Fabricating of integrated optics

Method for fabricating components for integrated optics are provided wherein a film of polystyrene doped with methyl red (PS-MR) on a glass substrate, is masked, e.g. with one or more metallic strips on a glass plate and such film is irradiated with, e.g. a UV lamp, tuned within the absorption band of such film, in air, to photobleach the film portions around channels covered by the masking strips and reduce the refractive index thereof below that of the unbleached film channels. The bleached film portions then provide reflective beam confining interfaces which define the unbleached channels and form waveguides within the film. The invention also provides for interfering two laser beams of like .lambda., at a coupling spot on the film, which beams are again tuned within the absorption band of such film, which bleach alternate lines or bars of such film to provide a phase grating therein. A waveguide can be inscribed in such film to optically communicate with such phase grating and other components can be added such as optical interconnects, to direct light on various paths into and out of such integrated optical film circuit. The thus-formed circuit is preserved by encapsulating the so-bleached film in a layer of, e.g. epoxy at a lower index of refraction than the unbleached film, while the glass substrate protects the reverse face of such film, which glass also has a lower index of refraction than the unbleached film so that the optical components of the invention, e.g. the waveguides, are enclosed on all sides by reflective interfaces of lower index of refraction to confine and channel transmitted light therein. The light can enter and exit the integrated optical circuit of the invention at a phase grating or where the waveguides reach the film edges. The invention takes advantage of photobleaching to change the index of refraction of portions of a thin film to define optical components therein and has located a class of films, including PS-MR, which, upon photobleaching, provide a considerable reduction in refractive index, enabling the definition of optical components in such film. The invention includes the methods of fabricating such integrated optics as well as the integrated optic products so-made.

BACKGROUND OF THE INVENTION 
1. FIELD OF THE INVENTION 
This invention relates to fabricating integrated optics, including optics 
in polymer films, particularly films of polystyrene doped with an azo dye. 
2. THE PRIOR ART 
Integrated optical circuits include optic components in a film, e.g., 
waveguides, optical interconnects, phase gratings and the like, which 
direct (or reflect) light beams in a desired direction. These components 
have been made in the past by a) ion exchange, a multi-step process 
wherein ions are exchanged in a glass substrate or b) ion milling, another 
multi-step process which requires a vacuum and an ion gun, both processes 
being relatively complex and cumbersome. 
In the photobleaching prior art, U.S. Pat. No. 4,677,049 to Gritting 
(1987), teaches interposing a photobleachable layer between a mask and a 
photo-resist layer and irradiating the layers through the mask. The 
exposed parts of the photobleachable layer bleach to form an in-situ mask 
on the photo-resist layer and enhances the contrast of the mask image on 
such photo resist layer. Such enhanced mask images on the photo resist 
layer are used for photolithography in the manufacture of integrated 
electric circuits. Also see German Patent DE 3346716 A1 to Wegner et al. 
which appears to disclose the bleaching of polydiacetlylenes into 
photo-resist layers to make masks therein. 
Also U.S. Pat. No. 4,808,285 to Chen et al. (1989) discloses the making of 
Y-couplers and gratings in polydiacetylene film by exposing same to a 
scanning electron beam or e-beam, which changes the index of refraction in 
such film to a certain depth. This is a cumbersome process which takes 
time and in which, variation in the scanning rate and/or beam intensity 
will vary the depth of penetration into such film. Further the scanning 
beam can define ragged diagonal edges in the Y-coupler 100 of FIG. 2, 
resulting in beam leakage thereat. Additional beam leakage can occur in 
the unirradiated core 310 below the Y-coupler pattern 400, as indicated in 
such FIG. 2. 
U.S. Pat. No. 4,270,130 to Houle et al. discloses the use of dyes in a 
record disk that is grooved by a laser beam that forms a deformation 
pattern therein, such dyes being deformed by ablation during groove 
formation so as to render it transparent to such recording laser beam. 
