Tunable wavelength laser diode, control method for tunable wavelength laser diode, and control program for tunable wavelength laser diode.

Abstract

A wavelength tunable laser diode is provided that does not rely on temperature control and can stabilize the oscillation wavelength against fluctuations in optical output. [Solution] An MMI 12 with an M×N port configuration, N reflective delay line arrays 13 connected to the N port side of the MMI 12, and an optical gain region 11 provided in at least one port on the M port side to amplify light are integrated on the same substrate, and a wavelength tunable laser diode is formed by a wavelength adjustment electrode group 18 that applies a voltage to the reflective delay line array 13 to adjust the wavelength of the oscillating light output from the optical gain region 11, a photocurrent detection electrode group 28 that detects the photocurrent generated from the reflective delay line array 13 to which a voltage is applied by the wavelength adjustment electrode group 18, and a control circuit 16 that controls the wavelength of the oscillating light based on the voltage applied by the photocurrent detection electrode group 28 and the current detected by the photocurrent detection electrode group 28.

Description

[Technical Field]

[0001] The present invention relates to a tunable laser diode (TLD), and more particularly to a tunable laser diode whose oscillation wavelength can be adjusted within a predetermined range. [Background technology]

[0002] A tunable laser diode (hereafter referred to as "TLD") is a variable wavelength light source that uses a semiconductor. Due to its compact size, TLDs are widely used in applications such as carrier light sources for optical communications and gas sensing. When using a TLD, wavelength stability is important. Wavelength stability here refers to the ability of a TLD to continue outputting the oscillation wavelength intended by the user over an extended period of time. Wavelength fluctuations include fluctuations over time and fluctuations in oscillation wavelength when only the optical output is changed.

[0003] One method for maintaining a constant oscillation wavelength of a TLD is a mechanism called a wavelength locker. A wavelength locker inputs a portion of the output light of the TLD into a filter (etalon) with an appropriate wavelength period (free spectrum range; FSR), and controls the oscillation wavelength of the TLD so that the output light of the etalon remains constant. Such a technique is described, for example, in Non-Patent Document 1. It is also known that in a wavelength locker, the temperature dependence of the wavelength period of the etalon is utilized to select an arbitrary wavelength period and control the oscillation wavelength.

[0004] To control the etalon temperature, a temperature control element separate from the semiconductor chip is required, which increases the number of components and the size of the device. Furthermore, temperature control can result in insufficient speed and stability when changing wavelengths, depending on the application.

[0005] To overcome the above problems, there is the RTF (Reflection-type Transversal Filter; RTF) laser, which does not use temperature-based wavelength control. RTF lasers are described in, for example, Patent Document 1 and Non-Patent Document 2. An RTF laser uses a tunable filter connected to the N-port side of an (M×N) multi-mode interference coupler (MMI) with M ports (each also referred to as an M-port) and N ports (each also referred to as an N-port). The tunable filter is composed of multiple optical waveguides functioning as reflectors, whose effective optical path length is controlled by applying voltage. The M-port side of the MMI is connected to an optical semiconductor waveguide with optical amplification gain characteristics by injecting current. This configuration is a laser structure consisting of a resonator with wavelength selectivity provided by the tunable filter. Because the oscillation wavelength of an RTF laser light source is controlled by applying voltage, it is preferable to wavelength lockers using etalons for its high speed and stability. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] International Publication WO2022 / 079814 [Non-patent literature]

[0007] [Non-Patent Document 1] Hiroyuki Ishii et al., "High-Performance Wavelength Tunable Light Source Technology," NTT Technical Journal, November 2007, p.66 [Non-patent document 2] Yuta Ueda, et al., "Electro-Optically Tunable Laser with <10-mW Tuning Power Dissipation and High-Speed ​​λ-Switching for Coherent Network", in Proc. of ECOC2019, PD.2.2. Summary of the Invention [Problem to be solved by the invention]

