JP2016046276A - Wavelength sweeping type semiconductor laser element and gas concentration measuring apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a small wavelength sweeping type semiconductor laser element capable of improving detection accuracy for detecting a gas concentration, and a gas concentration measuring apparatus.SOLUTION: A wavelength sweeping type semiconductor laser element comprises an active layer of which the DC component is changed with time, clad layers between which the active layer is sandwiched, and a diffraction grating layer on which the laser light generated in the active layer, impinges, and a relationship between the laser light wavelength λM selected based on a lattice width of the diffraction grating layer and a peak wavelength λp of the laser light generated in the active layer satisfies λp-2nm≤λM<λp, in the air.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a wavelength sweep type semiconductor laser element and a gas concentration measuring apparatus using the same.

  2. Description of the Related Art Conventionally, there is known a gas concentration measurement device that obtains a gas concentration by irradiating a measurement target gas with a laser beam and measuring the transmitted light intensity. Since the absorbance of laser light is proportional to the gas concentration and the optical path length (Lambert-Beer law), the gas concentration can be determined if the absorbance at the light absorption wavelength of the gas to be measured is known.

  Therefore, in the conventional gas concentration measuring device, an absorption spectrum corresponding to the gas is obtained by sweeping the wavelength of the laser beam in the vicinity of the light absorption peak wavelength of the gas to be measured, and the gas is obtained from the absorbance indicated by the absorption spectrum. The concentration is obtained (see Patent Document 1 and Patent Document 2). In addition, in the conventional wavelength sweep of the laser beam, a laser beam having a different wavelength is generated by generating a harmonic using a nonlinear optical crystal separately from the laser light source, and the waveform sweep is performed.

JP 2001-074654 A JP 2007-78566 A

  However, although the conventional gas concentration measuring apparatus uses a semiconductor laser element as a small laser light source, it is necessary to use a large number of harmonic generators in order to perform wavelength sweeping. There was also room for improvement in gas concentration detection accuracy.

  The present invention has been made in view of such problems, and an object of the present invention is to provide a wavelength-swept semiconductor laser element and a gas concentration measuring apparatus that are small in size and capable of improving gas concentration detection accuracy. To do.

  In order to solve the above-described problem, a wavelength-swept semiconductor laser device according to an aspect of the present invention includes an active layer to which a current whose DC component changes over time, a cladding layer sandwiching the active layer, and the active layer A diffraction grating layer on which the laser beam generated in the laser beam is incident, the wavelength λM of the laser beam selected based on the grating width of the diffraction grating layer, and the peak wavelength λp of the laser beam generated in the active layer The relationship is characterized by satisfying λp−2 nm ≦ λM <λp in air.

  The basic structure of a semiconductor laser device has a structure in which an active layer is sandwiched between clad layers. Carriers injected into the active layer through the clad layer recombine to emit light, and this light emission is caused by stimulated emission and resonance. Amplified and laser light is output to the outside. Laser light generated in the active layer enters the diffraction grating layer. The diffraction grating layer intensifies and selectively amplifies light having a wavelength twice that of the grating width, so that laser light having a selected single wavelength component is amplified and output to the outside.

  On the other hand, the wavelength of the laser beam generated uniquely in the active layer is determined based on the energy band gap of the active layer. When light is emitted due to a simple resonator structure in the active layer, the laser light in the active layer contains components that are generated when it functions as a Fabry-Perot laser. In addition to these wavelengths, a component having a plurality of wavelengths is included in addition to this wavelength as a central peak wavelength.

  These plural wavelength components are noises in the gas concentration measurement and are preferably reduced. In the present invention, the wavelength defined by the diffraction grating layer is set smaller than the peak wavelength of laser light oscillation in the active layer. In this case, a plurality of wavelength components of laser light generated as a Fabry-Perot laser is suppressed, and a single wavelength component defined by the diffraction grating layer tends to be dominant. This single wavelength component can be swept precisely as the direct current component of the current supplied to the active layer changes.

  Since the wavelength-swept semiconductor laser device having this structure can be swept independently, it is smaller than a product using a conventional harmonic generator and can precisely sweep a single wavelength component. Therefore, when this is used in a gas concentration measuring device, the detection accuracy of the gas concentration can be improved.

