Devices for optical applications

Abstract

A device for optical applications comprises an optical waveguide (10) to which a light source (11) can be connected. The optical waveguide (10) is designed such that light emitted from the connectable light source (11) propagates along a light propagation axis (12). A wavelength-sensitive grating structure (13) in the optical waveguide (10) comprises a detector (20) arranged such that it receives a portion of the light of the light source (11) scattered by the wavelength-sensitive grating structure (13). The grating structure (13) in the optical waveguide (10) is composed of periodically arranged ellipsoidal structural elements (14). The ellipsoidal structural elements (14) have a refractive index that differs from that of the material surrounding the structural elements of the optical waveguide (10). The ellipsoidal structural elements (14) have a major axis and a minor axis that are substantially perpendicular to the light propagation axis (12). Depending on the wavelength, a portion of the light scattered by the grating structure (13) is coupled out of the optical waveguide (10). This light falls on the detector (20). An absorptive or partially reflective filter (30) is arranged between at least one of the detectors (20) and the optical waveguide (10). The detectors (20) have a measuring element for the intensity of a portion of the light incident on the respective detector (20). An evaluation element is provided which determines the wavelength from the ratio of the intensities of the plurality of detectors (20). The detectors (20) are arranged such that they are arranged relative to one another on different sides of the major axis of an ellipsoidal structural element (14) in the grating structure (13) or on only one side of the major axis of an ellipsoidal structural element (14) of the grating structure (13), wherein the detectors are each positioned such that they detect differently scattered diffraction orders of the observed wavelength.

Description

Technical Field

[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 designed such that light emitted by the connectable light source propagates along a light propagation axis; a wavelength-sensitive grating structure in the optical waveguide; and detectors, which are arranged such that they receive a portion of the light of the light source that is scattered by the wavelength-sensitive grating structure. Background Art

[0002] Devices with optical waveguides for optical applications are known in various ways. Light fed in by a light source is guided in the optical waveguide and propagates therein along a propagation direction.

[0003] DE 42 09 672 C1 already describes a device for determining the wavelength of optical radiation, which operates using an optical waveguide with a wavelength-selective grating structure. The radiation is coupled out of the grating structure and impinges on associated detectors, which are part of a spectrometer. US Pat. No. 5,982,962 A also shows a similar structure.

[0004] A system for monitoring the wavelength of light in an optical fiber is known from US Pat. No. 6,885,792 B2, wherein light coupled out of a grating structure falls on a detector.

[0005] Another system for monitoring the wavelength of light in an optical waveguide having a grating structure is described in US 2007 / 0110367 A1.

[0006] Applications that require closer scrutiny of light from light sources that have a specific wavelength of interest but are already subject to specific influences are gaining increasing attention. In this case, for example, intensity minima or maxima should be closely scrutinized, and the spectral properties of the corresponding light within this wavelength range should be considered.

[0007] For example, DE 10 2017 11 9 810 B4 discloses a proposal in which such light is fed to an optoelectronic chip through a light inlet opening. A transmission filter is arranged in the chip at an angle to the light propagation axis. This filter reflects a portion of the incident light onto a first detector and allows another portion to pass through and fall onto a second detector. The measured values ​​of these two detectors can then be correlated and conclusions drawn.

[0008] The structure described there has previously generally eliminated the need for a separate beam splitter in such devices, as the filter simultaneously performs the task of splitting the beam into two parts. Despite the advantages of this approach, it still requires a relatively significant beam splitting and relatively large components, raising the question of whether alternative solutions could also be advantageous.

[0009] Other proposals for evaluating light wavelengths in optical applications are susceptible to thermal disturbances and provide fluctuating results depending on ambient conditions. Common problems also include evaluation speed, often complex optical structures, and excessively large structural components.

[0010] For other applications, devices are known for optical applications that have optical waveguides that operate with so-called fiber Bragg gratings, such as are generally known from WO 1998 / 44366 A1 and are also described in WO 2018 / 153868 A1. A special shape for a grating structure having a plurality of ellipsoidal individual structures in an optical waveguide is also described there. Summary of the Invention

[0011] In contrast, the object of the present invention is to provide a generic device for optical applications having an optical waveguide, which has the simplest possible structure, is insensitive to thermal disturbances and enables high-frequency wavelength evaluation.