[0008] However, in an RTF laser, further stabilization of the oscillation wavelength is desired against variations in the characteristics of the reflector over time, variations in the light emission efficiency in the optical gain region over time, or variations in the optical output due to intentional control. The present invention has been made in consideration of these points, and relates to a wavelength-tunable laser diode, a control method for a wavelength-tunable laser diode, and a control program for a wavelength-tunable laser diode that can stabilize the oscillation wavelength without relying on temperature control and against variations in the optical output. [Means for solving the problem]

[0009] In order to achieve the above object, one aspect of the present invention provides a wavelength tunable laser diode, which includes: a multimode interference waveguide having an M×N port configuration (M is a natural number of 1 or more, and N is a natural number of 2 or more), a reflective delay line array consisting of N reflective delay lines connected to the N-port side of the multimode interference waveguide; and an optical gain waveguide provided at at least one port on the M-port side of the multimode interference waveguide for amplifying light, all integrated on the same substrate; a wavelength tuning electrode that applies a voltage to the reflective delay line to adjust the wavelength of oscillating light output from the optical gain waveguide; a photocurrent detection unit that detects a photocurrent generated from the reflective delay line to which a voltage is applied by the wavelength tuning electrode; and a wavelength control unit that controls the wavelength of the oscillating light based on the voltage applied by the wavelength tuning electrode and the current detected by the photocurrent detection unit.

[0010] A control method for a wavelength tunable laser diode according to one aspect of the present invention is a control method for a wavelength tunable laser diode in which a multimode interference waveguide having an M×N port configuration (M is a natural number of 1 or more, and N is a natural number of 2 or more), a reflective delay line array consisting of N reflective delay lines connected to the N-port side of the multimode interference waveguide, and an optical gain waveguide provided at at least one port on the M-port side of the multimode interference waveguide for amplifying light, are integrated on the same substrate, the control method including the steps of: applying a voltage to the reflective delay line; detecting a photocurrent generated from the reflective delay line to which the voltage has been applied; and controlling the wavelength of oscillating light output from the optical gain waveguide based on the voltage applied to the reflective delay line and the current detected in the photocurrent detection step.

[0011] A control program for a wavelength tunable laser diode according to one aspect of the present invention is a control program for controlling a wavelength tunable laser diode in which a multimode interference waveguide having an M×N port configuration (M is a natural number of 1 or more, and N is a natural number of 2 or more), a reflective delay line array consisting of N reflective delay lines connected to the N-port side of the multimode interference waveguide, and an optical gain waveguide provided in at least one port on the M-port side of the multimode interference waveguide for amplifying light, are integrated on the same substrate, and the control program causes a computer to realize the following functions: applying a voltage to the reflective delay line; detecting a photocurrent generated from the reflective delay line to which the voltage has been applied; and controlling the wavelength of oscillating light output from the optical gain waveguide based on the voltage applied to the reflective delay line and the current detected by the function for detecting the photocurrent. [Effects of the Invention]

[0012] According to the above-described embodiments, the oscillation wavelength can be stabilized without relying on temperature control and against fluctuations in optical output, so that a tunable laser diode, a control method for a tunable laser diode, and a control program for a tunable laser diode can be provided. [Brief explanation of the drawings]

[0013] [Figure 1]FIG. 1 is a schematic top view showing the configuration of an RTF laser using a 5×5 port MMI. [Figure 2] 1A is a diagram showing the relationship between the injection current and the intensity of the oscillating light of the RTF laser of this embodiment, FIG. 1B is a diagram showing the relationship between the absorption current and the wavelength tuning voltage of the RTF laser of this embodiment, and FIG. 1C is a diagram showing the relationship between the intensity of the oscillating light and the oscillation wavelength of the RTF laser of this embodiment when the product of the absorption current and the wavelength tuning voltage is controlled to be a constant. [Figure 3] 1A is a diagram illustrating the relationship between the wavelength tuning voltage, the oscillation wavelength, and the injection current, and FIG. 1B is a diagram illustrating the relationship between the wavelength tuning voltage, the oscillation wavelength, and the injection current after a forced degradation test. [Figure 4] FIG. 2 is a schematic top view showing a modified example of the RTF laser shown in FIG. DETAILED DESCRIPTION OF THE INVENTION

[0014] An embodiment of the present invention will be described below with reference to the drawings. The drawings are intended to explain the configuration and function of the tunable laser diode (TLD) of the present invention, the positional relationship of each part, and the technical concept, but do not limit the dimensional ratio or specific shape. This embodiment will be described using the configuration of a reflection-type transversal filter (RTF) as the TLD.

