GB2459505A

GB2459505A - Optical component fabrication by depositing a polymerisation activator pattern

Info

Publication number: GB2459505A
Application number: GB0807583A
Authority: GB
Inventors: Susanne Martin; Vincent Toal
Original assignee: Dublin Institute of Technology
Current assignee: Dublin Institute of Technology
Priority date: 2008-04-25
Filing date: 2008-04-25
Publication date: 2009-10-28
Also published as: EP2277067A2; WO2009130333A3; WO2009130333A2; GB0807583D0

Abstract

A method, figure 4, of manufacturing an optical component e.g. a waveguide in an integrated optical circuit, comprises: the step 100 of providing a first layer of inactive photo polymerizable material comprising a monomer, e.g. acrylamide; the step 102 of depositing a patterned second layer comprising an activating component, e.g, a light activated free radical generating ink or dye, to define the shape or pattern of the optical component on the first layer and the step 104 of exposing the layers to a light sources to cause the polymerisation of the monomer in the region defined by the track or imprint of the second layer. Preferably the method is performed wherein the layers are overlaid onto a plastics substrate and the second layer is printed, e.g. using an inkjet printer. Optical components produced by the above method are also disclosed, figure 3.

Description

A METHOD OF MANUFACTURING AN OPTICAL COMPONENT

Field of the Application

The present application relates generally to the field of optical components and more particularly to optical components employing the principles of total internal reflection.

Background to the Application

Several existing patent documents describe making waveguides and similar structures using a printing process. In some, the ink that is deposited is transparent or is cured to become transparent. One example is GB2397897 from ZARLINK SEMICONDUCTOR LTD. Another example is JP2003202444 from OPTOQUEST CO LTD.

Other methods use a printed ink to pattern a mask so that the exposing light only reaches defined areas. For example, U55358827from AT & T BELL LAB uses printed ink as a mask to make a feature of a certain width in photoresist. In this arrangement, the dye is only used as a block or to change the phase of the exposing light.

KR20040056272 from KOREA ELECTRONICS TELECOMM discusses manufacturing an optical waveguide using a direct laser printing technology, in which the optical waveguide is formed using a laser beam.

U52007 172774 from PAOLO ALTO RES CT describes an etching process using a printed mask.

The fabrication of microscopic light-guiding devices at present is also possible by two photon polymerization [Kelemen L, Valkai 5, Ormos P, Integrated optical motor, APPLIED OPTICS 45 (12): 2777-2780 APR 20 2006]. This approach requires the use of an expensive femtosecond pulse laser system. The fabrication process consists of drawing the structure with a moving focused laser light along a preprogrammed trajectory. It also requires post exposure heat treatment and chemical treatments in order to dissolve the unused recording material.

The present application seeks to provide similar benefits to the described methods whilst providing greater versatility and\or reduced cost.

Summary

The present application provides a method of manufacture and an optical component in accordance with the claims which follow.

Description of Drawings

Figure 1 illustrates a simple waveguide produced according to the present invention, Figure 2 illustrates a light coupler produced according to the present invention, Figure, Figure 3 illustrates a light splitter produced according to the present invention, and Figure 4 illustrates an exemplary process flow for the present invention.

Detailed description of Drawings

The inventors of the present application have realized that a process previously employed for fabricating holograms (described in W02008003661, the entire contents of which are herein incorporated) may advantageously be modified and employed in the manufacture of optical components. In particular, the method may be employed to fabricate microscopic photonic devices using a process previously employed for manufacturing holograms.

In a conventional photopolymer hologram recording process a light pattern is produced by the interference of two or more beams of mutually coherent light incident upon a photosensitive layer containing a photosensitive dye. The photosensitive layer typically comprises a number of individual components including: a dye, a free radical generator, and a monomer and, optionally, a binder. Although, it will be appreciated that there may be more than one type of each component, e.g. several different monomers or several different dyes may be employed in the photosensitive material. The role and nature of these materials and the process by which they combine to record a hologram will now be discussed briefly. Although, it will be appreciated that these techniques would be well known and understood by those skilled in the art.

