Active optical vortex fiber

Abstract

Various exemplary embodiments relate to active optical fibers and devices using active optical fibers. An active optical fiber can include a central portion surrounded by a ring-shaped active core. The optical fiber can have a tapered longitudinal profile such that the optical fiber includes a single-mode portion and a multi-mode portion. The ring-shaped core can have a low birefringence, for example, obtained by rotating (fast-rotating) the optical fiber preform during the optical fiber manufacturing. The refractive index of the ring-shaped core can be higher than the refractive index of the central portion and the cladding(s) surrounding the ring-shaped core. The active optical fiber is capable of selectively generating or amplifying an optical mode field having orbital angular momentum (OAM). Furthermore, the optical fiber has a large mode field diameter (MFD) and is insensitive to internal heating or environmental influences.

Description

TECHNICAL FIELD

[0001] Various exemplary embodiments relate generally to the field of active optical fibers and devices using active optical fibers. In particular, some exemplary embodiments relate to the generation and amplification of optical signals having orbital angular momentum (OAM). BACKGROUND

[0002] Fiber laser and amplifier technology can be used for various applications. Some applications can employ orbital angular momentum (OAM) of optical signals. Optical signals having OAM can be generated in various ways. However, the achievable optical power and mode field contrast can be insufficient for all applications. SUMMARY

[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 active optical fiber segments suitable for applications using optical signals having OAM. Further implementations are provided in the dependent claims, the description and the drawings.

[0005] According to a first aspect, an active optical fiber segment can comprise: a central portion having a first refractive index n1, wherein a diameter of the central portion varies gradually along a length of the active optical fiber segment, thereby forming a tapered longitudinal profile; an annular core radially surrounding the central portion, the annular core being doped with at least one rare earth element and having a second refractive index n2, wherein n2 > n1 and wherein a birefringence of the annular core is less than 10 -5 ; a first cladding radially surrounding the annular core and having a third refractive index n3, wherein n3 < n2; and a second cladding radially surrounding the first cladding and having a fourth refractive index n4, wherein n4 < n3, wherein a first portion of the active optical fiber segment is configured to support single mode operation of the optical signal and a second portion of the active optical fiber segment is configured to support multi-mode operation of the optical signal.

[0006] Many of the attendant features will be more readily understood and appreciated by referring to the following detailed description, taken in conjunction with the attached drawings. Of course, the exemplary embodiments will also be apparent from the discussion, along with the accompanying BRIEF DESCRIPTION OF DRAWINGS

[0007] The accompanying drawings, which are included to provide a further understanding of the exemplary embodiments and are incorporated in and constitute a part of this specification, illustrate exemplary embodiments and together with the description serve to explain exemplary embodiments. In the drawings:

[0008] Figure 1 An example of a double-clad active tapered vortex optical fiber segment according to an exemplary embodiment is shown.

[0009] Figure 2 An example of a cross-section of a first cladding of a double-clad active tapered vortex optical fiber is shown in accordance with an example embodiment.

[0010] Figure 3 An example of a field distribution of an optical signal propagating in a segment of a double-clad active tapered vortex optical fiber is shown in accordance with an example embodiment.

[0011] Figure 4 An example of a master oscillator power amplifier (MOPA) including a double-clad active vortex optical fiber is shown in accordance with an example embodiment.

[0012] Figure 5 An example of a laser including a double-clad active vortex optical fiber is shown in accordance with an example embodiment.

[0013] Figure 6 Another example of a master oscillator power amplifier (MOPA) including a double-clad active vortex optical fiber is shown in accordance with an example embodiment.

[0014] Figure 7 Another example of a laser including a double-clad active vortex optical fiber is shown in accordance with an example embodiment.

[0015] In the drawings, like reference numerals are used to designate like parts. DETAILED DESCRIPTION

[0016] Reference will now be made in detail to example embodiments, examples of which are illustrated in the accompanying drawings. The detailed description provided below in connection with the appended drawings is intended as a description of the present examples and is not intended to represent the only forms in which the present examples can be constructed or used. The description sets forth the functions of the examples and the sequence of steps for constructing and operating the examples. However, the same or equivalent functions and sequences can be accomplished by different examples.

[0017] Generally, an optical fiber can include a core surrounded by at least one cladding having a refractive index lower than that of the core. The refractive indices of the core and cladding materials affect the critical angle of total internal reflection of light propagating in the core. The angle also defines a range of angles of incidence that enable light emitted at the end of the optical fiber to propagate within the core. The core can include a transparent material such as, for example, silica.