  As described above, when the wavelength specified by the diffraction grating layer is smaller than the peak wavelength of oscillation in the active layer, there is a possibility of oscillation with a single wavelength component due to the diffraction grating layer. However, if it is too small, the selected wavelength component is not emphasized by the diffraction grating layer, and the ratio of the laser light component generated as a Fabry-Perot laser may increase. Therefore, in the aspect of the present invention, the wavelength specified by the diffraction grating layer is set to be smaller than the peak wavelength of oscillation in the active layer by 2 nm or less in air, and λp−2 nm ≦ λM <λp is set. Satisfies.

  In this case, the ratio of laser light components generated as a Fabry-Perot laser can be suppressed, and wavelength sweeping can be performed at a single stable wavelength.

  A gas concentration measuring apparatus according to an aspect of the present invention includes the above-described wavelength sweep type semiconductor laser element, a drive circuit that supplies a current whose DC component changes over time to the wavelength sweep type semiconductor laser element, A photodetection element that detects laser light output from a wavelength-swept semiconductor laser element via a gas to be measured, and a control device that outputs the concentration of the gas based on the output of the photodetection element It is characterized by providing.

  The drive circuit supplies a current whose DC component changes over time to the wavelength-swept semiconductor laser element. When the direct current component of the current increases, the temperature of the wavelength-swept semiconductor laser element increases. In general, the energy band gap of a semiconductor decreases with increasing temperature. Since there is a relationship of λ = 1240 / Eg between the energy band gap Eg and the wavelength λ corresponding to Eg, the wavelength λ becomes longer as Eg becomes smaller. In addition, since the diffraction grating width of the diffraction grating layer increases as the temperature rises, the wavelength selected by the diffraction grating layer also increases. Therefore, when the direct current component of the current supplied from the drive circuit to the active layer is increased, the wavelength of the laser light can be increased and wavelength sweeping can be performed.

  When the laser light output from the wavelength sweep type semiconductor laser element passes through the gas to be measured, the wavelength component specific to the gas is absorbed by this gas. Since the absorbance of the laser light is proportional to the gas concentration, the transmitted light of the gas is detected by the light detection element. If a single wavelength can be swept linearly with respect to time, the concentration of gas can be accurately measured. Since the control device calculates and outputs the gas concentration based on the output of the light detection element, the gas concentration can be accurately obtained.

  According to the wavelength sweep type semiconductor laser device and the gas concentration measuring apparatus of the present invention, it is small and the accuracy of detecting the gas concentration can be improved.

It is a block diagram of a gas concentration measuring device. It is a front view of a wavelength sweep type semiconductor laser device. It is a figure which shows the cross-sectional structure along the III-III arrow of the wavelength sweep type semiconductor laser element shown in FIG. It is a graph which shows the relationship between the wavelength of the laser beam generate | occur | produced in an active layer, and a gain. It is a graph which shows the relationship between the electric current If [mA] supplied to the laser element, the output intensity Po [mW], and the output intensity differential value SE [W / A] of the photodetecting element in the example. In an Example, it is a graph which shows the laser beam spectrum (relationship of wavelength [nm] and intensity [dBm]) at the time of changing temperature. In Comparative Example 1, the current If [mA] and the fluctuation current Im supplied to the laser element, the output intensity Po [mW], the output intensity differential value SE [W / A], the voltage Vf [V], and It is a graph which shows the relationship of resistance Rd [ohm]. It is a graph which shows the relationship between the electric current If [mA] supplied to a laser element, the output intensity Po [mW] of an optical detection element, and output intensity differential value SE [W / A]. In Comparative Example 1, a graph showing the relationship between the horizontal spread angle [degree] of the far-field image (FFP) and the intensity of the laser light (FIG. 5A), and the vertical direction of the far-field image (FFP) 5 is a graph showing the relationship between the spread angle [degree] of the laser beam and the intensity of the laser beam ((B) in the figure). 6 is a graph showing a laser beam spectrum (relationship between wavelength [nm] and intensity [dBm]) in Comparative Example 1; 10 is a graph showing a relationship between a current If [mA] supplied to a laser element, an output intensity Po [mW] and an output intensity differential value SE [W / A] of a light detection element in Comparative Example 2. In comparative example 2, it is a graph which shows a laser beam spectrum (relationship of wavelength [nm] and intensity [dBm]) at the time of changing temperature. The current [mA], the LD output and the PD output [a. u. It is a graph which shows the relationship with]. It is a graph which shows the relationship between the electric current [mA] in an Example, and the value (normalized value) of the secondary differentiation of PD output. It is a graph which shows the relationship between the electric current [mA] in a comparative example, and the value (normalized value) of the secondary differentiation of PD output. It is a figure for demonstrating the manufacturing method of a wavelength sweep type semiconductor laser element.

