JP2011118045A - Laser light source and gas detection apparatus using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a laser light source for detecting with high accuracy the gas which is to be detected and the central wavelength of whose absorption line is shorter than 1.2 μm and to provide a gas detection apparatus using the laser light source. <P>SOLUTION: The laser light source includes: a semiconductor light emitting element 11 for emitting the laser light the wavelength of which is modulated to have predetermined wavelength width (modulation frequency); a light emitting element drive part 13 for applying a drive current I<SB>d</SB>to the semiconductor light emitting element 11 in order to generate the wavelength-modulated laser light; and a wavelength conversion element 12 for converting the wavelength of the wavelength-modulated laser light into the shorter wavelength by a nonlinear optical effect. The wavelength of the wavelength-modulated laser light is included only on the wavelength side longer or shorter than the wavelength when the conversion efficiency of the wavelength conversion element 12 becomes maximum. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a laser light source and a gas detection device using the same, and more particularly to a laser light source for performing gas detection using an absorption line of a detection target gas and a gas detection device using the same.

  In recent years, with respect to environmental problems, a technique for detecting a specific gas in the atmosphere is required for reducing greenhouse gas emissions, monitoring air pollutants, and the like. In detecting gas, chemical gas detectors such as gas chromatographs have been used conventionally. However, it has been difficult to perform simple measurement and quick measurement with a gas detector that performs such chemical measurement.

  By the way, it is known that gases such as methane, oxygen, carbon dioxide, acetylene, and ammonia absorb light having a specific wavelength according to molecular rotation, vibration between constituent atoms, and the like. In general, gas molecules absorb electromagnetic waves in the infrared region. For example, methane absorbs light with wavelengths of 1.6 μm, 3.3 μm, and 7 μm, and oxygen absorbs light with a wavelength of 760 nm.

  Therefore, a spectroscopic gas detector utilizing this phenomenon has been developed. Such a spectroscopic gas detection device can detect the presence and concentration of a detection target gas extremely simply and quickly by emitting a laser beam having a specific wavelength into the measurement target space and measuring the attenuation state thereof. it can.

  In general, the absorption line of gas is extremely steep, so in order to realize highly accurate gas detection, the laser light source used in the above-described spectroscopic gas detector can perform single longitudinal mode laser oscillation. Preferably there is.

  As a laser light source that meets such a demand, a distributed feedback (DFB) semiconductor laser can be cited. The distributed feedback semiconductor laser has a periodic structure along the light guiding direction in the optical waveguide, and realizes laser oscillation in a single longitudinal mode having a specific wavelength determined by the pitch length of the periodic structure and the refractive index of the optical waveguide. Is.

  As shown in FIG. 8, for example, a gas detection apparatus including a DFB semiconductor laser includes a semiconductor laser module 100, a light receiver 102, and a gas detection unit 103 (see, for example, Patent Document 1). In the gas detection device disclosed in Patent Document 1, laser light emitted from a semiconductor laser module 100 in which a DFB semiconductor laser is incorporated is transmitted through a measured gas 101 in a measurement atmosphere and is incident on a light receiver 102. The

Here, the gas to be measured 101 has, for example, an absorption characteristic A having a center wavelength λ ₀ shown in FIG. As shown in FIG. 9, the DFB semiconductor laser included in the semiconductor laser module 100 has a driving current modulated with an amplitude I _w and a modulation frequency f ₁ (for example, 10 kHz) around a center current value I ₀ (bias current value). By applying I _d, laser light that oscillates with an amplitude λ _w (for example, 10 pm) and a modulation frequency f ₁ around the center wavelength λ ₀ is emitted to the gas 101 to be measured.

Since the absorption characteristic A has a minimum point at the center wavelength λ ₀ , the laser light oscillating at the modulation frequency f ₁ passes through the gas to be measured 101, and thereby has a frequency f ₂ (= 20 kHz) twice the modulation frequency f _1. ) Vibration component.

