AU622523B2 - Optical subscriber loop system - Google Patents

Description

COMMONWEALTH OF AUSTRALIA PATENTS ACT 1952-1969 COMPLETE SPECIFICATION FOR THE INVENTION ENTITLED C ~5 Ct t t t6 C it "OPTICAL SUBSCRIBER LOP SYSTEM4" The following statement is a full description of this invention, including the best method of performing it known to us:i; This invention relates to an optical communication system for transmitting subscriber-assigned information signals, particularly telephone signals, in both directions between a centre and subscribers and for distributing information signals, particularly television signals, from the centre to subscribers, wherein the information signals to be distributed to the subscribers and the subscriber-assigned information signals are transmitted from the centre to the subscribers over first optical waveguides using frequencydivision multiplexing, and wherein the subscriber-assigned information signals are transmitted from the subscribers to the centre over second optical waveguides.

Such a system is known from IEEE Transactions on Communications, Vol.

COM-29, No. 6, 1981, pages 868 to 885, and in particular from Figure 1.3 on page 878 and its accompanying description.

:oao That describes a system for the distribution of TV signals from an exo change to subscribers connected to it, and for the two-way transmission of 0o0 telephone and data signals between the subscribers and the exchange, which in practice is the local telephone exchange. Every subscriber connected to the I l exchange is connected to it by means of two optical fibres. A first optical fibre (shown at the top of Figure 13 in the above article) transmits the "forward" direction signals to the subscribers, that is TV signals and the subscriber telephone and data signals; as shown in Figure 13(b), transmission uses frequency division multiplex techniques. A second optical fibre trans- 0o c °4 mits the "backward" direction signals, namely the telephone and data signals going from the subscribers to the exchange. Thus the system provides one optical fibre for the forward direction and one for the backward direction, for each subscriber.

Therefore, an optical transmitter and an optical receiver is required at the exchange for each subscriber, resulting in high costs.

It is, therefore, an object of the present invention to provide an optical subscriber loop system of this type (distribution of TV and sound pro- 2' t 1 I i 0 0o a o n 0o 06 0 O 0 000 grams, and a two-way telephone and data service) which can be built at a lower cost than the known system.

According to the invention there is provided an optical conmmunications system of the abovementioned type, wherein the subscribers are arranged in groups, and the signals to be transmitted from the centre to a group of subscribers are combined at the centre into a frequency-division multiplex signal having subscriber-assigned carrier frequencies for subscriber-assigned signals, which is transmitted to the vicinity of said subscribers over an optical waveguide common to said group of subscribers, said optical signal being distributed to the subscribers by optical means and over individual optical waveguides at each subscriber, means are provided for converting the received 0 optical signal into an electric signal and for extracting from the subscriberassigned signals the signal intended for said subscriber, the signals to be *9 transmitted from the subscribers of said group to the centre modulated carriers having subscriber-assigned carrier frequencies, said modulated carriers being converted into optical signals having approximately the same wavelengths, which are transmitted over individual optical waveguides to a common optical waveguide into which they are coupled by optical means and over which they are transmitted to the centre, and wherein the centre includes means for 2Q converting the received composite optical signal into a composite electric signal and for demodulating the latter.

It should be observed that the concept of optically distributing the for- Q0 ward transmission signals to a group of subscribers and optically collecting the signals to be sent from them to the exchange in the backward direction, is known, however, the forward transmission is digital and uses time division multiplex, which the backward transmission uses individual wavelengths for the subscribers and wavelength multiplex.

The invention will now be described in more detail with the aid of the drawings, in which:- Figure 1 shows the basic design of the system according to the invention; 00 00 06 a a 00 a a c 0 00« 0 o Or i 3

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s l- l^ Figure 2 shows a first alternative to Figure 1; Figure 3 shows modulation and multiplex equipment to produce a frequency band containing telephone and data signals for transmission from the exchange to the group of subscribers; Figure 4 shows a first design example of modulation and multiplex equipment for transmitting TV, telephone and data signals from the exchange to the group of subscribers; Figure 5. shows a first design example of demodulation equipment corresponding to the modulation equipment of Figure 4; Figure 6 shows modulation and demodulation equipment for telephone and data signals at the subscriber end; 0000 Figure 7 shows demodulation equipment for telephone and data signals 0* which is provided at the exchange for each subscriber; 0,4*09 0 0 Figure 8 shows a second design example of modulation and multiplex equipo0 ment for transmitting TV, telephone and data signals from the exchange to the 0 04 a group of subscribers; Figure 9 shows demodulation equipment corresponding to the modulation o0. equipment of Figure 8; 0.0:o Figure 10 shows a second alternative to Figure 1; Figure 11 shows schematically the subdivision of the frequency division multiplex signal into sub-bands; Figure 12 shows an arrangement of several converters for transforming the o sub-bands into optical signals; Figure 13 shows a first protection circuit for detecting a break in the optical fibre; Figure 14 shows a second, alternative protection circuit for detecting a break in the optical fibre.

In Figure 1, the left hand side shows the equipment provided at the exchange for a group of subscribers, while the right hand side shows the equipi 30ment provided for one subscriber in this group. The exchange is given the 4 reference 100, and all the equipment at the subs criber, shown as installed in a single house, is referenced together as 111. An optical fibre 112 which is common to a group of subscribers, leads to a coupler 113, and from there individual fibres L 1to L klead to the individual subscribers in the group. In this way, transmission takes place of all the signals transmitted from the exchange to the subscribers, that is the TV and sound program signals distributed to all subscribers, and the telephone and data signals for individual subscribers.

