GB2205767A

GB2205767A - Coated optical waveguides

Info

Publication number: GB2205767A
Application number: GB08714311A
Authority: GB
Inventors: Stephen Mann; George Elliott
Original assignee: General Electric Company PLC
Current assignee: General Electric Company PLC
Priority date: 1987-06-18
Filing date: 1987-06-18
Publication date: 1988-12-21
Anticipated expiration: 2007-06-18
Also published as: GB2205767B; GB8714311D0

Abstract

A method of producing an organic waveguide which comprises depositing on a substrate a solution comprising an inert solvent, an organic polymer and a non-linear optical organic material and eliminating said solvent to obtain a surface layer having a refractive index which is greater than that of the substrate.

Description

022CAN I c-oP'r 1 CAL WAVEGUIDES This invention relates to optical waveguides and particularly, but not. exclusively is concerned with a method of fabricating optical waveguides in organic materials.

Typically optical wavegtlides 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.

The transmission of data over optical fibres is now an established technology in the communications industry. For the encoding, transmitting, decoding and processing of signals on a light beam a variety of electro-optical devices are required.

Typical devices include linear modulators, phase shifters, beam deflectors, mixers and harmonic generators. Waveguides are in many types of electro-optic device in which the light signal is subjected to modulating fields.

For modulation of the light signal, the waveguide should be formed within a non-linear optical material. The guide should be of a higher refractive index than the surrounding medium in order to confine the light signal by total internal reflection. A popular material to date has been lithium niobate, in which the waveguide is formed by the indiffusion of titanium metal at 9000 10000 C, which produces material with a higher index of refraction. Such a waveguide need only be a few microns in depth and width; the length may be of the order of 1 to lOmm, depending on the interaction length required.

The second-order and third-order non linear optical effects, hereinafter referred to as NLO, required for the devices have in the past been exploited only in inorganic materials, such as lithium niobate, cadmium sulphide or potassium dihydrogen phosphate. However, more recentLy it has been discovered that some organic compounds give NLO effects. The molecules of these organic compounds are invariably highly polar, contnin an ele(^Lron-withdrawing group At one end of the molecule and an electron-donating group at the other end. These materials can be utilised as single crystals, which if 2nd order NLO effects are to be obtained should have a non-centrosymmetic structure.

However, organic compounds present a number of problems in utilisation for practical electro-optic devices (i) They are less stable than the corresponding inorganic materials, so that purification and crystal growth from the melt is more difficult.

(ii) The growth of crystals of good optical quality from solution is difficult, because of interaction and inclusion of the solvent, resulting in defects and scattering centres in the crystal.

(iii) When single crystals are obtained they are structurally weaker than inorganic crystals, as they are held together mainly by weak Van der Waals forces. Thus, cutting and polishing samples is more difficult.

The present invention seeks to provide an improved method of using these organic compounds in production devices and demonstrates a technique of device construction.

According to one aspect of the present invention there is provided a method of producing an organic optical waveguide which comprises depositing on a substrate a solution comprising an inert solvent, an organic polymer and an NLO organic material and eliminating said solvent to obtain a surface layer having a refractive index which is greater than that of the substrate.

According to another aspect of the invention there is provided a method of producing an organic optical waveguide which comprises depositing on a substrate a solution comprising an inert solvent, an organic polymerisable material and an NLO organic material and eliminating said solvent to obtain a surface layer having a refractive index which is greater than that of the substrate.

The invention also provides an organic optical waveguide incitiding n substrate and a surface lager comprising an organic polymer nnd an organic NLO material such that the surface layer has a ret-ractive index which is greater than that of the substrate.

The invention further provides a device incorporating a waveguide as defined above.

The invention also provides a method of producing an optical device in accordance with the first aspect of the invention.

The invention also provides a method of producing an optical device in accordance with the second aspect of the invention.

