Method for preparing photonic crystal fiber with sensing and filtering functions

Abstract

The invention provides a method for preparing a photonic crystal fiber with sensing and filtering functions. The method comprises the specific steps that a first layer of air holes are formed in the circle center of quartz; by taking the first layer of air holes as the center, two second type of air holes and three third type of air holes are formed in the periphery of the first layer of air holes along a regular hexagon; by taking the first layer of air holes as the center, two first type of air holes and nine third type of air holes are formed in the periphery of the first layer of air holes along a regular hexagon; thirteen third-type air holes are formed in the periphery of the first-layer air holes in a regular hexagon mode with the first-layer air holes as the center; and preparing a preform according to the arranged cladding, melting and drawing wires, and depositing gold on a polished plane to obtain the photonic crystal fiber, and immersing the photonic crystal fiber in a liquid to be detected when in use. According to the invention, integration of the optical fiber sensor and the filter is realized, high-sensitivity sensing is realized in the X polarization direction, and polarization filtering with adjustable bandwidth and high extinction ratio is realized in the Y polarization direction.

Description

Technical Field

[0001] The invention relates to the field of optical fiber sensing and filtering, and in particular to a method for preparing a photonic crystal optical fiber with sensing and filtering functions. Background Art

[0002] With the continuous development of optical communication and Internet of Things technology, high-sensitivity sensors and high-performance bandwidth-adjustable polarization filters based on photonic crystal fibers have attracted widespread attention. This is of great significance to the development of biomedicine, industrial safety, environmental protection and other fields.

[0003] By changing the structure of photonic crystal fiber or introducing metal plasma materials on the surface of the fiber, the performance of sensors and filters can be improved by using surface plasmon resonance (SPR) technology. When the liquid to be tested is adsorbed on the metal surface, the change in the refractive index of the liquid to be tested will change the resonance wavelength of the surface plasmon resonance. By measuring the change in the resonance wavelength, the change in the liquid to be tested can be detected, achieving a highly sensitive sensing function. In addition, by adjusting the propagation characteristics of light in the photonic crystal fiber, light in a certain polarization direction is filtered out within the near-infrared wavelength range, thereby achieving a single polarization direction output of light, thereby achieving a filtering effect.

[0004] Photonic crystal fiber sensors and filters each play an important role in optical applications. Chen Nan and others invented a D-type photonic crystal fiber sensor based on surface plasmon resonance, with a refractive index range of 1.26-1.29 and a sensitivity of 9000nm / RIU. Ma Ji and others invented a photonic crystal fiber filter with a bandwidth exceeding 800nm ​​and a maximum extinction ratio of 250dB. The optical devices disclosed above only have a single sensing or filtering function and are limited in application scenarios, which will increase the complexity of the optical system and waste resources. Therefore, there are still certain challenges in the integration and miniaturization of optical fiber devices. Summary of the invention

[0005] In view of the problems existing in the prior art, the present invention provides a method for preparing a photonic crystal fiber with sensing and filtering functions. With the first layer of air holes as the center, the second layer of air holes, the third layer of air holes and the fourth layer of air holes are arranged in a regular hexagonal arrangement inside the quartz, respectively, so as to improve the stability of the optical fiber signal; the first layer of air holes is located at the center of the quartz, which increases the limiting loss of the Y polarization mode and causes the loss difference between the X polarization mode and the Y polarization mode to increase, thereby improving the extinction ratio of the photonic crystal fiber and improving the filtering ability; the second layer of air holes eliminates one air hole on the basis of the hexagonal ring, increases the effective mode area of ​​the optical fiber, and obtains the core of the photonic crystal fiber, so that the optical fiber can The invention can accommodate more optical signals, and two second-class air holes are symmetrically arranged at the upper ends of both sides of the first-class air holes, which control the leakage path of light in the optical fiber, enhance the coupling strength between the Y polarization mode and the surface plasma SPP mode, and finally improve the filtering ability of the optical fiber; the two first-class air holes in the third layer of air holes reduce the limiting loss of the X polarization mode to maintain the intensity of the optical signal in the core and improve the measurement accuracy of the sensor. When the evanescent wave matches the phase of the free electrons on the gold surface, the core mode and the SPP mode are coupled and resonated, and an obvious resonance peak will appear in the spectrum. The position of the resonance peak is extremely sensitive to the change of the refractive index of the liquid on the metal surface, thereby realizing high-sensitivity sensing in the X polarization direction. Finally, the photonic crystal fiber prepared by the present invention can be used not only as a polarization filter, but also as a refractive index sensor.

