Ultra-wideband high-extinction-ratio dual-core photonic crystal fiber polarization beam splitter

Abstract

The invention provides an ultra-wideband high-extinction-ratio dual-core photonic crystal fiber polarization beam splitter, and particularly relates to the technical field of photonic crystal fibers (PCF), the ultra-wideband high-extinction-ratio dual-core photonic crystal fiber polarization beam splitter comprises a cladding and two fiber cores arranged at central symmetry positions, and the refractive index of the fiber cores is higher than that of the cladding. The cladding is composed of a background material and first oval air holes which are periodically distributed, the structure center of the polarization beam splitter comprises two third oval air holes which are symmetrically arranged in the vertical direction and are the same in structure size, and the two fiber cores are symmetrically arranged on the two sides of the two oval air holes according to the center. The optical fiber is flexible and compact in structural design and excellent in optical performance, has the short optical fiber length on the premise of having the high extinction ratio and the ultra-wide bandwidth range, can be widely applied to high-precision optical measurement, quantum communication, sensors and other fields needing high polarization purity, and has the advantage of being high in application value compared with the prior art. The invention has higher polarization selectivity and better environmental stability.

Description

Technical Field

[0001] The invention relates to the technical field of photonic crystal optical fiber (PCF), and in particular to an ultra-wideband high extinction ratio double-core photonic crystal optical fiber polarization beam splitter. Background Art

[0002] Photonic Crystal Fiber (PCF) is a type of optical fiber designed based on the principle of photonic crystals. It controls the propagation characteristics of light by introducing a periodic microstructure inside the optical fiber. This type of optical fiber has a wide range of applications in modern optical communications, sensors, and optical devices due to its unique optical properties and design flexibility.

[0003] A photonic crystal fiber beam splitter is an optical device used to distribute an input optical signal to multiple output channels. Compared with traditional fiber beam splitters, photonic crystal fiber beam splitters can provide higher performance and greater design freedom.

[0004] Bandwidth is the wavelength range in which the polarization beam splitter works, which determines the transmission capacity of the optical communication system to a certain extent. The wavelength range corresponding to the extinction ratio of 20dB is usually defined as the bandwidth of the polarization beam splitter. The wider the bandwidth, the larger the transmission range and the higher the capacity of the optical communication system. Ultra-wideband photonic crystal fiber beam splitters can maintain good beam splitting performance over a very wide wavelength range. This characteristic originates from the bandgap effect of photonic crystal fibers. By precisely designing the microstructure in the optical fiber, light propagation and beam splitting can be achieved almost over the entire spectrum. This broadband characteristic enables the beam splitter to support a variety of applications, including optical communications over a wide wavelength range, spectral analysis, and multi-functional sensors.

[0005] Extinction Ratio (ER) refers to the ratio of signal strength when a fiber optic splitter passes and blocks optical signals. An extremely high extinction ratio means that the fiber optic splitter can very effectively reduce light leakage when blocking unwanted optical signals, thereby improving signal quality and overall system performance. The key to achieving a high extinction ratio lies in the precise design and manufacturing process of the optical fiber structure to ensure that optical signals can minimize loss and interference during transmission.

[0006] In order to achieve ultra-wideband characteristics, the bandgap structure inside the optical fiber must be precisely designed to ensure that the propagation of light is effectively controlled over a wide wavelength range. Ensuring that the extinction ratio of the beam splitter remains at an extremely high level over the entire operating wavelength range requires optimizing the design parameters of the optical fiber, including aperture size, hole spacing, and fiber length. Summary of the invention

[0007] The purpose of the present invention is to provide an ultra-wideband high extinction ratio dual-core photonic crystal fiber polarization beam splitter to solve the above-mentioned shortcomings in the technology.

[0008] In order to achieve the above object, the present invention provides the following technical solutions:

[0009] An ultra-wideband high extinction ratio double-core photonic crystal fiber polarization beam splitter comprises a cladding and two fiber cores arranged symmetrically at the center, wherein the refractive index of the fiber core is higher than the refractive index of the cladding, wherein the cladding is composed of a background material and periodically distributed first elliptical air holes, the structural center of the polarization beam splitter comprises two third elliptical air holes arranged symmetrically in the vertical direction, the two fiber cores are arranged symmetrically at the center on both sides of the elliptical air holes, two second elliptical air holes arranged side by side in the horizontal direction are distributed on the left and right sides of the fiber core, four identical larger first circular air holes are arranged around the fiber core, and four identical smaller second circular air holes are respectively arranged directly above and directly below the fiber core.

