WO2001050169A1

WO2001050169A1 - Electrically terminated optical fibre cable

Info

Publication number: WO2001050169A1
Application number: PCT/EP2000/013369
Authority: WO
Inventors: Guido Oliveti
Original assignee: Pirelli Cavi E Sistemi S.P.A.
Priority date: 1999-12-31
Filing date: 2000-12-27
Publication date: 2001-07-12
Also published as: AU2844501A

Abstract

An electrically terminated optical cable (1) comprising: an optical cable (9) comprising an optical fibre, and an opto-electronic end portion (10) permanently connected to an end of said optical cable (9); said end portion (10) comprising, in turn an opto-electronic conversion device (12; 13) having an electric port and an optical port; a ferrule (5) firmly housing an end portion of said optical fibre so that said optical fibre has one end lying flush with an endface of said ferrule (5); an optical focusing lens (11) having an elongated shape and a first endface optically aligned with, and mechanically connected to, the optical port of said opto-electronic conversion device (12; 13), and a sleeve (6) tightly housing said ferrule (5) and said optical lens (11) with said endface of said ferrule (5) and a second endface of said optical lens (11) facing each other so as to optically align said optical lens (11) with said optical fibre housed into said ferrule (5).

Description

Electrically terminated optical fibre cable

** * * •** * *

DESCRIPTION

The present invention relates to an electrically terminated optical fibre cable. More m particular, the present invention relates to an electrically terminated optical fibre cable comprising, m turn, an optical fibre and an opto-elecronic end portion permanently connected to an end of the optical cable, a method for producing an electrically terminated optical cable and a unit comprising an opto-electromc conversion device and an optical lens.

Currently, m the field of telecommunications optical technology is mainly used for long-distance transmission of optical signals using the known properties of wide band provided by optical fibres. On the contrary, the most used technology for distributing signals to a plurality of users

(such as for example, analogue and/or digital television and/or telephone signals) and for transmitting digital data between electronic apparatuses (such as for example, the Personal Computers of a LAN network) makes use of electric cables such as, for example, coaxial cables or those made up of copper pairs .

Nevertheless, electric cables have a relatively narrow band, and they are becoming a bottleneck with respect to the band of signals to be transmitted. Moreover, they present problems of electromagnetic interferences, of impedance matching, and they are difficult to be introduced into the appropriate raceways of a building since they are stiff. In addition, being bulky, they significantly reduce the number of cables that can be inserted into a raceway. Moreover, due to electrical safety requirements, they require the arrangement of separate raceways from those used for distributing electric energy. Thus, the research is investigating the possibility of using optics not ust m the long-distance transmission of signals, but also m the signal distribution networks from a common branch point to a plurality of user apparatuses. In fact, optical -fibre cables are suitable to be inserted into the appropriate raceways of a building since they are not too bulky, they are flexible, light, free from electromagnetic interferences, and their bending loss is very low. Moreover, they are suitable to be inserted into the same raceways used for distributing electric energy. Additionally, optical fibres potentially have a very wide band, low attenuation values, and they are transparent to the bit rate, to the format and to the code of transmission.

Nevertheless, for connections to electronic apparatuses, optical-fibre cables require the use of opto-electromc conversion devices to convert electric signals into corresponding optical signals, and vice versa.

The conversion of an electric signal into a corresponding optical signal is conventionally carried out by modulating the intensity of a light emitted by an optical source. The modulation is associated with the information conveyed by the electric signal, and the conversion of an optical signal into a corresponding electric signal is conventionally carried out by means of a photodetector.

The connection of an optical cable to an optical source and/or to a photodetector is conventionally carried out by means of an optical connector. Typically, an optical connector is a device comprising two parts that can be repeatedly connected to and disconnected from one another, and which must be attached to an end of the optical cable and to the optical source or to the photo-receiver respectively.

For example, an installation of an optical cable suitable to connect, inside a building, a user apparatus to a central distribution unit (located, for example, m an office or m a flat and, respectively, m a cellar or m a loft) requires the implementation of the following steps: passing the optical cable along an appropriate raceway of the building; cutting the optical cable according to the desired length; fastening two parts of two connectors to the two ends of the cable m the proximity of the user apparatus and of the central unit; optionally fastening the other two parts of the two connectors to an optical source and to a photodetector (if the optical source and the photodetector are not already provided with connectors) ; and finally, connecting the two parts of each optical connector so as to connect, m a removable way, one end of the optical cable to the optical source and the other end of the cable to the photodetector, respectively m the proximity of the central unit and of the user apparatus.

Nevertheless, conventional optical connectors are disadvantageous to be used since their alignment with and fastening to the end of an optical-fibre cable (and optionally to the optical source and to the photodetector) are operations very delicate to be carried out on site, and require very high precision (m the range of micrometers) , the use of highly-specialised personnel and thus, involve high installation cost.

