Decrease in the thermal sensitivity of an active optical fiber

Abstract

A rare-earth doped optical fiber (active optical fiber) is presented that exhibits a thermal coefficient of optical refraction (dn / dT) of the core region that is reduced relative to the thermal coefficient of optical refraction (dn / dT) of the surrounding cladding. The reduction of the dn / dT of the core region has been found to reduce the impact of transverse mode instability (TMI) in the presence of high levels of pump power (or other conditions that can also increase the thermal load present in the core region). Specifically, it has been found that reducing the dn / dT of the core region relative to the dn / dT of the cladding by at least 10% is sufficient to extend the temperature range in which the active optical fiber is maintained in single mode operation. The reduction of the dn / dT can be provided by modifying the dopants introduced into the core region, where, for example, it is known to introduce boron into ytterbium doped optical fibers to reduce the dn / dT of the core region.

Description

[0001] Cross-references to related applications

[0002] This application claims the benefit of U.S. Provisional Application No. 63 / 527,976, filed July 20, 2023, which is incorporated herein by reference. Technical Field

[0003] This disclosure relates to the design of rare-earth-doped optical fibers (sometimes referred to as "active fibers") for use in high-power applications, and more specifically, to controlling the thermo-optic coefficient of the fiber core relative to the cladding to reduce the likelihood of triggering transverse mode instability (TMI) at elevated operating temperatures (e.g., in the presence of high levels of pump power). Background Technology

[0004] During the operation of fiber-based lasers and amplifiers, the temperature of the core glass composition (which includes rare-earth dopants) increases relative to the cladding glass. This correspondingly modifies the refractive index of each segment of the fiber, and for conventional laser fibers, ultimately causes the fiber to become multimode and reduces its ability to suppress higher-order modes (HOMs). The loss in single-mode operation eventually leads to transverse mode instability (TMI), which is considered one of the main mechanisms limiting the usable output power of fiber lasers and amplifiers.

[0005] Specifically, TMI occurs when the thermally induced refractive index grating within the active fiber reaches the level where the propagating fundamental mode (e.g., LP01) is coupled to the HOM. Generally, the LP11 mode is the primary HOM of interest. When modes are coupled together, the output from the active fiber randomly oscillates at kHz frequencies between the desired fundamental mode (e.g., LP01) and the undesired HOM (typically, the LP11 mode). This random coupling between modes leads to significant noise and reduced optical efficiency in the resulting output.

[0006] One mitigation strategy for reducing TMI is to modify the structure of the active fiber to include an enhanced waveguide with even greater HOM suppression. While these designs are useful, they can degrade in the presence of increased pump power because the refractive index distribution is perturbed by thermal effects.

[0007] Another mitigation strategy is to reduce the thermo-optical coefficient dn / dT of the fiber core to near zero, as this is theoretically predicted to eliminate TMI. However, this strategy is challenging from a materials perspective, especially when using silica-based materials in the fiber composition. Furthermore, the need for larger effective area fibers to suppress nonlinear effects, as fiber laser power increases, exacerbates this problem because such fibers are inherently more sensitive to small refractive index perturbations caused by thermal effects. Summary of the Invention

[0008] This invention addresses a need in the prior art. The invention relates to a rare-earth-doped (active) optical fiber configured to reduce the fiber's sensitivity to thermal fluctuations by introducing a limited reduction in the thermo-optic coefficient (dn / dT) of the core region relative to the dn / dT of the surrounding cladding.

