Apparatus and method for synthesizing wideband electromagnetic signals - Patents.com

Abstract

The present invention relates to an apparatus (1), a system (2), a method (200) and a computer program for synthesizing at least one targeted wideband electromagnetic signal (60), wherein the apparatus (1) comprises at least two electromagnetic signal generators (10, 11) configured to provide at least two tributary electromagnetic signals having at least partially overlapping spectra, at least one signal combining element (50) configured to merge the at least two tributary electromagnetic signals into at least one combined electromagnetic signal, at least one signal detecting element (90) configured to generate at least one detected output signal (100) by detecting at least a portion of at least one of the at least one combined electromagnetic signals, and at least one control element (110) configured to detect at least a portion of at least one of the at least one combined electromagnetic signals. at least one control element (110) configured to extract signal information relating to at least one of the relative amplitude or relative phase of the at least two tributary electromagnetic signals from the at least one detection output signal (100) to merge into at least one combined electromagnetic signal; and at least one actuating element (30) configured to modify at least one of the amplitude or phase of the at least one of the at least two tributary electromagnetic signals based on the signal information, wherein modifying at least one of the amplitude or phase of the at least one of the at least two tributary electromagnetic signals is performed such that at least one of the at least one combined electromagnetic signals corresponds to at least one targeted wideband electromagnetic signal (60).

Description

[Technical field]

[0001] The present invention is in the field of photonics and microwave engineering. The present invention relates to an apparatus, a system, a method, and a computer program for synthesizing at least one wideband electromagnetic signal. In particular, the at least one targeted wideband electromagnetic signal can be a wideband arbitrary electromagnetic signal, where the term "arbitrary signal" refers to a signal that can be freely defined, for example by digital signal processing, within the bandwidth provided by the signal generator system without the need to change the hardware configuration. [Background technology]

[0002] Several prior works in the field of optical arbitrary waveform generation (OAWG) are known.

[0003] Geisler et al., Demonstration of a Flexible Bandwidth Optical Transmitter / Receiver System Scalable to Terahertz Bandwidths, IEEE Photonics Journal 3(6), 2011, pp.1013-1022, speculate that the OAWG approach could be pushed to terahertz bandwidths, but typical experimental applications using discrete components are limited to bandwidths below 100 GHz. See, for example, Guan, B., et al., Optical Spectrally Sliced ​​Transmitter for High Fidelity and Bandwidth Scalable Waveform Generation, Journal of Lightwave Technology 34(2), 2016, pp.737-744.

[0004] Although implementations of OAWG schemes using separate components such as those described in Geisler et al. referenced above may vary, they typically suffer from random amplitude and / or phase variations between the tributary electromagnetic signals. If not monitored and controlled, these random variations can distort the combination of the desired signal, resulting in poor output quality. In particular, the OAWG scheme described in Geisler et al. referenced above is based on wavelength division multiplexing (WDM) transmitters, where a large number of alternating optical signals with overlapping frequency components are generated and then combined into a wideband optical output signal using an optical multiplexer. Although fiber-based devices are used, slow changes in the phase shift between the tributary electromagnetic signals are inevitable due to thermal and / or mechanical effects from the environment. Although it is stated in Geisler et al. referenced above that the phase between each spectral slice requires alignment for correct generation of the desired waveform, no attempt is made to monitor or adjust the phase between the tributary electromagnetic signals.

[0005] Feng et al., Rapidly reconfigurable high-fidelity optical arbitrary waveform generation in heterogeneous photonic integrated circuits, Optics Express 8884, Vol. 25(8), 2017, describes an example of an OAWG implemented on a multi-chip photonic module. Here, the authors declare that they succeeded in generating waveforms spanning a spectral bandwidth of 160 GHz. However, the waveforms in question are not truly arbitrary and cannot cover a continuous spectrum beyond 160 GHz with a well-defined phase. As used herein, the scheme is the same as that described in Geisler et al., referenced above, and implemented on a multi-chip module. The array of phase modulators implemented in InP technology is a Si 3 N 4This is combined with two arrayed-waveguide gratings implemented on the chip. Because pure phase modulation is used instead of IQ modulation, the waveforms produced in this manner cannot be truly arbitrary, as the amplitude and phase of the waveform cannot be simultaneously adjusted. Spectrally adjacent signals produced by the modulator can be superimposed on one another, but the resulting superposition cannot form a well-defined time-domain waveform, because neither the relative amplitude nor the relative phase of the superimposed tributary electromagnetic signals can be effectively controlled to compensate for random phase shifts in the underlying photonic circuit.

[0006] Combining an ultra-wideband optical signal starting from at least two band-limited tributary electromagnetic signals requires precise knowledge of the relative amplitudes and / or phases at which the tributary electromagnetic signals are superimposed. The main limitation in using state-of-the-art OAWGs, and especially OAWGs such as those mentioned in the previous paragraph, is that they do not allow monitoring and subsequent control of the stable amplitude and / or phase relationship between the tributary electromagnetic signals that are combined to generate a wideband output signal. Without some form of feedback signal configured to track the relationship between the tributary electromagnetic signals, it is not possible to reliably generate a specific time-domain output waveform with a well-defined spectral phase across the entire bandwidth of the THz range.

[0007] Generally, the phase of the tributary electromagnetic signal may be affected by environmental influences such as thermal fluctuations and / or minute changes in mechanical distortion. However, other types of influences on the phase of the tributary optical signal may also occur. As a result of phase and / or amplitude changes, the resulting superimposed waveform may be subject to time-varying and at least partially random disturbances. In the extreme case, where any of the tributary electromagnetic signals propagates through at least one optical fiber or waveguide before being superimposed with another signal, the phase changes may be of the order of 2π radians or even exceed it, such that the resulting wideband signal becomes almost random, but it becomes impossible to provide a well-defined waveform. Apart from phase changes, the tributary optical signal may also undergo amplitude changes, which may also prevent the synthesis of a well-defined target waveform by simple superposition.

[0008] Guan Binbin et al., Optical Spectrally Sliced ​​Transmitter for High Fidelity and Bandwidth Scalable Waveform Generation, J. Lightwave Techn. 34(2), 2016, pages 737-744, describes a single-carrier optical coherent transmitter that synthesizes high-fidelity waveforms from N spectral slices using state-of-the-art electrical drivers. The synthesis technique overcomes the electronic speed bottleneck and generates an optical waveform bandwidth that is N times the electrical bandwidth. Using two 32 GHz slices, the inventors synthesized and transmitted a 60-GBd polarization division multiplexed, quadrature phase shift keying (PDM-QPSK) waveform over 4480 km with a Q2 factor of 8.71 dB. To demonstrate high-fidelity waveform synthesis, the inventors generated a 60-GBd PDM 16-QAM and observed a 2.5-dB implementation penalty at a BER of 1 × 10-2. To address scalability, the inventors developed a phase mismatch correction algorithm for transmitters using photonic integrated circuits.

[0009] Nicolas Fontaine et al., Dynamic optical arbitrary waveform generation and detection in lnP photonic integrated circuits for Tb / s optical communications, Optics Comm. 284(15), 2011, pages 3693-3705 present the results of the development of an optical arbitrary waveform generation (OAWG) technique based on optical frequency combs and indium phosphide devices. A novel spectrally sliced ​​dynamic OAWG approach and a waveform shaper with customized spectral multiplexers and modulators allows the continuous generation of high-fidelity optical waveforms accessing bandwidths of over 1 THz. The inventors present results for two types of integrated waveform shapers: a 100 GHz electrically controlled device with 10 channels spaced at 10 GHz and a 1 THz optically controlled device with 100 channels spaced at 10 GHz. In addition, the inventors also include results from a 640 GHz waveform measurement device with 16 channels and 40 GHz spacing.

[0010] Ben Yoo et al., Terahertz Information and Signal Processing by RF-Photonics, IEEE Transactions on Terahertz Sc.& Techn.2(2),2012,pages 167-176, discusses THz bandwidth information and signal processing based on RF photonic technology. In particular, the inventors focus on integrated circuit approaches to RF photonics that can provide highly stable THz bandwidth information and signal processing. The inventors demonstrate the generation of THz signals by optical arbitrary waveform generation (OAWG) and describe the reverse process, optical arbitrary waveform measurement (OAWM). In addition, the inventors discuss RF photonic lattice filters useful for optical equalization and other THz signal processing, replacing traditional electronic digital signal processing that cannot currently be extended to THz. The inventors also cover future perspectives for THz information and signal processing by integrated RF photonic methods. Summary of the Invention [Problem to be solved by the invention]

[0011] It is an object of the present invention to provide an apparatus, a system, a method and a computer program for synthesizing at least one wideband electromagnetic signal, thereby at least partially overcoming the above-mentioned problems of the state of the art.

[0012] In particular, it is desirable to provide devices, systems, methods, and computer programs that enable superimposing tributary electromagnetic signals having a correct relationship between the amplitudes and / or phases of the tributary electromagnetic signals to form a well-defined wideband waveform. More particularly, it is desirable to use devices, systems, methods, and computer programs to establish and maintain the correct relationship between the amplitudes and / or phases even during operation by continuously monitoring and adjusting adjacent or overlapping tributary electromagnetic signals to enforce corresponding operating conditions over a useful time interval despite various adverse factors such as manufacturing tolerances or environmental effects. [Means for solving the problem]

[0013] This problem is solved by an apparatus, a system, a method and a computer program for synthesizing at least one targeted wideband electromagnetic signal having the features of the independent claims. Preferred embodiments implemented in a separate manner or in any arbitrary combination are recited in the dependent claims and throughout this specification.

[0014] In a first aspect, the present invention relates to an apparatus for synthesizing at least one wideband electromagnetic signal, in particular at least one arbitrary electromagnetic signal. As used herein, the terms "arbitrary signal" or "arbitrary waveform" refer to an electromagnetic signal or a waveform of an electromagnetic signal, respectively, that can be freely defined, in particular by digital signal processing, having a bandwidth provided by at least one electromagnetic signal generator, without the need to change the hardware configuration during operation.

[0015] According to the invention, an apparatus comprises: at least two electromagnetic signal generators configured to provide at least two tributary electromagnetic signals having at least partially overlapping spectra; at least one signal combining element configured to merge the at least two tributary electromagnetic signals into at least one combined electromagnetic signal; at least one signal detection element configured to generate at least one detected output signal by detecting at least a portion of at least one of the at least one combined electromagnetic signals; at least one control element configured to extract signal information relating to at least one of a relative amplitude or a relative phase of the at least two tributary electromagnetic signals merged into at least one combined electromagnetic signal from the at least one detection output signal; at least one actuation element configured to modify at least one of an amplitude or a phase of at least one of the at least two tributary electromagnetic signals based on the signal information; Altering at least one of the amplitude or phase of at least one of the at least two tributary electromagnetic signals is performed such that at least one of the at least one combined electromagnetic signal corresponds to the at least one targeted wideband electromagnetic signal.

[0016] As used herein, the terms "relative amplitude" and "relative phase" or any grammatical variations thereof refer to an amplitude relationship or a phase relationship, respectively, that superimposes two of the tributary electromagnetic signals to generate at least one targeted wideband electromagnetic waveform. In a preferred embodiment, the time domain signal can be described by a complex-valued Fourier transform characterized by a frequency-dependent spectral amplitude and a frequency-dependent spectral phase. The amplitude or phase relationship of the two signals can then be defined by the relationship of the complex-valued spectral components of the two signals at different frequencies, particularly via a difference or ratio, although further types of relationships are feasible. As an example, the relationship can be determined for complex-valued spectral components of the two signals that occur at a common frequency within the spectral overlap region of the two signals. In further embodiments, the relationship can be more general and may be frequency-dependent. In particular, the frequency-dependent spectral phase can be used to express the relative time delay of the two tributary signals in the time domain, which can be compensated for by the actuation element. In an alternative embodiment, the time domain signal can be described by a corresponding complex-valued analytic signal, which can be represented by a complex-valued continuous wave carrier that is modulated or multiplied with a time-dependent complex-valued envelope. The relationship between the amplitude or phase of the two signals can then be defined by representing both signals on the same wave and comparing the phase and amplitude of the complex-valued envelope at different times or frequencies. This approach is described in more detail below.

