JP2011119541A - Gas detecting sensor using optical fiber coupling external cavity semiconductor laser - Google Patents

Abstract

A fiber-coupled external resonator type semiconductor laser capable of increasing the coupling efficiency of a fiber and easily controlling the narrowing of the oscillation spectrum and the wavelength, and a trace gas detection using the fiber-coupled external resonator type semiconductor laser. Provide sensors for use.
According to the present invention, a semiconductor laser that oscillates laser light, an external resonator that amplifies the intensity of the incident laser light and emits feedback laser light, and the semiconductor laser and the external resonator are provided. An optical fiber disposed between and emitting the laser light to the external resonator and the feedback laser light entering the semiconductor laser, and an optical fiber Bragg grating disposed in the middle of the optical fiber. A fiber-coupled external cavity semiconductor laser is provided.
[Selection] Figure 1

Description

  The present invention relates to a trace gas detection sensor using an optical fiber coupled external resonator type semiconductor laser, and more particularly to a trace gas detection sensor in which the wavelength of an optical fiber coupled external resonator type semiconductor laser is stabilized.

  As a light source capable of obtaining a standing wave with a stable wavelength, an external cavity type semiconductor laser (External Cavity Diode Laser, hereinafter referred to as ECDL) is used in various directions. FIG. 10 shows a general configuration of an ECDL external cavity semiconductor laser. FIG. 10 is a general schematic configuration diagram of the external cavity semiconductor laser 100. As shown in FIG. 10, an external resonator type semiconductor laser 100 includes a non-reflective coating (hereinafter referred to as an AR coating) semiconductor laser 110, a collimating lens (here, an aspherical lens is used) 130, and an incident mirror. An external resonator 120 including a pair of highly reflective mirrors 121 and an output mirror 122 is provided. Of the light incident on the external resonator 120 from the semiconductor laser 110, only the component that matches the longitudinal mode of the resonator 120 is selected and fed back to the active layer of the semiconductor laser 110. If the intensity of this feedback light is sufficient, the semiconductor laser will produce stimulated emission at a passively fixed wavelength. Therefore, it is possible to stably obtain a standing wave in the external resonator without performing fine control of the resonator length or stabilizing the oscillation frequency of the semiconductor laser. However, it is not easy to sufficiently secure the intensity of the feedback light. Generally, in order to match the resonance frequency, fine control of the resonator length by the piezoelectric element and stabilization of the oscillation wavelength are required.

  Such an ECDL resonator can be used for absorption spectroscopy, which is a method for optically detecting a gas concentration. Here, the absorption spectroscopy is a method in which the measurement is performed using the light absorption effect by the laser wavelength tuned to the absorption line due to the electronic transition or vibrational rotation transition of the molecule.

The reason why ECDL is used for gas concentration detection by absorption spectroscopy is that a long optical path length can be obtained inside an ECDL resonator. That is, the intensity change of the transmitted light of the laser after passing through the sample cell where the absorbing medium is present can be expressed as the following equation (1) by Lambert-Beer law.

Here, I [W] is the transmitted light intensity, I ₀ [W] is the incident light intensity, α [m ⁻¹ ] is the absorption constant, and d [m] is the optical path length of the cell. As can be seen from Equation 1, the higher the optical path length, the more sensitive the measurement. In ECDL, since the laser beam reciprocates several thousand times in the external resonator, a long optical path length can be obtained. Therefore, by using ECDL, highly sensitive measurement can be performed at the time of gas detection. FIG. 11 schematically shows the intensity change of the transmitted light of the laser after passing through the sample cell where the absorbing medium is present.

  An ECDL resonator can also be used for cavity ring-down spectroscopy (hereinafter referred to as CRDS), which is a kind of absorption spectroscopy. CRDS is a method of measuring the concentration of a medium in an optical resonator (sample cell) by a time constant called a ring-down signal.

That is, by modulating the semiconductor laser and blocking the light incident on the external resonator, the transmitted light from the external resonator is attenuated by a first-order exponential function. The attenuation is defined by the following equation (2).

At this time, I [W] is the transmitted light intensity, I ₀ [W] is the incident light intensity, t [s] is the elapsed time, and τ [s] is the time constant. Further, when there is a medium that absorbs laser light in the external resonator, the laser light intensity of the transmitted light is attenuated according to the following equation (3).

Here, τ [s] is a time constant of a medium having absorption, and τ ₀ [s] is a time constant of a medium having no absorption. The time constant is also called a ring-down signal, and the value varies depending on the concentration of the medium in the external resonator. Therefore, it is possible to measure the concentration of the medium in the external resonator by obtaining the time constant.