[0015] [RTF laser configuration] The following description will explain the configuration of the RTF laser of this embodiment. FIG. 1 is a schematic top view showing the configuration of an RTF laser using a 5×5-port MMI. The RTF laser 100 has an M (a natural number equal to or greater than 1; 5 in this embodiment)×N (a natural number equal to or greater than 2; 5 in this embodiment) port MMI 12. A reflective delay line array 13 consisting of N reflective delay lines 13-1 is connected to the port MMI 12. An optical gain region 11 that amplifies light is connected to at least one of the M ports on the side of the port MMI 12 opposite to the side to which the reflective delay line array 13 is connected. The reflective delay line array 13 and the optical gain region 11 are integrated on the same substrate to form a single chip.

[0016] In this embodiment, hereinafter, a port to which the reflective delay line array 13 is connected is also referred to as an "N port," and the side to which the reflective delay line array 13 is connected is also referred to as an "N port side." Also, a port to which the optical gain region 11 or a photodiode (PD) described later is connected is also referred to as an "M port," and the side to which the optical gain region 11 or the PD is connected is also referred to as an "M port side."

[0017] The RTF laser 100 of this embodiment includes a wavelength adjusting electrode group 18 that applies a voltage to the reflective delay line array 13 to adjust the wavelength of the oscillating light 24 output from the optical gain region 11, and a photocurrent detecting electrode group 28 (photocurrent detecting unit) that detects a photocurrent generated by the application of a voltage by the wavelength adjusting electrode group 18. The RTF laser 100 further includes a control circuit 16 (wavelength control unit) that controls the wavelength of the oscillating light 24 output from the optical gain region 11 based on the voltage applied by the wavelength adjusting electrode group 18 and the current detected by the photocurrent detecting electrode group 28.

[0018] The above configuration will be explained below in order. (MMI) In the MMI 12, light input from the reflective delay line array 13 is split into multiple modes within the internal waveguide. The split light propagates through the waveguide and is recombined. During this process, light of different modes interferes with each other, amplifying or attenuating the light intensity under certain conditions. The interfered light is recombined at the exit of the waveguide. The intensity and phase of the oscillating light 24 change depending on the result of the interference. A phase adjustment electrode 17 is provided on the MMI 12, and the oscillation wavelength of the RTF 100 is adjusted by the phase adjustment electrode 17 and a group of wavelength adjustment electrodes 18 on the reflective delay line array 13. Details of this are publicly known, for example, as described in Non-Patent Document 2. While this embodiment uses a 5 × 5 port MMI 12, the MMI 12 is not limited to a 5 × 5 port configuration, and may be any configuration in which M is a natural number greater than or equal to 1 and N is a natural number greater than or equal to 2.

[0019] (Reflective delay line array) A reflective delay line array 13 is connected to the N port side of the MMI 12. The reflective delay line array 13 is composed of N reflective delay lines 13-1. The reflective delay lines 13-1 are optical waveguides of different lengths and equipped with mirrors 14-1 at their ends, forming round-trip optical paths of different optical path lengths between each of the M side ports of the MMI 12 and the mirror 14-1. The waveguides of the reflective delay line 13-1 are semiconductor optical waveguides with pn junctions.

[0020] (Optical gain area) The optical gain region 11 may be an optical gain waveguide including an optical gain region. The optical gain region 11 includes a gain medium. When light propagates through the gain waveguide, the gain medium absorbs the light and is internally excited. This excited light is emitted from the gain medium and amplifies the surrounding optical signal. The M port includes ports indicated by 1, 2, 3, 4, and 5 (referred to as port 1 to port 5) on the M port side in the figure. In FIG. 1, the optical gain region 11 that amplifies light is connected to port 3, but it may also be connected to other ports. Furthermore, as described in Non-Patent Document 2, the optical gain region 11 may be provided at multiple ports on the M port side. Furthermore, since an optical gain region can generally be used as a light absorption layer, for example, all of the M ports may be provided with an optical gain region, and the optical gain region that does not contribute to oscillation operation may be used as a photodetector. A current (injection current) is supplied to the optical gain region 11 from a power source (not shown).

