JP5261862B2 - Method and apparatus for measuring stray light of diffraction grating - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for measuring stray light of grating that figuring a quantity caused from grating of stray light of a spectroscope, and a device for achieving the method. <P>SOLUTION: By a constant deviation measuring method that a laser light source 4 and a photodetector 5 are fixed respectively to rotate the grating 1 arranged on a rotation stage 8, stray light at different rotation angles 14 is measured. Then, effect from other optical elements can be suppressed at a minimum and also measurement in a state approximate to an actual spectroscope can be performed. By determining a stray light value as a ratio with a laser input or the intensity of diffracted light, a stable measurement value can be obtained. Also, luminous flux is concentrated by a pinhole and the stray light near a diffraction angle can be measured easily and precisely. <P>COPYRIGHT: (C)2007,JPO&amp;INPIT

Description

  The present invention relates to a method for measuring stray light of a diffraction grating.

  A diffraction grating is an optical element for separating / selecting light into narrow-band wavelength elements used in spectroscopes, duplexers, etc., and most commonly on the surface of an optical material as shown in FIG. A large number of parallel grooves with extremely fine and uniform intervals are carved and a reflective metal film is deposited on the surface. The number of grooves is selected from the range of several tens to several thousand per mm depending on the wavelength region in which the diffraction grating is used. As a cross-sectional shape of the groove, a sawtooth waveform enlarged and displayed in a circle in FIG. 5 is widely used because of its high diffraction efficiency. A diffraction grating in which such a sawtooth cross section is formed on the surface by a holographic exposure method is called a blazed holographic diffraction grating. A grid is often used. An example of using a diffraction grating in a general spectrometer is shown in FIG.

  FIG. 6A shows a spectroscope called Littrow type, in which one off-axis paraboloidal mirror 32 serves as both a collimator mirror and a condenser mirror. ) Is a spectroscope having a structure called a Czerny-Turner type, which uses independent off-axis parabolic mirrors 32 and 33 as a collimator mirror and a condenser mirror, respectively. The diffraction grating 1 is held in such a direction that the direction of the groove is perpendicular to the paper surface of FIG. The entrance slit 30 and the exit slit 31 are both rectangular openings having a narrow width, and the long sides of the rectangles are installed so as to be perpendicular to the paper surface. After the incident light from the entrance slit 30 is converted into a parallel light beam by the off-axis parabolic mirror 32 and guided to the diffraction grating 1, in the configuration of (a), the diffracted light from the diffraction grating 1 is off-axis parabolic. The light is condensed on the exit slit 31 by the surface mirror 32. On the other hand, in the configuration of (b), the diffracted light from the diffraction grating 1 is collected on the exit slit 31 by the second off-axis parabolic mirror 33. The diffraction grating 1 can be rotated around a single axis parallel to the groove on the surface of the diffraction grating 1 by a rotary table not shown in FIG. And the wavelength of the light condensed on the exit slit 31 can be selected.

  In the spectroscope as shown in FIG. 6, it is ideal that the light emitted from the exit slit 31 to the outside is monochromatic light centered on the wavelength determined by the rotation angle of the diffraction grating 1, but in an actual spectroscope, Depending on the irregularities and surface roughness of the grooves of the diffraction grating 1, the light reflected and scattered in directions other than the diffraction angle, and the light scattered and multiply reflected by the mirror and lens used are set. Often mixed with light of a wavelength and emitted from the exit slit 31. Such light other than the set wavelength that is unnecessarily mixed is generally referred to as stray light. Various devices for minimizing the amount of stray light have been applied to the spectroscope design. (For example, see Patent Documents 1, 2, and 3)

  Measuring the amount of stray light for an actual spectrometer is important for evaluating the performance of the spectrometer. Japanese Industrial Standards define a method of measuring stray light using a solution filter that includes all the effects of optical elements such as lenses, mirrors, and diffraction gratings used therein, and the inner wall of the spectroscope housing. (See Non-Patent Document 1) In addition, a method of measuring the amount of stray light using a solid filter that is easier to handle is also widely used.

