Wavelength tunable semiconductor laser control method and control device, and wavelength tunable semiconductor laser

Abstract

According to the present invention, a method for controlling a wavelength tunable semiconductor laser (10) comprising DBR regions (106, 107) into which a first current (117) is injected and a phase adjustment region (108) into which a second current (118) is injected includes: a step for approximating a boundary line of a desired wavelength region in a wavelength map (21) by means of a first power function (211) and a second power function (212); a step for acquiring a relational expression between the first and second currents; a step for mapping the first and second power functions and the relational expression onto an SMSR map (22); a step for determining the relational expression such that a path (223) onto which the relational expression is mapped in the SMSR map passes through a region having a high SMSR; and a step for changing a combination of the first and second currents in accordance with the relational expression.　As a result, the present invention can provide a wavelength tunable semiconductor laser control method with which it is possible to sweep the wavelength of laser light over a wide band with a high signal-to-noise intensity ratio.

Description

Wavelength tunable semiconductor laser control method, control device therefor, and wavelength tunable semiconductor laser device

[0001] The present invention relates to a method for controlling a wavelength tunable semiconductor laser having a DBR structure, a control device therefor, and a wavelength tunable semiconductor laser device.

[0002] Tunable lasers are useful light sources that are used in a wide range of fields, such as wavelength division multiplexing transmission, optical measurement, optical frequency sweeping OCT, laser spectroscopy, and photosensitivity measurement. Among these, tunable semiconductor lasers that use semiconductors as gain media are widely used in various fields because they consume little power, are small, and are easy to handle.

[0003] Wavelength-tunable semiconductor lasers include distributed feedback (DFB) lasers, distributed Bragg reflector (DBR) lasers, and external cavity lasers. DFB lasers employ a simple structure in which a grating is formed on the active layer, and wavelength can be changed by adjusting the amount of injected current or the chip temperature. DBR lasers do not have a grating on the active region, but instead have a DBR grating on one or both sides of the active region. They usually have a phase adjustment region for phase matching. DBR lasers achieve wavelength tuning by using the carrier plasma effect that occurs when current is injected into the DBR region, which is independent of the active region. External cavity lasers achieve wavelength tuning by mechanically moving a mirror placed outside the active region. In the case of semiconductor lasers, Micro Electro Mechanical Systems (MEMS) are typically used to reduce the footprint.

[0004] These lasers are used for gas sensing. The laser most commonly used for gas sensing applications is the DFB laser. DFB lasers can achieve a narrow linewidth, so they are used by tuning their wavelength to the absorption line of the gas. As mentioned above, the wavelength can be changed by about 1 nm by changing the injection current and the temperature of the laser chip, but it takes more than 1 ms to sweep the wavelength.

[0005] Although DBR lasers have a complex structure, they can change the wavelength by about 5 nm by simultaneously changing the DBR current and the phase adjustment current. In addition, because the wavelength is changed based on the refractive index change induced by the injection current, they can achieve high-speed wavelength tuning on the order of microseconds or less.

[0006] External cavity lasers can tune the wavelength over a wide band by using a MEMS mirror. In principle, a tunable wavelength width of 100 nm can be achieved. However, when a semiconductor is used as the gain medium, the gain band is limited, so the tunable wavelength width is about 60 nm. Because external cavity lasers mechanically drive the MEMS mirror, it takes about milliseconds to sweep the wavelength.

[0007] Therefore, DBR lasers are expected to be applied to gas sensing because they can operate at high speeds and within a predetermined wavelength tuning range. In DBR lasers, the same power supply is resistively divided to inject currents into the DBR region and the phase adjustment region in a synchronized manner, achieving a wavelength tuning range of 5.6 nm (Non-Patent Document 1). It is also possible to control the DBR current and the phase adjustment current using separate synchronized power supplies (Non-Patent Document 2).

[0008] Takuya Kanai, et. al., “First Demonstration of 2 mm Wavelength Tunable Distributed Bragg Reflector Laser Diode,” The Proceedings of the 25th Int. Semiconductor Laser Conf., Kobe, Japan, TuB4, Sep. 2016. M. Abe, et. al., “4 nm continuous rapid sweeping spectroscopy in 2 μm band using distributed Bragg reflector laser,” Appl. Phys. B, 123:260 2017.

[0009] 9 shows an example of the configuration of a conventional DBR laser 40. The DBR laser 40 comprises, on a semiconductor substrate 401, an active region 405 having a multiple quantum well 402, an SOA region 409, a front DBR region 406 made of a bulk 403, a rear DBR region 407, and a phase adjustment region 408. Diffraction gratings 420 are formed in the front DBR region 406 and the rear DBR region 407.

