Optical transmitter, optical transceiver using the same, and wavelength control method

Abstract

To suppress crosstalk due to a high-order spectrum by an optical transmitter using an optical frequency COM as a light source.SOLUTION: An optical transmitter has: a frequency COM light source; a demultiplexer which demultiplexes light output from the frequency COM light source into n (n: an integer equal to or larger than 2) channels at wavelength intervals Δλ, and (n) filters which are connected to n output ports of the demultiplexer respectively and reduce high-order spectra, wavelength by wavelength.SELECTED DRAWING: Figure 3

Description

[Technical field]

[0001] The present disclosure relates to an optical transmitter, an optical transceiver using the same, and a wavelength control method. [Background technology]

[0002] In order to handle large-volume data communications, optical communications using the wavelength division multiplexing (WDM) method are being carried out. In particular, in dense WDM, many channels are densely arranged with narrow wavelength intervals. WDM requires a multi-wavelength light source that outputs light according to the wavelength of each channel. As a multi-wavelength light source, a light source using an optical frequency comb is promising (see, for example, Patent Document 1). An optical frequency comb has many longitudinal modes distributed on the frequency axis at a predetermined repetition frequency, and has a comb-like spectral shape. Each of these comb modes that appear at equal intervals is a continuous wave laser (amplified radiation of light by stimulated emission), and is suitable for WDM optical transmission.

[0003] As a multiplexer for multiplexing a large number of channels, a CAT (Cascaded AMZ Triplet) configuration in which a plurality of Asymmetric Mach-Zehnder (AMZ) interferometers are connected in multiple stages has been proposed (see, for example, Patent Document 2). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] U.S. Patent Application Publication No. 2018-0083599 [Patent Document 2] JP 2021-152569 A Summary of the Invention [Problem to be solved by the invention]

[0005] In a light source that uses an optical frequency comb (hereafter referred to as a "frequency comb light source"), higher-order spectra are generated in addition to the fundamental mode. When the light generated by the optical frequency comb is split by a splitter, the higher-order mode spectra are transmitted due to the periodicity of the filter, and crosstalk occurs with the signal wavelength for transmission. The OIF-400ZR standard to which digital coherent optical communication modules comply specifies that the crosstalk of an interferometer should be -35 dB or less. The higher the performance of the optical frequency comb light source, the greater the power of the higher-order spectra, which may result in the standard specifications not being met. A configuration in which a fixed filter is used to extract only the wavelength band for transmission from the output of the optical frequency comb cannot track the wavelength fluctuations of the optical frequency comb.

[0006] In one aspect, a configuration is provided for suppressing crosstalk due to higher-order spectra in an optical transmitter that uses an optical frequency comb as a light source. [Means for solving the problem]

[0007] The optical transmitter is A frequency comb source; a demultiplexer that demultiplexes the light output from the frequency comb light source into n channels (n is an integer equal to or greater than 2) with wavelength intervals of Δλ; n filters connected to the n output ports of the splitter, respectively, for reducing high-order spectral components for each wavelength; has. Effect of the Invention

[0008] An optical transmitter using an optical frequency comb light source can suppress crosstalk caused by higher-order spectra. [Brief description of the drawings]

[0009] [Figure 1] FIG. 1 is a diagram showing a general configuration of an optical transmitter using an optical frequency comb light source. [Diagram 2] FIG. 1 is a diagram showing technical issues that arise when using an optical frequency comb light source. [Diagram 3] FIG. 2 is a schematic diagram of an optical transmitter according to an embodiment. [Figure 4] FIG. 4 is a schematic diagram of a modified example of the optical transmitter of FIG. [Diagram 5] FIG. 2 illustrates an example of a duplexer. [Figure 6] 6 is a schematic diagram of a filter connected to some of the output ports of the duplexer of FIG. 5. [Figure 7] FIG. 4 is a diagram illustrating a filter characteristic according to the embodiment. [Figure 8] 13 is a flowchart of wavelength control of an optical transmitter. [Figure 9] FIG. 13 is a schematic diagram showing a modified example of the filter. [Figure 10] FIG. 10 is a diagram illustrating the filter characteristics of FIG. [Figure 11] FIG. 11 is a diagram illustrating an effect of the configuration of the embodiment. [Figure 12] 1 is a schematic diagram of an optical transceiver module using an optical transmitter according to an embodiment of the present invention; DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] Before describing the optical transmitter and wavelength control of the embodiment, new technical issues that arise when a frequency comb light source is used in optical communication will be described in more detail with reference to Figs. 1 and 2. Fig. 1 shows a general configuration of an optical transmitter using a frequency comb light source (denoted as "Comb" in the figure). Light output from the frequency comb light source is demultiplexed into multiple channels, i.e., wavelengths λ1 to λn, by a demultiplexer (denoted as "Dmux" in the figure). Light of each wavelength is input to a corresponding IQ modulator (denoted as "IQM" in the figure) and modulated by a data signal input to the IQ modulator. The modulated optical signals of n channels are multiplexed by a multiplexer (denoted as "Mux" in the figure) and a WDM signal is output.

