GB2264890A - Moulding of lenses and lenticular sheets - Google Patents

Abstract

A lens preferably having a diameter of the order of 1 or 2mm is fabricated by heating a body of lens material such that it softens, and pressing the softened lens material against a surface having one or more apertures 4 such that the softened lens material forms a protrusion or protrusions. The invention is particularly useful for fabricating arrays of lenses, or a lenticular sheet. Lens arrays or even single lenses made according to the invention might be used for instance in the packaging of optical devices, where input or output ports are needed for fibres, or in spatial modulators. <IMAGE>

Description

LENS FABRICATION The present invention relates to the fabrication of lenses, and finds particular application in fabricating arrays of lenses having diameters of the order of 1 or 2mm.

Techniques are known for making relatively small lens arrays, such as the technique disclosed in the article entitled "Microlens Arrays", by M Hutley, R Stevens and D Daly, published in Physics World, a publication of the Institute of Physics, and dated July 1991. In the technique disclosed in this article, small islands of photoresist are formed by photolithography and subsequently melted. Surface tension draws the islands of photoresist into the shape of small lenses. The size range of lenses made by this technique, as disclosed in the article, is from 5#m to 750cm.

Lenses with diameters of 2mm or greater can be moulded, but moulding cannot be used very easily to produce lenses of smaller sizes. Also, moulds, particularly of high quality, tend to be expensive in themselves.

An object of the present invention is to provide a method which can be used in fabricating lenses having diameters of the order of 1 or 2mm.

According to the present invention there is provided a process fcr fabricating lenses which comprises heating a body of lens material such that it softens, and pressing against a surface of that material a surface with one or more apertures therein such that the softened lens material forms a protrusion at the or each aperture.

There might be, for some applications, an array of apertures such that the lens material is pressed into an array of lenses, or a lenticular sheet.

Preferably, the material used to create the lens or array of lenses is amorphous. Otherwise, the lens or lenses so fabricated may suffer from scattering effects from crystalline features in the material.

The material used to fabricate the lens or lenses must necessarily either be initially soft, or be capable of being softened, for instance by heat, and should subsequently harden or re-harden, for instance on removal of a heat source. It will be understood that it may not be essential that the lens or lenses are rigid and, once hardened as described, may in practice have a resilient nature so that they regain a particular profile when relaxed but may be deformed temporarily therefrom by pressure.

Preferably, the means of softening the material should be a convenient means to apply, and an advantageous material for use in fabricating the lens or lenses comprises a polymer such as polystyrene or, more preferably, polycarbonate.

Polycarbonate is preferred over polystyrene because it does not adhere to stainless steel, and stainless steel is a convenient material for providing the surface with one or more apertures therein. That is, known etching techniques can be used to create apertures of predetermined shape in a sheet of stainless steel. Because polycarbonate does not adhere, once pressed to form the protrusion or protrusions, it can simply be parted from the stainless steel without damage. However, polystyrene has a slightly higher refractive index than polycarbonate and may be preferred in some circumstances.It is also process able at lower temperatures, for instance of the order of 15000, whereas polycarbonate tends to soften, and therefore be mouldable, at 20000. Conversely, because polycarbonate softens at a higher temperature, it may be more suitable for use in some environments or applications.

Another alternative material for the lens or lenses is glass. It would be necessary, however, to heat glass to higher temperatures than the polymers mentioned above in order to achieve softening.

A process according to an embodiment of the present invention will now be described, by way of example only, with reference to the accompanying figures, in which: Figure 1 shows, in diagrammatic and exploded form, the elements of a press for use in carrying out the process; and Figure 2 shows a single lens during fabrication.

Referring to Figure 1, a press arrangement for fabricating lenses comprises a heated base block 1 with a lcm diameter circular aperture 2 in its upper surface, a stainless steel plate 3 with an array of circular apertures 4 therethrough, and an uppermost heated plate 5 which can be moved n the vertical direction. To fabricate polycarbonate lenses, a polycarbonate sheet 6 is preheated at 1200C and positioned between the stainless steel plate 3 and the pre-heated plate 5, being separated from the preheated plate 5 by a protective sheet 7.

In a pressing operation, the uppermost plate 5, preheated at 200 C, is brought down so as to sandwich the polycarbonate sheet 6 between the stainless steel plate 3 and the protective sheet 7. On applying heat and pressure, the polycarbonate sheet 6 takes on a thick "treacle" like consistency and will "bulge" into each circular aperture 4 in the plate 3.

Once the polycarbonate sheet 6 has been pressed to a required shape, pressure on the uppermost plate 5 is released, the uppermost plate 5 withdrawn upwards, and the polycarbonate sheet 6 removed, together with the stainless steel plate 3. On cooling slightly, the polycarbonate sheet 6 and the stainless steel plate 3 tend to spring apart of their own accord, presumably because of a differential in cooling rate or coefficient of expansion.