In the backward direction, i.e. from the subscribers to the exchange, an optical fibre runs from each subscriber in a group to a coupler 114, as shown for subscriber 111. This couples the individual signals received from the subscribers (over individual optical fibres ELl to LI into an optical fibre

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115 which is common to the group of subscribers and carries the total signal to the exchange. The couplers 113 and 11)4 are in the simplest case the welltootI Ot known star couplers, constructed from optical fibres. The couplers can be parts of pre-assembled modules or can be considered cable accessories. Sometimes they are themselves considered as modules.

*0I4 Instead of simple star couplers, which can be considered as passive mod- Sules, active modules may also be used. These, for example, would contain one more optical amplifiers. An active module could also contain an optoelectric converter, an electrical amplifier and an electro-optic converter, the latter with branching into several outputs. In some cases, such an exam- Sple could include distribution of the electrical signal to several electrical o transmission lines, in which case several electro-optic converters would be required.

Star couplers or modules are required near a group of subscribers, which can mean that they will be installed in a cellar, for exaxmple, if the group of subscribers lives in a multi-family dwelling. This is particularly desirable in the case of active modules.

i~ In the following, the equipment in the exchange 100 is described. A cable TV terminal 116 delivers an FDM signal at its output, which preferably contains 35 TV and 30 short-wave programs in accordance with the current Deutsche Bundespost cable TV standard. This FDM signal, however, is not disj tributed to the subscribers over coaxial cables, but according to the inj vention is applied to modulator/multiplexer 117. This equipment 117 adds telephone and data signals to the signal from the cable TV terminal 116, as described below.

These telephone and data signals, which are to be transmitted to the group of subscribers, originate in a local exchange 118; they are applied to modem 119 which produces a frequency band which in turn is combined in the *coo S0000 modulator 117 with the FDM signal from the cable TV terminal.

S At the output of modulator/multiplexer 117 there is therefore an elec- S trical signal which contains the programs to be distributed and the subscriber-specific telephone and data signals to be transmitted to the group of subscribers. This signal is converted in electro-optic converter 120 to an optical signal (by intensity modulating the output light) and transmitted over the optical fibre 112 to coupler 113; there it is divided for, say, five subp scribers (k 5) and transmitted to the subscribers over individual optical fibres LI to Lk. In the drawing, optical fibre L 3 is shown connected to Subscriber 111.

At the subscriber, the received optical signal is converted into an electrical signal in an opto-electric converter 121. By means of filter 122, the y telephone signals are diverted to demultiplexer 123, whereas the TV and soundprogram signals are recovered in demodulator 124 to their original form, as produced at the cable TV terminal; they are then presented at an interface where the responsibility of the network operator stops and that of the subscriber starts. At this interface, the signals are presented in the same format and with the same quality as i3 currently required for cable TV in the Deutsche Bundespost or other telecommunication administration.

ii i L I The demultiplexer 123 separates the frequency band containing the telephone and data signals into the individual signals 1 to n n 64) and applies each signal to its own modem; only one modem 125 for one signal is shown in the drawing. This converts the signal into the appropriate standard format for the relevant terminal (telephone set or data or ISDN terminal).

The demultiplexer ensures that only the telephone and data signals intended for the subscriber's terminals are sent to them. In the simplest case, the subscriber has only a single terminal a telephone) and therefore a single modem 125 which receives one of the signals from the output of the Q demultiplexer 123.

j 1 In many subscribers live in the same house, the equiments 123, 125, 126 S and 127 are installed at the cable TV interface and the subscribers are connected via the existing cabling in the house.

The telephone or data signals being sent from the terminal to the ex- S change, are modulated onto a carrier with a subscriber-specific frequency by means of the modulator in modem 125. If the subscriber has several terminals, e.g. several telepones, then several modems modulate several different carriers and a multiplexer 126 combines the signals into a single frequency band.

S Either a single modulated carrier, or a frequency band containing several modulated carriers, is then converted by electro-optic converter 127 into an optical signal, which is then transmitted to the coupler 114 over an individual optical fibre 128 provided for each subscriber. The subscribers in a group use an individual carrier frequency for each terminal they have. For example, 64 such carrier frequencies may be provided, or some other number, say m. A total of m terminals (telephone or data) can therefore be operated by the subscribers in a group. Allocation to individual subscribers is in accordance with individual communication requirements.

The electro-optic converters 127 of the different subscribers all operate with the same wavelength i, so that the coupler 114 receives e.g. 64 optical PA_, signals with the same wavelength X 1 which, however, differ depending on the JfC, L frequency (frequencies) of the carrier (group of carriers) applied to each particular electro-optic converter 127.

This concept of transmitting different signals over a single optical fibre is already known.

The optical fibres which lead from the subscribers in the group to the coupler 114 are referenced LL1 to IL n. Their number does not have to match the number of optical fibres L I to Lk. There can be, say, up to m optical fibres; it is useful to make m as large as possible, e.g. m 64 to 128, and also to group as many as possible of the telephone or data signals together into a composite signal, after their modulation onto the carriers; this results in the smallest number of electro-optic converters and optical fibres o° L to 11 n for the transmission to coupler 114.