[0011] Figure 2 shows the comb spectrum generated by a frequency comb source. The comb spectrum in Figure 2 is obtained by Fourier transforming the ultrashort pulse train output from the frequency comb source on the frequency axis. Multiple comb lines are generated at a repetition rate of f r Each comb line can be used as a continuous wave laser beam.

[0012] In an optical frequency comb, a higher-order spectrum A is generated. For example, each comb line in the frequency band used for transmission is assigned to n channels. The solid lines represent the comb lines assigned to n channels. The m-th comb line of the optical frequency comb, i.e., the m-th comb mode, through the (m+n)-th comb line are assigned to n channels. The equally spaced dotted lines on both sides of the transmission frequency band represent comb lines outside the transmission frequency band. The comb lines are divided into two groups with a repetition rate of f r When the frequency is virtually extended to zero frequency, a frequency difference of f0 occurs between zero frequency and the first comb line (comb mode). This frequency difference f0 is called the carrier envelope offset frequency, and corresponds to the phase shift between the carrier wave and the envelope contained in the ultrashort pulse on the time axis. The frequency fm of the first channel to which the mth comb line is assigned is m·f r It is represented as +f0.

[0013] If the (n+m+1)th comb line is assumed to be virtual channel 1', a higher-order spectrum due to the second harmonic of channel number 1 appears near virtual channel 1'. The frequency of this higher-order spectrum is 2fm, twice that of channel number 1, and is expressed as 2m·f r It is represented as +2f0. If the power difference Δ between channel number 1 and the higher-order spectrum near virtual channel 1' is -35dB or less, the crosstalk requirement required by the standard is met. However, the difference between the comb line of channel number 1 and the higher-order spectrum near virtual channel 1' is generally about -10dB, and there is a high probability that the signal will be degraded by crosstalk. Due to the periodicity of the splitter filter, not only the second harmonic but also the third harmonic and higher-order spectral components pass through.

[0014] In the embodiment, high-order spectral components are reduced to suppress signal degradation due to crosstalk. Specific configurations and methods of the optical transmitter and wavelength control of the embodiment are described below with reference to the drawings. The embodiment shown below is an example for realizing the technical idea of ​​the present disclosure, and does not limit the disclosed contents. The size, positional relationship, etc. of the components shown in each drawing may be exaggerated to facilitate understanding of the invention. The same components or functions may be given the same name or symbol, and duplicate explanations may be omitted.

[0015] <Optical transmitter configuration> 3 is a schematic diagram of an optical transmitter 10 according to an embodiment. The optical transmitter 10 includes a frequency comb light source 11, a demultiplexing unit 13, a filter unit 4, modulators 17-1 to 17-n, and a multiplexer 19. The demultiplexing unit 13 includes a demultiplexer 131 that demultiplexes the light output from the frequency comb light source 11 into n channels (n is an integer of 2 or more) at a predetermined wavelength interval Δλ. The demultiplexer 131 may have any configuration as long as it can disperse light at a desired wavelength interval, but a waveguide-type demultiplexer formed by silicon photonics technology is suitable for integration. As the waveguide-type demultiplexer, the above-mentioned CAT-configured demultiplexer, an arrayed waveguide grating (AWG), or the like can be used.

[0016] The filter unit 4 has n filters 14-1 to 14-n connected to n output ports of the demultiplexer 131. Each of the filters 14-1 to 14-n reduces high-order spectral components for each wavelength and suppresses crosstalk with respect to the fundamental wave. The light transmitted through the filters 14-1 to 14-n is incident on the corresponding modulators 17-1 to 17-n. The light of each channel modulated by the modulators 17-1 to 17-n is multiplexed by the multiplexer 19 and output as a WDM signal.

[0017] The optical transmitter 10 includes a controller 12 that controls the frequency comb light source 11, a demultiplexing control unit 133 that controls the demultiplexer 131, and control units 15-1 to 15-n that control the filters 14-1 to 14-n. The controller 12, the demultiplexing control unit 133, and the control units 15-1 to 15-n may be realized by a single processor, or may be realized by individual microprocessors or logic devices.