[0009] Specifically, it has been found that reducing the dn / dT of the core region by only about 10% (relative to the dn / dT of the cladding) is sufficient to provide predictable operating characteristics across a wide range of operating conditions. Preferably, a reduction in the range of about 10%-25% is recommended to extend the temperature range over which active fibers maintain single-mode operation, thereby allowing the use of kilowatt-level pump bundles to generate high-power output signals. In other words, it has been found that modifying the composition of the core region of an active fiber to reduce the thermo-optic coefficient of the core region by at least about 10% relative to the surrounding cladding allows the operating range of high-power lasers and amplifiers to be extended beyond the operating range of the prior art. In one example embodiment, when using ytterbium-doped (Yb-doped) active fiber, it is recommended to use boron instead of fluorine as a dopant to reduce the refractive index, where boron has also been found to reduce the dn / dT of the core relative to the cladding. Other materials (and combinations thereof) may preferably be used in conjunction with different rare-earth dopants. More specifically, it should be understood that the principles of the present invention extend to other known types of rare-earth-doped fibers used in lasers and amplifiers (such as erbium (Er) doping, Er-Yb doping, thulium (TM) doping, etc.).

[0010] For certain types of optical fibers (especially large mode area fibers) and / or applications requiring extremely high power pumping, a 25%–60% reduction in the thermo-optical coefficient of the core region may be anticipated. As this percentage increases (i.e., as the dn / dT of the core region decreases), additional fiber modifications (such as including loops or trenches in the cladding, or using a pedestal-type core configuration) may be required to maintain single-mode operation. In some embodiments, the dn / dT of these additional features may also be adjusted to further improve single-mode operation of the device in the presence of temperature fluctuations, particularly in the presence of very high core temperatures.

[0011] The benefits of using the reduced dn / dT in the core region also apply to step-index fiber design and graded-index fiber design.

[0012] In one embodiment, the optical fiber can be in the form of an active fiber configured to generate amplification in the presence of a pump beam. The active fiber includes a core region, a cladding, and a coating material. The core region includes a rare-earth dopant, the cladding surrounds the core region, and the coating material surrounds the cladding. According to the principles of the invention, the core region includes at least one dopant selected to reduce the thermo-optical coefficient (dn / dT) of the core region relative to the cladding.

[0013] Other and further aspects and principles of the invention will become apparent in the following discussion and with reference to the accompanying drawings. Attached Figure Description

[0014] Referring now to the accompanying drawings, where similar numbers in several views denote similar elements:

[0015] Figure 1 Includes a simplified cross-sectional view of the active optical fiber, and a drawing of the temperature gradient along the axial direction of the fiber.

[0016] Figure 2 The illustration shows the change in the refractive index of the fiber core when using active optical fiber (i.e., when a pump bundle is injected into the fiber core).

[0017] Figure 3 It contains data depicting the dependence of the transverse mode instability (TMI) threshold on both HOM bending loss and cladding absorption, wherein plot A shows the change in the TMI threshold (measured in kW) associated with conventional fiber, and data points I and II are associated with two fiber designs formed according to the principles of the present invention.

[0018] Figure 4 The illustration shows simulated HOM bending losses of a set of four different optical fibers formed according to the principles of the present invention, as well as HOM bending losses of conventional prior art optical fiber designs (both at room temperature and under heating) for comparison purposes.

[0019] Figure 5 Includes and Figure 4 The plotted data is associated with the simulated bending loss of the fiber group in the basic LP01 mode.

[0020] Figure 6 An exemplary ring configuration of a large-area optical fiber and its associated refractive index distribution are illustrated.

[0021] Figure 7 It is a description Figure 6A graph showing the LP11 loss (at room temperature) of the ring-assisted fiber configuration; and

[0022] Figure 8 Includes and Figure 6 The LP11 ring-assisted structure is associated with a set of three diagrams, in which Figure 8 (a) is associated with the high-temperature operation of conventional optical fibers. Figure 8 (b) illustrates the improvement in LP11 loss of the ring-assisted fiber formed according to the present invention (wherein dn / dT is also adjusted), and Figure 8 (c) is associated with an alternative configuration of the ring-assisted fiber of the present invention, which also has a reduction in dn / dT in the core region, and dn / dT is adjusted to be slightly less than that of the core region. Figure 8 The value associated with the plot of (b). Detailed Implementation

[0023] Lateral mode instability (TMI) is one of the main mechanisms limiting the output power levels achievable in high-power fiber lasers and amplifiers. TMI occurs when the fiber can no longer maintain single-mode operation, rendering higher-order mode (HOM) suppression ineffective. As described above, TMI arises when the thermally induced refractive index grating within the active fiber reaches a level where the propagating fundamental mode (e.g., LP01) couples to a HOM. When modes are coupled together, the output from the active fiber randomly oscillates at kHz frequencies between the desired fundamental mode (e.g., LP01) and the undesired HOM (typically, the LP11 mode). This random coupling between modes leads to significant noise and reduced optical efficiency in the resulting output.