The Applicant faced the problem of reducing said costs by manufacturing an electrically terminated optical-fibre cable, that is to say, having at least one end optically aligned with and permanently connected to, an opto- electronic conversion device.

Electrically terminated optical cables have already been proposed.

US 3 792 284 describes an electro-optical link for transmitting signals comprising an optical-f bre guide, a light emitting diode (LED), and a photodiode. One end of the guide and the LED (or the photodiode) are mounted m fixed relationship with one another m a coupling unit having electrical connectors suitable to be inserted into a receptacle of an outer unit. The coupling between one end of the optical fibre and the LED (or photodiode) is carried out by placing them in front of one another.

US 3 878 397 describes an electro-optical transmission line wherein an electric signal is converted to an optical output signal by a LED and the optical signal is transmitted by an optical fibre bundle to a diode which converts the optical signal back to an electrical signal. The optical fibres are made of plastic or quartz. The coupling between one end of the optical fibre bundle and the LED (or photodiode) is carried out by placing them m front of one another.

US 4 595 839 describes an optical cable having two plastic or glass optical fibres and, at its two ends, two connectors. Each of the latter comprises a light source and a light detector respectively coupled to two ends of the optical fibres. The ends of the optical fibres are aligned with the respective light source or with the respective light detector through a ferrule housing the end of the fibre and an alignment guide which m turn houses the ferrule on one side, and the light source or the detector on the opposed side.

US 4 756 593 describes a connector comprising a plug and a socket. The plug is fastened to an end of an optical cable and comprises a LED for converting electric signals into optical signals and for sending them to the optical cable, and a photodiode for receiving the optical signals from the optical cable and converting them into electric signals.

US 4 767 168 describes a hybrid cable having at least one optical fibre, a metal conductor, an input connector and an output connector. One of the two input and output connectors is provided with a light source for converting electric signals into optical signals, and the other one is provided with a photodetector for converting optical signals into electric signals.

US 5 325 453 describes a signal transmission line comprising a polymeric optical wave guide and two connection elements connected to the two ends of the wave guide. In an embodiment, the transmission line is bi- directional; each connector comprises a light source, a photodiode (each having a portion of wave guide glued on their output) and a mixer. The light coming from the wave guide is sent by the mixer to the portion of wave guide glued to the output of the photodiode, whereas the light emitted by the light source on the portion of wave guide glued to it is sent by the mixer along the polymeric wave guide .

The Applicant faced the problem of providing an electrically terminated optical cable comprising an optical fibre and an opto-electromc conversion device that is relatively cheap and at the same time guarantees a precise alignment between the optical fibre and the opto-electromc conversion device.

Thus, m a first aspect thereof, the present invention relates to an electrically terminated optical cable comprising :

- an optical cable comprising an optical fibre, and

- an opto-electromc end portion permanently connected to an end of said optical cable; said end portion comprising, m turn

* an opto-electromc conversion device having an electric port and an optical port; * a ferrule firmly housing an end portion of said optical fibre so that said optical fibre has one end lying flush with an endface of said ferrule;

characterised m that said end portion also comprises

* an optical focusing lens having an elongated shape and a first endface optically aligned with, and mechanically connected to, the optical port of said opto-electromc conversion device, and

* a sleeve tightly housing said ferrule and said optical lens with said endface of said ferrule and a second endface of said optical lens facing each other so as to optically align said optical lens with said optical fibre housed into said ferrule.

In the present description and following claims, the expression "opto-electromc end portion or opto-electromc conversion device" respectively refers to an end portion or to a device carrying out an opto-electπc and/or electro- optical conversion.

Advantageously, the opto-electromc conversion device, the optical lens and the optical fibre housed into the ferrule are arranged with respect to one another so as to allow a transfer of optical power between the opto-electromc conversion device and the optical fibre equal to at least 5%. Typically, equal to at least 10%. Preferably, to at least 50%.

In the electrically terminated optical cable of the present invention, the alignment of the optical fibre with the optical port of the opto-electromc conversion device is obtained by means of a suitable coupling of the opto- electronic conversion device, of the optical fibre, of an optical lens, of a ferrule and of a sleeve. The precision of alignment is obtained thanks to the fact that the optical lens is optically aligned with and connected to, the optical port of the opto-electromc conversion device; that an end portion of the optical fibre is firmly housed into the ferrule, and that the ferrule and the lens are housed into the sleeve so as to optically align the optical fibre with the optical lens.

In the optical cable of the invention, the optical alignment between the optical fibre and the optical port of the opto-electromc conversion device is thus obtained with a single step of precise optical alignment (that one between the optical lens and the opto-electromc conversion device) and with simple steps of mechanical coupling (insertion of the end portion of the optical fibre into the ferrule and insertion of the ferrule and of the lens into the sleeve) . Considering that

- when assembling the various parts of the electrically terminated optical cable to reduce the times and costs required by the implementation of the step of precise optical alignment, said step itself can be carried out during the step of production of the opto-electromc conversion device m which, since a series of precision operations are needed for producing the conversion device, the presence of a suitable precision assembly bench is already provided; and that

ferrules, optical lenses and sleeves with manufacture tolerance up to one micrometer are available on the market at a low price,

the electrically terminated optical cable of the present invention advantageously allows obtaining a high precision coupling between optical fibre and opto-electromc conversion device at a relatively low cost.