  Hereinafter, the wavelength sweep type semiconductor laser device and the gas concentration measuring apparatus according to the embodiment will be described. In addition, the same code | symbol is used for the same element and the overlapping description is abbreviate | omitted.

  FIG. 1 is a block diagram of a gas concentration measuring apparatus.

  In a factory having a combustion furnace or a reaction furnace, the gas G is released from the chimney. If the concentration of each component of such a gas G can be measured, the operating conditions of the factory can be adjusted based on the measured gas concentration, and a clean outside air environment can be maintained. Although the gas concentration measuring apparatus of this embodiment is used for such a use, it is not limited to this.

  This gas concentration measuring apparatus includes a wavelength sweep type semiconductor laser element 1, a drive circuit 5 that supplies a current whose DC component changes over time to the wavelength sweep type semiconductor laser element 1, and the wavelength sweep type semiconductor laser element 1. A light detection element 2 that detects the output laser light via a gas to be measured, and a control device 4 that outputs the gas concentration based on the output of the light detection element 2 are provided.

  The laser light output from the wavelength sweeping type semiconductor laser device 1 enters the gas passage through one window 9 provided on the wall surface of the gas passage such as a chimney, travels in the gas, The light is output from the window 9 and enters the light detection element 2. The light detection element 2 is preferably a photodiode, but any element other than a photodiode may be employed as long as it is an element that outputs an electrical signal in response to incident laser light. For example, it is possible to employ a photomultiplier tube, a CCD image sensor, a MOS image sensor, or a photoconductive element whose resistance value changes according to the incident light intensity.

  The output of the light detection element 2 is input to the lock-in amplifier 3, amplified in a state where noise is reduced, and input to the control device 4. The control device 4 is a computer or the like, and an internal program operates in accordance with a command instructed from the input device 7 and outputs a control signal to an external device connected thereto. The output of the light detection element 2 can be monitored by directly connecting a display device such as an oscilloscope to this, but in this example, the output from the light detection element 2 is input to the control device 4. The gas absorption spectrum is obtained.

  Since the wavelength sweep type semiconductor laser element 1 performs wavelength sweep, the storage device in the control device 4 stores the relationship between the wavelength of the laser beam and the output of the light detection element 2 in association with each other. Become. Since the output of the light detection element 2 decreases as the gas concentration increases with respect to the same laser light intensity, the absorbance is indirectly indicated. The control device 4 calculates the gas concentration from the absorbance and inputs it to the display device 8. The display device 8 displays the specified gas concentration and the like.

  When the laser light output from the wavelength-swept semiconductor laser element 1 passes through the gas G to be measured, the wavelength component specific to the gas is absorbed by this gas. Since the absorbance of the laser light is proportional to the gas concentration, the light detection element 2 detects the transmitted light of the gas G. Here, if the single wavelength can be swept linearly with respect to time, the concentration of the gas G can be accurately measured. Since the control device 4 calculates and outputs the gas concentration based on the output of the light detection element 2, the gas concentration can be determined accurately. Since the absorbance of the laser beam is proportional to the concentration of the gas G and the optical path length (Lambert-Beer law), the gas concentration can be obtained if the absorbance at the light absorption wavelength of the gas G to be measured is known. Since the control device 4 stores the correlation between the absorbance and the gas concentration, the control device 4 can obtain the gas concentration and output it to the display device 8.

