WO1990002348A1

WO1990002348A1 - Organic optical waveguides

Info

Publication number: WO1990002348A1
Application number: PCT/GB1989/000990
Authority: WO
Inventors: Clive Trundle; Martin John Goodwin; Neil Carr
Original assignee: Plessey Overseas Limited
Priority date: 1988-08-25
Filing date: 1989-08-25
Publication date: 1990-03-08
Also published as: GB2222272A; GB8820211D0; JPH03501895A; EP0389592A1

Abstract

An optical device comprises a substrate film (1) of a polymer material, a region (4) of the film including a doping additive effective to locally increase the refractive index of the polymer material and define a waveguide channel, in which the polymer material is a nonlinear optical material. This can enable a waveguide channel to be occupied entirely by nonlinear material so that the maximum optical effect can be obtained in a device which still has a robust mechanical construction.

Description

ORGANIC OPTICAL WAVEGUIDES

This invention relates to organic optical waveguides and it includes a method of fabricating waveguide structures. One construction of nonlinear optical waveguide is disclosed in

British Patent Application No. 2189624, published on 28th October 1987. This specifies the formation of a linear polymer film layer which is deposited on a suitable support from a solution in an organic solvent. A portion of the film surface is then treated with a nonlinear doping additive to cause a local increase in the refractive index value as compared with that of the adjacent parts of the film. This gives a nonlinear channel waveguide structure which can be used for many optical logic and signal processing applications.

The doping additive which is used for this purpose is required to exhibit a high degree of nonlinearity in the molecule. However, the bulk effect of this property becomes very much reduced by the presence of the polymer substrate which tends to dilute the nonlinearity of the dopant material which is impregnated therein. A high proportion of the waveguide channel volume is thus occupied by a linear type of material and this material is unable to make any contribution to the optical control of a transmitted signal.

The present invention was devised to provide an alternative way of constructing an optical waveguide, and one in which a substantial proportion of the bulk effect of a nonlinear material could be used. In addition, the choice of possible materials for the doping addit e mav be able to be made somewhat wider. According to the invention, there is provided an optical device comprising a substrate film of a polymer material, a region of the film surface including a doping additive effective to locally increase the refractive index of the polymer material and define a waveguide channel, in which the polymer material is a nonlinear optical material.

One class of suitable nonlinear polymer materials is the chiral. soluble polydiacetylenes. A particular example is 10,12- Docosadiyne-1.22-diol bis (S-(-)-methylbenzylurethane), which will be referred to hereinafter as 9SMBU.

The preparation and properties of various polydiacetylenes and the relevant substrate films have been disclosed in the specification accompanying patent application No. GB871451 L filed on 20th June 1987. Some examples of suitable doping additives include nitroanilines, such as 2-methyl-4-nitroaniline.

According to a further aspect, the invention comprises a method of constructing an optical device comprising the steps of taking a support body, depositing a nonlinear polymer material on a surface thereof to form a substrate film, said deposition being effected from a solution of the material in an organic solvent, depositing a masking material on said film surface, removing a portion of a masked area in a region where the impregnation of a doping additive is required, and immersing the masked substrate body in a solution of a doping additive to promote diffusion of said additive into the exposed parts of the surface film. The deposition stage may be carried out by a dip coating operation in which the support body is withdrawn from a surface of said solution or by a spin coating process, so that a film of good optical quality will be formed.

At the end of a suitable treatment time, the support body carrying the substrate film is taken from the doping additive solution. The remaining masking material may then be removed, if necessary, from the film surface to allow completion of the device construction.

By way of example, some particular embodiments of the invention will now be described with reference to the accompanying drawings, in which:

Figure 1 to 3 show different constructions of optical waveguide, an d,

Figure 4 is a graph showing the absorbance of material in the polymer film.

The construction of the optical waveguide of the invention begins with the provision of a support body which in the present embodiment was a glass plate. The plate was then required to be coated with a substrate film of the 9SMBU compound and the necessary quantity of this material had been prepared previously by the method disclosed in the aforementioned patent application No. 871451 1 .

