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 waveguides.

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, US5358827from 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.

US2007172774 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 S, 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,

Figure 5 is an exemplary pattern for a Fresnel lens according to a further embodiment of the present invention,

Figure 6 is a photograph of a lens fabricated to the pattern of Figure 5, Figure 7 is a photograph of the light pattern from a light source passed through material which has not been polymerized using the techniques of Figure 4, and

Figure 8 is a photograph of the same light pattern as employed in Figure 7 after being passed through the lens of Figures 6 and 7.

Detailed description of Drawings

The inventors of the present application are the named inventors on WO2008003661, the entire contents of which are herein incorporated. This application employed photopolymeric materials to fabricating holograms by means of a recording process in which a light pattern was produced by the interference of two or more beams of mutually coherent light.

The inventors have realized that the photopolymer materials of that application may advantageously be employed using a different process to provide extremely complex non-holographic optical components directly from a computer using a simple printing process using a single light source for polymerization.

Specifically, the present application employs selective polymerization of a material to manufacture non-holographic optical components. More specifically, the method uses a layer of photopolymerisable material missing an essential component (which is necessary for photopolymerisation). The photopolymeric material is such that its refractive index changes between unpolymerised and polymerized states.

Referring to the previously identified application, In particular, the applicants have realized that the previously described components may advantageously be employed in the manufacture of an optical component, whereby an inactive recording material (as described above with respect to holograms) is selectively activated in a pattern defining the shape of a waveguide. Activation is achieved by 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 of being polymerized by exposure to light. In contrast to the holographic recording techniques, there is no need to use a light pattern produced by the interference of two or more beams of mutually coherent light incident upon a photosensitive layer containing a photosensitive dye.

An exemplary photopolymeric material which may be employed in the present application and discussed below in greater detail has been developed at the Centre for Industrial and Engineering Optics, Dublin Institute of Technology. This material may be provided as a layer of material, possibly on a solid substrate such as a glass slide or a sheet of plastics material. The material is provided without an essential component, for example the dye is omitted from the formulation of the 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

I Il

H O

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.

— (CH ₂ CH) _n - 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

I I

----- [CH ₂ -GH] _n - -[CH ₂ -CH] ₁₁ -- OH COO-CH,

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.

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 (CH ₂ CH ₂ OH) 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 2ml 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 mm ² 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 μm to 60 μm. 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 ₅ N -Diethyl acrylamide, Tradename : DEAA; N ₅ 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 ₅ N Dimethylaminoethyl Acrylate; or N ₅ N Dimethylaminoethyl Methacrylate

Exemplary alternative binders could include Poly vinylpirrolidone; a sol-gel; a hydrogel; an acrylate: Polyethyleneoxide: Polyethyleneglycol: 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.

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.

In the regions where the dye is present, 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. 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 a activating the photopolymer, since, if any are absent, photopolymerisation cannot take place. This photopolymerisation characteristic is employed in the manufacture of an optical component as set forth below in the present application.

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 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 including particularly waveguides for guiding light from at least one input to at least one output in a plane parallel to the planar layers and lenses defined as a pattern in the material.

One 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. It will be appreciated that a printing process allows for the rapid prototyping of optical devices designed on a computer. It will be appreciated that the dye molecues once deposited will diffuse into the unsensitised layer and that there will be a trade-off between sensitisation depth of the photopolymer layer and lateral resolution but it will be appreciated that optimum results may be obtained by experimentation and will depend upon factors include the nature and density of the dye.

One potential application is the use of the manufacturing process described herein to fabricate devices for guiding, coupling and splitting light beams as well as lenses for focusing\directing light. An example of such devices is presented in Figures 1 to 3. More particularly, the planar optical device 1 , of Figure 1 , has been fabricated using the above described methods and comprises a polymerized patterned area 4 defining a waveguide 2 having a first refractive index, whereas those areas 8 outside the patterned area (non polymerized areas) will have a second refractive index. The polymerized patterned area will have had dye deposited before the entire device was exposed substantially uniformly to a non structured light source, i.e. a spatially homogeneous light field. The refractive index of the photopolymer material in the polymerized area is substantially spatially uniform as is the non-polymerized area albeit with different refractive indexes. The waveguide is suitable for transmitting light in the primary plane of the sheet. In the example shown, the light is transmitted

from one side of the sheet in which the waveguide is formed to the opposite side.

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 non- holographic optical components, for example as shown in the exemplary light coupler of Figure 2, the exemplary splitter of Figure 3 or the Fresnel lens of Figure 5. In the case of Figure 5, a Fresnel lens is provided, allowing for the focusing of light passing through the lens. In contrast to the previously described waveguides, the lens directs\guides light in a direction transverse to the primary plane of the sheet of material in which the lens is formed.

The lens is formed as a series of alternating rings. The rings comprise alternating refractive indexes provided by the application and\non-application of the dye, on a sheet of material as described above, alternating rings and the subsequent exposure of the sheet of material to a source of light sufficient to cause polymerization of the rings.

An example of such a device was fabricated by dye deposition of a Fresnel phase plate pattern (Fig.4a,b) on a dry layer by means of a Dimatix Fujifilm of Santa Clara, California, USA material deposition system (DMP-2831). The layer was left in the dark for 60 min and illuminated by a laser light of wavelength of 532nm. The intensity of the light was 5mW/cm2 and the exposure time was 180s. The recorded structure was then illuminated by an attenuated 532 nm laser beam. The beam was focused after passing through the Fresnel plate as may seen in Fig. 8. This observation proves that the localised dye sensitised polymerisation has caused the expected patterned change of the refractive index. No focusing was observed when a printed but non-polymerised pattern was probed as is clear from Fig.7.

It will be appreciated that a key 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 by exposing all of the material to light sufficient to cause polymerization and thus define 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.

It will be appreciated that the advantages of the present application include the use of a uniform light source (there is no need for a laser or laser to define a pattern).