JP2013156113A - Laser type gas analyzer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a laser type gas analyzer which achieves highly accurate measurement by reducing an interference noise caused by a laser beam.SOLUTION: In a laser type gas analyzer 10, the light emitted by a laser element 204e of a laser source part 20 is applied to a measuring object space 1 in which measured gas exists by using a floodlighting optical fiber 205, and vibration means 203 is provided on an end part of the floodlighting optical fiber 205 so as to minutely vibrate the floodlighting optical fiber 205.

Description

  The present invention relates to a laser type gas analyzer that measures the concentration of a gas with a laser beam.

  It is known that each gaseous gas molecule has its own light absorption spectrum. Gas analyzers using laser light (hereinafter referred to as “laser-type gas analyzers”) utilize the fact that the absorption amount of laser light at a specific wavelength is proportional to the concentration of the gas to be measured (measurement target gas). It is a device that measures. Gas concentration measurement methods are roughly classified into a frequency modulation method (see Patent Document 1) and a two-wavelength difference method (see Patent Document 2).

  In a laser gas analyzer, interference light is generated due to multiple reflections of light between optical components such as between a laser element and a collimating lens, or between a condenser lens and a light receiving element, due to the coherent nature of the laser light. Then, it is superimposed on the received light signal as interference noise. Since this interference noise fluctuates due to changes in the external environment (particularly temperature), this hinders high sensitivity and high stability of the apparatus.

Therefore, in recent years, a laser-type gas analyzer provided with means for reducing interference light due to multiple reflection of light between the optical components described above has been studied (see Patent Document 3).
FIG. 7 shows a laser gas analyzer described in Patent Document 3. In the laser type gas analyzer 700, a light projecting / receiving unit 710 and a reflection unit 720 are arranged to face each other with a predetermined measurement optical path length in a measurement atmosphere (gas G).

  The light projecting / receiving unit 710 receives and detects the light source unit 730 including a laser element, the condensing lens 740 that condenses the light emitted from the light source unit 730 and passed through the gas, and the light collected by the condensing lens 740. A light receiving element 750A, a light receiving signal processing unit 750B that calculates a gas concentration by processing a light receiving signal detected by the light receiving element 750A, and a vibration unit 760 that slightly vibrates the condenser lens 740 in the optical axis direction. The vibration unit 760 includes various motors such as a vibration motor and a driving unit such as a piezoelectric element, and is directly attached to the condenser lens 740. On the other hand, the reflection unit 720 includes a reflection plate 790 that reflects the laser light from the light projecting / receiving unit 710 toward the measurement atmosphere again.

  The laser light emitted from the light source unit 730 is reflected by the reflection mirrors 770A and 770B, and then emitted from the glass window 780 provided at the center position of the condenser lens 740 toward the measurement atmosphere. The light emitted from the glass window 780 passes through the measurement atmosphere, is reflected by the reflection plate 790, passes through the measurement atmosphere again, is collected by the condenser lens 740, and is received by the light receiving element 750A. The received light is converted into an electrical signal and then subjected to signal processing by the received light signal processing unit 750B.

  In the laser-type gas analyzer 700 configured as described above, when the vibration unit 760 is operated during the gas detection operation, the condenser lens 740 and the window portion 780 in the light projecting / receiving unit 710 vibrate slightly in the optical axis direction. Thus, the distance D1 between the reflection mirror 770B and the window portion 780, the distance D2 between the window portion 780 and the reflection plate 790, the distance D3 between the reflection plate 790 and the condensing lens 740, and the condensing lens 740. And the distance between the optical members such as the distance D4 between the light receiving element 750A and the light receiving element 750A change. As a result, the reflection position of the interference light on the surface of the window portion 780, the surface of the condenser lens 740, and the surface of the reflection plate 790 is changed to cause a phase shift. As a result, the interference light generated at the distances D1 to D4 is averaged during multiple reflection, and interference light unnecessary for measurement can be reduced.

Japanese Patent Laid-Open No. 7-151681 (paragraph [0005], FIG. 4 etc.) JP 2001-235418 A (paragraphs [0012] to [0024], FIG. 2, FIG. 11 etc.) JP 2008-70314 A (paragraph [0020], FIG. 1 etc.)

  As shown in Patent Document 3, the method of minutely vibrating the condenser lens during gas detection is an effective method for reducing interference noise derived from laser light, which is a noise factor that hinders gas concentration measurement. There are the following problems.

