WO2021130578A1

WO2021130578A1 - Microwave photonic signal processor

Info

Publication number: WO2021130578A1
Application number: PCT/IB2020/061433
Authority: WO
Inventors: Miguel Vidal Drummond; Rui Luís VIEIRA OLIVEIRA; Rogério NUNES NOGUEIRA
Original assignee: Instituto De Telecomunicações
Priority date: 2019-12-23
Filing date: 2020-12-03
Publication date: 2021-07-01

Abstract

ln accordance with a described example, a microwave photonic signal processor (MPSP) is disclosed for processing a first and a second input wavelength-division multiplexing (WDM) signals, each comprising RF sidebands and optical carriers, which comprises a programmable photonic processor (PPP) for individually adjusting the magnitude and phase of each optical carrier and RF sideband of the first WDM signal; an optical combiner for combining the output signal of the PPP with the second WDM signal; and a photodetection stage for converting the signal output by the optical combiner to an electrical signal.

Description

DESCRIPTION

MICROWAVE PHOTONIC SIGNAL PROCESSOR

FIELD OF THE INVENTION

The present invention relates generally to the field of radio-frequency (RF) signal processing resorting to photonic means, and, in particular, to microwave photonic signal processing.

BACKGROUND ART

Microwave photonics is an interdisciplinary area that combines RF and optical signal processing techniques, for applications such as broadband wireless access networks, sensor networks, radar, satellite communications, instrumentation, and warfare systems.

An important part of microwave photonics is microwave photonic signal processing. A microwave photonic signal processor takes several input RF signals, converts these to optical signals, processes these using photonic techniques, and converts the processed optical signals to RF signals.

A microwave photonic signal processor (MPSP) can be broadly described according to the scheme of Figure 1. An MPSP takes M input RF signals 101, and converts these to optical signals in an electrical-to-optical conversion stage 102. The resulting signals 10S are then processed by a photonic processor 104 that resorts mainly to photonic technologies such as optical filters, optical fibers, photonic integrated circuits, variable attenuators, phase shifters, and passive splitters or combiners. The processed signals 105 are then converted to electrical signals in an optical-to-electrical conversion stage 106. The MPSP outputs at least one RF or baseband signal 107.

Although the concept of an MPSP is very broad, and thus may serve a wide range of applications, two particular applications have been the main target of MPSPs: microwave photonic filtering and photonic beamforming.

Microwave photonic filtering is typically used for channelization, in which a single input RF signal containing different RF carriers is channelized into at least one output RF signal, each with a set of RF carriers. As a result, microwave photonic filtering can be regarded as a MPSP targeting spectral shaping. An extensive review on microwave photonic filters is given in the article J. Capmany, B. Ortega and D. Pastor, "A tutorial on microwave photonic filters," in Journal of Lightwave Technology, vol. 24, no. 1, pp. 201- 229, Jan. 2006, doi: 10.1109/JLT.2005.860478.

Beamforming is a technique used to tailor the radiation diagram of a phased array antenna (PAA). A PAA is a set of multiple antenna elements, each of which typically has low directivity. The radiation diagram of the PAA as a whole depends on how the multiple antenna elements are arranged, as well as on the signals fed to or obtained from the antenna elements. Without loss of generality, let us consider a PAA receiver feeding a beamformer, and a single wireless beam impinging the antenna. Such a scenario is illustrated in Figure 2. Each antenna element of the PAA 201 captures a version of the impinging beam. Different versions thus have different amplitudes and time delays. The beamformer 202 receives the different captured versions of the impinging beam 201, equalizes them such that all become identical in amplitude and time delay, and finally combines the equalized versions into an output signal 203.

The same operation principle can be extended to multiple impinging beams, provided that the spatial resolution of the PAA is sufficiently high to properly separate any beam with negligible crosstalk from other beams. In addition, multiple outputs 203 can be added such that the beamformer 202 becomes capable of outputting multiple beams at the same time. Finally, given that the entire operation principle is linear, it is also applicable to a PAA transmitter. In this case, the beamformer receives individual beams and outputs the signals to be fed to the antenna elements of the PAA. A beamformer may thus be regarded as a multiple-input multiple output (MIMO) signal processor.

The bottom line is how to implement a MIMO signal processor. As with any signal processor, there are three main technologies upon which a signal processor can be built: digital, analogue (i.e., using RF electronics), and photonic. A photonic implementation can be broadly inspired in Figure 1. For instance, considering a PAA receiver, the signals provided by the antenna elements 101 are fed to a photonic processor 104. The processed optical signals 105 are converted to RF signals 107, each of which comprises a single beam.

An MPSP 104 comprises four basic components: signal splitters/combiners, attenuators/amplifiers, phase shifters and delay lines. Signal splitters are mandatory for creating several copies of various input signals. A signal combiner is required to combine the various processed input signals into one output signal. Attenuators or amplifiers are required to equalize and define the magnitude of the input signals. Phase shifters are required e.g., to guarantee that various processed input signals are combined into one output signal with correct phases. Delay lines are required to guarantee that various processed input signals are combined into one output signal with correct time delays. The following two patents are prior-art examples of MPSP targeting photonic beamforming.

US patent US89S4774B2 discloses a photonic beamformer suitable for a PAA transmitter. It is important for the present invention to note that the disclosed systems are based on direct detection of single sideband signals.

