Retroreflector communication system, especially for satellite communication

Abstract

Retroreflector communication system (1), in particular for satellite communication, comprising: - a transmitting unit (3) with an elastically suspended mirror element (31) which is designed to reflect an incoming light beam, and with a control unit (33) which is designed ◯ to excite the mirror element (31) to a continuous oscillation by means of an actuator (32) and ◯ to frequency-modulate a transmission signal (S) by assigning to each signal value of a transmission signal (S) a specific one of several oscillation frequencies of the continuous oscillation of the mirror element (31).

Description

Technical field

[0001] The invention relates to retroreflector communication systems for optical signal transmission, particularly between satellites and between satellites and ground-based base stations. The invention further relates to the provision of retroreflector communication systems for robust, high-bandwidth signal transmission. Technical background

[0002] In retroreflector communication, a signal receiver sends a light beam (e.g., a laser beam) toward a signal transmitter. The signal transmitter receives the transmitted light beam, modulates it, and reflects the light beam back to the signal receiver.

[0003] The modulation of the light beam by the signal transmitter can be achieved in a variety of ways. Common approaches involve a movable retroreflector that influences the direction of the reflected light beam, thus producing a correspondingly amplitude-modulated light beam on the receiver side.

[0004] Simple signal transmission methods provide for so-called on-off keying, in which data is transmitted by interpreting a light beam reflected onto the receiving unit as a binary 1 value and a light beam not reflected onto the receiving unit as a binary 0 value.

[0005] Various variations are used to modulate the light beam, with an approach using a movable retroreflector representing a low-cost implementation. Approaches that implement modulation directly through movement of the retroreflector generally use movable mirror elements miniaturized using microsystem technology. These can be moved using piezoelectric, electrostatic, or other physical principles, thus influencing the direction of reflection of the received light. Mirror elements implemented using microsystem technology can be deformed or deflected according to a control signal to change their reflection behavior. By switching between a deflected and a non-deflected state, binary data can be transmitted using on-off keying.

[0006] The movement of such a mirror element implemented using microsystem technology can be achieved very quickly. However, when data is transmitted during on-off keying, the acceleration of the mirror element for a state change results in an energy requirement that increases with the control frequency, i.e., an increase in the transmission rate.

[0007] Publication WO 2021 / 078487 A1 relates to a micromechanical oscillation system comprising a micromechanical oscillating body with at least one micromirror. The micromechanical oscillating body is configured to oscillate about an oscillation axis.

[0008] The document DE 10 2014 209 901 A1 discloses a communication device for the optical free-space transmission of data, with a retroreflector having an integrated modulation device with which an incident light beam is modulated.

[0009] The document US2007 / 0 189 779 A1 discloses a method for encoding an analog input signal for optical transmission, wherein a voltage-controlled oscillator is driven by an analog input signal to generate a frequency-modulated signal having a frequency proportional to the amplitude of the input signal, and wherein the frequency-modulated signal is applied to a retroreflector modulating a plurality of quantum wells.

[0010] Document DE 698 23 336 T2 discloses a signaling system, wherein a first signaling unit comprises a retroreflective modulator having a lens, a modulator, and a reflector, and wherein the retroreflective modulator is operable to receive a signal transmitted from the second signaling unit and to modulate and reflect the received signal to transmit a modulated signal to a second signaling unit. The second signaling unit comprises means for generating a signal; means for transmitting the generated signal to the first signaling unit; means for receiving the modulated signal back from the first signaling unit; and means for recovering modulation data from the modulated signal.

[0011] It is therefore an object of the present invention to provide an improved retroreflector communication which is not susceptible to interference and requires little energy for data transmission. Disclosure of the invention

[0012] This object is achieved by the retroreflector communication system, in particular for satellite communication according to claim 1, and by the method for operating a retroreflector communication system according to the independent claim.

[0013] Further embodiments are specified in the dependent claims.