A measurement example using ECDL for CRDS is shown below. FIG. 12 is a diagram showing an example of nitrogen dioxide measurement by CRDS using ECDL. In FIG. 12, pure nitrogen (Pure_N ₂ ) and 50 ppm concentration of nitrogen dioxide (NO ₂ ) were measured by CRDS using a laser beam having a wavelength of 638 nm, and changes in the respective time constants were shown. From FIG. 12, it is understood that the change in time constant differs between pure nitrogen and nitrogen dioxide. Moreover, FIG. 13 is a figure which shows the nitrogen dioxide measurement result by CRDS using ECDL. In FIG. 13, the attenuation rate of a predetermined concentration of nitrogen dioxide is measured in advance, and the result is displayed in a straight line. From FIG. 12 and FIG. 13, the difference in time constant between nitrogen dioxide and nitrogen and the change in attenuation rate due to the concentration of nitrogen dioxide can be grasped.

  As described above, it is understood that a minute amount of gas can be detected using an external resonator type semiconductor laser. However, when manufacturing a gas detection sensor using such an external resonator type semiconductor laser, there is a disadvantage that the entire apparatus becomes large because laser light is coupled in space. Furthermore, it is necessary to arrange the semiconductor laser, the collimating lens, and the external resonator linearly on the optical path of the laser beam, which lacks flexibility.

  Therefore, in order to ensure the miniaturization and flexibility of the apparatus, an optical fiber coupled external resonator type semiconductor laser (Fiber-coupled_external-cavity_diode_laser: which can be used more flexibly by separating the light source unit and the sensor unit by using an optical fiber. Hereinafter, a gas detection apparatus using FC-ECDL) is considered. An optical fiber coupled external resonator type semiconductor laser is disclosed in Non-Patent Document 1.

  FIG. 14 is a schematic configuration diagram illustrating an example of a gas detection device 200 using FC-ECDL. The light source is a non-reflective coated semiconductor laser 210. Light emitted from the laser is collimated by a collimator lens 230, shaped from an ellipse to a circle by an anamorphic prism pair 260, and an optical fiber (FIG. 14) using a coupling lens 240. In FIG. 2, a polarization maintaining fiber was used.) Light emitted from the optical fiber 250 is collimated by the coupling lens 240 and is incident on the external resonator 220. The non-reflective coated semiconductor laser 210 is connected to a semiconductor laser power source 211 that supplies power, and the external resonator 220 is connected to a power meter 280. Further, an optical spectrum analyzer 270 used for analyzing the spectrum of the laser light is connected between the anamorphic prism pair 260 and the coupling lens 240 on the semiconductor laser side. However, the arrangement position of the optical spectrum analyzer 270 is not limited to this, and may be between the collimating lens 230 and the anamorphic prism pair 260. Here, the collimating lens 230, the anamorphic prism pair 260 and the coupling lens 240 are devices for improving the coupling efficiency to the optical fiber 250.

  By using the optical fiber in this way, it is not necessary to arrange the light source unit and the sensor unit on a straight line, and both can be arranged more flexibly. Further, since the sensor itself is non-contact, explosion-proof measurement can be easily performed in an environment with a high explosion risk, and further the size of the entire apparatus can be reduced. Furthermore, by coupling the ECDL semiconductor laser and the external resonator with a fiber, it is possible to construct a sensor for detecting a trace gas that does not include any electrical device such as a piezoelectric element in the external resonator that is a sensor portion.

  This FC-ECDL can also stabilize the oscillation frequency of the semiconductor laser by the feedback light from the external resonator.

  Therefore, even with this FC-ECDL, the same measurement as the gas detection in the aforementioned ECDL is possible. A measurement example using FC-ECDL for CRDS is shown below. FIG. 15 is a diagram illustrating an example of nitrogen dioxide measurement by CRDS using FC-ECDL. Moreover, FIG. 16 is a figure which shows the nitrogen dioxide measurement result by CRDS using FC-ECDL. It can be understood that measurement can be performed in the same manner as in FIGS.

  However, when FC-ECDL is used, the fiber coupling efficiency is one of the parameters. The coupling efficiency of the fiber indicates the laser light intensity before and after the incidence of the fiber. When the coupling efficiency is low, sufficient feedback light cannot be obtained from the external resonator, and the semiconductor laser cannot oscillate. Therefore, the fiber coupling efficiency is one parameter for laser oscillation in FC-ECDL.

  Therefore, when a trace gas detection sensor is constructed using FC-ECDL, it is required to increase the fiber coupling efficiency. Also, since the oscillation spectrum depends on the feedback component from the external resonator (external resonator length, mirror reflectivity, laser medium), when building a trace gas detection sensor using FC-ECDL, the oscillation spectrum is In addition to depending only on the feedback component from the external resonator, it is required to narrow the oscillation spectrum and easily control the wavelength.

