JP6673674B2 - Spectrometer - Google Patents

Description

  The present invention relates to a spectrometer, and more particularly, to a wavelength scanning spectrometer (monochromator) mounted on an optical spectrum analyzer, for example.

  FIG. 4 is a configuration explanatory view showing an example of a conventional wavelength scanning type spectrometer using a plane diffraction grating (hereinafter, referred to as a diffraction grating). In FIG. 4, light that has passed through an entrance slit 2 from a light source 1 is converted into a parallel light by a first collimator 3 and is incident on a diffraction grating 4.

  The light diffracted after being incident on the diffraction grating 4 is converted into convergent light by the second collimator 5 and condensed on the exit slit 6. Thereby, the light receiver 7 can receive only light of a specific wavelength. The light source 1, the entrance slit 2, the first collimator 3, the diffraction grating 4, the second collimator 5, the exit slit 6, and the light receiver 7 that constitute the spectrometer are arranged on a base 8. .

  FIG. 5 is a configuration explanatory view showing a configuration example of a conventional two-stage spectrometer. In FIG. 5, the light source 1 and the light receiver 7 are arranged so as to be located at concentric positions. The light that has passed through the slit 2 from the light source 1 is converted into parallel light by a first collimator (off-axis parabolic mirror) 3 and is incident on a diffraction grating 4.

  The light diffracted after being incident on the diffraction grating 4 is converted into convergent light by a second collimator (off-axis parabolic mirror) 5, and is incident on a reflection block 9 composed of a plurality of reflection mirrors. The light incident on the reflection block 9 and reflected is shifted to the second collimator (off-axis parabolic mirror) 5 again in such a state that the position of the optical axis is shifted from the position of the optical axis of the light at the time of incidence. The light is reflected toward the diffraction grating 4 again along a path that reverses the path at the time of incidence from the light source 1.

  The light reentering the diffraction grating 4 is diffracted again and emitted toward the first collimator (off-axis parabolic mirror) 3. The light incident on the first collimator (off-axis parabolic mirror) 3 is reflected toward the light receiver 7, passes through the slit 2 again, and is received by the light receiver 7.

Generally, the relational expression between the wavelength of light and the angles of incident light and diffracted light is expressed by equation (1).
mλ = dcosθ (sinα + sinβ) (1)
m: diffraction order d: lattice constant λ: wavelength of light
θ: Incline of diffraction grating groove α: Angle between incident light and diffraction grating normal (incident angle)
β: Angle (diffraction angle) between the diffracted light and the diffraction grating normal

  According to the equation (1), the wavelength of the specific light received can be known based on the angle of the diffraction grating 4. If the slit width of the diffraction grating 4 is small, the wavelength separation ability is high, and it can be said that the spectrometer is a higher performance spectrometer.

In general, the wavelength resolution (RB) of a diffraction grating is expressed by equation (2).
RB = (d · S · cosβ) / (m · f) (2)
m: Diffraction order d: Lattice constant S: Emission slit size f: Focal length of spectrometer (monochromator) β: Angle between diffraction light and diffraction grating normal (diffraction angle)

Based on the equation (2), the wavelength resolution (RB) in the two-stage spectrometer as shown in FIG. 5 is expressed by the equation (3).
RB = 2 (d · S · cosβ) / (m · f) (3)
m: diffraction order d: lattice constant S: exit slit size f: focal length of spectrometer (monochromator) β: angle between diffracted light and normal to diffraction grating (diffraction angle)

  In the expression (3), the second time for determining the wavelength resolution, that is, by arranging the light so that the diffraction angle β is larger than the incident angle α when the light finally enters the diffraction grating and is diffracted, the higher wavelength is obtained. It is common practice to obtain resolution.

  In general, lenses and concave mirrors are used as collimators for these spectrometers.However, in order to obtain collimated light or aberration-free light over a wide wavelength range using lenses, multiple lenses must be used. It is said that concave mirrors are simple and superior because they need to be combined.