[0014] According to a first aspect, a retroreflector communication system, in particular for satellite communication, is provided, comprising: - a transmitting unit with an elastically suspended mirror element designed to reflect an incoming light beam, and with a control unit designed ◯ to stimulate the mirror element to a continuous oscillation using an actuator and ◯ to frequency-modulate a transmission signal by assigning a specific oscillation frequency of the continuous oscillation of the mirror element to each signal value of a transmission signal.

[0015] Furthermore, a receiving unit can be designed to direct a light beam onto the transmitting unit, to receive a frequency-modulated light beam and to decode the frequency-modulated light beam by assigning a specific signal value to specific frequencies of light pulses of the frequency-modulated light beam.

[0016] The retroreflector communication system can be based on directing a light beam, in particular a laser beam, at a receiving unit to a transmitting unit and receiving a modulated reflected light beam, in particular a modulated laser beam, wherein a data signal can be generated by demodulation of the modulated light beam.

[0017] The transmitting unit merely provides a movable mirror element, which can be in the form of a micromirror. The mirror element is elastically suspended, thus forming an oscillating mechanical system. The mirror element is preferably operated on the elastic suspension near its mechanical resonance frequency of oscillation. This allows the transmitting unit to be operated with relatively low energy consumption by only slightly influencing the oscillation frequency of the mirror element.

[0018] While the conventional method for data transmission uses on-off coding, the above retroreflector communication system envisions operating an elastically held mirror element in oscillation near its resonant frequency to achieve an oscillating modulation of the transmitted light beam. For this purpose, the mirror element can be periodically controlled by a suitable actuator, such as an electrostatic actuator with a counter electrode spaced from the mirror element or a piezo actuator, so that the actuator tilts the reflection surface of the mirror element in an oscillating manner and introduces energy to maintain continuous oscillation. These approaches are known, for example, from the publication WO2021078487 A1.

[0019] Frequency modulation is therefore implemented as a modulation method for transmitting the transmission signal. By operating close to the resonant frequency, the energy required to maintain continuous oscillation of the mirror element can be kept very low, since the mirror element does not need to be abruptly decelerated or accelerated, as is the case with on-off keying.

[0020] It can be provided that the oscillation frequencies for encoding the transmission signal lie in a frequency range of + / - 5% of a resonance frequency of the elastically suspended mirror element or its harmonics.

[0021] Discrete signal levels or signal values ​​of a transmitted signal can be encoded at different oscillation frequencies around the resonant frequency. For example, different signal levels or signal values ​​can be encoded within a range of ± 5% of the resonant frequency or its harmonics. In particular, with a digital transmitted signal, a first signal level can be encoded at an oscillation frequency below the resonant frequency and a second signal level at an oscillation frequency above the resonant frequency.

[0022] The transmission signal can be configured with temporally successive data elements so that each data element can assume one of two or more than two signal values. Thus, transmission signals with more than two possible signal values ​​per "data bit" or data element can also be provided.

[0023] The resonance frequency of a mirror element designed as a micromirror can be in the range of several 10 kHz, so that a correspondingly high data transmission rate is possible with low energy consumption, especially if the modulated oscillation frequencies are chosen close to the resonance frequency or the frequency of a harmonic.

[0024] The combination of a frequency-modulated signal transmission with an oscillating mirror element that can be actuated electrostatically or piezoelectrically enables signal transmission using an energy-efficiently maintained harmonic oscillation of the mirror surface.

[0025] As a result, the receiving unit receives a frequency-modulated light beam whose intensity changes periodically. The periodic amplitude change of the modulated light beam reflected back to the receiving unit can be demodulated accordingly to reconstruct the transmitted signal.