[0018] In an active optical fiber, the core can be doped with at least one rare earth element. Rare earth elements include 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 can be doped with one or more of these elements, for example, doped with Er or Yb, or a combination of Er and Yb. During operation of the active optical fiber, in addition to the optical signal, the rare earth ions absorb the pump radiation emitted in the active optical fiber. This enables the optical signal to be amplified by means of stimulated emission. Different rare earth elements can be used for different wavelengths. For example, Yb can be used for the 980-1100 nm wavelength range, and Er can be used for the 1535-1600 nm wavelength range.

[0019] An optical fiber can be configured to support single-mode or multi-mode operation. A single-mode optical fiber can be configured to transmit single-mode light, which can be understood as a single light ray propagating through the core of the optical fiber. However, a single-mode optical fiber can include one or more single-mode segments and multi-mode segments. For example, a single-mode optical fiber can include a tapered portion, such that at least one thinner portion of the active core can be configured to support single-mode operation through only the fundamental mode, while the thicker portion(s) of the active core can be configured to support multi-mode operation. However, the single-mode portion of the tapered core can also cause the thicker portion(s) to transmit single-mode optical signals.

[0020] Birefringence (B) is an optical property of a material, for example, the active core of an optical fiber. A material is birefringent if it has different refractive indices in different directions. Moreover, for example, a bent optical fiber can cause the refractive indices in the X and Y directions to become slightly different. A birefringent material has a refractive index that is different for different polarizations of the optical signal. The birefringence can be defined based on the maximum difference between the refractive indices of different polarizations: B = 2πΔn, where Δn is the maximum difference between the refractive indices of different polarizations, for example, the “fast” and “slow” modes.

[0021] Light beams with orbital angular momentum (OAM) can be applied to various applications, such as, for example, optical communications, optical tweezers, atomic manipulation, and material processing. Light beams with OAM can be generated, for example, by a bulk optics device, such as a cylindrical lens mode field converter, a spatial light modulator, or an integrated silicon device. Moreover, OAM light beams can be generated directly in a single-mode optical fiber, for example, based on a long-period fiber grating, and by controlled mode field coupling in a low-mode optical fiber (2-4 mode field). However, the optical power of the light beams obtained by these methods can be rather small, for example, on the order of a few mW.

[0022] To obtain a more powerful light beam with OAM, a master oscillator power amplifier (MOPA) system of large mode area (LMA) active fiber with a sufficiently large mode field spot size (e.g., 25 pm) can be used. Such a system can impose selective excitation of the required OAM mode field, while appropriately bending the active fiber. Another approach can include using a MOPA system containing a bulk OAM converter, such as an S-plate or Q-plate, in combination with a fiber amplification stage. However, these approaches can result in a limited size of the mode field diameter at the active fiber, which can also limit the achievable optical power. Moreover, the use of a low-mode fiber that does not preserve polarization in the last gain stage can be required. The presence of inherent residual random birefringence in such fibers can result in a deterioration of the mode field contrast, partial depolarization of the output beam, and ultimately a deterioration of the OAM beam quality. The mode field contrast can refer to the power distribution between different mode fields, such as a mode field with OAM and a mode field without OAM.

[0023] Moreover, the use of a low-aperture active few-mode LMA fiber can bring similar gain to all supported fiber mode fields (e.g., a ring mode field (due to OAM) as well as other mode fields). Thus, such a system can not provide selectivity to mode field amplification. This can not only result in the generation of the required light beam with OAM, but also can result in the transfer of pump radiation power to other (undesirable) mode fields. This can ultimately result in a deterioration of the contrast of the mode field content.

[0024] Accordingly, exemplary embodiments of the present disclosure can be used to increase the achievable average power and peak power of a light mode field with OAM, improve the mode field contrast by reducing the weight of undesirable light mode fields without OAM, and improve the stability of the generated light beam with OAM to environmental influences.

[0025] According to exemplary embodiments, an active optical fiber can include a central portion surrounded by a ring (circular) active core. The optical fiber can have a tapered longitudinal profile such that the optical fiber includes a single-mode portion and a multi-mode portion. The ring core can have a low birefringence, e.g., obtained by rotating (fast-rotating) the optical fiber preform during the optical fiber manufacturing. The refractive index of the ring core can be higher than the refractive index of the central portion and the cladding(s) surrounding the ring core. The active optical fiber is capable of selectively generating or amplifying a light mode field with OAM. Moreover, the optical fiber has a large mode field diameter (MFD) and is not sensitive to internal heating due to pumping or environmental influences. The active optical fiber can be applied to various devices, such as, for example, a laser or a master oscillator power amplifier (MOPA).

