A microcomb-based free space optical communication system

Abstract

Described herein is an optical free-space communications system (100). The system (100) includes one or more inputs (102) for receiving one or more input signals (104). Each input signal (104) includes a respective data stream to be transmitted. An optical carrier signal generator module (114) is configured to generate an optical carrier signal (106). The optical carrier signal generator module includes an optical microcomb generator (116) to generate a plurality of coherent discrete, equally spaced optical frequency channels (118) and an optical spectral shaper (116) configured to selectively apply respective amplitude weightings to the optical frequency channels (118). System (100) includes a plurality of coherent modulators (108, 128) configured to modulate an optical frequency channel (118) with a respective one of the one or more input signals (104) to generate an optical transmission signal (132). A signal transmission module (130) is configured to directionally transmit the optical transmission signal (132) across a region of free-space (102) to a target location. Finally, a coherent receiver module (112) is positioned at the target location and configured to receive the transmitted optical transmission signal (132) and to decode phase and amplitude information of the one or more input signals (104) from each of the modulated optical frequency channels (118).

Description

A MICROCOMB-BASED FREE SPACE OPTICAL COMMUNICATION SYSTEMFIELD OF THE INVENTION

[0001] The present application relates to optical communications and in particular to treespace optical communications.

[0002] Embodiments of the present invention are particularly adapted for transmitting high volumes of data across a tree-space optical network using an optical microcomb generator. However, it will be appreciated that the invention is applicable in broader contexts and other applications.BACKGROUND

[0003] Although terrestrial communications is now dominated by optical networks built by optical fiber, free-space applications still rely primarily on radio frequency communications. Communications in the optical domain provides for the possibility of significantly higher data rates than communications in the radio frequency range. Extending the optical domain into free space also provides the advantage of reduced latency as free space media have lower refractive indices than conventional optical media like optical fibers. As a comparison, optical signal propagation in a vacuum (refractive index of 1 ) provides 33% lower latency than the equivalent signal propagation in an optical fiber (refractive index of about 1 .5).

[0004] The move to the optical domain in free space communications is difficult for many reasons including the turbulent and lossy nature of the Earth’s atmosphere. Early free-space optical (FSO) systems employed simple modulation schemes short distances and incoherent receivers. These simple systems ignore many of the fundamental difficulties that arise when sending high data rates over large distances of free space.

[0005] More recently, Shao et al.

[0001] have demonstrated FSO communication based on microcombs. However, the distance of the transmission link was limited to 1 km and relies on incoherent receivers. As such, while this is a step in the right direction, this system is still not practical for transmission to / from satellites or over large distances.

[0006] Coherent detection methods, which capitalize fully on the available degrees of freedom to maximize data capacity, have the added complication of needing to couple the received signal into single-mode fiber. In terrestrial free-space media, single mode coupling, leads to power fluctuations, which requires active compensation through systems such as adaptive optics.

[0007] Any discussion of the background art throughout the specification should in no way be considered as an admission that such art is widely known or forms part of common general knowledge in the field.SUMMARY OF THE INVENTION

[0008] In accordance with a first aspect of the present invention, there is provided an optical free-space communications system including: one or more inputs for receiving one or more input signals, each input signal including a respective data stream to be transmitted; an optical carrier signal generator module configured to generate an optical carrier signal, the optical signal generator module including: an optical microcomb generator to generate a plurality of coherent discrete, equally spaced optical frequency channels; an optical spectral shaper configured to selectively apply respective amplitude weightings to the optical frequency channels; a plurality of modulators, each modulator configured to modulate an optical frequency channel with a respective one of the one or more input signals to generate an optical transmission signal comprising a plurality of modulated optical frequency channels, each modulated optical frequency channel having a respective input data stream encoded therein, wherein the optical frequency channels are modulated with the respective input signals using a coherent modulation scheme; a signal transmission module configured to directionally transmit the optical transmission signal across a region of free-space to a target location; and a coherent receiver module positioned at the target location and configured to receive the transmitted optical transmission signal and to decode phase and amplitude information of the one or more input signals from each of the modulated optical frequency channels.

[0009] In some embodiments, the system includes a polarization module for polarizing the optical frequency channels into orthogonal polarization components.

[0010] In some embodiments, the modulators are configured to perform polarization division multiplexing of the optical frequency channels.

[0011] In some embodiments, the optical microcomb generator includes a soliton crystalbased microcomb generator. In other embodiments, the optical microcomb generator includes a laser cavity-soliton microcomb generator. In some embodiments, the optical microcomb generator is adapted to generate optical frequency channels that are spaced apart at intervals of about 50GHz.

[0012] In some embodiments, the modulators are configured to perform quadrature amplitude modulation on the optical frequency channels. In some embodiments, the modulators are configured to perform phase modulation on the optical frequency channels. In some embodiments, the modulators are configured to perform single sideband modulation of an input signal with a corresponding optical frequency channel.

[0013] In some embodiments, the modulators are divided into a plurality of pairs of modulators, wherein each pair of modulators is configured to modulate two input signals onto a common optical frequency channel, wherein a first input signal is modulated to an upper sideband of the optical frequency channel and a second input signal is modulated to a lower sideband of the optical frequency channel.