  First, in a laser type gas analyzer, since the laser beam is normally expanded and irradiated on the measurement target space, the lens diameter of the condensing lens 740 becomes as large as about 25 to 30 mm. In addition, when measuring the gas concentration in a high temperature and highly corrosive atmosphere such as a flue, it is necessary to use quartz glass having heat resistance and corrosion resistance as a material for the condenser lens 740. The weight of the condensing lens 740 becomes heavier than when a lens is used. In order to vibrate such a large and heavy glass lens, the vibration means 760 also has a large and complicated structure, which causes an increase in cost.

  Further, as shown in FIG. 7, the condenser lens 740 is disposed in a state exposed to the outside of the light projecting / receiving unit 710, and has a structure in which the gas in the flue is directly touched. Usually, in order to prevent gas from leaking outside the flue, the inside of the flue, which is the measurement target space, has a negative pressure with respect to the outside air. For this reason, when the condensing lens 740 is vibrated, there is a concern that it may be difficult to vibrate the condensing lens 740 due to the pressure difference between the inside and outside of the flue. Further, it is not preferable from the viewpoint of structural reliability to vibrate the condensing lens 740 having the function of keeping the inside of the unit secret. Although it is conceivable to cover the front surface of the condensing lens 740 with a glass plate or the like so that the condensing lens 740 is not exposed to the outside, it is difficult to apply because this glass plate further increases interference light. .

  In addition, with regard to conventional laser gas analyzers, for example, it is conceivable to reduce interference noise by vibrating a laser light source section having a semiconductor laser element. In that case, thermistor or Peltier installed in the laser element is also conceivable. Since the element is vibrated together, the heat dissipation condition of the Peltier element may change due to vibration, and it may become impossible to control the temperature precisely for the laser element, or the Peltier element may be damaged. The measurement accuracy of the apparatus may be reduced.

  If an optical fiber type laser gas analyzer that irradiates laser light emitted from a laser light source (semiconductor laser element) using an optical fiber toward the measurement target space, a semiconductor laser is located near the measurement target space. Since there is no need to install an element, temperature control can be accurately performed, and interference noise can be suppressed to some extent by, for example, obliquely polishing the end face of the optical fiber. However, the end face of the optical fiber has a diameter of about 100 μm and has an area more than 10 times that of the exit end face of the semiconductor laser element, so that interference noise is likely to occur and high-precision measurement can be realized for low-concentration gas. However, it is not enough to suppress interference noise by oblique polishing.

  The present invention has been made in view of the above points, and reduces interference noise derived from laser light, which is a noise factor that hinders gas concentration measurement, so that even when the measurement target gas has a low concentration, high accuracy is achieved. It is an object of the present invention to provide a laser type gas analyzer capable of performing measurement.

A laser type gas analyzer according to claim 1 of the present invention comprises:
A laser light source unit having a laser element, a wavelength scanning drive signal for changing the emission wavelength of the laser element so as to scan the absorption wavelength of the gas to be measured, and a modulation signal for modulating the emission wavelength are synthesized. A laser drive signal generator that outputs a laser drive signal; a current controller that converts the signal output from the laser drive signal generator into a current to drive the laser element; and light emitted from the laser element. A laser beam emitting optical system for irradiating the gas to be measured; a condensing optical system for condensing the light transmitted through the gas to be measured; and a light receiving element for measuring the intensity of the light collected by the condensing optical system; A laser-type gas analyzer that includes a light-receiving signal processing unit that processes a light-receiving signal output from the light-receiving element, and that detects a concentration of a gas component to be measured based on a processing result of the light-receiving signal processing unit.
The laser light emitting optical system includes a light projecting optical fiber that guides light emitted from the laser element, and a first optical fiber that slightly vibrates the optical fiber at an end of the light projecting optical fiber. A vibration means is provided.

  A laser gas analyzer according to a second aspect of the present invention is the laser gas analyzer according to the first aspect, wherein the light reception signal output from the light receiving element is guided to the light reception signal processing unit via a light receiving optical fiber. A second optical fiber vibrating means for minutely vibrating the optical fiber is provided at the end of the light receiving optical fiber.

  In the laser type gas analyzer according to the present invention, the laser light emitted from the laser light source unit having the semiconductor laser element is irradiated toward the measurement target space through the optical fiber for projection, and the vibration means is used. The end of the light projecting optical fiber is minutely oscillated in the direction of the optical axis of the measurement light. This enables precise temperature control of the semiconductor laser element to obtain a stable laser output. The interference noise derived from the laser beam due to the increase in the area can be suppressed. Therefore, according to the laser gas analyzer of the present invention, it is possible to perform gas concentration analysis with high accuracy even when the measurement target gas has a low concentration.

