JP4345625B2 - Diffraction grating - Google Patents

Description

  The present invention relates to a reflective diffraction grating used in a spectrophotometer or the like.

  In general, in an optical measuring device such as a spectrophotometer, a diffraction grating is used to extract monochromatic light having a specific wavelength from light mixed with various wavelengths or decompose it into a spectrum. FIG. 3 is a cross-sectional view for explaining the principle of spectral spectroscopy of a conventionally known diffraction grating (for example, see Non-Patent Document 1). Here, a lamellar type (sometimes called a laminar type) diffraction grating in which the cross-sectional shape of the grating groove is rectangular is shown, but a blazed type (called an echelle type) in which the groove cross-sectional shape is a sawtooth shape. The principle of spectroscopy is basically the same even in a holographic type in which the groove cross-sectional shape is sinusoidal.

As shown in FIG. 3, the reflective diffraction grating 1 is an optical element in which grating grooves 11 having a period D are regularly arranged on a substrate 10. When light incident on the diffraction grating 1 at an angle θ1 formed with the normal line P of the grating groove 11 is diffracted at an angle θ2 formed with the normal line P, the optical path of the light beam in the adjacent grating groove 11 The difference ΔL1 is given by the following equation (1).
ΔL1 = D · (sin θ1-sin θ2) (1)
This optical path difference ΔL1 is an even multiple of 1/2 of the wavelength λ, that is,
(Λ / 2) × 2M = Mλ
However, M = 1, 2, 3, ...
The emitted light will strengthen each other. Therefore, light having a specific wavelength λ that satisfies the following equation (2) is emitted (diffracted) from the diffraction grating 1.
D · (sin θ1-sin θ2) = Mλ (2)

  For example, the wavelength λ of the diffracted light can be arbitrarily changed by changing the incident angle θ1 and the outgoing angle θ2 under the condition that θ1 + θ2 = α is constant. Even when the incident light is white light, light having a specific wavelength can be extracted as diffracted light by changing the emission angle θ2.

  However, the conventional diffraction grating as described above has a principle problem that undesired high-order light is emitted in the same direction together with light having a desired wavelength. That is, equation (2) means that the primary light having a wavelength of λ, the secondary light having a wavelength of λ / 2, the tertiary light having a wavelength of λ / 3, and so on are output in an intensified manner. Therefore, if the wavelength of the primary light to be extracted with the arrangement of the incident angle θ1 and the emission angle (diffraction angle) θ2 is λA, the light of λA / 2, λA / 3,. Will come out. In general, as the order increases, the intensity of the diffracted light decreases, but the secondary light often has an intensity that cannot be ignored.

  Thus, as long as spectroscopy is performed with a conventional diffraction grating, it is inevitable that light having a desired wavelength λA and unnecessary light having a wavelength λA / 2 come out simultaneously from the same direction. Therefore, in order to remove such unnecessary wavelength light, conventionally, a filter having optical characteristics that allows light of wavelength λA to pass while blocking light of wavelength λA / 2 is inserted into the optical path after the diffraction grating. Thus, the influence of higher-order light is removed (see, for example, Patent Document 1).

  However, when scanning a selected wavelength by rotating a diffraction grating in a spectrophotometer or the like, a single type of filter cannot be used to remove high-order light corresponding to each selected wavelength, and a plurality of optical characteristics differ. It was necessary to switch the kind of filter. Therefore, the configuration of the apparatus becomes complicated, and there are problems such as an increase in the size of the apparatus and an increase in cost. In addition, if an extra optical element such as a filter is inserted in the optical path, the light intensity of the desired wavelength light may be reduced or stray light may be caused. Is desired to be removed.

JP-A-5-133808 (paragraph 0002 etc.) “Applied Optics I” Baifukan, 6th edition, 1997, p. 295

  The present invention has been made in view of these points, and an object of the present invention is to provide a diffraction grating that can suppress unnecessary high-order light without using a filter for removing high-order light. It is in.

  The diffraction grating according to the present invention, which has been made to solve the above problems, extends in the same direction as the grating groove on the first grating groove whose groove period is D and has a groove period of D / 4. It is characterized in that the second grating grooves are superimposed and engraved.

