Patent Publication Number: US-2020295535-A1

Title: Semiconductor laser device, and method and program for driving the same

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a semiconductor laser device, a method for driving a semiconductor laser device, and a program for driving a semiconductor laser device. 
     Background Art 
     Conventionally, there has been a gas analyzer that analyzes, for example, a measurement target component in exhaust gas by using a semiconductor laser device (for example, Patent Document 1). The gas analyzer includes the semiconductor laser device that emits a laser light having an oscillation wavenumber according to an absorption wavenumber of the measurement target component. The oscillation wavenumber is designed with an accuracy of an absorption wavenumber of ±1 cm −1 . Because the oscillation wavenumber is changed depending on an operating temperature of the semiconductor laser device, the semiconductor laser device is controlled so that variations in oscillation wavenumber of the laser light fall within ±0.01 cm −1  by temperature control of the semiconductor laser device. In one which is designed to sweep the oscillation wavenumber of the laser light with respect to a predetermined central wavenumber, control is made so that variations in the central wavenumber falls within ±0.01 cm −1 . 
     A semiconductor laser element to emit laser light has conventionally been mounted on a cooling module including a Peltier element in order to perform the temperature control. A supply power to the Peltier element is controlled using a detected temperature obtained from a temperature sensor mounted on the cooling module. Thus, the operating temperature of the semiconductor laser element is adjusted to control so that the variations in the oscillation wavenumber of the laser light falls within ±0.01 cm −1 . 
     However, because the temperature of the semiconductor laser element is indirectly measured by the temperature sensor disposed separately from the semiconductor laser element, the temperature of the semiconductor laser element may not necessarily be identical with the detected temperature obtained from the temperature sensor even if the temperature control is performed using the detected temperature obtained from the temperature sensor. Specifically, even though the temperature sensor is subject to temperature control using the Peltier element, the temperature of the semiconductor laser element may be changed due to a circumferential temperature change. Consequently, variations larger than ±0.01 cm −1  may occur in the oscillation wavenumber of the laser light. 
     PRIOR ART DOCUMENT 
     Patent Document 
     
         
         Patent Document 1: Japanese Unexamined Patent Publication No. 2009-216385 
       
    
     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     The present invention has been made to solve the above problem and has for its main object to ensure that the oscillation wavenumber of the semiconductor laser element is less likely to vary under the influence of the circumferential temperature. 
     Means for Solving the Problems 
     A semiconductor laser device in one of embodiments of the present invention includes a semiconductor laser element, a temperature control part to control a temperature of the semiconductor laser element, a temperature sensor to detect a temperature of the temperature control part, and a temperature control device to control a supply signal to the temperature control part so that a detected temperature obtained from the temperature sensor reaches a predetermined target temperature. The temperature control device changes a target temperature for the temperature control part depending on a supply signal to the temperature control part. A supply current, a supply voltage or a supply power is usable as a supply signal to the temperature control part. 
     If a circumferential temperature change occurs, a supply signal to the temperature control part is changed so that the detected temperature obtained from the temperature sensor reaches the target temperature. At this point, a temperature of the semiconductor laser element is different from the detected temperature obtained from the temperature sensor, and an oscillation wavenumber is therefore changed. With the present invention, the temperature control device changes the target temperature for the temperature control part depending on the supply signal to the temperature control part, and the temperature of the semiconductor laser element is therefore less likely to change due to a circumferential temperature change, and the oscillation wavenumber of the semiconductor laser element is therefore less likely to vary under the influence of the circumferential temperature. 
     In a specific embodiment of the temperature control device, the temperature control device preferably includes a relationship data storage part, a supply signal acquisition part and a supply control part. The relationship data storage part stores therein relationship data indicating a relationship between a supply signal to the temperature control part and a target temperature for the temperature control part. The supply signal acquisition part acquires a supply signal to the temperature control part. The supply control part acquires a target temperature for the temperature control part from the relationship data depending on the supply signal thus acquired, and controls the supply signal to the temperature control part so as to achieve the target temperature thus acquired. With this configuration, the oscillation wavenumber of the semiconductor laser element is less likely to vary under the influence of the circumferential temperature. 
