Analyzer, absorption characteristic calculation circuit, and analysis method

An analyzer includes a quantum cascade laser that converts a cyclic driving signal to laser light; an optical receiver that receives the laser light having passed through a sample and outputs a detected signal depending on intensity of the laser light; and a data calculation portion that outputs information representing absorption characteristics of the sample. The data calculation portion includes a delaying unit that produces a time-delayed waveform by applying a time delay to a reference driving signal; an adding unit that produces a symmetrical waveform by adding the time-delayed waveform and the detected signal; a time inversion unit that produces a time-inverted waveform by time-inverting the symmetrical waveform; and a subtracting unit that produces a waveform difference between the time-inverted waveform and the symmetrical waveform. The data calculation portion repeatedly calculates the waveform difference by changing the time delay until the waveform difference is minimized.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an analyzer, an absorption characteristic calculation circuit, and an analysis method.

2. Description of the Related Art

A quantum cascade laser (QCL) is a laser that emits laser light having a wavelength in a mid-infrared range of 3 μm to 20 μm, for example. The QCL is highly expected as a small-sized and low-cost light source in various fields such as environmental gas analysis, medical diagnosis, and industrial processing. T. K. Subramaniam, “Quantum Cascade Laser in Atmospheric Trace Gas Analysis”, AASCIT Journal of Environment, April 2015, discloses a technique of analyzing components of environmental gas by irradiating the gas with mid-infrared light emitted from the QCL, and by measuring a spectrum of the light having passed through the gas. Jonas Kottmann et al., “Mid-Infrared Photoacoustic Detection of Glucose in Human Skin: Towards Non-Invasive Diagnostics”, Sensors, October 2016, discusses a feasibility of non-invasive diagnosis of a blood glucose value by utilizing absorption characteristics of glucose, etc. with respect to light of a particular wavelength in the mid-infrared range.

Other references are as follows:Jane Hodgkinson and Ralph P Tatam, “Optical gas sensing: a review”, Measurement Science and Technology, 24 (2013)Lionel Tombez, “Analysis and improvement of the spectral properties in mid-infrared semiconductor quantum cascade lasers”, doctoral thesis, UNIVERSITÉ DE NEUCHÂTEL, 2014Andreas Hangauer et al., “High-speed Modulation Characteristic of a Quantum Cascade Laser”, Conference Paper, ResearchGate, June 2013

SUMMARY OF THE INVENTION

In order to measure components of a gas sample and their concentrations, an absorption spectrum of the gas sample needs to be measured with high accuracy. For that purpose, a wavelength of mid-infrared light used in measurement needs to be changed accurately and continuously. It deems that the wavelength of the light emitted from the QCL can be changed by changing a temperature of the QCL with a thermoelectric cooler (TEC), and/or by changing an amount of injection current applied to the QCL. The following methods are known, by way of example. (a) According to one method, mid-infrared light changing in intensity and wavelength of the light is obtained by changing each of the temperature, the amount of injection current, and the length of a cavity formed by a reflective mirror disposed in the outside slowly to such an extent as maintaining a state in which a continuous wave of light is stably laser-oscillated from the QCL. (b) According to another method, the intensity of mid-infrared light with wavelength slowly changing in a stable laser oscillation state is changed by causing the light to pass through a chopper. (c) According to still another method, mid-infrared light changing in intensity and wavelength is obtained by modulating conditions of current injection to the QCL at a short time cycle.

With the above method (a), the wavelength of the light for use in measurement is stabilized, but a measurement time is prolonged. Another problem is that, with an increase of the measurement time, the influences of 1/f noise and drift caused in the QCL appear significantly and an S/N ratio decreases. With the above method (b), the influences of the drift, etc. can be reduced while the wavelength of the light is stabilized. However, there is a risk in regards to reliability and measurement accuracy because the mechanically-operated chopper is used.

With the above method (c), because the intensity of the light is electrically changed, the risk in regards to reliability is less in comparison with the above method (b). Furthermore, since the measurement can be performed in a shorter time than in the case of using the above method (a), the influences of the drift, etc. can be reduced. However, when the injection current to the QCL is changed at a short time cycle, a complicated wavelength shift from the expected wavelength change occurs due to the influence of frequency chirping and the influence of a temperature change of the QCL caused by a current change. In order to measure the absorption spectrum of the sample gas with high accuracy, it is desired to exclude the influence of the wavelength shift caused by the cyclic change of the injection current to the QCL as far as possible. Moreover, various conditions for the current injection are set on the QCL in order to measure a variety of gas samples having unknown components and concentrations. Because the wavelength shift changes depending on the conditions for the current injection, measuring the wavelength shifts under the various conditions for the current injection in advance takes a lot of effort and time.

To solve the above-described problem, an analyzer according to an embodiment includes an QCL that receives a driving signal including a cyclic waveform and converts the driving signal to laser light in a mid-infrared range, an optical receiver that receives the laser light having passed through a sample and outputs a detected signal depending on the intensity of the laser light, and a data calculation portion that outputs, on the basis of the detected signal, information representing absorption characteristics of the sample. The data calculation portion includes a delaying unit that produces a time-delayed waveform by applying, to one of a waveform of a reference driving signal and a waveform of the detected signal, a time delay in comparison with the other waveform, an adding unit that produces a symmetrical waveform by adding the time-delayed waveform to the other waveform, a time inversion unit that produces a time-inverted waveform by inverting a time dependency of the symmetrical waveform, and a subtracting unit that produces a waveform of a difference between the time-inverted waveform and the symmetrical waveform. The data calculation portion repeatedly calculates the waveform of the difference while the time delay applied in the delaying unit is changed, and outputs, as the information representing absorption characteristics of the sample, the symmetrical waveform or information obtained from the symmetrical waveform when an absolute value of the waveform of the difference is minimized.

