POLARIZATION MODULATOR WITH SEPARATE OPTICAL SECTIONS

Description

Technical area

[0001] This description relates to optical signal generation and signal transmission, in particular the transmission of information using polarization modulation. Specifically, the description relates to a modulator unit for modulating the polarization of an optical signal, an optical signal transmission path with such a modulator unit, and a system with such a modulator unit, for example in the form of a satellite. Technical background

[0002] Information can be transmitted using technical means by assigning a unit of information to a specific state of a carrier signal. The carrier signal is typically an electromagnetic wave from a specific spectral range. To imprint information on the carrier signal, a property of the carrier signal is changed. The change itself, or the state of the carrier signal after the change, corresponds to the information to be transmitted. The carrier signal is typically changed at intervals to transmit multiple units of information.

[0003] Depending on the carrier signal, various physical characteristics of the carrier signal can be considered as information carriers, such as amplitude, frequency, phase, and / or polarization. If one of these characteristics changes over time, this process is called modulation.

[0004] Various technical components are used in the signal processing path to process the carrier signal and incorporate the desired information into the carrier signal before the carrier signal is transmitted over the transmission path (wired or wireless).

[0005] The components used in the preparation and processing of the carrier signal serve to modulate the carrier signal accordingly so that the information to be transmitted is correctly applied to the carrier signal and is transmitted over the transmission path with as little interference and loss as possible.

[0006] However, it can happen that, in addition to the intended modulation (change in the carrier signal), unwanted changes to the carrier signal also occur, for example, due to parasitic effects or other undesired properties of the components involved in preparing the carrier signal. In this case, it is not always possible for a receiver of the carrier signal to determine which changes in the carrier signal are due to the intended modulation and which changes are caused by unwanted effects. This can negatively impact the signal quality.

[0007] Each of the following non-patent documents discloses a polarization modulator based on a Sagnac loop with a phase modulator and a Faraday rotator mirror: X. LIU, C. LIAO ET AL.: "Polarization coding and decoding by phase modulation in polarizing Sagnac interferometers," PROC. SPIE, Vol. 6827, November 26, 2007 (2007-11-26), pages 682701-1

[0008] COSTANTINO AGNESI ET AL: "All-fiber self-compensating polarization encoder for Quantum Key Distribution", ARXIV.ORG, CORNELL UNIVERSITY LIBRARY, 201 OLIN LIBRARY CORNELL UNIVERSITY ITHACA, NY 14853, March 2, 2019 (2019-03-02) Description

[0009] Based on this, the objective can be considered to reduce or eliminate the influence of unwanted effects of a modulator unit on a carrier signal modulated for transmission. In particular, the objective can be considered to prevent polarization errors in a polarization-modulated optical signal.

[0010] This problem is solved by the subject matter of the independent claim.

[0011] Further embodiments emerge from the dependent claims and from the following description.

[0012] According to one aspect, a modulator unit for modulating the polarization of an optical signal is specified. The modulator unit comprises a light source, a polarization component separator, a phase modulator (which is in particular a polarization-dependent phase modulator, but can also be a polarization-independent phase modulator), and a reflector. The light source is configured to output an optical signal and to emit it as an input signal in the direction of the polarization component separator, wherein the optical signal contains a first polarization component with a first polarization direction and a second polarization component with a second polarization direction.The polarization component separator is configured to separate the first polarization component from the second polarization component and to transmit the first polarization component via a first optical path and the second polarization component via a second optical path. A first signal propagation time is assigned to the first optical path, and a second signal propagation time is assigned to the second optical path, wherein the first signal propagation time differs from the second signal propagation time, i.e., the first signal propagation time is greater or less than the second signal propagation time.The polarization component separator is configured to introduce a temporal offset between the first polarization component and the second polarization component as a result of the difference between the first signal propagation time and the second signal propagation time, and to transmit the first polarization component and the second polarization component along a common optical path to the phase modulator. The phase modulator is configured to modulate a first phase of the first polarization component in the first polarization direction and to transmit the modulated first polarization component to the reflector.The reflector is configured to retroreflect both the first polarization component and the second polarization component toward the phase modulator, thereby changing the respective polarization (e.g., by 90°), so that the first polarization component with the first polarization direction receives the second polarization direction, and the second polarization component with the second polarization direction receives the first polarization direction. The phase modulator is configured to modulate a second phase of the retroreflected second polarization component in the first polarization direction and to transmit the retroreflected first polarization component and the retroreflected second polarization component to the polarization component separator.The polarization component separator is configured to eliminate the temporal offset between the retroreflected first polarization component and the retroreflected second polarization component and to output the retroreflected first polarization component and the retroreflected second polarization component without temporal offset via a common optical path. The modulator unit is configured to output the thus modulated optical signal as a polarization-modulated output signal.

[0013] The first polarization component and the second polarization component are separated in time when they first pass through the phase modulator. Therefore, the second polarization component can be modulated or unmodulated when it first passes through the phase modulator. Subsequently, both the first polarization component and the second polarization component are passed to the reflector.

[0014] The optical signal generated and output by the light source essentially represents a superposition of two orthogonal polarization components. The phase modulator is designed to apply a desired phase to each of the two polarization components.

