A compact dual-core photonic crystal fiber polarization beam splitter based on gold layer tuning

Abstract

The application discloses a compact double-core photonic crystal fiber polarization beam splitter based on gold layer tuning and belongs to the technical field of optical communication systems. The technical problems that the existing polarization beam splitter cannot consider high extinction ratio and device size are solved. The technical scheme is as follows: the photonic crystal fiber polarization beam splitter comprises a base material, a core area and a cladding area arranged on the base material, the cladding area comprises a plurality of small circular air holes, two large circular air holes and four elliptical air holes, the core area comprises a central elliptical air hole with an inner wall attached with a gold layer and two cores, and the two cores are formed by two regions with two continuous air holes missing. The photonic crystal fiber polarization beam splitter has the advantages of short length and high extinction ratio, the length is only 0.122 mm, the maximum extinction ratio is -65 dB, and the operation bandwidth reaches 100 nm.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of optical communication systems, in particular to a compact dual-core photonic crystal fiber polarization beam splitter based on gold layer tuning. BACKGROUND

[0002] With the rapid development of modern communication networks, people have entered the era of big data, and the demand for information resources is also increasing. The core of the single-mode optical fiber is very small, mainly made of glass or plastic, with large optical loss, easy to break, and the optical signal can only be transmitted in one waveguide mode, which is low in efficiency, and is far from enough as a signal carrier for optical communication. After half a century of development, the emergence of special optical fibers has made scholars become hot in the research of low-loss optical fibers. Among them, the photonic crystal fiber has attracted much attention due to its flexible structure design and performance far superior to traditional optical fibers. The photonic crystal fiber polarization beam splitter is an essential device in an optical communication system. The polarization beam splitter can split a beam of light into two beams of orthogonal polarized light and redistribute them into different optical fibers, realizing polarization state multiplexing, thereby improving the communication capacity of the optical fiber network and increasing the utilization rate of the optical fiber.

[0003] With the rapid development of materials science, semiconductor materials, liquid crystals, glasses and metal materials have been widely concerned due to their unique properties when integrated into optical devices, among which the tuning effect of metal materials has attracted much attention. There are some specific frequency electromagnetic waves in the plasmon of the metal surface, which can resonate with another frequency electromagnetic wave to enhance coupling and improve the performance of optical devices. Moreover, due to the tuning effect of metal, it is easier to adjust the parameters to obtain better performance. For example, CN112230328B and CN104749690A, the former introduces metal filling, and it is found that the extinction ratio of the former is significantly higher than that of the latter. In the research of polarization beam splitter, scholars often use the method of introducing metal film or metal filling to improve the performance of the beam splitter, obtain shorter length, higher extinction ratio and lower loss, such as CN112230328B and CN103091770A. However, the above designs cannot balance high extinction ratio and device size, and different sizes of air holes are introduced, which will have some difficulties in preparation, and it is difficult to prepare a compact beam splitter in a short time. SUMMARY

[0004] In view of the technical problem that the existing polarization beam splitter cannot balance high extinction ratio and device size, the present application provides a compact dual-core photonic crystal fiber polarization beam splitter based on gold layer tuning, which has the advantages of small size, high extinction ratio and low loss.

[0005] To achieve the above object, the application adopts the following technical scheme: a compact double-core photonic crystal fiber polarization beam splitter based on gold layer tuning, comprising a base material, a core region and a cladding region arranged on the base material, the cladding region comprising a plurality of small circular air holes, two large circular air holes and four elliptical air holes; the diameter of the small circular air hole is smaller than that of the large circular air hole.

[0006] The core region comprises a center elliptical air hole with an inner wall attached with a gold layer and two cores; the long axis of the center elliptical air hole is parallel to the y-axis, the two cores are formed by two regions where two continuous air holes are missing, and the two cores are distributed on both sides of the center elliptical air hole along the x-axis direction; the two cores, the four elliptical air holes, the two large circular air holes and the plurality of small circular air holes are arranged in multiple layers of hexagons around the center elliptical air hole.

