Method and system for optical computing

Abstract

The disclosure provides an optical data combiner. The optical data combiner is for use with a coherent light source which provides a frequency swept signal. The optical data combiner has a splitter for splitting the signal between a first arm and a second arm. A first phase modulator and a second phase modulator control a phase modulation of the signals in the first arm and the second arm. A light combiner combines the first light signal with the second light signal to produce a combined light signal which has: a frequency content resulting from an optical path length difference between the first arm and the second arm; and a phase based on the phase modulation of the first data stream and the second data stream. Also disclosed is an optical computing element, a method of optically combining data, and an apparatus comprising a plurality of optical data combiners.

Description

[0001]METHOD AND SYSTEM FOR OPTICAL COMPUTING Field of the invention The present invention relates to methods and systems for optical computing, and more particularly to methods and systems for optically combining streams of data. Background It is known to use optical systems instead of electronic systems for computation. In a conventional architecture, a Mach-Zehnder interferometer may be used to divide light into two arms and phase modulate the light according to an input data stream. By recombining the light, the combined light signal can be analysed to monitor the output power and infer characteristics of the input data stream. In this way, various logic gates can be implemented. It is an aim of the present invention to provide an improved optical computing system. Summary of the invention Embodiments of the disclosure provide an optical data combiner comprising any or all of the following features: a coherent light source configured to provide a frequency swept signal; a first arm for carrying a light signal; a second arm for carrying a light signal; a splitter configured to split the frequency swept signal to provide a first light signal in the first arm and a second light signal in the second arm; a first phase modulator configured to control a phase modulation of the first light signal based on a first data stream; a second phase modulator configured to control a phase modulation of the second light signal based on a second data stream; and a light combiner for combining the first light signal with the second light signal to produce a combined light signal which has: a frequency content resulting from an optical path length difference between the first arm and the second arm; and a phase based on the phase modulation of the first data stream and the second data stream. Therefore, the optical data combiner may provide an apparatus to facilitate using a photonic architecture to replace an electronic base GPU. This can result in lower power outputs compared to transistors. The optical data combiner may provide the advantage that by using a coherent light source to provide a frequency swept signal, optical computations can be performed faster, thereby increasing clock speed. The frequency content may comprise at least one frequency component and the at least one frequency component may have a phase based on the combination of a data element of the first data stream with a data element of the second data stream. The optical data combiner may comprise a delay component. The delay component may be configured to provide the optical path length difference between the first arm and the second arm. The delay component may comprise an optical delay line. The optical delay line may be arranged between the first phase modulator and the light combiner. The optical delay line may be configured to introduce a fixed delay to the first light signal. The splitter may be a 50:50 splitter. The coherent light source may be made and sold separately from the optical data combiner, so some embodiments do not include the light source itself but rather are configured for use with such a light source. For example, such embodiments may comprise a coupling for coupling the splitter to such a light source to receive the frequency swept signal – examples of such couplings include optical and / or mechanical elements arranged to guide a beam of light comprising the frequency swept signal to the splitter to provide the first light signal and the second light signal. Embodiments of the disclosure provide an optical computing element comprising: the optical data combiner as described hereinabove; and a controller configured to superpose a first interval of the combined light signal with a second interval of the combined light signal to produce a superposed signal. The controller may be further configured to determine frequency data representing frequency components of the superposed signal. The controller may be further configured to determine a temporal representation of the frequency data, which may be obtained by performing a fast Fourier transform (FFT). The controller may be further configured to compare the frequency data or its temporal representation with a stored association to perform one of: a logic operation and an elementary operation, for example an arithmetic operation such as addition, subtraction, multiplication, or division. The stored association may comprise a relation between said frequency data and a result of said operation. For example, the operation may comprise a logic operation, and the relation may comprise a truth table. For example, the logic operation may be a Boolean operation. The logic operation may be an arithmetic operation. The relation may comprise a mapping between frequency data and the arithmetic result of the combination. Embodiments of the disclosure provide a method of optically combining first data from a first data stream with second data from a second data stream, the method comprising any or all of the following steps: generating a frequency swept signal from a coherent light source; splitting the frequency swept signal to provide a first light signal in a first arm and a second light signal in a second arm; modulating a phase of the first light signal based on the first data; modulating a phase of the second light signal based on the second data; providing an optical path length difference between the first light signal and the second light signal; and combining the first light signal with the second light signal into a combined light signal. The frequency swept signal may comprise a repeating signal. The repeating signal may have a first part and a second part. The second part may have the same duration as the first part. The second part may have a different duration to the first part, for example wherein a variable duration is used to perform the weighted sum or multiplication (e.g. for summing two numbers the Result = w_1*cos(p1) + w_2*cos(p2), where w_1 and w_2 are weights). The combined light signal may have a first interval resulting from the first part of the repeating signal. The combined light signal may have a second interval resulting from the second part of the repeating signal. The method may comprise superposing the first interval with the second interval into a superposed signal. Modulating the phase of the first light signal and / or the second light signal may comprise applying a different phase modulation to the first part and the second part. Modulating the phase of the first light signal and / or the