Wavefront compensation system

Abstract

The invention provides an atmospheric turbulence-compensation system configurable as: receiver, transmitter, transceiver comprising a first optical array, a phase-shift modulator, a light-wave combiner, and a control system, wherein: (I) the first optical array is configured to receive and provide optical signals (II) the phase-shift modulator alters the optical signals to modified optical signals in dependence of phase-shift correction signals; (III) the light-wave combiner is configured to combine the modified optical signals and provide an electrical output signal; and (IV) the control system is configured to compensate for atmospheric disturbances by providing phase-shift correction signals based on the electrical output signal.

Description

[0001] WAVEFRONT COMPENSATION SYSTEM

[0002] TECHNICAL FIELD

[0003] The invention relates to an atmospheric turbulence-compensation system. The invention further relates to a method for atmospheric turbulence-compensation using the atmospheric turbulence-compensation system.

[0004] BACKGROUND ART

[0005] Wireless communication is established by transmitting information optically, wherein a beam of light is transmitted from a transmission station and be subsequently received by a receiver at a receiving station. However, the beam being transmitted through the atmosphere is distorted or undergoes aberrations as a consequence of traveling through an inhomogeneous medium i.e. the atmosphere. Furthermore, due to variations in the atmospheric conditions as a result of turbulence, there is a need to compensate for these aberrations to recover the original optical transmission.

[0006] Conventional systems for optical space-ground communication rely on adaptive optics (AO) systems to overcome disturbances in the atmosphere due to turbulence. Such systems are known in the art. For instance, US2016028479A1 describes an adaptive optics compensation approach for an orbital-angular-momentum (OAM) multiplexed free-space optical (FSO) communication system, in which a Gaussian beam is used to probe the turbulence -induced wavefront distortions and derive the correction pattern for compensating the OAM beams. Using this approach, it is demonstrated to have simultaneous compensation of multiple OAM beams each carrying a 100 Gbit / s data channel through emulated atmospheric turbulence. The results indicate that the turbulence-induced crosstalk and power penalty could be efficiently mitigated by -12.5 dB and -11 dB respectively.

[0007] These systems usually require telescopes to collect a sufficient amount of received irradiance at the interface between the atmosphere and the telescope (i.e., at the air interface), and dynamically adjust optical elements (e.g., an array of mirrors) located at the telescope’s back end, to compensate for distortions caused by atmospheric turbulence or other optical aberrations. Such systems may comprise arrays of optical elements such as mirrors or lenses that may have to be controlled in realtime in dependence of the atmospheric conditions. These systems may comprise mechanically complex elements that may have to be monitored in real-time.

[0008] As a consequence of this, these systems are relatively complex in their construction and operation. Furthermore, such systems require precise control and calibration to be used effectively. These systems, especially systems comprising adaptive optics, may also be relatively large in size and expensive to manufacture: Additionally, such systems are difficult to scale to match telescope sizes for different applications which require application-specific baseline architectures. Further, the serial functional chain for processing communication signals combines key components that constitute single point of failure risks. Therefore, it is desired to have a system that is simple in construction, robust and cost-efficient.

[0009] Hence, it is a goal of the present invention to provide an improved atmospheric turbulencecompensation system that allows for a scalable, cost-efficient baseline architecture and re-distributes the serial chain of functionalities to a parallel architecture.

[0010] SUMMARY OF INVENTION

[0011] According to a first aspect, the invention provides an atmospheric turbulence-compensation system comprising a first optical array, a phase-shift modulator, a light-wave combiner, and a control system, wherein: (I) the first optical array comprises nl first optical elements optically connected to the phase-shift modulator, wherein the nl first optical elements are configured to receive input free space optical signals (or “free space signals”) and to provide the received input optical signals as nl first optical signals respectively to the phase-shift modulator; (II) the phase-shift modulator comprises nl first optical input-output ports configured to receive the nl first optical signals, nl first electrical input ports configured to receive nl first phase-shift correction signals and nl first optical input-output ports configured to provide nl modified first optical signals to the light-wave combiner, wherein the phase-shift modulator alters the nl first optical signals to nl modified first optical signals in dependence of nl first phase-shift correction signals; (III) the light-wave combiner comprises nl first optical input ports configured to receive the nl modified first optical signals from the phase-shift modulator and to combine the nl modified first optical signals to a combined modified optical signal, wherein the light-wave combiner is configured to provide an electrical output signal based on the combined modified optical signal to the control system; and (IV) the control system comprises an electrical input port configured to receive the electrical output signal from the light-wave combiner and nl first electrical output ports configured to provide the nl first phase-shift correction signals to the phase-shift modulator, wherein the control system is configured to compensate for atmospheric disturbances by providing nl first phase-shift correction signals to the phase-shift modulator.

[0012] In contrast to conventional solutions, the present invention does not need specific photonic integrated circuits or conventional adaptive-optics systems. Instead the present invention relies on existing telecommunications components and applies customized firmware and software-algorithms that are developed to create a versatile wavefront compensator. This has the advantage of a flexible supply chain that may utilize a widely available and reliable telecom component base. The architecture provides scalability on effective diameter and power levels, allowing for versatile implementations and future upgrades via software and firmware. The wavefront compensation algorithm relies on reciprocity tracking and is upgraded, for instance via machine-learning algorithms, that are collecting data, training and learning during operation. Consequently, the aforementioned features also provide the benefit of reducing the complexity of the system, making it feasible to manufacture and maintain the system at relatively low cost. Here below, individual elements of the atmospheric turbulencecompensation system are discussed in further detail.

[0013] The first optical array comprises nl first optical elements. The nl first optical elements facilitate an optical connection between the first optical array and the phase-shift modulator. The nl first optical elements are configured to (i) receive input free space optical signals and (ii) provide the received input free space signals as the nl first optical signals to the phase-shift modulator. The input free space signals may be a single beam / signal that is transmitted from a transmission station. This input free space signal is incident on the nl first optical elements at a receiving station. Each of the nl first optical element elements receive at least a part of the optical signal incident on said first optical element. Therefore, each individual first optical element receives at least a part of the input free space signal, and collectively the nl first optical elements provide the received input free space signal as nl first optical signals to the phase-shift modulator. Here, the input free space signals refer to optical transmissions that are transmitted through the atmosphere and are subsequently received by the first optical array. For example, a transmission station (e.g., a satellite or an aircraft) beams optical signals which are then received by the first optical array located at a ground station. The first optical array captures the input free space signals and transmits these signals to the phase-shift modulators via the nl first optical elements. In an embodiment, the first optical element may be an optical fiber.

[0014] In an embodiment, the first optical array comprises at least two first optical elements. That is, nl is at least 2. However, to improve capture of the wavefront of the input free signals, additional first optical elements are desired. Hence, in an example hexagonal embodiment, nl is 7, 19, 37, 61, 91 and so on. In addition to the number of first optical elements, it may also be advantageous to arrange the first optical elements in arrays. In an embodiment, the first optical elements are arranged in a circumferential array. Circumferential arrays, refers to an arrangement where the nl first optical elements are arranged equidistant around a central point. The circumferential arrays may or may not be symmetric, however, symmetric circumferential arrays are preferred. Typically, such arrays comprise, in cross-section, a single optical element (e.g., an optical fiber with a collimator) in the center with a ring of optical fibers surrounding the central fiber. Alternatively, in an embodiment, the circumferential array may only comprise first optical elements arranged around a central obstruction or void. Here, circumferential array refers to first optical elements that in cross-section are arranged around a central point. For example, the first optical elements is configured in hexagonal arrays i.e., at positions of comers of a hexagon. In a further embodiment, the first optical elements are arranged in concentric hexagonal rings. That is, the first ring may comprise six elements around a first optical element in the center, the second ring surrounding the first ring may comprise twelve elements and so on. Hence, in an embodiment, the first optical array has one first optical element in the center surrounded by six first optical elements in the first hexagonal ring. This is followed by a second ring of twelve first optical elements, and so on. Hence, as mentioned above, in specific embodiments, nl is 7, 19, 37, 61 and 91 (this includes a single first optical element in the center of the array). Yet further, the first optical elements may (also) be arranged differently to suit any other predetermined geometry, e.g. rectangular, square, triangular.

[0015] In an embodiment, the phase-shift modulator comprises nl first optical input-output ports configured to receive the nl first optical signals. Especially, the phase-shift modulator is configured to receive the nl first optical signals from the first optical array. Atmospheric turbulence causes variations in the density of the atmosphere in the space between the transmission station and the first optical array, thereby introducing optical variations / aberrations (e.g., phase modulations) of the input free space optical signals. Since the input free space signals are captured by the first optical array, these variations / aberrations are also manifested in the transmitted nl first optical signals. In addition to the nl first optical signals, the phase-shift modulator additionally receives nl phase-shift correction signals. Therefore, the phase-shift modulator comprises nl first electrical input ports configured to receive the nl phase-shift correction signals. The nl phase-shift correction signals contain information to compensate for aberrations due to turbulent atmospheric conditions. Said information relates to modifications in the phase of each of the nl first optical signals. In this way, the incoming nl first optical signals are corrected (for variations due to atmospheric turbulence) in dependence of the nl phase-shift correction signals. Here, the phase-shift modulator is configured to individually alter the phase of the nl first optical signals. Especially, the nl first optical signals are altered to the nl modified first optical signals. Particularly, the phase of the nl first optical signals are each individually modified in dependence of the nl first phase-shift correction signals. Further, the phase-shift modulator comprises nl second optical input-output ports to provide the nl modified first optical signals to the light wave combiner. Note that the first (or second) input-output ports are bi-directional i.e., the inputoutput ports are configured to receive and / or transmit signals.

