WO2005114284A1

WO2005114284A1 - Optical coupling element and method of manufacturing the optical coupling element

Info

Publication number: WO2005114284A1
Application number: PCT/IB2004/001611
Authority: WO
Inventors: Cornel Marxer; Michael Zickar; Wilfried Noell; Nicolaas F. De Rooij
Original assignee: Sercalo Microtechnology Ltd.
Priority date: 2004-05-18
Filing date: 2004-05-18
Publication date: 2005-12-01

Abstract

An optical coupling element comprises a support having at least one groove for the reception of an optical element like a single mode optical fibre (SMF). The groove is U-shaped and has inwardly projecting spring elements formed thereon for clamping the fibres to be placed in the groove. The optical coupling element may be used for aligning graded index (GRIN) fibre pieces with optical single mode fibres/waveguides on a silicon motherboard. The silicon motherboard can include aligning structures, snap-in springs, micro-electro-mechanical systems (MEMS) actuators and other MEMS structures with an optical function such as mirrors, filters and absorbers.

Description

Optical coupling element and method of manufacturing the optical coupling element

Field of the Invention The present invention relates an optical coupling element and a method of manufacturing the optical coupling element according to the preamble of claims 1 and 12, respectively.

Prior Art

The conventional way of collimating light beams of optical single mode fibres (SMF) utilizes a short piece or rod of a GRIN fibre such as multimode fibres (MMF), which have the same diameter as SMF's of 125 μm, and placing them in front of the SMF. The length of the GRIN MMF and the distance in between the GRIN MMF and the SMF are chosen such that the light from the SMF is collimated after it has passed the GRIN rod. The typical lengths and distances are in the order of a few hundred micrometers.

EP-A-0370 663 discloses a light collimating system comprising an integrated optical device and an optical fibre coupled to the device. The optical device is mounted on a semiconductor substrate having a V-groove etched into its surface, in which the optical fibre is disposed. A lens fibre section, e.g. a multimode fibre, is interposed between the end of the optical fibre and the optical device for collimating the light emerging from the end of the optical fibre. Two spacer elements are used in order to maintain the correct spacing between the fibre and the collimating fibre section on the one hand and between the collimating fibre section and the device on the other hand.

WO 86/04156 discloses an optical coupler for a single mode light fibre, which comprises a multimode light fibre being - in a preferred embodiment - directly fused onto the end of a single mode communication fibre. The process of fusing involves placing the single mode and the graded index multimode fibres to be connected in an electric arc. In order to obtain the appropriate length the graded index multimode fibre is then scribed and broken at the desired location. This method, however, is difficult to control. It is therefore preferred that the fused fibres are placed in housing and then subsequently polished to the desired length. The housing can subsequently be removed, if the size of the coupler should be as small as possible. The fusion of the end faces of two fibres has the disadvantage that at the location of the fusion a change the optical properties and the diameter of the GRIN MMF occurs. In an alternate embodiment the WO 86/04156 proposes to connect the multimode fibre- lens to the single mode transmission fibre by a UV curing cement or by an epoxy resin.

A common problem of the proposed set-ups, which was not addressed in the prior art, is the ease of manufacture and assembly of the GRIN MMF rod with the SMF due to their small sizes and the high alignment accuracy requirements, which are in the micrometer range. The optically required alignment accuracy is as low as a few microns and can be even less if the application requires a perfect optical beam. The known methods use standard cleaving, splicing and polishing of the SMF, the undoped spacer fibre and the GRIN MMF rods. These methods require the use of macroscopic tools, which are not precise enough to obtain optimal fibre collimators. Additionally, the splicing can change the optical properties and the geometry of the GRIN MMF by changing the index profile during the heating of the fibre endface. Hence the optical properties are not well defined and can change considerably with the splicing parameters.

Object of the invention

It is therefore an object of the present invention to provide an improved optical coupling element and a method of manufacturing an optical coupling element on a silicon motherboard which allows for better beam collimation and beam manipulation than conventional devices. A still further object is to provide an optical coupling element for optically connecting fibres and optical micromachined (optical micro electro mechanical systems o-MEMS) elements like optical switches, filters and attenuators. A still further object of the inventin is to provide a coupling element which allow for better alignment and and fixing of the optical lenses and fibers. Another object is to provide a micro- optical device comprising an inventive element and an improved light collimating optical system.

