Method for providing a fiber interferometer

Abstract

The invention relates to a method for providing a fiber interferometer (100) on the basis of components (110, 120, 130, 150), wherein at least one component (150) comprises a light-guiding fiber (156) having a free end (158). The method comprises producing the fiber interferometer including preparing (S110) the components (110, 120, 130, 150) on the basis of mechanically cutting the fiber (156) having the free end (158), coupling (S120) the components (110, 120, 130, 150) by means of fiber couplings (101, 102) on the basis of the cut fiber (156), and applying (S130) static mechanical stress to at least one fiber coupling (102) in order to thus bring a length of the at least one fiber coupling (102) to a specified value and in this way provide the fiber interferometer (100).

Description

[0001] Method for providing a fiber interferometer

[0002] The present invention relates to a method for providing a fiber interferometer, to a fiber interferometer, and in particular to a method for manufacturing and adjusting fiber interferometers with precise arm length difference and phase control by temperature adjustment.

[0003] BACKGROUND

[0004] For applications in the field of quantum telecommunications, interferometers are used that guide light or photons in waveguides (especially glass fibers) and whose arm lengths or optical path lengths, OPL, must be determined with high precision.

[0005] As an example of such an application, Fig. 5 shows the basic structure of a quantum key distribution (QKD) system with phase-time coding. It depicts a pulse source io, a generator of 20 entangled photon pairs, and three fiber-based interferometers 30, 40, 50 in Mach-Zehnder design with single-photon detectors 31, 32, 41, 42, where α, β, γ denote phase settings of the interferometers 30, 40, 50. The Mach-Zehnder design serves only as an example; other interferometer types, such as Michelson interferometers with Faraday mirrors, can also be used. Important for the application, however, is that each interferometer 30, 40, 50 has a relatively large arm length difference, so that the arms can be referred to as a "long" and a "short" arm.The three interferometers 30, 40, and 50 can be located several dozen kilometers apart in different buildings and be at least partially connected by fiber optic cables. The fiber optic cables can run underground or via masts, for example.

[0006] With the illustrated combination of interferometers 30, 40, and 50, so-called two-photon interference according to Franson can be achieved and used to generate quantum keys. This requires that the arm length differences between individual interferometers 30, 40, and 50 differ only slightly, namely by significantly less than the coherence length of the photons. This means that ideally, the long arms of the three interferometers 30, 40, and 50 are the same length, as are the short arms. A further requirement is to keep the phase constant in all three interferometers 30, 40, and 50 relative to each other (e.g., a+ß+y=o).

[0007] The task of determining the arm lengths or OPLs with high precision for all participating interferometers 30, 40, and 50 can be divided into two sub-aspects: first, determining the arm lengths or OPLs on length scales significantly larger than the wavelength of light ("macroscopic control"), and second, determining the change in the arm lengths or OPLs on length scales smaller than the wavelength of light. The latter primarily serves to control the interferometer phase and is used in particular to achieve high interference contrast.

[0008] To achieve macroscopic control accuracy of the optical fibers in the millimeter range, geometric measurement of the optical fibers, for example, using a ruler, is sufficient. Several methods are known for achieving higher accuracy of the macroscopic arm lengths.

[0009] One method involves adjusting the fiber lengths in the interferometer arms to the correct lengths by polishing the fiber's facets. Regular length checks using white-light interferometry / optical coherence tomography (OCT) and the significant amount of polishing required make this method labor-intensive.

[0010] Another method involves manufacturing interferometers 30, 40, and 50 on an integrated optical chip instead of using glass fibers. This process requires complex lithographic technology. Due to the birefringence of the waveguides on the chips, interferometers manufactured in this way are polarization-sensitive. However, for applications in quantum telecommunications, such as quantum key distribution, polarization-insensitive interferometers are necessary, which cannot be easily manufactured in this way.

[0011] Another method involves integrating optical delay lines into the interferometer. These delay lines are based on the extraction of light from the fiber optic cable, a variable propagation path in air, and the recoupling of the light into the fiber optic cable. Due to the extraction and recoupling of the light, this solution is associated with comparatively high light losses.

[0012] Another method involves using electronic, usually electro-optical phase modulators or piezoelectric fiber clamps to change the OPL by mechanically stretching the glass fibers. To enable a sufficiently wide adjustment range of the OPL using these devices alone, these clamps require a minimum fiber length in the interferometers, which makes the interferometers susceptible to phase disturbances caused by external temperature influences.

[0013] In the current state of the art, the interferometer phase can be adjusted by controlling the interferometer temperature and the associated change in the OPL. It is also known to incorporate additional components for electronic phase control, such as electro-optical phase modulators or piezoelectric fiber clamps, into the interferometer arms.

[0014] In the prior art, it is also known to use methods for stabilizing the interferometer temperature in order to attenuate phase changes in the interferometer caused by temperature fluctuations when the interferometer is used to such an extent that the phase can be readjusted.