[0016] The light-wave combiner (e.g., a fiber-optic coupler) is configured to combine a plurality of optical signals into a single optical signal i.e., a combined modified optical signal. Especially, the lightwave combiner is configured to provide a combined optical signal at its output by optically summing the power of all the nl first input signals. In an embodiment, the light-wave combiner comprises nl first optical input ports configured to receive the nl modified first optical signals from the phase-shift modulator. Subsequently, the light-wave combiner is configured to combine the nl modified first optical signals to a combined modified optical signal. The light-wave combiner is further configured to provide an electrical output signal based on the combined modified optical signal. Furthermore, the light-wave combiner comprises an electrical output port to provide the electrical output signal. Especially, the light-wave combiner is configured to provide the electrical output signal to a control system via the electrical output port. Note that light-wave combiners are known in the art, wherein the light-wave combiner may also perform the function of splitting any combined optical signals to individual optical signals (in dependence of a predetermined splitting ratio). Further, the light-wave combiner may in an embodiment be selected from the group of a bare fiber optical combiner, a blockless fiber combiner and an ABS combiner.

[0017] In an embodiment, the atmospheric turbulence compensation system comprises an optical- electrical device (or “optoelectronic transducer” or “optical-electrical converter”) configured to convert the combined modified optical signal to the electrical output signal. It will be apparent to the skilled person that the optical-electrical device comprises an optical input port configured to receive the combined modified optical signal (from the light-wave combiner) and an electronic output port to provide the electrical output signal (to the control system).

[0018] Additionally or alternatively, in embodiments, the atmospheric turbulence-compensation system comprises an optical power detector. Especially, the light-wave combiner may be connected to the optical power detector. The optical power detector is configured to convert the combined modified optical signal into the electrical output signal. In a further embodiment, the optical power detector may be configured to optically filter and / or amplify the combined modified optical signal before conversion to the electrical output signal. It will be apparent to the skilled person that the optical power detector comprises an optical input port configured to receive the combined modified optical signal (from the light-wave combiner) and an electronic output port to provide the electrical output signal (to the control system). Alternatively, the optical power detector may be configured as a part of the light-wave combiner.

[0019] In an embodiment, the control system comprises an electrical input port configured to receive the electrical output signal from the light-wave combiner. The control system processes the electrical output signal by means of a phase-shift algorithm to provide the nl phase-shift correction signals. The nl phase-shift correction signals comprise information relating to the required shift or modulation of the phase of each of the nl first optical signals. Furthermore, the control system comprises nl first electrical output ports configured to provide the nl first phase-shift correction signals (back) to the phase-shift modulator. In this way, the control system is configured to compensate for atmospheric disturbances by providing nl first phase-shift correction signals (back) to the phase-shift modulator.

[0020] In a further embodiment, the control system may comprise one or more of an opto-electrical device, a tuneable optical filter, an optical (e.g. ASE-) noise blocker, an optical signal splitter and an optical signal amplifier. In yet further embodiments, the control system may comprise a tuneable electric filter and / or an electrical noise blocker. These are standard electrical components and the skilled person will be aware of their use in generating the required phase-shift correction signals from the electrical output signal.

[0021] In summary, the invention is configured to execute a control loop, wherein:

[0022] (i) the first optical array is configured to receive input free space optical signals and to provide nl first optical signals respectively to the phase-shift modulator;

[0023] (ii) the phase-shift modulator is configured to receive the nl first optical signals and the nl first phase-shift correction signals, alter the nl first optical signals to the nl modified first optical signals in dependence of the nl first phase-shift correction signals, and provide the nl modified first optical signals to the light-wave combiner;

[0024] (iii) the light-wave combiner is configured to receive the nl modified first optical signals, combine the nl modified first optical signals to the combined modified optical signal, and provide the electrical output signal based on the combined modified optical signal to the control system; and

[0025] (iv) the control system is configured to receive the electrical output signal and provide the nl first phase-shift correction signals to the phase-shift modulator.

[0026] Thereby, the iterative control loop is executed which facilitates continuously compensating for atmospheric disturbances by providing nl first phase-shift correction signals (to the phase-shift modulator) to alter / modulate the phase of the first input signals (received from the first optical array). The phase-shift algorithm used by the control system for this purpose is described further below.

[0027] As mentioned above, one of the benefits of the present system is the scalability of the system. In a further embodiment, the atmospheric turbulence-compensation system further comprises at least a second optical array. Here, the second optical array comprises n2 second optical elements optically connected to the phase-shift modulator. In some embodiments, the second optical array is identical to the first optical array. However, in other embodiments, the second optical array may also be different from the first optical array, with a different number of second optical elements. That is, nl is equal to n2 in some embodiments; and alternatively, nl is not be equal to n2 in other embodiments.

[0028] In an embodiment, the n2 second optical elements are configured to provide n2 second optical signals respectively to the phase-shift modulator. Here, the phase-shift modulator receives n2 second optical signals (from the second optical array) in addition to the nl first optical signals (from the first optical array). Hence, the phase-shift modulator comprises n2 second optical input-output ports configured to receive n2 second optical signals. Additionally, the phase-shift modulator is configured to receive n2 second phase-shift correction signals. Therefore, the phase-shift modulator (also) comprises n2 second electrical input ports configured to receive the n2 phase-shift correction signals. Furthermore, the phase-shift modulator is configured to alter the n2 second optical signals in dependence of the n2 phase-shift correction signals, to provide the n2 modified second optical signals. Subsequently, the phase-shift modulator is configured to provide the n2 second modified second optical signals to the light-wave combiner. Therefore, the phase-shift modulator comprises (additional) n2 second input-output ports configured to provide n2 modified second optical signals to the lightwave combiner.

[0029] The light-wave combiner comprises n2 second optical input ports configured to receive the n2 modified second optical signals from the phase-shift modulator. Here, the light-wave combiner is configured to combine both the nl modified first optical signals and the n2 modified second optical signals to provide the combined modified optical signal. As mentioned above, the light-wave combiner is configured to generate the electrical output signal based on the combined modified optical signal. Subsequently, the light-wave combiner provides the electrical output signal to the control system.

[0030] The control system is configured to receive the electrical output signal from the light-wave combiner. The control system comprises an electrical input port configured to receive the electrical output signal from the light-wave combiner. Subsequently, the control system executes the phase-shift algorithm to generate nl phase-shift correction signals and n2 phase-shift correction signals. Therefore, the control system comprises n2 second electrical output ports (in addition to the nl first electrical output ports) configured to provide the n2 phase-shift correction signals (and the nl phaseshift correction signals) to the phase-shift modulator, respectively. In this way, the control system is configured to compensate for atmospheric disturbances by providing n2 second phase-shift correction signals (in addition to the nl phase-shift correction signals). Subsequently, the nl phase-shift correction signals and the n2 phase-shift correction signals are (both) provided (back) to the phaseshift modulator.

[0031] The use of two optical arrays is an example. It will be apparent to the skilled person that the entire architecture may be scaled as an array of arrays, i.e. comprise multiple optical arrays configured to receive input free space optical signals. In a further embodiment, the atmospheric turbulencecompensation device may comprise a third optical array, a fourth optical array and so on. In a further embodiment, the atmospheric turbulence-compensation system may comprise a plurality of such optical arrays. Subsequent arrays are coupled to the system in a similar manner to the first optical array and the second optical array.

[0032] A further functionality of the present system is to transmit signals in addition to receiving signals. In a further embodiment, the atmospheric turbulence-compensation system comprises a multiport circulator and a high-power transmitting optical array (or “high-power optical array”).

[0033] The multiport-circulator comprises n3 transmission ports, n3 reception ports and n3 inputoutput ports. The multiport circulator facilitates bi-directional communication, (both) to receive and transmit signals. In such embodiments, the phase-shift modulator comprises n3 first optical inputoutput ports connected to the n3 input-output ports of the multiport-circulator. Further, the n3 reception ports of the multiport-circulator are connected to n3 first optical input ports of the light-wave combiner. Here, the phase-shift modulator is connected to the light-wave combiner via the multiport-circulator.

[0034] Further, the n3 transmission ports of the multiport-circulator are connected to a high-power optical array. The high-power optical array is configured to generate n3 optical transmission signals and provide said signals to the phase-shift modulator via the multiport circulator. Here, the phase-shift modulator is connected to the high-power optical array via the multiport-circulator. In this way, the multiport-circulator may facilitate bi-directional communication by (i) the transmission of optical transmission signals from the high-power optical array to the phase-shift modulator and (ii) the reception of signals from the phase-shift modulator to the light-wave combiner. Hence, in some embodiments nl=n3. However, alternatively, the multiport-circulator may comprise dedicated ports for the nl first optical signals and the n2 second optical signals, wherein n3=nl+n2. It will be apparent to the skilled person to configure the necessary number of ports. Yet further, in some embodiments it may apply that n3>nl+n2. In this way, the required number of ports for the operation of the system may be selected.

[0035] The ‘circulator’ may in an embodiment also be a directional coupler that separates a weak (receive) signal at a wavelength RX from a strong (transmit) signal at wavelength ATX. Depending on the separation width between the wavelengths Ai<xand ATX. different types of ‘directional couplers’ may be used (e.g. a wavelength splitter with filters for (receive) signals instead of a conventional ‘circulator’ devices).

[0036] In an embodiment, the atmospheric turbulence-compensation system executes the iterative control loop to continuously compensate for atmospheric disturbances in the n3 optical transmission signals. In order to facilitate the execution of the control loop, the system is configured as follows. The first optical array is configured to be operated in both a reception mode to receive the input free space signals and in a transmission mode to transmit optical transmission signals. Hence, in such embodiments, nl is selected to be equal to n3. Thereby, the same input-output ports of the phase-shift modulator are used to receive the nl first optical signals and to transmit the nl optical transmission signals.

[0037] In alternative embodiments, the atmospheric turbulence-compensation system comprises a dedicated optical array to connect with the high-power optical array. That is, the atmospheric turbulence-compensation system comprises a third optical array further comprising n3 third optical elements. The third optical array also allows for bi-directional operation, where the third optical array is configured to transmit the n3 optical transmission signals and to receive n3 third optical signals (both) by means of the n3 third optical elements. In this way, the atmospheric disturbances are discerned from the n3 third optical signals and the n3 optical transmission signals are corrected for disturbances in the atmosphere before transmission (in dependence of the (measured) n3 third optical signals).