[0012] The object is achieved by means of the invention in a device of this type in that a grating structure in an optical waveguide is formed by periodically arranged ellipsoidal structural elements, wherein the ellipsoidal structural elements have a refractive index different from that of the material of the optical waveguide surrounding the structural elements, wherein the ellipsoidal structural elements have a major axis and a minor axis, which are essentially perpendicular to the light propagation axis, so that a portion of the light scattered by the grating structure is coupled out of the optical waveguide depending on the wavelength and falls on detectors, an absorptive or partially reflective filter is arranged between at least one of the detectors and the optical waveguide, the detectors having a measuring element for the intensity of the portion of the light incident on the respective detector, an evaluation element is provided which determines the wavelength from the ratio of the intensities of a plurality of detectors, and The detectors are arranged such that they are arranged relative to each other on different sides of the major axis of the ellipsoidal structural elements in the grating structure, or on only one side of the major axis of the ellipsoidal structural elements of the grating structure, wherein the detectors are respectively positioned such that they detect different scattered diffraction orders of the observed wavelength, or two groups of detectors are provided, wherein one group consists of at least two detectors, the at least two detectors of the group being arranged relative to each other on different sides of the major axis of the ellipsoidal structural elements in the grating structure, and the other group of detectors consists of at least two detectors, the at least two detectors of the other group being arranged on only one side of the major axis of the ellipsoidal structural elements of the grating structure, wherein the detectors are respectively positioned such that they detect different scattered diffraction orders of the observed wavelength.

[0013] The present invention completely departs from the concept of DE 10 2017 11 9 810 B4. Instead of constructing and arranging a large transmission filter and a detector inside a housing and guiding the light to be examined as a whole to the detector via beam splitting with the aid of a transmission filter and partial transmission, a completely new approach is pursued and the light to be examined from the light source is scattered out of the optical waveguide with the aid of a grating structure, and the scattering pattern generated by the specific grating structure is used to guide a portion of the light in the direction of the detector.

[0014] This method allows for rapid, robust, and reproducible determination of wavelength changes in optical waveguides. The overall structure is very small and requires no complex optical structures. Instead, it only requires modifications to the optical waveguide itself and a few components applied alongside or as additional layers on it. This also means that the overall structure is less susceptible to thermal disturbances, as the very short paths and spacings result in virtually no thermal differences between locations in the device.

[0015] A specific technical effect is exploited here. Intensely focused femtosecond laser radiation can produce microstructures in optical waveguides (e.g., glass fibers, various polymers, or even flat layers of planar materials) that scatter the light guided therein. The typical, almost elliptical shape of the laser focus in such femtosecond laser radiation results in a significantly preferential scattering in the direction of the major axis of the ellipsoidal structures produced in this way.

[0016] If multiple such structures within the overall structure now also exhibit periodicity, the individual scattered components interfere with one another. Light of a specific wavelength propagating in the optical waveguide is then scattered by the multiple ellipsoidal structures almost exclusively within a specific solid angle. This solid angle corresponds to the structural interference of different levels of the Bragg condition.

[0017] Depending on the periodic structure of the grating, which is assembled from its individual elements, the direction of the resulting outcoupled radiation can be modified for a specific wavelength. With a period of exactly half a wavelength, the light is reflected only directly within the fiber of the optical waveguide, i.e., diffracted by exactly 180°. This special case corresponds to the conventional fiber Bragg grating. A grating structure with a spacing exactly equal to the wavelength also produces a beam that emerges perpendicularly from the fiber of the optical waveguide. If a grating structure with a periodic spacing of 1.5 times the wavelength of the propagated light is considered, a second beam is generated. If the individual elements of the grating structure are now considered with double the spacing, a third beam is generated, and so on.

[0018] Furthermore, by relatively simple modification of the period, the beam itself can be focused or defocused. A detailed review and modification of such a grating structure is given in WO 2018 / 153868 A1, albeit for other purposes.