It should be noted that the concentration C of the specific gas, the optical path length L1 of the laser beam (the optical path length from the wavelength sweep type semiconductor laser element 1 to the photodetection element 2), the absorption coefficient α of the specific gas, and the output intensity P of the photodetection element 2 _PD , the intensity P _{LD of the} laser beam output from the wavelength sweep type semiconductor laser element 1 satisfies the following relational expression. As a specific gas, oxygen is often used for measurement.

C = − (1 / αL1) × ln (P _PD / P _LD )

  The control device 4 can include a function generator. The function generator can generate an AC voltage signal having an arbitrary frequency and waveform. The frequency and waveform of the function generator can be set by the input device 7.

  The drive circuit 5 generates a drive current for the wavelength sweep type semiconductor laser device 1 based on a control signal from the control device 4. The drive current may be synchronized with the output of the function generator. The drive circuit 5 supplies a current whose DC component changes with time to the wavelength-swept semiconductor laser element 1. The drive current is a high-frequency pulse current, and the direct current component may increase with time or the direct current may increase with time.

  When the direct current component of the drive current increases, the temperature of the wavelength sweep type semiconductor laser device 1 rises. In general, the energy band gap of a semiconductor decreases with increasing temperature. Since there is a relationship of λ = 1240 / Eg between the energy band gap Eg and the wavelength λ corresponding to Eg, the wavelength λ of the laser light increases as Eg decreases. Further, since the diffraction grating width of the diffraction grating layer of the wavelength-swept semiconductor laser device 1 increases as the temperature rises, the wavelength selected by the diffraction grating layer also increases. Therefore, when the direct current component of the current supplied from the drive circuit 5 to the active layer of the wavelength sweep type semiconductor laser device 1 is increased, the wavelength of the laser beam can be increased and the wavelength sweep can be performed.

  The wavelength sweep type semiconductor laser device 1 is provided with a temperature control device 6. The temperature control device 6 is a Peltier element, a heat sink, or the like, and can perform temperature control based on a command from the control device 4, but does not perform rapid temperature increase control due to an increase in drive current. The temperature control is performed so that the temporal change for stabilizing the operation of the sweep type semiconductor laser device 1 is gentle.

  FIG. 2 is a front view of the wavelength sweep type semiconductor laser device 1, and FIG. 3 is a diagram showing a cross-sectional configuration along the III-III arrow of the wavelength sweep type semiconductor laser device 1 shown in FIG.

  The wavelength sweeping type semiconductor laser device 1 includes a buffer layer 11, a first cladding layer 12, a first light guide layer 13, an active layer 14, a second light guide layer 15, a second cladding layer 16, and a first layer on a support substrate 10. The first etching stop layer 17, the second etching stop layer 18, the third cladding layer 19, the diffraction grating layer 20, the fourth cladding layer 21, and the contact layer 22 are sequentially stacked.

  The region above the first etching stop layer 17 is etched so that a convex portion is formed (the width W of the convex portion is 4 μm), and a ridge type semiconductor laser element is formed. An insulating film 23 is formed on the first etching stop layer 17 and the side surfaces of the protrusions. An upper electrode layer 24 is formed on the insulating film 23, and the upper electrode layer 24 is in contact with the contact layer 22. . A back contact layer and / or a back electrode layer 25 is formed on the back surface of the support substrate 10.

  When a current is passed between the upper electrode layer 24 and the back electrode layer, hole-electron recombination occurs in the active layer 14 and the active layer 14 emits light. The active layer 14 of this example has a multiple quantum well structure, and is formed by sequentially laminating a first well layer 14a, a barrier layer 14b, and a second well layer 14c. The light generated in the active layer 14 also reaches the diffraction grating layer 20, and laser light having a specific wavelength corresponding to the diffraction grating width of the diffraction grating layer 20 is selectively amplified. Therefore, in the active layer 14, the laser beam having the wavelength selected in the diffraction grating layer 20 is finally output from the YZ end face.

  When the interval (grating width) along the X-axis direction of the diffraction grating layer 20 is Λd and the integer is m (see FIG. 3), m × 2 × Λd or m in the wavelength sweep type semiconductor laser device 1 The laser light having a wavelength of 1 / is amplified. A diffraction grating that is adjusted to a wavelength that is multiplied by m to a wavelength in which the order m is 1 is called an m-th order diffraction grating. Outside the wavelength-swept semiconductor laser device 1, assuming that the effective refractive index of the active layer 14 is neff, a laser beam of λM = 2 × Λd × neff / m is emitted.