US patent US10224628B2 discloses a photonic beamformer suitable for a PAA receiver. A noteworthy feature of the disclosed photonic beamformer is that the basic concept is based on a single wavelength, i.e., it does not resort to WDM. This means that the entire system is coherent, and that heterodyne detection is self-coherent, which in turn enables laser phase noise cancellation. It is important for the present invention to note that the systems disclosed in US patent US8934774B2 do not take any advantage in using WDM.

The first patent above described, US patent US8934774B2, is a paradigmatic example of an incoherent MPSP. An incoherent MPSP typically generates various copies of an input RF signal over multiple wavelengths. Each wavelength is then individually processed (i.e., the RF signal carried in that wavelength is delayed and/or phase shifted), directly detected by a photodiode, and routed to an output port. When considering beamforming for a PAA transmitter, the phase shifts and time delays added to each wavelength are defined such that the radiated signal propagates along a pre-defined direction. When considering beamforming for a PAA receiver receiving multiple beams, the RF signal from each antenna element modulates a different wavelength. Each wavelength is then individually processed (i.e., the RF signal carried in that wavelength is delayed and/or phase shifted), and multiplexed together with the other wavelengths. A single photodiode then directly detects the entire WDM signal. The phase shifts and time delays added to each wavelength are defined such that the output RF signal typically contains a single beam.

The second patent above described, US patent US10224628B2, is a paradigmatic example of a coherent MPSP. A coherent MPSP typically generates various copies of an input RF signal by modulating it onto a single optical carrier, and passively splitting the modulated optical signal into multiple copies. Each copy is then individually processed (i.e., the RF signal carried in that wavelength is delayed and/or phase shifted), coherently detected, and routed to an output port. When considering beamforming for a PAA receiver receiving multiple beams, the RF signal from each antenna element modulates one of the copies of a single optical carrier. Each modulated copy is then individually processed (i.e., the RF signal carried in that wavelength is delayed and/or phase shifted), and coherently added with the other copies. A single coherent receiver then detects the resulting optical signal. The phase shifts and time delays added to each copy are defined such that the output RF signal typically contains a single beam.

Even though an incoherent MPSP based on WDM involves straightforward setups, the limited capacity of a WDM signal limits the number of input and/or output RF signals. In addition, it does not resort to highly sensitive coherent detection, and consequently also does not support heterodyne reception. RF phase shifting is also not a trivial operation as it typically involves single sideband generation, followed by optical phase shifting applied to the remaining sideband.

A coherent MPSP enables building an optical signal processor identically to a typical analogue heterodyne RF processor. That is, signals are coherent among themselves, RF frequency conversion is achieved by means of heterodyne reception, optical phase shifting directly results in RF phase shifting, and RF phase shifting can be produced either by phase shifting the optical input signal or the optical local oscillator. The main drawback of a coherent MPSP is that a complex phase stabilization loop is required to ensure that relative phases amongst signals remain stable. An example of such a phase stabilization loop can be found in Duarte, V., et a I, Modular coherent photonic-aided payload receiver for communications satellites, vol. 10, no. 1, Nature Communications, doi:10.1038/s41467-019-10077-4.

The drawbacks pointed out on the two paragraphs above are manageable for small-scale MPSPs, such as single-input single-output (SISO) microwave photonic filters, or single-input multiple-output (SIMO) or multiple-input single-output (MISO) photonic beamformers. However, for large-scale MPSPs such as multi-beam photonic beamformers such drawbacks quickly become unmanageable, invalidating a feasible implementation. Without loss of generality, let us consider a multi-beam beamformer. A beamformer with full flexibility must have a number of phase shifters given by N_AE x N_B, where N_AE is the number of antenna elements and N_B is the number of beams. The same scale applies for delay lines, which are required if squint-free beamforming is targeted.

On one hand, when considering an incoherent MPSP for photonic beamforming, implementing these many phase shifters using single sideband generation, followed by optical phase shifting applied only to the remaining sideband would be cumbersome, as each phase shifter would be in fact a tunable optical filter. Such a tunable optical filter would require a frequency resolution at most identical to the RF signal frequency, meaning that path lengths of the phase shifter would be as long as path lengths of an RF phase shifter, thus resulting in a large frame.

On the other hand, when considering a coherent MPSP for photonic beamforming, such would not be a problem as an optical phase shifter is exactly equivalent to an RF phase shifter. As a result, phase shifters can be miniaturized from at least one RF wavelength down to a single optical wavelength. However, handling these many phase shifters requires an unmanageably complex phase stabilization loop.

The problem addressed by the present invention thus is how to design a MPSP with benefits from both coherent and incoherent MPSPs, however without significant drawbacks. In detail, this means that such an MPSP must have the following features:

— Supports coherent optical signal processing, which includes supporting coherent detection, and also means that optical phase shifting is equivalent to RF phase shifting; — Can be tolerant to phase noise, i.e., does not require narrow-linewidth laser sources;

— Does not require complex phase stabilization loops;

— Is compatible with WDM signals;

— If the number of WDM signals is insufficient to accommodate all input RF signals, the MPSP can be easily scaled to allow more WDM signals.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a microwave photonic signal processor capable of processing several RF signals with significant flexibility.

The present invention provides a microwave photonic signal processor (MPSP) and a method for processing a first and a second wavelength-division multiplexing (WDM) signals as recited in claims 1 and 19, respectively. Further advantageous features are as defined in the appended dependent claims.

DESCRIPTION OF FIGURES

Fig. 1 is a scheme of a generic MPSP.