[0007] The two large circular air holes are distributed on both sides of the core region along the x-axis direction, the four elliptical air holes and the four small circular air holes are distributed on both sides of the core region along the y-axis direction, two small circular air holes and two elliptical air holes are distributed on each side, and the small circular air holes are distributed on the inner side of the elliptical air holes, and the long axis of the elliptical air hole is parallel to the x-axis.

[0008] In addition to the four small circular air holes inside the four elliptical air holes close to the center elliptical air hole, the other plurality of small circular air holes are distributed on the outer side of the large circular air hole and the elliptical air hole.

[0009] Further, the center elliptical air hole with the inner wall attached with the gold layer is distributed at the structural center of the photonic crystal fiber polarization beam splitter.

[0010] Further, the hexagonal structure is a regular hexagonal structure.

[0011] Further, the plurality of small circular air holes are regularly distributed along the center of symmetry in the form of a regular triangle; the two large circular air holes are centrally symmetrically distributed on both sides of the core region along the x-axis direction; and the four elliptical air holes are centrally symmetrically distributed on both sides of the core region along the y-axis direction.

[0012] Further, the base material is N-FK51A glass.

[0013] Further, the long axis of the center elliptical air hole with the inner wall attached with the gold layer is 0.42 μm in length, and the short axis is 0.26 μm in length.

[0014] Further, the thickness of the gold layer is 70 nm.

[0015] Further, the ratio of the diameter of the large circular air hole to the diameter of the small circular air hole is 2, and the diameter of the small circular air hole is 0.35 μm.

[0016] Further, the length of the major axis of the elliptical air hole is 0.6 μm, and the length of the minor axis is 0.45 μm.

[0017] Further, the center distance between the large circular air hole and the small circular air hole and the center distance between two adjacent small circular air holes are the same, and the center distance is 1.2 μm.

[0018] Compared with the prior art, the present application has the following advantages:

[0019] (1) Based on the tuning property of the gold layer and the adjustment of the shape and size of the air hole, the fiber core and the surface plasmon mode resonate in the x and y polarization directions, affect the core coupling, and thus change the coupling length;

[0020] (2) The photonic crystal fiber polarization beam splitter has the advantages of short length, high extinction ratio and low loss. When the length of the beam splitter is 0.122 mm, the extinction ratio reaches -65 dB. The length of the device is reduced while the extinction ratio is improved. At the same time, the operating bandwidth of the beam splitter is 100 nm, covering the 1500 nm-1600 nm waveband near the general communication window. The small size and high extinction ratio are realized, which is more conducive to the application of the photonic crystal fiber polarization beam splitter. BRIEF DESCRIPTION OF DRAWINGS

[0021] Figure 1 Fig. 1 is a schematic diagram of the cross-sectional structure of the embodiment 1 of the present application;

[0022] Figure 2 Fig. 2 is a schematic diagram of the structure parameters of the embodiment 1 of the present application;

[0023] Figure 3 Fig. 3 is a mode field distribution diagram of the even mode in the x polarization direction (a);

[0024] Figure 3 Fig. 4 is a mode field distribution diagram of the odd mode in the x polarization direction (b);

[0025] Figure 3 Fig. 5 is a mode field distribution diagram of the even mode in the y polarization direction (c);

[0026] Figure 3 Fig. 6 is a mode field distribution diagram of the odd mode in the y polarization direction (d);

[0027] Figure 4 Fig. 7 is a diagram of the relationship between the coupling length ratio and the wavelength of the embodiment 1 of the present application and the comparative examples 1 and 2;

[0028] Figure 5This is a graph showing the relationship between the coupling length ratio of Embodiment 1, Comparative Examples 3 and 4 of the present invention and the wavelength.

[0029] Figure 6 This is a graph showing the relationship between the coupling length ratio of Embodiment 1, Comparative Examples 5 and 6 of the present invention and the wavelength.

[0030] Figure 7 This is a graph showing the relationship between the coupling length ratio of Embodiment 1, Comparative Examples 7 and 8 of the present invention and the wavelength.

[0031] Figure 8 This is a graph showing the relationship between the coupling length ratio of Embodiment 1, Comparative Examples 9 and 10 of the present invention and the wavelength.