[0015] Quantum technology, in particular, is still a young industry, with products based on the latest scientific findings. Many of these are not yet ready for series production, require expensive components, or are very labor-intensive in the manufacture and adjustment of the interferometers.

[0016] Here and in the following, the manufacture of an interferometer is understood to mean its assembly from components up to macroscopic control, i.e. the determination of the arm lengths or OPL on length scales larger than the wavelength of light.

[0017] Furthermore, here and in the following, interferometer adjustment is understood to mean setting the optical path length on length scales smaller than the wavelength of the light. Adjusting the interferometer phase can include fine-tuning the optical path length of the interferometer and / or checking the interferometer phase. Adjusting the interferometer typically follows manufacturing.

[0018] In addition, here and in the following, the provision of an interferometer shall be understood to mean at least its manufacture, but optionally also its adjustment.

[0019] There is a need for cost-effective and efficient solutions for producing interferometers with the highest possible precision and for adjusting their interferometer phase.

[0020] BRIEF DESCRIPTION OF THE INVENTION

[0021] This object is at least partially achieved by a method for providing a fiber interferometer according to claim 1 and by a fiber interferometer according to claim 13. The dependent claims relate to advantageous developments of the subject matter of the independent claims. The present invention relates to a method for providing a fiber interferometer, or fiber-based interferometer, based on components that are each designed to be connected to one another via fiber connections. The fiber interferometer can in particular be a Michelson interferometer or a Mach-Zehnder interferometer. At least one of the components has a light-conducting fiber, in particular a glass fiber, which has a free end at a starting point of the method. The method comprises producing the fiber interferometer.Manufacturing the fiber interferometer includes preparing the components based on mechanically cutting the fiber with the free end, connecting the components by fiber connections based on the previously cut fiber, and applying a static mechanical stress to the at least one fiber connection so as to approximate a length of the at least one fiber connection to a prescribed value and thus manufacture the fiber interferometer.

[0022] The method can, in particular, be based on prefabricated optical components. These components can include, for example, a mirror, a fiber coupler, or a beam splitter. Additional components can, for example, be mere sections of fiber. In exemplary embodiments, a Mach-Zehnder interferometer can be constructed from two beam splitters, for example, and need not include any mirror components.

[0023] The components can each have a component housing in which a mirror, fiber coupler or partial reflector is enclosed. The light-guiding fibers can in particular be glass fibers which are designed to guide light or photons into the interior of the component housing and / or out of the interior of the component housing. The light-guiding fiber is therefore not or not completely enclosed by the component housing. The light-guiding fiber on the respective component is usually surrounded by a coating, for example a polymer coating. The polymer coating can enclose a plurality of fibers or fiber strands. The fibers and components can be designed to preferentially guide light or photons of a certain polarization or to suppress the further transmission of light or photons of a certain polarization. Furthermore, the fibers and / or components can be designed as polarization-maintaining fibers orcomponents must be designed.

[0024] Advantageously, the preparation of the components does not involve any further adjustment of the length of the respective fiber other than mechanical cutting, i.e., in particular, no polishing or tempering of the fiber. In exemplary embodiments, it may be provided that the lengths of the optical fibers can be determined to within a tolerance of approximately 100 μm by preparing the components.

[0025] The components can be connected during assembly. This can involve fixing the component housings in a relative position and at least leading to a preliminary alignment of the first and second fibers. The fiber connections can be created from fibers contained in the components. In particular, the previously cut free end of the fiber of at least one component can be connected to another end of a fiber of another component. The other end or the fiber of the other component can also have been previously cut mechanically.

[0026] In exemplary embodiments, at least two components are present, each having a fiber with a free end. At least one of these is precisely cut in the process. The two free ends of the fibers of the components can then be spliced ​​together, so that the optical path length of a resulting fiber connection can be determined with an accuracy of approximately 100 pm. Other components of the interferometer do not necessarily have to have fibers with free ends.

[0027] Such components can also be connected using plug connections, for example.

[0028] The term "joining" should be understood broadly. In particular, it does not necessarily refer to the creation of a permanent connection; rather, the components can be connected or joined together in a temporary manner.

[0029] In exemplary embodiments, the lengths of the fiber connections can be determined by connecting them to within a tolerance of 100 μm. This can particularly apply to the length of the at least one fiber connection. The at least one fiber connection can be based on the previously cut fiber of the at least one component. In particular, the at least one fiber connection can be created by connecting previously cut free ends of fibers. Not all fiber connections have to be based on fibers with free ends.

[0030] Applying the static mechanical stress to the at least one fiber connection utilizes an optomechanical effect according to which the stress can change both a spatial length of the at least one fiber connection and a refractive index in the fiber. Both of these effects affect the optical path length. While the term “length of the at least one fiber connection,” and the lengths of fiber connections in general, can also refer to purely spatial dimensions, the term advantageously relates to an optical path length of the respective fiber connection. The prescribed value can, in particular, be a suitable target value for the length. By applying the static mechanical stress to the at least one fiber connection, the length can be set as an optical path length of the at least one fiber connection to the prescribed value within a tolerance of approximately 10 pm.