[0010] The ultra-wideband high extinction ratio dual-core photonic crystal fiber polarization beam splitter is characterized in that the first elliptical air holes arranged in the cladding are arranged in a regular hexagonal arrangement, the major semi-axis a5 and the minor semi-axis b5 of the first elliptical holes are 0.5 μm and 0.3 μm respectively, and the ellipticity η=a5 / b5, wherein η=1.67, the horizontal axis spacing is 0.7 μm, and the vertical axis spacing e1 is 1 μm.

[0011] The ultra-wideband high extinction ratio dual-core photonic crystal fiber polarization beam splitter is characterized in that the major semi-axis a1 and the minor semi-axis b1 of the third elliptical air hole are 1.6μm and 0.2μm respectively, and the spacing e1 is 0.5μm, the major semi-axis a2 and the minor semi-axis b2 of the second elliptical air hole are 1.2um and 0.2μm respectively, the spacing e1 is 0.45μm, and the lateral distance L2 of the center of the circle is 6μm, the diameter of the first circular air hole is 1.15μm, the lateral distance L1 of the center of the circle is 2.2μm, and the longitudinal distance L4 is 2.1.μm, the radius of the second circular air hole is 0.3μm, the spacing is 0.7μm, and the lateral distance L3 of the center of the circle is 2.2μm.

[0012] The ultra-wideband high extinction ratio dual-core photonic crystal fiber polarization beam splitter is characterized by: using finite element analysis software COMSOL Multiphysics for simulation, adding electromagnetic wave frequency domain (ewfd) and combining with perfect matching layer (PML) for calculation, to obtain the optical mode field distribution of the present invention. According to the electric field distribution diagram obtained by simulation, the birefringence, coupling length, normalized optical power and extinction ratio are calculated.

[0013] The ultra-wideband high extinction ratio dual-core photonic crystal fiber polarization beam splitter is characterized in that the birefringence coefficient calculation formula is:

[0014]

[0015] In the formula, Re represents the real part, and are the effective refractive indices for the x-polarization mode and the y-polarization mode, respectively.

[0016] The ultra-wideband high extinction ratio dual-core photonic crystal fiber polarization beam splitter according to claim 1 is characterized in that the coupling length formula is:

[0017]

[0018]

[0019] Where: λ is the wavelength of incident light; is the odd-mode propagation constant in the x direction; is the even mode propagation constant in the x direction; is the odd-mode propagation constant in the y direction; is the even-mode propagation constant in the y direction; is the odd-mode effective refractive index in the x direction; is the even-mode effective refractive index in the x direction; is the odd-mode effective refractive index in the y direction; is the even-mode effective refractive index in the y direction.

[0020] The normalized optical power formula of the two polarized lights x and y is:

[0021]

[0022]

[0023] Where: is the input power of the incident light in the x direction; is the input power of the incident light in the y direction; is the output power of the incident light in the x direction; is the output power of the incident light in the y direction.

[0024] The extinction ratio (ER) formula is:

[0025]

[0026] Where: is the optical power of x-polarized light at the output port; is the optical power of y-polarized light at the output port.

[0027] The ultra-wideband high extinction ratio dual-core photonic crystal fiber polarization beam splitter is characterized in that the photonic crystal fiber has a refractive index of 1.444 and a density of 2203kg / m3 of silicon dioxide.

[0028] The present invention obtains the best value of coupling length ratio of 2:1 when the incident wavelength is 1550nm, and the birefringence reaches 1.3×10 -3 The extinction ratio reaches 94.23dB, the extinction ratio is less than 20dB in the wavelength range of 1300-1850nm, the bandwidth can reach 550nm, and the fiber length L is only 407.89μm. It has a short fiber length, ultra-wide bandwidth and high extinction ratio, which meets the needs of future ultra-large capacity and easy-to-integrate all-optical networks. The invention has a simple structure, is easy to manufacture, and has low material cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the drawings required for use in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in the present invention. For ordinary technicians in this field, other drawings can also be obtained based on these drawings.

