Method and apparatus for implementing at least first recurrent unit of recurrent optical neural network

Abstract

The invention relates to a method for implementing at least a first circulating unit (1) of a circulating optical neural network by means of an optical waveguide (2), each subsequent laser pulse pair (P1-P3) consisting of a control signal pulse (C1-C3) and a data signal pulse (D1-D3) being propagated back through the waveguide (2), the waveguide (2) is configured such that, for a given pulse pair (P1-P3), a control signal pulse (C1-C3) is coupled to a first end (4) of the waveguide (2) and propagates towards a second end (5) of the waveguide (2), and a data signal pulse (D1-D3) is coupled into the second end (5) of the waveguide (2) and propagates towards the first end (4) of the waveguide (2), where, for the given pulse pair (P1-P3), the control signal pulse (C1-C3) is coupled to the first end (4) of the waveguide (2) and the data signal pulse (D1-D3) is coupled to the second end (5) of the waveguide (2) and propagates towards the first end (4) of the waveguide (2). The optical frequency of the data signal pulses (D1-D3) is set to be higher than the optical frequency of the control signal pulses (C1-C3) of the same pulse pair (P1-P3) in the bandwidth of the Brillouin frequency shift of the waveguide (2). And wherein a time delay (dT12, dT23) between the first pulse pair (P1) and a subsequent second pulse pair (P2) is set to be less than a decay time of the acoustic waves (10) generated by the stimulated Brillouin scattering process in the waveguide (2) such that a second data signal pulse (D2) of the second pulse pair (P2) propagates through the waveguide (2), a dependency of the second data signal pulse (D2) on the first data signal pulse (D1) of the first pulse pair (P2) is caused by a first acoustic wave (10) generated by the stimulated Brillouin scattering process.

Description

Technical Field

[0001] The present invention relates to a method for realizing at least a first recurrent unit of a recurrent optical neural network by means of an optical waveguide, wherein each subsequent laser pulse pair consisting of a control signal pulse and a data signal pulse is counter-propagated through the waveguide. The present invention also relates to an apparatus for realizing at least a first recurrent unit of a recurrent optical neural network. Background Art

[0002] In everyday life, there are many tasks that require memory to be solved correctly. This is one reason why recurrent neural networks (RNNs) have been developed as a branch of artificial neural networks (ANNs). RNNs are ANNs with so-called recurrent neurons, which allow them to memorize and store input signals for a specific period of time. This feature allows ANNs to process current input based on past input.

[0003] The recurrent neurons of RNNs have a self-referential component that acts as a neuron's memory, which enables RNNs to recognize and predict sequences. RNNs are used in many applications, including language, video, and image processing. For example, RNNs have been used to generate complex text.

[0004] However, the possibilities offered by ANNs and RNNs come at the cost of computationally intensive model training, ultimately leading to high energy consumption. Furthermore, the computing power of data centers used to train ANNs is limited in performance by the von Neumann processing units used to implement them. Consequently, researchers are investigating more specific non-von Neumann architectures designed specifically for machine learning and deep learning applications.

[0005] One possible solution to overcome current limitations could be to transfer electronic ANNs into the optical domain. The resulting optical neural networks (ONNs) have attracted significant attention over the past few decades due to their promise of high processing speed and bandwidth, as well as low dissipation losses at ambient temperature. In particular, the possibility of folding many computational steps into a single process—which is natural in most ONN designs—could lead to a strong increase in computational efficiency.

[0006] While the field of optical ANNs within ONNs has made tremendous progress in the past few years, the field of recurrent ONNs (RONNs) remains relatively narrow. Currently, there are only a few approaches to implement recurrent operations within ONNs, such as those based on optical reservoir computing, free-space optics, or inverse design. In particular, fully integrated solutions remain scarce. Summary of the Invention

[0007] It is an object of the present invention to present an implementation of the recurrent operation for a recurrent optical neural network in a way that allows integration into other systems, for scalability and with the best possible properties regarding the stability of the recurrent operation.

[0008] According to the invention, this object is achieved by a method for realizing at least a first recurrent unit of a recurrent optical neural network with the aid of an optical waveguide, wherein each subsequent laser pulse pair consisting of a control signal pulse and a data signal pulse is counter-propagated through the waveguide such that for a given pulse pair the control signal pulse is coupled into a first end of the waveguide and propagates towards a second end of the waveguide, and the data signal pulse is coupled into said second end of the waveguide and propagates towards said first end of the waveguide.

[0009] The method requires that, for a given pulse pair, the optical frequency of the data signal pulse is set higher than the optical frequency of the control signal pulse within the bandwidth of the Brillouin frequency shift of the waveguide, and / or the difference between the optical frequency of the data signal pulse and the sum of the optical frequency of the control signal pulse and the Brillouin frequency shift of the same pulse pair is set to be within the bandwidth of the Brillouin frequency shift, and the time delay between a first pulse pair and a subsequent second pulse pair is set to be less than the decay time of the acoustic wave generated by the stimulated Brillouin scattering process in the waveguide, so that by propagating the second data signal pulse of the second pulse pair through the waveguide, the dependence of the second data signal pulse on the first data signal pulse of the first pulse pair is caused via the first acoustic wave generated by the stimulated Brillouin scattering process. Particularly advantageous embodiments are outlined in the dependent claims and the following description, which embodiments may themselves be inventive.

[0010] The object of the present invention is also achieved by a device for implementing at least a first cyclic unit of a recurrent optical neural network, the device comprising: an optical processing element, the optical processing element comprising an optical waveguide; a device for generating subsequent laser pulse pairs, each subsequent laser pulse pair consisting of a control signal pulse and a data signal pulse, wherein the device for generating pulse pairs is configured to set the optical frequency of the control signal pulse of the pulse pair to be higher than the optical frequency of the data signal pulse of the same pulse pair within the bandwidth of the Brillouin frequency shift of the waveguide, and / or to set the difference between the optical frequency of the data signal pulse and the sum of the optical frequency of the control signal pulse of the same pulse pair and the Brillouin frequency shift to be within the bandwidth of the Brillouin frequency shift, and the device also includes a device for coupling the control signal pulse of the subsequent pulse pair to the first end of the waveguide, and a device for coupling the data signal pulse to the second end of the waveguide.

