DEVICE FOR OPTICAL APPLICATIONS

Description

[0001] The invention relates to a device for optical applications, comprising an optical waveguide to which a light source can be connected, wherein the optical waveguide is constructed such that light emitted from the connectable light source propagates along a light propagation axis, with a wavelength-sensitive grating structure in the optical waveguide, and with detectors arranged such that they detect subsets of the light from the light source scattered by the wavelength-sensitive grating structure.

[0002] Devices for optical applications using an optical waveguide are widely known. Light is guided within the optical waveguide and propagates in a direction of transmission that feeds into a light source.

[0003] German patent DE 42 09 672 C1 already describes a device for determining the wavelength of optical radiation, which uses an optical waveguide with a wavelength-selective grating structure. Radiation is coupled out at the grating structures and falls onto associated detectors that are part of a spectrometer. A similar design is also shown in US patent 5,982,962 A.

[0004] From US patent 6,885,792 B2, a system for monitoring light wavelengths in optical fibers is known. In this system, the light coupled out at the grating structures falls onto detectors.

[0005] Another system for monitoring light wavelengths in an optical waveguide with lattice structures is described in US 2007 / 0110367 A1.

[0006] Increasingly interesting are applications that involve examining light from a light source that has a specific wavelength of interest but has been subjected to certain influences. In such cases, the aim is to investigate intensity minima or maxima and to determine the spectral properties of the corresponding light in the range of that wavelength.

[0007] For example, DE 10 2017 119 810 B4 describes a proposal in which such light is directed through a light entry aperture to an optoelectronic chip. A transmission filter is arranged at an angle to the light propagation axis within this chip. The filter reflects a subset of the incident light onto a first detector and allows another subset to pass through and fall onto a second detector. The measured values ​​of the two detectors can then be compared to allow conclusions to be drawn.

[0008] The design described therein is intended to eliminate the need for separate beam splitters, which are currently standard in such devices, since the filters themselves perform this task of dividing the beams into two. Despite the advantages of this concept, it still involves a fairly pronounced beam splitting and relatively large components, raising the question of whether alternative solutions might also offer advantages.

[0009] Other proposals for evaluating light wavelengths in optical applications are susceptible to thermal disturbances and deliver fluctuating results depending on environmental conditions. Some general problems also include the speed of the evaluation, often complex optical structures, and excessively large components.

[0010] For other applications, devices for optical applications with optical waveguides are known that operate with so-called fiber Bragg gratings, as are known, for example, from WO 1998 / 44366 A1 and also described in WO 2018 / 153868 A1. A special form for grating structures with a multitude of ellipsoidal individual structures in optical waveguides is also described there.

[0011] Further state of the art is represented by US 2004 / 0561 83 A1, which deals, among other things, with filters.

[0012] In contrast, the object of the invention is to propose a generic device for optical applications with an optical waveguide that has the simplest possible structure, is less susceptible to thermal disturbances, and enables high-frequency evaluation of wavelengths.

[0013] This problem is solved by the invention in a generic device by the fact that the grating structure in the optical waveguide is composed of periodically arranged ellipsoidal structural elements, wherein the ellipsoidal structural elements have a different refractive index than the surrounding material of the optical waveguide, wherein the ellipsoidal structural elements have a longitudinal axis and a short axis that are essentially perpendicular to the light propagation axis, so that, depending on the wavelength, subsets of the light scattered by the grating structure are coupled out of the optical waveguide and fall onto the detectors, that an absorbing or partially reflective filter is arranged between at least one of the detectors and the optical waveguide, that the detectors have measuring elements for the intensity of the subset of light incident on the respective detector, and that an evaluation element is provided.that determines a wavelength from the intensity ratio of the several detectors, and that the detectors are arranged such that they are opposite each other on different sides of the long axes of the ellipsoidal structural elements in the lattice structure, or are arranged on only one side of the long axis of the ellipsoidal structural elements of the lattice structure, each positioned such that they detect a different scattered diffraction order of the wavelength under consideration, or two groups of detectors are provided, one group consisting of at least two detectors arranged opposite each other on different sides of the long axes of the ellipsoidal structural elements in the lattice structure, and the other group of detectors consisting of at least two detectors arranged on only one side of the long axis of the ellipsoidal structural elements of the lattice structure, each positioned such thatthat they detect a differently scattered diffraction order of the wavelength under consideration.

[0014] The invention completely departs from the concept described in DE 10 2017 119 810 B4. Instead of constructing and arranging large-format transmission filters and detectors within a housing and guiding the entire light to be examined to detectors via beam splitting using a transmission filter and partial transmission, a completely new approach is taken. The light to be examined from the light source is scattered out of the optical waveguide by means of a grating structure, and the resulting scattering pattern is used to guide the partial amounts of light towards the detectors.