  A parabolic mirror is one of the concave mirrors having a small aberration, but an off-axis parabolic mirror may be used from the viewpoint of using the mirror without generating extra aberration.

  FIG. 6 is a diagram illustrating the positional relationship between a collimator (off-axis parabolic mirror) and a slit. As shown in FIG. 6, the first collimator (off-axis parabolic mirror) 3 is obtained by extracting a portion near a specific angle called an off-axis angle θ ′ from the parabolic mirror, and forms a parallel light. Since the position of the part and the focal point do not overlap, parts can be easily arranged so as not to cause aberration. The slit 2 is used by being arranged at a position overlapping the focal position F.

  Patent Literature 1 discloses a two-stage spectrometer in which a sufficient wavelength resolution is obtained, the substantial length of the two-stage spectrometer can be shortened, the adjustment of the slit system is simplified, and a good neighborhood dynamic range is obtained. Has been proposed.

JP-A-2000-88647

  In a general use environment, in order to obtain accurate measurement results, it is required that the performance as a spectrometer does not change even when the ambient temperature changes. As described above, although the aberration in the spectrometer using an off-axis parabolic mirror as the collimator is small, depending on the material of the component parts, the angle change of the parallel light due to the temperature change and the shift of the focusing position may occur. As a result, there is a problem that the spectral characteristics of the spectrometer are deteriorated.

  Generally, an off-axis parabolic mirror used as a collimator is often formed of a glass material for reasons such as manufacturing accuracy. The base 8 on which the components constituting the spectrometer are arranged at predetermined positions is often made of a metal material due to ease of handling and restrictions on price. However, since the glass material and the metal material have different coefficients of thermal expansion, a change in the ambient temperature of the spectrometer causes a shift in the relative positional relationship, resulting in deterioration of the spectral characteristics. There is.

  FIG. 7 is a diagram illustrating a change in the positional relationship between a collimator (off-axis parabolic mirror) and a slit due to a temperature change. When the thermal expansion coefficient of the base 8 that fixes the first collimator (off-axis parabolic mirror) 3 and the slit 2 is larger than the thermal expansion coefficient of the first collimator (off-axis parabolic mirror) 3 3 shows a change in the positional relationship between the slit 2 and the first collimator (off-axis parabolic mirror) 3 when the temperature rises.

  As shown in FIG. 7, the position of the focal point F is moved by the thermal expansion of the first collimator (off-axis parabolic mirror) 3 with an increase in temperature. Further, the position of the slit 2 is different from the position of the slit 2 before the temperature rise due to the thermal expansion of the base 8 with a rise in temperature, and as a result, the position of the focus F is moved by a larger amount. For this reason, the relative position between the focal point F of the first collimator (off-axis parabolic mirror) 3 and the slit 2 also deviates, and the light emitted from the light source 1 is displaced by the first collimator ( The light is incident on the off-axis parabolic mirror 3 from a position shifted by x in the off-axis direction and by y in the focal direction.

For example, when the focal length f of the first collimator (off-axis parabolic mirror) 3 is 275 mm and the off-axis angle θ ′ is 7 °, the off-axis focal length f ′ is 279.1 mm. Then, soda glass (thermal expansion coefficient: 9.0 × 10 ⁻⁶ ) is used as a material of the off-axis parabolic mirror constituting the first collimator 3, and aluminum (thermal expansion coefficient: 23.8) is used as a material of the base 8. × 10 ^-6) in the case of using a + 1 if the temperature change of ℃ occurs, the focal position of the first collimator (off-axis paraboloidal mirror) 3, a slit 2, also the output slit 6 (emission slit 6 In addition, since it is arranged on the base 8, its position changes similarly to the slit 2 in accordance with the temperature change.) As a result, the positional relationship is about 0.004 mm in the direction of the focal length and about 0.2 mm in the off-axis direction. This results in a displacement of 001 mm.