[0018] The frequency comb light source 11 has an arbitrary configuration capable of generating an optical frequency comb in a desired wavelength band used in optical communications. For example, a mode-locked laser using an Er-doped fiber that generates a frequency comb in the 1550 nm band may be used. The mode-locked laser fixes the phase relationship between the longitudinal modes in the resonator (mode-locked state), and generates and outputs ultrashort pulses at peak positions where the phases of each wavelength are aligned. The optical frequency comb is the result of Fourier transforming this ultrashort pulse train on the time axis onto the frequency axis. The controller 12 controls the repetition frequency fr, carrier envelope offset frequency f0, output power, resonator temperature, etc. of the optical frequency comb.

[0019] The demultiplexing control unit 133 controls the phase of the light propagating through the waveguide of the demultiplexer 131 to adjust the effective optical path length, and separates the output light from the frequency comb light source 11 into n light beams at wavelength intervals of Δλ. The phase of the light propagating through the waveguide can be controlled by adjusting the electric field level applied to the waveguide or the temperature of a heater provided in the waveguide to change the refractive index of the waveguide.

[0020] The filter section 4 has photodetectors (PD) 16-1 to 16-2 provided corresponding to the filters 14-1 to 14-n, respectively, and controllers 15-1 to 15-n. In the filter section 4, solid lines indicate optical waveguides, and dotted lines indicate electrical signal lines. Couplers 18-1 to 18-n on the output side of the filters 14-1 to 14-n (hereinafter sometimes collectively referred to as "filters 14") split a part of the transmitted light of each filter 14, and the split light is detected by PDs 16-1 to 16-n. The detection results of PDs 16-1 to 16-n are input to controllers 15-1 to 15-n.

[0021] The control units 15-1 to 15-n (hereinafter, sometimes collectively referred to as "control unit 15") feedback-control the transmission characteristics of the filters 14-1 to 14-n based on the detection results of the PDs 16-1 to 16-n (hereinafter, sometimes collectively referred to as "PD16") to reduce high-order spectral components. The control unit 15 changes the center wavelength of the transmission spectrum of the filter 14 based on the detection results of the corresponding PDs 16, i.e., the power of the transmitted light of the corresponding filter 14, to reduce high-order spectral components while following the wavelength fluctuation of the frequency comb light source. Unlike a general LD ​​array, the frequency comb light source 11 shifts the entire comb line while maintaining the wavelength interval even if there is a wavelength fluctuation. Therefore, by making the filter unit 4 follow the wavelength fluctuation of the frequency comb light source 11, suppressing high-order spectral components and transmitting the fundamental wave, high-precision demultiplexing is possible. A specific configuration example and a control example of the filters 14-1 to 14-n will be described later.

[0022] The comb light of each wavelength from which higher-order spectral components have been removed by the filters 14-1 to 14-n is incident on the corresponding modulators 17-1 to 17-n. The light of each channel modulated by the modulators 17-1 to 17-n is multiplexed by the multiplexer 19 and output as a WDM signal.

[0023] Fig. 4 is a schematic diagram of an optical transmitter 10A which is a modified example of Fig. 3. The optical transmitter 10A has a demultiplexing unit 13A and a filter unit 4A. The demultiplexing unit 13A has a demultiplexer 131A of a CAT configuration and a demultiplexing control unit 133A which controls the demultiplexer 131A. The demultiplexing control unit 133A is indicated as "CAT-Dmux control unit" in the figure. The filter unit 4A has filters 14A-1 to 14A-n (hereinafter sometimes collectively referred to as "filters 14A") which are connected to the respective output ports of the demultiplexer 131A.

[0024] Each filter 14A has m filters 14(i,k) connected in series. Here, i is the number of the filter row connected to the n output ports of the demultiplexing unit 13A and is an integer from 1 to n. k is the number of the filter connected in series in each filter row and is an integer from 1 to m. The kth filter in the filter row connected to the ith output port is represented as 14(i,k). The transmission spectra of the m filters 14(i,k) each have an individual period. High-order spectral components are removed by connecting filters with different transmission spectra in series. The number m of filters connected in series may be determined according to the rise of the high-order spectrum.

[0025] In the first channel corresponding to wavelength λ1, filters 14(1,1), 14(1,2), ..., 14(1,m) are connected in series, and each filter is provided with a control unit 15(1,1), 15(1,2), ..., 15(1,m). The transmitted light of each filter 14(1,k) is branched by a coupler 18(1,k) and monitored by a PD(1,k), and the monitoring result is input to the corresponding control unit 15(1,k). The control unit 15(1,k) feedback controls the filter 14(1,k) based on the power of the monitored transmitted light. That is, the control unit 15(1,k) controls the filter 14(1,k) so that the peak of the transmission spectrum coincides with or is close to the wavelength of the output port of channel number 1 of the demultiplexing unit 13A. Similar filtering is performed on the second to n-th channels.

