Tunable wavelength light source

Abstract

Provided is a highly accurately controllable wavelength-variable light source. A wavelength-variable light source (200, 600) according to an embodiment comprises: a wavelength-variable laser (210); a demultiplexer (250, 650) which demultiplexes light output from the wavelength-variable laser into N portions, where N is an integer of 2 or more; N light-reception elements (260) that each convert the N demultiplexed portions of light into an electric signal indicative of the intensity of the corresponding light; a processing unit (270) which performs a predetermined computation on the electric signals from the N light-reception elements; and a control unit (280) which generates, in accordance with the result of the computation, a control signal for changing the drive state of the wavelength-variable laser.

Description

[Technical Field]

[0001] The present disclosure relates to a wavelength-tunable light source, and more particularly to a wavelength-tunable light source including a wavelength-tunable laser and a control device that controls the oscillation wavelength of the wavelength-tunable laser. [Background technology]

[0002] In recent years, advances in information and communication technology have led to a rapid increase in communication traffic. To meet this demand, efforts are being made to increase the speed and capacity of networks, not only by improving baud rates but also by using multiplexing technologies such as wavelength division multiplexing transmission.

[0003] A wavelength-tunable laser, whose oscillation wavelength can be adjusted arbitrarily within a certain range, is used as a light source for a wavelength division multiplexing transmission system.

[0004] Before use, wavelength-tunable lasers are calibrated, and the relationship between the applied voltage or current and the oscillation wavelength is recorded, and these are used as initial setting parameters to obtain the desired wavelength. However, due to aging and other factors, deviations occur between the wavelength control target value and the actual oscillation wavelength, so it is necessary to monitor the wavelength and perform feedback control. A method using a wavelength locker has been used to achieve this (Non-Patent Document 1).

[0005] A wavelength locker is a device that monitors the wavelength by passing light through a wavelength-selective filter such as an etalon, inputting it to a light-receiving element, and monitoring the current value.

[0006] In conventional wavelength lockers, the current value at the photodetector is detected to change periodically with respect to the oscillation wavelength, reflecting the periodicity of the transmittance of the etalon filter, as shown in Fig. 1 (Patent Document 1). In known wavelength tunable lasers, feedback control is performed so that the current value in Fig. 1 is maintained at half the amplitude (the difference between the maximum and minimum values). This is because more precise wavelength control is possible by controlling the current value at the point where the change in the current value relative to the change in the oscillation wavelength of the wavelength tunable laser is greatest. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2008-277644 [Non-patent literature]

[0008] [Non-Patent Document 1] Hiroyuki Ishii et al., "High-Performance Wavelength Tunable Light Source Technology," NTT Technical Journal, p. 66, November 2007. [Non-patent document 2] Yuta Ueda, Takahiko Shindo, Shigeru Kanazawa, Naoki Fujiwara, and Mitsuteru Ishikawa, "Electro-optically tunable laser with ultra-low tuning power dissipation and nanosecond-order wavelength switching for coherent networks," Optica Vol. 7, Issue 8, pp. 1003-1006, (2020). Summary of the Invention

[0009] However, the above method has a problem in that, near the maximum amplitude of the current value shown in FIG. 1, the change in the current value is small relative to the change in the oscillation wavelength, making it difficult to detect the wavelength shift.

[0010] The present disclosure provides a wavelength-tunable light source that solves these problems. One embodiment of the present invention is a wavelength-tunable light source comprising: a wavelength-tunable laser; a demultiplexer that demultiplexes light output from the wavelength-tunable laser into N parts, where N is an integer equal to or greater than 2; N light-receiving elements that convert each of the N demultiplexed parts of light into an electrical signal, where the electrical signal indicates the intensity of the corresponding light; a processing unit that performs a predetermined calculation on the electrical signals from the N light-receiving elements; and a control unit that generates a control signal that changes the driving state of the wavelength-tunable laser in accordance with the result of the calculation.