Thus, the present invention advantageously allows obtaining an optical cable aligned with and permanently connected to, an opto-electromc conversion device with a high alignment precision and at a very low cost with respect to the prior art wherein the optical cable is detachably connected to an opto-electromc conversion device by means of a conventional optical connector.

Moreover, since the optical cable of the invention guarantees an alignment precision also m the range of micrometers, at a low cost, said optical fibre can advantageously be single-mode.

This is a very advantageous aspect of the invention because, although a single-mode optical fibre presents several advantages with respect to a multi -mode optical fibre (it is much less sensitive to bending losses, and it is less expensive m itself, more rugged, has lower absorption loss, it is suitable to be used for a wavelength division multiplexing - or WDM - transmission and it has a wider band) , its use on systems for distributing signals to a plurality of users has been very limited to-date due to the high costs required for aligning it with an optoelectronic conversion device.

In fact, as known, since a single-mode optical fibre has a core with a very small diameter (typically, equal to 7 - 10 μm with respect to that, for example, of 50 - 70 μm of a glass multi-mode optical fibre, or more than 100 μm of a polymeric multi -mode optical fibre) , it requires an alignment with the opto-electromc conversion device m the range of one or some micrometers. This is especially true m the particular case of a single-mode optical fibre and an optical source (typically, a laser) since also the optical source has typical dimensions m the range of some micrometers, and the light propagating into a single-mode optical fibre has a spot size different from the light emitted by the optical source [Masatoyo Sum da et al . , ⁿLens coupling of laser diodes to single-modal fibers" , JOURNAL OF LIGHTWAVE TECHOLOGY, Vol. LT-2 , No. 3, June 1984, pages 305-311] . Since the optical cable of the invention is capable of guaranteeing at low cost an alignment precision m the range of one micrometer, it advantageously allows a low- cost use of single-mode optical fibres m signal distribution systems to a plurality of user apparatuses.

Advantageously, said optical lens has a substantially cylindrical shape. Preferably, it is a GRIN™ lens.

Advantageously, said GRIN^IM lens has such a pitch as to focus the light m output from the optical port of said opto-electromc conversion device to the input of said optical fibre housed into said ferrule, or vice versa, to focus the light m output from said optical fibre to the input of the optical port of said opto-electromc conversion device. For example, said GRIN^1M lens has a pitch equal to 0.5 or to an integer multiple of 0.5.

Typically, said ferrule has a substantially cylindrical shape with a substantially central hole suitable to house the end portion of said optical fibre.

Advantageously, said optical fibre has a glass core and cladding with an outer protective coating of a polymeric material. Preferably, the core of said optical fibre has a diameter smaller than or equal to, 20 μm. More preferably, said diameter is comprised between 3 and 15 μm. Even more preferably, it is comprised between 5 and 10 μm.

Typically, the diameter of the outer protective coating is equal to about 250 μm. In addition, the outer diameter of the cladding typically is of atiout 125 μm.

Typically, the end portion of said at least one optical fibre is stripped of the outer protective coating.

Preferably, the substantially central hole of said ferrule has a diameter substantially equal to the outer diameter of the end portion of the optical fibre. More preferably, the substantially central hole of said ferrule has a diameter substantially equal to the outer diameter of the end portion of the optical fibre, stripped of the outer protective coating.

Advantageously, the difference between the diameter of said substantially central hole of said ferrule and the diameter of the end portion of said optical fibre is smaller than or equal to, 1.5 μm. Preferably, said difference is less than or equal to, lμm. More preferably, it is less than or equal to, 0.5 μm.

Advantageously, between the axis of the substantially central hole and the axis of symmetry of the ferrule there is a deviation lower than or equal to, 1.5 μm. Preferably, said deviation is lower than or equal to, 1 μm. More preferably, it is lower than or equal to 0.5 μm.

Advantageously, the axes of symmetry of said optical lens and of said ferrule substantially coincide into said sleeve .

Advantageously, said ferrule and said optical lens have a substantially equal outer diameter.

Preferably, said sleeve has a substantially cylindrical hollow body with an inner diameter substantially equal to the outer diameter of the ferrule and of the optical lens. More preferably, said sleeve is made of an elastic material, with a diameter substantially smaller than the outer diameter of the ferrule and of the optical lens, and with a longitudinal slot extending from side to side. In this way, said sleeve is adapted to enlarge so as to receive said ferrule and said optical lens, holding them into position once they have been inserted into it with an endface of the one towards the endface of the other one.