[0024] This paper proposes reducing the thermo-optical coefficient dn / dT in the core material of active optical fibers relative to the cladding material, thereby minimizing the impact of high core temperatures on the waveguide properties of the fiber while maintaining single-mode operation under increased thermal load in the core region. As will be discussed below, the increased temperature can be largely attributed to the significant increase in pump power (e.g., kW-level pump bundles) required to provide the desired high-power amplified output signal. Reducing the core dn / dT allows the active fiber to exhibit improved HOM suppression relative to conventional fiber compositions, enabling further power ramps beyond those achievable with existing techniques.

[0025] The heat load present in the optical fiber used in fiber-based optical amplifiers (or lasers) is typically greatest in the core region because the heat is generated due to both quantum defects and absorption losses. Figure 1The diagram includes a simplified cross-sectional view of the active optical fiber 10, and a temperature gradient plot along the radial direction of the fiber 10. Here, the optical fiber 10 includes a relatively small core region 12 surrounded by a cladding 14, which itself is surrounded by a coating (sheath) material 16.

[0026] The temperature gradient is presented as a function of the fiber radius, with the center of the core region 12 aligned with the midpoint of the gradient. In practice, as described above, the thermal load in the active fiber used for amplification will be greatest in the core region 12, since most of the pump power resides in this region. Conversely, the fiber temperature drops significantly in the outer region of the coating 16. Figure 1 The temperature gradient depicted in the figure modifies the refractive index of the optical fiber in a manner proportional to both temperature and the thermo-optical coefficient (dn / dT) of the material, as characterized by the following relationship:

[0027] ,

[0028] Where n i n is the refractive index of the i-th region of the optical fiber. 0,i is the refractive index at the reference temperature T0, and T is the actual temperature in the region, and dn i / dT is the thermo-optic coefficient of the i-th region of the optical fiber. The third term in this defined relationship is used to account for the stress (ε) caused by the expansion or contraction of the region due to temperature changes. In most cases, this third term is considered a small perturbation compared to the second term.

[0029] For core compositions used in active optical fibers, the refractive index of the core will increase relative to the cladding during operation of devices that include active optical fibers (i.e., fiber lasers or fiber amplifiers). Figure 2 The diagram illustrates this change in the refractive index of the fiber core. As mentioned above, this change in refractive index leads to suppression of HOM loss in the fiber, which, with increasing thermal load, will eventually result in TMI.

[0030] According to the principles of the present invention, it is proposed to incorporate dopants known to reduce dn / dT by altering the composition of the core region, thereby reducing the thermo-optical coefficient (dn / dT) of the core region relative to the dn / dT of the surrounding cladding by at least 10%, wherein the amount of incorporated dopants is selected such that a reduction of at least 10% in dn / dT relative to the cladding is achieved. This level of reduction contrasts with conventional teachings that require the substantial elimination of dn / dT (i.e., no change in refractive index in the presence of temperature fluctuations) to provide a suitable TMI threshold for high-power / temperature applications.

[0031] It has been found that using a core composition that provides a reduction of at least 10% (preferably in the range of 10%-25%) in the dn / dT of the core region relative to the dn / dT of the cladding results in standard active fibers maintaining single-mode over a much wider power range than comparable fibers with conventional core compositions. Furthermore, it has been found that the resulting active fiber exhibiting this level of reduction in the dn / dT of the core region, along with the associated HOM suppression, allows for stable operation without being affected by phase correlation from any residual HOM content in the output.