The role of the photosensitive dye in the photosensitive layer is to absorb light and start the photochemical processes that lead to holographic recording. Examples of photosensitive dye would include erythrosine B, fluorescein, eosin and other xanthene dyes.

In bright regions of the produced interference pattern, the energy from photons of light raises the dye molecules to excited singlet states. Many of the singlet state excited molecules are then converted to triplet state excited molecules by intersystem crossing. In the triplet state, a dye molecule can interact with a free radical generator, for example, triethanolamine. This interaction produces an active free radical. The active free radical can, in turn, interact with a monomer molecule such as acrylamide creating a monomer radical. The creation of the monomer radical, results in free radical polymerization occurring in the polymer material.

Conversion of the carbon-carbon double bond to a single bond changes the molecular polarizability of the acrylamide and thus its refractive index. In addition, the depletion of monomer concentration in illuminated regions results in a spatial gradient of monomer concentration inducing diffusion of monomer from dark to bright regions.

This process also contributes to a local change in refractive index. By these mechanisms, the spatial variation in light intensity is recorded as a refractive index variation in the photopolymer layer, i.e. a hologram is produced.

The dye, free radical generator and monomer may all be considered as being necessary to the photosensitive material or more generally to the process of recording a hologram, since, if any are absent, photopolymerisation cannot take place. This characteristic is employed in the manufacture of an optical component as set forth below in the present application.

In particular, the applicants have realized that the previously described method may advantageously be employed in the manufacture of an optical component, whereby an inactive recording material (suitable for recording a hologram if activated) is selectively activated in a pattern defining the shape of a waveguide. Activation is the provision of one or more essential components substantially absent from the inactive recording medium, which are required to transform the inactive material into an active material capable which may be polymerized.

In particular, the present application uses the principle that only when a missing component, e.g. dye, monomer or free radical generator is introduced to an inactive layer containing all the other essential components does photopolymerisation become possible. In the event that the missing component is not introduced, the refractive index of the material will not change whereas in contrast if the missing component is present a change in the reflective index of the material will result (when exposed to a suitable light source). As a result, as shown in the planar optical device 1 of Figure 1, the polymerized patterned area 4 defining the waveguide 2 will have a first refractive index, whereas those areas 8 outside the patterned area (non polymerized areas) will have a second refractive index. Typically, the first refractive index may be of the order of 1.500, whereas the second refractive index may be of the order of 1.505. This difference in refractive index ensures that any light 10 entering the waveguide at the input 7 and having an angle of incidence at the front face within an angle defined by the numerical aperture of the guide, is subjected to total internal reflection within the waveguide and is restrained within the waveguide to exit at the output 8. It will be appreciated that the structure of Figure 1 is a simple one to illustrate the invention and that the present application may be employed for more complex optical components, for example as shown in the exemplary light coupler of Figure 2 and the exemplary splitter of Figure 3.

The photopolymerisation process described above is by nature an amplification process, insofar as one dye molecule can facilitate the polymerization of many monomer molecules. Accordingly, it is advantageous to select the dye or free radical generator as the activating component. Moreover, it has been experimentally determined that it is most advantageous to select the dye as the activating component.

The photopolymer (holographic) recording material (discussed below in greater detail) has been developed at the Centre for Industrial and Engineering Optics, Dublin Institute of Technology which may be provided as a layer of material, possibly on a solid substrate such as a glass slide or a sheet of plastics material, with the exception that the dye is omitted from the formulation of the recording material. The omission of the dye from the formulation renders the final layer inactive i.e. substantially incapable of responding to light. Efforts to effect a change in the reflective index of the material by illumination with visible light will not be successful unless dye is introduced.

In greater detail, the inactive medium suitably comprises a photopolymer layer composition comprising the following: a binder, which acts as a support medium or host matrix for monomers and a free radical generator.

In more detail, the structure of an exemplary medium, which has been found to provide good results, includes the following: Monomer: An exemplary monomer used in the photopolymer composition is acrylamide. The structure of the acrylamide molecule is shown below. The molecules contain a carbon-carbon double bond (C=C). This double bond is broken on polymerization resulting in two single bonds. In particular, electrophoresis grade acrylamide powder (for example, as available from Sigma Aldrich of St Louis, Missouri, USA) may be used.