[0031] Optionally, the method includes adjusting the interferometer. For this purpose, the interferometer can have a controllable device for adjustment, in particular for fine adjustment and / or for controlling the interferometer phase, after manufacture. The controllable device can be, for example, an electro-optical phase modulator or a piezoelectric or electromechanical fiber stretcher. The controllable device can be contained in one of the components. Optionally, adjusting the interferometer phase includes tempering the at least one fiber connection in order to bring the length closer to the specified value.

[0032] In further embodiments, at least one fiber connection is actively tempered, i.e. brought to a temperature such that the length or its length increases or decreases to the prescribed value.

[0033] Tempering utilizes a thermo-optical effect, according to which the optical path length, in particular both the spatial length of the corresponding fiber and the refractive index, can change due to a change in temperature. The change in the optical path length is reversible. Tempering can thus be carried out in particular to achieve a microscopic length change in the range of less than one wavelength to adjust the interferometer phase. At least one fiber connection, and possibly several or all fiber connections, can initially be exposed to target temperatures T, which cause changes in the corresponding lengths by values ​​greater than a typical wavelength of the light or photons.Small temperature changes ΔT in the environment of T, which affect the fiber connections, can also be used to adjust the optical path lengths of the corresponding fibers to a tolerance of significantly less than 0.1 pm, thereby controlling the interferometer phase. The optimal temperature of the fibers is then given by T + ΔT . Since the thermo-optical effect used is reversible, a different optical path length difference can be adjusted by changing T at a later time, and / or a different interferometer phase can be adjusted by changing ΔT.

[0034] In exemplary embodiments, the application of a static mechanical stress can be combined with the temperature control. For example, the stress can be applied while simultaneously changing the temperature, or the two steps can be repeated iteratively, individually or together. Between the repetitions, in particular, a test or check of the length of the fiber connections can take place. The method can also be combined with other techniques for controlling the optical path length and phase. In particular, additional phase adjustment devices or electronic fiber stretchers can be additionally installed in the interferometer.

[0035] In further embodiments of this method, a combination of precise mechanical preparation of the glass fibers used, the optomechanical effect, and the thermo-optical effect can be used. Pathlength difference accuracies of approximately 10 pm are achieved through mechanical preparation, accuracies of up to approximately 10 pm through the optomechanical effect, and accuracies of less than 10 pm through the thermo-optical effect. The static mechanical stress applied to the glass fibers, in combination with the active temperature adjustment, allows the phase to be controlled in the interferometer with a precisely adjusted pathlength difference. Optionally, however, the interferometer can be adjusted exclusively by temperature control.Advantageously, the method requires no other technique for determining the optical path length during or after the application of the static mechanical stress and the temperature control to adjust the optical path length and phase of the fiber interferometer. In particular, it has been found that adjusting the interferometer phase can advantageously be based solely on changing the temperature; thus, in particular, no further application of a mechanical stress is required.

[0036] Optionally, the preparation of the components comprises fixing the at least one component with the fiber with the free end in a holding device, pre-tensioning the fiber by applying a tensile force to the free end of the fiber and clamping the fiber by a clamping device in order to enable the mechanical cutting of the fiber.

[0037] Optionally, the at least one component comprising the free-ended fiber has a component housing, and securing the at least one component comprises inserting the component housing into the holding device to enable mechanical cutting of the fiber at a predetermined distance from the component housing. The component housing can be inserted, for example, into a recess or cutout in the holding device designed for this purpose.

[0038] Alternatively or additionally, securing the at least one component can also comprise clamping the fiber with the free end in a clamping device, wherein the clamping device is then inserted into the holding device. The at least one component comprising a fiber with a free end does not necessarily have to have a component housing; it can merely be a fiber section. The clamping device is to be distinguished from a fiber clamping device. The clamping device can be designed as a simple fiber holder.

[0039] The holding device can thus be designed to fix the at least one component via a component housing, or to fix the at least one component via the clamping device holding the fiber. This can be achieved, for example, by a suitable recess or cutout in the holding device into which the component housing or the clamping device can be inserted.

[0040] Optionally, the application of the static mechanical stress is based on a mechanical, electromechanical, or piezoelectric fiber tensioning device. The fiber tensioning device can be provided as a further component or as part of a further component and can be integrated into the structure of the fiber interferometer when the components are assembled. The fiber tensioning device or the component of the fiber interferometer which comprises the fiber tensioning device can in particular comprise a fiber with two loose ends. In other exemplary embodiments, however, the fiber tensioning device can also be used merely to apply the static mechanical stress to a fiber of a component or to a fiber connection. The fiber tensioning device can thus also be integrated into the fiber interferometer to be provided after the components have been connected.