Such beam can now be used, (even at an increased power level) for playback 
from such recording without destroying the recorded deformation pattern. 
The prior art discloses various uses of an azo dye, e.g. methyl red, in 
other fields. For example, U.S. Pat. No. 4,124,390 to Kohn (1978), 
discloses the use of methyl red for dye toning of black and white, 
photographic silver images. U.S. Pat. No. 4,818,660 to Blanchet-Fincher et 
al (1989), discloses the use of methyl red in a photo-hardenable master, 
for rendering faithful proofs in the graphic arts. U.S. Pat. No. 4,360,606 
to Tobias et al. (1982) discloses photo-degradeable polymer compositions 
based on, e.g. polystyrene, which also includes an organic 
photosensitizer, such as methyl red. 
Accordingly, the above prior art makes no suggestion of employing an 
uncomplex process for forming clearly defined components for integrated 
optics. However, there is need and market for a process that can fabricate 
integrated optics including structures, circuits and components thereof, 
that is streamlined rather than complex and otherwise overcomes the above 
prior art shortcomings. 
There has now been discovered a simplified (and reduced temperature) 
process for fabricating the above optics by locating a suitable polymer 
and then selectively changing the index of refraction thereof to obtain 
such optics. 
SUMMARY OF THE INVENTION 
Broadly the present invention provides a method of fabricating components 
for integrated optics comprising irradiating a portion of a film of PS-AD 
on a substrate, with light tuned within the absorption band of such 
polymer, in air, to photobleach such film portion while applying lesser or 
no bleaching to at least part of such film adjacent such film portion, to 
reduce the refractive index of such film portion below that of such film 
part, to define an optical component in such lesser bleached portion. 
The invention further provides an integrated optic comprising a film of 
PS-AD on a substrate, which film has a photobleached portion and a lesser 
or unbleached part, adjacent said portion, said portion having a lesser 
index of refraction than said part to provide a reflective interface or 
boundary therewith and to define an optical component in said part. 
By films of "PS-AD", as used herein, is meant films of polystyrene doped 
with an azo dye. 
By an "azo dye" as used herein, is meant, methyl red, methyl orange, methyl 
violet, methyl yellow and similar azo dyes. 
By films of "PS-MR", as used herein, is meant, films of polystyrene doped 
with methyl red. 
By "methyl red" as used herein, is meant, e.g. 
2-[4-(dimethylamino)phenylazo]benzoic acid 
By "lesser or unbleached (film) part", as used herein, is meant a film part 
that is, e.g. under the edges of a mask and is less bleached than 
irradiated film beyond the outlines of the mask but may be more bleached 
than unbleached film including film well under such mask.

DESCRIPTION OF PREFERRED EMBODIMENTS 
Referring in more detail to the drawings, argon laser 10 directs a laser 
beam 12 through a focusing lens 14, which focuses a beam 16 onto a spot 18 
of a polymer film 20 mounted on a glass substrate 22. The film is 
irradiated a sufficient time to photobleach it, e.g. after several minutes 
of irradiation, the laser beam bleaches the film and increasingly passes 
therethrough and is received by a detector 24, as shown in FIG. 1. 
For purposes of the method of the present invention, it has been determined 
that a certain type of polymer film lends itself to the formation of 
integrated optics by the above photobleaching process. It has been found 
that films of PS-AD are suitable, of which PS-MR is highly suitable. That 
is, it has been found that thin film samples of PS-MR become transparent 
or bleach when irradiated with light tuned to the absorption band of the 
sample. In addition the concomitant reduction in the refractive index 
thereof is appreciable, as discussed below. 
A suitable light is UV light which is broad-banded and can include 
wavelengths, .lambda., of 200 nm up to 440 nm or more, including 
.lambda.=405 nm. 
