Novel mid-infrared dual-core photonic crystal fiber polarization beam splitter based on aluminum wire filling

Abstract

The invention discloses a novel intermediate infrared double-core photonic crystal fiber polarization beam splitter based on aluminum wire filling, particularly relates to the technical field of photonic crystal fiber polarization beam splitters, and solves the technical problem that in the prior art, a compact photonic crystal fiber polarization beam splitter with a high extinction ratio and an ultra wide band in an intermediate infrared band is still scarce. According to the technical scheme, the optical fiber comprises two fiber cores and a cladding, wherein a cladding area is distributed in a hexagonal lattice mode; a plurality of circular air holes and two large circular filling holes are formed in the fiber core area, the distance between every two adjacent air holes is the same, the fiber core area comprises four small circular air holes and a central air hole, and the fiber cores are arranged in parallel and symmetrically distributed on the two sides of the central air hole. The As2S3 glass with high refractive index is used as a substrate material, aluminum is used as a metal filler, and the shortest length of 160 microns, the maximum extinction ratio of-72.1 dB and the operation bandwidth of 1020 nm are obtained at the mid-infrared band with the wavelength of 3.3 microns.

Description

Technical Field

[0001] The present invention relates to the technical field of photonic crystal fiber polarization beam splitters, and particularly relates to a novel mid-infrared dual-core photonic crystal fiber polarization beam splitter based on aluminum wire filling. Background Art

[0002] With the substantial growth of big data traffic services such as artificial intelligence, Internet of Things, and cloud computing in recent years, new requirements have been put forward for information transmission bandwidth and processing speed, which has stimulated people's interest in technologies such as polarization multiplexing, space division multiplexing, and frequency division multiplexing. As an important device in polarization division multiplexing technology, the polarization beam splitter plays an irreplaceable role in optical communication networks. It can split incident light with any polarization state, including polarized light, partially polarized light, and unpolarized light, into two beams of orthogonal polarized light, enabling the system to simultaneously transmit multiple signals, thereby improving the transmission capacity and efficiency of optical communication networks. However, the refractive index of traditional optical fibers is relatively small, and the degree of freedom for design is limited. The polarization beam splitters made of traditional optical fibers are too long to meet the requirements of all-optical network communication systems that are constantly evolving towards miniaturization and multifunctionality. It was not until the emergence of photonic crystal fibers that this problem was solved. Compared with traditional optical fibers, photonic crystal fibers have various advantages such as flexible structure design, high birefringence characteristics, single-mode transmission characteristics without cut-off, and low loss characteristics, providing a new direction for the design of polarization beam splitters.

[0003] With the improvement of micro-nano science theories and methods and the progress of manufacturing technologies, researchers have developed various polarization beam splitting carrier optical waveguides with excellent performance, including Y-shaped silicon ridge nanowires, photonic crystals, metasurfaces, and optical fibers. Using optical fibers as the waveguides of polarization beam splitters can be directly coupled with optical communication networks to reduce losses. In recent years, the method of applying metal coatings on the inner walls of air holes has developed quite rapidly. When incident light is coupled with the metal layer, surface plasmon polariton modes will be formed on the metal surface. When the phase of the core-guided mode and the surface plasmon polariton mode matches, they will resonate. Through this interaction between plasmon polaritons and the optical field, active control of light propagation can be achieved, which is important for the miniaturization and high-density integration of optical paths.

[0004] Through dense wavelength division multiplexing technology, frequency division multiplexing technology, time division multiplexing technology and space division multiplexing technology, the expansion in the traditional 1.55μm communication band has been widely studied. However, in the era of big data, the demand for information resources is increasing. The traditional communication band is always limited and cannot meet the requirements of high-speed and large-capacity information transmission. If the available spectrum can be further expanded, information transmission will be more efficient. The mid-infrared region of 3-5μm is one of the most important communication windows, and its unique spectral position and characteristics determine its wide applications in fields such as chemical sensing, medical diagnosis, imaging, and national defense. However, until now, compact photonic crystal fiber polarization beam splitters with high extinction ratio and ultra-wideband working in the mid-infrared band are still scarce. Summary of the Invention

[0005] Therefore, the present invention solves the technical problem that compact photonic crystal fiber polarization beam splitters with high extinction ratio and ultra-wideband in the mid-infrared band are still scarce in the prior art; the novel mid-infrared dual-core photonic crystal fiber polarization beam splitter based on aluminum wire filling provided by the present invention has the advantages of simple structure, small size, high extinction ratio and ultra-wideband, and can meet the growing demand for information transmission bandwidth and processing speed.