[0014] In some embodiments, the optical microcomb generator produces between 50-100 optical frequency channels.

[0015] In some embodiments, the signal transmission module includes a collimating optical telescope. In some embodiments, the collimating optical telescope supports single mode optical propagation.

[0016] In some embodiments, the signal transmission module includes an optical beam steering device adapted to selectively directionally transmit the optical signal to the target location. In some embodiments, the target location is a target spacecraft in orbit. In other embodiments, the target location is a terrestrial target.

[0017] In some embodiments, the optical carrier signal generator module and signal transmission module are located onboard a spacecraft in orbit.

[0018] In some embodiments, the signal transmission module and coherent receiver module include single mode optical fibers to transmit and receive the optical transmission signal across the region of free-space in single mode propagation.

[0019] In some embodiments, the coherent modulation scheme includes Quadrature Amplitude Modulation (QAM). In some embodiments, the coherent modulation scheme includes one of 16QAM, 32QAM, 64QAM or 256QAM.

[0020] In accordance with a second aspect of the present invention, there is provided an optical free-space transmitter including: an optical microcomb generator to generate a plurality of coherent discrete, equally spaced optical carrier frequency channels; an optical spectral shaper configured to selectively apply respective amplitude weightings to the optical frequency channels; one or more inputs for receiving one or more input signals, each input signal including a respective data stream to be transmitted; a plurality of modulators, each modulator configured to modulate an optical frequency channel with a respective one of the one or more input signals to generate a plurality of modulated optical frequency channels, each modulated optical frequency channel having a respective input data stream encoded therein, wherein the optical frequency channels are modulated with the respective input signals using a coherent modulation scheme; a signal combiner adapted to combine the modulated optical frequency channels into an optical signal to be transmitted; and a signal transmission module configured to directionally transmit the optical signal across a region of free-space to a coherent detector at a target location; and

[0021] In some embodiments, the signal transmission module includes a collimating optical telescope.

[0022] In some embodiments, the signal combiner includes an optical multiplexer. In some embodiments, the optical multiplexer includes a wavelength selective switch.

[0023] In some embodiments, the signal transmission module includes an optical beam steering device to point the beam to the target location.

[0024] In some embodiments, the transmitter includes a polarization splitting device to split each optical frequency channel into a pair of orthogonally polarized signals.

[0025] In some embodiments of the second aspect, the coherent modulation scheme includes Quadrature Amplitude Modulation (QAM). In some embodiments, the coherent modulation scheme includes one of 16QAM, 32QAM, 64QAM or 256QAM.BRIEF DESCRIPTION OF THE FIGURES

[0026] Example embodiments of the disclosure will now be described, by way of example only, with reference to the accompanying drawings in which:Figure 1 is a schematic diagram of an optical free-space communications system;Figure 2 is a schematic diagram of an optical signal generator module incorporating three pump lasers pumping three separate micro-ring resonators to produce a carrier signal comprising a spectrum of three microcombs; andFigure 3 is a schematic diagram of a modulator of the system of Figure 1 for modulating input signals onto an optical carrier signal for free-space transmission.DESCRIPTION OF THE INVENTIONSystem overview

[0027] Referring initially to Figure 1 , there is illustrated an optical free-space communications system 100. As will be described below, system 100 is inherently coherent in that the phase of light is preserved from inputs to detection at a receiver end. System 100 includes one or more inputs 102 for receiving one or more input signals 104. Each of the input signals contain information to be transmitted across system 100 and include a respective data stream to be transmitted. In some embodiments, the input signals 104 are in the form of optical signals such as signals transmitted along optical fibers. In these embodiments, the inputs 102 take the form of optical fibers and corresponding fiber ports. In other embodiments, the input signals 104 are in the form of electrical signals transmitted along inputs 102 in the form of electrical wires. Although illustrated as a single input in Figure 1 , it will be appreciated that inputs 102 may include a plurality of optical fibers and / or electrical wires to provide multiple input signals 104. Further, each optical fiber or electrical wire may include multiple input signals in a multiplexed manner.

[0028] The input signals 104 are modulated onto an optical carrier signal 106 using a signal modulator 108 for transmission by a signal transmission module 130 across a free-space communications link 1 10 to a receiver 1 12. Optical carrier signal 106 may be in the visible, infrared or ultraviolet wavelength ranges. Signal modulator 108 is preferably connected to signal generator module 1 14 by an optical fiber, which carries the optical carrier signal 106. Similarly, modulator 108 is preferably connected to signal transmission module 130 by an optical fiber. These optical fibers connecting the different modules of system 100 may be single mode or multimode fibers.

[0029] Although not illustrated, it will be appreciated that system 100 includes various coupling optics for coupling input signals 104 and optical carrier signal 106 to modulator 108. In some embodiments, this coupling may be a free-space coupling which uses one or more of lenses, mirrors, beamsplitters, prisms and other optical elements to direct optical beams into modulator 108. In other embodiments, the signals may be coupled to modulator 108 via direct coupling of optical fibers or via waveguides. Where input signals 104 comprise electrical signals, these signals may be coupled into modulator 108 via electrical wires connected directly with electrodes of modulator 108. Electrical inputs may also be coupled to modulator 108 via electrical waveguides, RF connections or surface mount technologies. The choice of which connection to use will depend on the application and system demand.