[0011] As described above, according to one embodiment of the present invention, it is possible to control the oscillation wavelength of the tunable laser of the tunable light source with higher precision. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 10 is a diagram showing the relationship between the current value monitored by the wavelength locker and the oscillation wavelength. [Figure 2] 1 is a diagram showing a schematic configuration of a wavelength-tunable light source according to one embodiment of the present invention; [Figure 3] 1 is a diagram showing a schematic configuration of a wavelength tunable laser of a wavelength tunable light source according to one embodiment of the present invention; [Figure 4] 10 is a diagram illustrating the target oscillation wavelength (target oscillation wavelength) of a wavelength tunable laser that is periodically changed with respect to time. FIG. [Figure 5] 3 is a diagram illustrating the oscillation wavelength (actual oscillation wavelength) of light detected by a light receiving element. FIG. [Figure 6] 1 is a diagram showing a schematic configuration of a wavelength-tunable light source according to one embodiment of the present invention; DETAILED DESCRIPTION OF THE INVENTION

[0013] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. The same or similar reference symbols in the drawings indicate the same or similar elements, and descriptions thereof may be omitted. The numerical values ​​in the following description are examples, and other numerical values ​​may be used as long as they do not deviate from the spirit of the present invention.

[0014] [First embodiment] A tunable light source according to a first embodiment of the present invention will be described with reference to Figures 2 and 3. Figure 2 is a diagram showing a schematic configuration of a tunable light source 200 according to this embodiment.

[0015] 2 includes a tunable laser 210, an arrayed-waveguide grating 250, N light-receiving elements 260, a processing unit 270, and a control unit 280. N can be an integer equal to or greater than 2. In the description of this embodiment, N=5.

[0016] The tunable laser 210 may be, by way of example and not limitation, an RTF laser, which is described below.

[0017] The arrayed waveguide grating 250 is, for example, an arrayed waveguide grating (AWG) having one input port and N output ports. The AWG is an example of a wavelength-dependent demultiplexer that demultiplexes light incident on an input port and outputs the demultiplexed light from different output ports according to the wavelength (optical frequency). Wavelength-dependent demultiplexers with other configurations may be used instead of the AWG 250.

[0018] The light receiving element 260 may be, for example, a photodiode.

[0019] Processing unit 270 can be any processor. The processor may include or be electrically coupled to memory. The processor may be an application specific integrated circuit (ASIC), a reduced instruction set computer (RISC), or a complex instruction set computer (CISC).

[0020] The control unit 280, like the processing unit 270, can be any processor, and may be an ASIC, RISC, or CISC (complex instruction set computer). The control unit 280 and the processing unit 270 may be separate or integrated.

[0021] Fig. 3 is a diagram showing a schematic configuration of the wavelength tunable laser 210. The wavelength tunable laser 210 shown in Fig. 3 is a reflection-type transversal filter (RTF) laser that uses an RTF.

[0022] The wavelength tunable laser 210 shown in FIG. 3 includes an MMI 312 connected to i optical gain regions and j reflective delay lines, where i is an integer greater than or equal to 1 and j is an integer greater than or equal to 2. Each of the reflective delay lines, except for 313-2, has a mirror 314. One of the reflective delay lines 313 has an optical output port 313-2, the end of which is a cleaved surface 314-2. The MMI 312 and the j reflective delay lines 313 form an RTF 310. FIG. 3 also shows the control unit 280 shown in FIG. 2. This embodiment will be described assuming i=5 and j=5. Various design arrangements are possible for determining which optical waveguide in the reflective delay line 313 serves as the optical output port 313-2.

[0023] The MMI 312 is provided with a phase adjustment electrode 317 that applies an electric field in response to a control signal 322 for phase control from the control unit 280 .

[0024] Each of the j reflective delay lines 313 has a delay line 313-j, which is an optical waveguide of a different length, and an end mirror 314-j. A round-trip optical path of a different optical path length is formed between each port on the optical gain region 311 side of the MMI 312 and the end mirror 314. The mirror 314 is made of, for example, a metal film. The end of the optical output port 313-2 is configured as a mirror, with the cleavage plane 314-2 serving as a mirror. This forms a round-trip optical path in which a portion of the light is reflected, and the transmitted light is extracted to the outside. Each of the j reflective delay lines 313 also has a wavelength tuning electrode 318 for applying an electric field in response to a wavelength control signal 323 from the control unit 280.