[0041] Optionally, the application of the static mechanical tension is based on a fiber tensioning device having a winding body configured to wind the at least one fiber connection around the winding body, and having a changing device configured to change a diameter of the winding body and thus apply the tension.

[0042] Optionally, the fiber interferometer to be manufactured has a tempering device which is designed to carry out the tempering based on the tempering device by changing a temperature.

[0043] The thermo-optical effect can be used for macroscopic control and in particular for adjusting the interferometer, i.e. for fine adjustment and / or for controlling the interferometer phase. This is particularly advantageous if the same temperature control device can be used for both applications, which can in particular be permanently connected to the actual interferometer or integrated into the fiber interferometer. It should be noted that macroscopic control of the lengths of the fiber connections requires temperatures or temperature differences T that can be several orders of magnitude greater than a temperature change ΔT required, for example, to adjust an interferometer phase. For example, macroscopic control of the length can be achieved by setting the temperature Tm in the range of a few degrees Celsius around room temperature, while ΔT is set in the range of millidegrees Celsius.

[0044] Optionally, the tempering for adjusting the interferometer comprises, in particular, changing a temperature within a housing or tempering housing that encloses the at least one fiber connection.

[0045] The housing can be part of the temperature control device of the fiber interferometer and, in advantageous embodiments, can also completely enclose the latter. The interferometer typically has two arms (for two different light paths), and in exemplary embodiments, it has proven advantageous to arrange both arms in the housing or controlled environment. In exemplary embodiments, it has also proven advantageous to be able to set a temperature with an accuracy of approximately 0.5 mK using the temperature control device. Optionally, the method comprises providing a reference interferometer and measuring the length of the at least one fiber connection based on a comparison with the reference interferometer.

[0046] The connection of the components, the application of the mechanical stress and the tempering can each be carried out over several stages or iterations, between which the measurement can be carried out.

[0047] In particular, when assembling the components, component housings can initially be fixed only loosely or easily detachably, and the trimmed ends of the fibers can be connected only loosely or easily separably. The length of the fiber connections can then be checked or inspected and, if necessary, corrected using the aforementioned measurement before the assembly of the components is completed and the components are firmly connected.

[0048] Likewise, the application of a voltage and / or the tempering can each be carried out in a suitable alternation with a test or control and, if necessary, a correction, whereby the said measurement and comparison with the reference interferometer is carried out.

[0049] Optionally, the measurement is based on white-light interferometry and / or optical coherence tomography. Both methods are known in the art for the purpose of comparing different interferometers and can be advantageously used here for testing or checking, and if necessary, for correcting the length of the fiber connections.

[0050] Embodiments also relate to a fiber interferometer made from components connected by fiber connections. The fiber interferometer comprises a fiber tensioning device having a winding body around which at least one fiber connection is wound, and having a fiber tensioning device configured to change a diameter of the winding body and thus apply mechanical tension to the at least one fiber connection. The fiber interferometer further comprises a temperature control device configured to temperature control the fiber connections based on the temperature control device by changing a temperature. The temperature control device can also be configured to adjust the interferometer, in particular to control an interferometer phase, in addition to macroscopically controlling the lengths of the fiber connections.

[0051] Important aspects of the presented method and the presented fiber interferometer can also be presented as follows.

[0052] The method combines several techniques and effects to enable the construction and operation of interferometers that are as identical as possible, with virtually identical path length differences between the arms of each interferometer, using standard fiber-optic components for optical communications technology. In exemplary embodiments, length differences in the fiber optic cables to be installed, which arise during component manufacture, are first measured and corrected by mechanical cutting. The components are connected, and any remaining inaccuracies are compensated for by mechanically stretching the fiber optic cables without compromising their service life. Adjustment, such as further fine-tuning, is reversible by adjusting the temperature.A central aspect of the process is to exploit the combination of the optomechanical and thermo-optical effect in the glass fibers to change the spatial length and refractive index or the optical path length of the fibers and thereby adapt the optical path lengths to each other.

[0053] The thermo-optical effect can also be used to control the interferometer phase. In general, the temperature range for the phase change in the fiber interferometer is several orders of magnitude smaller than the temperature range required to compensate for the residual errors.

[0054] In embodiments, the method can be divided into the following steps, for example: - Adjusting the fiber lengths on at least one of the interferometer components (accuracy in the millimeter range), embodiments comprising at least two components, each with a free end, in order to establish a connection between them by splicing, wherein at least one of the two components is adjusted in fiber length by mechanical cutting;

[0055] - Preparation of the components by precisely measuring the fiber lengths on the components and correcting length deviations using a specially developed mechanical holding device for precise mechanical cutting of defined fiber optic lengths (accuracy approx. 100 pm);

[0056] - Assembling the parts to form an interferometer and then mounting it on a mechanical fiber clamping device;

[0057] - Measuring the deviation in the optical path length and adjusting it by applying a mechanical voltage (accuracy of a few io pm); and

[0058] - Further fine adjustment of the optical path length by tempering, i.e. adjusting the interferometer temperature (accuracy better than oi pm).

