Fiber optic diffraction grating maker

A compact and portable diffraction grating maker comprised of a laser beam, optical and fiber optics devices coupling the beam to one or more evanescent beam splitters, and collimating lenses or mirrors directing the split beam at an appropriate photosensitive material. The collimating optics, the output ends of the fiber optic coupler and the photosensitive plate holder are all mounted on an articulated framework so that the angle of intersection of the beams can be altered at will without disturbing the spatial filter, collimation or beam quality, and assuring that the beams will always intersect at the position of the plate.

BACKGROUND OF THE INVENTION 
This invention relates to a device for the manufacture of diffraction 
gratings, and more particularly to a device which combines laser beam and 
fiber optic technology with a flexible mechanical design to facilitate the 
production of high quality holographic diffraction gratings. 
A diffraction grating can be used to cause an incident light beam to be 
deflected and dispersed according to the well known principles of 
diffraction and interference. These gratings have wide usage in modern 
scientific devices such as monochrometers, spectrometers and lasers, and, 
in addition, mass-produced grating replicas which are selected for their 
beauty and novelty are found in art, advertising, and displays. 
The manufacture of diffraction gratings using the holographic exposure 
method is well known in the prior art. A photographically sensitive 
material known as photo-resist is coated on a substrate (the grating 
blank) and located in a position at which two coherent beams of light 
intersect to create a three dimensional array of light and dark regions 
known as interference fringes. After exposure and development using 
certain etching chemicals, the pattern of regularly spaced bars and 
furrows on the grating surface will be directly related to the shape and 
intensity of the exposing interference fringes. 
The production of interference fringes requires two coherent beams of light 
which 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 
the prior art this requires the use of a complex assemblage of lasers, 
spatial filters, beam splitters and mirrors precisely located and oriented 
so as to create beams of the proper size and quality. Generally, one sets 
up and aligns the optics for one particular type of grating, and because 
maintaining alignment of the various components is difficult, if any 
changes are required the apparatus must be largely rebuilt. 
Problems of alignment and realignment which are incurred in the prior art 
are especially difficult when producing specialized gratings such as 
crossed-line gratings, dual frequency gratings and holographic optical 
elements. A crossed-line grating is one that has furrows along both 
orthogonal x and y directions, and it is produced by exposing the grating 
blank and then rotating it 90.degree. and exposing it again. Precise 
alignment is essential. 
A dual frequency grating is produced by superimposing two different 
diffraction gratings of slightly different groove spacing, with one 
frequency being slightly compressed or expanded in relation to the other. 
The process is tedious, requiring two exposures at different beam angles 
without disturbing the spatial filtering, collimation or beam quality. 
Holographic optical elements (HOE) often require the use of beams with 
other than plane wave fronts to obtain lensing effects. One such element 
has a groove pattern consisting of concentric circles, resulting from 
exposure by one plane wave and one spherical wave. When light strikes such 
a groove pattern it will be affected much as if it had struck a lens, but 
the HOE is only a few microns thick, and thus provides significant weight 
savings compared to a lens. Considerable care must be taken to properly 
align the optical components. 
It is therefore a primary object of this invention to provide an improved 
apparatus for the manufacture of diffraction gratings. 
In the accomplishment of the foregoing object, it is another important 
object of this invention to provide an apparatus which is compact, 
portable, and permits flexibility in alignment. 
It is another important object of this invention to provide an apparatus 
which is convenient to use for the manufacture of specialized gratings 
such as crossed-line gratings, dual frequency gratings and holographic 
optical elements. 
Additional objects, advantages and novel features of the invention will 
become apparent to those skilled in the art upon examination of the 
following and by practice of the invention. 
SUMMARY OF THE INVENTION 
To achieve the foregoing and other objects, this invention comprises a 
novel improved compact and portable diffraction grating maker. The 
improved grating maker is comprised of a laser beam, optical and fiber 
optics devices coupling the beam to one or more evanescent beam splitters, 
and collimating lenses or mirrors directing the split beam at an 
appropriate photosensitive material. The collimating optics, the output 
ends of the fiber optic coupler and the photosensitive plate holder are 
all mounted on an articulated framework so that the angle of intersection 
of the beams can be altered at will without disturbing the spatial filter, 
collimation or beam quality, and assuring that the beams will always 
intersect at the position of the plate.

DETAILED DESCRIPTION OF THE INVENTION 
Diffraction gratings are used to disperse the frequency components in an 
incident light beam into a spectrum. The exit angle is a function of the 
beam incident angle and its wavelength. As depicted in FIG. 1, for a laser 
with a single wavelength of emission, the incident beam 20 is simply 
redirected by the reflection grating 21 into new beams at fixed angles. 
The multiple beams generated are called diffraction orders. The zero order 
0 is the "normal" reflection, where the angle of incidence equals the 
angle of reflection. The various output diffraction orders are designated 
.+-.1,.+-.2, etc. Similar effects occur with transmission gratings. 
The usual apparatus for the manufacture of holographic diffraction gratings 
is depicted in FIG. 2. An incident beam 32, which may be a laser beam, is 
split into two beams by a beam splitter 33 and mirror 34, producing 
mutually coherent beams 35 and 36. 