[0006] The present invention provides a method for preparing a photonic crystal optical fiber with sensing and filtering functions, and the specific implementation steps are:

[0007] S1. Arrange the first layer of air holes: a first type of air hole with a diameter of 0.5 μm-0.7 μm is arranged at the center of the quartz circle, thereby increasing the limiting loss of the Y polarization mode, which will increase the loss difference between the X polarization mode and the Y polarization mode, and improve the extinction ratio of the photonic crystal fiber, so that it is easier to separate the Y polarization light from the mixed optical signal, and obtain the polarization filtering characteristics of the photonic crystal fiber. The limiting loss of the photonic crystal fiber is expressed as:

[0008]

[0009] Where Loss is the limiting loss of the photonic crystal fiber, in dB / m, Im[n eff ] is the imaginary part of the effective refractive index of the photonic crystal fiber, and λ is the wavelength;

[0010] S2. Arrange the second layer of air holes: With the first layer of air holes as the center, arrange two second-type air holes with diameters of 1.6 μm-2 μm and three third-type air holes with diameters of 1.4 μm-1.8 μm along the regular hexagonal arrangement on the periphery of the first layer of air holes, eliminate one air hole on the basis of the hexagonal ring, thereby increasing the effective mode area of ​​the optical fiber, obtaining the core of the photonic crystal fiber, so that the photonic crystal fiber can accommodate more optical signals, and the expression of the effective mode area of ​​the photonic crystal fiber is:

[0011]

[0012] In the formula, A eff is the effective mode area of ​​the photonic crystal fiber, E(x,y) is the electric field norm;

[0013] S21, symmetrically arranging two second-type air holes at the upper ends of both sides of the first layer of air holes at a preset interval, controlling the leakage path of light in the photonic crystal fiber, thereby increasing the limiting loss of the Y polarization mode, and at the same time introducing an additional refractive index difference, increasing the refractive index difference between the X polarization mode and the Y polarization mode, thereby improving the birefringence effect of the photonic crystal fiber;

[0014] S22, symmetrically arranging two third-type air holes at the lower ends of both sides of the first layer of air holes at a preset interval, and arranging the last third-type air hole at the bottom of the first layer of air holes at a preset interval;

[0015] S3, arranging the third layer of pores: with the first layer of pores as the center, two first-type pores with diameters of 0.5 μm-0.7 μm and nine third-type pores with diameters of 1.4 μm-1.8 μm are arranged in a regular hexagonal pattern on the periphery of the first layer of pores;

[0016] S31. Arrange two first-class air holes symmetrically on both sides of the top of the first layer of air holes at twice the preset spacing. The positions of the two first-class air holes and the positions of the first layer of air holes form an equilateral triangle, which reduces the limiting loss of the X-polarization mode to maintain the intensity of the optical signal in the fiber core and improve the measurement accuracy of the sensor. When the evanescent wave matches the phase of the free electrons on the gold surface, the core mode will couple and resonate with the SPP mode, and an obvious resonance peak will appear in the spectrum. The position of the resonance peak is extremely sensitive to the change in the refractive index of the liquid on the metal surface. The expression of the mode coupling equation is:

[0017]

[0018] Where E1 and E2 are the electric fields of the core mode and SPP mode, β1 and β2 are the propagation constants of the core mode and SPP mode, k is the coupling strength, and z is the transmission distance;

[0019] S32, symmetrically arranging the first and second third-type air holes at two sides of the bottom of the first layer of air holes at twice the preset spacing, and arranging the third third-type air hole at the bottom of the first layer of air holes at twice the preset spacing;

[0020] S33, symmetrically arranging the remaining six third-type air holes on both sides of the first layer of air holes at twice the preset spacing;

[0021] S4, arranging the fourth layer of pores: with the first layer of pores as the center, thirteen third-type pores with diameters of 1.4 μm to 1.8 μm are arranged in a regular hexagonal pattern on the periphery of the first layer of pores;

[0022] S41, arranging the first third-type air hole at the bottom of the first layer of air holes at a value three times the preset spacing, and symmetrically arranging the second, third, fourth and fifth third-type air holes at two sides of the bottom of the first layer of air holes at a value three times the preset spacing;

[0023] S42, symmetrically arranging the remaining eight third-type air holes on both sides of the first layer of air holes at a value three times the preset spacing;

[0024] S5, combining the fiber core and the cladding obtained in the above step to obtain a cladding with periodically arranged pores;

[0025] S6, using the cladding obtained in step S5 to prepare an initial preform rod, and placing the preform rod in a drawing tower, and performing melt drawing to obtain a prefabricated optical fiber by controlling the temperature and air pressure;

[0026] S7. A gold film is deposited on the polished surface of the prefabricated optical fiber obtained in step S6 by magnetron sputtering and the prefabricated optical fiber is immersed in a refractive index matching liquid. In the near-infrared wavelength range, the interaction between the incident light and the gold film and the refractive index matching liquid is maximized, thereby regulating the transmission characteristics of the photonic crystal fiber. By analyzing the transmission characteristics and calculating the wavelength sensitivity and extinction ratio, a photonic crystal fiber with sensing and filtering functions is obtained.

[0027] Preferably, the outer structure of the quartz consists of an arc surface and a polished plane, the cross section of the quartz is a D-shaped cross section, and the radius R of the arc surface is 7 μm.

[0028] Preferably, the fiber core is located directly above the first layer of pores, on a vertical line with the center of the quartz, and perpendicular to the polishing plane of the quartz.

[0029] Preferably, the preset spacing ranges from 1.8 μm to 2 μm.