[0010] In the DBR laser 40, light generated in the active region 405 is oscillated by a resonator formed by the front DBR region 406, phase adjustment region 408, and rear DBR region 407, amplified by the SOA region 409, and emitted from the output end (right in the figure). At this time, currents 410 to 413 are injected into the active region 405, front DBR region 406, rear DBR region 407, phase adjustment region 408, and SOA region 409, respectively. Here, a current 411 is injected into the front DBR region 406 and rear DBR region 407 in unison.

[0011] The oscillation wavelength of the DBR laser 40 is determined by a resonator consisting of the front DBR region 406, the rear DBR region 407, and the phase adjustment region 408. Therefore, the oscillation wavelength can be changed by changing the current 411 injected into the front DBR region 406 and the rear DBR region 407 and the current 412 injected into the phase adjustment region 408.

[0012] FIG. 10 shows an example of a wavelength map 51 of the DBR laser 40. The wavelength map 51 of the DBR laser is based on a measured oscillation spectrum, with the horizontal axis representing the current (DBR region injection current) 411 injected into the DBR regions 406 and 407 and the vertical axis representing the current (phase adjustment region injection current) 412 injected into the phase adjustment region 408. The oscillation wavelength band determined by the combination of these currents 411 and 412 is represented by regions with distinguishable display states. In the example shown in FIG. 10, each region is identified by assigning a letter (alphabet) a to p. Each region can also be identified by color. The DBR region injection current 411 is the total amount of current that flows when the front DBR region 406 and the rear DBR region 407 are electrically connected.

[0013] In the wavelength map 51, mode hopping does not occur even if the injection current is changed within each identified region. When the injection current is changed and the boundary between each identified region is crossed, mode hopping occurs because the wavelength changes discontinuously at this boundary.

[0014] As a result, the wavelength can be changed by injecting current only into the DBR region, and the wavelength can be changed by injecting current only into the phase adjustment region, but because mode hopping occurs, the range over which the oscillation wavelength can be continuously changed is narrow, at around 1 nm.

[0015] On the other hand, in a configuration in which a divided resistor is placed between the DBR region and the phase adjustment region to apply a current (Non-Patent Document 1), or in which a current is applied by synchronizing different power supplies (Non-Patent Document 2), the current can be changed along the trajectory 510 shown in FIG. 10 (indicated by the black arrow in the figure), and the wavelength can be continuously changed by 5 nm or more.

[0016] FIG. 11 shows an SMSR map 61 for the same DBR laser 40 as in FIG. 10 . The SMSR map 61 is based on the measured oscillation spectrum. The horizontal axis represents the current (DBR region injection current) 411 injected into the DBR regions 406 and 407, and the vertical axis represents the current (phase adjustment region injection current) 412 injected into the phase adjustment region 408. The SMSR (Side-Mode-Suppression-Ratio) of light oscillated by the combination of these currents 411 and 412 is represented by regions with distinguishable display states. In the example shown in FIG. 11 , as with the wavelength map, each region is identified by assigning a letter (alphabet). Each region can also be identified by color. The DBR region injection current 411 is the total amount of current that flows when the front DBR region 406 and the rear DBR region 407 are electrically connected.

[0017] When the DBR region injection current 411 and the phase adjustment region injection current 412 are changed in the same manner as the trajectory 510 in the wavelength map 51 shown in Figure 10, the trajectory 610 in the SMSR map 61 (black arrow in the figure) passes through a region with poor SMSR.

[0018] As described above, while the conventional control method makes it relatively easy to control the wavelength tuning operation of a DBR laser, it is not possible to maintain a good SMSR because the current is changed and controlled so as to form a linear trajectory on the wavelength map. As a result, when the trajectory during current control on the wavelength map passes through an area with poor SMSR, the oscillation state becomes unstable and mode hopping occurs.