[0015] Changes in the wavelength of light within an optical waveguide can be determined quickly, robustly, and reproducibly using this method. The entire setup is very small and requires no complex optical structures, but merely a modification of the optical waveguide itself and a few elements applied alongside or on top of the waveguide as additional layers. This also means that the overall design is not very susceptible to thermal disturbances, as the very short paths and distances result in virtually no thermal differences between positions within the device.

[0016] This process utilizes certain technical effects. A highly focused femtosecond laser beam can create microscopic structures in optical waveguides such as glass fibers, various polymers, or even planar layers made of planar materials, which scatter internally guided light. The typically highly elliptical shape of the laser focus in such femtosecond laser beams leads to a significantly favored scattering along the long axis of the ellipsoidal structure created in this way.

[0017] If a multitude of such structures within a larger assembly also exhibit periodicity, the individual scattered components interfere with each other. The light of a specific wavelength propagating through the optical waveguide is then scattered by this multitude of ellipsoidal structures almost exclusively within specific solid angles. These solid angles correspond to constructive interferences of the Bragg condition in various orders.

[0018] Depending on the periodic structure of a grating composed of individual elements, the directions of the resulting emitted radiation can be altered for a specific wavelength. With a period of exactly half the wavelength, only light directly within the optical fiber is reflected, i.e., diffracted by exactly 180°. This special case corresponds to a typical fiber Bragg grating. A grating structure with a spacing exactly equal to the wavelength additionally produces exactly one beam exiting perpendicular to the optical fiber. If we consider a grating structure with periodic spacings 1.5 times the wavelength of the propagated light, a second beam is generated. If we now consider twice the spacing of the individual elements of a grating structure, a third beam is generated, and so on.

[0019] With relatively simple modifications to the period, the beam itself can also be focused or defocused. A more detailed discussion of such a grating structure and the modifications, albeit for a different purpose, is given in WO 2018 / 153868 A1.

[0020] All the aforementioned rays, as well as the two preferred output directions for the propagated subsets of light, are generated by the same grating structure. This means that the directions correlate with each other in a defined way, and in particular, the ratio of the intensities of the subsets on all rays is constant.

[0021] According to the invention, two detectors are provided onto which the coupled-out light is to strike. The effect according to the invention occurs in two alternatively applicable arrangements.

[0022] In the first case, two different beams are required. In the other case, the two preferred coupling directions of a specific scattering order of the grating structure are used. In both cases, the detector measurements are independent of temperature influences, losses, or mechanical stresses in the waveguide, since the temperature influences affect both partial beams equally, and losses or mechanical stresses in the optical waveguide either do not occur at all or at least affect both partial beams equally.

[0023] The same applies to different polarization states that can occur in the optical waveguide.

[0024] Thus, a constant intensity ratio of the rays or subsets of the propagated light is formed.

[0025] The invention thus makes it possible to determine the wavelengths or changes in wavelength of the light guided and propagating in the fiber core of the optical waveguide. This light is scattered by the grating structure. Additionally, at least one wavelength-selective light filter is placed in one of the beam paths between the grating structure and one of the detectors. By utilizing the constant intensity ratio of the partial amounts of the propagated light or beams, a wavelength-dependent change in intensity relative to the two detectors is measured.

[0026] If the wavelength of light being examined changes, the ratio of the signals between the two detectors also changes. This is because the appropriately selected filter in the beam path transmits either more or less light, so that more or less light falls on the detector. Knowing the spectral properties of the selected filter, this ratio can be calculated as a change in the wavelength of the light.

[0027] A particularly advantageous feature of the solution according to the invention is its extreme miniaturization. The lattice structure required for this only needs to employ a few hundred of the aforementioned ellipsoidal structural elements. As a result, the two light beams or subsets of the propagated light can be separated from each other in a very small space. In this way, it becomes conceivable to achieve a total size of only a few millimeters for the analysis of a single wavelength of light.

[0028] Further advantages and preferred features are specified in the dependent claims and in the following figure description.

[0029] The drawing shows various embodiments of a device according to the invention in more detail.

[0030] They show: Figure 1 A schematic representation of a first embodiment of the invention; Figure 2a schematic representation of the second embodiment of the invention; Figure 3 a third embodiment of the invention; Figure 4 a fourth embodiment of the invention; Figure 5 a fifth embodiment of the invention; Figure 6 a sixth embodiment of the invention; and Figure 7 a seventh embodiment of the invention

[0031] In the Figure 1 A basic schematic representation of a simplified embodiment of the invention is shown. An optical waveguide 10 is shown running horizontally in the image plane.

