An asymmetrically core-partially doped active optical fiber and its manufacturing method

Abstract

The present invention relates to the field of optical fiber technology, and in particular to an active optical fiber with an asymmetric core partially doped and a preparation method thereof, which sequentially includes from inside to outside: a core and a cladding; the core includes a rare earth ion doped region and a region without doped rare earth ions, the rare earth ion doped region has an eccentric structure with respect to the core, and the offset direction of the rare earth ion doped region is the same as the bending direction of the active optical fiber. The core diameter is 20 - 150 μm, the offset amount of the rare earth ion doped region from the center point of the core to the bending direction is 0 - 50 μm, and the range of the numerical aperture of the optical fiber is: 0.05 - 0.2. By designing an asymmetric (eccentric) core partially doped optical fiber, the present invention can take into account the mode distortion caused by bending, so as to ensure that when the optical fiber is bent, both the gain of the fundamental mode is maintained and the gain of the high-order mode can be effectively reduced, thereby improving the beam quality of the optical fiber output and effectively suppressing the occurrence of transverse mode instability.

Description

Technical Field

[0001] The present invention relates to the technical field of optical fibers, and particularly to an active optical fiber with an asymmetric core partially doped and a preparation method thereof. Background Art

[0002] In a conventional active optical fiber, the core is entirely and uniformly doped. At this time, the overlapping degrees of the various modes transmitted in the optical fiber with the doped region are similar, so the gains of the various modes are similar, which results in more modes being excited and obtaining gain in the optical fiber, thereby degrading the beam quality of the output laser and making the optical fiber laser prone to transverse mode instability.

[0003] Since the core of the core-partially doped active optical fiber is partially doped, the overlapping degrees of the various modes transmitted in the optical fiber with the doped region will all decrease. Since the energy distribution of the fundamental mode is mainly concentrated in the central part, while the intensity distributions of most high-order modes are in the periphery, using core partial doping will cause the overlapping degree of most high-order modes with the doped region to decrease more significantly compared with the fundamental mode, so that the gain of the fundamental mode becomes larger than that of the high-order modes, and finally the beam quality of the output laser will be improved, and the occurrence of transverse mode instability can be effectively suppressed.

[0004] In the existing technology, ordinary active optical fibers do not consider controlling the gain of high-order modes, so ordinary active optical fibers are more prone to transverse mode instability and have poorer beam quality compared with symmetric core-partially doped optical fibers. Although symmetric core-partially doped optical fibers consider controlling the gain of high-order modes, they do not consider the fact that the active optical fiber is in a bent state during use, and each mode will be distorted under the bent state. Therefore, although symmetrically partially doped optical fibers can improve transverse mode instability, they do not achieve the best effect. For this reason, we propose an active optical fiber with an asymmetric core partially doped and a preparation method thereof. Summary of the Invention

[0005] Based on the technical problems existing in the background art, the present invention proposes an active optical fiber with an asymmetric core partially doped and a preparation method thereof. By designing an asymmetric (eccentric) core-partially doped optical fiber, the mode distortion caused by bending can be taken into account, so as to ensure that when the optical fiber is bent, both the gain of the fundamental mode is maintained and the gain of high-order modes can be effectively reduced, thereby improving the beam quality of the optical fiber output and effectively suppressing the occurrence of transverse mode instability.

[0006] The present invention provides the following technical solution: An active optical fiber with an asymmetric core partially doped, which sequentially includes from the inside to the outside: a core and a cladding;

[0007] The core includes a rare-earth ion doped region and an undoped rare-earth ion region. The rare-earth ion doped region has an eccentric structure with respect to the core, and the offset direction of the rare-earth ion doped region is consistent with the bending direction of the active optical fiber. The rare-earth ion is one or more of ytterbium, erbium, thulium, holmium, and neodymium.

[0008] Preferably, the core diameter is 20 - 150 μm.

[0009] Preferably, the offset of the rare-earth ion doped region from the center point of the core towards the bending direction is 0 - 75 μm.

[0010] Preferably, the numerical aperture of the optical fiber ranges from 0.05 to 0.2.

[0011] Preferably, the rare-earth ion doped region has a uniform refractive index distribution inside and maintains the same refractive index as the region of the core where rare-earth ions are not doped. The doping of the rare-earth ion doped region is step-type or graded.

[0012] Preferably, the offset of the rare-earth ion doped region from the center point of the core towards the bending direction is inversely proportional to the bending radius of the active optical fiber.

