SELF-COMPENSATING POLARIZATION MODULATOR

Description

Technical area

[0001] This description relates to optical signal generation and signal transmission, in particular the transmission of information by means of polarization modulation. Specifically, the description relates to a modulator unit for modulating the polarization of an optical signal, an optical signal transmission path comprising such a modulator unit, and a system comprising 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 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. Description

[0007] 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.

[0008] This object is achieved by the subject matter of the independent claim. Further embodiments emerge from the dependent claims and the following description.

[0009] 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-dependent 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-dependent phase modulator, 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-dependent phase modulator is configured to modulate a first phase of the first polarization component of the input signal in the first polarization direction and to forward the thus modulated input signal to the reflector.The reflector is configured to retroreflect the received optical signal toward the polarization-dependent phase modulator, thereby changing the polarization of the optical signal (for example, 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 polarization-dependent phase modulator is configured to modulate a second phase of the second polarization component of the retroreflected optical signal in the first polarization direction. The modulator unit is configured to output the thus modulated optical signal as a polarization-modulated output signal.

[0010] The optical signal represents a superposition of two orthogonal polarization components. The polarization-dependent phase modulator is designed to apply different phases to both polarization components.

[0011] 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. On the outward path, the optical signal travels from the light source through the polarization-dependent 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, rotating the polarization of the light by 45° (+45° or -45°). The optical signal is then mirrored and passes through the Faraday rotator again, rotating its polarization 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°.

[0012] Precisely because the optical signal traverses the same optical path with reversed polarization directions twice, parasitic influences of the modulator unit on the phase of the optical signal and error phases are eliminated, in particular phase errors which occur due to slow thermal and mechanical fluctuations of the modulator unit, in particular on the path from and including the polarization-dependent phase modulator up to and including the reflector, but also upstream of the polarization-dependent phase modulator.

[0013] On the outgoing path, 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 polarization-dependent 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.On the return path, the optical signal passes through the polarization-dependent phase modulator again, and the (now) horizontally polarized component (which corresponds to the vertically polarized component of the forward path) is phase modulated, while the (now) vertically polarized component passes through the polarization-dependent phase modulator without any further phase change. This means that the polarization-dependent phase modulator has only a single modulation axis, which remains the same on both the forward and return paths, thus acting on the same polarization direction.

[0014] If an error phase (or phase shift) is introduced into the optical signal on the outgoing path, 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 on the return path 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.

[0015] The phase applied to the horizontally polarized component of the return path is called the second phase because on the return path 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.

[0016] In the example given here, reference is made to horizontally and vertically polarized components of the optical signal in a specific way, namely that on the forward and return paths through the polarization-dependent phase modulator, the horizontally polarized component is phase-modulated. However, it should be understood that this example is not restrictive, and the polarization-dependent phase modulator can phase-modulate the vertically polarized component or any other component of the optical signal instead of the horizontally polarized component. The crucial point is that between the two processes in which the phase of a polarization component of the optical signal is modulated, the polarization of the optical signal is rotated by 90°.

[0017] As a result, in this setup, both the vertically polarized component and the horizontally polarized component of the optical signal are modulated in a modulator unit with a single optical path. The resulting optical signal has a desired polarization modulation overall by modulating the phase of the first polarization component with a first polarization direction on the forward path and the second polarization component, which has the first polarization direction on the return path, on the return path. However, the polarization of the optical signal on the return path is changed by 90° compared to the forward path.

[0018] The polarization-dependent phase modulator used here is a phase-changing modulator that acts on a polarization component of an optical signal, meaning that the polarization-dependent phase modulator changes the phase of two orthogonal polarization components of the optical light. For example, the polarization-dependent 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 polarization-dependent phase modulator.

[0019] For example, the polarization-dependent phase modulator has two orthogonally aligned optical axes. Applying electrical energy, such as a voltage, along one of these optical axes modifies its refractive index, resulting in a phase change of the polarization component of the optical signal relative to the other, or orthogonal, optical axis.

[0020] The use of a Faraday mirror makes the modulator unit very space-saving and compact. A polarization-dependent phase modulator can be switched very quickly, allowing the modulator unit described here to be used in high frequency ranges, for example, several tens of GHz, up to 30 to 40 GHz, or even higher.

[0021] 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 portion of the optical signal relative to the phase of the vertically polarized portion of the optical signal. However, the polarization of the optical signal can, in principle, be continuously changed as desired, without being limited to a specific number of polarization states.

