WO2006063807A2

WO2006063807A2 - Optical coupling device using a rod-shaped lens with aspeherical surfaces

Info

Publication number: WO2006063807A2
Application number: PCT/EP2005/013419
Authority: WO
Inventors: Bogdan Rosinki; Gnitaboure Yabre; Claude Casses
Original assignee: Fci
Priority date: 2004-12-13
Filing date: 2005-12-13
Publication date: 2006-06-22
Also published as: WO2006063807A3; FR2879307A1

Abstract

The invention relates to a device for optical coupling between an optical waveguide (1) and an optical or optoelectronic component (2), comprising a support (3) and a coupling element (5, 5'), the coupling element being a transparent optical rod comprising a light inlet face and a light outlet fac forming coupling faces (6, 7, 6', 7') for coupling between the optical waveguide and the component, in which the inlet face (6, 6', 7, 7') is shape so that the optical beam entering via this face propagates along an optical axis A and reaches the outlet face (7, 7', 6, 6') without essentially having been reflected on any surface between these inlet and outlet faces i.e. cylindrical beam there between.

Description

OPTICAL COUPLING DEVICE

The present invention relates to a device for optical coupling between a f irst optical waveguide and an optoelectronic component or a second optical waveguide .

More particularly, the present invention relates to a device for optical coupling between an optical waveguide and an optical or optoelectronic component , comprising a support and a coupling element , the coupling element being a transparent optical rod comprising a light inlet face and a light outlet face , forming faces for coupling between the optical waveguide and the component .

One embodiment of the optical rod of the prior art is described in the document US 6 491 443 .

According to said document , a f rustoconical rod centres a beam by reflections against the surface defined by a generatrix of the frustum.

The obj ect of the present invention is to improve the coupling performance of a device of the type mentioned above .

To do this , the present invention relates mainly to a coupling device comprising an optical rod, characterized in that the inlet face of the rod is shaped so that the optical beam entering via this face propagates along an optical axis and reaches the outlet face without essentially having been reflected on any surface between these inlet and outlet faces.

The device according to the invention makes it possible to prevent or at least to reduce losses at the faces and/or interfaces between the inlet and outlet faces.

According to one particular embodiment, the inlet face is an aspherical surface centred on an optical axis, this surface having a radius of curvature which increases as the distance from the optical axis increases .

According to another alternative or additional embodiment, the outlet face is an aspherical surface centred on an optical axis, this surface having a radius of curvature which increases as the distance from the optical axis increases.

According to the invention, the parameters of the inlet and outlet face of the lenses may be determined thanks to an equation which plots the z coordinate of the aspheric surface of the lens as a function of its radius of curvature, its conical constant and three aspherical parameters .

The optimization of the parameters is performed by means of an optical simulation software well know by the man in the art. The main criterion to meet is to minimize the losses between the inlet and the outlet optical signal, i.e. the inlet signal power should be substantially the same as the outlet signal power. The diameter of aspheric lenses will be chosen and optimized according to the diameter of the optical beam entering and exiting the optical device and consequently will also depend on the shape of the first optical waveguide and of the optoelectronic component or the second optical waveguide.

In one alternative or additional embodiment, at least one of the faces is a face having a network of diffraction patterns.

According to one particular embodiment of the invention, at least one of the inlet/outlet faces is surrounded by an annular rim.

More particularly, the annular rim is arranged on the side of the inlet/outlet face facing the component and comprises a stop flange which is inserted into a cavity of the component and forms centring means and means of adapting the focal distance between said facing face and the active element of the component.

According to the invention, the coupling element comprises means of retaining the coupling element in a support body, which means extend radially in a direction perpendicular to said optical axis.

The retaining means comprise in particular an annular ring.

In one particular embodiment, the support body is moulded onto the optical coupling element in a double injection-moulding operation. The coupling element more particularly includes a transparent body comprising a cylindrical inner section which ends in the inlet/outlet faces and a tubular outer section which surrounds the inner section and the ends of which form reference surfaces with respect to a focussing distance of the inlet/outlet faces.

Other features and advantages of the invention will be better understood upon reading the following description of a non-limiting example of embodiment of the invention with reference to the figures, in which:

Fig. 1 shows a schematic view of a first example of embodiment of an optical coupling element according to the invention;

Fig. 2a describes the shape of an aspheric surface (polynomial aspheric) of a lens according to the invention.

