JP3450946B2 - Laser light generating optical system and gas concentration measuring device using the same - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser light generating optical system and a gas concentration measuring apparatus using the same.

[0002]

2. Description of the Related Art Recently, the problem of environmental pollution has been widely taken up, but air pollution has become a major theme. Although NO _X and SO _X are particularly important among polluted gases, laser radar is conventionally used to measure the concentration of these polluted gases and a particular important gas of interest.

The gas concentration measurement by a laser radar is to measure the gas concentration by using a laser to generate a laser beam having an absorption wavelength of the gas to be measured and detecting the degree of absorption of the laser beam by the gas. Therefore, a laser that emits a laser beam having a specific wavelength (for example, about 450 nm for NO _{2 and about} 300 nm for O ₃ ) absorbed by the gas to be measured is used.

The laser includes a gas laser, a liquid laser, a solid-state laser, a semiconductor laser, etc., depending on the laser medium used, and is used depending on the application. For example, a kind of solid-state laser that is relatively unaffected by the environment. N
A typical d: YAG (neodymium: yttrium aluminum garnet) laser has a lasing wavelength of 1064 nm, and oscillates a wavelength of 1319 nm, but in any case, only discrete wavelengths can be obtained. Even if it is combined with a non-linear optical element, it is not possible to generate an absorption band wavelength of a gas to be measured other than a specific gas.

On the other hand, a dye laser is conventionally known as an example of a laser whose oscillation wavelength can be freely selected.
As an example, this dye laser is used for measuring the concentration of NO ₂ (published in 1981, "Applied Physics", Vol. 50, No. 9, pages 923 to 928). The dye laser selects the type of dye used for the excitation laser light source and adjusts the angle of the laser light applied to the prism or diffraction grating placed in the resonator to adjust the oscillation wavelength from the ultraviolet range to the near infrared range. Since it can be selected by, it is possible to generate laser light having absorption wavelengths of many measurement target gases.

However, the dye used in the dye laser stains the surroundings, strongly dyes the hands and clothes, and is troublesome to handle, so it takes time to prepare and the maintainability is poor. Since the dye laser deteriorates over time, it is necessary to replace the dye with a new one in a timely manner. In addition, some pigment materials are harmful to the human body. Furthermore, when the gas to be measured changes, the absorption wavelength also changes, so it is necessary to change the wavelength each time, but since the wavelength range in which one dye can oscillate is a finite range, the oscillation wavelength is changed beyond that range. If desired, the dye must be replaced by another type. Therefore, it is inconvenient in terms of time required for preparation and maintainability. In addition, a laser radar using a dye laser requires a pump and a tank for circulating the dye, so that the size of the device becomes large, and there is a problem that it takes up a lot of space.

Therefore, the present inventor, as of November 8, 1993, can generate a laser beam having an absorption band wavelength of a gas to be measured without using a dye laser, which is easy to handle and prepare.
We filed a patent application for a gas concentration measuring device incorporating a laser with a simple configuration.

According to this concentration measuring device, since a solid-state laser such as a YAG laser and a wavelength tunable solid-state laser such as a titanium sapphire laser are used without using a dye laser, there is no fear of contamination and the handling is simple. Therefore, it does not take much time to prepare for measurement, does not need to be replaced, and is excellent in maintainability. In addition, it does not require a pump or tank, so it has the advantage that it does not take up space in equipment or facilities.

[0009]

However, this concentration measuring device has the following two problems to be solved. The first issue is low sensitivity and the second issue is walk-off. KD ^* P or KDP for nonlinear optical crystal
In the above-mentioned concentration measuring device using, the walk-off occurs. Walk-off means that the emitted blue laser light has a different optical path from the incident laser light due to the birefringence of the crystal. This creates two problems. (1) It is a point that the conversion efficiency to blue does not increase when the sum frequency is generated. Theoretically, the conversion efficiency increases in proportion to the square of the length of the crystal, but if there is walk-off, the conversion efficiency does not rise above a certain limit no matter how long the length is.
(2) Walk-off is a point where the emission angle changes depending on the wavelength. When measuring a gas such as NO ₂ with a differential absorption lidar (DIAL), analysis is performed under the condition that two different wavelengths having different absorptivities with respect to the gas are used and the laser light is directed in the same direction. However, if there is a walk-off, even if the output angle is slightly shifted, it will be enlarged several Km ahead, resulting in a large error. (3) In the case of NO measurement, since the double wave of blue light used for NO ₂ measurement is used, if the intensity of blue light before conversion is weak, the ultraviolet light with the intensity required for NO measurement cannot be obtained.