[0038] Here, the n3 reception ports of the multiport-circulator are connected to the n3 first optical input ports of the light-wave combiner (e.g. a High-Power NxM Optical Coupler). In this way, the phase-shift modulator is optically connected to the light-wave combiner via the multiport-circulator. That is, the light-wave combiner is configured to provide the n3 modified third optical signals via the multiport-circulator to the light-wave combiner. In a similar manner to the above-mentioned embodiments, the light-wave combiner is configured to receive n3 third optical signals and combine said optical signals into a combined modified optical signal. Furthermore, the light-wave combiner is configured to provide an electrical output signal to the control system. The control system is configured to generate n3 phase-correction signals by means of the phase-shift algorithm. Subsequently, the n3 phase-correction signals are provided (back) to the phase-shift modulator. In this way, the n3 phaseshift correction signals are used to correct the n3 optical transmission signals to compensate for disturbances in the atmosphere. In this embodiment, it would be apparent to the skilled person that the number of ports on the one or more elements comprised by the system are selected to be consistent.

[0039] In a further embodiment, the phase-shift modulator may comprise (additional) n3 third inputoutput ports and the multiport-circulator may comprise (additional) n3 third optical input-output ports. In such an embodiment, nl is selected to be different from n3 as there are separate ports to facilitate the transmission of the n3 optical transmission signals.

[0040] Furthermore, note that it may not always be necessary that the system comprises the second optical array and the third optical array. In an embodiment, the first optical array may be used in the same capacity as the third optical array. In other words, the first optical array may be used to transmit the optical transmission signals and to receive the input free space signals.

[0041] Some further features of the elements of the atmospheric turbulence-compensation system are described here below.

[0042] In an embodiment, the high-power optical array comprises n3 light sources. In a further embodiment, the n3 light sources may be selected from monochromatic coherent light sources, preferably the light sources is laser-pumped. In some embodiments, a single coherent light source may be split into n3 individual beams, for instance a diffractive beam-splitter or a diffractive optical element (DOE) may be used to split the single light source into n3 (co-phased) light sources. Furthermore, the high-power optical array is configured to provide optical signals with sufficient power to be beamed into the atmosphere. This permits distributing a relatively high total power over n3 elements. The total optical power is at least 40 W, especially at least 80 W, preferably at least 100 W, or even higher.

[0043] The first optical array is positioned such that it is directed towards the source of the incoming transmission. In a further embodiment, the atmospheric turbulence-compensation system comprises a first tip tilt controller. Likewise, in a further embodiment comprising the second optical array or the third optical array, said optical arrays may (also) comprise an individual tip tilt controller. The first tip tilt controller is configured to individually control the orientation of each of the nl first optical elements, to achieve maximum single-mode fiber power coupling (‘hot spot tracking’). It will be apparent to the skilled person that Single-mode fiber power coupling, often referred to as "hot spot tracking," is a technique used in optical communication systems to maximize the efficiency of light transmission between optical components.

[0044] The tip tilt controllers provide the benefit of partially correcting the incoming wavefront of the input free space signals thereby improving the coherence and quality of the input free space signals. In reciprocity, the same tip tilt controllers are used to correct the wavefront of the transmitted optical transmission signals. Tip tilt controllers are known in the art, and are a standard element in a variety of alignment applications. Yet further, in an embodiment, the first optical array and / or the second optical array may comprise one or more elements from the group of wavefront correctors, collimators, gradient index rod lenses (GRIN) lenses and diffractors. These elements are configured upstream of the first optical array and / or the second optical array. Here, an upstream position relative to a downstream position is defined as a first position closer to the source of light than a second position along the direction of propagation of light. Such elements provide the benefit of improving the quality of the free space input signals, and thereby providing higher quality first optical signals. Particularly, the coherence of the input signals is improved, aberrations in the input signals is reduced and especially noise is reduced.

[0045] Here below, a description of the phase-shift algorithm to provide the phase-shift correction signals to correct the first optical signals for atmospheric turbulence is provided. Turbulence is characterized by the Kolmogorov scales, especially, the Kolmogorov time scale. This time scale relates to the time it takes for energy to cascade from large turbulent eddies to the smallest scales where viscosity dominates and turbulence kinetic energy dissipates into thermal energy. In other words, the Kolmogorov time scale is the smallest time scale over which any atmospheric change takes place. In contrast, the time required for a single iteration of the control loop is several orders of magnitude (i.e., 10 times, or 100 times, or 1000 times, or more) smaller than the Kolmogorov time scale for atmospheric turbulence. As a result of this, several hundred or more iterations of the control loop may be executed before any change to the atmospheric condition is discernible i.e., the atmosphere may be treated as a quasi ‘frozen turbulence’ state.

[0046] Further, the atmospheric fading information (i.e., information relating to a phase modulation corresponding to each first optical signal due to disturbances in the atmosphere) is known for each aperture (i.e., each of the nl first optical elements). Therefore, the first optical signals may each be adaptively processed, co-phased, and scaled before being summed. Thus mitigating signal fading caused by atmospheric turbulence. The outcome of this signal processing and linear combining is an optimal, channel-matched adaptive coherent receiver implemented using the first optical elements (i.e., “the receive apertures”).

[0047] In free-space wireless optical communications, a channel-matched adaptive coherent receiver is implemented using an array of first optical elements (or “receive apertures”). Here, the term “receive aperture” refers to a receiving means that captures the incoming electromagnetic signals, such as the first optical element (in the first optical array) which is configured to receive the input free space signals. The term “channel-matched” refers to the system’s ability to adapt to the specific characteristics of the communication channel, for example aberrations in the input free space signals due to atmospheric turbulence. The term “coherent receiver” refers to the system’s ability to extract information from modulated signals (i.e., phase modulate light signals) by maintaining phase coherence with the optical signals transmitted from a transmission station. Especially, the receiver i.e., the atmospheric turbulence-compensation system may dynamically adjust the phase modulation of the nl first optical signals to maintain said phase coherence and subsequently increase the total summed power of the nl first optical signals.

[0048] The working principle of the phase-shift algorithm aims to increase the total (coherent) power of the combined modified optical signal. The control system measures the increments to the total summed optical power of the combined modified optical signal as a function of individual phase shifts of the nl first optical signals. At every subsequent iteration of the phase-change algorithm, the phase of each individual first optical signal is altered such that the total summed optical power of the combined modified optical signal is increased. To that end, an efficient optimization algorithm for atmospheric-turbulence compensation (or “fading parameter adaptation”) of the nl first optical signals is used.

[0049] For the atmospheric turbulence-compensation system, the signals received by the nl first optical elements (in the first optical array) may exhibit random fluctuations in both envelope and phase over time, leading to destructive interference in the light-wave combiner. To characterize the noise affecting the nl first optical signals, optical fading of the nl first optical signals is aggregated into a complex channel column vector a G Cnl. A general entry of the atmospheric fading vector is denoted = \ai \exp <pi), where \a(| represents the fading envelope, and <p(represents the corresponding random phase of the Ithfirst optical signal G {1,2, ... , nl}. In this context, an adaptive linear combiner iv G Cnlis considered to compensate for fading effects. In this case, the fading vector becomes w a. and the complex weight (yet unknown) applied to the 1thfirst optical signal is characterized broadly as Wi = \wi \exp(J9l). where are the amplitude and phase, respectively.

[0050] Further, when the dominant noise source is local oscillator shot noise, the signal-to-noise ratio (SNR) y is expressed as the number of signal photons collected by the first optical elements y0multiplied by the fading envelope y = y0|w*a|2. Here, |w*a|2= y / y0acts as a signal combining efficiency indicator. When the signals are not properly combined, the contributions to the total power from different first optical signals may interfere destructively, reducing the instantaneous combining efficiency |w*a|2and causing a diminished SNR y < y0. The problem is defined as determining an optimal coefficient vector w to maximize the instantaneous output SNR for the current observed channel vector a. In an embodiment, a search is executed for identifying the optimum weight vector w by successively selecting the 1thfirst optical signal, evaluating different settings of the corresponding Ithphase-change signal, selecting the optimal weights i(to maximize the combined SNR, and repeating the procedure for every one of the nl first optical signals G {1,2, ... , L}. The search algorithm possesses the attractive property that the combiner weight estimates are obtained in closed form by optimizing a quadratic cost function based on the measurement SNR.

[0051] Quadratic optimization problems are convex, and a coordinate-wise ascent algorithm is considered for solving the convex optimization problem. The underlying coordinate ascent algorithm involves decomposing the overall detection problem into a set of smaller subproblems of decreasing dimension. Coordinate ascent is advantageous because scalar maximization is computationally less expensive than multivariate maximization. Here, the coordinate-ascend approach is based on the fact that the total optical field is expressed as a linear superposition of the contributions from the nl first optical signals. This means that the optimal linear is constructed by optimizing each of the weights individually.

[0052] For example, the coordinate-wise ascent algorithm may use four independent measurements of the instantaneous coherent SNR y obtained when the settings of the corresponding 1thfirst optical signal is sequentially adjusted to four different specific values. The relevant settings are updated sequentially by cycling through the variables for I = 1, ... , nl . A coordinate-wise ascent procedure is applied for each value of the parameter and use each solution as a starting point for the next estimate w(+1. The weight of each branch is set to its optimal value directly after each estimation. With this approach, the optimization process runs continuously and dynamically follows changes in the fading signal behavior. Furthermore, the combined signal starts to increase directly, which enhances the signal-to-noise ratio of successive measurements.