Further, as a method for evaluating the amount of stray light generated only by the diffraction grating by irradiating the surface of the diffraction grating with monochromatic light and measuring the amount of light reflected (scattered) at an angle other than the diffraction angle, the structure shown in FIG. The device has also been announced. Among them, 632.8 nm light from the light source HeNe laser 18 is shaped by the spatial filter 19, and then irradiated to the test piece 25 (measurement target diffraction grating) through the folding mirror 20, the collecting mirror 21, and the aperture 22. . The light diffracted by the test piece 25 is focused on the detector 27 placed in the diffraction angle direction. The detector 27 is mounted on a rotating arm 26 that is linked to the rotating stage 23, and can be rotated on the circumference around the test piece 25 independently of the sliding stage 24 on which the test piece 25 is mounted. The output of the detector 27 is sent to a PC 29 with an A / D card via a current amplifier 28, and the light quantity value is measured and recorded. With the test piece 25 fixed, the detector 27 is rotated to measure the amount of light at multiple points before and after the diffraction angle, so that the amount of light reflected and scattered in the angle direction other than the diffraction angle from the diffraction grating surface. Can be obtained. (See Non-Patent Document 2)
Japanese Patent Laid-Open No. 5-26728 Japanese Patent Laid-Open No. 7-55562 JP 9-89665 A Japanese Industrial Standard (JIS) K0115 (2004) Absorption Spectrometry General Rules “Spectra-Physics, Inc. Diffraction Grading Handbook (5th Edition)” Spectra-Physics, Inc. 2002

  It is well known that the irregularity of the groove shape of the diffraction grating, or its surface roughness, increases the stray light of the spectrometer. For example, fine irregularities in the flat portion 2 or the ridge portion 3 on the surface of the diffraction grating 1 shown in FIG. 5 affect the amount of stray light. FIG. 7 shows the relationship between the surface roughness of the planar portion 2 measured using an intermolecular force microscope (AFM) and the stray light amount of the spectroscope. The spectroscope used for the measurement was used for a commercially available liquid chromatograph detector, and the method for measuring stray light was based on a method using a solid filter. FIG. 7 clearly shows that there is a positive correlation between the surface roughness and the amount of stray light. Therefore, grasping the amount of stray light generated by the diffraction grating is very useful for the design and production of the diffraction grating itself and the design and production of the spectrometer. However, the stray light measurement method using a solid filter or the stray light measurement method specified in the Japanese Industrial Standard mentioned above is the total stray light amount including the effects of all optical elements including the diffraction grating and the inner wall of the spectroscope housing. It is difficult to grasp the stray light caused by the individual components of the spectrometer.

  The method of Non-Patent Document 2 shown in FIG. 8 is for the purpose of grasping the amount of stray light due to only the diffraction grating, but the diffraction grating (test piece 25) is fixed to the incident light, The amount of light reflected and scattered in an angular direction other than the diffraction angle is measured by moving the detector 27. However, in an actual spectroscope, as shown in FIG. 6, the diffraction grating 1 is rotated to obtain monochromatic light. Therefore, the method of Non-Patent Document 2 does not necessarily grasp the characteristics of stray light due to the diffraction grating in an actual spectroscope.

In order to solve the above-described problems, an object of the present invention is to provide a method for measuring stray light due to a diffraction grating under a situation very close to that in an actual spectrometer. In order to achieve this object, the first method provided by the present invention is to form light from a laser light source into a light beam having a predetermined diameter by a light beam shaping optical system, guide the light beam to a diffraction grating, and The reflected light intensity at an angle other than the diffraction angle determined by the characteristics of the diffraction grating and the wavelength of the laser light is measured by a photodetector at a constant declination, and the measured value is used. The amount of stray light is calculated. Further, according to a second method of the present invention, the stray light amount is calculated to an angle other than the diffraction angle determined by the incident light intensity from the laser light source to the diffraction grating, the characteristics of the diffraction grating, and the wavelength of the laser light. This is done by measuring the ratio of the reflected light intensity to the above. According to the third method provided by the present invention, the stray light amount is calculated by calculating the reflected light intensity (diffracted light intensity) to a diffraction angle determined by the characteristics of the diffraction grating and the wavelength of the laser light, and an angle other than the diffraction angle. This is done by measuring the ratio with the reflected light intensity. According to the fourth method of the present invention, the position of the laser light source and the position of the light detector are fixed, and the diffraction grating is on the diffraction surface and rotates one straight line parallel to the groove of the diffraction grating. The angle of reflection of light reflected by the diffraction grating from the laser light source incident on the photodetector is selected by rotating as an axis. In the fifth method provided by the present invention, when the diffraction angles of the n-th order diffracted light and the n + 1-order diffracted light of the diffraction grating are θn and θn + 1, respectively, the angles other than the diffraction angle measured using the photodetector are obtained. The reflection angle of the reflected light is set to (θn + θn + 1) / 2. Furthermore, the sixth method provided by the present invention is to set the reflection angle of the reflected light to an angle other than the diffraction angle measured using the photodetector near the diffraction angle.