[0019] All the mentioned beams and also the two preferred coupling-out directions for the propagated partial quantities of light are generated by the same grating structure. This means that the directions are mutually related in a defined manner and in particular the ratio of the intensities of the partial quantities in all beams to one another is also constant.

[0020] According to the invention, two detectors are now provided, on which the outcoupled light is to be incident. In two alternatively applicable arrangements, the effect according to the invention now occurs.

[0021] In the first case, two different beams are required. In the other case, two preferred coupling-out directions of a specific scattering order of the grating structure are used. In both cases, the detector measures independently of temperature influences, losses in the waveguide, or mechanical stresses, since temperature influences act equally on both partial beams, and no losses or mechanical stresses occur in the optical waveguide at all, or in any case both partial quantities appear identical.

[0022] The same applies to the different polarization states that may occur in the optical waveguide.

[0023] This results in a constant intensity ratio of the beam or of the partial quantities of propagated light.

[0024] The present invention thus makes it possible to determine the wavelength or wavelength change of light guided and propagated in the fiber core of an optical waveguide. This light is scattered by the grating structure. Furthermore, at least one wavelength-selective filter is introduced into the beam path between the grating structure and one of the detectors. By utilizing a constant intensity ratio of the propagated light sub-quantities or beams, the proportional, wavelength-dependent intensity change of the two detectors is measured.

[0025] If the wavelength of the light being examined changes, the ratio of the signals between the two detectors also changes. This is because the filters chosen in the beam path transmit more or less light, resulting in more or less light falling on the detectors. Knowing the spectral properties of the selected filters, the corresponding ratio can be calculated back to the change in light wavelength.

[0026] A particular advantage of the solution according to the invention is its extreme miniaturization. The grating structure required for this requires only a few hundred of the aforementioned ellipsoidal structural elements. This allows the two light beams or propagated light sub-quantities to be separated even in the smallest possible installation space. This makes it conceivable to achieve overall dimensions for analyzing light wavelengths of only a few millimeters.

[0027] Further advantages and preferred features are given in the dependent claims and in the following description of the figures. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] The figures show in detail different embodiments of the device according to the invention.

[0029] In the attached figure:

[0030] Figure 1 A schematic diagram showing a first embodiment of the present invention;

[0031] Figure 2 A schematic diagram showing a second embodiment of the present invention;

[0032] Figure 3 A third embodiment of the present invention is shown;

[0033] Figure 4 A fourth embodiment of the present invention is shown;

[0034] Figure 5 A fifth embodiment of the present invention is shown;

[0035] Figure 6 A sixth embodiment of the present invention is shown; and

[0036] Figure 7 A seventh embodiment of the present invention will be described. DETAILED DESCRIPTION

[0037] exist Figure 1 A basic schematic diagram of a simplified embodiment of the present invention is shown in FIG. Here, the optical waveguide 10 is shown extending horizontally in the plane of the drawing.

[0038] Schematically, a connectable light source 11 is conceivable at the end of the optical waveguide 10 , which is only schematically illustrated here.

[0039] Light is emitted from a light source 11 into the optical waveguide 10 and then propagates along a light propagation axis 12 in the optical waveguide 10 .

[0040] As can be seen further, the element essential to this embodiment of the invention is the grating structure 13 .

[0041] The grating structure 13 is arranged in the inner part of the optical waveguide 10, that is, in the fiber core 10a. The fiber core 10a is surrounded by a jacket 10b.

[0042] The grating structure 13 comprises a plurality of small ellipsoidal structural elements 14. These structural elements are only schematically shown here. The longitudinal axis (long axis) and the short axis of these ellipsoidal structural elements 14 are both perpendicular to the light propagation axis 12 in the optical waveguide 10.

[0043] It is further schematically shown that at least two detectors of a detector group are provided, which are generally designated by the reference numeral 20. The detectors 20 are located outside the optical waveguide 10 and also outside the housing of the optical waveguide 10.

[0044] Between the two detectors 20 and the optical waveguide 10, corresponding filters of a filter set 30 are present. At least one first filter 31 is present between the first detector 21 and the housing 10b of the optical waveguide 10. A possible second filter 32 is provided between the second detector 22 and the outside of the housing 10b of the optical waveguide 10. The portion of the propagating light scattered and refracted by the grating structure 13 is guided from the core 10a of the optical waveguide 10 through the housing 10b and the correspondingly located filter 31 or 32 to the detector 21 or 22 arranged outside the filter.