Fig. 2 is a scheme of a generic beamformer applied to a phase array antenna receiver. Fig. 3 is a basic scheme of the disclosed MPSP.

Fig. 4 illustrates the signal spectra at different points of the scheme of Figure 3.

Fig. 5 is a scheme of the disclosed MPSP that also includes optical modulation for generating more optical carriers or RF sidebands.

Fig. 6A illustrates the signal spectra at different points of the scheme of Figure 3, considering double sideband signals.

Fig. 6B illustrates the signal spectra at different points of the scheme of Figure 3, considering double sideband signals and a different interleaving function.

Fig. 7 is a scheme of the disclosed MPSP that also includes an optical interleaver for obtaining the signals depicted in Fig. 6B. Fig. 8 is a scheme of the disclosed MPSP that also includes two wavelength-dependent ODLs for delaying the first and second WDM signals.

Fig. 9 is a scheme of the disclosed MPSP that also includes a wavelength-dependent ODL for delaying the combined signal.

Fig. 10 shows how a wavelength-dependent ODL can be used to individually delay each optical carrier and each RF sideband.

Fig. 11 shows how a wavelength-dependent ODL can be used to individually delay a pair of adjacent signals, each pair comprising an optical carrier and RF sideband.

Fig. 12 is a scheme of the disclosed MPSP that also includes an optical filter for filtering the combined signal.

Fig. 13 is an alternative scheme of Fig. 12 in which the optical filter has multiple outputs, such as a channelizer.

Fig. 14 depicts a MIMO PPP.

Fig. 15 is a scheme of the disclosed MPSP that includes a MIMO PPP, and thus is capable of receiving multiple WDM signals and outputting multiple RF signals.

Fig. 16 is a variation of the scheme of Fig. 15, in which the WDM signals input to the programmable photonic processor are made identical.

Fig. 17 is a variation of the scheme of Fig. 15, in which the WDM signals output by programmable photonic processor are combined into a single WDM signal, which results in a single output RF signal.

Fig. 18 shows how two MPSPs can be combined, while maintaining the same number of output RF signals.

Fig. 19 shows how two MPSPs can be combined for doubling the number of output RF signals, wherein both MPSPs share the same second WDM signal.

Fig. 20 is a scheme of the disclosed MPSP that also includes a phase stabilization loop that, in turn, actuates on an optical phase modulator for stabilizing the phase of the output RF signal.

Fig. 21 is a scheme of the disclosed MPSP that also includes a phase stabilization loop that, in turn, actuates on the PPP for stabilizing the phase of the output RF signal. Fig. 22 is a scheme of the disclosed MPSP that also includes a phase stabilization loop that, in turn, actuates on a phase modulator for stabilizing the phase of the output RF signal.

DETAILED DESCRIPTION

The more general and advantageous configurations of the present invention are described in the Summary of the invention. Such configurations are detailed below in accordance with other advantageous and/or preferred embodiments of implementation of the present invention.

As will be clear to one skilled in the art, the present invention should not be limited to the embodiments described herein, and a number of changes are possible which remain within the scope of the present invention.

Of course, the preferred embodiments shown above are combinable, in the different possible forms, being herein avoided the repetition all such combinations.

Operation principle

The operation principle of the MPSP of the present invention can be explained resorting to Figures 3 and 4.

Figure 3 represents a MPSP, according to a preferred embodiment of the present invention, which receives a first and second WDM signals. Without loss of generality, let us consider that the first WDM signal 310 consists of two optical carriers 421, 431, and that the second WDM signal 311 consists of two RF sidebands 422, 432. The optical carriers 421, 431 of the first WDM signal 310 are processed by a programmable photonic processor (PPP) 301, which is capable of individually manipulating the magnitude and phase of each optical carrier. Even though the implementation of the PPP is not relevant for explaining the operation principle of the MPSP, a PPP can be implemented by a wavelength-selective switch based on a liquid crystal on silicon matrix (LCoS). However, if it turns out that all that is required is an adjustable phase difference between two consecutive optical carriers, then a much simpler tuneable optical delay line (TODL) can be used instead. The signal output by the PPP 312 is combined with the second WDM signal 311 using an optical combiner 302. This signal is then fed to a photodetection stage 303 that converts it to an output RF signal 314.

In the present invention, the photodetection stage 303 comprises at least one photodetector. If a single photodetector is used for each input optical signal, then photodetection is equivalent to single-ended detection. As an alternative, if the optical combiner 302 has two complementary outputs, a balanced photodetector can be used instead, resulting in differential detection.

The output RF signal 314 has multiple terms 441, 442, 443 and 444. The selfbeats of optical carriers or RF sidebands, only present in single-ended detection, result in terms centred at DC 441. The term between a corresponding pair of optical carrier and RF sideband, i.e., 421 with 422 and 431 with 432, results in a term at the lowest RF frequency 442. The remaining terms at higher RF frequencies are the result of noncorresponding pairs of optical carriers and RF sidebands, i.e., the beat between 421 and 432 results in term 443, and the beat between 431 and 422 results in term 444. From all these terms that do not result from self-beats, only the term 442 is the result of two identical beats, one from each corresponding pair of optical carrier and RF sideband.

