Wavelength-variable light source and its control method

Abstract

In the wavelength-variable light source and its control method of the present disclosure, the intensity of the oscillating light at multiple non-working ports of the MMI is utilized in consideration of the filter characteristics between the working port and the non-working ports that do not directly contribute to the oscillation operation. The RTF laser is controlled in such a way that the light intensity of the wavelength of the oscillating light at the monitored non-working ports becomes a desired relationship, thereby realizing a wavelength-variable light source that reflects the SMSR characteristics. The SMSR can be effectively controlled only by adding a photoreceptor to the non-working ports not considered in the prior-art RTF laser. The inspection of the SMSR and the monitoring of the SMSR during actual operation can be realized by a simple mechanism in the wavelength-variable light source.

Description

Technical Field

[0001] The present invention relates to a wavelength tunable light source and a control method thereof. Background Art

[0002] A wavelength tunable light source is widely used as a light source capable of arbitrarily adjusting an oscillation wavelength within a certain wavelength band range. A representative example of a wavelength tunable light source using a semiconductor is a tunable laser diode (TLD). The TLD is used in a wide range of applications such as a carrier light source for optical communication and gas sensing due to its small size. When using the TLD, the wavelength stability of the oscillating output light is important in various systems. The wavelength stability of the oscillating output light means, first, that the TLD continuously outputs an oscillation wavelength as expected by the user. Second, in addition to the accuracy and stability of the wavelength of the oscillating output light, it is also important that the side-mode suppression ratio (SMSR) is a certain value or more.

[0003] The SMSR is one of the indices indicating the quality of a laser and is defined as the intensity ratio between the peak (oscillation mode) and the second peak (sub-mode) of the spectral intensity output from the laser. For example, in optical communication, a light source with an SMSR of 40 dB or more is generally required when not modulated. The reason is that in an optical communication network using wavelength division multiplexing (WDM), the degradation of the SMSR may become noise light for other directly adjacent wavelength channels.

[0004] As a method for keeping the oscillation wavelength of the TLD constant, a method is adopted in which a part of the light output from the TLD is input to an appropriate wavelength filter, and the light output from the wavelength filter is monitored. Specifically, as disclosed in Non-Patent Document 1, the light from the TLD is input to an etalon having an appropriate free spectral range (FSR) to control the oscillation wavelength of the TLD so that the light output from the etalon is always constant.

[0005] Prior Art Documents

[0006] Non-Patent Document 1: Keiji Ishii et al., "High-Function Wavelength Tunable Light Source Technology", NTT Technical Journal, November 2007, p. 66

[0007] Non-Patent Document 2: Yuta Ueda, et al., “Electro-optically tunable laser with ultra-low tuning power dissipation and nanosecond-order wavelength switching for coherent networks”, Vol.7, No.8 / August 2020 / Optica Summary of the Invention

[0008] Problems to be Solved by the Invention

[0009] However, it has not been possible to implement the inspection of SMSR and the monitoring during actual operation in a wavelength-variable light source by a simple mechanism. The oscillation wavelength control mechanism disclosed in Non-Patent Document 1, also called a wavelength locker, can control the wavelength with high precision using an etalon with narrow-band transmission characteristics. The method using a wavelength locker is useful for keeping the wavelength of the laser constant, but it is difficult to know the state of SMSR. This is because the light output from the above-mentioned etalon is an output reflecting the wavelength of the oscillation mode of the TLD, and it is difficult to extract the wavelength information for the output of the sub-mode which is usually about 40 dB lower in intensity than the oscillating light.

[0010] In order to directly know the SMSR of the oscillating output light of the TLD, a spectral analyzer can be used. However, the spectral analyzer requires a mechanism for scanning the diffraction wavelength of the diffraction grating, and thus an additional scanning mechanism is provided in the TLD which is originally a wavelength scanning light source. For the inspection of the TLD performance or for the monitoring during the actual operation of the TLD, it is not practical to install a spectral analyzer in the TLD in terms of device size and cost. Therefore, there is a need for a mechanism and a control method for the oscillating output light that can extract the SMSR characteristics reflecting the oscillating output light of the wavelength-variable light source and have a high SMSR output.

[0011] The present invention has been completed in view of the above problems, and provides a mechanism for a wavelength-variable light source that can obtain oscillating output light reflecting SMSR and a control method therefor.

[0012] Solution to the Problem

[0013] One embodiment of the present invention is a method for controlling oscillating light in a wavelength-variable light source, the wavelength-variable light source comprising: a multimode interference waveguide (MMI waveguide) composed of M×N ports (M is an integer of 1 or more, and N is an integer of 2 or more); N reflective delay lines respectively connected to the N-port side of the MMI waveguide; and an optical gain waveguide connected to at least one port on the M-port side of the MMI waveguide. The method is characterized by comprising the following steps: detecting the intensity of light from the M-port side of the MMI waveguide other than the at least one port at the oscillation wavelength of the oscillating light; and generating a signal for controlling the oscillating light based on the detected intensity.

[0014] Another embodiment of the present invention is a wavelength-variable light source, which comprises: a multimode interference waveguide (MMI waveguide) composed of M×N ports (M is an integer of 1 or more, and N is an integer of 2 or more); N reflective delay lines respectively connected to the N-port side of the MMI waveguide; an optical gain waveguide connected to at least one port on the M-port side of the MMI waveguide; a light receiver for detecting the intensity of light from the M-port side of the MMI waveguide other than the at least one port at the oscillation wavelength of the oscillating light; and a controller for generating a signal for controlling the oscillating light based on the intensity detected by the light receiver.

