DEVICE AND METHOD FOR THE PHASE-STABLE TRANSMISSION OF OPTICAL SIGNALS

Description

Technical area

[0001] The present invention relates to a device and a method for the phase-stable transmission of optical signals, in particular for the transmission of a reference frequency or a reference signal from a reference or source location to one or more destination locations.

[0002] The transmission of reference frequencies from a reference location to a spatially spaced destination is described, for example, in DE 10 2008 062 139 B4. Known devices and methods, which are also described, for example, in PA Williams, WC Swann, and NR Newbury, "High stability transfer of an optical frequency over long fiber-optic links," J. Opt. Soc. Am. B 25, pp. 1284-1293, doi: 10.1364 / JOSAB.25.001284 (2008), are based exclusively on the so-called reciprocity of light propagation along the path between a source location and a destination location. This prohibits the insertion or provision of non-reciprocal elements in the path to be stabilized.

[0003] Furthermore, a stable transmission of radio frequency signals over fiber links using an interferometric delay sensing is known from WILCOX R ET AL: "Stable transmission of radio frequency signals on fiber links using interferometric delay sensing", OPTICS LETTERS, OPTICAL SOCIETY OF AMERICA, US, Vol. 34, No. 20, October 15, 2009 (2009-10-15), pages 3050-3052, XP001549119, ISSN: 0146-9592, DOI: 10.1364 / OL.34.003050.

[0004] Typically, a source can be, for example, a light field from a laser that has been stabilized to an ultrastable optical resonator or to an optical frequency standard, such as an optical clock. Such high frequency stability or frequency accuracy should be provided at one or more other, possibly distant, target locations with the same quality or stability. Inevitable length changes on the transmission path between the source location and the target location, for example, due to thermal influences or similar changing boundary conditions, lead to additional frequency and phase instabilities, which must be counteracted by appropriate measures.

[0005] Furthermore, especially for particularly long transmission distances, it is desirable to provide amplification of the optical signals along the optical path between the source and destination. This is particularly necessary to compensate for unavoidable losses over long distances, ranging from a few kilometers to several hundred kilometers. Furthermore, splitting the optical path among multiple destinations would be desirable in order to make the optical reference signal available not only at a single destination, but at multiple destinations simultaneously.

[0006] Optical amplifiers have so far been of limited use for amplifying optical signals between a source and a destination, as they would typically violate the reciprocity condition along the optical path between the source and destination. While bidirectional or reciprocal amplifiers also exist, their use would be associated with other problems, such as parasitic, i.e., unwanted lasing.

[0007] It is therefore an object of the present invention to provide an improved device and an improved method for the phase-stable transmission of optical signals, which are particularly simple and efficient to implement, which enable a phase-stable transmission of optical signals over comparatively long distances, which can further distribute the optical reference signal at the source location to several destination locations, and which can possibly also enable the use of non-linear processes, such as frequency doubling or supercontinuum generation.

[0008] This object is achieved by a device and a method according to the features of the independent patent claims. Advantageous embodiments are the subject of dependent patent claims.

[0009] In one aspect, a device for the phase-stable transmission of optical signals is provided. The device has a first light source for emitting a first light beam. The device further comprises a first interferometer arrangement into which the first light beam is coupled and which includes a first source location and a first target location spatially separated therefrom. A first detector can be used to determine the phase of the first light beam at the first source location relative to the phase of the first light beam at the first target location.

[0010] The device further comprises a second interferometer arrangement comprising a second source location and a spatially separated second target location for a second light beam. The first target location spatially overlaps with the second source location.

[0011] By means of a second detector, a phase of the second light beam at the second source location can be determined relative to the phase of the second light beam at the second destination location. The second interferometer arrangement is further optically coupled to the first interferometer arrangement. Furthermore, the device has at least one control loop, which is signal-coupled to the first detector and to the second detector. The control loop further has a first actuator, which is arranged in the beam path of the first light beam and / or in the beam path of the second light beam. The actuator is signal-coupled to the first detector and to the second detector.

[0012] It can be coupled directly or indirectly to these two detectors. In particular, the control loop can comprise one or more electronic controllers configured to process signals from the first and second detectors and, based on these signals, generate a control signal for the at least first actuator. By means of the first actuator arranged in the beam path of the first and / or second light beam, a fixed phase relationship between the phase of the first light beam at the first source location and the phase of the second light beam at the second target location can be set, controlled, or fixed.

[0013] Any phase or frequency instabilities of the first light beam between the first source location and the second light beam at the second target location can be detected by the first interferometer arrangement using the first detector, by the second interferometer arrangement using the second detector, and by the coupling of the first and second interferometer arrangements. Any phase and / or frequency instabilities are expressed as a beat signal on the respective detector, which can be electrically or electronically detected and evaluated.

[0014] Likewise, a phase and / or frequency instability between the first source location and the first target location, or between the second source location and the second target location, can also be interferometrically measured in the region of the first and / or second interferometer arrangement and thus recorded not only qualitatively but also quantitatively. By means of the at least one control loop and due to the optical coupling of the first and second interferometer arrangements, it is possible to quantitatively record any phase instabilities and / or frequency instabilities arising in the region of the first and second interferometer arrangements and to dynamically control the actuator arranged in the beam path of one of the said light beams, for example with the aid of the control loop, such that the phase of the second light beam at the second target location is in a fixed phase relationship to the phase of the first light beam at the first source location.

[0015] By means of the first interferometer arrangement and the second interferometer arrangement as well as by means of the optical coupling between the first and second interferometer arrangements, a cascade consisting of a first interferometer arrangement and a second interferometer arrangement, optionally with further interferometer arrangements connected thereto, can be provided, which enables a phase-stable transmission of the optical signal from the first source location to the second destination location over comparatively long or further distances.

[0016] In the same way, the device can also be supplemented with additional interferometer arrangements. Each additional interferometer arrangement has an additional source location and a spatially separated additional target location, as well as a detector by means of which the phase of the additional light beam at the additional source location can be determined relative to the phase of the additional light beam at the additional target location.

[0017] Coupling this additional detector with the control loop enables the appropriate inclusion of fluctuations or phase or frequency instabilities that may arise in the region of the additional interferometer arrangement. By appropriately controlling at least the first actuator, any phase or frequency instabilities can be actively compensated, namely by means of the control loop, so that a fixed phase relationship between the phase of the first light beam at the first source location and the phase of the additional light beam at the additional target location can be set, controlled, or fixed.

[0018] The spatial overlap of the first target location with the second source location can be achieved, in particular, with a partially reflecting or even a fully reflecting mirror. The spatial overlap of the first target location with the second source location is highly advantageous for seamless path stabilization from the first source location to the second target location and beyond. The entire optical path from the first source location to the second target location can be actively controlled in this way, or its phase or frequency stability can be controlled.

[0019] The first interferometer arrangement has at least one first detector. The second interferometer arrangement has at least one second detector. Each additional, optional interferometer arrangement has at least one further detector. Additional detectors, for example, for determining the optical coupling between the first and second interferometer arrangements, can also be provided.

[0020] The control loop can be implemented as a global control loop. It can process signals from multiple detectors of different interferometer arrangements and, based on this signal processing, control at least one actuator located in the beam path between the first source location and the second target location. In this respect, all signals detected by the detectors can be used to control the at least one actuator.

[0021] In further embodiments of the device, the control loop can be provided with multiple actuators. For example, the first interferometer arrangement can be provided with its own actuator. The second interferometer arrangement can also be provided with its own actuator, such as a second one. Furthermore, it is conceivable to also provide the optical coupling between the first interferometer arrangement and the second interferometer arrangement with a separate actuator.

[0022] In further embodiments, one actuator can be provided per control loop. Each control loop can be coupled to at least one or more detectors and use the signals from the respective detector(s) to control the associated actuator.

