Preparation device and preparation method of chirped fiber grating and optical fiber filter

Abstract

The invention provides a chirp fiber grating preparation device, a preparation method and an optical fiber filter, a femtosecond laser direct writing technology is utilized to prepare a chirp localization grating of an asymmetric structure in a fiber core of a single-mode fiber, and therefore the ultra-wide-band optical fiber filter is achieved. On one hand, chirp is introduced on the basis of the localized grating, on the other hand, inscribing of the point-by-point chirp grating is controlled, preparation of the chirp fiber grating with the controllable initial period and chirp rate is achieved, and the filter with the working bandwidth capable of being flexibly controlled is obtained. The optical fiber filter provided by the invention has larger bandwidth and more flexible bandwidth regulation and control capability, can be prepared conveniently, and is suitable for various broadband filtering application scenes such as a super-continuous light source, a Raman spectrum system, optical coherence tomography and the like.

Description

Technical Field

[0001] This invention relates to the field of optical fiber communication technology, and in particular to a fabrication apparatus, fabrication method, and optical fiber filter for a chirped fiber grating. Background Technology

[0002] Traditional fiber Bragg gratings (FBGs) are limited by their periodic modulation range and reflection spectrum characteristics, resulting in bandwidths typically within a few nanometers. While chirping can extend the bandwidth to tens of nanometers, problems such as decreased reflectivity, enhanced sidelobes, and excessive dispersion arise under large bandwidth conditions. Long-period fiber gratings offer some bandwidth adjustment capabilities for transmission-type filtering, but their sensitivity to environmental parameters leads to insufficient stability in practical filtering. Although interferometric structures can extend bandwidth by designing multiple cavity lengths or multi-path interference, their complex structure makes stable integration difficult, and they are also sensitive to environmental disturbances. Therefore, current fiber optic filters cannot yet achieve a wider range of filtering capabilities.

[0003] Therefore, the existing technology needs further improvement. Summary of the Invention

[0004] In view of the shortcomings of the prior art, the purpose of the present invention is to provide a fabrication apparatus, fabrication method and fiber filter for chirped fiber gratings, so as to overcome the defects of insufficient filtering function of fiber filters in the prior art.

[0005] In a first aspect, this application provides an apparatus for fabricating a chirped fiber Bragg grating, comprising:

[0006] Femtosecond lasers are used to output femtosecond laser pulse signals;

[0007] A power attenuation module is disposed in the optical path of the femtosecond laser and is used to adjust the power attenuation of the femtosecond laser pulse signal to obtain an adjusted modulated optical signal.

[0008] A focusing objective lens is placed in the optical path of the modulated optical signal to focus the modulated optical signal and obtain focused light;

[0009] A single-mode optical fiber is disposed in the optical path of the focused light to receive the focused light, which enters the core of the single-mode optical fiber.

[0010] A control component is used to control the movement position of the single-mode fiber so that the single-mode fiber moves along a preset movement path, and to control the frequency of the femtosecond laser pulse signal emitted by the femtosecond laser so that the focused light photolithographically etches at least one grating period in the optical core of the single-mode fiber to obtain a chirped fiber grating.

[0011] Optionally, it also includes an imaging camera located above the single-mode fiber and an illumination source disposed below the single-mode fiber;

[0012] The illumination source and imaging camera are used for imaging the core of a single-mode optical fiber.

[0013] Optionally, a semi-reflective lens is also provided in the optical path of the modulated optical signal; the semi-reflective lens is disposed between the focusing objective and the imaging camera, and is used to reflect the received modulated optical signal to the focusing objective while also receiving the illumination beam transmitted from the core of the single-mode optical fiber and transmitting the illumination beam to the imaging camera.

[0014] Optionally, the control components include: a host computer, a controller, and a three-axis displacement platform;

[0015] The single-mode optical fiber is fixed on the triaxial displacement platform;

[0016] The host computer is used to determine the starting period and linear increment of the grating in the single-mode fiber according to the preset control parameters, generate the grating spacing value, generate a control signal based on the grating spacing value, and send the control signal to the controller.