The present invention has been made in view of the above points, and is a laser light generating optical system which is easy to handle and has a simple structure, and which does not cause walk-off, and a gas of high sensitivity using the same. An object is to provide a concentration measuring device.

[0011]

The present invention has achieved the above object as follows.

(1) Compared with the above-mentioned concentration measuring device, it is necessary to measure NO ₂ with high sensitivity of ppb order in the natural atmosphere (that is, it is possible to measure even at low concentration) in order to take environmental measures. Need to get blue light with strong output. Therefore, in the present invention, the nonlinear optical crystal for generating the sum frequency is changed from KD ^* P to KNbO ₃ . This is KNb that changes the laser light to another wavelength called nonlinear optical constant.
This is because the ability of O ₃ is 40 times stronger than that of KD ^* P.

(2) Walk-off occurs in the conventional device using KD ^* P in the nonlinear optical crystal. In the present invention, NCPM (Non-Critical Phase Matching) is used to prevent this walk-off. It is obtained with a limited wavelength, temperature, angle, and polarization of very limited crystals. The NCPM at the sum frequency to obtain the blue light this time is K
NbO ₃ is the only possible. By using KNbO ₃ , the nonlinear optical constant of the crystal is much larger than other crystals, and since there is no walk-off, strong blue light can be obtained by using a crystal with a long length.

Further, in order to measure NO, it is necessary to have a wavelength of around 226 nm, which can be obtained by taking a double wave of the blue light obtained above by a non-linear optical crystal.

The inventor of the present invention made the following consideration until the achievement of the above means.

Up to now, the vicinity of 226 nm has been used for DIAL measurement of NO in the ultraviolet region. In the reports so far, Opt. Lett. 13 (1988) p. 704, Edner et al.
5 nm) and the sum frequency of the YAG laser and the Appl. Phys. Bol. (1992) page 89, Kolsch et al.
This wavelength is obtained by a harmonic wave, and NO measurement is performed.

The inventor of the present invention uses a KD ^* P crystal as a sum frequency of a titanium sapphire laser and another YAG laser as a laser light generating optical system of a lidar for measuring NO ₂ distribution, and a maximum of 20 mJ / pulse at a wavelength of 453 nm. Got the output of. Also, the wavelength is switched every pulse at 10 Hz operation by modifying the titanium sapphire laser resonator portion. Ultraviolet light can be obtained by adding a crystal for the second harmonic to this. N
In this case, for the O measurement, in the case of using a titanium sapphire laser as a fundamental wave, a wavelength around 791 nm is used. However, this wavelength has a large output because the gain is near the peak.
The blue light to be converted is larger as the intensity of the incident light is larger, so that more efficient wavelength conversion can be performed. On the other hand, a method of taking the fourth harmonic of a titanium sapphire laser may be considered, but the intensity of the obtained light becomes weak because the gain is small.

Therefore, according to the present invention, in the laser light generating optical system of the laser radar for measuring the distribution of the gas to be measured by the solid-state laser, the first solid-state laser, the first nonlinear optical element and the wavelength tunable solid-state laser are arranged in this order. The optical axes of the second solid-state laser and the laser light output from the wavelength tunable solid-state laser are aligned with each other by the dichroic mirror, and then arranged in series and the temperature of the crystal is adjusted. Two identical non-linear optical elements (KNbO ₃ ) phase-matched to different temperatures of ON wavelength (wavelength with high absorption rate for measurement gas) and OFF wavelength (wavelength with low absorption rate for measurement gas)
Then, a laser light generating optical system was constructed in which only the blue light was separated by a dichroic mirror and then emitted as laser light of a desired wavelength into the gas to be measured.

Further, according to the present invention, in the laser light generating optical system of the laser radar for measuring the distribution of the gas to be measured by the solid-state laser, the first solid-state laser, the first nonlinear optical element and the wavelength tunable solid-state laser are arranged in this order. Aligned optical axes, the laser light output from the second solid-state laser and the laser light output from the wavelength tunable solid-state laser are aligned by the dichroic mirror, and then arranged in series and phase-matched. Two non-linear optical elements (KN
bO ₃ ), after which only blue light is separated by a dichroic mirror, and the same two nonlinear optical elements (BB) are arranged in series so that the blue light is antiparallel to the optical axis of the crystal.
A laser light generation optical system was constructed in which only the ultraviolet light was passed through O), and thereafter, only the ultraviolet light was separated by a dichroic mirror and emitted as laser light of a desired wavelength into the gas to be measured.

