ORGANIC OPTICAL WAVEGUIDES

Abstract: Optical waveguides and a method of forming waveguides by diffusing an organic dopant into a plastics substrate. A method for producing organic waveguides is disclosed which comprises treating a transparent plastics substrate with a solution of an organic dopant in an inert solvent under conditions of temperature and treatment time such that a surface layer is formed having a refractive index which is greater than that of the bulk substrate.

Description: ORGANIC OPTICAL WAVEGUIDES This invention relates to optical waveguides and includes a method of fabricating waveguide structures. In principle, optical waveguides comprise a thin film or layer of material supported on a substrate of lower refractive index. A ray of light may be propagated within the thin film of higher refractive index material. The ray is confined by total internal reflection at the thin film/substrate interface and at the film/air space boundary. Waveguides are also used for roles additional to merely guiding light. For example, by combining the waveguide with a non-linear element, structures can be produced which are useful in optical logic and signal processing application. Traditionally, waveguides have been constructed from inorganic materials such as glasses which may be doped with inorganic materials, such as metal or semi-conductor compounds. In order to produce a device which combines waveguiding and non-linear optical properties, it has generally been the practice to grow a single crystal layer onto a suitable base substrate. Such techniques are laborious and complex. In contrast, the use of organic materials for waveguide construction has been considered to suffer from disadvantages such as lack of stability, insufficient purity, poor optical properties and the difficulty of controlling the incorporation of dopant materials. According to the present invention there is provided a method of producing an organic waveguide which comprises treating a transparent plastics substrate with a solution in an inert solvent of an organic dopant which is absorbed into the surface of the substrate, under conditions such that a surface layer is formed having a refractive index which is greater than that of the substrate. The process of the present invention enables a polymer substrate having the necessary optical properties, (including transparency , clarity and good surface properties), to be doped with a sufficient amount of a suitable organic compound to produce a local change in the refractive index of a surface region of the polymer. Provided that the resultant refractive index of the surface layer is greater than that of the bulk polymer, the resulting structures can be used as a waveguide. Moreover, by selecting organic dopant compounds which possess non-linear optical properties, a non-linear waveguide can be produced in a single production step. In essence, the process of the present invention, which we have termed "solvent-assisted indiffusion" or (SAID), involves the contacting of the polymer substrate in a two-phase system comprising the organic 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 m 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. Ideally, a large dopant concentration should be achieved near the surface but not penetrate far into the bulk polymer. This would give an effective waveguide with good non-linear properties (depending on the nature of the organic dopant), but would avoid the creation of a thick waveguide supporting many modes. In effect, a surface layer is formed in the polymer matrix forming the substrate which is doped with the organic 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 from a solution in an inert solvent can be controlled to enable the degree of change in refractive index and the depth of penetration of the organic compound to be varied. The main factors which determine these features of the resulting waveguide are:- (a) the nature of the organic compound 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 pretreatment and (g) the presence of other substances, such as surfactants, in the solution which assist the absorption of the organic dopant into the matrix. In general, organic compounds which have a higher affinity for the polymer matrix will be imbibed more quickly and to a greater depth in the substrate. Higher temperatures, higher concentration and longer treatment times all contribute to a greater degree of penetration of the compound into the polymer matrix. In practice, the plastics substrate 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 thermostatted bath or by reflux of the solvent. Sufficient dopant is added to give a saturated solution with a small excess and the system allowed to equilibrate. 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 may 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. In general, the waveguides produced exhibit low loss, good surfaces, little scatter (due to the diffuse nature of the edge of the dopant boundary) and may support one or more modes depending on the depth to which the dopant is diffused. Choice of Substrate In view of the intended purpose of producing a waveguide it is necessary to use a substrate polymer which has good optical properties. Clear, amorphous polymers of good thermal and optical stability are called for, preferably with hard surfaces which can be polished to a flat surface. The polymer substrate may be selected from any polymers which have good optical properties and into which the dopant may be diffused. Examples of polymers which may be selected are polycarbonates, polyesters, polystyrene, vinylidene ^ difluoride polymers and polyalkyl acrylates and methacrylates. The polymer material which is currently preferred is CR 39, poly diethylene glycol bis (allyl carbonate), which is obtainable from P.P.G. Limited as a curable polymer. CR 39 has shown the greatest uptake of the dopant materials so far tested. This may be partly explained by the fact that when cured, CR 39 is a cross-linked polymer having a loosely packed network of polymer