[0023] By means of a first actuator between the first source location and the first target location, any path instabilities, i.e. phase or frequency instabilities in the area of ​​the first interferometer arrangement, can be compensated. Similarly, by means of a second actuator in the area between the second source location and the second target location, any path instabilities in the area of ​​the second interferometer arrangement can be compensated. For the global compensation of any path instabilities between the first source location and the second target location, it may also be sufficient to only sensor the instabilities occurring in the area of ​​the first interferometer arrangement and the second interferometer arrangement, i.e.by means of first and second detectors and to control a single or a few actuators accordingly by means of appropriate processing of the detector signals thus obtained, so that a fixed phase relationship between the phase of the first light beam at the first source location and the phase of the second light beam at the second target location can be set, regulated or fixed.

[0024] According to a further embodiment of the device, the first interferometer arrangement has a first path section along which the first light beam propagates. The second interferometer arrangement has a second path section along which the second light beam propagates. The first path section and the second path section spatially overlap. The first light beam and the second light beam co-propagate in the region of the first and second path sections. They can interfere with each other in the region of those path sections. In particular, it is provided that the first light beam and the second light beam are capable of interference. This means that they have a coherence length that is longer than any path differences, in particular in the region of the first and second path sections.

[0025] An optical coupling between the first interferometer arrangement and the second interferometer arrangement is possible by means of a first path section and a second path section, which can be spatially overlapping or can be configured as a component of the first interferometer arrangement and also as a component of the second interferometer arrangement. Thus, the first interferometer arrangement can be optically coupled to the second interferometer arrangement. A partial region of a path of the first light beam of the first interferometer arrangement overlaps with the partial region of a path of the second interferometer arrangement.

[0026] According to a further embodiment, the first path section and the second path section are delimited by an at least partially reflecting mirror. The partially reflecting mirror is a component of the first interferometer arrangement and the second interferometer arrangement. In some embodiments, the partially reflecting mirror delimiting the first and second path sections can also be designed as a fully reflecting mirror. A partially reflecting mirror enables the implementation of a transmission geometry of the structure or topology of the device. By means of the partially reflecting mirror, for example, the first interferometer arrangement can be separated from the second interferometer arrangement or spatially decoupled from it.

[0027] According to a further embodiment, the at least partially reflecting mirror forms the first destination and / or the second source location. The first destination and the second source location can, for example, coincide. This is advantageous not only for a particularly simple design, but also for particularly efficient signal processing. The implementation of the first source location and / or second destination using at least one partially reflecting mirror enables, in particular, seamless path stabilization from the first source location to the second destination. In this respect, end-to-end path stabilization can be realized using the at least partially reflecting mirror, which coincides with or forms the first source location and the second destination location.

[0028] According to a further advantageous embodiment, the second light beam branches off from the first light beam by means of a beam splitter. The second light beam can be generated, in particular, by splitting the first light beam. This eliminates the need for an additional light source. The second light beam branched off from the first light beam can, for example, be fed to a light-amplifying element, such as a fiber amplifier, to amplify the optical power of the second light beam.

[0029] This proves advantageous for signal transmission over longer distances, such as several kilometers or even several hundred kilometers. The provision of a non-reciprocal element, such as an amplifier, in the beam path of the second light beam poses no obstacle to the phase-stable transmission of the optical signal from the first source location to the second destination location. Typically, the branch for the second light beam is located upstream of the second interferometer arrangement. Any optical amplifier is also located upstream of the second interferometer arrangement.

[0030] In this respect, according to a further embodiment, an optically non-reciprocal element can be provided in the beam path of the second light beam. It is typically located outside the second path section along which the first and second light beams of the first and second interferometer arrangements propagate. The non-reciprocal element can be implemented, for example, as an optical amplifier, such as an erbium-doped optical fiber amplifier, a semiconductor optical amplifier, or a Brillouin amplifier.

[0031] According to a further embodiment, an optical amplifier is arranged in the beam path of the second light beam. This is typically implemented as a non-reciprocal optical element. The optical amplifier can be, for example, an erbium-doped optical fiber amplifier, a semiconductor optical amplifier, or a Brillouin amplifier.

[0032] According to a further embodiment of the device for phase-stable transmission of optical signals, a third light beam with a fixed phase relationship to the second light beam is diverted or branched off from the second light beam by means of a frequency converter. The third light beam spatially overlaps with the second light beam at least in some regions. However, it has a different frequency than the first light beam and / or the second light beam. The frequency of the third light beam can, for example, be double the frequency of the second light beam. The conversion efficiency of the frequency converter is typically less than 100%. This means that only a portion of the second light beam can be converted into the third light beam at a different frequency.The second light beam and the third light beam copropagate downstream of the frequency converter, so that, for example, an interferometric detection and evaluation of the copropagating beam of the second and third light beams can be carried out solely or predominantly based on the component of the second light beam.

[0033] According to a further development, at least one partial beam of the first light beam and at least one partial beam of the second light beam propagate along a coupling path that leads to a third detector, by means of which a phase relationship between the first light beam and the second light beam can be determined in the region of the coupling path. For example, the coupling path can be used to relate a phase relationship in the region of the first interferometer arrangement to a phase relationship in the region of the second interferometer arrangement.

[0034] The implementation of a coupling path in combination with a third detector makes it possible to implement a non-reciprocal optical element in the beam path of the second light beam, or upstream of the second interferometer. Any phase or frequency instabilities in the region of the second light beam caused by the non-reciprocal optical element can be qualitatively and quantitatively detected by the coupling path and the third detector provided there, and effectively used to control the at least one actuator. Any phase or frequency instabilities caused by the non-reciprocal optical element can be compensated in this way.

[0035] According to a further embodiment, it is further provided that the at least one control loop is signal-coupled to the third detector and is further configured to set, regulate, or fix the fixed phase relationship between the phase of the first light beam at the first source location and the phase of the second light beam or the third light beam at the second target location by means of the at least first actuator, taking into account or using signals from the third detector. Typically, the control loop is configured to process signals from the first detector, the second detector, and signals from the third detector to control the at least first actuator in order to set, regulate, or fix the phase of the second light beam or the third light beam at the second target location with respect to the phase of the first light beam at the first source location.

[0036] According to a further embodiment of the device, it has a second light source that generates the second light beam. The second light source serves to generate the second light beam. The second light source is therefore arranged upstream of the second interferometer arrangement. The second light beam emitted by the second light source can be coupled into the second interferometer arrangement. The first and second light sources can, but do not have to, have the same frequency. However, the first and second light beams, which are generated by different light sources, namely the first and second light sources, must be capable of interfering with each other at least in the region of the coupling path.

[0037] The second light beam is typically provided with its own actuator, by means of which the phase of the second light beam can be adjusted as needed. In particular, a signal from the third detector can be used to control the actuator of the second light source. Thus, the phase and / or frequency of the second light source can be precisely adjusted to the phase and / or frequency of the first light beam of the first light source.

[0038] According to a further embodiment of the device, it is provided that the control circuit is designed to process signals from a plurality of detectors and to control at least one actuator arranged in the beam path between the first source location and the second target location.

[0039] According to a further embodiment, the first interferometer arrangement has a first polarizing beam splitter. The second interferometer arrangement has a second polarizing beam splitter. The polarizing beam splitters are both arranged in the beam path of the first light beam and / or in the beam path of the second light beam. Thus, the first polarizing beam splitter can be arranged in the beam path of the first and second light beams. Likewise, the second polarizing beam splitter can be arranged in the beam path of the first and second light beams.By means of the first and second polarizing beam splitters, in particular in combination with further elements in the beam path of the first light beam and / or the second light beam that rotate the polarization plane or filter polarization, crosstalk between the first interferometer arrangement and the second interferometer arrangement, and thus unwanted feedback of optical signals from the direction of the target location to the source location, can be suppressed.

[0040] Appropriate polarization techniques make it possible, for example, to prevent any light intensity of the second light beam from propagating in the direction of the first light source, or vice versa, to prevent any light intensity from propagating from the first light source to the second light source, or to the non-reciprocal optical element, and thus to prevent any instabilities from occurring there.

