Patent Publication Number: US-2020287349-A1

Title: Semiconductor laser device and analysis apparatus

Description:
TECHNICAL FIELD 
     The present invention relates to a semiconductor laser device and analysis apparatus used for optical analysis. 
     BACKGROUND ART 
     Usually, for example, in a gas analysis apparatus using a semiconductor laser element, a variation in the wavelength of laser light emitted from the semiconductor laser element affects analysis accuracy, and therefore the temperature of the semiconductor laser element as one of the factors of the variation in the wavelength is controlled to a desired temperature. 
     For example, in Patent Literature 1, a temperature control part including a Peltier element, a supporting substrate, and the like is mounted with a semiconductor laser element and a temperature detection part such as a thermistor to control the temperature control part on the basis of temperature obtained by the temperature detection part. In doing so, the temperature of the semiconductor laser element is controlled to a desired temperature to suppress a variation in the wavelength of laser light. A semiconductor laser light source in Patent Literature 1 includes the semiconductor laser element, the temperature detection part, the temperature control part in an internal space formed by a base member and a cap member. In addition, multiple pin terminals provided penetrating through the base member from outside the base member toward the internal space are electrically connected to the semiconductor laser element, temperature detection part, and temperature control part by electrically conductive wires. 
     However, along with a change in the ambient temperature of the semiconductor laser light source, the temperature of the pin terminals provided to the base member changes, and heat is transferred to the temperature detection part from pin terminals through electrically conductive wires. This causes the temperature of the temperature detection part to be varied by the heat from the electrically conductive wires. As a result, a measurement error occurs in the temperature of the semiconductor laser element, making it difficult to accurately control the temperature of the semiconductor laser element. 
     CITATION LIST 
     Patent Literatures 
     Patent Literature 1: International Publication No. 2013/084746 
     SUMMARY OF INVENTION 
     Technical Problem 
     Therefore, the present invention has been made in order to solve the above-described problem, and the main object thereof is to make it possible to accurately control the temperature of a semiconductor laser element by reducing a measurement error of a temperature detecting element for detecting the temperature of the semiconductor laser element. 
     Solution to Problem 
     That is, the semiconductor laser device according to the present invention is one used for optical analysis, and the semiconductor laser device includes: a semiconductor laser element; a temperature detecting element that detects the temperature of the semiconductor laser element; an output terminal that outputs the output of the temperature detecting element to the outside; a wire that electrically connects the temperature detecting element and the output terminal; and a heat capacity increasing part that is provided interposed between the temperature detecting element and the output terminal and contacts with at least a part of the wire to increase the heat capacity of the wire. 
     Such a semiconductor laser device is capable of reducing the amount of heat transferring through the wire and flowing into the temperature detecting element because there is provided the heat capacity increasing part that contacts with the wire connecting the temperature detecting element and the output terminal to increase the heat capacity of the wire. As a result, the temperature of the semiconductor laser element can be accurately detected by the temperature detecting element, making it possible to highly accurately control the temperature of the semiconductor laser element. 
     In order to preferably use the semiconductor laser element for optical analysis such as infrared spectroscopic analysis, the semiconductor laser element is desirably a quantum cascade laser. The quantum cascade laser requires large power consumption by one or more orders of magnitude as compared with normal semiconductor lasers and easily causes a temperature change, and is therefore significantly affected by wavenumber variation. According to the present invention, since the temperature of a quantum cascade laser can be highly accurately controlled, wavenumber variation due to a temperature change can be suppressed, making it possible to highly accurately perform the optical analysis. 
     As a specific embodiment for adjusting the temperature of the semiconductor laser element, it is desirably that the semiconductor laser device further includes: a cooling mechanism that is mounted with the semiconductor laser element and for cooling the semiconductor laser element; and a control part that uses the detected temperature by the temperature detecting element to control the cooling mechanism. 
     When directly mounting the semiconductor laser element on the cooling mechanism, the difference in thermal expansion coefficient between the cooling mechanism and the semiconductor laser element may cause strain in both or causes a gap between both to prevent heat transfer. 