Description

According to the invention said object is achieved by an optical coupling element having a U-shaped groove and inwardly projecting spring elements formed at distinct locations along the U-shaped groove for clamping the fibres placed in the groove. The spring elements are designed for clamping fibres, in particular single mode fibres (SMF) as well the graded index multimode fibre rods, which can be placed in the groove. By the provision of spring elements optical fibres can easily and precisely placed in the groove. The optical fibres, e.g. SMF and the grin lens, respectively, are held in defined positions in the groove of the inventive coupling element. Accordingly, the light loss is smaller than with conventional coupling elements. The U-shaped groove allows proper alignment and secure fixation laterally and vertically. By the provision of the inventive coupling element the coupling and adjustment of optical fibres are facilitated. The groove may also be formed as two essentially spaced apart and parallel bars, e.g. of rectangular or square cross section, projecting from the surface of the support.

Preferably, a short rod (typically between 500 and 700 μm of length) of graded index (GRIN) multimode fibre (MMF) is placed in the groove. The length of the GRIN MMF is smaller than the length of the groove so that at least one further fibre can be placed in a spaced apart relationship to the GRIN MMF. When placing e.g. a single mode fibre (SMF) into the same groove, the GRIN MMF and the SMF are aligned relative to each other by the groove and the collimator is formed. Because of the larger beam diameter of the collimated beam after passage of the GRIN MMF, the light throughput can be higher than with conventional coupling elements where two single mode fibres face each other. Due to the cpllimation of the light beam and the precise arrangement of the fibres in the coupling element only a minimum of light is lost. As the light is collimated, the tolerances for connecting two single mode fibres can be larger.

The distance between the end face of the single mode fiber and the GRIN MMF is the only variable distance, which is not defined by the groove. Therefore, according to a preferred embodiment of the invention alignment marks next to the groove are provided. These alignment marks may be formed by an etching process. By the provision of alignment marks at the edge of the grooves or close to the edge of the groove at distinct locations along the groove the adjustment of the GRIN MMF's and/or the SMF's is facilitated.

According to an other preferred embodiment of the invention mechanical stops projecting into the groove are provided at distinct locations along the groove in order to define the distance between two optical elements placed in the groove. The stops can serve as abutments for the end faces of the light transmitting /receiving conducting devices to be placed in the groove and define the distance between the single mode fibre and the GRIN MMF. It is feasible to provide stops projecting into the groove at distinct locations along the groove. In this way the GRIN MMF's and/or the SMF's can easily and accurately be placed in the groove. Preferably, the graded index (GRIN) multimode fibre (MMF) is either arranged in the groove in accordance with the alignment marks or arranged into the groove such that an end face abuts the stop.

Although the graded index (GRIN) multimode fibre (MMF) can be unreleasably fixed in the groove, e.g. by means of a glue or adhesive, it is preferred that the GRIN MMF is held or fixed in the groove by spring elements projecting into the groove. This has the advantage that the alignment of the GRIN MMF is maintained in the groove. According to a preferred embodiment of the invention the support is a silicon wafer and the groove is U-shaped. In this way a plurality of optical coupling elements may be manufactured by a single micro-photolithographic process followed by etching etc. and dicing.

Preferably, the support is a silicon wafer and in particular a silicon on insulator substrate (SOI). A silicon on insulator substrate comprises a SiO layer sandwiched between two silicon layers. According to a preferred embodiment of the invention a second graded index (GRIN) multimode fibre (MMF) is placed in the groove at a distance from the first graded index (GRIN) multimode fibre (MMF). Such a coupling element is perfectly suitable for coupling two single mode fibres. It is also feasible to provide between the end faces of the first and second GRIN fibres an other optical device, e.g. an attenuator, a filter element, a mirror, a prism or the like. In this way ready-to-use coupling elements with a further optical functionality are provided.

Each groove can be formed as two essentially spaced apart and parallel bars, e.g. of rectangular or square cross section, projecting from the surface of the support. Also a plurality of parallel and spaced-apart grooves with inserted GRIN MMF's can be provided.

According to the invention two or more optical coupling elements and further optical devices like attenuators, filter elements, mirrors, prisms or the like may be combined on one (single piece) motherboard (silicon wafer) to form a higher integrated optical device

According to the invention also a method of manufacturing an optical coupling element is proposed comprising the steps of providing a silicon support etching a U-shaped groove into the support surface and inwardly projecting spring elements at distinct locations along the U-shaped groove. The spring elements can be manufactured in the same etching process as the grooves. This is an efficient method for manufacturing high precision ready-to-use coupling elements.

