Wavelength-tunable laser diode, control method for wavelength-tunable laser diode, and control program for wavelength-tunable laser diode

Abstract

Provided is a wavelength-tunable laser diode capable of stabilizing the oscillation wavelength against fluctuations in optical output without relying on temperature control. An MMI (12) having 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) that is provided on at least one port on the M port side and amplifies light are integrated on the same substrate. The wavelength-tunable laser diode comprises: a wavelength adjustment electrode group (18) that applies a voltage to the reflective delay line arrays (13) to adjust the wavelength of oscillation light that is output from the optical gain region (11); a photocurrent detection electrode group (28) that detects a photocurrent generated from the reflective delay line arrays (13) to which the voltage has been applied by the wavelength adjustment electrode group (18); and a control circuit (16) that controls the wavelength of the oscillation light on the basis of the voltage applied by the photocurrent detection electrode group (28) and the current detected by the photocurrent detection electrode group (28).

Description

Wavelength-tunable laser diode, wavelength-tunable laser diode control method, wavelength-tunable laser diode control program

[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.

[0002] A tunable laser diode (hereinafter 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 a TLD is in operation, its wavelength stability is important. Here, wavelength stability refers to the ability of a TLD to continue outputting the oscillation wavelength intended by the user over a long period of time. Note that 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 from 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 from the etalon remains constant. Such a technique is described, for example, in Non-Patent Document 1. It is also known that a wavelength locker utilizes the temperature dependence of the etalon's wavelength period 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, for example, in 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 of which is also referred to as an M-port) and N ports (each of which is also referred to as an N-port). The tunable filter is composed of multiple optical waveguides that function as reflectors, with the effective optical path length controlled by the application of voltage. The M-port side of the MMI is connected to an optical semiconductor waveguide that has optical amplification gain characteristics through current injection. 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 the application of voltage, it is preferable to wavelength lockers that use etalons for control due to its high speed and stability.

[0006] International Publication WO2022 / 079814

[0007] Hiroyuki Ishii et al., "High-Performance Wavelength Tunable Light Source Technology," NTT Technical Journal, November 2007, p. 66. Yuta Ueda et al., "Electro-Optically Tunable Laser with <10-mW Tuning Power Dissipation and High-Speed ​​λ-Switching for Coherent Networks," in Proc. of ECOC2019, PD.2.2.

[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 view 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.

[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] According to one aspect of the present invention, there is provided a method for controlling a wavelength tunable laser diode in which a multimode interference waveguide having an M×N port configuration (where 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 least at one port on the M-port side of the multimode interference waveguide for amplifying light, are integrated on the same substrate, the method comprising 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.

[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.

[0013] 1 is a schematic top view showing the configuration of an RTF laser using a 5x5 port MMI. (a) is a diagram showing the relationship between injection current and oscillation light intensity in the RTF laser of this embodiment. (b) is a diagram showing the relationship between absorption current and wavelength tuning voltage in the RTF laser of this embodiment. (c) is a diagram showing the relationship between oscillation light intensity and oscillation wavelength in the RTF laser of this embodiment when the product of the absorption current and wavelength tuning voltage is controlled to be a constant. (a) is a diagram for explaining the relationship between wavelength tuning voltage, oscillation wavelength, and injection current. (b) is a diagram for explaining the relationship between wavelength tuning voltage, oscillation wavelength, and injection current after a forced degradation test is performed.

[0034] FIG. 1 is a schematic top view showing a modified example of the RTF laser shown in FIG. 1.

[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 a 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. In addition, this embodiment will be described using the configuration of a reflection-type transversal filter (RTF) as the TLD.

[0015] [Configuration of RTF Laser] The following describes 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 includes 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 connected to the reflective delay line array 13. The reflective delay line array 13 and the optical gain region 11 are integrated on the same substrate into a single chip.

[0016] In this embodiment, hereinafter, a port to which the reflective delay line array 13 is connected will also be referred to as an "N port," and the side to which the reflective delay line array 13 is connected will also be referred to as an "N port side." Furthermore, a port to which the optical gain region 11 or a photodiode (PD) described later is connected will also be referred to as an "M port," and the side to which the optical gain region 11 or the PD is connected will also be 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 described 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. At this time, light of different modes interferes with each other, amplifying or attenuating the intensity of the light under certain conditions. The interfered light is recombined at the exit of the waveguide. At this time, 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, as described, for example, in Non-Patent Document 2. Note that, in this embodiment, a 5x5 port MMI 12 is used as the MMI 12, but the MMI 12 is not limited to a 5x5 port MMI, and may be any M where 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, so that round-trip optical paths of different optical path lengths are formed 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 having pn junctions.

