FR2479618A1 - Bidirectional fibre=optic link for cable TV - has large dia. step refractive index fibres and gradient index fibre for carrier transmission - Google Patents

Abstract

The cable provides a link between two stations (A,B). The energy transmitter-receivers of each station are optically coupled to a single link cable (Ca) which comprises an optic fibre of step index (f2) (large dia. fibres) and an optic fibre of gradient index (f1), the latter being made e.g. by modified vapour chemical deposition. The cable has an axial fibre of step index forming the carrier element and one or more gradient index fibres colinear with the axial fibre. Alternatively the axial fibre is of step index while the gradient index fibre is coiled about the axial fibre. A plastics protective coating (1) is provided together with aromatic polyamide fibres embedded in the plastics parallel with the step index fibre.

Description

 The present invention relates to a system of bidirectional optical links by optical fiber cables and to a television distribution network comprising such a system.

Cable television, widely used in certain countries, will further develop in the next appendices. This development is facilitated by the use of free optical link cables.
In the simplest cable television networks, a central station (hereinafter called central) transmits by a connecting cable connected to subscriber stations (hereinafter called subscribers) one or more television programs, broadcasting programs
AM / FM, as well as possibly messages intended for particular subscribers. The selection is made by tuning with the help of filters the receiver which is equipped the subscriber on a determined frequency band, associated with a particular program In more elaborate networks, transmissions are bidirectional These systems allow, in addition to the facilities which have just been listed, the connection to the cable distribution system of devices such as telex, videophone, etc. The subscriber can then transmit to order center to choose a particular program (TV, AM / FM) which alone will be transmitted to it "or issue orders relating to the other available services. It can also transmit its own video signals, for example within the framework of a connection by telephone.

The transmission of video signals requires the implementation of high speed link or what is equivalent to wide bandwidth It was first of all used in the context of cable television, connections by electric cables, coaxial for example To ensure a higher speed, it is advantageous to replace these electrical connections by connections provided by optical fibers
The commonly used bidirectional optical transmissions can be classified according to two fundamental approaches.

According to the first approach, the connections between the exchange and the subscribers
are performed using a light source of a first length
wave modulated by the information to be transmitted. Reverse links
between subscribers and the exchange is made using a
second wavelength light, also modulated by informa
tions to be transmitted between subscribers and central This approach is, for
example, described in French patent application no. 74 Ol 800 filed
January 18, 1974 and published under number 22 587 51 or in the article by
TAKEUCHI and aI "OPTICAL DIRECTIONAL FILTER AND ITS APPLICA
TION TO THE BIDIRECTIONNAL TRANSMISSION SYSTEM "published in the
Dutch publication :: "Fifth European Conference on Optical Communi
cation, AMSTERDAM, 17-19 September 1979, pages 13. 2-1 to 13. 24.

According to the second approach, only one wavelength is used
works for transmissions, on the one hand, between the exchange and the subscribers and,
on the other hand, for reverse transmissions. Discrimination between
two directions of transmission are made using directional couplers.

This last approach allows the reversal of the transmission organs
and reception, however the risk of crosstalk is higher.

This approach is described for example in the patent application
French No 76 08 389, filed on March 23, 1976 and published under No 2 362
413 or in the article by ICHIDA et al: "BIDIRECTIONAL VIDEZ TRANSMIS
SION SYSTEM USING A SINGLE OPTICAL FIBER1, also published in the
above publication, pages 20.3 - 1 to 20.3 - 4.

In addition to the risk of crosstalk which has just been mentioned, the connections
carried out according to the two aforementioned approaches require to be implemented
work, the development of specific components. On the other hand,
couplings between the cable television cable and the transmitting devices
central office and subscriber receivers must enter the least
possible losses. It follows that high efficiency coupling members
should be used. If these bodies participate only for a small part
at the cost of central office installations, it is not the same for
devices installed at subscribers.

 A third approach would be to split the number of link cables to make bidirectional transmissions using two cables providing unidirectional links. This approach would, however, lead to a very sharp increase in the price of the installation. On the other hand, only the link in the central-subscriber direction is a high-speed link (or broad bandwidth). It follows that the reverse link would be used far below its capacity.