[0026] Figure 1 An example of a double-clad active tapered vortex optical fiber segment according to exemplary embodiments is shown. Figure 1A longitudinal cross-section (left) and a radial cross-section (right) of the active optical fiber segment 100 are shown. The active optical fiber segment 100 can comprise a central portion 101. The central portion 101 can have a first refractive index n1. The central portion 101 can be radially symmetric or substantially radially symmetric. The central portion 101 can comprise any suitable material such as, for example, silica. Although the central portion 101 is shown as a uniform portion, it is to be understood that the central portion 101 can be internally structured as one or more sub-portions. The diameter of the central portion 101 can gradually change along the length (L) of the active optical fiber segment 100, thereby forming a tapered longitudinal profile. For example, as shown in FIG. 1B, the diameter of the central portion 101 can increase from left to right along the length of the active optical fiber segment 100. The tapered longitudinal profile can comprise a linear profile or a parabolic convex profile, which is beneficial for the amplification of the mode field with OAM as they improve the pump absorption. Figure 1

[0027] The active optical fiber segment 100 can further comprise an annular core 102. The annular core 102 can radially surround the central portion 101. Thus, the annular core 102 can have a circular cross-section, as shown in the radial cross-section in FIG. 1C. The annular core 102 can have a second refractive index n2. The refractive index of the annular core can be higher than the refractive index of the central portion 101, n2> n1, as shown in the refractive index profile 105. Figure 1

[0028] The annular core 102 can further comprise at least one rare earth element. Thus, the annular core 102 can be active. The annular core 102 can be doped with the rare earth element(s), for example, capable of amplifying an optical signal launched in the annular core 102 when a pump radiation is launched in the active optical fiber segment 100. The birefringence of the annular core 102 can be less than 10 -5 . For example, the difference between the refractive index n slow of the slow polarization mode field and the refractive index n fast of the fast polarization mode field can be less than 10 -5 , i.e., B = n slow - n fast < 10 -5 . The annular core 102 can comprise any suitable material such as, for example, silica. The thickness of the annular core 102 can gradually change, for example, increase, along the tapered longitudinal profile. For example, the thickness of the annular core 102 can be proportional to the diameter of the central portion 101 along the length of the fiber segment. Increasing the thickness of the annular core 102 can be beneficial as it means a larger mode field area, a high threshold for nonlinear effects and a large storage of energy, for example, high peak or average power.

[0029] ​​The active optical fiber segment 100 can further comprise a first cladding 103. The first cladding 103 can have a third refractive index n3. The refractive index of the first cladding can be lower than the refractive index of the annular core 101, n3 < n2, as illustrated by the refractive index profile 105. The first cladding 103 can radially surround the annular core 102, for example as illustrated in the radial cross-section of Figure 1 .

[0030] The active optical fiber segment 100 can further comprise a second cladding 104. The second cladding 104 can have a fourth refractive index n4. The refractive index of the second cladding can be lower than the refractive index of the first cladding, n4 < n3, as illustrated by the refractive index profile 105. The second cladding 104 can radially surround the first cladding 103, for example as illustrated in the radial cross-section of Figure 1 . The first and second claddings can comprise any suitable material, such as for example silica. The thickness of the first cladding 103 and the second cladding can gradually change, for example increase, along the tapering longitudinal profile. For example, the thickness of the first cladding 103 and / or the second cladding 104 can be proportional to the diameter of the central portion 101. Similarly to the annular core 102, it can be beneficial to increase the thickness of the first cladding 103 and the second cladding 104.

[0031] The active optical fiber segment 100 can comprise a first portion configured to support single mode (SM) operation. The first portion can be located at a first end of the active optical fiber segment 100. The first end can comprise a narrow end of the fiber segment. At the narrow end of the fiber segment, the central portion 101 can have a radius a, as illustrated in Figure 1 . The annular core 102 can have an inner radius a and an outer radius b. The first cladding 103 can have an inner radius b and an outer radius c. The second cladding 104 can have an inner radius c and an outer radius d.

[0032] The annular core 102 can be configured to receive an optical signal, for example at the first portion of the fiber segment. In other words, the optical signal can be launched at the annular core 102 at the narrow end of the active optical fiber segment 100.

[0033] The first portion of the active optical fiber segment 100 can be configured to satisfy the following condition: 2a < λ s , and 2πbNA / λ s < 2.405, where a is the inner radius of the annular core 102, b is the outer radius of the annular core 102, λ s is the wavelength of the optical signal, and NA is the numerical aperture of the annular core. The numerical aperture NA can be defined based on the square root of the difference of the square of the second refractive index and the square of the third refractive index, for example Satisfying the above conditions enables the fundamental mode to propagate in the first portion of the active fiber segment 100. The length of the first portion (SM) can be determined based on satisfying both conditions simultaneously. The length of the first portion can be, for example, 0.1 m to 1 m. The length of the first portion can depend on the longitudinal shape of the tapered fiber.