In the exchange there is an opto-electric converter 129 which converts ,o the received composite optical signal having a single wavelength, into a como posite electrical signal with differing carrier frequencies and applies it to 0e o the input of modem 119. This separates the composite signal into its individual signals, demodulates them and provides them at its outputs 64 outputs) in a form suitable for application to the inputs of tbe local exchange 118. At the inputs of this local exchange 118, the telephone and data signals 2Q appear in the same format and with the same quality as is prescribed for the transmission of telephone or ISDN or data signals.

Figure 2 shows an alternative to Fig. 1 in which both the optical fibre Si 112 which is common to the subscribers, and the optical fibres k to Lkwhich are individual to the subscribers, are used for transmission in both directions. For transmission in the forward direction, a first wavelength Xo is used 1300 nm), and for transmission in the opposite direction a second wavelength Xi is used 800 nm); as in the system of Figure 1, the latter wavelength is common to all the subscribers in the group. Both at the exchange and at the subscribers, there are wavelength-selective couplers 220 and S 30 221 which can separate the two wavelengths X0 and Xi without any large loss A -i and with very low crosstalk. In other respects the system of Figure 2 does not differ from that of Fig. 1, so that the design examples described below apply to both systems.

With the aid of Figure 3 and Figure 4 the modulation and multiplex techniques will now be described which are used in the modem 119 and the modulation and multiplex equipment 117.

Fig. 3 shows the modulator/multiplex section of model 119 for telephone and data signals, as provided in the exchange. The phrase "telephone and data signals" is taken to include: analogue signals from a conventional telephone 1 0 set, 64 kbit/s digital signals from a digital telephone set, 144 kbit/s signals from an ISDN terminal and digital data signals of 2 to 34 Mbit/s.

0 o The signals coming from the local exchange (118 in Figure 1) over a normal 2-wire line 201 are transformed in a so-called adapter 202 into a form suited to transmission via a -wire system with freuenc modulation. The 0 suited to transmission via a 4-wire s.vstem with frecuency modulation. The o 0 0000 0 ®0 0 000 adapter 202 basically fulfills the function of a converter, e.g. for 2-wire/4-wire conversion, for dialling signals, ringing signals, etc. The signal resulting from this conversion is applied to the signal input of a modulator 203 and there is frequency modulated onto a carrier with frequency f This carrier frequency fl is individually allocated to a telephone connection of one of the subscribers in the group of subscribers shown in Figure 0 0* So a 1: SO F 0 0 00 0 0 ft04 as e inr As outlined above, the subscribers in this group can also have other con- 0o0°0 nections than to the telephone system, e.g. access to a digital network such S as ISDN (Integrated Services Digital Network). In that case, the digital ISDN signals are applied via a further 2-wire line 204 to an adapter 211 for ISDN signals; the digital output signals from this are sent to the signal input of a modulator 205 suitable for digital signals, where it is modulated onto a carrier with frequency f 3 2 which is individually allocated to this ISDN subscriber; either phase shift keying (PSK) or frequency shift keying (FSK) may be used.

9 In the example just described, a first group of 32 carriers is formed with different frequencies in the range 20 to 30 MHz, which are either frequency modulated with analogue telephone signals, or phase or frequency modulated with digital'] ISDN) signals. The example shows one group in which a carrier wiith frequency f 1is frequency modulated by telephone signals and a secoud group in which a carrier with frequency f 32 is phase modulated by ISDN signals. Any arbitrary combination is possible.

In the example shown, a first combiner 206 groups a maximum of 32 modulated carriers with frequencies f I to f 32 into a frequency band from 20 to MHz, and a second combiner 207 similarly groups a maximum of 32 modulated carriers with frequencies to f into the frequency band from 20 to 30 MHz.

32 oco4c a p By means of converter 208 and filter 209, one of these frequency bands is then 00 00 0 99 shifted to the frequency band from 30 to 40 MHz; both these frequency bands 000000 000-are then combined in combiner 210 into the band 20 to 40 Miz which can accom- 0modate up t6ktlpoeaddtsins.This output is the output signal of the equipment referenced as 119 in Figure 1, which is then applied to the modulation and multiplex equipment 117, which is also shown in Figure 1 and 00: whose operation will now be described with reference to Figure 4b.

ro: This equipment 117 (Figure 1) receives the composite cable TV signal In the form in which it is currently applied to the cable TV network of the 00)Deutsche Bundespost. It contains up to 35 TV signals which ara amplitude modulated onto different carriers, and the VHF' band 87 to 108 MHz. Altogether a S0frequency band from approximately 4b5 to 450 MHz is involved.

According to a first method -called single-channel FM the band is di-* vided by means of filters F into the WHT band and the 35 modulated carriers containing the TV signals (denoted TV1 to TV35), AM demodulators Dl to demodulate the anplitude-iodulated carriers and thereby recover the baseband signal. Following this, FM modulators Ml to M35 frequency modulate these baseband signals with different frequencies. By means of converter 300, the VHF band is shifted to a suitable band which is then modulated onto a further F't I r

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1) 4* 4, #1 01 #4! carrier by modulator D If the frequency conversion is suitably chosen, the modulator M may be omitted. The filters F may be included in the AM demodulators Dl to D35 or in the VHF band converter 300.

The frequency modulated carriers thus produced are combined in power combiner 301, resulting in a composite TV/radio signal which is then applied to several 10) combiners, of which only one has been shown, denoted 305.

