Multichannel waveguide print head with symmetric output

In a preferred embodiment of the present invention there is provided a substrate having formed therein a plurality of optical waveguides. The waveguides are formed in a fanned out pattern with each alternate waveguide having a different width. At the fanned out ends of each of the waveguides there is coupled an optical fiber for transmitting light to the optical waveguide. In another embodiment of the invention, a substrate block is formed of glass, plastic, or X-cut LiNbO.sub.3 material and each waveguide in LiNbO.sub.3 is formed by diffusing strips of Ti or other suitable optical waveguide material into the substrate in a desired fan pattern and by forming an overcoat with a material, such as MgO for LiNbO.sub.3, which has an index of refraction that causes the output beam to be symmetrically formed. In yet another embodiment of the invention each of the optical waveguides is formed with a narrow width portion and a wide width portion with the narrow width portions of each of the waveguides being adjacent the wide width portions of a neighboring waveguide.

BACKGROUND OF THE INVENTION 
1. Field of The Invention 
The present invention is directed to an optical print head of the type that 
utilizes optical fibers and a light source to record images and text onto 
a recording medium, and more particularly to an improved optical print 
head which utilizes optical fibers coupled to closely positioned optical 
waveguides for improving the speed of recording. 
2. State of The Prior Art 
One method of increasing the writing speed of an optical printing system is 
to increase the number of channels used to transfer information onto the 
recording medium and to have them write in parallel. If for some reason 
the channels cannot be separated by the desired distance, the print head 
containing the channels can be tilted at an angle and the electronic 
signals modulating the light to the channels delayed by the amount needed 
to compensate for the tilt. Large tilt angles are needed if the spacing 
between the channels is too large and the accurate setting of the print 
head then becomes difficult. 
A patent of interest for its teaching is U.S. Pat. No. 4,389,655, entitled 
"Optical Device For Non-Contact Recording And Particular Facsimile 
Reproduction Of Images And Test" by P. Baues. In that patent, optical 
fibers are held in close proximity by the use of grooves in a supporting 
structure. Another approach for decreasing the spacing between each 
printing element is to form the printing elements from a fanned out 
plurality of closely spaced optical waveguides. The fanned out end of each 
of the waveguides is connected to an individual optical fiber. The 
opposite ends of the waveguides are closely positioned, at distances which 
are smaller than the diameters of the optical fibers. Such a structure is 
suggested in FIG. 4 of a paper by J. T. Boyd and S. Sriram entitled 
"Optical Coupling from Fibers to Channel Waveguides formed on Silicon" 
Mar. 15, 1978/ Vol. 17, No. 6/Applied Optics. 
A number of problems occur when channel waveguides are used in a print 
head, particularly when optical waveguides are formed in substrate mediums 
such as glass, plastic or LiNbO.sub.3 with each optical waveguide 
positioned in close proximity to its neighbor. One of the problems is that 
the Gaussian distribution of the intensity of the light from the end of 
the waveguide is skewed in a direction perpendicular to the surface of the 
waveguide. This is caused by the presence of the material forming the 
optical waveguides such as Ti which increases the index of refraction at 
its location. The index of refraction then decreases monotonically from a 
maximum at the surface to a bulk value (associated with the particular 
medium used) at a distance of several microns below the surface. This 
causes the optical beam, formed on the recording medium in this direction, 
to take on a skewed gaussian shape when in fact a symmetric gaussian shape 
is preferred. A second problem is cross talk which occurs because the 
elements forming the optical waveguide are close to each other. 
The first problem, namely the one caused by the difference in the index of 
refraction of the materials used to form the print head may be minimized 
when using LiNbO.sub.3 by diffusing a thin layer of MgO over the Ti. A 
paper which addresses a solution to this problem is authored by J. Noda et 
al and is entitled "Effect of Mg Diffusion on Ti-diffused LiNbO.sub.3 
Waveguides", J. Appl. Phys., Vol. 49, No. 6, June 1978. 
A suggestion of a solution to the second problem is set out in a paper 
authored by J. L. Jackel et al, and entitled "Nonsymmetric Mach-Zehnder 
Interferometers Used as Low-Drive-Voltage Modulators". In that paper it is 
stated that optical decoupling of the waveguides may be effected by 
differing the propagation constants of the two guides. One method for 
accomplishing this difference is to provide different widths for each 
waveguide. 
In order to provide an optical head having multiple waveguide channels it 
is necessary to address and to solve the aforementioned problems. 
SUMMARY OF THE INVENTION 
In a preferred embodiment of the present invention there is provided a 
substrate, preferably of glass, having formed therein a plurality of 
optical waveguides. The waveguides are formed, by an ion exchange process 
for example, in a fanned out pattern with each alternate waveguide having 
a different width. At the fanned out ends of each of the waveguides there 
is coupled an optical fiber for transmitting light to the optical 
waveguide. In another embodiment of the invention, the substrate is formed 
of LiNbO.sub.3 and each waveguide is formed by diffusing strips of Ti into 
the substrate in a desired fan pattern and by forming an overcoat of MgO 
over the diffused strips so as to provide an output beam of light from the 
non-fanned end of the waveguides. In yet another embodiment of the 
invention, each of the optical waveguides is formed with a narrow width 
portion and a wide width portion, with the narrow width portion of one of 
the waveguides being adjacent the wide width portion of a neighboring 
waveguide. 
In yet another embodiment of the invention, each of the waveguides is 
formed having an equal total area of narrow and wide portions so as to 
maintain the transmission characteristics of each waveguide substantially 
equal. 