[0059] The interferometer phase can also be adjusted by adjusting the interferometer temperature.

[0060] An important aspect of the method is that, in exemplary embodiments, concrete values ​​for the temperature of the interferometer or for the mechanical stress are not required or need to be known numerically for the provision of the fiber interferometer. Rather, a known interferometer state, given by a fixed value of the optical path difference, can be specified, and the mechanical stress and temperature can then be optimized to achieve this state. The exact numerical values ​​for mechanical stress and temperature are irrelevant. In exemplary embodiments, the mechanical stress or any resulting change in length does not need to be known at all, and the temperature can be controlled to a target value using an uncalibrated sensor.This aspect also distinguishes the presented method from calibrating an interferometer for measurement purposes, in which known temperature, voltage, or load values ​​are specified and adjusted in order to then calibrate a state of the interferometer using the resulting interference pattern. Rather, the presented method allows for the adjustment of a predefined interferometer state defined by the path difference, which is important for quantum optical applications.

[0061] Another important aspect of component preparation is a specially developed holding device for precise mechanical cutting of the lengths of glass fiber and the glass fibers on optical components.

[0062] Another important aspect is the compensation of differences in optical path length through a combination of optomechanical and thermo-optical effects. This involves static mechanical tensioning of the fiber optic cable and temperature adjustment.

[0063] Another important aspect is that fine adjustment of the optical path length with phase stabilization and matching can be achieved solely based on the control and adjustment of the interferometer temperature.

[0064] Another important aspect is that a well-known method of white-light interferometry and / or optical coherence tomography in the frequency range of light or photons can be used to test, control, and, if necessary, correct the optical path length during the process. This method can also be used for optical components with non-negligible chromatic dispersion.

[0065] Another important aspect is that preparing the components for mechanically cutting the fibers can include securing the respective component in a holding device, pre-tensioning the fiber by applying a tensile force to the free end of the fiber, and clamping the fiber with a clamping device, thus enabling mechanical cutting of the fiber at a predetermined distance from a component housing. Another important aspect is that a purely mechanical pre-tensioning device can be used to apply the mechanical tension, thus eliminating the need for piezo-controlled fiber tensioning devices.

[0066] Another important aspect is that the adjustment of the interferometer phase can be based solely on changing a temperature, without the need to apply a mechanical voltage for this purpose.

[0067] Embodiments of the present invention offer, among other things, the following advantages.

[0068] The presented method enables the simple, precise, and reproducible construction of fiber optic interferometers and the adjustment of their optical path length (OPL). For applications such as quantum key distribution, the interferometer arms for these fiber interferometers must be manufactured with high precision in the range of a few micrometers. Furthermore, the interferometer phase must be controlled to achieve high interference contrast. The presented method allows the OPL of the components required for construction to be precisely measured and compensates for deviations that may still exist after assembly. The presented method then makes it possible to adjust the OPL through temperature changes down to the sub-micrometer range, thus controlling the interferometer phase.

[0069] The presented method makes it possible to reduce the manufacturing costs and time required for the construction of fiber interferometers with precisely tuned path length differences. Alignment to a reference interferometer enables faster, lower-cost production of larger quantities of such fiber interferometers.

[0070] Since neither a piezo fiber stretcher nor a phase modulator (e.g., based on the electro-optical effect) is required for fine adjustment and phase control, the complexity of the fiber interferometer is reduced, which reduces construction costs and operational error rates. Alternatively, the method can be used in combination with piezo-

[0071] For example, by reducing the required adjustment range of fiber stretchers or phase modulators, these components can be simplified or cheaper components can be used. One example is the reduction of the required minimum fiber length for piezoelectric fiber stretchers, which leads to increased stability of the interferometer against temperature fluctuations.

[0072] BRIEF DESCRIPTION OF THE CHARACTERS

[0073] The embodiments of the present invention will be better understood from the following detailed description and the accompanying drawings of the various embodiments, which, however, should not be construed as limiting the disclosure to the specific embodiments, but are for explanation and understanding only.

[0074] Fig. i illustrates an embodiment of the presented method for manufacturing a fiber interferometer.

[0075] Fig. 2 illustrates steps of an embodiment for preparing components.

[0076] Fig. 3 shows a fiber tensioning device for applying mechanical tension to a fiber.

[0077] Fig. 4 shows an embodiment of the proposed fiber interferometer. Fig. 5 shows a conventional system for quantum key exchange.

[0078] DETAILED DESCRIPTION

[0079] Fig. i illustrates an embodiment of the proposed method for manufacturing a fiber interferometer. A starting point of the method comprises provided optical components, such as mirror components and in particular Faraday mirrors, beam splitter components, and light-conducting fibers. A fiber clamping device may be present, which is installed in the fiber interferometer. At least one component has a light-conducting fiber with a free end for connection to other components. In the method, further devices, in particular a temperature control device, can also be integrated into the fiber interferometer.