Without loss of generality let us assume single-ended detection. The produced photocurrent is given by:

where x₄₂₄(t), x₄₂₂(t), x₄₃₄(t), and x₄₃₂(t) are the signals 421, 422, 431 and 432, respectively. The signal 422 is modulated in amplitude and phase by signals a₄₂₂(t) and θ₄₂₂ (t), and is centered at a frequency of ω₄₂₂. The signal 432 is modulated in amplitude and phase by signals a₄₃₂(t) and θ₄₃₂(t), and is centered at a frequency of ω₄₃₂. The tone 421 has an amplitude of A₄₂₁, frequency of ω₄₂₁ and phase θ₄₂₁. The tone 431 has an amplitude of A₄₃₁, frequency of ω₄₃₁ and phase θ₄₃₁. C is a constant that relates photocurrent amplitude with input optical power. Let us expand equation (1), but only consider the term 442:

The frequency of term 442 is given by ω₄₄₂ = ω₄₂₁ — ω₄₂₂ = ω₄₃₁ — ω₄₃₂. Thus:

The term 442 thus results from the coherent addition of beats 421 with 422 and 431 with 432. The complex weight of each beat can be adjusted solely by the PPP 301 by adjusting A₄₂₁ and A₄₃₁, for amplitude, and by adjusting θ₄₂₁ and θ₄₃₁, for phase. Therefore, thanks to the PPP 301, the disclosed coherent MPSP is able to combine the various input RF sidebands with complex weights individually adjusted for each RF sideband, by only adjusting magnitude and phase of the optical carriers 421 and 431.

However, being the MPSP coherent, it is important to know whether phase wandering between different paths, namely paths 311 and 312, affects the end result. While it can be assumed that different signals propagating overthe same path are phase modulated by the same slow-varying function describing phase wandering, such assumption does not hold for different paths. Considering equation (3), these assumptions can be modelled as

where θ_s(t) models the phase wandering that the optical path carrying the RF sidebands 311 experiences, whereas θ_OLO(t) models the phase wandering that the optical path carrying the optical carriers 312 experiences. Equation (4) shows that, although the photocurrent is phase modulated by θ_s(t) — θ_OLO(t), such a phase modulation does not affect the end result as the complex weights individually adjusted for each RF sideband are not affected by phase wandering θ_s(t) nor θ_OLO(t).

The MPSP, according to the present invention, takes two input WDM signals. Without loss of generality, let us assume that the first WDM signal only contains optical carriers, and the second only contains RF sidebands. The first WDM signal is processed such that the magnitude and phase of each optical carrier is individually adjusted. The processed first WDM signal is then combined with the second WDM signal. Finally, coherent detection is done using the second WDM signal as input signal, and the processed first WDM signal as local oscillator (LO). The resulting output RF signal is the combination of the various input RF sidebands, with complex weights individually adjusted for each RF sideband.

The concept of the MPSP of the present invention, is based on the fact that RF sidebands are wavelength-division multiplexed in a single WDM signal, thus always propagating along the same path, and the same is true for optical carriers. As a result, even though phase wandering between paths results in a slow-varying phase modulation of the output RF signal, it does not affect the end result as the complex weights individually adjusted for each RF sideband are not affected by phase wandering.

Hence, the present invention provides a MPSP that achieves the following advantages:

— It is a coherent MPSP, thus supporting coherent optical signal processing and coherent detection. As shown in equation (3), the optical phase shifts introduced by the PPP (θ₄₂₁ and θ₄₃₁) are equivalent to RF phase shifts;

— The MPSP can be tolerant to phase noise, e.g., provided that each optical carrier and RF sideband pair (421 and 422, and 431 and 432) is generated from the same laser source. If such is the case, coherent detection is in fact self-coherent detection, which enables laser phase noise cancellation; — The MPSP does not require complex phase stabilization loops, as phase wandering does not affect the complex weights individually adjusted for each RF sideband, but only produces a slow-varying phase modulation on the output RF signal;

— The MPSP is based on WDM signals;

— If the number of WDM signals is insufficient to accommodate all input RF signals, the MPSP can be easily scaled to allow more WDM signals. Implementation details are described in embodiments presented further below (Stacks of MPSPs).

Generation of more optical carriers (LOs)

The MPSP, of a further embodiment of the present invention, is shown in Figure 5 and comprises an optical modulator 501 in path 310. Such an optical modulator can be driven with an arbitrary RF signal in orderto generate optical carriers (e.g., 421 and 431). This can have different purposes such as:

• Having more than one RF signal at the output, i.e., different RF signals with different RF frequencies that result from the combination of the RF sidebands 422 and 432 with different complex weights;

• For generating multiple optical carriers from each input optical carrier 310;

• For frequency-shifting the input optical carriers 421 and 431.

In summary, adding the optical modulator 501 provides further flexibility to the MPSP.

Double sideband signals processing capability

The MPSP of the present invention also supports double sideband signals, such that each optical signal comprises upper and lower RF sidebands with associated optical carriers. This case is depicted in Figure 6A. Equation (2) can now be written as:

where ω₁ is the center frequency of signals 421I, 422I and 421u, 422u, and ω₂ is the center frequency of signals 431I, 432I and 431u, 432u. Without loss of generality, let us assume that signals 421I, 422I, 421u, 422u are very far apart from signals 4311, 4321, 431u, 432u, i.e., frequencies ω₁ and ω₂ are very different. Assuming that the photodiode has a limited bandwidth, one can write equation (5) as:

Let us expand equation (6), but once again only consider lower frequency terms resulting in term 442:

Equation (7) is exactly identical to equation (3) if the last term cos (θ_s(t) — θ_OLO(t)) equals 1, thus validating the statement that the disclosed MPSP also supports double sideband signals.

