JPH0399238A - High resolution spectroscope - Google Patents

Abstract

PURPOSE:To improve resolution with a compact, simple structure by providing a constitution wherein a plurality of diffraction gratings are arranged in a near field region and multiple diffractions are performed. CONSTITUTION:A light wave (b) is inputted through a slit S1 at the incident side and made to be the parallel luminous flux with a concave mirror M1. The luminous flux is diffracted through a diffraction grating G1. The light rays having various kinds of wavelengths contained in the light wave (b) are diffracted at diffracting angles corresponding to the wavelengths. The light rays are inputted into a diffraction grating G2 which is arranged in a near field region. The light having each wavelength is diffracted at the diffracting angle corresponding to the wavelength again. The diffracted light is converted to the converged light with a concave mirror M2. The light is converged at the position of a slit S2. When the grating G1 or G2 is turned by a minute angle, only the intended wavelength light can be selectively emitted to the outside from the light rays in the broad wavelength region. At this time, the wavelength of the emitted light can be computed based on the turning angle of the diffraction grating, and spectroscopic measurement can be performed. Thus, a high resolution spectroscope having a compact, simple structure is obtained without using a diffraction grating having a large diameter.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a high-resolution spectrometer that utilizes a plurality of diffraction gratings.

(Prior Art) With the development of optical communication and optical processing technology, there is a strong demand for the development of a spectroscopic device that can analyze the frequency of light waves with high resolution.

FIG. 8 is a diagram showing the basic configuration of a conventional spectrometer using a diffraction grating. A spectroscopic light wave is made to enter through the slit S1. Since this light wave is diffracted and spread, it is made into a parallel light beam by the concave mirror, and the parallel light beam is made incident on the diffraction grating G. This diffraction grating has grating grooves extending in a direction perpendicular to the plane of the paper, and is mounted so as to be rotatable within the plane of the paper. The incident light wave is diffracted by the diffraction grating G in a diffraction angle direction corresponding to the wavelength, focused by the concave mirror h, and exited through the slit sz. Since the diffraction angle of the light wave to be diffracted differs depending on its wavelength, the focusing position is formed to be shifted in the vertical direction of the plane of the paper perpendicular to the grating grooves depending on the wavelength.

As a result, only a certain wavelength of light is emitted from the slit S2. Therefore, by rotating the diffraction grating G and changing the angle of incidence on the diffraction grating, it is possible to selectively emit only light of a specific wavelength, and from this rotation angle, the wavelength of the light wave emitted from the slit St can be calculated. Spectroscopic measurements can be performed.

Next, the resolution of the conventional diffraction grating spectrometer mentioned above will be explained based on FIG. 9. When the pitch of the diffraction grating G1 is d and the incident angle is θ1, the diffraction angle θ2 of a light wave of wavelength λ is given by the following equation.

θt = sin-'(nλ/d-sinθ,)
・(1) Here, n is the order of diffraction ±1. ±2, ±3-11 (usually n=±1).

For simplicity, let us assume that the beam is irradiated over the entire width D of the diffraction grating to obtain the best wavelength resolution.If the focusing concave mirror M2 is sufficiently large compared to the beam diameter, the beam diameter will be approximately D cos θ2. Become.

When the focal length of the concave mirror M2 is f2, the beam has the smallest diameter at the focal position, and the beam diameter at the diffraction limit is given by the following equation.

,dX,. ．． ．． -ft ・λ/ (Dcosθt)・
(2) Furthermore, if it is assumed that the diffraction angle θ2 also deviates by Aθa because the wavelength differs from λ by Aλ, Aθ2 is given by the following equation.

Zθz 'in X lλba dcosθz)・(3)
As a result, the focal position also shifts, and this shift 1aX is given by the following equation.

? x=rz・Jθ. =nXf2・,jλ/ (dcos
θ2)...(4) If the deviation amount Ax is smaller than the value specified by equation (2), it cannot be resolved, so a wavelength difference equal to the value specified by equation (2) is theoretically possible. Corresponds to wavelength resolution. Therefore, from equations (2) and (4), the maximum spectroscopic resolution Aλ
．． l (theoretical limit) is determined, and its maximum resolution is given by the following equation.