[0034] The active fiber segment 100 can further include a second portion configured to support multi-mode (MM) operation. In the second portion, the above two conditions can not be satisfied, resulting in the second portion not being limited to single-mode operation and allowing several modes to propagate in the annular core 102. The second portion can include the wide end of the active fiber segment 100. At the wide end of the fiber, the diameter of the central portion 101, the thickness of the annular core 102, the thickness of the first cladding 103, and / or the thickness of the second cladding 104 can be higher than the corresponding measurements at the narrow end.

[0035] The first cladding 103 can be configured to receive the pump radiation at the first portion (e.g., the narrow end) of the active fiber segment 100. In addition, the second portion (e.g., the wide end) of the active fiber segment 100 can be configured to receive the pump radiation. Thus, the pump radiation can be launched into the first cladding 103 at one or both ends of the first cladding 103. According to exemplary embodiments, the power of the pump radiation launched at the second portion can be higher than the power of the pump radiation launched at the first portion. The wavelength λ p must be shorter than the wavelength λ s of the optical signal. The active fiber segment 100 can be used in various applications by itself or in combination with other similar or other types of fiber segment(s).

[0036] Figure 2 Examples of truncations of the first cladding of a double-clad active tapered vortex fiber according to exemplary embodiments are shown. The truncations of the first cladding 103 can be applied to improve the absorption of the pump radiation. The first cladding 103 can be truncated, for example, 1, 2, 3, 4, 5, 6, 7, or 8 times to form different radial cross-sections, such as Figure 2 a- Figure 2 shown in FIG. h. For example, one truncation can result in a D-shaped radial cross-section Figure 2 a). Five truncations can result in a pentagonal radial cross-section Figure 2 e), and eight truncations can result in an octagonal radial cross-section Figure 2 h). The truncations enable more pump radiation to pass through the annular core 102, thereby improving the amplification of the optical signal propagating in the annular core 102.

[0037] Figure 3Examples of field distribution of optical signals propagating in a double- cladding active tapered vortex fiber segment according to exemplary embodiments are shown. Experimental evidence shows that when the conditions for single mode operation are met at the first section, the fundamental mode field with a Gaussian field distribution 301 (M 2 ~1) propagates in the first section (single mode section) of the annular core 102 with an outer diameter of 2b. In the second section (multi-mode section), these conditions are no longer met and multiple mode fields can be supported in the annular core 102. Without strong mechanical perturbations that induce mode field coupling, the fundamental mode field can also propagate in the annular core 102 at the second section (M 2 ~1). The mode field can have an annular shape 302 in the near field region at the wide end of the fiber segment and a Gaussian shape 301 in the far field at the narrow end of the fiber segment. For example, for a passive tapered fiber with a 120 pm annular core, a diffraction-limited beam with sufficient quality (M 2 ~1.2) was experimentally obtained.

[0038] The low birefringence (B < 10 -5 ) of the annular core 102 can reduce the polarization state distortion during the propagation and amplification of the optical signal in the active tapered fiber segment 100. According to exemplary embodiments, manufacturing the active fiber segment 100 can include rotating the fiber preform during the drawing of the active fiber segment 100. The rotation can be used to obtain a spun fiber. The preform can be rotated, for example, at an angular velocity in the range of 300-1000 rpm. The resulting fiber pitch can be in the range of 2-15 mm at the wide part (second section) of the tapered fiber segment. The pitch of the fiber can refer to the length of time for a rotation of 360°, for example, of a spun fiber.

[0039] The intrinsic residual birefringence in a fiber is mainly determined by the geometrical imperfections of its core, such as ellipticity, frozen mechanical stresses, or local fiber bends. The local eigenstate of such birefringence is usually linear polarization. When the fiber preform is rotated during the drawing process, the geometrical anisotropy axis changes its angular position many times and the time spent by each polarization component propagating in the form of fast and slow waves is approximately equal. Therefore, the intrinsic birefringence of such a spun fiber can be very small (B < 10 -5 ). The intrinsic polarization eigenstate can be close to circular polarization. Therefore, the polarization of the light propagating through such a fiber can be only slightly perturbed. As a result, such a fiber can preferably not acquire any polarization and, therefore, reduce the distortion of the polarization during amplification. This is beneficial for the amplification of waves with OAM.

[0040] Low birefringence can also be alternatively obtained. One way to obtain low intrinsic birefringence is to make the fiber as close to ideal as possible, for example, by making the fiber substantially symmetric under low levels of internal stress. Another way to obtain low intrinsic birefringence is to apply a compensating fiber. For example, by choosing the fiber dopant material such that the stress birefringence (B s ) adds to the geometric birefringence (B c ) to zero, a low level of internal birefringence can be achieved.