[0021] Ports 1, 2, 4, and 5 on the M port side are inactive ports that do not contribute to oscillation. Ports 1, 2, 4, and 5 are connected to photodetectors (hereinafter also referred to as "PDs") 15-1, 15-2, 15-4, and 15-5, respectively. RTF laser 100 adjusts the voltages applied to phase adjustment electrode 17 and wavelength adjustment electrode group 18 based on the light reflected by PDs 15-1, 15-2, 15-4, and 15-5.

[0022] PDs 15-1, 15-2, 15-4, and 15-5 may be monolithically integrated on the same substrate as the substrate that constitutes the RTF laser 100, or may be provided outside the substrate and receive light from the M-side port of MMI 12 of the RTF laser.

[0023] (Wavelength adjustment electrode) The wavelength tuning electrode group 18 includes multiple wavelength tuning electrodes 18-1 and adjusts the reflection spectrum observed at the M port side of the MMI 12. That is, the reflection spectrum observed at each of ports 1 to 5 contains multiple components. Each wavelength tuning electrode 18-1 applies a voltage to the delay line 13-1 to independently adjust the multiple components contained in the reflection spectrum. The reflection spectrum is adjusted so that the total reflection spectrum obtained by adding the reflectances of the non-operating ports 1, 2, 4, and 5 is minimized.

[0024] (Photocurrent detection electrode) The photocurrent detection electrode group 28 includes a plurality of photocurrent detection electrodes 28-1 provided on each of the reflective delay lines 13-1, similar to the wavelength tuning electrodes 18. When the wavelength tuning electrodes 18 apply a voltage to the reflective delay line array 13, the reflective delay line 13-1, which is a semiconductor waveguide including a pn junction, absorbs propagating light in response to the application of an electric field. The photocurrent detection electrodes 28-1 detect (extract) the absorption of propagating light as an optical absorption current. The phenomenon of generating a photocurrent by applying a voltage to the reflective delay line 13-1 is caused by the action of the photoelectric effect and an electric field. The absorption current corresponds to a predetermined ratio of the intensity of the light propagating through the N-side port. This ratio is determined by the electric field strength generated by the application of a voltage by the wavelength tuning electrodes 18. Therefore, the absorption current reflects the voltage applied to the reflective delay line 13-1 and the intensity of the propagating light at that time. Therefore, the RTF laser 100 having the photocurrent detection electrode can indirectly monitor the intensity of the propagating light within itself without using an external configuration for monitoring the output intensity of the propagating light.

[0025] Furthermore, although the intensity of the oscillating light 24 output from the RTF laser 100 is basically determined by the current injected into the optical gain region 11 formed at the M-side port, the optical absorption that occurs in the reflective delay line array 13 results in a loss of propagating light, and therefore it also depends on the voltage applied to the wavelength tuning electrode 18. Since the degree of loss of the oscillating light 24 is reflected in the absorption current, it is clear that the relationship between the absorption current, the propagating light, and the applied voltage can be obtained by a preliminary experiment conducted in advance.

[0026] (Control circuit) First, the basic control of control circuit 16 in this embodiment will be described. Control circuit 16 receives optical intensity signals 21-1, 21-2, 21-3, 21-4, and 21-5 from PDs 15-1, 15-2, 15-4, and 15-5. Control circuit 16 then supplies control signals 22 and 23 to phase adjustment electrode 17 and wavelength adjustment electrode group 18, respectively. In this case, control circuit 16 controls the voltages applied to phase adjustment electrode 17 and wavelength adjustment electrode group 18 so that the total reflection spectrum obtained by adding the reflectances of non-operating ports 1, 2, 4, and 5 is minimized, as described above.

