WO2011013535A1

WO2011013535A1 - Laser system

Info

Publication number: WO2011013535A1
Application number: PCT/JP2010/062164
Authority: WO
Inventors: 秋山知之; 宇佐美真; 菅原充
Original assignee: 株式会社Ｑｄレーザ
Priority date: 2009-07-27
Filing date: 2010-07-20
Publication date: 2011-02-03
Also published as: JP2011029426A

Abstract

The laser system comprises a DFB laser (10) having a semiconductor layer containing an active layer (58) for emitting a laser beam (14) the intensity of which is modulated by driving power being modulated, and a heater (16) for keeping the active layer (58) at a constant temperature; and a controller (80) for applying, to the heater (16), heater power modulated by a signal derived by reversing a modulation signal of the driving power, so as to offset the temperature change in the active layer (58) caused by the modulated driving power.

Description

Laser system

The present invention relates to a laser system.

In recent years, laser systems that emit laser light have been used in various fields, and in particular, semiconductor lasers have been used in inexpensive laser systems. For example, Patent Document 1 describes a laser system that converts laser light emitted from a semiconductor laser into a harmonic using a nonlinear optical element and emits the same.

JP-A-6-132595

In laser systems, the intensity of laser light may be modulated. For example, in laser projection using laser light for projection, the intensity of the laser light is modulated at a high speed of about 500 MHz, for example.

The intensity modulation of the laser beam can be realized by modulating the driving power used for the oscillation of the laser beam. However, when the intensity of the laser beam is modulated by modulating the driving power, a temperature change occurs in the active layer, and as a result, the oscillation wavelength of the laser beam fluctuates.

An object of the present invention is to provide a laser system capable of suppressing fluctuations in the wavelength of laser light even when the intensity of the laser light is modulated by modulation of drive power.

The present invention provides a DFB laser having a semiconductor layer including an active layer that emits laser light whose intensity is modulated by modulating driving power, and a heater unit for maintaining the temperature of the active layer constant. A control unit that applies heater power modulated to the same frequency as the modulation frequency of the drive power to the heater unit so as to cancel the temperature change of the active layer due to the modulated drive power. This is a featured laser system. According to the present invention, even when the intensity of laser light is modulated by modulation of driving power, fluctuations in the wavelength of the laser light can be suppressed.

In the above configuration, the control unit may apply the heater power complementary to the modulated driving power to the heater unit. According to this configuration, it is possible to easily cancel the temperature change of the active layer due to the driving power, and to easily maintain the temperature of the active layer constant.

In the above configuration, the time constant of the temperature change of the active layer based on the driving power and the time constant of the temperature change of the active layer based on the heater power may be approximately the same. According to this configuration, it is possible to make the frequency response characteristic of temperature change due to drive power similar to the frequency response characteristic of temperature change due to heater power, and the heater power is modulated by a signal inverted in the same form as the modulation signal of drive power. By doing so, the temperature change of the active layer due to the driving power can be easily canceled.

In the above configuration, the semiconductor layer may have an isolated mesa portion, and the driving electrode to which the driving power is applied and the heater portion may be provided on the mesa portion. According to this configuration, the heat capacity of the active layer with respect to driving power and heater power can be made comparable. In addition, the time constant of the temperature change of the active layer based on the driving power and the heater power can be made comparable.

In the above configuration, the width of the heater portion may be smaller than three times the width of the mesa portion.

In the above configuration, the heater portion may be provided along the drive electrode.

In the above configuration, the DFB laser may be a quantum dot DFB laser. According to this configuration, the wavelength variation of the laser beam can be further reduced.

In the above configuration, the laser beam may be configured to have a harmonic generation element that converts the laser beam into visible light that is a harmonic of the laser beam. According to this configuration, the laser beam can be converted into a harmonic with high conversion efficiency.

In the above configuration, the harmonic element can be configured to convert the laser beam into a second harmonic of the laser beam. In the above configuration, the visible light may be green light.

According to the present invention, fluctuations in the wavelength of the laser light can be suppressed even when the intensity of the laser light is modulated by modulation of the driving power.