[0032] Figure 9 This is a graph showing the relationship between the coupling length ratio of Embodiment 1 of the present invention and Comparative Examples 11 and 12 as a function of wavelength.

[0033] Figure 10 This is a normalized power curve diagram of Embodiment 1 of the present invention;

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

[0035] Among them, 1 is the base material, 2 is a small circular air hole, 3 is a large circular air hole, 4 is an elliptical air hole, 5 is a central elliptical air hole, and 6 is a gold layer. Detailed Implementation

[0036] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the present invention will be further described below with reference to the accompanying drawings of the embodiments of the present invention.

[0037] The photonic crystal fiber polarization beam splitter of this invention is made of N-FK51A, which is an environmentally friendly glass that replaces traditional lead and arsenic-containing glass. It is more environmentally friendly and has a low specific gravity, high heat resistance, high refractive index, and stable optical properties. It has a relatively high refractive index of 1.476 at 1550nm.

[0038] This invention introduces a central elliptical air hole at the center of the structure and sets a gold layer on its inner surface. Based on the tunability of the gold layer, the central elliptical air hole resonates with the two polarized beams of light in the fiber core, enabling the device to achieve a length of 0.122 mm and an extinction ratio of -65 dB. The designed photonic crystal fiber is easy to fabricate and relatively easy to realize.

[0039] The refractive index n of FK51A N-FK51A (λ) can be obtained using the following Sellmeier formula (1):

[0040]

[0041] where B1=0.97124782, B2=0.216901417, B3=0.904654666, C1=0.00472301995, C2=0.0153575612, C3=168.6813300, and λ is the wavelength.

[0042] The coupling length CL in different polarization directions can be obtained by the following formula (2):

[0043]

[0044] where i=x or y, are the effective refractive indexes of even and odd modes respectively, and λ is the wavelength in vacuum.

[0045] When the incident wavelength is fixed, the relative coupling length ratio CLR, i.e. the coupling length ratio, can be defined as the following formula (3):

[0046]

[0047] where m and n are positive integers with opposite parity; CL x is the coupling length in x polarization direction, and CL y is the coupling length in y polarization direction.

[0048] Embodiment 1

[0049] The photonic crystal fiber polarization beam splitter of the embodiment, as shown in Figure 1 includes a base material 1, a core region and a cladding region arranged on the base material 1, and the cladding region is composed of eighty small circular air holes 2, two large circular air holes 3 and four elliptical air holes 4; the diameter of the small circular air hole 2 is smaller than that of the large circular air hole 3.

[0050] The core region includes a center elliptical air hole 5 provided with a gold layer 6 and two cores; the center elliptical air hole 5 is distributed at the structural center of the photonic crystal fiber polarization beam splitter, the long axis of the center elliptical air hole 5 is parallel to the y axis, and the two cores are composed of two regions in which two continuous air holes are missing, and the two cores are distributed on both sides of the center elliptical air hole 5 along the x axis direction; taking the center elliptical air hole 5 as the center, the two cores, the four elliptical air holes 4, the two large circular air holes 3 and the eighty small circular air holes 2 are arranged in five layers of hexagons around the center elliptical air hole 5.

[0051] Two large circular air holes are distributed on both sides of the fiber core region along the x-axis, with one large circular air hole on each side; four elliptical air holes 4 and four small circular air holes 2 are distributed on both sides of the fiber core region along the y-axis, with two small circular air holes and two elliptical air holes 4 on each side, and the small circular air holes 2 are distributed inside the elliptical air holes 4, with the major axis of the elliptical air holes 4 parallel to the x-axis; except for the four small circular air holes 2 located inside the four elliptical air holes and close to the central elliptical air hole 5, the other seventy-six small circular air holes are distributed outside the two large circular air holes and the four elliptical air holes 4.

[0052] Eighty small circular air holes 2 are arranged in an equilateral triangle periodic pattern and are symmetrically distributed along the center. Two large circular air holes 3 are symmetrically distributed on both sides of the outer side of the two fiber cores along the x-axis. Four elliptical air holes 4 are symmetrically distributed on both sides of the fiber core area along the y-axis.