[0015] Advantages of the Invention

[0016] According to the present invention, there is provided a mechanism for a wavelength-variable light source that obtains oscillating output light reflecting the SMSR and a control method therefor. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 It is a schematic diagram showing the configuration of an RTF laser using a 5×5 port MMI.

[0018] Figure 2 It is a diagram showing the wavelength selection filter characteristics in the RTF laser of the present disclosure.

[0019] Figure 3 It is a diagram showing an enlarged view of the reflectivity near a wavelength of 1.544 μm.

[0020] Figure 4 It is a diagram showing the relationship between the reflection spectrum and the longitudinal mode condition of the RTF laser.

[0021] Figure 5 It is a diagram for explaining the SMSR adjustment based on the intensity of the oscillating light at the non-operating port.

[0022] Figure 6 It is a diagram for explaining the optimization at the peak of adjacent fine spectra.

[0023] Figure 7 This is a diagram showing the configuration of a wavelength-variable light source having a cut-off unit that oscillates and outputs light.

[0024] Figure 8 This is a diagram showing the configuration of a modified example of the wavelength-variable light source of the present disclosure. Detailed implementation mode

[0025] In the RTF laser using a reflection-type transversal filter (RTF), focusing on the filter characteristics inherent in the RTF laser, the control of SMSR is achieved by a simple configuration having only a plurality of light receivers. The RTF laser is a form of wavelength-variable light source that has received attention in recent years and includes an RTF having a multi-mode interference (MMI) waveguide and a plurality of reflection-type delay lines. In the following description, for simplicity, the MMI waveguide is only referred to as "MMI".

[0026] The inventors focused on the wavelength selection filter characteristics represented by the reflection characteristics and transmission characteristics between ports in the MMI of the RTF laser, which can reflect the intensity difference between the oscillation wavelength and the wavelength of the sub-mode. In the RTF laser using the MMI as described later, there must be a port that is not connected to the optical gain medium that contributes to the oscillation operation. The intensity of the oscillating light at a plurality of non-working ports of the MMI is monitored by considering the filter characteristics between the working port connected to the optical gain medium that contributes to the oscillation operation and the non-working port that does not directly contribute to the oscillation operation. By controlling the RTF laser so that the monitored intensity of the oscillating light has a specified relationship, the control of the wavelength-variable light source reflecting the SMSR characteristics is achieved.

[0027] In the following description, first, the basic configuration of the RTF laser is described, and while focusing on the wavelength selection filter characteristics observed at the non-working ports of the MMI of the RTF laser, the basic mechanism and some embodiments of the control mechanism of the wavelength-variable light source are shown. First, the mechanism of monitoring the signal (information) reflecting the SMSR in the RTF laser and feeding it back to various wavelength control mechanisms of the RTF laser to control the SMSR is described.

[0028] [Configuration of RTF laser]

[0029] Figure 1It is a schematic diagram showing the configuration of an RTF laser using a 5×5 port MMI. The RTF laser 100 includes N reflective delay lines 13 connected to the N ports on one side of the M×N port MMI 12 and an optical gain region (optical gain waveguide) 11 connected to at least one port among the M ports on the other side of the MMI 12. The MMI 12 and the plurality of reflective delay lines 13 constitute a reflective transverse filter (RTF) 10. The plurality of reflective delay lines each have a delay line 13-1 as an optical waveguide with a different length and a mirror 14-1 at the end, forming a reciprocating optical path with different optical paths between each port on the optical gain region 11 side of the MMI and the mirror at the end.

[0030] In Figure 1 it, an optical gain region 11 is connected to port 3 of the MMI 12, and oscillating light 24 is output from the end of the optical gain region 11. The optical gain region 11 can be an optical gain waveguide including an optical gain region. Here, a detailed description of the oscillation mechanism of the RTF laser 100 will not be given, but laser oscillation will occur at a wavelength where the reflected lights from a plurality of RTFs with different lengths become a relationship of mutual enhancement at port 3 of the MMI 12. The oscillation wavelength is adjusted by the phase adjustment electrode 17 on the MMI 12 and the wavelength adjustment electrode 18 on the plurality of reflective delay lines 13. For details, refer to Non-Patent Document 2 for example.

[0031] In the RTF laser 100 of the present disclosure, in order to monitor and control the SMSR, photodetectors (PD1, PD2, PD4, PD5) 15-1 to 15-2, 15-4 to 15-5 are provided at ports on the optical gain region 11 side of the MMI that do not contribute to the oscillation operation (unused). In the prior art RTF laser as a wavelength-variable light source, the wavelength and intensity of the oscillating light itself from the optical gain region 11 are monitored to ensure its wavelength stability. The inventors obtained the following idea: using the optical intensity information of the wavelength of the oscillating light from non-working ports, that is, ports in the MMI that do not contribute to the oscillation operation, for the control of the SMSR. The optical intensity signals 21-1 to 21-5 from the photodetectors are supplied to the control unit (hereinafter referred to as the controller) 16. The controller 16 supplies control signals 22 and 23 to the phase adjustment electrode 17 and the wavelength adjustment electrode 18 respectively as described later, and controls the SMSR according to the control method of the present disclosure described later.

