Spot-adjustable high-backscattering light-guiding microwave fiber

Abstract

The application discloses a high-backlight-reflection-resistant light guide microwave fiber with adjustable light spot, and aims to solve the problems of high-backlight-reflection, light spot homogenization and light spot shape control. The light guide microwave fiber is composed of a fiber cladding, a fiber core, a fiber filtering area and a fiber output end face. The fiber cladding is divided into an input end part, a middle part and an output end part. The input end part is composed of a cladding corrosion gentle area and a cladding corrosion transition area. The middle part is composed of an uncorroded cladding and the fiber filtering area. The output end part is composed of a fiber tapering taper area and a fiber tapering waist area. The fiber filtering area is formed by means of fiber tapering and cladding filtering at a position close to the fiber output end of the fiber cladding. The application can reduce the back reflection of light from the light guide switch to the solid laser. The size and homogenization of the output light spot can be adjusted by adjusting the fiber tapering ratio of the fiber output end. The shape of the fiber core is circular or square, and different shaped light spots can be output.

Description

Technical Field

[0001] This invention relates to the field of high-power optical waveguides and microwaves, and more particularly to an optical waveguide and microwave fiber with adjustable beam size and resistance to high back reflection. Background Technology

[0002] Novel photoconductive microwave technology utilizes high-power tunable high-repetition-rate pulsed lasers as a light source, incident on a wide-bandgap photoconductive semiconductor switch (PCSS), and then amplified by a bias voltage to generate tunable high-power microwaves. Compared to the mechanism of microwave generation by traditional vacuum tube devices, it has advantages such as small size, tunable frequency, and array combining capability. It has great potential in power combining (modularization) and tunability (intelligentization) and has attracted widespread attention from researchers in the field. Currently, because high-power tunable pulsed laser sources mostly adopt solid-state power amplification structures, the discrete optical components required limit the miniaturization of high-power photoconductive microwave systems in terms of size and flexibility. Fiber optics, with their high transmission efficiency, high flexibility, and small size, have been widely used in high-power fiber lasers and fiber optic communications, and also have great potential in the application of high-power photoconductive microwave systems.

[0003] Despite the numerous advantages of optical fibers, several challenges remain in high-power microwave applications. Firstly, there's the issue of high back-reflection and the ability to handle high-power pulsed lasers. Because wide-bandgap semiconductors like silicon carbide (SiC) and gallium nitride (GaN) have low light absorption, highly reflective materials (such as Ag) are typically deposited on the lower surface of the semiconductor to increase its absorption rate. This creates a specular reflection, increasing the optical path length within the semiconductor and promoting absorption. However, strong specular reflection can also cause reflected light to couple back into the fiber and into the solid-state amplification system. When the reflected light reaches a certain threshold, the laser system will burn out. Spatial light coupling to optical fibers is generally achieved by focusing the collimated laser onto the fiber's end face. Increasing the fiber core diameter can effectively reduce the power density at the fiber end face, improving the ability to handle high-power pulsed lasers. However, increasing the core diameter also increases the risk of reflected light from the fiber output coupling back into the fiber core. In addition, the size and shape of the light spot output from the optical fiber to the photoconductive switch is also a major challenge. Because the light intensity withstand voltage of the transparent conductive film (aluminum-doped zinc oxide (ZnO:Al), AZO, commonly known as transparent conductive film) on the surface of the photoconductive switch is limited, the photoconductive switch will be damaged when the light power density input to the AZO surface exceeds its withstand voltage threshold. Therefore, the light input to the AZO surface should be as uniform as possible. Generally, the output light spot can be adjusted by adjusting the fiber core size, the numerical aperture of the optical fiber, etc. Due to the compact size of the photoconductive switch and considering the high back reflection problem, existing optical fibers are difficult to apply to complex application scenarios.

[0004] Related patents have publicly reported the application of optical fibers to photoconductive microwave systems. The 2018 US patent, *Photoconductive switch package configurations* (patent number: US2018 / 0012028), employs a photoelectric coupling structure with perpendicular light incidence on a semiconductor. This patent inserts the output end face of the optical fiber into a conductive pillar electrode. The scattered laser output from the optical fiber undergoes multiple reflections by the conductive pillar electrode before being incident on the photoconductive switch to achieve photoelectric coupling. This method assumes the cross-sectional area of ​​the copper pillar is the size of the incident laser spot, which easily leads to laser energy loss and leakage, and does not consider the issue of backlight reflection. A Chinese patent, "An Optical Structure for Improving the Photoelectric Conversion Efficiency of Silicon Carbide Photoconductive Switches" (patent publication number: CN111739953A), describes a photoconductive switch photoelectric coupling method in embodiment three where the optical fiber is inserted close to the SiC (the distance between the reflector and the SiC is 3.5 cm), forming multiple reflections of the laser to improve coupling efficiency. The optical fiber is a multimode fiber with a core diameter greater than 100 μm, and it also does not consider power carrying capacity and backlight reflection issues.

[0005] In summary, none of the currently reported patents and papers have solved the problem of backlighting when optical fibers are applied to optical waveguide microwave systems, and they have not considered application factors such as how to control the uniformity of the light spot input to the optical waveguide switch and the light spot morphology. Summary of the Invention:

[0006] The technical problem this invention aims to solve is the high back reflection issue in high-power pulsed laser loads when optical fibers are used in optically guided microwave systems, as well as the issues of beam uniformity and beam shape control at the input to the optical guide switch. This invention provides an anti-high back reflection optically guided microwave fiber with adjustable beam shape, reducing back reflection from the optical guide switch to the solid-state laser. Simultaneously, the fiber output end is tapered, and by adjusting the taper ratio at the output end, multiple parameters can be used to adjust the size and uniformity of the output beam. Furthermore, different beam shape outputs can be achieved by adjusting the fiber core shape, enabling flexible application of optical fibers in optically guided microwave systems.