[0027] The optical intensity controlled by the optical gain region 11 depends on the injection current. However, long-term operational stability depends on the control method. Known methods for maintaining a constant oscillation light intensity include automatic power constant (APC) control, which monitors and controls the optical intensity, and automatic current constant (ACC) control, which maintains a constant oscillation light intensity indirectly by maintaining a constant injection current. APC control directly monitors the optical intensity and controls the injection current to maintain a constant optical intensity. For this reason, APC control requires a device for monitoring the optical intensity and a feedback circuit for current injection. Such APC control has the drawback of increasing the number of components in the device and making the configuration complex. On the other hand, ACC control uses a constant current circuit for current injection, which simplifies the configuration, but it cannot directly control the optical intensity, which may result in insufficient stability.

[0028] This embodiment controls the intensity of the oscillating light 24 using the existing non-operating photodetectors of ports 1, 2, 4, and 5, without installing new equipment to observe the light intensity, without increasing the number of parts, and furthermore, it is possible to directly observe the oscillating light 24 and apply feedback.

[0029] Furthermore, the inventors have focused on the above point and provided the RTF laser 100 with a photocurrent detection electrode group 28, which allows the RTF laser 100 to monitor the intensity of the oscillating light by itself, without relying on the photodetectors at ports 1, 2, 4, and 5 used for signal detection for wavelength spectrum control. Therefore, the RTF laser 100 of this embodiment can control the wavelength of the oscillating light simply and without increasing the circuit scale, including the configuration for monitoring the intensity of the oscillating light.

[0030] That is, the control circuit 16 of this embodiment receives a monitor signal 26 indicating the absorption current Iref from the photocurrent detection electrode group 28. The control circuit 16 performs control so that the wavelength of the oscillating light is constant, using the absorption current Iref detected by the photocurrent detection electrode group 28, the voltage Vref (hereinafter also referred to as "wavelength tuning voltage") applied by the wavelength tuning electrode group 18, and the injection current Iact. At this time, the absorption current Iref, the wavelength tuning voltage Vref, and the injection current Iact may each be corrected depending on the element conditions, temperature, etc., and the corrected absorption current Iref, wavelength tuning voltage Vref, and injection current Iact may be used for control. Specific examples of control will be described later.

[0031] [control] Next, we will explain how to control the wavelength of the oscillating light (oscillation wavelength) in this embodiment. The RTF laser controls the oscillating light 24 by increasing or decreasing the injection current. Since the threshold carrier density, which is a laser oscillation condition, is theoretically constant, the oscillating wavelength theoretically does not fluctuate. However, in reality, there is a finite potential resistance between the electrodes of the RTF laser, and changing the current flowing between the electrodes changes the amount of Joule heat generated. The change in the amount of heat generated fluctuates the temperature of the semiconductor waveguide, changes the refractive index, and varies the oscillating wavelength.

[0032] FIG. 2(a) is a diagram illustrating the above points. The horizontal axis of FIG. 2(a) represents the injection current Iact, and the vertical axis represents the intensity Pout of the oscillating light 24, the absorption current Iref, and the oscillation wavelength λ. The dotted line parallel to the vertical axis represents the minimum value of the injection current. In FIG. 2(a), line a represents the oscillation wavelength, line b represents the intensity of the oscillating light 24, and line c represents the absorption current Iref. The graph in FIG. 2(a) was obtained by keeping the wavelength adjustment voltage Vref applied to the wavelength adjustment electrode group 18 constant. As shown in FIG. 2(a), the intensity Pout increases with an increase in the injection current Iact, but the oscillation wavelength of the oscillating light 24 also increases. The slope of the increase in the oscillation wavelength and the absorption current is constant, and lines a and c in FIG. 2(a) are shown as parallel lines.

[0033] Figure 2(b) is a graph showing the relationship between the wavelength tuning voltage Vref, the absorption current Iref, and the oscillation wavelength λ. The horizontal axis of Figure 2(b) represents the wavelength tuning voltage Vref, and the vertical axis represents the absorption current Iref and the oscillation wavelength λ. In Figure 2(b), the line d represents the oscillation wavelength, and the line e represents the absorption current Iref. The graph shown in Figure 2(b) is obtained with a constant injection current Iact. The oscillation wavelength of the RTF laser is controlled by controlling the refractive index of the reflective delay line 13-1 with the reverse bias voltage applied by the wavelength tuning electrodes 18, thereby changing the phase of the guided light. When the reflective delay line 13-1 is equipped with the photocurrent detection electrodes 28, the reflective delay line 13-1, which includes a pn junction, absorbs a portion of the light traveling through the waveguide and generates an optical absorption current Iref proportional to the intensity of the absorbed light. As shown in Figure 2(b), as the wavelength tuning voltage Vref increases, the absorption current Iref increases, and simultaneously the oscillation wavelength also increases.