 The invention provides a system of bidirectional links by optical means using only optoaelectronic components currently existing. It also does not require the use of high performance components to ensure the coupling between the transceiver installed at the subscriber and the optical link cable, thus minimizing the problems of optical connection at the subscriber and the like. made me reducing the cost of its coupling to the cable system.

 The subject of the invention is therefore. a system of bidirectional fiber optic links between a first station and a second station each station comprising opto-electronic means for transmitting and receiving radiant energy; system characterized in that the radiant energy transmission-reception means are optically coupled to a single connecting cable, connecting the first and second stations and comprising an index-hopping optical fiber and at least one gradient optical fiber index.

 The invention also relates to a cable television network implementing such a system.

The invention will be better understood and other advantages will become apparent from the following description, with reference to the appended figures:
- Figure 1 schematically illustrates a system of bidirectional optical links by optical fibers according to a first approach of the known art;
- Figure 2 illustrates a second approach to the known art;
- Figure 3 schematically illustrates the architecture of a cable network in which the invention can be implemented;
- Figure 4 is a first alternative embodiment of a cable used in the bidirectional fiber optic link system according to the invention;
- Figure 5 is a second alternative embodiment of this cable;
- Figure 6 is a sectional view of these cables;
- Figures 7 and 8 illustrate particular points of the invention.

 In what follows, elements common to two or more figures bear the same references and will only be described once.

 FIG. 1 illustrates an example of a system of bidirectional links by optical fibers between two stations A and B according to a first approach of the known art. To fix the ideas, it will be assumed that each station, A and B, comprises a transmitter of video signals, respectively S1 and S2, and a receiver of video signals symbolized by a television screen, respectively TV1 and TV2. According to this approach, two different wavelengths are used: a first wavelength for signal transmissions from station A to station B, on the one hand, and a second wavelength 2 for reverse transmissions The signal source Ss is associated with an optoelectronic organ E1 for emission, producing a light beam of the first wavelength 1 modulated by the signals of the emitter S1 and transformed using the lens L10 in a beam of parallel rays transmitted on a mirror M10 which reflects this beam of parallel rays towards a lens L1 of focusing on the termination tl of a connecting fiber f coupling station A to station B. The beam of parallel rays crosses previously a mirror (or a blade), semi-transparent M11 at the wavelength \ 1. This mirror M11 is used to reflect the radiation of the second wavelength ts 2 towards an optoelectronic detection member R1, via a focusing lens L11. The output of this opto-electronic detection device is connected to the TV1 display device. Station B has the same optical and opto-electronic elements as station A: E2, R2, L20, L21 'M20 M21L2 and t2. There is no need to rewrite them. It should however be noted that. the optoelectronic emitter E2 must produce a beam of light rays of wavelength 2 and the semi-transparent mirror M21 must reflect the light rays of wavelength # 1 and transmit the light rays of wavelength # 2. Another solution consists in using two identical mirrors, M11 and M21, and in inverting the respective places of the opto-electronic transmission and reception organs, E2 and R2.

 This approach is described in more detail in French patent application No. 74 018 00 and the article by TAKEUCHI et al above.

 A second approach of the known art is illustrated in FIG. 2. This approach implements only one and only wavelength whatever the direction of the transmissions considered. We find the main elements described in relation to Figure 1: S1 - E1, S2 - E2, R1 - TV1, R2 - TV2 and f which it is useless to rewrite. To obtain bidirectional connections, it is necessary in this case to use directional couplers, respectively DC1 and DC2. The directional coupler DC1 is connected on the one hand using two optical fibers f10 and fll 'respectively to transmitter E1 and receiver R1 on the one hand, and a connector C1 on the other hand, via fiber f12. The same is true for station B, in which the directional coupler DC2 is connected using the fibers f20 to f22 respectively to the transmitter E2, to the receiver R2 and to the connector C2. The bidirectional connections are made as before using an optical fiber f.