[0030] System 100 includes an optical carrier signal generator module 1 14, which is configured to generate the optical carrier signal 106 to be transmitted. The optical signal generator module 1 14 includes a number of elements as described below. An optical microcomb generator 1 16 is configured to generate a microcomb spectrum including a plurality of coherent discrete, equally spaced comb lines defining optical frequency channels 1 18, as illustrated in the inset in Figure 1 . A microcomb is a form of frequency comb that is generated within a microresonator as opposed to a large-scale optical system. Microcombs are also referred to as Kerr frequency combs or microresonator frequency combs.

[0031] Optical microcomb generator 1 16 includes a coherent light source in the form of a pump laser 120, which generates a laser beam 122 having a corresponding central frequency and spectral linewidth. The laser beam 122 is coupled into a microresonator in the form of a micro-ring resonator 124 to form a soliton and generate a frequency comb including a plurality coherent discrete and equally spaced spectral peaks. These peaks form the optical frequency channels 1 18. In some embodiments, pump laser 120 may be replaced by a plurality of lasers,each having a different peak spectral output. The laser beams from each of the pump lasers are coupled to a single common microresonator or respective micro resonators to generate respective microcombs in the spectral domain.

[0032] Incorporating multiple pump lasers into microcomb generator 1 16 can broaden the spectrum of the generated microcomb and provide more efficient and precise control over the spacing and positions of comb lines. Multiple pump lasers can also enable complex modulation schemes to be implemented, which can be beneficial for encoding information in telecommunications or creating tailored waveforms for specific scientific applications. Using multiple pump lasers coupled to a single microresonator can provide a more compact and simpler design and lead to efficient utilization of the single nonlinear medium as all the pumps interact within the same resonator. Using multiple pump lasers coupled into separate microresonators allows each microresonator to be optimized for the specific wavelength of the pump laser, providing more flexibility to achieve desired nonlinear properties and dispersion characteristics.

[0033] Pump laser 120 may include any laser that produces a continuous wave or phase stabilised pulsed laser output at a frequency within a predefined operable range that is tuned to the micro-ring resonator 124 characteristics. The preferred type of laser to use for pump laser 120 is dependent on the type of microcomb generation mechanism (discussed below). Pump laser 120 is preferably, but not necessarily tunable across a range of peak output wavelengths. In one embodiment, pump laser 120 is in the form of an external cavity laser configured to generate a tunable output across a bandwidth of 100 GHz or greater. The peak spectral wavelength of pump laser 120 should be chosen such that it is central to an operable band of interest as the microcomb output will be generated symmetrically about this central wavelength.

[0034] When microcomb generator 1 16 is in use, a primary comb is initially generated as the laser 120 is tuned into resonance with the micro-ring resonator 124. At certain resonant conditions, a microcomb forms, with spectral features based around the primary comb lines.

[0035] In some embodiments, the optical microcomb generator 116 is adapted to generate soliton crystal-based microcomb spectrum. In these embodiments, micro-ring resonator 124 is adapted to support soliton crystal oscillations. Soliton crystal microcombs may have an efficiency in the range of 40% to 70%. In other embodiments, the optical microcomb generator 1 16 is adapted to generate a microcomb spectrum based on laser cavity-solitongeneration. In these embodiments, micro-ring resonator 124 forms part of a system that supports the generation of laser cavity solitons, such as that described in Bao et al. Laser cavitysoliton microcombs. Nat. Photonics 13, 384-389 (2019).

[0036] Various forms of micro-ring resonator can be manufactured. In some embodiments, micro-ring resonator 124 is formed by etching a waveguide on a CMOS chip such as a silicon, silicon nitride, aluminium nitride, gallium arsenide or indium phosphide based substrate. The quality factor of the resonator is preferably greater than 1 million and include a free spectral range that closely matches a target spacing of the desired output microcomb spectrum. Although microcomb generator 1 16 will be described as incorporating a micro-ring resonator, it will be appreciated that other forms of microresonator such as Fabry-Perot cavities and photonic crystal cavities may be used.

[0037] In further embodiments, microcomb generator 1 16 is adapted to generate other soliton sources for the microcombs including dark solitons and dissipative Kerr solitons. The choice of microcomb source is made so as to provide a source having high per-line power, low noise (e.g. optical signal to noise ratio greater than 30 dB) and strong robustness to environmental variations. Preferably, the generated microcomb should be continuous and stable over a period of many hours without external feedback control. The microcomb generator 1 16 should generate a stable microcomb spectrum having a large enough total bandwidth to cover an appropriate bandwidth for operation, which may include a significant part (or all) of the available communications spectrum. The available microcomb spectrum may extend or be tunable substantially across the C+L optical telecommunications band or wider. However, the bandwidth should be contained somewhat to the desired operable range so that the comb energy lies predominantly in the wavelengths of interest. The operating bandwidth of the microcomb spectrum must also cover optical amplifier bandwidth ranges that are available.

[0038] The choice of which type of microresonator and which soliton source to use for generating the microcomb will depend on factors such as the desired operating output, robustness, cost, power levels available and the environment in which the system is used (e.g. terrestrial or space based). However, preferably, the microcomb generator 1 16 has the ability to initiate stable and continuous comb operation easily without elaborate or sophisticated ultrahigh precision and ultrafast tunable lasers.

