GB2212935A

GB2212935A - Holographic diffraction grating

Info

Publication number: GB2212935A
Application number: GB8727466A
Authority: GB
Inventors: Boris Ganievich Turukhano
Original assignee: LE I YADERNOI FIZ IM B P KONST
Current assignee: LE I YADERNOI FIZ IM B P KONST
Priority date: 1987-12-08
Filing date: 1987-11-24
Publication date: 1989-08-02
Also published as: DE3741594A1; CH673343A5; GB8727466D0; FR2623918B1; FR2623918A1

Description

HOLOGRAPHIC DIFFRACTION,4J, , 12 9 1 The present invention relates to

diffraction gratings, and more particularly it relat.es to holographic diffraction gratings.

The invention can be employed in optics, precision mecha nics and engineering as a measurement device, and also in met rology as a standard measure.

There is known a diffraction grating serving as a compo nent copy of measurement diffraction gratings ("Optiko-Mekha nicheskaya Promyshlennost" Magazine, Ko.9. 1975, Moscow, F.A.Mitina, A.M.Nizhin, G.N.Rassudova "Sostavnye kopii izmeri telnykh diffraktsionnykh reshetok". Composite copies of measu rement diffraction gratings, pp.47-50).

This diffraction grating comprises a backing having coat- ed thereon a polymer resin layer accommodating phased (phase- combined) portions of diffraction gratings produced as copies of original images. The grating is produGed by contact printing in the following procedure. 20 Original images of gratings are placed upon the backing and phase-related or phased in successive pairs with the aid of a third original image (transparency) by the moire fringes formed. As a result of this phasing step, the rulings or lines of the individual diffraction gratings are indexed parallel to one another, and the spacing of the lines of adjacent pairs of phased portions of these original gratings equals an integer of the constants (periods) of the grating. This done, the original gratings are removed, the resin 2 - is poured upon the backing, the original gratings are carefully repositioned and retained with spring-urged stops, the positioning is checked against the moirg fringe pattern and readjusted, if necessary. 5 In 2 hours superfluous resin is removed, and in 18 - 20 hours the copy is separated from the original gratings, and the latter are thoroughly washed with distilled water to remove any traces of chemicals used in the copying prosess. This technique can be employed for making composite co- pies of diffraction gratings made of any number of successive portions.

A diffraction grating thus produced as a succession of component copies of measurement diffraction gratings ia bound to have unruled or unlined stretches, i.e. areas where grat- ing rulings or lines are absent, as well as "steps" left in the areas of abutment of the original gratings.

Quite evident3y,-such unlined stretches and "steps" impair the accuracy of the diffraction gratings produced.

There is further known a holographic diffraction grating comprising a backing with a light-sensitive layer coated thereon, having formed therein a system of recurring rulings or lines grouped into portions arranged successively one after another in a direction perpendicular to the lines. and phased relative to one another (GB, B, 8624014).

In this holographic diffraction grating adjacent pairs of the portions are recorded in a contact-less procedure suc cessively in an interference field by shifting the backing in this field through a length equalling the field aperture.

Consequentlyg in the last-described holographic diffraction grating lines in the areas of matching of the adjacent 1 0 portions are spaced non-uniformly and intermittently, which cannot but impair the accuracy of the grating.

In order to record a continuous high-accuracy grating with uniform spacing of the lines throughout its length by the last-described technique, the backing has to be shifted with a precision in excess of a tenth of the recurrence period of the lines. which is virtually impossible with periods equalling mere fractions of a micron.

It is an objectof the present invention to enhance the 10 accuracy of a holographic diffraction grating.

The essence of the invention resides in a holographic diffraction grating comprising a backing and a light-sensitive layer coated on this backing. having formed therein a sys- tem of recurring lines grouped into portions successively arranged one after another in a direction perpendicular to the lines, and phased relative to one another, the border areas of adjacent pairs of these portions being arranged to overlap each other. defining overlapping zones. with the phases of the lines situated in.the overlapping zones coinciding.