[0026] <Example of splitter configuration> FIG. 5 shows an example of the configuration of a demultiplexer 131A in a CAT configuration. For simplicity, four wavelengths λ1, λ2, λ3, and λ4 are demultiplexed here. The demultiplexer 131A is made up of two L-stage (L is an integer equal to or greater than 1) tree-connected demultiplexers. L - One unit circuit UC. When splitting four wavelengths, two stages are connected. 2 -One unit circuit, that is, three unit circuits UC1, UC2, and UC3 are connected in a tree configuration. Each unit circuit UC has one input port and two output ports, and the next-stage unit circuit UC is connected in series to each of the output ports.

[0027] In each unit circuit UC, three asymmetric Mach-Zehnder interferometers (AMZ) with the same arm length difference are cascaded. Here, the arm length difference is the effective arm length difference taking into account the refractive index of the waveguide. Hereinafter, the term "arm length difference" refers to the effective arm length difference. The "same" arm length difference means that the arm length difference in design is the same, including manufacturing variations and tolerances. Each AMZ has one input port and two output ports, and the second and third AMZs are connected to the two output ports of the first AMZ.

[0028] The unit circuit UC1 has cascaded AMZs 101, 102, and 103, and each AMZ is provided with a phase shifter 111, 112, and 113. The arm length difference between the AMZs 101, 102, and 103 is set so that the incident light is separated by the wavelength of the adjacent channel interval. As a result, λ1 and λ3 are output to one output port, and λ2 and λ4 are output to the other output port. A monitor 121 is connected to one output port of the second AMZ 102, and a monitor 122 is provided to the other output port. The next-stage unit circuit UC2 is connected to the output port to which the monitor 121 is connected. Similarly, a monitor 123 is connected to one of the output ports of the third AMZ 103, and a monitor 124 is provided to the other output port. The next-stage unit circuit UC3 is connected to the output port to which the monitor 123 is connected.

[0029] The monitoring results of the monitors 121 and 123 are input to the controller 31. The controller 31 controls the phase shifter 111 of the AMZ 101, and controls the transmission characteristics of the AMZ 101 in a direction that increases the power detected by the monitors 121 and 123. In other words, the controller 31 controls the AMZ 101 in a direction that increases the optical power of λ1 and λ3 incident on the next-stage unit circuit UC2, and the optical power of λ2 and λ4 incident on the next-stage unit circuit UC3. In this sense, the controller 31 is denoted as "Inc."

[0030] On the other hand, the monitoring result of the monitor 122 is input to the controller 132. The controller 132 controls the phase shifter 112 of the AMZ 102 to control the transmission characteristics of the AMZ 102 in a direction in which the power detected by the monitor 122 decreases. That is, the controller 132 controls the AMZ 102 in a direction in which the wavelength components of λ2 and λ4 that are not required on the path on the AMZ 102 side decrease. In this sense, the controller 132 is denoted as "Dec". Similarly, the monitoring result of the monitor 123 is input to the controller 33. The controller 33 controls the phase shifter 113 of the AMZ 103 to control the transmission characteristics of the AMZ 103 in a direction in which the power detected by the monitor 123 decreases. That is, the controller 132 controls the AMZ 103 in a direction in which the wavelength components of λ1 and λ3 that are not required on the path on the AMZ 103 side decrease. In this sense, the controller 33 is denoted as "Dec".

[0031] The arm length difference of the three AMZs included in each of the next-stage unit circuits UC2 and UC3 is set to half the arm length difference of the AMZs in the first-stage unit circuit UC1. The wavelength separation interval is determined by the arm length difference, and with half the arm length difference of the first-stage UC1, the wavelength separation interval is doubled, and wavelengths are separated at every other channel interval.

[0032] The unit circuit UC2 has cascaded AMZs 201, 202, and 203, and each AMZ is provided with a phase shifter 211, 212, and 213. The arm length difference between the AMZs 201, 202, and 203 is set so that λ1 is output to one output port and λ3 is output to the other output port. A monitor 221 is connected to one output port of the second AMZ 202, and a monitor 222 is provided to the other output port. The output port to which the monitor 221 is connected is the output port of λ1. Similarly, a monitor 223 is connected to one of the output ports of the third AMZ 203, and a monitor 224 is provided to the other output port. The output port to which the monitor 223 is connected is the output port of λ3.

