CN112859245B - Dual Core Terahertz Fiber Coupler - Google Patents

Abstract

The invention discloses a double-core terahertz optical fiber coupler which comprises two first-class circular medium tubes, a second-class circular medium tube and two cylindrical fiber cores; the outer diameter of the first round medium pipe is smaller than the inner radius of the second round medium pipe, and the diameter of the fiber core is smaller than the inner diameter of the first round medium pipe; the first round medium pipe is internally tangent to the second round medium pipe, or the first round medium pipe is partially embedded into the second round medium pipe along the direction vertical to the tangent; a fiber core is internally tangent to each first circular medium tube, or each fiber core is partially embedded into one first circular medium tube along the direction perpendicular to the tangent. The invention can make the coupling lengths of the two polarization modes equal by adjusting the structural parameters of the optical fiber, and has the characteristic of polarization independence of the coupling lengths. The coupler is short in length, and the transmission loss of the fiber core module is small. The second circular medium tube isolates the fiber core conduction mode from the external environment, and the coupler can avoid the interference of the external environment in the operation process.

Description

Dual Core Terahertz Fiber Coupler

technical field

The invention relates to the field of optical fiber communication, in particular to a symmetrical suspension dual-core terahertz optical fiber coupler.

Background technique

Terahertz (Terahertz, THz) usually refers to electromagnetic waves with frequencies in the range of 0.1 to 10 THz. In the past ten years, terahertz waves have been widely used in various physical and biological fields, such as biotechnology, communications and spectroscopy. . A complete THz system generally consists of three main parts: THz radiation source, optical waveguide and detector. At present, THz radiation sources and detectors are getting closer and closer to practical use, but THz fibers and devices are still in the stage of theoretical and laboratory research, because the low-loss transparent materials used to compose fibers and fiber-optic devices in this band are relatively scarce. Therefore, the THz system mainly adopts the free-space propagation method to realize the transmission of THz waves, which makes the THz system bulky and requires precise maintenance and calibration. At the same time, the external environment will also cause a large loss of THz waves. At this stage, the THz wave The efficient transmission of terahertz has become a bottleneck affecting the development of terahertz technology. Therefore, it is of great scientific significance and practical value to develop high-quality THz fibers and their devices.

The dual-core THz fiber coupler is one of the important components in the terahertz system. The strong absorption of the THz wave by the material puts forward requirements for the design of the THz fiber coupler: (1) the length of the coupler should be short to reduce the transmission loss; (2) The conduction mode energy is mainly distributed in the air hole to reduce the absorption loss; (3) the coupling length of the two polarization modes is the same, which improves the quality of the coupler; (4) the fiber coupler has a simple structure and is easy to prepare.

Ming-Yang Chen et al. [Ming-Yang Chen, et al., "Design and analysis of a low-loss terahertzdirectional coupler based on three-corephotonic crystal fibre configuration," J.Phys.D:Appl.Phys, 2011,44:405104 ] proposed a pointing coupler based on a solid photonic crystal fiber, the coupling lengths of the x-polarization mode and the y-polarization mode are 1.968 cm and 2.193 cm, respectively, and the coupling length difference between the two polarization modes is greater than 10%. The core mode of this directional coupler is mainly distributed in the base material, which indicates that the transmission loss of the coupler can only be reduced by shortening the length of the coupler.

Zhong Hua et al. [Zhong Hua, Xu Yufu, Zhu Yuanfeng, Rao Chunfang, "A Dual-core Terahertz Fiber Coupler," Chinese Invention Patent, CN201910759385.2] designed a suspended subwavelength core Terahertz fiber coupler to adjust the structure The parameters can make the two polarization molds have the same coupling length, and the coupling length is shorter, but the support arm of the fiber core is a rectangular dielectric layer, which is easy to deform during the fiber preparation process.

SUMMARY OF THE INVENTION

The purpose of the present invention is to provide a dual-core terahertz optical fiber coupler, which can solve one or more of the above-mentioned problems in the prior art.