Generalization of the input WDM signals

In the paragraphs above, it is assumed that the first WDM signal 310 only contains optical carriers, and that the second WDM signal 311 contains only RF sidebands. In general, such is not required; it is only required that the optical carrier associated with one RF sideband is not in the same WDM signal as that RF sideband. This means that the WDM signals 310 and 311 may each comprise both optical carriers and RF sidebands. This is illustrated in Figure 6B. A similar derivation as done for equations (1) to (7) done for the WDM signals 610 and 611 depicted in Figure 6B shows that the output photocurrent is phase modulated by θ_s(t) — θ_OLO(t), in contrast to the WDM signals 310 and 311 depicted in Figure 6A, in which the output photocurrent is amplitude modulated by cos (θ_s(t) — θ_OLO(t)).

Figure 7 illustrates a further embodiment of the present invention having a simple way of obtaining WDM signals 610 and 611 from signals 310 and 311 of Figure 6A. A wavelength-dependent optical combiner 701 is used to transform signals 310 and 311 of Figure 6A into signals 610 and 611.

Even though the PPP 301 now has to handle both optical carriers and RF sidebands, such it not a problem as magnitude and phase adjustments imposed by the PPP to a given spectral slice is transparent to the spectrum of the input signal.

True-time delay capability

In the embodiments above, the MPSP is able to combine multiple RF signals with complex weights, i.e., with arbitrary magnitude and phase. However, some applications such as RF filtering and true-time delay beamforming require that input signals be not only combined with complex weights, but also with arbitrary time delays.

Further embodiments of the present invention are as illustrated in Figures 8 and 9. Specifically, Figure 8 shows how such a true-time delay capability may be added to the disclosed MPSP. The input WDM signals 310 and 311 are first processed by wavelength-dependent ODLs 801 and 802, respectively, being then processed by the MPSP as already described. Figure 9 shows an alternative implementation to Figure 8, in which a single wavelength-dependent ODL 901 is placed after the optical combiner 302.

As shown in Figure 10, each RF sideband and/or optical carrier, i.e., 421, 422, 431 and 432, fits in a spectral slot of the wavelength-dependent ODL, respectively 1001a, 1001b, 1001c and lOOld. The wavelength-dependent ODL allows defining the delay of each spectral slot.

In case RF sidebands and optical carriers form signal pairs, e.g., optical carrier 421 with RF sideband 422 and optical carrier 431 with RF sideband 432, each pair can be allocated to a single spectral slot of the wavelength-dependent ODL. Such a case is depicted in Figure 11. A particular advantage of this implementation is that the width of a spectral slot is defined by the spacing between pairs of optical carriers and RF sidebands, instead of being defined by the narrower spacing between one optical carrier and the adjacent RF sideband.

An additional feature of time delay adjustability can be achieved in two different ways. The first is to use a wavelength-dependent ODL 901 with adjustable phase response, such that the time delay of each individual spectral slot can be individually adjusted. The second is to tune the centre wavelength of each optical carrier, RF sideband or RF sideband and optical carrier pair, such that such a signal is allocated to a given spectral slot of the wavelength-dependent ODL 901 having the target time delay. Both options are complementary.

Optical filtering

The embodiments of the present invention disclosed above relate to a MPSP able to provide coloured operation, i.e., applying different attenuations to different spectral bands of a given signal, thanks to the PPP 301, as it allows defining the magnitude of each optical carrier and RF sideband of the first WDM signal 310.

Figure 12 discloses another embodiment of the present invention that includes an optical filter 1201 placed after the optical combiner 302. Such an optical filter 1201 enables a fully coloured operation as it filters the first WDM signal (after being processed by the PPP) combined with the second WDM signal, i.e., the signal that is originally fed to the photodetection stage BOB. As a result, the photodetection stage 303 detects a filtered version of the signal output by the optical combiner 302, for instance only comprising a subset of optical carriers and/or RF sidebands.

Figure 13 discloses an embodiment similar to Figure 12, with the difference that the optical filter 1201 has multiple outputs 1310. Each output 1310 is connected to a photodetection stage 303. Such an implementation includes the case of the optical filter 1201 being a demultiplexer or channelizer; in this case each output port of the optical filter 1201 carries a unique subset of optical carriers and/or RF sidebands.

Generalization to multiple-input multiple-output processing capability

All Figures except for Figure 13 describe SISO systems, as such systems involve a single first WDM signal 310, a single second WDM signal 311, and a single output RF signal 314.

Taking Figure 13 as a basis for multiple-output operation, a generalization for MIMO processing capability can be produced. In general, a MIMO PPP is required, as depicted in Figure 14. Such a PPP has multiple input ports and multiple output ports, and is capable of individually adjusting the magnitude and phase of each optical carrier and RF sideband from any given input WDM signal to any given output WDM signal while, of course, preserving conservation of energy.

Figure 15 shows a possible implementation, according to the present invention, of a MIMO MPSP based on the MIMO PPP depicted in Figure 14. A set of first WDM signals 310 is fed to the MIMO PPP 301 for processing optical carriers and RF sidebands as described in the last paragraph. Copies of the second WDM signal 311 are generated resorting to an optical splitter 1510. Each processed first WDM signal is combined with a copy of a second WDM signal 311 using an optical combiner 302a, 302b or 302c. Each combined signal is converted to an electrical signal using a photodetection stage 303. In summary, the set of first WDM signals 310 are the multiple inputs, whereas the multiple output RF signals 314 are the multiple outputs, thus forming a MIMO system.

