Optical characteristic control device

Abstract

An exemplary optical characteristic control device according to the present disclosure is provided with a storage device and a control circuit. The storage device has circuit parameters of an optical circuit stored therein. The control circuit derives a control value that, on the basis of the circuit parameters and information indicating a target output of the optical circuit, causes the output of the optical circuit to approach the target output, and that supplies the derived control value to the optical circuit.

Description

Optical property control device

[0001] This disclosure relates to an optical property control device.

[0002] With the expansion of data communication networks such as the internet, there is a growing demand for even greater capacity in optical communication networks. To meet this expanding network demand, multiband transmission methods that utilize multiple wavelength bands are being considered and adopted.

[0003] In multiband transmission systems, the use of O-band (Original-band), E-band (Extended-band), S-band (Short-wavelength-band), C-band (Conventional-band), L-band (Long-wavelength-band), and U-band (Ultra-long-wavelength-band) is expected. Of these various bands, the application of multiband systems utilizing the C-band and L-band is beginning in actual systems.

[0004] Japanese Patent Publication No. 2009-15156

[0005] Masashi Sakamoto et al., "Broadband Rare Earth Doped Fiber Amplification Technology - Broadbanding of C-band and L-band," NTT Technical Journal, p. 11, October 2004. Morimoto et al., "Variable Gain Equalizer Using a Lattice Filter Type Quartz Planar Optical Wave Circuit," Proceedings of the IEICE Annual Conference (CD-ROM) (IEICE Society Conference Proceedings (CD-ROM)), Volume: 2022, Issue: Society Conference Page: ROMBUNNO.C-3 / 4-15, Publication Date: August 23, 2022.

[0006] One exemplary purpose of this disclosure is to suppress or reduce the possibility of unexpected changes (e.g., spectral changes) occurring in the output of an optical circuit when controlling the output of the optical circuit to a target output.

[0007] Therefore, an optical characteristic control device according to one aspect of the present disclosure includes a memory device for storing circuit parameters of an optical circuit, and a control circuit for deriving a control value that brings the output of the optical circuit closer to the target output based on information indicating the target output of the optical circuit and the circuit parameters, and supplying the control value to the optical circuit.

[0008] This figure shows an example of a multiband transmission system. This figure shows an example configuration of an erbium-doped optical fiber amplifier (EDFA). This figure illustrates the outline of spectral control (e.g., equalization) using a lattice filter circuit. This block diagram shows an exemplary configuration of an optical signal processing device according to Embodiment 1. This figure shows an example configuration of the lattice filter circuit illustrated in Figure 4. This figure schematically shows an exemplary filter loss with respect to wavelength of the lattice filter circuit in the control process according to Embodiment 1. This is a flowchart illustrating an exemplary control flow according to Embodiment 1.

[0009] Embodiments will be described in detail below with reference to the drawings. However, the accompanying drawings and the following description are provided for the benefit of those skilled in the art to fully understand this disclosure and are not intended to limit the subject matter described in the claims. Furthermore, unnecessary details may be omitted. For example, detailed descriptions of already well-known matters or redundant descriptions of substantially identical configurations may be omitted.

[0010] Furthermore, in the drawings, identical or corresponding elements are appropriately denoted by the same reference numeral. The drawings are schematic, and the dimensional relationships or ratios of each element may differ from reality. There may also be differences in dimensional relationships or ratios between drawings. When numerical values ​​are given in the following explanation, these values ​​are merely examples, and other values ​​may be used additionally or as substitutes.

[0011] <Overview> In multiband transmission systems, the flatness of the signal light spectrum can be impaired during transmission due to wavelength-dependent loss in the optical fiber transmission medium or a nonlinear optical phenomenon called stimulated Raman scattering (SRS). When the flatness of the signal light spectrum is impaired, it can be difficult to obtain good transmission performance across the entire transmission wavelength band. SRS is a phenomenon in which the power of short-wavelength signal light is transferred to long-wavelength signal light.

[0012] Figure 1 shows an example of a multiband transmission system, illustrating an example of applying wavelength division multiplexing (WDM) in multiband transmission. In Figure 1, signals output from two different band (wavelength band) optical transmitters 101a and 101b are wavelength-multiplexed by an optical multiplexer 102 and propagate through a first transmission fiber 103.