J. Sato et al. “Flexible laser diode race gas sensor based on cavity ringdown spectroscopy with an optical fiber-coupled high-finesse external-cavity diode laser” Applied Physics B (2009) 96: 741-744

  The present invention relates to a fiber-coupled external resonator type semiconductor laser capable of increasing the coupling efficiency of a fiber and easily controlling the narrowing of the oscillation spectrum and the wavelength, and a trace gas using the fiber-coupled external resonator type semiconductor laser. An object is to provide a sensor for detection.

  According to an embodiment of the present invention, a semiconductor laser that oscillates a laser beam, an external resonator that amplifies the intensity of the incident laser beam and emits a feedback laser beam, the semiconductor laser and the external resonator, An optical fiber disposed between the optical fiber that emits the laser light to the external resonator and the feedback laser light is incident on the semiconductor laser, and an optical fiber Bragg grating disposed in the middle of the optical fiber. A featured fiber coupled external cavity semiconductor laser is provided.

  According to an embodiment of the present invention, a semiconductor laser that oscillates a laser beam, an external resonator that amplifies the intensity of the incident laser beam and emits a feedback laser beam, the semiconductor laser and the external resonator, A fiber comprising: an optical fiber disposed between the optical fiber that emits the laser light to the external resonator and the feedback laser light incident on the semiconductor laser; and an optical fiber Bragg grating disposed in the middle of the optical fiber A gas detection sensor comprising a coupled external cavity semiconductor laser is provided.

  The optical fiber Bragg grating may have a reflectance of 2% to 10%.

  The optical fiber Bragg grating may have a reflectivity of 5%.

  Both end faces of the optical fiber may be subjected to at least one of a PC polishing process, an APC polishing process, and an anti-reflection coating process.

  A collimating lens may be provided between the semiconductor laser and the optical fiber.

  A coupling lens may be provided at each end of the optical fiber.

  An anamorphic prism pair may be further provided between the semiconductor laser and the coupling lens located on the semiconductor laser side.

  According to an embodiment of the present invention, a fiber-coupled external resonator type semiconductor laser capable of increasing the coupling efficiency of a fiber and easily controlling the narrowing of the oscillation spectrum and the wavelength, and the fiber-coupled external resonator type semiconductor laser A sensor for detecting a small amount of gas is provided.

It is a schematic block diagram of FC-ECDL which raised the coupling efficiency of the fiber which concerns on one embodiment of this invention, and the sensor for trace gas detection using this FC-ECDL. It is a schematic diagram of FBG used for FC-ECDL which improved the coupling efficiency of the fiber which concerns on one embodiment of this invention. It is a figure which shows the wavelength dependence of the absorption cross section in the ultraviolet to visible region of nitrogen dioxide. It is a comparison figure of the oscillation spectrum of FC-ECDL (A) provided with FBG (reflectance 5%) which selects the wavelength of 638 nm, and FC-ECDL (B) which does not include FBG. It is a comparison figure of the oscillation spectrum of FC-ECDL (A) provided with FBG (reflectance 2%) which selects the wavelength of 638 nm, and FC-ECDL (B) without FBG. It is the figure which compared the oscillation spectrum of A in the case of reflectivity 5%, and B in the case of reflectivity 2% about F-FCDL provided with FBG which selects the wavelength of 638 nm. FC-ECDL (A) with FBG (reflectance 5%) to select 638 nm wavelength, FC-ECDL (B) and FBG with FBG (reflectance 2%) to select 638 nm wavelength It is a comparison figure of the oscillation spectrum of no FC-ECDL (C). It is the figure which showed the change of the spectrum in the elapsed time of FC-ECDL which provided FBG which concerns on one embodiment of this invention. It is the figure which showed the change of the spectrum in the elapsed time of FC-ECDL which provided FBG which concerns on one embodiment of this invention. It is the figure which showed the change of the spectrum in the elapsed time of FC-ECDL which provided FBG which concerns on one embodiment of this invention. It is a figure showing the wavelength range with absorption for every gas kind. 1 is a general schematic configuration diagram of an external cavity semiconductor laser 100. FIG. It is a schematic diagram which shows the intensity change of the transmitted light of the laser after passing the sample cell in which an absorption medium exists. It is a figure which shows the nitrogen dioxide measurement example by CRDS using ECDL. It is a figure which shows the nitrogen dioxide measurement result by CRDS using ECDL. It is a schematic block diagram which shows an example of the gas detection apparatus 200 using FC-EDCL. It is a figure which shows the nitrogen dioxide measurement example by CRDS using FC-ECDL. It is a figure which shows the nitrogen dioxide measurement result by CRDS using FC-ECDL.