The result of the above operation is a sheet of polycarbonate having part-spherical lenses in an array on a surface thereof. The lenses are part-spherical in that the radius of curvature is at least substantially constant over the whole diameter of each lens, the angle subtended by the ends of the diameter of each lens being determined by parameters of the pressing process.

Referring to Figure 2, during the pressing process, each lens 8 develops in a downward direction, into one of the circular apertures 4 in the stainless steel plate 3.

A diameter of the lens 8 so formed subtends an angle a which is determined by the time for which pressing is carried out, and by the viscosity of the polycarbonate sheet 6 during the pressing process, this being determined by the temperature at which pressing is carried out as well as by the characteristics of the polycarbonate material.

There is a tendency for edge effects to deform each lens 8 at its circumference, where it contacts the circular aperture A ln which it is formed. The surface of the circular aperture 4 may not be perfectly smooth and some distortion may be produced for instance by surface tension effects. However, in general, using a pressure of 2 atm (28 psi), a range of pressing times has produced the following characteristics in a lens array: Lens material Polycarbonate Diameter of lens elements in array 1. 5mm Pressing time range at 200 degC 30 secs to 60 secs Range of focal lengths 1. 9 - 3. 8mm Corresponding range of R 1. 1 - 2. 4mm Typical surface profile PV=0. 9, RMS 0.2, for a "D" of 0. 75mum Measured spot diameter 25 microns (Full Width at Half Maximum Intensity) VARIABLES:: PV = "Peak to Valley" measurement of number of wavelengths at 0. 633 pm over which lens does not conform to sphericity R = radius of curvature D = measured diameter RMS = root mean square, standard deviation The profile measurements were made using a "ZYGO" interferometer, a product of ZYGO CORPORATION in the US.

It might be noticed that the sphericity checks were made in terms of numbers of wavelengths (PV) at 0. 6 3 3 Cim. The lenses might normally be used with light of longer wavelength in optical communications, particularly 1. 3 or 1. 55 pm, in which case the PV values would be significantly lower.

The ZYGO interferometric technique is more suitable for use with smaller lenses and it might also be noted that only the central area of each lens was checked, D being significantly less In each case than the diameter of the circular apertures 4 (1. 5mm) in the plate 3 used to press the lenses. Alternative evaluation techniques include the use of a stylus, moving across the surface of each lens, or use of the lenses as optical elements to give a direct measure of their optical performance.

The testing of small lenses presents some problems however. A publication on this topic, for reference, is "The Testing of Microlens Arrays", by M C Hutley, D Daly and R v Stevens, published in the IOP (Institute of Physics) Short Meetings Series No 30, in 1991, starting at page 67. (The meeting concerned was a one-day seminar organised by the National Physical Laboratory in conjunction with the Institute of Physics and the Royal Photographic Society, 1. 5. 91.

MATERIALS: The polycarbonate material used for the polycarbonate sheet 6 is a commercial product of the company GENERAL ELECTRIC (GE), sold in the UK by the company GOODFELLOW under the GE trade mark "LEXAN". The sheet is 2mm thick and a convenient shape for use in a press as described above is a square in plan view, of side 2. 5cm. It is important to avoid contact between the polycarbonate surface and a liquid such as a solvent or water since these can cause crazing of the surface.

The stainless steel plate 3 is selected because it will not oxidise at the temperatures concerned and does not stick to polycarbonate material. A mask, having the circular apertures 4 therethrough, can be easily produced by known techniques as already used in photolithography for different purposes. A plate 3 of 0. 33mum thickness does not bow into the circular aperture 2 in the base block 1 at the relevant pressures and no untoward effects are produced by the heat.

It might be noted that, in the arrangement described above and providing the results for evaluation, the circular apertures 4 in the stainless steel plate 3 had a diameter of 1.5mm. The plate 3 used had a thickness of 0. 33mm to provide the strength against bowing, but, in different circumstances, the plate 3 may for instance be reinforced by some other structure to avoid distortion during pressing.

The protective sheet 7 comprises a sheet of glass or, alternatively, stainless steel. Neither material will stick to the polycarbonate material during or after the pressing process. The sheet 7 must necessarily be sufficiently thin to transmit heat from the uppermost heated plate 5 sufficiently well for the polycarbonate material to be softened. The use of glass is not essential but it is convenient particularly in that flat sheets are easily available. The surface quality of the protective sheet 7 must however be good since it lies in contact, during pressing, with the surface parts of the polycarbonate sheet 6 which will generally become the flat, rear faces of the lenses produced.