[0045] The scattered light components themselves constitute defined beams and propagate spatially separated from one another, so that only one scattered component always strikes each detector 20. The scattered components are shown in the diagram by different dashed lines.

[0046] This diagram also shows that fixing elements or spacers 37 can also be arranged between the filters 31 and 32 and the optical waveguide 10 , which can be identical for both detectors 21 and 22 .

[0047] This means that the detector is acted upon by at least one or both of the filters 31 and 32 and by different partial quantities of light and can thereby draw corresponding conclusions about the wavelength of the light propagating in the optical waveguide 10 .

[0048] The spacing between the individual ellipsoidal structural elements 14 of the grating structure 13 should be greater than 120% of the wavelength being observed. This relates to the refractive index in the optical waveguide 10. This ensures that both orders of magnitude of the propagated light are coupled out of the grating structure 13.

[0049] Furthermore, the distance between the optical waveguide 10 and the detector 20 should be selected so large that the individual beams of the two coupling-out stages used are also spatially completely separated. For this purpose, a distance as large as the entire grating structure 13 in the core 10a of the optical waveguide 10 should generally be selected.

[0050] To achieve this object, it is proposed that the above-mentioned spacer 37 be defined and selected accordingly.

[0051] exist Figure 2 A schematic diagram of an alternative arrangement with another embodiment of the invention can be seen in . The elements used are however essentially the same.

[0052] The optical waveguide 10 with a core 10a and a jacket 10b is again shown. Inside the core 10a there is again a grating structure 13 with ellipsoidal structural elements 14, wherein for a better overview Figure 2 These two reference numerals are omitted.

[0053] The outer shell 10 b surrounding the optical waveguide 10 is in turn provided with a layer having filters 31 and 32 , and the detectors 21 and 22 of the entire detector assembly 20 are mounted outside the filters 31 and 32 of the filter assembly 30 .

[0054] exist Figure 2 In the embodiment of FIG, two detectors 21 and 22 located opposite each other detect the incident light. The detectors compare the light coupled out of the optical waveguide 10 by means of the grating structure 13 and passing through different filters 31 and 32 .

[0055] Besides these schematic basic arrangements, more complex and therefore more efficient embodiments are also possible using the inventive concept.

[0056] Such an embodiment can be implemented, for example, in Figure 3This embodiment constitutes a multi-channel system with multiple scattered diffraction orders.

[0057] It is conceivable that grating structure 13 can be used to generate not only two levels, but three or more levels. These different levels can then be imaged onto not only two, but more detectors of detector array 20. Photodiodes, in particular arrays of a plurality of photodiodes, are proposed as detectors. However, pixels of an image sensor can equally well serve as detectors 20 if they are large enough to each receive only a scattered portion of the light.

[0058] By using one or different filters 30 , a plurality of different wavelengths of the light propagating in the optical waveguide 10 can also be evaluated with the detector array 20 .

[0059] Furthermore, additional filters 30 of different types, such as polarization filters, can be used if multiple levels are generated and imaged onto different detectors 20. Thus, in addition to the wavelength of interest and its changes, changes in polarization can also be determined.

[0060] exist Figure 4 A further development of this concept is shown in In this exemplary embodiment, the change in the wavelength of the light propagating in the optical waveguide 10 is determined using the first coupling-out stage and the intensity ratio between the measured values ​​of the detectors 21 and 23 , wherein a wavelength-selective filter 33 is positioned upstream of the detector 23 .

[0061] Furthermore, the polarization change can then be measured as a function of the wavelength using the intensity ratio of the second coupling-out stage and the detectors 22 and 24. For this purpose, two polarization filters 32 and 34 are positioned linearly and perpendicularly to one another.

[0062] exist Figure 5 A multi-channel system with a wavelength range and a plurality of detectors 20 is described in . For the sake of clarity, the reference numerals of the detector groups 20 are omitted.