second light signal may comprise applying a reference phase modulation to the second part. Modulating the phase of the first light signal and / or the second light signal may comprise phase modulating only the first part. The method may further comprise generating frequency data representing frequency components of the superposed signal, for example by performing a frequency transform such as a Fourier transform. The method may further comprise performing a logic operation based on the frequency data of the superposed signal. The logic operation may be performed by comparing the frequency data with a stored association. Embodiments of the disclosure also provide a system comprising at least two optical data combiners according to the present disclosure, coupled together to perform data processing operations. The plurality of optical data combiners may be arranged in series, so that an output of one data combiner is provided to an input of a subsequent optical data combiner in the series. For example, at least one output of a light combiner of a first optical data combiner may be connected to at least one input of a second optical data combiner. The at least one output may be either (a) the combined light signal or (b) a data stream carried by the combined light signal (e.g. a data stream which results from the phase modulation caused by the first data stream and the second data stream in the respective first and second arms of the first optical data combiner). The system may be configured so that the data stream carried by the combined light signal from the first optical data combiner is used to control a phase modulator of the subsequent optical data combiner. The system may be configured so that the combined light signal from the first optical data combiner is provided to the second optical data combiner (e.g. as the coherent light source for that second optical data combiner). For example, this combined light signal may be further modulated by further data stream(s) provided to either or both of the phase modulators of the subsequent optical data combiner. The system may comprise a plurality of optical data combiners arranged in parallel. A single (e.g. shared) coherent light source may be configured to provide light to a plurality of optical data combiners in parallel. In this arrangement, two or more pairs of data streams may be modulated onto the shared coherent light source in parallel. The system may be configured to recombine each combined light signal obtained from each of the plurality of optical data combiners. Accordingly, an aspect of the disclosure provides a system comprising a plurality of the optical data combiners arranged in parallel, in which the coherent light source of each of the plurality of optical data combiners is provided by a shared coherent light source. A first two data streams may be provided, one to each of the first and second phase modulators of a first one of the plurality of optical data combiners, and a second two data streams may be provided, one to each of the first and second phase modulators of a second one of the plurality of optical data combiners. The first two data streams may be different from the second two data streams. The system may be configured so that the combined light signal produced from the first one of the plurality of optical data combiners is optically combined with the combined light signal of the second one of the of the plurality of optical data combiners – for example the two combined light signals may be provided to a further light combiner to produce a further combined light signal, which encodes the four data streams in combination. It can therefore be seen that the apparatus of the present disclosure may provide a system configured to provide sequential and / or parallel data combination, for example by comprising a combination of sequential optical data combiners and / or parallel optical data combiners. Further embodiments are envisaged. Brief of the drawings Embodiments of the disclosure will now be described, by way of example only, with reference to the accompanying drawings, in which: Fig.1 is a schematic diagram of an optical computing element according to embodiments of the disclosure; Fig.2 is a flowchart of a method of optically combining data according to embodiments of the disclosure; Fig.3 is a series of four plots of a superposed light based on phase differences as provided in embodiments of the disclosure; Fig. 4 is a schematic diagram of a sequential optical computing system according to embodiments of the disclosure; Fig. 5 is a schematic diagram of a sequential optical computing system according to embodiments of the disclosure; and Fig. 6 is a schematic diagram of a parallel optical computing system according to embodiments of the disclosure. Detailed description of the drawings Embodiments of the disclosure relate to an optical data combiner. The optical data combiner shares some structural features with a typical Mach-Zehnder interferometer, in that the optical data combiner includes a light source, a beam splitter to split the light into two paths, and a combiner. In examples of the disclosure, the light source is a coherent light source, such as a laser. The light source is configured to provide a frequency swept signal. This means that at a given time, the light source emits light at a single frequency (and therefore a single wavelength), but the frequency of light emitted varies with time. For example, the frequency may vary periodically by sweeping up from a minimum frequency to a maximum frequency before sweeping back down to the minimum frequency and repeating the cycle. The splitter splits the light signal so that it travels through a first arm and a second arm. Each arm includes a respective phase modulator to control a phase modulation of the light signal in the arm. The degree of phase modulation is controlled by data streams. In particular, the light signal in the first arm is phase modulated based on a first data stream and the light signal in the second arm is phase modulated based on a second data stream. In this way, the data from each data stream is encoded in phase shifts of the light signals in the first arm and the second arm. The optical data combiner also has a delay component to provide an optical path length difference between the first arm and the second arm. In an example, the delay component is provided in the first arm to increase the optical path length travelled by the light signal in the first arm before reaching the combiner. In this way, light arriving at the combiner from the first arm will be delayed compared to light arriving at the combiner from the second arm. Given that the light source provides a frequency swept signal, the light arriving at the combiner from the first arm will have a different frequency to that from the second arm, resulting in a combined signal of beats. As such, the combined signal can be analysed, for example digitally, for the purpose of performing computational operations with the data streams. Figure 1 shows an optical data combiner 110. The optical data combiner 110 comprises a coherent light source 101, a splitter 102, a first arm 121, a second arm 122, a first phase modulator 131, a second phase modulator 132, a delay component 150 and a light combiner 160. The optical data combiner is an arrangement of optical components which is configured to guide a light signal from the light source 101 to the combiner 