Organic polymers are good constructional materials and many can be prepared in a high optical quality. They therefore provide a good host material for the incorporation of organic SLO compounds. Thin layers, a few microns in thickness, can be deposited from an organic solvent containing the polymer and a controlled quantity of the NLO organic dopant.

A wide range of materials may be employed to form the substrate. Preferred substrate materials for supporting the surface layer include glass (R.T. 1.55), where R.I. is the Refractive index, or methylmethacrylate (R.I. 1.49). To obtain a guide, the polymer film must have a higher refractive index than the substrate, therefore using a glass substrate, suitable polymers are polystyrene (R.I. 1.59), poly (o-chlorostyrene) (R.I. 1.61), poly (ethyleneterephthalate) (R.I. 1.64) and polycarbonates (R.I. 1.58-1.65 depending on composition). With a substrate of poly-methylmethacrylate, other polymers are also suitable, for example, nitrocellulose (R.I. about 1.51) and polyacrylonitrile (R.I. 1.51).

In a preferred embodiment the solvent is evaporated to leave a clear glassy film without any precipitation or crystallisation of the NLO compound. In order to achieve this the following criteria must be observed in the selection of materials: (i) The polymer should be highly soluble in the solvent.

(ii) The NLO organic compound must also be highly soluble the solvent.

(iii) The 8'l.O organic compound must be highly soluble in the polymer itself.

rt is preferable to trav a high concentration of NLO compound in the film as a low concentration of a few percent will confer little practical advantage even if the NLO compound shows good NLO effects. In tests concentrations up to about 65% by weight have been obtained.

Suitable non-linear organic compounds include, 3-methyl-4nitro-pyridine oxide (PON), (I); 4-(1-(2-hydroxymethyl) pyrrolidinyl) nitrobenzene (NPP), (II); 2-methyl-4-nitroaniline (MNA), (III);2-(N-methylbenzyl) amino-5-nitropyridine (MBA-NP), (IV); 2-(pyrrolidinyl)-5-nitroacetanilide (PAN), (V); 2-(N, N dimethylamino)-D-nitroacetanilide (DAN), (VI)

l'reforably the polymer should be used with u solvent or combination or solvents in which a clear solution is obtained and pr.fernbly the polymer should be used with a solvent or combination of solvents which will evaporate to provide a clear glassy film.For example, polystyrene will dissolve in chloroform or a chioroform-toluene mixture, while preferred solvents for the potycarbonates are ortho-dichlorobenzene, methylene chloride and dimethyl-formamide. For nitrocellulose, the preferred solvent is a 3:1 mixture of ethylacetate and butanone.

These solvents have been found to be effective in dissolving many of the organic NLO materials and give a high concentration, up to 65% W/W, of the organic NLO material in the final film.

The preparation of an organic waveguide according to the invention is illustrated in the following Examples: EXAMPLE 1 2g of polystyrene and 2g NPP were dissolved in 40ml chloroform or 40ml methylene chloride. This stock solution was diluted 2 to 10 times with solvent, depending on the thickness of film required. The solution was flowed onto a glass substrate in the quantity of about lml per 40cm2 of surface, and allowed to evaporate slowly. The final glassy film contained 50X W/W of NPP.

EXAMPLE 2 1.05 Bis-phenol A Polycarbonate (Malrrolon 2400) and195 MBA-NP were dissolved in 40ml O-dichlorobenzene. This stock solution was diluted 2 to 10 times with solvent, depending on the thickness of film required. The solution was flowed onto a glass substrate in the quantity of about lml per 40cm2 of surface, and allowed to evaporate at a temperature of 90 -110oC. The final glassy film contained 65% IV/W NBA-NP.

fs R Bis-phenol A Polycarbonate and 2g MBA-NP were dissolved in 50ml of methylene chloride. This stock solution was diluted 2 to 10 times with solvent, depending on the thickness of film required. The solution ras flowed onto a glass substrate in the quantity of about Iml per 40cm2 of surface, and allowed to evaporate slowly. The final glassy film contained 50% WW MBA-NP.