[0026] The mirror element used can, for example, be suspended in a torsionally elastic manner and, in a central position around which the mirror element oscillates when excited, reflect the modulated light beam back toward the transmitting unit. If the mirror element is set into oscillation, the reflected light beam can be received in the receiving unit twice per oscillation period. By measuring the time between two consecutive pulses of the reflected light beam, decoding to a signal level or signal value can be performed. In principle, the transmission frequency achievable in this way roughly corresponds to the oscillation frequency of the mirror element, whereby two to five half-periods should preferably be used for each transmitted signal level or signal value to ensure proper data transmission.

[0027] The light beam should be reflected back to the receiving unit at a zero point position of the mechanical oscillation or the center position of the mirror element, so that a light pulse can be received at each half-oscillation and the oscillation amplitude has only a minimal influence on the quality of the data transmission. For this purpose, the mirror element can be elastically suspended, for example, on a torsion beam or a web structure, whereby the deflection can occur in two directions starting from the zero point position or center position. This enables the mirror element to oscillate independently around the zero point position or center position after excitation.

[0028] A telecentric lens system can be positioned in front of the mirror element in the beam direction, allowing light rays arriving from different directions of incidence to be directed onto the mirror element, so that an incoming light beam and an outgoing light beam coincide twice per oscillation period. This allows for a higher angular tolerance with respect to the orientation between the optical axis of the transmitting unit and the distance between the transmitting unit and the receiving unit, enabling data transmission even in the event of misalignment, particularly in the orientation of the transmitting unit.

[0029] Furthermore, the mirror element can be designed as a micromirror having a mirror surface suspended from a torsion beam or having a mirror surface suspended from an elastic web.

[0030] According to a further aspect, a method for operating the above retroreflector communication system, in particular for satellite communication, is provided, comprising the following steps: - Operating an actuator to excite the mirror element to a continuous oscillation; and - Frequency modulating a transmission signal with successive signal values ​​by assigning to each signal value of a transmission signal a specific one of several oscillation frequencies of the continuous oscillation of the mirror element. Brief description of the drawings

[0031] Embodiments are explained in more detail below with reference to the attached drawings. They show: Fig. 1 a schematic representation of a retroreflector communication system with a transmitting unit and a receiving unit for unidirectional data transmission; and Fig. 2a and Fig. 2b various embodiments of the spring-elastically mounted mirror element; Fig. 3 an amplitude curve of an exemplary frequency-modulated light signal. Description of embodiments

[0032] Fig. 1 shows schematically the structure of a retroreflector communication system 1 with a receiving unit 2 and a transmitting unit 3.

[0033] The receiving unit 2 has a laser source 21 that directs a laser beam (for example, a light beam) toward the transmitting unit 3. Furthermore, the receiving unit 2 has a photodetector 22 or another sensor element sensitive to the laser beam to receive a reflected laser beam and provide an electrical signal to a demodulation unit 23. The electrical signal obtained from the photodetector 22 is demodulated in the demodulation unit 23 to obtain a data stream of received data E.

[0034] The transmitting unit 3 has a movable mirror element 31 with a reflector surface. The mirror element 31 is elastically suspended so that it can oscillate around a center position when excited. The mirror element 31 is preferably designed as a micromirror so that high oscillation frequencies in the range of several tens of kHz can be achieved. The surface of the micromirror can, for example, be between 0.1 and 1 mm 2 be.

[0035] The micromirror can be excited to oscillate using a suitable actuator 32. The actuator 32 can be piezoelectric or electrostatic and is controlled by a control unit 33.

[0036] In Fig. 2a and Fig. Figure 2b shows variants of the elastically suspended micromirror (mirror element 31). Fig. Figure 2a shows a micromirror 42 suspended from a torsion beam 41, so that it can tilt due to elastic torsion and forms an oscillating system. The embodiment of the Fig. Figure 2b shows a micromirror 42 suspended from an elastic web-like structure 43 so that it is pivoted about its suspension.