FIG. 1 is a schematic cross-sectional view showing the overall configuration of the laser type gas analyzer of the present embodiment. FIG. 2 is a diagram showing a schematic configuration of the laser light source unit. 3A is a waveform diagram of the scanning drive signal output from the wavelength scanning drive signal generator, FIG. 3B is a waveform diagram of the modulation signal output from the harmonic modulation signal generator, and FIG. c) is a waveform diagram of a drive signal output from the current control unit. FIG. 4 is a diagram illustrating a configuration of the light reception signal processing unit. FIG. 5 is a diagram illustrating the light reception signal, the output signal of the synchronous detection circuit, and the trigger signal. FIG. 6 is a diagram illustrating an example of a signal waveform when there is no vibration and a signal waveform when there is a signal. FIG. 7 is a diagram showing a configuration of a conventional laser gas analyzer equipped with a vibrating means for minutely vibrating the condenser lens.

  Exemplary embodiments of a laser type gas analyzer according to the present invention will be described in detail with reference to the accompanying drawings. Hereinafter, the present invention will be described based on an example in which the present invention is applied to a frequency modulation type laser gas analyzer.

FIG. 1 is a schematic cross-sectional view showing a configuration of a laser type gas analyzer 10 of the present embodiment. A laser type gas analyzer 10 illustrated in FIG. 1 is installed on walls W1 and W2 of piping such as a flue, which is a measurement target space 1 through which a gas to be measured G passes.
The pair of flanges 11a and 11b are formed in a cylindrical shape with both ends opened, and are fixed to the flue walls W1 and W2 by welding or the like. The light projecting unit housing 13 and the light receiving unit housing 14 are formed in a bottomed cylindrical shape with one end opened and the other end closed. Installed.

  The light emitted from the laser light source unit 20 having the laser element is guided into the light projecting unit housing 13 by the light projecting optical fiber 205, converted into parallel light by the optical system including the collimating lens 30, and then measured. The laser beam L is irradiated toward the measurement target space 1. The measurement laser beam L is absorbed by the gas when passing through the measurement gas existing in the flue. The measurement laser light L that has passed through the flue is collected by the condenser lens 40 disposed in the light receiving unit housing 14, then received by the light receiving element 50, and received by the light receiving optical fiber 601. To 60.

  FIG. 2 is a diagram showing a configuration of the laser light source unit 20 shown in FIG. The laser light source unit 20 is configured as a unit in which a plurality of components such as a semiconductor laser element described below are accommodated in a package. As shown in FIG. 2, the laser light source unit 20 includes a wavelength scanning drive signal generation unit 204a and a high frequency modulation signal generation unit 204b as a wavelength control unit 204s, a current control unit (laser element driving unit) 204c, and a laser. The element 204e, the thermistor 204f, the Peltier element 204g, and the temperature controller 204d are accommodated.

  The wavelength scanning drive signal generator 204a generates a wavelength scanning signal that makes the emission wavelength of the laser element 24e variable so as to scan the absorption wavelength of the gas to be measured. The high-frequency modulation signal generation unit 204b generates a sine wave signal of, for example, about 10 kHz and frequency-modulates the wavelength in order to detect the gas absorption waveform. The current control unit 204c drives a laser element using a signal (laser drive signal) obtained by synthesizing the wavelength scan signal generated by the wavelength scan drive signal generation unit 204a and the sine wave signal generated by the high frequency modulation signal generation unit 204b. The laser element 204e is driven by converting into current.

  The laser element 204e is a semiconductor laser (LD: laser diode), and emits laser light by a driving current supplied from the current control unit 204c. The thermistor 204f is a temperature detection element for detecting the temperature of the laser element 204e, and is disposed in contact with the laser element 204e. The Peltier element 204g is a heating / cooling means for the laser element 204e, and is disposed in contact with the thermistor 204f. The temperature control unit 204d controls the Peltier element 204g based on the temperature measured by the thermistor 204f, and thereby maintains the temperature of the laser element 204e at a constant temperature, thereby controlling the oscillation wavelength of the laser light.