[0022] According to one embodiment, the polarization-dependent 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 of the optical signal. However, the refractive index can also be changed by applying a mechanical voltage.

[0023] The polarization-dependent 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.

[0024] According to a further embodiment, the modulator unit is designed to vary the electrical voltage applied to the crystal over time.

[0025] 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 polarization-dependent phase modulator. The time-varying electrical voltage at the polarization-dependent phase modulator changes the phase between differently polarized components of the optical signal.

[0026] 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.

[0027] In other words, this means that the polarization-dependent phase modulator modulates a different phase on the horizontally polarized component of the optical signal on the forward path than on the horizontally polarized component of the retroreflected signal on the return path. This changes the polarization of the output signal of the modulator unit.

[0028] According to a further embodiment, the polarization-dependent phase modulator is configured to change a difference between the first phase and the second phase over time.

[0029] By changing the difference between the phases on the forward and return paths over time, the polarization of the output signal is also changed over time.

[0030] Overall, the polarization of the modulator unit's output signal is freely adjustable between two linear polarizations (horizontal, vertical) and two circular polarizations (Z+ and Z-). However, the polarization can also be continuously adjusted to any elliptical state between the discrete states.

[0031] 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.

[0032] 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.

[0033] According to a further embodiment, the modulator unit is designed to control the light source such that the light source emits pulsed optical signals.

[0034] A light pulse passes through the polarization-dependent phase modulator, and the phase of a first polarization component is 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 polarization-dependent phase modulator. On the return path, the polarization-dependent phase modulator again modulates the phase of a polarization component of the light pulse.

[0035] For example, the light pulse is designed in such a way that in the polarization-dependent phase modulator the light on the way to the reflector and the light on the way back from the reflector do not overlap.

[0036] Apart from the use of pulsed light signals, the light source can also emit a continuous light signal whose polarization is changed as described.

[0037] 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.

[0038] According to a further embodiment, the modulator unit further comprises a beam splitter which is arranged between the light source and the polarization-dependent phase modulator and is designed to direct at least a portion of the retroreflected optical signal phase-modulated by the polarization-dependent phase modulator in a predetermined direction.

[0039] The beam splitter is arranged to at least partially structurally direct the optical signal output by the polarization-dependent phase modulator in a desired direction, so that the optical signal output by the polarization-dependent phase modulator is not emitted exclusively in the direction of the light source that emits the input signal to the polarization-dependent phase modulator. The output signal thus generated carries information in its polarization that can be read and processed by a receiver.

[0040] In one variant, a circulator can be used instead of the beam splitter.

[0041] 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.

[0042] 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.

[0043] The modulator unit described herein is implemented as part of an optical signal transmission link. The optical signal transmission link is thus configured to transmit information by polarization modulation of an optical carrier signal. The polarization modulation is introduced into the optical carrier signal by the modulator unit. The modulator unit intrinsically compensates for phase errors due to the components involved in the modulation because the optical signal traverses the same optical path twice, and the orthogonal polarization components of the optical signal are sequentially modulated.

[0044] According to one embodiment, the modulator unit is arranged in a satellite. However, the modulator unit can be arranged in any other communication system.

[0045] According to a further aspect, a satellite having a modulator unit as described herein is provided.

[0046] 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

[0047] 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: A schematic representation of a modulator unit. Fig. 2: A schematic representation of the optical path through a modulator unit. Fig. 3: A schematic representation of a modulator unit. Fig. 4: A schematic representation of an optical signal transmission path. Detailed description of implementation examples

[0048] Fig. 1 shows the structural design of a modulator unit 100. The modulator unit 100 contains a light source 110, a beam splitter 120, a beam absorber 125, a polarization-dependent 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 and the Faraday mirror 140 may collectively be referred to as a polarization modulator 105. Even if reference is made here, for example, to a Faraday mirror 140, the corresponding explanations generally apply to a reflector mentioned herein.

[0049] The light source 110, which is, for example, a laser, emits an optical signal in the form of the input signal 111. This input signal is fed to the other components and its polarization is modulated in order to transmit information via the optical signal.

[0050] The information to be transmitted is modulated on the output signal 118 in the polarization of the optical signal.

[0051] 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.

[0052] 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.

[0053] First, the input signal 111 hits the beam splitter 120. In this example, the beam splitter 120 is designed 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 EOM 130.

[0054] The EOM 130 then modulates a polarization component of the second part 113 of the optical signal by changing the phase of the polarization component of the signal 113. At the output of the EOM, the optical signal is now present as a singly modulated signal 114, i.e., as an optical signal in which a polarization component is phase-modulated.