This approach, described for example in French patent application No. 76 083 89 and in the article by ICHIDA et al, cited above. This approach presents higher risks of crosstalk between upstream A A # B and downlink B # A.

 These two approaches also require the development of specific components. on the other hand, although not limited to this single application, the invention is particularly advantageous in the context of a cable television network. As will be described later, depending on whether the transmissions are considered in the uplink or downlink, the information rate is very different. It is therefore not necessary to offer the same transmission capacity in both directions. Finally, if the organs intended for coupling the central station of a cable television network ensuring the connections represents only a short installation, it is not the same for the organs intended for coupling subscriber stations on this me cable. In addition, this coupling must be able to be carried out easily, without requiring delicate development. The invention proposes to meet these needs.

Before describing a first variant embodiment of the invention, it
It is useful to recall the architecture of a cable television network, in which the invention can be implemented. FIG. 3 illustrates an example of architecture of such a network. A central station A, called in what follows in a more concise central manner, communicates with a subscriber or subscriber station B, by a cable C a ensuring bidirectional links by two unidirectional links, symbolized by L1 and L2. Although only one subscriber has been represented, in practice the number of subscribers can be very high. To fix the ideas, exchange A ensures the simultaneous transmission of three video channels, corresponding to two TVI TV programs and TV2, and to a videophone connection VP, as well as a corresponding channel to which it will be called in the following "other services These services can include, for example, telex, teletext transmissions, or the broadcasting of musical programs AMiFM in stereophony All these services are simultaneously available and can be chosen by a subscriber B. The transmission of the signals corresponding to the services which have just been listed is ensured by the devices A1 to A4. These devices are well known to the skilled in the art and depart from the scope of the invention. Subscriber B, in order to choose a particular program, or possibly several programs, can transmit to central exchange A switching order signals of the video channels: CVD, CTV or its own telex or videophone signals, represented under the unique reference CSV.

 The channels or channels corresponding to "other services" and the control orders occupy a narrow frequency band: a few tens of KHZ, while the channels corresponding to video signals require approximately 6 MHZ of base band.

 Several coding and multiplexing methods can be used, but one of the most commonly used is frequency modulation multiplexing. In all cases, it is certain that the links in the downlink direction, central A to subscriber B, must be able to ensure a transmission rate much higher than the uplink, subscriber B to central switching office A.

The unidirectional links L1 and L2 shown separately by way of illustration can be merged into a single bidirectional link, as in the case of the approaches of the known art which have been described in relation to FIGS. 1 and 2. bidirectional Ca transmission cable
To fix an order of magnitude, and in the case of frequency multiplexing of the signals passing through the links L1 and L2, the bandwidth required in the downward direction AB is of the order of 100
MHz, while in the upstream direction B> A, it is less than 90 MHz, the signal-to-noise video ratio being greater than 40 dB and the central distance A subscriber B being of the order of 3 km.

 As is well known, an optical fiber is constituted by a central light guide called a heart surrounded by a sheath intended to prevent light rays from leaving the fiber, the assembly being protected by an additional sheath, for example made of silicone . The core has a different refractive index than that of the cladding. There are essentially two types of optical fibers: high quality gradient index fibers of the so-called "telecommunication" type and index jump fibers.

 With regard to the fibers of the first category, with an index gradient, the refractive index of the core of the fiber varies according to a profile of parabolic shape, if one moves on a straight line passing through the center of symmetry of the fiber, the index being minimum at the periphery and maxirnum at the center. This type of fiber is obtained for example by the process known as "MCVD" of Anglo-Saxon rexpression: "modified chemical vapor deposition", which can be translated by "modified gas phase deposition process". The core is generally based on different silica compounds and the sheath surrounding it in pure silica.

 Regarding the index hopping fibers, on the contrary, the core has a constant refractive index whatever the place where one is placed. These fibers generally have a larger diameter than the fibers of the first type and are of a lower cost price than these. The core can be made of pure silica and the sheath of silicone.