[0033] The monitoring results of the monitors 221 and 223 are input to a control circuit 231. The control circuit 231 controls the phase shifter 211 of the AMZ 201, and controls the transmission characteristics of the AMZ 201 in a direction that increases the power detected by the monitors 221 and 223, that is, the optical power of λ1 and λ3 output from the demultiplexer 131A. In this sense, the controller 231 is denoted as "Inc."

[0034] Meanwhile, the monitoring result of the monitor 222 is input to the controller 232. The controller 232 controls the phase shifter 112 of the AMZ 202 to control the transmission characteristics of the AMZ 202 in a direction in which the power detected by the monitor 222, i.e., the wavelength component of λ3 mixed into the output port of λ1, decreases. In this sense, the controller 232 is denoted as "Dec". Similarly, the monitoring result of the monitor 223 is input to the controller 233. The controller 233 controls the phase shifter 213 of the AMZ 203 to control the transmission characteristics of the AMZ 203 in a direction in which the power detected by the monitor 223, i.e., the wavelength component of λ1 mixed into the output port of λ3, decreases. In this sense, the controller 233 is denoted as "Dec".

[0035] The unit circuit UC3 also performs the same operation as the unit circuit UC2 for λ2 and λ4, outputting λ2 and λ4 from the demultiplexer 131A, respectively. In this CAT configuration, each AMZ constituting the demultiplexer 131A is controlled to increase the target wavelength component and reduce the unnecessary wavelength component, so that even if demultiplexing as designed cannot be performed due to manufacturing variations, refractive index variations, etc., the accuracy of demultiplexing can be improved.

[0036] <Filter configuration example> Fig. 6 is a schematic diagram of a filter connected to an output port of the unit circuit UC2 of the duplexer 131A in Fig. 5. The filter section 4B has a filter 14B-1 connected to one output port of the unit circuit UC2 and a filter 14B-3 output to the other output port. Similarly, two filters are connected to the two output ports of the unit circuit UC3.

[0037] The filter 14B-1 has m filters 14B(1,1) to 14B(1,m) connected in series. Here, the first to third filters 14B(1,1), 14B(1,2), and 14B(1,3) are shown. As described with reference to FIG. 4, a corresponding monitor 16B(1,k) is provided for each filter 14B(1,k), and the transmission spectrum of the filter 14B(1,k) is controlled by the control unit 15B(1,k) based on the monitoring result.

[0038] In the example of FIG. 6, each filter 14B(i,k) is formed of an AMZ, and the arm length difference of the m AMZs connected in series is different. In the filter 14B-1, the arm length of the first filter 14B(1,1) is set so that λ1 is transmitted to the first output port connected to the second filter 14B(1,2), and the second harmonic component of λ1 is output to the monitor output port connected to the monitor 16B(1,1). The control unit 15B(1,1) controls the transmission characteristic of the filter 14B(1,1) so that the power detected by the monitor 16B(1,1) is minimized. The transmission characteristic of the filter 14B(1,1) is controlled, for example, by adjusting the temperature of a phase shifter such as a heater provided in the arm of the AMZ.

[0039] The transmission characteristics of the second filter 14B(1,2) are controlled so that λ1 is transmitted to the first output port connected to the third filter 14B(1,3) and the third harmonic component of λ1 is output to the monitor output port connected to the monitor 16B(1,2). The control unit 15B(1,2) controls the transmission characteristics of the filter 14B(1,2) so that the optical power detected by the monitor 16B(1,2) is minimized. By repeating this, the peak value of the transmission spectrum can be adjusted to the wavelength λ1 of the output port of this channel. In general, in each channel i, the peak of the transmission spectrum can be adjusted to the wavelength λi of that channel while reducing the higher order spectral components. This filter configuration can suppress crosstalk due to higher order spectra.

[0040] Fig. 7 shows the filter characteristics of the embodiment. The horizontal axis is frequency, and the vertical axis is power. Focus on λ1 of channel number 1. The solid line shows the transmission characteristics of the first filter 14(1,1) of filter 14-1 connected to the output port of λ1 of demultiplexing unit 13. If the channel spacing is Δλ, the transmission spectrum of filter 14(1,1) for k=1 has a period of 1×2n×Δλ. Due to this period, second-order spectral components appearing at positions "1'" are removed without being transmitted.