[0080] For producing the interferometer, the method comprises preparing Sno the components by mechanically cutting the fibers, connecting S120 the components by fiber connections, which can be created by connecting the free ends of the fibers, and applying S130 a static mechanical stress to at least one fiber connection in order to approximate a length of the fiber connection to a prescribed value and thus produce the fiber interferometer. In the exemplary embodiment presented here, the method additionally comprises adjusting the interferometer, which includes tempering S140 the at least one fiber connection in order to approximate the length of the at least one fiber connection to a prescribed value and thus, overall, provide the fiber interferometer.

[0081] The method may, in particular, also comprise providing a reference interferometer and measuring path length differences between the interferometer to be provided and a reference interferometer at various points, in particular during the application S130 of the mechanical stress, but also during the temperature control S140. In this way, the interferometer can be manufactured and / or provided with precisely the same construction as the reference interferometer.

[0082] Embodiments of the method are characterized by a combination of a simplified setup for Michelson interferometers and a method for their manufacture. Both allow for the uncomplicated production of Franson interferometers for two-photon interference and provide a solution for the efficient manufacture of identical, highly precise interferometers, as well as for adjusting the interferometer phase. In the method, the components, i.e., the individual optical interferometer components, are prepared by precisely and easily cutting the corresponding fibers mechanically. Later, the remaining deviations in the optical path length of fiber connections are eliminated through a combination of optomechanical and thermo-optical effects. The phase of the fiber interferometers can also be controlled through the thermo-optical effect, which can lead to a significant reduction in the complexity of the setup.

[0083] The optomechanical and thermo-optical effects describe changes in the refractive index of the fiber optic cable as a function of the applied mechanical stress or temperature, respectively. Static mechanical stretching is applied to the fiber to adjust the length with an accuracy of approximately 10 pm. Fine adjustment of the OPL and interferometer phase is achieved by temperature changes and corresponds to a length change on the order of less than 10 pm. Phase adjustment requires a length change on the order of less than 10 pm.

[0084] An embodiment of the method characterized by the following steps has proven to be particularly advantageous:

[0085] - Construction of a reference interferometer relative to which the fiber interferometer to be manufactured or all other fiber interferometers to be manufactured are aligned;

[0086] - precise cutting of the fiber lengths on the components or parts by a mechanical device as described in Fig. 2;

[0087] - Assembling these components into a temporary interferometer without permanently connecting the parts;

[0088] - Measuring differences between the temporary interferometer and the reference interferometer using white-light interferometry / optical coherence tomography (OCT) in the frequency domain; - Re-cutting the fiber lengths of specific components with correction of the measured deviations;

[0089] - S120 firmly joining the components and subsequent assembly on a mechanical fiber tensioning device;

[0090] - Measurement of the difference between the combined interferometers and the reference interferometer using white light interferometry / OCT;

[0091] - Rough adjustment of a path length difference by applying S130 or adjusting a static mechanical tension of the glass fiber in one of the arms of the fiber interferometer, based on a fiber tensioning device as described in Fig. 3 and based on the results from the previous step. By alternately repeating this step and the previous step, the path length difference of the interferometers is minimized;

[0092] - Fine adjustment of the path length of the fiber interferometer by tempering S140 or adjusting the temperature of the fiber interferometer.

[0093] Fig. 2 schematically illustrates steps of an advantageous embodiment for precise mechanical cutting of the fiber optic coupled components.

[0094] Shown is a component 150, specifically a fiber optic component (for example, with a Faraday mirror or a beam splitter), which is partially enclosed in a component housing 154. The optical fiber 156, in this case a glass fiber, which is to be precisely cut, extends from the component housing 154. This fiber has a free end 158.

[0095] For the sake of simplicity, only one optical fiber 156 is shown; however, the embodiment can also be implemented for components with one or more incoming and outgoing optical fibers of a different type by performing the steps illustrated here for all fibers individually and sequentially.

[0096] Also shown is a clamping device 210 or a fiber holder for the fiber 156, relative to which the fiber 156 is clamped during mechanical cutting or

[0097] Cleaving, for example with a commercial fiber cleaver, reproducibly separates the fiber.

[0098] Before the cutting process, the end of the glass fiber 156 must usually be freed from a polymer coating and the required length of the glass fiber 156 must be determined.

[0099] The steps described below are illustrated in the figure, numbered accordingly, in a sequence from top to bottom. Arrows F1, F2, and F3 indicate the direction in which a force should be applied during the process. Arrows F1, F2 pointing downward indicate the fixation of a component, while arrows F3 pointing to the right indicate the tensioning of the fiber optic cable 156. The steps include:

[0100] 1. Fasten the component housing 154 to a holding device, such as a prepared base such as a metal plate with a suitable recess or cutout for the component housing 154. This ensures that the component housings 154 of different components 150 of the same type are reproducibly fastened in one location. The component housing 154 is inserted into the cutout and secured against displacement and tilting, for example, by screwing or clamping. If the fiber optic component 150 is a loose fiber optic cable without a component housing, one end of the fiber can be clamped into a fiber holder, and this fiber holder can be used in the described process instead of the fiber component (not shown).

