JPS5854310A - Optical system for littrow spectroscope - Google Patents

Abstract

PURPOSE:To hold bright and high wavelength resolution and space resolution, by constituting a collimator lens with three lenses at least and allowing the length between the collimator lens and an entrance slit, the area of an exit slit, etc. to satisfy a specific conditional inequality. CONSTITUTION:The luminous flux from an entrance slit S1 becomes a parallel luminous flux through a collimator lens L and reaches a reflective diffraction grating G, and the reflected diffracted light is condensed on an exit slit S2 by the collimator lens L, and a spectrum image is formed thereon. In this case, this optical system is so constituted that a length d0 between the collimator lens L and the extrance slit S1, a length l between the exit slit S2 and an image S'1 of the entrance slit formed by the light which is reflected by the surface of an optional lens, for example, the j-th lens of the collimator lens L, and an area S of the exit slit S2 satisfy the relation of expression I.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a beam-draw type spectrometer, particularly to improvements in its optical system.

A Littrow-type spectrometer basically consists of a combination of a reflective dispersion element such as a diffraction grating or prism and a collimator lens system, and the light from the entrance slit is converted into a parallel beam by the collimator lens system. The reflected and diffracted light from the dispersive element passes through the frimeter lens system again to form a spectral image on the exit slit. This type of spectrometer has the advantages of being easy to fix aberrations because the angle of view of the lens system is small, and having a simple configuration because a desired spectrum can be obtained with a single lens system. However, it has a serious drawback that the reflected light on the lens surface becomes stray light and impairs the contrast of the spectral image. In other words, if you increase the number of lenses in the collimator lens in order to improve the wavelength resolution and spatial resolution (resolution in the longitudinal direction of the slit) and create a bright lens system, the amount of light reflected from each lens surface increases as the number of lens surfaces increases. As a result, stray light increases, and there is a fear that the performance of the spectrometer may deteriorate.

An object of the present invention is to provide an excellent optical system for a draw-type spectrometer that can maintain brightness, high wavelength resolution, and spatial resolution while retaining the advantages of a draw-type spectrometer.

In the optical system for a Littrow type spectrometer according to the present invention, the collimator lens is composed of at least three lenses, and the distance between the collimator lens and the entrance slit is d. Closing, the distance between the entrance slit image formed by the light reflected from any surface of the collimator lens and the exit slit! , where S is the area of the exit slit, the configuration satisfies the following relationship.

Here, the area of the exit slit does not necessarily refer to the entire slit surface, but when the spectral image plane is divided, it refers to the divided aperture area.

The present invention will be described in detail below.

Stray light from each lens surface, which degrades the performance of the spectrometer, can be quantitatively evaluated as follows. First, in Littrow-type spectrometers, the entrance slit and exit slit are generally placed off the optical axis of the collimator lens, but because the angle of view is small, it is almost impossible to calculate the paraxial region of a thin lens system. It is possible to make it correspond to Now 41
As shown in the figure, the collimator lens system (L) is composed of multiple lenses, and the entrance slit (S,)
Consider the reflected light at the j-th lens surface from the side. In Fig. 1, the light flux from the entrance slit (S,) passes through the collimator lens (L), becomes a parallel light flux, and reaches the reflective diffraction grating (G), and the reflected diffracted light is transmitted through the collimator lens (L). L) is focused onto the exit slit (S2) where a spectral image is formed. In the figure, only the optical path of the light beam reflected by the first surface of the collimator lens (L) is shown. Here, the focal length of the collimator lens system is f, and the distance between the collimator lens system and the entrance slit and exit slit is d. Then, do is the back focus of the collimator lens system, and f and d.

are almost equal. Then, assume that the solid angle of incidence is Ω0, the area of the Ross slit is SO%, and the Romero slit product is S, and the image of the entrance slit by the light reflected on the first surface is formed at a distance l from the exit slit. When the image plane is divided in one spectral plane, let S be the area of one divided aperture.

The image of the incident loss slit due to the reflected light from the first surface is refracted from the first surface to the (j-1)th surface, reflected by the first surface, and reflected by the (j-1)th surface.
Since it is formed by sequentially passing through refraction from the J −,1) surface to the first surface, the first to (j−1) surfaces can be regarded as one lens system, and this lens system If the focal length of is f, then a reflected image of the entrance slit is formed at a position J away from the exit slit. At this time, the imaging magnification β is O2.

The ratio of the amount of light incident from the entrance slit to the amount of reflected light on the first surface that reaches the exit slit is given by the surface reflectance R, the amount of reflected light that reaches the exit slit, the amount of incident light from the entrance slit, f entrance slit 78°″0 (3) ΩO S is usually negative, but it is usually -.) It is given by 2.

Since the amount of stray light in a normal spectrometer is 10-4 to 1O'-5, if the amount of stray light in the reflected light is 10-5 or less, it can be assumed that there is no deterioration of stray light due to surface reflection.

Ω0 need to be satisfied.

Now, the purpose of the present invention is to maintain brightness and excellent resolution, and for this purpose, it is desirable to maintain the F number of the relay lens system at about 3. Therefore, if F3 is used as a reference, this solid angle of incidence is Ω. =0
．． 0855 str. The reflectance R of the lens surface is approximately 0.005 or less over a wide wavelength range.

