Planar waveguide film structure for an integrated optical system

730~ The present invention relates to a planar film waveguide and/or components for the integrated optical system consisting of a carrier substrate, a film applied thereon and guiding the optical waves and at least one optically effective coating (overlay) which is arranged on the film and which is either weakly or strongly dispersive, whereby the refractive index ~ the carrier substrate is smaller than that of the film and the refractive index of the film is smaller -than that of the coatincJ.

Such a planar film waveguide with integrated lenses and a prism is known from the Canadian patent 1,108,903 of MacDonald, issued September 15, 1981.

The lenses are thereby formed from an optically effective coating with low dispersion which is applied to a film guiding the optical waves and the coating forming the prism has a high dispersion.

The desired dispersive behaviour of the coatings and of the film is achieved by means of specific materials such as, for example, As2S3 for a high dispersion and Ta2O5 for a low dispersion.

This is said to be a so-called material dispersion.

An exact adjustment of the desired dispersion values of the optically effective layers is achieved only with great difficulty through the composition of the materials.

The invention is therefore based on the object of providing a planar film waveguide and/or components of the kind named at the beginning, the dispersive behaviour of which can be brought very precisely to the desired values in order to thus be able to realize integrated components with exact optical properties.

Thus, in accordance with a broad aspect of the invention there is provided planar film wave guide structure for an integrated optical sys-tem consisting of a carrier substrate, a film applied thereon for guiding optical waves and at least one optica:lly effective overlay, which is arranged on t~le film and is weakly dispersive, wherein the refractive index of the carrier substrate is smaller than that of the film and the refractive index of the film is smaller than that of the overlay, characterised in that, for minimum dispersion in the case of a refractive index (ns) of the carrier substrate o:E 1.4 and a refractive index (nF) of the fi.lm o:E 1.51 as well as a refractive index (n~) of the overlay of 1.53 and a ratio o:E ~he thickness (d) oE the overlay to the thickness (D) oE the .Eilm oE 1:10, the thickness (d) of the overlay is chosen to be about 0.65 times the wavelength~ of the optical radiation so that a film paramater V = ~_ .d ~ nA ~ nS assumes the value of approximately 2.7.

In accordance with another broad aspect of the invention there is provided planar film wave guide structure for an integrated optical system consisting of a carrier substrate, a film applied thereon for guiding optical waves and at least one optically effective overlay, which is arranged on the film and is weakly dispersive, wherein the refractive index of the carrier substrate is smaller than that oE the film and the refractive index of the film is smaller than that of the overlay, characterised in that~ for minimum dispersion in the case of a refractive index (ns) of the carrier substrate of 1.4 and a refractive index (nF~ of the film of 1.51 as well as a refractive index (nA) of the overlay of 1.53 and a ratio of the thickness (d) of the overlay to the thickness (D) of the film of 1:5, the thickness (d) ~3~ 6 of the overlay is chosen to be about 0.8 times the wavelength~ of the optical radiation so that a film par~meter V = ~ .d n~\ - nS2 assumes the value of approximately 3Ø In accordance with another broad aspect of the invention there is provided planar f:ilm wave guide structure for an integrated optica:l system consisting oE a carrier substrate, a film applied thereon Eor guiding the optical waves and at least one opticnlly eEEective overlay which is arrange(l on the film and us strongly dispersive, ~herein the reEractive index of the carrier substrate is smaller than that of the film and the reEractive index of the film is smaller than that of the overlay, characterised in that, for maximum dispersion in the case of a refractive index (ns) of the carrier substrate of 1.4 and a refractive index (nF) of the film of 1.41 as well as a refractive index (nA) of the overlay of 1.68 and a ratio of the thickness (d) of the overlay (A) to the thickness (D~ of the film of 1:10, the thickness (d) of the overlay is chosen to be about 0.34 times the waveleng.h ~ of the optical radiation so that a film parameter 2~ d ~n 2 n 2 assumes the value of approximately 2Ø B~IEF DESC~IPTION OF TIIE DR~WING Figure 1 is a cross-sectional side view of one preferred embodiment of a waveguide film structure according to the invention.

Figure 2 is a plan view of a waveguide component according to the invention.

Figure 3 is a cross-sectional side view of another - 3a 3~ preferred embodiment of the invention.

Figures 4-6 are diagrams illustrating the behavior of various embodiments of the invention.

