D-type single-mode optical fiber surface integrated tapered polymer waveguide structure and processing method

Abstract

The application discloses a D-type single-mode optical fiber on-surface integrated taper polymer waveguide structure and a processing method, and belongs to the field of all-fiber integrated optical devices. The application optimizes the side wall inclination angle of a polished area of a D-type optical fiber and the length of a taper optical fiber gradual change area by using an intrinsic mode expansion method, improves the coupling efficiency from a fiber core to a micro-nano waveguide, and optimizes the transmittance from 0.06 to 0.64. The micro-nano waveguide is supported by the waveguide of the gradual change area to be suspended, and mode crosstalk is reduced. The fiber core is completely exposed by polishing the D-type single-mode optical fiber, and a sufficient flat polished area is reserved as a substrate, so that the optical structure expandability and robustness are significantly improved. Femtosecond laser ablation is used to polish the side of the single-mode optical fiber, the D-type single-mode optical fiber with different side wall inclination angles is manufactured, and the femtosecond laser two-photon polymerization technology can be used to print the taper waveguide with high precision in the polished area. The application has the advantages of high integration, high coupling efficiency, strong expandability and high robustness.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of all-fiber integrated optics, and particularly relates to a D-shaped single-mode fiber surface-integrated tapered polymer waveguide structure and a processing method. BACKGROUND

[0002] As a technology to integrate micro / nano-scale functional materials with different physical, chemical and biological properties on the surface of a single optical fiber, all-fiber integrated optical design has the advantages of small size, light weight, low cost and low energy consumption in the design of fiber micro / nano integrated devices. It is the key development direction of the new generation of miniaturized all-fiber technology platform, and is becoming the key and supporting technology behind many devices, components and systems in modern industry and scientific research system. It has broad application prospects in signal processing, environmental monitoring, life science and even national defense security. Improving the integration of multi-functional structures on the surface of a single optical fiber and further improving the modulation capacity of signals in the fiber have become an important issue for the development of future functional fiber micro / nano integrated devices. Among them, the design of waveguide structure in the fiber is the basis of fiber micro / nano integration. The literature "Femtosecond Laser Microprinting of a Fiber Whispering Gallery Mode Resonator for Highly-Sensitive Temperature Measurements" and the literature "Micro-Ring Resonator Devices Prototyped on Optical Fiber Tapers by Multi-Photon Lithography" both propose a structure based on taper micro / nano fiber as waveguide and resonant cavity coupling. However, the taper micro / nano fiber waveguide structure is fragile and has no natural substrate support. In the fabrication of micro / nano fiber functional structures, a substrate other than the fiber is often needed, which increases the size and difficulty of the overall structure and restricts the expansion of the multi-functional integration on the surface of the fiber. The literature "In-fiber whispering-gallery-mode resonator fabricated by femtosecond laser micromachining" and the literature "In-fiber Mach–Zehnder interferometer and sphere whispering gallery mode resonator coupling structure" use the core of shallow polished D-shaped fiber as waveguide coupling microcavity, which cannot realize the strong light field localization effect of micro / nano fiber waveguide. The literature "Two-Dimensional Tapered Optical Fiber Core for Whispering Gallery Mode Excitation" designs a femtosecond laser subtractive waveguide on the surface of D-shaped fiber, but the waveguide structure surface is rough and the coupling efficiency is low.The document "In-Fiber Polymer Microdisk Resonator and Its Sensing Applications of Temperature and Humidity" designs a micro-nano waveguide in a hollow optical fiber by using two-photon polymerization, but the optical fiber structure has poor expandability, and direct coupling of the optical fiber core and the micro-nano waveguide will cause large mode mismatch and reduce the coupling efficiency.

