Diffraction gratings

A method, and apparatus for performing the method, of manufacturing a blazed diffraction grating by making a photographic record of the interference fringes produced at the intersection of two coherent secondary light beams derived from a common coherent primary light beam, in which one of the two secondary light beams is compressed or expanded in at least one dimension before it is incident on the region where intersection with the other beam takes place, the angle of intersection between the two secondary beams being such that they are perfectly in register across a certain plane in the region of intersection, the interference fringes formed at the plane being recorded photographically by exposing a photo-sensitive material, preferably an etchable photo-resist, at this plane. Expansion can be achieved by passing one of the recording beams through a prism or by effecting the separation of the primary beam into two secondary beams by means of a blazed diffraction grating.

The present invention relates to diffraction gratings, and particularly to 
diffraction gratings of the type produced as a photographic record of 
interference fringes obtained by intersecting two coherent beams of light. 
The present invention also comprehends a method of making such diffraction 
gratings and is concerned, in particular, with the manufacture of blazed 
diffraction gratings which, in use, throw the majority of the diffracted 
light energy into one, or a small number, of orders of diffraction. 
Techniques for the photographic production of diffraction gratings as a 
record of interference fringes are known as such. For the production of 
such gratings a photographically sensitive material known as photo-resist 
is coated on a substrate (the grating blank) and located in a position at 
which the interference fringes can be formed. The resistance of the 
photo-resist to certain etching chemicals is dependent, when developed, on 
the intensity of light received upon exposure of the photo-resist, and 
thus, by developing and etching the film of photo-resist a diffraction 
grating can be produced the topography of which is directly related to the 
shape and intensity of the interference fringes to which the photo-resist 
was exposed. 
It is well known that for the production of interference fringes it is 
necessary to intersect two coherent beams of light, and such coherent 
beams of light are usually produced as secondary beams by the separation 
of a primary beam of coherent light from a suitable source such as a 
laser. In practice, however, static dust on the optical elements through 
which the beam passes causes the production of a number of subsidiary 
interference patterns which are not wanted in the ultimately produced 
interference fringes. Furthermore, any small imperfections in the optical 
components used in the apparatus for producing the fringes will also give 
rise to a series of spurious interference fringes. This is sometimes 
referred to as the "orange peel" effect. Such imperfections in the 
interference fringes, which are to be photographically recorded in the 
production of the diffraction grating, causes unwanted scattering in a 
grating so produced. 
In order to overcome this problem the light in the primary beam can be 
rendered spatially non-coherent by passing it through a rotating diffuser 
before it is separated into the two secondary beams which will be used to 
generate the desired interference fringes to be recorded. There is a 
drawback to this arrangement, however, in that the secondary beams, since 
their spatial coherence is reduced to an arbitrarily low level must be in 
exact positional register in order to provide fringes which can be 
observed. This means, in practice, that the two secondary beams must be of 
exactly the same cross-sectional size and the angle of incidence of the 
two secondary beams on the plane at which the interference fringes are to 
be produced must be exactly equal to one another and must be parts of a 
single amplitude divided beam: This means that for a given wavelength of 
light and a given photo-resist, only a single blaze wavelength can be 
obtained irrespective of the grating pitch. The production of blazed 
diffraction gratings having different blaze wavelengths, but a common 
grating pitch, cannot thus conveniently be made if the system of reducing 
spatial coherence by means of a rotating diffuser is utilised. This system 
of reducing spatial coherence, however, is extremely effective in reducing 
"noise" due to the above mentioned speckle effect, and accordingly some 
method of changing the blaze wavelength while nevertheless making use of a 
diffuser in the primary beam has been sought. 
A solution to this problem has been found by the method of the present 
invention, according to which there is provided a method of producing a 
blazed diffraction grating by making a photographic record of the 
interference fringes produced at the intersection of two coherent 
secondary light beams derived from a common coherent primary beam, in 
which the spatial coherence of the primary beam is at least reduced by 
passing it through a rotating diffuser, and one of the secondary beams is 
laterally compressed or expanded in relation to the other by an amount 
related to the angle of intersection at the region of interference between 
the two secondary beams such that both beams are in register across at 
least one plane in the region of intersection of the two beams, a 
photographically sensitive surface being located at this plane to record 
the interference fringes so produced. 
Reference herein to a plane where two intersecting light beams are "in 
register" will be understood to refer to a plane where a wavefront of one 
beam interfers with only one wavefront of the other beam. It is only at 
such a plane that interference fringes can be observed even though 
interference as such takes place over the whole of the region where the 
two beams intersect. 
Various procedural modifications can be incorporated into the method 
defined above. For example, the primary beam may be passed through two 
diffusers, one sthationary and one rotating, placed closely adjacent one 
another. 
The lateral compression of one of the interfering secondary beams may be 
effected in any one of a number of ways. For example, one of the beams may 
be passed through a prism located so that the optical path is asymmetric, 
whereby the output beam is reduced, in one dimension of the cross section, 
in relation to the input beam. 
