Single-mode high-nonlinearity photonic crystal fiber capable of maintaining circular polarization transmission and preparation method of single-mode high-nonlinearity photonic crystal fiber

Abstract

A single-mode high-nonlinearity photonic crystal fiber capable of maintaining circular polarization transmission comprises a solid fiber core and a porous cladding structure arranged periodically, and the nonlinearity of the fiber is improved by optimizing the size of the fiber core and the duty ratio of cladding air holes. The torsion rate alpha introduced in the axial direction of the optical fiber is 0-8 rad / mm to introduce 10 <-3 >-order circular birefringence to maintain the circular polarization state, and effective refractive index change curves in different modes are changed. A plurality of cladding air holes are selected on the cross section of the optical fiber for selective filling or doping to realize phase matching of a fiber core high-order mode and a plurality of cladding modes, so that the fiber core high-order mode with the refractive index changing under torsion is filtered out by means of intermode coupling, the loss of the filtered high-order mode is increased by more than two orders of magnitude, and the high-order mode loss is reduced by more than two orders of magnitude. And single-mode transmission is effectively realized. Meanwhile, the characteristics of circular polarization state transmission and high nonlinearity are kept, and the optical fiber can be applied to application research of high-precision optical fiber sensing, optical fiber lasers with special circular polarization state output and the like.

Description

Technical Field

[0001] The present invention belongs to the field of optics and laser optoelectronics technology, and specifically relates to a single-mode highly nonlinear photonic crystal optical fiber that maintains circular polarization state transmission and a preparation method thereof. Background Art

[0002] Photonic Crystal Fiber (PCF), also known as Micro-Structured Fibers or holey fiber, is a new type of optical fiber with a periodic air hole structure. Periodically arranged air holes are embedded in its cladding, thereby introducing a periodic refractive index distribution on the cross section of the optical fiber. Compared with traditional optical fibers, photonic crystal fibers have many advantages. For example, through the reasonable design of the optical fiber structure, non-stop single-mode transmission can be achieved, the signal quality and transmission rate of optoelectronic devices can be improved, and mode dispersion can be reduced. It is very important for scenarios that require high-quality and high-speed optical signal transmission. For example, the cladding structure of photonic crystal fibers can regulate the dispersion and nonlinear characteristics of optical fibers by changing the structural parameters such as the arrangement, shape, radius and spacing of air holes, which is crucial for the development of supercontinuum light sources or ultrafast lasers.

[0003] Polarization is a basic property of light waves, and the transmission of circularly polarized light has significant applications in the fields of fiber optic sensing and fiber optic communication. Twisted fiber is a good medium for the transmission of circularly polarized light, and its polarization state can be well maintained by this fiber. The twisting of the optical fiber is a special structural design. In the twisted fiber, the two polarization components of the light are coupled to each other due to the spiral twisting of the optical fiber structure along the axial direction. This coupling effect can offset the interference of external factors (such as bending of the optical fiber, temperature changes, etc.) on the polarization state. In some optical fiber sensors based on polarization characteristics, a stable polarization state is required to improve the accuracy and reliability of the sensor. For example, in optical fiber sensors that measure physical quantities such as magnetic fields, electric fields, temperature, and pressure, twisted optical fibers that maintain circular polarization can be used as sensitive elements to achieve high-precision measurement of physical quantities by detecting small changes in the polarization state.

[0004] In addition, the nonlinear coefficient of highly nonlinear photonic crystal fibers is much higher than that of conventional optical fibers, usually several orders of magnitude higher. Under low optical power conditions, the nonlinear effect in conventional optical fibers is weak, while highly nonlinear photonic crystal fibers can exhibit significant nonlinear effects even at lower optical powers, which gives them significant advantages in nonlinear optical applications. In order to enhance the nonlinear effect, it is usually necessary to increase the degree of restriction of the core to the light field. In the design of highly nonlinear photonic crystal fibers, in order to achieve high nonlinearity, the ratio of the air hole diameter d to the air hole spacing Λ d / Λ is required to be close to 1 or the core is small. A small core will increase the loss of fiber splicing in the all-fiber system or reduce the laser damage threshold, etc.; while non-cutoff single-mode transmission requires d / Λ<0.42. Therefore, a specially designed waveguide structure is required to achieve both high nonlinearity and single-mode transmission in twisted photonic crystal fibers. A common method to optimize single-mode transmission is to filter out high-order modes by constructing phase matching conditions for the core-clad mode. However, in twisted fibers that maintain circular polarization state transmission, the difference in effective refractive index of different polarization states of the same mode increases due to the twist effect, which increases the complexity of mode matching.