     For this reason, it is conceivable that the semiconductor laser device further includes a sub-mount that is provided interposed between the cooling mechanism and the semiconductor laser element. It is also conceivable to set the thermal expansion coefficient of the sub-mount between the thermal expansion coefficient of the cooling mechanism and that of the semiconductor laser element. In addition, as the material of the sub-mount, it is conceivable to use ceramic such as aluminum nitride (AlN) or silicon carbide (SiC). 
     In order to increase the heat capacity of the wire and also positively make the temperature of the wire equal to that of the semiconductor laser element, it is desirable that the heat capacity increasing part is in contact with the sub-mount. In this case, by configuring the heat capacity increasing part to cover part of the wire, the volume of the heat capacity increasing part can be decreased as compared with a configuration in which the wire is entirely covered, and therefore temperature adjustment based on heat transfer from the sub-mount can be facilitated to shorten a time required for stabilizing the temperature of the wire (e.g., a warming-up time). 
     It is desirable that the heat capacity increasing part is formed of silicone that is an insulating material. In this configuration, the heat capacity can be increased without preventing an electrical signal from being transmitted through the wire. Also, since silicone has flexibility, the wire becomes unlikely to be broken, and the heat capacity increasing part also plays a role in protecting the wire. 
     In order to configure the heat capacity increasing part using an existing configuration, it is desirable that the wire is embedded in the sub-mount, and the sub-mount functions as the heat capacity increasing part. 
     In addition, it is desirable that the sub-mount is a multilayer ceramic substrate, and the wire is configured using a wire in the multilayer ceramic substrate. 
     Light may leak out of an end surface on the side opposite to the light emitting surface of the semiconductor laser element and the light leaking out may serve as stray light. Therefore, in order to reduce the stray light, it is desirable that the semiconductor laser device further includes a light shielding part that faces the end surface on the side opposite to the light emitting surface of the semiconductor laser element to shield the light coming out of the end surface. 
     Also the temperature detecting element is provided facing the end. surface on the side opposite to the light emitting surface of the semiconductor laser element, and thereby the stray light can be reduced by the temperature detecting element. 
     Also, the analysis apparatus according to the present invention is one that analyzes a measurement target component contained in fluid, and the analysis apparatus includes: a measurement cell into which the fluid is introduced; the above-described semiconductor laser device that irradiates the measurement cell with laser light; a light detector that detects laser light passing through the measurement cell; and an analysis part that uses a detected signal by the light detector to analyze the measurement target component. 
     Such an analysis apparatus can highly accurately control the temperature of the semiconductor laser element, and therefore accurately analyze the measurement target component contained in the fluid. 
     Advantageous Effects of Invention 
     According to the present invention configured as described, a measurement error of the temperature detecting element that detects the temperature of the semiconductor laser element can be reduced, making it possible to accurately control the temperature of the semiconductor laser element. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is an overall schematic view of an exhaust gas analysis apparatus using a semiconductor laser device according to the present embodiment; 
         FIG. 2  is a plan view schematically illustrating the overall configuration of the semiconductor laser device according to the same embodiment; 
         FIG. 3  is a cross-sectional view schematically illustrating the overall configuration of the semiconductor laser device according to the same embodiment; 
         FIG. 4  is a plan view schematically illustrating the overall configuration of a semiconductor laser device according to a variation; 
         FIG. 5  is a cross-sectional view schematically illustrating the overall configuration of the semiconductor laser device according to the variation; 
         FIG. 6  is a plan view schematically illustrating the overall configuration of a semiconductor laser device according to a variation; and 
         FIG. 7  is a cross-sectional view schematically illustrating the overall configuration of the semiconductor laser device according to the variation. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     In the following, one embodiment of the semiconductor laser device according to the present invention will be described with reference to the drawings. 
     As illustrated in  FIG. 1 , a semiconductor laser device  100  of the present embodiment is one used for, for example, an exhaust gas analysis apparatus  10  that analyzes a measurement target components (e.g., CO, CO 2 , H 2 O, NO, NO 2 , N 2 O, NH 3 , a hydrocarbon component such as HC, or an oxygen-containing hydrocarbon component such as HCHO) in exhaust gas discharged from an internal combustion engine. The exhaust gas analysis apparatus  10  includes: a multireflection measurement cell  11  into which the exhaust gas is introduced; the semiconductor laser device  100  that irradiates the measurement cell  11  with laser light X; a light detector  12  that detects laser light X passing through the measurement cell  11 ; and an analysis part  13  that analyzes the measurement target component using a detected signal by the light detector  12 . Note that the measurement cell  11  is not limited to the multireflection measurement cell but may he a single reflection measurement cell or a one-pass measurement cell not using reflection. 