Preferably, a photolithographic process is used to form alignment marks at or close to the edge of the U-shaped grooves. Stops projecting into the groove can be formed at the same time and with the same photolithographic process as for the formation of the U-shaped grooves. Further advantages of the invented method are defined in the dependent claims.

Preferably, the graded index (GRIN) multi mode fibre (MMF) inserted into the coupling element is manufactured comprising the steps of - forming a V- or U-groove in a substrate, e.g. into a wafer substrate by an etching process; placing GRIN fibres of a first length in the groove; fixing the GRIN fibres in the groove by a glue, wax or resist; and and cutting the GRIN fibres into smaller pieces of the desired second length. In this way GRIN fibre pieces of an exact length can be manufactured. The method of manufacturing GRIN fibre pieces can be used also independently of process of manufacturing an optical coupling element. Preferably, a plurality of parallel grooves is formed and GRIN fibres of about the length of the grooves are placed in the grooves. This allows the simultaneous manufacture of a large number of GRIN fibre pieces.

According to another aspect of the invention there is provided a light collimating optical system comprising an optical coupling element according to any of claims 1 to 12 and at least first and second light transmitting /receiving conducting devices. The first light transmitting /receiving/conducting devices can be a single mode fibre (SMF) and the second light transmitting /receiving/conducting devices can be graded index fibre (GRIN lens), i.e. a section of an optical fibre having a refractive index which changes gradually in a radial direction. In a light collimating optical system of the present invention a SMF and a GRIN lense are received in the U-groove in a spaced relationship with respect to each other and clamped by at least one spring element projecting into the groove. In a preferred embodiment a GRIN lense is interposed between two single mode fibres, which are received in the U-groove and arranged at a distance from the end faces of the GRIN lense.

The invention is hereinafter described by way of example and with reference to the enclosed Figures in more detail wherein like reference numerals are used for like parts: The Figures show:

Fig.1 Schematically, a plan view of a first embodiment of an optical bench used as an optical coupling element;

Fig.2 A perspective view of a second embodiment of an optical coupling element with additional functionality;

Fig. 3 Illustration of the beam propagation in a two-lens system

Fig.4 Illustration of the beam propagation in a three-lens system

Fig. 5 A collimator array with parallel grooves for coupling a plurality of fibres.

Fig. 6 The different steps of the manufacturing process of the coupling element;

Fig. 7 Schematically, the manufacturing steps of forming GRIN rods from GRIN fibres.

The schematic representation of Figure 1 shows a support 11 onto which a first fibre 13 and a second fibre 15 are mounted in a spaced-apart relationship. The first fibre 13 is a graded index (GRIN) fibre, and the second fibre is a single mode fibre (SMF). The fibres 13,15 are arranged in a U-groove 17 formed in the surface of the support 11. Stops or spacer pins 19 project into the groove 17 and serve as abutments for the fibres 13 and 15. The spacer pins 19 determine the distance between the single mode fibre (SMF) 15 and the GRIN lens 13. Said distance determines the beam waist of the collimated beam 21. Since diffraction increases quadratically with the decreasing beam waist a large beam is desirable in order to achieve long coupling distances. This beam diameter should be chosen to have about 2/3 of the size of the GRIN lens core. The length of the GRIN lens determines the focus. A slightly f ocalised beam is desired to have a large coupling length. Springs 23 formed at selected positions clamp the graded index (GRIN) fibre and the single mode fibre (SMF) 15 in the groove 17.

Figure 2 shows an optical coupling element composed of 2 coupling elements of Figure 1. In the embodiment shown in Figure 2 the groove 17 is formed by two longitudinal bars 27 projecting from the surface of the support 11. Said embodiment further differs from that of Figure 1 in that a second GRIN lens 13' couples the beam 21 into an outgoing SM fibre 15'. In principle, the lengths of the GRIN lenses 13,13' and the spaces between the SM fibres and the GRIN lenses do not have to be the same. However, the design and calculation is easier for a symmetrical set-up. Optical elements or devices 25, e.g. an attenuator, a filter element, a mirror, a prism or the like, may be placed in the path of the collimated beam 21. Such optical elements and micromachined actuators supporting these can be manufactured in the one and the same manufacturing process as for the silicon U- grooves. (see or example: C. Marxer et al "Vertical Mirrors Fabricated by Deep Reactive Ion Etching For Fiber Optic Switching Applications", IEEE J. of Micro- Electro- Mechanical Systems, vol. 6, no 3, Sept. 1997, pp 227 - 285)

The regions where the beam does not travel in a fibre may be filled with a gas or gas mixture other than air or by index matching materials like an oil or glue. An index matching material improves the coupling efficiency because it can smooth the rough surface of the diced GRIN fibre piece. Additionally Fresnel reflection can be avoided. Diffraction decreases due to the higher refraction index.