[0013] In the following description, it is assumed that optical transmitter 101a transmits, for example, C-band signal light, and optical transmitter 101b transmits, for example, L-band signal light. The number of optical transmitters 101a and 101b is one or more, and Figure 1 shows, as an example, two optical transmitters 101a and two optical transmitters 101b.

[0014] The transmission wavelengths of the two optical transmitters 101a are, for example, different wavelengths in the C band, and the transmission wavelengths of the two optical transmitters 101b are, for example, different wavelengths in the L band. In other words, in the example in Figure 1, the wavelength multiplexing number is "2" for both the C band and the L band.

[0015] The signal light propagating through the first transmission fiber 103 is attenuated in intensity due to wavelength-dependent losses in the transmission fiber 103, and deviations in signal light intensity may occur between wavelengths within the C band and between wavelengths within the L band.

[0016] The signal light propagating through the first transmission fiber 103 is then separated into a C-band component and an L-band component by an optical filter 104 that separates signal light of different wavelength bands. The separated C-band signal light is amplified in intensity by a C-band optical amplifier 105a. The L-band signal light is amplified in intensity by an L-band optical amplifier 105b. When optical amplifiers 105a and 105b are not distinguished from each other, they may be abbreviated as "optical amplifier 105".

[0017] The C-band and L-band signal light, amplified by the optical amplifiers 105a and 105b respectively, are combined by an optical filter 106 that combines signal light of different wavelength bands, propagate through the second transmission fiber 107, and are input to the optical demultiplexer 108.

[0018] The optical demultiplexer 108 separates the input signal light from the second transmission fiber 107 by wavelength. Of the separated signal light of each wavelength, the signal light with C-band wavelengths is input to the C-band optical receiver 109a, and the signal light with L-band wavelengths is input to the L-band optical receiver 109b.

[0019] Note that the number of optical receivers 109a and 109b is one or more, and in Figure 1, as an example, two optical receivers 109a and 109b are provided, corresponding to the number of optical transmitters 101a and 101b. For example, the receiving wavelengths of the two optical receivers 109a are different in the C band, and the receiving wavelengths of the two optical receivers 109b are different in the L band.

[0020] In the above example, the optical amplifier 105 may be an optical fiber amplifier in which rare earth ions such as praseodymium (Pr), erbium (Er), thulium (Tm), and ytterbium (Yb) are added to the optical fiber, which is the amplification medium.

[0021] The following describes an example in which an erbium-doped optical fiber amplifier (EDFA) is applied to the optical amplifier 105. EDFAs amplify signal light by stimulated emission from a higher level to a lower level of the excited population inversion distribution, but the gain spectrum is not necessarily flat with respect to the signal light wavelength.

[0022] Therefore, an EDFA may be equipped with a gain flattening filter (GFF) to shape (e.g., flatten) the gain spectrum. The GFF may have fixed filter characteristics (or transmission characteristics) optimized to match the amplification characteristics of the EDFA.

[0023] Figure 2 shows an example of the configuration of the EDFA 105. The signal light input to the input port 201 of the EDFA 105 is combined with the excitation light from the excitation light source 202 of the EDFA 105 in the WDM coupler 203 and input to the EDF 204.

[0024] In the EDF 204, the energy of the excitation light is transferred to the signal light, amplifying the signal light intensity. The signal light output from the EDF 204 has the residual excitation light component 207 removed in the WDM coupler 206, and then propagates to the output port 210 via the isolator 208 and GFF 209. The isolator 208 and GFF 209 can be swapped in order in the direction of light propagation.

[0025] The GFF209 is designed and installed to have a loss spectrum that compensates for the gain deviation of the output signal light that may arise due to the stimulated emission gain spectrum in the EDF204. For example, the GFF209 is designed to compensate for a profile typical of the gain spectrum of the EDF204 and is formed by a dielectric multilayer film deposited on a substrate.

[0026] In an optical communication system using EDFA 105, when long-distance transmission is performed, multiple optical amplifiers 105 are installed in multiple stages in the transmission direction of the signal light. To compensate for transmission fiber loss corresponding to the spacing between them, the output signal light level of each optical amplifier 105 can be adjusted.

[0027] For example, in Figure 1, the C-band optical amplifier 105a and the L-band optical amplifier 105b each amplify the signal light to an intensity that can compensate for the losses occurring in the second transmission fiber 107 for the C-band and L-band, respectively.

