, resulting in strong deformation or delocalization.

[0017] According to the invention, the double-jacketed fiber can be doped, at least partially, particularly in the core region, with rare-earth ions. It can also be designed to preserve polarization or to selectively modify the polarization. Doping with appropriate elements also enables index matching or mismatching of the fiber regions, which in turn can lead to increased delocalization. Furthermore, the fiber can be designed to be mechanically stiff or particularly flexible for specific applications.

[0018] According to the invention, the core region can be separated from the other regions of the fiber by a refractive index step. Alternatively, the core region can also be defined by an index gradient.

[0019] According to the invention, the internal structure is formed by channels extending essentially in the longitudinal direction of the fiber. The described properties of the delocalization according to the invention are dominant in the hexagonal structures of the channels, with the distance between two channels being more than 20 times the wavelength of the propagating light. Furthermore, the ratio of the diameter d of the channels to the center-to-center spacing A of adjacent channels is less than 0.5, preferably less than 0.3. The core region of the structured double-clad fiber is formed by the absence of at least one channel. This is particularly easy to manufacture. This design, in which the diameters of the channels are relatively small compared to the spacing between them, has proven to be particularly advantageous in practice.

[0020] Another conceivable embodiment involves surrounding the inner structure with an additional region that can be used to enhance delocalization or for shielding. Enhanced delocalization can be achieved, in the simplest case, by an unstructured region that can facilitate mode interaction by having the same or a higher refractive index than the rest of the inner region. Shielding can be achieved, for example, by giving the additional region a lower refractive index compared to the rest of the inner region. This advantageous effect arises because the region with the higher refractive index, which contains the inner structure (e.g., channels), acts as a barrier, effectively dividing the fiber into two regions with different refractive indices. This prevents interaction between the modes of different regions.In this way, it is possible to increase the diameter of the outer structure without the fundamental mode experiencing interaction with other modes, for example through avoided crossing, which could lead to reduced overlap of the fundamental mode with the core region. This additional area of ​​shielding or enhanced delocalization can also be achieved through a targeted choice of channel structure, for example, additional rings with particularly densely arranged channels or increased channel size. Thus, the outer structure can primarily effect the delocalization of higher-order modes from the core region, or it can support the effect of the inner structure.

[0021] The structured double-clad fiber according to the invention is particularly suitable for use in fiber laser systems where laser light is to be guided and amplified at high intensity. In this sense, the invention expressly includes laser systems that use a double-clad fiber according to the invention.

[0022] Exemplary embodiments of the invention are explained in more detail below with reference to the figures. The figures show: Figure 1: Cross-sectional view of an embodiment of a double-sheath fiber according to the invention; Figure 2: Representation of the basic mode and four different higher-order modes within the double-sheath fiber according to Figure 1 Figure 3: Representations of the basic mode and four different higher-order modes within a double-sheath fiber according to Figure 1with an increased diameter of the outer region; Figure 4: Variant with a spiral arrangement of channels (not an embodiment of the invention); Figure 5: Representations of the basic mode and two higher-order modes within the double-sheath fiber according to Figure 4 Figure 6: another embodiment with an arrangement of the internal structure in a region of the fiber with a refractive index different from the other regions; Figure 7: representations of the fundamental mode and four different higher-order modes within the double-sheath fiber according to Figure 6 .

[0023] Figure 1Figure 1 shows a cross-section of a double-sheathed fiber with a core region 1 and an inner region 2. The inner region has an internal structure 4 and is bounded by an outer region 3. In this example, the inner structure 2 is formed by channels parallel to the fiber axis, which have a refractive index different from that of the core region 1.

[0024] A double-sheathed fiber can, in principle, be considered a normal step-index fiber with a very large core. The core of this equivalent step-index fiber extends over the entire inner region 2 and the core region 1 and is surrounded by the outer region 3 as the sheath of the step-index fiber. The local distribution of modes within such an unstructured, large-diameter step-index fiber is nearly homogeneous, meaning that each region of the fiber core is, on average, permeated by the same number of modes 5, 6. A delocalization of modes 5, 6 according to the invention does not occur in such a step-index fiber.

[0025] However, if, as according to the Figure 1Since the fiber core is surrounded by an internal structure 4, it is possible to delocalize some modes 6 from the core region 1 and concentrate other modes 5 (fundamental mode) within the core region 1. The hexagonal arrangement of the channels 4 within the double-clad fiber is designed such that the higher-order modes 6 are delocalized from the core region 1, while only the desired fundamental mode 5 remains within the core region 1. This results in good overlap of the fundamental mode 5 with the core region 1 and simultaneously reduces the overlap of the higher-order modes 6 with the core region 1. In the case of a passive fiber, this leads to a less pronounced excitability of the higher-order modes 6 within the core region 1. In an active fiber, this configuration additionally exhibits reduced overlap with the doped region and thus a lower amplification of the higher-order modes 6.Thus, the double-jacketed fiber exhibits effectively single-mode behavior.

[0026] According to Figure 1 The structured double-sheath fiber has a hexagonal arrangement of air channels 4, which have a center-to-center spacing A that is less than 0.5 relative to the channel diameter d. Specifically, the fiber possesses according to Figure 1 Two concentric rings of air channels 4, which have a center-to-center distance of 30 µm and a ratio d:A=0.22. The outer region 3 of the fiber sheath has a diameter of 150 µm.