Spatial filters 37 and 38 serve to focus and filter beams 35 and 36 
respectively, reducing each beam to its TEM.sub.00 mode, and removing any 
intensity variations across the beam. The filtered beams 35 and 36 are 
then reflected by parabolic mirrors 39 and 40 to expose the photosensitive 
plate 41. 
The angle between beams 35 and 36 reflected from mirrors 39 and 40 is set 
coarsely to conform to the following formula: 
EQU sin.theta.=F.lambda./2 (1) 
where .theta. is the half angle between the beams, F is the spatial 
frequency of the desired grating in lines or grooves per mm, and .lambda. 
is the wavelength of the light source in mm. 
Once coarsely aligned, one method for fine adjustment is to insert in place 
of the photosensitive plate 41 a commercial reflective diffraction grating 
which has a groove spacing exactly twice that which is desired (i.e. 
spatial frequency half that desired). Beams 35 and 36 will interact with 
the reference grating and generate two or more diffracted beams. 
The zero order beam generated by incident beam 35 corresponds to the 
normally reflected beam, and will exit the grating more or less coincident 
with but counterpropagating relative to beam 36. Also produced are two 
first order beams +1 from beam 35 and -1 from beam 36. These first order 
beams will interfere, with the interference pattern being more complex the 
further the beam angles deviate from the ideal or the more aberrations 
which the beams have due to misalignment or other defects in the optical 
system. 
As the angles of incidence of beams 35 and 36 are adjusted and approach 
proper alignment the +1 and -1 diffraction orders are converged by lens 42 
to two bright spots on screen 43. Adjustment is achieved when the two 
bright spots are superimposed. Removal of lens 42 will reveal an 
interference pattern which should be made as coarse as possible for ideal 
alignment. 
One can now produce a diffraction grating having exactly twice the number 
of grooves per millimeter as the reference grating. This is accomplished 
by positioning a photosensitive material such as a photographic emulsion 
or a photoresist in place of the reference grating. The laser is shuttered 
and the recording material exposed for an appropriate time. 
If the arrangement of laser, beam splitter, spatial filters, and mirrors 
depicted in FIG. 2 is ad hoc, setting up and aligning the optics to 
produce a desired grating is onerous. If the components must be rearranged 
for multiple exposures as required for specialized gratings the apparatus 
must be largely rebuilt. 
FIG. 3 is a schematic diagram of the preferred embodiment of the present 
invention. A laser 50 directs a laser beam 51 to a laser-to-fiber optic 
coupler 52, and the beam emerges through single mode polarization 
maintaining optical fiber 53. The signal is then split by an evanescent 
wave fiber optic beam splitter 54 (also called a coupler) and the 
resulting two beams are routed respectively through optical fiber 55 to 
fiber mount 56 and through optical fiber 57 to fiber mount 58. 
The output end of optical fiber 55 is mounted on fiber mount 56 so as to 
direct its output light beam at a collimating mirror 59, and the output 
end of optical fiber 57 is mounted on fiber mount 58 so as to direct its 
output light beam at a collimating mirror 60. Collimating mirrors 59 and 
60 direct the collimated beams through a mask 61 to intersect in the 
location of a photosensitive plate holder 62, where interference fringes 
result and expose the photographic plate. 
Fiber mount 56 and collimating mirror 59 are rigidly fixed relative to one 
another on optical rail 63, and fiber mount 58 and collimating mirror 60 
are rigidly fixed relative to one another on optical rail 64. Optical 
rails 63 and 64 rotate in a plane about a common axis, designated in FIG. 
2 as a pivot pin 65, which serves as a hinge and is located directly below 
plate holder 62. Photosensitive plate holder 62 is positioned so that the 
plane of the plate is perpendicular to the plane defined by the optical 
rails 63 and 64, and intersects that plane at pivot pin 65. 
In the preferred embodiment, the laser 50 is a Lexel Model 95 4 watt Argon 
ion laser; any laser beam of adequate power, polarization, wavelength and 
coherence to permit exposure of the photosensitive material may be used. 
Using fiber optics, the laser 50 may be mounted remotely, using a fiber 
optic link (not shown). Mounting the laser 50 remotely will facilitate the 
use of a more powerful laser, if that is necessary in a particular 
application. Also, removing the laser 50 from the optical table may 
improve the product grating by eliminating vibration due to laser coolant 
flow. 
Shutter 67 lies between the output port of laser 50 and laser-to-fiber 
optic coupler 52; the shutter 67 is used to control the exposure of the 
photosensitive plate 62 to the laser light. 
Laser-to-fiber optic coupler 52 serves to focus the output beam of the 
laser 50 down to the 5 micrometer spot needed to enter optical fiber 53 
efficiently. In combination fiber 53 and laser-to-fiber coupler 52 perform 
a spatial filtering function which was provided in the prior art by 
spatial filters 37 and 38 depicted in FIG. 2. In the present invention, 
since the single mode fiber 53 has a core only 5 micrometer in diameter, 
only the TEM.sub.00 mode is captured by the fiber 53 and transmitted along 
its length. Thus, the beam entering fiber 53 has fewer transverse 
intensity variations and is therefore very clean. 