Further, according to the present invention, each of the laser light generating optical systems described above includes a light receiving means for receiving scattered laser light from a direction to be measured, and a photoelectric conversion means for photoelectrically converting the scattered laser light received by the light receiving means. A gas concentration measuring device provided with recording means for recording light reception signal data from the photoelectric conversion means and calculation means for calculating the concentration of the gas to be measured based on the light reception signal data recorded in the recording means. Configured.

[0021]

DETAILED DESCRIPTION OF THE INVENTION The present invention will be described below with reference to the drawings.

FIG. 1 is a block diagram of a NO ₂ concentration measuring apparatus according to the present invention.

The portion surrounded by the broken line in FIG. 1 is NO ₂
Major components become NO ₂ distribution measurement laser beam generating optical system use concentration measuring device is (NO ₂ distribution measuring lidar source) 1A, 447.9nm of the absorption wavelength of NO ₂ is a measuring object gas Laser light and non-absorption wavelength 4
It is generated by switching the laser light of 46.5 nm.

The laser light generating optical system 1A has the YAG laser 12A and the wavelength of the input laser light,

[0025]

[Equation 1] The non-linear optical element (for example, KD ^* P) 13 for generating the second harmonic, which outputs 1/2 as a result of the relationship, a titanium sapphire laser 14, and another YAG laser 1
2B, a mirror 15 for reflecting the laser light from the YAG laser 12B, a dichroic mirror 16 for aligning the laser light from the titanium sapphire laser 14 and the laser light from the YAG laser 12B in the same optical path, and an input laser light. From the wavelength of

[0026]

[Equation 2] The two nonlinear optical elements (KNbO ₃ ) 17A and 17B arranged in series to obtain a new wavelength, the dichroic mirror 21 that extracts only the laser light (blue light) of the required wavelength, and the rest of the laser light are stopped. There is provided a beam dump 22 that causes the laser beam to be emitted, and a mirror 24 that reflects the output laser beam and emits the laser beam to a front measurement target region. The non-linear optical elements 17A and 17B are placed in an oven.

The above-mentioned nonlinear optical element (KNbO ₃ ) is used as a crystal for sum frequency in order to improve conversion efficiency. This KNbO ₃ has a large nonlinear optical constant,
Further, since NCPM (non-critical phase matching) is possible by the temperature controller 19, stable output and high conversion efficiency can be expected. Since KNbO ₃ has a small temperature matching tolerance, two crystals 17A and 17B phase-matched to ON and OFF wavelengths are used during DIAL measurement.

On the other hand, an aerosol (dust, dust, dust, etc.) on the optical axis passes through a region where NO _{2 as} a gas to be measured exists.
A Cassegrain telescope 2 that receives laser light scattered by water droplets) with the primary mirror 10 and the secondary mirror 11 and the laser light that has passed through the diaphragm 3 is a transmission wavelength for NO ₂ measurement 44.
A switching type interference filter 4 that passes both wavelengths near 7.9 nm and 446.5 nm, a photomultiplier tube 5 that photoelectrically converts an extremely weak laser light output and outputs it as a light reception signal, and a weak light reception signal is high. The preamplifier 6 that amplifies with the amplification factor, and the entire waveform of the amplified received light signal are A / D
A transient recorder 7 which is recorded by exchange, a personal computer 8 which processes the data of the transient recorder 7 to calculate the gas concentration, and a display 9 which displays the calculation result are provided.

Nonlinear optical element 1 of laser light generating optical system 1
This nonlinear optical element 17 is provided on the output side of 7A and 17B.
A photodiode 20 for detecting laser light output from A and 17B is provided, and its output signal is input to the transient recorder 7 as a trigger signal.

Next, the measurement of NO ₂ concentration according to this embodiment will be described. Laser light of 1064 nm is generated from the YAG lasers 12A and 12B. YAG laser 12
The laser light output from A is the nonlinear optical element 13 (KD ^*
It is converted into laser light of 532 nm by P), and the titanium sapphire laser 14 is excited. In the titanium sapphire laser 14, the angle of the diffraction grating of the internal resonator is switched for each laser pulse, and the wavelength of the generated laser light is set to 77.
Alternately switch to 3.5 nm and 769.4 nm.