[0017] The controller is used to control the movement of the three-axis displacement platform according to the received control signal, so as to drive the movement of the single-mode optical fiber, and to control the femtosecond laser to emit femtosecond laser pulse signals.

[0018] Optionally, it also includes an amplified spontaneous emission source and a spectrometer;

[0019] The beam emitted by the amplified spontaneous emission source is incident on the interior of the single-mode fiber, modulated by the grating inside the single-mode fiber, and outputs a wide spectrum to the spectrometer.

[0020] The spectrometer is used to receive the broadband signal and display the spectrum of the broadband signal.

[0021] Secondly, this application also provides a method for fabricating a chirped fiber grating, wherein the fabrication apparatus used for the chirped fiber grating includes:

[0022] Based on the preset grating parameters, the starting period and linear increment of the grating are determined to obtain the grating spacing value;

[0023] A control signal is generated based on the grating spacing value;

[0024] By using position synchronization triggering technology, the movement position of the single-mode fiber and the emission time of the femtosecond laser pulse are controlled according to the control signal, so as to realize the processing of at least one grating period in the core of the single-mode fiber.

[0025] Optionally, the step of determining the starting period and linear increment of the grating according to preset grating parameters to obtain the grating spacing value includes:

[0026] Based on the target filter parameters, determine the initial grating period corresponding to the threshold wavelength, and determine the linear increment of the grating period based on the target chirp rate;

[0027] The grating spacing value is generated based on the initial grating period and the linear increment.

[0028] Optionally, the steps of controlling the movement position of the single-mode fiber and the emission time of the femtosecond laser pulse according to the control signal using position synchronization triggering technology include:

[0029] The control component controls the single-mode fiber to move along the fiber axis at a preset distance, and when the single-mode fiber moves to the designated position, it controls the femtosecond laser to emit a pulse signal.

[0030] When the femtosecond laser receives the pulse signal, it emits a laser pulse; the laser pulse is focused into the core of the single-mode fiber to form a grating period.

[0031] Thirdly, this application also discloses an ultra-wideband fiber filter, comprising: a single-mode fiber and a chirped fiber grating disposed inside the single-mode fiber; the chirped fiber grating is fabricated using the same fabrication method as the chirped fiber grating.

[0032] Optionally, the chirped fiber grating of one cycle is asymmetric to the core of the single-mode fiber.

[0033] Beneficial effects:

[0034] This invention provides a fabrication apparatus, method, and fiber filter for chirped fiber gratings. By introducing chirp into a localized grating and controlling the writing of the point-by-point chirped grating, the fabrication of chirped fiber gratings with controllable initial period and chirp rate is achieved, resulting in a filter with flexibly controllable operating bandwidth. The fiber filter provided by this invention has a larger bandwidth, more flexible bandwidth control capability, and is easy to fabricate, making it suitable for various broadband filtering applications such as supercontinuum light sources, Raman spectroscopy systems, and optical coherence tomography. Attached Figure Description

[0035] Figure 1 This is a schematic diagram of the structure of the chirped fiber grating manufacturing apparatus provided by the present invention;

[0036] Figure 2 This is a flowchart of the manufacturing method of the chirped fiber grating provided by the present invention;

[0037] Figure 3 This is a flowchart illustrating an application embodiment of the chirped fiber grating manufacturing method provided by the present invention;

[0038] Figure 4 This is a schematic diagram of the fiber optic filter provided by the present invention;

[0039] Figure 5 This is a top-view microscopic image of the chirped localization grating provided by the present invention;

[0040] Figure 6 This is a microscopic image of the end face of the chirped localization grating provided by the present invention;

[0041] Figure 7 The transmission spectra of chirped localized gratings under different periods provided by this invention;

[0042] Figure 8 This invention provides the transmission spectra of chirped localized gratings of different lengths. Detailed Implementation

[0043] To make the objectives, technical solutions, and advantages of this invention clearer and more explicit, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

[0044] Fiber optic filters are passive devices that use optical fibers as a carrier to achieve selective transmission or reflection of specific wavelengths. They are widely used in fiber optic communication, fiber optic sensing, laser systems, spectral measurement, and nonlinear optics. Since the first proposal and application of fiber Bragg gratings (FBGs), fiber optic filters have gradually become an important alternative to traditional bulk optical filters. Compared with traditional free-space optics-based filters such as interferometric filters, prisms, and dispersive elements, fiber optic filters have significant advantages such as small size, low insertion loss, strong anti-interference capability, and high compatibility with fiber optic systems, making them particularly suitable for high-density integration and long-range distributed optical systems.