     When controlling the temperature control part, it is conceivable that the supply signal varies slightly even in a stable state of the temperature control part. In this case, the target temperature may be changed in spite of that no circumferential temperature change occurs. Hence, the supply control part preferably acquires a target temperature for the temperature control part from the relationship data depending on an average value of the supply signal thus obtained, per predetermined period. The supply control part preferably does not change a target temperature for the temperature control part if a variation width of the average value of the supply signal thus obtained is less than a predetermined value. 
     If a setting value of an oscillation wavenumber of the semiconductor laser element is changed and used, the relationship data storage part stores therein relationship data indicating a relationship between a supply signal to the temperature control part and a target temperature for the temperature control part per oscillation wavenumber of the semiconductor laser. 
     In one of embodiments of the present invention, a method for driving a semiconductor laser device is intended to be applied to a semiconductor laser device. The semiconductor laser device includes a semiconductor laser element, a temperature control part to control a temperature of the semiconductor laser element, and a temperature sensor to detect a temperature of the temperature control part. The method includes controlling a supply signal to the temperature control part so that a detected temperature obtained from the temperature sensor reaches a predetermined target temperature. The method includes changing a target temperature for the temperature control part depending on a supply signal to the temperature control part. 
     In one of embodiments of the present invention, a driving program is used for a semiconductor laser device. The semiconductor laser device includes a semiconductor laser element, a temperature control part to control a temperature of the semiconductor laser element, a temperature sensor to detect a temperature of the temperature control part, and a control device to control a supply signal to the temperature control part so that a detected temperature obtained from the temperature sensor reaches a predetermined target temperature. The driving program causes the control device to perform a function of changing a target temperature for the control temperature part depending on a supply signal to the temperature control part. 
     Effect of the Invention 
     With the present invention as described above, the oscillation wavenumber of the semiconductor laser element is less likely to vary under the influence of the circumferential temperature because the target temperature of the temperature control part is changed depending on the supply signal to the temperature control part. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a general schematic diagram of an exhaust gas analyzer for which a semiconductor laser device in one of embodiments is used; 
         FIG. 2  is a general schematic diagram of the semiconductor laser device in the embodiment; 
         FIG. 3  is a sectional view orthogonal to an optical waveguide direction of a semiconductor laser element part in the embodiment; 
         FIG. 4  is a sectional view taken along line A-A in the semiconductor laser element part in the embodiment; 
         FIG. 5  is a diagram illustrating light emitting principle of quantum cascade laser; 
         FIG. 6  is a functional block diagram of the semiconductor laser device in the embodiment; and 
         FIG. 7  is a schematic diagram illustrating operation contents during a circumferential temperature change in the embodiment. 
     
    
    
     DESCRIPTION OF THE REFERENCE CHARACTERS 
     
         
         
           
               10  gas analyzer 
               11  measurement cell 
               12  photodetector 
               13  analysis part 
               100  semiconductor laser device 
               2  semiconductor laser element 
               32  Peltier element (temperature control part) 
               4  temperature sensor 
             temperature control device 
               51  relationship data storage part 
               52  supply signal acquisition part 
               53  supply control part 
           
         
       
    
     MODE FOR CARRYING OUT THE INVENTION 
     A semiconductor laser device in one of embodiments of the present invention is described below with reference to the drawings. 
     The semiconductor laser device  100  in the embodiment is intended to be used for a gas analyzer  10  that analyzes a measurement target component in exhaust gas discharged from, for example, an internal combustion engine or an external combustion engine, and in atmosphere as illustrated in  FIG. 1 . The gas analyzer  10  includes a multiple reflection type measurement cell  11  to permit introduction of the exhaust gas, the semiconductor laser device  100  to emit a laser light to the measurement cell  11 , a photodetector  12  to detect the laser light after passing through the measurement cell  11 , and an analysis part  13  to analyze the measurement target component by using a detection signal obtained from the photodetector  12 . 
     Specifically, the semiconductor laser device  100  is designed to emit a laser light having an oscillation wavenumber of approximately ±1 cm −1  with respect to an absorption wavenumber of the measurement target component. As illustrated in  FIG. 2 , the semiconductor laser device  100  includes a semiconductor laser element  2 , a temperature control module  3  to control a temperature of the semiconductor laser element  2 , a temperature sensor  4  disposed in the temperature control module  3 , and a temperature control device  5  to control the temperature control module  3  on the basis of a detected temperature obtained from the temperature sensor  4 . 