  At this time, regarding the displacement of the position in the focal direction, since the beam diameter becomes larger by being shifted from the condensing position where the beam diameter at the slit position becomes the minimum, there is a problem that a narrow slit cannot be used. If the width is used, this effect can be neglected, but a new problem that the wavelength separation performance is deteriorated occurs.

  Further, regarding the shift from the focal position of the first collimator (off-axis parabolic mirror) 3 to the off-axis direction, the light is incident from the shifted position, so that the angle of the parallel light changes. As a result, since the incident angle of light on the diffraction grating 4 changes, the diffraction angle of the diffracted light also changes.

  In addition, if the exit slit is displaced in the off-axis direction from the focal position of the collimator (off-axis parabolic mirror), the light cutout position changes according to the displacement, resulting in a shift in the selected wavelength. sell.

Therefore, when used as a spectrometer, both the positional deviation of the entrance slit and the positional deviation of the exit slit that occur in response to a temperature change affect the measurement environment, and the result appears as, for example, a wavelength error. Will be.

  In addition, the diffraction grating 4 may be made of glass as a material for reasons of manufacturing accuracy and the like. However, relatively easily available and inexpensive glass tends to have a large thermal expansion coefficient.

For example, the lattice constant d of the diffraction grating is generally represented by the following equation (4).
d = (1 + Δt · K) / N (4)
d: lattice constant Δt: temperature change K: thermal expansion coefficient of diffraction grating N: number of grooves per 1 mm of diffraction grating

  Based on the equation (4), the value of the lattice constant d changes according to the change in temperature. Therefore, when the temperature changes, there is a problem that the diffraction angle β of the diffracted light by the diffraction grating 4 also changes.

Here, a diffraction grating made of borosilicate glass will be described as an example. Examples of the borosilicate glass include Tempax Float (registered trademark). The borosilicate glass has a coefficient of thermal expansion of about 3 × 10 ⁻⁶ .

Under the condition that the number of grooves of the diffraction grating is 650 / mm and the inclination (θ) of the grooves of the diffraction grating is 16 °, when a monochromator as shown in FIG. , The light receiver 7 receives the incident light under the angle condition of the diffraction grating 4 such that the incident angle α２．５22.5773 ° and the diffraction angle β ≒ 38.5773 °.

  Here, for example, when a temperature rise of 1 ° C. occurs, other conditions do not change, and only the diffraction grating 4 has an influence. Considering the case where the diffraction angle of the diffracted light is β．38.5771 °, the diffraction angle is narrow. When a slit is used, diffracted light cannot pass through the slit, and the light receiver 7 does not receive incident light.

  Under this condition, the light receiver 7 can receive the incident light by slightly changing the angle of the diffraction grating 4 so that the incident angle α ≒ 22.5772 ° and the emission angle β ≒ 38.5772 °. The wavelength calculated from the angle of the diffraction grating 4 is 1549.9949 nm, and a wavelength error occurs with respect to the true wavelength.

  Therefore, when used in a spectrometer, a change in the diffraction grating caused by a change in temperature also affects the measurement environment, and the result appears as, for example, a wavelength error.

  SUMMARY OF THE INVENTION The present invention has been made to solve these problems, and an object of the present invention is to realize a spectrometer capable of suppressing a wavelength change of light due to a temperature change to be low, and having a stable wavelength accuracy.

That is, of the present invention, the invention described in claim 1 is:
A spectrometer comprising a collimator, a plane diffraction grating and a base, wherein the incident light is configured as a multi-stage type diffracted a plurality of times by the plane diffraction grating ,
The collimator comprises an off-axis parabolic mirror,
The plane diffraction grating is such that a diffraction angle when light is first incident on the plane diffraction grating and is diffracted is larger than an incident angle when light is first incident on the plane diffraction grating and is diffracted. Arranged so that the diffraction angle when light is finally incident on the plane diffraction grating and diffracted is smaller than the incident angle when light is finally incident and diffracted on the plane diffraction grating. It is characterized by being.

