Low birefringence active optical fiber

Abstract

Various exemplary embodiments relate to active optical fibers and devices including active optical fibers. A section of the active optical fiber may comprise an active core doped with at least one rare earth element. The active core may have a first refractive index and be configured to support single-mode operation of the optical signal. The section of the active optical fiber may further comprise at least one cladding layer having a second refractive index. The second refractive index may be less than the first refractive index. The birefringence of the active core may be greater than 10 -5 Also disclosed are fiber lasers and power amplifiers comprising sections of active optical fiber.

Description

[Technical Field]

[0001] Various exemplary embodiments relate generally to the field of active optical fibers and devices that use active optical fibers. In particular, some exemplary embodiments relate to improving the stability of polarization states in active optical fibers. [Background technology]

[0002] Fiber laser and amplifier technology can be used in a variety of applications. In some applications, it is desirable for the state of polarization (SOP) of the output radiation of an active optical fiber to be stable. An ideal active optical fiber would not distort the SOP. However, real optical fibers can be bent or subjected to various environmental influences, which can cause the SOP to become unstable. Summary of the Invention

[0003] This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter.

[0004] Exemplary embodiments provide a section of an active optical fiber that allows having a sufficiently stable polarization state regardless of internal heating of the active optical fiber during operation. Further embodiments are provided in the dependent claims, the description and the drawings.

[0005] According to a first aspect, the section of active optical fiber may comprise an active core. The active core may be doped with at least one rare earth element. The active core may have a first refractive index. The active core may be configured to support single-mode operation of the optical signal. The section of active optical fiber may further comprise at least one cladding layer having a second refractive index. The second refractive index may be less than the first refractive index. The birefringence of the active core may be greater than 10 -5 It may be less than.

[0006] According to a second aspect, an apparatus may comprise a section of active optical fiber according to the first aspect. The apparatus may further comprise at least one pump radiation source optically connected to at least one pump radiation coupler. The pump radiation coupler may be configured to couple radiation from the pump radiation source into the active optical fiber. The apparatus may be embodied, for example, as a fiber laser or a fiber master oscillator power amplifier (MOPA).

[0007] Many of the attendant features will become better understood and more readily appreciated by reference to the following detailed description considered in connection with the accompanying drawings. [Brief explanation of the drawings]

[0008] The accompanying drawings are included to provide a further understanding of the exemplary embodiments, are incorporated into and constitute a part of this specification, illustrate exemplary embodiments, and together with the description, serve to aid in understanding the exemplary embodiments.

[0009] [Figure 1] 1 illustrates an example of a model of an ideal optical fiber in accordance with an example embodiment. [Figure 2] 1 illustrates an example of an actual optical fiber according to an exemplary embodiment. [Figure 3] 1 illustrates an example of the temperature of an active optical fiber versus pump power, according to an example embodiment. [Figure 4]An example of an experiment for measuring polarization stability is shown. [Figure 5] An example of the polarization stability versus temperature for pumping power of a PANDA-type active optical fiber is shown. [Figure 6] An example of the polarization extinction ratio of a PANDA type active optical fiber relative to the pumping power is shown. [Figure 7] 1 shows an example of polarization stability versus temperature versus pump power for a spun active optical fiber with low birefringence, according to an exemplary embodiment. [Figure 8] 1 shows an example of polarization extinction ratio versus pump power for a spun active optical fiber with low birefringence, according to an exemplary embodiment. [Figure 9] 1 illustrates an example of a section of an active single-clad optical fiber according to an exemplary embodiment. [Figure 10] 1 illustrates an example of a section of an active double-clad optical fiber according to an exemplary embodiment. [Figure 11] 1 illustrates an example of a section of an active tapered single-clad optical fiber according to an exemplary embodiment. [Figure 12] 1 illustrates an example of a section of an active tapered double-clad optical fiber according to an exemplary embodiment. [Figure 13] 1 illustrates an example of a fiber laser device according to an exemplary embodiment. [Figure 14] 1 illustrates another example of a fiber laser device according to an exemplary embodiment. [Figure 15] 1 illustrates another example of a fiber laser device according to an exemplary embodiment. [Figure 16] 1 illustrates another example of a fiber laser device according to an exemplary embodiment. [Figure 17] 1 illustrates an example of a fiber master oscillator power amplifier device according to an exemplary embodiment.

[0010] Like reference numerals are used to designate like parts in the accompanying drawings. DETAILED DESCRIPTION OF THE INVENTION

[0011] Reference will now be made in detail to exemplary embodiments, examples of which are illustrated in the accompanying drawings. The detailed description provided below in connection with the accompanying drawings is intended to describe the embodiments and is not intended to represent the only manner in which the embodiments may be constructed or utilized. The description sets forth the functions of the embodiments and the sequence of steps for constructing and operating the embodiments. However, similar or equivalent functions and sequences may be accomplished by different embodiments.

[0012] Exemplary embodiments relate generally to the field of optical fibers. An optical fiber may include a core surrounded by at least one cladding layer having a refractive index lower than that of the core. The refractive indices of the core and cladding material affect the critical angle for total internal reflection of light propagating within the core. This critical angle also defines the range of incident angles over which light emitted from the end of the optical fiber can propagate within the core. The numerical aperture (NA) of the fiber may be defined as the sine of the maximum angle through which light can propagate within the core. The core may be composed of a transparent material, such as silicon dioxide.