[0063] In this embodiment, the other scattered partial quantities and their coupling-out angles are particularly related to the wavelength range. For example, light in the wavelength range of 820 nm to 825 nm and light in the wavelength range of 830 nm to 835 nm can be scattered on the grating structure 13 in such a way that they can be measured separately from each other as defined beams by different detectors 20. In one embodiment, it can be provided that a plurality of detectors 20 are arranged side by side and slightly offset from each other. The arrangement is implemented in such a way that the detectors only receive the coupling-out of a specific wavelength range. Each pair of detectors thus acts as a separate measurement channel and can be used in parallel. One of the two detectors in a pair is used as a reference, while the other detector of the pair is used as the signal to be evaluated.

[0064] exist Figure 6 In another particularly preferred embodiment, the grating period (not indicated by reference numerals) of the grating structure 13 is selected such that light propagating in opposite directions through the optical waveguide 10 propagates in different directions for all orders of wavelength under consideration and can therefore be distinguished from one another.

[0065] This is possible for grating point spacings in the grating structure 13 with a refractive index in the reference optical waveguide 10 of 50 to 90% and also 110 to 140% and 170 to 180% of the observed wavelength.

[0066] Here, light from light source 11 can be directly coupled in at one end of optical waveguide 10, and the initially scattered light is eliminated by beam trap 42. Further along in optical waveguide 10, only the initially transmitted portion of the light is reflected, for example, at fiber Bragg grating 41. Detectors 21 and 22 are oriented so that they can detect the structured scattering direction of the reflected signal. This embodiment forms a very 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.

[0067] exist Figure 6 , a simplified sensor system can be seen, which comprises a light source 11 and a fiber Bragg grating 41 at the end of an optical waveguide 10, with a grating structure 13, a photodiode pair as detector 20, an optical filter 30, and a scattered light trap 42. Utilization is made of the spatial asymmetry of the scattered light depending on the direction of light propagation.

[0068] Furthermore, it is particularly advantageous to illuminate the grating structure 13 with the light source 11 from one end of the optical waveguide 10 and to position one or more fiber Bragg gratings 41 along the optical waveguide 10 at the other end.

[0069] As the light source 11 , an LED, an SLED, a micro LED, a tunable laser, or other light sources may be used.

[0070] exist Figure 7 , a multi-channel evaluation is shown in which a so-called multi-core fiber is used as the optical waveguide 10 .

[0071] In a multi-core fiber as optical waveguide 10, multiple channels can be evaluated in parallel. For this purpose, a separate grating structure 13 is associated with each core 10a of the optical waveguide 10. Furthermore, at least two detectors 20 and at least one filter 30 are respectively arranged.

[0072] In an advantageous embodiment, such a multi-channel evaluation of at least one fiber Bragg grating 41 for measuring each core 10a of the optical waveguide 10 is performed as in Figure 6 The light propagating from the light source 11 and coupled out through the corresponding ellipsoidal structural elements 14 of the grating structure 13 of the plurality of cores 10 a of the optical waveguide 10 is eliminated in the scattered light trap 42 .

[0073] The light reflected back again by the corresponding fiber Bragg gratings 41 and coupled out at the grating structure 13 is emitted to the detectors 20 arranged in pairs.

[0074] exist Figure 7 , a multi-channel sensor system is shown, which has a light source 11 and respectively at least one fiber Bragg grating 41 and a grating structure 13 , as well as a photodiode pair with a filter 30 as a detector 20 and a scattered light trap 42 for each core 10 a of the optical waveguide 10 .

[0075] By means of a corresponding intensity ratio of the detector 20 or the photodiode pair and the wavelength-selective filter 30 preceding it, the change in the wavelength of the fiber Bragg grating 41 of each glass fiber core 10a can be measured.

[0076] exist Figure 7 Detectors 21 and 22 show the change in wavelength of light passing through the fiber Bragg grating 41'" by their measured values ​​or ratios of their measured values. Detectors 23 and 24 show the change in wavelength of light passing through the fiber Bragg grating 41'" by ratios of their measured intensities. Detectors 25 and 26 show the change in wavelength of light passing through the fiber Bragg grating 41' by their measurements of the ratio of the intensities.