[0017] According to the present invention, at least one targeted wideband electromagnetic signal is synthesized using a superposition of at least two different tributary electromagnetic signals. As generally used, the term "target" refers to at least one desired electromagnetic signal generated by using at least one of the devices, systems, methods, or computer programs for synthesizing at least one targeted wideband electromagnetic signal according to the present invention. Thus, the at least one targeted wideband electromagnetic signal corresponds to the combined electromagnetic signal generated by the at least one signal combining element in the event that, based on at least one detection output signal, at least one actuating element is triggered by at least one control element to modify at least one of the amplitude or phase of at least one of the at least two tributary electromagnetic signals before entering the at least one signal combining element, so that the deviation between the at least one targeted wideband electromagnetic signal and the combined electromagnetic signal can be maintained below a defined threshold.

[0018] In particular, the purpose of superimposing tributary electromagnetic signals may include increasing the bandwidth of a continuous arbitrary signal, even when using band-limited or non-arbitrary electromagnetic signal generators, thereby making it possible to obtain truly arbitrary signals with bandwidths in the GHz range, preferably in the THz range. As commonly used, the terms "THz frequencies" and "THz range" refer to frequencies or bandwidths above 100 GHz, respectively, while the terms "THz signals" or "THz waveforms" refer to electromagnetic signals or forms of electromagnetic signals with bandwidths in the THz range, respectively. The terms "arbitrary waveforms" or "arbitrary signals" refer to electromagnetic waveforms with an arbitrary time dependence of a complex-valued signal spectrum, or equivalently, an arbitrary frequency dependence, which may be obtained, preferably by using a Fourier transform. In particular, the spectrum of the arbitrary signal shall not suffer from uncontrolled discontinuities in frequency-dependent phase and amplitude, which may arise, for example, as a result of the spectral stitching of band-limited tributaries whose relative phases and / or amplitudes are uncontrolled or poorly controlled.

[0019] For the purpose of synthesizing at least one targeted wideband electromagnetic signal by superposition of at least two tributary electromagnetic signals according to the invention, at least one relationship between at least one amplitude and at least one phase of the at least two tributary electromagnetic signals is known for at least one time point of superposition. Generally, errors in the characteristics of the at least two tributary electromagnetic signals at the time of superposition may lead to serious disturbances of the waveform of the at least one synthesized wideband electromagnetic signal or may prevent the synthesis of the targeted waveform. Establishing and maintaining at least one relationship between at least one amplitude and at least one phase of the at least two tributary electromagnetic signals during the superposition of the at least two tributary electromagnetic signals is difficult, especially in the optical frequency range, especially when the carrier frequency used for the at least two tributary electromagnetic signals is high, since optical signals are particularly susceptible to undesirable phase drifts and amplitude changes when propagating in a waveguide. As used herein, the terms "light" and "optical frequency range" refer to electromagnetic signals whose wavelength spectrum covers the optically visible range (wavelength 400 nm≦λ<800 nm), and in addition, any type of electromagnetic wave that can be guided in a waveguide, preferably the UV range (10 nm≦λ<400 nm), the infrared range (800 nm≦λ<1 mm), the THz and millimeter wave range (30 μm≦λ≦3 mm). Preferably, each of the at least two tributary electromagnetic signals and the at least one targeted broadband electromagnetic signal may have a wavelength in the optical frequency range, more preferably 10 nm to 3 mm, in particular 100 nm to 1 mm.

[0020] The signal portion detected by the detection unit can be derived from any of the at least one combined electromagnetic signal. In particular, the detected signal portion can be derived from the combined electromagnetic signal corresponding to the targeted wideband electromagnetic signal or from any other combined electromagnetic signal at any of the at least one output of the combiner unit. In a special embodiment, the combined electromagnetic signal from which the detection output signal is derived, and / or the detection output signal itself, can be much narrower band than the targeted wideband electromagnetic signal. This is the case, for example, when the transmission characteristics of the signal combining unit are highly frequency-dependent, and the detection output signal can then be derived from a spectral portion of the combined electromagnetic signal that includes only a portion of the overlap region of the two tributary electromagnetic signals. This can alternatively occur when the detection element can be configured such that the detection output signal can be generated only by a small portion of the combined electromagnetic signal, while irrelevant portions of the combined electromagnetic signal are not converted to the detection output, preferably by using a balanced photodetector for the optical signal.

[0021] The invention presented herein may be particularly useful for the synthesis of spectrally broadband waveforms. Thus, the bandwidth of the targeted electromagnetic signal may preferably exceed 20 GHz, 40 GHz, 75 GHz, 150 GHz, 200 GHz, or 300 GHz. Similarly, the bandwidth of each tributary electromagnetic signal may preferably be between 1 GHz and 500 GHz, more preferably between 10 GHz and 200 GHz, and most preferably between 20 GHz and 150 GHz. The number of tributary electromagnetic signals that are merged into the targeted broadband electromagnetic signal may preferably be 2 or more, more preferably 4 or more, and most preferably 8 or more. For embodiments based on spectrally sliced ​​tributary electromagnetic signals, the bandwidth of the portion of the combined electromagnetic signal that may be used to generate the detection output signal may preferably be less than 50%, more preferably less than 30%, and most preferably less than 10% of the bandwidth of the spectral slice. In absolute terms, for embodiments based on spectrally sliced ​​tributary electromagnetic signals, the bandwidth of the portion of the combined electromagnetic signal that can be used to generate the detection output signal can be preferably between 1 MHz and 50 GHz, more preferably between 100 MHz and 20 GHz, most preferably between 1 GHz and 20 GHz, in particular between 2 GHz and 10 GHz. In further embodiments in which the targeted electromagnetic signal can be generated not by spectral slicing but by superposing two or more wideband tributary electromagnetic signals, the bandwidth of the portion of the combined electromagnetic signal that can be used to generate the detection output signal can correspond to the bandwidth of the targeted electromagnetic signal. Regardless of the embodiment, the power of the portion of the tributary electromagnetic signals that can be used to generate the detection output signal can be lower than the total power of the respective tributary electromagnetic signals, preferably between 10 dB and 60 dB, more preferably between 15 dB and 40 dB, most preferably between 20 dB and 30 dB. Generally, the actuation bandwidth of the drive signal configured to drive at least one actuation element may preferably exceed 1 kHz, 10 kHz, 100 kHz, 1 MHz, or 10 MHz, where the actuation element may preferably be configured to support the selected actuation bandwidth.

[0022] In a preferred embodiment, the overlapping portion of the spectrum of the at least two tributary electromagnetic signals may include at least one reference signal that may be suppressed in the at least one targeted wideband electromagnetic signal upon combination of the tributary electromagnetic signals in the signal combining element. Due to the inaccuracies and tolerances of the components used in the device, this suppression may be incomplete, thus resulting in a residual reference signal in the combined wideband electromagnetic signal. Preferably, the power level of the residual reference signal is lower than the total power of the combined electromagnetic signals, and may be preferably 10 dB, 20 dB, 30 dB, 40 dB, or 50 dB.

[0023] In order to properly combine the tributary electromagnetic signals and maintain a high quality of the generated wideband electromagnetic waveform, undesirable time-varying and at least partially random amplitude and / or phase offsets are corrected by using a feedback control loop according to the present invention. As used herein, the terms "amplitude offset" and "phase offset", or any grammatical variations thereof, refer to an unintended or random amplitude or phase difference between two of the tributary electromagnetic signals, respectively, that prevents the combination in the two combiner units of the tributary electromagnetic signals to generate a targeted output signal at an intended output port. The feedback signal of the feedback loop is derived by detecting at least one of the combined signal outputs in at least one signal combining element. Information regarding at least one relative amplitude and / or at least one relative phase of the superimposed tributary electromagnetic signals to form at least one combined electromagnetic signal is extracted by detecting at least one combined electromagnetic signal, specifically at least a portion of at least one of the combined electromagnetic signals. Here, the extraction of the signal information is based on characteristics of the tributary electromagnetic signals, characteristics of the at least one signal combining element, and characteristics of the at least one signal detecting element. By using the extracted signal information, the at least one control element may establish a real-time feedback control loop to modify the amplitude and / or phase of at least one of the tributary electromagnetic signals. As used herein, the term "real-time" or "real-time feedback loop" refers to the operation of an apparatus designed to provide a response within a specified time that is short enough to maintain a predefined relationship between the relative amplitude and / or phase of two tributary electromagnetic signals that combine to form at least one combined electromagnetic signal. The response time may be less than, preferably more than 2 times, more preferably more than 5 times, and most preferably more than 10 times, the time scale associated with distortions that lead to phase and / or amplitude errors in the tributary electromagnetic signals. For digital signal processors based on field programmable gate arrays (FPGAs), the response time is typically in the range of 100 ns to 100 μs.Faster response times can be achieved by using application specific integrated circuits (ASICs).As will be described in more detail below, at least one actuating element is used to modify the amplitude and / or phase of at least one of the at least two tributary electromagnetic signals.

[0024] As mentioned above, the apparatus for synthesizing at least one targeted broadband electromagnetic signal according to the present invention comprises at least two electromagnetic signal generators. As used herein, the term "electromagnetic signal generator" refers to an apparatus configured to generate and provide at least one tributary electromagnetic signal. As further used herein, the term "tributary electromagnetic signal" refers to an initial electromagnetic signal, which may be in the optical frequency range, provided as an input signal for further processing by the apparatus. Furthermore, the at least two electromagnetic signal generators are selected and operated such that the resulting two or more tributary electromagnetic signals have at least partially overlapping spectra.

[0025] As commonly used, the term "spectrum" refers to the spectral range covered by the corresponding electromagnetic signals. As used herein, the term "spectral overlap" refers to the observation that the spectral ranges of two or more individual tributary electromagnetic signals are not separated, but have a common overlapping range constituted by the spectral ranges of at least two of the individual tributary electromagnetic signals. In general, the spectral overlap may be very small and only facilitates a detectable signal for the extraction of signal information regarding the relative amplitude and / or relative phase of the two or more individual tributary electromagnetic signals. Alternatively, the spectral overlap may be substantial such that the common overlapping range covers at least a significant portion of the spectrum of at least two of the individual tributary electromagnetic signals that interfere across the overlapping range to generate at least one wideband electromagnetic signal. As a further alternative, the tributary electromagnetic signals may overlap in their entirety. The superposition of at least two tributary electromagnetic signals may be used to generate a completely arbitrary signal, as long as the conditions regarding the amplitude and / or phase of the superposition are satisfied in at least one signal combining element.

[0026] Generally, the at least two electromagnetic signal generators can be implemented in various ways. In a preferred embodiment, particularly relating to signal generation in the optical frequency range, at least one electro-optic modulator can be used, preferably implemented as a separate fiber-based component or as an integrated optical component that is part of a more complex Photonic Integrated Circuit (PIC). In particular, the at least one electro-optic modulator can be implemented using LiNbO 3 , or BaTiO 3The electromagnetic signal generator may rely on a technology that can be selected from at least one of second-order nonlinear organic or inorganic materials such as, for example, InP, GaAs, silicon photonics, organic materials, silicon-organic hybrids (SOH), or plasmonic organic hybrids (POH). In particular, a phase modulator, a Mach-Zehnder modulator, or an IQ modulator can be used, which can be supplied by at least one common optical signal generator, such as a laser source or a frequency comb generator. The electromagnetic signal generator may further include an electrical signal generator that can be configured to generate an electrical radio frequency (RF) drive signal, preferably an optical carrier signal, that can be modulated onto an optical waveform by using one of the aforementioned devices. The electrical signal generator may be implemented by using an RF signal generator or an electronic digital-to-analog converter (DAC). However, further types of electromagnetic signal generators may also be feasible.