chains allowing more space for the entry of dopant molecules. Preparation of optically clear flat sheets in CR 39 polymer is described in U.S. Patent No. 2,542,386. Other factors which influence the choice of the polymer include the ability of the polymer to be cut and polished. In general, cross-linkable polymers such as CR 39, can be polished to provide satisfactory thin flat substrates. Polymers which can be shaped, e.g. by extrusion or by moulding or casting are also advantageous. Thus, for example, polyvinylidene difluoride and copoly ers of vinylidene difluoride with trifluoroethylene can be extruded or moulded and also cast from solution in common solvents, such as acetone and di methylformam ide. Vinylidene difluoride polymers exhibit piezoelectric and pyroelectric properties when films of the material are poled (see British Patent No.1,589,786). These properties can be of value in connection with alignment of dopant materials as described subsequently herein. Another factor which should be taken into account in selecting the polymer material is its behaviour in contact with the solvent. Polymers which are dissolved or swelled excessively so as to cause distortion in the substrate are to be avoided. Choice of Dopant The dopant selected should primarily be capable, on diffusion into the polymer substrate, of raising the refractive index. In general, we prefer dopants which exhibit intrinsic non-linearity. Advantageously, the dopants are selected from organic compounds with high values of quadratic or cubic hyperpolar isabili ty. A large value for quadratic or hyperpolar i s abi 111 y , which generally implies a corresponding high value for cubic hyperpolarisability, is easily measured (see Schmidt et al Appl. Phys. Lett. 25, 458 (1974). The organic dopants which are most effective tend to be compounds having large dipole moments. For example, the dopants may be selected from aryl compounds having a ino, nitro, hydroxy and/or alkoxy subst tuents, such as anilines and aniline derivatives. Specific examples of this class of compounds are, m-nitroani line (mNA), o- mtroaniline (oNA), 2-chloro-4-nitroanilme (C1NA) and 1- di ethylamino-2-acetamιdo-4-nιtrobenzene (DAN). Dopants may also be selected from compounds having a heterocyclic nitrogen-containing ring, such as pyrazoline derivatives, and from compounds being extended conjugated systems extending between dipoles, such as azo dyes. The book entitled "Non-linear optical properties of organic and polymer materials" edited by D.J. Williams and published 1983 by the American Chemical Society as No.233 in the A.C.S. Symposium series may be consulted for additional examples of compounds and polymers which exhibit non-linear optical properties. Chapter 3 of this book written by Robert J. Twieg and Kauti Jain may be consulted for further examples of compounds which exhibit high beta values . 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 transferance 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 perfluorinated 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 commercially available from 3M Corporation (FC range of perfluorinated solvents) and from ISC Chemicals Ltd. ( P range). The following Examples will illustrate the process of the present invention:- Examples A piece of cured CR 39 sheet material abo.ut 1 mm thick was immersed in saturated solution of m-nitroaniline (mNA) in a perfluorinated hydrocarbon sold by 3M Corporation under the trade name FC 72. The CR 39 sample was refluxed (56°C) in the solution for 10 minutes. After removal and drying, the optical properties of the resulting waveguides was tested, light being coupled into the waveguide by prism coupling. Similar pieces of cured CR 39 sheet were treated with l-dimethyl-2-acetamido-4-ni trobenzene (DAN), ortho- nitroaniline (o-NA) and 2-chloro-4-nitroaniline (C1NA). The process parameters, depth of diffusion and number of modes is shown in Table 1 below:- TABLE 1 Material Solvent* Temper¬ Immer¬ Surface Number (see text) ature sion n of time modes-)- C s DAN FC40 155 15 0.001 2 oNA FC40 155 15 0.01 5 C1NA FC40 155 30 0.006 8 NA FC72 56 600 0.-005 2 *FC series, 3M Corporation " t " Measured at 633 nm wavelength Concentration refractive index profiling was achieved by an optical method, determining the refractive index profile from measurements of the mode propagation constants. A typical profile (for C1NA) is shown in Figure 1, and was determined at 633 nm using the method described by J.M. White and P.F. Heidrich, Appl. Opt. 1976, 1_5_, pp 151-155. Waveguiding quality was good with losses of only a few dBcm ~ . Depths of a few microns were used, with surface refractive index changes of 0.001 to 0.01 with respect to the undoped substrate. Non-linearity was investigated using an argon laser at 514.4nm. The response of a DAN waveguide is shown in Fig. 2; there was a reduction in coupling efficiency as input light intensity was increased. This was recoverable by realignment of the beam, as shown for the highest intensity. These results give an exceptionally large intensity-dependent index of ~0.5 (MWcm~ 2 )~ , which was confirmed by experiments with DAN samples mounted in a Fabry-Perot cavity. These experiments revealed an associated time constant of ~2ms, which implies a thermal origin, unrelated to any effect. An almost identical response was obtained with m-NA waveguides. Similar experiments were carried out using the pulsed output from a passively Q-switched Nd:YAG laser of 1.06 jum wavelength. Fig. 3 shows the result obtained with an o-NA waveguide, similar to that described above but with a much _*7 lower non-linear coefficient of refractive index of ~10 (MWcm ~) 1, and a much faster response time, comparable with the laser pulse rise time of 5ns. Using the technique described in the above Examples, 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 ind ' iffused, 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 4(a), (b) and (c) and in the paper by Bennion et al published in the Radio & Electronic Engineer, volume 53, No. , 