According to an aspect of the present invention, a dual-core terahertz optical fiber coupler is provided, which mainly includes: two first-type circular dielectric tubes, one second-type circular dielectric tube and two cylindrical fiber cores; The outer diameter of the first type circular medium tube is smaller than the inner radius of the second type circular medium tube, and the diameter of the fiber core is smaller than the inner diameter of the first type circular medium tube; the first type circular medium tube is inscribed in the second type circle or the first type of circular dielectric tube is partially embedded in the second type of circular dielectric tube along the direction perpendicular to the tangent; each first type of circular dielectric tube has a core cut in it, or each core is vertically A first type of circular dielectric tube is partially embedded in the direction of the tangent; the centers of the first type of circular dielectric tube, the second type of circular dielectric tube and the fiber core are on the same straight line; The side of the first-class circular medium tube away from the second-class circular media tube; the first-class air hole is formed between the outer surface of the fiber core and the inner surface of the first-class circular media tube; the first-class circular media tube A second-type air hole is formed between the outer surface of the second-type circular medium pipe and the inner surface of the second-type circular medium pipe; the first-type circular medium pipe, the second-type circular medium pipe, the first-type air hole and the second-type air hole It is the cladding of the optical fiber, and the cladding of the optical fiber has a total internal reflection structure as a whole.

In the dual-core terahertz optical fiber coupler of the present invention, the optical fiber coupler is mainly composed of a first-type circular dielectric tube, a second-type circular dielectric tube and a dielectric cylinder. The boundaries of the circular dielectric tube and the cylinder are all circular. During the fiber preparation process, the shape of the fiber tends to remain unchanged. The fiber core is arranged inside the optical fiber through the first-type circular dielectric tube, and the coupling length of the two polarization modes can be made equal by adjusting the structural parameters of the optical fiber, so that the coupling length is independent of polarization. The length of the coupler is short, and the transmission loss of the core mode is small. The second type of circular dielectric tube isolates the core conduction mode from the external environment, and the coupler can be free from the interference of the external environment during operation, which is convenient for operation and application.

In some embodiments, the thickness t of the first type of circular media tubes of the present invention is ≥ 10 μm. Thereby, the shape of the optical fiber can be kept unchanged during the production process.

In some embodiments, the inner radius R ₂ of the second type of circular medium pipe of the present invention satisfies: 6mm≧R ₂ ≧1mm. Thus, the cross section of the device will not be too large.

In some embodiments, the distance δ between the outer walls of the two first type circular media tubes of the present invention is greater than 260 microns. If the distance between the two fiber cores is too close, the optical field separation in the two fiber cores cannot be effectively achieved, which will reduce the coupling beam splitting characteristics.

Description of drawings

FIG. 1 is a schematic structural diagram of a dual-core terahertz fiber coupler according to an embodiment of the present invention.

FIG. 2 is a schematic structural diagram of a dual-core terahertz fiber coupler according to another embodiment of the present invention.

3 shows the coupling length of the x-polarization mode and the coupling length of the y-polarization mode with the core radius when the thickness t of the first type of circular dielectric tube of the dual-core terahertz fiber coupler according to an embodiment of the present invention is different Schematic diagram of the change curve of R.

4 shows the coupling length of the x-polarization mode and the coupling length of the y-polarization mode when the radius R ₂ of the second type of circular dielectric tube 2 of the dual-core terahertz fiber coupler according to an embodiment of the present invention varies with the fiber Schematic diagram of the variation curve of the core radius R.

5 shows the coupling length of the x-polarization mode and the coupling of the y-polarization mode when the distance δ between the outer walls of the two first-type circular dielectric tubes of the dual-core terahertz fiber coupler according to an embodiment of the present invention is different Schematic diagram of the variation curve of the length with the distance δ between the outer walls of the two first-type circular media tubes.

FIGS. 6 to 9 are the electric field intensities of the x-polarized odd supermode, x-polarized even supermode, y-polarized odd supermode, and y-polarized even supermode of a dual-core terahertz fiber coupler according to an embodiment of the present invention. Distribution diagram.

Detailed ways

In order to make the purposes, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments These are some embodiments of the present invention, but not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative efforts shall fall within the protection scope of the present invention.

It should be noted that the embodiments in the present application and the features of the embodiments may be combined with each other in the case of no conflict.

Finally, it should also be noted that in this document, relational terms such as first and second, etc., counterclockwise and clockwise are used only to distinguish one entity or operation from another, not necessarily Any such actual relationship or order between these entities or operations is required or implied. Moreover, the terms "comprising" and "comprising" include not only those elements, but also other elements not expressly listed, or elements inherent to such a process, method, article or apparatus. Without further limitation, an element defined by the phrase "comprises" does not preclude the presence of additional identical elements in a process, method, article, or device that includes the element.

The present invention will be described in further detail below in conjunction with the accompanying drawings.