[0028] Therefore, each optical amplifier 105 may be set to have a gain that compensates for transmission fiber loss in each of the multiple stages. Here, the distance of the transmission fiber may vary depending, for example, on the location where the optical amplifier 105 (or optical node containing the optical amplifier 105) can be installed.

[0029] The locations where the optical amplifier 105 can be installed may vary depending on, for example, the location of the telecommunications carrier's central office, and therefore the distance of the transmission fiber may also vary depending on the location of the central office. Consequently, the transmission fiber loss compensated by the optical amplifier 105 may also vary for each optical amplifier 105.

[0030] Losses that may vary from one transmission fiber section to another can be appropriately compensated, for example, by adjusting the excitation light intensity of the optical amplifier 105 at each node. However, since the excited state of the optical amplifier 105 may change in response to the adjustment of the excitation light intensity, the gain spectrum may also change in response to the change in the excited state. Therefore, the GFF 209, which flattens the gain spectrum of the EDF 204, has limitations in flattening the signal light spectrum.

[0031] Furthermore, in transmission systems that transmit multiband signal light such as C-band and L-band, the wavelength-dependent loss spectrum of the transmission fiber may differ for each wavelength band. Moreover, because light energy is transferred from the short-wavelength band to the long-wavelength band due to the influence of SRS, the excitation light control of EDFA 105 may also become more complex. As a result, the flatness of the signal light spectrum output from EDFA 105 may be impaired. Furthermore, the excitation state and gain spectrum may also fluctuate depending on the ambient temperature in which EDF 204 is installed.

[0032] For gain equalizers such as the GFF209, a lattice filter circuit constructed using an optical waveguide can be applied, for example (see Non-Patent Document 2).

[0033] Another point of discussion is that, in short-distance optical communication, the IMDD (Intensity Modulation / Direct Detection) method can be used from an economic standpoint. However, in the IMDD method, the waveform of the signal light is easily distorted due to the dispersion of optical fibers, which can limit the transmission distance.

[0034] Therefore, for example, by installing a dispersion compensator having characteristics opposite to those of the dispersion characteristics of an optical fiber at the wavelength used for optical communication in the optical fiber transmission line, it is possible to extend the transmission distance. For the dispersion compensator, for example, a lattice filter circuit or a transversal filter circuit configured using an optical waveguide is applicable (for example, Patent Document 1).

[0035] As described above, a lattice filter circuit or a transversal filter circuit configured using an optical waveguide may be useful for compensating for a deterioration in the quality of signal light, such as equalization of the gain spectrum or compensation for dispersion. On the other hand, controlling a lattice filter circuit or a transversal filter circuit configured using an optical waveguide may involve a certain degree of difficulty.

[0036] FIG. 3 is a diagram for explaining an outline of spectrum control (for example, equalization) using a lattice filter circuit 3000. In FIG. 3, the lattice filter circuit 3000 equalizes (or shapes) a non-flat input optical spectrum 3001 and outputs a flat output optical spectrum 3002.

[0037] The lattice filter circuit 3000 has, for example, two waveguides 3003 and 3004. Spectrum shaping is achieved by the wavelength dependence when the signal lights propagating through the waveguides 3003 and 3004 interfere with each other in the optical coupler 3005.

[0038] Here, since all the signal light components of the input optical spectrum 3001 propagate through each of the waveguides 3003 and 3004, it is not easy to partially or selectively change the power of the signal light components of a specific wavelength.

[0039] For example, in FIG. 3, even if the phase shifter 3006 is controlled to change the interference state of the signal light components for the purpose of attenuating the signal light having a wavelength corresponding to the peak 3001a of the input optical spectrum 3001, it is difficult to selectively attenuate the signal light components having a wavelength corresponding to the peak 3001a.

[0040] In other words, even when attenuation control is applied to signal light of a specific wavelength, the attenuation control may also affect signal light of wavelengths other than the target wavelength. This phenomenon can be called a "hit" on different wavelengths. A "hit" can affect the entire transmission signal spectrum, so it is a phenomenon that should be avoided.

[0041] Therefore, the following describes several exemplary embodiments of controlling optical circuits (in other words, controlling optical characteristics) that can reduce the effects of "hits".