[0007] The technical solution of this invention is:

[0008] This invention comprises an optical fiber cladding, an optical fiber core, an optical filtering region within the optical fiber cladding, and an optical fiber output end face. The optical fiber cladding surrounds the optical fiber core, and the refractive index of the optical fiber core is greater than that of the optical fiber cladding. The optical filtering region within the optical fiber cladding is formed near the output end of the optical fiber by tapering and filtering the cladding. The optical fiber output end face refers to the end face formed by cutting a flat angle and coating the optical fiber cladding along its cross-sectional direction.

[0009] The fiber cladding has a circular cross-section, enclosing the fiber core. The outer diameter of the fiber cladding (called the outer diameter because the fiber cladding is cylindrical; its inner diameter is equal to the diameter of the fiber core) is D. 22 D 22 The range is 250–1400 μm, D 22 The diameter D0 of the optical fiber core is larger than that of the fiber cladding. The cladding is made of fluorine-doped silica material. Fluorine doping reduces the refractive index of the silica material. The refractive index of the fiber cladding 11 is represented by n. cladding The optical fiber cladding is divided into an input section, a middle section, and an output section. The input section consists of a cladding corrosion gradient region and a cladding corrosion transition region. The middle section consists of an uncorroded cladding and an optical fiber cladding light filtering region. The output section consists of a tapered fiber region and a tapered fiber waist region.

[0010] The fiber core cross-section is circular, but it can also be square or other shapes depending on the actual needs of the photoconductive switch. Different core shapes result in different light spot shapes output to the photoconductive switch. The fiber core is made of germanium-doped silica, with germanium doping increasing the refractive index of the silica. When the fiber core cross-section is circular, the input end diameter D0 ranges from 100 to 1400 μm. When the fiber core cross-section is square, the side length D3 of the fiber core ranges from 100 to 800 μm, and the outer diameter of the fiber cladding D... 22 Greater than D3. The fiber core is coaxially nested within the fiber cladding. The refractive index n of the fiber core. core The refractive index n greater than that of the fiber cladding cladding Numerical aperture of optical fiber The NA value determines how easily spatial light can couple into an optical fiber. To reduce coupling difficulty and improve coupling efficiency, the NA value should be between 0.22 and 0.5.

[0011] The cladding corrosion smooth zone and cladding corrosion transition zone of the optical fiber cladding are formed by etching the optical fiber cladding. The cladding corrosion smooth zone and cladding corrosion transition zone are integrated. At room temperature (20-25℃), the optical fiber cladding is placed in a hydrofluoric acid (HF) solution with a mass concentration of 50%, the immersion length in HF is 5cm, and the immersion time is 20-50min. This forms the cladding corrosion smooth zone and cladding corrosion transition zone at the input end.

[0012] The cladding corrosion plateau region is located at the leftmost end of the optical fiber. The length L5 of this plateau region is 5-10 mm. The core diameter of the plateau region is equal to D0, and the outer diameter D of the cladding in the plateau region is... 00 Satisfy D 22 >D 00 >D0 and D 00 = D0 + α, where α is the threshold, ranging from 10 to 20 μm; the outer diameter of the input end of the fiber cladding 11 is from D... 22 Corrosion reduced to D00 On the one hand, it can protect the fiber core from the influence of the environment such as air humidity; on the other hand, when high peak power spatial light couples into the fiber from the input end face of the fiber corrosion smooth region, the cross-sectional area of ​​the fiber cladding is reduced, which can reduce some of the high NA stray laser coupling into the fiber cladding; the input end of the fiber corrosion smooth region is the input end of this invention, and the input end is a flat angle.

[0013] The shape of the cladding corrosion transition region is determined by the fiber cladding corrosion rate and can be tapered or stepped. The length L1 is 2-5 mm, and L1 mainly depends on the HF volatilization during the fiber cladding corrosion process. The length of the cladding corrosion transition region does not affect the fiber performance.

[0014] The fiber taper region and the fiber taper waist region are achieved through tapering. The tapering process at the output end is as follows: the cladding of the fiber output end is cleaned with alcohol. The lengths of the two taper regions (i.e., the lengths of the fiber taper regions) L2 are set to 20mm, and the length of the fiber taper waist region L4 is set to 25mm. The fiber is placed on the tapering machine for tapering. During the tapering process, the fiber core and fiber cladding decrease proportionally, forming a fiber taper waist region. The two fiber taper regions on the left and right, centered on the fiber taper waist region, are the middle area between the two fiber taper regions. The length L2 of the fiber taper region satisfies the adiabatic tapering condition, i.e., L2 is greater than 10mm. Considering that the fiber core waveguide structure changes from large to small when the fiber laser transmission passes through the fiber taper region, L2 is set to 20mm to reduce the transmission loss of the laser in the taper region. The divergence angle of the high-power pulsed laser increases after passing through the fiber taper region of length L2.

