JP2011232239A - Optical system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To eliminate reflected scattered light at the same wavelength as the wavelength of excitation light with high efficiency.SOLUTION: The optical system (7) includes: a first filter (11) which receives light (5), which is reflected scattered light from a sample (4) irradiated with excitation light (3) and includes light at a wavelength different from the wavelength of the excitation light (3), and prevents transmission of light in a wavelength region including the wavelength of the excitation light (3); an absorption filter (12) which receives the light transmitted through the first filter (11), and absorbs and attenuates the light; and a second filter (13) which receives the light transmitted through the absorption filter (12), is disposed parallel to the first filter (11), and prevents transmission of light in a wavelength region including the wavelength of the excitation light (3).

Description

  The present invention relates to a measurement optical system for weak light such as Raman light from a sample irradiated with excitation laser light.

In a Raman spectroscopic analyzer that measures Raman light, a sample is irradiated with excitation laser light, and reflected / scattered light from the sample is detected. At this time, reflected / scattered light from the sample includes Rayleigh light, which is reflected / scattered light having the same wavelength as that of the excitation laser light, and vibration inherent to the sample with respect to the wavelength of the excitation laser light. And Raman light whose wavelength is shifted according to the number. Therefore, spectroscopic analysis of Raman light makes it possible to analyze the sample. However, Raman light is extremely weak, 10 ^-6 to 10 ^-12 times the intensity of Rayleigh light. In order to measure light, it is necessary to remove and separate Rayleigh light with a filter or the like.
Regarding the technique for separating Rayleigh light, the techniques described in Patent Documents 1 to 4 below are conventionally known.

Japanese Patent Application Laid-Open No. 6-230219 as Patent Document 1 discloses a multilayer dielectric in which a first material and a second material having different refractive indexes are alternately coated to form a multilayer film in order to cut Rayleigh scattered light. It describes a technology that uses a filter and a technology that uses a holographic notch filter that cuts light of a specific wavelength by fixing by periodically changing the refractive index due to light interference in a thick gelatin film. Yes.
In JP-A-9-145619 as Patent Document 2, a holographic notch filter (34) for removing an excitation light component is used between a camera lens (12) and a condenser lens (14). The technology is described.

In Japanese Patent Laid-Open No. 9-184809 as Patent Document 3, a bandpass filter that transmits Raman scattered light to be measured from scattered light generated from a sample is used between two converging lenses (5a, 5b). The technology is described.
In Japanese Patent Application Laid-Open No. 2008-191041 as Patent Document 4, a pair of silicons (302) as a high refractive index material film are arranged in parallel, and an air layer (301) as a low refractive index material film is sandwiched between them. A pair of parallel optical resonators (330) with an optical film thickness set to ½ the wavelength at which the desired film thickness is to be cut, and an oxide film (402) as a high-refractive index material film, with a low refractive index in between. The Rayleigh scattered light is transmitted or reflected by the optical resonator (430) having the optical film thickness set to 1/4 of the wavelength to be cut with the air layer (401) as the material film interposed therebetween. A technique for cutting is described. Patent Document 4 discloses an optical resonator (730) in which a pair of aluminum (702, 703) as high reflection layers are arranged in parallel, an air layer (701) is sandwiched between them, and the distance between the high reflection layers is movable. Describes a technique for transmitting and cutting Rayleigh scattered light.

Japanese Patent Laid-Open No. 6-230219 (“0003” to “0008”) Japanese Patent Laid-Open No. 9-145619 (“0026”, FIG. 2) JP-A-9-184809 ("0032", FIG. 6) JP 2008-191041 A (“0041” to “0050”, “0060” to “0062”, FIGS. 3, 4, and 7)

(Problems of conventional technology)
In the conventional techniques described in Patent Documents 1 to 4, an attempt is made to remove Rayleigh light by using one filter or an optical resonator. As described above, the intensity of Rayleigh light is 10 times that of Raman light. The intensity is ⁶ times or more, and in reality, it is often impossible to remove with one filter. Therefore, it has been difficult for conventional techniques to block strong Rayleigh light.