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 4(b) has been formed with a 'J' shaped waveguide channel by confining the solvent indiffusion to the area 3. Figure 4(c) shows a 'Y' 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 4(c), for example, light directed along channel 4 will divide into two paths 5 and 6. The Bennion et al article describes basic principles of the fabrication and use of organic optical waveguides. Although the process of the present invention has been illustrated above with particular reference to the indiffusion of nitroanilines, other organic dopants may be used, especially compounds having extensively delocalised T -electron systems. Dipolar organic compounds having extended conjugated systems tend to display greater non- linear optical effects. They also tend to diffuse less within the substrate and therefore form more stable waveguide structures. For example, a number of ortho- nitro, or ho-cyano-phenyl, and ortho-methoxy pyrazolines have been investigated and shown to solvent indiffuse with CR 39 plastics to form waveguides. The following specific pyrazoline derivatives have been successfully incorporated into CR 39 plastic using perfluorinated hydrocarbon solvents. i ) ( II ) ( III ) 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.85 05363 (Publication No.2155194). Multilayer waveguide structures can also be fabricated, e.g. 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 depositing 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 structure can be produced by standard photolithographic methods to define desired patterns on the plastics substrate. Such methods have been used to define windows of the order of 3 to 7 urn in a metallic mask (e.g. aluminium) evaporated onto the plastics substrate. Solvent indiffusion into the substrate followed by removal of the aluminium left well-defined channel waveguides. Metallic layers deposited onto the plastics substrate 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 indif fused dopant causing alignment of polar dopant molecules. This last-mentioned technique is particularly valuable in the production of optically non-linear 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 a wide range of non-linear 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 , non-linear 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. Stiger an and CT. Seaton published in Journal Appl. Phys. 58, R57 (1985).

Claims: CLAIMS 1. A method of producing an organic waveguide which comprises treating a transparent plastics substrate with a solution of an organic dopant in an inert solvent under conditions such that a surface layer is formed having a refractive index which is greater than that of the bulk substrate. 2. A method according to claim 1 in which the organic dopant is an organic compound having intrinsically non-linear optical properties. 3. A method according to claim 2 in which the organic dopant is selected from compounds exhibiting large dople moments. 4. A method according to any one of the preceding claims in which the organic dopant is an aryl compound having amino, hydroxy, nitro and/or methoxy substituents . 5. A method according to any one of claims 1 to 3 in which the organic dopant includes a nitrogen-containing heterocyclic ring having aromatic character. 6. A method according to claim 4 in which the organic dopant is a nitroaniline or a derivative thereof. 7. A method according to any one of the preceding claims in which the affinity of the dopant for the solvent is lower than its affinity for the plastics substrate so as to aid in the diffusion of the dopant into the plastics substrate. 8. A method according to any one of the preceding claims in which the solvent is a high boiling inert solvent which does not swell the plastics substrate. 9. A method according to claim 8 in which the solvent is one or more fluorinated hydrocarbons. 10. A method according to any one of the preceding claims which includes the step of masking the surface of the plastics substrate prior to contacting the surface of the substrate with the solution of the dopant, thereby defining a pattern of doped areas m the surface of said substrate. 11. A method according to claim 10 in which the substrate is formed with a metallic mask and an electrical field is applied to areas between said metallic mask portions during treatment of said substrate with said solution of the organic dopant. • 12. An organic waveguide which comprises a transparent plastics substrate having a doped surface layer having a refractive index which differs from that of the bulk plastics substrate and contains an organic dopant dispersed in the surface layer of the plastics matrix. 13. A non-linear organic waveguide according to claim 12 wherein the surface layer exhibits incident intensity dependent birefringence. 14. A waveguide according to claim 13 which is in the form of a channel waveguide and the organic dopant is substantially confined to surface layers of the channel portions. 15. A waveguide according to any one of claims 12 to 14 which is in the form of a laminate, the plastics substrate being an element in the laminate. 16. A waveguide according to claim 15 in which a material which is impermeable to the organic dopant is used to form a layer which defines the depth of the doped layer . 17. A waveguide according to any one of claims 12 to 16 in which the plastics substrate comprises a polycarbonate, a polyester, a vinylidene difluoride polymer a polyalky lac ry la te or methacrylate or polystyrene. 18. A waveguide according to claim 17 in which the plastics substrate comprises a polymer of diethylene glycol bis(allyl carbonate). 19. A Fabry-Perot resonator which contains within its cavity a waveguide structure as claimed in any one of claims 12 to 18.

Inventors: BENNION IAN, BRETTLE JACK, GLENN ROBERT, GOODWIN MARTIN JOHN, TRUNDLE CLIVE, BENNION, IAN, BRETTLE, JACK, GLENN, ROBERT, GOODWIN, MARTIN, JOHN, TRUNDLE, CLIVE
Applicants: PLESSEY CO PLC, THE PLESSEY COMPANY PLC
Application year: 1987
Main classification: G02B6
IPC codes: C08J7, C09K9, G02B6, G02F1