FIG. 1 and FIG. 2 schematically show the structures of two-core terahertz fiber couplers according to two embodiments of the present invention.

Referring to FIG. 1 , the dual-core terahertz fiber coupler includes: two first-type circular dielectric tubes 1 , one second-type circular dielectric tube 2 and two fiber cores 3 .

The core 3 has a cylindrical shape. The outer diameter of the first type circular medium tube 1 is smaller than the inner radius of the second type circular medium tube 2, and the diameter of the fiber core 3 is smaller than the inner diameter of the first type circular medium tube 1.

The first type circular medium tube 1 is inscribed in the second type circular medium tube 2; each first type circular medium tube 1 has a fiber core 3 inscribed in it.

The centers of the first type circular dielectric tube 1, the second type circular dielectric tube 2 and the fiber core 3 are on the same straight line, and each fiber core 3 is located in the first type circular dielectric tube 1 away from the second type circle. one side of the shaped medium tube 2.

A first-type air hole 4 is formed between the outer surface of the fiber core 3 and the inner surface of the first-type circular dielectric tube 1 , and the outer surface of the first-type circular dielectric tube 1 and the inner surface of the second-type circular dielectric tube 2 A second type of air holes 5 are formed between the surfaces. The first type circular dielectric tube 1 , the second type circular dielectric tube 2 , the first type air hole 4 and the second type air hole 5 are the core cladding, and the fiber core cladding has a total internal reflection structure as a whole.

When using the dual-core terahertz fiber coupler, the cross-sectional radius R of the core 3 should preferably satisfy:

R/λ<1/3, λ is the working wavelength.

The boundaries of the first type circular dielectric tube 1 , the second type circular dielectric tube 2 and the fiber core 3 are all circular, which provides a strong guarantee for the shape retention during the optical fiber preparation process.

Core radius - R;

The inner radius of the first type of circular medium pipe - R ₁ , the thickness - t;

The inner radius of the second type of circular medium pipe - R ₂ ;

The distance between the outer walls of the two first-type circular media tubes—δ=R ₂ −2*(R ₁ +t).

Referring to Fig. 2, the structure of the dual-core terahertz fiber coupler of the present invention can also be: the first type circular dielectric tube 1 is partially embedded in the second type circular dielectric tube 2 along the direction perpendicular to the tangent; The core 3 is partially embedded in a first-type circular dielectric tube 1 in a direction perpendicular to the tangent.

3 schematically shows the coupling length of the x-polarization mode and the coupling length of the y-polarization mode when the thickness t of the first type of circular dielectric tube of the dual-core terahertz fiber coupler according to an embodiment of the present invention is different Variation curve with core radius R.

The double-core terahertz fiber coupler with the structure shown in FIG. 1 is adopted, the refractive index of air is n _air =1.0, the refractive index of the matrix material of the optical fiber is n = 1.5258, and the center frequency of the coupler is 1THz.

Referring to Figure 3, the solid line is the change curve of the coupling length of the x-polarization mode with the radius R of the core 3, the dotted line is the change curve of the coupling length of the y-polarization mode with the radius R of the fiber core 3, the solid line and the The intersection of the dotted lines is the point where the coupling lengths of the two polarization modes are equal, where the coupler length has polarization-independent characteristics.

In this embodiment, R ₂ =3.0 mm, and δ = 280 μm. When t=20 μm, 25 μm and 30 μm, and the coupling lengths of the two polarization modes are equal, the corresponding radii R of the core 3 are 52.1 μm, 53.2 μm and 54.0 μm, respectively, and the coupling lengths of the corresponding couplers are 1.07027 cm, 1.26482 cm and 1.49134cm. It can be seen that the thinner the thickness t of the first type of circular dielectric tube 1, the shorter the coupling length, but in order to keep the shape of the optical fiber unchanged during the optical fiber preparation process, t≥10 μm is required here.

It can be seen that when the working wavelength λ and the distance δ between the outer walls of the two first-type circular dielectric tubes 1 are kept constant, when the radius R of the core 3 is small, the coupling length of the x-polarization mode is greater than that of the y-polarization mode. Coupling length: As the radius R of the core 3 increases, the coupling lengths of the two polarization modes can be equal, and if the radius R of the core 3 continues to increase, the coupling length of the y-polarization mode will be greater than the coupling length of the x-polarization mode.

Moreover, when the first type of circular dielectric tube 1 selects different thickness t, the change of the coupling length of the two polarization modes with the radius R of the fiber core 3 has the above-mentioned change trend.