[0032] Schematically, one can imagine a connectable light source 11 at the end of the optical waveguide 10, which is only indicated here.

[0033] Light from the light source 11 is emitted into the optical waveguide 10 and then propagates along a light propagation axis 12 in the optical waveguide 10.

[0034] As can be further seen, a central element for the embodiment of the invention is a lattice structure 13.

[0035] The grid structure 13 is arranged in the inner part of the optical waveguide 10, i.e., in a fiber core 10a. The fiber core 10a is surrounded by a cladding 10b.

[0036] The lattice structure 13 consists of a multitude of small ellipsoidal structural elements 14. These are only indicated here. Both the long axis and a short axis of the ellipsoidal structures 14 are perpendicular to the light propagation axis 12 within the optical waveguide 10.

[0037] The schematic diagram further shows that at least two detectors are provided in a group of detectors designated with the reference numeral 20. The detectors 20 are located outside the optical waveguide 10 and also outside the cladding 10b of the optical waveguide 10.

[0038] Between each of the two detectors 20 and the optical waveguide 10, there is a filter from a group 30 of filters. Between a first detector 21 and the cladding 10b of the optical waveguide 10, there is at least one first filter 31. Between a second detector 22 and the outer surface of the cladding 10b of the optical waveguide 10, there is a possible second filter 32. The subsets of the propagating light scattered and refracted by the grating structure 13 are guided from the core 10a of the optical waveguide 10 through the cladding 10b and a filter 31 or 32 located at the respective position to the detectors 21 or 22 arranged outside the filter.

[0039] The subsets of scattered light, considered individually, represent defined rays and propagate spatially separated from one another, so that only one scattered subset reaches each detector 20. The scattered subsets are represented in the schematic diagram as differently dashed lines.

[0040] The schematic representation also shows that a fastening element or placeholder 37 can be arranged between the filters 31 and 32 and the optical waveguide 10, which can be identical for both detectors 21 and 22.

[0041] The detectors are thus acted upon based on at least one or both filters 31 and 32 and different subsets of the light and can draw corresponding conclusions about the wavelength of the propagating light in the optical waveguide 10.

[0042] The spacing between the individual ellipsoidal structural elements 14 of the lattice structure 13 should be greater than 120% of the wavelength under consideration. This value is based on the refractive index in the optical waveguide 10. This ensures that two orders of propagating light are coupled out of the lattice structure 13.

[0043] Furthermore, the distance between the optical waveguide 10 and the detectors 20 should be chosen to be large enough so that the individual beams of the two output coupling orders used are completely spatially separated. Typically, a distance should be chosen that is as large as the entire grating structure 13 within the core 10a of the optical waveguide 10.

[0044] To achieve this goal, it is advisable to define the aforementioned placeholder 37 and select it accordingly.

[0045] In the Figure 2The figure shows a schematic representation of an alternative arrangement with a different embodiment of the invention. However, the basic elements used are the same.

[0046] One can again see an optical waveguide 10 with a core 10a and a cladding 10b. Inside the core 10a is a lattice structure 13 with ellipsoidal structural elements 14, where, for better clarity, these two reference symbols are shown in the Figure 2 omitted.

[0047] Around the outer sheath 10b of the optical waveguide 10, a layer with filters 31 and 32 is provided, and outside of filters 31 and 32 of the filter group 30, detectors 21 and 22 of the entire detector group 20 are attached.

[0048] In the embodiment of the Figure 2Two opposing detectors 21 and 22 detect the incident light. They compare the light coupled out of the optical waveguide 10 by means of the grating structure 13 through different filters 31 and 32.

[0049] In addition to these basic schematic arrangements, more complex and therefore more efficient embodiments are also possible, which make use of the inventive ideas.

[0050] One such embodiment is found, for example, in the Figure 3 to recognize. It represents a kind of multi-channel system with several scattered diffraction orders.

[0051] It is conceivable to generate not just two orders, but three or more orders using the grid structure 13. These different orders can then be mapped onto not just two, but more detectors of the detector group 20. Photodiodes, especially an array of several photodiodes, are suitable detectors. However, pixels of an image sensor can also serve as detectors 20, provided these pixels are large enough to each receive only a scattered subset of the light.

[0052] By using one or more filters 30, the detector group 20 can also evaluate several different wavelengths of the propagating light in the optical waveguide 10.