An absorption characteristic calculation circuit according to an embodiment is to output information representing absorption characteristics of a sample on the basis of a detected signal depending on the intensity of laser light in a mid-infrared range, the laser light being generated in accordance with a driving signal including a cyclic waveform and passing through the sample. The absorption characteristic calculation circuit includes a delaying unit that produces a time-delayed waveform by applying, to one of a waveform of a reference driving signal and a waveform of the detected signal, a time delay in comparison with the other waveform, an adding unit that produces a symmetrical waveform by adding the time-delayed waveform and the other waveform, a time inversion unit that produces a time-inverted waveform by inverting the time dependency of the symmetrical waveform, and a subtracting unit that produces a waveform of a difference between the time-inverted waveform and the symmetrical waveform. The absorption characteristic calculation circuit repeatedly calculates the waveform of the difference while the time delay applied in the delaying unit is changed, and outputs, as the information representing the absorption characteristics of the sample, the symmetrical waveform or information obtained from the symmetrical waveform when an absolute value of the waveform of the difference is minimized.

An analysis method according to an embodiment includes a light generation step of supplying a driving signal including a cyclic waveform to a QCL, and converting the driving signal to laser light in a mid-infrared range, a light detection step of producing a detected signal depending on the intensity of the laser light that has passed through a sample, and a data calculation step of producing, on the basis of the detected signal, information representing absorption characteristics of the sample. The data calculation step includes a delaying step of applying, to one of a waveform of a reference driving signal and a waveform of the detected signal, a time delay in comparison with the other waveform, an adding step of producing a symmetrical waveform by adding the one time-delayed waveform and the other waveform, a time inversion step of producing a time-inverted signal by inverting a time dependency of the symmetrical waveform, and a subtracting step of producing a waveform of a difference between the time-inverted waveform and the symmetrical waveform. In the data calculation step, the waveform of the difference is repeatedly calculated while the time delay applied in the delaying step is changed, and the symmetrical waveform or information obtained from the symmetrical waveform when an absolute value of the waveform of the difference is minimized is given as the information representing the absorption characteristics of the sample.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Description of Embodiment of Present Invention

First, features of an embodiment of the present invention are described. An analyzer according to an embodiment includes an QCL that receives a driving signal including a cyclic waveform and converts the driving signal to laser light in a mid-infrared range, an optical receiver that receives the laser light having passed through a sample and outputs a detected signal depending on the intensity of the laser light, and a data calculation portion that outputs, on the basis of the detected signal, information representing absorption characteristics of the sample. The data calculation portion includes a delaying unit that produces a time-delayed waveform by applying, to one of a waveform of a reference driving signal and a waveform of the detected signal, a time delay in comparison with the other waveform, an adding unit that produces a symmetrical waveform by adding the time-delayed waveform and the other waveform, a time inversion unit that produces a time-inverted waveform by inverting a time dependency of the symmetrical waveform, and a subtracting unit that produces a waveform of a difference between the time-inverted waveform and the symmetrical waveform. The data calculation portion repeatedly calculates the waveform of the difference while the time delay applied in the delaying unit is changed, and outputs, as the information representing absorption characteristics of the sample, the symmetrical waveform or information obtained from the symmetrical waveform when an absolute value of the waveform of the difference is minimized.

An analysis method according to an embodiment includes a light generation step of supplying a driving signal including a cyclic waveform to a QCL, and converting the driving signal to laser light in a mid-infrared range, a light detection step of producing a detected signal depending on the intensity of the laser light that has passed through a sample, and a data calculation step of producing, on the basis of the detected signal, information representing absorption characteristics of the sample. The data calculation step includes a delaying step of applying, to one of a waveform of a reference driving signal and a waveform of the detected signal, a time delay in comparison with the other waveform, an adding step of producing a symmetrical waveform by adding the one time-delayed waveform and the other waveform, a time inversion step of producing a time-inverted signal by inverting a time dependency of the symmetrical waveform, and a subtracting step of producing a waveform of a difference between the time-inverted waveform and the symmetrical waveform. In the data calculation step, the waveform of the difference is repeatedly calculated while the time delay applied in the delaying step is changed, and the symmetrical waveform or information obtained from the symmetrical waveform when an absolute value of the waveform of the difference is minimized is given as the information representing the absorption characteristics of the sample.

According to the analyzer and the analysis method described above, the QCL receives the driving signal that is modulated so as to include the cyclic waveform. The QCL converts the driving signal to the laser light in the mid-infrared range where absorption wavelengths of many substances are present. The laser light is applied to the sample and passes through the sample while the laser light is partly absorbed at wavelengths corresponding to components of the sample. In the optical receiver (light detection step), the detected signal is produced depending on the intensity of the laser light that has passed through the sample. In the data calculation portion (data calculation step), the detected signal is processed, and the information representing the absorption characteristics of the sample is produced. A measurement operator can recognize the components of the sample and their concentrations on the basis of the produced information.