[0027] To provide an optical signal that can be used to drive an optical device provided in the electromagnetic signal generator, the optical signal generator may include a light source selected from a separate continuous wave (cw) laser source, an optical frequency comb generator, preferably in combination with at least one optical filter for selecting a specific optical tone. The optical frequency comb generator may preferably be based on a mode-locked semiconductor laser, a quantum dash laser, a quantum dot laser, a microresonator with second-order and / or third-order optical nonlinearity, an electro-optical modulator, a highly nonlinear optical fiber, an integrated waveguide with second-order and / or third-order optical nonlinearity, or a combination thereof. Alternatively, the desired electromagnetic tributary electromagnetic signal may be generated by directly modulating the light source. Further types of light sources and optical or electromagnetic signal generators may also be envisaged.

[0028] In certain embodiments, the specific characteristics of the electromagnetic signal generator and associated signal chain can be taken into account and compensated for, in particular by pre-distorting the electrical RF drive signal by digital or analog signal processing techniques. In particular, the pre-distortion can be configured to take into account the characteristics of the electromagnetic signal generator, such as the radiation power or pulse shape, or the frequency-dependent transfer function of the RF signal generator or other RF components, the modulator, the transmission fiber, and / or the at least one signal coupling element, thereby ensuring that the at least one coupled electromagnetic signal corresponds to the at least one targeted wideband electromagnetic signal. The pre-distortion can further compensate for nonlinear impairments of the signal processing chain, such as the specific transfer function of the electro-optic modulator.

[0029] Furthermore, the apparatus for combining at least one targeted wideband electromagnetic signal according to the present invention comprises at least one signal combining element. As used herein, the term "signal combining element" refers to a device configured to merge at least two tributary electromagnetic signals into at least one combined electromagnetic signal. As further used herein, the term "merging" or any grammatical variant refers to assembling at least two tributary electromagnetic signals, in particular by superposition, as facilitated by using at least one signal combining element, thereby obtaining at least one combined electromagnetic signal. To this end, the signal combining element may have a selected transfer characteristic configured to support achieving and maintaining desired characteristics of the amplitude and / or phase of the tributary electromagnetic signals at at least one output port of the at least one signal combining element.

[0030] The at least one signal coupling element can be based on a wide range of technical concepts. The coupling between the at least two input ports and the at least one output port of the waveguide coupler can be embodied by any structure configured such that the input signals are split and / or combined in a predictable manner. In a preferred embodiment, the at least one signal coupling element does not exhibit a strongly frequency-selective transfer characteristic in the bandwidth of the at least two tributary electromagnetic signals. Here, the term "not strongly frequency-selective" refers to signal coupling elements such as directional couplers, beam splitters, or multimode interference couplers (MMIs) that are not specifically designed to exhibit a frequency-dependent transfer characteristic, but may still be subject to unavoidable frequency-dependent variations in the coupling ratio. In such an embodiment, the transfer function between any pair of input and output ports may vary preferably by less than 6 dB, more preferably by less than 3 dB, and most preferably by less than 1 dB over the frequency range covered by the tributary electromagnetic signals at the respective input ports. In alternative embodiments, signal coupling elements with strongly frequency-selective transfer characteristics may be used, where each input port may have a purposefully designed band-limited frequency response to at least one of the output ports with a bandwidth comparable to or slightly larger than the tributary electromagnetic signal fed to the corresponding input port. Examples of such embodiments include optical filters based on, for example, gratings, thin films, waveguide-based devices such as arrayed waveguide gratings, waveguide-based resonators or interferometers, or other integrated optical circuits. In such embodiments, the transfer function between at least one input port and the output port preferably varies by more than 10 dB, more preferably more than 15 dB, and most preferably more than 20 dB over the frequency range covered by the targeted electromagnetic signal.

[0031] In a preferred embodiment, the at least one signal coupling element may have at least one first output port and at least one second output port, where the at least one targeted broadband electromagnetic signal may preferably be provided to the at least one first output port, while the at least one signal detection element may preferably be connected to the at least one second output port.

[0032] In a further preferred embodiment, at least one signal coupling element may comprise a photonic integrated circuit (PIC), which may be based on glass-based waveguide structures, silicon photonic waveguide structures, silicon nitride-based waveguide structures, waveguide structures based on III-V compound semiconductors such as InP, GaAs, InGaAsP, or other binary, ternary, or quaternary compounds, or waveguide structures based on any other material with sufficiently low losses.

[0033] In a further preferred embodiment, the at least one signal combining element may comprise a multimode interference coupler (MMI). As commonly used, the term "multimode interference coupler" or "MMI" refers to a wide multimode waveguide connected to several narrower, typically single-mode, input and output waveguides. As a result, MMIs function as power splitter elements and operate on the principle of self-imaging. Hence, MMIs are typically used to split or combine optical signals with precisely defined phase and amplitude relationships. Alternatively or additionally, the at least one signal combining element may comprise a network of directional couplers or a network of MMI couplers.

[0034] In a further preferred embodiment, the at least one signal-combining element comprises a directional coupler or a network thereof. An example of such an embodiment is a so-called "90° optical hybrid", which can be used as or part of the at least one signal-combining element. The at least one signal-combining element may alternatively or additionally comprise a waveguide-based Y-junction.

[0035] Further alternatively or additionally, the at least one signal coupling element may include a non-waveguide based broadband signal coupling element, such as an optical free space beam splitter or grating. The beam splitter may be selected from at least one of a pellicle beam splitter, a beam splitter cube, or a partially transmitting mirror or a thin film filter, among others.

[0036] Further alternatively or additionally, the at least one signal combining element may comprise an arrayed waveguide grating, a network of delay interferometers, one or more ring resonators, a coupled resonator optical waveguide (CROW) filter, an optical lattice filter, a discrete thin film filter, a prism, or a grating, where each input port may be characterized by a strongly frequency-selective, e.g., band-limited, transfer function to at least one common output port having a bandwidth comparable to or larger than the bandwidth of the tributary electromagnetic signal supplied to the respective input port, and where the transfer function from all input ports to the at least one output port may ideally lead to a substantially flat transmission.

[0037] Particularly preferably, the excess loss of at least one signal coupling element is kept low. In this context, the term "excess loss" quantifies the percentage of power lost in the device, i.e. the difference between the power fed into a particular input port of the device and the sum of the powers measured at all output ports. Typically, the excess loss is expressed as the ratio of the power loss to the input power and quantified in decibels (dB). In the case of signal coupling elements that do not exhibit a strongly frequency-selective transfer characteristic, such as MMIs or directional couplers, the excess loss is preferably less than 3 dB, more preferably less than 2 dB, most preferably less than 1 dB or less than 0.5 dB. In the case of implementing signal coupling elements that exhibit a strongly frequency-selective transfer characteristic, such as AWGs or other filters, the excess loss is preferably less than 6 dB, more preferably less than 3 dB, most preferably less than 2 dB, in particular less than 1 dB. In a preferred embodiment, the power transfer functions between the input ports and any of the output ports may be widely different, where for a given frequency, the output port with the highest power transfer coefficient is used to generate at least one targeted broadband electromagnetic waveform, while the output port with the lower power transfer function is used to extract information regarding the relative amplitude and / or relative phase of the at least two combined tributary electromagnetic signals. However, additional types of signal combining elements may be feasible.

[0038] In a further embodiment, the tributary electromagnetic signal may be preconditioned to provide enhanced monitoring information for at least one output signal of the at least one signal-combining element, which may be specifically dedicated for monitoring the superposition of the tributary electromagnetic signals at the signal-combining element. Preconditioning the tributary electromagnetic signal may include adding auxiliary signal components, such as time-harmonic pilot tones, that provide the desired enhanced monitoring information to the at least one output signal without distorting the targeted electromagnetic waveform to any relevant extent. Adding auxiliary signals, such as pilot tones, may be useful when any one targeted wideband electromagnetic signal has no signal components or only very weak signal components in the spectral overlap region between the at least two tributary electromagnetic signals. In this embodiment, unless the spectral overlap region is filled with auxiliary signals, interference signals between the spectral overlap signal components are not present or are too weak to be detected. In a preferred embodiment, these auxiliary signal components do not impair the generated targeted wideband electromagnetic signal by destructive interference at the corresponding output port of the at least one signal-combining element.

[0039] Furthermore, the apparatus for synthesizing at least one targeted wideband electromagnetic signal according to the present invention comprises at least one signal detection element. As used herein, the term "signal detection element" refers to a device configured to generate at least one detection output signal by detecting at least a portion of at least one of the combined electromagnetic signals. To this end, at least a portion of the at least one combined electromagnetic signal may be provided by or taken from any output port of the at least one signal combination element. As further used herein, the term "detection output signal" refers to an electromagnetic signal based on the at least one combined electromagnetic signal as an input signal, where at least one characteristic of the detection output signal depends on at least one characteristic of the superposition of the tributary electromagnetic signals in the signal combination element. Specifically, by analyzing the at least one detection output signal and / or extracting certain characteristics thereof, the apparatus can be configured to extract information regarding the relative amplitude and / or phase of the at least two tributary electromagnetic signals superimposed in the signal combination element, and later to use this information to automatically and autonomously counteract undesirable deviations. As alluded to above, the at least one detected output signal may be generated by using at least one signal detection element, in particular to detect at least a portion of at least one of the coupled electromagnetic signals as provided by at least one output port of the at least one signal coupling element.

[0040] In a particular embodiment, the multiple output ports of the at least one signal coupling element may have different input-to-output transfer functions, which allows the tributary electromagnetic signals to be modified such that at least one targeted arbitrary electromagnetic waveform is provided to at least one use signal output port, while an additional signal or a component thereof is provided to another output port of the at least one monitor signal. Based on knowledge of the various input-to-output transfer functions, the monitor signal or signal components may be exploited to extract desired information regarding the relative amplitude and / or relative phase of at least two tributary electromagnetic signals superimposed to form at least one targeted electromagnetic waveform at the use signal output. This concept may be particularly useful for establishing robust feedback circuits that are sensitively dependent on the relative amplitude and / or relative phase of the tributary electromagnetic signals and that can rely on strong indicator properties contained in the at least one monitor signal, while the targeted waveform of the at least one use signal output port is rather insensitive to small deviations in amplitude and / or phase from the targeted relationship. In a preferred embodiment, a tributary electromagnetic signal or a component thereof may be subject to constructive or destructive interference of a particular signal component at at least one of the at least one used signal output ports, while the same or other signal components are subject to partial interference at at least one of the at least one monitor signal components. This feature may be used in combination with the aforementioned preconditioning of the tributary electromagnetic signal to provide enhanced monitor information at at least one output signal. Specifically, the preconditioning of the tributary electromagnetic signal may include adding an auxiliary signal component, such as a time harmonic pilot tone, which is eliminated by destructive interference in the targeted wideband electromagnetic signal provided at a first output port of the signal combining element, while the desired enhanced monitor information is obtained by constructive, destructive, or partial interference of the auxiliary signal at at least one second output port of the signal combining element. As used herein, the terms "constructive interference" or "destructive interference" refer to the superposition of two signals or signal components having a phase difference close to 0 or π, whereas the phase difference of partial interference is not close to 0 or π.The term "close to" is used to describe a deviation preferably less than π / 3, more preferably less than π / 4 or π / 5, most preferably less than π / 10, especially less than π / 20. The present embodiment may preferably be implemented using directional or MMI couplers with known or pre-characterized phase relationships between the various input and output ports.