[0025] 3, optical gain regions 311-1 to 311-5 are connected to ports 1 to 5 on the left side of the MMI 312. The optical gain region 311 can be an optical gain waveguide including an optical gain region. An electrode (not shown) is provided in the optical waveguide including the optical gain region, and by passing a current through the electrode, carriers are injected into the optical gain region, generating optical gain.

[0026] Here, a detailed explanation of the oscillation mechanism of the wavelength tunable laser 210 will be omitted, but laser oscillation occurs at a wavelength where the reflected light from each of the multiple reflective delay lines 313 of different lengths is in a constructive relationship at the left port 3 of the MMI 312 (see Non-Patent Document 2). The oscillation wavelength is adjusted by control signals 322 and 323 applied to the phase adjustment electrode 317 on the MMI 312 and the wavelength adjustment electrodes 318 on the multiple reflective delay lines 313. As exemplified in Non-Patent Document 2, the wavelength adjustment electrode 318 is composed of a coarse electrode for coarse adjustment of the oscillation wavelength and a fine electrode for fine adjustment. The relationship between the coarse voltage V applied to the coarse electrode and the oscillation wavelength λ is expressed as λ=aV using an arbitrary coefficient. 2 +bV+λ0, where λ0 is the oscillation wavelength when V=0 [V]. The wavelength tunable laser 210 determines the final control voltage by adding corrections based on measurements to the design values ​​of the control voltage and oscillation wavelength based on the above formula.

[0027] The light 324 output from the optical output port 313-2 of the wavelength tunable laser 210 is input to an input port of the arrayed-waveguide grating 250. The light input to the arrayed-waveguide grating 250 is output from one of N output ports depending on its wavelength, and is further detected by a light-receiving element 260 connected to each output port. That is, the light is converted into an electrical signal by the light-receiving element 260, such as a photodiode, and output. The electrical signals output from the N light-receiving elements 260 are input to the processing unit 270.

[0028] In this embodiment, the wavelength channel spacing of the arrayed waveguide grating 250 is set to 25 GHz as an example, and the spectral linewidth of the wavelength tunable laser is designed to be 350 kHz or less.

[0029] With this configuration, an oscillation wavelength is associated with each light receiving element 260. That is, the light receiving wavelength set in the light receiving element 260 that generated the detection signal is the oscillation wavelength of the wavelength tunable laser 210.

[0030] By synchronizing the operation timing of these N light receiving elements 260, the oscillation wavelength can be plotted on the time axis. Note that, since the optical path lengths of the ports of the arrayed-waveguide grating 250 are different along the way, the processing unit 270 corrects this so that light simultaneously input to the input ports of the arrayed-waveguide grating 250 is simultaneously detected at different ports.

[0031] The control unit 280 generates a control signal 322 and a control signal 323 based on the N electrical signals corrected by the processing unit 270, and supplies them to the phase adjustment electrode 317 and the wavelength adjustment electrode 318, thereby controlling the oscillation wavelength of the wavelength-tunable laser 210.

[0032] The advantage of this embodiment is that the relationship between the control parameters (voltage, current, etc.) and the oscillation wavelength appears in a plot on the time axis.

[0033] Figure 4 shows an example in which the oscillation wavelength of the control target value (hereinafter referred to as the target oscillation wavelength) is changed periodically over time. The change is such that it shows a sawtooth wave with a frequency of 10 kHz. Note that the waveform is just an example, and the control method described below can be performed in the same way as in this example even with a triangular wave, for example.

[0034] Similarly, FIG. 5 shows the oscillation wavelength detected by the light receiving element 260 (hereinafter referred to as the actual oscillation wavelength). As an example, it shows a case where there is a deviation between the target oscillation wavelength and the actual oscillation wavelength. The frequency of the sawtooth wave is reduced by about 10% compared to FIG. 4. As can be seen from these examples, with the configuration of this embodiment, the deviation between the target oscillation wavelength and the actual oscillation wavelength can be detected as the difference in frequency of the sawtooth wave. The frequency of such an electrical signal can be easily measured. The oscillation wavelength can be controlled by changing the control voltage so that the difference in frequency of the sawtooth wave obtained in this way is minimized.