 According to one of the most important aspects of the invention, the cable ensuring the bidirectional connections comprises two fibers, one with an index gradient of small diameter, and the other with jump index of large diameter.

 The downlink connection, central A-subscriber B, is carried out by index gradient fibers, capable of providing high-speed information transmission, the uplink connection, subscriber B-central A being provided by a fiber of the index jump type.

 A cable cable arriving at the subscriber must also be low cost while having sufficient mechanical strength for the handling of installation it will have to bear. According to the invention, the large diameter index hopping fiber is used as a central carrier, because of its high tensile strength, the index gradient fiber being collinear, according to a first variant, to the carrying fiber , or arranged in a helix around this fiber, according to a second variant.

 Figures 4 and 5 illustrate, respectively, the first and second alternative embodiments of a cable according to the invention.

 In FIG. 4, the cable C a ensuring the bidirectional links between a center A and a subscriber B comprises two optical fibers fl, with index gradient, and f2, with index jump, the latter fiber being used as a carrier element . These two fibers fl and f2 are surrounded by a protective sheath 1 which will be described in relation to FIG. 6. The light emission is ensured, for example, by a semiconductor laser La for uplink transmissions A> B. The emitted radiation is detected in the subscriber by a photodiode D2 of the "PIN" type. For downlink transmissions BA, the light emission can be ensured by a simple light-emitting diode LED, the radiation emitted is also detected by a photodiode of the "PIN" type ", D1.

In FIG. 5, only the fibers fl and f2 have been shown to illustrate an exemplary embodiment of cable according to a second variant of
The invention.

 References 10 and 11 identify in Figures 4 and 5, respectively the core and the cladding of the jump fiber of index f2.

 FIG. 6 illustrates in section in more detail the structure of the transmission cable according to the invention, whatever the variant chosen. This figure shows the two optical fibers f1 and f2, the latter serving as a support element. The fiber f1 is sheathed with silicone and the fiber f2 is coated with a copolymer of ethylene and tetrafluoroethylene, the assembly of these two fibers being itself surrounded by a sheath 2 of the same material. Finally, Ensemble is protected by a sheath 1 made of polyvinylidene chloride PVC. Within this sheath, aromatic polyamide fibers can be provided to increase the mechanical resistance of the Ca cable.

These fibers 3 have a mechanical resistance comparable to that of steel, with a very low elongation factor
The advantages of the invention can be summarized as follows:
All the optical components: fibers, opto-electronic components, transmitters and detectors, are components in common use OR in the course of industrialization. There are therefore no specific components in the system of the invention, which tends to decrease the cost of connections
The large diameter jump optical index f2 'also serves as a carrier element, which simplifies the structure of the Ca cable.
The optical connection devices required to couple the subscriber station to the network have simplified and therefore reduced the cost of coupling. Indeed, the connection of the gradient index fiber, whose core diameter is d about 50 m with the "PIN" photodiode, the active diameter of which is several hundred micrometers, poses no alignment problem due to the wide margin of mechanical tolerances available. One can use for example in the optical connection member performing the coupling between the cable of bidirectional connections Ca used by the invention and the photodiode "PIN", an intermediate fiber having a core diameter of several hundreds of micrometers.

This coupling between the light-emitting diode LED and the jump fiber of index f2 can be compared to a coupling between two identical fibers f, f, illustrated in FIG. 8, having a large core diameter. Effects by placing oneself in a case unfavorable, the active diameter of the core of a fiber with index jump is greater than or equal to 200 m and the light-emitting diode
LED has an active diameter of around 200 m. The coupling losses Pe in decibels are then analogous to the losses due to the misalignment of two identical fibers. The losses P e in decibels as a function of the axial misalignment in m are plotted on the curve of FIG. 7. In a more favorable case, the diode having an active diameter of the order of 100 m for example for example, there follows a margin of mechanical tolerances of about 50 m. Connection to the subscriber therefore requires only inexpensive connectors and easy installation, without requiring delicate adjustments by experienced tradespeople.

 To illustrate the invention, a cable distribution network incorporating the two-way optical fiber optic link system of the invention was tested and the main characteristics of the system are given in the table at the end of this description.