[0041] In certain embodiments, the at least one signal combining element may be an MMI or an optical 90° hybrid. In this embodiment, the tributary electromagnetic signals may be generated such that their combination at one of the output ports of the at least one signal combining element may be equal to the targeted broadband electromagnetic signal, but the at least one combined signal at another output port of the at least one signal combining element may then be approximated by a relationship proportional to the amplitude and / or phase offset between the at least two tributary electromagnetic signals.

[0042] In a further embodiment, the at least two tributary electromagnetic signals may be designed by starting with the spectrum of the at least one targeted wideband electromagnetic signal and filtering it into different band-limited spectral regions, where each tributary electromagnetic signal may have only a portion of the total targeted signal frequency content, and the at least two tributary electromagnetic signals may reconstruct the at least one targeted wideband electromagnetic signal when added together. To detect the amplitude and / or phase offset between the at least two tributary electromagnetic signals, an auxiliary signal component, such as a pilot frequency tone, may be added to one or more of the at least two tributary electromagnetic signals within the spectral overlap range of the at least two tributary electromagnetic signals. In a preferred embodiment, the tones may be selected to produce a combined signal that cancels out due to destructive interference in the targeted electromagnetic signal, but is indicative of the amplitude and / or phase relationship between the at least two tributary electromagnetic signals at another output port of the signal combining element.

[0043] In a preferred embodiment, at least one of a single-ended photodetector (PD) or a balanced photodetector (BPD) may be used to detect at least a portion of at least one combined signal at at least one output port of at least one signal combining element, specifically by using the secondary detection properties of the PD and / or BPD. In a possible embodiment, for the purpose of optical detection, a single-ended photodiode may be used to function as a PD. As typically used, the term "single-ended photodiode" refers to a semiconductor device having a p-n junction that can generate an electrical RF signal proportional to the power of an incident optical signal. In optical communications, a single-ended photodiode has a slightly tuned pin structure, where an intrinsic semiconductor material is inserted between two doped semiconductor layers. In contrast to a single-ended diode, a BPD comprises two photodiodes connected in series, where the photodiodes are oriented such that, under illumination by an optical signal, the difference between the photocurrents generated by each diode is used to generate a difference current. A BPD is "properly balanced" when the two photocurrents exactly cancel each other out, resulting in zero difference current under the same incident optical signal incident on both photodiodes. For any photodiode, the photocurrent generated is proportional to the instantaneous power and therefore to the square of the amplitude of the electric or magnetic field of the optical signal. Due to this quadratic detection property, using a single-ended diode to directly detect the photo-mixing product of at least two tributary electromagnetic signals results in an RF signal that includes signal-signal mixing terms, which makes it more difficult to extract the desired signal information of the relative amplitude and / or phase of the tributary electromagnetic signals. With a BPD, the information of the relative amplitude offset and / or phase offset between the tributary electromagnetic signals can be more easily extracted, because the signal-signal beating terms are eliminated or strongly suppressed in the RF detection signal.In addition, by using a BPD in combination with properly designed tributary electromagnetic signals, it is possible to greatly simplify the information extraction process by directly providing an output signal that may be roughly proportional to the phase error of the tributary electromagnetic signals that are superimposed in a signal combining element to form one targeted wideband electromagnetic signal.

[0044] In further embodiments, at least one conversion element such as an ultra-wideband photodetector may be used to convert at least a portion of the at least one combined electromagnetic waveform at at least one output port of the at least one signal combining element, in particular to convert at least a portion of the generated optical waveform into an ultra-wideband converted electromagnetic waveform having a different center frequency, which may be, for example, in the mm-wave or THz frequency range. As used herein, the term "ultra-wideband photodetector" refers to a photodetector configured to be able to generate a wideband electrical signal from an associated optical signal. The electro-optical bandwidth of the ultra-wideband photodetector is preferably greater than 20 GHz, 50 GHz, 100 GHz, 200 GHz, or 300 GHz. The ultra-wideband photodetector may rely on high-speed photodiodes, in particular conventional pin photodiodes, triple transit region photodiodes (TTR-PDs), and / or uni-traveling carrier (UTC) photodiodes, or other optical signal detection concepts such as plasmonic internal emission photodetectors (PIPEDs). Depending on the device concept and the electrical circuitry and connectors of the photodetector, the spectrum of the generated electromagnetic waveform may not extend down to DC and may cover a frequency band with a center frequency other than zero. This occurs, for example, in a UTC photodiode coupled to a rectangular THz or sub-THz waveguide. Further types of signal detection elements may also be feasible.

[0045] Thus, complementing the device by using at least one conversion element can open an attractive route towards targeted synthesis of electromagnetic waveforms in the mm-wave, sub-THz or THz frequency range. In a first step, the device can be used to synthesize at least one targeted electromagnetic waveform implemented as an optical waveform from at least two tributary electromagnetic signals generated by at least two different electromagnetic signal generators. The broadband optical waveform or a part thereof can then be converted by an optical detector into an associated converted electromagnetic waveform, where the conversion can be, for example, a frequency shift, in case of homodyne, intradyne or heterodyne detection of the broadband optical waveform in combination with an additional optical local oscillator (LO) tone, or can include more complex operations such as squaring of the optical field, in case of direct detection. In case of direct detection, the broadband optical waveform can be designed to generate the desired electromagnetic waveform upon detection.

[0046] Furthermore, the apparatus for synthesizing at least one targeted wideband electromagnetic signal according to the present invention comprises at least one control element. As used herein, the term "control element" refers to a device configured to extract signal information regarding the relative amplitude and / or relative phase of at least one of the at least two tributary electromagnetic signals in the at least one combined electromagnetic signal from at least one detection output signal, and generate at least one actuator drive signal to act on the at least one tributary electromagnetic signal via at least one actuation element, thereby autonomously countering deviations from a desired operating parameter. As generally used, the term "extract" or any grammatical variant thereof refers to generating at least one of the signal information from at least one input signal. As further used herein, the term "deviations from a desired operating parameter" or any grammatical variant thereof refers in particular to amplitude and / or phase errors of the tributary electromagnetic signals that cause impairment of the generation of the at least one targeted electromagnetic signal by the at least one signal combining element. Also, as further used herein, the term "act on" or any grammatical variations thereof, specifically refers to modifying at least one of the amplitude and / or phase of at least one tributary electromagnetic signal.

[0047] In comparison with approaches known from the prior art aimed at generating a wideband electromagnetic signal, the present device comprises a feedback control system configured to monitor and control in real time the amplitude offset and / or phase offset between at least two tributary electromagnetic signals, thereby ensuring a correct and reliable generation of a targeted wideband electromagnetic signal. Here, signal information regarding the relative amplitude and / or relative phase of the at least two tributary electromagnetic signals can be extracted from the at least one detection output signal, in particular by using interference characteristics of the preconditioned tributary electromagnetic signals, which may, but need not necessarily, be enhanced by additional auxiliary signal components such as pilot tones. The extracted signal information regarding the relative amplitude and / or relative phase of the at least two electromagnetic tributary electromagnetic signals can be used to drive at least one actuating element, as will be explained in more detail below, in particular by using at least one actuator drive signal, in order to enable the at least one actuating element to modify the amplitude and / or phase between the at least two electromagnetic tributary electromagnetic signals in a feedback loop. Here, the at least one control element may assume the role of extracting signal information, in particular with respect to relative amplitude and / or relative phase, determining the current state of the device, and, if necessary, carrying out at least one corrective action via at least one actuation element to close the feedback loop, thereby actively stabilizing the amplitude and / or phase of the coupling of the at least two tributary electromagnetic signals to the at least one targeted wideband arbitrary signal. In a preferred embodiment, the at least one control element may be implemented by using a digital control circuit, specifically selected from at least one of a microcontroller, a field programmable gate array (FPGA), and an electrical application specific integrated circuit (ASIC). Alternatively, a purely analog or a hybrid digital-analog feedback control circuit may also be feasible, depending on the characteristics of the at least one detection output signal. However, further types of control elements may also be feasible.

[0048] Furthermore, the apparatus for synthesizing at least one targeted wideband electromagnetic signal according to the present invention comprises at least one actuation element. As used herein, the term "actuation element" refers to a device configured to modify at least one of the amplitude or phase of at least one of the at least two tributary electromagnetic signals based on signal information generated by a control element. As used herein, the term "modify" or any grammatical variation thereof refers to generating a corrected tributary electromagnetic signal, particularly by using at least one actuator drive signal.

[0049] In particular, the at least one actuating element may act on one or more of the at least two tributary electromagnetic signals to generate at least one corresponding corrected tributary electromagnetic signal that may be used as an input signal to the at least one signal combining element to generate a targeted wideband electromagnetic signal by merging the at least two tributary electromagnetic signals in the signal combining element. In this way, a predefined relative amplitude and / or relative phase of the at least one tributary electromagnetic signal in the at least one combined electromagnetic signal may be achieved and maintained as desired.

[0050] In a further preferred embodiment, the at least one actuation element may be partially or fully integrated into at least one of the at least two electromagnetic signal generators. This may be the case for electromagnetic signal generators based on digital signal processing, where real-time phase and / or amplitude adjustments can be implemented in the signal generation algorithm. Alternatively or additionally, an analog signal generation element may be directly controlled to provide the desired actuation function.

[0051] In a further preferred embodiment, the at least one actuating element may comprise at least one further element selected from at least one of a thermo-optical phase shifter, an electro-optical phase shifter, an acousto-optical phase shifter or an elasto-optical phase shifter, which may in particular be driven by a piezoelectric actuator. As commonly used, the term "thermo-optical phase shifter" refers to a phase shifter used mainly in photonic integrated circuits (PICs) that is based on the thermo-optical effect, where the optical properties of an optical waveguide structure are modified by temperature changes. In particular, a temperature-dependent phase shift can be induced in the waveguide by heating, in particular by using heat dissipated through a conductive metal structure in the vicinity of the waveguide. As more commonly used, the term "acousto-optical phase shifter" is used to refer to a device that introduces an optical phase or frequency shift, usually into a free-space optical beam, based on the acousto-optical effect, where a traveling optical signal interacts with an acoustic wave. As more commonly used, the term "elasto-optical phase shifter" refers to a device that can induce a phase shift in an optical signal propagating in a waveguide by exploiting the elasto-optical effect, which changes the optical properties of the waveguide by applying a strain, resulting in a change in the optical phase accumulated by the traveling optical signal. This strain can be induced, for example, by a piezoelectric phase shifter that is integrated on top of the waveguide structure. As more commonly used, the term "electro-optical phase shifter" refers to a phase shifter in which the optical properties of the underlying material are changed by an applied voltage and / or an injected current. This change in optical properties can be the result of optical nonlinearities, such as, for example, the linear electro-optical effect (Pockels effect) or the electro-optical Kerr effect, or can result from a change in the refractive index due to the injection of free carriers. Specifically, the Pockels effect is a well-known phenomenon in LiNbO 3 , BaTiO 3Phase shifters occur in non-centrosymmetric media such as quartz crystals, or in electrically poled organic materials. Such phase shifters may be part of a more complex configuration for at least one actuating element, including an electro-optic modulator capable of modifying both the amplitude and phase of an associated tributary electromagnetic signal, such as a Mach-Zehnder Modulator (MZM) or an IQ Modulator (IQM). As a further alternative, the actuating element may be selected from infinite phase shifters. As commonly used, the term "infinite phase shifter" refers to a device that can provide a continuously increasing phase shift even for a range-limited electrical drive signal. Specifically, infinite phase shifters do not introduce discontinuities that are typically caused by the 2π phase shift ("unwrapping") required to stay within the operating range of the device. Infinite phase shifters can be implemented, in particular, by using a network of IQ or Mach-Zehnder modulators and phase shifters. However, further types of actuating elements may also be feasible.