FIG. 4 schematically shows the coupling length of the x-polarization mode and the coupling length of the y-polarization mode when the radius R ₂ of the second type of circular dielectric tube 2 of the dual-core terahertz fiber coupler according to an embodiment of the present invention is different. The curve of the coupling length as a function of the core radius R.

The double-core terahertz fiber coupler with the structure shown in FIG. 1 is adopted, the refractive index of air is n _air =1.0, the refractive index of the matrix material of the optical fiber is n = 1.5258, and the center frequency of the coupler is 1THz.

Referring to Figure 4, the solid line is the change curve of the coupling length of the x-polarization mode with the radius R of the core 3, the dotted line is the change curve of the coupling length of the y-polarization mode with the radius R of the fiber core 3, the solid line and the The intersection of the dotted lines is the point where the coupling lengths of the two polarization modes are equal, where the coupler length has polarization-independent characteristics.

In this embodiment, t=20 μm and δ=280 μm. When R ₂ =3.0mm, 2.6mm and 2.2mm, and the coupling lengths of the two polarization modes are equal, the corresponding radii R of the core 3 are 51.9 μm, 52.6 μm and 53.7 μm, respectively, and the corresponding coupling lengths are 1.0658 cm, 1.1213cm and 1.1756cm. It can be seen that the smaller the inner radius R2 of the second type of circular dielectric tube ₂ , the larger the radius R of the fiber core 3 corresponding to the point where the coupling lengths of the two polarization modes are equal, and the coupling length also increases accordingly. In order to reduce the length of the coupler, the coupling length should be shorter, but in order to reduce the length of the coupler, R ₂ ≥ 1mm is required here, and in order that the cross section of the device is not too large, R ₂ ≤ 6mm is required here.

It can be seen that, keeping the working wavelength λ and the distance δ between the outer walls of the two first-type circular dielectric tubes 1 unchanged, when the inner radius R ₂ of the second-type circular dielectric tubes 2 is larger, the x-polarization mode The coupling length of the y polarization mode is greater than the coupling length of the y polarization mode; with the decrease of the inner radius R ₂ of the second type of circular dielectric tube 2, the coupling lengths of the two polarization modes can be equal, and continue to reduce the second type of circular dielectric tube 2. With the inner radius R ₂ , the coupling length of the y-polarized mode will be greater than that of the x-polarized mode.

When the second type of circular dielectric tube 2 selects different inner radii R ₂ , the coupling lengths of the two polarization modes with the radius R of the fiber core 3 all have the above changing trend.

To sum up, selecting different thickness t of the first type circular dielectric tube 1 or different inner radius R ₂ of the second type circular dielectric tube 2 can make the coupling lengths of the two polarization modes equal. The length of the 3dB coupler is equal to half the coupling length of the polarization mode. Based on this method, the length of the coupler is short, so the transmission loss of the terahertz wave through the coupler can be reduced.

In addition, the radius of the fiber core 3 is less than 1/3 of the working wavelength, so that a large amount of core mode energy can be distributed in the air holes, which reduces the material absorption loss and further reduces the device loss. This twin-core fiber design can be used to realize low-loss THz coupling devices that are polarization-independent on the coupling length.

FIG. 5 schematically shows the coupling length of the x-polarization mode and the coupling length of the y-polarization mode when the core radius R of the dual-core terahertz fiber coupler according to an embodiment of the present invention varies with the two first types Variation curve of the distance δ between the outer walls of the circular medium tube.

The double-core terahertz fiber coupler with the structure shown in FIG. 1 is adopted, the refractive index of air is n _air =1.0, the refractive index of the matrix material of the optical fiber is n = 1.5258, and the center frequency of the coupler is 1THz.

Referring to Fig. 5, the solid line is the change curve of the coupling length of the x-polarization mode with the distance δ between the outer walls of the two first-type circular dielectric tubes 1, and the dashed line is the coupling length of the y-polarization mode with the two thirds. The change curve of the distance δ between the outer walls of a type of circular dielectric tube 1, the intersection of the solid line and the dotted line is the point where the coupling lengths of the two polarization modes are equal, where the coupler length has polarization-independent characteristics.