[0053] The photonic crystal fiber is made of N-FK51A glass. The size ratio of the central elliptical air hole 5 is η = 1, with a major axis length a1 = 0.42 μm and a minor axis length b1 = 0.26 μm. The gold layer thickness is t = 70 nm. The diameter ratio of the large circular air hole to the small circular air hole is δ = d2 / d1 = 2, with a diameter d2 = 0.7 μm for the large circular air hole and a diameter d1 = 0.35 μm for the small circular air hole. The major axis length of the elliptical air hole is a2 = 0.6 μm, and the minor axis length is b2 = 0.45 μm. The center-to-center distance between adjacent large and small circular air holes is the same as the circular distance between two adjacent small circular air holes, with a center-to-center distance Λ = 1.2 μm.

[0054] like Figure 1 As shown, the fiber core region of this embodiment is composed of a central elliptical air hole 5 with an inner gold layer 6 and two fiber cores. The central elliptical air hole 5 is located between the two fiber cores. The cladding region is composed of an elliptical air hole 4 and two types of large circular air holes 3 and small circular air holes 2 of different diameters arranged in a periodic hexagonal pattern. The center-to-center distance between adjacent large circular air holes 3 and small circular air holes 2 is equal to the center-to-center distance between two adjacent small circular air holes 2. The four elliptical air holes 4 are symmetrically distributed on both sides of the fiber core region along the y-axis direction. Each fiber core has elliptical air holes 4 on both sides along the y-axis direction. Each fiber core has a central elliptical air hole 5 on one side and a large circular air hole 3 on the other side along the x-axis direction.

[0055] Figure 2is a structural parameter schematic diagram of embodiment 1 of the present application, the space coordinates are xyz, wherein d1 and d2 respectively represent the diameters of the large circular air hole and the small circular air hole, Lambda represents the center distance between the adjacent large circular air hole and the small circular air hole and the center distance between the adjacent two small circular air holes, t represents the thickness of the gold layer, a1 and b1 respectively represent the major axis and the minor axis of the central elliptical air hole, and a2 and b2 respectively represent the major axis and the minor axis of the elliptical air hole.

[0056] Figure 3 (a) is the mode field distribution diagram of the even mode in the x polarization direction, Figure 3 (b) is the mode field distribution diagram of the odd mode in the x polarization direction, Figure 3 (c) is the mode field distribution diagram of the even mode in the y polarization direction, Figure 3 (d) is the mode field distribution diagram of the odd mode in the y polarization direction. The arrows in the figure represent the electric field direction under different modes, the core and surface plasmon mode resonate in the x and y polarization directions, and the resonance is more obvious in the x polarization direction. The resonance will affect the coupling of the core, thereby changing the length of the device.

[0057] Comparative example 1

[0058] The same as example 1, the difference is that the small circular air hole diameter d1=0.325μm.

[0059] Comparative example 2

[0060] The same as example 1, the difference is that the small circular air hole diameter d1=0.375μm.

[0061] Comparative example 3

[0062] The same as example 1, the difference is that the center distance between the adjacent large circular air hole and the small circular air hole and the center distance between the adjacent two small circular air holes Lambda=1.15.

[0063] Comparative example 4

[0064] The same as example 1, the difference is that the center distance between the adjacent large circular air hole and the small circular air hole and the center distance between the adjacent two small circular air holes Lambda=1.25.

[0065] Comparative example 5

[0066] The same as example 1, the difference is that the large circular air hole diameter d2=0.65μm.

[0067] Comparative example 6

[0068] The same as example 1, the difference is that the large circular air hole diameter d2=0.75μm.

[0069] Comparative example 7

[0070] Same as Example 1, except that the minor axis length of the elliptical air hole is b2 = 0.40 μm.

[0071] Comparative Example 8

[0072] Same as in Example 1, except that the minor axis length of the elliptical air hole is b2 = 0.50 μm.

[0073] Comparative Example 9

[0074] Same as in Example 1, except that the size multiple of the central elliptical air hole η = 0.9.