Figure 7 is a cross-sectional side view of still another preferred embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT .. ...

Figure 1 is a cross-sectional view of a waveguide film member 3b ~3~3~ constructed according to the invention. It includes a substrate S having the index of refraction nS and serving as the carrier for a film F through which the lightwaves are essentially conducted. Film F has a thickness D and an associated index of refraction nF. In the illustrated embodiment, an optically effective coating A which is relatively thin is placed onto film F. Coating A has a thickness d and an index of refraction nA. This coating A, if dimensioned properly, gives the illustrated waveguide ~ilm low dispersion.

Coating A can be applicd to film F to extent in the longitudinal direction of the Ellm strlp, i.e. perpcllclicular to the planc of ~igure 1.

Furthermore, coating A can be completely embedded into film F so that the exposed surface of coating A is flush, i.e. its surface forms a continuous plane with, the surface of ilm F.

In the illustrated embodiment, coating A is only partially embedded in film F.

If the following relations are established, the resulting dispersion behavior of the member is uniform over the broadest possible frequency band:

0.05 D < d < 1.00 D nS ' nF ' nA According to a modification of the illustrated example, coating A can of course also be designed exactly like film F therebelow.

Within the scope of the above conditions it is possible to make the waveguide film together with its coating either low in dispersion or highly dispersive.

In the latter case, the waveguide film with coating may be used in a film prism for spectral separation and may be employed in multiplexers or demultiplexers.

The coating A to be designed exactly like film F there below means that it can also have exactly the same thickness as the film there below.

Figure 2 shows a particular embodiment of a device according to the invention. A film F is again applied onto a substrate S as in Figure 1.

In order to produce components _ ~ _ ~3~ which influence the propagation of light in the desired manner, one coating A of this embodiment is designed on film F in the form of a lens Ll. Further in Figure 2 a coating A as shown in Figure 1 is provided in the form of a prism P. The above mentioned limit values apply for the parameters of each component. To again restrict the transverse spreading of the lightwave after it passes the prism P, a further lens L2 identical to lens Ll is placed onto the arrangement shown in Figure 2. Thus, a wave deflectorr multiplexer or demultiplexer can be constructed on the waveguide ilm F with elements Ll, P and L2.

The light signal to be processed at a point SE may be fed into the arrangement by a suitable transmitter.

Within the arrangement, the path of the waves is indicated by lines extending between components Ll, P and L2.

At the exit point E of the arrangement, the individual channels of the transmitted frequency band which have been fed in at SE can be coupled out.

Figure 3 is a longitudinal sectional view of a prism coupler in film technique according to the invention.

The basic film Fl having the index of refraction nl is again applied to à carrier substrate S having the index of refraction ns.

Film Fl has a thickness a.

It is given such dimensions thàt the waveguide film has the lowest possible dispersion.

At a suitable location, the basic film Fl is interupted by a , for example, lens-shaped section Ll as the coating.

This section includes a lower film portion having a thickness al and an index of refraction nll which clearly produces the film for the coating and an upper portion forming the coating, having a thickness dl and an index of refraction _ 5 _ ~'~ ~ .. .. .

~23~ nld. The lens portion, L1 projects above the film F1. At the right-hand end of the illustrated ~aveguide film Fl, a further film embedded partially in the film waveguide Fl and having -the shape of a prism creates a prism Pl with the index of refraction nd and a film thickness dp. The thickness of the waveguide film Fl underneath the prism Pl is ap.

According to the invention the structure in Figure 3 can be dimensioned as follows:

The substrate S has the index reEraction ns = 1.40.

The basic film Fl with index oE refraction nl = Nl = 1.41 and the thickness a = - 4.1 X has an effective index of refrac~ion neffl=1.407 for the fundamental wave.

According to the invention the lens-film is designed to have low dispersion, i.e. with V = 3 in Figure 5 the dispersion is X.dneff = 0.01 while the effective index of refraction for the dA fundamental filmwave is neffl = 1.51.

To obtain these characteristics, the coating with nld = ND = 1.53 should have the thickness dQ = D = 0.8X and the underlying film with nQl = 1.51 has the thickness al = A = 5 D = 4A.

Yor the prism Pl to be used for spectral decomposition of the through passing wave, a high dispersive structure according to Figure 6 is chosen.