     The semiconductor laser element  2  is, for example, a quantum cascade laser (QCL) element, and oscillates laser lights of, for example, mid-infrared (4-10 μm). The semiconductor laser element  2  is capable of modulating (changing) an oscillation wavenumber (oscillation wavelength) depending on an applied current (or voltage). The semiconductor laser element  2  is subject to current (voltage) control by the laser control device  6 . 
     The semiconductor laser element  2  is, for example, a distributed feedback laser (DFB laser), and includes an optical waveguide  2 A composed of a clad layer and a core layer, each disposed on a semiconductor substrate  20  as illustrated in  FIGS. 3 and 4 . A light passes through the core layer due to a difference between a refractive index of the clad layer and a refractive index of the core layer in the optical waveguide  3 A. 
     Specifically, the semiconductor laser element  2  is one in which a buffer layer  21 , a core layer  22 , an upper clad layer  23  and a cap layer  24  are formed in this order on an upper surface of the semiconductor substrate  20 . These layers  21  to  24  are individually extended in an identical direction. The optical waveguide  2 A extended in one direction can be formed by causing side surfaces of these layers in their width direction to be covered with a protection film  25 . The protection film  25  is an inorganic film and may be composed of, for example, SiO 2  or a combination of SiO 2  and Si 3 N 4 . 
     The buffer layer  21  and the upper clad layer  23  are layers, each composed of InP. A lower clad layer composed of InP may be disposed between the buffer layer  21  and the core layer  22 . Alternatively, the buffer layer  21  may serve as a clad layer. 
     The cap layer  24  is a layer composed of InGaAs, and a part of an upper surface of the layer (both sides in a width direction) is covered with the protection layer  25 . Other parts of the upper surface of the cap layer  24  (a middle part in the width direction) are covered with an upper electrode  26 . 
     The core layer  22  includes a lower guide layer  221  composed of InGaAs, an active layer  222  that emits light upon current injection, and an upper guide layer  223  composed of InGaAs. 
     The active layer  222  is composed of a multi quantum well structure including a plurality of well layers, in which a plurality of semiconductor layers serving as a light-emitting region and a plurality of semiconductor layers serving an injection region are alternately laminated one upon another. These well layers may be different in thickness. The semiconductor layer serving as the light-emitting region is formed by alternately laminating InGaAs and InAlAs. The semiconductor layer serving as the injection region is formed by alternately laminating InGaAs and InAlAs. 
     A semiconductor layer element part thus configured is a quantum cascade laser, in which a plurality of cell layers are subjected to multistage connection as illustrated in  FIG. 5 , is designed to emit a light by optical transition between subbands formed in quantum cells. The semiconductor laser  2  may be a distributed bragg reflector laser (DBR laser). 
     A diffraction grating  2 B is formed between the core layer  22  and the upper clad layer  23 , namely, on the upper guide layer  223  in the semiconductor laser element  2  (refer to  FIG. 4 ). The diffraction grating  2 B is composed of concave parts and convex parts alternately formed on the upper guide layer  223 , and the concave parts and the convex parts are extended in a width direction of the upper guide layer  223 . Lights having a predetermined oscillation wavenumber are mutually intensified and selectively amplified by the diffraction grating  2 B. The predetermined oscillation wavenumber is prescribed by a pitch of the diffraction grating  2 B. 
     A lower electrode  37  is disposed on a lower surface of the semiconductor substrate  20 . A laser light having a predetermined oscillation wavenumber prescribed by the diffraction grating  2 B is emitted by applying a current (voltage) for laser oscillation to an upper upper electrode  26  and a lower electrode  27 . A current source (or voltage source) is coupled to the upper electrode  26  and the lower electrode  27  that are intended for laser oscillation, and the laser control device  8  controls the current source (voltage source). 