[0006] To achieve the above object, the present invention is realized through the following technical solutions: A novel mid-infrared dual-core photonic crystal fiber polarization beam splitter based on aluminum wire filling, comprising a cladding region and a core region. The cladding region includes a plurality of cladding air holes and two large circular filling holes filled with aluminum wire, and the diameter of the cladding air holes is smaller than that of the large circular filling holes.

[0007] The core region is formed by removing a total of four cladding air holes on the left and right respectively, and four small circular air holes with smaller diameters are arranged to modulate the dual-core mode field.

[0008] Further, the central air hole is located at the structural center of the photonic crystal fiber polarization beam splitter.

[0009] Further, the two large circular filling holes filled with aluminum wire are located on both sides of the central air hole along the Y-axis direction.

[0010] Further, the distance between the four small circular air holes and the x-axis is 1.39μm, and the distance from the y-axis is 1.75μm.

[0011] Further, the base material is As 2 S 3 glass.

[0012] Further, the diameter of the cladding air holes is 1.6μm.

[0013] Furthermore, the diameter of the large circular filling hole filled with aluminum wire is 2.0 μm.

[0014] Furthermore, the diameter of the central air hole is 1.2 μm.

[0015] Furthermore, the diameter of the small circular air hole is 0.7 μm.

[0016] Furthermore, the pitch between adjacent cladding air holes is 2.1 μm.

[0017] Furthermore, the pitch between the large circular filling hole filled with aluminum wire and the cladding air hole is also 2.1 μm.

[0018] In the above technical solution, the technical effects and advantages provided by the present invention are as follows:

[0019] 1. For the novel mid-infrared dual-core photonic crystal fiber polarization beam splitter based on aluminum wire filling provided by the present invention, four small circular air holes are arranged in the core region, which can increase the asymmetry of the structure, thereby reducing the coupling length.

[0020] 2. For the novel mid-infrared dual-core photonic crystal fiber polarization beam splitter based on aluminum wire filling provided by the present invention, As 2 S 3 glass is used as the substrate material, which is beneficial to extending the working wavelength to the mid-infrared band. The aluminum wire provides a surface plasmon polariton effect to enhance the birefringence of the photonic crystal fiber polarization beam splitter and reduce the length of the device.

[0021] 3. For the novel mid-infrared dual-core photonic crystal fiber polarization beam splitter based on aluminum wire filling provided by the present invention, most photonic crystal fiber polarization beam splitters work in the conventional communication window with a wavelength of 1550 nm. With the continuous growth of communication services, the capacity of the traditional communication window is approaching its limit. Therefore, it is necessary to explore new communication bands. The photonic crystal fiber polarization beam splitter in this application works in the mid-infrared band with a wavelength of 3300 nm. Compared with the conventional communication band, the mid-infrared band has advantages such as low transmission loss and wide gain spectrum range, and is expected to become the next communication window for optical fiber communication and space laser communication.

[0022] 4. The novel mid-infrared dual-core photonic crystal fiber polarization beam splitter based on aluminum wire filling provided by the present invention has the advantages of simple structure, short length, high extinction ratio, and ultra-wideband. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the drawings required for use in the embodiments. Obviously, the drawings described below are only some embodiments recorded in the present invention. For those of ordinary skill in the art, other drawings can also be obtained based on these drawings.

[0024] Figure 1 It is a schematic cross-sectional structure diagram of the present invention.

[0025] Figure 2 It is a schematic diagram of the structural parameters of the present invention.

[0026] Figure 3 It is the mode field distribution diagram of the present invention.