CH C -C-NH 2 2 H 0

Binder: A suitable binder used in the photopolymer layer is polyvinyl alcohol (PVA) (for example from Sigma Aldrich or Riedel De Haen). The chemical formula for pure polyvinyl alcohol (100% hydrolyzed) binder is shown below.

-(CH2CH)-

OH

A low percentage hydrolysis binder may also be used. The chemical formula of an alternative lower percentage hydrolyzed polyvinyl alcohol in which a second polymer (generally polyvinyl acetate, from which the polyvinyl alcohol is synthesized) is as follows polyvinyl alcohol polyvinyl acetate Jr 1 [ CH2-CH] [ CH2-CH]11 OH COO-CR3 Crosslinking monomer: A second monomer employed in the exemplary photopolymer layer composition acts as a crosslinking monomer, for example NN'methylenebisacrylamide (available from Sigma Aldrich). The structure of the molecule is shown below. It is a symmetric molecule of two acrylamide molecules attached with a methyl group in the middle.

HHH

H\ I I I,H CHC-C-N-C-N-C-CH=C

H II I II o H 0

Free radical generator: An exemplary free radical generator comprises Triethanolamine (TEA) (available from Sigma Aldrich). As explained above, the free radical generator plays a significant role in the generation of free radicals to initiate a polymerization reaction.

The chemical formula of TEA is shown below.

N (CH2CH2OH)3 A method of preparation of an exemplary suitable (inactive) polymer recording layer comprises some or all or the following steps: Stock solution of polyvinyl alcohol (PVA): 10 grams of PVA of specified molecular weight and hydrolysis is dissolved in 100 ml of water to prepare a 9.1% by weight or 10% w/v PVA solution.

Composition of photosensitive medium: A composition of the photosensitive medium is prepared by adding 2m1 of triethanolamine to 0.25 grams of NN'methylenebisacrylamide (crosslinking monomer) and 0.8 grams of acrylamide (monomer). To this mixture, 17.5 ml of stock solution of 9.1% polyvinyl alcohol is then added and the total solution is stirred thoroughly, to ensure the monomer and crosslinking monomer are completely dissolved to obtain a homogenous solution.

The method of manufacturing an optical component using the previously described materials will now be described with reference to Figure 4, in which the process commences with the provision 100 of a layer of inactive material.

Layer preparation: 0.5 to 1 ml of photopolymer solution is spread uniformly on a 25x75 mm2 glass plate placed on a leveled surface and allowed to dry forming a film. The drying time is usually 24 hours. The thicknesses of the photopolymer film layers thus formed are approximately 30 pm to 60 pm. The layer may also be used in liquid form, when higher concentrations may be used There are many variations in concentration and volume of the above formula, which also work well. For example, where more brightness is required in the final component more acrylamide can be used It will be appreciated that alternative monomers, free radical generators and binders may be employed depending on the particular requirements of the application.

Similarly, additional components, for example, nanoparticles may be added for improved performance.

Alternative monomers would include any suitable monomers such as acrylamides, for example: N,N -Diethyl acrylamide, Tradename: DEAA; N,N Dimethyl acrylamide, Tradename: NNDMA; N-Isopropyl acrylamide, Tradename: NIPAM; N-(2-Hydroxyethyl acrylamide), Tradename: HEAA; or 20Hydroxyethyl methacrylate, Tradename-HEMA. Similarly, the monomer may comprise an acrylate such as: N,N Dimethylaminoethyl Acrylate; or N,N Dimethylaminoethyl Methacrylate Exemplary alternative binders could include Poly vinylpirrolidone; a sol-gel; a hydrogel; an acrylate: Polyethyleneoxide: Po lyethyleneglyco 1: and Polyethyloxazine.

Exemplary alternative free radical generators may include N-phenilglycine (NPG) which may be used in combination with Diphenyliodonium Hexafluorophosphate (DPI).

Once the layer of inactive recording material has dried (if not being used in liquid form or gelatinous form), the next step 102 in the process comprises depositing sensitizer in the pattern of the desired waveguide(s) for the optical component.

