GB2212938A

GB2212938A - Apparatus for recording a radial diffraction grating

Info

Publication number: GB2212938A
Application number: GB8727969A
Authority: GB
Inventors: Boris Ganievich Turukhano; Sergei Vladimirovich Gordeev
Original assignee: LE I YADERNOI FIZ IM B P KONST
Current assignee: LE I YADERNOI FIZ IM B P KONST
Priority date: 1987-12-10
Filing date: 1987-11-30
Publication date: 1989-08-02
Also published as: CH675164A5; DE3741956A1; GB8727969D0; FR2624623A1; FR2624623B1

Description

1 1 APPARATUS FOR RECORDING A RADIAL DIFFRACTION MATING The present

invention relates to apparatus for record ing a diffraction grating, and more particularly it relates to apparatus for recording a radial diffraction grating.

The invention can be employed in metrology and in sys tems of high-precision mesearement of angular displacement.

There is known an apparatus for recording a radial dif fraction grating, comprising-a diamond cutting tool and a metal-coated glass disc for cutting in its coating a radial diffraction grating, the disc being mounted for rotation about its axis by a circular dividing machine (L.P.Presnukhin '"Fotoelektricheskiye preobrazovateli informatsii", 1974, Mashinostroyenuye (Moscow), pp.319- 320).

In this apparatus a radial diffraction grating is'pro- duced by cutting radial grooves or lines in the metal coating of the disc with the diamond tool.

The apparatus is operatedgas follows.

The disc is successively rotated through predetermined angles by the circular dividing machine, and after each ro- tation step a groove of the grating-to-be is engraved by removing a layer of the metal coating by the diamond cutting tool.

This apparatus, however, is uncapable of producing gratings with a high frequency or density of the grooves. This is explained by the inevitable wearing out of the cutting tool, as it has to pass relatively great distances over the surface of the disc, which limits the number of the grooves that can be out and their length. Moreoverg the same shortcoming affects the shape of the grooves or lines and the accuracy of their location from the beginning of the cutting operation through its final phase, to say nothing of the strictest requirements of vibration stability and thermal etability put befor the apparatus of the prior art. Furthermore, this apparatus would not provide for a high rate of groovescutting (the time of cutting a single disc stretching to dozens of hours), as each groove has to be out individually.

There is further known an apparatus for recording a radial diffraction grating, comprising a successive arrangement of an illuminating system and of a disc having its surface coated with a light-sensitive layer for recording a radial diffraction grating thereon, mounted for rotation about its axis (Applied Optiosq V.20, No.23. 1981. December 1. US. D.P.Horne "Optical Scales. Reticles. Gratings, Masks and Standards"q PP.4000-4008, 4003-4006).

In this apparatus the illuminating system includes a light source in the form of a mercury-arc lamp. deflecting mirrors mounted in the path of the light beam. a collimating system and a mask in the form of an opaque flat screen with five transparent radial elite arranged at predetermined angular spacing relative to the axis of the disc.

This apparatus of the prior art operates. as follows.

A light beam from the light source passes through the mask and throws images of the lines upon the light-sensitive layer of the disc. thus imprinting these latent images upon this layer. The disc with the light-sensitive layer is rotated about its axes through angular steps equalling the angular spacing of the slits of the mask, with the illuminating eyetea being continuously activated. The time of rest of the disc in its successive stationary position is far greater 1 than the time of its rotation from one position to the sucoessive one. During each rest period (positioning) of the disc, five lines are simultaneously registered by its light-sensitive layer. This mode of operation of the apparatus allows to minimize the Influence of errors in the angular positioning of the disc upon the accuracy of the recording of the radial grating on the disc.

This apparatus. however. in uncapable of recording a radial grating with a high density of the lines or grooves. for when the density of the lines (the frequency of the grating) is Increased to hundreds of lines per millimetre, the contrast of the images of the slits of the mask on the surface of the light-sensitive layer of the disc becomes exceedingly dependent on the value and stability of the clearance between the mask and the disk, which results in substantial variatiions of the diffraction efficiency along the aperture of the recorded grating.

Moreover. the very technology of the manufacture of the mask involving photographic recording of a single line with the aid of a coordinatograph on a magnified scale, its photographic reauction and subsequent recording of five radial slits with the aid of a circular dividing machine would not provide for obtaining a mask with a high density of the alitag whereas the frequency of a grating recoeded in the above apparatus is defined by the slit density or frequency of the mask.