The composite signal at the output of combiner 301 is preferably arranged to have all the carrier frequencies lie within one octave. Each such combiner 305 adds to the composite TV/radio signal a telephone/ISDN band with bandwidth 20 to 40 MHz; thus each combiner produces a composite signal destined for a group of subscribers, containing the TV and radio signals sent equally to all subscribers, and the telephone and or signals specific to a group of subscribers. For example the composite signal transmitted to a particular group of subscribers over the path shown in Figure 1, appears at the output marked of the combiner 305; other composite signals appear at the outputs of other a combiners for transmission to other groups of subscribers over correspsonding transmission paths.

All these composite signals are so arranged that the mnodulated carriers p, have frequencies which lie within an octave. This may mean that the composite signal of telephone and data signals has to be transferred to a much higher band than 20 to 40 MHZ before it is added to the TV/radio signal.

The transmission of a number of TV signals by frequency-modulating dif- S' ferent carriers and then intensity-modulating this composite signal in an optical transmitter is known from "Electronics Letters", 22 October 1987, Vol.

23, No. 22, pages 1196 to 1197. The transmission of telephone and data signals is not mentioned there.

With the aid of Figure 5, we will now describe how the composite signal received by a subscriber is converted back to the standard frequency range.

First, the telephone and data signals are separated from the composite signal by filter 122 of Figure 1 (not shown in Figure the remaining signal, e 4 4 r, i i _i _i i ~I ~L;I containaing the TV and radio signals, is applied to an arrangement of FM demodulators FDl to FD35 and FDr, whereby the demodulators FD1 to FD35 convert the TV signals back to the baseband. The baseband signals are then modulated, using vestigial sideband modulation in amplitude modulators AMI to AM35, onto the different carriers as specified currently for cable TV. One FM demodulator FD demodulates the VHF-band carrier and a converter 401 converts the signal back to the normal VHY band. The resultant signals TV1 to TV35 and the VHF band are then grouped together by combiner 400 into the standard cable TV signal format.

With the aid of Figure 6, the processing will now be described of the telephone and data signals which were separated out from the original signal.

o O The frequency band 20 to 40 MHz containing these signals is pre-filtered by filter 500 which is set to the individual frequency allocated to the subscriber being considered; the desired carrier is thus selected from the composite signal, but only to an inadequate degree, i.e. the other carriers are still present but at low level. In frequency converter 501 this signal is mixed with local carrier HT, this frequency being so chosen that the desired carrier is converted to the same standard intermediate frequency for all carriers, preferably 10.7 MHz. The subsequent IF filter 502, set to this intermediate frequency, then isolates the desired signal adequately from all the other interfering carriers. After demodulation by demodulator 503 the telephone or data signal is finally made available in its original baseband form.

r 4° Q *In the case of telephone access the demodulator 503 is an FM demodulator, S*i while an ISDN connection uses a PSK or FSK demodulator.

The conversion to an intermediate frequency of 10.7 MHz and subsequent filtering and FM-demodulation corresponds to the signal processing in today's conventional VHF broadcast receivers; it therefore has the advantage of being able to use proven, low-cost equipment. A further advantage lies in the fact that all subscriber connections can use standard demodulators and IF filters *q 30 and, apart from the local carrier frequency, standard frequency converters.

12 k L -I The telephone or ISDN signal appearing at the output of demodulator 503 is finally so transformed in adaptor 504 (whose function was already described in connection with Figure 3) that signals meeting the normal standards are applied to the subscriber's equipment (telephone set or ISDN terminal).

The signals to be transmitted to the exchange from the subscriber are so transformed in adaptor 504 and the modulator 505 that a modulated carrier is formed that has a frequency fl allocated specifically to that subscriber in that local group; the modulator is either an FM modulator (for telephone signals) or a phase shift keying or frequency shift keying modulator (for data signals). Tis modulated carrier is then in the simplest case applied directly to the electro-optic converter 127 shown in Figure 1 for this subscriber; if the subscriber has several terminals, the signals are combined in multiplexer 126, so that in tnis case the laser in the converter 127 is intensity modulated by a composite signal comprising several modulated carriers.

The modulators and demodulators shown in Figure 3 and Figure 6 can be switchable so that they can process either telephone signals by frequency modulation, or data signals by frequency or phase shift modulation.

I ;Figure 7 shows the demodulator section of the equipment 119 of Figure 1.

The optical signals received over the optical fibre 115 in Figure 1 or the optical fibre 112 in Figure 2, after conversion to an electrical signal in the opto-electric conveerter 129, is distributed to ran separate lines (eg. m= 6 4), since it may contain up to 64 telephone or data signals. This composite sig- 1 nal is applied to several demodulation circuits, of which only one is shown.

t Demodulation takes place using pre-filter 600, IF converter 601, IF filter 602 and demodulator 603, as already explained in connection with correspondinglynamed circuits in Figure 6. The telephone or data signal at the output of demodulator 603 is finally transformed in adaptor 202 (already described in connection with Figure 3),in such a way that it may be sent over the 2-wire line leading to the local exchange.