A preferred composition range of PS-AD in such films, is 20 to 30 wt.% of 
an azo dye mixed with 80 to 70 wt.% of polystyrene. Relative to the graph 
of FIG. 5, 28.5 wt.% of methyl red mixed with 71.5 wt.% of polystyrene, 
was employed in a film about 1 um thick. 
The method of the present invention takes advantage of at least two 
scientific principles. That is, 1) according to the invention, a selected 
thin film is photobleached to reduce the index of refraction in a portion 
thereof and 2) the so-reduced portion or portions of the film, provide 
reflective boundaries or interfaces with the unbleached part or parts of 
the film so as to define said parts as optical components, including 
waveguides and gratings which are integral with such film. Such film can 
be photobleached in a pattern e.g. with portions bleached on both sides of 
an unbleached part, to define a channel or waveguide in such part. Other 
optical components including phase gratings, can be formed in such film, 
as more fully described below. 
By "thin film(s)", as used herein, is meant films that are from 0.2 to 5.0 
um. thick, with a preferred range of 0.5 to 1.5 um, including about 1 um 
thick. 
Here it is noted that the arrangement shown in FIGS. 1 and 4, is suited for 
examining the change of physical properties in such film, before and after 
photobleaching, of which, absorption changes are measured and shown in the 
graph of FIG. 5, as more fully discussed below. 
In an another embodiment of the present invention, UV lamp 30 directs a 
beam 32 through a collimating lens 34, forming a collimated beam 36, which 
irradiates a PS-MR film 38, mounted on a glass substrate 40, which film is 
mounted below two optical channel-defining metal masks 42 and 44 on a 
glass plate 37, as shown in FIG. 2. Here, after sufficient irradiation, 
the film 38 is photobleached on both sides of the masking elements 42 and 
44 reducing the index of refraction of the film, n.sub.f, and providing 
reflective interfaces for the unbleached film parts under the masking 
strips 42 and 44. These unbleached parts now have a higher index of 
refraction than the bleached film portions and thus define optical 
channels 80 and 82, as shown in the integrated film circuit 78 of FIG. 8. 
Such film circuit 78 is more fully discussed below. 
A light beam can enter a waveguide in a film through the film edge as 
discussed below with respect to FIG. 8. A light beam can also enter a 
waveguide in a film, coming in at an angle with the surface of such film, 
provided a coupling means or grating is provided in such film. In the 
prior art grating couplers have been formed by, e.g. ion milling of ridges 
and grooves either into the surface of the film above a channel or the 
surface of such film or substrate below a channel, which grating can 
couple a major portion of an incoming incident beam into the film channel, 
the rest of the beam being reflected or transmitted as is known in the 
prior art. Thus a grating 50 of ridges and grooves, having been milled 
into a glass substrate 52, is covered with a PS-MR film 54, as shown in 
FIG. 3. Such ridged and grooved phase grating is known as a surface relief 
grating. When a beam 56 of a suitable wavelength, e.g. .lambda.=700 nm, 
per the graph of FIG. 5, more fully discussed below, is directed at the 
grating 50 at a suitable incident angle .theta., most of the beam will be 
coupled into a waveguide 58 of the film, the remainder of the beam being 
reflected, absorbed or transmitted as noted above. 
When the film above the grating 50 is irradiated by a light beam 60 (per 
FIG. 4), tuned to the absorption band of such polymer film, that film 
portion is photobleached (and made more transparent), which induces a 
reduction in the refractive index of the PS-MR film. 
This drop in refractive index can be measured since the incident angle at 
which the above light beam 56 will couple into the waveguide of the film 
54, is now reduced to angle .beta. with the incoming beam moved to the 
position of arrow 61, as shown in FIG. 4. 
Thus the refractive index of the film is reduced with increasing exposure 
to irradiation, as shown in FIG. 4 and the coupling incident angle of the 
incoming beam to the grating 50, is accordingly reduced. 