  As described above, the basic structure of the wavelength-swept semiconductor laser device 1 includes the active layer 14 to which a current whose DC component changes over time, the clad layer 12.16 sandwiching the active layer 14, and the active layer 14. And a diffraction grating layer 20 on which the generated laser light is incident.

  The basic structure of the semiconductor laser element has a structure in which the active layer 14 is sandwiched between the clad layers 12 and 16, and carriers injected into the active layer 14 through the clad layers 12 and 16 recombine to emit light, This light emission is amplified by stimulated emission and resonance (resonance length L = 1000 μm), and laser light is output to the outside. Laser light generated in the active layer 14 enters the diffraction grating layer 20. Since the diffraction grating layer 20 selectively amplifies the light having a wavelength 2 × neff / m times the grating width Λd, the laser light having the selected single wavelength component is amplified and output to the outside. The

  Here, the relationship between the wavelength λM of the laser beam selected based on the grating width Λd of the diffraction grating layer 20 and the peak wavelength λp of the laser beam generated in the active layer 14 is λp−2 nm ≦ λM in the air. <Λp is satisfied.

  FIG. 4 is a graph showing the relationship between the wavelength of laser light generated in the active layer and the gain. For the structure of each layer, refer to FIGS. 2 and 3 as appropriate.

  The wavelength of the laser beam generated uniquely in the active layer 14 is determined based on the energy band gap of the active layer 14. When light is emitted due to a simple resonator structure in the active layer 14, the laser light in the active layer 14 includes a component that is generated when it functions as a Fabry-Perot laser. In addition to the 14 unique wavelengths, a component having a plurality of wavelengths is included in addition to this wavelength as a central peak wavelength.

  These plural wavelength components are noises in the gas concentration measurement and are preferably reduced. In this embodiment, the wavelength λM defined by the diffraction grating layer 20 is set to be smaller than the peak wavelength λp of laser light oscillation in the active layer 14.

  When the wavelength λH defined by the diffraction grating layer 20 is equal to or greater than the peak wavelength λp of the laser light oscillation in the active layer 14, laser light having a plurality of wavelength components is generated as a Fabry-Perot laser. On the other hand, when the wavelength defined by the diffraction grating layer 20 is λM, a plurality of wavelength components of the laser light generated as a Fabry-Perot laser is suppressed, and the single wavelength component λM defined by the diffraction grating layer 20 dominates. Tend to be. This single wavelength component can be swept precisely as the direct current component of the current supplied to the active layer 14 changes.

  Since the wavelength-swept semiconductor laser device 1 having this structure can be swept independently, it is smaller than a product using a conventional harmonic generator and can accurately sweep a single wavelength component. Therefore, when this is used for a gas concentration measuring device, the detection accuracy of the gas concentration can be improved.

  When the wavelength λM defined by the diffraction grating layer 20 is smaller than the oscillation peak wavelength λp in the active layer 14, there is a possibility of oscillation with a single wavelength component due to the diffraction grating layer 20, When it is too small (when the wavelength specified by the diffraction grating layer 20 is λL), the selected wavelength component is not emphasized by the diffraction grating layer 20 and the ratio of the laser light component generated as a Fabry-Perot laser increases. There is. Therefore, the wavelength λM defined by the diffraction grating layer 20 is set to be 2 nm or less smaller than the oscillation peak wavelength λp in the active layer 14 in the air, and satisfies λp−2 nm ≦ λM <λp. That is, Δλ = 2 nm in FIG.

  In this case, the ratio of laser light components generated as a Fabry-Perot laser can be suppressed, and wavelength sweeping can be performed at a single stable wavelength.