[0053] In the above mentioned embodiment, the coordinate-wise ascent algorithm (i.e., an example of the phase-shift algorithm) is used to solve the quadratic optimization problem to iteratively increase the total summed power of the nl first optical signals. However, it will be apparent to the skilled person that alternative algorithms to the coordinate-wise ascent algorithm may also be used to solve the quadratic optimization problem. For instance, it will be apparent to the skilled person to (i) define an objective function to increase the total summed power, and (ii) use a machine learning based training algorithm (based on atmospheric data) to iteratively update the above-mentioned weights to optimize the quadratic optimization problem. Hence, in other embodiments, the phase-shift algorithm may also be selected from the group comprising a machine-learning algorithm, a coordinate ascent algorithm and a stochastic algorithm.

[0054] In a further aspect, the invention provides a method for atmospheric turbulencecompensation using the atmospheric-turbulence compensation system. The method comprises the following steps: a signal acquisition step, a signal modification step, a signal processing step and a phase modulation step.

[0055] In an embodiment, the signal acquisition step further comprises recording nl first optical signals from input free space optical signals (i.e., incoming transmission signals) using nl first optical elements. Following which, the signal acquisition step comprises providing the received input free space signals to the phase-shift modulator as the nl first optical signals.

[0056] Subsequently, the signal modification step further comprises receiving the nl first optical signals at the phase-shift modulator from the first optical array. Additionally, the signal modification step comprises receiving nl first phase-shift correction signals at the phase-shift modulator from the control system. Furthermore, the signal modification step comprises individually modifying the phase of each of the nl first optical signals to the nl modified first optical signals in dependence of nl first phase-shift correction signals. Following which, the signal modification step comprises providing the nl modified first optical signals to the light-wave combiner.

[0057] In an embodiment, the signal processing step comprises receiving nl modified first optical signals. The signal processing step further comprises combining the nl modified first optical signals using the light-wave combiner to provide a combined modified optical signal. As mentioned above, here, the individual nl modified first optical signals are summed to provide a combined modified optical signal. In the event that phase coherence is achieved with the nl modified first optical signals, the nl first optical signals will interfere constructively, resulting in the maximization of the power of the combined modified optical signal. However, improving the phase coherence is an iterative process and a combined modified optical signal is generated at each iteration. Furthermore, the signal processing step further comprises providing an electrical output signal based on said combined modified optical signal to the control system.

[0058] In a further embodiment, the signal processing step may further comprise altering the electrical output signal by performing one or more operations of filtering a predetermined range of frequencies, reducing noise, signal splitting and signal amplification. It will be apparent to the skilled person to perform the aforementioned operations to improve the quality of the nl modified first optical signals and / or to prepare the nl modified first optical signals for further processing by the control system.

[0059] In an embodiment, the phase modulation step comprises receiving the electrical output signal. Further, the phase modulation step comprises generating nl first phase-shift corrections signals. Especially, the nl phase-shift correction signals are generated in dependence of the electrical output signal using a phase-shift algorithm in the control system to compensate for atmospheric disturbances. Next, the phase modulation step comprises providing the nl first phase-shift correction signals to the phase-shift modulator.

[0060] In this way, the iterative loop is complete. By the execution of the phase-shift algorithm in the control system the phase of each of the nl first optical signals are modulated individually by the nl phase-shift correction signals. Subsequently, the increment to the total power is measured from the electrical output signal based on the combined modified optical signal. In dependence of the total power, the phase-shift algorithm is used to generate phase-shift correction signals to further increase the total power of the summation of the nl first optical signals. The optimization is performed iteratively to compensate for aberrations in the nl first optical signals due to atmospheric disturbances.

[0061] In a further embodiment, the method may also facilitate scaling the atmospheric turbulencecompensation system to accommodate a plurality of optical arrays. For example, the method may comprise facilitating atmospheric-turbulence compensation of the received nl first optical signals and (additionally) n2 second optical signals. The signal acquisition step comprises recording n2 second optical signals from input free space optical signals using n2 second optical elements. Additionally, the signal acquisition step comprises providing to the phase-shift modulator the n2 second optical signals.

[0062] The signal modification step further comprises receiving n2 second optical signals and n2 second phase-shift correction signals using the phase-shift modulator. Next, the signal modification step comprises modifying the phase of the n2 second optical signals to n2 modified second optical signals in dependence of n2 second phase-shift correction signals. Furthermore, the signal modification step comprises providing the n2 modified second optical signals to the light-wave combiner.

[0063] Further, in an embodiment, the signal processing step comprises receiving n2 modified second optical signals using the light-wave combiner. The signal processing step comprises combining the n2 modified second optical signals using the light-wave combiner to provide a combined modified optical signal. Here, the combined modified optical signal is generated from a combination of both the nl modified first optical signals and the n2 modified second optical signals. Further, the signal processing step comprises providing an electrical output signal based on the combined modified optical signal to the control system.

[0064] Next, the phase modulation step comprises receiving the electrical output signal. Following which, the phase modulation step comprises generating n2 second phase-shift correction signals in dependence of the electrical output signal using the phase-shift algorithm in the control system to compensate for atmospheric disturbances. Lastly, the phase modulation step comprises providing n2 second phase-shift correction signals to the phase-shift modulator. In this way, the phase-shift algorithm is used to compensate for aberrations due to atmospheric disturbances in the nl first optical signals and the n2 second optical signals. In an embodiment of the method, the phase-shift algorithm may be selected from the group comprising a machine-learning algorithm, a coordinate ascent algorithm and a stochastic algorithm.

[0065] Furthermore, the method is not only used to correct for disturbances in the input free space signals received, but in addition the method may facilitate correcting for aberrations in the transmitted optical signals (using the atmospheric turbulence-compensation system in reciprocity).

[0066] In a further embodiment, the signal modification step comprises providing n3 modified first optical signals to the light-wave combiner via the multiport-circulator to the light-wave combiner. Furthermore, the method comprises a transmission step comprising generating n3 optical transmission signals. The transmission step further comprises providing the n3 optical transmission signals to the phase-shift modulator via the multiport-circulator. Lastly, the transmission step comprises transmitting the n3 optical transmission signals via the first optical array. In this embodiment, the phase-shift algorithm may yet be used to modulate the phase of the transmitted n3 optical transmission signals. Here, the method provides a transmission through turbulent atmospheric conditions such that the n3 optical transmission signals are received as a coherent transmission. Further, the signal modification step may comprise splitting a relatively high-power laser signal into n3 optical transmission signals. The power of a laser required to transmit an optical signal to a satellite may typically range from a few watts to several hundred watts, depending on the distance to the satellite, atmospheric conditions, and desired data transmission rate. The n3 optical transmission signals may also be statically co-phased (calibrated) by Variable Optical Delay Lines (VODL). VODL work by changing the optical path length that a light signal travels. VODLs help in synchronization, signal processing, and compensation for time delays, ensuring efficient data transmission. Furthermore, this facilitates achieving laser safety in airspace for the atmospheric turbulencecompensating system 1000 by design and thus avoids laser safety measures in airspace.

[0067] In a further embodiment, the method comprises a beam-correction step further comprising controlling the first optical array and / or the second optical array to transmit the n3 optical transmission signals at a predetermined point-ahead angle (0) with respect to the incoming angle of reception, using the beam steering principle of phased arrays. For long distance optical transmission, either due to a moving transmission station (such as a satellite), the transmitted n3 optical transmission signals may have to be beamed to a point ahead of the present position of the target receiving station. The point ahead angle is calculated based on the relative position and relative velocity of the target receiving station and the atmospheric turbulence-compensation system. In an embodiment, the beam correction step uses the beam steering principle of phased arrays to obtain a two-dimensional point ahead angle (PAA) in Far Field.

[0068] As mentioned above, the n3 optical transmission signals are transmitted via n3 first optical elements. Hence, nl is at least larger than n3 i.e., nl > n3. Each individual optical transmission signal is also referred to as beamlet. Typically, due to the arrangement of the first optical elements on the surface of the first optical array, there is a finite distance between the first optical elements. As a result of this gap or distance between two first optical elements and the relative motion of the target receiving station with respect to the first optical array, the optical transmission signals transmitted via said optical elements are received by a target receiving station at two different instants of time. This delay between when two optical signals or beamlets are received results in a phase difference between the two received optical signals at the target receiving station. Hence, to facilitate transmitting the n3 optical transmission signals such that there is no phase difference between the said n3 optical transmission signals, a phase shift Acp is introduced to each individual optical transmission signal.

[0069] The phase of each of the individual n3 optical transmission signals is altered by a phase shift in dependence of the point ahead angle (0) and an offset distance d. The point ahead angle (0) is an angular displacement ahead of a target receiving station along a trajectory of the target receiving station. The offset distance is a distance measured between a reference first optical element and each of the individual n3 optical elements are transmitted. The reference first optical element is an outermost first optical element of the first optical array closest to a projection of the trajectory’s starting point on the surface of the first optical array. For instance, the first optical array is focussed on a satellite and the satellite traverses over the first optical array from the west to the east. In such a scenario, the outermost first optical element located on the west side of the first optical array is the reference first optical element. The offset distances are measured from said reference first optical element (in a direction parallel to a projection of the trajectory of the satellite on the surface of the first optical array).

[0070] Once the point ahead angle 0 and the offset distance of each individual first optical element are determined, the phase shift Acp of the optical transmission signal beamed via said first optical element can be calculated using the relation Acp = 2n / X * d * sin(0) ~ 2n / X * d * 0. Here, X is the wavelength of the optical transmission signal.

[0071] In a similar vein, in an embodiment, the method comprises altering a phase of each of the individual n3 optical transmission signals by a unique phase shift Acp in dependence of a point ahead angle 0 and an offset distance d.

[0072] In a yet further embodiment, the method comprises individually tilting the tip of the nl first optical elements using a first tip tilt controller. Hence, the atmospheric turbulence-compensation system in an embodiment comprises a tip tilt controller to control each of the nl first optical elements. Here, the tip tilt controller determines a dedicated position signal corresponding to each first optical element, which consists of a dynamic signal (baseline, ‘hot spot tracking’) plus optionally a quasistatic offset signal.