160. In the arrangement of Figure 1, the optical data combiner 110 provides two paths for such a light signal, one through the first arm 121 and one through the second arm 122. The coherent light source 101 is configured to provide temporally coherent light to the splitter 102. The coherent light source 101 may be a laser, such as a tuneable laser. The coherent light source 101 is configured to provide a frequency swept signal. In this respect, the coherent light source 101 provides coherent light at a frequency or wavelength that varies with time, for example in a periodic manner. The frequency swept signal may have a triangular profile, wherein the frequency increases linearly over a time period from a minimum frequency to a maximum frequency before decreasing to the minimum frequency, for example at the same linear rate. The splitter 102 is configured to split the frequency swept signal to provide a first light signal in the first arm 121 and a second light signal in the second arm 122. Typically, the splitter 102 is a beam splitter configured to receive the frequency swept signal from the coherent light source 101 and divide the signal between the first arm 121 and the second arm 122. The beam splitter may be a 50:50 beam splitter so as to provide an equal division of the signal power between the first arm 121 and the second arm 122. The first phase modulator 131 is configured to modulate the phase of the light signal in the first arm 121. In this respect, the first phase modulator 131 is configured to control a phase modulation of the first light signal received from the splitter and direct the phase modulated light towards the light combiner. The first phase modulator 131 is configured to control the phase modulation based on a first data stream 141. As such, the phase of the first light signal can be modulated based on data in the first data stream 141. The second phase modulator 132 is configured to modulate the phase of the light signal in the second arm 122. In this respect, the second phase modulator 132 is configured to control a phase modulation of the second light signal received from the splitter and direct the phase modulated light towards the light combiner. The second phase modulator 132 is configured to control the phase modulation based on a second data stream 142. As such, the phase of the second light signal can be modulated based on data in the second data stream 142. The first phase modulator 131 and / or the second phase modulator 132 may be one of: a liquid crystal spatial light modulator, an acousto-optic modulator, and an electro-optic modulator, or any other device capable of performing suitable phase and / or amplitude modulation. The delay component 150 is configured to provide an optical path length difference between the first arm 121 and the second arm 122. The delay component 150 can be configured to receive light from the splitter 102 and direct a delayed light signal towards the light combiner 160. In the illustrated arrangement, the delay component 150 is configured to delay the first light signal in the first arm 121. In particular, the delay component 150 is configured to delay the phase modulated light signal from the first phase modulator 131 and direct the delayed, phase modulated light signal to the light combiner 160. In this way, the delay component 150 is configured to increase the distance travelled between the splitter 102 and the combiner 160 of the first light signal compared to the distance travelling between the splitter 102 and the combiner 160 of the second light signal. The delay component 150 may be provided by increasing the length of the first arm 121, for example between the first phase modulator 131 and the combiner 160. The delay component may comprise an optical delay line between the first phase modulator 131 and the light combiner 160. The delay component 150 may be provided by arranging the first arm 121 such that the first light signal travels through a different material compared to the second arm, such as a material having a different optical density. In this way, the optical delay line can be configured to introduce a fixed delay to the first light signal relative to the second light signal. The delay component 150 may be operable to introduce a slight phase difference between the light received on the first arm 121 and the second arm 122 when the signals entering the combiner 160 may otherwise be in phase, as perfectly in-phase signals entering the combiner 160 may be reflected in a DC signal component after combining. The DC signal component of the combiner 160 may often be removed for practical considerations. The delay component 150 may be implemented with e.g., an extra length of fibre or may be implemented by ensuring that the light path from the splitter 102 through the first arm 121 and the second arm 122 are not the same length. The light combiner 160 is configured to combine the first light signal with the second light signal to produce a combined light signal. In particular, the light combiner 160 is configured to receive light from the first arm 121 and the second arm 122, and combine the received light into a combined light signal. The light combiner 160 may be provided by a beam splitter, such as a 50:50 beam splitter. The optical data combiner 110 is arranged such that light is guided from the coherent light source 101 to the light combiner 160. This may be performed using a series of waveguides as illustrated in Figure 1 by the lines between the optical components, namely the coherent light source 101, the splitter 102, the first phase modulator 131, the second phase modulator 132, the delay component 150 and the light combiner 160. The coherent light source 101 is connected to the splitter 102. The splitter 102 is connected to the first phase modulator 131 and the second phase modulator 132. As such, the splitter 102 is arranged between the coherent light source 101, the first phase modulator 131 and the second phase modulator 132. The optical path length from the splitter 102 to the first phase modulator 131 and from the splitter 102 to the second phase modulator 132 may be the same or may be different. The first phase modulator 131 is arranged in the first arm 121 and is connected between the splitter 102 and the light combiner 160. The second phase modulator 132 is arranged in the second arm 122 and is connected between the splitter 102 and the light combiner 160. In the example shown in Figure 1, the delay component is arranged in the first arm 121 and is connected between the first phase modulator 131 and the light combiner 160. It will be appreciated that such connections are optical connections which facilitate light signals to travel between the optical components, for example along a waveguide. In operation, a frequency swept signal is generated by the coherent light source 101. The frequency swept signal is directed to the splitter 102. The splitter 102 splits the frequency swept signal between a first light signal in the first arm 121 and a second light signal in the second arm 122. The first light signal is phase modulated by the first phase modulator 131 based on data from the first data stream 141. The first light signal is then directed to the delay component 150 which delays the first light signal, before directing the first light signal to the combiner 160. Meanwhile, the second light signal is phase modulated by the second phase modulator 132 based on data from