According to a second aspect of the present invention, in the spectrometer according to the first aspect ,
The off-axis parabolic mirror has a coefficient of thermal expansion of 7 × 10 ⁻⁶ or more and 9 × 10 ⁻⁶ or less.

According to a third aspect of the present invention, in the spectrometer according to the second aspect ,
The main component of the off-axis parabolic mirror is glass.

According to a fourth aspect of the present invention, in the spectrometer according to the third aspect ,
The main component of the off-axis parabolic mirror is soda glass.

According to a fifth aspect of the present invention, in the spectrometer according to any one of the first to fourth aspects,
The planar diffraction grating has a coefficient of thermal expansion of 3 × 10 ⁻⁶ or more and 6 × 10 ⁻⁶ or less.

According to a sixth aspect of the present invention, in the spectrometer of the fifth aspect ,
The main component of the plane diffraction grating is glass.

According to a seventh aspect of the present invention, in the spectrometer according to the sixth aspect ,
The main component of the planar diffraction grating is one of borosilicate glass and soda glass.

The invention according to claim 8 is the spectrometer according to any one of claims 1 to 7 ,
The thermal expansion coefficient of the base is 22 × 10 ⁻⁶ or more and 26 × 10 ⁻⁶ or less.

According to a ninth aspect of the present invention, in the spectrometer of the eighth aspect ,
The main component of the base is a metal.

According to a tenth aspect , in the spectrometer according to the ninth aspect ,
The main component of the base is aluminum.

  As a result, a wavelength change due to a temperature change can be suppressed low, and a spectrometer with stable wavelength accuracy can be realized.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a configuration explanatory view showing one embodiment of the present invention. FIG. 10 is a configuration explanatory view showing another embodiment of the present invention. FIG. 10 is a configuration explanatory view showing another embodiment of the present invention. It is a structural explanatory view showing an example of a conventional spectrometer. FIG. 2 is a configuration explanatory diagram showing an example of a configuration of a conventional multi-stage spectrometer. FIG. 5 is an explanatory diagram of a positional relationship between an off-axis parabolic mirror and a slit in FIG. 4. FIG. 9 is a diagram illustrating a change in the positional relationship between the off-axis parabolic mirror and the slit due to a temperature change.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 is an explanatory diagram showing the configuration of an embodiment of the present invention, and portions common to FIG. 4 are denoted by the same reference numerals.

  In FIG. 1, the spectrometer includes the same components as in FIG. 4, but the diffraction grating 4 is arranged so that the diffraction angle β is smaller than the light incident angle α (α> β).

  Also in the spectrometer of FIG. 1, the wavelength shift due to the temperature change of the diffraction grating 4 and the positional relationship between the slit 2, the exit slit 6, and the first collimator (off-axis parabolic mirror) 3 due to the temperature change. Of the optical axis caused by the change of the optical axis, the temperature change of the optical axis caused by the first collimator (off-axis parabolic mirror) 3 is caused by the change of the diffraction angle of the diffraction grating 4 arranged to invert the diffraction order. The change in temperature is offset by the change in temperature, and the change in wavelength of the spectrometer as a result of the change in temperature can be reduced.

  The components that make up the spectrometer may have different coefficients of thermal expansion due to differences in the main components of the materials that make up each.

In the case of the present invention, the off-axis parabolic mirror constituting the collimator may be one conventionally used in a spectrometer, but its thermal expansion coefficient is 7 × 10 ⁻⁶ or more and 9 × 10 ⁻⁶ or less. It is preferred that In addition, from the viewpoint of more appropriately exhibiting the effects of the present invention, the main component of the material is more preferably glass, and more preferably soda glass.