[0034] FIG. 2(c) is a graph showing the relationship between the optical intensity of the oscillating light and the oscillation wavelength λ when the absorption current Iref of this embodiment is constant. The horizontal axis of FIG. 2(c) represents the optical intensity Pout of the oscillating light, and the vertical axis represents the oscillation wavelength λ. The line f shows the dependence of the oscillation wavelength λ on the optical intensity Pout. As is clear from FIG. 2(c), when the absorption current Iref is constant, the oscillation wavelength remains constant regardless of the optical intensity Pout. For this reason, this embodiment controls the wavelength tuning voltage Vref and the injection current Iact while monitoring the absorption current Iref so that the intensity of the monitored light remains constant. In this way, the oscillation wavelength can be stabilized regardless of the intensity Pout of the oscillating light 24.

[0035] Furthermore, the inventors have confirmed that when the absorption current Iref fluctuates, the fluctuation of the absorption current Iref can be suppressed and a constant absorption current Iref can be obtained by varying the wavelength tuning voltage Vref so that the product of the wavelength tuning voltage Vref and the absorption current Iref (wavelength tuning voltage Vref × absorption current Iref) is constant C. Note that the constant C may take on a different value depending on the set oscillation wavelength λ. For this reason, the control circuit 16 shown in FIG. 1 detects the absorption current from the photocurrent detection electrode group 28 and controls the wavelength tuning electrode group 18 to apply a voltage corresponding to the absorption current.

[0036] As described above, this embodiment is configured to store the relationship shown in Figure 2(c) in the control circuit 16, for example. Then, when an unintended change occurs, such as a change in the intensity of the oscillating light or deterioration in characteristics, the wavelength tuning voltage Vref can be changed to maintain a constant oscillation wavelength. Furthermore, as described below, this embodiment may use an injection current Iact to maintain the absorption current Iref constant with higher precision.

[0037] Figure 3(a) is a diagram illustrating the relationship between the wavelength tuning voltage Vref, the oscillation wavelength λ, and the injection current Iact. Figure 3(b) is a diagram illustrating the relationship between the wavelength tuning voltage Vref, the oscillation wavelength λ, and the injection current Iact after a 600-hour forced degradation test was performed on the RTF laser showing the relationship in Figure 3(a). In both Figures 3(a) and 3(b), the horizontal axis represents the wavelength tuning voltage Vref, and the vertical axis represents the wavelength λ. Among the plots shown in Figures 3(a) and 3(b), the + and ■ plots represent an injection current Iact of 40 mA, the × and - plots represent an injection current Iact of 80 mA, and the ● plot represents an injection current Iact of 120 mA.

[0038] In the RTF laser of this embodiment, as shown in FIGS. 3(a) and 3(b), there is a wavelength tuning voltage Vref (absolute value) that changes stepwise to lengthen the wavelength. Here, the absolute value of this wavelength tuning voltage Vref is referred to as the "mode hop voltage." As shown in FIGS. 3(a) and 3(b), the mode hop voltage is relatively low when the injection current Iact is large, and relatively high when the injection current Iact is small. Furthermore, comparing FIGS. 3(a) and 3(b), the mode hop voltages before degradation are 6V and 13V (injection currents Iact: 120mA and 40mA), while the mode hop voltages after degradation are 7V and 14V (injection currents Iact: 120mA and 40mA).

[0039] From this point of view, it is clear that the value of the wavelength tuning voltage Vref, which is changed to suppress fluctuations in the absorption current Iref, should also take into consideration the injection current Iact and the degree of deterioration (operating time) of the RTF laser. Specifically, in this embodiment, the value of the wavelength tuning voltage Vref may be controlled to be small when the injection current Iact is relatively large, and to be large when the operating time of the RTF laser is relatively long. A quantitative value of the wavelength tuning voltage Vref can be obtained by preliminary experiments, simulations, etc. In this embodiment, such a value may be stored in, for example, the control circuit 16 and used.