the second data stream 142. The second light signal is then directed to the combiner 160. The combiner 160 combines the first light signal from the first arm 121 with the second light signal from the second arm 122 into a combined light signal. As such, the combined light signal will have a frequency content based on the optical path length difference between the first light signal and the second light signal introduced by the delay component 150 (or a difference in the length of the arms 121, 122). In addition, the phase of the combined light signal depends on both the phase of the first light signal and the phase of the second light signal – for example, the combination of the phase modulation caused to each signal by each of the two data streams 141, 142 is therefore encoded in the combined light signal. For example the combined signal may comprise sine components O1 and cosine components O2 (typically O2 is identical to O1 phase shifted by π due to the construction of the combiner): where λ(t) is the wavelength; ΔL is the path length difference or path delay 150 between the first arm and the second arm; and Δφ(t) is the phase shift introduced by the combination of the two phase-modulated data streams in the combiner 160. The phase of the combined light signal may be used to perform data operations for the two data streams, as described below. These data operations may comprise elementary operations on pairwise elements of the two data streams – such as logic operations and arithmetic operations. Figure 2 shows a method 200 of optically combining data from a first data stream with data from a second data stream. Any of the steps of the method 200 may be performed using the optical computing element 100, in particular the optical data combiner 110, described in relation to Figure 1. The method 200 comprises generating 202 a frequency swept signal. In the example shown, this step 202 comprises generating a frequency swept signal using a coherent light source, such as a laser, for example a tuneable laser. This step 202 can be carried out by the coherent light source 101 described above. The frequency swept signal may include a plurality of pulses (or sweeps) wherein each pulse has a first part and a second part. The first part may be any (strictly) monotonically increasing frequency function. The second part may be any (strictly) monotonically decreasing frequency function. The first part may be identical to or symmetrical to the second part. In an example of a frequency swept signal, the first part includes a linear increase in frequency over a time interval and the second part, which is generated sequentially after the first part, includes a linear decrease in frequency over the same length of time. The step 202 may further comprise directing the frequency swept light signal to a splitter, for example along a waveguide to the splitter 102. The method 200 comprises splitting 204 the frequency swept signal between a first arm and the second arm. In this way, the step 204 comprises generating a first light signal and a second light signal from the frequency swept signal. The step 204 can be carried out by the splitter 102 described above. The method 200 comprises modulating 206 a phase of the light signal in each arm based on a data stream associated with each arm. As such, this phase modulating step 206 comprises modulating a phase of the first light signal based on data of the first data stream and modulating a phase of the second light signal based on data of the second data stream. The step 206 can be carried out by the first phase modulator 131 and the second phase modulator 132, and the first data stream 141 and the second data stream 142, as described above. The phase modulating step 206 may comprise phase modulating only the first part or only the second part of the frequency swept signal. In an example of such a step 206, only the first part of the frequency swept signal is phase modulated by the first phase modulator and the second phase modulator. In particular, in view of the frequency swept signal having a first part and a second part as described above, it will be appreciated that each of the first light signal in the first arm and the second light signal in the second arm has a first part and a second part. The first phase modulator modulates the phase of the first part of the first light signal based on the first data but does not apply any modulation to the second part of the first light signal. Similarly, the second phase modulator modulates the phase of the first part of the second light signal based on the second data, but does not apply any modulation to the second part of the second light signal. In view of the periodic nature of the frequency swept signal, it will be appreciated that this step may be repeated for each successive period of the repeating signal. In other words, the first part of a period is phase modulated while the second part of the period is not phase modulated. The method 200 comprises providing 208 an optical path length difference between the first light signal and the second light signal. This step 208 can be performed by a delay component configured to provide an optical path length difference between the first arm 121 and the second arm 122, such that the first light signal and the second light signal travel different optical distances therein. This may be achieved using the delay component 150 described above. This step 208 may be performed after or before the phase modulation step 206. In the illustrated example, the method includes delaying the first light signal in the first arm after the first light signal is phase modulated and before the step of combining the first light signal with the second light signal. The method 200 comprises combining 210 the first light signal with the second light signal. This step 210 comprises creating a combined light signal based on the light signals in the first arm and the second arm. This step 210 may be performed using the light combiner 160 described above. Overall, the method 200 of optically combining data from a first data stream with data from a second data stream comprises generating 202 a frequency swept signal; splitting 204 the frequency swept signal between a first arm and a second arm; modulating 206 a phase of the light signal in each arm based on a data stream associated with each arm; providing 208 an optical path length difference between the first arm and the second arm; and combining 210 the first light signal with the second light signal. These steps of the method 200 may be performed using the optical data combiner 110 described above. The disclosure also provides an optical computing element for combining and performing operations on data from the first data stream and the second data stream. In this respect, Figure 1 also shows an optical computing element 100. The optical computing element 100 comprises an optical data combiner 110, for example as describe above, and an analyser 170. The analyser 170 may be configured to detect or otherwise suitably process the combined light signal produced by the combiner 160. The analyser 170 may comprise a controller. The controller may receive a first output and a second output based on the combined signal. In the arrangement shown, the controller receives a first output 161 and a second output 162. The controller can be configured to superpose a first interval of the combined light signal with a second interval of