[0041] One solution for amplifying and generating OAM beams is to use step-index LMA active fibers. For example, a PANDA (polarization maintaining and absorption reducing) type LMA fiber with a 25 pm core can be applied. However, this approach can not be optimal for beam formation. For example, the fast and slow waves of a high birefringence PANDA fiber can differ greatly in propagation constant, and in order to form a ring-shaped OAM beam, the active fiber can need to be bent in an unpredictable manner. A high birefringence LMA fiber can also be highly sensitive to temperature. When the pump radiation is absorbed, such a fiber can heat up, causing a change in the polarization state at the fiber output.

[0042] Therefore, another solution is to use a regular isotropic active LMA fiber to amplify beams with OAM. This can simplify the formation of the OAM mode field. However, when using an isotropic active fiber, the presence of random birefringence can cause depolarization of the radiation, and the presence of random birefringence can in turn be caused by fiber bending. The respective disadvantages of a strong birefringence with a step profile and a regular LMA fiber can be that the integral of the profile of the refractive index (distribution of the dopant) is roughly the same for all supported mode fields. Therefore, the amplification of the optical signal is not optimal with respect to the mode field contrast, because the undesired mode fields that exist in the low mode fiber will also experience amplification. This can limit the power of the OAM beam. This can also deteriorate the quality of the OAM beam. Therefore, exemplary embodiments of the present disclosure provide an active tapered fiber with a ring-shaped core that has a significant amplification only for a ring-shaped mode field (e.g., a mode field with OAM). The disclosed active fiber segment can be referred to as a SPUN ring core tapered double-clad fiber (SPUN rcT-DCF).

[0043] Exemplary embodiments of the present disclosure provide at least the following benefits:

[0044] 1) Due to the annular doped core, the SPUN rcT-DCF has a spatially selective amplification of the annular mode field. The SPUN rcT-DCF effectively only acquires mode fields with substantial overlap integral, i.e. the annular mode field. This provides a good modal contrast, enabling excitation and amplification for OAM beams only or predominantly. In practice, mode fields with maximum field in the central portion 101 can not provide gain, and thus, for example, over-amplification of the fundamental mode field can be avoided.

[0045] 2) The SPUN rcT-DCF has a large mode field diameter (MFD), e.g. at least 120 pm in the wide portion, while maintaining high brightness (close to the diffraction-limited beam quality) of the amplified light. This enables high average and peak power.

[0046] 3) The SPUN rcT-DCF has a low intrinsic birefringence (<10 -5 ), and thus, it does not practically perturb the polarization state of the amplified light. Therefore, amplification and propagation of OAM beams occur with minimal distortion. Due to the low birefringence, the SPUN rcT-DCF is also not sensitive to heating caused by pump absorption. Furthermore, the SPUN rcT-DCF is not sensitive to environmental influences such as vibrations and temperature changes.

[0047] Even though not shown in Figure 1 , the active fiber segment 100 can also include additional structures such as, for example, one or more claddings radially surrounding the cladding(s). The cladding(s) can for example comprise a polymer cladding. The cladding(s) can be configured to reduce environmental influences that can cause the introduction of external birefringence at the annular core 102 having a low intrinsic birefringence. Thus, the low intrinsic birefringence coupled together with the one or more claddings provides an active optical vortex fiber that provides stable but selective propagation and amplification of mode fields with OAM under varying (internal / external) temperature and other environmental influences such as mechanical bending. In the above exemplary embodiment, the pump radiation can be configured to propagate in the first cladding 103 in the same or substantially the same direction as the optical signal and / or in the opposite or substantially opposite direction as the optical signal.

[0048] Figure 4 An example of a master oscillator power amplifier (MOPA) comprising a double-clad active vortex fiber according to an exemplary embodiment is shown. The MOPA 400 can comprise a light source 401 such as, for example, a seed laser (master oscillator) configured to generate an optical signal, e.g. a seed laser beam. The optical signal can have a wavelength l sThe MOPA 400 can further include an optical converter 402 configured to impart or introduce orbital angular momentum to the optical signal. The optical (beam) converter 402 can include a free-space bulk optical scheme to form a beam having OAM based on the seed laser beam. The optical converter 402 can for example include an S-plate or a Q-plate. The MOPA 400 can further include the active fiber segment 100.

[0049] The MOPA 400 can further include a first dichroic mirror 403. The first dichroic mirror 403 can be configured to couple a first portion of the pump radiation from the first pump radiation source 404 to the first portion of the active fiber segment 100, for example at the first cladding 103 at the narrow end of the fiber segment. The first dichroic mirror 403 can be further configured to couple the optical signal from the optical converter 402 to the first portion of the active fiber segment 100, for example at the ring core 102 at the narrow end of the fiber segment.