13 Using Figures 8 and 9, a second version will now be described of the modulation and multiplex equipment 117 of Figure 1, and the corresponding equipment at the subscriber end. The composite signal containing the TV and radio programs in the form currently specified for cable TV, is distributed to lines (as shown in the example of Figure if the exchange is to supply subscriber groups as shown in Figure 1. For each line, there is a combiner which adds to the composite signal a signal consisting of m. telephone and data signals produced as shown in Figure 3. The m telephone and data signals occupy the band 20 to 40 MHz, as explained above, which is outside the cable TV band. The output of each combiner (only one is shown, marked 705) now has a composite signal consisting of the TV and VHF radio programs and the combined telephone and data signals for one group cf subscribers. Each such composite signal occupies a band from 20 to 450 MHz.

Each such composite signal is applied to a broadband FM modulator 706 which has a carrier of 5 GHz, for example, and uses a deviation of, say, GHz. The outputs of these broadband FM modulators are the outputs of 'the modulator 117 shown in Figure 1; for example, at output A5 there appears the t t4A f e u n y m d l t d c r i r w i h co t i s t eT.n B a i r g a s a wel asthetelephone and ISDN signals for the group of subscribers shown in Figue 1.Thisfrequency-mnodulated carrier, for example with a bandwidth of .5 ~z owintensity modulates the electro-optic converter 120 shown in Figure 1 or Figure 2.

As shown in Figure 9, at the subscriber end the composite signal delivj eed b theopto-electric converter 121 is demodulated in a broadband FM demoulaor 00;the telephone and data signals are selected by filter 801 s0 that the cable TV signal can then be provided at the interface point TfulI in the correct standard format.

In accordance with a third method referred to as "AMv-IM"1 there is a third version of the modulation and multiplexing equipment 117, and the corresponding subscriber equipment. It differs from that in Figures 8 and 9 in that the broadband FM modulators and demodulators are omitted. In this method, the cable TV band is combined with the m telephone and data signals and then applied without any further conversion directly to the electro-optic converter to intensity-modulate the output. The modulation signal is, therefore, in this case a composite signal in the band from 20 to 450 MHz containing amplitude-modulated carriers for the TV programs, frequency-modulated carriers for the VIH radio programs and either frequency- or phase-modulated carriers for telephone and data signals.

In the following, some modifications and supplementary information are discussed for the system of Figure 1.

Figure 10 shows a modification concerning the transmission from the exchange to the subscribers in the group. Instead of the coupler of Figure 1, which distributes the optical signal for the subscribers to separate optical fibres LI to Lk (Figure the modification shown in Figure 10 provides the module 900; in this, the optical signal is transformed into an electrical signal, converted into a signal of correct standard format (apart from the telephone and data signals) and the transmitted to the subscribers over electrical transmission lines, eg. coaxial lines. To achieve this, the module 900 t contains an opti-electric converter 901, and a demodulator 902 which demodu- 2 lates the FM signal to receiver the standard format signal (apart from the telephone and data signals), as originally produced at the cable TV terminal; in other words, the module fulfils the function of the demodulator in Figure or that of the broadband FM demodulator 800 in Figure 9. With amplitudemodulation/intensity-modulation the demodulator 902 would be omitted. After demodulation, the signal is distributed to the coaxial lines K 1 to K 7 which are equal in number to that of the subscribers in the group, and carry the signal to the subscribers. Figure 10 shows that one coaxial line, K 4 leads to Subscriber 111. There a filter 122 merely selects the telephone and data signals out of the frequency division multiplex signal; that is, no processing of the actual cable TV signals is done at the subscriber end, having been 1 I carried out for all the subscribers in the group, in a single module 900. In other respects the modified version of Figure 10 does not differ from the system of Figure 1, so that no further explanations are needed.

Naturally the module 900 can, for example, be installed in the cellar of a multi-family dwelling.

One variation concerns the choice of the frequencies for the transmission of the telephone or data signals, both in the forward and backward direction.

Included in the term "telephone or data signal" are all the signals specified above. As mentioned above, the carriers for the transmission of these signals lie, for example, in the frequency range 20 to 40 MHz. The carrier frequen- Scies used for the transmission of both the analogue telephone signals and the digital signals with bitrates from 64 to 144 kbit/s should preferably have a uniform space, i.e. the carrier frequency spacing should be 300 kHz. In this way, the provden circuits from VHF radio systems can be used for the trans- 0: mission of the analogue signals by frequency modulation. The carrier frequencies for transmitting digital data with 2 Mbit/s are preferably placed in the upper part of the frequency band from 20 to 40 MHz for example, at 38 MHz; 0 ao the modulation for transmitting a 2 Mbit/s digital signal requires a bandwidth of 1 to 2 MHz and therefore a separation from adjacent carriers of 1 to 2 MHz.

o*o24 A further variation concerns the electro-optic converters used in the system. If their characteristics are not sufficiently linear, they produce o a 0 intermodulation products at frequencies in the transmission band, causing 0*0 problems thereby. These problems become more serious the wider the frequency spectrum of the input signal. In order to improve the linearity of the electro-optic converters, each of the converters is equipped with a linearising circuit in which the linearising results from negative feedback of the optical signal, such as is known from eg. US-PS 3 996 526.

A further modification concerns the transmission of the optical signal which contains a number of modulated carriers. In the above discussion of the example of equipment 117 (Figure it is assumed that a single electro-optic 16 i. i I I converter 120 receives at its input a frequency division multiplex signal containing a number of modulated carriers. This either consists of a number of frequency modulated carriers, produced as shown in Figure 4, or is simply the standard cable TV band with the telephone and data signals added-in, as discussed above. In both these cases, the input signal to the electro-optic converter 120 is a broadband composite signal containing many carriers.