Hereinafter, an FC-ECDL with enhanced fiber coupling efficiency according to an embodiment of the present invention, and a trace gas detection sensor using the FC-ECDL will be described in detail with reference to the drawings. In the embodiment, an example of FC-ECDL with improved coupling efficiency of the fiber of the present invention and a sensor for detecting a trace gas using the FC-ECDL is shown, and the coupling efficiency of the fiber of the present invention is shown. The enhanced FC-ECDL and the sensor for detecting a trace gas using the FC-ECDL are not limited to those embodiments.

(Fiber-coupled external cavity semiconductor laser: FC-ECDL)
FIG. 1 is a schematic configuration diagram of FC-ECDL with enhanced fiber coupling efficiency according to an embodiment of the present invention, and a trace gas detection sensor using the FC-ECDL. As shown in FIG. 1, an FC-ECDL 2 with improved fiber coupling efficiency according to an embodiment of the present invention is roughly shown as an anti-reflection coated semiconductor laser 10, an external resonator 20, a collimating lens 30, an optical fiber 50, and an optical fiber. A pair of coupling lenses 40 disposed at both ends of the optical fiber, an anamorphic prism pair 60, a semiconductor laser power source 11 that supplies power to the semiconductor laser, and an optical fiber Bragg grating (Fiber_Bragg_Grating: hereinafter referred to as FBG). It is composed of 90. However, the collimating lens 30, the coupling lens 40, and the anamorphic prism pair 60 may be provided as necessary.

  The non-reflective coated semiconductor laser 10 is electrically connected to the semiconductor laser power source 11 and emits laser light upon receiving power from the semiconductor laser light source 11. The end surface on the emission side of the non-reflective coating semiconductor laser 10 is anti-reflective coated (AR coating). In the present embodiment, a non-reflective coated semiconductor laser is used as the semiconductor laser. However, the reflectance of the AR-coated end face is approximately 0.01%, which makes the semiconductor laser 10 substantially non-reflective. This is because there is no laser oscillation inside.

  The non-reflective coated semiconductor laser 10 feeds back the reflected light from the external resonator 20 to the active layer of the semiconductor laser, thereby eliminating the need to control the resonator length and oscillation frequency. In the non-reflection coated semiconductor laser 10, the laser oscillation wavelength is passively controlled to the resonance frequency of the external resonator 20.

  In the present embodiment, the non-reflection coated semiconductor laser 10 having an oscillation wavelength of 638 nm is used, but the oscillation wavelength is not limited to this, and can be appropriately selected according to the type of gas to be detected.

  The external resonator 20 includes a pair of high reflection mirrors, that is, an entrance mirror and an exit mirror (both not shown). The entrance mirror and the exit mirror are arranged to face each other, and the reflectance of the reflection surface of each mirror needs to be set very high. As a result, the laser light incident from the incident mirror side is reciprocated several thousand times between the output mirror and the incident mirror, and the intensity of the laser light is amplified. Therefore, the reflectance of the exit mirror is preferably set to 99.99% or more, more preferably 99.999% or more. The reflectance of the incident mirror is set lower than the reflectance of the output mirror, preferably 99.94% or more, more preferably 99.97% or more. However, the reflectance of each mirror is not limited to this.

  The external resonator 20 can be a gas cell that can be sealed by introducing gas, and the external resonator length can be appropriately selected in consideration of the size of the entire apparatus.

  The collimating lens 30 is disposed on the non-reflective coated emission side of the non-reflective coated semiconductor laser 10. The collimating lens 30 converts the laser light, which is the divergent light emitted from the non-reflective coated semiconductor laser 10, into parallel light and focuses it on the anamorphic prism pair 60. In the present embodiment, an aspherical lens is used as the collimating lens 30, but the collimating lens 30 is not limited to this.

  The coupling lens 40 is disposed at both ends of the optical fiber 50, couples the laser light emitted from the anamorphic prism pair 60 to one end of the optical fiber 50, and couples the laser light emitted from the optical fiber 50 to an external resonator. 20.

  The optical fiber 50 is a cylindrical laser beam transmission path. The laser light emitted from the non-reflective coated semiconductor laser 10 is transmitted to the external resonator 20, and the reflected laser light amplified by reciprocating several thousand times in the external resonator 20 is used as the active layer of the non-reflective coated semiconductor laser 10. To give feedback.