[0073] BRIEF DESCRIPTION OF DRAWINGS

[0074] Embodiments will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts;

[0075] Fig. 1 schematically shows an embodiment of the atmospheric turbulence-compensation system 1000;

[0076] Fig. 2 schematically shows another embodiment of the atmospheric turbulencecompensation system 1000;

[0077] Fig. 3 schematically shows another embodiment of the atmospheric turbulencecompensation system 1000;

[0078] Fig. 4A, 4B, 4C schematically show three arrangements of the nl first optical elements 1110 of the first optical array 1100;

[0079] Fig. 5 schematically shows an embodiment of the method 1 of compensating for atmospheric turbulence;

[0080] Fig. 6 schematically shows another embodiment of the method 1 of compensating for atmospheric turbulence; and Fig. 7A-7D schematically show the operation of the atmospheric turbulence-compensation system 1000.

[0081] The depicted figures are schematic representations of some embodiments of the invention, and do not serve as restriction of the scope or the protection laid down by the claims. The figures are intended to provide an illustration only, and are not necessarily to scale.

[0082] DETAILED DESCRIPTION OF DRAWINGS

[0083] The following is a description of certain embodiments of the invention, given by way of example only and with reference to the figures.

[0084] Fig. 1 schematically shows an embodiment of an atmospheric turbulence-compensation system 1000. The depicted embodiment comprises a first optical array 1100, a phase-shift modulator 200, alight-wave combiner 400 and a control system 300. The first optical array 1100 comprises seven first optical elements (i.e., lenses) 1110 arranged in a hexagonal array. The seven first optical elements 1100 are optically connected to the phase-shift modulator 200. Here, seven first optical elements 1110 in the depicted embodiment are arranged circumferentially on a pentagon around one of the first optical elements 1110. In this embodiment nl is seven. It will be apparent to the skilled person that there are at least two, such as at least seven, especially at least ten first optical elements 1110 in a further embodiment. The nl first optical elements 1110 are configured to receive input free space optical signals 11. In this embodiment, each of the first optical elements 1110 is an optical fiber or a lens attached to an optical fiber. The input free space signals 11 are signals transmitted by a transmission station to the atmospheric turbulence-compensation system 1000 positioned at a receiving station. That is, the atmospheric turbulence-compensation system 1000 is installed at the receiving station to receive the input free space signals (transmitted by a transmission station). Further, the first optical array 1100 is configured to provide the received input free space signals 11 as the nl first optical signals 1101 respectively to the phase-shift modulator 200.

[0085] In the depicted embodiment, the phase-shift modulator 200 is optically connected to the first optical array 1100. The phase-shift modulator 200 comprises nl first optical input-output ports 2110 (i.e., seven first optical input-output ports 2110) configured to receive nl first optical signals 1101. In this embodiment, nl first optical elements 1110 (which are optical fibers) are connected to the nl first optical input-output ports 2110. In this way, the phase-shift modulator 200 receives nl first optical signals 1101 from the first optical array 1100.

[0086] Further, the phase-shift modulator 200 comprises nl electrical input ports 2120 to receive nl first phase-shift correction signals 301. In this embodiment, nl is seven. So, the phase-shift modulator comprises seven electrical input ports 2120. Subsequently, the phase-shift modulator 200 alters the nl first optical signals 1101 to (in dependence of the nl first phase-shift correction signals 301) to provide the nl modified first optical signals 1102. The nl modified first optical signals 1102 are provided to the light-wave combiner 400. Especially, the phase-shift modulator 200 comprises nl second optical input-output ports 2130 configured to provide the nl modified first optical signals 1102 to the lightwave combiner 400.

[0087] The light-wave combiner 400 comprises nl first optical input ports 4110 configured to receive the nl modified first optical signals 1102 from the phase-shift modulator 200. The light-wave combiner 400 is configured to combine the nl modified first optical signals 1102 into a combined modified optical signal 1601. Further, the light-wave combiner 400 is configured to provide an electrical output signal 1602 based on the combined modified optical signal 1601. Therefore, the lightwave combiner 400 comprises an electrical output port 420 for providing the electrical output signal 1602 to the control system 300. To facilitate the conversion of the combined modified optical signal 1601 to the electrical output signal 1602, the light-wave combiner 400 further comprises an optical- electrical device 450. Such optical -electrical devices are known in the art. Further, the light-wave combiner 400 is connected to an optical power detector 430 that filters optically, optionally preamplifies and converts the combined modified optical signal 1601 into the electrical output signal 1602. Once the nl first input signals have been corrected for aberrations due to atmospheric turbulence, the (clean) combined modified optical signal 1601 is extracted from the system via the optical power detector 430. Further, the optical power detector 430 splits the combined modified optical signal 1601 into two components, where the first component comprising at least 80%, such as 85%, especially 90% of the total power of the combined modified optical signal is provided to the optical power detector 430 and the second component is used to generate the electrical output signal 1602. The optical power detector comprises an optical input port 4510 to receive at least a part of the combined modified optical signal 1601. The electrical output signal 1602 is subsequently provided to the control system 300.

[0088] The control system 300 comprises an electrical input port 310 configured to receive the electrical output signal 1602 from the light-wave combiner 400. In dependence of the electrical output signal 1602 provided to the control system 300, the control system 300 is configured to utilize a phaseshift algorithm to generate nl phase-shift correction signals 301. The phase-shift algorithm is configured to compensate for atmospheric disturbances. The phase-shift algorithm may be selected from the group comprising a machine-learning algorithm, a coordinate ascent algorithm and a stochastic algorithm.

[0089] The control system comprises nl first electrical output ports 3120 to provide the nl phaseshift correction signals 301. The nl first electrical output ports 3120 are electrically connected to the nl electrical input ports 2120 of the phase-shift modulator 200. The nl phase-shift corrections signals 301 provided to the phase-shift modulator 200 contain information on modulating the nl first optical signals 1101 based on the calculated individual phase shift corrections to provide the nl modified first optical signals 1102. It will be apparent to the skilled person that the control system 300 may comprise one or more additional components from a standard electronic component base such as an optoelectrical detector, a tuneable optical signal fdter, an tuneable electrical signal fdter, an optical noise blocker, an electrical noise blocker, an optical signal splitter and an optical signal pre-amplifier.

[0090] Fig. 2 schematically shows another embodiment of the atmospheric turbulencecompensation system 1000. The embodiment depicted here is a further embodiment of that depicted in Fig. 1. A difference between this embodiment and the embodiment of Fig. 1 is the second optical array 2100 connected to the phase shift modulator 200. This embodiment may be operated in the same manner as the embodiment of Fig. 1 by disabling the second optical array 2100.

[0091] In this embodiment, the atmospheric turbulence-compensation system 1000 comprises the first optical array 1100 and further comprises a second optical array 2100. In the depicted embodiment the first optical array 1100 comprises seven first optical elements 1110 and the second optical array 2100 also comprises seven second optical elements 2510. Here, nl is seven and n2 is seven. However, it will be apparent to the skilled person that it is not necessary to select nl and n2 to be equal, nl and n2 may also be different in other embodiments.

[0092] In the depicted embodiment, the atmospheric turbulence-compensation system 1000 further comprises a tip tilt controller 121. The tip tilt controller 121 is configured to individually control the orientation of the nl first optical elements 1110. Especially, each first optical element is mechanically connected to an individual tilt-actuator to change the orientation of the corresponding first optical element 1110. The first optical array 1100 further comprises nl tilt-actuators wherein the tip tilt controller 121 is configured to control the nl tilt-actuators. The tip tilt controller 121 is capable of making adjustments to the orientation of each individual first optical elements 1110 to optimally position the first optical elements 1110 to receive the input free space signals 11. That is, the tip tilt controller 121 individually controls the azimuthal angle and the polar angle of inclination of each of the first optical elements. Thereby, the alignment or the direction towards which the nl first optical elements 1110 point towards are controlled.

[0093] The first optical array 1100 is connected to the phase-shift modulator 200 that is subsequently connected to the light-wave combiner 400 in a similar manner to that described in Fig. 1 and the descriptions relating to the first optical array 1100 are hence not repeated. Additionally, in this embodiment, in addition to the connection of the nl first optical elements to the phase shift modulator 200, the n2 second optical elements 2510 are optically connected to the phase-shift modulator 200. The n2 second optical elements 2510 are configured to provide n2 second optical signals 2101 respectively to the phase-shift modulator 200. In the depicted embodiment, the phase-shift modulator 200 comprises n2 second optical input-output ports 2210 configured to receive n2 second optical signals 2101. Further, the phase-shift modulator 200 further comprises n2 second electrical input ports 2220 configured to receive n2 second phase-shift correction signals 302 and n2 second optical inputoutput ports 2230 configured to provide n2 modified second optical signals 2102. The n2 second optical input-output ports 2230 are optically connected to the n2 second optical input ports 4210 of the light-wave combiner 400.

[0094] The light-wave combiner 400 is configured to receive the n2 modified second optical signals 2102 from the phase-shift modulator 200 via the n2 second optical input ports 4210. The light-wave combiner 400 is also configured to receive the nl modified second optical signals 1102 from the phaseshift modulator 200 via the n 1 first optical input ports 4110. The light-wave combiner 400 is configured to generate a (single) combined modified optical signal 1601 by combining both the nl modified first optical signals 1102 and the n2 modified second optical signals 2102. Furthermore, the light-wave combiner 400 is configured to provide an electrical output signal 1602 based on the combined modified optical signal 1601. The light-wave combiner 400 comprises the electrical output port 420 electrically connected to electrical input port 310 of the control system 300.