[0030] Preferably, the diameter of the first type of pores is smaller than the diameter of the third type of pores, and the diameter of the third type of pores is smaller than the diameter of the second type of pores.

[0031] Preferably, in step S6, the air pressure control range is 1-3500 Pa, and the temperature control range is 1750-2050°C.

[0032] Preferably, in step S7, the thickness of the gold film is 25nm-35nm.

[0033] Preferably, in step S7, the refractive index of the refractive index matching liquid is in the range of 1.450 to 1.480.

[0034] Preferably, in step S7, the dielectric constant of the gold film is expressed as:

[0035]

[0036] Among them, ε m is the dielectric constant of gold, ε ∞ is the dielectric constant of gold at high frequency, Δε is the weighting factor, ω is the angular frequency, ω D and γ D are the plasma frequency and damping frequency, Ω L and Γ L represent the frequency and spectral width of the Lorentz oscillator respectively.

[0037] Preferably, in step S7, the specific expression of the extinction ratio (ER) is:

[0038] ER=20lg{exp[(α1-α2)L]}

[0039] Wherein, ER is the extinction ratio of the optical fiber, in dB, α1 and α2 are the limiting losses of the X polarization mode and the Y polarization mode, respectively, and L is the length of the optical fiber.

[0040] Preferably, in step S7, the specific expression of the wavelength sensitivity is:

[0041]

[0042] Among them, S λ Δλ is the wavelength sensitivity of the optical fiber, in nm / RIU. peak is the shift of the resonance wavelength, Δn a is the change in the refractive index of the liquid.

[0043] Compared with the prior art, the present invention has the following advantages:

[0044] 1. The cladding of the photonic crystal fiber prepared by the present invention adopts a multi-layer air hole arrangement, and this structure forms a periodic refractive index distribution. The second layer of air holes, the third layer of air holes and the fourth layer of air holes are in the shape of a hexagonal ring, the purpose of which is to reduce light leakage. The second layer of air holes eliminates one air hole on the basis of the hexagonal ring, thereby increasing the effective mode area of ​​the optical fiber, so that the optical fiber can accommodate more optical signals, and the leakage path of the light is controlled by two second-type air holes. In the third layer of air holes, two first-type air holes are used to reduce the limiting loss of the X polarization mode, and the light is limited to propagate in the core to maintain the intensity of the optical signal, thereby improving the measurement accuracy of the sensor. The two second-type air holes introduced around the core can effectively control the leakage path of light, increase the coupling strength between the Y polarization core mode and the SPP mode, and improve the extinction ratio of the optical fiber, thereby improving the filtering ability of the optical fiber. Two first-class air holes are set in quartz to reduce the light leakage of the X-polarized core mode to the D-type surface. When the evanescent wave matches the phase of the free electrons on the gold surface, the core mode will be coupled and resonated with the SPP mode, and an obvious resonance peak will appear in the spectrum. The position of the resonance peak is extremely sensitive to the change of the refractive index of the liquid on the metal surface, thereby realizing the sensing function in the X-polarization direction.

[0045] 2. The photonic crystal fiber prepared by the present invention can be used not only as a polarization filter but also as a refractive index sensor without any external adjustment or structural modification. Compared with optical fiber sensors and filters with only a single function, the waste of resources is reduced.

[0046] 3. The photonic crystal fiber prepared by the present invention has good sensing performance, with a wavelength sensitivity of up to 13072nm / RIU and a liquid refractive index range of 1.450-1.480, and is widely used in the detection of high refractive index materials.

[0047] 4. The photonic crystal fiber prepared by the present invention has excellent filtering performance. In the wavelength range of 650nm-2000nm, the extinction ratio of the filter is greater than 20dB, with a transmission bandwidth of 1350nm. At a wavelength of 1.31μm, the extinction ratio peak can reach -827dB. By adjusting the bandwidth, different types of polarized light can be selected, which has higher flexibility and stronger functionality.

[0048] 5. The photonic crystal fiber prepared by the present invention is a quartz polished surface coated with a gold film, which reduces the overall manufacturing difficulty compared to coating the gold film on the inner wall of the pore and makes the coating of the gold film more uniform.

[0049] 6. When the photonic crystal fiber prepared by the present invention is used, the sensing performance can be tested by directly contacting the refractive index matching liquid with the outer surface of the photonic crystal fiber, and the detection is more accurate and efficient. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] Figure 1 A flow chart of a method for preparing a photonic crystal optical fiber having sensing and filtering functions according to the present invention;

[0051] Figure 2 A cross-sectional view of a photonic crystal fiber obtained in the method for preparing a photonic crystal fiber with sensing and filtering functions of the present invention;

[0052] Figure 3a The electric field distribution diagram of the core fundamental mode of the photonic crystal fiber in the X polarization direction obtained in the method for preparing the photonic crystal fiber with sensing and filtering functions of the present invention;

[0053] Figure 3b The electric field distribution diagram of the core fundamental mode of the photonic crystal fiber in the Y polarization direction obtained in the method for preparing the photonic crystal fiber with sensing and filtering functions of the present invention;