[0011] The device is configured to set a time delay between a first pulse pair and two subsequent second pulse pairs to be less than a decay time of phonons generated by stimulated Brillouin scattering in the waveguide, so that a second data signal pulse of the second pulse pair propagates through the waveguide, causing dependence of the second data signal pulse on the first data signal pulse of the first pulse pair via a first acoustic wave generated by the stimulated Brillouin scattering process.

[0012] The device for implementing at least a first recurrent unit of a recurrent optical neural network shares the advantages of the corresponding previously described method. The features, advantageous embodiments and specific assets of the described method for implementing at least a first recurrent unit of a recurrent optical neural network can be directly transferred to the device, mutatis mutandis.

[0013] While the concept of an artificial neural network (ANN) generally refers to the abstract computational concept of information processing via a specific algorithm with specific properties, regardless of its physical implementation, the concept of an optical neural network (ONN) should be understood here to refer to any device designed and configured to process optical information (such as light pulses, preferably laser pulses, particularly with phase and / or amplitude modulations for encoding said information) comprising a plurality of different nodes (or neurons), each configured for interaction with a different entity of said optical information, and the nodes being connected via optical waveguides, wherein each node is connected to at least one other node. In this regard, a recurrent optical neural network (RONN) shall refer to an ONN comprising at least one recurrent node, which, by definition, is a node configured to store output information of the node for subsequent input operations. A recurrent node (or recurrent neuron) comprises a recurrent unit configured to perform a recurrent operation, i.e., an operation using any kind of memory of previously stored inputs, and may also comprise a module configured to implement a mapping, particularly a nonlinear mapping, of received inputs to outputs, particularly with the aid of so-called (nonlinear) activation functions. In this respect, the method for implementing at least a first cyclet unit of a RONN may in particular be used for implementing a cycle operation in a RONN and / or an implementation of a cycle node of a RONN.

[0014] The method implements such a circulating unit by using subsequent pairs of laser pulses, each of which includes a corresponding control signal pulse and a data signal pulse, and by causing the two pulses to propagate in opposite directions through a waveguide. That is, the two pulses of each pulse pair are coupled into the waveguide at opposite ends thereof so that the propagation direction of the control signal pulse is opposite to the propagation direction of the data signal pulse in the waveguide. An optical waveguide, preferably an optical fiber, is used as the underlying physical device. Preferably, the control signal pulse and the data signal pulse of each pulse pair are polarization-matched.

[0015] For a given pulse pair, the condition that the optical frequency of the data signal pulse is set higher than the optical frequency of the control signal pulse within the bandwidth of the Brillouin frequency shift of the waveguide is preferably understood as follows: the optical frequency v of the data signal pulse of the pulse pair d should be set to the optical frequency v of the control signal pulse c Add another frequency v a Given, with a Brillouin frequency shift v b Bandwidth Δ b Inside, that is, v d =v c +v a , where |v a -v b |≤Δ b / 2, or |v d -v c -v b |≤Δ b / 2. Brillouin frequency shift v b Bandwidth Δ b represents the frequency range that the acoustic waves generated by the Brillouin scattering process can reach and may depend in particular on the geometry and / or material properties of the waveguide and / or the pulse length of the control and / or data signal pulses. In particular, the Brillouin frequency shift v b Bandwidth Δ b can be obtained in the frequency domain by the convolution bandwidth of the waveguide with the spectral bandwidth due to the finite pulse length. Preferably, the Brillouin frequency shift v b It can be in the range of 1GHz to 25GHz.

[0016] The term Brillouin scattering denotes the scattering of light waves on acoustic waves, or equivalently, the scattering of photons with acoustic phonons (hereinafter, the term "phonon" shall denote acoustic phonons). Acoustic waves are propagating pressure variations that produce periodic oscillating density and, therefore, periodically varying refractive index in the medium. The inelastic interaction of light waves with these refractive index variations, i.e. the above-mentioned inelastic interaction of photons and phonons, results in a frequency shift of the scattered light that depends on the speed of the acoustic waves. Details on the principle of Brillouin scattering can be found in C. Wolff, M. Smith, B. Stiller, and C. Poulton, "Brillouin scattering-theory and experiment: tutorial", JOSA B, vol. 38, no. 4, pp. 1243-1269, 2021.

[0017] In the present case of stimulated Brillouin scattering (SBS), instead of a single pump beam, a control signal pulse and a counter-propagating data signal pulse result in interference between the control signal pulse and the data signal pulse, i.e. the amplitude variation of the generated superposition signal oscillates with the difference frequency, which is set in a small bandwidth window around the Brillouin frequency shift frequency. If the correct phase matching conditions are met, the counter-propagating control signal pulse and the data signal pulse may result in density oscillations (i.e. pressure / density waves) due to electrostriction of the fiber medium, i.e. due to the excitation or enhancement of the acoustic waves, by adding phonons in specific acoustic modes. The acoustic field / pressure wave changes the dielectric field and, in particular, generates a refractive index grating inside the medium. The data signal pulse is scattered by the refractive index grating, generating a phonon. Due to energy conservation, the energy of this phonon is then taken from the data signal pulse, reducing its frequency by the amount of the frequency of the generated or excited phonon, i.e. the Brillouin frequency shift. The addition of the phonon to the density pool enhances the process and thus additionally drives it to produce stimulated Brillouin scattering. The so-called Brillouin frequency shift can be derived as v b =2nV a / λ c , where V a represents the speed of sound in the optical fiber, n is its refractive index, and λ c is the wavelength of the control signal pulse. Furthermore, momentum conservation conditions apply to the phonons generated by both the data and control signal pulses. The initial and backscattered light fields interfere with each other, accelerating the formation of acoustic waves, which in turn increases the amount of backscattered light. This creates a feedback loop that significantly enhances the efficiency of Brillouin scattering.

[0018] Because the SBS process is coherent, the optical information encoded in the first data signal pulse (i.e., its amplitude and / or phase of the first pulse pair) can be transferred to the first acoustic wave generated by the first pulse pair (i.e., to its phonons) via the SBS process. Therefore, the phonons generated by the SBS can serve as a memory for the optical information encoded in the phase and / or amplitude of the data signal pulse.