[0041] The dashed line shows the transmission characteristics of the second filter 14(1,2). The transmission spectrum of filter 14(1,2) where k=2 has a period of 2×2n×Δλ. Due to this period, the third-order spectral components that appear at the position of "1'' are not transmitted but are removed. The dashed and dotted line shows the transmission characteristics of the third filter 14(1,3). The transmission spectrum of third filter 14(1,3) has a period of 4×2n×Δλ. Due to this period, even higher-order components are not transmitted but are removed. Generalizing, the transmission spectrum of the kth filter 14(1,k) has a period of 2 k-1 ×2n×Δλ, effectively removing higher order spectral components at the valleys of the period.

[0042] <Wavelength control> FIG. 8 is a flowchart of wavelength control of the optical transmitter 10A. The optical transmitter 10A has a filter unit 4A having a series-connected filter row as shown in FIG. 4, and a CAT-configured demultiplexer 131A as shown in FIG. 5. First, the frequency comb light source 11 is turned on (S1), and wavelengths are assigned to each output port of the CAT-configured demultiplexer 131A (S2). Light of wavelengths λ1 to λn is sequentially input to the demultiplexer 131A, and the transmission characteristics are adjusted by controlling a phase shifter provided on a waveguide between the input port of the demultiplexer 131A and the output port of the selected wavelength. That is, the monitor power is controlled so that it is increased at the position where the light of the target wavelength is output, and is reduced at the monitor positions of the other wavelength components. When the output is stabilized at the selected wavelength, the monitor power is controlled for the next wavelength so that it is maximized at the output position of the target wavelength, and is minimized at the monitor positions of the other wavelength components.

[0043] Once control of the phase shifter for each wavelength is complete, signals of all wavelengths from λ1 to λn are input to demultiplexer 131A at once, and the phase shifters on the waveguide are controlled again. When the outputs of all channels are stabilized with signals of all wavelengths input, port assignment of demultiplexer 131A in the CAT configuration is complete.

[0044] Next, the filter unit 4A is optimized (S3). The filter optimization is a process of adjusting the peak of the transmission spectrum of the m filters 14A(i,k) connected in series to the target wavelength λi for each of the filters 14A-i (i is an integer from 1 to n) connected to each output port of the demultiplexer 131A. The filter optimization may be performed in parallel for n ports. Each filter 14A-i controls each of the m filters 14(i,k) (S31). The output power of the kth filter (k is an integer from 1 to m) connected in series is monitored by the monitor 16(i,k) (S32). The control unit 15(i,k) controls so that the monitored value becomes the minimum (S33). Steps S31 to S33 are repeated until the monitored value becomes the minimum for the m filters.

[0045] When the monitored values ​​of all the m filters become minimum, the peak of the transmission spectrum of the filter 14A-i coincides with or is close to the wavelength λi assigned to the corresponding port, and high-order spectral components are reduced, thereby extracting the target wavelength λi from the filter 14A-1.

[0046] After the filter optimization, a process may be performed to follow the wavelength fluctuation of the frequency comb light source 11. The wavelength of the frequency comb light source 11 fluctuates due to temperature changes and the like. Even if the wavelength fluctuates in the frequency comb light source 11, the repetition frequency fr of each comb line is kept constant, so that the entire power spectrum shifts in the same direction while maintaining the wavelength interval. The CAT-structured splitter 131A is dither-controlled (S4) to follow the wavelength fluctuation of the frequency comb light source 11, and the filter section 4A is dither-controlled (S5).

[0047] A dither signal is input to splitter 131A, and a change in the monitor light due to the dither signal is observed to confirm whether the direction of control to track the wavelength fluctuation of frequency comb light source 11 is correct. Next, filters 14A-1 to 14A-n connected to each output port of splitter 131A are each made to track the wavelength fluctuation by dither control (S5). The wavelength fluctuation tracking process of filter unit 4A may be performed in parallel for n ports.

[0048] A dither signal is input to the filter 14A-i of each port, and dither control is performed by the m filters 14A(i,k) connected in series (S51). A change in the dither component contained in the monitor light is observed (S52), and the control direction of the filter 14A(i,k) is determined, for example, so that the dither component decreases (S53).

[0049] The wavelength fluctuation tracking process (S4) of the demultiplexer 131A and the wavelength fluctuation tracking process (S5) of the filter unit 4A are repeatedly performed during the operation of the optical transmitter 10A. As a result, even if wavelength fluctuation occurs in the frequency comb light source 11, the wavelength fluctuation can be tracked to accurately separate the light of each wavelength, and crosstalk of high-order spectral components can be suppressed.

