JP2000321135A - Spectrometer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To realize a spectrometer in which moisture resistance and temperature characteristics can be enhanced. SOLUTION: The spectrometer comprises a lens 2 for collimating an incident light, a wavelength dispersion element 9, a beam shape correcting means 10 integrated with the wavelength dispersion element 9 and directing the collimated light from the collimating lens 2 to the wavelength dispersion element 9 thus refracting the outgoing light beam from the wavelength dispersion element 9, a lens 4 for focusing the output from light beam from the beam shape correcting means 10, and a photodetector 5 for detecting the output light beam from the focus lens 4.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a spectrometer using a wavelength dispersive element, and more particularly to a spectrometer capable of improving environmental characteristics and temperature characteristics.

[0002]

2. Description of the Related Art In a conventional spectroscope, incident light is irradiated on a diffraction grating or the like which is a wavelength dispersive element, and wavelength-dispersed light is received by a photodetector to separate and detect light for each wavelength. It is.

FIG. 4 is a block diagram showing an example of such a conventional spectroscopic device. In FIG. 4, reference numeral 1 denotes an incident end where output light from a light source or light emitted from an optical fiber is incident from outside, 2 is a collimating lens, 3 is a wavelength dispersion element such as a diffraction grating, 4 is a focusing lens, and 5 is a focusing lens. Is a photodetector using a photodiode array or the like.

The output light from the incident end 1 is converted into parallel light by a collimating lens 2 and is incident on a wavelength dispersion element 3. The wavelength-dispersed light from the wavelength dispersion element 3 is condensed by the focusing lens 4 and is incident on the photodetector 5.

FIG. 5 is a structural sectional view showing an example of a diffraction grating which is the wavelength dispersion element 3. In FIG. 5, 6 is a substrate made of glass or the like, 7 is a resin replica provided with a large number of gratings for causing diffraction, and 8 is a metal reflection film.

A replica 7 of a resin having a large number of lattices is formed on a substrate 6, and the surface of the replica 7 is covered with a reflection film 8.

Here, the operation of the conventional example shown in FIG. 4 will be described. Since the light incident on the wavelength dispersion element 3 such as a diffraction grating has a different diffraction angle depending on the wavelength, the light is emitted as diffracted light in different directions, and is focused by the focusing lens 4 on each light receiving element constituting the photodetector 5. Be lighted.

For example, "FP01", "FP0" in FIG.
Light of different wavelengths is collected by the light receiving elements located at 2 "and" FP03. "In the conventional example shown in Fig. 6, there is no need to rotate the wavelength dispersion element 3 such as a diffraction grating, so that high speed and reliability are achieved. Are better.

For example, the order of diffraction of the wavelength dispersion element 3 such as a diffraction grating is "m", the lattice constant of the wavelength dispersion element 3 such as a diffraction grating is "d", and the angle of incidence on the wavelength dispersion element 3 such as a diffraction grating. If the emission angles are “i” and “θ” and the wavelength is “λ”, then mλ / d = sini + sinθ (1)

[0010] A spectroscopic device as shown in FIG.
If the wavelength division multiplexing signal is designed to handle a narrow wavelength range, such as a system monitoring monitor, the optical path spread due to chromatic dispersion becomes smaller than the focal length of the focusing lens 4 and the photodetector When a one-dimensional array of photodiode arrays is used as 5, the position of each element and the emission angle have a substantially proportional relationship.

However, the relationship between the wavelength and the emission angle is dλ / dθ | _i = (d / m) · cosθ (2) obtained by differentiating equation (1).

As can be seen from equation (2), the wavelength and the dispersion angle are proportional to the cosine of the emission angle. This emission angle can be obtained from equation (1) using the wavelength range of the spectroscope, the lattice constant of the diffraction grating used, the focal length of the focusing lens 4, and the like.