[0019] A second pulse pair following the first pulse pair is then coupled into the waveguide. Specifically, the second control signal pulse of the second pulse pair is preferably coupled into the first end of the waveguide, and the second data signal pulse of the second pulse pair is coupled into the second end of the waveguide, so that the second data pulse propagates counterclockwise through the waveguide relative to the second control signal pulse. The delay time between the first and second pulse pairs, preferably defined as the time between two interactions of the two pulses of each pulse pair within the spatially confined region, is set to be less than the decay time of the SBS-generated phonons in the waveguide. The second data signal pulse can then interact with the first acoustic wave generated by the SBS process induced by the first pulse pair, thereby retrieving the optical information of the first data signal pulse stored in the first acoustic wave. The optical information of the outgoing second data signal pulse, i.e., the second data signal pulse after interacting with the first acoustic wave, has then been modified by the first acoustic wave and, in turn, by the optical information of the first data signal pulse. In this way, an optical recycling operation is achieved, and thus a RONN recycling unit is realized. The dynamics of SBS can be described by coupled partial differential equations for the electric field and acoustic waves. By applying appropriate approximations to the current situation (in particular, the subsequent pulse pair, the rotating frame, and the slowly varying wave), these are transformed into coupled partial differential equations for the two optical modes and for the acoustic wave. The presence of a non-zero acoustic wave in the second pulse pair is then an initial condition that drastically changes the dynamics of the entire system.

[0020] The cycle cell implemented as described above is very stable to temperature changes and strain changes, which only result in small changes in the Brillouin frequency shift. The decay time is on the order of 10 ns (and slightly above, i.e., possibly around 12 ns), making it possible to generate subsequent pulse pairs of short pulses (e.g., 2 ns short), which is fully achievable using the present technology for implementation.

[0021] Preferably, for a given pulse pair, the intensity of the interaction between the corresponding control signal pulse and the data signal pulse, which generates the acoustic wave via the stimulated Brillouin scattering process, is controlled by the amplitude and / or envelope and / or optical power of the control signal pulse. This allows adjustment of how much optical information of the first data signal pulse will be transferred to the first acoustic wave and will be available for the subsequent second data signal pulse.

[0022] In one embodiment, for each pulse pair, the optical power of the control signal pulse is at least 10 dB, preferably at least 15 dB, and most preferably at least 20 dB higher than the optical power of the data signal pulse, and / or the optical power of the control signal pulse varies by at most + / - 10 dB, preferably at most + / - 5 dB, and most preferably at most + / - 3 dB, for controlling the intensity of the interaction between the control signal pulse and the data signal pulse. Preferably, the optical power of the control signal pulse is at most 30 dB higher than the optical power of the data signal pulse. The relationship between the optical powers given here ensures a high efficiency of the SBS process and, therefore, the generation of the acoustic waves required for the cycle cell. The given value of the variation of the optical power for controlling the intensity of the interaction ensures that the efficiency of the interaction is not impaired.

[0023] In one embodiment, for a given pulse pair, the difference between the optical frequency of the data signal pulse and the sum of the optical frequency and the Brillouin frequency shift of the control signal pulse of the same pulse pair is set to match the Brillouin frequency shift, i.e., v d -v c -v b = 0, where the optical frequency of the data signal pulse v d and the optical frequency v of the control signal pulse c , and the Brillouin frequency shift v b In this way, SBS has the highest efficiency.

[0024] In an alternative or additional embodiment, for at least one given pulse pair, the difference between the optical frequency of the data signal pulse and the sum of the optical frequency and the Brillouin frequency shift of the control signal pulse of the same pulse pair is set to be different from the Brillouin frequency shift, i.e. |v d -v c -v b |>0.

[0025] This means that, at least for a particular pulse pair, the difference in optical frequency of the data signal pulse and the control signal pulse has a "detuning" relative to the Brillouin frequency shift, in that it deviates from the exact Brillouin frequency shift that yields the highest efficiency. The detuning causes a phase shift to be "imprinted" on the acoustic wave, said phase shift propagating through the waveguide depending on the detuning. Subsequent acoustic waves generated by subsequent pulse pairs will then interfere with the phase-shifted acoustic wave, either only in phase (if the subsequent pulse pairs have the same detuning) or in both phase and frequency (if the subsequent pulse pairs have a different frequency difference, for example, if their optical frequency difference lies within the bandwidth of the Brillouin frequency shift).

[0026] The possibility of acoustic interference of acoustic waves generated by two subsequent optical pulse pairs via detuning of at least one of the pulse pairs provides an additional degree of freedom and introduces further nonlinearity into the system, increasing system complexity. This nonlinearity can be exploited to implement "extreme learning machines" or other types of machine learning by optical means.

[0027] In an embodiment, sets of subsequent pulse pairs are prepared in different optical frequency bands, each frequency band containing a different set of subsequent pulse pairs, wherein for each frequency band, the difference between the optical frequencies of the control signal pulses and the data signal pulses of the corresponding pulse pairs in that frequency band is set within the bandwidth of the Brillouin frequency shift of the waveguide so as to realize multiple parallel cycle cells in the frequency domain. For example, a "red" set of subsequent lower frequency pulse pairs - each of which includes their corresponding control signal pulse and data signal pulse (separated by the Brillouin frequency shift in the frequency domain) - as well as a "green" set of subsequent intermediate frequency pulse pairs and a "blue" set of subsequent higher frequency pulse pairs (i.e., red / green / blue first / second / ... control / data signal pulses) can be prepared so as to realize a "red" cycle cell, a "green" cycle cell, and a "blue" cycle cell in parallel in the waveguide. Thus, both the high frequency stability and frequency selectivity of the SBS process are optimally exploited.

[0028] Preferably, for this purpose, the bandwidth of the optical frequency band, which is given by the difference between the frequencies of two adjacent control signal pulses and / or the frequencies of two adjacent data signal pulses, is set as a function of the Brillouin frequency shift in the waveguide and / or the pulse width of the control signal pulses and / or the data signal pulses of a pulse pair, respectively. This includes in particular that "stacking" of parallel cyclets in the frequency domain can be performed as a function of the required space in the frequency domain for each of the cyclets, which is in particular given by the Brillouin frequency shift, but may also be influenced by the pulse width (for ultrashort pulses).

