CN110361813A - A kind of twin-core Hz optical fiber coupler - Google Patents

Abstract

The invention discloses a double-core terahertz optical fiber coupler. Two parallel first-type rectangular dielectric layers intersect with two parallel second-type rectangular dielectric layers, and two dielectric cylinders are respectively suspended on the first-type rectangular dielectric layer and the second-type rectangular dielectric layer. At the intersection of the left and right sides of the rectangular-like dielectric layer, two cores of the optical fiber are formed. The first-type rectangular dielectric layer, the second-type rectangular dielectric layer, the dielectric circular tube and the air hole are the cladding of the fiber core, forming a total internal reflection structure. . The invention effectively reduces the absorption loss of the material, the size of the fiber core is much smaller than the operating wavelength, and most of the conduction mode energy of the fiber core can be distributed in the air, thereby reducing the absorption loss of the material. The distribution of the core mode field can be adjusted by changing the angle between the two types of rectangular dielectric layers, so that the coupling lengths of the x‑ and y‑ polarization modes are the same, and a polarization-independent low-loss dual-core THz fiber coupler is obtained.

Description

A dual-core terahertz fiber coupler

technical field

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

Background technique

Terahertz (THz) usually refers to electromagnetic waves with a frequency in the range of 0.1 to 10 THz, and its band is located between microwave and infrared in the electromagnetic spectrum. THz radiation has potential applications in many fields, such as communications, sensing, imaging, spectroscopy, and medicine. In the near-infrared band, directional coupling devices based on microstructured fibers have been widely studied. In theory, these directional coupling device structures can be applied to the THz band, that is to say, increasing the size of the fiber proportionally can realize the operation in the near-infrared. The coupler structure in the infrared band is applied to THz wave coupling. For example, if the operating wavelength of the coupler is 1.55μm, and the structural parameters of the coupler are expanded to 300/1.55 times of the original, the directional coupling with a wavelength of 300μm can be realized, but unfortunately The good thing is that the coupler length is also enlarged to 300/1.55 times of the original. Therefore, the length of this coupler is very long, which will cause excessive device transmission loss in the THz band. Ming-Yang Chen et al. [Ming-Yang Chen, et al., “Design and analysis of a low-loss terahertz directional coupler based on three-core photonic crystal fibre configuration,” J. Phys. D: Appl. Phys, 2011, 44 : 405104] proposed a directional coupler based on solid photonic crystal fiber. This coupling device can only reduce the transmission loss by shortening the length of the coupler. The reason is that the core conduction mode is mainly transmitted in the material, and there is a large absorb losses. A. Dupuis et al. [A. Dupuis, et al., “Fabrication and THz lossmeasurements of porous subwavelength fibers using a directional couplermethod,” Opt. Express, 2009, 17, 8012] proposed a directional coupler based on subwavelength fibers, optical The transmission process is susceptible to interference from the external environment. K. Nielsen et al [K. Nielsen, H. K. Rasmussen, P. U. Jepsen, and O. Bang, “Broadband terahertz fiber directional coupler,” Opt. Lett., 35, 2879~2881 (2010)] theoretically studied a low-doped The core THz dual-core photonic crystal fiber coupler uses a low-doped core to shorten the coupler length, and the coupling length is 20 cm at the center wavelength of 1.4 THz.

The above couplers ignore the influence of the different coupling lengths of the x-polarization mode and the y-polarization mode, and the coupling quality is restricted by the different coupling lengths of the x-polarization mode and the y-polarization mode.

SUMMARY OF THE INVENTION

In view of the above deficiencies, the present invention provides a dual-core terahertz (THz) fiber coupler, which realizes low loss, short length, and protection from external interference and polarization during optical transmission.

The present invention achieves the above objects through the following technical solutions. A dual-core terahertz optical fiber coupler, two parallel first-type rectangular dielectric layers intersect with two parallel second-type rectangular dielectric layers, the included angle between the first-type rectangular dielectric layer and the horizontal direction is θ, and the second-type rectangular dielectric layer is θ. The angle between the dielectric layer and the horizontal direction is π-θ; two dielectric cylinders are respectively suspended at the intersections of the left and right sides of the first-type rectangular dielectric layer and the second-type rectangular dielectric layer to form two cores of the optical fiber; The outer ends of the rectangular dielectric layer and the second type of rectangular dielectric layer are connected to the inner wall of the medium tube; The dielectric layer, the second type of rectangular dielectric layer, the dielectric circular tube and the air hole are the cladding of the fiber core, forming a total internal reflection structure.