The material / thickness of each of the above layers is as follows. Note that the same effect can be achieved with respect to thickness and composition even when there is an error of ± 10%.
Support substrate 10: N-type GaAs
Buffer layer 11: N-type GaAs (Si doped) /1.0 μm
First clad layer 12: N-type AlGaAs (Si doped) /2.0 μm
First light guide layer 13: P-type AlGaAs (Al composition 38%: non-doped) / 60 nm
First well layer 14a: P-type AlGaAs (Al composition 20%: non-doped) / 9 nm
Barrier layer 14b: P-type AlGaAs (Al composition 38%: non-doped) / 9 nm
Second well layer 14c: P-type AlGaAs (Al composition 20%: non-doped) / 9 nm
Second light guide layer 15: P-type AlGaAs (Al composition 38%: non-doped) / 60 nm
Second clad layer 16: P-type AlGaAs (Al composition 48%: Zn doped) / 70 nm
First etching stop layer 17: P-type AlGaAs (Al composition 70%: Zn doped) / 20 nm
Second etching stop layer 18: P-type AlGaAs (inclined toward the top so that the Al composition becomes 70% to 48%: Zn doped) / 50 nm
Third cladding layer 19: P-type AlGaAs (Al composition 48%: Zn doped) / 140 nm
Diffraction grating layer 20: P-type AlGaAs (Al composition 30%: Zn doped) / 50 nm
Fourth cladding layer 21: P-type AlGaAs (Al composition 48%: Zn doped) /1.2 μm
Contact layer 22: P-type GaAs (Zn doped) /0.2 μm
Insulating film 23: made of SiNx, but other insulating materials can also be used.
Upper electrode layer 24: AuGe or the like can be used, but other insulating materials can also be used. The back electrode layer can also be composed of the same or similar material such as Ag.

Regarding the impurity concentration of these compound semiconductor layers, the concentration of Si added in the case of N type is 1 × 10 ¹⁸ / cm ³ in each layer, and the concentration of Zn added in the case of P type is in each layer. The impurity concentration is 1 × 10 ¹⁸ / cm ³ , and the same effect can be obtained even when these impurity concentrations have an error of ± 30%. Further, even if the P-type and N-type conductivity types are switched, the laser element operates.

  FIG. 5 shows the current If [mA] supplied to the laser element, the output intensity Po [mW] and the output intensity differential value SE [W / W] supplied to the laser element in the example (selected wavelength in the diffraction grating layer = λM). It is a graph which shows the relationship of A]. When no gas is introduced, these values are the same as the output intensity and output intensity differential value of the laser beam.

  In the embodiment, λM smaller than λp is adopted as the selection wavelength in the diffraction grating layer 20 (see FIG. 4). Specifically, λp = 760.6 nm and λM = 759.2 nm.

  As the current If increases, the temperature of the wavelength sweep type semiconductor laser device 1 rises. It is preferable that the output intensity increases linearly with respect to the current If. That is, the output intensity differential value SE is desirably a constant value after the rise of the laser beam. In the example shown in FIG. 5, the output intensity differential value SE is almost constant. Within the temperature range of 15 ° C. to 35 ° C., the fluctuation range of SE after rising of the laser beam is 0.73 W / A to 0.74 W / A at 15 ° C., and 0.62 W / A to 0.64 W / A at 35 ° C. A.

  Further, even in a device having a wavelength difference of λM = 766.2 nm, which is exactly λp−2 nm, the fluctuation range of the output intensity differential value SE excluding noise is 0.65 W / A to 0.76 W at 15 ° C., respectively. / A at 35 ° C. was obtained from 0.50 W / A to 0.60 W / A.

  FIG. 6 is a graph showing a laser light spectrum (relationship between wavelength [nm] and intensity [dBm]) when the temperature is changed in the example.

  It can be seen that the peak wavelength of the laser light spectrum increases as the temperature rises. Thus, the wavelength of the laser beam can be swept by the temperature rise accompanying the increase in current. Specifically, when the temperature rises to 15.0 ° C., 20.2 ° C., 25.2 ° C., 29.9 ° C., 25.3 ° C., 40.3 ° C., the wavelengths are 758.62 nm, 758.94 nm, 759.18 nm, 759.44 nm, 759.78 nm, and 760.14 nm. The wavelength distribution is symmetric with respect to the peak position.