[0101] 2. Pre-tensioning the optical fiber 156 with a reproducible force F3. One possible embodiment for this step is based, for example, on a tensioning device with a weight attached to the optical fiber 156, so that the reproducible force F3 is applied by a weight. Other methods for reproducibly applying the force F3 can also be used. 3- Attaching the clamping device 210 or a fiber holder used during splicing. The clamping device 210 can be part of the holding device and is advantageously positionable at a fixed, finely adjustable distance from the attachment of the component housing 154 to ensure the mechanical cutting or cleaving of the fiber 156 at a reproducible length. This allows small, defined differences in the fiber length to be compensated or generated, such as those required, for example, to compensate for length variations in the optical fiber 156.In exemplary embodiments, it has proven particularly advantageous to install a mechanical micrometer displacement unit (not shown) with which the clamping device 210 can be precisely positioned. The micrometer displacement unit can, in particular, be attached to the prepared base and be part of the holding device. The clamping device 210 can be detachably connected to the micrometer displacement unit.

[0102] 4. Release the tensile stress on fiber 156 and loosen the fastening of clamping device 210. The clamping device 210 can then be removed from the micrometer displacement unit or from the prepared support, and the fiber 156 can be mechanically cut with a cleaver at a length now precisely defined by the clamping device 210 (not shown). This cleaving process produces fiber ends with a very precisely reproducible length relative to the clamping device 210 using most known cleaver machines. After the cleaving process, the component 150 can be measured or installed directly.

[0103] Fig. 3 shows on the left a schematic and on the right a perspective view of a particularly advantageous fiber tensioning device 140, as it can be used for applying S130 the mechanical tension.

[0104] In the schematic representation on the left in the figure, the fiber tensioning device 140 comprises a winding body 143 around which a fiber 146 is wound, and a changing device 145 configured to change a diameter of the winding body 143 by exerting a force F4 spreading the winding body 143 and thus determining a tension in the glass fiber 146. To allow the winding body 143 to be spread, it has an opening or gap 144.

[0105] After the precise cutting of the fibers 156 and the joining of the components 150, 140, any remaining fiber length differences can be further reduced with the mechanical fiber tensioning device 140 by stretching the fiber 146. Tensioning the fiber 146 leads to a change in the OPL (optomechanical effect) through a geometric change in length and a stress-induced change in the refractive index of the fiber 146.

[0106] Overall, small fiber length differences that arise during cutting can be compensated for by the mechanical tension. In this way, in particular, the length or optical path length of the at least one fiber connection wound around the fiber tensioning device 140 can be approximated to the corresponding prescribed value.

[0107] The fiber tensioning device 140 can initially be present as a component in the method and comprise a fiber with one or two free ends. The respective fiber can be mechanically cut during preparation S110. In advantageous embodiments, however, the fiber tensioning device 140 itself does not need to have a fiber and can only be integrated into the fiber interferometer 100 during or after connecting the components S120. For example, a fiber of a component can be so long that it is wound around the winding body 143 of the fiber tensioning device 140 and then its free end is spliced ​​to a free end of a fiber of another component. This can be advantageous because it eliminates the need for splices.

[0108] An advantageous technical implementation of the fiber tensioning device 140 is shown in the right-hand part of the figure. The fiber tensioning device 140 comprises a copper or brass ring as a winding body 143, which is semi-open and can be expanded from the inside by a screw in a change device 145. The material can advantageously be selected so that the thermal conductivity of the ring 143 is as high as possible. A counter screw 147 secures the expansion. The glass fiber 146 can be wound externally around the ring 143 in specially provided grooves and can be attached with adhesive at specific points opposite the opening 144 of the ring 143.

[0109] Fig. 4 shows an embodiment of a fiber interferometer 100, such as can be manufactured using the presented method. Its structure is that of a Michelson interferometer. The fiber interferometer 100 is composed of components (110, 120, 130) connected by fiber connections 101, 102. In particular, the fiber interferometer 100 in the present embodiment comprises a first mirror component 110, a second mirror component 120, and a beam splitter component 130. The beam splitter component 130 is connected to the first mirror component 110 via a first fiber connection 101 of a first length and to the second mirror component 120 via a second fiber connection 102 of a second length.

[0110] The fiber interferometer 100 comprises a fiber tensioning device 140 with a winding body around which at least one fiber connection, namely the second fiber connection 102, is wound. The second fiber connection 102 is wound around the fiber tensioning device 140 as shown in Fig. 3. The fiber tensioning device 140 can generate a static tension in the glass fiber 102 and thus adjust the OPL. For this purpose, the fiber tensioning device 140 comprises a changing device 145, which is designed to change a diameter of the winding body 143 and thus apply mechanical tension to the second fiber connection 102. The fiber tensioning device 140 can be part of a separate component.