  The light receiving element 102a of the light receiver 102 receives the above-described laser beam that has passed through the gas 101 to be measured and converts it into a received light current. The gas detection unit 103 converts the light reception current obtained by the light receiver 102 into a voltage signal by the current-voltage converter 104.

Further, the gas detection unit 103 includes a fundamental wave voltage signal h ₁ that is a signal component of the modulation frequency f ₁ = 10 kHz included in the voltage signal, and a frequency f ₂ (= 20 kHz) that is twice the modulation frequency f ₁ = 10 kHz. ) is adapted to detect respectively the second harmonic voltage signal h ₂ is the signal component by the fundamental wave signal detector 105 and the second harmonic signal detector 106. Then, the fundamental wave the second harmonic divider section 107, the ratio of the amplitude H ₁ of the second harmonic voltage signal h ₂ amplitude H ₂ and the fundamental wave voltage signal _{h 1 (H 2 / H 1} ) in the gas concentration A corresponding detection value D (= H ₂ / H ₁ ) is output.

JP 11-326199 A

  In the conventional gas detector disclosed in Patent Document 1, in order to detect, for example, an oxygen absorption line (760 nm), a DFB semiconductor laser having an oscillation wavelength of 760 nm made of a GaAs substrate and a material that can be epitaxially grown thereon is used. Can be considered.

  However, such a GaAs-based DFB semiconductor laser has a semiconductor crystal structure caused by generation of crystal defects as compared with an InP-based DFB semiconductor laser having an oscillation wavelength of 1.2 μm or more made of an InP substrate and a material that can be epitaxially grown thereon. Sudden degradation of optical characteristics is likely to occur. Therefore, when a GaAs-based DFB semiconductor laser is used as the light source of the gas detector, there is anxiety in terms of reliability.

  The present invention has been made to solve such a conventional problem, and uses a laser light source for detecting a detection target gas having a center wavelength of an absorption line shorter than 1.2 μm with high accuracy, and the same. An object of the present invention is to provide a gas detection device.

  The laser light source of the present invention includes a semiconductor light emitting element that emits laser light that is wavelength-modulated with a predetermined wavelength width, a wavelength conversion element that converts the wavelength of the wavelength-modulated laser light into a short wavelength by a nonlinear optical effect, And the wavelength of the wavelength-modulated laser light is included only on the long wave side or the short wave side from the wavelength at which the conversion efficiency of the wavelength conversion element is maximized.

  With this configuration, generation of a frequency component twice the modulation frequency in the laser light converted to a short wavelength by the wavelength conversion element can be suppressed. Therefore, the center wavelength of the absorption line can be obtained using an InP-based semiconductor light emitting element. Can realize a laser light source for a gas detection device capable of detecting a detection target gas shorter than 1.2 μm.

  The laser light source of the present invention may have a configuration in which the wavelength conversion element is an SHG element that converts the wavelength-modulated laser light into a second harmonic.

The laser light source of the present invention has a configuration in which the semiconductor light emitting element oscillates in a single longitudinal mode.
With this configuration, it is possible to realize a laser light source for a gas detection device for detecting a detection target gas with high accuracy.

  The gas detector of the present invention includes a light source unit that emits laser light, which is wavelength-modulated with a predetermined wavelength width around the wavelength of one absorption line of a plurality of absorption lines of the detection target gas, to a measurement atmosphere; A light receiving unit that receives the wavelength-modulated laser light transmitted through the measurement atmosphere as measurement light and outputs a light reception signal corresponding to the measurement light; and a modulation frequency of the wavelength modulation detected from the light reception signal. A gas detection device comprising: a gas detection unit that calculates a gas concentration of the detection target gas contained in the measurement atmosphere based on a ratio of a fundamental wave signal and a second harmonic signal. It has the structure which consists of a laser light source.

  With this configuration, since the laser light source is provided, it is possible to detect the detection target gas whose center wavelength of the absorption line is shorter than 1.2 μm with high accuracy.