[0042] <Embodiment 1> In this embodiment 1, an example of the configuration of the optical signal processing device 40 will be described with reference to Figure 4, using the case where the signal optical spectrum is controlled (e.g., equalized or shaped) using a lattice filter circuit as an example. In the following descriptions, including embodiments 2 and 3 described later, "spectrum" may include light intensity information or phase information. For example, as in the case of optical dispersion compensation, it can be understood that the "spectrum" may include not only light intensity information but also light phase information.

[0043] Figure 4 is a block diagram showing an exemplary configuration of the optical signal processing device 40 according to Embodiment 1. As shown in Figure 4, the optical signal processing device 40 includes, for example, a lattice filter circuit 410, an optical monitor 412, and a control circuit 413. Note that the optical monitor 412 is not essential and may be omitted.

[0044] The lattice filter circuit 410 is a non-exclusive example of an "optical circuit." An "optical circuit" is not limited to the lattice filter circuit 410, but may include a transversal filter circuit or other optical circuits or devices capable of shaping the input optical spectrum. Furthermore, in the optical signal processing device 40, the configuration excluding the lattice filter circuit 410 may correspond to an "optical characteristic control device" that controls the optical characteristics of the optical circuit. The term "optical characteristic control device" may be interchangeable with other terms such as "optical spectrum control device" or "optical intensity control device."

[0045] The signal light input to the input port (or input waveguide) 400 is output as output signal light 401 after its spectrum is controlled (e.g., equalized) by, for example, the lattice filter circuit 410. The light input to the lattice filter circuit 410 through the input port 400 may be, but is not limited to, the output light of the optical amplifier 105 (e.g., the output light of the GFF 209). For example, the light input to the optical amplifier 105 may be input to the input port 400, or the input or output of other optical circuits or optical devices may be input to the lattice filter circuit 410.

[0046] Figure 5 shows an example of the configuration of the lattice filter circuit 410. The lattice filter circuit 410 can be formed using, for example, a silica-based planar optical circuit (PLC) or various optical waveguide materials such as silicon photonics or lithium niobate.

[0047] The lattice filter circuit 410, as an example, includes two parallel optical waveguides 502 and optical waveguide 504, and has a configuration in which multiple optical interferometers 505 are connected in series, with each optical interferometer (e.g., MZI) 505 being composed of an optical coupler 506 and an interference arm 507. The "interference arm" may also be referred to as an "arm waveguide."

[0048] Both (or either) of the optical waveguides 502 and 504 constituting each optical interferometer 505 may be loaded with, for example, a phase shifter 503. The phase shifter 503 may, exemplary, be a phase shifter based on the thermo-optic effect or the electro-optic effect (in the case of the thermo-optic effect, for example, a heater). A control signal generated in the control circuit 413 is applied to the phase shifter 503, for example.

[0049] Here, the order of the lattice filter circuit 410 can be such that it can compensate for non-flatness less severe than the change in the gain spectrum dependent on the excitation light intensity, the wavelength-dependent loss spectrum of the optical fiber, and / or the non-flatness of the optical spectrum caused by SRS.

[0050] For example, in the lattice filter circuit 410, the order of the filter's spectral shape is determined by the number of stages of the multiple optical interferometers 505 connected in series. Therefore, the lattice filter circuit 410 may be configured with an optical interferometer 505 with a number of stages that can compensate for a non-flatness less severe than that caused by the aforementioned phenomena.

[0051] For example, the lattice filter circuit 410 should be equipped with an optical interferometer 505 with a number of stages sufficient to compensate for changes in the gain spectrum dependent on the excitation light intensity, the wavelength-dependent loss spectrum of the optical fiber, and / or spectral non-flatness that may occur due to SRS.

[0052] As illustrated in Figure 4, a portion of the output signal light 401 of the lattice filter circuit 410 is branched by a tap 402, and this branched light is input to the optical monitor 412. The optical monitor 412 monitors the spectrum of the branched light (in other words, the output signal light 401) input from the tap 402 and outputs the monitoring result to the control circuit 413 (for example, the processor 403). Note that "monitor" may be replaced with other terms such as "measure" or "detect".

[0053] As shown in Figure 4, the control circuit 413 includes, as an example, a processor 403, a storage device 404, and a driver 405. The processor 403 may be a central processing unit (CPU), an MPU (Micro Processing Unit), or any other processing unit with computing capabilities.