According to a seventh aspect of the present invention, there is provided a stray light measuring apparatus including at least one laser light source and a diffraction grating for measuring stray light, and being in a diffraction plane of the diffraction grating and having grooves in the diffraction grating. A rotation stage that rotates the diffraction grating about a single straight line parallel to the light axis, a photodetector that detects the intensity of reflected light from the diffraction grating, and a light beam having a predetermined diameter from the laser light source. A light beam shaping optical system for shaping and guiding the light beam to the surface of the diffraction grating, and a condensing optical system for condensing the reflected light of the diffraction grating onto the photodetector, and freely rotating the diffraction grating It is possible to measure the intensity of reflected light at various reflection angles.

  The stray light amount caused by the diffraction grating itself can be grasped by the stray light measurement method and stray light measurement apparatus for the diffraction grating according to the present invention. This makes it possible to evaluate the total stray light of the spectrometer separately from that caused by the diffraction grating and those caused by other optical elements, and more accurately design and manufacture the low stray light spectrometer. Makes it possible to do efficiently. Further, since it is not necessary to mount the diffraction grating in an actual spectrometer in the stray light inspection process for manufacturing the diffraction grating, the process can be greatly shortened.

  The diffraction grating stray light measurement method provided by the present invention is characterized in that the stray light of the diffraction grating can be measured under conditions very close to those installed in an actual spectrometer. The second feature is that reflected / scattered light in directions other than the diffraction angle can be measured at a plurality of angles. In order to realize both of these, the present invention uses a method of selecting a reflection / scattering angle by fixing a detector and rotating a diffraction grating as one main component.

  A description will be given below in accordance with examples. FIG. 1 shows an apparatus for measuring stray light of a diffraction grating according to the present invention. This apparatus includes a laser light source 4, a laser driving power source 7 for driving the laser light source 4 with a constant output, a rotating stage 8 on which the diffraction grating 1 is mounted and rotating the same, a stage driving unit 13 for driving the rotating stage 8, and a rotating stage. 8, a control unit 12 that controls the rotation angle 14 of the light source 8, a light detector 5 that detects light diffracted or reflected / scattered by the diffraction grating 1, and a light amount received by the light detector 5 from the output of the light detector 5. A light amount measuring device 11 for measuring and displaying is provided. The diameter of the output light beam of the laser light source 4 is enlarged by the light beam shaping optical system 6 and irradiates the diffraction grating 1 held on the rotary stage 8. In order to change the diameter of the light beam according to the purpose, the light beam shaping optical system 6 has a variable range of a wide range of diameters. Further, if necessary, the light beam shaping optical system 6 can be removed and a pinhole can be inserted into the light side. The photodetector 5 includes a condenser lens 9 and a photodiode 10 and detects the intensity of the collected light. In order to measure light diffracted or reflected / scattered by the diffraction grating 1 at a free angle, the rotary stage 8 is driven by the stage drive unit 13 so as to be set to the angle input to the control unit 12. Further, the photodetector 5 has a structure capable of freely adjusting and fixing its position. In this embodiment, a silicon photodiode having a small dark current is used as the photodiode 10.