[0027] Figure 4 It is a graph showing the variation of the coupling length ratio with wavelength of the present invention under different cladding air hole diameters (d 1 = 1.4 μm, 1.6 μm, 1.8 μm).

[0028] Figure 5 It is a graph showing the variation of the coupling length ratio with wavelength of the present invention under different central air hole diameters (d 2 = 1.0 μm, 1.2 μm, 1.4 μm).

[0029] Figure 6 It is a graph showing the variation of the coupling length ratio with wavelength of the present invention under different small circular air hole diameters (d 3 = 0.5 μm, 0.7 μm, 0.9 μm).

[0030] Figure 7 It is a graph showing the variation of the coupling length ratio with wavelength of the present invention under different large circular filling hole diameters (d 4 = 1.9 μm, 2.0 μm, 2.1 μm).

[0031] Figure 8 It is a graph showing the variation of the coupling length ratio with wavelength of the present invention under different cladding air hole pitches (Λ = 2.05 μm, 2.10 μm, 2.15 μm).

[0032] Figure 9 It is a graph showing the variation of the coupling length ratio with wavelength of the present invention under different filling materials (gold, aluminum, silver).

[0033] Figure 10 It is the normalized power curve diagram of Embodiment 1 of the present invention.

[0034] Figure 11 It is the extinction ratio curve of Embodiment 1 of the present invention Figure 1 ;

[0035] Figure 12 It is the extinction ratio curve of Embodiment 1 of the present invention Figure 2 。

[0036] Explanation of reference numerals:

[0037] 1. Cladding air hole; 2. Central air hole; 3. Small round air hole; 4. Large circular filling hole; 5. Cladding air hole spacing; 6. Substrate material. Detailed implementation mode

[0038] In order to enable those skilled in the art to better understand the technical solution of the present invention, the present invention will be further introduced in detail below with reference to the accompanying drawings.

[0039] Example 1:

[0040] This example provides a novel mid-infrared dual-core photonic crystal fiber polarization beam splitter based on aluminum wire filling. Figure 1 is a cross-sectional schematic diagram of an embodiment of the present invention, including a substrate material 6, a cladding region and a core region. The cladding region is composed of three layers of cladding air holes 1 arranged periodically in a regular hexagonal structure. The core region is composed of a central air hole 2, four small round air holes 3 and two large circular filling holes 4 filled with aluminum wire. The two cores are distributed in parallel on both sides of the central air hole 2. The two aluminum wires can enhance the birefringence effect, thereby improving the performance of the photonic crystal fiber polarization beam splitter.

[0041] Figure 2 is a schematic diagram of the structural parameters of an embodiment of the present invention, where d 1 is the diameter of the cladding air hole 1, d 2 is the diameter of the central air hole 2, d 3 is the diameter of the small round air hole 3, d 4 is the diameter of the large circular filling hole 4 filled with aluminum wire. Λ is the distance between adjacent two cladding air holes 1, that is, the cladding air hole spacing 5, and the distance refers to the distance between the centers of the circles. Considering comprehensively, we select d 1 = 1.6 μm, d 2 = 1.2 μm, d 3 = 0.7 μm, d 4 = 2.0 μm, Λ = 2.1 μm as the optimal structural parameters of the embodiment of the present invention.

[0042] Figure 3 is the four optical mode field distribution diagrams of the embodiment of the present invention at a wavelength of 3.3 μm. The direction of the arrow represents the polarization direction. The optical modes with the same arrow direction are called even modes, and the optical modes with opposite arrow directions are called odd modes.

[0043] The substrate material 6 of the embodiment of the present invention is chalcogenide glass As 2 S 3 , The chalcogenide glass material can increase the infrared transmission range from 0.5 - 1 μm to 12 - 25 μm according to different composition materials. In addition, the chalcogenide glass also has a high linear refractive index and non-linear refractive index. From As 2 S 3A photonic crystal fiber polarization beam splitter made of glass has stable physical properties and low drawing costs.

[0044] As 2 S 3 The refractive index of glass can be obtained by the following Sellmeier formula (1):

[0045]

[0046] The specific coefficients in the formula are A 1 = 1.8983678, A 2 = 1.9222979, A 3 = 0.8765134, λ 1 = 0.0225μm 2 , λ 2 = 0.0625μm 2 , λ 3 = 0.1225μm 2 .