Furthermore, according to the analyzer and the analysis method described above, the data calculation portion (data calculation step) includes the delaying unit (delaying step), the adding unit (adding step), and the subtracting unit (subtracting step). The delaying unit (delaying step) applies, to one of a waveform of the driving signal and a waveform of the detected signal, a time delay in comparison with the other waveform. A phase difference is thus generated between the driving signal and the detected signal. The adding unit (adding step) adds both the waveforms having the phase difference therebetween. The subtracting unit (subtracting step) produces a waveform of a difference between a waveform obtained by time-inverting a waveform after the addition and the waveform after the addition. The waveform of the difference is repeatedly calculated while the time delay is changed. With the above-described configuration, the time delay when the absolute value of the waveform of the difference is minimized represents the magnitude of a time shift in wavelength. Accordingly, the accurate wavelength at which the absorption has occurred can be recognized in consideration of the time delay when the absolute value of the waveform of the difference is minimized. Stated in another way, according to the analyzer and the analysis method described above, the influence of the wavelength shift, which is caused by cyclic change of an injection current to the QCL, can be eliminated, and the information regarding the absorption characteristics of the sample can be accurately extracted from the detected signal. As a result, the absorption characteristics of the sample can be obtained with high accuracy using the measurement method of modulating the injection current to the QCL.

In the analyzer and the analysis method described above, the data calculation unit (data calculation step) may further include a waveform correction unit (waveform correction step) that produces, using parameters prepared in advance and the waveform of the reference driving signal, a corrected waveform by superimposing a distortion on the waveform of the driving signal to provide the corrected waveform closer to a waveform of the laser light output from the QCL. With that feature, since the above-described processing can be performed in the data calculation unit (data calculation step) in consideration of a variation in the light intensity (i.e., a distortion of the waveform of the laser light) caused by temperature changes, the absorption characteristics of the sample can be obtained with higher accuracy.

In the analyzer and the analysis method described above, the cyclic waveform may be given by a sinusoidal wave. With that feature, the driving signal including the cyclic waveform can be easily produced.

An absorption characteristic calculation circuit according to an embodiment is to output information representing absorption characteristics of a sample on the basis of a detected signal depending on the intensity of laser light in a mid-infrared range, the laser light being generated in accordance with a driving signal including a cyclic waveform and passing through the sample. The absorption characteristic calculation circuit includes a delaying unit that produces a time-delayed waveform by applying, to one of a waveform of a reference driving signal and a waveform of the detected signal, a time delay in comparison with the other waveform, an adding unit that produces a symmetrical waveform by adding the time-delayed waveform and the other waveform, a time inversion unit that produces a time-inverted waveform by inverting the time dependency of the symmetrical waveform, and a subtracting unit that produces a waveform of a difference between the time-inverted waveform and the symmetrical waveform. The absorption characteristic calculation circuit repeatedly calculates the waveform of the difference while the time delay applied in the delaying unit is changed, and outputs, as the information representing the absorption characteristics of the sample, the symmetrical waveform or information obtained from the symmetrical waveform when an absolute value of the waveform of the difference is minimized.

The absorption characteristic calculation circuit includes, as in the data calculation portion of the above-described analyzer, the delaying unit, the adding unit, and the subtracting unit. Accordingly, the influence of the wavelength shift, which is caused by the cyclic change of the injection current to the QCL, can be eliminated, and the information regarding the absorption characteristics of the sample can be accurately extracted from the detected signal. As a result, the absorption characteristics of the sample can be obtained with high accuracy using the measurement method of modulating the injection current to the QCL.

Details of Embodiment of Present Invention

Practical examples of the analyzer, the absorption characteristic calculation circuit, and the analysis method according to the embodiment of the present invention will be described below with reference to the drawings. The present invention is not limited to those practical examples, and the present invention is purported to include all modifications that are equivalent in meaning to the scope of Claims, and that fall within the scope of Claims. In the following description, the same elements in the drawings are denoted by the same reference signs, and duplicate description of those elements is omitted. It is to be noted that, in the following description, the term “waveform” stands for time dependency of a signal.

FIG. 1is a block diagram illustrating a configuration of a gas analyzer1A that is an analyzer according to the embodiment of the present invention. As illustrated in FIG.1, the gas analyzer1A according to this embodiment includes a QCL module10serving as a light source, a sample cell20containing sample gas, i.e., a gas sample to be measured, a mercury-cadmium-telluride: HgCdTe (MCT) module30serving as an optical receiver, a spectrum processing circuit40serving as a data calculation portion, a driver circuit70driving the QCL module10, and a display60. Of those components, the QCL module10, the sample cell20, the MCT module30, and the spectrum processing circuit40are accommodated in a common housing3. Although the driver circuit70and the display60are disposed outside the housing3in the illustrated example, they may be disposed inside the housing3. At least one of the QCL module10, the sample cell20, the MCT module30, and the spectrum processing circuit40may be disposed outside the housing3.

The driver circuit70is a circuit for producing an electrical driving signal Sd that is supplied to the QCL module10. The driving signal Sd produced by the driver circuit70includes a cyclically changing voltage signal. A part of the driving signal is branched off from the driving signal Sd, and is sent to the spectrum processing circuit40as a reference driving signal Sd0.FIGS. 2A, 2B and 2Care graphs depicting various examples of time dependency of the driving signal Sd.FIG. 2Adepicts a waveform of a signal with a voltage changing in the form of a sinusoidal wave at a certain time cycle.FIG. 2Bdepicts a waveform of a triangular wave signal with a voltage repeatedly increasing and decreasing in a linear form at a certain time cycle.FIG. 2Cdepicts a waveform of a signal with a voltage repeatedly increasing and decreasing in a semicircular form at a certain time cycle. Desirably, the time dependency of the driving signal Sd has time reversal symmetry in one cycle. Here, the term “time reversal symmetry” implies that, in a graph with a horizontal axis representing time, waveforms in time spans before and after a center time of one cycle are linearly symmetrical to each other. The driving signal Sd has a frequency of not less than 10 Hz and not more than 1 MHz, for example. The gas analyzer1A may be supplied with the driving signal Sd from the outside of the gas analyzer1A without including the driver circuit70. The driving signal Sd0has the same waveform as the driving signal Sd.