     Specifically, the semiconductor laser device  100  is one that emits the laser light X having oscillation wavelengths including an absorption wavelength of the measurement target component, and as illustrated in  FIGS. 2 and 3 , includes: a semiconductor laser element  2 ; a temperature detecting element  3  that detects the temperature of the semiconductor laser element  2 ; a cooling mechanism  4  that is mounted with the semiconductor laser element  2  and the temperature detecting element  3  and for cooling the semiconductor laser element  2 ; a sub-mount  5  provided between the semiconductor laser element  2  and temperature detecting element  3 , and the cooling mechanism  4 ; and a hermetic container  6  that contains them. 
     The semiconductor laser element  2  is, for example, a distributed feedback laser (DFB laser) that emits the laser light X having infrared wavelengths, and in particular, in the present embodiment, a quantum cascade laser in which multiple well layers are connected in a cascaded manner and which emits light by optical transitions between subbands formed in the quantum wells. 
     The temperature detecting element  3  is one that detects the temperature of the semiconductor laser element  2  by detecting the temperature of the sub-mount  5  mounted with the semiconductor laser element  2 . The temperature detecting element  3  in the present embodiment is a thermistor, and provided in the vicinity of the semiconductor laser element  2  in order to reduce the difference from the temperature of the semiconductor laser element  2 . The temperature detecting element  3  in  FIG. 2  is provided laterally to the semiconductor laser element  2 , but may be provided in another position. In addition, by providing the temperature detecting element  3  oppositely to a rear end surface on the side opposite to the light emitting surface of the semiconductor laser element  2 , light leaking out of the rear end surface of the semiconductor laser element  2  can be shielded to reduce stray light. 
     The cooling mechanism  4  is one configured using a Peltier element, and the upper surface thereof is adapted to be a heat absorption surface. In addition, the sub-mount  5  is provided in contact with the heat absorption surface. The cooling mechanism  4  is controlled by a control part COM having acquired the detected temperature by the temperature detecting element  3 . Specifically. the control part COM uses the detected temperature by the temperature detecting element  3  to control electric power to be provided to the Peltier element and makes the detected temperature by the temperature detecting element  3  equal to a desired temperature. 
     The sub-mount  5  is a ceramic substrate made of, for example, aluminum nitride (AlN), silicon carbide (SiC), or the like. On the upper surface of the sub-mount  5 , the semiconductor laser element  2  and the temperature detecting element  3  are provided. In addition, on the upper surface of the sub-mount  5 , metal layers  51 ,  52 , and  53  electrically connected to the respective elements  2  and  3  are formed. 
     The hermetic container  6  is one that forms a hermetic space such as a butterfly package, and a side wall facing the light emitting surface of the semiconductor laser element  2  is formed with a light lead-out part  61  for leading the laser light X to the outside. The light lead-out part  61  is provided with an optical window member  62 , and the optical window member  62  is tilted slightly (e.g., at an angle of two degrees) so as to prevent the laser light X reflected by the optical window member  62  from returning to the semiconductor laser element  2  again. 
     Also, the hermetic container  6  is provided with output terminals T 1  and T 2  for outputting the output of the temperature detecting element  3  to the outside and feed terminals T 3  and T 4  for feeding power to the semiconductor laser element  2 . Which of the temperature detecting element  3  and the semiconductor laser element  2  is connected to the output terminals T 1  and T 2  or the feed terminals T 3  and T 4  is arbitrarily determined. In addition, the hermetic container  6  is also provided, with feed terminals T 5  and T 6  for feeding power to the Peltier element of the cooling mechanism  4 . 
     The output terminals T 1  and T 2  are connected with temperature detecting wires L 1  and L 2  electrically connected to the temperature detecting element  3 . There are two output terminals T 1  and T 2 , and one T 1  of the output terminals is connected with the wire L 1  connected to the metal layer  51  electrically connected to one electrode of the temperature detecting element  3 , whereas the other output terminal T 2  is connected with the wire L 2  electrically connected to the other electrode of the temperature detecting element  3 . These wires L 1  and L 2  are, for example, gold wires. 