[0041] The polarization techniques described here also effectively prevent light from the second interferometer array from reaching one or more upstream detectors. Otherwise, too many beat signals or beat frequencies would occur, which would be quite difficult to separate from each other. Furthermore, this prevents the potential saturation of an amplifier associated with the detector, which would otherwise lead to the generation of various mixed frequencies and harmonic multiples thereof.

[0042] According to a further embodiment of the device, the second interferometer arrangement has a first target path and at least one second target path, which branches off from the first target path by means of a beam splitter. The second target location is arranged in the first target path. Furthermore, a further second target location is located in the second target path. At least one of the first and second target paths has its own actuator, signal-coupled to the second detector, for adjusting, regulating, or fixing the phase of the second light beam at the downstream second target location relative to the phase of the first light beam at the first source location.

[0043] The device is by no means limited to the branching of just a first and a second target path. Far more target paths can be branched off from the first target path. Implementing a branch from the second target path enables division, such as a type of star-shaped distribution of the reference from the first source location to several second target locations. The first target path and the second target path, as well as additional target paths, can in principle be aligned parallel to one another or optically coupled in parallel. Each of the target paths can be provided with its own actuator in order to adjust and regulate any path instabilities in the area of ​​the respective branch up to the respective target location as required. The actuator is, of course, also implemented in the control loop.

[0044] According to a further embodiment of the device for phase-stable transmission of optical signals, it comprises a further light source for generating at least one further light beam. The further light beam is frequency-shifted relative to the first light beam and superimposed on the first light beam at a detector. This detector can generate a beat signal that is spectrally separable by means of an RF signal processing unit of the at least one control loop. Based on this beat signal, the fixed phase relationship between the phase of the first light beam at the first source location and the phase of the second light beam at the second destination location can be adjusted, controlled, or fixed using the at least one actuator.

[0045] In this way, frequency fluctuations of multiple laser light sources, namely the first light source and the additional light source, can be reduced on average, for example in the form of ultra-stable lasers. The light beams of the first light source and the additional light source exhibit a frequency offset from each other. This can be achieved, for example, by using suitable frequency shifters before superimposing the corresponding light beams.

[0046] By means of a weighted addition or subtraction of the respective frequency components of the resulting beat signal, a signal can be generated for at least the first actuator, so that a type of weighted averaged fluctuation of several light sources can be used as a source signal for the source location and can be transmitted accordingly to one or more target locations.

[0047] By using multiple light sources with a frequency offset from each other, and by electronically evaluating a resulting beat signal in the radio frequency range, a composite source signal can be used at the first source location, which, in isolation, exhibits even greater stability than the individual light sources. For example, long-term drift of a resonator can be eliminated, with the control signal being determined by comparison with a long-term stable frequency standard, such as an atomic clock.

[0048] According to a further embodiment, the first interferometer arrangement and / or the second interferometer arrangement can be implemented using fiber optics. In this case, one or more optical paths, for example from the first source location to the first target location or from the second source location to the second target location, and thus also the entire optical path from the first source location to the second target location, can be implemented using fiber optics. The optical fibers can be configured as non-polarization-maintaining fibers. Using the control loop, the first detector, the second detector, and the actuator, it is possible to effectively compensate for the fiber-induced phase instabilities between the source and target locations through active control.

[0049] It is possible that a comparatively long fiber path is implemented in the optical path of the first interferometer arrangement and / or the second interferometer arrangement, which can be several kilometers, such as 50 km, 100 km or more.

[0050] According to a further embodiment, an amplifier unit is provided which has a sealed, i.e., moisture- and / or dust-proof housing. Major parts and components of the first and / or second interferometer arrangement can be arranged in the housing. In particular, a non-reciprocal optical element, for example in the form of a light source or an optical amplifier for the second light beam, as well as detectors, control circuits, and actuators downstream in the respective optical path, can be arranged in the housing.

[0051] In this respect, and apart from a corresponding power supply, this system can be designed to be completely self-sufficient and integrated into a submarine and / or long-distance cable connection. This allows for relatively long paths and large distances between the source and destination locations, for example, several hundred kilometers. Between a first source location and an nth destination location, corresponding amplifier units can be provided at regular intervals, approximately every 50 to 200 kilometers.

[0052] Finally, according to a further aspect, the present invention relates to a method for the phase-stable transmission of optical signals. The method can preferably be implemented and realized with a previously described device for the phase-stable transmission of optical signals. In this respect, all features, advantages, properties, and effects previously described and explained with regard to the device for the phase-stable transmission of optical signals also apply equally to the method; and vice versa.

[0053] For a basic implementation, however, the method does not necessarily have to be operated with the device described here. In other words, it is not limited to the implementation of the device described above. The method for the phase-stable transmission of optical signals comprises several method steps that can run simultaneously or continuously. In a first method step, for example, a first light beam is generated using a first light source. The first light beam is then coupled into a first interferometer arrangement, which comprises a first source location and a first destination location spatially separated therefrom.

[0054] A first detector is used to determine the phase of the first light beam at the first source location relative to the phase of the first light beam at the first target location. A second light beam is coupled into a second interferometer arrangement, which is optically coupled to the first interferometer arrangement and which comprises a second source location and a second target location spatially separated therefrom. Similar to the first interferometer arrangement, a second detector is used in the second interferometer arrangement to determine the phase of the second light beam at the second source location relative to the phase of the second light beam at the second target location.

[0055] By means of at least one first actuator of a control circuit arranged in the beam path of the first light beam and / or the second light beam, which is signal-coupled to the first detector and to the second detector, a fixed phase relationship between the phase of the first light beam at the first source location and the phase of the second light beam at the second destination location is then set, regulated or fixed.

[0056] As already explained, the method described here can be implemented with a previously described device for phase-stable transmission of optical signals. Advantageous embodiments, effects, effects, and features previously described for the device also apply equally to the present method, which is why reference can be made to the present explanations of the device for further method steps.

[0057] In summary, the device described here and the method in question can solve previously unsolved problems in the transmission of optical carrier or differential frequencies over a transmission link between two or more locations. In particular, the device enables true end-to-end stabilization between a source location and a destination location, allowing for unlimited scalability and cascading depending on specific requirements, as well as the possibility of implementing non-reciprocal optical elements for path stabilization. Phase and frequency distribution can also be enabled over comparatively long distances. This enables, in particular, phase- and / or frequency-stable overseas and / or intercontinental transmission of highly stable or highly accurate frequencies.

[0058] The project 18SIB05 ROCIT, which led to the present invention and the present patent application, was funded by the EMPIR programme of the European Union's Horizon 2020 research and innovation programme, co-financed by the participating states.

[0059] Further objectives, features, and advantageous applications are explained below using specific embodiments with reference to the drawings. Herein: Fig. 1 is a schematic representation of an interferometer arrangement, Fig. 2 is a schematic representation of a device for the phase-stable transmission of optical signals according to a first embodiment of the invention, Fig. 3 is a further embodiment of the device for the phase-stable transmission of optical signals, Fig. 4 is a schematic representation of a control loop, Fig. 5 is a further embodiment of the device, Fig. 6 is a schematic representation of a control loop, Fig. 7 is a schematic representation of a further embodiment of the device for the phase-stable transmission of optical signals, Fig. 8 is a further embodiment of a device for the phase-stable transmission of optical signals with multiple target paths, Fig. 9 is a further embodiment of such a device with a further light source, in particular for generating a composite control signal, Fig.Fig. 10 shows a further embodiment with a frequency converter as a non-reciprocal optical element, Fig. 11 shows a flowchart of a method for the phase-stable transmission of optical signals, and Fig. 12 shows a further embodiment of the device in which the optical paths of the interferometer arrangements are implemented using fiber optics. Detailed description

[0060] In Fig. 2 A first exemplary embodiment of a device 1 for the phase-stable transmission of optical signals is shown schematically. The device 1 comprises a first interferometer arrangement 20 and a second interferometer arrangement 120. The device 1 further comprises a first light source 10. The light source 10 is designed to generate a first light beam 11. This propagates from the light source 10 via a beam splitter 63 to a mirror 70, which is implemented as the first source location Q1. Furthermore, the first light beam 11, or a partial beam thereof, propagates from the beam splitter 63 via further mirrors 60, 61 and a polarizing beam splitter 51 to a further mirror 50, which is implemented as the first target location Z1. The beam splitter 63 forms a Michelson interferometer with the mirrors 50, 70 provided at the source location Q1 and target location Z1.