  FIG. 7 shows the current If [mA] and the fluctuation current Im supplied to the laser element, the output intensity Po [mW], the output intensity differential value SE [W / A], and the voltage Vf supplied to the laser element in Comparative Example 1. It is a graph which shows the relationship between [V] and resistance Rd [Ω].

  In Comparative Example 1, λH larger than λp is adopted as the selection wavelength in the diffraction grating layer 20 (see FIG. 4). Specifically, λp = 755.6 nm and λH = 758.5 nm.

  The output intensity differential value SE cannot hold a constant value after the rise of the laser beam. Further, the voltage Vf [V] and the resistance Rd [Ω] indicate 1.68 V and 1.6 Ω, respectively, and it can be seen from the graph that there is no change in the voltage and resistance value corresponding to the wavelength parameter change.

  As shown in FIG. 8, when λH = λp, the fluctuation range of the output intensity differential value SE is 0.71 W / A to 0.91 W / A at 15 ° C., and 0.52 W / A to 0.71 W at 35 ° C. / A.

  FIG. 9 is a graph (FIG. 9A) showing the relationship between the horizontal spread angle [degree] of the far-field image (FFP) and the intensity of the laser light in Comparative Example 1, and the far-field image (FFP). ) Is a graph showing the relationship between the vertical spread angle [degree] and the intensity of laser light ((B) in the figure). In Comparative Example 1, a laser beam according to a Gaussian distribution was obtained. Also in the examples, laser light having a similar distribution was obtained.

  FIG. 10 is a graph showing a laser beam spectrum (relationship between wavelength [nm] and intensity [dBm]) in Comparative Example 1.

  From this graph, it can be seen that in Comparative Example 1, the ratio of EL emission in the spectrum is high. From the above, it was found that the example was superior to the comparative example 1 in terms of spectral wavelength unity.

  FIG. 11 is a graph showing the relationship between the current If [mA] supplied to the laser element, the output intensity Po [mW] and the output intensity differential value SE [W / A] of the light detection element in Comparative Example 2. .

  In Comparative Example 2, λL, which is significantly lower than λp, is adopted as the selected wavelength in the diffraction grating layer 20 (see FIG. 4). Specifically, λp = 763.5 nm and λL = 759.6 nm.

  As the current If increases, the temperature of the wavelength sweep type semiconductor laser device 1 rises. It is preferable that the output intensity increases linearly with respect to the current If. That is, the output intensity differential value SE is desirably a constant value after the rise of the laser beam, but in the example shown in FIG. 11, the output intensity differential value SE gradually decreases.

  FIG. 12 is a graph showing a laser beam spectrum (relationship between wavelength [nm] and intensity [dBm]) when the temperature is changed in Comparative Example 2.

  As the temperature rises, the peak wavelength of the laser light spectrum increases. Specifically, when the temperature rises to 15.0 ° C., 20.2 ° C., 25.2 ° C., 30.0 ° C., the wavelengths become 759.02 nm, 759.32 nm, 763.52 nm, and 765.38 nm. Thus, the wavelength of the laser light can be swept by the temperature rise accompanying the increase in current, but the wavelength distribution is asymmetric with respect to the peak position. In this case, there is a problem that the wavelength sweep in the oxygen absorption measurement becomes discontinuous.

FIG. 13 shows the current [mA], the output (LD output) P _{LD of} the wavelength-swept semiconductor laser device 1 and the output (PD output) P _PD [a. u. It is a graph which shows the relationship with].

  The PD output decreases when the current supplied to the wavelength sweep type semiconductor laser device 1 is in the vicinity of 80 mA, and it is considered that there is a gas absorption band in the vicinity of the corresponding wavelength.

  FIG. 14 is a graph showing the relationship between the current [mA] and the secondary differential value (standardized value) of the PD output in the example.

The PD output is symmetrical about the vicinity of 80 mA. The amount of change in the transmission optical density corresponds to the absorbance, and the gas concentration C is calculated from the absorbance. The conversion formula is given by C = − (1 / αL1) × ln (P _PD / P _LD ). In order to obtain the gas concentration C from the graph shown in FIG. 14, the measurement data at a known oxygen concentration may be compared with the change in signal intensity from the background of the measurement data. The type of gas is oxygen, which can be identified by comparing the wavelength at which absorption occurs with the absorption wavelength inherent in the gas. In this measurement, a TDLAS (wavelength tunable semiconductor laser absorption spectroscopy) method can be used.

