A system for fabricating fiber gratings using an ultrafast laser direct writing method with monitoring capabilities

Abstract

This invention relates to the field of fiber grating fabrication technology, specifically to a system for fabricating fiber gratings using an ultrafast laser direct writing method with monitoring capabilities. The system includes a femtosecond laser, a dichroic mirror, a first CCD, an objective lens, and a monitoring module. The monitoring module includes a precision translation stage, an optical fiber, a light source, and a second CCD. The femtosecond laser emits femtosecond pulsed laser light, which is reflected by the dichroic mirror and enters the objective lens. The objective lens focuses the femtosecond pulsed laser light. The optical fiber is fixed on the precision translation stage, and the femtosecond pulsed laser light irradiates the fiber core. The light source emits laser light, which is coupled into the optical fiber. The second CCD is connected to the other end of the optical fiber. The reflected light from the optical fiber passes through the objective lens and the dichroic mirror before entering the first CCD. This invention utilizes the second CCD to detect the light field distribution within the optical fiber, enabling real-time and precise monitoring of the position of the femtosecond pulsed laser focus point relative to the fiber core, allowing for timely adjustments and thus improving the quality of the written fiber grating.

Description

Technical Field

[0001] This invention relates to the field of fiber grating fabrication technology, and specifically to a system for fabricating fiber gratings using an ultrafast laser direct writing method with monitoring function. Background Technology

[0002] A fiber grating (FBG) is a one-dimensional periodic structure in which the refractive index of an optical fiber core is periodically perturbed, resulting in a periodic distribution of refractive index. Its working principle is to achieve wavelength selectivity by coupling between resonant wavelength modes under the premise of phase matching. FBG technology has greatly expanded the application scope of optical fiber technology. Due to the enormous practical and potential application value of FBGs in optical fiber sensing, communication technology, and industrial manufacturing, FBG research has attracted widespread attention from scientists worldwide and will remain a hot research topic in optics and communication for a considerable period of time. Traditional methods for fabricating gratings include standing wave writing and interferometric exposure. For example, Kohl and his colleagues at the Canadian Centre for Communications Research first discovered the photosensitivity of optical fibers and used standing wave writing to obtain self-sensing gratings. G. Meltz et al. invented a technique for writing photosensitive gratings using 244nm ultraviolet dual-beam interferometric exposure. By adjusting the angle between two coherent beams or changing the wavelength of the laser source, the resonant wavelength of the grating can be controlled, leading to significant progress in optical fiber fabrication technology, which is gradually maturing and becoming commercialized. Traditional methods for fabricating gratings often suffer from drawbacks such as low writing efficiency, high dependence on laser source, and high stability of the overall optical path system.

[0003] Ultrafast lasers are a relatively new technology that has emerged in recent years. Their pulse widths range from several ultrafast to tens of picoseconds, with extremely high peak power. Their interaction mechanism with matter is fundamentally different from other types of lasers, representing a form of laser "cold processing," and has had a disruptive impact on industries such as laser chemistry and precision micro / nano fabrication. Fiber gratings fabricated using ultrafast lasers do not require phase masks, allowing for flexible control of grating parameters. By controlling the exposure width and the relative widths between exposure points, fiber gratings with different characteristics can be created. This characteristic offers significant advantages in ultrafast laser production lines, which require a wide variety of gratings in small batches. Furthermore, they can be directly written into ordinary single-mode fibers without removing the coating, eliminating the need for special fiber treatment and greatly improving the fiber's mechanical strength. They also exhibit extremely high thermal stability, with an operating temperature reaching 1000℃. This characteristic holds immense potential in manufacturing high-reflectivity devices and partially reflective gratings for high-power fiber lasers. Researchers have achieved some research results in this area. For example, SJ Martinez et al. in the UK used infrared ultrafast laser point-by-point writing to realize first-order and third-order Bragg fiber gratings on common communication optical fibers and phase-shifting optical fibers; GD Marshall et al. in Australia used 800nm ​​ultrafast laser to write point-by-point second-order Bragg fiber gratings with a linewidth of 0.15nm and a reflectivity of up to 99.99% on standard single-mode optical fibers.

