Semiconductor laser element, laser device, and method for controlling semiconductor laser element

Abstract

A semiconductor laser element according to an aspect of the present invention comprises: an optical resonator having a first reflection portion and a second reflection portion; a gain unit disposed in the optical resonator; and a loss amount control unit provided in the optical resonator and capable of switching between a first state in which an optical loss in the optical resonator is a first loss amount, and a second state in which the optical loss is a second loss amount greater than the first loss amount. In a state in which a predetermined drive current is being supplied to the gain unit, the semiconductor laser element performs laser oscillation when the loss amount control unit is in the first state, and may not perform laser oscillation when the loss amount control unit is in the second state.

Description

Semiconductor laser element, laser device, and method for controlling semiconductor laser element

[0001] The present invention relates to a semiconductor laser element, a laser apparatus, and a method for controlling a semiconductor laser element.

[0002] In some cases, it is desired to switch a semiconductor laser element between a state in which laser light is output to the outside (on state) and a state in which laser light is not output to the outside (off state). One method for achieving this switching is to supply a drive current to a gain section of the semiconductor laser element to set it in a laser oscillation state when it is in the on state, and not supply a drive current to the gain section when it is in the off state (Patent Documents 1 and 2).

[0003] Patent No. 7060395 Patent No. 6943150

[0004] The above method has a problem in that when switching from the off state to the on state, it may take time for the laser oscillation state to stabilize after switching. One way to solve this problem is to provide an optical switch that can be switched between a light transmitting state and a light blocking state on the output side of the semiconductor laser element, outside the optical resonator. In this case, the semiconductor laser element can be switched between the on state and the off state by switching the state of the optical switch while keeping the drive current supplied to the gain section.

[0005] However, when an optical switch is provided on the output side of the semiconductor laser element, outside the optical resonator, the extinction ratio of the optical switch between the light transmitting state and the light blocking state must be relatively high, which poses a problem of requiring an optical switch with a high extinction ratio.

[0006] The present invention has been made in view of the above, and has an object to provide a semiconductor laser element, a laser apparatus, and a method for controlling a semiconductor laser element that can more suitably switch between an on state and an off state.

[0007] In order to solve the above-mentioned problems and achieve the object, one aspect of the present invention is a semiconductor laser element including: an optical resonator having a first reflecting portion and a second reflecting portion; a gain portion disposed within the optical resonator; and a loss amount control portion provided within the optical resonator and switchable between a first state in which an optical loss within the optical resonator is a first loss amount and a second state in which an optical loss within the optical resonator is a second loss amount greater than the first loss amount.

[0008] When a predetermined drive current is supplied to the gain section, laser oscillation may occur when the loss amount control section is in the first state, and laser oscillation may not occur when the loss amount control section is in the second state.

[0009] The loss amount control section may include any of a Mach-Zehnder interferometer, a microring switch having a ring resonator structure, an optical switch using an electroabsorption effect, and a Michelson interferometer.

[0010] The semiconductor laser element may further include two wavelength selection filters having comb-shaped reflectance or transmittance spectra, and may be configured as a vernier-type tunable laser element.

[0011] The gain section and at least one of the first reflecting section and the second reflecting section of the optical resonator and the loss amount controlling section may be made of different semiconductor materials.

[0012] One aspect of the present invention is a laser apparatus comprising the semiconductor laser element and a control unit, wherein the control unit controls a drive current supplied to the gain unit and controls the loss amount control unit to switch between the first state and the second state.

[0013] One aspect of the present invention is a control method for a semiconductor laser element, the control method comprising: measuring a first voltage value which is a voltage value of the gain section when the loss amount control section is in the first state and the semiconductor laser element is oscillating, and a second voltage value which is a voltage value of the gain section when the loss amount control section is in the second state and the semiconductor laser element is not oscillating; and controlling the spectra of the two wavelength-selective filters so that the first voltage value takes a minimum value when the first voltage value is equal to or less than the second voltage value; and controlling the spectra of the two wavelength-selective filters so that the first voltage value takes a maximum value when the first voltage value is greater than the second voltage value.

[0014] According to the present invention, it is possible to realize a semiconductor laser element that can be switched between an on state and an off state more suitably.

[0015] FIG. 1 is a schematic diagram of a laser device according to embodiment 1. FIG. 2 is a schematic diagram of a laser device according to embodiment 2. FIG. 3 is a schematic diagram of a laser device according to embodiment 3. FIG. 4 is a schematic diagram of a laser device according to embodiment 4. FIG. 5 is a schematic diagram of a laser device according to embodiment 5. FIG. 6 is a schematic diagram of a laser device according to embodiment 6. FIG. 7 is a schematic diagram of a laser device according to embodiment 7. FIG. 8 is a schematic diagram of a laser device according to embodiment 8. FIG. 9 is an explanatory diagram of control of laser oscillation wavelength. FIG. 10 is a diagram showing an example of voltage characteristics of active layers of gain sections of different semiconductor laser elements. FIG. 11 is a flowchart showing an example of a control method.