[0047] When the input light is incident on core A or B, the energy of the X-polarized light and the Y-polarized light will be periodically transferred between the two cores. The propagation distance required for the energy of the polarized light to be completely transferred from one core to the other is called the coupling length. The coupling length CL formulas for the X-polarization and Y-polarization directions are as follows:

[0048]

[0049] In the formula and respectively represent the effective refractive index of the x-polarized even mode, the effective refractive index of the x-polarized even mode, the effective refractive index of the x-polarized odd mode, the effective refractive index of the y-polarized even mode, and the effective refractive index of the y-polarized odd mode. λ is the wavelength in vacuum.

[0050] When the coupling length ratio is equal to 0.5 or 2, the light in the two polarization directions can be completely separated. The formula for the coupling length ratio CLR is as follows:

[0051]

[0052] Figures 4 to 9 is the curve graph of the coupling length ratio varying with the wavelength for the embodiments of the present invention and Comparative Examples 1 to 12. It can be seen from the figure that all the coupling length ratio curves increase with the increase of the wavelength, and as the diameter d 1 of the cladding air hole 1 increases, the wavelength at which the coupling length ratio is equal to 2 will undergo a red shift, and the overall curve shows high regularity. The change in the diameter d 2 of the central air hole 2 has a more obvious influence on the coupling length ratio, and the spacing between the three curves becomes significantly wider. The diameter d of the small circular air hole 33 Changes will directly affect the size of the core region, resulting in a huge change in birefringence, which has a significant impact on the beam splitting performance of the polarization beam splitter. As can be seen from Figure 6 , reducing the diameter of the small circular air holes 3 appropriately is beneficial to realizing a polarization beam splitter operating in the mid-infrared band. The distance Λ between adjacent cladding air holes 1, also known as the lattice constant, can be used to adjust the beam propagation mode in the photonic crystal fiber polarization beam splitter. As Λ increases, it becomes more difficult for the input light to be transmitted between the two cores, which means that reducing Λ can obtain a shorter coupling length. However, the beam splitting performance obtained only by adjusting the structure in the polarization beam splitter is very limited. Therefore, we fill aluminum wires in the large circular filling holes 4 on both sides along the Y-axis direction of the central air hole 2. Compared with coating an aluminum layer in the air holes, filling aluminum wires can reduce the direct contact with air, thereby reducing oxidation. Most photonic crystal fiber polarization beam splitters usually select gold with excellent chemical stability and silver with high SPR excitation efficiency as filling materials. Therefore, we Figure 9 compared the effects of these three filling materials on the coupling length ratio. The results show that in the mid-infrared band, due to the stronger influence of SPR, the polarization beam splitters filled with gold or silver have a higher coupling length ratio. In contrast, aluminum is a more suitable photon modulation material in the mid-infrared band.

[0053] The embodiment of the present invention obtains the shortest beam splitting length of 160 μm, the maximum extinction ratio of -72.1 dB, and an ultra-wide band of 1020 nm under the optimal structural parameters.

[0054] As the light propagates in the core, the optical power transmitted between the two cores continuously undergoes periodic exchange. When the power of the incident light in core A is P in , the beam splitting length is L, and assuming P in = 1, the normalized output powers of cores A and B can be obtained. and can be obtained through formulas (5) and (6):

[0055]

[0056] The extinction ratio ER is an important parameter for evaluating the beam splitting effect of the photonic crystal fiber polarization beam splitter, which represents the separation degree between different polarized lights in the two cores. A higher ER means a greater ability to separate polarized lights into the required channels and reduce the interference between them. The ER formulas for cores A and B can be described as:

[0057]

[0058] where respectively represent the output powers in the X polarization direction and the Y polarization direction in core A and core B. The wavelength range where ER is greater than 20 dB can be considered as the splitting bandwidth of the PBS. In this range, the X-polarized and Y-polarized components can be completely separated.