Aλraw = l X r*s/ (n X ft
/ (dCosθ2))=λ(d/n)/D
...(5) In a normal spectrometer, there is a slit in the focusing part.
Sz is placed to selectively detect only light of a specific wavelength. In this case, if the width W of the slit l-sz is (2)2 or less (
5) A resolution close to the theoretical limit equation can be obtained. On the other hand, generally, a slit with a wide width (for example, 10DII1 or more) is used, so the resolution Aλ'■3 is the slit width W
and is given by the following equation.

Aλ” rat = W/ ( n X f !/ (
a cosθ2))=(dcosθz/n)W/fz ≧λ, . ．． (6) From equation (6), it can be understood that increasing the focal length f2 of the focusing concave mirror h further increases the resolution.

However, in reality, even if f2 is increased, it will not ultimately become larger than the theoretical limit given by (5) 2. Also, from equation (5), the relative resolution of the spectrometer Aλr1.
/λ is determined only by the grating number (D/d) of the diffraction grating and the diffraction order n. Furthermore, since the grating pitch d does not function as a diffraction grating unless it is larger than n×λ/2, the following equation holds true.

Aνras l”: lν(Aλ,.3/λ)=vl
d/nl/D≧νλ/ (2D)=C/ (2D) (7) Here, the equal sign stands when d=nλ/2, and C is the speed of light.

Therefore, when d and n are optimally selected, the best frequency resolution is approximately the reciprocal of the time it takes for light to make one round trip across the entire width of the diffraction grating. As a result, the possible resolution of conventional spectrometers is ultimately determined by the size of the diffraction grating.

(Problems to be Solved by the Invention) From the above studies, it has been found that what determines the resolution of conventional spectrometers is the number of gratings and the order of diffraction, and if the grating pitch and order of diffraction are optimally selected, the diffraction It can be concluded that the aperture of the grating (total width D of the diffraction grating).

Therefore, in order to obtain high resolution, a large-diameter diffraction grating is required, but it is difficult to manufacture a large-diameter diffraction grating, and moreover, if it is 20 to 30 cu+, it becomes extremely expensive. To overcome these drawbacks, spectrometers called double monochromators and triple monochromators, in which spectrometers are arranged in two or three stages in series, have been proposed. However, these spectrometers are just wavelength filters connected in series, and although the overall resolution increases, the overall device size not only increases, but there are also difficulties in operability. In addition, the degree of improvement in resolution is only 5 times and F times, respectively, even with double and triple structures, and even with the highest grade 3m double monochromator in practical use, it is only 0. LA (5~10GHz
).

As other high-resolution spectrometers, the sweep type Fabry
Examples include Perot interferometer and Fourier transform spectrophotometer.

This swept Fabry-Perot interferometer has a high resolution on the order of MHz and is suitable for stationary spectrum observation.
Since one wavelength has an almost infinite number of transmission regions, if several wavelengths coexist in the light source to be measured, it not only becomes impossible to measure, but also makes it impossible to measure the absolute wavelength. Therefore, measurements are limited to a narrow free spectral range.
Furthermore, since it takes more than 1 millisecond to sweep, there is a drawback that high-speed real-time measurement cannot be performed. Furthermore, the Fourier transform spectrophotometer has a resolution close to 1 kHz at the research stage. However, since it is necessary to sweep the arm length of the interferometer slowly, the measurement takes a long time. Therefore, although it is suitable for long-term time-averaged spectrum measurement, it has the disadvantage that it cannot be applied to real-time spectrum measurement of signals. Also,
If the light source does not have long-term wavelength stability, there is also the drawback that it is not possible to distinguish between the broadening of the signal spectrum and the long-term fluctuation of the light source.

Therefore, an object of the present invention is to eliminate the above-mentioned drawbacks and provide a spectrometer that can achieve high resolution with a small and simple structure without using expensive large-diameter diffraction gratings or double monochromators. .