  According to the diffraction grating according to the present invention, the secondary light generated by strengthening the first grating groove is weakened by the second grating groove, so that the secondary light is greatly reduced in principle. Will be. Therefore, it is not necessary to use a filter for removing secondary light, which has been almost essential in the past. For example, when the present invention is applied to a spectrophotometer or the like, the configuration of the optical system is simplified, and the size and cost of the apparatus are reduced. Significantly contributes to reduction. Further, by not inserting the filter into the optical path, it is possible to achieve an increase in the amount of desired light and a decrease in stray light, thereby improving the analysis accuracy.

  Note that the primary light that reinforces in the second grating groove is emitted from the diffraction grating. However, when the diffraction grating is used in the visible region, the wavelength of such light should be 190 nm or less, which is highly absorbed in air. In fact, it can be ignored. However, when the diffraction grating according to the present invention is used in the infrared region or used in the air other than the air (for example, in a vacuum), undesired diffracted light is absorbed by the second grating groove as described above. However, even in such a case, there is no need to switch between various types of filters according to the selected wavelength as in the prior art. For example, one filter for removing a predetermined wavelength region is prepared. If so, it is possible to cope with it, and the configuration of the apparatus can be greatly simplified as compared with the conventional case.

  The principle of spectroscopy of the diffraction grating according to the present invention will be described with reference to FIGS. FIG. 1 is a cross-sectional perspective view of a diffraction grating according to the present invention, and FIG. 2 is an optical path diagram for explaining the operating principle of spectroscopy of this diffraction grating. As shown in FIG. 1, this diffraction grating 1 extends on a conventional grating groove 11 having a groove period D (referred to as a first grating groove) 11 in the same direction as that and has a groove period of D / 4. A certain second grating groove 12 is formed in an overlapping manner.

The diffraction conditions in the first grating groove 11 are as described above. On the other hand, when considering the diffraction condition of the second grating groove 12, similarly to the first grating groove 11, the incident light at an angle θ1 formed with the normal line P is determined at an angle θ2 formed with the normal line P. Then, the optical path difference ΔL2 between the light beams in the adjacent grating grooves 12 is given by the following equation (3).
ΔL2 = (D / 4) · (sin θ1−sin θ2) (3)
The conditions for strengthening the second grating grooves 12 are the following equation (4) as in the equation (2).
(D / 4) · (sin θ1−sin θ2) = Nλ (4)
However, N = 1, 2, 3,...

Assuming that the wavelength of the primary light to be extracted by the first grating groove 11 is λA, from the equation (2),
D · (sin θ1−sin θ2) = λA
Therefore, in the second grating groove 12 having a groove period of D / 4, the primary light having a wavelength of λA / 4, the secondary light having a wavelength of λA / 8, and the wavelength of λA / 12 3 You can see that the next light is strengthened. On the other hand, the condition for weakening is when the following equation (5) is satisfied.
(D / 4) · (sin θ 1 −sin θ 2) = (2N−1) · (λ / 2) (5)
That is, primary light having a wavelength of λA / 2, secondary light having a wavelength of λA / 6, tertiary light having a wavelength of λA / 10, etc. are weakened. Incidentally, there is no strengthening or weakening at wavelengths longer than λA / 2.

  That is, in the first grating groove 11 having the groove period D, the secondary light having the wavelength λA / 2 should come out as undesired diffracted light, but the second grating groove having the groove period D / 4. 12, the light of this wavelength λA / 2 is weakened as primary light, and as a result, the emission of the diffracted light of wavelength λA / 2 from the diffraction grating 1 is suppressed. In this way, the diffraction grating according to the present invention can reduce the second-order light of the first-order diffracted light having the desired wavelength λA.

  The diffraction grating according to the present invention will be described with specific examples. Here, the second grating grooves 12 having a groove number density of 2400 lines / mm (D = 417 nm) are overlapped on the first grating grooves 11 having a groove number density of 600 lines / mm (D = 1667 nm). Consider an engraved example.