[0003] Therefore, designing an optical fiber internal or a surface waveguide with optical fiber integration, good expandability, high coupling efficiency and strong light field localization effect has become a key problem for further development of all-optical fiber integrated optical devices. SUMMARY

[0004] In order to overcome the deficiencies of the integration, expandability, coupling efficiency and light field localization effect of the waveguide in the all-optical fiber integrated optical device, the main purpose of the present application is to provide a D-type single-mode optical fiber surface integrated tapered polymer waveguide structure and a processing method, which realizes three-dimensional processing of the tapered waveguide on the D-type optical fiber by using two-photon polymerization technology, uses the surface area of the D-type optical fiber as the substrate of the optical structure, and significantly improves the expandability of the optical structure; the waveguide has the size of a micro-nano optical fiber, has strong light field localization effect, and is beneficial to the integration of the micro-nano optical structure; the tapered region can significantly improve the coupling efficiency; and the tapered waveguide is suspended, which can reduce mode crosstalk.

[0005] The purpose of the present application is realized by the following technical solutions.

[0006] The D-type single-mode optical fiber surface integrated tapered polymer waveguide structure disclosed by the present application directly couples the micro-nano waveguide on the D-type optical fiber, uses the surface area of the D-type optical fiber as the substrate of the optical structure; the waveguide has the size of a micro-nano optical fiber, the tapered region with strong light field localization effect is added to the waveguide, which significantly improves the coupling efficiency of the optical fiber core and the tapered waveguide; the micro-nano waveguide is suspended by the tapered region waveguide, and the mode crosstalk is reduced by the suspended tapered waveguide.

[0007] The processing method of the D-type single-mode optical fiber surface integrated tapered polymer waveguide structure disclosed by the present application is as follows:

[0008] Step one: single-mode optical fiber side polishing is realized by using femtosecond laser ablation to make a D-type single-mode optical fiber. The D-type single-mode optical fiber is polished, the polishing depth is 71-73 microns, the core is completely exposed, and enough flat surface area is reserved as a substrate. The side wall of the polishing area at both ends is processed at an angle of 28-32 degrees.

[0009] Preferably, in step one, the D-type single-mode fiber is polished to a depth of 72.5 μm to fully expose the fiber core and retain a sufficiently flat polished surface area as a substrate; the sidewall tilt angle at both ends of the polished area is 29.74° to improve the coupling efficiency between the fiber core and the tapered waveguide.

[0010] Step 2: Immerse the polished area in photoresist and dry it.

[0011] Step 3: A tapered waveguide is printed in the polished area using femtosecond laser two-photon polymerization technology. The waveguide cross-section connecting the sidewalls of the polished area is 6μm~9μm×19μm~21μm, used to improve the coupling efficiency between the waveguide and the fiber core. The tapered gradient region has a length of 10μm~20μm, used to reduce losses caused by mode mismatch. Before the gradient is completed, the waveguide is attached to the polished substrate, and the width of the waveguide cross-section perpendicular to the substrate remains unchanged to ensure the stability of the waveguide. After the gradient, the waveguide cross-section is 0.6μm~1μm×8μm~10μm, and the waveguide is suspended, that is, the micro / nano waveguide is supported by the gradient region waveguide to achieve suspension.

[0012] Preferably, in step three, the waveguide cross-section connecting the sidewalls of the polished area is 8μm × 20μm. The length of the tapered gradient region is 14μm. The cross-section of the waveguide after the gradient is 1μm × 10μm.

[0013] Step 4: Immerse the optical fiber in alcohol to remove the uncured photoresist, and obtain the integrated tapered polymer waveguide structure on the surface of the processed D-type single-mode optical fiber.

[0014] Furthermore, the sidewall tilt angle of the polished region of the D-type fiber is optimized using the intrinsic mode expansion (EME) method to improve the coupling efficiency from the fiber core to the wide waveguide; the length of the tapered waveguide gradient region is optimized to improve the coupling efficiency from the wide waveguide to the micro / nano waveguide.

[0015] The coupling efficiency mainly depends on the overlap integral formula of the waveguide cross-section optical field distribution of each cell in the simulation. The overlap integral formula is as follows:

[0016]

[0017] in These are the electric and magnetic field distributions of the m-th mode in cell 1 and the n-th mode in cell 2, respectively, in two adjacent cells. This is the simulation region along the waveguide cross-section. The final transmission transmittance is optimized through continuous iteration.