[0029] In a preferred embodiment, the pulse length of the control signal pulse and / or data signal pulse, e.g. the first and / or second control and / or data signal pulse, is set to at least 10 ps, ​​preferably at least 100 ps, ​​most preferably at least 500 ps, ​​and / or wherein a time delay (i.e. a dead time) between two subsequent pulse pairs, e.g. between the first pulse pair and the second pulse pair, is set depending on the decay time of the acoustic wave in the waveguide. In particular, the dead time can be selected to be at most half of the decay time, preferably 1 / 4 of the decay time. In particular, the dead time between two subsequent pulse pairs is set to the order of magnitude of the pulse length of the corresponding pulse. In particular, the pulse length of the mentioned pulses can be set to at most 100 ns, preferably at most 10 ns, and most preferably at most 1 ns. These pulse lengths and dead times are technically feasible with reasonable effort and ensure that at least two subsequent pulse pairs can be achieved before the acoustic wave generated by the SBS decays.

[0030] In an embodiment, a reset operation is performed on at least one cycle cell by erasing the acoustic wave in the waveguide with the aid of a unique control signal pulse without a corresponding data signal pulse. Sometimes, it may be useful to "clear" the memory of the cycle cell, which can be achieved by this reset operation. In cases where it is desired that a particular data signal pulse pass through the cycle cell without being affected by the cycle cell, i.e., without interacting with optical information already stored in the acoustic wave by an earlier data signal pulse, it is sufficient to propagate the particular data signal pulse through the waveguide without a counter-propagating control signal pulse, so that there is no SBS interaction of the particular data signal pulse with the acoustic wave generated from the earlier data signal pulse and the control signal pulse.

[0031] Preferably, an optical fiber is used as the waveguide, in particular a polarization preserving optical fiber is used as the waveguide.The polarization degree of freedom can then be used to encode further optical information.

[0032] In this regard, in another embodiment, a first pulse pair having corresponding first control signal pulses and first data signal pulses is prepared in one of the two retained polarizations, and a reset operation is performed by a control signal pulse in the other retained polarization. Although the second control signal pulse does not interact with the first pulse pair given in the other polarization, it still interacts with the acoustic wave generated by the first pulse pair to perform the reset operation.

[0033] In a further preferred embodiment, a cyclic unit chain is realized in the waveguide by means of a plurality of subsequent pulse pairs of corresponding control signal pulses and data signal pulses, wherein the number of pulse pairs is set as a function of the pulse length, in particular as a function of an upper limit on the pulse length of the control and / or data signal pulses, and / or as a function of an upper limit on the dead time between two subsequent pulse pairs, and / or as a function of the decay time of the acoustic waves generated by the stimulated Brillouin scattering process.

[0034] In a preferred embodiment, the optical waveguide allows for interaction of multiple "cross pulse pairs" along its length, i.e., the length of the optical waveguide is selected depending on the time delay and / or the decay time of the acoustic waves generated in the waveguide by stimulated Brillouin scattering, such that a control signal pulse of a first pulse pair first interacts with a corresponding data signal pulse of the first pulse pair and, after further propagation through the waveguide, interacts with a data signal pulse of a subsequent second pulse pair, and / or a data signal pulse of a first pulse pair first interacts with a corresponding control signal pulse of the first pulse pair and, after conversion (by generating acoustic waves via an SBS process) and further propagation through the waveguide, interacts with a control signal pulse of a subsequent second pulse pair. In this way, more complex systems incorporating the desired cyclic operation can be realized, in particular (but not limited thereto) in combination with the aforementioned detuning of at least one of the pulse pairs.

[0035] The present invention also discloses a RONN comprising a device for implementing at least a first recurrent unit of a recurrent optical neural network as described above, and / or comprising a recurrent unit implemented by the device as described above. Preferably, the RONN is implemented as an integrated design on a single chip. Most preferably, the RONN comprises an erbium-doped fiber amplifier (EDFA). Specifically, the RONN comprises a device for generating subsequent laser pulse pairs as described above, preferably at an input layer, wherein the processing element comprises a device for retrieving the subsequent laser pulse pairs, for example at an inner layer (but this should not constitute any limitation). Preferably, the RONN comprises a plurality of optical processing elements, each optical processing element implementing a recurrent unit as described above.

[0036] The RONN according to the invention shares the advantages of the above-described method and device for realizing at least the first circulation unit of the RONN. The features, advantageous embodiments and specific assets of the described method and device for realizing at least the first circulation unit of the RONN can be directly transferred mutatis mutandis to the RONN itself. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] The characteristics, properties and advantages of the invention described above will now be described with the aid of drawings illustrating exemplary embodiments.

[0038] Figure 1Snapshots at four different times of the schematic signal flow of a method for implementing a recurrent unit for RONN are shown,

[0039] Figure 2 A schematic block diagram of a device for implementing a recurrent unit for RONN including a processing element is shown,

[0040] Figure 3 Shown includes Figure 2 A schematic block diagram of RONN with a processing element of

[0041] In each case where they appear in all the figures, corresponding components and variables are provided with the same reference numerals. Detailed Description

[0042] In Figure 1 A method for implementing recurrent unit 1 for RONN is shown by four different-time snapshots of the schematic signal flow of the method. The snapshots are shown at time t0 (upper left), t1 (upper right), t2 (lower left), and t3 (lower right), where t0 < t1 < t@2 < t3. The method employs an optical waveguide 2, which in this example is given by an optical fiber 3. The optical fiber includes a first end 4 and a second end 5.