  FIG. 15 is a graph showing the relationship between the current [mA] and the second derivative value (standardized value) of the PD output in the comparative example.

  The PD output is asymmetrical about the vicinity of 80 mA. Therefore, the precise gas concentration C cannot be obtained according to the logic shown in FIG.

  FIG. 16 is a diagram for explaining a method of manufacturing a wavelength sweep type semiconductor laser device.

When manufacturing the above-described wavelength-swept semiconductor laser device, first, as shown in (A), a buffer layer 11, a first cladding layer 12, a first light guide layer 13, an active layer 14, The second light guide layer 15, the second cladding layer 16, the first etching stop layer 17, the second etching stop layer 18, the third cladding layer 19, and the layer that is the source of the diffraction grating layer 20 (the same as the diffraction grating layer) Are sequentially laminated. For the growth of each layer, MOCVD (metal organic chemical vapor deposition) is used. Since each semiconductor layer is formed by an element selected from Al, Ga, and As, TMA (trimethylaluminum), TMG (trimethylgallium), and arsine (AsH ₃ ) can be used as respective raw materials. The growth temperature of GaAs is 700 ° C., and the growth temperature of AlGaAs is 700 ° C. For the dopants Zn and Si, DMZn (dimethyl zinc) and monosilane (SiH ₄ ) were used as raw materials, respectively.

  Next, as shown in (B), a photoresist is applied on the layer that is the base of the diffraction grating layer 20, and this is exposed using an interference exposure method, and then developed, and the diffraction grating is applied to the photoresist. Form a pattern. Next, the diffraction grating layer 20 is formed by etching the original layer of the diffraction grating layer 20 using this diffraction grating pattern. The etchant is bromomethanol.

  Thereafter, as shown in (C), the fourth cladding layer 21 and the contact layer 22 are sequentially laminated. For the growth of each layer, the above-described MOCVD method is used, the growth temperature of AlGaAs constituting the cladding layer is 700 ° C., and the growth temperature of GaAs constituting the contact layer is 700 ° C.

  As described above, the above-described wavelength-swept semiconductor laser element is a distributed feedback (DFB) laser using a diffraction grating layer. When the current was increased in a ramp shape, kink did not occur until the LD output was about 40 mW, and the current-output efficiency was constant. Moreover, in Comparative Example 2, the distortion (FIG. 15) that appeared in the absorption curve could be eliminated, and a highly accurate absorption curve (FIG. 14) could be obtained. The above-described element can be applied to a 760 nm DFB-LD for oxygen absorption analysis, but can also be applied to other current sweep type DFB-LDs.

  The above numerical values can have an error of ± 10% unless otherwise specified.

  DESCRIPTION OF SYMBOLS 10 ... Support substrate, 11 ... Buffer layer, 13 ... 1st light guide layer, 14 ... Active layer, 15 ... 2nd light guide layer, 12, 16 ... Cladding layer, 17 ... 1st etching stop layer, 18 ... 2nd Etching stop layer, 19 ... cladding layer, 20 ... diffraction grating layer, 21 ... cladding layer (buried layer), 22 ... contact layer, 23 ... insulating film, 24 ... upper electrode layer.

Claims (2)

An active layer to which a current whose DC component changes over time is provided;
A clad layer sandwiching the active layer;
A diffraction grating layer on which laser light generated in the active layer is incident;
With
The relationship between the wavelength λM of the laser beam selected based on the grating width of the diffraction grating layer and the peak wavelength λp of the laser beam generated in the active layer satisfies λp−2 nm ≦ λM <λp in the air. A wavelength-swept semiconductor laser device characterized by that. A wavelength-swept semiconductor laser device according to claim 1,
A drive circuit for supplying a current whose DC component changes over time to the wavelength-swept semiconductor laser element;
A light detection element for detecting the laser beam output from the wavelength-swept semiconductor laser element via a gas to be measured;
A control device for outputting the concentration of the gas based on the output of the light detection element;
A gas concentration measuring device comprising: