JP2002250708A - Laser measuring instrument - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an inexpensive laser measuring instrument of high accuracy. SOLUTION: The laser measuring instrument is constituted so as to detect the physical or chemical change generated by irradiating a sample with acting beam using detection beam to perform the measurement of the sample and equipped with a beam source 1 for acting beam having a wavelength different from that of the detection beam and capable of being modulated in its output, a beam source 3 for the detection beam constituted of a semiconductor laser emitting means, a detection means 9 for detecting the detection beam and a quantity-of-beam reducing means 5 for reducing the quantity of the acting beam incident on the beam source 3 for the detection beam.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser measuring apparatus for measuring a sample using a laser beam.

[0002]

2. Description of the Related Art Conventionally, in a measuring method using light,
Lasers have been frequently used as light sources by taking advantage of features such as high brightness, high light condensing, and high directivity. For example, there are rare gas lasers such as an argon ion laser and a helium neon laser, solid lasers such as a YAG laser, and dye lasers. In particular, semiconductor lasers have recently been used in products instead of gas lasers because stable manufacturing processes that achieve high output and long life have been established in addition to being cheap and easy to handle. Is coming.

[0003] When a semiconductor laser is used in a measuring apparatus, the output is easily gigahertz (GHz).
It can be modulated at any frequency up to the level. The fact that the output can be easily modulated electrically is that in a measuring device using light, in signal processing after a photodetector, the S / N (Sig
nal-to-noise ratio), and high-precision signal processing such as a lock-in amplifier that extracts only components having the same cycle as the modulation frequency from the measurement signal with low noise can be performed. It can be a big feature.

In general, a measuring device often uses one semiconductor laser to measure a chemical change or a physical change of an object to be measured. However, if two lasers are used, the above-described modulation capability is utilized. Further, highly accurate measurement can be performed. As such an example, there is a thermal lens spectroscopic analyzer that measures the concentration of a gas or liquid sample with high sensitivity.

When a laser is condensed by a condenser lens and is incident on an object to be measured, heat is generated near a focal point (condensing position), and a temperature rise occurs. Since the spatial intensity distribution at the focus of the laser is generally Gaussian, the calorific value distribution generated in proportion to the intensity distribution and the resulting temperature distribution are also Gaussian. Then, since the refractive index of the solvent decreases as the temperature rises, the refractive index distribution becomes inverted Gaussian.
This refractive index distribution can be regarded as optically equivalent to a concave lens, and such a refractive index distribution is called a thermal lens. This thermal lens spectroscopy has an excellent feature of being 100 times or more more sensitive than the absorbance method (for example,
Manabu Tokeshi et al., J. Lumin.Vol.83-84, 261-26
4, 1999).

In this example, a laser beam (excitation light) adjusted to the wavelength of an object to be measured has its output modulated at a kilohertz level and is incident on a sample, a thermal lens is periodically generated, and a laser of another wavelength is generated. Light enters this thermal lens,
Only components having the same period as the excitation light are extracted by the photodiode and the lock-in amplifier. In this way, the system (measurement target) is stimulated by the laser light (action light) modulated at a certain cycle to cause a physical change or a chemical change, and the laser light (detection light) of another wavelength is used. Measuring the component having the same period as the action light in the change is a highly accurate and low-noise measurement method capable of obtaining a wide dynamic range since the background of the measurement is zero.

On the other hand, it is generally known that when the output is finally detected by a photodiode or the like using a semiconductor laser, light interference occurs due to return light from the system, resulting in noise. In this case, measures have been taken to reduce the return light itself by using a wave plate and a polarization dependent beam splitter, or to reduce the interference due to the return light by lowering the interference capability by high frequency superposition.

[0008]

When a measuring device is constituted by two semiconductor lasers, as described above, measures are taken to prevent light of the same wavelength from returning in order to reduce noise due to return light. Is usually the case. In fact, the present inventors have constructed an optical system for thermal lens spectroscopic analysis by using two semiconductor lasers to modulate working light. At this time, in order to reduce noise due to return light of the same wavelength, an attempt was made to measure the thermal lens of the liquid sample with low noise by taking measures against the return light.