[0043] For this method, subsequent laser pulse pairs P1, P2, P3 (each pulse pair P1 - P3 consisting of a respective control signal pulse C1 - C3 and data signal pulse D1 - D3) propagate in the reverse direction through the optical fiber 3, i.e., the first control signal pulse C1 of the first pulse pair P1 is coupled into the first end 4 of the optical fiber 3 and propagates towards its second end 5, while the first data signal pulse D1 of the first pulse pair P1 is coupled into the second end 5 of the optical fiber 3 and propagates towards the first end 4. For this example embodiment, each of the control signal pulses C1 - C3 and data signal pulses D1 - D3 should have the same pulse length dL, but this is not a necessary requirement, and the mentioned pulses can be relaxed to have pulse lengths of the same order of magnitude, preferably varying by at most a factor of 2 (i.e., preferably, max(dL) < 2min(dL)). The first pulse pair P1 and the second pulse pair P2 have a delay dT12, i.e., the delay between the first data signal pulse D1 and the second data signal pulse D2 of the second pulse pair P2 is dT12, and the delay between the first control signal pulse C1 and the second control signal pulse C2 of the second pulse pair P2 is also dT12. For this embodiment, the pulse length dL should be 500 ps, and the delay dT12 should be 1.5 ns.

[0044] As will be described in more detail below, the optical frequency v of any one of the data signal pulses D1 - D3 d (in this embodiment, v dshould be the same for all data signal pulses D1-D3, but this condition can also be relaxed) is set to be higher than the optical frequency v of any of the control signal pulses C1-C3 c The Brillouin frequency shift v b (In this embodiment, it should also be the same for all control signal pulses C1-C3, but this condition can also be relaxed), that is, v d =v c +v b The frequency relationship (at the Brillouin frequency shift v b is a condition for obtaining a reliable and efficient SBS process. However, a small detuning as described above can also be induced for at least some pulse pairs.

[0045] At time t0 (i.e., the upper left snapshot), the first control signal pulse C1 and the first data signal pulse D1 of the first pulse pair P1 are coupled to their respective first and second ends 4 and 5 of the optical fiber 3. Once the first data signal pulse D1 and the first control signal pulse C1 meet within the optical fiber 3, the SBS process transforms the coherent optical energy of the data signal pulse D1 into an acoustic wave, generating an acoustic wave 10 within the optical fiber 3. This acoustic wave 10 resides within the optical fiber 3 and is quasi-static compared to the laser pulse. Furthermore, the acoustic wave 10 (i.e., the acoustic wave) decays exponentially with time, with the acoustic lifetime depending on the material properties. Furthermore, the described process is a high-frequency selective process, which also allows for multi-frequency cycling operation. Through the SBS process, the first data signal pulse D1 depletes and loses intensity, and is thus transformed into a transformed first data signal pulse D1*, as can be seen in the upper right snapshot, i.e., at time t1 (when the first control signal pulse C1 and the first data signal pulse D1 have already propagated through the optical fiber 3). The first control signal pulse C1 does not undergo any significant transformation from the propagation.

[0046] In this example, the acoustic wave 10 has a decay time of approximately 10-12 ns, so that due to the relatively short delay dT12 between the first pulse pair P1 and the subsequent second pulse pair P2, the second control and data pulses C2, D2 are coupled into the optical fiber and propagate through the optical fiber in opposite directions (relative to each other) while the acoustic wave 10 is still in the optical fiber. The second data signal pulse D2 not only interacts with the second control signal pulse C2 during its propagation through the optical fiber 3, but also interacts with the acoustic wave 10 carrying the optical information 12 stored by the first data signal pulse D1 (e.g., encoded in the phase and / or amplitude of the first data signal pulse D1). Thus, the acoustic wave 10 can serve as a memory for the optical information 12 of the first data signal pulse D1, and this optical information 12 can then interact with the optical information encoded in the second data signal pulse D2 as the second pulse pair P2 propagates through the optical fiber.

[0047] Upon propagating through the optical fiber 3 and interacting with the optical information 12 stored in the acoustic wave 10, the second data signal pulse D2 also depletes and is transformed into a transformed second data signal pulse D2*, while the second control signal pulse C2 does not undergo any significant transformation from the propagation. This is shown in the lower left snapshot (t2). Note that the transformed second data signal pulse D2* depends on the first data signal pulse D1, i.e., D2*=D2*(D1).

[0048] Finally, at t3, the third control signal pulse C3 and the third data signal pulse D3 of the third pulse pair P3 have propagated through the optical fiber 3 with a delay of dT23 relative to the previous second pulse pair P2 (here, dT23 should be equal to dT12=1.5ns). Again, the corresponding data signal pulse (here, the third data signal pulse D3) has previously interacted with the acoustic wave 10 and the optical information 12 stored therein through the previous first data signal pulse D1 and the second data signal pulse D2, and has been transformed by this interaction into a transformed third data signal pulse D3*=D3*(D2, D1), while the corresponding control signal pulse (here, the third control signal pulse C3) has not undergone any significant transformation from the propagation (see time t3, i.e., the lower right snapshot).

[0049] exist Figure 2 In FIG. 1 , a schematic block diagram of a device 15 for implementing a cyclic unit of RONN is shown. A laser device 20 generates a continuous wave (CW) laser 22. The CW laser 22 is split into a data branch 24 and a control branch 26 via a beam splitter (not shown). Figure 1 For each pulse pair P1, P2, P3 shown, a corresponding control signal pulse C1-C3 is prepared in the control branch 26 of the device 15, and a corresponding data signal pulse D1-D3 is prepared in the data branch 24 of the device 15. For this purpose, the optical frequency v of the CW laser 22 in the control branch 26 is cw The Brillouin frequency shift v through waveguide 2 b The resulting optical frequency v towards the control signal pulse c Such a frequency shift can be achieved in different ways, for example using a high frequency radio frequency (RF) source and an optical IQ modulator 28. In this case, the optical frequency v of the CW laser 22 is cw The optical frequency v of the data signal pulse is taken as d .

[0050] Alternatively, the optical frequency v of the CW laser in the data branch 24 cw The Brillouin frequency shift v through waveguide 2 b Optical frequency v towards the data signal pulse dShift up (dashed box 28’ and dashed arrow). In this case, the optical frequency v of the CW laser 22 cw is taken as the optical frequency of the control signal pulse v c . For the multi-frequency implementation of the loop unit 1, a multi-tone RF source can be used to generate the required multi-tone, and additional spectral filtering can be added to the optical path to ensure a clean multi-tone spectrum.