On the other hand, the laser light with a wavelength of 1064 nm output from the YAG laser 12B is reflected by the mirror 15, and the dichroic mirror 16 aligns the laser light with the above-mentioned two wavelengths output from the titanium sapphire laser 14 to the nonlinear optical element. (KNbO ₃ ) 17A, 1
Sent to 7B. The non-linear optical element converts the laser light having a wavelength of 773.5 nm outputted from the titanium sapphire laser 12 into the laser light having a wavelength of 447.9 nm, and the laser light having a wavelength of 769.4 nm is 446.5 n.
The laser light converted into m is output alternately. The output from the nonlinear optical elements 17A and 17B is the wavelength before conversion 7
Since the laser light of 73.5 nm or 769.4 nm and 1064 nm is included, the dichroic mirror 2
The wavelength required for measuring NO ₂ (44
Only the laser beams of 7.9 nm and 446.5 nm) are extracted, reflected by the mirror 24, and emitted toward the front measurement target region.

The scattered laser light from the measurement target region is received by the Cassegrain telescope 2, is reflected by the main mirror 10 and the sub mirror 11, passes through the diaphragm 3, and is transmitted by the interference filter 4 to the NO ₂ measurement wavelength 447.9 nm. And 446.5n
Only wavelengths near m are transmitted. Interference filter 4
The light passing through is photoelectrically converted by the photomultiplier tube 5,
It is amplified by the preamplifier 6 and input to the transient recorder 7. In the transient recorder 7, the output signal from the photodiode 20 is used as a trigger signal to perform A / D conversion, and a waveform is recorded with respect to scattered light by one laser light pulse emitted from the laser light generation optical system 1. The data recorded in the transient recorder 7 is transferred to the personal computer 8 via the GPIB cable, where the following density calculation is performed.

Scattered light data when the laser light is greatly absorbed by NO ₂ (laser light having a wavelength of 447.9 nm) and scattered light data when it is small absorbed (wavelength is 4
The concentration can be obtained by taking the ratio with the laser light of 46.5 nm) for a large number of minute elapsed times and differentiating it with respect to the distance. In this case, the distance can be easily calculated based on the elapsed time because the laser light propagates in the atmosphere at the speed of light.

As a second embodiment of the present invention, a concentration measuring device for measuring the concentration of NO will be described.

FIG. 2 is a block diagram of the NO concentration measuring apparatus according to the present invention.

A portion surrounded by a broken line in FIG. 2 is a laser light generating optical system for NO distribution measurement (NO distribution lidar light source) 1B which is a main constituent element of the NO concentration measuring apparatus. This 1B is shown in FIG. The difference from 1A is that the wavelengths are halved at the output side of the dichroic mirror 21 that extracts only the laser light of the required wavelength. It is connected in series at the phase matching angle,
Others are the same as FIG. In the figure, the same reference numerals as in FIG. 1 indicate the same components. ON wavelength is 2
It is 26.812 nm and the OFF wavelength is 226.824 nm. Therefore, the titanium sapphire laser 14 is 79
Alternately switching between 0.752 nm and 790.825 nm.

The BBO crystal is the most used non-linear optical crystal for generating ultraviolet light. This BB
Since the O crystal has a large walk-off, the optical axes of the two crystals are arranged antiparallel to each other in order to reduce the influence of the walk-off. Even this is ON wavelength and OFF
Since the wavelengths are distant from each other, the emission angles are different, but the wavelength difference is 0.012 nm, which is negligible.

A dichroic mirror 21B is connected to the output sides of the non-linear optical elements 23A and 23B, and only the necessary ultraviolet laser light is taken out of the laser light emitted from the non-linear optical elements 23A and 23B to the front measurement target area. Emit. Beam dump 22B for stopping the remaining laser light
Is provided after the dichroic mirror 21B.
Between the output side of the non-linear optical elements 23A, 23B and the dichroic mirror 21, the non-linear optical elements 23A,
A photodiode 20 for detecting the laser beam output from 23B is provided, and its output signal is input to the transient recorder 7 as a trigger signal.

Although the YAG laser is used as the solid-state laser and the titanium sapphire laser is used as the wavelength tunable solid-state laser in the above embodiments, the present invention is not limited to these lasers and can be realized by using other lasers. . Further, the application of the laser light generated by the present invention is not limited to the concentration measurement.