[0013] In an active optical fiber, the core may be doped with at least one rare-earth element. Rare-earth elements are comprised of a group of materials including cerium (Ce), dysprosium (Dy), erbium (Er), europium (Eu), gadolinium (Gd), holmium (Ho), lanthanum (La), lutetium (Lu), neodymium (Nd), praseodymium (Pr), promethium (Pm), samarium (Sm), scandium (Sc), terbium (Tb), thulium (Tm), ytterbium (Yb), and yttrium (Y). The core of an active optical fiber may be doped with one or more of these elements, for example, with Er or Yb, or a combination of Er and Yb. During operation of the active optical fiber, the rare-earth ions absorb the optical signal as well as the pump radiation launched into the active optical fiber. This allows the optical signal to be amplified by stimulated emission of light. The rare earth elements can be selected depending on the wavelength. For example, Yb may be used in the wavelength range of 980 to 1100 nm, and Er may be used in the wavelength range of 1535 to 1600 nm.

[0014] Optical fibers may be configured to support single-mode or multimode operation. Single-mode fibers may be configured to propagate light in a single mode, which may be understood as a single ray of light propagating through the core of the optical fiber. Single-mode fibers may have a relatively thin core. The single-mode regime of propagation occurs when the so-called normalized frequency V<2.405 (where

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[0015] A single-mode fiber may comprise one or more single-mode and multimode sections. For example, a single-mode fiber may comprise a tapered section in which at least one thinner portion of the active core is configured to support single-mode operation, passing only the fundamental mode, and a thicker portion of the active core is configured to support multimode operation. However, the single-mode portion of the tapered core may also transmit a single-mode optical signal through the thicker portion.

[0016] Birefringence (B) is an optical property of a material, such as the active core of an optical fiber. A material is birefringent if it has different refractive indices in different directions. Furthermore, for example, bending an optical fiber can cause the refractive indices in the X and Y directions to be slightly different. A birefringent material is one that has different refractive indices for the polarization of an optical signal. Birefringence can be defined based on the maximum difference between the refractive indices for different polarizations: B = 2πΔn, where Δn is the maximum difference between the refractive indices for different polarizations (e.g., "fast" and "slow" modes). Linear birefringence may refer to the difference between the refractive indices for different linear polarizations of an optical signal. Circular birefringence may refer to the difference between the refractive indices for different circular polarizations (left and right) of an optical signal.

[0017] According to an exemplary embodiment, the section of active optical fiber may comprise an active core doped with at least one rare earth element. The active core may have a first refractive index and may be configured to support single-mode operation of the optical signal. The section of active optical fiber may further comprise at least one cladding layer having a second refractive index that may be lower than the refractive index of the active core. The birefringence of the active core may be greater than 10 -5 This allows the active optical fiber to provide a sufficiently stable polarization state even under internal heating caused by pumping. Such thermally stable active optical fibers can be used in various applications, such as fiber lasers and amplifiers.

[0018] Figure 1 shows an example of a model of an ideal optical fiber. The model of an ideal optical fiber may consist of a straight fiber of length L. The ideal optical fiber has a perfectly round core 102 and at least one cladding layer 104 that is aligned in a strict axial symmetry without mechanical stress. X and Y polarization modes

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[0019] FIG. 2 shows an example of a real optical fiber according to an exemplary embodiment. A real fiber, which may have a length L, e.g., more than a few centimeters, may be subjected to environmental influences, such as mechanical vibrations, stresses, temperature gradients, etc. A real fiber may be bent, and various sections along the length of the real fiber may be bent differently. This may result in tension and compression along the fiber, as shown in FIG. 2. Furthermore, a real fiber may not have a perfect core and cladding shape. For example, the core 202 of a real fiber may be slightly non-circular and eccentric. A real fiber may have X-polarized and Y-polarized modes.

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[0020] To make the polarization state of light passing through an optical fiber more stable and predictable, fibers with large intrinsic birefringence can be used. Strong intrinsic birefringence can be achieved by various means, such as elliptical-core fibers and side-pit fibers consisting of stressors embedded in the fiber cladding, such as tension rods or bow-tie glass sections. Strong internal birefringence induced by any suitable method can exceed the birefringence induced by environmental influences. As a result, the intrinsic birefringence of the fiber makes it less susceptible to environmental influences. Therefore, the state of polarization at the output of such fibers remains stable even under environmental influences.

[0021] This approach to stabilizing the polarization state may be suitable for passive optical fibers intended for use in applications in telecommunications and sensor systems. Passive fibers can be long, for example, hundreds of kilometers for telecommunications purposes and hundreds of meters for sensing systems, and due to the nature of their applications, they may be subject to primarily mechanical perturbations (e.g., bending, stretching, compression). Stabilizing the polarization state through strong internal birefringence may be effective for fibers subjected to such mechanical perturbations.

[0022] The strong internal birefringence approach may also be applied to active optical fibers, examples of which include bowtie or PANDA (Polarization Maintaining and Absorption Reduced) type fibers with stressed cladding on either side of the core.