[0015] The length L4 of the tapered waist region of the optical fiber is 25mm. The diameter of the fiber core after tapering, i.e., the diameter D1 of the fiber output end, is determined according to the required output spot size of the photoconductive switch. D1 = D0 / δ, where δ is the fiber tapering ratio. δ is determined according to the required output spot size of the photoconductive switch, and its range is 1-20. The diameter D1 of the tapered fiber core and the outer diameter of the cladding at the output end, i.e., the outer diameter D of the fiber cladding in the tapered waist region, are... 11By proportionally reducing δ, compared to untapered optical fibers, the smaller cross-sectional area of ​​the fiber core results in less reflected light entering the fiber core at the same distance. The laser beam exits from the output end face after passing through the tapered region and the waist region of the fiber. The exit divergence angle θ1 = θ0 × δ satisfies the conservation of luminance, where θ0 is the laser input reception angle at the input end of the fiber core before tapering. θ0 and the numerical aperture NA of the fiber satisfy NA = n0 × sinθ0, where n0 is the air refractive index. Therefore, the larger the NA of the fiber, the larger θ0, and the larger the tapering ratio δ and θ0, the larger the divergence angle θ1 at the output of the fiber. By setting the tapering ratio δ and the NA value of the fiber, the size and uniformity of the output spot can be adjusted. Compared to untapered optical fibers, this invention provides a more uniform spot from the fiber output to the photoconductive switch at the same distance. A flat angle is cut at the junction of the fiber tapered waist region and the fiber tapered cone region to the right of the fiber tapered waist region. The fiber tapered cone region to the right of the cut is discarded as waste. The cut end face is coated with an antireflection film to form the output end face. The thickness of the antireflection film is d. Where n1 is the refractive index of the antireflection coating material, and λ is the wavelength of the laser; the antireflection coating material is a conventional optical antireflection coating material, such as Al2O3, and the purpose of the coating is to reduce the back reflection from the fiber end face caused by Fresnel reflection at the output fiber end face. D 11 =D 22 / δ.

[0016] The optical fiber cladding optical filtering area covers part of the fiber tapered waist region, the entire fiber tapered cone region, and part of the uncorroded fiber cladding. The total length of the optical fiber cladding optical filtering area is L3 = L2 + L6 + L7 = 40-50 mm, where L7 is the length of the fiber tapered waist region within the optical fiber cladding optical filtering area, L7 < L4, and is generally 5-10 mm. L6 is the length of the uncorroded fiber cladding portion within the optical fiber cladding optical filtering area, and L6 is 10-20 mm. The processing of the optical fiber cladding optical filtering area involves covering the required length of the fiber tapered cone region, part of the fiber tapered waist region, and part of the uncorroded fiber cladding with an optical frosting solution containing HF (HF mass concentration of 40%). Combined with the ambient temperature (20-25℃) and etching time (approximately 20-30 minutes), the filtration intensity of the optical fiber cladding optical filtering area is made greater than 20 dB. The fiber cladding light filtering region can filter out cladding light coupled from the gently corroded cladding region on the left end, especially cladding light with low NA. It can also filter out the back reflection light from the highly reflective material on the surface of the photoconductive switch that enters the cladding through the fiber output end face.

[0017] The following technical effects can be achieved by using this invention:

[0018] 1. High-power pulsed laser carrying capacity with reduced back reflection. In this invention, the fiber core diameter D0 is relatively large, effectively reducing the power density of spatial light coupled into the fiber end face under high power, thus enabling the carrying capacity of high-power pulsed lasers. Furthermore, reducing the cladding corrosion at the fiber input end reduces the coupling of some high-NA stray laser light into the fiber cladding after focusing; simultaneously, reducing the tapering size of the fiber output end and implementing fiber filtering effectively reduces back reflection caused by highly reflective materials on the photoconductor switch surface coupling into the fiber core and cladding. Additionally, the filter at the output end filters out cladding back reflection, further reducing fiber back reflection.

[0019] 2. More uniform light spot output. This invention introduces a taper ratio δ at the output fiber of the optical fiber. By changing the taper ratio δ, the divergence angle of the output laser can be significantly adjusted. By combining this with the setting of the NA value of the fiber core, a more uniform light spot can be obtained at a shorter distance between the output fiber and the photoconductive switch (especially when the photoconductive switch is a small-volume device).

[0020] 3. Variable output spot shape. By changing the core shape of the optical fiber, different output spot shapes can be obtained; combined with the technical effect of point 2, this invention can achieve uniform spot shape with different spot shapes; combined with the technical effect of point 1, at high power, it is no longer necessary to use spatial optical elements to shape the laser and process spatial light return, which can adapt to various application scenarios of optical guiding microwave systems. Attached image description:

[0021] Figure 1 This is a schematic diagram of the overall structure of the present invention;

[0022] Figure 2 This is a schematic diagram of the fiber taper drawing steps of the present invention;

[0023] Figure 3 These are cross-sectional views of the optical fiber at different locations in Example 1; Figure 3 (a) is a cross-sectional view of the fiber optic input end. Figure 3 (b) is a cross-sectional view of the fiber optic output end;

[0024] Figure 4 These are cross-sectional views of the optical fiber at different locations in Example 2; Figure 4 (a) is a cross-sectional view of the fiber optic input end. Figure 4 (b) is a cross-sectional view of the fiber optic output end;

[0025] Figure 5 This is a schematic diagram comparing the simulated output spot size and uniformity of Example 1 and ordinary circular fiber under the same conditions; Figure 5 (a) is a diagram showing the simulated output spot size and uniformity of a typical circular optical fiber. Figure 5 (b) is a diagram showing the size and uniformity of the simulated output light spot in Example 1;