[0045] In recent years, with the rapid development of high-speed, high-capacity optical communication and precision optical measurement technologies, higher performance requirements have been placed on filters, including wider filtering bandwidth, steeper edges, higher suppression ratios, lower losses, and better environmental stability. Against this backdrop, various novel fiber optic filter technologies have emerged, such as chirped fiber Bragg gratings (FBGs), tilted fiber gratings (FBGs), long-period fiber gratings (LPFGs), interferometric fiber filters, Sagnac structures, fiber Mach-Zehnder interferometers (MZIs), and fiber Fabry-Perot cavities (FPIs). These filter structures, each with its own characteristics, have played important roles in various application scenarios.

[0046] Existing fiber optic filters still face several challenges in achieving ultra-wideband filtering. Traditional fiber Bragg gratings (FBGs) are limited by the modulation range of the grating period and the characteristics of the reflection spectrum, typically limiting their bandwidth to within a few nanometers. While chirping can extend the bandwidth to tens of nanometers, problems such as decreased reflectivity, enhanced sidelobes, and excessive dispersion easily arise under large bandwidth conditions. Long-period fiber gratings (LPFGs) offer some bandwidth adjustment capability for transmission-type filtering, but their sensitivity to environmental parameters leads to insufficient stability in practical filtering. Interferometric structures, although capable of bandwidth extension through the design of multiple cavity lengths or multi-path interference, are structurally complex, difficult to integrate stably, and similarly sensitive to environmental disturbances.

[0047] A crucial current research focus is on achieving filtering capabilities across a range of hundreds of nanometers or even wider, while maintaining the excellent integration, stability, and low-loss characteristics of fiber optic filters—that is, realizing ultra-wideband fiber optic filters. These filters have potential applications in spectral compression lasers, spectral selective communication systems, broadband noise filtering, supercontinuum processing, and multichannel optical sensing. Particularly in femtosecond laser systems and Raman spectroscopy systems, the ability to selectively process energy or information across a wide wavelength range is becoming a key performance bottleneck.

[0048] The existing technologies for implementing ultra-wideband fiber optic filters mainly fall into the following categories:

[0049] (1) Based on the large-chirped fiber Bragg grating structure, a broadband reflection spectrum is achieved through nonlinear modulation, but its bandwidth is usually only tens of nanometers, and there are problems such as many side lobes and uneven spectral lines.

[0050] (2) Based on composite grating structures (such as multi-segment splicing or multi-layer stacking), although the bandwidth can be expanded, the process is complex, the cost is high, and it will lead to excessive insertion loss.

[0051] (3) Wide-spectrum transmission filtering is achieved based on tilted long-period fiber gratings (LPFG), chirped long-period fiber gratings (LPFG) or high-order mode coupling structures, but its stability is insufficient and its compatibility is poor.

[0052] (4) Broadband filtering is carried out using microstructured optical fibers or non-standard refractive index profile design. Although theoretically the bandwidth can be adjusted in a wide range, the manufacturing cost is high and the practical application is limited.

[0053] To overcome the problems of insufficient bandwidth, high insertion loss, complex structure, and poor stability in existing ultra-wideband fiber filters, this application provides a fabrication apparatus, fabrication method, and fiber filter for a chirped fiber grating. This application utilizes femtosecond laser direct writing technology to fabricate an asymmetric localized grating in the core of a single-mode fiber, and then linearly increases its period and introduces spectral chirp. This allows light with a bandwidth of hundreds of nanometers in the core to be extracted into the cladding and dissipated, thus realizing an ultra-wideband, low-loss, simple, and stable fiber filter, providing a new fabrication scheme for ultra-wideband optical filtering.

[0054] The following description, in conjunction with the accompanying drawings, provides a more detailed account of the fabrication apparatus, fabrication method, and fiber optic filter for a chirped fiber grating provided in this application.