[0027] Figure 2 shows different modes 5, 6 of the double-jacket fiber according to Figure 1, which exhibit varying degrees of overlap with the core region 1. The core region 1 is represented as a dashed circle. As can be seen, the Gaussian fundamental mode 5 has the greatest overlap with the core region 1, namely approximately 85%. Therefore, most of the energy of the coupled beam propagates in this mode 5. The higher-order modes 6 have an overlap with the core region 1 that is in any case less than 55%. In contrast to an ordinary LMA step-index fiber, the overlap of the higher-order modes 6 in the double-clad fiber is significantly lower. In the step-index fiber, the proportion of higher-order modes 6 within the core region 1 would not be significantly lower than that of the fundamental mode 5. If the double-clad fiber according to the invention is simultaneously doped with rare-earth ions in the core region 1, the higher-order modes 6 have a correspondingly small overlap with the active region of the core region 1.When the modes propagate within the fiber, they are subject to different gain ratios, resulting in a correspondingly lower proportion of higher-order 6 modes at the fiber exit. Although this latter effect plays an important role in active fibers, it is not a necessary condition for the invention. Since the reduced overlap of the higher-order 6 modes with the core region 1 of the fiber is present regardless of any doping, the delocalization of the higher-order 6 modes occurs directly from the moment the pump light enters the fiber, as the higher-order 6 modes cannot be efficiently excited.

[0028] Figure 3Figure 1 shows the representations of the basic mode 5 and four different higher-order modes 6 within a double-sheathed fiber with an increased diameter of the outer region 3. The arrangement of the channels 4 within the inner structure 2 is the same as in the arrangement according to Figure 2. Figure 2 Only the diameter of the outer region 3 is increased and is, for example, 181 µm. Increasing the diameter of the outer region 3 utilizes the avoided-crossing effect. This effect occurs primarily in structured double-sheath fibers and can be controlled by appropriately selecting the diameter of the outer region 3. In the given embodiment according to Figure 3 With an outer region diameter 3 of 181 µm, the effective refractive indices of two higher-order modes 6 are very close to each other. This leads to an interaction between the two modes 6 with strong deformation, as shown in Figure 3The overlap of the basic mode 5 with the core region 1 remains approximately unchanged at about 84%, while the overlap of the higher-order modes 6 is now significantly lower at less than 25% than in the first implementation variant according to the Figure 1 or 2.

[0029] Figure 4 Figure 1 shows a variant, which is not an embodiment of the invention, with a spiral arrangement of the channels 4. This asymmetric arrangement of the air channels 4 – or alternative arrangements with a reduced number of axes of symmetry – also leads to a strong delocalization of the higher-order modes 6 from the core region 1. Although the spiral arrangement of the channels 4 does not have axes of symmetry, it is sufficiently similar to a symmetric arrangement to ensure the guidance of the localized fundamental mode 5.

[0030] Figure 5The overlap of the basic mode 5 with the core region 1 is on the order of 76%, while the overlap of the higher order modes 6 is less than 15%.

[0031] Figure 6Figure 1 shows a double-clad fiber with a shield of the inner structure 2. In this embodiment, the shielding is achieved by a different refractive index in a region 7. If this refractive index is higher than the refractive index of the surroundings, it is possible to increase the diameter of the outer region 3 without the fundamental mode 5 experiencing avoided crossing with another mode. This is because the region 7 of the higher refractive index, which has the channels 4, acts like a barrier, dividing the fiber into two regions with different effective refractive indices. Thus, the propagating modes are also divided into two separate groups with different refractive indices, preventing avoided crossing between modes from different groups. As shown in Figure 2, the propagating modes are separated into two groups with different refractive indices, preventing avoided crossing between modes from different groups. Figure 6As shown, the area of ​​higher refractive index 7 completely surrounds the inner structure 2 of channels 4. Therefore, in comparison with the arrangement according to Figure 1 no influence on the shape of the fashions or the overlap with core area 1. This leads according to Figure 7 The basic mode 5 overlaps with the core region 1 by approximately 85%, while the higher order modes 6 have an overlap of a maximum of 54%.

[0032] As an alternative to Figure 7 It is also conceivable that the region 7 with higher refractive index, which contains the air channels 4, is enclosed by a second arrangement of more densely arranged air channels 4. This leads to equivalent shielding of the modes as shown in Figure 7 . In principle, the shielding of the modes within the inner structure 2 can be achieved by any design and geometric form of the arrangement of the air channels 4.

Claims

1. Double-sheath fibre having a core region (1) and a sheath region, wherein the sheath region has an inner region (2) and an outer region (3) with a refractive index that is lower with respect to that of the inner region (2) and the core region (1), wherein the outer region (3) surrounds the inner region (2), wherein the inner region (2) has an inner structure (4) that effects a spatial overlap of modes of higher order with the core region (1), which is lower than the spatial overlap of a fundamental mode with the core region (1), wherein the inner structure (4) is formed by channels mainly extending in fibre length direction, wherein the ratio of the diameter d of the channels to the centre distance Λ of adjacent channels is less than 0.5, preferably less than 0.3, wherein the centre distance Λ is greater than 20 times the wavelength of an electromagnetic radiation propagating in the fibre, wherein the diameter (3) of the outer region is selected such that the fundamental mode has a spatial overlap with the core region (1) that is at least 25% greater than all other modes of the electromagnetic radiation propagating in the double-sheath fibre, characterised in that the inner structure (4) consists of two concentric rings of hexagonally arranged channels, wherein the core region (1) is formed by the absence of a channel.

2. Double-sheath fibre according to claim 1, characterised in that the core region (1) is doped with rare earth.

3. Use of a double-sheath fibre according to claim 1 or 2 in a laser system, in particular in a fibre laser system.

4. Laser system, in particular a fibre laser system, having a double-sheath fibre according to claim 1 or 2.