[0032] In an exemplary embodiment of the invention, since boron also reduces the dn / dT of the core relative to the cladding, the composition of the ytterbium-doped fiber is modified to utilize boron instead of fluorine as a refractive index-reducing dopant in the core region. Specifically, the percentage reduction in dn / dT will be a function of the amount of boron introduced. In one particular embodiment, both GeO2 and B2O3 can be introduced into the core, where GeO2 is used to control the actual refractive index of the core material itself. It can be found that the resulting core composition exhibits performance comparable to conventional optical fibers (e.g., in terms of laser generation efficiency and the absence of photodarkening effects), while allowing for higher power performance and still maintaining single-mode operation.

[0033] For an exemplary application of ytterbium-doped active fiber, it is assumed that the refractive index contribution from ytterbium is dn ~ 1 × 10⁻⁶. -3 It has been proposed to find a structure with dn=1×10⁻⁶ relative to silicon dioxide. -3 The design. Typical preform formulations for achieving low thermo-optical coefficients, achievable through MCVD, utilize combinations of B₂O₃, P₂O₅, Al₂O₃, GeO₂, and fluorine. It has been found that by replacing fluorine with additional B₂O₃ (and possibly GeO₂), the refractive index value can be maintained while reducing dn / dT by at least 10% (and more typically in the range of about 10%–25%). dn / dT can be tuned within relevant ranges by adding both B₂O₃ and GeO₂ in an easily manufacturable manner.

[0034] Compositions suitable for higher ytterbium contents can be achieved by increasing the content of AlPO4 and either Al2O3 or P2O5, which are considered to have a negligible effect on the total dn / dT (in the case of AlPO4) or only cause a relatively small decrease in dn / dT (for P2O5). The dn / dT values ​​of different glass regions (such as the core, cladding, trench, ring, etc.) of active optical fibers can be calculated based on the content of each dopant and their dn / dT values. A reference was found that provides the following values:

[0035]

[0036] dn / dT can also be determined by measuring optical properties as a function of temperature (such as bending losses in different optical modes) and inferring the change in the refractive index distribution required to produce the observed change.

[0037] Figure 3 This includes data depicting the dependence of the TMI threshold on both HOM bending loss and cladding absorption. Figure A shows how the TMI threshold (measured in kW) changes with increasing HOM bending loss and cladding absorption in conventional fiber. Clearly, it is unlikely that a TMI threshold greater than 4 kW can be obtained using conventional fiber. Also in Figure 3 Data points I and II, associated with an active optical fiber designed according to the principles of the present invention, are shown, demonstrating a reduction in the dn / dT of the core relative to the dn / dT of the cladding. The core region of prior art drawing A may exhibit a dn / dT on the order of approximately 9.3e-6, while the estimated value for the fiber of the present invention has been determined to fall within the range of 8.5e-6 to 8.3e-6 (in both cases, using a conventional silica cladding with a dn / dT of 10e-6).

[0038] The increase in the TMI threshold obtained through the fiber with the characteristics shown in data points I and II is significant; in fact, an increase of approximately 50% in the TMI threshold relative to the conventional value in plot A was observed. According to the principles of the invention, the increase in the TMI threshold is associated with an increased ability of the fiber of the invention to maintain strong HOM suppression even with increased pump power.

[0039] Considering the power guiding characteristics under high-power operation, the simulations already developed can further support the conclusion that the optical fiber formed according to the present invention exhibits a reduction in dn / dT in the core region relative to the dn / dT in the cladding region, indicating a performance improvement. Figure 4 This is a plot of simulated HOM bending loss associated with a high-power pump operating at 4.2 kW. For comparison, the HOM bending loss of conventional fiber (with a core dn / dT of 9.3e-6) operating at room temperature (RT) and at elevated temperatures with a 4.2 kW pump power is shown. As expected, HOM suppression is significantly reduced for conventional fiber operating with a high-power pump (compare curve 1 with curve 2).