FIG. 1 is a block diagram of a laser system according to a comparative example. FIG. 2 is a schematic diagram for explaining the oscillation spectrum of laser light according to the driving power, the wavelength of the laser light, and the conversion efficiency of the harmonic generation element. FIG. 3 is a block diagram of a laser system for explaining the principle of the present invention. FIG. 4 is a schematic diagram for explaining the relationship between the driving power and the output intensity of the laser beam, and the relationship between the driving power and heater power, and the temperature change of the active layer. FIG. 5 is a schematic top view of the laser system according to the first embodiment. FIG. 6A is a schematic cross-sectional view of a DFB laser in a direction parallel to the laser beam emission direction, and FIG. 6B is a schematic cross-sectional view of a DFB laser in the vertical direction. FIG. 7 is a schematic diagram for explaining the heater power. FIG. 8 is a schematic cross-sectional view illustrating the width of the heater portion. FIG. 9A is a conceptual diagram illustrating an example of a first control method of heater power by the control unit, and FIG. 9B is a conceptual diagram illustrating an example of a second control method. FIG. 10 is a schematic top view of the laser system when no harmonic generation element is provided.

First, a problem in the case of intensity modulation of laser light by modulation of driving power will be described in detail by taking a laser system that converts laser light into a harmonic and emits it as an example. FIG. 1 is a block diagram of a laser system according to a comparative example. As shown in FIG. 1, the laser system according to the comparative example includes a DFB (distributed feedback type) laser 11 and a harmonic generation element 30. The DFB laser 11 is a laser (for example, QD (quantum dot) -DFB laser) that has a corrugation and emits laser light 14 having a single wavelength from an active layer when a driving power is applied to the driving electrode 12. . The DFB laser 11 emits laser light 14 having a wavelength of 1064 nm, for example.

The harmonic generation element 30 is a nonlinear optical element, for example, PPLN (PeriodicallydPoled Lithium Niobate). The harmonic generation element 30 converts the laser light 14 having a wavelength of, for example, 1064 nm into green light (visible light) 32 having a wavelength of, for example, 532 nm, which is the second harmonic, and emits it.

The laser system of the comparative example is used for laser projection, for example. Laser projection requires that the intensity of laser light be modulated. The intensity modulation of the laser beam can be realized by modulating the driving power used for the oscillation of the laser beam. FIG. 2 shows the oscillation spectrum of the laser beam when the magnitude of the drive power is changed (the upper diagram in FIG. 2), and the relationship between the wavelength of the laser beam and the conversion efficiency of the harmonic generation element (the lower diagram in FIG. 2). FIG.

As shown in the upper diagram of FIG. 2, when the driving power increases, the oscillation wavelength of the laser light shifts to the longer wavelength side. For example, when the driving power is small, the laser light oscillation spectrum 13 is obtained, whereas when the driving power is increased, the laser light oscillation spectrum 15 is obtained, and the laser light oscillation wavelength is shifted to the longer wavelength side. . This is because the driving power affects the temperature of the active layer, and the temperature of the active layer changes based on the magnitude of the driving power.

As described above, when the driving power is modulated in order to modulate the intensity of the laser beam, the temperature of the active layer changes with time as the magnitude of the driving power changes. As a result, the oscillation wavelength of the laser beam Fluctuates.

On the other hand, as shown in the lower diagram of FIG. 2, when the conversion from the fundamental wave to the harmonic by the harmonic generation element 30 is performed with high conversion efficiency, the allowable wavelength range is a narrow wavelength range such as the region 17. .

As described above, when the intensity of the laser beam is modulated by modulating the driving power, the wavelength of the laser beam is changed. As a result, for example, it becomes difficult to set the wavelength of the laser light within a wavelength range that can be converted into a harmonic with high conversion efficiency by the harmonic generation element, and the laser light cannot be converted into harmonic light with high conversion efficiency. . Therefore, a laser system that can suppress fluctuations in the wavelength of laser light even when the intensity of the laser light is modulated by modulation of drive power will be described.

The principle of the present invention will be described with reference to FIGS. FIG. 3 is a block diagram of a laser system 100 for explaining the principle of the present invention. As shown in FIG. 3, the laser system 100 includes a DFB laser 10 and a harmonic generation element 30. The DFB laser 10 is provided with a heater unit 16 to which heater power is applied to maintain the temperature of the active layer constant, in addition to the drive electrode 12 to which drive power to oscillate the laser light 14 is applied. Yes.