[0016] Hereinafter, embodiments of the present invention will be described with reference to the drawings. Note that the present invention is not limited to the embodiments described below. Furthermore, in the description of the drawings, identical parts are appropriately designated by the same reference numerals, and duplicate explanations will be omitted as appropriate. Furthermore, the drawings are schematic, and the dimensional relationships and ratios of each element may differ from the actual situation. Furthermore, the drawings may include parts whose dimensional relationships and ratios differ from each other.

[0017] 1 is a schematic diagram showing the configuration of a laser device according to embodiment 1. The laser device 1000 includes a semiconductor laser element 100 and a control unit 200 that controls the operation of the semiconductor laser element 100.

[0018] [Configuration of Laser Device] The semiconductor laser element 100 is configured to output laser light L having a wavelength in a communication band, such as the 1.55 μm band. The semiconductor laser element 100 is a semiconductor laser element having an optical waveguide structure formed on a substrate 101 made of, for example, an InP-based semiconductor material. The semiconductor laser element 100 includes an optical resonator 110, a gain section 120, a loss amount control section 130, and a semiconductor optical amplifier (SOA) 140. The optical resonator 110, the gain section 120, the loss amount control section 130, and the SOA 140 are optically connected by an optical waveguide.

[0019] The optical resonator 110 has a first reflecting portion 111 and a second reflecting portion 112. The first reflecting portion 111 and the second reflecting portion 112 are, for example, mirrors having a DBR (Distributed Bragg Reflector) structure, and reflect light of the wavelength of the laser light L with a predetermined reflectance.

[0020] The gain section 120 is disposed within the optical resonator 110 so as to be sandwiched between the first reflecting section 111 and the second reflecting section 112. The gain section 120 has an optical waveguide made of an active layer that emits light with the wavelength of the laser light L when supplied with a drive current. The active layer is made of, for example, GaInAs.

[0021] The loss amount control unit 130 includes a Mach-Zehnder (MZ) interferometer 131 and a heater 132 that heats a portion of an arm waveguide of the MZ interferometer 131, and is disposed within the optical resonator 110. The MZ interferometer 131 functions as an optical switch by heating a portion of the arm waveguide with the heater 132 to change the refractive index of the arm waveguide. As a result, the loss amount control unit 130 operates to switch between a state in which light having the wavelength of the laser light L is transmitted and a state in which it is blocked. Here, the optical loss within the optical resonator 110 when the loss amount control unit 130 transmits light having the wavelength of the laser light L is defined as a first loss amount, and the optical loss within the optical resonator 110 when the loss amount control unit 130 blocks light having the wavelength of the laser light L is defined as a second loss amount. The loss amount control unit 130 then operates switchably between a first state in which the optical loss within the optical resonator 110 is the first loss amount and a second state in which the optical loss is the second loss amount. The MZ interferometer 131 has the advantage of being broadband and having low loss.

[0022] The SOA 140 is disposed outside the optical resonator 110. The SOA 140 has an optical waveguide made up of an active layer that emits light with the wavelength of the laser light L when supplied with a drive current. The active layer is made of, for example, GaInAs.

[0023] The control unit 200 controls the drive current supplied to the gain unit 120 and the SOA 140. The control unit 200 also supplies a heater current to the heater 132 of the loss amount control unit 130, and controls the loss amount control unit 130 to switch between a first state and a second state. Furthermore, the control unit 200 can also measure the voltage of the gain unit 120.

[0024] The control unit 200 includes hardware such as a processor, memory, and peripheral devices such as an input / output interface and a current supply unit. The processor, such as a central processing unit (CPU), digital signal processor (DSP), or graphics processing unit (GPU), performs various arithmetic operations for the functions implemented by the control unit 200. The memory includes a portion configured, for example, as read-only memory (ROM) and a portion configured, for example, as random access memory (RAM). The ROM stores various programs and data used by the processor for arithmetic operations. The RAM is used as a workspace for the processor when performing arithmetic operations and for storing the results of the processor's arithmetic operations. The control unit 200 may also include a computer-readable recording medium. The functions of the control unit 200 are realized by a combination of hardware and software, for example, when the processor executes a program read from memory. The control unit 200 may also include an FPGA (field programmable gate array).