[0050] <Modifications of Filter> Fig. 9 is a schematic diagram showing a modified example of the filter section. The filter section 4C uses a ring resonator instead of the AMZ. Fig. 9 is based on the premise of the duplexer 131A with the CAT configuration shown in Fig. 5. The filter 14C-1 and the filter 14C-3 are connected to two output ports of the unit circuit in the final stage of the duplexer 131A (unit circuit UC2 in the example of Fig. 5), respectively, and λ1 and λ3 are extracted. Similarly, the filters 14C-2 and 14C-4 are connected to two output ports of another unit circuit in the second final stage (for example, unit circuit UC3), respectively, and λ2 and λ4 are extracted.

[0051] The output waveguide of the AMZ202 of the unit circuit UC2 becomes the input waveguide 34-1 of the filter 14C-1. A part of the light of λ1 guided through the input waveguide 34-1 is coupled to the ring resonator 20-1, circulates around the waveguide, and is coupled to the output waveguide 30-1. The ring resonator 20-1 is designed so that the product of its circumference and effective refractive index is an integer multiple of λ1. Of the light propagating through the input waveguide 34-1, the light corresponding to the resonant wavelength of the ring resonator 20-1 is coupled to the ring resonator 20-1, and the other light travels straight through the input waveguide 34-1 and is monitored by the monitor 411. The monitor result of the monitor 411 is supplied to the controller 511. The controller 511 controls the phase shifter 210-1 provided in the ring resonator 20-1 so that the power detected by the monitor 411 decreases. In this sense, the controller 511 is denoted as "Dec." This allows λ1 to be output with maximum power.

[0052] The ring resonator 20-1 has periodic resonance peaks. In this embodiment, the FSR of the ring resonator 20-1, i.e., the resonance peak interval, is set to be greater than n times the channel interval Δλ (FSR>nΔλ), where n is the number of channels. By designing the resonance characteristics of the ring resonator 20-1 in this way, high-order spectral components can be effectively removed.

[0053] The filter 14C-3 has a similar configuration to the filter 14C-1, but the circumferential length of the ring resonator 20-3 is different. The circumferential length of the ring resonator 20-3 is designed so that the product of the circumferential length and the effective refractive index is an integer multiple of λ3. The operation of the monitor 431 and the controller 531 is the same as that of the monitor 411 and the controller 511. The ring resonator 20-3 is also designed so that its FSR satisfies FSR>nΔλ.

[0054] FIG. 10 shows the transmission characteristics of the filter 14C-1 in FIG. 9. The dotted line is the transmission spectrum of the ring resonator 20-1. Focus on the wavelength λ1. The transmission band for n channels is n×Δλ. We want to remove higher-order spectrum that appears outside this band. Therefore, the transmission characteristics are designed so that the transmission peak of the ring resonator 20-1 appears at a position that does not transmit higher-order spectral components. This makes it possible to effectively remove higher-order spectral components in the λ1 channel.

[0055] In n-channel WDM transmission with a channel spacing of Δλ, by aligning the peak position of the transmission spectrum of ring resonator 20-i with λi and setting the FSR of that ring resonator 20-i to FSR>nΔλ, it is possible to follow the wavelength fluctuations of frequency comb light source 11 and effectively remove high-order spectral components.

[0056] Figure 11 shows the effect of the configuration of this embodiment. In n-channel WDM transmission, the power spectrum occurring on both sides of the wavelength band used in transmission can be significantly reduced, and the difference between the target wavelength and the higher-order spectral components of that wavelength can be reduced to less than -35 dB. This satisfies the crosstalk regulations and improves signal quality.

[0057] When a CAT configuration is adopted for the demultiplexer 131, the filter characteristics can be made to follow the wavelength fluctuation of the frequency comb light source 11, just like a CAT. The CAT-structured multiplexer and the periodic filter section 4 can be integrated on the same substrate by silicon photonics technology, which allows the optical transmitter 10 to be miniaturized.

[0058] In the optical transmitter 10, The light output from the frequency comb light source 11 is demultiplexed by the demultiplexer 131 into n channels (n is an integer of 2 or more) with wavelength intervals of Δλ. Filters 14-1 to 14-n are provided at the n output ports of the demultiplexer 131 to reduce higher-order spectral components of each channel; Each filter 14-i is controlled to bring the peak of the transmission spectrum of that filter 14-i closer to the wavelength that appears at the corresponding output port of the demultiplexer 131. This control makes it possible to make the transmission spectrum of the filter follow the fluctuations of the frequency comb light source 11 and to suppress crosstalk caused by high-order components.