[0015] In principle, the light source can be a light emitter (e.g., a laser) or the output of a light-guiding element. In one embodiment, the optical signal travels through the same optical path of the modulator unit twice in opposite directions. During the first pass, the optical signal, or rather its polarization components, travels from the light source through the polarization component separator and the phase modulator to the reflector. The reflector retroreflects the optical signal, rotating the phase of the optical signal by 90° (either +90° or -90°). For example, the reflector is implemented as a Faraday mirror, which is a retroreflector based on the Faraday effect. It contains a crystal to which a magnetic field is applied and which rotates the polarization depending on the direction of propagation of the magnetic field.For example, if horizontally polarized light hits the reflector, it is retroreflected as vertically polarized light. The reflector is therefore a combination of a Faraday rotator and a mirror. The optical signal first passes through the Faraday rotator, and the polarization of the light is rotated by 45° (+45° or -45°). The optical signal is then mirrored and passes through the Faraday rotator again, where its polarization is rotated by another 45° (in the same direction as in the first step). In total, the polarization of the retroreflected light differs from the polarization of the incident light by 90°.

[0016] Precisely because the optical signal or its polarization components pass through the phase modulator twice with reversed polarization directions and a temporal offset between the polarization components, the phase modulator first acts on the polarization component to be modulated. The delayed polarization component either passes through the phase modulator without actively influencing its phase or can be modulated during the first pass. With a polarization-dependent phase modulator, the second polarization component is not modulated initially (because the second polarization component has a different polarization direction than the first polarization component). However, with a polarization-independent phase modulator, the second polarization component can also be modulated during the first pass.

[0017] Below, some examples are described with reference to a polarization-dependent phase modulator. However, it should be understood that the principle described here can also be implemented with a polarization-independent phase modulator.

[0018] As a result, parasitic influences of the modulator unit on the polarization components of the optical signal and error phases are eliminated.

[0019] During the first pass, for example, the component of the optical signal with horizontal polarization (assumed to be the first polarization direction for the example) is modulated (a first phase is modulated onto this component), and the component with vertical polarization (assumed to be the second polarization direction for the example) passes through the phase modulator without any changes to the phase of this component. The optical signal is then retroreflected by the reflector, and the polarization is rotated by 90°; the component with horizontal polarization is now vertically polarized, and the component with vertical polarization is now horizontally polarized.During the second pass, the optical signal and its polarization components pass through the phase modulator again, and the (now) horizontally polarized component (which corresponds to the vertically polarized component of the first pass) is phase modulated, while the (now) vertically polarized component passes through the phase modulator without any further phase change. This means that the phase modulator has only a single modulation axis, which remains the same for the first and second passes of optical signals, thus acting on the same polarization direction.

[0020] If an error phase (or phase shift) is introduced into the optical signal during the first pass, for example, by parasitically influencing the phase of the horizontally polarized component in a different way than the phase of the vertically polarized component, this error phase cancels out during the second pass because the optical signal travels the same optical path with a polarization rotated by 90°, and the same error phase is now applied to the other polarization axis. This introduces all relative phase errors equally into both polarization axes. Furthermore, the time delay between the two polarization components prevents any parasitic influence on the polarization component that lies outside the polarization axis of the phase modulator.

[0021] The phase applied to the horizontally polarized component of the second pass is called the second phase because in the second pass the first phase is contained in the vertically polarized component of the optical signal and the first phase may be different from the second phase.

[0022] In the example given here, reference is made in a specific way to horizontally and vertically polarized components of the optical signal, namely that the horizontally polarized component is phase-modulated during the first and second pass through the phase modulator. However, it should be understood that this example is not restrictive and the phase modulator can phase-modulate the vertically polarized component or any other component of the optical signal instead of the horizontally polarized component. What is crucial is that the polarization components are offset from one another in time, and between the two processes in which the phase of a polarization component of the optical signal is modulated, the polarization of the two polarization components is rotated by 90° each.

[0023] As a result, with this setup, both the vertically polarized component and the horizontally polarized component of the optical signal are modulated in one modulator unit. The resulting optical signal has a desired polarization modulation overall, with the first polarization component having a first polarization direction being phase modulated during the first pass, and the second polarization component, which has the first polarization direction during the second pass, being phase modulated during the second pass. However, the polarization of the optical signal is changed by 90° during the second pass compared to the first pass.

[0024] The phase modulator used here is, for example, a phase-changing modulator that acts on a polarization component of an optical signal, meaning that the phase modulator changes the phase of two orthogonal polarization components of the optical light. For example, the phase modulator is an electro-optic modulator (EOM). It should be understood that any reference to an EOM in this description is merely exemplary and generally applies to a phase modulator that can be polarization-dependent or polarization-independent.

[0025] For example, the phase modulator has two orthogonally aligned optical axes. Using electrical energy, such as an applied voltage along one of these optical axes (the polarization axis), its refractive index is modified, resulting in a phase change of the polarization component of the optical signal relative to the other, or orthogonal, optical axis.

[0026] The use of a Faraday mirror makes the modulator unit very space-saving and compact. A phase modulator can be switched very quickly, so the modulator unit described here can be used up to high frequency ranges, for example, several tens of GHz, such as up to 30 to 40 GHz, or even higher.

[0027] The modulator unit described here allows different polarization states of an optical signal to be set. For example, different discrete polarization states can be set by changing the phase of the horizontally polarized polarization component of the optical signal with respect to the phase of the vertically polarized polarization component of the optical signal. However, the polarization of the optical signal can, in principle, be continuously varied as desired as a linear combination of the two polarization components, without being limited to a specific number of polarization states.