[0111] The figure shows fiber connection points 107 in the first fiber connection 101 and the second fiber connection 102, where the individual components 110, 120, 130 were joined. The illustrated parts of the fiber interferometer 100 can be housed in a housing or temperature control housing with active temperature stabilization (not shown). This temperature control device is designed to perform temperature control S140 of the second fiber connection 102 and advantageously also of the first fiber connection 101 by changing a temperature, in order to thus adjust the interferometer 100.

[0112] The features of the invention disclosed in the description, the claims and the figures may be essential for the realization of the invention both individually and in any combination.

[0113] LIST OF REFERENCE SYMBOLS

[0114] IO pulse source

[0115] 20 Generation of entangled photon pairs

[0116] 30, 40, 50 interferometers

[0117] 31, 32, 41, 42 Single-photon detectors

[0118] 100 fiber interferometers

[0119] 101 first fiber connection

[0120] 102 second fiber connection

[0121] 107 Fiber connection point

[0122] 110 component (first mirror component)

[0123] 120 component (second mirror component)

[0124] 130 Component (beam splitter component)

[0125] 140 fiber clamping device

[0126] 143 winding bodies

[0127] 144 Opening

[0128] 145 Change facility

[0129] 146 fiber

[0130] 147 Screw

[0131] 150 component (with free-ended fiber)

[0132] 154 component housings

[0133] 156 fiber

[0134] 158 free end of the fiber

[0135] 210 clamping device

[0136] Fi, F2, F3 forces

[0137] S110, S120, S130, S140 process steps

Claims

CLAIMS 1. A method for providing a fiber interferometer (100) starting from components (no, 120, 130, 150), wherein at least one component (150) has a light-conducting fiber (156) with a free end (158), the method comprising producing the fiber interferometer, which comprises the following steps: Preparing (S110), based on a mechanical cutting of the fiber (156) with the free end (158), the components (110, 120, 130, 150); Connecting (S120), based on the cut fiber (156), the components (110, 120, 130, 150) by fiber connections (101, 102); and Applying (S130) a static mechanical stress to at least one fiber connection (102) so as to approximate a length of the at least one fiber connection (102) to a prescribed value and thus produce the fiber interferometer (100).

2. The method of claim 1, wherein the method comprises adjusting the interferometer.

3. The method according to claim 2, wherein adjusting the interferometer comprises tempering (S140) the at least one fiber connection (102) so as to further approximate the length to the prescribed value.

4. The method according to claim 3, wherein the adjustment of the interferometer is carried out exclusively by tempering (S140).

5. The method according to any one of the preceding claims, wherein the preparing (S110) comprises: Fixing the at least one component (150) in a holding device; Pre-tensioning the fiber (156) with the free end (158) by applying a tensile force (F3) to the free end (158); and Clamping the fiber (156) by a clamping device (210) to enable mechanical cutting of the fiber (156).

6. The method of claim 5, wherein fixing the at least one component (150) comprises one of the following: Inserting a component housing (154) into the holding device, and / or Clamping the fiber (156) with the free end (158) in a clamping device and inserting the clamping device into the holding device.

7. The method according to any one of the preceding claims, wherein the application (S130) of the static mechanical stress is based on at least one of the following: - a mechanical fiber tensioning device, - an electromechanical fiber tensioning device, - a piezoelectric fiber tensioning device.

8. The method according to any one of the preceding claims, wherein the application (S130) of the static mechanical tension is based on a fiber tensioning device (140) comprising: a winding body (143) configured to wind the at least one fiber connection (102) around the winding body (143), and a changing device (145) which is designed to change a diameter of the winding body (143) and thus apply the voltage.

9. The method according to one of claims 3 to 8, wherein the fiber interferometer (100) to be produced has a tempering device which is designed to carry out the tempering (S140) based on the tempering device by changing a temperature.

10. The method according to any one of claims 3 to 9, wherein the tempering (S140) comprises changing a temperature within a housing enclosing the at least one fiber connection (102).

11. The method according to any one of the preceding claims, further comprising: Providing a reference interferometer; and Measuring the length of the at least one fiber connection (102) based on a comparison with the reference interferometer.

12. The method according to claim 11, wherein the measuring is based on one of the following methods: white light interferometry, and / or optical coherence tomography.

13. Fiber interferometer (100) made from components (110, 120, 130, 150) connected by fiber connections (101, 102), the fiber interferometer (100) comprising: a fiber tensioning device (140) with a winding body (140) around which at least one fiber connection (102) is wound, and with a changing device (145) which is designed to change a diameter of the winding body (143) and thus apply a mechanical tension to the at least one fiber connection (102); and a tempering device which is designed to temper (S140) the fiber connections (101, 102) in order to adjust the interferometer (100).

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