[0054] The information that can be input to the processor 403 may, for example, include measurement results from the optical monitor 412 and / or user input. The measurement results from the optical monitor 412 (e.g., spectral information) are input to the processor 403 as a digital signal via, for example, an analog-to-digital converter (ADC) (not shown) in the input IF 406. User input may, for example, include information indicating a target spectrum for the output of the lattice filter circuit 410, which will be described later.

[0055] The memory device 404 stores information or data used in calculations by the processor 403, such as the loss spectrum of the lattice filter circuit 410 and / or circuit parameters.

[0056] Circuit parameters may include, as a non-limiting example, the group refractive index, refractive index dispersion, temperature dependence of the equivalent refractive index of the interference arm 507 in the lattice filter circuit 410, and / or the wavelength dependence of the coupling ratio of the optical coupler 506 constituting the interferometer. The circuit parameters may define or specify a hypothetical model representing the transmission or loss characteristics of the lattice filter circuit 410.

[0057] The processor 403 generates control signals for the lattice filter circuit 410 (e.g., one or more phase shifters 503) based on information or data stored in the memory device 404, measurement results from the optical monitor 412, and / or user input.

[0058] For example, the processor 403 derives a new target loss spectrum for the lattice filter circuit 410 based on the difference between the current loss spectrum of the lattice filter circuit 410, stored in the memory device 404, and the output optical spectrum obtained by the optical monitor 412.

[0059] As a non-limiting example, the processor 403 may calculate an intermediate spectrum between the current loss spectrum of the lattice filter circuit 410 and the target loss spectrum. For example, the processor 403 may derive a number of control values ​​{C1, C2, ... Cn} that give the intermediate spectrum of the lattice filter circuit 410, based on the circuit parameters of the lattice filter circuit 410 stored in the memory device 404. The "intermediate spectrum" will be described later with reference to Figure 6.

[0060] The control values ​​{C1, C2, ..., Cn} obtained in the processor 403 are supplied to the driver 405, which generates the drive signal for the lattice filter circuit 410, in the order of C1, C2, ..., Cn.

[0061] The driver 405 can be configured in a way that is suitable for driving the lattice filter circuit 410. For example, it could be a current driver that drives a phase shifter (e.g., a heater) using a thermo-optic effect with current.

[0062] Figure 6 schematically shows an example of the filter loss (in other words, "loss spectrum") of the lattice filter circuit 410 with respect to wavelength during the control process described above. Figure 6 shows an example in which the initial loss spectrum 601 of the lattice filter circuit 410 is changed to the target spectrum 604 in the wavelength band to which the signal light is assigned.

[0063] Furthermore, the loss spectrum (intermediate spectrum) in the intermediate state between the loss spectrum 601 and the target spectrum 604 is represented by reference numerals 602 and 603 in Figure 6. Note that "loss spectrum" may be read interchangeably with "transmission spectrum," and can represent the transmission or loss characteristics of the lattice filter circuit 410 with respect to wavelength.

[0064] In the example in Figure 6, if the filter loss at the wavelength indicated by point 600 corresponds to the expected value at that wavelength, it is expected that the fluctuation in the filter loss at point 600 will be suppressed as the loss spectrum 601 is changed to the target spectrum 604.

[0065] For example, when the control circuit 413 (e.g., the processor 403) changes the loss spectrum 601 to the target spectrum 604, it may apply a control signal to the actual lattice filter circuit 410 and control the spectrum of the output signal light 401 monitored in the optical monitor 412 to approach the target spectrum 604.

[0066] Alternatively, the control circuit 413 may optimize the control by, for example, changing the control signal given to the lattice filter circuit 410 during the adaptive control process, and evaluating whether the loss spectrum approaches or moves away from the target spectrum 604.

[0067] Therefore, in adaptive control to bring the output light spectrum of the lattice filter circuit 410 closer to the target spectrum 604, a "hit" may occur at the wavelength indicated by point 600. To suppress or reduce such "hits," for example, the processor 403 derives intermediate spectra 602 and / or 603 based on a virtual model of the lattice filter circuit 410 (hereinafter also referred to as the "virtual model") defined by the circuit parameters stored in the memory device 404.