The DFB laser 10 emits intensity-modulated laser light 14 from the active layer by applying modulated drive power to the drive electrode 12. The wavelength of the laser beam 14 is a single wavelength of 1064 nm, for example. The harmonic generation element 30 converts the intensity-modulated laser light 14 into green light (visible light) 32, which is the second harmonic, and emits it. The wavelength of the green light 32 is, for example, 532 nm.

Figure 4 shows the relationship between the output intensity of the driving power P _DFB laser light and green light, the relationship between the temperature change of the driving power P _DFB and the active layer, the relationship between the temperature change of the heater power P _Heater and the active layer described It is a schematic diagram to do.

First, the output intensities of the laser light 14 and the green light 32 will be briefly described. As shown in FIG. 4, the output intensity of the laser beam 14 increases as the drive power _PDFB applied to the drive electrode 12 increases (broken line in FIG. 4). The harmonic generation element 30 emits light having an intensity squared with respect to the intensity of the incident light. Therefore, the output intensity of the green light 32 emitted from the harmonic generation element 30 increases as the drive power _PDFB increases (thin solid line in FIG. 4).

Next, the temperature change of the active layer due to the driving power P _DFB and the heater power P _heater will be described. As shown in FIG. 4, the temperature of the active layer is affected by the application of the driving power P _DFB and the heater power P _heater . The temperature change ΔT _DFB of the active layer caused by the driving power P _DFB changes, for example, linearly with respect to the driving power P _DFB and changes as ΔT _DFB = aP _DFB (a chain line in FIG. 4). a is a proportionality coefficient. The temperature change [Delta] T _Heater active layer due to the heater power _{P Heater,} for example changes linearly with respect to the heater power _{P Heater,} changes as _{_ΔT} heater = _bP heater (two-dot chain line in FIG. 4). b is a proportionality coefficient. As described above, generally, the temperature of the active layer changes in proportion to the drive power P _DFB and the heater power P _heater .

Therefore, the temperature change ΔT of the active layer due to the application of the driving power P _DFB and the heater power P _heater is ΔT = ΔT _DFB + ΔT _heater = aP _DFB + bP _heater . Therefore, if aP _DFB + bP _heater has a constant power P _fix , the temperature change ΔT of the active layer becomes constant, and as a result, the temperature of the active layer becomes a constant temperature T _fix (thick solid line in FIG. 4).

Here, the driving power P _DFB is modulated. That is, the drive power _PDFB changes according to the modulation frequency. Therefore, if the heater power P _heater is modulated at the same frequency as the modulation frequency of the drive power P _DFB so that the aP _DFB + bP _heater is constant in time, the temperature change of the active layer due to the drive power P _DFB is canceled out. The temperature of the active layer can be made constant over time. Thereby, the wavelength fluctuation | variation of the laser beam 14 radiate | emitted from an active layer can be suppressed. An embodiment of the laser system of the present invention using this principle will be described below.

FIG. 5 is a schematic top view of the laser system 200 according to the first embodiment. As shown in FIG. 5, the laser system 200 includes the DFB laser 10, collimating lenses 40 and 42, the harmonic generation element 30, and the control unit 80. The DFB laser 10 is, for example, a QD (quantum dot) -DFB laser. The harmonic generation element 30 is, for example, PPLN.

On the upper surface of the DFB laser 10, heater power modulated to the same frequency as the modulation frequency of the drive power is applied to cancel the temperature change of the active layer due to the drive power and the drive electrode 12 to which the modulated drive power is applied. The heater portion 16 is formed. The modulation frequency of drive power and heater power is, for example, 500 MHz. The heater portion 16 is formed to extend along the drive electrode 12 above the drive electrode 12. That is, the drive electrode 12 and the heater portion 16 are formed at the same position.

On the end face of the DFB laser 10 where the laser beam 14 is not emitted, an HR (high reflection) film 18 for light having a wavelength of 1064 nm, which is the oscillation wavelength of the DFB laser 10, is formed. An AR (antireflection) film 20 for light having an oscillation wavelength (eg, 1064 nm) of the DFB laser 10 is formed on the end face of the DFB laser 10 that emits the laser light 14.