  FIG. 3A shows an example of a current waveform output from the wavelength scanning drive signal generator 204a of FIG. The wavelength scanning drive signal S1 for scanning the light absorption characteristics of the gas to be measured changes the emission wavelength gradually by linearly changing the drive current value of the laser element 204e, for example, scanning the light absorption characteristics of about 0.2 nm. . On the other hand, the signal S2 is for maintaining the drive current value to be equal to or higher than the threshold current at which the laser element 204e is stable and to emit light at a constant wavelength. Further, in the signal S3, the drive current value is set to 0 mA. The trigger signal is a signal synchronized with the signal S3.

FIG. 3B is a waveform diagram of the modulation signal output from the high frequency modulation signal generator 204b of FIG. 2, and the signal S4 for detecting the light absorption characteristics of the measurement target gas is, for example, a sine wave having a frequency of 10 kHz. And the wavelength width is modulated by about 0.02 nm.
FIG. 3C is a waveform diagram of the drive signal output from the current control unit 204c of FIG. 2. When this drive signal S5 is supplied to the laser element 204e, the laser element 204e receives 0. 0 of the measurement target gas. Modulated light capable of detecting light absorption characteristics of about 2 nm with a wavelength width of about 0.02 nm is output.

  FIG. 4 is a diagram showing a schematic configuration of the received light signal processing unit 60 in FIG. The light receiving element 50 in FIG. 1 is, for example, a photodiode, and an element having sensitivity to the emission wavelength of the laser element 204e is applied. When the output of the light receiving element 50 is sent to the light receiving signal processing unit 60 via the light receiving optical fiber 601, it is converted into a voltage output by the IV converter 61 of FIG. 4, and a high frequency noise component is removed by the low pass filter 64A. Is done. The output signal from the low-pass filter 64A is input to the synchronous detection circuit 63 to which the 2f signal (double wave signal) from the oscillator 62 is added, and only the amplitude of the double frequency component of the modulation signal of the laser beam is extracted. The output signal of the synchronous detection circuit 63 is subjected to noise removal and amplification by the low-pass filter 64B, and is sent to the arithmetic unit 65. The calculation unit 65 performs calculation processing for gas concentration detection.

  A gas concentration detection method using the laser gas analyzer 10 configured as described above will be described. First, the temperature of the laser element 204e is detected in advance by the thermistor 204f. Further, the temperature control unit 204d controls the energization of the Peltier element 204g so that the temperature of the laser element 204e is set to a desired temperature so that the measurement gas G can be measured at the central portion of the wavelength scanning drive signal S1 shown in FIG. keep.

  The laser element 204e is driven by changing the drive current while maintaining the temperature of the laser element 204e at a desired temperature, and the measurement laser light L is irradiated toward the flue where the gas G to be measured exists, and the light receiving element Light is incident on 50. When laser light is absorbed by the gas to be measured, the second harmonic signal is detected by the synchronous detection circuit 63, and an absorption waveform of the gas appears. FIG. 5 shows an output waveform of the synchronous detection circuit 63 when the gas is detected.

  Subsequently, the trigger signal a is input to the calculation unit 65 from the wavelength scanning drive signal generation unit 204a. The trigger signal is a signal that is output every cycle including the above-described S1, S2, and S3, and is output from the wavelength scanning drive signal generation unit 204a of the laser light source unit 20 and received through a communication line (not shown). The signal is input to the calculation unit 65 of the signal processing unit 60. The trigger signal is synchronized with S3 of the wavelength scanning drive signal described above. In FIG. 5, a region A surrounded by a dotted line is an output waveform obtained when the measurement gas G exists. As shown in FIG. 5, when a predetermined time tb, tc, td elapses from the trigger signal, the minimum value B at point B, the maximum value C at point C, and the minimum value D at point D in the output waveform of the synchronous detection circuit 63 are as follows. Detected. These predetermined times tb, tc, td are experimentally calculated in advance and registered in the memory before factory shipment or at the time of calibration.

  The calculation unit 65 reads and stores the value of the output waveform of the synchronous detection circuit 63 when a predetermined time tb, tc, td has elapsed from the trigger signal, and then performs a process of calculating the concentration. This synchronous detection circuit output waveform outputs the maximum value as the concentration, for example, because the maximum value at the peak of the waveform directly represents the gas concentration. Alternatively, the difference value obtained by subtracting the minimum value from the maximum value is used as the density.

  Although the gas concentration can be detected by the above-described method, since the laser element 204e is used as a light source, for example, between the laser element 204e and the incident end face of the optical fiber 205 due to its extremely high coherence, A part of the laser beam is multiple-reflected between the end face on the emission side of the optical fiber 205 and the collimating lens 30, and this multiple reflected light becomes interference noise.