[0005] Therefore, how to achieve single-mode transmission and high nonlinear characteristics of photonic crystal fibers while ensuring stable transmission of circular polarization states remains an important scientific problem that needs to be solved urgently in this field. Summary of the invention

[0006] In view of the shortcomings of the above problems, the present invention provides a single-mode high nonlinear photonic crystal fiber that maintains circular polarization state transmission and a preparation method thereof, which improves nonlinearity by optimizing the solid core size and the duty cycle of the cladding air holes, introduces a twist rate of 0-8rad / mm to produce circular birefringence to maintain the circular polarization state, and changes the mode refractive index curve. Selectively fill or dope part of the cladding air holes to achieve phase matching, filter out high-order modes with refractive index changes to achieve single-mode transmission, and maintain the circular polarization state and high nonlinearity. The optical fiber is suitable for fields such as high-precision optical fiber sensing and special circular polarization state optical fiber lasers.

[0007] The technical solution of the present invention is as follows:

[0008] A single-mode and highly nonlinear photonic crystal optical fiber that maintains circular polarization state transmission is characterized in that it includes a core region with a high refractive index and a porous cladding region with a low refractive index, wherein the cladding region surrounds the core region, and the cladding region is composed of multiple layers of regularly arranged air holes, and the ratio of the diameter of the air holes to the air hole spacing is between 0.7 and 0.98; wherein at least one air hole is selectively selected in the cladding region for doping or filling, and the filler is a liquid, gas or glass with a different refractive index, and the refractive index is between that of air and a base material; and periodic torsion is introduced in the axial direction of the optical fiber.

[0009] Preferably, the twist rate α is 0-10 rad / mm to introduce circular birefringence up to the order of 10^-3 to maintain the polarization state of circularly polarized light.

[0010] Preferably, the base material of the optical fiber cladding region and the material of the core region are silicon dioxide, doped glass or plastic high molecular polymer material.

[0011] Preferably, the number of cladding holes for selective filling of the cladding is not limited to 4, and may be 1, 2, 3 or n.

[0012] Preferably, the cladding is used to selectively fill the air holes with various doped glasses, high and low refractive index glasses or various liquids and gases, whose refractive index is between that of air and the substrate material and close to the effective refractive index of the high-order mode to be filtered out at a specific rotation rate.

[0013] The base material of the optical fiber cladding and the material of the fiber core are silicon dioxide, doped glass or plastic high molecular polymer material.

[0014] Preferably, the twist rate α of the optical fiber axial direction is 0-10 rad / mm, and a circular birefringence of the order of 0 to 10-3 can be obtained. There are two ways to introduce twist in the optical fiber axial direction. One is to introduce twist during the optical fiber preparation process, mainly by rotating the optical fiber preform rod at high speed during the drawing process. The other is to introduce twist by introducing laser heating technology to post-process the optical fiber.

[0015] Preferably, in order to adjust the refractive index change curves of different optical fiber modes under torsion, the shape of the optical fiber core can be circular or elliptical. When the core is circular, the number of the first circle of air holes around the core is 4 to n, so that it presents an equilateral distribution from quadrilateral to n-gon; the diameter of the circular core and the length of the major axis of the elliptical core are 2 or 4 times the air hole spacing (Λ), and the length of the minor axis of the elliptical core is between the major semi-axis and the major axis.

[0016] Preferably, the arrangement of the photonic crystal structure in the optical fiber cladding region may be, but is not limited to, a hexagonal lattice arrangement, or may be a regular arrangement of quadrilateral lattices to form a photonic crystal cladding.

[0017] Preferably, the air hole spacing (Λ) in the cladding region is 2-10 microns, and the number of air hole layers in the cladding can be arranged in 2-8 layers.

[0018] The present invention also provides a method for preparing the above-mentioned photonic crystal optical fiber, which is characterized in that it comprises:

[0019] making an optical fiber preform including a high refractive index core region and a low refractive index porous cladding region;

[0020] Selectively selecting at least one air hole in the cladding region of the optical fiber preform for doping or filling;

[0021] During the process of drawing the optical fiber, the optical fiber preform is rotated at high speed, or the optical fiber is post-processed using laser heating technology after the optical fiber is drawn, so as to introduce periodic torsion.

[0022] Preferably, the method further includes performing a performance test on the filled or doped optical fiber to verify the realization of its circular polarization state maintenance, single-mode transmission and high nonlinear characteristics.