[0095] The control system 300 is configured to receive the electrical output signal 1602 from the light-wave combiner 400. Here, the control system 300 is configured to compensate for atmospheric disturbances by executing the phase-shift algorithm. Particularly, the phase-shift algorithm is executed to generate the nl first phase-shift correction signals 301 and the n2 second phase-shift correction signals 302. Further, the control system 300 comprises the nl first electrical output ports 3120 and the n2 second electrical output ports 3220 configured to provide the nl first phase-shift correction signals 301 and the n2 second phase-shift correction signals 302 to the phase-shift modulator 200, respectively. In this way, the control loop is executed iteratively to improve the coherence and minimize the aberrations in the nl first optical signals and the n2 second optical signals.

[0096] Fig. 3 schematically shows another embodiment of the atmospheric turbulencecompensation system 1000. The depicted embodiment facilitates generation and transmission of optical transmission signals 612. Furthermore, the optical transmission signals 612 are transmitted such that a coherent beam is received at a target receiving station 50. In this embodiment, the atmospheric turbulence-compensation system 1000 further comprises a multiport-circulator 500 and a high-power optical array 600. The multiport-circulator 500 facilitates receiving nl modified optical signals 1102 as well as transmitting n3 optical transmission signals 611.

[0097] The embodiment depicted in this figure is a further embodiment of that depicted in Fig. 1 and / or Fig. 2. In this embodiment, the atmospheric turbulence compensation system comprises a multiport-circulator 500 which facilitates a two way communication of optical signals. The depicted embodiment may be operated in the same manner as the embodiment depicted in Fig. 1 by disabling the high-power optical array 600. Furthermore, the depicted embodiment may also be operated in the same manner as the embodiment depicted in Fig. 2 by the introduction of the second optical array 2100. In this way, existing telecommunications components and customized firmware and softwarealgorithms can be used to create a versatile wavefront compensator. In a first mode of operation, the system 1000 is configured to receive free space input signals 11. Further, in this mode, the input free space signals 11 are recorded as the nl first optical signals 1101. These nl first optical signals 1101 are corrected for aberrations due to atmospheric turbulence. In this system, different to other embodiments, the phase-shift modulator 200 is connected to the lightwave combiner 400 via the multiport-circulator 500. That is, the nl second optical input-output ports 2130 of the phase-shift modulator 200 are connected to the n3 second optical input-output ports 530 of the multiport-circulator 500. Subsequently, the n3 reception ports 520 of the multiport-circulator 500 are connected to the nl first optical input ports 4110 of the light-wave combiner 400. In this way, the phase-shift modulator 200 is connected to the light-wave combiner 400 via the multiport-circulator 500. In this embodiment, nl is selected equal to n3 for compatibility of the elements in the system. The remainder of the connections are configured in an analogous manner to the embodiment in Fig. 1 and are hence not repeated. The system 1000 executes the phase-shift algorithm in the control system 300 to correct the incoming input free space signals 11 for aberrations due to atmospheric turbulence. Furthermore, this same information is also utilized in correcting the n3 optical transmission signals 611 prior to transmission. In this way, the signal received at a receiving station 50 is coherent and is already corrected for aberrations due to atmospheric turbulence.

[0098] In a second mode of operation, the multiport-circulator 500 comprises n3 transmission ports 510 connected to n3 light sources 610 of the high-power optical array 600. The n3 light sources 610 are configured to generate n3 optical transmission signals 611. In this embodiment, the n3 light sources 610 are monochromatic coherent light sources.

[0099] Subsequently, the high-power optical array 600 is configured to provide the n3 optical transmission signals 611 to the phase-shift modulator 200 via the multiport-circulator 500.

[0100] In this way, the n3 optical transmission signals 611 are provided to the phase-shift modulator 200 via the multiport-circulator 500. The phase-shift modulator 200 alters the n3 optical transmission signals 611 (in dependence of the nl phase-shift correction signals 301) to the n3 modified optical transmission signals 612 (‘reciprocity’). Next, these n3 modified optical transmission signals 612 are transmitted as the output free space signal 12 via the nl first optical elements 1110 of the first optical array 1100.

[0101] In some instances, a phase delay or phase shift may be needed to compensate for relative motion of the receiving station and physical distance between the first optical elements 1110 arranged on the first optical array 1100. Note that the phase of the optical transmission signals are shifted or altered in addition to the phase delay or compensation for atmospheric turbulence.

[0102] As a consequence of the relative motion of the target receiving station 50 (e.g., a satellite) relative to the first optical array 1100 (situated on the ground), the output free space signals 12 (comprising the optical transmission signals) may be beamed ahead of the target receiving station. This angle by which the transmission is offset is referred to as the point ahead angle 0. Hence, in a further embodiment, the high-power optical array 600 is configured to generate n3 optical transmission signals using quasi-static, individual phase shift offsets between all n3 light sources in case a point ahead angle is required and to provide the n3 optical transmission signals 611 to the phase-shift modulator 200 via the multiport-circulator 500. The phase delay introduced in each output free space signal (or beamlet) from the corresponding first optical element is Aq>k where k is the index of the specific n3 light source. The calculation of the phase delay Aq>k is explained in Fig. 7.

[0103] The process of generating an optical signal begins with the Serializer, which organizes digital data. This data is then framed, error-corrected (FEC), and interleaved to enhance transmission reliability. Digital Signal Processing (DSP) further refines the signal for quality and consistency before it's converted to analog through a Digital -to-Analog Converter (DAC). At the front end, the analog signal modulates the laser, adjusting its intensity or phase according to the encoded data. Moving to the front end, the modulated laser signal is directed to a high-power optical array (600), where it undergoes amplification to strengthen the signal for long-distance transmission. Subsequently, the signal passes through a fiber splitter, which divides it into multiple paths for distribution across the network.

[0104] Fig. 4A, 4B, 4C schematically shows three arrangements of the nl first optical elements 1110 of the first optical array 1100. The first optical elements 1110 are arranged in a plurality of different ways on the first optical array 1100.

[0105] In Fig 4A, the system 1000 comprises fourteen first optical elements 1110 arranged in a circumferential array about a central obstruction 1111. In further embodiments, this configuration may use a reflective telescope as an interface to the turbulent environment. In such embodiments, the first optical array 1100 is then located in the pupil plane behind the telescope. The pupil plane behind a telescope is where incoming light converges before forming the image.

[0106] Fig 4B is analogous to embodiment I with the difference that the central obstruction 1111 is relatively smaller than the first optical elements 1110. Further, in embodiment II, the system 1000 comprises only seven first optical elements arranged in a hexagonal arrangement surrounding the central obstruction 1111.

[0107] Fig 4C depicts an arrangement of first optical elements 1110 on the first optical array 1100. In Fig. 4C, the first optical elements are arranged in a square array and the system 1000 only comprises four optical elements 1110. In this case the first optical array 1100 is realized either as an array of self- contained collimators with autonomous tip-tilt tracking functionality (‘hot spot tracking’ into single mode fiber) or the first optical array 1100 is arranged in the pupil plane of a refractive or a reflective off-axis telescope, both without central obstruction.

[0108] It will be apparent to the skilled person that the nl first optical elements 1110 may also be arranged in patterns other than the circumferential array in a further embodiment, for instance like hexagons and with other amouts of elements 1110. Fig. 5 schematically shows an embodiment of the method 1 of compensating for atmospheric turbulence. In an embodiment, the invention provides a method 1 for atmospheric turbulencecompensation using the atmospheric turbulence-compensation system 1000. The method 1 is configured to execute a control loop comprising one or more steps.

[0109] The method comprises a signal acquisition step 110 further comprising recording nl first optical signals 1101 and n2 second optical signals 2101 from input free space optical signals 11,21 using nl first optical elements 1110 and n2 second optical elements 2510, respectively. The signal acquisition step 110 further comprises providing to the phase-shift modulator 200 the nl first optical signals 1101 and the n2 second optical signals 2101.

[0110] Next, the method 1 comprises a signal modification step 120. The signal modification step 120 comprises receiving the nl first optical signals 1101 and nl first phase-shift correction signals 301 using the phase-shift modulator 200. Additionally, the signal modification step 120 further comprises receiving the n2 second optical signals 2101 and the n2 second phase-shift correction signals 302 using the phase shift modulator 200. Further, the signal modification step 120 comprises modifying individually the phase of each of the nl first optical signals 1101 to nl modified first optical signals 1102 in dependence of nl first phase-shift correction signals 301. Likewise, the signal modification step 120 further comprises modifying individually the phase of each of the n2 second optical signals 2101 to n2 modified second optical signals 2102 in dependence of n2 second phase-shift correction signals 302. The modified first optical signal 1102 and the modified second optical signal 2102 are provided to the light-wave combiner 400.

[0111] Next, the method 1 comprises a signal processing step 130. The signal processing step 130 further comprises receiving nl modified first optical signals 1102 and the n2 modified second optical signals 2102. The signal processing step 130 comprises combining the nl modified first optical signals 1102 and the n2 modified second optical signals 2102 using the light-wave combiner 400 to provide a combined modified optical signal 1601. Subsequently, the signal processing step 130 comprises providing an electrical output signal 1602 based on the combined modified optical signal 1601 to the control system 300. In a further embodiment, the signal processing step 130 may also comprise altering the electrical output signal 1602 by performing one or more operations of filtering a predetermined range of frequencies, reducing noise, signal splitting and signal amplification.

[0112] Lastly, the method 1 comprises a phase modulation step 140 further comprising receiving the electrical output signal 1602 and generating (both) the nl first phase-shift correction signals 301 and the n2 second phase-correction signals in dependence of the electrical output signal 1602 using a phase-shift algorithm 1610 in the control system 300. The nl first phase-shift correction signals 301 and the n2 second phase-shift correction signals 302 are provided back to the phase-shift modulator 200. The method 1 is applied iteratively, wherein the each correction provided by the phase-shift correction signals 301,302 serve to iteratively improve the received optical signals 1101 and 2101, respectively. This embodiment describes the method 1 of processing input free signals 11,21 from the first optical array 1100 and the second optical array 2100. It will be apparent to the skilled person that the method 1 may also be used when the atmospheric turbulence-compensation system 1000 comprises only the first optical array, see Fig. 6 for such an embodiment.