[0004] In the fabrication of fiber gratings using the ultrafast laser direct writing method, the focusing position of the ultrafast laser significantly affects the quality of the fiber grating: when the grating is written near the center of the fiber core, the spectral quality of the grating is high; when the grating is written near the edge of the fiber core, the reflectivity of the grating is very low; and when the grating is written outside the fiber core, the grating has no transmission spectrum. Therefore, real-time monitoring of the focusing position of the ultrafast laser is crucial for fabricating high-quality fiber gratings. Traditional methods involve writing the fiber grating under a microscope and using imaging equipment to visually determine the focusing position of the ultrafast laser, which results in large errors and poor quality fiber gratings. Summary of the Invention

[0005] To address the above problems, this invention provides a system for fabricating fiber Bragg gratings using an ultrafast laser direct writing method with monitoring capabilities. The system includes a femtosecond laser, a dichroic mirror, a first CCD, an objective lens, and a monitoring module. The monitoring module includes a precision translation stage, an optical fiber, a light source, and a second CCD. The femtosecond laser emits femtosecond pulsed laser light, which is reflected by the dichroic mirror and enters the objective lens. The objective lens converges the femtosecond pulsed laser light. The optical fiber is fixed on the precision translation stage, and the femtosecond pulsed laser light irradiates the fiber core. The light source emits laser light, which is coupled into the optical fiber. The second CCD is connected to the optical fiber and is used to detect the light field distribution within the fiber. The reflected light from the optical fiber passes through the objective lens and the dichroic mirror before entering the first CCD.

[0006] This invention uses a second CCD to observe the distribution of the emitted light field at the fiber end face, thereby determining the focusing position of a femtosecond pulse laser and achieving real-time monitoring of fiber grating fabrication. Specifically, after the femtosecond pulse laser is focused onto the fiber, it causes a change in the fiber's refractive index. This change in refractive index within the fiber leads to a change in the emitted light field distribution at the fiber end face, and the focusing position of the femtosecond pulse laser is determined by this change in light field distribution. If the focusing position of the femtosecond pulse laser deviates from the center of the fiber core, it is adjusted in real time.

[0007] Furthermore, a second CCD is connected to the other end of the optical fiber. That is, a laser is input at one end of the optical fiber, and a second CCD is connected to the other end of the optical fiber to detect changes in the light field distribution. This connection method is simple.

[0008] Furthermore, it also includes a coupler and a spectrometer. The coupler is a four-port fiber optic coupler, with its four ports connected to the optical fiber, the spectrometer, and the second CCD, respectively. This provides another way to detect the light field distribution: it can detect not only the light field distribution in the optical fiber but also the reflection spectrum of the optical fiber, thus detecting the quality of the fabricated fiber grating from two aspects.

[0009] Furthermore, the light source is a broadband light source.

[0010] Furthermore, the precision translation stage is a three-dimensional precision translation stage. The precision translation stage can not only move in the horizontal plane, but also be adjusted in the vertical direction, so that the femtosecond pulse laser can be focused onto the core position of the optical fiber.

[0011] Furthermore, a fixing component is fixed on the precision translation stage. The surface of the fixing component is provided with a V-shaped groove, and the optical fiber is placed in the V-shaped groove to facilitate the stable fixing of the optical fiber on the fixing component and its movement with the precision translation stage.

[0012] Furthermore, a reflective layer made of aluminum is provided on the side of the V-groove. The reflective layer partially reflects the femtosecond pulse laser back into the optical fiber, thereby creating a stronger optical field within the fiber and causing the region of refractive index change within the fiber to become more concentrated.

[0013] Furthermore, it also includes a waveplate and a prism, after which the femtosecond pulsed laser passes through the waveplate and prism in sequence before illuminating the dichroic mirror.

[0014] Furthermore, the prism is a Gran Thompson prism.

[0015] Furthermore, the waveplate is a half-wave plate.

[0016] The beneficial effects of this invention are:

[0017] (1) The present invention uses a first CCD to monitor the position of the femtosecond pulse laser relative to the optical fiber and adjusts it in real time according to the actual situation, which is equivalent to achieving coarse adjustment of the optical fiber position, and the adjustment is convenient.

[0018] (2) The present invention uses a second CCD to detect the optical field distribution in the optical fiber, determine the position of the femtosecond pulse laser focusing point relative to the fiber core, and adjust the position of the optical fiber in a timely manner, thereby achieving fine adjustment of the optical fiber position; since the position of the femtosecond pulse laser focusing point relative to the fiber core can be easily determined through the optical field distribution, the adjustment is precise.

[0019] (3) The present invention places the optical fiber in a V-groove, which makes it easier to fix the optical fiber and makes the fiber grating writing system more stable.

[0020] (4) The present invention sets a reflective layer in the V-groove, which facilitates the focusing of femtosecond pulse laser, not only improving the interaction between the femtosecond pulse laser and the fiber core, but also preventing the femtosecond pulse laser from damaging the fixed components.