[0050] The MOPA 400 can further include a second dichroic mirror 405. The second dichroic mirror 405 can be configured to couple a second portion of the pump radiation from the second pump radiation source 406 to the second portion of the active fiber, for example at the first cladding 103 at the wide end of the fiber segment. The second dichroic mirror 405 can be further configured to provide an output optical signal from the second portion of the active fiber segment. The second dichroic mirror 405 can for example be configured to pass the optical signal received from the ring core 102 at the wide end of the fiber 100 to an output of the MOPA 400. The output optical signal can include an amplified version of the optical signal in which the OAM mode field has been selectively amplified. The MOPA 400 can include at least one of the pump radiation sources 404, 406. Alternatively, the MOPA 400 can be configured to be coupled to an external pump radiation source(s). Thus, the MOPA 400 can not include the pump radiation sources 404, 406.

[0051] Figure 5 An example of a laser including a double-clad active vortex fiber is shown in accordance with an example embodiment. The laser 500 can be configured to generate an OAM beam intra-cavity, the cavity being formed by at least two mirrors and a SPUN rcT-DCF. The laser 500 can include a first mirror 501. The first mirror 501 can be highly reflective, for example substantially 100% reflective. The first mirror 501 can be optically connected to an optical converter 502, which can be similar to the optical converter 402. The first mirror 501 can be configured to reflect an optical signal. For example, the first mirror 501 can be configured to reflect an optical signal received from the optical converter 502 back to the optical converter 502. The optical signal can have a wavelength λ sThe laser 500 can further include the active fiber segment 100.

[0052] The laser 500 can further include a second mirror 505. The second mirror 505 can be optically connected and / or physically coupled to a second portion of the active fiber segment 100, such as a wide end of the fiber segment. For example, the second mirror 505 can be deposited at a wide end face of the SPUNrcT-DCF. The second mirror 505 can be partially transparent such that a portion of the optical signal exiting the annular core 102 of the fiber segment can pass through the second reflective mirror 505. Thus, the second mirror 505 can be configured to partially reflect the optical signal. Accordingly, the reflectivity of the first mirror 501 can be higher than the reflectivity of the second mirror 505.

[0053] The laser 500 can further include a first dichroic mirror 503. The first dichroic mirror 503 can be configured to couple a first portion of the pump radiation from the first pump radiation source 504 to a first portion of the active fiber segment 100, such as at the first cladding 103 at the narrow end of the fiber segment. The first dichroic mirror 503 can be further configured to couple the optical signal from the optical converter 502 to the first portion of the active fiber segment 100, such as at the annular core 102 at the narrow end of the fiber segment. In addition, the first dichroic mirror 503 can be configured to couple the optical signal from the first portion of the active fiber segment 100, such as from the annular core 102 at the narrow end of the fiber segment, to the optical converter 502.

[0054] The laser 500 can further include a second dichroic mirror 507. The second dichroic mirror 507 can be configured to couple a second portion of the pump radiation from the second pump radiation source 506 to a second portion of the active fiber segment 100, such as at the first cladding 103 at the wide end of the fiber segment. The second dichroic mirror 507 can be further configured to provide the output optical signal from the second portion of the active fiber segment through the second mirror 505. The second dichroic mirror 507 can be, for example, configured to pass the optical signal that exits the annular core 102 and propagates through the second mirror 505 at the wide end of the fiber 100 to an output of the laser 500. The laser 500 is capable of generating a laser beam having a high power OAM mode field.

[0055] Figure 6 Another example of a master oscillator power amplifier (MOPA) including a double- cladding active vortex fiber according to an example embodiment is shown. The MOPA 600 can be similar to the MOPA 400. However, in this example embodiment, the OAM converter 603 can be integrated within the active fiber segment 100.

[0056] MOPA 600 can include a light source 601, such as a seed laser, similar to light source 401. MOPA 600 can also include a first dichroic mirror 602 configured to couple a first portion of pump radiation from a first single-pump radiation source 604 to a first portion of the active fiber segment 100, such as to the first cladding 103 at the narrow end of the fiber segment. The first dichroic mirror 602 can also be configured to couple the optical signal from the light source 601 to the first portion of the active fiber segment 100, such as to the annular core 102 at the narrow end of the fiber segment. Similar to MOPA 400, the pump radiation sources 604, 606 can be internal or external to the MOPA 600.

[0057] The optical converter 603 can include an all-fiber device, such as, for example, a mechanical device for controlled mode field coupling, a device for providing a controlled bend of the optical fiber, or an acousto-optic device. The optical converter 603 can be integrated within the second (multimode) portion of the active fiber segment 100. The optical converter 603 can be located in proximity to the first (single mode) portion of the fiber segment 100. Similar to the optical converter 402, the optical converter 603 can be configured to impart orbital angular momentum (OAM) to the optical signal. The optical converter 603 can be located at the beginning of the multimode portion, where the optical fiber has already satisfied two conditions, 2a < λ s and 2πbNA / λ s <2.405, and thus supports several modes of the mode field that can form a mode field with OAM.