[0051] Next, both the CW laser 22 in the data branch 24 and the downshift CW laser 22’ in the control branch 26 are transformed into synchronous pulse sequences, thereby generating subsequent pulse pairs Pj (in the Figure 1 example shown, j = 1, 2, 3) via intensity modulators IMc, IMd driven by the RF pulse generator 30. The individual control signal pulses Cj and data signal pulses Dj will contain information for later calculation steps and can be separated by a dead time (i.e., delay dTj,j+1 < Tac), where Tac is the decay time of the acoustic wave in the optical fiber 3, which serves as the optical waveguide 2 for SBS interaction. For example, the second pulse pair P2 can be separated from the first pulse pair P1 by a dead time (delay) dT12 < Tac, while the third pulse pair P3 can be separated from the second pulse pair P2 by another dead time (delay) dT23 and dT23 ≠ dT12, or by the same dead time (delay) dT23 = dT12.

[0052] In a practical implementation, the RF pulse generator 30 preferably includes two independent but synchronous outputs to drive the intensity modulators IMc, IMd. Independence is important to match the data control interaction in the waveguide 2 and possibly compensate for different optical path lengths in the data and control branches 24, 26. Synchronization of the outputs of the intensity modulators IMc, IMd with a common frequency reference given by the RF pulse generator 30 helps to ensure the same behavior of the control signal pulses and data signal pulses in the frequency domain. Note that in an alternative embodiment (not shown), this can also be achieved using two RF sources configured in a pilot and tracking configuration. Additionally, the pulses generated by means of the RF pulse generator 30 and the corresponding intensity modulators IMc, IMd can have any envelope shape, while a rectangular shape proves to be the most suitable. The data signal pulses Dj and control signal pulses Cj generated in this way do not necessarily have the same pulse width; however, better SBS efficiency is achieved if the control signal pulses Cj have the same or shorter pulse width than the corresponding data signal pulses Dj. Similarly, the control signal pulses Cj should have an optical power preferably 10 - 20 dB higher than the optical power of the corresponding data signal pulses Dj to ensure high SBS processing efficiency.

[0053] Next, the prepared data signal pulses Dj can now be directed to an optical data processor unit 32, such as an ONN, where they are manipulated in amplitude and / or phase in order to encode the data to be transmitted to the optical data processor unit 32 by means of an SBS process. Figure 1 The optical information of the acoustic wave 10 is encoded. Alternatively (not shown), information encoding can also be performed at an earlier stage by the intensity modulator IMd (and / or possibly by the optical IQ modulator 28' of the data branch 24). The control signal pulses Cj are directed to a high-power erbium-doped fiber amplifier (EDFA) 34, a 1 nm bandpass filter 36, and a polarization controller (not shown). The bandpass filter 36 reduces the amplification noise introduced by the amplified spontaneous emission (ASE) of the EDFA 34.

[0054] Finally, both the control signal pulse Cj and the data signal pulse Dj are simultaneously coupled into their respective first and second ends 4 and 5 of the waveguide 2 (with the aid of an optical circulator 38 for separating the incoming signal pulses from the counter-propagating outgoing signal pulses that have left the waveguide). Once they interact in the waveguide 2, they induce an SBS process. It is important to note that there will be no interaction between the data signal pulse Dj and the control signal pulse Cj outside the waveguide 2 because the frequency detuning of the control signal pulse and the data signal pulse is specifically set to the Brillouin frequency shift v of the waveguide 2. b The optical circulator 38 provides access not only to the post-SBS, transformed data signal pulses Dj*, but also to the post-SBS control signal pulses Cj, which can then be reused for another interaction (not shown).

[0055] In addition, for a simple implementation of the cyclic unit 1, the length of the waveguide 2 should be selected so that the data signal pulse Dj interacts only with its corresponding control signal pulse Cj and not with any other control signal pulse Ck, with k≠j. For more complex optical manipulations, such as one data signal pulse interacting with more than one control signal pulse during propagation through the waveguide, the length of the waveguide 2 can be increased to induce several, but spatially separated, SBS processes.

[0056] One possible application of the recurrent unit 1 is in optical neural networks (ONNs), as it can directly promote "traditional" neurons (without any memory) to recurrent neurons. The output of the output neuron module can be fed (on-chip) into a recurrent unit with synchronized recurrent neurons. The synchronization of the individual recurrent neurons enables the reuse of a single control signal pulse for multiple recurrent neurons. Therefore, each recurrent neuron should have a delay line to ensure that the control pulse arrives at the next recurrent neuron at the same time as the input of the corresponding recurrent neuron. This means that the synchronization required to use only one control signal pulse in different recurrent neurons (i.e., different recurrent units) is provided by the control signal pulse itself. Alternatively, the delays in the different input paths to the individual recurrent neurons can be matched to the propagation time of the control signal pulse. With this synchronization via the data signal pulse, there is no need for control signal pulse delay. In a multi-frequency embodiment, the control signal pulse also contains several frequency components, each of which specifies the strength of a separate SBS process in each frequency channel. After the input of the recurrent neuron is processed by SBS and the recurrent neuron's internal nonlinear activation function (NLA), it can be passed to the next layer of the network.

[0057] Figure 3 A block diagram of a RONN 40 implemented on a single photonic chip 42 is schematically shown. RONN 40 comprises N layers X1, ..., XN, connected by corresponding edges ejk. Each set of edges ejk between two consecutive layers Xk, Xk+1 (k=1, ..., N-1) is represented by a matrix operation Wk (the matrix Wk contains the corresponding weights for propagating information from layer Xk to layer Xk+1). Optical information in the form of laser pulses Q1-Q4 is input to input layer X1 and propagates along edges ejk through RONN 40 to output layer XN. The propagated optical information processed by RONN at output layer XN is retrieved as output pulses R1-R4. It should be noted that the number of input pulses Q1-Q4 and / or the number of output pulses R1-R4 can be different from four (in particular, more than four), and in particular, the number of input pulses Q1-Q4 and the number of output pulses R1-R4 can be different from each other.