[0021] In summary, this invention has promising applications in the field of ultrafast laser direct writing fabrication of fiber gratings.

[0022] The present invention will now be described in further detail with reference to the accompanying drawings. Attached Figure Description

[0023] Figure 1 This is a schematic diagram of a system for fabricating fiber gratings using an ultrafast laser direct writing method with monitoring capabilities.

[0024] Figure 2 The optical field distribution at the fiber output end is as follows when the refractive index change region is located at different positions in the fiber core: (a) the distance between the small cylinder and the fiber core axis is 0 nanometers, (b) the distance between the small cylinder and the fiber core axis is 100 nanometers, and (c) the distance between the small cylinder and the fiber core axis is 180 nanometers.

[0025] Figure 3 This is a schematic diagram of yet another monitoring module.

[0026] Figure 4 This is a schematic diagram of another system for fabricating fiber gratings using an ultrafast laser direct writing method with monitoring capabilities.

[0027] In the diagram: 1. Femtosecond laser; 2. Dichroic mirror; 3. First CCD; 4. Objective lens; 5. Precision translation stage; 6. Optical fiber; 7. Light source; 8. Second CCD; 9. Waveplate; 10. Prism; 11. Coupler; 12. Spectrometer. Detailed Implementation

[0028] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided with reference to the accompanying drawings and embodiments.

[0029] Example 1

[0030] This invention provides a system for fabricating fiber Bragg gratings using an ultrafast laser direct writing method with monitoring capabilities, such as... Figure 1 As shown, the system includes a femtosecond laser 1, a dichroic mirror 2, a first CCD 3, an objective lens 4, and a monitoring module. The monitoring module includes a precision translation stage 5, an optical fiber 6, a light source 7, and a second CCD 8. The optical fiber 6 described in this invention is the optical fiber to be written.

[0031] During the fabrication of the fiber Bragg grating, the femtosecond laser 1 emits femtosecond pulsed laser light. This pulsed laser light is reflected by the dichroic mirror 2 and enters the objective lens 4, where it is focused. The objective lens 4 is preferably a microscope objective lens, and specifically a long-focal-length objective lens. Due to its long focal length, a larger space is left between the objective lens 4 and the optical fiber 6, facilitating the adjustment of the optical fiber 6's position. The pulse width of the femtosecond laser 1 is precisely controlled by an autocorrelation meter.

[0032] Fiber 6 is fixed on a precision translation stage 5, and computer control enables fiber 6 to move precisely at a specific speed along with the high-precision translation stage. A femtosecond pulsed laser irradiates the core of fiber 6; that is, the femtosecond laser 1 emits laser light and couples it into fiber 6, aligning it with the core for marking. A light source 7 emits laser light and couples it into fiber 6. A second CCD 8 is connected to the other end of fiber 6. The reflected or scattered light from fiber 6 passes through objective lens 4 and dichroic mirror 2 before entering the first CCD 3. The first CCD 3 is used to monitor the adjustment process and roughly observe the marking status online. In summary, the first CCD 3 not only monitors the adjustment process but also preliminarily determines the position of the femtosecond pulsed laser focus point relative to the fiber core and implements adjustments, offering the advantage of convenient adjustment.

[0033] When the precision translation stage 5 moves along the fiber 6 at a set speed, the focused ultrafast pulsed laser interacts with the fiber core, changing the contact point between the ultrafast laser beam and the fiber 6. Each pulse generates a point with refractive index modulation and a modulation period within the fiber core, and this sequence of points forms a fiber Bragg grating. The grating period can be set by changing the ratio between the moving speed of the precision translation stage 5 and the pulse repetition rate.

[0034] This invention uses a second CCD8 to observe the distribution of the emitted light field at the end face of fiber 6, determining the focusing position of a femtosecond pulse laser, thereby achieving real-time monitoring of fiber grating fabrication. Specifically, after the femtosecond pulse laser is focused onto fiber 6, it causes a change in the refractive index in certain areas of fiber 6. This change in refractive index within fiber 6 leads to a change in the emitted light field distribution at the fiber end face, and the focusing position of the femtosecond pulse laser is determined by this change in light field distribution. If the focusing position of the femtosecond pulse laser deviates from the center of the fiber core, it is adjusted in real time. In summary, this invention uses a second CCD8 to detect the light field distribution within fiber 6, monitoring the position of the femtosecond pulse laser focusing point relative to the fiber core in real time. It identifies any deviation in any direction and makes corresponding real-time adjustments, achieving fine-tuning and precise adjustment of the fiber position.

