Method for manufacturing an optical component using laser radiation

Abstract

Method for manufacturing an optical component (1) using laser radiation, comprising the following process steps: - Generation of a structure in the material of the component (1) which gives the component (1) an optical functionality, wherein the optical functionality of the component (1) is that of an optical grating, in particular a fiber Bragg grating, an aperiodic fiber Bragg grating, a long-period grating or a bulk Bragg grating, - Determination of deviations of the generated structure in the material of the component (1) from specified target parameters, wherein the specified target parameters determine a central operating wavelength and / or a dispersion of the component (1), - Modification of the refractive index in the material of the component (1) by means of laser beams in a pre- and / or post-processing step, i.e. before or after the creation of the structure, in order to correct the deviations from the specified target parameters, characterized in that in the pre- and / or post-processing step a beam shaping of the laser radiation directed on the component (1) takes place in order to generate a spatially variable modification of the refractive index in the material of the component (1), wherein the beam shaping is carried out by means of an adaptive optics (4) which dynamically controls the wavefront and intensity profile of the laser radiation in order to flexibly address local deviations in the material of the component (1) individually.

Description

[0001] The invention relates to a method for manufacturing an optical component using laser radiation.

[0002] Various approaches exist for manufacturing optical components and equipping them with specific functionalities. The use of short or ultrashort laser pulses (pulse duration in the femtosecond to picosecond range) for modifying transparent, partially transparent, or absorptive materials, either within the bulk or on the surface, has proven particularly effective as a key tool. The high power of the laser pulses heats the component material locally, either up to or below the threshold at which a single laser pulse generates a plasma within the material. The result is a structure created at the focus of the laser radiation, representing a corresponding localized modification of the refractive index within the component material. This structure forms the basis for the desired function, for example, as an optical grating.

[0003] Regardless of the chosen method for generating the refractive index modifications that give the component its functionality, deviations from the target parameters can occur. These target parameters determine the optical function of the generated structure; for example, for a Bragg grating, they determine the dispersion and the central operating wavelength, i.e., the wavelength of maximum reflection (or minimum transmission). Possible reasons for deviations include material inhomogeneities or, in the case of optical waveguides (e.g., optical fibers), material variations between different waveguides (in multicore fibers or waveguide systems) or along the individual waveguide. The very process of generating the structure that determines the optical functionality can also lead to deviations from the specified target parameters (e.g., due to heat input and the resulting material stresses).Such deviations can hardly be compensated for or corrected in the production of optical components using state-of-the-art technology.

[0004] Several approaches to addressing these problems are known from the prior art. For example, in the manufacturing process of waveguides or waveguide systems, such as multicore fibers, considerable effort is invested to produce waveguides that are as uniform as possible with regard to both symmetry and material properties. For waveguide systems with only one waveguide, material deviations can be corrected within certain limits both thermally and by applying mechanical force. This is hardly feasible when the waveguide system has more than one waveguide, since all waveguides are affected in a similar way.

[0005] Methods are known from US 2003 / 0 186 142 A1, US 2003 / 0 161 579 A1, DE 10 2015 009 610 A1 WO 2016 / 123 719 A1 which use femtosecond laser pulses or UV laser radiation to create and / or adjust optical structures, e.g. fiber Bragg gratings, in optical components such as optical fibers.

[0006] In US 2003 / 0 186 142 A1, a closed feedback mechanism with iterative correction steps is used to make a precise adjustment of, for example, grid parameters, so that deviations in the generated structure can be corrected.

[0007] US 2003 / 0161579A1 describes the optimization of a filter response of an arrayed waveguide grating (AWG). The refractive properties of the waveguide are adjusted by the targeted application of UV laser energy to correct the optical path length.

[0008] WO 2016 / 123 719 A1 describes a method for adapting waveguides. This involves using external laser radiation – e.g., from a femtosecond laser – to generate a permanent, spatially controlled change in the refractive index in or near an existing waveguide, in order to realize, for example, multimode interference (MMI) devices with a matched number and distribution of modes. DE 10 2015 009 610 A1 describes the use of a femtosecond laser in combination with OCT-based and confocal detection systems to modify an intraocular lens by selectively writing or erasing refractive index patterns.