  The present invention provides a laser light source for detecting a detection target gas having a center wavelength of an absorption line shorter than 1.2 μm with high accuracy and a gas detection device using the laser light source.

The block diagram which shows the structure of the laser light source of the 1st Embodiment of this invention. Sectional drawing along the waveguide direction of the light of the semiconductor light-emitting device used for the laser light source of the 1st Embodiment of this invention The perspective view of the wavelength conversion element used for the laser light source of the 1st Embodiment of this invention The side view which shows the structure of the laser light source of the 1st Embodiment of this invention. The side view which shows the other structure of the laser light source of the 1st Embodiment of this invention Graph showing wavelength dependence of conversion efficiency of wavelength conversion element, and waveform diagram showing waveform of transmitted light of wavelength conversion element The block diagram which shows the structure of the gas detection apparatus which concerns on this invention Block diagram showing the configuration of a conventional gas detector Graph showing the relationship between the absorption characteristics of the gas to be measured and the modulation signal

  Hereinafter, embodiments of a laser light source and a gas detector using the same according to the present invention will be described with reference to the drawings.

(First embodiment)
A first embodiment of a laser light source according to the present invention will be described with reference to FIGS. FIG. 1 is a block diagram showing a configuration of a laser light source 1 of the present embodiment.

That is, as shown in FIG. 1, a laser light source 1 includes a semiconductor light emitting element 11 that emits laser light that is wavelength-modulated with a predetermined wavelength width (modulation frequency), and a laser light source that generates the wavelength-modulated laser light. A light emitting element driving unit 13 that applies a driving current I _d to the semiconductor light emitting element 11 and a wavelength conversion element 12 that converts the wavelength of the wavelength-modulated laser light into a short wavelength by a nonlinear optical effect are provided. Here, the drive current I _d is a current modulated with the amplitude I _w and the modulation frequency f ₁ (for example, 10 kHz) around the center current value I ₀ .

  FIG. 2 is a cross-sectional view of the semiconductor light emitting device 11 taken along the light guiding direction. As shown in FIG. 2, the semiconductor light emitting element 11 is a DFB semiconductor laser that oscillates in a single longitudinal mode. For example, an n-type semiconductor substrate 111 made of n-type InP (indium phosphorus) is n-type. InP cladding layer 112, SCH layer (light confinement layer) 113 made of n-type InGaAsP (indium, gallium, arsenic, phosphorus), active layer 114 including a multiple quantum well layer (MQW) made of InGaAsP, and SCH made of p-type InGaAsP A layer 115, a p-type InP cladding layer 116, and a contact layer 117 made of InGaAs (indium gallium arsenide) are sequentially stacked.

  Further, a diffraction grating 118 is formed between the SCH layer 115 and the p-type InP cladding layer 116, an upper electrode 119 is formed on the contact layer 117, and a lower electrode 120 is deposited on the lower surface of the n-type semiconductor substrate 111. ing.

The semiconductor light emitting element 11 is formed by cleavage and includes a rear end face 11a and an emission end face 11b that constitute an optical resonator. The active layer 114 is provided from the rear end surface 11 a to the emission end surface 11 b, and generates a laser beam when the driving current I _d is supplied between the upper electrode 119 and the lower electrode 120 by the light emitting element driving unit 13. The generated laser light is emitted from the emission end face 11b. A low reflection film may be formed on the emission end face 11b, and a high reflection film may be formed on the rear end face 11a.

The wavelength λ of the laser light emitted from the semiconductor light emitting element 11 varies according to the drive current I _d described above. As a result, the light source unit 1 outputs laser light that vibrates at the center wavelength λ ₀ with the amplitude λ _w (for example, 10 pm) and the modulation frequency f ₁ . Here, the center wavelength λ ₀ is an oscillation wavelength when the center current value I ₀ is applied to the semiconductor light emitting element 11.