[0035] Example 2

[0036] Based on Example 1, to verify the core concept of this invention, numerical software was used to calculate the change in the optical field at the output end of fiber 6 when the refractive index changed at different locations in the fiber core. In the simulation, the diameter of the fiber core was 1000 nm, and the refractive index of the fiber core material was 1.45. The wavelength of the incident light was 633 nm, and the effective refractive index of the base film was 1.2959. A small cylinder was placed in the fiber core to simulate the region where the refractive index changed due to the femtosecond pulsed laser. The small cylinder was along the direction of the fiber core, with a length of 200 nm and a radius of 100 nm. After irradiation by the femtosecond pulsed laser, the refractive index became 2.0. Figure 2 As shown, different light field distributions appear at the exit end of the fiber core when the small cylinder is in different positions relative to the fiber core axis. Figure 2 (a) is the light field distribution at the output end when the axis of the small cylinder is aligned with the axis of the fiber core; Figure 2 (b) shows the light field distribution at the output end when the distance between the axis of the small cylinder and the axis of the fiber core is 100 nanometers. Figure 2 (c) shows the light field distribution at the output end when the distance between the axis of the small cylinder and the axis of the fiber core is 180 nanometers. Figure 2 It can be seen that when the region of refractive index change changes, the intensity and specific distribution of the light field change significantly. Therefore, it is possible to determine whether the region of refractive index change deviates from the central axis of the fiber core by observing the characteristics of the light field distribution.

[0037] Example 3

[0038] Based on Example 1, such as Figure 3As shown, the system also includes a coupler 11 and a spectrometer 12. The coupler 11 is a four-port fiber optic coupler, with its four ports connected to the fiber optic cable 6, the spectrometer 12, and the second CCD 8, respectively. The light source 7 is a broadband light source. The light source 7 emits broadband laser light, which enters the fiber optic cable 6 through the coupler 11. The grating region in the fiber optic cable 6 reflects the laser light, and the reflected laser light reaches the spectrometer 12 and the second CCD 8 via the coupler 11. The spectrometer 12 measures the reflection spectrum of the grating region and determines whether the focusing position of the femtosecond pulse laser has deviated relative to the fiber core by analyzing the width and intensity of the resonance peaks in the reflection spectrum. The second CCD 8 detects the light field distribution of the reflected light and determines the direction in which the focusing position of the femtosecond pulse laser deviates relative to the center of the fiber core, thereby allowing for adjustments in different directions.

[0039] Example 4

[0040] Based on Embodiments 1 and 3, the precision translation stage 5 is a three-dimensional precision translation stage. The precision translation stage 5 can not only move in the horizontal plane but also be adjusted in the vertical direction to facilitate the focusing of the femtosecond pulse laser onto the core position of the optical fiber 6. The direction of the optical fiber 6 is along the direction of translation of the precision translation stage 5. A fixing component is fixed to the precision translation stage 5, and the fixing component is adhered to or clamped onto the precision translation stage 5. The fixing component is made of metal to ensure secure fixation to the precision translation stage 5. A V-shaped groove is provided on the surface of the fixing component, and the optical fiber 6 is placed within the V-shaped groove, with the optical fiber 6 positioned along the direction of the V-shaped groove. Specifically, the optical fiber 6 is adhered to the V-shaped groove, which restricts the lateral movement of the optical fiber 6, facilitating stable fixation of the optical fiber 6 to the fixing component and its movement with the precision translation stage 5. At both ends of the V-shaped groove, the optical fiber 6 is more securely adhered to the fixing component to prevent traction from adjacent areas on the entire optical fiber 6. Furthermore, a reflective layer is provided on the side of the V-shaped groove. The reflective layer is made of aluminum or silver. The reflective layer partially reflects the femtosecond pulsed laser back into fiber 6, thereby creating a stronger optical field within fiber 6 and concentrating the area of ​​refractive index variation within fiber 6. This not only concentrates the area of ​​refractive index variation but also reduces damage to fixed components caused by the femtosecond pulsed laser, while also reducing the amount of pulsed laser light emitted by femtosecond laser 1 returning to the original optical path and eliminating stray light.

[0041] Example 5

[0042] Based on Examples 1-4, such as Figure 4 As shown, it also includes a waveplate 9 and a prism 10. The femtosecond pulsed laser passes sequentially through the waveplate 9 and the prism 10 before illuminating the dichroic mirror 2. The waveplate 9 can be a half-waveplate. The prism 10 is a polarizing beam splitter prism.