[0028] The polarization component separator helps prevent parasitic effects of the phase modulator on a non-modulated polarization component. In principle, a phase modulator can also have parasitic or unwanted effects on optical signals with a polarization direction that lies outside the polarization axis of the phase modulator, which the phase modulator should, in principle, allow to pass unaffected.

[0029] Thus, it may happen that a polarization-dependent phase modulator, which is only intended to modulate horizontally polarized light, also influences vertically polarized light. The latter is an unwanted / parasitic effect. However, this effect can lead to inaccuracies and errors in the modulation. To eliminate this effect, the present proposal is to split an optical signal into polarization components with different polarization directions, to introduce a temporal offset between these two polarization components, and then to pass them sequentially through the phase modulator. The phase modulator is controlled such that it only modulates the polarization component that is to be modulated at a time. For example, the phase modulator is controlled such that it applies modulation when horizontally polarized light passes through the phase modulator.However, when vertically polarized light passes through the phase modulator, the phase modulator is inactive, meaning it does not modulate the corresponding signal. This prevents unwanted modulation or corruption from being introduced into the vertically polarized polarization component. The polarization components are polarized in different directions and pass through the phase modulator with a time gap. The phase modulator applies modulation to only one polarization component. When the polarization component not to be modulated passes through the phase modulator, no modulation is applied, or the phase modulator is inactive. The reflector reflects the optical signals back to the phase modulator, changing the polarization direction of the polarization components.The first polarization component receives the second polarization direction, and the second polarization component receives the first polarization direction. Both polarization components pass through the phase modulator again. This time, however, the phase modulator acts on the other polarization component because the respective polarization direction has changed and the phase modulator maintains its polarization axis. The temporal offset is then eliminated, and the two polarization components are output via a common optical path. Both polarization components now contain the desired modulated phase information, and an optical signal containing the two polarization components is output.

[0030] In other words, and as a non-limiting example, the operation of the modulator unit can be described as follows: The phase modulator is configured to apply a phase modulation exclusively to the first polarization component before the two polarization components strike the reflector, and to apply a phase modulation exclusively to the second polarization component after the reflector reflects the two polarization components. During the first pass, i.e., when the optical signal passes through the phase modulator for the first time, the phase modulator modulates the first polarization component with the first polarization direction (e.g., horizontal polarization direction), and the second polarization component with the second polarization direction (e.g., vertical polarization direction) is transmitted unchanged.During the second pass, after the reflector has reflected the polarization components and changed their polarization direction, the phase modulator modulates the second polarization component, which now has the first polarization direction (in this example, horizontal polarization direction), and the second polarization component, which now has the second polarization direction (in this example, vertical polarization direction), is transmitted unchanged.

[0031] According to one embodiment, the first optical sub-path has a first optical path length and the second optical sub-path has a second optical path length, wherein the first optical path length differs from the second optical path length.

[0032] The time delay between the two polarization components can fundamentally be introduced in different ways. In general, this is achieved by a different signal propagation time over the two optical sections. This different signal propagation time can be introduced, for example, by using a delay element. However, it can also be introduced by physically designing the optical sections differently, for example, by having a different optical path length. For a fiber-based optical section, fibers of different lengths can be used. It is also conceivable to use a single polarization-maintaining optical fiber for both polarization components, provided that this optical fiber has different refractive indices on the different polarization axes, which ensures the time delay.In a free beam, the elements used for transmitting the optical free beam can be positioned at different distances from each other in order to achieve different optical propagation times for the optical signals over the optical sections.

[0033] According to a further embodiment, the polarization component separator comprises a first polarizing beam splitter and a second polarizing beam splitter, wherein the first optical sub-path and the second optical sub-path are arranged between the first polarizing beam splitter and the second polarizing beam splitter.

[0034] Each polarizing beam splitter has a first interface and a second interface. Optical signals that hit the polarizing beam splitter at the first interface are split into polarization components based on their polarization direction. These components are then directed at the second interface via the first optical path or the second optical path, so that polarization components with different polarization directions travel through different optical paths. Conversely, optical signals that hit the corresponding polarizing beam splitter at the second interface with the corresponding polarization direction via the two optical paths are combined and output via a common optical path at the first interface.

[0035] An optical signal from the light source first strikes the first polarizing beam splitter in the polarization component separator. This first polarizing beam splitter splits the optical signal into the first polarization component with a first polarization direction and the second polarization component with a second polarization direction. The first polarization component is guided over one optical path, for example, the first optical path. The second polarization component is guided over the other optical path, for example, the second optical path. Due to the different signal propagation times over the optical paths, a relative temporal offset is introduced between the two polarization components.The two polarization components now reach the second beam splitter, which recombines the incoming polarization components and transmits them via a common optical path to the phase modulator. The polarization components pass through the phase modulator one after the other at a time interval. One polarization component is modulated, the other is transmitted unchanged. The polarization components then strike the reflector, which reflects the polarization components and changes their polarization direction. For example, an optical signal with a horizontal polarization direction is changed to be vertically polarized, and vice versa. The reflector provides the first polarization component with the second polarization direction and the second polarization component with the first polarization direction.On the return path, the phase modulator now modulates the second polarization component and allows the first polarization component to pass through unchanged. There is still a temporal offset between the two polarization components. This temporal offset is eliminated when the polarization components pass through the polarization component separator for the second time and in the opposite direction. The polarization components hit the second polarizing beam splitter. This guides the polarization components, which now have changed polarization directions, over the optical sections in such a way that the polarization components now pass through the optical section that they did not pass through on the way to the phase modulator. For example, the first polarization component is delayed during the first pass, and the second polarization component is delayed during the second pass.Because the polarization components pass through the polarization component separator in opposite directions, they are each subject to the same time delay. After the polarization components have passed through the polarization component separator twice and in opposite directions, the relative time offset is eliminated.