[0025] [Operation of Laser Device] Next, the operation of the laser device 1000 will be described. First, with the control unit 200 setting the loss amount control unit 130 in the first state, the control unit 200 supplies a predetermined drive current to each of the gain unit 120 and the SOA 140. Then, laser oscillation occurs due to the light emission and optical amplification action of the gain unit 120 and the action of the optical resonator 110, and laser light is output from the second reflector 112 to the SOA 140. The SOA 140 optically amplifies the input laser light and outputs it to the outside of the semiconductor laser element 100 as laser light L.

[0026] In this manner, when a predetermined drive current is supplied to the gain section 120, laser oscillation occurs when the loss amount control section 130 is in the first state. When the control section 200 switches the loss amount control section 130 to the second state in this state, laser oscillation does not occur. However, when the control section 200 subsequently switches the loss amount control section 130 back to the first state, laser oscillation occurs again.

[0027] In this way, the semiconductor laser device 100 can switch between a laser oscillation state and a non-laser oscillation state by switching the state of the loss amount control unit 130, even when a drive current continues to be supplied to the gain unit 120. As a result, even when the semiconductor laser device 100 switches between an on state in which laser light L is output to the outside and an off state in which laser light L is not output to the outside, the time from switching until the laser oscillation state stabilizes can be relatively short. Furthermore, since the loss amount control unit 130 controls the loss applied to the inside of the optical resonator 110, switching between the on state and the off state can be performed with a lower extinction ratio (the difference between the first loss amount and the second loss amount) than when an external optical switch is provided.

[0028] As described above, the semiconductor laser device 100 can more suitably switch between the on state and the off state.

[0029] 2 is a schematic diagram of a laser device according to embodiment 2. The laser device 1000A has a configuration in which the semiconductor laser element 100 of the laser device 1000 shown in Fig. 1 is replaced with a semiconductor laser element 100A. The semiconductor laser element 100A has a configuration in which the loss amount control unit 130 of the semiconductor laser element 100 is replaced with a loss amount control unit 130A.

[0030] The loss amount control unit 130A includes a microring switch 131A having a ring resonator structure and a heater 132A that heats a portion of the ring waveguide of the microring switch 131A. The microring switch 131A is arranged so that a transmission port is connected to the first reflecting unit 111 and the gain unit 120. The microring switch 131A functions as an optical switch by heating a portion of the ring waveguide with the heater 132A to change the refractive index of the arm waveguide. Specifically, when the resonance wavelength of the microring switch 131A matches the wavelength of the laser light L, the loss amount control unit 130A transmits light of the wavelength of the laser light L. When the resonance wavelength of the microring switch 131A deviates from the wavelength of the laser light L, the loss amount control unit 130A blocks light of the wavelength of the laser light L.

[0031] Regarding the loss amount control unit 130A, if the optical loss in the optical resonator 110 in a state where the loss amount control unit 130 transmits light of the wavelength of the laser light L is defined as a first loss amount, and the optical loss in the optical resonator 110 in a state where the loss amount control unit 130 blocks light of the wavelength of the laser light L is defined as a second loss amount, the loss amount control unit 130A operates switchably between a first state in which the optical loss in the optical resonator 110 is the first loss amount, and a second state in which the optical loss is the second loss amount. The microring switch 131A has the advantages of being smaller and consuming less power than an MZ interferometer.

[0032] The control unit 200 supplies a heater current to the heater 132A of the loss amount control unit 130A and controls the loss amount control unit 130A to switch between a first state and a second state. The control unit 200 also controls the drive currents supplied to the gain unit 120 and the SOA 140. Furthermore, the control unit 200 can also measure the voltage of the gain unit 120.

[0033] In the semiconductor laser element 100A, when a predetermined drive current is supplied to the gain section 120, laser oscillation occurs when the loss amount control section 130A is in the first state, but when the control section 200 switches the loss amount control section 130A to the second state in this state, laser oscillation does not occur.

[0034] The semiconductor laser device 100A configured as above can also be switched between the on state and the off state more suitably.

[0035] 3 is a schematic diagram of a laser device according to embodiment 3. The laser device 1000B has a configuration in which the semiconductor laser element 100 of the laser device 1000 shown in FIG. 1 is replaced with a semiconductor laser element 100B. The semiconductor laser element 100B has a configuration in which the loss amount control unit 130 of the semiconductor laser element 100 is replaced with a loss amount control unit 130B.

[0036] The loss amount control unit 130B is configured as an optical switch using the electroabsorption (EA) effect. The loss amount control unit 130B functions as an optical switch by changing the electric field applied to it. Specifically, when no electric field is applied to the loss amount control unit 130B, the loss amount control unit 130B transmits light of the wavelength of the laser light L, and when an electric field of a predetermined strength is applied, the loss amount control unit 130B absorbs light of the wavelength of the laser light L.