The QCL module10receives the driving signal Sd supplied from the driver circuit70and outputs laser light LS in a mid-infrared range. The QCL module10includes a package11, a TEC12, a submount13, and a QCL14. The package11air-tightly seals off the TEC12, the submount13, and the QCL14. An exit window11ais formed in part of the package11. The exit window11ais made of a material transparent to the wavelength of the laser light LS. The laser light LS produced inside the package11is emitted to the outside of the package11through the exit window11a.

The QCL14is dynamically and sinusoidally driven upon receiving the driving signal Sd, and converts the driving signal Sd to the laser light LS in the mid-infrared range. In more detail, a current corresponding to the magnitude of a voltage of the driving signal Sd is injected to the QCL14, and the QCL14is laser-oscillated by the injected current. A wavelength of the laser light output from the QCL14is longer than 3 μm and more preferably longer than 4 μm, for example. In addition, the wavelength of the laser light output from the QCL14is shorter than 10 μm and more preferably shorter than 8 μm, for example. Within such a wavelength range, the QCL14enables the laser oscillation wavelength to be changed over a range of about 0.01 μm (wavelength sweep) depending on change of the magnitude of the injection current. A spectrum width of the laser oscillation light from the QCL14when not subjected to the wavelength sweep is very narrow. Static characteristics of the QCL14, i.e., changes of the intensity and the wavelength of the light from the QCL14with respect to the injection current when the injection current is statically changed, are previously measured and known.

The submount13is a rectangular parallelepiped member on which the QCL14is mounted. The submount13contains a material having good thermal conductivity, such as AlN. The QCL14is fixed onto a front surface of the submount13with, for example, a solder or a thermally conductive adhesive interposed between them. The QCL14generates heat due to the current injection, and the heat generated from the QCL14dissipates to the submount13. The TEC12is placed on a bottom surface of the package11. The submount13is mounted onto a stage of the TEC12. The TEC12dissipates heat of the submount13to the package11upon receiving a driving current supplied from the outside of the package11. The magnitude of the driving current supplied to the TEC12is controlled such that a temperature of the QCL14is kept constant. The temperature of the QCL14is not lower than 0° C. and not higher than 40° C., for example. The temperature of the QCL14is detected using a thermistor (not illustrated) that is arranged near the QCL14, and a detected result is fed-back to the driving current of the TEC12.

The sample cell20has a light entry window optically coupled to the QCL module10, and a light exit window positioned opposite to the light entry window. The sample cell20further has a space21containing sample gas G, the space21being communicated with an inlet port22and an outlet port23. The sample gas G is introduced into the space21through the inlet port22from the outside of the gas analyzer1A. Moreover, the sample gas G is discharged to the outside of the gas analyzer1A from the space21through the outlet port23. In a box-like member defining the space21, at least the light entry window and the light exit window are each made of a material transparent to the wavelength of the laser light LS. The laser light LS output from the QCL module10is applied to the sample gas Gin the space21after passing through the light entry window. The laser light LS is absorbed at a wavelength corresponding to a certain component in the sample gas G. An amount by which the laser light LS is absorbed depends on a concentration of the relevant component in the sample gas G. After passing through the sample gas G while being absorbed, the laser light LS penetrates the light exit window and then goes to the outside of the space21. The gas analyzer1A may have a structure of emitting the laser light LS to an external space without including the sample cell20.

The MCT module30receives the laser light LS having passed through the sample gas G, and outputs a detected signal Sb with a voltage depending on the intensity of the laser light LS that has reached the MCT module30. The amplitude of the detected signal Sb depends on an amount by which the laser light LS is absorbed by the sample gas G. The MCT module30is optically coupled to the light exit window of the sample cell20. The MCT module30includes a semiconductor MCT detector and has sensitivity in a wavelength range of 2 μm to 10 μm, for example. The MCT module30may further include a cooler for cooling the MCT detector to the temperature of liquid nitrogen, and an amplifier for amplifying a voltage output from the MCT detector and producing the detected signal Sb. The laser light LS having entered the MCT module30is converted to the detected signal Sb in the MCT detector.

FIGS. 3A, 3B, 4A and 4Bare each a graph depicting exemplary waveforms of the driving signal Sd and the detected signal Sb. InFIGS. 3A, 3B, 4A and 4B, a graph G1represents a waveform of the driving signal Sd, and a graph G2represents a waveform of the detected signal Sb. In the depicted examples, the waveform of the driving signal Sd has a sinusoidal shape. A horizontal axis indicates time (unit: msec), while a vertical axis on the left indicates a voltage value of the driving signal Sd (i.e., a voltage applied to the QCL14, unit: V) and a vertical axis on the right indicates a voltage value of the detected signal Sb (i.e., a voltage output from the MCT detector, unit: V).FIGS. 3A and 3Brepresent respectively the cases in which the frequency of the driving signal Sd is set 50 Hz and 100 Hz.FIGS. 4A and 4Brepresent respectively the cases in which the frequency of the driving signal Sd is set 1 kHz and 10 kHz. A center voltage of the driving signal Sd is 13.7 (V), and a voltage amplitude is 1.5 (V). A peak-to-peak value of the driving current flowing in the QCL14is 0.4 (A) in each case.