However, contrary to the intention, when the detection light is a semiconductor laser light, a large background signal is generated even when the measurement sample is not installed, because the working light having a different wavelength enters the light source of the detection light. Was found to reduce the accuracy of the measurement. In a conventional thermal lens spectroscopic analysis system, gas laser light is used for both excitation light and detection light, or semiconductor laser light,
Gas laser light is used as the detection light, and no example using semiconductor laser light as the detection light has been reported. Therefore, the above-mentioned problem that a large background signal is generated when modulated laser beams having different wavelengths are incident on the semiconductor laser light-emitting means, which is a light source of the detection light, is a phenomenon first discovered by the present inventors.

Although this phenomenon occurs when the incident laser light is modulated, it is completely unrelated to the presence or absence of the thermal lens effect in the sample. Further, this phenomenon is completely different from noise that occurs when light interference occurs in a semiconductor laser due to return light from a conventionally known system. The present inventors have found that this problem can be solved by providing means for reducing the amount of excitation light incident on the light source of the detection light, such as an optical filter, and made the present invention.

Further, the present inventor has found that this solution can be widely applied to not only the thermal lens spectroscopic analyzer but also a laser measuring apparatus having two lasers, and completed the present invention. I came to.

[0012]

The present invention has the following configuration. That is, the laser measuring device according to claim 1 of the present invention is a laser measuring device that measures a sample using laser light, wherein the light source of the laser light configured by a semiconductor laser light emitting unit and the laser light A light source for a second laser light having a wavelength different from that of the laser light and whose output can be modulated, and a light amount of the second laser light incident on the light source for the laser light. Light quantity reducing means.

When the second laser light, which is a modulated laser light, is used together with the laser light in a laser measuring device, the second laser light enters a light source of the laser light, There is a possibility that the accuracy of the measurement of the sample by the laser light is reduced. However, if a light amount reducing unit that reduces the amount of the second laser light incident on the light source of the laser light is provided, the laser light is not affected by the second laser light (true light). (Only the signal) can be detected by the detection means.

[0014] Examples of the second laser light include an operation light that is irradiated on the sample to cause a physical change (such as a photothermal conversion phenomenon described later) or a chemical change, and the laser light is completely different from the laser light. Detection light or the like that performs another measurement independently. Further, in the laser measuring device according to claim 2 of the present invention, in the laser measuring device according to claim 1, the sample is irradiated with the second laser light to cause a physical change or a chemical change. The laser light may be detection light for detecting the physical change or the chemical change.

With such a configuration, the output of the second laser light (working light) is modulated,
Since the physical change or the chemical change is a periodic change, the output of the laser light (detection light) is also modulated in the same cycle by the action, thereby reducing the S / N of the electrical output from the detection means. , Can be increased by signal processing such as integration.

In addition, since the light quantity reducing means for reducing the quantity of the modulated working light incident on the light source of the laser light is provided, the detection means can reduce the background signal generated despite the absence of the sample. In addition, measurement accuracy can be improved. In addition, the present invention is applicable to all the laser measurement devices as long as the physical change and the chemical change are changes that have some optical effect on the detection light, such as reflection, refraction, scattering, rotation of the polarization plane, and the like. It is valid. As a particularly useful application example, the sample absorbs the action light to generate heat, changes the refractive index by thermal expansion, changes the optical path of the detection light, and changes the sample concentration etc. with high sensitivity based on the degree of the change. Photothermal conversion spectroscopy to be analyzed. Further, as an example of a chemical change, there is an example in which photoisomerization is caused by irradiation of a working light having a certain wavelength as seen in a system such as azobenzene. In this case, the change in absorbance due to photoisomerization may be measured using detection light.

Further, a laser measuring apparatus according to a third aspect of the present invention is the laser measuring apparatus according to the first or second aspect, wherein the detecting means is a synchronous detecting means. With such a configuration, even more accurate measurement can be performed by using a synchronous detection unit such as a lock-in amplifier that detects only components having the same period as the modulation frequency of the second laser light with low noise and high sensitivity. It can be a device.

According to a fourth aspect of the present invention, in the laser measuring apparatus according to any one of the first to third aspects, the light source of the second laser light is changed to the light source of the laser light. And a semiconductor laser emitting means different from the above. With such a configuration, the light source of the second laser light in addition to the light source of the laser light is also a semiconductor laser light emitting unit, so that a less expensive measuring device can be provided.

Further, a laser measuring apparatus according to a fifth aspect of the present invention is the laser measuring apparatus according to any one of the first to fourth aspects, wherein the light amount reducing means is an optical filter. . With such a configuration, utilizing the fact that the laser light and the second laser light have different wavelengths, the light is incident on the light source of the laser light by an optical filter such as an absorption filter or an interference filter. The amount of the second laser light to be emitted can be reduced. As a result, the background signal in the detection means can be reduced, and the measurement accuracy can be increased.