Typically, said sleeve is made of a metal material (for example, steel) or of zirconia. Advantageously, said opto-electromc conversion device comprises a laser source. Typical examples of suitable laser sources are Fabry-Perot lasers, VCSEL (vertical cavity surface emitting laser) lasers, and single- frequency lasers such as, for example, DFB or DBR semiconductor lasers .

Advantageously, it is a single transversal mode laser source .

According to an alternative embodiment, said opto- electronic conversion device comprises a photodetector. Typically, said photodetector is a photodiode. Preferably, it is a PIN photodiode.

Typically, considering the expected use for the distribution of signals to final users starting from a common branch point, said optical cable is shorter than 2 km. Advantageously, it is arranged m a plurality of standard lengths such as, for example, 20, 50, 100, 500 and 1000 m.

Advantageously, said optical cable also comprises a plastic sheath suitable to contain said optical fibre.

Advantageously, said optical cable also comprises a strength member. Preferably, said strength member comprises a plurality of conventional longitudinal yarns, flexible and with tensile strength, arranged between said optical fibre and said sheath of said optical cable. More preferably, said yarns are made of Kevlar™. Advantageously, said yarns allow withstanding possible outer tractive forces applied to the cable.

Advantageously, said plurality of longitudinal yarns exiting from said optical cable is arranged so as to coat said opto-electromc end portion. This advantageously allows protecting it from possible tractive forces applied to the electrically terminated optical cable. Advantageously, the maximum transverse dimension of the opto-electronic end portion is greater than the diameter of the outer sheath of the optical cable by at most 4 mm. More preferably, by at most 3 mm. Even more preferably, by at most 1 mm.

Preferably, the opto-electronic end portion of the electrically terminated optical cable has a transverse dimension smaller than 7 mm. More preferably, smaller than 5 mm. Even more preferably, smaller than 4 mm.

Advantageously, the outer sheath of the optical cable has a diameter comprised between 2 and 7 mm. More preferably, said diameter is comprised between 2 and 5 mm.

This advantageously allows facilitating the passage of the opto-electromc end portion of the electrically terminated optical cable of the invention along a raceway of a building .

Typically, said optical cable also comprises an additional optical fibre. Advantageously, in this case, said opto- electromc end portion of said electrically terminated optical cable also comprises:

- an additional opto-electronic conversion device having an electric port and an optical port ^•

- an additional ferrule firmly housing an end portion of said additional optical fibre so that said additional optical fibre has one end lying flush with an endface of said additional ferrule;

- an additional optical focusing lens having an elongated shape and a first endface optically aligned with, and mechanically connected to, the optical port of said additional opto-electromc conversion device; and

- a sleeve tightly housing said additional ferrule and said additional optical lens with said endface of said additional ferrule and a second endface of said additional optical lens facing each other so as to optically align said additional optical lens with said additional optical fibre housed into said additional ferrule.

As regards the structural and functional characteristics of said additional optical fibre, said additional optoelectronic conversion device, said additional optical lens, said additional ferrule and said additional sleeve, reference shall be made to what previously described.

According to an embodiment, besides the first optoelectronic end portion, the electrically terminated optical cable of the invention also comprises a second optoelectronic end portion permanently connected to an opposed end of said optical cable.

The structural and functional characteristics of said second opto-electromc end portion are totally similar to those of the first opto-electromc end portion; thus, reference shall be made to what previously described.

In this last embodiment, the optical cable of the invention is electrically terminated at both ends.

In a second aspect thereof, the present invention also relates to a unit comprising an opto-electromc conversion device, with an optical port and an electric port, and an optical focusing lens, with a substantially cylindrical shape, characterised m that said optical lens is optically aligned with and fastened to, the optical port of said opto-electronic conversion device.

Said unit advantageously allows optically aligning and connecting an opto-electronic conversion device and an optical fibre with high precision and very easily.

As regards the structural and functional characteristics of said opto-electromc conversion device and of said optical lens, reference shall be made to what described above relating to the electrically terminated optical cable of the invention.

In a third aspect thereof, the present invention relates to a method for producing an electrically terminated optical cable comprising an optical fibre and an opto-electronic conversion device permanently connected to said optical fibre, said method comprising the following steps:

a) firmly housing an end portion of said optical fibre into a ferrule so that said optical fibre has one end lying flush with an endface of said ferrule;

characterised m that it also comprises the following steps :

b) optically aligning a first endface of an optical focusing lens, having an elongated shape, with an optical port of said opto-electronic conversion device;

c) mechanically connecting the first endface of the optical lens with the optical port of the opto-electromc conversion device, optically aligned at step b) ;

d) tightly housing said ferrule and said optical lens into a sleeve, with said endface of said ferrule and a second endface of said optical lens facing each other so as to optically align said optical lens with said optical fibre housed into said ferrule at said step a) .