Fig. 2b shows a schematic view of the interception of an optical beam by a face of a coupling element of Fig. 1;

Fig. 3 shows a view of one variant of the coupling element of Fig. 1;

Fig. 4A shows a view from the side and in section of one example of embodiment of an optical coupling device according to the invention, comprising the coupling element of Fig. 3;

Fig. 4B shows a view from the side and in section of one example of embodiment of an optical coupling device according to the invention, comprising the coupling element of Fig. 3 associated with a first optical waveguide;

Figs. 5A and 5B show an alternative example of embodiment of an optical coupling element according to the invention and an enlarged view of a coupling face of this element.

The optical coupling device according to the invention aims to improve the transmission of optical signals in a link between a transmitter and a receiver and in particular to limit the losses at an optical connection between an optoelectronic component and an optical waveguide or between two optical waveguides.

A first example of embodiment of a coupling element 5 is shown in Fig. 1. This coupling element comprises inlet/outlet faces at two ends of a transparent body which is designed to be inserted into a housing of a coupler box.

The inlet/outlet faces of this element are aligned with the active element of an optoelectronic component 2 and with the waveguide 1 which is intended to be connected to the component.

The optoelectronic component 2 may be a transmitter such as a light-emitting diode or a laser diode or may be a receiver such as a phototransistor or a photodiode, whereas the waveguide is an optical fibre and in particular a monomode or multimode plastic fibre. According to this embodiment, the coupling element 5 comprises a rod provided at its end with curved inlet/outlet faces 6, 7, the two faces in this case being shaped so as to concentrate the optical beam at the core of the rod along a longitudinal optical axis A of the rod and shaped so as to focus the optical beam at a given distance (Dl, D2) from the rod.

According to this configuration, the face 6 may be an inlet face and the face 7 an outlet face, or vice versa. The faces 6 and 7 will be referred to in general as inlet/outlet faces.

The curvatures of the inlet/outlet faces may thus be adapted in each case to their waveguide 1 and component 2 counterparts.

The coupling element 5 of Fig. 1 comprises a transparent body including a cylindrical inner section 13 which ends in the inlet/outlet faces 6, 7 and an outer section 14 which surrounds the inner section and the ends 15, 16 of which extend the rod so as to form reference surfaces with respect to the focussing distance of the curved faces.

According to this example, the inlet/outlet faces 6, 7 are not necessarily in contact on the one hand with the component 2 and on the other hand with the waveguide 1, but rather are remote from these elements respectively by a distance Dl and by a distance D2, which distances are optimal with respect to the inlet/outlet faces for focussing the beam with respect to the waveguide and the component. The use of these inlet/outlet faces 6, 7 makes it possible to deflect the light rays forming the optical beam so that they do not substantially intercept the periphery of the rod in its longitudinal direction.

Parameters of aspherical lenses may be determined and optimized via equation (1) ,

axr²

(1) z= , +δxr +φxr +γxr +σxr l+sjl-a²(β+l)r²

which plots the z values of the lens ^■ aspheric surface shape (Fig. 2a) as a function of the following parameters:

- α: inverse of the radius of curvature of the lens surface;

- β: conic constant;

- r: minimum distance of any point of the optical aspheric surface to the axis of revolution of this optical aspheric surface. It is mentioned that the diameter of the aspheric lens corresponds to twice the maximum value of the parameter r.

- δ, φ, γ and σ: aspheric parameters (also known as coefficient of deformation of the lens with respect to a spherical lens) .

The parameters of the lenses will of course depend on the characteristics of the inlet and the outlet components (e.g. geometry, material) , and on the wavelength of the optical beam as well.

The other distances, namely Dl, D2 and L are provided via the same optical simulation software.

It is described hereunder, as an example, the optimized parameters of the aspheric lenses for coupling a FOT (Fibre Optic Transceiver) , capable of emitting or receiving an optical beam of 650nm wavelength, with a plastic optical fibre of lmm diameter.

According to this example, the parameters for the lens facing the FOT component will be comprised in the following ranges:

- α: 1,0 to 1,6 mm^" , more preferably between 1,2 and 1, 4 mm^"

- β: -1,05 to -0,65, more preferably between -0,85 and -0,80;

- δ: 4.10^~ to 6,6.10^" mm^" , more preferably between 5.10^"23 to 5,5.10^"23 mm^"3

- (p: 8,8.10^"23 to l,4.10^"22 mm^"5, more preferably between l,l.lθ^"22 to l,2.10^"22 mm^"5; - γ: -7.10 to -4.10^" mm^" , more preferably between -5.10^"23 to -6.10^"23 mm^"7;

29 29 9

- and σ: 2,2.10 to 3,6.10 mm^" , more preferably between 2,5.10^"29 to 2,9.10^"29 mm^"9.

- Diameter of the lens: 1,95 to 3,25 mm, more preferably between 2,25 and 2,75 mm.