Further, the laser measuring device according to claim 6 of the present invention is characterized in that, in the laser measuring device according to any one of claims 2 to 5, the physical change is a change due to a photothermal conversion phenomenon. And With such a configuration, it is possible to perform a highly sensitive analysis of the characteristic of the concentration of the liquid sample. In addition, since the measurement method using the photothermal conversion phenomenon generally has higher sensitivity when two different lasers having different wavelengths are used, the present invention has the feature of high sensitivity, and is inexpensive and highly accurate. It is possible to simultaneously have the feature of being.

[0021]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the laser measuring apparatus according to the present invention will be described in detail with reference to the drawings. FIG. 1 is a configuration diagram illustrating a configuration of a laser measurement device according to one embodiment of the present invention. Note that the present embodiment is an example of the present invention, and the present invention is not limited to the present embodiment. In the present embodiment, a case will be described in which the second laser light is a working light that causes a physical or chemical change in the sample. Further, the physical change and the chemical change to be measured are not limited in the present embodiment, and have a minimum configuration for implementing the effects of the present invention. If a specific physical change or chemical change is to be measured, necessary optical components and the like may be added to this embodiment.

As the light source 1 for the working light, it is desirable to use a semiconductor laser emitting means because it is inexpensive. In the case of a semiconductor laser light emitting means, the output can be easily modulated simply by inputting an electric modulation signal from the outside. Although not shown, the control of the semiconductor laser light emitting means may be an output control type or a current control type. further,
As a countermeasure against noise due to return light, it is desirable to superimpose a high frequency of 100 MHz level. Further, the wavelength of the working light source 1 may be selected such that the change becomes large depending on what physical change and chemical change occur in the sample. When the photothermal conversion phenomenon occurs, the absorption of the sample is used, so that it is desirable to select a wavelength that maximizes the absorption of the sample.

When a semiconductor laser light emitting means is used as the light source 1 for the working light, the output light is usually divergent light, and therefore it is desirable to use a collimator lens 2 for making it closer to parallel light. The collimator lens 2 only needs to have a positive focal length, and for example, a convex lens or a GRIN lens having a refractive index distribution may be used. Also,
When wavelength accuracy is required, another type of semiconductor laser, distributed feedback (D
The FB) type and the distributed reflection (DBR) type are preferable because the spectral width can be narrowed and the wavelength can be stabilized.

The light source 1 of the working light is not particularly limited to a semiconductor laser light emitting means, but may be a gas laser, a solid laser, a dye laser or the like. In this case, since electrical modulation is not possible, mechanical modulation means such as a chopper is used in the optical path. In this case, since the output light is close to parallel light, the collimator lens 2 may be omitted.

The semiconductor laser light emitting means used as the light source 3 for the detection light can be used without any problem as long as the wavelength is different from that of the excitation light. The control may be an output control type or a current control type. Like the light source 1 for the working light, it is desirable to superimpose a high frequency to reduce the noise of the return light. When wavelength accuracy is required, a distributed feedback (DFB) type or a distributed reflection (DBR) type in which a diffraction grating is formed in a resonator as another type of semiconductor laser can reduce the spectral width. It is desirable because the wavelength can be stabilized.

Further, since the light source 3 of the detection light is a semiconductor laser light emitting means, it is desirable to use a collimator lens 4 to make the light close to parallel light. The collimator lens 4 only needs to have a positive focal length, and for example, a convex lens or a GRIN lens having a refractive index distribution may be used. The beam splitter 6 was used to make the working light and the detection light coaxial. Beam splitter 6
May be made coaxial by using the difference in the wavelength of the working light and the detection light, or may be made coaxial by using the difference in the polarization plane. It is desirable that the beam splitter 6 has as high a reflectivity as possible with respect to the working light and, if necessary, a high transmittance with respect to the detection light. In the present embodiment, the working light and the detection light are made coaxial. However, the present invention is not limited to this. The light may intersect and be incident on the sample cell 7 or may face each other.

As described above, when the working light source 1 is modulated at a certain frequency and the modulated working light is incident on the detection light source 3 which is a semiconductor laser light emitting means, the detecting means 9
It was found that, despite the absence of the sample cell 7, a background signal was generated and the accuracy of the measurement was significantly reduced. This incidence may be due to light reflected from the surface of the configured optical component, or stray light due to scattered light.