The laser light 14 emitted from the DFB laser 10 is incident on one end face of the harmonic generation element 30 by the collimating lenses 40 and 42. The surfaces of the collimating lenses 40 and 42 are coated with an AR film for the wavelength of the laser light 14 (for example, 1064 nm).

An AR film 34 for the wavelength of the laser light 14 (for example, 1064 nm) is formed on the end face of the harmonic generation element 30 on which the laser light 14 is incident. The harmonic generation element 30 converts the incident laser light 14 into green light 32 that is the second harmonic and emits it. An AR film 36 for the wavelength of the green light 32 of, for example, 532 nm is formed on the end face of the harmonic generation element 30 from which the green light 32 is emitted.

The control unit 80 applies the modulated drive power to the drive electrode 12 using the wire 70. Further, the control unit 80 applies the heater power modulated to the same frequency as the modulation frequency of the driving power to the heater unit 16 using the wire 70 in order to cancel the temperature change of the active layer due to the driving power.

6A is a schematic cross-sectional view of the DFB laser 10 in a direction parallel to the emission direction of the laser light 14, and FIG. 6B is a schematic cross-sectional view of the DFB laser 10 in the vertical direction. As shown in FIG. _6A , an n-type cladding layer 52 made of n-type Al _0.35 Ga _0.65 As is formed on an n-type GaAs substrate 50. An electrode 69 is formed under the substrate 50. On the n-type cladding layer 52, a quantum dot active layer 58 having a quantum dot 56 made of InAs in a base layer 54 made of GaAs is formed. A p-type layer 60 made of p-type GaAs is formed on the quantum dot active layer 58. A p-type cladding layer 62 made of p-type InGaP is formed on the p-type layer 60. Between the p-type layer 60 and the p-type cladding layer 62, a corrugation 64 that determines the wavelength of the oscillating laser light is formed. As described above, the semiconductor layer 61 including the n-type cladding layer 52, the p-type layer 60, and the p-type cladding layer 62 including the quantum dot active layer 58 that oscillates laser light is provided.

A contact layer 66 made of p ⁺ GaAs is formed on the p-type cladding layer 62. A drive electrode 12 is formed on the contact layer 66. An insulating film 68 made of a silicon oxide film is formed on the drive electrode 12. On the insulating film 68, for example, a heater portion 16 made of Pt (platinum) is provided. Drive power and heater power are applied to the drive electrode 12 and the heater unit 16 by the control unit 80. The electrode 69 is connected to a constant potential and is grounded, for example.

As shown in FIG. 6B, the p-type cladding layer 62 and the contact layer 66 have isolated mesa portions 72. The drive electrode 12 is formed on the mesa portion 72. The heater unit 16 is formed on the drive electrode 12 with an insulating film 68 interposed therebetween. The width W1 of the heater part 16 is narrower than the width W2 of the mesa part 72.

When drive power is applied to the drive electrode 12, a drive current flows between the drive electrode 12 and the electrode 69. As a result, stimulated emission occurs in the quantum dot active layer 58, and the laser beam 14 is emitted from the active layer 58. By applying the modulated drive power to the drive electrode 12, the intensity of the laser beam 14 emitted from the active layer 58 can be modulated.

As described above, the laser system 200 according to the first embodiment includes the DFB laser 10 and the control unit 80. The DFB laser 10 emits intensity-modulated laser light 14 from the active layer 58 by applying a modulated drive power to the drive electrode 12. The heater power 16 modulated to the same frequency as the modulation frequency of the drive power is applied to the heater unit 16 of the DFB laser 10 so that the temperature change of the active layer 58 due to the drive power is canceled and the temperature of the active layer 58 is maintained constant. Applied by the controller 80. Thereby, even when the intensity of the laser beam is modulated by modulating the driving power, the temperature of the active layer 58 can be kept constant, and the fluctuation of the wavelength of the laser beam 14 can be suppressed.

Therefore, the wavelength of the laser beam 14 can be kept within a wavelength range in which the conversion from the fundamental wave to the harmonic by the harmonic generation element 30 can be performed with high conversion efficiency. Therefore, the harmonic generation element 30 can convert the laser light 14 into green light 32 that is the second harmonic of the laser light 14 with high conversion efficiency.