[0035] Specifically, the arbitrary coefficients a and b in the relationship between the oscillation wavelength λ of the wavelength tunable laser 210 and the control voltage V are adjusted to correct the relationship so that the values ​​shown in Figures 4 and 5 match. The residual sum of squares of the sawtooth waves in Figures 4 and 5 is calculated, and the arbitrary coefficients a and b are determined so that the residual sum of squares is minimized. Correction amounts Δa and Δb are added to the arbitrary coefficients a and b, and the correction amounts Δa and Δb are set to a + Δa and b + Δb. By repeatedly performing a sequence of changing the correction amounts Δa and Δb in the direction that reduces the residual sum of squares of the sawtooth wave, the coefficients a and b that minimize the deviation between the target frequency and the actual frequency can be determined. Alternatively, the oscillation light frequencies in FIGS. 4 and 5 may be Fourier transformed to determine Δa and Δb perturbatively so that the actual frequency of the sawtooth wave coincides with the target frequency.

[0036] [Second embodiment] A wavelength-tunable light source according to a second embodiment of the present invention will be described with reference to Fig. 6. Fig. 6 is a diagram showing a schematic configuration of a wavelength-tunable light source 600 according to this embodiment.

[0037] The tunable light source 600 shown in FIG. 6 includes a tunable laser 210, a demultiplexer 659, N optical bandpass filters 660, N light receiving elements 260, a processing unit 270, and a control unit 280.

[0038] The wavelength-tunable light source 600 of this embodiment has a configuration in which the arrayed waveguide grating 250 in the wavelength-tunable light source 200 of FIG. 2 is replaced with a demultiplexer 650 and N optical bandpass filters 660.

[0039] Demultiplexer 650 is, for example, a wavelength-independent demultiplexer having one input port and N output ports, configured by connecting multiple 1-input, 2-output optical couplers in multiple stages. Demultiplexer 650 may also be configured by connecting multiple 1-input, 2-output optical couplers in multiple stages.

[0040] The light 324 output from the optical output port 313-2 of the wavelength tunable laser 210 is input to an input port of the demultiplexer 650. The light input to the demultiplexer 650 is branched into N and output from N output ports. The light output from the N output ports is input to N optical bandpass filters 660.

[0041] The N optical bandpass filters 660 have different transmission optical frequency bands. Therefore, the wavelengths of the light transmitted through the N optical bandpass filters 660 are different from one another. The light transmitted through the N optical bandpass filters 660 is detected by the N light receiving elements 260, respectively. That is, the light is converted into an electrical signal by the light receiving elements 260, such as photodiodes, and output. The electrical signals output from the N light receiving elements 260 are input to the processing unit 270.

[0042] In this embodiment, the full width at half maximum (FWHM) representing the wavelength transmission width of the optical bandpass filter 660 is set to 25 GHz as an example, and the spectral linewidth of the wavelength tunable laser 210 is designed to be 350 kHz or less.

[0043] As described above, with the configuration of this embodiment, an oscillation wavelength is associated with each light receiving element 260. That is, the light receiving frequency width set for the light receiving element 260 that generates the detection signal is the oscillation wavelength of the wavelength tunable laser 210.

[0044] By synchronizing the operation timing of these N light receiving elements 260, the oscillation wavelength can be plotted on the time axis.

[0045] In this embodiment, the wavelength transmission width of the N optical bandpass filters 660 indicates the detection accuracy of the oscillation wavelength (optical frequency) of the wavelength tunable laser 210. For example, as shown in the previous example, when the wavelength transmission width of the optical bandpass filter 660 is 25 GHz, the true frequency of light observed as 193 THz will be between 193 THz ±12.5 GHz.

[0046] The larger this wavelength transmission width (FWHM), the wider the range in which the residual between the target frequency and the oscillation wavelength is detected as 0, making it possible to shorten the time required to converge to the control target value.