A problem with this method of transmission is the noise of the system, ie. with many carriers being transmitted simultaneously over an optical transmitter, the signal-to-noise ratio can become too low in each channel. This is partly due to the fact that the large number of carriers in the composite signal results in a low modulation factor for each carrier, if overloading of the BoQ e laser in the electro-optic converter is to be avoided. For a signal consisting of about 35 carriers modulated by TV programs and about 30 carriers modulated by VHF radio programs, only a small modulation factor is possible, eg.

o" 1.5% per TV signal carrier. If the modulation factor is increased, the input signal to the laser has large amplitude peaks, leading to overloading. Interaction between the desired signals then causes interference products which seriously degrade the quality of the signals.

According to one advantageous version of the invention, provision is o *2 therefore made to divide the total frequency band being transmitted into several sub-bands, each of which only contains a part of all the modulated carriers to be transmitted; each sub-band is then converted to an optical signal in its own electro-optic converter, with the same wavelength being used for all optical signals, and these optical signals are then coupled into the optical fibre 112 (Figure 1) connecting the group of subscribers to the exchange.

Figure 11 shows how the frequency band being transmitted is subdivided into the sub-bands. A frequency band containing a number of carriers is so divided into n sub-bands that each sub-band contains a group of m carriers and il 30 that thereby the total number of m x n carriers in the original frequency band i g17 i is distributed to the sub-bands as shown in Figure 11. The sub-bands can therefore have the same width as the original transmission band. However, they contain a much smaller number of carriers than the original frequency band.

Figure 12 shows that each of the n sub-bands is applied to its own electro-optic converter Wi to Wn and that their output signals are combined into a composite optical signal by means of star coupler SK and then coupled into the optical fibre 112 (Figure 1) leading to the group of subscribers. In this way each electro-optic converter, or more precisely the laser therein, can be dirven in an optimal manner with respect to intermodulation and noise effects. Because of the smaller number of carriers, the modulation factor is now significantly higher for each carrier, so that the signal-to-noise ratio 0 tot is significantly better.

8 t ,The wavelength of the converters W I to Wn is preferably chosen to be the ^l same, eg. 1300 nm. As shown in Figure 1, at the receiving end the transmitted composite optical signal is converted to an electrical signal, using a single converter 121; this signal contains all the original carriers, but with a far better signal quality than with the method shown in Figure 1.

t In connection wi..th this transmission method it should be noted that the separation into sub-bands of a broadband frequency division multiplex signal, and the separate conversion of the sub-bands into optical signals, is known.

However, optical signals with different wavelengths are produced and transmitted over a single optical fibre. On the other hand, several non-overlapping frequency bands are converted, each by its own electro-optic converter, into optical signals with the same wavelength; these signals are then combined using a star coupler and transmitted over a single optical fibre.

Concerning the choice of wavelengths, it is also possible to choose a different wavelength for the TV signals than for the telephone and data signals, ie. to transmit the different types of signal by wavelength multiplex; this means that at the receiving end an optical receiver must be provided for the TV signals as well as for the telephone and data signals. A suitable wavelength for the telephone and data signals is 800 nm.

The use of several lasers, as just described, has a further advantage, namely a higher optical power level at the output of the star coupler; this allows larger optical attenuations to be overcome and simpifies the distribution of optical signals to several subscribers by means of a star coupler.

In the following, an additional variation is described that concerns the safety of system users in the event of a break in the optical fibre. Since lasers are preferably used as optical transmitters in the system, they must be switched off in the event of interruptions, eg. if the cable is broken by •o earth-moving equipment, in order to avoid the risk of eye injury and conseo quent damages claims.

With the optical conmmunication system according to this invention it may be necessary to work with a high transmitter power (more than -6 dBm) in order to achieve certain system requirements, such as for example the spanning of a minimum distance between exchange and subscribers, or the achievement of a given signal-to-noise ratio. There is no backward channel available so that ott it is not possible to send an alarm signal from the subscriber to the exchange S to switch off the laser. Also, sending back an alarm signal is impossible if the cable is defective, since each subscriber is only connected to the exchange by one cable or one fibre.

O 0 According to the invention, one of the protective circuits described be- S low can be used.

Figure 13 shows a protection circuit which provides an electrical or optical loop for the supervision of a length of optical fibre. To detect a break in a fibre between two points A and B (eg. between exchange 100 and coupler 113 as shown in Figure 13), an optical or electrical loop 181 is placed in the immediate vicinity of the fibre. For example, from the supervisory S: equipment 180 a copper wire can be run alongside the fibre 112 to the coupler 113 and back to the supervisory equipment. The latter sends a steady current 2-d

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around this loop in the normal operating state. If the cable is broken, this loop is also broken and the supervisory equipment switches off the laser in the electro-optic converter 120.

It is also possible to use the power-feeding conductors in this way to detect cable breaks. The copper conductors can either be placed in the core of the cable, or. a thick copper cable can be placed in parallel with the optical fibre cable.

Instead of copper wires, optical fibres can also be used which form part of the cable containing the transmission fibre. In that case, the auxiliary fibres are spliced together near the coupler and a fibre break is detected by the break in the optical loop from the exchange to the coupler and back.

If a copper wire loop is used and there are optical connectors in the transmission path, then they can be so constructed that the electrical loop is Sclosed when the connectors are mated.