[0030] Figure 1 is a schematic diagram of a photonic crystal fiber beam splitter of the present invention;

[0031] Figure 2 It is a schematic diagram of the dimensions of the photonic crystal fiber beam splitter of the present invention;

[0032] FIG3( a ) is one of the electric field mode distribution diagrams of the photonic crystal fiber beam splitter of the present invention in the x transmission direction;

[0033] FIG3( b ) is a second diagram of the electric field mode distribution of the photonic crystal fiber beam splitter of the present invention in the x transmission direction;

[0034] FIG3( c ) is one of the electric field mode distribution diagrams of the photonic crystal fiber beam splitter of the present invention in the y transmission direction;

[0035] FIG3( d ) is a second diagram of the electric field mode distribution of the photonic crystal fiber beam splitter of the present invention in the y transmission direction;

[0036] Figure 4 A graph showing the relationship between the refractive index of each polarization mode of the photonic crystal fiber beam splitter of the present invention and the incident wavelength;

[0037] Figure 5 A graph showing the relationship between the x and y polarization coupling lengths of the photonic crystal fiber beam splitter of the present invention and the incident wavelength;

[0038] Figure 6 This is a graph showing the variation of x- and y-polarized light powers of the photonic crystal fiber beam splitter with fiber length;

[0039] Figure 7 The figure is a graph showing the relationship between the extinction ratio of the photonic crystal fiber beam splitter of the present invention and the incident wavelength.

[0040] Description of reference numerals:

[0041] 1. The third elliptical air hole; 2. The second elliptical air hole; 3. The first circular air hole; 4. The second circular air hole; 5. The first elliptical air hole. DETAILED DESCRIPTION

[0042] In order to make the purpose, technical scheme and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0043] In order to achieve the above-mentioned purpose, the terms used in the embodiments of the present invention are only for the purpose of describing specific embodiments, and are not intended to limit the present invention. The singular forms "a", "said" and "the" used in the embodiments of the present invention and the appended claims are also intended to include plural forms, unless the context clearly indicates other meanings, and "multiple" generally includes at least two.

[0044] It should be understood that the term "and / or" used in this article is only a description of the association relationship of associated objects, indicating that there can be three relationships. For example, A and / or B can represent: A exists alone, A and B exist at the same time, and B exists alone. In addition, the character " / " in this article generally indicates that the associated objects before and after are in an "or" relationship.

[0045] It should be understood that, although the terms first, second, third, etc. may be used to describe in the embodiments of the present invention, it should not be limited to these terms. It should also be noted that the terms "include", "comprise" or any other variants thereof are intended to cover non-exclusive inclusion, so that a commodity or device including a series of elements includes not only those elements, but also includes other elements not explicitly listed, or also includes elements inherent to such commodity or device. In the absence of further restrictions, the elements defined by the sentence "comprise a ..." do not exclude the presence of other identical elements in the commodity or device including the elements.

[0046] The present invention provides Figure 1-Figure 6 The specific application of an ultra-wideband high extinction ratio dual-core photonic crystal fiber polarization beam splitter is shown:

[0047] Select a refractive index of 1.444 and a density of 2203 kg / m 3Silica is used as the basic material of photonic crystal fiber. The first elliptical air holes are arranged in a regular hexagonal pattern, the long semi-axis a5 and the short semi-axis b5 of the first elliptical holes are 0.5 μm and 0.3 μm respectively, the ellipticity η=a5 / b5, wherein η=1.67, the horizontal axis spacing is 0.7 μm, and the vertical axis spacing e1 is 1 μm.

[0048] The major semi-axis a5 and the minor semi-axis b5 of the first elliptical hole are 0.5 μm and 0.3 μm respectively, and the ellipticity η=a5 / b5, wherein η=1.67, the horizontal axis spacing is 0.7 μm, and the vertical axis spacing e1 is 1 μm.

[0049] The major semi-axis a1 and the minor semi-axis b1 of the third elliptical air hole 1 are 1.6 μm and 0.2 μm respectively, and the spacing e1 is 0.5 μm. The major semi-axis a2 and the minor semi-axis b2 of the second elliptical air hole 2 are 1.2 μm and 0.2 μm respectively, and the spacing e1 is 0.45 μm. The lateral distance L2 of the center of the circle is 6 μm. The diameter of the first circular air hole 3 is 1.15 μm, the lateral distance L1 of the center of the circle is 2.2 μm, and the longitudinal distance L4 is 2.1. μm. The radius of the second circular air hole 4 is 0.3 μm, the spacing is 0.7 μm, and the lateral distance L3 of the center of the circle is 2.2 μm.