[0037] Regarding the loss amount control unit 130B, if the optical loss in the optical resonator 110 in a state where the loss amount control unit 130 transmits light of the wavelength of the laser light L is defined as a first loss amount, and the optical loss in the optical resonator 110 in a state where the loss amount control unit 130 absorbs light of the wavelength of the laser light L is defined as a second loss amount, the loss amount control unit 130B operates switchably between a first state in which the optical loss in the optical resonator 110 is the first loss amount, and a second state in which the optical loss is the second loss amount. The loss amount control unit 130B using the EA effect has the advantage of being smaller in size than an MZ interferometer.

[0038] The control unit 200 applies a voltage to the loss amount control unit 130B and controls the loss amount control unit 130B to switch between a first state and a second state. The control unit 200 also controls the drive current supplied to the gain unit 120 and the SOA 140. The control unit 200 can also measure the voltage of the gain unit 120.

[0039] In the semiconductor laser element 100B, when a predetermined drive current is supplied to the gain section 120, laser oscillation occurs when the loss amount control section 130B is in the first state, but when the control section 200 switches the loss amount control section 130B to the second state in this state, laser oscillation does not occur.

[0040] The semiconductor laser device 100B configured as above can also be switched between the on state and the off state more suitably.

[0041] 4 is a schematic diagram of a laser device according to embodiment 4. The laser device 1000E has a configuration in which the semiconductor laser element 100 of the laser device 1000 shown in FIG. 1 is replaced with a semiconductor laser element 100E. The semiconductor laser element 100E has a configuration in which the optical resonator 110 of the semiconductor laser element 100 is replaced with an optical resonator 110E, and the loss amount control unit 130 is deleted.

[0042] The optical resonator 110E has a configuration in which the first reflector 111 of the optical resonator 110 is replaced with a first reflector 111E. The first reflector 111E includes a Y-branch optical waveguide and two DBR mirrors 111E1 and 111E2 connected to the Y-branch, forming a Michelson interferometer configuration. The mirrors 111E1 and 111E2 are each equipped with a heater. When a heater current is supplied to the heater, the DBR mirrors 111E1 and 111E2 of the first reflector 111E are heated, causing a change in the phase of the reflected light. When the phase of the reflected light from the mirror 111E1 and the phase of the reflected light from the mirror 111E2 match, the reflectivity of the first reflector 111E increases; when they do not match, the reflectivity decreases. Therefore, the first reflector 111E also functions as a loss amount control unit. The first reflecting section 111E is an example of an element in which the first reflecting section and the loss amount control section are integrated.

[0043] The control unit 200 supplies a heater current to the heater of the first reflector 111E and controls the first reflector 111E to switch between a first state and a second state. The first state of the first reflector 111E is, for example, a state in which the phase of the light reflected by the mirror 111E1 and the phase of the light reflected by the mirror 111E2 are the same. The second state is, for example, a state in which the phase of the light reflected by the mirror 111E1 and the phase of the light reflected by the mirror 111E2 are out of phase with each other, and the optical loss of the optical resonator 110E is a second loss amount that is greater than the first loss amount in the first state. The control unit 200 also controls the drive current supplied to the gain unit 120 and the SOA 140. The control unit 200 can also measure the voltage of the gain unit 120.

[0044] In the semiconductor laser element 100E, when a predetermined drive current is supplied to the gain section 120, laser oscillation occurs when the first reflecting section 111E is in the first state, but when the control section 200 switches the first reflecting section 111E to the second state in this state, laser oscillation does not occur.

[0045] The semiconductor laser device 100E configured as above can also be switched between the on state and the off state more suitably.

[0046] 5 is a schematic diagram showing the configuration of a laser device according to embodiment 7. A laser device 1000F has a configuration in which the semiconductor laser element 100 of the laser device 1000 shown in FIG.

[0047] The semiconductor laser device 100F is configured to output laser light L having a wavelength in a communication band, such as the 1.55 μm band. The semiconductor laser device 100F is a so-called hybrid integrated semiconductor laser device having a configuration in which a substrate 102F made of, for example, an InP-based semiconductor material is mounted on a substrate 101F made of, for example, a silicon-based material. An optical resonator 110F and a loss control section 130F are formed on the substrate 101F. In addition, a gain section 120 is formed on the substrate 102F. The optical resonator 110F, the gain section 120, and the loss control section 130F are optically connected by optical waveguides.

[0048] The optical resonator 110F has a first reflecting portion 111F and a second reflecting portion 112F. The first reflecting portion 111F and the second reflecting portion 112F are, for example, mirrors having a DBR structure, and reflect light of the wavelength of the laser light L with a predetermined reflectance.