[0040] Furthermore, the RTF laser 100 of this embodiment may include a power supply that injects an injection current Iact into the optical gain region 11, or the power supply may be configured separately, and the control circuit 16 may control the power supply to inject the injection current Iact to be supplied into the optical gain region 11.

[0041] 1 of this embodiment shows an example in which the wavelength adjustment electrode group 18 and the photocurrent detection electrode group 28 are configured independently. Here, "independent" means that the wavelength adjustment electrode group 18 and the photocurrent detection electrode group 28 are configured separately. If the wavelength adjustment electrode group 18 and the photocurrent detection electrode group 28 are configured separately, it is not necessary for each of the wavelength adjustment electrode group 18 and the photocurrent detection electrode group 28 to have both the functions of applying a voltage and measuring an absorbed current, and an element optimal for each function can be selected.

[0042] However, this embodiment is not limited to this configuration, and the wavelength adjustment electrode group 18 and the photocurrent detection electrode group 28 may be integrally configured. This reduces the number of parts in the RTF laser and simplifies the device configuration. Furthermore, the wavelength adjustment electrode group 18 and the photocurrent detection electrode group 28 can be integrally configured by using a photoelectric effect device, a photoconductive element, or a PIN photodiode.

[0043] The photoelectric effect is a phenomenon in which photons incident on a semiconductor material excite electrons, generating a current. A photoelectric effect device applies a voltage to the reflective delay line array 13 and detects the resulting photocurrent using the same electrode. Examples of photoelectric effect devices include photodiodes and phototransistors. A photoconductive element applies a voltage to the reflective delay line array 13 and detects changes in photocurrent using the same electrode. A PIN photodiode has electrodes on both ends, and a reverse bias voltage is applied to the electrodes. The electrode to which the reverse bias voltage is applied is the same as the electrode that detects the photocurrent generated when light is incident.

[0044] (Variation) Furthermore, in this embodiment, the configuration of the RTF laser is not limited to the RTF laser 100 shown in Fig. 1. Fig. 3 is a top view for explaining an RTF laser 300, which is a modified example of the RTF laser of this embodiment. In Fig. 3, the same components as those shown in Fig. 1 are denoted by the same reference numerals, and their description will be omitted.

[0045] In the RTF laser 300 shown in FIG. 4, of the inactive ports, ports 1 and 2 are connected to PD 40-1, and ports 3 and 4 are connected to PD 40-2. Therefore, the RTF laser 300 is configured with two PDs. Optical intensity signals 41-1 and 41-2 output from the optical receivers PD 40-1 and PD 40-2 are supplied to the control circuit 16. The optical receiver PD 40-1 monitors the intensity of light including that from ports 1 and 2 and the leakage light. Furthermore, the optical receiver PD 40-2 monitors the intensity of light including that from ports 4 and 5 and the leakage light. In other words, in the modified RTF laser 300, the wavelength of the oscillating light is controlled based on the intensity of the leakage light from the portion of the oscillating light excluding the port on the M port side. This type of RTF laser 300 can also achieve the wavelength control of the oscillating light in the RTF laser 100 described above.

[0046] (RTF laser control method and control program) The above-described control method for an RTF laser is a control method for a wavelength tunable laser diode in which a multimode interference waveguide having an M×N port configuration, a reflective delay line array consisting of N reflective delay lines connected to the N-port side of the multimode interference waveguide, and an optical gain waveguide provided at at least one port on the M-port side of the multimode interference waveguide for amplifying light are integrated on the same substrate, and includes the steps of applying a voltage to the reflective delay line, detecting a photocurrent generated from the reflective delay line to which the voltage has been applied, and controlling the wavelength of the oscillating light output from the optical gain waveguide based on the voltage applied to the reflective delay line and the current detected by the photocurrent detection unit.