A method for measuring stray light of a diffraction grating using the above-described apparatus will be described below.
After measuring and recording the output light intensity Pin of the laser beam in advance using the photodetector 5, the stray light of the diffraction grating 1 is measured according to the following procedure.
(1) The diffraction grating 1 to be measured is placed on the rotary stage 8.
(2) The light beam shaping optical system 6 is adjusted so that the light beam diameter irradiates a wide portion of the surface of the diffraction grating 1.
(3) A position where the rotation angle 14 is finely adjusted around 0 °, and the regular reflection light (0th order diffracted light) of the laser beam from the diffraction grating 1 travels backward in the optical path and matches the exit hole of the laser light source 4. The control unit 12 is initialized so that the rotation angle 14 at this time is 0 °.
(4) The rotation stage 8 is rotated to rotate the rotation angle 14 by a constant deflection angle.
(5) The photodetector 5 is installed at a position where the 0th-order diffracted light of the laser beam is accurately incident on the light receiving surface of the photodiode 10, and the photodetector 5 is fixed at this position.
(6) The rotation stage 8 is driven, and the rotation angle 14 is adjusted so that the first-order diffracted light from the diffraction grating 1 accurately enters the photodetector 5. The value θ ₁ of the rotation angle 14 at this time is recorded.
(7) Further, the rotation stage 8 is driven to adjust the rotation angle 14 so that the second-order diffracted light from the diffraction grating 1 is accurately incident on the photodetector 5. The value θ ₂ of the rotation angle 14 at this time is recorded.
(8) The rotary stage 8 is driven and adjusted so that the value of the rotation angle 14 becomes (θ ₁ + θ ₂ ) / 2.
(9) The light intensity Psl is measured from the output of the photodetector 5 at this position.
(10) Psl / Pin is calculated as a stray light value from Pin and Psl.
The stray light values obtained by applying the above measurement method to a plurality of diffraction gratings were compared with the total stray light values measured using a solid filter with these diffraction gratings mounted on an actual spectrometer. The result is shown in FIG. The diffraction grating used for the measurement has a groove number of 1600 / mm and a blaze wavelength of 250 nm. The used laser light source is a solid-state laser having a wavelength of 473 nm. 2A and 2B show the results of measurement using two spectroscopes of the same type. It can be seen that both results show a very good correlation between the result of the stray light measurement method according to the present invention and the result of stray light measurement by an actual spectroscope. As a result, the stray light measurement method according to the present invention is extremely effective in grasping the stray light caused by the diffraction grating, and does not require a method for mounting on an actual spectrometer as in the prior art, and greatly reduces the inspection process. it can. Also, the points P and Q shown in FIGS. 2A and 2B, respectively, are indicated by the conventional method when the stray light value according to the method of the present invention, that is, the stray light value caused only by the diffraction grating is zero. It represents the stray light value. That is, if the method according to the present invention and the conventional method are used in combination, it becomes possible to separate and grasp the stray light amount caused by the optical system other than the diffraction grating.

Another embodiment of the present invention is different from the first embodiment in that stray light appearing in the vicinity of the diffraction angle of the laser beam by the diffraction grating is measured. It is known that stray light appears at the bottom of the peak of diffracted light due to imperfect shape of the groove of the diffraction grating and surface roughness. However, in order to accurately measure stray light in the vicinity of the diffraction angle, it is necessary to measure with a small diameter of the light beam. An example of a device that makes this possible is shown in FIG. In this example, the light beam shaping optical system 6 is removed from the configuration shown in FIG. 1, and instead, a pinhole 16 is placed in order to narrow the light beam, and the condenser lens 9 is removed. One example of the measuring method is performed by the following procedure. After measuring the intensity Pin of the laser beam after the pinhole 16 in advance,
(1) The diffraction grating 1 is installed on the rotary stage 8.
(2) The rotation angle 14 is finely adjusted in the vicinity of 0 °, the laser beam is regularly reflected on the diffraction grating 1 and travels backward in the optical path, and the position that exactly matches the pinhole 16 is obtained. The control unit 12 is initialized so as to be.
(3) The rotation stage 8 is rotated to rotate the rotation angle 14 by a constant deflection angle.
(4) The photodiode 10 is installed at a position where the 0th-order diffracted light of the laser beam is accurately incident on the light receiving surface, and the photodiode 10 is fixed at this position.
(5) The rotation stage 8 is driven, and the rotation angle 14 is adjusted so that the first-order diffracted light from the diffraction grating 1 is accurately incident on the photodiode 10. The value θ ₁ of the rotation angle 14 at this time is recorded.
(6) The light intensity Psl at a rotation angle in the vicinity of θ ₁ is measured.
(7) The stray light amount Psl / Pin is calculated.
As enlarged and displayed in the circle of FIG. 3, the laser beam irradiation spot 17 moves in the direction of the arrow according to the rotation of the rotary stage 8 on the light receiving surface of the photodiode 10. When the diameter of the laser beam irradiation spot 17 is A, the diameter of the light receiving surface of the photodiode 10 is B, the distance from the diffraction grating 1 to the photodiode 10 is L, and the magnitude of the deflection angle 15 is 2K, the laser beam irradiation spot The angle dθ until 17 passes through the light receiving surface of the photodiode 10 is