[0037] A telecentric lens system 4 is positioned in front of the mirror element 31 in the optical beam path to direct an incoming laser beam onto the mirror surface of the mirror element 31. The laser light is only reflected back in the direction of incidence if the surface normal of the mirror surface is aligned at a specific angle. If the mirror surface is deflected as a result, the laser beam does not return to the receiving unit 2.

[0038] The control unit 33 controls the actuator 32 so that the mirror element 31 oscillates at a predetermined frequency. The oscillation frequencies are selected so that they are close to the resonant frequency of the mechanical system of the mirror element and its elastic suspension.

[0039] A transmission signal S, which is to be transmitted from the transmitting unit 3 to the receiving unit 2, is frequency modulated so that a signal level corresponds to a specific oscillation frequency of the mirror element 31.

[0040] Fig. 3 shows schematically the coding of two signal values ​​“0” and “1” in two oscillation frequencies f niedrig , f hoch .

[0041] The excitation of the mirror element 31 preferably occurs close to the resonant frequency or a harmonic, so that the oscillation of the mirror element 31 / micromirror 42 can be maintained with low energy expenditure. The oscillation frequencies can preferably be in a range between - and + 5% of the resonant frequency or one of its harmonics.

[0042] Bits of a data stream can thus be encoded using different oscillation frequencies of the laser beam modulation. Receiving unit 2 can then receive a periodic signal from the reflected laser beam and assign it to the corresponding signal value using a corresponding frequency measurement. List of reference symbols 1 retroreflector communication system 2 receiving unit 21 Laser source 22 Photodetector 23 Demodulation unit 3 Transmitter unit 31 Mirror element 31 32 Actuators 33 Control unit 4 telecentric lens system E Reception data L light beam S transmission signal

Claims

[1] Retroreflector communication system (1), in particular for satellite communication, comprising: - a transmitting unit (3) with an elastically suspended mirror element (31) which is designed to reflect an incoming light beam, and with a control unit (33) which is designed ◯ to excite the mirror element (31) to a continuous oscillation by means of an actuator (32) and ◯ to frequency-modulate a transmission signal (S) by assigning to each signal value of a transmission signal (S) a specific one of several oscillation frequencies of the continuous oscillation of the mirror element (31). [2] Retroreflector communication system (1) according to claim 1, wherein a receiving unit (2) is designed to direct a light beam (L) onto the transmitting unit (3), to receive a light beam frequency-modulated by the transmitting unit (3) and to decode the frequency-modulated light beam by assigning a signal value to specific frequencies of light pulses of the frequency-modulated light beam. [3] Retroreflector communication system (1) according to claim 1 or 2, wherein the oscillation frequencies for encoding the transmission signal (S) lie in a frequency range of + / - 5%, preferably of + / - 2%, of a resonance frequency or a harmonic of the elastically suspended mirror element (31). [4] Retroreflector communication system (1) according to one of claims 1 to 3, wherein the transmission signal (S) is designed with temporally successive data elements in order to assume one of two or more than two signal values ​​per data element. [5] Retroreflector communication system (1) according to one of claims 1 to 4, wherein the mirror element (31) is designed as a micromirror having a mirror surface suspended from a torsion beam or having a mirror surface suspended from an elastic web. [6] Retroreflector communication system (1) according to one of claims 1 to 5, wherein a telecentric optical lens system (4) is arranged in front of the mirror element (31) in order to direct an incident light beam (L) onto the mirror element (31). [7] Retroreflector communication system (1) according to one of claims 1 to 6, wherein a signal value of the transmission signal (S) is encoded by 2 to 10, preferably by 3 to 5 half-periods of the mechanical oscillation. [8] Method for operating a retroreflector communication system (1), in particular for satellite communication, according to one of claims 1 to 7, comprising the following steps: - operating an actuator (32) to excite the mirror element (31) to a continuous oscillation; and - Frequency modulating a transmission signal (S) with successive signal values ​​by assigning to each signal value of a transmission signal (S) a specific one of several oscillation frequencies of the continuous oscillation of the mirror element (31).

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