FIG. 7 is an explanatory diagram relating to a conventional method of removing Rayleigh light, FIG. 7A is an explanatory diagram of a state in which two filters are arranged in parallel, FIG. 7B is a graph showing wavelength characteristics of filter transmittance, and FIG. It is a graph which shows the wavelength characteristic of a rate.
Here, in order to weaken the intensity of Rayleigh light, it is conceivable to reduce the intensity of Rayleigh light by arranging a plurality of filters and passing the Rayleigh light multiple times. As shown in FIG. 7A, the excitation laser light 01 having the wavelength λ ₁ is irradiated on the sample 02 and the light obtained by collimating the reflected / scattered light 03 from the sample 02 by the collimator 04 is the first high-pass filter in the example of FIG. Incident at 06. Here, as shown in FIG. 7B, the first high-pass filter 06 reflects light having a wavelength of λ _c or less, which is slightly longer than the wavelength λ _1, and prevents transmission, and has a wavelength of λ _c or more. Has the property of transmitting light. Therefore, Raman light having a wavelength longer than the wavelength λ _c passes through the first high-pass filter 06, but also transmits Rayleigh light that cannot be removed. This transmitted component is incident on the second high-pass filter 07 arranged in the same manner as the first high-pass filter 06, and the Raman light is transmitted and the Rayleigh light is reflected.

However, in the configuration shown in FIG. 7, reflection is repeated between the first high-pass filter 06 and the second high-pass filter 07, and so-called multiple reflection occurs. Therefore, even if a plurality of filters are arranged side by side, the Rayleigh light to be removed is at most about 1/2, and the remaining half of the Rayleigh light is transmitted. That is, if the intensity of Rayleigh light is to be reduced by, for example, about 3 orders (1000 times), about 10 high-pass filters ((1/2) ¹⁰ = 1/1024) must be arranged.

FIG. 8 is an explanatory view showing a conventional method for removing Rayleigh light, FIG. 8A is an explanatory view showing a state in which two filters are inclined, and FIG. 8B is a wavelength characteristic graph of filter transmittance when inclined. .
As shown in FIG. 8A, when the second high-pass filter 07 is tilted with respect to the first high-pass filter 06, multiple reflection does not occur. When inclined and arranged in this manner, the light reflected by the second high-pass filter 07 is obliquely incident on the first high-pass filter 06, so that the number of multiple reflections is limited and the Rayleigh light removal rate is improved. On the other hand, if the second high-pass filter 07 is tilted, the incident angle of the incident light is increased, so that the thickness of the second high-pass filter 07 with respect to the incident light appears to decrease, as shown in FIG. 8B. The edge wavelength λ _c of the high pass filter is shifted to the short wavelength side. Therefore, there is a problem that Rayleigh light having the wavelength λ ₁ leaks and the Rayleigh light cannot be sufficiently removed even though the second high-pass filter 07 is disposed.

  In view of the above circumstances, an object of the present invention is to efficiently remove reflected / scattered light having the same wavelength as that of excitation light.

In order to solve the technical problem, an optical system according to claim 1 is:
Reflected / scattered light from a sample irradiated with excitation light, and reflected / scattered light including light having a wavelength different from the wavelength of the excitation light is incident, and the wavelength of the excitation light is changed. A first filter that prevents transmission of light in a wavelength region including:
An absorption filter that receives light transmitted through the first filter and absorbs and attenuates the light;
A second filter that receives light that has passed through the absorption filter and is arranged in parallel with the first filter, and prevents transmission of light in a wavelength region including the wavelength of the excitation light;
It is provided with.

The invention according to claim 2 is the optical system according to claim 1,
A collimator that collimates the reflected / scattered light from the sample and enters the first filter;
It is provided with.

In order to solve the technical problem, the optical system according to claim 3 is:
Two filters that transmit the wavelength component reflected / scattered from the sample by irradiation of the excitation light and reflect the wavelength component of the excitation light are arranged in parallel, and the transmission of each filter is between the two filters. An absorption filter that absorbs and attenuates components is provided.

According to the first and third aspects of the invention, the reflected / scattered light having the same wavelength as that of the excitation light can be removed with high efficiency compared to the conventional configuration in which the absorption filter is not disposed between the two filters. be able to.
According to the second aspect of the present invention, even if collimated light is incident on the first filter, the influence of multiple reflection can be reduced.