[0049] The loss amount control unit 130F includes an MZ interferometer 131F and a heater 132 that heats a part of the arm waveguide of the MZ interferometer 131F, and is disposed in the optical resonator 110F. Similar to the loss amount control unit 130, the loss amount control unit 130F operates switchably between a first state in which the optical loss in the optical resonator 110F is a first loss amount, and a second state in which the optical loss is a second loss amount.

[0050] As described above, in the semiconductor laser device 100F, the gain section 120, the first reflecting section 111F and the second reflecting section 112F of the optical resonator 110F, and the loss control section 130F are made of different semiconductor materials.

[0051] The control unit 200 controls the drive current supplied to the gain unit 120. The control unit 200 also supplies a heater current to the heater 132 of the loss amount control unit 130F and controls the loss amount control unit 130F to switch between a first state and a second state. Furthermore, the control unit 200 can also measure the voltage of the gain unit 120.

[0052] In the semiconductor laser element 100F, when a predetermined drive current is supplied to the gain section 120, laser oscillation occurs when the loss amount control section 130F is in the first state, but when the control section 200 switches the loss amount control section 130F to the second state in this state, laser oscillation does not occur.

[0053] The semiconductor laser device 100F configured as above can also be switched between the on state and the off state more suitably.

[0054] Sixth Embodiment Fig. 6 is a schematic diagram showing the configuration of a laser device according to a sixth embodiment. A laser device 1000G has a configuration in which the semiconductor laser element 100F of the laser device 1000F shown in Fig. 5 is replaced with a semiconductor laser element 100G.

[0055] The semiconductor laser device 100G is also a so-called hybrid integrated semiconductor laser device having a configuration in which a substrate 102G made of, for example, an InP-based semiconductor material is mounted on a substrate 101G made of, for example, a silicon-based material. An optical resonator 110F is formed on the substrate 101G. In addition, a gain section 120 and a loss control section 130 are formed on the substrate 102G.

[0056] As described above, in the semiconductor laser device 100G, the gain section 120 and the first and second reflecting sections 111F and 112F of the optical resonator 110F are made of different semiconductor materials.

[0057] In the semiconductor laser element 100G, when a predetermined drive current is supplied to the gain section 120, laser oscillation occurs when the loss amount control section 130 is in the first state, but when the control section 200 switches the loss amount control section 130 to the second state in this state, laser oscillation does not occur.

[0058] The semiconductor laser device 100G configured as above can also be switched between the on state and the off state more suitably.

[0059] Seventh Embodiment Fig. 7 is a schematic diagram showing the configuration of a laser device according to a seventh embodiment. A laser device 1000H has a configuration in which the semiconductor laser element 100G of the laser device 1000G shown in Fig. 6 is replaced with a semiconductor laser element 100H.

[0060] The semiconductor laser device 100H is also a so-called hybrid integrated semiconductor laser device having a configuration in which a substrate 102H made of, for example, an InP-based semiconductor material is mounted on a substrate 101H made of, for example, a silicon-based material. A second reflector 112F of an optical resonator 110H is formed on the substrate 101G. Furthermore, a first reflector 111, a gain section 120, and a loss control section 130 of the optical resonator 110H are formed on the substrate 102H.

[0061] As described above, in the semiconductor laser device 100G, the gain section 120 and the second reflector 112F of the optical resonator 110H are made of different semiconductor materials.

[0062] In the semiconductor laser element 100H, when a predetermined drive current is supplied to the gain section 120, laser oscillation occurs when the loss amount control section 130 is in the first state, but when the control section 200 switches the loss amount control section 130 to the second state in this state, laser oscillation does not occur.

[0063] The semiconductor laser device 100H configured as above can also be switched between the on state and the off state more suitably.

[0064] 8 is a schematic diagram of a laser device according to embodiment 8. The laser device 1000I has a configuration in which the semiconductor laser element 100 of the laser device 1000 shown in FIG. 1 is replaced with a semiconductor laser element 100I. The semiconductor laser element 100I is configured as a vernier-type tunable laser element, and is configured to output laser light L having a wavelength in a communication band such as the 1.55 μm band.

[0065] The semiconductor laser device 100I has a configuration in which the optical resonator 110 of the semiconductor laser device 100 is replaced with an optical resonator 110I. The optical resonator 110I has a first reflector 111I and a second reflector 112I. The first reflector 111 and the second reflector 112 are wavelength-selective filters whose reflectance spectra have comb-shaped peaks. The first reflector 111I and the second reflector 112I have different peak periods along the wavelength axis. The first reflector 111I or the second reflector 112I is formed, for example, by a sample-type DBR mirror or a ring resonator-type reflecting mirror. Furthermore, the first reflector 111I and the second reflector 112I are each provided with a heater. The first reflector 111I and the second reflector 112I have a characteristic in which, when power is supplied to the heater and the first reflector 111I and the second reflector 112I are heated, the reflectance spectrum shifts in the wavelength axis direction.