[0077] Reference Signs List

[0078] 10 Optical waveguide

[0079] 10a Fiber core

[0080] 10b: housing of the optical waveguide 10

[0081] 11 Light Source

[0082] 12 Light propagation axis

[0083] 13 Grating structure

[0084] 14 Structural elements of an ellipsoid

[0085] 20 detector groups

[0086] 21 Detector

[0087] 22 detectors

[0088] 23 Detector

[0089] 24 detectors

[0090] 25 detectors

[0091] 26 detectors

[0092] 30 filter sets

[0093] 31 filter

[0094] 32 filters

[0095] 37 Placeholders

[0096] 41 Fiber Bragg Grating

[0097] 41' Fiber Bragg Grating

[0098] 41" Fiber Bragg Grating

[0099] 41'" Fiber Bragg Grating

[0100] 42 Scattered Light Trap

Claims

1. A device for optical applications, the device comprising: An optical waveguide (10) to which a light source (11) is connected, wherein: The optical waveguide (10) is configured such that light emitted from a connected light source (11) propagates along a light propagation axis (12); A wavelength-sensitive grating structure (13) in the optical waveguide (10); a detector (20) arranged such that it receives a portion of the light of the light source (11) scattered by the wavelength-sensitive grating structure (13), It is characterized by: The grating structure (13) in the optical waveguide (10) is formed by periodically arranged ellipsoidal structural elements (14). The ellipsoidal structural element (14) has a refractive index that is different from the material of the optical waveguide (10) surrounding the structural element. The ellipsoidal structural element (14) has a major axis and a minor axis which are substantially perpendicular to the light propagation axis (12), so that a portion of the light scattered by the grating structure (13) depending on the wavelength is coupled out of the optical waveguide (10) and falls onto the detector (20). An absorptive or partially reflective filter (30) is arranged between at least one of the detectors (20) and the optical waveguide (10), The detectors (20) have measuring elements for the intensity of a partial quantity of light incident on the respective detector (20), An evaluation element is provided, which determines the wavelength from the intensity ratio of the plurality of detectors (20), and The detector (20) is arranged to: Make the detector a) are arranged opposite one another on different sides of the major axis of the ellipsoidal structural elements (14) in the grating structure (13), or b) is arranged on only one side of the major axis of the ellipsoidal structural element (14) of the grating structure (13), wherein the detectors are each positioned such that they detect different scattered diffraction orders of the observed wavelength, or c) Two groups of detectors (20) are provided, one group consisting of at least two detectors (20), the at least two detectors of the group being arranged opposite each other on different sides of the major axis of the ellipsoidal structural element (14) in the grating structure (13), and the other group of detectors (20) consisting of at least two detectors (20), the at least two detectors of the other group being arranged on only one side of the major axis of the ellipsoidal structural element (14) of the grating structure (13), wherein the detectors are respectively positioned such that they detect different scattered diffraction orders of the observed wavelength.

2. The device for optical applications according to claim 1, characterized in that More than two detectors (20) are provided and are arranged such that light of different orders scattered by the grating structure (13) falls on a detector (20) having at least one filter (30).

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

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

5. The device for optical applications according to claim 2 or 3, characterized in that Some of the filters (30) are polarization filters (32, 33, 34), so that a change in the polarization of the light in the optical waveguide (10) is measured.

6. The device for optical applications according to any one of claims 1 to 3, characterized in that The spacing of the individual ellipsoidal structural elements (14) within the grating structure (13) is such that the refractive index in the optical waveguide (10) is greater than 120% of the wavelength under consideration.

7. The device for optical applications according to any one of claims 1 to 3, characterized in that At least one fiber Bragg grating (41) is introduced along the optical waveguide (10).

8. The device for optical applications according to any one of claims 1 to 3, characterized in that The optical waveguide (10) is a single-mode glass fiber, a multi-mode glass fiber, a double-clad glass fiber or a multi-core glass fiber.

9. The device for optical applications according to any one of claims 1 to 3, characterized in that The light source (11) is an LED or a laser.

10. The device for optical applications according to claim 9, characterized in that The LED is a SLED or micro-LED.

11. The device for optical applications according to claim 9, characterized in that The laser is a tunable laser.

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