[0055] In a first aspect, this application provides an apparatus for fabricating a chirped fiber Bragg grating, such as... Figure 1 As shown, it includes:

[0056] Femtosecond laser 1 is used to output femtosecond laser pulse signals. The femtosecond laser used in this embodiment is a laser device capable of generating femtosecond-level ultrashort pulses. It has high precision and high stability, and therefore can provide photolithography light for the fabrication of chirped fiber gratings processed inside single-mode fibers.

[0057] Power attenuation module 2, disposed in the optical path of the femtosecond laser, is used to adjust the power attenuation of the femtosecond laser pulse signal to obtain an adjusted modulated optical signal. The power attenuation module is an electronic component used to adjust signal power. It can reduce the signal power to the required level by introducing a predetermined attenuation, thereby adjusting the intensity of the optical signal received inside the single-mode fiber. Excessive laser pulse signal power can cause equipment damage or an excessively large fused zone in the fiber, making it impossible to photolithographically create the required chirped fiber grating.

[0058] A focusing objective 5 is positioned in the optical path of the modulated optical signal to focus the modulated optical signal, obtaining focused light. The focusing objective receives the adjustment optical signal and focuses the modulated optical signal onto the single-mode fiber. In this embodiment, a high numerical aperture focusing objective is used to ensure that the single-mode fiber receives a laser signal of sufficient power.

[0059] A single-mode fiber 6 is disposed in the optical path of the focused light and is used to receive the focused light, which enters the core of the single-mode fiber.

[0060] In this embodiment, a single-mode fiber is used as the substrate, and a chirped fiber grating is fabricated within the substrate. In one implementation, the fiber parameters corresponding to the single-mode fiber are a cladding diameter of 125 μm and a core diameter of 9 μm.

[0061] A control component is used to control the movement position of the single-mode fiber so that the single-mode fiber moves along a preset movement path, and to control the frequency of the femtosecond laser pulse signal emitted by the femtosecond laser so that the focused light photolithographically etches at least one grating period in the core of the single-mode fiber to obtain a chirped fiber grating.

[0062] In this embodiment, the control component is used to control the movement of the single-mode fiber and to control the laser to emit pulse signals. Therefore, this control component needs to output two different sets of control signals to achieve these two aspects of control. Figure 1 As shown, the control component includes a host computer 12, a controller 11, and a three-axis displacement platform 8. A single-mode optical fiber 6 is fixed on the three-axis displacement platform 8. The host computer 12 is used to determine the initial period and linear increment of the grating in the single-mode optical fiber according to preset control parameters, generate a grating spacing value, generate a control signal based on the grating spacing value, and send the control signal to the controller 11. The controller 11 is used to control the movement of the three-axis displacement platform 8 according to the received control signal, thereby moving the single-mode optical fiber, and to control the femtosecond laser 1 to emit femtosecond laser pulse signals.

[0063] In detail, the three-axis displacement platform used in this embodiment is a high-precision air-floating three-axis displacement platform. A high-precision air-floating three-axis displacement platform is a precision motion device based on gas hydrostatic bearing technology. It uses high-pressure gas to form a micron-level gas film, suspending the moving parts above the guide rail or base, completely eliminating solid friction. It possesses core advantages such as frictionless operation, zero wear, high precision, high speed, and high stability, and is therefore widely used in high-end fields such as semiconductor manufacturing, optical engineering, precision testing, and aerospace.

[0064] A single-mode fiber is fixed on a high-precision air-floating triaxial displacement platform, allowing the fiber to move in nanometer-scale three dimensions along with the platform. Since the position of the focused light on the single-mode fiber remains constant, the movement of the fiber allows it to be positioned at different points, thereby enabling the photolithography of multiple modulation structures within the fiber. These modulation structures form an asymmetric grating structure relative to the fiber core.

[0065] Furthermore, in order to have a clearer understanding of the photolithographically formed grating structure, combined with Figure 1As shown, the fabrication apparatus also includes an imaging camera 3 located above the single-mode fiber 6 and an illumination source 7 positioned below the single-mode fiber 6. The illumination source 7 and the imaging camera 3 are used for imaging the core of the single-mode fiber.