[0068] The processor 403 then derives, for example, a target spectrum 604 and n control values ​​{C1, C2, ... Cn} that give (in other words, reproduce) n intermediate spectra. The obtained control values ​​{C1, C2, ... Cn} are then provided, for example, from the processor 403 to the actual lattice filter circuit 410 via the driver 405. The control values ​​{C1, C2, ... Cn} can, as an example, be provided sequentially to the lattice filter circuit 410.

[0069] Figure 6 shows, as a non-limiting example, that the initial loss spectrum 601 is obtained by the initial control value C0, the intermediate spectra 602 and 603 are obtained by the control values ​​C1 and C2, respectively, and the target spectrum 604 is obtained by the control value C3.

[0070] The intermediate spectra 602 and 603 can be derived, for example, in the processor 403 by complementing the initial loss spectrum 601 with the target spectrum 604. Note that the target spectrum 604 of the lattice filter circuit 410 is not limited to a loss spectrum intended to equalize the input signal light. For example, a loss spectrum in which the loss on the short-wavelength side is increased compared to the loss on other wavelengths, or vice versa, may be used as the target spectrum 604.

[0071] For example, the target spectrum 604 may be set as information indicating the target spectrum 604 by user input, or it may be derived in the control circuit 413 based on the current output light spectrum measured by the optical monitor 412 and the current loss spectrum of the lattice filter circuit 410.

[0072] Figure 7 shows a flowchart illustrating an example of the control described above. As shown in Figure 7, the control circuit 413 (for example, the processor 403) derives the target spectrum and intermediate spectrum of the lattice filter circuit 410 based on the measurement results from the optical monitor 412 or user input (S701).

[0073] Next, the control circuit 413 derives control values ​​that give the target spectrum and each of the n intermediate spectra based on a virtual model of the lattice filter circuit 410, for example by optimization calculation (S702).

[0074] The control circuit 413 then applies the obtained control values ​​to the lattice filter circuit 410 (for example, the phase shifter 503) (S703). For example, the control circuit 413 may sequentially supply each control value to the lattice filter circuit 410 in the order of a control value that gives an intermediate spectrum close to the current output light spectrum of the lattice filter circuit 410 to a control value that gives an intermediate spectrum close to the target spectrum.

[0075] As described above, according to Embodiment 1, the output (e.g., spectrum) of the lattice filter circuit 410 is controlled to the target output based on the circuit parameters (or virtual model) of the lattice filter circuit 410, making it possible to suppress or reduce the effects of "hits". Therefore, for example, it is possible to suppress or reduce the loss of continuity of the signal light due to "hits" (e.g., momentary interruption).

[0076] <Embodiment 2> In Embodiment 2, another method for determining the control value (Vi) of the lattice filter circuit 410 based on the circuit parameters of the lattice filter circuit 410 stored in the memory device 404 will be described.

[0077] For example, the control circuit 413 can derive a control value Vi based on the current loss spectrum of the lattice filter circuit 410 and the wavelength-specific measurements taken by the optical monitor 412. "Vi" represents, for example, a control value (e.g., a voltage value) applied to the phase shifter 503 of the optical interferometer 505 located at the i-th position (where i is a non-negative integer) from the input side of the lattice filter circuit 410.

[0078] The control circuit 413 can derive, for example, the transmission characteristic Ic of the lattice filter circuit 410 when a control value Vi is applied, based on the circuit parameters stored in the memory device 404. For example, the vector E representing the output electric field of the lattice filter circuit 410 illustrated in Figure 5 is expressed by the following equation (1).

[0079] In equation (1), "Ci" (where i is a non-negative integer) is a 2x2 transfer matrix representing the i-th directional coupler (optical coupler) 506 located from the input side of the lattice filter circuit 410, and can be expressed, for example, by the following equation (2).

[0080] Furthermore, in equation (1), "Ai" is a 2x2 transfer matrix representing the optical propagation in the i-th interference arm 507 located from the input side of the lattice filter circuit 410, and can be expressed, for example, by the following equation (3).

[0081] In equation (2), "φ" represents the phase angle that changes with the coupling length of the directional coupler 506, and in equation (3), "β" represents the propagation constant of the interference arm 507, and "L" represents the path length difference of the interference arm 507.