For a symmetrical set-up the best coping efficiency is achieved (theoretical loss of 0 dB) if the beam at the emitting end of the GRIN lens has the same diameter as the beam at the entrance of the receiving GRIN lens.

Figure 3 illustrates the beam propagation in a two-lens system as shown in Figure 2: In Figure 3 the basic collimator set-up is sketched. The beam diameter is represented for having index matching fluid between the optical fibres (i.e. nl = n2 = n3 « nO fibre). A material, having another refraction index could be chosen instead.

Beam propagation in a three-lens system (Fig. 4): If additional GRIN lenses 13 are placed in the light path of the collimated beam, the diverging beam is re-focalised and an extended region with a collimated beam 21 is obtained. If the lenses 13 are placed at the position for a symmetrical set-up, the collimated region may be extended theoretically to infinity. This can lead to a cascade of optical benches.

In Figure 5 a connector for coupling a plurality of fibres (collimator array) is shown. It comprises single mode fibres and spaced apart grin lenses held in the groove by spring elements.

Microfabrication of the lens supports 11 by well-established photolithographic processes enables to make arrays of self-aligned collimators having a high precision of the pitch between them. This configuration is perfectly suitable to build an optical connector array having the same pitch as commercial fibre ribbons. Due to the large diameter of the light beams the alignment tolerances of such a connector are much larger than having only two SM fibres facing each other. A stack of those collimator arrays could lead to a two- dimensional connector array.

Manufacturing process of the optical coupling element (Figures 6a to 6d): The optical coupling element (which can be considered as an optical bench) providing self aligned holding structures for the GRIN rods as well as for other optical components, is fabricated on a silicon on insulator substrate (SOI). As a first step a photoresist layer is spun on the wafer. The holding structures (U-groove and spring elements designed as parallel tongues to the longitudinal groove and being integral with the remaining substrate material) are transferred to the photo resist using a photolithographic mask and preferably UV light (Figure 6a). Once the photo resist is developed, U-grooves are etched into the device layer of the SOI wafer (Figure 6b). Together with the formation of the U- grooves also the elastic springs for clamping the GRIN rods and optical fibres are formed by removing the substrate material between the tongue and remaining substrate. Optionally, spacers or stops can be etched into the device layer of the SOI wafer. This can be done together with the formation of the U-groove and the spring elements. In a further step the elastic springs are released from the insulator layer by means of hydrofluoric acid, which under-etches the thin spring structures (Figure 6c: removal of the sandwiched insulator material). Finally the GRIN rods are assembled to the optical bench and covered with a suitable lid, which is sealed e.g. by using an epoxy glue (Figure 6d). The spaces between the mounted optical elements on the optical bench may be filled with an index matching fluid, a glue, or any other material to fulfil any functionality.

Manufacturing process of the GRIN lenses (Fig. 7):

The fabrication of the GRIN rods requires a rigid support, preferably a silicon wafer that can be diced using established wafer dicing machines and blades. Parallel V- or U- grooves 41 are either diced or etched into this support to assure precise mounting of the GRIN fibre pieces, which can have a length of roughly 10 cm. Additionally, alignment marks 43 can be deposited, etched or diced onto the support. The length of GRIN fibre pieces is about the same as the groove length. After placing the pieces into the grooves they are fixed to the support by glue, wax or resist 45. A simple way is to use hot melt glue. After hardening of the adhesive the GRIN fibre pieces are cut into GRIN rods of a very precise length using a wafer dicing machine. The length of the pieces is given by the lateral displacement of the blade 47 minus the width of the blade. A precision well below 5 um was obtained using a standard dicing saw for silicon wafers.

The support can either be diced completely in vertical direction or only partially. Latter has the advantage that no particles being broken off the lower part of the support are torn up the sidewalls of the diced trench resulting in a better sur ace quality of the GRIN rods. The release of the GRIN rods is accomplished by dissolving the adhesive in a solvent, typically alcohol or acetone.