[0053] If multiple orders are generated and imaged onto different detectors 20, additional different types of filters 30 can also be used, for example, polarization filters. This allows not only the wavelength of interest and its change to be determined, but also a change in polarization.

[0054] In the Figure 4 A further development of this idea is shown. An exemplary embodiment shows that the change in the wavelength of the propagating light in the optical waveguide 10 is determined using the first coupled order and the intensity ratio between the measured values ​​of detectors 21 and 23, whereby a wavelength-selective filter 33 has been positioned in front of detector 23.

[0055] Furthermore, a polarization change as a function of the wavelength change can then be measured using a second coupled order and the intensity ratio of detectors 22 and 24. For this purpose, two linearly perpendicular polarization filters 32 and 34 are positioned.

[0056] In the Figure 5 A multi-channel system with wavelength ranges and several detectors 20 is described. The reference symbol of the detector group 20 has been omitted for clarity.

[0057] In this embodiment, the further scattered subsets and their coupling angles depend in particular on the wavelength ranges. For example, light in the wavelength range of 820–825 nm and light in the wavelength range of 830–835 nm can be scattered by the grating structure 13 in such a way that they can be measured separately as defined beams by different detectors 20. In one embodiment, several detectors 20 can be arranged side by side and slightly offset from one another. The arrangement is such that the detectors only receive the coupling from a specific wavelength range. Each pair of detectors thus functions as its own measurement channel and can be used in parallel. One of the two detectors in a pair serves as a reference, and the other detector in the pair serves as the signal to be evaluated.

[0058] In a further, particularly preferred embodiment of the Figure 6 The lattice period of the lattice structure 13 (the reference symbol is not entered) is chosen such that light propagating in opposite directions through the optical waveguide 10 is propagated in different directions for the wavelength under consideration for all orders, and these can thus be distinguished from one another.

[0059] This is possible for the lattice point spacings in the lattice structure 13 from 50 to 90% of the considered wavelength as well as from 110 to 140% and from 170 to 180%, based on the refractive index in the light guide 10.

[0060] Light from the light source 11 can be directly coupled into one end of the optical waveguide 10, and the initially scattered light is eliminated via a beam trap 42. Only a portion of the initially transmitted light is reflected further along the optical waveguide 10, for example, by a fiber Bragg grating 41. The detectors 21, 22 are aligned such that they can detect constructive scattering directions of the reflected signal. This embodiment represents a highly simplified and advantageous sensor system with an additional fiber Bragg grating 41. Such a system can also be referred to as an FBG sensor system.

[0061] A simplified sensor system consisting of a light source 11 and a fiber Bragg grating 41 at the ends of the optical waveguide 10, as well as with a grating structure 13, a pair of photodiodes as detectors 20 and with filters 30, and a stray light trap 42 is in the Figure 6to recognize. This utilizes the spatial asymmetry of the scattered light depending on the direction of light propagation.

[0062] Additionally, illuminating the grating structure 13 from one end of the optical waveguide 10 with the light source 11 and positioning one or more fiber Bragg gratings 41 along the optical waveguide 10 at the other end is particularly advantageous.

[0063] LEDs, SLEDs, microLEDs or tunable lasers or other light sources can be used as light source 11.

[0064] In the Figure 7 A multi-channel evaluation is shown, in which 10 so-called multi-core fibers are used as optical waveguides.

[0065] In a multi-core optical fiber 10, several channels can be evaluated in parallel. For this purpose, a separate grating structure 13 is inscribed for each core 10a of the optical fiber 10. In addition, at least two detectors 20 and at least one filter 30 are placed in each core.

[0066] In an advantageous embodiment, such a multi-channel evaluation for measuring at least one fiber Bragg grating 41 per core 10a of the optical waveguide 10 is bidirectional as in the embodiment shown. Figure 6 designed. Light which is propagated from the light source 11 and coupled out through the respective ellipsoidal structural elements 14 of the lattice structures 13 of the several cores 10a of the optical waveguide 10 is destroyed in scattering light traps 42.

[0067] Light which is reflected back from the respective fiber Bragg gratings 41 and coupled out at the grating structures 13 is emitted onto paired detectors 20.

[0068] One can see in the Figure 7 the multi-channel sensor system, which includes a light source 11 and at least one fiber Bragg grating 41 and a grating structure 13, as well as a photodiode pair as detectors 20 with filters 30 and a stray light trap 42 for each core 10a of the optical waveguide 10.

[0069] Changes in the wavelengths of the fiber Bragg gratings 41 per glass fiber core 10a can be measured via the respective intensity ratios of the detectors 20 or photodiode pairs and wavelength-selective filter 30 in front of them.