[0082] The transmission characteristic Ic of the lattice filter circuit 410 can be derived, for example, from equation (1) to equation (4). However, (1) in equation (4) represents the first component of the electric field vector E in equation (1), ＊ The symbol represents the complex conjugate.

[0083] Next, the control circuit 413 derives the loss spectrum It of the lattice filter circuit 410, which can equalize the current optical spectrum, based on the wavelength measurements taken by the optical monitor 412.

[0084] For example, the control circuit 413 calculates the evaluation function F, expressed by the following equation (5), as a variable Vi (i = 1 to K), and performs calculations sequentially so that the evaluation function F is minimized or the error ε is within an acceptable range.

[0085] "K" represents the number of optical interferometers 505 constituting the lattice filter circuit 410 (in other words, the number of stages in the lattice filter circuit 410), and "T" represents the temperature of the lattice filter circuit 410 (in other words, the ambient temperature). Therefore, the control value Vi can be generated for each different temperature T. Temperature T information can, for example, be acquired by a temperature sensor (not shown) provided in the lattice filter circuit 410 and provided to the control circuit 413.

[0086] For the optimization calculation of equation (5), example optimization methods such as the steepest descent method and / or the annealing method can be applied. Furthermore, not limited to this example, the evaluation function F may also be weighted by equation (6) below, for example, for wavelengths where the optical signal-to-noise ratio (OSNR) is prone to degradation (e.g., wavelengths where the input optical power is relatively small). In equation (6), W = Σw(i).

[0087] The control circuit 413 repeats the optimization calculation until the evaluation function F is minimized or the error ε is within an acceptable range. When the evaluation function F is minimized or the error ε is within an acceptable range, the control circuit 413 provides the finally obtained set of control values ​​Vi to the lattice filter circuit 410 as a control signal.

[0088] Thus, in Embodiment 2, the control circuit 413 derives control values ​​based on a virtual model of the lattice filter circuit 410 pre-stored in the memory device 404, and provides control values ​​to the actual lattice filter circuit 410.

[0089] Therefore, unlike when determining the target spectrum by trial and error while changing the control value given to the lattice filter circuit 410, it is not necessary to control the actual lattice filter circuit 410 for the purpose of searching for the control value. Thus, there is the advantage that it does not affect the actual signal light during optical communication, in other words, no hits occur.

[0090] <Embodiment 3> In Embodiment 3, the method for deriving (or identifying) the circuit parameters described above will be explained. In the control method described in Embodiment 2, it is expected that the circuit parameters of the lattice filter circuit 410 will be derived with high accuracy and used to derive the control value. Therefore, it is expected that a precise virtual model of the lattice filter circuit 410 will be prepared.

[0091] In deriving the circuit parameters, for example, an appropriate control signal is applied to at least one of the control terminals of the manufactured lattice filter circuit 410, and its output optical spectrum is measured. Here, in order to identify multiple parameters such as the thermo-optic constants of the optical waveguide, the group refractive index, the refractive index dispersion, and the wavelength characteristics of the directional coupler, it is preferable to evaluate by applying multiple control signals. This set of control signals is given by the set of vectors c {c j This is expressed as} (j = 0 to p).

[0092] c j In the example in Figure 7, there are 10 phase shifters 503, so it is represented as a vector with 10 elements. j In the following, the process of evaluating the light spectrum by applying a given signal will be referred to as a trial.

[0093] The number of trials is expected to be as few as possible from a manufacturability standpoint. Furthermore, if the lattice filter circuit 410 is constructed using a silica-based PLC or silicon photonics, the equivalent refractive index of the optical waveguide may change via thermo-optic effects depending on the ambient temperature, so it is expected that trials will be conducted at multiple ambient temperatures.

[0094] Therefore, a set with fewer elements {c} jIt is expected to accurately identify (or parameter fit) parameters by {}. To accurately identify circuit parameters, it is preferable that the functions representing the optical spectra in each trial are in a state close to being independent of each other. An example of a method for expressing this state close to independence is shown below.

[0095] Let the vector with the circuit parameters to be identified as elements be represented by n (assuming the number of elements is r, that is, the number of parameters to be identified is r), and the control value c j The spectrum obtained in the trial when is given is represented by S(λ l , c j ).

[0096] S(λ l , c j ) represents a vector with elements being the light intensities measured (sampled) by the optical monitor 412 when control values c0, c1, c2,..., c q are given for each of wavelengths λ0, λ1, λ2,..., λ p . Note that in one trial (spectrum scan), q elements are obtained.