(Means for Solving the Problems) A high-resolution spectrometer according to the present invention includes a first optical element that converts a light beam to be separated into a parallel light beam, a second optical element that focuses a multiple diffracted light beam, The optical path between these optical elements is provided with a plurality of diffraction gratings arranged so that the grating grooves are parallel to each other, and the diffracted light from the diffraction grating located on the incident side is transmitted within the near field of this diffraction grating. The present invention is characterized in that it is configured to perform multiple diffraction by sequentially irradiating the diffraction grating at the next stage located on the exit side.

Further, the high-resolution spectrometer according to the present invention includes an optical element that converts the light beam to be separated into a parallel beam, a first diffraction grating that diffracts the parallel beam, and a near-field image of the l-th diffraction grating. an imaging optical system; a second diffraction grating that is disposed at or near the imaging position of the near-field image and whose grating grooves extend parallel to the grating grooves of the first diffraction grating; and a second diffraction grating. and an optical element that focuses the diffracted light from the diffraction grating, and is configured to cause multiple diffraction by the first and second diffraction gratings.

(Function) FIG. 1 is a schematic diagram for explaining the principle of a high-resolution spectrometer according to the present invention. Spectroscopic special incident beam b has wavelength λ
and λ+Aλ, and this beam b is passed through a first diffraction grating G whose diffraction grooves extend in a direction perpendicular to the plane of the paper.
1 and the incident angle θ. The beam is incident on the beam and diffracted. The difference in diffraction angles 2θ2l between a light wave with wavelength λ and a light wave with wavelength λ+Aλ is given by the following equation. Aθz+=n+Jλ/ (a cosθZ
+ > ・(S) When these diffracted lights are diffracted by a second diffraction grating Gt arranged in the near-field region and whose diffraction grooves extend perpendicularly to the plane of the paper, the angle of incidence on the second diffraction grating Gt becomes Since they are different, the difference in diffraction angles Aθ. due to the second diffraction grating. is a quantity that directly depends on the wavelength difference 2λ and the second diffraction grating due to the diffraction angle difference Aθffi+ of the first grating? This is the sum of the difference 2θ1■ in the angle of incidence on G2, and is given by the following equation.

1 11 z*=l J:}e zd3A +l fl
I-fl zz/aθ12=ntXJλ/ (aco
sθ. t) − Jθ, gcOsθ+ t/cosθ
zt””nt/nIAθ. ．． (cosθ2+/COS
θ0−Cn. lθz+/nzJθ+z)cosθ+z
/cosθ2z} where Aθ,2 and 2θ. have the same absolute value, and if the two degrees of diffraction are taken to have the same order, regardless of the positive or negative sign, the absolute values of nl+ng will also be equal. Moreover, this sign can be selected by setting the diffraction grating, etc. (n, aθ+z/
nzJθ21) can be set to -1. In this case Aθ. ．． l=1,6θt+l (cosθg,+CO
Sθ+ z) /cosθ2■...(9) θ, 2, θ2. The absolute value of? Become. Therefore, if θ2■ is chosen to be approximately the same, Aθzzl~2xl, dθ. l·”00) is obtained, and it can be seen that the difference in diffraction angles in the double path configuration is approximately twice that in the case of single diffraction.

Therefore, after two diffraction steps, if the concave mirror is used to focus the spectrometer as in a normal spectrometer, the points with a wavelength difference of 2λ will be focused twice as far apart spatially as in the case of a single diffraction (Equation (4) ), it is easy to understand if you consider that 2θ2 has doubled.

On the other hand, if the aperture of the diffraction grating is about the same, the width of the beam will hardly change, so the focusing width will also remain about the same as in the case of one-time diffraction. In other words, the resolvable wavelength difference is halved, and the resolution is doubled. Similarly, if the beam is diffracted three times, the resolution will improve three times. This is natural considering the residence time. The other configurations are basically the same, and the resolution improves by 2 times, 3 times, 4 times, etc. depending on the number of times of diffraction.

An example of design numerical values will be described below.

Method: 12cm diameter diffraction grating (2400 lines/InII1
) using the 41i path (four diffraction steps shown in Figure 4b and Figure 7) Wavelength meter used = 400~800nII1 Resolution: 2λ/λ~Aν/ν~0.87×1016 Wavelength 500nm (frequency 6007Hz) and frequency Resolution ~521M}lz, wavelength resolution ~0. OO043nm Wavelength 800nm (frequency 375THz), frequency resolution ~326MHz, wavelength resolution ~0. OO069nn
+ It should be noted that although the wavelength resolution is higher at a wavelength of 800 nm, the practical resolution and frequency resolution are higher.