[0066] Furthermore, such as Figure 1 As shown, a semi-reflective lens 4 is also provided in the optical path of the modulated optical signal; the semi-reflective lens 4 is disposed between the focusing objective lens 5 and the imaging camera 3, and is used to reflect the received modulated optical signal to the focusing objective lens 5, and at the same time, to receive the illumination beam transmitted from the core of the single-mode optical fiber 6, and transmit the illumination beam to the imaging camera 3.

[0067] In detail, the fabrication apparatus provided in this embodiment has two light paths with different transmission paths: one is the femtosecond laser emitted by a femtosecond laser, and the other is the imaging light emitted by an illumination source. The femtosecond laser emitted by the femtosecond laser is reflected by a semi-reflective mirror and transmitted to a focusing objective lens. After being focused by the focusing objective lens, it is transmitted into the core of a single-mode fiber. The imaging light emitted by the illumination source is incident on the core of the single-mode fiber, transmitted through the core to the focusing objective lens, then transmitted from the focusing objective lens to the semi-reflective mirror, and finally transmitted into the imaging camera. These two light paths have different wavelengths, so the semi-reflective mirror can reflect the femtosecond laser on one hand and transmit the illumination light on the other, thus achieving a semi-reflective and semi-transparent function.

[0068] Furthermore, combined Figure 1 As shown, the preparation apparatus further includes an amplified spontaneous emission light source 9 and a spectrometer 10; the light beam emitted by the amplified spontaneous emission light source 9 is incident on the interior of the single-mode fiber 6, and after being modulated by the grating inside the single-mode fiber 6, a broadband spectrum is output to the spectrometer 10; the spectrometer 10 is used to receive the broadband signal and display the spectrum of the broadband signal.

[0069] Amplified Spontaneous Emission (ASE) is a broadband light source based on rare-earth-doped fiber (such as erbium-doped fiber) and a high-performance pump laser. After the interior of a single-mode fiber is illuminated by the ASE light source, the light is input into a spectrometer. The spectrometer displays the spectral information of the received light signal and can detect the spectral information of the chirped fiber grating inside the single-mode fiber.

[0070] The following is combined with Figure 1 The preparation apparatus of this embodiment will be described in further detail.

[0071] A femtosecond laser 1 emits a femtosecond laser beam, which passes through a power attenuation module 2 and a semi-reflective lens 4 before entering a focusing objective lens 5. This beam is then focused into the core of a single-mode fiber 6. An illumination source 7 and an imaging camera 3 image the interior of the fiber core to observe the fabrication process. Amplified light emitted from a spontaneous emission source 9 is input into the single-mode fiber 6 and ultimately into a spectrometer 10, allowing for the detection of the spectral characteristics of the fabricated grating. The single-mode fiber 6 is fixed to a triaxial displacement platform 8, enabling nanoscale three-dimensional movement along with the platform. The movement of the triaxial displacement platform 8 is controlled by a controller 11, which, along with the femtosecond laser 1, is controlled by a host computer 12. Therefore, programming in the host computer 12 allows for the simultaneous control of the movement of the triaxial displacement platform 8 and the emission of light from the femtosecond laser 1, thereby achieving the fabrication of a chirped localized grating.

[0072] Furthermore, when the focused beam enters the interior of the single-mode fiber, the focused region of the beam forms an approximately elliptical modulation structure. Each time the single-mode fiber moves to a different position, the focused beam forms a modulation structure within it. Therefore, after the single-mode fiber has moved a series of positions, a series of modulation structures are formed within it, which together form the chirped fiber grating to be fabricated in this embodiment. Since this modulation structure can excite a large number of cladding modes in the single-mode fiber, light meeting the coupling conditions in the fiber core can be introduced into the cladding and dissipated. Based on this, by introducing chirp, the spectral bandwidth of the excited cladding modes can be broadened, thus broadening and overlapping the loss peaks they form, ultimately forming a smooth loss envelope with a bandwidth of several hundred nanometers, which can be used for filtering.