As noted above, photobleaching of PS-MR films by the method of the present 
invention, renders film that was opaque to light at certain wavelengths, 
increasingly transparent, that is, the absorbance of such wavelength is 
decreased by irradiation of such film by light tuned within the absorption 
band thereof. Such change in absorbance is shown in the graph of FIG. 5 
wherein absorbance on the ordinate is plotted against wavelength on the 
abscissa and curve 64 is the absorbance of such film before irradiation 
and curve 66 is the absorbance of such film after photobleaching. It can 
be seen from the curve of 64 that such film is absorbant to light at 
wavelengths of about 400 to 600 nm, i.e. is opaque to such wavelengths. 
Accordingly such wavelengths cannot be transmitted in the unbleached 
channels or waveguides of the film employed in the present invention since 
they would be absorbed, as indicated in FIG. 5. However, light of 
wavelengths of 600 nm and above, can be transmitted in the (unbleached) 
film waveguides of the invention since the unbleached film is transparent 
thereto. However, the remainder of the film ,e.g. film 71 of FIG. 8, is 
bleached on both sides of the respective waveguides 80 and 82, to provide, 
as noted above, reflective interfaces or boundaries of lower indices of 
refraction to keep the transmitted light in the respective waveguide 80 or 
82. 
Returning now to FIG. 4, when the film 54, above the grating 50 is 
irradiated by laser beam 60 at, e.g. 514.5 nm to photobleach such film, 
the index of refraction thereof is progressively lowered for all 
wavelengths, including those above 600 nm. Thus the incoming light beam 
incident angle at a coupling grating, per FIG. 4, will be reduced for all 
wavelengths as said grating is photobleached per the method of the present 
invention. Accordingly progressive measurements of the reduction of 
incident angle of a selected wavelength, e.g. .lambda. =1064 nm, can be 
made to calculate progressive reductions in the index of refraction of 
such film, during irradiation, as discussed more fully in Example II 
below. 
Thus it has been found that the refractive index of PS-MR film, n.sub.f (of 
the composition described with respect to FIG. 5 above), when 
photobleached, e.g. at between 400 to 600 nm, (such n.sub.f) is reduced 
from 1.633 to 1.610 for a drop in n.sub.f of 0.020. 
In addition to defining optical channels by boundary photobleaching of such 
film, the invention provides other optical components, including a grating 
that does not require a physical reshaping, e.g. ion milling of film or 
substrate. That is, the method of the invention provides for 
photobleaching of a coupling grating in such film, herein denoted a phase 
grating. 
Thus laser diode 70 directs laser beam 65 through a lens combination 71 to 
produce a collimated light beam 72. The collimated beam 72 is split into 
two separate beams of equal intensity 74 and 75 by an optical beam 
splitter 73. Two mirrors 77 and 79 redirect the light beams to optically 
interfere at a spot 81 on PS-MR film 76 which has been deposited onto a 
glass substrate 78. The relative angle of the interfering beams is 
adjusted in order to define the periodicity of the constructive and 
destructive interference zones, which provide a phase grating of modulated 
refractive index 84, as shown in FIG. 6. 
Thus a light beam 88, incoming at a suitable incident angle .alpha., is 
coupled by the phase grating 84 into a waveguide in the film, as indicated 
by the arrow 90, shown in FIG. 7. 
However, the phase grating embodying the invention does not change its 
coupling angle by photobleaching in the manner described above with 
respect to FIGS. 3 and 4, since such photobleaching is detrimental to the 
phase grating of the invention, itself formed by photobleaching of two 
interfering beams, as noted above. 
An integrated optical circuit according to the invention, is shown in FIG. 
8, wherein PS-MR film 71 is mounted on glass substrate 92 and has lesser 
or unbleached waveguides 80 and 82, defined in such film 71. A phase 
grating 94 in the film 71 optically connects with the waveguide 82, as 
shown in FIG. 8. Preferably the phase grating 94 is fabricated in the film 
71, e.g. in the manner shown in FIG. 6, before the waveguides 80 and 82, 
which are then fabricated, e.g. per the method shown in FIG. 2. 