Absolute wavelength accuracy: 0.001 nm (with calibration light) was used.

Output: Frequency distribution display of light intensity was used on a CRT monitor.

(Example) FIG. 2 is a diagram showing the basic configuration of a high-resolution spectrometer according to the present invention. In this example, two reflection gratings G1 and G
We will explain an example of spectroscopy using 2. The light wave b to be separated is made incident through the slit SI on the incident side, and is made into a parallel light beam by the first concave mirror M1.

This parallel light beam is diffracted by a first diffraction grating G1 whose grating grooves extend in a direction perpendicular to the plane of the paper. The various wavelengths contained in the light wave b are diffracted at diffraction angles corresponding to the wavelengths, and are incident on the second diffraction grating G8 arranged in the near-field region. These diffraction gratings G and G. The lattice grooves of the lattice grooves extend parallel to each other. Each wavelength light is diffracted again by the second diffraction grating G2 at a diffraction angle corresponding to the wavelength. This diffracted light is converted into a convergent beam by the second concave mirror h, converged at the position of the slit S2 on the exit side, and emitted to the outside. Then, by rotating the first or second diffraction grating G or G2 by a minute angle,
Only desired wavelength light can be selectively emitted to the outside from light in a wide wavelength range. In this case, the wavelength of the emitted light can be calculated from the rotation angle of the diffraction grating, and spectroscopic measurement can also be performed. In addition, since a spot is formed in the focusing part according to the wavelength difference along the diffraction direction, a TV camera, CCD array, or photodiode array is placed at the focusing position of the concave focusing mirror H in place of the slit S2. It is also possible to perform spectroscopic measurements by arranging a spatially resolved image sensor such as the one shown in FIG. By using a spatially resolved image sensor instead of the slit StO, high-speed real-time spectral measurement can be performed for light in a relatively narrow wavelength range. Therefore,
By providing both the exit slit and the image sensor, it is possible to perform both measurement over a wide wavelength range and real-time measurement over a narrow wavelength range.

FIG. 3 is a diagram showing the configuration of a modified example of the diffraction section of the high-resolution spectrometer according to the present invention. In this example, three diffraction gratings G. An example is shown in which multiple diffraction is performed by diffracting three times using Gt and Gx. By performing diffraction multiple times in this way, the resolution can be further improved.

Figures 4a and 4b are diagrams showing the configuration of another modification.
In this example, the beam to be separated is a flat beam that spreads in a direction perpendicular to the extending direction of the grating grooves, and is made obliquely incident on the diffraction grating G for multiple diffraction. This flat beam can be easily shaped by using a cylindrical lens, cylindrical mirror, etc. In the embodiment shown in FIG. 4a, a flat incident beam b is made incident on the diffraction grating G1, its reflected diffracted light is made incident on a plane mirror placed opposite to it, and the reflected light is made incident on the diffraction grating G again. and then diffract it again for multiple diffraction. In the embodiment shown in FIG. 4b, a second diffraction grating G2 is used instead of the plane mirror 12 to cause the light to be diffracted four times. With this configuration, the great advantage of multiple diffraction can be achieved by just using two diffraction gratings, and the structure of the spectrometer can be made even more compact. Furthermore, by repeating the reflection alternately between the diffraction grating and the diffraction grating or the plane ξ beam, it is possible to perform multiple diffraction not only two or four times but many times.