As the diffraction grating, a diffraction grating conventionally used in a spectrometer may be used, but its thermal expansion coefficient is preferably 3 × 10 ⁻⁶ or more and 6 × 10 ⁻⁶ or less. Then, from the viewpoint of more appropriately exerting the effects of the present invention, the main component of the material is more preferably glass, and further preferably borosilicate glass or lanthanum crown glass.

As the base, those conventionally used in spectrometers may be used, but the thermal expansion coefficient is preferably 22 × 10 ⁻⁶ or more and 26 × 10 ⁻⁶ or less. Then, from the viewpoint of more appropriately exhibiting the effects of the present invention, the main component of the material is more preferably a metal, and further preferably aluminum.

For example, similarly to FIG. 4, as the first collimator (off-axis parabolic mirror) 3, the off-axis angle θ is 7 °, the focal length f is 275 mm, and the main component is soda glass (thermal expansion coefficient 9.0 ×). 10 ⁻⁶ ), and a base 8 on which aluminum (thermal expansion coefficient: 23.8 × 10 ⁻⁶ ) is used as a main component and a diffraction grating 4 Is made mainly of borosilicate glass (coefficient of thermal expansion 3.3 × 10 ⁻⁶ ).

  In the case of the spectrometer configured as described above, when a temperature change of + 1 ° C. occurs, the incident angle (α) of the incident light becomes α ≒ 38.5775 °, and the diffraction angle of the diffracted light on the diffraction grating 4 (β ) Becomes β ≒ 22.5770 ° due to thermal expansion. The diffracted light is condensed by the second collimator (off-axis parabolic mirror) 5, but the light condensing position is about the focal point of the first collimator (off-axis parabolic mirror) 3 whose temperature has changed. The position is shifted by 0.001 mm, and the position of the exit slit 6 is also changed by the change due to the thermal expansion of the base 8 so as to match this shift, and as a result, the arrangement is such that the influence of the temperature change is eliminated.

  At this time, the light of the true wavelength of 1550 nm can be extracted while the angle of the diffraction grating 4 allows the light of 1550 nm to pass, and as a result, a wavelength error hardly occurs.

  FIG. 2 is a structural explanatory view showing another embodiment of the present invention, and is a two-stage spectrometer similar to FIG. Parts common to those in FIG. 5 are denoted by the same reference numerals.

  In FIG. 2, the spectrometer is composed of the same components as those in FIG. 5, but the diffraction grating 4 is diffracted from the diffraction grating at the second time, that is, at the angle of incidence α of the light finally entering the diffraction grating. Are arranged so that the diffraction angle β of the light to be emitted is small (α> β).

  Also in the spectrometer of FIG. 2, the wavelength shift due to the temperature change of the diffraction grating 4 and the light due to the change in the positional relationship between the slit 2 and the first collimator (off-axis parabolic mirror) 3 due to the temperature change. Although the axis shift occurs, the temperature change of the optical axis caused by the first collimator (off-axis parabolic mirror) 3 is more than the incident angle α of the light finally incident on the diffraction grating. The diffraction angle of the diffraction grating 4 arranged so that the diffraction angle β of the light diffracted from the diffraction grating becomes small (α> β) is offset by the temperature change, and the wavelength change as a spectrometer due to the temperature change Can be kept low.

  FIG. 3 is an explanatory view showing the configuration of another embodiment of the present invention, and portions common to those in FIGS. 1 and 2 are denoted by the same reference numerals. The diffraction grating 4 in FIG. 3 is also arranged such that the diffraction angle β is smaller than the light incident angle α (α> β).

  In FIG. 3, light that has passed through a slit 2 from a light source 1 is converted into a parallel light by a first collimator (off-axis parabolic mirror) 3 and is incident on a diffraction grating 4.

  The light that enters the diffraction grating 4 and is diffracted here is converted into convergent light by a second collimator (off-axis parabolic mirror) 5, and the convergent light is composed of plane reflecting mirrors M1 to M5. The light is reflected by the reflection block 9 and reenters the second collimator (off-axis parabolic mirror) 5.