Figure 16 shows a further embodiment of the present invention which defines a degenerate case of the MIMO MPSP shown in Figure 15. All WDM signals of the set of first WDM signals BIO are made identical, as a single first WDM signal 310 is split into multiple copies using an optical splitter 1610, which in turn form the set of first WDM signals. As a result, the MIMO MPSP degenerates into a SIMO MPSP.

Figure 17 shows yet another embodiment of a degenerate case of the MIMO MPSP shown in Figure 15. The processed first WDM signals are combined into a single processed first WDM signal 312 using an optical combiner 1710. That signal is combined with a second WDM signal 311, and the resulting signal is converted to an output RF signal 314 by a photodetection stage 303. As a result, the MIMO MPSP degenerates into a MISO MPSP.

In summary, the MIMO MPSP presented in Figure 15 can degenerate straightforwardly into SISO, SIMO and MISO MPSPS.

Stack of MPSPs

The processing capacity of the disclosed MPSP is limited both by the spectral width of the WDMs signal, which in turn limits the number of optical carriers and RF sidebands, and by the number of output RF signals. If more input or output signals are needed, several identical MPSPs can be stacked as shown in Figures 18 and 19.

Figure 18 shows an example, according to the present invention, of how to stack two identical MPSPs 1801 and 1802. Twice as many input signals can be accommodated in comparison with a single MPSP. The output RF signal 314 is obtained by combining the output RF signal of MPSP 1801 with the one of MPSP 1802. The phases of each corresponding pair of output RF signals from both MPSPs 1801 and 1802 should match such that the RF signals are combined in phase. Such can be achieved by resorting to phase stabilization loops (PSLs), described further below.

Figure 19 shows another example, according to the present invention, of how to stack two identical MPSPs 1901 and 1902. Twice as many output RF signals 314 can be accommodated in comparison with a single MPSP. The second WDM signal is split into two copies 311, with each copy 311 serving as input to one MPSP of the stack of MPSPs 1901 and 1902. Contra rily to Figure 18, as the output RF signals are not combined phase stabilization loops are optional. Phase stabilization

Phase wandering between an optical path carrying RF sidebands and an optical path carrying optical carriers, modelled by θ_s(t) — θ_OLO(t) in the equations above, affects the output RF signal. Equation (4) shows that if the output RF signal results from the addition of multiple beats, with each beat resulting from a given optical carrier and a given RF sideband, the output RF signal, I(t), is phase modulated by θ_s(t) — θ_OLO(t). Equation (7) shows that if double sideband signals are considered instead, the output RF signal is amplitude modulated by cos (θ_s(t) — θ_OLO(t))·

Such a phase or amplitude modulation can be ameliorated if one resorts to a phase stabilization loop (PSL) for stabilizing pairs of optical paths, in the present case paths 310 and 311. According to the invention, there are several possible implementations, as described in the embodiments of Figures 20, 21 and 22.

In Figure 20, a phase monitor 2001 estimates θ_s(t) — θ_OLO(t) from the output RF signal 314. Based on such an estimation, the phase monitor 2001 produces a feedback signal 2010 that feeds a phase modulator (PM) 2002. Such a phase modulator tunes θ_s(t), and therefore also θ_s(t) — θ_OLO(t). The phase monitor operates continuously such that θ_s(t) — θ_OLO(t) is set to a constant value, and preferably 0.

Figure 21 shows yet another implementation to Figure 20, however not resorting to a PM 2002. Instead, phase modulation is produced directly by the PPP 301. As a result, the phase shift set by the PPP 301 to each optical carrier or RF sideband is no longer static, but dynamic. More specifically, the difference between phase shifts is kept constant (modulo 2p), but a dynamic common term is added to all phase shifts such that θ_s(t) - θ_OLO(t) is set to a constant value, e.g., [θ₄₂₂, θ₄₃₂, ..] → [θ₄₂₂, θ₄₃₂, ...] + (θ_s(t) — θ_OLO(t) + Const. ). Such an implementation is practical only if the PPP 301 is able to tune quickly enough such that θ_s(t) — θ_OLO(t) can be tracked.

Figure 22 shows yet another implementation to Figure 20, however resorting to a RF PM 2201 for stabilizing the phase of the output RF signal 314. This scheme is not applicable for double sideband signals, as for such signals amplitude modulation of the photocurrent occurs before phase modulation.

In terms of scale, only one PSL is required for a single MPSP. In contrast, a typical coherent MPSP requires one PSL per output RF signal and per input signal. In order to facilitate the estimation of θ_s(t) — θ_OLO(t), one of the input RF sidebands may comprise an unmodulated pilot tone with an RF frequency chosen such that the output photocurrent contains not only the main term at a frequency of ω₄₄₂, but also an out of band tone at ω_pilot· For input single sideband signals and pilot tone, the pilot tone at the output photocurrent is phase modulated only by θ_s(t) — θ_OLO(t) + θ_PSL(t), where θ_PSL(t) is the phase modulation produced by the phase monitor 2001 with the purpose of making θ_s(t) — θ_OLO(t) + θ_PSL(t) a constant value over time. For input double sideband signals and pilot tone, the pilot tone at the output photocurrent is amplitude modulated only by cos (θ_s(t) — θ_OLO(t) + θ_PSL(t)). As a result, monitoring θ_s(t) — θ_OLO(t) + θ_PSL(t) from the pilot tone is straightforward when compared with processing the amplitude and phase modulated main term at a frequency of ω₄₄₂.