FIGS. 5a to 5c show a configuration in which an almost complete near-field region is formed using a certain imaging optical system from a combination of lenses and concave mirrors to perform multiple diffraction. As shown in FIG. 8, a convex lens L1 with a focal length f is arranged at a distance fI from the diffraction grating G, and further distances f, +f from the convex lens L+.
! A second convex lens L2 having a focal length f2 is disposed with a distance of f2 from the second convex lens L2. In this case, the distance ft from the second convex lens L2
A diffraction grating G. The inverted real image of
Moreover, the parallel light rays near the diffraction grating G1 become parallel light rays with their vertical relationship reversed at the formation position of this inverted real image. In other words, the position where the inverted solid line is formed and its vicinity is a nearly complete near-field region where the vertical relationship of the diffraction grating G is reversed. Therefore, lenses L+ and ! constitutes an imaging optical system, and by arranging the second diffraction grating at the imaging position of this inverted real image, almost perfect multiple diffraction with almost no deviation can be realized. In this case, if the first and second convex lenses and the focal length of L2 are set to f,=f, the magnification is 1.
This makes the configuration easier. FIG. 5b shows f. =f2=
An example where f is shown. FIG. 5C shows an example in which a complete near-field region is formed using an imaging optical system using a concave mirror instead of a convex lens.

In addition, since lens media generally have a wavelength dispersion effect and affect the performance of a spectrometer, it is preferable to use a concave mirror rather than a convex lens when dispersing light waves in a wide wavelength range. A perfectly parallel beam of spectroscopy is made incident on a first diffraction grating G, and is made incident on a first concave mirror MCI having a focal length f arranged in the direction of diffraction. A second concave mirror MCz having a focal length f is arranged at a distance of 2f in the reflection direction of the first concave mirror, and the reflected light from the l-th concave mirror is made incident thereon. Then, the second diffraction grating G2 is arranged at a distance f in the reflection direction of the second concave mirror. The reflected light from the second concave mirror MCz becomes a parallel light beam, enters the second diffraction grating Gt, and is diffracted again. With this configuration, multiple diffraction can be performed using a diffraction grating arranged in an almost perfect near field with a simple configuration, and therefore, almost perfect multiple diffraction without deviation can be performed.

Note that a spectrometer with even higher resolution can be realized by arranging a plurality of optical systems in series, each consisting of two diffraction gratings and an imaging optical system.

FIG. 6 shows a modification of the embodiment shown in FIG. 5C, in which a plane; IM
An example of arranging F is shown. With this configuration, the optical path space can be further reduced.

Furthermore, FIG. 7 is a diagram showing the overall configuration of a spectrometer that uses a set of optical systems arranged in perfect near-field positional relationship to form a round trip optical path and perform multiple diffraction.

A light beam to be separated is made incident through the slit S, reflected by the first plane mirror MF+, and is reflected by the cylindrical lens L3.
The light beam passes through the first concave mirror M1 and becomes a parallel light beam with a flat cross section. This flat parallel light beam is diffracted by the first diffraction grating G, and the diffracted light is reflected by a second concave mirror (focal length f) MC. Inject it into the A third concave mirror (focal length f
) Place Mcz. The reflected light from this second concave mirror is made incident on the second diffraction grating G2 arranged at a distance f, and the diffracted light is again transmitted to the third concave mirror MC. Inject it into the The reflected light is made to enter the second concave mirror MC+ again, the reflected light is made to enter the first diffraction grating G, and then the concave mirrors MCI and MC. The light is then made incident on the second diffraction grating Ct again. Furthermore, the diffracted light is transferred to a fourth concave mirror M.
2 to focus the plane mirror MP. and then passes through the slit St. In this case, it is necessary to set a zigzag-shaped optical path by shifting the optical paths from each other so that the round-trip optical paths do not overlap with each other. By using the round trip optical path in this way, it is possible to diffract the light four times and perform spectroscopy. Note that this example is just an example, and it is also possible to diffract the light a large number of times, such as 6 or 8 times, by forming multiple sets of zigzag-like reciprocating optical paths. In addition, the diffraction grating G. , G, or other optical elements by a small angle, light of specific wavelengths can be sequentially and selectively extracted.

Note that various orders of light are generated during diffraction, and the diffraction angles differ depending on the orders of diffraction. When performing spectroscopy by adding the difference in diffraction angles revealed to the wavenumber difference due to multiple diffraction as in the present invention, the optical path must be set or a light shielding unit must be used to prevent diffracted light of different orders from leaking to the output side (so that it does not become stray light). It is desirable to provide

The present invention is not limited to the embodiments described above, and various changes and modifications are possible. For example, in the embodiments described above, a reflection type diffraction grating was used, but it is of course possible to use a transmission type diffraction grating, and it is also possible to perform multiple diffraction by combining a reflection type diffraction grating and a transmission type diffraction grating. in this case,
It is desirable to set the optical path so that the optical path forms a loop.