[0047] The above method can also be automatically executed by a computer program. The computer includes known hardware such as a CPU (Central Processor Unit), memory, and user interface, reads the control program into the memory, and outputs a control signal to the RTF laser. Such a computer may be included in the control circuit 16. The program causes the computer to perform the following functions: apply a voltage to the reflective delay line; detect a photocurrent generated from the reflective delay line to which the voltage has been applied; and control the wavelength of the oscillating light output from the optical gain waveguide based on the voltage applied to the reflective delay line and the current detected by the photocurrent detector. [Explanation of symbols]

[0048] 11 Optical gain region 13 Reflective delay line array 14-1 Mirror 15-1,15-2,15-4,15-5 Receiver 16 Control circuit 17 Phase adjustment electrode 18 Wavelength adjustment electrode group 24 Oscillation light 26 Monitor Signal 28 Photocurrent detection electrode group 100,300 RTF laser

Claims

1. a multimode interference waveguide having an M×N port configuration (M is a natural number of 1 or more, and N is a natural number of 2 or more); a reflective delay line array consisting of N reflective delay lines connected to the N-port side of the multimode interference waveguide; an optical gain waveguide that is provided at at least one port on the M port side of the multimode interference waveguide and amplifies light; a wavelength adjusting electrode that applies a voltage to the reflective delay line to adjust the wavelength of the oscillation light output from the optical gain waveguide; a photocurrent detection unit that detects a photocurrent generated from the reflective delay line to which a voltage is applied by the wavelength adjustment electrode; a wavelength control unit that controls the wavelength of the oscillated light based on the voltage applied by the wavelength adjustment electrode and the current detected by the photocurrent detection unit; 1. A tunable laser diode comprising:

2. 2. The wavelength tunable laser diode according to claim 1, wherein the wavelength control unit controls the voltage applied by the wavelength adjustment electrode so that the current detected by the photocurrent detection unit becomes constant.

3. 3. The wavelength tunable laser diode according to claim 2, wherein the wavelength control unit controls the voltage applied by the wavelength adjustment electrode so that the product of the voltage applied by the wavelength adjustment electrode and the current detected by the photocurrent detection unit is constant.

4. 3. The wavelength tunable laser diode according to claim 2, wherein the wavelength control section further controls an injection current injected into the optical gain waveguide to control the wavelength of the oscillated light.

5. 2. The wavelength tunable laser diode according to claim 1, wherein the photocurrent detection section is a photocurrent detection electrode that detects a photocurrent generated from the reflective delay line, and the wavelength adjustment electrode and the photocurrent detection electrode are integrally configured.

6. 2. The tunable laser diode according to claim 1, wherein the photocurrent detection section and the wavelength adjustment electrode are independent of each other.

7. A method for controlling a wavelength tunable laser diode in which a multimode interference waveguide having an M×N port configuration (M is a natural number of 1 or more, and N is a natural number of 2 or more), a reflective delay line array consisting of N reflective delay lines connected to the N port side of the multimode interference waveguide, and an optical gain waveguide provided in at least one port on the M port side of the multimode interference waveguide for amplifying light are integrated on the same substrate, comprising: applying a voltage to the reflective delay line; detecting a photocurrent resulting from the reflective delay line with an applied voltage; and controlling the wavelength of the oscillation light output from the optical gain waveguide based on the voltage applied to the reflective delay line and the current detected in the step of detecting the photocurrent. A method for controlling a wavelength-tunable laser diode.

8. A control program for controlling a wavelength tunable laser diode in which a multimode interference waveguide having an M×N port configuration (M is a natural number of 1 or more, and N is a natural number of 2 or more), a reflective delay line array consisting of N reflective delay lines connected to the N port side of the multimode interference waveguide, and an optical gain waveguide provided in at least one port on the M port side of the multimode interference waveguide for amplifying light are integrated on the same substrate, On the computer, applying a voltage to the reflective delay line; a function of detecting a photocurrent generated from the reflective delay line when a voltage is applied; and a function of controlling the wavelength of the oscillation light output from the optical gain waveguide based on the voltage applied to the reflective delay line and the current detected by the function of detecting the photocurrent. Control program for wavelength tunable laser diodes.

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