[0058] The MOPA 600 can also include a second dichroic mirror 605. The second dichroic mirror 605 can be configured to couple a second portion of pump radiation from a second pump radiation source 606 to the second portion of the active fiber segment 100. Similar to the dichroic mirror 405, the second dichroic mirror 605 can also be configured to provide an output optical signal from the second portion of the active fiber segment. The MOPA 600 is capable of producing a powerful OAM beam between the light source 601 and the active fiber segment 100 without the need for an external optical converter.

[0059] Figure 7 Another example of a laser including a double-clad active vortex fiber is shown in accordance with an example embodiment. The laser 700 can be similar to the laser 500. However, in this example embodiment, the OAM converter 703 can be integrated within the active fiber segment 100.

[0060] Laser 700 can include a first mirror 701 similar to mirror 501. Similar to mirror 501, first mirror 701 can be optically connected to a first portion of active fiber segment 100 and configured to reflect an optical signal. Laser 700 can also include a second mirror 705 similar to mirror 505. Reflectivity of first mirror 701 can be higher than reflectivity of second mirror 705. Similar to mirror 505, second mirror 707 can be optically connected to a second portion of active fiber segment 100 and configured to reflect an optical signal.

[0061] Laser 700 can also include a first dichroic mirror 702 configured to couple a first portion of pump radiation from a first pump radiation source 704 to a first portion of active fiber segment 100, for example to first cladding 103 at a narrow end of fiber segment. First dichroic mirror 702 can also be configured to couple an optical signal from the first portion of active fiber segment 100 to first mirror 701, and to couple a reflected optical signal from first mirror 702 to the first portion of active fiber segment. First dichroic mirror 702 can be configured to receive an optical signal from annular core 102 of fiber segment. First dichroic mirror 702 can be configured to couple a reflected optical signal to annular core 102 of fiber segment.

[0062] Laser 700 can also include an optical converter 703 similar to optical converter 603 and integrated in active fiber segment 100 in a similar manner. Laser 700 can also include a second dichroic mirror 707, which can be similar to and have similar functionality as dichroic mirror 507. For example, dichroic mirror 507 can be configured to couple pump radiation from a second pump radiation source 706 to active fiber segment 100 and provide an output from laser 700. Laser 700 is capable of generating a laser beam with a high-power OAM mode field between first mirror 701 and active fiber segment 100 without the need for an external optical converter.

[0063] Exemplary embodiments disclosed herein provide an active fiber and a device suitable for generation and amplification of an OAM mode field of an optical signal.

[0064] Any ranges or apparatuses given herein can be extended to include any and all sub-ranges thereof, and other equipment thereof, unless otherwise indicated. Furthermore, any embodiments given herein can be combined with one another, unless otherwise indicated.

[0065] Although the subject matter has been described in language specific to structural features and / or methodological acts, it is to be 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 implementations of implementing the claims and other equivalent features and acts are intended to be within the scope of the claims.

[0066] It should be appreciated that the above benefits and advantages can relate to one embodiment or can relate to several embodiments. Embodiments are not limited to those that solve any or all of the problems described above or that have any or all of the benefits and advantages described above. It should also be appreciated that a reference to“a” item can refer to one or more of those items.

[0067] The term“comprising” is used herein to mean including the recited blocks or elements, but not excluding other blocks or elements. Thus, a device can include additional blocks or elements.

[0068] Although the subject matter can be described as“first” or“second” subject matter, this does not necessarily indicate any order or importance of the subject matter. Rather, such attributes can merely be used to distinguish one subject matter from another.

[0069] It should be understood that the foregoing description is only illustrative of the example embodiments and that various modifications can be made by those skilled in the art without departing from the scope of the disclosure. The foregoing description, examples, and data provide exemplary embodiments for understanding the claims. Although various embodiments have been described above with a certain level of particularity, it is to be understood that the above description is merely an example and that many changes can be made to the embodiments described without departing from the scope of the disclosure.

Claims

1. An apparatus comprising: An active optical fiber segment, the active optical fiber segment comprising: The central portion has a first refractive index n1, wherein the diameter of the central portion gradually varies along the length of the active fiber segment forming a tapered longitudinal profile. A ring-shaped core, radially surrounding the central portion, is doped with at least one rare earth element and has a second refractive index n2, wherein n2 > n1, and wherein the birefringence of the ring-shaped core is less than 10. -5 ; A first cladding layer, radially surrounding the annular core, and having a third refractive index n3, where n3 < n2; and A second cladding, radially surrounding the first cladding and having a fourth refractive index n4, where n4 < n3, wherein a first portion of the active fiber segment is configured to support single-mode operation of the optical signal, and a second portion of the active fiber segment is configured to support multimode operation of the optical signal, the device further comprising: A dichroic mirror is used to provide the output optical signal from the ring core of the second section of the active optical fiber to the output of the device. The first part includes the narrow end of the active optical fiber segment, and the second part includes the wide end of the active optical fiber segment.