(Effects of the Invention) As explained above, according to the present invention, since a plurality of diffraction gratings are arranged in the near-field region to perform multiple diffraction, it is possible to achieve a compact and simple structure without using a large-diameter diffraction grating. A high-resolution spectrometer can be realized with this structure.

In addition, when performing spectrum measurements, it is possible to measure over a wide wavelength range by rotating optical elements such as diffraction gratings or mirrors.On the other hand, when measuring a narrow wavelength range, a spatial image sensor is used instead of the exit slit. High-resolution, high-speed real-time spectral measurement can be performed by arranging .

Furthermore, by arranging a second diffraction grating at the imaging position of the first diffraction grating and performing multiple diffraction, highly accurate multiple diffraction with almost no deviation can be achieved.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram for explaining the principle of a high-resolution spectrometer according to the present invention, FIG. 2 is a diagram showing the configuration of an example of a spectrometer according to the present invention, and FIGS. 3 and 4 are according to the present invention. Diagrams showing modified examples of the spectrometer; FIGS. 5 to 7 are diagrams showing the configuration of other embodiments of the spectrometer according to the present invention; FIG. 8 is a diagram showing the basic configuration of a conventional spectrometer. , FIG. 9 is a schematic diagram for explaining the resolution of the spectrometer. GI+ GZI G! ...Diffraction grating M+, Mz, MC. MC1...Concave mirror S,, St...
...Slit figure 2 figure l figure 4 figure a

Claims (1)

[Claims] 1. A first optical element that converts the light beam to be separated into a parallel light beam, a second optical element that focuses the multiple diffracted light beam, and a grating in the optical path between these optical elements. a plurality of diffraction gratings arranged such that the grooves are parallel to each other;
The diffraction light from the diffraction grating located on the input side is sequentially incident on the next stage diffraction grating located on the output side located within the near field of this diffraction grating to perform multiple diffraction. High-resolution spectrometer. 2. The light beam to be separated is a flat parallel light beam whose longitudinal axis extends in a direction perpendicular to the grating grooves, and is configured to be reflected multiple times alternately between the diffraction gratings to perform multiple diffraction. The high-resolution spectrometer according to claim 1, characterized in that: 3. An optical element that converts the light beam to be separated into a parallel beam with a flat cross section, a diffraction grating that diffracts this parallel beam, and an optical element that is arranged at a distance smaller than half the distance of the near-field limit in the direction of this diffraction. A high-resolution spectroscopy system comprising: a plane mirror that has a flat mirror; and an optical element that focuses the multiple diffracted beam, and is configured to perform multiple diffraction by alternately reflecting the beam multiple times between the diffraction grating and the plane mirror. vessel. 4. An optical element that converts the light beam to be separated into parallel light beams;
A first diffraction grating that diffracts this parallel light beam; an imaging optical system that forms a near-field image of the first diffraction grating; a second diffraction grating that extends parallel to the grating grooves of the first diffraction grating; and an optical element that focuses diffracted light from the second diffraction grating, and the first and second diffraction gratings perform multiple diffraction. A high-resolution spectrometer characterized in that it is configured to 5. Claim 4 characterized in that it has a plurality of optical systems each including the first and second diffraction gratings and an imaging optical system, and is configured to perform multiple diffraction using these plurality of optical systems.
High-resolution spectrometer described in . 6. The light beam to be separated is a flat parallel light beam whose longitudinal axis extends in a direction perpendicular to the grating grooves, and an imaging optical system is used between the first diffraction grating and the second diffraction grating. 5. The high-resolution spectrometer according to claim 4, wherein the high-resolution spectrometer is configured to alternately reflect a plurality of times to cause multiple diffraction. 7. The high-resolution spectrometer according to claim 4, 5 or 6, wherein the optical system is composed of two concave mirrors arranged opposite to each other.