Further relevant features

Besides all the embodiments described above, many other features can be included in the MPSP of the present invention. The following is highlighted:

• In general, each WDM signal may comprise one or two RF sidebands, but may also comprise more than one optical carrier. Employing more than one optical carrier can be advantageous for example for including more than one combination of input RF signals in one output RF signal, such that different combinations are allocated to different spectral bands of the output RF signal;

• Any term of the output photocurrent (442, 443, 444, etc.) may comprise the output RF sideband.

The source of the input WDM signals depends on the application:

• In case the input is a free-space optical link, the input signal may already be a WDM signal comprising optical carriers and RF sidebands. An optical interleaver may therefore be used to generate the first and second WDM signals input to the MPSP;

• In case the MPSP is to serve as a microwave photonic filter, the RF sidebands can originate from an unrelated set of RF signals; • If the MPSP is to be used as a photonic beamformer for a PAA receiver, each RF signal originates from an antenna element of the PAA;

• If the MPSP is to be used as a photonic beamformer for a PAA transmitter, each RF signal originates from a beam to be emitted by the PAA.

Beamforming methods

The generic ability that the MPSP has to operate as a MIMO RF signal processor can be used for a specific application: PAA beamforming, in the context of filtering schemes, be it a PAA transmitter or receiver. This section details preferred implementations to achieve so, according to the present invention.

Considering a PAA transmitter, it is fed by a beamformer, which in turn takes as input multiple beams to be emitted by said PAA transmitter, and from such beams generates a set of RF signals that feed the PAA transmitter. Such a method is summarily referred to as beamforming multiple beams emitted by a PAA transmitter, and may comprise the following steps:

• A set of first WDM signals is obtained as follows: o A comb signal is obtained from a comb source; o Multiple copies of the comb signal are generated; o Each copy of the comb signal is modulated by a beam, resulting in a modulated comb signal comprising optical carriers and RF sidebands; o The modulated comb signals comprise a set of first WDM signals;

• Each modulated comb signal is processed by a PPP such that the amplitude and phase of each optical carrier and RF sideband are individually adjusted;

• The processed modulated comb signals are combined into a single output signal of the PPP;

• The output signal of the PPP is combined with a second WDM signal, wherein the second WDM signal is a copy of the comb signal, resulting in a combined signal;

• The combined signal is optically filtered and converted to at least one RF signal;

• Each resulting RF signal is fed to an antenna element of the PAA. This method may further include the modulation of the second WDM signal with an RF signal.

This method may further comprise the additional following feature: the combined signal is first processed by a wavelength-dependent ODL.

Alternatively, a PAA receiver may be considered. In this context, multiple RF signals from a PAA receiver are fed to a beamformer, which in turn separates and outputs multiple beams contained in the multiple RF signals. Such a method is summarily referred to as beamforming multiple beams from the RF signals produced by PAA receiver and may comprise the following steps:

• A first and second WDM signals are obtained as follows:

• A comb signal is obtained from a comb source;

• Each comb signal is modulated by the RF signal produced by an antenna element of the PAA, resulting in a modulated comb signal comprising optical carriers and RF sidebands;

• The modulated comb signals are combined into a first combined signal;

• The first combined signal is channelized by an optical filter, resulting in a first and a second WDM signals;

• Multiple copies of the first WDM signal are generated, forming a set of first WDM signals;

• Multiple copies of the second WDM signal are generated;

• Each WDM signal of the set of first WDM signals is processed by a PPP such that the amplitude and phase of each optical carrier and RF sideband are individually adjusted;

• Each processed WDM signal is combined with a copy of the second WDM signal, resulting in a second combined signal;

• Each second combined signal is converted to an RF signal, wherein such RF signal contains a number of beams defined by how each WDM signal of the first set of WDM signals is processed.

This method may further include the modulation of the second WDM signal with an RF signal. This method may further comprise the additional following feature: the first combined signal is first processed by a wavelength-dependent ODL.

This method may further comprise the additional following feature: the first and second WDM signals are processed by a wavelength-dependent optical combiner.

Modifications are possible in the embodiments described above, and other additional variations and combinations that form additional embodiments are possible within the scope of the claims.

Acronyms

Claims

1. A microwave photonic signal processor (MPSP) for processing a first and a second wavelength-division multiplexing (WDM) signals, each signal comprising a plurality of radio-frequency (RF) sidebands and optical carriers; the MPSP characterized by comprising:

— A programmable photonic processor (PPP) programmed for individually adjusting the magnitude and phase of each optical carrier and RF sideband of the first WDM signal;

— An optical combiner configured for combining an output signal of the PPP with the second WDM signal;

— A photodetection stage adapted for converting a signal outputted by the optical combiner to an electrical signal.

2. A MPSP according to claim 1 wherein the photodetection stage comprises a single-ended photodetector;

3. A MPSP according to claim 1 wherein:

— The optical combiner comprises two outputs; and

— The photodetection stage comprises a balanced photodetector connected to the two outputs of the optical combiner.

4. A MPSP according to any of the previous claims, wherein the PPP is a tuneable optical delay line.

5. A MPSP according to any of the previous claims further comprising an electro optic modulator connected to the input of the PPP; the electro-optic modulator being configured for modulating the first WDM signal.