[0026] Figure 6 This is a schematic diagram comparing the simulated output spot size and uniformity of Example 2 and ordinary square fiber core optical fiber under the same conditions; Figure 6 (a) is a diagram showing the simulated output spot size and uniformity of a typical square optical fiber. Figure 6 (b) is a diagram showing the size and uniformity of the simulated output spot in Example 2;

[0027] The annotations in the attached figures are explained as follows:

[0028] 1. Optical fiber; 11. Optical fiber cladding; 12. Optical fiber core; 13. Optical fiber cladding light filtering region; 111. Cladding corrosion smooth region; 112. Cladding corrosion transition region; 113. Optical fiber tapered region; 114. Optical fiber tapered waist region; 115. Uncorroded cladding; 14. Optical fiber output end face. Detailed Implementation

[0029] Specific embodiments of the present invention will now be described with reference to the accompanying drawings.

[0030] For ease of description, let the laser be input from the left end of the optical fiber, let the left end of the optical fiber be the input end, and let the right end of the optical fiber be the output end; the part of the cladding at the input end after etched treatment is called the cladding etch smooth region 111; the part of the output end after tapering is divided into the tapered region 113 and the tapered waist region 114; the tapered region 113, part of the tapered waist region 114, and part of the cladding 11 constitute the output end optical cladding light filtering region 13 (i.e., Figure 1 (The part marked in black).

[0031] Figure 1 This is a schematic diagram of a microwave optical fiber structure with adjustable beam size and resistance to high back reflection, according to the present invention. The present invention consists of an optical fiber cladding 11, an optical fiber core 12, an optical fiber cladding light filtering region 13, and an optical fiber output end face 14. The optical fiber cladding 11 encloses the optical fiber core 12, and the refractive index of the optical fiber core 12 is greater than that of the optical fiber cladding 11. The optical fiber cladding light filtering region 13 is formed by tapering and filtering the cladding near the output end of the optical fiber cladding 11. The optical fiber output end face 14 refers to the end face formed by cutting a flat angle and coating along the cross-sectional direction of the optical fiber cladding 11.

[0032] like Figure 1 and Figure 3 As shown in (a), the cross-section of the fiber cladding 11 is annular, and the fiber cladding 11 encloses the fiber core 12; the outer diameter of the fiber cladding 11 is D. 22 D 22 Size range 250–1400 μm, D 22The diameter D0 of the fiber core 12 is larger than that of the fiber cladding 11, which is made of fluorine-doped silica. The fiber cladding 11 is divided into an input end section, an intermediate section, and an output end section. The input end section consists of a cladding corrosion smooth region 111 and a cladding corrosion transition region 112. The intermediate section consists of an uncorroded cladding 115 and a fiber cladding light filtering region 13. The output end section consists of a fiber tapered region 113 and a fiber tapered waist region 114.

[0033] like Figure 1 , Figure 3 (a) and Figure 3 As shown in (b), the cross-section of the fiber core 12 is circular. The fiber core 12 is made of germanium-doped quartz material; the refractive index of the quartz material is increased by doping with germanium. The input end diameter D0 of the fiber core 12 ranges from 100 to 1400 μm. Figure 4 (a) and Figure 4 As shown in (b), the cross-section of the fiber core 12 is square, and the side length D3 of the fiber core 12 is 100-800 μm. The outer diameter D of the fiber cladding (11) is... 22 Greater than D3. The fiber core 12 is coaxially nested within the fiber cladding 11. The refractive index n of the fiber core 12... core The refractive index n greater than that of the fiber cladding 11 cladding The NA value is between 0.22 and 0.5.

[0034] The cladding corrosion smooth region 111 and the cladding corrosion transition region 112 of the optical fiber cladding 11 are formed by etching the optical fiber cladding 11. The cladding corrosion smooth region 111 and the cladding corrosion transition region 112 are integrated. At room temperature (20-25℃), the optical fiber cladding 11 is placed in a hydrofluoric acid HF solution with a mass concentration of 50%, the immersion length in HF is 5cm, and the immersion time is 20-50min, thus forming the cladding corrosion smooth region 111 and the cladding corrosion transition region 112 at the input end.

[0035] like Figure 1 As shown, the cladding corrosion smoothing region 111 is located at the leftmost end of the optical fiber. The length L5 of the cladding corrosion smoothing region 111 is 5-10 mm. The core 12 diameter of the cladding corrosion smoothing region 111 is equal to D0, and the outer diameter D of the cladding of the cladding corrosion smoothing region 111 is... 00 Satisfy D 22 >D 00 >D0 and D 00 = D0 + α, where α is the threshold, ranging from 10 to 20 μm; the outer diameter of the input end of the fiber cladding 11 is from D... 22 Corrosion reduced to D 00On the one hand, it protects the fiber core 12 from the influence of the environment such as air humidity; on the other hand, when high peak power spatial light is coupled into the fiber 1 from the input end face of the cladding corrosion smooth region 111, the cross-sectional area of ​​the fiber cladding 11 is reduced, which can reduce some of the high NA stray laser coupling into the fiber cladding 11; the input end of the cladding corrosion smooth region 111 is the input end of the present invention, and the input end is a flat angle.

[0036] The shape of the cladding corrosion transition zone 112 is determined by the corrosion rate of the fiber cladding 11, and is either conical or stepped. The length L1 is 2-5 mm.