The deposition of the substrate film on the plate was effected by withdrawing the plate from a solution of the 9SMBU in chloroform where the concentration of solids present amounted to about ten percent by weight, the rate of withdrawal of the plate was arranged to be 280 millimetres per minute. The deposit which resulted was in the form of a film having a thickness of 0.4 micrometres and it exhibited a red/orange colouration. This effect indicated that the dip coating process had provided some degree of order in the arrangement of the polymer molecules on the plate surface. The step of incorporating the required doping additive in the prepared film was carried out by a process of "solvent-assisted indiffusion" (SAID), which involves the contacting of the polymer substrate in a two-phase system comprising the dopant and a saturated solution of the dopant. The solvent acts as a transfer medium which allows dopant molecules to come into contact with the surface of the substrate at a constant rate while at the same time giving an even distribution over the substrate. Presence of solid dopant suspended in the solution ensures a constantly saturated solution and steady state conditions. Dopant molecules reaching the surface of the substrate face a choice between continued solvation and entry into the substrate to form a solid solution. Molecules which do enter the surface may then diffuse further into the matrix. The concentration of dopant should be sufficient to create a change in refractive index, the depth of diffusion being determined by the film thickness. This would give an effective waveguide with good nonlinear properties, but would avoid the formation of a thick waveguide supporting many modes.

In effect, a surface layer is formed in the polymer matrix of the substrate which is doped with an organic or organometallic compound to such an extent that there is a local increase in refractive index in the thickness of the doped layer. The technique of introducing a dopant into a polymer matrix by imbibition frcrr. a solution in an inert solvent can be controlled to enable the degree of change in refractive index to be varied. The main factors which determine these features of the resulting waveguides are:- (a) the nature of the dopant and of the solvent, (b) the solution concentration, (c) the treatment time, (d) the type of polymer matrix and the relative affinity of the dopant for the solvent and the polymer matrix, (e) the temperature of treatment, (f) the nature of any prεtreatment and (g) the presence of other substances, such as surfactants, in the solution which assist the absorption of the dopant into the matrix. In general, compounds which have a higher affinity for the polymer matrix will be imbibed more quickly.

In practice, the supported polymer film is normally immersed in a saturated solution of the dopant. A vessel is charged with a sufficient amount of solution to cover the substrate and heated with stirring to the required temperature. A condenser may be necessary if the temperature used is likely to cause excessive solvent evaporation. The temperature is best maintained by means of a thermostat-controlled bath or by reflux of the solvent. Sufficient dopant is added to give a saturated solution with a small excess and the system is allowed to reach equilibrium. The substrate is immersed in the solution for the required time. Provided the solution is stirred solid dopant particles in suspension do not normally affect the process, but if the dopant is molten at the temperature used, care must be taken that the substrate is not wetted directly by dopant droplets; otherwise an uneven indiffusion is obtained. Usually, swirling will cause any droplets to adhere to the sides of the vessel: the substrate mav then be introduced. Where uneven indiffusion does occur because of particles sticking to the substrate surface, the cause is often static electricity. Pretreatment of the substrate before immersion with an anti-static gun is normally effective in overcoming this problem. After immersion, the substrate may be cooled, washed and dried.

Choice of Dopant

The dopant selected should primarily be capable, on diffusion into the polymer substrate, of raising the refractive index. Therefore, any non-reactive material that will diffuse into the nonlinear polymer substrate forming a stable solid solution may be used providing it does not absorb the guided beam.

The dopant may be organic such as known nonlinear compounds such as mNA, oNA, C1NA, DAN or linear such as benzophenonε, substituted succinic anhydrides, for example fulgidεs or organometallic compounds, such as scandium tris heptafiuorodimethyl acetylacetonate.