[0023] Compared with the prior art, the present invention has the following beneficial effects:

[0024] The present invention enhances circular birefringence by twisting photonic crystal fiber, thereby maintaining the circular polarization state; achieves high nonlinearity by controlling the ratio of the diameter of cladding air holes to the air hole spacing; and filters out high-order modes by flexibly selecting air holes to be filled or replaced with media of different refractive indices and utilizing the phase matching between the filling mode of the cladding space and the high-order modes of the core, thereby ensuring the single-mode performance of the optical fiber.

[0025] Taking the filtering out of different vector modes of the LP11 core high-order modes under twisted optical fiber as an example, but not limited to the LP11 mode, the disclosed idea can be extended to filtering out any core high-order modes. It has many adjustable degrees of freedom and strong performance optimization potential, and has research value in the fields of optical fiber sensing and optical fiber lasers. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 Schematic diagram of an embodiment of a single-mode and highly nonlinear photonic crystal fiber that maintains circular polarization state transmission according to the present invention, wherein (a) is a schematic diagram of an end face cross section, and (b) is a three-dimensional diagram under torsion;

[0027] Figure 2 The following is a graph showing the variation of the effective refractive index of different modes with the twist rate at 1550nm and 0-8rad / mm twist in the embodiment, where (a) is the fundamental mode and LP11 mode of the core. It can be seen that the effective refractive index of the fundamental mode is significantly higher than that of the higher-order mode; and (b) is the LP11 mode of the core and the fundamental mode of the selectively filled / doped cladding holes. It can be seen that at a twist rate of 6rad / mm, different polarization states of the core LP11 mode are close to the effective refractive index of the fundamental mode of the selectively filled cladding, thereby achieving filtering of the LP11 mode.

[0028] Figure 3 3 is a mode field distribution diagram of the mutual coupling between the core high-order mode and the cladding mode under 6 rad / mm torsion of the embodiment.

[0029] Figure 4This is the mode field distribution diagram of the embodiment in which the core fundamental mode remains well confined under 6 rad / mm torsion.

[0030] Figure 5 is the change of the circular birefringence value of the optical fiber fundamental mode with twisting.

[0031] Figure 6 The cross-sectional structure diagram of the single-mode and highly nonlinear photonic crystal optical fiber embodiment 2 of the present invention that maintains circular polarization state transmission;

[0032] Figure 7 The cross-sectional structure diagram of the single-mode and highly nonlinear photonic crystal optical fiber embodiment 3 of the present invention that maintains circular polarization state transmission;

[0033] Figure 8 The cross-sectional structure diagram of embodiment 4 of the single-mode and highly nonlinear photonic crystal optical fiber maintaining circular polarization state transmission of the present invention;

[0034] Fig. 9 This is a cross-sectional structural diagram of a single-mode and highly nonlinear photonic crystal fiber embodiment 5 of the present invention that maintains circular polarization state transmission;

[0035] Fig.10 This is a cross-sectional structural diagram of Example 6 of a single-mode and highly nonlinear photonic crystal fiber that maintains circular polarization state transmission according to the present invention. DETAILED DESCRIPTION

[0036] The specific implementation of the present invention is further described in detail below in conjunction with the accompanying drawings and examples. The following examples are used to illustrate the present invention, but are not intended to limit the scope of the present invention.

[0037] See also Figure 1 , Figure 1 Schematic diagram of a single-mode and highly nonlinear photonic crystal fiber embodiment of the present invention that maintains circular polarization state transmission, as shown in FIG. Figure 1 As shown, the waveguide characteristics of maintaining circular polarization state transmission, single mode and high nonlinearity are introduced simultaneously in the photonic crystal fiber. The cross section of the optical fiber consists of a core region 1 with a high refractive index and a porous cladding region 5 with a low refractive index.

[0038] The cladding region is composed of multiple layers of regularly arranged air holes, and the ratio of the diameter d of the air holes 4 to the air hole spacing Λ is between 0.7 and 0.98, so as to improve the nonlinear characteristics of the optical fiber.

[0039] In the cladding region 5, a plurality of (such as in the present embodiment) Figure 1 As shown in (a), 4 air holes are doped or filled. The fillers can be liquids, gases or glasses with different refractive indices to filter out high-order modes in the fiber core and ensure the single-mode performance of the optical fiber.