[0097] Here, the Jacobian J obtained by partially differentiating each element arranged with S(λ l , c j ) for the r elements of the vector n is defined as in the following equation (7).

[0098] The Jacobian J is a matrix with q×p rows and r columns. Using this matrix, the condition number κ is defined by the following equation (8).

[0099] Note that in equation (8), represents the norm of the vector. S corresponds to a matrix formed by arranging S(λ ｌ , c ｊ ) in the column direction and connecting them.

[0100] Here, the smaller the condition number κ (in other words, the less large it is), the higher the accuracy of the solution (in other words, the smaller the error). Therefore, for example, a set of control values {c such that the condition number κ becomes small to the extent that the reproduction accuracy of the optical spectrum measured by the optical monitor 412 is satisfied}j Circuit parameters are identified using}.

[0101] For example, the control circuit 413 determines a set of control values ​​{c} such that the condition number κ is sufficiently small, based on known values ​​of circuit parameters such as group refractive index, dispersion, and temperature coefficient of refractive index derived by mode calculations, etc. j Let's find the set {c} and j Based on}, each c j A trial is performed on this. Then, using the same method as described in Embodiment 2, a list of circuit parameters that reproduce the measured optical spectrum is derived by optimization.

[0102] Note that the circuit that derives the circuit parameters is not limited to the control circuit 413. For example, the circuit parameters may be derived by a separate computing device, such as a personal computer, and provided to the control circuit 413 (for example, stored in the memory device 404).

[0103] <Supplement> The optical spectrum control described herein is not limited to control that compensates for or equalizes the non-flatness of the optical spectrum; for example, any control that minimizes the deviation from the expected optical spectrum in a particular wavelength band is acceptable.

[0104] As used in this disclosure, the term “connection” may be replaced with “joining.” “Connection” or “joining” may be understood to mean any direct or indirect “connection” or “joining” between two or more elements. For example, an indirect “connection” or “joining” between two elements that are mutually “connected” or “joined” may be understood to be included in the term.

[0105] Any reference to elements with designations such as "first..." or "second..." does not limit the quantity or order of those elements. These designations are merely a convenient way of distinguishing between two or more elements. For example, references to the first and second elements do not imply that only two elements can be adopted, nor do they imply that the first element must take precedence over the second element in any physical quantity.

[0106] While the Disclosure has been described in detail above, it will be apparent to those skilled in the art that the purpose and scope of this Disclosure are not limited to what has been described herein. This Disclosure can be implemented in modified and altered forms without exceeding the purpose and scope of this Disclosure as defined by the claims. Therefore, the descriptions in this Disclosure are for illustrative purposes only and are not intended to be restrictive in any way to the purpose and scope of this Disclosure.

[0107] This disclosure is useful, for example, in optical communication technology.

Claims

1. An optical characteristic control device comprising: a memory device for storing circuit parameters of an optical circuit; and a control circuit that derives a control value to bring the output of the optical circuit closer to the target output based on information indicating the target output of the optical circuit and the circuit parameters, and supplies the control value to the optical circuit.

2. The optical characteristic control device according to claim 1, wherein deriving the control values ​​includes deriving control values ​​that provide one or more intermediate spectra that complement the current output optical spectrum of the optical circuit and the target spectrum as the target output, the output optical spectrum, the target spectrum, and the intermediate spectra are spectra that include light intensity information or phase information, and supplying the control values ​​to the optical circuit includes supplying the control values ​​to the optical circuit sequentially in the order of a control value that provides an intermediate spectrum close to the current output optical spectrum to a control value that provides an intermediate spectrum close to the target spectrum.

3. The optical characteristic control device according to claim 2, wherein the circuit parameters are identified from the output optical spectrum of the optical circuit measured when the optical circuit is controlled using the set of control values, and the set of control values ​​is a set such that the number of conditions relating to the transmission characteristics of the optical circuit when the set of control values ​​is applied to the optical circuit is small enough to satisfy the reproducibility accuracy of the measured output optical spectrum.

4. The optical characteristic control device according to claim 1, wherein the optical circuit is a lattice filter circuit including a plurality of optical interferometers, and the interference state of the optical interferometers is controlled by the control value.

Images