[0061] The optical path between the beam splitter 63 and the first source location Q1 functions as a reference path. The path between the beam splitter 63 and the mirror 50, thus the first target location Z1, functions as a signal path. The partial beams of the first light beam 11 reflected at the source location Q1 and the target location Z1 recombine at the beam splitter 63 and are directed from there onto the first detector 21. The first detector 21 can detect an interference signal, thus a beat signal, which provides information about the phase of the first light beam 11 at the first source location Q1 relative to the phase of the first light beam 11 at the first target location Z1.

[0062] A control loop 30 is also provided. The control loop 30 is signal-coupled to the first detector 21 and comprises an actuator 31 arranged in the beam path of the first light beam 11. The first actuator 31 can be implemented, for example, as a frequency or phase modulator, such as an acousto-optical or electro-optical modulator. Any phase fluctuations between the phase of the first light beam at the first source location Q1 and the phase of the first light beam 11 at the first target location Z1 can be signal-detected by the first detector 21. By means of a control loop coupled to the first detector 21 or a corresponding control loop 30, any phase fluctuation can be compensated by appropriately controlling the first actuator 31 such that the phase of the first light beam 11 at the first target location Z1 is in a fixed relationship to the phase of the first light beam 11 at the first source location Q1.

[0063] The device 1 described here further comprises a second interferometer arrangement 120. This has a second source location Q2 and a second target location Z2 spatially separated therefrom. In the present exemplary embodiment, the second source location Q2 coincides with the first target location Z1, thus with the mirror 50. The second interferometer arrangement 120 can also be implemented as a Michelson interferometer. Other interferometer designs, such as a Mach-Zender interferometer, are equally conceivable. A second light beam 111, which is coupled into the second interferometer arrangement 120 via a polarizing beam splitter 52, is split at the polarizing beam splitter 52. A portion of the second light beam propagates toward the second target location Z2, which is implemented as a mirror 71.Another part of the second light beam 111 is deflected at the polarizing beam splitter 52 in the direction of the first destination location Z1, thus to the second source location Q2.

[0064] The partial beams of the second light beam 111 reflected from the second source location Q2 and the second destination location Z2 recombine at the polarizing beam splitter 52 and copropagate to the second detector 121. Behind the polarizer 53, the phase of the second light beam 111 at the second source location Q2 relative to the phase of the second light beam 111 at the second destination location Z2 can be determined by means of the second detector 121. The second interferometer arrangement 120 is equipped with its own, thus with a second control loop 130. Signals from the second detector 121 can be electronically processed and used to control a second actuator 131 in the beam path of the second light beam 111. The second actuator 131 is located upstream of the second destination location Z2. By means of the second control loop 130, any fluctuations, i.e.Phase and / or frequency instabilities of the second light beam 111 are interferometrically detected and compensated accordingly by targeted control of the actuator 131, which is also designed as a modulator.

[0065] In particular, it is provided that the first interferometer arrangement 20 and the second interferometer arrangement 120 are optically coupled to one another. The optical coupling can be implemented in different or diverse ways. On the one hand, the first light beam 11 can propagate along a first path section P1, which spatially overlaps at least partially or in sections with a second path section P2, along which the second light beam 111 propagates. In the illustrated embodiment, such first and second path sections P1, P2 are located between the polarizing beam splitter 51 and the mirror 50, thus between the beam splitter 51 and the first target location Z1 and the second source location Q2.

[0066] Furthermore, it can be provided that the first light beam 11 and the second light beam 111 propagate along a coupling path K, which leads to a further detector, thus to a third detector 221. By means of the third detector 221, a phase relationship between the first light beam 11 and the second light beam 111 in the region of the coupling path can be determined. The signals of the third detector 221 can be processed by means of a third control loop 230, which includes a third actuator 231. In the embodiment of the Fig. 2 The third actuator 231 is arranged in the beam path of the first light beam 11. However, it could also alternatively be arranged in the beam path of the second light beam 111.

[0067] Any fluctuations or frequency and / or phase instabilities that may occur in the region of each individual interferometer arrangement 20, 120 can be interferometrically detected and / or quantitatively measured with the first detector 21 and the second detector 121. Coupling between the first and second interferometer arrangements 20, 120 can be measured, in particular, by implementing the coupling path K and by interferometrically superimposing partial beams from the first interferometer arrangement 20 and the second interferometer arrangement 120.

[0068] By means of the third detector 221 and an actuator 231 coupled thereto or controllable by the third detector, a fixed phase relationship can ultimately be established between the first light beam 11 in the region of the first interferometer arrangement 20 and the second light beam 111 in the region of the second interferometer arrangement 120. Thus, a fixed phase relationship between the phase of the first light beam 11 at the first source location Q1 and the phase of the second light beam at the second target location Z2 can ultimately be set, regulated, or fixed.

[0069] In particular, since the first target location Z1 coincides with the second source location Q2, a multi-stage and thus cascading arrangement of several such interferometer arrangements 20, 120 can be realized. If necessary, the second target location Z2 can also function as a third source location Q3, so that the optical signals can also be transmitted accordingly to a further third target location Z3, as is the case, for example, in the embodiment according to Fig. 8 is the case.

[0070] In the Fig. 2 In the embodiment shown, a non-reciprocal optical element 40 can also be arranged in the beam path of the second light beam 111. This can be implemented, for example, as an optical amplifier 41. The non-reciprocal optical element 40 can, for example, be fed by the first light beam 11 and thus generate an amplified second light beam 111. Thus, the second light beam 111 can be the result of an amplification of a branch of the first light beam 11.

[0071] Advantageously, the non-reciprocal optical element 40 is provided and arranged outside both the first interferometer arrangement 20 and the second interferometer arrangement 120. Therefore, the non-reciprocal optical element cannot have a detrimental effect on the phase-stable transmission of the optical signal. The second light beam 111, which in the exemplary embodiment Fig. 2 emanating from the non-reciprocal optical element 40, can be coupled to the first light beam 11 with regard to its phase and / or frequency via the coupling path K and by means of the third detector 221.

[0072] This connection is also shown again in the separate presentation according to Fig. 1 Instead of the polarizing beam splitter 51 made of Fig. 2 A conventional beam splitter 65 is provided. The first light beam 11 and the second light beam 111 copropagate along the coupling path K. There, they can interfere with each other. A corresponding interference pattern, or a temporal beat of the resulting light intensity formed by the beam interference, can be determined at the third detector 221 and used to control an actuator 231. In this respect, the first light beam 11 and the second light beam 111 can be coupled to each other in a phase-locked manner. The actuator 231 can alternatively also be implemented in the beam path of the second light beam 111.

[0073] In the example of Fig. 2 Furthermore, crosstalk of unwanted signals between the individual partial paths is suppressed by a polarization technique. In particular, it is provided that the polarization of the second light beam 111 is adjusted such that the largest portion of the light beam 111 is transmitted through the polarizing beam splitter 52. Only a small portion of the second light beam 111 is reflected at the beam splitter 52 in the direction of the first beam splitter 51. The portion of the second light beam 111 reflected at the beam splitter 52 is tilted by a predetermined angle with respect to its plane of polarization by means of a delay element 56, which can be designed, for example, as a λ / 2 or λ / 4 plate.