FIG. 6 is a table showing one design example of such a spectroscopic device, and FIG. 7 is a table showing emission angles for respective wavelengths. In this case, for example, “λ = 1.55 [μm]”, “
If there are "190" light receiving elements in a wavelength range of 900 [l / mm] "and" 32 [nm] ", the average chromatic dispersion is" 32/190 = about 0.17 [nm] ". .

If a lens having a focal length f2 of "50 mm" is used as the collimating lens 2, the use area of the wavelength dispersive element 3 such as a diffraction grating becomes the numerical aperture of the incident end 1 and the incident angle to the wavelength dispersive element 3. "11.1 [m
m] ".

Theoretical resolution based on Reileigh criterion ”
Since λ / Δλ ″ is determined by the total number of grooves of the diffraction grating that is the wavelength dispersion element 3, “900 × 11.1 ≒ 10000 (3), and λ / Δλ = 1.55 / Δλ = 10000 (4) ∴Δλ = 1.55 / 10000 / 0.15 [nm] (5)

The image size "ω" is such that the beam width of the diffracted light is "3.4 [mm]" and the focusing lens 4
Assuming that the ratio between the radius of the light incident on and the focal length is “NA”, ω = 2 · λ / (π · NA) (6)

From the equation (6), the size of the image is "59 [μ]
m] ”and the resolution is average chromatic dispersion“ 0.17 nm / 5
The product of “0 μm” is “0.2 [nm]”, which is slightly below the theoretical resolution “Δλ = 0.15 [nm]” and is an appropriate value.

[0018]

However, in the conventional example shown in FIG. 4, the resolution depends on the size of the area used in the wavelength dispersion element 3 such as a diffraction grating as can be seen from the equation (5). However, in order to improve the resolution, it is difficult to reduce the size of the optical components constituting the optical system, and it is difficult to reduce the size of the device.

Further, as shown in FIG. 5, the replica portion of the diffraction grating, which is the wavelength dispersive element 3, is inferior in moisture resistance to optical components such as lenses and prisms formed only of glass or the like. There was a problem.

Further, the diffraction grating shown in FIG.
If the air has a refractive index of "n _air"Temperature"
T ”, the lattice constant of the diffraction grating is“ D ”, and the wavelength is“ λ ”.
Then, the temperature characteristic of the emission angle “θ” is dθ / dT = −λ / (D · cos θ) × ｛dD / (D · dT) + (1 / n_air) (Dn_air/ DT)｝ (7)

In equation (7), the first term in “｛｝” is the linear expansion coefficient of the diffraction grating that is the wavelength dispersion element 3, and the second term is
The term is the temperature coefficient of the refractive index of air. The temperature coefficient of the wavelength is: dλ / dT = (dλ / dθ) · (dθ / dT) = − λ · {dD / (D · dT) + (1 / n _air ) (dn _air / dT)} (8 ).

For example, when a diffraction grating having a wavelength of “1.55 μm” and Pyrex glass as the substrate 6 is used in the air, the temperature coefficient thereof becomes “about 3.7 pm / ° C.”.
That is, there is a problem that the wavelength characteristic of the spectrometer has a temperature characteristic caused by the linear expansion coefficient of the material of the diffraction grating. Therefore, an object of the present invention is to realize a spectroscopic device capable of improving humidity resistance and temperature characteristics.

[0023]

In order to achieve the above object, according to a first aspect of the present invention, a collimator for converting incident light into parallel light in a spectrometer using a wavelength dispersive element is provided. A wavelength dispersing element, a beam integrated with the wavelength dispersing element, the parallel light from the collimating lens being incident on the wavelength dispersive element, and refracting and emitting the light emitted from the wavelength dispersive element. By providing a shape correcting means, a focusing lens for condensing the output of the beam shape correcting means, and a photodetector for detecting the output light of the focusing lens, a beam such as a prism is provided on the replica of the diffraction grating surface. Since the shape correcting means is provided, the moisture resistance is improved.