[0032] In Figure 1 the RTF laser 100, the MMI 12 is configured as a 5×5 port, but it is not limited to this configuration. The number M of ports on the optical gain region side can be an integer of 1 or more, and the number N of ports on the RTF side can be an integer of 2 or more, so that an M×N port configuration is generally adopted. In addition, although in Figure 1The optical gain region 11 that generates and amplifies light is connected to port 3, but it can also be connected to other ports. In addition, the optical gain region 11 can also be provided in multiple ports on the M port side as described in Non-Patent Document 2. Moreover, generally, the optical gain region can be used as a light absorption layer. Therefore, for example, the optical gain region can be provided in all of the M ports, and the optical gain region that does not contribute to the oscillation operation can be used as a light receiver. In addition, it can also be configured to output the oscillating light from one or more ends (mirrors) among the multiple reflective delay lines of the RTF laser 100.

[0033] Figure 1 The photodetectors (PD1 to PD5) connected to the non-operating ports in can be monolithically integrated on the same substrate as the substrate constituting the RTF laser, or can be provided outside the substrate to receive light from the MMI port of the RTF laser. Next, focusing on the characteristics of the wavelength selection filter in the RTF laser 100, the control actions in the control method of the wavelength tunable light source of the present disclosure will be described.

[0034] [Control of SMSR in RTF Laser]

[0035] Figure 2 is a diagram showing the characteristics of the wavelength selection filter in the RTF laser of the present disclosure. Figure 2 Many of the waveforms in are in Figure 1 In the RTF laser having the configuration shown, the reflection spectra of ports 1 to 5 (M side) observed from port 3 (hereinafter referred to as the working port) that is connected to the optical gain region 11 and operates for the oscillation operation are shown. The wavelength (μm) is shown on the horizontal axis, the reflectance is shown on the vertical axis, and the corresponding ports 1 to 5 are shown by the display of #1 to #5.

[0036] Here, it should be noted that the "reflectance" in the following description represents the reflection spectrum of the entire RTF 10 composed of the MMI12 and the multiple reflective delay lines 13 observed from the working port 3. Regarding the working port 3, it is shown by the label #3 in Figure 2 and represents the reflectance of the working port 3 as it is. The reflectance at the working port 3 is the same as the reflectance of light at a specific port generally used in the optical path, and the reflection loss can also be obtained from the value of this reflectance. In the state of generating laser oscillation, ideally, the reflectance of the working port 3 is 1.

[0037] On the other hand, Figure 2The waveform curves labeled #1, #2, #4, and #5 respectively represent the reflectivities at the non-working ports 1, 2, 4, and 5 when observing the entire RTF10. The following points should be noted: It substantially represents the "transmission characteristics" between different ports that reflect all the optical paths of the RTF10. All the optical paths of this RTF10 are composed of the forward paths and the return paths formed by the mirrors at the ends of the respective delay lines folding back. For example, in Figure 2 the reflectance spectral curve labeled #1 represents the transmission characteristics between port 3 and port 1. In Figure 2 it is possible to confirm reflectance spectra #1 to #5 that are at different positions on the wavelength axis and show waveforms of substantially similar shapes. These reflectance spectra represent the interference states of N reflective delay lines of different lengths in the RTF10 based on Figure 1 and are observed as different filter characteristics corresponding to the M ports of the MMI. Note that Figure 2 the reflectance characteristics observed at each port of the MMI represent the "wavelength selection filter characteristics" of the entire RTF10 for generating laser oscillation at a specific wavelength. In the following description, for simplicity, the reflectance characteristics or transmission characteristics observed at each port on the optical gain region 11 side of the MMI are referred to as reflectivity or reflectance spectrum.

[0038] Looking at it in more detail Figure 2 , the reflectance spectra #1 to #5 observed at each port of the MMI are composed of components with a short period where the FSR is less than 2 nm and components with a long period that are the envelopes thereof. Here, the spectrum of the short-period component is called the fine spectrum 31, and the long-period component shown by the dashed line is called the coarse spectrum 30. The fine spectrum 31 and the coarse spectrum 30 can be adjusted independently by applying appropriate electrical signals to the wavelength adjustment electrodes 18 on the N reflective delay lines shown in Figure 1 (Non-Patent Document 2). For example, it is also possible to control the position of the fine spectrum 31 on the wavelength axis while keeping the position of the coarse spectrum 30 on the wavelength axis at the same position. At this time, the fine spectrum 31 is controlled to shift its peak position while being inscribed within the dashed line representing the coarse spectrum 30.

[0039] Figure 3 is a diagram that magnifies and shows the reflectivity near a wavelength of 1.544 μm. It shows the reflectance spectrum in the wavelength range where the reflectivity of the working port 3 labeled #3 has a peak near 1.544 μm on the horizontal axis in Figure 2 . In the RTF laser in Figure 1 , the optical gain region 11 is connected to port 3. Therefore, in Figure 3Laser oscillation is achieved near the peak wavelength of the narrow spectrum of #3. Subsequently, the oscillation generated near the peak in the narrow spectrum that contributes to laser oscillation is referred to as the oscillation narrow mode.

[0040] The more precise laser oscillation wavelength under the oscillation narrow mode is the wavelength that satisfies the resonator longitudinal mode condition. The resonator longitudinal mode condition is the condition for the light reciprocating in the resonator formed by RTF10 to form a standing wave within the resonator. When Figure 1 the refractive index of the RTF laser 100 as the resonator is set to n and the length is set to L, the wavelength λ (m is a natural number) that satisfies the following formula is the wavelength λ that satisfies the longitudinal mode condition.