[0075] Comparative Example 10

[0076] Same as in Example 1, except that the size multiple of the central elliptical air hole η = 1.1.

[0077] Comparative Example 11

[0078] Same as Example 1, except that the gold layer thickness t = 60 nm.

[0079] Comparative Example 12

[0080] Same as Example 1, except that the gold layer thickness t = 80 nm.

[0081] The relationship between the coupling length ratio and wavelength in Example 1 and Comparative Examples 1-12 is as follows: Figures 4 to 9 As shown. Figures 4 to 9 As shown, these represent the coupling length ratios of devices under different structural parameters, thus indirectly reflecting the performance of the devices. Figure 4 and Figures 6 to 7 As shown, in Examples 1, 1, 2, 5 to 8, changing the diameter d1 of the small circular air hole, the diameter d2 of the large circular air hole, and the minor axis b2 of the elliptical air hole can change the wavelength band when the coupling length ratio is 2. The operating wavelength is near the commonly used communication window of 1550nm. Preferably, d1 = 0.35μm, d2 = 0.70μm, and b2 = 0.45μm. Figure 5 As shown, in Example 1, Comparative Example 3, and Comparative Example 4, changing the center-to-center distance between adjacent large and small circular air holes and the center-to-center distance between two adjacent small circular air holes can also change the wavelength band when the coupling length ratio is 2. Preferably, Λ = 1.20 μm; Figure 8 As shown, in Example 1, Comparative Example 9, and Comparative Example 10, changing the size factor η of the central ellipse can significantly alter the coupling length. At 1550 nm, a coupling length of 2 effectively ensures a short beam splitter length; preferably, η = 1.0. Figure 9As shown in Embodiment 1, Comparative Example 11 and Comparative Example 12, the coupling length ratio difference can be changed by changing the thickness t of the gold layer, and preferably t = 70 nm.

[0082] Under the splitter structure parameters of Embodiment 1, the optical fiber length L of the splitter is 0.122 mm, the maximum extinction ratio reaches -65 dB, and a relatively wide 100 nm operating bandwidth is obtained.

[0083] As shown in Embodiment 1, Comparative Example 11 and Comparative Example 12, the coupling length ratio difference can be changed by changing the thickness t of the gold layer, and preferably t = 70 nm. Figures 10 to 11 The normalized power and extinction ratio curves of the splitter under the basic structure data are sequentially shown, the length of the splitter is determined by the peak value of the two-core normalized power difference, the extinction ratio reflects the pros and cons of the device performance, and is quantified by the calculation formula.

[0084] On the basis of ignoring the transmission loss, assuming that the optical power incident into the double-core PCF is P in , the output power in the left core A is , and the output power in the right core B is The normalized powers P i,A and P i,B in the x and y directions corresponding to the cores A and B can be obtained by formulas (4) and (5):

[0085]

[0086]

[0087] The extinction ratios ER A and ER B in the x and y directions of the two cores can be calculated by formulas (6) and (7):

[0088]

[0089]

[0090] wherein P and P are the output powers in the x and y directions of the core A and the core B, respectively.

[0091] Figure 10 The normalized power under the basic structure data is shown, the normalized power of one beam reaches the highest, the normalized power of the other beam reaches the lowest, and the normalized power difference between the two beams also reaches the maximum value at the transmission distance of 0.122 mm, i.e., the length of the splitter. Figure 11 The extinction ratio curve with the wavelength change is shown, and it is found that the highest extinction ratio can reach -65 dB, and the bandwidth is 100 nm.

[0092] By comparing and analyzing the performance of the polarizing beam splitter of example 1 with that of comparative examples 1-12, and considering the possible errors in the preparation process and the material loss in the use process, the basic structural parameters d1=0.35 μm, d2=0.7 μm, Λ=1.2 μm, t=70 nm, a2=0.6 μm, b2=0.45 μm, a1=0.42 μm, b1=0.26 μm of the present application are determined.