[0019] In order to solve the above-mentioned problems, a control method for a wavelength tunable semiconductor laser according to the present invention is a control method for a wavelength tunable semiconductor laser including an active region having optical gain, a DBR region into which a first current is injected, and a phase adjustment region into which a second current is injected, the control method comprising the steps of: approximating a boundary line of a desired wavelength region by a first power function and a second power function in a wavelength map in which the oscillation wavelength of the wavelength tunable semiconductor laser is represented by a wavelength region in which the oscillation wavelength of the wavelength tunable semiconductor laser is distinguishable, with the first current and the second current serving as coordinate axes; adding the first power function and the second power function using a weighting coefficient to obtain an injection current relational expression that represents the relationship between the first current and the second current; the power function of (x), the second power function, and the injection current relational expression onto an SMSR map expressed by an SMSR region in which SMSR of the oscillation spectrum of the wavelength-tunable semiconductor laser can be identified, with the first current and the second current serving as coordinate axes; determining the weighting coefficients to determine the injection current relational expression so that a trajectory onto which the injection current relational expression is mapped passes through a region of the SMSR region in which SMSR is high; and changing a combination of the first current and the second current according to the injection current relational expression, and applying the current to the wavelength-tunable semiconductor laser to change the oscillation wavelength of the wavelength-tunable semiconductor laser.

[0020] Furthermore, a control device for a wavelength tunable semiconductor laser according to the present invention is a device for controlling a wavelength tunable semiconductor laser including an active region having optical gain, a DBR region into which a first current is injected, and a phase adjustment region into which a second current is injected, the device comprising a storage unit and a calculation unit, wherein the storage unit stores a wavelength map in which an oscillation wavelength of the wavelength tunable semiconductor laser is represented in a wavelength region in which the oscillation wavelength is distinguishable, with the first current and the second current as coordinate axes, respectively, and an SMSR map in which an SMSR of an oscillation spectrum of the wavelength tunable semiconductor laser is represented in an SMSR region in which the SMSR of the oscillation spectrum of the wavelength tunable semiconductor laser is distinguishable, with the first current and the second current as coordinate axes, respectively, and the calculation unit calculates a wavelength map in which an oscillation wavelength of the wavelength tunable semiconductor laser is represented in a wavelength region in which the oscillation wavelength is distinguishable, with the first current and the second current as coordinate axes, respectively, and The boundary of the desired wavelength region is approximated by a first power function and a second power function, and the first power function and the second power function are added using a weighting coefficient to obtain an injection current relational expression that expresses the relationship between the first current and the second current. The first power function, the second power function, and the injection current relational expression are mapped onto the SMSR map. The weighting coefficient is determined so that, in the SMSR map, a trajectory onto which the injection current relational expression is mapped passes through a region of the SMSR region where SMSR is high, thereby determining the injection current relational expression. A combination of the first current and the second current is determined according to the injection current relational expression.

[0021] According to the present invention, it is possible to provide a method for controlling a wavelength tunable semiconductor laser that can sweep the wavelength of laser light over a wide band with a high signal-to-noise intensity ratio, a control device therefor, and a wavelength tunable semiconductor laser device.

[0022] FIG. 1 is a block diagram showing the configuration of a wavelength tunable semiconductor laser device according to a first embodiment of the present invention. FIG. 2 is a schematic diagram showing the configuration of a wavelength tunable semiconductor laser in the wavelength tunable semiconductor laser device according to the first embodiment of the present invention. FIG. 3 is a block diagram showing the configuration of a wavelength tunable semiconductor laser control device according to the first embodiment of the present invention. FIG. 4 is a flowchart illustrating a wavelength tunable semiconductor laser control method according to the first embodiment of the present invention. FIG. 5 is a diagram illustrating a wavelength tunable semiconductor laser control method according to the first embodiment of the present invention. FIG. 6 is a diagram illustrating a wavelength tunable semiconductor laser control method according to the first embodiment of the present invention. FIG. 7 is a diagram illustrating the effects of the wavelength tunable semiconductor laser control method, the control device thereof, and the wavelength tunable semiconductor laser device according to the first embodiment of the present invention. FIG. 8 is a diagram illustrating the effects of the wavelength tunable semiconductor laser control method, the control device thereof, and the wavelength tunable semiconductor laser device according to the first embodiment of the present invention. FIG. 9 is a schematic diagram showing the configuration of a conventional wavelength tunable semiconductor laser. FIG. 10 is a diagram illustrating a conventional wavelength tunable semiconductor laser control method. FIG. 11 is a diagram illustrating a conventional wavelength tunable semiconductor laser control method.

[0023] First Embodiment A method for controlling a wavelength tunable semiconductor laser, a control device therefor, and a wavelength tunable semiconductor laser device according to a first embodiment of the present invention will be described with reference to FIGS. 1 to 8. FIG.