Also an optical fiber 104 connects with waveguide 82 and an optical fiber 
106 connects with waveguide 80 at the film edge 73, as shown in FIG. 8. 
For refractive index durability, the integrated optic circuit 78 is 
encapsulated With an epoxy layer 108, which has a refractive index below 
that of the lesser or unbleached waveguides 80 and 82 as does the glass 
substrate 92, so that such waveguides are enclosed on all sides by 
reflective interfaces of lower indices of refraction, as indicated in FIG. 
8. 
In operation, a laser diode 110 directs through a lens 112 a laser beam 114 
into the phase grating 94 to couple into the waveguide 82 and thence to 
the optic fiber 104, as shown in FIG. 8. Concurrently or separately, laser 
diode 116 directs through lens 118, laser beam 120 into waveguide 80, at 
the edge of the film 77 and thence to optic fiber 106, as shown in FIG. 8. 
Accordingly, the methods of the present invention provide for fabricating 
an integrated optical circuit of considerable compactness and versatility. 
Thus the invention provides methods to produce integrated optical 
components including waveguides and phase gratings by irradiating PS-AD 
films, directly, with or without masks through photolithographic 
processing. The photobleaching process renders transparent such films and 
induces reduction in the refractive index thereof as noted above. After 
exposure the photobleached product can be stabilized by encapsulating the 
material as noted above. 
The following examples are intended to illustrate the invention and should 
not be construed in limitation thereof. 
Example I 
PS-MR was obtained commercially. Methyl red and polystyrene (28.5 wt.% MR & 
71.5 wt% PS) were dissolved at 10% by wt. into chlorobenzene by standing, 
e.g., 6 to 10 hours at e.g. 20.degree. C. The molecular weight of the 
polystyrene was 20,000. The solution was spin-coated onto fused silica 
substrates at 1000 to 2000 RPM, for 60 seconds to form thin films thereon. 
The thin film thicknesses measured 0.5 to 1.5 um thick. The film samples 
were allowed to sit overnight (8 to 10 hours) to facilitate the 
evaporation of excess solvent. 
The spin-coated PS-MR samples were exposed to a collimated beam from a UV 
lamp (at a power density of 16 mW/cm.sup.2 for 3 hours. Absorption spectra 
of the above film for various exposure energies are displayed in FIG. 5. 
This Figure shows that the absorption band in the range of 400 to 600 nm, 
is reduced after irradiating such film samples for the 3 hour period. 
The refractive index of the bleached sample was found to be reduced by 0.02 
from the refractive index of the unbleached PS-MR film as more fully 
discussed in Example II below. 
When a mask was placed in close contact over the film in the path of the UV 
beam ,e.g. as shown in FIG. 2, the observed transition boundaries between 
bleached and unbleached regions of the film were sharp. 
EXAMPLE II 
A 365 nm period grating was ion-milled into fused silica substrate slides 
through a photoresist mask to form relief gratings. The 10% by weight 
solution of PS-MR/chlorobenzene was spin-coated onto such slides, as 
described in Example I, above and allowed to sit until the excess solvent 
evaporated (8 to 10 hours), to provide solution cast film on such relief 
grating slides such as shown in FIG. 3 hereof. Again the films were 0.5 to 
1.5 um thick. 
Coupling angles were measured using a computer-controlled rotation stage 
and were obtained for three TE polarized waveguide modes at 0.6328 um or 
632.8 nm. The three coupling angles were used to measure the film 
thickness and refractive index of the PS-MR film both before and after 
photobleaching thereof. 
The coupling spot on the film above the grating was irradiated with a 
focused laser beam at 514.5 nm in the manner shown in FIG. 4 hereof. 