Advantageously, step b) is carried out m an active way. That is to say, it is carried out by measuring the optical power transmitted by the optical port of the opto- electromc conversion device to the optical lens or, vice versa, by the optical lens to the optical port of tne opto- electronic conversion device, reciprocally arranging said optical lens and said optical port so as to obtain a suitable transfer of optical power from the one to the other one .

Typically, step c) is carried out by means of gluing.

Advantageously, m step d) , the endface of said ferrule and the second endface of said optical lens are arranged, inside said sleeve, one against the other one.

As regard the structural and functional characteristics of said opto-electromc conversion device, of said optical lens, of said ferrule, of said sleeve and of said optical fibre, reference shall be made to what described above relating to the electrically terminated optical cable of the invention.

In a further aspect thereof, the present invention relates to an electrically terminated optical cable comprising:

- an optical cable comprising an optical fibre and a strength member comprising a plurality of longitudinal yarns, flexible and with tensile strength, and

- an opto-electromc end portion permanently connected to an end of said optical cable, said end portion comprising an opto-electromc conversion device ,- characterised m that said plurality of longitudinal yarns is arranged so as to coat said opto-electromc end portion and is fastened to it.

In a further aspect thereof, the present invention also relates to an electrically terminated optical cable comprising:

- an opto-electronic end portion permanently connected to an end of said optical cable, said opto-electronic end portion comprising a unit comprising an opto-electromc conversion device, having an optical port and an electric port, and an optical focusing lens, with a substantially cylindrical shape, said optical lens being optically aligned with and fastened to, the optical port of said opto-electronic conversion device; wherein said plurality of longitudinal yarns is arranged so as to coat said opto-electronic end portion and is fastened to it.

Further features and advantages of the present invention will appear more clearly from the following detailed description of a preferred embodiment, made with reference to the attached drawings . In such drawings :

- Figure 1 shows a portion of an electrically terminated optical cable of the invention;

- Figure 2 shows elements comprised m the end portion of Figure 1.

Figure 1 shows an electrically terminated optical cable 1 according to an embodiment of the invention, comprising an optical cable 9 having two ends and an opto-electromc end portion 10 permanently connected to one of the two ends.

In the embodiment shown, the optical cable 9 comprises two single-mode optical fibres (not shown) , a plastic coating to cover both the fibres, and an outer plastic sheath.

Each optical fibre s of the SMR model produced by the manufacturing firm FIBRE OTTICHE SUD F.O.S. S.p.A., with a core, a cladding with an outer diameter equal to 125 μm, and an outer acrylate coating with outer diameter of 250 μm. In addition, they have a nominal attenuation equal to 0.2 dB/Km.

Moreover, the optical cable 9 comprises a plurality of longitudinal yarns made of Kevlar™ (not shown) that are flexible and tensile resistant.

Said Kevlar^1M yarns are arranged between the plastic coating of the two optical fibres and the outer plastic sheath. For example, the optical cable 9 is of the type described m the Patent Application EP 0 829 742 filed by the Applicant .

The opto-electromc end portion 10 comprises a container 2 of which a first portion 17 is suitable to house one of the two ends of the optical cable 9, two end portions of the two optical fibres and two ends of two ferrules 5, whereas a second portion 18 is suitable to house the two ferrules 5, two optical lenses 11, two sleeves 6, a first and a second opto-electromc conversion device 12 and 13, an electronic card 14 and two electric cables 15 (Figure 2)

Advantageously, said container 2 is made of plastic, and it is suitable to house the above components m a reciprocally fixed position. Moreover, m the embodiment shown, it has a substantially parallelepiped shape, a length of about 40 mm, a height of about 3 mm and a width of about 5 mm.

The first portion 17 of said container 2 has a groove 3 for housing an end of the optical cable, two grooves 4 for housing the ends of the two optical fibres exiting from the optical cable 9, and two holes 16 for housing two ends of the ferrules 5.

In addition, the second portion 18 of said container 2 is provided with two longitudinal holes 19 suitable to house the two sleeves 6, and with a support base 20 for housing the electronic card 14, the two opto-electromc conversion devices 12 and 13 and the two electric cables 15.

The two ferrules 5 have a cylindrical shape with a substantially central hole 8 suitable to nouse two end portions of the two optical fibres

For example, said ferrules 5 are produced and marketed by the firm HIROSE, JP, and their manufacture tolerance is m the range of 1 μm. The two optical lenses 11 are conventional GRIN™ lenses with a cylindrical shape.

For example, said optical lenses 11 are produced and marketed by the firm NSG America, and their manufacture tolerance is m the range of 1 μm.

In the embodiment shown, both ferrules 5 and the optical lenses 11 have an outer diameter of about 1.249 mm, and a length of about 5 mm.