[0054] Figure 4 The transmission loss diagram of the core fundamental mode of the photonic crystal fiber in the X-polarization direction under different refractive index matching liquids obtained in the method for preparing the photonic crystal fiber with sensing and filtering functions of the present invention;

[0055] Figure 5 A relationship diagram between the resonance wavelength of a photonic crystal fiber and the refractive index of a refractive index matching liquid obtained by the method for preparing a photonic crystal fiber with sensing and filtering functions of the present invention;

[0056] Figure 6 Transmission loss diagram of the core fundamental mode of the photonic crystal fiber in the X polarization direction and the Y polarization direction at a refractive index of 1.46 obtained by the method for preparing the photonic crystal fiber with sensing and filtering functions of the present invention;

[0057] Figure 7 This is a graph of the extinction ratio of a photonic crystal fiber at a refractive index of 1.46 obtained by the method for preparing a photonic crystal fiber with sensing and filtering functions of the present invention.

[0058] Main reference numerals:

[0059] Matching layer 1, cladding 2, core 3, quartz 4, refractive index matching liquid 5, gold film 6, first layer of pores 15, second layer of pores 16, third layer of pores 17, fourth layer of pores 18, first type of pores 21, second type of pores 22, third type of pores 23. DETAILED DESCRIPTION

[0060] In order to fully describe the technical content, structural features, objectives and effects of the present invention, the following will be described in detail with reference to the accompanying drawings.

[0061] like Figure 1 As shown, the method for preparing a photonic crystal fiber with sensing and filtering functions comprises the following specific implementation steps:

[0062] S1. Arrange the first layer of air holes 15: a first type of air hole 21 with a diameter of 0.5 μm-0.7 μm is arranged at the center of the quartz 4 to reduce the light flow of the core mode to the cladding 2, increase the coupling strength of the Y polarization mode and the surface plasma mode, and thus increase the limiting loss of the Y polarization mode, which will cause the loss difference between the X polarization mode and the Y polarization mode to increase, improve the extinction ratio of the photonic crystal fiber, and thus make it easier to separate the Y polarization light from the mixed optical signal, and obtain the polarization filtering characteristics of the optical fiber. The limiting loss of the photonic crystal fiber is used to represent the loss caused by mode coupling or mode conversion in optical fiber transmission, and the specific expression is:

[0063]

[0064] Where Loss is the limiting loss of the photonic crystal fiber, in dB / m, Im[n eff ] is the imaginary part of the effective refractive index of the optical fiber, and λ is the wavelength.

[0065] Specifically, the outer structure of quartz 4 is composed of an arc surface and a polished plane. The cross section of quartz 4 is a D-shaped cross section, and the radius R of the arc surface is 7 μm. Furthermore, the first type of air hole 21 is set at the center of quartz 4 in order to make the core 3 of the photonic crystal fiber appear below the D-shaped cross section and control the interaction between the light of the core 3 and the D-shaped cross section. If the first type of air hole 21 is removed, the limiting loss of the Y polarization mode will be reduced and the filtering ability will be weakened. In addition, increasing the number of the first type of air holes 21 will make the structure of the cladding 3 more complicated and increase the difficulty of production.

[0066] S2. Arrange the second layer of air holes 16: With the first layer of air holes 15 as the center, two second-type air holes 22 with diameters of 1.6μm-2μm and three third-type air holes 23 with diameters of 1.4μm-1.8μm are arranged along a regular hexagon on the periphery of the first layer of air holes 15. One air hole is eliminated on the basis of the hexagonal ring, thereby increasing the effective mode area of ​​the optical fiber and obtaining the core of the photonic crystal fiber, so that the optical fiber can accommodate more optical signals. The regular hexagonal structure will make the air holes in the cladding 2 more compact, thereby limiting the light energy in the core 3.

[0067] S21. Two second-type air holes 22 are symmetrically arranged at the upper ends of both sides of the first layer of air holes 21 with a spacing of 1.8μm-2μm, so as to control the leakage path of light, enhance the interaction between the light in the Y polarization mode and the metal on the D-shaped cross section, increase the limiting loss of the Y polarization mode, and introduce an additional refractive index difference, increase the refractive index difference between the X polarization mode and the Y polarization mode, thereby improving the birefringence effect of the photonic crystal fiber.

[0068] S22, symmetrically arrange two third-type pores 23 at the lower ends of both sides of the first-layer pores 15 with a spacing of 1.8 μm-2 μm, and at the same time arrange the last third-type pore 23 at the bottom of the first-layer pores 15 with a spacing of 1.8 μm-2 μm.

[0069] S3. Arrange the third layer of pores 17: With the first layer of pores as the center, arrange two first-type pores 21 with a diameter of 0.5 μm-0.7 μm and nine third-type pores 23 with a diameter of 1.4 μm-1.8 μm in a regular hexagonal arrangement around the first layer of pores 15 .