[0018] Beneficial effects:

[0019] 1. The present invention discloses a tapered polymer waveguide structure and fabrication method integrated on the surface of a D-type single-mode optical fiber, which directly couples micro-nano waveguides onto the D-type optical fiber, resulting in high integration and high structural strength.

[0020] 2. The application discloses a D-type single-mode optical fiber surface integrated tapered polymer waveguide structure and a processing method, which takes a D-type optical fiber polishing area as an optical structure substrate to improve the expansibility.

[0021] 3. The application discloses a D-type single-mode optical fiber surface integrated tapered polymer waveguide structure and a processing method, which adds a tapered gradient area in the waveguide to obviously improve the coupling efficiency of the fiber core and the tapered waveguide, and the coupling efficiency is improved by one order of magnitude compared with direct coupling without the gradient area.

[0022] 4. The application discloses a D-type single-mode optical fiber surface integrated tapered polymer waveguide structure and a processing method, which suspends the waveguide after the gradient to reduce the mode crosstalk. BRIEF DESCRIPTION OF DRAWINGS

[0023] Figure 1 It is a schematic diagram of a D-type single-mode optical fiber surface integrated tapered polymer waveguide structure.

[0024] Figure 2 (a) is a D-type optical fiber polishing vertical side wall design, and a fiber core and a micro-nano waveguide directly coupled electric field distribution diagram.

[0025] Figure 2 (b) is a D-type optical fiber polishing angle design side wall and a waveguide gradient area introduced after the coupling electric field distribution diagram from the fiber core to the micro-nano waveguide.

[0026] Figure 3 (a) is a transmission spectrum of a structure varying with a D-type optical fiber inclination angle when there is no gradient area between a wide waveguide and a micro-nano waveguide, and the length in the figure represents the length of the inclined area in the direction of light propagation.

[0027] Figure 3 (b) is a transmittance of different waveguide gradient area lengths based on a D-type optical fiber side wall inclination angle of 29.74°

[0028] Figure 4 It is a femtosecond laser processing trajectory diagram of a micro-ring resonant cavity side-coupled tapered waveguide.

[0029] Figure 5 It is a confocal microscopic imaging diagram of the application of the design of the application in micro-ring resonant cavity side coupling. DETAILED DESCRIPTION

[0030] In order to make the purpose, technical scheme and advantages of the application clearer and more apparent, the application is further described in detail below with reference to the embodiments and the accompanying drawings. It should be understood that the specific examples described herein are only used to explain the application and do not limit the application.

[0031] Example 1:

[0032] As Figure 1 shown in the embodiment, a D-type single-mode optical fiber surface-integrated tapered polymer waveguide structure is disclosed, a micro-nano waveguide is directly coupled on the D-type optical fiber, and the polished area of the D-type optical fiber is used as the substrate of the optical structure; the waveguide has the size of the micro-nano optical fiber, a tapered gradient region with strong light field localization effect is added in the waveguide, and the mode crosstalk is reduced by suspending the waveguide after the gradient.

[0033] The parameters of the side wall of the D-type optical fiber and the tapered waveguide structure are optimized by EME calculation, and the implementation method is as follows:

[0034] The waveguide cross-section center is aligned with the fiber core by polishing the side surface of the D-type single-mode optical fiber. The coupling efficiency is improved by optimizing the parameters of the side wall inclination angle of the D-type optical fiber and the length of the waveguide gradient region through EME simulation calculation. The coupling efficiency mainly depends on the overlap integral formula of the light field distribution of each cell in the simulation, and the overlap integral formula is as follows:

[0035]

[0036] Among them, are the electric field and magnetic field distribution of the mth mode of cell 1 and the nth mode of cell 2 in two adjacent cells, is the simulation area in the direction of the waveguide cross-section. The final transmission transmittance is calculated by iteration.

[0037] The polishing depth of the side surface of the D-type single-mode optical fiber is 72.5 μm, and the height of the wide waveguide is 20 μm.