[0113] Fig. 6 schematically shows another embodiment of the method 1 of compensating for atmospheric turbulence. In this embodiment, the atmospheric turbulence-compensation system 1000 comprises a first optical array 1100 configured to receive input free space signals 11. Subsequently, the control loop as described in Fig. 5 is executed to correct the first optical signal 1101 iteratively for aberrations due to atmospheric turbulence. For the sake of brevity, these features are not repeated again.

[0114] In this embodiment of the method 1, in contrast to the embodiment in Fig. 5, the atmospheric turbulence-compensation system 1000 comprises a multiport-circulator 500. Here, the signal modification step 120 comprises providing n3 modified first optical signals 1102 from the phase-shift modulator 200 to the light-wave combiner via the multiport-circulator 500 to the light-wave combiner 400. In this way, the multiport-circulator 500 facilitates reception of the first optical signals 1101.

[0115] Additionally, the multiport-circulator also facilitates transmission. In this embodiment, the method 1 comprises a transmission step 150 comprising generating n3 optical transmission signals 611. The n3 optical transmission signals 611 are provided to the phase-shift modulator 200 via the multiport-circulator 500. Subsequently, the phase-shift modulator 200 is configured to (generate n3 optical transmission signals using quasi-static phase shift offsets between all n3 light sources in case a point ahead angle is required and to) provide the n3 optical transmission signals 611 to the first optical array 1100. The n3 optical transmission signals 611 are altered in dependence of the n3 phaseshift correction signals 301 to provide the n3 modified optical transmission signals 612. Subsequently, the n3 modified transmissions signals 612 are transmitted as output free space signals 12 via the first optical array 1100 to a target receiving station 50. Note that in an embodiment, the phase-shift correction signal 301 provided to the phase-shift modulator 200 in the phase modulation step 140 is used to alter the n3 optical transmission signals 611 to (eventually) provide the output free space signal 12. The output free space signal 12 is altered such that the receiving station receives the output free space signal 12 as a coherent signal (even through turbulent atmospheric conditions).

[0116] Fig. 7A-7D schematically show the operation of the atmospheric turbulence-compensation system 1000.

[0117] Fig. 7A depicts the atmospheric turbulence-compensation system 1000 transmitting the output free space signals 12 to a target receiving station 50. In the figure, the output free space signals 12 travel via the atmosphere to the target receiving station 50. Here, the target receiving station 50 is in motion (for example a satellite). In such instances, it is necessary to transmit the output free space signal 12 comprising the n3 modified optical transmission signals 612 ahead of the target receiving station 50 to compensate for the relative motion of the target receiving station 50 relative to the atmospheric turbulence-compensation system 1000 during the time of travel of the signals. Hence, in this embodiment, the method 1 further comprises a beam -correction step 170 further comprising controlling the first optical array 1100 to transmit the n3 modified optical transmission signals 612 at a predetermined point-ahead angle 0 with respect to the incoming angle of reception, for example using the beam steering principle of phased arrays. The dashed outline of the target receiving station 50 indicates the current position and the solid outline indicates the position of the target receiving station 50 when it receives the output free space signals 12. In this context, the atmospheric turbulencecompensation system 1000 is a transceiver and the target receiving station 50. For the sake of illustrating the point ahead angle, in the depicted embodiment, the atmospheric turbulencecompensation system 1000 is the transmitter and the satellite is the receiver. However, the atmospheric turbulence-compensation system 1000 and the target receiving station 50, in embodiments, can both transmit and receive signals i.e., the atmospheric turbulence-compensation system 1000 is a transceiver and the target receiving station 50 (e.g., the satellite 50) is also a transceiver.

[0118] Fig. 7B schematically depicts a three dimensional view of the first optical array 1100. In this embodiment, the system 1000 comprises a gimbal 1105 to orient the first optical array 1100 in the direction of the satellite 50. The trajectory of the satellite 50 is predetermined and the skilled person operating the atmospheric turbulence-compensation system 1000 would be aware of the position of the satellite 50 in space. The gimbal 1105 stabilizes the first optical array 1100 by counteracting unwanted motion and maintaining its orientation, allowing the first optical array 1100 to remain focused in the direction of the satellite 50, even as the satellite 50 is in motion. Further, the orientation of each individual nl first optical element 1110 is controlled by the tip tilt controller 121 to precisely orient the nl first optical elements 1110 towards the moving satellite 50. In this way, the gimbal 1105 facilitates making coarse adjustments to the orientation of the first optical array 1100 and the tiltactuators (controlled by the tip tilt controller 121) facilitate fine alignment of the first optical elements 1110 to the satellite 50.

[0119] Fig. 7C shows the operation of the atmospheric turbulence-compensation system 1000 when transmitting output free space signals 12 to the satellite 50 which is in the Far Field (e.g., relatively far away from the optical arrays 1100). When transmitting to a moving target (e.g., the satellite 50), the spacing between the first optical elements 1110 and relative motion between the satellite 50 and the first optical array 1100 may result in a phase delay Acp between the output signals 12 from the aforementioned optical elements 1110. The delay is due to the satellite 50 receiving (at two different instants in time) the output free space signal 12a, 12b from two different first optical elements 1110a, 1110b (spaced a finite distance ‘d’ apart). In the depicted embodiment, the output signal 12a from the first optical element 1110a positioned in the left extremity of the first optical array 1100 will reach the satellite 50 first. Hence, in a 1-dimensional example a phase delay of Acp is introduced to the optical transmission signals (i.e, the output signal 12b) transmitted from the first optical element 1110b (positioned a distance ‘d’ away from the first optical element 1110a). Next, a phase delay of 2Acp is introduced to the output free space signal 12c transmitted from the first optical element 1110c positioned a distance ‘2d’ away from the (left extreme) first optical element 1110a.

[0120] Fig. 7D shows a cross-sectional view of the first optical array 1100 depicting the offset distance ‘d’ between different first optical elements. The offset distance is measured along a projection of the trajectory of the target (satellite) as it is traversing relative to the first optical array.

[0121] The phase offset Acp is determined in dependence of the point ahead angle 0, the wavelength Z. the inter-beamlet spacing d according to the relation: Acp = 2TT / X * d * sin(0). This feature allows for operating dynamic phase- and tip-tilt adaptation per output free space signal 12 in full reciprocity and avoids non-common path aberrations.

[0122] The point ahead offset angle 0 on the uplink beams is then achieved by adding (quasi-)static phase delays on the outgoing path of each beamlet 1110, with respect to each other in an optical array 1100. Further, it will apparent to the skilled person that this is an example for one specific scenario of the satellite 50 traversing over the first optical array 1100 in one particular trajectory. In a scenario where the satellite is traversing over the first optical array 1100 in a different direction or trajectory, the offset distance “dk” of each first optical element from the reference optical element has to be taken into account when calculating the unique phase offset Ac k corresponding to the output signal (or beamlet) transmitted from each first optical element. Here ‘k’ is the index of the first optical element in the first optical array 1100 and the reference optical element is the outermost optical element closest to the starting point of the projection of the trajectory of the target receiving station 50 on the surface of the first optical array 1100.

[0123] For a 2-dimensional decomposition of a point ahead angle, an array of unique phase offsets Acpkj needs to be calculated accordingly.

[0124] Here, the design of the conformal antenna array allows it to achieve a high signal strength (antenna gain) directly in front of it (on-axis) when measured far from the array (in the Far Field). The gain approaches the maximum possible value dictated by the size of the array's outer diameter, while the mean variance of the signal received in Far Field stays limited to that resulting from a single beamlet (i.e., the optical element), which provides significant advantage especially in presence of strong angular anisoplanatism. Dividing a high-power input laser into an optical array of n3 co-phased (coherent) light sources provides in its baseline laser-safety in airspace by design, complying with the AS6029B standard, thus avoiding entirely the otherwise required significant operational effort for aircraft monitoring and uplink power control (i.e., switch off the uplink in case of an aircraft intersection with the uplink beam).

[0125] Here, each optical array beams the same signal, however, a constant quasi-static phase offset is implemented between two consecutive optical arrays. Especially, the quasi-static phase shift offset are implemented among all n3 light sources 610 of the optical array such that the phase offset is applied to each of the n3 optical transmission signals 611 of each light source (of the optical array). This may be static phase shifts in case of uplinks to geostationary Earth orbit (GEO) satellites, or nearly static phase shifts with gradual variation in case of uplinks to medium Earth orbit (MEO) or low Earth orbit (LEO) satellites.

[0126] Note that the phase delay Acp introduced in the optical transmission signals transmitted via a corresponding first optical element 1110 is an additional phase delay or phase shift to compensate for the delays due to the finite spacing between the first optical elements 1110 comprised by the first optical array 1100 and the relative motion between the receiving station 50 and the first optical array 1100. This is not the same as the corrections to the optical transmission signals 612 to compensate for atmospheric turbulence-compensation. To clarify, the control system 300 first generates nl phase-shift correction signals to compensate for atmospheric turbulence during transmission. Additionally, the phase delay Ac k (as described above) may be introduced to the nl phase shift correction signals to compensate for a point ahead angle 0 if needed (in dependence of the finite spacing between the first optical elements, the trajectory of the satellite and the relative velocity between the satellite and the first optical array).

[0127] The invention has been described with reference to some embodiments. Obvious modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims.