The two sleeves 6 are made of metal, and they have a cylindrical shape Moreover, each sleeve has an inner diameter smaller than the outer diameter of ferrules 5 and of the optical lenses 11 so as to tightly house them inside it. In the example shown, the sleeve has an inner diameter of about 1.2 mm. A longitudinal groove allows sleeves 6 to enlarge so as to receive the ferrules 5 and the optical lenses 11 inside it.

Additionally, m each sleeve 6, ferrule 5 has an endface m contact with an endface of the optical lens 11.

According to a variant, m each sleeve 6, ferrule 5 and the optical lens 11 are interspaced by means of a transparent element (for example, of glass) or of a mechanical abutment surface inside the sleeve 6.

In both cases, the GRIN™ optical lens 11 optically aligned with and fastened to, the optical port of the laser source 12 has a selected pitch so as to focus the light exiting from said optical port onto the end of the corresponding optical fibre.

Moreover, the GRIN^rM optical lens 11 optically aligned with and fastened to, the optical port of photo-receiver 13 has a selected pitch so as to focus the light exiting from said optical fibre onto the optical port of photo-receiver 13.

Both the first and the second opto-electromc conversion device 12 and 13 have an optical port and an electric port (not shown) .

In the embodiment shown, the first opto-electromc conversion device 12 is a laser source of the VCSEL type, with emission at wavelength of about 1300 nm, and having a diameter of about 1.5 mm and length of about 3 mm.

Advantageously, the laser source 12 is housed into a PILL package, and the optical lens is glued directly onto the package .

In turn, the second opto-electronic conversion device 13 is a PIN photodiode produced by the manufacturing firm EPITAXY with a reception band of about 1 GHz, and an active area of about 100 μm.

The electronic card 14 is a conventional device the characteristics of which will be obvious to the man skilled m the art on the basis of the present description and without departing from the scope of the invention.

The electric cables 15 are, for example, conventional coaxial cables. Typically, they are less than 2 m long. Preferably, their length is comprised between 0.5 and 1 m.

Alternatively, when the transmission band is lower than about 10 Mbit/s, the electric cables 15 can be copper pairs .

When assembling the various elements of the end portion 10 of the electrically terminated optical cable 1 of the invention, each optical lens 11 is optically aligned with and fastened to, the optical port of the respective optoelectronic conversion device 12, 13.

The optical alignment, for example, can be conventionally carried out m an active way whereas the fastening can be carried out through gluing. Moreover, sleeves 6 are inserted into the longitudinal holes 19 of the second portion 18 of container 2.

On the side of the optical cable 9, said holes 19 have apertures with such dimensions as to prevent the passage of sleeves 6. Thus, the latter are introduced into holes 19 through apertures located on the opposed side with respect to cable 9 and once inserted into said holes 19, they are locked inside them by the apertures of the holes 19 located on the side of the optical cable 9, on the one side, and by the electronic card 14, which is afterwards laid onto the support base 20, on the opposed side.

In addition, the end portions of the optical fibres exiting from the optical cable 9 are stripped of their outer acrylate coating, they are made pass through the two holes 16 of the first portion 17 of container 2; they are coated with a glue and inserted into the substantially central hole 8 of ferrules 5, where they are shortly let slide backwards and forwards so as to evenly distribute the glue inside the hole 8. Once the end portions of optical fibre have been glued inside hole 8, they are cut lying flush the endfaces of ferrules 5 located at the opposed side of the optical cable 9, and said faces are polished through lapping .

Then, ferrules 5 and the optical lenses 11 are inserted into the second portion 18 of container 2, inside sleeves 6, with the endfaces of ferrules 5, lying flush with which are the ends of the optical fibres, m contact with the endfaces of the respective optical lenses 11.

The second portion 18 of container 2 is suitable to loosely house sleeves 6 so as to allow them to slightly enlarge during the introduction of ferrules 5 and of the optical lenses 11.

Then, the ends of ferrules 5 located on the side of the optical cable 9 are glued into holes 16; the two optical fibres exiting from the optical cable 9 are arranged into the respective grooves 4, and an end portion 44 of the outer plastic coating, with the two optical fibres housed into it, is inserted into groove 3.

After having inserted sleeves 6 and the optical lenses 11, with the respective opto-electronic conversion device 12, 13, fastened to them, into container 2, the electronic card

14 is arranged onto the support base 20; the electrical connections between the opto-electronic conversion devices 12 and 13, the electronic card 14 and the electric cables

15 are carried out and, finally, the first and the second portion 17 and 18 of container 2 are sealed with two respective covers 21 and 22, and they are fastened to one another so as to obtain the end portion 10 shown in Figure 1.

Finally, the plurality of Kevlar™ yarns exiting from the optical cable 9 is wound around and fastened to the end portion 10 so as to impart tensile strength to it.

Optionally, the Kevlar™ fibres continue beyond the end portion 10 and they are wound also around the electric cables 15.