[0070] S31. In order to reduce the limiting loss of the X-polarization mode and realize the sensing function of the X-polarization mode, two first-class air holes 21 are symmetrically arranged on both sides of the top of the first layer of air holes 15 with a spacing of twice the value of 1.8μm-2μm. The positions of the two first-class air holes 21 and the positions of the first layer of air holes 15 are equilateral triangles. The positions of the two first-class air holes 21 are on a horizontal straight line, which reduces the limiting loss of the X-polarization mode to maintain the intensity of the optical signal in the fiber core and improve the measurement accuracy of the sensor. When the evanescent wave matches the phase of the free electrons on the gold surface, the core mode will couple and resonate with the SPP mode, and an obvious resonance peak will appear in the spectrum. The position of the resonance peak is extremely sensitive to the change of the refractive index of the liquid on the metal surface. The expression of the mode coupling equation is:

[0071]

[0072] Where E1 and E2 are the electric fields of the core mode and SPP mode, β1 and β2 are the propagation constants of the core mode and SPP mode, k is the coupling strength, and z is the transmission distance;

[0073] S32, symmetrically arrange the first and second third-type pores 23 on both sides of the bottom of the first layer of pores 15 at twice the spacing of 1.8μm-2μm, and at the same time arrange the third third-type pore 23 at the bottom of the first layer of pores 15 at twice the spacing of 1.8μm-2μm.

[0074] S33 , symmetrically arrange the remaining six third-type pores 23 on both sides of the first-layer pores 15 at a spacing of twice the value of 1.8 μm-2 μm.

[0075] S4. According to the limiting loss, filtering effect and overall manufacturing complexity of the Y polarization mode, prepare the fourth layer of pores 18: with the first layer of pores 15 as the center, thirteen third-type pores 23 with a diameter of 1.4μm-1.8μm are arranged along a regular hexagon on the periphery of the first layer of pores 15.

[0076] S41. Arrange the first third-type pore 23 at the bottom of the first layer of pores 15 with a spacing of three times 1.8μm-2μm, and arrange the second, third, fourth and fifth third-type pores 23 symmetrically on both sides of the bottom of the first layer of pores 15 with a spacing of three times 1.8μm-2μm.

[0077] S42 , symmetrically arrange the remaining eight third-type pores 23 on both sides of the first-layer pores 15 at a spacing of three times the value of 1.8 μm-2 μm.

[0078] S5, combining the core 3 with the cladding 2 obtained in the above step to obtain a cladding 2 with periodically arranged pores. Specifically, the core 3 is located directly above the first layer of pores 15, on a vertical line with the center of the quartz 4, and perpendicular to the polished plane of the quartz 4.

[0079] S6, using the cladding obtained in step S5 to prepare an initial preform rod, and placing the preform rod in a drawing tower, and performing melt drawing by controlling the temperature and air pressure to obtain a prefabricated photonic crystal fiber;

[0080] S7, depositing a gold film on the polished surface of the prefabricated photonic crystal fiber obtained in step S6 by magnetron sputtering and immersing the prefabricated photonic crystal fiber in a refractive index matching liquid, maximizing the interaction between the incident light and the gold film and the refractive index matching liquid in the near-infrared wavelength range, thereby regulating the transmission characteristics of the photonic crystal fiber, and obtaining a photonic crystal fiber with sensing and filtering functions by analyzing its characteristics and calculating the wavelength sensitivity and extinction ratio, such as Figure 2 As shown. Specifically, the gold film 6 provides an inducing medium for the surface plasmon resonance effect. The material of the gold film 6 is gold. The thickness of the gold film 6 is 25nm-35nm. The refractive index of the refractive index matching liquid 5 ranges from 1.450 to 1.480. The matching layer 1 is located outside the refractive index matching liquid 5. It is a good absorption boundary condition that can absorb light energy from all directions radiated from the calculation area without reflecting energy to the calculation area, further improving the calculation accuracy. It is a calculation boundary added when simulating the performance of the optical fiber. The scattering boundary condition is used in combination with the matching layer 1 to reduce the reflected energy.

[0081] Furthermore, in order to improve the transmission characteristics of the photonic crystal fiber, the gold film 6 and the single polarized light in the core 3 produce an SPR (surface plasma resonance) effect, that is, the SPP mode is generated on the interface between the gold film 6 and the dielectric under the stimulation of light, and a resonance phenomenon occurs. The dielectric constant of the gold film 6 is expressed as:

[0082]

[0083] Among them, ε m is the dielectric constant of the gold film 6, ε ∞is the dielectric constant of gold at high frequency, Δε is the weighting factor, ω is the angular frequency, ω D and γ D are the plasma frequency and damping frequency, Ω L and Γ L represent the frequency and spectral width of the Lorentz oscillator respectively.

[0084] The specific expression of extinction ratio (ER) is:

[0085] ER=20lg{exp[(α1-α2)L]}

[0086] Wherein, ER is the extinction ratio of the optical fiber, in dB, α1 and α2 are the limiting losses of the X polarization mode and the Y polarization mode, respectively, and L is the length of the optical fiber.

[0087] The specific expression of wavelength sensitivity is:

[0088]

[0089] Among them, S λ Δλ is the wavelength sensitivity of the optical fiber, in nm / RIU. peak is the shift of the resonance wavelength, Δn a is the change in the refractive index of the liquid.