[0038] Figure 3 (a) is the transmission spectrum of EME calculation structure changing with the inclination angle of the D-type optical fiber when there is no gradient region between the wide waveguide and the micro-nano waveguide, and the length in the figure represents the length of the inclined region in the direction of light propagation. When the length is 35 μm, the transmittance reaches the maximum, and at this time the inclination angle is 29.74°. On this basis, the length of the waveguide gradient region is optimized, and it is found that the highest transmittance is obtained when the length of the waveguide gradient region is 14 μm, as shown in Figure 3 (b). By changing the inclination degree of the side wall and the length of the waveguide gradient region, the transmittance is improved from 0.06 to 0.64, which is an order of magnitude. The wide waveguide cross-section connected to the side wall of the polished region is 8 μm×20 μm, which is used to improve the coupling efficiency with the fiber core. The length of the tapered gradient region is 14 μm, which is used to reduce the loss caused by mode mismatch. Before the gradient is completed, the waveguide is attached to the substrate formed by the polishing surface, and the width of the waveguide cross-section in the vertical substrate direction is unchanged, which ensures the stability of the waveguide. The cross-section of the waveguide after the gradient is 1 μm×10 μm, which is suitable for the size of the 50x objective focusing femtosecond laser guided two-photon polymerization processing, and the mode crosstalk is reduced by suspending the waveguide after the gradient.

[0039] Embodiment 2:

[0040] AsFigure 1 As shown in the embodiment, the processing method of the D-shaped single-mode optical fiber surface-integrated tapered polymer waveguide structure is that the D-shaped single-mode optical fiber tapered polymer waveguide is side-coupled with the micro-ring resonant cavity.

[0041] Sample preparation: The cut silicon wafer is pasted to the glass slide with double-sided tape for improving the quality of reflective imaging during processing. The optical fiber is fixed on the silicon wafer with high-temperature-resistant tape.

[0042] Laser ablation preparation of D-shaped single-mode optical fiber: The femtosecond laser generates a wavelength of 1030 nm Gaussian beam, which is frequency-doubled through the OPA crystal to generate a wavelength of 515 nm Gaussian beam. The energy is adjusted by an electric attenuator, and the laser is turned on and off by an electronic shutter. The 20X objective lens is focused. The higher the frequency of the laser focus, the smaller the spot, and the higher the precision of processing the D-shaped optical fiber. The sample is placed on a three-dimensional moving stage so that the upper surface of the cylindrical surface of the optical fiber is located in the focal plane. The laser is vertically scanned along the direction perpendicular to the core extension, with a scanning speed of 1 mm / s, an adjacent track spacing of 1 μm, and a laser power of 8 mW. Each scanning layer moves down 2 μm to continue scanning, a total of 35 planes are scanned, and an inclined side wall is processed by changing the length of each plane in the direction of the optical fiber. After processing, the sample is cleaned in the ultrasonic cleaning machine for 5 minutes with deionized water, and then dried.

[0043] Two-photon polymerization processing of D-shaped optical fiber polished surface polymer waveguide and micro-ring resonant cavity: EME is used to simulate the coupling efficiency of fiber-tapered waveguide, and the cross-section of the wide waveguide is designed to be 6 μm x 20 μm to match the micro-ring resonant cavity. The length of the tapered region is designed to be 10 μm. The cross-section of the waveguide after the taper is designed to be 1 μm x 10 μm. The photoresist SZ2080 is dropped on the polished area of the femtosecond laser using a dropper, and a cover glass is covered. The sample is placed in a heating furnace at 95 degrees Celsius for one hour to remove moisture. The femtosecond laser generates a wavelength of 1030 nm Gaussian beam, the energy is adjusted by an electric attenuator, the laser is turned on and off by an electronic shutter, and the 50X objective lens is focused. The sample is placed on a three-dimensional moving stage so that the polished surface of the optical fiber is located in the focal plane. The designed Figure 4 processing track realizes the printing of the structure. The scanning speed is 0.2 mm / s, the adjacent track spacing is 0.3 μm, and the laser power is 4 mW. After printing is completed, the sample is immersed in alcohol until the photoresist that is not photo-cured is completely removed, and the D-shaped single-mode optical fiber tapered waveguide and micro-ring resonant cavity side-coupling integrated structure sample is obtained, as shown in Figure 5 .