Further, the included angle θ between the first type of rectangular dielectric layer and the horizontal direction satisfies: 0<θ<45 ^o .

Further, the widths t of the first type of rectangular dielectric layer and the second type of rectangular dielectric layer are both: t≦100 μm.

Further, the diameter of the section of the medium cylinder is d, which satisfies: d/λ<2/3.

In the present invention, the diameter of the fiber core is much smaller than the operating wavelength, and the core mode energy can be more distributed in the air holes adjacent to the fiber core, thereby effectively reducing the material absorption loss. Adjusting the size of the included angle between the first type of rectangular dielectric layer and the horizontal direction can make the coupling length of the x-polarization mode equal to the coupling length of the y-polarization mode. When the fiber is equal to the coupling length of the x-polarization mode (that is, the coupling length of the y-polarization mode), the two polarization modes of the fiber are simultaneously coupled from the input core to the output core, achieving polarization-independent coupling. The length of the coupler is also the coupling length of the two polarization modes. When the coupler length is short, the transmission loss of the core mode is very small. In addition, the core conduction mode is effectively confined around the core by the fiber cladding, the change of the surrounding environment does not affect the coupling performance of the coupler, and the device is easy to operate.

Description of drawings

1 is a schematic structural diagram of a dual-core fiber THz coupler of the present invention;

Fig. 2 is a graph showing the variation of the x- and y-polarization mode coupling lengths with θ in the embodiment of Fig. 1;

Fig. 3 is the electric field intensity distribution diagram of the x-polarization coupled supermode of the embodiment of Fig. 1;

Fig. 4 is the electric field intensity distribution diagram of the x-polarization odd supermode of the embodiment of Fig. 1;

Fig. 5 is the y-polarization coupled supermode electric field intensity distribution diagram of the embodiment of Fig. 1;

Fig. 6 is the electric field intensity distribution figure of the y-polarized odd supermode electric field of the embodiment of Fig. 1;

Fig. 7 is the graph of the variation curve of the normalized power of the x-polarization mode of the embodiment of Fig. 1 with the transmission distance;

FIG. 8 is a graph showing the variation of the normalized power of the y-polarization mode with the transmission distance in the embodiment of FIG. 1;

In the figure: 1- the first type of rectangular dielectric layer, 2- the second type of rectangular dielectric layer, 3- medium cylinder, 4- medium round tube, 5- air hole.

Detailed ways

The present invention will be further described below with reference to the accompanying drawings and embodiments. 1 shows a schematic cross-sectional view of the dual-core optical fiber THz coupler of the present invention. Two parallel first-type rectangular dielectric layers 1 intersect with two parallel second-type rectangular dielectric layers 2, and the first-type rectangular dielectric layers 1 and The included angle in the horizontal direction is θ, and the included angle between the second-type rectangular dielectric layer 2 and the horizontal direction is π-θ; the two dielectric cylinders 3 are respectively suspended on the first-type rectangular dielectric layer 1 and the second-type rectangular dielectric layer 2 At the intersection of the left and right sides, the two cores of the optical fiber are formed; the outer ends of the first-type rectangular dielectric layer 1 and the second-type rectangular dielectric layer 2 are connected to the inner wall of the dielectric circular tube 4; the first-type rectangular dielectric layer 1, the second-type rectangular dielectric layer 1 The space between the rectangular dielectric layer 2 and the dielectric circular tube 4 constitutes an air hole 5, and the first type of rectangular dielectric layer 1, the second type of rectangular dielectric layer 2, the dielectric circular tube 4 and the air hole 5 are the cladding of the fiber core, constitute a total internal reflection structure.

As shown in FIG. 1 , the thicknesses of the first type of rectangular dielectric layer 1 and the second type of rectangular dielectric layer 2 are both t, the inner diameter of the dielectric circular tube 4 is D, the distance between the two dielectric cylinders 3 is D ₁ , and the dielectric cylinder 3 The cross-sectional diameter is d, the angle between the first type of rectangular dielectric layer 1 and the horizontal direction x is θ, and the angle between the second type of rectangular dielectric layer 2 and the horizontal direction x is π-θ.

The principle of the present invention is to adjust the θ angle to make the fiber core have birefringence characteristics, and then to control the coupling length of the two polarization modes. At a certain value of θ, the coupling lengths of the two polarization modes are equal, so the length of the coupler is equal to one coupling length of the polarization mode, and a coupler with a shorter length can be realized; in addition, the core size is smaller than the operating wavelength, which can make a large number of The core mode energy is distributed in the air hole, which reduces the material absorption loss, and the combination of the two can realize a THz fiber coupler with low transmission loss.