[0024] <Configuration of Wavelength Tunable Semiconductor Laser Device> As shown in FIG. 1, a wavelength tunable semiconductor laser device 1 according to this embodiment includes a wavelength tunable semiconductor laser 10, a wavelength tunable semiconductor laser control device 11, and an arbitrary waveform generator 12.

[0025] An example of the configuration of a wavelength tunable semiconductor laser is shown in Fig. 2. The wavelength tunable semiconductor laser 10 is a wavelength tunable semiconductor laser having a DBR structure (hereinafter referred to as a "DBR laser").

[0026] The DBR laser 10 comprises, on a semiconductor substrate 101, an active region 105 consisting of a multiple quantum well 102, an SOA region 109, a front DBR region 106 consisting of a bulk 103, a rear DBR region 107, and a phase adjustment region 108. An overclad 104 is provided on these regions.

[0027] A lower electrode 110 is arranged on the back surface of the semiconductor substrate 101, an active region electrode 111 is arranged above the active region 105, an SOA region electrode 115 is arranged above the SOA region 109, a front DBR region electrode 112 is arranged above the front DBR region 106, a rear DBR region electrode 113 is arranged above the rear DBR region 107, and a phase adjustment region electrode 114 is arranged above the phase adjustment region 108. A diffraction grating 120 is formed in the front DBR region 106 and the rear DBR region 107.

[0028] In the DBR laser 10, light generated by injecting a current 116 into the active region 105 is oscillated as a laser by a resonator formed by the front DBR region 106, the phase adjustment region 108, and the rear DBR region 107, and is amplified by injecting a current 119 into the SOA region 109, and is emitted from the emission end (right in the figure).

[0029] The oscillation wavelength is determined by a resonator consisting of the front DBR region 106, the rear DBR region 107, and the phase adjustment region 108. The oscillation wavelength changes and is swept by changes in the current 117 injected into the front DBR region 106 and the rear DBR region 107 (hereinafter referred to as the "DBR region injection current" or "DBR current") and the current 118 injected into the phase adjustment region 108 (hereinafter referred to as the "phase adjustment region injection current" or "phase adjustment current").

[0030] In the wavelength-tunable semiconductor laser device 1, the DBR region injection current 117 and the phase adjustment region injection current 118 determined by the wavelength-tunable semiconductor laser control device 11 are generated by the arbitrary waveform generator 12, and injected into the wavelength-tunable semiconductor laser 10, causing the wavelength-tunable semiconductor laser 10 to operate and sweep the wavelength of the output laser light.

[0031] <Configuration of Wavelength Tunable Semiconductor Laser Control Device> As shown in FIG. 3, the wavelength tunable semiconductor laser control device 11 includes an input unit 1101, a storage unit 1102, a calculation unit 1103, and an output unit 1104.

[0032] Measurement data such as the oscillation spectrum, wavelength map, and SMSR map of the DBR laser 10 is input to the input unit 1101. The wavelength map and SMSR map may be created by the calculation unit 1103 based on the measured oscillation spectrum.

[0033] The storage unit 1102 stores data such as a wavelength map, an SMSR map, etc. It also stores a DBR region injection current 117 and a phase adjustment region injection current 118. It may also store data such as the oscillation spectrum of the DBR laser 10.

[0034] The calculation unit 1103 determines a combination of the DBR region injection current 117 and the phase adjustment region injection current 118 in the trajectory along which the current is swept for wavelength sweeping, based on the wavelength map and the SMSR map.

[0035] The output unit 1104 outputs the combination of the DBR region injection current 117 and the phase adjustment region injection current 118 determined by the calculation unit 1103 to the arbitrary waveform generator 12 .

[0036] The arbitrary waveform generator 12 generates the DBR region injection current 117 and the phase adjustment region injection current 118 according to the combination of the DBR region injection current 117 and the phase adjustment region injection current 118 determined by the calculation unit 1103, and outputs them as injection currents to the DBR regions 106, 107 and the phase adjustment region 108 of the DBR laser 10, respectively.

[0037] In the wavelength tunable semiconductor laser device 1, an arbitrary waveform generator 12 is connected to each of the DBR regions 106 and 107 and the phase adjustment region 108 of the DBR laser 10. An output section 1104 of the wavelength tunable semiconductor laser control device 11 is connected to the arbitrary waveform generator 12.

[0038] In the wavelength tunable semiconductor laser device 1, the DBR laser 10 is controlled by a wavelength tunable semiconductor laser control method described below, thereby achieving wavelength tunable operation.