Moreover, the end of the optical cable 9, the end portion 10 and, in case, the electric cables 15, enveloped m the plurality of Kevlar-"⁴ yarns, are covered with a sheath made of a heat-shnnkable material. The latter is then heated so as to make the sheath adhere to the end of the optical cable 9, to the end portion 10, and m case, to the electric cables 15 so as to make them suitably impermeable to outer agents.

According to an alternative, the optical cable 9 comprises one or more electric conductors, for example, adapted to transport a supply current .

Nevertheless, the variant of the optical cable 9 without electric conductors is preferred since it has the advantage that it can be inserted into the same raceways used for distributing electric energy without any problems of electric safety.

As already said, the GRIN™ optical lens 11, optically aligned with and fastened to, the optical port of the laser source 12 has a selected pitch so as to focus the light exiting from said optical port onto the end of the corresponding optical fibre. Moreover, the GRIN™ optical lens 11 optically aligned with and fastened to, the optical port of photo-receiver 13 has a selected pitch so as to focus the light exiting from the corresponding optical fibre onto the optical port of photo-receiver 13.

The opto-electromc end portion 10 of the electrically terminated optical cable 1 thus guarantees a high precision of optical coupling between the opto-electro c conversion devices 12, 13 and the corresponding optical fibres.

Although m the embodiment shown the bi -directional optical cable 9 comprises two optical fibres respectively connected to the laser sources 12 and to photo-receiver 13, according to a variant (not shown) the optical cable 9 can comprise a singe optical fibre connected, by means of a conventional optical coupler or an optical circulator, both to the laser source 12 and to photo-receiver 13. According to another variant, the optical signals travelling into the two directions have two different wavelengths and the couplers are advantageously wavelength selective, such as for example, conventional wavelength multiplexing/ demultiplexing devices.

In the embodiment shown, the electrically terminated optical cable 1 has a single opto-electromc end portion 10. That is to say, the optical cable 9 is permanently electrically terminated only at one end.

According to an alternative embodiment of the invention, besides the first end portion 10 permanently connected to an end of the optical cable 9, the electrically terminated optical cable 1 has a further opto-electronic end portion

(not shown) which is permanently connected to the opposed end of said optical cable 9.

As regards the structural and functional characteristics of said additional opto-electromc end portion, reference shall be made to what described above relating to the first end portion 10.

For example, m the case that the optical cable is electrically terminated at the two ends; the laser source

12 of an end portion 10 emits optical power equal to 1 mW; the coupled power m optical fibre is equal to -10 dBm

(equal to 10% of the power emitted by the laser source) and the receiver comprising the photodiode 13, m the opposed end portion 10, has a sensitivity equal to -30 dBm, the electrically terminated optical cable of the invention has a power budget equal to 20 dB. In other words, power losses up to a maximum of 20 dB are allowed along the electrically terminated optical cable.

The electrically terminated optical cable 1 of Figure 1 can be used for a bi-directional transmission of optical signals between an optical apparatus and an electric apparatus .

For example, it can be used for connecting an optical distribution unit and an electric user apparatus m a signal distribution system to a plurality of users. In this case, the electric carjles 15 of the opto-electromc end portion 10 of the cable of the invention can be directly connected to the electric connections of the user apparatus .

In addition, the electrically terminated optical cable 1 can be made pass along the raceway of a building, which connects the electric user apparatus to the distribution unit, from the optical end not connected to the optoelectronic end portion 10 since the dimensions of said optical end are typically smaller than those of said optoelectronic end portion 10.

On the contrary, when the two ends of the optical cable 9 are permanently connected to two opto-electronic end portions 10, the electrically terminated optical cable of the invention can be directly connected to an electric user apparatus and to an electric distribution unit without having to use any optical connector for the connection to the opto-electronic conversion devices.

In this last case, the electrically terminated optical cable 1 must be made pass along the appropriate raceway through one of the opto-electronic end portions 10. This is not an obstacle because, as described above, said end portions of the electrically terminated optical cable of the invention can be miniaturised. In particular, they can have transverse dimensions in the range of some millimetres .

Claims

1. An electrically terminated optical cable (1) comprising:

- an optical cable (9) comprising an optical fibre, and

- an opto-electronic end portion (10) permanently connected to an end of said optical cable (9) ; said end portion (10) comprising, m turn

* an opto-electromc conversion device (12; 13) having an electric port and an optical port;

* a ferrule (5) firmly housing an end portion of said optical fibre so that said optical fibre has one end lying flush with an endface of said ferrule (5) ;

characterised m that said end portion (10) also comprises

*an optical focusing lens (11) having an elongated shape and a first endface optically aligned with, and mechanically connected to, the optical port of said optoelectronic conversion device (12; 13), and

* a sleeve (6) tightly housing said ferrule (5) and said optical lens (11) with said endface of said ferrule (5) and a second endface of said optical lens (11) facing each other so as to optically align said optical lens (11) with said optical fibre housed into said ferrule (5) .