the combined light signal to produce a superposed signal. The superposition may be achieved by e.g., an integrating functionality over a certain time interval. In accordance with various embodiments, the superposition may be obtained with a photodetector or another form of light sensor. Thus, the analyser may be operable to convert an optical signal to a related electrical signal. In one example, the controller receives a combined light signal over a time interval T and separates the combined signal into a first interval of the combined light signal between 0 and T / 2 and a second interval of the combined light signal between T / 2 and T. The controller may superpose the first interval with the second interval to produce a superposed signal. For example, the first interval of the combined light signal may correspond to the first part of the frequency swept signal and the second interval of the combined light signal may correspond to the second part of the frequency swept signal. For example, assume that a sweep duration of the frequency swept signal lasts 1,000 ns. The first part of the signal in which the frequency linearly increases lasts 500 ns. This first part of the signal is split at the splitter 102 and proceeds to the first phase modulator 131 and the second phase modulator 132, where its phase is modulated based on the respective data streams. After the first part of the signal, the coherent light source 101 generates the second part of the signal, in which the frequency linearly decreases, and which also lasts 500 ns. The second part of the signal is split at the splitter and proceeds to the first phase modulator and the second phase modulator but no phase modulation takes place. Then, at the light combiner 160, the first part of the signal from the first arm is combined with the first part of the signal from the second arm. This forms the first interval of the combined signal, which lasts 500 ns (0 to T / 2) in this example. After, the second part of the signal from the first arm is combined with the second part of the signal from the second arm at the light combiner 160. This forms the second interval of the combined signal, which lasts 500 ns (T / 2 to T) in this example. Similarly, the superposition time T may be larger than a complete sweep period and may be multiple sweep periods, for example two sweep periods. Then, a first interval may be one complete sweep period and a second interval may be a complete sweep period. In the example of a frequency swept signal having a triangular profile, the second interval of the combined signal can be reversed before being superposed with the first interval of the combined signal. This may be performed so that the two signals being superposed both relate to the same positive rate of frequency increase. In other words, if the first part of the frequency swept signal is an up-sweep and the second part is a down-sweep, then reversing one of the intervals of the combined signals will ensure that the time-varying nature of the frequency in each signal is matched before superposition. In view of the above, the method 200 may further comprise analysing the combined signal. In the method of Figure 2, the method 200 comprises superposing 212 a first interval of the combined signal with a second interval of the combined signal. This step 212 may be performed after detecting and digitising the combined signal. Figure 3 illustrates an example of how the combined signal in the first interval can be superposed with the combined signal in the second interval, for example during the superposing step 212 of the method 200. In this respect, Figure 3 shows four plots A, B, C and D. In each plot, the Y-axis represents the signal amplitude and the X-axis represents time. Given that the signal is a frequency swept signal such that the frequency and wavelength are a function of time, the X-axis also represents wavelength. In each plot, the second interval of the combined signal is generated with zero phase modulation, such that the second interval may act as a reference signal. In view of the delay of the first light signal compared to the second light signal, the frequency of each signal arriving at the combiner is different, which results in a combined signal of beats. Plot A represents the first interval signal 301a when the phase modulation is zero. In this case, the first interval signal 301a is identical to the second interval signal 302a because there is no phase difference. As such, the superposed signal 303a is equivalent to doubling the amplitude of the signals. Plot B represents the first interval signal 301b when the phase modulation is π / 2. In this case, the first interval signal 301b is phase shifted with respect to the second interval signal 302b. As such, the superposed signal 303b of plot B is attenuated compared to that of plot A. Because 301b and 302b are out of phase by exemplary π / 2, the summation obtained in the superposition may be partly constructive only, resulting in a lower combined amplitude in the superposed signal 303b. Plot C represents the first interval signal 301c when the phase modulation is 5π / 6. In this case, the first interval signal 301c is phase shifted further with respect to the second interval signal 302c. As such, the amplitude of the superposed signal 303c of plot C is further diminished compared to that of plot B. Plot D represents the first interval signal 301d when the phase modulation is π. In this case, the first interval signal 301d is phase shifted so as to be completely out of phase with the second interval signal 302d. As such, this results in destructive superposition such that the superposed signal 303d of plot D is completely nullified. Because 301b and 302b are out of phase by exemplary π, the summation obtained in the superposition may be wholly destructive, resulting in a zero combined amplitude in the superposed signal 303b. As demonstrated by the above example, the combined signal can be controlled by changing the phase shift difference. The output signals from the combiner 160 can be described as follows: where λ(t) is the wavelength; ΔL is the delay between the first arm and the second arm; and Δφ(t) is the combined phase difference between the two input signals to the combiner 160, based on the phase modulation in the first arm 121 and the second arm 122. The first output O1may correspond to the first output 161 of the optical computing element 100, and the second output O2may correspond to the second output 162 of the optical computing element 100. The described functionality can be used to construct logic gates, such as an AND gate, as described below. The following truth table provides an example of how the phase shifts can be set for each combination of input data. In the example, Z denotes the amplitude of the superposition, for example 303a, 303b, 303c. X Y Δφ Z X&Y = Z > 1.5 1 1 π 2 1 0 / 1 1 / 0 + / - π / 2 1.25 0 0 0 0 0 0 For inputs X=0, Y=0, a phase difference of 0 provides an output signal of 0. For inputs X=0, Y=1, a phase difference of π / 2 provides an output signal of 1.25. For inputs X=1, Y=1, a phase difference of π provides an output signal of 2. Therefore, by monitoring the signal at the first output port 161 to require that Z > 1.5 to generate 1, and 0 otherwise, an AND gate can be implemented. It will be appreciated that other logic gates can be implemented by modifying the phase shifts in phase modulators 131, 132 and the threshold for Z as