[0041] mλ = 2nL Equation (1)

[0042] The wavelength that satisfies the resonator longitudinal mode condition of the above formula is determined by the number, length, structure of the delay line formed by the optical waveguide of RTF10, the structure of the MMI waveguide, and the refractive index of the materials of each part, and can be adjusted by the phase adjustment electrode 17.

[0043] Figure 4 is a diagram showing the relationship between the reflection spectrum and the longitudinal mode condition in the RTF laser. Figure 4 (a) of Figure 3 is a diagram obtained by overlapping and writing the longitudinal mode period with FSR = 0.3 nm on an enlarged view near the wavelength of 1.544 μm shown. Therefore, Figure 4 the reflection spectrum shown in (a) of Figure 3 is the same as the reflection spectrum shown. Figure 4 (b) of

[0044] In Figure 4 (a) of Figure 1 , for the reflection spectrum 32a of the working port 3 of the MMI, the equally spaced lines represent the wavelengths that satisfy the longitudinal mode condition. Near the peak wavelength of the narrow spectrum 32a of the working port 3, the oscillation longitudinal mode line 33a closest to the peak of the oscillation narrow mode among the oscillation longitudinal mode lines 33a, 33b, 33c becomes Figure 4 the oscillation wavelength of the RTF laser 100 of

[0045] In Figure 4 (b) of Figure 4The reflection spectra at the four non-operating ports in (b) have different values at the wavelength of the oscillating longitudinal mode line 33a of the oscillating wavelength. In the oscillating state satisfying the longitudinal mode condition, at the four non-operating ports, light of the oscillating wavelength is observed at intensities corresponding to the reflectivities in (b). Figure 4 In the wavelength locker described in Non-Patent Document 1 as an example of the prior art, fine tuning of the oscillating wavelength is mainly achieved by controlling the wavelength of the longitudinal mode. In the RTF laser 100 shown in

[0046] , an appropriate electrical signal is applied to the phase adjustment electrode 17 to finely adjust the refractive index n in Equation (1), thereby achieving fine tuning of the oscillating wavelength. At this time, the electrical signal applied to the phase adjustment electrode 17 is equivalent to adjusting the oscillating longitudinal mode lines 33a, 33b, and 33c with respect to the reflection spectrum at the working port 3 on the Figure 1 wavelength axis of Figure 2 .

[0047] Here, when considering the SMSR in the RTF laser 100, in the state of oscillating at the wavelength of the oscillating longitudinal mode line 33a within the longitudinal mode condition in (b), the longitudinal mode reflectivity difference 35 between the two oscillating longitudinal mode lines 33a and 33c determines the SMSR. In the oscillating state, most of the energy supplied to the optical gain region is consumed at the oscillating wavelength of the longitudinal mode wavelength, but an oscillating state is also observed at the wavelength of the next oscillating longitudinal mode line 33c with a high reflectivity after the oscillating longitudinal mode line 33a. Therefore, if the oscillating longitudinal mode line 33a coincides with the peak wavelength of the reflectivity 32a of the working port 3 in (a) of Figure 4 , the longitudinal mode reflectivity difference 35, which is the intensity difference between adjacent longitudinal modes, becomes the maximum, and the SMSR becomes the maximum. Figure 4 Even if the position of the oscillating longitudinal mode line is adjusted in the RTF laser 100 as described above, only the position of the envelope of the fine spectrum is relatively adjusted together with the coarse spectrum 30, and sometimes the peak of the reflection spectrum 32a does not exactly coincide with the oscillating longitudinal mode line 33a. It can be considered that the prior art RTF laser is equivalent to the state where the peak of the fine spectrum 32a does not exactly coincide with the oscillating longitudinal mode line 33a as shown in (a) of

[0048] . Figure 4 The inventors believe that in addition to adjusting the relative position of the oscillating longitudinal mode line and the coarse spectrum to adjust the longitudinal mode oscillating wavelength on the wavelength axis, it is also necessary to adjust the fine spectrum in order to maximize the SMSR. From

[0049] Figure 4 ​The relationship between the reflection spectra #3 of the working port 3 in (a) and (b) and the reflection spectra #1, #2, #4, #5 is also obvious: the wavelength of the peak of the narrow spectrum 32a is roughly the same as the wavelength of the minimum value of the total reflection spectrum 34a obtained by adding the reflectivities of the 4 non-working ports. Therefore, if the intensity of the light with the wavelength of the oscillating light observed at the non-working ports is monitored in the MMI 11 of the RTF laser 100 while adjusting Figure 4 the reflection spectra #1, #2, #4, #5 shown in (b), the SMSR can be maximized.

[0050] Figure 5 FIG. is a diagram for explaining the adjustment of SMSR based on the intensity of the oscillating light at the non-working ports in the control method of the wavelength-variable light source of the present disclosure. Figure 5 FIG. (a) shows the reflection spectrum obtained by further adjusting the narrow spectrum after adjusting the longitudinal mode oscillation wavelength. Figure 5 FIG. (b) is a diagram further magnifying the wavelength range near the oscillating narrow mode of the reflection spectrum in (a) and showing the reflectivities of the non-working ports 1, 2, 4, 5 near the reflectivity of 0.

[0051] In Figure 5 FIG. (a), only the reflection spectrum of the working port 3 before adjusting the narrow spectrum is shown by the dotted line 32a, and the dotted line 32a is the same as the reflection spectrum 32a in Figure 4 FIG. (a). The solid line represents the state where the narrow spectrum is slightly shifted to the longer wavelength side and the peak of the oscillating narrow mode coincides exactly with the oscillating longitudinal mode line 33a. At this time, the longitudinal mode reflectivity difference 35 obtains a value more than 3 times larger than that in the case of Figure 4 FIG. (a), and an improvement in SMSR can be expected.