[0059] Figure 10 is the normalized output power curve of the embodiment of the present invention. Through periodic coupling changes, when the energy of one polarization state reaches its first maximum value of 1 and the energy of the other polarization state happens to be at its minimum value of 0, the propagation distance at this time is the minimum coupling length of the PBS and also the minimum length of the device. As shown in the figure, when the propagation distance is 160 μm, the two polarized lights in cores A and B are completely separated.

[0060] Figure 11 and Figure 12 are the curves of the extinction ratio versus wavelength of cores A and B respectively. When the shortest length of the polarization beam splitter is set to 160 μm, core A obtains a maximum extinction ratio of -72.1 dB at 3.3 μm, and the bandwidth is 1020 nm, while the maximum extinction ratio of core B at 3.3 μm is only -44.3 dB, and the bandwidth is only 140 nm. Obviously, using core A as the polarization splitting channel of the present invention has better performance.

[0061] The above are only the preferred embodiments of the present invention, and do not limit the patent scope of the present invention accordingly. Any equivalent structure or equivalent process transformation made by using the content of the specification and drawings of the present invention, or directly or indirectly applied in other related technical fields, shall be included in the patent protection scope of the present invention by the same token.

Claims

1. A novel mid-infrared dual-core photonic crystal fiber polarization beam splitter based on aluminum wire filling, characterized in that: The invention comprises a base material (6), wherein a central air hole (2) is arranged in the central region of the base material (6), two large circular filling holes (4) filled with aluminum wires are located on both sides of the central air hole (2); two small circular air holes (3) are arranged on both sides of the large circular filling hole (4); and a plurality of cladding air holes (1) are arranged outside the large circular filling hole (4) and the small circular air hole (3).

2. The novel mid-infrared double-core photonic crystal fiber polarization beam splitter based on aluminum wire filling according to claim 1 is characterized in that: The diameters of the large circular filling hole (4), the cladding air hole (1), the central air hole (2), and the small circular air hole (3) decrease in sequence.

3. The novel mid-infrared double-core photonic crystal fiber polarization beam splitter based on aluminum wire filling according to claim 2 is characterized in that: The base material (6) includes a cladding region and a core region; the cladding region is composed of the base material (6) and three layers of inner and outer cladding air holes (1) periodically arranged according to a regular hexagonal lattice; two large circular filling holes (4) filled with aluminum wire are distributed on both sides of the central air hole (2) along the Y-axis direction; the region surrounded by the two small circular air holes (3), the large circular filling hole (4) and the cladding air holes (1) on both sides of the central air hole (2) is the core region; and the core region is divided into two cores, namely A and B.

4. The novel mid-infrared double-core photonic crystal fiber polarization beam splitter based on aluminum wire filling according to claim 3 is characterized in that: The four small circular air holes (3) are distributed in the core region in a rectangular structure, and the upper spacings of the four small circular air holes (3) on the X axis and the Y axis are 1.39 μm and 1.75 μm respectively.

5. The novel mid-infrared double-core photonic crystal fiber polarization beam splitter based on aluminum wire filling according to claim 4 is characterized in that: The diameter of the cladding air hole (1) is 1.6 μm.

6. The novel mid-infrared double-core photonic crystal fiber polarization beam splitter based on aluminum wire filling according to claim 5 is characterized in that: The diameter of the central air hole (2) is 1.2 μm.

7. The novel mid-infrared double-core photonic crystal fiber polarization beam splitter based on aluminum wire filling according to claim 6 is characterized in that: The diameter of the small round air hole (3) is 0.7 μm.

8. The novel mid-infrared double-core photonic crystal fiber polarization beam splitter based on aluminum wire filling according to claim 7 is characterized in that: The diameter of the large circular filling hole (4) filled with aluminum wire is 2.0 μm.

9. The novel mid-infrared double-core photonic crystal fiber polarization beam splitter based on aluminum wire filling according to claim 8 is characterized in that: The substrate material (6) is As2S3 glass.

10. The novel mid-infrared double-core photonic crystal fiber polarization beam splitter based on aluminum wire filling according to claim 9, characterized in that: The distance between adjacent cladding air holes is 2.1 μm.

Images