[0050] The finite element analysis software COMSOL Multiphysics was used to simulate and analyze the manufactured photonic crystal fiber, adding the electromagnetic wave frequency domain (ewfd) and combining the perfect matching layer (PML) for calculation. The optical mode field distribution was obtained through simulation, and the performance indicators such as birefringence, coupling length, normalized optical power and extinction ratio were calculated based on the electric field distribution diagram. The calculation results were compared with the theoretical design values ​​to verify the accuracy and performance of the optical fiber structure.

[0051] The formula for calculating the birefringence coefficient is:

[0052]

[0053] In the formula, Re represents the real part, and are the effective refractive indices for the x-polarization mode and the y-polarization mode, respectively.

[0054] The coupling length formula is:

[0055]

[0056]

[0057] Where: λ is the wavelength of incident light; is the odd-mode propagation constant in the x direction; is the even mode propagation constant in the x direction; is the odd-mode propagation constant in the y direction; is the even-mode propagation constant in the y direction; is the odd-mode effective refractive index in the x direction; is the even-mode effective refractive index in the x direction; is the odd-mode effective refractive index in the y direction; is the even-mode effective refractive index in the y direction.

[0058] The normalized optical power formula of the two polarized lights x and y is:

[0059]

[0060]

[0061] Where: is the input power of the incident light in the x direction; is the input power of the incident light in the y direction; is the output power of the incident light in the x direction; is the output power of the incident light in the y direction.

[0062] The extinction ratio (ER) formula is:

[0063]

[0064] Where: is the optical power of x-polarized light at the output port; is the optical power of y-polarized light at the output port.

[0065] like Figure 5 As shown, it can be seen that the relationship between the variation of the bias coupling length and the wavelength in this embodiment. From the mode coupling theory, it can be known that there are 4 supermodes in the dual-core photonic crystal fiber, namely the odd mode and the even mode in the x polarization direction, and the odd mode and the even mode in the y polarization direction. The odd and even modes of the same polarization state are coupled along the propagation direction of the optical fiber, so that the polarized light energy is transferred from one core to another. When the light energy of a certain polarization state in the incident core is 0, the corresponding propagation distance is called the coupling length, and the ratio of the coupling lengths in the two polarization directions is called the coupling length ratio. The coupling length and the coupling length ratio determine the transmission distance of the light coupling in the optical fiber, that is, determine the size of the polarization beam splitter. The coupling length ratio is closest to 2 at a wavelength of 1550nm. The coupling length ratio close to 2 is a crucial factor in the performance of the optical fiber polarization beam splitter.

[0066] like Figure 6 As shown in FIG. 1 , it can be seen that the normalized energy of the polarization beam splitter in different polarization directions in this embodiment varies with the length of the polarization beam splitter. Figure 4It can be seen that when the optical fiber length is 407.89 μm, the energy of the x-polarized light at a wavelength of 1550 nm reaches a maximum in one fiber core, while the energy of the y-polarized light reaches a maximum in another fiber core, and the two lights are completely separated.

[0067] like Figure 7 As shown, it can be seen that the extinction ratio of the polarization beam splitter in this embodiment varies with wavelength. From the figure, it can be seen that the extinction ratio at a wavelength of 1550nm can be 94.23dB, the wavelength range where the extinction ratio is less than 20dB is 1.30μm to 1.85μm, and the bandwidth can reach 550nm, which has a large extinction ratio and broadband.

[0068] From the above analysis, it can be concluded that the present invention obtains the best value of coupling length ratio of 2:1 when the incident wavelength is 1550nm, and the birefringence reaches 4.3×10 -3 The extinction ratio reaches 94.23dB, the extinction ratio is less than 20dB in the wavelength range of 1300-1850nm, the bandwidth can reach 550nm, and the fiber length L is only 407.89μm. It has a short fiber length, ultra-wide bandwidth and high extinction ratio, which meets the needs of future ultra-large capacity and easy-to-integrate all-optical networks. The invention has a simple structure, is easy to manufacture, and has low material cost.