[0039] In some embodiments, characteristics of the micro-ring resonator 124 are tunable so as to vary the comb wavelengths across a bandwidth range. By way of example, this tuningmay be performed by controlled temperature adjustments of the micro-ring resonator 124, which selectively varies the length of the resonator cavity. Tuning of micro-ring resonator 124 may also be performed by mechanical tuning of the resonator by selectively applying mechanical stress / strain or by changing the effective refractive index of the resonator through electro-optic tuning or nonlinear optical effects.

[0040] The line spacing of the frequency channels 118 should be close enough that each individual comb line can be independently modulated with data to occupy all of the spectrum between the comb lines. By way of example, the free spectral range of the microcomb sources is preferably in the range of 25 GHz to 100 GHz to interface with currently available transceiver technology. Conventional dense wavelength divisional multiplexing systems rely on 12.5 GHz, 25 GHz, 50GHz or 100GHz channel spacings and microcomb generator 1 16 can be configured to generate microcombs with comb lines of this spacing. In some embodiments, the spacing and wavelengths of the microcombs may be configured so as to match standardised values set by the International Telecommunication Union (ITU).

[0041] The linewidth of the comb lines is preferably less than 100 kHz but may be wider than this in some embodiments. In some embodiments, modulator 108 is adapted to produce upper and lower sideband pairs for each frequency channel and combine the upper and lower sideband pairs into an optical super-channel of interleaved sub-bands in a similar manner to that described in PCT Patent Application Publication WO 2022 / 133517 to Corcoran et al. entitled “Improved Data Transmission Rates From Optical Sources” (hereinafter “Corcoran et al"). The contents of this publication are incorporated herein by way of cross-reference.

[0042] Microcomb generator 1 16 may generate a large number of comb lines and frequency channels 1 18 such as 10 to 300 channels. In one embodiment, microcomb generator 1 16 is configured to generate 80 comb lines over the telecommunications C band. By utilising the method described in Corcoran et al., the number of channels can effectively be doubled to 160. In other embodiments, microcomb generator 1 16 is capable of generating a plurality of comb spectra from multiple microcombs, with each comb covering a different spectrum for extending the spectral range of system 100. In various embodiments, microcomb generator 116 may generate fewer than 10 or greater than 300 comb lines.

[0043] Referring to Figure 2, there is illustrated an alternate signal generator module 1 14A including three independent laser sources 120A-120C that input respective laser beams 122A- 122C to respective independent micro-ring resonators 124A-124C to generate separatemicrocomb spectra. Although Figure 2 illustrates three pump lasers and three micro-ring resonators, it will be appreciated that any number of pump lasers and micro-ring resonators can be used to produce a carrier signal 106 having a spectrum comprising a plurality of microcombs. In some embodiments, separate spectral shaper devices are used for each of the microcombs generated from microcomb generator 116. In some embodiments, signal generator module 114 comprises 2, 3, 4, 5, 6, 7, 8, 9 or 10 pump lasers.

[0044] Using multiple microcomb sources, microcomb generator 1 16 may generate an optical carrier signal 106 having a comb spectrum that extends across any optical communication band such as one or more of the C, L and S optical communication bands (as standardised by the ITU and IEEE organisations). The C band may be defined as the frequency range between 191 .6 to 195.9 THz. The L band may be defined as the frequency range between184.5 to 191 .6 THz and the S band may be defined as the frequency range between 196.1 to206.5 THz. The primary limitation on the operable bandwidths are the availability and efficiency of the various components such as detectors, modulators and optical amplifiers. Each of these components have limited operating spectral ranges. Thus, microcomb generator 116 is capable of generating a microcomb having comb peaks extending across any optical communications band that is supported by network components.

[0045] An optical spectral shaper 126 illustrated in Figures 1 and 2 is configured to selectively apply respective amplitude weightings to the optical frequency channels. In particular, the optical frequency channels 1 18 output from microcomb generator 1 16 have different amplitudes, as illustrated in the inset, and spectral shaper 126 acts to flatten the microcomb spectrum such that each optical frequency channel includes a substantially similar amplitude. Spectral shaper 126 may comprise any optical device that is capable of independently manipulating the amplitude of each of the optical frequency comb channels. By way of example, spectral shaper 126 may include a Waveshaper 1000 device developed by Coherent Corp. This device utilises liquid crystal on silicon phase modulators to independently manipulate the amplitude and / or phase of optical channels which are separated from incident light. However, in other embodiments, spectral shaper 126 may include other programmable optical filter devices.

[0046] Referring now to Figures 1 to 3, the optical carrier signal 106 generated by generator module 1 14 is input to signal modulator 108. As illustrated in Figure 3, modulator 108 includes a demultiplexer 140 for initially spectrally splitting the frequency channels 1 18 from the opticalcarrier signal 106 along separate optical paths for input into respective modulator devices 128. The optical paths may comprise free space or may comprise waveguides such as optical fibers. Demultiplexer 140 may include an arrayed waveguide, wavelength selective switch (WSS) such as a Liquid Crystal on Silicon (LCOS) based switch or a micro-electromechanical (MEMS) based switch, integrated Echelle gratings, fiber Bragg gratings, thin film filters and integrated ring-based demultiplexers.