[0036] In summary: To modulate the polarization components of an optical signal, the polarization components are first separated, and a temporal offset is introduced between them. The polarization components pass through the phase modulator twice, each time with a different polarization direction. During the first pass, the first polarization component is modulated; during the second pass, the second polarization component is modulated. The temporal offset is then eliminated, and the two polarization components are combined to be output as a phase-modulated optical signal.

[0037] According to a further embodiment, the phase modulator comprises a crystal configured to be subjected to an electrical voltage, thereby changing its refractive index, thereby changing the phase of the first polarization component and / or the second polarization component of the optical signal. However, the refractive index can also be changed by applying a mechanical voltage.

[0038] The phase modulator can, for example, contain a birefringent medium which, when a voltage is applied, changes the phase of an optical signal passing through the optical medium.

[0039] According to a further embodiment, the modulator unit is designed to vary the electrical voltage applied to the crystal over time.

[0040] For example, the modulator unit contains a power supply that provides a predefined electrical voltage. A control unit controls the power supply so that it supplies a desired electrical voltage to the phase modulator. The electrical voltage at the phase modulator, which varies over time, changes the phase between differently polarized components of the optical signal.

[0041] According to a further embodiment, an amount of the first phase of the first polarization component of the input signal in the first polarization direction differs from an amount of the second phase of the second polarization component of the retroreflected optical signal in the first polarization direction.

[0042] In In other words, this means that the phase modulator modulates a different phase onto the horizontally polarized component of the optical signal during the first pass than it modulates onto the then horizontally polarized component of the retroreflected signal during the second pass. This changes the polarization of the output signal of the modulator unit.

[0043] According to a further embodiment, the phase modulator is configured to change a difference between the first phase and the second phase over time.

[0044] By changing the difference between the phases in the first pass and the second pass over time, the phase modulator also changes the polarization of the output signal over time.

[0045] Overall, the polarization of the modulator unit's output signal is freely adjustable between two linear polarizations (diagonal / anti-diagonal, or, with the use of an additional waveplate at the polarization modulator's output, also horizontal and vertical) and two circular polarizations (Z+ and Z-). However, the polarization can also be continuously adjusted to any elliptical state between the discrete states.

[0046] According to a further embodiment, the light source is configured to emit or output light with a well-defined optical mode. In this context, the polarization and coherence properties of the optical signal are important. For example, the light source is a laser.

[0047] Lasers are characterized by their ability to emit optical signals with a well-defined polarization. This makes them particularly suitable for applications such as those described here.

[0048] According to a further embodiment, the modulator unit is designed to control the light source such that the light source emits pulsed optical signals.

[0049] A light pulse passes through the phase modulator, and a first polarization component is phase modulated. The light pulse is then passed on to the reflector, where the polarization of the light pulse is changed and reflected back to the phase modulator. During the second pass, the phase modulator again modulates the phase of a polarization component of the light pulse.

[0050] For example, a light pulse is designed (e.g. with regard to its duration) in such a way that in the phase modulator the light or one of its polarization components in the first pass (to the reflector) does not overlap with another polarization component of the same light pulse or another light pulse when these are in the second pass (from the reflector).

[0051] In another embodiment, the modulator unit may be configured to modulate and emit pulsed optical signals. In this example, the modulator unit may receive the pulsed optical signals from another source.

[0052] According to a further embodiment, the modulator unit further comprises a beam splitter which is arranged between the light source and the polarization component separator and is designed to direct at least a part of the retroreflected optical signal phase-modulated by the polarization component separator in a predetermined direction.

[0053] The beam splitter is arranged to at least partially structurally direct the optical signal output by the polarization component separator after the second pass in a desired direction, so that the optical signal output by the polarization component separator is not emitted exclusively in the direction of the light source that emits the input signal to the polarization component separator. The output signal thus generated carries information in its polarization that can be read and processed by a receiver.

[0054] In one variant, a circulator can be used instead of the beam splitter.

[0055] According to a further aspect, an optical signal transmission path is provided. The optical signal transmission path comprises a modulator unit as described herein and a receiver. The modulator unit functions as a signal source or part of a transmission unit, which outputs an optical signal on which information is applied. The signal source transmits the modulated optical signal toward the receiver. The receiver is configured to receive optical signals. The modulator unit is arranged to emit the output signal toward the receiver.

[0056] The signal transmission path can be designed for unidirectional or bidirectional signal transmission. In the case of bidirectional signal transmission, there are at least two communication units, each of which has a modulator unit and a receiver unit.

[0057] The modulator unit described herein is implemented, for example, as part of an optical signal transmission link. The optical signal transmission link is thus configured to transmit information by means of polarization modulation of an optical carrier signal. The polarization modulation is introduced into the optical carrier signal by means of the modulator unit. The modulator unit prevents phase errors from being introduced into a polarization component by the phase modulator if this polarization component has a polarization direction that lies outside the polarization axis of the phase modulator. Furthermore, the modulator unit intrinsically compensates for phase errors due to the components involved in the modulation because the polarization components traverse the same optical path twice, and the orthogonal polarization components of the optical signal are modulated one after the other.

[0058] According to one embodiment, the modulator unit is arranged in a satellite. However, the modulator unit can be arranged in any other communication system.

[0059] According to a further aspect, a satellite having a modulator unit as described herein is provided.

[0060] A modulator unit as described herein can be used, for example, on optical signal transmission links which are used between two mobile units (air, water or land vehicles or satellites), between a mobile unit and a counterpart on the earth's surface, or between two stationary units. Short description of the characters

[0061] The following examples are described in more detail with reference to the accompanying drawings. The illustrations are schematic and not to scale. Like reference numerals refer to like or similar elements. They show: Fig. 1 is a schematic representation of a modulator unit. Fig. 2 is a schematic representation of the optical path through a modulator unit. Fig. 3 is a schematic representation of a polarization component separator. Fig. 4 is a schematic representation of a modulator unit. Fig. 5 is a schematic representation of an optical signal transmission path. Detailed description of implementation examples

[0062] Fig. 1 shows the structural design of a modulator unit 100. The modulator unit 100 includes a light source 110, a beam splitter 120, a beam absorber 125, a polarization component separator 170, a (polarization-dependent or polarization-independent) phase modulator in the form of an electro-optical modulator, EOM, 130, and a reflector in the form of a Faraday mirror 140. The EOM 130, the Faraday mirror 140, and the polarization component separator 170 may collectively be referred to as polarization modulator 105. Although reference is made here, for example, to a Faraday mirror 140, the corresponding explanations generally apply to a reflector mentioned herein.

[0063] The light source 110, which is, for example, a laser, emits a pulsed optical signal in the form of the input signal 111. This input signal is fed to the other components and the phase of its polarization components is phase-modulated in order to transmit information in the polarization of the optical signal resulting from the superimposed polarization components.

[0064] The information to be transmitted is modulated on the output signal 118 in the polarization of the optical signal.

[0065] How the polarization of the optical signal is modulated will now be explained with reference to Fig. 2 , which complements Fig. 1 to be considered.

[0066] In Fig. 2 The path of the optical signal through the modulator unit 100 is described. Reference is made to the state of the optical signal at different times or at different points in the modulator unit 100.

[0067] First, the input signal 111 hits the beam splitter 120. In this example, the beam splitter 120 is configured as a non-polarizing beam splitter. A portion of the input signal 111 is directed toward the beam absorber 125 as the first portion 112 of the split input signal, and another portion of the input signal 111 passes through the beam splitter 120 as the second portion 113 of the split input signal toward the polarization component separator 170 and the EOM 130.

[0068] The function of the polarization component separator 170 will be explained with reference to Fig. 3 described in more detail. In general, the polarization component separator 170 has the function of splitting an optical signal 113 (or in the opposite direction the optical signal 115A) into its polarization components and transmitting them over two different optical paths (see Fig. 3 ) with different signal propagation times, whereby a relative time delay is introduced between the two polarization components (in the first pass through the polarization component separator) or eliminated (in the second pass through the polarization component separator).

[0069] The optical signal 113A thus contains two polarization components on its way to the EOM 130 that are offset in time from one another. From a physical perspective, two signals are transmitted here, each polarization component representing an optical signal. The first polarization component contains light with a first polarization direction (e.g., horizontal polarization), and the second polarization component contains light with a second polarization direction different from the first polarization direction (e.g., vertical polarization).

[0070] The EOM 130 now modulates a polarization component of the optical signal 113A by changing the phase of this polarization component of the signal 113A. At the output of the EOM 130, the optical signal 114 is present such that the first polarization component with the first polarization direction is phase-modulated and the second polarization component with the second polarization direction is not modulated (if the EOM is a polarization-dependent phase modulator).

[0071] The Faraday mirror 140 retroreflects the signal 114 and changes the polarization of both polarization components by 90° each, so that the optical signal 114 with its two polarization components is reflected as a mirrored signal 115 to the EOM 130.

[0072] The optical signal 115 therefore also contains two polarization components that are delayed relative to one another. The optical signals 113, 113A, 114 contain a first polarization component with a first polarization direction and a second polarization component with a second polarization direction. In contrast, in the optical signal 115, the polarization directions of the two polarization components are changed: in the optical signal 115, the first polarization component contains light with the second polarization direction, and the second polarization component contains light with the first polarization direction. Thus, the first and second polarization components pass through the EOM 130. However, the second polarization component is now modulated because the second polarization component contains the light with the first polarization direction, whereas the first polarization component, which contains light with the second polarization direction, is not modulated.

[0073] For example, if the horizontally polarized pulse of the optical signal was modulated during the first pass between signals 113 and 114 through the EOM, then during the second pass this modulation is now present in the vertically polarized pulse of the optical signal 115 because the Faraday mirror 140 has changed the polarization by 90°. If the optical signal 115 now passes through the EOM 130 again during the second pass, the phase of the now horizontally polarized pulse (corresponding to the vertically polarized pulse during the first pass, which did not undergo any phase change during the first pass) is changed.

[0074] The optical length between the EOM 130 and the Faraday mirror 140 is dimensioned such that an optical signal during the second pass does not overlap an optical signal during the first pass within the EOM while one of these optical signals is being modulated. In a polarization-dependent phase modulator, polarization components of different polarization directions may overlap, since only one polarization direction is modulated. Generally speaking, the length of the optical path between the EOM and the Faraday mirror is dimensioned such that a polarization component during the second pass does not overlap a polarization component during the first pass. In particular, the length of the optical path is matched to the duration of a light pulse from the light source 110 and the transmission rate.