required and / or using the second output of the combiner 160. Similarly, the superposition time T may be adjusted as desirable. As such, the analyser 170 can be configured to implement one or more logic gates, for performing logic operations on the data X and Y, based on a threshold and the output of the combiner. In other examples, the analyser 170 is configured to determine frequency data representing frequency components of the superposed signal, for example using a Fourier representation of the signals received. The controller may be further configured to compare the frequency data with a stored association, which may be stored in a memory of the controller. In this way, the controller can be configured to perform a logic operation and / or an elementary operation, such as an arithmetic operation. The arithmetic operation may be addition, subtraction, multiplication, or division, for example. The stored association may comprise a relation between the frequency data and a result of the operation. In some examples, the operation comprises a logic operation and the relation comprises a truth table. The logic operation may be a Boolean operation. The logic operation may be an arithmetic operation and the relation may comprise a mapping between frequency data and the arithmetic result of the combination. The controller may be configured to perform a frequency transformation of the output signal. For example, the analyser may be configured to perform a Fourier transformation on the output signal. The magnitude of the Fourier transform will vary based on the phase shift. As such, similarly to the method described above, specific thresholds can be applied to implement various logic gates. The controller can be configured to perform arithmetic operations for real numbers in a selected range, such as [-1,1]. It will be appreciated in the context of the present disclosure that the controller may be configured to perform additions and multiplications, for example by appropriately configuring the analyser 170. In this respect, the analyser 170 may use the product-to-sum identities for trigonometric functions. Any number in the range [-1,1] can be expressed using trigonometric functions. A worked example of this is provided in the following. Specifically, for x and y in this range, we can find phases ^^,^^ such that: ^^ = cos^^^^ => ^^ = acos^^^^^^ = cos^^^^ => ^^ = acos^y^.Then we express the product of cosines in terms of sums of cosines: +^^^ ^ .To achieve this using the optical combiner we add phase shift ^^^ெ^(^^^) using the first phase modulator 131, and phase shift using the second phase modulator 132, where may correspond to sweep interval I, ΔL is assumed to be zero for simplicity. So, the phasedifference can be written as ∆^^(^^^) = ^^^ெ^(^^^) − ^^^ெଶ(^^^), and the outputs of the opticalprocessor are: Now, to perform multiplication we use ^^ଶ(^^, ^^^) such that at: By setting the superposition time T in the analyser 170 to operate over two sweeps, i.e.,^^^ and ^^ଶ, the outputs O2 are integrated: For example, if we want to multiply 0.5 by -0.5, we set: 0.5 −0.5 = ^^ = cos^∆^^(^^ଶ)^ => ∆^^(^^ଶ) = 2^^ / 3.Hence, Using the identity, we get: It will be appreciated in the context of the present disclosure that the controller can be configured to receive an input based on the combined signal, perform a transformation (such as a Fourier transform) on the combined signal, and use a mapping (a lookup table or pattern recognition, for example), to obtain a result of the multiplication operation based on a component of the transformation, such as the amplitude. The addition or subtraction of two numbers is implemented using superposition. By adjusting the phases of the first light signal and the second light signal to correspond to different data values, the light signals can be made to interfere constructively or destructively when recombined, effectively adding the values they represent. To this end, the light signal in each arm can be sequentially modulated such that they add during the integration on the detector. In this case, the modulation may happen over a shorter timescale than the detector integration time. While the frequency swept signal has been described as having a triangular profile, it will be appreciated that other profiles can be envisaged while remaining compatible with the functionality described above. For example, the frequency swept signal may have a sawtooth profile. In a sawtooth profile, the frequency increases linearly over a time period from a minimum frequency to a maximum frequency before a step change to the minimum frequency. It will also be appreciated that the increase (or decrease) of the frequency need not be linear. In such a sawtooth profile, a single period may include two upsweeps, such that the first part of a frequency swept signal corresponds to a first up-sweep and the second part of the frequency swept signal corresponds to a second up-sweep, wherein the second up-sweep is identical to the first up-sweep. While the delay component 150 has been described as being located in the first arm 121, it will be appreciated that in other arrangements the delay component 150 is located in the second arm 122. Furthermore, each of the first arm and the second arm may include a delay component each configured to provide a different delay to the first light signal and the second light signal, in order to provide a net delay between the first light signal and the second light signal. While the optical data combiner 110 has been described as having a first phase modulator 131 and a second phase modulator 132, a single phase modulator may be employed instead of having two phase modulators. The single phase modulator may be configured to impose a phase difference between the first light signal and the second light signal, wherein the phase difference may be calculated based on the two input data streams. Use of a single phase modulator can reduce the power requirements. While the method 200 has been described as including a step of modulating only the first part of the first light signal and the second light signal, it will be appreciated that in other arrangements this is reversed such that only the second part of the first light signal and the second part of the second light signal is phase modulated based on the data streams. Furthermore, the first part or the second part may be subjected to a reference phase modulation instead of a zero phase modulation. A plurality of optical data combiners may be arranged in series, so that an output of one data combiner is provided to an input of a subsequent optical data combiner in the series. For example, at least one output of a light combiner of a first optical data combiner may be connected to at least one input of a second optical data combiner. The at least one output may be either (a) the combined light signal or (b) a data stream carried by the combined light signal (e.g. a data stream which results from the phase modulation caused by the first data stream and the second data stream in the respective first and second arms of the first optical data combiner). Figure 4 shows an example of the first of these two possibilities, in which the combined light signal from the first optical combiner is used as the light source for the second optical combiner. Figure 4 illustrates an apparatus comprising a first optical combiner 110 and a second optical combiner 110′ arranged in sequence. The first optical