In the first embodiment, the harmonic generation element 30 describes the case where the laser light 14 is converted into the green light 32 that is the second harmonic of the laser light 14, but the present invention is not limited thereto. For example, the harmonic generation element 30 may convert the laser light 14 into higher-order harmonics of the laser light 14, and the harmonic light may be visible light other than green light. However, from the viewpoint of conversion efficiency, the harmonic generation element 30 preferably converts the laser light 14 into the second harmonic of the laser light 14. In addition, since a semiconductor laser that emits green light is not realized, it is preferable that the harmonic light emitted from the harmonic generation element 30 is green light.

6A and 6B, the drive electrode 12 and the heater unit 16 are provided on the mesa unit 72. Thereby, the drive electrode 12 and the heater unit 16 which are heat sources can be made small with respect to the spread of heat caused by the drive power and the heater power, and the heat capacity of the active layer 58 with respect to the drive power and the heater power can be made comparable. it can.

Further, by reducing the drive electrode 12 and the heater unit 16 that are heat sources with respect to the spread of heat, the time constant of the temperature change of the active layer 58 based on the drive power and the temperature change of the active layer 58 based on the heater power are reduced. The time constant can be made comparable. When the modulation frequency of the driving power and the heater power is, for example, 500 MHz, the response of heat is slow. Therefore, the time constant of the temperature change of the active layer 58 based on the driving power and the heater power is in the range of, for example, 10 kHz to 100 kHz. For example, 25 kHz. As described above, since the time constants of the temperature change of the active layer 58 based on the drive power and the heater power are approximately the same, the frequency response characteristics of the temperature change of the active layer 58 due to the drive power and the temperature of the active layer 58 due to the heater power. It becomes possible to make the frequency response characteristics of the changes similar, and by modulating the heater power with a signal inverted in the same form as the modulation signal of the driving power, it is possible to easily cancel the temperature change of the active layer due to the driving power.

Therefore, for example, the control unit 80 applies the heater power P _heater complementary to the modulation signal of the drive power P _DFB to the heater unit 16 as shown in FIG. In other words, the control unit 80 supplies the heater power P _heater obtained by inverting the drive power P _DFB to the half of the power P _fix (P _fix / 2) at which the aP _DFB + bP _heater described above is constant. Apply to. Thereby, the temperature change of the active layer 58 due to the driving power can be canceled, and the temperature of the active layer 58 can be kept constant.

As described above, it is preferable that the drive electrode 12 and the heater unit 16 are formed at the same position and are sufficiently small with respect to the heat spread due to the drive power and the heater power. Thereby, the heat capacity of the active layer 58 with respect to the driving power and the heater power can be made comparable. In addition, the time constant of the temperature change of the active layer 58 based on the driving power and the heater power can be made comparable.

The drive electrode 12 and the heater part 16 are preferably provided in the same shape. For example, when the drive electrode 12 extends and is provided, the heater part 16 extends along the drive electrode 12. It is preferable. Further, as shown in FIG. 8, when the drive electrode 12 is formed only on the upper surface of the mesa portion 72, the width of the heater portion 16 is preferably smaller than three times the width W of the mesa portion 72. More preferably, the width of the heater portion 16 is smaller than twice the width W of the mesa portion 72. More preferably, the width of the heater portion 16 is smaller than the width W of the mesa portion 72.

In the first embodiment, the DFB laser 10 is a QD-DFB laser as an example. However, the present invention is not limited to this. For example, a QW (quantum well) -DFB laser may be used. Even in the case of the QW-DFB laser, the temperature change of the active layer due to the modulated driving power can be canceled, and the effect of suppressing the wavelength variation of the laser light can be obtained.

In the QW-DFB laser, not only the temperature of the active layer but also the number of carriers affects the oscillation wavelength of the laser beam. When modulating the driving power to modulate the intensity of the laser light, according to the first embodiment, the temperature of the active layer can be kept constant, but it is difficult to control the number of carriers to be constant. Therefore, in the case of the QW-DFB laser, the wavelength fluctuation of the laser beam can be suppressed, but it is difficult to completely eliminate the wavelength fluctuation. On the other hand, in the QD-DFB laser, the main factor affecting the oscillation wavelength of the laser light is the temperature of the active layer. Therefore, in Example 1 in which the temperature of the active layer can be kept constant, it is preferable to use a QD-DFB laser as the DFB laser 10. By using the QD-DFB laser, it is possible to further reduce the wavelength fluctuation of the laser beam when the drive power is modulated to modulate the intensity of the laser beam.