[0052] In a further preferred embodiment, the device for synthesizing at least one targeted broadband electromagnetic signal according to the present invention may further comprise at least one conversion element. As used herein, the term "conversion element" refers to a device configured to convert at least a portion of at least one targeted broadband electromagnetic signal covering a certain frequency range into another signal, typically in a different frequency range. As used herein, the term "conversion" or any grammatical variant thereof refers to modifying at least a portion of the at least one targeted broadband electromagnetic signal to generate a converted electromagnetic signal being output by the device. As explained above, the at least one conversion element may be based on a high-speed photodiode, in particular a conventional pin photodiode, a triple transit region photodiode (TTR-PD), and / or a uni-traveling carrier (UTC) photodiode, or other optical signal detection device such as a plasmonic internal emission photodetector (PIPED).

[0053] According to the invention, a device for synthesizing at least one targeted broadband electromagnetic signal may be arranged in a preferred embodiment to comprise at least the elements shown above individually. In an alternative embodiment, the device may comprise a number of elements, which may preferably be cascaded one after the other. In such a cascade, the functions of combining, amplitude actuation, phase actuation and signal detection may be intertwined. In particular, by using integration technologies such as photonic integrated circuits (PICs), which allow the integration of multiple component functions on a common semiconductor chip, it may be difficult to clearly separate the individual elements of the device. Integrated photonic solutions are obtained on monolithic photonic chips or photonic multichip modules (MCMs). In multichip modules (MCMs), multiple chips, which may be based on different integration technologies, are bonded and optically connected to each other by precise alignment and / or by using 3D printed optical coupling structures such as microlenses or photonic wire bonds. Optimized signal combining elements may be provided that can incorporate at least one of the amplitude actuation elements, phase actuation elements or signal detection elements while maintaining functionality.

[0054] Also, at least one of the at least one actuating element may be at least partially constituted by at least one of the at least two electromagnetic signal generators. Alternatively or additionally, at least one of the at least one signal detection element may be integrated into at least one of the at least one signal coupling element or may be coupled with at least one signal coupling element in at least one photonic integrated circuit. Still alternatively or additionally, at least one of the at least one actuating element may be integrated into at least one of the at least one signal coupling element or may be coupled with at least one signal coupling element in at least one photonic integrated circuit. Still alternatively or additionally, at least one signal coupling element, at least one signal detection element and at least one actuating element may be implemented together in at least one photonic integrated circuit. It is emphasized that this list is not exhaustive and further types of integration of at least two elements may be envisaged.

[0055] In a further aspect, the present invention relates to a system for synthesizing at least one targeted wideband electromagnetic signal, the system comprising a plurality of devices as described herein, the plurality of devices being arranged in a cascaded manner. As commonly used, the term "cascaded" refers to an arrangement in which at least one output signal of at least one element assigned to a particular stage of the arrangement is used as at least one input signal of at least one further element assigned to a successive stage following the particular stage. Thus, a plurality of devices according to the present invention may generally be arranged one after the other in a cascaded manner.

[0056] In a particularly preferred embodiment, at least one first stage device may comprise all the elements shown above, including at least two electromagnetic signal generators, where at least one targeted broadband electromagnetic signal output by at least one first stage device may be used as at least one of the at least two electromagnetic signal generators in at least one second stage device. Similarly, at least one targeted broadband electromagnetic signal output by at least one second stage device may be used as at least one of the at least two electromagnetic signal generators in at least one third stage device, and so on. Generally, cascaded arrangements with further successive stages may also be envisaged.

[0057] In a further preferred embodiment, the system for synthesizing at least one targeted wideband electromagnetic signal according to the present invention may further comprise at least one transducer element, where the at least one transducer element is configured to convert at least a part of the at least one targeted wideband electromagnetic signal covering a certain frequency range, as defined above, into another signal, typically in a different frequency range. For further information regarding the at least one transducer element, reference may be made to the above and / or below description.

[0058] In a further aspect, the present invention relates to a method for synthesizing at least one targeted wideband electromagnetic signal, in particular by using a device or a system as disclosed herein, said method comprising the following steps a) to d): a) providing at least two tributary electromagnetic signals having at least partially overlapping spectra; b) merging the at least two tributary electromagnetic signals into at least one combined electromagnetic signal; c) detecting at least a portion of at least one of the at least one combined electromagnetic signal and extracting signal information related to at least one of the relative amplitude or relative phase of combining the at least two tributary electromagnetic signals into the at least one combined electromagnetic signal; d) modifying at least one of the amplitude or phase of at least one of the at least two tributary electromagnetic signals based on the signal information; The step of modifying at least one of the amplitude or phase of at least one of the at least two tributary electromagnetic signals is performed such that at least one of the at least one combined electromagnetic signal corresponds to the at least one targeted wideband electromagnetic signal.

[0059] In particular, according to the present invention, the step of merging the at least two tributary electromagnetic signals is performed by modifying at least one of the amplitude or phase of at least one of the at least two tributary electromagnetic signals such that at least one of the one or more combined electromagnetic signals corresponds to the at least one targeted wideband electromagnetic signal, whereby the at least one targeted wideband electromagnetic signal is combined.

[0060] Here, the steps shown may preferably be performed in a given order, starting from step a) and ending with step d). In a particularly preferred embodiment, the steps shown may be performed simultaneously and successively, in particular by successively repeating steps b), c) and d). As used herein, the term "closed-loop control" refers to an operation mode in which the control action of a control element depends on signal information extracted by at least one signal detection element. In particular, closed-loop control may be used within the method of the present invention by using signal information regarding at least one relative amplitude and / or at least one relative phase of the at least two tributary electromagnetic signals determined during step c) for modifying at least one of the amplitude or phase of at least one of the at least two tributary electromagnetic signals in step d) before or during the merging of the at least two tributary electromagnetic signals into at least one combined electromagnetic signal according to step b).

[0061] In a further aspect, the present invention relates to a computer program for synthesizing at least one targeted wideband electromagnetic signal. Wherein the computer program comprises instructions that, when the program is executed by a computer, cause the computer to carry out the method for synthesizing at least one targeted wideband electromagnetic signal as disclosed herein. In particular, the computer program may be stored on a computer-readable, non-transitory data carrier. Thus, in particular, any one or all of the steps of the above method may be carried out by using a computer or a computer network, preferably by using a computer program.

[0062] For further details regarding the system, method and computer program for synthesizing at least one targeted wideband electromagnetic signal, reference may be made to the description of the apparatus for synthesizing at least one targeted wideband electromagnetic signal above and / or below.

[0063] With respect to the known prior art, the device, system, method and computer program for synthesizing at least one targeted wideband electromagnetic signal exhibit the following advantages: In particular, the active amplitude and / or phase stabilization of the involved tributary electromagnetic signals is advantageous: The generation of extractable signal information regarding amplitude and / or phase characteristics without interfering with the generation of any signal allows the application of feedback control systems configured to stabilize the signal generation under the influence of non-ideal system components and environmental changes, thereby opening up new possibilities for reliable wideband signal synthesis systems that are currently considered unattainable.

[0064] In particular, to Geisler et al., referenced above, the tributary electromagnetic signals need to be combined with the correct phase offset between each other to generate the correct waveform at at least one output port of the OAWG. If left uncontrolled, the combination of the tributary electromagnetic signals can result in the random generation of any output signal that does not resemble the desired target signal. As seen in the Geisler et al. paper, this results in only 1% of the measured output signals coincidentally matching the correct waveform. Without the feedback control system disclosed herein, it is virtually impossible to generate wide optical signals using OAWGs. Geisler et al., referenced above, ensure that by implementing the OAWG scheme at an integrated photonic chip level, the effects of phase offsets can be minimized, resulting in better performance. Since the effects of phase instability are less in chip-scale OAWGs, the randomly varying amplitude and phase between the tributary optical signals are a smaller issue, but still remain an open problem. In contrast, the present invention completely solves this problem for both chip-scale systems and implementations based on discrete free-space or fiber-based optical components by actively modifying the amplitude and / or phase.

[0065] Combining multiple tributary electromagnetic signals to generate a specific waveform without knowledge of their amplitude and / or phase relationships would remain entirely contingent without the solution provided by the present invention. Other current state-of-the-art OAWG technologies have no monitoring or control mechanisms, automated or not, that ensure that the output waveform is generated according to the user's specifications. This makes OAWG implementation difficult, especially when two or more tributary electromagnetic signals are merged to form a desired wideband arbitrary signal. The solution for reliable generation of high quality wideband signals over long periods of time is to propose the apparatus disclosed herein, which can monitor the relationships between the tributary electromagnetic signals and adjust accordingly.

[0066] As a result, the apparatus for synthesizing targeted broadband electromagnetic signals disclosed herein goes beyond all comparable apparatuses developed to date by its ability to generate and use feedback signals that provide real-time signal information regarding the relationship between the at least two tributary electromagnetic signals, thereby allowing offsets between the at least two tributary electromagnetic signals in both amplitude and phase to be corrected, thus ensuring that the desired correct output signal is generated. The adjustment between the at least two tributary electromagnetic signals is performed via a feedback control loop, thereby allowing correction of amplitude and / or phase offsets autonomously during normal operation of the apparatus over extended periods of time.

[0067] As used herein, the terms "have", "comprise" or "include", or any grammatical variants thereof, are used in a non-exclusive manner. Thus, these terms may refer both to the situation in which no further features are present in the entity described in this context, other than the features introduced by these terms, and to the situation in which one or more further features are present. As an example, the expressions "A has B", "A comprises B", and "A includes B" may refer both to the situation in which no other elements are present in A, other than B (i.e., A consists solely and exclusively of B), and to the situation in which one or more further elements are present in the entity A, other than B, such as element C, elements C and D, or further elements.

[0068] As further used herein, the terms "specifically", "preferably", "more preferably", "particularly", "more particularly", or similar terms are used in conjunction with optional features without limiting alternative possibilities. Thus, features introduced by these terms are optional features and are not intended to limit the scope of the claims in any way. The invention may be implemented by using alternative features, as recognized by those skilled in the art. Similarly, features introduced by "in an embodiment of the invention" or similar expressions are intended to be optional features, without any limitations on alternative embodiments of the invention, without any limitations on the scope of the invention, and without any limitations on the possibility of combining the features thus introduced with other features of the invention. [Brief description of the drawings]

[0069] Further optional features and embodiments of the present invention are disclosed in more detail in the following description of the preferred embodiments, preferably in conjunction with the dependent claims, where each optional feature may be implemented in an independent manner as well as in any feasible combination, as will be understood by those skilled in the art. It is emphasized here that the scope of the present invention is not limited by the preferred embodiments.