[0039] <Method for Controlling Wavelength-Tunable Semiconductor Laser> A method for controlling a wavelength-tunable semiconductor laser according to this embodiment will be described with reference to Figures 4 to 6. Figure 4 shows a flowchart for explaining the method for controlling a wavelength-tunable semiconductor laser.

[0040] 5 and 6 show a wavelength map 21 and an SMSR map 22 of the DBR laser 10, respectively. The wavelength map 21 and the SMSR map 22 of the DBR laser 10 are based on the measured oscillation spectrum. The horizontal axis represents the current (DBR region injection current) 117 injected into the DBR regions 106 and 107, and the vertical axis represents the current (phase adjustment region injection current) 118 injected into the phase adjustment region. The oscillation wavelength band and SMSR of the light oscillated by the combination of these currents 117 and 118 are represented by regions with distinguishable display states. In FIGS. 5 and 6, each region is identified by assigning a letter (alphabet) a to p. The regions can also be identified by color. The DBR region injection current 117 is the total amount of current that flows when the front DBR region 106 and the rear DBR region 107 are electrically connected.

[0041] First, as shown in FIG. 5, two boundary lines of a desired region in the wavelength map 21 where wavelength sweeping is to be performed are approximated by power functions 211 and 212 (dotted lines in the figure) (step S1).

[0042] Next, the power functions 211 and 212 are weighted by a coefficient k (hereinafter referred to as the "weighting coefficient") and added together to obtain a relational expression (hereinafter referred to as the "injection current relational expression") between the DBR region injection current 117 and the phase adjustment region injection current 118. In the trajectory expressed by the injection current relational expression, a region (trajectory) 213 (solid arrow in the figure) for sweeping the wavelength is determined (step S2).

[0043] Next, the power functions 211 and 212 and the trajectory 213 based on the injection current relational expression are mapped onto the SMSR map 22 as shown in Fig. 6 (step S3). More specifically, the DBR region injection current 117 and the phase control region injection current 118 corresponding to the power functions 211 and 212, respectively, are extracted from the wavelength map 21, and the SMSR corresponding to the DBR region injection current 117 and the phase control region injection current 118 are plotted on the SMSR map 22. As a result, the power functions 211 and 212 and the trajectory 213 on the wavelength map 21 are mapped onto the SMSR map 22 as power functions 221 (white solid lines in the figure), power functions 222 (white solid lines in the figure), and trajectory 223 (black arrows in the figure).

[0044] As a result, in the SMSR map 22, a trajectory 223 based on the injection current relation is located between a function (power function) 221 onto which the power function 211 is mapped and a function (power function) 222 onto which the power function 212 is mapped.

[0045] Next, a coefficient k is determined (weighted) to determine an injection current relational expression so that the trajectory 223 passes through a high SMSR region in the SMSR map 22 (step S4). Here, the high SMSR region is preferably a region having an SMSR higher than the average value of the SMSR in the region between the power function 221 and the power function 222. Alternatively, the trajectory 223 is preferably set so that the highest SMSR is obtained between its start point and end point.

[0046] Here, the display state of the identified regions in the SMSR map 22 changes at 5 dB intervals. Regions with high SMSR values ​​in the SMSR map 22 indicate regions with good signal-to-noise ratios (SNRs). Since the SMSR map 22 has a 5 dB width within regions with the same display state, it is desirable to determine a wavelength sweep route so that the trajectory due to current change passes through regions with high SMSR.

[0047] Next, in accordance with the determined injection current relational expression, the DBR region injection current 117 and the phase adjustment region injection current 118 are combined and changed and applied to the DBR laser 10 to change and sweep the wavelength of the laser light from the DBR laser 10 (step S5).

[0048] This allows the DBR laser 10 to maintain a high SMSR and continuously sweep the wavelength over a wide band.

[0049] <Effects> The effects of the method for controlling a wavelength tunable semiconductor laser, the control device therefor, and the wavelength tunable semiconductor laser device according to this embodiment will be described with reference to FIGS. 7, 8, and 10. FIG.

[0050] First, for comparison, a conventional method for controlling a wavelength tunable semiconductor laser will be described with reference to FIG.

[0051] First, as shown in FIG. 10, pairs of DBR current and phase adjustment current (current pairs) for each of the start wavelength and end wavelength of the desired wavelength range to be swept are extracted from the wavelength map 51.

[0052] Next, the pair of the DBR current and the phase adjustment current is connected by a straight line 510 (black arrow in the figure) on the wavelength map 51. This is equivalent to changing the current pair on the wavelength map 51 in accordance with a linear function.