Next, the wavelength conversion element 12 used in the gas detection device of the present embodiment will be described with reference to the drawings. The wavelength conversion element 12 is an SHG (Second harmonic generation) element made of a nonlinear optical crystal having a birefringence such as LiNbO ₃ (lithium niobate), LiTaO ₃ (lithium tantalate), etc., having a domain-inverted structure. The incident laser beam is converted to the second harmonic.

FIG. 3 is a perspective view showing a schematic configuration of the wavelength conversion element 12. That is, the wavelength conversion element 12 is, for example, a substrate 121 made of a nonlinear optical crystal MgO is doped LiNbO _3, a plurality of polarization inversion portion 122a that reverses the direction of the Z-axis parallel to the spontaneous polarization of the substrate 121, A periodic polarization inversion structure 122 in which 122b, 122c,... Are periodically formed and an optical waveguide 123 extending along the periodic polarization inversion structure 122 are formed.

  Here, a specific arrangement example of the semiconductor light emitting element 11 and the wavelength conversion element 12 in the laser light source 1 is shown in FIG. As shown in FIG. 4, the laser light source 1 includes a semiconductor light emitting element 11, a wavelength conversion element 12, a light emitting element driving unit 13, collimating lenses 14 and 15, and a condenser lens 16. Light emitted from the active layer 114 of the light emitting element 11 enters the optical waveguide 123 of the wavelength conversion element 12.

  Alternatively, as another arrangement example, as illustrated in FIG. 5, the light source unit 1 does not include the collimating lens 14 and the condenser lens 16, and the emission end face 11 b of the semiconductor light emitting element 11 and the incident end face of the wavelength conversion element 12. The active layer 114 of the semiconductor light emitting element 11 and the optical waveguide 123 of the wavelength conversion element 12 may be directly optically coupled to each other by bonding to 12a.

FIG. 6 is a graph showing the wavelength dependence of the conversion efficiency of the wavelength conversion element 12 and a waveform diagram showing the waveform of the transmitted light of the wavelength conversion element 12. (1) to (5) in the upper graph of FIG. 6 indicate the center wavelength λ ₀ of the laser light emitted from the semiconductor light emitting element 11, and the lower waveform diagrams are the above (1) to (5). The corresponding waveform is shown.

As shown in (1) and (5) of FIG. 6, the center wavelength λ _{0 of the} laser light emitted from the semiconductor light emitting element 11 is different from the phase matching wavelength λ _p and the amplitude λ _w is | λ _p −λ _0. If it is smaller than | × 2, the laser light passes through the wavelength conversion element 12 in the same phase or opposite phase as before transmission.

As shown in (3) of FIG. 6, when the center wavelength λ _{0 of the} laser light emitted from the semiconductor light emitting element 11 coincides with the phase matching wavelength λ _p , the laser light includes a vibration component having the frequency f _2. It becomes light and passes through the wavelength conversion element 12.

As shown in (2) and (4) of FIG. 6, the center wavelength λ _{0 of the} laser light emitted from the semiconductor light emitting element 11 is different from the phase matching wavelength λ _p and the amplitude λ _w is | λ _p −λ _0. When larger than | × 2, the laser light is transmitted through the wavelength conversion element 12 in the same phase or opposite phase as before transmission, but includes a vibration component of the frequency f ₂ .

By the way, when the laser light source 1 of this embodiment is used as the light source of the gas detection device, the laser light transmitted through the wavelength conversion element 12 includes a vibration component having a frequency f _{2 that} is twice the modulation frequency f _1. This is not preferable because the S / N ratio of the detection of light including the vibration component having the double frequency f ₂ derived from the detection target gas is deteriorated.

Therefore, as shown in (1) and (5) of FIG. 6, the wavelength of the laser light emitted from the semiconductor light emitting element 11 and subjected to wavelength modulation is more than the phase matching wavelength λ _p where the conversion efficiency of the wavelength conversion element 12 is maximized. Is preferably included only on the long wave side or the short wave side.

