Patent Publication Number: US-2021181782-A1

Title: Optical Signal Processing Device

Description:
TECHNICAL FIELD 
     The present invention relates to an optical signal processing device that can be applied to optical reservoir computing. 
     BACKGROUND ART 
     In recent years, an environment has been constructed to acquire a large amount of data from various sensors via the Internet, and research and business for analyzing the large amount of acquired data and performing highly accurate knowledge processing and future prediction has been actively carried out. In general, because analysis of enormous data requires time and cost such as power consumption, a computing device having high speed and high efficiency is required. As a computing scheme for such information processing, an optical computing technique called reservoir computing (RC), which imitates signal processing of a cerebellum, has been proposed. Optical computing devices using a dynamical system are attracting attention because the devices are likely to have both high speed and high efficiency. 
     In an example of an application of optical RC in the related art, an example of solving a one-dimensional input and output problem such as a chaos approximation problem and NARMA10 has been mainly reported (for example, see Non Patent literature 1). Further, it is necessary to improve computing accuracy in order to further widen a range of applications of optical RC. In RC, it is generally known that the computing accuracy is improved by an increase in the number of nodes of a reservoir layer. However, in the case of optical RC, because the nodes of the reservoir layer are represented by the number of optical pulses that circulate around a fiber ring, a device having a longer fiber ring is required in order to increase the number of nodes and improve the computing accuracy. 
     CITATION LIST 
     Non Patent Literature 
     Non Patent Literature 1: L. Larger, et al., “Photonic information processing beyond Turing: an optoelectronic implementation of reservoir computing,” Optics Express Vol. 20, Issue 3, pp. 3241-3249 (2012) 
     SUMMARY OF THE INVENTION 
     Technical Problem 
     There are three problems when the fiber ring is lengthened. A first problem is an increase in manufacturing cost of a device. Because a price of a fiber is determined depending on the length, a cost of the fiber increases when the fiber ring is lengthened. A second problem is an increase in a size of the device. Because the fiber cannot be bent at an acute angle in view of loss, a volume for accommodation of the fiber increases when the fiber ring is lengthened. A third problem is unstable operation of the device. Because an optical pulse signal in the fiber easily changes due to an influence of a vibration or temperature change, an operating environment satisfying severe environmental conditions is required as the fiber ring is lengthened. 
     Means for Solving the Problem 
     An object of the present invention is to provide an optical signal processing device capable of improving computing accuracy without increasing the number of nodes of a reservoir layer. 
     In order to achieve such an object, an aspect of the present invention is an optical signal processing device for converting an input one-dimensional signal to an optical signal and performing signal processing, the optical signal processing device including: an input unit configured to perform linear processing on the input one-dimensional signal to convert the one-dimensional signal to an optical signal; a reservoir unit connected to an output of the input unit and configured to perform linear processing and nonlinear processing on the optical signal; an output unit connected to an output of the reservoir unit and configured to convert the optical signal to an electrical signal, perform linear processing to output a one-dimensional output; and a determination unit configured to determine whether the one-dimensional output from the output unit is to be output or to be input as the one-dimensional signal to the input unit. 
     Effects of the Invention 
     According to the present invention, a multi-layer structure in which a computing result calculated by optical RC is input to the input unit again is adopted by including the determination unit that is connected to the output unit and determines whether or not the output from the output unit is fed back as an input signal again, and it is possible to improve computing accuracy without increasing the number of nodes of a reservoir layer corresponding to one layer. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram illustrating an overall configuration of an optical signal processing device according to an embodiment of the present invention. 
         FIG. 2  is a diagram illustrating a configuration of an input unit of the optical signal processing device according to the embodiment. 
         FIG. 3  is a diagram illustrating a configuration of a reservoir unit of the optical signal processing device according to the embodiment. 
         FIG. 4  is a diagram illustrating a specific operation example of the reservoir unit. 
         FIG. 5  is a diagram illustrating a configuration of an output unit of the optical signal processing device according to the embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. 
       FIG. 1  illustrates an overall configuration of an optical signal processing device according to an embodiment of the present invention. The optical signal processing device includes an input unit  11  that performs linear processing on an input one-dimensional signal to convert the signal into an optical signal, a reservoir unit  12  that is connected to an output of the input unit  11  and performs random linear processing and nonlinear processing on the optical signal, and an output unit  13  that is connected to an output of the reservoir unit  12 , converts an optical signal into an electrical signal to perform linear processing to output a one-dimensional output. Further, the optical signal processing device includes a determination unit  14  that determines whether the one-dimensional output from the output unit  13  is output or the output is input as the one-dimensional signal to the input unit. 
     The optical signal processing device according to the embodiment adopts a multi-layer structure in which a computing result calculated by optical RC is input to the reservoir unit  12  again by including the determination unit  14  that determines whether or not the output from the output unit  13  is fed back as an input signal again, and it is possible to effectively increase the number of nodes and improve computing accuracy. In other words, it is possible to improve the computing accuracy without increasing the number of nodes in each reservoir unit using a device configuration in which the numbers of nodes of the reservoir units  12  are the same. 
     Input Unit 
       FIG. 2  illustrates a configuration of the input unit of the optical signal processing device according to the embodiment. The input unit  11  has a function of receiving the one-dimensional signal of a problem to be solved, converting the one-dimensional signal to a predetermined optical signal (an optical pulse train), and propagating the optical signal to the reservoir unit  12 . The one-dimensional input signal to be input to a signal processing unit  111  differs depending on the number of layers of optical RC. A round of an input layer, a reservoir layer, and an output layer, which is a configuration of normal RC, is counted as one layer. Assuming deep optical RC of C layers, the one-dimensional signal of the problem to be solved by the optical RC is input to the input unit  11  in a first layer. In an A-th (1&lt;A≤C) layer, the one-dimensional signal is first input to the deep optical RC, and then the one-dimensional time-series signal propagating from the determination unit  14  is input to an (A−1)-th layer. 
     A case in which the input signal is distributed from one input channel to m nodes of the reservoir layer in normal RC is considered. Here, the input channel corresponds to a sampling number (voice data or the like) or the number of pixels (image data or the like) of one piece of data (which is represented as a pulse train in which one pulse is arranged in  FIG. 2 ). The signal processing unit  111  generates a time-series signal obtained by extending the one-dimensional input signal K-fold in a time axis direction for each pulse. For example, when a pulse width of one input channel is one second, a pulse having a pulse width of K seconds is generated. A subscript K is 1≤K≤m, and is intended to cause K nodes to be selected from among m nodes of normal RC and cause a one-dimensional input signal to be input. 
     A randomly determined weight w in   lm  of the input unit is then multiplied with the extended time-series signal. A subscript  1  is a type of the one-dimensional signal in the first layer corresponding to the input channel, and is N in the second and subsequent layers. This allows a pulse extended to K seconds to be a modulation signal having different intensity for one second. An optical modulation unit  112  modulates an optical signal from a light source  113  with a modulation signal having information of w in   lm ·um. 
     Thus, the input unit  11  outputs, to the reservoir unit  12 , a pulse train in which K pulses having light intensity corresponding to a magnitude (intensity) of the input signal are connected by the number of 1 types of one-dimensional signals for each time step u m . 
     For the light source  113 , an incoherent light source or a coherent light source can be used. When the former is used, the light source can be operated relatively stably because only intensity information is used. When the latter is used, an amount of information can be doubled because both intensity information and phase information are used. 
     For the optical modulation unit  112 , an optical attenuator such as an LN modulator or an optical amplifier such as a semiconductor optical amplifier can be used. When the former is used, it is possible to shorten a computing time because modulation can be performed at a high speed. When the latter is used, it is possible to curb deterioration of computing capability due to a loss because a signal can be amplified. 
     A weight w in  of the input unit is given before training of optical RC starts, and a value of the weight is not updated through training or a determination. All values of respective elements of the weight w in  (weight w in ≥0) may be different values, or may be the same value for the same m. Although the number of weights differs between the first layer and the second layer, the weights may be different or some of the weights may be the same. 
     Specific Operation Example 
     The one-dimensional signal is one-dimensional time-series data, such as a transition in a stock price of a certain company or a transition in temperature at a certain weather station. A case of utilization for temperature prediction in a specific weather station will be described herein. 
     Time-series data of temperature during a predetermined period of time is divided by certain periods of time, and data is created for each of the divided periods. For example, the predetermined period of time is divided into nine (1=9), and an average temperature during the divided period of time is calculated and used as the one-dimensional input signal. The signal processing unit  111  generates a time-series signal obtained by extending the one-dimensional input signal K-fold in order to perform processing m times in the reservoir unit  12 , and multiplies the weight w in  of the input unit with the time-series signal to generate a modulation signal. The weight w in  may be a random value among K values. 
     The optical modulation unit  112  modulates the optical signal from the light source  113  with a modulation signal, outputs K optical pulses having different light intensities from one input signal, and combines all of the divided one-dimensional signals to output 9 K optical pulse trains. 
     Reservoir Unit 
       FIG. 3  illustrates a configuration of the reservoir unit of the optical signal processing device according to the embodiment. In the reservoir unit  12 , a merging unit  121 , an optical computation processing unit  122 , and a branch unit  123  are provided on a ring waveguide  124  around which an optical pulse train circulates. Going from the merging unit  121  to the ring waveguide  124  and back to the merging unit  121  is counted as one round. After a first pulse train is input to the ring waveguide  124 , a one-dimensional input signal propagating from the input unit  11  at a t-th round and a one-dimensional input signal circulating around the ring waveguide  124  and returning to the merging unit  121  at a (t−1)-th round are combined by the merging unit  121 . The optical computation processing unit  122  performs computation processing on the combined one-dimensional input signal, and the branch unit  123  causes the processed one-dimensional input signal (optical pulse train) to branch to output the branching one-dimensional input signal to the output unit  13  and the merging unit  121 . 
     A dynamic system in the reservoir unit  12  is shown in Equation (1). 
       Math. 1 
         x   1 ( t )=cos 2 (Σ m   K   w   lm   in   ·u   m ( t )+Σ k   K   w   Ik   r   ·x   k ( t− 1))  (1)
 
     Here, u m  is a time step of the input unit  11  and corresponds to a node of the input layer, w in   lm  represents a weight of the input unit, x k (t−1) corresponds to a node of the reservoir layer when a pulse circulates around the waveguide  124  t−1 times, w r   ik  represents a weight of the reservoir unit, and x 1  corresponds to m nodes of the reservoir layer. Among components input to a cos square function of Equation (1), a first term indicates a signal coupled from the input unit  11 , and a second term indicates a signal coupled from the reservoir unit  12 . A weight w r  of the reservoir unit is a fixed value that is randomly determined, like the weight of the input unit. The optical computation processing unit  122  performs linear processing for multiplying the weight w r  of the reservoir unit and nonlinear processing for performing computation of a nonlinear function (a cos square function). The weight w r  of the reservoir unit is a fixed value that is randomly determined, as in the input unit. 
     For the optical computation processing unit  122 , methods of performing linear processing include a method of using an LN modulator and a delay circuit, and a method of demodulating a signal with an electrical signal once, performing electrical computation processing using a PC, an FPGA, or the like, and then performing return to an optical signal. When the former is used, a processing speed becomes higher because the calculation is performed at the speed of light. When the latter is used, it is possible to ensure the computing accuracy because signal compensation can be performed when conversion to electricity is performed. 
     The optical computation processing unit  122  can use, for example, a Mach-Zehnder interferometer or a semiconductor optical amplifier in order to perform the nonlinear processing. When the former is used, power consumption is reduced because nonlinear processing is performed with only passage through the Mach-Zehnder interferometer without using a control signal. When the latter is used, it is possible to change a form of the nonlinear function from the cos square function by changing a current value injected into the semiconductor optical amplifier, and to perform adjustment to an appropriate nonlinear function for a problem to be solved. 
     For the merging unit  123 , for example, a planar optical waveguide (PLC) or a fusion-extended fiber coupler can be used. When the former is used, it is possible to reduce a connection loss and construct a device with a low loss. When the latter is used, it is possible to easily construct the device by combining commercially available products. 
     Specific Operation Example 
     A specific operation example of the reservoir unit will be described with reference to  FIG. 4 . The ring waveguide  124  of the reservoir unit  12  is set to have a length in which K pulses circulate once at equal intervals. When pulse trains are input sequentially from K pulse trains of a first type from the input unit  11 , the pulse trains sequentially circulate around the ring waveguide  124 , and when K pulses of a 9-th type are input, all nine types of pulses are caused to overlap, as illustrated in  FIG. 4 . In the optical computation processing unit  122 , the linear processing for multiplying the weight w r  of the reservoir unit and the nonlinear processing for passing a cos square function are performed for each circulation of the pulse, and adjustment of light intensity of the pulse that is an appropriate fixed value is performed. K pulses, each including nine overlapping pulses, are output from the branch unit  123  to the output unit  13 . 