[0047] Modulator 108 includes a plurality of modulator devices 128, with one or more modulator device preferably being provided for each microcomb frequency channel 1 18 of carrier signal 106. However, in some embodiments, two modulator devices are provided for each microcomb frequency channel 1 18 so as to be able to reuse each frequency channel 118 to transmit different data in each of an upper and a lower modulation sideband in a manner described in Corcoran et al.

[0048] This is achieved by each modulator of a pair utilizing a single sideband modulation scheme (e.g. single sideband suppressed carrier scheme) with one of the pair modulating an input signal to a lower sideband and the other modulator of the pair modulating an input signal to an upper sideband. This modulation of two input signals onto a single microcomb frequency channel increases the capacity of the overall system. For example, a microcomb having 80 microcomb channels can be used to modulate 160 input signals for transmission. An optical interleaver or multiplexer may be used to combine the upper and lower sidebands of each channel into a superchannel for subsequent transmission. Optical filtering may be performed to remove or reduce power relating to neighbouring channel sidebands.

[0049] In addition, some frequency channels may be used for reference and noise reduction as mentioned below. Thus, the number of modulator devices 128 may be slightly less than the number of frequency channels 1 18. Each of the modulator devices 128 are configured to modulate an optical frequency channel 1 18 with a respective one of the one or more input signals 104 to generate an optical transmission signal 132 after multiplexing by a signal combiner in the form of multiplexer 144. The optical transmission signal 132 comprises a plurality of modulated optical frequency channels. Each modulated optical frequency channel includes a respective input data stream encoded therein.

[0050] Where inputs 104 are wavelength division multiplexed optical inputs, an optical demultiplexer such as an arrayed waveguide 142 separates out the input wavelength channelsinto individual signals for subsequent modulation with frequency channels 1 18 of optical carrier signal 106 by modulator devices 128.

[0051] Where inputs 104 are electrical signals, modulator devices 128 include complex electro-optic modulators to modulate the amplitude and phase of frequency channels separated by demultiplexer 140 with the electrical signals using an electro-optic effect. Modulation may be via single sideband or double sideband modulation.

[0052] To achieve coherent transmission and detection, modulator devices 128 are configured to modulate the input signals 104 onto the optical frequency channels 118 using a coherent modulation scheme. Such modulation schemes include Polarization multiplexed Quadrature Amplitude Modulation (QAM) schemes such as 16QAM, 32QAM, 64QAM and 256QAM or other M-QAM schemes, 4D constellation shaping or set partitioning. In QAM, the input signals 104 are modulated together with an in-phase and a quadrature (phase shifted) version of the respective optical frequency channels 1 18. The information from the input signals 104 is then encoded by modulating the amplitude and phase of the signals before transmission, wherein information symbols are represented as combinations of amplitude and phase according to a modulation scheme.

[0053] A complex field modulator device used in modulator 108 takes in two electrical signals, which correspond to the in-phase and quadrature parts of the optical field that is to be modulated. In some embodiments, the incoming light is split to two Mach-Zehnder interferometers, and the electrical signals each drive a separate Mach-Zehnder interferometer. This modulates the light on either the in-phase or quadrature axis of the optical field, and the in- phase and quadrature parts of the field are then recombined to enable arbitrary modulation in both phase and amplitude. In some embodiments, two of these modulator devices are placed in parallel, and the output of one is rotated in polarization to an orthogonal state to the other, enabling arbitrary modulation or multiplexing in polarization.

[0054] After modulation of input signals 104 with frequency channels 1 18, multiplexer 144 combines the modulated signals to generate optical transmission signal 132. Like demultiplexer 140, multiplexer 144 may include an arrayed waveguide, or a wavelength selective switch such as a Liquid Crystal on Silicon (LCOS) based switch or a microelectromechanical (MEMS) based switch, integrated Echelle gratings, fiber Bragg gratings, thin film filters and integrated ring-based multiplexers.

[0055] In some embodiments, multiplexer 144 or the broader modulator 108 includes an optical interleaver to operate in the de-interleaving mode for combining the modulated signals in an interleaved manner to generate a super-channel in the form of optical transmission signal 132. This interleaving may act to combine upper and lower sideband pairs into an optical superchannel of interleaved sub-bands in the manner described in Corcoran et al.

[0056] In some embodiments, system 100 includes a polarization module (not shown) for polarizing the optical frequency channels into orthogonal polarization components. Where a polarization module is included, modulator devices 128 are configured to perform polarization division multiplexing of the optical frequency channels 1 18. In some embodiments, the polarization module is incorporated into the modulator devices 128. Some commercially available modulators incorporate polarization diversity by splitting an input signal into orthogonal polarization components and then rotating one component by 90 degrees before performing modulation on each component independently. In some embodiments, modulator devices 128 are configurable in phase, amplitude and polarization for full control of the optical field.