Examples 1, 2 and 3 when using a dilution of 3 times with solvent give polymer films of thickness in the range of 3 to 4 microns.

In another embodiment of the invention, films are also prepared by spinning the substrate. The undiluted solution is flowed onto the substrate, which is then spun at about 500 rpm to produce a uniform film. The solution concentrations and spinning speed can be varied to obtain the desired film thickness.

The films as prepared in the above Examples contain polar molecules of the NLO material which are randomly orientated within the polymer matrix (i.e. the sum of the individual molecular vectors of the polar molecules is zero). The refractive index of the polycarbonate films being typically about 1.62. In this form they are suitable for 3rd order non-linear optical applications such as Kerr effect devices, phase conjugate reflectors, or 3-wave mixing. To obtain 2nd order non-linear effects for frequency doublers, mixers or electro-optic modulators, some orientation must be imposed on the molecules of the NLO material.In a preferred method this is obtained by applying a high electric field to the film while it is held above its softening temperature, so that the polar NLO molecules can rotate into alignment (i.e. the vectors along the axes of polarisation in the molecules are substantially parallel and in the same sense) . Preferably, the organic polymer and organic compound layer are deposited on two electrodes. The temperature of the layer is then raised to its softening point and a D.C.

electric field applied to produce orientation of the molecules of the organic compound. The device is then cooled rapidly to fix the orientation of the molecules in position, with the electric field still applied.

A preferred device structure to achieve this alignment of molecules will now be described by wny of example with reference to the accompanying drawings, in which Figure 1 is a plan view of the device, showing two metallic parallel electrodes which are deposited on a substrate with a gap of 5-30 microns between them.

Figure 2 is a side view of the device, shown in Figure 1.

Figure 3 is a plan view of a modified form of the device.

Figure 4 is a side view of the device as shown in Figure 3.

The two electrodes 2a and 2b are prepared on a glass substrate 1 by vacuum evaporation of copper or gold to a thickness of 3 to 4 microns to form a metallised area.

Alternatively, a thinner layer of the metal can be evaporated and the thickness built up by electroplating. The metallised area is preferably 10mum long X 5mm wide. Electrical connecting leads 3a and 3b connect the two electrodes to a power supply. A gap 4 of about 15film wide is etched longitudinally in the metal, using known photolithographic techniques, to give the two separate electrodes. For example, the size and shape of the electrodes is determined by the device. For example, the size and shape of the electrodes and the deposition of a ground plane on the opposite side of the substrate are employed to obtain the correct terminating impedance for the modulating signal source used with the device (e.g. 50 ohms).

A layer 5 of bis-phenol A polycarbonate (Makrolon 2400)/MBA NP, 50/50 is deposited on the substrate to a thickness of 3-4 microns, as described in Example 3 to form an intimate coating.

The structure is then heated to 1200-1300C to bring the polycarbonate layer to its softening point and an electric field of 600-800V is applied across the electrode gap 4 in order to align the molecules of the organic compound. The temperature is maintained for 3-5 minutes and then the assembly is rapidly tooled with the electric field still applied.

n example of an optical device in accordance with the invention is shown in Figures 3 and 4. The optical organic waveguide comprises a substrate 1 and a surface layer 9 comprising an organic polymer and an organic NLO material. Two electrodes 2a and b connected via connecting leads 3a and 3b to a power source, not shown, are arranged with one electrode either side of the waveguide to form an active waveguide gap. Light is coupled in and out of the waveguide by an optical fibre 6, which is positioned in a groove on the substrate. The two electrodes are covered by a thin glass plate 8 which gives an improved top surface to the waveguide, The temperature of the device is raised to 120 -130 C which is the softening temperature of the NLO material within a press, not shown.During this pressing an electric field is applied to bring the molecules of the NLO material into alignment. At the same time the optical fibre coupling is introduced into the device. In operation light from a laser beam is passed through the active waveguide gap and, if required, a modulating signal is applied to the device by applying a voltage across the electrodes. Using this device a second harmonic coefficient of the order of 10-6 esu is obtained. Light may also be coupled in and out of the waveguide using a prism.