[0013] A method for preparing an asymmetric core partially doped active optical fiber includes the following steps: First, use the MCVD method to prepare a symmetric core partially doped optical fiber. The refractive index of the undoped region of the core is controlled to match the refractive index of the doped region of the core by doping one or more of Al, P, and Ge. Then, use the MCVD method to prepare a preform of an optical fiber without doped rare-earth ions with the same refractive index as the undoped region. Use a combination of mechanical processing and acid treatment to process the preform of the core partially doped optical fiber to take out the core rod with the undoped region, and then use a combination of mechanical processing and acid treatment to process the preform of the optical fiber without doped rare-earth ions to obtain a sleeve. Finally, use the tube-in-rod method to prepare the final optical fiber.

[0014] Preferably, before drawing the optical fiber, it is necessary to mark the asymmetric position on the preform for positioning before loading it onto the drawing tower.

[0015] Preferably, the winding direction of the finally prepared optical fiber is consistent with the direction of asymmetric doping, where the direction of asymmetric doping refers to the offset direction of the rare-earth ion doped region.

[0016] An optical fiber laser including the above-mentioned asymmetric core partially doped active optical fiber.

[0017] An optical fiber amplifier including the above-mentioned asymmetric core partially doped active optical fiber.

[0018] The present invention provides an asymmetric core partially doped active optical fiber and its preparation method, with the following effects:

[0019] 1. By adopting an asymmetrically partially doped fiber core structure, the situation where the optical fiber is bent during the construction of an actual fiber laser is considered. The eccentric asymmetrically doped fiber core takes into account the mode distortion under fiber bending, so that the fundamental mode content in the output laser can be effectively increased under the condition of fiber bending.

[0020] 2. By adopting a step-by-step method for preparing an asymmetrically partially doped fiber core, and ensuring that the marked direction of the preform asymmetry is consistent with the wire drawing direction, it can be guaranteed that during the use of this optical fiber, the bending direction is consistent with the doping offset direction, making the asymmetric optical fiber more convenient and accurate to use. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 It is a schematic diagram of the asymmetrically partially doped fiber core structure of the present invention;

[0022] Figure 2 It is a flowchart of the preparation of the fiber with an asymmetrically doped fiber core of the present invention;

[0023] Figure 3 It is a bar chart showing the gain of each mode in the embodiment of the present invention; Figure 1 ;

[0024] Figure 4 It is a bar chart showing the gain of each mode in the embodiment of the present invention; Figure 2 ;

[0025] Figure 5 It is a curve of the bending radius and the optimal offset in the embodiment of the present invention; Figure 1 ;

[0026] Figure 6 It is a curve of the bending radius and the optimal offset in the embodiment of the present invention; Figure 2 ;

[0027] Figure 7 It is a curve of the bending radius and the optimal offset in the embodiment of the present invention; Figure 3 . DETAILED DESCRIPTION OF THE EMBODIMENTS

[0028] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0029] As Figure 1 shown, the present invention provides a technical solution: an active optical fiber with an asymmetrically partially doped fiber core, which sequentially includes from the inside out: a fiber core and a cladding;

[0030] The core includes a rare-earth ion doped region and a region without rare-earth ion doping. The rare-earth ion doped region has an eccentric structure with the core, and the offset direction of the rare-earth ion doped region is consistent with the bending direction of the active optical fiber. The rare-earth ion is one or more of ytterbium, erbium, thulium, holmium, and neodymium.

[0031] The core diameter is 20 - 150 μm; the diameter of the rare-earth ion doped region is less than the core diameter and greater than 0; the offset of the core doped region is less than 75 μm and greater than 0; the range of the numerical aperture of the optical fiber is 0.05 - 0.2. The offset of the rare-earth ion doped region from the center point of the core in the bending direction is inversely proportional to the bending radius of the active optical fiber. The refractive index distribution inside the rare-earth ion doped region is uniform and the same as that of the region in the core without rare-earth ion doping. The doping of the rare-earth ion doped region is step-type or graded.

[0032] First, use the MCVD method to prepare a symmetrically core-partially doped optical fiber. The refractive index of the undoped region in the core is controlled to match that of the rare-earth ion doped region in the core by doping one or more of Al, P, and Ge. Then, use the MCVD method to prepare an undoped optical fiber preform with the same refractive index as the undoped region. Use a combination of mechanical processing and acid treatment to process the core-partially doped optical fiber preform to take out the core rod with the undoped region. Then, use a combination of mechanical processing and acid treatment to process the optical fiber preform without rare-earth ion doping to obtain a sleeve. Finally, use the tube-rod method to prepare the final optical fiber. Since the optical fiber has an asymmetric structure, it is necessary to mark the asymmetric position on the preform before drawing the optical fiber for positioning before putting it on the wire drawing tower. The wire winding direction of the finally prepared optical fiber is consistent with the direction of asymmetric rare-earth ion doping, where the direction of asymmetric doping refers to the offset direction of the rare-earth ion doped region.