  As described above, in the laser light source of the present embodiment, the wavelength of the laser light emitted from the semiconductor light emitting element and wavelength-modulated is only on the long wave side or the short wave side from the wavelength at which the conversion efficiency of the wavelength conversion element is maximized. By being included, generation of a frequency component twice the modulation frequency can be suppressed. Thereby, the laser light source of this embodiment is applicable as a light source for gas detection apparatuses.

  In particular, the laser light source of the present embodiment includes an InP-based semiconductor light emitting element and a wavelength conversion element that converts the wavelength of the laser light emitted from the semiconductor light emitting element to a short wavelength, whereby the center of the absorption line The present invention can be applied as a laser light source for a gas detection device that can detect a detection target gas having a wavelength shorter than 1.2 μm with high accuracy.

(Second Embodiment)
An embodiment of a gas detection apparatus provided with the laser light source of the first embodiment will be described with reference to FIG. FIG. 7 is a block diagram showing the configuration of the gas detection device of this embodiment.

That is, as shown in FIG. 7, the gas detection apparatus of the present embodiment, at a predetermined wavelength width centered on the wavelength of one absorption line of the plurality of absorption lines included in the detection target gas (modulation frequency f ₁₎ A laser light source 1 as a light source unit that emits wavelength-modulated laser light to the measurement atmosphere 2, and the wavelength-modulated laser light that has passed through the measurement atmosphere 2 is received as measurement light, and received according to the measurement light A light receiving unit 3 for outputting a signal.

In addition, the gas detection device of the present embodiment is a detection included in the measurement atmosphere 2 based on the ratio of the fundamental wave signal and the second harmonic signal of the modulation frequency f ₁ detected from the light reception signal output from the light receiving unit 3. A gas detection unit 4 that calculates the gas concentration of the target gas.

  The light receiving unit 3 is composed of a light receiving element such as a photodiode, for example, receives light collected by a lens (optical system) (not shown), and outputs a light reception signal corresponding to the amount of received light. ing.

The gas detection unit 4 converts a light reception signal, which is a current signal output from the light reception unit 3, into a voltage signal, and a modulation frequency f ₁ included in the voltage signal converted by the current voltage converter 41. A fundamental wave signal detector 42 for detecting a fundamental wave voltage signal h ₁ that is a signal component of the second harmonic wave, and a second harmonic voltage signal h that is a signal component of a frequency f ₂ that is twice the modulation frequency f ₁ included in the voltage signal. a second harmonic signal detector 43 for detecting a _2, the second harmonic signal is detected by the fundamental wave signal detector 42 the amplitude of H ₂ were detected second harmonic voltage signal h ₂ by the detector 43 the fundamental wave voltage signal Dividing the value obtained by dividing the amplitude H ₁ of h ₁ into a detection value D (= H ₂ / H ₁ ) corresponding to the gas concentration of the detection target gas contained in the measurement atmosphere 2, the fundamental wave double wave division is performed. Unit 44.

  This signal operation (division) is performed because the light received by the light receiving unit 3 causes a level fluctuation to some extent for reasons other than the transmission through the measurement atmosphere 2. This is for detecting the gas concentration.

The current-voltage converter 41 includes a preamplifier having a function of converting the received light signal output from the light receiving unit 3 into a voltage signal and amplifying it. In the preamplifier included in the current-voltage converter 41, the fundamental wave signal signal h ₁ and the second harmonic wave signal h ₂ are at the same detectable level in the fundamental wave signal detector 42 and the second harmonic signal detector 43. Thus, it is preferable that the amplification degree for each signal is set to an appropriate value.

Next, the operation of the gas detector of the present embodiment configured as described above will be described.
First, the light emitting element driving unit 13 between the upper electrode 119 and lower electrode 120 of the semiconductor light emitting element 11, the central current value I _0, the amplitude I _w, by the drive current I _d of the modulation frequency f ₁ is applied The inside of the active layer 114 is in a light emitting state (see FIG. 4).