[0075] Thus, the EOM applies a phase to both polarization components of the optical signal on the same optical path. By specifying the corresponding phase for the first polarization component during the first pass and for the second polarization component during the second pass, the polarization of the resulting optical signal 116 can be varied.

[0076] The optical signal 116 is now phase-modulated in both polarization components. The superposition of these two modulations results in the polarization of the optical signal 116, which contains the information to be transmitted.

[0077] The optical signal 116 now encounters the beam splitter 120 again. A portion 117 of the optical signal passes through the beam splitter, and another portion 118 is deflected in a different direction and corresponds to the output signal to be transmitted, the polarization of which contains the information to be transmitted. Alternatively, it is conceivable that the output signal passes through the beam splitter and the deflected signal is discarded.

[0078] Fig. 3 shows a schematic structure of a polarization component separator 170. The polarization component separator 170 is designed to separate differently polarized components of an optical signal from one another and transmit them over different optical paths. The polarization component separator 170 operates bidirectionally, meaning that optical signals can pass through the polarization component separator 170 in two directions (i.e., from left to right, signals 113 and 113A, and from right to left, signals 115A, 116), with the polarization component separator 170 performing the same function for optical signals in both directions.

[0079] The following describes the path of the optical signal 113 through the polarization component separator 170. This path is referred to as the first pass. The same applies to the path of the optical signal 115A through the polarization component separator 170, whereby this path is referred to as the second pass.

[0080] First, an optical signal 113 containing two polarization components (e.g., horizontal polarization component and vertical polarization component) strikes the first interface 171 of the first polarizing beam splitter 172. The first polarizing beam splitter 172 separates the two polarization components from one another. An optical signal 113 fed into the polarizing beam splitter 172 is split and results in two optical polarization components, which are output at the second interface 173 of the first polarizing beam splitter 172 and each guided via one of the two separate optical sections 174, 176. The first polarization component is guided, for example, via the first optical section 174 and the second polarization component is guided via the second optical section 176.The optical sub-links 174, 176 have different signal propagation times for optical signals, or different optical path lengths. Assuming an identical or nearly identical propagation speed of optical signals over the two optical sub-links 174, 176, the propagation times of optical signals over the optical sub-links 174, 176 vary. For example, the second optical sub-link 176 is longer than the first optical sub-link 174.

[0081] Thus, the two polarization components into which the optical signal 113 was split experience a relative temporal offset through the first and second optical sections 174, 176. Preferably, the temporal offset between the two polarization components is dimensioned such that there is no temporal overlap between the two polarization components after they are passed back onto a common optical path at the output side, i.e., at the second polarizing beam splitter 178. This means that the first polarization component arrives at the second polarizing beam splitter 178 via the first optical section 174, and the second polarization component arrives at the second polarizing beam splitter 178 via the second optical section 176, such that the second polarization component only arrives after the first polarization component has passed the second interface 179 of the second polarizing beam splitter 178.So there is a real time offset between the two polarization components.

[0082] The two polarization components reach the second interface 179 of the second polarizing beam splitter 178 via the first and second optical sections 174, 176. The two polarization components are combined by the second polarizing beam splitter and output at the first interface 177 via a common optical path in the direction of the phase modulator 130.

[0083] This function, which was described for the optical signal 113 through the polarization component separator 170, also applies analogously to the optical signal 115A, which passes through the polarization component separator 170 in the opposite direction (compared to the optical signal 113).

[0084] The optical signal 113 is transmitted as a pulsed signal. The light pulses have a specific pulse duration with a start and end time. Between the light pulses, there is a predetermined time, depending on the transmission frequency, during which no optical signal is transmitted. Thus, the individual polarization components are also transmitted as light pulses. After the polarization components have passed through the optical sections 174, 176, there is a time offset between the light pulses of one polarization component and the light pulses of the second polarization component.

[0085] In the example of Fig. 3 The light pulse of the second polarization component, which is transmitted via the second optical path 176, can be referred to as a delayed light pulse, whereas the light pulse of the first polarization component, which is transmitted via the first optical path 174, can be referred to as a non-delayed light pulse. The temporal offset between the light pulses of the two polarization components is preferably configured such that the start time of the delayed light pulse lies after the end time of the non-delayed light pulse, and there is a time period between the start time of the delayed light pulse and the end time of the non-delayed light pulse that is, for example, greater than 0 seconds.

[0086] The temporal offset is defined by the respective propagation time of an optical signal across the two optical sections 174, 176. Thus, the temporal offset between the two polarization components can be influenced by selecting the signal propagation times or optical lengths of the two optical sections 174, 176 accordingly.

[0087] The optical sections 174, 176 can be implemented as fiber-based or free-beam optical sections. For fiber-based optical sections, for example, the optical fibers are polarization-maintaining optical fibers. Furthermore, the optical fiber of the first optical section 174 has a first length that differs from a second length of the optical fiber of the second optical section 176. This difference in length between the optical sections 174, 176 is Fig. 3 shown as a loop in the second optical section 176. In other words, the fiber of the second optical section 176 is longer than the fiber of the first optical section 174.

[0088] The optical sections 174, 176 can also be implemented as a free beam. In this case, an optical beam is guided over a predetermined distance using mirrors or other optical deflection elements, for example. The length of the optical sections can be influenced by the position of the mirrors.