FIG. 1 is a schematic explanatory diagram of a Raman analyzer including the optical system according to the first embodiment of the present invention. FIG. 2 is an explanatory diagram for explaining the characteristics of the absorption filter of the first embodiment. FIG. 3 is an explanatory diagram of the operation of the first embodiment, FIG. 3A is a schematic diagram of the arrangement relationship of three filters, FIG. 3B is an explanatory diagram of an example of scattered light before being incident on the filter group, and FIG. It is explanatory drawing of an example of the scattered light inject | emitted from. FIG. 4 is an explanatory diagram of symbols used for numerical calculation of Rayleigh light removal in the configuration of the embodiment, FIG. 4A is an explanatory diagram of the configuration of the optical system, and FIG. 4B is a schematic diagram of transmittance characteristics of the high-pass filter. FIG. 5 is an explanatory diagram of the transmittance of transmitted light. FIG. 6 is a graph showing the relationship between the transmittance of the absorption filter and the overall transmittance. The horizontal axis represents the transmittance of the absorption filter, and the vertical axis represents the ratio of the overall transmittance. FIG. 7 is an explanatory diagram regarding a conventional method of removing Rayleigh light, FIG. 7A is an explanatory diagram of a state in which two filters are arranged in parallel, FIG. 7B is a filter wavelength and transmittance characteristic, and FIG. It is a graph which shows the characteristic of a wavelength and a reflectance. FIG. 8 is an explanatory view showing a conventional method for removing Rayleigh light, FIG. 8A is an explanatory view showing a state in which two filters are inclined, and FIG. 8B is a wavelength characteristic graph of filter transmittance when inclined. .

Next, examples which are specific examples of embodiments of the present invention will be described with reference to the drawings, but the present invention is not limited to the following examples.
In addition, in the description using the following drawings, illustrations other than the members necessary for the description are omitted as appropriate for easy understanding.

FIG. 1 is a schematic explanatory diagram of a Raman analyzer including the optical system according to the first embodiment of the present invention.
In FIG. 1, in the Raman analyzer 1 according to the first embodiment, a laser beam 3 for excitation as an example of light is irradiated onto a sample 4 from a laser light source 2. The reflected / scattered light 5 from the sample 4 irradiated with the laser light 3 is collimated (collimated) by the collimator 6, and is a filter as an example of an optical system for removing Rayleigh light contained in the reflected scattered light. Incident into the group 7. The light transmitted through the filter group 7 passes through the spectroscope G, is collected by the condenser lens 8, and is measured by a detector such as a CCD camera 9 as an example of a measuring apparatus.

The filter group 7 includes a first high-pass filter 11 as an example of a first filter to which light from the collimator 6 is incident, an absorption filter 12 to which light from the first high-pass filter 11 is incident, and an absorption filter And a second high-pass filter 13 as an example of a second filter on which light from 12 enters. The filter group 7 of Example 1 is unitized by making the high-pass filters 11 and 13 on the surface of the absorption filter 12 or by forming a film. 13 may be arranged side by side at intervals.
The high-pass filters 11 and 13 of the first embodiment are configured by the same high-pass filter, and, as shown in FIG. 7B, a wavelength region having a wavelength λ _c that is greater than the wavelength λ ₁ of the excitation laser beam 3. Transmission of light in the wavelength region of wavelength λ _c or more. As the first filter and the second filter, it is possible to use a notch filter that transmits light outside the wavelength region in the vicinity of the wavelength λ ₁ of the excitation laser beam 3.

FIG. 2 is an explanatory diagram for explaining the characteristics of the absorption filter of the first embodiment.
In addition, the absorption filter 12 of Example 1 is an absorption type filter in which a light absorbing material that absorbs transmitted light is dispersed. As shown in FIG. 2, the absorption type ND that absorbs light in all wavelength regions. (Neutral Density: Absorption) Absorption by the filter 21, the absorption high-pass filter 22 that absorbs light in the wavelength region on the short wavelength side including the wavelength λ ₁ of the excitation laser beam 3, the absorption band-pass filter, etc. A type of filter can be used.
In FIG. 1, in the filter group 7 of the first embodiment, a first high-pass filter 11, an absorption filter 12, and a third high-pass filter 13 are arranged in parallel.