[0090] In a preferred embodiment of the present invention, the diameter of the first type of pores 21 is smaller than the diameter of the third type of pores 23, and the diameter of the third type of pores 23 is smaller than the diameter of the second type of pores 22. The first type of pores 21 and the second type of pores 22 control the leakage path of light and the size of the fiber core, and generate an evanescent electric field reaching the plasma material to excite free electrons to generate surface plasma resonance. The first type of pores 21 are located near the polishing plane in the first layer of pores 15 and the third layer of pores 17, the third type of pores 23 are located below the center of the quartz 4 in the third layer of pores 17, the fourth layer of pores 18, and the second type of pores 22, and the second type of pores 22 are located near the polishing plane in the second layer of pores 16. The fiber core 3 is located in the middle area between the first type of pores 21 and the third type of pores 23, and is on a vertical line with the center of the quartz 4, and is perpendicular to the polishing plane.

[0091] The following is a further description of a method for preparing a photonic crystal fiber with sensing and filtering functions according to the present invention in conjunction with an embodiment:

[0092] In this specific embodiment, the method for preparing a photonic crystal fiber with sensing and filtering functions is achieved as follows:

[0093] S1 . Arrange the first layer of pores 15 : arrange a first type of pore 21 with a diameter of 0.6 μm at the center of the quartz 4 .

[0094] S2. Arrange the second layer of pores 16: With the first layer of pores 15 as the center, arrange two second-type pores 22 with a diameter of 1.8 μm and three third-type pores 23 with a diameter of 1.8 μm on the periphery of the first layer of pores 15 along a regular hexagonal arrangement.

[0095] S21 , symmetrically arrange two second-type pores 22 at upper ends of both sides of the first-layer pores 21 with a spacing of 2 μm.

[0096] S22, symmetrically arrange two third-type air holes 23 at the lower ends of both sides of the first-layer air holes 15 with a spacing of 2 μm, and at the same time arrange the last third-type air hole 23 at the bottom of the first-layer air holes 15 with a spacing of 2 μm.

[0097] S3. Arrange the third layer of pores 17: With the first layer of pores as the center, arrange two first-type pores 21 with a diameter of 0.6 μm and nine third-type pores 23 with a diameter of 1.8 μm in a regular hexagonal arrangement around the first layer of pores 15 .

[0098] S31. In order to reduce the limiting loss of the X-polarization mode and realize the sensing function of the X-polarization mode, two first-type air holes 21 are symmetrically arranged on both sides of the top of the first-layer air holes 15 with a spacing of twice 2μm. The positions of the two first-type air holes 21 and the positions of the first-layer air holes 15 form an equilateral triangle, and the positions of the two first-type air holes 21 are on a horizontal straight line.

[0099] S32, symmetrically arrange the first and second third-type pores 23 at two sides of the bottom of the first layer of pores 15 at a spacing of twice 2 μm, and arrange the third third-type pore 23 at the bottom of the first layer of pores 15 at a spacing of twice 2 μm.

[0100] S33 , symmetrically arrange the remaining six third-type air holes 23 on both sides of the first-layer air holes 15 at a spacing twice as large as 2 μm.

[0101] S4. Arrange the fourth layer of pores 18 according to the limiting loss, filtering effect and overall manufacturing complexity of the Y polarization mode: with the first layer of pores 15 as the center, arrange thirteen third-type pores 23 with a diameter of 1.8 μm in a regular hexagonal arrangement on the periphery of the first layer of pores 15.

[0102] S41, arrange the first third-type pore 23 at the bottom of the first layer of pores 15 with a spacing three times that of 2 μm, and at the same time arrange the second, third, fourth and fifth third-type pores 23 symmetrically in pairs on both sides of the bottom of the first layer of pores 15 with a spacing three times that of 2 μm.

[0103] S42 , symmetrically arrange the remaining eight third-type air holes 23 on both sides of the first-layer air holes 15 at a spacing three times of 2 μm.

[0104] S5. Combining the fiber core 3 with the cladding 2 obtained in the above steps to obtain the cladding 2 with periodically arranged pores.

[0105] S6. Using the cladding obtained in step S5, an initial preform is prepared, and the preform is placed in a drawing tower to perform melt drawing to obtain a prefabricated photonic crystal fiber by controlling the temperature and air pressure.

[0106] S7, on the polished surface of the prefabricated photonic crystal fiber obtained in step S6, a gold film 6 with a thickness of 25 nm and a refractive index matching liquid 5 with refractive indices of 1.450, 1.455, 1.460, 1.465, 1.470, 1.475 and 1.480 are deposited by magnetron sputtering, and the transmission characteristics are analyzed and the extinction ratio is calculated to obtain a photonic crystal fiber with sensing and filtering, such as Figure 2 As shown, the mode fields of the core mode in the X-polarization direction and the Y-polarization direction are as follows Figure 3a and Figure 3b shown.