[0037] The taper region 113 and the waist region 114 of the optical fiber are achieved through a taper mechanism. (See attached image) Figure 2 This is a schematic diagram of the tapered output end of the present invention, combined with... Figure 2 The tapering process at the output end is described below. The cladding 11 of the output fiber is cleaned with alcohol. The lengths of the two tapered regions (i.e., the lengths of the tapered regions 113) L2 of the fiber tapering machine (such as the 3SAE LDS model) are set to 20 mm, and the length of the waist region 114 L4 is set to 25 mm. The fiber is placed on the tapering machine for tapering. During the tapering process, the fiber core 12 and the fiber cladding 11 decrease proportionally, forming a tapered waist region 114. The two tapered regions 113 on the left and right sides, centered on the waist region 114, are the middle area of ​​these two regions. The length L2 of the tapered region 113 satisfies the adiabatic tapering condition, i.e., L2 is greater than 10 mm. To reduce laser transmission loss in the tapered region, L2 is set to 20 mm. After passing through the L2-length tapered region 113, the divergence angle of the high-power pulsed laser increases.

[0038] The length L4 of the tapered waist region 114 of the optical fiber is 25mm, and the diameter of the tapered fiber core 12 is the diameter D1 of the fiber output end (e.g., Figure 2 (As shown) The output spot size required by the photoconductive switch is determined by D1 = D0 / δ, where δ is the fiber taper ratio. The fiber taper ratio is determined by the output spot size required by the photoconductive switch, and its range is 1 to 20. The diameter D1 of the fiber core 12 after tapering is equal to the outer diameter of the cladding at the output end, i.e., the outer diameter D1 of the fiber cladding 11 in the tapered waist region 114 of the fiber. 11As δ decreases proportionally, compared to untapered optical fibers, the cross-sectional area of ​​the fiber core 12 becomes smaller, resulting in less back-reflected light entering the fiber core 12 at the same distance. The laser beam exits from the output end face 14 after passing through the tapered region 113 and the tapered waist region 114 of the fiber. The divergence angle θ1 = θ0 × δ satisfies the conservation of light intensity, where θ0 is the laser input receiving angle at the input end of the fiber core 12 before tapering. θ0 and the numerical aperture NA of the fiber satisfy NA = n0 × sinθ0, where n0 is the air refractive index. Therefore, the larger the NA of the fiber, the larger θ0, and the larger the tapering ratio δ and θ0, the larger the divergence angle θ1 output through the fiber. By setting the tapering ratio δ and the NA value of the fiber, the size and uniformity of the output spot can be adjusted. Compared to untapered optical fibers, the present invention provides a more uniform spot output from the fiber to the photoconductive switch at the same distance. A flat angle is cut at the junction of the fiber tapered waist region 114 and the fiber tapered cone region 113 to the right of the fiber tapered waist region 114 (the cut is as follows). Figure 2 At section AA, the fiber tapered region 113 to the right of the cut is discarded as waste. The cut end face (i.e., section AA) is coated with an anti-reflection film to form the output end face 14. The thickness of the anti-reflection film is d. Where n1 is the refractive index of the antireflection coating material, and λ is the wavelength of the laser; the antireflection coating material is a conventional optical antireflection coating material, such as Al2O3. The D... 11 =D 22 / δ.

[0039] like Figure 1 As shown, the optical fiber cladding optical filtering region 13 covers part of the fiber tapered waist region 114, the entire fiber tapered region 113, and part of the uncorroded fiber cladding 115. The total length of the optical fiber cladding optical filtering region 13 is L3 = L2 + L6 + L7 = 40~50mm. L7 is the length of the fiber tapered waist region 114 located in the optical fiber cladding optical filtering region 13. L7 < L4, and generally L7 is 5~10mm. L6 is the length of the part of the uncorroded fiber cladding 115 located in the optical fiber cladding optical filtering region 13. L6 is 10~20mm. The processing of the optical fiber cladding optical filtering region 13 involves covering the required length of the fiber tapered region 113, part of the fiber tapered waist region 114, and part of the uncorroded fiber cladding 115 with an optical frosting solution containing HF (HF mass concentration of 40%). Combined with the ambient temperature (20–25°C) and etching time (approximately 20–30 minutes), the filtration intensity of the optical fiber cladding optical filtering region 13 is made greater than 20 dB. The optical fiber cladding optical filtering region 13 can filter out cladding light coupled from the gently etched cladding region 111 at the left end, especially cladding light with low NA. It can also filter out back-reflected light from the highly reflective material on the surface of the photoconductive switch, which enters the cladding through the fiber output end face 14.