[0055] The Faraday mirror 140 retroreflects the singly modulated signal 114 and changes its polarization by 90°, so that the optical signal is reflected again to the EOM 130 as a mirrored signal 115.

[0056] For example, if the horizontally polarized portion of the optical signal was modulated by the EOM on the outward path between signals 113 and 114, then on the return path this modulation is now present in the vertically polarized portion 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 on the return path, the now horizontally polarized portion (corresponding to the vertically polarized portion on the outward path, which did not experience any phase change on the outward path) is phase-changed.

[0057] The optical length between the EOM 130 and the Faraday mirror 140 is dimensioned such that an optical signal on the return path does not interfere with an optical signal on the forward path within the EOM. Generally speaking, the length of the optical path between the EOM and the Faraday mirror is dimensioned such that a light pulse on the return path does not interfere with a light pulse on the forward path. In particular, the length of the optical path is matched to the duration of a light pulse and the transmission rate.

[0058] Thus, the EOM applies a phase to both polarization components of the optical signal on the same optical path. By specifying the corresponding phase on the forward and return paths, the polarization of the optical signal can be varied.

[0059] 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.

[0060] 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.

[0061] Fig. 3 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.

[0062] 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.

[0063] The power supply 160 supplies the light source 110 with electrical energy so that the light source generates the optical signal that 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 portion of a passing optical signal that is polarized in a certain way 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 optical signal passing through the EOM in the desired manner on the outward and return paths 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.

[0064] 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°.

[0065] 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.

[0066] Fig. 4 shows 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.

[0067] 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. The receiver 220 can be located on the Earth's surface or on board another satellite.

[0068] 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

[0069] 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 Branching signal 118 Output signal 120 Beam splitter 125 Beam absorber 130 Polarization-dependent phase modulator, electro-optical modulator 135 Crystal 140 Reflector, Faraday mirror 143 Faraday rotator 145 Mirror 150 Control unit 160 Power supply 200 Optical signal transmission path 210 Transmission path, optical signal 220 Receiver 300 Satellite

Claims

1. Modulator unit (100) for modulating the polarization of an optical signal, including: a light source (110); a polarization-dependent phase modulator (130); a reflector (140); wherein the light source (110) is designed to output an optical signal and emit it as an input signal (111) in the direction of the polarization-dependent phase modulator (130), wherein the optical signal contains a first polarization component having a first polarization direction and a second polarization component having a second polarization direction; wherein the polarization-dependent phase modulator (130) is designed to modulate a first phase of the first polarization component of the input signal (111) in the first polarization direction and to pass on the input signal thus modulated to the reflector (140); characterized in that the reflector (140) is designed to retroreflect the received optical signal in the direction of the polarization-dependent phase modulator (130) and at the same time to change its polarization, so that the first polarization component having the first polarization direction receives the second polarization direction and the second polarization component having the second polarization direction receives the first polarization direction; wherein the polarization-dependent phase modulator (130) is designed to modulate a second phase of the second polarization component of the retroreflected optical signal in the first polarization direction; wherein the modulator unit (100) is designed to output the optical signal thus modulated as a polarization-modulated output signal (118).

2. Modulator unit (100) according to Claim 1, wherein the polarization-dependent phase modulator (130) includes a crystal (135), which is designed to be subjected to an electrical voltage and in this case to change its index of refraction, by which the phase of the first polarization component of the optical signal is changed.

3. Modulator unit (100) according to Claim 2, wherein the modulator unit (100) is designed to vary the electrical voltage applied to the crystal (135) over time.

4. Modulator unit (100) according to any one 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.

5. Modulator unit (100) according to Claim 4, wherein the polarization-dependent phase modulator (130) is designed to change a difference between the first phase and the second phase over time.

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

7. Modulator unit (100) according to any one of the preceding claims, wherein the modulator unit (100) is designed to activate the light source (110) so that the light source (110) emits pulsed optical signals.

8. Modulator unit (100) according to any one of the preceding claims, furthermore including a beam splitter (120), which is arranged between the light source (110) and the polarization-dependent phase modulator (130) and is designed to guide at least a part of the retroreflected optical signal phase-modulated by the polarization-dependent phase modulator (130) in a specified direction.

9. Optical signal transmission link (200), including a modulator unit (100) according to any one of Claims 1 to 8; a receiver (220), which is 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) .

10. Satellite (300) comprising a modulator unit (100) according to any one of Claims 1 to 8.