  In order to reduce this interference noise, in this embodiment, the vibration means 203 (first optical fiber vibration means) provided in the light projecting section housing 13 shown in FIG. The end portion is minutely vibrated to change the distance between the end face of the light projecting optical fiber 205 and the collimating lens 30. The vibration means 203 includes a piezoelectric vibration unit 201 and a vibration control unit 202 configured using piezoelectric elements. An end of a projecting optical fiber 205 is connected to the piezoelectric vibration unit 201, and the piezoelectric vibration unit 201 vibrates at a vibration frequency and amplitude (amount of displacement in the optical axis direction) based on a command signal from the vibration control unit 202. As a result, the end of the light projecting optical fiber 205 slightly vibrates in the optical axis direction of the measurement laser light L. The amplitude of this minute vibration is about several μm to several tens of μm. Further, the vibration frequency of the piezoelectric vibration unit 201 is set to be 10 times or more (for example, 100 kHz or more) or 1/10 times or less (for example, 1 kHz) of the frequency (for example, 10 kHz) of the sine wave signal by the high frequency modulation signal generating unit 204a. Is desirable. When the vibration frequency of the piezoelectric vibration unit 201 is greater than 1/10 times and less than 10 times the frequency of the sine wave signal, it is difficult to separate the laser modulation frequency (for example, 10 kHz) from the vibration frequency, and it is difficult to remove interference noise. Become.

  When the distance between the end face of the light projecting optical fiber 205 and the collimating lens 30 is changed by the vibration means 203, it is generated by the multiple reflected light between the end face of the light projecting optical fiber 205 and the surface of the collimating lens 30. The intensity of the interference light is fluctuating because the conditions for generating interference change. The period of the interference light fluctuation is equal to the vibration frequency of the piezoelectric vibration unit 201. Therefore, it is possible to remove interference noise from the detection signal by filter processing such as a low-pass filter that can remove the vibration frequency component.

  FIG. 6 shows an example of an output signal waveform by the received light signal processing unit when the vibration unit 203 is operated and not operated. As shown in FIG. 6, uneven interference noise is generated in the signal waveform when the vibration means 203 is not operated (no vibration). On the other hand, when the vibration means 203 is operated (with vibration), it can be seen that the interference noise can be greatly reduced.

  Incidentally, the interference light is also generated between the condenser lens 40 and the light receiving element 50. Accordingly, second optical fiber vibrating means (not shown) for vibrating the incident side end of the light receiving optical fiber 601 coupled to the light receiving element 50 may be provided. This second optical fiber vibrating means is configured in the same manner as the vibrating means 203.

DESCRIPTION OF SYMBOLS 10 Laser type gas analyzer 11a, 11b Flange 12a, 12b Mounting seat 13 Light emission part housing | casing 14 Light receiving part housing | casing 20 Laser light source part 201 Piezoelectric vibration part 202 Vibration control part
203 Vibrating means 205 Light projecting optical fiber 30 Collimating lens 40 Condensing lens 50 Light receiving element 60 Light receiving signal processing unit 601 Light receiving optical fiber

Claims (2)

A laser light source unit having a laser element;
A laser drive signal for synthesizing a wavelength scanning drive signal for changing the emission wavelength of the laser element so as to scan the absorption wavelength of the gas to be measured and a modulation signal for modulating the emission wavelength and outputting as a laser drive signal Generating part,
A current controller that converts the signal output from the laser drive signal generator into a current to drive the laser element;
A laser beam emission optical system for irradiating a gas to be measured with light emitted from the laser element;
A condensing optical system for condensing the light transmitted through the measurement gas;
A light receiving element for measuring the intensity of the light collected by the condensing optical system;
A light receiving signal processing unit for processing a light receiving signal output from the light receiving element,
In the laser type gas analyzer that detects the concentration of the gas component to be measured based on the processing result of the light receiving signal processing unit,
The laser light emitting optical system includes a light projecting optical fiber that guides light emitted from the laser element,
A laser type gas analyzer characterized in that a first optical fiber vibrating means for minutely vibrating the optical fiber is provided at an end of the light projecting optical fiber. In the laser type gas analyzer according to claim 1,
A second optical fiber vibration that guides the light reception signal output from the light receiving element to the light reception signal processing unit through the light receiving optical fiber and causes the end of the light receiving optical fiber to vibrate slightly. A laser type gas analyzer characterized by comprising means.