[0052] In Figure 5 FIG. (b), the reflection spectra #1, #2, #4, #5 at the non-working ports are shown, and the total reflection spectrum 34b obtained by adding the reflectivities of the 4 non-working ports is also shown. Here, the wavelength of the minimum point of the total reflection spectrum 34b coincides with the oscillating longitudinal mode line 33a. Therefore, it is only necessary to control the wavelength-variable light source so that the total amount of the signal intensities detected by the light receivers 15-1 to 15-5 at the non-working ports #1, #2, #4, #5 becomes the minimum at the specified laser oscillation wavelength (oscillating longitudinal mode line 33a).

[0053] Therefore, the method for controlling the oscillating light in the wavelength-variable light source of the present disclosure includes the following steps: detecting the intensities 21-1 to 21-5 of the light from the M-port side of the MMI waveguide except for at least one port. It also includes the following steps: the controller 16 generates signals 22, 23 for controlling the oscillating light 24 based on the detected intensities. The control signals 22, 23 act on the wavelength tuning electrode 18 to control the positions on the wavelength axes of the fine spectrum and the coarse spectrum.

[0054] As already described, the adjustment of the reflection spectrum on the wavelength axis is achieved by the wavelength tuning electrode 18. The wavelength tuning electrode 18 is a plurality of electrodes formed on the plurality of reflective delay lines 13. There is no limitation in the present invention on the specific method of applying what voltage to the wavelength tuning electrode 18 and how to change the reflection spectrum. That is, the method for controlling the oscillating light in the wavelength-variable light source of the RTF laser has the characteristics in the following steps and points: the step of detecting the intensity of the light from the M-port side of the MMI waveguide except for at least one port to which the optical gain waveguide is connected and the point of generating a signal for controlling the oscillating light based on the detected intensity. As long as the wavelength tuning electrode 18 can be controlled in such a way that the total reflection spectrum 34b obtained by adding the reflectivities of the reflection spectra #1, #2, #4, #5 at the non-operating ports is minimized.

[0055] Therefore, the present invention can be implemented as a method, which is a method for controlling the oscillating light in a wavelength-variable light source, the wavelength-variable light source comprising: a multimode interference waveguide (MMI waveguide) composed of M×N ports (M is an integer of 1 or more, N is an integer of 2 or more); N reflective delay lines respectively connected to the N-port side of the MMI waveguide; and an optical gain waveguide connected to at least one port of the M-port side of the MMI waveguide, and the method is characterized by comprising the following steps: detecting the intensity of the light from the M-port side of the MMI waveguide except for the at least one port at the oscillating wavelength of the oscillating light; and generating a signal for controlling the oscillating light based on the detected intensity.

[0056] Refer again to Figure 1 , the light intensity signals 21-1 to 21-5 are supplied from the light receivers 15-1 to 15-5 to the controller 16, and the controller 16 generates a control signal 23 for the wavelength tuning electrode 18 based on the received light intensity signals 21-1 to 21-5. Each light intensity signal is an electrical signal corresponding to the reflectivity of the reflection spectra #1, #2, #4, #5, and the total reflection spectrum 34b is a spectrum obtained by adding these four electrical signals. In Figure 1In this case, only the light intensity signals 21-1 to 21-5 supplied to the controller 16 are shown, and there is no limitation on how to obtain the combined signal corresponding to the combined reflection spectrum 34b. It can be either physically adding the four electrical signals or performing arithmetic processing after converting each electrical signal into a digital signal to obtain it.

[0057] Therefore, the present invention can be implemented as a wavelength-variable light source, which includes: a multimode interference waveguide (MMI waveguide 12) composed of M×N ports (M is an integer of 1 or more, and N is an integer of 2 or more); N reflective delay lines 13, respectively connected to the N-port side of the MMI waveguide; an optical gain region (optical gain waveguide) 11, connected to at least one port on the M-port side of the MMI waveguide; light receivers 15-1 to 15-5, detecting the intensity of light from the M-port side of the MMI waveguide other than the at least one port at the oscillation wavelength of the oscillating light; and a controller 16, generating a signal for controlling the oscillating light based on the intensity detected by the light receiver.

[0058] As described above, in the wavelength-variable light source of the present disclosure, that is, the RTF laser and its control method, the intensity at the oscillation wavelength of the non-working ports of the RTF laser that do not contribute to the oscillation operation and are other than the at least one port connected to the optical gain region is detected and monitored by the light receiver. In the wavelength-variable light source of the present disclosure, the following mechanism is characteristic: based on the intensity of the light observed at the non-working port obtained by the light receiver, a signal for controlling the oscillating output light in the wavelength-variable light source is generated by the controller. The light receivers connected to the non-working ports detect the light of all wavelengths that appear at the non-working ports. However, the following points need to be noted: from Figure 5 The reflection spectra #1, #2, #4, #5 of (b) show that in the state where laser oscillation occurs at port 3 of MMI11, the signal intensity of the oscillation wavelength observed at ports 1, 2, 4, and 5 is 0.01 or less, and the "leakage light" of the oscillating output light in port 3 is measured by the light receiver. In the RTF laser of the prior art, it is quite different from the RTF laser of the present disclosure that utilizes the intensity of the oscillating light from the non-working port in terms of detecting the oscillating light itself from the working port connected to the optical gain region that contributes to the oscillation operation. According to the signal from the controller, the positions on the wavelength axis of the fine spectrum and the coarse spectrum of the oscillating output light are controlled, thereby controlling the wavelength-variable light source in a manner that maximizes the SMSR.