Further, by correcting the light emitted from the wavelength dispersive element by the beam shape correcting means, the nonlinearity caused by the cosine component of the emission angle of the wavelength dispersive element is compensated by the nonlinearity due to the cosine component of the beam shape correcting means. As a result, the wavelength dispersion characteristics can be flattened. Further, by correcting the temperature coefficient when the wavelength dispersion element and the beam shape correcting means are integrated with the temperature coefficient due to refraction at the incident surface of the beam shape correcting means, the temperature characteristics can be improved. Further, by increasing the refractive index of the medium of the beam shape correcting means such as a prism, the size of the spectroscopic device can be reduced without sacrificing the wavelength resolution.

According to a second aspect of the present invention, in the spectral device according to the first aspect of the present invention, the wavelength dispersion means is a diffraction grating. Is provided, the moisture resistance is improved. Further, by correcting the light emitted from the wavelength dispersion element by the beam shape correction means, the nonlinearity caused by the cosine component of the emission angle of the wavelength dispersion element is compensated by the nonlinearity due to the cosine component of the beam shape correction means. Thus, the wavelength dispersion characteristics can be flattened. Further, by correcting the temperature coefficient when the wavelength dispersive element and the beam shape correcting means are integrated with the temperature coefficient due to refraction at the incident surface of the beam shape correcting means, the temperature characteristics can be improved. Further, by increasing the refractive index of the medium of the beam shape correcting means such as a prism, the size of the spectroscopic device can be reduced without sacrificing the wavelength resolution.

According to a third aspect of the present invention, in the spectral device according to the first aspect of the present invention, since the beam shape correcting means is a prism, a beam shape correcting means such as a prism is provided on a replica of a diffraction grating surface. Is provided, the moisture resistance is improved. Also, by correcting the light emitted from the wavelength dispersion element by the beam shape correction means,
Non-linearity due to the cosine component of the emission angle of the wavelength dispersion element is compensated for by non-linearity due to the cosine component of the beam shape correction means, and the chromatic dispersion characteristics can be flattened.
Further, by correcting the temperature coefficient when the wavelength dispersion element and the beam shape correcting means are integrated with the temperature coefficient due to refraction at the incident surface of the beam shape correcting means, the temperature characteristics can be improved. Further, by increasing the refractive index of the medium of the beam shape correcting means such as a prism, the size of the spectroscopic device can be reduced without sacrificing the wavelength resolution.

[0027]

Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 1 is a configuration diagram showing one embodiment of the spectrometer according to the present invention. 1, 2, 4,
Reference numerals 5 and 5 denote the same components as those in FIG. 4, reference numeral 9 denotes a wavelength dispersion element such as a diffraction grating, and reference numeral 10 denotes a beam shape correcting means such as a prism.

The output light from the incident end 1 is converted into parallel light by the collimating lens 2 and is incident on a wavelength dispersion element 9 such as a diffraction grating via a beam shape correcting means 10 such as a prism. Diffracted light from a wavelength dispersive element 10 such as a diffraction grating is condensed again by the focusing lens 4 via the beam shape correcting means 9 and is incident on the photodetector 5.

Here, the embodiment shown in FIG. 1 will be described with reference to FIG. FIG. 2 is an explanatory diagram for explaining an optical path in the wavelength dispersion element 9 and the beam shape correcting means 10. In FIG.
“01” is incident light and “OL01” is outgoing light.
The basic operation is the same as that of the conventional example shown in FIG.

As shown in FIG. 5, the replica on the surface of the diffraction grating is provided with a beam shape correcting means such as a prism, so that the moisture resistance is improved.

The wavelength dispersion characteristic of the spectrometer is given by the following equation (2).
When this is deformed, dλ = (d / m) · cos θ · dθ (9), and if the light receiving elements constituting the photodetector 5 are equally spaced, the wavelength due to the cosine component (cos θ) Non-uniformity in dispersion results. In other words, there is non-linearity.