[0066] The semiconductor laser element 100I oscillates at the wavelength of a peak when one of the reflectance peaks in the first reflector 111I overlaps with one of the reflectance peaks in the second reflector 112I. Changing the value of the current supplied to the heater of at least one of the first reflector 111I and the second reflector 112I shifts the wavelength of the overlapping peaks and changes the laser oscillation wavelength, thereby realizing vernier-type wavelength tunability.

[0067] The control unit 200 controls the laser oscillation wavelength by supplying heater currents to the heaters of the first reflecting unit 111I and the second reflecting unit 112I. The control unit 200 controls the drive currents supplied to the gain unit 120 and the SOA 140. The control unit 200 also supplies heater currents to the heater 132 of the loss amount control unit 130, and controls the loss amount control unit 130 to switch between a first state and a second state. Furthermore, the control unit 200 can also measure the voltage of the gain unit 120.

[0068] In the semiconductor laser element 100I, when a predetermined drive current is supplied to the gain section 120, laser oscillation occurs when the loss amount control section 130 is in the first state, but when the control section 200 switches the loss amount control section 130 to the second state in this state, laser oscillation does not occur.

[0069] The semiconductor laser device 100I configured as above can also be switched between the on state and the off state more suitably.

[0070] [Control of Laser Oscillation Wavelength] The control of the laser oscillation wavelength by the control unit 200 will be described in more detail. Fig. 9 is an explanatory diagram of the control of the laser oscillation wavelength. In Fig. 9, the first control parameter is the value of current or power supplied to the first reflector 111. The second control parameter is the value of current or power supplied to the second reflector 112.

[0071] Cell C is a range defined by a combination of the first and second control parameters, and is a range in which the laser oscillation wavelengths are approximately equal. Therefore, to set the laser oscillation wavelength to a certain target wavelength, the first and second control parameters are set so that the combination of the first and second control parameters is included in cell C corresponding to the target wavelength.

[0072] However, even for combinations of the first control parameter and the second control parameter included in the same cell C, the threshold current for laser oscillation differs depending on the position within the cell C. A smaller threshold current for laser oscillation is preferable. Within each cell C, there exists an operating point P where the threshold current is minimal within that cell C. Therefore, the control unit 200 executes control to detect the operating point P by slightly varying the first control parameter and the second control parameter to shift the spectra of the first reflecting unit 111I and the second reflecting unit 112I. This type of control is called dither control.

[0073] In order to examine the threshold current, it is effective to measure the voltage of the gain section 120 while a certain driving current is flowing. Here, through intensive studies by the present inventors, it has been found that the sign of the difference in the voltage of the gain section between the state in which the semiconductor laser element is oscillating and the state in which it is not oscillating can change depending on, for example, the type of semiconductor laser element and the state of the semiconductor laser element (environmental temperature and aging).

[0074] Specifically, the inventors prepared a certain type of semiconductor laser device, device A1, which oscillated when supplied with the same driving current (for example, 160 mA) as a test current, and device A2, which did not oscillate, and measured the voltages of the gain sections of devices A1 and A2 when the test current was supplied. Also, as device B, a different type from device A, they prepared device B1, which oscillated when supplied with a test current, and device B2, which did not oscillate, and measured the voltages of the gain sections of devices B1 and B2 when the test current was supplied.

[0075] Fig. 10 is a diagram showing the voltage characteristics of the active layer of the gain section of different semiconductor laser elements A and B. In Fig. 10, the horizontal axis represents the drive current supplied to the semiconductor laser element, and the vertical axis represents the voltage difference. Here, the voltage difference is defined as (active layer voltage when the element is oscillating) - (active layer voltage when the element is not oscillating).

[0076] 10, when the test current is 160 mA, the voltage difference is negative for Device A, whereas the voltage difference is positive for Device B. In this way, it was found that the sign of the potential difference can change depending on the type of semiconductor laser device.

[0077] According to the inventor's investigation, the following conclusion can be drawn: In the gain section, before laser oscillation, when the current is increased, the increase is converted into an increase in the carrier number density, whereas after laser oscillation, the increase in current is converted into light, leaving the carrier number density almost unchanged, so that the differential resistance of the gain section decreases, and as a result, the applied voltage, which is the integral of the current, generally decreases. However, in reality, due to factors such as differences in the degree of temperature rise at the junction of the active layer caused by the current, the differential resistance characteristic during laser oscillation may become larger than when not oscillating, and the voltage may instead increase.