[0040] A periodic twist 6 is introduced in the axial direction of the optical fiber, and the twist rate α is 8 rad / mm, such as Figure 1 As shown in (b), it aims to enhance the circular birefringence of the optical fiber, thereby achieving stable transmission of circularly polarized light.

[0041] There are two ways to introduce axial twist in the optical fiber. One is to rotate the optical fiber preform at high speed during the optical fiber preparation process to introduce different degrees of twist online. The other is to use laser heating technology to post-process the optical fiber and introduce twist. This method can achieve a higher twist rate.

[0042] The shape of the fiber core can be circular or elliptical, which helps to adjust the effective refractive index change curve of different optical fiber modes under torsion. The diameter of the circular fiber core and the long axis length of the elliptical fiber core are 2 or 4 times the air hole spacing (Λ), and the short axis length of the elliptical fiber core is between the long semi-axis and the long axis, aiming to optimize the transmission performance of the optical fiber.

[0043] The cladding is used to selectively dope or fill air holes with various doped glasses or various liquids or gases, whose refractive index is between that of air and the substrate material and close to the effective refractive index of the high-order mode that needs to be filtered out at a specific rotation rate.

[0044] Different degrees of twist are introduced into the optical fiber, thereby introducing circular birefringence of up to 10-3 order of magnitude, so as to achieve polarization state maintenance of circularly polarized light under interference. The twist rate α of the optical fiber axial direction is 0-10rad / mm, such as Figure 1 As shown in 6, a circular birefringence of the order of 0-10-3 can be obtained. The base material 5 of the optical fiber cladding and the material of the fiber core are silicon dioxide, doped glass or plastic polymer materials, and the unfilled air holes are filled with air, vacuum or other low refractive index gases.

[0045] like Figure 2 As shown, at a rotation rate of 0-8rad / mm, Figure 1 The effective refractive index variation curve of the optical fiber in the embodiment shown in the 1550nm core LP01 mode and LP11 mode. It can be obtained that the effective refractive index of the LP01 mode and the LP11 mode is greatly different, and the effective refractive index difference between different polarization states of each mode increases with the increase of the twist rate. Therefore, effectively filtering out each LP11 mode under twist requires effective phase matching design for each polarization mode.

[0046] like Figure 3 As shown, at a torsion rate of 6 rad / mm, Figure 1The core LP01 mode 7 of the optical fiber in the embodiment shown is well confined in the core at 1550nm. The LP11 mode is close to the effective refractive index of the fundamental mode in the selectively filled holes in the cladding. The fundamental mode refractive indices in the four cladding holes are 1.3304, 1.3334, 1.338 and 1.3466 respectively. At this time, the liquid filling the hole can be selected as an aqueous solution or a sodium chloride solution of different concentrations. When the torsion rate is 6rad / mm, the four different polarization states 8 of the core LP11 are cross-coupled with the mode 9 selectively filled by the cladding, and the effective loss increases the LP11 high-order mode loss by more than two orders of magnitude, as shown in FIG. Figure 4 shown.

[0047] Different degrees of fiber twisting will significantly change the effective refractive index of different modes. At the same time, the distribution of the first ring of capillaries around the fiber core, the size of the core, and the periodic arrangement of the cladding holes will also significantly affect the values ​​of the effective refractive index of different modes under twisting. We modeled and analyzed the above, and the results are as follows Figure 5 shown.

[0048] It can be seen that in order to adjust the refractive index change curve of different optical fiber modes under torsion, the shape 2 of the optical fiber core can be circular or elliptical. When the core is circular, the number of the first circle of air holes around the core is 4, 5, 6 or 7, so that it presents an equilateral distribution from quadrilateral to n-gon, such as Figure 7 , Figure 8 , Figure 1 as well as Fig. 9 The corresponding embodiments are shown.

[0049] The arrangement of the photonic crystal structure in the optical fiber cladding region may be, but is not limited to, a hexagonal lattice arrangement, or may be a regular arrangement of quadrilateral lattices to form a photonic crystal cladding, such as Figure 8 The corresponding embodiments are shown in the figure. The base material of the optical fiber cladding and the material of the fiber core are silicon dioxide, doped glass or plastic polymer materials. These materials have good optical properties and stability and are suitable for making high-performance optical fibers.

[0050] The air hole spacing (Λ) in the cladding region is 2-10 microns, and the number of air hole layers in the cladding can be arranged in 2-8 layers, such as Fig.10 The corresponding embodiments are shown.