On the other hand, the equation of refraction indicates that the refractive index of the medium is "n ₁ ".
And “n ₂ ”, and when the incident angle and the outgoing angle are “φ” and “ψ”, n ₁ · sin φ = n ₂ · sinψ (10), and when differentiated by “φ”, n ₁ · cos φ · dφ = n ₂ · cosψ · dψ (11)

As can be seen from equation (11), the angle of refraction also depends on the cosine component. Therefore, the nonlinearity due to the cosine component of the emission angle of the wavelength dispersion element 9 can be compensated for by the nonlinearity due to the cosine component of the refraction (beam shape correcting means 10).

In FIG. 2, the incident angle and the outgoing angle of the wavelength dispersive element 9 are “θ ₁ ” and “θ ₂ ”, and the beam shape correcting means 1
The incident angle and the outgoing angle of 0 are “θ ₃ ” and “θ ₄ ”, the refractive index of the beam shape correcting means 10 is “n”, and the wavelength is “λ”.
If, the _{sinθ 1 + sinθ 2 = λ /} (n · d) (12) dθ 2 / d = -dθ 3 / dλ (13) n · sinθ 3 = sinθ 4 (14).

Then, the average chromatic dispersion can be obtained by differentiating and rearranging the equation (14) from the equation (12), and dθ ₄ / dλ−cos θ ₃ / (d · cos θ ₂ · cos θ ₄ ) (15) Become.

Furthermore, by modifying the equation _{^{(15), d 2 θ 4}} / dλ 2 = (dθ 4 / dλ) 2 × {sinθ 4 / cosθ 4 - (sinθ 2 · cosθ 4) / (n · cosθ 2 Cos θ ₃ )-(sin θ ₃ cos θ ₄ ) / (n cos ² θ ₃ ) ₃ (16)

Here, in order for this characteristic to be linear, “d ² θ ₄ / d ² = 0”, so equation (16) is modified to obtain tan θ ₃ / (1−n ² · sin). ² θ ₃ ) = n · tan θ ₂ / (n ² −1) (17)

As a result, the non-linearity caused by the cosine component of the emission angle of the wavelength dispersion element 9 is corrected by the beam shape correction means 10 by correcting the light emitted from the wavelength dispersion element 9 by the beam shape correction means 10. The compensation is performed by the nonlinearity, and the chromatic dispersion characteristics can be flattened.

Further, since the temperature coefficient of the refractive index of the medium such as a prism is the beam shape correcting means 10 are present, the relative value of the temperature coefficient for air of the medium "n _r", the absolute value "n _a" Assuming that the refractive index of _air is “n _air ”, n _r = n _a / n _air (18) (1 / n _r ) (dn _r / dT) = (1 / n _a ) (dn _a / dT ) - (expressed as _{1 / n air) (dn air} / dT) (18) n air ≒ 1, n r ≒ n a.

The wavelength dispersion element 9 and the beam shape correcting means 10
With reference to FIG. 3, a description will be given of a temperature coefficient in the case of integrating
FIG. 3 is an explanatory diagram for explaining an optical path in the wavelength dispersion element 9 and the beam shape correcting means 10. "IL12" and "IL22" in FIG. 3 are incident light, and "OL12" and "O" in FIG.
L22 "is the outgoing light, and FIG. 3A is the incoming light" IL2 ".
FIG. 3B shows a case where 1 "is refracted clockwise with respect to the surface of FIG. 3 on the surface of the beam shape correcting means 10, and FIG.
Reference numeral 22 "denotes a case where the surface of the beam shape correction means 10 refracts counterclockwise with respect to the surface of FIG.