     The feed terminals T 3  and T 4  for the semiconductor laser element  2  are connected with wires L 3  and L 4  for the semiconductor laser element  2 , which are electrically connected to the semiconductor laser element  2 . There are two feed terminals T 3  and T 4 , and one T 3  of the feed terminals is connected with the wire L 3  connected to the metal layer  52  electrically connected to one electrode of the semiconductor laser element  2 , whereas the other feed terminal T 4  is connected with the wires L 4  electrically connected to the other electrode of the semiconductor laser element  2  via the metal layer  53 . Similarly, the Peltier element feed terminals T 5  and T 6  are connected with wires L 5  and L 6  for the Peltier element, which are electrically connected to the Peltier element. 
     Further, the semiconductor laser device  100  of the present embodiment includes a heat capacity increasing part  7  that is provided interposed between the temperature detecting element  3  and output terminal T 1 , and the output terminal T 2 , and contacts with at least part of the temperature detecting wires L 1  and L 2  to increase the heat capacity of the wire L 1  or L 2 . 
     The heat capacity increasing part  7  is provided in contact with part of the wire L 2  directly connecting the output terminal T 2  and the temperature detecting element  3 . The heat capacity increasing part  7  is one that reduces heat transfer from the output terminal T 2  while keeping the electrical conduction of the wire L 2 . Specifically, the heat capacity increasing part  7  is configured of an insulating material, and the material desirably has flexibility because it covers the wire L 2  and is flexible resin such as silicone. Also, the heat capacity increasing part  7  is provided so as to cover the periphery of part of the temperature detecting wire L 2 . Further, the heat capacity increasing part  7  is provided in contact with the sub-mount  5  as well. 
     In addition, the heat capacity increasing part  7  may be provided so as to cover at least part of the temperature detecting element  3  together with at least part of the temperature detecting wire L 2 . When silicone is used for the heat capacity increasing part  7 , siloxane gas may be produced, and since the siloxane gas causes a failure in electronic equipment, such as a contact failure, the heat capacity increasing part  7  made of silicone is desirably decreased in its volume. 
     Effect of Present Embodiment 
     In such a semiconductor laser device  100 , since there is provided the heat capacity increasing part  7  that contacts with the wire L 2  connecting the temperature detecting element  3  and the Output terminal T 2  to increase the heat capacity of the wire L 2 , the amount of heat transferring through the wire L 2  and flowing into the temperature detecting element  3  can be reduced. As a result, the temperature of the semiconductor laser element  2  can be accurately detected by the temperature detecting element  3 , and the temperature of the semiconductor laser element  2  can be highly accurately controlled. That is, the difference between the detected temperature by the temperature detecting element  3  and the temperature of the semiconductor laser element  2  can be reduced to highly accurately control the temperature of the semiconductor laser element  2 . The gas analysis apparatus  10  using such a semiconductor laser device  100  is capable of suppressing variations in the oscillation wavelengths of the semiconductor laser element  2  and accurately analyzing the measurement target component contained in the gas. 
     Other Embodiments 
     Note that the present invention is not limited to the above-described embodiment. 
     For example, the above-described embodiment is configured to connect the output terminals T 1  and T 2  and the temperature detecting element  3  via the wires L 1  and L 2  above the sub-mount  5 , but as illustrated in  FIGS. 4 and 5 , may be configured to embed the temperature detecting wires L 1  and L 2  in the sub-mount  5 . The sub-mount  5  illustrated in  FIGS. 4 and 5  is configured of a multilayer ceramic substrate. In this configuration, the temperature detecting wires L 1  and L 2  are partly configured of wires  5   a  and  5   b  in the multilayer ceramic substrate. 
     Specifically, the one electrode of the temperature detecting element  3  is electrically connected to the wire  5   a , and the wire  5   a  is electrically connected to a metal layer  54 . Further, the metal layer  54  is electrically connected to the output terminal T 1  by a lead wire L 11  such as a gold wire. On the other hand, the other electrode of the temperature detecting element  3  is electrically connected to a metal layer  55  formed in the vicinity of the temperature detecting element  3 , and the metal layer  55  is electrically connected to the wire  5   b . Further, the wire  5   b  is electrically connected to a metal layer  56 , and the metal layer  56  is electrically connected to the output terminal T 2  by a lead wire L 2  such as a gold wire. 