[0059] Regarding the wavelength-variable light source of the present disclosure and its control method, a more specific control method will be further described in the following embodiments.

[0060] (Embodiment 1)

[0061] In the above-described wavelength-variable light source and its control method of the present disclosure, control is performed to maximize the SMSR of the oscillating output light by minimizing the total amount of the intensity signal measured by the light receiver connected to the non-operating port. The maximization of the SMSR can be achieved by shifting the fine spectrum in the reflection spectrum of the non-operating port on the wavelength axis and finely adjusting the wavelength selection filter characteristics of the RTF. Here, when controlling the spectrum of the RTF, information for determining the control direction of the spectrum on the wavelength axis is required. For example, when comparing (a) of Figure 4 with (a) of Figure 5 , the fine spectrum is shifted to the long-wavelength side in order to make the peak wavelength of the oscillating fine mode of the reflection spectrum 32a of port 3 coincide with the longitudinal mode wavelength (oscillating longitudinal mode line 33a). Therefore, as long as information in the following direction can be obtained: In the RTF laser of the prior art, an appropriate electrical signal is applied to the phase adjustment electrode 17 to finely adjust the oscillation wavelength, and in a subsequent stage, the direction in which the fine spectrum should be further shifted on the wavelength axis. With this information, the control process performed by the controller 16 in the RTF laser of Figure 1 can be simplified, and the optimization of the SMSR can be implemented more simply. Therefore, an embodiment for determining the adjustment direction on the wavelength axis of the reflection spectrum of the RTF will be described by focusing on the magnitude relationship of the intensity of the oscillating output light observed at the non-operating port.

[0062] Here, again, focus on the reflection spectra #1, #2, #4, #5 at the non-operating port of (b) of Figure 4 after the fine adjustment of the oscillation wavelength is performed. It can be seen that the intensities of the light observed on the long-wavelength side and the short-wavelength side with respect to the peak wavelength of the reflectivity 32a of port 3 (which is approximately the wavelength of the minimum value of the total reflection spectrum 34a) serving as the operating port vary depending on the port. In the case of the 5×5 MMI12 of the RTF laser of Figure 1 , as in (b) of Figure 4 , on the long-wavelength side of the peak of the reflectivity 32a of port 3 (e.g., the oscillating longitudinal mode line 33a), the relationship of reflectivity #2, #4 > reflectivity #1, #5 holds. On the other hand, on the short-wavelength side of the peak of the reflectivity 32a of port 3 (e.g., the oscillating longitudinal mode line 33c), conversely, the relationship of reflectivity #2, #4 < reflectivity #1, #5 holds.

[0063] For example, when actually using the RTF laser, in the case where the relationship of the intensities of the lights from the photoreceivers 15-1 to 15-5 is reflectance #2, #4 > reflectance #1, #5, it can be determined that the peak wavelength of the oscillating fine mode 32a is on the short-wavelength side with respect to the desired oscillating longitudinal mode peak wavelength (oscillating longitudinal mode line 33a). On the other hand, in the case where reflectance #2, #4 < reflectance #1, #5, it can be determined that the peak wavelength of the oscillating fine mode 32a is on the long-wavelength side with respect to the desired oscillating longitudinal mode peak wavelength (oscillating longitudinal mode line 33a). By comparing the magnitude relationships of the intensities of the lights in each of the photoreceivers 15-1 to 15-5 for the given longitudinal mode wavelength (oscillating wavelength), information on the adjustment direction regarding which side, the long-wavelength side or the short-wavelength side, the fine mode peak wavelength, i.e., the reflectance 32a of port 3, should be shifted to can be obtained.

[0064] To determine the adjustment direction on the wavelength axis of the reflection spectrum of the above-mentioned RTF, as long as based on the known magnitude relationships in advance, it is only necessary to compare the optical intensity signals from the photoreceivers 15-1 to 15-5 in Figure 1 respectively. Therefore, Figure 1 the configuration of the RTF laser in Figure 4 remains unchanged, and only the determination process of the control signal 23 of the controller 16 is changed. The magnitude relationships between the ports of the reflection spectra #1, #2, #4, #5 described in (b) of the above-mentioned Figure 1 are the relationships in the configuration of the MMI 11 where the optical gain region is connected to port 3, and vary depending on the configuration of the MMI and the position of the working port to which the optical gain region is connected. Therefore, as long as based on the configuration of the RTF laser including the MMI used, the relationship of the intensities of the lights of the oscillating wavelengths observed between specific ports within the non-working ports is known in advance. In short, as long as the wavelength selection filter characteristics shown in Figure 2 are grasped in advance and the relationship for determining the adjustment direction on the wavelength axis of the reflection spectrum is known. There is no limitation on the non-working ports for comparing the magnitude relationships of the intensities of the lights in the photoreceivers, and regarding the number of ports for which the intensities are compared, it is not limited to the relationship between the above two ports and the other two ports, and is arbitrary.