[0074] As a result, a certain portion of the second light beam 111, which strikes the further polarizing beam splitter 51, can propagate through it to the second source location Q2. There, the corresponding signal portion is reflected and propagates back to the polarizing beam splitter 52. The portion of the second light beam 111 transmitted through the polarizing beam splitter 52 and previously reflected by Q2 propagates through a polarizer 53 and is detected in the second detector 121.

[0075] The portion of the second light beam 111 transmitted at the polarizing beam splitter 52 propagates through a polarization rotator 55, which can be implemented, for example, as a Faraday rotator, and which rotates the polarization plane of the incident light by approximately 45° in the same direction, regardless of the beam direction. The transmitted portion of the second light beam 111 then propagates through the actuator 131 and is reflected back at the second target location Z2, which is implemented as a mirror 71. On the return path, the polarization plane of the reflected second light beam 111 is again rotated accordingly, so that the light previously transmitted through the polarizing beam splitter 52 is now almost completely reflected by the polarizing beam splitter 52 and redirected toward the second detector 121.By means of the polarizer 53, the intrinsically non-interference-capable portions of the components of the second light beam 111 transmitted and reflected at the polarizing beam splitter can be polarization-filtered in such a way that portions of the same polarization of the two superimposed beam parts are superimposed at the detector 121 in a manner capable of causing interference.

[0076] In this way, an interference signal can be measured by the second detector 121, which provides information about the relative phase relationship between the second light beam at the second source location Q2 and the second destination location Z2. The rotator 55 can also ensure that all of the power of the second light beam 111 reflected at the second destination location Z2 is completely or at least predominantly reflected by the polarizing beam splitter 52 and directed to the detector 121. In this way, unwanted feedback of light intensity into the optical amplifier 41 can be reduced to a minimum, or completely suppressed or eliminated. This contributes to the stability of the operation of the optical amplifier 41.

[0077] A portion of the light of the first light beam 11 transmitted through the polarizing beam splitter 51, as well as a portion of the second light beam 111 reflected by the same beam splitter 51, propagates along the coupling path K. The partial beams of the first light beam 11 and the second light beam 111, which originally follow the beam splitter 51 due to their different polarization directions, can be made capable of interference by filtering at the polarizer 54 aligned at a predetermined angle. The appropriate position of the polarizer 54, implemented, for example, as a linear polarizer, makes it possible to project the respective polarization components from the respective superimposed light beams onto one another in the region of the coupling path K, so that interference between the first light beam 11 and the second light beam 111 can be measured with the third detector 221.

[0078] The medium in which the partial beams propagate between the beam splitter 51 and the detector 221 is isotropic. This means that neither linear nor circular dichroism or corresponding birefringence is present there. The fields of the first and second light beams 11, 111 therefore do not experience any relative phase fluctuations to one another. The measurement of a beat phase at the third detector 221 thus corresponds to the relative phase at all other locations behind the polarizing beam splitter 51. Likewise, the optical paths between the polarizing beam splitters 51, 52, the polarizers 53, 54, and the first target location Z1 should also be isotropic, since otherwise, for example, temperature dependence in the birefringence could lead to undesirable phase fluctuations between the partial beams polarized approximately perpendicular to one another.

[0079] Isotropic fluctuations in these optical paths, for example, due to pressure fluctuations, are eliminated by the active stabilization provided by the control loop 30, 130. The isotropic paths are typically implemented as free-space optics. All other paths can be implemented, for example, as fiber optics.

[0080] The second interferometer arrangement 120, with the mirrors 50, 71 in the region of the second source location Q2 and the second target location Z2, forms a Michelson interferometer. Due to the described polarization technique or polarization selection, ideally no light from the second interferometer arrangement 120 returns to the first interferometer arrangement 20. Therefore, the partial optical paths formed by the first interferometer arrangement 20 and the second interferometer arrangement 120 are essentially separated from each other. Crosstalk can only occur due to imperfect polarization extinction at the polarizing beam splitter 51.

[0081] Typically, the polarization of the first light beam is adjusted so that a portion is transmitted through the polarizing beam splitter 51 to the third detector 221. The other portion is reflected to the target location Z1, returns from there with the same polarization, and is therefore reflected at the polarizing beam splitter 51 via the quasi-reciprocal arm of the first interferometer arrangement 20 back to the source location Q1.

[0082] One advantage of this approach is that the assembly between elements 31 and 55 can be constructed as a self-contained, locally operated unit that acts like a unidirectional isolation amplifier. This is an essential prerequisite for use in intercontinental optical fiber links for the transmission of highly stable and highly accurate optical frequencies and corresponding optical signals.

[0083] The further embodiment of the Fig. 3 is conceptually very similar to that of the Fig. 2 , which is why only the differences and the resulting effects will be discussed here. The first target location Z1 is implemented as a partially transparent mirror 50 and also defines the second source location Q2. The 45° rotator 55 is located between the two polarizing beam splitters 51, 52. The design according to Fig. 3 differs from that according to Fig. 2 furthermore, in the manner in which the first and second light beams 11, 111 are guided to the first destination Z1, and how the light is extracted for the detector 121 and for the path to the second destination Z2.

[0084] In contrast to Fig. 2 the second light beam 111 emitted by the non-reciprocal element 40 is not split at the polarizing beam splitter 52, but transmits it unhindered in the forward direction and then reaches the polarizing beam splitter 51. It also transmits a large part of this and reaches the partially reflecting mirror 50. The advantageously larger portion of the light beam 111 transmitted there in the direction Z2 is reflected at the mirror 71, or the second target Z2, and superimposed at the partially reflecting mirror with the portion of the beam 111 reflected there. The rays thus superimposed propagate back through the polarizing beam splitter 51. By means of the rotator 55, the polarization plane is rotated on the return path such that the light reflected at the target Z2 and at the partially reflecting mirror 50 is efficiently reflected at the polarizing beam splitter 52 and reaches the detector 121.There, that part of the second light beam 111 which was reflected on the one hand at the partially reflecting mirror 50 of the first target location Z1, which is identical to the second source location Q2, and that part of the second light beam 111 which was reflected at the second target location Z2 can be superimposed interferometrically so that a corresponding interference signal can be detected.

[0085] The coupling path K also extends here, similar to Fig. 2 , between the polarizing beam splitter 51 and the third detector 221. A polarizer 54 is again provided between the beam splitter 51 and the third detector 221 in order to make the components of the partial beams, which are reflected on the one hand and transmitted on the other hand and which otherwise do not interfere with each other, capable of interference and thus to enable an optical coupling between the first interferometer 20 and the second interferometer 120.

[0086] In the example of Fig. 4 It is schematically shown that the signals from the third detector 221 can also be used for the phase-stable coupling of another light source 110. The second light source 110 is provided for generating the second light beam 111. It can have an integrated actuator 231, which can be coupled via a third control loop 230 using signals from the third detector 221 for the phase-stable connection of the light source 110 to the light field of the first light source 10.

[0087] In the example of Fig. 5 Such an application of a light source 110 is in the embodiment of the Fig. 2 integrated. Here, the non-reciprocal optical element 40 is replaced by a separate light source 110. The first interferometer arrangement 20 also extends here via a Michelson interferometer from the first source location Q1 to the first target location Z1. Instead of an optical amplifier 41 in the embodiment of the Fig. 2 is now in the embodiment of the Fig. 5 a light source 110 is provided which is phase-locked to the third detector 221 via the third control loop 230.

[0088] This can typically be implemented as a laser. The beat signal, which is detectable at the third detector 221, can be used for a so-called offset frequency phase lock of the laser frequency of the light source 110, whereby the second light beam can be phase-locked to the first light beam 11 with a frequency offset. Here, too, the mirror 50 functions equally as the first destination Z1 and the second source Q2. Crosstalk between the individual partial paths is also prevented here by implementing the previously described Fig. 2 suppressed by the polarization techniques described.

[0089] In this design of the Fig. 5 It is particularly advantageous that a phase-locked laser 110 enables signal regeneration compared to a broadband optical amplifier 41. Unwanted parasitic spectral components in the field of the first light beam 11 are filtered out outside the control bandwidth of the phase-locked laser connection.