With regard to the parameters of the lens facing the optical fibre, they are preferably comprised in the following ranges :

- α: 1,0 to 1,6 mm , more preferably between 1,2 and 1,4 mm^"1

- β: -1,05 to -0,65, more preferably between -0,9 and -0,8;

- δ: 2,7.10^"21 to 4,6.10^"21 mm^"3, more preferably between 3.10^"21 to 4.10~²¹ mm^"3 - φ: 5,3.10 to 8,9.10 mm^" , more preferably

-99 -99 ς between 6,5.10 to 7,5.10 mm ;

- γ: -3,0.10^"23 to -l,8.10^"23 mm^"7, more preferably between -2,5.10^"23 to -3,5.10^"23 mm^"7; - and σ: 1,05.1029 to 1,75.10 29 mm 9, more preferably

OQ _OQ _Q between 1,20, .10 to 1,50.10 mm ;

According to this example, the distance Dl is around 0.15 mm, the distance D2 is about 0,82 mm and the length of the inner section 13 being around 4.0 mm depending on the dimensions of the support body.

More specifically, the radius of curvature increases as the distance from the optical axis A towards the outside increases in the radial direction. This arrangement makes it possible to bring the beam towards the centre of the coupling element 5 and to intercept a greater part of the incident beam, as shown in Fig. 2b. In this Fig. 2b, portions of the intercepted beam are shown by hatched surfaces. These portions would not be intercepted by spherical surfaces having a radius of curvature corresponding to the centre of the lens.

Different focal distances between Dl and D2 for example are possible in the case of optoelectronic components having a lens, or in which the optoelectronic chip is more or less embedded in its casing.

More particularly, as shown in Fig. 1, the curved faces are formed so as to guide the light into the rod in the form of a cylindrical beam F, the beam leaving the curved faces 6, 7 in a convergent manner.

Moreover, in the embodiment of Fig. 1, the ends 15, 16 of the tubular outer section 14 extend beyond the curved faces 6, 7 and form stops on the one hand for the component and on the other hand for the waveguide 1, which prevents any mutual contact between the optical faces and separates the optical function from the mechanical centring and positioning function.

The coupling element is made of an optical material and moulded for example by a micromoulding operation.

Fig. 3 shows a variant embodiment of a coupling element 5', in which the coupling element comprises two curved inlet/outlet faces 6, 7, only one of which is surrounded by an annular rim 8.

Like in the example of Fig. 1, this annular rim 8 comprises a stop flange 9 which is inserted into a cavity 10 of the optoelectronic component, as shown in Fig. 4B, and forms centring means and means of adapting the focal distance between the curved face 7 and the active element of the component 2.

The mounting of the coupling element 5' in a support body 3, which in this case forms an optical coupling box, is shown in Fig. 4A.

According to this figure, the device for optical coupling between an optical waveguide 1 and an optical or optoelectronic component 2 comprises a support body 3 having a socket 4 for receiving one end of the first waveguide and a channel for receiving the coupling element 5' . The coupling element 5' shown in Fig. 4A is the coupling element of Fig. 3 and is of the type comprising a transparent optical rod of cylindrical overall shape, comprising a first curved light inlet/outlet face 6 and a second curved light inlet/outlet face 7, said faces forming faces for coupling between the optical waveguide 1 and the component 2.

The curved faces 6, 7 of the coupling element have a curvature which focuses an optical beam at given distances from the rod and in particular the distance Dl from the curved face 6 to the inlet/outlet face of the waveguide 1 and the distance D2 from the curved face 7 to the active element of the optoelectronic component 2.

According to the example of Fig. 4A, only the curved face 7 on the component side is surrounded by an annular rim 8 provided as explained above with a stop flange 9 which makes it possible to centre the rod with respect to the component 2.

The coupling element 5' comprises an optical axis A between the two curved faces and means 12 for retaining the coupling element in the support body 3. These means 12, which extend radially in a direction perpendicular to said optical axis, comprise according to the example an annular ring.

On account of the curvature of the faces which contain the beam F in the core of the rod, there is no or at least little loss of signal due to reflections in the annular ring for keeping the rod in the support body.

The rod may thus be made of a transparent plastic material without requiring the use of a graded-index material.

According to the example of Fig. 4A, the support body 3 may be moulded onto the optical coupling element 5' in a double injection-moulding operation or else moulded separately, the rod 5' then being inserted from outside the support body until the annular rim abuts against a shoulder within the channel for receiving the rod in the support body.

According to this example, the support body comprises a stop face 17 for the waveguide 1, this stop face defining the distance D2 from the curved face 7 to the waveguide 1, which means that there is no need to provide the rod with two annular rims.