From the foregoing it can be seen that it is a primary object of the 
present to provide on optical print head with increased writing speed. 
It is a further object of the present invention to provide an optical 
fiber/waveguide print head which forms output beams having gaussian shapes 
which may differ in the vertical and horizontal directions. 
Another object of the present invention is to minimize the cross-talk 
between optical elements of a multi-element optical print head. 
These and other objects of the present invention will become more apparent 
when taken in conjunction with the following description and drawings 
wherein like characters indicate like parts and which drawings form a part 
of the present specification.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1, wherein one embodiment of a print head 10 is 
illustrated connected to one end of a bundle of optical fibers 30. Each 
fiber in the bundle is connected at its opposite end to individual high 
intensity light sources 40, e.g. laser diodes. The print head 10 is formed 
from a block 12 of glass, LiNbO.sub.3 or plastic into which a plurality of 
channel waveguides 20 are formed. The waveguides 20 are positioned closely 
together at a printing end 26 and thereafter fan apart to make the 
connection with the optical fibers 30 at an end labeled 24. The print head 
10 functions by being scanned over the surface of a recording medium while 
the appropriate ones of the laser diodes 40 are energized so as to cause 
recording spots to appear on the recording medium. 
In the preferred embodiment of the invention the spacing of the channel 
waveguides 20, at the printing end 26, is 0.00907 mm. The spacing at the 
other end 24 is 0.0900 mm. The distance between the two ends is 20 mm. 
Each end of an optical fiber is centered with respect to the end of an 
associated channel waveguide and is connected thereto by means of an 
adhesive. A suitable adhesive is supplied by the Norland Company under the 
name Norland 61. However, a preferred adhesive is Lamdek U V Adhesive, 
Catalog No. 177-6921, obtainable from Dymax Engineering Adhesives, a 
division of American Chemical and Engineering Company, Torrington, CT. 
Other means may be used to optically couple the optical fibers to the 
optical waveguides e.g. mounting the fibers in etched v-grooves in the 
material 12 which grooves are aligned with the center of an associated 
channel waveguide. The channel waveguides 20 may be formed of diffused 
areas of Ti and the block 12 may be formed from an X-cut of LiNbO.sub.3 
material, or other acceptable material. The channels in LiNbO.sub.3 are 
formed photolithographically using approximately 300 to 400 angstroms of 
Ti that is diffused at approximately 1025 degrees C. for approximately 8 
hours. Mg ions are diffused into the LiNbO.sub.3 to decrease the 
refractive index at the waveguide surface. By proper adjustment of the 
amount of Mg that is deposited the waveguide's depth index may be 
symmetrised. One paper of interest for its teaching in this area is 
entitled "Titanium/Magnesium Double Diffusion Method for Efficient 
Fibre-LiNbO.sub.3 Waveguide Coupling" by K. Komatsu Electronics Letters, 
14th Aug. 1986, Vol. 22, No. 17, pgs. 881-882. 
FIG. 2, illustrates, in detail, one preferred embodiment of the invention. 
The channel waveguides 20 are formed on the surface of the block 12 with 
widths that alternate so as to minimize the cross-coupling of light 
between two adjacent waveguides. As an example, the width of the channel 
waveguides 20A are selected to be 3 .mu.m and the width of the channel 
waveguides 20B are selected to be 4 .mu.m. It is to be recognized that 
other widths may be used and that it is only necessary that adjacent 
waveguides not have the same width when positioned in close proximity. 
FIG. 3, illustrates a second embodiment of the invention wherein each of 
the channel waveguides 20 is formed with one section 20C of equal width 
and an equal length with it's neighbors. A second section of each 
waveguide, denoted 20D and 20E for alternate waveguides, is formed with 
different widths so as to minimize their cross-coupling. In the preferred 
embodiment of the invention the width of the sections 20C is 3.6 .mu.m, 
the width of sections 20D is 4.0 .mu.m, and the width of sections 20E is 
3.2 .mu.m. The spacing on centers, at the printing end 26, is 9 .mu.m with 
the spacing at the receiving end 24 being 90 .mu.m. 
Referring to FIG. 4, the print head 10 is formed with alternate ones of the 
waveguides 20 having a first section 20F which is narrow and a second 
section 20G which is wider. Intermediate to these waveguides are 
positioned waveguides having a wide section 20H and a narrower section 
20I. The transitions between the widths is selected such that the total 
area of each of the channel waveguides is the same. By forming each of the 
waveguides with equal areas the light path for each of the waveguides is 
maintained substantially constant. 
In FIG. 5 each of the waveguides 20 is provided with an equal width 
termination section 20K irrespective of the remainder of its width. For 
example, sections 20J and 20L are shown with widths that are smaller than 
and greater than the width of section 20K, respectively. This provides a 
degree of uniformity in the light pattern emitted from the ends of the 
channel waveguides. 
In summary, in order to reduce the cross coupling between channel 
waveguides the widths of adjacent waveguides are made non-alike. In order 
to increase the symmetry of the output light intensity from a channel 
waveguide a layer of MgO is deposited over the waveguide. 
While there has been shown what are considered to be the preferred 
embodiments of the present invention it will be manifest, to a person 
skilled in the art, that many variations of the present invention may be 
had in light of Applicant's teachings without departing from the essential 
spirit of the invention. It is intended, therefore, in the annexed claims, 
to cover all such changes and modifications as may fall within the true 
scope of the invention.