[0023] However, the conditions of use for active and passive fibers can be very different. The active fiber in a laser or amplifier is relatively short, e.g., less than 20 m, well isolated from vibrations, and, contrary to passive fibers, may be internally heated during operation. For example, there may be two wavelengths propagating in an active optical fiber: the signal wavelength λ (to be amplified) and s and the shorter wavelength λ pump The signal may propagate in the core. The pump radiation may propagate in the core or in the cladding. The rare earth ions emit pump photons λ pump When an electron absorbs an electron, energy equal to the energy difference between the excitation and signal photons (quantum decay) can be released as heat.

[0024] FIG. 3 shows an example of the temperature of an active optical fiber versus pump power, according to an exemplary embodiment. The solid curve 301 represents the fiber center temperature for a fiber with an active core radius of 4.6 μm and a cladding thickness of 200 μm. The dashed curve 302 represents the fiber center temperature when the cladding thickness is 315 μm. The dotted curve 303 represents the fiber center temperature when the cladding thickness is 500 μm. The convection coefficient is 1×10 -3 W / (m 2 As can be seen from Figure 3, the core temperature increases linearly with pump power. Increasing the cladding thickness reduces the temperature change, but even with the thickest cladding of 500 μm, the temperature change is still significant. Therefore, internal heating due to pump absorption makes the active optical fiber susceptible to temperature-dependent changes.

[0025] The retardance in an optical fiber, i.e., the phase shift between the "fast" and "slow" waves, is given by the following equation:

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[0026] Some applications focus on the phase change of optical radiation, mechanical stress in the fiber, or the degradation of pump absorption due to heating of the optical fiber. However, as can be seen from Equations 1 and 2, heating the fiber can cause large changes in birefringence, which can then lead to large changes in the polarization state.

[0027] 4 shows a scheme of an experiment for measuring the temperature sensitivity of a birefringent core material. The setup comprises a laser diode 401 configured to launch radiation (optical signal) into an active fiber under test 403 via an isolator 402. The setup further comprises a pump diode 404 configured to launch pump radiation into the active fiber 403 via a dichroic mirror 405, and a polarimeter 406 (PAX1000IR1 / m) for analyzing the polarization of the amplified radiation exiting the active fiber 403.

[0028] In the first experiment, 100% linearly polarized semiconductor fiber-coupled laser diode radiation at 1064 nm was launched (by splicing) into a PANDA-type birefringent double-clad ytterbium-doped tapered fiber such that a single polarization mode (one eigenstate) was excited. The length of the birefringent active tapered fiber was 5 m, and the fiber was wound into a 35 cm ring with a polarization beat length of 25 mm. The core birefringence was B=0.4*10 -4 The pump light with a wavelength of 976 nm was launched into the wide cladding of the active ytterbium-doped tapered fiber using a lens and dichroic mirror 405. The polarization state (azimuth angle, ellipticity, and polarization extinction rate) of the amplified radiation was analyzed using a polarimeter 406. In this experiment, the dependence of the polarization state of the amplified radiation was measured as a function of the pump power radiation launched into the cladding. The temperature was measured at a distance of 5 cm from the wide end of the fiber. No special measures were taken to cool the fiber during the experiment. The results are shown in Figures 5 and 6.

[0029] Figure 5 shows an example of the polarization stability versus temperature for a PANDA-type active tapered optical fiber. The black dotted line represents the polarization state when the pump power is increased, and the white dotted line represents the polarization state when the pump power is decreased. As can be seen from the experimental results, increasing the pump power from 0 to approximately 22.5 W increases the fiber temperature by 2°C (from 24°C to 26°C), and the SOP orientation changes periodically with a dispersion of 9.41° and a standard deviation of 3.07° (top graph). The mean orientation value is -22.69°, and the minimum and maximum values ​​are -26.02° and -17.21°, respectively. The ellipticity (bottom graph) also changes simultaneously with a dispersion of 7.97° and a standard deviation of 2.82°. The mean ellipticity value is -5.68°, and the minimum and maximum values ​​are -10.07° and -1.8°, respectively.

[0030] Figure 6 shows an example of the polarization extinction ratio (PER) versus pump power for a PANDA-type active tapered optical fiber. The polarization extinction ratio (PER) is an index that compares the power of the desired polarization with the power of the undesired polarization. As shown in Figure 7, a change in the fiber temperature of just 2°C changes the PER with a dispersion of 6.11 dB and a standard deviation of 2.47 dB. The average value of the PER was 10.63 dB, and the minimum and maximum values ​​were 7.51 dB and 15.03 dB, respectively.

[0031] The difference in polarization state between increasing and decreasing pump power indicates that the change in polarization state is accompanied by hysteresis, and active fibers with such stress-induced birefringence exhibit memory for the history of the launched pump power. Based on this measurement, it was observed that internal heating due to the absorption of 22 W of pump power causes the drift of the polarization state.

[0032] Therefore, based on this experiment, the following observations can be made: 1) The polarization state of the amplified light in a strongly birefringent active fiber is highly dependent on the launched pump power. 2) The polarization state changes with hysteresis and has a memory related to the history of the launched pump power. Therefore, the behavior of the polarization state is unpredictable.

[0033] Based on Equation 2, when the intrinsic birefringence is small (B→0), (dB / dt)*ΔT<<B, and as a result, the temperature sensitivity of the polarization state tends to be zero (i.e., dR→0). Therefore, the smaller the intrinsic birefringence of the fiber, the lower the polarization sensitivity of the fiber. For example, the change in retardance is small when the fiber is excited. In contrast, high-birefringence fibers may have strong temperature sensitivity.