The surface quality of the GRIN rods depends strongly on the roughness of the blade. A typical surface roughness in the micron range has been measured. To decrease the surface roughness, polishing steps can be added either before or after the release of the GRIN rods. Polishing can either be done using hydrofluoric acid, which is an isotropic etchant for glass, smoothing the surface or mechanical polishing of a diced support - GRIN rod bar, both enabling parallel treatment of GRIN rods. The manufacturing process of the GRIN rods can also be used independently of the optical coupling element and is therefore a separate or additional independent aspect of the invention.

Summary

The silicon chip acting as a support and optical bench, respectively, can be desi ned or equipped with exactly etched U-shaped grooves and clamping springs, and, optionally, stoppers, actuators and other alignment features, and micro optical or mechanical components. Therefore, one can consider the silicon chip (support) as a micro optical bench with precise alignment and device features, where the fibres and fibre pieces can be put into with exact relative distances in the micron or even sub-micron range from each other. Surface roughness of the fibres or components due to etching or dicing can be smoothed out optically by an index matching glue or liquid.

Reference structures on the chip (support) make it easy to align the assembly with other internal or external components of optical or mechanical nature. The silicon chip (support) can also contain actuators and/or passive components to manipulate the optical beam or the relative position of the fibre or integrated components to each other. Hence the SMF or the GRIN MMF piece can be displaced to further manipulate the beam to either change the degree of focus (i.e. positive or negative focal length) or collimation of the optical beam in order to optimize the beam shape for a given application. Particularly, this manipulation feature was not considered in the prior art, giving the novel configuration much more flexibility and many more possibilities for previously not considered applications and devices. Optical components, which can be integrated in the silicon chip with the alignment and placement features, can be gratings, mirrors, shutters, or thin film layers, whose position or performance can be changed or tuned, respectively, to further increase the integration level of the micro-optical device. The tuning or manipulation of the components on or in the optical bench can be done electrically, magnetically or by internal or external actuation of one or more components. Integrated actuators such as comb drive actuators can be as precise as a few nm or better giving the optical bench extremely precise alignment and tuning possibilities, which was completely impossible in the prior art, where all components are fixed by their packaging such as gluing or splicing.

The microf abrication of the silicon chip is based on standard processes of silicon and glass micromachining. In particular, deep reactive ion etching (DRIE) can very precisely etch U-grooves and other components into different substrates, typically silicon or glass. Recent advances in DRIE allow to transfer more complex and 3-dimensional structures such as prisms or lenses into the substrate. Therefore, it is also feasible to direct the optical beam out of the ibre-chip plane, i.e. changing the direction of the optical axis, by a slanted or tilted wall working as a mirror. More complex wall shapes will allow to achieve defined light intensity distributions at a certain distance either above or next the micro optical chip.

Organic films, thin films or metallic coatings deposited onto the components of the micro optical bench allow a wavelength sensitive performance, which can also be optically tuned by mechanical actuators, electric and/or magnetic fields. The actuator or field generating components can be integrated or external.

An optical comprises a support having at least one groove for the reception of at least an optical element (e.g. light transmitting /receiving/conducting devices). The groove is U- shaped and has inwardly projecting spring elements formed thereon for clamping the fibres to be placed in the groove. The optical coupling element may be used for aligning graded index (GRIN) fibre pieces with optical single mode fibres/waveguides on a silicon motherboard. The silicon motherboard can include aligning structures, snap-in springs, micro-electro-mechanical systems (MEMS) actuators and other MEMS structures with an optical function such as mirrors, filters and absorbers.

Claims

1. An optical coupling element comprising - a support - having at least one groove 17 for the reception of an optical element like a single mode optical fibre (SMF) (15) or a graded index (GRIN) multimode fibre (MMF) (13) , characterized in that - the groove 17 is U-shaped and that at distinct locations of the U-shaped groove (17) inwardly projecting spring elements (23) are formed for clamping the fibres placed in the groove (17).

2. Optical coupling element according to claim 1, characterized in that a rod of graded index (GRIN) multimode fibre (MMF) (13), whose length is smaller than the length of the groove (17), is placed in the groove (17)

3. Optical coupling element according to claim 2, characterized in that the graded index (GRIN) multimode fibre (MMF) is fixed in the groove (17) by means of at least one spring element (23).

4. Optical coupling element according to any of claims 1 to 3, characterized in that the support (11) is a silicon wafer or a piece of a silicon wafer, in particular a silicon on insulator substrate (SOI). .