  In this embodiment, the end face of the optical fiber 50 is PC polished (Physical_Contact, and the end face of the optical fiber is polished to have a convex spherical surface in order to reduce loss at the time of connection. The return loss (the ratio of the reflected wave to the incident wave) The reciprocal number) ≧ 27 dB.) Was used. The alignment on the fiber incident side (that is, the non-reflective coated semiconductor laser 10 side) was intentionally shifted. This is because, in the optical fiber polished by PC, the surface reflectivity of the end face and the reflectivity of the FBG are close, so that resonance occurs between the end face of the optical fiber and the semiconductor laser, and oscillation does not occur at the original FBG wavelength. This is because of this. Therefore, in order to prevent resonance, the alignment on the optical fiber incident side is intentionally shifted to suppress the resonance. This makes it possible to oscillate at the original FBG wavelength. Since the cause of resonance is that the surface reflectance of the end face and the reflectance of the FBG are close, the end face of the optical fiber is APC-polished (Angle_Physical_Contact, the return light is further reduced by making the polishing surface oblique to the PC. Return loss: ≧ 60 dB) By processing or AR coating, resonance can be suppressed without shifting the alignment, and oscillation at the original FBG wavelength can be achieved. Therefore, it is preferable to perform at least one of the above three processes as the end face processing of the optical fiber.

  The anamorphic prism pair 60 includes two prisms 61 and 62 inside the housing, and expands or contracts one of the beam axes of the elliptical laser light emitted from the non-reflective coated semiconductor laser 10. Mold into a circle. The emitted light is emitted in parallel to the incident light. The anamorphic prism pair 60, the collimating lens 20 and the coupling lens 40 described above are all devices for improving the coupling efficiency to the optical fiber 50. Therefore, when the coupling efficiency of the non-reflective coated semiconductor laser 10, the external resonator 20, and the optical fiber 50 can be maintained high, any or all of the anamorphic prism pair 60, the collimating lens 20, and the coupling lens 40 are omitted. You can also. In particular, as described above, when the coupling efficiency is increased by, for example, APC polishing or non-reflective coating on the end face of the optical fiber, it can be omitted.

  The FBG 90 is a device in which a diffraction grating 91 is formed in the core of an optical fiber, and is an element that selects a specific wavelength. That is, it has a function as an optical filter. FIG. 2 is a schematic diagram of an FBG used for FC-ECDL with enhanced fiber coupling efficiency according to an embodiment of the present invention. As shown in FIG. 2, in order to form the diffraction grating 91 in the core 92 of the optical fiber, the incident light is reflected according to the reflectance of the diffraction grating, and only a certain amount of incident light is transmitted through the diffraction grating as transmitted light. pass. As a result, the band of the oscillation spectrum can be narrowed and the wavelength can be easily controlled.

  Since the FBG 90 has the above-described configuration, only a certain amount of incident light passes through the diffraction grating 91 as transmitted light, so that the oscillation spectrum can be narrowed. Here, a filter may be used instead of the FBG 90. However, when the filter is used, the oscillation spectrum cannot be narrowed. Therefore, it is necessary to use FBG90.

  In the present embodiment, FBG (manufactured by Tatsuta Electric Wire Co., Ltd.) that selects a wavelength of 638 nm is used as FBG90. The reflectivity of the FBG 90 is 5%. Further, as a comparative example, an FBG of 2% reflectivity (also manufactured by Tatsuta Electric Cable Co., Ltd.) that selects the same wavelength of 638 nm was used. When constructing a sensor for detecting a very small amount of gas, an FBG 90 that appropriately selects an appropriate wavelength in accordance with the wavelength of the laser light to be used can be used.

  The FBG 90 is disposed between both end faces of the optical fiber 50. The arrangement position is not limited to the middle of the optical fiber 50, and may be arranged so as to be deviated toward any end face side. The optical fiber 50 and the FBG 90 may be integrally formed by irradiating the core of the optical fiber 50 with ultraviolet rays. That is, a refractive index change is caused in the core of the optical fiber by light-induced refractive index change (Photosensitivity), and a kind of diffraction grating is formed inside the core by periodically making this refractive index change. Can do. For example, a UV laser (excimer laser, etc.) is used for the ultraviolet light to be irradiated, and a diffraction grating is formed in the core by inserting a transmissive diffraction grating (phase mask) to cause laser diffraction between the laser and the irradiated surface. can do. By such a method, the optical fiber 50 and the FBG 90 can be integrally formed.

  As described above, by using the FBG, it is possible to narrow the oscillation spectrum and easily control the wavelength, which cannot be realized by the conventional FC-ECDL.