[0044] The Light Conversion pH2 femtosecond laser is used, the laser wavelength is 1030 nm, the repetition frequency is 200 kHz, and the light intensity distribution is Gaussian. The 20X and 50X objective lenses used are Olympus achromatic reflective imaging objective lenses.

[0045] The above detailed description of the specific description, the purpose, technical scheme and beneficial effects of the application are further described in detail, it should be understood that the above description is only a specific embodiment of the application, and is not used to limit the protection scope of the application, any modification, equivalent replacement, improvement, etc. made within the spirit and principles of the application shall be included in the protection scope of the application.

Claims

1. A method for fabricating a tapered polymer waveguide structure integrated on the surface of a D-type single-mode optical fiber, characterized in that: It comprises the following steps, Step one: single mode fiber side polishing is realized by femtosecond laser ablation to make D type single mode fiber; through polishing of the D type single mode fiber, the polishing depth is 71-73 μm, the core is completely exposed and enough flat polishing surface area is reserved as a substrate; the side wall of the polishing area at both ends is processed with an inclination angle; Step two: the polishing area is immersed in photoresist and dried; Step three: a tapered waveguide is printed on the polishing area by using femtosecond laser two-photon polymerization technology; The waveguide cross section connecting the side wall of the polishing area is 6-9 μm x 19-21 μm, which is used to improve the coupling efficiency of the waveguide and the core; the length of the tapered transition region is 10-20 μm, which is used to reduce the loss caused by mode mismatch; before the transition is completed, the waveguide is attached to the substrate formed by the polishing surface, the waveguide cross section width in the vertical substrate direction is unchanged, which ensures the stability of the waveguide; after the transition, the waveguide cross section is 0.6-1 μm x 8-10 μm, and the waveguide is suspended after the transition; Step four: the fiber is immersed in alcohol to remove the photoresist that has not been photo-cured, and a D type single mode fiber surface integrated tapered polymer waveguide structure is obtained; The D type single mode fiber surface integrated tapered polymer waveguide structure directly couples the micro-nano waveguide on the D type fiber, and uses the polishing surface area of the D type fiber as the substrate of the optical structure; the waveguide has the size of micro-nano fiber, and the tapered transition region with strong light field localization effect is added to the waveguide, which significantly improves the coupling efficiency of the waveguide core and the tapered waveguide; The micro-nano waveguide is supported by the transition region waveguide to realize suspension, and the mode crosstalk is reduced by the suspended waveguide after the transition.

2. A method for fabricating a D-shaped single-mode optical fiber on-chip tapered polymer waveguide structure as claimed in claim 1, wherein: In step one, the polishing depth of the D type single mode fiber is 71-73 μm, the core is completely exposed and enough flat polishing surface area is reserved as a substrate; the inclination angle of the side wall of the polishing area at both ends is 28-32°, which is used to improve the coupling efficiency from the fiber core to the wide waveguide.

3. A method for fabricating a D-shaped single-mode optical fiber on-chip tapered polymer waveguide structure as claimed in claim 1, wherein: In step three, the waveguide cross section connecting the side wall of the polishing area is 6-9 μm x 19-21 μm; the length of the tapered transition region is 10-20 μm; the waveguide cross section after the transition is 0.6-1 μm x 8-10 μm, which is used to improve the coupling efficiency from the wide waveguide to the micro-nano waveguide.

4. A method of fabricating a D-shaped single-mode optical fiber on-chip tapered polymer waveguide structure according to claim 2 or 3, characterized in that: The inclination angle of the side wall of the D type fiber polishing area and the length of the tapered waveguide transition region are optimized by the eigenmode expansion method to improve the coupling efficiency from the core to the micro-nano waveguide.

5. A method of fabricating a D-shaped single mode optical fiber on-chip tapered polymer waveguide structure as claimed in claim 4, wherein: The coupling efficiency mainly depends on the overlap integral formula of the waveguide cross section light field distribution of each cell in the simulation: wherein and E1m, E2nare the electric field, magnetic field distribution of the mth mode of cell 1 and the nth mode of cell 2 in the two adjacent cells respectively, is the waveguide cross-section direction simulation region.

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