     The laser control device  6  is designed to control the current source (or voltage source) of the semiconductor laser element  2  by outputting a current (voltage) control signal. Specifically, the laser control device  6  controls the current source (or voltage source) of the semiconductor laser element  2  by outputting a current (or voltage) control signal, thereby causing the semiconductor laser element  2  to be subjected to continuous wave operation (CW) or pulse-like oscillation (pulse driving). Ones which cause the pulse driving include pseudo continuous wave operation (pseudo CW). 
     The temperature control module  3  includes a substrate  31  having the semiconductor laser element  2  mounted on one surface thereof, namely, a front surface thereof, and a Peltier element  32  that is a temperature control part whose endothermic surface is contactedly disposed on the other surface, namely, rear surface of the substrate  31 . The Peltier element  32  is a semiconductor element capable of controlling heat generation and heat absorption by using current. The Peltier element  32  has such a configuration that a plurality of alternately disposed N-type semiconductors and P-type semiconductors are connected in series by using a metal electrode, and they are held between a pair of ceramic substrates. Alternatively, the temperature control module  3  may not include the substrate  31 , but may include a semiconductor laser element mounted on the ceramic substrate on a heat absorption side of the Peltier element  32 . As a temperature control part, it is possible to employ, for example, one which uses a compressor, one which uses a heating wire, one which uses a fan, or one which employs a water-cooled system. 
     The temperature sensor  4  is disposed on a surface of the substrate  31  and intended to detect a temperature of the substrate  31  with the semiconductor laser element  2  mounted thereon. The temperature sensor  4  is, for example, a thermistor. Although the temperature sensor  4  is disposed away from the semiconductor laser element  2  in the embodiment, the temperature sensor  4  may be disposed in contact with the semiconductor laser element  2 . 
     The semiconductor laser element  2  and the cooling module  3  are accommodated in a tight container  7  in the embodiment. A light outlet part  71 , through which a laser light passes to the outside, is formed at a part of the tight container  7  which is opposed to a light emission part of the semiconductor laser element  2 . An optical window member  8  is disposed on the light outlet part  71 , and the optical window member  8  is inclined slightly (for example, 2 degrees) in order to prevent a laser light reflected from the optical window member  8  from returning to the semiconductor laser element  2 . A light emitting module is constituted by the tight container  7  accommodating therein the semiconductor laser element  2  and the cooling module  3  or the like. 
     The temperature control device  5  is intended to perform feedback control of a supply current, a supply voltage or a supply power to the Peltier element  32  on the basis of a detected temperature obtained from the temperature sensor  4 . Specifically, the temperature control device  5  controls the current source (voltage source) of the Peltier element  32  by outputting a current (or voltage) control signal. 
     The temperature control device  5  is a general-purpose or special-purpose computer including, as a structure thereof, CPU, memory, input/output interface and AD converter. The temperature control device  5  controls a supply power to the Peltier element  32  by causing the CPU and peripheral devices thereof to operate according to a driving program stored in the memory. 
     Specifically, the temperature control device  5  includes a relationship data storage part  51 , a supply data acquisition part  52  and a supply control part  53  as illustrated in  FIG. 6 . The relationship data storage part  51  stores therein relationship data indicating a relationship between a target temperature for the temperature sensor  4  and a supply power to the Peltier element  32  if an oscillation wavenumber of the semiconductor laser element  2  is made constant. The supply data acquisition part  52  acquires supply data indicating a supply power to the Peltier element  32 . The supply control part  53  performs feedback control of the supply power to the Peltier element  32 . 
     The relationship data storage part  51  stores therein relationship data indicating a relationship between a target temperature T for the temperature sensor  4  and a supply power P to the Peltier element  32  if the oscillation wavenumber (oscillation wavelength λ 1 ) of the semiconductor laser element  2  is made constant. 
     A method for generating the relationship data is, for example, as follows. 
     A light-emitting module of the semiconductor laser device  100  as a specimen is accommodated in a thermostatic container (not illustrated), and circumferential temperatures (ambient temperatures) are changed, for example, from 5° C. to 45° C. The Peltier element  32  is controlled so that the oscillation wavelength of the semiconductor laser element  2  becomes constant (λ 1 ) at the above circumferential temperatures. 