As described above, the provision of the light amount reducing means 5 for reducing the amount of the working light incident on the light source 3 of the detection light is provided.
This is a very important point in improving measurement accuracy. An optical filter is used as the light amount reducing unit 5 by utilizing the fact that the wavelengths of the working light and the detection light are different. As the optical filter, an absorption filter that absorbs only the working light can be used. In this case, the absorption filter cuts the light of the working light as much as possible, and if it is desired not to reduce the output of the detection light, it is desirable that the absorption filter transmit the detection light as much as possible.

Further, an interference filter may be used as another optical filter. In this case, a filter that selectively transmits only the detection light can be used,
As in the case of the absorption filter, the light of the working light is cut as much as possible, and if it is desired not to reduce the output of the detection light, it is desirable that the detection light be transmitted as much as possible. Sample cell 7
For a liquid or gas, a cell made of glass or a transparent resin may be used, and for a solid, a cell may not be used. When the physical change phenomenon is a photothermal conversion phenomenon, the amount of light absorption is measured with high sensitivity. Therefore, it is desirable that the absorption of glass or transparent resin at the wavelength of the working light be as small as possible, preferably 1% or less. Desirably, the absorption rate.

The working light cut filter 8 is for cutting the working light and selectively transmitting the detection light.
As the working light cut filter 8, there is no problem if a filter similar to the light amount reducing means 5 is used. Further, spectral means such as a spectroscope may be used. A photodiode is mainly used as the detection means 9. There is no problem if the photodiode has sensitivity to the wavelength of the detection light, but it is desirable that the photodiode have sufficient sensitivity when the output of the detection light is small.

Further, in order to measure a periodic output change of the detection light due to a periodic physical change and a chemical change in the sample cell 7, the detection means 9 may be provided with an oscilloscope or a lock-in amplifier. In the case of an oscilloscope, it is desirable to use a digital oscilloscope capable of integrating periodic waveforms. In addition, if a lock-in amplifier is used, it is possible to extract a component having the same cycle as the modulation frequency of the working light, so that high-precision measurement with low noise is possible. If the sensitivity of the oscilloscope or the lock-in amplifier is insufficient, the current signal or the voltage signal may be amplified by an amplifier or the like in advance.

(Example) Next, measurement using the laser measuring apparatus according to the present invention will be described in detail with reference to an example.
FIG. 2 is a configuration diagram showing the configuration of the laser measurement device used in the present embodiment, specifically, the configuration of a thermal lens spectrometer. Note that the configuration of the laser measurement device of FIG. 2 is substantially the same as the configuration of the laser measurement device of FIG. 1, and therefore only different portions will be described, and description of the same portions will be omitted. In FIG. 2, the same or corresponding parts as those in FIG.
The same reference numerals are given as. Further, although there are self-made parts in the embodiments, there is no problem even if a commercially available part having the same specifications is used.

The light source 1 of the working light includes a semiconductor laser light emitting device (LT051 having a wavelength of 635 nm and a rated output of 30 mW).
(PS, manufactured by Sharp Corporation), and the output was adjusted to 3.5 mW after the lens 12. Also,
The light source 3 of the detection light has a wavelength of 780 nm and a rated output of 50 m.
A W semiconductor laser light emitting device (ML60114R, manufactured by Mitsubishi Electric Corporation) was used. The output of the detection light was adjusted to be 3.5 mW after the lens 12.

The output and current of these semiconductor lasers can be controlled by a commercially available LD driver (ALP-6323CA, manufactured by Asahi Data Systems) not shown. The LD driver is a PCI (not shown)
It is connected to a personal computer via a card (NIPCI-6025E, manufactured by National Instruments), and the output, current, and modulation frequency of the semiconductor laser can be set from the personal computer.
The modulation frequency of the excitation light was set to about 1 kHz. Further, a high frequency of 350 MHz was superimposed on both the operation light and the detection light to reduce the influence of the return light.

As the collimator lens 2 for working light, a self-made lens having a numerical aperture of 0.34 and a focal length of 8 mm was used.
As the collimator lens 4 for probe light, a self-made lens having a numerical aperture of 0.39 and a focal length of 7 mm was used. Further, the beam splitter 6 for making the working light and the detection light coaxial has a transmittance for p-polarized light of 100%,
A self-made polarization dependent beam splitter having a reflectance of 100% for s-polarized light was used. In this case, since the working light is s-polarized light and the detection light is p-polarized, the power loss in this beam splitter is almost zero.