  In addition, since the FBG can easily control the wavelength by applying temperature change and tension, the FC-ECDL according to the embodiment of the present invention using the FBG controls the oscillation spectrum. It can be done easily.

  That is, the oscillation spectrum of the conventional FC-ECDL depends on the feedback component from the external resonator (external resonator length, mirror reflectivity, laser medium) and therefore cannot be easily controlled. The oscillation spectrum is a parameter that determines whether or not measurement is possible when detecting a trace gas using absorption spectroscopy. Please refer to FIG. FIG. 3 is a graph showing the wavelength dependence of the absorption cross section of nitrogen dioxide in the ultraviolet to visible range. As shown in FIG. 3, the absorption cross-sectional area of nitrogen dioxide depends on the wavelength and varies greatly even when the wavelength is slightly shifted. Although it becomes nearly flat from a wavelength of about 630 nm to 700 nm, it is not flat at all. Therefore, when using absorption spectroscopy, if the oscillation wavelength is not a narrow band with respect to the absorption spectrum, the absorption coefficient depends on the frequency, so the ring-down signal is not constant, and concentration measurement is impossible. Become. Therefore, if not only the oscillation spectrum depends on only the feedback component from the external resonator but also the narrowing of the oscillation spectrum and the control of the oscillation spectrum become possible, it becomes advantageous for gas detection by absorption spectroscopy. Further, even in a region where there are many gas absorption lines such as the infrared region, gas detection can be effectively performed by absorption spectroscopy using FC-ECDL.

In the FC-ECDL according to one embodiment of the present invention, it is possible to narrow the oscillation spectrum. This will be described with reference to the drawings. 4 to 7 are diagrams comparing the oscillation spectra of FC-ECDL using FBG and FC-ECDL not using FBG. FIG.
It is a comparison figure of the oscillation spectrum of FC-ECDL (A) provided with FBG (reflectance 5%) which selects the wavelength of 638 nm, and FC-ECDL (B) which does not include FBG. FIG. 5 is a comparison diagram of oscillation spectra of FC-ECDL (A) having FBG (reflectance 2%) for selecting a wavelength of 638 nm and FC-ECDL (B) not having FBG. FIG. 6 is a diagram comparing oscillation spectra of A when the reflectivity is 5% and B when the reflectivity is 2% for the FC-ECDL including the FBG that selects a wavelength of 638 nm. FIG. 7 shows FC-ECDL (A) with FBG (5% reflectivity) for selecting a wavelength of 638 nm and FC-ECDL (B) with FBG (2% reflectivity) for selecting a wavelength of 638 nm. 2 is a comparison diagram of oscillation spectra of FC-ECDL (C) without FBG and FBG.

  As shown in FIG. 4, the oscillation spectrum of FC-ECDL (A) with FBG draws a steep curve across the wavelength of 638 nm, but is almost flat in other wavelength regions. In other words, it can be understood that by providing the FBG, the band of the oscillation spectrum of the FC-ECDL is narrowed. On the other hand, the oscillation spectrum in the case where no FBG is provided has many curves drawn over a wavelength of 636 nm to 637 nm over 1 nm, and the width of the oscillation spectrum is wide. Further, referring to FIG. 5, the oscillation spectrum of FC-ECDL (A) provided with an FBG having a reflectance of 2% is lower in intensity than the oscillation spectrum of FC-ECDL (B) without FBG, but has a wavelength of 636 nm. A plurality of spectra appear over a wide band from 638 nm to 638 nm, and the oscillation spectrum is not so narrow compared to FC-ECDL provided with FBG having a reflectance of 5%. The achievement status of the narrowing of the oscillation spectrum due to the difference in the reflectance of the FBG is well understood from FIG. 6, and the fact that the oscillation spectrum of the FC-ECDL equipped with the FBG having the reflectance of 5% is narrowed to a high bandwidth. Be grasped. This can also be understood from FIG. 7 which compares three oscillation spectra.

Coupling efficiency is also a parameter as a condition for oscillation of FC-ECDL. FIG. 6 and FIG. 7 show comparison of oscillation spectra due to the difference in FBG reflectivity. The following equation (4) is given as an equation for obtaining the threshold current of FC-ECDL.

Where I _th is the threshold current, l is the laser medium length, s is the stripe width, d is the thickness of the active layer, η is the quantum efficiency, J ₀ is the threshold current density, β is the gain factor, and Γ is the confinement The coefficient, α is the absorption coefficient, R _F is the fiber coupling efficiency, R _c is the reflectance of the FBG, and R _HR is the reflectance of the rear end face of the semiconductor laser.