[0078] The inventors have considered that when the sign of the voltage difference in the active layer is negative, if an operating point at which the voltage is minimal is detected by dither control, this operating point is the operating point at which the threshold current is minimal (operating point P in FIG. 11), and when the sign of the voltage difference is positive, if an operating point at which the voltage is maximal is detected by dither control, this operating point is the operating point at which the threshold current is minimal (operating point P in FIG. 9). However, with ordinary semiconductor laser elements, when a certain drive current is supplied, it is not possible for one element to be in a state where it is lasing and a state where it is not lasing.

[0079] In contrast to this, the semiconductor laser device 100I can have one device in a state where it is oscillating or not oscillating, depending on the function of the loss amount control unit 130. As a result, the semiconductor laser device 100I can determine, using only one device, whether it is necessary to detect an operating point where the voltage is at a minimum or a maximum by dither control.

[0080] 11 is a flowchart showing an example of a control method executed by the control unit 200 during dither control. First, in step S101, the control unit 200 supplies a drive current (Gain current) to the gain unit 120 while not supplying a drive current to the SOA 140. This Gain current has a current value such that the semiconductor laser element 100I oscillates when the loss amount control unit 130 is in the first state, and such that the semiconductor laser element 100I does not oscillate when the loss amount control unit 130 is in the second state.

[0081] Next, in step S102, the control unit 200 supplies a first heater current to the heater of the first reflecting unit 111, and supplies a second heater current to the heater of the second reflecting unit 112. The first heater current and the second heater current have values ​​according to the cell corresponding to the target wavelength of the laser oscillation wavelength.

[0082] Next, in step S103, the control unit 200 switches the loss amount control unit 130 between a first state and a second state, and measures the voltage (gain voltage) of the gain unit 120 in each state. Here, the value of the gain voltage when the semiconductor laser device 100I is oscillating in the first state is defined as a first voltage value, and the value of the gain voltage when the semiconductor laser device 100I is not oscillating in the second state is defined as a second voltage value.

[0083] Next, in step S104, the control unit 200 determines whether the first voltage value is equal to or less than the second voltage value. If the first voltage value is equal to or less than the second voltage value (Yes in step S104), in step S105, the control unit 200 performs dither control of the first heater current and the second heater current (i.e., controls the spectra of the first reflector 111I and the second reflector 112I) so that the first voltage value becomes a minimum value.

[0084] Next, in step S106, the control unit 200 determines whether the first voltage value is near the minimum value. Near the minimum value means that the change (differential coefficient) in the first voltage value with respect to the change in the first heater current and the second heater current is within an allowable range. If the first voltage value is near the minimum value (Yes in step S106), the flow proceeds to step S109. If the first voltage value is not near the minimum value (No in step S106), the flow returns to step S105.

[0085] On the other hand, if the first voltage value is greater than the second voltage value (step S104, No), in step S107, the control unit 200 dithers the first heater current and the second heater current so that the first voltage value takes on a maximum value (i.e., controls the spectra of the first reflector 111I and the second reflector 112I).

[0086] Next, in step S108, the control unit 200 determines whether the first voltage value is near the maximum value. Near the maximum value means that the change (differential coefficient) in the first voltage value with respect to the change in the first heater current and the second heater current is within an allowable range. If the first voltage value is near the maximum value (Yes in step S108), the flow proceeds to step S109. If the first voltage value is not near the maximum value (No in step S106), the flow returns to step S107.

[0087] Next, in step S109, the control unit 200 supplies a drive current (SOA current) to the SOA 140. This causes the SOA 140 to optically amplify the laser light input from the second reflector 112I and output it as laser light L to the outside of the semiconductor laser device 100I. Here, the SOA 140 generates heat when an SOA current is supplied, and therefore whether it is necessary to detect an operating point where the voltage is at a minimum or a maximum by dither control may differ from when an SOA current is not supplied. Therefore, after step S109, the control unit 200 executes the same processes as steps S103 to S108.

[0088] Specifically, in step S110, the control unit 200 switches the loss amount control unit 130 between a first state and a second state and measures the gain voltage in each state. Next, in step S111, the control unit 200 determines whether the first voltage value is equal to or less than the second voltage value. If the first voltage value is equal to or less than the second voltage value (Yes in step S111), the control unit 200 performs dither control of the first heater current and the second heater current so that the first voltage value becomes a minimum value in step S112. Next, in step S113, the control unit 200 determines whether the first voltage value is near the minimum value. If the first voltage value is near the minimum value (Yes in step S113), the process ends. If the first voltage value is not near the minimum value (No in step S113), the flow returns to step S112.