In the case of FIG. 3A, the incident angle of the incident light “IL12 (or“ IL22 ”)” with respect to the beam shape correcting means 10 is “θ _i ”, and the incident light “IL12 (or“ IL12 ”) of the beam shape correcting means 10 is set. , “IL22”), the refraction angle is “θ ₀ ”, the incident angle to the wavelength dispersion element 9 is “θ ₁ ”, the diffraction angle at the wavelength dispersion element 9 is “θ ₂ ”, and the outgoing light “OL”.
12 (or “OL22”) ”, the angle of incidence of the diffracted light on the exit surface of the beam shape correcting means 10 from which the beam is emitted is represented by“ θ ”.
₃ ", and the angle of refraction at the exit surface" if _{θ 4 ", n r · sinθ} 0 = sinθ i (19) dθ 0 = dθ 1 (20) sinθ 1 + sinθ 2 = λ / (d · become _{n a) (21) dθ 2} = -dθ 3 (22) n r · sinθ 3 = sinθ 4 (23).

If the equations (19) to (23) are differentiated with respect to the temperature “T”, then (dn _r / dT) · sin θ ₀ + n _r · cos θ ₀ · (dθ ₀ / dT) = 0 (24) dθ _{0 / dT = dθ 1 / dT (} 25) cosθ 1 · (dθ 1 / dT) + cosθ 2 · (dθ 2 / dT) = -λ / (d · n a) × {(1 / d) (dd / dT) _{+ (1 / n a) (} dn a / dT)} (26) dθ 2 / dT = -dθ 3 / dT (27) (dn r / dT) · sinθ 3 + n r · cosθ 3 · (dθ 3 / dT ) = Cos θ ₄ · (dθ ₄ / dT) (28)

When Equations (24) to (28) are arranged, dθ ₄ / dT = (sin θ ₃ / cos θ ₄ ) (dn _r / dT) + (n _r · cos θ ₃ / cos θ ₄ ) (dθ ₃ / dT _{) = (sinθ 3 / cosθ 4} ) (dn r / dT) - (n r · cosθ 3 / cosθ 4) × [-λ / (d · n a) (1 / cosθ 2) × {(1 / d) (dd / dT) + (1 / n a) (dn a / dT)} - (cosθ 1 / cosθ 2) (dθ 1 / dT)] = (sinθ 3 / cosθ 4) (dn r / dT) + λ · _{cosθ 3 / (d · cosθ 2} · cosθ 4) × {(1 / d) (dd / dT) + (1 / n a) (dn a / dT)} - (sinθ 0 · cosθ 1 · cosθ 3) / (cos θ ₀ · cos θ ₂ · cos θ ₄ ) × (dn _r / dT) = tan θ ₄ · (1 / n _r ) · (dn _r / dT) -λ · (dθ ₄ / dλ) × {(1 / d) (dd / dT) + (1 / n a) (dn a / dT)} - (sinθ i · cosθ 1 · cosθ 3) / (cosθ 0 · cosθ 2 · cosθ 4) × (1 / n r) · ( dn _r / dT) becomes (29).

Here, the temperature coefficient of the medium is "(1 /
n _r ) · (dn _r / dT) ”is a positive value in a medium that is usually used. At this time, dθ ₄ / dλ <0 (1 / d) (dd / dT)> 0 (1 / n _a ) (dn _a / dT)> 0 ° <θ _j <90 ° (j = i, 1,2,3,4) Therefore, the first and second terms of the equation (29) are positive values. It is.

On the other hand, since the third term of the equation (29) is a negative value, the temperature coefficient “dθ ₄ / dT” when the wavelength dispersion element 9 and the beam shape correcting means 10 are integrated can be reduced. it can.

In the equation (29), the first term is refraction at the exit surface of the beam shape correcting means 10, the second term is diffraction at the wavelength dispersive element 9, and the third term is the beam shape correcting means 10. This is due to refraction at the entrance surface.

As a result, the temperature coefficient in the case where the wavelength dispersion element 9 and the beam shape correcting means 10 are integrated is corrected by the temperature coefficient due to refraction at the incident surface of the beam shape correcting means 10, thereby improving the temperature characteristics. Will be possible.

On the other hand, in the case of FIG. 3B, the sign of the third term in the equation (29) changes from "-" to "+", so that only the term which increases the temperature coefficient "dθ ₄ / dT" is obtained. Would.
However, the temperature coefficient of the medium is “(1 / n _r ) · (dn _r / d
Since some media have a negative value of “T)”, the use of such a media can reduce the temperature coefficient “dθ ₄ / dT”.