  From Equation 4 above, the threshold current of FC-ECDL depends on the coupling efficiency of the fiber and the reflectance of the FBG, so these values are also considered to be parameters related to oscillation. 4 and 5 described above indicate that the reflectivity of the FBG contributes to stabilization of the oscillation spectrum.

In addition, when gas is detected by absorption spectroscopy using FC-ECDL, measurement is performed in microsecond time, but by using FBG, wavelength can be stabilized for longer time than measurement time. Therefore, a great effect can be expected also in gas detection.

  Furthermore, changes in the spectrum over time are shown in FIGS. 8A, 8B, and 8C. 8A, 8B, and 8C are diagrams showing changes in the spectrum in the elapsed time of FC-ECDL provided with the FBG according to one embodiment of the present invention, and the spectrum of the case where the reflectance of the FBG is 5% is shown. Changes in elapsed time are indicated by (a), (c), (e), (g), (i), (k), (m), (o), (q), and the reflectance of the FBG is 2%. (B), (d), (f), (h), (j), (l), (n), (p), (r) are shown in FIG. Each spectrum has an elapsed time of 30 seconds.

  As shown in FIGS. 8A, 8B, and 8C, when the reflectance of the FBG is 5%, (a), (c), (e), (g), (i), (k), (m), ( In o) and (q), there is almost no change in the elapsed time of the spectrum. On the other hand, when the reflectance of the FBG is 2%, the change in the elapsed time of the spectrum is (b), (d), (f), (h), (j), (l), (n), (p). , (R) does not change greatly, but changes appear partially. From these results, it is understood that stable oscillation is possible by providing the FBG, but if the reflectivity of the FBG is 5%, more stable oscillation is possible over time. .

  As described above, it is possible to narrow the oscillation spectrum by providing the FBG in the FC-ECDL, and it is preferable to use an FBG having a reflectance of more than 2%, more preferably a reflectance of 5%. A narrow band can be achieved. Note that the reflectance of the FBG can be, for example, a reflectance of more than 10%, but in this case, the light incident on the external resonator is reduced and the output is reduced. Therefore, the reflectance is preferably 10% or less. If the reflectance of the FBG is 5%, stable oscillation is possible over time as shown in FIGS. 8A, 8B, and 8C.

  As described above, the oscillation wavelength can be narrowed by the FC-ECDL including the FBG according to the embodiment of the present invention. In addition, the oscillation wavelength can be stabilized and more stable oscillation can be achieved.

Furthermore, as described above, the FC-ECDL provided with the FBG according to one embodiment of the present invention can narrow the oscillation wavelength band, so that the ring-down signal can be made constant. Therefore, by using the FC-ECDL equipped with the FBG according to one embodiment of the present invention for the gas detection sensor, gas detection can be performed by absorption spectroscopy even in a region where there are many absorption lines of gas such as the infrared region. Can be performed. FIG. 9 is a diagram showing a wavelength region with absorption for each gas type. A portion shown in black in FIG. 9 is an absorption wavelength region. As shown in FIG. 9, for example, N ₂ O (nitrous oxide) has absorption in the wavelength ranges of 2 μm to 3.2 μm and 4 μm to more than 10 μm. In addition, CH ₄ (methane) has absorption in the wavelength range of about 2.2 μm to 2.8 μm, 3 μm to 3.7 μm, and around 10 μm. Thus, all of the harmful gases shown in FIG. 9 are absorbed in the wavelength range of 2 μm to 5 μm. Absorption spectroscopy can be used as long as there is absorption. Therefore, by using the FC-ECDL equipped with the FBG according to one embodiment of the present invention capable of narrowing the oscillation wavelength and making the ring-down signal constant, in the infrared region or the like, Gas detection can also be performed. In addition, since the wavelength stabilization in milliseconds can be achieved by using the FBG, and the oscillation wavelength can be stabilized in a time longer than the measurement time, the FBG according to one embodiment of the present invention can be stabilized. By using the provided FC-ECDL as a gas detection sensor, trace gas detection can be performed appropriately. In the above-described embodiment, an FBG that selects a wavelength of 638 nm is used. However, an FC-ECDL having an appropriate oscillation wavelength and an FBG that selects an appropriate wavelength are used depending on the type of target gas. Thus, an appropriate sensor can be constructed according to the type of gas.

(Small gas detection sensor using fiber coupled external resonator type semiconductor laser)
Next, a trace gas detection sensor using FC-ECDL equipped with FBG according to an embodiment of the present invention will be described. In the description, the description of the components described in the FC-ECDL including the FBG described above will be omitted in order to avoid overlapping description.