  Assume that the diffraction grating 1 is used under the condition of α (= θ1 + θ2) = 90 °. First, when the incident angle θ1 is 54.77 °, the first grating groove 11 diffracts and emits primary light having a wavelength of 400 nm, secondary light having a wavelength of 200 nm,. On the other hand, the first grating light having a wavelength of 200 nm is emitted from the second grating groove 12 while the primary light having a wavelength of 100 nm, the secondary light having a wavelength of 50 nm,... , 67 nm secondary light, etc. are weakened and canceled out. Therefore, when considering the first and second grating grooves 11 and 12 together, the primary light having a wavelength of 400 nm, the primary light having a wavelength of 100 nm, the secondary light having a wavelength of 50 nm, and the like are diffracted. . Here, when the diffraction grating 1 is used in the air, the light having a wavelength of about 190 nm or less is greatly attenuated in the air. Therefore, the primary light having a wavelength of 100 nm, the secondary light having a wavelength of 50 nm, and Light with shorter wavelengths can be ignored. Thus, it can be seen that the secondary light of 200 nm, which should appear in the conventional diffraction grating, is suppressed and does not come out of the diffraction grating 1 (or the intensity is very weak).

  Next, when the incident angle θ1 = 62.28 °, the first grating groove 11 diffracts and emits the primary light having a wavelength of 700 nm, the secondary light having a wavelength of 350 nm,. On the other hand, the first grating light having a wavelength of 175 nm, the secondary light having a wavelength of 87.5 nm, and the like are diffracted and emitted by the second grating groove 12, while the primary light having a wavelength of 350 nm is emitted at 117 nm. Some secondary light ... is weakened and offset. Accordingly, when considering the first and second grating grooves 11 and 12 together, the primary light having a wavelength of 700 nm, the primary light having a wavelength of 175 nm, the secondary light having a wavelength of 87.5 nm, and the like are diffracted. become. Again, considering attenuation in the air, the primary light having a wavelength of 175 nm, the secondary light having a wavelength of 87.5 nm, and light having a shorter wavelength can be ignored. Therefore, it can be seen that the secondary light of 350 nm, which should appear in the conventional diffraction grating, is suppressed and does not come out of the diffraction grating 1 (or the intensity is very weak).

  When this diffraction grating is not used in air, for example, when it is used in a vacuum, attenuation of light with a wavelength of 190 nm or less cannot be expected. In this case, an ultraviolet light removal filter instead of this is used after the diffraction grating. Can be inserted into the optical path. Even when wavelength scanning and wavelength selection are performed by rotating the diffraction grating, switching of the ultraviolet light removal filter is not necessary.

  Further, when this diffraction grating is to be used in a longer wavelength region (infrared region), the first-order diffracted light of the second grating groove 12 is larger than 190 nm, and even if it is used in the air. No longer attenuated. Therefore, when this diffraction grating is used in such a long wavelength region, a filter for removing ultraviolet light having a wavelength of about 400 nm or less may be inserted.

  The diffraction grating 1 as described above can be manufactured by using a manufacturing process similar to the conventional one. More specifically, first, a grating groove (first grating groove 11) formed by two-beam interference of a laser and having a groove density of 600 / mm is exposed to a photoresist coated on a substrate by a holographic exposure method. . The pattern thus formed is transferred to the substrate by an ion beam etching method or the like to form lattice grooves, and the same process may be repeated again with an exposure arrangement with a groove number density of 2400 lines / mm. However, the manufacturing method is not necessarily limited to this, as long as a lattice groove shape that satisfies the above-described conditions can be formed.

  In the above embodiment, a lamellar diffraction grating having a rectangular cross-sectional shape of the grating groove has been described. However, a blazed diffraction grating having a sawtooth-shaped groove cross-section or a hollow having a sinusoidal groove cross-sectional shape. A similar technique can be adopted for graphic diffraction gratings. In addition, it is apparent that other points are included in the scope of the claims of the present application even if appropriate changes, corrections, and additions are made within the scope of the present invention.

1 is a cross-sectional perspective view of a diffraction grating that is one embodiment of the present invention. The optical path figure for demonstrating the operation | movement principle of spectroscopy of the diffraction grating by a present Example. The optical path figure for demonstrating the operation | movement principle of the spectroscopy of the conventional diffraction grating.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Diffraction grating 10 ... Board | substrate 11 ... 1st grating groove (groove space | interval: D)
12 ... 2nd lattice groove (groove interval: D / 4)

Claims (1)

  A second grating groove extending in the same direction as the grating groove and overlaid with a second grating groove having a groove period of D / 4 is formed on the first grating groove having a groove period of D. Diffraction grating.

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