[0093] Finally, it should be noted that: the above examples are only used to illustrate the technical solutions of the present application, but not to limit it. Although the present application has been described in detail with reference to the above examples, it should be understood by those skilled in the art that the specific embodiments of the present application can be modified or replaced by equivalents without departing from the spirit and scope of the present application, and any modification or equivalent replacement without departing from the spirit and scope of the present application should be covered in the scope of the claims of the present application.

Claims

1. A compact dual-core photonic crystal fiber polarization beam splitter based on gold layer tuning, characterized in that, It includes a base material (1) and a core region and a cladding region disposed on the base material (1). The cladding region includes a plurality of small circular air holes (2), two large circular air holes (3), and four elliptical air holes (4). The diameter of the small circular air holes (2) is smaller than the diameter of the large circular air holes (3). The fiber core region includes a central elliptical air hole (5) with a gold layer (6) attached to its inner wall and two fiber cores; the major axis of the central elliptical air hole (5) is parallel to the y-axis, and the two fiber cores are formed by two regions that are missing two consecutive air holes. The two fiber cores are distributed on both sides of the central elliptical air hole (5) along the x-axis direction; the two fiber cores, four elliptical air holes (4), two large circular air holes (3) and multiple small circular air holes (2) are arranged in a multi-layer hexagonal pattern around the central elliptical air hole (5); Two large circular air holes (3) are distributed on both sides of the fiber core region along the x-axis direction, and four elliptical air holes (4) and four of them small circular air holes (2) are distributed on both sides of the fiber core region along the y-axis direction. Two small circular air holes (2) and two elliptical air holes (4) are distributed on each side, and the small circular air holes are distributed inside the elliptical air holes. The major axis of the elliptical air holes (4) is parallel to the x-axis. In addition to the four small circular air holes located inside the four elliptical air holes and close to the central elliptical air hole (5), a number of other small circular air holes are distributed outside the large circular air hole and the elliptical air hole.

2. The compact dual-core photonic crystal fiber polarization beam splitter based on gold layer tuning according to claim 1, characterized in that, The central elliptical air hole (5) with a gold layer (6) attached to the inner wall is distributed at the structural center of the photonic crystal fiber polarization beam splitter.

3. The compact dual-core photonic crystal fiber polarization beam splitter based on gold layer tuning according to claim 1, characterized in that, The hexagonal structure is a regular hexagonal structure.

4. The compact dual-core photonic crystal fiber polarization beam splitter based on gold layer tuning according to claim 1, characterized in that, The multiple small circular air holes (2) are arranged in an equilateral triangle periodic pattern and are symmetrically distributed along the center; the two large circular air holes (3) are symmetrically distributed on both sides of the fiber core region along the x-axis; the four elliptical air holes (4) are symmetrically distributed on both sides of the fiber core region along the y-axis.

5. The compact dual-core photonic crystal fiber polarization beam splitter based on gold layer tuning according to claim 1, characterized in that, The substrate material (1) is N-FK51A glass.

6. The compact dual-core photonic crystal fiber polarization beam splitter based on gold layer tuning according to claim 1, characterized in that, The major axis of the central elliptical air hole (5) of the inner wall with gold layer (6) is 0.42 μm and the minor axis is 0.26 μm.

7. The compact dual-core photonic crystal fiber polarization beam splitter based on gold layer tuning according to claim 1, characterized in that, The thickness of the gold layer is 70 nm.

8. The compact dual-core photonic crystal fiber polarization beam splitter based on gold layer tuning according to claim 1, characterized in that, The diameter ratio of the large circular air hole to the small circular air hole is 2, and the diameter of the small circular air hole is 0.35 μm.

9. The compact dual-core photonic crystal fiber polarization beam splitter based on gold layer tuning according to claim 1, characterized in that, The major axis of the elliptical air hole (4) is 0.6 μm and the minor axis is 0.45 μm.

10. The compact dual-core photonic crystal fiber polarization beam splitter based on gold layer tuning according to claim 1, characterized in that, The center-to-center distance between adjacent large circular air holes and small circular air holes is the same as the center-to-center distance between two adjacent small circular air holes, and the center-to-center distance is 1.2 μm.

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