The light generated in the active layer 114 propagates along the active layer 114 and oscillates at a wavelength λ corresponding to the drive current I _d in the resonator constituted by the rear end face 11a and the emission end face 11b. Here, the center wavelength λ ₀ of the wavelength λ is an oscillation wavelength when the center current value I ₀ is applied to the semiconductor light emitting element 11.

As a result, a laser beam oscillating with an amplitude λ _w (for example, 10 pm) and a modulation frequency f ₁ around the center wavelength λ ₀ is emitted from the semiconductor light emitting element 11 to the wavelength conversion element 12. In the wavelength conversion element 12, the center wavelength lambda ₀ and the amplitude lambda _w of the incident laser beam are both half-wavelength. The half-wave laser beam is emitted to the measurement atmosphere 2.

  The laser light emitted from the wavelength conversion element 12 to the measurement atmosphere 2 is transmitted through the measurement atmosphere 2 and received by the light receiving unit 3 to be converted into a light reception signal that is a current signal. The received light signal is input to the gas detector 4.

The received light signal is input to the current-voltage converter 41 and converted into a voltage signal. Voltage signal is input to the fundamental wave signal detector 42 and the second harmonic signal detector 43 of the gas sensing portion 4, the fundamental wave voltage signals h ₁ and the second harmonic voltage signal h ₂ is detected. The fundamental wave the second harmonic divider section 44, the detection object value obtained by dividing the second harmonic amplitude of H ₂ voltage signal h ₂ with an amplitude H ₁ of the fundamental wave voltage signal h ₁ is included in the measurement atmosphere 2 The detection value D (= H ₂ / H ₁ ) corresponding to the gas concentration of the gas is output.

For example, when the detection target gas is oxygen (the center wavelength of the absorption line is 760 nm), the composition ratio of each layer constituting the semiconductor light emitting element 11 so that the center wavelength λ ₀ of the laser light is 1520 nm, The gas concentration of oxygen can be detected by adjusting the type and amount of impurities to be doped.

  As described above, since the gas detection device of the present embodiment includes the laser light source of the first embodiment, it is possible to detect a detection target gas having a center wavelength of the absorption line shorter than 1.2 μm with high accuracy. .

1 Laser light source (light source)
2 Measurement Atmosphere 3 Light Receiving Unit 4 Gas Detection Unit 11 Semiconductor Light Emitting Element 12 Wavelength Conversion Element

Claims (4)

A semiconductor light emitting device (11) for emitting laser light that is wavelength-modulated with a predetermined wavelength width;
A wavelength conversion element (12) for converting the wavelength of the wavelength-modulated laser light into a short wavelength by a nonlinear optical effect,
The laser light source characterized in that the wavelength of the wavelength-modulated laser light is included only on the long wave side or the short wave side of the wavelength at which the conversion efficiency of the wavelength conversion element is maximized.   The laser light source according to claim 1, wherein the wavelength conversion element is an SHG element that converts the wavelength-modulated laser light into a second harmonic.   The laser light source according to claim 1, wherein the semiconductor light emitting element oscillates in a single longitudinal mode. A light source unit (1) for emitting laser light, which is wavelength-modulated with a predetermined wavelength width around the wavelength of one of the plurality of absorption lines of the detection target gas, to the measurement atmosphere (2);
A light receiving unit (3) that receives the wavelength-modulated laser light transmitted through the measurement atmosphere as measurement light and outputs a light reception signal corresponding to the measurement light;
A gas detector (4) for calculating a gas concentration of the detection target gas contained in the measurement atmosphere based on a ratio between a fundamental wave signal and a second harmonic signal of the modulation frequency of the wavelength modulation detected from the light reception signal In a gas detection device comprising:
The gas light source device, wherein the light source unit includes the laser light source according to any one of claims 1 to 3.

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