[0070] [Figure 1a] 1 illustrates generally an exemplary embodiment of an apparatus for synthesizing at least one targeted wideband electromagnetic signal according to the present invention; [Figure 1b] 1 illustrates generally an exemplary embodiment of an apparatus for synthesizing at least one targeted wideband electromagnetic signal according to the present invention; [Figure 1c] 1 illustrates generally an exemplary embodiment of an apparatus for synthesizing at least one targeted wideband electromagnetic signal according to the present invention; [Diagram 2]1 illustrates generally an exemplary embodiment having a common electromagnetic signal source. [Diagram 3] 1 illustrates generally an exemplary embodiment of a system for synthesizing at least one targeted wideband electromagnetic signal according to the present invention, where the system comprises a plurality of devices arranged in a cascaded manner. [Figure 4] 1 illustrates generally an exemplary embodiment of a device element having multiple elements arranged in a cascaded manner. [Diagram 5] 1 shows a diagram illustrating an example embodiment of two tributary electromagnetic signals having at least partially overlapping spectra. [Figure 6] 1 shows a diagram illustrating an example embodiment of a transfer function from three input ports to one output port of a frequency selective signal combiner element. [Figure 7] 1 illustrates generally an exemplary embodiment of a method for synthesizing at least one targeted wideband electromagnetic signal according to the present invention; [Figure 8] 2 illustrates diagrammatically a further exemplary embodiment of an apparatus for synthesizing at least one targeted wideband electromagnetic signal according to the present invention; [Figure 9] 1 illustrates a schematic diagram of an exemplary spectral structure of signals a s1 (t) and a s2 (t), each having contributing signals called us(t) and r(t). [Figure 10] FIG. 13 illustrates a diagram illustrating the time course of measured signals during a proof-of-concept operation. [Figure 11a] FIG. 13 illustrates a diagram illustrating measurements without phase stabilization. [Figure 11b] FIG. 13 illustrates a diagram illustrating measurements with phase stabilization. [Figure 12] 1 illustrates generally an exemplary embodiment of an apparatus having multiple signal combining elements arranged in a cascaded manner. [Figure 13] 1A and 1B are schematic diagrams illustrating example embodiments of four tributary electromagnetic signals having at least partially overlapping spectra; [Figure 14]10 illustrates, in a simplified manner, a further exemplary embodiment of an apparatus for combining at least one targeted wideband electromagnetic signal with tributary electromagnetic signals that individually cover a full range of targeted wideband electromagnetic waveforms. [Figure 15] FIG. 1 illustrates a diagram illustrating an exemplary embodiment of four tributary electromagnetic signals having spectra covering the entire bandwidth of a targeted wideband electromagnetic waveform. [Figure 16] 1A and 1B are schematic diagrams illustrating exemplary embodiments of optical frequency tones used as phase-controlled local oscillators in conjunction with wideband optical signals; [Figure 17] 10 illustrates generally a further exemplary embodiment of an apparatus for synthesizing a wideband electrical signal using a wideband optical signal with a wideband balanced photodetector functioning as a phase-controlled local oscillator and a conversion element; DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0071] Detailed Description of the Embodiments 1 illustrates, in a schematic manner, an exemplary embodiment of an apparatus 1 for synthesizing at least one targeted broadband electromagnetic signal 60 according to the present invention, where the apparatus 1 can be used as an optical arbitrary waveform generator (OAWG).

[0072] The exemplary apparatus 1 shown in Figure 1a comprises two electromagnetic signal generators 10, 11 configured to generate two tributary electromagnetic signals 20, 21 having at least partially overlapping frequency spectra as shown diagrammatically in Figure 5. However, three, four or more tributary electromagnetic signals having at least partially overlapping frequency spectra may also be feasible.

[0073] In embodiments in which any of the targeted broadband electromagnetic signals has no or very weak signal components in the spectral overlap region between at least two tributary electromagnetic signals, an auxiliary signal configured for reliable detection of the interfering signal can be added without compromising the generated targeted broadband electromagnetic signal, an example of which is provided in FIG.

[0074] In the exemplary embodiment of Fig. 1a, an actuation element 30 is disposed in at least one of the tributary electromagnetic signal paths before the signal-combining element 50, where the at least one actuation element 30 is configured to modify the tributary electromagnetic signal 20 to compensate for relative amplitude and / or phase errors and variations between the tributary electromagnetic signals that may occur within the apparatus 1, thereby generating at least one compensated tributary electromagnetic signal 40. In alternative embodiments (not shown herein), it may also be feasible to place more than one actuation element 30 before the signal-combining element 50.

[0075] The signal combining element 50 shown in FIG. 1a is configured to merge the corrected tributary electromagnetic signal 40 with the tributary electromagnetic signal 21 using the actuation element 30. Here, it is shown that in this particular embodiment, no further tributary electromagnetic signals are corrected before the signal combining element 50, since all corrections are applied to the remaining tributary electromagnetic signal 20. In an alternative embodiment (not shown here), two corrected tributary electromagnetic signals 40 are merged in the signal combining element 50, where the two corrected tributary electromagnetic signals 40 may be generated by two associated actuation elements 30. The two signals 40, 21 are now combined by superposition with a predefined amplitude and phase relationship, such that two combined electromagnetic signals 60, 61 are generated at the output of the signal combining element 50, where the signal 60 also corresponds to the targeted broadband electromagnetic signal. In general (although not shown here), the number of combined electromagnetic signals generated at the output of the signal combining element may vary depending on the number of tributary electromagnetic signals.

[0076] The targeted broadband electromagnetic signal 60 may be output by the device 1 directly. However, in the exemplary embodiment illustrated in FIG. 1a, the targeted broadband electromagnetic signal 60 may be directed through a conversion element 70 to generate a converted output waveform 80 that may cover a different frequency range than the targeted broadband electromagnetic signal 60 before being output by the device 1. In particular, the targeted broadband electromagnetic signal 60 may be an optical signal, while the converted output waveform 80 may be an electromagnetic signal selected from a microwave signal, a millimeter wave signal, a sub-THz signal, or a THz signal, generated by a sufficiently fast optical detector 70.

[0077] As further shown in Fig. 1a, the combined electromagnetic signal 61 is guided to a signal detection element 90 configured to generate a detection output signal 100 including signal information regarding the relative amplitudes and phases with which the at least two tributary electromagnetic signals 40, 21 are superimposed at the output of the signal combining element 50. Furthermore, the control element 110 is configured to use the signal information regarding the relationship between the amplitudes and phases of the two superimposed tributary electromagnetic signals 40, 21 to drive the at least one electromagnetic driving element 30 by using at least one actuator driving signal 120 configured to modify at least one of the tributary electromagnetic signals 20. To this end, the actuator driving signal 120 may include a feedback amplitude correction signal and / or a feedback phase correction signal, at least one of which may ensure that the relationship between the amplitudes and phases of the two superimposed tributary electromagnetic signals 40, 21 is fulfilled and thus maintained at the respective output of the signal combining element 50 to successfully superimpose and combine the targeted wideband electromagnetic signal 60.

[0078] In an alternative embodiment shown in Fig. 1b, the device 1 is configured to merge the N tributary electromagnetic signals 20-1...20-N generated by the associated signal generators 10-1...10-N into K combined electromagnetic signals 61-1...61-K, including one or more targeted electromagnetic signals 60. A group of M < N tributary signals is routed through one or more actuation elements 30-1...30-L, where L < M, to generate M corresponding corrected electromagnetic signals, which together with the remaining (NK) uncorrected tributary electromagnetic signals form the input signals 41-1...41-N of the signal combining element 50. The signal combining element 50 has N input ports and K output ports and generates K combined electromagnetic signals 61-1...61-K from its N input signals 40-1...40-N. Here, not all tributary electromagnetic signals 20-1...20-N need to be routed through an associated actuation element, i.e. M < N, and each actuation element can process one or more tributary electromagnetic signals, i.e. L < M. The actuation elements 30-1...30-L are controlled by one or more control elements 110, which are fed by one or more signal detection elements 90 configured to generate one or more detection output signals 100 containing information about the relative amplitudes and phases of the tributary electromagnetic signals 20-1...20-N superimposed at the output of the signal combination element 50. At least one signal detection element 90 is fed by at least one group of combined electromagnetic signals 61-1...61-K generated at the output of the signal combination element 50. Similar to FIG. 1a, the actuator drive signal 120 may include amplitude and / or phase correction signals that ensure that the relative amplitudes and phases of the tributary electromagnetic signals 20-1...20-N superimposed at the output of the signal combining element 50 are maintained, thereby enabling targeted combination of the at least one wideband electromagnetic signal 60.

[0079] 1c illustrates diagrammatically a further exemplary embodiment of the device 1 according to the invention, in which actuation elements 30, 31 are integrated into the corresponding electromagnetic signal generators 10, 11. For this purpose, a digital or analog signal generator with an additional input port may be used here to receive actuator drive signals 120, 121, which are converted into real-time phase and / or amplitude adjustments during signal generation. In a further alternative (not shown here), only a subset of the electromagnetic signal generators 10, 11 are equipped with internal actuation elements 30, 31.

[0080] Fig. 2 illustrates a schematic example embodiment of a common electromagnetic signal source. While the example embodiment shown in Figs. 1a and 1b comprises two signal generators 10, 11 based on independent electromagnetic signal sources, a further example embodiment illustrated in Fig. 2 comprises a common electromagnetic signal source 130 configured to provide at least one reference signal 140, 141 to each of the signal generators 10, 11 to generate two tributary electromagnetic signals 20, 21.

[0081] FIG. 3 illustrates in schematic form an exemplary embodiment of a system 2 for synthesizing a targeted wideband electromagnetic signal 60, which can also be used as an optical arbitrary waveform generator (OAWG). The exemplary system 2 illustrated herein comprises a number of devices 150, 151, 152, 153, 160, 161, 170 arranged in a cascaded manner having three consecutive stages. The first stage devices 150, 151, 152, 153 as used herein have their own signal generators (not shown herein) that may use a common electromagnetic signal source 130 as illustrated in schematic form in FIG. 2. The output signals of the first stage devices 150, 151, 152, 153 may replace the electromagnetic signal generators of the second stage devices 160, 161. Similarly, the output ports of the second stage devices 160, 161 may be configured to feed a single third stage device 170. In general, there is no theoretical limit to the number of stages.

[0082] FIG. 4 illustrates, in schematic form, an exemplary embodiment of an integrated circuit, in particular a photonic integrated circuit (PIC) 3, with a number of elements arranged in a cascaded manner. Here, signal-combining elements 50, 51, 52 are integrated together with driving elements 30, 31, 32 and signal-detecting elements 90, 91, 92. The exemplary embodiment of the integrated circuit 3 shown in FIG. 4 illustrates an embodiment of combining four tributary electromagnetic signals 20, 21, 22, 23 with a number of signal-combining elements 50, 51, 52, each configured to combine two of the four tributary electromagnetic signals 20, 21, 22, 23 at a time. In further embodiments (not shown here), functionally equivalent circuit variations can be provided in which the individual elements can be placed in different positions without changing the core functionality of the integrated circuit 3. In the exemplary embodiment of FIG. 4, the actuating element 32 is positioned to operate on the output of the signal-combining element 52, but functionally it can be placed before the signal-combining element 51 to operate on the tributary electromagnetic signal 23. Depending on the position of the actuation elements 30, 31, 32, the corresponding actuator drive signals (not shown here) may vary.

[0083] FIG. 5 shows a diagram illustrating an example embodiment of amplitude spectra A over frequency f of two tributary electromagnetic signals 20, 21 having at least partially overlapping spectra within a common overlap range 180.

[0084] 6 shows a diagram illustrating an exemplary embodiment of the amplitude spectrum A over frequency f of the transfer function from three input ports 190, 191, 192 to one output port of a frequency selective signal combining element. For optical signals, such a transfer function may be provided, for example, by using an arrayed waveguide grating (AWG).

[0085] 7 illustrates generally an exemplary embodiment of a method 200 for synthesizing a single targeted wideband electromagnetic signal according to the present invention, where the method 200 may be implemented by, among other things, using an apparatus 1 or a system 2 as disclosed herein.

[0086] In the providing step 205, two tributary electromagnetic signals having at least partially overlapping spectra within a common overlap range 180 are provided, in particular by using the output signals of two electromagnetic signal generators 10, 11 or devices of a previous stage as explained in more detail above. However, it may also be feasible to provide three, four or more tributary electromagnetic signals, where two frequency adjacent signals have at least partially overlapping spectra, possibly mediated by a suitable auxiliary signal.

[0087] In a merging step 210, the at least two tributary electromagnetic signals are merged into at least one combined electromagnetic signal, particularly by using one or more signal combining elements as described above.

[0088] In a detecting step 215, at least a portion of at least one of the combined electromagnetic signals is detected, particularly by using one or more signal detection elements 90, 91, 92 as described above. Furthermore, signal information regarding the amplitude and phase relationship that causes the two or more tributary electromagnetic signals to be combined in the at least one signal combining element is extracted from the at least one combined electromagnetic signal 61, particularly by using signal detection element 90 as described in more detail above.

[0089] In a modifying step 220, the relationship between the amplitude and phase of the two or more tributary electromagnetic signals is modified, particularly by using one or more actuating elements as described in more detail above.

[0090] According to the present invention, all steps are performed in parallel and sequentially, so that the merging step 210 is performed by using the relationship between the amplitudes and phases of two or more tributary electromagnetic signals after they have been modified by applying the modifying step 220 such that the targeted wideband electromagnetic signal is combined.

[0091] FIG. 8 illustrates a schematic demonstration setup as a further exemplary embodiment of the device 1 for synthesizing wideband electromagnetic signals according to the present invention. Herein, two optical signals are generated as tributary electromagnetic signals 20, 21 by modulating two optical carriers using optical in-phase / quadrature (I / Q) modulators 260, 261, which may be driven from any RF waveform generator with one or more digital-to-analog converters (DACs). The corresponding optical carriers are generated from an optical comb source by separating two optical tones from the optical comb by using a wavelength selective switch (WSS). Preferably, the tones of the optical comb are phase-locked, thereby minimizing random phase variations between them. In the illustrated embodiment, the comb itself is generated by modulating a laser source L using an MZM. In the example illustrated in FIG. 8, the comb provides only two optical carriers. However, it is also possible to extract more than two phase-locked comb tones from a single comb source, for example by operating the MZM with a large amplitude sinusoidal electrical drive signal, by using a cascade of amplitude and phase modulators, or by relying on other comb generation concepts known in the literature, such as the Kerr comb source. When used in conjunction with a modulator, a large amplitude drive signal means a drive signal whose amplitude swing is large enough to push the MZM into the nonlinear operating region of its transfer function. Herein, a number of Erbium-Doped Fiber Amplifiers (EDFAs) are used to boost the optical signal power in the device 1.

[0092] Both tributary electromagnetic signals 20, 21 are filtered after the optical in-phase / quadrature (I / Q) modulators 260, 261 by using filters Filt.1 and Filt.2, especially to suppress the wideband noise generated by the EDFA. Herein, the fibers are length-matched by using a delay line DL. One of the signal paths further includes at least one actuating element 30, which is here implemented by using an all-fiber piezo phase shifter PS.

[0093] As shown in FIG. 8, here a 90° optical hybrid is used as the signal combining element 50. In this implementation, two of the four output ports of the signal combining element 50 are used as inputs to a balanced photodetector connected to an analog-to-digital converter ADC, which together function as a signal detection element 90. Furthermore, a field programmable gate array (FPGA) or some other electronic circuit providing the required functionality is used to implement a control element 110, which is configured to extract signal information regarding the relative phase of the two tributary optical signals combined in the signal combining element, and control the relative phase to the required state by using the actuation element 30. The targeted composite wideband optical signal 60 is directed to an optical spectrum analyzer OSA, which is configured to monitor the optical spectrum of the composite wideband optical waveform.

[0094] TIFF2025514901000002.tif45164 TIFF2025514901000003.tif27164

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[0103] TIFF2025514901000016.tif26164 TIFF2025514901000017.tif32164

[0104] TIFF2025514901000018.tif40164

[0105] FIG. 10 illustrates diagrams illustrating the time evolution of the amplitude A of the measured detection output signal 100 and the actuator drive signal 120 over a period t during the proof-of-concept operation of the setup shown in FIG. 8. A first set of signals illustrates the time course of the amplitude A of the detection output signal 100 generated by the signal detection element 90 of FIG. 8 and according to the characteristics described in equation (1.16). Herein, the change in the amplitude A of the detection output signal 100 represents the change in the relative phase between the tributary electromagnetic signals 20, 21 at the signal coupling element 50. In a first period t<5s, the feedback control loop is disabled and the system is left as is, which can be recognized by the occurrence of phase variations of the detection output signal 100 and corresponding variations in the uncontrolled system. A further set of signals shows the time course of the amplitude A of the actuator drive signal 120 configured to drive the actuation element 30 implemented here by using a phase shifter as explained in more detail above. The amplitude A of the actuator drive signal 120 is maintained at a constant level for a first period of time t<5s, in particular because the control element 110 is disabled for the first period of time t<5s.

[0106] As further shown in FIG. 10, the control element 110 is initiated at a time t<5s. As a result, it can be observed that the amplitude A of the detection output signal 100, which represents the change in the relative phase between the tributary electromagnetic signals 20, 21, converges to and maintains a constant level set by the control element 110. Furthermore, the operation of the control element 110 can be observed by observing the time course of the amplitude A of the actuator drive signal 120 for a second period t<5s. As shown therein, the control element 110 is actively controlling the relative phase between the tributary electromagnetic signals 20, 21 to meet the desired condition. Due to the limitations of the selected setup, the range of the actuating element 30 used for the proof-of-concept operation as illustrated in FIG. 10 is limited, so the device 1 needs to reset the actuating element 30 after a period of approximately 8.5 to 10 seconds. In an improved setup, the range of the actuating element 30 can be extended, thus limiting the phase variation over a longer period, thereby avoiding one or more resets. Moreover, by using the infinite phase shifter thus shown, the limitations imposed by the currently used actuation elements 30 can be completely eliminated.

[0107] Fig. 11 shows two diagrams each illustrating, in a schematic manner, measurements obtained from the setup shown in Fig. 8, where Fig. 11a illustrates a first measurement without phase stabilization, while Fig. 11b illustrates a second measurement with phase stabilization. Here, each measurement is based on a targeted electromagnetic signal 60 generated by the signal combining element 50, recorded here by an optical spectrum analyzer OSA.

[0108] The proof-of-concept operation is carried out with two tributary electromagnetic signals 20, 21 in the optical frequency range, where the two tributary electromagnetic signals 20, 21 overlap here only at a single frequency f0 in the center of the measured frequency range. The broadband content of one tributary electromagnetic signal 20 is at a higher frequency, whereas the broadband content of the other tributary electromagnetic signal 21 is at a higher frequency. The spectral overlap region between both broadband contributions does not contain the signal components of the targeted broadband electromagnetic signal, so an auxiliary signal 230, 231 needs to be added to each of the tributary electromagnetic signals 20, 21 to generate the detection output signal 100, which relies on the interference of signal components occupying the same spectral range. This auxiliary signal component can be observed near the frequency f0 in the intermediate spectral overlap region between the two signal slices corresponding to the tributary electromagnetic signals 20, 21.

[0109] TIFF2025514901000019.tif74164

[0110] In an exemplary embodiment, an optical arbitrary waveform generator (OAWG) as disclosed herein can be implemented in a photonic integrated chip (PIC) structure, thereby reducing its footprint as well as limiting undesirable effects resulting from optical path mismatches and phase variations. In a preferred embodiment, the PIC OAWG can include an electro-optic modulator, using one of the implementations discussed above.

[0111] In a further exemplary embodiment, a four-slice OAWG can be implemented as a PIC structure, as shown in FIG. 12. Such a scheme is similar to that used in fiber-based setups, but since there are four tributary electromagnetic signals, two steps may be required for the interleaving process, leading to a cascaded structure of signal combining elements 50, 51, 52. The OAWG presented herein can be scaled up to a larger number of tributary electromagnetic signals by using more cascaded stages, leveraging the compactness of the PIC. In the implementation shown in FIG. 12, three individual 3×3 MMIs are used as signal combining elements 50, 51, 52, where each 3×3 MMI is implemented as a balanced pair of PIN diodes and connected to a photodetector that serves as a corresponding signal detection element 90, 91, 92. The output signal of each photodetector is directed to a corresponding control element 110, 111, 112 that resides on a printed circuit board (PCB) attached to the photonic integrated circuit (PIC). The electrical connection between the PIC and the PCB can preferably be achieved by electrically packaging the PIC here, for example by flip-chip wire bonding coupling the output of the photodetector to the control element 110, 111, 112 on the PCB. Based on the corresponding detection output signal 100 from the balanced photodetector as the corresponding signal detection element 90, 91, 92, the corresponding control element 110, 111, 112 actuates the corresponding actuation element 30, 31, 32, which may be a phase shifter implemented on the PIC. In particular, the phase shifter can be implemented as a thermal phase shifter. Although a thermal phase shifter has a slow response time, mainly due to a slow cooling rate, this is ultimately not an issue since most effects that lead to a phase offset between tributary electromagnetic signals on the PIC are relatively slow. However, this assumes that the optical tones originate from a phase-locked comb source and therefore do not suffer from strong relative phase drift. Alternatively, the phase shifter can also be implemented as an electro-optical phase shifter. The implementation of control elements on a PIC provides a particularly attractive route towards using endless phase shifters that rely on more complex cascading for Mach-Zehnder type structures with phase shifters.

[0112] In a further exemplary embodiment, the input of the electro-optical modulator of the OAWG may be an optical signal originating from an optical frequency comb OFC. The optical frequency comb OFC may be fed into an optical splitting element, which may preferably be implemented as a multi-mode interference (MMI) coupler or an arrayed waveguide grating. The optical frequency comb (OFC) entering the optical splitting element may be generated on-chip or may be provided by an independent off-chip comb source. In particular, the OFC may be generated by using a Kerr nonlinear ring resonator structure, which may generate a Kerr comb. The OFC is guided to the optical splitting element, and four individual frequency components are selected to be filtered and forwarded to each electro-optical modulator individually. The bandwidth of the modulated signal may be larger than the free spectral range FSR of the optical frequency comb OFC, which leads to a spectral overlap between the frequencies of adjacent tributary electromagnetic signals.

[0113] Chip-scale implementation of OAWGs can be performed in either a single homogenous PIC or a hybrid multi-chip module (MCM), where different components can be implemented using different wafer technologies, thereby leveraging the individual advantages of the different implementations. As discussed above, in the hybrid approach, different PICs need to be bonded and optically connected to each other by precise alignment and / or by using 3D printed optical coupling structures such as microlenses or photonic wire bonds.

[0114] FIG. 13 illustrates a diagram illustrating an exemplary embodiment of the amplitude A over frequency f of four tributary electromagnetic signals 20, 21, 22, 23 having at least partially overlapping spectra. The tributary electromagnetic signals 20, 21, 22, 23 are arranged here from left to right according to the order of their frequency spectra. As further shown in FIG. 13, the spectral arrangement of the signals leads to three spectral overlap regions 181, 182, 183, where adjacent tributary electromagnetic signals may interfere to generate a detection output signal. The tributary electromagnetic signals 21, 22, 23, 24 are injected into the 3×3 MMI such that the tributary electromagnetic signals 20 and 21, as well as the tributary electromagnetic signals 22 and 23, are paired together, respectively (see FIG. 12).