[0033] An optical fiber laser including the above-mentioned active optical fiber with an asymmetric core partially doped.

[0034] An optical fiber amplifier including the above-mentioned active optical fiber with an asymmetric core partially doped.

[0035] Since fiber lasers need to be bent during actual use and, in order to save space, have a small bending radius. Fiber bending can cause mode distortion in the fiber, and the smaller the bending radius, the greater the mode distortion. For the core-partially doped fiber mentioned above, if the doped region is in the center, when the fiber is bent, the gain effect of the fundamental mode will decrease, which is harmful to the generation of laser and the beam quality. The core-asymmetric partial doping proposed in this solution can effectively solve this problem. By designing an asymmetric core-partially doped fiber, the mode distortion caused by bending can be taken into account, so as to ensure that when the fiber is bent, both the gain of the fundamental mode is maintained and the gain of the high-order mode can be effectively reduced, thereby improving the beam quality of the fiber output and effectively suppressing the occurrence of transverse mode instability.

[0036] The preparation method of the above active fiber is as follows:

[0037] As Figure 2 shown, generally speaking, the chemical vapor deposition method (MCVD) can only prepare fibers with a symmetric structure. Therefore, this technology proposes a two-step method to prepare the above fibers. First, use the MCVD method to prepare a symmetric core-partially doped fiber. The refractive index of the undoped region of the core is mainly controlled by doping one or more of Al, P, and Ge to match the refractive index of the core doped with rare earth ions. Then, use the MCVD method to prepare an undoped fiber preform with the same refractive index as the undoped region. Use a combination of mechanical processing and acid treatment to process the core-partially doped fiber preform to take out the core rod with the undoped rare earth ion region. Finally, use the tube-rod method to prepare the final fiber. In addition, since the fiber has an asymmetric structure, it is necessary to mark the asymmetric position on the preform before drawing the fiber for positioning before putting it on the wire drawing tower. The winding direction of the finally prepared fiber and the direction of asymmetric rare earth ion doping need to be the same to ensure the matching of fiber design and actual application.

[0038] The relevant results were calculated for the above asymmetric core-partially doped fiber structure:

[0039] In order to suppress the transverse mode instability effect in the fiber, the asymmetric core-partially doped fiber structure was calculated. Figure 5 The offset of the fundamental mode of an active fiber with a ratio of 30 / 250 (numerical aperture (NA) = 0.09) at different bending radii. As Figure 3 can be seen, as the bending radius decreases, the offset of the fundamental mode increases. Therefore, in the process of designing the fiber, the offset of the partially doped region needs to be determined according to the fiber structure and the bending radius of the fiber.

[0040] Figure 3 and 4For the symmetric and asymmetric core - part doping structures, when the fiber bending radius is 8 cm, the overlap factors of each mode and the doping region are shown. The overlap factor is used to characterize the gain of each mode. From Figure 3 it can be seen that when the fiber bending radius is 8 cm, the overlap factor between the fundamental mode (mode number 1) of the fiber with an asymmetric core - part doping structure and the doping region increases significantly, while the change in the overlap factors of other higher - order modes is not obvious. Then, the fiber with an asymmetric core - part doping can effectively increase the content of the fundamental mode in the finally output laser, thereby increasing the threshold of transverse - mode instability. From Figure 4 it can be seen that for the same fundamental - mode gain, the rare - earth - ion - doped region with asymmetric doping slightly decreases. When maintaining the same fundamental - mode gain, the gains of all higher - order modes in the asymmetric - doping structure decrease. Therefore, the asymmetric - doping structure can effectively reduce and suppress the occurrence of transverse - mode instability compared with the symmetric structure under bending.

[0041] In summary, using a fiber with an asymmetric core - part doping can effectively increase the content of the fundamental mode in the finally output laser, reduce the content of higher - order modes, improve the beam quality of the laser, increase the transverse - mode instability threshold of the laser, and better meet the usage conditions of the fiber in the fiber laser.

[0042] Offset corresponding to the core region under bending (embodiments with different core diameters)

[0043] As Figure 5 shown, the optimal offsets corresponding to different bending radii of a fiber with a core diameter of 25 μm and a numerical aperture of 0.065. When the fiber bending radius is 5 μm, the optimal offset is 4.7 μm; when the bending radius is 6 μm, the optimal offset is 4.3 μm; when the bending radius is 8 μm, the optimal offset is 3.6 μm; when the bending radius is 10 μm, the optimal offset is 2.9 μm; when the bending radius is 12 μm, the optimal offset is 2.3 μm; when the bending radius is 14 μm, the optimal offset is 1.9 μm; when the bending radius is 15 μm, the optimal offset is 1.7 μm. It can be seen that the offset is inversely proportional to the bending radius. The larger the bending radius, the smaller the offset. The offsets for different bending radii can refer to the curve in the figure.