2. The apparatus of claim 1, wherein the first portion of the active optical fiber segment is configured to satisfy the following condition: 2a<λ s ,as well as 2πbNA / l s <2.405, Where a is the inner radius of the ring core, b is the outer radius of the ring core, and λ s NA is the wavelength of the optical signal, and NA is the numerical aperture of the ring core, where NA = .

3. The device according to claim 1, wherein the thickness of at least one of the annular core, the first cladding, or the second cladding gradually varies along the longitudinal profile of the tapered shape.

4. The apparatus according to any one of claims 1-3, wherein the tapered longitudinal profile comprises a parabolic convex profile.

5. The apparatus according to any one of claims 1-3, wherein the central portion is substantially radially symmetrical.

6. The apparatus according to any one of claims 1-3, wherein the apparatus is obtainable by rotating the fiber preform during stretching of the active fiber segment.

7. The apparatus according to claim 6, wherein at the second portion of the active fiber segment, the pitch of the active fiber segment is in the range of 2-15 mm.

8. The apparatus of claim 6, wherein the rotational angular velocity during stretching of the active fiber segment is in the range of 300-1000 rpm.

9. The apparatus according to any one of claims 1-3, wherein the ring core is configured to receive an optical signal at a first portion of the active optical fiber segment.

10. The apparatus of claim 1, wherein the first cladding is configured to receive pump radiation at a first portion of the active fiber segment and / or at a second portion of the active fiber segment.

11. The apparatus of claim 1, wherein the wavelength λ of the pump radiation p Shorter than the wavelength λ of the optical signal s .

12. The apparatus according to claim 10 or claim 11, further comprising: The light source is configured to generate light signals; An optical converter is configured to generate an optical signal with orbital angular momentum; A first dichroic mirror is configured to couple a first portion of pump radiation from a first pump radiation source and an optical signal from an optical converter to a first portion of an active fiber segment, wherein the dichroic mirror is configured to couple a second portion of pump radiation from a second pump radiation source to a second portion of the active fiber segment and provide an output optical signal from the second portion of the active fiber segment.

13. The apparatus according to claim 10 or claim 11, further comprising: A first reflecting mirror is configured to reflect a light signal, wherein the first reflecting mirror is optically connected to an optical transducer configured to generate a light signal having orbital angular momentum; The second reflector is optically connected to the second part of the active fiber segment, wherein the reflectivity of the first reflector is higher than that of the second reflector. A first dichroic mirror is configured to couple a first portion of pump radiation from a first pump radiation source and an optical signal from an optical converter to a first portion of an active fiber segment, wherein the dichroic mirror is configured to couple a second portion of pump radiation from a second pump radiation source to a second portion of the active fiber segment and to provide an output optical signal from the second portion of the active fiber segment through a second reflector.

14. The apparatus of claim 12, wherein the optical converter comprises an S-plate or a Q-plate.

15. The apparatus according to claim 10 or claim 11, further comprising: The light source is configured to generate light signals; The first dichroic mirror is configured to couple a first portion of the pump radiation from the first pump radiation source and the optical signal from the light source to a first portion of the active optical fiber segment. An optical transducer, integrated within a second portion of an active fiber segment near a first portion of the fiber segment, wherein the optical transducer is configured to generate orbital angular momentum (OAM) for an optical signal, wherein the dichroic mirror is configured to couple a second portion of pump radiation from a second pump radiation source to the second portion of the active fiber and to provide an output optical signal from the second portion of the active fiber segment.

16. The apparatus according to claim 10 or claim 11, further comprising: The first reflector is optically connected to the first part of the active fiber segment and is configured to reflect light signals; The second reflector is optically connected to the second part of the active fiber segment and is configured to reflect light signals, wherein the reflectivity of the first reflector is higher than that of the second reflector. The first dichroic mirror is configured to couple a first portion of the pump radiation from the first pump radiation source to a first portion of the active fiber segment. An optical transducer, integrated within a second portion of an active fiber segment near a first portion of the fiber segment, wherein the optical transducer is configured to generate orbital angular momentum (OAM) for an optical signal, wherein the dichroic mirror is configured to couple a second portion of pump radiation from a second pump radiation source to the second portion of the active fiber and to provide an output optical signal from the second portion of the active fiber segment via a second reflector.

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