[0020] (Optical Gain Region) 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 in 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] The 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 the MMI 12 of the RTF laser.

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

[0024] (Photocurrent Detection Electrode) The photocurrent detection electrode group 28, like the wavelength adjustment electrode group 18, includes multiple photocurrent detection electrodes 28-1 provided on each of the reflective delay lines 13-1. When the wavelength adjustment electrode group 18 applies 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 electrode 28-1 is an electrode that detects (extracts) the absorption of propagating light as a photoabsorption current. The phenomenon of generating a photocurrent by applying a voltage to the reflective delay line 13-1 occurs due to the action of the photoelectric effect and an electric field. The absorption current corresponds to a predetermined ratio of the intensity of 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 adjustment electrode group 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 electrodes 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 the control circuit 16 of this embodiment will be described. The control circuit 16 receives optical intensity signals 21-1, 21-2, 21-3, 21-4, and 21-5 from the PDs 15-1, 15-2, 15-4, and 15-5. The control circuit 16 then supplies control signals 22 and 23 to the phase adjustment electrode 17 and the wavelength adjustment electrode group 18, respectively. At this time, as described above, the control circuit 16 controls the voltages applied to the phase adjustment electrode 17 and the wavelength adjustment electrode group 18 so that the total reflection spectrum obtained by adding the reflectances of the non-operating ports 1, 2, 4, and 5 is minimized.

[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 intensity of oscillating light include automatic power constant (APC) control, which monitors and controls the optical intensity, and automatic current constant (ACC) control, which maintains a constant injection current and indirectly maintains a constant intensity of oscillating light. APC directly monitors the optical intensity and controls the injection current to maintain a constant optical intensity. For this reason, APC 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, so that the RTF laser 100 itself can monitor the intensity of oscillated light, 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 oscillated light simply and without increasing the circuit scale, including the configuration for monitoring the intensity of oscillated 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, control of the wavelength of the oscillating light (oscillation wavelength) in this embodiment will be described. 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 electrodes 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] FIG. 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 FIG. 2(b) represents the wavelength tuning voltage Vref, and the vertical axis represents the absorption current Iref and the oscillation wavelength λ. The line d in FIG. 2(b) represents the oscillation wavelength, and the line e represents the absorption current Iref. The graph shown in FIG. 2(b) is obtained by keeping the injection current Iact constant. 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 provided 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 FIG. 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. Based on this, the control circuit 16 shown in FIG. 1 detects the absorption current from the photocurrent detection electrodes 28 and controls the wavelength tuning electrodes 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, this embodiment may use the injection current Iact to maintain the absorption current Iref constant with higher precision, as described below.

[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. Furthermore, 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] As shown in FIGS. 3( a) and 3(b), the RTF laser of this embodiment has a wavelength tuning voltage Vref (absolute value) that changes stepwise to increase 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 were 6 V and 13 V (injection currents Iact: 120 mA and 40 mA), whereas the mode hop voltages after degradation were 7 V and 14 V (injection currents Iact: 120 mA and 40 mA).

[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. In this way, the number of parts in the RTF laser can be reduced and the device configuration can be simplified. 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 the 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] (Modification) 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 modification 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 photodetectors PD 40-1 and PD 40-2 are supplied to the control circuit 16. The photodetector PD 40-1 monitors the intensity of light including that from ports 1 and 2 and the leakage light. Furthermore, the 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] (Method and program for controlling an RTF laser) The method for controlling an RTF laser described above is a method for controlling 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 Processing 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.

[0048] 11 Optical gain region 13 Reflective delay line array 14-1 Mirror 15-1, 15-2, 15-4, 15-5 Photodetector 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 wavelength tunable laser diode comprising: 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.

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

3. A wavelength tunable laser diode as described in 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. The tunable laser diode according to claim 2, wherein said wavelength control section further controls the wavelength of said oscillated light by controlling an injection current injected into said optical gain waveguide.

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

6. The tunable laser diode according to claim 1, wherein said photocurrent detection section and said wavelength adjustment electrode are configured independently of each other.

7. A method for controlling a wavelength tunable laser diode in which an M×N-port multimode interference waveguide (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 least at one port on the M-port side of the multimode interference waveguide for amplifying light are integrated on the same substrate, the method comprising the steps of: applying a voltage to the reflective delay line; detecting a photocurrent generated from the reflective delay line to which a 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 in the photocurrent detection step.

8. A control program for controlling a wavelength tunable laser diode in which a multimode interference waveguide with an M×N port configuration (M is a natural number of 1 or more, and N is a natural number of 2 or more) is integrated on the same substrate, 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 that is provided in at least one port on the M port side of the multimode interference waveguide and amplifies light, the control program causing 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 a voltage has been applied; 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 by the photocurrent detection function.

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