[0009] US Patent 2004 / 0252364A1 discloses methods for improving the optical properties of Bragg gratings, in particular by correcting defects in the refractive index profile. Two approaches are described: a post-correction, in which defects in the grating—such as period errors or apodization errors—are compensated for after its fabrication by a spatially variable, photoinduced change in the mean refractive index; and a pre-correction, in which systematic errors are identified using test gratings and the fabrication parameters for subsequent gratings are adjusted. The defects are detected by spectroscopic measurements (e.g., reflection, transmission, or group transit-time spectra) and analyzed using reconstruction algorithms (e.g., layer peeling).The correction is achieved by superimposing a UV-induced, location-dependent refractive index profile, which optimizes the optical properties of the grating.

[0010] From the prior art (A. Halstuch, A. Ishaaya, “Fine-tuning the fiber Bragg grating wavelength by femtosecond photo-treatment,” in: Advanced Photonics 2018, OSA Technical Digest paper Jtu2A.11; T. Goebel et al., “Realization of aperiodic fiber Bragg gratings with ultrashort laser pulses and the line-by-line technique,” ​​Opt. Lett., Vol. 43, No. 15, pp. 3794–3797, August 1, 2018), systems suitable for producing various types of fiber Bragg gratings are known. Using ultrashort laser pulses, the corresponding grating structures are generated in a suitable optical component, such as an optical fiber, and corrected according to requirements in a pre- / post-processing step.

[0011] WO 2019 / 030 521 A1 concerns a method for the laser modification of optical fibers, in particular for the fabrication of fiber Bragg gratings (FBGs) using adaptive optics to correct aberrations. By employing adaptive optics, the laser wavefront is adjusted to compensate for aberrations caused by the cylindrical geometry of the fiber (e.g., astigmatic or spherical aberration, coma). This enables precise focusing of the laser within the fiber core.

[0012] WO 2018 / 002061A1 describes a method and system for the optical functionalization of semiconductor materials by laser-induced, spatially selective modification of the refractive index at depth within the material. Using an infrared laser with pulse durations between 1 ps and 100 ns, multiphoton absorption is triggered in the transparent spectral range of the semiconductor to selectively generate local changes in the refractive index. A measurement system (e.g., phase microscope or dark-field microscope) records the actual change in refractive index at each point of a predefined pattern, while a control unit regulates the number of laser pulses per point to achieve a gradual, controlled modification of the refractive index. Adaptive optics are used to correct aberrations (e.g., astigmatic or spherical aberration) that arise from focusing the laser into the semiconductor material.

[0013] Against this background, the object of the invention is to provide a method improved compared to the prior art, which enables the correction of deviations in the optical functionality of the component from predetermined target parameters.

[0014] The invention solves this problem by means of a method according to claim 1.

[0015] According to the invention, pre- and post-processing is carried out to reduce undesirable deviations and to achieve the desired target parameters as precisely as possible. The invention is suitable for the production of components with various functionalities, with periodic or aperiodic structures, mostly in transparent components, such as optical fibers.

[0016] According to the invention, deviations from target parameters in an already structured component are first determined, for which microscopy methods such as phase-contrast or nonlinear microscopy (SHG, THG) are suitable. Spatially resolved Raman spectroscopy can also be used to determine material deviations from the desired structure. In particular, spectroscopy can be used to determine deviations in the spectral properties. If an interferometer is additionally used, the dispersive function can also be measured. Based on this, refractive index modifications can then be introduced in the post-processing step according to the invention in order to correct the detected deviations from the target parameters in a targeted and precise manner.

[0017] For modifying the component material in the pre- or post-processing step, pulsed laser radiation is advantageously used, with a pulse duration of 10 fs to 10 ps and a central wavelength in the range of 150 nm to 10 µm. A short-pulse laser (or ultrashort-pulse laser) of a known and commercially available type, such as a titanium-sapphire laser or a mode-locked fiber laser, serves as the source for generating such laser radiation. In the latter, a rare-earth-ion-doped optical fiber is used as the laser medium, which is optically pumped by a laser diode. To achieve the required power, the generated laser radiation is advantageously amplified by one or more optical amplifiers, also of a known and commercially available type.