[0073] Since the chirp introduced in this application is achieved through Position Synchronization Trigger (PSO) functionality, the laser can be programmed to emit laser pulses when the three-axis stage moves to each specific position, thus enabling high-precision chirped grating writing. Furthermore, the fabrication process for the grating structure in this application is simple and highly efficient. For example, the typical length of this grating device is 10 mm, and fabrication takes only 100 seconds at a processing speed of 0.1 mm / s. Therefore, this grating device has significant advantages in terms of structural complexity, fabrication efficiency, and cost.

[0074] Secondly, this application also provides a method for fabricating a chirped fiber grating, such as... Figure 2 As shown, the fabrication apparatus for the chirped fiber grating includes:

[0075] Step S1: Determine the starting period and linear increment of the grating according to the preset grating parameters to obtain the grating spacing value.

[0076] To photolithographically print a chirped fiber grating of a specified size, it is first necessary to determine the starting period and linear increment of the grating based on the required grating parameters, so as to determine the grating spacing value required during photolithography.

[0077] Furthermore, this step includes: determining the initial grating period corresponding to the threshold wavelength based on the target filter parameters, and determining the linear increment of the grating period based on the target chirp rate; generating the grating spacing value based on the initial grating period and the linear increment.

[0078] The target filter parameters include the required filtering bandwidth. Based on the filtering bandwidth, the threshold wavelength of the target filter can be determined. The initial period of the grating is calculated using the determined threshold wavelength and the core refractive index of the single-mode fiber. Then, the linear increment of the grating period is calculated based on the target chirp rate and the total length of the grating. Once the linear increment of the target chirp rate and the grating period is calculated, the grating spacing value can be obtained.

[0079] Step S2: Generate a control signal based on the grating spacing value.

[0080] Based on the calculated grating spacing value, the host computer generates control signals to control the position movement of the single-mode fiber and the emission of laser pulses by the femtosecond laser.

[0081] In practice, the grating interval values ​​are saved as an array and imported into the controller, serving as the distance for the three-dimensional displacement platform to execute the next set of movement commands.

[0082] Step S3: Using position synchronization triggering technology, the movement position of the single-mode fiber and the emission time of the femtosecond laser pulse are controlled according to the control signal, so as to realize the processing of at least one grating period in the core of the single-mode fiber.

[0083] This step utilizes position synchronization triggering technology to achieve synchronous control of the three-axis displacement platform and the laser, specifically including:

[0084] The control component controls the single-mode fiber to move along the fiber axis at a preset distance, and when the single-mode fiber moves to the designated position, it controls the femtosecond laser to emit a pulse signal; when the femtosecond laser receives the pulse signal, it emits a laser pulse; the laser pulse is focused into the core of the single-mode fiber to form a grating period.

[0085] In detail, upon receiving a movement command, the controller controls the three-axis displacement platform to move along the X-axis (fiber axis) at a preset distance. At each position, the controller controls the displacement stage to move along the X-axis (fiber axis) at a preset distance and sends a pulse signal to the femtosecond laser. When the femtosecond laser receives a pulse signal from the controller, it emits a laser pulse, which is focused into the fiber core to form a grating period. Therefore, through the above process, a complete single chirped localized grating is finally fabricated.

[0086] like Figure 3 The diagram shows the steps of the preparation method provided in this embodiment in a specific application, which include the following:

[0087] Step H1: Based on the required filter parameters, design the initial grating period corresponding to the threshold wavelength, and determine the linear increment of the grating period based on the required chirp rate. Input these two quantities into the control software, and a series of linearly increasing values ​​will be generated.

[0088] In step H2, the control software saves these generated values ​​as an array and imports them into the controller, which will serve as the distance for the displacement platform to execute the next set of movement commands.

[0089] Step H3: In the control software, click to execute the program. The controller controls the displacement stage to move along the x-axis (fiber axis) at a preset distance, and when it moves to each position, it sends a pulse signal to the femtosecond laser.

[0090] In step H4, when the femtosecond laser receives a pulse signal from the controller, it emits a laser pulse, which is focused into the fiber core to form a grating period, and finally a chirped localized grating is prepared.