[0107] like Figure 4 As shown, curves 1, 2, 3, 4, 5, 6 and 7 are respectively the transmission loss of the photonic crystal fiber in the X polarization mode changing with the wavelength when the liquid refractive index is 1.450, 1.455, 1.460, 1.465, 1.470, 1.475 and 1.480. As the refractive index increases, the resonance peak redshifts. When the core mode and the SPP mode are phase-matched, the light in the core 3 and the free electrons on the metal surface are strongly coupled and resonated, so that the transmission loss reaches a peak value, and the wavelength corresponding to the loss peak is the resonance wavelength. When the refractive index of the refractive index matching liquid 5 changes, the resonance wavelength will also change accordingly. Therefore, the change in the refractive index of the liquid to be measured can be accurately obtained according to the change in the resonance wavelength, thereby realizing the monitoring and sensing functions of the liquid analyte.

[0108] Figure 5 is the relationship curve between the resonance wavelength and the refractive index of the liquid in this specific embodiment. The relationship curve in the refractive index range of 1.44 to 1.48 is λ res =-176190n 2 +524023n-387980, the sensitivity under different refractive indices can be obtained by deriving the above curve. The sensitivity curve is S=-352380n+524023. The sensitivity decreases with the increase of the refractive index of the liquid to be measured, and the highest sensitivity reaches 13072nm / RIU. The corresponding fitting linear coefficients are 0.99725. res represents the resonance wavelength, n represents the refractive index of the liquid, and S represents the sensitivity.

[0109] Figure 6 Curves A and B are the transmission loss diagrams of the X-direction polarization mode and the Y-direction polarization mode of the photonic crystal fiber when the liquid refractive index is 1.460. Figure 6 As shown, the loss peak of the Y polarization mode appears at 1.31μm. At this time, the loss of the Y polarization mode is 96905dB / m, and the loss of the X polarization mode is 1583dB / m. The difference in loss between the two is large. The large loss difference is conducive to the polarization filter filtering out light in one polarization direction. Therefore, the filter can well filter out the light in the Y polarization direction at 1.31μm. Figure 7 The extinction ratio diagram of the photonic crystal fiber obtained by measuring this specific embodiment shows that when the ER in a certain polarization direction is less than -20dB or greater than 20dB, the photonic crystal fiber can filter out the light in that direction. The higher the extinction ratio, the better the filtering effect of the photonic crystal fiber. Figure 6 As shown, within the wavelength range of 800nm-1800nm, the extinction ratio of the photonic crystal fiber is less than -20dB, and it has the advantage of adjustable bandwidth. Different types of polarized light selection can be achieved by adjusting the bandwidth. This photonic crystal fiber has higher flexibility and functionality, and the extinction ratio peak is -827dB at 1310nm, -474dB at 1550nm, and 152-dB at 800nm. Therefore, the photonic crystal fiber can be used in the communication windows of 800nm, 1310nm and 1550nm, respectively, for transmitting data / voice and video.

[0110] Therefore, the photonic crystal fiber of the present invention has improvements in sensing sensitivity, adjustable polarization filtering bandwidth and high extinction ratio, miniaturization of optical devices, etc., has better performance and wider application potential, and can meet the needs of different application fields.

[0111] The above embodiments are only descriptions of the preferred implementation modes of the present invention, and are not intended to limit the scope of the present invention. Without departing from the design spirit of the present invention, various modifications and improvements made to the technical solutions of the present invention by ordinary technicians in this field should all fall within the protection scope determined by the claims of the present invention.