[0018] According to the invention, in the pre- and / or post-processing step, the laser radiation directed at the component is shaped to selectively generate a spatially variable modification of the refractive index in the component's material. This beam shaping is achieved using adaptive optics. As a result of the inventive method, the spatially variable modification of the refractive index is advantageously superimposed on the structure in the component's material that determines its optical functionality, so that the finished component meets the target specifications with high precision. According to the invention, adaptive optics are suitable for deflecting and focusing the laser radiation. The adaptive optics are used to modify the intensity profile across the laser beam's cross-section and thus shape the beam.

[0019] A necessary change in the direction of the laser beam can be achieved using deflection and focusing optics, which are conveniently controlled by a computer during the pre- and / or post-processing steps. In the simplest case, a combination of deflection mirrors and focusing optics (e.g., in the form of an adjustable arrangement of spherical or cylindrical lenses, or even freeform optics and / or curved mirrors) is used. Alternative implementations are possible, for example, based on diffractive optics. This allows for targeted local as well as large-area pre- and / or post-processing, e.g., by guiding (scanning) the laser beam used for refractive index modification across the component. By combining static optical components with adjustable optical components for pre- and / or post-processing, flexible local and large-area modification can be achieved.

[0020] According to the invention, beam shaping is achieved using adaptive optics. Adaptive optical elements are known in the prior art, for example, in the form of mechanically deformable or adjustable mirrors or lenses. The adaptive optical element enables static or dynamic control of the beam shape. For the purposes of the invention, an adaptive optical element is any element that enables adjustable control of the wavefront and intensity profile of the laser radiation. This allows for precise control of the intensity and wavefront profile within the material of the component. Any dynamically adjustable reflective or transmissive element known in the prior art that modifies the beam shape is suitable as an adaptive optical element.The adaptive optics used according to the invention make it possible to selectively influence the resulting modification, since, for example, unwanted local material deviations in the material can be addressed individually and flexibly by using permanent or dynamically adaptive mirrors.

[0021] Advantageously, in the pre- and / or post-processing step for generating the spatially variable modification, the pulse energy, the repetition rate, and / or the number of laser pulses applied to the component material per unit volume or per unit area can be varied. For this purpose, the laser (or an associated pulse picker or attenuator) can be easily controlled accordingly using the control computer employed for the pre- and / or post-processing of the component.

[0022] In a further preferred embodiment of the method according to the invention, the component is clamped in a holder during the modification of the refractive index, and / or an immersion fluid is used to couple the laser radiation into the component material. The holder allows any potential surface curvature or distortion of the component (e.g., curvature of the fiber surface) to be overcome. An immersion fluid improves the coupling of the laser radiation into the component material.

[0023] The method according to the invention is advantageously suited for the production of optical components such as optical waveguides or optical waveguide systems, in particular single- or multi-core optical fibers (with or without coating). In the case of fibers with a coating (e.g., made of polymer material), the laser radiation used to modify the refractive index during post-processing can also be coupled axially into the fiber.

[0024] According to the invention, the optical functionality of the component is that of an optical grating, in particular a fiber Bragg grating, an aperiodic fiber Bragg grating, a long-period grating, or a bulk Bragg grating. The target parameter to be set according to the invention is a central operating wavelength and / or a dispersion of the component.

[0025] Exemplary embodiments of the invention are explained in more detail below with reference to the figures. The figures show: Fig. 1 Schematic illustration of the refractive index modification according to the invention: a) uniform modification, b) linearly increasing modification of the refractive index, c) variable modification; Fig. 2 Schematic representation of an optical arrangement used for the method according to the invention.

[0026] The diagrams of Fig. Figure 1 shows different refractive index profiles n(x) along the longitudinal axis x of an optical waveguide. The solid curve represents the refractive index profile n(x) initially created as a structure within the material of component 1 to impart its optical functionality; in this case, a periodic structure (Bragg grating) acting as a narrowband reflector. The arrow in each diagram indicates how the refractive index is modified in a post-processing step, resulting in the refractive index profile n(x) shown by the respective dashed curve. The local change in the refractive index is not necessarily always positive.

[0027] The Fig. Figure 2 schematically shows an arrangement with which, according to the invention, a refractive index modification can be introduced into the material of the component in a pre- or post-processing step.