[0091] The fabrication method provided in this embodiment enables nanoscale three-dimensional movement of the triaxial displacement platform, allowing the laser to precisely emit a laser pulse at each specific position of the displacement stage. This achieves the fabrication of a chirped localized grating with controllable initial period and chirp rate, thus allowing for flexible control of the filter's operating bandwidth. Furthermore, this fabrication method is simple, highly efficient, and cost-effective, making it highly practical.

[0092] Thirdly, this application also discloses an ultra-wideband fiber optic filter, such as... Figure 4 As shown, it includes: a single-mode optical fiber and a chirped fiber grating disposed inside the single-mode optical fiber; the chirped fiber grating is fabricated using the same method as described above. Specifically, in conjunction with... Figure 4A cross-sectional view of the target region 101 inside the single-mode fiber shown illustrates the location of the chirped fiber grating. This target region contains a one-period chirped fiber grating 1013, which is asymmetric to the core 1012 of the single-mode fiber.

[0093] The present invention provides an ultra-wideband fiber optic filter that can achieve a maximum filtering bandwidth of over 600nm, and the bandwidth is flexibly adjustable. Figure 5 The filter is shown in a top view under a microscope. A femtosecond laser is incident along the plane of the view and inscribes a chirped localized grating structure in the middle position off the fiber core. The grating is a first-order grating with a typical period size of about 500 nm. Figure 6 The microstructure of the end face of the grating is shown. The magnified image shows that the modulation structure of the grating is elliptical and located in the upper part of the fiber core. Figure 7 This demonstrates the filter's flexible bandwidth control capability. By setting the initial grating period to 0.414µm, 0.483µm, 0.552µm, and 0.621µm, the Bragg wavelengths are approximately 1200nm, 1400nm, 1600nm, and 1800nm, respectively. Therefore, within the 1100nm-1700nm spectral range (covering the main operating band of single-mode fiber), the filter bandwidths are 100nm, 300nm, 500nm, and 600nm, respectively. Thus, the filter bandwidth can be flexibly controlled by adjusting the Bragg wavelength. Furthermore, its typical insertion loss is <1dB, and the average filtering depth is >10dB (90%). Figure 8 The transmission spectra of the filter at different lengths are shown. It can be seen that as the length increases from 5 mm to 10 mm, 20 mm, and 30 mm, the average filtering depth of the filter increases from 5 dB to 10 dB, 20 dB, and a maximum of 40 dB (99.99%), demonstrating that the filter possesses not only an ultra-large filtering bandwidth but also ultra-high filtering efficiency. At a typical fabrication speed of 0.1 mm / s, fabricating a 30 mm filter sample only requires 300 s. Experimental results verify that the ultra-wideband filter designed in this invention simultaneously possesses the characteristics of low insertion loss, large bandwidth, adjustable bandwidth, high filtering efficiency, simple fabrication process, and high fabrication efficiency.

[0094] This invention provides a fabrication apparatus, method, and fiber filter for chirped fiber gratings. The fabrication apparatus utilizes femtosecond laser direct writing technology to fabricate asymmetric chirped localized gratings in single-mode fiber. Compared to other traditional structures, this novel ultra-wideband fiber filter offers greater bandwidth, lower insertion loss, more flexible bandwidth control, and a simpler fabrication process. Furthermore, thanks to the characteristics of fiber Bragg gratings, it also possesses advantages such as high stability and compatibility with general fiber optic systems, making it suitable for various broadband filtering applications, including supercontinuum light sources, Raman spectroscopy systems, and optical coherence tomography.

[0095] Other embodiments of the invention will readily occur to those skilled in the art upon consideration of the specification and practice of the invention disclosed herein. This invention is intended to cover any variations, uses, or adaptations of the invention that follow the general principles of the invention and include common knowledge or customary techniques in the art not disclosed herein.

[0096] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of this application. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0097] It is understood that the above embodiments are exemplary and should not be construed as limiting the present application. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present application.