The coupling element 5' comprises a transparent body including a cylindrical inner section 13 which ends in the curved faces and a tubular outer section 14 which surrounds the inner section and the ends 15, 16 of which form reference surfaces with respect to the focussing distance of the curved faces.

As seen above, the coupling element 5' retained in the support body 3 does not require two annular rims as in the example of Fig. 1, one of the stop flanges for the component or the fibre being directly formed in the support body 3. The stop face 17 of the support body protects the optical face when the rod is inserted into the support body 3.

The support body 3 forms an optical coupler socket provided with holding means 18 for the optoelectronic component .

According to the example, these holding means are a metal cover which comes to bear against the support body and comprises a spring tab 19 which presses the component 2 against the coupling element 5' .

The metal cover serves in particular as a casing for the electronic part of the optoelectronic component which provides shielding against electromagnetic interference (EMI) .

The support body 3 forms a socket for receiving a plug 20, said plug being known per se and only the general profile thereof being shown.

The plug 20 is provided with the optical fibre 1 which forms the optical waveguide and is inserted into the receiving socket in order to make the optical connection.

The examples of Figs . 1 to 4B are embodiments in respect of which at least one of the inlet/outlet faces 6, 7 is a curved face.

As seen above, the use of curved faces makes it possible to shape the beam by light refraction at the air/plastic interface and to deflect the beam so that the light rays do not substantially intercept the periphery of the rod in its longitudinal direction.

The example of Fig. 5A shows a coupling element which also has two light inlet/outlet faces shaped so as to concentrate the optical beam at the core of the rod along a longitudinal optical axis A of the rod, but in this case the principle used is a light diffraction principle, the faces 6' and 7' in this case having a network of diffraction pattern 21, one example of which is shown on an enlarged scale in Fig. 5B, which make it possible to recombine the waves once they have travelled a different path and to direct them into the waveguide or into the optoelectronic component.

The light emitted by the face will be the combination of the recombined beams as shown in Fig. 5B on the graph of the intensity I and the distance D.

A combination of the examples of Fig. 1 and Fig.

5A is possible, one of the faces then being a curved face while the other face is a face having a network of diffraction patterns.

The invention is not limited to the examples shown and in particular can be applied to the connection of two optical fibres to one another, one of the fibres being received in place of the component 2, the support in this case being a support with two sockets back-to-back. The support may also be a connector element provided with electrical contacts for the simultaneous connection of an electrical connector in particular in the context of applications such as audiovisual devices installed in motor vehicles or public transport vehicles.

Claims

1. Device for optical coupling between an optical waveguide (1) and an optical or optoelectronic component (2), comprising a support (3) and a coupling element (5, 5'), the coupling element being a transparent optical rod comprising a light inlet face and a light outlet face, forming coupling faces (6, 7, 6', T) for coupling between the optical waveguide and the component, characterized in that the inlet face (6, 6', 7, T) is shaped so that the optical beam entering via this face propagates along an optical axis (A) and reaches the outlet face (7, T, 6, 6') without essentially having been reflected on any surface between these inlet and outlet faces.

2. Device according to Claim 1, in which the inlet face (6, 7) is an aspherical surface centred on an optical axis, this surface having a radius of curvature which increases as the distance from the optical axis increases.

3. Device according to either of the preceding claims, in which the outlet face (7, 6) is an aspherical surface centred on an optical axis, this surface having a radius of curvature which increases as the distance from the optical axis increases.

4. Coupling device according to any of Claims 1 to 3, in which at least one of the faces (6', T) is a face having a network of diffraction patterns (21) .

5. Optical coupling device according to any of the preceding claims, in which at least one of the faces (6, 7, 6', T) is surrounded by an annular rim (8) .

6. Optical coupling device according to Claim 5, in which the annular rim (8) is arranged on the side of the face (7) facing the component and comprises a stop flange (9) which is inserted into a cavity (10) of the component and forms centring means and means of adapting the focal distance between said facing face (7) and the component (2) .

7. Optical coupling device according to any of the preceding claims, in which the faces (6, 6', 7, T) are formed so as to guide the light into the rod in the form of a cylindrical beam (11) .

8. Optical coupling device according to any of the preceding claims, in which the coupling element comprises means (12) of retaining the coupling element in a support body, which means extend radially in a direction perpendicular to said optical axis (A) .

9. Optical coupling device according to Claim 8, in which the retaining means (12) comprise an annular ring.

10. Optical coupling device according to any of the preceding claims, in which the coupling element (5, 5') includes a transparent body comprising a cylindrical inner section (13) which ends in the inlet/outlet faces and an outer section (14) which surrounds the inner section and the ends (15, 16) of which extend the rod so as to form reference surfaces with respect to a focussing distance of the inlet/outlet faces.