dθ = tan ^-1 [(A + B) / L] (1)

And when using a diffraction grating with a constant declination, the wavelength width dλ of dθ output is

dλ = 2 · sin (dθ) · cos (K) / N · m (2)

It becomes. Here, N is the number of grooves of the diffraction grating per mm, and m is the order of diffraction.
FIG. 4 shows the result of calculating the resolution from the equations (1) and (2) assuming that the number of grooves N = 1600 / mm, the order m = 2, K = 13.5 °, the wavelength λ = 473 nm, and B = 1 mm. However, the diameter of the light beam is assumed to be constant after the pinhole. As a result, it is possible to measure the vicinity of the diffracted light with a resolution of about 1 nm when the diameter A of the laser beam irradiation spot 17 is 0.5 mm and L = 200 mm. However, the resolution (λ / dλ) of the diffraction grating is, according to Rayleigh's standard,

λ / dλ = m · N × W (3)

It can be expressed as. Here, W represents the width of the diffraction grating. N = 1600 / mm,
When W = 0.5 mm is applied, dλ = 0.3 [nm], and the decomposition at 4.6 nm is sufficiently possible by the equation (3).
Further, the value of the inverse dispersion is expressed by the following equation (4).

D = cos (β) / N · m · f (4)
(D: inverse dispersion, β: diffraction angle, f: focal length)

Therefore, the wavelength resolution (D × B) at the light receiving surface B of the photodiode 10 is about 1.24 nm when the conditions of N = 1600 / mm, W = 0.5 mm, and m = 1 are applied, and can be sufficiently resolved.

  The method described above performs constant declination measurement in which the laser light source 4 and the photodetector 5 or the photodiode 10 are fixed and stray light is measured while the diffraction grating 1 is rotated. Since this is the same as the operation of a diffraction grating in a general spectrometer, it is possible to perform measurements closer to the actual situation, and at the same time, the components other than the diffraction grating do not change, so they are not affected by these. Measurement is possible. Further, an example in which the ratio with the input intensity of the laser beam is used for calculating the stray light amount has been described. This method is advantageous in that it is not affected by changes in the intensity of the laser beam.

It is also possible to change one part of the above procedure and use the ratio with the diffracted light intensity for calculating the stray light amount. Thereby, the relative intensity with respect to the diffracted light intensity can be known, and signal-to-noise evaluation can be performed.
The features of the present invention are as described above. However, the present invention is not limited to the above and illustrated examples, and includes various modifications. The configuration of the photodetector 5 is a combination of a photodiode 10 and a condensing lens 9 in the embodiment, but a photomultiplier tube is used instead of the photodiode 10 and a concave condensing mirror is used instead of the condensing lens 9. Is also possible.

  The present invention relates to stray light measurement of a diffraction grating.