Choice of Solvent The function of the solvent used in the process of this invention is to act essentially as a transport medium. It should be inert and should not dissolve or swell the substrate significantly. Although the dopant must be soluble in the -solvent in order to make transference possible, the solubility product should be small. There are two reasons for this. First, transfer of the dopant from the solvent to the substrate depends upon its relative solubility in the solvent and polymer. A low affinity for the solvent coupled with a high affinity for the polymer substrate should encourage dopant molecules at the interface to enter the substrate. Secondly, a high solubility in the solvent would mean that a large amount of dopant would be taken up, whereas a low solubility means that a saturated solution may be maintained with only a little dopant.

A high-boiling solvent is also desirable, both for high temperature diffusion of the dopant, and for operation at low temperature (where vapour loss is minimised).

It has been found that perfluorenated organic solvents are advantageous solvents in the process of the invention. Most organic compounds are sparingly soluble in these solvents at elevated temperatures and a wide range of such solvents is available. They are inert, non-flammable and non-toxic and are commerically available from 3M Corporation (FC range of perfluorenated solvents) and from ISC Chemicals Ltd. (PP range). However, the technique is not limited to the use of perfluorenated solvents other solvents such as high boiling alkenes for example tetradecane, and silicone oils such as the Dow Corning 200 series may be used.

The following Example will illustrate the process of the present invention :-

Example

The indiffusion was carried out by immersing the dip coated 9SMBU into a saturated solution of 2-methyl-4-nitroaniline (MNA) in PP9 (a commercial fluorocarbon solvent consisting predominantly of an isomεric mixture of perfluoromethyldecalin and boiling at 160°C ) at a temperature of 120°C. Using the technique described in the above Example, more complicated waveguide structures can be formed by predefining a mask on the surface of the matrix material by standard photolithographic processes. Apertures defined in this mask enable selected areas of the surface of the matrix material to be solvent indiffused, resulting in channel waveguide structures. A wide variety of active and passive channel waveguide structures can be formed by this technique, including curved waveguides, branching waveguides, directional couplers and interferometers. Typical waveguide structures are illustrated in Figures 1 , 2 and 3 and in the paper by Bennion et al published in the Radio & Electronic Engineer, Volume 53, No.9, pp 313-320, September 1983. Figure 4(a) shows a planar waveguide, 4(b) a curved stripe waveguide and 4(c) a Y- junction broadening stripe waveguide. In Figure 4(a), a plastics substrate 2 has a waveguiding surface layer 1 formed by solvent indiffusion of a dopant which raises the refractive index within the layer 1. The waveguide shown in Figure 2 has been formed with a 'J' shaped waveguide channel by confining the solvent indiffusion to the area 3. Figure 3 shows a 'Y^r shaped channel waveguide structure formed by similarly restricting the solvent indiffusion dopant to a defined pattern in the surface layer 1. Thus in the waveguide of Figure 3, for example, light directed along channel 4 will divide into two paths 5 and 6. The Bennion et al paper describes basic principles of the fabrication and use of organic optical waveguides. Figure 4 is a graph which shows the absorbance of the dopant material in the polymer film. The horizontal axis gives the wavelength in nanometres of light transmitted by the film sample and the vertical axis shows Absorbance. The sample related to the 9SMBU film supported on a glass substrate and the dopant material was MNA. The uptake of dopant material with increasing time of immersion in the solution was monitored by UV VIS spectroscopy and an absorption peak for MNA was expected at 384 nanometres with similar peaks for 9SMBU being at 482 and 530 (doublet line) nanometres.

The curves of the graph show the results for zero time of immersion of the film in the dopant solution together with those for immersions of one, two, four and eight minutes. Two minutes was observed as being the optimum time for giving a maximum uptake of the dopant material. The curve which is depicted as a dotted line gives the result for a maximum uptake of doping material in the film but after an ageing period of fourteen days has elapsed. This indicates the stability of the doping material in the polymer film layer.