Claims

CLAIMS1. An atmospheric turbulence-compensation system (1000) comprising a first optical array (1100), a phase-shift modulator (200), a light-wave combiner (400), and a control system (300), wherein: the first optical array (1100) comprises nl first optical elements (1110) optically connected to the phase-shift modulator (200), wherein the nl first optical elements (1110) are configured to receive input free space optical signals and to provide the nl first optical signals (1101) respectively to the phase-shift modulator (200); the phase-shift modulator (200) comprises nl first optical input-output ports (2110) configured to receive the nl first optical signals (1101), nl first electrical input ports (2120) configured to receive nl first phase-shift correction signals (301) and nl second optical input-output ports (2130) configured to provide nl modified first optical signals (1102) to the light-wave combiner (400), wherein the phase-shift modulator (200) alters the nl first optical signals (1101) to the nl modified first optical signals (1102) in dependence of the nl first phase-shift correction signals (301); the light-wave combiner (400) comprises nl first optical input ports (4110) configured to receive the nl modified first optical signals (1102) from the phase-shift modulator (200) and to combine the nl modified first optical signals (1102) to a combined modified optical signal (1601), wherein the light-wave combiner (400) is configured to provide an electrical output signal (1602) based on the combined modified optical signal (1601), wherein the light-wave combiner (400) comprises an electrical output port (420) for providing the electrical output signal (1602) to the control system (300); and the control system (300) comprises an electrical input port (310) configured to receive the electrical output signal (1602) from the light-wave combiner (400) and nl first electrical output ports (3120) configured to provide the nl first phase-shift correction signals (301) to the phase-shift modulator (200), wherein the control system (300) is configured to compensate for atmospheric disturbances by providing the nl first phase-shift correction signals (301) to the phase-shift modulator (200).

2. The atmospheric turbulence-compensation system (1000) according to any one of the preceding claims, further comprises at least a second optical array (2100), wherein: the second optical array (2100) further comprises n2 second optical elements (2110) optically connected to the phase-shift modulator (200), wherein the n2 second optical elements (2110) are configured to provide n2 second optical signals (2101) respectively to the phase-shift modulator (200); the phase-shift modulator (200) comprises n2 second optical input-output ports (2210) configured to receive the n2 second optical signals (2101), n2 second electrical input ports (2220)configured to receive n2 second phase-shift correction signals (302) and n2 second optical input-output ports (2230) configured to provide n2 modified second optical signals (2102); the light-wave combiner (400) comprises n2 second optical input ports (4210) configured to receive the n2 modified second optical signals (2102) from the phase-shift modulator (200); and the control system (300) comprises n2 second electrical output ports (3220) configured to provide the n2 second phase-shift correction signals (302) to the phase-shift modulator (200), wherein the control system (300) is configured to compensate for atmospheric disturbances by providing the n2 second phase-shift correction signals (302) to the phase-shift modulator (200).

3. The atmospheric turbulence-compensation system (1000) according to any one of the preceding claims, further comprises a multiport-circulator (500) and a high-power optical array (600), wherein: the multiport-circulator (500) comprises n3 transmission ports (510), n3 reception ports (520) and n3 input-output ports (530), wherein the n3 second optical input-output ports (2130) of the phase-shift modulator (200) are connected to the n3 input-output ports (530) of the multiport-circulator (500); the n3 reception ports (520) of the multiport-circulator (500) are connected to the n3 first optical input ports (4110) of the light-wave combiner (400), wherein the phase-shift modulator (200) is optically connected to the light-wave combiner (400) and is configured to provide the n3 modified first optical signals (1102) via the multiport-circulator (500) to the light-wave combiner (400); and the high-power optical array (600) comprises n3 light sources (610) configured to generate n3 optical transmission signals (611), wherein the n3 light sources (610) are optically connected to the n3 transmission ports (510) of the multiport-circulator (500), wherein the high-power optical array (600) is configured to provide the n3 optical transmission signals (611) to the phase-shift modulator (200) via the multiport-circulator (500).

4. The atmospheric turbulence-compensation system (1000) according to claim 3, wherein: a phase of each of the individual n3 optical transmission signals (611) is altered by a unique phase shift Acp in dependence of a point ahead angle (0) and an offset distance d, wherein the point ahead angle (0) is an angular displacement ahead of a target receiving station (50) along a trajectory of the target receiving station (50); and the offset distance d is a distance measured between a reference first optical element and the optical element via which the optical transmission signal (611) is transmitted.

5. The atmospheric turbulence-compensation system (1000) according to any one of the preceding claims, wherein the first optical array (1100) and / or the second optical array (2100) further comprises a tip tilt controller (121).

6. The atmospheric turbulence-compensation system (1000) according to any one of the preceding claims, wherein the first optical array (1100) and / or the second optical array (2100) further comprises one or more elements from the group of wavefront correctors, collimators, gradient index rod lenses and diffractors.

7. The atmospheric turbulence-compensation system (1000) according to any one of the preceding claims, wherein the nl first optical elements (1110) are arranged in a circumferential array, preferably the nl optical elements (1110) are arranged in concentric hexagonal rings.

8. The atmospheric turbulence-compensation system (1000) according to any one of the preceding claims, wherein nl is selected from 7, 19, and 37.

9. The atmospheric turbulence-compensation system (1000) according to any one of the preceding claims, wherein the light-wave combiner is selected from the group of a bare fiber optical combiner, a blockless fiber combiner and an ABS combiner.

10. The atmospheric turbulence-compensation system (1000) according to any one of the preceding claims, wherein the atmospheric turbulence-compensation system comprises an optical power detector.

11. The atmospheric turbulence-compensation system (1000) according to claim 10, wherein the optical power detector is configured as a part of the light-wave combiner.

12. The atmospheric turbulence-compensation system (1000) according to any one of the preceding claims, wherein the control system (300) comprises one or more of an opto-electrical device, a tuneable optical filter, an optical noise blocker, an optical signal splitter and an optical signal amplifier.

13. The atmospheric turbulence-compensation system (1000) according to claims 3-12, wherein the n3 light sources are selected from monochromatic coherent light sources,14. The atmospheric turbulence-compensation system (1000) according to claim 13, wherein the light sources are laser-pumped.

15. The atmospheric turbulence-compensation system (1000) according to claims 3-14, wherein the high-power optical array comprises a diffractive beam-splitter for providing n3 light sources by splitting a single coherent light source into n3 individual beams.

16. A method (1) for atmospheric turbulence-compensation using the atmospheric turbulencecompensation system (1000) according to any one of the preceding claims, comprising: a signal acquisition step (110) further comprising recording nl first optical signals (1101) from input free space optical signals (11) using the nl first optical elements (1110) and providing to the phase-shift modulator (200) the nl first optical signals (1101); a signal modification step (120) further comprising receiving the nl first optical signals (1101) and nl first phase-shift correction signals (301) using the phase-shift modulator (200), comprising modifying individually the phase of each of the nl first optical signals (1101) to nl modified first optical signals (1102) in dependence of the nl first phase-shift correction signals (301), comprising providing the nl modified first optical signals (1102) to the light-wave combiner (400); a signal processing step (130) further comprising receiving the nl modified first optical signals (1102), comprising combining the nl modified first optical signals (1102) using the light-wave combiner (400) to provide a combined modified optical signal (1601), providing an electrical output signal (1602) based on the combined modified optical signal (1601) to the control system (300); and a phase modulation step (140) further comprising receiving the electrical output signal (1602), comprising generating the nl first phase-shift correction signals (301) in dependence of the electrical output signal (1602) using a phase-shift algorithm (1610) in the control system (300) to compensate for atmospheric disturbances, comprising providing nl first phase-shift correction signals (301) to the phase-shift modulator (200).

17. The method (1) according to the claim 16, wherein: the signal acquisition step (120) further comprises recording n2 second optical signals (2101) from input free space optical signals (21) using n2 second optical elements (2110) and providing to the phase-shift modulator (200) the n2 second optical signals (2101); the signal modification step (120) further comprises receiving the n2 second optical signals (2101) and n2 second phase-shift correction signals (302) using the phase-shift modulator (200), comprising modifying the phase of the n2 second optical signals (2101) to n2 modified second optical signals (2102) in dependence of the n2 second phase-shift correction signals (302), comprising providing the n2 modified second optical signals (2102) to the light-wave combiner (400); the signal processing step (130) further comprises receiving the n2 modified second optical signals (2102), comprising combining the n2 modified second optical signals (1102) using the lightwave combiner (400) to provide a combined modified optical signal (1601), providing an electricaloutput signal (1602) based on the combined modified optical signal (1601) to the control system (300); and a phase modulation step (140) further comprises receiving the electrical output signal (1602), comprising generating the n2 second phase-shift correction signals (301) in dependence of the electrical output signal (1602) using the phase-shift algorithm (1610) in the control system (300) to compensate for atmospheric disturbances, comprising providing the n2 second phase-shift correction signals (302) to the phase-shift modulator (200).

18. The method (1) according to any one of the claims 16-17, wherein: the signal modification step (120) comprises providing n3 modified first optical signals (1102) to the light-wave combiner via the multiport-circulator (500) to the light-wave combiner (400); and the method (1) comprises a transmission step (150) comprising generating n3 optical transmission signals (611), comprising providing the n3 optical transmission signals (611) to the phase-shift modulator (200) via the multiport-circulator (500), comprising transmitting the n3 optical transmission signals (611) via the first optical array (1100).

19. The method (1) according to claim 18, wherein the signal modification step (120) comprises splitting a laser signal into n3 optical transmission signals (611), wherein the n3 optical transmission signals (611) are statically co-phased using variable optical delay.

20. The method (1) according to any one of claims 18-19, wherein the method (1) comprises a beamcorrection step (170) further comprising controlling the first optical array (1100) and / or the second optical array (2100) to transmit the n3 optical transmission signals (611) at a predetermined point- ahead angle (0) with respect to the incoming angle of reception.

21. The method (1) according to any one of the claims 16-20, wherein the phase-shift algorithm (1610) is selected from the group comprising a machine-learning algorithm, a coordinate ascent algorithm and a stochastic algorithm.

22. The method (1) according to any one of the claim 16-21, comprising individually controlling the orientation of the nl first optical elements (1110) using the tip tilt controller (121).

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