[0040] Example 1

[0041] A microwave optical fiber with adjustable beam spot and resistance to high back reflection has a circular annular cross-section for its cladding layer 11, with an outer diameter of D. 22 =440μm, the fiber cladding 11 is made of fluorine-doped silica material. The fiber core 12 has a circular cross-section and is made of germanium-doped silica material. The diameter D0 of the fiber core 12 is 400μm. The fiber cladding 11 encloses the fiber core 12, and the refractive index of the fiber core 12 is greater than that of the fiber cladding 11. (where n) core n is the refractive index of fiber core 12. cladding (Refractive index of cladding 11). The cladding corrosion smooth region 111 and cladding corrosion transition region 112 of the optical fiber cladding 11 are achieved by etching the optical fiber cladding 11. The optical fiber cladding 11 is wiped clean with alcohol and placed in a 50% HF solution, with the immersion length in the HF solution being 5 cm. The ambient temperature is approximately 25°C, and the etching time is 20 min. This forms the cladding corrosion smooth region 111 and the cladding corrosion transition region 112 at the input end. The length L5 of the cladding corrosion smooth region 111 is 10 mm, and the diameter D of the cladding 11 after etching is... 00 =420μm; The input end of the cladding corrosion smooth region 111 is the input end of this invention, and the input end is a flat angle, as shown in 3(a). The length L1 of the cladding corrosion transition region 112 is 2mm. The tapered region 113 and the fiber tapered waist region 114 are realized by tapering. Using a fiber tapering machine (3SAE, model: LDS), the length L2 of the left and right tapered regions 113 is set to 20mm, and the length L4 of the fiber tapered waist region 114 is 25mm. The fiber is placed on the tapering machine for tapering. During the tapering process, the fiber core 12 and the cladding 11 are reduced proportionally. After tapering, the left and right fiber tapered regions 113 and the fiber tapered waist region 114 are formed. The middle area of ​​the left and right tapered regions 113 is the fiber tapered waist region 114. The length L2 of the fiber tapered region 113 is 20mm. The length L4 of the tapered waist region 114 of the optical fiber is 25mm, and its tapering ratio δ=D0 / D1=4, that is, the diameter D1 of the fiber core 12 after tapering is 100μm, and the outer diameter D of the cladding 11 at the output end is... 11 =110μm. A flat angle is cut at the junction of the fiber tapered waist region 114 and the right tapered cone region 113, as shown in the figure. Figure 2 As shown at section AA, the cut right-hand tapered region 113 is discarded as waste. The cut end face is coated with an optical antireflection film of the wavelength required for laser treatment. For a 1064nm laser, the film material is Al2O3, and the film thickness is 86.6nm, forming the output end face 14. The cross-section of the output end face 14 is shown below. Figure 3As shown in (b), the optical fiber cladding optical filtering removal area 13 covers the entire tapered region 113 of the optical fiber and part of the tapered waist region 114 of the optical fiber, with a length of L6 = 5 mm and part of the uncorroded cladding 115, with a length of 10 mm, and its total length is L3 = 40 mm.

[0042] Compared to similar optical fibers on the market, this embodiment 1 can effectively homogenize the light spot output to the photoconductive switch under the same application conditions. Using Trace Pro (version 7.4) under the same simulation conditions (consistent optical path system and distance from the optical fiber to the photoconductive switch), the diameter and intensity distribution of the two types of optical fiber light spots were detected on a photoconductive switch with dimensions of 3*3*1mm. Figure 5 As shown, Figure 5 The horizontal axis corresponds to the length of the photoconductive switch, and the vertical axis corresponds to its width. The length and width of the photoconductive switch are 3*3mm. The light intensity scale on the left is in W / m. 2 That is, the smaller the color contrast between the center and the edge of the light spot, the more uniform the light spot; the larger the light spot size, the more uniform the light spot. Figure 5 (a) The optical fiber spot diameter shown in the market for the same type is 2mm, and the spot diameter of the present invention is as follows: Figure 5 As shown in (b), the optical fiber spot diameter is greater than 2.8 mm. Therefore, the optical fiber of the present invention has a larger and more uniform spot, which can achieve an effective effect.

[0043] Example 2

[0044] A microwave optical fiber with adjustable beam spot and resistance to high back reflection has a circular cross-section for the fiber cladding 11, with a diameter D. 22 The fiber has a diameter of 600 μm, and the cladding 11 is made of fluorine-doped silica. The fiber core 12 has a square cross-section and is made of germanium-doped silica. The side length D3 of the fiber core 12 is 400 μm. The cladding 11 encloses the fiber core 12, and the refractive index of the fiber core 12 is greater than that of the cladding 11. (where n) core n is the refractive index of fiber core 12. cladding (Refractive index of cladding 11). The cladding corrosion smooth region 111 and cladding corrosion transition region 112 of the optical fiber cladding 11 are achieved by etching the optical fiber cladding 11. The optical fiber cladding 11 is placed in a 50% HF solution, with the immersion length in the HF solution being 5 cm. Combined with an ambient temperature of approximately 25°C and an etching time of 25 min, the cladding corrosion smooth region 111 and the optical fiber cladding 11 corrosion transition region 112 are formed at the input end. The length L5 of the cladding corrosion smooth region 111 is 10 mm, and the outer diameter D of the cladding corrosion smooth region 111 is... 00 =580μm; Cladding corrosion smooth zone 111 The input terminal is the input terminal of this invention. The input terminal is a flat angle, and the cross-section of the input terminal is as shown. Figure 4 As shown in (a), the cladding corrosion transition region 112 has a length L1 of 3 mm. The tapered region 113 and the fiber tapered waist region 114 are achieved by tapering. (See attached diagram.) Figure 2 As can be seen, the length L2 of the left and right tapered regions 113 is set to 20mm, and the length L4 of the fiber tapered waist region 114 is set to 25mm. The fiber is placed on a tapering machine for tapering. During the tapering process, the fiber core 12 and cladding 11 decrease proportionally, resulting in the formation of the fiber tapered regions 113 and 114. The middle region of the left and right tapered regions 113 is the fiber tapered waist region 114. The length L2 of the left and right tapered regions 113 satisfies the adiabatic tapering condition for fiber tapering, i.e., L2 is 20mm. The length L4 of the fiber tapered waist region 114 is 25mm, and its tapering ratio δ = D3 / D4 = 1.33 (D3 and D4 are the side lengths of the fiber core 12 before and after tapering, respectively). After tapering, the side length D4 of the fiber core 12 is 300μm, and the outer diameter D of the output fiber cladding 11 is... 11 =450μm. At the junction of the waist region 114 and the right tapered region 113 of the optical fiber (i.e. Figure 2 The cross-section AA) is cut with a flat angle. The cut end face is then coated with an antireflective film of the required laser wavelength to form the output end face 14. For a 1064nm wavelength laser, the antireflective film material is Al2O3, and the thickness is 86.6nm. The cross-section of the output end is shown below. Figure 4 As shown in (b), the optical fiber cladding optical filtering removal area 13 covers the entire optical fiber tapered area 113 and part of the optical fiber tapered waist area 114, with a length of L6 = 10 mm and part of the uncorroded cladding 115, with a length of L7 = 15 mm, and its total length is L3 = 50 mm.