     As described, embedding parts of the temperature detecting wires L 1  and L 2  (in particular, the wire L 2  connected to the other electrode of the temperature detecting element  3 ) allows the sub-mount  5  itself to function as a heat capacity increasing part  7 , thus making it possible to reduce the amount of heat transferring through the wire L 2  and flowing into the temperature detecting element  3 . 
     Also, the one electrode of the semiconductor laser element  2  is electrically connected to a wire  5   c  in the multilayer ceramic substrate, and the wire  5   c  is electrically connected to a metal layer  57 . Further, the metal layer  57  is electrically connected to the feed terminal T 3  by a lead wire L 31  such as a gold wire. On the other hand, the other electrode of the semiconductor laser element  2  is electrically connected to the feed terminal T 4  by the wire L 4  such as a gold wire via a metal layer  58 . In addition, it is not required that the wires L 3  and L 4  for the semiconductor laser element  2  are configured to be embedded in the sub-mount. 
     As a configuration for embedding the wires L 1  and L 2  in the sub-mount  5 , in addition to the configuration using the wires  5   a  and  5   b  in the multilayer ceramic substrate as described above, it may be configured to form grooves in the sub-mount  5 , contain the wires L 1  and L 2  in the grooves, and seal them with resin or the like. Besides, the wires L 1  and L 2  may be laid in contact with the surface of the sub-mount  5 . 
     Also, as illustrated in  FIGS. 6 and 7 , the sub-mount  5  may be formed with a recess part M 1  for containing the semiconductor laser element  2  and formed with a recess part M 2  for containing the temperature detecting element  3 . In this case, the inner surface of the recess part M 1  faces the rear end surface of the semiconductor laser element and functions as a light shielding part that shields the light leaking out of the rear end surface to reduce stray light. Also, the light shielding surface of the light shielding surface is tilted slightly (e.g., at an angle of two degrees) so as to prevent reflected light from returning to the semiconductor laser element  2  again. In addition, in  FIGS. 6 and 7 , the sub-mount  5  is configured of the multilayer ceramic substrate, but may be configured of a simple ceramic substrate. 
     Further, a step part may be formed facing the rear end surface of the semiconductor laser element  2 . The height dimension of the step part is desirably made higher than the height dimension of the semiconductor laser element  2 , and desirably made equal to or higher than, for example, 2 mm. 
     In the above-described embodiment, described is the semiconductor laser device having the quantum cascade laser element; however, the semiconductor laser device may be one having another semiconductor laser element (e.g., a distributed Bragg-reflector laser (DBP laser)). 
     A method for driving the semiconductor laser element  2  may be a continuous oscillation (CW) method, a pseudo-continuous oscillation (pseudo-CW) method, or a pulse oscillation method. 
     In the above-described embodiment, described is an example of applying the semiconductor laser device to the gas analysis apparatus; however, the semiconductor laser device may be applied to another optical analysis apparatus. 
     Also, the analysis apparatus using the semiconductor laser device in the above-described embodiment may be one that, for example, in addition to being used to analyze the exhaust gas of a vehicle, analyzes a measurement target component (e.g.,. CO, CO 2 , H 2 O, NO, NO 2 , N 2 O, NH 8 , a hydrocarbon component such as HC, or an oxygen-containing hydrocarbon component such as HCHO) in various gases (e.g., environmental gas) such as the atmosphere, or analyze liquid.  
     Besides, it should be appreciated that the present invention is not limited to the above-described embodiment, but can be variously modified without departing from the scope thereof. 
     REFERENCE SIGNS LIST 
       100 : Semiconductor laser device 
       2 : Semiconductor laser element 
       3 : Temperature detecting element 
     T 1 , T 2 : Output terminal 
     L 1 , L 2 : Wire 
       7 : Heat capacity increasing part 
       4 : Cooling mechanism 
     COM: Control part 
       5 : Sub-mount 
       5   a,    5   b : Wire in multilayer ceramic substrate