The quarter wave plate 11 (02WRM0)
19, manufactured by Meles Griot) and the polarization plane of the return light is 9
By changing the angle by 0 degrees, the return light of the same wavelength was reduced to 1/100 in combination with the beam splitter 6 described above.
Further, as the condenser lens 12, a numerical aperture of 0.4 and a focal length of 4.5 mm (350022, manufactured by GELTECH) were used.

Further, the same light receiving lens 13 as the condenser lens 12 was used. In this embodiment, the sample cell 7 is not described because no sample is used.
Further, the aperture 14 has a diameter of 0.8 mm to 25 mm.
An iris diaphragm (BJ35110, manufactured by Edmund Scientific Japan KK) that can be adjusted to a maximum is used.

Further, the working light cut filter 8 includes:
A laser line interference filter (03FIL050, manufactured by Meles Griot Co., Ltd.) having a center wavelength of 780 nm and a half width of 10 nm was used. The detection light transmitted through the filter was guided to the detection means 9 and was converted into an electric signal. Detecting means 9
, A 4-division photodiode (S6344, manufactured by Hamamatsu Photonics) was used. Note that it is not necessary to use a four-division photodiode for the detection means 9, and a non-division photodiode may be used. In the present embodiment, the sum of the four electric signals of the four-division photodiode is calculated and used as a signal.

The output from the four-division photodiode was converted from a current to a voltage by a self-made circuit. The conversion magnification from current to voltage was 1000 times. Furthermore, the converted voltage signal is a low noise preamplifier (L
(I-75A, manufactured by NF Corporation) (not shown), and further guided to a two-phase lock-in amplifier (5610B, manufactured by NF Corporation) (not shown) to synchronize with the modulation frequency of the working light. Only electric signals were extracted and used as signal values.

Further, the light amount reducing means 5 for reducing the amount of the working light incident on the light source 3 of the detection light is cut into 1/1000 of the working light by one sheet, and 70% of the detection light is cut.
A transmitting absorption filter (IR76, manufactured by Fuji Film Co., Ltd.) was used. Using the apparatus of this embodiment, the output from the lock-in amplifier was measured when the number of the absorption filters of the light amount reducing means 5 was changed without the sample cell 7.

In the state without any filter, a background signal of 0.5 V was generated in the lock-in amplifier despite the absence of the sample cell 7. This means that the detection light is modulated at the same frequency as that of the operation light because the operation light is incident on the light source 3 of the detection light. If one filter is used and the incidence of the working light is made 1/1000 or less, this background signal becomes 0.00
The voltage dropped to 1/50, that is, 9 V, and was 0.006 V in two sheets.

As described above, by reducing the amount of the working light incident on the light source 3 of the detection light by using the light amount reducing means 5, the background signal is reduced and the measurement accuracy is remarkably improved.

[0043]

As described above, according to the present invention, an inexpensive and highly accurate laser measuring apparatus can be realized. Further, the present invention has been shown to be applicable to any laser measuring device using two separate lasers and at least the light source of the detection light is constituted by a semiconductor laser emitting means.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing one embodiment of a laser measurement device of the present invention.

FIG. 2 is a configuration diagram illustrating a configuration of a laser measurement device according to an embodiment.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Light source of working light 3 Light source of detection light 5 Light intensity reduction means 7 Sample cell 9 Detection means

Claims (6)

[Claims] 1. A laser measuring apparatus for measuring a sample using a laser beam, comprising: a light source of the laser beam configured by a semiconductor laser light emitting unit; a detecting unit for detecting the laser beam; And a light source of a second laser light having a different wavelength and whose output can be modulated, and a light amount reducing unit that reduces the light amount of the second laser light incident on the light source of the laser light. Laser measuring device. 2. The method according to claim 1, wherein the second laser light is used as an operating light that irradiates the sample to cause a physical change or a chemical change, and the laser light detects the physical change or the chemical change. The laser measurement device according to claim 1, wherein the detection light is used as the detection light. 3. The laser measuring apparatus according to claim 1, wherein said detecting means is a synchronous detecting means. 4. The laser measuring apparatus according to claim 1, wherein the light source of the second laser light is constituted by a semiconductor laser light emitting means different from the light source of the laser light. . 5. The laser measuring apparatus according to claim 1, wherein said light amount reducing means is an optical filter. 6. The laser measuring apparatus according to claim 2, wherein the physical change is a change due to a photothermal conversion phenomenon.