As seen fromFIGS. 3A, 3B, 4A and 4B, the waveform of the detected signal Sb is greatly distorted in comparison with that of the sinusoidal driving signal Sd. Such a distortion includes not only a reduction of the signal due to the absorption of the laser light by the sample gas G, but also influences of changes in the intensity and the wavelength of the laser light LS, which are caused by changing the driving signal Sd supplied to the QCL14. More specifically, when the driving signal Sd is supplied to the QCL14, the intensity of the laser light LS increases in proportion to the driving current that is in proportion to the driving signal Sd. On the other hand, the temperature of the QCL14rises with the supply of the driving current, and such a temperature rise reduces the light intensity. The wavelength of the laser light LS also changes in a similar manner. Thus, when the driving current increases, the wavelength of the laser light LS is prolonged. On the other hand, the temperature of an active layer in the QCL14rises, and this temperature rise shortens the wavelength. In addition, a temperature change of the active layer in the QCL14is delayed in time relative to a change of the driving current. Accordingly, when the driving signal Sd having the cyclic waveform is supplied to the QCL14, the changes of the light intensity and wavelength caused by the temperature rise are superimposed in a delayed relation on the changes of the light intensity and wavelength caused by the change of the driving current. Therefore, the light intensity and wavelength change in a complicated way. In addition, a magnitude and a delay time of the temperature change of the QCL14further depend on the heat capacities of both the submount13onto which the QCL14is mounted and the TEC12, and the ambient temperature of the gas analyzer1A. For that reason, it is difficult to accurately estimate the magnitude and the delay time of the temperature change of the QCL. Moreover, as seen fromFIGS. 3A, 3B, 4A and 4B, as the frequency of the driving signal Sd increases, the influence of the delay of the temperature change appears more significantly, and a distortion of the wavelength of the detected signal Sb increases. From the viewpoint of shortening the measurement time, it is preferable that the frequency of the driving signal Sd is as high as possible.

In consideration of the above-described point, in this embodiment, the spectrum processing circuit40described below excludes the influences of the changes of the light intensity and wavelength, which are caused by the change of the driving current supplied to the QCL14, and extracts only absorption characteristics of the sample gas G. As illustrated inFIG. 1, the spectrum processing circuit40is electrically connected to the MCT module30, and it receives the detected signal Sb from the MCT module30. The spectrum processing circuit40then outputs, on the basis of the detected signal Sb, information representing the absorption characteristics of the sample gas G. The spectrum processing circuit40may be implemented with an analog and/or digital electronic circuit, for example. Alternatively, the spectrum processing circuit40may be implemented with a computer that includes a CPU and a memory, and that is operated by executing a predetermined program. The information representing the absorption characteristics of the sample gas G is output to a display60from the spectrum processing circuit40, and is displayed on the display60. A measurement operator can analyze gas components of the sample gas G on the basis of the information displayed on the display60.

A practical exemplary configuration of the spectrum processing circuit40will be described below.FIG. 5is a block diagram illustrating the exemplary configuration of the spectrum processing circuit40. As illustrated inFIG. 5, the spectrum processing circuit40includes a filter circuit41, a comparator circuit42, a waveform correction circuit43, a delaying circuit44, an adding circuit45, a time inverting circuit46, a signal control circuit47, a subtracting circuit48, a peak detection circuit49, a phase adjusting circuit50, a background subtracting circuit51, and an output circuit52. Furthermore, the spectrum processing circuit40receives the detected signal Sb from the MCT module30and receives the reference driving signal Sd0from the driver circuit70. The spectrum processing circuit40uses the driving signal Sd0in deciding a frequency of a corrected waveform given in the waveform correction circuit43described later, and in deciding a reference time to establish synchronization between the driving signal Sd0and the corrected waveform. The driving signal Sd0is introduced to the spectrum processing circuit40through, for example, a line branched from a wiring extending from the driver circuit70to the QCL module10.

The filter circuit41performs filtering of the driving signal Sd0and removes a noise component. The filter circuit41is constituted, for example, by a low pass filter including a resistance and a capacitor. Furthermore, the filter circuit41performs normalization of the driving signal Sd0after the filtering. The term “normalization” stands for processing to adjust the amplitude of each input signal to be matched with a predetermined level. With the provision of the filter circuit41, subsequent electrical processing steps performed by various circuits can be carried out with high accuracy. The filter circuit41may perform only one of the filtering and the normalization. Alternatively, the filter circuit41may be omitted. The driving signal Sd0passing through the filter circuit41is sent to the comparator circuit42and the waveform correction circuit43.

The waveform correction circuit43is a waveform correction unit in this embodiment, and it performs, using parameters prepared in advance, correction to make the waveform of the driving signal Sd0closer to the waveform of the laser light LS output from the QCL14. By way of example, the waveform correction circuit43produces, on the basis of the driving signal Sd0, a driving signal Sd1that is a voltage signal having a primary corrected waveform δ(t) expressed by the following mathematical expression (1). The primary corrected waveform δ(t) is resulted from making the waveform of the driving signal Sd0closer to the waveform of the laser light LS from the QCL14by superimposing a predetermined waveform distortion component on the driving signal Sd0.