[0044] As Figure 6As shown, the core diameter is 30 μm, the numerical aperture is 0.09, and the optimal offset corresponding to different bending radii. When the bending radius of this optical fiber is 5 μm, the optimal offset is 6.0 μm; when the bending radius is 6 μm, the optimal offset is 5.5 μm; when the bending radius is 8 μm, the optimal offset is 4.4 μm; when the bending radius is 10 μm, the optimal offset is 3.6 μm; when the bending radius is 12 μm, the optimal offset is 3.2 μm; when the bending radius is 14 μm, the optimal offset is 2.5 μm; when the bending radius is 15 μm, the optimal offset is 2.3 μm. The offsets for different bending radii can be referred to the curve in the figure.

[0045] As Figure 7 shown, the core diameter is 40 μm, the numerical aperture is 0.09, and the optimal offset corresponding to different bending radii. When the bending radius of this optical fiber is 5 μm, the optimal offset is 11.2 μm; when the bending radius is 6 μm, the optimal offset is 10.8 μm; when the bending radius is 8 μm, the optimal offset is 9.8 μm; when the bending radius is 10 μm, the optimal offset is 8.5 μm; when the bending radius is 12 μm, the optimal offset is 7.8 μm; when the bending radius is 14 μm, the optimal offset is 6.8 μm; when the bending radius is 15 μm, the optimal offset is 6.5 μm. From Figure 6 and 7 the comparison, it can be seen that the larger the core diameter, the larger the offset, and the offset is proportional to the core diameter. The offsets for different bending radii can be referred to the curve in the figure.

[0046] In the present invention, by designing an asymmetric (eccentric) core partial-doped optical fiber, the mode distortion caused by bending can be taken into account, so as to ensure that when the optical fiber is bent, both the gain of the fundamental mode is maintained and the gain of the high-order modes can be effectively reduced, thereby improving the beam quality of the optical fiber output and effectively suppressing the occurrence of transverse mode instability.

[0047] The above are only the preferred specific embodiments of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present invention, according to the technical solution and inventive concept of the present invention, makes equivalent substitutions or changes, and all should be covered within the protection scope of the present invention.

Claims

1. An active optical fiber with an asymmetrically doped core section, characterized in that: it sequentially includes from inside to outside: a core and a cladding; the core includes a rare earth ion doped region and an undoped rare earth ion region, the rare earth ion doped region has an eccentric structure with the core, and the offset direction of the rare earth ion doped region is consistent with the bending direction of the active optical fiber. The rare earth ion is one or more of ytterbium, erbium, thulium, holmium, and neodymium; the offset amount of the rare earth ion doped region from the center point of the core to the bending direction is inversely proportional to the bending radius of the active optical fiber; the core diameter is 20 - 150 μm; the offset amount of the rare earth ion doped region from the center point of the core to the bending direction is 0 - 75 μm.

2. The active optical fiber with an asymmetrically doped core section according to claim 1, characterized in that: the inside of the rare earth ion doped region has a uniform refractive index distribution and has the same refractive index as the region of the core where rare earth ions are not doped. The doping of the rare earth ion doped region is step - type or graded - type.

3. The active optical fiber with an asymmetrically doped core section according to any one of claims 1 - 2, characterized in that: the active optical fiber is used for fiber lasers and fiber amplifiers.

4. A method for preparing an active optical fiber with an asymmetrically doped core section as claimed in claim 1, characterized in that: it includes the following steps: First, use the MCVD method to prepare a symmetrically doped core section optical fiber. The refractive index of the region of the core where rare earth ions are not doped is controlled to match the refractive index of the region of the core doped with rare earth ions by doping one or more of Al, P, and Ge. Then, use the MCVD method to prepare a preform of an optical fiber without doped rare earth ions with the same refractive index as the undoped region of the core. Use a combination of mechanical processing and acid treatment to process the preform of the optical fiber with a partially doped core to take out the core rod with the undoped region. Then, use a combination of mechanical processing and acid treatment to process the preform of the optical fiber without doped rare earth ions to obtain a sleeve. Finally, use the tube - rod method to prepare the final optical fiber.

5. The method for preparing an active optical fiber with an asymmetrically doped core section according to claim 4, characterized in that: before drawing the optical fiber, it is necessary to mark the asymmetric position on the preform for positioning before putting it on the wire - drawing tower.

6. The method for preparing an active optical fiber with an asymmetrically doped core section according to claim 4, characterized in that: the wire - winding direction of the finally prepared optical fiber is consistent with the direction of asymmetric doping, where the direction of asymmetric doping refers to the offset direction of the rare earth ion doped region.

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