It is a conceptual diagram of the stray light measuring method and apparatus for a diffraction grating according to the present invention. It is a graph of the comparison of the measurement result by the stray light measuring method of this invention, and the result of having mounted in the actual spectrometer and measuring the stray light. It is a figure which shows another Example of this invention. It is a figure which shows the relationship between the resolution and focal length which used the diameter of the light beam of the laser beam as a parameter. It is the external appearance and partial enlarged view of a reflective holographic diffraction grating. It is a figure which shows the structure of the common spectrometer which uses a diffraction grating. It is a correlation diagram of the measured value of the surface roughness of the diffraction grating by an atomic force microscope and the stray light measured value by an actual spectroscope. 6 is a schematic diagram of a scattered light intensity measurement device for a diffraction grating described in Non-Patent Document 2. FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Diffraction grating 2 Flat part 3 Edge part 4 Laser light source 5 Photo detector 6 Light beam shaping optical system 7 Laser drive power supply 8 Rotating stage 9 Condensing lens 10 Photodiode 11 Light quantity measuring device 12 Control part 13 Stage drive part 14 Rotation angle 15 Deflection angle 16 Pinhole 17 Laser light irradiation spot 18 HeNe laser 19 Spatial filter 20 Folding mirror 21 Condensing mirror 22 Opening 23 Rotating stage 24 Sliding stage 25 Test piece 26 Rotating arm 27 Detector 28 Current amplifier 29 A / D card PC with
30 Entrance slit 31 Exit slit 32, 33 Off-axis parabolic mirror

Claims (5)

The light from the laser light source is formed into a light beam of a predetermined diameter by a light beam shaping optical system,
Guiding the luminous flux to a diffraction grating,
Of the reflected light from the diffraction grating, it is caused by the diffraction grating itself by using the measured value of the reflected light intensity to an angle other than the diffraction angle determined by the characteristics of the diffraction grating and the wavelength of the laser light. it a stray light measurement method of a diffraction grating, characterized in that for calculating the amount of stray light that, further, the stray light measurement, the position of the light detector and the position of the laser light source is fixed, respectively, the said diffraction grating Reflection of reflected light by the diffraction grating from the laser light source incident on the photodetector by rotating about a single straight line on the diffraction surface of the diffraction grating and parallel to the groove of the diffraction grating. A method for measuring stray light of a diffraction grating by constant declination measurement, wherein the method is performed by selecting an angle. The calculation of the stray light amount caused by the diffraction grating itself, the reflected light intensity to an angle other than the diffraction angle determined by the incident light intensity from the laser light source to the diffraction grating, the characteristics of the diffraction grating and the wavelength of the laser light, and The stray light measurement method for a diffraction grating according to claim 1, wherein the stray light measurement method is performed by measuring the ratio. Calculate the amount of stray light caused by the diffraction grating itself, and measure the ratio of the reflected light intensity to a diffraction angle determined by the characteristics of the diffraction grating and the wavelength of the laser light and the reflected light intensity to an angle other than the diffraction angle The method for measuring stray light of a diffraction grating according to claim 1, wherein: When the diffraction angles of the n-th order diffracted light and the n + 1-order diffracted light of the diffraction grating are θn and θn + 1, respectively, the reflection angle of the reflected light to an angle other than the diffraction angle measured using the photodetector is (θn + θn 4. The method for measuring stray light of a diffraction grating according to claim 1, wherein +1) / 2 is set. At least one laser light source;
A rotating stage that mounts a diffraction grating for measuring stray light, and that rotates the diffraction grating about a single straight line that is in the diffraction plane of the diffraction grating and parallel to the groove of the diffraction grating;
A photodetector for detecting the intensity of reflected light from the diffraction grating, a light beam shaping optical system for shaping laser light from the laser light source into a light beam having a predetermined diameter, and guiding the light beam to the surface of the diffraction grating; A condensing optical system for condensing the reflected light of the diffraction grating on the photodetector,
It is possible to measure the intensity of reflected light at a free reflection angle by rotating the diffraction grating with the laser light source, the light beam shaping optical system, the condensing optical system, and the photodetector fixed. ,
Of the reflected light from the diffraction grating, the measured value of the reflected light intensity at an angle other than the diffraction angle determined by the characteristics of the diffraction grating and the wavelength of the laser light is used to determine the diffraction grating itself. An apparatus for measuring stray light of a diffraction grating with a constant declination, comprising means for calculating the amount of stray light caused.

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