[0115] In the first combining stage, the phase offset between tributary electromagnetic signals 20 and 21 is corrected so that tributary electromagnetic signals 20 and 21 have a desired phase relationship as shown by detected output signal 100 produced by interference of signal components in spectral overlap region 181. As a result, signals 20, 21 produce a desired combined signal at the output of signal combining element 51. Herein, the correction may be performed by using a phase shifter PS to act on tributary electromagnetic signal 20 to match the phase of tributary electromagnetic signal 21. Similarly, tributary electromagnetic signals 22 and 23 have a desired phase as shown by detected output signal 110 produced by interference of signal components in spectral overlap region 183. Again, the correction may be performed by using a phase shifter PS to act on tributary electromagnetic signal 22 to match the phase of tributary electromagnetic signal 23. In the second combining stage, the phase shifter PS can act on the already combined tributary electromagnetic signals 20 and 21 such that the already combined tributary electromagnetic signals 20 and 21 are also phase-aligned with the already combined tributary electromagnetic signals 22 and 23, thus leading to the desired overall waveform at the output 60 of the signal combining element 53.

[0116] In this manner, the three feedback control loops can operate using mutually independent control elements 110, 111, 112 to precisely align and maintain the phase of all four tributary electromagnetic signals 20, 21, 22, 23. However, this procedure is not limited to independent control elements 110, 111, 112. In an alternative embodiment, the feedback control loops may use interdependently driven control elements 110, 111, 112, which further improves the quality and accuracy of the phase control.

[0117] TIFF2025514901000020.tif193164

[0118] TIFF2025514901000021.tif122164

[0119] TIFF2025514901000022.tif38164

[0120] In alternative embodiments, the various tributary spectra may have a greater overlap or may be separate, covering the full range of targeted broadband electromagnetic waveforms. Such broadband tributary electromagnetic signals may be generated by simultaneously modulating a number of two or more continuous wave (cw) electromagnetic tones with the same modulation signal. In this embodiment, the device 1 may be configured to include at least one frequency comb generator 240 for generating at least two continuous wave electromagnetic tones 250, 251, at least one first electromagnetic signal generator 10 configured to generate at least a first tributary electromagnetic signal by modulating the two continuous wave electromagnetic tones 250, 251 in concert with a first modulation signal, and at least one second electromagnetic signal generator 11 configured to generate at least a second tributary electromagnetic signal by modulating the two continuous wave electromagnetic tones 250, 251 in concert with a second modulation signal. The two tones 250, 251 and / or the at least two tributary electromagnetic signals thus generated are subjected to various frequency-dependent transfer functions before being modulated by the electromagnetic signal generator or superimposed by the signal combining element.

[0121] Examples of such embodiments are illustrated generally in Figures 14 and 15. In the illustrated example, the four tributary electromagnetic signals are optical signals and are generated by modulating four differently delayed copies of the output of an optical frequency comb generator 240 which provides four continuous wave electromagnetic tones 250, 251, 252, 253.

[0122] In the implementation illustrated diagrammatically in FIG. 14, the optical frequency comb is split into four copies by a power splitter 290, and then delays are provided by a number of delay elements 280, 281, 282, which may be implemented by fixed or adjustable optical delay lines. Modulation is provided by four optical in-phase / quadrature (I / Q) modulators 260, 261, 262, 263 that are part of the corresponding electromagnetic signal generator for the tributary electromagnetic signals. The tributary electromagnetic signals are then superimposed by an implementation of the device similar to that shown in FIG. 12. The modulation signals are selected such that the superposition of the four signals at the output of the device leads to the desired targeted wideband electromagnetic waveform 60. The relative phase and amplitude of the superimposed tributary electromagnetic signals is again maintained by a control loop that depends on the detection output signal 100 derived from the output of the pair of MMI-based signal combining elements 50, 51, 52. In a preferred embodiment, optional optical filters 270, 271, 272 are inserted between the outputs of the signal combining elements 50, 51, 52 and the corresponding signal detecting elements 90, 91, 92. Obviously, other signal combining elements may be used. In a preferred embodiment, the relative delay between the four copies of the comb generator output is approximately equal to T=1 / (4×FSR), where FSR corresponds to the free spectral range of the comb source.

[0123] 15 illustrates diagrammatically the spectrum of tributary electromagnetic signals 20, 21, 22, 23, each covering the full bandwidth of a targeted broadband electromagnetic waveform. These tributary electromagnetic signals are generated by modulating continuous wave electromagnetic tones 250, 252, 252, 253 generated by a frequency comb generator 240. This scheme can be generalized to a larger number of tributary electromagnetic signals, but in that case the delays need to be adapted accordingly.

[0124] TIFF2025514901000023.tif118164 TIFF2025514901000024.tif39164 [Explanation of symbols]

[0125] 1. Equipment 2. System 3. (Optical) Integrated Circuits 10, 11. Electromagnetic signal generator 20, 20-1, ...20-N, 21, 22, 23. Tributary electromagnetic signal 30, 30-1, ... 30-L, 31, 32. Operating elements 40. Corrected tributary electromagnetic signal 41-1...41-N. Signal coupling element input signal 50, 51, 52. Signal coupling elements 60. Targeted broadband electromagnetic signals 61, 61-1, ... 61-K, 62, 63. Combined Electromagnetic Signals 70. Conversion element 80. Converted output waveform 90, 91, 92. Signal detection element 100.Detection output signal 110, 111, 112. Control elements 120, 121. Actuator drive signal 130. Common Electromagnetic Signal Sources 140, 141. Reference signal 150, 151, 152, 153. First stage device 160, 161. Second stage device 170. Third stage device 180, 181, 182, 183. Overlapping range 190, 191, 192. Input port 200. Method 205.Providing process 210. Merging Process 215. Detecting process 220. Change process 230, 231. Auxiliary signals 240. Frequency Comb Generator 250, 251, 252, 253. Continuous wave electromagnetic tones 260, 261, 262, 263. Optical In-phase / Quadrature (I / Q) Modulators 270, 271, 272. Optical filters 280, 281, 282. Delay elements 290. Power splitter 300. Local oscillator tone 310. Electrical Waveform Generator 320. Frequency shifter 330.90° Optical Hybrid 340. Wideband balanced photodetector

Claims

1. 1. An apparatus (1) for synthesizing at least one targeted wideband electromagnetic signal (60), said apparatus (1) comprising: at least two electromagnetic signal generators (10, 11), the at least two electromagnetic signal generators (10, 11) configured to provide at least two tributary electromagnetic signals having at least partially overlapping spectra; at least one signal combining element (50), configured to merge the at least two tributary electromagnetic signals into at least one combined electromagnetic signal; at least one signal detection element (90) configured to generate at least one detected output signal (100) by detecting at least a portion of at least one of the at least one coupled electromagnetic signals; at least one control element (110) configured to extract signal information relating to at least one of the relative amplitude or relative phase of the at least two tributary electromagnetic signals merged into the at least one combined electromagnetic signal from the at least one detection output signal (100); at least one actuation element (30) configured to modify at least one of the amplitude or phase of at least one of the at least two tributary electromagnetic signals based on the signal information; The apparatus (1), wherein modifying at least one of the amplitude or phase of at least one of the at least two tributary electromagnetic signals is performed such that at least one of the at least one combined electromagnetic signal corresponds to the at least one targeted broadband electromagnetic signal (60).

2. 2. The apparatus (1) of claim 1, wherein the at least one signal coupling element (50) has at least one first output port and at least one second output port, the at least one targeted broadband electromagnetic signal (60) being provided to the at least one first output port, and at least a portion of the at least one coupled electromagnetic signal (61) being provided to the at least one signal detection element (90) by the at least one second output port.

3. at least one of said at least one actuating element (30) is at least partially integrated in at least one of said at least two electromagnetic signal generators (10, 11); or At least one of said at least one actuation element (30) is at least partially integrated with at least one of said at least one signal coupling element (50); or The device (1) of claim 1, wherein at least one of the at least one signal detection elements (90) is at least partially integrated with at least one of the at least one signal coupling elements (50).

4. At least one of said at least one actuation element (30) and at least one of said at least one signal coupling element (50); or at least one of said at least one signal detection element (90) and at least one of said at least one signal coupling element (50); or At least one of said at least one actuating element (30) and at least one of said at least one signal detecting element (90); or At least one of the at least one actuating element (30), at least one of the at least one signal coupling element (50), and at least one of the at least one signal detection element (90) The device (1) according to claim 1, which is implemented together in at least one photonic integrated circuit (3).

5. The at least one actuating element (30) Piezoelectric actuators, Photonic integrated circuits (3), or 3. The device (1) according to claim 1, wherein the phase shifter is selected from at least one of:

6. The at least one signal coupling element (50) a multi-mode interference (MMI) coupler or network thereof; a directional coupler or a network thereof, or 2. The device (1) according to claim 1, comprising at least one of: a network of ring resonators;

7. The apparatus (1) of claim 1, wherein the at least one signal detection element (90) comprises a balanced photodetector.

8. The device (1) of claim 1, wherein at least one of the electromagnetic signal generators (10) is configured to shape at least a spectral portion of at least one tributary electromagnetic signal in a spectral overlap region with at least one other tributary electromagnetic signal such that a feedback signal generated by interference of the two electromagnetic signals is independent of the targeted wideband electromagnetic signal (60).

9. 2. The apparatus (1) of claim 1, wherein at least a first tributary electromagnetic signal of the at least two tributary electromagnetic signals is a broadband electromagnetic signal, and a second tributary electromagnetic signal of the at least two tributary electromagnetic signals is a single optical frequency tone configured to function as a local oscillator tone for frequency downconversion.

10. 2. The apparatus (1) of claim 1, wherein at least one of the at least two electromagnetic signal generators (10, 11) is configured to generate at least one of the at least two tributary electromagnetic signals by modulating at least one of a single tone or multiple tones of an optical frequency comb.

11. 2. The apparatus (1) of claim 1, further comprising: at least one frequency comb generator (240), wherein the at least one frequency comb generator (240) is configured to generate at least two continuous wave electromagnetic tones (250, 251); at least one first electromagnetic signal generator (10) is configured to generate at least one first tributary electromagnetic signal by modulating the at least two continuous wave electromagnetic tones (250, 251) in cooperation with a first modulation signal; and at least one second electromagnetic signal generator (11) is configured to generate at least one second tributary electromagnetic signal by modulating the at least two continuous wave electromagnetic tones (250, 251) in cooperation with a second modulation signal, and the resulting two tributary electromagnetic signals are superimposed by the signal combining element (50).

12. 1. A system (2) for synthesizing at least one targeted wideband electromagnetic signal (60), said system (2) comprising a plurality of devices (1) according to claim 1, said plurality of devices (1) being arranged in a cascade manner.

13. The device (1) of claim 1 or the system (2) of claim 12 further comprising at least one signal conversion element (70), the at least one signal conversion element (70) configured to convert at least a portion of the at least one targeted broadband electromagnetic signal (60) into a converted electromagnetic waveform.

14. 1. A method (200) for synthesizing at least one targeted wideband electromagnetic signal (60), said method (200) comprising: a) providing at least two tributary electromagnetic signals having at least partially overlapping spectra; b) merging the at least two tributary electromagnetic signals into at least one combined electromagnetic signal; c) detecting at least a portion of at least one of the at least one combined electromagnetic signal and extracting signal information related to at least one of the relative amplitudes or relative phases of the at least two tributary electromagnetic signals merging into the at least one combined electromagnetic signal; d) modifying at least one of the amplitude or phase of at least one of the at least two tributary electromagnetic signals based on the signal information; The method (200), wherein modifying at least one of the amplitude or phase of at least one of the at least two tributary electromagnetic signals is performed such that at least one of the at least one combined electromagnetic signal corresponds to the at least one targeted wideband electromagnetic signal.

15. 15. A computer program for synthesizing at least one targeted wideband electromagnetic signal (60), said computer program comprising instructions that, when executed by a computer, cause said computer to perform the method (200) of claim 14.

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