[0090] In Fig. 6 A somewhat simplified example of a control loop 30 is shown schematically. Starting from a detector 21, the fluctuations measured there, such as a beat signal, can be mixed with a reference, such as the reference frequency of a reference oscillator 33, by means of a phase comparator 35. The low-frequency component of the output signal of the phase comparator is proportional to the phase difference between its input signals and can then be used by means of a voltage-controlled oscillator 34 to control the actuator 31 arranged in the beam path of the light beam 11.

[0091] The measured fluctuations and the electrical signal derived therefrom can be used by RF signal processing to control the actuator 31 in such a way that resulting frequency and / or phase fluctuations in the area between the source location Q1 and the target location Z1 can be actively eliminated.

[0092] In the example of Fig. 7 is based on the example of the Fig. 5 It has been shown that the device 1 for the phase-stable transmission of optical signals can, in principle, also be equipped with only a single actuator 31. In this case, the signals from the first detector 21 and the second detector 121 are processed in such a way that only a single actuator 31 needs to be supplied with a corresponding control signal in order to compensate for the overall phase and / or frequency instabilities occurring in the system.

[0093] In terms of control technology, it is provided to couple the first detector 21 and the second detector 121 to a frequency mixer 32. The frequency mixer 32 is signal-coupled and connected to the first detector 21 and the second detector 121. An output of the frequency mixer 32 leads to a phase comparator 35, which, as already described above, Fig. 6 described, which compares the signal provided by frequency mixer 32 with the signal of a reference oscillator. From the comparison, a control signal for actuator 31 can then be generated using a voltage-controlled oscillator 34, and actuator 31 can be controlled accordingly.

[0094] The embodiment of the Fig. 8 is to that of the Fig. 2 It comprises a further cascade and thus a further interferometer arrangement 320, which is connected to the second interferometer arrangement 120. On the one hand, this example shows that the device for phase-stable transmission 1 is cascaded multiple times and can therefore be extended in length as desired. On the other hand, this example also shows a segmentation or the implementation of multiple target paths 122, 124, 126, which enable a division or distribution of the optical signal to be transmitted.

[0095] In the example of Fig. 8 is similar to the example of Fig. 2 a generation of the second light beam 111 by means of an optical amplifier 41, thus by means of a non-reciprocal optical element 40. Unlike in the example of Fig. 2 Here, the third detector 221 is coupled to the third control circuit 230, whose third actuator 231 is located in the beam path of the first light beam 11, which is branched off from the first light beam 11 via the beam splitter 60. Unlike in the example of the Fig. 2 The non-reciprocal arm of the Mach Zehnder interferometer is actively controlled here.

[0096] The destination Z2 is similar to the example of the Fig. 2 implemented. It is located downstream of the second actuator 131. A beam splitter 123 is provided upstream of the actuator 131, by means of which several target paths 124, 126 branch off from the original target path 122. The target path 126 can, in particular, be branched off from the target path 124 by means of a further beam splitter 125, 127. The target paths 124, 126 each have their own actuator 131', 131". These are coupled to the second detector 121 via control loops 130, 130', 130" that are not explicitly shown here but merely indicated.

[0097] Several parallel target locations Z2, Z2', Z2" are provided. Each of these target paths 122, 124, 126 can represent its own isolation amplifier stage. The individual actuators 131, 131', 131" can generate a type of coded beat frequency in the individual target paths 122, 124, 126, which can be correspondingly detected on a corresponding detector 121. By means of the coded beat frequencies, each of the individual actuators 131, 131', 131" can be specifically adjusted or controlled by appropriate spectral signal filtering at the detector 121.

[0098] In the design according to Fig. 8 The second target location Z2, which is implemented by a mirror 150, simultaneously functions as the third source location Q3. The second light beam 111, generated by the optical amplifier 41, passes through a polarizing beam splitter 151 to the second target location Z2 and is reflected from there back to the polarizing beam splitter 52. The portion of the second light beam 111 transmitted at the polarizing beam splitter 151 propagates through the polarizer 154 to a fourth detector 321.

[0099] A part of the beam reflected from the third source location Q3 transmits through the polarizing beam splitter 151 and through a downstream delay element 156. Similar to the previous design of the Fig. 5 A second light source 110 is provided, which is phase-locked to the fourth detector 321 of the coupling path K' via a fourth control loop 330. The coupling path K' extends between the polarizing beam splitter 151 and the fourth detector 321.

[0100] In this respect, the third interferometer arrangement 320 essentially repeats the structure of the second interferometer arrangement 120. The polarizing beam splitter 152 corresponds to the beam splitter 52. The rotator 155 corresponds to the rotator 55. The polarizer 153 corresponds to the polarizer 53, and the further detector, ie, the fifth detector 421, functionally corresponds to the second detector 121.

[0101] From this, in turn, several control loops 430, 430', 430" originate, each of which is designed to control an actuator 431, 431', 431". Here, too, the further actuators 431, 431', 431" are located in individual target paths 322, 324, 326, which branch off from one another in parallel and each have or form a further target location Z3, Z3', Z3".

[0102] In the further embodiment of the Fig. 9 Several sources Q1, Q1' are provided, each fed, for example, by a light source 10, 10'. The light sources 10, 10' can be ultra-stable lasers. Overall, many more sources Q1, Q1', Q1" can be coupled to one another, each with a light source, in particular to compensate for fluctuations in the individual source locations Q1, Q1' or the corresponding light sources. The additional light source 10', together with an additional beam splitter 63' and an additional mirror 63', essentially forms an additional source location Q1'. Beam splitter 63', source location 1', and target location Z1 again form a type of Michelson interferometer.

[0103] The partial beams reflected from the target location Z1 and the further source location Q1' are detected at the further detector 21' and used by a further control circuit 30' to control an actuator 31'. The beams from the light sources 10, 10' are superimposed in a copropagating manner via the mirrors 60 and the beam splitter 61. The optical frequencies of the first light beams 11, 11' thus formed have a predetermined frequency offset from one another. A corresponding frequency shift can typically be adjusted with at least one of the actuators 31, 31'.

[0104] This results in multiple beat signals on the third detector 221. These can first be spectrally separated from one another. Furthermore, a second light beam 111 is branched off from the further light beam 11' by means of an optical amplifier 41. The co-propagating light beams 11, 11', 111 are superimposed in the region of the coupling path K. Corresponding fluctuations or interferences can be detected by the third detector 221. The corresponding beat signals, which each contain information about the fluctuations of the first light beams 11, 11' relative to the second light beam 111, can be spectrally separated and then used by a signal processing unit 236 by weighted addition or subtraction of the frequencies to generate a control signal for the third actuator 231.

[0105] In this way, weighted average fluctuations of the individual light sources 10, 10' acting on the second light beam 111 can be imposed. This ultimately allows a type of composite wanted signal, or a stabilization of the second light beam 111, to be generated and further distributed. The wanted signal, i.e., the second light beam 111, is more stable at the second target location Z2 than the individual light sources 10, 10' themselves, provided they fluctuate uncorrelated with each other.

[0106] In a similar way, the long-term drift of a resonator can be eliminated, except that the control signal is determined from a comparison with a long-term stable frequency standard, such as an atomic clock.

[0107] Specifically, the first light beam 11 can be defined as cos{ υ 2 t + φ 2 ( t )} and the further first light beam 11' as cos{ υ 1 t + φ 1 ( t)}, which have a fixed frequency or phase offset from each other. The resulting beat frequency at the detector 221 can be expressed as follows: f Schwebung = cos {( υ 2 - υ 1 ) t + φ 2 ( t ) - φ 1 ( t )}. The weighted addition / subtraction with a weighting factor k and the phase comparison with a reference signal cos { f ref t + φ ref } ultimately leads to a signal of the following structure at the actuator 231, or at the second light beam 111: cos{[(1 - k ) υ 1 + kυ 2 ± f ref ] t + (1 - k ) φ 1 ( t ) + kφ 2 ( t ) ± φ ref }, where (1 - k ) υ 1 + kυ 2 a weighted average frequency and ( 1 - k )φ 1 ( t ) + kφ 2 ( t ) represent a weighted average phase.