[0089] In general, the function of the polarization component separator 170 can be described as follows: an optical signal with two polarization components is split into these two polarization components. The polarization components are transmitted via separate optical sections. A polarization component experiences a delay along its optical section. This delay can arise from different optical lengths of the optical sections. However, it is also conceivable to integrate an optical delay element into an optical section in order to increase the propagation time of one polarization component compared to the propagation time of the other polarization component. The polarization components thus provided with a temporal offset are subsequently recombined onto a common optical path and transmitted. A polarization component can now be modulated.The reflector reflects the polarization components back to the phase modulator, changing their polarization direction. The phase modulator then modulates the other polarization component. After the two polarization components have each passed through the phase modulator twice (once from left to right and once from right to left), both polarization components have undergone modulation. At this point, however, there is still a temporal offset between the two polarization components. This temporal offset is eliminated by the polarization components passing through the polarization component separator again. The polarization components strike the second polarizing beam splitter 178. The second polarizing beam splitter 178 functions in such a way that the polarization components are guided via the first optical path 174 or the second optical path 176, depending on their polarization direction.However, because the reflector 140 changed the polarization direction of the polarization components during the reflection process, the polarization components pass through the other optical path during the second pass (from right to left, optical beams 115A, 116). This means that the polarization component that passed through the first optical path 174 during the first pass is transmitted over the second optical path 176 during the second pass, and vice versa. Thus, during the second pass, the polarization component that was not delayed during the first pass is delayed, and vice versa. In summary, after the two polarization components have passed through the polarization component separator 170 and the phase modulator 130 in both directions, both polarization components are modulated and no longer have a temporal offset from one another.Each polarization component experiences the same degree of relative time delay in one direction by the polarization component separator 170, so that in the optical signal 116 the two polarization components coincide again in time.

[0090] On the first pass, i.e. on the way from left to right in the representation of the Fig. 1 bis 3 and relative to the path of the optical signal 113, the polarization-dependent phase modulator 130 applies a phase modulation to a first polarization component. The other (second) polarization component is not modulated. During the second pass, i.e., on the path from right to left in the representation of the Fig. 1 bis 3 and relative to the path of the optical signal 115A, the polarization-dependent phase modulator 130 applies phase modulation to the second polarization component. The first polarization component is not modulated.

[0091] The phase modulator 130 is preferably controlled such that it applies phase modulation only when a first polarization component with a corresponding polarization direction passes through the phase modulator. If a second polarization component with a different polarization direction passes through the phase modulator, the phase modulator does not apply any modulation. This ensures that the second polarization component is neither intentionally nor inadvertently subjected to phase modulation, e.g., due to parasitic effects. This has the advantage that the device described here can perform highly precise phase modulation with a low error rate.

[0092] After the phase modulator 130 has applied phase modulation to the first polarization component in this way, the two polarization components reach the reflector 140. The reflector 140 reflects the two polarization components, changing their polarization directions in the process, so that during the second pass (from right to left, i.e., signal direction 115, 115A, 116), the first polarization component adopts the polarization direction of the second polarization component of the first pass, and the second polarization component adopts the polarization direction of the first polarization component of the first pass. The phase modulator 130 now applies phase modulation to the second polarization component without applying any modulation when the first polarization component is transmitted through the phase modulator 130.

[0093] Fig. 4 shows how the components of the modulator unit 100 are controlled in order to incorporate the information to be transmitted into the polarization of the optical signal.

[0094] The modulator unit 100 includes a power supply 160 and a control unit 150. Both the power supply 160 and the control unit 150 are connected to the light source 110, the EOM 130, and the Faraday mirror 140. However, the control unit 150 can also be connected directly to the power supply 160 in order to specify an electrical voltage output by the power supply 160 at the respective terminal.

[0095] The power supply 160 supplies the light source 110 with electrical energy so that the light source generates the optical signal, which acts as the input signal. The power supply 160 also supplies the EOM 130 with electrical energy, for example, an electrical voltage applied to a crystal 135. This electrical voltage influences the crystal 135 such that the phase of a polarization component of a passing optical signal is changed. For example, variations in the electrical voltage can change the phase to different extents. The control unit 150 and the power supply 160 control the EOM such that it acts on the polarization component passing through the EOM in the desired manner during the first and second pass of the optical signal and changes the phase of the influenced polarization component accordingly and as desired.The control unit and the power supply must switch quickly and control the EOM.

[0096] The polarization of a passing optical signal is changed in the Faraday rotator 143, in this example by 45°. The optical signal then hits the mirror 145, is reflected, and passes through the Faraday rotator 143 again. The polarization of the optical signal is now changed again by 45° in the same direction, so that the polarization of the optical signal incident on the Faraday mirror and the polarization of the optical signal output by the Faraday mirror differ by 90°.

[0097] The control unit 150 is configured to control the power supply 160 and / or each of the components 110, 130, 140 such that these components are supplied with the energy required for their function. For this purpose, the control unit 150 can send control commands to the components 110, 130, 140 and / or control commands to the power supply 160.

[0098] The polarization component separator 170 is arranged between the light source 110 and the phase modulator 130. The polarization component modulator can be a passive component that does not require separate control by the control unit 150. However, it is also conceivable for the polarization component separator 170 to be an active component that is supplied with power by the power supply 160 and controlled by the control unit 150. For example, the polarization component separator 170 can be configured to vary the signal propagation time of a polarization component, and the control unit 150 can be configured to specify the extent of the time delay of a polarization component by the polarization component separator 170.

[0099] Fig. 5shows an optical signal transmission path 200. A modulator unit 100 acts as the signal source or transmitter. The modulator unit 100 modulates the polarization of an optical signal as described above and transmits the modulated optical signal via a transmission path 210. The transmission path 210 is, for example, a wireless optical path.