Claims

1. A method for preparing a photonic crystal fiber with sensing and filtering functions, characterized in that: It includes the following steps: S1. Arrange the first layer of air holes: Arrange a first type of air hole with a diameter of 0.5μm-0.7μm at the center of the quartz circle to form the first layer of air holes, increase the limiting loss of the Y polarization mode, and obtain the polarization filtering characteristics of the photonic crystal fiber. The expression of the limiting loss of the photonic crystal fiber is: Where Loss is the limiting loss of the photonic crystal fiber, in dB / m, Im[n eff ] is the imaginary part of the effective refractive index of the photonic crystal fiber, and λ is the wavelength; S2. Arrange the second layer of air holes: With the first layer of air holes as the center, arrange two second-type air holes with a diameter of 1.6μm-2μm and three third-type air holes with a diameter of 1.4μm-1.8μm along the regular hexagonal arrangement on the periphery of the first layer of air holes. The second-type air holes and the third-type air holes constitute the second layer of air holes that can increase the effective mode area of ​​the photonic crystal fiber, wherein the expression of the effective mode area of ​​the photonic crystal fiber is: In the formula, A eff is the effective mode area of ​​the photonic crystal fiber, E(x,y) is the electric field norm; It includes the following sub-steps: S21, symmetrically arranging two second-type air holes at the upper ends of both sides of the first layer of air holes at a preset interval; S22, symmetrically arranging the first third-type air hole and the second third-type air hole of the three third-type air holes at the lower ends of both sides of the first layer of air holes at a preset interval, and arranging the third third-type air hole at the bottom of the first layer of air holes at a preset interval; S3, arranging the third layer of pores: with the first layer of pores as the center, two first-type pores and nine third-type pores are arranged along a regular hexagon on the periphery of the first layer of pores. The specific steps are as follows: S31. Arrange two first-type air holes symmetrically on both sides of the top of the first layer of air holes at twice the preset spacing. The positions of the two first-type air holes and the positions of the first layer of air holes are respectively located at the vertices of an equilateral triangle. The expressions of the coupling mode equations of the core mode and the SPP mode are respectively: Where E1 and E2 are the electric fields of the core mode and SPP mode, β1 and β2 are the propagation constants of the core mode and SPP mode, k is the coupling strength, and z is the transmission distance; S32, symmetrically arranging two of the nine third-type air holes at two sides of the bottom of the first layer of air holes at twice the preset spacing, and arranging the third of the nine third-type air holes at the bottom of the first layer of air holes at twice the preset spacing; S33, symmetrically arranging the fourth to ninth third-type air holes among the nine third-type air holes on both sides of the first layer of air holes at twice the value of the preset spacing; S4, arranging the fourth layer of pores: arranging thirteen third-type pores in a regular hexagonal pattern around the first layer of pores, with the first layer of pores as the center; S41, arranging the first third-type air hole among the thirteen third-type air holes at the bottom of the first layer of air holes at a value three times the preset spacing, and symmetrically arranging the second third-type air hole, the third third-type air hole, the fourth third-type air hole and the fifth third-type air hole among the thirteen third-type air holes at two times the preset spacing on both sides of the bottom of the first layer of air holes; S42, symmetrically arranging the sixth to thirteenth third-type air holes among the thirteen third-type air holes on both sides of the first layer of air holes at a value three times the preset spacing; S5, combining the fiber core and the cladding obtained in the above step to obtain a cladding with periodically arranged pores; S6, using the cladding obtained in step S5 to prepare an initial preform rod, and placing the preform rod in a drawing tower, and performing melt drawing by controlling the temperature and air pressure to obtain a prefabricated photonic crystal fiber; S7. The polished surface of the prefabricated photonic crystal fiber obtained in step S6 is deposited with a gold film by magnetron sputtering and immersed in a refractive index matching liquid. In the near-infrared wavelength range, the interaction between the incident light and the gold film and the refractive index matching liquid is maximized, and the transmission characteristics of the photonic crystal fiber are regulated. By analyzing the transmission characteristics of the photonic crystal fiber and calculating the wavelength sensitivity and extinction ratio, a photonic crystal fiber with sensing and filtering functions is obtained.

2. The method for preparing a photonic crystal fiber with sensing and filtering functions according to claim 1, characterized in that: The outer shape structure of the quartz includes an arc surface and a polished plane. The cross section of the quartz is a D-shaped cross section. The radius R of the arc surface is 7 μm.

3. The method for preparing a photonic crystal fiber with sensing and filtering functions according to claim 1 or 2, characterized in that: The fiber core is located directly above the first layer of air holes, is located on a vertical line with the center of the quartz and is perpendicular to the polishing plane of the quartz.

4. The method for preparing a photonic crystal fiber with sensing and filtering functions according to claim 1, characterized in that: The preset spacing ranges from 1.8 μm to 2 μm.

5. The method for preparing a photonic crystal fiber with sensing and filtering functions according to claim 1, characterized in that: The diameter of the first type of pores is smaller than the diameter of the third type of pores, and the diameter of the third type of pores is smaller than the diameter of the second type of pores.

6. The method for preparing a photonic crystal fiber with sensing and filtering functions according to claim 1, characterized in that: In step S6, the air pressure control range is 1-3500 Pa, and the temperature control range is 1750-2050°C.

7. The method for preparing a photonic crystal fiber with sensing and filtering functions according to claim 1, characterized in that: In step S7, the thickness of the gold film is 25 nm-35 nm, and the refractive index of the refractive index matching liquid is in the range of 1.450 to 1.

480.

8. The method for preparing a photonic crystal fiber with sensing and filtering functions according to claim 7, characterized in that: In step S7, the dielectric constant of the gold film is expressed as: Among them, ε m is the dielectric constant of the gold film, ε ∞ is the dielectric constant of gold at high frequency, Δε is the weighting factor, ω is the angular frequency, ω D and γ D are the plasma frequency and damping frequency, Ω L and Γ L represent the frequency and spectral width of the Lorentz oscillator respectively.

9. The method for preparing a photonic crystal optical fiber with sensing and filtering functions according to claim 7, characterized in that: In step S7, the specific expression of the extinction ratio ER is: ER=20lg{exp[(α1-α2)L]} Wherein, ER represents the extinction ratio of the photonic crystal fiber in dB, α1 and α2 are the limiting losses of the X polarization mode and the Y polarization mode, respectively, and L is the length of the photonic crystal fiber.

10. The method for preparing a photonic crystal optical fiber with sensing and filtering functions according to claim 7, characterized in that: In step S7, the specific expression of the wavelength sensitivity is: Among them, S λ is the wavelength sensitivity of the photonic crystal fiber, in nm / RIU, Δλ peak is the shift of the resonance wavelength, Δn a is the change in the refractive index of the liquid.

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