[0051] The present invention enhances circular birefringence by twisting photonic crystal optical fiber, thereby maintaining the circular polarization state; realizes high nonlinearity by controlling the ratio of the diameter of cladding air holes to the air hole spacing; and filters out high-order modes by flexibly selecting cladding air holes for doping or filling with media of different refractive indices and utilizing the filling mode of the cladding space to match the phase of the high-order modes of the core, thereby ensuring the single-mode performance of the optical fiber.

[0052] The present invention uses the example of filtering out different polarization modes of the LP11 high-order mode in the core of a twisted optical fiber to illustrate the idea of ​​the present invention, but is not limited to the LP11 mode as a group of high-order modes. The disclosed idea can be extended to any core high-order mode, has many adjustable degrees of freedom, and has strong performance optimization potential.

Claims

1. A single-mode and highly nonlinear photonic crystal fiber that maintains circular polarization state transmission, characterized in that: The invention comprises a core region with a high refractive index and a porous cladding region with a low refractive index, wherein the cladding region surrounds the core region, and the cladding region is composed of multiple layers of regularly arranged air holes, and the ratio of the diameter of the air holes to the air hole spacing is between 0.7 and 0.98; wherein at least one air hole is selectively selected in the cladding region for doping or filling, and the filler is a liquid, gas or glass with a different refractive index, and the refractive index thereof is between that of air and that of the substrate material; and a periodic twist is introduced in the axial direction of the optical fiber.

2. The single-mode and highly nonlinear photonic crystal fiber maintaining circular polarization state transmission according to claim 1, characterized in that: The twist rate α is 0-10 rad / mm to introduce circular birefringence up to the order of 10^-3 to maintain the polarization state of circularly polarized light.

3. The single-mode and highly nonlinear photonic crystal fiber maintaining circular polarization state transmission according to claim 1, characterized in that: The base material of the optical fiber cladding region and the material of the core region are silicon dioxide, doped glass or plastic high molecular polymer material.

4. The single-mode and highly nonlinear photonic crystal fiber maintaining circular polarization state transmission according to claim 1, characterized in that: The filler is various doped glasses, high and low refractive index glasses, aqueous solutions, sodium chloride solutions of different concentrations or other liquids and gases, and its refractive index is close to the effective refractive index of the high-order mode to be filtered out at a specific rotation rate.

5. The single-mode and highly nonlinear photonic crystal fiber that maintains circular polarization state transmission according to any one of claims 1 to 4, characterized in that: Twist is introduced during the fiber preparation process, or the fiber is post-processed and twisted using laser heating technology.

6. The single-mode and highly nonlinear photonic crystal fiber that maintains circular polarization state transmission according to any one of claims 1 to 4, characterized in that: The shape of the fiber core is circular or elliptical; when the fiber core is circular, the number of air holes in the first circle around the fiber core is 4 to n, presenting an equilateral distribution from a quadrilateral to an n-gon, the diameter of the circular fiber core and the long axis length of the elliptical fiber core are 2 times or 4 times the air hole spacing (Λ), and the short axis length of the elliptical fiber core is between the major semi-axis and the major axis.

7. The single-mode and highly nonlinear photonic crystal fiber that maintains circular polarization state transmission according to any one of claims 1 to 4, characterized in that: The photonic crystal structure in the cladding region is arranged in a hexagonal lattice arrangement or a quadrilateral lattice arrangement.

8. A single-mode and highly nonlinear photonic crystal fiber that maintains circular polarization state transmission according to any one of claims 1 to 4, characterized in that: The circular birefringence introduced by the twisting can maintain the polarization state of circularly polarized light under interference; high nonlinearity can be achieved by controlling the ratio of the diameter of the cladding air holes to the air hole spacing; and the air holes can be flexibly filled or replaced with media of different refractive indices, and the filling mode of the cladding space is matched with the phase of the high-order modes of the core to filter out the high-order modes, thereby ensuring the single-mode performance of the optical fiber.

9. A method for preparing a photonic crystal fiber according to any one of claims 1 to 8, characterized in that: include: making an optical fiber preform including a high refractive index core region and a low refractive index porous cladding region; Selectively selecting at least one air hole in the cladding region of the optical fiber preform for doping or filling; During the process of drawing the optical fiber, the optical fiber preform is rotated at high speed, or the optical fiber is post-processed using laser heating technology after the optical fiber is drawn, so as to introduce periodic torsion.

10. The method for preparing a photonic crystal optical fiber according to claim 9, characterized in that: It also includes performance testing of filled or doped optical fibers to verify the maintenance of circular polarization state, single-mode transmission and the realization of high nonlinear characteristics.

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