[0034] Due to the strong temperature sensitivity, the birefringence changes dramatically when the temperature changes. Furthermore, as described above, the change in internal birefringence occurs irreversibly with hysteresis. During annealing, both an increase and a decrease in internal birefringence may occur. Since the change in birefringence occurs with hysteresis, high-birefringence fibers will remember the birefringence with respect to the heating history of the fiber. Nevertheless, depending on the nature of the application (e.g., a transparent medium for optical transmission), high-birefringence optical fibers may not be exposed to significant temperature changes, and thus, the above characteristics generally do not prevent their use, for example, as passive optical fibers or active optical fibers with relatively low excitation power.

[0035] Fibers with low intrinsic birefringence can be manufactured in various ways. One way to obtain low intrinsic birefringence is to make the optical fiber as close to ideal as possible, for example, making the fiber extremely symmetric with low internal stress. Another way to obtain low intrinsic birefringence is to apply a compensating fiber. A low level of internal birefringence can be achieved, for example, by selecting the fiber dopant material such that the sum of the stress birefringence (B s ) and the geometric shape birefringence (B c ) is zero. Another way to obtain low intrinsic birefringence is to use a spun fiber. When the fiber preform is spun at high speed while being pulled, the internal birefringence becomes small. When the preform is spun, the fast and slow birefringence axes periodically alternate along the fiber, and the relative phase delay between the polarization eigenmodes is fragmentarily compensated.

[0036] According to an exemplary embodiment, an active optical fiber with low intrinsic birefringence is provided. The SOP stability of such a fiber was verified using the experimental setup shown in FIG. 4. For experimental verification of the SOP stability, a Yb-doped spun active double-clad tapered fiber was fabricated. The spun fiber was fabricated by drawing the active tapered fiber while rotating the fiber preform at an angular velocity of 600 rev / min. In this experiment, emission from a linearly polarized semiconductor laser with a wavelength of 1064 nm was spliced ​​into the spun tapered fiber via a fiber-coupled isolator 402. The spun tapered fiber was 2.8 m long and had a 6 mm pitch at its widest point. The pitch may refer to the period of rotation, e.g., the length of a 360° rotation of the spun fiber. The pitch may depend on the fiber pulling speed and the angular velocity of rotation. The residual linear birefringence of the fiber was 3.21*10 -6 , the circular birefringence is 6.88*10 -6 The fiber was wound into a ring with a diameter of 35 cm. The 976 nm pump light was launched into the cladding of the active fiber via a lens and a dichroic mirror 405.

[0037] The polarization state of the amplified radiation (azimuth angle, ellipticity, PER) was analyzed using a polarimeter 405. The dependence of the polarization state of the amplified radiation was again investigated as a function of the launched pump power. No measures were taken to force cooling of the fiber during the experiment. The results are shown in Figures 7 and 8.

[0038] Figure 7 shows an example of polarization stability versus temperature for a spun-active tapered optical fiber with respect to pump power. As can be seen from the experimental results, increasing the pump power from zero to approximately 20 W resulted in a 2°C increase in fiber temperature (from 24°C to 26°C), and the orientation changed periodically with a variance of 0.12° and a standard deviation of 0.35° (top graph). The average orientation was 1.39°, with minimum and maximum values ​​of 0.11° and 1.97°, respectively. At the same time, the ellipticity (bottom graph) changed with a variance of 0.03° and a standard deviation of 0.18°. The average ellipticity was 1.15°, with minimum and maximum values ​​of 0.93° and 1.48°, respectively.

[0039] Figure 8 shows an example of the polarization extinction ratio versus pump power for a spun active tapered optical fiber. As shown in Figure 8, when the fiber temperature changes by 2°C, the PER changes with a dispersion of 0.43 dB and a standard deviation of 0.65 dB. The average value of the PER is 17.01 dB, and the minimum and maximum values ​​are 15.88 dB and 17.90 dB, respectively.

[0040] Based on the results in Figures 5-8, the change in polarization state for the active spun fiber is significantly smaller for the low-birefringence spun fiber compared to the PANDA-type fiber. Table 1 shows the comparative data for the PANDA-type fiber and the spun fiber. As can be seen from Table 1, the polarization state stability (e.g., azimuth and ellipticity deviation) is one order of magnitude better for the spun active tapered fiber. The ellipticity dispersion is two orders of magnitude better.

[0041] Table 1. Comparison of SOP fluctuations between PANDA-type active fiber and spun active fiber (excitation power 22 W). [Table 1]

[0042] The above experiments demonstrate that the low-birefringence active optical fiber is significantly superior in terms of SOP stability compared to amplifiers using high-birefringence fibers, such as PANDA-type fibers.