Fiber optic beam splitter 54 has a 3 dB (50/50) split and its function is 
to distribute the beam power equally between two polarization maintaining, 
single optical fibers 55 and 57. In doing so, the device does not distort 
the beam optical wavefront excessively, and does not disturb the 
polarization of the output at fiber mounts 56 and 58. 
Beam splitter 54 is variable which permits the adjustment of the relative 
intensity of the output beams. This is useful since different 
photosensitive materials may require different wavelengths for proper 
exposure. For example, photographic or silver based materials have good 
sensitivity at the 514 nm laser line, while photoresist materials require 
exposure more in the blue end of the spectrum (for example, the 488 nm 
line of the Argon Ion Laser). 
In the preferred embodiment, collimating mirrors 59 and 60 are off-axis 
paraboloid mirrors with a diameter of 6 inches and focal length of 25 
inches. The optical axis of the paraboloid is about 3 inches from the edge 
of the mirror. The diameters and focal length of collimating mirrors 59 
and 60 must be chosen to satisfy the requirements of the final collimated 
beam diameter. The use of an off-axis paraboloid eliminates certain 
wavefront distortions (aberrations) which are produced by the more 
commonly used on-axis parabolic or spherical collimating mirrors. 
Laser light exits the output end of fiber 55 at fiber mount 56, and the 
output end of fiber 57 at fiber mount 58 in a very clean wavefront due to 
the spatial filtering properties and the single mode nature of the optical 
fiber. The light expands at an angle determined by the Numerical Aperture 
of the fiber (NA=0.1 in the preferred embodiment). The output ends of 
fibers 55 and 57 must lie at the focal points of collimating mirrors 59 
and 60 respectively. In reflecting from collimating mirrors 59 and 60, 
light is both collimated (given a plane wavefront which implies that it is 
neither diverging nor converging) and redirected towards the plate holder 
62. 
As noted above, the relationships between optical rails 63 and 64, plate 
holder 62, and collimating mirrors 59 and 60 are fixed, except that rails 
63 and 64 are rotatable about a common axis. Pivot pin 65 serves as an 
axis for rails 63 and 64, so that if optical rails 63 and 64 are adjusted 
to any relative angle, the beams emanating from collimating mirrors 59 and 
60 will always intersect at the position of the photosensitive plate 
holder 62. 
Plate holder 62 can be rotated so as to permit exposure of the grating with 
grooves at arbitrary angles relative to the plate edge. Mask 61 in front 
of the plate holder permits generation of crossed-line gratings, and dual 
frequency gratings by multiple exposures. 
The present invention may be used to make holographic optical elements by, 
for example, replacing one of the collimating mirrors 59 and 60 with a 
plane or flat mirror. The photosensitive plate will then be exposed by one 
plane wave and one spherical wave, resulting in a groove pattern of 
concentric circles. 
FIG. 4 is a schematic diagram of an alternate embodiment of the present 
invention, showing collimating lenses directing mutually coherent beams to 
create interference fringes. The output end of optical fiber 55 is mounted 
on fiber mount 56 so as to direct its output light beam at collimating 
lens 68, and the output end of optical fiber 57 is mounted on fiber mount 
58 so as to direct its output light beam at collimating lens 69. 
Collimating lenses 68 and 69 transmit the collimated beams through a mask 
61 to intersect in the location of a photosensitive plane holder 62, where 
interference fringes result and expose the photographic plate. 
The benefits derived from use of the present invention are significant. 
Most importantly, because there is a fixed relationship between the fiber 
ends, the collimating mirrors and the region of intersection, the 
collimated beams will always intersect at the position of the 
photosensitive plate, and the angle of intersection of the collimated 
beams can be altered at will without disturbing the spatial filtering, 
collimation or beam quality, making the invention particularly useful for 
the manufacture of specialized diffraction gratings. 
In addition, the use of fiber optics for transmission and splitting of the 
laser beam virtually eliminates alignment problems in the spatial 
filtering, and splitting of the primary beam; movement of the fiber is 
immaterial as long as the positions of the fiber ends relative to other 
optical components is maintained. The use of fiber optics also permits 
remote location of the laser, reducing vibration due to coolant flow. 
Finally, the present invention can be made very compact for storage and 
transport by detaching the laser 50 and folding in the optical rails 63 
and 64. When this is done the device will occupy a space which is only 
16.times.46.times.12 inches. The device can be rapidly set up; the only 
realignment necessary is that between the laser and the laser-to-fiber 
optic coupler 52. 
The foregoing description of a preferred embodiment of the invention has 
been presented for purposes of illustration and description. It is not 
intended to be exhaustive or to limit the invention to the precise form 
disclosed, and obviously many modifications and variations are possible in 
light of the above teaching. The embodiments described explain the 
principles of the invention and practical applications and should enable 
others skilled in the art to utilize the invention in various embodiments 
and with various modifications as are suited to the particular use 
contemplated. It is intended that the scope of the invention be defined by 
the claims appended hereto.