     The ring waveguide  124  may be extended in length so that K or more pulses can circulate at the same time in consideration of extensibility. In this case, in the pulse train output from the input unit  11 , an idle period of time corresponding to a time corresponding to the extended length is inserted into every K pulses. Thus, the temperature data divided in nine is processed by the m nodes of the reservoir layer. 
     Output Unit 
       FIG. 5  illustrates a configuration of the output unit of the optical signal processing device according to the embodiment. The output unit  13  processes the optical pulse train emitted from the reservoir unit  12  to generate a one-dimensional output. The output unit  13  includes a demodulation unit  131  that converts the optical pulse train emitted from the reservoir unit  12  to an electrical signal, and an electrical computation processing unit  132  that extracts m one-dimensional signals at a time from the converted electrical signal and performs linear processing of m input N output. 
     A dynamic system in the output unit  13  is shown in Equation (2). 
       Math. 2 
         y   j ( t )= k   m   w   jk   o   ·x   k ( t )  (2)
 
     Here, y j  corresponds to a node of the output layer, and w o   jk  is a weight of the output unit. The electrical computation processing unit  132  extracts m of the one-dimensional signals x k (t) output from the demodulation unit at a time and calculates the linear combination shown in Equation (2). The calculation is repeated a number of times corresponding to the number N of categories to be classified, and N one-dimensional outputs are generated from m signals. A weight w o  of the output unit is a value that is calculated by a pseudo-inverse matrix method using a node x k (t) of the reservoir unit and a desired result of the problem to be solved. This value is different for each layer. 
     For the demodulation unit  131 , a light receiver is used. For the electrical computation processing unit  132 , a PC and an FPGA, for example, can be used. When the former is used, it is possible to implement a dynamical system relatively easily. When the latter is used, it is possible to increase a computing speed because dedicated machine can be manufactured. 
     Specific Operation Example 
     In the reservoir unit  12 , the temperature data divided in nine is processed by the m nodes of the reservoir layer. In the output unit  13 , N candidate values for a transition in the predicted temperature are obtained from K pieces of data processed in the reservoir layer. 
     Determination Unit 
     The determination unit  14  determines whether the one-dimensional signal output from the output unit  13  is to be read as a computing result or to be propagated as an input signal to the input unit  11  again. For example, when data having a length of L input  seconds of the one-dimensional signal input to the input unit  11  is solved by RC of A layers, data up to L input ×(A−1) seconds after the signal first propagates to the determination unit  14  propagates to the input unit  11 , and data after L input ×(A−1) seconds is read as a computing result. 
     For the determination unit  14 , a switch can be used. An operation timing of the switch is synchronized with a device that generates the modulation signal of the input unit  11 . 
     Specific Operation Example 
     As described above, in the first layer, the one-dimensional signal of the problem to be solved by the optical RC is input to the input unit  11 , and in the A-th (1&lt;A≤C) layer, the one-dimensional signal is first input to the deep optical RC and then the one-dimensional time-series signal propagating from the determination unit  14  is input at an (A−1)-th round. That is, the determination unit  14  causes the output from the output unit  13  to be input as a one-dimensional signal to the input unit  11  again when the number of times the output unit  13  performs output is smaller than A, and causes the output from the output unit  13  to be output as a one-dimensional output when the number of times the output unit  13  performs output is A. Thus, deep optical RC of the C layers is executed, and the most probable candidate value of the temperature is obtained in the temperature prediction. 
     With the optical signal processing device of the embodiment, it is possible to improve the computing accuracy using a device configuration in which the numbers of nodes are the same by adopting a multi-layer structure in which the computing result calculated by optical RC is input to optical RC again, instead of increasing the number of nodes. In optical RC in the related art, the number of nodes of the reservoir layer is increased so that the computing accuracy is improved, whereas in the embodiment, it is possible to improve the computing accuracy with the same number of nodes, it is not necessary to lengthen the fiber ring in the reservoir layer, and it is possible to curb a manufacturing cost of the device and reduce a size of the device. Further, in the embodiment, it is possible to stabilize an operation of the device because the computation can be performed with a short fiber ring. 
     REFERENCE SIGNS LIST 
     
         
           11  Input unit 
           12  Reservoir unit 
           13  Output unit 
           14  Determination unit 
           111  Signal processing unit 
           112  Optical modulation unit 
           113  Light source 
           121  Merging unit 
           122  Optical computation processing unit 
           123  Branch unit 
           124  Ring waveguide 
           131  Demodulation unit 
           132  Electrical computation processing unit