[0053] More specifically, in a wavelength tunable semiconductor laser having a DBR structure, a current of 100 mA is applied to the active region and a current of 100 mA is applied to the SOA region.

[0054] Next, while bias currents are applied to the DBR region and the phase adjustment region, they are modulated by a periodically changing current such as a sine wave or sawtooth wave, and currents of the same phase and with a constant amplitude ratio are applied.

[0055] For example, the DBR current is set to a bias current of 4 mA and an amplitude of 3 mA, and the phase adjustment current is set to a bias current of 10 mA and an amplitude of 9 mA, and is changed as a cosine wave with a period of 0.1 ms. In this case, the trajectory 510 resulting from the change in the DBR current and phase adjustment current is linear, and the oscillation wavelength varies in the range of 2010.5 to 2012.5 nm. In this case, the minimum SMSR of the laser oscillation light is 20 dB.

[0056] Next, a method for controlling the wavelength tunable semiconductor laser according to this embodiment will be described with reference to FIGS.

[0057] First, as shown in FIG. 7, pairs of DBR current and phase adjustment current (current pairs) for each of the start wavelength and end wavelength of the desired wavelength range to be swept are extracted from the wavelength map 31.

[0058] Next, the boundary between the region to which the current pair belongs and one of the adjacent regions (for example, the boundary on the left side in the figure) is calculated using a power function 311, for example, a quadratic function y=a １ x ２ +c １ (The dotted line in the figure) where x represents the DBR current and y represents the phase adjustment current. a>0 and the power function has a downward convex shape.

[0059] Next, the boundary between the region to which the current pair belongs and the other adjacent region (for example, the boundary on the right side in the figure) is calculated using a power function 312, for example, a quadratic function y=a ２ x ２ +c ２ Fitting is performed using the dotted line in the figure.

[0060] Next, the two quadratic functions are weighted and added together. Specifically, this is approximated by equation (1) to obtain the injection current relational equation. In the trajectory expressed by the injection current relational equation, a region (trajectory) 313 (solid arrow in the figure) in which the wavelength is swept is determined.

[0061] y = {ka １ +(1-k)a ２ x ２ +kc １ +(1-k)c ２ (1)

[0062] Here, 0<k<1.

[0063] 8, the power functions 311, 312, and trajectory 313 are mapped onto the SMSR map 32 to produce power functions 321 (white solid line), 322 (white solid line), and trajectory 323 (black arrow). In the region between the power functions 321 and 322, fitting is performed so that the SMSR is optimal, i.e., the highest value, on the wavelength sweep route (trajectory 323). In detail, k in equation (1) is changed to determine the trajectory 323 that passes through the optimal SMSR on the SMSR map 32, and the injection current relational expression is determined.

[0064] Next, a current of 100 mA is applied to the active region of the DBR laser, and a current of 100 mA is applied to the SOA region. Furthermore, for the DBR region and the phase adjustment region, the independent variable x is set to the DBR current in the injection current relationship expressed by Equation (1), and the dependent variable y (phase adjustment current) is determined using the k determined as described above. This dependent variable y (phase adjustment current) is generated by an arbitrary waveform generator from the start current to the end current on the independent variable side.

[0065] Specifically, the DBR current is set to a bias current of 4 mA, and the phase adjustment current is set to a bias current of 10 mA, and is varied every 0.1 ms according to the wavelength sweep route function (injection current relational expression) expressed by equation (1). In this case, as shown in Figure 7, in wavelength map 31, trajectory 313 (indicated by the black arrow in the figure) resulting from the variation of DBR current 601 and phase adjustment current 602 is an trajectory from which part of the injection current relational expression is extracted, and the oscillation wavelength varies in the range of 2010.5 to 2013 nm. The minimum SMSR of the laser oscillation light in this case is 45 dB.

[0066] As described above, the control method for a wavelength tunable semiconductor laser according to this embodiment enables wavelength sweeping over a variable wavelength width of 2.5 nm with an SMSR (45 dB) higher than the SMSR (20 dB) achieved by the conventional control method.

[0067] The wavelength tunable semiconductor laser control method, the control device thereof, and the wavelength tunable semiconductor laser device according to the present embodiment can suppress the degradation of SMSR in a DBR laser, thereby enabling the laser beam to be oscillated with a higher signal-to-noise ratio (SNR), and the wavelength to be continuously changed and swept over a wide band, thereby enabling the absorption lines of multiple gases to be detected with high accuracy.