Alternatively, a diffraction grating may be used to effect the separation 
of the primary beam into the two secondary beams, and in this case, one 
order beam will be compressed or expanded in relation to the other. Of 
course, a diffraction grating which throws the incident light into two 
orders (or at least the majority of the incident light into two orders) 
should be selected. Such a grating acts, therefore, both as means to 
separate the primary beam into secondary beams, and also as means for 
relatively compressing or expanding one of the secondary beams in relation 
to the other. 
The photographic record is obtained by locating a film or layer of 
photographic material in the plane in which the two secondary light beams 
interfere. Because of the reduction in spatial coherence of the primary 
beam, although the two beams will interfere in the whole of the 
intersecting region, interference fringes will only be observable in a 
single plane, that is the plane in which both beams are exactly in 
register. This plane may be parallel or perpendicular to the plane 
bisecting the lines of the two incident beams depending on whether the two 
secondary beams are laterally inverted with respect to one another or not. 
If each secondary beam is reflected the same number of times between the 
beam separation means and the region of intersection then the two beams 
will be in "normal" register whereas if one beam is reflected an odd 
number of times more than the other the beams will be in "inverted" 
register at the intersection region. 
It is preferred that the primary beam is collimated, and this may be 
effected in any of a number of ways, although conveniently the light in 
the primary beam is focused at the diffuser, so that it is divergent away 
therefrom. In such a case a concave mirror may be used to reflect the 
light towards the beam separating means and at the same time to collimate 
the beam. Alternatively, a convergent lens system may be used. 
In order to obtain high blaze angles a grating blank carrying the said 
photograhically sensitive surface, may be located in a cell containing a 
suitable liquid, the cell having two transparent windows through which 
respective interfering secondary beams are incident on the 
photographically sensitive surface. The suitable liquid may be carbon 
tetrachloride, liquid paraffin, silicone oil or the like. 
Another way in which high blaze angles can be produced, particularly if a 
relatively coarse pitch is required, is to reflect the two secondary beams 
produced by the beam separating device back on either side of the beam 
separating device to intersect at a small angle to one another. 
The present invention also comprehends apparatus for performing the method 
defined above, comprising a source of a primary beam of coherent light, a 
rotary diffuser for at least reducing the spatial coherence of the primary 
beam, means for separating the primary beam into two secondary beams, 
means for laterally compressing or expanding one of the two secondary 
beams with respect to the other, and means for directing the two secondary 
beams along respective paths along at least one point of which they 
intersect, the said lateral compression or expansion means operating to 
cause such a degree of lateral compression or expansion that the two 
secondary beams are exactly in register in one plane passing through the 
region of intersecting whereby they interfere to produce a pattern of 
interference fringes observable along the said plane, and means for 
photographically recording the pattern of interference fringes formed in 
the said plane, passing through the region of intersection whereby they 
interfere to produce an interference pattern observable along the said 
plane, there being provided means for photographically recording the 
pattern of interference fringes formed in this plane.

Referring now to FIG. 1 there are shown two coherent light beams 11, 12 
which have been produced by splitting a primary beam of coherent 
radiation. The arrows 13, 14 illustrate the relative orientation of the 
two beams with respect to their orientation in the primary light beam. If 
the two coherent light beams are unmodified, interference fringes will be 
produced throughout the whole of the intersecting region indicated by the 
quadrangle of points A, B, C, D, and a suitably placed photographic film 
can be used to record the interference fringes within this region; the 
fringes extend in the direction of the arrow BD. If, on the other hand, 
the primary beam was modified by passing it through a diffuser to reduce 
the spatial coherence of this beam, interference fringes can only occur 
where the two secondary beams 11, 12 are in exact register; in the 
situation illustrated in FIG. 1 this is only along a plane indicated by 
the arrow BD. Interference fringes are only observable along this plane, 
therefore, and thus a photographic film must be placed in this plane in 
order to record the fringes. 
The solution provided by the invention to this problem, to enable the 
production of blazed gratings having different blaze wavelengths from the 
same wavelength (and therefore the same grating pitch or fringe spacing) 
is achieved by compressing the beam 11 with respect to the beam 14. If, as 
shown in FIG. 2, the angles of intersection remain unchanged, the plane at 
which interference fringes can be observed is turned through an angle from 
the plane indicated by the line BD in FIG. 1 to the plane indicated by the 
line BD in FIG. 2. Since the angle of incidence of the two light beams, 
and the wavelength of the light, are the same, the interference fringes 
extend in the same direction as in FIG. 2, and have the same spacing 
(these being indicated by the broken lines within the quadrilateral 
A,B,C,D, as shown in FIG. 2), but the form of the quadrilateral A,B,C,D is 
modified by the reduction in width of the beam 11. 