The disclosed structure of a holographic diffraction grating provides for continuous arrangement of periodically recurring spaced lines lengthwise of the backing of the grating with any length and frequency of the grating. with uniform spacing of the lines yielding higher accuracy of the 25 grating.

The invention will be further described in connection with its embodiment and appended drawings, wherein:

Fig. 1 is a longitudinally sectional schematic view of a holographic diffraction grating embodying the invention; Fig. 2 is a plan view of the grating shown in Pig. 1; 1 Fig. 3 illustrates the sinusoidal distribution of the intensity of blacking/transparency of the lines in.one of the overlapping zones of the holographic diffraction grating shown in Figs 1 and 2; Pig. 4 is a chart illustrating the variation of the depth of modulation of the interference pattern in the overlapping zone and in the adjoining zones. shown in Fig. 3.

The holographic diffraction grating embodying the invent ion comprises a backing 1 (Fig. 1) having coated thereon a light-sensitive layer 2 in which a system of periodically recurring rulings or lines 3 (Fig. 2) is formed, grouped into portions 4, 5 and 6 arranged successively one after another in a direction A perpendicular to the lines 3. and phased relative to one another.

The border areas of adjacent pairs 4 and 5, 5 and 6 of the portions 4, 5 and 6 are arranged to overlap each other. defining overlapping zones 7 and 8, respectively. The phases of the lines 3 situated within these overlapping zones 7 and 8 coincide. The overlapping zones 7 and 8 are conditionally indicated in the appended drawings. Figs 1 and 2 by thicker lines as symbolic means for their better appreciation.

It can be seen from Fig. 3 that the phases of the lines 3 in the overlapping zones 7 (8) coincide. with the dotted lines showing-that the actual number of the lines 3 falling within these overlapping zones 7 (8) depends on the width of these zones.

A holographic diffraction grating in accordance with the invention is prepared. as follows.

The grating embodying the invention is produced by se- quential interference-wise copyiiig of a single stretch of an 17 interference field in successive portions lengthwise of the grating. Relative phasing of the portions 4 (Fig. 2)9 5 and 6 is preferably achieved with the aid of an additional original diffraction grating of the same-frequency, using'the pattern of moir6 fringes yielded by the interference field and the additional grating. The copying is carried out so that adjacent pairs 4 and 5, 5 and 6 of the portions 4, 5 and 6 should overlap by their border areas, defining the overlapping zones 7 and 8.

- The frequency of a diffraction grating thus produced equals the frequency of the interference field being copied, and can be as high as several thousand lines per one millimetre in the optical range. The disclosed grating features uniform spacing of the rulings or lines 3 throughout the length of the successive portions 4, 5 and 6, their overlapping zones 7 and 8 included, where uniform distribution of diffraction efficiency is also retained. The uniformity of the spacing or distribution of the lines 3 in any area of the grating is determined by the unifDrmity of the bands of the stretch of the interference field being copied. Practically, this attainable uniformity of distribution can be as high as /100 over a grating length of at least 70 Mm, or even higher, where A is the wavelength of the source of coherent light. In the embo- k, diment being described A- 0.6328.

The uniformity of distribution of the lines 3 within the overlapping zones 7 and 8 is additionally defined by pre cise phasing of the lines 3 in these overlapping zones 7 and 8.

Fig. 4 of the appended drawings presents a chart show- 1 ing the variation of the depth Q of modulation of the interference pattern or fringe (Y-axis, per cent) over one pair of the adjacent portions 4 and 5 (or 5 and 6) of the disclosed diffraction grating versus its length L (X-axis, mm), inclUding the 3 mm wide overlapping zone 7 (or 8). It can be seen that within the overlapping zone 7 or 8 the depth Q of modulation varies by 3.6% from the maximum value, whereas outside this zone 7 or 8 the magnitude of variation is within 2.6%.