δ⁡(t)=-h·sin⁢⁢ω⁢⁢t·{1-4⁢α′·cos2⁡(ω⁢⁢t2)}(1)
Here, ω denotes a frequency extracted from the driving signal Sd0by the waveform correction circuit43. Furthermore, h and α′ denote constants (parameters) of which initial values are input by the measurement operator in advance. Moreover, t denotes time. The mathematical expression (1) is based on a model representing that, when the driving signal Sd0is sinusoidal, a delay of thermal response also becomes sinusoidal. The waveform distortion component in the mathematical expression (1) is determined by the constants h and α′ related to the delay of thermal response. The constants h and α′ are related to a delay time of the thermal characteristics and the magnitude of the temperature change of the QCL14. Once the heat capacities of the QCL14, the submount13, and the TEC12inside the QCL module10are determined, values of h and α′ are substantially determined incidental to those heat capacities. In the case of using the QCL modules10having the same configuration, the same values can be used as the initial values of h and α′. The waveform correction circuit43sends the produced driving signal Sd1to the delaying circuit44.

The delaying circuit44is a delaying unit in this embodiment, and it applies, to the waveform of the driving signal Sd1, a time delay in comparison with the waveform of the detected signal Sb. In other words, the delaying circuit44produces a driving signal Sd2that is a voltage signal having a secondary corrected waveform δ(t−Δτi). Here, Δτidenotes an i-th delay time (i=1, 2, etc.). The delay time Δτiis set by the phase adjusting circuit50, described later, in sequential order starting from i=1. The delaying circuit44sends the produced driving signal Sd2to the adding circuit45.

The adding circuit45is an adding unit in this embodiment. The adding circuit45adds the time-delayed driving signal Sd2and the detected signal Sb, thereby producing a voltage signal Sd3having a symmetrical waveform. The term “symmetrical waveform” stands for a waveform of the type that a waveform resulting from time-inverting an original waveform with respect to a certain point in time becomes the same as the original waveform. The adding circuit45sends the produced voltage signal Sd3to the time inverting circuit46, the signal control circuit47, and the background subtracting circuit51.

The above-described driving signal Sd0output from filter circuit41is sent to the comparator circuit42as well. The comparator circuit42compares the magnitude of the driving signal Sd0with a predetermined threshold, and produces a push signal Spush and a pop signal Spop. The produced push signal Spush and pop signal Spop are each delayed by the delaying circuit44through the same delay time Δτias that applied to the driving signal Sd1. The delayed push signal Spush is sent to the time inverting circuit46. The delayed pop signal Spop is sent to the time inverting circuit46and the signal control circuit47.

The time inverting circuit46is a time inversion unit in this embodiment, and it produces a voltage signal Sd4having a time-inverted waveform using the waveform (symmetrical waveform) of the voltage signal Sd3sent from the adding circuit45. The time inverting circuit46inverts a time dependency of the voltage of the signal Sd3. More specifically, the time inverting circuit46produces the voltage signal Sd4by repeatedly executing, on the voltage signal Sd3, a pushing process and a popping process in accordance with timings of the push signal Spush and the pop signal Spop with the aid of a Last In Last Out (LILO) buffer. The time inverting circuit46sends the produced voltage signal Sd4after the time inversion to the subtracting circuit48.

The signal control circuit47receives the voltage signal Sd3from the adding circuit45and the delayed pop signal Spop from delaying circuit44. The signal control circuit47sends the voltage signal Sd3to the subtracting circuit48in accordance with timing of the pop signal Spop.

The subtracting circuit48is a subtracting unit in this embodiment. The subtracting circuit48calculates a difference between the voltage signal Sd3and the voltage signal Sd4, thereby producing a voltage signal Sd5having a waveform of the difference between the waveform of the voltage signal Sd3and the waveform of the voltage signal Sd4. The voltage signal Sd5represents an error value due to an inappropriate choice of the delay time Δτi. The subtracting circuit48sends the produced voltage signal Sd5to the peak detection circuit49.

The peak detection circuit49accumulates an absolute value of the waveform of the difference, which is included in the voltage signal Sd5, in a memory as a value corresponding to the delay time Δτi. The absolute value of the waveform of the difference represents an error value. The peak detection circuit49compares the present error value with each of the error values corresponding respectively to the delay times Δτ1to Δτi−1, which have been accumulated so far. If the delay time Δτk(k is an integer of not smaller than 1 and not larger than i) at which the error value is minimized can be determined as a result of the comparison, the peak detection circuit49determines the relevant delay time Δτkas the final delay time Δτ. The final delay time Δτ represents a phase shift in the wavelength of the laser light LS. In other words, assuming the delay time Δτ to be a reference time, the wavelength can be recognized on the basis of a time difference from the reference time. After determining the final delay time Δτ, the peak detection circuit49outputs a timing signal St. If the delay time Δτ at which the error value is minimized cannot be determined, the peak detection circuit49accumulates the relevant error value as the error value at the relevant delay time Δτi. Then, the peak detection circuit49sends, to the phase adjusting circuit50, a signal Sc instructing generation of a next delay time Δτi+1. At that time, the phase adjusting circuit50generates the new delay time Δτi+1different from the delay times Δτ1to Δτi, which have been generated so far, and sends the generated delay time to delaying circuit44. The delaying circuit44, the adding circuit45, the time inverting circuit46, the signal control circuit47, the subtracting circuit48, and the peak detection circuit49repeat the above-described operations until the delay time Δτkat which the error value is minimized can be determined. Stated in another way, the spectrum processing circuit40repeatedly calculates the waveform of the difference while the delay time Δτiapplied in the delaying circuit44is changed.