As the apparatus is operated in the mode of successive rotation through angular steps equalling the angular spacing of the lines of the grating to be recorded. alternating with rest periods of the disc, the recording of a grating takes considerable time proportional to the total number of its lines. which means that the rate of recording a grating In this apparatus is relatively low. Thus. some 9 hours are required to record a grating with 8000 lines. 5 To summarize. the last-described apparatus of the prior art Is operable exclusively for recording diffraction gratings of relatively low line frequencies, and that at a relatively low rate of recording. It is an object of the present invention to provide for atepping up the frequency of recorded radial diffraction gratings.

It is another object of the present invention to provide for increasing the rate of recording a radial diffraction grating.

With these and other objects in view, the essence of the present invention resides in an apparatus for recording a radial diffraction grating, compriBing an illuminating system including a laser beam source and a double-beam interferometer arranged in the path of the laser beam, a disc position- ed behind the double-beam interferometer in the path of the laser beam, mounted for rotation about its axis and having its surface coated with a light-sensitive layer for recording a radial diffraction grating thereon. the arms of the doublebeam interferometer being so arranged with respect to the surface of the disc that their respective optical lines intersection point belonging to the surface of the light-sensitive layer of the disc in the area of the radial diffraction grating to be recorded, symmetrically with respect to a plane including the point of intersection of their respective opti cal lines and the axis of the disc, the arms having optical 2 elements for shaping in each respective arm a divergent beam of laser radiation with a spherical wavefront, passing through the light-sensitive layer of the disc in the area of the radial diffraction grating to be recorded, in a direction from the axis of the disc toward the periphery thereof, the respective optical elements of the arms being arranged symmetrically with respect to the said plane.

The optical elements of the arms of the double-beam interferometer are preferably in the form of objective lenses hav- ing their main foci belonging to the optical lines of the respective arms of the interferometer, at equal distances from the point of intersection of these optical lines, and from this point to the axis of the disc.

The disclosed structure of an apparatus in accordance with the invention allows to record radial diffraction gratings with a high frequency of the lines, at an increased rate of recording.

The abovementioned and other objects of the present invention will be better understood from the following des- cription of its embodiment in an apparatus for recording a radial diffraction grating, with reference being made to the accompanying drawings, wherein:

Fit. I shows schematically a perspective view of an apparatus for recording a radial diffraction grating, embodying the invention; Fig. 2 shows schematically on a larger scale a perspect-, ive view illustrating the arrangement of the optical elements of Fig. 1 relative to the disc; Fig. 3 is a partly cross-sectional view on a larger scale of the assembly for securing the disc in the apparatus illustrated in Pig. 1; Fig. 4 illustrates the geometric locations of the centres S 1 and 52 of the spherical wavefronts of divergent beams of laser radiation. shaped by the optical elements of the inter- ferometer in the apparatus of Fig. 1, in accordance with the invention.

The apparatus for recording a radial diffraction-grating, embodying the invention. comprises a baseplate 1 (Fig. 1) hayIng mounted thereon, successively arranged one in front of the other. an illuminating system 2 and a disc 3 having a light-senaitive layer 4 coated on its surface for recording thereon a radial diffraction grating 5.

The illuminating system 2 Includes an individual mount 6 secured on the baseplate 1 for a source 7 of a laser beam.

which in the presently disclosed embodiment is a laser 7, the mount 6 further supporting in the path A of the laser beam a double-beam interferometer 8. An individual mount 9 also secured to the baseplate 1 supports in the path A of the laser beam after the interferometer 8 a segment-shaped diaphragm 10. 20 The double-beam interferometer 8 includes a cubic prisis 11 arranged after the source 7 in the path A of the laser beam, for dividing the beam into two split laser beam directed into the respective arms 12 and 13 of the interferometer 8. The arms 12 and 13 of the interferometer 8 are so arranged with respect to the surface of the disc 3 that their respective optical lines 14 and 15 intersect in an intersection point Q belonging to the surface of the light-sensitive layer 4 of the disc 3, in the area of the diffraction grating 5 being recorded. the arms 12 and 13 being also arranged symmet- rically with respect to a plane including the point Q of intersection of their optical lines 14 and 15 and the axis 16 (Pig. 2) of the disc 3.