The absorption loss of the core mode will increase with the increase of the width t of the rectangular dielectric layer, so the width t of the rectangular dielectric layer is required to be less than or equal to 100 μm.

The present invention can realize two θ angle values with equal coupling lengths of the two polarization modes, one angle value satisfies: 0<θ<45 ^o , and the other angle value satisfies: 45 ^o <θ<90 ^o , in order to obtain a small coupling length, where θ is required to satisfy: 0<θ<45 ^o .

The coupler works in the THz band, and the core size is required to be smaller than the working wavelength, so that more energy of the fundamental mode of the core can be distributed in the air holes around the core. Here, the diameter of the cylindrical section of the medium is required to satisfy: d/λ< 2/3.

Example:

The structure of the dual-core THz fiber coupler is shown in Figure 1. 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 thickness t of the first type of rectangular dielectric layer 1 and the second type of rectangular dielectric layer 2 are both is 30 μm, the distance between the two dielectric cylinders 3 is D ₁ =300 μm, and the cross-sectional diameter of the dielectric cylinder 3 is d=140 μm. At a frequency of 1 THz, the coupling lengths of the x- and y-polarized modes as a function of θ are shown in Fig. 2. It can be seen from the figure that the coupling length curves of the two polarization modes have two intersection points. At the intersection point, the coupling lengths of the two polarization modes are equal, and the corresponding values of θ are 30.5 ^o and 53.2 ^o respectively. Obviously, θ=30.5 ^o is smaller than the coupling length corresponding to θ=53.2 ^o , so it is required here that θ satisfies: 0<θ<45 ^o .

Fig. 3, Fig. 4, Fig. 5 and Fig. 6 show the electric fields of x-polarization odd supermode, x-polarization coupled supermode, y-polarization odd supermode and y-polarization coupled supermode when θ= ^30.5o , respectively Strong distribution map. It can be seen from the figure that a large part of the mode energy is distributed in the air holes around the fiber core, which can reduce the material absorption loss. In order to meet the low material absorption loss, it is required that the diameter of the section of the dielectric cylinder 3 is d to satisfy: d/ λ<2/3, the width of the rectangular dielectric layer t≤100μm is required.

Figures 7 and 8 show the normalized power variation curves of the x- and y-polarization modes with the transmission distance, respectively. It can be seen from the figures that the coupling lengths of the two polarization modes are equal, both 0.421 cm. Due to the short coupling length and part of the energy distributed in the air hole, the transmission loss of the coupling device is small. The calculated coupling transmission losses of the x- and y-polarized modes are 0.30dB and 0.27dB, respectively.

The above-mentioned drawings are only schematic diagrams for illustration, and do not limit the protection scope of the present invention. It should be understood that this example is only intended to illustrate the invention, and not to limit the scope of the invention in any way.

Claims (4)

1. A dual-core terahertz fiber coupler, characterized in that two parallel first-type rectangular dielectric layers intersect with two parallel second-type rectangular dielectric layers, and the angle between the first-type rectangular dielectric layer and the horizontal direction is θ, the angle between the second type of rectangular dielectric layer and the horizontal direction is π-θ; two dielectric cylinders are respectively suspended at the intersections of the left and right sides of the first type of rectangular dielectric layer and the second type of rectangular dielectric layer, forming the two sides of the optical fiber. Fiber core; the outer ends of the first type of rectangular dielectric layer and the second type of rectangular dielectric layer are connected to the inner wall of the dielectric circular tube; the gaps between the first type of rectangular dielectric layer, the second type of rectangular dielectric layer and the dielectric circular tube constitute air holes , and the first type of rectangular dielectric layer, the second type of rectangular dielectric layer, the dielectric circular tube and the air hole are the cladding of the fiber core, forming a total internal reflection structure. 2 . The dual-core terahertz fiber coupler according to claim 1 , wherein the angle θ between the first type of rectangular dielectric layer and the horizontal direction satisfies: 0<θ<45 ^o . 3 . 3 . The dual-core terahertz fiber coupler according to claim 1 , wherein the widths t of the first rectangular dielectric layer and the second type of rectangular dielectric layer are both: t≦100 μm. 4 . 4 . The dual-core terahertz fiber coupler according to claim 1 , wherein the diameter of the cross-section of the dielectric cylinder is d, which satisfies: d/λ<2/3. 5 .