[0070] In the Figure 7Detectors 21 and 22, with their measured values ​​and the ratio of their measured values, respectively, show the change in wavelength through the fiber Bragg grating 41 III<. Detectors 23 and 24, with the ratio of their measured intensities, show the change in wavelength through the fiber Bragg grating 41". Detectors 25 and 26, with their measured intensities in the ratio, show the change in wavelength through the fiber Bragg grating 41 I<. Reference symbol list

[0071] 10 Optical fiber 10a Fiber core 10b Optical fiber cladding 10 11 Light source 12 Light propagation axis 13 Grating structure 14 Ellipsoidal structural elements 20Group of detectors 21Detector 22Detector 23Detector 24Detector 25Detector 26Detector 30 Group of filters 31 Filter 32 Filter 37 Placeholder 41Fiber Bragg grating 41 I< Fiber Bragg grating 41 II< Fiber Bragg grating 41 III< Fiber Bragg grating 42Scattered light trap

Claims

1. A device for optical applications, having an optical waveguide (10), to which a light source (11) can be connected, wherein the optical waveguide (10) is designed in such a way, that light emitted by the connectable light source (11) propagates along a light propagation axis (12), having a wavelength-sensitive grating structure (13) in the optical waveguide (10), having detectors (20) that are arranged in such a way that they absorb partial amounts of the light of the light source (11) that is scattered by the wavelength-sensitive grating structure (13), wherein the grating structure (13) in the optical waveguide (10) is constructed from periodically arranged ellipsoid structural elements (14), wherein the ellipsoid structural elements (14) have an index of refraction that is different from that of the surrounding material of the optical waveguide (10), wherein the ellipsoid structural elements (14) have a longitudinal axis and a short axis that are substantially perpendicular to the light propagation axis (12), so that, depending on the wavelength, partial amounts of the light from the optical waveguide (10) that is scattered by the grating structure (13) will be coupled out and impinges on the detectors (20), wherein an absorbing or partially reflecting filter (30) is arranged between at least one of the detectors (20) and the optical waveguide (10), wherein the detectors (20) have measuring elements for the intensity of the partial amount of light that impinges on the detector (20) in question, wherein an evaluation element is provided, which determines a wavelength from the intensity ratio of the plurality of detectors (20), and wherein the detectors (20) are arranged in such a way that they a) are arranged opposite one another on different sides of the long axes of the ellipsoid structural elements (14) in the grating structure (13), or b) are arranged on only one side of the long axes of the ellipsoid structural elements (14) of the grating structure (13), wherein they are positioned in each case so that they capture a different scattered diffraction order of the examined wavelength, or c) two groups of detectors (20) are provided, of which one group is composed of at least two detectors (20), which are arranged opposite each another on different sides of the long axes of the ellipsoid structural elements (14) in the grating structure (13), and the other group of detectors (20) is composed of at least two detectors (20), which are arranged on only one side of the long axes of the ellipsoid structural elements (14) of the grating structure (13), wherein they are each positioned in such a way that they capture a different scattered diffraction order of the examined wavelength.

2. The device for optical applications according to claim 1, further characterized, in that more than two detectors (20) are provided and are arranged in such a way that light of different orders that is scattered by the grating structure (13) impinges on the detectors (20) with their at least one filter (30).

3. The device for optical applications according to claim 1, further characterized, in that more than two detectors (20) are provided and are arranged in such a way that light of different wavelength ranges that is scattered by the grating structure (13) impinges on the detectors (20) with their at least one filter (30).

4. The device for optical applications according to claim 2 or claim 3, further characterized, in that different filters (30) are provided in order to be able to evaluate a plurality of wavelengths from the measured values of the detectors (20) that result therefrom.

5. The device for optical applications according to any of claims 2 to 4, further characterized, in that some of the filters (30) are polarization filters (32, 33, 34), so that a measurement of the change in polarization of the light in the optical waveguide (10) is produced.

6. The device for optical applications according to any of the preceding claims, further characterized, in that the spacing of the individual ellipsoid structural elements (14) inside of the grating structure (13) is larger than 120 % of the examined wavelength, in relation to the index of refraction in the optical waveguide (10).

7. The device for optical applications according to any of the preceding claims, further characterized, in that at least one fiber Bragg grating (41) is placed along the optical waveguide (10).

8. The device for optical applications according to any of the preceding claims, further characterized, in that the optical waveguide (10) is a single-mode, multimode, double-clad, or multicore glass fiber.

9. The device for optical applications according to any of the preceding claims, further characterized, in that the light source (11) is a LED, SLED, micro LED, laser, or tunable laser.