[0043] Exemplary embodiments provide different types of active optical fiber that allow for stable polarization states that are substantially independent of the launched pump power. Exemplary embodiments provide, for example, sections of single-clad or double-clad active optical fiber with or without a tapered longitudinal profile in combination with low intrinsic birefringence in the core. According to exemplary embodiments, the birefringence of the active core is less than 10 -5 According to an exemplary embodiment, the linear birefringence of the active core may be less than 10 -5 According to an exemplary embodiment, the circular birefringence of the active core may be less than 10 -5 According to an exemplary embodiment, both the circular and linear birefringence of the active core may be less than 10 -5 Based on experiments, it may be less than 10 -5 A birefringence value of less than 10 can provide a polarization state that is sufficiently stable against temperature changes due to internal heating of the active optical fiber. In general, the smaller the birefringence, the better the stability of the polarization state. For example, -5 Less than, e.g., 10 -6 <B<10 -5 Birefringence values ​​in the range of 3.2*10 provide more stable polarization states, which may be beneficial, for example, with longer fiber lengths L or higher pump powers. According to exemplary embodiments, the birefringence of the active core may follow the active spun fiber described in connection with Figures 6 and 7. For example, the linear birefringence of the active core may be 3.2*10 -6 The circular birefringence of the active core may be 6.7*10 -6 may be.

[0044] FIG. 9 shows an example of a longitudinal cross section of an active single-clad optical fiber, according to an exemplary embodiment. The section of active optical fiber may comprise an active core 901. The core may be composed of any suitable material, such as, for example, silicon dioxide. The active core 901 may further comprise at least one rare earth element. The active core 901 may be doped with a rare earth element to enable amplification of an optical signal launched into the active core 901 when pump radiation is launched into the active core 901. The section of active optical fiber may further comprise a cladding layer 902. The active core 901 has a first refractive index n core The cladding layer 902 may have a second refractive index n clad As shown in the cross-sectional refractive index profile 903, the second refractive index n clad is the first refractive index n clad The birefringence of the active core may be smaller than 10 -5 For example, the refractive index of the slow polarization mode n slow and the refractive index of the fast polarization mode, n fast The difference is 10 -5 may be smaller, i.e., B=n slow -n fast <10 -5 may be.

[0045] The active core 901 may be configured to support single-mode operation. For example, the active core 901 may satisfy propagation conditions for single-mode operation of an optical signal. The propagation conditions may include 2πrNA / λ<2.405, where r is the radius of the active core, NA is the numerical aperture of the active core, and λ is the wavelength of the optical signal. As shown in FIG. 9 , the active core 901 may be configured to receive an optical signal and pump radiation. In other words, the optical signal may be launched at the active core 901, for example, at one end of the active core 901. The pump radiation may be configured to be received or launched at either end or both ends of a section of the active core 901.

[0046] 10 shows an example of a longitudinal cross section of an active double-clad optical fiber, according to an exemplary embodiment. A section of the active optical fiber may be comprised of an active core 1001. The active core 1001 has a first refractive index n core The section of active optical fiber may further comprise an inner cladding layer 1002 around the active core 1001. The inner cladding layer 1002 may have a second refractive index n clad1 The section of active optical fiber may further comprise an outer cladding layer 1003 around the inner cladding layer 1002. The outer cladding layer 1003 may have a third refractive index n clad2 As shown in the cross-sectional refractive index profile 1004, the first refractive index n core is the second refractive index n clad1 may be smaller than the third refractive index n clad2 is the second refractive index n clad1 The birefringence of the active core 1001 may be less than 10 -5 The active core 1001 may be configured to receive an optical signal. In other words, the optical signal may be launched at the active core 1001. The inner cladding layer 1002 may be configured to receive pump radiation from either one or both ends of the section of the active optical fiber. In other words, pump radiation may be launched into the inner cladding layer 1002 at either one or both ends of the section of the active optical fiber.

[0047] A low birefringence in the active core improves resistance to internal heating caused by pumping. A low birefringence in a non-tapered single-mode active core can be beneficial, as a relatively thin single-mode core may be more susceptible to internal heating due to pump power than a wider multimode core. For example, a small diameter single-mode core reduces its surface area, which in turn limits its ability to dissipate heat. A low birefringence in a single-mode core allows for higher-power pump radiation to be launched within a single-mode fiber, resulting in better amplification of the optical signal while maintaining a sufficiently stable polarization state.

[0048] FIG. 11 illustrates an example of a longitudinal cross-section of an active tapered single-clad optical fiber, according to an exemplary embodiment. The section of active optical fiber may comprise an active core 1101 and a cladding layer 1102, which may be similar to the active core 901 and cladding layer 902 of FIG. 9. However, the section of active optical fiber may have a tapered longitudinal profile such that the diameter d of the active core 1101 gradually changes along the length L of the section of active optical fiber, thereby forming a tapered longitudinal profile. As a result, the section of active optical fiber may comprise a first portion and a second portion, with the radius of the first portion of the active core being smaller than the radius of the second portion of the active core. Furthermore, the thickness of the cladding layer 1102 may gradually change along the tapered longitudinal profile. For example, the thickness of the cladding layer 1102 may be proportional to the diameter d of the corresponding portion of the active core 1101.

[0049] The first portion of the active core may be configured to satisfy propagation conditions for single-mode operation of the optical signal. The remaining portion of the active core, e.g., the second portion, may be configured to support multi-mode operation of the optical signal. The propagation conditions may include 2πrNA / λ<2.405, where r is the radius of the first portion of the active core (d / 2), NA is the numerical aperture of the first portion of the active core, and λ is the wavelength of the optical signal. The first portion of the active core may be configured to receive an optical signal. In other words, the optical signal may be launched at the first portion of the active core 1101. The first portion and / or the second portion of the active core 1101 may be configured to receive pump radiation. In other words, the pump radiation may be launched at the first portion and / or the second portion of the active core 1101.