The background subtracting circuit51is a filter for removing background noise that is included in the voltage signal Sd3received from the adding circuit45and having the symmetrical waveform. The filter is a high pass filter, for example. The background subtracting circuit51sends, to the output circuit52, a voltage signal Sd6obtained after removing the background noise.

The output circuit52receives, from the background subtracting circuit51, the voltage signal Sd6obtained after removing the background noise, and further receives the timing signal St from the peak detection circuit49. The output circuit52produces a voltage signal Sd7having a waveform resulting from time-inverting the waveform of the voltage signal Sd6. Upon receiving the timing signal St sent from the peak detection circuit49, the output circuit52outputs a voltage signal Sd7, as information (absorption spectrum) representing the absorption characteristics of the sample gas G, to the display60at the timing of receiving the timing signal St. In other words, the output circuit52outputs, as the information representing the absorption characteristics of the sample gas G, information (voltage signal Sd7) that is obtained from the symmetrical waveform (voltage signal Sd3) after the addition when the error value is minimized. The output circuit52may output directly the symmetrical waveform (voltage signal Sd3or Sd6) after the addition when the error value is minimized, or may output different information (e.g., absorption wavelength data) that is obtained from the symmetrical waveform (the voltage signal Sd3) after the addition when the error value is minimized. The measurement operator can recognize the components of the sample gas G on the basis of the output information.

FIGS. 6A, 6B, 7A and 7Bdepict examples of the symmetrical waveform (graph G3) given by the voltage signal Sd3, the time-inverted waveform (graph G4) given by the voltage signal Sd4, and the error value (graph G5) given by the voltage signal Sd5at a certain delay time Δτi.FIGS. 8A, 8B, 9A and 9Bdepict examples of the symmetrical waveform (graph G6) given by the voltage signal Sd3, the time-inverted waveform (graph G7) given by the voltage signal Sd4, and the error value (graph G8) given by the voltage signal Sd5at the optimum delay time Δτkat which the error value is minimized. In each ofFIGS. 6A to 9B, a horizontal axis indicates time (unit: msec), and a vertical axis indicates a voltage value of the detected signal Sb (i.e., an output voltage from the MCT detector, unit: V).FIGS. 6A and 8Arepresent the cases in which the frequency of the driving signal Sd is set to 50 Hz.FIGS. 6B and 8Brepresent the cases in which the frequency of the driving signal Sd is set to 100 Hz.FIGS. 7A and 9Arepresent the cases in which the frequency of the driving signal Sd is set to 1 kHz.FIGS. 7B and 9Brepresent the cases in which the frequency of the driving signal Sd is set to 10 kHz.

Referring toFIGS. 6A to 9B, at the optimum delay time Δτkat which the error value is minimized, medium-cycle undulations in the error value (graph G8) are smaller than those (namely, the error value is smaller than that) in the case (graph G5) in which the delay time is not optimum. Thus, in the symmetrical waveform (graph G6), steep variations having very short cycles (i.e., variations attributable to absorption by the sample gas G) can be identified with high accuracy. Furthermore, long-cycle undulations (waves) in the error value (graph G8) can be reduced (namely, the graph G8can be made closer to a flat line) by appropriately adjusting the parameters h and cc′ in the mathematical expression (1) in the waveform correction circuit43. Alternatively, the long-cycle undulations may be reduced using the background subtracting circuit51.

FIG. 10is a graph depicting absorption characteristics (absorption spectrum) extracted from the symmetrical waveform after optimizing the delay time Δτ and removing the long-cycle undulations. InFIG. 10, a horizontal axis indicates wavelength converted from time, and a vertical axis indicates voltage (unit: V) depending on an amount of light absorbed by the sample gas G. As seen fromFIG. 10, sharp absorption peaks appear at certain wavelengths corresponding to components contained in the sample gas G. The components contained in the sample gas G can be recognized from both numerical values of those wavelengths and absorption intensities of individual peaks (i.e., magnitudes of the peaks).

A gas analysis method using the gas analyzer1A according to this embodiment will be described below.FIG. 11is a flowchart representing individual steps of the gas analysis method according to this embodiment. First, the driving signal Sd and Sd0including the cyclic waveform is produced in the driver circuit70(signal production step S11). Then, the driving signal Sd is supplied from the driver circuit70to the QCL14, and the driving signal Sd is converted to the laser light LS in the mid-infrared range (light production step S12). Then, the sample gas Gin the sample cell20is irradiated with the laser light LS (irradiation step S13). Then, the detected signal Sb depending on the intensity of the laser light LS after passing through the sample gas G is produced in the MCT module30(light detection step S14).