The arm 12 (Fig. 1) further includes a mirror 17 arranged 5, after the cubic prism 11 in the path of one of the split laser beams. and each arm 12 and 13 has, arranged after the mirror 17 in the path of the respective split laser beam. a mirror 18 or 19 and a housing 20 or 21 enclosing an optical element 22 or 23 for shaping in the respective arm 12 and 13 a divergent beam of laser radiation with a spherical wayefront, passing through the light-sensitive layer 4 of the disc 3 in the area of the recording of the diffraction grating 5, the elements 22 and 23 being arranged symmetrically with respect to the abovesaid plane, and being associated with their respective spot diaphragms 24 and 25.

In the embodiment being described, the optical elements 22 (Fig. 2) and 23 are short-focus objective lenses having their main foci 26 and 27 belonging to the respective optical lines 14 and 15 of the arms 12 and 13 of the interferometer 8,, at equal distances 1110 from the point Q of intersection of these lines 14 and 15, and from the point Q to the axis 16 of the disc 3.

Altermatively,, the same function in an apparatus embodying the invention could be performed by optical elements in the form of long focal-length lenses, diverging lenses or spherical mirrors. The spot diaphragms 24 (Fig. 1) and 25 are positioned for their apertures to coincide with the respective main foci 26 and 27 of the optical elements-22 and 23. 30 The diaphragm 10 is positioned above the area of the light-sensitive layer 4 of the disc 3 where the radial diffraction grating 5 la to be recorded.

The disc 3 (Pig. 3) is supported on a massive platform 28 whichq in its turn. is mounted on the shaft 29 of a circular 5 dividing machine 30 which is fixed on the baseplate 1.

The operating principle of the disclosed apparatus for recording a radial diffraction grating isp as follows.

A laser beam generated by the laserbeam source 7 (Fig.1) is divided by the cubic prism 11 into two split beam. These split beams are directed by the mirrors 17, 18, 19 onto the respective optical elements 22 and 23 - the short-focus lenses in the presently described embodiment - which focus them into the respective apertures of the spot diaphragms 24 and 25.

In this way there are created two divergent beam of laser radiation with spherical wavefronts having their respective centres S 1 (Fig. 4) and S2 coinciding with the centres of the apertures of the spot diaphragms 24 (Fig. 1) and 15. and, consequently, with the respective main foci 26 (Fig. 2) and 27 of the short-focus lenses. Within the area of intersection of the two divergent beams, of which the centre coincides with the point Q of intersection of the optical lines 14 and 15 of the respective arms 12 and 13 of the interferometer 8 (Fig. 1), there is created an interference field which,, when projected upon the surface of the light- sensitive layer 4 of the disc 3, forms a radial fringe structure of interference bands. This field will be hereinafter referred to as the radial interference field.

The light-sensitive layer 4 of the disc 3 thus becomes exposed to this radial interference field within a segment- shaped zone of the angular dimension defined by the dia- T phragm 10.

The disc 3 is rotated by the circular dividing machine 30 through successive angular stops equalling c>,' m /k, where I-IC is an integer. The interferometer 8 is so adjusted that the disc 39 when rotated through each angular step of the abovedescribed magnitudeg advances by a whole number of the steps or pitches of the interference field, i.e. the ratio n where 0 is the angular pitch of the radial interference field is positively an Integer. the angular pitch a corresponding to the angular pitch of the diffraction grating 5 being recorded.

It can be seen that 2-1, / N. where I'Z. equals 3.14.., and W is the total number of the lines of the radial dif- fraction grating 5 being recorded.

The time t 1 of rotating the disc 3 through a successive angular step is substantially smaller than the time t2 of rest of the disc 3 in each successive angular position. Thus, the disc 3 is successively exposed to continuous radiation coming from the illuminating system 2 (i.e. to light in the form of a radial interference field) throughout its rotation through 3600. whereafter the recording of the radial diffaction grating 5 is terminated. Hence. the abovedescribed operating mode of the apparatus embodying the invention provides for recording the radial diffraction grating 5 over the ro- tation of the disc 3 through 3600.

As can be seen in Fig. 4, the coordinates S 1 (a, b. h) and S 2 (-a. b. h) of the centres S 1 and S 2 of the spherical wavefronts of the divergent beams of laser radiation in a system M of Cartesian rectangular coordinates where the plane XOY belongs to the surface of the light-sensitive layer 4 of the disc 3, the axis OY includes the point Q of intersection of the optical lines 14 (rig. 1) and 15 of the interferometer 89 and the axis OZ (Pig. 4) is aligned with the axis 16 (Fig. 2) of the disc 4, can be found from expressions:

a C 1 P_.

where ? 14 -2. 02 (2) and c 2 (3 b ( 1 Al'a 2, p 2).