[0065] (Embodiment 2)

[0066] In Figure 4 and Figure 5 in the basic control method of the SMSR in the RTF laser described, only the reflectances of each port at the wavelength of the oscillating longitudinal mode line 33a near the peak of the oscillating fine mode are considered. However, when optimizing the SMSR and focusing on the relative relationship between the coarse spectrum and the fine spectrum, an index effective for optimizing the SMSR can also be found at the peak of the adjacent fine spectrum far from the oscillating longitudinal mode line 33a.

[0067] Figure 6 It is a diagram explaining the optimization at the peaks of adjacent narrow spectra. Figure 6 (a) and (b) show that the peak wavelength of the oscillating fine mode satisfies the longitudinal mode condition. Figure 5 Starting from the states (a) and (b), the reflectivity of the adjacent fine mode is further adjusted through the coarse filter to optimize the state of SMSR. Figure 5 same, Figure 6 (a) shows the reflection spectrum obtained by adjusting the reflectivity of adjacent fine modes. Figure 6 (b) is an enlarged view showing the reflectance of non-operating ports 1, 2, 4, and 5 with a reflectance near 0 in the wavelength range near the oscillation fine mode of the reflection spectrum of (a).

[0068] When the general Figure 6 (b) and Figure 5 When comparing (b) Figure 5 In (b), at the wavelength of the oscillation longitudinal mode, i.e., the oscillation longitudinal mode line 33a, the total reflection spectrum 34b of the non-working port takes an extreme value. However, the individual reflection spectra #1, #2, #4, and #5 of the non-working port are not extreme values. On the other hand, in this embodiment, the SMSR is optimized at the peak of the adjacent narrow spectrum. Figure 6 In (b), the total reflection spectrum 34c of the non-working port and the individual reflection spectra #1, #2, #4, #5 all take extreme values ​​at the wavelength of the oscillation longitudinal mode line 33a. That is, the wavelength adjustment electrode 18 can be controlled in a manner that not only minimizes the total reflection spectrum of the non-working port but also minimizes the individual reflection spectra #1, #2, #4, #5 of the non-working port. It is known that the method of controlling the individual reflection spectra #1, #2, #4, #5 independently on the wavelength axis is known, and which electrode of the wavelength adjustment electrode 18 is applied with what voltage depends on the specifications of the wavelength adjustment electrode 18.

[0069] The difference between the basic control method of SMSR described above and this embodiment lies in the point that the relative relationship between the coarse spectrum and the fine spectrum is reflected. Figure 6 (a), in the narrow spectrum of the working port 3, the two peaks on both sides adjacent to the peak that coincides with the oscillation longitudinal mode line 33 have the same intensity. At this time, the intensity difference between the peak of the narrow spectrum of the working port 3 and the adjacent peak, that is, the narrow mode reflectivity difference 36 is the maximum. Figure 5 The difference in fine mode spectra is obvious when compared with the fine mode reflectivity difference 36 of (a). Figure 6 As shown in (b), the individual reflection spectra #1, #2, #4, and #5 of the non-working ports are minimized. Figure 2As understood from the relationship between the fine spectrum 31 and the coarse spectrum 30 described in, it can be seen that in the state where the adjustment of (a) of Figure 6 is performed, the peaks of the coarse spectrum and the fine spectrum are adjusted to be consistent.

[0070] In Figure 1 , the wavelength adjustment electrode 18 can be controlled in such a way as to minimize each of the light intensity signals 21-1 to 21-5 from the light receivers 15-1 to 15-5. At this time, the coarse spectrum and the fine spectrum are adjusted, and the SMSR degradation due to modes (adjacent fine modes) different from the oscillating fine mode can also be reduced. Also in this embodiment, Figure 1 the configuration of the RTF laser 100 of

[0071] (Embodiment 3)

[0072] In a system using a wavelength-variable light source, there may be cases where the difference between the wavelength requested by the user and the wavelength of the actually output oscillating light is larger than a certain value, or the SMSR of the laser oscillating light is lower than a certain value. In such a state, when observing at a wavelength channel other than the wavelength channel expected in the wavelength-variable light source, wavelength crosstalk occurs, and interference and obstruction occur. For example, in an optical communication network, in a wavelength division multiplexing (WDM) system that transmits information over different wavelength channels, the degradation of the SMSR of a certain wavelength-variable light source may directly become noise light when observed from other wavelength channels. Since it directly leads to a reduction in communication quality, it is ideal to cut off the light output itself from the wavelength-variable light source when the SMSR of the wavelength-variable light source is becoming below a certain level.

[0073] Figure 7 FIG. is a diagram showing the configuration of a wavelength-variable light source having a light output cut-off unit. Figure 7 The wavelength-variable light source of Figure 1 is the RTF laser 200, which is generally the same as the RTF laser 100 shown in Figure 1 in its basic configuration. Therefore, only the differences will be described here. For the RTF laser 200 of Embodiment 3, the configurations of the RTF10, the optical gain region 11, the light receivers 15-1 to 15-5, and the phase adjustment electrode 17 and the wavelength adjustment electrode 18 are the same as those of the RTF laser 100 of Figure 1The controller 16 of the RTF laser 100 can be a general-purpose controller or a separate dedicated controller.