[0050] According to an exemplary embodiment, a first portion of the active core 1101 may be disposed at a first end of a section of active optical fiber, and a second portion of the active core 1101 may be disposed at a second end of the section of active optical fiber. According to an exemplary embodiment, the first portion of the active core 1101 may constitute a narrow end of the active core 1101. The second portion of the active core 1101 may constitute a wider end of the active core 1101.

[0051] By launching an optical signal in the first portion of the tapered active core 1101, propagation of only the fundamental mode can be arranged in the second (multimode) portion of the active core 1101. The larger diameter of the second portion of the active core 1101 allows for efficient launch of pump radiation from a high-power, low-intensity pump source into the active tapered fiber. The smaller birefringence of the tapered core of the active optical fiber allows for the benefit of the higher pump power launch capability of the second portion while maintaining a sufficiently stable polarization state for single-mode optical signals. According to an exemplary embodiment, for example, approximately 90% of the pump radiation may be launched into the second portion of the active core 1101 to achieve a desired gain with low nonlinearity. For example, approximately 10% of the pump radiation may be launched into the first portion of the active core 1101 to induce saturation of the active core 1101.

[0052] FIG. 12 shows an example of a longitudinal cross-section of an active tapered double-clad optical fiber, according to an exemplary embodiment. The section of the active optical fiber may comprise an active core 1201, an inner cladding layer 1202, and an outer cladding layer 1203, similar to the active core 1001 and cladding layers 1002 and 1003 of FIG. 10. However, the section of the active optical fiber may have a tapered longitudinal profile. For example, the diameter d of the active core 1201 may vary gradually along the length L of the section of the active tapered optical fiber. Furthermore, the thickness of the inner and / or outer cladding layers may vary gradually along the tapered longitudinal profile. For example, the thickness of the inner and / or outer cladding layers may be proportional to the diameter d of the corresponding portion of the active core 1201.

[0053] According to an exemplary embodiment, the active core 1201 may comprise first and second portions similar to the active core 1101 of FIG. 11 . According to an exemplary embodiment, a section of the active optical fiber may comprise a first portion of an inner cladding layer 1202 around the first portion of the active core 1201 and a second portion of the inner cladding layer 1202 around the second portion of the active core 1201. The thickness of the first portion of the inner cladding layer 1202 may be less than the thickness of the second portion of the inner cladding layer 1202. The first and / or second portions of the inner cladding layer 1202 may be configured to receive pump radiation. In other words, the pump radiation may be launched in the first and / or second portions of the inner cladding layer 1202. A larger thickness of the second portion of the inner cladding layer 1202 may enable a higher power of pump radiation to be launched into the tapered fiber. The low birefringence of the tapered core of the active optical fiber allows it to benefit from the higher pump power launch capability of the second portion of the inner cladding layer 1202 while maintaining a sufficiently stable state of polarization for a single-mode optical signal.

[0054] According to an exemplary embodiment, a first portion of the inner cladding layer 1202 may be disposed at a first end of a section of the active optical fiber, and a second portion of the inner cladding layer 1202 may be disposed at a second end of the active optical fiber. According to an exemplary embodiment, the first portion of the inner cladding layer 1202 may constitute a narrow end of the inner cladding layer. The second portion of the inner cladding layer 1202 may constitute a wide end of the inner cladding layer.

[0055] Although not shown in FIGS. 9-12 , a section of the active optical fiber may further comprise additional structures, such as one or more coating layers around the cladding layer. The coating layers may be composed of, for example, a polymer coating. The coating layers may be configured to reduce environmental influences that may introduce external birefringence in the active core 901, 1001, 1101, 1201, which has low intrinsic birefringence. Thus, the low internal birefringence combined with the one or more coating layers together provide an active optical fiber that provides a sufficiently stable polarization state under varying (internal / external) temperatures and other environmental influences, such as mechanical bending. In the above exemplary embodiments, the excitation radiation may be configured to propagate in the same or substantially the same direction as the optical signal and / or in the opposite or substantially opposite direction to the optical signal.

[0056] FIG. 13 shows an example of a fiber laser device 1300, according to an exemplary embodiment. The fiber laser device 1300 may comprise an active optical fiber 1301. The active optical fiber 1301 may comprise any of the different types of active optical fiber, or sections thereof, described above. The fiber laser device 1300 may be configured to provide amplified output radiation within the active optical fiber 1301 as it travels back and forth between a pair of reflecting mirrors. The fiber laser device 1300 may comprise a pump radiation source 1305. The pump radiation source may be optically coupled to a pump radiation coupler 1304. The pump radiation source may be configured to generate pump radiation at an appropriate power. The pump radiation coupler 1304 may be configured to couple radiation from the pump radiation source 1305 into the active optical fiber 1301. The pump radiation coupler 1304 may comprise, for example, a multimode pump combiner, a free-space lens system, and / or a wavelength-dependent multiplexer (WDM) for single-clad fiber. The multimode pump combiner may be of type (1+n)*1, which may indicate that one input signal fiber and n pump fibers are coupled together into one signal output fiber, for example, by a tapered ring. An example of such a multimode pump combiner is a (1+6)*1 combiner, which couples six pump fibers and one signal fiber together. The pump radiation coupler 1304 may be configured to launch pump radiation into an appropriate portion and / or layer of the active optical fiber 1301. For example, in the case of a single-clad fiber, the pump radiation coupler 1304 may be configured to launch pump radiation emitted from the pump source 1305 into the core of the active optical fiber 1301. In the case of a double-clad fiber, the pump radiation coupler 1304 may be configured to launch pump radiation emitted from the pump source 1305 into the core of the active optical fiber 1301. The pump radiation coupler 1304 may be optically connected to a first end of the active optical fiber 1301. An optical connection may allow light to propagate between two optically connected or optically coupled components.An optical connection may include a direct optical connection such that there are no intermediate components, such as mirrors or excitation radiation couplers, between the optically connected components.