Therefore, from equation (4), it becomes, and from equation (2), it is given as follows. Therefore, by adopting a configuration that satisfies this relationship, stray light due to reflection on the fifth surface can be substantially eliminated, and similarly stray light due to all lens surfaces can be eliminated.

Exit slit has a width of 0.1 frrrn and a length of 10++a.
In this case, S=1, and the configuration that should be satisfied by each lens surface of the collimator lens in this case can be obtained from equation (5). Multiple reflected light can also be a stray light component, but since its intensity is a power of R, it can be ignored compared to single reflection.

Now, if we look at the first and second lens surfaces of the lens closest to the entrance slit in the collimator lens system, the radius of curvature of each surface is rl, r
2, since the reflected light on the first surface is the same as that of a spherical mirror, the following equation is obtained. From this condition (6) and the above-mentioned equations (2) and (5), the conditions for fi rl can be found.

Regarding reflection on the second surface, the composite focal length fl of the lens reflecting on the back surface is 1 n-12n □= 2 ・□ −□ f 1 p rI, where n is the refractive index of the lens. By substituting into equation (1), we get dociO+1.

Therefore, rl+''2 may be determined so as to satisfy the stray light removal conditions (5) and 7).

1 in Equation (1) has a sign.) means that a reflected image of the entrance slit is formed on the side opposite to the lens from the exit slit, and (→ means that a reflected image is formed on the side of the lens from the exit slit) means.

Also, the focal length f of the first lens is determined from the power arrangement of the entire lens.
rl that also satisfies r2 at the same time
, r2. Then, rl'+r2 is determined without performing overall aberration correction. Similarly, the conditions to be satisfied for the radius of curvature of the subsequent lenses are determined.

Examples according to the present invention will be described below. The specifications of the collimator lens system obtained by the present invention as described above are shown in the table below. In the table, rI, r2. r3.・
... is the radius of curvature of each lens surface from the entrance slit side, d
l+'2+... is the center thickness and air spacing of each lens, n
d.

νd represents a refractive index and a number, respectively.

do =347.8+a+ ,,I
J βr, = 489.76mm d, = 1
7.5mand=1.62041603-491m
O,413r,,--697,98=28.7
=1.0 122 0.659rs=317.
01 =17.5 =1.62041603-
641 0.6421. - of =33
=1.0 -181 0.551rs= 1
53.09 =25.4 =1.620416
03 591 0.450r6--1963.9
=23.0 = 1.7495035.2 33
0 0.235rt= ncigs
660 0.44Of=300 In the table, 1 is the distance from the exit slit of the reflected image of each surface, β is the imaging magnification of the reflected image, and J and β are each obtained by ray tracing. It is. (The symbol → indicates that the lens is located closer to the lens than the exit slit.) FIG. 2 shows an optical path diagram of a Littrow-type spectrometer using this embodiment.

In this embodiment, as shown in the figure, the collimator lens is composed of two biconvex lens components, a positive lens and a negative lens, and a meniscus lens component with the convex surface facing the entrance slit side, in order from the entrance slit side. It is becoming more and more.

Here, when do''347.8, the condition of equation (5) is j
<-63.0 or j)98.76, so it is clear that this example fully satisfies the conditions of the present invention. FM, when calculating the amount of stray light for each lens surface using equation (3), for F number 3, Ω0=
0.0855 str, and S = 0. IX
10 + mn2 is as follows, which is a sufficiently small value.

In actual measurements, the amount of stray light was about the above value, and the lens of the above example had sufficient aberration correction to satisfy the performance as a collimator lens, and the resolution was around 1200. 0.05 nm using a diffraction grating
A good value has been obtained.

As described above, according to the configuration of the optical system of the present invention, stray light due to surface reflection is substantially sufficiently removed, and by configuring the lens system with multiple lenses, it is possible to correct aberrations over a wide angle of view. In addition, a bright spectrometer can be obtained.

Therefore, it is possible to photometer a wide wavelength range at the same time, and the slit length can be increased.
The slit length can be increased to 70fi.

Since the resolution in the slit length direction is also good, it can be divided in the length direction and two-dimensionally in the wavelength direction.
It is possible to obtain excellent performance that cannot be obtained with conventional Retro-type spectrometers, such as being able to perform spectrophotometry with good wavelength resolution and spatial resolution.

[Brief explanation of drawings]

Fig. 1 shows an optical path diagram of only the rays reflected by the first surface of the collimator lens of the glue-draw type spectrometer of the present invention, and Fig. 2 shows an optical path diagram of a draw-type spectrometer using the embodiment of the present invention. shows. [Explanation of symbols of main parts] SI... Entrance slit S2... Exit slit G... Reflection type diffraction grating L... Collimator lens

Claims (1)

[Claims] In a Littrow spectrometer having an entrance slit, an exit slit, a reflective dispersion element and a collimator lens,
The collimator lens is composed of at least three lenses, and the distance between the collimator lens and the entrance slit is 11 [Iwod. , characterized in that the distance between the image of the entrance slit formed by the light reflected by an arbitrary lens surface of the collimator lens and the exit slit satisfies 11, where S is the area of the exit loss slit. Optical system for suri-draw type spectrometer.