[0068] In the embodiment of the present invention, an example in which a sine wave is applied to the DBR region and the phase adjustment region has been shown, but this is not limiting. Other waveforms, such as a triangular wave or a sawtooth wave, which have the same phase-amplitude relationship, may also be used. Since the trajectories caused by changes in the DBR current and the phase adjustment current do not change, the same effect can be obtained.

[0069] In the embodiment of the present invention, an example has been shown in which a DBR laser having a DBR structure on both ends is used, but this is not limiting. A DBR laser having a DBR structure on only one side may also be used. A semiconductor laser having a DBR region and a phase adjustment region and showing a wavelength map and an SMSR map may also be used.

[0070] In the embodiment of the present invention, an example has been shown in which a quadratic function is used as the power function, but this is not limitative, and a group of higher-order power functions such as a cubic function or a quartic function may also be used.

[0071] In the embodiments of the present invention, examples of the structure, dimensions, materials, etc. of each component in the configuration of the wavelength tunable semiconductor laser control method, the control device therefor, and the wavelength tunable semiconductor laser device have been shown, but the present invention is not limited to these examples. Anything that can demonstrate the functions and effects of the wavelength tunable semiconductor laser control method, the control device therefor, and the wavelength tunable semiconductor laser device may be used.

[0072] It should be noted that the present invention is not limited to the above-described embodiments, and it is clear that many modifications and combinations can be made by a person having ordinary knowledge in the art within the technical concept of the present invention.

[0073] A part or all of the above-described embodiment or an example thereof can be described as, but is not limited to, the following supplementary notes.

[0074] (Supplementary Note 1) A method for controlling a wavelength tunable semiconductor laser including an active region having optical gain, a DBR region into which a first current is injected, and a phase adjustment region into which a second current is injected, the method comprising the steps of: approximating a boundary line of a desired wavelength region in a wavelength map expressed in a wavelength region in which an oscillation wavelength of the wavelength tunable semiconductor laser can be identified, with the first current and the second current as coordinate axes; adding the first power function and the second power function using a weighting coefficient to obtain an injection current relational expression that expresses the relationship between the first current and the second current; and approximating the boundary line of the desired wavelength region in a wavelength map expressed in a wavelength region in which an oscillation wavelength of the wavelength tunable semiconductor laser can be identified, with the first power function and the second power function as coordinate axes. a step of mapping the injection current relational expression and the injection current relational expression onto an SMSR map, the SMSR map being represented by an SMSR region where SMSR of the oscillation spectrum of the wavelength tunable semiconductor laser can be identified, using the first current and the second current as coordinate axes, respectively; a step of determining the injection current relational expression by determining the weighting coefficients so that a trajectory onto which the injection current relational expression is mapped passes through a region of the SMSR region where SMSR is high, in the SMSR map; and a step of changing a combination of the first current and the second current in accordance with the injection current relational expression, and applying the current to the wavelength tunable semiconductor laser to change the oscillation wavelength of the wavelength tunable semiconductor laser.

[0075] (Supplementary Note 2) When the first current is x, the second current is y, and the weighting coefficient is k, the first power function is expressed as y=a １ x ２ +c １ and the second power function is expressed as y=a ２ x ２ +c ２ 2. The method for controlling a wavelength tunable semiconductor laser according to claim 1, wherein the injection current relational expression is expressed by the following expression (A):

[0076] y = {ka １ +(1-k)a ２ x ２ +kc １ +(1-k)c ２ (A)

[0077] Here, 0<k<1.

[0078] (Supplementary Note 3) An apparatus for controlling a wavelength-tunable semiconductor laser including an active region having optical gain, a DBR region into which a first current is injected, and a phase adjustment region into which a second current is injected, the apparatus comprising: a storage unit; and a calculation unit, wherein the storage unit stores a wavelength map in which an oscillation wavelength of the wavelength-tunable semiconductor laser is represented in a wavelength region in which the oscillation wavelength is identifiable, with the first current and the second current as coordinate axes, and an SMSR map in which an SMSR of an oscillation spectrum of the wavelength-tunable semiconductor laser is represented in an SMSR region in which the SMSR of the oscillation spectrum of the wavelength-tunable semiconductor laser is identifiable, with the first current and the second current as coordinate axes, and the calculation unit calculates a boundary line of a desired wavelength region in the wavelength map by dividing the boundary line by a first current. and a second power function, and add the first power function and the second power function using a weighting coefficient to obtain an injection current relational expression that expresses the relationship between the first current and the second current, map the first power function, the second power function, and the injection current relational expression onto the SMSR map, determine the weighting coefficient so that a trajectory onto which the injection current relational expression is mapped passes through a region of the SMSR region where SMSR is high, thereby determining the injection current relational expression, and determine a combination of the first current and the second current in accordance with the injection current relational expression.