FIG. 9 is a conceptual diagram illustrating an example of a method for controlling the heater power by the control unit 80. FIG. 9A is a conceptual diagram illustrating an example of the first control method, and FIG. 9B is a conceptual diagram illustrating an example of the second control method. As shown in FIG. 9A, the first control method uses a multiplier 90 and an adder 92, and aV _DFB is calculated from the driving voltage V _DFB , the driving current I _DFB , the heater voltage V _heater , and the heater current I _heater. I _DFB + bV _heater _Determines the current size of I _heater . Then, the potential difference is obtained by the differential circuit 94 from the difference from the power P _fix for setting the active layer 58 to a constant temperature T _fix . The potential difference obtained by the differential circuit 94 is fed back, the magnitude of V _heater is controlled, and the heater power is applied to the heater section 16. Thereby, the magnitude of aV _DFB I _DFB + bV _heater I _heater can be made constant at P _fix , and the temperature of the active layer 58 can be kept constant at T _fix .

As shown in FIG. 9B, in the second control method, a calculation table for calculating the heater voltage V _heater and the heater current I _heater from the values of the drive voltage V _DFB and the drive current I _DFB is created in advance. Then, from the driving power applied to the driving electrode 12 (P _DFB = V _DFB I _DFB ), using the calculation table, V _heater and I _heat are calculated, and the heater power (P _heater = V _heater I _heater ) is applied. . This also makes it possible to make the size of aV _DFB I _DFB + bV _heater I _heater constant at P _fix and keep the temperature of the active layer 58 constant at T _fix . Examples of the calculation table include V _heater = (P _fix -aV _DFB I _DFB ) / bR _heater and I _heater = V _heater / R _heater . R _heater is the resistance of the heater section 16. a and b are the proportional coefficients described in FIG.

In the laser system 200 according to the first embodiment, the case where the harmonic generation element 30 is provided as illustrated in FIG. 5 has been described as an example, but the present invention is not limited thereto. As shown in FIG. 10, the laser system may include the DFB laser 10 and the control unit 80 without the harmonic generation element 30. Even in this case, the intensity of the laser beam can be modulated and the wavelength variation of the laser beam can be suppressed. Therefore, the laser system as shown in FIG. 10 can also be used as a communication laser system in a 1.55 μm band or a 1.3 μm band, for example.

The preferred embodiments of the present invention have been described in detail above, but the present invention is not limited to such specific embodiments, and various modifications can be made within the scope of the gist of the present invention described in the claims.・ Change is possible.

Claims

A DFB laser having a semiconductor layer including an active layer that emits laser light whose intensity is modulated by modulating driving power, and a heater unit for maintaining the temperature of the active layer constant;
A control unit that applies a heater power modulated to the same frequency as the modulation frequency of the drive power to the heater unit so as to cancel the temperature change of the active layer due to the modulated drive power. A laser system.
The laser system according to claim 1, wherein the controller applies the heater power complementary to the modulated driving power to the heater section.
3. The laser system according to claim 1, wherein the time constant of the temperature change of the active layer based on the driving power and the time constant of the temperature change of the active layer based on the heater power are approximately the same.
The semiconductor layer has an isolated mesa portion;
4. The laser system according to claim 1, wherein the driving electrode to which the driving power is applied and the heater unit are provided on the mesa unit. 5.
The laser system according to claim 4, wherein a width of the heater portion is smaller than three times a width of the mesa portion.
6. The laser system according to claim 4, wherein the heater section is provided along the drive electrode.
The laser system according to any one of claims 1 to 6, wherein the DFB laser is a quantum dot DFB laser.
The laser system according to claim 1, further comprising a harmonic generation element that converts the laser light into visible light that is a harmonic of the laser light.
9. The laser system according to claim 8, wherein the harmonic element converts the laser beam into a second harmonic of the laser beam.
10. The laser system according to claim 8, wherein the visible light is green light.