[0040] The simulated HOM bending loss of a set of four different optical fibers formed according to the principle of this invention is also... Figure 4As shown, the dn / dT ranges from 8.5e-6 (corresponding to curve 3) to 7.5e-6 (corresponding to curve 6). As expected, all four curves confirm a significant improvement in HOM suppression when compared to prior art fibers operating at the same temperature (i.e., as shown in curve 2). This simulation is considered to affirm the hypothesis that the reduction in the dn / dT of the fiber core, associated with the principles of the present invention, sufficiently increases the threshold at which TMI is triggered, thereby enabling single-mode operation. It should be understood that the range of dn / dT analyzed in this simulation is merely exemplary, and values ​​outside this range are also acceptable for improving HOM suppression.

[0041] The relevant consideration is that the optical fiber of the present invention, with its reduced dn / dT, will be able to adequately guide the fundamental mode (LP01) under thermal load. In fact, since the dn / dT of the fiber core will be close to zero, it is known that the fundamental mode becomes extremely lossy under high power operation, ultimately leading to equipment failure. However, it has been found that as long as the reduction in dn / dT is maintained within a reasonable percentage, the core region can still support the propagation of the fundamental mode. Figure 5 Simulation data supporting this conclusion shows that, in most cases, the fundamental mode loss is minimal. Again, signal loss associated with conventional prior art fibers is shown for the RT case (curve 1A) and the elevated temperature case (curve 2A). In many applications, a fundamental mode loss close to 0.40 associated with a core of 7.5e-6 dn / dT (curve 6A) remains problematic and can unacceptably degrade device performance. In such cases, fiber designs can include additional features, as described below, to maintain the fundamental mode confined within the core region.

[0042] As the core size and required mode field diameter (MFD) of doped gain fibers become increasingly large, reducing the dn / dT in the core region may be insufficient to maintain single-mode operation at elevated operating temperatures. Core sizes can become as large as 50 µm (or larger), with MFDs greater than 35 µm. For example, high-energy pulsed fiber lasers require extremely large cores to suppress nonlinear damage such as self-phase modulation, stimulated Raman scattering, and stimulated Brillouin scattering. At these larger diameters, maintaining fundamental mode output and diffraction-limited single-mode performance (even at room temperature or relatively low pump power) is considered extremely difficult.

[0043] Therefore, it can be anticipated that the reduction in dn / dT of the core region as described above may not be sufficient to maintain single-mode operation. In this case, it is recommended to combine the teachings on reducing the dn / dT of the core with other types of HOM suppression known in the art. For example, features including rings or grooves in the cladding or utilizing a base core structure may need to be combined with a core region having a reduced dn / dT in order to maintain acceptable high power performance with appropriate HOM suppression.

[0044] Furthermore, it is anticipated that the performance of these large-area fibers can be further improved by reducing the dn / dT of other features in the fiber structure. For example, in so-called ring-assisted design, modifying the dn / dT of the ring and possibly the dn / dT of the cladding material can be beneficial. By reducing the dn / dT of the core and optimizing the dn / dT of the cladding and ring, effective coupling can be achieved at very high pump power levels, thus enabling higher-order mode suppression. Figure 6 An exemplary ring configuration of a large-area optical fiber 60 and its associated refractive index distribution are illustrated. The optical fiber 60 is shown as including a core region 62, an inner cladding 64, a ring 66, and an outer cladding 68.

[0045] Figure 7 Is it like this? Figure 6 The diagram illustrates a plot of the LP11 (HOM) loss at room temperature for an exemplary ring-assisted large-area fiber configuration. Specifically, this plot is associated with a high-absorption, ring-assisted ytterbium-doped active fiber operating in a counter-pumped configuration. It is known that counter-pumped configurations can be very prone to drastically reducing HOM loss at the output end because the thermal load is relatively high at the fiber's output termination, and the bending radius is not sharp enough to provide high HOM loss. By optimizing the dn / dT of the core region 62, the inner cladding 64, and the ring 66, effective coupling between the core 62 and the ring 66 can be maintained even at pump power levels of 4 kW (or higher).