[0058] can be detected from one or more laser devices located on the photonic chip 42 (see Figure 2The input pulses Q1-Q4 are generated by a laser device 20 in the photonic chip 42, or one or more of the input pulses Q1-Q4 can be fed externally into the photonic chip 42, possibly after passing through a previous optical processing operation. The output pulses R1-R4 can be measured (e.g., by direct measurement using a photodiode located on the photonic chip 42, or by homodyne or heterodyne measurement using a local oscillator) to access the information content encoded therein and convert it into an electrical signal, or one or more of the output pulses R1-R4 can be forwarded to further optical processing operations that may be performed external to the photonic chip 42.

[0059] exist Figure 3 In the embodiment shown, the inner layer X2 ... XN-1 includes loop nodes Xrn21-Xrn24, Xrn31-Xrn34. Different embodiments are possible in which not all nodes of the inner layer are implemented as loop nodes. The loop node Xrn34 of the third layer X3 is shown in the highlighted portion of the image. The loop node Xrn34 contains loop unit 1 (see Figure 2 ) and activation function 44. The recurrent unit 1 is implemented by means of a processing element 46 of the RONN 42. The processing element 46 comprises an optical waveguide 2 (see Figure 2 ), and is configured to perform the above-mentioned loop operation, retrieving a pulse pair Pj consisting of a control signal pulse Cj and a data signal pulse Dj from the previous layer X2 via different edges ejk, thereby generating a corresponding acoustic wave 10 inside the optical waveguide 2. The loop operation implemented by the loop unit 1 causes the data signal pulse Dj to be transformed towards Dj* depending on information from previous data signal pulses Dj-1, Dj-2, etc. (not shown), which have been stored in the acoustic wave via corresponding control signal pulses Cj-1, Cj-2, etc. The transformed data signal pulse Dj* is then processed by the activation function 44 (i.e., by its physical implementation), generating a node output Ej that is a function f(Dj*) of the transformed data signal pulse Dj*.

[0060] Another output of the processing element 46 is given by a control signal pulse Cj. This control signal pulse Cj can be dumped or used as an input to another loop node, for example the input to the loop node Xrn31.

[0061] Although the present invention has been described in detail with the aid of a preferred embodiment example, the present invention is not limited to this example. Other variations may be made by those skilled in the art without departing from the scope of protection of the present invention.

[0062] Reference numerals

[0063] 1 cycle unit

[0064] 2 Optical waveguides

[0065] 3. Fiber Optic

[0066] 4 First end

[0067] 5 Second end

[0068] 10 Sound Waves

[0069] 12 Optical Information

[0070] 15 Equipment

[0071] 20 Laser equipment

[0072] 22 CW laser

[0073] 22. Move down the CW laser

[0074] 24 Data Branch

[0075] 26 Control Branch

[0076] 28(') Optical IQ Modulator

[0077] 30 RF pulse generator

[0078] 32 Optical Data Processor Units

[0079] 34 Erbium-doped fiber amplifier (EDFA)

[0080] 36 Bandpass filter

[0081] 38 Optical Circulator

[0082] 40 Recurrent Optical Neural Network (RONN)

[0083] 42 Photonic Chip

[0084] 44 Activation Function

[0085] 46 (Optical) Processing Components

[0086] C1-C3 The first to third control signal pulses

[0087] Cj (pulse to Pj) control signal pulse

[0088] D1-D3 First to third data signal pulses

[0089] D1*-D3* The first to third data signal pulses of the transition

[0090] Dj (pulse to Pj) data signal pulse

[0091] dL pulse length

[0092] dT12 / 23 delay time

[0093] Ej node output

[0094] f(Dj*) function that transforms the data signal pulse

[0095] IMc / d Intensity Modulator

[0096] P1-P3 The first to third pulse pairs

[0097] Pj pulse pair

[0098] Q1-Q4 input pulse

[0099] R1-R4 output pulse

[0100] t0-t3 snapshot time

[0101] W1-WN-1 matrix operation / edge (between layers)

[0102] X1-XN (RONN's) layer

[0103] Xrn21-34 loop node

Claims

1. A method for realizing at least a first recurrent unit (1) of a recurrent optical neural network (40) by means of an optical waveguide (2), in, Subsequent pairs of laser pulses (P1-P3), each consisting of a control signal pulse (C1-C3) and a data signal pulse (D1-D3), counter-propagate through the waveguide (2) in the following manner: for a given pair of pulses (P1-P3), - the control signal pulses (C1-C3) are coupled into the first end (4) of the waveguide (2) and propagate towards the second end (5) of the waveguide (2), and - the data signal pulses (D1-D3) are coupled into the second end (5) of the waveguide (2) and propagate towards the first end (4) of the waveguide (2), Where, for a given pulse pair (P1-P3), - setting the optical frequency of the data signal pulses (D1-D3) to be higher than the optical frequency of the control signal pulses (C1-C3) of the same pulse pair (P1-P3) within the bandwidth of the Brillouin frequency shift of the waveguide (2), and / or - setting the difference between the optical frequency of the data signal pulse (D1-D3) and the sum of the optical frequency and the Brillouin frequency shift of the control signal pulse (C1-C3) of the same pulse pair (P1-P3) to be within the bandwidth of the Brillouin frequency shift, and The time delay (dT12, dT23) between a first pulse pair (P1) and a subsequent second pulse pair (P2) is set to be less than the decay time of the acoustic wave (10) generated by the stimulated Brillouin scattering process in the waveguide (2), so that by causing the second data signal pulse (D2) of the second pulse pair (P2) to propagate through the waveguide (2), the dependence of the second data signal pulse (D2) on the first data signal pulse (D1) of the first pulse pair (P2) is caused via the first acoustic wave (10) generated by the stimulated Brillouin scattering process.

2. The method according to claim 1, in, For a given pulse pair (P1-P3), the intensity of the interaction between the corresponding control signal pulses (C1-C3) and the data signal pulses (D1-D3) for generating an acoustic wave (10) by a stimulated Brillouin scattering process is controlled by the amplitude and / or envelope and / or optical power of the control signal pulses (C1-C3).

3. The method according to claim 1 or claim 2, in, For each pulse pair (P1-P3), - the optical power of the control signal pulses (C1-C3) is at least 10 dB higher than the optical power of the data signal pulses (D1-D3), and / or - the optical power of the control signal pulses (C1-C3) varies by at most + / - 10 dB for controlling the intensity of the interaction between the control signal pulses (C1-C3) and the data signal pulses (D1-D3).