In this embodiment, R ₂ =3.0mm and t=20μm. When δ=280 μm, 320 μm and 360 μm, and the corresponding core radii R are 52.1 μm, 54.9 μm and 56.6 μm, respectively, the coupling lengths of the two polarization modes are equal, and the coupling lengths are 1.07 cm, 1.8 cm and 2.96 cm, respectively. It can be seen from the results that as the distance δ between the outer walls of the two first-type circular dielectric tubes 1 decreases, the coupling length decreases, which is beneficial to reduce the transmission loss of the coupling device. If the distance between the two fiber cores 3 is too close, the separation of the optical fields in the two fiber cores 3 cannot be effectively achieved, and the coupling and beam splitting characteristics will be reduced. The distance δ between the outer walls of the two first-type circular media tubes is preferably greater than 260 microns.

It can be seen that when the radius R ₂ of the second type circular dielectric tube 2 and the thickness t of the two first type circular dielectric tubes 1 are kept unchanged, when the radius R of the core 3 is larger, the coupling of the x-polarization mode The length is less than the coupling length of the y polarization mode; as the radius R of the core 3 decreases, the coupling lengths of the two polarization modes can be equal, and if the radius R of the core 3 continues to decrease, the coupling length of the y polarization mode will be less than the coupling length of the x polarization mode .

The fiber core 3 selects different radii R, and the coupling length of the two polarization modes with the change of the distance δ between the outer walls of the two first-type circular dielectric tubes 1 has the above changing trend.

In addition, using the dual-core terahertz fiber coupler with the structure shown in FIG. 2 , the coupling lengths of the two polarization modes also have the changing trends in FIGS. 3 to 5 . The refractive index of air is n _air = 1.0, the refractive index of the matrix material of the optical fiber is n = 1.5258, and the center frequency of the coupler is 1 THz.

6-9 schematically show the electric field fields of the x-polarized odd supermode, x-polarized even supermode, y-polarized odd supermode, and y-polarized even supermode of a dual-core terahertz fiber coupler according to an embodiment of the present invention Schematic diagram of a strong distribution.

In this embodiment, the thickness t=25 μm of the first type circular dielectric tube 1, the radius R=53.2 μm of the fiber core 3, the inner radius R ₂ =3.0 mm of the second type circular dielectric tube 2, and the two The distance between the outer walls of a type of circular medium tube 1 is δ=280 μm.

Referring to FIGS. 6-9, it can be seen that a large part of the mode energy is distributed in the air holes around the fiber core 3, which can reduce the material absorption loss.

In order to satisfy the low material absorption loss, when using the dual-core terahertz fiber coupler of the present invention, the radius R of the fiber core 3 can preferably satisfy: R/λ<1/3.

The foregoing are merely some of the embodiments of the present invention. For those of ordinary skill in the art, without departing from the inventive concept of the present invention, several modifications and improvements can be made, which all belong to the protection scope of the present invention.

Claims (4)

1. Two core terahertz fiber coupler now, its characterized in that includes: the fiber core comprises two first-class circular medium tubes, a second-class circular medium tube and two cylindrical fiber cores;

the outer diameter of the first round medium pipe is smaller than the inner radius of the second round medium pipe, and the diameter of the fiber core is smaller than the inner diameter of the first round medium pipe;

the first round medium pipe is internally tangent to the second round medium pipe, or the first round medium pipe is partially embedded into the second round medium pipe along the direction vertical to the tangent;

a fiber core is internally tangent to each first circular medium tube, or each fiber core is partially embedded into one first circular medium tube along the direction perpendicular to the tangent;

the centers of the first round medium tube, the second round medium tube and the fiber core are positioned on the same straight line; each fiber core is positioned on one side, far away from the second round medium tube, in the first round medium tube;

a first air hole is formed between the outer surface of the fiber core and the inner surface of the first circular medium tube;

a second air hole is formed between the outer surface of the first round medium pipe and the inner surface of the second round medium pipe;

the first round medium tube, the second round medium tube, the first air hole and the second air hole are fiber core cladding, and the fiber core cladding is of a total internal reflection structure.

2. The dual-core terahertz optical fiber coupler of claim 1, wherein the thickness t of the first-class circular medium tube is greater than or equal to 10 μm.

3. The dual-core terahertz fiber coupler of claim 1, wherein the inner radius R of the second-kind circular dielectric tube₂Satisfies the following conditions: r is more than or equal to 6mm₂≥1mm。

4. The dual-core terahertz fiber coupler of any one of claims 1 to 3, wherein the distance δ between the outer walls of the two first-type circular dielectric tubes is greater than 260 microns.

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