[0057] The fiber laser device 1300 may further comprise a second excitation radiation source 1307 and a second excitation radiation coupler 1306, which may be similar to the excitation coupler 1304 and the excitation radiation source 1305, respectively. However, the excitation radiation coupler 1306 may be optically connected to a second end, e.g., an output end, of the active optical fiber 1301. Furthermore, the excitation radiation source 1307 may be configured to generate excitation radiation having a different power level compared to the excitation radiation originating from the excitation radiation source 1305. For example, in the case of an active tapered optical fiber, the excitation radiation source may be optically connected to a first end of the active optical fiber 1301 and may be thinner than the second end of the active optical fiber 1301. The power level of the second excitation radiation source 1307 may be higher than the power level of the excitation radiation source 1305, as described above.

[0058] The fiber laser device 1300 may further include a first reflector 1302, which may be optically connected to a first end of the active optical fiber 1301. The first reflector 1302 may be configured to transmit pump radiation from the pump coupler 1304 to the active optical fiber 1301. The first reflector 1302 may be configured to reflect a majority of light propagating in the active optical fiber 1301 toward the first reflector 1302. The first reflector 1302 may comprise, for example, a free-space bulk dielectric or metal-coated mirror, a fiber Bragg grating (FBG) written in another optical fiber spliced ​​to the first end of the active optical fiber 1301, a fiber loop mirror, or a fiber-coupled Faraday rotator mirror. Alternatively, a fiber Bragg grating may be written in the first end of the active optical fiber 1301. The reflectivity of the first reflector may be, for example, 90% or greater.

[0059] The fiber laser device 1300 may further comprise a second reflector 1303, which may be optically connected to a second end, e.g., an output end, of the active optical fiber 1301. The second reflector 1303 may be configured to transmit pump radiation from the pump coupler 1306 to the active optical fiber 1301. The second reflector 1303 may be configured to pass a portion of light propagating within the active optical fiber towards the second reflector 1303 so as to enable amplified light to be output from the fiber laser device 1300. The second reflector 1303 may comprise, for example, a free-space bulk dielectric or metal-coated mirror, a fiber Bragg grating (FBG) written or spliced ​​to the second end of the active optical fiber 1301, or a fiber loop mirror. The reflectivity of the second reflector may be, for example, less than 90%.

[0060] 14 shows another example of a fiber laser device 1400 according to an exemplary embodiment. The fiber laser device 1400 may include similar components to the fiber laser device 1300. However, some of the components may be arranged in a different order. For example, the first reflecting mirror 1302 may be optically connected to the pump radiation coupler 1304, which may be optically connected to a first end of the active optical fiber 1301. Furthermore, the second reflecting mirror 1303 may be optically connected to the pump radiation coupler 1306, which may be optically connected to a second end of the active optical fiber 1301. The pump radiation couplers 1304 and 1306 may be configured to carry light that is reflected between the reflecting mirrors 1302 and 1303 to enable amplification of the light in the active optical fiber 1301.

[0061] FIG. 15 shows another example of a fiber laser device 1500 according to an exemplary embodiment. The fiber laser device 1500 may include similar components to the fiber laser device 1300. However, some of the components may be arranged in a different order. In this example, similar to FIG. 14, the first reflector 1302 may be optically connected to the pump radiation coupler 1304, which may be optically connected to a first end of the active optical fiber 1301. On the output side, similar to FIG. 13, the second reflector 1303 may be optically connected to a second end of the active optical fiber 1301, and the pump radiation coupler 1306 may be coupled to the second reflector 1303.

[0062] FIG. 16 shows another example of a fiber laser device 1600 according to an exemplary embodiment. The fiber laser device 1500 may include similar components to the fiber laser device 1300. However, some of the components may be arranged in a different order. In this example, the first reflector 1302 may be optically connected to the pump radiation coupler 1304, which may be optically connected to a first end of the active optical fiber 1301, similar to FIG. 13 . On the output side, the second reflector 1303 may be optically connected to a second end of the active optical fiber 1301, and the pump radiation coupler 1306 may be coupled to the second reflector 1303.