Before bleaching, the refractive index of such grating samples were found 
to be 1.633 (measured at .lambda.=0.6328 um). After irradiating the 
coupling spot as described above, the refractive index of such film (due 
to a change in incident angle, as discussed above with respect to FIG. 4) 
was found to be 1.610 so that a change in refractive index of 0.02 was 
obtained. Such change (in n.sub.f) is sufficient to write channels and 
phase gratings in films of PS-MR in the manner shown in FIGS. 2 and 6 
hereof. As indicated above, the channels are written by irradiating and 
photobleaching films of PS-MR with portions thereof being masked to 
provide unbleached channels or waveguides, as discussed above. The phase 
gratings are formed by directing interfering laser beams to a coupling 
spot on such film, as previously discussed. 
EXAMPLE III 
A phase grating with period .lambda.=1.7 um is formed in spin-coated PS-MR 
thin film. This is done by directing a pair of interfering argon laser 
beams at a coupling spot on the film, each at an opposite angle of 19 
degrees with the film surface, at .lambda.=514.5 nm, with an intensity of 
30 mW for 300 seconds. Thus by the method shown, e.g. in FIG. 6, a phase 
grating is quickly and readily inscribed in such film. 
Thereafter, a channel or waveguide connecting with such grating is formed 
in the PS-MR film by covering a portion thereof connecting with such 
grating, with a metalized mask and exposing the film with an ultraviolet 
source(HTG LS-60 UV light source system), for 15 minutes at about 20 
W/cm.sup.2 in the manner indicated in FIG. 2. 
Thus, in the manner shown in FIG. 6, a phase grating is readily and quickly 
inscribed in such polymer film, followed by installation of a connecting 
waveguide (e.g. per FIG. 2) to quickly provide integrated optical 
components per the method of the present invention. 
As noted above, the invention provides for photobleaching PS-AD film to 
provide integrated optics at low cost. 
Such films are preferably deposited on transparent substrates such as 
glass. Such films can be irradiated per the method of the invention by 
laser beam, white light or UV light depending upon the application 
thereof. Of course gratings are formed in such film by interfering two 
laser beams at a coupling spot on the film, as discussed above. 
The benefit of the photobleaching of such film is that it lowers the 
refractive index of such film on both sides of the unbleached channel so 
as to keep an applied or coupled light beam in such channel by reflectance 
at the interfaces thereof. And gratings are added to such integrated 
circuit as discussed above. 
Per the graph of FIG. 5, such unbleached channels will accept and not 
absorb wavelengths greater than 600 nm which render such integrated 
optics, including phase gratings, waveguides and optical interconnects, 
highly suitable for communication wavelengths, which are within the range 
of, e.g. 1300 to 1500nm. 
As noted above, various lights can be employed to bleach such film provided 
it be tuned within the absorption band of such film. Thus, looking at FIG. 
5, it can be seen that a UV beam, including .lambda.s of 405 to 440 nm or 
more has suitable wavelengths to bleach such film. 
Of course as the channels are lesser or unbleached portions of the film, 
the light transmitted therein must be of a wavelength greater than 600 nm 
to render such channels transparent thereto, as discussed above. 
Thus photo-induced bleaching can occur by exposing spin-coated PS-MR films 
in open air, to light tuned to within the adsorption band of the polymer. 
This effect is believed caused by oxidation of the PS-MR film. This effect 
can be quite useful because refractive index changes of 0.020 (or more) 
allow integrated optic components and circuits to be formed directly, by 
proper exposure of such polymer film. The novel methods of the invention 
also provide novel products, i.e. the integrated optics including the 
components thereof, such as phase gratings and optical channels or 
waveguides. Encapsulating the exposed film after removal of residual gas 
therein, stabilizes the integrated optical products from further 
bleaching. 
Thus the methods of the invention can lead to low cost, low temperature, 
single step fabrication of integrated optical patterns for integrated 
optics, optical interconnects, non linear, integrated optics and passive 
linear, integrated, optical circuits. In addition there are applications 
in the area of low-cost polymer, read-write heads for optical data storage 
.