[0043] The probability of multiphoton absorption in fiber 6 is proportional to the higher-order terms of the photoelectric field; therefore, only beams with extremely high energy density can exhibit multiphoton absorption. However, the energy density cannot exceed a certain threshold. Under ultrafast laser irradiation, when the density of transformed free electrons exceeds the plasma density, the material begins to absorb a large amount of laser energy, leading to material damage. The higher the nonlinear absorption order of the material, the smaller the damage area. For example, at a laser pulse width of 200 fs, the damage threshold for fused silica is 2.2 J / cm². 2 The above analysis shows that controlling the power of the femtosecond pulsed laser is crucial when fabricating fiber gratings using the femtosecond pulsed laser direct writing method.

[0044] In this embodiment, the femtosecond pulsed laser, after passing through waveplate 9, forms polarized light. After passing through prism 10, the power of the transmitted light is adjustable, which facilitates the adjustment of the size of the region with refractive index change and the magnitude of the refractive index change within the fiber core.

[0045] Furthermore, prism 10 is a GranThompson prism. A GranThompson prism is made by grinding two right-angled triangular prisms (calcite or quartz) into edges with parallel optical axes, and then bonding them together with Canada balsam. An air gap exists between the two prisms. Light is incident perpendicularly from the end face; the o-ray is totally reflected at the adhesive surface, while the e-ray is transmitted. Because the light is incident perpendicularly to the end face, reflection is minimal, and transmission is strong. Moreover, rotating the prism does not cause lateral movement of the emitted image. This invention requires precise positioning of the femtosecond pulsed laser; the fact that the emitted image of the GranThompson prism does not move laterally when rotated is of great significance for improving the quality of the fabricated fiber grating.

[0046] In summary, this invention provides a system for fabricating fiber gratings using an ultrafast laser direct writing method with monitoring capabilities. By using a second CCD8 to detect the optical field distribution within the fiber, the position of the femtosecond pulse laser focusing point relative to the fiber core is realized, enabling fine adjustment of the fiber position. The position of the femtosecond pulse laser focusing point relative to the fiber core can be easily determined through the optical field distribution, allowing for precise adjustment and facilitating the fabrication of high-quality fiber gratings.

[0047] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the scope of protection of this application.

Claims

1. A system for fabricating fiber gratings using an ultrafast laser direct writing method with monitoring function, characterized in that, The system includes a femtosecond laser, a dichroic mirror, a first CCD, an objective lens, and a monitoring module. The monitoring module includes a precision translation stage, an optical fiber, a light source, and a second CCD. The femtosecond laser emits femtosecond pulsed laser light, which is reflected by the dichroic mirror and enters the objective lens. The objective lens converges the femtosecond pulsed laser light. The optical fiber is fixed on the precision translation stage, and the femtosecond pulsed laser light irradiates the core of the optical fiber. The light source emits laser light and couples it into the optical fiber. The second CCD is connected to the optical fiber and is used to detect the light field distribution in the optical fiber. The reflected light from the optical fiber passes through the objective lens and the dichroic mirror before entering the first CCD. The system also includes a waveplate and a prism. The femtosecond pulsed laser light passes through the waveplate and the prism in sequence before irradiating the dichroic mirror. The prism is a GranThompson prism, and the waveplate is a half-waveplate. A fixing component is fixed on the precision translation stage. The surface of the fixing component has a V-shaped groove. The optical fiber is adhered to the V-shaped groove. A reflective layer made of aluminum is provided on the side of the V-shaped groove.

2. The system for fabricating fiber Bragg gratings with monitoring function using ultrafast laser direct writing as described in claim 1, characterized in that: The second CCD is connected to the other end of the optical fiber.

3. The system for fabricating fiber Bragg gratings with monitoring function using ultrafast laser direct writing as described in claim 1, characterized in that: It also includes a coupler and a spectrometer. The coupler is a four-port fiber optic coupler, and the four ports of the coupler are respectively connected to the optical fiber, the spectrometer, and the second CCD.

4. The system for fabricating fiber Bragg gratings with monitoring function using ultrafast laser direct writing as described in claim 3, characterized in that: The light source is a broadband light source.

5. The system for fabricating fiber Bragg gratings with monitoring function using ultrafast laser direct writing as described in claim 1, characterized in that: The precision translation stage is a three-dimensional precision translation stage.

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