A practical system for achieving this is shown in FIG. 3 in which light 
from a laser 15, which is in a narrow substantially collimated beam, is 
focused by a lens 16 onto a rotating diffuser 17 which is driven to rotate 
by a motor 18. Immediately behind the diffuser 17 is a stationary diffuser 
19 from which the diverging light beam, indicated 20, is directed towards 
a concave mirror 21 which is positioned at a distance corresponding to its 
focal length from the focal point of the lens 16, but inclined with 
respect to the axis of the light beam 20 so that the reflected light is 
directed by the mirror 21 as a collimated beam 22 towards a diffraction 
grating 31. 
The grating 31 is one in which the majority of the incident light energy is 
thrown into a zero order beam and a first order beam (24 and 23 
respectively), and as will be seen from FIG. 3 since the angles which 
these beams make with the plane of the grating are different, the width of 
the zero order beam and the first order beam are different. The zero order 
beam is directed towards a reflector 26, and then onto a reflector 32 
which directs the beam, now indicated 28, to interfere with a beam 27 
which is the light from the beam 23 reflected by a reflector 25. 
Again, the region of intersection is indicated by the quadrangle A, B, C, 
D, and the plane in which the interference fringes produced by the light 
beams 27, 28 are observable is indicated by the line BD. A grating blank 
29, carrying a photographically sensitive film 30, is placed so that the 
film lies in the plane indicated by the line BD. The direction of the 
interference fringes is indicated by the broken lines within the 
quadrangle A, B, C,D. As will be seen these extend at a small angle to the 
plane of the photographically sensitive material 30. 
Turning now to FIG. 4 there is shown a modification to the system 
illustrated in FIG. 3 for producing higher blaze angles. For this, the 
grating blank 29 carrying the photosensitive material 30 is located in a 
cell 34 at the intersection of the two beams 27, 28. The cell 34 contains 
a suitable liquid such as carbon tetrachloride, liquid paraffin, silicone 
oil or the like, and has two plane glass windows 35, 36 forming the two 
sides of the cell on which the light beams 27, 28 are incident, and two 
light absorbing or transmitting walls 37, 38. Again, the direction of the 
fringes is as in the system illustrated in FIG. 3, and is shown in FIG. 4 
by the broken arrow O. 
An alternative system for photograhically recording the fringes is 
illustrated in FIG. 5. In this system the substantially collimated beam of 
coherent light 22 is obtained as in the system of FIG. 3, by passing light 
from a laser 15 through a converging lens 16 focussed at a diffuser 17 
driven to rotate by a motor 18. The divergent beam 20 passing from the 
diffuser 17 is directed through a stationary diffuser 19 and impinges on a 
concave mirror 21 spaced from the focal point of the lens 16 by a distance 
equal to its own focal length and inclined in such a way as to reflect the 
parallel collimated beam 22 towards a beam splitter 38 which, in this 
embodiment, is in the form of a semi-reflecting mirror. The incident 
primary beam 22 is separated by the beam splitter 38 into two secondary 
beams 42, 43 which are incident, respectively, on two mirrors 39, 40. The 
mirror 39 reflects the incident beam 42, as beam 45 which is incident on a 
triangular prism 41 which, by refraction, serves to compress the incident 
beam laterally producing an output beam 46 the transverse dimension of 
which is reduced in relation of the beam 44 reflected by the mirror 40. 
The two secondary beams 44, 46 intersect over a region indicated by the 
quadrilateral ABCD and a grating blank 29 having a photosensitive layer 30 
thereon is placed such that the photosensitive layer lies in the plane 
indicated by the line BD where the two incident beams 44, 46 are exactly 
in register and, therefore, at which the interference fringes produced by 
the two beams are observable. 
Finally, FIG. 6 illustrates a simple system for obtaining diffraction 
gratings which a coarse pitch and high blaze angles. A collimated primary 
beam 22 is obtained in the same way as in the system of FIG. 5 and is 
incident on a diffraction grating 47 which throws the light into two 
orders, the respective beams being indicated 48 and 49. The beam 49, which 
is the second order beam, is expanded in relation to the beam 48, and the 
two beams 48, 49 are incident on respective mirrors 50, 51 which are so 
positioned that they reflect the incident beams, as beams 52, 53, closely 
past the grating, one on either side thereof, so that they intersect at a 
region indicated by the quadrilateral ABCD at a shallow angle to one 
another. In this case, since both beams are reflected only once from the 
beam splitting device, the two beams are not laterally inverted with 
respect to one another and accordingly the plane at which fringes are 
observable, that is where the two beams are exactly in register, extends 
generally transverse this bisector, as indicated by the line AC. The 
direction of the fringe is, in this case, not at a small angle to the 
plane at which fringes are observable, but rather at an angle approaching 
90.degree. to this plane. Again, a grating blank 29 having a 
photosensitive surface 30 is placed at the appropriate plane, in this case 
the plane indicated by the line AC. 
In all the systems described, the photosensitive surface of the grating is 
exposed for a given time, and then the photo-resist material is developed 
to a shape in accordance with the pattern of fringes recorded thereon.