[0069] The above description is only by way of illustration of certain exemplary embodiments of the present invention. It is undoubted that those skilled in the art can modify the described embodiments in various ways without departing from the spirit and scope of the present invention. Therefore, the above drawings and descriptions are illustrative in nature and should not be construed as limiting the scope of protection of the claims of the present invention.

Claims

1. An ultra-wideband high extinction ratio dual-core photonic crystal fiber polarization beam splitter, comprising a cladding and two cores arranged symmetrically in the center, wherein the refractive index of the core is higher than the refractive index of the cladding, wherein: The cladding is composed of a background material and periodically distributed first elliptical air holes (5); the structural center of the polarization beam splitter includes two third elliptical air holes (1) arranged symmetrically in the vertical direction; two fiber cores are arranged symmetrically on both sides of the elliptical air holes; two second elliptical air holes (2) arranged in parallel in the horizontal direction are distributed on the left and right sides of the fiber core; four identical larger first circular air holes (3) are arranged around the fiber core; and four identical smaller second circular air holes (4) are respectively arranged directly above and directly below the fiber core.

2. The ultra-wideband high extinction ratio dual-core photonic crystal fiber polarization beam splitter according to claim 1, characterized in that: The first elliptical air holes arranged in the cladding are arranged in a regular hexagonal pattern, with the major semi-axis a5 and the minor semi-axis b5 of the first elliptical holes being 0.5 μm and 0.3 μm respectively, and the ellipticity η=a5 / b5, wherein η=1.67, the horizontal axis spacing is 0.7 μm, and the vertical axis spacing e1 is 1 μm.

3. The ultra-wideband high extinction ratio dual-core photonic crystal fiber polarization beam splitter according to claim 1, characterized in that: The major semi-axis a1 and the minor semi-axis b1 of the third elliptical air hole (1) are 1.6 μm and 0.2 μm respectively, and the spacing e1 is 0.5 μm. The major semi-axis a2 and the minor semi-axis b2 of the second elliptical air hole (2) are 1.2 μm and 0.2 μm respectively, and the spacing e1 is 0.45 μm. The lateral distance L2 of the center of the circle is 6 μm. The diameter of the first circular air hole (3) is 1.15 μm, the lateral distance L1 of the center of the circle is 2.2 μm, and the longitudinal distance L4 is 2.1 μm. The radius of the second circular air hole (4) is 0.3 μm, the spacing is 0.7 μm, and the lateral distance L3 of the center of the circle is 2.2 μm.

4. The ultra-wideband high extinction ratio dual-core photonic crystal fiber polarization beam splitter according to claim 1, characterized in that: The finite element analysis software COMSOL Multiphysics was used for simulation, and the electromagnetic wave frequency domain (ewfd) was added and combined with the perfect matching layer (PML) for calculation to obtain the optical mode field distribution of the present invention. The birefringence, coupling length and extinction ratio were calculated based on the electric field distribution diagram obtained by simulation.

5. The ultra-wideband high extinction ratio dual-core photonic crystal fiber polarization beam splitter according to claim 4, characterized in that: The formula for birefringence is: In the formula, Re represents the real part, and are the effective refractive indices for the x-polarization mode and the y-polarization mode, respectively.

6. The ultra-wideband high extinction ratio dual-core photonic crystal fiber polarization beam splitter according to claim 4, characterized in that: The coupling length formula is: Where: λ is the wavelength of incident light; is the odd-mode propagation constant in the x direction; is the even mode propagation constant in the x direction; is the odd-mode propagation constant in the y direction; is the even-mode propagation constant in the y direction; is the odd-mode effective refractive index in the x direction; is the even-mode effective refractive index in the x direction; is the odd-mode effective refractive index in the y direction; is the even-mode effective refractive index in the y direction. The normalized optical power formula of the two polarized lights x and y is: Where: is the input power of the incident light in the x direction; is the input power of the incident light in the y direction; is the output power of the incident light in the x direction; is the output power of the incident light in the y direction. The extinction ratio (ER) formula is: Where: is the optical power of x-polarized light at the output port; is the optical power of y-polarized light at the output port.

7. The ultra-wideband high extinction ratio dual-core photonic crystal fiber polarization beam splitter according to claim 1, characterized in that: The photonic crystal fiber beam splitter has a refractive index of 1.444 and a density of 2203 kg / m 3 of silicon dioxide.

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