[0057] Referring again to Figure 1 , the optical transmission signal 132 is passed to signal transmission module 130, which is configured to directionally transmit the optical transmission signal 132 across a free-space link 1 10 comprising a region of free-space to a target location. By way of example, the region of free-space may be a terrestrial region such as between towers in a telecommunications system. The region of free-space may also be between a terrestrial station and a satellite or between two satellites in space.

[0058] Signal transmission module 130 may include one or more optical amplifiers 148 for amplifying the optical transmission signal 132, a beam steering device 149 for selectively steering the optical transmission signal 132 and a collimating optical telescope 150 for transmitting the optical transmission signal 132 over the free-space link 110. Module 130 may also include other components such as gain flattening filters to equalize the amplitude of the different optical channels after amplification. Additional coupling elements may be included to couple optical transmission signal 132 from the input optical fiber to telescope 150. These coupling elements may include optical couplers, lenses, objectives or GRIN lenses. The fiber coupling system may be multimode or single mode or include multimode fiber coupled to single mode fibers.

[0059] Telescope 150 may comprise one or more optical elements and, overall, must have sufficient diameter and focal power to direct the beam across the required free-space distanceof free-space link 102. In some embodiments, telescope 150 supports single mode optical propagation.

[0060] Beam steering device 149 is adapted for selectively steering the direction of the transmission signal 132 through free-space to point the beam through turbulent air and / or redirect the optical transmission signal 132 to different target locations. In some embodiments, beam steering device 149 includes one or more mechanical or electromechanical actuators. Beam steering device 149 may be responsive to a control signal from a control system (not shown) to adjust the direction of telescope 150. This control signal may be configured to track the movement of a target location such as a satellite in low-Earth orbit (LEO). LEO satellites move across the horizon quite quickly so beam steering device 149 is preferably capable of rapidly steering the optical transmission signal 132 to scan the whole horizon over a period of a few minutes.

[0061] In some embodiments, beam steering element 149 includes a directional mount connected to telescope 150 for directionally steering the direction of transmission of signal 132. In other embodiments, beam steering device 149 may include a tiltable mirror or deformable mirror, MEMS device, spatial light modulator, mode selective devices with modal phase manipulation such as photonic lanterns or spatial mode multiplexers.

[0062] The beam steering device may form part of an adaptive optics system 153 for selectively modifying the wavefront of the transmitted signal 132 to account for atmospheric distortions in free-space. Alternatively, a separate adaptive optics system may be incorporated into system 100.

[0063] At the target location at the other end of free-space link 102, a receiving telescope 152 is positioned to receive the transmitted optical signal 132. Receiving telescope 152 must have a diameter sufficient to capture the beam as translated over the required distance. Receiving telescope 152 may be mounted to a directional mount or other beam steering device 151 for directionally steering the direction of the receiving telescope to receive signal 132 from different angles.

[0064] An adaptive optics system 153 is configured to perform wavefront modifying device is provided to selectively and dynamically modify the wavefront based on control input from a control feedback loop. Exemplary wavefront modifying devices for adaptive optics include a tiltable mirror, a deformable mirror, a spatial light modulator (such as a LCOS or MEMS device)or a set of modulators disposed on arms of a photonic lantern. In space-to-space transmissions such as between satellites, where atmospheric effects are not as prominent, adaptive optics system 153 may be optional.

[0065] A coherent receiver module 1 12 is connected to receiving telescope 152 via optical fiber 154. Coherent receiver module 112 is configured to receive the optical transmission signal 132 and to decode phase and amplitude information of the one or more input signals 104 from each of the modulated optical frequency channels 1 18. Prior to the coherent detection, an optical amplifier 156 is provided for amplifying the transmitted signal 132.

[0066] For coherent detection, a reference source 160 such as a laser, bank of lasers or a microcomb source provides a local oscillator signal 162 to a polarization diverse complex field detector 164, which is capable of detecting phase, amplitude and polarization of each of the modulated frequency channels of optical transmission signal 132. This detection process allows the extraction of the data from input signals 104. Digital signal processing may be performed on the recovered signal by a digital signal processor 166 to recover the signal information from noise. The detection process also allows for the extraction of full field information of the incoming light, enabling the cancellation of channel distortions (e.g. phase noise).

[0067] A coherent receiver device used in module 1 12 takes in two optical fields, commonly known as a signal (the received optical transmission signal 132) and a local oscillator signal, and uses photodetectors to detect the interference between these fields. This provides an electrical output with is representative of the in-phase and quadrature parts of the modulated signal. In some embodiments, the received signal and local oscillator are coupled into an interferometric device known as a 90 degree optical hybrid. The outputs of this hybrid are measured with balanced photodiodes. In the case where the carrier phase and frequency of the optical signal match the local oscillator, the hybrid will produce two electrical signals that will correspond to the in-phase and quadrature parts of the optical field. This enables measurement of the phase and amplitude state of the optical field. In some embodiments, two optical hybrids are used, where the inputs to those hybrids are orthogonal polarization states of the signal, and matching polarization states of the local oscillator. This enables measurement of polarization state of the signal, as well as the amplitude and phase.

[0068] In some embodiments, one or more of the comb frequency channels are used as reference channels allow for the measurement of phase noise. This can then be compensated optically, electro-optically or digitally. In the optical domain compensation, a phase modulator isdriven with the opposite phase to perform compensation. In electro-optic compensation, a comb is recovered with the precise phase noise on it from the reference tone, and then this is used as a local oscillator in coherent detection, which then beats out the phase. In digital compensation, the phase noise is measured on a reference channel, and then the inverse of this phase noise is applied electrically or electronically to all other channels.

[0069] In addition to very high data rates, using microcomb generators provides additional advantages in terms of small size and weight, low power and possessing the same phase noise across all channels. This allows one channel of the microcomb to be used as a phase tracking channel to track phase across all optical channels and apply phase compensation. This compensation is possible in free-space but not in optical fiber links as dispersion in the fiber destroys the phase relationship.

[0070] The use of microcombs is advantageous for free-space transmission as the comb lines do not drift relative to each other, which is a big problem for multiple discrete laser systems. This means that wavelength guard bands can be made to be a lot tighter, leading to higher spectral efficiency.

[0071] In some embodiments, optical carrier signal generator module 1 14 is incorporated onto an integrated optical chip.References1. Wen Shao, Yang Wang, Shuaiwei Jia, Zhuang Xie, Duorui Gao, Wei Wang, Dongquan Zhang, Peixuan Liao, Brent E. Little, Sai T. Chu, Wei Zhao, Wenfu Zhang, Weiqiang Wang, and Xiaoping Xie, "Terabit FSO communication based on a soliton microcomb," Photon. Res. 10, 2802-2808 (2022).Interpretation

[0072] Throughout this specification, use of the term “optical” in the context of an optical signal or the like is intended to refer to electromagnetic radiation in one or more of the visible, infrared or ultraviolet wavelength ranges.

[0073] Unless specifically stated otherwise, as apparent from the following discussions, it is appreciated that throughout the specification discussions utilizing terms such as "processing," "computing," "calculating," “determining”, analyzing” or the like, refer to the action and / or processes of a computer or computing system, or similar electronic computing device, thatmanipulate and / or transform data represented as physical, such as electronic, quantities into other data similarly represented as physical quantities.

[0074] In a similar manner, the term “controller” or "processor" may refer to any device or portion of a device that processes electronic data, e.g., from registers and / or memory to transform that electronic data into other electronic data that, e.g., may be stored in registers and / or memory. A “computer” or a “computing machine” or a "computing platform" may include one or more processors.

[0075] Reference throughout this specification to “one embodiment”, “some embodiments” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present disclosure. Thus, appearances of the phrases “in one embodiment”, “in some embodiments” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures or characteristics may be combined in any suitable manner, as would be apparent to one of ordinary skill in the art from this disclosure, in one or more embodiments.

[0076] As used herein, unless otherwise specified the use of the ordinal adjectives "first", "second", "third", etc., to describe a common object, merely indicate that different instances of like objects are being referred to, and are not intended to imply that the objects so described must be in a given sequence, either temporally, spatially, in ranking, or in any other manner.

[0077] In the claims below and the description herein, any one of the terms comprising, comprised of or which comprises is an open term that means including at least the elements / features that follow, but not excluding others. Thus, the term comprising, when used in the claims, should not be interpreted as being limitative to the means or elements or steps listed thereafter. For example, the scope of the expression a device comprising A and B should not be limited to devices consisting only of elements A and B. Any one of the terms including or which includes or that includes as used herein is also an open term that also means including at least the elements / features that follow the term, but not excluding others. Thus, including is synonymous with and means comprising.

[0078] It should be appreciated that in the above description of exemplary embodiments of the disclosure, various features of the disclosure are sometimes grouped together in a single embodiment, Fig., or description thereof for the purpose of streamlining the disclosure andaiding in the understanding of one or more of the various inventive aspects. This method of disclosure, however, is not to be interpreted as reflecting an intention that the claims require more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive aspects lie in less than all features of a single foregoing disclosed embodiment. Thus, the claims following the Detailed Description are hereby expressly incorporated into this Detailed Description, with each claim standing on its own as a separate embodiment of this disclosure.

[0079] Furthermore, while some embodiments described herein include some but not other features included in other embodiments, combinations of features of different embodiments are meant to be within the scope of the disclosure, and form different embodiments, as would be understood by those skilled in the art. For example, in the following claims, any of the claimed embodiments can be used in any combination.

[0080] In the description provided herein, numerous specific details are set forth. However, it is understood that embodiments of the disclosure may be practiced without these specific details. In other instances, well-known methods, structures and techniques have not been shown in detail in order not to obscure an understanding of this description.

[0081] Similarly, it is to be noticed that the term coupled, when used in the claims, should not be interpreted as being limited to direct connections only. The terms "coupled" and "connected", along with their derivatives, may be used. It should be understood that these terms are not intended as synonyms for each other. Thus, the scope of the expression a device A coupled to a device B should not be limited to devices or systems wherein an output of device A is directly connected to an input of device B. It means that there exists a path between an output of A and an input of B which may be a path including other devices or means. "Coupled" may mean that two or more elements are either in direct physical, electrical or optical contact, or that two or more elements are not in direct contact with each other but yet still co-operate or interact with each other.

[0082] Embodiments described herein are intended to cover any adaptations or variations of the present invention. Although the present invention has been described and explained in terms of particular exemplary embodiments, one skilled in the art will realize that additional embodiments can be readily envisioned that are within the scope of the present invention.