[0074] The RTF laser 200 of this embodiment also includes an optical intensity adjuster 19 on the output side of the optical gain region 11. The optical intensity signals 21-1 to 21-5 from the non-working ports observed by each photoreceiver are provided to the controller 16-1. As in the above-described Embodiment 1 and Embodiment 2, the optical intensity signals 21-1 to 21-5 from the non-working ports reflect the SMSR of the oscillating output light and can be used to optimize the SMSR. Therefore, as long as the control method of the SMSR in the above-described RTF laser and the optical intensity signals 21-1 to 21-5 used in Embodiment 1 and Embodiment 2 are used, when a certain degree of reduction in the SMSR is confirmed, the laser output light can be cut off or attenuated by the optical intensity adjuster 19. By turning off or significantly reducing the intensity of the laser output light, the influence on other wavelength channels can be minimized. The optical intensity adjuster 19 can be any device as long as it can change the output intensity of the laser output light. For example, it can be a mechanism that amplifies an optical signal like a semiconductor optical amplifier, or an optical modulator originally intended for generating an optical signal such as an electro-absorption optical modulator or a Mach-Zehnder optical modulator.

[0075] As described above, in the wavelength-variable light source and its control method of the present disclosure, by utilizing the properties of the wavelength selection filter of the RTF laser and focusing on the filter characteristics between the working port and the non-working ports that do not directly contribute to the oscillation operation, the intensity of the light of the wavelength of the oscillating light observed at the non-working ports is monitored. The above-described wavelength selection filter characteristics of the RTF laser are originally based on the characteristics of the intensity of the light of the oscillating wavelength observed at the M ports defined in the M×N MMI. That is, in Figure 1 the MMI 12, including the optical waveguide connected to the optical gain region, the light from the "M ports" connected to the optical waveguides is monitored by photoreceivers. However, even by using the intensity including the leakage of the oscillating light from the "portion other than the ports" on the M port side, which is outside the portion defined as the ports by the optical waveguides of a certain width connected in the MMI, information reflecting the SMSR can be obtained.

[0076] Figure 8 This is a modified example of the wavelength-variable light source of the present disclosure and is a diagram showing a form in which light from the "portion other than the ports" other than the waveguide connected to the optical gain region is also utilized. In Figure 8In the RTF laser 300 according to a modified example, the light receivers are constituted by PD A 40a and PD B 40b, and only the light intensity signals 41a and 41b from the two light receivers are supplied to the controller 16. Among the two light receivers, the light intensity including the ports 1 and 2 and the light leakage is monitored by the light receiver PD A 40a, and the light intensity including the ports 4 and 5 and the light leakage is monitored by the light receiver PD B 40b. That is, in the RTF laser according to the modified example, the SMSR is controlled based on the light leakage intensity of the oscillating light from the portion other than the ports on the M port side. Even in the RTF laser 300 in such a configuration, the control of the SMSR in the above-described RTF laser and the basic mechanisms of the first to third embodiments can be applied.

[0077] As described in detail above, in the wavelength variable light source and its control method of the present disclosure, the light intensity of the wavelength of the oscillating light at the plurality of non-working ports of the MMI is utilized in consideration of the filter characteristics between the working port and the non-working ports that do not directly contribute to the oscillation operation. The RTF laser is controlled such that the monitored light intensity at the non-working ports has a desired relationship, thereby realizing the control of the wavelength variable light source reflecting the SMSR characteristics. The SMSR can be effectively controlled only by adding a light receiver to the non-working ports not considered in the conventional RTF laser. The inspection of the SMSR and the monitoring during actual operation can be achieved by a simple mechanism in the wavelength variable light source.

Claims

1. A method for controlling oscillating light in a wavelength-variable light source, the wavelength-variable light source comprising: A multimode interference waveguide formed by M×N ports, i.e., an MMI waveguide, wherein, M is an integer of 1 or more, and N is an integer of 2 or more; N reflective delay lines, respectively connected to the N-port side of the MMI waveguide; and An optical gain waveguide, connected to at least one port on the M-port side of the MMI waveguide, The method is characterized by comprising the following steps: Detecting the intensity of light from the M-port side of the MMI waveguide other than the at least one port at the oscillation wavelength of the oscillating light; And Generating a signal for controlling the oscillating light based on the detected intensity, The method includes: The intensity is determined by the sum of the intensities from two or more ports that do not contribute to the oscillation operation, and the sum is minimized; Or, the intensity is the intensity from two or more ports that do not contribute to the oscillation operation, and the intensities from the two or more ports are minimized respectively.

2. The method according to claim 1, characterized in that, The intensity is the intensity from a port that does not contribute to the oscillation operation or the intensity of the leakage light of the oscillating light from a part other than the port on the M-port side.

3. The method according to claim 1, characterized in that, Generating the signal based on the magnitude relationship of the intensities from two or more ports on the M-port side that do not contribute to the oscillation operation.

4. The method according to any one of claims 1 to 3, characterized in that, The signal includes a control signal for an optical intensity modulator that changes the output of the wavelength-variable light source.

5. A wavelength-variable light source, characterized in that, comprising: A multimode interference waveguide formed by M×N ports, i.e., an MMI waveguide, wherein M is an integer of 1 or more, and N is an integer of 2 or more; N reflective delay lines, respectively connected to the N-port side of the MMI waveguide; An optical gain waveguide, connected to at least one port on the M-port side of the MMI waveguide; A light receiver, detecting the intensity of light from the M-port side of the MMI waveguide other than the at least one port at the oscillation wavelength of the oscillating light; and A controller, generating a signal for controlling the oscillating light based on the intensity detected by the light receiver, The intensity is determined by the sum of the intensities of the oscillating lights from two or more ports that do not contribute to the oscillation operation, and the controller minimizes the sum, Or, the intensity is the intensity from two or more ports that do not contribute to the oscillation operation, and the controller is configured to minimize the intensities from the two or more ports respectively.

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