[0046] In comparison, Figure 8 A plot of LP11 loss including heated optical fiber (e.g., under high pump power conditions), where Figure 8 The plot of (a) is associated with a conventional ring-assisted fiber, in which the core dn / dT is 9.3e-6 (the inner cladding and the ring both have dn / dT of 1.0e-5). Figure 8 (b) is a plot of the LP11 loss of a ring-assisted fiber formed according to the principles of the present invention, wherein the dn / dT of the core region is reduced to 8.3e-6. In this example, the dn / dT of the ring is increased (relative to the prior art) to a value of 1.1 dn / dT. Reducing the dn / dT to a value of 1.02e-5 yields... Figure 8The drawing of (c) is noteworthy, Figure 8 The scaling on the y-axis of (c) is different from the scaling of the other plots. Figure 8 (c) extends between 400 dB / m and 800 dB / m (with) Figure 8 (a) and Figure 8 (b) The range of 1 dB / m to 1000 dB / m is relative.

[0047] Designs with a core featuring a step-index or graded-index refractive index can be combined with surrounding trenches that aid in the separation of the fundamental mode and the HOM (Hyper-Oriented Mode). The performance of the resulting fiber under varying operating / thermal load conditions can also be optimized by adjusting the dn / dT of the inner cladding and trenches, combined with a reduction in the dn / dT of the core.

[0048] The use of substrate design differs somewhat because the substrate relies on the refractive index difference between the core and the extended, raised inner cladding (which may or may not have additional cladding features). In this case, a reduction in the dn / dT of the core region may be of particular interest for applications using any known rare-earth dopant, where these fibers are designed to use optimal glass compositions with inherently high refractive indices (relative to silicon dioxide), and without a reduction in the dn / dT of the core, it may be difficult to maintain its large mode area in single-mode operation.

[0049] While the foregoing invention has been described with reference to the embodiments discussed above, many variations are possible. Indeed, although specific reference has been made to the fabrication of ytterbium-doped active fibers, the premise of reducing dn / dT in the core region of active fibers also applies to active fibers formed to include other rare-earth dopants (or combinations thereof). Therefore, modifications and alterations such as those suggested above (but not limited thereto) are considered to be within the scope of the following claims.

Claims

1. An optical fiber configured to generate amplification in the presence of a pump bundle, comprising: The fiber core region includes rare earth dopants. Cladding, the cladding surrounding the core region; and A coating material surrounding the cladding, wherein the core region includes at least one dopant, the at least one dopant being selected to reduce the thermo-optical coefficient dn / dT of the core region relative to the thermo-optical coefficient dn / dT of the cladding.

2. The optical fiber of claim 1, wherein the at least one dopant is selected to reduce the dn / dT of the core region by at least about 10% relative to the dn / dT of the cladding.

3. The optical fiber of claim 2, wherein the dn / dT of the core region is reduced to a value in the range of 10%-25% of the dn / dT of the cladding.

4. The optical fiber of claim 2, wherein the dn / dT of the core region is reduced by no more than 60% relative to the dn / dT of the cladding.

5. The optical fiber of claim 1, wherein the core region comprises ytterbium (Yb) as the rare earth dopant and boron as the at least one dopant for reducing dn / dT.

6. The optical fiber of claim 4, wherein the addition of boron is controlled to provide a flux of 8.5 e.g. -6 - 8.0e -6 The value of dn / dT within the range.

7. The optical fiber of claim 1, wherein the optical fiber further comprises... At least one additional feature, said at least one additional feature for controlling the suppression of higher-order modes.

8. The optical fiber of claim 7, wherein the dn / dT of the at least one additional feature is modified to improve the separation between the propagating fundamental mode and undesirable higher-order modes.

Images