In cases where a monomer is readily obtainable in a convenient form, for example, styrene monomer, it is possible to use the monomer to prepare the coating. This has the advantage of the orientation of the molecules of the NLO material is more rapid and a smaller electric field is necessary to align the molecules than was required if the molecules of the NLO material are aligned in a polymeric layer. The monomeric film is polymerised after alignment by the application of heat and/or U.V. light. The following Example illustrates the formation of the coating using monomers: EXANIPI,E Sl 2g of high purity styrene monomer and 2g of MBA-NP were dissolved in 40ml chloroform. The soLution was diluted 3 times with chloroform and flowed onto a glass substrate, provided with metal electrodes as previously described, using a quantity of Lml per 40cm2 of surface. The liquid fiLm was allowed to evaporate slowly inside a light-tight enclosure. Once the solvent had evaporated (30-40 minutes), an electric field was applied across the electrodes, using 200-500V. D.C. After 2 minutes, the film was irradiated from a strong U.V. lamp with the field still applied. After 20-30 minutes irradiation, an aligned polymer film was obtained.

Shaped waveguide structures could be obtained in this way by using masking techniques during the U.V. illumination and afterwards dissolving away the unexposed monomer in a suitable solvent.

Claims

CLAIMS:

1. A method of producing an optical waveguide which comprises depositing on a substrate a solution comprising an inert solvent, an organic polymer and an organic non-linear optical material and eliminating said solvent to obtain a surface layer having a refractive index which is greater than that of the substrate.

2. A method of producing an optical waveguide which comprises depositing on a substrate a solution comprising an inert solvent, an organic polymerisable material and a organic non-linear optical organic material and eliminating said solvent to obtain a surface layer having a refractive index which is greater than that of the substrate.

3. A method according to Claim 1 or 2 whereby the solvent is eliminated by evaporation.

4. A method according to Claims 1, 2 or 3 whereby the percentage concentration of the non-linear optical material in the surface layer is up to 65% W/W.

5. A method according to Claims 1, 2, 3, or 4 whereby the solvent is chosen so that a clear solution is obtained.

6. A method according to any one of claims 1 to 5 whereby the solvent is chosen so that it will leave a clear, glassy film when eliminated.

7. A method according to any one of claims 1 to 6 whereby the solution is deposited on the substrate by flowing the solution onto the substrate, the substrate being spun to produce a uniform film.

8. A method according to any one of claims 1 to 7 whereby the non-linear optical material comprises molecules which are polar.

9. A method according to claim 8 whereby the method includes raising the temperature of the said layer and applying a d.c.

electric field under such conditions as to align the polar molecules of the organic non-linear optical material and subsequently lowering the temperature to fix the mdlecules in alignment.

10. A method according to claim 9 whereby the temperature of the layer is raised to 1200-130C for 3-5 minutes and an electric field of 600-800V is applied.

11. A method according to claims 9 or 10 when dependent on 2 whereby the method includes polymerising the said organic polymerisable material after the alignment of said polar molecules.

12. A method according claim 2 or any one of claims 3 to 7 when dependent thereon which further comprises polymerising the organic polymerisable material.

13. An optical waveguide including a substrate and a surface layer, the surface layer comprising an organic polymer and an organic non-linear optical material such that the surface layer has a refractive index which is greater than that of the substrate.

14. An optical waveguide according to claim 13 whereby the organic non-linear optical material comprises molecules which are polar and aligned.

15. A method of producing an optical waveguide substantially as described herein with reference to Examples 1 to 4 and as illustrated in the accompanying drawings.

16. An optical organic waveguide substantially as described herein with reference to Examples 1 to 4 and as illustrated in the accompanying drawings.