  A trace gas detection sensor 1 using FC-ECDL equipped with an FBG according to an embodiment of the present invention is roughly shown as an antireflective coated semiconductor laser 10, an external resonator 20, a collimator lens 30, an optical fiber 50, and both ends of the optical fiber. A pair of coupling lenses 40, an anamorphic prism pair 60, an optical spectrum analyzer 70, a power meter 80, a semiconductor laser power source 11 for supplying power to the semiconductor laser, and an FBG 90. In other words, the FC-ECDL 2 including the FBG according to the embodiment of the present invention described above is further provided with an optical spectrum analyzer 70 and a power meter 80.

  The optical spectrum analyzer 70 is an apparatus used for analyzing the spectrum of laser light, analyzes the received laser light, and displays the wavelength of the laser light on the horizontal axis and the voltage of the laser light on the vertical axis. In the present embodiment, the optical spectrum analyzer 70 is disposed between the collimating lens 30 and the anamorphic prism pair 60. The arrangement position of the optical spectrum analyzer 70 is not limited to this. For example, the optical spectrum analyzer 70 may be arranged between the coupling lens 40 and the anamorphic prism pair 60 on the non-reflection coated semiconductor laser 10 side. Other positions may also be used.

  The power meter 80 is a device for measuring the output light intensity of the laser light, and is connected to the exit mirror side of the external resonator 20. With the above configuration, a trace gas detection sensor 1 using FC-ECDL provided with the FBG according to one embodiment of the present invention is obtained.

(Effect of gas detection sensor)
With the configuration as described above, the trace gas detection sensor using the FC-ECDL equipped with the FBG according to one embodiment of the present invention can separate the light source unit and the sensor unit, and can provide a more flexible gas. A sensor for detection can be provided. In addition, since the sensor itself is contactless, it is possible to easily perform explosion-proof measurement in an environment with a high explosion risk. Furthermore, the sensor can be miniaturized.

In addition, a trace gas detection sensor using FC-ECDL equipped with FBG enables gas detection by absorption spectroscopy even in a region where there are many gas absorption lines such as the infrared region. Further, the use of the FBG makes it possible to stabilize the wavelength over a long period of time and to stabilize the oscillation wavelength over a longer time than the measurement time. Therefore, the FBG according to an embodiment of the present invention is provided. By using FC-ECDL as a gas detection sensor, trace gas detection can be performed appropriately.

1: Trace gas detection sensor 2: FC-ECDL (Optical fiber coupled external resonator type reaction body laser)
10: Non-reflective coated semiconductor laser 20: External resonator 30: Collimating lens 40: Coupling lens 50: Optical fiber 60: Anamorphic prism pair 70: Optical spectrum analyzer 80: Power meter 90: FBG (optical fiber Bragg grating)

Claims (10)

A semiconductor laser that oscillates laser light;
An external resonator that amplifies the intensity of the incident laser beam and emits a feedback laser beam;
An optical fiber disposed between the semiconductor laser and the external resonator, emitting the laser light to the external resonator, and inputting the feedback laser light to the semiconductor laser;
An optical fiber Bragg grating disposed in the middle of the optical fiber;
A fiber-coupled external resonator type semiconductor laser comprising:   The fiber-coupled external resonator type semiconductor laser according to claim 1, wherein the optical fiber Bragg grating has a reflectance of 2% to 10%.   The fiber-coupled external cavity semiconductor laser according to claim 1, wherein the optical fiber Bragg grating has a reflectance of 5%.   4. The fiber-coupled external resonator semiconductor according to claim 2, wherein both end faces of the optical fiber are subjected to at least one of a PC polishing process, an APC polishing process, and an anti-reflection coating process. laser.   5. The fiber coupled external resonator type semiconductor laser according to claim 4, further comprising a collimating lens between the semiconductor laser and the optical fiber.   5. The fiber-coupled external resonator type semiconductor laser according to claim 4, further comprising coupling lenses at both ends of the optical fiber.   The fiber coupled external resonator type semiconductor laser according to claim 6, further comprising an anamorphic prism pair between the semiconductor laser and the coupling lens located on the semiconductor laser side. A semiconductor laser that oscillates laser light;
An external resonator that amplifies the intensity of the incident laser beam and emits a feedback laser beam;
An optical fiber disposed between the semiconductor laser and the external resonator, emitting the laser light to the external resonator, and inputting the feedback laser light to the semiconductor laser;
An optical fiber Bragg grating disposed in the middle of the optical fiber;
A gas detection sensor comprising: a fiber-coupled external resonator type semiconductor laser comprising:   The gas detection sensor according to claim 8, wherein the optical fiber Bragg grating has a reflectance of 2% to 10%.   The gas detection sensor according to claim 8, wherein the optical fiber Bragg grating has a reflectance of 5%.