[0045] Compared to similar optical fibers on the market, this implementation case 2 can effectively homogenize the light spot output to the photoconductive switch under the same application conditions. Using Trace Pro (version 7.4) under the same simulation conditions (consistent optical path system and distance from the optical fiber to the photoconductive switch), the light spot size and intensity distribution of the two types of optical fibers output to the photoconductive switch with dimensions of 3*3*1mm were detected as follows. Figure 6 As shown, Figure 6 The horizontal axis corresponds to the length of the photoconductive switch, and the vertical axis corresponds to the width of the photoconductive switch. The dimensions are 3*3mm, and the unit of the light intensity scale is W / m. 2 That is, the smaller the color contrast between the center and the edge of the light spot, the more uniform the light spot; the larger the light spot size, the more uniform the light spot. Figure 6 (a) The effective side length of the optical fiber spot in the same market is 0.8 mm, and the diameter of the optical fiber spot in this invention is as follows: Figure 6 As shown in (b), the effective side length of the fiber spot is 1.2 mm. In comparison, the spot of the present invention is larger and more uniform, which can achieve an effective effect.

Claims

1. A microwave optical fiber with adjustable beam spot and resistance to high back reflection, comprising an optical fiber cladding (11) and an optical fiber core (12), wherein the cross-section of the optical fiber cladding (11) is annular, the optical fiber cladding (11) encloses the optical fiber core (12), and the optical fiber core (12) is coaxially nested within the optical fiber cladding (11); characterized in that The anti-high back-reflection light-guided microwave optical fiber with adjustable light spot further comprises an optical fiber cladding light filtering region (13) and an optical fiber output end face (14); the refractive index of the optical fiber core (12) is greater than the refractive index of the optical fiber cladding (11); the optical fiber cladding light filtering region (13) is a part formed by processing the position of the optical fiber cladding (11) close to the optical fiber output end through tapering and cladding filtering, and the optical fiber output end face (14) refers to an end face formed by cutting the optical fiber cladding (11) flat along the cross-sectional direction and coating with a film; laser is input from the left end of the optical fiber, the left end of the optical fiber is defined as the optical fiber input end, and the right end of the optical fiber is defined as the optical fiber output end; The cross-section of the fiber core (12) is circular, and the fiber core (12) is made of germanium-doped quartz material; the refractive index of the fiber core (12) is... Larger than the fiber cladding (11) ; Fiber cladding (11) outer diameter D 22 The diameter D0 of the fiber core (12) is larger than that of the fiber core (12). The fiber cladding (11) is made of fluorine-doped silica material. The fiber cladding (11) is divided into an input end part, a middle part and an output end part. The input end part consists of a cladding corrosion smooth area (111) and a cladding corrosion transition area (112). The middle part consists of an uncorroded fiber cladding (115) and a fiber cladding light filtering area (13). The output end part consists of a fiber tapered taper area (113) and a fiber tapered waist area (114). The cladding corrosion gentle region (111) and the cladding corrosion transition region (112) of the optical fiber cladding (11) are formed by corroding the optical fiber cladding (11), and the cladding corrosion gentle region (111) and the cladding corrosion transition region (112) are integrated as a single body; The cladding corrosion smoothing region (111) is located at the leftmost end of the optical fiber. The length of the cladding corrosion smoothing region (111) is L5, and the outer diameter D of the cladding corrosion smoothing region (111) is... 00 Satisfy D 22 >D 00 >D0; The outer diameter of the fiber cladding (11) input end is from D 22 Corrosion reduced to D 00 The input end of the cladding corrosion smooth zone (111) is the input end of the anti-high back-reflection optical fiber with adjustable light spot, and the input end is flat. The cladding corrosion transition region (112) is conical or stepped, with a length of L1; The optical fiber tapering cone region (113) and the optical fiber tapering waist region (114) are realized by a tapering method; the length L2 of the optical fiber tapering cone region (113) satisfies the adiabatic tapering condition of the optical fiber, L2 is greater than 10mm, and the divergence angle of the high-power pulsed laser becomes larger after passing through the optical fiber tapering cone region (113) with the length L2; The length of the tapered waist region (114) of the optical fiber is L4, the diameter of the tapered core (12) of the optical fiber is D1, and the outer diameter of the cladding of the tapered waist region (114) of the optical fiber is D. 11 Cut a flat angle at the junction of the fiber tapered waist region (114) and the fiber tapered cone region (113) to the right of the fiber tapered waist region (114), and coat the cut end face with an anti-reflection film to form the fiber output end face (14). The optical fiber cladding light filtering region (13) covers part of the optical fiber tapering waist region (114), the entire optical fiber tapering cone region (113) and part of the uncorroded optical fiber cladding (115). The total length L3 of the optical fiber cladding light filtering region (13) is L2+L6+L7, where L2 is the length of the optical fiber tapering cone region (113), L7 is the length of the part of the optical fiber cladding light filtering region (13) located in the optical fiber tapering waist region (114), L7<L4, and L6 is the length of the part of the optical fiber cladding light filtering region (13) located in the uncorroded optical fiber cladding (115); the filtering strength of the optical fiber cladding light filtering region (13) is greater than 20dB, which can filter out the cladding light coupled in from the cladding corrosion gentle region (111) at the left end, and filter out the back-reflected light of the laser reflected by the high-reflection material on the surface of the light-guided switch that enters the cladding through the optical fiber output end face (14).