Claims

1. An apparatus for fabricating a chirped fiber grating, characterized in that, include: Femtosecond lasers are used to output femtosecond laser pulse signals; A power attenuation module is disposed in the optical path of the femtosecond laser and is used to adjust the power attenuation of the femtosecond laser pulse signal to obtain an adjusted modulated optical signal. A focusing objective lens is placed in the optical path of the modulated optical signal to focus the modulated optical signal and obtain focused light; A single-mode optical fiber is disposed in the optical path of the focused light to receive the focused light, which enters the core of the single-mode optical fiber. A control component is used to control the movement position of the single-mode fiber so that the single-mode fiber moves along a preset movement path, and to control the frequency of the femtosecond laser pulse signal emitted by the femtosecond laser so that the focused light photolithographically etches at least one grating period in the core of the single-mode fiber to obtain a chirped fiber grating, wherein the grating period is asymmetrically set relative to the core of the single-mode fiber.

2. The fabrication apparatus for chirped fiber gratings according to claim 1, characterized in that, It also includes an imaging camera located above the single-mode fiber and an illumination source disposed below the single-mode fiber; The illumination source and imaging camera are used for imaging the core of a single-mode optical fiber.

3. The fabrication apparatus for chirped fiber gratings according to claim 2, characterized in that, A semi-reflective lens is also provided in the optical path of the modulated optical signal. The semi-reflective mirror is disposed between the focusing objective and the imaging camera. It is used to reflect the received modulated light signal to the focusing objective and to receive the illumination beam transmitted from the core of the single-mode fiber and transmit the illumination beam to the imaging camera.

4. The fabrication apparatus for chirped fiber gratings according to claim 1, characterized in that, The control components include: a host computer, a controller, and a three-axis displacement platform; The single-mode optical fiber is fixed on the triaxial displacement platform; The host computer is used to determine the starting period and linear increment of the grating in the single-mode fiber according to the preset control parameters, generate the grating spacing value, generate a control signal based on the grating spacing value, and send the control signal to the controller. The controller is used to control the movement of the three-axis displacement platform according to the received control signal, so as to drive the movement of the single-mode optical fiber, and to control the femtosecond laser to emit femtosecond laser pulse signals.

5. The fabrication apparatus for chirped fiber gratings according to claim 1, characterized in that, It also includes amplified spontaneous emission sources and spectrometers; The beam emitted by the amplified spontaneous emission source is incident on the interior of the single-mode fiber, modulated by the grating inside the single-mode fiber, and outputs a broadband signal to the spectrometer. The spectrometer is used to receive the broadband signal and display the spectrum of the broadband signal.

6. A method for fabricating a chirped fiber grating, characterized in that, An apparatus for fabricating a chirped fiber grating as described in any one of claims 1-5, comprising: Based on the preset grating parameters, the starting period and linear increment of the grating are determined to obtain the grating spacing value; A control signal is generated based on the grating spacing value; By using position synchronization triggering technology, the movement position of the single-mode fiber and the emission time of the femtosecond laser pulse are controlled according to the control signal, so as to realize the processing of at least one grating period in the core of the single-mode fiber.

7. The method for fabricating a chirped fiber grating according to claim 6, characterized in that, The step of determining the starting period and linear increment of the grating according to preset grating parameters to obtain the grating spacing value includes: Based on the target filter parameters, determine the initial grating period corresponding to the threshold wavelength, and determine the linear increment of the grating period based on the target chirp rate; The grating spacing value is generated based on the initial grating period and the linear increment.

8. The method for fabricating a chirped fiber grating according to claim 6, characterized in that, The steps of using position synchronization triggering technology to control the movement position of the single-mode optical fiber and the emission time of the femtosecond laser pulse according to the control signal include: The control component controls the single-mode fiber to move along the fiber axis at a preset distance, and when the single-mode fiber moves to the designated position, it controls the femtosecond laser to emit a pulse signal. When the femtosecond laser receives the pulse signal, it emits a laser pulse; the laser pulse is focused into the core of the single-mode fiber to form a grating period.

9. An ultra-wideband fiber optic filter, characterized in that, include: A single-mode optical fiber and a chirped fiber grating disposed inside the single-mode optical fiber; the chirped fiber grating is fabricated using the method for fabricating a chirped fiber grating as described in any one of claims 6-8.

10. The ultra-wideband fiber optic filter according to claim 9, characterized in that, At least one period of the chirped fiber grating is asymmetric to the core of the single-mode fiber.

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