[0040]

As described above, according to the present invention, the following excellent effects can be obtained. (1) Since a solid-state laser such as a YAG laser and a wavelength tunable solid-state laser such as a titanium sapphire laser are used without using a dye laser, the life is long, there is no fear of contamination, and handling is easy. (2) The conventional KD is used as a nonlinear optical crystal for generating the sum frequency.
^* Changed from P to KNbO ₃ . KNbO ₃ has the ability to convert laser light into another color called the nonlinear optical constant KD ^*
Since it is about 40 times stronger than P, blue light can be obtained with a strong output, and NO ₂ can be measured with high sensitivity. (3) Walk-off occurs in the conventional device using KD ^* P for the nonlinear optical crystal, but in the present invention, the nonlinear optical constant of the crystal is extremely larger than that of other crystals, KNbO.
By using ₃ , it is possible to eliminate walk-off and obtain intense blue light. (4) By taking a double wave of blue light with a BBO crystal, ultraviolet light necessary for NO measurement can be generated and NO measurement can be performed.

[Brief description of drawings]

FIG. 1 is a block diagram of a NO ₂ concentration measuring device according to the present invention.

FIG. 2 is a block diagram of the NO concentration measuring apparatus according to the present invention.

[Explanation of symbols]

1A Laser light generation optical system for NO ₂ distribution measurement (Lider light source for NO ₂ distribution measurement) 1B Laser light generation optical system for NO distribution measurement (Lider light source for NO distribution measurement) 2 Cassegrain telescope 3 Aperture 4 Interference filter 5 Photoelectron enhancement Double tube 6 Preamplifier 7 Transient recorder 8 Personal computer 9 Display 10 Telescope primary mirror 11 Telescope secondary mirror 12A YAG laser 12B YAG laser 14 Titanium sapphire laser 15 Mirror 16 Dichroic mirror 17A Nonlinear optical element 17B Nonlinear optical element 18 Oven 19 Temperature controller 20 Photo Diode 21 Dichroic mirror 21B Dichroic mirror 22 Beam dump 22B Beam dump 23A Nonlinear optical element 23B Nonlinear optical element 24 Mirror

Continuation of the front page (56) Reference JP-A-7-128232 (JP, A) JP-A-4-15976 (JP, A) JP-A-7-104334 (JP, A) Appl Phys Lett, March 1987 9th, VOL. 50 NO. 10, PAGE. 554-556 REPORTS RES. LAB. A SAHI GLASS CO. LT D. , 1992, VOL. 42, NO. 1, p67-79 (58) Fields investigated (Int.Cl. ⁷ , DB name) G01N 21/00-21/61 G01J 3/00-3/52 JISST file (JOIS) Practical file (PATOLIS) Patent file ( PATOLIS)

Claims (3)

(57) [Claims] 1. A laser light generation optical system of a laser radar for measuring a distribution of a measurement target gas by a solid-state laser,
The first solid-state laser, the first nonlinear optical element, and the tunable solid-state laser are arranged in this order with their optical axes aligned, and the second solid-state laser is arranged.
After the optical axes of the laser light output from the solid-state laser and the laser light output from the wavelength tunable solid-state laser are aligned by a dichroic mirror, the same two nonlinear optical elements KNbO ₃ arranged in series and phase-matched are used. Through
After that, a laser light generating optical system is characterized in that only blue light is separated by a dichroic mirror and emitted as laser light of a desired wavelength into a gas to be measured. 2. A laser light generation optical system of a laser radar for measuring the distribution of a gas to be measured by a solid laser,
The first solid-state laser, the first nonlinear optical element, and the tunable solid-state laser are arranged in this order with their optical axes aligned, and the second solid-state laser is arranged.
After the optical axes of the laser light output from the solid-state laser and the laser light output from the wavelength tunable solid-state laser are aligned by a dichroic mirror, the same two nonlinear optical elements KNbO ₃ arranged in series and phase-matched are used. Through
After that, only the blue light is separated by a dichroic mirror, and the blue light is passed through the same two nonlinear optical elements BBO arranged in series so that the optical axes of the crystals are antiparallel, and then only the ultraviolet light is separated by the dichroic mirror. A laser light generation optical system, which emits laser light of a desired wavelength into a gas to be measured. 3. The laser light generation optical system according to claim 1, a light receiving means for receiving scattered laser light from a direction to be measured, and photoelectric conversion of the scattered laser light received by the light receiving means. A photoelectric conversion means, a recording means for recording the light reception signal data from the photoelectric conversion means, and a calculation means for calculating the concentration of the gas to be measured based on the light reception signal data recorded in the recording means. A gas concentration measuring device.