Figure 14 shows a protection circuit which has an optical coupler 183 spliced between the electro-optic converter 120 and the optical fibre cable; the optical signal being transmitted is coupled into the transmission fibre 112 by means of this coupler. The signals reflected and scattered back from this are detected at one end of the coupler 183 by means of opto-electric converter 184. A supervisory circuit 182 processes the electrical output signal from the converter 184 and switches off the laser in the electro-optic converter 120 as soon as the detected light exceeds a given threshold. The basis of this is that an excessive level of reflected light points to a break in the optical fibre.

According to a further development of the protective circuit of Figure 14, it is possible to connect an opto-electric converter to the unused end of coupler 183. This then detects a part of the light sent out from the electrooptic converter and provides an electrical output signal that can be used as negative feedback to the laser in order to linearise its characteristic; also the low-frequency component can be used to set the laser operating point.

3 It should be pointed out that the protective circuits just described are not only useable with the system according to the invention, but can be independently considered as solutions whenever it is necessary to guard against breaks in an optical transmission line between two points A and B.

Finally one other modification will be described which concerns the twoway transmission of telephone and data signals between the exchange and the subscribers. The transmission of these signals has been described above as taking place by a frequency division multiplex method, using carrier frequencies specific to each subscriber.

However, the signals can instead also be transmitted by time division or code multiplex methods.

If a time division multiplex system is used for transmitting from the exchange to subscribers, the signals to be transmitted to the subscribers are grouped together at the exchange into a digital time division multiplex signal 0 with a bitrate of eg. 8 Mbit/s; this is then added to the cable TV signal in the modulation and multiplex equipment 117 (Figure 1) and transmitted, possibly being modulated onto a carrier and/or limited to a suitable frequency band g by means of a transmission code. Analogue telephone signals are converted to digital signals before the formation of the time division multiplex signal.

'C At every subscriber this RDM signal is selected by means of filter 122 and every subscriber has appropriate demultiplexers which select the signals intended for him.

If code multiplexing is used to transmit the telephone and data signals from exchange to subscribers, the digital (or digitised) signals for each group are multiplied by an address code and grouped into a composite code multiplex signal by means of an adder, which is added to the cable TV band in the modulation and multiplex equipment 117 (Figure At the subscriber end this signal is selected by the filter 122 (Figure 1) and every subscriber has appropriate demultiplexers which select the signal intended for him.

Code multiplex or time division multiplex can also be used for the transmission of the telephone and data signals in the opposite direction. The subscribers' multiplexers then ensure that the telephone or data signals to be sent from a local group of subscribers are either assigned to a subscriberspecific time slot in a TDM frame, or multiplied by a subscriber-specific address code, and converted into an optical signal with wavelength X1. A group of optical signals is then transmitted to the exchange which differ, not in their wavelength, but in the time slots allocated to the telephone or data signals, or in the codes of the signal (or signals) modulating the relevant elctro-optic converter 127 (Figure The exchange contains appropriate demultiplexers to separate the composite electrical output signal from the opto-electric converter 129 (Figure 1) into its individual components.

The preferred combination uses time division multiplex for the forward direction (ie. from the exchange) and code multiplex for the backward direc- S' tion.

So s2 )7 4 _*22 11 9 0 0

Claims (10)

1. An optical communication system for transmitting outgoing subscriber- assigned informniation signals from a cntre to subscribers, incoming subscriber- assigned information signals fronm tihe plu rality of subscribers to the centre and for distributing information signals from the cen ric to subscribers, wherein the informa- tion signals to be distributed to the subscri bers and the outgoing subscriber-assigned information signals are transmitted from t11he coIltic to the subscribers as a first optical signal over first optical wavegu ides using frcq uency-division multiplexing, the first optical signal being intensity rnmodulated, and whercin the incoming subscriber- assigned information signals ar transmittcd from the subscribers to the centre over second optical waveguides, whercin the subscribels are arranged in groups, and the V. S signals to be transmitted frorn the ccntr-e to a group of subscribers are combined at the centre into a frequency-division multiplex signal having, for each suscriber of a group, a dedicated subscriber-assigned carrier frcqucnicies for outgoing subscriber- assigned signals, which is transmnitcd to the \iciniity of said subscribers over an op- tical waveguide common to said group of subscribCi-s, said first optical signal being distributed to the subscribers by optical distribution means and over individual op- tical waveguides at each subscriber location, whcrein first transducer means are pro- vided for converting the received optical ignrial into an electric signal and wherein first S 20 filter means are provided for extracting from the subscriber-assigried signals the signal intended for said subscriber, the systerm including at each subscriber location elec- trical modulation means whcrcin the incominiig signals to be transmitted from the subscribers of said group to t(.he ccntrce modulatc carriers having subscriber-assigned carrier frequencies, the modulaIted carriers bcinig converted into optical signals having approximately the same wavelengths, \\wih i iarc tra nsmiittcd o\lvec individual optical waveguides to a commnion optical wavcguide into which they are coupled by optical means and over which they are transmitted to the ccntre, and wherin the centre in- eludes means for converting the rcceived composite optical signal into a composite electric signal and for denmodulating the latter.