[0079] (Supplementary Note 4) A wavelength-tunable semiconductor laser device comprising: a control device for a wavelength-tunable semiconductor laser according to Supplementary Note 3; an arbitrary waveform generator; and the wavelength-tunable semiconductor laser, wherein an output section of the control device is connected to the arbitrary waveform generator, and the arbitrary waveform generator outputs the first current to the DBR region of the wavelength-tunable semiconductor laser and outputs the second current to the phase adjustment region of the wavelength-tunable semiconductor laser.

[0080] The present invention relates to a method and device for controlling a wavelength tunable semiconductor laser, and can be applied to gas sensing systems, gas sensing devices, optical communication systems, and optical communication devices.

[0081] 10 Tunable wavelength semiconductor laser 106, 107 DBR region 108 Phase adjustment region 117 DBR region injection current (first current) 118 Phase adjustment region injection current (second current) 21 Wavelength map 211, 212 Power function 22 SMSR map 223 Trajectory

Claims

1. A method for controlling a wavelength tunable semiconductor laser having an active region having optical gain, a DBR region into which a first current is injected, and a phase adjustment region into which a second current is injected, the method comprising the steps of: approximating a boundary line of a desired wavelength region in a wavelength map expressed in a wavelength region in which an oscillation wavelength of the wavelength tunable semiconductor laser can be identified, with the first current and the second current as coordinate axes, using a first power function and a second power function; adding the first power function and the second power function using a weighting coefficient to obtain an injection current relational expression that expresses the relationship between the first current and the second current; and mapping the first power function, the second power function, and the injection current relational expression onto an SMSR map expressed in an SMSR region in which an SMSR of an oscillation spectrum of the wavelength tunable semiconductor laser can be identified, with the first current and the second current as coordinate axes; determining the injection current relational expression by determining the weighting coefficients so that a trajectory onto which the injection current relational expression is mapped passes through a region of the SMSR region where SMSR is high in the SMSR map; and changing a combination of the first current and the second current in accordance with the injection current relational expression, and applying the current to the wavelength-tunable semiconductor laser to change an oscillation wavelength of the wavelength-tunable semiconductor laser.

2. Let the first current be x, the second current be y, and the weighting coefficient be k, and the first power function be y=a １ x ２ +c １ and the second power function is expressed as y=a ２ x ２ +c ２ 2. The method for controlling a wavelength tunable semiconductor laser according to claim 1, wherein the injection current relational expression is expressed by the following expression (A): y={ka １ +(1-k)a ２ x ２ +kc １ +(1-k)c ２ (A) where 0<k<1.

3. A device for controlling a wavelength-tunable semiconductor laser having an active region having optical gain, a DBR region into which a first current is injected, and a phase adjustment region into which a second current is injected, comprising: a memory unit; and a calculation unit, wherein the memory unit stores a wavelength map in which the oscillation wavelength of the wavelength-tunable semiconductor laser is expressed in a wavelength region in which the oscillation wavelength is identifiable, with the first current and the second current as coordinate axes, and an SMSR map in which the SMSR of the oscillation spectrum of the wavelength-tunable semiconductor laser is expressed in a SMSR region in which the SMSR is identifiable, with the first current and the second current as coordinate axes, the calculation unit approximates a boundary line of the desired wavelength region in the wavelength map by a first power function and a second power function, adds the first power function and the second power function using a weighting coefficient to obtain an injection current relational expression that expresses a relationship between the first current and the second current, maps the first power function, the second power function, and the injection current relational expression onto the SMSR map, determines the weighting coefficient so that a trajectory onto which the injection current relational expression is mapped passes through a region of the SMSR region where SMSR is high, thereby determining the injection current relational expression, and determines a combination of the first current and the second current according to the injection current relational expression.

4. A wavelength tunable semiconductor laser device comprising: a wavelength tunable semiconductor laser control device according to claim 3; an arbitrary waveform generator; and the wavelength tunable semiconductor laser, wherein an output section of the control device is connected to the arbitrary waveform generator, and the arbitrary waveform generator outputs the first current to the DBR region of the wavelength tunable semiconductor laser and outputs the second current to the phase adjustment region of the wavelength tunable semiconductor laser.

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