(Operation of Example 1)
FIG. 3 is an explanatory diagram of the operation of the first embodiment, FIG. 3A is a schematic diagram of the arrangement relationship of three filters, FIG. 3B is an explanatory diagram of an example of scattered light before being incident on the filter group, and FIG. It is explanatory drawing of an example of the scattered light inject | emitted from.
In the Raman spectroscopic device 1 of Example 1 having the above-described configuration, the reflected / scattered light 5 from the sample 4 irradiated with the excitation laser light 3 is incident on the first high-pass filter 11. In FIG. 3, when the reflected / scattered light 5 incident on the first high-pass filter 11 passes through the first high-pass filter 11, a part of the Rayleigh light having a high intensity is cut and cannot be cut. The remaining Rayleigh light and Raman light are transmitted through the absorption filter 12. When passing through the absorption filter 12, Rayleigh light and Raman light are attenuated and incident on the second high-pass filter 13. The Raman light incident on the second high-pass filter 13 is transmitted as it is and measured by the CCD camera 9. The Rayleigh light incident on the second high-pass filter 13 is partially reflected by the second high-pass filter 13 and passes through the absorption filter 12 again. Therefore, the reflected Rayleigh light is absorbed by the absorption filter 12 and attenuated. Therefore, the light that has undergone multiple reflection between the first high-pass filter 11 and the second high-pass filter 13 is absorbed by the absorption filter 12 and attenuated. Therefore, as shown in FIG. 3C, the light transmitted through the filter group 7 includes light including Raman light and Rayleigh light that is sufficiently attenuated compared to the conventional case, and is measured by the CCD camera 9.

(Numerical calculation for the removal of Rayleigh light)
FIG. 4 is an explanatory diagram of symbols used for numerical calculation of Rayleigh light removal in the configuration of the embodiment, FIG. 4A is an explanatory diagram of the configuration of the optical system, and FIG. 4B is a schematic diagram of transmittance characteristics of the high-pass filter.
Next, numerical calculation is performed on the efficiency of removing Rayleigh light.
First, in FIG. 4A, let us consider a state in which Rayleigh light of wavelength λ ₁ and Raman light of wavelength λ ₂ are transmitted through the filter group 7. Here, as shown in FIG. 4B, the high-pass filter (edge filter) 11 and 13, which wavelength lambda ₁ is reflection band, the wavelength lambda ₂ corresponding to the transmission band, each of the transmittance, T _1, Let T ₂ . In this numerical calculation, the transmittance of the high-pass filters 11 and 13 is T ₁ = 10 ⁻⁵ and T ₂ = 1. That is, it is assumed that Rayleigh light is transmitted for 10 ⁻⁵ minutes with respect to the intensity of incident light.
Next, when the internal transmittance of the absorption filter 12 and T _a, T _a <1 if there is absorption, if there is no absorption becomes T _a = 1. Assuming that the transmittance of the entire filter group 7 including the absorption filter is I ₁ and I ₂ with respect to light of wavelengths λ ₁ and λ ₂ , in order to detect weak Raman scattered light, I ₁ / I ₂ << 1 is a requirement.

FIG. 5 is an explanatory diagram of the transmittance of transmitted light.
Here, the transmittances I ₁ and I ₂ are calculated as follows.
In FIG. 5, when light having an intensity X of wavelength λ ₁ is incident, the intensity of the light transmitted through the first high-pass filter 11 is represented by X · T ₁ . The intensity of the light transmitted through the high pass filter 13 is
X ・ T ₁・ T _a , X ・ T ₁ ²・ T _a
It is represented by
The intensity of the light reflected by the second high pass filter 13 is
X ・ T ₁・ T _a (1-T ₁ )
When this reflected light is transmitted through the absorption filter 12, the intensity is
X ・ T ₁・ T _a ² (1-T ₁ )
It becomes.
When this light is reflected by the first high-pass filter 11, the intensity of the light is
X ・ T ₁・ T _a ² (1-T ₁ ) ²
When this light passes through the absorption filter 12 and the second high-pass filter 13, the intensity of the light is
X ・ T ₁・ T _a ³ (1-T ₁ ) ² , X ・ T ₁ ²・ T _a ³ (1-T ₁ ) ²
It becomes.
Similarly, each time reflection is repeated, the transmission component from the second high-pass filter 13 decreases by _Ta ² (1−T ₁ ) ² .