4. The method according to any one of the preceding claims, in, For each pulse pair (P1-P3), information is encoded in the phase and / or amplitude of the corresponding data signal pulse (D1-D3).

5. The method according to any one of the preceding claims, in, For a given pulse pair (P1-P3), the difference between the optical frequency of the data signal pulse (D1-D3) and the sum of the optical frequency and the Brillouin frequency shift of the control signal pulse (C1-C3) of the same pulse pair (P1-P3) is set to match the Brillouin frequency shift.

6. The method according to any one of the preceding claims, in, For at least one given pulse pair (P1-P3), the difference between the optical frequency of the data signal pulse (D1-D3) and the sum of the optical frequency and the Brillouin frequency shift of the control signal pulse (C1-C3) of the same pulse pair (P1-P3) is set to be different from the Brillouin frequency shift.

7. The method according to any one of the preceding claims, in, preparing sets of subsequent pulse pairs (P1-P3) in different optical frequency bands, each frequency band containing a different set of subsequent pulse pairs (P1-P3), wherein, for each frequency band, the difference between the optical frequencies of the control signal pulses (C1-C3) and the data signal pulses (D1-D3) of the corresponding pulse pairs (P1-P3) in the frequency band is set within the bandwidth of the Brillouin frequency shift of the waveguide (2) so as to realize a plurality of parallel cyclic units (1) in the frequency domain.

8. The method according to claim 7, in, The bandwidth of the optical frequency band given by the difference between two adjacent control signal pulse frequencies and / or two adjacent data signal pulse frequencies is set depending on the Brillouin frequency shift in the waveguide (2) and / or the pulse width of the control signal pulses (C1-C3) and / or the data signal pulses (D1-D3) of the pulse pairs (P1-P3), respectively.

9. The method according to any one of the preceding claims, in, Setting the pulse length (dL) of the control signal pulses (C1-C3) and / or the data signal pulses (D1-D3) to at least 10 ps, ​​and / or Therein, the time delay (dT12, dT23) between two subsequent pulse pairs is set depending on the decay time of the acoustic wave (10) in the waveguide (2).

10. The method according to any one of the preceding claims, in, A reset operation is performed on the at least one cycle cell (1) by erasing the acoustic wave (10) in the waveguide (2) by means of a single control signal pulse (C1-C3) without a corresponding data signal pulse (D1-D3).

11. The method according to any one of the preceding claims, in, An optical fiber (3) is used as the waveguide (2).

12. The method according to claim 11, in, A polarization-preserving optical fiber is used as the waveguide (2).

13. The method according to claim 12 when appended to claim 10, in, said first pulse pair (P1) having a corresponding first control signal pulse (C1) and a first data signal pulse (D1) being prepared in one of said two retained polarizations, and The reset operation is performed by another control signal pulse in the preserved polarization.

14. The method according to any one of the preceding claims, in, A chain of cyclic cells (1) is realized in the waveguide (2) by means of a plurality of subsequent pulse pairs (P1-P3) of corresponding control and data signal pulses (C1-C3, D1-D3), and The number of pulse pairs (P1-P3) is set according to the following: - the pulse length (dL) of the control and / or data signal pulses (C1-C3, D1-D3), and / or - the dead time (dT12, dT23) between two subsequent pulse pairs (P1-P3), and / or - decay time of the acoustic wave (10) generated by the stimulated Brillouin scattering process.

15. A device (15) for implementing at least a first recurrent unit (1) of a recurrent optical neural network, comprising: - an optical processing element (46), said optical processing element (46) comprising an optical waveguide (2), - means for generating subsequent pairs of laser pulses (P1-P3), each pair (P1-P3) consisting of a control signal pulse (C1-C3) and a data signal pulse (D1-D3), wherein the means for generating the pulse pairs (P1-P3) are configured to set the optical frequency of the data signal pulses (D1-D3) of a pulse pair (P1-P3) to be higher than the optical frequency of the control signal pulses (C1-C3) of the same pulse pair (P1-P3) within the bandwidth of the Brillouin frequency shift of the waveguide and / or to set the difference between the optical frequency of the data signal pulses (D1-D3) and the sum of the optical frequency of the control signal pulses (C1-C3) of the same pulse pair (P1-P3) and the Brillouin frequency shift to be within the bandwidth of the Brillouin frequency shift, - means for coupling the control signal pulses (C1-C3) of a subsequent pulse pair (P1-P3) into a first end (4) of the waveguide (2), and means for coupling the data signal pulses (D1-D3) into a second end (5) of the waveguide (2), wherein the device (15) is configured to set a time delay (dT12, dT23) between a first pulse pair (P1) and a subsequent second pulse pair (P2) to be less than a decay time of an acoustic wave (10) generated by stimulated Brillouin scattering in the waveguide (2), so that a dependence of the second data signal pulse (D2) on the first data signal pulse (D1) of the first pulse pair (D1) is caused by propagating the second data signal pulse (D2) of the second pulse pair (P2) through the waveguide (2) via the first acoustic wave (10) generated by the stimulated Brillouin scattering process.

16. The device (15) according to claim 15, in, The length of the optical waveguide (2) is selected depending on the time delay (dT12, dT23) and / or the decay time of the acoustic wave (10) generated by stimulated Brillouin scattering in the waveguide (2) so that - a control signal pulse (C1) of a first pulse pair (P1) first interacts with a corresponding data signal pulse (D1) of said first pulse pair (P1) and, after further propagation through said waveguide (2), interacts with said data signal pulse (D2) of said subsequent second pulse pair (P2), and / or - A data signal pulse (D1) of a first pulse pair (P1) first interacts with a corresponding control signal pulse (C1) of said first pulse pair (P1) and, after transformation and further propagation through said waveguide (2), interacts with said control signal pulse (C2) of said subsequent second pulse pair (P2).

17. A recurrent optical neural network comprising a device (15) according to claim 15 or claim 16.

18. The recurrent optical neuronal network of claim 17, implemented as an integrated design on a single chip.

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