Further, in order to reduce the size without sacrificing the wavelength resolution, it is necessary to reduce the grating constant "d" of the diffraction grating. However, since the upper limit of the right side of the equation (1) is "2", the wavelength When "λ" is determined, the minimum value of the lattice constant "d" is naturally found.

On the other hand, when the wavelength dispersion element 9 such as a diffraction grating and the beam shape correcting means 10 such as a prism are integrated, the refractive index of the medium is expressed by the equations (12) to (14). Since this is the product of "n" and the lattice constant "d", the lattice constant "d" can be reduced as the refractive index "n" increases.

In other words, by increasing the refractive index of the medium of the beam shape correcting means 10 such as a prism, the size of the spectrometer can be reduced without sacrificing the wavelength resolution.

The beam shape correcting means 10 such as a prism is provided on the replica of the diffraction grating surface as the wavelength dispersive element 9. In particular, if some window is provided on the surface of the diffraction grating instead of the beam shape correcting means such as the prism, the moisture resistance is improved. It is possible to improve the performance.

When the wavelength dispersive element 9 and the beam shape correcting means 10 are integrated, even if the wavelength dispersive element 9 is attached to the beam shape correcting means 10,
0 may be directly formed.

Although a diffraction grating has been exemplified as the wavelength dispersion element, not only a diffraction grating but also an Eschler grating can be used in the same manner.

[0055]

As is apparent from the above description,
According to the present invention, the following effects can be obtained. According to the first, second and third aspects of the present invention, the replica on the surface of the diffraction grating is provided with a beam shape correcting means such as a prism, so that the moisture resistance is improved.

Further, by correcting the light emitted from the wavelength dispersion element by the beam shape correction means, the nonlinearity caused by the cosine component of the emission angle of the wavelength dispersion element is compensated by the nonlinearity due to the cosine component of the beam shape correction means. As a result, the wavelength dispersion characteristics can be flattened.

Further, by correcting the temperature coefficient when the wavelength dispersion element and the beam shape correcting means are integrated with the temperature coefficient due to refraction on the incident surface of the beam shape correcting means, the temperature characteristics can be improved.

Furthermore, by increasing the refractive index of the medium of the beam shape correcting means such as a prism, the size of the spectrometer can be reduced without sacrificing the wavelength resolution.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing one embodiment of a spectroscopic device according to the present invention.

FIG. 2 is an explanatory diagram illustrating an optical path in a wavelength dispersion element and a beam shape correction unit.

FIG. 3 is an explanatory diagram illustrating an optical path in a wavelength dispersion element and a beam shape correction unit.

FIG. 4 is a configuration diagram illustrating an example of a conventional spectroscopic device.

FIG. 5 is a structural sectional view showing an example of a diffraction grating that is a wavelength dispersion element.

FIG. 6 is a table showing a design example of a spectroscopic device.

FIG. 7 is a table showing emission angles for respective wavelengths.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Incident end 2 Collimating lens 3, 9 Wavelength dispersion element 4 Focusing lens 5 Photodetector 6 Substrate 7 Replica 8 Reflective film 10 Beam shape correcting means

Claims (3)

[Claims] 1. A spectroscopic device using a wavelength dispersive element, a collimating lens for converting incident light into parallel light, a wavelength dispersive element, and the parallel light from the collimating lens integrated with the wavelength dispersive element. Beam shape correcting means for making light incident on the wavelength dispersive element, refracting and emitting light emitted from the wavelength dispersive element, a focusing lens for condensing an output of the beam shape correcting means, and output light of the focusing lens A spectroscopic device comprising: a photodetector that detects light. 2. The spectroscopic device according to claim 1, wherein said wavelength dispersion means is a diffraction grating. 3. The spectroscopic apparatus according to claim 1, wherein said beam shape correcting means is a prism.