combiner 110 and the second optical combiner 110′ may be the same as or similar to the optical data combiner 110 described in relation to Figure 1. To simplify the drawing, Figure 4 shows the splitter (e.g.102), the phase modulators (e.g.131 and 132) and the light combiner (e.g. 160) of the first and second optical combiners as a single element 180 referred to as a first phase modulation assembly 180 and a second phase modulation assembly 180’, respectively. This is indicated by the dashed line shown in Figure 1. A difference from the arrangement shown in Figure 1 is that instead of the outputs 161, 162 of the light combiner 160 being detected by the analyser 170, the system is arranged such that an output 190 of the first optical combiner 110 is directed to the second optical combiner 110′. This output 190 is provided by the combined light signal which, in Figure 1, is provided by the light combiner 160. This output 190 provides the light source for the second phase modulation assembly 180’. In this way, light encoding data from the first data stream 141 and the second data stream 142 can be provided to the second optical combiner 110’. The second optical combiner 110’ can then encode further data from a third data stream 141’ and a fourth data stream 142’ onto that same light signal. In this way, an output 190’ of the second optical combiner 110’ may encode data from all four data streams 141, 142, 141’, 142’. Figure 5 shows an example of the second of the two possibilities mentioned above, in which the data stream carried by the combined light signal is used to control a phase modulator of the second data combiner. A difference from the arrangement shown in Figure 4 is that light is provided to the second phase modulation assembly 180’ by a second light source 110’ and that the output 190 of the first optical combiner 110 is provided to the second optical combiner 110’ as a data input. In other words, the output 190 may be used to control a phase modulator (e.g. the first phase modulator 131, not shown in Figure 5) of the second optical combiner 110’. The apparatus can be configured such that the combined light signal provided by the light combiner 160 is received by an analyser 170. The analyser 170 is configured to demodulate the data. The apparatus can be configured to use the data from the combined light signal to control a phase modulation of one of the arms (e.g., the first arm 121 as shown in Figure 1). In the illustrated example, the output 190 can be considered to take the place of the first data stream 141’. In some arrangements, the second light source 101’ may originate from the first light source 110, for example as a branch (not shown) of the first light source 101. In either of the above examples illustrated in Figures 4 and 5, it will be appreciated that further optical combiners may be provided in series, such that the output of one optical combiner can be used as an input of a subsequent optical combiner, whether as a light source input or as a data input. The disclosure also encompasses a plurality of optical data combiners that can be parallelized to combine data in parallel. For example, light from the light source 101 can be split into multiple paths, and then directed to the plurality of optical combiners, wherein each optical combiner may be an optical data combiner 110 as described in relation to Figure 1. In this way, the initial data streams can be combined in parallel by the plurality of optical combiners. Figure 6 shows an example of such a parallelized arrangement comprising a first optical combiner 110 and a second optical combiner 110′ arranged in parallel. The components of the first optical combiner 110 and the second optical combiner 110′ may be the same or similar, and may be configured in the same or a similar manner, to the corresponding components described in relation to Figure 1. A difference from the arrangement in Figure 1 is that in the illustrated example the coherent light source 101 is configured to provide coherent light to a primary splitter 101a, and the primary splitter 101a is configured to split the coherent light between the first optical combiner 110 and the second optical combiner 110′. In particular, the system is arranged such that part of the coherent light is directed to the first phase modulation assembly 180 and part of the coherent light is directed to the second phase modulation assembly 180’. In this way, two data streams 141, 142 may be encoded on a light signal to provide a first output 190, and another two data streams 141’, 142’ may be encoded on a light signal to provide a second output 190’, which will be coherent with the first output 190. The parallelized system (e.g. described in relation to Figure 6) can be combined with the sequential system (e.g. described in relation to Figures 4 and 5) to enable parallel sequential data combination. It will be appreciated from the discussion above that the embodiments shown in the Figures are merely exemplary, and include features which may be generalised, removed or replaced as described herein and as set out in the claims. With reference to the drawings in general, it will be appreciated that schematic functional block diagrams are used to indicate functionality of systems and apparatus described herein. It will be appreciated however that the functionality need not be divided in this way, and should not be taken to imply any particular structure of hardware other than that described and claimed below. The function of one or more of the elements shown in the drawings may be further subdivided, and / or distributed throughout apparatus of the disclosure. In some embodiments the function of one or more elements shown in the drawings may be integrated into a single functional unit. In some examples the functionality of the controller described herein may be provided by a general purpose processor, which may be configured to perform a method according to any one of those described herein. In some examples the controller may comprise digital logic, such as field programmable gate arrays, FPGA, application specific integrated circuits, ASIC, a digital signal processor, DSP, or by any other appropriate hardware. In some examples, one or more memory elements can store data and / or program instructions used to implement the operations described herein. Embodiments of the disclosure provide tangible, non-transitory storage media comprising program instructions operable to program a processor to perform any one or more of the methods described and / or claimed herein and / or to provide data processing apparatus as described and / or claimed herein. The controller may comprise an analogue control circuit which provides at least a part of this control functionality. An embodiment provides an analogue control circuit configured to perform any one or more of the methods described herein. The above embodiments are to be understood as illustrative examples. Further embodiments are envisaged. It is to be understood that any feature described in relation to any one embodiment may be used alone, or in combination with other features described, and may also be used in combination with one or more features of any other of the embodiments, or any combination of any other of the embodiments. Furthermore, equivalents and modifications not described above may also be employed without departing from the scope of the invention, which is defined in the accompanying claims.