     A detected temperature obtained from the Peltier element  32  when the oscillation wavelength of the semiconductor laser element  2  became constant (λ 1 ) at the individual circumferential temperature and a supply power at that time are recorded. In terms of the supply power, a supply current and a supply voltage, each supplied to the Peltier element  32 , are detected, and the supply power is calculated from the supply current and the supply voltage. 
     Thus, the relationship between the target temperature and the supply power when a circumferential temperature change occurs can be found in cases where the oscillation wavelength of the semiconductor laser element  2  is made constant (λ 1 ). The relationship data indicating this relationship is stored in the relationship data storage part  51 . The relationship data may be data indicating a relationship between absolute values at individual target temperatures (T 1 , T 2 , T 3 , . . . ) and absolute values of individual supply powers (P 1 , P 2 , P 3 , . . . ) or, alternatively, data indicating a relationship between a variation relative to a target temperature (T 1 ) serving as a criterion (ΔT 1 (=T 2 −T 1 ), ΔT 2 (=T 3 −T 1 ), . . . ) and a variation relative to a supply power (P 1 ) serving as a criterion (ΔP 1 (=P 2 −P 1 ), ΔP 2 (=P 3 −P 1 ), . . . ). 
     In the case of generating relationship data relating not only a single oscillation wavelength (λ 1 ) but also a different oscillation wavelength (λ 2 ), the same operation as described above is carried out. The number of the relationship data is not limited to two, but it may be configured to have relationship data respectively relating to a plurality of oscillation wavelengths. 
     A circumferential temperature is detectable from a supply power by linking the circumferential temperature to the relationship data. That is, the Peltier element  32  is usable as a circumferential temperature sensor. 
     The supply data acquisition part  52  is designed to acquire a supply current and a supply voltage supplied to the Peltier element  32  and then calculate a supply power by using the supply current and the supply voltage in a state in which the Peltier element  32  is controlled so that a detected temperature obtained from the temperature sensor  4  reaches a constant target temperature. 
     The supply current supplied to the Peltier element  32  is detected by a current sensor, and the supply voltage is detected by a voltage sensor. If the Peltier element  32  is subjected to current control, a setting value for the current control is usable. If the Peltier element  32  is subjected to voltage control, a setting value of the voltage control is usable. The supply data acquisition part  52  outputs the calculated supply power data to the supply control part  53 . 
     The supply control part  53  receives the relationship data from the relationship data storage part  51 , and also receives the supply power data from the supply data acquisition part  52 . Then, the supply control part  53  changes a target temperature for the temperature sensor  4  so that the oscillation wavenumber (oscillation wavelength), from the supply power data and the relationship data. The supply control part  53  then controls a supply power to the Peltier element  32  so that the detected temperature obtained from the temperature sensor  4  reaches a target temperature after being changed. The supply control part  53  is designed not to change the target temperature for the Peltier element  32  if a variation range of the acquired supply power is less than a predetermined value. The term “predetermined value” denotes a threshold value for distinguishing a variation width of the supply power generated in a state in which the Peltier element  32  is controlled so as to achieve the target temperature when no circumferential temperature occurs, and a variation width of a supply power which occurs when a circumferential temperature change occurs. 
     &lt;Operations of Semiconductor Laser Device  100 &gt; 
     Operations performed to ensure that the oscillation wavenumber (oscillation wavelength) of the semiconductor laser device  100  becomes constant are described below with reference to  FIG. 7 . The following temperatures are for the purpose of description, which are different from actual values. 
     For example, suppose that the semiconductor laser element  2  is oscillated in a situation where a circumferential temperature (ambient temperature) is 27° C., and the Peltier element  32  is controlled so that a detected temperature obtained from the temperature sensor  4  reaches 25.0° C. It is also supposed that an actual temperature of the semiconductor laser element  2  is, for example, 70.0° C. 
     In this state, the laser control device  6  controls a current (or voltage) applied to the semiconductor laser element  2  so as to cause the semiconductor laser element  2  to emit a laser light having a predetermined wavenumber (predetermined wavelength) (refer to  FIG. 7 ( 1 )). 