2. A system as claimed in claim I, wherciin to transmit the optical signals in both directions, a single optical waveguicle comnimon to said group of subscribers and a single optical waveguide for each individual subscriber are used, the transmission in one direction being effected at a first wavelength and that in the other direction at a second wavelength. i 24

3. A system as claimed in claim I or 2, wherein to form the frequency-division multiplex signal to be transmitted fron the ccntrc to a group of subscribers, the centre includes modulator means for frcq ucncy-modiulating the information signals to be distributed to the subscribers onto different carriers, modulator means for modulating the subscriber-assigned information signals to be transmitted to the subscribers onto different carriers having subscriber-assigncd carrier frequencies, and mniultiplexer means for combining the modulated carriers into the frequency-division multiplex signal (Figure 3, Figure the centre iIIncluding an electric-to-optical transducer which converts the frequenicy-division multiplex signal into an optical signal by intensity-modulating the light produced by il, and at the su bscriber, democldula tors, 0*r* and converter means are provided for recovering from the frequency-division multi- S plex signal the information signals to be distributed and the subscriber-assigned in- formation signals intended for said subscriber and for converting the recovered information signals into the original form. 15 4. A system as claimed in claimn I or 2, wihcrcin to form the frequency-division multiplex signal to be transmitted from the centre to a group of subscribers, the centre includes modulator means for modulating the subscribcr-assigned information signals to be transmitted to the subscribers of said group onto different carriers having subscriber-assigned carrier frequencies, and multiplexer means for combining said modulated carriers with a composite signal con taining a plurality of carriers :amplitude-modulated with the inforniation signals to be distributed to form an overall composite signal. A system as claimned in claim 4, whrcin the centre includes a broadband fre- quency mod ulator which modulates the overall composite signal onto a. carrier by frequency modulation; the ctrec incl Lding n ectric-to-optical transducer which converts the modulated carricr into a n optical signal by intensity-miodulating the light produced by it, and at the subscriber, a broadband frequency demodulator, demodulators and converters are provided, the broadband frequency demodulator frequency-demodulating the reccived signal a ind thereby recovering the overall com- posite signal, the demodulators recovering the subscriber-assigned information signals intended for said subscriber, and the converters converting the recovered information signals into the original form.

6. A system as claimed in claim 4, wherein the centre includes an electric-to- optical transducer which converts the overall composite signal into an optical signal 44 by intensity-modulating the light produced by it, and wherein at the subscriber, demodulators and converters arce provided For recovering the subscriber-assigncd in- formation signals intended for said su bsc1ribel and for converting the recovered in- formation signals into the original form.

7. A system as claimed in any of the preceding claims, whecrein the optical signal transmitted to the vicinity of the subscribers is converted into an electric signal, de- modulated and distributed to the subscrilcrs over individual electric lines.

8. A system as claimed in any of the preceding claims, wherein the electro-optic converters contain circuits to linearise (hei'r output characteristics.

9. A system as claimed in any one of claims I to 4, 6 to 8, wherein the frequency division multiplex signal is divided into severnll sub-bands, that for each of the sub- bands an electro-optic converter is provided which converts it to an optical signal, and that the resulting optical signals are coupled into an optical fibre and transmitted S by it. system as claimed in any of the preceding claims, including an optical or elec- trical loop in parallel with an optical fibre carrying a high-level optical signal, and a supervisory circuit for one of the electro-optic converters connected to the optical Sfibre, the supervisory circuit sending light or an electric current around this loop and S switches off the electro-optic converter in the event of an interruption of the light or 0 current path. 1 1. A system as claimed in any one of the claims 1 to 9, wherein a coupler is in- serted between an electro-optic converter t'ransmitting light into an optical fibre and S the optical fibre, which couplr copl cples out reflected light from the optical fibre, and wherein a. detector and a supervisory circuit is provided which switch off the electro- optic converter when the level of the coupled-out reflected light exceeds a given threshold.

12. A system as claimed in claim I I, wherein the coupler also couples out a part of the light transmitted by the electro-optic converter, a detector being provided which transforms the coupled-out light into an electrical signal, and wherein circuits are provided which use the electrical signal for negative feedback in the electro-optic converter, the low frequency part of the electrical signal setting the operating point of the electro-optic converter.

13. A system as claimed in any of the preceding claims, wherein the multiplex signal formed at the centre is a time-division multiplex signal having subscriber- assigned time slots for the subscriber-assigned signals instead of a frequency-division WA T multiplex signal having subscriber-assigned carrier frequencies. 0 y 26

14. A system as claimed in any one o0 clainms I to 12, wherein the multiplex signal fo'med at the centre is a code multiplex suil signal having subscriber-assigned codes for the subscriber-assigned signals instead or a frequency-division multiplex signal having subscriber-assigned carricr freq ucncies. 15 A system as claimed in any of the precding claims, wherein the signals to be tansmitted from the subscribers of said group to the centr are converted into digital Fig tals having subscriber-assigned codes which are converted into optical signals in- stead of and analogously to the mod ulated carricrs, and transm itteC to the centre. 16 A system as claimed in any one of ciinims I to 14, wherein the signals to be transmitted from the subscribers of said 1group to the centre are converted into digital signals having 7ubscriber-assigncd active time periods relative to a frame which are converted into optical signals instead of anad 1anlogously to the modulated carriers, and transmitted to the centre. i r 17. An optical communication system substantially as herein described with ref- erence to the accompanying drawings. DATED THIS TWENTY-THIRD DAY OF JANUARY, 1992 20 ALCATEL, N.V. toot 4, NT0nrfi