  The light that is incident again on the second collimator (off-axis parabolic mirror) 5 and reflected is incident on the diffraction grating 4 again, is diffracted, and is transmitted from the diffraction grating 4 to the first collimator (off-axis paraboloid). (Mirror) 3.

  The light emitted from the first collimator (off-axis parabolic mirror) 3 is condensed on the emission slit 6, and the light receiver 7 receives only light of a specific wavelength. A light source 1, a slit 2, a first collimator (off-axis parabolic mirror) 3, a diffraction grating 4, a second collimator (off-axis parabolic mirror) 5, a light receiver 7, The reflection block 9 is arranged on the base 8.

In the case of the two-stage spectrometer shown in FIG. 3, the number of times light is reflected by the collimator (off-axis parabolic mirror) is four times in total, so that the influence of deviation due to temperature change is also cumulative. Become larger. However, for example, soda glass (thermal expansion coefficient: 9.0 × 10 ⁻⁶ ) is mainly used as the first collimator (off-axis parabolic mirror) 3 and the second collimator (off-axis parabolic mirror) 5. When an off-axis parabolic mirror having a focal length of 275 mm and an off-axis angle of 7 ° is used as a component and the base 8 is mainly composed of aluminum (coefficient of thermal expansion 23.8 × 10 ⁻⁶ ), a lanthanum crown By placing a diffraction grating mainly composed of glass (coefficient of thermal expansion 5.5 × 10 ^-6 ) at a position where the diffraction angle β is smaller than the incident angle α, a spectrometer with almost no wavelength error is constructed. can do.

  As described above, according to the present invention, a wavelength change due to a temperature change can be suppressed to a low level, and a spectrometer with stable wavelength accuracy can be realized.

Reference Signs List 1 light source 2 emission slit 3 first collimator (off-axis parabolic mirror)
4 Diffraction grating 5 Second collimator (off-axis parabolic mirror)
6 Emission slit 7 Receiver 8 Base 9 Reflection block

Claims (10)

A spectrometer comprising a collimator, a plane diffraction grating and a base, wherein the incident light is configured as a multi-stage type diffracted a plurality of times by the plane diffraction grating ,
The collimator comprises an off-axis parabolic mirror,
The plane diffraction grating is such that a diffraction angle when light is first incident on the plane diffraction grating and is diffracted is larger than an incident angle when light is first incident on the plane diffraction grating and is diffracted. Arranged so that the diffraction angle when light is finally incident on the plane diffraction grating and diffracted is smaller than the incident angle when light is finally incident and diffracted on the plane diffraction grating. Yes,
A spectrometer that is characterized in that: 2. The spectrometer according to claim 1 , wherein the off-axis parabolic mirror has a coefficient of thermal expansion of 7 × 10 ⁻⁶ or more and 9 × 10 ⁻⁶ or less. The spectrometer according to claim 2 , wherein a main component of the off-axis parabolic mirror is glass. The spectrometer according to claim 3 , wherein a main component of the off-axis parabolic mirror is soda glass. The spectrometer according to any one of claims 1 to 4 , wherein a thermal expansion coefficient of the plane diffraction grating is 3 × 10 ⁻⁶ or more and 6 × 10 ⁻⁶ or less. The spectrometer according to claim 5 , wherein a main component of the plane diffraction grating is glass. The spectrometer according to claim 6 , wherein a main component of the plane diffraction grating is one of borosilicate glass and lanthanum crown glass. The spectrometer according to any one of claims 1 to 7 , wherein a thermal expansion coefficient of the base is 22 × 10 ⁻⁶ or more and 26 × 10 ⁻⁶ or less. The spectrometer according to claim 8 , wherein a main component of the base is a metal. The spectrometer according to claim 9 , wherein a main component of the base is aluminum.

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