[0047] In this way, by adjusting the wavelength transmission width of the optical bandpass filter 660 according to the wavelength accuracy required for the wavelength-tunable light source for each application, it is possible to control the wavelength-tunable laser so that the wavelength converges to the control target value in a shorter time. [Industrial Applicability]

[0048] According to one embodiment of the present invention, it is possible to control the oscillation wavelength of the wavelength tunable laser of the wavelength tunable light source with higher precision, and it is possible to increase the speed and capacity of networks through wavelength division multiplexing transmission. [Explanation of symbols]

[0049] 200 Tunable wavelength light source 210 Tunable Laser 250 Arrayed Waveguide Grating 260 Photodetector 270 Processing Section 280 Control Unit 310 RTF 311 Optical gain region 312 MMI 313 Reflective Delay Line 314 Mirror 317 Phase Adjustment Electrode 318 Wavelength adjustment electrode 321 Light Intensity Signal 322 Control Signal 600 wavelength tunable light source 650 Duplexer 660 Optical Bandpass Filter

Claims

1. a tunable laser; a demultiplexer that demultiplexes the light output from the wavelength tunable laser into N components, where N is an integer of 2 or greater; N light receiving elements, each of which converts the N light beams into an electrical signal, each of which indicates the intensity of the corresponding light beam; a processing unit that performs a predetermined operation on the electrical signals from the N light receiving elements; a control unit that generates a control signal for changing the driving state of the wavelength tunable laser according to the result of the calculation; Equipped with The processing unit is configured to adjust the relationship between the electrical signals from the N light receiving elements and the times at which the electrical signals are generated.

2. 2. The wavelength-tunable light source according to claim 1, wherein the demultiplexer is a wavelength-dependent demultiplexer having one input port and N output ports, and is configured to demultiplex light input from the one input port to any one of the N output ports according to an optical frequency.

3. the optical frequency bands received by the N light-receiving elements are different from one another and correspond to the optical frequencies of the light demultiplexed into corresponding output ports among the N output ports of the wavelength-dependent demultiplexer; the control unit is configured to generate the control signal so that an oscillation wavelength of the wavelength-tunable laser is periodically changed with respect to time, the N light receiving elements are configured to be driven in synchronization with each other and convert light having different optical frequencies into the electrical signals; the control unit is configured to correct the control signal so that a residual sum of squares between the oscillation wavelength that changes periodically with respect to time and the optical frequency width of the light receiving element from which the electrical signal is generated is minimized. The wavelength-tunable light source according to claim 2 .

4. A wavelength tunable laser; a demultiplexer that demultiplexes the light output from the wavelength tunable laser into N components, where N is an integer of 2 or greater; N light receiving elements, each of which converts the N light beams into an electrical signal, each of which indicates the intensity of the corresponding light beam; a processing unit that performs a predetermined operation on the electrical signals from the N light receiving elements; a control unit that generates a control signal for changing the driving state of the wavelength tunable laser according to the result of the calculation; Equipped with The demultiplexer is a wavelength-dependent demultiplexer having one input port and N output ports, and is configured to demultiplex light input from the one input port to any one of the N output ports depending on the optical frequency.

5. the optical frequency bands received by the N light-receiving elements are different from one another and correspond to the optical frequencies of the light demultiplexed into corresponding output ports among the N output ports of the wavelength-dependent demultiplexer; the control unit is configured to generate the control signal so that an oscillation wavelength of the wavelength-tunable laser is periodically changed with respect to time, the N light receiving elements are configured to be driven in synchronization with each other and convert light having different optical frequencies into the electrical signals; the processing unit is configured to adjust a relationship between the electrical signals from the N light receiving elements and the times at which the electrical signals are generated; the control unit is configured to correct the control signal so that a residual sum of squares between the oscillation wavelength that changes periodically with respect to time and the optical frequency width of the light receiving element from which the electrical signal is generated is minimized. The wavelength-tunable light source according to claim 2 or 4.

6. 5. The wavelength-tunable light source according to claim 2, wherein the wavelength-dependent demultiplexer is an arrayed waveguide grating having one input port and N output ports.

7. The wavelength-dependent demultiplexer comprises: a wavelength-independent demultiplexer having one input port and N output ports; N optical bandpass filters having different transmission optical frequency widths connected to the outputs of the N output ports of the wavelength-independent demultiplexer; The tunable light source according to claim 2 or 4, comprising:

8. 8. The wavelength-tunable light source according to claim 7, wherein the control unit is configured to change a transmission light frequency width of the N bandpass filters to adjust the accuracy of the oscillation wavelength of the wavelength-tunable light source and a feedback time until the accuracy of the oscillation wavelength is obtained.

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