[0063] FIG. 17 shows an example of a fiber master oscillator power amplifier device (MOPA) 1700 according to an illustrative embodiment. The fiber master oscillator power amplifier device 1700 may comprise any of different types of active optical fiber, or sections thereof, as described above. Additionally, the fiber master oscillator power amplifier device 1700 may comprise an excitation radiation source 1305 and / or an excitation radiation source 1307 similar to those in FIG. 13 . The fiber master oscillator power amplifier device 1700 may further comprise an excitation radiation coupler 1304 and / or a second excitation radiation coupler 1306 similar to those in FIG. 13 . The excitation radiation coupler 1304 may be coupled to a first end of the active optical fiber 1301 and configured to launch excitation radiation generated by the excitation radiation source 1305 into the active optical fiber 1301. The second excitation radiation coupler 1306 may be optically coupled to a second end of the active optical fiber 1301 and configured to launch excitation radiation generated by the excitation radiation source 1307 into the active optical fiber 1301. The second pump radiation coupler 1306 may be further configured to provide output radiation from the active optical fiber 1301. The fiber master oscillator power amplifier device 1700 may further comprise a seed laser source 1701 optically connected to the pump radiation coupler 1304. The seed laser source 1701 may be configured to provide a seed laser signal for amplification in the active optical fiber 1301. The pump coupler 1304 may be configured to couple light from the seed laser source 1701 into the active optical fiber 1301.

[0064] Exemplary embodiments provide thermally stable sections of active optical fiber that can be used in a variety of applications, such as fiber lasers and fiber master oscillator power amplifiers, where higher gain is possible due to, for example, high resistance to pump radiation-induced internal heating.

[0065] Any range or device value given herein may be expanded or modified without losing the desired effect, and any embodiment may be combined with another embodiment unless expressly prohibited.

[0066] Although the subject matter has been described in language specific to structural features and / or acts, it is understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or acts described above. Rather, the specific features and acts described above are disclosed as example ways of implementing the claims, and other equivalent features and acts are intended to be within the scope of the claims.

[0067] It will be understood that the benefits and advantages described above may relate to one embodiment or to multiple embodiments. Embodiments are not limited to those that solve any or all of the stated problems or that have any or all of the stated benefits and advantages. It will be further understood that reference to "an" or "an" item may refer to one or more of those items.

[0068] As used herein, the term "comprising" is used to mean including the specified blocks or elements, but that such blocks or elements do not constitute an exclusive list. Thus, a device may include additional blocks or elements.

[0069] Although a subject may be referred to as a "primary" subject or a "secondary" subject, this does not necessarily indicate the order or importance of the subject. Rather, such attributes may be used solely to distinguish between subjects.

[0070] It will be understood that the above description is provided by way of example only, and that various modifications may be made by those skilled in the art. The above specification, examples, and data provide a complete description of the structure and use of the exemplary embodiments. While various embodiments have been described above with a certain degree of particularity, or with reference to one or more individual embodiments, those skilled in the art may make numerous modifications to the disclosed embodiments without departing from the scope of the present specification.

Claims

1. A section of active optical fiber for amplifying an optical signal, comprising: an active core doped with at least one rare earth element, the active core having a first refractive index and configured to support single-mode operation of the optical signal; an inner cladding layer around the active core and having a second refractive index, the second refractive index being less than the first refractive index; an outer cladding layer surrounding the inner cladding layer and having a third refractive index less than the second refractive index; Equipped with The birefringence of the active core is 10 －５ a diameter of the active core forming a tapered longitudinal profile that varies gradually along the length of the section of the active optical fiber, a thickness of the inner cladding layer that varies gradually along the tapered longitudinal profile, a narrow end of the active core configured to receive the optical signal and a wide end of the inner cladding layer configured to receive pump radiation.

2. The section of active optical fiber of claim 1 , wherein a radius of the first portion of the active core is smaller than a radius of the second portion of the active core.

3. 3. The section of active optical fiber of claim 2, wherein the first portion of the active core satisfies propagation conditions for the single-mode operation of the optical signal, and the second portion of the active core supports multi-mode operation of the optical signal.

4. 4. The section of active optical fiber of claim 3, wherein the propagation conditions include 2πrNA / λ<2.405, where r is the radius of the first portion of the active core, NA is the numerical aperture of the first portion of the active core, and λ is the wavelength of the optical signal.

5. The section of active optical fiber according to any one of claims 2 to 4, wherein the first portion of the active core is configured to receive the optical signal.

6. 6. The section of an active optical fiber according to claim 2, wherein the first portion of the active core is configured to receive the excitation radiation and / or the second portion of the active core is configured to receive the excitation radiation.

7. 7. The section of active optical fiber according to claim 6, wherein the excitation radiation is configured to propagate substantially in the same direction as the optical signal and / or substantially in the opposite direction to the optical signal.

8. A device comprising a section of active optical fiber according to any one of claims 1 to 6, further comprising: at least one excitation radiation source optically connected to at least one excitation radiation coupler, the at least one excitation radiation coupler configured to couple radiation from the excitation radiation source into the active optical fiber; An apparatus comprising:

9. the apparatus comprises a fiber laser device, and a first reflector optically connected to a first end of the active optical fiber, or a first excitation-emission coupler optically connected to the first end of the active optical fiber; The apparatus of claim 8 , comprising:

10. 10. The apparatus of claim 9, further comprising: a second reflector optically connected to a second end of the active optical fiber; or a second excitation-emission coupler optically connected to the second end of the active optical fiber.

11. the apparatus comprising a fiber master oscillator power amplifier; a seed laser source optically connected to a pump coupler, the pump coupler configured to couple light from the seed laser source into the active optical fiber; The apparatus of claim 8 , comprising:

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