[0100] The modulated optical signal is received and processed by a remote station. The remote station is receiver 220. The modulator unit 100 can be located on board a satellite or on the Earth's surface. The receiver 220 can be located on the Earth's surface or on board another satellite.

[0101] Additionally, it should be noted that "comprising" or "having" does not exclude other elements or steps, and "a" or "an" does not exclude a plurality. Furthermore, it should be noted that features or steps described with reference to one of the above embodiments may also be used in combination with other features or steps of other embodiments described above. Reference signs in the claims are not to be considered as limitations. List of reference symbols

[0102] 100 Modulator unit 105 Polarization modulator 110 Light source 111 Input signal 112 First part of the split input signal 113 Second part of the split input signal 114 Single-modulated signal 115 Mirrored signal 116 Double-modulated signal 117 Retrograde signal 118 Output signal 120 Beam splitter 125 Beam absorber 130 Phase modulator 135 Crystal 140 Reflector, Faraday mirror 143 Faraday rotator 145 Mirror 150 Control unit 160 Power supply 170 Polarization component separator 171 First interface 172 First polarizing beam splitter 173 Second interface 174 First optical section 176 Second optical section 177 First interface 178 Second polarizing Beamsplitter 179Second interface 200Optical signal transmission path 210Transmission path, optical signal 220Receiver 300Satellite

Claims

1. Modulator unit (100) for modulating the polarization of an optical signal, comprising: a light source (110); a polarization component separator (170); a phase modulator (130); a reflector (140); wherein the light source (110) is designed to output an optical signal and emit the latter as an input signal (111) in the direction of the polarization component separator (170), wherein the optical signal contains a first polarization component with a first polarization direction and a second polarization component with a second polarization direction; wherein the polarization component separator (170) is designed to separate the first polarization component from the second polarization component and transmit the first polarization component via a first optical section (174) and the second polarization component via a second optical section (176); wherein the first optical section (174) is assigned a first signal propagation time and the second optical section (176) is assigned a second signal propagation time, the first signal propagation time differing from the second signal propagation time; wherein, as a result of the difference between the first signal propagation time and the second signal propagation time, the polarization component separator (170) is designed to introduce a time offset between the first polarization component and the second polarization component and transmit the first polarization component and the second polarization component on a joint optical path to the phase modulator (130); wherein the phase modulator (130) is designed to modulate a first phase of the first polarization component in the first polarization direction and transfer the modulated first polarization component to the reflector (140); wherein the reflector (140) is designed to retroreflect both the first polarization component and the second polarization component in the direction of the phase modulator (130) and in the process modify the respective polarization such that the first polarization component with the first polarization direction receives the second polarization direction and the second polarization component with the second polarization direction receives the first polarization direction; wherein the phase modulator (130) is designed to modulate a second phase of the retroreflected second polarization component in the first polarization direction and transmit the retroreflected first polarization component and the retroreflected second polarization component to the polarization component separator (170); wherein the polarization component separator (170) is designed to eliminate the time offset between the retroreflected first polarization component and the retroreflected second polarization component and output the retroreflected first polarization component and the retroreflected second polarization component without a time offset via a joint optical path; wherein the modulator unit (100) is designed to output the optical signal modulated thus as polarization-modulated output signal (118).

2. Modulator unit (100) according to Claim 1, wherein the first optical section (174) has a first optical path length and the second optical section (176) has a second optical path length, the first optical path length differing from the second optical path length.

3. Modulator unit (100) according to Claim 1 or 2, wherein the polarization component separator (170) comprises a first polarizing beam splitter (172) and a second polarizing beam splitter (178); wherein the first optical section (174) and the second optical section (176) are arranged between the first polarizing beam splitter (172) and the second polarizing beam splitter (178).

4. Modulator unit (100) according to any of the preceding claims, wherein the phase modulator (130) comprises a crystal (135) designed to have a voltage applied thereto and designed to change its refractive index in the process, whereby the phase of the first polarization component and / or second polarization component of the optical signal is modified.

5. Modulator unit (100) according to Claim 4, wherein the modulator unit (100) is designed to vary the voltage applied to the crystal (135) over time.

6. Modulator unit (100) according to any of the preceding claims, wherein an absolute value of the first phase of the first polarization component of the input signal (111) in the first polarization direction differs from an absolute value of the second phase of the second polarization component of the retroreflected optical signal in the first polarization direction.

7. Modulator unit (100) according to Claim 6, wherein the phase modulator (130) is designed to change a difference between the first phase and the second phase over time.

8. Modulator unit (100) according to any of the preceding claims, wherein the light source (110) is designed to output light with a defined optical mode.

9. Modulator unit (100) according to any of the preceding claims, wherein the modulator unit (100) is designed to control the light source (110) such that the light source (110) emits pulsed optical signals.

10. Modulator unit (100) according to any of the preceding claims, further comprising a beam splitter (120) arranged between the light source (110) and the polarization component separator (170) and designed to steer, in a given direction, at least a portion of the retroreflected optical signal phase-modulated by the polarization component separator (170).

11. Optical signal transmission path (200), comprising a modulator unit (100) according to any of Claims 1 to 10; a receiver (220) designed to receive optical signals; wherein the modulator unit (100) is arranged to emit the output signal (118) in the direction of the receiver (220).

12. Satellite (300) having a modulator unit (100) according to any of Claims 1 to 10.