[0108] The optical output signal, i.e., the second light beam 111, thus receives a weighted average frequency and phase of the two input beams 11, 11' behind the actuator 231. This approach can, of course, also be implemented for more than just two light sources. The reference signal can be an RF signal, which can also be generated from the other beat signals for interferometric path stabilization if necessary.

[0109] In the further embodiment of the Fig. 10 , which is analogous to the design of the Fig. 2 If implemented, in addition to the optical amplifier 41, a frequency converter 42 can be provided and arranged in the beam path of the second light beam 11 upstream of the polarizing beam splitter 52. The efficiency of the wavelength conversion in the region of the frequency converter 42 is less than 100%. This means that only a partial beam, thus a third light beam 112 at a different frequency, is branched off from the second light beam 111, or frequency-converted.

[0110] After frequency conversion, the fundamental field 111 and the converted optical field 112 propagate along a common path and thus experience no relative phase fluctuations. For example, a first partial path can be stabilized using the fundamental light beam at the wavelength of the second light beam 111. Another path, namely the path between the first target location Z1 and the second target location Z2, can be stabilized using the wavelength-converted third light beam 112 at a different wavelength. The second detector 121 detects a beat signal generated solely by the second interferometer 120 and based on the third light beam 112 propagating in the second interferometer arrangement 120.

[0111] The optical or phase-technical coupling between the first interferometer arrangement 20 and the second interferometer arrangement 120 is still achieved by copropagation of the first light beam 11 and the second light beam 111 between the polarizing beam splitter 51 and the third detector 221.

[0112] In the flowchart according to Fig. 11 Finally, individual method steps of a method for the phase-stable transmission of optical signals are explained, which can be carried out with the device 1 described here. In a first step 100, a first light beam is generated by means of a first light source 10. In a further step 101, the first light beam 11 is coupled into a first interferometer arrangement 20, which comprises a first source location Q1 and a first target location Z1 spatially separated therefrom. In step 102, a phase of the first light beam at the first source location relative to the phase of the first light beam at the first target location is determined by means of a first detector. Similarly, in step 103, a second light beam is coupled into a second interferometer arrangement 120, which is optically coupled to the first interferometer arrangement 20 and which comprises a second source location Q2 and a second target location Z2 spatially separated therefrom.In step 104, a phase of the second light beam 111 at the second source location Q2 is determined by means of a second detector relative to the phase of the second light beam 111 at the second destination location Z2.

[0113] Finally, in step 105, a fixed phase relationship between the phase of the first light beam 11 at the first source location Q1 and the phase of the second light beam 111 at the second destination location Z2 is set, controlled or fixed by means of at least one first actuator 31, 131 of a control circuit 30, 130 arranged in the beam path of the first light beam 11 and / or in the beam path of the second light beam 111, which in turn is signal-coupled to the first detector 21 and to the second detector 121.

[0114] It goes without saying that the steps 100-105 described here successively run simultaneously and permanently in order to ensure a phase-stable transmission of optical signals from the first source location Q1 to at least the second destination location Z2.

[0115] The further embodiment of a device 1 for phase-stable transmission of optical signals according to Fig. 12 is conceptually identical, or largely analogous to the embodiment of the Fig. 2 Insofar as and unless otherwise specified, identical or similar reference symbols are used for functionally identical or similar components, which is why the description of the Fig. 2 can be referred to.

[0116] The embodiment of the Fig. 12 is implemented predominantly using fiber optics, possibly even entirely using fiber optics. The optical paths between the light source 10 and the first target location Z1, as well as the optical paths between the first target location Z1 and the second target location Z2, are provided by optical fibers 80. In particular, comparatively long fiber lengths 81, which may be several kilometers long, i.e., up to 50 km, up to 100 km, or more, can be implemented within the first and / or second interferometer arrangement 20, 120.

[0117] The comparatively long fibre lines, which can be several kilometres long, are Fig. 12 only shown schematically.

[0118] The beam splitters 63, 60, 52 and 51 are in the embodiment according to Fig. 12 have been replaced by fiber optic couplers 63', 60', 52', 51'. However, they fulfill a largely similar or identical function with regard to the deflection of the individual light beams 11, 111.

[0119] As already mentioned Fig. 2 As explained, a non-reciprocal optical element 40 in the form of an optical amplifier 41 is provided to generate the second light beam 111. The amplification of the first light beam 11 provided by the optical amplifier 41 to generate the second light beam 111 is advantageous for transmitting optical signals over long distances. The second light beam 111 is comparatively powerful compared to the first light beam 11.

[0120] In contrast to the embodiment according to Fig. 2 is according to the embodiment according to Fig. 12 It is provided that, in order to stabilize the optical path extending from the first source location Q1 to the first destination location Z1, instead of the first beam 11 reflected at the mirror 50, the portion 111' of the second light beam 111 reflected at the mirror 50 and thus at the first destination location Z1 and still relatively powerful is used.

[0121] The reflected portion 111' of the second light beam 111 is path- or phase-stabilized at the first target location Z1 relative to the first light beam 11. This ensures that the reflected portion 111' of the powerful second light beam 111 not only propagates to the second target location Z2, but also propagates in the opposite direction back to the first detector 21 via the first target location Z1 and the path along which the first light beam 11 propagates in the forward direction to the first target location Z1.

[0122] Thus, inevitable intensity losses on the return path from the first destination location Z1 to the first source location Q1 can be effectively compensated within the first and / or second interferometer arrangement 20, 120. This makes it possible to implement a comparatively long fiber path 81 in the first interferometer arrangement 20 and / or in the second interferometer arrangement 120.

[0123] The relative phase position of the light beams 11 and 111' in the optical paths between the fiber couplers 61', 51' can be adjusted as already described above for the embodiment of the Fig. 2 explained, are stabilized relative to each other by means of the third detector 221 and the third actuator 231.

[0124] A beat signal on the first detector 21, which results from the interference between the first beam 11 reflected by the mirror 70, or at the first source location Q1, and the beam portion 111' reflected by the mirror 50, or at the first target location Z1, can be distinguished from the other signals on this detector 21 by its beat frequency and separated by spectral filtering before further processing to generate a signal for controlling the actuator 31 or the actuators.

[0125] For beams 11 and 111' to interfere on detector 221, their polarizations must be projected onto one another. For this purpose, a polarization controller 94 can be provided in the first and / or second interferometer arrangement 20, 120, for example. Using polarization controller 94, the polarization plane of second light beam 111 can be rotated or changed as needed on its way to the first target location Z1. Polarization controller 94 can be coupled, for example, to the third detector 221 for signal transmission.

[0126] A signal for controlling the polarization can be derived from the amplitude of the beat signal of the third detector 221.

[0127] The embodiment according to Fig. 12 solves previously unsolved problems in the transmission of optical signals with highly stable optical frequencies over long-distance fiber connections, especially over intercontinental submarine cables.

[0128] Long-distance fiber connections, and in particular submarine cables, are predominantly designed as non-polarization-maintaining fibers. Due to the high losses on the long fiber links 81, the signals of the second light beam 111, which would otherwise be detectable on the first detector 21 and reflected at the second destination Z2, are already so strongly attenuated that they do not cause interference there. Therefore, in the embodiment according to Fig. 12 to a in the embodiment of the Fig. 2 The beam suppression implemented using the polarization technique described there can be dispensed with. For the embodiment of the Fig. 12 In this respect, no free beam propagation is required.

[0129] The second interferometer arrangement 120, as well as parts of the first interferometer arrangement 20, can be arranged largely within a housing 91 of an amplifier unit 90. The housing 91 can be implemented as a sealed, i.e., water- and dust-tight housing and, apart from a power supply, for example, for the optical amplifier 41, for the polarization controller 94, for the detectors 121, 221, for the associated control circuits 130, 230, and / or for actuators 131, 231, can be designed to be completely self-sufficient. This makes it possible to integrate the amplifier unit 90, for example, into a long-distance transmission system, such as a submarine cable arrangement.