[0089] On the other hand, if the first voltage value is greater than the second voltage value (step S111, No), the control unit 200 performs dither control of the first heater current and the second heater current so that the first voltage value reaches a maximum value in step S114. Subsequently, in step S115, the control unit 200 determines whether the first voltage value is near the maximum value. If the first voltage value is near the maximum value (step S115, Yes), the process ends. If the first voltage value is not near the maximum value (step S113, No), the flow returns to step S114.

[0090] In the semiconductor laser device 100I of the eighth embodiment, a vernier-type wavelength-tunable laser device is configured using the first reflecting portion 111 and the second reflecting portion 112, which are wavelength-selective filters whose reflectance spectrum has a comb-shaped peak. However, the vernier-type wavelength-tunable laser device according to the embodiment of the present invention may be configured using a wavelength-selective filter whose transmittance spectrum has a comb-shaped peak.

[0091] Furthermore, although the semiconductor laser element of the fourth embodiment has a structure in which the first reflecting section and the loss amount control section are integrated, a structure in which the second reflecting section and the loss amount control section are integrated may also be employed.

[0092] In the loss amount control unit having the MZ interferometer or the microring switch and the heater in the above-mentioned embodiment, the heater may be replaced with an electrode, and the loss amount may be controlled by applying an electric field to the MZ interferometer or the microring switch through the Pockels effect or the electroabsorption effect. In this case, high-speed switching between the first state and the second state is possible.

[0093] Furthermore, the present invention is not limited to the above-described embodiments. The present invention also includes configurations in which the above-described components are appropriately combined. Furthermore, further effects and modifications can be easily derived by those skilled in the art. Therefore, the broader aspects of the present invention are not limited to the above-described embodiments, and various modifications are possible.

[0094] As described above, the present invention is suitable for realizing a semiconductor laser element that can be switched between an on state and an off state more suitably.

[0095] 100, 100A, 100B, 100E, 100F, 100G, 100H, 100I: semiconductor laser element 101, 101F, 101G, 101H, 102F, 102G, 102H: substrate 110, 110E, 110F, 110H, 110I: optical resonator 111, 111E, 111F, 111I: first reflecting section 111E1, 111E2: mirror 112, 112F, 112I: second reflecting section 120: gain section 130, 130A, 130B, 130F: loss amount control section 131, 131F: MZ interferometer 131A: microring switch 132, 132A: heater 200 : Control unit 1000, 1000A, 1000B, 1000E, 1000F, 1000G, 1000H, 1000I: Laser device C: Cell L: Laser light P: Operating point

Claims

1. A semiconductor laser device comprising: an optical resonator having a first reflecting portion and a second reflecting portion; a gain portion disposed within the optical resonator; and a loss amount control portion disposed within the optical resonator, switchable between a first state in which the optical loss within the optical resonator is a first loss amount and a second state in which the optical loss is a second loss amount greater than the first loss amount.

2. The semiconductor laser element according to claim 1, wherein, when a predetermined drive current is supplied to the gain section, laser oscillation occurs when the loss amount control section is in the first state, and laser oscillation does not occur when the loss amount control section is in the second state.

3. The semiconductor laser device according to claim 2, wherein the loss control section includes any one of a Mach-Zehnder interferometer, a microring switch having a ring resonator structure, an optical switch using the electroabsorption effect, and a Michelson interferometer.

4. The semiconductor laser device according to claim 1, further comprising two wavelength-selective filters having comb-shaped reflectance or transmittance spectra, and configured as a vernier-type tunable laser device.

5. The semiconductor laser device according to claim 1, wherein the gain section and at least one of the first reflecting section and the second reflecting section of the optical resonator and the loss amount control section are made of different semiconductor materials.

6. A laser device comprising: a semiconductor laser element according to any one of claims 1 to 5; and a control section, wherein the control section controls the drive current supplied to the gain section and controls the loss amount control section to switch between the first state and the second state.

7. A method for controlling a semiconductor laser element as defined in claim 4, comprising measuring a first voltage value, which is the voltage value of the gain section when the loss amount control section is in the first state and the semiconductor laser element is oscillating, and a second voltage value, which is the voltage value of the gain section when the loss amount control section is in the second state and the semiconductor laser element is not oscillating; when the first voltage value is equal to or less than the second voltage value, controlling the spectra of the two wavelength selection filters so that the first voltage value takes a minimum value; and when the first voltage value is greater than the second voltage value, controlling the spectra of the two wavelength selection filters so that the first voltage value takes a maximum value.

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