The invention is not limited to the exemplary embodiments which come
to be described. In particular, the optical link cable used by the invention
tion may include more than two fibers and, for example not limiting,
several fibers with an index gradient f1 allowing to carry a greater
number of information in the downward direction AB.

A. OPTICAL CABLE Ca
AI MECHANICAL CHARACTERISTICS
- outside diameter # mm
- maximum radius of curvature # 50 mm
- maximum authorized traction 20 kg
- number of fibers: two - a gradient gradient fiber f
a jump fiber index f2
A.2. GRADIENT WITH INDEX f1 (type "MCVD")
- optical attenuation at 840 nm <3db / km
- digital aperture + 0.2
- core diameter &num; 50 m
- diameter of the optical sheath &num; 123 m
-diameter of the protective silicone sheath w 400 m
- bandwidth> 800 MHz.km
A.3. Fiber with index jump f2 (silicone-silicone type)
-attenuation at 840 nm <5bd / km
- digital aperture + 0.25
- core diameter &num; 200 m
- diameter of the optical sheath &num; 400 m
- diameter of the protective sheath &num; 600 m
- bandwidth &num; 20 MHz.km
B. TELEVISION NETWORK
B.1. General
- wavelength used # = 840 nm
- cable length 3 km
B.2. Central switching link A - Subscriber B
- bandwidth required> 300MHz
- optical signal level available at the subscriber> -15 dbm
B.3. subscriber link B - switching center A
- bandwidth required> 10 MHz
- optical signal level available
at the switching center> - 30 dBm

Claims (7)

 1. System of bidirectional links by optical fibers between a first station (A) and a second station (B), each station comprising opto-electronic means for transmitting and receiving radiant energy; system characterized in that the radiant energy transmission-reception means are optically coupled to a single connection cable (Ca), connecting the first (A) and second (B) stations and comprising a jump optical fiber index (f2) and at least one optical fiber with an index gradient (f1).  2. System according to claim 1, characterized in that the cable (Ca) comprises an axial fiber, with index jump (f2), constituting the carrying element of the cable and at least one index gradient fiber (fl ), collinear to the index hopping fiber (f2).  3. System according to claim 1, characterized in that the cable (Ca> comprises an axial fiber, with index jump (f2), constituting the carrying element of the cable and at least one fiber with index gradient (fl ), wound in a helix around the index hopping fiber (f2).  4. System according to any one of claims 1 to 3, characterized in that the cable (Ca) further comprises at least one protective sheath (1) made of plastic material surrounding the optical fibers and in that fibers (3) made of aromatic polyamide material, embedded in the plastic material of the sheath (1), are arranged parallel to the indicelf jump fiber2).  5. System according to claim 4, characterized in that the cable comprises a first protective sheath (2) made of a copolymer of ethylene and tetrafluoroethylene surrounded by a second sheath (1) made of polyvinylidene chloride.  6. Cable television network comprising a first central station (A) comprising at least transceiver devices (A1, A2, A3) video signals (TV1, TV2, VP); at least one second station constituted by the station of a subscriber connected to the cable television network, comprising receivers of these video signals (TV1, TV2, VP) as well as at least transmitters of command signals (CTV, CVP) and a bidirectional fiber optic link system comprising optoelectronic means for transmitting and receiving radiant energy and a link cable (Ca) according to any one of claims 1 to 5; network characterized in that the radiant energy transmission-reception means of the first station (A) comprise a laser diode (La) and a photodiode (D1), coupled respectively to the index gradient fiber (f1) and to the index hopping fiber (f2) of the connecting cable (Ca); and in that the radiant energy transmission-reception means of the second station (B) comprise a light-emitting diode (LED) and a photodiode (D2), coupled respectively to the index-hopping fiber (f1) and to the index gradient fiber (f2).  7. Cable (Ca) for bidirectional optical links between a first station (A) and a second station (B) characterized in that it contains an index-hopping optical fiber (f2), constituting the carrying element of the cable, and at least one optical fiber with an index gradient