Claims

CLAIMS:

1. An optical data combiner, for use with a coherent light source configured to provide a frequency swept signal, the optical data combiner comprising: a first arm for carrying a light signal; a second arm for carrying a light signal; a splitter configured to split the frequency swept signal to provide a first light signal in the first arm and a second light signal in the second arm; a first phase modulator configured to control a phase modulation of the first light signal based on a first data stream; a second phase modulator configured to control a phase modulation of the second light signal based on a second data stream; and a light combiner for combining the first light signal with the second light signal to produce a combined light signal which has: a frequency content resulting from the optical path length difference between the first arm and the second arm; and a phase based on the phase modulation of the first data stream and the second data stream.

2. The optical data combiner of claim 1, wherein the frequency content comprises at least one frequency component and the at least one frequency component has a phase based on the combination of a data element of the first data stream with a data element of the second data stream.

3. The optical data combiner of claim 1 or claim 2, further comprising a delay component configured to provide the optical path length difference between the first arm and the second arm.

4. The optical data combiner of claim 3, wherein the delay component comprises an optical delay line between the first phase modulator and the light combiner.

5. The optical data combiner of claim 4, wherein the optical delay line is configured to introduce a fixed delay to the first light signal.

6. The optical data combiner of any preceding claim, wherein the splitter is a 50:50 splitter.

7. The optical data combiner of any preceding claim, wherein at least one of the first phase modulator and the second phase modulator is one of: a liquid crystal spatial light modulator, an acousto-optic modulator, and an electro-optic modulator.

8. An optical computing element comprising: the optical data combiner of any preceding claim; and a controller configured to superpose a first interval of the combined light signal with a second interval of the combined light signal to produce a superposed signal.

9. The optical computing element of claim 8, wherein the controller is further configured to: determine frequency data representing frequency components of the superposed signal; and compare the frequency data with a stored association to perform one of: a logic operation and an elementary operation, for example an arithmetic operation such as addition, subtraction, multiplication, or division.

10. The optical computing element of claim 9, wherein the stored association comprises a relation between said frequency data and a result of said operation, for example wherein the operation comprises a logic operation, and the relation comprises a truth table, for example wherein the logic operation is a Boolean operation or wherein said operation is an arithmetic operation and the relation comprises a mapping between frequency data and the arithmetic result of the combination.

11. A method of optically combining first data from a first data stream with second data from a second data stream, the method comprising: generating a frequency swept signal from a coherent light source; splitting the frequency swept signal to provide a first light signal in a first arm and asecond light signal in a second arm; modulating a phase of the first light signal based on the first data; modulating a phase of the second light signal based on the second data; providing an optical path length difference between the first light signal and the second light signal; and combining the first light signal with the second light signal into a combined light signal.

12. The method of claim 11, wherein the frequency swept signal comprises a repeating signal having a first part and a second part, and wherein the combined light signal has a first interval resulting from the first part of the repeating signal and a second interval resulting from the second part of the repeating signal, wherein the method further comprises superposing the first interval with the second interval into a superposed signal.

13. The method of claim 12, wherein modulating the phase of the first light signal and / or the second light signal comprises applying a different phase modulation to the first part and the second part.

14. The method of claim 12 or claim 13, wherein modulating the phase of the first light signal and / or the second light signal comprises applying a reference phase modulation to the second part.

15. The method of any of claims 12 to 14, wherein modulating the phase of the first light signal and / or the second light signal comprises phase modulating only the first part.

16. The method of any of claims 12 to 15, further comprising generating frequency data representing frequency components of the superposed signal, for example by performing a frequency transform such as a Fourier transform.

17. The method of claim 16, further comprising performing a logic operation based on the frequency data of the superposed signal.

18. The method of claim 17, wherein the logic operation is performed by comparing thefrequency data with a stored association.

19. An apparatus comprising a plurality of optical data combiners according to any of claims 1 to 7 arranged so that the combined light signal from a first one of the optical data combiners provides the coherent light source of a second one of the optical data combiners, such that the first optical data combiner and the second optical data combiner are arranged in series.

20. An apparatus comprising a plurality of optical data combiners according to any of claims 1 to 7 arranged so that a data stream carried by the combined light signal from a first one of the optical data combiners provides a data input to at least one of the phase modulators of a second one of the optical data combiners, such that the first optical data combiner and the second optical data combiner are arranged in series.

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