Since the total intensity Y of the light transmitted through the second high-pass filter 13 is the sum of these, Y is expressed by the following equation (1).
Y = X ・ T ₁ ²・ T _a + X ・ T ₁ ²・ T _a ³ (1-T ₁ ) ² + X ・ T ₁ ²・ T _a ⁵ (1-T ₁ ) ⁴
+ X ・ T ₁ ²・ T _a ⁷ (1-T ₁ ) ⁶ +…
= X ・ T ₁ ²・ T _a [1 + T _a ² (1-T ₁ ) ² + _Ta ⁴ (1-T ₁ ) ⁴ +…] Equation (1)
Where z = T _a ² (1-T ₁ ) ² <1, the formula for the sum of an infinite geometric series of z with the first term = 1
1 + z + z ² + z ³ +… = 1 / (1-z)
Equation (1) is expressed by (2).
Y = X · T ₁ ² · T _a / [1-T _a ² (1-T ₁ ) ² ] ... Formula (2)

Therefore, the transmittances I ₁ and I ₂ are expressed by the following formulas (3) and (4).
I ₁ = Y / X = T ₁ ² · T _a / [1-T _a ² (1-T ₁ ) ² ] ... Formula (3)
I ₂ = T ₂ ² · T _a / [1-T _a ² (1-T ₂ ) ² ] ... Formula (4)
Here, the ratio between I ₁ and I ₂ is considered as an index for considering the removal efficiency of the Rayleigh line.
I ₁ / I ₂ = {[1-T _a ² (1-T ₂ ) ² ] / [1-T _a ² (1-T ₁ ) ² ]} (T ₁ / T ₂ ) ² ... Formula (5)

Here, calculation is performed for the case where there is no absorption in the absorption filter 12, that is, the conventional case shown in FIG. 7 in which the absorption filter 12 is not disposed. In this case, since T _a = 1, Expression (5) becomes the following Expression (6).
I ₁ / I ₂ = {[1-(1-T ₂ ) ² ] / [1-(1-T ₁ ) ² ]} (T ₁ / T ₂ ) ²
≒ (1 / 2T ₁ ) (T ₁ / T ₂ ) ² = (1/2) ・ T ₁ / T ₂ ² ... Formula (6)
Here, Equation (6) is derived with T ₁ << 1 and (1 − T ₂ ) << 1. For example, substituting T ₁ = 10 ⁻⁵ and T ₂ = 1 into equation (6) yields the following equation (7).
I ₁ / I ₂ = (1/2) × 10 ^-5 Equation (7)
Therefore, when two filters are arranged as shown in FIG. 7, due to the influence of multiple reflection, the attenuation effect 10 ^-5 for one filter is only ½ and is a power, that is, ( 10 ^-5 ) It is confirmed that the effect of ² cannot be obtained.

Next, the case where there is absorption in the absorption filter 12, that is, the case of Example 1, is calculated. In this case, similarly to the derivation of Expression (6), when T ₁ and 1−T ₂ are handled as minute amounts, the following Expression (8) is obtained from Expression (5).
I ₁ / I ₂ = {[1-T _a ² (1-T ₂ ) ² ] / [1- _Ta ² (1-T ₁ ) ² ]} (T ₁ / T ₂ ) ²
= [1 / (1-T _a ² )] ・ (T ₁ / T ₂ ) ² ... Formula (8)
Therefore, since there is a factor of the final term (T ₁ / T ₂ ) ^{2 on} the right side of Equation (8), substituting the numerical values of T ₁ and T ₂ results in the effect of (10 ^-5 ) ² = 10 ^-10 As a result, it is confirmed that the Rayleigh light is efficiently removed by the two high-pass filters 11 and 13.

6, equation (6) is a graph showing the value of the transmittance T _a of the absorption filter 12 from the equation (8), the ratio I ₁ / I ₂ of the transmittance of the two wavelengths by an index.
As apparent from the graph, the transmittance T _a of the absorption filter 12 it can be seen that the ratio I ₁ / I ₂ of less becomes transmittance than 1 suddenly decreases. That is, it has been confirmed that the Rayleigh light removal efficiency is rapidly increased, and even when the transmittance T _{a is} about 0.8, it is on the order of 10 ^−9, and it has been confirmed that sufficiently high Rayleigh light removal efficiency is achieved.