The polycarbonate sheet 6 can be cut from a larger sheet. It may be found preferable to protect the surface of the polycarbonate sheet during cutting, this being possible by applying for instance a sheet of plastic protective film (not shown). A suitable commercial product available is known as "NITTO" dicing tape, used otherwise in the semiconductor industry for dicing. This prevents surface damage and can be peeled away from the polycarbonate sheet without leaving adhesive behind on its surface. NITTO dicing tape has "tacky" rather than adhesive properties and therefore is easily applied and removed with respect to the polycarbonate sheet 6. In particular, such a protective film avoids liquid takelp by the polycarbonate material which can cause the crazing referred to above.

Another polymer material might replace the polycarbonate material described above. For instance, polystyrene (Styrene) might be used, which could be pressed at a lower temperature, such as 150do, and is less prone to take up moisture from the air. Polystyrene however, being softened at lower temperatures, may lose its shape, or suffer otherwise, in certain environments or applications.

Other alternative materials include polymethyl methacrylate (Acrylic), Styrene/Acrylic copolymer, Styrene acrylonitrite (SAN), methylpentene (TPX), ABS or nylon.

These are all principal optical plastics and some relevant properties are listed in CLEO Issue PHOTONICS SPECTRA, May 1991, Plastics Optics, pages 120 to 128 by Claude Tribastone and Chuck Teyssier. This paper also covers practical information such as coating techniques to protect these materials which tend to be difficult to coat.

Notably, polycarbonate has greatest transmission for optical radiation at 1. 3 or 1. 55ism, this matching the transmission "window" for silica fibres.

Polycarbonate has a potential disadvantage in that it takes up water to 0. 15% which can create bubbles if the temperature is raised too quickly, too high. It is a recommendation of the manufacturer therefore that water be driven off before use and the procedure in the method for carrying out the present invention described above is to leave the polycarbonate sheet at 1200C overnight prior to use. This drives off the water apparently slowly enough to avoid the bubbling which occurs at 2000C.

It is not essential to use an optical plastics material and another alternative is glass. The softening temperature for glass however is significantly higher.

Although as described above, the apertured stainless steel plate 3 is removed after pressing, in some circumstances it may be found preferable to leave it in place to provide diaphragms for the lenses. Alternatively, a sheet with slightly smaller apertures might replace it, so as to mask edge distortion in the lenses. In either case, the diaphragm sheet might be simply keyed in place with respect to the lenses.

The circular apertures 4 through which the lenses are pressed might in practice have a different shape, such as rectangular. This might produce a preferred lens profile such as cylindrical.

Lens arrays, or even single lenses, made as embodiments of the present invention, might find application in several arrangements, including packaging of optical devices which require input or output ports to fibres, or in spatial modulators. Microlenses must generally meet stringent optical requirements but lenses of the order of 1 or 2mm diameter may not have to meet such requirements and it is in this size range that embodiments of the present invention are most likely to be found useful.

It might be found beneficial to use additional process steps in embodiments of the invention, such as an additional annealing step applied to the lenses to remove possible surface defects such as scratches. This has been carried out with polycarbonate lenses, raising them to a temperature of 1700C for a period of 5 mins. An effect was to decrease the depth of the lenses.

As well as simply pressing the softened lens material through one or more apertures, it may be advantageous, for instance to achieve a particular optical characteristic, partially to extrude the or each lens. This might be done by using an apertured plate 3 which has significant thickness so that each aperture 4 is relatively long. The result might be that the or each lens produced was supported on a rod of polycarbonate, forming a curved end thereto.

Claims (12)

1. A process for fabricating lenses which comprises pressing a surface of a body of deformable lens material against a surface with one or more apertures therein, such that the lens material forms a protrusion at the or each aperture.

2. A process according to claim 1 wherein there is an array of apertures and the softened lens material forms a plurality of protrusions such as to provide a lenticular sheet.

3. A process according to any one of the preceding claims wherein said body of deformable lens material is heated prior to said pressing to bring the material to the deformable state.

4. A process according to any preceding claim wherein said lens material hardens or is hardened subsequent to said pressing so that said protrusion or protrusions is or are relatively undeformable, or become resiliently deformable so as to regain their shape when relaxed.

5. A process according to claim 4 wherein the deformable lens material hardens on cooling subsequent to said pressing.

6. A process according to any preceding claim, wherein the body of lens material is amorphous.

7. A process according to any preceding claim wherein the body of lens material comprises an optical plastic.

8. A process according to any preceding claim, wherein the body of lens material comprises polycarbonate.

9. A lens comprising a protrusion from a surface of a body of material, said lens having a diameter or equivalent dimension which lies in a range of the order of from lmm 2mm inclusive.

10. A lenticular sheet comprising a plurality of lenses according to claim 9.

11. A lens, or plurality of lenses providing a lenticular sheet, according to either one of claims 9 or 10, wherein the protuberant surface of the lens is a virgin surface, formed without contact with other surfaces.

12. A process, lens or lenticular sheet, according to any of the preceding claims, wherein the or each aperture is circular in plan view and the or each lens has a partspherical profile, at least in its central region.