With the overlapping zone 7 (or 8) being made as narrow as possible, the above difference between the values of Q can be made even smaller.

This small variation of the value of Q allows, when checking a ruled or lined measure, to sight on the lines of the diselossed diffraction grating employed as the standard measure with the same accuracy both within and outside the overlapping zones 7 or B. The presently available standard measures for checking other measures with rulings or slits offer relatively low frequencies (within 100 lines/mm). The desirability of making available diffraction gratings of the disclosed kind is explained by the advent of high-accuracy, high-frequency diffraction gratings as measuring implements, requiring corresponding standard measures for their checking and inspection. These standard measures should feature high accuracy, uni- formity and continuity of the distribution of their slits or rulings, i.e. lines, in addition to having high frequency of i:

these lines, of the order of magnitude of the frequency of the rulings or slits of a grating to be checked or inspected.

By employing the present invention, there has been pro- duced a holographic diffraction grating 1 metre long, of 1000 lines/mm frequency with continuous distribution of its rulings or lines 3 (Fig. 3) throughgut its length. owing to the overlapping of theqe lines 3 in the border areas of the adjacent component portions of the grating. The grating was 5 recorded on a glass backing coated with a high- resolution ligh -sensitive layer. The grating produced features high uni formity of distribution of the lines 3 throughout Its length. including the overlapping zones 7 and 8. The error of the grating is within 1 jd/m, which corresponds to the "0" class of accuracy of standard lined or ruled measures.

The disclosed grating offers enhanced accuracy as on outcome of the continuity and uniformity of distribution of its lines 3 throughout its length. The continuity of distribution of the lines 3 is ensured by the existence of the overlapping zones in the border areas of adjacent pairs 4 and 5. 5 and 6 of the portions 4, 5 and 6 making up the grating, these overlapping zones 7 and 8 retaining the accuracy and uniformity of the distribution and spacing of the lines and the diffraction efficiency attained outside these zones 7 and 8. 20 Gratings of the disclosed kind can be employed for producing continuous high-accuracy standard measures. A diffraction grating of the kind disclosed. produced by copying an interference pattern or field, can have its frequency as high as 5000 lines/mm and accuracy equalling

0.2 j4/m. In this case the recurrence period of the lines equals 0.2 g/m, i.e. the same value as the accuracy of the grating itself. If a grating of this kind is used as the measuring means in a linear displacement sensor, the incremental accuracy or discreteness of 0.2.4m is attainable without any electronic interpolation by simple counting of the interference fringes or bands, representing an optimized situation where discreteness equals accuracy.

WHAT WE CUIM IS:

1. A holographic diffraction grating comprising a backing and a lightsensitive layer coated on this backing, having formed therein a system of recurring lines grouped into portions successively arranged one after another in a direction perpendicular to the lines, and phased relative to one another; the border areas of adjacent pairs of these portions being arranged to overlap each other. defining overlapping zones. with the phases of the lines situated in the overlap10 ping zones coinciding.

2. A holographic diffraction grating, substantially as hereintofore described with reference to the appended drawings.

1 J Amendments to the claims have been filed as follows - IY- 1. A holographic diffraction grating comprising a backing and a lightsensitive layer coated on this backing during preparation of the grating, having formed therein a system of recurring lines grouped into portions successively arranged one after another in a direction perpendicular to the lines, and phased relative to orie another; the border areas of adjacent pairs of these portions being arranged to overlap each other, defining overlapping zones, with the phases of the lines situated in the overlapping zones coinciding. 2. A holographic diffraction grating, substantially as hereintofore described with reference to the appended drawings.

Published 1989 at The Patent Office. State House, 66'71 High Holborn.LondonWCIR4TP. Further copies maybe obtained from The Patent OfficeSales Branch, St Mary Cray, Orpington, Kent BR5 3RD. Printed by Multiplex teChniques Itd, St Mary Cray, Kent, Con. 1/87