  Therefore, in the filter group 7 of the first embodiment, by arranging the absorption filter 12 between the high-pass filters 11 and 13, the attenuation that has been about ½ in the past can attenuate the Rayleigh light with power efficiency. This makes it possible to significantly improve the attenuation efficiency of Rayleigh light. In the conventional Raman spectroscopic analysis, it is common technical knowledge for those skilled in the art to prevent the Raman light from being attenuated as much as possible, and the absorption filter 12 that is attenuated to the Raman light has not been disposed. In the Raman spectroscopic device 1 of Example 1, the efficiency of the high-pass filters 11 and 13 could be remarkably improved by arranging the absorption filter 12.

(Example of change)
As mentioned above, although the Example of this invention was explained in full detail, this invention is not limited to the said Example, A various change is performed within the range of the summary of this invention described in the claim. It is possible. Modification examples (H01) to (H08) of the present invention are exemplified below.
(H01) In the above-described embodiments, the specific numerical values and materials illustrated are not limited to the illustrated values and materials, and can be changed as appropriate according to the design, specifications, usage, and the like.
(H02) In the above-described embodiment, the present invention can be applied to configurations other than the illustrated Raman spectroscopic device 1, and an optical system such as a mirror, a half mirror, a diaphragm, a plurality of lenses, or a spectroscope depending on the design, specifications, etc. As described above, a known spectroscope other than the configuration exemplified in the embodiments, for example, a wavelength tunable bandpass filter may be provided, or a detector may be changed to a measuring device other than a CCD camera.

(H03) In the above-described embodiment, the filter group 7 has a configuration including the two high-pass filters 11 and 13. However, the configuration is not limited to this configuration, and three or more high-pass filters 11 and 13 and two or more absorption filters are used. A configuration having 12 is also possible.
(H04) In the above embodiment, the high-pass filter is exemplified as the filter. However, the present invention is not limited to this, and any filter capable of removing Rayleigh light and transmitting Raman light, such as a notch filter, a low-pass filter, and a bandpass. Arbitrary filters such as filters and band elimination filters can be used.
(H05) In the above embodiment, an absorption high-pass filter or an absorption band-pass filter can be used in addition to the ND filter as the absorption filter.
(H06) In the above embodiment, the present invention can be used not only for Raman measurement but also for an optical apparatus that measures a very weak signal in the presence of a strong background signal such as fluorescence measurement. The present invention relates to Rayleigh light and background, such as Raman light and background (reflected / scattered light having the same wavelength as excitation light (electromagnetic waves) such as X-rays, ultraviolet rays, and visible light). If is not removed, it can be applied to an optical system in a measuring apparatus that measures weak light that is difficult to measure.
(H07) In the above embodiment, the filter group 7 is shown as a combination of two two high-pass filters 11 and 13 and the absorption filter 12 arranged in a line. However, a high-pass filter is deposited on both sides of the absorption filter. It is also possible to use an integrated filter.
(H08) In the above embodiment, the filter group 7 is shown as a combination of the two high-pass filters 11 and 13 and the absorption filter 12 arranged in a line, but an absorption film is deposited between the pair of high-pass filters. It is also possible to use an integrated filter.

3 ... Laser light for excitation,
4 ... Sample,
5 ... light,
6 ... Collimator,
7 ... Optical system,
11 ... first filter,
12 ... Absorption filter,
13: Second filter.

Claims (3)

Reflected / scattered light from a sample irradiated with excitation light, and reflected / scattered light including light having a wavelength different from the wavelength of the excitation light is incident, and the wavelength of the excitation light is changed. A first filter that prevents transmission of light in a wavelength region including:
An absorption filter that receives light transmitted through the first filter and absorbs and attenuates the light;
A second filter that receives light that has passed through the absorption filter and is arranged in parallel with the first filter, and prevents transmission of light in a wavelength region including the wavelength of the excitation light;
An optical system comprising: A collimator that collimates the reflected / scattered light from the sample and enters the first filter;
The optical system according to claim 1, further comprising:   Two filters that transmit the wavelength component reflected / scattered from the sample by irradiation of the excitation light and reflect the wavelength component of the excitation light are arranged in parallel, and the transmission of each filter is between the two filters. An optical system comprising an absorption filter that absorbs and attenuates components.