[0130] For non-polarization-maintaining optical paths in the region of the first and / or second interferometer arrangement 20, 120, the source locations Q1, Q2 and the target locations Z1, Z2 are advantageously designed as Faraday mirrors. These typically have a Faraday rotator, such as a 45° Faraday rotator, in front of a mirror. List of reference symbols

[0131] 1 Device 10 Light source 11 Light beam 20 Interferometer arrangement 21 Detector 30 Control loop 31 Actuator 32 Frequency mixer 33 Reference oscillator 34 Voltage-controlled oscillator 35 Phase comparator 40 Non-reciprocal optical element 41 Optical amplifier 42 Frequency converter 50 Mirror 51 Beam splitter 52 Beam splitter 53 Polarizer 54 Polarizer 55 Rotator 56 Delay element 60 Beam splitter 61 Mirror 63 Beam splitter 64 Mirror 65 Beam splitter 70 Mirror 71 Mirror 80 Fiber 81 Fiber link 90 Amplifier unit 91 Housing 94 Polarization controller 110 Light source 111 Light beam 112 Frequency-converted light beam 120 Interferometer arrangement 121Detector 122Target path 123Beam splitter 124Target path 125Beam splitter 126Target path 127Beam splitter 130Control loop 131Actuator 150Mirror 151Beam splitter 152Beam splitter 153Polarizer 154Polarizer 155Rotator 156Delay element 221Detector 230Control loop 231Actuator 236Signal processing unit 320Interferometer arrangement 321Detector 322Target path 324Target path326Target path 430Control loop 431Actuator Q1, Q2Source location Z1, Z2Destination location P1, P2Path section KCoupling path

Claims

1. Device for the phase-stable transmission of optical signals comprising: - a first light source (10) for emitting a first light beam (11), - a first interferometer arrangement (20), into which the first light beam (11) is coupled and which comprises a first source location (Q1) and a first target location (Z1) spatially separated therefrom, wherein a phase of the first light beam (11) at the first source location (Q1) relative to the phase of the first light beam (11) at the first target location (Z1) is determinable by means of a first detector (21), - a second interferometer arrangement (120), which comprises a second source location (Q2) and, spatially separated therefrom, a second target location (Z2) for a second light beam (111), wherein a phase of the second light beam (111) at the second source location (Q2) relative to the phase of the second light beam (111) at the second target location (Z2) is determinable by means of a second detector (121), wherein the second interferometer arrangement (120) is optically coupled to the first interferometer arrangement (20) and wherein the first target location (Z1) spatially overlaps the second source location (Q2), and comprising - at least one closed-loop control circuit (30, 130) coupled to the first detector (21) and to the second detector (121) in terms of signalling and having at least one first actuator (31, 131) which is arranged in the beam path of the first light beam (11) and / or of the second light beam (111) and by means of which a fixed phase relationship between the phase of the first light beam at the first source location (Q1) and the phase of the second light beam (111) at the second target location (Z2) is adjustable, controllable or fixable.

2. Device according to Claim 1, wherein the first interferometer arrangement (20) has a first path section (P1), along which the first light beam (11) propagates, and wherein the second interferometer arrangement (120) has a second path section (P2), along which the second light beam (111) propagates, and wherein the first path section (P1) and the second path section (P2) spatially overlap.

3. Device according to Claim 2, wherein the first path section (P1) and the second path section (P2) are delimited by an at least partially reflective mirror (50), which is part of the first interferometer arrangement (20) and of the second interferometer arrangement (120).

4. Device according to Claim 3, wherein the at least partially reflective mirror (50) forms the first target location (Z1) and / or the second source location (Q2).

5. Device according to any of the preceding claims, wherein the first source location (Q1) and / or the second target location (Z2) have / has an at least partially reflective mirror (70, 71).

6. Device according to any of the preceding claims, wherein the second light beam (111) is branched off from the first light beam (11) by means of a beam splitter (61).

7. Device according to Claim 6, furthermore comprising an optically non-reciprocal element (40) in the beam path of the second light beam (111).

8. Device according to Claim 6 or 7, wherein an optical amplifier (141) is arranged in the beam path of the second light beam (111).

9. Device according to any of the preceding claims, wherein a third light beam (112) with a fixed phase relationship with respect to the second light beam (111) is derived from the second light beam (111) by means of a frequency converter (42), wherein the third light beam (112) at least regionally spatially overlaps the second light beam and has a different frequency from that of the first light beam (11) and / or the second light beam (111).

10. Device according to any of the preceding claims, wherein at least one partial beam of the first light beam (11) and at least one partial beam of the second light beam (111) propagate along a coupling path (K) leading into a third detector (221), by means of which a phase relationship between the first light beam (11) and the second light beam (111) in the region of the coupling path (K) is determinable.

11. Device according to any of the preceding claims, wherein the closed-loop control circuit (30, 130) is configured for processing signals of a plurality of detectors (21, 121, 221) and for controlling at least one actuator (31, 131) arranged in the beam path between the first source location (Q1) and the second target location (Z2).

12. Device according to any of the preceding claims, wherein the first interferometer arrangement (20) has a first polarizing beam splitter (51) and wherein the second interferometer arrangement (120) has a second polarizing beam splitter (52), which are both arranged in the beam path of the first light beam (11) and / or in the beam path of the second light beam (111).

13. Device according to any of the preceding claims, wherein the second interferometer arrangement (120) has a first target path (122) and at least one second target path (124, 126) branching off from the first target path (122) by means of a beam splitter (123, 125, 127), wherein the second target location (Z2) is arranged in the first target path (122) and a further second target location (Z2', Z2") is arranged in the second target path (124, 126) and wherein at least one of the first and second target paths (122, 124) has its own actuator (131', 131") coupled to the second detector (121) in terms of signalling and serving for adjusting, controlling or fixing the phase of the second light beam (111) at the downstream second target location (Z2', Z2") relative to the phase of the first light beam (11) at the first source location (Q1).

14. Device according to any of the preceding claims, which has at least one further light source (10') for generating at least one further light beam (11') which, frequency-shifted relative to the first light beam (11) and superimposed with the first light beam (11) at a detector (121, 221), generates a beat signal which is spectrally separable by means of an RF signal processing unit (236) of the at least one closed-loop control circuit (230) and on the basis of which the fixed phase relationship between the phase of the first light beam (11) at the first source location (Q1) and the phase of the second light beam (111) at the second target location (Z2) is adjustable, controllable or fixable by the at least one actuator (231).

15. Method for the phase-stable transmission of optical signals, wherein: - a first light beam (11) is generated by means of a first light source (10), - the first light beam (11) is coupled into a first interferometer arrangement (20), which comprises a first source location (Q1) and a first target location (Z1) spatially separated therefrom, - wherein a phase of the first light beam (11) at the first source location (Q1) relative to the phase of the first light beam (11) at the first target location (Z1) is determined by means of a first detector (21), - a second light beam (111) is coupled into a second interferometer arrangement (120), which is optically coupled to the first interferometer arrangement (20) and which comprises a second source location (Q2) and a second target location (Z2) spatially separated therefrom, wherein the first target location (Z1) spatially overlaps the second source location (Q2), - wherein a phase of the second light beam (111) at the second source location (Q2) relative to the phase of the second light beam (111) at the second target location (Z2) is determined by means of a second detector (121), and - wherein a fixed phase relationship between the phase of the first light beam at the first source location (Q1) and the phase of the second light beam (111) at the second target location (Z2) is adjusted, controlled or fixed by means of at least one first actuator (31, 131) - arranged in the beam path of the first light beam (11) and / or of the second light beam (111) - of a closed-loop control circuit (30, 130) coupled to the first detector (21) and to the second detector (121) in terms of signalling.