5. Optical coupling element according to any of claims 1 to 4, characterized in that alignment marks are provided at the edge of the grooves (17) and at distinct locations along the groove (17). _

6. Optical coupling element according to any of claims 1 to 3, characterized in that stops (19) projecting into the groove (17) are provided at distinct locations along the groove.

7. Optical coupling element according to claim 5 or 6, characterized in that tb oraΛeA index (GRIN) multimode fibre (MMF) (13) is either arranged in the groove (17) in accordance with the alignment marks or inserted into the groove (17) such that an end face abuts the stop (19).

8. Optical coupling element according to any of claims 1 to 7, characterized in that a second graded index (GRIN) multimode fibre (MMF) (13') is placed in the groove (17) at a distance from the first graded index (GRIN) multimode fibre (MMF (13).

9. Optical coupling element according to any of claims 1 to 8, characterized in that each groove (17) is formed as two essentially spaced apart and parallel bars (17), e.g. of rectangular or square cross section, projecting from the surface of the support.

10. Optical coupling element according to any of claim 1 to 9, characterized in that a plurality of parallel and spaced-apart grooves (17) with inserted GRIN MMF's (13) are provided.

11. Optical coupling element according to any of claim 1 to 10, characterized in that between the end faces of the first and second GRIN fibres an optical device (25), e.g. an attenuator, a filter element, a mirror, a prism or the like, is provided.

12. Optical coupling element according to any of claim 1 to 11, characterized in that the spring elements (23) are integral with the support (11) and preferably along the same edge of the groove (17).

13. Micro-optical device comprising at least an optical coupling element according to any of claims 1 to 12 and, additionally, a graded index (GRIN) multi mode fibre (MMF).

14. Method of manufacturing an optical coupling element for coupling at least two optical fibres, comprising the steps of . - providing a silicon support (11) - etching an U-shaped groove (17) into the support surface and inwardly projecting spring elements at distinct locations along the U-shaped groove (17).

15. Method according to claim 14, characterized in that a graded index (GRIN) multi mode fibre (MMF) is placed in the groove (17).

16. Method according to claim 14, characterized in that one or more alignment marks are formed at the edge of the U-shaped grooves (17).

17. Method according to claim 14, characterized in that stops (19) projecting into the groove (17) are formed.

18. Method according to any of claims 14, to 17 characterized in that a first graded index (GRIN) fibre piece is placed in the U-shaped groove (17) such that its end face is aligned with a first alignment mark or abuts a first projecting stop (19).

19. Method according to claim 18, further comprising the step of - placing a second graded index (GRIN) fibre piece (13') at a distance from the first graded index (GRIN) fibre piece (13) such that one end face is aligned with the end face of the first GRIN fibre.

20. Method according to claim 19, characterized in that that the second graded index (GRIN) fibre piece (13^ is aligned wit a second alignment mark or abuts a second projecting stop(19).

21. Method according to any of claims 18 to 20 characterized in that the space between the opposite end faces of the GRIN MMF's are filled with an index-matching medium.

22. Method according to claim 14, characterized in that the inwardly projecting spring elements (23) are formed by an etching process.

23. Method according to any of claims 14 to 22, characterized in that each groove (17) is formed as two essentially spaced apart and parallel bars (27), e.g. of rectangular or square cross section, projecting from the surface of the support.

24. Method according to any of claims 14 to 23, characterized in that the support extends laterally beyond the groove (17).

25. Method according to any of claims 14 to 24, characterized in that an optical device or element (25), e.g. an attenuator, a filter element, a mirror, a prism or the like, is placed in the optical path between the GRIN MMFs.

26. Method according to any of claims 14 to 25, characterized in that the space between the second end face of the GRIN and the optical element is filled with an index- matching medium.

27. Method according to any of claims 15 to 26, characterized in that the graded index (GRIN) multi mode fibre (MMF) is manufactured comprising the steps of - forming a V- or U-groove (17) in a substrate, e.g. into a wafer substrate by an etching process; - placing GRIN fibres of a first length in the groove (17); - fixing the GRIN fibres in the groove (17) by a glue, wax or resist; and - and cutting the GRIN fibres into smaller pieces of the desired second length.

28. Method according to claim 27, characterized in that a plurality of parallel grooves (17) is formed and GRIN fibres of about the length of the grooves are placed in the grooves.

29. Light collimating optical system comprising an optical coupling element according to any of claims 1 to 12 and at least first and second light transmitting /receiving/conducting devices.