Although the process of the present invention has been illustrated above with particular reference to the indiffusion of nitroanilines, alternative organic dopants may be used. One specific application of channel waveguide structures of this kind is to provide optical interconnections between semiconductor components as described, for example in our British Patent Application No. 2155194.

Multilayer waveguide structures can also be fabricated, for example by depositing one or more layers of polymer material onto the original substrate and subjecting the structure (after curing, if necessary ) to solvent indiffusion treatment prior to depositiπ z the subsequent layer of polymer. Spin-coating or dip-coating techniques are suitable procedures for depositing two or more superposed layers of polymer matrix.

As indicated above, channel waveguide structures can be 5 produced by using standard photolithographic methods to define desired patterns on the polymer film. Such methods have been used to define windows of the order of three to seven micrometres in width in a metallic mask (for example, aluminium) evaporated onto the polymer film. Solvent indiffusion into the substrate followed by 0 removal of the aluminium left well-defined channel waveguides.

Metallic layers deposited onto the polymer film by photolithography or other standard techniques need not be removed after solvent indiffusion but can be used as a means for applying an electrical field to the indiffused dopant causing alignment of polar dopant molecules. 5 This last-mentioned technique is particularly valuable in the production of optically nonlinear waveguide structures which make use of the large second and third order susceptibilities and intensity dependent refractive indices of many organic molecules. Waveguides fabricated with such organic molecules will be capable of performing 0 a wide range of nonlinear functions. They can take the form of the planar structures and the numerous channel waveguide structures, described above, as well as multilayer and distributed feedback structures.

Organic, nonlinear waveguide devices formed by solvent

"> <^*. indiffusion can perform a wide range of high speed optical signal processing functions including optical bistability. logic gating, inversion, optical limiting, pulse shaping, correlation and convolution by degenerate four-wave mixing, phase conjugation, harmonic generation, frequency mixing, frequency shifting and high speed all optical switching. Devices performing these functions will have widespread applicability in the fields of optical computing, signal processing and high data rate optical communications systems. Details of construction and operation of optical switching devices can be found in the review paper of _. G.I. Stegeman and C.T. Seaton published in Journal Appl. Phys. 58.. R57 (1985).

Claims

CLAIMS:

1 . An optical device comprising a substrate film of a polymer material, a region of the film surface including a doping additive effective to locally increase the refactive index of the polymer material and define a waveguide channel, in which the polymer material is a nonlinear optical material.

2. A device as claimed in Claim 1 , in w^;hich the said polymer material is a chiral, soluble polydiacetylene.

3. A device as claimed in Claim 1 or 2, in which the said polymer material is 9SMBU, as hereinbefore defined.

4. A device as claimed in any one of Claims 1 to 3, in which the said doping additive is an organic compound which is capable of entering the film surface by a diffusion process.

5. A device as claimed in any one of Claims 1 to 4, in which the doping additive is 2-methyl-4-nitroaniline.

6. A method of constructing an optical device, comprising the steps of taking a substrate body and depositing a nonlinear polymer material on a surface thereof to form a surface film, said deposition ^ being effected from a solution of said material in- ah organic solvent, depositing a masking material on said film surface, removing a portion of a masked area in a region where the impregation of a doping additive is required, and immersing the masked substrate body in a solution of a doping additive to promote diffusion of said additive into the surface film.

7. A method as claimed in Claim 6, in which said doping additive comprises a 2-methyl-4-nitroaniline compound.

8. A method as claimed in Claim 7, in which the 2-methyl-4- nitroaniline compound is added as a solution in perfluoromethyldecalin .

9. A method as claimed in any one of Claims 6 to 8, in which the diffusion operation is effected at a temperature in the range of 80°C to 160°C.

10. A method as claimed in Claim 9, in which the temperature range is 1 15°C to 125°C.

1 1 . A method of constructing an optical device, substantially as hereinbefore described.

12. An optical device substantially as hereinbefore described with reference to any one of the accompanying drawings.