GB1598040A - Catadioptric objective - Google Patents

Description

(54) IMPROVEMENTS IN AND RELATING TO A CATADIOPTRIC OBJECTIVE (71) We, INSTRUMENTS S.A., a French Corporate Body of 15 rue Jean Jaures, 94201 Ivry-Sur-Seine, France, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention is concerned with improvements in the modifications of the catadioptric objective disclosed in Patent Application No. 6861/76 (Serial No.

1,542,879).

In the above referred to patent application it is shown that a concave holographic grating can be used as a catadioptric objective, i.e. made up entirely of reflecting surfaces, and the grating can even be the only optical component of the objective, thus limiting the losses of light.

It is stated in the above referred to patent application that the conditions for registering a holographic grating on a concave spherical surface can be determined so that the grating comprises pairs of conjugate stigmatic working points. According to the invention of the above referred to patent application, the conjugate stigmatic points are used as conjugate object-image points for the objective.

The embodiment disclosed in the above referred to patent application is limited to the case of a holgraphic grating registered by placing one of the point sources at the centre of the sphere bearing the concave grating. This application illustrates another embodiment of the invention and gives additional conditions for choosing the conjugate object-image points in order to obtain the best aplanatism conditions.

The stigmatism conditions of holographic gratings have been studied and the research has resulted in the definition of two classes of holographic gratings which have conjugate stigmatic working points. Using the conventional notation, let C and D be the point sources of two laser beams of wavelength A, the interference fringes of the beams being reproduced on the support to form the lines of the grating.

In the first class of gratings, the point source D is at the centre 0 of the supporting sphere and the point source C is at any point If point C is defined by the straight line joining it to 0 and by the distance R OC= m as a function of the radius R of the sphere, it is possible to define a point H along the line OC and at a distance such that OH=mR. It is thus shown that the three points 0, C and H form pairs of stigmatic working points for the grating, but this applies only to a specific working wavelength for each pair. Table I gives the wavelengths in accordance with the positions of the object and image points and the order K of diffraction considered.

TABLE 1

Image/Object O C H Ao O # = O # = # = m K K C # = #o # = 2 #o # = (1+m) #o K K K #o #o #o H # = m # = (1+m) # = 2m K K K This class of gratings can also include a modification in which one of the registering point sources is virtual and situated behind the grating. The results given above, including the data in Table 1, remain valid provided m is replaced by -m.

In the second class of gratings, the point sources C and D, on the same diametrical line of the sphere, are conjugate and harmonic with respect to the sphere. Consequently, the three points 0, C and D form pairs of stigmatic working points for the grating. As before, however, this applies only to a working wavelength specific to each pair. If the parameter m is defined by R OC=- m Table 2 gives the working wavelength in accordance with the positions of the object and image points and in accordance with the order K of diffraction considered.

TABLE 2

.Image/Object O C D 1 Ao m rn O A=O # = A - - 1-m K 1m K 1 Ao 2 Ao 1em Ao C A - - A - - - 1-m K 1-m K bm K m -Ao l+m Ao 2m #o D A = # = # = 1-m K 1-m K 1-m K As before, the second class of gratings can also include a modification in which one of the registering point sources is virtual and behind the grating. The above results remain valid, including the data in Table 2, provided m is replaced by -m.

The invention of this application applies to a catadioptric objective having a concave spherical reflecting surface bearing a holographic grating of either of the aforementioned classes, comprising three points forming pairs of conjugate stigmatic working points. According to the invention, the object-image pair used is that single pair of conjugate stigmatic points C D which is harmonic with respect to the spherical surface bearing the grating.

The example given in the above referred to patent application corresponds to this condition in the case of a grating of the first class having a registration point source D at its centre. In this example the points B and C, which are taken as the point image and the point object respectively, are conjugate and harmonic with respect to the sphere of radius B. They correspond to the pair H in Table 1 and the working wavelength is given in the Table.

An embodiment comprising a grating of the second class3 wherein the registration points C and D are conjugate and harmonic with respect to the sphere bearing the grating is given below with reference to the accompanying drawing of which the single Figure shows the working assembly diagrammatically.

The third stigmatic point is the centre of the sphere, but in order to obtain the best aplanatism conditions, the conjugate object-image points will be placed at the same positions as the registration points, so that they also are conjugate and harmonic with respect to the sphere. Accordingly Table 2 us used.

Using a registration wavelength of 4579 A corresponding to an intense ionizedargon laser beam, it is desired to construct an objective for operating at a krypton laser wavelength of 3507 A, in the order of interference K=2. The resulting ratio m, which is also the reduction ratio, is approximately 0.21. If the angle of incidence for the registration beam from point C is 30 , the resulting positions of points C and D are defined, in dependence on the radius R of the sphere, by C=5.60 R and D=1.17 R. The number of lines in the grating is approximately 862 per mm.

As shown in the drawing, the ultraviolet beam from a krypton laser is sent to grating 10 via a "mask" 11, e.g. the large-scale trace of an integrated circuit. The "mask" 11 has its centre at the point C, i.e. at one of the sources for registering the grating, and is disposed perpendicular to the radial line OC through the sphere bearing the grating. Accordingly, a reduced image of the " mask " forms on the plate 12, which is centred at the point D, i.e. at the other source for registering the grating. Plate 12 is likewise disposed perpendicular to the radical lines OD, i.e. parallel to the "mask" 11.

As in the example in the above referred to patent application, it is possible to calculate the resolution of the objective and check its aplanatism by determining the dimensions aX and AY in two perpendicular directions of the image of a point object when the point object is successively taken at the following positions: at the centre of a square object centred at the stigmatic point; at the centre of the top side; at the top right corner; and at the centre of the right side. Table 3 gives the dimensions AX and AY in microns for two square objects having sides 5.95 and 11.9 mm long respectively, using an objective 260 mm in diameter and having a radius of curvature of 165.8 mm.

TABLE 3

o; (f ~ A X = O EX ^ EX = 1-7 l\X = O 1L o=5mm AAXy==OO AAyX==O%i OX=17 AAXV=OO19LI o=fl9mm AX=O EX=211 AX-68 AX'-O7 AY=O mm I ,y = 0 AV=36 In the examples herein and in the above referred to patent application, ultraviolet is used but, of course, the invention is not limited to a particular range of wavelengths of light The image definition will be increased if light of even shorter wave lengths is used. If the wavelength is approximately 500 , the aforementioned grating will give a reduction ratio of approximately 0.64.

Although the examples relate to reducing objectives, the invention is similarly applicable to magnifying objectives. The above referred to patent application discloses an application of the last mentioned kind to a microscope objective. The invention can also be applied to magnifying objectives for projectors such as microfilm readers or cross-section projectors for checking dimensional accuracy.

Claims (5)

WHAT WE CLAIM IS:

1. A catadioptric objective having a concave reflecting surface bearing a holographic grating of the kind comprising three points forming pairs of conjugate stigmatic working points, wherein the object-image pair used is that single pair of conjugate stigmatic points C, D which is harmonic with respect to the spherical surface bearing the grating.

2. A catadioptric objective according to claim 1, wherein the object plane extends through the stigmatic point C and the image plane extends through the point D, both planes being perpendicular to the radius of the sphere passing through points C and D.

3. A highly-magnifying microscope having a long front, including a catadioptric objective according to either claim 1 or claim 2.

4. A magnifying projecting device including a catadioptric objective according to either claim 1 or claim 2.

5. A catadioptric objective according to claim 1 substantially as herein described with reference to the accompanying drawing.