If the coating with an index of refraction nd = ND = 1.68 has the thickness dp = D = 0.34X and if the remaining underlyincJ film has the thickness a = A = 10 D = 3.42, then, according to Figure 6, the parameter V has the value V = 2 and the dispersion is as high as ~dneff = 0.16 in the prism region.

d,~ The effective index of refrac-tion in the prism is n ffp= 1.503.

Due to the different dimensioning possibilities, it is thus possible to produce a low dispersion waveguide film, for example for use in broad-band lenses.

Figures 4 and 5 show diagrams of the dispersion curves of a film structure in -- 6 - dependence on the film parameter V for the fundamental lightwave. The wave length ~ is plotted on the perpendicular axis, multipled by -the derivation of the effective refractive index neff of the fundamental film wave from the wave length.

The lower portion of each curve shows a relatively broadband range wherein the dispersion has a low, approximately constant value.

This structure is suitable for use as Eilm lens or for a low dispersion Eilm prism in a wave deflec-tor.

If the present invention is to be used for film prisms having high dispersion, the film parameters V of the arrangement are selected so that a highly dispersive film structure results.

E`igure 6 shows a dispersion curve which has a broad maximum.

The indices of refraction of the illustrated examples correspond to those of glasses or transparent plastics as they are customarily used for the assembly of such film structures.

The legends employed in the diagrams of Figures 4 through 6 have the following meaning:

Nl index of refraction nF of film F in Figure 1 ND index of refraction nA of coating A in Figure 1 NS index of refraction nS of sustrate S in Figure 1 Film thickness A- thickness of film F in -the region below coating A in Figure 1 Film thickness D-thickness d of coating A in Figure 1 Lambda wavelength of the optical radiation ln free space DN/DLAMBDA derivation of the effective index of refraction N of the fundamental film wave from the wavelength of the optical radiation in free space r the letters D preceding N and LAMBDA indicate the mathematical -- 7 - ~, operation of differentiation V film parameter U as defined in Figures 4, 5 and KD wave number times thickness of coating A the wave number is 2~ divided by the wavelength lambda of the optical radiation in free space - 7a 73~6 According to the invention the basic structure of a coated film on a substrate can be designed to have either low or high dispersion. As can be secn from the dispersion characteristics for the specific embodiments of the invention in Figures 4 and 5 for low dispersion the film parameter V = 2~ D ~ must have values in the range from 2.5 < V < 3.5 for these embodiments ~i~ meaning that for thc embodiment of Figure 4 the coating should have a thickness in the range oF 0.6A < 1) ~ 0.9 ~ where as the underlying film should be A ten times as thick meaning a film thickness in the range of 6 ~ < ~ < 9 ~.

For the embodiment of Figure 5 in order to achieve the lowest possible dispersion over a wide spectral range the coating should be D = 0.8 ~ thick while the underlying film should have a thickness of A = 5D = 4 ~.

In contrast to this a design of the basic structure according to the invention for high dispersion requires in general a relatively lower value of the film parameter V.

For the specific embodiments the dispersion characteristic of which appears in Figure 6 the film parameter V should be V = 2 meaning that for this embodiment the coating should be D = 0.3~ ~ thick and the underlying fi.lm sllould have a thic~ness of A = 10 D = 3.~ ~.

All three embodiments with dispersion characteristics according to Figures 4, 5 and 6 respectively have the same refractive index nS = 1.~ of the substrate.

The embodiments for low dispersion have a film with the refractive index nl = nF = 1.51 and a coating on top of the film with a refractive index nD = nA = 1.53.

In contrast to these index values the embodiment for high dispersion has a film with the lower refractive index of nl = nF = 1.41 but a coating with the higher refractive index of ~3730~ nD = nA = 1.68. Although different in refractive index the substrate as well as the film and coating materials can be made and deposited out of glass with the refractive index adapted to the required values by choosing a suitable mixture of glass constituents.

Figure 7 is a cross-sectional side view of a wave~uide film member constructed according to the invention. The film F has a thickness D and an associated index of refraction nF. It is deposited on a substrate 5 having the index of refraction nF. The coating A has a thickness d and an index oE refraction n~\ and is completely embedded into film F, so that the exposed surface of coating i~ is Elush, i.c. its surface forms a continuous plane with tlle surface of film P.

It will be understood that the above description of the present invention is susceptiblc to various modifications, changes and adaptations, and the same are intended to be comprehended within the meaning and range of equivalents of the appended claims.