[0028] The laser source is an ultrashort pulse laser 2 with a central wavelength in the range of 150 nm to 10 µm, with possible pulse durations in the range of 10 fs to 10 ps. All types of transparent, semi-transparent, or absorptive materials (for the respective laser central wavelength used) are suitable for the component 1 to be processed. These materials can be, for example, optical fibers with and without coatings, bulk materials with and without waveguides, etc. To overcome any potential surface curvature or other distortion of the component (e.g., curvature of the fiber surface), it can also be placed in a suitable holder (not shown), optionally supplemented by an immersion fluid for coupling the laser radiation used for refractive index modification.

[0029] The use of ultrashort laser pulses enables the local modification of the material. This allows for a highly localized change in the refractive index. Furthermore, the ultrashort laser pulses enable the modification of transparent (or semi-transparent) materials. According to the invention, the area to be processed within the material of the component is addressed by beam shaping of the laser beam. Addressing by scanning would also be conceivable. The magnitude of the refractive index change can be controlled, among other things, by the pulse energy, the number of pulses per area or per volume, and the repetition rate of the laser.

[0030] With a uniform change in the refractive index, as in Fig. As shown in Figure 1a, the centrally reflected wavelength of a Bragg grating can be changed.

[0031] A modification of the refractive index that increases (or decreases) towards one side of the component, as in Fig. As shown in 1b, it can be used to change the dispersive and reflective properties.

[0032] Furthermore, nonlinear profiles of the refractive index modification are conceivable in order to obtain specifically desired complex dispersion and reflection profiles. An example of what such a nonlinear profile, imprinted on a periodic structure, can look like is shown in Fig. 1c shown.

[0033] Various optical arrangements can be used to carry out the pre- or post-processing according to the invention. For example, as shown in Fig.As indicated in Figure 2, an imaging focusing optic 3 (comprising spherical or cylindrical lenses, freeform optics, curved mirrors, etc.) can be used, if required also in combination with a flexible adaptive optic 4 for beam shaping for the purpose of targeted local modification. This enables both large-area and local pre- and / or post-processing. In the case of post-processing structures within or in the effective range of an optical waveguide (e.g., within a coated fiber), the laser radiation can also be coupled into it.

Claims

[1] Method for manufacturing an optical component (1) using laser radiation, comprising the following process steps: - Generation of a structure in the material of the component (1) which gives the component (1) an optical functionality, wherein the optical functionality of the component (1) is that of an optical grating, in particular a fiber Bragg grating, an aperiodic fiber Bragg grating, a long-period grating or a bulk Bragg grating, - Determination of deviations of the generated structure in the material of the component (1) from specified target parameters, wherein the specified target parameters determine a central operating wavelength and / or a dispersion of the component (1), - Modification of the refractive index in the material of the component (1) using laser beams in a pre- and / or post-processing step, i.e. before or after the creation of the structure, in order to correct the deviations from the specified target parameters, characterized by , that in the pre- and / or post-processing step a beam shaping of the laser radiation directed at the component (1) takes place in order to generate a spatially variable modification of the refractive index in the material of the component (1), wherein the beam shaping is carried out by means of an adaptive optics (4) which dynamically controls the wavefront and intensity profile of the laser radiation in order to flexibly address local deviations in the material of the component (1) individually. [2] Method according to claim 1, characterized by, that the laser radiation used to modify the refractive index in the pre- and / or post-processing step is pulsed, with a pulse duration of 10 fs to 10 ps and a central wavelength in the range of 150 nm to 10 µm. [3] Method according to claim 2, characterized by , that to generate the spatially variable modification, the pulse energy, the repetition rate and / or the number of laser pulses applied in the material of the component (1) per volume or per area is varied. [4] Method according to any one of claims 1 to 3, characterized by , that in the pre- and / or post-processing step, a beam deflection of the laser radiation directed at the component (1) also takes place in order to generate the spatially variable modification of the refractive index in the material of the component (1). [5] Method according to any one of claims 1 to 4, characterized by, that the spatially variable modification of the refractive index is superimposed on the structure determining the optical functionality in the material of the component (1). [6] Method according to any one of claims 1 to 5, characterized by that the component (1) is clamped in a holder during the modification of the refractive index and / or an immersion fluid is used to couple the laser radiation into the material of the component (1). [7] Method according to any one of claims 1 to 6, characterized by , that the optical component (1) is an optical waveguide or an optical waveguide system, in particular an optical single-core or multi-core fiber.

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