Claims

What is claimed is:

1. An optical free-space communications system including: one or more inputs for receiving one or more input signals, each input signal including a respective data stream to be transmitted; an optical carrier signal generator module configured to generate an optical carrier signal, the optical signal generator module including: an optical microcomb generator to generate a plurality of coherent discrete, equally spaced optical frequency channels; an optical spectral shaper configured to selectively apply respective amplitude weightings to the optical frequency channels; a plurality of modulators, each modulator configured to modulate an optical frequency channel with a respective one of the one or more input signals to generate an optical transmission signal comprising a plurality of modulated optical frequency channels, each modulated optical frequency channel having a respective input data stream encoded therein, wherein the optical frequency channels are modulated with the respective input signals using a coherent modulation scheme; a signal transmission module configured to directionally transmit the optical transmission signal across a region of free-space to a target location; and a coherent receiver module positioned at the target location and configured to receive the transmitted optical transmission signal and to decode phase and amplitude information of the one or more input signals from each of the modulated optical frequency channels.

2. The system according to claim 1 including a polarization module for polarizing the optical frequency channels into orthogonal polarization components.

3. The system according to claim 2 wherein the modulators are configured to perform polarization division multiplexing of the optical frequency channels.

4. The system according to any one of the preceding claims wherein the optical microcomb generator includes a soliton crystal-based microcomb generator.

5. The system according to any one of claims 1 to 3 wherein the optical microcomb generator includes a laser cavity-soliton microcomb generator.

6. The system according to any one of the preceding claims wherein the optical microcomb generator is adapted to generate optical frequency channels that are spaced apart at intervals of about 50GHz.

7. The system according to any one of the preceding claims wherein the modulators are configured to perform quadrature amplitude modulation on the optical frequency channels.

8. The system according to any one of the preceding claims wherein the modulators are configured to perform phase modulation on the optical frequency channels.

9. The system according to any one of the preceding claims wherein the modulators are configured to perform single sideband modulation of an input signal with a corresponding optical frequency channel.

10. The system according to claim 9 wherein the modulators are divided into a plurality of pairs of modulators, wherein each pair of modulators is configured to modulate two input signals onto a common optical frequency channel, wherein a first input signal is modulated to an upper sideband of the optical frequency channel and a second input signal is modulated to a lower sideband of the optical frequency channel.1 1 . The system according to any one of the preceding claims wherein the optical microcomb generator produces between 50-100 optical frequency channels.

12. The system according to any one of the preceding claims wherein the signal transmission module includes a collimating optical telescope.

13. The system according to claim 12 wherein the collimating optical telescope supports single mode optical propagation.

14. The system according to claim 12 wherein the signal transmission module includes an optical beam steering device adapted to selectively directionally transmit the optical signal to the target location.

15. The system according to any one of the preceding claims where the target location is a target spacecraft in orbit.

16. The system according to claim 15 wherein the optical carrier signal generator module and signal transmission module are located onboard a spacecraft in orbit.

17. The system according to any one of the preceding claims wherein the signal transmission module and coherent receiver module include single mode optical fibers to transmit and receive the optical transmission signal across the region of free-space in single mode propagation.

18. The system according to any one of the preceding claims wherein the coherent modulation scheme includes Quadrature Amplitude Modulation (QAM).

19. The system according to claim 18 wherein the coherent modulation scheme includes one of 16QAM, 32QAM, 64QAM or 256QAM.

20. An optical free-space transmitter including: an optical microcomb generator to generate a plurality of coherent discrete, equally spaced optical carrier frequency channels; an optical spectral shaper configured to selectively apply respective amplitude weightings to the optical frequency channels; one or more inputs for receiving one or more input signals, each input signal including a respective data stream to be transmitted; a plurality of modulators, each modulator configured to modulate an optical frequency channel with a respective one of the one or more input signals to generate a plurality of modulated optical frequency channels, each modulated optical frequency channel having a respective input data stream encoded therein, wherein the optical frequency channels are modulated with the respective input signals using a coherent modulation scheme; a signal combiner adapted to combine the modulated optical frequency channels into an optical signal to be transmitted; and a signal transmission module configured to directionally transmit the optical signal across a region of free-space to a coherent detector at a target location; and21 . The optical free-space transmitter according to claim 20 wherein the signal transmission module includes a collimating optical telescope.

22. The optical free-space transmitter according to claim 20 wherein the signal combiner includes an optical multiplexer.

23. The optical free-space transmitter according to claim 22 wherein the optical multiplexer includes a wavelength selective switch.

24. The optical free-space transmitter according to any one of claims 20 to 23 wherein the signal transmission module includes an optical beam steering device to point the beam to the target location.

25. The optical free-space transmitter according to any one of claims 20 to 23 including a polarization splitting device to split each optical frequency channel into a pair of orthogonally polarized signals.

26. The optical free-space transmitter according to any one of claims 20 to 25 wherein the coherent modulation scheme includes Quadrature Amplitude Modulation (QAM).

27. The optical free-space transmitter according to claim 26 wherein the coherent modulation scheme includes one of 16QAM, 32QAM, 64QAM or 256QAM.

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