In this respect, the sensitizer (also referred to as a dye or as a dye sensitizer) is suitably a photosensitive dye, for example Erythrosin B (available from Sigma Aldrich chemicals). Erythrosin B is green light sensitive dye having a complex structure with four benzene rings. The structure of the molecule is shown below.

I I

N a 0 0 1o H COONa

HH

H

Alternatively, a wide variety of sensitizers may be used, including for example: Erythrosin B; Methylene blue; Eosin; Eosin yellowish; Fluorescein; any xanthene dye; or a thionine dye. The dye can be introduced to the polymer layer by deposition in solution or solid form, by contact with a dye impregnated dry layer, or by a printing or spraying process.

After the sensitizer has been deposited on the layer of inactive material in the shape of the desired waveguide for the optical component, a final step 104 in the process exposes the layers to a suitable light source, such as for example a laser light (or a lamp) with appropriate wavelength, absorbed by the sensitizing dye for a period sufficient to cause polymerization to occur in the region defined by deposition of the dye.

The idea of this invention is that by selectively depositing the dye on the surface of the otherwise inactive photopolymer layer photoinduced changes can only be introduced in these strictly predetermined areas, the main interest being in photoinduced refractive index change. A precise control over the dye deposition location can provide the possibility to create a large variety of patterns of refractive index modulation, which may be selected to define optical components and more particularly waveguides for guiding light from at least one input to at least one output in a plane parallel to the planar layers.

The simplest approach towards realization of the dye deposition is using a printer. It will be appreciated by those skilled in the art that lateral diffusion of the deposited dye molecules and the spatial resolution of the selected printer (currently approaching 5 micrometers) will impose a limitation on the minimum size of the fabricated microscopic photonic devices. However, as printer technology or other deposition techniques improve so will the scale of optical components that may be fabricated.

One potential application is the use of the manufacturing process described herein to fabricate devices for guiding, coupling and splitting light beams. An example of such devices is presented in Figures 1 to 3.

It will be appreciated that a fundamental aspect of this application is the bringing together of an inactive recording material with one or more missing components required to activate the recording material and the subsequent polymerization of an area defining an optical component. Accordingly, whilst the above exemplary embodiments describe providing the inactive recording material as a first step and the subsequent provision of the missing components, it will equally be appreciated that the missing component, e.g. dye may be provided as a dry layer for example on a substrate as a first step with the inactive polymer provided on a second substrate which is brought into contact with the first substrate and their subsequent exposure to light to cause polymerization and the forming of the optical component.

Similarly, it will be appreciated, that the missing component, e.g. dye may be provided as a dry layer for example on a substrate, as shown in 8, with the inactive photopolymer recording material subsequently being brought into contact with it (e.g. by pouring a liquid mixture of monomer and free radical generator onto a surface to which the dye is bound.

Claims

Claims: 1. A method of manufacturing an optical component comprising at least one waveguide for providing an optical path between at least one input and at least one output, the method comprising the steps of: providing a first layer of inactive photopolymerisable material comprising a monomer, depositing a patterned second layer comprising an activating component to define the shape of the at least one waveguide of the optical component on the first layer, exposing the layers to a light source to cause the polymerisation of the monomer in the region defined by the pattered second layer.
2. A method according to claim 1, wherein the layer of inactive material is provided as a sheet.
3. A method according to claim 2, wherein the sheet comprises a layer of plastics material on which the layer of inactive material has been deposited.
4. A method according to anyone of claims 1 to 3, wherein the activating component is provided in an ink.
5. A method according to claim 4, where the ink is provided by a printing process.
6. A method according to claim 5, wherein the printing process comprises a printer.
7. A method according to claim 6, wherein the printer is an ink-jet printer.
8. A method according to anyone of claims 1 to 7 wherein the inactive recording material further comprises a free radical generator.
9. A method according to any preceding claim wherein the activating component is a dye.
10. An optical component as produced by the method of anyone of claims 1 to 8.
11. A method for fabrication of optical component by localised light induced change of the refractive index of a photopolymerisable material achieved by controlled activation of the otherwise inactive photopolymer mixture by depositing an activating component to define the shape of the optical component.