2. The adjustable beam spot anti-high back-reflection optical fiber of claim 1, characterized in that... The outer diameter D of the optical fiber cladding (11) 22 The size range is 250–1400 μm; the input end diameter D0 of the fiber core (12) ranges from 100 to 1400 μm, and the numerical aperture of the fiber is... The value is between 0.22 and 0.

5.

3. The adjustable beam spot anti-high back-reflection optical fiber of claim 1, characterized in that... The length L5 of the cladding corrosion smooth zone (111) is 5-10 mm, and the outer diameter D of the cladding corrosion smooth zone (111) is... 00 = D0+α, where α is the threshold, α is 10~20μm, and the length L1 of the cladding corrosion transition zone (112) is 2~5mm.

4. The adjustable beam spot anti-high back-reflection optical fiber of claim 1, characterized in that... The process of realizing the optical fiber tapering cone region (113) and the optical fiber tapering waist region (114) by the tapering method is as follows: wipe the optical fiber cladding (11) at the output end of the optical fiber clean with alcohol, set the length L2 of the left and right cone regions of the optical fiber tapering machine to 20mm, set the length L4 of the optical fiber tapering waist region (114) to 25mm, place the optical fiber on the tapering machine for tapering. During the tapering process, the optical fiber core (12) and the optical fiber cladding (11) are reduced in equal proportion. After tapering, one optical fiber tapering waist region (114) is formed, and the two left and right optical fiber tapering cone regions (113) centered on the optical fiber tapering waist region (114), that is, the middle region of the two left and right optical fiber tapering cone regions (113) is the optical fiber tapering waist region (114).

5. The adjustable beam spot anti-high back-reflection optical fiber of claim 4, characterized in that... The optical fiber tapering machine is a model LDS optical fiber tapering machine manufactured by 3SAE Corporation.

6. The adjustable beam spot anti-high back-reflection optical fiber of claim 1, characterized in that... The diameter D1 of the tapered fiber core (12) is D0. , Fiber taper ratio, The size of the output light spot depends on the required size of the photoconductive switch. The outer diameter D of the cladding of the fiber tapered waist region (114) ranges from 1 to 20. 11 =D 22 .

7. The adjustable beam spot anti-high back-reflection optical fiber of claim 1, characterized in that... The thickness of the antireflective film ,in To improve the refractive index of the antireflective coating, The wavelength of the laser; the antireflective coating uses Material.

8. The adjustable beam spot anti-high back-reflection optical fiber of claim 1, characterized in that... The total length L3 of the optical fiber cladding light filtering region (13) is 40 to 50mm, L7 is 5 to 10mm, L6 is 10 to 20mm, and L2 is 20mm.

9. The adjustable beam spot anti-high back-reflection optical fiber of claim 1, characterized in that... The formation process of the cladding corrosion smooth zone (111) and cladding corrosion transition zone (112) of the optical fiber cladding (11) is as follows: the optical fiber cladding (11) is placed in a hydrofluoric acid HF solution at room temperature, the mass concentration of HF is 50%, the immersion length of HF is 5cm, and the immersion time is 20-50min; the processing process of the optical fiber cladding light filtering zone (13) is as follows: an optical frosting solution containing HF at room temperature is applied to the optical fiber tapered region (113), the optical fiber tapered waist region (114) with a length of L7, and the uncorroded optical fiber cladding (115) with a length of L6, and the corrosion time is 20-30min, so that the optical fiber cladding light filtering zone (13) has a cladding light filtering capability greater than 20dB; the mass concentration of HF in the optical frosting solution containing HF is 40%; the room temperature refers to 20-25℃.

10. The adjustable beam spot anti-high back-reflection optical fiber of claim 1, characterized in that... The fiber core (12) has a square cross-section, and the outer diameter D of the fiber cladding (11) is... 22 The side length D3 of the fiber core (12) is greater than that of the fiber core (12), where D3 is 100-800 μm, and the outer diameter of the cladding in the cladding corrosion smooth zone (111) is D. 00 = D3+α, where α is the threshold value and α is 10~20μm; the outer diameter D of the cladding in the tapered waist region (114) of the fiber. 11 Satisfy D 11 >D4 and D 11 =D 22 D4 = D3 / , The side length of the fiber core (12) after tapering. Fiber taper ratio, The size of the output light spot depends on the required size of the photoconductive switch. The range is 1 to 20.

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