2. An electrically terminated optical cable (1) according to claim 1, wherein said optical fibre is single-mode.

3. An electrically terminated optical cable (1), according to claim 1 or 2 , wherein said optical lens (11) has a substantially cylindrical shape.

4. An electrically terminated optical cable (1) according to claim 3, wherein said optical lens (11) is a GRIN^1M lens .

5. An electrically terminated optical cable (1) according to any of the preceding claims 1 to 4 wherein said optoelectronic conversion device (12; 13) is a photo-receiver (13) .

6. An electrically terminated optical cable (1) according to any of the preceding claims 1 to 4, wherein said optoelectronic conversion device (12; 13) is a laser source (12) .

7. An electrically terminated optical cable (1) according to any of the preceding claims 1 to 6, wherein said opto- elecronic end portion (10) has transverse dimensions smaller than 7 mm.

8. An electrically terminated optical cable (1) according to claim 7, wherein, said opto-elecromc end portion (10) has transverse dimensions smaller or equal to 5 mm.

9. An electrically terminated optical cable (1) according to any of the preceding claims 1 to 8 , also comprising a second opto-electronic end portion (10) , permanently connected to an opposite end of said optical cable (9) .

10. An electrically terminated optical cable (1) according to any of the preceding claims 4 to 9, wherein said GRIN™ lens (11) has a pitch equal to 0,5 or to an integer multiple of 0.5.

11. An electrically terminated optical cable (1) according to any of the preceding claims 1 to 10, wherein said endface of said ferrule (5) and said second endface of said optical lens (11) tightly housed in said sleeve (6) are m contact .

12. An electrically terminated optical cable (1) according to any of the preceding claims 1 to 10, wherein said ferrule (5) and said optical lens (11) tightly housed m said sleeve (6) are interspaced by means of a transparent element .

13. An electrically terminated optical cable (1) according to any of the preceding claims 1 to 12, wherein said optical cable (9) also comprises a strength member comprising a plurality of longitudinal yarns, flexible and with tensile strength, said plurality of longitudinal yarns being arranged so as to coat said opto-electromc end portion (10) and being fastened to it.

14. A method for producing an electrically terminated optical cable (1) comprising an optical fibre and an optoelectronic conversion device (12; 13) permanently connected to said optical fibre, said method comprising the following steps :

a) firmly housing an end portion of said optical fibre into a ferrule (5) so that said optical fibre has one end lying flush with an endface of said ferrule (5) ;

characterised m that it also comprises the following steps :

b) optically aligning a first endface of an optical focusing lens (11), having an elongated shape, with an optical port of said opto-electromc conversion device (12; 13),

c) mechanically connecting the first endface of the optical lens (11) with the optical port of the opto-electromc conversion device (12; 13), optically aligned at step b) ,

d) tightly housing said ferrule (5) and said optical lens (11) into a sleeve (6) with said endface of said ferrule (5) and a second endface of said optical lens (11) facing each other so as to optically align said optical lens (11) with said optical fibre (5) housed into said ferrule at

15. A unit comprising an opto-electromc conversion device (12; 13), with an optical port and an electric port, and an optical focusing lens (11) , with a substantially cylindrical shape, characterised m that said optical lens

(11) is optically aligned with and fastened to, the optical port of said opto-electromc conversion device (12; 13) .

16. A unit according to claim 15 wherein said optical lens

(11) is a GRIN™ lens.

17. A unit according to claim 15 or 16, wherein said optoelectronic conversion device (12; 13) is a laser source (12) .

18. A unit according to claim 17, wherein said laser source

(12) is a VCSEL laser.

19. A unit according to claim 16, 17 or 18, wherein said GRIN™ lens (11) has a pitch equal to 0,5 or to an integer multiple of 0.5.

20. An electrically terminated optical cable (1) comprising:

- an optical cable (9) comprising an optical fibre and a strength member comprising a plurality of longitudinal yarns, flexible and with tensile strength, and

- an opto-electromc end portion (10) permanently connected to an end of said optical cable (9) , said opto-electromc end portion (10) comprising an opto-electronic conversion device (12 ; 13) ; characterised m that said plurality of longitudinal yarns is arranged so as to coat said opto-electromc end portion (10) and is fastened to it.

21. An electrically terminated optical cable (1) comprising : - an optical cable (9) comprising an optical fibre and a strength member comprising a plurality of longitudinal yarns, flexible and with tensile strength, and

- an opto-electromc end portion (10) permanently connected to an end of said optical cable (9) , said opto-electronic end portion (10) comprising a unit comprising an optoelectronic conversion device (12; 13), having an optical port and an electric port, and an optical focusing lens (11) , with a substantially cylindrical shape, said optical lens (11) being optically aligned with and fastened to, the optical port of said opto-electronic conversion device (12; 13); wherein said plurality of longitudinal yarns is arranged so as to coat said opto-electronic end portion (10) and is fastened to it.