Subsequently, in the spectrum processing circuit40, the information representing the absorption characteristics of the sample gas G is produced on the basis of the detected signal Sb (data calculation step S15). The waveform of the driving signal Sd0is used as a base waveform to be modified through the calculation steps. In the data calculation step S15, a filtering process is performed on the driving signal Sd0in the filter circuit41(filtering step S16). Then, correction of the driving signal Sd0is performed in the waveform correction circuit43such that the predetermined distortion is superimposed on the driving signal Sd0to produce the driving signal Sd1having the corrected waveform made closer to the waveform of the laser light LS from the QCL14(waveform correction step S17). Then, in the delaying circuit44, the delay time Δτ in comparison with the waveform of the detected signal Sb is applied to the waveform of the driving signal Sd1to produce the driving signal Sd2(delaying step S18). Then, in the adding circuit45, the waveform of the time-delayed driving signal Sd2and the waveform of the detected signal Sb are added to produce the symmetrical waveform Sd3(adding step S19). Then, the time inverting circuit46produces the waveform Sd4resulting from time-inverting the symmetrical waveform Sd3after the addition, and the subtracting circuit48produces a waveform Sd5representing the difference between the time-inverted waveform Sd4and the symmetrical waveform Sd3(inversion and subtracting step S20). Then, the peak detection circuit49determines whether the absolute value of the waveform of the difference Sd5, i.e., the error value, is a minimum (determination step S21). If the error value is not a minimum (determination step S21: NO), the delay time Δτ in the delaying step S18is changed to another delay time Δτi, and the waveform of the difference Sd5is obtained again by repeating the delaying step S18, the adding step S19, and the inversion and subtracting step S20. If the error value is a minimum (determination step S21: YES), the background noise is removed in the background subtracting circuit51from the symmetrical waveform after the addition, and the resulting symmetrical waveform is output from the output circuit52as the information representing the absorption characteristics of the sample gas G (output step S22).

The advantageous effects obtained with the above-described gas analyzer1A and gas analysis method according to the embodiment will be described below. In one of methods for carrying out gas analysis with the use of the QCL, the wavelength and the intensity of the laser light are cyclically changed by modulating the injection current to the QCL. In such a method, however, the wavelength chirping of the QCL and the wavelength shift due to heating of the QCL (i.e., dynamic characteristics of the QCL) affect the signal detected by the MCT. On the other hand, in the gas analyzer according to this embodiment, only the absorption characteristics of the sample gas G are taken out in the spectrum processing circuit by subtracting the influence of the dynamic characteristics of the QCL14from the waveform of the detected signal Sb (in which the absorption characteristics of the sample gas G and the dynamic characteristics of the QCL14are mixed together).

More specifically, in the gas analyzer1A and the analysis method according to the embodiment, the spectrum processing circuit40(data calculation step S15) includes, as described above, the delaying circuit44(delaying step S18), the adding circuit45(adding step S19), and the subtracting circuit48(subtracting step S20). The delaying circuit44(delaying step S18) applies, to the waveform of the driving signal Sd0, the delay time Δτiin comparison with the waveform of the detected signal Sb. A phase difference is thus generated between the driving signal Sd and the detected signal Sb. Then, the adding circuit45(adding step S19) adds both the waveforms having the phase difference to each other. Furthermore, the subtracting circuit48(subtracting step S20) produces the waveform of the difference between the waveform obtained by time-inverting the waveform after the addition, and the waveform after the addition (i.e., the symmetrical waveform). The waveform of the difference is repeatedly calculated while the delay time Δτiis changed.

In the above-described configuration, the delay time Δτkwhen the absolute value of the waveform of the difference is minimized represents the magnitude of a time shift in wavelength. Accordingly, the accurate wavelength at which the absorption has occurred can be recognized in consideration of the delay time Δτkwhen the absolute value of the waveform of the difference is minimized. Stated in another way, with the gas analyzer1A and the gas analysis method according to this embodiment, the influence of the wavelength shift, which is caused by the cyclic change of the injection current to the QCL14, can be eliminated, and the information regarding the absorption characteristics of the sample gas G can be accurately extracted from the detected signal Sb. As a result, the absorption characteristics of the sample gas G can be obtained with high accuracy in the measurement method of modulating the injection current to the QCL14.

Furthermore, with the spectrum processing circuit40in the embodiment, since details of the processing are comparatively simple and the number of necessary circuits is relatively small, the spectrum processing circuit40can be constituted in a small size. Thus, the size of the housing3can be reduced, and portability of the analyzer can be increased.

Moreover, as in the embodiment, the spectrum processing circuit40(data calculation step S15) may include the waveform correction circuit43(waveform correction step S17) that corrects, using the parameters (h and α′) prepared in advance, the waveform of the driving signal Sd to become closer to the waveform of the laser light LS output from the QCL14. With that feature, since the above-described processing can be performed in the spectrum processing circuit40(data calculation step S15) in consideration of the variation in the light intensity (i.e., the distortion of the waveform of the laser light LS) caused by temperature changes, the absorption characteristics of the sample gas G can be obtained with higher accuracy. In addition, with the gas analyzer1A according to this embodiment, since the number of parameters having arbitrary properties is small, namely just h and α′, wavelength characteristics can be extracted with high reliability.

As in the embodiment, the cyclic waveform may be given by a sinusoidal wave. By using a sinusoidal wave, the driving signal Sd including the cyclic waveform can be easily produced. Furthermore, since the distortion of the waveform of the laser light LS can be expressed by a simple expression using trigonometric functions, such as the mathematical expression (1) in the embodiment, the number of parameters can be reduced.

The analyzer, the absorption characteristic calculation circuit, and the analysis method according to the present invention are not limited to the above-described embodiment, and they may be modified in various ways. For instance, while gas (gas sample) is used, by way of example, as the sample to be measured in the above-described embodiment, the present invention can be applied to a liquid or solid sample as well. Moreover, while, in the above-described embodiment, the delaying unit applies the time delay to the corrected waveform produced from the driving signal, the delaying unit may apply the time delay to the waveform of the detected signal because the purpose of the delaying unit is to apply a phase difference between the driving signal and the detected signal. Moreover, the corrected waveform given in the waveform correction unit is not limited to the waveform expressed by the mathematical expression (1) including the parameters (h and α′), a mathematical expression including other appropriate parameters may be used as required.