A2 where i is the wavelength of the source 7 of laser radiation; r 0 is the mean radius of the grating 5 i.e. the dietance OQ (Fig. 4).

Prom (1) we can derive limitations of the permissible value of the coordinate "h":

v 2; 2 A 2 2 - -c- 1) - 4 2. (p 2 '4Y 12 ( a 0 0 CorresPondinglYs from (5) we can derive a limitation concerning the values r 0 and 0: 2, (5) (6) Thus, with provisions for expressions (5) and (6), expressions (1) and (4) allow to determine the coordinates "a" and Ob" recording a radial diffraction grating 5 with an angular pitch and the mean radius rog with a selected value of "h".

The value of "h" is selected with provision for expression (5) so that the angle defined by each respective_optical line 14 and 15 with the surface of the light-sensitive layer 4 of the disc 3 should be sufficiently greater than 00 to reduce the share of reflected light. andg consequently, to enhance the effect of interaction of the incident light beams with the light-sensitive layer 4 of the disc 3.

To increase the rate of the recording of the radial dif fraction grating 5, the active segment of the radial interfe rence field should include a sufficiently big angle. This_ angular dimension of the segment of the interference field produced by the two split beams of laser radiation with sphe- rical wavefronts is related to the required accuracy ASO of the angular positions of the lines of the radial interference field by the following expression:

2 z 1/3 3 ( Ck'- 4 P7)!- vi (7) where "a" and "b" are determined from expressions (1) to (4).

The disclosed structure of an apparatus embodying the invention provides for recording radial diffraction gratings with a high mean frequency of lines (as high as 1000 lines/mm or higher. with an angular pitch or spacing of the lines of the grating equalling. correspondingly. 20 or less with the mean radius ro of the grating equalling 100 mm), while at the same time reducing the time "T" of recording a grating by 8 to 12 times, by way of increasing by 8 to 12 times the angular step of intermittent rotation of the disc. The rate "V" of recording the lines of a grating is stepped up 300 to 500 times to a value of 4000-7000 lines/min.

Thus. the disclosed apparatus provides for significantly increasing the frequency of the lines of a recorded radial diffraction grating and the rate of its recording. while maintaining an adequate recording accuracy.

A prototype of the disclosed apparatus has been operated to record a radial diffraction grating with the following characteristics: 1 - 324, 000;. 1.9.10-5 rad m C; r 0 0 =; Do= 516 linesImm.

The parameters of the apparatus were: h m 25 om; a m a - 15.5 mm., b - 9. 6 mm; WP m 0.211; r m 20; Clv - 301; n m 450; k = 4. The operating mode was: ti = 4 a; t2 m 1 T - 1 hour; V a 5400 lineslmin.

z b

Claims

WE CIAIM:

1. An apparatus for recording a radial diffraction grating, comprising an illumination system including a laser beam source and a double-beam interferometer arranged in the path of the laser beam; a disc positioned behind the double-beam interferometer in the path of the laser beam, mounted for rotation about its axis and having its surface coatedwith a light-sensitive layer for recording a radial diffraction grating thereon; the arms of the double-beam interferometer being so arranged with respect to the surface of the disc that their respective optical lines intersect in an intersection point belonging to the surface of the light-sensitive layer of the disc in the area of the radial diffraction grating to be recordedg symmetrically with respect to a plane including the point of intersection of their respective optical lines and the axis of the disc; the arms having optical elements for shaping in each respective arm a divergent beam of laser radiation with a spherical wavefront passing through the lightsensitive layer of the disc in the area of the radial dif- fraction grating to be recorded, in a direction from the axis of the disc toward the periphery thereof. the respective optical elements of the arms being arranged symmetrically with respect to the said plane.

2. An apparatus for recording a radial diffraction gratIng as claimed in Claim 1, wherein the optical elements of the arms of the double-beam interferometer are in the form of objective lenses having their foci belonging to the optical lines of the respective arms of the interferometerg at equal distances from the point of intersection of these optical lines, and from this point to the axis of the disc.

- 14

3. An apparatus for recording a radial diffraction grating as claimed inany one.of Claims 1 and 29 substantially as hereintofore described with reference to the appended drawings.

Published 1989 at The Patent Office, State House, W71 EUh Holborn, London WCIR 4TP. Further copies may be obtained from The Patent Ofnee. Wes Branch, St Mary Cray, Orpington. Rent BR5 3RD. Printed by Multiplex tecliniques ltd, St Mary Cray, Kent, Con. 1/87