     If the circumferential temperature is changed from 27° C. to 30° C., the temperature control device  5  controls the Peltier element  32  so that a detected temperature obtained from the temperature sensor  4  reaches 25° C. At this point, a supply power to the Peltier element  32  is increased due to a circumferential temperature change, and the semiconductor laser element  2  is excessively cooled and changed to, for example, 69.6° C. The oscillation wavenumber (oscillation wavelength) varies accordingly (refer to  FIG. 7 ( 2 )). 
     The supply data acquisition part  52  of the temperature control device  5  acquires supply power data for the Peltier element  32 . The supply control part  53  of the temperature control device  5  changes a target temperature so that the oscillation wavenumber (oscillation wavelength) of the semiconductor laser element  2  becomes constant, from the supply power data and the relationship data. For example, the target temperature for the temperature sensor  4  is changed to 24.5° C. The supply control part  53  therefore controls a supply power to the Peltier element  32  so that the detected temperature obtained from the temperature sensor  4  reaches 24.5° C. 
     Effects of Embodiments 
     With the semiconductor laser device  100  of the above embodiment, the oscillation wavenumber of the semiconductor laser element  2  is less likely to vary under the influence of a circumferential temperature. This is because a target temperature is changed so that the oscillation wavenumber of the semiconductor laser element  2  becomes constant by using the relationship between a target temperature and a supply power to the Peltier element  32  if an oscillation wavenumber (oscillation wavelength) of the semiconductor laser element  2  is made constant. 
     For example, in cases where the semiconductor laser device  100  analyzes a measurement target component in exhaust gas, the circumferential temperature of the semiconductor laser element  10  may vary significantly due to a temperature influence of the exhaust gas as being a sample gas and an atmospheric temperature influence in addition to the influence of heat generated from the semiconductor laser element  2  itself. The semiconductor laser device  100 , to which the present invention is already applied, is capable of highly precisely analyzing the exhaust gas because the oscillation wavenumber of the semiconductor laser element  2  is less likely to vary under the influence of the circumferential temperature. In cases where the semiconductor laser device  100  analyzes atmospheric pollutant or the like, there is some possibility of, for example, continuously analyzing the atmospheric pollutant in the open air in a range from several hours to several days, the semiconductor laser device  100  is susceptible to the atmospheric temperature influence. The semiconductor laser device  100 , to which the present invention is already applied, is capable of highly precisely analyzing the atmospheric pollutant because the oscillation wavenumber of the semiconductor laser element  2  is less likely to vary under the influence of the circumferential temperature. 
     Other Modified Embodiments 
     The present invention is not limited to the foregoing embodiments. 
     For example, even though the power control part  53  of the temperature control device  5  is designed to change a target temperature by using the supply power data and the relationship data, the power control part  53  may be designed to adjust the supply power by using these data. For example, the power control part  53  adjusts a supply power by using a relationship of ΔWN corr =a×P pel +b, where ΔWN corr  is a correction amount of an oscillation wavenumber (a reciprocal of the oscillation wavenumber, P pel  is a supply power to the Peltier element  32 , and coefficients “a” and “b” are values included in the relationship data or values obtainable from the relationship data. 
     The supply signal acquisition part may be designed to acquire a supply signal indicating a supply current or a supply voltage to the temperature control part in the above embodiment. In this case, the relationship data storage part stores therein relationship data indicating a relationship between a target temperature and a supply current or supply voltage for the temperature control part when an oscillation wavenumber of the semiconductor laser element is made constant. The supply control part controls the supply current or supply voltage for the temperature control part so that a detected temperature obtained from the temperature sensor  4  reaches a target temperature after being changed by changing the target temperature for the temperature sensor  4  so that the oscillation wavenumber (oscillation wavelength) becomes constant, from the supply signal and the relationship data. 
     While the above embodiments have described the semiconductor laser devices including the quantum cascade laser element, the semiconductor laser device may include another semiconductor laser element  2 . 
     The temperature control device  5  may be designed to determine deterioration and lifetime of the Peltier element by monitoring a change in resistance value of the Peltier element from a supply current and a supply voltage to the Peltier element. 
     Besides the above, various modifications and combinations of embodiments may be made as long as they do not depart from the spirit and scope of the present invention. 
     INDUSTRIAL APPLICABILITY 
     With the present invention, the oscillation wavenumber of the semiconductor laser element is less likely to vary under the influence of the circumferential temperature.