[0059] <Application to optical transceivers> Fig. 12 is a schematic diagram of an optical transceiver 1 using an optical transmitter 10 according to an embodiment. The optical transceiver 1 includes an optical transmitter 10 and a receiver RX. The optical transmitter 10 has the configuration described with reference to Figs. 3 to 11, and includes a frequency comb light source 11, a controller 12, a demultiplexing unit 13, a filter unit 4, modulators 17-1 to 17-n, and a multiplexer 19. The filter unit 4 is provided to remove high-order spectral components of the light of each wavelength transmitted by the demultiplexing unit 13 due to the periodicity of the transmission characteristics.

[0060] The receiver RX has a splitter 6 and coherent detectors 8-1 to 8-n connected to the outputs of the splitter 6. A part of the output light of each channel of the filter unit 4 of the optical transmitter 10 may be split and used as a local light LO. The local light LO is supplied to the coherent detectors 8-1 to 8-n of the receiver RX and used to detect the received signal of each channel. The signals detected by the coherent detectors 8-1 to 8-n and converted into electrical signals are input to a digital signal processor (DSP) for signal processing. The DSP may be implemented in the optical transceiver 1 or may be provided outside the optical transceiver 1.

[0061] By using the frequency comb light source 11, the optical transceiver 1 can be made smaller and its power consumption reduced to support DWDM optical transmission. By providing the filter unit 4 at the output port of the demultiplexer 13, it is possible to reduce high-order spectral components in each channel and suppress crosstalk. [Explanation of symbols]

[0062] 1 Optical transceiver 4, 4A, 4B, 4C Filter section 10, 10A Optical Transmitter 11 Frequency comb source 12 Controller 13, 13A branch 131, 131A splitter 133, 133A Branching control section 14-1~14-n, 14A-1~14A-n, 14B-1, 14B-3, 14C-1, 14C-3, 14(1,1)~14(n,m) Filter 15-1 to 15-n, 15(1,1) to 15(n,m) Control section 16-1~16-n, 16(1,1)~16(n,m) PD (photodetector) 20-1, 20-3 Ring resonator UC1, UC2, UC3 Unit Circuit

Claims

1. A frequency comb source; a demultiplexer that demultiplexes the light output from the frequency comb light source into n channels (n is an integer equal to or greater than 2) at wavelength intervals of Δλ; n filters connected to the n output ports of the splitter, respectively, for reducing high-order spectral components for each wavelength; An optical transmitter having

2. A control unit that controls a peak wavelength of a transmission spectrum of each of the n filters; 2. The optical transmitter of claim 1, comprising:

3. each of the n filters includes one or more asymmetric Mach-Zehnder interferometers connected in series; 3. The optical transmitter according to claim 2.

4. The kth (k is an integer equal to or greater than 1) asymmetric Mach-Zehnder interferometer is ｋ－１ × 2n × Δλ periodic transmission characteristic, 4. The optical transmitter according to claim 3.

5. Each of the n filters has a resonator resonating with a corresponding channel wavelength.

3. The optical transmitter according to claim 2.

6. the free spectral range of the resonator is greater than n×Δλ; 6. The optical transmitter according to claim 5.

7. The splitter is a two-stage splitter connected in a tree in L stages (L is an integer of 1 or more). Ｌ -having one unit circuit, 2 Ｌ Each of the unit circuits has one input port and two output ports; The n filters are connected to the output ports of the L-th stage unit circuit, 2. The optical transmitter according to claim 1.

8. 2 Ｌ Each of the one unit circuits includes a first asymmetric Mach-Zehnder interferometer, a second asymmetric Mach-Zehnder interferometer, and a third asymmetric Mach-Zehnder interferometer having an equal arm length difference; the arm length difference of the L-th unit circuit is 1 / 2 of the arm length difference of the L-1-th unit circuit, 8. The optical transmitter according to claim 7.

9. n optical modulators provided corresponding to the n filters; a multiplexer for multiplexing output light from the n optical modulators; 2. The optical transmitter of claim 1, comprising:

10. An optical transmitter according to any one of claims 1 to 9; An optical receiver; having the optical receiver demultiplexes the received wavelength multiplexed signal into optical signals of each wavelength, and detects the demultiplexed optical signals of each wavelength by using a part of the light transmitted through the n filters of the optical transmitter as a local light. Optical transceiver.

11. The light output from the frequency comb light source is split into n channels (n is an integer equal to or greater than 2) with wavelength spacing of Δλ by a splitter. providing a filter at each of the n output ports of the splitter to reduce higher order spectral components in each channel; controlling said filter so that a peak of the transmission spectrum of said filter approaches a wavelength appearing at a corresponding output port of said demultiplexer; Wavelength control method.

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