6. A MPSP according to any of the previous claims further comprising a wavelength- dependent optical combiner; the wavelength-dependent optical combiner being configured for combining first and second WDM signals.

7. A MPSP according to any of the previous claims further comprising two wavelength-dependent optical delay line (ODLs) configured for processing respectively first and second WDM signals.

8. A MPSP according to any of the previous claims further comprising one wavelength-dependent ODL configured for processing the signal output by the optical combiner.

9. A MPSP according to any of the previous claims further comprising an optical filter configured for filtering the signal output by the optical combiner.

10. A MPSP according to claim 9 wherein:

— The optical filter comprises multiple outputs;

— Each output of the optical filter is connected to the photodetection stage.

11. A MPSP according to any of the previous claims wherein the PPP comprises multiple input ports and multiple output ports; the MPSP further comprising:

— A set of first WDM signals, wherein each first WDM signal of said set is fed to an input port of the PPP;

— An optical splitter for splitting the second WDM signal into multiple copies;

— A set of optical combiners; wherein:

— Each output signal of the PPP is combined with one copy of the second WDM signal using one optical combiner of the set of optical combiners;

— The output of each optical combiner is connected to the photodetection stage.

12. A MPSP according to claim 11 further comprising:

— An additional optical splitter fed with a first WDM signal; wherein:

— The output signals of the additional optical splitter comprise the set of first WDM signals.

13. A MPSP according to claims 11 or 12 further comprising:

— An additional optical combiner configured for combining the output signals of the PPP into a single output signal.

14. A MPSP according to any of the previous claims further comprising additional first and second MPSPs and a RF combiner, wherein:

— The RF combiner is fed with an output RF signal of the first MPSP and with an output RF signal of the second MPSP.

15. A MPSP according to any of the previous claims further comprising additional first and second MPSPs and an optical splitter, wherein:

— The optical splitter is fed with a second WDM signal, and thereby outputs two copies of the second WDM signal;

— The first copy of the second WDM signal is fed to the first MPSP;

— The second copy of the second WDM signal is fed to the second MPSP.

16. A MPSP according to any of the previous claims further comprising a phase monitor configured to:

— sample the output signal of the photodetection stage;

— estimate a slow-varying phase modulation from the sampled signal.

17. A MPSP according to claim 16 further comprising an optical phase modulator wherein:

— The optical phase modulator modulates the second WDM signal; — The phase monitor feeds the optical phase modulator with a driving signal.

18. A MPSP according to claims 16 or 17 wherein the phase monitor is configured to program the PPP with a driving signal.

19. A MPSP according to any of the previous claims 16 to 18 further comprising a RF phase modulator wherein:

— The RF phase modulator modulates the output signal of the photodetection stage;

— The phase monitor feeds the RF phase modulator with a driving signal.

20. Method for operating the MPSP of claims 1 to 19 comprising the steps of:

— Processing a first and a second input WDM signals, each WDM signal comprising a set of RF sidebands and optical carriers;

— Individually adjusting the magnitude and phase of each optical carrier and RF sideband of the first WDM signal, resulting in a processed first WDM signal;

— Combining the processed first WDM signal with the second WDM signal, resulting in a combined signal;

— Converting the combined signal to an electrical signal.

21. Method according to claim 20 adapted to perform beamforming of multiple beams emitted by a phased array antenna (PAA) transmitter; the method further comprising the steps of:

— A comb signal is obtained from a comb source;

— Multiple copies of the comb signal are generated;

— Each copy of the comb signal is modulated by a beam, resulting in a modulated comb signal comprising optical carriers and RF sidebands; — Each modulated comb signal is processed by a PPP such that the amplitude and phase of each optical carrier and RF sideband are individually adjusted;

— The processed modulated comb signals are combined into a single output signal of the PPP;

— The output signal of the PPP is combined with a second WDM signal, wherein the second WDM signal is a copy of the comb signal, resulting in a combined signal;

— The combined signal is optically filtered and converted to at least one RF signal;

— Each resulting RF signal is fed to an antenna element of the PAA.

22. Method according to claim 21 further comprising the step of modulating the second WDM signal with an RF signal.

23. Method according to any of the claims 21 or 22 wherein the combined signal is processed by a wavelength-dependent ODL.

24. Method according to claim 20 adapted to perform beamforming of multiple beams from the RF signals produced by a PAA receiver; the method further comprising the steps of:

— A comb signal is obtained from a comb source;

— Each comb signal is modulated by the RF signal produced by an antenna element of the PAA, resulting in a modulated comb signal comprising optical carriers and RF sidebands;

— The modulated comb signals are combined into a first combined signal;

— The first combined signal is channelized by an optical filter, resulting in a first and a second WDM signals;

— Multiple copies of the first WDM signal are generated, forming a set of first WDM signals;

— Multiple copies of the second WDM signal are generated; — Each WDM signal of the first set of WDM signals is processed by a PPP such that the amplitude and phase of each optical carrier and RF sideband are individually adjusted;

— Each processed WDM signal is combined with a copy of the second WDM signal, resulting in a second combined signal;

— Each second combined signal is converted to an RF signal, wherein such RF signal contains a number of beams defined by how each WDM signal of the first set of WDM signals is processed.

25. Method according to claim 24 further comprising the step of modulating the second WDM signal with an RF signal.

26. Method according to claim 24 wherein the first combined signal is processed by a wavelength-dependent ODL.

27. Method according to claim 24 wherein the first and second WDM signals are processed by a wavelength-dependent optical combiner.